From the design, provided by an engineer or designer, a craftsperson called a patternmaker produces a master of the object to be produced,using wood, plaster or plastic. The patternmaker will also produce patterns from existing cast metal, particularly in the renovation of antique ironwork.
As the metal to be cast will shrink somewhat between the time it first solidifies and the time it is cool the master must be made slightly larger than the finished product.
To simplify the making of the pattern the patternmaker will use an appropriately scaled oversize ruler—called a shrink rule—specific to the type of metal to be cast.
Additional paths for the entrance of metal—the sprue—and the exiting of gas—the riser—are added to the pattern.
When the pattern is completed, the next step is to make the sand mould in readiness for the casting.