Thermal Boundary Layer Instabilities and Viscoelastic Melt Degradation during High Throughput Molding
High throughput molding requires balancing injection velocity against viscous dissipation to prevent polymer chain scission and wall slip defects.
Wall Slip Transition
Meaning
Rheological phenomena occurring during polymer extrusion mark the shift where molten polymer transitions from sticking to metal die walls to sliding along die surfaces. The wall slip transition occurs when interfacial shear stress exceeds a critical threshold, altering fluid flow profiles and causing surface defects such as sharkskin melt fracture. Polymer processing engineers measure critical shear stresses to set extrusion speed limits and select appropriate processing additives.
Laboratory capillary rheometers measure slip velocity curves across temperature ranges.
Velocity Discontinuity
High shear rates generated at die walls cause polymer molecules to detach from metal tooling surfaces. Crossing the wall slip transition alters output flow rates and causes pressure oscillations inside extrusion dies. Process engineers add fluoropolymer processing aids to smooth boundary layer flows.
Interfacial Shear
Capillary flow tests determine the precise shear stress threshold where wall adhesion fails. Identifying the wall slip transition helps engineers optimize die land length and barrel temperature profiles.
Surface Instability
Uncontrolled slip behavior during polymer processing generates periodic surface roughness and dimensional non-uniformity in extruded pipes and films. Reaching the wall slip transition forces operators to reduce screw RPM or raise melt temperature to preserve surface quality standards. Tooling designers apply specialized non-stick surface coatings to delay slip onset.
Extrusion lines run at optimized speeds below critical slip thresholds to maximize product yield.