July 14, 2022

Injection molded parts problems and solutions

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Plastic parts produced by the injection molding process are usually long lived consumer products. It’s suitability depends on the properties of the finished part and surface quality, it required a lot of experiences of raw materials, processing condition, injection molding parameter to get optimum results. The reasons to cause problems could not be a certain answer as the processing condition is always different, on the basis of an individual case a decision has to be made as to whether simple and rapid fault correction is possible or whether intervention is the design of the parts or the layout of the mold and gating is necessary.

In conclusion the molded plastic parts surface problems occurring most commonly in polymers are noted and described and recommendations for their removal are set out.

Weld line
Problem: hairlines.
Reason: different mold flow meet, decorative effect pigments such us material with glass fibers.
Solution: as far as possible, position weld lines where they have no visual or mechanical impact. If possible, enlarge the sprue channel, optimization of the injection mold temperature, mold surface temperature and injection speed.

Air entrapments/bubble formation
Problem: the air is trapped into the injection molded parts.
Reason: during mold filling air is entrapped, it could be bubbles.
Solution: as far as necessary optimize the geometry of the molding with the aid of a mold flow calculation. Check design and condition of the mold vents.

Sink marks
Problem: dents or sink marks on the surface of the molded parts.
Reason: sink marks are always produced in regions f accumulation of material when the contraction in volume arising during the cooling phase cannot be adequately compensated for by the holding pressure.
Solution: Avoid large differences in wall thickness. Optimize melt and mold temperature, set the holding pressure, holding pressure time and melt cushion to adequate dimensions.

Problem: flash existed around the mold parting surface.

  • fitting tolerance of the halves of the injection mold are too big or the mold parting line is damaged.
  • clamp force of injection machine is inadequate or is set too low.
  • melt temperature, injection speed or internal mold pressure is too high.


  • fit the injection mold again.
  • set the clamp force higher or change to larger machine.
  • reduce melt temperature and injection mold temperature.

Problem: deformations and twists, deviation from designed shape.

  • Unsuitable molding design and different wall thickness of parts cause different shrinkage in different direction.
  • Incorrect location of the sprue.
  • Incorrect setting of molding temperature and injection speed.


  • avoid internal stresses by correct the part design, do a mold flow study.
  • check sprue location.
  • optimization of processing conditions.

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