- August 01, 2008, By Timothy J. Walker Contributing Editor
Winding is an amazingly complex process. Our understanding of winding can be broken into three areas: the winding process, winder design, and wound roll quality. For each of these three areas, there are some things that are well or generally understood. There are also some mysteries for which we still need enlightenment.
One measure of understanding is whether something can be modeled (and verified) from 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 called squeezably soft — relieves in-roll tensions and keeps roll pressures low.
The high load, precision strain stack test rarely is done, and without it, you are left either modeling with great assumptions or resorting to understanding by experiment only. We would go a long way toward understanding the differences in winding our various products if we would stack test everything.
With stack modulus knowledge, we can know the pressures and tensions within our rolls and can modify the models with the real world variables: air, dimensional changes, and profile. The effects of air are fairly well known, including the air entrained with and without a nip, side leakage during winding, and the loss of roll tightness as air escapes over time.
The effects of dimensional changes due to viscoelasticity, temperature, and moisture are well understood, However, similar to stack modulus, the coefficients of these effects are largely unmeasured.
The third dimension — crossweb thickness variations — greatly challenges our ability to model real world winding. Work is underway in this area, but predicting when crossweb defects, such as bagginess or regional buckling, will or will not form is not here…yet.
In winder design, we have a good handle on winding single rolls; understanding the differences in driving from the center, surface, or both; nipping or not nipping; and tapering tension or nip load versus roll radius. The effect of nip position — above, below, after initial contact — is a little muddier, including the many perturbations of two-drum surface winding.
For post-slitting winding, we have a good handle on locked versus differential center winding. The use of individual or common nip rolls is understood.
Regarding roll quality, most sources of laterally shifted layers are well-understood, including from upstream web handling or telescoping from air lubrication, cinching, and roll handling. Less understood are shifted layers from dishing, creep of adhesives, and sawtooth-shaped edge patterns.
Most buckling defects are at least partially understood, including starring, spoking, sagging, and tin canning. All buckling defects in adhesive products (gapping, delaminating) — since they are dependent on viscoelastic creep as a function of temperature and humidity — are difficult to fully grasp, but the general direction away from the defects is known.
Other roll defects moving from more to less understood include: slitter rings, blocking, dimples/pimples, slip knots/wrinkles, and crepe wrinkles.
Now you know what we do and don't know. Take time to learn and use what is known. If you've figured out the unknown, use it as a competitive advantage until the rest of us get to know it, too.
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; firstname.lastname@example.org; www.webhandling.com.