This paper presents a methodology to improve quality of a display product produced by press machines. The presented methodology is focusing on the pooling problem, which shows a wave pattern on a display screen. To solve the pooling problem, the joint six sigma technique has been employed. To find the source of the wave pattern, we identify six potential factors; the box bending components, the pressure of pad spring, the blending angle, the stripper, the amount of chamfer, and different environment of production lines. For the six potential factors, we performed extensive experiments and analyzed the results by using statistical techniques. Through the analysis we extracted five key factors; the pressure of pad spring, the blending angle, the stripper, the amount of chamfer, and different environment of production lines. For each of the factors, we propose a remedy. As a result, the pooling problem has been solved and the quality of the product has been improved significantly.