Meaning
Rheological flow behavior at the convergence of a capillary die exhibits a significant drop in pressure as the fluid transitions from a wide reservoir to a narrow channel. This phenomenon, known as the entrance pressure loss, occurs due to the acceleration and elastic deformation of the polymer melt. It represents a substantial portion of the total pressure drop in short dies and must be accounted for to obtain accurate viscosity measurements.
Flow Restriction
Polymer melts flowing from an extruder barrel into a capillary die experience rapid acceleration and stretching. The entrance pressure loss reflects the energy spent in organizing the random polymer coils into a highly oriented state as they enter the restriction. This elastic energy is stored in the polymer and can lead to swelling or surface defects after the material exits the die.
Energy Dissipation
Capillary rheometers use dies of different lengths to isolate the pressure loss associated with flow through the capillary from the pressure loss at the entrance. Calculating the entrance pressure loss requires a Bagley correction, which involves plotting total pressure drop against the length to diameter ratio of the die. This correction ensures that the calculated shear viscosity represents only the steady flow region within the capillary, preventing serious errors in die design and flow modeling.
It provides the accurate rheological data needed to design complex extrusion profiles that avoid costly trial and error during tooling development.
Process Limit
High pressure requirements at the die entry can limit the throughput of an extrusion line by exceeding the pressure limits of the extruder or the die assembly. Minimizing the entrance pressure loss involves designing dies with gradual convergence angles rather than abrupt steps.
