processes for general shapes

Plaster Molding - Copper alloys can also be cast in plaster molds to produce precision products of near-net shape. Plaster-molded castings are characterized by surface finishes as smooth as 32 gin rms and dimensional tolerances as close as _+ 0.005 in (+ 0.13 mm), and typically require only minimal finish machining. In some cases, rubber patterns can be used. These have the advantage of permitting re-entrant angles and zero-draft faces in the casting's design.

Gypsum plaster (CaSO4) is normally mixed with refractory or fibrous compounds for strength and specific mechanical properties. The plaster must be made slightly porous to allow the escape of gases as the castings solidify. This can be achieved by autoclaving the plaster molds in steam, a technique known as the Antioch process. This produces very fine cast surfaces suitable for such precision products as tire molds, pump impellers, plaques and artwork. It is relatively costly.

Foaming agents produce similar effects at somewhat lower costs. Labor cost remains relatively high, however. Foamed plaster molds produce very fine surface finishes with good dimensional accuracy, but they are better suited to simple shapes.

Most plaster mold castings are now made using the Copaco process, which utilizes conventional wood or metal patterns and gypsum-fibrous mineral molding compounds. The process is readily adapted to automation; with low unit costs, it is the preferred plaster mold method for long production runs. On the whole, however, plaster molding accounts for a very small fraction of the castings market.