What tension is best for laminators?
David Roisum, Ph.D. Consulting Technical Editor -- Converting Magazine, 12/1/2008 12:00:00 AM
Setting and controlling tension is more challenging on laminators than almost any other machine. The tensions at the two unwinds are set first by the guidelines of web handling. Most webs like to run at tensions of 10 percent to 25 percent of their MD breaking strengths. The laminate also would like to run in a similar range based on the strength of the laminate. You often hear that the laminate tension should be set as the “sum of the tensions of the two plies.” Generally, this is not true because both plies do not carry full load at failure.
Consider bonding paper to poly. The paper may yield at 1 percent while the poly might yield at 10 percent. Thus, when the laminate is stressed/strained to the point of failure, the paper will break at 1 percent. True, the paper will carry its full breaking strength load, but the poly will carry only one-tenth of its breaking strength. The “sum of the ply tension” fallacy becomes worse with more plies and with a wider range of modulii.
If this were all we had for tension concerns, the challenges presented would be quite enough. However, controlling tension on center-driven unwinds is not trivial. Roll diameters, widths and inertias test both extremes of the drive's torque range. Add to that the typically much wider range of webs that must be run on a laminator than many other machines, and you already have the ingredients for an extensive drive-control challenge.
The biggest challenge remains that tension control on laminators is over-constrained. Along with keeping both plies happy with appropriate tensions, the strains of those plies must be equal or the material will curl in the MD, CD or both directions. Thus, we have two constraints from web handling and two more from conventional laminate curl. We may even have constraints on tension if we have baggy webs, webs that are dimensionally unstable after manufacturing (such as crystallization) and webs that are dimensionally unstable in environments where moisture or temperature change. Thus, we may have to solve a half dozen or more problems with only two knobs: the tension of both plies.
One way to add a much-needed range extension is to drive both laminating rollers. Given the challenges listed above, it's expected. The principle is very similar to how motors on a center-surface winder give a wider range of winding tightness than the one of a center winder (with/without a lay-on roller) or surface winder.
So how do you control the two motors? The motor that is connected to the steel roller is in speed control. In fact, it should be the master speed reference for the line. The other motor that is connected to the rubber-covered roller is in torque-difference control. Here, we pit one motor against the other in a last ditch effort to control curl given all of the other constraints. Thus, one motor may be running at -100 amps (regenerating) and the other at +200 amps (motoring) to adjust curl in one direction and vice versa to adjust curl in the other direction.
So now we see that if any web machine needed a gold-plated drive, the laminator would be first in line. Anything less will exacerbate the already challenging tension-control and curl problems.
920/312-8466, drroisum@aol.com, www.roisum.com
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