Meaning
Rheological phenomena during uniaxial or biaxial elongational deformation appear as a steep increase in transient extensional viscosity beyond linear viscoelastic predictions. Viscoelastic strain hardening stabilizes polymer melt stretching during blow moulding and film blowing processes. Polymer characterization labs measure extensional rheology to predict parison wall thickness uniformity and bubble stability during processing of branched polyolefins or high melt strength polyesters.
The behavior stops governing melt performance under pure shear flow or at deformation rates below the molecular relaxation threshold.
Extensional Stabilization
Polymer chain entanglements and long-chain branching resist rapid elongation, causing local melt viscosity to rise during stretching. Localized strain hardening prevents localized necking and parison wall thinning during container stretch blow moulding. Resins lacking extensional hardening experience melt fracture and non-uniform wall thickness distribution across complex moulded geometries.
Branching Architecture
Long-chain branching increases extensional viscosity under rapid elongation rates.
Process Optimization
Blow moulders select resin grades with pronounced strain hardening to optimize container weight and mechanical strength. Material selection balances melt strength against injection moulding flowability, as highly branched structures increase extrusion pressure drops. Extensional rheology measurements guide resin modification strategies, such as reactive extrusion or irradiation, to introduce controlled long-chain branching.
Maintaining consistent extensional behavior reduces scrap rates and stabilizes wall thickness across high-speed packaging production lines.
