Establishing Standard Shear Viscosity Tolerances for Polyolefin Sourcing
Single-point melt index testing misses high-shear flow variance; enforce multi-point ISO 11443 capillary viscosity limits at 5,000 s⁻¹ in resin contracts.
Wall Shear Stress
Meaning
Frictional force exerted by a moving polymer melt against the boundary walls of a die or runner channel governs the velocity profile and pressure drop in flow channels. This rheological value, defined as wall shear stress, is proportional to the melt viscosity and the shear rate at the wall boundary. It defines the limit for stable polymer flow, determining when surface defects or flow instabilities will occur during extrusion.
The boundary of this stress lies at the critical shear stress of the resin, above which the polymer melt slips at the wall, causing melt fracture.
Flow Resistance
Viscous drag resists the movement of polymer chains next to the steel walls of the die. In high-speed flow, wall shear stress increases as the velocity gradient becomes steeper. This increase raises the pressure required to push the polymer through the die.
Melt Distortion
Excessive drag forces stretch the polymer molecules beyond their elastic limit. When the wall shear stress exceeds a critical threshold, the polymer exhibits surface roughness known as sharkskin.
Process Adjustment
Die designers polished the flow surfaces or added slip agents to the polymer formulation to lower flow friction. Utilizing processing aids reduces the adhesion between the melt and the metal, which lowers the required injection pressure. This adjustment increases the maximum defect-free production rate.