Casting

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					GATING SYSTEM:-

As molten metal is poured into a mold, the gating system conveys the material and
delivers it to all sections of the mold cavity. The speed or rate of metal movement is
important as well as the degree cooling that occurs while it is flowing. Show filling and
High loss of heat can result it microns and cold shuts. Rapid rates of filling, on the other
hand, can produce corrosion of the gating system and mold cavity and mighty results in
the entrapment of mold material in the final casting. The cross-sectional areas of the
Various channels can be selected to regulate flow. In addition, the shape and length of the
channels are influent in controlling temperature loss. When heat loss are to be minimized,
short channels with round or square cross section are the most desirable. The
Gates are usually attached to the thickest or heaviest sections of a casting to control
shrinkage and to the bottom of the casting to minimize turbulence and splashing. For
large casting, multiple gates and runners may be used to introduce metal to more than
One point of the mold cavity.

CASTING:-

In the casting processes a solid material is first melted, heated to proper temperature, and
sometimes treated to modify its chemical composition. The molten material generally
metal is then poured into a cavity or mold that contains it in the desired shapes during
subsequent cool-down and solidification. Thus, in a single step. Simple or complex
shapes can be made from any material that can be melted. The product can have virtually
any configuration the designer desires. In addition, the resistance to working stress can be
optimized, directional properties can be controlled, and a pleasing, appearance can be
product.

ADVANTAGES OF CASTING:-

   1- Size
   2- Intricate
   3- Low cost

   In the casting process a solid material is first method the molten material generally
   metal is purred into a mold cavity that contains it in the desired shape during
   subsequently cool down and solidification.
PATTERNS:-

A pattern is a shaped form of wood or metal around which sand is packed in the mold.
When the pattern is removed the resulting cavity is the exact shape of the object to be
cast.
The pattern must be designed to be easily removed without damage to the mold. It must
be accurately dimensioned and durable enough for the use intended. Either one time use
or production runs. PATTERN MAKING
Each different item we wish to cast presents unique problems and requirements. In a
large foundry there is a close relationship between the pattern maker and the molder.
Each is aware of the capabilities and limitations of his own field.
Throughout the industry, pattern making is a field and an art of its own. The pattern
maker is not a molder or the molder a pattern maker. This is not to imply that the pattern
maker cannot make a simple mold or the molder make a simple pattern but each may
soon reach a point in the other's field beyond his own skill and experience.
In the hobby or one man shop, however, pattern and mold making are so closely
interrelated as to become almost one continuous operation. This chapter will acquaint you
with some of the various types of patterns and their requirements.


PATTERNS ALLOWNSES:-


DRAFT:-

In order to illustrate some of the important pattern characteristics we will use as an
example a simple disc pattern. The object we want to cast is 12 inches in diameter and 1
inch thick. The edge of the disc is tapered from the top face to the bottom face. See Fig.
5-1. This taper is known as the pattern draft. This draft is necessary in order that the
pattern can be removed easily from the mold causing no damage to the sand. Pattern draft
is defined as the taper on vertical elements in a pattern which allows easy withdrawal of
the pattern from the mold. The amount of draft required will vary with the depth of the
pattern. The general rule is VA inch taper to the foot which comes out to about 1 degree
and on shallow patterns such as our disc 1/16 inch taper or 0.5 degree is sufficient.

Shrinkage:-

Now back to our simple disc pattern. If we wish the casting to come out as cast to the
dimensions we show of 12 inches in diameter 1 inch thick, we must make the pattern
larger and thicker than 12 inches x 1 inch to compensate for the amount that the metal
will shrink when going from a liquid to a solid. This is called pattern shrinkage. This
varies with each type of metal and the shape of the casting.
The added dimensions are incorporated into the pattern by the pattern maker by using
what is called shrink rulers. These rulers are made of steel and the shrinkage is
compensated for by having been worked proportionately over its length. Thus a 3/16 inch
shrink rule 12 inches long will be actually 12 3/16 inches long, but for all appearances
will look like a standard rule. But, when layer out against a standard ruler it will project
3/16 inch past the standard ruler. These rulers come in a large variety of shrinks.
Generally the
Shrinkage allowance for brass is 3/16 inch per foot, VA inch per foot for cast
iron, VA inch per foot for aluminum and VI inch for steel. This would hold true for most
small to medium work, for larger work the shrinkage allowance is less, in some cases 50
percent less. Where a small steel casting in steel would require VT inch per foot shrinkage
allowance, a very large steel casting might require only VA inch per foot shrinkage
allowance. So, from this we see that if we wish to cast a bar in brass 1 foot long we must
make the pattern 1 foot and 3/16 inches long to start with.

