Machinery's Handbook, 31st Edition
1506 PRECISION INVESTMENT CASTING pressure. The procedure is governed by the kind of alloy, the size of the investment cavity, and its contours or shape. Investment Removal.— When the casting has solidified, the investment material is re - moved by destroying it. Some investments are soluble in water, but those used for ferrous castings are broken by using pneumatic tools, hammers, or by shot or abrasive blasting and tumbling to remove all particles. Gates, sprues, and runners may be removed from the castings by an abrasive cutting wheel or a band saw according to the shape of the cluster and machinability of the material. Accuracy of Investment Castings.— The accuracy of precision investment castings may, in general, compare favorably with that of many machined parts. The overall tolerance varies with the size of the work, the kind of metal and the skill and experience of the operators. Under normal conditions, tolerances may vary from ± 0.005 or ± 0.006 inch per inch (οr mm/mm), down to ± 0.0015 to ± 0.002 inch per inch(or mm/mm), and even smaller tolerances are possible on very small dimensions. Where tolerances applying to a lengthwise dimension must be smaller than would be normal for the casting process, the casting gate may be placed at one end to permit controlling the length by a grinding opera tion when the gate is removed. Casting Weights and Sizes.— Investment castings may vary in weight from a fractional part of an ounce up to 75 lb (34 kg) or more. Although the range of weights representing the practice of different firms specializing in investment casting may vary from about 1 ∕ 2 pound up to 10 or 20 lb (4.5 or 9.1 kg), a practical limit of 10 or 15 lb (4.5 or 6.8 kg) is common. The length of investment castings ordinarily does not exceed 12 or 15 in. (30 or 38 cm), but much longer parts may be cast. It is possible to cast sections having a thickness of only a few thousandths of an inch, but the preferred minimum thickness, as a general rule, is about 0.020 in. (0.50 mm) for alloys of high castability and 0.040 in. (1 mm) for alloys of low castability. Design for Investment Casting.— As with most casting processes, best results from in- vestment casting are achieved when uniform wall thicknesses between 0.040 and 0.375 in. (1 and 10 mm) are used for both cast components and channels forming runners in the mold. Gradual transition from thick to thin sections is also desirable. It is important that molten metal should not have to pass through a thin section to fill a thick part of the casting. Thin edges should be avoided because of the difficulty of producing them in the wax pattern. Fil - lets should be used in all internal corners to avoid stress concentrations that usually accom- pany sharp angles. Thermal contraction usually causes distortion of the casting, and should be allowed for if machining is to be minimized. Machining allowances vary from 0.010 in. (0.25 mm) on small, to 0.04 in. (1 mm) on large parts. With proper arrangement of castings in the mold, grain size and orientation can be controlled and directional solidification can often be used to advantage to ensure desired physical properties in the finished components. Casting Milling Cutters by Investment Method.— Possible applications of precision in- vestment casting in tool manufacture and in other industrial applications are indicated by its use in producing high-speed steel milling cutters of various forms and sizes. Removal of the risers, sand blasting to improve the appearance, and grinding the cutting edges are the only machining operations required. The bore is used as cast. Numerous tests have shown that the life of these cutters compares favorably with high-speed steel cutters made in the usual way. Hard Mold Casting Hard mold casting is permanent mold casting. Two halves of a mold are made from ma- terials such as cast iron, steel, bronze, graphite, or refractory metal alloys. The mold cavity and gating system are machined into the mold and become an integral part of it. To produce castings with internal cavities, cores made of metal or sand aggregate are placed in the mold prior to casting. Typical core materials are oil-bonded or resin-bonded sand, plaster, graphite, gray iron, low-carbon steel, and hot-work die steel.
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