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Injection molding products’ shrinkage analysis

Shrinkage is a common phenomenon in injection molding process.There are main reasons for this situation:

  1. Machine reason:
  • The nozzle hole is too large, resulting in the melting material reflux and shrinkage, and too small resistance plus large material quantity to result in shrinkage.
  • Lack of clamping force will cause the flash edge to shrink. Check the clamping system.
  • If the plasticizing amount is insufficient, the machine with large plasticizing amount should be selected to check whether the screw and cylinder are worn


  1. Mold Reason:
  • Product design to uniform wall thickness, to ensure consistent shrinkage
  • The cooling and heating system of the mold should ensure the same temperature in all aspects
  • the gating system should be kept smooth, the resistance should not be too large, such as the size of the mainstream channel, the shunt channel and the gate should be appropriate, the finish should be enough, and the transition zone should be circular transition
  • For thin parts, the temperature should be raised to ensure smooth material, and for thick-walled products, the mold temperature should be lowered
  • The gate should be set symmetrically, located in the thick-walled part of the product as far as possible, and the volume of the cold material well should be increased


  1. Plastic Material Reason:
  • Crystalline plastic than non-crystalline plastic shrinkage severe, processing should be appropriate to increase the amount of material, or in the plastic to add a change agent, in order to accelerate crystallization, reduce shrinkage depression
  1. Manufacturing Reason:
  • The temperature of the cylinder is too high, and the volume changes greatly, especially the temperature of the front furnace. The temperature of the plastic with poor fluidity should be appropriately raised to ensure smooth flow
  • The injection pressure, speed, back pressure is too low, back pressure is too large, time is too long cause the flash edge and appear contraction
  • When the buffer pad is too large, the injection pressure will be consumed. When the buffer pad is too small, the injection pressure will be insufficient
  • For products that do not require precision, after the completion of injection and pressure retention, the outer layer is basically condensed and hardened and the sandwich part is still soft and can be ejected. As early as possible, the mold can be released and cooled slowly in air or hot water, so that the contraction sag can be gentle and not so conspicuous and does not affect the use

Gmolding is professional plastic injection molding manufacturer in China with 20 years experience engineer team and factory has passed BSCI audit by SGS.

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