With shell mould casting and sand mould casting the mould has to be broken up after each casting operation. In the process known as die casting the mould known as a die is made of metal and used a large number of times. It is of course more expensive to produce an expendable once only mould.
The most widely used materials used for die making are cast iron, steel and heat resisting alloys of iron.For specific purposes other materials are sometimes used for the dies, and these can include copper, graphite and aluminium. A die can produce castings with clean and smooth surface finishes and a high dimensional accuracy, and usually require little or no final machining, or other finishing treatments.
The service life of a die depends on such factors as the thermal shock resistance of the die material, the casting material, the temperature at which it is poured and the casting method which is being used.
There are many details which have to be taken into consideration when designing the pattern from which the die is made. In designing the risers and pouring gate mechanism it should be remembered that the walls of the mould should exert a quenching action upon the molten metal, so that it solidifies more quickly than in sand casting. Also the die must be provided with channels at the joints, and with air vent holes, to enable the air displaced by the casting metal to escape from the interior of the die. The die must also be constructed so that it does not restrain the shrinkage that occurs when the metal cools and solidifies, and so allow the casting to be easily removed. Shrinkage can present particular problems in the designing of the cores which form the cavities and recesses in the casting. Usually the cores are made from steel or special alloys and sometimes compressible sand or shell cores are used.
To prevent the metal from sticking to the die, it may be given an internal coating of chalk, clay or bone ash with waterglass as a binder. This mixture is applied to the die by immersion, brushing or spraying.
With simple castings the molten metal may be poured into the open die from the top. Usually the die is a closed and complex assembly of two or more parts. It must be designed so that the molten metal will flow quickly without hinderance into every part of the die.