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Visual Six Sigma, 2nd Edition by Mia L. Stephens, Marie A. Gaudard, Ian Cox

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Chapter 9Improving a Polymer Manufacturing Process

Your employer, a British company called MoldMat Ltd., manufactures granulated white plastic at a plant in Britain and supplies it to a molding plant in Italy, where it is made into white garden chairs and tables. However, the molding process goes through intermittent phases when its product quality drops, leading to yield losses at both the polymer and the molding plants. When a crisis occurs, teams are formed to tackle the problem, but the problem usually disappears for no apparent reason.

After yet another mysterious crisis occurs and resolves itself, you are selected to solve the problem once and for all. Your selection is due in large part to your recent Six Sigma black belt training. Together with a small project team, you identify two characteristics (Ys) that are of paramount importance relative to quality and yield: the polymer's melt flow index (MFI) and its color index (CI).

Together with your team, you reanalyze the most recent crisis team's data. The analysis fails to reveal suspected relationships between the two responses and eight process factors. This leads you to suspect that measurement variation may be clouding results. Consequently, you and your team conduct Measurement System Analysis (MSA) studies on the measured Ys and Xs. The problematic variables turn out to be MFI (one of the two Ys) and filler concentration (one of the Xs).

Once the repeatability and reproducibility issues for these two variables are ...

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