Meaning
Plastic impact resistance finds its primary numerical definition in ISO 180, which specifies the Izod method for determining pendulum impact strength under defined notch conditions. Injection moulded bars face a pendulum strike on a clamped cantilever specimen, returning a energy absorption figure expressed in kilojoules per square metre. That protocol applies strictly to rigid thermoplastics and filled composite compounds, while flexible elastomers and films fall outside the scope because their high elongation prevents clean fracture.
Standardised testing conditions govern notch geometry, striking velocity, and ambient conditioning parameters to eliminate variance across laboratory facilities.
Impact Dynamics
Pendulum velocity drops upon fracturing the moulded bar, and the residual kinetic energy read on the dial reveals the resistance capacity of the polymer matrix. Specimen thickness dictates whether plane stress or plane strain dominates the fracture mechanics, shifting brittle failures toward ductile tearing when wall dimensions exceed critical thresholds. Molecular weight distribution governs chain entanglement density, which directly controls the propagation energy required once a microcrack initiates at the machined notch tip.
Tooling Variance
Melt temperature during injection and mould cavity cooling rates alter crystalline morphology, changing measured impact values independently of raw material grades. Shear forces inside the runner system orient polymer chains along flow lines, creating anisotropic mechanical properties that cause transverse test bars to yield lower energy absorption figures than parallel samples. Processing technicians must lock barrel heats and injection speeds to hold consistent impact metrics across production runs, because excessive shear degrades polymer backbones and destroys notch sensitivity margins.
Regrind Economics
Recycled polymer streams accumulate thermal history through successive melting cycles, shortening average chain lengths and degrading Izod performance below virgin baseline thresholds. Moulding shops blend regrind into virgin resin up to specific percentage limits, but exceeding those thresholds introduces microvoids that concentrate stress and trigger catastrophic part failure under dynamic loads. Supplier datasheets quote impact strength derived from pristine injection moulded test bars, leaving parts manufacturers to verify that shop floor scrap incorporation does not depress finished component toughness below structural design requirements.
