Casting is a fabrication process whereby a totally molten metal is poured into a mold cavity having the desired shape; upon solidification, the metal assumes the shape of the mold but experiences some shrinkage. Casting techniques are employed when (1) the finished shape is so large or complicated that any other method would be impractical, (2) a particular alloy is so low in ductility that forming by either hot or cold working would be difficult, and (3) in comparison to other fabrication processes, casting is the most economical. Furthermore, the final step in the refining of even ductile metals may involve a casting process. A number of different casting techniques are commonly employed, including sand, die, investment, and continuous casting. Only a cursory treatment of each of these is offered.
SAND CASTING
With sand casting, probably the most common method, ordinary
sand is used as the mold material. A two-piece mold is formed by packing sand
around a pattern that has the shape of the intended casting. Furthermore, a gating
system is usually incorporated into the mold to expedite the flow of molten
metal into the cavity and to minimize internal casting defects. Sand-cast parts
include automotive cylinder blocks, fire hydrants, and large pipe fittings.
DIE CASTING
In die casting, the liquid metal is forced into a mold under pressure and at a relatively high velocity, and allowed to solidify with the pressure maintained. A two-piece permanent steel mold or die is employed; when clamped together, the two pieces form the desired shape. When complete solidification has been achieved, the die pieces are opened and the cast piece is ejected. Rapid casting rates are possible, making this an inexpensive method; furthermore, a single set of dies may be used for thousands of castings. However, this technique lends itself only to relatively small pieces and to alloys of zinc, aluminum, and magnesium, which have low melting temperatures.
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