Thin Webs without Wrinkles
- Published: October 01, 2002, By Jeff Damour, Converter Accessory Corp.
Any converter running webs of thin- and ultra-thin-gauge material, including foil, may experience wrinkle problems. In general, there are at least two excellent solutions to wrinkle problems, but only one — expanding surface spreader rolls — is the preferred choice for converters running thin gauges.
Expanding surface spreader rolls have proven to provide the best wrinkle removal for virtually any application in web widths 3-120 in. They are effective in any printing, coating, or laminating operation, and their performance is especially noteworthy when applied to thin-gauge film and foil.
Studying the Systems
An impartial examination and evaluation of the performance of all wrinkle-removing systems came in a definitive study published in a bound edition, edited by J.K. Good, of the Fourth Intl. Conference on Web Handling, held June 1-4, 1997, at the Web Handling Research Center, Oklahoma State Univ., Stillwater, OK.
The specific study, “Testing and Analysis of Web Spreading and Anti-Wrinkle Devices” by R.P. Swanson, 3M Co., (pp414-428), found “the expanding surface rollers did not wrinkle under any test conditions. The regime I performance was excellent. This roller is an aggressive web-spreading device. An expanding surface roller would be insensitive to wrinkles at low, medium, and high tensions. This roller is an excellent, high drag, web spreading and anti wrinkle device.” (p421).
Of the ten types of systems tested, only the expanding surface spreader rolls and the “curved axis” or bowed roll systems were described as “excellent” systems according to the study.
The single distinguishing drawback of bowed spreader rolls, compared to expanding surface spreader rolls, is the potential web-distorting effect built into the design. (See Figures 1 and 2.)
The arc itself, the core of the design principle, has the potential of stretching and even tearing the center of the web. With many materials, depending on the amount of distortion, this may not be an issue. But with very thin gauges, as is often the case with foils and films, the capacity to absorb any distortion of material is limited. Tears and stretching may result, and the impaired quality may be more noticeable than it might be with other materials.
When compared to bowed spreader rolls, the only other system to receive an “excellent” in the Swanson study, the expanding surface spreader roll is distinguished by its ability to maintain a constant tension profile while acting on the web.
Example Makes the Point
Let's take a look at a typical expanding surface spreader roll.
This example (same example as the Swanson study) employs a stretchable rubber sleeve supported by a series of rubber support disks. On each end of the sleeve, there are end collars mounted on bearings and an adjustable axle. The end collars clamp and hold the sleeve in place. Importantly, however, the end collars are canted to a mechanically adjustable degree, and it is this cant that provides the contracted and expanded pattern of the stretching section of the roll. (See Figures 3 and 4).
Essentially and simply, during production the web continually stretches from the short side of the rubber sleeve to the long side. The material enters the roll at the point where sleeve expansion begins and exits at a point prior to sleeve contraction. The desired spreading action is achieved without distortion of the web.
Wrinkle removal comes from the expanding action evenly distributed across the web. The amount of expansion may be adjusted at each end of the web — continually, if necessary — to compensate for distortions derived from changing upstream conditions.
This type of spreader is an aggressive wrinkle-removal device. Spreading amount is adjustable from 0%-100%. Adjustments can be made from each end of the roll while the machine is running. The recommended wrap angle for this roll is from 90°-180°; the greater the wrap angle, the greater the stretch.
For converters of thin-gauge materials, or where consistent gauge and unmarred surfaces are especially important, the expanding surface spreader roll maintains an even tension across the surface and does not distort or tear the center or edges of a web. Also, the smooth surface does not mar, scratch, or distort web surfaces.
Benefits Outweigh Flaws
With a couple of minor caveats (the system does not distribute the stretch across the web perfectly, and its effectiveness may be inhibited at extremely fast web speeds), the expanding surface spreader roll has proven to be the most effective wrinkle-removal system available for thin-gauge materials.
It is true almost every spreader roll will remove some level of wrinkles, and a converter's selection of a system should be guided, in part, by the degree of wrinkle removal desired and, in part, by the amount of distortion converters and their customers can tolerate. While both bowed roll and expanding surface spreader rolls offer an excellent performance, when it comes to thin- and ultra-thin-gauge materials and when quality is paramount, the expanding surface spreader roll is the preferred choice.
Jeff Damour is the engineering manager at Converter Accessory Corp., Wind Gap, PA. He has been with CAC for 17 years and has published several technical articles covering a wide range of web handling applications. Jeff has the honor of presenting web spreading technology for converters at the Slitting and Rewinding Fundamentals and Coating and Laminating seminars run by the Converting Equipment Mfrs. Assn. (CEMA). Contact Jeff at 800/433-2413; This email address is being protected from spambots. You need JavaScript enabled to view it..
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