Black dot defect reduction in plastic injection moulding process

Abstract

The objective of this research is to reduce black dot defect in plastic injection moulding process. The total amount of black dot defect before process improvement in plastic injection moulding machine number P24 and P25, was 0.65%.

Six Sigma DMAIC methodology is applied as the approach for quality improvement in this research. The methodology consists of 5 phases, comprised of define, measure, analyse, improve, and control phases. In define phase, statement of problem, project team, objective, and scope of the research are identified and are used to emphasise on the reduction of black dot defect on moulded parts in plastic injection machine number P24 and P25. In measure phase, accuracy and precision is assessed by Gage Repeatability and Reproducibility. Then, the potential causes are brainstormed through Causes-and-Effect Diagram and are prioritised and selected by Cause-and-Effect Matrix, Failure Mode and Effects Analysis. In analyse phase, Why-Why Diagram were developed to identify the root causes of the problem and to recognise corrective action. The selected factors were then tested for reliability by hypothesis testing on statistical significance. The test results show that there are 4 factors influencing black dot defect, including carbonised and dirty barrel and screw, previous material trapped inside the barrel, raw material degradation, and contamination in the hopper. In improve phase, those 4 factors were tested to identify the optimum process conditions of the process by Design of Experiment. Next, the confirmation test is performed before implementing optimum condition in actual production. Finally, in control phase, work instruction, control plan, and control charts are constructed to ensure that the optimum process conditions are sustained over time.

The results after implementing Six Sigma DMAIC Methodology reveals that the total proportion of black dot defect in plastic injection moulding machine number P24 and P25 are reduced from 0.65% to 0.34%, a 47.69% reduction.