Non Isothermal Flow Dynamics and Viscous Heat Generation in Polymer Melt Extrusion
Non-isothermal viscous dissipation drives core melt temperature spikes that cause extrudate defects if die land geometry is not scaled to shear rates.
Viscous Heat Generation
Meaning
Kinetic conversions resulting from internal friction within a moving fluid raise the temperature of the material independently of external heaters. In high-speed extrusion, viscous heat generation can account for more than eighty percent of the energy entering the polymer melt. This phenomenon is caused by the work performed to shear the viscous liquid as it passes through the screw and die.
Dissipation Rate
The amount of energy converted into heat depends on the viscosity of the material and the square of the shear rate. Viscous heat generation is highest in areas where the flow is most restricted, such as the metering zone of a screw or a narrow die opening. Managing this energy is a requirement for preventing the material from overheating.
Temperature Rise
Sudden spikes in the melt temperature often indicate that the internal friction is exceeding the cooling capacity of the barrel. While external heaters can be turned off, viscous heat generation continues as long as the screw is turning. This makes it a primary constraint on the maximum production speed for heat-sensitive materials.
Stability Constraint
Achieving a steady state requires a balance between the energy added by the motor and the energy removed by the cooling system. If viscous heat generation is not controlled, it leads to thermal runaway where the viscosity drops, the shear increases and the temperature climbs uncontrollably. This instability limits the capability of the process to maintain uniform dimensions over long runs.