Machinery's Handbook, 31st Edition
1510 DIE CASTING in the chamber to fill the cavity of the mold. In preparation for the next cycle of casting, the plunger travels back upward and opens the intake port in the hot chamber, again al- lowing the chamber to fill with molten metal. Hot-chamber die casting has a high rate of productivity. Typical materials used in hot-chamber die casting are zinc, lead, and magnesium alloys and other low melting alloys. Common applications of hot-chamber die casting parts in- clude housings with thin walls, often requiring many ribs. Cold-Chamber Process.—Cold-chamber die casting is a permanent mold casting pro- cess. Large, robust machines are used to exert the great clamping force necessary to hold the mold closed against very high pressure during the manufacturing process. A metal cold chamber is located at the entrance of the mold. A piston is connected to this chamber, which, in turn, is connected to a power cylinder. At the start of the process, the correct amount of molten metal for a single part is poured into the shot chamber from an external source holding the metal for the cast. When the power cylinder forces the piston forward in the chamber, it cuts off the in- take port. The piston forward forces molten metal into the casting mold cavity with high pressure, which is maintained during solidification. After solidification, the pressure is released, the mold is opened, and the casting is removed using ejector pins. In comparison with hot-chamber die casting, cold-chamber die casting uses higher pressure to force molten metal into the die cavity. Pressures as high as 3000–50000 psi (21 MPa–345 MPa) may be used. Fig. 13 illustrates the cold-chamber die casting process. Cold-chamber die casting is ideal for metals with high melting points and corrosive prop- erties, such as aluminum alloys and other alloys with high melting points. This process is used for several automobile components, including pistons, cylinder heads, and engine blocks. Other common die cast parts are propellers, gears, bushings, pumps, and valves.
Fig. 13. Cold-Chamber Die Casting Process: a) Die is Closed, Plunger Withdrawn, and Molten Metal is Poured into Chamber; b) Plunger Forces Metal in Chamber to Flow into Die Cavity, Maintaining Pressure During Solidification; After Plunger is Withdrawn, Die is Opened, and Casting is Ejected Centrifugal Casting As the name suggests, this process involves using a centrifugal force (high-velocity spinning action) to exert pressure on molten metal as it is poured into a die. The die can be set to spinning on a horizontal or vertical axis, depending on the structure required. Usu- ally, a horizontal axis machine is used for long, thin cylinders, and vertical axis machines are used for rings. This method of casting, also known as liquid forging, is used to manufacture high- integrity cylindrical components, and therefore is a technology of choice for pipes, tube, bearings, rolls, bushes, ball valves, and cylinder liners. Notably, it is used to cast cylinder liners and sleeve valves for piston engines, parts that cannot reliably be manufactured otherwise. Wall thickness of parts can be varied to suit specific needs; this thickness is controlled by the amount of molten metal that is poured, making the centrifugal casting system highly flexible to individual requirements.
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