Injection molding quality control by integrating weight feedback into a cascade closed-loop control system

Quality control plays a crucial role in injection molding control to meet stringent tolerance requirements and to facilitate automation. Previous work has tackled this challenging subject mainly through consistent machine operations or process variable control. In this paper, a direct quality feedback control system has been proposed and developed. The system has a cascade structure and combines both feedback and feedforward controls. An important quality index, namely, part weight, is measured in each molding cycle. The difference between this measurement and a quality target is used to adjust the mold separation at the process level. The mold separation is controlled via both a cycle-to-cycle mass-based switchover point and a within-cycle holding pressure control. Molding experiments have been conducted using different mold geometries and resins. Compared with the cavity pressure based control system currently used in industry, the control system in this study results in a significant improvement of both long-term and short-term consistencies in part quality. In addition, this direct quality feedback control has other benefits, such as 100% quality inspection and automatic process tuning.