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There are many types of casting materials used in the production of sculpture or mechanical parts that may be too complicated to make using another technique. Various types of material in liquid or liquefied form are poured into a cast, or mold, containing a hollowed out area in the shape of the part or item desired, and then allowed to harden into a solid form of the mold. One of the oldest types of casting materials is sand which is used to make a single large molded item. Urethane rubber is used in the process of casting concrete and other aggregate materials for use in both indoor and outdoor applications. Plaster is the main ingredient used for casting materials made from various types of low-melting-point metals.
Probably, the least complicated and most widely used of all the different casting processes, sand casting allows cast items to made at a very low cost. Since the mold is destroyed after each use, this type of casting is best for manufacturers who only need a few molded items. The sand used in sand casting is held together by use of chemical binders, clay or certain types of polymerized oils. Sand casting molds are also used in casting very large parts. For example, an entire steel frame for a railroad car, bulldozer or ship may be made from a single sand casting mold.
Urethane rubber is one of the most durable of all casting materials and is used primarily for items made of concrete or other aggregates that will be exposed to the elements. As with casting materials such as plaster, a model is made and the urethane rubber poured or formed around the model. After being allowed to dry, the urethane is then carefully extracted from the model creating a negative form of the item. After concrete is poured into the mold, it is subjected to vibration that draws air bubbles away from the surface of the concrete, creating a smooth finish that allows the urethane to be pulled away easily. The wetcast concrete process uses rubber molds to create smaller, decorative items such as retaining wall blocks, medallions, cornice molding, balusters, and pavers.
Similar to sand casting, plaster mold casting uses the same basic techniques except that plaster of paris is used instead of sand. In certain instances, plaster casting materials allow for more than just a single cast to be made. A model of the part or item is created by pouring plaster around the part, resulting in a negative mold or plaster casting. Molten metal, or other liquid material, is then poured into the mold to make the final part. Excess plaster is removed by mechanical knock-out and pressurized water jets. Casting materials of this sort are only able to handle non-ferrous metals with a low melting point such as copper, aluminum, and zinc.