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1ODesign for Die Casting10e

1 | 10 Design for Die Casting 10.1 INTRODUCTION The die casting process also called pressure die casting is a molding process in which molten metal is injected under high pressure into cavities in reusable steel molds called dies and held under pressure during solidification. In principle the process is identical to injection molding with a different class of materials. Die casting can in fact produce parts that have identical geometries to injection-molded ones. The reverse is also true and much of the increase in the use of injection molding since the mid 1980s has been as a substitute for part types that were previously die cast. In many cases this has been a wise substitution resulting in decreased parts costs. However for structural parts particularly those for which thick-wall injection moldings are required die casting can often be the better selection. The analysis of die casting costs in this chapter closely parallels the early costing procedure for injection molding given in Chapter 8. This is intended to allow comparisons of the two processes to be made with a minimum of redundant effort. 10.2 DIE CASTING ALLOYS The four major types of alloys that are die-cast are zinc aluminum magnesium and copper-based alloys. The die casting process was developed in the 19th century for the manufacture of lead tin alloy parts. However lead and tin are now very rarely die-cast because of their poor mechanical properties. A tabulation of the specific gravity mechanical properties and cost of commonly used examples of the four principal alloy groups is given in Table 10.1. 423 Marcel Dekker Inc. 270 Madison Avenue New York New York 10016 424 Chapter 10 TABLE 10.1 Commonly Used Die Casting Alloys AUoya Specific gravity Yield strength MN m2 Elastic modulus GN m2 Cost kg Zamalf1 6.60 220 66 1.78 Zamak 5 1 6.60 270 73 1.74 A13 2 2.66 130 130 1.65 A360 2 2.74 170 120 1.67 ZA8 3 6.30 290 86 1.78 ZA27 3 5.00 370 78 1.94 Silicon brass 879 4 8.50 240 100 6.60 Manganese 4-1 8.30 190 .