Several special casting processes have been developed to meet the specific casting requirements. These special processes can be classified into two categories based on the use of mould as below:

• Expendable-Mould Special Casting Processes
• Multiple-Use-Mould Special Casting Processes

Expendable-Mould Special Casting Processes

Under this category, the special casting processes in which mould is made for every casting and broken after solidification of casting are covered. Some of these processes are listed as below:

1. Investment Casting
2. Plaster Mould casting
3. Ceramic Shell Moulding
4. Evaporative Pattern Casting
5. Graphite Mould Casting
6. Rubber Plaster Mould Casting

Multiple-Use-Mould Special Casting Processes

Under this category, the special casting processes with multiple use moulds are covered. Some of these are listed as below:

1. Centrifugal Casting
2. Continuous Casting
3. Squeeze Casting
4. Vacuum Sealed Moulding
5. Slush Casting
6. Semi Solid Metal Casting

Investment casting is a casting process based on lost-wax, one of the oldest known metal-forming techniques. Investment casting was developed over 5500 years ago and can trace its roots back to both ancient Egypt and China. Parts manufactured in industry by this process include dental fixtures, gears, cams, ratchets, jewellery, turbine blades, machinery components and other parts of complex geometry.

Products such as rocket components and jet engine turbine blades required the fabrication of high precision complex shapes from high-melting-point metals that are not easily machined. Investment casting offers almost unlimited freedom in both the complexity of shapes and the types of materials that can be cast. Many materials are suitable for investment casting; examples are aluminium, copper, and steel; also performed with stainless steel, nickel, magnesium, and the precious metals.

Investment casting derives its name from the pattern being invested (surrounded) with a refractory material. The following steps describe the investment casting process, which can take two to seven days to complete.

1. Produce a master pattern: An artist or mould-maker creates an original pattern from wax, clay, wood, plastic, or another material.
2. Create a mould From Master Pattern: A mould, known as the master die, is made to fit the master pattern. If the master pattern was made from steel, the master die can be cast directly from the pattern using metal with a lower melting point. Rubber moulds can also be cast directly from the master pattern. Alternatively, a master die can be machined independently—without creating a master pattern.
3. Produce wax patterns: Although called wax patterns, pattern materials may also include plastic and frozen mercury. Patterns are made by pouring molten wax into the master die, or injecting it under pressure (injection moulding), and allowing it to harden. Release agents, such as silicone sprays, are used to assist in pattern removal. The polystyrene plastic may be preferred for producing thin and complex surfaces, where its higher strength and greater durability are desired. Frozen mercury is seldom used because of its cost, handling problems, and toxicity. If a core is required, there are two options: soluble wax or ceramic. Soluble wax cores are designed to melt out of the investment coating with the rest of the wax pattern; ceramic cores are removed after the product has hardened.
4. Assemble the wax patterns onto a common wax sprue: Multiple wax patterns can be created and assembled into one large pattern to be cast in one batch pour. Using heated tools and melted wax, patterns are attached to a central wax sprue to create a pattern cluster, or tree. As many as several hundred patterns can be assembled into a tree. Wax patterns can also be chased, which means parting lines or flashings are rubbed out using the heated metal tool. Finally, patterns are dressed (by removing imperfections) to look like finished pieces. If the product is sufficiently complex that its pattern could not be withdrawn from a single master die, the pattern may be made in pieces and assembled prior to attachment.
5. "Investing'', or covering the pattern assembly with refractory slurry: The ceramic mould, known as the investment, is produced by repeating a series of steps—coating, stuccoing, and hardening—until a desired thickness is achieved. Coating involves dipping a pattern cluster into a slurry of fine refractory material and then draining to create a uniform surface coating. Fine materials are used in this first step, also called a prime coat, to preserve fine details from the mould. Stuccoing applies coarse ceramic particles by dipping patterns into a fluidised bed, placing it in a rainfall-sander, or by applying materials by hand. Hardening allows coatings to cure. These steps are repeated until the investment reaches its required thickness—usually 5 to 15 mm. Investment moulds are left to dry completely, which can take 16 to 48 hours. Drying can be accelerated by applying a vacuum or minimizing environmental humidity. Investment moulds can also be created by placing the pattern clusters into a flask and then pouring liquid investment material from above. The flask is then vibrated to allow entrapped air to escape and help the investment material fill any small voids. Common refractory materials used to create the investments are: silica, zircon, various aluminium silicates, and alumina. Silica is usually used in the fused silica form, but sometimes quartz is used because it is less expensive.
6. Dewax: Once ceramic moulds have fully cured, they are turned upside-down and placed in a furnace or autoclave to melt out and/or vaporize the wax. Most shell failures occur at this point because the waxes used have a thermal expansion coefficient that is much greater than the investment material surrounding it, as the wax is heated it expands and introduces stress. To minimize these stresses, the wax is heated as rapidly as possible so that outer wax surfaces can melt and drain quickly, making space for the rest of the wax to expand. In certain situations, holes may be drilled into the mould before heating to help reduce these stresses. Any wax that runs out of the mould is usually recovered and reused.
7. Burnout preheating: The mould is then subjected to a burnout, which heats the mould to between 550 °C and 1100 °C to remove any moisture and residual wax, and to sinter the mould. Sometimes this heating is also used to preheat the mould before pouring, but other times the mould is allowed to cool so that it can be tested. Preheating allows the metal to stay liquid longer so that it can better fill all mould details and increase dimensional accuracy. If the mould is left to cool, any cracks found can be repaired with ceramic slurry or special cements.
8. Pouring: The investment mould is then placed open-side up into a tub filled with sand. The metal may be gravity poured or forced by applying positive air pressure or other forces. Vacuum casting, tilt casting, pressure assisted pouring and centrifugal casting are methods that use additional forces and are especially useful when moulds contain thin sections that would be otherwise be difficult to fill.
9. Knockout, cut-off and finishing: After solidification, techniques such as mechanical chipping or vibration, high-pressure water jet, sand blasting, or chemically dissolving (sometimes with liquid nitrogen) are used to release the casting. The sprue is cut off and recycled. The casting may then be cleaned up to remove signs of the casting process, usually by grinding.

