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Pattern Allowance

For making pattern, allowance is take in consideration for perfect sand casting.

1. Shrinkage or Contraction allowance

In cooling process, all metal shrinkage during solidification and contract. To compensate this problem, some allowance is provided in pattern so that final casting is as per required. This type of allowance is known as Shrinkage or Contraction allowance.

This allowance is further classified in two type.

i. Liquid Shrinkage

Reduction in volume of metal during change from liquid to solid form. To compensate this problem, riser is use. Riser feed liquid material when required.

ii. Solid Shrinkage

Reduction in volume when metal loose temperature. To compensate this problem, allowance is provided in pattern.

2. Draft or Taper allowance

This allowance is provided in all vertical surface of pattern so that pattern is remove easily without tear or break the side of mold.

3. Distortion or Camber allowance

During cooling process, stress will generate. Due to this stress bending or distortion is generate in casting. This can be eliminate by adding opposite distortion in pattern.

4. Machining or Finish allowance

Finishing and accuracy in sand casting is not so good. To achieve finishing and accuracy in casting some extra machining need to required. For extra machining extra allowance is require at machining surface. This type of allowance is known as machining allowance.

5. Shake or Rapping allowance

During pattern remove from mold cavity, sometime some dimension is enlarge. To compensate enlarge, pattern dimension need to reduce. This type of allowance is known as shake or rapping allowance. 
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