Die Casting

Die casting is a manufacturing process used to produce metal parts with precise dimensions, clear contours, smooth or textured surfaces. This is done by pressing molten metal under high pressure into a reusable metal mold. This process is usually described as the shortest distance between the raw material and the finished product. The term "die casting" is also used to describe the finished product.

The term "gravity die casting" refers to a casting made in a metal mold under a gravity head. In the United States and Canada, this is called a permanent mold. The so-called "die casting" here is called "die casting" in Europe.

The Way Die Castings Produced

First, a steel mold capable of producing tens of thousands of castings in rapid succession must be made in at least two sections to allow the castings to be removed. These sections are securely mounted on the machine and are arranged so that one is stationary (fixed die half) and the other is movable (injector die half). To start the casting cycle, the two die halves are clamped firmly together by the die casting machine. Molten metal is injected into the die cavity where it solidifies quickly. The die halves are moved apart and the casting is ejected. Die casting dies can be simple or complex, with movable sliders, cores or other sections, depending on the complexity of the casting.

The complete die casting process cycle is by far the fastest known manufacturing process for precision non-ferrous metal parts. This is in stark contrast to sand casting which requires a new sand mold for each casting. While the permanent mold process uses iron or steel molds instead of sand, it is considerably slower, and not as precise as die casting.


Types of Machines for Die Casting

Regardless of the type of machine used, it is essential that the die halves, cores and / or other moving parts are securely locked in place during the casting cycle. Generally, the clamping force of the machine is governed by (a) the projected surface of the casting (measured at the die cutting line) and (b) the pressure used to inject metal into the die. Most machines use toggle mechanisms actuated by hydraulic cylinders (sometimes air pressure) to achieve lockout. Others use direct hydraulic pressure. Safety interlock systems prevent die opening during casting cycles.

Die casting machines, large or small, differ essentially only in the method used to inject molten metal into the mold. They are classified and described as hot or cold chamber die casting machines.


Hot Chamber Machines

Hot chamber machines are mainly used for zinc and low melting alloys which do not attack easily and do not erode metal pots, cylinders and pistons. Advanced technology and the development of new higher temperature materials have extended the application of this equipment to magnesium alloys.

In a hot chamber machine, the injection mechanism is immersed in molten metal in a furnace connected to the machine. As the plunger is lifted, a port opens, allowing the cylinder to be filled with molten metal. As the plunger moves down to seal the port, it forces molten metal through the neck and nozzle into the die. After the metal solidifies, the plunger is pulled out, the die opens and the resulting casting is ejected.

Hot chamber machines run fast. Cycle times range from less than one second for small parts under one ounce to thirty seconds for castings weighing several pounds. The dies are filled quickly (typically five to forty milliseconds) and the metal is injected under high pressure (1,500 to over 4,500 psi). Nevertheless, modern technology gives tight control over these values, creating castings with fine details, tight tolerances and high strength.


Cold Chamber Machines

Cold chamber machines differ from hot chamber machines mainly in one respect; the injection piston and cylinder are not immersed in the molten metal. The molten metal is poured into the "cold chamber" through the fill opening or slot either manually or with an automatic ladle. A hydraulically operated forward piston seals the port forcing metal into a locked die under high pressure. Injection pressures range from 3,000 to over 10,000 psi for both aluminum and magnesium alloys, and from 6,000 to over 15,000 psi for copper-based alloys.

In the cold chamber machine, more molten metal is poured into the chamber than is needed to fill the die cavity. This helps to maintain enough pressure to reliably fill the cavity with casting alloy. The excess metal is ejected with the casting and forms part of the full shot.

The operation of the "cold chamber" machine is slightly slower than that of the "hot chamber" machine due to the incense operation. The cold chamber machine is used for casting high melting alloys because the piston and cylinder assemblies are less vulnerable to attack because they are not immersed in the molten metal.

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