There are two kinds of air flotation: passive and active. Passive flotation is usually called air entrainment. It is a fact of physics that a moving web and rotating roller will bring in a tiny amount of air with them. The amount of air increases with diameter and speed but decreases with tension (and with nip during winding). We should expect then, that our large process rollers may be more problematic than the smaller idler rolls. We should expect that air entrainment might limit speeding up a line beyond a few hundred or a few thousand feet/min. However, web and roller roughness greatly affect the tolerance to air entrainment. Glossy paper and especially film are much more trouble in that regard than newsprint or nonwovens.

This hairsbreadth of passive air entrainment has both good and bad aspects. The good aspect is that it can increase the wrinkle tolerance of that roller location. Obviously, this applies only when you are at the edge of rejectable wrinkles at that spot. The bad aspects: Passive air flotation decreases the effective coefficient of friction. A certain amount of friction is required to control edge position and traction. If you are at the edge of slipping at low speeds, it would be quite easy to lose it as the line speed increases.

This small amount of air has similar effects on winding. It may make the wound roll more tolerant. We have all heard the teaching to wind as loosely as possible. Yet looseness in film rolls is largely related to the amount of air in the roll. Loose rolls have relatively more air, and tight rolls less. If, however, we want to float the web more aggressively or more reliably or at any speed, then passive flotation may not be enough. We will have to force air in to “lubricate” bars so that the web touches lightly or more often not at all.

Air-greased bars are found commonly in two applications: the oscillating bars at the top of blown-film lines, and in the turnover (X-bars) to flip the web so it can be coated or printed on both sides. The bars are usually piped from both sides and have hundreds of drilled holes over the wrapped arc. Sometimes the air is supplied by compressors, but this is expensive and wasteful. We don't want the high pressure and low volume that is the characteristic of compressors. What we want, in fact, is low pressure (tension/radius) and high volume. This is the characteristic of blowers. You can check whether an air bar is floating by bluing the surface to see if the coloring stays or is scrubbed off.

Sometimes even a light touch on a bar is unacceptable, such as with wet coatings and inks. A touchdown here would cause a rejectable defect and probably downtime as well for cleaning. To be sure to float, we would use air turns to serve as a non-touch roller function, or air-float ovens to dry. There is a balance to be achieved. Too little air will allow touchdown. Increasing air on air turns increases the risk of wrinkles but decreases the risk on air-float ovens. Increasing air on ovens is, however, expensive and decreases the edge control of a long web. This at the very least will work the next edge guide. However, in a more extreme example the web may crash into the edge of the oven. Flotation, like everything else, is gray rather than black-and-white. There is an ideal amount.

920/725-7671, drroisum@aol.com, www.roisum.com