- February 01, 2004, by Timothy J. Walker TJWalker & Assoc. Inc.
Web handling is about transport and delivery, getting from point A to point B, without damaging the product. It is relatively easy to move a web across a free span or over an unnipped roller, but getting into and through a nip can be a show stopper for many processes or products. As a web handler, you should consider nips your tough customers.
Nips are the most threatening point in a process for man or web. A nip's power to crush and pull endangers body parts. The nature of a nip's threat to a web is different. Nips are intended to pressure and pull a web. The source of most nip-web problems is cross-web variations in either the nip or the web. Process nips are used to meter material uniformity. In extrusion or coating, the nip is expected to redirect and smooth out the fluid polymer or solution to a uniform cross-web thickness and flow rate. This assumes uniform nip geometry and pressure and the uniform web.
Uniformity is like perfection: always desired but seldom delivered. Once again, we have the recurring theme in web handling of dealing with imperfection (a.k.a. making the silk purse from the sow's ear). By following a few tips, you can optimize your web-nip interactions:
Minimize nip pressures
Excessive forces lead to deflection and wear. Determine what is needed for the process. Don't assume that more is better; usually, it is not.
Increase nip system stiffness
Start with sufficient roller diameters, shell thicknesses, and material strengths. Continue good stiffness into the nip loading structure, framework, and loading system.
Use tension to create a uniform web
Baggy webs and nips don't play well together. Use enough tension to pull the bagginess out of the web. The metering nature of a nip wants to feed a uniform length of web over all cross-web lanes. Any untensioned baggy lanes will accumulate any excess length upstream of the nip until it folds over and passes through as a wrinkle.
Avoid 0-deg entry angles to reduce wrinkles
Even a small wrap angle of 5-10 deg will create a shape-stiffening benefit to prevent tracking and shear wrinkles.
Avoid 90-deg wrap angles to avoid wrinkles
End-loaded nip deflections can turn a cylinder into a banana. Wrapping a nip roller with near zero or 180 deg of wrap orients the entry span perpendicular to any deflections, avoiding “negative bow” tracking wrinkles.
Make nip load independent of web tension
Most nip systems move a nipping roller into a fixed roller. The nip pressure is created by controlling the two rollers' relative position through load or engagement. If you are working with a single web, avoid wrapping the loading roller. Engagement or negative gap controlled nips are insensitive to tension and wrap angles. For load-controlled nips, use low or 180-deg wrap angles to make nip load independent of tension.
Minimize entry span length to reduce wrinkles
Free web spans are controlled by the downstream roller. Nips are notorious for subtle non-uniformities that create tracking or shear wrinkles. Both of these wrinkle mechanisms are less likely with shorter spans.
Adjustable rollers can save the day
When high web stiffness prevents pulling out web bagginess with web tension, consider installing an adjustable roller upstream of the nip. Though a skewing roller can compensate only for left-right variations, this tool can be a web-saver, getting bad webs to run wrinkle-free through a nip. If you use an adjustable roller, make sure to include a position indicator, allowing you to move back to a trammed and level position.
Keep away from nips
No discussion of nip points is complete without reviewing safety. Coating, laminating, winding, and slitting nips are all hazardous pinch points and should be guarded to prevent human access. There are standard OSHA guidelines to nip guarding based on access slot gaps and their distance from the nip point.
Don't use nips
The nip that isn't there can't damage the web. If at all possible, design a process that doesn't require nips. For wrinkle-sensitive webs, tensioning nips should be replaced with S-wrap or vacuum pull rollers.
Nips are required for many processes, but they have unintended potential for web handling disasters. Follow these nip tips for successful web handling, not web mangling.
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; email@example.com; tjwa.com