Regardless of the type of machine used, it is essential that die halves, cores, and/or other moveable sections be securely locked in place during the casting cycle. Generally, the clamping force of the machine is governed by:

• the projected surface area of the casting (measured at the die parting line)

• the pressure used to inject metal into the die.

Most machines use toggle-type mechanisms actuated by hydraulic cylinders (sometimes air pressure) to achieve locking. Others use direct acting hydraulic pressure. Safety interlock systems are used to prevent the die from opening during the casting cycle.

Die casting machines, large or small, vary fundamentally only in the method used to inject molten metal into the die. These are classified and described as either hot or cold chamber die casting machines.

Hot chamber machines are used primarily for zinc and low melting point alloys which do not readily attack and erode metal pots, cylinders and plungers. Advanced technology and development of new higher temperature materials has extended the use of this equipment for magnesium alloys.

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 After the metal has solidified, the plunger is withdrawn, the die opens, and the resulting casting is ejected.

Hot chamber machines are rapid in operation. Cycle times vary from less than one second for small components weighing less than one ounce to thirty seconds for a casting of several pounds in weight. Dies are filled quickly (normally between five and forty milliseconds) and metal is injected at high pressures (1,500 to 4,500 psi). Nevertheless, modern technology gives close control over these values, thus producing castings with fine detail, close tolerances and high strength.

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 10,000 psi for both aluminum and magnesium alloys, and from 6,000 to 15,000 psi for copper base alloys.

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.

Operation of a "cold chamber" machine is a little slower than a "hot chamber" machine because of the ladling operation. A cold chamber machine is used for high melting point casting alloys because plunger and cylinder assemblies are less subject to attack since they are not submerged in molten metal.