Metal Casting Processes

Chapter 6 Metal Casting Processes 6.1 INTRODUCTION As with polymer processing, there are also a number of metal casting processes. Although there are distinct differences between these many casting processes, there are also many common characteristics. For example, in all casting processes, a metal alloy is melted and then poured or forced into a mold where it takes the shape of the mold and is allowed to solidify. Once it has solidified, the casting is removed from the mold. Some castings require finishing due to the cast appearance, tolerance, or surface finish requirements. During solidification, most metals shrink (gray cast iron is an exception) so molds must be made slightly oversize in order to accommodate the shrinkage and still achieve the desired final dimensions. There are two basic types of molds used in castings, namely, expendable molds (sand casting and investment casting) that are destroyed to remove the part, and permanent molds (die casting). Expendable molds are created using either a permanent pattern (sand casting) or an expendable pattern (investment casting). Permanent molds, of course, do not require a pattern. Two of the major advantages for selecting casting as the process of choice for creating a part are the wide selection of alloys available and the ability, as in injection molding, to create complex shapes. However, not all alloys can be cast by all processes. The most common metal casting processes are sand casting, investment casting, and die casting. These are described in Sections 6.2 to 6.4 below. Other casting processes are more briefly described in Section 6.5. The nature of the molds used and the method for removing the part from the mold differs for the various processes. The tolerances and surface finishes achievable are also different. For more information on casting processes and technology, consult ASM Metals Handbook in the reference list. 6.2 SAND CASTING Sand casting is a process in which a sand mold is formed by packing a mixture of sand, a clay binder, and water around a wood or metal pattern that has the same external shape as the part to be cast. A pattern can come in two halves: a top half (called a cope) and a bottom half (called a drag) (Figure 6.1). Each half is placed in a molding box, and the sand mixture is then poured all around the pattern. After the sand is packed, holes, which are used to pour the molten metal into the mold (sprue) and to be used as a reservoir of molten metal (risers), are formed in the sand. Vents are also created in order to allow the escape of gases from the melt. Then the pattern is removed and a runner system or small 115

Metal Casting Processes

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