This process lies between gravity die casting and high-pressure die casting. In a low-pressure casting process, the molten metal is forced upward by gas or air pressure into a graphite or metal mould. The mould is located just above the furnace. It may take half a minute to fill up the mould, so any entrapped gas has time to escape; the casting part cools from the top to down. The applied pressure continually feeds molten metal to compensate for shrinkage and the unused metal in the feed tube drops back into the crucible after the pressure is released. When the metal is completely solidified in the mould, the upper half of the mould is lifted up with the casting entrapped in it. An ejector pin drops the casting in a hopper.
Advantages
- High pressure die castings have a porosity problem that is non-existent in the low pressure system.
- The casting is dense, free from inclusions and oxidation, and of good dimensional accuracy.
- Mechanical properties of casting are improved as close control of temperature and pressure.
- Cost of the machine is less than that of high pressure die casting.
- Die life is longer than the high pressure die casting.
- Wider tolerance is required.
- Casting modifications are tough.
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