Meaning
Time-dependent strain energy release mechanisms drive polymer molecules to return toward their unstressed equilibrium orientation following deformation during melt processing. Viscoelastic recovery occurs after polymer melts experience intense shear deformation inside extrusion dies and mold cavities. As orienting shear forces cease, macromolecular chains recoil, causing dimensional changes such as post-extrusion die swell and post-molding part distortion.
The magnitude of structural recovery depends on melt temperature and cooling rates during phase transition. Molders and extruders must account for these delayed elastic forces to maintain tight part dimensions after demolding.
Stress Relaxation
Polymer chains frozen under high orientation stress gradually relax over time as stored elastic energy dissipates. During viscoelastic recovery, residual internal stresses rearrange molecular structures, leading to subtle dimensional changes days after processing. Annealing molded parts above their glass transition temperature accelerates stress relaxation, stabilizing dimensions prior to final assembly.
Die Swell
Extrudate emerging from a die land expands in cross-sectional area as frozen elastic strain rebounds. Managing viscoelastic recovery in profile extrusion requires land length adjustments and vacuum sizing tanks to control extrudate swelling. Higher melt temperatures reduce elastic memory, decreasing swell ratios at the die exit.
Molded Warpage
Asymmetric cooling across part wall thickness creates uneven recovery rates between core and skin layers. Differential viscoelastic recovery distorts flat surfaces and alters spring-back angles in flexible hinge designs.
