A Hybrid Framework to Investigate Selected Interfacial Phenomena in Plastics Co-Extrusion ? Interfacial Flow Instabilities, Interdiffusion, and Layer Adhesion
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Co-extrusion is the most important processing technology for producing multilayer structures in the plastics industry. Machine manufacturers and processors face the ever-growing demand in increasing output rates and production capacity utilization at consistent product quality. Steady optimization of co-extrusion processes requires thorough understanding of transport phenomena and physical processes. This thesis aims at the development of novel methods and approaches to investigate both theoretically and experimentally two major processes at polymer-polymer interfaces that govern the quality of co-extruded products: interfacial flow instabilities and interdiffusion. Using industrially relevant material combinations, the applicability of the developed methods is shown.
The first part of this thesis addresses the development of prediction models for two-layer co-extrusion flows by applying a hybrid modeling approach. This approach includes: (i) definition of the governing flow equations, (ii) identification of all independent influencing parameters by applying theory of similarity, (iii) implementation of the shooting method to numerically solve the dimensionless governing equations, (iv) variation of all independent influencing parameters in a full factorial design study, and (v) approximation of the numerical data by means of symbolic regression analysis based on genetic programming. As a result, accurate, numerically stable, and simple algebraic relationships between influencing parameters and target quantities were obtained. With particular emphasis on the interface, a set of eleven target quantities ? such as interfacial shear stress, position of the interface, interfacial flow velocity, pressure gradient ? was defined. The accuracy of the models was confirmed both numerically and experimentally.
The second part of this work involved the development of a two-layer co-extrusion demonstration die ? instrumented with comprehensive sensor technology ? and the digitalization of the co-extrusion process. The novelty of this die lies in the possibility to systematically investigate the effect of co-extrusion flow conditions on interfacial phenomena. This is attributed to the special design of the flow domain and the symbolic regression flow models constituting a digital process twin to represent the co-extrusion process. Depending on the interfacial phenomena investigated, operating points were adjusted and evaluated according to a particular design of experiments, including the variation of (i) total throughput, (ii) ratio of individual throughputs (i.e., the layer distribution), (iii) interfacial contact time, (iv) interfacial shear stress, and (v) melt temperatures.
To detect interfacial flow instabilities, two in situ systems based on ultrasound technique and optical coherence tomography were developed and installed. An evaluation procedure was implemented to determine the magnitude of interfacial flow instability and indicate operating points with class labels: (i) stable, (ii) transition (commencing wave formation), (iii) unstable (wave-type flow instability), and (iv) mixing (flow with interlayer mixing, i.e., zig-zag-type flow instability). A huge number of co-extrusion experiments was carried out to identify novel criteria for the onset of interfacial flow instabilities. These are physically based and thus not restricted to this particular experimental die but also applicable to multilayer flows with an arbitrary number of layers and any kind of cross-sectional channel shapes. The third part of...