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.