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  1. 1. Solidification Processes - CastingFigure 01: Classification of solidification processes1.0 IntroductionSolidification processes are manufacturing processes which create a part by solidification of thematerial in a liquid or highly plastic stage. Moulding and casting are the main types of solidificationprocesses. The solidification processes can be categorised by the materials that are processed –metals, ceramics (especially glasses. Other ceramics are rarely made using these processes), andpolymers and polymer matrix composites (PMCs).Casting is a process where molten metal flows by gravity or some other force into a mould where itsolidifies in the shape of the mould cavity. The term casting is also applied to parts made by thisprocess. It is one of the oldest shaping processes available (dating back 6000 years). The principle ofcasting is simple – melt the metal, pour it into the mould, and allow it to cool and solidify.Nevertheless, there are many other factors to be considered during the casting process.Casting includes both the casting of ingots and the casting of shapes. Shape casting involves theproduction of complex geometries close to the required final geometry of the part.A variety of shape casting methods are available, making it one of the most versatile of allmanufacturing processes. Its advantages include: • It can be used to create complex part geometries, including both external and internal shapes. • Some casting processes can produce parts to net shape. No further manufacturing operations are required to achieve the required geometry and the dimensions of the parts. Other casting processes produce near net shape, where some additional processing (usually machining) is required to achieve dimensions and details. 1
  2. 2. • Casting can be used to produce very large parts. Castings weighing more than 100 tons have been made. • They can be performed on any metal that can be heated to liquid state. • Some casting methods can be used for mass production.There are disadvantages associated with casting, which are different for different casting methods.These disadvantages include limitations on mechanical properties, porosity, poor dimensionalaccuracy and surface finish, safety hazards to humans when processing hot molten metals andenvironmental problems.Parts made by casting may vary from small components weighing only a few ounces to huge partsweighing tons. The list of parts that are cast includes dental crowns, jewellery, statues, wood burningstoves, engine blocks and heads, machine frames, railway wheels, frying pans, pipes, and pumphousings. A wide variety of metals, ferrous and nonferrous, can be cast.Plastics and ceramics can also be cast, but the methods and details of the processes differ.2.0 Overview of casting technologyCasting is usually carried out in the foundry. The foundry is a factory equipped for making moulds,melting and handling metal in molten form, performing the casting process, and cleaning the finishedcasting. The workers who perform casting are called foundrymen.2.1 Casting processThe mould (used for casting) contains a cavity whose geometry determines the shape of the cast part.The actual size and shape of the cavity must be slightly oversized to allow for shrinkage that occurson the metal during solidification and cooling. Different metals undergo different amounts ofshrinkage. Therefore, if dimensional accuracy is required, the mould must be made for the particularmetal used. moulds may be made of a variety of materials – sane, plaster, ceramic, and metal. Castingprocesses are often classified according to the type of mould.For casting, the metal is first heated to a temperature high enough to completely transform the metalto liquid state. It is then poured or otherwise directed into the cavity of the mould. In an open mould,the liquid metal is simply poured till it fills the cavity. In a closed mould, a passageway, called thegating system, is provided t permit the molten metal to flow from the outside the metal into the cavity.The closed is by far an important category in casting operations.As the metal is poured into the mould, it begins to cool. When the temperature drops sufficiently, (tothe freezing point of the pure metal), solidification begins. Solidification is a change of phase, andtime is required to complete it. During this process, the metal assumes the shape of the mould cavityand many of the properties and characteristics of casting are established.Once the casting has cooled sufficiently, it is removed from the mould. Depending on the castingmethod used, further processing many be required. These processing may include trimming metalfrom the cast part, cleaning the surface, inspecting the product, and heat treatment to enhanceproperties. In addition, machining may be required to achieve close tolerances on certain part featuresand to remove the cast surface. 2
  3. 3. Casting processes are divided into two main categories depending on the type of mould used –expendable mould casting and permanent mould casting. An expendable mould in which the metalsolidifies should be destroyed to remove the casting. These moulds are made of sane, plaster, orsimilar materials, whose form is maintained by binders of various kinds. Sand casting is the mostprominent example of expendable mould processes. In sand casting, liquid metal is poured into amould made of sand. After the metal hardens, the mould must be sacrificed to recover the casting.A permanent mould can be used repeatedly to produce many castings. It is usually made of metalthough ceramic is sometimes used (which can withstand the high temperatures of casting. Inpermanent mould casting, the mould consists of two or more sections that can be opened to permit theremoval of the casting. Die casting the most commonly used of this group.More intricate shapes are usually possible with expendable moulds. Part shapes in permanent mouldprocess are limited by the need to open the mould. However, permanent mould operations haveeconomic advantages over high production operations.2.2 Sand casting mouldsThis is usually the most important casting process. Many of the features of a sand casting mould arecommon to other mould types. The mould consists of two halves – the cop and the drag. The cope isthe upper half and the drag is the lower half. The two parts are contained in a box, called a flask,which is also divided into two halves, one each for the cope and the drag. The two halves of themould separate at the parting line.In sand casting the mould cavity is formed by the means of a pattern, which is made of wood, metal,plastic, or other materials and has the shape of the part to be cast. The cavity is formed by packingsand around the pattern, about half of each in the cope and drag, so that when the pattern is removed,the remaining void is the desired shape of the cast part. The pattern is usually oversized to allow forshrinkage. The sand used is usually moist and contains a binder to help it maintain its shape.The cavity in the mould provides the external surface f the cast part. Casting may also have internalsurfaces. These are determined by the means of a core, a form places inside the mould cavity to definethe interior geometry of the part. In sand casting, although other materials can be used, though othermaterials, such as metals, plastics and ceramics.The gating system is the channel or the network of channels by which molten metal flows into thecavity from inside the mould. The gating system typically consists of a downsprue (sometimes calledsprue) through which the metal enters a runner that leads into the main cavity. At the top of thedownsprue, the pouring cup is often used to minimise splash and turbulence as the metal flows intothe downsprue. The pouring cup may be a simple cone shaped funnel, but some are designed in theshape of a bowl which has an open channel leading to the downsprue. 3
  4. 4. Figure 02: Two forms the moulds: (a) Open moulds, simply a container in the shape of the desiredpart, and (b) closed mould, in which mould geometry is more complex and required a gating system(passway leading into the cavity.In addition to this, any casting with significant shrinkage required a riser connected to the maincavity. The riser is a reservoir in the mould that serves as a source of liquid metal for casting tocompensate for shrinkage during solidification. The riser must be designed to freeze after the maincasting in order to satisfy its function.As the metal flows into the mould, the air that previously occupied the cavity as well as hot gasesformed by reactions of the molten metal must be evacuated so that the metal can fill the cavity In sandcasting, the natural porosity of the sand mould permits the air and gases to escape through the walls ofthe cavity. In permanent metal moulds, small vent holes are drilled into mould or machined into theparting line to permit the removal of air gases.3.0 Heating and pouringTo perform casting, the metal must be heated to a temperature somewhat above its meltingtemperature and then poured into the mould cavity to solidify.3.1 HeatingVarious furnaces may be used to heat the metal to a molten temperature sufficient for casting. Theheat energy required is the sum of the heat required to • Raise the temperature of the meta to melting point • Heat of fusion to convert it from solid to liquid • Heat to raise the molten metal to the desired temperature for pouring.This can be expressed as: H = ρV{Cs(Tm – To) + Hf + Cl(Tp – Tm)}Where H is the total energy required to heat the metal to casting temperature (J), ρ is the density(g/cm3), Cs is the the weight specific heat for the solid metal (J/(gK)), Tm is the melting point (K), Tostarting temperature (usually ambient) (K), Hf the heat of fusion (J/g), Cl the weight specific heat 4
