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Metal Casting Processes

• satyendra
• May 19, 2024
• 25 Comments
• binder, centrifugal casting, Ceramic casting, Cope, core, Die casting, Drag, Investment casting, Metal casting, mould, Riser, sand casting, Shell moulding,

Metal Casting Processes

Metal casting process is the simplest and most direct route to a near net shape product and frequently the least expensive process. This process in its fundamental form needs a mould cavity of the desired shape and liquid metal to pour into the mould cavity. Metal castings are being produced for thousands of years, most frequently pouring liquid metal into moulds made of sand. The basic components of a mould cavity are cope, drag, parting line, riser, sprue (a channel through which the liquid metal is poured into a mould), and pouring basin etc., as well as the liquid metal handling system known as a ladle. The production of liquid metal and moulds to make castings has traditionally been an art form, an expression of human creativity carried out both for aesthetic and practical reasons.

The objective of metal casting has been to produce useful implements for human consumption as well as beautiful works of art. It is clear on examination of ancient art castings as well as modern industrial castings that their production needs considerable skill as well as technological know-how. The ancient artisan used traditions and learned skills passed down through the ages, as well as experience to produce acceptable castings. The modern producer of industrial castings makes use of these same skills, but supplements them with an understanding of the fundamental principles of fluid flow, heat transfer, thermo-dynamics, and metallurgical micro-structural development.

Today, metal casting is a complex and intricate process which needs exact chemistry and flaw-less execution. While the present methods can be relatively new when compared to the history of human civilization, the first casting of metals can actually be traced all the way back to around 4000 BCE (before common era). In those times, gold (Au) was the first metal to be cast because of its malleability, and back then, metal from tools and decoration was reused because of the complications of getting pure ore. Metal casting, on the other hand, dates back to 5000 BCE and 3000 BCE, which refers to the Chalcolithic period during which metals were melted for castings together with the experimentation of smelting copper. Initial moulds were made from smooth textured stones, resulting in fine cast products which can be witnessed in the museums and archeological exhibitions.

Metal castings form integral components of devices which perform useful functions. The cast component has a shape, size, chemical composition and metallurgical micro-structure which is determined by engineering decisions taken by (i) design engineers, (ii) pattern makers, (iii) casting personnel, and (iv) production personnel. All these personnel work together, and share information so that the casting performs as intended in a timely and cost-effective manner.

A large number of metal components are made by casting. This is because (i) casting process can produce very complex geometry parts with internal cavities and hollow sections, (ii) casting process can be used to make small (few hundred grams) to very large size parts (thousands of kilograms), (iii) casting process is economical, with very little wastage since the extra metal in each casting is re-melted and re-used, and (iv) cast metal is isotropic i.e., it has the same physical properties / properties along any direction.

Fluid life is the ability of the liquid metal to fill the mould cavity, flow through thin narrow channels to form thin walls and sections, and conform to fine surface detail. In addition to the temperature of the liquid metal, fluid life also depends on chemical, metallurgical, and surface tension factors. Hence, the fluid life of each metal is different. Fluid life determines the minimum wall thickness and maximum length of a thin section. It also determines the fineness of cosmetic detail which is possible. Hence, knowing that a metal has limited fluid life suggests that the part is to feature softer shapes (i.e., generous radii etc.), larger lettering, finer detail in the bottom portion of the mould, coarser detail in the upper portions of the mould, more taper leading to thin sections, and so forth.

Solidification shrinkage occurs in three distinct stages namely liquid shrinkage, liquid-to-solid shrinkage, and solid shrinkage. Liquid shrinkage is the contraction of the liquid before solidification. Liquid-to-solid shrinkage or solidification shrinkage is the shrinkage which occurs as the liquid’s disconnected atoms and molecules form into the crystals of atoms and chemical compounds which comprise the solid metal. Solid shrinkage is the shrinkage which occurs as the solid metal casting cools to ambient temperature.

Although liquid shrinkage is important to the metal casting process, it is not an important design consideration. Solidification shrinkage and solid shrinkage, on the other hand, are extremely important and are to be carefully considered during casting design. Different metals have differing quantities of liquid-to-solid shrinkage. Most importantly, there are three different types of solidification shrinkage namely directional, eutectic, and equiaxed. In alloy metals such as malleable iron and carbon steel, which solidify directionally, solidification moves along predictable pathways determined by the casting geometry and thermal gradients in the mould. For example, solidification typically begins at the mould wall and move perpendicularly toward the centre of the part. This is called progressive solidification.

Solidification also begins in cooler regions where the mould surface area to metal volume ratio is large and travel toward the hotter regions of the casting. This is called directional solidification. The key is to configure the part geometry so that directional solidification can occur before progressive solidification shuts off the source of liquid metal supply (the riser). Without proper path-way geometry (e.g., risering and tapering), voids or pores because of the isolated internal shrinkage can result.

In eutectic-type solidification, the liquid metal cools and then solidifies very quickly all over. This behaviour minimizes internal shrinkage and the need for risers and makes this type of alloy metal the most forgiving of the three. Eutectic-type materials which have very little solidification shrinkage like gray iron frequently need no risering at all. The key geometric concern for eutectic-type solidifying alloy metals which have small but appreciable solidification shrinkage is to ensure that the path of liquid metal supply stays open and functioning all the way to final solidification.

In addition to solidifying both progressively and directionally from the mould walls, alloy metals which show an equiaxed solidification behaviour also begin to solidify throughout the liquid, forming mushy regions consisting of equiaxed islands of solid. These equiaxed islands can block the paths of liquid metal supply making these alloy metals difficult to feed. For offsetting this tendency, regions which solidify in an equiaxed-type manner are to be designed to have small thermal gradients, i.e., to be as thermally neutral as possible. Hence, thermal mass in these regions is to be spread out and distributed uniformly throughout the region. This causes the shrinkage to be distributed as microscopic pores throughout the volume of the casting.

