Plaster Mold Casting
Introduction
In plaster mold casting, a plaster, usually gypsum or calcium sulfate, is mixed with talc, sand, asbestos, and sodium silicate and water to form a slurry. This slurry is sprayed on the polished surfaces of the pattern halves (usually brass). The slurry sets in less than 15 minutes to form the mold. The mold halves are extracted carefully from the pattern, and then dried in an oven.
The mold halves are carefully assembled, along with the cores. The molten metal is poured in the molds. After the metals cools down, the plaster is broken and the cores washed out.
Parts cast are usually small to medium size, ranging in weight from 30 g (1 oz) to 7 kg (15 lb). The section thickness can be as small as 0.6 mm (0.025 in) and tolerances are 0.2 % linear. The draft allowance is 0.5-1.0 degree. The surface finish is 1.25 µm to 3 µm (50 µin to 125 µin) rms. Low temperature melting materials such as aluminum, copper, magnesium and zinc can be cast using this process. This process is used to make quick prototype parts as well as limited production parts.
