A Pattern is often made larger than final casting as a result of it carries certain allowances due to metallurgical and mechanical reasons.
The following are the pattern allowances
Shrinkage allowance
Draft allowance
Finish allowance
Shake allowance
Distortion allowance
SHRINKAGE ALLOWANCE:
During cooling from pouring temperature to room temperature all the metals that are used in the casting work will shrink or contract.
This contraction takes places in three forms. These are
Liquid contraction
Solidifying contraction
Solid contraction
To compensate liquid and solidifying contraction, gates and riser are provided in the mould. Solid contraction adequate allowances are provided on the pattern.
The different metals shrink at different rates as a result of shrinkage is that the metal property. For Aluminium permissible shrinkage allowance is 17 mm/metre.
DRAFT ALLOWANCE:
Withdrawing the pattern from the sand mould, the vertical faces of the pattern are in continual contact with the sand, which may damage the mould cavity.
To reduce the posibilities of this happening the vertical faces of the pattern are always tapered from the parting line.
Suggested draft values for wood pattern are
Outer 0.25 to 3.0 degree
External surfaces 10 mm to 25 mm per metre
internal surface 40mm to 65 mm.
FINISH ALLOWANCE:
Extra material is to be provided that is to be subsequently removed by machining or cleaning process.
Suggested machining allowance or pattern for non ferrous metal up to 3 mm.
SHAKE ALLOWANCE:
Before withdrawal from the sand mould.
To enlarge the mould cavity slightly.
Since it enlarges the final casting made.
It is fascinating that the original pattern dimensions ought to be reduced to account for this increase.
DISTORTION ALLOWANCE:
Distortion allowance applies only to those castings of irregular shape or U & V shape.
Which are distorted within the method of cooling because of metal shrinkage.
The amount of distortion allowances varies from 2mm to 20 mm as per size shape and casting material.
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