Gravity Die Casting uses a permanent mould usually made from iron, metal being poured into the mould and using gravity only to fill the cavity. Sand or steel cores can be used in the mould to create the shape of the casting required and make highly intricate designs possible. This process generally offers better surface finishes than sand processes and by virtue of the faster metal cooling rate, tighter grain structures and hence, far higher mechanical properties within the material. Text can be engraved into the mould surface - to give highly detailed annotation - on the final parts. In some instances, it is beneficial to tilt the mould itself during the filling process to reduce turbulence within the metal filling the mould – especially with aluminium that oxidises rapidly. Any oxides formed during the filling being greatly detrimental to the finished part.
In addition to the mechanical properties, the castings have lower finishing costs and normally a higher quality finished product. With the output being faster, this also means higher demands can be met easily in shorter periods of time.
Despite the initial costs for making the tools being higher, the lower on-going costs of manufacture make this process suitable for higher production volumes. Gravity die castings also react well to heat-treatment, with substantially greater properties possible within some alloys.
Although usually a manual process with the molten metal added by use of a hand-held ladle, automation can be added to pour the molten metal into the mould, where higher volumes make it cost effective.
The main ways in which gravity die casting differs from sand (aside from the reusability of the mould) are that the mould needs to be coated with a sprayed-on refractory coating. This will assist the release of the casting from the die and also can be used to control the cooling characteristics of certain areas of the casting, to ensure that any design features which are inconsistent with that of an ‘ideal casting’ can be overcome.
The coating also protects the die mould from contact with the molten metal which can adhere to, and, also attack the surface of the mould itself.
After the casting has solidified the die is opened and the part removed or ejected and made ready for fettling and finishing. In the same way as sand casting, the runners and risers are removed, sharp edges fettled off and the part made ready for final finishing - such as machining or painting.
Gravity die casting enables us to supply high quality parts to the highest levels with fine details and finishes in casting sizes from a few grams, up to in excess of 30 kg in weight.