Determining Hydrolytic Degradation Thresholds in Virgin Resin Feedstocks
Hydrolytic degradation thresholds require drying virgin polyesters below 0.02 percent and polyamides below 0.10 percent water before melt processing.
Impact Strength Drop
Meaning
Thermal degradation during injection moulding reduces the molecular weight of polymers through chain scission, which directly causes a measurable impact strength drop in finished components. Long polymer chains provide the energy absorption necessary to resist brittle failure under sudden force, so their fragmentation during processing leads to lower ductility. This phenomenon often occurs when residence time in the heated barrel exceeds the material thermal stability limits.
Thermal Mechanism
Higher processing temperatures accelerate the oxidation and hydrolysis of polymer chains, creating shorter fragments that fail to entangle effectively. Recycled material content adds to this vulnerability because each prior cycle of melting contributes to the progressive shortening of molecular chains. High shear rates inside the nozzle or gate further amplify this breakage, particularly when resin flow meets high resistance in thin wall sections.
Cooling water temperature in the mould also governs the crystalline structure, since rapid cooling may lock in internal stresses that work against the remaining toughness.
Economic Consequence
Virgin resins carry a premium price partly because their molecular weight distribution ensures predictable energy dissipation upon impact. Regrind usage requires strict control of these additive and polymer breakdown factors, or the final parts will consistently show failure under standard drop testing. Moulding operations that ignore these limits face high scrap rates and liability risks when structural parts shatter prematurely in the field.
Material Validation
Datasheet values represent ideal conditions using optimal injection speeds and low heat, so these figures rarely hold true across a full production run. Practitioners verify stability by comparing the notched izod toughness of samples taken from the start and end of a production lot. Measuring this divergence helps determine whether the processing window stays within a safe range for the specific resin grade.
The ultimate energy threshold of a moulded part depends more on the thermal history of the melt than on the initial grade specification.