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The Casting Process

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					The Casting
Process

   Casting is the process of causing liquid metal to fill
a cavity and solidify into a useful shape. It is a basic
method of producing shapes. With the exception of a
very small volume of a few metals produced by elec-
trolytic or pure chemical methods, all material used
in metal manufacturing is cast at some stage in its
processing. Castings of all kinds of metals, in sizes
from a fraction of an ounce up to many tons, we
used directly with or without further shape process-
ing for many items of manufacture. Even those ma-
terials considered to be wrought start out as cast
ingots before deformation work in the solid state puts
them in their final condition.
   A vast majority of castings, from a tonnage stand-
point, are made from cast iron. A relatively small num-
ber of these are subjected to NDT. In most cases they
are designed for non-critical applications with princi-
pally compressive loading and oversize dimensions to
eliminate the problem effect of the innumerable discon-
tinuities inherent in the material. However, some of
these castings and many others made of different ma-
terial may be used in such a way that careful inspec-
tion is essential for satisfactory service. Penetrant
testing may be in order for surface examination. Radio-
graphy or ultrasonic testing may be needed to detect
internal defects regardless of the material or type of
casting. Ultrasonic methods are difficult to use with
some castings because of noise created by grain struc-
ture. The rough surfaces of many castings also can pro-
duce problems in transducer coupling, but ultrasonic
testing is used extensively in the examination of criti-
cal coolant passages in turbine engine blades to meas-
ure thickness. Eddy current and penetrant methods
are also used to detect leading and trailing edge cracks
before and during service of turbine blades.
 80 Materials and Processes for NDT Technology

THE PROCESS                                                                        Mold Cavity Filled with Molten Material. Liquic
    The Process Starts with a Pattern. The casting, or                         metal is poured through the channels to fill the cavit:
founding, process consists of a series of sequential                           completely. After time has been allowed for solidifj
steps performed in a definite order, as shown in                               cation t o occur, the mold is opened. The product i
 Figure 8-1. First, a pattern to represent the finished                        then ready for removing the excess metal that ha
product must be chosen or constructed. Patterns can                            solidified in the runners, cleaning for removal of an:
be of a number of different sytles, but are always the                         remaining mold material, and inspecting to determin
shape of the finished part and roughly the same size                           if defects have been permitted by the process. Th
as the finished part with slightly oversized dimensions                        casting thus produced is a finished product of th
to allow for shrinkage and additional allowances on                            foundry. This product occasionally may be used i~
surfaces that are to be machined. In some casting                              this form, but more often than not needs furthe
processes, mainly those performed with metal molds,                            processing, such as machining, to improve surfac
the actual pattern may be only a design consideration                          qualities and dimensions and, therefore, becomes ra.i
with the mold fulfilling the function of a negative of                         material for another processing area.
the pattern as all molds do. Examples would be molds                               Casting Is a Large Industry. The tonnage o u t p ~
for ingots, die casting, and permanent mold castings.                          of foundries throughout the United States is ver
 Most plastic parts are made in molds of this type, but                        large, consisting of close to 20 million tons (18 mi
with plastics, the process is often called molding                             lion tonnes) per year. Foundries are scattered all ovc
rather than casting.                                                           the United States but are concentrated primarily i
                                                                               the eastern part of the nation with a secondary cor
   A Mold Is Constructed from the Pattern. In some
casting processes, the second step is to build a mold                          centration on the west coast in the two areas wher
of material that can be made to flow into close con-                           the main manufacturing work is carried on.
tact with the pattern and that has sufficient strength                             Foundries Tend to Specialize. Because of diffe:
t o maintain that position. The mold is designed in                            ences in the problems and equipment connected wit
such a way that it can be opened for removal of the                            casting different materials, most foundries specializ
pattern. The pattern may have attachments that make                            in producing either ferrous or nonferrous casting:
grooves in the mold t o serve as channels for flow of                           Relatively few cast both kinds of materials in appn
material into the cavity. If not, these channels, or                           ciable quantities in the same foundry.
                                                                                   A few foundries are large in size, employing sever:
runners, must be cut in the mold material. In either
case, an opening to the outside of the mold, called a                          thousand men, but the majority are small with fror
sprue, must be cut or formed.                                                  one t o one hundred employees. Most large foundaric
                                                                               are captive foundries, owned by parent manufactu~
                                                                               ing companies that use all, or nearly all, of th
                                                                               foundry's output. More of the small foundaries ar
                                                                               independently owned and contract with a number c
                                                                               different manufacturers for the sale of their casting:
                                                                               Some foundries, more often the larger ones, ma
                                                                               produce a product in sufficient demand that thej
                                                                               entire facility will be devoted to the making of tha
                                                                               product with a continuous production-type operl
                                                                               tion. Most, however, operate as job shops tha
                                                                               produce a number of different things at one time an1
                                                                               a r e continually changing from one product t'
                                                                               another, although the duplication for some parts ma,
          PATTERN                           PATTERN IN S A N D M O L D
                                                                               run into the thousands.

                                                                                          SOLIDIFICATION O F METALS
                                                                                  The casting process involves a change of state o
                                                                               material from liquid to solid with control of shap
                                                                               being established during the change of state. Th
                                                                               problems associated with the process, then, ar
                                                 COMPLETE CASTING WlTH
                                                 ATTACHED G A T I N G SYSTEM   primarily those connected with changes of physice
                                                                               state and changes of properties as they may be i n f l ~
   M O L D CAVITY WlTH G A T I N G SYSTEM
                                                                               enced by temperature variation. The solution t      c
                                                                               many casting problems can only be attained with a   1
                               Figure 8-1                                      understanding of the solidification process and th
                    Casting steps for a pulley blank                           effects of temperature on materials.
                                                                                              The Casting Process 81

                                                               Second Phase Slower. After formation of the
   Energy in the form of heat added to a metal changes      solid slrin, grain growth is likely t o be more orderly,
,he force system that ties the atoms together. Eventu-      providing the section thickness and mass are large
~lly, heat is added, the ties that bind the atoms are
       as                                                   enough t o cause a significant difference in freezing
proken, and the atoms are free to move about as a li-       time between the outside shell and the interior metal.
pid. Solidification is a reverse procedure, a s shown in    Points of nucleation will continue t o form around the
2igure 8-2, and heat given up by the molten material        outside of the liquid as the temperature is decreased.
nust be dissipated. If consideration is being given         The rate of decrease, however, continues t o get lower
mly to a pure metal, the freezing point occurs a t a        for a number of reasons. The heat of fusion is added.
iingle temperature for the entire liquid. As the temper-    The heat must flow through the already formed solid
iture goes down, the atoms become less and less             metal. The mold mass has been heated and has less
nobile and finally assume their position with other         temperature differential with the metal. The mold
itoms in the space lattice of the unit cell, which grows    may have become dried out t o the point that it acts
nto a crystal.                                              as an insulating blanket around the casting.
   Ciystal Growth Starts a t the Surface. In the case          Second Phase Also Directional. Crystal growth
l a casting, the heat is being given up t o the mold
f                                                           will have the least interference from other growing
laterial in contact with the outside of the molten          crystals in a direction toward the hot zone. The
lass. The first portion of the material t o cool to the     crystals, therefore, grow in a columnar shape toward
reezing temperature will be the outside of the liquid,      the center of the heavy sections of the casting. With
nd a large number of these unit cells may form              the temperature gradient being small, growth may
imultaneously around the interface surface. Each            occur on the sides of these columns, producing struc-
 nit cell becomes a point of nucleation for the             tures known as dendrites (Figure 8-3).This pine-tree-
rowth of a metal crystal, and, as the other atoms           shaped first solidification seals off small poclrets of
 001, they will assume their proper position in the         liquid t o freeze later. Evidence of this kind of crystal
pace lattice and add t o the unit cell. As the crystals     growth is often difficult t o find when dealing with
Drm, the heat of fusion is released and thereby in-         pure metals but, as will be discussed later, can readily
reases the amount of heat that must be dissipated           be detected with most alloy metals.
  efore further freezing can occur. Temperature                Third Phase. As the wall thickness of frozen metal
 radients are reduced and the freezing process re-          increases, the cooling rate of the remaining liquid
 ~ r d e d .The size of crystal growth will be limited by   decreases even further, and the temperature of the
 lterference with other crystals because of the large       remaining material tends t o equalize. Relatively
 umber of unit cell nuclei produced at one time with        uniform temperature distribution and slow cooling
 ~ n d o morientation. The first grains to form in the      will permit random nucleation at fewer points than
 kin of a solidifying casting are likely t o be of a fine   occurs with rapid cooling, and the grains grow t o
 quiaxed type with random orientation and shapes.           large sizes.

