Machinery's Handbook, 31st Edition
HARD MOLD CASTING 1507 To increase life of permanent molds, surfaces of the mold cavity usually are coated with refractory slurry (such as sodium silicate and clay) or sprayed with graphite every few castings. These coatings also serve as parting agents, as well as thermal barriers, control- ling the rate of cooling of the casting. The molds are clamped together by mechanical means and heated to about 302–392°F (150–200°C) to facilitate metal flow and reduce thermal damage to the dies due to high temperature gradients. Then molten metal is poured through the gating system. After so- lidification, the molds are opened, and the casting is removed. Water or fins may be used in cooling the mold, and mechanical ejectors may be needed for removal of complex castings. Hard mold casting is used mostly for aluminum, magnesium, copper alloys, and gray iron because of their generally lower melting points. Steels also can be cast using graphite or heat-resistant metal molds. This process produces castings with good surface finish, close dimensional tolerances, and uniform and good mechanical properties, and supports high production rates. Typical parts made are automotive pistons, cylinder heads, con- necting rods, gear blanks for appliances, and kitchenware. Vacuum Casting Vacuum permanent mold casting (not to be confused with vacuum molding) is similar to low-pressure permanent mold casting, except for the step of filling the mold. In this case, molten metal is sucked upward into the mold by a vacuum pump (Fig. 10). The permanent mold is enclosed in an airtight bell housing with two openings: the sprue at the bottom, through which molten metal enters the mold, and the vacuum outlet at the top. The sprue opening is submerged below the surface of the molten metal, and the vac- uum is drawn within the housing, creating a pressure differential between the mold cavity and the molten metal in the crucible. This pressure differential causes the molten metal to flow up the sprue and into the mold cavity, where it solidifies. The mold is removed from the housing and opened, and the casting is ejected. By controlling the vacuum, the pressure differential between the mold cavity and the molten metal can be varied, allowing for different fill rates necessitated by certain part designs and gating requirements. This results in tight control of the fill rate, which directly influences soundness of the casting. Because the sprue opening is submerged beneath the surface of the molten metal, only pure alloy, free from oxides and dross, can enter the die cavity. This helps to produce clean, sound castings with minimal foreign materials that detract from strength, appearance, and machinability. With proper part and mold design, this process can greatly reduce or eliminate issues such as voids, shrinks, and gas pockets in critical areas. Typical parts made by vacuum casting include vacuum pump covers, turbine wheels, intake manifolds, and train accessories.
Fig. 10. Vacuum Casting Process a) Before Flow Up of Molten Metal into Cavity; b) After Flow Up of Molten Metal
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