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June 28, 2005
When do roller grooves get to be a problem?
I've never seen where a groove is a problem because it is too small.
I have seen grooves that are too big, usually big meaning too wide, sometime meaning they are too deep.
I've never seen the math on this, but at some point a web will begin to fall into a groove. For films, this usually happens when the web thicknes is less than 0.002" for stiffer polymers or less than 0.004" for softer polymers. For these materials, I think 0.25" is definitely too wide. As you go thinner, less than 0.001" thick films, even 0.1" wide grooves may be too much. The groove sensitivity drops with lower tension and when the wrap angle is near zero.
How about really wide grooves? (continued)
I've worked with some roller where there is a section missing or where only the edges make contact. Segmented rollers can be used to tranport a web with an attached feature, such as a nozzle or hook. An edge-only contact roller may be used for narrow, stiff, low tension processing where you can touch the web outside the margins and have too many turns to make to do it all with air floating turn bars.
The limit is any case is at some point the tension differential from the supported to unsupported web creates a shear stress and internal compressive stress leading to a wrinkled web.
Much of these "rules of thumb" on grooving are from experience. If anyone knows of a good model to predict the limited of unsupported webs on grooves, I'd love to hear about it.
tjw
Posted by Tim Walker at June 28, 2005 07:23 PM
Comments
As a roller manufacturer, we are asked this type of question all the time. I agree with the film thickness as being the main factor. Deciding what the groove width, if any, should be used is the question of the day. It is my belief that we should partner with OEM's and develop some standards.
Posted by: Randy Apperson at August 22, 2005 10:09 AM
Why groove? To prevent lubrication.
When are grooves bad? When they wrinkle or catch wrinkles in the web.
The first question has been answered. The second is unanswered, to date.
There has been some good work presented at IWEB 2003 on modeling web-roller air lubrication as a function of roller (and web) surfaces. Brian Rice of Kodak present two papers at IWEB 2003 on "A Simple Model to Predict Web-to-Roller Traction". His paper outlined the calculations to determine the air lubrication of a roller surface from it groove geometry. I'm not sure 'simple' best describes the model, but it is relatively straightforward.
This model can predict lubrication. Usually, rollers are aggressively grooved to ensure no traction loss. Rarely are roller surfaces 'dialed in' to provide some lubrication, but not so much that they cause slip and scratching. Finding the right surface is a complex matter. Too little traction = scratching. Too much traction = more wrinkle holding friction. We need Goldilocks to figure out the roller surface that is 'just right'.
Concerning thin films and wrinkles falling into grooves. I haven't thought much about it, but from my experience... 0.5-1.0 mil polyesters are OK with groove width of 10-20 mils, but 100 mils is too much. 3-5 mil films have no problem on 100-200 mils wide grooves. So can we say the rule of thumb is the groove width should be less than 10-20 times the product thickness squared?
tjw
Posted by: tjwalker at August 23, 2005 12:23 PM
