Viscoelastic simulation of film casting process for a polymer melt

Abstract

Numerical simulations of the film casting process were performed using a finite element method for Newtonian and viscoelasttc fluids. We simplified the governing equations by the assumption that the stress and velocity gradients in the thickness direction were negligible, and obtained the film thickness and mean value of stress and velocity components in the thickness direction as variables. Viscoelasticity was described by the Larson model with multiple relaxation times. Non‐isothermal conditions were considered by applying the time‐temperature superposition law. The simulation results for the several kinds of commercial low‐density polyethylenes wore compared to the experimental data for a laboratory‐scale process at l90°C and a commercial‐scale process at 310°C. The film width and film thickness distribution at chill roll, and the change of film width were in good agreement for the laboratory‐scale process, but the agreement for the commercial‐scale process was; not as good. In the simulation of the commercial‐scale process at high temperature, the value predicted by the use of the Viscoelasticity for the original pellet showed poor agreement owing to the change of Viscoelasticity in the process. The agreement was improved by the use of the Viscoelasticity for the processed resin, which was changed from the original one. Next, viscoelastic effects on neck‐in and edge bead phenomena were investigated. The neck‐in and edge bead phenomena were considered to be affected by both the uniaxial elongational viscosity and planar elongational viscosity.

Relationship between Neck-in Phenomena and Rheological Properties in Film Casting

“…5) It was confirmed that the final film shape at the chill roll was not influenced by the boundary conditions at the die exit. 2,3) The die width, die thickness and air gap used in the computations were 250 mm, 0.8 mm and 88 mm, respectively.…”

“…The validity of this assumption has been verified by Satoh et al 5) by comparing the numerical results of fully-three-dimensional simulations. The effect of gravity force can be negligible over 2 in draw ratio.…”

“…Debroth and Erwin 6) and Satoh et al 5) have suggested that the film edge elongates in uniaxial stress while the film center does in plane stress. Because the planar elongational viscosity rather than the uniaxial one is expected to play an important role for the film deformation at the film center, it is important to take into account both the uniaxial/ planar elongation kinetics in order to understand the essential mechanism of the polymer deformations in the film casting.…”

“…3) For viscoelastic fluids, the neck-in value, defined as the difference between the die width and the film width, depends upon not only the operating conditions but also upon material properties of polymers such as elongational viscosities, relaxation times and relaxation modulus. Debbaut et al 4) showed the quasi-three-dimensional simulations using Upper A branched LDPE showed a smaller neck-in value due to its stronger extension-thickening property , 5) while a linear HDPE exhibiting smaller uniaxial elongational viscosity leads to a larger neck-in.…”

Planar Extensional Viscosity Measurements for LDPE Polymer Melt by Using Novel Orifice Die Design and Cogswell Model

“…Interestingly, the level of the strain hardening in uniaxial extensional viscosity is much higher than in the planar extensional viscosity, which is more clearly visible in Figure 5 where normalized uniaxial and planar extensional viscosities are plotted as the function of the extensional strain rate. Recently, it has been revealed [17][18][19][20][21] that the ratio of planar to uniaxial extensional viscosity, Ș E,P /Ș E,U , (i.e. the difference between the strain-hardening nature in planar and uniaxial elongation) controls the level of the unwanted neck-in phenomena (film width reduction occurring during the film casting process between the flat die and the chill/nip rolls) due to the fact that the middle of the film undergoes planar elongation whereas the material at the edge undergoes uniaxial elongation.…”