  HEAT ADDED                       HEAT REMOVED
                                                      -2

  AT CONSTANT KATE             I   AT CONSTANT RATE
                               I
                               I




  MELTING TEMPERATURE




                        TIME



                      Figure 8-2
 Heating and cooling curves for temperature increase                             Figure 8-3
         above the melting point for a metal                         Schematic sketch of dendritic growth
82 Materials and Processes for NDT Technology

   G r a i n C h a r a c t elistics Influenced by Coding             alloys, the noneutectic alloys freeze over a temper?
Rates. As shown in Figures 8-4 and 8-5, it would be                  ture range. As the temperature of the molten materiz
expected in castings of heavy sections that the first                is decreased, solidification starts at the surface ant
grains t o form around the outside would be fine                     progresses toward the interior where the metal i
equiaxed. Columnar and dendritic structure would be                  cooling more slowly. Partial solidification may prc
present in directions toward the last portions t o cool              gress for some distance before the temperature a t th
for distances depending upon the material and the                    surface is reduced low enough for full solidificatio
cooling rate under which it is solidified. Finally, the              t o take place. The material at temperatures betwee
center of the heavy sections would be the weakest                    those at which solidification begins and ends is part
structure made up of large equiaxed grains. Changes                  ally frozen with pockets of liquid remaining t
in this grain-growth pattern can be caused by a num-                 produce a mixture that is of mushy consistency an
ber of factors affecting the cooling rate. Thin sections             relatively low strength. Figure 8-6 is a graphic reprc
that cool very quickly will develop neither the colum-                                                                 s e n t a t i o n of - t h
nar nor the coarse structure. Variable section sizes                                  FINE IQUIAXIAL GRAINS            kind of freezing. Th
and changes of size and shape may cause interference                                                     DENDqlrEs     d u r a t i o n o f thi
and variations of the grain-structure pattern. Dif-                                                                    condition and th
ferent casting procedures and the use of different                                                                     dimensions of t h
                                              m o l d materials                                                        space between th
                                              can affect grain                                                         start and finish c
                                              s i z e a n d shape                                             ..       freezing are fun1
                                                                          ., . . .    ..
                                                                                                                   I , tions of the solidif
                                                                         ,
                                              t h r o u g h their    I
                                                                        . .        . . .

FINE EQUIAXED                                 influenci on the               i_i                         u             cation temperatu~
                               ,O L U M N A R
                                 C
                                                                                     Figure 8-5                        range of the allo
                                              cooling rate.
                                                  Results of NDT         Grain formation in a heavy                    material and tk
                                              for internal de-                     sand casting                        t h e r m a l gradien
                                              fects may be diffi-    The greater the solidification temperature range (i
                                              cult t o analyze be-   most cases meaning the greater the variation awa:
                                              cause of effects       from the eutectic composition)
                                              from variable          and the smaller the temperature
                                              grain size in mas-     gradient, the greater the size
                                              sive castings.         and duration of this mushy
                                              Large        grains    stage.
                                              cause diffraction
                                              efects with radio-        Segregation. Dendritic grain
                                              graphic methods        growth is much more evident in
                                              a n d reflection       the noneutectic alloy metals than       :';
                     \ COARSE EQLIIAXED       from grain boun-       in pure metal. When more than               Figure 8-6
                                              daries causes          one element is present, segrega- Process of freezinc
                 Figure 8-4                   problems with ul-
                                                                     tion of two types occurs during in a noneutectic allc
       Typical grain structure from           trasonic testing.
                                                                     solidification. The first solids t o freeze will be riche
     solidification of a heavy section Special              tech-    in one component than the average composition. Th
niques which minimize these effects may be necessary                 change caused by this ingot-type segregation is smal
t o test large grained castings.                                     but as the first solids rob the remaining material,
   Eutectics Similar to Pure Metals. Eutectic alloys                 gradual change of composition is caused as freezin
freeze m much the same manner as a pure metal. Solidi-               progresses t o the center. The other type of segrt
fic,,ion takes place a t a single temperature that is                gation is more localized and maltes the dendriti
lower than that for the individual components of the                 structure easy t o detect in alloy materials. The sma
alloy. The grain size produced with an eutectic alloy is             liquid pockets, enclosed by the first dendritic solid:
smaller than the grain size of a pure metal under the                have supplied more than their share of one con
same conditions. It is believed that this is due t o a               ponent t o the already frozen material. This diffe
smaller temperature gradient and the formation of a                  ence in composition shows up readily by difference i
greater number of points of nucleation for the start of              chemical reaction if the material is polished an
grains.                                                              etched for grain examination.
  N o n e u t eetics Freeze through a Temperature
Range. The majority of products are made from                        SHRINKAGE
noneutectic alloys. Instead of freezing at a single                    Shrinltage Occurs in 'I'hree Stages. Some of tk
temperature as does the pure metal and the eutectic                  most important problems connected with the castir
                                                                                                               The Casting Process 83

roccss are those of shrinkage. The amount of shrink-                                       TABLE 8-1
ge that occurs will, of course, vary with the material                    Approximate solidification shrinkage of some
eing cast, but it is also influenced by the casting                                    common metals
 rocedure and techniques. The three stages of con-
faction that occur as the temperature decreases from                                                       Percent
i e temperature of the molten metal t o room                                      Metal               Volumetric Shrinkage
2mperature are illustrated in Figure 8-7.                                         Gray iron . . . . . . . . . . . . 0-2
   first Stage Shrinkage in the Liquid. In the melt-                              Steel . . . . . . . . . . . . . . . 2.5-4
lg procedure, preparatory to pouring castings, the                                Aluminum . . . . . . . . . . 6.6
letal is always heated well above the melting temp-                               Copper . . . . . . . . . . . . 4.9
                                                                                                                                  --
rature. The additional heat above that necessary for
~ e l t i n g called superheat. It is necessary t o provide
            is                                                       porosity causes a reduction in density and tends to re-
                                                                     duce the apparent shrinkage that can be seen on the
luidity of the liquid t o permit cold additives t o be
                                                                     surface of a casting.
~ i x e d with the metal before pouring. Superheat
                                                                        The shrinkage that occurs during solidification and
llows the metal t o be transferred and t o contact cold
quipment without starting t o freeze, and insures that               the microporosity that often accompanies it are
ufficient time will elapse before freezing occurs t o                minimized in materials that are near eutectic compo-
llow disposal of the material. Some superheat is lost                sition. This seems to be due t o more uniform freezing
 uring transfer of the liquid metal from the melting                 with lower temperature gradients and more random
quipment t o the mold. However, as the metal is                      nucleation producing finer grain structure. Micro-
loured into the mold, some superheat must remain t o                 shrinkage is often a problem in aluminum or magne-
                                                                     sium castings.
nsure that the mold will fill. Loss of superheat results
                                                                        Macroporosity. The porosity of a casting may be
n contraction and increased density but is not likely
                                                                     amplified by the evolution of gas before and during
o cause serious problems in casting. The volume                      solidification. Gas may form pockets or bubbles of its
hange can be compensated for by pouring additional                   own or may enter the voids of microporosity t o en-
naterial into the mold cavity as the superheat is lost.
                                                                     large them. The evolved gas is usually hydrogen,
111 exception exists when the cavity is of such design
                                                                     which may combine with dissolved oxygen t o form
hat part of it may freeze off and prevent the flow of                water vapor. These randomly dispersed openings of
he liquid metal for shrinkage replacement.                           large size in the solid metal are referred t o as
   Solidification Shrinkage. The second stage of                     macroporosity .
hrinlcage occurs during the transformation from
                                                                                                /   MACROPOROSITY
 quid t o solid. Water is an exception t o the rule, but
nost materials are more dense as solids than as
iquids. Metals contract as they change from liquid
o solid. The approximate volumetric solidification
hrinlcage for some common metals is shown in Table
;-I.Contraction a t this stage can be partially replaced
       LIQUID               SOLIDIFICATION                SOLID
    CONTRACTION             CONTRACTION                CONTRACTION




