The commonly used plunger injection system in the microinjection molding (mu IM) process has separate screw melting, metering, and injection units. As a result, extra operating parameters and complexity are introduced, in comparison with conventional injection molding. In this study, a mu IM machine was used to obtain micromoldings of polyoxymethylene, high-density polyethylene, and polycarbonate. A data acquisition system was developed to record traces of data regarding the evolution of process variables with time. Cavity filling was followed, at the millisecond time scale, using short-shot experiments and traces of injection pressure, runner pressure, and plunger position. Six characteristic process parameters (CPPs) were defined to characterize both the cavity filling and packing stages. The method of design of experiments was used to investigate the effects of machine settings on the CPPs. Metering size, which was optimized for each set of machine variables, was also used as a CPP. Injection speed was the most significant factor affecting plunger velocity and injection pressure during cavity filling, while the effects of mold and melt temperature varied with the material and machine settings. POLYM. ENG. SCI., 50:1214-1225, 2010. (C) 2010 Society of Plastics Engineers