Although the thought of having microscopic holes in the casting is disturbing, the effect on mechanical properties is greatly minimized by the small size, rounded shape, and uniform dispersion produced by using the proper geometry for the type of alloy metal. Also, a uniform dispersion of very small pores is clearly preferable to large, irregular pores concentrated in possibly critical regions of the casting which can result from less appropriate part geometries.

Design is the critical first step in the development of cost-effective, high-quality castings. Designing of a successful casting needs an integrated, concurrent engineering approach. It also needs systematic and structured use of sophisticated computer-aided design software and casting analysis and simulation software.

In casting, the part being produced and the tooling used to produce the part interact in complex ways, which affect both the quality and the cost of the casting. This suggests that the design of a successful casting needs an integrated approach which considers both functional and process needs simultaneously. In the traditional setting, however, the design engineer typically first decides the geometry of a casting and then a casting engineer independently develops the mould and the process. This decoupled arrangement frequently results in more costly castings with larger safety factors than necessary. In addition, lead times for casting development using this approach can be excessive.

As the producers seek to reduce weight and cost of the products, casting has re-emerged as the manufacturing process of choice in several situations. This is since casting offers the important advantage of being able to produce highly complex functional shapes quickly and easily. Cost is reduced since several parts and complex construction and processing typically associated with built up structures and weldments can be replaced by a single cast part. Weight is reduced since material can be distributed to where it is needed and since sections can be thinner since load does not transfer across part interfaces (i.e., through fasteners or welds).

For taking advantage of these unique capabilities, castings are to be properly conceived and designed from the start. This means that design engineers and casting engineers need to change the casting design and development process from the historical sequential, de-coupled approach to a more integrated and concurrent approach. This new approach is to be based on the recognition that the part geometry not only affects the load carrying functionality of the casting, but also the mould construction, mould filling, and material solidification processes involved in producing the casting. These processes in turn affect cycle time, casting quality, and material properties such as yield strength, ultimate tensile strength, and fatigue resistance.

Hence, the casting geometry is to be determined based on both functional and processing needs. Hence, design engineers are to become more knowledgeable of the casting process, and casting engineers need to have a better understanding of the functional requirements which drive the design. In short, the new design philosophy is not only to facilitate good part and process design, it is also needed to teach it.

There are a number of metal casting processes. Two of the major advantages for selecting casting as the process of choice for producing a part are the wide selection of alloys available and the ability, as in injection moulding, to produce complex shapes. However, not all alloys can be cast by all processes.

Although there are distinct differences between the different casting processes, there are also several common characteristics. For example, in all casting processes, a metal or a metal alloy is melted and then poured or forced into a mould where it takes the shape of the mould and is allowed to solidify. Once it has solidified, the casting is removed from the mould. Some castings need finishing because of the cast appearance, tolerance, or surface finish requirements. During solidification, majority of metals shrink (gray cast iron is an exception) so moulds are to be made slightly oversize in order to accommodate the shrinkage and still achieve the desired final dimensions.

The most common metal casting processes are sand casting, investment casting, and die casting. The nature of the moulds used and the method for removing the part from the mould differs for the different processes. The tolerances and surface finishes achievable are also different. Tab 1 gives some common types of metal casting processes, their advantages, disadvantages and examples of the parts made by the processes.

Metal casting process begins with the making of a mould, which is the ‘reverse’ shape of the part which is to be cast. The mould is made from a refractory material, for example, sand. The metal is heated in a furnace until it melts, and then the liquid metal is poured into the mould cavity (Fig 1a). The liquid takes the shape of cavity, which is the shape of the part. It is cooled until it solidifies. Finally, the solidified metal part is removed from the mould.

Fig 1 Pouring of liquid metal and assembly of cope and drag

There are two basic types of moulds used in castings, namely, expendable moulds (sand casting and investment casting) which are destroyed to remove the part, and permanent moulds (die casting). Expendable moulds are made using either a permanent pattern (sand casting) or an expendable pattern (investment casting). Permanent moulds, of course, do not need a pattern.

Casting design and quality – Several factors affect the quality / performance of cast parts. Hence, the design of parts which is to be produced by casting, as well as the design of casting moulds and dies, is to account for these. These can be considered as design guidelines, and their scientific basis lies in the analysis of the strength and behaviour of materials. Corners, angles, and section thickness are important parameters.

Several metal casting processes lead to small surface defects (e.g., blisters, scars, scabs, or blow-holes etc.), or tiny holes / impurities in the interior (e.g., inclusions, cold-shuts, or shrinkage cavities etc.). These defects are an issue, if the part with such a defect is subject of varying loads during use. Under such conditions, it is likely that the defects act like cracks, which propagate under repeated stress causing fatigue failure. Another possibility is that internal-holes act as stress concentrators and reduce the actual strength of the part below the expected strength of the design.

For avoiding such issues, it is necessary that (i) sharp corners are avoided since these behave like cracks and cause stress concentration, (ii) section changes are blended smoothly using fillets, and (iii) rapid changes in cross-section areas are avoided. If unavoidable, the mould is designed for ensuring that liquid metal can flow to all regions, and mechanism is provided for uniform and rapid cooling during the solidification. This can be achieved by the use of chills or incorporating fluid-cooled tubes in the mould.

