Meaning
Dynamic behaviours characterise the reduction in viscosity that occurs in many polymer melts as the rate of deformation increases. The shear thinning regime is the range of processing speeds where the polymer chains align with the flow, making it easier for the material to move through narrow channels. This characteristic is what makes it possible to fill complex, thin-walled moulds at high speeds without requiring extreme pressures.
Viscosity Reduction
Molecular alignment reduces the number of entanglements and friction points between the long chains. Within the shear thinning regime, doubling the injection speed does not double the required pressure because the material becomes more fluid as it moves faster. This non-Newtonian behaviour is a primary design consideration for the screws and runners used in modern moulding equipment.
Processing Speed
Efficiency in the factory depends on operating within the most effective part of this flow region. The shear thinning regime allows for shorter cycle times by enabling rapid mould filling and cooling. However, if the shear rate becomes too high, the polymer can suffer from mechanical degradation or thermal breakdown due to the heat generated by the internal friction of the molecules.
Molecular Alignment
Orientation of the polymer backbone during flow has a permanent effect on the properties of the final part. As the melt moves through the shear thinning regime, the chains are stretched out in the direction of the flow. This creates a part that is much stronger in the direction of the alignment but potentially weaker in the transverse direction, which must be accounted for in the structural design of the component.
The transition into this flow state is what separates plastics from simpler Newtonian fluids like water or oil.
