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Basic Principles of Gating and Risering

CHAPTER 1—Filling Science
A basic understanding of fluid flow is necessary before an in-depth discussion of gating system design occurs. A fluid is
a substance that has particles which easily move and change their relative position without separation of the mass and
which easily yield to pressure. A fluid is also defined as a substance, which tends to flow or conform to the outline or
shape of its container. Fluids are generally thought of as substances that are in the liquid or gaseous state.

Molten metal is a liquid that observes the same fluid flow principles exhibited by other liquids (such as water). However,
the fluidity (ability to flow) of molten metals is affected by several factors—superheat, metal composition, metal
viscosity, surface tension, surface oxide films, absorbed gas films, suspended inclusions and inclusions precipitated
during freezing. All these factors need to be considered when designing a gating system. An understanding of the basic
scientific principles that govern fluid flow is essential to basic gating design.

BERNOULLI'S THEOREM

Bernoulli's theorem is one of the most useful theorems that relate to the design of gating systems. This theorem states
that the sum of the potential energy, kinetic energy, pressure energy and frictional energy of a flowing liquid is equal to a
constant when the entire system is completely filled. This energy balance may be expressed as follows:

Potential energy + Pressure energy + Kinetic energy + Frictional energy=Constant
Z+ P+V2/2g+F=K

where:
Z = potential energy; height of liquid, in. (cm)
P = pressure energy
P = static pressure in the liquid, lb/in.2 (kg/cm2)
 = specific volume of the liquid, in.3/lb (cm3/kg)
2
V /2g = kinetic or velocity energy
V = velocity, in./s (cm/s)
g = acceleration due to gravity, 384 in/s2 (981 cm/s2)
F = frictional energy (losses), in. (cm)
K = a constant

The application of Bernoulli's theorem is illustrated in a horizontal gating system as shown in Fig.1-1. Bernoulli's
theorem is an aid in explaining the variation in potential head of the metal, molten metal pressures, velocity of molten
metal flow and friction losses in molten metal.

Fig. 1-1. An illustration of a basic horizontal gating system.

The potential energy is at a maximum when the molten metal enters the pouring basin (Fig. 1-1). Potential energy then
changes rapidly to kinetic energy and pressure energy as the flow is established. Once the flow is established, the
potential and frictional heads are relatively constant. It is at this point that the kinetic and pressure energy becomes
significant, i.e. when the velocity is high the pressure is low and vice versa.

The behavior of molten metal in a horizontal runner can be interpreted by simplifying Bernoulli’s theorem. Assuming
that potential energy and friction losses are relatively constant, the kinetic and pressure energy in the system becomes
important. Figure 1-2 shows how this theorem is capable of explaining different observed flows from two gates located
at the top of the runner. When the runner is of uniform size along the entire length, as in Fig. 1-2, the velocity is high and
pressure is low at the first gate, and vice versa at the second gate. This condition results in an unequal volume of molten
metal to flow through the gates. In other words, more molten metal flows through the gate furthest from the sprue. The
velocity and pressure energy is equalized when the runner cross-sectional area is reduced by an amount equal to the
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cross-sectional area of the first gate. In theory, this means that an equal amount of molten metal flows through each gate,
and that the flow starts into each gate almost simultaneously, as seen in Fig. 1-3.

Fig.1-2. Uniform runner cross section is shown.

Fig.1-3. A stepped runner cross section is shown.

Bernoulli's theorem also states that if the potential and pressure energy remain constant and a sudden friction factor is
encountered, the velocity of the molten metal decreases. This means that if the molten metal stream flowing through a
gating system encounters a sudden change in direction, such as a 90o corner, the velocity of the stream will be drastically
reduced (see Fig. 1-4 and the video clip for Fig.1-4a). If the sharp corner changes to a round corner, far less velocity will
be lost in turning at the corner. This is due to the lower friction factor.

o
Fig.1-4. A 90 corner in a gating system results in high friction factor, causing a sharp loss in metal flow
velocity—(a) iron flow pattern shown using massless particles; (b) higher velocity shown in lighter colors. (See
video clip for Fig. 1-4a).

At this point this information may seem arbitrary. However, an understanding of the second major principle of fluid
flow, called the Law of Continuity, will help in understanding this theorem.

LAW OF CONTINUITY

A second fundamental principle of fluid flow is a mass balance equation called the "Law of Continuity." This law states
that the volume of fluid flowing in a full channel is the same at all points in the channel. A simple way to think of this is,
"what goes in must come out." The mathematical representation of this law is:
Q = a1v1 = a2v2
where:
Q = volumetric flow rate, in.3/s (cm3/s)
a1 = area at point 1, in.2 (cm2)
v1 = velocity at point 1, in./s (cm/s)
a2 = area at point 2, in.2 (cm2)
v2 = velocity at point 2, in./s (cm/s)

If a given volume of fluid flows past one point in a channel in a given period of time, then that same volume flows past
another point down stream in the channel in the same time period. The velocity and area may be different at both points,
but the volume (amount) is the same. If the cross-sectional area of the channel is constant, then the velocity of the fluid
in the channel also remains constant. However, if the cross section of the channel changes, the velocity of the fluid also
changes reciprocally. If the cross-sectional area of the channel is increased, the velocity of the fluid decreases and vice
versa.

PRACTICAL APPLICATION
An example is a runner in which the cross-sectional area is changed. Figure 1-5 illustrates a runner with a cross-sectional
area of 2 in.2 (12.9 cm2). The molten metal flowing in this part of the runner has a velocity of 20 in./s (50.8 cm/s). If the
cross sectional area of the runner is then reduced to 1 in.2 (6.54 cm2), the molten velocity increases to 40 in./s (101.6
cm/s) based on the Law of Continuity.
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Fig.1-5. A change in velocity and flow lines are visible—(a) metal flow pattern shown using massless
particles; (b) higher velocity shown in lighter colors.

This example illustrates the relationship between velocity and cross-sectional area. By reducing the cross-sectional area
of the channel by one half, the velocity of the fluid is increased by a factor of two (in other words, it is doubled). This
then becomes the basis for the control of molten metal flow. In order to reduce the velocity of a molten metal stream, the
runner or channel needs to be enlarged as shown in Fig. 1-6.

Fig.1-6. Change in velocity and flow lines are visible—(a) metal flow pattern shown using massless particles;
(b) higher velocity shown in lighter colors.

Thus far the discussion is focusing on the Law of Continuity as it applies to fluids that are flowing in a horizontal plane.
Fluids often are required to flow or fall in a vertical direction. Vertical drops or falls create several unique effects on the
falling stream, which can also be predicted through the study of basic fluid flow. The velocity of a stream at the start of
the fall will be quite low and increase as the stream continues to fall. This is a result of gravity acting upon the stream.
Gravity pulls the stream toward the earth and accelerates the stream as it falls. The velocity of a free falling body or
stream is given as:

v = 2 gh
where:
g = acceleration due to gravity, 384 in./s2 (981 cm/s2)
h = distance of fall, in. (cm)

The greater the distance a stream falls, the greater its velocity. This equation is theoretical and frictional losses are not
considered. Even air can impart a velocity loss on a free-falling stream. Typically, a body will fall until it reaches a
maximum velocity, a point at which the friction of the air actually impedes the acceleration, and causes it to remain
constant. This is called terminal velocity. In a typical metal casting operation, terminal velocity is rarely reached. Figure
1-7 depicts a free-falling metal stream.

Fig.1-7. A free-falling metal stream is shown.

Since the Law of Continuity states that the volumetric flow rate (amount) of the fluid remains constant, the cross-
sectional area of the falling fluid stream decreases as it falls. The reason for this is that the velocity is increasing
throughout the fall, thus accounting for the tapered shape of a free-falling stream as shown in Figs.1-8a and 1-8b (see
video clips for Figs. 1-7 and 1-8a and Fig. 1-8b).

Fig.1-8. Illustrations show the flow of metal in (a) a straight downsprue and (b) a tapered downsprue. (See
video clips for Figs. 1-7, 1-8a and 1-8b).

Table 1-1 shows the velocities that can be attained in falling streams before stream impinges on the sprue well
(conversion of 1.00 in. = 25.4 mm).
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Table 1-1. Free Fall Height and Corresponding Velocity of Liquid at Base of Column

The velocity of a falling stream is independent of the alloy poured. The volumetric flow rate (Q), expressed as in.3/s
(cm3/s) and given by Q = av (area x velocity), is also independent of the alloy. Most foundries, however, state their flow
rate in lbs/s (kg/s), which will vary based on the alloy used.

Example of iron and aluminum:
area = 1 in.2 (6.54cm2)
free fall height = 10 in. (25.4 cm)
velocity = 87.63 in./s (222.58 cm/s)
volumetric flow rate = area x velocity
weight of metal flowing =  x volumetric flow rate, lb/s (kg/s)

Fe = 0.26 lb/in.3 (0.007 kg/cm3)
Al = 0.09 lb/in.3 (0.002 kg/cm3)

Pour Rate Aluminum = 8 lb/s (3.63 kg/s)
Pour Rate Iron = 22.8 lb/s (10.34 kg/s)

MOMENTUM

In working with fluid flow, another law of physics must be considered if gating systems are to be scientifically designed.
Sir Isaac Newton's first law states, "Bodies in motion in a particular direction tend to stay in motion in that direction
unless acted upon by opposing forces." Objects or a liquid stream, which are moving in a particular direction do not
change their direction easily. To make them stop or change direction, an opposing force has to be applied. This tendency
of objects to continue to move in straight lines and their resulting opposition to changes in direction is a result of
momentum.

PRACTICAL APPLICATION
Experienced drivers and highway engineers are aware of the effects of momentum, whose effects can be lessened if
curves are gradual rather than sharp. The effects of sharp corners in a gating system can be just as dangerous as
attempting to make a 90o turn at 70 mph in an automobile. A sharp 90o turn in a gating system is shown in Fig. 1-9. Note
that the fluid stream causes a high-pressure area where it runs into an opposing force. Conversely, a low-pressure area is
formed opposite the high-pressure area. These areas are created due to momentum of the fluid stream. In a gating
system, a sharp corner of sand could be eroded and entrained into the stream of molten metal. The low-pressure area can
also cause air and mold gasses to be drawn into the stream.

Fig.1-9. Flow behavior, sharp vs. rounded corner (velocity and flow lines), is illustrated—(a.) metal flow pattern
shown using massless particles; (b.) higher velocity shown in lighter colors.

Two other situations that warrant consideration are shown in Figs. 1-5 and 1-6. In Fig. 1-5, there is a large channel
connected to a small channel. The fluid flowing in the channel has momentum, and when the smaller channel is reached,
the fluid that fills the corners of the large channel forms a high-pressure area in those corners. This action forces the flow
toward the centerline of the channel. As a result of the Law of Continuity, the velocity in the smaller channel is higher
than the larger channel. This higher velocity, and the fact that the flow was forced toward the centerline of the channel,
causes the fluid stream to deviate from the walls of the smaller channel just beyond the junction. This causes a low-
pressure area to be formed. If the fluid in this channel is a liquid, there is a good chance that air will become entrained in
the stream at this point. Here again there is a sharp corner of sand or other mold material which can be eroded, and
entrained in the metal.
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A similar low-pressure area is developed when a fluid stream flows from a small channel into a larger one as shown in
Fig. 1-6. Due to momentum effects, the stream wants to continue in a straight line when it leaves the small channel. This
forms a low-pressure area in the corners as the stream leaves the small channel and enters the larger channel.

Fig. 1-10. The inertia of the melt in a basin initially doesn’t allow for good filling of a tapered downsprue (see
video clip for Fig. 1-10).

Fig.1-11. The inertia of the melt in a basin doesn’t allow for good filling of a back tapered downsprue (see
video clip for Fig. 1-11).

FRICTIONAL FORCES

In all cases of fluid flow through a channel, the fluid stream is subject to the action of frictional forces. These forces
cause the velocity of the stream to be reduced.

Some of the areas where the effects of frictional forces are important are:
 velocity loss at the entrance of the sprue
 velocity loss due to the friction of the walls of the sprue
 velocity loss due to the bend at the base of the sprue
 velocity loss due to friction of the runner walls
 velocity loss due to friction in filters
 velocity loss due to turns in runners and ingates

These frictional losses may be considered as individual components. However, since the frictional losses are essentially
constant once flow is established, the overall system loss is usually consistent. A detailed discussion of frictional loss
calculations is beyond the scope of this text. However, industry literature contains numerous examples.1 The effects of
various types of turns on the stream velocity are shown in Fig. 1-12.

Fig. 1-12. Friction factors for various shapes of runner systems are illustrated.

REYNOLDS NUMBER

Reynolds number applies to a runner system where the metal in the flowing section is full. Molten metal can flow either
in a quiet, streamlined, laminar manner or with turbulence of varying amounts. In laminar flow, the particles follow well
defined, non-intersecting paths with minimal turbulence. In turbulent flow, the path of the liquid particles cross and re-
cross one another in an intricate pattern of interlacing lines with eddies (circular currents). The degree of turbulence can
vary widely from very slight to fairly violent, depending on conditions of flow.

The characterization of the flow of liquids, including molten metal in a channel is accomplished with a dimensionless
number called Reynolds number. This number is obtained by using the formula:

NR = vd/

where:
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v = velocity of liquid, in./s (cm/s)
d = diameter of the channel, in. (cm)
 = kinematic viscosity of the liquid, in.2/s (cm2/s)

In the case of channels with non-circular cross sections, d is taken to equal four times the hydraulic radius or:

4 x cross sectional area
d=
perimeter of cross section

The representative values of kinematic viscosity of various metals and of water are given in Table 1-2. Kinematic
viscosity is a property of the liquid, which is defined, as the viscosity divided by the density.

Table 1-2. Kinematic Viscosities

Examples of NR in Iron and Aluminum
For 10 lb/s (4.54 kg/s) pour rate with runner area=1.0 in.2 (6.54 cm2)
runner velocity = pour rate/density x runner area
runner velocity Al = 10/0.09 x 1= 111.11 in./s (282.22 cm/s)
runner velocity Fe =10/.26 x 1=38.46 in./s (97.69 cm/s)

For hydraulic diameter = 1 in. (2.54 cm)
Fe = 0.0006 in.2/s (0.004 cm2/s)
Al = 0.002 in.2/s (0.0127cm2/s)

NR Iron = 38.46 x 1/.0006= 64,100 (corresponds to 10 lb/s [4.54kg/s])
NR Al = 111.11 x 1/0.002=55,555 (corresponds to 10 lb/s [4.54kg/s])
By definition both will be turbulent flow.

Experimental work has shown that for NR of 2,000 or less, flow invariably is smooth or laminar. When the NR is greater
than 2,000 but less than 20,000, the flow is turbulent in nature. If the NR exceeds 20,000, severe turbulence occurs. These
variations in flow are illustrated in Fig.1-13. For most practical situations in pouring metal castings, the NR is
considerably greater than 2,000, and therefore turbulent flow occurs because velocities are high in real applications.
There are some different critical velocities or a critical Reynolds number for each material, which would cause potential
defects.

Fig.1-13. Laminar vs. turbulent flow is illustrated.

Fortunately for the metal caster, it has been recognized that two levels of turbulence occur. When the NR is less than
about 20,000, a relatively undisturbed boundary layer of metal exists on the surface of the stream. The turbulence is
confined to the central portion of the stream. This type of turbulent flow is considered relatively harmless because the
surface layer is not ruptured and air or gas entrainment is avoided. Also, erosion of molding sand is reduced or
eliminated. When the NR exceeds 20,000, the surface layer of the stream is ruptured and severe turbulence takes place.
Under these flow conditions, dross formation, entrainment of air, mold gasses, slag and dross are likely to occur. In sand
molds, the chance of eroding the sand is greatly increased.

The degree to which frictional forces affect the nature of laminar or turbulent flow is shown in Fig. 1-14. The velocity at
which the break in the curve occurs is frequently referred to as the ―critical velocity.‖ This usually occurs at a
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corresponding NR of about 2,000. This may vary dependent on different metal alloy systems. The application of critical
velocity to sand and dross defects needs to be researched more in detail.

Fig.1-14. The Reynolds number is illustrated.

Since the Reynolds number is used to predict the degree of turbulence in a fluid stream, this parameter is used to design
effective and efficient gating systems. Gating systems are designed to minimize turbulence, thus an NR as low as possible
is desired. This is especially true for those alloy families, which are very sensitive to any turbulence. Some of these
alloys include aluminum bronzes, aluminum, magnesium and the high alloy steels.

The shape of gating system components has a significant effect on the Reynolds number. Figure 1-15 illustrates the
variation in Reynolds numbers for various alloy families flowing through different shaped channels. The velocity is
constant at 20 in./s (50.80 cm/s) and that the cross-sectional area of each shape is the same, 1 in.2 (6.54 cm2). However,
the hydraulic diameter of each shape differs.

Fig.1-15. Channel shape affects the Reynolds number.

WEBERS NUMBER

Webers number applies to a partially filled runner system where the metal in the flowing section is not full with open
free surface. It is a measure of surface turbulence where the liquid metal has surface tension forces. During filling of
the casting cavity, the liquid metal can have enough energy to deviate from the flowing stream at a critical velocity.
This critical velocity corresponds to a Webers number close to one.

Webers number is proportional to inertial force/surface tension force and is used in momentum transfer in general
and bubble/droplet formation and breakage of liquid jets calculations in particular. It is defined as follows:

We = DV2/gc
where:
D = length
V = velocity
 = density
gc = dimensional constant
 = surface tension

For example, the critical velocity for liquid magnesium is 1.97 ft/s (0.6 m/s), for aluminum it is 1.64 ft/s (0.5 m/s),
and for steel it is about1.31 ft/s (0.4 m/s).

Generally the surface tension forces become more significant when there are free surfaces associated during filling
of the mold. Typically in a gating system where the choke is near the sprue, the runners and ingates do not fill
immediately. These unfilled runners and ingates have free surfaces where the forces of surface tension become
important.

METALLOSTATIC PRESSURE

There is tremendous amount and variation of metallostatic pressure in the system during filling. Foundry engineers
design the pressure relief to lower the pressure build up in the system. The pressure differential keeps on changing
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during the fill when the metal height changes in the system. In the initial filling the full height is high and pressure
differential is high as well, but during later filling when cavities are almost full the pressure differential decreases
making it harder to fill. The metallostatic pressure is given by the formula:

P = gH
where:
P = metallostatic pressure
 = density
G = gravitational acceleration
H = height

Effective pressure height (EPH) is a function of potential head effects on fluid flow. It is needed to calculate
velocities. The EPH drastically affects the flow patterns and velocity of metal. In a horizontal gating system, the
flow rate declines and metal velocities slow during cope filling. In a vertical gating system the velocities are
different at different levels of the gate. The EPH equations for the three basic systems are shown in Table 1-3 and
Fig. 1-16.

Table 1-3. EPH Equations for Three Basic Systems

Fig.1-16. Effective pressure height is illustrated.

Cope lifting is a common phenomenon when pour rates become bigger. This increases when upward forces are
larger than the downward forces acted upon the cope. The significant upward forces comprise of static force on
mold, core and dynamic peak pressure due to weight of gating system, mold compressibility retardation, etc. The
significant downward forces are weight of sand in the cope, weight of the flask, etc. These forces are calculated
based on the following equations and the dimensions represented in Fig. 1-17.

F(Mold) = (A1[H1] + A2H2)m
F(Core) = m(M/c) - M
F(Total Static) = F(Mold) + F(Core)
F(Dynamic) = weight of the gating system(R)
where:
F(Mold) = static force by mold
F(Core) = static force by core
F(Total Static) = total static forces on cope
H1 = height of projection area-1
H2 = height of projection area-2
A1 = projection surface area-1
A2 = projection surface area-2
M = weight of core
m = density liquid metal
c = density core
R = retardation
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Fig.1-17. Diagram illustrates a casting shared in cope and drag

Foundries calculate how much additional cope weight to add (increase down pressure) based on the uppressure
calculated. The horizontal cores submerged totally by metal around it create high uppressures. In some cases the
core prints are also designed specially to minimize floating up.

The metallostatic pressure is an important factor in gating systems, however the cope float and core float are not
explored enough in terms of designing of gating systems and are not discussed in this text.

FLUIDITY

Fluidity is an important variable relating to the flow of molten metal in a gating system. To the foundry engineer, this
does not mean the reciprocal of viscosity but rather the ability to fill the mold cavity. Fluidity is generally measured by
pouring metal into a standard fluidity spiral or a serpentine type of pattern and measuring the distance that the fluid
travels before it is solidified. The distance the molten metal flows in the spiral or serpentine gives a good indication of
the fluidity under those given conditions. Typical fluidity test patterns are shown in Fig. 1-18. Alloys with low fluidity
values will have trouble filling thin casting sections and must be poured at increased pouring rates to insure complete
filling of the part.

Fig.1-18. Fluidity spiral is illustrated.

Fluidity tests do not need to be run for every heat poured, but can be used to check against an internal standard. One key
point is that identical fluidity test patterns and mold materials should be used for all tests. Both metal and mold
characteristics affect the fluidity behavior. Various characteristics of metal affect fluidity, such as:
 superheat
 alloy viscosity
 metallostatic pressure
 alloy composition
 surface tension
 surface oxide
 absorbed gas films
 suspended inclusions
 inclusions precipitating freezing

Superheat and alloy composition, are the most important. Molten metal heated to a higher temperature has a longer
period in the mold before it freezes; therefore, it flows farther than an alloy heated to a lower temperature. Alloy
composition affects fluidity and it depends on the freezing characteristics of the alloy and the chemical elements, which
make up the alloy. The best fluidity is observed in narrow freezing ranges like pure metals and eutectic compositions.
These alloys have little or no mushy region during the solidification process. However, an alloy with a long freezing
range shows a condition during flow through the fluidity test mold where the alloy is in a mushy condition. This is
characterized by interlacing dendrites and, in some cases, many islands of solid material, surrounded by liquid near the
freezing temperature. With these types of alloys it is natural that fluidity is restricted.

Certain chemical elements, when added to different alloy families, help to increase their fluid life. When silicon is added
to carbon steel and aluminum alloys, their fluidity is increased. Phosphorous, on the other hand, helps to increase the
fluidity of gray cast iron and certain copper base alloys. The foundry metallurgist should be consulted concerning
chemical elements which can be used (and how much) for the alloys poured in the foundry.

WETTING
Wetting is a phenomenon, which can describe the wetting and non-wetting characteristics of mold media to slag
(Fig.1-19). The rate of wetting is dependent on viscosity, surface tension and contact angle.
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Wetting Rate = cos/2
where:
 = surface tension
 = viscosity
 = contact angle

Fig.1-19. Wetting and non-wetting are illustrated.

If molten metal wets it slows the flow. Wetting also assists in metal/slag retention. If filled slowly, it is possible to
make the slag adhere to sand core surfaces, etc.

STOKES LAW
Stokes law (Fig. 1-20) calculates the vertical velocity of slag particles. It assumes that slag separation is possible if
flow conditions are laminar. This condition normally occurs in the pouring basin. These conditions also occur when
the filling of the mold is completed. Slag floats to the top of casting and gating system. The vertical velocity of a
slag particle in the pouring basin should not exceed 1.97 in./s (50 mm/s).

Fig.1-20. Stokes Law is illustrated.

V = D2(metal-slag)g / 18metal
where:
V = vertical velocity of slag particle
D = diameter of slag particle
metal = density of metal
slag = density of slag
g = gravitational acceleration
 = kinematic viscosity

NON-STEADY FLOW
In addition to simple steady flow, where the flow has established and stabilized in the filling system, there are many
things that happen during filling of the mold, which makes it more time dependent and complicated. Steady flow
assumes that the flow is translating not rotating, is moving in straight-line streams and is conserving angular
momentum. All the convective terms are not included. The steady flow velocity—V = V(s)— is dependent on
location (s) and acceleration—as = V V/s— is due to change in velocity resulting from change in position of the
particle. The variation of velocity with respect to time is omitted.

In reality all flows are unsteady because velocity does vary with time (t). In metal casting, most flow is usually
turbulent and random. In non-steady flow, the velocity is dependent on the location and time—V = V(st).
Acceleration—as = V/t +V V/s—is due to the variation of velocity with respect to time and the acceleration
V/t term becomes a significant factor.

The streamline may change shape with time. If flow is unsteady its parameter values (velocity, temperature, density,
etc.) at any location may change with time. Since flow within the cavity is transient, the amount of liquid, location
of liquid and free surface orientation rapidly changes. The interior and surface flow behavior is slightly different.
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The surface is the interface between the atmosphere and molten metal, which is constantly changing and the free
surface boundary should be considered. In these cases, the tangential stress is omitted but normal stress balances the
applied pressure and surface tension.

REFERENCE

1.   Basic Principles of Gating and Risering, American Foundry Society Cast Metals Institute, Des Plaines, IL
(1973).
CHAPTER 3—Horizontal Filling Applications
INTRODUCTION

Historically gating systems are divided into two types. Because the names traditionally associated with these two
types, pressurized and non-pressurized, do not accurately describe these systems, these terms are not used in this
chapter. Instead, the terms, gate choke and sprue choke, are used. The location of the choke determines the flow
characteristics of a system and must be considered when determining gating geometry. (See Fig. 3-1.)

Fig. 3-1. Types of gating systems are illustrated.

Although some of the parts and functions of gating systems are the same for both vertically and horizontally parted
molds, there are important differences in terminology and methods for calculating the size of the components. The
information is divided into two chapters: horizontal filling application and vertical filling application. Within each
chapter, the parts (Fig. 3-2) and functions of a gating system are identified. Flow control is discussed, different types
of gating systems are examined and specific methods for calculating the size of a gating system are explained. In
discussing the gating system, it is assumed that the gating system has been properly designed so that all the
components are completely filled during pouring.

(Note: It has been a common mistake to use the term ―feeding‖ when discussing gating systems. Gating systems are
used for filling the mold. Runners fill ingates, ingates fill castings. To help eliminate confusion, feeding is a term
reserved for solidification and riser design.)

Fig. 3-2. Components of a horizontal gating system are illustrated.

COMPONENTS OF A HORIZONTAL GATING SYSTEM

POURING CUP / POURING BASIN
Although they differ in design and effectiveness, the pouring basin and the pouring cup both serve the same
function. They both receive the molten metal stream from the pouring ladle and direct it to the vertical shaft or
sprue. If this enlargement is offset from the sprue and is of sufficient size to allow the metal pourer to avoid pouring
directly down the sprue, then the enlargement is referred to as the pouring basin (Fig. 3-3).

Fig. 3-3. A pouring basin is illustrated.

The pouring cup is often used because it is more easily formed in the top of the mold than a properly designed
pouring basin. In some foundries the pouring cup is formed by a loose piece integral with a tapered sprue. In other
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instances the squeeze plate or board forms it when the mold is compacted. Also, the pouring cup might be hand cut
or cut by a rotating bit that is part of the molding machine.

The pouring basin is formed in the top of the mold in many of the same methods as the pouring cup or it is made
separate and placed on top of the mold before pouring.

When a pouring basin is used, the pourer directs the molten metal into the portion of the basin away from the sprue.
This prevents the first metal that might drip from an overly full ladle from entering the rest of the gating system and
possibly the casting. It also gives the pourer time to gain control of the pour, quickly fill the gating system and allow
the gating system to take control of the pour.

A properly designed pouring basin has sufficient cross sectional area to allow the vertical velocity of the liquid
metal to be low enough for slag to float to the top and not enter the sprue. The pouring basin should also be of
sufficient depth and size with a flat bottom to prevent the molten metal from rolling out of the basin rather than
flowing down the sprue. A rectangular shaped basin of sufficient size also aids in preventing a vortex from forming
that interferes with flow down the sprue and causes the lighter slag to be drawn down into the rest of the gating
system.

A skimmer core (not to be confused with filters) is also incorporated in the pouring basin to help hold the slag back
and keep it from entering the rest of the gating system. If a skimmer core is used, one precaution is that the clearance
between the bottom of the skimmer core and surface of the pouring basin does not control the flow rate of the liquid
metal.

