- August 01, 2007, Timothy J. Walker, TJWalker & Assoc. Inc.
What does minimizing a baggy web mean? Two things. First, we would like to minimize the level of bagginess in our webs. Second, we would like to minimize the waste associated with running the inevitable baggy web in our processes.
To minimize the creation of baggy webs, we must take on their causes. In last month’s column I said, “The biggest cause of bagginess is the web’s response over storage time to cross-roll stress variations created by magnifying the effect of crossweb thickness variations.” This statement is basically a flow diagram of what creates most bagginess.
How to make a baggy web:
- Make a web with crossweb thickness variations and stiff properties in the thickness direction.
- Magnify the thickness variations by winding with no lateral oscillation and ensure the final roll is many times the diameter of the core.
- Wind the roll with high tension, no taper, and at low speed to avoid entrained air softening the roll tightness.
- Ensure the product will be easy to yield or flow visco-elastically by promoting high moisture in papers and elevated temperatures in films.
- Store the roll for a long time. With film, make sure to store it in a hot environment like a summer warehouse or semi-trailer.
- Unwind the roll to enjoy the bagginess. Make sure you don’t measure it.
This is the recipe to maximize bagginess. But how do you minimize it? Just reverse all of the above steps.
None of these steps alone is responsible for creating a baggy web. So reversing one step will not fix the problem. Reducing bagginess is often a long, tough project, but taking a multi-pronged attack on this baggy creation process will reap benefits.
What can you do as a converter to desensitize your machines to baggy webs?
- Increase Tension—Pull out length variations. Bagginess that is pulled out with tension usually is not a problem.
- Eliminate Nips—If tension doesn’t pull out your bagginess, you will almost assuredly see wrinkling at all your nipped processes. Nips are needed for many processes, such as coating, laminating, and calendering, but avoid nipped rollers for web handling only.
- Apply Spreader Rollers—Use spreaders judiciously, such as just upstream of nipped processes, slitting, and winding. The best spreaders for baggy webs are bowed rollers, expanding surface rollers, edge nips, and D-bars.
- Consider an Adjustable Roller—In the case of left-right bagginess, a manually skewed roller can change the side-to-side web path length and eliminate slack web and related wrinkles. I haven’t seen an automatic system do this successfully, but a well-trained operator can save the day. If you choose to use an adjustable roller, place it as close to the nip as possible, wrap it 90 deg, skew it in the plane of the web entering the nip, and have an indicator to return it to the parallel position.
- Add a Bagginess: Removing Process—This is a little dangerous, so I don’t want to advocate this too much, but some film bagginess can be greatly reduced with a heat, stretch, cool process. Why is this dangerous? You may change the characteristics of your film, such as heat stability or stiffness.
I would have loved to title this column “Eliminating Baggy Webs” or “Eliminating Bagginess-Related Waste,” but I don’t want to get your hopes up too high. I think minimizing is the best many of us can hope for.
Timothy J. Walker has 20+ years of experience in web handling processes. He specializes in web handling education, process development, and production problem solving. Contact him at 651/686-5400; email@example.com; tjwa.com.