                                                                                               ' MICROPOROSITY
                                                                                             RANDOMLY DISTRIDUTED VOIDS
                                             CAVITY                                              O F SMALL SIZE
     SHRINK PERCENTAGES APPROXIMATE O N L Y FOR CAST IRON
                                                                                                Figure 8-8
                        Figure 8-7                                                               Porosity
             Three stages of metal contraction
                                                                                                              /     PATTERN

because the entire metal is not yet frozen. If a suit-
able path can be kept open, liquid metal can flow
from the hot zones t o replace most of the shrinkage.
It will be remembered, however, that in the forma-
tion of a dendritic grain structure, small pockets have                                                                       1
been left completely enclosed with solid material.
                                                                                                                  FINAL CASTING
Depending upon the characteristics of the material                                        PATTERNMAKER'S
                                                                                            ALLOWANCE
and the size of the liquid enclosures, localized shrink-
ing will develop minute random voids referred to as                                         Figure 8-9
microporosity or microshrinkage (Figure 8-8). Micro-                               Pattern shrinkage allowance
 84 Materials and Processes for NDT Technology

    Contraction in the Solid State. The third stage of                                                                  cause the metal farthes
shrinltage is that occurring after solidification takes                                                                 from the point of entr:




                                                                       +
place and is the primary cause of dimensional change                                                                    t o freeze first with solidi
t o a size different form that of the pattern used to                                                                   fication moving toward r
make the cavity in the mold. Although contraction of                                                                    feed head, which may bl
                                                                                                                        at the point where meta
solidification may contribute in some cases, the                            lnfcrrecling Ribr
solid metal contraction is the main element                                              POOR DESIGN
                                                                                                       H c a v y Bolr
                                                                                                                        is poured into the molc
of patternmaker's shrinkage, which must be allowed                                                                      or can be located at othe
for by making the pattern oversize.                                                                                     points where liquid c a
                                                                                                                        be stored t o feed into thl
                                                                                                                        casting proper.
                                                                        Oifset Rib5 IMPROVED DESIGN C o r e d Hole


                                                                                                                           Hot Spots Are Foca
          POURING AND FEEDING CASTINGS                                           Figure 8-11
                                                                                                                        P o i n t s f o r Solidifica
                                                                             Hot spot elimination                       tion. The highest temp
CASTING DESIGN
                                                                      erature areas immediately after pouring are called 120
   The first consideration that must be given t o                     spots and should be located as near as possible tc
obtain good castings is t o casting design. It should be              sources of feed metal. If isolated by sections tha
remembered that although volumetric shrinltage of                     freeze early, they may disturb good directional solidi
the liquid is thought of as being replaced by extra                   fication with the result that shrinks, porosity, craclte
metal poured in the mold and by hydraulic pressure                    ruptures, or warping will harm the casting quality. 1  1
from elevated parts of the casting system, this can be                is not always necessary to completely inspect somc
true only if no parts of the casting freeze off before                castings when the vulnerable spots can be determine(
replacement takes place. Except for the small pockets                 by visual inspection. Defects are most likely a t ho
completely enclosed by solid metal in the develop-                    spots created by section changes or geometry of thc
ment of dendritic structures, the shrinkage of solidifi-              part and where gates and risers have been connected t c
cation can be compensated for if liquid metal can be                  the casting.
progressively supplied t o the freezing face as it ad-                   Control of Hot Spots Usually by Proper Desigr
vances.                                                               Hot spots are usually located a t points of greates
   Progressive versus Directional Solidification. The                 sectional dimensions. Bosses, raised letters, nor
    FEED I i f i i D                     PROGPESSIVE SOLIDIFICATION
                                                                      uniform section thicknesses, and intersecting member
                                                                      are often troublemakers in the production of higl
                                                                      quality castings. Solution t o the problem involve
                                                                      changing the design, as shown in Figure 8-11, or poul
                                                                      ing the casting in such a way that these spots cease t~
                                                                      be sources of trouble. Changing the design migh
                                                                       include coring a boss t o make it a thin-walled cylir
                                                                       der, relieving raised letters or pads on the bacltsidc
                                                                      proportioning section thicknesses t o uniform change
                                                                      of dimensions, using thin-ribbed design instead o
                                                                      h e h y sections, spreading and alternating intersectin
                                                                      members, and making other changes that will no
                                                                      affect the function of the part but will decrease th
                                                                      degree of section change.
                             Figure 8-10
               Progressive and directional solidification                Uniform Section 'I'hicknesses Desirable. As :
                                                                      general rule, section changes should be minimized a:
term progessiue solidification, the freezing of a liquid              much as possible in order to approach uniform cool
from the outside toward the center, is different from                 ing rates and reduce defects. When pouring iron, heavy
directional solidification. Rather than from the sur-                 sections tend t o solidify as gray iron with precipitated
face t o the center of the mass, directional solidifica-              graphite. Thin sections of the same material cooling at
t i o n is used t o describe the freezing from one part of            higher rates tend to hold the carbon in the combined
a casting t o another, such as from one end t o the                   state a s iron carbide with the result that these sections
other end, as shown in Figure 8-10. The direction of                  turn out to be hard, brittle white iron. Since it is clearly
freezing is extremely important t o the quality of a                  impossible to design practical shapes without section
casting because of the need for liquid metal t o com-                 changes, the usual procedure calls for gradual section
pensate for the contraction of the liquid and that dur-               size changes and the use of liberal fillets and rounds
ing solidification. Casting design and procedure should               Some section changes are compared in Figure 8-12.
                                                                                                                        The Casting Process 85

