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. 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. Shadows are wrinkles waiting to happen when a slight change in product grade or web tension pushes the shadow into a crease. A portable lamp helps bring the shadows out more clearly. 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. 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.
Wrinkles come with lots of colorful nicknames, but they usually fall into one of five categories: MD or diagonal, symmetrical or asymmetrical, or CD.
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Symmetrical MD wrinkles are nearly parallel and uniformly spaced, with narrower spacing between wrinkles in thin film and wider in thick film. They are 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. Constant tension helps to eliminate wrinkles. Thin idler rollers with a lot of deflection can also cause film to stretch. The use of an idler roller with less deflection or shallower grooves helps to reduce wrinkles.
Sometimes the problem is part of the process. If film is sent through an oven, heat causes the film to expand. 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 these rollers. As a last resort, spreading can be used- not merely installing a spreader, but also adjusting it properly. |
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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. |
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Symmetrical diagonal wrinkles (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. 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. Larger-diameter roller helps in reducing the wrinkles. Mismatch of the nip-roller crown to tension load can also cause bowing. Spreading or flattening can also be an effective remedy. But spreading with too much bow or improper orientation can also cause these wrinkles. The spreaders should be adjusted properly. |
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Asymmetrical diagonal wrinkles (lightning-bolt wrinkles) are a single band of wrinkles oriented at an angle to the machine direction. 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. |
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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. |
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CD wrinkles, known as buckles, occur primarily during rewinding. They are 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. (Extracted from article by David Roisum) |
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