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.

MOLDING SHRINKAGE PHENOMENON

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)

Conclusion:

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.