At its most basic level, die casting is a process where non-ferrous molten metal (such as aluminium or zinc) is introduced to a metal mould. However, as individual castings can require diverse specifications depending on their final application, variations to the die casting process are needed in order to achieve different results.
Two of the most popular die casting processes are gravity and pressure die casting.
What’s the difference between gravity die casting and pressure die casting?
The key difference between gravity and pressure die casting is the way that molten metal is received by the mould.
For gravity die casting, the metal is poured into the mould from above. The only force which fills the mould (from the bottom up) is gravity.
Pressure die casting, however, uses molten metal which is injected into the mould by force (between 1500 and 25,400 psi). This pressure is maintained until the casting is solid.
The benefit of this high-pressure injection is predominantly speed. Injecting molten metal into a mould is very quick, as opposed to pouring it in and letting gravity do the work.
Pressure die casting is also an automated process. This means that production is both accurate and fast, with the risk of human error reduced; however, we do need to bear in mind that automation is an advanced technology which can lead to expensive set-up and tooling costs.
By contrast, gravity die casting relies on gravity alone to fill the mould. This makes the process slower than pressure casting and, by association, less suited to long production runs. On the other hand, if only a small batch is required, gravity die casting is often more cost-effective than the pressure casting equivalent.
Do pressure die casting and gravity die casting have a difference in quality?
Pressure die casting can make very precise and complex castings achievable. Due to the rapid filling of the mould under pressure, castings are dimensionally accurate with thin walls which are also very smooth, reducing the need for secondary machining.
Although gravity die casting will not match these benefits, the slower speed at which molten metal enters the mould does mean that less folding and turbulence occurs. As a result, less air is trapped in the casting, offering a distinct advantage if it needs future heat treatment.
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