SPRUE
The sprue is a vertical channel, delivering liquid from the pouring basin or cup to the rest of the gating system. 1 (Fig.
3-4). When the liquid metal drops vertically, the flow velocity increases causing the free stream to decrease in cross-
section. Although the ideal geometry for a sprue is a tapered rectangular shape with the small end at the bottom,
molding methods generally dictate the shape of the sprue If the molding method allows for a loose sprue to be drawn
from the top of the mold then it is relatively easy to use the ideal geometry. However, if the sprue is mounted on the
pattern plate as in the case of most automated molding machines, a straight or reverse taper sprue is used (Fig. 3-5).
Because the stream is decreasing in size as it falls, the liquid metal draws away from the wall of the sprue and allows
a negative pressure to form around the stream of metal. The region of negative pressure allows air and other gases to
be drawn into the stream and may a cause defect in the casting. However, if the gating system is correctly designed
to allow for quick filling or back-up of the gating system, the negative effect is short lived and usually does not
cause any problems. (See Fig. 3-6)

Fig. 3-4. A sprue is illustrated.

Fig. 3-5. A reverse tapered sprue is illustrated.

Fig. 3-6. The difference between straight and tapered sprues is depicted.

A sprue with a rectangular cross-section practically eliminates the possibility of a vortex from forming which can
restrict the flow in the sprue and draw in gases and inclusions. (Fig. 3-7) Careful attention is given to the design of
the top of the sprue to make sure it does not control the flow in the gating system. When the top of the sprue is
allowed to control the flow, the sprue can never be completely filled until the total mold is full. (Fig. 3-8)
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Fig 3-7. The shape of the sprue influences vortex formation.

Fig. 3-8. Sprue radius affects frictional losses.

Sprue Base or Well
The sprue base or well is an enlargement at the bottom of the sprue and is intended to reduce turbulence by
dissipating some of the kinetic energy developed in the vertical drop down the sprue. When the molten metal leaves
the sprue, it is traveling at a high velocity due to the influence of gravity during its fall. Also, at the sprue outlet, the
molten metal changes its direction of flow. When combined, these two things can cause extreme turbulence (Fig. 3-
9). The bottom of the sprue base should be flat and not concave in shape. The same action that occurs in and around
a soup ladle when it is held under a running faucet takes place in the sprue base, if it has a concave surface.

Fig. 3-9. Turbulence is illustrated.

RUNNER
The main horizontal passageways through which the molten metal flows are called the runners. A single runner is
used but often several runners extend from the sprue base to supply metal to various parts of the mold. The runner
not only serves to carry the liquid metal from the sprue well to the gates, it is also designed in such a way to aid in
trapping slag and other impurities thus preventing them from entering the casting. This is accomplished by
controlling the velocity in the runner through proper sizing of the cross-sectional area. By slowing the velocity of the
molten metal in the runner, slag is allowed to float to the top of the runner. The type of system and the available
room on the pattern plate determine the geometry of the runner.

The runner geometry produces a smooth flow with as little turbulence as practical. One common error when
designing a gating system is allowing the gates to lap across the runner in the drag when the runner is placed totally
in the cope. This practice produces a washboard effect generating a great deal of turbulence. When this occurs, it is
very likely that impurities are drawn into the gate and enter the casting.

If at all possible runners are kept straight. Any turns in a runner causes turbulence and unwanted inclusions can be
deflected into the gates. If the liquid metal stream is required to change direction, it is done with the least amount of
turbulence. Thus if a right angle turn is made, it is done by using a generous radius. A right angle turn causes erosion
and low pressure pockets to form, which can aspirate mold gases and entrain sand into the flowing stream of liquid
metal.

Another key job for the runners is to evenly distribute the molten metal into the mold cavity. Ideally, each gate
passes the same volume of molten metal into the mold cavity. Or, in a multiple cavity mold, each cavity fills at the
same flow rate and at the same time. In order to do this, the cross sectional area of the runner should be reduced as
each gate is past. This reduction is equal to the cross-sectional area of each gate. If two gates are taken off the runner
at the same location, then their total cross-sectional area is used for the reduction of the cross-sectional area in the
runner.

Runner Extension and Sump
The runner extension is that part of the runner which extends past the last gate. It is used to prevent the first metal from
entering the casting. The leading edge of the stream of molten metal takes a severe beating coming down the sprue and
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turning the corner to enter the runners. Also, as it fills the runners, the leading edge entrains any loose material that is in
the runner. This part of the metal stream must not enter the mold cavity, and thus, is trapped with an extension or sump.
Runner extensions are tapered to prevent the first metal from being directed back to the gate when it impacts the end of
the runner. The taper deflects the wave and reduces the wash back effect. The use of an extension sump, rectangular
shapes in the drag that allow the metal to drop below the runner, is even more effective at preventing the metal from
washing back into the gate.

Runner Vent
A runner vent is the passageway which allows air and mold gases to escape ahead of the liquid metal stream as it
fills the runner. The usual location is at the end of each runner.

GATE OR INGATE
The gate or ingate is the short passageway which connects the runner to the mold cavity or riser. The geometry of the
gate is dependant on the type of system design selected, the size of the casting and the number of impressions. The size
of the gate is as small and thin as possible to promote easy gating removal and reduce the chances of casting breakage.
However, there other factors such as velocity control and shrinkage that may require larger gates.

Gating Ratio
The term "gating ratio" refers to the ratio of the total cross sectional area of the sprue outlet, runner and gates. The
key point is always to compare total cross-sectional areas of the gating system components feeding the castings.
Some typical gating ratios used are 1:2:2, 1:3:3, 4:4:1 and 4:8:3. There are many factors to consider when
determining an appropriate gating ratio to be used in the design of the gating system. This is discussed in a
subsequent section of this chapter that deals with design.

CHOKE
The choke controls the flow of the molten metal and is located in several different places outside of the gating
system or in the gating system. There are several possible choke locations. The locations that should be avoided are:
the pourer (in the case of manual pouring); the lip or nozzle of the pouring ladle; the top of the sprue in the case of
reversed tapered sprues and undersized filters. The acceptable locations are: the bottom of the sprue in the case of
tapered sprues small end down; choke cores such as ―strainer cores‖ or single hole cores in the outlet of the sprue;
and the runner and the gate(s) (Fig. 3-10)

Fig. 3-10. Choke locations are possible in various areas.

The pourer (in the case of manual pouring) and the lip or nozzle of the pouring ladle are often overlooked by the foundry
worker when trying to determine the location of the choke. If the pourer doesn't keep the sprue full of molten metal, then
the individual is the choke. On the other hand, if pourer is not given the proper tools, such as well-maintained pouring
lips or spouts in the pouring ladle, then the pourer can't be expected to keep the sprue full of molten metal. In the case of
automatic pouring, the nozzle becomes the choke if the gating system is designed for a higher flow rate than the nozzle
delivers.

The choke is usually defined as the smallest total cross-sectional area of any component in the gating system.
However, consideration is given to the following factors:
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● metallostatic head height
● momentum of melt, i.e., in transitions and corners
● friction, and
● permeability
These factors can lead to changes in the cross sections used by the melt stream.

A very simple example of a gating system composed of one sprue, one runner and one gate could have the choke
located in any one of these three components. For example, if the area of the sprue and runner were each two square
inches and the ingate was one square inch, then the ingate would control the flow and is considered the choke. In
another way using the concept of gating ratios, this example has a gating ratio of 2:2:1 and the smallest number in
this ratio indicates the location of the choke. On the other hand, if the area of the sprue was one square inch and the
area of the runner and the area of the gate were each two square inches, then the sprue would control the flow and
would be considered the choke. Again the smallest number in the gating ratio of 1:2:2 indicates the location of the
choke. In a more complex gating system, it is important to understand the concept of total area. In the case of a
gating system composed of one sprue, two runners and four gates, the task of locating the flow control is more
difficult. If a value of two square inches is assigned to the sprue, three square inches is assigned to each of the
runners and one square inch is assigned to each of the gates, then the choke is located at the sprue assuming that this
is a symmetric system. This is because both the total runner area of six square inches and the total gate area of four
square inches are each greater than the area of the sprue. The gating ratio for this system is 1:3:2. These examples
assume that the gating system is properly designed so that all the components of the gating system are completely
filled during pouring.

Friction can also affect the location of the choke and an area larger than the smallest area of the system can become
the choke, if there is sufficient friction developed in that component to restrict the flow of metal. One very effective
method for determining the location of the choke in a gating system is to use computer flow simulation. Based on
the Law of Continuity, the choke is also the point of highest velocity in the gating system; therefore, graphical
output of the flow simulation can very quickly indicate where the choke develops when the system is analyzed.

Because the highest velocity occurs at the choke, the placement of the choke determines the flow characteristics of
the metal in the gating system and in the casting. If the choke is the gate, then the metal enters the casting at a higher
velocity than in a system in which the choke occurs at the bottom of the sprue or early in the runner system. There is
no clear-cut answer for determining where the choke is located. It is obvious that the choke should occur somewhere
in the system and not be left to the pourer or to the ladle design. This is referenced as taking the art out of pouring.
In other words, a designed gating system controls the flow. The best location for the choke really depends on the
type of metal being poured and the geometry of the mold and mold cavity. In all cases the goal is to reduce
turbulence and achieve a calm fill at the highest rate that produces acceptable castings.

SYSTEMS CHOKED AT THE GATE

In a gate choke system, the choke is located at or near the casting ingate. With this design, the molten metal stream
is forced to back up in the gating system (sprue and runners) and permit slag, dross, sand, etc. to separate from the
stream prior to the entry of molten metal into the gates. When using this design, the gates are positioned so as to
remove metal from the lower portion of the runner. If the runner is sized correctly for its length, the metal at this
level is relatively clean and any slag or dross has migrated to the top of the runner. This geometry is obtained by
placing both the runner and gates in the cope. If a gate is required to connect to the casting in the drag, then a portion
of the runner, the same thickness as the gate, is placed in the drag. It is also a good practice to place a thin portion of
the runner in the drag when mold seal is to be applied to the drag mold. This prevents the mold seal from being
placed in the area of the runner.

The shape of the runner is narrow and tall while the gate is relatively thin as compared to the runner. The area of the
runner is significantly larger than that of the gate(s). This allows the velocity of the molten metal to be low and give
time for the less dense slag and dross to float to the top if the runner is of sufficient length. The shape of the runner
as well as the lower velocity also helps in preventing excessive turbulence that interferes with the separation of the
slag and dross from the metal.
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Straight runners should be used. If bends in the runner are unavoidable, the gate is placed branching off as far past
the bend as conditions permit. The reason behind this recommendation is that liquid metal in the runner wants to run
straight and in so doing, bypasses gate openings (Newton’s First Law). This may cause some of the initial, slag-rich
metal to enter the gates. The bottom of the runner and the bottom of the gates should be in the same plane. If the
gate is placed in the drag and allowed to lap across the runner that is in the cope, a washboard effect is set up
increasing turbulence and deflecting slag-rich metal into the gates.

It is desirable to provide properly designed runner extensions and or sumps when ample space is available. With this
design, the highest velocity in the system is directed into the casting. Therefore, careful attention is given to how the
mold cavity fills. If the casting is entirely in the cope and the geometry is one that allows the cavity to fill with
minimal turbulence and splatter, this type of system is generally a good choice. However, if there is a drag cavity
that allows the metal to fall or if the casting geometry is one that allows the metal to strike the wall of the mold or
core and splatter, slag and dross can be generated in the mold cavity, especially in ductile metal. If this case, other
measures, such as bottom gating through a core or ceramic tile, are considered. Otherwise, a sprue choke system
needs to be used that allows metal to be introduced into the mold cavity at a lower velocity.

ADVANTAGES
1. It is easier to keep the basin full of molten metal during pouring. This promotes a constant head pressure that
assists in consistently filling the mold.
2. Ingates are smaller than a sprue-choked system, thus reducing cleaning costs.
3. Generally there is a better mold yield than sprue-choked systems.
4. More effective risering is evident. Smaller size ingates freeze off sooner and isolate the riser from any
influences from the runners, sprue or pour basin.

DISADVANTAGES
1. High pressure at the ingates may cause squirting or fountain effects that increase the potential for dross defects
and mold erosion.

SYSTEMS CHOKED AT OR NEAR SPRUE

In a sprue choke system, the choke is located at the outlet of the sprue or early in the runner. With this design, the
molten metal stream backs up quickly in the sprue and pouring cup or pouring basin. The highest velocity in this
type design occurs early in the gating system. Consequently, the velocity of the metal as it enters the mold cavity
can be lowered to reduce turbulence and splatter.

With the choke early in the system, the gates must remove the metal from the topmost portion of the runner.
Otherwise, the runner never becomes completely filled. The runners are always in the drag, while the gates are
located in the cope to allow the liquid level in the runner to reach the parting plane before the mold cavity begins to
fill. The metal is cleaned in this type of system by slowing velocity and allowing less dense slag and dross to adhere
to the cope side of the runner. The runner must be of sufficient length to allow the particles to reach the top and the
velocity must be low enough to allow the particles to remain stuck to the sand mold at the top of the runner.

ADVANTAGES
1. Lower runner and gate velocity than gate choked systems are evident
2. Less mold/core erosion exhibited
3. Less gas entrapment develops
4. Best metal cleaning effects without using a filter result

DISADVANTAGES
1. Higher cleaning costs result

REFERENCE

1.   Karsay, Stephen I., Ductile Iron III Gating and Risering, p.19, QIT–Fer Et Titane Inc., (1981).
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CHAPTER 4—Vertical Filling Applications

This chapter will explain the application of fluid theories to designing a gating system for molds with a vertical
parting plane. Friction, gravity, static pressure, density and velocity influence the movement of molten metal. The
major difference between horizontal gating systems and vertical gating systems is the influence of gravity. If metal
is moving in the same or opposite direction as gravity, the energy of the metal increases or decreases If metal is
moving in a direction perpendicular to gravity, the energy is not affected.

Horizontal gating systems have the majority of the gating components located at the main parting plane or within a
relatively short perpendicular distance from the main parting plane. The parting plane is perpendicular to the
direction of gravity. Because there is relatively little change in the height of the molten metal (direction
perpendicular to the main parting plane and in the same direction as gravity), the metal flow is primarily moving
perpendicular to gravity. Gravity does not increase or decrease the energy of the metal stream for a horizontal gating
system very much.

Vertical gating systems have the majority of the gating components located at the main parting plane or within a
relatively short perpendicular distance from the main parting plane; however the parting plane is in the same
direction as gravity. The majority of the time the metal movement is in the same or opposite direction as gravity.
Gravity increases or decreases the energy of the metal as it flows from the top of the mold to the bottom of the mold
and vise versa.

The change in energy that results from the action of gravity is often referred to as head pressure or metallostatic
pressure. If the metal flow is moving in a direction opposite to gravity (up toward the top of the mold as in up
runners, some ingates or a large cavity during filling) the change in energy is exerted opposite to the direction of the
moving metal. It acts to slow the metal flow and is often referred to as backpressure. Backpressure causes a change
in pour rate. At the beginning, a pourer pours ―hard‖ but must ―back off‖ near the end of the pour to avoid metal run
out on the top of the mold.

COMPONENTS OF A VERTICAL GATING SYSTEM

POUR CUP (BASIN)
The pour cup has several functions. It provides a target for the pourer to hit and is used to direct the metal as it first
enters the mold. A well-designed pour cup helps reduce splashing, promotes smooth filling and allows for floatation
of slag and dross. It also helps absorb the changes in pour rate that result from backpressure during filling.

The pour cup is located on one half of the mold or split between both halves of the mold (Fig. 4-1). The specific size
and shape of the cup depends upon the type of pouring system, the total pour weight and the overall pour rate. A
starting size in cubic inches that is equal to the overall pour rate is recommended. For example, if the overall pour
rate is 10 in.3/s (163.87cm3/s) the minimum pour cup size should be 10 in.3 (163.87cm3). This is a starting point and
the cup needs to be adjusted in size for yield and to work well with pouring operations. Horizontal placement of the
cup varies depending upon the limitations of the pouring system and the pattern cavity placement.

Fig. 4-1. The location of the pour cup is illustrated.

Vortexing is a concern when using a cylindrical or conical shaped pour cup. It often occurs when molten metal is
poured directly into the down runner. This is more prevalent when a cylindrical or conical cup is combined with a
single round down runner. Vortexing is also more prevalent with long pour times. Pouring observations are needed
to determine the specific pour time when vortexing begins for any given combination of pour weight, pouring
equipment and gating design. Changing the shape of the down runner from round to rectangular also minimizes
vortexing (Fig. 4-2). Vortexing is eliminated by not pouring directly into the down runner. This is easily
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accomplished with a vertical gating system by mounting the pour cup on one half of the mold and mounting the
down runner so that it laps over the back of the cup (Fig. 4-3).

Fig. 4-2. (a)Vortexing is due to a conical pour cup and a round down runner. (b)Vortexing is reduced by using
a round pour cup and a round tapered down runner.

Fig. 4-3. A runner overlapping the back of the pour cup is illustrated.

Pour basins are also used with vertical gating systems. Additional information is found in the previous chapter on
horizontal filling applications.

DOWN RUNNERS, HORIZONTAL RUNNERS, UP RUNNERS
Runner Function
Runners are channels in the mold that carry molten metal from the pour cup to the ingates. Runners have several
functions, which include minimizing turbulence, temperature loss and velocity. Reducing the velocity of the metal
flow allows for flotation of slag. Another function of the runners is to evenly distribute molten metal to the ingates.
This promotes simultaneous filling of all ingates. Simultaneous filling of ingates (rather than sequential filling) helps
decrease the likelihood of gas aspiration and can reduce undesirable thermal gradients. This is true for a single
pattern cavity with multiple ingates or multiple cavities, each with one ingate.

Runner Terminology
The runner terminology for vertically parted gating systems is slightly different than for horizontally parted gating
systems, although the actual components of the gating system look very similar. The distinguishing factor is how the
component is oriented in relation to the parting plane—orientation as compared to the top of the mold and what direction
the molten metal is flowing through the component. If the component is parallel with the parting plane, it is generally
referred to as a runner. If the component extends down toward the bottom of the mold and carries metal to the bottom of
the mold, it is referred to as a down runner. If the component is directly below the pour cup, extends down toward the
bottom of the mold and carries metal to the bottom of the mold, it is referred to as a central down runner. A component,
which is parallel to the parting plane, parallel to the top of the mold and located near the top of the mold, is referred to as
a top runner or top horizontal runner (Fig. 4-4). Runners that carry metal from the bottom of the mold up toward the top
of the mold are often referred to as up runners. The term sprue is reserved for components that run perpendicular to the
parting plane and is most often used when describing horizontal gating systems.

Fig. 4-4. Various types of runners are illustrated.

Runner Size
Runner size is determined by the desired pour time, pour weight, type of metal being poured and number of ingates
that need to be supplied with molten metal. Due to elevation changes inherent in vertical gating systems, head
pressure continuously changes from the top of the mold to the bottom of the mold. If a consistent pour rate is
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desired, changes in head pressure are taken into account when determining the size of runners for a vertical gating
system.

Runner Shape
The principles for determining the general shape of runners for a vertical gating system are similar to those used for
a horizontal gating system. The use of a tapered down runner prevents gas aspiration during pouring. The taper can
be height, width or a combination of both dimensions. The shape of the runners varies from drafted rectangles and
squares to half rounds, full rounds and triangles (Fig. 4-5). Triangular runners are helpful in areas of difficult
moldability. Metal in the pointy ends of the cross section of a triangular runner does not flow as efficiently as the
rest of the cross section of the runner. This results in the use of a larger triangular runner to obtain the same flow as
round or drafted runners. A runner with a small surface area to volume ratio helps reduce temperature loss during
pouring (Fig. 4-6). The surface area to volume ratio for a runner is estimated by dividing the cross-sectional
perimeter of the runner by its area. Runners in a vertical gating system do not always behave the same as runners in
a horizontal gating system. At sufficiently slow velocities, runners with the same height and width assist in
floatation of slag. This is because the slag had a shorter distance to float before it touches and sticks to the mold
surface. Tall runners used as down runners do not help float out slag. The slag remains within the molten metal
stream (Fig. 4-7). Tapered and stepped down horizontal runners promote even distribution of molten metal through
multiple ingates.

Fig. 4-5. Typical runner bar shapes are pictured.

Fig. 4-6. All of the above runners have the same area and carry the same flow at a given location, however
the different surface area to volume ratios affect the amount of temperature loss during filling.

Fig. 4-7. Both runners have the same cross sectional area and carry the same flow at a given location,
however the distance the slag has to float up and out of the melt and stick to the mold wall is different for
each runner. The runner on the left is more effective on removing slag than the runner on the right.

Changes in Direction
Like horizontal gating systems, changes in direction need special attention. Fluid flowing around a corner produces
a low-pressure area (Figs. 4-8a and 4-8b). The low-pressure area aggravates gas aspiration. To avoid gas aspiration,
a generous fillet and corner rounding should be used at the corner. Sharp curves also produce low-pressure areas.
Straight runners are used when ever possible, however if room does not permit, a large gentle radius is preferred to a
sharp curve.

Fig. 4-8. The velocity and flow lines for abrupt changes in cross section are illustrated.

Changes in Cross Section
Runner transitions in size affect the quality of castings. Abrupt changes in cross sectional area causes areas of low
pressure that results in aspiration and an increase in turbulence. A change in cross sectional area is done over a
specific distance. This produces a taper in the runner. The taper helps reduce gas aspiration and turbulence.

Simultaneous filling of a mold cavity or cavities is promoted when an even distribution of metal flows into each
ingate. The relationship between the size of the ingate and the size of the runner has a great influence on
simultaneous filling. Both metallostatic pressure and energy losses due to changes in direction affect the size
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proportion between the ingate and the runner. When there is more than one ingate per down runner, the flow in the
down runner directly in front of an ingate is able to provide flow for that ingate and all ingates downstream. There
are two methods of distributing the amount of flow needed in the down runners to promote simultaneous filling. The
first method is to size the flow in the down runner to be the same at both the top and the bottom of the down runner.
The flow is the sum of the ingate flow plus 10-20% for energy losses. When considering metallostatic pressure, the
runner has a straight taper. The down runner has a total of two different cross sections, one at the top and one at the
bottom but has the same flow at both ends (Fig. 4-9a.). The other method used to promote simultaneous filling is to
step down the down runner after each ingate. Metallostatic pressure is also considered with a stepped-down runner
(Fig. 4-9b.). The result is several more cross sections however this method produces more control over the flow in
the down runner.

Fig. 4-9. Two down runners are illustrated: (a) straight tapered and (b) stepped down central

Runner Extensions
Similar to horizontal gating systems, flow at the ends of runners needs special attention. Runner extensions are a
change in cross section at the end of a runner. Turbulence and aspiration usually oxidize the first metal through the
gating system. In addition, the metal may have picked up loose sand grains. Runner extensions are used to trap this
metal and prevent it from entering the casting cavity. The extensions typically extend past the ingates. They are used
to quietly stop the flow of molten metal. A properly designed runner extension prevents molten metal from bouncing
back toward the ingates. The lengths of runner extensions are determined by experience. Sumps or slag traps are
types of runner extensions that extend across the parting line to the mold half opposite the runner (Fig. 4-10).

Fig. 4-10. These pictures illustrate: (a) a typical runner extension; (b) a runner extension with slag trap on
same side of parting plane; and (c) a runner extension with slag trap that crosses the parting plane.

INGATES
Ingates are the components of the gating system that connect the casting or riser to the runner system. The ingates
are sufficiently thin to allow quick-freezing to promote independent solidification of the casting (or riser) without
influence from the rest of the gating system. However, the ingates are not too thin as to cause misruns or cold metal
defects. Ingate design also helps reduce turbulence, promotes quiet filling of the mold cavity (or riser) and promotes
consistent backpressure in the runner system while pouring. The shapes of the ingates are influenced by the desired
pour time, pour weight, moldability, knock-off and grinding. Generous radius and fillets are used where ingates
meet runners and castings or risers.

VENTS
Vents allow mold cavity gases to escape to the atmosphere. Vents are located at the parting line, connected to a
casting cavity, runner system or a cored surface. Depending upon design, the vents may or may not fill with molten
metal. The size and shape of vents are influenced by the desired pour time and cleaning operations (Fig. 4-11).

Fig. 4-11. A typical cavity gas vent is illustrated.

GATING RATIO
Because pressurized and non-pressurized is not defined in the same manner as horizontal gating, gating ratios are
not typically used to describe vertical gating system.
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FLOW CONTROL: CHOKE OF GATING SYSTEM

Like horizontal gating systems, the cross sectional area that controls the flow rate of molten metal in a mold is called the
choke. Metallostatic pressure changes throughout the mold in a vertical gating system. This causes the flow rate to
change. For vertically parted molds, the choke is generally discussed in terms of flow rate in mass per second or volume
per second—for example: lb/s (kg/s) or in.3/s (cm3/s). The choke is always located in the component with the smallest
flow rate; not necessarily, the smallest cross-sectional area.

VELOCITY VS. RATE
Velocity is speed, it is distance over time. Rate is volume over time. For example, in filling a 100-pound casting it
may be desirable to fill the mold cavity in 10 seconds. The mold can fill gently in 10 seconds or in can fill
turbulently in 10 seconds (spray). Both cases have a pour rate of 10 pound per second; however the velocity
differences cause turbulence. Velocity is controlled by the shape of the cross section through which the molten metal
flows.

CHOKE LOCATION
The portion of the gating component that contains the choke is oriented parallel or perpendicular to the top of the

mold. The choke is located in a length of a runner or at a specific point in a runner. It is placed near or far from the

top of the mold, near or far from the casting or riser, split between several ingates or split between ingates and

runners. There is a great deal of flexibility for choke location using vertically parted molds. Unlike horizontal

systems, there is a variation in head pressure throughout a vertical gating system. This head pressure must be

considered when determining the size of the components of the gating system and the placement of the choke.

TYPES OF VERTICAL GATING SYSTEMS

There are no hard and fast rules that apply to vertical gating design for all instances. Designing a gating system
often results in a series of compromises. A good practice for slag may also be a poor practice for solidification.
Placing the choke at the ingates may be a good practice for one size ingate, but not for a different size or shaped
ingate. Different types of vertical gating systems are defined, the advantages and disadvantages are explained and
examples of good and poor design practices are offered.

CHARACTERISTICS OF FILLING FOR VERTICAL GATING SYSTEMS
If certain characteristics are not present, most of the rules of gating presented are not applicable. These general
characteristics of vertical gating systems may not be met when any of the following occur—results of pouring
may seem contradictory to what the theories predict; the pour time may be longer or shorter than predicted; the
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pour rate consistently causes short pours or excessive metal on top the mold; mold and/or core erosion occurs;
and there are excessive casting defects such as gas defects, cold shuts, misruns, sand, and inclusions. When
looking at correcting casting defects through design changes to a gating system, the first place to begin is by
checking to ensure that the following points are fulfilled:
     The gating system should fill within the first few seconds of the pour time. Depending upon the size of the
mold, this period could range from 2 to 4 seconds.
     The level of metal in the cup should remain constant throughout filling.
     Once the gating system is full, it should remain full during the entire filling time. There should be no voids
formed from mold gas within the cavity being filled. Gas cavities can result from inadequate mold gas
venting or excessive generation of mold gas from core or mold materials.

PRESSURIZED VS. NON-PRESSURIZED GATING SYSTEMS
Traditionally, the defining characteristics of pressurized versus non-pressurized gating systems are shape of the runners,
location of the ingates (cope or drag) and placement of the choke. This is very straightforward for horizontal gating
systems. The terms pressurized and non-pressurized are misleading and difficult to apply to vertically parted gating
systems. Generally, gravity acts parallel to the metal flow and does not assist floatation of slag as it does with horizontal
gating systems. Gravity also exerts different head pressures on metal flow at different elevations throughout a vertically
parted gating system, no matter what component of the system is being examined. When gravity acts in the same
direction as the flow, head pressure has a greater influence on turbulence and velocity than the shape of the runners or
location of the ingates (ram or swing). Of these three traditional characteristics, used to distinguish pressurized versus
non-pressurized gating systems, only placement of the choke is used to define these terms for a vertically parted gating
system. Since there is no clearly defined ―sprue‖ for vertical gating systems and the choke is placed in any component or
combination of components, the use of the terms pressurized and non-pressurized is not applied to vertical gating
systems. Similar to horizontal gating systems, it makes more sense to define the gating system in descriptive terms of
choke location.

