Injection moulding is one of the most productive methods in plastics processing allowing the production of parts with complex geometries in a single production step. To stay competitive, part quality and its reproducibility remain a major issue for injection moulding manufacturers. Since modern machine technology allows an ample reproducibility of machine values like axis movements and pressures within the plasticising unit, the compensation of inevitable disturbances gains importance. Differences in the residual moisture of the plastics materials, the use of reground material, batch variations or interruptions in the production process may cause fluctuations in the melt viscosity. These fluctuations can result in short shots or flash formation. However, in any case the pressure distribution within the cavity and consequently the part warpage are affected. In this paper a mould using pressure sensors within the hot runner manifold is used to analyse melt viscosity fluctuations during the injection moulding process. Similar to the principle of a high pressure capillary rheometer, the viscosity can be analysed based on the pressure loss between two sensors. In contrast to the steady rheometer the transience of the injection moulding process has to be considered. Therefore, different indicators are analysed regarding their ability to distinguish polypropylene (PP) grades with different flowability. Finally, an indicator for the current melt viscosity is selected which enables an inline control of melt viscosity during injection moulding.