Analysis and design of annular dies for mono- and multi-layer polymer flows

Abstract

A review of the current die designs used to produce annular flows is presented. An analysis of the performance of the two most commonly used designs in the coextrusion industry showed that one design exposed the polymer melt to a much lower wetted surface area thereby resulting in lower pressure and residence time within the die. This analysis lead to the development of a new type of annular die which also minimized the exposed wetted surface area in a multi-layer construction. A mathematical model based on the Control Volume Method (CVM) was developed and used to design a prototype die which was subsequently manufactured and tested. The thickness variation that was predicted by the model was slightly higher than that which was measured from samples obtained experimentally. The prototype die was also successfully used to produce film.

A smaller die was also designed using the CVM and tested. The experimental data were in good agreement with the predictions of the CVM model. The small die was also analysed using a 3D Finite Element Method (FEM) simulation. The isothermal FEM analysis predicted a lower variation than what was experimentally observed. Subsequent non-isothermal analyses could not be performed at the isothermal flow rates, due to solution instability problems, but the predictions at lower flow rates indicated larger thickness variations.

The small die was reproduced from clear acrylic and used in a visualization experiment. The experiment involved the injection of a tracer at certain points within the flow field and observing the flow path. These visualization experiments were then simulated with a particle path analysis from the FEM results. The agreement between the calculated particle path s and the observed tracer paths was good indicating that the FEM analysis was a viable method of analysing and improving the design of this type of die.

A fundamental investigation into the interfacial instability phenomenon was also performed. A series of coextrusion experiments were conducting using a carefully selected set of polymers for the purpose of differentiating the effects of Molecular Weight (MW) and Molecular Weight Distribution (MWD) on the occurrence of an interfacial instability. It was concluded that there are essentially two types of interfacial instability and that the MW had the strongest effect on the occurrence of the 'zig-zag' instability due to high interfacial stress while the MWD had a strong affect on the appearance of the 'wave' instability. The actual source of the wave instability or the mechanism could not be confirmed but the experiments did provide some new insight for future study.