Special Casting Processes:
The sand moulds may be used for casting ferrous and non-ferrous metals, but these moulds can be used only once, because the mould is destroyed after the metal has solidified. This will increase the cost of production. The sand moulds also, can not maintain the tolerance and smooth surface finish. In order to meet these requirements, following casting method may be use:
1: Permanent Mould Casting
A casting made by pouring molten metal by gravity into a mould made of some metallic alloy or other material of permanence is known as permanent mould casting.
2: Slush Casting
The slush casting is a special application involving the used of permanent mould. It is used for casting low melting temperature alloys. This method is only adopted for ornaments and toys of non-ferrous alloys.
3: Die Casting
The die casting (also known as pressure die casting) may be defined as that casting which uses the permanent mould(called die) and the molten metal is introduced into it by means of pressure, following are two type of die casting machines commonly used for die casting:
(a) Hot chamber die casting machine
In a hot chamber die casting machine, the melting pot is an integral part of the machine. The molten metal is forced in the die cavity at pressure from 7 to 14 MPa. The pressure may be obtained by compressed air or by hydraulically operated plunger. The hot chamber die casting machine is use for casting zinc, tin, lead and other low casting melting alloys.
(b) Cold chamber die casting machine
In a cold chamber die casting machine , the melting pot is usually separate from the machine and the molten metal is not transferred to injection mechanism by ladle. The pressure on the casting metal may vary from 21 to 210 MPa and in same cases may reach 700 MPa. This process is used for casting aluminum, magnesium, copper, brass alloys and other high melting alloys.
4: Centrifugal Casting
A casting process in which the molten metal is poured and allowed to solidify while the mould is revolving, is called centrifugal process. The casting produced under this centrifugal force is called centrifugal casting. This process is especially designed for casting of symmetrical shape. The ferrous and the non-ferrous metals can be obtained by this process. The casting produced by this process have dense and fine grained structure.
5: Investment Casting
It is also known as lost wax process or precision casting. The casting produced by this method are within very close tolerance(±0.05mm).
6: Shell Moulding Process
The shell moulding process is also called croning process. The shell cast part can be produced with dimensional tolerance of ±02 mm.