tailieunhanh - 12Design for Sand Casting

. | 12 Design for Sand Casting INTRODUCTION Sand casting is a process in which molten metal is poured into sand molds which are broken away from the cast part after the metal has solidified. The sand molds are made of two halves in which impressions are made by a pattern. When the two halves are assembled the impressions form a cavity into which the molten metal is poured. The pattern resembles the part but is slightly larger to accommodate for the metal shrinkage that occurs during cooling. Patterns may contain multiple impressions to form multicavity sand molds for the economical production of large quantities of castings. A mixture of sand and a binding agent is compacted around the pattern to form the two mold halves in separate metal flasks. Historically the lower mold half is named the drag and the upper mold half is referred to as the cope. Channels are formed in the drag to feed the molten metal to the cavity or cavities in a multicavity mold. The opening of a feed channel to its cavity is the gate but often the complete set of feed channels is referred to as the gating system. A pouring cup and vertical tapered channel in the cope are used to deliver molten metal to the feed channels. Two additional features may sometimes be required in a sand mold. First one or more separate sand cores may be placed in formed locations called core prints during assembly of the mold halves. Second an additional cavity called a riser may be formed in the cope to supply molten metal pressure during solidification and shrinkage of the casting. Sand cores are used in sand casting to produce internal cavities or undercuts. Since the cores are broken out of the casting after solidification complex internal cavities are readily producible. The main requirement is that the internal surfaces 517 Marcel Dekker Inc. 270 Madison Avenue New York New York 10016 518 Chapter 12 FIG. Assembled cope and drag mold. be accessible for the subsequent cleaning processes to remove adhering