Web Lines | Action Plan for Baggy Webs
- Published: August 17, 2016, By Tim Walker
Bagginess is not easily measured, but here are some methods to compare low, moderate, and high levels of bagginess.
Baggy webs are a major source of waste or customer complaints in many paper, film, and foil processes. Baggy webs will not lay flat or they sag at the web’s edges or in machine direction (MD) lanes. Baggy webs essentially have crossweb direction (CD) length variations, with longer edges or lanes achieving tension only if the short lanes are stretched out to equal the longer lanes.
When tension does not pull out the bagginess, baggy webs wrinkles at nearly any nipped roller process. Bagginess can contribute to other defects, including coating thickness variations, curl, and CD tracking errors (especially in forced air ovens).
Defect Mechanism
Though not the only bagginess mechanism, many films and foils form bagginess due to stress and geometry variations within a roll. CD thickness profile, especially above average thickness bands (hardbands), will strongly correlate to bagginess.
In lanes where the profile is thick, the roll builds to larger-than-average diameter or stresses. Over time, material in these lanes will creep or distort beyond recovery, revealing CD length variations (bagginess) upon unwinding.
Proposed Action Plan
Start by assessing your bagginess baseline. Bagginess is not easily measured, but there are some methods to easily compare low, moderate, and high levels of bagginess.
Bagginess is assessed at unwinding, after the normal delay time and storage conditions between winding and unwinding. Changes in storage conditions (less time, lower temperatures) may change bagginess magnitude.
Pick one product (thickness, width, material, film maker).
Baseline: Assess 5 to 10 rolls for their levels of bagginess, thickness profile, and runability (wrinkles).
- Pull out a 5–6 ft sample at the outside of the roll.
- Hang up the sample vertically or lay it out on the floor or table with no tension and take a photo (take all the photos with lighting that highlights the bagginess and be consistent in angle and lighting of all photos for fair comparison).
- Repeat with another sample from near the core.
- The long baggy lanes will buckle. The short lanes will lay flat.
- Grade the sample on level of bagginess.
- Unwind and process the rolls, making notes (and photos or videos) of wrinkles, of how bad the wrinkles are, and any adjustments made to reduce wrinkles.
- See if wrinkles correlate to the bagginess (magnitude or position).
- Measure the thickness profile of the rolls (physical measurement is best with a caliper gauge).
- See if bagginess position correlates to thickness deviations.
Assessing Bagginess Reduction Options
Wind 5 to 10 rolls under different winding conditions, preferably lower winding tension. Do any or all of these changes:
- Wind the rolls when extrusion thickness profile is great (flat).
- Wind the rolls with lower starting tension.
- Wind the rolls with more taper tension.
- Wind the rolls with less nip load.
- Wind the rolls with a softer nip roller.
- Wind the rolls on a bigger core.
- Wind the rolls to a shorter yardage.
- Quickly process the rolls from winding to unwinding.
Repeat the bagginess quantifying experiment. Check for changes in bagginess and wrinkles. Also, note if other problems or defects occur under the new conditions.
Compare the bagginess and wrinkles of the baseline and altered conditions.
Other Things You Could look Into
Here are a few more things to try:
- Different tension (higher) at the unwinding and processing step.
- Add a bowed roller ahead of nipped rollers where wrinkles occur.
- Measure nipped rollers for footprint uniformity.
- Use a spring scale to measure the force to turn the unwind brake.
Make a chart or table of brake setting to brake torque. This bagginess baseline and improvement trial will take some time and diligent measurements, but it should show that bagginess and related wrinkling can be changed.
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.
