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Theoretical Considerations

The expandable mold is formed of refractory materials. The thermal conductivity of the mold is a property of the material selected. It is also a function of particle size and distribution. The thermal conductivity influences the rate of transfer through the mold and therefore the rate of solidification which, in turn, influence the metallurgical integrity of the casting.

When choosing the mold material, it must be sufficiently refractory to withstand the pouring temperature of the particular metal being cast without melting or softening. Providing the material is pure, the melting point value is a good guide to refractoriness. However, the melting points can be reduced severely by adding very small amounts of alkali metal salts or iron oxide.

The refractory selected should exhibit stability. It should not be susceptible to hydration, or reaction with other materials in the mold or the metal.

It should also be thermally stable, i.e., expansion should be predictable, constant and reproducible. It can be taken into account when estimating the contraction allowance for the casting.

Following is a table of some refractory material characteristics:

Information source: Clegg, Precision Casting Processes.