Die casting is the procedure of pushing molten metal under high pressure, into mold cavities or dies. It is best suited for applications where a large quantity of small to medium-sized parts are required. Here's more...
Die casting was invented by Elisha K. Root, an American inventor, in the 1830s. The process is similar to permanent mold casting, with the only difference being that 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.
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Die casting can be performed using the cold chamber or the hot chamber process. In the cold chamber process, molten metal is distributed in chambers for each shot.
There is very little exposure of the melt to the plunger walls or the plunger. It is mainly useful for metals like aluminum and 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 depressurized 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:
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.
The molten metal is charged into the die under high pressure; between 10-175 MPa (1,5025,000 psi).
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.
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 the scrap 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 other 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 AA 380 (UNS A03800), AA 360 (UNS A03600) is formed with an improved chemical resistance, and hence, can be used in marine environments.
Advantages
Excellent dimensional accuracy and smooth cast surfaces.
In place of sand and permanent mold casting, thinner walls can be cast.
Inserts can be cast, 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.
Disadvantages
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, injection-molded plastic parts have replaced some die castings, as they tend to be cheaper and take short cycles to build.