A thin-shell two-phase microstructural model for blown film extrusion

A two-phase microstructural constitutive relation is combined with the thin-shell model for the simulation of blown film extrusion. This combination includes equations for momentum conservation, flow-enhanced crystallization, viscoelasticity, and bubble-tube cooling. Consistent with typical blown film operation, the simulations set the bubble air mass and take-up ratio as constants, while treating the machine tension and inflation pressure as dependent variables. In all the simulations performed, the high degree of crystallization, and subsequent system stiffening, located the freeze-line naturally. Bubble geometry, temperature, and crystallinity were fitted to experimental data using material and kinetic parameters mostly obtained by a simpler quasi-cylindrical model. The thin-shell microstructural model was compared to a modified quasi-cylindrical model. The models predict similar responses to operational changes, including axial locked-in stresses at the freeze-line, but have significant differences in the locked-in stresses in the transverse direction, which were attributable to the use of different momentum equations. Either model can be used for data fitting, parameter estimation, and prediction of most process responses to upsets.