Don’t Wind Wrinkles Into Your Film
Wrinkles are a major cause of defects in extrusion winding and converting, especially with thinner films, which are much more wrinkle-prone than thick ones. The cause of most wrinkles is located close to the roller where the wrinkle first appears and can be identified by looking at the orientation and pattern of the wrinkle. First, find the cause; then try to eliminate or at least reduce it.
Start by “walking the line” from upstream to downstream. Find and clear upstream wrinkles first, which are fewer and easier to diagnose. By the time the web gets to the winder, it may have wrinkles from several different sources, though it’s not unusual for 90% of wrinkles to be caused by one or two machine components.
An experienced troubleshooter can spot wrinkles long before they actually fold over by looking for shadows in the web. Shadows are wrinkles waiting to happen when a slight change in product grade or web tension pushes the shadow into a crease. As you walk the line, you’ll want to make any such shadows more visible. A portable lamp helps bring them out more clearly. Be prepared to crouch in some places and climb onto a stepladder in others—following common sense and safety procedures, of course!
Wrinkles are tension-sensitive, though tension is seldom the root cause. (The few wrinkles caused by tension are primarily in very low-modulus elastomeric films.) Try both decreasing and increasing tension at the point where the wrinkle forms.
Another powerful diagnostic technique is to slow the machine down to thread-up speed, if the process allows it. Operators often ignore wrinkles that occur during thread-up because “they go away when you speed up,” because air lubricates webs, reducing traction on rollers. The important thing to note is that the causes of wrinkling at slow speed are usually the same ones that cause wrinkles at running speed. The wrinkles are just easier to see at low speed.
Once you determine which roller or other machine component is initiating wrinkles, sketch a side view and top view of the local area of the line. A side view should show the routing of the web, including at least one roller upstream and downstream from the suspected source. A top view should show two vital features: the angle of the wrinkle—i.e., MD, CD, or diagonal—and its location on the web.
Your accompanying description might say that the “No. 13 idler has a diagonal wrinkle on the back side.” Creases form on top going over a roller and on the back in a span between rollers, so crease location is also a clue to the cause or source.
COMMON WRINKLE TYPES
Wrinkles come with lots of colorful nicknames, but they usually fall into one of five categories: MD or diagonal, symmetrical or asymmetrical, or CD.
- Symmetrical MD wrinkles (aka tin canning, tin roof, or curtain wrinkles) are nearly parallel and uniformly spaced, with narrower spacing between wrinkles in thin film and wider in thick film. These wrinkles seldom “walk” much. They’re common in flexible films when the web becomes wider than it originally was. In very stretchy materials, this expansion can be caused by a tension drop in one web span vs. the preceding span. To fix it, keep tension constant, if possible. Thin idler rollers with a lot of deflection, or roller grooving of over 20 times film thickness, can also cause film to stretch. To fix this, use an idler roller with less deflection or shallower grooves. Sometimes the problem is part of the process. If film is sent through an oven, heat causes the film to expand. That can’t be changed. And for hygroscopic films like nylon or PET, high humidity or water-based inks or adhesives also can cause expansion. The least aggressive option, which is often adequate, is to flatten the web by routing it over a large-diameter roller, a lightly tensioned roller, a slippery roller, or some combination of the above. As a last resort, use spreading. This doesn’t mean merely installing a spreader, but also adjusting it properly.
- Asymmetrical MD wrinkles are caused by the same things as symmetrical MD wrinkles, except that the even pattern is disrupted somewhere, so several MD troughs collect in an open span into a single bulge or crease. Asymmetrical MD wrinkles retain their orientation, but may move around and even cross over each other.
- Typically, more things are happening to cause asymmetrical wrinkles than symmetrical ones. A single wrinkle that stubbornly stays in one place, as if it were stuck in a groove, may indeed be stuck in a valley or bulge in a roller. Such roller variations are correctible with better maintenance and housekeeping. Lack of gauge uniformity in the web can also be a cause. Take measures to improve film gauge and quality, such as adding automatic gauge control.
- Symmetrical diagonal wrinkles (aka bow wrinkles) are oriented inward from the edge of the film at an angle. They fade and disappear in the center of the web, forming a symmetrical arrowhead or chevron pattern pointing downstream. The steeper the angle, the more uneven the pulling forces. The cause commonly is something such as a roller or the web itself that is “smile” shaped.
- The curve may come from excessive roller deflection, caused by a roller too small in diameter for the web width or tension. Try replacing it with a larger-diameter roller. Mismatch of the nip-roller crown to tension load can also cause bowing. If you’re not sure which component has a bow, try adjusting tension both higher and lower to help identify the source. When you have identified a suspect, adjust that particular component and look for a response in the wrinkle pattern.
- Spreading or flattening can also be an effective remedy. But spreading with too much bow or improper orientation can also cause these wrinkles. So be sure spreaders are adjusted properly.
- Asymmetrical diagonal wrinkles (aka lightning-bolt wrinkles) are a single band of wrinkles oriented at an angle to the machine direction. The steeper the angle, the greater the problem. They all point the same way and may favor one side of the web. They tend to be evenly spaced and sometimes “walk” sideways. The usual cause is something such as a roller or the film itself that is crooked.
- A misaligned web-handling roller is a common cause of asymmetrical diagonal wrinkles. These wrinkles point toward the narrower side of the misalignment and walk toward the wide side. Roller diameter variation from side to side, nip-pressure variation, and uneven pull from narrow drive rollers or edge-trim tension can also cause these wrinkles. If you try a remedy, and the wrinkle angle is reduced, you’re on the right track.
- If you’re not certain of the root cause of the crookedness, change tension both higher and lower to reveal the location of the misalignment. Use spreading or flattening only as a last resort. If none of these steps work, look at film quality. Uneven film temperature or thickness profile, whether inadvertent or by design, can also cause these wrinkles.
- CD wrinkles, known as buckles, occur primarily during rewinding. They’re caused by loose winding followed by tighter winding and become apparent in the edge of a roll as a compressed wave pattern. The problem may be insufficient drum torque, temporary loss of web tension (possibly at a splice), malfunctioning tension control, too rapid braking of roll speed, or binding of the core shaft or rider roll slides. The solutions are to provide smooth tension as roll radius changes; to start, stop, or make splices under tension; and to ensure that slides move freely.
A machine audit can’t diagnose issues like bagginess, which is caused by film quality problems. In most cases it comes from bands of uneven gauge that stretch differently during winding. Spreading isn’t likely to help bagginess, since the root cause is in the extrusion process. So fix the process or return the film to the supplier.
About the Author
David Roisum is president of Finishing Technologies Inc., a consulting firm in Neenah, Wis. He consults, writes and lectures extensively on web-handling problems. This article is excerpted from his new chapter on wrinkles in the recently published Second Edition of Roll and Web Defect Terminology, edited by R. Duane Smith and published by TAPPI Press, Norcross, Ga. (order # 0101R318, www.tappi.com). Contact the author at firstname.lastname@example.org, www.roisum.com.
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