Meaning
A localized flow instability occurs along die walls when polymer melt alternates between wall adhesion and rapid slippage above a critical wall shear stress. Extrusion defect analyses track the stick slip transition to identify the origin of periodic surface defects like sharkskin or gross melt fracture. The transition produces characteristic pressure oscillations and alternating smooth and rough extrudate bands.
It defines the maximum output rate achievable before extrudate visual quality degrades.
Wall Kinematics
Adhesion failure occurs when wall shear stress exceeds critical intermolecular bonding forces between melt and die metal. Dynamic switching between slip and stick states creates periodic relief and buildup of wall boundary layer stresses.
Defect Diagnostic
High-frequency pressure monitoring coupled with optical surface analysis pinpoints the threshold shear rate for flow instability. When the stick slip transition occurs inside die land geometry, extrudate surface roughness increases without changes in bulk melt temperature. Relying solely on visual inspection without pressure frequency analysis fails to separate slip instabilities from upstream melt fracture.
Throughput Constraint
Production speed limits enforced by flow instability restrict line output on narrow tolerance pipe and film extrusion lines. Die land fluoropolymer coatings or processing aids shift boundary limits to allow higher volumetric throughput without extrudate distortion.
