A model for the two-layer blown film process with flow-enhanced crystallization

A 2-D model for non-isothermal bi-layer film blowing is developed based on the 1-D film blowing model of Henrichsen and McHugh [2007a. Analysis of film blowing with flow-enhanced crystallization: part 1. Steady-state behavior. International Polymer Processing XXII (2), 179–189] that accounts for viscoelasticity and flow-enhanced crystallization. Numerical results demonstrate the role of rheological, thermal, and crystallization properties on the development of crystallinity and stresses in a bi-layer system consisting of two crystallizable polymers. For a two-layer film consisting of the same materials, the evolution of the stress in an individual layer can be significantly different due to the temperature difference. Varying the material properties in a given layer, such as the plateau modulus and the maximum crystallization rate, not only leads to the corresponding responses in its own layer, but also influences stresses and crystallinity in the other layer through heat transfer between two layers. Results further demonstrate that stresses in the film after the frost line will be borne primarily by the layer that solidifies first, while the second, molten component will have a tendency to relax. The layer arrangement is also shown to have direct impact on the stresses and semi-crystalline phase orientation at the freeze point which will impact the final properties of the film.