Plaster MoldingThe most common specialty casting process employs plaster molds. This process is a refinement of sand casting, in that the sand is replaced by plaster, giving the finished casting a smoother surface and allowing greater accuracy in the dimensions of the molded part. A plaster mold is used for just one casting, since it is necessary to destroy the mold to remove the casting. The process is usually confined to castings under 2 lb. Gypsum plasters are the type most often used to make plaster molds. It is essential that all water be removed from the molds before casting.
The equipment required for plaster molds is more expensive than that for sand molds. Metal match plates and metal core boxes of extreme accuracy are used. Most plaster castings are poured by gravity, but moderate pressure gives improved casting detail.
The aluminum alloys used with plaster molds must be carefully selected. The refractory nature of the plaster results in a slow solidification time, with resultant lowering of mechanical properties. This refractory quality, however, enables thin and intricate sections to be cast. Due to their excellent fluidity, aluminum-silicon alloys are best for plaster casting.
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