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A lesson in Defects from Mark Miller, Co-Founder of Coating Tech Slot Dies ( Adapted from Mark's Coating Matters, a featured column of Paper Film & Foil Converter Online (

Mark's Coating Matters | Defects Transcript

In the fluid coated industry, one of the most important things you need to look at is what happens with the fluid after it exits the slot die. As a process engineer you're concerned about how that final product looks. So, when you get the fluid onto the web you need to make sure it's the final product in the final form exactly how you expected it. So, what happens when you have a defect?

Well, I look at it as a three step process. The first step being visualization. When you have the fluid coming out and you end up with a streak, a mis-coating, a herringbone issue, something where you have a defect you want to analyze the first step is to visualize what that defect is. So, what way do you visualize it? You need to look at it with the naked eye. You need to look at it under a microscope. Use your online inspection equipment and that way you're going to get a full grasp of what it is.

As an example, when I was looking at a fluid coated product in the past, I was a process engineer that had a streak. I thought for sure that streak was going to be the problem of the slot die. Because the fluid comes out of the slot die we tend to blame the last place that fluid was. After inspection under a microscope we realized that it was actually debris from the web on the web. So, once we implemented cleaning capabilities on the web then we got rid of the defect.

So what's the next step in the process? Once you've analyzed and visualized what that is you need to go forward and create a library of those defects so the same mistake is not made again. So what you need to do is take each defect, document it, and make sure the keeper of that document has a full listing of what not only occurred but how it was fixed. Then once you have looked and discovered if that defect has occurred in the past then you have to go through the third step which is the design of experiments.

The design of experiments is going to help you not only look at that defect but understand your process overall. Typically, when I look at a DOE or design of experiments, I like to look at a two to the five or five minus one fractional factorial statistical analysis to that design. What that does is it allows you to look at all five possible factors that may be affecting your product with the least number of runs making it a more efficient and a more effective way to understand your process and to look at the defects that are there.

So, after you've visualized, after you've created this library and done your analysis, and after you've done your experimentation, I would add that you need to make sure that everyone is aware of what's going on and that you have a control document. Most people miss this final step. This control document is what's going to make sure that the next person in your place or the next person on the line is doing exactly what they need to do to avoid the defects in fluid coating.

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