Meaning
Internal mechanical tension frozen into a plastic component results from unequal cooling rates and density changes during the solidification of the polymer melt. Residual moulding stress exists as a persistent field of force within the solid matrix once the part leaves the cavity. High levels of this stored energy cause unpredictable dimensional change or premature structural failure under load.
Processing Impact
Packing pressure variations during the holding phase often dictate the magnitude of the strain across a complex geometry. Operators adjust cooling times to modulate the rate of molecular relaxation before the part reaches the ejection temperature. Excessive speed in the injection cycle prevents the polymer chains from assuming a random orientation, which creates localized zones of density variation.
Thicker sections cool slower than thin walls, creating a thermal gradient that pulls the material inward during crystallization.
Performance Consequences
Brittle fracture occurs when environmental exposure or chemical contact triggers the release of this trapped energy. Small cracks appear in the surface of the article where the density mismatch reaches a threshold. Chemical resistance of a part drops sharply if solvents penetrate these areas of high tension.
Material Specification
Resin datasheets provide an elastic modulus determined under controlled laboratory conditions that assume an absence of internal load. Moulders differentiate this theoretical baseline from the actual capacity of a part by measuring the birefringence of the finished item. Regrind material alters the flow behaviour and increases the likelihood of uneven shrinkage compared to virgin feedstocks.
Consistent tooling temperature remains the primary method for maintaining the stability of a production run.
