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The Unsupported Web

In document FREE ebook on Converting Technology (Page 24-38)

6. The Unsupported Web

Too little web support can result in bagging and wrinkling (see Figure 4). Unsupported webs also are susceptible to other environmental factors, such as the drafts from nearby equipment or normal plant ventilation. Even an open door can cause enough draft to result in wrinkling.

Idlers typically are used to provide needed web support. The very act of wrapping a web around a roller does increase the web’s lateral rigidity. The strength and weight of the web will have a lot to do with the number and location of idlers needed. An applications engineer can better troubleshoot your problem when provided an elevation side view indicating rollers and location(s) where wrinkling occurs.

If wrinkling occurs on a longer web span between rollers, additional idler support may be needed. When evaluating web span, consider the width of the web. A rule of thumb is short = less than the width of your web; long = 3x the width of your web.

7. Improper Web Tension Control

Excessive web tension can wrinkle, stretch, or break the web and cause unwanted roll deflection.

Excessive tension can come from too much drag on idler rolls (bearing friction, roll weight, etc.) or from too many idler rolls between drive sections. Stretched edges and slack centers on the web are common with excessive tension.

Loose webs also can result in wrinkles. Lack of enough tension can come from drive sections being out of sync or a poorly designed or calibrated tension control system.

Lighter weight rolls may be the solution for tension problems experienced during line speed changes.

These idlers can supply support without excessive drag. If the problem persists once the web is up to speed, an idler designed with special free-running bearings may be needed.

If you suspect tension problems are from too many rolls or rolls that are too heavy, try experimenting.

Spin individual idlers by hand. Are they hard to turn? Can you selectively remove or replace some rolls and see problems can be reduced?

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If you are running a light web at light tension, and all your rolls are heavy steel idlers with sealed bearings, you could try lighter-weight idlers with free-running bearings to reduce roll inertia and drag. Lighter materials, such as aluminum or carbon fiber with free-running bearings, can significantly change operating performance.

Regardless of the many precautions and cures you might apply to your web converting operation, sometimes wrinkling is just going to happen. In some instances, a special roller will help. In others, it won’t. The problem may be in how the parent roll was wound. It might be in the makeup of the material itself. The cause may be a combination of things, some of which may be out of your control.

There is no magic cure for wrinkling. Do your homework and then look to the experience and advice of a qualified roller manufacturer to help work through the options.

Web Lines | Why Is Winding Optimization Difficult?

Web Handling for Winding

Before you can wind it, you must 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.

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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 cross web 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.

A lot of science goes into winding. So-called TNT (tension, winding, torque) winding principles are well established and have been proven and modeled at the Web Handling Research Center at Oklahoma State University, Stillwater, Okla.

Yet determining which of these principles, or the proportional amount of each, to use for proper winding of different types of materials under specific sets of conditions is still considered “art.” This article is intended to help processors select the best type of winder to use in order to consistently produce high-quality, defect-free rolls of web material. We will discuss the winding principles, how they are used on different types of winders, and present product parameters for which each type of winder is best suited.

Roll hardness is developed in different ways on different types of winders but the basic principles of how to build roll hardness stay the same. To remember these principles, just remember that to consistently wind “dynamite rolls” you need TNT:

o Tension: The winding web tension.

o Nip: The nip of the pressure roll or drum.

o Torque: From the center drive or torque drum

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2.Web Roll Making History

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In document FREE ebook on Converting Technology (Page 24-38)

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