Fluctuations in temperature and flow rate of the polymer injected into an injection moulding cavity, as well as changes in the heat transfer, may cause alterations in the cavity pressure profile from cycle to cycle during the filling stage. This is due to the fact that the process is usually fast, time varying, and nonlinear. Thus, considering the time-varying characteristics of this process, a self-tuning-control strategy was selected to control the cavity pressure profile during the filling stage. A first-order model is employed to simplify the controller design. The model parameters are determined on-line using the recursive least-squares estimation algorithm, and the controller parameters are calculated using the pole location procedure. In order to reduce controller saturation, the self-tuning algorithm is implemented along with a first-order observer and state feedback. Open-loop experiments are performed to verify the model order, and closed-loop experiments were used to show the effectiveness of the proposed strategy.