Sand casting, also called sand-molded casting, is a popular industrial process used by foundries to manufacture different parts for machines and instruments using different metals. Foundries prefer sand casting to other methods of mold casting, because sand is extremely cheap and can withstand high temperatures. Sand casting is used to make a broad variety of metal parts, ranging from simple ones like gears and pulleys, to bigger and more complex ones like engine blocks and cylinder heads. The sand-casting molds can be used to make parts that weigh anywhere from 1 ounce to 450 tons.
Sand
Sand casting makes use of silica-based sand that is combined with a chemical to help bind the sand particles together. The quality of the sand used to make molds is critical because the quality of the finished product depends on it. The quality of sand is based on the following parameters: strength (ability to hold its shape), permeability, thermal strength (ability to withstand high temperatures), collapsibility (ability to compress properly), and re-usability.
Mold Types
The type of sand mold used depends on the accuracy and the quality of surface finish required. The four types of sand molds are: green sand, skin-dried, dry sand and no-bake. Green-sand mold is the cheapest and most commonly used mold type. It is a mixture of sand (90 percent), clay or binder (7 percent), and water (3 percent). The skin-dried mold is green-sand mold that has a higher percentage of binder. It is costlier and takes longer to set, but provides improved accuracy. The dry-sand mold has no water content and is baked in an oven for strengthening. It is the most accurate and the most expensive type of molding. The no-bake mold is a mixture of sand and a liquid resin that doesn't require any baking.
Process
The sand-casting process involves six steps: placing the required pattern in the sand to form a mold; clamping the mold properly so there is no leakage; filling up the mold with the molten metal; cooling the metal in the mold; breaking the sand mold and removing the metal casting; and trimming any excess metal from the casting. The cooling process is the most important, because improper cooling can cause shrinkage or cracking in the final product.
Advantages
Sand casting is preferred to other methods of casting because: it can create big components with complex shapes; it can be used for variety of metals; the cost associated with tools and equipment involved in the casting process is relatively lower; the scrap metal can be used again; and the preparation time is relatively shorter.
Disadvantages
The materials created by sand casting are not as strong as those created by other techniques. The rate of production is relatively low for sand casting because one mold only can be used once. The preparation of the final product requires additional machinery and labor, which adds to the cost.
