Casting

Investment Casting - Very fine surface finishes and excellent reproduction of detail are some characteristics of investment casting or lost wax process. The process was practiced by several ancient cultures and has survived virtually without modification for the production of artwork, statuary and fine jewelry. Today, the process's most important commercial application is in the casting of complex, net shape precision industrial products such as impellers and gas turbine blades. The process first requires the manufacture of an intricate metal die with a cavity in the shape of the finished product (or parts of it, if the product is to be assembled from several castings). Special wax, plastic or a low melting alloy is cast into the die, then removed and carefully finished using heated tools. Clusters of wax patterns are dipped into a refractory/plaster slurry, which is allowed to harden as a shell or as a monolithic mold. The mold is first heated to melt the wax (or volatilize the plastic), then fired at a high temperature to vitrify the refractory. metal is introduced into the mold cavity and allowed to cool at a controlled rate. Investment casting is capable of maintaining very high dimensional accuracy in small castings, although tolerances increase somewhat with casting size. Dimensional consistency ranks about average among the casting methods; however, surface finishes can be as fine as 60 gin (1.5 gm) rms, and the process is unsurpassed in its ability to reproduce intricate detail. Investment casting is better suited to castings under 100 lbs (45 kg) in weight. Because of its relatively high tooling costs and higher than average total costs, the process is normally reserved for relatively large production runs of precision products, and is not often applied to copper alloys.