The sand casting process uses a pattern to make a hollow cavity in the sand, which is the shape of the finished part required. The pattern equipment is generally made from wood or resin often with metal inserts to protect from abrasive wear.
The pattern itself is slightly larger than the finished part, to allow for the contraction of the metal during cooling. Patterns can be made using traditional manual pattern making skills or more often now, using 3D models through CAD/CAM to a CNC machine, which cuts a resin material to shape. Additional tooling is usually needed to make cores, which are used to create undercuts, hollows or cavities within the final part itself.
In order to get the metal into the mould, a runner system is added to the pattern. It is designed to make sure once the metal has flowed through the down sprue, it will be filtered and its flow controlled to avoid turbulence whilst the mould is filled. Any thick sections can be chilled to promote accelerated solidification of material, by the addition of pieces of iron or steel, which preferentially cool where required. Risers are added to create reservoirs of molten metal to feed the casting, as it shrinks during the cooling/solidification stage.
The sand system employed will depend on the requirements of the final part but greensand or air set sand systems are most common. Greensand uses a fine sand with a clay binder for mould strength and air-set sand moulding uses a sand with chemical binder and acid catalyst to create a hard sand mould.
The sand is rammed around the pattern to give a compact surface (in the case of the air-set process allowed to chemically harden) and when ready, the mould is stripped from the pattern and then assembled with cores, to produce an enclosed cavity ready for casting.
Once the mould has been assembled, molten metal is then poured through the runner system into it to fill and create the final casting.
The stage of introducing metal into the mould is the most critical part of the casting manufacture. The metal must be introduced cleanly at the correct temperature to completely fill the mould without forming cold shuts, or miss-runs and without being excessively hot, which could affect metal integrity and final mechanical results. Turbulence must be avoided as this causes air entrainment and oxide formation.