Sudden Section Chonae   Lorac Rodii   Gradual T a ~ e r   N o Section Chonoe
                                                                               will be completely filled with a uniform flow of
                                                                               metal.
  POOR DESIGN            GOOD            BETTER                BEST
                                                                                  Superheat Affects Casting Quality. As mentioned
                           Figure 8-12                                         earlier, metals are superheated from 100" t o 530"
                Section changes in casting design                              above their melting temperature to increase their
                                                                               fluidity and t o allow for heat losses before they are in
                                                                               their final position in the mold. For good castings,
POUIEING                                                                       the metal must be at the correct superheat at the time
   Most Pouring Done from Ladles. Pouring is usu-                              it is poured into the mold. If the temperature is too
ally performed by using ladles to transport the hot                            low, misruns and cold shuts will show up as defects in
netal from the melting equipment to the molds. Most                            the casting, or the metal may even freeze in the ladle.
molds are heavy and could be easily damaged by jolts                           If the temperature at pouring is too high, the metal
md jars received in moving them from one place to                              may penetrate the sand and cause very rough finishes
another. Exceptions exist with small molds or with                             on the casting. Too high pouring temperatures may
7eavier molds, with which special equipment is used,                           cause excessive porosity or increased gas development
that can be conveyorized and moved to a central                                leading t o voids and increased shrinkage from thermal
pouring station. Even with these, the hot metal is                             gradients that disrupt proper directional solidifi-
usually poured from a ladle, though some high pro-                             cation. High pouring temperature increases the mold
duction setups make use of an automatic pouring                                temperature, decreases the temperature differential,
station where spouts are positioned over the mold                              and reduces the rate at which the casting cools. More
and release the correct amount of metal t o fill the                           time at high temperature allows greater gain growth
zavity .                                                                       so that the casting will cool with a weaker, coarse
   Turbulent Flow Harmful. Casting quality can be                              grain structure.
significantly influenced by pouring procedure. Tur-
bulent flow, which is caused by pouring from too
great a height or by excessive rates of flow into the
mold, should be avoided. Turbulence will cause gas to                          THE GATING SYSTEM
be picked up that may appear as cavities or pockets in
                                                                                  Metal is fed into the cavity that shapes the casting
the finished casting and may also oxidize the hot                              through a gating system consisting of a pouring basin,
metal to form metallic oxide inclusions. Rough, fast                           a down sprue, runners, and ingates. Some typical
flow of liquid inetal may erode the mold and result in                         systems are shown in Figure 8-13. There are many
loss of shape or detail in the cavity and inclusion of                         special designs and terminology connected with these
sand particles in the metal. Cold shots are also a result                      channels and openings whose purpose is that of
of turbulent flow. Drops of splashing metal lose heat,                         improving casting quality. Special features of a gating
freeze, and are then entrapped as globules that do not                         system are often necessary to reduce turbulence and
join completely with the inetal which freezes later                            air entrapment, reduce velocity and erosion of sand,
2nd are held partly by mechanical bond.                                        and remove foreign matter or dross. Unfortunately,
   Pouring Rate. The pouring rate used in filling a                            no universal design is satisfactory for all castings or
mold is critical. If metal enters the cavity too slowly,                       materials. There are no rules that can be universally
it may freeze before the mold is filled. Thin sections
that cool too rapidly in contact with the mold walls                                                    J   POURING BASIN

may freeze off before the inetal travels its complete
path, or metal flowing in one direction may solidify
and then be met by metal flowing through another
path to form a defect known as a cold shut. Even
though the mold is completely filled, the cold shut                                           RUNNER
                                                                                 KNIFE GATE            HORSESHOE GATE
shows the seam on the surface of the casting, and the
metal is not solidly joined and is therefore subject to                                                                     MULTIPLE INGATE WITH
                                                                                                                               TAPERED RUNNER
easy breakage.
   If the pouring rate is too high, it will cause erosion                                              Figure 8-13
of the mold walls wit11 the resulting sand inclusions                                            Typical gating systems
and loss of detail in the casting. High thermal shock
to the mold may result in cracks and buckling. The                             depended upon, and experimentation is commonly a
rate of pouring is controlled by the mold design and                           requirement for good casting production.
the pouring basin, sprue, runner, and gate dimensions.                           The location of the connection for the gate, or gates,
The gating system should be designed so that when                              can usually be determined visually. These spots are
the pouring basin is kept full, the rest of the system                         possible concentration points for defects.
 86 Materials and Processes for N D T Technology

RISERS                                                            before the chills have time t o collect moisture from
   Risers Ase Multipurpose. Risers, feeders, or feed              condensation. In addition to helping with directional
heads serve as wells of material attached outside the             solidification, chills may also improve physical
casting proper to supply liquid metal as needed to                properties. Fast cooling during and after solidification
compensate for shrinkage before solidification is                 retards grain growth and thus produces a harder,
complete. Although most liquid contraction is taken               stronger structure.
care of during pouring, a riser may supply replace-                  Choice of Intelnal Chills Critical. Internal chills
ment for some of this contraction after parts of the              that become an integral part of the casting are occa-
casting have frozen solid, as shown in Figure 8-14.               sionally used to speed solidification in areas where
However, the principal purposes of risers are t o re-             external chills cannot be applied. The design and use
place the contraction of solidification and to promote            of internal chills is critical. Usually this type of chill is
good directional solidification. The need for risers              made of the same material as the casting. The chill
varies with the casting shape and the metal being                 must be of such size that it functions as a cooling
poured.                                                           device, but at the same time it must be heated enough
                   Liquid metol wpply to compcnrate for liquid
                                                                  that it fuses with the poured material t o become an
                      ond ~ o l i d i f i e a t i o nshrinkage    integral and equally strong part of the casting.
                                                                     Nondestructive testing is often used to detect un-
                                                                  fused internal chills and adjacent defects that may be
                                                                  caused by the change in cooling rate created by the pre-
                                                                  sence of the chill.


                                                                               FOUNDRY TECHNOLOGY
                                                                    Although the casting process can be used to shape
                                                                  almost any metal, it has been necessary to develop a
                                                                  number of different methods to accommodate differ.
                                                                  ent materials and satisfy different requirements. Each
                          Figure 8-14                             method has certain advantages over the others, but all
                 Risers for shrinkage control                     have limitations. Some are restricted to a few special
                                                                  applications.

                                                                                     SAND MOLDING
CHILLS
                                                                      Sand is the most commonly used material for
   Chills Initiate Solidification. Help in directional            construction of molds. A variety of sand grain sizes,
solidification can also be obtained in a reverse manner           combined and mixed with a number of other mater-
by the use of chills, which are heat-absorbing devices            ials and processed in different ways, causes sand to
inserted in the mold near the cavity (Figure 8-15). To            exhibit characteristics that make it suitable for several
absorb heat rapidly, chills are usually made of steel,            applications in mold making. A greater tonnage of
cast iron, or copper and designed to conform t o the              castings is produced by sand molding than by all
casting size and shape. Because chills must be dry 'to            other methods combined.
avoid blowhole formation from gases, it is sometimes
                                                                      P r o c e d u r e for Sand Molding. The following
necessary to pour a mold soon after it has been made,
                                                                  requirements are basic to sand molding, and most of
                                                                  them also apply for the construction of other types
                                                                  of molds.
                                                                  1.Sand - To serve as the main structural material for
                                                                      the mold
                                                                  2. A pattern - To form a properly &aped and sized
                                                                      cavity in the sand
                                                       INTERNAL   3 . A flask - To contain the sand around the pattern
                                                        CHILL
                                                                      and to provide a means of removing the pattern
                                                                      after the mold is made
              v     EXTERNAL CHILL
                                                                  4. A ramming method - To compact the sand around
                                                                      the pattern for accurate transfer of size and shape
                                                                  5. A core - To form internal surfaces on the part
                        Figure 8-15                                   (usually not required for castings without cavitie2
       Chills as an aid to directional solidification                 or holes)
                                                                                                      The Casting Process 87

