Continuing our discussion on various causes of wrinkles in overlamination, this next topic focuses on machine alignment, tension variation and temperature. Our mission is to educate our customers on the factors that may play a role in the formation of wrinkles throughout the value stream, including film production, adhesive coating and converting.
The web can be moved out of its normal running plane by a number of machine issues, including, idler roll deflection, parent roll deflection and idler/driven roll misalignment. Wrinkles result from these issues because as the web seeks to align itself perpendicular to a roll it will move or bend out of its normal running plane. The web will either ‘walk’ as in the case of misalignment or create a situation of uneven tension across the web as in the case of deflection. Either situation will cause wrinkles due to lateral compressive stresses being formed across the web. For example, when roll deflection occurs, the film will wrinkle because it will compress toward the deflection point.
Varying tension through a process can cause wrinkles. For example, an adhesive coating line may contain 4-5 or more distinct zones of tension control. Another example is a press which generally has tension zones between the unwind (break) & the laminator nip, the overlaminate break & laminator nip and the laminator nip & rewind. Tension may vary through each of these zones. Also as material type changes, the effect of tension may change as well. As the web stretches and recovers through these various tension zones, lateral compressive forces (due to high tension) may cause wrinkles. Tension control through these sections can be further affected by machine condition. A defective roller or bearing will cause tension variations as the web is flowing through the process.
Certain film such as polypropylene may shrink when exposed to drying temperatures. Although drastic shrinkage will be easily detectable, small amounts shrinkage may go unnoticed. Cross directional shrinkage of the web may create lateral compressive forces and lead to wrinkles.
References MD Wrinkle Formation Causes and Cures Nov 1, 2001, William E. Hawkins, Film Handling Solutions Ltd. http://pffc-online.com/mag/paper_md_wrinkle_formation/
WEB SPREADING- By Jeff Damour CONVERTER ACCESSORY CORPORATION http://www.converteraccessory.com/products/wrinkle/PDF/spreader2.pdf