Meaning
Thermal degradation pathways generate hydroxyl point defects inside condensation polymers during high temperature injection moulding. Polymer sourcing requires strict monitoring of moisture levels before pellet plasticisation because bound water molecules cleave polymer backbones. Hydroxyl point defects reduce tensile strength across finished moulded parts by shortening average molecular chain length.
Material specifications enforce strict limits on incoming moisture content while part specifications govern structural integrity under load. Virgin resin tolerates minimal moisture before degradation accelerates whereas regrind incorporates prior thermal history and accumulates higher concentrations of chain scission sites. Moulders hold tighter melt temperatures across a production run to prevent polymer breakdown but machine display values often diverge from actual melt thermocouple readings inside the barrel.
Polymer Scission Mechanism
Water molecules react with ester linkages during high pressure barrel compression stages. Chemical bonds rupture when residual moisture interacts with molten chains at elevated processing temperatures. Molecular weight distribution shifts downward as shorter chains proliferate throughout the melt pool.
Melt flow rate climbs rapidly while intrinsic viscosity drops due to accelerated chain breakage. Viscosity shifts alter cavity filling dynamics and induce flash around parting lines when injection pressures remain unadjusted.
Resin Thermal Degradation
Bound moisture drives hydrolytic cleavage independently of mechanical shear forces. Dried pellets prevent premature polymer breakdown before the material enters the nozzle assembly. Melt temperature fluctuations exacerbate chemical degradation inside the heating cylinder zones.
Screw design influences residence time distribution and localized overheating spots accelerate chain scission rates. Material suppliers publish baseline moisture thresholds for virgin resin processing but regrind batches require lower drying temperatures to avoid excessive thermal exposure.
Moulding Process Control
Clamping tonnage parameters compensate poorly for structural weakening caused by degraded molecular chains. Part deflection increases noticeably when short shots trigger compensatory packing pressure adjustments. Dimensional stability degrades across prolonged production runs as material viscosity declines steadily.
Moulders verify incoming resin dryness using Karl Fischer titration methods rather than relying entirely on hopper sight glass inspections. Structural failure occurs beneath calculated load limits whenever unchecked hydroxyl point defects proliferate throughout the moulded cross section.