The process can be used for both small castings of a few grams and large castings weighing up-to 5 Kg. It can be more expensive than die casting or sand casting, but per-unit costs decrease with large volumes. Investment casting can produce complicated shapes that would be difficult or impossible with other casting methods.

The fragile wax patterns must withstand forces encountered during the mould making. Much of the wax used in investment casting can be reclaimed and reused.
Due to the hardness of refractory materials used, investment casting can produce products with exceptional surface qualities, which can reduce the need for secondary machine processes

Plaster mould casting is a manufacturing process having a similar technique to sand casting. Plaster of Paris is used to form the mould for the casting, instead of sand. In industry parts such as valves, tooling, gears, fittings, ornaments and lock components may be manufactured by plaster mould casting.

Like sand casting, plaster mould casting is an expendable mould process, however, it can only be used with non-ferrous materials due to low refractoriness of plasters. At the high temperatures of ferrous metal casting, the plaster would first undergo a phase transformation and then melt, and the water of hydration can cause the mould to explode. It is used for castings as small as 30 g to as large as 45 kg.

Moulding Process

In plaster moulding, the mould material is plaster of pairs (also known as calcium sulphate or gypsum), combined with various additives to improve green strength, dry strength, permeability, and castability. Talc or magnesium oxide are added to prevent cracking and reduce setting time, lime and cement limit expansion during baking, glass fibres increase strength, and sand can be used as a filler. The ratio of ingredients is 70-80\% gypsum and 20-30\% additives.

The mould material is first mixed with water, and the pattern is sprayed with a thin film of the parting compound to prevent the plaster from sticking to the pattern. The creamy slurry is then poured over a metal pattern (wood patterns tend to warp or swell) and the unit shook so that the plaster fills any small features. The plaster sets, usually in about 15 minutes, and the pattern is removed. Hydration of the plaster produces a hard mould that can be easily stripped from the pattern. The mould is then baked, between 120 \(^{\circ}\)C and 260 \(^{\circ}\)C, to remove any excess water. The dried mould is then assembled, preheated, and the metal is poured. Finally, after the metal has solidified, the plaster is broken from the cast part. The used plaster cannot be reused.