6. A mold grating system - To provide a means of            ture and the types of sand and clay may be varied t o
   filling the mold cavity with metal a t the proper rate   change the properties of the molds to suit the ma-
   and to supply liquid metal to the mold cavity as the     terial being poured. To produce good work consis-
   casting contracts during cooling and solidification      tently, it is important that advantage be talten of the
   The usual procedure for making a simple green            properties that can be controlled by varying the con-
sand casting starts with placing the pattern t o be         stituents of the sand mixture.
copied on a pattern, or follower, board inside one-            Sand Grains Held Together by Qay. In a mold,
half of the flask, as shown in Figure 8-16.Sand is then     the sand particles are bound together by clay that is
packed around the pattern and between the walls of          combined with a suitable quantity of water. The most
the flask. After striking off excess sand, a bottom         commonly accepted theory of bonding is that as pres-
board is held against the flask and sand and the            sure is applied t o the molding sand, clay, coating each
assembly turned over. Removal of the pattern board          sand particle, deforms and flows t o wedge and lock
exposes the other side of the pattern. A thin layer of      the particles in place. The clay content can be varied
parting compound (dry nonabsorbent particles) is            from as little as 2% or 3% t o as high as 50%, but the
dusted on the pattern and sand t o prevent adhesion.        best results seem to be obtained when the amount of
Addition of the upper half of the flaslt allows sand t o    clay is just sufficient t o coat completely each of the
be packed against the pattern.                              sand grains.
                                                               Water Conditions the Clay. Water is the third
F L A S K (Drag)
                    PATTERN                                 requisite for green sand molding. The optimum quan-
   I                /
                                                            tity will vary from about 2% t o 8% by weight, de-
                                                            pending largely upon the type and quantity of clay
                                                            present. Thin films of water, several molecules in
                                                            thickness, are absorbed around the clay crystals. This
                   'FOLLOWER               ' SAND
                                                            water is held in fixed relationship t o the clay by
            STEP I                             STEP 2
                                 F LASI<                    atomic attraction and is described as rigid water, or
  PARTING COMPOUND
                                                            tempering water. The clays that have the greatest
                                                            ability t o hold this water film provide the greatest
                                                            bonding strength. Water in excess of that needed t o
                                                            temper the molding sand does not contribute t o
                                                            strength but will improve the flowability that permits
                                                            the sand t o be compacted around the pattern.
             STEP 3                             STEP 4


                        RUNNER




              STEP 5                            STEP 6


                            Figure 8-16
              Principal steps for making a sand mold
                                                                                                        PARTING PLANE

  After the sprue is cut to the parting line depth, the
upper half of the mold can be removed, the pattern
withdrawn, and the gating system completed. Reas-
sembly of the mold halves completes the task, and
the mold is ready for pouring.                                                   SPLIT



GREEN SAND
   T11e Word Green Refers t o Moisture. The majority
of castings are poured in molds of green sand, which                        IRREGULAR P A R T I N G
is a mixture of sand, clay, and moisture. The ma-
terials are available in large quantities, are relatively
inexpensive, and except for some losses that must be                           Figure 8-17
replaced, are reusable. The proportions of the mix-                      Common loose pattern types
88 Materials and Processes for NDT Technology

PATTERNS                                                     into place in a mold is one of the greater labor and
  By most procedures, patterns are essential for pro-        time-consuming phases of malting castings. It also has
ducing castings. I n occasional emergency situations an      considerable influence on the quality of finished cast-
original part, even a broken or worn part, may be used       ings produced. Sand that is paclted too lightly will be
as a pattern for making a replacement, but consider-         weak and may fall out of the mold, buckle, or crack,
able care and skill is necessary when this is done.          which will cause casting defects. Loosely packed
  Patterns are made of various materials: principally        grains a t the surface of the cavity may wash with tho
wood, metal, plastic, or plaster, depending on the           metal flow or may permit metal penetration with a
shape, size, intricacy, and amount of expected use.          resulting rough finish on the casting. Sand that is toc
They are constructed slightly larger than the expected       tightly compacted will lack permeability, restrict ga:
resulting part t o allow for shrinkage of the liquid         flow, and be a source of blowholes, o r may even pre.
metal, during and after solidification, t o room tempera-    vent the cavity from completely filling. Too tightly
ture size. Extra matrial is also left on surfaces to be      paclted sand may also lack collapsability so that a.
machined or finished to provide removal material on          solidification occurs, cracks and tears in the casting
the casting. Patterns also must be contructed with           may be caused by the inability of the sand t o get out
suitable draft angles t o facilitate their removal from      of the way of the shrinking metal. Each of the several
the mold medium. Patterns may be designated as flat-         available methods for compacting sand has advantage:
back where the largest two dimensions are in a single        over the others and linlitations that restrict its use.
plane, split which effectively separates to form flat-          Butt Ramming Involves Human Effort. Peen and
back patterns, or irregular parting which requires sep-      butt rammers may be used on a bench or on the flool
aration along two or more planes for removal of the          by manual operation, or, in the case of large molds
pattern t o produce the casting cavity. Any of these         the work may be done with pneumatic rainmers simi,
pattern types can be mounted on a matchplate for im-         lar t o an air hammer. Peen ramming involves the usc
proved accuracy and faster production if justified by        of a rib-shaped edge t o develop high impact pressure:
the needed quantity of castings. Some pattern types of       and is used principally t o pack sand between narroti
the loose variety are shown in Figure 8-17.                  vertical walls and around the edges of the flask. Bull
                                                             ramming is done with a broader-faced tool for more
FLASKS                                                       uniform compaction of the sand throughout the
                                                             mold.
  Flasks are open faced containers that hold the mol-           Jolting and Squeezing Use Mechanical Energy
ten medium as it is packed around the pattern. They          Most production work and a large part of work done
are usually contructed in two parts: the upper half cope    in small quantities is performed by use of molding
and the lower half drag (see Figure 8-16) which are          machines whose principal duty is that of sand com.
aligned by guide pins t o insure accurate positioning.      paction. They are designed to compact sand by e i t h e ~
The separation between the cope and drag establishes        jolting or squeezing, or both methods may be com.
the parting line and when open permits removal of the       bined in a single machine.
pattern to leave the cavity whose walls form the cast-          Jolt compaction involves the lifting of the table
ing when liquified material solidifies against it.          carrying the mold and dropping it against a solid
   Some flasks, used most for small quantity casting,       obstruction. With the sudden stop, inertia forces
are permanent and remain around the sand until after        cause the s8nd particles t o compress together. Joli
pouring has been completed. Others used for higher          compaction tends t o pack the sand more tightly n e a
production quantities are removable and can be used         the parting surface. For this reason, it is usually no1
over and over for construction of a number of molds be-
                                                            too satisfactory when used alone with patterns that
fore pouring is required. The removable flasks are of
                                                            are high and project close t o the mold surface.
three styles: snap flasks, having hinged corners, that
                                                                On the other hand, squeeze compaction, applied
can be unwrapped from the mold; pop-off flasks that
                                                            by pushing a squeeze plate against the outside of the
can be expanded on two diagonal corners to increase
                                                            sand, tends t o pack the sand more tightly at the sur.
the length and width t o allow removal; and slip flasks
                                                            face. The combination of jolting and squeezing i:
that are made with movable sand strips that project in-
                                                            frequently used t o take advantages of each method
side t o obstruct sliding of the mold medium until they
                                                            although when both the cope and drag are being
are withdrawn t o permit removal of the flask from the
                                                            made on the same machine, it may be impossible tc
mold. When molds are constructed with removable
                                                            jolt the cope half (the second half constructed) with
flasks, jackets are placed over them t o maintain align-
                                                            out damage t o the drag.
ment during pouring.
                                                               Sand Slinging Limited t o Large Molds. Foundrie:
SAND COMPACTION                                             that manufacture quantities of large castings ofter
  Gas t i n g Quality Dependent on Proper Com-              use sand slingers t o fill and compact the sand in largc
paction. Compaction, packing, o r ramming of sand           floor molds. The sand is thrown with high velocity ir
                                                                                              The Casting Process 89

