Meaning
Rheological phenomena characterized by a reduction in the flow resistance of a molten polymer during thermal processing indicate a decline in the material’s molecular weight. The occurrence of melt viscosity degradation is usually driven by polymer chain scission under high thermal or shear stress. It alters the processing behavior of the resin in both extrusion and injection molding.
This flow change suggests a reduction in the physical properties of the finished part.
Chemical Cause
Hydrolysis of moisture-sensitive polymers like polyesters remains the primary pathway for this molecular breakdown. In bioplastics, melt viscosity degradation occurs rapidly if the resin has not been dried to moisture levels below two hundred parts per million. Shear-induced chain cleavage also occurs in long-chain polymers when screw speeds are set too high.
Thermal decomposition of the polymer backbone represents another major driver, making pre-drying the most effective prevention strategy.
Process Monitoring
Moulders monitor the change in viscosity during a production run by tracking the hydraulic injection pressure or the screw recovery time. A steady drop in these indicators reveals that melt viscosity degradation is occurring within the barrel. Inline rheometers can provide real-time melt flow index measurements to detect this instability early.
Adjusting the barrel temperature profiles can reduce thermal stress.
Moulding Defect
Lower viscosity causes the polymer melt to seep into the parting lines of the tooling, creating part flash. This flash increases manual trimming requirements. In thin-walled parts, melt viscosity degradation leads to structural weak spots.
