Analysis of Product Quality Defects and Root Cause Investigation in a Plastic Pipe Manufacturing Plant in Onitsha, Anambra State

The quality of plastic pipe products manufactured in Onitsha, Anambra State is a matter of public concern given the widespread use of these products in water distribution, sewerage, and building services across South East Nigeria, where substandard pipes have been implicated in repeated infrastructure failures. This study analyses the nature, frequency, and root causes of product quality defects in a plastic pipe manufacturing plant in Onitsha using a structured quality improvement methodology. Defect data were extracted from production quality inspection records over a 12-month period, and defect categories including ovality, wall thickness variation, surface roughness, and colour streaking were classified and stratified using Pareto analysis. Statistical process control charts were constructed for key dimensional quality characteristics, and the process capability indices were calculated. Root cause analysis for the three highest-frequency defect types was conducted using fishbone diagrams and the five-whys methodology. Findings showed that 83 percent of all defects were accounted for by wall thickness non-conformance, ovality, and surface finish defects. Process capability analysis revealed that the wall thickness process operated at a Cpk of 0.78, indicating insufficient capability to meet specification limits consistently. Root causes included inconsistent raw material resin viscosity from multiple suppliers, operator-adjusted extrusion parameters deviating from standard settings, and inadequate die temperature control. The study proposes targeted process control interventions and a supplier quality development programme. Keywords: quality defects, root cause analysis, plastic pipe manufacturing, statistical process control, Onitsha.