PFFC - August 2008 - (Page 12)

WEB LINES Winding: What We Know & What We Don’t Know W By Timothy J. Walker Contributing Editor inding is an amazingly complex or regional buckling, will or will not form is not process. Our understanding of here…yet. winding can be broken into three In winder design, we have a good handle on areas: the winding process, winder winding single rolls; understanding the differences design, and wound roll quality. For each of these in driving from the center, surface, or both; three areas, there are some things that are well or nipping or not nipping; and tapering tension generally understood. There are also some mysteries or nip load versus roll radius. The effect of nip for which we still need enlightenment. position—above, below, after initial contact—is a One measure of understanding is whether little muddier, including the many perturbations of something can be modeled (and verified) from two-drum surface winding. engineering principles. Winding process models are aimed at predicting the layer-by-layer pressures and residual tensions (or lack thereof) within a wound roll and, from the stresses and strain, predict defects. The simplest models find roll stresses from web and core properties, tensioning profile, and roll geometry. Though a winding roll is a continuous spiral, almost all models treat a wound roll as a series of pre-tensioned elastic rings or hoops. The greatest hurdle to useful winding models is measuring the mechanical properties of a stack of material such as we would find in a wound web. Low stack modulus—what Mr. Wiffle would have For post-slitting winding, we have a good handle called squeezably soft—relieves in-roll tensions and on locked versus differential center winding. The use keeps roll pressures low. of individual or common nip rolls is understood. The high load, precision strain stack test rarely is Regarding roll quality, most sources of done, and without it, you are left either modeling laterally shifted layers are well-understood, with great assumptions or resorting to understanding including from upstream web handling or by experiment only. We would go a long way toward telescoping from air lubrication, cinching, and understanding the differences in winding our roll handling. Less understood are shifted layers various products if we would stack test everything. from dishing, creep of adhesives, and sawtoothWith stack modulus knowledge, we can know shaped edge patterns. the pressures and tensions within our rolls and can Most buckling defects are at least partially modify the models with the real world variables: understood, including starring, spoking, sagging, air, dimensional changes, and profile. The effects and tin canning. All buckling defects in adhesive of air are fairly well known, including the air products (gapping, delaminating)—since they entrained with and without a nip, side leakage are dependent on viscoelastic creep as a function during winding, and the loss of roll tightness as air of temperature and humidity—are difficult to escapes over time. fully grasp, but the general direction away from The effects of dimensional changes due to the defects is known. viscoelasticity, temperature, and moisture Other roll defects moving from more are well understood, However, similar to less understood include: slitter to stack modulus, the coefficients of rings, blocking, dimples/pimples, slip these effects are largely unmeasured. knots/wrinkles, and crepe wrinkles. The third dimension—crossweb Now you know what we do and thickness variations—greatly don’t know. Take time to learn WEB challenges our ability to model real and use what is known. If you’ve ACCUMULATORS world winding. Work is underway figured out the unknown, use it as in this area, but predicting when a competitive advantage until the crossweb defects, such as bagginess rest of us get to know it, too. All buckling defects in adhesive products are difficult to fully grasp. COMING NEXT MONTH Web handling expert Tim Walker, president of TJWalker+Assoc., has 20+ years of experience in web processes, education, development, and production problem solving. Contact him at 651-686-5400; tjwalker@tjwa.com; www.webhandling.com. 12 | AUGUST 2008 WWW.PFFC-ONLINE.COM http://www.webhandling.com http://WWW.PFFC-ONLINE.COM

Table of Contents Feed for the Digital Edition of PFFC - August 2008

PFFC - August 2008 Table of Contents First Glance Calendar of Events From the Editor Web Lines Process Management News Clips Carton & Box Reporter Carton & Box What’s New Products Narrow Web & Label Reporter Narrow Web & Label What’s New Products Cover - Show Preview - Labelexpo Labelexpo Floor Plan Conference Program Digital Printing Gravure Printing Bagmaking What’s New Products Services Directory Classified Marketplace Equipment Buyers & Sellers Advertisers Index Experience Speaks

PFFC - August 2008

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