Split Patterns

Figure 1

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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.