High-Performance Extrusion under Consideration of Extensional and Wedge Flows

The aim of the present work is to improve the performance of plastication in extrusion lines by employing extensional and wedge flows. Extensional flows occur in many process steps of polymer processing, such as fiber spinning, production of blown films or stretching of flat films or flows through extrusion dies. Extensional and wedge flows are essential in the conveying and mixing processes of single and multi-screw extruders. The steady growth of the polymer processing industry and the competition between machine suppliers has driven the development and efficiency increase of extruders, i.e. the increase of output whilst ensuring or improving material quality. A thorough understanding of the physical transport processes, accurate calculation models and valid process measurement techniques are crucial for the design process of machine parts, process optimization and quality control. In the first part of this thesis, an on-line rheometer for simultaneously measuring the extensional and shear viscosity of polymer melts and filled compounds during processing is developed. Extensional viscosity determination is realized by means of the converging flow method in a novel and patented hyperbolic slit contraction that enables constant extensional rates along the contraction. Due to the unique design, pressure transducers can be incorporated directly into the two slit sections, which prevents material from accumulating in pressure holes. The results obtained with the developed rheometer are validated by well-established measurement methods, such as plate-plate rheometry, high-pressure capillary rheometry and the SER (Sentmanat Extensional Rheometer). Furthermore, the applicability for condition monitoring is demonstrated based on several examples. By measuring the extensional viscosity, material effects and behavior are observed which cannot solely be detected by means of shear rheometry. In the second part, the thesis focusses on modeling and experimental investigation of doublewave and energy-transfer screws. In general, wave or wave dispersion screws exhibit wave-like channel depth profiles (sequences of compression and decompression in channel depth) and flight undercuts. The main purpose of the wave screw design is to induce solid bed break-up, dispersion of solid agglomerates and enhance dispersive mixing by creating extensional and wedge flows in the tapered screw channels and flight undercuts. The main objective is to model the non-isothermal pressure-throughput behavior of wave zones by means of network analysis. To this end, heuristic melt conveying and dissipation models, which describe the pressurethroughput relation for isothermal, three-dimensional flow of shear-thinning fluids in rectangular screw channels, are used. In addition, melt conveying models considering the effect of flight undercuts in wave screw channels are developed by means of symbolic regression based on heuristic methods. In further consequence, experimental data are validated against CFD simulations. Experimental investigations demonstrate the potential of wave screws to increase extrusion performance compared to barrier screws.