Meaning
Polymer density transition defines the contraction of a moulded component during the cooling phase inside a tool cavity. Volume shrinkage represents a physical reduction in exterior dimensions occurring as the molten material loses thermal energy and shifts into a solid state. Crystallinity levels and molecular orientation dictate the total magnitude of this event for a given resin.
Thermal Calculation
Designers adjust the tool dimensions to compensate for these contraction characteristics to ensure the part reaches the required final tolerances. Molten plastic enters the cavity at an elevated temperature and occupies the full space provided by the steel. As the material cools, the mass density climbs while the overall volume drops.
Moulders account for this gap by increasing cavity size beyond the nominal print dimensions to anticipate the contraction. Regrind usage alters the flow properties and affects the consistency of this cooling curve compared to virgin resin performance.
Production Variability
Injection parameters influence the final part size through holding pressure and gate freeze times. High packing pressure forces additional material into the cavity to offset the cooling contraction before the gate seals. Variations in the cooling cycle cause the part to solidify at different rates across a production run.
Stable gate freeze times provide the control necessary to prevent internal voids or sink marks from forming as the core cools.
Specification Distinction
Laboratory tests establish the shrinkage data on simplified specimens while industrial parts encounter complex stress profiles. Datasheet values provide a baseline for material selection based on standardized cooling rates in a controlled environment. Real world moulded parts exhibit differential shrinkage patterns because of non uniform wall thickness and fiber alignment within the tool.
Final part dimensions reflect the interaction between the material contraction potential and the physical constraints imposed by the tooling geometry.
