A new approach for controlling part cooling in plastic injection molding is developed using a PI controller and coolant flow rate as the manipulated variable. The method uses an average part surface temperature within the mold as the setpoint parameter. A mechatronic control system was developed for providing variable coolant flow rates. The control strategy was simulated using plant models, implemented and tested on a 50-tonne plastic injection-molding machine with good closed loop responses. (C) 2006 Wiley Periodicals, Inc.