BOTTOM FILLING VS. TOP FILLING
A bottom filling gating system is characterized by a runner system and ingate placement that fills the casting from
the bottom (Fig. 4-12). The runner system has several components. In the simplest case, it has a pour cup, down
runner, horizontal runner with some type of runner extension or sump and an ingate into the bottom of the casting.
Gating components on either side of the choke (upstream or downstream) are sized to handle 10% to 20% more flow
than the choke. This ensures the choke is in the intended component and reduces gas aspiration. Typically, the choke
is placed in a component in the lower part of the mold. This helps reduce gas aspiration and helps keep the entire
gating system full during pouring.

Fig. 4-12. Bottom filling with multiple cavities is illustrated.

A top filling gating system is characterized by a runner system and ingate placement that fills into the top of the
casting (Fig. 4-13). Generally, a top filling system has the choke located at the ingate(s) with the runner(s) sized
10%-50% larger than the choke. The oversized runners allow for overpressure in the system. This helps ensure the
runners stay full during the pour and reduce the tendency for gas aspiration. The overpressure also helps keep the
ingate full to completely utilize the entire size of the ingates (Figs. 4-14a and b).
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Fig. 4-13: Top filling with multiple cavities is illustrated.

Fig. 4-14. (a) Runner did not fill completely and (b) runner is correctly sized for top ingates.

Advantages of Bottom Filling:
Less Turbulence
With properly shaped, located and sized ingates, filling into the bottom of the casting is less turbulent than dropping
metal into the top of the casting. Taking metal off the top of the runner ensures the runner stays full during the pour.
This reduces gas aspiration and the formation of dross resulting in cleaner castings. (See Fig. 4-15).

Reduced Grinding Costs Due to Smaller Ingates
When ingates are sized taking head pressure into account, ingates in the lower part of the mold have a smaller cross
section than an ingate of the same flow located higher in the mold.

Fig. 4-15. Various flow patterns are illustrated: (a) for bottom filling; (b) for top filling; and (c) for top and
bottom filling.

Disadvantages of Bottom Filling:
Pouring Problems Due to Changing Pour Rate throughout Filling
When the choke is located in the lower part of the mold, particularly at the ingates to the casting, backpressure
during filling causes a change in pour rate. Many manual and automatic pouring systems cannot consistently handle
this change in pour rate. The change in pour rate leads to short poured castings and cold shut defects. If the top edge
of the castings and/or risers are below the bottom of the pour cup, the changing pour rate may be overcome by using
a larger than ordinarily necessary pour cup. The pour stops when the pour cup is full, however as the mold proceeds
down the line; the metal in the cup draws down to complete the filling of the mold.

High Velocity Using Thin Ingates
Often to accommodate knock-off, ingates to the casting are made with a cross section that is thin and wide. When
this shaped ingate is the choke, the velocity at the ingates increases dramatically. The increased velocity leads to
dross formation due to a fountaining effect as the metal enters the cavity. It also leads to mold and core erosion.

Thermal Gradients That Work Against the Riser
When filling is complete, generally, the last metal to enter a mold has the hottest temperature. When the ingate is
located at the bottom of the casting and the riser is located at the top of the casting, the hottest part of the casting is
away from the riser. This leads to shrinkage defects. The use of larger than normal contacts and risers is needed to
compensate for temperature gradients set up during filling.

Advantages of Top Filling
High Mold Yield
This is accomplished by one or a combination of the following factors: fewer runners, smaller runners, smaller risers
when filling through the risers into the top of the casting and riserless castings.

Top Filling Can Promote Directional Solidification
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This can be an advantage or a disadvantage depending upon casting geometry. Ingate placement promotes
directional solidification solely based on the placement of the ingate at the top of the casting. Riserless casting for
metal is accomplished by using top filling and placing the ingates at the top of the castings. During solidification,
graphite separation of the metal in the mold cavity exerts an internal pressure that helps eliminate microporosity.

Promotes Uniform Filling of Mold Cavity as Compared to Side Ingates
This is due to the metallostatic pressure remaining constant within the ingate.

Disadvantages of Top Filling:
Higher Sensitivity to Size of the Ingate
The size of the ingate influences feeding from the runner. For example, when filling riserless castings with metal,
the ingates must freeze off before the castings reach the eutectic temperature. If the ingates do not freeze at the
correct time, proper feeding does not occur. This may result in draw downs on the casting surface or internal
porosity.

Increased Turbulence in the Casting Cavity When Molten Metal Falls through the Casting Cavity
This can result in dross type defects for metals and alloys that are sensitive to dross formation.

Increased Sensitivity to Casting Size
Due to increased tendencies for dross formation, top filling gating systems may not be well suited for large castings
or castings with severe offset partings.

FILLING INTO RISERS VS. FILLING INTO CASTING
The direction from which a riser fills is known by several terms. ―Live‖ or ―hot‖ risers occur when molten metal fills
the riser from an ingate off the gating system. The amount of filling may be enough to fill just the riser for both the
riser and the casting. ―Dead‖ or ―cold‖ risers occur when molten metal flows into the riser through the contact from
the casting (4-16).

Fig. 4-16. (a) In live or hot risers, metal fills from the riser into the casting; (b) in dead or cold riser, metal fills
from the casting into the riser; (c) in dead or cold riser, metal fills from the casting into the riser; and (d) in
live or hot risers, metal fills from the riser into the casting.

Advantages of Filling through the Riser
Possibility of Reduced Contact Size
The longer metal flows through the contact, the more the mold material temperature around the contact increases.
This is often referred to as superheating. The excess heat can be used to reduce the size of the contact.

Possibility of Smaller Riser
The last metal to fill a riser/casting combination has the hottest temperature. If the riser fills from the gating system,
it contains the last metal. Some of the literature reports a decrease in riser size up to 20% when using a hot riser
rather than a cold riser.

Reduced Cleaning Costs
If the casting is filled entirely through the riser, there are no ingates to grind. Eliminating the ingates into the casting
along with potentially small contacts produces considerable cleaning cost reductions.

Disadvantages of Filling through the Riser.
Potential Increase in Dross Defects
A casting design with a lot of offset parting planes or a large cavity causes excess falling of the molten metal stream
which increases the potential for dross formation.

Potential Core Erosion
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If a core is located near the riser contact, the core may be subject to erosion during filling.

FILLING INTO HEAVY CASTING SECTIONS VS. THIN CASTING SECTIONS (TEMP GRADIENTS,
MOLD EROSION, CHOKE)
Depending upon casting design, a casting may fill by placing an ingate into a heavy section or a thin section of the
casting.

Advantages of Filling into a Heavy Section
Possibility of Less Turbulence
Often an ingate into a thin section causes increased dross formation because the molten metal impacts the mold
opposite mold wall and increases the turbulence and splashing during filling. Splitting the flow between several
ingates reduces the turbulence to the thin section

Less Mold Erosion
Often an ingate into a thin section causes mold or core erosion due to the impact of the flow on the wall opposite the
ingate. Splitting the flow between several ingates reduces mold or core erosion.

Superheating of Mold or Core Material
Often an ingate into a thin section causes the mold or core material to superheat the area of the ingate. This results in
shrinkage defects. Splitting the flow between several ingates reduces superheating the mold or core materials.

Disadvantages of Filling into a Heavy Section
Possibility of More Turbulence
If the choke is placed at the ingates, there is a potential for a fountain effect or splashing in front of the ingate. This
causes dross formation. Changing the shape of the ingate reduces this potential.

Excessive Heat in the Casting Section
Filling into a heavy casting section increases the heat in that section. It becomes isolated from the riser and results in
a shrinkage defect. Splitting the flow between several ingates reduces the tendency for the formation of isolated
heavy sections.

Filling into Multiple Cavities
The best casting cost often requires making more than one casting in a mold. Filling multiple casting cavities
introduces more variables into the gating design. Some concerns are: how should the cavities fill; what order
should the cavities fill; where should the ingates be located; and what adjustments in the gating system are
needed to accommodate multiple cavities.

Simultaneous vs. Sequential Filling:
Sequential filling occurs when the lower cavities begin filling before the runner system is full. Simultaneous filling
occurs when all the cavities begin and finish filling at approximately the same time (Fig. 4-17).

Fig. 4-17. The sequential filling of cavities is illustrated by three rows of graphics.

Sequential filling can result in the lower cavities filling first, then the middle cavities and then the top cavities.
Sequential filling can occur throughout the entire pour time or for part of the pour time. The runners and pour cup
that are not full during the entire pour characterize sequential filling. This leads to aspiration of mold gases through
the empty ingates into the metal stream in the runner (Fig. 4-18). When the pour cup does not contain a cushion of
molten metal during the majority of the pouring, it results in mold erosion in the pour cup causing casting defects.
Sequential filling produces high velocities in the lower part of the mold. This leads to increase the turbulence, which
causes casting defects such as metal penetration, pinholes and interdendritic gas porosity. Sequential filling is caused
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by both runners and ingates that are too large for the capabilities of the pouring system. Sequential filling also
occurs when ingates to the lower cavities are too large or metallostatic pressure is not included when calculating the
size of the ingates. Sometimes dynamic pressures must also be taken into account to produce simultaneous filling of
cavities at different levels in the mold. This is often accomplished by increasing the flow rate into the top cavities by
as much as 10-30%.

Fig. 4-18. The simultaneous filling of cavities is illustrated by three rows of graphics.

Both ingate flow and the proportion between the ingate flow and the runner flow influence the filling sequence of
multiple cavities. The amount of ingate flow must be adjusted to accommodate different head pressures that result
from different vertical locations (distance from the top of the mold) within the mold. Even if the ingate flow is
adjusted for head pressure, if the proportion of flow between the ingates and the runner filling the ingate is incorrect,
the runners may not fill before the cavities begin filling. If the proportion is not correct there is uneven filling and
draw down in the pour cup at the end of pouring. Often a straight taper from the top of the down runner to the
bottom of the down runner may not have enough taper to produce the correct proportion of fill rate between the
ingate and the runner. Specifically stepping down the runner insures that the correct proportion is maintained (Fig.
4- 19).

Fig. 4-19. (a) The original runner is illustrated. (b) A stepped-down runner ensures that correct filling occurs.

Hybrid Filling Systems
Hybrid gating systems are generally a compromise or combination of other types of gating systems. For example,
when looking at top filling versus bottom filling, a hybrid system may be a gating system that fills the casting from
the side. It may also be a system that has both an ingate in the bottom of the casting and an ingate into the top of the
casting or the riser. The ingate into the bottom of the casting provides for a pool of metal in the casting cavity.
Generally, it is a poor practice to have molten metal filling a cavity from both the top and bottom (increased
turbulence and flow type defects). Sometimes soundness requirements dictate the use of a hot riser and cleanliness
requirements dictate the use of bottom filling. The ingate into the riser or top of the casting helps to set up thermal
gradients to aid in solidification. The ingate into the bottom of the casting provides a cushion of molten metal in the
casting cavity because mold erosion and dross is less likely to occur if the metal from the top of the casting falls into
a pool rather than on the mold surface. This compromise is considered a hybrid gating system.

Another hybrid system consists of a primary choke and a secondary choke. The primary choke is located at the base
of the down runner and functions to backup the down runner and keep the pour cup full during pouring. The
secondary choke, which has the same flow rate or slightly larger (5%) than the primary choke, is located at the
ingates. The secondary choke functions to backup metal in the runners to allow for slag flotation.
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CHAPTER 5—Special Filling Applications
SPECIAL GATING SYSTEMS AND GATING AIDS

During the history of the casting industry, many gating techniques have been tried and have obtained various levels of
success. Present manufacturing processes and quality levels forced a reevaluation of some of these methods with many
of them being ineffective and in some cases harmful. In the interest of providing complete information, some non-
standard gating designs are examined in the following sections. Full evaluation of these gating designs should be
completed before they are applied.

The principles presented in the previous chapters are applicable to most situations encountered in the production of
castings. However, several specialized gating systems have been developed for specific applications and have proven
useful in these restricted situations. Although limited in their use, these systems illustrate several important points.

REACTIVITY OF METALS WITH OXYGEN
While certain alloys tolerate more deviation from optimum flow conditions, it is emphasized that quality improvements
are realized by the application of proper gating techniques. The sensitivity of metals regarding fluid flow turbulence
varies. Certain metals take considerable punishment in the gating system and still produce a good casting. On the other
hand, there are those that react very highly to any turbulence in the gating system. Therefore, knowledge of the metal
alloy that is poured becomes important in designing a gating system. Aluminum alloys are generally very sensitive to
turbulence, whereas gray iron is less sensitive. The reason for the sensitivity to turbulence is the ability of the alloy to
react with oxygen. The alloys that easily react with oxygen to form stable oxides are more susceptible to turbulence
related defects. Also, if the turbulence is greater, the amount of the alloy that is in contact with air that contains oxygen is
larger and therefore the potential for the alloy to form an oxide is higher.

One other factor in oxide formation is pouring temperature. All alloys become more reactive with oxygen as the pouring
temperature is raised. Therefore, when high turbulence is combined with higher pouring temperatures the possibility of
turbulence related oxide defects increases dramatically.

The following is a general classification of turbulence sensitivity for a sampling of alloys that are poured in the foundry.
 Magnesium alloys–high
 Aluminum alloys–high
 Aluminum and manganese bronze–high
 Carbon steels–medium
 Ductile iron–medium
 High alloy steels–medium
 Gray iron–low
 Red brass–low

This is a general classification and a more thorough search of literature is required to determine their exact sensitivity and
the sensitivity of other alloys.

WHIRL GATES
The whirl (or swirl) gate was developed as a means to entrap slag, dross and other inclusions, which are carried by the
molten metal. When a liquid spins in a circular motion (Fig. 5-1), heavier liquids are forced to the outer perimeter and
lighter material remains close to the center. In order to get sufficient spinning action, higher velocities at the ingate are
required. In general, the inclusions are both capable of being mixed and are lower in weight than the molten metal. These
properties cause the particles to be forced to the center of the spinning mass of molten metal. The particles also float to
the top of the whirl gate and are removed from the flowing stream. However, momentum and turbulence act to carry the
particles into the mold cavity and eliminate some of the benefits. Due to the turbulence, foundry workers that pour very
sensitive alloys or even those less sensitive, usually refrain from using the whirl gate. In many cases, the inclusion is the
same weight or even heavier than the alloy being poured.
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Fig. 5-1. Flow lines in whirl gate are illustrated.

The design of the whirl gate is extremely important. To ensure that the whirl gate is doing its job, samples are sectioned
to see that the inclusions are entrapped in the center or top section of the body. Several designs are proposed with the
ingates and exit gates at 90º, 180º or 270º apart. Also, various height-to-diameter ratios for the body of the gate are
proposed. A summary of the experimental work of Trojan, et al, is presented below.1

The use of opposing tangents for the ingate and exit gate with a height/diameter ratio of 0.6 for the body of the whirl gate
gives the most efficient inclusion removal. This relationship is illustrated in Fig. 5-2. Due to energy losses in the whirl
gate, flow rate is somewhat reduced. In addition, casting yield is reduced and cleaning costs increase. The foundry
worker must weigh these disadvantages to determine whether the inclusion removal efficiency over the conventional
gating system is economically justified.

Fig. 5-2. A typical whirl gate is illustrated.

KISS OR LAP GATING
The kiss gating technique (Fig. 5-3) is developed for brass castings but is used in other metals. This type of gating
involves forming a gate by just "kissing" or touching the side of the casting at the parting line.

Fig. 5-3. A typical kiss gate technique is illustrated.

The main factors in the design of this type of gate are:
 Controlling the gate or touch area in order to provide sufficient volume and velocity to rapidly fill the mold
cavity and prevent misruns;
 providing proximity of metal source (runner) to the casting to keep the gate open until the casting is filled and
solidified; and
 placing the runner, if it is to serve as a source of feed metal, in the cope and placing the casting in the drag; and
using a gate choke gating system.

Kiss gating is kept open because the sand, which is at the corners between the runner, gate and casting, is heated
sufficiently to alter heat transfer characteristics and allow the runner to feed the casting.

Kiss gating has been used on several types of castings:
 True kiss or top kiss on castings such as small nuts
 Side kiss gate (knife gate) on small or medium weight castings to eliminate grinding or machining
 Gate choke used on problem castings or on larger castings. In some instances, reduction of gate size alone
produces remarkable quality improvement and loss reduction.
 Bottom kiss, in which most of the casting lies in the cope above the gate and runner. This is used on certain
castings, which will be highly polished. This gating minimized polishing defects caused by casting these
surfaces in the drag.

The advantages of kiss gating are:
 simplicity in gating
 mold making advantages
 lower scrap due to misruns
 increased yield in castings per mold
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    lower cleaning costs
    lower direct and indirect material costs
    castings having closer dimensional tolerances
    improved machining due to grain structure

Problems That Can Occur
Several precautions must be noted in regard to kiss gating. The flask equipment must be in top condition. Sloppy pins
and bushings cause shifting that can either close off or expand the gates. Many foundry technicians and engineers believe
that kiss gating automatically provides the equivalent of a gate choked system. This is not always true. Gating ratios
always use total cross-sectional areas. Therefore, to get the ratio number for the gate area, all of the gate cross-sectional
areas are added up. When this is done, many times the total cross-sectional area of the ingates is greater than the sprue
exit area or the runner cross-sectional area, thus the system is an expanding or sprue choke gating system.

If the runner is located in the cope and the castings are located in the drag, the runner(s) does not fill completely until the
castings are full. This practice makes it impossible to float out any inclusions in the runner. This material therefore ends
up in the casting. In bottom kiss gating, the system works because the runner fills before any metal enters the mold
cavities. Also, having a large runner, as most kiss gating systems do, the molten metal stream's velocity is slowed down
sufficiently to allow inclusions to float and be trapped on the cope surface of the runners.

Another condition to consider is when using a gate choke gating system with a cope runner; the runner does not always
fill completely. The runner is not completely filled until the casting is filled or nearly filled. Thus, the chances of floating
out inclusions are again hindered in the early stages of pouring. X-ray films back up this action in the runner.

This type of gating is promoted for red brasses that are used to pour plumbing fittings and also for gray cast iron.
However, this type of gating should not be used with alloys that are more sensitive.

A judicious use of kiss gating produces quality castings. However, as in all gating system designs, a thorough study
should be made to insure the production of quality castings.

METAL INGATE RESTRICTIONS
An extension of the whirl gate concept is the use of a slot dam of refractory coated metal to control the exit flow. An
extension analysis of the materials and design of these slot dams is given by Wildermuth.2

Sheet metal slot dams coated with fireclay refractory demonstrate the ability to:
 permit more effective use of the whirl riser principle from the Steel Founders' Society design
 permit control of metal flow at the riser-casting connection
 not restrict the feeding ability of a given riser design
 facilitate riser knock-off by effectively reducing the riser connection

Several materials are investigated based on the following criteria:
 The slot dam is sufficiently thin or has as high a thermal conductivity as possible so that it does not act as a heat
sink or heat dissipater and does not inhibit the flow of feed metal from the riser to the casting.
 The material is either inexpensive or reusable and has sufficient refractoriness to withstand temperatures of
3000F (1649C) without eroding.
 It must be sturdy enough to facilitate rapid placement by the molder.

The material that proved to be ideal was fireclay washed 30-gage sheet steel. A circular orifice is found to be
satisfactory. As with other special systems, the slot dam design is examined utilizing fluid flow principles to determine
its effect on casting quality.

REFRACTORY GATING MATERIALS
The use of a special refractory or tile gating systems can be beneficial to improve casting quality levels. They are
traditionally examined when severe runner system applications exist. This is especially true when large quantities of a
high temperature alloy, such as cast iron, steel, alloyed steels or other exotic alloys, pass through the gating system. A tile
gating system helps prevent erosion by replacing the molding sand, which would normally form the gating system.
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Various types and shapes of refractory tile are available and used. Gating tile is also made from a heavy-duty core sand
mix, which allows flexibility of design. Core sand tile proves to be very efficient in trapping oxide materials such as
those found in manganese steel and other metals. The use of the gating principles also applies to tile gating systems.

HORN GATING
This type of gating is used when pouring hub type castings as shown in Fig. 5-4.

Fig. 5-4.A typical horn gate with a smaller area attached to casting is illustrated.

With the larger cross sectional area of the horn gate attached to the casting, as shown in Fig. 5-5, the velocity, geysering
and turbulence are reduced.

Fig. 5-5. A horn gate with larger area attached to the casting is illustrated.

There are several areas in fluid flow principles that are observed when using horn gating. If the horn gate is connected
directly to the sprue, the first metal down the sprue goes into the mold cavity. Unless the horn gate is made of refractory
tile, the pattern piece used to form the horn gate is removed from the mold. Due to the shape of the horn gate,
considerable draft is required. Also, many times the horn gate pattern cannot be removed through the mold cavity. In this
case, the smallest cross sectional area of the horn gate is connected to the casting. This means that there is considerable
velocity, geysering and turbulence if this cross sectional area is the choke in the gating system. Many other types of
special gating systems are specifically designed for a particular purpose. A good understanding of fluid flow principles
and alloy properties is necessary in these designs in order to obtain a gating system that produces quality castings.

VENTING
The least understood aspect of gating involves the use of vents to relieve the pressure of gases, generated during the
filling of the mold. Along with these gases, there is also air, which occupies the mold cavity but must be displaced by the
molten metal. The gases are generated when the hot metal makes contact (or even before) with the mold and cores.
Venting is particularly significant in high density and chemically bonded molds, in permanent molds poured statically
and in low pressure permanent molds. This is because of the high density of the sand molds and their reduced
permeability, and in the case of the permanent molds, the solid metal mold.

When the mold is filled, the heat wave, preceding the molten metal and the transfer of heat to the mold and cores, causes
gases to be generated. This is also true in permanent molds using sand cores. This is due to the evaporation of water and
the decomposition of binders and additives. In general, these gases are liberated at low temperatures and tend to fill the
mold cavity since this offers the path of least resistance to their flow. The gas pressures within the mold cavity are
therefore increased and must be offset by the entering metal. This is particularly significant during the latter stages of
pouring, when the pressure head and thus, the flow rate, is substantially less than during the initial stages of pouring.

If open risers are used, gas pressures during the early stages of pouring are reduced. However, many times there are
sections of the casting located higher than the highest riser to the riser connection. Thus, the air and gases are trapped in
this area. A study of the casting design and riser placement helps to determine those areas where a vent must be placed in
these situations.

The incorporation of vents in the system minimizes the pressure build-up by offering a path for the air and gases to
escape. Unfortunately, there does not appear to be any specific rules to apply to the design and placement of mold vents.
A search of the AFS library and other sources for information on venting and gas pressure should provide a basis for
vent size and location selection.
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Many times, casting misruns are blamed on the gating system when the problem really is a poor venting practice. Figure
5-6 shows the shape of the leading edge of two misruns. The leading edge in Fig. 5-7 is concave in shape. This shape
normally denotes that entrapped air or mold gases prevented the molten metal from filling that part of the casting. On the
other hand, Fig. 5-8 shows the leading edge being convex in shape. In most cases, this edge is smooth and shiny. This is
normally an indication that the molten metal ran out of fluid life and did not fill out the casting. One of the causes of the
misrun shown in Fig. 5-8 is a poorly designed gating system. Proper venting serves as an aid to the gating system.

Fig 5-6. Misruns from two foundries are illustrated.

Fig. 5-7. Leading edge is concave in shape.

Fig. 5-8. Leading edge is convex in shape.

REFERENCES

1.       Trojan, P. K., Guichelarr, P. J. and Flinn, R. A., ―An Investigation of Entrapment of Dross and Inclusions Using
Transparent Whirl Gate Models,‖ AFS Transactions, vol. 74, pp 462-469 (1966).

Wildermuth, J. W., Lutz, P. H. and Loper, C. R., ―A Study of Thin Metal Ingate Restrictions in Steel and Ductile
Iron Castings,‖ AFS Transactions, vol. 76, pp 258-263 (1966)
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CHAPTER 6—Filtration

INTRODUCTION

Non-metallic inclusions can form both outside and inside the mold. While inclusions that occur before a filter can be
trapped, even the most effective filtration system is not a substitute for good foundry practices (Fig. 6-1). The use of
filters is not a replacement for good melting, handling and pouring techniques; but rather a valuable tool to improve total
process control.

Fig 6-1. An unfiltered casting is illustrated.

SUMMARY OF FILTRATION BENEFITS

By removing entrained slag and other contaminants, both the exterior casting appearance and internal structure are
greatly improved. Fewer castings are scrapped in the foundry and fewer customer returns are caused by internal defects
discovered during machining. As a result, machining allowance requirements and machining costs are reduced. Test
machining of identical filtered and unfiltered castings have shown reduced tooling consumption of both single-point
tools and drills (Fig. 6-2).

Fig. 6-2. Machining test results are graphed.

Molding line productivity is increased because of improved pattern plate utilization. Because filtration traps slag and
other inclusions, there is no need for long runners that allow time for entrained slag and other inclusions to float and
adhere to the gating system surfaces (Fig. 6-3). In addition, reduced gating increases mold yield and reduces cleaning
costs and scrap melting/energy costs.

Fig. 6-3. A conventional gating system is illustrated.
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The need for extended gating to reduce molten metal turbulence (and secondary oxidation) prior to reaching the casting
cavity, is also eliminated by proper filter application. A further result is shorter pouring times and reduced pouring-time
standard deviation—critical issues for automated molding operations. Shorter pouring times also lead to smaller
temperature differences between the riser and the casting, an important consideration in the pouring of thin-wall castings.
Because filtration eliminates non-metallic inclusions, other casting defects can be more readily isolated and diagnosed.

SOURCES OF INCLUSIONS

An understanding of filter performance is aided by an understanding of the types and sources of inclusions found in
castings. While "metallic" inclusions are also found in castings (undissolved alloys, treatment materials, etc.), it is the
nonmetallic inclusions that pose the greatest concerns and offer the best opportunity for removal by filtration.

Inclusions arise from conditions and/or materials found both outside and inside the mold. Inclusions, formed outside the
mold, (which are carried inside the mold by the flowing metal) commonly derive from:
    melting furnace slag
    ladle/launder refractory
    desulphurization/deoxidation slag
    treatment slag
    ladle slag
    oxidized alloy
    reacted/nonreacted slag coagulant
    slag from chill castings
    oxidation products
    contaminants and foreign objects

Inside the mold, inclusions may be caused by:
                                   loose sand
                                   mold erosion
                                   foreign matter in the molding sand
                                   oxidation from turbulent entry of metal into the mold
                                   oxidation as metal passes through the gating system
                                   mold or core coating particles
                                   mold treatment reaction products
                                   oxidized mold treatment alloys contaminants
                                   ceramic inclusions
                                   reaction of organics in the mold
                                   a non-metallic rejected from solution

While chemical composition of these inclusions is important, their existence rather than their analysis is generally of
greater interest to foundry worker until defect diagnoses are required. Visual inspection, after shakeout and before shot
blasting, reveals many problem inclusions. Close inspection may reveal loose sand grains or particles of refractory
coating. Foreign substances from the molding sand are often seen on the casting surface as well. Foreign objects, core
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materials and inclusions from chills or chaplets are also recognized. Inspecting the gating before removing it from the
casting discloses loss of molding material, which has moved into the casting.

Sectioning the defects, polishing and etching them prior to examination with an optical microscope can identify smaller
inclusions. However, care must be taken in sample preparation to avoid dislodging and losing inclusion material. This
makes it difficult to determine whether a cavity was caused by slag or gas. When the source of slag inclusions identified
by metallography is required, Scanning Electronic Microscope (SEM) analysis is often employed.

TYPES OF CERAMIC FILTERS

Various ceramic forms have been introduced as filtration devices, some being more effective than others. They include
extruded cellular ceramic filters, ceramic foam filters and pressed parts.

