Sand Casting

Sand casting is a process in which molten metal is poured into a sand mold to produce an object of a desired shape. It is by far the most widely used method of producing castings in quantities. The sand casting process is exceptionally flexible. A variety of metals can be sand cast, and the process can be used for unlimited designs of castings in sizes from ounces to many thousands of pounds.

Pattern costs for sand castings are lower than for any other casting method. Also, the tolerances on sand castings are usually wider than for other methods. These factors combined have considerable effect on making sand castings the least costly and most commonly used process for casting metals.

A casting may be defined as a " metal object obtained by allowing molten metal to solidify in a mold ", the shape of the object being determined by the shape of the mold cavity.

Certain advantages are inherent in the metal casting process. These often form the basis for choosing casting over other shaping processes such as machining, forging, welding, stamping, rolling, extruding, etc. Some of the reasons for the success of the casting process are:

The most intricate of shapes, both external and internal, may be cast. As a result, many other operations, such as machining, forging, and welding, can be minimized or eliminated.

Because of their physical properties, some metals can only be cast to shape since they cannot be hot-worked into bars, rods, plates, or other shapes from ingot form as a preliminary to other processing.

Construction may be simplified. Objects may be cast in a single piece which would otherwise require assembly of several pieces if made by other methods.

Metal casting is a process highly adaptable to the requirements of mass production. Large numbers of a given casting may be produced very rapidly. For example, in the automotive industry hundreds of thousands of cast engine blocks and transmission cases are produced each year.

Extremely large, heavy metal objects may be cast when they would be difficult or economically impossible to produce otherwise. Large pump housing, valves, and hydroelectric plant parts weighing up to 200 tons illustrate this advantage of the casting process.

Some engineering properties are obtained more favorably in cast metals. Examples are:

More uniform properties from a directional standpoint; i.e., cast metals exhibit the same properties regardless of which direction is selected for the test piece relative to the original casting. This is not generally true for wrought metals.

Strength and lightness in certain light metal alloys, which can be produced only as castings.

Good bearing qualities are obtained in casting metals.

A decided economic advantage may exist as a result of any one or a combination of points mentioned above. The price and sale factor is a dominant one which continually weighs the advantages and limitations of the process used in a competitive enterprise.

There are many more advantages to the metal-casting process; of course it is also true that conditions may exist where the casting process must give way to other methods of manufacture, when other processes may be more efficient. For example, machining produces smooth surfaces and dimensional accuracy not obtainable in any other way; forging aids in developing the ultimate of fiber strength and toughness in steel; welding provides a convenient method of joining or fabricating wrought or cast products into more complex structures; and stamping produces lightweight sheet metal parts. Thus the engineer may select from a number of metal processing methods that one or combination, which is most suited to the needs of his work.