Non Isothermal Melt Rheology in Single Screw Extrusion
Non-isothermal melt rheology decouples drag and pressure flow through shear heating, requiring thermal mixing to stabilize viscosity and output dimensions.
Viscoelastic Stress Recovery
Meaning
Time-dependent deformation recovery driven by stored elastic energy occurs after oriented polymer chains exit high-shear flow channels. Material characterization protocols measure viscoelastic stress recovery to predict post-extrusion dimensional changes, commonly observed as extrudate swell and recoil. Tensile stresses accumulated in the die land relax upon exiting the die face, causing lateral expansion and axial contraction.
Molecular relaxation mechanisms depend heavily on polymer chain branching and temperature. The phenomenon governs extrudate behavior immediately downstream of the die exit, dissipating as the material cools below its solidification point.
Elastic Memory
Polymer molecules stretch and align under shear forces within narrow die passages. Quantifying viscoelastic stress recovery indicates the magnitude of stored elastic strain that drives dimensional recoil upon exit. Higher molecular weight and molecular weight distribution increase elastic memory effects.
Rheological testing measures primary normal stress differences to evaluate this elastic behavior during resin selection.
Die Swell
Unconstrained fluid expansion occurs at the die exit as orientation stresses relax without boundary wall restraint. Monitoring viscoelastic stress recovery allows tool designers to compensate die land dimensions to achieve desired profile final dimensions. Higher extrusion speeds increase strain rates and swell magnitude, requiring precise line speed synchronization.
Calibration sizing sleeves pull the expanding extrudate to fix dimensions before cooling.
Warpage Control
Overlooking elastic recovery forces in profile extrusion causes dimensional warpage and fitment failures in assembly applications. When viscoelastic stress recovery is unmeasured, differential relaxation across complex profiles distorts cross-sectional geometry during vacuum calibration. Scrap rates escalate as operators struggle to balance cooling rates against elastic recoil forces.
Accurate elastic recovery characterization ensures right-first-time tooling design for precision profiles.