The ability to trap inclusions depends partly on the size of the openings. Cellular filters are measured by cells per square
inch (csi), foam filters by pores per linear inch (ppi) and pressed parts by hole size. The filter element must be large
enough to allow the desired metal flow. Flow is determined by the effective open frontal area and affected by the amount
of supporting area required. Filter elements are normally positioned in filter prints, molded into the sand (Fig. 6-4). They
may also be used in "direct pour" practices where a refractory sleeve and filter replace conventional gating.

Fig. 6-4. A filter gating system is illustrated

CELLULAR CERAMIC FILTERS
Cellular filters are extruded in different grades and cross-sectional dimensions, using a mixture of quality, high-
temperature ceramic material (see Fig. 6-5). Extruded lengths are sliced into elements and fired to withstand
temperatures up to 2700F (1482C). They are available in grades of 100 csi, 200 csi and 300 csi. Normally 0.5 in. (1.3
cm) thick, the elements range from 1.5 x 1.5 in. to 5.25 x 5.25 in. (3.8 x 3.8 cm to 13.3 x13.3 cm). Their composition
and physical structure provide a high ratio of open area to total surface area and high strength to resist the force of the in-
rushing metal.

FOAM CERAMIC FILTERS
Foam filters begin as individual pieces of plastic foam, which are dipped in a slurry. The type of slurry is determined by
the final application of the filter. The slurry is then dried and the pieces are fired. The plastic foam skeleton burns off
during the firing process, leaving a ceramic foam element (Fig. 6-5). Foam filters are available from 10 ppi through 30
ppi, with a nominal thickness of 0.87 in. (22 mm). Standard elements range from 1.97 x 1.97 in. to 2.95 in. x 2.95 in. (50
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x 50 mm to 75 x 75 mm). Unlike other devices, the area required to support the filter in the filter print does not have to
be subtracted from the active filter area since the liquid metal is able to circulate throughout the foam structure.

PRESSED PARTS
Pressed parts are individually pressed from clay-based materials and fired (Fig. 6-5). Round holes in the core range from
.060 in. to .100 in. (1.52 mm to 2.54 mm) in diameter. Nominal core thickness is .5 in. (1.3cm), with sizes ranging from
1.5 x 1.5 in. to 5.25 x 5.25 in. ((3.8 x 3.8 cm to 13.3 x13.3 cm). The percentage of open area to total surface area is
generally less than that found in cellular and foam filters.

Fig. 6-5. A cellular filter, a strainer core and a foam filter are shown.

FILTER DYNAMICS

The filtering ability of the various devices depends on ―sieving‖ action (determined by cell, pore, or hole size); "filter
cake" buildup on the entry face; and ―deep-bed‖ filtration. Deep-bed filtration results from the amount of internal surface
area available to attract or trap inclusions and the dynamics of the metal as it passes through the device.

Proper filter orientation with relation to metal flow can generate a "self-cleaning" action at the entry face of the filter,
increasing the amount of metal that passes through the filter before blockage ultimately occurs. An important benefit of
certain filters is the elimination of turbulence at the exit face of the filter. The metal leaves these filters in a laminar flow,
reducing chances of re-oxidation and improving fill time consistency.

CELLULAR FILTERS.
Cellular ceramic filters provide high flow rate and a large internal surface area, which attracts and holds non-metallic
inclusions (Fig. 6-6). The linear pathways through the element and the thin cell walls allow the filter to act as a flow
control, causing the metal to exit without turbulence and minimizing secondary re-oxidation. The cellular filter is quickly
primed; 90% of the heat needed to bring the filter up to temperature comes from the first "cell volume" to pass through
the filter.

Fig. 6-6. A sectioned cellular filter is pictured.
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FOAM FILTERS
A foam filter forces the metal through a tortuous path (Fig. 6-7). Oxides and other suspended non-metallics are trapped
on the surface of the filter or become entrained in the flow patterns, which develop within the filter body pores. Particles
escaping one pore travel on to another, etc. As a result, foam filters trap inclusions significantly smaller than the open
area of the pores and trap fluid inclusions as well. The foam filter effectively eliminates all turbulence in the molten
metal flow beyond the filter.

Fig. 6-7. The flow-through foam filter is illustrated.

PRESSED PARTS.
The reduced open area of the core face tends to increase the velocity of the metal as it passes through the core holes. This
may reduce the separation capabilities of pressed parts, potentially limiting their use to removing coarse impurities.
Because of the liquid metal velocity, a pressed part has a tendency to spray the stream as it exits, thereby entraining air
bubbles and causing re-oxidation.

IMPLICATIONS FOR CASTING

Filtration is particularly ideal for improving ductile iron cleanliness because large amounts of slag may be formed from
base-metal sulphur, magnesium, silicon and rare earths introduced during the nodularizing process. Other metals obtain
similar improvements by the removal of their specific slag. The melting point of the slag constituents, which makes the
general slag more or less fluid, determine the ease of its removal though proper filtration. Filtration generally has no
effect on the metal structure because the structure does not form until the metal passes through the filter and begins to
solidify.

Temperature control during metal casting is necessary for maximum filtration effectiveness. The tendency of high
volatility alloying agents to form slag at higher temperatures must be balanced with the tendency of other oxide reactions
to occur at lower temperatures. When coarse particle blockage occurs prematurely, a larger filter area is needed. When
internal blockage occurs, higher pouring temperature or less alloying agent may be required. Also, the cleanliness of the
alloying agent itself is critical; some materials are known to produce the cleanest metal.

GUIDELINES FOR FILTER APPLICATION
When implementing a filtration system for any foundry practice, the filtration specialist considers these basic factors:
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    molding method
    alloying practice
    treatment practice
    desired filtration effectiveness
    pouring temperature and
    metal flow rate

For maximum filtration benefit, the specialist also designs a gating system that provides:
    simple filter insertion
    proper choke location
    consistent mold fill times
    filter location for optimum effectiveness
    minimum turbulence and erosion beyond the filter
    minimum gating system for maximum yield and
    worst-case blockage determination to guide filter sizing

The obvious benefits of castings with fewer inclusions are reduced foundry scrap and fewer customer returns. Not so
obvious, are improved yields from labor, materials and equipment. Ceramic filters, located as close as possible to the
casting cavity, are a ―last chance‖ to trap inclusions.

By remembering that filtration is not a replacement for good foundry practice and by including it in the gating system
design, maximum results and major contributions to total process control can be obtained.

Author’s Note

The benefits of using filtration in ductile iron casting were the focus of a recent quality improvement program conducted
at a major midwestern automotive foundry. This highly automated operation casts ductile iron crankshafts. A new
casting procedure including filtration reduced annual customer scrap rates from 1.89% to 0.12%, a 93% reduction. The
modified gating system reduced the total pour weight of each mold by 9 pounds, for a total iron saving of 354 tons per
year.

The calculated pouring-time standard deviation was reduced by 43% and the modified gating system led to a more
uniform average pouring time. The improved casting practice was also credited for a productivity increase in the
automaker's machining line, with fewer crankshafts rejected due to internal defects.

FILTER APPLICATION
While adding filter elements to conventional gating has proven beneficial, designing gating systems specifically for
filtration greatly enhances filter performance and substantially increases yields. These systems afford simple filter
placement, consistent fill times, optimum filtration and minimum gating size.

THE IMPORTANCE OF FILTER POSITIONING

Locating the filter as close to the casting cavity as possible, offers an opportunity to trap most of the inclusions
formed within the runner system itself. However, the molding method used (vertical or horizontal), automatic
equipment restrictions, the alloying treatment and in iron inoculation method affect the filter position.
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Since the filter must withstand the full force of the molten metal flow, adequate support must be provided (without
detracting too much from the active filter area). This is particularly critical where larger filters are used and direct
vertical impingement is unavoidable. Also, the orientation of the filter to the molten metal stream has a significant
effect on how much metal is filtered before blockage occurs.

With horizontally parted molds, four filter positions may be used. Positioning horizontally in the runner, with metal
flow from cope to drag, allows large inclusions to float up before they reach the filter (Fig. 6-8a). Conversely,
allowing metal to flow from drag to cope forces the large inclusions to move up against the filter and quickly restrict
the passage of additional metal (Fig. 6-8b). Vertical positioning in the runner with a slag trap at the entry face also
allows inclusions to float above the filter (Fig. 6-9). Another alternative, vertical positioning with the filter placed at
the base of the sprue creates a scouring action, which discourages larger inclusions from prematurely blocking the
entry face (Fig. 6-10).

Fig. 6-8. (a) Horizontal filter placement with metal flow from cope to drag and (b) horizontal filter placement with metal flow
from drag to cope are illustrated.

Fig. 6-9. Vertical filter placement with a slag trap at the entry face is illustrated.

Fig. 6-10. Vertical filter placement at the base of the sprue is shown.

In vertically parted automatic molding, filters are often positioned at the base of the pouring cup. While this
simplifies filter placement, it places the filter at a considerable distance from the casting cavity and limits its ability
to trap any inclusions formed downstream within the mold. Placing the filter at the base of the sprue with automatic
core-setting equipment is much preferred and also allows the use of multiple filters if necessary. Proper orientation
of the filter to the molten metal stream maximizes performance and minimizes the filter's effects on pour time and
pour-time variations.

MODIFIED MOLDED GATING

Conventional gating systems include long runners, slag traps, swirl bobs and runner extensions, which give lower-
density, non-metallic inclusions time to float up out of the metal stream and adhere to the mold surface. However,
adequate casting quality is not always obtained and these extensive systems consume valuable pattern plate space
and sacrifice mold yield (Fig. 6-11).

Fig. 6-11. Conventional gating consumes excessive pattern plate space.
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Molded gating systems properly designed for filtration are more effective in controlling inclusions. They allow more
parts on the same pattern plate or reduce pour weights (Fig. 6-12). Where less metal is required, pouring rates are
reduced to avoid turbulence and secondary oxidation.

Fig. 6-12. Filter gating optimizes pattern plate utilization.

As mentioned previously, filters are placed as close as possible to the casting cavity to minimize the opportunity for
inclusions to form downstream of the filter. Also, gating is designed so that it completely fills and maintains a metal
head before metal begins to enter the casting cavity. This prevents air aspiration, turbulence beyond the filter and
secondary oxidation.

Metal flow characteristics not only affect filter performance, but consistency of mold pour times as well. This makes
the relationship of filter size to other gating cross-sectional areas critical, especially in automated operations. The
filter must not be so small that it becomes the controlling choke, causing varied pour times as large non-metallic
inclusions block the entry face.

As a broad rule of thumb for iron and a lesser amount for other metals, it is possible that 2/3 of the filter entry face
eventually becomes blocked by inclusions during the pour, leaving 1/3 for continued metal flow. Thus, the active
filter area (total area minus support area) should be at least three times the area of the controlling choke (smallest
runner cross section). See Fig. 6-13 for typical gating ratios using ceramic filters.

Fig. 6-13. Typical gating ratios for filter gating systems are shown.

The cleanliness of metal (and the amount of filter blockage which occurs) can vary greatly from foundry to foundry
and from heat to heat. Therefore, it is suggested that a filter four to six times the area of the controlling choke should
be used at the beginning. Later the size of the filter could be reduced as warranted. If premature coarse-particle
blockage is a problem, a larger filter area is needed. If internal oxide blockage occurs, higher pouring temperature or
less alloying agent is indicated

The location of the system-controlling choke relative to the filter also affects filtration, especially in horizontal
parting applications. Pressurized gating systems (choke at ingate) which are common in conventional systems, do
not offer the best environment for optimum filtration. They usually require rapid metal flow switching from cope to
drag, which causes turbulence and can contribute to mold erosion and sand inclusions. In addition, these systems
often include large slag traps or runner bars, which reduce yield. As a general guideline, all runners and filter prints
should be molded into the drag with the exception of a small cavity to support the top side of the filter. The ingates
should be located in the cope.

In vertical parting systems, casting size and casting location create unique conditions from plate to plate; therefore,
each case must be considered separately to determine the best location of the filter in relation to the choke.
Numerous tests and production experience have shown, however, that the filter works best in molded gating when
placed near to the bottom of the pattern plate. This allows the casting to fill from the bottom up, eliminating
turbulence within the casting cavity.

Proper filter print design is needed to assure adequate filter support, especially on the downstream side, to resist the
pressure of the molten stream and assure optimum flow through the filter. Adequate recessing of the filter on the
entry or upstream side is necessary to create a full seal and assure that metal does not "go around" the filter. Filter
manufacturers recommend the amount of support required for each filter size and offer filter prints, which meet their
specifications.
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Vertical-parting filter prints are split in half (a "diamond split"), placing half of the filter in the ram and half in the door.
One side includes a "crush strip" which is marginally undersized to allow secure placement of the filter. In automatic
insertion, this holds the filter in place as the core mask pulls away and assures that the filter is presented at exactly 90 as
the next mold closes (Fig. 6-14).

Fig. 6-14. Diamond split filter print places half of the filter in the ram and half in the door.

Another configuration, a "full-ram" print, places all but the small support recess mentioned above in one half of the
mold (Fig. 6-15). In this position, direct metal impact on the filter can be avoided by placing the sprue on one side of
the parting line and transferring iron to the filter on the other side.

Fig. 6-15. Full-ram filter print avoids direct impact of metal on filter.

Computer software that determines optimum filter size and CAD systems that assist in pattern plate layout are used.
This technology has simplified the design of molded filter gating systems and optimized pattern plate utilization.

The use of ceramic filters in the casting process to remove non-metallic inclusions is a proven way to improve
casting quality and improve yields from labor, materials and equipment. By trapping furnace and ladle slag,
treatment slag, mold materials and secondary oxidation products, cellular and foam filters are the last chance at
process control before solidification occurs.

DIRECT POUR FILTER/SLEEVE ASSEMBLIES

The direct pouring of castings completely eliminates molded gating and permits direct pouring into insulated
refractory sleeves. First developed for non-ferrous and steel casting, patented direct pour filter/sleeve assemblies
have been successfully trialed and are now being utilized in major high-production ductile iron foundries.

Direct pour assemblies incorporate refractory insulating sleeves with ceramic filter inserts and function as a
combination pouring cup, filter and riser sleeve. They incorporate or eliminate all the parts of a conventional gating
system, including pour cup, downsprue, risers, runners, filter prints and ingates (Fig. 6-16).

Fig. 6-16. Direct pour filter/sleeve assemblies incorporate all gating functions in one refractory unit.

The insulating sleeve directs the metal to the casting and acts as a feeder during solidification. The filter traps
furnace and ladle slag and other inclusions and controls the metal flow into the casting cavity. The sleeve unit can be
rammed up in position or inserted into a cavity of suitable design. With certain provisions, the unit can also be used
in automated matchplate and vertical parting applications, including high-pressure jolt-squeeze. The unit should be
placed as close to the casting cavity as possible and, to minimize turbulence within the casting cavity, the distance
between the bottom of the filter and the impact area should be limited.
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By providing filtration, eliminating the runner system and keeping the riser metal molten longer, direct pouring can
offer major product quality and production efficiency benefits to the foundry in many applications. Cleaner castings
reduce foundry scrap, lower customer return rates and decrease tool wear during machining. Increased volumetric
feed characteristics and enhanced directional solidification result from the improved thermal gradient and, with less
shrinkage, castings are nearer to net shape.

The size of the sleeve creates a larger "pouring basin" or target area. In automated operations, even if the
filter/sleeve is placed by hand, cycle times do not increase because pouring less metal reduces pouring time. The
elimination of the runner system often increases pattern plate utilization which increases yield. Shakeout, despruing,
cleaning room and inspection costs may also be reduced.

FACTORS TO CONSIDER

When planning a molded filter gating or contemplating a direct pouring system for a casting application, these basic
factors must be considered.

Molded gating:
 Ease of filter insertion
 Filter size vs. pour weight
 Controlling choke location
 Consistent mold fill times
 Optimum filter effectiveness
 Minimum turbulence and mold erosion beyond filter
 Minimum gating size for productivity and yield considerations
 Compatibility with treatment methods

Direct pour filter/sleeves:
 Cleanliness of metal from ladle
 Analysis of metal
 Pouring temperature of metal
 Free-fall of metal after leaving filter
 Flow rate and head height vs. filter porosity
 Head height vs. turbulence
 Location of unit on casting cavity for best results

Filtration is a vital step toward total process control and careful planning of the gating system or direct pour unit is
the first step toward optimum filter utilization.
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CHAPTER 7—Solidification Science
Heat transfer is the very basis of the metalcasting process. A large part of everyday work in the foundry relates to
transferring heat in and out of the metal used to produce a quality casting at a profit. The efficiency with which this is
accomplished is dependent upon the knowledge of established practices, both basic and highly theoretical. The basics of
heat transfer as they apply to the production of metal castings are essential to design proper gating and risering systems.

Heat transfer is the energy being transferred from a hotter material to a colder material. In melting, the heat is being
transferred to the colder solid charge materials. The temperature of the charge material is raised to the point where it
changes phases of matter and turns liquid. Extra heat is then added so that the metal stays liquid until the pouring of the
casting is completed. After the molten metal is in the mold, the process of heat transfer is reversed. The heat in the
molten metal is removed by being transferred to the cooler mold material (cores, if they are being used) and the
surrounding air. When enough heat has been removed, solidification begins and the molten metal turns into a solid
casting. Before the metal casting is handled, more heat has to be removed by heat transfer.

The method by which the contained heat in the metal is removed between the furnace and the shipping department helps
govern the quality of the casting. All decisions in regard to ladles, mold and core materials, pouring, shakeout, cleaning
and heat treatment reflect casting design and heat content for optimum casting quality. The focus of this chapter is the
control of heat in metals between the melting unit and the shakeout.

PHASES OF MATTER AND ENERGY

Materials in nature can exist in four different states—solid, liquid, gas and plasma. The difference between these states is
a result of the way the atoms making up the material arrange themselves. If the atoms are packed together tightly with
minimum distance between each one, the material exhibits the properties of a solid. As the distance between the atoms
increases because of their movement due to an elevated temperature, the material starts to change from a solid to liquid,
and finally it can change to gas or even plasma as a final chaotic state. This behavior is temperature dependent. The
temperature level itself is material dependent, i.e., mercury is liquid at room temperature while all other metals are solid.
The state in which a material exists is a result of the type of substance being considered, the heat content in the material
and the atmospheric pressure. Since the pressure in the foundry is commonly that of the atmosphere (14.3 lbs/in2 [1.01
kg/cm2]), this factor is of minor concern. Thus the affects of atmospheric pressure are not discussed in detail. It is
sufficient to state that the lower pressures reduce the amount of heat required to cause boiling.

Heat is a form of energy (the ability to do work), but to understand its effects on materials, it is important to understand
the effect of this energy on the atoms of a material. An atom only moves when it has sufficient energy. As the material
heats up, the atoms begin to move in straight lines until they strike another atom or bounce off of the container walls. The
atoms bounce off one another, and some energy of the first atom is transferred to the second, third, atoms, etc. This is
very similar to the "break" in playing pool. The cue ball is the first atom containing the energy it received from the cue
stick, and then it strikes another pool ball, etc. This continues until the heat is uniformly distributed throughout the
container. This is known as heat transfer. When each atom in the container has the same amount of energy, the container
is said to be in thermal equilibrium.

The atoms in the solid charge material are nearly at rest and are located very close together, forming a dense material
known as a solid. When heat (energy) is added, the atoms begin to move about and strike other atoms, which were not
previously affected by the heat. Now these atoms begin striking other atoms. Adding heat to the metal keeps the atoms
moving about. Eventually all of the atoms of the metal charge are moving the same amount thus establishing thermal
equilibrium within the metal. Since the atoms are now moving about, they occupy a greater amount of space. Thus, when
metals are heated, they increase in volume because the movement and vibration of the atoms increases. The same
increase in volume that is added to a substance occurs in liquids and gases for the same reason (See Fig. 7-1).

Fig. 7-1. The movement of atoms in a gas, liquid and solid is illustrated.
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In order to understand the effects of heat on various materials, the following terms are defined:
Temperature—a measure of the amount of heat in an object whether it be solid, liquid or gas; not the same as heat;
measured in degrees Fahrenheit or Celsius.
Specific Heat —the amount of heat required to raise one unit of a solid or liquid material one degree of temperature;
measured in Btu/lb/ F (cal/g/C).
Latent Heat of Fusion—the amount of heat required to melt one unit of material at a constant temperature; measured in
Btu/lb (cal/g).
Latent Heat of Vaporization—the amount of heat required to transform liquid to a gas at its boiling point at a constant
temperature; measured in Btu/lb (cal/g).
Superheat —the amount of heat used to raise the temperature of a metal above its melting point; in reverse, the amount
of heat that must be removed to bring the material from the liquid state to the solid state.

A very important aspect of changing the phases of matter is the change of the specific weight (density) that a material
encounters. With some exceptions, materials shrink when they are cooled from their gaseous state to liquid and further
down to solid. Density changes not only occur at phase changes, but within a phase, as well. Essentially the density of a
material is dependent on its temperature. In metal castings three density changes are important during the solidification
and cooling process—the density change in the liquid state; the density change during solidification; and the density
change during the cool down of the solidified casting (Fig.7-2).

Fig. 7-2. A temperature-density graph is illustrated.

EXAMPLE
Beginning with two pounds of ice, the goal is to melt the ice and heat the water to make a cup of coffee. The ice is at 32F
(0C). The latent heat of fusion for ice (water) is 144 Btu/lb (80.06 cal/g). In order to convert two pounds of ice to water,
288 Btu are required to be added to the ice. This energy converts the ice into two pounds of water, but the temperature of
the water is still 32F. Latent heat only affects the state of matter, not the temperature of matter (see Fig. 7-3).

Fig. 7-3. Energy graph depicts the additional energy necessary to change from a liquid to a solid (latent heat).

More heat has to be added to the water until it reaches the desired temperature, 100F. The amount of energy required is
defined by the specific heat requirements of the water, which is 1 Btu/lb/F (1 cal/gC). The difference between 32F and
100F is 68 degrees. For two pounds of water, the amount of specific heat to be added is 136 Btu. When this requirement
is reached, the water has a temperature of 100F, and it is ready to make the cup of coffee. The amount of heat, which is
added to raise the temperature of the water from 32F to 100F, is called superheat. Adding heat to the water results in the
water reaching 212F, or the boiling point of water. At this point, after the latent heat of vaporization has been met, the
water would turn to steam. In the foundry, the metal won’t be heated to the point where it begins to vaporize. However,
the metal is "superheated" so that it is transported from the furnace to the mold so that it can be poured. In between the
furnace and the mold, depending on the alloy, certain process steps are performed, i.e. inoculation, Mg-treatment,
degassing, grain refinement or modification of the alloy in order to pour the quality alloy required for the casting. These
procedures must also be considered when deciding how much superheat is to be added to the alloy. Also, the fluidity of
the alloy is greatly affected by the superheat.

This is a very simple illustration of what happens when metal is molten and poured into the mold. The relative
importance of the properties varies with different metallic materials, i.e. it takes approximately the same number of Btu
to melt one ton of aluminum as it does one ton of iron. There is a big difference in Btu required to superheat aluminum
versus iron. This is due to the properties of latent heat of fusion and specific heat of the two metals. Once the metal is
poured, the heat, which is added, needs to be removed so that it solidifies. This part of the metalcasting process is of
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major importance since it greatly affects the mechanical properties of the casting and also the elimination of some
casting defects.

HEAT TRANSFER AND THERMAL GRADIENT

Heat always flows from a hot object to a cold object. Nature strives for matter to maintain a constant state of balance
therefore it seeks to equalize the temperature difference between two objects. In order for heat to flow there must be
a driving force, the difference in temperature between the mold and cores and the molten metal. This temperature
difference occurs over an extremely small distance. The measurement of the heat loss over a unit of distance is
called a thermal gradient (Fig.7-4). A thermal gradient is measured in degrees over a given unit of distance (ΔT/ΔD).
This is also called a temperature gradient.

Fig. 7-4. A thermal gradient depicting the energy flow between two materials is shown in the graph above.

In order to increase or make the thermal gradient steeper several options are available. The material between the two
temperatures can be changed. The higher temperature can be increased or the lower temperature can be decreased. The
steeper the thermal gradient the more rapid the heat transfer and the faster initial solidification takes place. On the other
hand, the thermal gradient can be decreased by using materials in/on the molds or cores that act as insulators and reduce
the speed at which heat is transferred. Hence the solidification is sped up or slowed down. The solidification speed is also
called the cooling rate and is measured in degrees over a unit of time (ΔT/Δt).

There are three basic methods by which heat is transferred (Fig. 7-5).
1. Conduction— heat transferred through material either in the solid, liquid or gaseous state
2. Radiation—heat transferred through space in the form of waves or particles
3. Convection—heat transferred by fluid motion between regions of unequal density that result from nonuniform
heating

Fig. 7-5. Conduction, convection, and radiation are illustrated.

An example of conduction is holding the end of a metal rod and placing the other end of the rod in a gas flame. After a
period of time, the rod feels hot. Heat is transferred by conduction through the rod. Using this same gas flame, and even
the heated rod, heat can be felt being radiated away from the flame and rod. The measure of the intensity of the radiation
of heat from a material is called its emissivity. For instance, aluminum has poor emissivity, whereas iron has good
emissivity. Using the same flame, a fan is placed on one side of the flame and blows air over the flame. The air is heated
and transported through the room. In a ladle of molten metal, convection currents cause the hotter molten metal to rise
towards the top of the ladle and the colder metal on the surface to sink towards the bottom of the ladle. These are both
examples of heat transfer by convection.

In the metalcasting process, all three methods of heat transfer take place. From the minute the molten metal is tapped
from the melting furnace until it reaches the pouring station, heat is transferred and the molten metal is beginning to cool
down. As the molten metal is poured into the mold and once it enters the mold, heat is transferred by one, two or all three
methods of heat transfer. When defining the pouring temperature, it is the metal temperature at the mold and not at the
spout of the furnace or some transfer point. When studying casting defects it is also good to know if the defective casting
is poured from the first metal out of the pouring ladle or towards the end of the ladle (that is if more than one casting is
poured from the same ladle).

HEAT TRANSFER AND HEAT LOSS
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During the process of transporting the molten metal from the furnace to the mold, heat is transferred and lost. When
tapping molten metal from the furnace into a ladle, the stream is exposed in most cases to the atmosphere, and heat is lost
to the surrounding air. The bulk of the heat loss from the tapping stream is due to radiation, although a portion is due to
convection, especially if it is an area with air currents. The amount of heat loss varies with different alloys because of
their relative ability to radiate heat (emissivity).

Open risers (risers whose top surface is exposed to the air) in a mold into which cast iron, steel or brass is poured, are
affected by the emissivity of these alloys. The heat is being radiated, or given off, by the molten metal exposed to the air.
When tapping a furnace, the heat transfer by radiation or heat loss with brass, cast iron and steel alloys is larger than in
aluminum alloys.

The next heat transfer occurs in the ladle. Heat transfer takes place using any or all three of the methods. Conduction
takes place with molten metal to ladle wall contact. Radiation is due to the emissivity of the molten metal in the case of
open ladles. Convection occurs when the cooler air passes over an open ladle, and also when convection currents are
present in the molten metal bath.

The heat transfer in the ladle is controlled in different ways. Molten metal should be transferred as few times as possible
between the furnace and pouring station. A rule of thumb states that every time molten metal is transferred from the
furnace to the ladle and then again to other ladles as much as 25F (15C) is lost with each transfer. This is especially true
for alloys having high emissivity such as cast iron and steel alloys. Another way to control the heat transfer in the ladle is
to get the molten metal from the furnace to the pouring station as quickly as possible. As far as the ladle is concerned,
there are several procedures to try to help reduce the amount of this heat transfer. If the ladle is preheated, not only does
this reduce the amount of heat transfer due to conduction, but also drives off any moisture in the ladle lining. Moisture in
the ladle lining or coating is a source of gas pickup in the molten metal. This is especially true in the case of aluminum
alloys. Besides reducing the amount of heat loss due to conduction, the ladle lining material is conserved if the
preheating is done properly. By heating the lining, the thermal gradient between the molten metal and the ladle lining is
reduced. The smaller the difference in temperature between the molten metal and the ladle lining, the shallower the
thermal gradient, and consequently the amount of heat lost is reduced. As more heat of molten metal is passed through
the pouring ladle, the temperature between the molten metal and the ladle lining becomes less as the lining heats up.
Therefore, it can be concluded that the first molten metal from an improperly preheated ladle is colder than that from a
properly preheated ladle. This is important to remember when pouring castings, since fluidity is greatly influenced by the
temperature of the molten metal poured.

