01 Jun 2013 How can I predict wrinkles in my web?
Wrinkles in the web are a puzzle to most converters and cause problems at coating, slitting and laminating stations. Some days they are there, sometimes they’ve gone. Occasionally the wrinkles are so bad that they form a permanent crease as they go over the roller.
Although the root cause of wrinkles is well-known to be misalignment between two rollers, the physics of whether the wrinkles appear or not is quite complex. Fortunately you don’t need to know the physics: you just need to play with the Wrinkle Predictor App on your tablet, phone or computer.
First enter some basic numbers about your setup (you can choose Metric or US units): web Tension and Width, Span between the rollers of interest, the Thickness of the web, its Modulus (click the ? button to find typical values for common webs). To know whether you are going to get a soft wrinkle which passes harmlessly over the roller or a hard one which forms a crease you need to provide the roller Diameter, the Wrap angle and an estimate of the friction, typically something like 0.3. Lower friction reduces the risk of hard wrinkles, but you might get TD scratches instead.
Finally, you need to know the Misalignment. In this example the rollers are misaligned by 400µm, i.e. 0.4mm over a span of 2500mm, an error of 0.016%. If your rollers have been aligned by your own engineers the chances are that the misalignment is much larger than that, simply because accurate alignment is very hard to achieve.
In this example the outputs tell you that the misalignment is not large enough to give a slack edge, that the downstream web will move 333µm in the TD and, sadly, that you have a wrinkle at 5.7° which is “hard” and therefore likely to cause a permanent crease in the web.
Many of you will have spotted that the tension is very low for a 2m web. Increasing it to 100N removes the wrinkle. The default action for most users is to increase the tension, and often it works. But the app lets you play with scenarios. Suppose your misalignment is 1mm (not unusual) and the span is only 1500mm. Now it requires a tension of 500N before the wrinkle disappears. You can also use the app in reverse. If you know your tension, span etc. and can measure the angle of the wrinkle it tells you what the misalignment is likely to be, so you can tell whether you can re-align it yourselves or need to call in professional aligners with more sophisticated tools – because aligning one roller might solve one problem but cause a problem in the next span.
This blog first appeared in the Converting Quarterly Magazine from AIMCAL and I am grateful to the Editor, Mark Spaulding, for permission to re-use the text.