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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 to as large as . 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.〔 ==Details== 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 s. For example, if the casting includes reentrant 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 for the first 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 micrometres (50–125 μin).〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Plaster mold casting」の詳細全文を読む スポンサード リンク
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