A casting pattern is basically a shaped form of either metal or wood where sand is packed around it in the mold. When this pattern is removed, the resulting cavity would be exactly the shape of the pattern object that was to be cast. The casting pattern should however be designed in order to be easily removed without any damage done to the mold. The pattern should be also be perfectly dimensioned and should be very durable for the intended use. The patterns that are used in sand casting can be made of plastic, metal, wood or any other material. The casting patterns are made to the right standards of construction this is basically because they can last for a reasonable time. The durability of the pattern would however depend according to the quality of grade in the casting pattern. If the pattern is made of right quality then they would be constantly providing dimensionally perfect casting patterns.
The process: The process of making casting patterns is actually called as pattern making which is in fact a skilled trade which is related to the trades of die and tool making and mold making. However this also sometimes incorporates the elements of fine wood making. The makers of casting patterns basically learn their pattern making skills through trade schools and apprenticeships over a number of years of experience. There is also an engineer who a few times helps in making the casting patterns but in fact it is the pattern maker who carries out the design.
Design: The casting patterns incorporate proper allowances for the shrinkage and this process is called as contraction allowance. Their accurate values however depend on the alloys that are being used and the accurate sand casting methods that is used. There are a few alloys that have the overall linear shrinkage of nearly 2.5 percent as other alloys might actually experience 0 percent shrinkage or even a light positive shrinkage or it might also increase the size in the casting process. The amount of the shrinkage is also dependant on the sand casting process that is employed for instance chemical bonded sands, clay bonded sands or other bonding materials.
The casting patterns should also include proper allowance for the draft. This means that the sides would be tapered so that when it would be taken out from the sand it will not tend to drag out sand of place along with it. This process is also called as taper that is normally done in the range of one to three degrees. The foundry engineers or the pattern maker of the casting patterns actually decide where the gating systems, risers and the sprues are places in terms of casting patterns. In this pattern a hole is desired in the casting pattern that uses a core which defines the location and volume of in the casting where the metal would not flow into. Chills are also used sometimes located on the casting patterns surface and these are then formed into sand molds. These casting patterns however continue to be a vital requirement for sand casting of metal.