The pattern is usually made from metal, however, rubber patterns may be used for complex geometry. For example, if the casting includes re-entrant angles or complex angular surfaces then the rubber is flexible enough to be removed, unlike metal.

The process of ceramic mould casting is like the process of plaster mould casting but can cast materials of much higher melting temperatures. Instead of using plaster to create the mould, ceramic moulding uses refractory ceramics as the mould material. Parts such as machining cutters, dies for metalworking, metal moulds, and impellers can be manufactured by this process.

There are two types of ceramic shell moulding: The Shaw process and the Unicast process

Evaporative-pattern casting is a type of casting process that uses a pattern made from a material that will evaporate when the molten metal is poured into the moulding cavity. In this method, expanded polystyrene is used to prepare the complete pattern including the gates and risers.

The two major evaporative-pattern casting processes, which are widely used because intricate design can be cast with relative ease and with reasonable expense are:

• Lost-foam casting
• Full-mould casting

The main difference is that lost-foam casting uses an unbounded and full-mould casting uses a bonded sand (or green sand). Because this difference is quite small there is much overlap in the terminolog.

Graphite mould casting technique is a modern affordable, quick and perfect casting methodology to produce graphite moulds. Aluminium alloy and special zinc-aluminium alloys like ZA-12 can be the best cast using this casting process. For metals such as titanium, which tend to react with many of the more common mould materials this is the suitable process. Graphite mould casting is a cheaper alternative to other casting methods which helps mass production of quality casting like camera housings.

Graphite Mould for Continuous Casting

Graphite mould is used for the continuous casting process. In a continuous casting process, the pouring and solidification go non-stops with the casted product being continuously withdrawn from the open end of the mould.

Applications

Railroad car wheels are cast in graphite mould accurately that requires no further machining. High-tech camera housings are perfectly cast using ZA-12 as alloy and graphite mould for the casting procedure. Graphite is widely used as a material for Continuous casting moulds.

Rubber Plaster Moulding (RPM), also known as plaster mould casting, is an appropriate substitute for investment casting, sand casting and prototype die casting. This process is suitable for both prototype and short run production quantities of aluminium and zinc parts. Rubber plaster moulding is proffered to another casting for its speed and the superior quality that is achieved in the castings. Due to the fragility of the mould, only nonferrous metals like aluminium, zinc casting alloys and some copper based alloys can be cast by this technique.

Applications of Rubber Plaster Moulds

Rubber plaster moulds process is particularly suitable for castings with extremely thin sections. This process is suitable for both prototype and short run production quantities of aluminium and zinc parts. Castings for telecommunications, business machines, medical equipment, computers, automotive, aerospace, electronics and robotics etc. are cast with accuracy and excellent surface finish

In this process, the mould is rotated rapidly about its central axis as the metal is poured into it. Because of the centrifugal force, a continuous pressure will be acting on the metal as it solidifies. The slag, oxides and other inclusions being lighter, get separated from the metal and segregate towards the centre.

There are three different types of centrifugal casting process as listed below:

1. True Centrifugal Casting Process
2. Semi-Centrifugal Casting Process
3. Centrifuging

True Centrifugal Casting Process

In true centrifugal casting, a dry-sand, graphite, or metal mould is rotated about either a vertical, horizontal or an inclined axis or about its horizontal and vertical axes simultaneously at pre-defined speed. As the molten metal is introduced, it is flung to the surface of the mould, where it solidifies into some form of the hollow product. The exterior profile is usually round, but hexagons and other symmetrical shapes are also possible. 

The positioning of the axis of the rotation is affected by the radial forces and length of the product. If the product is long, where length is more than four times the bore diameter, the axis of rotation is kept horizontal.

Maintenance of proper spinning speed is important. A lower speed will result in slipping and raining of the metal which will not adhere to the mould surface. If the speed is higher than the required speed it will cause hot tears on the wall of the product. The speed of rotation is calculated to obtain a radial force of the molten metal which is equal to 75-100 times the force of gravity in the case of sand moulds and about 60 times in case of metal moulds.

