Meaning
Thermal or chemical cleavage of the repeating acetal oxymethylene molecular backbone breaks high-molecular-weight polymer chains down into volatile formaldehyde monomer gas and shorter oligomers during processing. This polyoxymethylene scission alters the melt flow index and severely weakens the impact resistance and load-bearing performance of the final moulded component. The boundary of application encompasses degradation reactions occurring within the plasticising barrel and mould runner systems, terminating where solid-state oxidation takes place over years in service.
Degradation Pathway
Elevated barrel residence times and intense shear heating trigger unzipping of unstable polymer chain ends if end-capping chemistries fail under excessive temperature. Acidic contaminants and residual moisture catalyse internal polyoxymethylene scission, fracturing the central molecular chain and releasing gaseous formaldehyde into the melt stream. The resulting gas creates visible splay marks on component surfaces and forms corrosive acidic residues on internal tooling steel surfaces.
Degraded polymer experiences a sudden, uncontrolled drop in melt viscosity.
Thermal Audit
Moulding validation protocols monitor barrel dwell times across intended cycle profiles to prevent prolonged polymer exposure to critical degradation thresholds. Measuring the melt flow rate of purged resin against virgin raw pellet baseline values verifies that scission has not compromised structural integrity during plasticisation. Releasing high-cavitation tooling without assessing resin thermal sensitivity risks catastrophic mould venting deposits and severe brittle part failures in production.
Suppliers running oversized injection barrels on slow cycle times inadvertently expose polyoxymethylene to thermal scission through excessive residence durations, masking degradation by masking visual defects behind heavy mold cooling.
Mitigation Protocol
Screw geometry selection and barrel purge procedures serve as primary controls against mechanical and thermal molecular cleavage. Low-compression screws reduce shear stress peaks, while generous mold gating prevents localised shear spikes during the filling phase. Acidic additives, cross-contaminating polymers such as polyvinyl chloride, and residual cleaning agents must be rigorously excluded from the process equipment.
Regular mould venting maintenance prevents gaseous decomposition byproducts from etching precision core pins and cavity cavities.
