Meaning
Polymer flow properties under extreme pressures are characterized by an exponential sensitivity parameter known as the piezoviscosity coefficient. This constant, also called the barus coefficient, defines how the viscosity of a polymer melt rises when it is subjected to the high hydrostatic pressures found in injection molding nozzles and extruder dies. When the melt experiences these pressures, its free volume decreases, causing the molecular chains to lock together and increase flow resistance.
Viscosity Scaling
High pressures can raise viscosity by several orders of magnitude, making accurate pressure-dependence data essential for mold design. Using the barus coefficient allows simulation software to predict the pressure drop along the flow path during the filling phase. If a calculation ignores this behavior, the predicted injection pressure will fall far short of the actual force needed, resulting in incomplete parts or short shots.
The coefficient varies with temperature and polymer structure, with linear chains showing different sensitivity than branched ones. For instance, polycarbonate displays a much higher sensitivity to pressure than polyethylene, meaning its flow behavior changes more dramatically as it travels through the restricted channels of a runner system.
Flow Modeling
Accurate molding simulations require continuous updates to the viscosity equations used in the solver. Integrating the barus coefficient into the rheological model ensures that the local shear rates and pressures are accurately calculated at every grid point. Process engineers use these outputs to locate gates and determine the required clamp force of the molding machine.
This modeling is especially valuable when working with highly viscous resins like polycarbonate or thick-walled parts.
Material Verification
Laboratory measurements of this sensitivity constant require high-pressure capillary rheometers equipped with dual dies or a pressurized chamber. The computed barus coefficient serves to evaluate the processability of new resin formulations before they are run on the factory floor. Variations in the coefficient between batches can explain unexplained shifts in part dimensions or surface defects.
This metric is a fundamental property for quality control in high-performance polymer processing.
