Pattern making is the production of a replica of the part to be cast. The pattern is used to form the cavity in the mould that the molten metal fills.
Patterns have historically been hand-produced in wood for sand casting, or in wax for investment casting. Modern approaches include CNC machining the pattern from a computer model of the part into polymers.
How a pattern works in sand casting
In sand casting, the oldest process, the pattern is placed into a box that is packed with sand. Either the pattern is the shape of one half of the component, or only half of the pattern is packed in sand. Repeating this for the other half leaves two boxes which, once the pattern is removed and the halves are placed together, hold a void in the shape of the desired component.
The join line between the two halves of the casting is called the 'witness mark'. Gates, runners and feeders are also added to the sand pattern box to control the flow of molten metal to the shape being cast.
Pattern making requires significant expertise, allowing for the flow of metal to all parts of the mould, accommodating anticipated shrinkage, and avoiding voids and other quality issues. Many of these steps can be computer modelled, but human experience and judgement remain valuable given the many variables involved.