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Investment casting is also known as the lost wax
process. This process is one of the oldest manufacturing
processes. The Egyptians used it in the time of
the Pharaohs to make gold jewelry (hence the name
Investment) some 5,000 years ago. Intricate shapes
can be made with high accuracy. In addition, metals
that are hard to machine or fabricate are good
candidates for this process. It can be used to
make parts that cannot be produced by normal manufacturing
techniques, such as turbine blades that have complex
shapes, or airplane parts that have to withstand
high temperatures.
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The
mold is made by making a pattern using wax or
some other material that can be melted away. This
wax pattern is dipped in refractory slurry, which
coats the wax pattern and forms a skin. This is
dried and the process of dipping in the slurry
and drying is repeated until a robust thickness
is achieved. After this, the entire pattern is
placed in an oven and the wax is melted away.
This leads to a mold that can be filled with the
molten metal. Because the mold is formed around
a one-piece pattern, (which does not have to be
pulled out from the mold as in a traditional sand
casting process), very intricate parts and undercuts
can be made. The wax pattern itself is made by
duplicating using a stereo lithography or similar
model-which has been fabricated using a computer
solid model master.
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The
materials used for the slurry are a mixture of
plaster of Paris, a binder and powdered silica,
a refractory, for low temperature melts. For higher
temperature melts, sillimanite an alumina-silicate
is used as a refractory, and silica is used as
a binder. Depending on the fineness of the finish
desired additional coatings of sillimanite and
ethyl silicate may be applied. The mold thus produced
can be used directly for light castings, or be
reinforced by placing it in a larger container
and reinforcing it more slurry. |
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Just
before the pour, the mold is pre-heated to about
1000 ºC (1832 ºF) to remove any residues
of wax, harden the binder. The pour in the pre-heated
mold also ensures that the mold will fill completely.
Pouring can be done using gravity, pressure or
vacuum conditions. Attention must be paid to mold
permeability when using pressure, to allow the
air to escape as the pour is done.
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Tolerances of 0.5 % of length are routinely
possible, and as low as 0.15 % is possible for
small dimensions. Castings can weigh from a
few grams to 35 kg (0.1 oz to 80 lb), although
the normal size ranges from 200 g to about 8
kg (7 oz to 15 lb). Normal minimum wall thicknesses
are about 1 mm to about 0.5 mm (0.040-0.020
in) for alloys that can be cast easily.
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The types of materials that can be cast are
Aluminum alloys, Bronzes, tool steels, stainless
steels, Stellite, Hastelloys, and precious metals.
Parts made with investment castings often do
not require any further machining, because of
the close tolerances that can be achieved.
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