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December 22, 2006
Surface Winding = Looser?
My last column in Paper, Film, and Foil Converter magazine was on how to drive a winding roll, comparing and contrasting surface and center winding. One main point I make is surface winding tends to make a looser roll.
This prompted the following advice from one of my past clients.
He said that I should be careful when I say that a surface winder winds a looser roll. He has a surface winder that can't wind a roll loose enough. The rolls are so tight on his thin film product that it magnifies the gauge bands to point where the customer rejects the product. The winder is a true surface driven winder with no drive on the core and the roll weight creates the surface winding nip pressure. (So as the roll diameter and weight increases, the surface winding nip pressure goes up.)
My reply:
But I’m still going to stick to my ‘surface winding will be less tight’ statement. If you compare winding the same roll on a pure surface winder and a pure center winder using the same winding tension and winding nip force, the surface wound roll will be looser. In surface winding, the nip load is the #1 variable to determine roll tightness for a given material and roll geometry.
Why are your surface-wound rolls extra hard? I believe it is due to excessive nip load. It sounds like the roll’s weight controls your surface winding nip load (pounds per inch of width that the roll presses against the surface driving roll). I’m guessing your surface winder has the core positioned on the top or 12:00 position of the surface drive roll. Sometime the top load surface winders will have a ‘rider roller’ that allows the surface winding nip to be set above the roll’s weight, but don’t really have an option to relieve the gravity effect or run a nip load less than the roll’s weight.
Another surface winder design loads the core from the side (the 3:00 or 9:00) position and pneumatically controls the load to the optimum value, independent of roll weight. In this design, the surface winding nip load can be less, sometime much less, than the roll’s weight.
What is too high in surface winding nip load? The first estimate of surface winding tension is the nip load multiplied by the web-web friction coefficient. So if you roll weighs 10 pounds per inch of width (say winding polyester, density of 0.05 lbs/in^3, on a 4-inch OD core to a 9-in final diameter), then the effective winding tension is 10 x 0.25 (typical PET friction coefficient) or 2.5 PLI. If this was a 3-mil film, no problem, but if you are winding 0.5 or 1 mils film, the 2.5 PLI is way too high.
Posted by Tim Walker at December 22, 2006 10:00 AM
