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.
In next blog, we have discussed about the other casting processes..
you may read the blog in detail
https://academyofengineers.blogspot.com/2020/06/what-is.html
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