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What Is Investment Casting?
The term "investment" may bring
dollar signs before the eyes of those who are not familiar with
the process. Actually, it refers to the ceramic materials that
are used to build a hollow shell into which the molten metal is
poured to make the castings. The origin of the term investment
comes from the solid mold process where a plaster type material
is poured or "invested" into a container that holds a clustered
tree of small plastic patterns that are identical to the casting
being produced. After the plaster has set, the disposable
patterns are burned out leaving a hollow cavity into which the
metal is poured.
The same holds true for the
investment or "lost wax" casting process. Wax is injected into
an aluminum die to produce a pattern that is an exact replica of
the part to be produced. For every casting, a wax pattern must
be manufactured. The patterns are clustered around a coated
sprue and repeatedly dipped into an agitated vat of ceramic and
allowed to dry. After a shell thickness of approximately 3/16"
has been built; the molds are dewaxed by either flash firing at
high heat (1400 F.) or autoclaving (pressure and steam). The
hollow shells are then preheated to 800-2000 F. depending on the
alloy to be poured and the molten metal cast immediately into
the hot shell. After cooling, the ceramic is vibrated and
blasted off the metal parts and discarded. The balance of the
cleaning operations (cut off, grind, heat treat, straightening,
blast) are straight forward and quite similar to the other
casting processes.
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