A conventional control of the injection moulding process is based on machine values like the position and speed of the plasticising screw. Modern machine technology can reproduce these values with a high precision. However, systematic disturbances like fluctuating environmental conditions or variations in material viscosity still affect the reproducibility of the injection moulding process and the resulting part quality. Consequently adaptive and self-optimising control algorithms have to be applied for a further improvement of product quality. A pvT-optimisation allows a material-tailored online control of the holding pressure. Based on an online determination of the temperature state of the melt in the cavity, the cavity pressure can be closed loop controlled in order to achieve a constant specific volume of the moulded parts. Therefore a pvT-optimisation enables a compensation of thermal disturbances during injection moulding. However, fluctuations of material viscosity have a strong impact on the resulting profiles of the cavity pressure. Therefore under the influence of viscosity disturbances the specific volume and other related quality values cannot be reproduced homogenously using a pvT-optimisation. For the compensation of viscosity fluctuations an online measurement of the current viscosity in the process is necessary. For this purpose an indicator based on the pressure loss within the mould cavity is defined and analysed. On this basis a control of the viscosity is implemented. Within this concept the injection speed is adapted to the current process viscosity. First results of compensating viscosity fluctuations inflicted by the use of recycled material show the feasibility of the concept. In the future this concept has to be validated in combination with the pvT-optimisation. This enables an injection moulding process which is less sensitive to changing boundary conditions.