Pattern (casting)

From Wikipedia, the free encyclopedia

The top and bottom halves of a sand casting mould showing the cavity prepared by patterns. Cores to accommodate holes can be seen in the bottom mould, the drag.

In the process of casting, a pattern is a replica of the object to be cast, used to prepare the cavity into which molten material will be poured during the casting process. The pattern needs to incorporate suitable shrinkage allowances depending on material flow and heat transfer considerations. Pattern making is a skilled technique that was highly valued in the different cultures practising casting through the centuries.

In sand casting, the pattern is usually of wood, whereas it may be of metal or other materials in pressure or centrifugal casting. The patternmaker also decides where the sprues and risers (inlet and outlet for molten material) will be placed with respect to the pattern. Parts of the object which have holes or depressions are handled by inserting cores defining volumes that the material will not flow into. Sometimes patterns may also accommodate chills, solid pieces of the final material, to enable rapid cooling, resulting in martensitic hardening in the neighbourhood of the chill.

As sand casting methods have been overtaken by die casting and thin mould casting (the mould is made of a thin, workable refractory material), patterns are not widely used today in mass production, though they may still be used for small or specialized jobs.