Learn about Casting Patterns and the different types of
Casting Patterns
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.
