Meaning
Polymer flow stability is the processing variable that dictates how uniformly a thermoplastic melt fills a mould cavity during injection. Recycled polymer migration describes the localized concentration shift of varying chain lengths and additive packages within a heterogeneous regrind melt stream. Heterogeneity arises when post consumer flakes mix with virgin pellets of different melt mass flow rates.
Viscosity gradients across the barrel cause lower molecular weight fractions to travel faster than higher molecular weight fractions during high pressure injection phases. Shear heating amplifies this velocity split within the runner system. Tooling designers must account for this phenomenon when balancing multi cavity hot runner drops.
Part specifications often fail structural drop tests if segregated polymer domains freeze unequally across the wall section.
Resin Viscosity Drift
Material sourcing introduces batch to batch variability that directly alters shear stress profiles inside the reciprocating screw. Virgin pellets exhibit predictable melt flow indexes established during synthesis. Regrind stocks suffer chain scission from prior thermal cycles, yielding broader molecular weight distributions.
Melt temperature spikes during plastication accelerate the separation of short chains from long chains before the nozzle opens. Injection speed profiles must compensate for the resulting density fluctuations to prevent surface blemishes on finished components.
Thermal History Gradient
Temperature control across the barrel heating zones dictates how rapidly distinct polymer fractions mobilize. Molecular weight dispersion broadens when degradation products mix with untouched virgin feedstock. Processing windows shrink because degraded fractions soften at lower thresholds than virgin material.
Pressure drops across the gate fluctuate when uncoordinated polymer fractions arrive out of sequence. Moulding technicians adjust back pressure settings to force mechanical homogenization before the shot delivers.
Defect Formation Cost
Part specifications define structural integrity limits that surface segregation frequently violates. Weld lines become acute points of mechanical failure when migrating additives concentrate at the intersection of two melt fronts. Scrap rates climb rapidly when differential shrinkage warps flat housings beyond acceptable tolerance bands.
Material cost savings from regrind incorporation evaporate if sorting rejects exceed allowable shop floor thresholds. Component performance relies entirely on maintaining uniform molecular distribution throughout the entire moulding cycle.
