Meaning
Polymer deformation within a processing die is driven by the velocity gradient developed across the flow channel during molten transport. This localized flow intensity is represented by the intrinsic shear rate, which depends on the die geometry and the volumetric flow rate of the melt. It defines the flow conditions that the polymer molecules experience at any specific point within the die channel, determining their orientation and viscosity.
The value is calculated using fluid dynamics models that account for the non-Newtonian behavior of polymer melts.
Rheological Behavior
Molecular chains of a polymer disentangle when subjected to high shear, causing the viscosity of the melt to drop. The intrinsic shear rate determines the extent of this shear thinning, which directly affects the pressure drop across the extrusion die. Melt flow indexing or capillary rheometry measurements allow processors to map this relationship before production starts.
Understanding how the material thins helps in choosing the proper process temperatures.
Tooling Design
Die geometry must be tailored to the melt viscosity to prevent excessive heat generation from viscous dissipation. When the intrinsic shear rate in the gate or die land is too high, the resulting frictional heating can degrade shear-sensitive resins such as polyvinyl chloride or polycarbonate. Tooling designers utilize flow channel tapers to gradually transition the melt, keeping the shear within safe operating limits.
This reduces melt fracture and improves surface finish.
Part Defect
High shear conditions can trigger surface imperfections when the melt exits the die. If the intrinsic shear rate exceeds the critical value for the polymer, the sudden release of pressure causes surface tearing or sharkskin. This defect reduces the mechanical strength and aesthetic appeal of the extruded profile, leading to part rejection.
