Pattern (casting)
[1][2] Patterns used in sand casting may be made of wood, metal, plastics or other materials.
Patternmakers (sometimes styled pattern-makers or pattern makers) learn their skills through apprenticeships and trade schools over many years of experience.
Sugar pine wood is the most commonly used material for patterns, primarily because it is soft, light, and easy to work.
Fiberglass and plastic patterns have gained popularity in recent years because they are water proof and very durable.
Metal patterns are long lasting and do not succumb to moisture, but they are heavier, more expensive and difficult to repair once damaged.
[5] Plaster of Paris is usually used in making master dies and molds, as it gains hardness quickly, with a lot of flexibility when in the setting stage.
[5] The patternmaker or foundry engineer decides where the sprues, gating systems, and risers are placed with respect to the pattern.
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.
It is a replica of the desired casting—usually in a slightly larger size to offset the contraction of the intended metal.
Skeleton pattern comes into play when the entire setup made of wood or metal is costlier.
Turbine Casing, Soil and Water pipe bends, valve bodies and boxes.
To compensate for any dimensional changes which will happen during the (solid) cooling process, allowances are usually made in the pattern.
Type Metal is also known, and used, for its ability to hold a true and sharp cast, and retain correct dimensions after cooling.
A mould has been made to cast a 100 kg block, based on the solid density of steel.
When the pattern is to be removed from the sand mold, there is a possibility that any leading edges may break off, or get damaged in the process.
[7] The surface finish obtained in sand castings is generally poor (dimensionally inaccurate), and hence in many cases, the cast product is subjected to machining processes like turning or grinding in order to improve the surface finish.
To compensate for this, a machining allowance (additional material some times referred to as green) should be given in the casting.
Shaking of the pattern causes an enlargement of the mould cavity and results in a bigger casting.
[7] During cooling of the mould, stresses developed in the solid metal may induce distortions in the cast.
The exact process and pattern equipment is always determined by the order quantities and the casting design.
Wherever it provides suitable material properties at competitive unit cost, it will remain in demand.
