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Eliminate Molded Parts Deformation

Causes of molded parts deformation
  • Because of the shape of plastic parts, especially the residual stress in forming shrinkage rates and product thickness.
  • Residual stress generated in injection moulding
  • Residual stress in demoulding
  • Deformation caused by insufficient cooling time
Associate knowledge about deformation
Reason of molded parts deformation (warping, bending, small wrinkles) is the same to cracks. That is the residual stress within the product. Moulding conditions should be set towards to elimination of stress. That is to increase cylinder and mould temperature, and when the flow ability of the material becomes a good state, the injection pressure should not be too high.
Molded Plastic Parts Deformation
To reduce the residual stress, the injection mould should be annealed. Heat distortion temperature below 10℃, heated over 2 hours to eliminate the effect of internal stress. However, due to the high cost, this method is not widely used.

If the cooling water holes of the mould cannot uniformly cool the product, and it also have residual stress, which means that the cooling water hole cannot be too shallow.

Solutions
Instant: Sufficiently cooled and solidified the product in the injection mould (extended cooling timer), raise the barrel temperature, decreasing injection pressure.
Short-term: uniform cooling of the injection mould.
Long-term: Avoid different thickness in product design, and design a sprue in the thickness parts (see the picture in sink marks). Because the straight line is easy to cause warping, so we made large R curve (Figure A), or products reversible bending mould (Figure B), increase the number of the ejection lever, increased stripping slope.
Eliminate Molded Parts Deformation

Differences in materials
Crystalline materials, like polyethylene, polypropylene, acetal and nylon, have high moulding shrinkage and prone to bias. Non-crystalline material, like polystyrene and ABS, are easily lead to residual stresses.

Reference
After moulding aligner kept at room temperature, it can be a little to prevent deformation, but does not have high expectations.
In a thin box-shaped moulding, thermal expansion may cause bending due to temperature. (Figure C)

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