Gravity die casting

In pressure die casting the metal is injected so fast that air is entrained and stays trapped in the section. The part is dense and strong enough for most purposes, but heat it in a furnace and the gas expands and raises blisters under the surface. That is why die cast parts normally are not heat treated.

Gravity die casting fills the mould calmly, under gravity. Less air is entrained, and the part can be heat treated. That is where the strength comes from: a heat treated gravity die cast aluminium part gains higher tensile and ultimate strength than the same part as cast. Read more about heat treatment.

One of the brackets we show is exactly this: a gravity die cast bracket that is heat treated to take more load. We choose the process from what the part has to withstand.

Gravity die casting is not pressure die casting in slow motion. It is a different process, and the part should be designed for it from the start.

Heavier sections, not thinner. The melt fills the mould under gravity alone, with no pressure behind it. Tight, thin features do not fill reliably. Where die casting handles thin walls, gravity needs more material for the metal to reach before it freezes.

A rougher surface. A gravity cast part comes out with more visible texture than a die cast one. If the face has to look good, it must be blasted, machined or coated. Allow for it in the costing.

Direction of solidification matters more. With no pressure, the part has to be fed by gravity as it freezes. That places demands on how heavy and light sections sit relative to each other, and it is one of the things we look for in the review.

The alloy is often a different one. The usual gravity alloys are chosen precisely because they can be heat treated. Which alloy your part should have, we decide together in the design review.

For a closer look at the choice, see the decision guide to die casting and gravity die casting.