The gypsum model points are heat-resistant, inexpensive, have a small thermal conductivity, and have good copy ability. Generally used to make master molds. Easy to manufacture, suitable for large products. But not durable, afraid of impact, slow drying. It is mostly used for single products and complex linear products, such as embossing, small and medium-size round sculptures. Most of the gypsum used is hemihydrate gypsum, that is, plaster of Paris. Like the cement mold, the brick base and wood can be used as the skeleton foundation and then covered with a plaster layer. In order to increase the rigidity and prevent cracks, it can be added to the gypsum.
Sufficient fillers, such as quartz, reduce shrinkage and cracking, and add cement (gypsum: cement = 7:3) to increase strength. It has also been proposed to add a proper amount of latex with gypsum, dilute with water to make a mold, and the strength is good, and powder is not obtained.
The plaster mold can be used as a master mold for a low melting point alloy mold to cast the alloy in a hot state. When casting a gypsum mold (sub-mold) with a gypsum master mold, the surface of the master mold should be coated with a separating agent such as potassium soap solution, transformer oil, edible oil, 20% stearic acid, 80% kerosene, or gasoline ten Vaseline… The plaster mold can be repaired by taking a gypsum powder with a water pan. The gypsum mold drying process is: (60 ~ 120) ° C / (4 ~ 5) L, after natural cooling, gently polished with metallographic sandpaper, and then (100 ~ 150) ° C / (8 ~ 10) L, ( 200 ~ 230) ° C / (20 ~ 24) L drying.