Different Types of Pattern Allowances in Casting

Pattern allowances:

There are some allowances which are responsible for the difference in the dimensions of the casting and the pattern. These allowances are considered when a pattern is designed for casting. In this article we will discuss those allowances -

1.     Shrinkage allowance:

After solidification of the metal from further cooling (room temp.) dimensions  of the patterns increases. So pattern size is bigger than that of the finished cast products. This is known as shrinkage allowance. 

It depends on:

a)      Dimensions of casting

b)      Design and intricacy of casting

c)       Resistance of mol to shrinkage

d)      Molding materials used

e)      Method of molding used

f)       Pouring temp of the molten metal

2.     Draft or taper allowance:

Pattern draft is the taper placed on the pattern surfaces that are parallel to the direction in which the pattern is withdrawn from the mould (that is perpendicular to the parting plane), to allow removal of the pattern without damaging the mould cavity.

It depends on:

a)      the method of molding

b)      the sand mixture used

c)       the design (shape and length of the vertical side of the pattern)

d)      economic restrictions imposed on the casting

e)      intricacy of the pattern

3.     Distortion allowance:

This allowance is taken into consideration when casting products of irregular shapes. When these are cooled they are distorted due to metal shrinkage.

4.     Finishing or machining allowance:

Machining allowance or finish allowance indicates how much larger the rough casting should be over the finished casting to allow sufficient material to insure that machining will "clean up" the surfaces.

This machining allowance is added to all surfaces that are to be machined. Machining allowance is larger for hand molding as compared to machine molding.

It depends on:

a)      Machining operation

b)      Characteristics of metal

c)       Methods of castings

d)      Size, shapes and volumes of castings

e)      Degree of finish required in castings

f)       configuration of the casting

5.     Shaking or rapping allowance:

To take the pattern out of the mould cavity it is slightly rapped to detach it from the mould cavity. So the cavity is increased a little.  

Please Read :

• Different types of casting defects 
• What is casting ? Main application of castings 
• different types of patterns that are used on the sand casting process. 
• different types of molding sand
• properties of various natural and synthetic molding sands
• Comparison of different casting processes. 
• Advantages and disadvantages of centrifugal castings.  
• Different Materials Used for Patterns