Meaning
An abrupt rheological excursion during polymer processing marks a sudden drop in melt viscosity that disrupts extrusion pressure profiles. This melt flow rate spike signals severe molecular weight degradation or unexpected moisture flashing inside the barrel heaters. Extrusion lines processing polyethylene and polypropylene monitor this rheological instability to prevent severe dimensional distortion in extruded profiles.
Thermal overload in the compounding screw generates localized shear heating that triggers the anomaly without warning. Tooling pressure drops instantly as the polymer chains undergo excessive scission under uncontrolled thermal stress.
Shear Instability
Rotational speed increases inside the compounding extruder push polymer chains beyond their critical shear rate limit. Viscous dissipation accelerates rapidly when barrel temperature controllers fail to dissipate frictional energy from high throughput rates. Operators adjust screw geometry to mitigate thermal runaway during high speed compounding runs.
Virgin polymer pellets tolerate higher shear stress before degradation occurs compared to recycled flake batches containing degraded polymer chains. Granule feed inconsistency exacerbates the thermal imbalance by starving specific flight zones of solid conveying capacity.
Resin Degradation
Molecular chain scission occurs when processing temperatures exceed the thermal stability threshold of the base resin. Additive packages fail prematurely under prolonged residence time inside the heating cylinder, releasing volatile byproducts. Moulders purchasing virgin material rely on certified melt flow index values supplied in the resin datasheet.
Recycled regrind exhibits wider molecular weight distributions that elevate the probability of sudden viscosity drops during plastication. Blending regrind with virgin feedstock requires strict screw temperature reduction to avoid exceeding the operational ceiling of the polymer.
Dimensional Defect
Part weight variance increases dramatically when low viscosity resin floods the mould cavity during the injection phase. Clamping tonnage fails to contain flash formation along the parting line when material pressure surges unexpectedly. Finished components suffer from localized sink marks and structural weakness due to incomplete packing of the volumetric shrinkage.
Quality control laboratories measure density shifts in moulded samples to verify material consistency against the baseline specification. Production managers absorb high scrap rates when unstable resin batches ruin entire moulding shifts before manual intervention halts the press.
