Determining Polyolefin Melt Flow Changes from Repeated Shear Exposure
Polyolefin melt flow shifts reveal chain scission or crosslinking under repeated shear, requiring strict multi-point viscosity and antioxidant monitoring.
Polymer Thermal History
Meaning
Cumulative exposure to heat and shear cycles during processing defines polymer thermal history, governing molecular weight degradation and crystalline morphology across subsequent moulding stages. This property dictates the final mechanical performance of a moulded component by establishing baseline molecular orientation before the melt enters the tool. Virgin pellets and regrind material exhibit divergent profiles because repeated heating reduces chain length, shifting viscosity curves away from the resin specification.
Thermal Burden
Molecular chain scission accelerates when barrels operate above recommended temperature thresholds during compounding, lowering impact strength in finished parts. Residence time inside the screw channels amplifies this degradation, converting steady energy input into uncontrolled chain cleavage. Extrusion lines mitigate such damage through barrel cooling loops, but excessive residence creates a narrow processing window that causes flash and short shots during injection.
Crystalline Profile
Nucleation rates depend directly on how slowly the molten material cools inside the cavity, dictating shrinkage rates and warpage tendencies in semicrystalline resins. Fast quenching freezes amorphous regions, whereas controlled mould temperatures promote spherulitic growth, raising tensile modulus at the expense of elongation. Datasheet values assume standard cooling rates, but local variations in tool geometry alter the actual morphology, producing anisotropic mechanical properties across structural ribs.
Economic Cost
Scrap rates climb when degraded regrind enters precision tooling, forcing processors to adjust holding pressures to compensate for lowered melt viscosity. Part specifications demand tight control over supplier lots to prevent dimensional drift, which incurs financial losses from rejected shipments. Moulders absorb these expenses through increased sampling frequency, balancing virgin resin costs against the processing instability introduced by cumulative heat exposure.