a steady stream by a rotating impeller and is coin-        relatively free passage is essential for the gases t o
pacted by impact as it fills up in the mold. Figure        escape through core prints or other small areas.
8-18 illustrates the common compaction methods.               Gollapsability is likewise important because of this
                                                           metal enclosure. Ideally, a core should collapse
                                                           immediately after metal solidification takes place. In
                                                           addition t o not interfering with shrinkage of the cast-
                                                           ing, it is important in many cases that cores collapse
                                                           completely before final cooling so that they can be
                                                           removed from inside castings in which they are al-
                                                           most totally enclosed. For example, cores used t o
                                                           form the channels in a hot-water radiator or the water
                                                           openings in an internal combustion engine would be
     HAND R A M M I N G
                                                           almost impossible to remove unless they lost their
                                   JOLT RAMMING            strength and became free sand grains. The casting
                                                           metal must supply the heat for the final burning out
                                                           of the additives and the binding material.
                                                              When a substantial portion of a core is enclosed in a
                                                           casting, radiography is frequently used to determine
                                                           whether or not the core shifted during casting, or t o be
                                                           certain that all the core material has been successfully
                                                           removed after casting.
                                                              Chaplets. Very large or long slender cores that
                                                           might give way under pressure of the flowing metal
     SQUEEZE RAMMING               SAND S L I N G I N G
                                                           are sometimes given additional support by the use of
                       Figure 8-18                         chaplets. Chaplets are small metal supports with
             Common sand-compaction methods                broad surfaced ends, usually made of the same metal
                                                           as that t o be poured, that can be set between the
                                                           mold cavity and the core. Chaplets become part of
CORES                                                      the casting after they have served their function of
    Cores are bodies of mold material, usually in the      supporting cores while the metal is liquid.
form of inserts that exclude metal flow t o form in-         NDT may be necessary for castings requiring the use
ternal surfaces in a casting. The body is considered t o   of chaplets. Not ony must the chaplets be chosen of
be a core when made of green sand only if it extends       suitable material t o fuse with the base metal, but
through the cavity to form a hole in the casting.          shrink cavities may form during the cooling, porosity
Green sand cores are formed in the pattern with the        may form from moisture condensation, and non-fusing
regular molding procedure.                                 may occur from too low a pouring temperature t o melt
    Cores Need Strength for Handling. The vast             the surface of the chaplet. Radiography of the finished
majority of cores are made of dry sand and contain         casting can reveal discontinuities surrounding chaplet
little or no clay. A nearly pure sand is combined with     regions and can indicate whether the chaplets com-
additives that bum out after pouring t o promote col-      pletely fused with the base metal.
lapsability and with binders to hold the particles
together until after solidification takes place.
    Final Core Properties Very Important. The prop-
erties needed in core sand are similar t o those re-
quired for molding sand, with some taking on greater
importance because of differences in the cores' posi-
tion and use. Most cores are baked for drying and
development of dry strength, but they must also have
sufficient green strength t o be handled before baking.                          Figure 8-19
    The dry strength of a finished core must be suf-       Slender core supported by chaplets to aid core location
ficient that it can withstand its own weight without         and prevent sagging of its own weight or springing,
sagging in the mold, and it must be strong enough                     possibly floating, during pouring
that its own buoyancy, as liquid metal rises around it,
will not cause it t o break or shift.                      GREEN SAND ADVANTAGES AND LIMITATIONS
    Permeability is important with all molding sands          Green Sand Process Extremely Flexible. For most
but is especially so with core sand because cores are      metals and most sizes and shapes of castings, green
often almost completely surrounded by metal, and a         sand molding is the most economical of all the mold-
90 Materials and Processes for NDT Technology

ing processes. Green sand can be worked manually or         FLOOR AND PIT MOLDS
mechanically and, because very little special equip-           Large Molds Difficult t o Handle. Although he
ment is necessary, can be easily and cheaply used for       number of extremely large castings is relatively small,
a great variety of products. The sand is reusable with      molds must be constructed for one, five, ten, and
only slight additions necessary t o correct its com-        occasionally, even as much as several hundred ton
position. In terms of cost, the green sand process can      castings. Such molds cannot be moved about, and the
be bested only when the quantity of like castings is        high h y d r o static pressures established by high
large enough that reduced operational costs for some        columns of liquid metal require special mold "con-
other processes will more than cover higher original        struction stronger than that used for small castings.
investment or when the limitations of the green sand        Floor molds made in the pouring position are built in
process prevent consistent meeting of required quali-       large flasks. The mold can be opened by lifting the
ties.                                                       cope with an overhead crane, but the cope flask
   Green Sand Not Universally Applicable. One of            usually must be constructed with special support bars
the limitations of green sand is its low strength in        to prevent the mold material from dropping free
thin sections. It cannot be used satisfactorily for cast-   when it is lifted.
ing thin fins or long, thin projections. Green sand also       Drag of Pit Molds Below Floor Level. Pit molds
tends to crush and shift under the weight of very           use the four walls of a pit as a flask for the drag
heavy sections. This same weakness makes the casting        section. The cope may be an assembly of core sand or
of intricate shapes difficult also. The moisture present    may be made in a large flasli. similar to that used for a
in green sand produces steam when contacted by hot          floor mold. The mold material for these large sizes is
metal. Inability of the steam and other gases t o           usually loam, 50% sand and 50% clay, plus water. The
escape causes problems with some casting designs,           mold structure is often strengthened by inserting
and blowhole damage results. The dimensional ac-            bricks or other ceramic material as a large part of its
curacy of green sand castings is limited. Even with         substance.
small castings, it is seldom that dimensions can be
held closer together than rt 0.5 millimeter (0.02 inch);
with large castings, r t 3 millimeters (118 inch) or        SHELL MOLDS
greater tolerances are necessary.                              Shell molding is a fairly recent development that,
                                                            as far as casting is concerned, can be considered a
                                                            precision process. Dimensions can be held within a
                                                            few thousandths of an inch in many cases to elimi-
                                                            nate or reduce machining that might be necessary
DRY SAND MOLDS                                              otherwise and t o decrease the overall cost of manufac-
                                                            turing. The cost of the process itself, however, is
   Elimination of Moisture Reduces Casting Defects.
Improvement in casting qualities can sometimes be           relatively high, and large quantities are necessary for
obtained by use of dry sand molds. The molds are            economical operation.
made of green sand modified to favor the dry prop-             Sand Bonded with Thermosetting Plastic. The
erties and then dried in an oven. The absence of            mold is made by covering a heated metal pattern with
moisture eliminates the formation of water vapor and        sand that is mixed with small particles of a thermoset-
reduces the type of casting defects that are due t o gas    ting plastic. The heat of the pattern causes the
formation. The cost of heat, the time required for          mixture to adhere and semicures the plastic for a
drying the mold, and the difficulty of handling heavy       short depth. The thin shell thus made is baked in
molds without damage make the process expensive             place or stripped from the pattern, further cured by
compared to green sand molding, and it is used              baking at 300" C and then cemented to its mating
mostly when steam formation from the moisture               half to complete the mold proper. Because the shell is
present would be a serious problem.                         thin, approximately 3 millimeters, its resistance to
                                                            springing apart is low; it may be necessary to back it
   Skin Drying - Substitute for Oven Drying. Most           up with loose sand or shot to take the pressures set
of the benefits of dry sand molds can be obtained by
                                                            up by filling with liquid metal. The sand particles are
shin drying molds to depths from a fraction of an
                                                            tightly held in the plastic bond. As erosion and metal
inch t o an inch. With the mold open, the inside sur-
                                                            penetration are minor problems, high quality surface
faces are subjected t o heat from torches, radiant
                                                            finishes, in addition to good dimensional control, are
lahps, hot dry air, or electric heating elements to
                                                            obtained from shell molding.
form a dry insulating skin around the mold cavity.
Skin-dried molds can be stored only for short periods
of time before pouring, since the water in the main             METAL MOLD AND SPECIAL PROCESSES
body of the mold will redistribute itself and remois-         Metal patterns and metal core boxes are used in
turize the inside skin.                                     connections with molding whenever the quantities
                                                                                              The Casting Process 91

