| This work developed several constitutive models for describing steady state and transient behavior of the polymer film blowing process. Each model uses two phases, one each for the amorphous and semi-crystalline phases, and includes the effect of both thermal and flow-enhanced crystallization. The model has been modified to allow both phases to be present from the extrusion die, thus eliminating the discontinuities found in prior work.;Using the Giesekus/Rigid rod model, experimental blown film data are fit for two polyethylenes, LLDPE and LDPE. Using a fixed set of parameters, the model produces excellent fits of radial and temperature data, and very good fits of crystallinity for most processing conditions. The required inflation pressure predicted by the model agrees with the qualitative experimental trends, although not the exact values. Predicted model stresses at the freeze line are correlated with real mechanical properties of elongation at break and yield stress.;Variations of the model were developed by substituting alternate fluid models for each phase, including the Extended Pom-pom and Newtonian models. The key finding is that the overall two-phase model structure leads to freeze line formation, not the specific fluid models used. The transition from a low relaxation time material to a high relaxation time material, with crystallinity acting as the switch, causes the radius to lock in.;Perturbation analysis, which has never been applied to film blowing, was used to explore the effect of system disturbances on the process. Results showed that perturbations related to heat transfer and inflation pressure were more significant than the effect of die swell. In addition, crystallinity is shown to have a stabilizing effect on the system, with more crystallinity dampening the effect of perturbations. Stability diagrams for each material and model show reasonable results, with higher blow-up ratios becoming unstable for all draw ratios. |
