A pattern may be defined as a replica or facsimile model of the desired casting which, when packed or embedded in a suitable moulding material, produces a cavity called mould. This cavity, when filled with molten metal, produces the desired casting after solidification of the poured metal. Since it is a direct duplication, the
pattern very closely conforms to the shape and size of the desired casting, except for a few variations due to the
necessary allowances. The ways in which a pattern differs from an actual component are :
1. It carries an additional allowance to compensate for metal shrinkage.
2. It carries additional allowances over those portions, which are to be machined or finished otherwise.