Meaning
Excessive polymer fluid velocity gradients at internal metal die surfaces cause physical surface distortions on extruded plastic products. During high-speed plastic extrusion, a shear rate defect produces sharkskin, melt fracture or severe surface roughness across extruded profiles. The condition arises when wall shear stress exceeds polymer critical melt flow limits inside die land channels.
Physical scope focuses on extrusion die land boundaries, terminating once smooth laminar flow re-establishes downline.
Flow Mechanism
Polymer molecules along die channel walls slip and stick rapidly when wall shear rates exceed critical material thresholds. In high-output profile lines, a shear rate defect appears as periodic surface ridges as the polymer melt exits the die land under extreme wall friction. Pilot runs conducted at low screw speeds hide melt fracture tendencies that emerge during full commercial production rates.
Pushing output beyond wall shear stress limits degrades optical clarity and reduces tensile strength in thin-walled tubing. Fluid friction mitigation requires modifying die entrance angles or lowering melt viscosity through temperature adjustments.
Remediation Method
Processing additives introduce microscopic fluoropolymer lubrication layers along internal die steel walls. Incorporating these additives eliminates a shear rate defect without requiring costly extrusion die tooling redesigns.
Velocity Threshold
Critical shear rate boundaries depend on polymer molecular weight distribution and melt temperature parameters. Exceeding critical shear thresholds forces line speed reductions to restore surface smoothness.