Large, flat regions are avoided, since they tend to warp because of the residual stresses. Further, it is not good for a casting to have surfaces having normal which is perpendicular to the direction along which the part is to be ejected from the mould. This can cause the part to stick in the mould and forceful ejection can cause damage to the part, and the mould if the mould is re-usable. Hence, all such surfaces are tilted by a small angle (between 0.5-degree and 2-degree) so as to allow easy ejection. Draft angles on the inner surfaces of the part are higher, since the cast part also shrinks a little bit towards the core during the solidification and cooling.

As the casting cools, the metal shrinks. For common cast metals, a 1 % shrinkage allowance is designed in all linear dimensions (namely, the design is scaled up by around 1 %). Since the solidification front, i.e., the surface at the boundary of the solidified and the liquid metals, travels from the surface of the mould to the interior regions of the part, the design is to ensure that shrinkage does not cause cavities.

The parting line is the boundary where the cope, drag and the part meet. If the surface of the cope and drag are planar, then the parting line is the outline of the cross-section of the part along that plane. The parting line can be easily seen for several cast and moulded parts which are normally used. It is conventional that the parting line is planar, if possible. A very small of metal always ‘leak’ outside the mould between the cope and the drag in any casting. This is called the ‘flash’. If the flash is along an external surface, it is to be machined away by some finishing operation. If the parting line is along an edge of the part, it is less visible which is preferred.

Sand casting – Sand casting process is one of the most versatile of metal-forming processes. It is typically used to produce large parts such as machine-tool bases and components, structures, large housings, engine blocks, transmission cases, connecting rods, and other large components which, because of their size, cannot be cast by other processes. These castings are not normally used for the production of parts which need high production volumes. They are sometimes used to produce proto-type parts and at times they are the only method available for the creation of large parts which need a large crane capacity to remove from the mould. Although almost any metal which can be melted can be sand cast, sand castings have (as a result of the sand mould) a grainy surface with large dimensional variations. Hence, sand castings frequently need local finish machining operations in order to get the necessary surface finishes and dimensional tolerances.

Green sand moulding is one of several methods available for making a mould into which liquid metal can be poured. Green sand moulding and chemically bonded sand moulding are considered to be the most basic and widely used mould making processes. The moulding media for the two methods are prepared quite differently. Chemically bonded media are prepared by coating grains of sand with a binder which is later cured by some type of chemical reaction. Green sand media are prepared by coating the grains of sand with binder which is later shaped into a rigid mass by the application of force.

Green sand moulding is the least expensive, fastest, and most common of all the presently available moulding methods. The mixture of sand and binder can be used immediately after the mixing process which coats the sand grains. Although the time taken to shape the mould is of importance in some cases the forming process can be considered to be almost instantaneous.

Green sand as a moulding medium consists of a number of different materials which are to be present in varying quantities and grades in order to produce the desired results for a specific type of casting. The increased demands for casting accuracy and integrity have caused an increase in the use of high-pressure, high-density moulding machines. Except for relatively few applications (such as thin-section castings), fireclay (kaolin) is unsuitable for this type of moulding. The montmorillonite, or bentonite, clays are used mainly because of their increased durability when heated, higher bonding strength, and plasticity.

The bentonites can be classified as two distinct types namely sodium bentonite and calcium bentonite. The properties derived from the use of each vary widely. The use of one in preference to the other depends on the castings to be made, the system being run, and the economics of the total situation. Fortunately, these bentonites are compatible and can be blended in any ratio to tailor sand properties to the specific requirements of a system (casting condition).

Sand casting process provides tremendous freedom of design in terms of size, shape, and product quality. Sand casting processes are classified as per the way in which the sand is held (bonded). Sand casting process have been categorized into three types. Resin binder processes are organically bonded systems which include no-bake binders, heat-cured binders (the shell process and warm box, hot box, and oven-bake processes), and cold box binders. Bonded sand moulds processes are based on inorganic bonds and include such processes as green sand moulding, dry sand moulding, skin dried moulding, and loam moulding, sodium silicate-carbon dioxide systems, and phosphate bonded moulding. With unbonded sand moulding processes, dry, unbonded, free-flowing sand surrounds the pattern. Lost foam processing, which uses expandable polystyrene patterns, and vacuum moulding, are examples of unbonded sand moulding processes. Lost foam moulds for large castings are sometimes backed up with a no-bake binder system.

In sand casting process, a pattern is used to make the mould. The pattern is a form made of wood, metal, plastic, or composite materials around which a moulding material (normally prepared sand) is formed to shape the casting cavity of a mould. Majority of the patterns are removed from the completed mould halves and used repeatedly to make several duplicate moulds. Expendable patterns of such materials as wax or expanded polystyrene, are made in quantity and are used only once to produce an individual mould. The pattern equipment (tooling) needed to make a casting includes the pattern and can also include one or more core boxes. Core boxes are used to make refractory inserts or cores which are placed within a mould cavity to form internal cavities or passage-ways within the casting.

The selection of the type of pattern equipment used to make a casting depends on several factors, including the number of castings to be produced, the size and shape of the casting, the moulding or casting process to be used, and other special requirements such as the dimensional accuracy needed. Pattern-making begins with the dimensions of the casting needed. However, the design and dimensions of the resultant pattern equipment is to incorporate other features and take into account different pattern allowances. Patterns are to be oversized to correct for the contraction of the liquid metal upon cooling and any extra metal to be removed from the machined surfaces of a casting.

A mould parting line is to be selected, and taper or draft is to be included on the vertical faces of the pattern to permit the removal of reusable patterns from the mould. Patterns also incorporate provisions for gating and risering (rigging), which facilitate the flow (feeding) of liquid metal into the mould cavity. For castings to be made with cores, the pattern and core boxes are to include projections called core prints, which are used to support and locate the cores in the mould cavity.

