AbstractProduct quality of injection‐moulded' plastics greatly depends upon the injection flow rate of molten polymer during filling. In this paper, an optimal control of the filling process is shown to improve the properties of the moulded parts. Our goal is to provide an efficient basis for designing a supervisory computer‐controlled injection moulding system. A dynamic model of the filling process is developed. It takes into account the effect of molten polymer on the dynamics of the injection ram. Based upon this model, an optimal control input, which is a control current actuating the injection ram‐screw assembly, is derived from the minimization of a quadratic performance index which serves to keep the front velocity of molten polymer flow into the cavity at a desired value. The optimal input thus obtained has been implemented via a microcomputer for on‐line control of the injection process. The experimental results show that surface blemish and residual stress of the moulded parts can be reduced via this optimal