How patterns are prepared for casting, types of patterns and pattern allowance

What is Casting?

Patterns and Pattern making

Types of patterns

• Single piece pattern

  
  As the name states it is made of a single piece. It is the simplest form of parent used in casting. This type of pattern is used for the castings that are simple and symmetric in shape. Single piece patterns are usually made of wood and are used for small scale production. This type of pattern is also known as Solid piece pattern.






• Split piece pattern

  
  The patterns which are complicated in shape and whose depth is too high are split in to two or more parts to have an ease in withdrawing the pattern from the mould. For two piece patterns one piece is placed in the cope box and the other in drag box, these two boxes are aligned by using dowel pins. When shape is more complicated we can even use three piece patterns. This is also known as 'multi piece pattern'.






• Loose piece pattern of casting

  
  Loose piece patterns are used when some irregularity is present on the contour of the pattern, making a difficulty in withdrawing the pattern from the mould. So to overcome this difficulty the irregular part of the pattern is held loose against the contour of the pattern using dowel pins or wire. By using such arrangement during the withdrawal we can first withdraw the main pattern which is regular in shape and then the loose pieces can be removed without damaging the contour of the casting from the gap provided by the main pattern.






• Match plate pattern of casting

  
  In this type the actual pattern is made in two halves and is then mounted on the two faces of a universal match plate, which acts as the parting line between the two pattern pieces. Match plate is also fitted with gating system making as ease for moulder, and reducing the mounding time. When casting is very small many patterns can be mounted on the same match plate reducing the number of moulds to be prepared. Match plate is usually made of metal like aluminium. When the match plate is withdraw the cope and drag boxes can be joined to form the complete mould cavity.






• Sweep pattern of casting

  
  Sweep patterns are used for the castings which are symmetrical and regular in shape like cylinder or pipe etc. This type of pattern is used when the castings required are very large. In sweep pattern a simple sweep which is rotated about a central axis is used to produce a mould cavity. A sweep pattern is shown in the figure below.

Materials used for pattern making in casting

Wood:

• It is available very easily.


• It has low weight.


• It is easy to work with wood.


• It is relatively cheap.

• It shrinks when exposed to moisture.


• More wear & tear due to moisture.


• No proper surface finish when used in large scale production.

Metals and alloys:

Different pattern allowances in casting

• Shrinkage allowance:

  As we know, when the metal is heated to its liquid state and then allowed to cool after pouring it into a mould cavity, it shrinks from its actual size due to phase transformation from liquid state to solid state. To overcome this shrinkage the patterns are provided with shrinkage allowance. Table below shows the allowance required for different metals.






• Draft allowance:

  The draft allowance is defined as the tapering which is provided on the pattern. When the sides of the pattern are deeper, it may damage the walls of the pattern while with drawing it from the mould cavity. To overcome this problem a taper or slight inclined surface is furnished on the sides of the pattern for the easy withdrawal of pattern from the mould. Table below shows the values of taper to be provided for easy removal.








• Machining allowance:

  when an object is prepared using casting, the surface finish we get is very bad and rough. To overcome this issue some extra metal is provided on the surface to machine and make the surface smooth. This allowance is known as machining allowance.