Machining Allowance:-

Now the plot thickens, say the disc we want in brass requires that the outer diameter of
the casting is to be machined (the 12 inch dimension is a machined dimension). We must
then allow for machining to our 12 inch dimension. This allowance must be in addition to
the shrinkage and draft allowance, taken at the short side of the pattern or smallest
diameter. See Fig. 5-2.
We must have a pattern dimension of 12 3/16 inches the 3/16 inches to allow for
shrinkage plus 1/16 inch for metal to come off. So we need an actual diameter on the
small end of our pattern of 12% inches.
If we dimension our layout as 12 and 1/16 inches (the 1/16 inch for machining) and we
use a 3/16 inch shrink ruler to measure this dimension, then when you build the pattern it
will come out fine. Or, make your pattern layout read 12 inches in diameter taking the 12
inch dimension off of a % inch shrink ruler.
Approximate finish allowances including the draft are, brass 1/16 inch,
aluminum Va inch, cast iron Va inch, cast steel V* inch.
On a blue print given to the pattern maker all finishes should be noted (turned, ground
etc.) he will then know from experience how much to allow and how much shrinkage to
add to the pattern

Production Pattern:-

In the case of our disc, say we are only going to make a casting from the pattern now and
again one at a time, we dimension as above. The pattern is called a production pattern,
one from which the actual castings are produced.

Rolling operation:-

A method of forming to shape an initially annular work piece in which the work piece is
rotated and squeezed between a pair of opposed forming rolls at least one of which is
advanced relatively towards another. A pair of opposed growth control rolls are situated
at right angles to the forming rolls, contacting the outer surface of the work piece during
at least a part of the rolling by the forming rolls. When the forming rolls have advanced
to a predetermined depth of roll the force exerted by the growth control rolls on the outer
surface of the work piece is increased and the rolls continue to move towards one another
until an adjustable dead stop is reached.
FORGING:-

Forging is the shaping of metal using localized compressive forces. Forging is often
classified according to the temperature at which it is performed: '"cold," "warm," or "hot"
forging. Forged parts can range in weight from less than a kilogram to 170 metric
tons.[1] Forged parts usually require further processing to achieve a finished part.

TYPES OF FORGING:-

   1-   Open – die drop- hammer forging
   2-   Impression – die drop – hammer forging
   3-   Press forging
   4-   Upset forging
   5-   Automatic forging
   6-   Roll forging
   7-   Swaging

Open – die drop – hammer forging:-

Open-die forging is also known as smith forging.[5] In open-die forging, a hammer strikes
and deforms the work piece, which is placed on a stationary anvil. Open-die forging gets
its name from the fact that the dies (the surfaces that are in contact with the work piece)
do not enclose the work piece, allowing it to flow except where contacted by the dies.
Therefore the operator needs to orient and position the work piece to get the desired
shape. The dies are usually flat in shape, but some have a specially shaped surface for
specialized operations. For example, a die may have a round, concave, or convex surface
or be a tool to form holes or be a cut-off tool.

Impression – die drop – hammer forging:-

Impression-die forging is also called closed-die forging. In impression-die work metal is
placed in a die resembling a mold, which is attached to the anvil. Usually the hammer die
is shaped as well. The hammer is then dropped on the work piece, causing the metal to
flow and fill the die cavities. The hammer is generally in contact with the work piece on
the scale of milliseconds. Depending on the size and complexity of the part the hammer
may be dropped multiple times in quick succession. Excess metal is squeezed out of the
die cavities, forming what are referred to as flash. The flash cools more rapidly than the
rest of the material; this cool metal is stronger than the metal in the die so it helps prevent
more flash from forming. This also forces the metal to completely fill the die cavity.
After forging the flash is removed.
HOT WORKING:-

Hot working refers to processes where metals are plastically deformed above their
recrystallization temperature. Being above the recrystallization temperature allows the material to
recrystallize during deformation. This is important because recrystallization keeps the materials
from strain hardening, which ultimately keeps the yield strength and hardness low
and ductility high. This contrasts with cold working.

Some of the hot – working processes that are of major importance in modern
manufacturing are:

    1- Rolling

    2- Hot rolling

    3- Extrusion

    4- Pipe welding

    5- Forging

    6- Drawing

    7- Rotary piercing



COLD WORKING:-

Altering the shape or size of a metal by plastic deformation. Processes include
rolling, drawing, pressing, spinning, extruding and heading, it is carried out below
the recrystallization point usually at room temperature. Hardness and tensile
strength are increased with the degree of cold work whilst ductility and impact values
are lowered. The cold rolling and cold drawing of steel significantly improves surface
finish.
RING ROLLING:-
The ring rolling process efficiently forms seamless circular products with constant cross
section. These products are used in bearings and as components for aero-engines,
vehicles, wind turbines, pipelines - anything in fact which requires round products. A
wide variety of ring profiles are manufactured but complex shapes require the
development of dedicated tooling. A significant production run is needed to recoup the
tool cost.
EXTRUSION:-

Extrusion is a process used to create objects of a fixed cross-sectional profile. A material is
pushed or drawn through a die of the desired cross-section. The two main advantages of this
process over other manufacturing processes are its ability to create very complex cross-sections
and work materials that are brittle, because the material only encounters
                                                                                                  [1]
compressive and shear stresses. It also forms finished parts with an excellent surface finish.

Extrusion may be continuous (theoretically producing indefinitely long material) or semi-
continuous (producing many pieces). The extrusion process can be done with the material hot or
cold.

Commonly extruded materials include metals, polymers, ceramics, concrete and foodstuffs.

Hollow cavities within extruded material cannot be produced using a simple flat extrusion die,
because there would be no way to support the center barrier of the die. Instead, the die assumes
the shape of a block with depth, beginning first with a shape profile that supports the center
section. The die shape then internally morphs along its length into the final shape, with the
suspended center pieces supported from the back of the die.

				
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posted:6/16/2011
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