Evaluation of a new cavity-pressure-based controller for injection molding

A new cavity-pressure-based controller was successfully incorporated in an open loop point-controlled injection-molding machine. Set points (cavity pressures) for the controller were determined experimentally. The resultant cavity pressure switchover from the injection stage to second stage produced more controllable and uniform part dimensions than the time and position switchover typically employed in the machine. This was true for three materials: polypropylene, polystyrene, and polycarbonate. These set points, however, seemed to depend on the mold and machine rather than the processing conditions and material type. Moreover, the controller algorithm, however, could not compensate for changes in injection speed, melt temperature, hydraulic oil temperature, and cycle time. Such variations produced substantial changes in average part dimensions, but similar deviation in those dimensions. Since the magnitude of these dissimilarity correlated with material properties, this suggests that incorporation of a material model might improve the controller. The controller's ability to switch from pack to hold was not evaluated in this study.