DIFFERENCES BETWEEN METAL FORMING PROCESS AND CASTING PROCESS

Metal casting process Differences Metal forming processCasting is a manufacturing process where a solid is melted, heated to proper temperature (sometimes treated to modify its chemical composition), and is then poured into a cavity or mould, which contains it in the proper shape during solidification. Thus, in a single step, simple or complex shapes can be made from any metal that can be melted. The resulting product can have virtually any configuration the designer desires.

Working Principle

The plastic deformation of a metal in order to produce a useful shape. Metal forming is done at elevated or hot-working temperatures with processes such as forging, or it is done at cold-working room temperatures with processes such as stamping or bending.

OBTAINING THE CASTING GEOMETRY-The traditional method of obtaining the casting geometry is by sending blueprint drawings to the foundry. This is usually done during the request for quotation process. However, more and more customers and foundries are exchanging part geometry via the exchange of computer aided design files.

PATTERNMARKING-The pattern is a physical model of the casting used to make the mould.The mould is made by packing some readily formed aggregate material, such as moulding sand, around the pattern. When the pattern is withdrawn, its imprint provides the mould cavity, which is ultimately filled with metal to become the casting.If the casting is to be hollow, as in the case of pipe fittings, additional patterns, referred to as cores, are used to form these cavities.

COREMAKING-Cores are forms, usually made of sand, which are placed into a mould cavity to form the interior surfaces of castings. Thus the void space between the core and mould-cavity surface is what eventually becomes the casting.

MOULDING-Moulding consists of all operations necessary to prepare a mould for receiving molten metal. Moulding usually involves placing a

Procedures Metal-forming processes: Processes that cause changes in the shape of solid metal articles via plastic (permanent) deformations.

Drawing: Metal forming process whereby the workpiece is a shaped longitudinal prism that undergoes a reduction and change in its cross section area and shape while being pulled through a shaped converging die.

Extrusion: Metal forming process whereby the workpiece is placed in a chamber with an opening and is forced to escape through the opening, usually being pushed out by a mandrel.

Forging: Metal forming process whereby the workpiece is placed between an anvil and a hammer and subjected to compressive force between them.

Rolling: Metal forming process whereby the workpiece is a longitudinal prism, which is placed between two opposing circular rolls that rotate in opposite directions, drag the workpiece along, and force it to reduce in cross section.

Metal forming is normally performed after the primary

moulding aggregate around a pattern held with a supporting frame, withdrawing the pattern to leave the mould cavity, setting the cores in the mould cavity and finishing and closing the mould.

MELTING AND POURING-The preparation of molten metal for casting is referred to simply as melting. Melting is usually done in a specifically designated area of the foundry, and the molten metal is transferred to the pouring area where the moulds are filled.

CLEANING-Cleaning refers to all operations necessary to the removal of sand, scale, and excess metal from the casting. The casting is separated from the mould and transported to the cleaning department. Burned-on sand and scale are removed to improve the surface appearance of the casting. Excess metal, in the form of fins, wires, parting line fins, and gates, is removed. Castings may be upgraded by welding or other procedures. Inspection of the casting for defects and general quality is performed.

OTHER PROCESS-Before shipment, further processing such as heat-treatment, surface treatment, additional inspection, or machining may be performed as required by the customer's specifications.

processes of extraction, casting, and powder compaction and before the finishing processes of metal cutting, grinding, polishing, painting, and assembly. With few exceptions, the bulk of the products of the metal fabrication industry are shaped by forming or a combination of forming and other processes like metal cutting or joining. Forming operations are classified as those processes where the desired shape is achieved by imparting plastic deformation to the workpiece in the solid state. Classification by (1) product, (2) material, (3) forming temperature, and (4) Nature of deformation (sheet metal versus bulk deformation) can also be helpful.

However, the boundaries between categories are not perfectly defined. 

