- July 01, 2007, By Timothy J. Walker, TJWalker & Assoc. Inc.
After two columns on baggy webs, we know they are a problem and we know we can measure them. What do we do next? First, we need to find a way to fix it in the short term. Second, we can work on a long-term plan to prevent baggy webs from happening.
One of the hardest things to fathom about a baggy web is “Why would my web supplier send this material to me?” The answer: They didn’t know they were sending you a baggy web; it was fine before they wound it up.
In my experience touring paper, film, and foil suppliers, the sheet or web quality as the web enters the winding process usually has only a small amount of bagginess. Yes, if there are gross errors in the webmaking processes, if film heating, stretching, and cooling are uneven, if paper forming or moisture content is uneven, if foil rolling and decurling processes are uneven, then the sheet quality of the web just prior to winding may be so bad that nominal tension won’t pull the web flat. But in a vast majority of cases, this just isn’t so.
So if the web wasn’t baggy before winding but was baggy at unwinding, clearly something has happened to the web while stored within the structure of the wound roll.
We’ve found the crime scene, now we just have to figure out how it happened.
How does a web become baggy while it resides in the wound roll? A baggy web is one with non-uniform geometry in the plane of the web, essentially crossweb variations in web length.
If the product had uniform length entering the winding roll but not as it is unwound, then clearly some lanes grew longer, some lanes got shorter, or a combination of the two. If we evaluate the mechanism of growing and shrinking webs, we will identify the cause of web bagginess.
Why do webs grow or shrink? The big three variables causing dimensional change in webs are stress (force over area), temperature, and moisture. In tensile testing, a web usually reacts to stress in one of two ways. It may stretch elastically, fully recovering when the stress is removed. If a web is elongated past its yield point, it will deform permanently.
Variations in web temperature and moisture are double-edged swords. Changing a web’s temperature or moisture will create dimensional changes, but more importantly, it also will change the web’s mechanical properties and how the web reacts to stress.
Beyond stress, temperature, and moisture, we need to add the fourth dimension—time—to complete the baggy web motion picture. In many cases the stress variations within a wound roll would not yield the web if the time periods were short. But given the hours, days, or months of storage, the web has a chance to creep or relax according to the roll stresses, causing a bag-inducing dimensional change—a change that can’t be undone in the short time of unwinding and processing a web after roll storage.
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. Reducing baggy web created in wound rolls will require a combination of improved thickness control, winding geometry, processes, and conditions to minimize internal roll stress variations, and controlling material properties that affect dimensional change.
Not an easy row to hoe.
Next month: Stop the bleeding. Since eliminating baggy webs is hard, desensitizing your process to baggy webs may be critical to making a silk purse (good converted product) out of a sow’s ear (baggy web).
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; firstname.lastname@example.org; tjwa.com.