Process Parameters and Scrap Rates in Plastic Injection Moulding: A Six Sigma DMAIC Case Study

This paper reports an exploratory Six Sigma DMAIC case study of scrap generation in plastic injection moulding, based on six months (October 2025–March 2026) of production and quality-control records from a manufacturer of household products. Thirteen high-volume products were analysed. Product-level total scrap rates ranged from 0.12% to 4.47%, with the Defne Double Dustpan Body a clear outlier; the pooled scrap rate of 0.55% corresponds to about 5480 defective parts per million (≈4.0 sigma). Pearson correlations between ten nominal machine parameters and the total scrap rate were weak to moderate, and none was statistically significant (largest: Injection Speed, r = 0.40, 95% CI −0.19 to 0.78; n = 13). The three parameters with the largest absolute coefficients—Injection Speed, Injection Temperature and Ejector Pressure—were examined descriptively against the Short Shot, Damaged Part and Surface Defect rates using parameter–response charts. The elevated Short Shot and Damaged Part rates observed at particular parameter levels all came from a single product, and the ranking of the parameters changed when that product was excluded. Because the data are product-level and observational, these levels cannot be separated from product, mould and machine effects or from operator adjustments, so they are reported as hypotheses for confirmation, not as process thresholds. The Improve and Control phases are accordingly presented as recommendations: a confirmatory designed experiment, cycle-level parameter logging and chronological p-chart monitoring. The study shows both the practical value and the evidential limits of applying DMAIC to routinely collected, aggregated factory data.