— Casting Processes —

Plaster Mould Casting

Similar to sand casting, the mould is made of Plaster of Paris, rather than sand, and is used when an excellent surface finish and good dimensional accuracy is required. A more unusual process, and only applicable for lower melting point alloys, the process is more expensive.

In the conventional process, plaster of Paris is mixed with water to produce a slurry which is poured over a permanent pattern contained within a moulding box. Upon setting a rigid mould is produced which, after pattern stripping, is dried at an elevated temperature to remove free and chemically combined water before the metal is cast into the mould. This method produces a strong, dense, but inherently impermeable mould. Hence, metal casting must be conducted using vacuum and/or pressure assistance to ensure complete filling of the mould by the metal. The insulating nature of the plaster improves the fluid life of the metal that aids mould filling and thin section production.

Typical applications include large pumps, valve bodies, ship propellers, large engineering castings

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