Machinery's Handbook, 31st Edition
DIE CASTING
1509
Die Casting Die casting is a commonly used type of permanent mold casting process used for pro- ducing many components of home appliances, such as stoves, fans, washing machines, dryers, refrigerators; various kinds of motors; toys; and hand tools. Die casting molds, commonly called dies , are expensive and require significant lead time to fabricate. How - ever, surface finish and tolerance of die cast parts are so good that there is almost no post- processing required. There are two common types of die casting: hot- and cold-chamber die casting. Advantages of Die Casting: Depending on the casting material, excellent dimensional accuracy can be achieved—typically within 0.005 in. for the first inch (0.127 mm for the first 25 cm) and 0.002 in. for each additional inch (0.051 mm for additional 25 cm). Compared with sand and permanent mold casting, parts can have thinner walls (approxi- mately 0.030 in. or 0.76 mm). Inserts, such as threaded inserts, heating elements, and high-strength bearing surfaces, can be cast in. Casting tensile strength may be as high 60 ksi (414 MPa). Smooth cast surfaces of 40–100 μin (1–2.5 μm) Ra reduce or eliminate secondary machining operations. And rapid production rates can be achieved. Disadvantages of Die Casting: Both the casting equipment required and the die com- ponents are very costly, so large production volume is needed to make this process eco- nomical. The process is limited to high-fluidity metals. Casting weight must be under 24 in. (600 mm) in size and weight between 1 oz (28 g) and 20 lb (9 kg). Parts that need to be hardened and tempered cannot be cast in dies. Another consideration of the standard die casting process is that the final casting will have a small amount of porosity. This does not accommodate heat treatment or welding, because heat causes gas in the pores to expand, which causes micro-cracks inside the part and exfoliation of the surface. Thus, a related disadvantage of die casting is that it is only suitable for parts in which softness is acceptable. Hot-Chamber Process.—Hot-chamber die casting is one of the two main techniques in the manufacturing process of die casting. A characteristic common to both die casting processes is the use of high pressure to force molten metal through a mold. Advantageous characteristics of manufacturing by die casting, such as a smooth casting surface, can be attributed to the use of pressure to ensure the flow of metal through the die. In hot-chamber die casting (Fig. 12), the die is attached to the casting machine and an integral part of the casting equipment.
Fig. 12. Hot-Chamber Die Casting Process: a) Die Is Closed, Plunger Is in Contact with Molten Metal in Start Position; b) Plunger Forces Metal in Chamber to Flow into Die Cavity, Maintaining Pressure During Solidification; After Plunger Is Withdrawn, Die Is Opened, and Casting Is Ejected The hydraulic cylinder of the casting machine over the plunger rod provides power to the plunger, which is in contact with the molten metal. At the start of the casting cycle, the plunger is at the top of the hot chamber. As the cycle begins, the hot chamber is filled with liquid metal through the intake port. When the casting machine’s hydraulic cyl- inder forces the plunger downward, it travels past the intake port, cutting off the flow of molten metal to the hot chamber and ensuring the correct amount of molten metal is
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