manufactured justify the additional expense of the          permanent molding. It is made of metal, again usually
longer wearing patterns. The metal mold process             cast iron or steel; has parting lines along which it can
refers not t o the pattern equipment but t o a reusable     be opened for extraction of the casting; and is con-
metal mold that is in itself a reverse pattern in which     structed with small draft angles on the walls t o reduce
the casting is made directly.                               the work of extraction and extend the life of the die.
   Special Processes Receive Limited Use. In addi-          Vents, in the form of grooves or small holes, also are
tion t o the metal mold processes, there are special        present t o permit the escape of air as metal fills the
processes involving either single-use or reusable           die.
molds. Their use is limited t o a comparatively small          Hot Chamber Die Casting. The machines in which
number of applications in which the processes, even         the dies are used, however, are quite different be-
though more costly, show distinct advantages over           cause, in addition t o closing and opening the die
the more commonly used methods.                             parts, they must supply liquid metal under pressure
                                                            t o fill the cavity. The hot chamber die-casting
PERMANENT MOLD CASTING                                      machine, a s shown in Figure 8-20, keeps metal melted
   Metal Molds Used Mostly for Low Melting Point            in a chamber through which a piston moves into a cy-
Alloys. Permanent molds may be reused many                  linder t o build up pressure forcing the metal into the
times. The life will depend, t o a large extent, upon       die.
the intricacy of the casting design and the temp-
erature of the metal that is poured into the mold.
Cast iron and steel are the most common materials
with which the mold is made. Permanent mold cast-
ing is used most for the shaping of aluminum, copper,
magnesium, and zinc alloys. Cast iron is occasionally
poured in permanent molds that have much lower
mold life because of the higher operating tempera-
ture. Satisfactory results require operation of the
process with a uniform cycle time t o maintain the
operating temperature within a small range. Initial use
of new molds often demands experimentation t o
determine thc most suitable pouring and operating
temperatures as well as to correct the position and
size of thc small vent grooves cut at the mold parting
line t o allow the escape of gases.
   I-Iigh Accuracies and Good Finishes. The cost of                               Figure 8-20
the molds, sometimes referred t o as dies, and the                          Hot chamber die casting
operating mechanism by which they are opened and
closed is high, but permanent mold casting has several         Machines Limited t o Low Pressures. Because the
advantages over sand casting for high quantity pro-         piston and the portions subjected t o pressure are
duction. Dimensional tolerances are more consistent         heated t o the melting terrperature of the casting
and can he held to approximately t 0 . 2 5 millimeter       metal, hot chamber machiiles are restricted t o lower
(0.1 inch). The higher cond~ictanceof heat through          pressures than those with lower operating tempera-
the metal mold causes a chilling action, producing          tures. Although it is a high speed, low cost process,
finer grain structure and harder, stronger castings.        the low pressures d o not produce the high density,
   T h e minimum practical section thickness for            high quality castings often desired. In addition, iron
permanent molding is about 3 millimeters (118 inch).        absorbed by aluminum in a hot chamber machine
The majority of castings are less than 30 centimeters       would be detrimental t o its properties. Pressures as
(12 inches) in diameter and 1 0 kilograms (22 pounds)       high as 1 4 MPa (2,000 psi) are used in the hot cham-
in weight. The process is used in the manufacture of        ber process t o force fill the mold.
automobile cylinder heads, automobile pistons, low            Cold Chamber Die Casting. With cold chamber
horsepower engine connecting rods, and many other           equipment, a s shown in Figure 8-21, molten metal is
nonferrous alloy castings needed in large quantity.         poured into the shot chamber, and the piston ad-
                                                            vances t o force the metal into the die. Aluminum,
DIE CASTING                                                 copper, and magnesium alloys a-e die cast by this
                                                            method with liquid pressures as high as 210 MPa
  Die casting differs from permanent mold casting in
                                                            (30,000 psi).
that pressure is applied to the liquid metal t o cause it
to flow rapidly and uniformly into the cavity of the          Casting Quality High. Sections as thin as 0.4
mold, or die. The die is similar t o that used for          millimeter (1164 inch) with tolerances as small as
92 Materials and Processes for NDT Technology

                                                DIE CAVITY




                                                                                                                   I
                                                                  WAX PATTERN             COAT WITH REFRACTORY         R E I N F O R C E WITH
                                                                                               SLURRY
                                                                                                                       PLASTER BACKING
                                                                                                                         (INVESTMENT)




                       Figure 8-21
                Cold chamber die casting
2.05 millimeter (0.002 inch) can be cast with very            O V E N DRY TO LlQUlFY OR
                                                              VAPORIZE PATTERN ALSO
good surface finish by this pressure process. The                     DRY M O L D
                                                                                             POUR ( A N Y METAL)        REMOVE INVESTMEN1
material properties are likely to be high because the                                                                      MATERlAl
pressure improves the metal density (fewer voids),                                          Figure 8-22
and fast cooling by the metal molds produces good                                  Steps for investment casting
strength properties. Other than high initial cost, the
principal limiting feature of die casting is that it can-    heated t o suitable temperatures for pouring, usually
not be used for the very high strength materials.            between 600" C and 1,100" C, depending upon the
However, low temperature alloys are continually              metal that is to fill the mold. After pouring and
being developed, and with their improvement, die             solidification, the investment is broken away to free
casting is being used more and more.                         the casting for removal of the gating system and final
                                                             cleaning.
 INVESTMENT CASTING                                             Process Limited to Small Castings. Investment
   The Working Pattern Destroyed During Investment           casting is limited to small castings, usually not over 2
Casting. Investment casting (Figure 8-22) is also            kilograms (4.4 pounds) in weight. The principal ad-
known as precision casting and as the lost wax               vantage of the process is its ability to produce intri-
process. The process has been used in dentistry for          cate castings with close dimensional tolerances. High
many years. A new wax pattern is needed for every            melting temperature materials that are difficult to
piece cast. For single-piece casting, the wax pattern        cast by other methods can be cast this way because
may be made directly by impressions as in dentistry,         the investment material of the mold can be chosen
by molding or sculpturing as in the making of                for refractory properties that can withstand these
statuary, or by any method that will shape the wax to        higher temperatures. In many cases, pressure is
the form desired in the casting. Shrinkage allowances        applied t o the molten metal to improve flow and
must be made for the wax, if it is done hot, and for         densities so that very thin sections can be poured by
the contraction of the metal that will be poured in          this method.
the cavity formed by the wax. Reentrant angles in the           High Quality at High Cost. It can easily be rea-
casting are possible because the wax will not be             lized, by examination of the procedures that must be
removed from the cavity in solid form. Variations of         followed for investment molding and casting, that the
this process involve the use of frozen mercury or low        costs of this process are high. Accuracy of the fin-
melting point thermoplastics for the pattern.                ished product, which may eliminate or reduce ma-
   Duplicate Parks Start with a Master Pattern. Mul-         chining problems, can more than compensate for the
tiple production requires starting with a master pat-        high casting cost with some materials and for some
tern about which a metal die is made. The metal die          applications.
can be used for making any number of wax patterns.              A number of important parts, some of new or exotic
A gating system must be part of the wax pattern and          materials, are presently manufactured by investment
may be produced in the metal die or attached after           casting. Many of these, such as high strength alloy tur-
removal from the die. When complete, the wax pat-            bine buckets for gas turbines, require NDT inspection
tern is dipped in a s l u ~ ~ y fine refractory material
                            of                               by radiographic and penetrant methods to insure that
and then encased in the investment material (plaster         only parts of high quality get into service.
of paris or mixtures of ceramic materials with high
refractory properties). The wax is then removed from         PLASTER MOLD CASTING
 the mold by heating to liquify the wax and cause it to         Molds made of plaster of paris with additives, such
 run out to be reclaimed. Investment molds are pre-          as talc, asbestos, silica flour, sand, and other materials
                                                                                               The Casting Process 93

