Distortion Allowance or Camber Allowance In Casting
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| Distortion Allowance Diagram |
Distortion Allowance
What is distortion allowance or camber allowance in casting?
Distortion allowance can be defined as an allowance given to the pattern to compensate for the change in shape and size of the casting during the solidification process due to non-uniform cooling in uneven sections of the casting.
Allowance given to the pattern is counter and opposite equivalent to the amount of distortion casting will go through during the solidification process. This is also called inverse reflection of the pattern.
Distortion of casting takes place during the solidification process due to the internal stress and unequal heat transfer in the casting with irregular complex thickness and shape of the casting.
Allowance should be higher if there is a sudden change in the thickness of the casting causing non-uniform solidification of the casting, allowance is kept in the range of 2mm to 20mm per section of the casting.
Distortion allowance is a positive allowance and is also called camber allowance, deformation allowance & bend allowance. This is one of the allowances among the 6 types of pattern allowance in casting.
As shown below in diagram (a) desirable sand casting is to be produced. Distortion during the solidification process will tend to bend thin sections of the casting.
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| (a) Expected Casting Without Distortion Defect |
In order to avoid the distortion, deformation or bend of casting the distortion allowance in the pattern is given in the opposite direction as seen in the diagram below (b) to get the desirable casting we saw above.
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| (b) Distortion Allowance In Sand Casting |
Now during cooling, the casting can deform or bend but will distort towards the desired shape.
It is important to bend the required casting in the exact opposite direction by using a bend pattern with an inward angle.
Casting with thin irregular shapes such as U-shapes, W-shapes, L-shapes, V-shapes and T-shapes is more prone to distortion and bending during the solidification process.
Long horizontal sections of casting need support as they distort and sag in the centre.
How to prevent distortion of casting?
Distortion in casting sections takes place during the cooling process. Thermal internal stress generated due to non-uniform cooling in irregular sections increases the chances of distortion in casting sections.
Distortion is avoided by giving allowance to the pattern on the opposite side where distortion is going to happen as shown in the diagram (c- 2 and c-3 notation).
If we give distortion allowance as shown in c-2 and c-3 we can produce sand casting as shown in c-1.
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| (c) Distortion Allowance In Casting |
Distortion allowance cannot be prevented by shrinkage allowance.
Finite element analysis (FEA) is used to simulate distortion during to solidification process giving accurate distortion values.
Factors To Consider While Selecting Distortion Allowance
What does distortion allowance depend on?
Distortion or camber allowance depends upon the following factors:
- Solidification rate and phase transformation during cooling.
- Internal stress is induced in the casting during cooling.
- Intersections of the casting sections and cooling rate of those sections.
- Uneven thickness of the casting.
- Complex design and irregular shape of the casting.
- Cooling pads were used in the process.
- Heat transfer during the solidification process.
- Sudden change in geometry of the casting sections.
- Change in thermal gradient during solidification.
- Thinner casting has U, V, T, L, X and W shapes.
- Shrinkage characteristic of molten metal.
- Type of metal used for casting.
- Cores rigidity and residual stress concentration in contraction area.
- Resistance of mould during contraction of the casting.
- Location of the core and core properties.
- A number of chills and padding are used in the casting process.
- Desirable dimensional accuracy and tolerance of the final casting to be achieved.
- Type of gating system used to feed molten metal to the casting during the solidification process.
- Solidification of all sections of the casting and combined cooling rate.
- The support is given to casting during solidification.
- Amount of the inverse reflection given to the pattern.
Reducing Distortion Allowance
Camber or distortion allowance can be reduced by taking the following steps to avoid distortion.
Allowance can be reduced in casting by doing the following steps:
- Provide proper machining allowance to casting sections which tend to distort.
- Design sections of casting with uniform thickness.
- Eliminate sudden changes in the sections of the casting.
- Provide padding for uniform solidification of molten metal.
- Incorporate proper, continuous and uniform feeding of molten metal to the casting.
- Manufacture casting and weld complex casting sections later.
- Provide support for long heavy bars in the mould cavity to reduce buckling and sagging of bars.
- The core should have compressibility properties during the solidification of molten metal.
- Use casting metals that have minimum contraction properties.
Advantage of Distortion Allowance In Casting
- Reduces warping and deformation defects in longer casting sections.
- Long casting sections without buckle and sagging defects are produced.
- Casting components with uniform thickness can be produced.
- Rejection of bend casting is minimized.
- Complex shape castings such as U, V, W, L, Y, F and T sections can be manufactured by providing allowance.
- Non-distorted internal cavities, recesses and holes can be manufactured with distortion.
- Allowance is applied on casting sections that will bend during cooling.
Distortion Allowance Conclusion
Allowance given to the pattern on exactly the opposite side of the distortion to compensate for the bend, sagging and hogging of the thin sections of the casting during the solidification process is called camber or distortion allowance.
This allowance is mainly given to thinner and more complex sections.
| Allowance Name | Reason For Allowance | Allowance Sections |
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| Camber Allowance or Distortion Allowance | Allowance is given to the thinner, weaker and non-supported sections of casting which tend to bend or distort during the solidification process. | Allowance is given to sections such as U,X,V,T,Y and M. |
This allowance is different from the shrinkage allowance where allowance is given to the pattern, making the pattern larger in size as metal shrinks during the cooling process.