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What is sand Casting

Source:Renyi    Time:11-08-24

Sand casting is a type of casting in which a metal part (itself called a casting) is cast in an expendable mold made from sand packed around a replica of the object to be cast (called a pattern). After the sand is packed around it, the pattern is removed, and the molten metal is poured. After the metal has solidified and cooled, the sand is swept away, freeing the casting. 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 may be extra material to be removed by grinding or machining, more than is required by other, more accurate casting processes. Furthermore, because the mold is destroyed in order to retrieve the object, a new mold must be made for each casting. (The pattern is reusable.) However, sand casting offers the advantage of allowing the casting of iron and steel, whose melting points are too high for processes such as die casting.

From the design, provided by an engineer or designer, a skilled pattern maker builds a pattern of the object to be produced, using wood, metal, or plastic; other materials such as expanded polystyrene. Sand can even be ground, swept or even strickled into shape. The metal to be cast will contract during solidification, and this may be non-uniform due to uneven cooling. Therefore, the pattern must be slightly larger than the finished product, a difference known as contraction allowance. Pattern-makers are able to produce suitable patterns using 'Contraction rules' (these are sometimes called "shrink allowance rulers" where the ruled markings are delibeartley made to a larger spacing according to the percentage of extra length needed). Different scaled rules are used for different metals because different metals / alloys contract by differing amounts. Patterns also have core prints; these create registers within the molds, into which are placed sand cores. Sand cores are used to create under cut profiles and holes which cannot be molded.

Paths for the entrance of metal, during the pouring (casting) process into the mold cavity constitute the runner system and include the sprue, various feeders which maintain a good metal 'feed' and 'runners', and in-gates which attach the runner system to the casting cavity. Gas and steam generated during casting exit through the permeable sand or via the riser, are added either in the pattern itself, or as separate pieces.

A multi-part molding box (known as a casting flask, the top and bottom halves of which are known respectively 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.