Meaning
Thermal conditions encountered by polymer resins during processing and storage alter the physical and molecular structure of the material. Excess temperature exposure causes polymer degradation, which leads to discolored parts, reduced mechanical properties or brittle failures. This thermal damage can occur inside the heated barrel of an injection moulding machine or during long periods of storage in hot warehouses.
Thermal Degradation
Long exposure to high temperatures breaks the long molecular chains of the polymer into shorter segments. This chain scission reduces the molecular weight of the resin and increases the melt flow rate. The degraded material flows more easily but loses its impact strength and toughness.
If the resin remains in the injection cylinder too long during a cycle interruption, the polymer can char and create black specks in subsequent moulded parts.
Physical Consequence
Components that have suffered thermal damage will fail prematurely under mechanical stress. These parts often show surface defects like silver streaks, gas bubbles or severe discoloration. The loss of molecular weight also increases the susceptibility of the polymer to environmental stress cracking.
Products used in demanding applications like automotive engine compartments are particularly vulnerable to these thermal failures.
Process Control
Operators must closely monitor both barrel temperatures and resin residence times to prevent degradation. Using a screw design that minimizes frictional shear heating helps to control the actual melt temperature. Proactive monitoring of melt temperature keeps the polymer chains intact and ensures consistent mechanical performance of the moulded components.
