Some unwinds are not capable of switching the unwinding direction from the top to the bottom of the unwinding roll or vice versa. Some customers or products require a certain side of the web be up. However, in the majority of situations it is at least possible to flip the unwinding roll around. It is interesting to see what happens.
The expected result is to more clearly see profile issues with the web. If a defect is found on the front side of the machine, for example, flipping the unwind will tell if it is due to the customer’s machine or your material. If the defect flips to the other side, you (the web supplier) are in trouble. If the defect stays on the same side, you are both in trouble. You because you supplied a material that was not infinitely tolerant and they because they own the non-uniformity that was responsible for the defect. This is one of the most powerful binary search troubleshooting methods. I use it whenever I can.
What you would NOT expect to see, however, is a level material to wind differently. In one case you have the surfaces ABABAB and so on while flipping the unwind would give BABABA and so on. We would be surprised to see a difference in winding behavior because the material and the interfaces between the plies are the same. However, I have heard of a couple of paper cases where it appeared that the winding might be different. I have not personally seen anything like that on paper. However, I was on a specialty film winder recently where the difference was not subtle, it was night and day. The rolls with the A side in bore no visual resemblance to those with the B side in. I had never seen anything like it. Nothing in winding theory, my personal experiences or even my imagination can explain why. Mysteries are good. They keep us busy and humble.
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