P T. Maju Jaya Plasindo is a manufacturing company that manufactures HDPE plastic bags. HDPE plastic bag production activities are not separated from the quality problems, one of which is a high defect rate on the plastic produced. It is known that this defect has a defect percentage of 52.9% compared to other types of defects. The first step taken to remedy this problem is to find out the effect of the color of the plastic bag that the company produces on the appearance of spots using the t-test on the proportion of defects in both colors. Then use Ishikawa Diagram to find out the cause of the problem and analyze it using Failure Mode and Effect Analysis (FMEA). Results and FMEA are known to be the major failure modes that cause freckle defects and then follow up using Simple Comparative Experiments and Taguchi Experimental Design. Improvement effort using Simple Comparative Experiments did not give significant influence on washing process, while using Taguchi Experimental Design obtained optimal oven machine setting that is 105 ° C oven temperature, 7 minutes oven time and 3500 RPM blower speed. Classification using Data Mining is then performed to evaluate the results of the Taguchi Experimental Design, resulting in a decision tree and a rule if if rules function. After the research, the percentage decrease in the loss rate of 33.458% obtained and the percentage of decrease in freckle defects obtained by 3.05%.