Types of Pattern Allowance
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| Types of Pattern Allowances |
6 Types of Pattern Allowance In Casting
- Draft allowance.
- Shrinkage allowance.
- Distortion allowance or Shake allowance.
- Machining allowance or Finishing allowance.
- Shake allowance or Rapping allowance.
- Movement of mould wall allowance.
Tpes of the pattern allowances are given to patterns so casting engineers can manufacture casting with the exact desirable shape, size surface finish, dimensional accuracy and tolerance.
Pattern allowances are provided on the pattern for the changes that happen during the casting process such as metal shrinkage, warpage or distortion, damage to the mould while drawing patterns out of the mould and requirements of the final casting such as elimination of surface roughness and improving the surface finish of the casting.
For example, during pattern withdrawal pattern is shaken, enlarging the mould cavity and resulting in larger defective casting, to compensate for this shake allowance or rapping allowance is provided to make a casting of the exact size desired.
When molten metal is poured into the mould cavity care must be taken to ensure the mould wall does not expand causing the enlarged casting. To solve this problem of mould movement and enlarged casting mould movement allowance is provided on the pattern to make it smaller in size.
As shake allowance is given to the pattern to compensate for the enlarging casting care must be taken that casting does not solid shrink to form small casting. To take care of this shrinkage allowances are provided on the pattern to compensate for changes in the size of casting due to molten metal contracting during the solidification process.
While removing the pattern from the mould, the pattern needs to be easily withdrawn without damaging the mould cavity. In order to solve the problem pattern is provided with a draft allowance to take care of the mould getting damaged.
After the casting has been taken out of the mould, the casting needs to be machined to make a good surface finish and dimensionally accurate. To take care of this machining allowance is provided on the pattern.
Draft Allowance or Taper Allowance
Draft allowance is a taper angle given to the vertical sides of the pattern to avoid damage to the mould cavity while removing the pattern from the mould, this is done by providing a taper to the pattern.
Draft allowance in casting is discussed in the draft allowance in the casting article with a diagram, advantages, draft allowance for various materials with internal and external surfaces, problems, and factors considering the selection of camber, taper or draft allowance.
Shrinkage Allowance or Contraction Allowance
Shrinkage allowance is a type of pattern allowance given to compensate for the reduction in the size of the casting during the solidification process.
Metal shrinks during the solidification process causing the final casting to be undersized than the desired casting.
To make a casting of the exact size the pattern is made larger in size so after solidification, the final casting will be the same size as on the drawing.
Shrinkage allowance in casting is explained in detail with types, diagrams, advantages, factors on which shrinkage allowance is dependent and examples.
Distortion Allowance
Distortion allowance is a type of pattern allowance given to compensate for changes in the dimensions of casting because of uneven solidification during the process.
In distortion allowance, the allowance pattern is shaped in the opposite direction of the distortion so when casting actually goes through distortion because of its complex shape and size during the uneven solidification process it will get back to its desired shape.
I have written a detailed article explaining distortion allowance in casting with a diagram explaining how to prevent distortion of casting.
Shake Allowance or Rapping Allowance
A shake allowance or rapping allowance is a type of pattern allowance given to compensate for the change in dimensions of the mould cavity while withdrawing the pattern from the sand mould.
When the pattern is withdrawn from the green sand mould, the mould cavity is enlarged making the final casting larger than the required size. To compensate for this enlarged casting shake allowance or rapping allowance in casting is provided.
I have written a detailed article on shake allowance or rapping allowance in casting explaining the allowance with diagrams, shrinkage chart, advantages, and factors on which shake or rapping allowance is dependent.
Machining Allowance or Finishing Allowance
Machining allowance is a pattern allowance given to the pattern to achieve a better surface finish with dimensional accuracy which is difficult to achieve by sand casting alone.
This allowance is provided more on sand casting because sand casting has a rough texture compared to other casting processes such as die casting.
I have discussed machining allowance in casting with diagrams, problems, advantages, and factors on which machining allowance is dependent.
Movement Of Mould Wall Allowance
When molten metal enters the mould cavity, the mould cavity tends to increase in size because of heat and metallostatic forces acting on the mould wall. This makes the final casting larger in size than the required casting.
To solve this problem of mould expansion, the pattern is made undersize than the required size to take care of mould movement problems.
- The metallostatic force acting on the mould wall.
- The high temperature of molten metal while entering the mould cavity.
- The velocity of molten metal entering the mould cavity.
- The density of the molten metal.
- Green and dry strength of the mould.
- Mould sand composition.
- Use of additives and binders.
- Mould sand resistance.
Type Of Pattern Allowance Depends Upon Following Factors
All 6 types of pattern allowances widely depend upon the following factors:
- Parting sand is used to part the pattern from sticking the moulding sand. (In case of draft allowance)
- Type of mould used in the process. (Die casting has less machining allowance compared to sand casting)
- Heat transfer during the casting process. (Uniform heat transfer reduces shrinkage of casting)
- Pouring temperature of molten metal. (Very high temperature expands mould producing larger casting)
- Sand mould making process.
- Gating system in the moulding process.
- Position of chaplet and cores.
- Mould sand properties.
- Mould material and type of moulding process.
- Amount of sand ramming done during the mould-making process.
- Type of moulding machine and technique used.
- Type of pattern material used and pattern maker's skill.
- Properties of pattern material in the casting process.
- Final casting dimensions, shape, size and thickness of the casting desired.
- Minimum, maximum and uniformity of thickness of the casting.
- Type of metal used in the casting process.
- Casting defects, finishing value of the casting, the surface finish of casting, solidification process, thermal padding done in mould, machines and equipment used during and after the casting process.
Types of Pattern Allowance In Casting Summary
How many types of pattern allowances are there in casting?
6 Types of pattern allowance are explained in the simple table below:
| Pattern Allowance Type | Pattern Allowance Reason |
|---|---|
| 1) Machining Allowance or Finishing Allowance | Positive allowance is provided on the pattern to improve the surface finish or finishing of the final casting. |
| 2) Draft Allowance | Positive allowance provided for easy withdrawal of pattern from the sand mould without damaging mould cavity. |
| 3) Distortion or Camber Allowance | Positive allowance is provided on the pattern to take care of distortion in the casting during the solidification process due to uneven cooling and internal stress in sections of the casting. |
| 4) Shrinkage Allowance | Positive allowance is given to take care of shrinkage of casting by making the pattern larger in size than the required casting. |
| 5) Shake Allowance or Rapping Allowance | Negative allowance was given to the pattern by making the pattern undersize to take care of the mould cavity getting larger while pattern withdrawal. |
| 6) Mould movement allowance | Negative allowance was given to the pattern by making the pattern smaller in size to take care of the mould cavity getting large due to metallostatic force acting on the mould wall at high temperature |
What will happen if these 6 types of pattern allowance are not given on the pattern in the casting process?
If these 6 different types of pattern allowance are not given on the pattern the following result will take place:
- Rejection of the casting will increase.
- Increase chances of defective casting.
- The cost of inspection and testing will increase.
- There will be an increase in the waste of resources. (Material, time, energy and tools)
- Undersized and oversized casting will be produced. (If shrinkage allowance and rapping allowance are not given)
- Casting with distortion and warpage will be produced. (If distortion allowance is not provided)
- Reduction in productivity and profit of the foundry.
- The need for quality control will increase.
- The mould will be damaged. (In case of draft allowance)
- The cost of manufacturing casting products will increase.