The next area in which heat loss can be controlled is in covering the ladle top. Here heat loss is due mainly to radiation;
however, some heat loss occurs as convection currents move across the top of the ladle, especially near open doors. This
heat loss is of course greater for those alloys that have good emissivity characteristics. These include the cast iron, steel
and copper-base alloys. This means that the aluminum foundry worker can reduce heat loss due to radiation and
convection by covering the ladle. One side benefit of a covered pouring ladle is the comfort provided to the pourer.

When the molten metal is in the mold, it releases heat so that it returns to the solid state. By controlling this heat transfer,
the quality of the casting is controlled, as well. Heat transfer through permeable sand molds occurs by all three methods
of transfer—conduction, convection and radiation. Research shows that when the molten metal strikes the sand mold
surface, there is a rapid rate of heat extraction for a period of approximately one second. This is due to the steep thermal
gradient and also the moist sand at the molten metal/sand interface. During this period, the water (in the case of green
sand molds) is changed to steam, and extra heat is removed by convection of steam through the air spaces between the
sand grains in the sand mold. This reaction also occurs in chemically bonded sands where the binders generate gases,
which move back through the permeable mold. In green sand molds, the solidification of large, thick section castings is
relatively unaffected by water content in the sand. The solidification of thin section castings, however, is influenced to
some extent by the presence of water.

Heat is also transferred by conduction and radiation within the sand mold and core. The foundry worker can choose from
a variety of molding and core sands, which have different physical properties. The properties that are of more concern
regarding heat transfer are thermal conductivity, density, specific heat, thermal diffusivity and heat diffusivity. For
instance, chromite and zircon sands have high densities and a higher heat diffusivity than silica sand Once their specific
heat content is reached, which is rather rapid, their heat conductivity is not much better than silica sand. Chromite and
zircon sand remove and disperse the heat from the molten metal faster into the mold and core than silica sand. Therefore,
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the judicious use of these "specialty sands" aids in removing heat from the molten metal at faster rates than normal when
necessary.

Research has shown that the conductivity of sands increases markedly with temperature in the range of 1000-2300ºF
(537-1260ºC). Also, increasing grain size results in a more rapid increase of conductivity with temperature. This effect is
due to heat transfer by radiation from grain to grain in the voids between the sand grains. Thus, filling these voids
between the sand grains with fines results in decreased conductivity, since the effective path for radiation is diminished.
The paths for the convection of steam are also diminished.

In the case of permanent molding, where metal or graphite molds are used, the main method of heat transfer is by
conduction. The different materials, used to construct the mold, determine the rate at which heat is conducted. Most
permanent mold operations require a coating to be placed on the mold surface. This coating serves several purposes.
From a heat transfer standpoint, it serves as an insulator or as a conductor, depending on the coating material used. This
controls the heat transfer once the molten metal enters the mold cavity. With heat transfer under control, the
solidification patterns in the casting are predictable. When designing gating and risering systems, these basic principles
are essential. A properly designed gating and risering system, with heat transfer taken into consideration, aids the
foundry in producing quality castings.

SOLIDIFICATION OF METALS

In normal foundry practice, alloys are used, not pure metals. Alloys are the combination of two or more elements at least
one of which is a metal. For example cast iron contains iron, carbon and silicon. Different alloys are created to improve
the metal, to change its characteristics or lower costs. In most cases, a liquid metal requires only a few seconds or
minutes to go from the liquid to the solid state and become a metal casting. During this brief period of time, many
important characteristics affecting the casting quality are determined. The original crystal structure is established, upon
which many physical and mechanical properties depend. It is also during this time that many gating and risering related
defects are formed. Thus, it is important to consider the mechanisms by which a metal solidifies and the techniques,
which a metalcaster can use to control the solidification process.

This discussion of the solidification of metals is very basic and related to how it affects the design of a gating and
risering system. In order to describe the solidification of a metal, cooling curves are frequently used. Cooling curves are
plots of temperature versus time. They are achieved by placing thermocouples (sensors) in the casting allows the foundry
worker to follow the solidification and cooling process of a metal casting.

In binary (two-component) alloys, solidification occurs by one of three modes (see Fig. 7-6):
 At a constant temperature (pure metals and eutectic alloys—this last term will be explained later);
 Over a temperature range (alloys); and
 By a combination of freezing over a temperature range followed by solidification at a constant temperature
(proeutectic, plus eutectic solidification).

Fig. 7-6. Cooling curves of pure metal (left) vs. alloy (right) are illustrated..

Solidification is principally a process of nucleation and growth. Nucleation describes the process of very small grains or
crystals of solid matter forming or nucleating in the liquid metal. In other words, liquid metal contains small grains or
crystals. This usually occurs at the mold or core surfaces that are cooler than the liquid metal. These surfaces serve as
nucleation sites. As more heat is removed from the metal, the grains or crystals serve as nuclei (centers) onto which more
solid material attaches itself. The growth of these grains is influenced by the crystallographic and thermal conditions that
prevail, the chemical characteristics of the metal, and the cooling rate or rate of heat transfer.
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The driving force behind solidification is the extraction of heat from the liquid metal, which causes the solid state to be
more stable than the liquid state. The speed at which the heat is removed is dependent on the thermal gradient. The
steeper the thermal gradient, the more rapid the heat transfer. Under these conditions, there is a rapid nucleation of small
grains. However, as the heat is removed from the metal, the surrounding mold and encased core(s) temperatures increase
and the thermal gradient is not as steep. This "slow down" in heat transfer allows the grains that are forming, to grow
larger than those formed during the steep thermal gradients. The amount of heat that can leave the casting is controlled
by the relationship between its volume and the available cooled surface. The larger the surface through which the energy
can flow and the smaller the volume of the heat containing metal, the faster is the solidification. There is a big difference
in the conductivity of the metal versus the conductivity of the sand, especially in sand castings. The heat travels very fast
through the already solidified metal, and very slowly through the sand. That means, the liquid kernel of the metal
―considers‖ the contact surface between the casting and the sand as its cooled surface. Hence the cooled surface is
constant, while the overheated volume decreases. The last solidifying metal encounters a more rapid change of volume
than the earlier solidifying melt. This effect leads to a finer grain structure in the last solidifying sections. The way, that
the liquid metal solidifies after the initial skin of solid metal is formed, is determined by the crystallographic and thermal
conditions that prevail and the chemical characteristics of the metal.

The effects of corners on the rate of solidification depends on the cooled surface area to volume ratio. Protruding or
external corners of castings have a greater volume of mold material to extract heat thus skin formation occurs faster.
Inside or internal corners (reentrant angles) have more metal and less sand available to extract heat, and therefore,
solidify at a slower rate due to a shallow thermal gradient. Examples of this situation are shrink, blows, hot tears and
various sand related defects found in internal corners (see Fig. 7-7).

Fig. 7-7. Photos showing the sand corner effect (left: pool of liquid melt; right: shrinkage prediction).

FREEZING RANGE
The type of solidification pattern of an alloy is also determined by the temperature range over which solidification takes
place (see Fig. 7-8). This is referred to as "freezing range." This temperature range varies with the particular alloy
poured. The distinction between the freezing temperature ranges in degrees Fahrenheit or Celsius are not universally
agreed upon. Thus a freezing temperature range of 100ºF (38ºC) is considered by some to be a narrow freezing range, by
others a medium freezing range and by some a wide freezing range alloy (see Fig. 7-9).

Fig. 7-8. Cooling curves of metals A and B at different compositions show different freezing ranges.

Fig. 7-9. Freezing range at an identical cooling time (left: narrow; right: wide) is illustrated.

In general, pure metals and those alloys with a wide temperature freezing range are the more difficult to gate and riser.
Alloys with a narrow or medium temperature freezing range are easier to work with from the gating and risering point of
view. The pure metals or eutectic alloys do not have a temperature range over which solidification takes place. The
casting is liquid (except for the skin of the casting) and the instant that the solidification temperature is reached, it turns
solid. Therefore, there is only a very little chance of feeding any shrinkage. In these cases, the gating, risering and
chilling of these castings becomes very critical.

In the case of wide temperature freezing range alloys, the situation is just the opposite. These alloys normally have poor
fluidity and are difficult to feed because of the "mushy" type of solidification pattern in which there is both liquid and
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solid present. Risers with these alloys are used primarily to maintain metallostatic pressure against the atmospheric
pressure to avoid surface shrinks or "sinks."

Alloys that possess a narrow or medium temperature freezing range over which solidification takes place offer the
metalcaster the best chance of producing quality castings free of shrinkage. Due to these freezing temperature
ranges, there is an opportunity to gate and riser the casting so that feeding of shrinkage takes place

PURE METALS AND EUTECTIC ALLOYS
The solidification (or melting) temperatures of pure metals and eutectic alloys are accurately established. When a pure
metal is allowed to solidify in a mold, that portion of the liquid that first reaches the solidification temperature begins to
solidify. Due to the steep thermal gradient, this usually occurs next to the mold wall. This chilling action results in the
formation of a thin skin or shell of solid metal, which contains the remaining liquid metal. With sufficient heat extraction
through the thin wall of solid metal, the liquid metal begins to freeze onto it and the wall increases in thickness. This
growth continues progressively inward to the center and is governed by the existing thermal gradient. The interface
between the liquid and solid metal is relatively smooth because the metal is freezing at a constant temperature. Actually,
there is a mild change in the character of the interface as the front advances. The liquid metal near the mold wall is
supercooled and solidifies as small equiaxed grains. These grains are formed and grow by attachment of atoms from the
liquid onto foreign particles in the liquid. Small foreign particles are usually present in the melt (oxide inclusions, etc.)
and the mold surface also provides numerous nucleation sites. The best nucleus is a fine particle of the metal itself.
During the time the first skin of fine, equiaxed grains is forming, latent heat of fusion is released which tends to reduce
the amount of undercooling (or amount of thermal gradient) in the remaining liquid. The result is a tendency to stop
further nucleation. Growth continues, however, on some of the grains already formed. This growth is controlled by the
heat transfer from the casting into the mold. Since growth is also dependent on crystal orientation, only those grains
which are favorably oriented continue to grow with less favorably oriented grains being pinched off. This gives a
structure consisting of columnar grains with an outer skin of fine equiaxed grains. The columnar zone usually extends to
the center of the casting in the case of pure metals. However in some cases, it is followed by another zone of equiaxed
randomly oriented grains in the center region of solid solutions, caused by the fast heat transfer out of the last solidifying
areas (see Fig. 7-10).

Fig. 7-10. Local grain size in a sphere (white: large grains; center: small grains) is illustrated.

SOLIDIFICATION OF ALLOYS
Metalcasters are primarily concerned with the solidification of alloys. The solidification of alloys differs from pure
metals in three principle ways.
1. Solidification of alloys usually occurs over a temperature range.
2. The composition of the solid which first forms and separates from the liquid is different than that of the liquid.
3. There may be more than one solid phase separating from the liquid.

Figure 7-11 shows an alloy having two principal elements (binary alloy) having solute B dissolved in solvent A. In most
alloy systems, the solidification temperature is lowered with the addition of B, as indicated by the liquidus line in a phase
diagram. This diagram also shows that an alloy of composition, C, does not freeze at a single temperature, but over a
temperature range. The temperature at which freezing begins is called the liquidus and the temperature at which freezing
is complete is called the solidus. As stated earlier, the zone between the liquidus and solidus is called the freezing or
solidification range of the alloy. Another important consideration is the fact that, as the metal solidifies, the composition
of the solid forming is not the same as the parent liquid. In this example, it is richer in component, A. It starts as a
homogeneous melt of composition, C1, but the first solid crystals to form have a composition, C2. If solidification is
fairly rapid so that no diffusion occurs, the liquid at the interface becomes richer in component, B, than the bulk of the
liquid. Because an increase in the amount of B lowers the liquidus temperature, the liquidus profile is depressed.
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Fig. 7-11. A phase diagram is illustrated.

Plotting the possible temperature gradients on a curve illustrates a depressed liquidus. When the slope of the liquidus
curve is steeper than the thermal gradient, the liquid is said to be constitutionally supercooled. The effect of
constitutional supercooling is small. Certain preferred regions of the interface grow more rapidly forming "spikes" which
protrude into the liquid (see Fig. 7-12). This is because the driving force for solidification is greater in the supercooled
liquid, as well as because the spikes reject their solute at their sides, thus delaying solidification of these regions.
This type of solidification results in a honeycomb or cored structure. If the amount of supercooling is greater, the
spikes form side arms, resulting in the familiar dendrite structure. When the amount of supercooling is extreme, the
temperature difference reaches a maximum, which is large enough to lead to independent crystallization. In this
way, randomly oriented (equiaxed) grains are produced toward the center of the casting section. If the thermal
gradient is steeper, then the tendency toward columnar growth is greater (see Fig.7-13). This tends to minimize the
amount of dendritic growth and equiaxed crystallization, and facilitates metal flow for feeding solidification shrinkage.

Fig. 7-12. Shaded area shows constitutional undercooling.

Fig. 7-13. Different solidification modes are illustrated—planar (left), dendritic (center); and equiaxed (right).

The other common type of alloy used in metalcasting is the eutectic alloy. In this case, under equilibrium conditions, two
different solids are formed from the liquid at the same time at a constant temperature. If the composition lies to the left of
composition (CE), it is called hypoeutectic. On the other hand, if the composition lies to the right of composition (CE) it is
called hypereutectic. Should the alloy have the composition (CE), it is called eutectic composition. A eutectic alloy
solidifies as a pure metal. In other words, it does not freeze over a temperature range. At composition (L), the alloy is all
liquid until it reaches the solidus line, at which time solidification takes place at a constant temperature.

Note that as you move either to the left or to the right of the eutectic point (CE), that there is a zone in which both liquid
and solid particles are found. This zone widens to a given point, and then begins to narrow until it reaches a pure metal
such as A or B. The point within these zones, determines the temperature range over which an alloy solidifies.

GRAIN SIZE
Normally, the wider the temperature range over which the alloy solidifies the larger the grain size. The mechanical
properties of the finished casting are strongly dependent on the as-cast grain size. The methods that are used to control
grain size are of considerable importance. The refinement of grain size requires solidification to start from a large
number of nucleation sites, thus inhibiting excessive growth of any single grain. In general, this may be accomplished
by:
 rapid cooling,
 mechanical vibration, or
 addition of inoculants or grain refiners

A rapid cooling rate cools a large portion of the liquid metal to a speed where the rate of nucleation is high, particularly
near the mold wall. In pure metals, this results in a finer, though still columnar, structure.

Little is known about the exact mechanism of grain refinement brought about by mechanical vibration. It is suggested
that the energy (vibration) imparted to the liquid metal alters the energy requirements for nucleation, thus producing a
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finer grain size. Other work on this subject also suggests that the use of ultrasonic sound produces the same results;
however, results of this work are not conclusive. Another theory is that the mechanical vibration breaks small particles of
solid material from the grains and columnar growth. These small particles then act as nuclei for the liquid metal to
contact and freeze onto.

The first two methods of grain refinement have the disadvantage of either being limited in applicable casting size, or they
require extensive equipment. A more common method is the addition of foreign particles (inoculants or grain refiners) to
the liquid metal to provide nuclei. The materials added have a higher melting point than the liquid metal to which it is
added. It also has a favorable crystallographic orientation to permit adhesion of atoms from the liquid. Examples are the
use of inoculants to control eutectic cell size in cast irons, and grain refiners in aluminum alloys to control the size of
grains or crystals.

DIRECTIONAL SOLIDIFICATION
In the production of sound castings, the shrinkage which occurs when the last remaining liquid metal solidifies is the
main concern. When the metal begins to solidify at the mold wall and progresses more or less toward the center of the
casting it is called progressive solidification. When the solidification fronts meet at the thermal center, shrinkage
normally accompanies the solidification of the remaining liquid. Since any channels for additional feed metal are closed
off, centerline shrinkage occurs. Thus, centerline shrinkage is normally a good indication that a pure metal or an alloy
with a very narrow freezing range was used to make the casting.

On the other hand, when solidification begins at a point farthest from the source of feed metal and moves uniformly in
the direction towards this source, it is called directional solidification or unidirectional solidification. The source of feed
metal can be a riser or, in some cases runners. Because of the method by which this type of solidification takes place,
feed metal normally reaches this area and centerline shrinkage is eliminated.

Both types of solidification take place in a casting (Fig. 7-14). The "big race" in a casting is for directional solidification
to get to the source of feed metal before progressive solidification shuts the door. If this occurs, the chances of centerline
shrinkage are virtually eliminated.

Fig. 7-14. Basic solidification modes (progressive, directional, directional + progressive) are illustrated.
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CHAPTER 8—Solidification Applications
INTRODUCTION

Gating and risering systems are designed to promote directional solidification. In following this practice, there is a ready
supply of feed metal available to feed shrinkage. Several different feeding concepts are discussed as well as feeding aids.
Providing feed metal during the solidification period involves several variables:
 riser shape
 riser size as a function of casting shape
 location of riser
 grouping of castings
 riser connection to casting
 use of chills
 use of riser aids
 special conditions that develop from joining sections

FEEDING CONCEPTS

The primary function of a riser (feeder) is to provide liquid metal to compensate for shrinkage, which takes place in both
the casting and riser. The riser is a reservoir from which liquid metal is obtained to feed shrinkage. Once the mold is
filled with liquid metal and pouring stops, shrinkage begins to occur. The first shrinkage that occurs while the metal is
still liquid is called liquid shrinkage. Although it probably is not that noticeable in smaller molds, it is very obvious in
large floor and pit molds. Then, as the liquid metal begins to turn solid, solidification shrinkage takes place. Finally, once
the metal in the mold has turned solid, it shrinks further until it reaches ambient temperature. This final contraction is
called solid shrinkage and is considered when the pattern is built by making it slightly larger than the planned dimensions
of the castings (see Fig. 8-1). Table 8-1 shows the typical shrinkages allowances for various alloys.

Table 8-1. Typical Shrinkage Allowances

Fig. 8-1. The three phases of shrinkage are illustrated.

Of the three types of shrinkage—liquid, solidification and solid—occurring in the mold, the risers take care of the first
two types. Actually, risers are normally designed and sized to compensate for solidification shrinkage or to ―feed‖
solidification shrinkage. The ability of a riser to meet its requirements depends on the alloy that is poured. Thus, a good
understanding of the way the alloy(s) solidifies is important when considering the use and design of risers. Also, in some
cases mold wall movement is considered.

Only a small percentage of feed metal is required to compensate for solidification shrinkage. This may lead to the false
idea that small risers are all that are required to provide the necessary feed metal. However, risers are subject to the same
rules of solidification as the casting. The riser has to remain liquid long enough to feed the casting and itself. The casting
and the riser lose heat to the mold and cores by conduction, convection and radiation. With all three mechanisms, it is
apparent that the surface area of the casting or casting section and riser is important in establishing the rate of heat
transfer. Chvorinov1 expressed this relationship as follows:

t = V²/ A²
where:
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t = solidification time
V2 = volume squared
A2 = surface area squared

Based on this ratio, in order for a riser to remain liquid longer than the casting or casting section it is to feed, the riser’s
ratio must be larger than the casting or section to be fed. An easier approach is to use the ratio between the volume (V)
and the surface available for cooling, called modulus.

The modulus can also help to determine the best riser shape to use. Of the four basic geometric shapes, the most efficient
shape based on the modulus, and thus solidification standpoint is a sphere. A sphere does not readily lend itself to
horizontal sand molding, although it can be used in vertical molding. Thus for horizontal molding, the next best shape is
the cylinder. It should be mentioned that where severe compaction of the molding sand is not required, some foundries
are using expandable polystyrene spheres as risers successfully. The square or rectangular shapes come in a poor third
and fourth. Again, in some instances, it is necessary to use square or rectangular risers. This is especially true where open
risers are used in flasks, which have flask bars. If this becomes the case, the foundry worker must be aware of this in
order to design a riser, which has a modulus less than the casting or section to be fed. If not, the casting or casting section
feeds the riser, in which case there is a sound riser and shrinkage in the casting.

Another precaution about flask bars—if the risers are located too close to the bars, the bars act as chills, and thus cause
the risers to solidify much more quickly. If this happens, the casting or casting section may feed the riser.

When using cylindrical risers, several factors are considered to help to approximate a spherical shape. The riser's height
to diameter ratio is maintained within a 1:1 or 1:1.5 ratio. A hemispherical bottom is attached to the bottom of the riser.
In the case of blind risers, both a hemispherical bottom and top are used on the riser. Blind risers, properly designed,
have a minimum SA/V ratio which improves their efficiency. A blind riser that is gated has the hottest metal in the riser
and the coldest metal in the casting, this promotes directional solidification. A blind riser can be smaller than a
comparable open riser. This is due to the minimum SA/V ratio if the riser is properly designed. Another advantage of a
blind riser is that there is very little heat loss to the atmosphere when compared to an open riser.

Risers must always be located so they feed casting sections that solidify last. A thermal design simulation of the casting
helps to determine which sections of the casting solidify first, and in turn are fed by heavier sections that solidify last.
This promotes directional solidification. The use of casting process simulation programs is highly recommended. When
used correctly, these programs take much of the guess work out of locating risers.

The heat loss of the liquid metal in the casting is governed by the shape of the casting. All castings have a thermal design
that determines the order in which the various casting sections solidify. Armed with this knowledge, the foundry worker
can more accurately locate risers and might eliminate other risers. This information may also answer questions why hot
tears, gas defects, sand related defects and shrink are prevalent in certain casting sections.

Understanding the thermal design of the casting also aids in locating the feed path(s) in the casting. With this information
the number and placement of risers needed for a given casting is determined. When the mold contains multiple castings
which are the same, there is the possibility that one riser may feed more than one casting. Attaching multiple castings on
a riser reduces the total riser volume that is required and increases the mold yield while still producing sound castings.
However, riser removal needs to be considered. The cost of this operation may exceed the dollars saved by grouping
castings around a common riser.

There are two basic types of risers, hot (live) or cold (dead). The location of the riser normally determines the riser type.
If the liquid metal passes through the riser in filling the casting, it is a live riser or hot riser. On the other hand, if the
liquid metal passes through the casting and then into the riser, it is a dead riser or cold riser (see Fig. 8-2).

Fig. 8-2. Riser nomenclature is illustrated.
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A good rule to follow is to gate through the riser into the casting whenever possible. One of the main requirements of the
riser is that it remains liquid longer than the casting or casting section it is to feed. In order to do this, the riser has the
hottest metal at the end of the pour. If the casting is gated through the riser, this requirement is met.

Risers can also be named by their location on the casting. Side risers are attached to the sides of the casting, whereas top
risers are found on the top surfaces of the casting. In both cases, these risers are either hot or cold, based on the gating
system. Normally, top risers are cold and side risers are hot due to the way the liquid metal is delivered to them. In
vertical gating, the top riser is a hot riser if the gating system delivers metal to the casting through the riser. Another
breakdown of riser names is whether or not the top of the riser is open to the atmosphere. The term open riser indicates
that the top of the riser is open to the atmosphere; a blind riser is completely enclosed by the mold material and not open
to the atmosphere.

There are times when it is not possible to use hot risers. At times such as these, the coldest metal is in the riser and thus
the casting feeds the riser. Several options can help this type of riser fulfill its duty. In an open riser, hot topping the riser
by pouring hot molten metal into the partially filled riser is an option. In some cases, this is done at intervals to
accommodate liquid shrinkage taking place in a very large casting and in the risers. One precaution, when doing this, is
to ensure that there is a sufficient "heel" of molten metal in the riser to prevent the falling stream of molten metal from
entering the casting. If this occurs, slag, dross and air are drawn down and trapped in the casting.

The use of riser aids extends the life of a dead or cold riser. Riser aids are broken down into two main categories,
insulating and exothermic. An insulating sleeve acts similar to insulation in a house. It reduces the amount of heat
transferred between the riser and mold material. Exothermic sleeves rely on an exothermic reaction, set off by the heat of
the molten metal, to raise the temperature of the sleeve and reduce the thermal gradient between it and the riser. This
action in turn slows down heat transfer. In all cases, whether an insulating or exothermic sleeve is used, precautions must
be taken to insure that the material used in the sleeves is compatible with the alloy poured. The effectiveness and
efficiency of these sleeves depends on the casting size and geometry, alloy type and sleeve material. Guides are available
from sleeve manufacturers to determine the optimum sleeve for the application.

Riser sleeves are used with hot or live risers. The main purpose of their use in this case is to help reduce the amount of
liquid metal needed in the riser or reduce the size of the riser. The foundry must calculate the value of the metal saved
versus the cost of the sleeve. Sleeves also help to reduce the amount of heat transferred to the mold material. The
efficiency of riser sleeves, whether insulating or exothermic, is reduced if the risers are open and if the alloy radiates heat
very readily. To overcome this loss of heat, a topping material is used to cover the open riser. These cover materials are
sometimes called anti-piping compounds, since they cause the riser to "dish" rather than "pipe." Materials used in anti-
piping compounds include proprietary exothermic and insulating materials, powdered charcoal, graphite, fly ash, rice or
oat hulls, refractory powders and dry unbonded silica sand. One problem encountered with the use of exothermic
materials is the evolution of gas during the exothermic reaction. Considerable attention must be directed toward venting
in order to prevent gas defects in the castings. Gas evolution is also a consideration with respect to occupational safety
and health. The use of insulating materials instead of exothermic materials minimizes these problems.

The use of these materials must be carefully evaluated from an economic standpoint. The reduction in cleaning and
finishing costs and the improved yield are opposed by the additional cost of the materials, handling and storage. Some of
the benefits attributed to the use of these materials include: reduced amount of molten metal which must pass through
the gating system, reduced metallostatic pressure and the ability to pour larger castings than possible with conventional
risers (based on maximum metal availability).

There are three conditions that affect riser feed: metallostatic pressure, partial vacuum in the casting and atmospheric
pressure. The metallostatic pressure is the weight of the metal in the riser. Normally risers are taller than the section of
the casting they are to feed to take advantage of metallostatic pressure. As the casting solidifies it tends to form a partial
vacuum, especially in the case of the skin forming alloys. Atmospheric pressure is probably the most important of the
three.

In the case of skin forming alloys the gates solidify early and the risers and casting form a closed system. A partial
vacuum develops when shrinkage occurs in the casting and feed metal is pulled into the casting. This occurs only if
atmospheric pressure is allowed to act upon the liquid metal remaining in the riser. This is accomplished by using a core
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(i.e., pencil core) to break the riser skin and introduce atmospheric pressure to the remaining liquid metal. This type of
core has many names—Williams core, atmospheric core, firecracker core and pencil core.

The positioning of this core in the riser is very important. The core is placed so that the atmospheric pressure reaches the
thermal center of the riser. If this is not done, the metal in the riser solidifies around the core and atmospheric pressure is
not able to reach the last remaining liquid metal in the riser. There is no advantage in providing atmospheric pressure
feeding for alloys that do not readily form a solid skin. The partial vacuum required for the success of this method is not
developed. Many cast iron foundries use a dimpled riser to carry out the same basic job of the atmospheric core. In order
to use this type of riser, a blind riser is required. When molding this riser a sharp dimple is molded into the top of the
riser. The theory behind this riser is that the sharp point of molding sand becomes very hot due to the surrounding molten
metal. The sand, being a good insulator and poor conductor of heat, remains hot and retards the formation of a solid skin
of metal, thus allowing atmospheric pressure to help force liquid metal into the casting from the riser.

The correct design and placement of risers is very critical if they are to perform their intended duties. Time allotted to
this area of gating and risering is time well spent and provides considerable savings to the foundry.

