<p>The performance of nonreturn valves (NRVs) in injection molding significantly affects melt delivery and part quality, particularly under wear conditions that induce melt backflow. Although pressure-based monitoring methods have been widely studied, most existing approaches rely on instantaneous indicators and are limited in capturing time-dependent sealing degradation. In this study, a pressure-integral-based approach is proposed to diagnose NRV wear and evaluate its impact on melt backflow and part quality. The pressure integral, defined as the time integration of nozzle pressure over the injection and packing stages, reflects the cumulative pressure transmission applied to the melt and is therefore sensitive to both the magnitude and duration of pressure evolution. Experimental investigations were conducted under different NRV wear modes (axial and radial), material flowabilities (virgin and recycled polypropylene), and processing conditions (injection speed and packing pressure). The results show that the pressure integral exhibits consistent relationships with part weight and effectively distinguishes between wear mechanisms. Axial wear primarily causes delayed pressure buildup, whereas radial wear leads to continuous leakage and unstable pressure behavior. In addition, higher material flowability increases melt backflow under wear conditions. A simplified compensation strategy based on the pressure integral was implemented, and a reduction in part weight variation was observed under mild wear conditions. These findings indicate that the proposed approach provides a physically meaningful and practically applicable framework for NRV wear diagnosis and process stabilization.</p>