Numerical modelling and experimental verification of blown film processing

A non-isothermal viscoelastic Kelvin model was developed to simulate the film blowing process. The predictions made by the Kelvin model for processing characteristics, such as bubble diameter, film thickness and strain rate profiles were compared to predictions from the non-isothermal Newtonian model and actual experimental film blowing data of polypropylene (PP). Reasonable agreement was found between the non-isothermal Newtonian model and experimentally observed bubble characteristics. However, by incorporating the elasticity of the polymer elasticity, using the Kelvin model, predictions were found to significantly improve. The reasonably good agreement between the theoretical predictions from the non-isothermal Kelvin model and actual experimental data may be due to the relatively small Hencky strains and strain rates used in the film blowing conditions in this work. Temperature was found to be the most critical parameter which influenced the film blowing characteristics of PP. A correct estimate of the relaxation time of the polymer is particularly important in giving a reasonably accurate fit with the experimentally observed processing behaviour of the polymer. Experimentally observed bubble stability measurements indicated that the stable operating window for PP increased up to a frost line height of 210 mm beyond which the stability decreased.