The solidification begins at the outer surface. Centrifugal force continues to feed molten metal as solidification progresses inward. Since the process compensates for shrinkage, no risers are required. Due to centrifugal force, the metal is forced against the outer wall of the mould and the final product has a strong, dense exterior.

The slag, oxides and other inclusions being lighter, get separated from the metal and segregate towards the centre. This process is normally used for the making of hollow pipes, tubes, hollow bushes, pressure vessels, cylinder liners, brake drums, the starting material for bearing rings etc., which are axisymmetric with a concentric hole. Since the metal is always pushed outward because of the centrifugal force, no core needs to be used for making the concentric hole.

When rotation is about the horizontal axis, the inner surface is always cylindrical. If the mould is oriented vertically, gravitational forces cause the inner surface to become parabolic, with the exact shape being a function of the speed of rotation. Wall thickness can be controlled by varying the amount of metal that is introduced into the mould.

The equipment is rather specialized and can be quite expensive for large castings. The permanent moulds can also be expensive, but they offer a long service life, especially when coated with some form of refractory dust or wash. Since no sprues, gates, or risers are required, yields can be greater than 90\%. Composite products can also be made by centrifugal casting of a second material on the inside surface of an already-cast product.

Semi-Centrifugal Casting Process

The semi-centrifugal process is used for products which are more complex than the products possible in the true centrifugal process but are symmetrical about their axis, for example, pulleys, wheels, gears and propellers etc. The mould rotates about the vertical axis and the metal is poured to the central pouring basin from where it flows to central sprue and enters to the hub and then it is forced outwards to the rim by centrifugal force.

If the central bore is required in the casting, a core is placed at the centre.  The rotational speeds are usually lower than for true centrifugal casting.

For larger production rates, the moulds can be stacked one over other, so they can be fed by a common pouring basin and sprue. The central sprue acts as a riser and must be large enough to ensure that it will be the last material to freeze. Since the lighter impurities concentrate in the centre, however, the process is best used for castings where the central region will ultimately be hollow.

Centrifuging

Centrifuging, or centrifuge centrifugal casting uses centrifugal action to force metal from a central pouring reservoir or sprue, through spoke-type runners, into separate mould cavities that are offset from the axis of rotation. Relatively low rotational speeds are required to produce sound castings with thin walls and intricate shapes. Centrifuging is often used to assist in the pouring of multiple-product investment casting trees.

The castings produced are not spun about their own axis and the pouring pressure used is not the same for all castings. Thus it is not a true centrifugal casting process. When a large number of small-sized castings are required, stack moulding is advantageous. The arrangement of castings in such a case is called as “Christmas Tree Formation”.
 

Continuous Casting is most commonly used in steel industries. In this process, the molten steel is solidified into a "semi-finished'' billet, bloom, or slab for subsequent rolling in the finishing mills. Prior to the introduction of Continuous Casting in the 1950s, steel was poured into stationary moulds to form "ingots''. Since then, "continuous casting'' has evolved to achieve improved yield, quality, productivity and cost efficiency.

Process

Molten metal is tapped into the ladle from furnaces. After undergoing any ladle treatments, such as alloying and degassing, and arriving at the correct temperature, the ladle is transported to the top of the casting machine. Usually, the ladle sits in a slot on a rotating turret at the casting machine. One ladle is in the 'on-cast' position (feeding the casting machine) while the other is made ready in the 'off-cast' position, and is switched to the casting position when the first ladle is empty.

From the ladle, the hot metal is transferred via a refractory shroud (pipe) to a holding bath called a tundish. The tundish works as a reservoir of metal to feed the casting machine while ladles are switched, thus acting as a buffer of hot metal, as well as smoothing out flow, regulating metal feed to the moulds and cleaning the metal.

Metal is drained from the tundish through another shroud into the top of an open-base copper mould. The mould is usually 300-350 mm long and the internal shape corresponds to that of the cross section of the casting required. The mould is water-cooled to solidify the hot metal directly in contact with it, this is the primary cooling process. The mould also oscillates vertically (or in a near vertical curved path) to prevent the metal sticking to the mould walls. A lubricant (either powder that melts on contact with the metal, or liquids) is added to the metal in the mould to prevent sticking. In some cases, shrouds may not be used between tundish and mould ('open-pour' casting), in this case, interchangeable metering nozzles in the base of the tundish direct the metal into the moulds. Some continuous casting layouts feed several moulds from the same tundish.

