Meaning
Material flow resistance defined as the ratio of shear stress to applied shear rate governs polymer melt deformation inside processing equipment. Measuring shear viscosity across varied shear rates provides tooling designers and process engineers with fundamental flow data required to size runner systems and mold gates. Injection molders monitor this property to predict cavity filling pressures and prevent short shots or flashing.
The measurement loses validity under extensional flow conditions or at extreme shear rates where viscous heating distorts local temperature fields.
Shear Thinning
Non-Newtonian polymer melts exhibit decreasing flow resistance as shear rate increases during forced flow through narrow channels. Unraveling entangled molecular chains under high shear drops shear viscosity sharply inside injection mold gates and extrusion die gaps. Polypropylene displays pronounced shear thinning, allowing fast cavity filling at manageable injection pressures.
Virgin resin thins predictably, while degraded regrind demonstrates erratic shear responses due to molecular weight reduction.
Temperature Dependence
Polymer melt flow resistance drops exponentially as thermal energy increases thermal chain mobility inside the processing barrel. Thermally activated flow behavior alters shear viscosity according to the Arrhenius relationship at temperatures well above glass transition boundaries. Raising barrel temperatures reduces filling pressure requirements, though excessive heat increases thermal degradation risk.
Processing datasheets specify temperature sensitivity coefficients to assist simulation software calculations.
Mould Filling
Lower flow resistance facilitates rapid cavity filling. Precise viscosity control prevents flash defects and dimensional variance.
