Precision injection molding of thin-wall parts has become an important concern in 3C (computer, communication, and consumer electronics) plastics industry. Previous studies in precision injection molding control focused on the injection screw, the hydraulic system control and the associated operations. In the present study, the influence of relevant parameters, including injection speed, melt temperature, mold temperature, filling-packing switchover, and packing pressure, on the mold plate separation under different clamping pressures was investigated as part of precision molding control. A two-cavity tensile test specimen mold equipped with four linear variable displacement transducers (LVDTs) on mold exteriors and across the parting surface was used for experiments. A PC-based monitoring system was also built to detect the mold separation signals. Mold separation can also be identified from part weight variation and exhibits relevant correspondence with part weight. It was found that because of the high injection speed required for thin-wall molding, mold separation is not negligible. In all situations, mold separation decreases with increasing clamping pressure. As melt temperature and mold temperature increase, mold separation increases, resulting in an increase of part weight. Similarly, when packing pressure and injection speed increase, mold separation also increases. Earlier switchover from filling to packing can decrease mold separation as well as part weight. Among all parameters, packing pressure exhibits the greatest influence on mold separation and the associated part weight change. Variation of melt temperature and mold temperature also changes the gate freezing time and the associated packing pressure distribution and mold separation. The influence also becomes larger when molding thin-wall parts as compared with conventional injection molded parts because of the larger injection pressure required for molding. Simulations also show good correspondence with experimental measurements. (C) 2003 Wiley Periodicals, Inc.