The Fundamentals
Introduction:
In metal casting,
metal is melted, heated to the proper temperature, and poured into a cavity or mold of the
desired shape or form. Before the metal can be melted and poured, the casting has to be
designed, a pattern made, and a mold produced.
Pattern making:
The
pattern is a replica of the casting to be poured, and is used to create
the cavity in the mold. It can be made of wood, metal, plastic, composite,
or a combination of all of the above. Patterns can even be made of wax
(used in lost wax casting). Patterns must be made to allow for the
shrinkage of the metal when it cools and they can include channels for the
metal to flow into the mold.
Molding:
The
mold is made when the pattern is packed in the molding material, usually
some type of sand. The molds can be made by hand or machine. If the
process is die casting, the mold is made from a heat resistant metal.
Other molds are made by melting a wax pattern that has been covered by an
investment or slip material. The cavity that results from melting out the
wax becomes the mold. Sand molds are usually made in a two part box (cope
and drag) called a flask. . Different types of sand are used in molding
and are made up of sand, clay, and other additives like water or oil to
make the sand hold its shape.
Melting:
The
metal is melted by heating it to the proper temperature. The melting
process provides molten metal at the proper temperature, quantity and
quality. The metal is usually melted in a crucible which is placed inside
a furnace for the melting process. The furnace that I use is designed to
use propane gas to provide the heat to melt the metal. The design can be
found back on the Foundry Home Page.
Pouring:
A
method of pouring must be provided to introduce the molten metal into the
mold cavity. Air and gasses must be allowed to escape from the mold. The
molten metal must be able to completely fill the mold cavity and
allowances made for shrinkage caused by the cooling metal. Hobby casters
usually use tongs to transport the crucible of molten metal from the
furnace to the mold.
Shakeout:
After
the metal is allowed to cool and solidify the casting is then removed from
the mold. If the casting is removed from the mold too soon, it may
imperfections or crack. The sand used in molds can be reused after
retempering.
Finishing:
After
the casting is removed from the mold, the sprues, risers and parting lines
are removed and the surface of the casting can further finished by
polishing or adding surface finishes.
Aluminum can be anodized, and patinas can be added to brass and bronze.
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