High Pressure Die Casting
Excerpt from the Encyclopedia of Materials: Science and Technology 2001
High-pressure die casting is a process in which molten metal is forced under pressure into a securely locked metal die cavity, where it is held by a powerful press until the metal solidifies. After solidification of the metal, the die is unlocked, opened, and the casting ejected. After removal of the casting, the die is closed and locked again for the next cycle. The injection of metal into the die cavity is completed in a fraction of a second. Often, while the molten metal is still held in the die, extremely high pressure is applied (called intensification pressure). This high pressure compresses any gas entrapped in the metal and feeds additional metal into the cavity to compensate for the shrinkage of the metal as it solidifies.
Two types of systems are used for injecting the molten metal into the die. The hot chamber system is used with metals such as zinc, magnesium, and lead. The injection system of a hot chamber machine is immersed in the molten metal bath of the melting furnace. As the shot plunger moves, it forces metal through the nozzle and into the die.
The cold chamber system is used for metals that melt at high temperatures, such as aluminum, brass, and magnesium. Magnesium parts can be produced using both systems, though usually small parts are produced in hot chamber machines and large parts in cold chamber machines since hot chamber machines are limited in size. There are also two injection systems used in the cold chamber process, horizontal and vertical injection.
In the cold chamber process, the molten metal is poured, by hand or by automatic means, into a port of the cold chamber sleeve. A hydraulically operated plunger advances through this steel sleeve, sealing off the port, and forcing the metal into the die at high speed and pressure. After solidification of the casting, the plunger is retracted, the die opened, the casting ejected, and the system is then ready for the next shot cycle. Higher pressure is used in this system than the hot chamber process. The production rate of a hot chamber machine is higher than that of a cold chamber machine because of the shorter time required during the pour operation. Typical zinc castings produced by the high-pressure die casting process are shown in Fig. 2.
In addition to the conventional high-pressure die casting processes, several enhancements to the process have been developed in recent years. These enhancements include the use of vacuum systems to reduce entrapped gas, slower fill processes to eliminate turbulence during fill and permit the use of heat treatment to enhance mechanical properties of the castings, and the application of semisolid metal processing to produce pressure tight parts not normally able to be produced in high-pressure die casting. Each of these processes utilizes the fundamentals of high-pressure die casting, but with additional capabilities to produce high integrity parts. They also have developed unique names for the purpose of distinguishing themselves from the conventional processes of hot chamber and cold chamber die casting.
Vacuum die casting utilizes a vacuum system on the die cavity to remove gas from the cavity prior to injection of the molten metal. The result is a high integrity part with very low levels of porosity and high mechanical properties. The process is used to produce critical components in light alloys, such as structural and safety components for motor vehicles. The process has higher costs than conventional die casting, but the parts produced justify the additional cost and they cannot be produced successfully in conventional die casting.
Slow fill die casting is often called squeeze casting. This process is widely practiced throughout the world today for producing parts that must be heat treated to achieve the required mechanical properties. Many automotive components that have been converted from iron castings or weldments of castings and/or stampings are now produced in light alloys in the squeeze casting process. This process utilizes the advantages of the low-pressure die casting process, controlled filling, and directional solidification of the molten metal, as well as the advantages of high-pressure die casting, high-pressure solidification, and fast cycle times. Parts produced in the squeeze casting process include steering and suspension components, e.g., alloy wheels, steering knuckles, and control arms, and air conditioning parts, e.g., compressor scrolls. Parts are produced with both vertical and horizontal shot systems. A schematic diagram of a vertical shot horizontal squeeze casting machine is shown in Fig. 3.