Expendable Casting Processes

Green (Moist) Sand Mold: In this method clay is used to bond sand around reusable patterns to form the mold. The moisture remains in the mold until the molten metal is poured. Cast iron is the primary material used, but steel and aluminum alloys can also be cast in this method. Consistent tolerances are possible, but machining is necessary for dimensional accuracy. This cheap, quick, most widely used method is helpful for single castings or mass production but porosity can develop due to the gases evolved from the mold. The part's surface is also rough and tolerances are poor.

Investment Casting (Lost Wax Method): In this processes, wax patterns made from a metal die are mounted on a 'tree'. A ceramic shell is formed around the tree through dipping and stuccoing. The wax pattern is then melted out and the shell is fired. The liquid metal is poured into the shell and solidifies. Advantages of investment casting include: the capability to cast parts of great complexity, intricacy, and diversity; close dimensional control; good surface finish; dies used to make the wax patterns are made from aluminum alloy rather than expensive die steel; the wax can be recovered and reused and additional machining is not normally required. The process is meant for quantity production, however, so a high production rate is necessary for the process to be economical.

Shell Casting: In shell casting, sand coated with thermosetting resin is dropped onto a heated metal pattern where it melts and the resin cures, forming a shell whose halves are stripped from the mold. This process allows for good surface quality and reproduction of detail but expensive metal patterns are required so high production rates must be run for it to be economical.

Vacuum Casting (The V-Process): In the vacuum casting process the two halves of a sand mold are covered with two plastic films through vacuum forming. The pattern is removes and the formed plastic sheets are put together to form a mold cavity that is surrounded by a flask filled with sand. The mold cavity is kept in a vacuum as the molten metal is poured into it to ensure easy flow. This process eliminates the need for special molding sands with binders as well as the problems associated with green sand molding such as gas bubbles caused by excess humidity. It also increases the efficiency of material utilization and provides a good surface finish and dimensional tolerances. The process is restricted, however, to plate like castings.

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