Web Lines | Why Is Winding Optimization Difficult?
- Published: April 22, 2014, By Timothy J. Walker
Winding is hard work—hard on your brain. In your education of web handling, nearly all learned out of the university, your Bachelor’s Degree is when you understand web tensioning, roller design, and guiding. Wrinkling and spreading get you your Master’s Degree program in web handling. Working with winding, winders, and roll quality is your Ph.D. in web handling. Are you ready for your winding Ph.D.?
Web Handling for Winding
Before you can wind it, you have to get it to the winder. It’s a good idea to have your B.A. and M.A. in web handling before you go for your Ph.D. Besides getting a good web to the winder, your B.A./M.A. in web handling will help you understand winding, too. This list shows where web handling principles apply to winding.
- Web Properties: As with web handling, strain is the secret to winding. Strain is the stretch in the web from tension, usually a small number, and is easily lost inside a winding or wound roll as layers compress and wound-in air escapes. Your specific web’s mechanical properties are critical to any winding project. Other key web properties are thickness profile, bagginess, thermal or moisture expansion, frictional properties, surface roughness, and viscoelasticity. (Winding is getting complicated already.)
- Tension Control: How is tension created and what is it as every layer makes initial contact with the winding roll? Understanding tensioning systems and torque limits is vital.
- Roller Design: You can think of the winding roll as the last roller in your process—and what an ugly roller it is. Before winding, think of the core as this last roller. Compared to your idler rollers, your core or winding roll are likely out of spec in terms of diameter uniformity, alignment, and deflection.
- Traction: In center winding, we hope the layers of the winding roll can transmit the torque from the core to create tension at the roll’s outer diameter. Can it? If no, cinching and telescoping is in your future (or present). Air management is critical in high speed winding. Get enough air out to have friction between layers, but don’t take it all out, since entrapped air helps hide some of your thickness profile variations.
- Nipping: Most winders need and have nips. Winding nips provide tensioning, lateral control, air management, and anti-wrinkle benefits. But nipping can be hard at winding, especially when the roll is a moving target (e.g., turret winder roll transfers) or prolific (e.g., many rolls of a slitter/rewinder).
- Guiding: Many winders cheat by slitting immediately upstream of winding. Guiding into winding can be challenging from moving rolls (turret winders), diameter variations (especially with long entry spans). Keeping the layers aligned or positioning the core properly can shift layers within the roll even when pre-winding alignment is perfect.
- Buckling: Getting wrinkle-free into winding is challenging, especially with at-speed core starts. Even after a perfect landing at winding, layers may buckle later, forming buckled patterns running machine direction, cross-roll, or form in shapes that look like worms, screw threads, and honeycombs.
Interestingly, some rewinders are called "doctoring machines." Do you want to be Dr. Winder? Beware.
Winding Process
How the winding process contributes to roll and web quality is complex. If that isn’t enough, some aspects of roll and web quality are difficult to quantify. Try improving something you can’t measure— yes, challenging.
- Winding process is complex with many variables to consider.
- Winding quality can be hard to quantify (buckling, bagginess).
- Roll and web quality can be highly dependent on crossweb thickness profile.
- Many winding defects have at low occurrence rates (sometimes less than 5%), making it difficult, expensive, and risky to evaluate winding process change.
- Winding stress and pressure models are complex and are poor at predicting many defects (cinching, buckling).
- Winding-critical web properties can be difficult to measure (e.g., thickness profile, stack compressibility).
- Winding models rely on seldom measured variables (stack modulus, coefficient of friction, surface roughness).
- Winding measurements rarely provide feedback of the dynamic winding process. Most winding and roll quality measurement occur after winding is complete.
Equipment
Winding equipment is vastly different. It can be difficult to determine the best winder design for a given product. It is more difficult to optimize winding when there is a poor match between product and winder. Product-winder mismatches are more obvious when trying to wind a giant roll on a tiny core.
- Driving and entry span options are varied and significant (center vs. surface, gapped vs. nipped).
- Winding equipment suppliers are often poor at providing winding process variables in pertinent engineering units and many have poor documentation of how the controls work.
- There is no universal standard for taper tension. In linear taper tension system, the target percent taper is often not the true taper percent if winding stops before maximum roll diameter.
- At-speed roll transfer systems are prone to starting rolls with wrinkled or folded over layers near the core.
- Zero speed roll transfer systems may have control variations from inertia of the roll or rollers during acceleration and deceleration and have changing air lubrication with speed changes.
- Winding may be dependent on variations in core properties (especially some slip shaft winding systems) and how cores are supported (chucks vs. shafts).
- Winding may be dependent on winding position (spindle A vs. B on turret winders, top vs. bottom shaft on slitters, inboard vs. outboard position on slitters).
Time and Money
Winding is time-consuming. Roll quality is time-delayed. Winder replacements or upgrades are big, expensive, time-consulting projects.
- Each data point is costly in terms of time, material, and customer happiness.
- Time to wind a single roll may be long.
- Some winding defects are immediately known (lateral shifting); other defects are only revealed at unwinding, often after a lengthy storage and transport time—and likely at a customer location.
- Winding equipment mechanical or control changes are expensive and time consuming.
Is There Any Good News?
A lot of the heavy brain work in understanding the winding process has been done. Though challenging, most critical winding variables can be measured and modeled. The options for modeling and measurement may be expensive, but they are available. Winder design and control is much better. I think it is an exciting time to work in winding.
Are you ready for your next patient, Doc?
Web handling expert Tim Walker, president of TJWalker+Assoc., has 25 years of experience in web processes, education, development, and production problem solving. Contact him at 651-686-5400; This email address is being protected from spambots. You need JavaScript enabled to view it.; www.webhandling.com.
