In gravity die casting molten aluminium is poured into a metallic tool. The casting temperature is about 750°C. The tolerances and surface finish are good. The use of sand cores in gravity die casting enables casting of very complex components.
Gravity die casting method is competitive casting method when production quantity is relatively small or when heat treatment is needed to improve the mechanical properties. This casting method gives better tolerances and surface finish than sand casting. The tooling costs are somewhat higher than by sand casting.
Using sand cores in gravity die casting makes it possible to manufacture items with complicated inner shapes in a cost effective way in small and medium quantities. Typical products produced this way are e.g. air intake components for diesel engines, valves for flow systems, tanks for air conditioning systems etc. Typically they have curved inner shapes which are optimized for gas or liquid flow. Using shell sand cores gives close tolerances and relatively good surface quality also for the surfaces solidifying against the sand core.
GDC is suited to medium to high volumes products and typically parts are of a heavier sections than HPDC, but thinner sections than sand casting.
There are three key stages in the process.
- The heated mould [Die or Tool] is coated with a die release agent. The release agent spray also has a secondary function in that it aids cooling of the mould face after the previous part has been removed from the die.
- Molten metal is poured into channels in the tool to allow the material to fill all the extremities of the mould cavity. The metal is either hand poured using steel ladles or dosed using mechanical methods. Typically, there is a mould “down sprue” that allows the alloy to enter the mould cavity from the lower part of the die, reducing the formation of turbulence and subsequent porosity and inclusions in the finished part.
- Once the part has cooled sufficiently, the die is opened, either manually or utilising mechanical methods.