High pressure die casting die design complete process highlight-2

Описание: 3.7.4 High pressure die casting
HPDC is the most common process for the production of aluminum castings. The range of utilization extends from parts as light as 100 g or even less, to very complex engine blocks, powertrain, and suspension components. As sketched in Fig. 3.8, molten metal is dosed into the “shot chamber” and pushed by a hydraulic ram into the die cavity at a controlled speed. The mold is filled rapidly, and a very high pressure (up to 20 MPa) is maintained during the solidification process. Because of this high-velocity filling, HPDC can produce shapes that are more complex than permanent mold casting with much thinner walls. The thin walls freeze extremely quickly when the metal comes in contact with the die walls, forming a skin responsible for high fatigue properties.

One problem with the rapid filling of the mold is the entrapment of air. Although this trapped gas might not be visible, as any porosity is closed due to the high pressure maintained during solidification, it renders the castings not heat treatable, difficult to weld and therefore not suitable for structural applications. Also, contrary to the low pressure method, the dies are not coated with a ceramic lining to maximize heat transfer and the formation of the skin. Therefore it is necessary to use high levels of Fe in HPDC alloys to avoid die soldering (mold sticking). High levels of Fe promote the formation of undesirable Al–Fe–Si intermetallic phases that affect ductility, a requirement of structural castings.

Die Casting (Permanent Mold) Materials: Science and Technology, 2001

3 High-pressure Die Casting
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 aluminium, 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