Sequential injection molding using a valvegate-controlled hot runner system has attracted attention for industrial applications in recent years. Because of the complexity of the operation mechanisms, a commercial valve gate usually delays for about 0.3-0.5 s once the valve-opening command is given. The signal-to-operation delay is acceptable for the conventional injection molding of large parts. However, this operation delay limits its application to thin-wall molded parts for computer, communication, and consumer electronics, for which the required filling time is very short. In this study, a gas-driven fast-response sequential-valve-gate system was developed for thin-wall injection molding by the adoption of valve-gate control performance. The characteristics and verifications of the valve-gate opening were monitored with a charge-coupled device (CCD) camera (nonmelt condition) and cavity pressure transducers and an accelerometer (melt-filled condition). The influence of the tolerance between the inner piston and cylinder and the gas pressure on the valve-gate opening was investigated in detail. Tensile bar parts 1 mm thick were used for the molding experiments. The delay time has been found to be intimately related to the response of the gas-pressure delivery controlling the valve-gate movement. In a nonmelt environment, the delay time of the valve-gate opening decreases with increasing driven gas slightly. In a melt-filled environment, the delay time is quite sensitive to the operating gas pressure because of the extra resistance between the shaft and the melt. A threshold pressure as high as 100 bar is required to keep the delay time below 15 ms. With the proper choice of the piston size and driven gas pressure, the delay time can be reduced to about 8 ms in a nonmelt environment and to about 12 ms in a melt-filled environment. Molding using this improved system for sequential valve opening can provide thin-wall injection parts without a weld line, and good cosmetic quality and better tensile strength require a lower injection pressure than molding using single-gate and concurrent-valve-gate opening. (c) 2005 Wiley Periodicals, Inc.