With sand casting the mold is split up after each casting operation, but with the process known as gravity die casting, the mold also called a 'die' is made from metal, and can be utilised a large number of times. This suggests that the die is far more expensive to make, than an expendable 'one use only' mold. An intermediate method gets use from semipermanent molds, which are made of gypsum plaster or fireclay, which can be employed repeatedly for a limit limited number of castings. With gravity die casting, the most generally used materials for die-making are cast iron, steel, and heat resisting alloys of iron. For some specific purposes other materials are used to make the dies, and these can include, aluminum. Copper or graphite. A metal die can produce smooth castings with a clean surface, and a particularly high dimensional accuracy. These castings require extraordinarily little or no last machining or other finishing treatment. The service life of metal dies can change in terms of the amount of castings it can produce, and this relies on certain considerations such as the casting material, the thermal metal shock resistance of the die material, the temperature at which it is poured, and the casting method employed.
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Many different details have to be taken into consideration when designing the pattern from which the die is made. For example the pouring-gate system and risers need to be considered so that the walls of mold allow a quenching action on the molten metal so it van firm up more rapidly than in sand casting. Also the die must be supplied with channels at the joints and air vent holes to permit air from the hot metal to escape from the interior of the die. The die must also be built so it won't prohibit the shrinkage that happens, when the metal cools. Shrinkage can present problems when designing the cores which form the casting. Sometimes the cores are made of steel or special alloys, and sometimes compressible sand or shell cores are used.
To prevent the casting metal from sticking to the die, the die can be given an internal coating of chalk, clay, or bone ash with water glass as a binder. This mixture can be applied to the die by spraying, brushing or immersion.
With straightforward castings the molten metal might be poured in at the top. It should be designed to permit the molten metal to flow quickly without turbulence into all bits of the die. For metals with low softening points the die is sometimes heated to prevent early solidification, and for metals with a high melting point, the die might have to be artificially cooled after each casting operation.
Slowly moving or leaning the die while casting can reduce turbulence and enable the metal to flow more smoothly, particularly when heavy castings are being produced. For awkwardly formed castings, a vacuum might be applied to help the filling of the die. Slush casting, is a technique used for producing ornamental or hollow castings : the molten metal is poured into the die, and when a solid shell of acceptable thickness has formed, the remaining liquid is poured out.
Though die castings are cheaper than sand castings, the die tooling is dearer, and a perfect number of castings need to be produced to make the process cost-effective.
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