RFD 2, Box 470, Knightstown, Indiana 46148
Casting has been popular for many years. It provides uniform
grain structure superior in strength to a similar part machined
from the same type of metal. Production procedures for casting are
normally quicker and cheaper due to lack of machining time and
little material waste.
Metal casting is a process whereby liquid metal is poured into a
mold. The metal cools and solidifies in the mold, taking on its
shape. The metal discussed hereafter in relation to casting will be
aluminum. It works best because of its low (1218°) melting
temperature. However, the casting process is very much the same for
other metals.
The first step is to make the pattern. An original part makes a
good pattern or patterns may be made from wood, aluminum and even
styrofoam. However, styrofoam patterns may be used only once, for
they are destroyed when the molten metal contacts them. The wood or
aluminum pattern may be reused. There are a number of different
types of patterns-the one-piece or solid being the simplest.
One-piece patterns are used for parts having simple shapes or when
only a few castings are desired. Split patterns are used for parts
of greater complexity, or those which cannot conveniently be molded
from a one-piece pattern, a split pattern is made in two parts
(Figure 1).
After the pattern is made, it is necessary to give it a light
dusting with a parting compound. This will let the pattern be
easily removed from the sand without sticking.
A pattern must also have draft which enables it to be lifted
from the mold without breaking the mold (Figure 2).
A sturdy steel box called a flask is used to hold the sand. The
flask is made up of two halves, called the cope and drag. The two
halves fit together perfectly even though the sand is packed in
each half independently. The one-piece pattern is packed in only
the one half of the flask, whereas the split pattern has one half
the pattern in both halves of the flask.
The aluminum may be purchased commercially or may be scrap from
old storm doors, windows, etc. The aluminum is melted in a crucible
or equivalent container that can be poured from. The pouring
temperature should be about 1418° (even though the aluminum melts
at 1218°) so that the aluminum will completely fill the mold before
it starts to solidify. A special thermometer called a pyrometer
would be helpful in this process.
The aluminum is poured into the mold through a special
passageway called a sprue hole, located at one edge of the pattern.
Another passage way called a riser is located in the sand mold
opposite the sprue hole (Figure 3). The aluminum is poured in the
sprue hole, completely filling the cavity made by the pattern and
then finally filling the riser, enabling one to tell when the mold
is full. After the aluminum cools, the casting is removed from the
sand and the sprue and riser can be hack sawed from the finished
casting. A file is helpful to smooth up the edges.
Remembering that metals shrink as they cool, one should allow
for this in pattern making. Allow for 5/32′ shrinkage per foot
for aluminum and 3/16′ shrinkage per foot for brass.
An ideal place to get further information on this subject would
be your local high school industrial arts department.
And last, but not least, is Safety! Persons
doing foundry work should always wear safety glasses and avoid
having the molten metal contact bare skin.
