Meaning
Molecular weight degradation during polymer processing creates a viscosity shift that alters how molten resin fills a mould cavity. Injection moulding operations experience this rheological phenomenon when thermal history or excessive shear forces break polymer chains. Virgin material maintains a predictable melt flow index specified by the resin supplier, whereas regrind batches often exhibit downward flow anomalies due to repeated heat exposure.
Production engineers must monitor these rheological changes to prevent flash formation and short shots across multi-cavity tooling.
Resin Degradation
Thermal overexposure inside the plasticating barrel breaks long molecular chains into shorter segments. Extrusion and injection moulding machines set barrel temperatures to soften pellets without inducing polymer scission. Excessive residence time traps material in hot zones, which accelerates molecular breakdown and reduces melt strength.
Lower molecular weight directly increases melt flow rate, creating dimensional instability in moulded parts.
Part Specification
Finished components require structural integrity that depends entirely on stable polymer chain length. Part specifications dictate mechanical properties like tensile strength and impact resistance, which drop sharply when degraded resin fills the cavity. Moulders must distinguish between raw material data sheets and actual shop floor measurements because pellet testing ignores thermal changes imparted by the screw.
Dimensional tolerances collapse when unexpected melt variations alter volumetric shrinkage inside the mould.
Economic Penalty
Financial losses accumulate rapidly when unstable melt behavior forces frequent machine adjustments or scrap generation. Virgin pellets cost significantly more than regrind material, but mixing recycled regrind without proper drying and compounding introduces severe batch inconsistency. Production lines suffer extended downtime while technicians purge degraded polymer and recalibrate injection pressure profiles.
Scrap parts resulting from flow anomalies increase material waste and directly inflate manufacturing overhead costs.