-the most intricate of shapes, both external and internal, may be cast. As a result, many other operations, such as machining, forging and welding can be minimized or eliminated.-due to their physical properties, some metals can only be cast to shape (since they cannot be hot-worked into bars, rods, plates, or other shapes) from ingot form as a preliminary to other processing.-construction may be simplified. Objects may be cast in a single piece which would otherwise require assembly of several pieces if made by other methods.-casting is a process highly adapted to the requirements of mass

Advantages -no or very small loss of material-little or no scrap-increase in ductility (hot forming of cast ingots)-increase in strength and hardness-high production rate, generate final shape in short time-better mechanical and metallurgical properties (strength, toughness, grain size)

production. Large numbers of a given casting may be produced very rapidly. -large and heavy objects may be cast when they would be difficult or uneconomic to produces otherwise. -limitation on mechanical properties-porosity-surface finish-dimensional accuracy-safety hazards to humans-environmental concerns

Limitations -equipment expensive because of the large forces involved-suited for a large number of parts only -Large capital expenditure because of heavy presses and die

-more uniform properties from a directional standpoint-strength and lightness in certain light metal alloys, which can be produced only as castings.-good bearing qualities are obtained in casting metals

Quality -near net-shaping forming

METAL CASTING (EXPANDABLE AND NON EXPANDABLE MOULD)

Expandable mouldExpendable mould casting or “one use mould” is a generic classification that includes sand, plastic, shell, plaster, and investment (lost-wax technique) mouldings. All of these involve the use of temporary and non-reusable moulds, and need gravity to help force molten fluid into casting cavities. In this process the mould is used only once.

Non-Expandable mouldNon-expendable mould casting differs from expendable processes in that the mould need not be reformed after each production cycle. This technique includes at least four different methods: permanent, die, centrifugal, and continuous casting. This form of casting also results in improved repeatability in parts produced and delivers Near Net Shape results.

TYPES OF CASTING PROCESS FOR EXPANDABLE AND NON EXPANDABLE MOULD CASTING

Expandable mould

Sand CastingA sand casting or a sand moulded casting is a cast part produced by forming a mould from a sand mixture and then pouring molten liquid metal into the cavity in the mould. The mould is then cooled until the metal has solidified. In the last stage the casting is separated from the mould. There are six steps in this process:

1. Place a pattern in sand to create a mould.2. Incorporate a gating system.3. Remove the pattern.4. Fill the mould cavity with molten metal.5. Allow the metal to cool.6. Break away the sand mould and remove the casting

Figure: Sand casting overview Equipment

Mould  

In sand casting, the primary piece of equipment is the mould, which contains several components. The mould is divided into two halves - the cope (upper half) and the drag (bottom half), which meet along a parting line. Both mould halves are contained inside a box, called a flask, which itself is divided along this parting line. The mould cavity is formed by packing sand around the pattern in each half of the flask. The sand can be packed by hand, but machines that use pressure or impact ensure even packing of the sand and require far less time, thus increasing the production rate. After the sand has been packed and the pattern is removed, a cavity will remain that forms the external shape of the casting. Some internal surfaces of the casting may be formed by cores.

Cores are additional pieces that form the internal holes and passages of the casting. Cores are typically made out of sand so that they can be shaken out of the casting, rather than require the necessary geometry to slide out. As a result, sand cores allow for the fabrication of many complex internal features. Each core is positioned in the mould before the molten metal is poured. In order to keep each core in place, the pattern has recesses called core prints where the core can be anchored in place. However, the core may still shift due to buoyancy in the molten metal. Further support is provided to the cores by chaplets. These are small metal pieces that are fastened between the core and the cavity surface. Chaplets must be made of a metal with a higher melting temperature than that of the metal being cast in order to maintain their structure. After solidification, the chaplets will have been cast inside the casting and the excess material of the chaplets that protrudes must be cut off.  

