Meaning
Fluid transport regime describes the movement of molten polymer where temperature varies across spatial coordinates and changes over processing time. In polymer processing operations such as injection moulding, film extrusion and blow moulding, non-isothermal flow governs the interaction between conductive heat loss to cold mould walls and internal viscous shear heating within the flowing melt. The changing temperature field continuously alters local viscosity, velocity profiles and pressure gradients.
The regime stops applying once the polymer freezes solid across the entire cross-section.
Thermal Mechanics
Injected polymer melt encounters tool cavity walls maintained below the resin freezing point. Conduction rapidly drops the temperature of the outer polymer layer, creating an insulating frozen skin layer adjacent to the steel. The molten core continues to flow through this constricted channel, generating significant viscous dissipation heating under high shear rates.
Energy conservation equations must couple velocity fields directly with temperature-dependent viscosity models to predict overall flow resistance.
Pressure Consequence
Channel constriction from growing skin layers increases the pressure gradient required to maintain a constant volumetric flow rate. In thin-wall injection moulding, rapid heat loss can cause premature freeze-off and short shots before the cavity fills completely. Fast injection speeds generate intense shear heating that lowers local viscosity, offsetting conductive cooling and allowing the melt to travel farther.
Process engineers optimise injection velocity profiles to balance shear heating against wall freezing during cavity filling.
Moulding Defects
Imbalanced heat transfer during flow generates localized residual stress profiles across part walls. Melt freezing under high shear orientation freezes stretched polymer chains into place, increasing anisotropic shrinkage and causing out-of-plane component warpage. Semi-crystalline polymers show pronounced variations in crystalline morphology from the rapidly quenched skin to the slowly cooled core.
Understanding temperature gradients across the flowing melt enables mould designers to position cooling channels and gates to prevent structural distortion.
