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Sand casting is a means of producing rough metal Castings that are further refined by any or all of Hammer Peening , Shot Peening , polishing, forging, plating, rough grinding, Machine Grinding or Machining . 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, more than is required by other more accurate casting processes. PATTERNS 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 Riser —are added to the pattern. MOLDING BOX AND MATERIALS A multi-part molding box (known as a Casting Flask , sometimes referred to as the Cope And Drag ) 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 unlatched, 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. In some cases, the sand may be oiled instead of moistened, which makes possible casting without waiting for the sand to dry. Sand may also be bonded by chemical binders, such as furane resins or amine-hardened resins. CHILLS 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. CORES To produce cavities within the casting—such as for liquid cooling in Engine blocks and Cylinder Head s—negative forms are used to produce ''cores''. Usually sand-molded, cores are inserted into the casting box after removal of the pattern. Whenever possible, designs are made that avoid the use of cores, due to the additional set-up time and thus greater cost. 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 , Brass , Aluminum alloy, or various Pot Metal alloys, which often include Lead , Tin , and Zinc . 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. When casting with metals like iron or lead, which are significantly heavier than the casting sand, the casting flask is often covered with a heavy plate to prevent a problem known as ''floating the mold.'' Floating the mold occurs when the pressure of the metal pushes the sand above the mold cavity out of shape, causing the casting to fail. After casting, the cores are broken up by rods or shot and removed from the casting. The metal from the sprue and risers is cut from the rough casting. Various Heat Treatment s 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. DESIGN REQUIREMENTS 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. A slight Taper , known as draft, must be used on surfaces perpendicular to the parting line, in order to be able to remove the pattern from the mold. This requirement also applies to cores, as they must be removed from the core box in which they are formed. The sprue and risers 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 embedded in the final casting, forming a ''sand pit'', which may render the casting unusable. Gas pockets can cause internal voids. These may be immediately visible or may only be revealed after extensive machining has been performed. For critical applications, or where the cost of wasted effort is a factor, non-destructive testing methods may be applied before further work is performed. DECORATIVE USE OF PATTERNS Old wood-patterns, once used to make molds for casting machine parts, are sought out and collected by some for use as interior decorations. ALTERNATIVE CASTING METHODS Sand casting for mass production has largely been superseded by other methods.
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