In addition to the external and internal features of the casting, other features must be incorporated into the mould to accommodate the flow of molten metal. The molten metal is poured into a pouring basin, which is a large depression in the top of the sand mould. The molten metal funnels out of the bottom of this basin and down the main channel, called the sprue. The sprue then connects to a series of channels, called runners, which carries the molten metal into the cavity. At the end of each runner, the molten metal enters the cavity through a gate which controls the flow rate

and minimizes turbulence. Often connected to the runner system are risers. Risers are chambers that fill with molten metal, providing an additional source of metal during solidification. When the casting cools, the molten metal will shrink and additional material is needed. A similar feature that aids in reducing shrinkage is an open riser. The first material to enter the cavity is allowed to pass completely through and enter the open riser. This strategy prevents early solidification of the molten metal and provides a source of material to compensate for shrinkage. Lastly, small channels are included that run from the cavity to the exterior of the mould. These channels act as venting holes to allow gases to escape the cavity. The porosity of the sand also allows air to escape, but additional vents are sometimes needed. The molten metal that flows through all of the channels (sprue, runners, and risers) will solidify attached to the casting and must be separated from the part after it is removed.

Sand  

The sand that is used to create the moulds is typically silica sand (SiO2) that is mixed with a type of binder to help maintain the shape of the mould cavity. Using sand as the mould material offers several benefits to the casting process. Sand is very inexpensive and is resistant to high temperatures, allowing many metals to be cast that have high melting temperatures. There are different preparations of the sand for the mould, which characterize the following four unique types of sand moulds.

o Greensand mould - Greensand moulds use a mixture of sand, water, and a clay or binder. Typical composition of the mixture is 90% sand, 3% water, and 7% clay or binder. Greensand moulds are the least expensive and most widely used.

o Skin-dried mould - A skin-dried mould begins like a greensand mould, but additional bonding materials are added and the cavity surface is dried by a torch or heating lamp to increase mould strength. Doing so also improves the dimensional accuracy and surface finish, but will lower the collapsibility.  Dry skin moulds are more expensive and require more time, thus lowering the production rate.

o Dry sand mould - In a dry sand mould, sometimes called a cold box mould, the sand is mixed only with an organic binder. The mould is strengthened by baking it in an oven. The resulting mould has high dimensional accuracy, but is expensive and results in a lower production rate.

o No-bake mould - The sand in a no-bake mould is mixed with a liquid resin and hardens at room temperature.

The quality of the sand that is used also greatly affects the quality of the casting and is usually described by the following five measures:

o Strength - Ability of the sand to maintain its shape.

o Permeability - Ability to allow venting of trapped gases through the sand. A higher permeability can reduce the porosity of the mould, but a lower permeability can result in a better surface finish. Permeability is determined by the size and shape of the sand grains.

o Thermal stability - Ability to resist damage, such as cracking, from the heat of the molten metal.

o Collapsibility - Ability of the sand to collapse, or more accurately compress, during solidification of the casting. If the sand can not compress, then the casting will not be able to shrink freely in the mould and can result in cracking.

o Reusability - Ability of the sand to be reused for future sand moulds.

Advantages

1. Can produce very large parts2. Can form complex shapes3. Many material options4. Low tooling and equipment cost5. Scrap can be recycled6. Short lead time possible

Disadvantages

1. Poor material strength2. High porosity possible3. Poor surface finish and tolerance4. Secondary machining often required5. Low production rate6. High labour cost

Shell Casting

Shell mould casting is a metal casting process similar to sand casting, in that molten metal is poured into an expendable mould. However, in shell mould casting, the mould is a thin-walled shell created from applying a sand-resin mixture around a pattern. The pattern, a metal piece in the shape of the desired part, is reused to form multiple shell moulds. A reusable pattern allows for higher production rates, while the disposable moulds enable complex geometries to be cast. Shell mould casting requires the use of a metal pattern, oven, sand-resin mixture, dump box, and molten metal. 

Shell mould casting allows the use of both ferrous and non-ferrous metals, most commonly using cast iron, carbon steel, alloy steel, stainless steel, aluminium alloys, and copper alloys. Typical parts are small-to-medium in size and require high accuracy, such as gear housings, cylinder heads, connecting rods, and lever arms. 

The shell mould casting process consists of the following steps:

1. Pattern creation - A two-piece metal pattern is created in the shape of the desired part, typically from iron or steel. Other materials are sometimes used, such as aluminium for low volume production or graphite for casting reactive materials.

