Meaning
Polymer chain degradation occurs through ester linkage cleavage when thermal energy and moisture break the condensation bonds holding polyester resins together. Hydrolysis accelerates during high temperature processing whenever dried pellets retain excess water prior to injection moulding. Extrusion barrels subject vulnerable molecular structures to mechanical shear stress that compounds moisture damage.
Material specifications define virgin resin melt flow rates under controlled drying conditions. Part specifications govern final mechanical strength tolerances that drop rapidly when polymer chains shorten inside the barrel. Regrind economics suffer because repeated thermal cycles weaken the ester bonds further.
Moulders must maintain strict dew point controls inside material hoppers to prevent molecular weight reduction before the melt enters the mould cavity.
Thermal Degradation
Moisture trapped inside polyethylene terephthalate pellets reacts at processing temperatures to sever backbone connections and shorten polymer chains. Injection machines push softened plastic through narrow nozzles where friction generates excess heat. Screw speeds must remain low enough to avoid overheating sensitive formulations during compounding.
Datasheet values assume bone dry conditions that production floors rarely achieve during high humidity shifts. Moulders hold tighter viscosity targets by venting barrel gases continuously throughout the plastication phase. Production supervisors adjust barrel heating zones downward whenever barrel residence times increase due to machine idling.
Resin Viscosity
Molecular weight drops directly affect melt flow behavior and final part performance across the entire production run. Shortened polymer chains flow too easily under clamping pressure and create flash along tool parting lines. Finished components exhibit brittle failure modes under drop testing because degraded chains lack sufficient entanglement length.
Material suppliers publish broad melt index ranges that laboratory tests verify under ideal conditions. Shop floor technicians measure actual melt flow indexes from molded parts to detect early chain scission before structural failures occur. Cooling rates change when resin viscosity drops and lead to warping inside thin walled enclosures.
Processing Yield
Scrap rates climb steeply when degraded material loses structural integrity during ejection from complex steel tooling. Quality inspectors reject moulded parts showing surface blistering caused by trapped steam expanding during cooling stages. Regrind incorporation percentages decrease total processing yield because recycled polymer batches accumulate excessive cleavage history.
Tooling designers specify larger gate dimensions to minimize high shear rates that break polymer backbones prematurely. Production planners balance regrind ratios against mechanical property requirements to maintain acceptable manufacturing margins.
