All of the various casting processes are methods of injecting molten metal into molds, where it hardens into the desired form. The various processes differ principally in the material of the molds and the method of injecting the metal, such as pouring or forcing under pressure.
Some processes can be used with any castable metal, while others are adapted to only a few of the lower melting point alloys. The resulting pieces differ greatly in accuracy* finish, complexity, and cost. No one process is best for every part, but each is best and most economical for certain types of cast pieces.
The principal casting processes are sand casting, shell mold casting, plaster mold casting, permanent mold casting, investment casting, and die or pressure casting.
This is the earliest, and still the most commonly used, of all cast- ing processes. It can be used with all castable metals: iron, steel, brass, aluminum, bronze, copper, and magnesium. The pattern cost is low, wood patterns often being used, although metal patterns are preferred for long runs.
Intricate pieces can be cast by the use of sand cores which can be broken out after the metal has hardened. The piece cost is fairly low, and pieces of almost any size can be cast. Large machine frames weighing many tons are made regularly, as well as small parts weighing only a few ounces.
Although it is adapted primarily to small- or medium-quantity production, many large production parts, like automobile cylinder blocks and heating radiators for the home, are regularly sand-cast. The principal advantages of sand casting are low cost of pattern and low piece cost; also, intricate pieces with elaborate interior cavities can be made by the use of sand cores.
The chief disadvantages are the rough surface and the low ac- curacy of the castings. This often means that expensive finishing and machining operations are required that might be eliminated by some of the other casting processes. The process is basically simple, although great skill and long experience are required to apply it successfully to difficult jobs and materials. The pattern is made of wood or metal in the form of the piece desired.
Allowance must be made in the dimensions for the shrinkage of the metal, but this usually is done by the pattern maker, the drawings showing the dimensions of the desired casting. Usually the pattern and the mold are made in two parts to allow for removal of the pattern from the mold, which is made of a special mixture of moist sand and clay packed firmly around the pattern in a wood or metal box called a flask.
The upper mold is called the cope; the lower, the drag. For more accurate work, the two halves of the pattern may be mounted on match plates which provide for exact alignment of the two halves. In this case, the pat- tern and the plates usually are made of metal. The mold material may be moist “green” sand, or may be held together by a binder and dried or baked (dry sand). Dry-sand molds cost more, but they give a better finish, sharper detail, and greater accuracy.
Holes or cavities are formed by cores which are molded separately of special sand mixed with a binder and baked for added strength, and are located in recesses in the mold before the metal is poured. One or more openings, called gates, are pro- vided into which, the metal is poured.
Sometimes additional pas- sages, called risers, are required to let the metal flow properly to all parts of the mold. The excess metals that harden in these gates and risers are called sprues. The location of the gates and the risers is usually determined by the foundry. Proper design at this stage contributes materially to the soundness of the final casting. After the metal has hardened, the sand mold is broken away and the cores are broken out; then, after the piece is cool, the gate and the riser sprues are cut off roughly and the casting is cleaned, by sand blasting or tumbling, of any adhering sand. The rough casting is then ready for delivery.
Since tolerances in sand castings have to be very large, any surfaces requiring a smooth finish or a close fit require machining. Usually % in. to % in. excess material is allowed, to be removed by the machine operations. If too little is allowed, the cutting tools wear very fast, because of abrasive particles in the surface layer of the casting.