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Casting Processes

Foundries and castings have made industrial progress possible. The idea of melting metal and shaping it has been instrumental in the making of many a tools that aided manufacturing. Today, there are many types of casting processes available; we will cover the basic ones here. Casting process is the most direct method of producing a product from the chosen material. And while metals still are a popular choice for casting, ceramics and polymers can be produced by this method as well.

Sand Casting
By far, sand casting is the most extensively used casting process that is undertaken in a foundry. Of all the casting processes, sand castings occupy a lion's share in manufacturing - exceeding 50% market share. The sand casting process involves the use of a furnace, metal, pattern, and sand mould. In sand casting, moist bonding sand is packed around a pattern. Patterns in sand casting are typically made out of wood, plastic, or metal. It is an expendable mould-permanent pattern casting process. Once the molten metal is cooled, the pattern is removed to create the mould and molten metal poured into the cavity. Risers supply necessary molten material during solidification. While the earlier processes relied on cupola furnaces, improvements in furnace technology have permitted the use of induction furnaces, crucible furnaces and electric arc furnaces as well. If needed, some post processing is done to achieve the desired finish. Sand casting foundries are very popular in India, and are famous in countries like Germany, U.K and U.S. for the quality of their castings and machined components. Sand casting is used in manufacturing of parts and components that use aluminum, copper and zinc based alloys. Typical applications include pumps and valves, agricultural equipment, construction equipment and plant machineries.

Permanent Mould Casting / Gravity die Casting
Permanent mould casting, also called as gravity die casting, involves pouring molten metal under gravity into a pre-heated die. It is a metal casting process that shares similarities to both sand casting and die casting. Just like sand casting, molten metal is poured into a mould which is clamped shut until the material cools and solidifies. Unlike sand casting (which used an expendable mould), permanent mould casting involves using a permanent metal mould that can be used repeatedly. The process is also called gravity die casting as the liquid metal is allowed to flow into the die under the force of gravity, and not by any force. Low-pressure casting is a variation of the process, and is completed by forcing the mould via low-pressure air rather than the use of gravity, which can reduce shrinkage during the cooling process. Gravity die casting is predominantly used in the automotive sector for gears, pistons and cylinder heads, in lighting components and to manufacture kitchen utensils.

Investment Casting / Lost Wax Casting
Developing investment castings involves creating an original wax pattern, coating the image with plaster and building of successive layers until a strong shell envelopes the model. The wax is then melted out and the metal cast in the ceramic mould. The mould is then destroyed to remove the casting. Because it involves melting the wax, investment casting is also called as the ‘lost wax’ casting process. After melting out the wax, molten steel, stainless steel or bronze will be poured into the mould, creating a perfect duplicate of the original wax pattern. Investment casting is used to manufacture very complex castings with unusual internal configurations. Typical uses of this process include jewellery, decorative work like figurines, intricate aerospace and auto components, etc.

Centrifugal Casting
As the name implies, this kind of casting involves using centrifugal force rather than gravity or pressure. Essentially, the centrifugal casting process involves rotating the mould at a high speed so that the centrifugal force distributes the molten metal to outer regions of the die casting in an uniform manner. A careful consideration of process parameters related to the centrifugal forces is necessary in order to achieve even distribution of the molten metal. There are three distinct types of centrifugal casting processes that foundries in India and other countries use - True centrifugal casting process, Semi centrifugal casting process, and Centrifuge casting process. Each of them involves slight variation in the way the mould is rotated. The axis of rotation is usually horizontal, but may be vertical for short work pieces. Unlike most other casting processes, centrifugal casting does not need any runners or risers. It is mostly used where the mechanical properties of thick-walled tubes are important and high alloy grades of steel are required. Typical applications include pipes, brake drums, flywheels, train wheels, etc.

Shell Mould Casting
Shell mold casting or shell molding is a foundry casting process. In this process, the mould is a thin hardened shell of sand and a heated metal pattern is placed over a box of thermosetting resin-coated sand, backed up by some other material. The box is inverted for a fixed time to cure the sand. The box is re-inverted and the excess sand falls out. The shell is then removed from the pattern and joined with the previously made other half. They are supported in a flask by an inert material ready for casting. For good dimensional accuracy, patterns are generally made of iron or steel. Since this process generates minimal scrap, it is considered one of the best low-cost casting methods for manufacturing large quantities of a componet. Shell mould casting can also produce complex shapes, especially when using composite moulds. This permits good variations in cross-section. Shell mould casting is a popular process to produce small mechanical parts that require high precision, transmission components and gear housings.

There are many other variants in the casting process that include vacuum casting, slush casting and pressure casting. Irrespective of the casting process used, it must be remembered that there are merits and demerits to using a particular casting process. For example, while sand casting is the most popular foundry process, it has low strength on account of higher porosity and has low dimensional accuracy.  Choosing the right process is one of the important considerations for successful casting. That and the choice of right foundry and right material can make the difference between success and failure.