Plaster mold casting

Plaster mold casting is a metalworking casting process similar to sand casting except the molding material is plaster of Paris instead of sand. Like sand casting, plaster mold casting is an expendable mold process, however it can only be used with non-ferrous materials. It is used for castings as small as 30 g (1 oz) to as large as 45 kg (99 lb). Generally, the form takes less than a week to prepare. Production rates of 1–10 units/hr can be achieved with plaster molds.

Parts that are typically made by plaster casting are lock components, gears, valves, fittings, tooling, and ornaments.

The plaster is not pure plaster of Paris, but rather has additives to improve green strength, dry strength, permeability, and castability. For instance, talc or magnesium oxide are added to prevent cracking and reduce setting time; lime and cement limit expansion during baking; glass fibers increase strength; sand can be used as a filler. The ratio of ingredients is 70–80% gypsum and 20–30% additives.

The pattern is usually made from metal, however rubber molds may be used for complex geometry; these molds are called Rubber plaster molds. For example, if the casting includes angles or complex angular surfaces then the rubber is flexible enough to be removed, unlike metal. These molds are also inexpensive, reusable, more accurate than steel molds, fast to produce, and easy to change.

Typical tolerances are 0.1 mm (0.0039 in) for the first 50 mm (2.0 in) and 0.02 mm per additional centimeter (0.002 in per additional inch). A draft of 0.5 to 1 degree is required. Standard surface finishes that are attainable are 1.3 to 4 micrometers (50–125 μin).

First, the plaster is mixed and the pattern is sprayed with a thin film of parting compound to prevent the plaster from sticking to the pattern. The plaster is then poured over the pattern and the unit shaken so that the plaster fills any small features. The plaster sets, usually in about 15 minutes, and the pattern is removed. The mold is then baked, between 120 °C (248 °F) and 260 °C (500 °F), to remove any excess water. The dried mold is then assembled, preheated, and the metal poured. Finally, after the metal has solidified, the plaster is broken from the cast part. The used plaster cannot be reused.

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