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Cold Chamber Diecasting Machines

Cold chamber machines differ from hot chamber machines primarily in one respect; the injection plunger and cylinder are not submerged in molten metal. The molten metal is poured into a "cold chamber" through a port or pouring slot by a hand or automatic ladle. A hydraulically operated plunger, advancing forward, seals the port forcing metal into the locked die at high pressures. Injection pressures range from 3,000 to over 10,000 psi for aluminium alloys.

Figure 2: Cold Chamber Machine. Diagram illustrates die, cold chamber and horizontal ram or plunger (in charging position).

In a cold chamber machine, more molten metal is poured into the chamber than is needed to fill the die cavity. This helps sustain sufficient pressure to pack the cavity solidly with casting alloy. Excess metal is ejected along with the casting and is part of the complete shot.

Hot Chamber Diecasting Machines

Hot chamber machines (Fig.1) are used primarily for zinc, and low melting point alloys which do not readily attack and erode metal pots, cylinders and plungers.

Figure 1: Hot Chamber Machine. Diagram illustrates the plunger mechanism which is submerged in molten metal. Modern machines are hydraulically operated and equipped with automatic cycling controls and safety devices.

In the hot chamber machine, the injection mechanism is immersed in molten metal in a furnace attached to the machine. As the plunger is raised, a port opens allowing molten metal to fill the cylinder. As the plunger moves downward sealing the port, it forces molten metal through the gooseneck and nozzle into the die. After the metal has solidified, the plunger is withdrawn, the die opens, and the resulting casting is ejected.

Why Pressure Diecasting?

The decision to choose Pressure Diecasting as the preferred production method is generally driven by a requirement for high annual volume. As the annual demand increases, the lower piece part price offered by the pressure diecasting route results in a far cheaper "Total Project Cost" than other methods of casting Aluminium.

The diagram indicates that should the volumes required be low, then the case for gravity diecasting or sand casting becomes increasingly strong, due to the low start up costs.