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Permanent mold casting is a process in which molds constructed
from a chill material, such as certain metal alloys or graphite,
are used repeatedly for the volume production of castings of the
same shape. The term “permanent mold” is somewhat misleading,
since the molds will not last forever. However, the service life
of a permanent mold can be as high as 5,000 pours, or more, depending
on the complexity of the part, the mold material used, and the
alloy being cast.
Permanent molds may have metal or sand cores. A semi-permanent
mold is a permanent mold in which sand cores are used, and the
cores are destroyed in the casting process. In addition, permanent
mold casting is sometimes referred to as gravity die casting,
since the metal is poured under gravity without external pressure
being applied. Or it may be called chill casting, because of the
materials used for the mold.
A permanent mold may have a number of cores, depending on the
design of the component to be cast. Gates and risers are designed
to minimize turbulence during pouring and provide reservoirs of
molten metal which reduce the risk of shrinkage and porosity.
Once solidification has occurred, the casting is removed from
the mold by ejector pins or by retraction of movable cores.
Although permanent molds are more expensive than sand molds, the
process offers a number of advantages, including higher production
per unit time per unit of equipment, improved mechanical properties,
finer grain size, smooth surface finish, closer dimensional tolerances
and lower machining costs. The process is used for casting of
copper and certain copper alloys, including yellow brass, silicon
brass, manganese bronze, nickel silver, and aluminum bronze.
Permanent mold casting ranks second to sand casting in popularity.
However, the tonnage produced by the process is only a small percentage
of that made by sand casting. About 500 units is generally considered
to be the minimum number of castings for which a permanent mold
may be built economically.
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