Molding Shrinkage Phenomenon

Using the phenomena of mold shrinkage, thermoplastic plastics can be injection molded to produce products with the appropriate dimensions. When molten plastic is injected into a mold cavity, a phenomenon known as mold shrinkage occurs where the volume of plastic decreases as it cools and solidifies.

Last Updated: September 18th, 2023 First Published :



It is feasible to shape the molded item so that it has the intended dimensions by preparing the mold and making the dimensions of the cavity a little larger by the amount of shrinkage, provided that the “molding shrinkage factor” is known with accuracy from both scientific and practical experience. Molding shrinkage factor values typically fall between 2/1000 and 20/1000 (or between 0.2 and 2%). The molding shrinkage factor is also affected by the below-mentioned factors:

  • The variety of plastic material that is being utilized affects the fundamental shrinkage factor’s range.
  • During injection molding, the cavity surface temperature affects the molding shrinkage factor.
  • Maintained pressure time spent maintaining pressure
  • The amount of pressure held after plastic injection and the length of time held at that pressure determine the molding shrinkage factor.
  • The shrinkage factor changes according to the molded item’s wall thickness as well.
  • The gate size and form have an impact on the shrinkage factor.
  • Whether the molding material contains additives or not.

Video 01: Shrinkage Allowance (Animation)


Molding shrinkage is a common phenomenon in plastic injection molding that occurs due to the cooling and solidification of molten plastic inside a mold. It is caused by the contraction of the plastic material as it cools and solidifies, resulting in a decrease in overall part dimensions. Molding shrinkage is a critical factor that affects the dimensional accuracy and quality of plastic parts.

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