This bachelor thesis does a theoretic approach to the basics of Viscosity, Fluids, Plastics, Polymers, Rheology and Rheometry before explaining, defining and studying the results of the carried out measurement procedures. The studied plastics are Lupolen 1800 H (HDPE), Moplen 340 K (PP), and Polystyrene 158 K (PS) at ranges of temperatures between 180ºC and 240ºC and apparent shear rates mainly between 100 and 10000s - 1 . All measurements have been mad e with the Haake Rheocap T100, a High Pressure Capillary Viscosimeter at the Rheology laboratory of the Hochschule Darmstadt. At the end of the project, the flow curves of all materials have been analyzed. This analysis includes a comparison between measu rements with different die angle but same other test conditions, comparison between the results of the Rheotens and the High Pressure Capillary Viscosimeter and a determination of the critical shear rates for each material under these conditions.