Meaning
The percentage of average chain length that survives thermal processing defines the stability of a material during manufacturing. High molecular weight retention indicates that the polymer has withstood the heat and shear of injection moulding without significant chain scission. This parameter is critical for ensuring that finished parts meet their mechanical specifications.
Thermal Stability
Chain scission occurs when polymers are exposed to temperatures above their thermal degradation limits. To maximize molecular weight retention, moulders must carefully control melt temperatures and residence times in the barrel. When resin stays in the heated barrel too long, thermal degradation accelerates, which reduces the molecular weight and causes the polymer to lose its impact strength.
This degradation is often visible as a discolouration of the melt or as a decrease in the viscosity of the flowing resin.
Processing Integrity
Incorporating regrind into virgin material requires careful monitoring to ensure that the blended resin does not suffer a decline in property values. Each time a polymer is reprocessed, its molecular weight retention decreases, meaning that a high ratio of regrind can lead to brittle parts. Molders typically limit regrind content to a fixed percentage to maintain the required mechanical performance.
Performance Limit
Low values of chain preservation are often linked to early part failure due to stress cracking. Achieving high molecular weight retention ensures that the polymer chains remain long enough to form effective entanglements, which gives the part its high impact resistance.
