High Shear Rheology Calibration Thresholds for Recovered Polyolefin Industrial Blends
Calibrating high-shear capillary rheology via Bagley and Weissenberg-Rabinowitsch corrections prevents off-spec polyolefin lot processing failures.
Sharkskin Melt Fracture
Meaning
Periodic surface roughness defects occurring at high extrusion rates emerge as fine, high-frequency ridges perpendicular to the flow direction on extrudate profiles. Tensile stress concentrations at the exit lip of extrusion dies trigger sharkskin melt fracture as the surface layer accelerates exiting the land. Linear low density polyethylene exhibits surface matte and ridges when wall shear stress exceeds approximately zero point one megapascals.
Fluoropolymer processing aids migrate to die land walls to reduce friction and eliminate surface tearing.
Exit Kinematics
Melt exiting the die undergoes rapid surface acceleration from zero velocity at the wall to average velocity in the free stream. High localized tensile stresses stretch the outer polymer skin beyond its tensile strength limit. Microscopic surface ruptures form periodic transverse ridges across the extrudate.
Surface Defect
The defect alters surface appearance and clarity without affecting bulk structural strength. High-frequency ridges reduce transparency in blown films and create rough surface textures on extruded pipes. Operating below critical shear stress boundaries avoids surface distortion.
Process Modification
Die land heating lowers surface melt viscosity, reducing localized exit stress. Internal processing aids form low energy coating layers on die metal surfaces. Increasing die gap opening lowers local shear rates at required volumetric throughputs.