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Shell mold casting is a process particularly suitable for producing
castings with thin sections, intricate details, close dimensional
tolerances, and excellent surface finish. The process lends itself
best to relatively large volume castings, because of the comparatively
high cost of the patterns and the set-up charges.
A metal pattern is produced first, and the metal used must have
good heat capacity and thermal conductivity, as well as a good
surface finish and the ability to withstand the abrasion of sand
mixes. A shell mold is formed by heating the metal pattern and
then bringing a sand and resin mixture into contact with the pattern.
The heat from the pattern cures the resin in the mixture for a
predetermined time, and any further curing needed is carried out
in an oven. When fully cured, the rigid shell is ejected from
the pattern plate.
Cope and drag shell sections are made for each mold, and they
are joined together with bonding resin or clamps to form the complete
mold. Any cores required are placed inside the cope and drag before
they are joined. When heavy castings are being produced, the thin
shell mold is usually supported with metal shot or other back-up
material. Shells for light castings do not require support.
With proper care and maintenance, well constructed metal patterns
will last indefinitely, and they can be used repeatedly in shell
molding machines to produce large volumes of molds. The process
can be used for castings ranging from a fraction of an ounce to
more than 50 lbs., and most copper-base materials can be cast
successfully in shell molds.
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