2. Mould creation - First, each pattern half is heated to 175-370°C (350-700°F) and coated with a lubricant to facilitate removal. Next, the heated pattern is clamped to a dump box, which contains a mixture of sand and a resin binder. The dump box is inverted, allowing this sand-resin mixture to coat the pattern. The heated pattern partially cures the mixture, which now forms a shell around the pattern. Each pattern half and surrounding shell is cured to completion in an oven and then the shell is ejected from the pattern.

3. Mould assembly - The two shell halves are joined together and securely clamped to form the complete shell mould. If any cores are required, they are inserted prior to closing the mould. The shell mould is then placed into a flask and supported by a backing material.

4. Pouring - The mould is securely clamped together while the molten metal is poured from a ladle into the gating system and fills the mould cavity.

5. Cooling - After the mould has been filled, the molten metal is allowed to cool and solidify into the shape of the final casting.

6. Casting removal - After the molten metal has cooled, the mould can be broken and the casting removed. Trimming and cleaning processes are required to remove any excess metal from the feed system and any sand from the mould.

Figure: Shell moulding overview

Advantages

1. Can form complex shapes and fine details2. Very good surface finish3. High production rate4. Low labour cost5. Low tooling cost6. Little scrap generated

Disadvantages

1. High equipment cost2. Since the tooling requires heat to cure the mould, pattern costs and pattern

wear can be higher.3. Energy costs are higher.4. Material costs are higher than those for green sand moulding.

Investment Casting

Investment casting is one of the oldest manufacturing processes, dating back thousands of years, in which molten metal is poured into an expendable ceramic mould. The mould is formed by using a wax pattern - a disposable piece in the shape of the desired part. The pattern is surrounded, or "invested", into ceramic slurry that hardens into the mould. Investment casting is often referred to as "lost-wax casting" because the wax pattern is melted out of the mould after it has been formed. Lox-wax processes are one-to-one (one pattern creates one part), which increases production time and costs relative to other casting processes. However, since the mould is destroyed during the process, parts with complex geometries and intricate details can be created. 

Investment casting can make use of most metals, most commonly using aluminium alloys, bronze alloys, magnesium alloys, cast iron, stainless steel, and tool steel. This process is beneficial for casting metals with high melting temperatures that cannot be moulded in plaster or metal. Parts that are typically made by investment casting include those with complex geometry such as turbine blades or firearm components. High temperature applications are also common, which includes parts for the automotive, aircraft, and military industries. 

Investment casting requires the use of a metal die, wax, ceramic slurry, furnace, molten metal, and any machines needed for sandblasting, cutting, or grinding.

The process steps include the following:

1. Pattern creation - The wax patterns are typically injection moulded into a metal die and are formed as one piece. Cores may be used to form any internal features on the pattern. Several of these patterns are attached to a central wax gating system (sprue, runners, and risers), to form a tree-like assembly. The gating system forms the channels through which the molten metal will flow to the mould cavity.

2. Mould creation - This "pattern tree" is dipped into a slurry of fine ceramic particles, coated with more coarse particles, and then dried to form a ceramic shell around the patterns and gating system. This process is repeated until the shell is thick enough to withstand the molten metal it will encounter. The shell is then placed into an oven and the wax is melted out leaving a hollow ceramic shell that acts as a one-piece mould, hence the name "lost wax" casting.

3. Pouring - The mould is preheated in a furnace to approximately 1000°C (1832°F) and the molten metal is poured from a ladle into the gating system of the mould, filling the mould cavity. Pouring is typically achieved manually under the force of gravity, but other methods such as vacuum or pressure are sometimes used.

4. Cooling - After the mould has been filled, the molten metal is allowed to cool and solidify into the shape of the final casting. Cooling time depends on the thickness of the part, thickness of the mould, and the material used.

5. Casting removal - After the molten metal has cooled, the mould can be broken and the casting removed. The ceramic mould is typically broken using water jets, but several other methods exist. Once removed, the parts are separated from the gating system by either sawing or cold breaking (using liquid nitrogen).

