Volumetric Shrinkage

Meaning

Percentage decrease in the total volume of a plastic part as it transitions from a hot melt to a cool solid inside the tool. The volumetric shrinkage dictates the final size of the moulded component and determines how well the plastic fills the cavity corners. It results from the contraction of polymer chains as they lose thermal energy and occupy less space.

Mould Geometry

Design of the cavity must compensate for the predictable loss of size by being slightly larger than the target dimensions. Since volumetric shrinkage varies between crystalline and amorphous polymers, switching resins often requires an entirely new set of tooling. This calculation ensures that assembly points like screw holes or snap fits align correctly after ejecting from the press.

Processing Pressure

Maintenance of high pack pressure during the cooling phase is the primary method used to minimize volume loss. When volumetric shrinkage remains high, parts develop internal voids or surface sink marks that ruin the structural integrity. Operators must balance the time spent under pressure against the need for short cycle times to keep the project profitable.

Cooling Profile

Variation in cooling rates across a large part creates uneven shrinkage and leads to significant warping or bending. Because volumetric shrinkage follows the path of least resistance, thin walls and thick walls will contract at different rates. Proper thermal management inside the tool helps normalize these effects to produce straight and square components.

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