Before discussing the types and design of risers, a discussion of the procedure to follow in risering a casting is essential.
1. It should be determined if the castings must be perfectly sound, or if there can be some dispersed
microshrinkage. This is a matter to be discussed with the customer. If the end use of the casting is such that
dispersed microshrinkage is not harmful, then multiple ingates can be used and risers eliminated.
2. It should be determined whether or not a riser is needed. Too often, risers are used when they aren't necessary.
Sometimes it is possible that the casting will be sound without risers. However, very seldom is a useless riser
removed, and therefore money is lost every time the pattern is run.
3. A riser seldom feeds an entire casting. In the majority of cases, the riser feeds only a section of a casting.
Therefore, the feed paths in the casting should be found, since they may be used to reduce the number of risers.
Handy tools that aid in finding feed paths are computerized casting process simulation programs.
4. Calculating riser size has to be used considering the modulus (volume to surface area ratio) and the volume of
feed metal required.
5. The cleaning room is a cost factor because the casting design may dictate that the risers be placed in an
inconvenient area for removal. The contact area between the riser and the casting is an important factor with
regard to riser removal.

There are times when a riser cannot be placed in a specific area because the design dictates that a locating tool or tooling
point must be located in this area. The same is true for the placement of ingates. It is essential to discuss these topics with
the casting designer to achieve the best performing casting at the lowest possible cost.

FEEDING DISTANCE AND CHILLS

The ability of a riser to feed depends on what is called feeding distance. It is obvious that for a given section size,
geometry, alloy, etc. there is some finite distance that the riser is able to feed liquid metal. If this distance is exceeded,
shrinkage cavities result. Many times the feeding distance concept is not understood. If there are shrinkage cavities some
distance from the risers the first idea is to increase the size of the risers. Unfortunately, the shrinkage remains, because
the feeding distance is exceeded.

In the previous chapter, the concepts of progressive and directional solidification were discussed. Progressive
solidification is the uniform advancement of the solidification front towards the thermal centerline of the casting section.
When the parallel advancing fronts meet, centerline shrinkage occurs. Directional solidification is the solidification from
a region of colder metal toward the area of hotter metal. The mode by which solidification occurs depends on many
factors such as alloy type, freezing range, etc. The thermal conditions have a most pronounced effect on the behavior,
however, and the steeper the thermal gradient in the section, the better the conditions for feeding.

A simple example is a bar-shaped casting with an end riser see (Fig. 8-3). Solidification occurs slightly faster at the end
of the bar opposite the riser. This phenomenon is called end effect and is due to the greater surface area of the casting in
this location as compared to other parts of the casting, causing greater heat extraction. A comparable effect is noted at the
edges of a plate casting. At the end of the casting where the riser is located, skin formation is retarded due to the mass of
heat contained in the riser. This effect also promotes directional solidification and is referred to as riser effect or riser
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contribution. If the bar is sufficiently long with a section in the center where neither the end effect nor riser effect are
operative, progressive solidification occurs and centerline shrinkage results. The feeding distance for this particular
section is exceeded.

Fig. 8-3. End effect and riser effect are illustrated.

It is important that the thermal gradient in the bar be relatively uniform with the end of the bar being at the lowest
temperature and the riser at the highest temperature. This is accomplished by gating into the riser. If the bar is gated from
the opposite end from the riser adverse thermal gradients are established and shrinkage usually results. In fact, in this
case the casting could actually feed the riser.

CHILLS
When the feeding distance for a given casting section exceeds that attained under normal circumstances, chills are
effectively used. This is due to the higher thermal conductivity of the chill material, which results in a greater rate of heat
extraction. This in turn establishes a steeper thermal gradient promoting directional solidification. If the chill is placed on
the end of the casting section opposite the riser, it increases the amount of ―end effect.‖ This in turn allows the directional
solidification to beat out progressive solidification before it can ―close the door‖ to the source of feed metal (see Fig. 8-
4).

Fig. 8-4. Chill effect is illustrated.

Chills do not increase the feeding distance of a riser. This is only done by changing the section geometry or size, and in
particular, thickness. Chills increase end effect and promote directional solidification. Chills are used to create an
―artificial end effect.‖ An example is placing a chill between two risers where no end effect normally occurs. In the case
of a ring type casting such as gears, chills are used to create an end effect and thus help promote directional
solidification. Many alloys are virtually impossible to feed properly without the use of chills; some examples of these are
wide-freezing-range alloys. Mechanical properties are also greatly improved with the use of chills.

There are essentially three basic types of chills.
1. External Chills—are used in almost any location or area of a casting where accelerated cooling is desired.
These chills are imbedded in the mold and lay up against the surface of the casting. If the surface is contoured,
chills are cast to accommodate the contour. Cracking brackets that are used on castings as strengtheners are
cooling ribs or ―pseudo-chills.‖
2. Internal Chills—are used primarily in positions of deep recesses or positions where external chilling is difficult
to apply. They are used in locations where they are removed by machining, i.e. in areas to be drilled, bosses or
bolting faces.
3. Chill Aggregates—are specialty sands such as chromite, zircon, or steel shot-sand mixtures. These are used on
a selective basis for fillets and pockets of castings for a quick skin formation. Many times, chromite or zircon is
used in washes or coatings that are applied to molds and/or cores.

The most common materials that are used for external chills are graphite, copper, steel or cast iron. Internal chills must
be metallurgically compatible with the alloy being poured. Usually internal chills are approximately the same
composition of the alloy in which it is used. Chill materials have a higher volumetric heat capacity than the alloy to be
chilled. (Volumetric heat capacity = volume x density x specific heat). They also have a relatively high thermal
conductivity and ability to distribute absorbed heat. Table 8-2 shows the relative chilling ability of various materials used
for external and aggregate chills.
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Table 8-2. Relative Chill Ability

There are some problems to be aware of when using external chills. The chills may "weld" or cling to the surface of the
casting. To prevent this, the chill must be properly prepared before using. The surface of the chill in contact with the
casting surface is ground and shot or sand blasted. A thin chill coating is then applied to keep the surface clean and free
from rust and moisture.

Cracking around chills or adjacent to chills is the primary reason most foundries reject their use. Many times this is due
to a ―severe chill.‖ Chill dimensions play an important role in this problem. Guidelines to be used in sizing external chills
are:
 Thickness = 0.5t to 1t (t = chilled section thickness)
 Width = 1t to 2t
 Length = 2t or 3t

These guidelines help to minimize cracking but do not control the problem completely.

The chill extracts heat from a localized area of the casting very quickly. This area then contracts too rapidly for the skin
of the casting adjacent to the chill to stretch fast enough to keep from tearing. This occurs on a microscopic thickness
level and it begins it progresses deeper into the casting, causing a hot tear. This problem is reduced and/or eliminated by
placing a layer of chromite or zircon sand next to the chill. These sands have a higher heat conductivity (or diffusivity)
than the silica sand in the mold, but a lower conductivity than the metal chill. Thus, the cooling effect is ―blended‖ at the
sand/metal interfaces.

When external chills do not work as intended, often times the placement of the chill in the mold is the problem. Chills
placed in the cope and sidewalls of the mold are not as effective as those placed in the drag. The reasons for this are
obvious because of what is happening as the mold fills and the metal begins to solidify. As the molten metal fills the
mold cavity, it is radiating heat upwards and sideways. This radiant heat ―preheats‖ the chill to some degree. Also as the
metal cools and begins to solidify, it contracts and pulls away from the chill in the cope and sidewalls. This action creates
an air gap between the chill and the casting. This air gap does not allow the conduction of heat from the casting to the
chill as readily as when the casting is in contact with the chill. When chills are placed in the cope or sidewalls of the
mold, they should have almost twice the surface area contacting the casting to do the same job as a full contact chill in
the drag of the mold.

Gas holes in and around the chilled area are usually caused by dirty and/or oily chills. Oxide films on the surfaces of the
chills cause gas problems. Also, cool chills placed in warm or hot molds and then rest for a period of time before pouring
begin to ―sweat‖ (form a moisture layer). When the molten metal hits these chills they ―kick‖ and cause gas holes to
form. When chills don't work at all, the problem is generally because they are too small. Another reason may be that the
chills are placed outside the effective feeding distance, recommended for the casting section.

Internal chills can also cause problems if not properly used. To achieve complete fusion of the chill into the casting, the
chill must be designed small enough. It is recommended that the weight of the chill be 2-5 % of the weight of the section
to be chilled; 4-5 % of the metal volume is commonly used. This estimated value is determined from the Btus or the
amount of heat that is required to melt the chill surface and still have the chill extract sufficient heat from the casting
section to set up an acceptable solidification front. The internal chill's surface area to volume ratio should be less than
14:1.

Gas defects associated with an internal chill are a result of different conditions. These chills condense moisture on their
surface (i.e., external chills). A localized pocket of gas forms around the chill when molten metal contacts the chill. This
gas condition may also be a result of iron oxide that forms on steel or iron chills. If possible copper coated chills are used
to minimize condensation or rusting. As with external chills, internal chills are placed in the mold as close to the time of
pouring as possible. The temperature of the chill is equal to or slightly higher than that of the mold material.
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Internal chills are placed so that they stand vertically; the use of horizontal surfaces is restricted. Also, the chills are
placed so that they are not strongly preheated prior to being enclosed by the molten metal. For internal chills, which are
not machined out of the casting, it is important to check the chill's chemistry. The chemistry in these cases must be
compatible with the metal alloy that is poured. For instance in carbon and low alloy steel castings steel chills are used. In
high alloy or stainless steel castings, chills should be of matching chemistry, otherwise incompatible microstructures
(causing cracking or corrosion failures) could result.

PADDING
Another method of promoting directional solidification and improving the feeding characteristics of the riser is the use of
padding. Padding is extra metal added to the casting, which in some cases has to be removed from the casting by
machining. This padding is best incorporated into the design of the casting, since it is not removed during cleaning and
finishing. The padding is used to set up feed paths into sections of the casting where it is impossible to place a riser. The
use of padding can eliminate the use of chills in some cases. If metal padding presents a problem, then an exothermic
or insulating material is used. The padding reduces heat transfer in the area where applied, which creates a favorable
thermal gradient and promotes directional solidification. In this case, the section of the casting is made to seem
thicker, than it really is, and is tapered thermally. The same economic and metallurgical considerations, like those
for exothermic and insulating sleeves, still apply. This material is purchased in sheet form and cut to the size and
shape desired. The padding is then attached to the mold so that it comes in direct contact with the casting.

DESIGN CHANGES
Failure to recognize the impact of liquid-to-solid (solidification) shrinkage is a common error that many design
handbooks make. In some alloys, disregard for this type of shrinkage results in voids within the casting. Both the
design and foundry engineer have the tools to combat this problem, but the designer has the most cost-effective
tool—geometry. The key parameter to a good casting design is the understanding of how solidification occurs. This
calls for a significant difference in design strategy in choosing geometry.

The metallurgical quality and mechanical properties of directional solidification are outstanding, but it must be
designed into the casting. A taper designed into the casting can be the key. In addition, the size of the riser for a
directionally solidifying alloy is larger than the riser for the eutectic-type or equiaxed solidifying alloys. The reason
is that the riser itself and the riser contact are trying to freeze off progressively. The riser must be considerably larger
to remain liquid long enough to feed the casting’s shrinkage.

The unique difficulty of equiaxed solidifying alloys is that it is not always possible to feed the solidification
shrinkage sufficiently to eliminate shrinkage in the casting. In some castings, no matter how large the riser or how
many of them are used, the feed metal avenues are shut off by the equiaxed solidification occurring in the middle of
the liquid. The design concept for geometry choice with equiaxed alloys is to spread out large thermal masses in the
casting. The objective is to have the casting sections as thermally equal as possible. This technique distributes
shrinkage microscopically over the inside of the casting. Although this sounds undesirable, it is effective. The
microscopic holes only have minimal effect on the mechanical properties of the alloy because of their size and
dispersion. If the design is not thermally neutral, chills are used to accomplish the dispersion. This can detract from
the casting’s surface finish. This thermally equal design concept is the reason some handbooks recommend avoiding
heavy casting sections and emphasize thin sections reinforced by ribs and gussets. This is fine for equiaxed
solidifying alloys, but the thermally equal approach does not work with directionally solidifying alloys.

―Soft shapes‖ are essential when designing castings for alloys with poor fluid life. In some cases, alloys do not flow
easily into the nooks and crannies of the mold, even when well above the liquidus temperature. The alloys tend to
soften the shape even if the mold is sharply detailed. It must be noted, however, that molding processes that provide
mold cavity refractory surfaces that are dry enable more detail to be cast in alloys with poor fluid life. Another
physical characteristic that requires soft shape design is the combination of large directional solidification shrinkage
volume and equiaxed solidifying alloys with high pouring temperature (with or without good fluid life). If there isn’t
sufficient use of soft shape geometry, the result is one or more defects including hot tears, pulls (small shrinkage
voids open to the surface) in sharp internal corners, hot spots, and/or cold cracks. The reasons for these defects are
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readily seen from heat transfer patterns in the molds. Good or bad heat transfer patterns are the direct result of the
design configuration of the mold cavity.

Evaluating heat transfer patterns in a casting design is easier than it sounds. Convection takes place in green sand
molds when the moisture is converted to steam and moves out through the sand mold (which is permeable). In
conduction and radiation, heat moves in straight lines, always perpendicular to the casting surface. Therefore, any
narrow peninsulas of mold material, surrounded by liquid metal, quickly become hot. The straight lines of heat
transfer run into each other, creating a hot spot and resulting in hot tears and pulls. Conversely, sharp external
corners or narrow peninsulas of liquid metal, extending into the mold material, cool quickly. In these cases, the
straight lines of heat transfer do not run into each other, with the only problem being that such areas on the casting
may cool too quickly. Cooling that is too rapid causes cold cracks and areas of high hardness in the casting.

The use of computerized solidification modeling programs (from conception to final design) helps design engineers
and metalcasters work together to solve possible casting problems due to heat transfer and geometry. Whether heat
transfer patterns result in hot tears, pulls and/or cracks depends on the pouring temperature and solidification
shrinkage volume resultant from the alloy and casting process. Both the designer and the tooling engineer must be
aware of the surfaces where risers, when required, are attached. The removal of risers and the subsequent grinding of
their contact areas have an affect on the casting dimensions. Risered surfaces should not be used for tooling targets
because they have been hand-ground and are not as consistent as cast surfaces.

REFERENCE

1.   Campbell, J. Castings, 2nd ed., Elsevier Butterworth Heineman, p 125 (2005).
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A
absolute temperature — temperature measured on the Kelvin temperature scale.
Admiralty brass (naval brass) — an alpha brass (typically 70% Cu, 29% Zn, 1% Sn) in which some of the zinc is replaced by
tin to increase strength and corrosion resistance.
Admiralty gunmetal — (government bronze) alloys of 87.5–88.5% Cu, 10.5% Sn and 1.5–2.5% Zn.
adnic alloy — a copper nickel with composition 70% Cu, 29% Ni and 1% Sn.
after expansion — permanent linear expansion of a material found after reheating to a specified temperature for a given time.
Expressed as a percentage of its original length.
alloy — 1) a substance composed of two or more elements at least one of which is a metal (for example, cast iron contains iron,
carbon and silicon) created to improve the metal, to change its characteristics or to lower its cost. An alloy usually possesses
metallic properties that differ from the properties of its individual components. 2) In minting, the base metal added.
alloy cast iron — cast iron that contains sufficient amounts of alloying elements (normally one or more of the elements Cr, Ni,
Cu, Mn and Mo) to produce measurable modifications of physical and mechanical properties by altering matrix structure
while not appreciably affecting graphite morphology. Si, Mn, S and P in normal amounts obtained from raw materials are
not considered alloy additions.
alloy steel — steel containing significant quantities of alloying elements other than carbon and the commonly accepted amounts
of Mn, Si, S and P.
alloying — adding elements to an alloy (other than the elements that usually comprise it) or to a metal to change its properties.
alloying element — chemical element(s) added to a metal in a bath or ladle to change its properties or to bring it to
specifications. The element(s) and the metal together constitute an alloy. In metals, alloying elements are usually limited to
metallic or metalloid elements that are added to modify the properties of the base metal.
Alpax alloys — proprietary eutectic-type Al-Si alloys composed of 87% aluminum and 13% Si.
alpha alloy — a single-phase solid solution.
alpha beta brass — a brass consisting of two solid solution phases, alpha and beta.
alpha brass — a copper-zinc alloy with greater than 64% copper, can be used in plumbing fitting applications.
alpha ferrite — see ferrite.
alpha iron — an allotropic (polymorphic) form of iron that is stable below the critical temperature for pure iron 1670F (910C). It
is ferromagnetic below 1414F (768C) and is characterized by a body-centered cubic crystal structure. Alpha iron is soft,
ductile and of fair strength and is capable of dissolving a few hundredths of a percent of carbon to form a solid solution.
alpha martensite — a form or stage of martensite of somewhat arbitrary distinction, probably representing the least-developed
and most-distorted stage in the transformation of austenite to martensite at ordinary temperatures.
alsifier — a ferroalloy with approximately 20% Al, 40% Si and 40% Fe, used in steelmaking for final deoxidation and for grain
size control.
aluminum — the most abundant metal in nature, a metallic element of 2.7 specific gravity, atomic wt 26.97, atomic no. 13,
melting point 1220F (660C). Extensively used for castings, foundry patterns and core driers and as a deoxidizer in iron- and
steel-making.
aluminum — (Al) a metallic element. The atomic weight is 26.98 and melting point is 660C.
aluminum brass — 1) In metal casting, an alpha brass with 2% Al to improve resistance to improve casting properties. Lead
(Pb) may be added to improve machining properties. 2) In wrought alloys, Al can be added to improve forging and extrusion
properties as well as oxidation resistance.
aluminum bronze — a copper-base alloy that contains 5–15% aluminum as the major addition element, may also contain iron,
manganese, nickel or zinc.
aluminum silicon — in metalcasting, an alloy of Al containing 5–22% silicon and small amounts of other alloys, Considered
lightweight and possesses high corrosion resistance.
ambient air — the surrounding air.
ambient temperature — temperature of the surrounding air.
ambrose alloys — a range of Cu, Ni, Mn and Zn alloys. See nickel silver.
apparent contraction — the net contraction of a casting dimension due to true metal contraction, mold wall movement, and
restraint during solidification and cooling.
apparent density — the weight per unit of apparent volume. Apparent Density ÷ Density = Percent Solid
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applied risering — various risering methods applicable to graphitic cast irons. The methods take advantage of the expansion
during solidification of this family of alloys.
AQL — Acceptable Quality Level. A quality level established on a prearranged system of inspection using samples selected at
random.
arrest points — horizontal sections of a time-temperature curve showing constant temperature on heating and cooling, which
indicates a phase change. See transformation temperature.
as cast — referring to a casting which has not received finishing (beyond gate removal or sandblasting), or treatment of any kind
including heat treatment after casting. See finish.
as-cast structure — matrix of castings as taken directly from the mold, without subsequent heat treatment, usually consists of
ferrite, pearlite and possibly carbides.
atmospheric riser — a riser that uses atmospheric pressure to aid feeding. Essentially, a ―blind riser‖ into which a small core or
rod protrudes. The function of the core or rod is to provide an open passage so the molten interior of the riser will not be
under a partial vacuum when metal is withdrawn to feed the casting but will always be under atmospheric pressure, thus
forcing metal into the casting cavity.
atomic      heat      —     the     heat    required     to  raise   the    temperature     of     one     gram-atom     of     an
element by 1˚ C.

B
back draft — a reverse taper from the designed or required direction of the draw of a pattern, corebox, die or similar forming
medium that prevents removal of a pattern from the mold or the core from the corebox without damaging the mold or core.
back gate — in centrifugal casting, a gate that enters the mold cavity from the part of the cavity nearest the perimeter of the
mold.
back vent — in centrifugal casting, a vent that exhausts the mold cavity from the part of the cavity nearest the mold perimeter.
base metal — principal metal of an alloy.
basin — a cavity on top of the cope into which metal is poured before it enters the sprue. See pouring basin.
bath — molten metal on the hearth of a furnace, in a crucible or in a ladle.
Bernoulli’s theorem — a theorem that states that in a stream flowing without friction, the total energy in a given amount of the
fluid is the same at any point in its path of flow. For example, in gating, the sum of the potential head, pressure head and
velocity head is a constant for all locations in a flowing system. See fluid flow principles.
blind riser — an internal riser that does not reach to the exterior of the mold and, therefore, is entirely surrounded by sand; often
combined with skim gates to form an efficient method of gating and feeding a casting. Contrast with atmospheric riser.
bottom gate — any gating system by which metal enters the mold cavity at its lowest level.
bottom pour ladle — ladle in which metal, usually steel, flows through a nozzle in the bottom. When the ladle is being filled, a
stopper blocks the nozzle. Upon removing the stopper, metal flows from the ladle.
bottom pour mold — mold gated at the bottom.
bottom running or pouring — filling a mold cavity from the bottom by means of gates from the runner.
branch gate — two or more gates leading into the casting cavity.
branch gating — see branch gate.
breakoff core — a thin core that is located between the riser and the casting. The breakoff core allows a thermally large but
physically small riser contact and does not impede the flow of metal. The core serves as a notch to assist in removing the
riser.
breakoff notch — a thin section of a gate or riser that facilitates and ensures a clean breakoff of the gate or riser while cleaning
the casting.
bypass gate — a gate that allows metal to feed around to other castings.

C
C — degrees centigrade or Celsius
cast — 1) To pour molten metal into a mold and allowing it to solidify in the mold. 2) The product of a single charge of an
electric or open-hearth furnace or a single blow of a Bessemer furnace.
castability — 1) a combination of liquid-metal properties and solidification characteristics that promotes accurate and sound
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final castings. 2) The relative ease with which a metal flows through a mold or casting die. See fluidity, fluidity test.
casting — (noun) metal object cast to the required shape by pouring or injecting liquid metal into a mold, as distinct from one
shaped by a mechanical process. The metal shape, exclusive of gates and risers that is obtained as a result of pouring molten
metal into a mold.
casting — (verb) the act of pouring molten metal into a mold to produce an object of desired shape. Specific methods of casting
include: centrifugal (liquid forging), centrifuge, continuous, die, investment, machine, permanent mold, plaster, plaster
mold, precision, pressure, sand, semi-centrifugal, static, tilt, vacuum and vibrational.
casting ladle — refractory-lined ladle used to transport molten metal from the melting furnace to the mold.
casting shrinkage — The reduction in volume of a casting during solidification.
casting volume — the total cubic units (expressed in mm3 or in.3) of cast metal in a casting.
casting yield — the weight of casting or castings divided by the total weight of metal poured into the mold, expressed as a
percent. See yield.
cavity — 1) the impression in a mold or die produced by withdrawal of the pattern to be filled by molten metal to give a casting
its external shape. 2) A hollow, sunken space or a void in the interior of a casting.
Celsius — temperature scale in which the temperature range between the boiling point and freezing point of water is divided into
99.99 degrees, with 0.01°C representing the freezing point (equivalent to 32F) and 100C the boiling point (equivalent to
212F). Formerly known as the centigrade temperature scale.
centerline shrinkage — shrinkage or porosity that occurs along the central plane or axis of a casting because of progressive
solidification or that occurs between casting sections (particularly those with plate-like or bar-like contours) that solidify
simultaneously from two faces and cut off feeding in the central portion.
centigrade — 1/100 of the difference between the temperature of melting ice and that of boiling water under standard
atmospheric pressure; °C = 5/9 (°F - 32) See Celsius.
chamfer — breaking or beveling the sharp edge or angle formed by two faces of a piece of wood or other material.
chill — (external) metal, graphite, or carbon blocks that are incorporated into the mold or core to locally increase the rate of heat
removal during solidification and reduce shrinkage defects.
chill — (internal) a metallic device/insert in molds or cores at the surface of a casting or within the mold to increase the rate of
heat removal, induce directional solidification and reduce shrinkage defects. The internal chill may then become a part of the
casting.
chill — (noun) 1) a device (usually metal, sometimes graphite) embedded in the surface of a sand mold or in a core to increase
the cooling rate at that point, thereby assisting in controlling shrinkage and in achieving directional solidification. See
external chill, internal chill. 2) The formation of white iron, which contains iron carbide (cementite) in lieu of graphite, in
cast irons by rapid solidification. Contrast with inverse chill.
chill — (verb) 1) to hasten metal solidification in certain segments of a casting by using materials with thermal conductivity
greater than that of the mold next to the casting surface. 2) To add solid metal to molten metal in a ladle to reduce its
temperature before pouring.
chill — a white iron structure that is produced in iron castings by rapid solidification.
chill block — a cast iron test block in which the depth of chill, as determined by fracture, is used as an estimate of the chilling
propensity of the metal.
chill coating — 1) (noun) a material applied to metal chills to prevent oxidation or other deterioration of the surface that might
result in blows when molten metal comes in contact with the chills. 2) (verb) Applying a coating to a chill that forms part of
the mold cavity so that the metal does not adhere to it or applying a special coating to the sand surface of the mold, which
causes iron to undercool.
chill coils — chills made of steel wire formed into helical coils or spirals.
chill depth — the distance into the casting to which white iron extends.
chill nails — special steel nails with heavy heads that are placed in certain mold sections to hasten solidification of the metal.
Also used to support the sand facing against which the metal will lie.
chill oil — see chill coating.
chill pin — one or more small pins cast onto selected surfaces to accelerate cooling.
chill structure — a matrix in which substantially all of the carbon is in solution in the combined form (carbides).

chill wash — a coating used on chills and metal-pouring basins to reduce defects from gas evolved when molten metal comes
directly in contact with these surfaces.
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chill zone — an area of a casting in which chilling occurs, such as long, sharp edges or exterior corners.
chilled edge or surface (crust) — a hard, brittle surface condition caused by rapid cooling of thin sections; can be detrimental to
machining.
chilling — 1) a method of compensating for unevenness of section in a casting by the use of chills. 2) A method of chilling iron
in the ladle by adding cold metal.
chills — metal inserts in molds or cores at the surface of a casting or within the mold which serve to hasten solidification of
heavy sections and cause the casting to cool at a uniform rate.
choke — 1) the smallest cross-sectional area in a gating system used to control the flow of metal beyond that point. The choke
often is located at the bottom of the sprue, but it also can be located in the runner(s) between the sprue base and the first
gate. See primary choke. 2) A restriction in the gating system to keep dirt, dross or slag from entering the casting proper.
choke area — restricted input area of a gated system. The choke area (A) can be determined for a given pouring time by: A =
W/[Ct ÷ 2gH)]; where W is weight of the casting,  is density of the molten metal, C is the flow efficiency factor, t is
pouring time, g is acceleration of gravity, and H is the effective metal above choke area A.
choke core — a core used to reduce the cross section at the bottom of a straight sprue in a nonpressurized gating system, forming
a choke at this point. This method is used when it is impractical to use a tapered sprue, as is the case with some highly
automated systems. Using a strainer core as a choke core is not recommended because the strainer core breaks the stream of
molten metal into several smaller streams, increasing turbulence and the chance for oxidation.
choked runner system — a runner system with a restriction placed in it (usually at the exit area of the sprue) to help maintain a
full sprue.
Chvorinov’s rule — the time (t) a metal body remains molten is directly proportional to the square of the volume of the metal
(V) and inversely proportional to the surface area (A) of the body. It can be expressed: t = KV2/A2, where K is a constant
cold shut — a casting defect caused by imperfect fusing or discontinuity of molten metal coming together from opposite
directions in a mold, or due to folding of the surface. It may have the appearance of a crack or seam with smooth, rounded
edges.
conduction — the transmission of heat, sound, etc., by the transferring of energy from one particle to another.
conductivity — (electrical) the quantity of electricity that is transferred through a material of known cross section and length.
The reciprocal of resistivity.
conductivity — (thermal) the quantity of heat that flows through a material measured in heat units per unit time per unit of cross-
sectional area per unit of length.
controlled cooling — process by which a metal object is cooled from an elevated temperature; a predetermined manner of
cooling to avoid hardening, cracking or internal damage. See cooling, controlled.
convection — the motion in a fluid resulting from the differences in density. In heat transmission, this meaning has been
extended to include either forced and natural motion or circulation.
cooling curve — a curve showing the relationship between time and temperature during the solidification and cooling of a metal
sample. Since most phase changes involve evolution or absorption of heat, there may be abrupt changes in the slope of the
curve.
cooling fin — see cracking strip.
cooling stresses — stresses developed by uneven contraction or external constraint of metal during cooling, generally due to
non-uniform cooling.
cooling, controlled — a process of cooling from an elevated temperature in a pre determined manner used to avoid hardening,
cracking or internal damage, or to produce a desired microstructure. This cooling usually follows the final hot forming
operation.
cracking strip — a fin of metal molded on the surface of a casting to prevent hot tearing, cracking.
critical points — temperatures at which phase changes occur in metals. See transformation temperature range, and iron-iron
carbide diagram.
critical temperature — 1) the temperature at which some change occurs in a metal or alloy during heating or cooling, 2) The
temperature at which alpha iron loses its magnetic properties. 3) The temperature above which a given gas cannot be
liquefied by pressure.
cross gate — see runner.
cross section — a view of the interior of an object that is represented as being cut in two, the cut surface presenting the cross
section of the object.
cutter, gate — a piece of sheet metal or other tool for removing a portion of the sand in a mold to form the gate or metal entrance
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into the casting cavity. See gate cutter.
cutter, sand — see sand cutter.
cutter, sprue — a piece of metal tubing or other tool used to remove a portion of the sand from a mold to form the sprue or
passage from the exterior of the mold to the gate. In addition, a machine used for shearing sprues and gates from castings.
See sprue cutter.