  5. 5. capacity of the metal (J/(gK)), Tp the pouring temperature (K), and V the volume of the metal beingheated.The computational value of the above equation is limited. The use of the equation is complicated dueto the following factors: • Specific heat and thermal properties of a solid material may vary with the temperature, especially if it undergoes a change of phase during heating. • A metal’s specific heat may be different in the solid and liquid states. • Most casting metals are alloys, and alloys melt over a range of temperatures between the solidus and the liquidus rather than at a single point. Therefore, the heat of fusion cannot be applied so simply. • The property values required in the equation for a particular alloy may not be available in most cases. • There are significant heat losses to the environment during heating.3.2 Pouring of the metalAfter heating, the metal can be poured. Introduction of the molten metal into mould, including itsflow through the gating system and into the cavity is a critical step in the casting process. For this tobe successful, the metal must flow into all regions of the mould before solidifying. Factors that affectthis operation include pouring temperature, pouring rate, and turbulence.The pouring temperature in the temperature of the molten metal as it is introduced into the mould. Thedifference between the pouring temperature and the melting temperature of the metal is important(liquidus for an alloy). This temperature difference is sometimes referred to as superheat. This alsorefers to the amount of heat that must be removed between pouring and when solidificationcommences.The pouring rate refers to the volumetric rate at which the molten metal is poured into the mould. Ifthe rate is too slow, the metal will chill and freeze before filling the cavity. If the pouring ate isexcessive, turbulence can be a serious problem. Turbulence is characterised by erratic variations in themagnitude and direction of the velocity through the fluid. The flow is agitated and irregular ratherthan smooth and streamlined, as in laminar flow. The turbulent flow should be avoided for severalreasons. It tends to accelerate the forming of metal oxides that can be entrapped during solidification,degrading the quality of the casting. This also aggravates mould erosion, the gradual wearing away ofthe mould surfaces due to the flowing of the molten metal. The densities of most molten metals aremuch higher that water. Consequently wear caused by the flow of this metal in the mould issignificant, especially under turbulent conditions. Erosion is especially serious when it occurs in themain cavity because the geometry of the cast part is affected.3.3 Engineering analysis of pouringThere are several relationships that govern the flow of liquid metal through the gating system into themould. An important relationship is the Bernoulli’s theorem, which states that the sum of energies(head, pressure, kinetic and friction) at any two points in a flowing liquid are equal. This can bewritten as: 5
  6. 6. h1 + (p1/ρ1) + v12/(2g) + F1 = h2 + (p2/ρ2) + v22/(2g) + F2where h = head (cm), p = pressure on the liquid (N/cm2), ρ = density (g/cm3), v=flow velocity(cm/s),gravitational acceleration constant (cm/g2), F = Head loss due to friction (cm). Subscripts indicate(two) locations in the liquid flow.The equation can be simplified in several ways. If friction losses are ignored (though it will obviouslyaffect the flow in a sand mould) and assume that the system remains at atmospheric pressurethroughout, the equation can be reduced to: h1 + v12/(2g) = h2 + v22/(2g)This can be used to determine the velocity of the molten metal at the base of the sprue. If point 1 is atthe top of the sprue and point 2 at the base, If point 2 is used as reference, head at that point = 0, andh1 us the height of the sprue. When metal is poured into the pouring cup and overflows down thesprue, the initial velocity at the top of the sprue is zero (v1 = 0). Hence the equation further simplifiesto h1 = v22/2gwhich gives v = √(2gh)where v is the velocity of the metal at the base of the sprue and h is the height of the sprue.Another important relation in pouring is the continuity law, which states that the volume of flowremains constant throughout the liquid. The volume flow rate is equal to the velocity multipled by thecross-sectional area of the flowing liquid. This can be expressed as Q = v1A1 = v2A2Where Q = volumetric flow rate (cm3/s), A = cross sectional area of liquid (cm2)Therefore, anincrease in area results in a decrease in velocity and vice versa.The sprue could be tapered. As the metal accelerates during its descent into the sprue opening, thecross sectional area of the channel must be reduced. Otherwise, as the velocity of the flowing metalincreases towards the base of the sprue, air can be aspirated into the liquid and conducted into themould cavity. To prevent this, the sprue is designed with a taper, so that the volume flow rate vA isthe same at the top and bottom of the sprue.Assuming the runner from the sprue base to the mould cavity is horizontal ( and therefore head h isthe same as the sprue base), then the volume rate of flow through he gate and into the mould cavityremains to vA at the base. Accordingly, the time required to fill a mould cavity of volume V can beestimated as TMF = V/QWhere TMF = the mould filling time (s), V = volume of the mould cavity (cm3), Q = is the volume flowrate. The mould filling time is computed by the above equation is the minimum. This is becausefrictional losses and possible constriction of the flow in the gating system.3.4 Fluidity 6
  7. 7. The molten metal flow characteristics are described by the term fluidity, which is a measure of thecapability of a metal to flow into and fill the mould before freezing. Fluidity is the inverse ofviscosity. Standard testing methods are available to asses fluidity. One is the spiral mould test shownbelow. The fluidity is indicated by the length of the solidified metal in the spiral. The longer castspiral means greater fluidity of the molten metal.Figure 03: Spiral mould test for fluidity, where fluidity is measured as the length of the spiral channelthat is filled by the molten metal prior to solidification.Factors affecting fluidity include pouring temperature relative to melting point, metal composition,viscosity of the liquid metal, and heat transfer to surrounding. A higher pouring temperature relativeto the freezing point of the metal increases the time it remains in liquid state, allowing it to flowfurther before freezing. This tends to aggravate certain casting problems such as oxide formation,porosity, and penetration of liquid metal into interstitial spaces between the grains of sand forming themould. The last problem causes the surface of the casting to contain embedded sand particles, makingit rougher and abrasive than normal.Composition also affects fluidity, particularly with respect to the metal’s solidification mechanism.The best fluidity is obtained by metals that freeze at a constant temperature (pure metals and eutecticalloys). When solidification occurs over a temperature range (most alloys), the partially solidifiedportion interferes with the flow of the liquid portion, reducing fluidity. In addition to the freezingmechanism, this also determines the heat of fusion – the amount of heat required to solidify the metalon the liquid state. A higher heat of fusion tends to increase the measured fluidity in casting.4.0 Solidification and coolingAfter pouring into the mould, the molten metal cools and solidifies. Issues associated withsolidification include the time for the metal to freeze, shrinkage, directional solidification, and riserdesign.4.1 Solidification of metalsThe solidification process depends on whether the metal is a pure element or an alloy.4.1.1 Pure metalsA pure metal solidifies at a constant temperature equal to its freezing point, which is the same as itsmelting point. The melting point of pure metals are well known an documented. The process occursover time as shown in the cooling curve below. The actual freezing takes time, known as localsolidification time in casting, during which the metal’s latent heat of fusion is released into thesurrounding mould. The total solidification time is the time taken between pouring and complete 7
  8. 8. solidification. After casting has completely solidified, cooling continues at a rate indicated by thedownward slope of the cooling curve.Because of the chilling action of the mould wall, a thin skin of solid metal is initially formed at theinterface immediately after pouring. The thickness of the skin increases to form a shell around themolten metal as solidification progresses inwards towards the centre of the cavity. The rate at whichfreezing proceeds depends on the thermal properties of the metal.The metal that forms the initial skin is cooled rapidly by the extraction of heat through the mouldwall. This cooling action causes the grains of the skin to be fine, equiaxed, and randomly oriented. Ascooling continues, further grain formation and growth occur in the direction away from heat transfer.Since heat transfer Since the heat transfer is through the skin and the wall, grains grow inwards asneedles or spines of solid metal. As these spines enlarge, lateral branches form at right angles to thefirst branches. This type of grain growth is referred to as dendritic growth, and it occurs not only inthe freezing of pure meals but alloys as well. These treelike structures are gradually filled in duringfreezing as additional metal is continually deposited on the dendrites until complete solidification hasoccurred. The grains resulting form dendritic growth take on a preferred orientation, tending to becoarse, columnar grains aligned towards the centre of the casting. Figure 04:Cooling curve for a pure metal during casting Figure 05:Characteristic grain structure in a casting of a pure metal, showing randomly oriented grains of smallsize near the mould wall, and large columnar grains oriented toward the centre of the casting.4.1.2 Most alloys 8
  9. 9. Most alloys freeze over a temperature range rather than at a single temperature. The exact rangedepends on the alloy system and the particular composition. Solidification of an alloy can beexplained with reference to figure 06 which shows the phase diagram for an alloy system and thecooling curve for a given composition. As the temperature drops, freezing begins at the liquidustemperature and completes when the solidus temperature is reached. The start of freezing is similar toa pure metal. A thin skin is formed on the surface due to the large temperature gradient of the surface.The freezing the continues as before by the growth of dendrites from the walls. However, owing to thetemperature spread between the liquidus and solidus, the nature of dendritic growth is such that anadvancing zone is formed where both liquid and solid states coexist. The solid portions are dendritestructures that have formed sufficiently to trap liquid metal in the matrix. This region has a softconsistency that has motivated its name as the mushy zone. Depending on the conditions of freezing,the mushy zone can be relatively narrow, r it can exist throughout most of the casting. The lattercondition is promoted by factors such as slow heat transfer out of the metal and a wide differencebetween liquidus and solidus temperatures. Gradually, the liquid islands in the dendrite matrixsolidify as the temperature of the casting drops to the solidus for given alloy compositions.Another factor that complicates solidification is the composition at which the dendrites start to formfavours the metal with the higher melting point. As freezing continues and dendrites grow, theredevelops an imbalance in composition between the metal that has solidified and the remaining moltenmetal. This composition imbalance is finally manifested in the completed casting in the form ofsegregation of elements. Segregation can be microscopic and macroscopic. At microscopic level, thechemical composition varies throughout each individual grain. This is due to the fact that thebeginning spine of each dendrite has a higher proportion of one element in the alloy. As the dendritegrown in its local vicinity, it must expand using the remaining liquid metal that has been partiallydepleted of the first component. Finally, the last metal to freeze in each grain is which has beentrapped by the branches of the dendrites, and its composition is even further out of balance. Therefore,there are variations of composition within a single grain of casting.Figure 06: (a) Phase diagram for copper-nickel alloy system, (b) associated cooling curve for a 50%Ni-50%Cu composition during casting.The composition also varies at macroscopic level throughout the entire casting. The regions of thecasting that freeze first are richer in one component than the other, the remaining molten metal isdeprived of one component when freezing occurs in the interior. Therefore, there is a generalsegregation throughout the cross-section of the casting, sometime called ingot segregation. 9
