<p>Micro-injection molding of ultra-thin polymer components is highly sensitive to transient filling behavior, often leading to unstable part quality and low production yield. Conventional parameter-based process control is insufficient for capturing the extremely short and dynamic filling stage involved in micro-scale molding. In this study, in-mold pressure curves were systematically analyzed to investigate their relationship with process stability and part completeness in micro-injection molding. Multi-point pressure sensors were installed along the melt flow path to capture transient pressure responses during cavity filling. The results reveal that part quality is predominantly governed by pressure evolution at the instant of cavity filling rather than by the subsequent holding stage. In particular, the pressure rise rate during filling was identified as a critical indicator for melt penetration into ultra-thin regions, whereas peak pressure alone was insufficient to fully characterize filling performance. In addition, the appearance of pressure signals at the end-of-flow location directly corresponds to complete cavity filling. By optimizing the pressure curve profile during the filling stage, shot-to-shot pressure fluctuations were significantly reduced, resulting in improved molding stability and quality consistency. These findings offer a physically interpretable framework for process monitoring and stability control in micro-injection molding of ultra-thin polymer components.</p>