Why does plastic warp after processing?
When we process plastic, we sometimes find that components change shape or warp after a while. We recognise that this can be problematic, particularly in applications requiring high precision. But what causes this phenomenon?
A common reason why plastic components deform is the internal stresses present in the material. These stresses can arise during manufacture, for example during extrusion when the plastic is shaped. If the plastic is not processed correctly, these internal stresses may remain and cause problems later on.
Internal Stresses from Extrusion
During extrusion, the plastic is subjected to various types of stresses, such as thermal and mechanical stresses. These can lead to deformation. If the plastic isn't handled correctly, these stresses can remain. For example, HDPE and UHMWPE may be more prone to developing internal stresses during extrusion.
Asymmetrical machining can also cause deformation. When machining plastic components, we may inadvertently create an imbalance in the material, which in turn can cause it to warp. A component cut with a circular saw may be more prone to deformation if it is not symmetrical.
Annealing as a Solution
One way to reduce internal stresses in plastic parts is through annealing. This means heating the plastic to a specific temperature and then cooling it down slowly. This can help reduce internal stresses and improve the material's properties. PA6 and POM-C, for example, can benefit from annealing to reduce stresses.
When the plastic is heated, the molecules begin to move more freely. This helps reduce internal stresses and improve the properties. When the plastic is then cooled down slowly, the molecules can return to their original position, which increases stability.
The specific properties of the material
Different plastics have different properties. PA, for example, is sensitive to moisture and can swell if exposed to too much moisture. This can cause deformation. POM is more stable and less sensitive to moisture.
Another important factor is hardness. PE-1000 has a hardness of 65° Shore D, which affects its behaviour during processing. If we do not take hardness into account, we may inadvertently create problems.
What can we do to prevent plastic components from deforming? Choosing the right material is crucial. PE-HD may be a better choice than PA for certain applications. It is also important to machine the components correctly, by utilising symmetry and avoiding asymmetry.
- Choose the right material.
- Machine the parts correctly.
- Use annealing to reduce stresses.
- Take into account the hardness and properties of the material.
By following this advice, we can reduce the risk of plastic components warping. If we have already produced components that have warped, we may need to re-machine them or manufacture new ones.
We hope this article has given you a better understanding of why plastic components deform and how you can prevent this. Please don’t hesitate to contact us if you have any further questions. We’d be happy to help!
Would you like to know more about our various plastic materials and how they can be used? You’ll find plenty of information on our website.
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