In the mould, a thin shell of metal next to the mould walls solidifies before the middle section, which is called a strand, exits the base of the mould into a spray chamber. The bulk of metal within the walls of the strand is still molten. The strand is immediately supported by closely spaced, water-cooled rollers which support the walls of the strand against the Ferro static pressure (compare hydrostatic pressure) of the still-solidifying liquid within the strand. To increase the rate of solidification, the strand is sprayed with water as it passes through the spray-chamber, this is the secondary cooling process. Final solidification of the strand may take place after the strand has exited the spray-chamber.

The process is carried out either wholly along the vertical axis or partly along vertical and partly along the horizontal axis which is called as “curved apron”. In a curved apron casting machine, the strand exits the mould vertically (or on a near vertical curved path) and as it travels through the spray-chamber, the rollers gradually curve the strand towards the horizontal. In a vertical casting machine, the strand stays vertical as it passes through the spray-chamber. Moulds in a curved apron casting machine can be straight or curved, depending on the basic design of the machine.

In a true horizontal casting machine, the mould axis is horizontal and the flow of steel is horizontal from liquid to thin shell and then to solid (no bending). In this type of machine, either strand or mould oscillation is used to prevent sticking in the mould.

After exiting the spray-chamber, the strand passes through straightening rolls (if cast on other than a vertical machine) and withdrawal rolls. There may be a hot rolling stand after withdrawal to take advantage of the metal's hot condition to pre-shape the final strand. Finally, the strand is cut into predetermined lengths by mechanical shears or by travelling oxyacetylene torches, is marked for identification, and is taken either to a stockpile or to the next forming process.

In many cases, the strand may continue through additional rollers and other mechanisms which may flatten, roll or extrude the metal into its final shape.

 

Squeeze casting is a combination of casting and forging process. The process can result in the highest mechanical properties attainable in a cast product. This process enables the production of high quality, near-net-shape, thin-walled parts with good surface finish and dimensional precision as well as properties that approach those of forgings. While the majority of applications involve alloys of aluminium, each of the processes has been successfully applied to magnesium, zinc, copper, and a limited number of ferrous alloys.

Process

The molten metal is poured into the die while the punch is initially separated. Then the punch is lowered to form a tight seal. The punch portion of the upper die is then forced to squeeze the casting while it solidifies. The pressure is applied on to the molten metal at the precise time when the metal temperature at the metal and die interface has reached the solidus temperature. Delay may necessitate the use of higher pressure, and premature application may cause coarse and uneven surface and ragged edges. The time for compression should be such that complete solidification takes place without any air gap. After withdrawal from the die, the casting is cooled in hot sand.

While the squeeze casting process is most commonly applied to aluminium and magnesium castings, it has also been adapted to the production of metal-matrix composites where the pressurized metal is forced around or through foamed or fibre reinforcements that have been positioned in the mould.

In low-pressure and vacuum permanent-mould casting, the mould is turned upside down and positioned above a sealed, airtight chamber that contains a crucible of molten metal. A small pressure difference then causes the molten metal to flow upward into the die cavity.

In the low-pressure permanent-mould (LPPM) process  a low-pressure gas (3 to 15 psi) is introduced into a sealed chamber, driving molten metal up through a refractory fill tube and into the gating system or cavity of a metal mould. This metal is exceptionally clean, since it flows from the centre of the melt and is fed directly into the mould (a distance of about 10 cm, or 3 to 4 in.), never passing through the atmosphere. Product quality is further enhanced by the non-turbulent mould filling, which helps to minimize gas porosity and dross formation.

Through design and cooling, the products directionally solidify from the top to down. The molten metal in the pressurized fill tube acts as a riser to continually feed the casting during solidification. When solidification is complete, the pressure is released and the unused metal in the feed tube simply drops back into the crucible. The reuse of this metal, coupled with the absence of additional risers, leads to yields that are often greater than 85%.