  10. 10. Figure 07: Characteristic grain structure in an alloy casting, showing segregation of alloyingcomponents in the centre of the casting.4.1.3 Eutectic alloysThese constitute an exception to the general process by which alloys solidify. Eutectic alloys have thesame liquidus and solidus temperatures, therefore solidification occurs at a constant temperature. Theeffect can be seen in the lead tin phase diagram below. The composition of 61.9% tin and 38.1% leadhas a melting point of 183oC (which is lower than the melting point of both pure metals. This isknown as the eutectic composition of the system and the melting point the eutectic temperature. Leadtin alloys are not commonly used in casting, but lead tin combinations near eutectic are used forsoldering, where low melting point is an advantage. Eutectic alloys used in casting include aluminiumsilicon - 911.6% Si and cast iron (4.3%C).4.2 Solidification timeRegardless of whether a casting is a pure metal or alloy, solidification takes time. The totalsolidification time is the time required for the casting to solidify after pouring. The time is dependenton the size and shape of the casting by an empirical relationship known as Chvorinov’s rule, whichstates TTS = Cm (V/A)nWhere TTS is the total solidification time (min), V = volume of casting (cm2), n is usually take to havethe value n, Cm is the mould constant. When n = 2, units of C are min/cm2 and its value depends onthe particular conditions of the casting operation, including mould material (specific heat, specificconductivity), thermal properties of the cast metal ( heat of fusion, specific heat, thermalconductivity), and pouring temperature relative to the melting point of the metal. The value of Cm fora particular casting can be based on experimental data from previous operations carried out using thesame mould material, metal, and pouring temperature, even though the shape of the part may bedifferent.This equation predicts that a casting with a higher volume to surface ratio will cool and solidify moreslowly than one with a lower ratio. This is put to good use when designing the riser in the mould. Tofeed molten metal into the casting, the riser must remain liquid longer than the rest of the casting (TTSfor the casting for the riser must be larger for the riser). As the mould conditions are similar for both,Cm will be same. By designing the riser to have a larger volume to area ratio, the main casting can bedesigned to solidify first and the effect of shrinkage are minimised.4.3 Shrinkage 10
  11. 11. Shrinkage occurs during cooling an freezing. It occurs in three steps – liquid contraction duringcooling (before solidification), Contraction during phase change (solidification shrinkage, and thermalcontraction of solid cast.These can be explained with reference to a cylindrical casting in an open mould. The cooling of theliquid causes the height of liquid in the mould to decrease. The amount of liquid contraction is about0.5%.Solidification shrinkage has two effects. It censuses a further reduction of the height of thecasting, and the amount of liquid metal available to feed the top centre portion on the casting becomesrestricted. This is usually the last region to freeze, and the absence of metal creates a void in thecasting in this location. This is called a pipe by foundrymen. Once solidified, the casting experiencesfurther contraction in height and diameter while cooling. This is determined by the metal’s coefficientof thermal expansion, which is applied to reverse the determine contraction.The table below gives the volumetric contraction for casting metals due to solidification shrinkageand solid contraction. Solidification contraction occurs in nearly all metals as the solid phase has ahigher density than the liquid phase. The phase transformation that accompanies solidification causesa reduction in the volume per unit weight of metal. The exception is cast iron containing a highcarbon content, whose solidification is complicated by a period of graphitization during the finalstages of freezing, which tends to counteract the volumetric decrease associated with solidification.Figure 08: Shrinkage of a cylindrical casting during solidification and cooling: (0) Starting level ofmolten metal immediately after pouring, (1) Reduction in level caused by liquid contraction duringcooling, (2) reduction in level caused by liquid contraction during cooling, (3) further reduction inheight and diameter due to thermal contraction during cooling of the solid metal. For clarity,dimensional reductions are exaggerated. 11
  12. 12. Table 01: Volumetric contraction of different casting metals due to solidification shrinkage and solidcontractionMetal Volumetric contraction due to, % Solidification shrinkage Solid thermal conductionAluminium 7.0 5.6Al alloy (typical) 7.0 5.0Gray cast iron 1.8 3.0Gray cast iron, high C 0 3.0Low C cast steel 3.0 7.2Copper 4.5 7.5Bronze (Cu-Sn) 5.5 6.0Pattern makers account for shrinkage by making oversized mould cavities. The amount the mouldshould be made larger relative to the final casting is called pattern shrinkage allowance. Althoughshrinkage is volumetric, the dimensions of the casting are almost always expressed linearly, so theallowances must be applied accordingly. Special shrink rules with slightly elongated scales are usedto make moulds larger than the desired casting by the appropriate amount. Depending on the metal tobe cast, the shrink rules are between 1% and 5% longer.4.4 Directional solidificationTo minimise the effects of shrinkage, it is desirable for the regions of the casting most distant fromthe liquid metal supply to freeze first and for solidification to progress from these remote regions tothe risers, to ensure that liquid metal are available from the risers to prevent shrinkage voids duringfreezing. The term directional solidification is used to describe this aspect of casting and the methodby which it is controlled. The desired directional solidification is achieved by observing Chvorinov’srule in designing the casting, its orientation and the riser system that feeds it. For example, areas withlow V/A can be located away from the riser, so that these regions freeze first and the supply of liquidmetal for the rest of the casting will remain open until the bulkier sections solidify.Another possible method is the usage of chills – internal or external heat sinks that cause rapidfreezing in certain regions of the casting. Internal chills are small metal parts placed inside cavitybefore pouring so that molten metal will solidify first around these parts. The internal chill shouldhave a chemical composition similar to the metal being poured, most readily achieved by making thechills out of the same material as the casting.External chills are metal inserts in the walls of the mould cavity that can remove heat from the moltenmetal more rapidly than the surrounding sand in order to promote solidification. They are often usedeffectively in sections of the casting difficult to feed with liquid metal, thus encouraging rapidfreezing in this sections while connection to the liquid metal is still open. 12
  13. 13. Figure 09: (a) external chill to encourage rapid freezing of the molten metal in a thin section of thecasting, (b) likely result if the external chill were not used. It is also important to avoid premature solidification in regions near the riser. Of particular concern isthe passway between the riser and the main cavity. This connection is designed in such a way that itdoes not freeze before casting, which does not isolate the casting from the molten metal in the riser.Though it is generally desirable minimise the volume in connection (to reduce waste), the cross-sectional area must be sufficient to delay the onset of freezing. This is usually aided by making thepassway short in length, so that it absorbs heat from the molten metal in the riser and the casting.4.5 Riser designRisers can also be designed in several forms. A side riser is attached to the side of the casting by themeans of a small channel. A top riser is connected to the top surface of the casting. Risers can be openor blind. An open riser is exposed to the outside at the top surface of the cope. This has thedisadvantage of allowing more heat to escape, promoting faster solidification. A blind riser is entirelyenclosed in the mould.5.0 Sand castingMetal casting processes can be divided into two categories – expendable mould and permanent mould.In expendable mould casting, the mould must be sacrificed to remove the cast part. As a new mouldeach required for each casting, production rates depend on the time required to make the mould ratherthan time required for the casting itself. However, for some parts, moulds can be produced and castingmade at a rate of 400 parts per hour or higher.Sand casting is an expendable mould casting process. It is also the most widely used casting process,accounting for a significant majority of all parts cast. Nearly all alloys can be sand cast, and is one ofthe few processes that can be used for metals with high melting temperatures such as steel, nickel andtitanium. Its versatility permits casting of parts ranging in size from small to large in productionquantities from one to millions.Sand casting consists of pouring the molten metal into a sad mould, allowing the metal to solidify,and then breaking the mould to remove the casting. The casting must then be cleaned and inspected,and heat treatment is required to improve metallurgical properties. The cavity in sand casting isproduced by packing sand around a pattern (an approximate duplicate of the part to be cast) andremoving the pattern by separating the mould to two halves. The mould also contains a gating andriser system. In addition, if the casting has internal surfaces (hollow parts or parts with holes, a coremust be included in the loud. As the mould is sacrificed to remove the casting new mould must be 13
