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Sand casting consists of placing a pattern (having the shape of the desired casting) in sand to make an imprint, incorporating a gating system, filling the resulting cavity with molten metal, allowing the metal to cool until it solidifies, breaking away the sand mold, and removing the casting.

Sand casting is still the most prevalent form of casting. In the United States alone, about 15 million tons of metal are cast by this method each year.

The Sand Mold

Most sand casting operations use silica sands. Sand is inexpensive and is suitable as mold material because of its resistance to high temperatures. There are two general types of sand: naturally bonded (bank sands) and synthetic (lake sands). Because its composition can be controlled more accurately, synthetic sand is preferred by most foundries. Several factors are important in the selection of sand for molds. Sand having fine, round grains can be closely packed and forms a smooth mold surface. Good permeability of molds and cores allows gases and steam evolved during casting to escape easily. The mold should have a good collapsibility (the casting shrinks while cooling) to avoid defects in the casting, such as hot tearing and cracking. The selection of sand involves certain tradeoffs with respect to properties. For example, fine-grained sand enhances mold strength, but the fine grains also lower mold permeability. Sand is typically conditioned before use. Mulling machines are used to uniformly mull (mix thoroughly) sand with additives. Clay is used as a cohesive agent to bond sand particles, giving the sand strength.

Source: Kalpakjian, Manufacturing Processes for Engineering Materials.