The pattern is used to form the mould cavity, the core print for locating the core, the gate, the runner, the riser, and the sprue. A separate core box is used to make the sand core which is inserted into the parted mould before pouring. The cost of pattern equipment for a given casting can vary greatly, depending on pattern material, type of pattern, and sometimes the dimensional accuracy needed. However, since the pattern equipment is only a part of the mould making process, the least expensive pattern is not necessarily the most economical. Additional pattern cost and quality frequently lead to lower end costs by a reduction in moulding costs and pattern repair costs and by an improvement in overall casting quality.

Sand casting uses natural or synthetic sand (river sand) which is mostly a refractory material called silica (SiO2). The sand grains are to be small enough so that it can be packed densely, however, the grains are to be large enough to allow gasses formed during the metal pouring to escape through the pores. Larger sized moulds use green sand (mixture of sand, binder, and some water). Sand can be re-used, and excess metal poured is cut-­off and re-used also. Fig 2 shows work flow in a typical foundry for sand-casting.

Fig 2 Work flow in a typical sand-casting foundry.

In sand casting process, sand mould is formed by packing a mixture of sand, a binder, and water around a wood or metal pattern which has the same external shape as the part to be cast. The pattern can come in two halves namely (i) a top half (called a cope), and a bottom half (called a drag) (Fig 1b). The liquid metal flows into the gap between the two parts, called the mould cavity. The geometry of the cavity is created by the pattern. The shape of the pattern is (almost) identical to the shape of the part which is needed to be made. Each half is placed in a moulding box, and the sand mixture is then poured all around the pattern.

There is a funnel shaped cavity. The top of the funnel is the pouring cup, The pipe-shaped neck of the funnel is the sprue. The liquid metal is poured into the pouring cup, and flows down the sprue. The runners are the horizontal hollow channels which connect the bottom of the sprue to the mould cavity. The region where any runner joins with the cavity is called the gate. Gates are the sections where the liquid metal enters the cavity. Hence, sprues feed the runners, and the runners feed the gates.

Some extra cavities are made connecting to the top surface of the mould. Excess metal poured into the mould flows into these cavities, which are called risers (Fig 1). They act as reservoirs, as the metal solidifies inside the cavity, it shrinks, and the extra liquid metal from the risers flows back down to avoid holes in the cast part. Vents are narrow holes connecting the cavity to the atmosphere to allow gasses and the air in the cavity to escape.

Several cast parts have interior holes (hollow parts), or other cavities in their shape which are not directly accessible from either piece of the mould. Such interior surfaces are generated by inserts called cores. Cores are made by baking sand with some binder so that they can retain their shape when handled. The mould is assembled by placing the core into the cavity of the drag, and then placing the cope on top, and locking the mould. After the casting is done, the sand is shaken off, and the core is pulled away and normally broken off.

Vents are also created in order to allow the escape of gases from the liquid metal. Then the pattern is removed and a runner system or small passage-way is created inside the die through which the liquid metal can flow and be distributed.

For facilitating the removal of the pattern from the sand mould, the pattern is to be provided with an angle or taper called draft. If possible, parts are to be designed so that natural draft is provided (Fig 3). If the part to be cast has one completely flat surface, then the pattern can be made in one piece. If the production volume is sufficiently large, the two halves of the pattern are normally mounted on opposite sides of a single board or metal plate to form what is called a match-plate. For avoiding the necessity of forming the runner system by hand, the patterns which form the runners can also be mounted on the match-plate. For large castings a match-plate becomes very large and heavy for convenient handling and the cope and drag half approach is used.

Fig 3 Parts with draft and cores needed with increasing geometric complexity

If the casting is to be a hollow shape, such as a thin-walled cylinder, then a separate sand core is placed in position so that the liquid metal cannot fill what is to be the open portion of the casting (e.g., the inside of the cylinder). In sand casting, these sand cores provide a function similar to those provided by side cavities and side cores in injection molding, and die casting, i.e., they provide those geometric features not easily achievable using a conventional two-piece mould or pattern. As the part geometry becomes more complex, the number of cores needed to provide the geometric shape increases (Fig 3b).

Once the core or cores are in place, the cope half of the mould box is then placed on top of the drag half. The liquid metal is then poured, and the casting left to solidify. Once the casting has cooled, the sand mould is destroyed and the casting removed. Fig 4 shows steps in the sand-casting process.

Fig 4 Steps in the sand-casting process

Sand casting results in parts with internal porosity which causes leaking and reduces part strength. Porosity is the result of voids or pores caused by trapped air, liquids, or gases which come about during freezing of the liquid metal. Trapped air and liquids are a result of the dendrites (a crystal which has a tree-like branching pattern) which occur when the cooling rate is relatively slow, as in sand casting. Because the trapped liquids and gases continue to freeze and shrink, holes are created.

Because of the shrinkage which occurs before and during solidification, risers which contain a reservoir of liquid metal, are connected to the casting. Hence, as the casting shrinks the riser supplies additional liquid metal. To be effective, the risers are required to freeze last, otherwise the supply of liquid metal to the mould is shut off and shrink holes are created (Fig 5). The location of risers is also critical, and if they are properly located, they can reduce the number and size of shrink holes. In order to produce a sound casting, the number and location of risers is important. However, since time is needed for providing and removing the risers, as well as by providing the risers, the yield of the liquid metal poured is reduced, risers add to the cost of a casting. Hence, the number of risers is to be kept to a minimum. Frequently re-design of a casting by the addition of webs and ribs permits the reduction in the number and / or placement of risers.

