Cavity pressure plays an important role in determining the quality of injection molded articles. The non-linear and time-varying nature of the process difficulties in implementing efficient control systems, which usually employ linear, time-invariant design strategies. conversely, advanced control methods, such as self-tuning control, do not provide physical information about the process. A physically-based adaptive controller has been developed for cavity pressure control during the filling phase. Different control algorithms, employing different tuning criteria, were implemented on a microcomputer to control the cavity pressure. Experimental results indicate that this control yields accurate results for the injection molding of polyethylene. The results also show that this method, using the IMC algorithm, is superior to the classical optimal controller tuning approach.