6. Finishing - Often times, finishing operations such as grinding or sandblasting are used to smooth the part at the gates. Heat treatment is also sometimes used to harden the final part.

Figure: Investment casting overview

Advantages

1. Can form complex shapes and fine details2. Many material options3. High strength parts4. Very good surface finish and accuracy5. Little need for secondary machining

Disadvantages

1. Time-consuming process2. High labour cost3. High tooling cost4. Long lead time possible

Plaster Mould Casting

A durable plaster intermediate is often used as a stage toward the production of a bronze sculpture or as a pointing guide for the creation of a carved stone. With the completion of a plaster, the work is more durable (if stored indoors) than a clay original which must be kept moist to avoid cracking. With the low cost plaster at hand, the expensive work of bronze casting or stone carving may be deferred until a patron is found, and as such work is

considered to be a technical, rather than artistic process, it may even be deferred beyond the lifetime of the artist.In waste moulding a simple and thin plaster mould, reinforced by sisal or burlap, is cast over the original clay mixture. When cured, it is then removed from the damp clay, incidentally destroying the fine details in undercuts present in the clay, but which are now captured in the mould. The mould may then at any later time (but only once) be used to cast a plaster positive image, identical to the original clay. The surface of this plaster may be further refined and may be painted and waxed to resemble a finished bronze casting.The process steps include the following:

1. First, the plaster is mixed and the pattern is sprayed with a thin film of parting compound to prevent the plaster from sticking to the pattern.

2. The plaster is then poured over the pattern and the unit shaken so that the plaster fills any small features.

3. The plaster sets, usually in about 15 minutes, and the pattern are removed. 4. The mould is then baked, between 120 °C (248 °F) and 260 °C (500 °F), to remove

any excess water. 5. The dried mould is then assembled, preheated, and the metal poured. Finally, after

the metal has solidified, the plaster is broken from the cast part. 6. The used plaster cannot be reused.

Advantages

1. excellent surface finish and good dimensional accuracy2. can produces complex parts3. produces minimal scrap material

Disadvantages

1. can only be used with lower melting temperature non-ferrous materials2. long cooling times restrict production volume

Non-expandable Mould

Permanent Mould CastingPermanent mould casting is a metal casting process that shares similarities to both sand casting and die casting. As in sand casting, molten metal is poured into a mould which is clamped shut until the material cools and solidifies into the desired part shape. However, sand casting uses an expendable mould which is destroyed after each cycle. Permanent mould casting, like die casting, uses a metal mould (die) that is typically made from steel or cast iron and can be reused for several thousand cycles. Because the molten metal is poured into the die and not forcibly injected, permanent mould casting is often referred to as gravity die casting. 

Permanent mould casting is typically used for high-volume production of small, simple metal parts with uniform wall thickness. Non-ferrous metals are typically used in this process, such as aluminium alloys, magnesium alloys, and copper alloys. However, irons and steels can also be cast using graphite moulds. Common permanent mould parts include gears and gear housings, pipe fittings, and other automotive and aircraft components such as pistons,

impellers, and wheels. 

The permanent mould casting process consists of the following steps:

1. Mould preparation - First, the mould is pre-heated to around 300-500°F (150-260°C) to allow better metal flow and reduce defects. Then, a ceramic coating is applied to the mould cavity surfaces to facilitate part removal and increase the mould lifetime.

2. Mould assembly - The mould consists of at least two parts - the two mould halves and any cores used to form complex features. Such cores are typically made from iron or steel, but expendable sand cores are sometimes used. In this step, the cores are inserted and the mould halves are clamped together.

3. Pouring - The molten metal is poured at a slow rate from a ladle into the mould through a sprue at the top of the mould. The metal flows through a runner system and enters the mould cavity.

4. Cooling - The molten metal is allowed to cool and solidify in the mould.5. Mould opening - After the metal has solidified, the two mould halves are opened and the

casting is removed.6. Trimming - During cooling, the metal in the runner system and sprue solidify attached to

the casting. This excess material is now cut away.

