What is Die Casting

Die casting was invented by an American inventor, Elisha K. Root, in the 1830s. It is similar to permanent mold casting but the only difference, is that in die casting, the metal is injected into the mold under extremely high pressure, which results in the formation of uniform parts with good dimensional accuracy. Most of the time, the need of post machining is eliminated or very light machining may be needed to bring the dimensions to size.

Die-casting is performed using the cold chamber or the hot chamber process. In the cold chamber process, the molten metal is distributed into the cold chambers for each shot. There is less exposure of the melt, to the plunger walls or the plunger. It is mainly useful for metals like aluminum, copper and elements that alloy easily with iron at high temperatures. In the hot chamber process, the pressure chamber is attached to the die cavity, which is immersed permanently in the molten metal. The inlet port of the pressurizing cylinder is exposed, as the plunger moves to the unpressurized position. This allows the molten metal to fill the cavity faster than the cold chamber process. This process is used for metals with low melting points and high fluidity, like tin, zinc, and lead that do not alloy with steel at their melting points.

Process
The die casting process consists of four major steps:

1. The mold is sprayed with a lubricant and closed. This lubricant checks for the temperature of the die and helps in the removal of the casting.
2. The molten metal is charged into the die under high pressure; between 10-175 MPa (1,5025,000 psi).
3. When the die is filled, the pressure is kept constant, until the casting solidifies. The die is then opened and the shot is released by the ejector pins.
4. And finally, the scrap including the gate, sprues, runners and flash, are separated from the casting. This is done using a special trim die in a power press or hydraulic press. Another method is to separate it by hands or tumble the shots, if the gates are thin and can be easily broken. This scrap can be reused by remelting.

Common Alloys in Die Casting
Aluminum, zinc, copper, lead and tin are the materials mainly used in die-casting. Some of the dies casting alloys include: AZ91D magnesium, ZAMAK, AA 380, AA 384, AA 386, AA 390 and zinc aluminum. Before casting, aluminum is alloyed with copper 3.5% and silicon 9% to form the Aluminum Association 380 alloy (UNS A03800). Copper increases the hardness and decreases the ductility and corrosion resistance. Similarly, silicon increases the melt fluidity and decreases the machinability. By reducing the amount of copper (less than 0.6%) in the AA 380 (UNS A03800), AA 360 (UNS A03600) is formed with an improved chemical resistance and hence, can be used in marine environment.

Advantages and Disadvantages of Die Casting
The advantages of die casting are:

• Excellent dimensional accuracy and smooth cast surfaces.
• In place of sand and permanent mold casting, thinner walls can be cast.
• Inserts can be cast-in, which include high strength bearing surfaces, heating elements and threaded inserts.
• Casting tensile strength is high, nearly 415 MPa (60 ksi).
• Reduces or eliminates the need of secondary machining operations, hence increasing the production rate.

The disadvantages of die casting are:

• Casting weight should be between 30 grams (1 oz) and 10 kg (20 lb).
• High initial cost and limited to high fluidity metals only.
• Casting should be smaller than 600 mm (24 in.) and the thickest section must be less than 13 mm (0.5 in.).
• A large production volume is required to establish it as an economical alternative to other processes.

Due to their versatility, die castings have been ranked amongst the highest volume products made in the metalworking industry. But in recent years, the injection-molded plastic parts have replaced some die castings, as they tend to be expensive and take long cycles to build.