Meaning
Constitutive rheological equations predict non-linear viscoelastic behaviour in polymer melts by modeling transient tube segment stretching under extensional deformation. In film blowing and blow moulding, molecular stress function theory describes how entangled polymer chains strain-harden under rapid elongation. The strain-dependent stretch variable modifies the linear viscoelastic relaxation spectrum to account for tube diameter reduction during deformation.
Polyolefin melts with long-chain branching exhibit strong non-linear responses that prevent localized necking during rapid draw-down steps. The model applies strictly to molten states above the melting temperature or glass transition point.
Strain Hardening
Extensional viscosity increases sharply beyond the linear viscoelastic limit when molecular chains resist stretching. Within constitutive modeling frameworks, the molecular stress function calculates the maximum stretch ratio reachable before chain segment disentanglement occurs. Linear polyethylenes exhibit minimal strain hardening, whereas branched low-density grades display pronounced resistance to extensional thinning.
Melt Strain
Non-linear extensional flow behavior governs bubble stability in film blowing operations and parison sag during blow moulding. Quantifying molecular stress function values enables process engineers to simulate melt strength during complex die flow sequences. Accurate strain predictions prevent film flutter and uneven wall thickness distribution in blown container geometry.
Processing Boundary
Model parameters require calibration through transient extensional viscosity data measured on filament stretching rheometers. High deformation rates cause chain slip at die walls, violating the continuous tube assumptions of the molecular stress function. Numerical calculations lose precision when melt temperatures fluctuate rapidly during processing.
