More about Sand Castings

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Sand casting is a widely used method of producing rough metal castings. Unfinished, these castings have an easily recognizable, grainy texture, although most foundries will perform finishing services that yield a smoother surface.

Sand castings are made by first starting with a wood or metal representation of the part, called a pattern. The pattern is made slightly larger than the desired size of the final casting to account for shrinkage during cooling. This pattern is placed in a molding box, which is then half filled with a mixture of sand (crushed rock), clay and water. The sand is tightly packed around the pattern, and then the box is turned and the process is repeated with the top half of the pattern. The two mold halves are then separated and the pattern is removed. This pattern can then be used to generate additional molds, if necessary. A hole, called a sprue, is punched into the top of the mold to allow molten metal to be poured into the mold. Additional holes, known as risers, are also punched into the pattern to allow gas to escape. Eventually, the metal is added and, after drying, the casting is removed.

The sand casting process is often far less expensive than other techniques, and is often one of the fastest methods available. It does suffer from some disadvantages, however. First, it typically yields tolerances of .030 inches for the first foot, and then and additional .010 for each additional three inches. It also produces a product that requires further machining. At the very least, the casting must be separated from the risers and the sprue. Most applications also require additional finishing services, such as shot and hammer peening, grinding, forging or plating. These processes create a much finer surface, and are particularly important any time that the casting will be used as a visible component.

The advantages of sand casting often outweigh its drawbacks. It produces less waste than a number of other machining processes, especially since unused material can simply be added to the next mold. It can achieve a wide range of shapes with a brief turnaround, especially when based on previously used molds or standard designs. It is also useful with many aluminum alloys, and can be effective with components weighing ounces or tons.