The Common Plastic Resins and What Each One Is For
Polymer grade selection fixes mechanical properties, tool shrinkage, cycle time, compliance limits, and final landed part cost across production runs.
Charpy Test
Meaning
High-rate impact evaluation of plastic specimens using a weighted pendulum measures the resistance of a material to sudden fracture. The charpy test employs a supported beam configuration to determine the energy absorbed during failure under dynamic flexural loading. It is performed on either notched or unnotched samples to assess the sensitivity of the resin to localized stress concentrators, providing a critical baseline for selecting materials for rugged applications.
Specimen Preparation
Notch geometry represents the most critical variable in determining the reproducibility of the impact data. Cutting the V-notch with a radius of exactly 0.25 millimeters concentrates the stress and forces a brittle fracture mode during the pendulum swing. If the notch is poorly machined or contains micro-scratches, the measured impact energy will show wide scatter, masking the true toughness of the resin.
Fracture Mechanism
Energy absorption during the impact event occurs through both plastic deformation and the creation of new fracture surfaces. Ductile polymers absorb substantial energy as the material yields and deforms before the crack propagates, while brittle polymers fail rapidly with minimal deformation. High-speed cameras show that the rate of crack propagation is highly sensitive to the temperature of the specimen during testing.
Lowering the test temperature often triggers a transition from ductile to brittle behavior, which is a critical limitation for automotive parts exposed to winter conditions. This transition point is a key factor when evaluating alternative resin formulations for outdoor service.
Injection Factor
Weld lines in molded parts significantly reduce the measured impact strength when they align with the notch location. Molders must position gates to ensure that flow fronts do not meet in high-stress areas where the charpy test would show a dramatic loss of performance. This is particularly true for glass-fiber filled resins, where the fibers fail to cross the weld line, creating a localized plane of weakness.