The die casting process, developed in the early 1900s, is a further example of
permanent mold casting. In Europe, the process is also known as pressure die casting.
When die casting is born
Die casting equipment was invented in 1838 to produce movable characters for the printing industry. The first die casting patent was granted in 1849 for a small hand-operated machine for the mechanized production of printing characters.
In the die casting process, molten metal is forced into the mold cavity at pressures ranging from 0.7 to 700 MPa. There are two basic types of machines: hot chamber and cold chamber machines.
The hot chamber process (Fig. 1) involves the use of a piston, which pushes a certain volume of metal into the mold cavity through a gooseneck and nozzle. Pressures vary up to 35 MPa, with an average of about 15 MPa.
The metal is kept under pressure until it solidifies in the mold. To improve mold life and to help the metal quickly cool (thus reducing cycle times), molds are usually cooled by circulating water or oil through slits in the mold. Low-melting point alloys (such as zinc, magnesium, tin, and lead) are usually used in this process. Cycle times usually range from 200 to 300 injections per hour for zinc, although for very small components, such as zip teeth, you can get to speeds of 18,000 injections per hour.
The hot chamber machine contains the alloy melting pot as part of the machine itself and uses a gooseneck to inject the material into the mold.
When the door seals, the metal remains in the cavity to cool and solidify. After that, the plunger retracts, and the casting itself can be removed once the mold is opened. This design allows hot chamber die casting to be a continuous process.
Since the melting pot is internal, hot chamber machinery is used for materials with lower melting points. Alloys are also limited to materials that do not erode or dissolve the metal of the machine when put under heat or high pressure. The typical materials used in hot chamber die casting are zinc, lead, and magnesium alloys.
In the cold chamber process (Fig. 2), the molten metal is poured into the injection cylinder (firing chamber). The room is not heated – hence the cold attribute for the room. The metal is forced into the mold cavity at pressures that usually range from 20 to 70 MPa, although they can reach 150 MPa.
Molten metal must be inserted directly into the chamber, either through a ladle system or manually. Once the molten metal is loaded into sufficient volume, it will be injected into the mold through a high-pressure hydraulic plunger. The pressure requirements for cold chamber jets are generally higher than those of hot chamber die casting.
The machines can be horizontal (as shown in the figure) or vertical, in which case the shooting chamber is vertical. This method uses high melting point alloys such as aluminum, magnesium, or copper alloys, although other metals (including ferrous metals) can also be used. Temperatures of molten metal start at around 600°C for aluminum and some magnesium alloys and rise considerably for iron-based alloys.
Die casting has the ability to produce high-quality and complex-shaped parts, in particular using aluminum, brass, magnesium, or zinc. Die casting also produces parts with good dimensional precision and surface details, so that they require little or no subsequent processing or finishing operations.
Due to the high pressures involved, thin thicknesses of up to 0.38 mm can be produced, thinner than those obtained by other casting methods.
Due to high pressures, die casting molds tend to open unless they are held together tightly. The die casting machines are then evaluated based on the locking force that can be exerted to keep the molds closed. The capacity of the machines available on the market ranges from about 23 to 2700 metric tons. Other factors involved in the selection of die casting machines are mold size, piston stroke, firing pressure, and cost.
Comparison of the hot chamber and cold chamber die casting
In conclusion, a nice video by Dynacast summarizes the main features and differences between the two processes.