and nickel-base alloys-including vac- uum melted alloys. Maximum metallurgical control. So- lidification patterns and grain size can be determined to some degree by con- trolling the ratio of mold temperature to pouring temperature. This insures castings of superior density and in- tegrity, and permits very thin sections with exceptionally sharp detail. Gating is extremely flexible, since it can be made as part of the final pattern. Molds can be poured in air, in an inert atmosphere, or under vacuum, and feed can be gravity, centrifugal, or pressure. Superior finish. Surface finish of the


wax or plastic patterns is naturally high, and the ceramic walls of the in- vested mold can be of porcelain qual- ity. As-cast surfaces range from 60 to 125 rms, and finishes as smooth as 32 rms and better can be obtained with secondary finishing operations. Generally, 125 rms is considered stand- ard for the industry. Precision. Critical tolerances can be


held within a practical range of ±0.003 to 0.005 in.fin., without re- gard to parting line location. General tolerances, applied to nonfunctional areas are more generous: ±0.015 in. for dimensions over 2 in. is generally desirable, although it can be held closer at increased cost. On very small


parts (under 1


4 in.), critical dimensions


can sometimes be held to ±0.002 in. Adaptability to quantity. The proc-


ess permits the average investment casting foundry to accept orders in almost any quantity-small runs, of less than I 00 pieces; medium runs, up to about 2000 pieces; long runs of 25,000 to 50,000 pieces and more. In many cases, permanent tooling for short runs and most medium runs is inexpensive, because adequate injec- tion molding pattern dies can be made from relatively soft materials.


These relative advantages over


other castings can prove absolute ad- vantages where investment castings are compared with machined or fabricated parts designed to perform the same function. Because the castings are shaped by freezing liquid metal in a sculptured cavity, the laws that apply are exactly the opposite of those that obtain for alternative parts shaped by removing metal from stock primary forms. Investment casting is used to its maximum advantage where a hard- to-machine alloy is cast to a hard-to- machine geometry; conversely, it is an uneconomical and generally un- satisfactory process for the production of parts that are simple and easy to machine.


Investment castings have an advan-


a


Stainless steel investment castings made by Hitchiner Manufacturing Co Inc, Milford, NH. Elbow is type 347, valve housing (center) and bracket are type 304.


tage over machined parts of equiva- lent material where the strength-to- weight ratio is important. With ma- chined parts, economy lies in remov- ing as little metal as possible; with in- vestment castings, you just don't put in metal that you don't need. Comparisons of strength are tricky,


because the destruction of test bars tells you little of the serviceability of a complex part under complex load- ing. Purely as metal-test bar for test bar-investment castings have lower mechanical properties than forgings measured in a longitudinal direction, and higher properties than forgings measured at the perpendicular. The casting, however, can be considered equally strong in all directions, be- cause of its random grain structure. Values for castings fall around the mean of the two values for forgings. The strength of the actual part is something else. Here, an investment casting has a


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