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Analysis and Optimization of Single Screw Extruders with High Speed Rotating Screws

In the focus of this dissertation stands the experimental and theoretical investigation of the operating performance of mixing elements in single-screw extruders with high rotating screws.The investigation focuses on widespread polyolefin and a screw diameter of 35 mm. The size of the screw results from the in practice often required throughput range from about 200 to 300 kg/h. For the experiments mixing elements are chosen, which differ greatly because of their designs and the operating performance.The investigated mixing elements run at speeds of up to 1000 rpm and throughputs of up to 325 kg/h.

The experimental analysis includes the evaluation of pressure, melt temperature and material homogeneity, depending on the screw speed, the specific flow rate and back pressure. In the experimental investigation the influence of the different mixing geometries are visible. The material homogeneity improves with increasing screw speed for all analyzed systems. Beside the evaluation of typical processing parameters, like pressure and melt temperature a new method for the practical investigation of the thermal homogeneity is presented. The comparison with various temperature measuring techniques shows the enormous benefits of this procedure. This is especially interesting with regard on the investigation aims. In this work influence on the thermal and material homogeneity are discussed and the correlation between these two dimensions are worked out.

The experimental investigation is supported by three-dimensional, non-isothermal and non-newtonian CFD-Simulations. Furthermore, numerous influences on the calculation quality are discussed, including software-internal settings, as well as material parameters and thermal boundary conditions. Considering the fact that thermal boundary conditions are not available, the thermodynamic behavior can be partly estimated or determined iteratively. Are the required variables known, the system behavior can be calculated very accurately. The simulations are validated with the help of the experimentally ascertained data (pressure and temperature) and show good agreement.

From the simulation results, different mixing criteria are derived and discussed. These system variables such as residence time, shear and tensile stress are evaluated along the path lines. From all experimental and theoretical investigations, some interesting approaches result for the design of screws for high rotating screw extruders.

Beside the calculation of the mixing elements, some geometrical variations of the reference screw are examined. By use of simulations, for high-speed ranges basic interrelations between the screw flight deep and pitch and the processing behavior are investigated.

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