Fig 5 Creation of shrink hole because of lack of proper feeding

Shell-mould casting – Shell-mould casting yields better surface quality and tolerances. In this process, the 2-piece pattern is made of metal (e.g. aluminum or steel), it is heated to between 175 deg C to 370 deg C, and coated with a lubricant, e.g. silicone spray. Each heated half-pattern is covered with a mixture of sand and a thermoset resin / epoxy binder. The binder glues a layer of sand to the pattern, forming a shell. The process can be repeated to get a thicker shell. The assembly is baked to cure it.  The patterns are removed, and the two half-shells joined together to form the mould and liquid metal is poured into the mould. When the liquid metal is solidified, the shell is broken to get the part. Fig 6 shows the shell mould casting process.

Fig 6 Shell mould casting

Expendable-pattern casting (lost foam process) – The pattern used in this process is made from polystyrene (a light, white packaging material). Polystyrene foam is 95 % air bubbles, and the material itself evaporates when the liquid metal is poured on it. The pattern is made by moulding. The polystyrene beads and pentane are put inside an aluminum (Al) mould, and heated. It expands to fill the mould, and takes the shape of the cavity. The pattern is removed, and used for the casting process as given below.

The pattern is dipped in a slurry of water and clay (or other refractory grains). It is dried to get a hard shell around the pattern. The shell-covered pattern is placed in a container with sand for support, and liquid metal is poured from a hole on top. The foam evaporates as the liquid metal fills the shell. After cooling and solidification, the part is removed by breaking the shell.

The process is useful since it is very cheap, yields good surface finish and can be used for complex geometries. There are no runners, risers, gating or parting lines and hence the design process is simplified. The process is used to manufacture crank-shafts for engines, aluminum engine blocks, and manifolds etc. Fig 7 shows expendable mould casting.

Fig 7 Expendable mould casting

 Plaster mould casting – Plaster mould casting is a specialized casting process used to produce non-ferrous castings which have high dimensional accuracy, smoother surfaces, and more finely reproduced detail than can be achieved in sand moulds or coated permanent moulds. The four normally recognized plaster mould processes are (i) conventional plaster mould casting, (ii) match plate pattern plaster mould casting, (iii) the Antioch process, and (iv) the foamed plaster process.

The Antioch process has, as its special feature, the requirement to process the moulds in a steam autoclave. This produces a unique granular structure, which provides mould permeability. The moulds produced by the Antioch process are denser than foamed plaster moulds and weaker than conventional plaster moulds.

In all of the processes for making plaster moulds and cores, the principal mould ingredient is calcium sulphate (CaSO4, also known as ‘Plaster of Paris’), along with talc and silica flour. This is a fine white powder, which, when mixed with water gets a clay-like consistency and can be shaped around the pattern. Other materials used to improve such mould properties as green and dry strength, permeability, and castability include cement, ceramic talc, fiber-glass, sand, clay, Wollastanite, pearlite, and fly ash. Cores are typically made of the same material and by the same process as moulds, but cores are sometimes made of other materials, such as shell molding sand. Cores for the match plate plaster mould process are made of sand.

The cast plaster can be finished to yield very good surface finish and dimensional accuracy. However, it is relatively soft and not strong enough at temperature above 1,200 deg C, so this method is mainly used to make castings from non-ferrous metals, e.g. zinc (Zn), copper (Cu), aluminum, and magnesium (Mg). Since the cast plaster has lower thermal conductivity, the casting cools slowly, and hence has more uniform grain structure (i.e. less warpage, and less residual stresses).

Ceramic mould casting – Ceramic mould casting techniques are based on proprietary processes which use permanent patterns and fine-grain zircon (ZrSiO4) and calcined, high-alumina (Al2O3) mullite slurries for moulding. Except for distribution of grain size, the zircon slurries are comparable in composition to those used in ceramic shell investment moulding. Like investment moulds, ceramic moulds are expendable. However, unlike the monolithic moulds got in investment moulding, ceramic moulds consist of a cope and a drag or, if the casting shape permits, a drag only.

Ceramic casting is similar to plaster-mould casting, except that ceramic material is used (e.g., silica or powdered zircon). Ceramics are refractory which have higher strength than plaster. The ceramic slurry forms a shell over the pattern. It is dried in a low temperature oven, and the pattern is removed. Then it is backed by clay for strength, and baked in a high temperature oven to burn off any volatile substances. The liquid metal is cast in the same way as in plaster casting.

Ceramic moulding can produce castings with fine detail, smooth surfaces, and a high degree of dimensional accuracy. The ceramic mould surface has refractory properties which enables it to withstand high metal pouring temperatures (such as those necessary for steel and heat-resistant alloys), give it excellent thermal stability, and do not permit burn-in.

Ceramic moulding is intended for the production of castings of high quality, not only in terms of their dimensional accuracy and surface finish but also in terms of soundness and freedom from non-metallic inclusions. In general, the capabilities of ceramic moulding are similar, and the selection of one process over the other is largely dependent on the size of the casting, the quantities needed, and the moulding costs involved. In some applications, depending on casting shape, permanent patterns used in ceramic moulding can provide higher dimensional accuracy than wax patterns, mainly since wax expands during melt-out. Permanent patterns are also less susceptible to damage and distortion in handling than wax or plastic patterns. This process can be used to make very good quality castings of steel or even stainless steel. It is used for parts such as impellor blades (for turbines, pumps, or rotors for motor-boats).

