High Shear Rheology Calibration Thresholds for Recovered Polyolefin Industrial Blends
Calibrating high-shear capillary rheology via Bagley and Weissenberg-Rabinowitsch corrections prevents off-spec polyolefin lot processing failures.
Frictional Heating
Meaning
Thermal energy is generated when adjacent layers of a polymer melt slide past one another at high shear rates. In injection moulding, frictional heating occurs primarily in high velocity regions such as gates, runners and thin wall sections. This internal heat generation can raise the temperature of the melt far beyond the setpoints of the barrel heater bands.
Shear Behavior
Rapid flow through narrow channels forces polymer chains to slide past each other with high resistance. This viscous dissipation converts mechanical work from the injection ram directly into thermal energy within the flow stream. The resulting temperature increase is concentrated near the channel walls where shear rates are highest.
This localized heating can reduce the apparent viscosity of the resin, allowing it to flow more easily into thin cavities. In crystalline polymers like polyamides, this rapid heat generation can also prevent premature crystallization, ensuring that the material fills the farthest corners of the cavity.
Thermal Risk
Extreme shear energy can degrade the polymer structure if the melt velocity is too high. If frictional heating raises the temperature of the resin past its thermal stability limit, chain scission and discoloration occur. This degradation results in weak spots, splay marks and cosmetic defects on the surface of the finished part.
Gate Control
Restricting the gate dimensions increases the localized shear rate and heightens the heating effect. This mechanism can be used to fill difficult mould geometries, but it must be calibrated to avoid burning the polymer. Proper design of the gate landing length limits the time the material spends in the high shear zone, keeping the thermal rise within safe limits.