Ahmad K Elshennawy, George F Schrader
Published By: SME
Other casting processes that produce more uniformly, more precisely, or at lower costs will be described in this chapter. One group utilizes metal molds that are not destroyed in service as sand molds are, but can be used repeatedly. Where metal molds are not suitable, precision casting is done in plaster and ceramic molds. Another group of processes casts metal continuously rather than in individual pieces. Two processes that use metal molds are permanent mold casting and die-casting. Although applied to other kinds of molds, centrifugal casting is often done with metal molds and is also included in this section. When fluid metal is poured into metal molds and subjected only to hydrostatic pressure, the process is called permanent mold casting. Other forms of reusable molds are low-pressure casting, slush casting, die casting, and centrifugal casting. Disposable or semi-permanent plaster molds or cores for metal molds are made from plaster of paris (gypsum) with added talc, silica flour, asbestos fiber, and other substances to control setting time and expansion. Investment casting, also called the lost wax process, is an ancient process that utilizes an expendable pattern of wax or plastic material. Continuous casting consists of pouring molten metal into one end of a metal mold open at both ends, cooling rapidly, and extracting the solid product.