D
density — mass per unit volume of a substance, usually expressed in grams per cubic centimeter or in pounds per cubic foot.
directional properties — (directionality) Anisotropic relationship of mechanical and physical properties with respect to the
direction or axis in which they are observed.
directional solidification — refers to the arrangement of a solidification pattern in a casting by establishment of high
temperature gradients, whereby solidification of the metal begins at the point farthest from the metal entrance or sprue and
the metal progressively freezes or solidifies to and including the sprue.
dispersed shrinkage — small shrinkage cavities dispersed through the casting, which are not necessarily cause for rejection.
downgate — see downsprue.
downsprue (sprue, downgate) — the first channel, usually vertical, which the molten metal enters: so called because it conducts
metal down into the mold.
draft —1) (pattern) the taper on the sides of pattern which is perpendicular to the parting plane that allows the pattern to be
withdrawn from the mold without breaking the edges of the mold. 2) (permanent mold) The taper in the mold cavity that
allows the casting to be removed easily. 3) Taper on the vertical sides of a pattern or corebox that permits the core or sand
mold to be removed without distorting or tearing of the sand.
draw — 1) term employed to denote shrinkage that appears on the surface of a casting. 2) Formerly used to describe tempering.
3) To remove the pattern from the mold. 4) A form of porosity defect due to insufficient venting at the corners of castings.
dross — metal oxides in, or on the surface of, molten metal, or trapped in the casting.

E
emissivity — ratio of the rate of loss of heat per unit area of a surface at a given temperature to the rate of loss of heat per unit
area of a black body at the same temperature and with same surroundings.
enthalpy — the heat content per unit mass expressed in Btu per pound.
eutectic — 1) isothermal reversible reaction of a liquid that forms two different solid phases (in a binary alloy system) during
cooling. 2) The alloy composition that freezes at constant temperature, undergoing the eutectic reaction completely. 3) The
alloy structure of two or more solid phases formed from the liquid eutectically. 4) The lowest melting (or freezing)
combination of two or more metals.
eutectic alloy — in an alloy system, the composition at which two descending liquidus curves in a binary system or three
descending liquidus surfaces in a ternary system, meet at a point. Such an alloy has a lower melting point than neighboring
compositions. More than one eutectic composition may occur in a given alloy system.
eutectic temperature — the lowest melting temperature in a series of mixtures of two or more components.
external chill — a piece of heat-conducting material (usually metal) placed in the mold in such a way that it forms part of the
mold wall. An external chill is located at a point in the mold where rapid cooling of the metal is desired. The chill must have
a melting point substantially higher than that of the metal being cast and should be coated with a wash to keep it from fusing
with or sticking to the casting.
exudation — the escape of a liquid phase from the solidified surface of a casting.

F
Fahrenheit — 1/180 of the difference between the temperature of melting ice and that of boiling water under standard
atmospheric pressure: °F = 1.8°C + 32
feed head — a reservoir of molten metal provided to compensate for contraction of metal as it solidifies, by the feeding down of
liquid metal to prevent voids. Also called a riser.
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feed metal — the liquid metal that passes from the riser to the casting to make up for the volume decrease on cooling and
solidification of the metal, thus eliminating shrinkage from the casting.
feeder — sometimes referred to as a riser, it is part of the gating system that forms the reservoir of molten metal necessary to
compensate for losses due to shrinkage as the metal solidifies.
feeder, feed head (riser) — a reservoir of molten metal attached to a casting to compensate for the contraction of metal as it
solidifies, thus preventing voids in the casting. Also known as a riser.
feeder, feeder head (hot top, sink head, shrink head, feed head) — 1) a reservoir of molten metal designed to compensate for
the contraction of the molten metal as it solidifies. Molten metal flowing from the feed head (also known as a riser) prevents
voids in the casting. 2) the reservoir of molten metal from which the casting feeds as it contracts during solidification. See
riser.
feeding — pouring additional molten metal into a freshly poured mold to compensate for volume shrinkage while the casting is
solidifying; the continuous supply of molten metal, as from a riser, to the solidifying metal in the casting; keeping risers
open by manipulation of feeding rods.
ferrite — iron practically carbon-free. It forms a body-centered-cubic (bcc) lattice and may hold in solution considerable
amounts of silicon, nickel, or phosphorus; hence, the term is also applied to solid solutions in which alpha or delta iron is the
solvent.
fillet — a concave corner piece used to radius a corner on patterns and core boxes, a radiuses joint replacing sharp inside corners.
May be of wax, plastic, leather or wood. Fillets are used to avoid shrinkage cracks and eliminate concentrations of stress. A
struck fillet is one that is dressed to shape in place, usually of wax. A planted fillet is one made separately and affixed in
place. Fillets used at reentrant angles in cast shapes lessen the danger of cracks and avoid fillet shrinkage.
fillet leather — leather strip of triangular cross section used for filleting patterns.
fillet wax — beeswax in the form of sheet, rod, or triangular ribbon, used for filleting patterns.
filter — a material that separates solids from liquids or gases.
filter medium — a section of the filtration system that provides the liquid-solid separation, such as textiles, metal screens, paper,
porous media or granular bed materials.
finger gate — used on thin castings to allow rapid filling of mold. It is wedge-shaped with thin edge divided vertically to
produce several members or fingers. Metal flows into mold in several thin streams. Facilitates distribution of metal
horizontally in a mold over a wide area and breaking gate from a thin or delicate casting.
finish — 1) the handwork on a mold after the pattern has been withdrawn. 2) Surface condition, quality or appearance. 3) The
amount of metal allowed for machining.
finish allowance — the amount of stock left on the surface of a casting for machining.
finish grinding — the final grinding operation to achieve the desired surface finish and dimensional specifications.
finish mark — a symbol (f, f1, f2, etc.) appealing on the line of a drawing that represents the edge of the surface of the casting to
be machined or otherwise finished.
finished machine drawing — an engineering drawing for producing accurate finished castings that satisfy quality control;
displaying ± tolerances or dimensions.
finite element analysis — (FEA) a computerized numerical analysis technique used for solving differential equations to
primarily solve mechanical engineering problems relating to stress analysis.
fissure defect — any discontinuity in the casting surface; crack, tear, blow, shrink.
fit — amount of clearance or interference between mating parts.
flash — (fin, shift, twist) a thin section of metal formed at the mold, core or die joint or parting in a casting due to the cope and
drag not matching properly or where the core and coreprint do not match.
flask — metal or wood frame without top and without fixed bottom used to hold the sand of which a mold is formed; usually
consists of two parts, cope and drag. Some types include slip flasks, snap flasks, tight flasks.
flask, tight — a type of molding flask that remains on the mold during pouring.
flat back — a pattern with a flat surface at the joint of the mold. It lies wholly within the drag and the joint of the cope is a plane
surface.
flat gate — a wide gate with a narrow opening into the mold, used to pour thin flat castings.
flaw — discontinuity in a material that exceeds acceptable quality standards.
flow rate — the quantity or volume of a molten metal flowing past a given point in the gating system. Usually measured in
lb/sec. Compare to velocity.
flowability — the property of a foundry sand mixture that enables it to fill pattern recesses and move in any direction against
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pattern surfaces under pressure.
flow-off (pop-off, strain relief) — a large vent, usually located at the high point of the mold cavity. In addition to letting air and
mold gases escape as metal fills the mold cavity, the flow off fills with metal and acts to relieve the surge of pressure near
the end of the pouring.
fluid flow principles — laws derived from Bernoulli’s Theorem that apply to any fluid material, whether water or molten metal.
These principles should be considered when gating systems are being designed. See velocity and law of continuity.
fluidity — the ability of molten metal to flow. Common devices used to measure fluidity are spiral casting and the Chinese
Puzzle.
fluidity test — measuring fluidity by pouring a standard mold. Usually, a long, thin casting is poured in the form of a spiral, and
the length of the spiral serves as the measure of fluidity. Factors like metal composition and superheating will influence
fluidity as will other factors, such as metal viscosity and surface tension.
fluorescent crack detection — application of penetrating fluorescent liquid to a part, then removing the excess from the surface,
which is then exposed to ultraviolet light. Cracks appear as fluorescent lines.
fluorescent penetrant inspection — a nondestructive test for detection of surface discontinuities with fluorescent material
viewed under black (ultraviolet) light.
fluoroscopy — examination of a radiographic image on a fluorescent screen.
fold — a casting surface defect, as in cold shut and cold lap.
foundry nails — coated steel nails with heavy heads, inserted in the mold wall to hasten chilling action of the metal at that
particular point, as well as to aid in holding the sand metal interface and prevent it from spalling; used to prevent cuts,
washes or scabs at the gate.
foundry returns — metal in the form of sprues, gates, runners, risers and scrapped castings, with known chemical composition,
that are returned to the furnace for remelting. Sometimes referred to as ―revert.‖
freezing range — that range of temperature between liquidus and solidus temperatures where molten and solid constituents
coexist.
friction — force acting between two bodies in contact, opposing their relative motion.
air pollutants released to the air other than those from stacks or vents; typically small releases from
furnace — an enclosed structure in which heat is produced for one purpose or another. Some (but not all) include: barrel, bell,
channel type induction, charge resistance, continuous, continuous annealing, crucible, dipout, double chamber, dry hearth,
melting and holding, electric arc, electric resistance, electric rocking, electron beam, high frequency, holding, incinerator,
induction, kettle, liftout (pit) crucible type, low (line) frequency, muffle, multiple-hearth (in hearth, out hearth), periodic
annealing, plasma arc, reducing, regenerative, resistance, reverberatory, rocking, rotary, shaker-grate, short annealing,
special atmosphere, stack charge, stationary pot, tilting, vacuum (consumable electrode).
furnace atmosphere — gases with which metal is in contact during melting or heat treating.
fusible alloy — low melting point alloy, usually of bismuth, cadmium, lead and tin, which melts at temperatures as low as 160F
(70C).
fusion — the change from a solid to a fluid state resulting from the application of heat alone or through the action of heat and a
flux. Also, a casting defect due to sand fused to the casting surface.
fusion point — 1) metals, that temperature at which a solid substance changes to a liquid. See melting point. 2) Nonmetals, that
temperature reached in the heating of foundry sand, clay, or refractory materials at which the specimen no longer holds its
shape, due to its softening under heat.

G
gage — also spelled gauge, a device for determining the dimensional size of an object.
gas holes — rounded cavities caused by generation or accumulation of gas or entrapped air in a casting; holes may be spherical,
flattened or elongated.
gas porosity — a condition existing in a casting caused by the trapping of gas in the molten metal or by mold gases evolved
during the pouring of the casting.
gassing — 1) absorption of gas by a metal. 2) The evolution of gas from a metal during melting or solidification. 3) Cure by
applying gas to sand mold or core.
gassy — metal made porous by gas inclusions.
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gate (ingate) — that portion of the runner in a mold where molten metal enters the casting or mold cavity; sometimes applied to
entire assembly of connected channels, to the pattern parts which form them or to the metal which fills them, and sometimes
is restricted to mean the first or main channel. Other gates: bottom, branch, bypass, finger, horn, horseshoe, knife, lap (kiss,
touch, pressure), parting, pencil, reverse horn gate, ring, shower (pencil gate, pop gate), skim, slot, step, strainer, top,
vertical, wedge, whirl (swirl).
gate core — core used for forming a gate in the mold.
gate cutter — a simple hand tool (scoop) made usually of thin sheet brass bent in the form of a U, employed to cut or form the
gate in the sand of a mold.
gate tile — ceramic components used in gating systems, particularly for steel castings.
gate, well — see pouring basin.
gated patterns — one or more patterns with gates or channels attached.
gating system — the complete assembly of sprues, runners, gates and individual casting cavities in the mold. Term also applies
to similar portions of master patterns, pattern die, patterns, investment mold and the finished casting.
geometry — in metalcasting, the dimensions of the sizes and shapes of patterns, cores and castings.
gradient, temperature — see temperature gradient.
grain — an individual crystal in a metal or alloy: also, an individual or component aggregate when referring to core or molding
sands.
grain boundaries — metal between individual grains.
grain growth — an increase in the grain size of metal by a reduction in the number of grains.
grain refiner — any material added to a liquid metal for producing a finer grain size in the subsequent casting.
grain size — the average size of the crystals or grains in a metal.
gravity pour — pouring of molten metal into the mold assisted by gravity.
grinding — removing gates, fins and other projections from castings by means of an abrasive grinding wheel.
growth, cast iron — permanent increase in dimensions of cast iron resulting from repeated or prolonged heating at temperatures
over 900F (482.2C). This growth is due to 1) graphitization of carbides, and 2) Internal oxidation.

H
h — symbol for hardness of a material.
hardness — defined in terms of the method of measurement. 1) resistance of a material to indentation as measured by such
methods as Brinell, Rockwell and Vickers. 2) Stiffness of tamper of wrought products. 3) Also refers to the ability of the
metal to resist scratching, abrasion or cutting. 4) Relative specification of the quality or penetrating power of x-rays. In
general, the shorter the wavelength, the harder the radiation.
hardness, Rockwell — the relative hardness value of metal determined by measuring the depth of penetration of a steel ball
(1/16 in. dia. for B Scale) or a diamond point (C Scale) with controlled loading, the Rockwell number being the difference
between                                                          the                                                       depth
obtained                 with                 a                 minor                and                a                 major
loading.
head — pressure exerted by a fluid such as molten metal. Also used as a term for a riser.
head metal — the reservoir of metal in the feeder of a mold.
head pressure — static pressure generated within a filled mold by virtue of the height of the highest column of molten metal
present. Metallostatic head pressure (in psi) is equivalent to column height (inches) multiplied by metal density (lbs/in).
heat — 1) an added energy that causes material to rise in temperature, fuse, expand, evaporate, volatize or undergo other related
changes, from another material by contact or radiation, or can be produced within a material, as by friction or compression.
2) The entire period of operation of a continuous melting furnace such as a cupola from light up to finish of melting. Can
also mean the total metal tonnage from one such operation.
heat (radiant) shield — a protective device, usually of metal or asbestos, to prevent excessive radiant heat from reaching a
human or a material that might be damaged.
heat affected zone — the area (or zone) on a metal part that undergoes structural changes but does not melt during welding or
cutting.
heat transfer — transfusion of heat from a body of higher heat content to one of lower heat content by means of radiation,
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convection, or conduction.
holding furnace — 1) a furnace for maintaining molten metal at the proper pouring temperature, supplied from a larger melting
furnace, generally used in conjunction with diecasting and permanent mold casting. 2) Versatile furnace used for aluminum
alloys 3) Any furnace into which the molten metal is poured to maintain its temperature at a desired measured degree until
used
holding ladle — heavily lined and insulated ladle in which molten metal is placed until it can be used. see holding furnace.
homogeneous structure — a microstructure consisting of only one phase.
homogenize — see homogenizing.
homogenizing — a process of heat treatment at high temperature intended to eliminate or decrease chemical segregation by
diffusion.
horizontal gating system — a gating system in which runners and gates lie in a horizontal plane.
horn gate — a semicircular gate to convey a molten metal over or under certain parts of a casting so that it will enter the mold at
or near the center or from under the casting; also used as a skim gate.
horseshoe gate — a grating system where a runner and two ingates are combined in the shape of a horseshoe. This gating system
is easy to hand cut and remove from the casting.
hot metal — metal hot enough to flow easily.
hot shortness — brittleness in hot solidified metal, usually in steel or wrought iron. Has been linked to high sulfur content.
hot spots — 1) localized areas of a mold or casting where higher temperatures are reached or where high temperature is
maintained for an extended period. 2) Term applied to gray iron castings to denote chilled areas or inclusions that are harder
than the surrounding iron and that cause machining difficulties.
hot spruing — removing castings from gates before the metal has completely solidified. Hot spruing is performed on light-
section or intricate castings which might crack if cold sprued.
hot top — 1) insulated reservoir to hold molten metal on top of a mold, ensuring complete filling of casting cavity and
elimination of pipes or voids. 2) A refractory lined steel or iron casting inserted into the top of a mold to feed the ingot as it
solidifies.
hot topping — the practice of pouring molten metal directly into partially-filled open risers, in order to promote better feeding of
the casting, or covering open risers with exothermic materials to delay freezing of riser.

hypereutectic alloy — an alloy containing more than the eutectic amounts of the solutes. Analogous to hypereutectoid.
hypereutectoid — an alloy containing less than the eutectoid composition.
hypereutectoid steel — a steel containing more than the eutectoid percentage of carbon.
hypoeutectic alloy — an alloy containing less than the eutectic amounts of the solutes.
hypoeutectoid — an alloy containing less than the eutectoid composition.
hypoeutectoid steel — a steel containing less than the eutectoid percentage of carbon.
hysteresis — 1) (cooling lag) difference between the critical points on heating and cooling due to tendency of physical changes
to lag behind temperature changes. 2) Energy that is converted to heat in an elastic or magnetic energizing and de-energizing
cycle. 3) see hysteresis, magnetic.
hysteresis curve (loop) — a curve showing relationship between magnetizing force and flux density in a sample of iron or steel.
hysteresis loss — energy lost when a magnetic material is subjected to a complete cycle of magnetism.
hysteresis, magnetic — property of a magnetic material by virtue of which magnetic induction for a given magnetizing force
depends on the previous condition of magnetization.

I
impingement — the act of being struck, as by a gas or flame.
impingement attack — corrosion associated with turbulent flow of a liquid; considerably accelerated by entrained bubbles in the
liquid.
impurity — any element unintentionally alloyed in a metal or alloy. Some impurities have little effect on properties; others will
grossly damage the alloy.
inclusions — particles of slag, sand or other impurities, such as oxides, sulfides, silicates, etc., trapped mechanically in the
casting during solidification.
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inert gas — a gas that will not support combustion or sustain any chemical reaction (i.e., argon or helium).

ingate — see gate
insulating pads and sleeves — as opposed to chills, insulating material, such as gypsum, diatomaceous earth, etc., used to lower
the rate of solidification. As sleeves on open risers, they are used to keep the metal liquid, thus increasing the feeding
efficiency.
internal chill — a special shape incorporated into a core to speed the heat extraction from a selected area of a casting during
cooling. The material should have a higher thermal conductivity and heat capacity than the sand core; it can be made from
gray iron, carbon, graphite, zircon or chromite. see also chill and inverse chill.
internal shrinkage — a void or network of voids within a casting caused by improper feeding of that section during
solidification.
inverse chill — the condition in an iron casting section where the interior is mottled or white, while the other sections are gray
iron. Also known as reverse chill, internal chill and inverted chill.
inverted chill — see inverse chill.
iron oxide — 1) a chemical compound of iron and oxygen. Three oxides are known, FeO, Fe 2O3 (hematite) and Fe3O4
(magnetite). The latter two ores of iron can be added to metal baths to produce oxidation. 2) Also used in core and molding
sand mixtures to give desirable properties. This material as prepared for foundry use generally contains about 85% ferric
oxide and is produced by pulverizing a high grade of pure iron ore. It can be added to core sand mixes to assist in keeping
the core from cracking before the metal solidifies during the casting operation and helps to resist metal penetration during
this period. It is also added to molding sand mixtures for control of finning and veining.

K
Kelvin temperature scale — measurement scale in which temperature increments are measured in units equal to Celsius degrees
but whose starting point is absolute zero (a theoretical temperature characterized by the complete absence of heat, equivalent
to –273.15C or –459.67F).
kish — free graphite that separates upon slow cooling of molten hypereutectic iron.
knife gate — a slit opening 3/16- to 1/4-in., thick and any length through which metal enters the mold cavity. These gates are
usually poured on downhill tilt to provide progressive filling of mold cavity. Knife gates are easy to remove.

L
ladle — metal receptacle usually lined with refractories used for transporting and pouring molten metal. Types: straight-sided,
tapered or cylindrical, include hand, bull, crane, bottom-pour, holding, teapot, trolley, shank, lip-pour, buggy, truck, mixing,
reservoir.
ladle addition — the addition of alloying elements to the molten metal in the ladle.
ladle brick — brick for lining ladles which might otherwise be prone to bulge under temperature of molten metal.
ladle liner — a preformed shape made from the same materials used for graphite crucibles and steel ladles for carrying metal
from the melting unit to the molds.
ladle lip — portion of the ladle over which the metal is poured.
ladle wash — a clay slurry in which steel ladies are dipped to coat them so that metal will not adhere.
ladle, bull — large ladle for transporting and pouring molten metal.
ladle, casting — a crucible or iron vessel lined with refractory material for conveying molten metal from the furnace and pouring
it into the mold.
ladle, crane — a mechanically operated ladle used for large quantities of metal.
ladle, hand — a hand-held vessel with pouring lip used by metalcasters to pour small castings or to ladle molten metal from pot
to pot.
ladle, radial — a ladle suspended from and traveling along overhead circular rail track, used for filling a circular group of
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centrifugal casting molds.
ladle, sludge — a hemispherical steel ladle used to remove sludge from the bottom of a melting pot.
ladle, teapot — a ladle in which, by means of an external spout, metal is removed from the bottom rather than the top of the
ladle.
lap (kiss, touch, pressure) gate — a slit opening 1/32- to 3/18-in. thick, usually less than 3 in. in length, formed by overlapping
the runner with an edge of the mold cavity. Lap gates are easy to remove. Knife, lap, pressure, and finger gates are similar in
that they are designed to leave minimum metal at breakoff points on castings
latent heat — thermal energy absorbed or released when a substance changes state; that is, from one solid phase to another, or
from solid to liquid or the like.
law of continuity — a basic fluid-flow equation, based on Bernoulli’s theorem, which allows the velocity and flow rates of
molten metal in a gating system to be calculated. The equation is written Q = AV where Q = flow rate (cu in./sec), A =
cross-sectional at a given point in the gating system (in.2), and V = velocity at that point (in./sec). see Bernoulli’s theorem
and fluid flow principles.
law of Hess — the total heat of a chemical reaction is the same whether the reaction takes place in one or several steps.
layout — full-sized drawing of a pattern showing its arrangements and structural features and overall geometry.
layout board — a board upon which a pattern layout is made.
leaker — 1) foundry term for a casting that fails to meet liquid or gas pressure tests. 2) Foundry term for a sand mold where
metal leaks out at the parting line because there is insufficient weight on the cope. 3) Investment casting term for a cracked
mold from which metal leaks out on pouring.
liquation — 1) separation of fusible metals from less fusible metals by the application of heat. 2) Partial melting of an alloy (or a
phase in an alloy) which results in a separation of two or more constituents.
liquid — material neither solid nor gaseous that readily changes its shape, the volume remaining constant at constant
temperature. Molecules of a liquid do not tend to separate as in gases.
liquidus — a line on a binary phase diagram or a surface on a ternary phase diagram, representing the temperatures at which
freezing begins during cooling, or melting ends during heating under equilibrium conditions.
live riser — (hot riser) a riser that is filled directly from the gating system rather than from metal that has flowed through the
casting cavity.
locating pad — a projection on a casting to assist in maintaining alignment of the casting for machining operations.
locating surface — a casting surface to be used in making secondary machining operations.
loose molding — the molding process utilizing unmounted pattern. Gates and runners are usually cut by hand.
loose piece — part of a pattern that remains in the mold and is taken out after the body of the pattern is removed. It can be
indicated as: 1) Corebox part that remains embedded in the core and is removed after lifting off the corebox. 2) Pattern
laterally projecting part of a pattern so attached that it remains in the mold until the body of the pattern is drawn. Back-draft
is avoided by this means. 3) Permanent mold part that remains on the casting and is removed after the casting is ejected from
the mold. 4) In diecasting, a type of core that forms undercuts positioned in, but not fastened to, a die and so arranged as to
be ejected with the diecasting from which it is removed and used repeatedly for the same purpose.

M
machinability — index or rate of metal removal by various methods with machine tools such as turning, boring facing, etc.,
usually expressed as cutting speed in surface feet per minute, depth of cut, etc.
machine allowance — stock added to the part to permit machining to final dimensions.
machine casting — process of casting by machine.
machine finish — 1) allowance of stock on the surface of the pattern in order to permit machining of the casting to the required
dimensions. 2) Operation of turning or cutting from the surface of metal an amount of stock in order to produce a finished
surface.
malleability — the property enabling a metal to be mechanically deformed under compression, as in hammering or rolling into
sheets without cracking.
malleable — capable of withstanding plastic deformation without rupture.
master pattern — a pattern embodying a contraction allowance in its construction, used for making castings to be employed as
patterns in production work. In investment castings, the object from which a die can be made, generally a metal model of the
part to be cast, with process shrinkage added.
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matched parting — forming a projection on the parting surface of a cope half of a pattern and a corresponding depression in the
surface of the drag.
matchplate — a plate of metal or wood on which patterns and gating systems, split along the parting line, are mounted back to
back to form an integral piece. This type of pattern is used when large quantities of small castings are desired, often with
some type of molding machine.
melt — a heat or charge of metal or the region above liquidus on a phase diagram.
melt, cold — 1) a bath of molten metal tapped from the melting furnace at a temperature lower than that considered desirable. 2)
A batch of molten metal which was charged in the furnace as solid metal and all of the melting done in one unit, in contrast
to duplex or triplex melting.
melting loss — weight loss of metal in the charge during the operation of melting, usually due to oxidation or volatilization.
melting point (mp) — temperature that metal begins to liquefy. Pure metals, eutectics and some intermediate phases, melt at a
constant temperature. Alloys generally melt over a range of temperature.
melting pot — metal, graphite-clay or ceramic vessel in which metal is melted.
melting range — pure metals melt at one definite temperature, but constituents of alloys melt at different temperatures and the
variation from the lowest to the highest is called the melting range.
melting rate — amount of metal melted in a given period of time, usually one hour.
melting ratio — the proportion of the weight of metal to the weight of fuel used in melting.
melting zone — the portion of the furnace in which the metal melts.
melting, continuous — an operation in which a furnace produces molten metal in a continuing stream, as distinguished from
batch melting.
metal — an opaque lustrous elemental substance. It conducts electricity and can be heated to a melting point and will form
positive ions in solution. As temperature increases conductivity decreases.
metal casting —metal object cast to the required shape by pouring or injecting liquid metal into a mold, as distinct from one
shaped by a mechanical process. The metal shape, exclusive of gates and risers that is obtained as a result of pouring molten
metal into a mold. see metalcasting.
metalcasting — pouring molten metal into a mold to solidify, the shape of the solidified object being determined by the shape of
the mold cavity.
metallurgy — the science and technology of metals. A broad field that includes but is not limited to the study of internal
structures and properties of metals and the effects on them of various processing methods, from ore refining to consumer
product. Its principal branches are process metallurgy and physical metallurgy.
misrun — denotes an irregularity of the casting surface caused by incomplete filling of the mold. A casting not fully formed.
mold weight — a weight that is applied to the top of a mold to keep the mold from separating to offset internal or ferrostatic
pressure.

