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Investment
casting is also known as the lost wax process. This process is one of
the oldest Casting processes dated backed to the Egyptian
Age. The
Egyptian Pharaohs used forms of Investment Casting to make gold jewelry
(and hence rumor has it came the name Investment) some 5,000
years ago. Using
Investment Casting, intricate shapes can be made with high accuracy. In addition,
metals that are hard to machine or fabricate are good candidates for
this process. Investment
Casting 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. How
it all Works
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text is below 
How
it all Works Text
The
investment 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.
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), this way very intricate parts and undercuts can be made.
The actual Wax Pattern itself is made by duplication process, using a stereo lithography
or similar model-which has been fabricated using a computer solid model
master. Then wax is poured into this pattern, making the wax patterns,
that are a fixed to the Investment Casting Tree.
The materials
used for the slurry are a mixture of plaster of Paris, a binder and
powdered silica, a refractory, for low temperature melts.
Just before
the pouring of molten metal, the mold is pre-heated to about 1000 ºC (1832 ºF) to remove
any residues of wax, harden the binder.
The pouring
of molten metal into a pre-heated mold
also ensures that the mold will fill completely, eliminating chances for
Porosity.
The Molten
Metal Pouring can be done
using gravity, pressure or vacuum conditions.
Extremely
Close Tolerances are Achieved
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.
Acceptable
Alloys
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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