A method is proposed for real-time process monitoring for expanded polystyrene (EPS) injection molding systems. The method employs measurement of two variables: vacuum pressure in the EPS supply hose and phase difference between two points along an acoustic standing wave generated within the EPS flow path. High-speed videography is utilized as a secondary means of monitoring the injection molding process. Video data are correlated with pressure and acoustic data to substantiate those variables’ validity as indicators of intended molding system performance. Data show recorded parameter curve shapes to be indicative of key injection molding milestone events, such as valve timing and changes in flow regime.