If two or more plies are tightly bonded there is little risk in running that laminate over a roller. However, if the plies are lightly bonded or not bonded at all, there is a huge risk of bunching (CD wrinkles) on the ingoing side of the roller. The mechanism is simple to describe.
When the plies go over a roller they act as a unit or a solid body, at least over the wrapped area. As a solid body they would have the same RPM. However, because the outer ply has a larger effective radius it will travel at a faster surface speed. The problem is on the inner ply. It travels slower and does not keep up with the material coming in. It bunches up just before the incoming tangent if the interply bond is inadequate.
The best thing is to avoid the problem altogether by running each ply separately until they are firmly bonded together such as at a laminating nip. Occasionally you can muscle you way through by increasing web tension. Alternatively, you can design a fluted roller that allows the upper ply to go straight while the lower ply gets metered through as a tiny pucker that is take up in the next span. While CD grooves are expensive to cut, CD oriented slats may be bonded onto a plain roller with a bit less effort.
My 3/8/07 post gives even more reasons not to share rollers.
In blown film it is common to handle thick tubing (up to 6-8 mil) and this phenomenon is common. I saw the use of a fluted idler at a customer plant, but was not allowed to take measurements. Is there a rule for determining the proper size and spacing of grooves or slats and the overall diameter of the roller? I have been searching the web for a supplier of fluted idlers. I have found 3 but I want to be sure they know how to make the roller properly for the application and width.
Posted by: Bill Hellmuth | May 24, 2007 at 01:40 PM