Investment casting – In investment casting, a ceramic slurry is applied around a disposable pattern, normally wax, and allowed to harden to form a disposable casting mould. The term disposable means that the pattern is destroyed during its removal from the mould and that the mould is destroyed to recover the casting. There are two distinct processes for making investment casting moulds namely the solid investment (solid mould) process, and the ceramic shell process. The ceramic shell process has become the predominant technique for engineering applications, displacing the solid investment process. Today the solid investment process is primarily used to produce dental and jewelry castings and has only a small role in engineering applications, mostly for non-ferrous alloys.

Investment casting can produce parts of similar geometric shapes and size. Since the disposable pattern is made by injecting wax into a mould, features which are difficult or costly to injection mould or die cast (e.g., undercuts) are also costly to investment casting. Investment casting is typically used when low production volumes are expected (e.g., less than 10,000 pieces).

Investment cast parts can be made of a wide range of alloy metals including aluminum and copper alloys, carbon (C) and low alloy steels, stainless steels, tool steels, and nickel (Ni) and cobalt (Co) alloys. In investment casting (Fig 8), a metal die or mould is made by either machining or casting. The more complicated is the shape (because of undercuts, for example), the more costly is the metal dies.

Fig 8 Investment casting process

After the mould is formed, wax is injected to form a pattern. The external shape of the wax pattern resembles the internal shape of the mould. The wax pattern is removed from the mould and attached to a wax base which contains a gate. If the production volume is large enough several wax patterns are attached to a tree which contains the runners, gates, and other features which feed and distribute the liquid metal. A metal hollow tube is now placed over the wax patterns and a slurry, such as Plaster of Paris, is poured to entirely cover the patterns. The completed mould is placed in an oven and the wax is removed by melting and evaporation. Following this, the mould is normally placed in a second oven to cure for 12 hours to 24 hours.

To make parts, the mould cavity is filled with liquid metal which is allowed to solidify. For facilitating filling of the mould, the liquid metal is poured while the mould is still hot. When the part has cooled, the mould is destroyed and the part removed. The tolerances and surface finishes achievable by investment casting are such that machining is normally not needed.

Vacuum casting – This process is also called counter-gravity casting. It is basically the same process as investment casting, except for the step of filling the mould. In this case, the material is sucked upwards into the mould by a vacuum pump. The Fig 8 shows the basic idea. It can be noticed how the mould appears in an inverted position from the normal casting process, and is lowered into the flask with the liquid metal. One advantage of vacuum casting is that by releasing the pressure a short time after the mould is filled, one can release the un-solidified metal back into the flask. This allows the person to create hollow castings. Since the majority of the heat is conducted away from the surface between the mould and the metal, hence the portion of the metal closest to the mould surface always solidifies first and the solid front travels inwards into the cavity. Hence, if the liquid metal is drained in a very short time after the filling, then a person can get a very thin-walled hollow object etc. (Fig 9).

Fig 9 Vacuum casting

Permanent mould casting – Here, the two halves of the mould are made of metal, normally cast iron, steel, or refractory alloys. The cavity, including the runners and gating system are machined into the mould halves. For hollow parts, either permanent cores (made of metal) or sand-bonded ones can be used, depending on whether the core can be extracted from the part without damage after casting. The surface of the mould is coated with clay or other hard refractory material. This improves the life of the mould.

Before moulding, the surface is covered with a spray of graphite or silica, which acts as a lubricant. This has two purposes namely (i) it improves the flow of the liquid metal, and (ii) it allows the cast part to be withdrawn from the mould more easily. The process can be automated, and hence yields high through-put rates. Also, it produces very good tolerance and surface finish. It is normally used for producing pistons used in car engines, gear blanks, cylinder heads, and other parts made of low melting point metals, e.g., copper, bronze, aluminum, and magnesium etc.

Die casting – Die casting is characterized by a source of hydraulic energy which imparts high velocity to liquid metal to provide rapid filling of a metal die. The die absorbs the stresses of injection, dissipates the heat contained in the metal, and facilitates the removal of the shaped part in preparation for the next cycle. The hydraulic energy is provided by a system which permits control of actuator position, velocity, and acceleration to optimize flow and force functions on the liquid metal as it fills the cavity and solidifies.

The variety in die casting systems results from trade-offs in metal fluid flow, elimination of gas from the cavity, reactivity between the liquid metal and the hydraulic system, and heat loss during injection. The process varieties have several features in common with regard to die mechanical design, thermal control, and actuation. Four principal alloy families which are normally die-cast are aluminum, zinc, magnesium, and copper-base alloys. Lead, tin, and, to a lesser extent, ferrous alloys can also be die-cast. The three main variations of the die casting process are the hot chamber process, the cold chamber process, and direct injection. Fig 10 shows two main die casting processes.

Fig 10 Die casting processes

In a hot chamber process (used for zinc alloys, and magnesium) the pressure chamber connected to the die cavity is filled permanently with the liquid metal (Fig 10a). The basic cycle of operation is (i) die is closed and goose-neck cylinder is filled with the liquid metal, (ii) plunger pushes the liquid metal through goose-neck passage and nozzle and into the die cavity where the liquid metal is held under pressure until it solidifies, (iii) die opens and cores, if any, are retracted, casting stays in ejector die, plunger returns, pulling the liquid metal back through nozzle and goose-neck, (iv) ejector pins push casting out of ejector die. Since the plunger uncovers the in-let hole, liquid metal refills goose-neck cylinder. The hot chamber process is used for metals which have low melting points and which do not alloy with the die material (steel). Common examples are tin, zinc, and lead.

