This study presents the application of a 32 full factorial design of experiments to reduce burn mark defects in the injection molding process of bottle caps. The research focuses on optimizing two critical process parameters: melt temperature and injection pressure, each evaluated at three levels. Initial defect identification was conducted using a fishbone diagram based on the 4 M principle, followed by quantitative analysis through analysis of variance. A total of 27 experimental runs were performed, with defect rates measured and statistically analyzed to determine the main and interaction effects of the selected variables. The results identified that higher melt temperatures and injection pressures significantly reduced the incidence of burn marks, with the optimal condition found at 190 °C and 85 bar. This setting achieved a 34.94% reduction in defect rate from 7.24% to 4.71%—demonstrating compliance with industrial quality standards. The findings validate the effectiveness of factorial design in defect minimization and process control, offering a structured methodology for quality improvement in plastic injection molding operations.