Reprinted from Product Engineering. Copyright 1964 by McGraw-Hi//.


When to consider Investment casting


To update two previous reports, here's a fresh look at the limitations as well as the advantages of investment casting


bred on a rich diet of aerospace-atomic R & D, and now establishing itself as a competitive source of custom parts for commercial and consumer prod- ucts. It is a national industry, based primarily on a multiplicity of locally owned, job-shopping foundries scat- tered through the outskirts of the ma- jor manufacturing areas. There are a few giants-heavily capitalized opera- tions with plants and licensed repre- sentatives across the country, all tooled up to handle big pieces and big contracts. There are a number of spe- cialists: Some work only in miniature; some cast only nonferrous alloys; some limit their services to special industries; some specialize in vacuum-melted met- als. But it is still basically an industry of versatile general practitioners, pre- pared to quote on any job that comes in over the transom, whatever the metals or quantities involved. This means that investment castings are easily available from many independ- ent suppliers-an important consid- eration when comparing processes. What's new is the successful indus- trialization of an ancient craft: As a production tool for industry, invest- ment casting is strictly post-war, but the process itself has been in continu- ous use since at least the 16th Century


6


JOHN PROCTOR, Co11trib11ti11g Editor T HE investment casting industry is


comparatively new-a war baby, All photos by Sid Carson, McGraw-Hill Studio.


(it was described by Cellini), and its origins probably go back to prehistory. It is a basic process, not a modification of another casting technique. It has been known by other names: "preci- sion casting" (still the listing in classi- fied telephone directories), "lost-wax", "cire perdue"-but the variations are only nomenclatural. Whatever you call it, the process is unique. It involves a sort of transmutation of materials through the use of expend- able tooling. To obtain a part of a given shape in a metal of specified properties, investment casting starts by producing the shape in a material of totally different properties. A high- quality injection molding of wax or polystyrene is produced and finished as carefully as if it were the final prod- uct-more carefully, because it must be held to even closer tolerances. It is then destroyed. But the shape is salvaged. Before it


is burned, the pattern is carefully in- vested (that is, enveloped) in a refrac- tory ceramic material. When the pat- tern is burned out, a perfect negative impression of its shape remains in the one-piece mold. The mold is then cured as carefully as if it were the final prod- uct-and then it, too, is destroyed, in the casting process. The metal part that remains has approximately the shape and the surface quality of the


"lost" pattern. Because of the tight processing controls permitted, it also has superior mechanical properties.


Advantages


All other casting techniques make compromises to salvage or simplify tooling, and to permit access to the cavity. Investment casting, by sacri- ficing both the pattern and the mono- lithic mold, obtains certain advantages: Intricacy. There is no parting line


in the mold itself, and parting line problems with the injection-molded patterns can be eliminated. With a few limitations, the process permits you to cast in metal any shape that can be molded in wax or plastic. This permits the use of collapsible coring, and even the inclusion of molded-in soluble cores, which can be removed from the pattern to form a hollow interior or other opening. By simple "wax weld- ing," two or more patterns can be assembled into complicated structures that could not be molded as a single piece, because of parting line difficul- ties. Draft is not required, and under- cuts and reentrants are possible. Materials. The selection of alloys for investment casting is almost unlimited. It includes: heat-treatable aluminum, cobalt-base alloys, brasses, copper, car- bon and low-alloy steels, a variety of stainless steels, tool steels, magnesium,


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