What is pattern making in casting?

A pattern is a replica of the part to be produced by casting. It has historically been hand-produced in wood when sand casting, or in wax for investment casting, but modern approaches include the CNC machining from a computer model of the part into polymers.

Focusing on sand casting as the oldest process, the pattern is placed in to a box that is packed with sand. Either the pattern will be the shape of one half of the component only, or only half of the pattern is packed in sand. When the process is repeated for the other half you are left with two boxes that when the pattern is removed and the halves placed together will have a void in the centre that takes 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 to be cast.

The role of the pattern maker requires significant expertise, to allow for the flow of metal to all parts of the mould, to accommodate anticipated shrinkage and avoid any voids and other quality issues. Although many of these process steps can be computer modelled, human experience and judgement is still of benefit given the many variables that come in to consideration.

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