"The Influence of Shear and Elongation Forces in Extrusion Processes"
The Influence of Shear and Elongation Forces in Extrusion Processes
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The use of elongation and shear flows in extrusion is of major interest for science and industry. With the following thesis we want to show new developments and applications of shear and elongation flows in extrusion.
In the beginning of the thesis, we investigated the process properties of starve extrusion on materials containing a very high molecular weight polyethylene (VHMWPE) fraction. With starve fed extrusion of these materials significant elongation of VHMWPE particles which developed through starve fed extrusion, was discovered. Since various attempts to deform and decrease the size of the particles through extrusion have already been tried, with no success, this work represents a first promising way for processing these materials. Since elongation forces can influence the size and shape of VHMWPE particles we developed a new mixing device, the so-called segment mixer with constant channel depth and decreasing channel depth and compared them to commercially available mixing devices. The big goal of these mixing devices is to introduce significant elongation forces to the processed material. Additionally, a new parameter for the characterization of mixing devices the so called ?Stress-ratio weighted mixing index (??????)? was introduced. This parameter is the first combination of mixing index (??????) and shear stress which is essential for the characterization of mixing devices. Additionally, the mixing devices show some significant improvement in distributive, dispersive and elongational mixing compared to the commercially available mixing devices. Before CFD simulation can be carried out, rheology data are needed. For this the applicability of the Cox-Merz rule for HDPE materials with different molecular masses was studied. This research showed that the applicability of the Cox-Merz rule is only possible to HDPEs to a certain molecular weight. If the molecular weight is too high the Cox-Merz rule does not apply anymore. Additionally, the work showed that with increasing elastic behaviour of the material the failure of the applicability of the Cox Merz rule decreases. With increasing viscous behaviour of the material, the failure of the Cox Merz rule decreases. An error in the measurement of the rheological properties of the material can result in significant error of CFD simulations.
Additionally, shear flows were used in coextrusion, to introduce flow instabilities at the interface of two layers. The more flow instabilities are at the interface of both layers as higher is the adhesion of both layers to each other. When the flow instabilities are big enough so-called Kelvin Helmholtz flow instabilities can occur. These flow instabilities can develop a multilayer at the interface of both layers. A multilayer at the interface increases the adhesion significantly.