Transient Solids Conveying in Single-Screw Machines

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A large amount of plastic products produced today are processed in the course of their manufacturing cycle on a single-screw machine and prepared for a subsequent casting process. This includes both the continuous production of pipes, profiles or films using a single-screw plasticizing extruder in the extrusion process and the discontinuous production of moulded parts using a plasticizing unit in the injection moulding process. Regardless of the mode of operation, the task of the single-screw machine is to feed the raw material, which is usually in granular form, to convey it as a solid, to plasticize it and to convert it into a homogeneous melt.In case of single-screw machines with smooth barrels, it is common practice when designing the screws to assume that the throughput is determined by the melt conveying sections. However, this assumption no longer applies at high screw speeds, low bulk densities of the feedstock and small screw diameters, as found in plasticizing units in an injection moulding machine. Instead, the functional sections of the screw where solids conveying takes place must be considered in more detail. In addition to the throughput rate, the solids conveying section can also decisively reduce the quality of the manufactured products if it is incorrectly designed, for example if air that is fed into the process cannot escape again and is found in the product as air inclusion. This is particularly true for injection moulding plasticizing units, since the axial movement of the screw during the metering phase impedes the intake of the granules.The aim of this research project is to significantly increase the quality of the mathematical description and the general process understanding of solids conveying in injection moulding plasticizing units with the aid of numerical DEM simulations.This is to be achieved by answering unresolved questions, which are due to the process engineering characteristics of the injection moulding plasticizing unit in comparison to a plasticizing extruder. Essentially, the different kinematics of the screw and the resulting uncertainties in the description of the flow processes of the pellets in the feed section should be mentioned here. By taking these kinematic characteristics into account, by systematically varying influencing variables by means of statistical design of experiments in the DEM simulations and by modelling based on this, solids conveying in injection moulding plasticizing units can be described holistically and with significantly fewer simplifications than before.