- September 25, 2012, Timothy J. Walker
I recently posted my top three blind recommendations for any process. If you are a long-time reader of Web Lines, you will know why to avoid nips and use biggers cores, but if not, here is a quick refresher on those topics.
Avoid nips? Why?
Nipped rollers are a top source of wrinkles and creases. Nipped rollers have the friction and pressure needed to shift, buckle, and fold your web. Uniform nips want to feed a uniform amount length of web per revolution. If you have baggy web (web with crossweb length variations not pulled out with tension), your nip will accumulate excess material until it folds over and passes through as a wrinkle. Non-uniform nips want to feed non-uniform amounts of web per revolution. If you have good web but uneven nips (either off left to right or edge to center), you will see the nips pull all or portions of your web, leading to shifting, troughing, and possibly wrinkles.
Nips also can promote wrinkles immediately downstream of the nip point. The same problems of uneven nips and web shear that occurs on the input side also can occur on the output of the nipped rollers. Many nipped systems include a spreader roller to avoid nip infeed wrinkles, but an outfeed spreader may be needed for nipped systems with poor uneven nipping.
Uneven nipping? Do I have that? Why? Uneven nipping is created by many factors, including roller loading variations, roller diameter variations, product thickness variations, roller deflection, and roller misalignment. Having one of the nipped roller elastomer covered helps with these variations, but it may not be enough.
- How can I wind at high speed without a nip?
- How can I laminate without air bubbles without a nip?
- How can I roll coat without a nip?
Answer: You can’t. I’m not trying to eliminate nips, just avoid unnecessary nips. The largest category of unneeded nips are drive or pull rollers. In pulling a web, many people naturally think of nipping the web. This is how we grab things with our hands; we pinch our thumb to forefinger or our fingers to our palms and hold on tight. But in web handling, we have two tricks that our hands don’t have.
First, we have tension. If you are carrying a bucket of water, you don’t have to pinch the handle in your fist to hold on. You can simply form a ‘J’ with your fingers and hang the bucket on your hook-shaped hand. The friction to keep the bucket in place is created by gravity. Similarly, the web wrapped around a roller has tension pulling it down to the roller’s surface creating friction without nipping. More tension, more wrap angle, and higher coefficient of friction will all increase the no-slip condition of a web on a roller. In many, many cases, nipping can be avoided on drive or pull rollers.
Second, rollers can suck. I don’t mean they are uncool or bad news, but a drive roller can have special design where the roller surface is porous and is coupled to a fan to create negative pressure, sucking the web down to the roller independent of, but supplementary to, web tension. Your hand can’t do that (unless you are a tree frog, but I haven’t met any web handlers that are tree frogs, yet). Use of vacuum or suction drive rollers is a great way to have no slip, nip-free web handling.
Go ahead and nip where you have to: coating, laminating, embossing, winding, etc. But avoid unnecessary nips.
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; firstname.lastname@example.org; www.webhandling.com.