Meaning
Flow behavior of polymer melts characterized by a viscosity that changes with the rate of shear deformation distinguishes these fluids from simple liquids. Most thermoplastics exhibit a decrease in viscosity when subjected to high shear forces. This characteristic of non Newtonian rheology allows polymers to flow easily through thin gates.
Shear Thinning
Aligning of polymer chains along the flow direction reduces the resistance to movement as the velocity gradient increases. This aspect of non Newtonian rheology is highly advantageous because it lowers the injection pressure required to fill a mould cavity. While a static material has a high viscosity, the high shear rates experienced during injection cause the melt to behave like a much thinner fluid.
A moulder can use this thinning effect to fill thin-walled sections that would otherwise freeze off prematurely.
Process Influence
Temperature and molecular weight distribution modify how a polymer melt responds to shear forces during processing. When non Newtonian rheology is not factored into the process design, the actual pressure drop across the runner system will deviate from predictions. Virgin resin with a broad molecular weight distribution exhibits a more gradual viscosity change than narrow-distribution resins or highly sheared regrind materials.
These differences affect the consistency of the part weight and the level of residual stresses in the moulded part.
Molecular Mechanism
Entanglement density of the polymer chains determines the transition from a constant-viscosity flow to a shear-thinning behavior. Under low shear rates, the chains remain tangled and resist flow, but as the shear rate exceeds a critical threshold, the chains disentangle and slide past one another. This molecular transition occurs during the high-speed filling stage, where the polymer experiences its highest shear rates.
Controlling this transition allows the moulder to optimize cycle times and maintain consistent part dimensions without causing molecular degradation of the polymer matrix during processing.