Figure: Permanent mould casting overview

Using these basic steps, other variations on permanent mould casting have been developed to accommodate specific applications. Examples of these variations include the following:

1. Slush Casting - As in permanent mould casting, the molten metal is poured into the mould and begins to solidify at the cavity surface. When the amount of solidified material is equal to the desired wall thickness, the remaining slush (material that has yet to completely solidify) is poured out of the mould. As a result, slush casting is used to produce hollow parts without the use of cores.

2. Low Pressure Permanent Mould Casting - Instead of being poured, the molten metal is forced into the mould by low pressure air (< 1 bar). The application of pressure allows the mould to remain filled and reduces shrinkage during cooling. Also, finer details and thinner walls can be moulded.

3. Vacuum Permanent Mould Casting - Similar to low pressure casting, but vacuum pressure is used to fill the mould. As a result, finer details and thin walls can be moulded and the mechanical properties of the castings are improved.

Advantages

1. Can form complex shapes2. Good mechanical properties3. Many material options4. Low porosity5. Low labour cost6. Scrap can be recycled

Disadvantages

1. High tooling cost2. Long lead time possible

Die Casting

The die casting process forces molten metal under high pressure into mould cavities (which are machined into dies). Most die castings are made from nonferrous metals, specifically zinc, copper, and aluminium based alloys, but ferrous metal die castings are possible. The die casting method is especially suited for applications where many small to medium sized parts are needed with good detail, a fine surface quality and dimensional consistency. There are two main types of die casting machines - hot chamber machines (used for alloys with low melting temperatures, such as zinc) and cold chamber machines (used for alloys with high melting temperatures, such as aluminium). In both machines, after the molten metal is injected into the dies, it rapidly cools and solidifies into the final part, called the casting.

Figure: Die casting cold and hot chamber machine overview

The process cycle for die casting consists of five main stages, which are explained below. The total cycle time is very short, typically between 2 seconds and 1 minute.

1. Clamping - The first step is the preparation and clamping of the two halves of the die. Each die half is first cleaned from the previous injection and then lubricated to facilitate the ejection of

the next part. The lubrication time increases with part size, as well as the number of cavities and side-cores. Also, lubrication may not be required after each cycle, but after 2 or 3 cycles, depending upon the material. After lubrication, the two die halves, which are attached inside the die casting machine, are closed and securely clamped together. Sufficient force must be applied to the die to keep it securely closed while the metal is injected. The time required to close and clamp the die is dependent upon the machine - larger machines (those with greater clamping forces) will require more time. This time can be estimated from the dry cycle time of the machine.

2. Injection - The molten metal, which is maintained at a set temperature in the furnace, is next transferred into a chamber where it can be injected into the die. The method of transferring the molten metal is dependent upon the type of die casting machine, whether a hot chamber or cold chamber machine is being used. The difference in this equipment will be detailed in the next section. Once transferred, the molten metal is injected at high pressures into the die. Typical injection pressure ranges from 1,000 to 20,000 psi. This pressure holds the molten metal in the dies during solidification. The amount of metal that is injected into the die is referred to as the shot. The injection time is the time required for the molten metal to fill all of the channels and cavities in the die. This time is very short, typically less than 0.1 seconds, in order to prevent early solidification of any one part of the metal. The proper injection time can be determined by the thermodynamic properties of the material, as well as the wall thickness of the casting. A greater wall thickness will require a longer injection time. In the case where a cold chamber die casting machine is being used, the injection time must also include the time to manually ladle the molten metal into the shot chamber.

3. Cooling - The molten metal that is injected into the die will begin to cool and solidify once it enters the die cavity. When the entire cavity is filled and the molten metal solidifies, the final shape of the casting is formed. The die cannot be opened until the cooling time has elapsed and the casting is solidified. The cooling time can be estimated from several thermodynamic properties of the metal, the maximum wall thickness of the casting, and the complexity of the die. A greater wall thickness will require a longer cooling time. The geometric complexity of the die also requires a longer cooling time because the additional resistance to the flow of heat.