  14. 14. made for each part Therefore, sand casting seems to include not only the casting operation, but alsothe fabrication of patter and making the mould.5.1 Patterns and coresSand casting requires a pattern, which is a full side model of the part, enlarged to account forshrinkage and machining allowances in the final casting. Materials used to make patterns includewood, plastics and metals. Wood is a common pattern material because it is easily worked into shape.Its disadvantages are the tendency to warp, and it is abraded by the sand being compacted about it,which limits the number of times it can be reused. Metal patterns are more expensive, but they lastlonger. Plastics are a compromise between wood and metal. Selection of the appropriate materialmostly depends on the total quality of the castings to be made.The simplest patterns are made of one piece, called a solid pattern. Although it is the easiest pattern tofabricate, it is not the easiest to use in making the sand mould. Determining the location of the partingline between the two halves can be a problem, and incorporating a gating system and sprue into themould is left to the judgement and skill of the foundry worker.Split patterns usually consist of two pieces, dividing the part along a plane coinciding with the partingline of the mould. Split patterns are appropriate for complex part geometries and moderate productionquantities. The parting line of the mould is predetermined by the two pattern halves, rather than byoperator judgement.For higher production rates, match plate or cope and drag patterns are employed. In match platepatterns, the two pieces are attached to the opposite sides of a wood or metal plate. Holes in the plateallow the cope and drag (top and bottom sections of the mould) to be aligned accurately. Cope anddrag patterns are similar expect that the split halves are attached to separate patterns, so that the copeand drag can be fabricated independently, instead of using the same tooling for both.Figure 10: Types of pattern used in sand casting (a) solid pattern, (b) split pattern, (c) match-platepattern, (d) cope and drag patternIf the casting is to have internal surfaces, a core is required. A core is a full-scale model of the interiorsurfaces of the part. It is inserted into the mould cavity prior to pouring so that the molten metal willflow and solidify between the moulding cavity prior to pouring, so that the molten metal will flow andsolidify between the mould cavity and the core to form the casting’s external and internal surfaces.The core is usually made of sand and compacted to the desired shape. As with the pattern, the actualsize of the core must allow for shrinkage and machining. Depending on the geometry of the part thecore may or may not require supports to hold it in position in the mould cavity during pouring. Thesesupports, called chaplets, are made of a metal with a higher melting temperature than the castingmetal. On pouring and solidification, the chaplets are bonded into the casting. The portion of chapletprotruding from the casting is subsequently cut off. 14
  15. 15. Figure 11: (a) Core held in place in the mould cavity by chaplets, (b) possible chaplet design, (c)casting with internal cavity.5.2 Moulds and mould makingFoundry sands are silica (Si2O) or silica mixed with other minerals.. The sand should posses goodrefractory properties – capacity to stand up under high temperatures without melting or otherwisedegrading, Important features of sand include grain size, distribution of grain size in the mixture theshape of individual grains. Small grains provide a better surface finish on the cast part, but large grainsizes are more permeable to allow the escape of gases during pouring. Most moulds made from grainsof irregular shape tend to be stronger that moulds of round grains due to interlocking but it tends torestrict permeability.When making the mould, the grains of sand are held together by a mixture of water and bonding clay.A typical mixture by volume is 90% sand, 3% water, and 7% clay. Other binding agents (other thanclay, such as organic resins (phenolic resins) and inorganic binders (e.g., sodium silicate andphosphate) may be used. In addition to the sand and the binder, additives are sometimes added toenhance properties such as strength and/or permeability of the mould.To form the mould cavity, the traditional method is to compact the moulding sand around the patternfor both cope and drag in a container called flask. The packing process is performed by variousmethods. The simplest is hand ramming, accomplished manually by a foundry worker. In addition,various machines have been developed to mechanise the packing procedure. These machines operateby several mechanisms, including squeezing sand around the pattern by pneumatic pressure, a joltingaction in which sand which is contained in a flask with the patter is dropped repeatedly in order topack it into place, or a slinging action where the sand grains are impacted against the pattern at highspeed.An alternative to traditional flasks for each sand mould is flaskless moulding, which refers to the useof one master flask in a mechanised system of mould production. Each sand mould is produced usingthe main master flask. Mould production rates up to 600 per hour are claimed for this automatedmethod.Several indicators are used to determine the quality of the sand mould: • Strength: the mould’s ability to maintain its shape and resist erosion caused by the flow of molten metal. It depends on grain shape, adhesive qualities of the binder, and other factors • Permeability: Capacity of the mould to allow hot air and gases from the casting operation to pass through the voids in the sand. 15
  16. 16. • Thermal stability: ability of the sand at the surface of the mould cavity to resist cracking and buckling upon contact with molten metal • Collapsibility: Ability of the mould to give away and allow the casting to shrink without cracking the casting. It also refers to the ability to remove sand from the casting during cleaning • Reusability: whether the sand from the broken casting be reused for other moulds.These parameters may be incompatible – for example, a strong mould may be less collapsible.Sand moulds can be classified as green-sand, dry-sand, or skin-dried moulds. Green sand moulds aremade of a mixture of sand, clay and water, and the word green refers to the fact that the mouldcontains moisture at the time of pouring. Green sand moulds generally have sufficient strength formost applications, good collapsibility, good permeability, good reusability, and are the least expensiveof all moulds. They are the most widely used mould type, but then have their problems. The moisturein the mould may cause defects in some castings, depending on the metal and the geometry of thepart. A dry sand mould is made using organic binders rather than clay, and the mould is baked in alarge oven at temperatures ranging from 200 to 320oC. Oven baking strengthens the mould andhardens the cavity surface. A dry-sand mould provides better dimensional control in the productcompared to green moulding. However, dry sand is more expensive and the production rate is reducesbecause of the drying time. Applications are generally limited to medium and large casting in low tomedium production rates. In a skin dried mould, the advantaged of a dry sand mould partiallyachieved by drying the surface of a green sand mould to a depth of 10 to 25 mm at the mould cavitysurface, using torches, heating lamps, or other means. Special bonding materials must be added to thesand mixture to strengthen the cavity surface.The preceding mould classifications refer to the use of conventional binders consisting of either clay-and-water or those that require heating to cure. In addition to these, chemically bonded moulds havebeen developed that are not based on either of these traditional binding ingredients. Some of thebinders used in these no bake systems include furan resins (consisting of furfural alcohol, urea, andformaldehyde), phenolics, and alkyd oils. No-bake moulds are growing in popularity due to their gooddimensional control in high production applications.5.3 The casting operationAfter the core is positioned (if one is used) and the two halves are clamped together, and then castingis performed. Casting consists of pouring, solidification, and cooling of the cast part. The gating andthe riser system must be designed to deliver liquid metal into the cavity and provide for sufficientreservoir of molten metal during solidification shrinkage. Air and gases must be allowed to escape.One of the hazards during pouring is that the buoyancy of the molten metal can displace the coreaccording to Archimedes’ principle. The force tending to lift the core is equal to the weight of thedisplaces liquid less than the weight of the core: Fb = Wm – WcWhere Fb is the buoyancy force, Wm is the weight of molten metal displaced, and Wc the weight of thecore. The weights are determines as volume of the core multiplied by the density of the core materialand the metal being cast. The density of the sand core is around 1.6 g/cm3. 16
  17. 17. Table 02: Density of selected casting alloysMaterial Density (g/cm3) Material Density (g/cm3)Aluminium (99% pure) 2.70 Cast iron, gray 7.16Aluminium silicon alloy 2.65 Copper (99% pure) 8.73Aluminium copper (92% Al) 2.81 Lead pure 11.30Brass 8.62 Steel 7.82Following solidification and cooling, the sand mould is broken away from the casting to remove thepart. Then it is cleaned, gating and riser systems separated, and sand is removed. The casting is theninspected.6.0 Other expendable mould casting processesThough sand casting is versatile, other casting processes have been developed for special needs. Thedifference between these methods are in composition of the mould material, or the manner in whichthe mould is made, or the way the pattern is made.6.1 Shell mouldingThis is casting process where the mould is thin shell (~9 mm) made of sand held together bythermosetting resin binder. It was developed in the early 1940s in Germany.There are many advantages of the shell moulding process. The surface of the shell mould cavity issmoother than a conventional green sand mould and this permits easier flow of molten metal duringpouring and better surface finish on the final casting. Finished of 2.5 μm can be achieved, as well asgood dimensional accuracy (with tolerances of ±0.25 mm possible on small to medium sized parts).The good finish and accuracy often preludes the need for further machining. Collapsability of themould is generally sufficient to avoid tearing and cracking of the casting.Disadvantages include a more expensive metal pattern than used for green sand moulding. This makesthe process difficult to justify for small quantities of parts. Shell moulding can be mechanised formass production and is very economical for very large quantities. It is particularly suited to steelcastings led that 20 lb. Examples of parts made using shell moulding include gears, valve bodies,bushings, and camshafts. 17
  18. 18. Figure 12: Steps in shell moulding. (1) A match plate or cope-and-drag pattern is heated and placedover a box containing sand mixed with thermosetting resin, (2) box is inverted so that the sand andresin fall onto the hot pattern, causing a layer of mixture to partially cure on the surface to form a hardshell, (3) the box is repositioned so that the loose, uncured particles drop away, (4) Sand shell isheated in oven for several minutes to complete the curing, (5) Shell mould is stripped from thepattern, (6) Two halves of the shell mould are assembled, supported by sand or metal shot in a box,and pouring is accomplished. The finished casting with sprue is removed as shown in (7).6.2 Vacuum mouldingVacuum moulding, also called V-process, was developed in Japan around 1970. It uses a sand mouldheld together by vacuum pressure rather than by a chemical binder. Recovery of sand is one of theadvantages of vacuum moulding, as no binders are used. Also, the sand does not require extensivemechanical reconditioning normally done when binders are used to mould sand. As no water is mixedwith the sand, moisture related defects are absent. Disadvantages of the V-process are it is relativelyslow and not adaptable to mechanisation. 18
