Meaning
Measurement of polymer melt resistance to flow when stretched in two directions simultaneously. Biaxial extensional rheometry provides data on strain hardening during processes like blow moulding or film stretching. It defines the limits of material stability before the onset of localized thinning.
Flow Characterization
Behavior under multi-directional stress differs from standard shear flow because the molecular chains orient differently. Testing reveals how the resin maintains integrity as it expands into a cavity. Stable expansion is required.
Engineers use these results to select grades that resist tearing during high-speed inflation. The stretching of a melt in two axes forces the entanglements to resist deformation in a way that single-axis tests cannot capture.
Process Application
Blow moulding machines rely on the strain-hardening properties of the melt to ensure uniform wall thickness. If the biaxial extensional rheometry values are too low, the parison may sag or pop. Problems arise.
This data informs the adjustment of die temperatures and inflation pressures. High-speed film lines also utilize these parameters to prevent web breaks during the orientation phase of production. The relationship between the expansion rate and the resisting force determines the maximum drawdown ratio achievable on the line.
Processors use these curves to predict the performance of a new resin batch without wasting expensive machine time on trial runs.
Quality Variable
A shift in the stretching limit signals a need for immediate process correction.
