<P> The pattern is usually made from metal, however rubber molds may be used for complex geometry; these molds are called Rubber plaster mold 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 . </P> <P> 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). </P> <P> 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 . </P> <P> Plaster mold casting is used when an excellent surface finish and good dimensional accuracy is required . Because the plaster has a low thermal conductivity and heat capacity, the metal cools more slowly than in a sand mold, which allows the metal to fill thin cross-sections; the minimum possible cross-section is 0.6 mm (0.024 in). This results in a near net shape casting, which can be a cost advantage on complex parts . It also produces minimal scrap material . </P>

How is a plaster of paris impression produced
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