Meaning
Dimensionless grouping quantifies elastic stress dominance over viscous forces within flowing polymer melts during extrusion and compounding operations. The weissenberg number governs normal stress differences driving extrudate swell and melt fracture phenomena at high shear rates. Calculations multiply relaxation time by shear rate to evaluate molecular stretching inside die lands.
Material specifications establish base viscoelastic behavior under laboratory conditions, whereas actual processing environments subject polymer chains to complex thermal and mechanical histories. Bounded by continuum mechanics assumptions, the ratio fails when molecular weights drop below entanglement thresholds or when strain frequencies exceed chain relaxation frequencies.
Relaxation Time
Molecular dynamics determine how long polymer chains store elastic energy before thermal motion randomises their conformations. High molecular weight distributions lengthen this recovery phase, creating severe processing instability during high output compounding runs. Extruders generate intense shear fields that stretch entangled macromolecules far beyond equilibrium states inside metering zones.
Cooling schedules freeze these strained configurations into finished shapes unless relaxation occurs prior to final solidification. Virgin polymers exhibit predictable recovery profiles, while regrind batches contain shorter chain fragments that dampen overall elasticity and alter melt strength.
Die Swell
Elastic recovery forces expand polymer extrudates beyond internal orifice dimensions upon exit from shaping dies. Moulders adjust thermal profiles and throughput speeds to control this radial expansion across production shifts. Excessive stress memory produces distorted profiles, surface roughness, and dimensional rejections that exceed part specification tolerances.
Datasheet values derived from capillary rheometers rarely match the actual swell observed on shop floor machinery due to pressure drops across tooling geometries. Uncontrolled expansion rates increase scrap volumes and drive up material costs when setup technicians fail to balance line speeds with polymer cooling rates.
Melt Fracture
Surface tearing occurs when boundary shear stresses exceed critical thresholds for specific molecular weight distributions. Processing temperatures must remain high enough to prevent premature solidification while avoiding thermal degradation inside runner systems. Moulders struggle to maintain stable output windows because regrind incorporation narrows the allowable processing envelope between smooth flow and melt distortion.
Part specifications dictate strict visual standards that reject mouldings displaying sharkskin defects caused by periodic slip stick transitions at die walls. Proper barrel temperature profiling minimizes entrance pressure losses and preserves structural integrity throughout continuous extrusion runs.
