Meaning
Thermal energy generated internally within a flowing polymer melt when rapid deformation occurs under high shear rates without time for heat dissipation represents a primary mechanism of localized temperature rise. High shear rates during injection moulding force polymer chains to slide past each other at extreme speeds, turning kinetic energy into heat. This phenomenon of adiabatic shear heating is particularly pronounced in thin-walled sections or narrow gates where the flow path experiences severe restrictions.
High velocity flow triggers a temperature increase that does not rely on barrel heater bands, raising the actual melt temperature well above the planned threshold. Since the heat cannot escape into the cold steel of the mould during the microsecond flow duration, the localized area behaves adiabatically, keeping the energy trapped in the melt stream and destabilizing the polymer chains.
Shear Generation
Frictional resistance between highly oriented polymer molecules creates thermal energy when the flow speed exceeds the rate of thermal conduction. Fast injection velocities concentrate this adiabatic shear heating at the gate region, reducing the local viscosity and causing a temporary drop in flow resistance. The rapid rise in temperature can exceed the thermal limit of the resin, causing molecular chain scission in sensitive polymers such as polycarbonate or acetal.
A moulder cannot hold the datasheet viscosity under these high-shear conditions because the local shear rate alters the flow behavior of the material.
Mold Defect
Localized thermal degradation of the polymer matrix from excessive shear during the fill phase produces visible cosmetics flaws or structural weaknesses. When adiabatic shear heating is uncontrolled, it leads to splay marks and gas burns on the surface of the moulded part. These defects force the rejection of finished products, increasing the cost of production by generating unusable scrap.
In extreme cases, the degraded polymer releases volatile gases that can corrode the mould steel or cause micro-voids in the part.
Control Strategy
Mitigating the risk of excessive shear heating relies on optimizing the gate design and the injection speed profile of the moulding cycle. Rounded gate profiles or wider flow channels reduce the shear rates experienced by the melt during the transition from the runner to the cavity. Adjusting the fill speed to a lower rate decreases the intensity of the shear-induced heating.
Balancing the use of regrind material can also stabilize the molecular weight distribution, helping to maintain a predictable viscosity across different production runs.
