Machinery's Handbook, 31st Edition
1508
SLUSH CASTING Slush Casting
Slush casting is a traditional method of permanent mold casting process in which the molten metal is not allowed to completely solidify in the mold. When the desired thick- ness is obtained, the remaining molten metal is poured out. This permanent mold casting process can be used to create hollow castings with thin walls, based on the principle that a solidified skin develops first in a casting and becomes thicker with time. A disadvantage of slush casting is that the casting internal geometry cannot be effec- tively controlled. But good surface finish and accurate external geometry are possible. Suitable for small production runs, slush casting generally is used for making ornamental and decorative objects, including lamp bases and stems, and toys from metals with low melting temperatures, such as zinc, tin, and lead alloys. The sequence of operations in slush casting is as follows: 1) The molten metal is poured into the metal mold. 2) After the desired thickness of the solidified skin is obtained, the mold is inverted, and the remaining molten metal is poured out. 3) Upon solidification and cooling, the mold halves are opened, and the casting is removed. Pressure Casting Another variation of permanent mold casting is pressure casting, also known in industry as low-pressure casting or pressure pouring (Fig. 11). Instead of pouring molten metal into the mold and gravity distributing it throughout the mold, pressure casting uses air (gas) pressure to force the liquid metal through the gating system and the metal casting’s cavity. As in other permanent mold methods, the mold needs to be able to open and close for removal of the workpiece, so complicated casting geometry is limited. However, since the refractory tube is submerged in the molten metal and the metal drawn up into the casting comes from well below the surface, the risk of gas entrapment in the metal and oxidation are greatly reduced. The sequence of operations in pressure casting is as follows: 1) The molten metal is forced upward by gas pressure into the graphite or metal mold through the refractory tube, using an overpressure by air of about 12 to 15 psi (0.08 to 0.1 MPa). 2) Pressure is maintained until the metal has solidified completely in the mold. Pressure casting can be used to produce metal castings with superior mechanical proper- ties, good surface finish, and close dimensional accuracy. High setup costs make pressure casting inefficient for mass runs, but excellent productivity rates make it suitable for batch manufacture. This process can be used to cast high-quality manufactured parts. Steel cast- ings, such as railroad car wheels, often are cast in graphite molds using this method.
Fig. 11. Low-Pressure Casting Process
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