Slitting & Rewinding
Layer-to-layer coefficient of friction films and the effect of this on winding
and converting customers produce defect-free products. Roll density or hardness is the key factor for consistently producing quality rolls of web materials. The amount of inwound tension or residual stress wound into the roll is obtained by using one or more of the T.N.T. (tension/nip/torque) winding principles. The film’s layer- to-layer coefficient of friction properties has a major effect on the ability to apply the T.N.T. principles to produce the desired roll hardness without roll defects. In general, films that have a
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layer-to-layer coefficient of friction (COF) value of 0.2 to 0.7 will wind well. However, consistently winding defect-free rolls of high slip or low slip (low COF or high COF) films usually presents major winding challenges.
LOW COEFFICIENT OF FRICTION FILMS High slip paper, film and laminate web products have low layer-to- layer coefficient of friction (generally COF < 0.2). These high slip materials will often have inner web slippage or cinching problems when they are winding and/or during subsequent unwinding operations; or will have roll handling problems in between these operations. This inner web slippage can result in defects such as web scratching, dishing,
18 June 2017
very web manufacturer’s goal is to consistently wind quality rolls to ensure their printing
telescoping and/or starring roll defects. Low COF products need to be wound as tight as possible at the core, usually with high torque. The winding torque needs to be tapered at a minimum of 4-5 times the core OD, and the desired roll hardness needs to be built using the nip winding principle. Air is never an aid when winding high slip films. These films always need to be wound with uniform nip loading to prevent air from entering the roll during the winding process. The resulting torque of the motor
is proportional to the winding tension multiplied by the web width, multiplied by the radius of the winding roll, multiplied by the taper, set by the operator. For constant tension winding, the torque increases proportionally to the radius roll. For constant torque winding, the torque stays constant throughout the winding cycle and the inwound tension decreases proportionally to the roll radius. Normally rolls are wound somewhere in between the constant tension and constant torque winding to achieve the proper roll hardness taper. High
slip (Low COF) products typically need to be run with tension tapers greater than 50 per cent from the start of the wind until the finished wound roll diameter.
HIGH COEFFICIENT OF FRICTION FILMS Low slip paper, film and laminate web products have high layer-to- layer coefficient of friction (generally COF >0.7). These films will often have blocking and/or wrinkling problems. When surface winding these high COF materials, the out- of-round rolls can be expected at low winding speeds. When winding at higher speeds, roll bouncing can be a problem. In a paper titled ‘The
Coefficient of Winding Trouble,’ consultant and columnist Tim Walker of TJWA noted that this is due to the outer layers of a winding roll requiring a small amount of sliding action as the layers first enter the winding roll. He added: “This sliding action produces inwound tension as the air following the web is ejected from the nip, or out the sides. If the full width slides, this is not a problem. But, if one lane or spot sticks and the rest slide, then a local shear stress will develop near the sticking point. This local shear may form a small buckle or soft wrinkle in the top layer. In some products, a small bump or ripple can be wound over and ignored, but in other products, especially optically clear films, the next layer will not smoothly wind over a bump or ripple, but will instead conform over the bump, producing a slightly larger bump or ripple. Like a rolling snowball, as additional layers are added, the defect will often get bigger with each turn.” These defects are commonly called slipknots or slip wrinkles. High COF paper, film and laminate web products are best gap wound with a minimum gap between the following roll and the winding roll. Spreading needs to be provided as close to the winding point as possible. A FlexSpreader covering on the following roll has proven to be successful on a number of high COF winding applications.
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