High pressure die casting (HPDC) is a precision casting technique that uses a permanent metal mould, or die, into which molten metal is injected under pressure forced by a plunger. The metal is injected into the die at a high velocity and solidifies under the applied pressure. The high velocity of the molten metal provides rapid filling of the metal die; the filling being accomplished in times of the order of 0.05–0.15 s. Because of the high conductivity of the die, the castings solidify very rapidly in comparison to the expendable mould castings. It is due to these facts and also due to the high level of automation and the small number of production steps that productivity of this casting method is very high – the production cycle often requires only a few seconds. The process is the fastest known method of producing non-ferrous metal parts, and there is a little similarity with other metal casting methods. The pressure die casting is a highly distinctive process which might be held to have more in common with injection moulding of plastics than with other metal casting processes. The process was patented for the first time in the United States of America towards the second half of the 19th century.
High-pressure die casting is most suitable for the non-ferrous metals with relatively low melting points, such as zinc, aluminium, magnesium and copper-based alloys. The most important alloys in use for pressure die casting are the very fluid alloys of the aluminium-silicon family thanks to their high castability. Smaller but significant quantities of castings are produced of the magnesium, zinc and copper alloys. The die and chamber materials are the limiting factor in the extension of pressure die casting to other groups of alloys. It has been proved that the application of the process to steels and other alloys of higher melting point is technically feasible, but economically inefficient, which is due to limited working lifetime of the die.
The machines for die casting can be divided into two basic groups depending on the design of the metal injecting system. This is based on the location of the furnace containing the melt in relation to pressure chamber:
- if the furnace is part of the casting machine, it is referred to as a hot-chamber system;
- if the furnace is located outside the casting machine and is not directly related to the machine design, it is referred to as a cold-chamber system.
In the first case, the pressure chamber is placed directly into the molten metal reservoir (Fig. 6.7) and is filled with the melt through an inlet opening, and the dose of liquid metal is injected into the die by the injection plunger. The hot chamber machines are used exclusively for the production of the low melting point alloys, usually zinc-based alloys. They are unsuitable for the aluminium and other alloys of higher melting point. The highest achievable pressure is limited to about 20 MPa. The hot chamber process typically produces castings at a faster rate and lower cost.
a) molten metal is poured into the shot chamber, b) pressing the melt by the plunger: injecting metal into the die and compression, c) opening the die, d) ejecting the casting out of the die;
1 – fixed die half, 2 – movable die half, 3 – fixed platen, 4 – movable platen,
5 – clamping bars, 6 – casting cavity, 7 – ingate, 8 – shot chamber, 9 – injection
plunger, 10 – ladle, 11 – ejector pins, 12 – ejector system, 13 – casting;
FIN – injection force, FCL – clamping force
1 – shot chamber, 2 – injection plunger, 3 – inlet opening, 4 – crucible of the furnace,
5 – molten metal, 6 – gooseneck, 7 – nozzle
The dies usually consist of two steel blocks, each containing part of the cavity, which are tightly pressed and locked together during the casting process (Fig. 6.6). One half of the die is fixed to the machine and the other one is mounted on the moving platen connected to the closing mechanism. After the solidification of the casting, the die halves are opened and the casting is ejected from the mould cavity using an ejector pin system which is a part of the moving die block assembly. Dies can be of a simple design, or very complex, having movable slides or sections and cores, depending on complexity of the part.