Since the higher temperatures used in casting aluminum and copper alloys considerably shorten the life of hot chamber machines, cold chamber process (Fig 10b) is frequently used. In a cold chamber process, the liquid metal from a separate holding furnace is ladled into the cold chamber sleeve after the mould is closed. The liquid metal is then forced into the mould, and after solidification, the mould is opened and the part ejected. The operating cycle is (i) die is closed and the liquid metal is ladled into the cold chamber cylinder, (ii) plunger pushes liquid metal into die cavity where the metal is held under high pressure until it solidifies, (iii) die opens and plunger follows to push the solidified slug from the cylinder, if there are cores, they are retracted away, (iv) ejector pins push casting off ejector die and plunger returns to the original position. Injection pressures in this type of process normally range from 17 MPa to 41 MPa. Pressures as high as 138 MPa are possible.

Direct injection extends the technology used for lower-melting polymers to metals by taking the hot chamber intimacy to the die cavity with small nozzles connected to a manifold, hence eliminating the gating and runner system. This process, however, is still under development.

In the die casting process, a liquid metal is injected under pressure into a metal mould. The liquid metal then cools, shrinks, and solidifies, taking on the shape of the mould. The mould then opens and the part is ejected. Since the processes are so similar, much of what has been said concerning the influence of part geometry and tolerance specifications on injection molding lower melting polymers, tooling costs, and processing costs applies equally well to die casting process. For example, moulds contain as few moving parts as possible, hence, the mould closure direction and the parting surface location are important and the part is easy to eject. For ease of flow of the liquid metal, be it an alloy or a resin, smooth paths and low flow resistance are important.

Thin uniformly thick walls are desirable to shorten cycle times and solidification. In die casting, as in injection molding, the three major cost components of a part are material cost, tooling cost, and processing cost. These three cost components are influenced, to varying degrees, by the geometry, size, and material of the part as well as by subsidiary factors such as part quality requirements. The same coding system for tooling costs applies, with only minor modifications, to die casting.

As in injection molding, as the part geometry becomes more complex, the cost of the mould increases. Also, as the wall thickness increases, the cycle time needed to produce the part also increases. While the thin film, called flashing (Fig 10c), which extrudes out through the spaces between parts of a mould is easily removed by hand in the case of injection-molded parts, the same cannot be said for die-cast parts. Hence, because of the difficulty of flash removal, internal under-cuts are not normally die-cast. However, die casting can economically produce parts of high complexity.

Since the moulds used in die casting are made of steel, only metals with relatively low melting points can be die cast. The large majority of castings are made of either zinc alloys or aluminum alloys. Zinc alloys are used for the majority of the ornamental or decorative objects, while the aluminum alloys are used for the majority of the non-decorative parts.

Centrifugal casting – In centrifugal casting, liquid metal is poured into a mould which is revolving about a horizontal or vertical axis. Horizontal centrifugal casting is used to produce rotationally symmetric parts, such as pipes, tubes, bushings, and other parts. Vertical centrifugal casting can be used to produce both symmetrical as well as non-symmetrical parts. However, since only a reasonable quantity of imbalance can be tolerated for a non-symmetrical part, the most common shapes produced are cylinders and rotationally symmetric flanged parts. Centrifugal casting of metal produces a finer grain structure and thinner ribs and webs than can be achieved in ordinary static mould casting.

Centrifugal casting is used to cast pieces having an axis of revolution. The technique uses the centrifugal force generated by a rotating cylindrical mould to throw the liquid metal against the mould wall and form the desired shape. Horizontal centrifugal casting was first used mainly to manufacture thin-wall gray iron, ductile iron, and brass tubes. Improvements in equipment and casting alloys made possible the development of a flexible and reliable process which is both economical and capable of meeting stringent metallurgical and dimensional requirements. Cylindrical pieces produced by centrifugal casting are now used in several industries. Of particular importance are large-diameter thick-wall bi-metallic and specialty steel tubes used in the chemical processing, pulp and paper, steel, and offshore petroleum production industries. Fig 11 shows schematics of horizontal centrifugal casting.

Fig 11 Schematics of horizontal centrifugal casting

The centrifugal casting machine is required to perform four operations accurately and with repeatability. These are (i) the mould is required to rotate at a pre-determined speed, (ii) there is required a means to pour the liquid metal into the rotating mould, (iii) once the liquid metal is poured, the proper solidification rate is to be established in the mould, and (iv) there is required means of extracting the solidified casting from the mould.

Centrifugal casting uses a permanent mould which is rotated about its axis at a speed between 300 revolutions per minute (rpm) to 3,000 rpm as the liquid metal is poured. Centrifugal forces cause the liquid metal to be pushed out towards the mould walls, where it solidifies after cooling. Parts cast in this method have a fine grain micro-structure, which is resistant to atmospheric corrosion, hence this method has been used to manufacture pipes. Since metal is heavier than impurities, most of the impurities and inclusions are closer to the inner diameter and can be machined away, surface finish along the inner diameter is also much worse than along the outer surface.

Moulds consist of four parts namely the shell, the casting spout, roller tracks, and end heads. The mould assembly is placed on inter-changeable carrying rollers which enable the use of different mould diameters and fine adjustments. Moulds are cooled by a water spray, which can be divided into several streams for selective cooling.

Liquid metal can be introduced into the mould at one end, at both ends, or through a channel of variable length. Pouring rates vary widely as per the size of the casting being produced and the metal being poured. Pouring rates which are too slow can result in the formation of laps and gas porosity, while excessively high rates slow solidification and are one of the main causes of longitudinal cracking.