4. Ejection - After the predetermined cooling time has passed, the die halves can be opened and an ejection mechanism can push the casting out of the die cavity. The time to open the die can be estimated from the dry cycle time of the machine and the ejection time is determined by the size of the casting's envelope and should include time for the casting to fall free of the die. The ejection mechanism must apply some force to eject the part because during cooling the part shrinks and adheres to the die. Once the casting is ejected, the die can be clamped shut for the next injection.

5. Trimming - During cooling, the material in the channels of the die will solidify attached to the casting. This excess material, along with any flash that has occurred, must be trimmed from the casting either manually via cutting or sawing, or using a trimming press. The time required to trim the excess material can be estimated from the size of the casting's envelope. The scrap material that results from this trimming is either discarded or can be reused in the die casting process. Recycled material may need to be reconditioned to

the proper chemical composition before it can be combined with non-recycled metal and reused in the die casting process.

Advantages

1. Can produce large parts2. Can form complex shapes3. High strength parts4. Very good surface finish and accuracy5. High production rate6. Low labour cost7. Scrap can be recycled

Disadvantages

1. Trimming is required2. High tooling and equipment cost3. Limited die life4. Long lead time

Centrifugal Casting

Centrifugal casting is both gravity- and pressure-independent since it creates its own force feed using a temporary sand mould held in a spinning chamber at up to 900 N. Lead time varies with the application. Semi- and true-centrifugal processing permit 30-50 pieces/hr-mould to be produced, with a practical limit for batch processing of approximately 9000 kg total mass with a typical per-item limit of 2.3-4.5 kg.

Industrially, the centrifugal casting of railway wheels was an early application of the method developed by German industrial company Krupp and this capability enabled the rapid growth of the enterprise.

Small art pieces such as jewellery are often cast by this method using the lost wax process, as the forces enable the rather viscous liquid metals to flow through very small passages and into fine details such as leaves and petals. This effect is similar to the benefits from vacuum casting, also applied to jewellery casting.  The casting process is usually performed on a horizontal centrifugal casting machine (vertical machines are also available) and includes the following steps:

1. Mould preparation - The walls of a cylindrical mould are first coated with a refractory ceramic coating, which involves a few steps (application, rotation, drying, and baking). Once prepared and secured, the mould is rotated about its axis at high speeds (300-3000 RPM), typically around 1000 RPM.

2. Pouring - Molten metal is poured directly into the rotating mould, without the use of runners or a gating system. The centrifugal force drives the material towards the mould walls as the mould fills.

3. Cooling - With all of the molten metal in the mould, the mould remains spinning as the metal cools. Cooling begins quickly at the mould walls and proceeds inwards.

4. Casting removal - After the casting has cooled and solidified the rotation is stopped and the casting can be removed.

5. Finishing - While the centrifugal force drives the dense metal to the mould walls, any less dense impurities or bubbles flow to the inner surface of the casting. As a result, secondary processes such as machining, grinding, or sand-blasting, are required to clean and smooth the inner diameter of the part.

Figure: Centrifugal casting overview Advantages

1. Can form very large parts2. Good mechanical properties3. Good surface finish and accuracy4. Low equipment cost5. Low labour cost6. Little scrap generated

Disadvantages

1. Limited to cylindrical parts2. Secondary machining is often required for inner diameter3. Long lead time possible

Continuous Casting

Continuous casting is a refinement of the casting process for the continuous, high-volume production of metal sections with a constant cross-section. Molten metal is poured into an open-ended, water-cooled copper mould, which allows a 'skin' of solid metal to form over the still-liquid centre. The strand, as it is now called, is withdrawn from the mould and passed into a chamber of rollers and water sprays; the rollers support the thin skin of the strand while the sprays remove heat from the strand, gradually solidifying the strand from the outside in. After solidification, predetermined lengths of the strand are cut off by either mechanical shears or travelling oxyacetylene torches and transferred to further forming processes, or to a stockpile. Cast sizes can range from strip (a few millimetres thick by about five metres wide) to billets (90 to 160 mm square) to slabs (1.25 m wide by 230 mm thick). Sometimes, the strand may undergo an initial hot rolling process before being cut.

