Capillary Rheometer End Correction Data Processing Workflows
End-corrected capillary rheology isolates entry losses and non-Newtonian wall shear rates to establish accurate melt viscosity curves for compounding quality control.
Melt Elasticity
Meaning
Viscoelastic properties govern the reversible storage and recovery of mechanical energy in sheared polymer fluids. Under high deformation rates, melt elasticity produces first normal stress differences and die swell upon exit from tooling. These elastic phenomena dictate parison sag in blow molding and extrudate shape retention in profile production.
The concept ceases to dominate flow behavior at ultra-low deformation rates where macromolecular chains relax completely into purely viscous dissipation.
Recoverable Compliance
Molecular orientation under shear generates stored elastic strain that recoils when confining boundaries disappear. Quantifying melt elasticity through recoverable shear compliance or storage modulus measurements isolates chain entanglement density. Resins with identical melt flow rates often exhibit divergent elasticity due to differences in high-molecular-weight fractions or long-chain branching.
High recoverable compliance expands the elastic memory of the fluid. Mismatched compliance between virgin and recycled polymer lots creates unstable flow patterns in co-extrusion feedblocks.
Swell Distortion
Extrudate dimensions expand immediately after exiting a die as stored normal stresses relax perpendicular to flow. In profile extrusion, pronounced melt elasticity causes non-uniform die swell, warping cross-sectional geometry and pulling parts out of drawing calibration. Tool designers counter swell distortion by cutting dies with dimensions smaller than the intended part profile.
Prototype single-die setups rarely expose how resin batch elasticity shifts under production line speeds. Calling an extrusion resin qualified based on low-speed lab trials leads to line shutdowns when factory draw-off equipment cannot pull swollen extrudate through calibrator sleeves. Surface matte defects and periodic sharkskin ruptures occur when elastic recovery exceeds the tensile strength of the exiting melt skin.
Uncontrolled elastic recovery caps line haul-off velocity.
Gate Clearance
Injection molding applications encounter melt elasticity when forcing resin through restricted gates at extreme velocities. Elevated elastic stresses induce gate jetting and severe residual stresses around the entry point. Supplier resin data sheets rarely report normal stress differences, forcing tooling engineers to infer elastic risks from entrance pressure drop measurements.
Demonstrated mold trials must audit part flatness and optical birefringence under maximum fill rates. Melt elasticity dictates the ultimate geometric stability of high-speed injection components.