The art of pattern making is a skilled trade that requires the utmost craftsmanship, patience, and quality control. Such attention must be given to the pattern as it is the start of your product and essential to the quality of your casting. The attention to the detail of the pattern is the most important factor in producing a product to meet or exceed our customer demands.
The pattern needs to incorporate suitable allowances for shrinkage; these are called contraction allowances, and their exact values depend on the alloy being cast and the exact sand-casting method being used. Some alloys will have overall linear shrinkage of up to 2.5%, whereas other alloys may actually experience no shrinkage or a slight "positive" shrinkage or increase in size in the casting process. The shrinkage amount is also dependent on the sand-casting process employed, for example clay-bonded sand, chemical bonded sands, or other bonding materials used within the sand.
The pattern needs to incorporate suitable allowances for draft, which means that its sides are tapered so that when it is pulled from the sand, it will tend not to drag sand out of place along with it. This is also known as taper which is normally between 1 and 3 degrees.
The patternmaker or foundry engineer decides where the sprues, gating systems, and risers are placed with respect to the pattern. Where a hole is desired in a casting, a core may be used which defines a volume or location in a casting where metal will not flow into. Sometimes chills may be located on a pattern surface, which are then formed into the sand mould. Chills are heat sinks which enable localized rapid cooling. The rapid cooling may be desired to refine the grain structure or determine the freezing sequence of the molten metal which is poured into the mould.
Wooden foundry moulds and patterns are wooden tools, carved out or built up to create negative space, which in turn is used to make the inverse form or shape to be used for the casting of metal. To make a wooden mould, we stack the wood up, like a relief, or cut out, like a sunken relief, with handles or other pieces to grab and pick up the mould from the moulding material. Individual pattern pieces that get packed into the moulding material are carved and shaped. The space or forms that these pieces make in the moulding material will be the inverse of the final cast piece.