With sand casting the mold is broken up after each casting operation, but with the process known as gravity die casting, the mold also called a ‘die’ is manufactured from metal, and can be used a large number of times. This means that the die is much more expensive to make, than an expendable ‘one use only’ mold. An intermediate technique makes use of semipermanent molds, which are made of gypsum plaster or fireclay, which can be used repeatedly for a limited number of castings. With gravity die casting, the most widely used materials for die-making are cast iron, steel, and heat resisting alloys of iron. For some specific purposes other materials are used to manufacture the dies, and these can include, aluminum. copper or graphite. A metal die can produce smooth castings with a clean surface, and a very high dimensional accuracy. These castings require very little or no final machining or other finishing treatment. The service life of metal dies can vary in terms of the number of castings it can produce, and this depends on certain factors 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.
Many different details need to be taken into consideration when designing the pattern from which the die is made. For instance the pouring-gate system and risers need to be considered so that the walls of mold allow a quenching action upon the molten metal so it van solidify more rapidly than in sand casting. Also the die must be provided with channels at the joints and air vent holes to allow air from the hot metal to escape from the interior of the die. The die must also be constructed so it will not restrict the shrinkage that occurs, when the metal cools. Shrinkage can present difficulties when designing the cores which form the casting. Usually the cores are made from 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 simple castings the molten metal may be poured in at the top. It should be designed to allow the molten metal to flow quickly without turbulence into all parts of the die. For metals with low melting points the die is sometimes heated to prevent premature solidification, and for metals with a high melting point, the die may have to be artificially cooled after each casting operation.
Slowly moving or tilting the die while casting can reduce turbulence and enable the metal to flow more smoothly, particularly when heavy castings are being produced. For awkwardly shaped castings, a vacuum may 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 sufficient thickness has formed, the remaining liquid is poured out.
Although die castings are cheaper than sand castings, the die tooling is more expensive, and an optimum number of castings need to be produced to make the process cost effective.