Web Lines | War on Bagginess

Published: February 27, 2014, By Timothy J. Walker

Bagginess is an area that many paper, film, and foil converters struggle with. Due to many factors—including the lack of good bagginess quantified measurements, need for feedback from roll unwinding measurements/observations (usually at customer processing), and complexity of process and material properties that create bagginess forms—bagginess continues to plague many web producers as an uncontrolled variation in product quality.

I have worked closely with Dr. Kevin Cole of Optimation Technology Inc. (formerly the winding and web handling corporate expert at Kodak) on understanding bagginess in films and foils. It is truly a highly complicated defect, but it's one that we now have many tools in place to address.

Here is combination of product and process variables needed for a full bagginess reduction program:

• On-line thickness profile measurement confirmed with off-line thickness measurement to understand the profile variation in the time span of winding a single roll.

• Understanding of material properties, including coefficient of friction, surface roughness, stack modulus pressure-compression behavior, and visco-elastic creep function (on a days or month scale, though this can be estimated from fitting bagginess measurements onto model predictions).

• Three-dimensional winding model to predict interaction of roll geometry, material properties, winding process variables (tension, nip, speed, taper), and transverse direction thickness profile.

• The 3D winding model predicts machine direction strains at all radial and transverse positions in the wound roll, then maps how much of these strains will become non-elastic permanent strains in the product upon unwinding, where transverse direction permanent strain variations will be observed as bagginess.

• Some bagginess trends that are common in films and predicted by these models include:

1. Loose baggy lanes will align to thickness variations "up" features. Quantifying thickness variations with a simple range (minimum to maximum) statistic will be a somewhat poor predictor of bagginess magnitude. Peak to average statistics, emphasizing thick bands over thin bands, will be a better bagginess predictor.
2. Bagginess will be greatest in a roll’s outer wraps.
3. If compressible cores are used, the high pressure thick lanes can indent the core, creating an opposite bagginess trend of tight lanes corresponding to thickness ‘up/thick’ features.
4. Winding with more tension and nip load will have high average strain and strain variations within a roll, creating more permanent strain variations and bagginess.

• Bagginess measurement is the final key tool and one that remains either difficult or expensive. Even qualitative bagginess can be quite time consuming. There are many options to measure bagginess, but the best systems will quantify bagginess in terms of the length change from baggy lanes to non-baggy lanes, much like a thickness gauge documents crossweb thickness profile. In units of percent length differential, these measurements then can be mapped onto the 3D winding model predictions and confirm the combination of winding, film, and thickness variations required to lower bagginess to a desire level (i.e., pulled out with a given tension).

Reducing bagginess is a challenging program, but it begins with quantifying bagginess (to measure when you have improved), reducing thickness variations, and minimizing winding stresses and strains.

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.

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