Die casting low pressure

The low-pressure die-casting process involves holding the molten metal to be cast in a crucible around which are arranged the heating elements necessary to keep the metal molten. The crucible and elements are contained inside a sealed pressure vessel in the top cover of which is a riser tube whose lower end is immersed in the molten metal. Low-pressure air (0.75 bar to 1.0 bar) is introduced into the crucible above the surface of the molten metal and forces the metal up the riser tube and out through the top end. This top end is attached to the bottom or fixed half of the die, which is designed to allow air to escape as the molten metal enters. When the metal has solidified in the die, which is aircooled, the air pressure is released and the remaining molten metal in the riser tube drops. 

The moving platen is then reversed to close the two halves of the die ready for the next casting to be produced. 

A diagrammatic view of the machine set up with the dies in the closed position is shown in Fig. 18.2. 

The dies are cheaper than for high-pressure methods since they can usually be made of cast iron, but they are sometimes more expensive than for gravity die-casting. The capital ouday for machines is higher than for gravity but lower than for the high-pressure method. 

---

Casting in permanent moulds 2.6.2.1 Gravity and low-pressure die-casting... 

Figure 2.44 The principle of a low-pressure die-casting machine [175, Brown, 1999]...

A mass balance for a hot-box core shop is given in Table 3.42. Data were collected from a core shop of a low-pressure die-casting brass foundry [177, Silva Ribeiro, 2002]. 

Permanent mould casting involves the injection of molten metal into a metal mould. The mould is opened after solidification and the easting is taken out for finishing. Chemieally-bonded sand cores are used to a limited extent in gravity and low-pressure die-casting. 

An open AZ91D scaffold was formed by (i) slowly infiltrating molten AZ91D at 600-670 °C into dry NaCl crystals in a core box placed in the chill mould of a low-pressure die-casting system, (ii) machining to shape, and (iii) dissolving the NaCl crystals in a sodium hydroxide solution with a pH above 11.5 which protected the AZ91D. The scaffolds were implanted... 

Low pressure die casting uses low-pressure (1 bar) air to force the molten metal into the die cavity. Less popular than gravity die casting, and tends to be used purely for the production of car wheels. Gives lower production rates. 

Recently, a low-pressure die casting process using pressures of only a few pounds per square inch has been developed, primarily for use with aluminium alloys. This has an airtight furnace chamber with the mould mounted above it. Gas is admitted to the chamber and used to force the molten metal in a crucible up a feed tube into the mould cavity. This has the advantage of very line control. It is, however, essentially a variant on gravity die casting. 

---