Shrinkage allowance

Shrinkage allowance

Molten metal supplied inside the mould at high temperature in as it comes in contact with mould wall the solidification process will be initiated. During solidification, the liquid metal first converted into the solid metal and due to change in density of the material it will be subjected to shrinkage. 

the shrinkage in metal is compromised by two ways-

1. By providing shrinkage allowance over the pattern
2. By providing a riser

In metal casting, As cooling or solidification takes place in three stages also take place in three-stages-

stage 1:- Liquid in this printed material solid from the liquid at the following temperature to liquid at melting temperature

Stage 2:- Shrinkage during phase temperature in this age of material during solidification from the liquid at melting temperature to solid at melting temperature 

Stage 3:- Solid sinkage shrinkage of material during solidification from melting temperature to solid at room temperature

         Out of these three stages of shrinkage, the first two stages are compensated by providing a Riser were as the third stage of Shrinkage i.e. solid shrinkage is allowance is compensated by providing a shrinkage allowance to the pattern.

Some important points regarding the allowance-

The shrinkage allowance provided to the pattern is governed by the rule of metal casting and the rule is shrink rule.

   The shrink rule state that the sinkage in casting is volumetric in nature but shrinkage allowance provided in a linear dimension, not on the volumetric dimension. 

In most of the material of the case during solidification and in order to compensate this shrinkage a +ve allowance is provided to the pattern (the pattern is made larger than the final product)

   In the case of grey cast iron(GCI) due to the formation of carbon flacks during solidification the cast iron is subjected to expansion in order to compensate the expansion of casting pattern is made smaller than the final product. This is known as a negative(-ve) allowance.

In the case of grey cast iron casting, there is no need of providing any riser along with the casting.

The pattern in foundry two types of pattern material are used first and second metal pattern.

• Wooden pattern- 

1. Sensitive to moisture but stable at high temperature
2. Cause wear and tear due to continuous use
3. Largely used pattern

• Metallic pattern

1. Long life and not sensitive to moisture
2. it produces by master pattern
3. Double allowances are provided to the master pattern(master pattern generally made up of wood)