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In Permanent
Molding Process, instead of
using sand as the mold material, a metal is used as a mold.
There is a
right hand and a left hand, to the Permanent Mold Tool or pattern. These
rights and lefts are the equal to the Cope and the Drag that were
discussed in Sand Casting Process. See
the diagram below for details on operation.
Typically
cast iron or Meehanite (a dense cast iron) is used as the mold material
and the cores are made from metal or sand. Cavity surfaces are coated
with a thin layer of heat resistant material such as clay or sodium
silicate.
The molds are
pre-heated upto 200 ºC (392 ºF) before the metal is poured into the
cavity. The cavity design for these molds do not follow the same rules
for shrinkage as in sand casting molds, due to the fact that the metal
molds heat up and expand during the pour, so the cavity do not need to
be expanded as much as in the sand castings. However, care has to be
taken to ensure proper thermal balance, by using external water cooling
or appropriate radiation techniques.
Permanent
mold castings, while not as flexible as sand castings in allowing the
use of different patterns (different part designs), lower the cost of
producing a part. At a production run of 1000 or more parts, permanent
mold castings produce a lower piece cost part. Of course, the break-even
point depends on the complexity of the part. More complex parts being
favored by the use of permanent molds.
The usual
considerations of minimum wall thicknesses (such as 3mm for lengths
under 75 mm), radius (inside radius = nominal wall thickness, outside
radius = 3 x nominal wall thickness), draft angles (1 to 3º on outside
surfaces, 2 to 5º on inside surfaces) etc all apply. Typical tolerances
are 2 % of linear dimensions. Surface finish ranges from 2.5 µm to 7.5
µm (100 µin to 250 µin).
Typical part
sizes range from 50 g to 70 kg (1.5 ounces to 150 lb). Typical materials
used are small and medium sized parts made from aluminum, magnesium and
brass and their alloys. Typical parts include gears, splines, wheels,
gear housings, pipe fittings, fuel injection housings, and automotive
engine pistons.
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