  19. 19. Figure 13: Steps in vacuum moulding: (1) A thin sheet of preheated plastic is drawn over a matchplate or cope-and-drag pattern by vacuum. The pattern has small vent holes to facilitate vacuumforming. (2) A specially designed flask is placed over the pattern plate and filled with sand, and asprue and pouring cup are formed in the sand. (3) Another thin plastic sheet is placed over the flask,and a vacuum is drawn that causes the sand grains to be held together, forming a rigid model. (4) thevacuum on the mould pattern is released to permit the pattern to be stripped from the mould. (5) Thismould is assembled with its matching half to form the cope and drag, and with vacuum maintained onboth halves, pouring can be achieved. The plastic sheets burns quickly on contacting the metal Aftersolidification, nearly all the sand be recovered for reuse.6.3 Expanded polystyrene processThe expanded polystyrene casting process uses a mould of sand packed around a polystyrene foampattern that vaporises when the molten metal is poured into the mould. This process is also known aslost-foam process, lost pattern process, evaporative foam process, and full mould process (this is thetrade name). The polystyrene pattern includes the sprue, risers, and gating system and it may alsocontain internal cores, eliminating the need for a separate core. Also, since, the foam pattern itselfbecomes cavity in the mould, considerations of draft and parting lines can be ignored. The mould doesnot have to be opened into cope and drag sections. The sequence in this casting process is illustratedin figure 13. Various methods for making a pattern are used, depending on the quantities of castingsto be produced. For one-of-a-kind castings, the foam is manually cut from large strips and assembledto form the pattern. For large production runs, and automated moulding operation can be set up tomould the patterns prior to casting. The pattern is usually coated with a refractory compound toproduce a smoother surface on the pattern and to improve high temperature resistance.Moulding sands usually include bonding agents. However, dry sand is used in certain processes in thisgroup, which aids recovery and reuse.A significant advantage is that the pattern need not be removed from the mould. This simplifies andexpedites mould making. In a conventional green sand mould, two halves are required with properparting lines, draft allowances must be made, cores must be inserted, and the gating and riser system 19
  20. 20. must be built. With expanded polystyrene, these steps are added into the pattern itself. Thedisadvantage is that a new pattern in needed for every casting. The economic justification is highlydependent on the cost of producing patterns. This has been applied to mass produce castings forautomobile engines. Automated production systems are installed to mould the polystyrene foampatterns for these applications.Figure 14: Expanded polystyrene casting process: (1) pattern of polystyrene is coated with refractorycompound, (2) foam pattern is placed in mould box, (3) Molten metal is poured into portion of thepattern that forms pouring cup and sprue. As the metal enters the mould, the polystyrene is vaporisedahead of the advancing liquid, allowing the resulting the mould cavity to be filled.6.4 Investment castingIn investment casting, a pattern made of wax is coated with a refractory material to make the mould,after which wax is melted away prior to pouring the metal. It is a precision casting process because itis capable of making castings of high accuracy and intricate detail. It is also known as lost waxcasting, as the wax pattern is lost before the casting.The lost wax casting process was developed by ancient Egyptians about 3500 years ago. Althoughwho invented this process is not recorded, historians speculate that it resulted from the closeassociation between pottery and moulding in early times. It was the potter who crafted the mouldsused for casting. The core was made of clay in the general shape of the piece and then a wax coatingwas given. The wax proved to be an easy material to form intricate designs and shapes could becreated. On the surface, several layers of clay were carefully plastered to hold the resultingcomponents together. Then the mould was baked in a kiln, so that the clay hardened and the waymelted and drained away to forma cavity. At last, molten bronze was poured into the cavity. Aftercasting, the mould was broken away to retrieve the casting.Steps in modern investment casting is shown in figure 15. As the wax pattern in lee doss after therefractory pattern is made, a separate pattern is required for every casting. Pattern production isusually accomplished by a moulding operation – pouring or injection hot wax into a master die thathas been designed with proper allowances for both wax shrinkage and subsequent metal casting. Incases where the part geometry is complicated, several wax pieces must be joined together to make thepattern. In high production operations, several patterns are attached to a sprue, also made of wax, toform a pattern tree, which is the geometry which will be cast out of metal.Coating with refractory is usually accomplished by dipping the pattern tree into a slurry of very finegrained silica or other refractory (almost powder form) mixed with plaster to bond he mould intoshape. The small grain size of the refractory material provides a smooth surface and captures theintricate details of the wax pattern. The final mould is accomplished by repeatedly dipping the tree 20
  21. 21. into the refractory slurry or by gently packing the refractory around the tree in a container. The mouldis allowed to air dry for about 8 hours to harden the binder.Figure 15: Steps in investment casting: (1) wax patterns are produced, (2) Several patterns areattached to a sprue to form a pattern tree, (3) The pattern tree s coated with a thin layer of refractorymetal, (4) the full mould is formed by covering the coated tree with sufficient refractory material tomake it rigid, (5) The mould is held in an inverted position and heated to melt the wax and permit it todrip out of the cavity, (6) the mould is preheated to a high temperature, which ensures that allcontaminants are eliminated from the mould. It also permits liquid metal to flow more easily into thedetailed cavity. The molten metal is poured and it solidifies, (7) the mould is broken away from thefinished casting. Parts are then separated from the sprue.Advantages of investment casting are: • Parts of great complexity and intricacy can be cast • Close dimensional control – tolerances of ±0.075 mm are possible • Good surface finish is possible • Additional machining is not normally required as it is a net shape process.Because many steps are involved in this casting operation, it is relatively expensive. Parts made byinvestment casting are generally small in size, tough parts with complex geometries weighing up to 75lb have been successfully cast. All types of metal, including steels, stainless steels, and other hightemperature alloys can be investment cast. Examples include complex machinery parts, blades, andother components for turbine engines, jewellery, and dental fixtures.6.5 Plaster-mould and ceramic mould castingPlaster mould casting is similar to sand casting except that the mould is mad of plaster of Paris(gypsum – CaSO4-2H2O) instead of sand. Additives such as talc and silica flour are mixed with plasterto control contraction and setting time, reduce cracking, and increase strength. To make the mould, 21
  22. 22. the plaster mixture combined with water is poured over a plastic or metal pattern in a flask andallowed to set. Wood patterns are generally unsatisfactory due to extended contact with water in theplaster. The fluid consistency permits the plaster mixture to readily flow around the pattern, capturingits details and surface finish. Thus the cast product in plaster moulding is noted for these attributes.Curing us one of the disadvantages of this method, at least for high production. The mould must be setfor about 20 minutes before the pattern is stripped. The mould is then baked for several hours toremove moisture. Even with baking, not all of the moisture is removed. The problem is that the mouldstrength is reduced when the plaster is too dehydrated but moisture content can cause defects in theproduct. A balance must be achieved between these. Another disadvantage is that the mould is notpermeable, limiting the escape of gases from the mould cavity. This can be solved in several ways: • Evacuating the mould cavity before pouring • Aerating the plaster slurry prior to mould making so that the resulting hard plaster contains finely dispersed voids. • Using a special mould composition and treatment known as Antioch process. This involves using about 50% sand mixed with plaster, heating the mould in an autoclave, (an oven that uses superheated steam under pressure), and then drying. The resulting mould has considerably greater permeability that the conventional plaster mould.Plaster moulds withstand the same high temperature as sand moulds. They are therefore limited to thecasting of lower-melting point alloys, such as aluminium, magnesium, and some copper based alloys.Applications include metal moulds for plastic and rubber moulding, pump and turbine impellers, andother parts of relatively intricate geometry. Castings range from 20 g to more than 100 kg. Partsweighing less than 10 kg are common. Advantages of plaster moulding for these applications are goodsurface finish and dimensional accuracy and the capability of thin cross-sections in casting.Ceramic mould casting is similar to plaster mould casting, except that the mould is made of refractoryceramic materials that can withstand higher temperature that plaster. Thus ceramic moulding can beused to cast steels, cast irons, and other temperature alloys. Its applications (mould and relativelyintricate parts)are similar to those of plaster mould casting except for the metals cast. Its advantages(good accuracy and finish) are also similar.7.0 Permanent mould casting processesIn this group, the mould is fabricated out of metal or some other durable metal and is used for manycastings. Permanent mould casting can considered the basic in a group of casting processes that usereusable moulds. Others in this group include die casting and centrifugal casting.7.1 The basic permanent mould processThis uses a metals mould constructed of two sections that are designed for easy, precise opening andclosing. The moulds are commonly made of steel or cast iron. The cavity, with the gating systemincluded, is machined into the two halves to produce accurate dimensions and good surface finish.Metals commonly cast in permanent moulds include Aluminium, Magnesium, copper-base alloys, andcast irons. However, cast iron requires a high pouring temperature (1250 – 1500oC) which affectsmould life. The very high pouring temperatures of steel makes permanent moulds unsuitable for it,unless it is made of refractory material. 22