N
nails, chill — see chill nails.
neck down —1) a thin core or tile used to restrict the riser neck, making it easier to break or cut off the riser from the casting.
(knock-off, wafer core, Washburn, Cameron core) 2) Localized area reduction of a test specimen during plastic deformation.
nickel silver— a silver-white copper-base alloy that contains 10 to 32% zinc and 10 to 30% nickel and may also contain Sn, Pb
or Fe. It contains no silver. Also called German silver, nickel brass.
non-fill — surface defects resulting in an incomplete casting or irregular surface.
nonpressurized gating system — a gating system in which the choke is located at or near the base of the sprue. The runner(s)
and gates become larger in total cross-sectional area after the choke is passed. This enlarging of the gating system reduces
the velocity of the molten metal and, thus, reduces turbulence. A common gating ratio is 1:4:4, meaning that, for example, if
the choke is 1 sq in. in cross section, the cross-sectional area of the runner(s) should equal 4 sq in., and the cross-sectional
area of all the gates (combined) should equal 4 sq in.
nozzle — pouring spout of the bottom-pour ladle.
nozzle brick — a thick-walled tubular refractory shape set in bottom of a ladle through which steel is teemed.
nozzle pocket brick — a refractory shaped set in bottom of a ladle containing a recess in which nozzle is set.
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O
one-piece pattern — solid pattern, not necessarily made from one piece of material. May have one or more loose pieces.
open riser — conventional form of riser usually located at the heaviest section of the casting and extending through the entire
height of the cope open to the atmosphere. Compare with blind riser.
open sand casting — a casting produced in an open mold; poured in the drag, with no cope or other top covering.
open-face mold — see open sand casting, Wagner casting machine.
oxidation — 1) A reaction of an element with oxygen. In a narrow sense, oxidation means the taking on of oxygen by an element
or compound and, based on the electron theory, it is a process in which an element loses electrons. 2) Any process which
increases the proportion of oxygen or acid-forming element or radical in a compound.
oxidation losses — reduction in amount of metal or alloy through oxidation. Such losses usually are the largest factor in melting
loss. see dross.
oxide — binary compound of an element and oxygen (i.e., rust).
oxide buildup — formation of oxides on the sidewalls or any metal-immersed object such as a thermocouple tube in a furnace.
oxidizing agent — compound that causes oxidation, thereby becoming reduced itself.
oxidizing atmosphere — an atmosphere resulting from the combustion of fuels in an atmosphere where excess oxygen is
present, and with no unburned fuel lost in the products of combustion. — furnace atmosphere which gives off oxygen under
certain conditions or where there is an excess of oxygen in the product of combustion, or the product of combustion are
oxidizing to the metal being heated.
oxygen — (O) an odorless, colorless gas comprises about 20% of the earth’s atmosphere, atomic weight is 15.9.

P
pareto principle or diagram — the principle states that, for example, a relatively large number of quality problems (approx. 80–
90%) will be caused by a few (10–20%) of the related factors or defect types. Sometimes referred to as the 80–20 rule. The
principle is illustrated by means of a Pareto diagram. Factors (such as defect types) are first ordered by number or percent
occurrence. The highest number is plotted on a bar chart as the first bar and will continue in that manner. This principle is
important in engineering since most problems are caused by a few factors, this principle allows more rapid means to
separate these sources from the remainder.
parted pattern — a pattern made in two or more parts.
parting — the joint where mold separates to permit removal of pattern.
parting gate — gate entering the mold cavity along the parting line separating cope from drag.
parting line — 1) a line on a pattern or casting corresponding to the separation between the cope and drag portions of a sand
mold. 2) Mark left on casting at die joint.
pattern — the wood, metal, plaster, foam or plastic shape used to form the cavity in the sand. A pattern may consist of one or
many impressions and would normally be mounted on a board or plate complete with a runner system for casting metals.
pattern board — board having a true surface upon which a pattern is placed preparatory to making a mold of the pattern.
pattern checking — verifying dimensions of a pattern with those of the drawing.
pattern draft — the taper allowed on the vertical faces of a pattern to permit easy withdrawal of the pattern from the mold or
die. see draft.
pattern layout — full-sized drawing of a pattern showing its arrangement and structural features.
pattern plate — 1) a plate upon which a pattern is usually fastened to facilitate sand molding. 2) In shell molding, a heated plate;
sand containing a thermosetting resin is blown or dumped against the hot plate to form a mold.
pattern, loose piece — see loose piece.
pattern, master — see master pattern.
pattern, split — pattern usually made in two parts, sometimes in more than two parts.
pattern, split plate — method in which cope and drag of a mold are made from the same plate by placing the cope on one side
of the plate center line and the drag portion on the other side.
pattern, standard — see standard pattern.
patternmaker — a craftsman engaged in production of foundry patterns from wood, plastic or metals, such as aluminum, brass,
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etc.
patternmaker’s contraction — the shrinkage allowance made on all patterns to compensate for the change in dimensions as the
solidified casting cools in the mold from freezing temperature of the metal to room temperature. The pattern is made larger
by the amount of shrinkage characteristic of the particular metal in the casting and the amount of resulting contraction to be
encountered. Also, the final stage of solidification when a casting is cooling, called ―solid shrinkage.‖ See patternmaker’s
shrinkage.
patternmaker’s shrinkage — contraction allowance made on patterns to compensate for the decrease in dimensions as the
solidified casting cools in the mold from freezing temperature of the metal to room temperature. Pattern is made larger by
the amount of contraction that is characteristic of the particular metal to be used. (Also called patternmaker’s contraction).
patternmaking — skilled craft of modeling in wood, metal, plastic, plaster, or other materials objects to be cast in molten metal:
patternmaking wood — synthetic wood, pine, mahogany or redwood most commonly used in making patterns; selected for
dimensional stability and resistance to abrasion.
patterns, card of — a number of patterns fastened together.
peg gate — a round downsprue leading from a pouring basin in the cope to a basin in the drag from which runners lead to the
mold.
pencil gate — 1) gating directly into the mold cavity through the cope by means of one or more small vertical downsprues
connecting the pouring basin and mold cavity. 2) A series of small round or rectangular gates entering the mold cavity from
above and coming from a common pouring basin.
phase diagram — a graphic representation of the equilibrium temperature and composition limits of phase fields and phase
reactions in an alloy system. In a binary system, temperature is usually the ordinate and composition the abscissa. Ternary
and more complex systems require several two-dimensional diagrams to show the temperature-composition variables
completely. In alloy systems, pressure is usually considered constant, although it may be treated as an additional variable.
pinhole porosity — very small holes scattered throughout a casting, possibly caused by microshrinkage or gas evolution during
solidification.
pins, flask — hardened steel locating pins used on flasks to ensure proper register of cope and drag molds.
pop up — slang for a blind riser.
pour — discharge of molten metal from the ladle into the mold.
poured short — casting that lacks completeness due to the cavity not being filled with molten metal.
pouring — transfer of molten metal from furnace to ladle, ladle to ladle, or ladle into molds.
pouring basin — the enlarged mouth of the sprue into which molten metal is poured. Because the metal must first fill the basin
before it can flow into the sprue, the pourer can more easily maintain a near-constant metallostatic head within the mold.
The pouring basin can be made from core sand and placed above the sprue or be cut into the cope alongside the sprue.
pouring cup — an enlarged, cup-like section at the top of the sprue that acts as a funnel to channel molten metal from the ladle
into the sprue. A pouring cup does not fulfill the functions of a good pouring basin because the pourer may be tempted to
pour directly down the sprue, thereby causing vortexing and turbulence of the molten metal. A pouring cup can be cut into
the cope, be a shaped part of the pattern used to form the sprue, or be a sand or ceramic cup placed on top of the cope over
the sprue.
pouring device — mechanically operated device with a ladle set for controlling the pouring operation.
pouring ladle — ladle used to pour metal into the mold.
pouring station — in production pouring, that central location where molds are brought to be filled with molten metal.
pressurized gating system — a gating system in which the choke is located at or near the gates. The idea behind this design is to
cause the metal stream to back up in the gating system, permitting dross to separate from the metal prior to the metal
entering the mold cavity. However, because most gating systems do not allow sufficient runner length and area for the metal
to slow and the nonmetallic to separate, this approach usually is not successful. This method can create high turbulence at
the gates, resulting in turbulence in the mold cavity and mold or core erosion where the metal stream impinges. Common
gating ratios (sprue: runners: gates) are 4:4:1 and 2:3:1 with this system.

R
radiant heat — heat communicated by radiation and transmitted by electro-magnetic waves.
radiation — emission of radioactive energy.
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rapid prototyping — (RP) refers to the physical modeling of component or tooling geometry using layered manufacturing.
These technologies make it possible to quickly generate polymer, wax or paper-based prototype parts from three-
dimensional solid model computer-aided design (CAD) representations. Parts are typically generated by building up one
layer at a time with the thickness of each layer determining the accuracy of the part and the time required to make it.
Rapid Tooling — (RT) This term evolves from rapid prototyping technology and its application. It means to use Rapid
Prototyping (RP) to produce 1) master patterns for making molds and dies, or 2) Molds and dies directly. Companies that
produce molds and dies quickly using CNC machining believe that RT includes CNC, particularly high-speed machining.
Generally, RT includes these two aspects. The essential point is to use computer numeric technology to produce the master
pattern or mold.
receiving ladle — a ladle placed in front of the cupola into which all metal is tapped. It acts as a mixer and reservoir and to
smooth out metal flow to the pouring area.
rechucking — reversing a pattern upon a face plate to permit turning the opposite face to the required shape.
recovery — the amount of metal remaining after melting, compared to the initial quantity, commonly given as a weight-
percentage.
recovery rate — ratio of the number of saleable parts to the total number of parts manufactured, expressed as a percentage.
red shortness — brittleness in metal when red hot.
refractoriness — the ability of a material to withstand high temperatures.
refractory — 1) (noun) Heat resistant materials, usually nonmetallic, used for furnaces and ladle linings, etc. 2) (adjective) The
quality of resisting heat.
relief sprue — a vertical channel in a mold, the approximate size of the downsprue connected to the runner to relieve pressure
surge during pouring. It functions similar to a standpipe in a plumbing system. See sprue.
Reynolds number — the ratio of the inertia forces in a flowing fluid to the viscous force. Inertia force is the product of mass and
acceleration and viscous force is equal to the shear stress multiplied by the area. Hence, Reynolds number: NR = Vd/v
where V = mean velocity of flow, d = diameter of the channel and v = kinematic viscosity of the fluid.
rigging — gates, risers, loose pieces, etc., needed on the pattern to deliver the metal to the mold cavity and produce a sound
casting.
ring gate — a gate so formed that a number of small gates conduct the metal from a circular runner to a mold in the center.
riser — an opening in the top of a mold that acts as a reservoir for the molten metal and prevents cavities in the casting as it
contracts on solidification. It allows gases to escape as the metal rises in the mold and indicates when the mold is full. See
feeder.
riser base — (drag bob) the shape of the bottom of a side riser.
riser distance — the length of the riser neck. The term is applied to aid risers only.
riser gating — gating system in which molten metal from the sprue enters a riser closest to the mold cavity and flows into the
mold cavity.
riser height — the distance from the top of the riser when liquid to the top of the riser neck Riser height when solid is usually
several inches less than when liquid because of contraction and loss of feed metal to the casting.
riser neck — the connecting passage between the riser and casting. Usually only the height and width or diameter of the riser
neck are reported, although the shape can be equally important.
riser pad — (riser contact) an enlargement of the riser neck where it joins the casting. The purpose of the pad is to prevent the
riser from breaking into the casting when it is struck or cut from the casting.
riser, blind — a riser that does not extend through the top of the cope and is entirely surrounded by sand. See atmospheric riser.
riser, dished — a riser with a concave top surface.
riser, open — conventional form of riser usually located at the heaviest section of the casting and extending through the entire
height of the cope.
riser, side — see side riser.
riser, top — see top riser.
riser-gating — practice of running metal for the casting through the riser to help directional solidification.
runner — the portion of the gate assembly that connects the downgate (sprue) with the casting ingate or riser. The term also
applies to that part of the pattern that forms the runner.
runner — (crossgate) the second channel, usually horizontal, through which molten metal or slag is passed from one receptacle
to another; in a mold, the portion of the gate assembly that connects the downgate or sprue with the casting ingate or riser.
The term also applies to similar portions of dies, master patterns, pattern dies, patterns, investment molds and finished
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castings.
runner box — device for distributing molten metal around a mold by dividing it into several streams.
runner brick — refractory shape with holes or hole to convey molten metal during teeming of bottom-poured ingots or certain
types of large castings.
runner extension — in a mold, a part of a runner that extends beyond the farthest gate as a blind end and functions as a ―dirt
trap.‖ The first rush of molten metal that enters the gating system, which tends to pick up loose sand and other foreign
materials, is carried into the runner extension instead of through the gates into the mold cavity. The extension must be of the
same cross section as the runner under the last gate and should be vented. Contrast with runner sump.
runner riser — a conventional runner, usually in the horizontal plane, which permits flow of molten metal to the ingate and is
large enough to act as a reservoir to feed the casting.
runner sump — a reservoir at the end of a runner, used in a mold that is not large enough to accommodate a properly designed
runner extension. The runner sump captures the first rush of molten metal that enters the gating system, as does a runner
extension, keeping the loose sand and other foreign materials picked up by this metal from entering the mold cavity.
runner system or gating — the set of channels in a mold through which molten metal is poured to fill the mold cavity. The
system normally consists of a vertical section (downgate or sprue) to the point where it joins the mold cavity (gate) and
leads from the mold cavity through vertical channels (risers or feeders).
runout — 1) a casting defect caused by incomplete filling of the mold due to molten metal draining or leaking out of some part
of the mold cavity during pouring; escape of molten metal from a furnace, mold or melting crucible. 2) The actual piece of
metal that ―runs out‖ of the mold.

S
scrap — 1) Any scrap metal melted (usually with suitable additions of pig iron or ingots) to produce castings. 2) Defective
castings..
shortness — a term loosely applied in the foundry to indicate brittleness in a metal.
shrink hole — a hole or cavity in a casting resulting from shrinkage and insufficient feed metal, and formed during
solidification.
shrink mark — see shadow mark.
shrink rule — special scales used by patternmakers in which each unit of measure is a designated amount larger than standard,
so that the resulting patterns are larger than indicated to compensate for the contraction of the metal in cooling from the
solidification temperature. A one percent linear shrinkage allowance is typical for iron.
shrinkage — 1) liquid, contraction in volume as metal cools to solidification. 2) Solidification, contraction in volume when the
metal passes from the liquid to the solid at the freezing point (may extend over a range). 3) Solid, the contraction on cooling
from freezing point to normal temperature. 4) The decrease in dimension in clays occurring when drying at 100C (212F) and
even more so on firing. 5) Reduction in dimensions of refractory material during heating. 6) The term also is used to
describe the casting defect, such as shrinkage cavity, which results from poor design, insufficient metal feed or inadequate
feeding.
shrinkage cavity — a void left in cast metals as a result of solidification shrinkage and the progressive freezing of metal toward
the center.
shrinkage cracks — cracks that form in metal because of contraction pulling apart the grains before complete solidification.
shrinkage spalling — shearing off refractory from stresses set up within the refractories by shrinkage of exposed face.
shrinkage, centerline — shrinkage occurring in the center of casting sections, particularly with plate-like or bar-like contours,
which solidify simultaneously from two faces and cut off feeding in the central portion.
shrinkage, patternmaker’s — a scale, divided in excess of standard measurement to allow for the difference in size between the
casting and the corresponding mold cavity. Used by patternmakers to avoid calculation for shrinkage.
shrinkage, process — the total corrections for shrinkage, occurring in the making of a precision investment casting, applied to
the master pattern.
side riser (side head) — a riser attached to the side of a casting.
skim gate — a gating arrangement that changes the direction of flow of molten metal and prevents the passage of slag and other
undesirable materials into the mold cavity.
skimmer — 1) A tool used for pulling slag and dross from the surface of molten metal. 2) A core in the gating system for tapping
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slag.
skimming — removing or holding back dirt or slag from the surface of the molten metal before or during pouting.
slag — a nonmetallic covering on molten metal as the result of the combining of impurities contained in the original charge, such
as ash from the fuel and any silica and clay eroded from the refractory lining. Can be a product resulting from the action of a
flux on the oxidized non-metallic constituents of molten metals. Also includes slags intentionally added for refining a metal
or as a blanket or cover. It is skimmed off prior to tapping the heat.
slag hole — an opening in the front or back of a cupola or other furnace through which the slag is drawn off.
slag inclusions — casting surface imperfections similar to sand inclusions but containing impurities from the charge materials,
silica and clay eroded from the refractory lining and ash from the fuel during the melting process. May also originate from
metal-refractory reactions occurring in the ladle during pouring of the casting.
slag trap — an enlargement, dam, or extrusion in the gating or runner system in a mold for the purpose of preventing molten slag
particles from entering the mold cavity. See dirt trap.
slot gate — a gate used on vertical cylindrical castings in which the downsprue and castings are connected over a large part or all
of the height of the casting.
solidification — process of a metal (or alloy) changing from the liquid to the solid state.
solidification range — only pure metals solidify or freeze at one definite temperature. Alloys contain different constituents that
solidify at different temperatures and the various temperatures from that of the first constituent to solidify to that of the last
constituent to freeze is called the solidification range.
solidification shrinkage — the decrease in volume accompanying the freezing of a molten metal.
solidification, directional — see directional solidification.
solidifying contraction — shrinkage or contraction as a metal solidifies.
solidus — a line on a phase diagram representing the temperature which freezing ends on cooling, or melting begins on heating.
specific heat — equivalent to thermal capacity, or quantity of heat required to produce a unit change in the temperature of a unit
mass.
split pattern — pattern made in two or more parts for convenience of molding. Also called parted pattern.
sprue — (downsprue, downgate) the channel, usually vertical, connecting the pouring basin with the skimming gate, if any, and
the runner to the mold cavity, all of which together is the gating system. In top-poured castings, the sprue may also act as a
riser. Sometimes used as a generic term to cover all gates, risers, etc. returned to the melting unit for remelting. Also applies
to similar portions of master patterns, pattern die, patterns, investment mold, and the finished casting. In diecasting, the
metal that fills the conical passage (sprue hole) connecting the nozzle with runners.
sprue base — (also called a sprue well) an enlarged section at the bottom of the sprue, used to lower the velocity of the molten
metal before it enters the runners, thereby reducing turbulence. The sprue base should have a flat bottom and can be
rectangular or cylindrical. Runners should come off the upper part of the sprue base.
sprue button — a print attached to the top or squeeze board of a mold to make an impression in the cope indicating where the
sprue should be cut.
sprue cutter — a piece of tubing that cuts the sprue hole through the cope. In addition, a shear-type machine for removing the
sprue and gates from the casting.
sprue hole — the opening through which the metal is poured into the cope to run into the casting cavity.
sprue pin — in diecasting, a tapered pin with a rounded end projecting into a sprue hole acting as a core that deflects the metal
and aids in removal of the sprue from the diecasting
sprue plug — a tapered wood, metal casting, graphite or refractory pin used to form the sprue opening in a mold. In addition, a
metal or other stopper used in a pouring basin to prevent molten metal from flowing into the sprue until a certain level has
been reached. It prevents entry of dirt and dross.
sprue well — see sprue base.
sprue, relief — see relief sprue.
sprue-master — in diecasting, a tool to replace modified battery (alligator) pliers in removing castings from a die.
spruing — removing gates from castings after the metal has solidified usually by tapping or cutting with an abrasive wheel or
torch.
spruing station — in the cleaning room, where sprues are removed from castings.
spruing, hot — removing gates from castings before the metal has completely solidified. Hot spruing is particularly necessary on
light-sectioned or intricate castings which might be cracked if cold sprued.
step gate — a vertical sprue containing a number of side branches or entries at different levels into the casting cavity.
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Basic Principles of Gating and Risering

stepped-down runner — a runner that has been sized down after passing a gate, forming what resembles a step. The step down
is equal to the cross-sectional area of the gate just passed. This is done so that all the gates in the system will feed at the
same time. Contrast with tapered runner.
stereolithography (STL) — computerized building of three-dimensional models and patterns. Enables the data representation of
a CAD solid model to be directly converted into a plastic model of a casting.
stereolithography apparatus — (SLA) equipment used for computerized building of three-dimensional models and patterns.
Enables the data representation of a CAD solid model to be directly converted into a plastic model of a casting.
STL Format — A data format widely used in rapid prototyping industry. STL is abbreviation from Stereolithography. A de facto
standard interface between CAD packages and rapid prototyping.
stopper rod — a device in a bottom-pour ladle for controlling the flow of metal through the nozzle into the casting. The stopper
rod consists of a steel rod, protecting sleeves, and a graphite stopper head.
strainer core — a perforated core placed at the bottom of a sprue or in other locations in the gating system to control the flow of
the molten metal. To some extent, it prevents coarse particles of slag and dross from entering the mold cavity.
strainer gate — a gate designed to prevent slag and dirt from entering the mold and to control the rate at which metal enters the
mold cavity.
supercooling — (undercooling) cooling below the temperature at which an equilibrium phase transformation can take place
without actually obtaining the transformation.
superheat — any increment of temperature above the melting point of a metal; sometimes construed to be any increment of
temperature above normal casting temperature introduced for refining, alloying or improving fluidity. Also called
superheating.
swirl gate — a device in the gating system for trapping slag and dross before it gets to the mold cavity.

T
tap — to withdraw a molten charge from the melting unit.
tapered runner — a runner that is sized down gradually, reducing its cross-sectional area by the cross-sectional area of the
gates. This is done so that all the gates in the system will feed at the same time. Contrast with stepped-down runner.
tapping — 1) removing molten metal from the melting furnace by opening the tap hole, allowing the metal to run into a ladle. 2)
Formation of an internal screw thread.
teapot ladle — ladle with an external spout wherein the molten metal is poured from the bottom rather than from the top.
temperature — degree of warmth or coldness in relation to an arbitrary zero measured on one or more of accepted scales, as
Centigrade, Fahrenheit, etc.
temperature conversion — to convert Fahrenheit to centigrade, subtract 32, multiply by 5 and divide by 9; to revert, multiply
centigrade by 9, divide by 5, add 32.
temperature gradient — temperature difference between two or more locations expressed in degrees per unit of distance.
temperature, holding — 1) temperature above the critical phase transformation range at which castings are held as a part of the
heat treatment cycle. 2) The temperature maintained when metal is held in a furnace, usually prior to pouring.
temperature, pouring — the temperature of the metal as it is poured into the mold.
temporary pattern — a pattern used to produce a limited number of castings and made as cheaply as permissible.
test bar — standard specimen bar designed to permit determination of mechanical properties of the metal from which it was
poured.
test coupon — see coupon.
test lug — a small projection (lug) on a casting that is removed for testing purposes (fractured to test the ductility of the metal)
without destroying the casting itself.
test sprue — sample bar cast at intervals by furnace attendant and quickly cooled and broken to observe fracture for mottle in
white iron or to determine melt quality gas content in copper and aluminum alloys. See chill test.
thermal analysis — the determination of equilibrium conditions and phase relationships in metals by means of thermal arrests as
shown on heating and cooling curves.
thermal conductivity — the property of matter by which heat energy is transmitted through particles in contact. For engineering
purposes, the amount of heat conducted through refractories is usually given in Btu or HB per hour for one square foot of
area, for a temperature difference of one degree Fahrenheit and for a thickness of one inch, Btu/hr - ft2 - F/in.
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Basic Principles of Gating and Risering

thermal contraction — the decrease in linear dimensions of a material accompanying a decrease in temperature.
thermal expansion — the increase in a linear dimension and volume of a material accompanying a change of temperature.
thermal fatigue — failure resulting from rapid cycles of alternate heating and cooling.
thermal shock — stress developed by rapid and uneven heating of a material.
thermocouple — a device for measuring temperatures by the use of two dissimilar metals in contact; the junction of these metals
gives rise to a measurable electrical potential which varies with the temperature of the junction. Thermocouples are used to
operate temperature indicators or heat controls. Most common metal combinations: chromel-alumel; copper-constantan;
iron-constantan and platinum-platinum rhodium.
tile — rectangular refractory shape larger than brick and usually comparatively thin.
top gate — a gate entering casting near the top of the drag half.
top riser (top head) — a riser attached to the top surface of a casting.
transfer ladle — a ladle that may be supported on a monorail or carried in a shank and used to transfer metal from the melting
furnace to the holding furnace or from furnace to pouring ladles.
transformation temperature the critical temperature at which a change in phase occurs. To distinguish between the critical
points in heating and cooling those in heating are referred to as the Ac points (c for chauffage or heating) and Ar for cooling
(r for refroidissement or cooling).
transition temperature — temperature where a fracture changes from tough to brittle during materials testing (i.e., notched-bar
impact testing).
trim die — a press die for trimming flash and excess metal from casting.
trimming — 1) removing fins and gates from castings with hammers. 2) Removing fins and gates from diecastings, usually on a
power-operated press, by means of a trimming die, punch, etc. See chip or chipping.

U
undercooled — the transformation of a material below its normal transformation temperature because of rapid cooling and
insufficient nuclei for the new phase. It can result in a different structure.
unpressurized gating system — see nonpressurized gating system.
uprunner — in a vertical gating system, a riser oriented vertically in the mold. An uprunner can be attached to the casting cavity
by using a web gate or by using individual gates, which are stepped up in size the higher they are located in the mold in
order to compensate for the change in metallostatic pressure.

V
velocity — in a gating system, the speed at which the molten metal is flowing past a given point in the system. Normally
measured in in./sec. Compare to flow rate. Initial velocity of molten metal can be determined by the velocity formula:
velocity = (2gH)1/2; where g is gravity and H is the distance the molten metal falls as it is poured (usually the distance from
the lip of the pouring device to the bottom of the sprue).
velocity pressure — measure of the energy a mass of air or gas possesses by virtue of its motion and amount of energy that must
be imparted to a stiff mass of air or gas to accelerate it to a given velocity.
vent — a small opening or passage in a mold or core to facilitate escape of gases when the mold is poured.
vertical gating system — a gating system used for vertically parted molds and some horizontal molds. If the cavities are all
located at the same level, the same rules apply as for horizontal gating systems. For molds where a cold riser on top of the
casting would be impractical, a side riser or ―uprunner‖ can be used to introduce metal into the casting cavity and to feed
shrinkage in the casting. A web gate can be used to connect the uprunner to the casting cavity. If individual gates are used
instead of a web gate, they should increase in cross section as they go up in the mold to help maintain the same flow rate
through all of the gates.

W
wash — 1) a casting defect resulting from erosion of sand by metal flowing over the mold or cored surfaces. Appear as rough
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Basic Principles of Gating and Risering

spots and excess metal on the casting surface. Also called cuts. 2) Term for coating materials applied to molds, cores, etc.
Washburn Core — A thin core that constricts the riser at the point of attachment to the casting. The thin core heats quickly and
promotes feeding of the casting. Riser removal cost is minimized.
web gate — In vertical gating systems, a gate that connects a riser (uprunner) to a casting cavity along the riser’s entire length.
Momentum of the metal tends to fill the riser before the metal flows through the web gate into the casting. The gate should
be approximately 3/8 to 1/2 in. thick. Thinner webs tend to make the metal cascade as it passes through the web, causing
severe dross formation. Thicker webs allow the metal to flow continuously into the mold cavity at the lowest point of entry,
forcing cold metal from above back into the riser and causing unfavorable temperature distributions.
wedge gate — a gate the shape of a wedge feeding directly into the mold cavity.
whirl (swirl) gate — a gate and sprue arrangement which tangentially introduces molten metal into a riser so the centrifugal
action forces dirt or slag to the center of the riser and away from the riser connection as the metal enters the casting cavity.
whirlpool effect — the aspiration of air and metal oxides resulting in a casting defect caused by a poorly designed pouring basin
or sprue.

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