Nearly all low-pressure permanent-mould castings are made from aluminium or magnesium, but some copper-based alloys can also be used. Mechanical properties are typically about 5% better than those of conventional permanent-mould castings. Cycle times are somewhat longer than those of conventional permanent moulding.

A similar variation of permanent-mould casting, where a vacuum is drawn on the die assembly and atmospheric pressure in the chamber forces the metal upward it is known as Vacuum Permanent-Mould Casting.

All of the benefits and features of the low-pressure process are retained, including the subsurface extraction of molten metal from the melt, the bottom feed to the mould, the minimal metal disturbance during pouring, the self-rising action, and the downward directional solidification. Thin-walled castings can be produced with high metal yield and excellent surface quality. Because of the vacuum, the cleanliness of the metal and the dissolved gas content are superior to that of the low-pressure process. Final castings typically range from 0.2 to 5 kg and have mechanical properties that are even better than those of the low-pressure permanent-mould products.

Slush casting is a variant of permanent moulding casting to create a hollow casting or hollow cast. In the process, the material is poured into the mould and allowed to cool until a shell of material forms in the mould. The remaining liquid is then poured out to leave a hollow shell. The resulting casting has good surface detail but variable wall thickness. The process is usually used to cast ornamental products, such as candlesticks, lamp bases, and statuary from low-melting-temperature materials.

Similarly, a process called slush moulding is used in automotive thermoplastic dashboard manufacture, where a liquid resin is poured into a hot, hollow mould and viscous skin forms; excess slush is drained off, the mould is cooled, and the moulded product is stripped out.

For most alloy compositions, there is a range of temperatures where liquid and solid coexist, and several techniques have been developed to produce shapes from this semisolid material.

Semi-solid metal casting (SSM) is a near net shape variant of die casting. The process is used with non-ferrous metals, such as aluminium, copper, and magnesium. The process combines the advantages of casting and forging. The process is named after the fluid property thixotropy, which is the phenomenon that allows this process to work. Simply, thixotropic fluids shear when the material flows, but thicken when standing. There are four different processes:

1. Rheocasting
2. Thixocasting
3. Thixomolding, and
4. SIMA.

Semi-solid casting is typically used for high-end castings. For aluminium alloys, typical parts include engine suspension mounts, air manifold sensor harness, engine blocks and oil pump filter housing.

Rheocasting Process

In the rheocasting process, molten metal is cooled to the semisolid state with constant stirring. The stirring or shearing action breaks up the dendrites, producing a slurry of rounded particles of solid in a liquid melt. This slurry, with about a 30\% solid content, can be readily shaped by high-pressure injection into metal dies. Because the slurry contains no superheat and is already partially solidified, it freezes quickly.

Thixocasting Process

In the thixocasting variation, there is no handling of molten metal. The material is first subjected to special processing (stirring during solidification as in Rheocasting) to produce solid blocks or bars with a nondendritic structure. When reheated to the semi-solid condition, the thixotropic material can be handled like a solid but flows like a liquid when agitated or squeezed. The solid material is then cut to prescribed length, reheated to a semisolid state where the material is about 40\% liquid and 60\% solid, mechanically transferred to the shot chamber of a cold-chamber die-casting machine, and injected under pressure.

The main disadvantage is that it is expensive due to the special billets that must be used. Other disadvantages include a limited number of alloys, and scrap cannot be directly reused.

Thixomolding

Thixomolding uses a machine similar to injection moulding. In a single step process, solid metal granules or pellets at room temperature are fed into a barrel chamber, where rotating screw shears and advance the material through heating zones that raise the temperature to the semisolid region. When a sufficient volume of thixotropic material has accumulated at the end of the barrel, a shot system drives it into the die or mould at velocities of 1 to 2.5 m/sec. The injection system of this process is a combination of the screw feed used in plastic injection moulding and the plunger used in conventional die casting.

SIMA

In the SIMA method, the material is first heated to the semi-solid temperature. As it nears the solidus temperature the grains recrystallize to form a fine grain structure. After the solidus temperature is passed the grain boundaries melt to form the SSM microstructure. For this method to work, the material should be extruded or cold rolled in the half-hard tempered state. This method is limited in size to bar diameters smaller than 37 mm (1.5 in), because of this only smaller parts can be cast.