  23. 23. Cores can be used in permanent moulds to form interior surfaces in the cast product. The cores can bemade of metal, but either their shape must allow for removal from the casting pr they must bemechanically collapsible to permit removal. If the withdrawal of a metal core is difficult, a sand corecan be used, in which case the process becomes semi-permanent mould casting.Figure 16: Steps in permanent mould casting: (1) mould is preheated and coated, (2) cores (if used)are inserted, and mould is closed, (3) molten metal is poured into the mould, and the mould is opened.(5) shows the finished part.Steps in the permanent mould casting process are shown in figure 16. In preparation for casting, themould is first preheated and one or more coatings are sprayed into the cavity. Preheating facilitatesmetal flow through the gating system and into the cavity. The coating aid heat dissipation andlubricate the mould surfaces for easier separation of the cast product. After pouring, as soon as themetal solidifies, the mould is opened and the casting is removed. Unlike expendable moulds,permanent moulds do not collapse, so the mould must be opened before appreciable coolingcontraction, occurs in occurs in order to prevent cracks from developing in the casting.Advantages of permanent mould casting include close dimensional control. Also, he more rapidsolidification caused by the metal mould results in a finer grain structure, so the castings are stronger.This is generally limited to metals of lower melting points. Other limitations include simple partgeometries compared to sand casting (because of the need to open the mould. Because mould cost issubstantial, the process is best suited to high-volume production and can be automated accordingly.Atypical parts include automotive pistons, pump bodies, and certain castings for aircraft and missiles.7.2 Variations of permanent mould castingSeveral casting processes are similar to the permanent mould method. These include slush casting,low pressure casting, and vacuum permanent mould casting. 23
  24. 24. 7.2.1 Slush castingThis is a permanent mould casting method in which a hollow casting is formed by inverting the mouldafter partial freezing at the surface to drain out the liquid metal in the centre. Solidification beginsmould walls because they are relatively cool, and it progresses over time towards the middle of thecasting. Thickness of the shell is controlled by the length of time allowed before draining. Slushcasting is used to make statues, lamp pedestals, and toys out of low melting point metals such as lead,zinc, and tin. In these, external appearance is important, but strength and interior geometry are minorconsiderations.7.2.2 Low pressure castingIn basic permanent mould casting and slush casting, the flow of metal into the mould is caused bygravity. In low pressure casting, the liquid metal is forces into the cavity under low pressure,approximately 0.1 MPa from beneath so that the flow is upwards. The advantage is the clean moltenmetal from the centre of the ladle is introduced to the mould, rather than those exposed to air. Gasporosity and oxidation defects are minimised and mechanical properties are improved.Figure 17: Low-pressure casting. The diagram shows how air pressure is used to force the moltenmetal in the ladle upwards into the mould cavity. Pressure is maintained until the casting hassolidified.7.2.3 Vacuum permanent mould castingThis is a variation of low-pressure casting in which vacuum is used to draw the molten metal into themetal cavity. The general configuration is similar to the low pressure casting operation. Thedifference is that reduced air pressure from the vacuum in the mould is used to draw the liquid metalinto the cavity, rather than forcing it by positive air pressure from below. There are several befits ofthe vacuum technique relative to low pressure casting – air porosity and related effects are reduced,and greater strength is given to the cast product.7.3 Die castingThis is a permanent mould casting operation where the molten metal is injected into the mould underhigh pressure. Typical pressures are 7 to 350 MPa. He pressure is maintained during solidification,after which the mould is opened and the part is removed. The moulds in this operation are called dies 24
  25. 25. (hence the name die casting). The use of high pressure is to force the metal into the die cavity in themost notable feature that distinguishes this process from others in the permanent mould category.Die casting operations are carried out in special die casting machines. Modern die casting machinesare designed to hold and accurately close the two halves of the mould, and keep them closed while theliquid metal is forces into the cavity. There are two main types of die casting machines: (1) hotchamber and (2) cold chamber, differentiated by how the molten metal is injected into the cavity.Figure 18: General configuration of a cold chamber die casting machineFigure 19: Cycle in hot chamber casting: (1) with die closed and plunger withdrawn, molten metalflows into the chamber, (2) plunger forces metal into to flow into die, maintaining pressure duringcooling and solidification, (3) Plunger is withdrawn, die is opened, and solidified part is ejected.Finished part is shown in (4).In hot chamber machines, the metal is melted in a container attached to the machine, and a piston isused to inject the liquid metal under high pressure into the die. Typical injection pressure are 7-35MPa. The casting cycle is shown in figure 19. Production rates up to 500 parts per hour is common.Hot chamber die casting imposes a special hardship in the injection system because much it issubmerged in molten metal. The process is therefore limited to low melting point metals that do notattack the plunger and other mechanical components chemically. The metals include zinc, lead, tin,and sometimes, magnesium. 25
  26. 26. In cold chamber die casting machines, the molten metal is poured into an unheated chamber from anexternal melting container, and a piston is used to inject the metal under high pressure into the diecavity. The production cycle is explained in figure 20. Injection pressures very from 14 to 140 MPa.Compared to hot chamber machines, the cycle rates are usually not that fast because of the need toladle the liquid metal into the chamber from an external source. Nevertheless, this casting process is ahigh production operation. Cold-chamber machines are typically used for casting aluminium, brass,and magnesium alloys. Low melting point alloys (zinc, tin, lead) can also be cast, but the advantagesof the hot chamber process favour its use with these metals. Figure 20:Cycle in cold chamber casting: (1) with die closed and ram withdrawn, molten metal is poured intothe chamber, (2) Ram forces metal to flow to die, maintaining pressure during cooling andsolidification, (3) ram is withdrawn, die is opened, and part is ejected.Moulds used in die casting machines are usually made of tool steel, mould steel, or maraging steel.Tungsten and molybdenum with good refractory qualities are also being used, especially in attemptsto die cast steel and cast iron. Dies can be single cavity or multiple cavity. Ejector pins are required toremove the part form the die as it open. These pins push away from the mould surface so that it can beremoved. Lubricants must also be sprayed into cavities to prevent sticking.As die casting materials have no natural porosity and molten metal flows rapidly into the die duringinjection venting holes and passways are built into the dies at the parting line to evacuate gases in thecavity. The vents are usually small, but they fill with metal during injection. This must be trimmedfrom the part. Also, flash formation is common, where the liquid metal under high pressure squeezesinto the small space between die halves at the parting line or into the clearances around cores andejector pins. This also must be trimmed from the casting along with the sprues and the gating system.Advantages of die casting include: • High production rates possible • Economical for large production quantities • Close tolerances possible, on the order of ±0.076 mm • Good surface finish 26
  27. 27. • Thin sections are possible, down to about 0.5 mm • Rapid cooling provides small grain size and good strength to the castingThe limitation in this process, other than for metals, is the shape restriction. The part geometry mustallow for the removal from the die cavity.7.4 Centrifugal castingThis refers to several casting methods in which the mould is rotated at high speed so that centrifugalforce distributes the molten metal to the outer regions of the die cavity. This includes true centrifugalcasting, semi centrifugal casting, and centrifuge casting.7.4.1 True centrifugal castingIn true centrifugal casting, molten metal is poured into a rotating mould to produce a tubular part.Examples of parts made by this process include pipes, tubes, bushings, and rings. A possible approachis shown below. Molten metal is poured into a horizontal rotating mould at one end. In someoperations, mould rotation commences after pouring has occurred rather than beforehand. The highspeed rotation produces centrifugal forces that cause the metal to take the shape of the mould cavity.Thus, the outside shape of the casting can be round, octagonal, hexagonal, etc. The inside shape isperfectly round (theoretically) due to the radially symmetric forces on the work.The orientation of the mould may be horizontal or vertical, the former being common. Consideringthe speed required to produce a horizontal centrifugal casting, the centrifugal force is given by theequation F = mv2/RWhere F is the force (N), m = mass (kg), R = inside radius of the mould (m), The force of gravity isits weight W = mg where W is the weight (N), and g = 9.81 m/s-2. The G factor (GF) is the ratio of thecentrifugal force divided by the weight: GF = mv2/Rmg = v2/RgThe velocity v can expressed as 2πRN/60 = πRN/30, where N is the rotational speed (rev/min).Substituting this, GF = [R(πN/30)2]/gRearranging for rotational speed, N = (30/π)√[(2gGF)/D]Where D is the diameter of the mould. If GF is too low, the liquid metal will not remain forcedagainst wall but instead will ‘rain’ into the cavity. Slipping occurs between the molten metal and thewall, which means the rotational speed of the metal is less than that of the mould. On an empiricalbasis, values of GF = 60 to 80 are found to be appropriate for horizontal centrifugal casting thoughthis may depend on the metal being cast. 27
