Special Casting Methods:
1. Die casting : Die casting is a permanent mould casting techniques in which molten metal is forced in the mould cavity under a pressure of 700 to 2500 Kg/cm2 . However the mould used is much more expensive. It is called a die and a complex machine is employed to produce castings at a very high rate. Die casting have excellent surface finish and dimensional accuracy, but volume production is necessary for a low cost per casting. Given figure shows the function of die casting.
2. Centrifugal casting : Casting that have rotational symmetry, such as long cylinders, are conveniently made by pouring the casting alloy into a metal, graphite or sand mould rotating above its axis of symmetry. Non-metallic inclusions and slag particles, being less dense than the liquid metal, are forced to the inner surface of the casting and are removed in a latter machining operation. The centrifugal casting methods can be classified as :
(i) True centrifugal casting, (ii) Semi-centrifugal casting,
(iii) Centrifugal casting or pressure casting.
3. Investment casting : It is also known as the lost wax process or precision casting. In this method, a wax pattern of the part of the made is invested in a fluid ceramic material that subsequently becomes solid. This mould is heated, causing the wax to melt and flow out, leaving a cavity of the desired shape. Molten metal is poured into the mould cavity and after solidification get final casting. It has excellent surface finish and dimensional accuracy. It is used for parts made of non-forgeable alloys.
4. Continuous casting : This process essentially consists of pouring the molten metal into the upper opening of a vertical metal mould, open at both ends, cooling it rapidly and removing the solidified product in a continuous length from the lower end of the mould. This process is largely applicable to brass, bronze, copper and aluminium etc.
5. Slush Casting : It is a type of permanent mould casting process used for producing hollow casting in metal moulds with out the use of cores molten metal is poured into the mould, which is turned over immediately so that the metal remaining as liquid runs out. A thin walled casting results, the thickness depending on the chilling effect from the mould and the time of operation. The casting is removed by opening the halves of the mould. The process is suitable for small production runs and is generally used for making ornamental and decorative objects and toys from low meting point metals, such as zinc, tin and lead alloys. parts cast in this fashion are either painted or finished to represent bronze, silver or other more expensive metals.