- July 16, 2013
Here's a question over which many of you may have labored many hours to find an answer:
Is there any new technology or wisdom for the elimination of boundary layer air in winding rolls of film, especially regarding winding lay-on roller positioning and durometer?
Your comments are welcome in response to my suggested answers:
The lay-on roller at winding is pretty much the only option to eliminate the boundary layer at winding (unless you decide to wind in a vacuum chamber). Coating and extrusion technology use vacuum boxes or electrostatic pinning in some cases, but these are usually weak effects in web winding.
What's my experience and advice with durometer and positioning? Positioning is the easier of these two.
The first choice is to have the winding nip control the web's initial contact to the winding roll. This means the web wraps the winding nip roller at least a few degrees prior to contacting the winding roll. The two most common wraps on winding nips are nearly-tangential (1-10 deg) or 180-deg. These options keep the web tension perpendicular to the winding nip load, making the nip load independent of tension and keeping high tension from opening the winding nip.
Besides air management, the winding nip is helpful in preventing web shifting and wrinkling from the lumpy (non-cylindrical) winding roll pulling on a long free entering span. Free span winding is a bad idea for wrinkle sensitive webs. In nipped winding, if the web is placed on the winding nip roller in position, wrinkle free, then this condition will be passed on to the winding roll.
The other winding nip position option is 'late'–meaning after the web contact point. Some people have good luck with this. I feel the air management function of a winding nip must be diminished since air rejection now has a more tortuous path to avoid passing through the nip roller, but I've also had others try to convince me that the late nip is fine as air rejection. The late winding nip is common on turret winders as an auxiliary nip roller used during the turret indexing cycle. (On this point, I prefer not to see two nips used at once on any roller or winding roll. When the auxiliary nip contacts, take the other one off.)
As for durometer? I would yield to others who have kept track of this more. There are likely many answers, but I'll offer a few points. Hard nips are used on soft materials (no need for a rubber nip roller on toilet paper winding). Soft nips are used on hard materials. Hardness and softness are not just about durometer. It is really about how much the roller can conform around the process sins of diameter variations (mostly of the winding roll) and deflection. Indentation will be a function of load, durometer (really modulus), covering thickness, effective radius (a factor of both roller and roll radii), and Poisson's ratio (of the rubber).
There's one more option–dual durometer. A few companies can make a roller with two different rubber coverings. The first covering is soft, even an open foam rubber with low Poisson's effect. The second covering is thin and hard to prevent wear and be cut resistant. Dual durometer rollers will not create very high pressure, since they indent a lot, but they have some interesting characteristics. They may have little or negative added nip induced tension. They will conform around big diameter variations. More work needs to be completed to understand these rollers as winding nips.
Anyone want to contribute advice on their standards for winding nip roller coverings?
BTW: The roller pressing against a winding roll has a long list of aliases. Most swindlers have fewer assumed names: winding nip, pack roller, press roller, surface roller, ironing roller, lay-on roller, pinch roller, contact roller.