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Sand casting is a means of producing rough metal castings that are further refined by peening, forging or machining, or both. Sand castings not further worked by polishing or peening are readily recognized by the sand-like texture imparted by the mold. As the accuracy of the casting is limited by imperfections in the mold making process there will be extra material to be removed by grinding or machining (fettling) more than is required by other more accurate casting processes.
From the design, provided by an engineer or designer, a craftsperson called a patternmaker produces a master of the object to be produced, often using wood. As the metal to be cast will shrink somewhat between the time it first solidifies and the time it is cool the master must be made slightly larger than the finished product. To simplify the making of the pattern the patternmaker will use an appropriately scaled oversize ruler—called a shrink rule—specific to the type of metal to be cast. Additional paths for the entrance of metal—the sprue—and the exiting of gas—the vents—are added to the pattern.
A multi-part molding box is prepared to receive the pattern. Molding boxes are made in segments that may be latched to each other and to end closures. For a simple object—flat on one side—the lower portion of the box, closed at the bottom, will be filled with prepared casting sand or green sand—a slightly moist mixture of sand and clay. The sand is packed in through a vibratory process called ramming and, in this case, periodically screeded level. The surface of the sand may then be stabilized with a sizing compound. The pattern is placed on the sand and another molding box segment is added. Additional sand is rammed over and around the pattern. Finally a cover is placed on the box and it is turned and unlached, so that the halves of the mold may be parted and the pattern with its sprue and vent patterns removed. Additional sizing may be added and any defects introduced by the removal of the pattern are corrected. The box is closed again. This forms a "green" mold which must be dried to receive the hot metal. If the mold is not sufficiently dried a steam explosion can occur that can throw molten metal about.
If it is desired to have most of the—iron or steel—casting in a tough, ductile, state but with a few surfaces hard, it is possible to introduce, into the mold, metal plates—chills—where the metal is to be hardened. The associated, local, rapid, cooling will form a finer-grained and harder metal at these locations.
To produce cavities within the casting—such as for liquid cooling in engine blocks and cylinder heads)—negative forms are used to produce cores. These, usually sand-molded, cores are inserted into the casting box after removal of the pattern.
With a completed mold at the appropriate moisture content, the box containing the sand mold is then positioned for filling with molten metal—typically iron, steel, bronze or aluminum alloy. After filling with liquid metal the box is set aside until the metal is sufficiently cool to be strong. The sand is then removed revealing a rough casting that, in the case of iron or steel, may still be glowing red.
After casting the cores are broken up by rods or shot and removed from the casting. The metal from the sprue and vents is cut from the rough casting. Various heat treatments may be applied to relieve stresses from the initial cooling and to add hardness—in the case of steel or iron, by quenching in water or oil. The casting may be further strengthened by surface compression treatment—like shot peening—that adds resistance to tensile cracking and smoothes the rough surface.
The part to be made and its pattern must be designed to accommodate each stage of the process, as it must be possible to remove the pattern without disturbing the molding sand and to have proper locations to receive and position the cores. The sprue and vents must be arranged to allow a proper flow of metal and gasses within the mold in order to avoid an incomplete casting. Should a piece of core or mold become dislodged it may be imbedded in the final casting, forming a sand pit. Gas pockets can cause internal voids. These may be immediately visible or may only be reveled after extensive machining has been performed. For critical applications, or where the cost of waste is a factor, non-destructive testing methods may be applied before further work is performed.
Modern mass production methods can produce thin but accurate molds—superficially resembling paper mache, such as is used in egg cartons, but that is refractory in nature—that are then supported by some means—such as dry sand surrounded by a box—during the casting process. Due to the higher accuracy it is possible to make thinner and hence lighter castings—extra metal does not have to be present to allow for variations in the molds—these thin-mold casting methods have been used since the 1960s in the manufacture of cast-iron blocks and heads.
Old wood-patterns, once used to make molds for casting machine parts, are sought out and collected by some for use as interior decorations.
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