Continuous casting is used due to the lower costs associated with continuous production of a standard product, and also increases the quality of the final product. Metals such as steel, copper and aluminium are continuously cast, with steel being the metal with the greatest tonnages cast using this method.The continuous casting process includes:

1. Molten steel is poured continuously into a bottomless mould. As it drawn, the steel comes into contact with the water-cooled interior surface of the mould, and begins to solidify. The cast metal is then drawn downwards guided by a series of rollers, while it continuous to cool. By the time it reaches the end, the steel is completely solidified, and is immediately cut into the required lengths.

2. Molten metal flows from ladle, through a tundish, to a bottomless mould.3. Mould cooling controlled such that outside has solidified before the metal exits4. Further cooling to ensure solidification5. Cut to required length

Figure: Continuous casting overview Advantages

1. Continuous casting helps in saving many steps that would be required in the convention casting technologies. Therefore, capital investment and the cost of operation can be reduced substantially.

2. Improved Quality, less variability in chemical composition both along the thickness and along the length and surface has fewer defects.

3. Increased Yield, since it is not necessary to crop the ends of continuously cast slabs.

4. Reduced Energy costs. The slabs are sent directly to hot rolling and do not require pits for reheating and also the thickness of continuously cast slabs is half the thickness of ingot castings and thus require lower energy for hot rolling

5. Less pollution6. Product has good consistent soundness.

Disadvantages1. Not suitable for small quantity production.2. Continuous and efficient cooling of moulds is required, else, center-line

shrinkage develops.3. Requires large floor space.

APPLICATION OF METAL CASTING

Casting Process

Applications

Sand casting

Motorcycles engines parts (aluminium)

(retrieved from http://www.alibaba.com/product-gs/227685441/Motorcycle_engine_parts_alumi

num_sand_casting.html )

Handwheel (iron)

(Retrieved from http://www.qrbiz.com/front.do?method=search&searchType=product&keyword=Green-Sand-

Casting&pager.offset=40 )Shell mould casting

Farm machinery parts

(retrieved from http://www.alibaba.com/product-gs/423338881/farm_machinery_part_shell_cast

ing_cast.html)

Gear blanks

(retrieved from http://www.cbfcomponents.com/products.php?id=11)Investment casting

Pump impeller

(retrieved from http://www.castingquality.com/casting-products/pump-fittings-casting-casting-products/pump-impeller.html)

Jewellery

(retrieved from http://www.alibaba.com/promotion/promotion_investment-jewelry-casting-promotion-list.html)

Plaster mould casting

Wheel rim

(retrieved from http://www.armstrongmold.com/pages/photo-gallery/wheel_rim.html)

Permanent mould casting

Engine Cylinder Head and Cylinder Block

(retrieved from http://www.metals-b2b.com/b2b/steel_pipes/1/engine_cylinder_head_30.html)

Gear box

(retrieved from http://www.best-b2b.com/Sub-cat/973/982/gear-transmission_34.html)

Die casting

Engine valve covers

(retrieved from http://www.manufacturer.com/map_buyeroffers/Zinc_Die_Casting.html)

Bearing caps for automobile

(retrieved from http://www.syegd.com/en/product-info.php?cid=8)Centrifugal casting

Bronze bushing

(retrieved from http://njsupermann.en.made-in-china.com/product/foEJeDmdgHhs/China-

Centrifugal-Casting-Bronze-Bushing.html)

Pipe mould

(retrieved from http://hnxinghua.en.alibaba.com/product/300863561-209989974/Centrifugal_Casting_Pipe_Mould.html)

Continuous casting

Iron hollow bar

(retrieved from http://www.dhcasting.com/pages/2-continuous-cast-iron-hollow-bar.htm)

Iron bar

(Retrieved from http://www.dhcasting.com/pages/1-continuous-cast-iron-bar.htm)