Sand casting

Sand casting is a means of producing rough metal castings that are further refined by peening, forging and/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 machining, more than is required by other more accurate casting processes.

From the design provided by an engineer or designer a craftsperson called a paternmaker will produce a master of the object to be produced, often of 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 (called the sprue) and the exiting of gas (called the vents) are added to the pattern.

A multi–part molding box will be 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 (a slightly moist mixture of sand and clay). The sand is packed in through a 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 then placed upon the sand and another molding box segment is added. Additional sand is rammed over and around the pattern. Finally a cover is placed upon the box and it is turned, 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. 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 which can throw molten metal about.

With a completed mold at the appropriate moisture content, the box containing the sand is then positioned for filling with molten metal (typically iron, steel, bronze, or aluminum). 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, which if iron or steel may still be glowing red hot.

Where it is desired to have most of the casting in a tough, ductile state but a few surfaces must be harder, it is possible (for iron and steel) to introduce into the mold metal plates, called chills where the metal is to be hardened. The rapid cooling will form a finer grained and harder metal at these locations.

For the producing of cavities within the casting (such as for liquid cooling in motor blocks and cylinder heads), negative forms are used to produce cores. These cores are inserted into the casting box after removal of the pattern. After casting the cores are broken up by rods or shot and removed from the casting. The metal from the sprue and vents are cut from the rough casting. Various heat treatments may be applied to relieve stresses from the initial cooling and to add hardness by quenching in water or oil. The casting may be further strengthened by shot peening, which places the surface under compression and so adds resistance to tensile cracking while also smoothing 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. This may be immediately visible or may only be reveled after extensive machining has been performed. For critical applications modern non-destructive testing methods may be applied before further work is performed.

Modern variations on this process use mass production methods to produce thin but accurate molds (superficially resembling paper mache such as is used in egg cartons, but which is refractory in nature) which are then supported by some means (such as dry sand surrounded by a box) during the casting process. Owing 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 engine 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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