to vary the mold properties, are used only for casting     time the principal product was cast iron sewer pipe,
nonferrous metals. Plaster molds will produce good         but present day uses of centrifugal castings include
quality finish and good dimensional accuracy as well        shafts for large turbines, propeller shafts for ships, and
as intricate detail. The procedure is similar to that      high pressure piping. Because of the critical nature of
used in dry sand molding. The plaster material must         some applications NDT may be necessary to check the
be given time to solidify after being coated over the      wall thickness and quality of the product material. The
pattern and is completely oven dried after removal         columnar grain structure may produce problems in ap-
before it is poured.                                       plying nondestructive tests.
   Casting Cools Slowly. The dry mold is a good                Semicentrifugal Casting - Solid Product. A simi-
insulator, which serves both as an advantage and as a      lar process, which may be termed semicentrifugal
disadvantage. The insulating property permits lower        casting, consists of revolving a symmetric mold about
pouring rates with less superheat in the liquid metal.      the axis of the mold's cavity and pouring that cavity
These contribute to less shrinkage, less gas entrap-        full. The density of a casting made in this way will
ment from turbulence, and greater opportunity for          vary, with dense, strong metal around the outside and
evolved gases to escape from the metal before solidifi-    more porous, weaker metal at the center. The varia-
cation. On the other hand, because of slow cooling,        tion in density is not great, but the fast filling of the
plaster molds should not be used for applications in       external portion of the mold cavity produces particu-
which large grain growth is a serious problem.             larly sound metal. Wheels, pulleys, gear blanks, and
                                                           other shapes of this kind may be made in this way t o
CENTRIFUGAL CASTING                                        obtain maximum metal properties near the outside
   Several procedures (Figure 8-23) are classed a s cen-   periphery.
trifugal casting. All of the procedures make use of           Centrifuge Casting - Multiple Produet. A third
a rotating mold t o develop centrifugal force acting on    type of casting using centrifugal force can be termed
the metal to improve its density toward the outside        centrifuge casting. In this process, a number of
of the mold.                                               equally spaced mold cavities are arranged in a circle
                 SAND OR OTHER REFRACTORY LINING           about a central pouring sprue. The mold may be sin-
                      CAST TUBING
                                                           gle or stacked with a number of layers arranged ver-
                                                           tically about a common sprue. The mold is revolved
                                                           with the sprue as an axis and when poured, centrifu-
                                                           gal force helps the normal hydrostatic pressure force
                                                           metal into the spinning mold cavities. Gases tend t o
                                                           be forced out of the metal, which improves metal
                                                           quality.
                 MACHINE DRIVE ROLLERS

                      CENTRIFUGAL                          CONTINUOUS CASTING
                                                              Although only a small tonnage of castings are pro-
                                                           duced by continuous casting, it is possible to produce
                                                           two-dimensional shapes in an elongated bar by draw-
                                                           ing solidified metal from a water-cooled mold.
                                                              Special Equipment and Skills Required. As shown
                                                           schematically in Figure 8-24, molten metal enters one
                                                           end of the mold, and solid metal is drawn from the
                                                           other. Control of the mold temperature and the speed
                                                           of drawing is essential for satisfactory results.
                                                           HOLDING
                                                           ctiAM,BER F O R
                                                           MOLTEN METAL
                                                           \
                                                                             /kBURyR
 I 11                                                I
    SEMICENTRIFUGAL                  CENTRIFUGE                                              SHUTOFF V A L V E


                       Figure 8-23
                                                                                                    WATER-COO L E D
                   Centrifugal casting                                                          H   MOLD
   True Centrifugal Casting-Hollow Product. The
true centrifugal casting process shapes the outside of
the product with a mold but depends upon centrifugal                                        CONTROLLED DRAW
force developed by spinning the mold to form the in-                                        OF SOLID BAR

side surface by forcing the liquid metal to assume a cy-                         Figure 8-24
lindrical shape symmetric about the mold axis. At one          Schematic diagram of continuous casting process
94 Materials and Processes for NDT Technology

   Good Quality Castings Possible. Exclusion of                formed in lift out crucibles constructed of graphite, sil-
contact with oxygen, while molten and during solidi-           icon carbide, or other refractory material. Gas or oil is
fication produces high quality metal. Gears and other          combined with an air blast around the crucible to pro-
shapes in small sizes can be cast in bar form and later        duce the melting heat. Unless a cover is placed on the
sliced into multiple parts.                                    crucible, the melt is exposed to products of combustion
   An automotive manufacturer makes use of the con-            and is susceptible to contamination that may reduce
cept as a salvage procedure for saving bar ends of alloy       the quality of the final castings. This is true of all the
 steel. The waste material is melted and drawn through         natural fuel fired furnaces.
 the mold in bar form. Subsequently, the bars are cut in-
 to billets that are suitable for processing into various      POT FURNACES
 automotive parts.                                               Quantities of non-ferrous materials to several hun-
                                                               dred pounds may be melted in pot furnaces that con-
                   MEETING EQUIPMENT                           tain a permanently placed crucible. Metal is ladled di-
                                                               rectly from the crucible, or in the larger size equip-
  The volume of metal needed a t any one time for cast-        ment, the entire furnace is tilted to pour the molten
ing varies from a few pounds for simple castings to            metal into a transporting ladle.
several tons in a batch type operation with a continu-
ous supply, usually of iron, being required by some            REVERBERATORYFURMACES
large production foundries. The quantity of available             Some of the largest foundries melt non-ferrous
metal can be varied by the size and type of melting            metals in reverberatory furnaces that play a gas-air or
equipment as well as the number of units in operation.         oil-air flame through nozzles in the side walls of a brick
The required melting temperature which varies from             structure, directly on the surface of the charged mate-
about 200°C (390°F)for lead and bismuth to as high as          rial. Gas absorption from products of combustion is
1540°C (2400°F) for some steels also influences the            high but the large capacity available and high melting
type of melting equipment that will serve best.                rate provide economics that help compensate for this
                                                               fault. Smaller tilting type reverberating furnaces are
CUPOLA                                                         also available for fast melting of smaller quantities of
  A considerable amount of cast iron is melted in a spe-       metal.
cial chimney-like furnace called a cupola. I t is similar
to a blast furnace (described in Chapter 5) used for re-       ELECTRIC ARC FURNACES
fining iron ore. The cupola (Figure 8-25) is charged             The electric arc provides a high intensity heat source
through a door above the melting zone with layers of           that can be used to melt any metal that is commonIy
coke, iron, and limestone and may be operated continu-         cast. Since there are no products of combustion and
ously by taking off melted iron as it accumulates in the       oxygen can be largely excluded from contact with the
well a t the bottom.                                           melt, quality of the resulting cast metal is usually
                                                               high.
CRUCIBLE FURNACES                                                The arc may be direct (between an electrode and the
  Melting of small quantities (1to 100 pounds) of non-         charged metal) or indirect (between two electrodes
ferrous materials for small volume work is often per-          above the charge).
            REFRACTORY
              LINING
                                                               INDUCTION FURNACES
                                                                  Induction furnaces melt materials with the heat dis-
             STEEL                     CHARGING
              SHELL                      DOOR                  sipated from eddy currents. Coils built into the furnace
                                                               walls set up a high frequency alternating magnetic
                                                               field which in turn causes internal eddy currents that
                                                               heat the charge to its melting point. Rapid heating and
                                                  limestone)
                                                               high quality resulting from the absence of combustion
                                                               products help offset the high cost of the equipment and
                                                               power consumed.
             AIR

                                                                          FOUNDRY MECHANIZATION
           S LAG
           HOLE                                                  The preceding pages briefly describe the most com-
                                                               mon foundry techniques for producing castings. Most
                                                               are performed largely by manual effort, resulting in
                                                               relatively slow production. However, a t any time the
                                 \                             production quantities justify the needed expenditure
                              BOTTOM DOORS
                                                               for equipment, these same techniques are subject to al-
                         Figure 8-25                           most complete mechanization resulting in higher pro-
                           Cupola                              duction rates and improved consistency.

				
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