  28. 28. In vertical centrifugal casting, the effect of gravity acting on the liquid metal causes the casting to bethicker at the bottom. The inside profile will take a parabolic shape. The difference in the insideradius between the top and bottom is related to the speed of rotation as: N = (30/π)√[(2gL)/(Rl2 – Rb2)]This equation can be used to determine the speed required for vertical centrifugal casting given thespecifications in the inside radii at the top and bottom. If Rl = Rb the speed will have to be infinite,which is impossible. Practically, parts made by vertical centrifugal casting are usually no more thanabout twice their diameter. This is sufficient for bushings and other parts that have large diametersrelative to length, especially is machining is used to accurately size the inside diameter.Castings made by true centrifugal casting re characterised by high density, especially in the outerregions of the part where F is the greatest. Solidification shrinkage at the exterior of the cast tube isnot a factor, because centrifugal force continually reallocates molten metal towards the mould wallduring freezing. Any impurities in casting tend to be on the inner wall and can be removed bymachining if necessary.7.4.2 Semicentrifugal castingIn this method, centrifugal force is used to produce solid castings rather than tubular parts. Therotation speed is set to GF about 15. The moulds are designed with risers in the centre to supply thefeed material. Density of the metal in the final casting is greater than the outer sections at the centre ofthe rotation. This process is usually used in components where the centre of the casting is machinedaway, eliminating the part of the casting where the quality is the lowest. Wheels and pulleys areexamples. Expendable moulds are often used in semicentrifugal casting. Figure 21: Semicentrifugal casting7.4.3 Centrifuge casting 28
  29. 29. Figure 22: (a) Centrifuge casting – centrifugal forces causes metal to flow to the mould cavities awayfrom the axis of rotation, (b) the castingIn centrifuge casting, the mould is designed with part cavities located away from the axis of rotation,so that the molten metal poured into the mould is distributed to these cavities by centrifugal force. Theprocess is used for smaller parts, and radial symmetry of the part is not a requirement as it is for theother two centrifuge casting methods.8.0 Foundry practiceIn all casting processes, the metal must be heated to the molten state and then poured or otherwiseforce into the mould. Heating and melting are accomplished in a furnace.8.1 FurnacesThe types of furnaces commonly used in foundries are cupolas, direct-fuel-field furnaces, cruciblefurnaces. Electric-arc furnaces and induction furnaces. Selection if the most appropriate furnace typedepends on factors such as casting alloy, its melting and pouring temperatures, pouring temperatures,capacity requirements, costs if investment, operation and maintenance, and environmental pollutionconsiderations.8.1.1 CupolasIt is a vertical cylindrical furnace equipped with a tapping spout near its base. Cupolas are used onlyfor melting cast irons, and although other furnaces are used, the largest tonnage of cast iron is meltedin cupolas. General construction and operating features are shown in figure 23. 29
  30. 30. Figure 23: Cupola used for melting cast iron. Furnace shown is typical for a small foundry and omitsdetails of emissions control system required in a modern cupola.The cupola consists of a large shell of steel plate lined with refractory. The charge consists of iron,coke, flux, and possible alloying elements. It is loaded through a charging door located less thathalfway up the height of the cupola. The ironis usually a mixture of pig iron and scrap iron (whichincludes risers, runner, sprues, etc. From old castings). Coke is used as fuel. Forces air is introduces,through openings near the bottom of the shell for combustion of coke. The flux is a basic compoundsuch as limestone that reacts with coke ash and other impurities to form slag. The slag covers themelt, protecting it from reaction and environment inside the cupola and reducing heat loss. As themixture is heated inside the cupola, and reduce heating loses. As the mixture is heated and melting ofiron occurs, and the furnace is periodically tapped to provide liquid to pour.8.1.2 Direct fuel-fired furnacesA direct fuel first furnace contains a small open hearth, in which metal charge is heated by fuelburners on the side of the furnace. The roof assists the heating action and by reflecting the flamedown against the charge. Typical fuel are natural gas, and the combustion products exit from thefurnace through a stack. At the bottom if the hearth is a tap hole to release the molten metal. Directfuel-fired furnaces are generally used in casting for melting nonferrous metals such as copper basealloys and aluminium.8.1.3 Crucible furnacesThese melt the metal without direct contact with a burning fuel mixture (they are sometimes calledindirect fuel furnaces). These types of crucible furnaces are used in foundries: lift-out type, stationary,and tilting. 30
  31. 31. Figure 24: Three types of crucible furnaces: (a) lift-out crucible, (b) stationary pot, (c) tilting potfurnaceThese utilise a container (crucible), made out of a suitable refractory material (e.g., clay-graphitemixture) or high-temperature steel alloy to hold the charge. In a lift-out crucible furnace, the crucibles placed in a furnace and heated sufficiently to melt the metal charge. Oil, gas, or powdered coals aretypical fuels for these furnaces. When the metal is melted, the crucible is lifted out of the furnace andused at a pouring ladle. The other two types, sometimes referred to as pot furnaces have the heatingfurnace and container as one unit. In the stationary pot furnace, the furnace is stationary and themolten metal is ladled out of the container. In the tilting pot furnace, the entire assembly can be tiltedfor pouring. Crucible furnaces are used for nonferrous metals such as bronze, brass, and alloys of zincand Aluminium. Furnace capacities are generally limited to several hundred pounds.8.1.4 Electric arc furnacesIn this furnace type, the charge is melted by the heat generated by an electric arc. Variousconfigurations are available, with two or three electrodes. Power consumption is high, but electric arcfurnaces are designed for high melting capacity (23000-45000 ton/hr) and they are used primarily forcasting steel.8.1.5 Induction furnacesThis type of furnace uses an ac current trough a coil to develop a magnetic field in the metal. Theresultant induced current causes rapid heating and melting. Features of an induction furnace areshown in figure 25. The electromagnetic force field causes a mixing action to occur in the liquidmetal. Also, as the metal does not come into direct contact with the heating elements, the environmentin which melting takes place can be closely controlled. All this result in molten metal of high qualityand purity and induction furnaces are used for nearly every casting alloy when these requirements areimportant. Melting steel, cast iron, and aluminium alloys are common applications in foundry work. Figure 25: Induction furnace8.2 Pouring, cleaning, and heat treatment 31
  32. 32. Moving molten metal from the melting furnace to the mould is sometimes done using crucibles. Moreoften the transfer is accomplished by ladles of various kinds. They receive the metal from the furnaceand allow convenient pouring into moulds. Two types of ladle are illustrated below – one for movinglarge volumes of molten metal using an overhead crane and a two man ladle for manually moving andpouring smaller amounts.Figure 26: two common types of ladles: (a) crane ladle and (b) two-man ladleOne of the problems in pouring is that oxidised metal can be introduced into the mould. These reduceproduct quality, perhaps making the casting defective, so measures must be taken to minimise theentry of these oxides into the mould during pouring. Filters are sometimes used to catch the oxidesand other impurities as the metal is poured from the spout, and fluxes are used to cover the moltenmetal to retard oxidation. In addition, ladles have been designed to pour the liquid metal from thebottom, since the top surface is where the oxides accumulate.After the casting has solidified and been removed from the mould, a number of additional steps arerequired. These are: • Trimming • Removing the core • Surface cleaning • Inspection • Repair • Heat treatment (if required)Steps 1 to 5 are referred to as cleaning. The extent to which these additional operations are requiredvaries with the casting process and metals. When required, they tend to be labour intensive and costly.Trimming involves removal of sprues, runners, riser, parting-line flash, fins, chaplets and any otherexcess material. In the case of brittle casting alloys and when the cross sections are relatively small,these can be broken off. Otherwise hammering, shearing, hack sawing, abrasive wheel cutting, orvarious torch cutting methods are used.If cores were used, they must be removed. Most cores are chemically bonded or oil-bonded sand, andthey often fall out of the casting as the binder deteriorates. In some cases, they are removed byshaking the casting, either mechanically or manually. In rare instances, cores are removed by 32
  33. 33. chemically dissolving the bonding agent used in the sand core. Solid cores must be hammered orpressed out.Surface cleaning is the most important in case of sand casting. In any of the other casting methods,especially the permanent mould processes, this step can be avoided. Surface cleaning involves theremoval of sand from the surface of the casting or otherwise enhancing the appearance of the surface.Methods used to clean the surface include tumbling, air-blasting with coarse sand grit or metal shot,wire brushing, buffing, and chemical pickling.Defects are possible in casting, and inspection is needed to detect their presence.Castings are often heat treated to enhance their properties, either for subsequent processing operationssuch machining or to bring out the desired properties for application of the part in service.9.0 Casting qualityThere are numerous for defects in a casting operation, resulting in quality defects in the product.9.1 Casting defectsSome defects are common to any casting process. These defects are described below • Misruns: which are castings that solidify before filling the mould cavity completely. Typical causes include insufficient fluidity of molten metal, too low pouring temperatures, too slow pouring or too thin cross section of the cavity. • Cold shuts: these occur when two portions of metal flow together but do not fuse properly due to premature freezing. The causes are similar to a misrun. • Cold shots: these are caused due to splattering during pouring, causing the formation of solid globules of metal that become entrapped in the casting. Pouring procedures and gating system designs that avoid splattering can prevent the defect. • Shrinkage cavity: is a depression in the surface or an internal void in the casting caused by solidification shrinkage that restricts the amount of molten metal available in the last region to freeze. It often occurs near the top of the casting, in which case it is referred to as a ‘pipe’. This can be solved by proper riser design. • Microporosity: this consists of a network of small voids distributed throughout the casting caused by localised solidification shrinkage of the final molten metal in the dendritic structure. This is usually associated with alloys, because of the protracted manner in which freezing occurs in these metals. • Hot tearing: also called hot cracking, occurs when a casting is restrained from shrinking by an unyielding mould during the final stages of solidification. This causes separation of the metal at a point of high tensile stress caused by the metal’s inability to shrink naturally. In sand casting and other expendable mould processes, it is prevented by compounding the mould to be collapsible. In permanent moulding processes, hot tearing is reduced by removing the part from the mould immediately after solidification. 33
