clear advantage, since it can eco- nomically incorporate strengthening design values-stress-relieving corner fillets, streamlined transitions, rein- forcing ribs-and distribute the mass where it matters. Machined pieces may be weakened by notches created in sharp corners, or by tool marks. When investment castings are com-


pared to alternative custom parts, the great advantages appear: • A complex, one-piece casting can


replace a multipiece assembly or weld- ment, with substantial improvement in quality and appearance, and usu- ally at a fraction of the alternative cost. • Alloys that cannot be econom-


ically machined because of tool wear and high spoilage can be economically investment cast, with a precision that requires little or no secondary finish- ing. • Shapes that could be obtained


only by hogging out a solid block of expensive metal can be investment cast in quantity. • Investment casting produces easily


the shapes that are hardest to ma- chine, such as compound, three-di- mensional curved surfaces. • Investment casting involves a minimum of waste. • There are aesthetic considerations


that may favor investment castings. Because they are essentially metal replicas of molded structures, they are strikingly compatible with the highly styled thermoplastic injection moldings that mark a conspicuous trend in contemporary design. The present industry developed from a jeweler's craft, and jewelry effects are still readily obtained.


Limitations


The basic limitation on the use of investment castings is, of course, eco- nomic. They are more expensive than castings produced by any other proc- ess, on a price-per-piece basis. The question is whether they are more or less expensive than equivalent finished parts produced by an alternative proc- ess. Or, if costs are at a standoff, whether they offer more or less real value for the price. Decisions must be made in terms of the true, ulti- mate cost of the finished product, and include the costs of secondary opera- tions, finishing, and assembly, amorti- zation of tooling, and all of the ac- cumulating hidden factors that can be isolated.


Consider the overhead equipment


and operations tha·t could be elimi- nated by the quantity purchase of highly finished


prec1s1on castings. There may be quite a number: set- 8


ups, jigs and fixtures; cutter inven- tory and maintenance; redundancy of drawings; operation-by-operation in- spection and attrition rate; handling and paperwork-all the nickels and dimes and couple-of-bucks that you would rather overlook. But you can't compare costs with-


out inviting bids. When is it worth while even to consider investment cast- ing as an alternative process? There are no fiat answers, of course, but there are generally applicable rules of thumb. Inquiries may not be a total waste of everybody's time when the logical alternative is: • Hogging a quantity of intricate


parts from the solid. • Finishing a forging or rough cast-


ing in more than five operations, espe- cially where repeated heat cycling and straightening may be involved. • High-skill machining, on the tool-


and-die shop level, where anticipated machining time exceeds one hour. • Assembly of high-strength parts


into a rigid structure of critical func- tion. • Die casting or powder metallurgy


on low and medium runs that can- not justify tooling costs. • Die casting, on production runs,


where properties of die casting alloys are inadequate. If investment casting can be con- sidered as an economical alternative,


there are further limitations that you must work within. If you understand the limitations of the process and work within them, you can do a great deal to mitigate the cost of the castings without compromising the value of the ultimate product. There are a few key facts that you


must keep clamped in your mind when you design and specify for investment casting. One is that the process demands


high skill in three technologies: plas- tic injection molding; ceramics; metal- lurgy. In all cases, shrinkage must be predicted from the pattern die to the pattern, from the pattern to the heated mold, from the heated mold to the cooled metal of the ultimate casting. (On short runs, there may be still an- other step, at the beginning, from a master pattern to a soft metal die.) There is obviously some loss of resolu- tion at each development from nega- tive to positive. This means that final casting toler-


ances are reflected backward through the process, so that each ±0.005 in tolerance on the drawing can impose a ±0.0005 in. tolerance on the pat- tern die. Even the general, fractional tolerance of 1I64 in. per 2 in. can hold the diemaker to less than one-mil-per- inch. Exactly how tight die toler- ances are to be is determined by a precise knowledge of the molding,


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