A Lean Six Sigma Framework for Reducing Defects in High-Volume Plastic Injection Moulding: A Case Study of Automotive Component Manufacture

Abstract

This paper presents a structured application of the Lean Six Sigma (LSS) DMAIC (Define- Measure- Analyse- Improve- Control) framework to reduce defect rates in a high- volume plastic injection moulding line producing automotive dashboard trim components. The case study was conducted at a Tier- 1 automotive supplier in Gurugram, India, operating six 500- tonne injection moulding machines at 24- hour production capacity. Prior to intervention, the process operated at 2.8σ with a defect rate of 11,200 parts- per- million (PPM). Root cause analysis using Ishikawa diagrams, Pareto analysis, and regression modelling identified material moisture content, injection speed, and cooling time as the critical- to- quality (CTQ) parameters. Design of Experiments (DOE) with a 2³ full- factorial design was used to quantify interactions and establish optimal process windows. Following implementation of process controls and operator training, defect PPM was reduced to 1,840 — a 83.6% improvement — and sigma level improved to 4.4σ. Annual cost savings of ₹47.3 lakhs were realized through scrap reduction and rework elimination. The study demonstrates the industrial applicability of LSS tools in Indian automotive manufacturing and provides a replicable methodology for process improvement in injection moulding operations.