Machinery's Handbook, 31st Edition
1516
Design Considerations for Casting
Fig. 20. Redesigned Casting Parts to Avoid Hot Spot and Fillet Joint Design Consideration.—The design of a junction in a casting can cause shrinkage cavities to form. In Fig. 20, L, T, V, and Y junctions are shown. The thickness of the casting here is consistent, though at the center of the junction, there is an area where the casting’s solidification is slower than the rest of the junction. These hot spots, circled in the figure, are located such that the material around them, which will undergo solidification first, will cut off the hot spots from the flow of molten metal. The flow of casting material must be carefully considered when manufacturing such junctions. If there is flexibility in the design of the casting, the designer may want to think about redesigning the junction. Some possible design alternatives are shown in Fig. 20 that should reduce formation of hot spots and shrinkage porosity. Location of the Parting Line.—A parting line in industrial casting of molds is the bor- derline in which draft angles change direction. That is, it is the dividing line that splits the core and cavity halves of a molded part and is sometimes a starting point for the mold part- ing surface. In engineering drawings, a parting line is often abbreviated as PL; ASME’s Y14.8 standard specifies a symbol. The parting line of a casting must be determined early in the design processes. In de- ciding on its location, the part designer and die cast engineer should work together to consider its influence on part functionality, draft angle, wall thickness, metal flow, cast - ing integrity, and tolerances. Often, it is useful to have most geometry in the ejector half; this prevents the casting from sticking in the cover half. Cosmetic surfaces typically are placed on the cover side. Gates, overflows, and vents will connect to the casting at the parting line. Also, flash will form at the parting line when the pressure from the injected metal tries to force the die halves apart. During the trimming process, the gates, overflows, vents, and flash will have to be removed by the trim die; a well-designed parting line will ease the trimming process. When flash occurs, it increases the thickness of the parting line, altering dimensions measured across this line. For critical dimensions, it is important to have measurements in only one die half to reduce dimensional tolerance. The parting line also can affect the life of the die. Metal injected straight onto a surface will erode that surface faster. Designating a parting line that follows the contours of the design and so that metal will flow more smoothly helps to optimize die life, casting qual - ity, and aesthetics of a die cast part. Since parting lines will be noticeable as a bump on the surface of the part, they should not be located on a sliding surface. Similarly, parting lines should not be located on a sealing surface, where a bump or mismatch would prevent the seal from making complete
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