Meaning
Viscoelastic fluids subjected to steady shear flow generate forces perpendicular to the direction of flow rather than just parallel to it. This phenomenon is characterized by the normal stress difference, which is a measure of the elasticity of the polymer melt. It influences the behavior of the melt during processing, particularly when it exits a die.
Rheological Force
Measuring the forces within a flowing polymer involves analyzing the first and second normal stress differences using a rotatory or capillary rheometer. The first normal stress difference is typically positive and much larger than the second normal stress difference, which is negative. These forces are generated by the orientation and stretching of the polymer chains as they are sheared.
They represent the elastic energy stored within the flowing melt.
Die Swell
Exiting the confinement of the die allows the stretched polymer chains to relax and return to a more random conformation. This relaxation causes the extrudate to expand in the direction perpendicular to the flow, a behavior directly driven by the normal stress difference. If this swell is not accurately predicted and controlled, it leads to oversized profiles and out-of-tolerance products.
Tooling must be designed to account for this recovery behavior.
Polymer Behavior
Controlling the elastic response of the melt involves adjusting the temperature and the molecular weight distribution of the resin. High temperatures increase the rate of molecular relaxation, which reduces the normal stress difference and the subsequent die swell. In contrast, polymers with broad molecular weight distributions or long-chain branching store more elastic energy during flow.
This behavior must be managed to maintain dimensional stability in production. By carefully selecting the processing window, engineers can minimize these elastic effects and achieve stable, high-speed output with minimal geometric distortion of the finished part.
