How do I prevent wrinkles from forming?

So much has been written about wrinkles (I plead guilty), that one could become mired in details before seeing the overall philosophy. The most obvious approach would be to identify the root cause and remove it. However, root cause analysis, or RCA for short, may be unnecessarily restricting, as we will see.

Root causes are plentiful and include roller problems such as misalignment and diameter variations across the width. Misalignment is easily recognized by wrinkle troughs which are at an angle with respect to the machine direction and usually "walk" sideways. Roller-diameter variations, on the other hand, make wrinkle troughs in the machine direction and favor certain locations despite changing materials. If a roller is not right, the obvious next step is to fix it.

However, root causes also include processing which may be much harder to prevent. For example, heating and cooling can cause expansion wrinkles. However, turning off the oven to "prevent" this root cause will not be looked upon favorably by either your boss or your customer. Same thing with the application of many adhesives, coatings, metallizing or print: they can make wrinkles.

If you want to avoid the root cause you may have to work with process and product developers so that they understand the risks they incur when they design a web for end use without considering design for manufacturability.

The root cause may also be thin and/or baggy webs. Thin webs are unforgiving of variation in material or machine that may be below our threshhold of detection or remedy. If the web is baggy, heaven help you. Few companies are equipped with the expertise necessary to troubleshoot this thorny problem.

Let's take a different approach. Let's identify the component that makes a specific wrinkle. Note that there are different types and causes for wrinkles. In some cases, the source is where the trough, soft wrinkle or hard wrinkle is first seen. Beware, however, that in severe situations a wrinkle may propagate upstream one or more rollers from its source. Also, many wrinkles form in one unforgiving location but in fact had invisible "help" from some upstream source. An example is wrinkling on a gage band on a wound roll.

If the source is a process and the wrinkles are roughly uniform in spacing, you will need to: live with wrinkles, use a different material, tone down the process or apply effective spreading immediately after the process and before the wrinkles initiate on a roller. If the source is a transport roller, first make sure geometry is impeccable. If so, you next determine whether the web is tracking, sliding, or floating. You must apply a spreader that is compatible with that state of traction. For example, in the case of tracking you may start with a concave roller (bands of tape on the ends of the roller) and proceed to stronger spreading—such as the bowed roller if needed, and if applicable.

In the case of sliding, you will probably use a roller with a slight barrel shape to flatten the web.

In the case of floating, you will manage air-film height for maximum wrinkle resistance. In the case of air bars, it will probably mean more air than the current system can put out. In the case of air turns, it will probably mean less air than you are comfortable with, perhaps to the occasional touchdown of the web.

First, go to wrinkle school so you can identify the root cause and eliminate it. However, you may not always find the cause, or you may not be able to undo it. In that case, you will need to go to spreader school.

David Roisum, Ph.D., Consulting Technical Editor, 920/725-7671, DRroisum@aol.com, www.roisum.com