![]() ![]() ![]() 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). These molds are also inexpensive, reusable, more accurate than steel molds, fast to produce, and easy to change. For example, if the casting includes reentrant angles or complex angular surfaces then the rubber is flexible enough to be removed, unlike metal. The pattern is usually made from metal, however rubber molds may be used for complex geometry these molds are called Rubber plaster molds. The ratio of ingredients is 70–80% gypsum and 20–30% additives. 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 plaster is not pure plaster of Paris, but rather has additives to improve green strength, dry strength, permeability, and castability. Parts that are typically made by plaster casting are lock components, gears, valves, fittings, tooling, and ornaments. Production rates of 1–10 units/hr can be achieved with plaster molds. Generally, the form takes less than a week to prepare. It is used for castings as small as 30 g (1 oz) to as large as 7–10 kg (15–22 lb). Like sand casting, plaster mold casting is an expendable mold process, however it can only be used with non-ferrous materials. Plaster mold casting is a metalworking casting process similar to sand casting except the molding material is plaster of Paris instead of sand. ![]()
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