The degree of super-heat needed to produce a casting is a function of the metal or alloy being poured, mould size, and physical properties of the mould material. The empirical formula which has been suggested as a general guide-line to determine the degree of super-heat needed is ‘L = 2.4 (delta T) + 110’ (equation 1), where ‘L’ is the length of spiral fluidity (in millimeters) and ‘delta T’ is the degree of super-heat (in degrees centigrade). The use of equation 1 for ferrous alloys results in casting temperatures which are 50 deg C to 100 deg C above the liquidus temperature. In practice, casting temperatures are kept as low as possible without the formation of defects resulting from too low a temperature. A high casting temperature needs higher speeds of rotation for avoidjbg sliding, while the low casting temperatures can cause laps and gas porosity. Casting temperature also influences solidification rates and hence affects the quantity of segregation which takes place.

Carbon di-oxide (CO2) mould casting – Carbon di-oxide mould casting is a sand-casting process where the sand mould is hardened by blowing carbon-di oxide gas over it. This allows for dimensionally accurate castings with fine surfaces at a lower cost than green sand casting. In this process, moulding sand is mixed with sodium silicate (water glass). Sodium silicate (Na2SiO3) is used as a binder in place of the clay binders used in conventional sand moulds and cores. The mix is loosely rammed in the mould around the pattern. When carbon di-oxide gas is sent through the mixture, the mould is hardened. The pattern can then be removed and the liquid metal is poured. Carbon di-oxide moulds are used when closer tolerances than those achievable through sand casting are needed.

Carbon di-oxide when mixed with sodium silicate forms silica gel as per the equation ‘Na2SiO3 + H2O + CO2 = Na2CO3 + SiO2 + H2O’ (equation 2). Silica gel is a product which is frequently found in reactions caused by different hardeners. In silica gel, the dispersed phase is silica. Carbon di-oxide moulds are useful for pouring alloys of high density like steel. The mould in high hardness and strength. High hardness of mould resist penetration of liquid metal, normally of high density, in to mould wall. Fig 12 shows carbon di-oxide mould casting.

Fig 12 Carbon di-oxide mould casting

Qualitative DFM (design for manufacturing) guide-lines for casting – All casting processes are internal flow processes in which liquid metal flows into and fills a die cavity. Then the liquid is cooled to form a solid, and finally the part is removed from the mould by either destroying the mould or, as in the case of die casting, ejecting the part from the mould. The physical nature of these processes e.g., flow, cooling to solidify, and, in the case of die casting, ejection, provides the basis for a number of the qualitative DFM (design for manufacturing) guide-lines or rules of thumb which have been established. Several of these rules are similar to the rules applicable the processes of injection moulding, compression moulding, and transfer moulding. For example, parts are required to be ideally designed so that (i) the flow of liquid metal can be smooth and fill the cavity evenly, (ii) cooling, and hence solidification, can be rapid to shorten cycle time and uniform to reduce warpage, and (iii) if ejection is needed, it can be accomplished with as little tooling complexity as possible.

As in injection moulding, to design parts properly for die casting, designers are to consider the effect of mould closure direction and parting surface location on tooling costs. The location of the parting surface, the direction of closure, and the design of the part are required to be considered simultaneously in order to provide for ejection of the part from the mould after solidification. Knowing the mould closure direction enables designers to recognize and, hence, possibly avoid designing unnecessary under-cuts. The six important DFM guide-lines are given below.

Guide-line number 1 – In designing parts to be made by die casting, designers are to keep in mind, as a part of the design, the direction of mould closure and the location of the parting surface. Advice can be sought from a die-casting specialists, since it is really impossible to do much design for manufacturing in this process without considering the mould closure direction and parting surface location.

Guide-line number 2 – An easy to manufacture parts are to be easily ejected from the die, and dies are to be less expensive if they do not need special moving parts (such as side cores) which are to be activated in order to allow parts to be ejected. Since under-cuts need side cores, parts without undercuts are less costly to cast. With knowledge of the mould closure direction and parting surface, designers can make tentative decisions about location(s) of features (holes, and projections etc.) in order to avoid under-cuts wherever possible.

Guide-line number 3 – Because of the need for metal to flow through the die cavity, parts which provide relatively smooth and easy internal flow paths with low flow resistance are desirable. For example, sharp corners and sudden changes or large differences in wall thickness are to be avoided since they both create flow problems. Such features also make uniform cooling difficult and result in the development of shrink cavities.

Guide-line number 4 – Thick walls or heavy sections slow the cooling process. Hence, parts with no thick walls or other thick sections are less costly to produce. Although reducing wall thickness normally does reduce strength, decreasing section thickness in die casting does not proportionately reduce casting strength. Reducing wall thickness reduces cycle time. This rapid cooling rate for thin sections yield castings with better mechanical properties. Thick sections, on the other hand, suffer from a coarse crystalline structure which results in internal voids and porosity which reduces strength.

Guide-line number 5 – In addition, every effort is to be made to design parts of uniform, or nearly uniform, wall thickness. If there are both thick and thin sections in a part, solidification can proceed unevenly causing difficult to control internal stresses and warping. It is also to be remembered that the thickest section largely determines solidification time, and hence the total cycle time.

Guide-line number 6 – In large or complex parts, two or more gates can be needed through which metal flows in two or more streams into the mould. There are hence fusion lines in the part where the streams meet inside the mould. The line of fusion can be a weak region, and it can also be visible. Hence, designers who suspect that multiple gates are needed for a part, are to discuss these issues with casting specialists as early as possible in the design process. With proper design and planning, the location of the fusion lines can normally be controlled as needed for appearance and functionality.

The above six DFM guidelines are not absolute, rigorous laws. If there are designs which have high advantages for function or marketing, then those designs can be given special consideration. Manufacturing engineers can sometimes solve the problems which can be associated with highly desirable functional but difficult to manufacture designs at a cost low enough to justify the benefit. However, relatively easy to manufacture designs are always to be sought. More frequently than not, a design can be found which is both efficient from a functional view point and relatively easy to manufacture.