Degradation of impact strength through polymer chain scission occurs when moisture attacks amorphous engineering resins during high temperature processing. Polycarbonate embrittlement destroys ductility by reducing molecular weight below the critical threshold required for crazing and plastic deformation. This structural failure mode governs drying protocols, barrel residence times, and maximum allowable melt temperatures in injection moulding equipment.
Moulding shops must maintain dew points below negative forty degrees Celsius to prevent polymer hydrolysis during processing. Hydrolytic attack cuts polymer chains, causing catastrophic loss of elongation at break even when tensile strength appears unaffected on standard certificates of analysis. The degradation mechanism stops applying once the resin is cooled below its glass transition temperature and isolated from humidity.
Excessive barrel residence times compound thermal stress independently of moisture content, accelerating polymer breakdown through free radical generation. Moulding machines running low shot capacities relative to barrel volume expose the melt to prolonged heat histories, driving polymer degradation forward. Screw design dictates shear heating levels, where high compression ratios generate localized hot spots that trigger chain scission.
Resin suppliers publish maximum melt temperatures, yet actual stock temperatures frequently exceed these limits due to frictional heating within the nozzle. Thermal history leaves no visual trace on the moulded article until mechanical testing reveals a sudden drop in dart drop impact performance. Processors mitigate this risk by purging barrels during extended machine stoppages and matching barrel size directly to shot volume requirements.
Virgin pellets carry higher initial material costs but provide guaranteed molecular weight distributions necessary for structural applications. Purchasing regrind material introduces unknown thermal histories and variable moisture levels that accelerate polycarbonate embrittlement during subsequent processing cycles. Moulding shops balancing material budgets must restrict regrind addition rates to single digit percentages to protect impact resistance specifications.
Datasheet values represent virgin resin performance under ideal laboratory conditions, whereas production parts experience property degradation from shear and heat during filling. Part specifications dictate whether recycled content is permissible, separating housings and structural brackets from non-critical enclosures. Quality control departments verify incoming resin batches using melt flow rate testing as a proxy for molecular weight retention.
Injection speed and mould temperature dictate residual stress profiles within the moulded component, influencing susceptibility to post-moulding failure. High injection speeds create molecular orientation that relaxes unevenly, leaving internal stresses that compound the effects of chain scission. Cooling channel design determines the rate of heat removal from thick wall sections, where slow crystallization or uneven cooling sets up differential shrinkage.
Mould temperature controllers must maintain steady fluid circulation to prevent localized overheating of the polymer melt during cavity packing. Process technicians adjust packing pressures to minimize sink marks without over-packing the gates, which introduces excessive frozen-in stress. Part geometry containing sharp internal corners concentrates stress under load, accelerating mechanical failure when material ductility has been compromised by thermal abuse.
Delivered resin moisture guarantees require coulometric Karl Fischer verification under ISO 15512 Method B1 paired with machine downtime chargeback clauses.
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