- April 01, 2003, Timothy J. Walker, TJWalker & Assoc. Inc.
More than a decade ago, rapper MC Hammer made famous forever the line “U Can't Touch This.” I've worked with converters who use this phrase to describe their product. When facing “no touch” challenges, start by understanding the most common reasons “U can't touch this.”
“Wet Paint” — Topping the charts of “no-contact” webs is the freshly coated web still wet on one side. Just like we should do if we see a park bench with a “wet paint” sign, we need to wait before any roller “sits” on the fresh paint. The touch-free zone starts at the coating head and extends into the drying or curing oven. Here “no contact” is qualified as one-sided and temporary.
“How long?” is a question of time and length. The chemistry will dictate the untouchable time. Multiply this time by the line speed to get the “no-touch” length. A 30-sec drying time doesn't seem long until you want to run 600 fpm and have to install an oven the length of a football field. Converters' need for 300 ft (and more) of touch-free web handling has funded today's advanced understanding of air flotation web handling. Where once a 300-ft air flotation oven was feared for the technical challenge, now only the price causes fret.
The “wet paint” scenario requires us to address another touch-free challenge: tension control. Coating and drying differ in their tension needs, especially with films. When coating, high tension is good to deliver a taut web. In drying, high tension and high temperature will turn coated films into taffy, or worse yet, into rope.
How will we isolate the high-low tension change with single-sided contact web? Wrap angle limits prevent significant unnipped tension change. Nipping the wet web is a no-go. The preferred choice employs vacuum-assisted, driven rollers or roller sets.
“Precious” — A touch-free policy also may be insurance against undue damage. To prevent scratching, gouging, or contamination, the safety motto “the person who isn't there can't be hurt” is translated into “the web that isn't touched, can't be damaged.”
“Beware of Web” — With precious webs we are concerned about damaging the web. The reverse can be true. We may worry about the damage the web can do by shedding and cross-contaminating subsequent products or personnel. (I once toured an estrogen patch coater and felt touching the web wasn't a good idea.)
“Bumpy and Jagged” — Some webs are more “difficult to touch” than “can't be touched.” These include profiled, bumpy, sharp, abrasive, or hot webs where contact leads to wrinkles, abrasion, deformation, or breakage.
“Exposed” — Like an orchid needs sunlight, air, and attention, some web processes require exposure. Contact by rollers, belts, or carrier webs will block the web from exposure to radiation or special atmosphere. Obstructing the line of sight to a web may prevent optical inspections.
“Don't Tread on Me” — Crushable webs are vulnerable to the pressure created by a tensioned web wrapping a roller. Web compression or density increase may change the pressure drop of a filter product, the diffusion properties of a medical test strip, or the absorbency of a paper towel.
You can probably imagine other scenarios of “don't touch” webs (for example, I don't advocate licking a frozen web).
With our untouchable challenges now defined, it is time to review the equipment options, but I can't touch that without more space. Join us next month to review the no-touch qualifiers (time, sides, edges, and pressure) and review the equipment solutions to the touchy subject of no-contact web handling.
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 404/373-3771; firstname.lastname@example.org; tjwa.com.