Fluid Mechanics of Polymer Melt Filtration

Polymer melt filtration is a crucial part of thermoplastic extrusion systems, including film casting, as the process removes contaminants from polymer melts which ensures product quality and process robustness. Millions of tonnes of plastic waste are generated every year. Ambitious goals have been proposed by leading organisations to reduce such waste through recycling. Melt filtration is crucial in processing with recycled material as it ensures that contaminants from recycled plastic have been removed and the final product is suitably clean.

There is little academic research tailored to melt filtration. Before computationally modelling melt filtration, it is crucial to determine the nature of the physical mechanisms driving particle transport, particle deposition and flow across a porous medium for potentially viscoelastic, non-Newtonian polymer melt flows in filter systems. An experiment is undertaken and clarifies that the most common polymer used in film production by Mylar obeys Darcy's law through an in-line screen filter.

Computational fluid dynamics modelling is used to investigate flow across the simple filter system; an experimental setup is used to test the validity of the porous media model in representing filters in polymer melt systems. Computational models of geometrically complex filter systems are created. These models are used to investigate and visualize initial flow patterns in each system.

Filter blocking models are fitted to experimental or production run data to investigate the blocking mechanisms taking place for each filter element. A novel filter blocking model is developed by modifying the blocking models to allow for permeability coupling with velocity. The model is implemented into the computational models of the filter systems created. The blocking model is shown to fit well to pressure drop evolution data from production runs with the filter systems. Results from the model offer insight into flow pattern alteration as filters block. Proposed design improvements to filter systems are suggested based on this model.