We numerically demonstrate that the ratio of uniaxial to planar elongational viscosity controls the neck-in phenomena of the casting polymer films, i.e., the decrease in film width value for both viscous and viscoelastic fluids. Quasi-three-dimensional numerical simulations of an isothermal film casting process were performed using a finite element method for two viscoelastic fluids using Larson and PTT models, purely-viscous non-Newtonian fluids using Cross model and Newtonian fluids. The increase in take-up velocity relative to that in extrusion showed particular film deformations of neck-in depending upon the rheological properties in the models. The results indicate that the film width is determined by the ratio of uniaxial to planar elongational viscosity rather than the extension-thickening nature in uniaxial elongation. The computations using different fluid models show that the viscosity ratio is universal for predicting the neck-in value of the stretching polymer films not only in viscoelastic but also in pure viscous fluids.
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