  34. 34. Figure 27: Some common defects in castings: (a) misrun, (b) cold shut, (c) cold sot, (d) shrinkagecavity, (e) microporosity, (f) hot tearingSome defects are related to the use of sand moulds and therefore occur only in sand castings. To acertain degree, other expendable mould processes are also vulnerable. The defects include: • Sand blow: is a defect consisting of a balloon shaped gas cavity caused by the release of mould gases during pouring. It occurs at or below the surface near the top of the casting. The usual causes are lo permeability, poor venting, and high moisture content of the sand mould. • Pinholes: these are also caused by the release of gases during pouring, and consist of many small gas cavities formed at or slightly below the surface of the casting. • Sand wash: is an irregularity in the surface caused by the erosion of the sand mould during pouring, and the contour of erosion is formed in the surface of the final cast part. • Scabs: these are rough areas on the casting due to encrustations of sand and metal. It is caused by the portions of the mould surface flaking off during solidification and becoming embedded to the casting surface. • Penetration: this refers to a surface defect that occurs when the fluidity of the liquid metal is high and it penetrates into the sand mould or sand core. Upon freezing, the casting surface consists of a mixture of sand grains and metal. Harder packing of sand mould helps to alleviate this condition. • Mould shift: this is a defect caused by sideways displacement of the cope relative to the drag, which results in a step in the cast at the parting line. • Core shift: this is similar to mould shift but the core is displaced, and the displacement is usually vertical. Core shift and mould shit are caused by the buoyancy of the metal. • Mould crack: this occurs when the strength of the mould is insufficient and a crack develops. The liquid metal can then seep into the crack, forming a ‘fin’. 34
  35. 35. Figure 28:Common defects in san castings: (a) sand blow, (b) pin holes, (c) sand wash, (d) scabs, (e)penetration, (f) mould shift, (g) core shift, (h) mould crack.Foundry inspection procedures include • Visual inspection to detect obvious defects such as misruns, cold shuts, and severe surface flaws • Dimensional measurements to ensure that tolerances have been met • Metallurgical, chemical, physical, and other tests concerned with the inherent quality of the cast metal. Tests in this category include o Pressure testing to locate leaks in the casting o Radiographic methods, magnetic particle tests, the use of fluorescent penetrates, and supersonic testing to detect either surface or internal defects in the casting o Mechanical testing to determine properties such as tensile strength and hardness.If defects detected are not too serious, it is often possible to save the casting by welding, grinding, orsome other method to which the customer has agreed.10.0 Metals for castingMost common casting are made using alloys rather than pure metals. Alloys are generally easier cast,and properties of the resulting product are better. Casting alloys can be classified as ferrous ornonferrous. The ferrous category is divided into cast iron and cast steel.10.1 Ferrous casting alloys10.1.1 Cast ironThis is the most important of all casting alloys. The tonnage of cast iron castings is several times thatof all other metals combined. There are several types of cast iron – gray cast iron, nodular iron, white 35
  36. 36. cast iron, malleable iron, and alloy cast irons. Typical pouring temperatures are around 1400oCdepending on composition.10.1.2 SteelThe mechanical properties of steel make it an attractive engineering material and the capability tocreate complex geometries makes casting an appealing process. However, great difficulties are facesby the foundry specialising in steel. The melting point of steel is considerably higher than that of mostmetals cast. Solidification range for low carbon steels begins just under 1540oC. Therefore therequired pouring temperatures are very high, at about 1650oC.At these temperatures, steel is veryreactive chemically and oxidises readily. Special procedures are required to isolate the molten metalfrom air. Also, it has relatively poor fluidity, which limits the design of thin sections.Several characteristics make it worthwhile solving these problems. The tensile strength is higher thanmost casting materials, ranging upwards from about 410 MPa. They also have better toughness thanmost other casting alloys. Properties of castings are isotropic – strength is virtually the same in alldirections (mechanically formed parts by forging, rolling, etc. Display directional properties).Depending on the requirement, the isotropic behaviour might be desirable. Another advantage is theease of welding. They are welded readily without a significant loss of strength to repair the casting orto fabricate structures with other steel components.10.2 Nonferrous casting alloysNonferrous casting alloys include alloys of Al, Mg, Cu, Zn, Ni, and Ti. Aluminium alloys aregenerally considered very castable. The melting point of pure Al is 660oC so pouring temperatures forAl casting alloys are low compared to cast iron and steel. The properties that make Steel attractive tocastings – light weight, wide range of strength properties possible thorough sheat treatment, and theease of machiningMagnesium alloys are the lightest of casting metals. Other properties include corrosion resistance,, aswell as high strength-to-weight and stiffness-to-weight ratiosCopper alloys include bronze, brass, and aluminium bronze. Properties that make these attractiveinclude corrosion resistance, attractive appearance, and good bearing qualities. The high cost ofcopper is a limitation on the use of its alloys. Application include pipe fittings, marine propellerblades, pump components, and ornamental jewellery.Tin has the lowest melting point of the casting metals. Tin-based alloys are generally easy to cast.They have good corrosion resistance but poor mechanical strength, which limits their application topewter mugs and similar products not requiring high strength. Zinc alloys are commonly used in diecasting. Zinc has a low melting point and good fluidity, making it highly castable. Its major weaknessis low creep strength, so its castings cannot be subject to prolonged high stress.Nickel alloys have good hot strength and corrosion resistance, which makes them suited to hightemperature applications such as jet engine and rocket components, heat shields, and similarcomponents. Nickel alloys also have high melting point and are not easy to cast.Titanium alloys used for casting are corrosion resistant and posses high strength-to-weight ratios.However, titanium has a high melting point, low fluidity, and a propensity to oxidise at hightemperatures. These properties make is and its alloys difficult to cast. 36
  37. 37. 11.0 Product design considerationsIn casting is used as the primary manufacturing process for a product, some guidelines must beobserved to facilitate production and avoid defects: • Geometric simplicity: Although casting is a process that can be used to produce complex part geometries, simplifying the part design will improve its castability. Avoiding unnecessary complexities simplifies mould making reduces the need for cores, and improves the strength of the casting. • Corners: Sharp corners and angles should be avoided as they are sources of stress concentration and may cause hot tearing and cracks in the casting. Generous fillers should be design on inside corners, and sharp edges should be blended. • Section thickness: Section thickness must be uniform to avoid shrinkage cavities. Thicker sections create hot spots in the casting as more time is needed for solidification and cooling. These are likely locations of shrinkage cavities. • Draft: Part sections that project into the mould should have a draft or taper. In expendable mould casting, the purpose of this is to facilitate removal of the pattern from the mould. In permanent mould casting, the purpose is to help remove the part from the mould. Similar tapers should be allowed if solid cores are used in the casting process. The required draft need only be about 1o for sand casting and 2-3o for permanent mould processes. • Use of cores: minor design changes can reduce the need for coring. • Dimensional tolerances: There are significant differences in the dimensional accuracies that can be achieved in castings, depending on the process used. • Surface finish: Typical surface roughness achieved in sand casting is around 6μm. Equally poor finishes are obtained in shell moulding while plaster mould and investment casting produce much better roughness values (0.75μm). Among the permanent mould processes, die casting is noted for good surface finishes at around 1μm. • Machining allowances: tolerances achievable in many casting processes are insufficient to meet functional needs in many applications. sand casting is the most prominent example of this deficiency. In these cases, portions of the casting must be machined to the required dimensions. Almost all sand castings must be machined to a certain extent in order for the part to be made functional. Therefore, additional material, called, the machining allowance, is left on the casting for machining those surfaces where necessary. Typical machining allowances for sand castings are between 1.5 mm and 3 mm.Figure 29: (a) thick section at intersection can result in shrinkage cavity. Remedies include (b)redesign to reduce thickness and (c) use of a core. 37
  38. 38. Figure 30: Design change toeliminate the need for using a core: (a) original design and (b) redesignTable 03: Typical dimensional tolerances for various casting processes and metalsCasting process Part size Tolerance/mm Casting process Part size Tolerance/mmSand casting Permanent mouldAluminium Small ±0.5 Aluminium Small ±0.25Cast iron Small ±1.0 Cast iron Small ±0.8 Large ±1.5 Copper alloys Small ±0.4Copper alloys Small ±0.4 Steel Small ±0.5Steel Small ±1.3 Die casting Large ±2.0 Aluminium Small ±0.12Shell moulding Copper alloys Small ±0.12Aluminium Small ±0.25 InvestmentCast iron Small ±0.5 Aluminium Small ±0.12Copper alloys Small ±0.4 Cast iron Small ±0.25Steel Small ±0.8 Copper alloys Small ±0.12Plaster mould Small ±0.12 Steel Small ±0.25 Large ±0.4 38