Notched Izod Impact

Thermal Degradation

Excessive barrel temperatures during compounding destroy molecular chain integrity and drastically reduce impact strength before pellets enter the injection unit. Residence time inside the plasticating screw dictates polymer thermal history, so prolonged heat exposure causes chain scission that destroys fracture resistance. Screw geometry and compression ratios directly influence shear heating rates, meaning incorrect profile selection accelerates polymer breakdown.

Processors prevent molecular weight reduction by monitoring melt temperatures continuously and purging stagnant material during production halts.

Regrind Economics

Incorporating post industrial scrap alters impact performance through cumulative thermal cycles that degrade polymer chains across successive reprocessing runs. Virgin feedstock exhibits high energy absorption, but blending recycled fractions introduces low molecular weight fragments that initiate premature cracking. Moulders balance cost reduction against structural reliability by establishing strict percentage limits for regrind addition in non critical cavities.

Component wall thickness determines allowable regrind ratios because thin sections require virgin material to survive drop tests without brittle failure.

Crystalline Morphology

Cooling rates inside the mould cavity dictate spherulite size and therefore control the final impact behaviour of semi crystalline thermoplastics. Rapid quenching yields smaller spherulites that enhance toughness, whereas slow cooling produces large structures susceptible to catastrophic cleavage under sudden loads. Tooling water temperature management dictates heat extraction speed, meaning improper channel design creates localized variations in mechanical performance across a single moulded part.

Injection speed adjustments alter shear induced crystallization patterns, shifting the balance between ductile failure and brittle shattering during end use events.