Gap winding is used for thin, wrinkle-prone material. It also may be useful for adhesive-coated webs which wind lumpy if a nip is used. Gap winding is a method where the winding roll and a roller are kept apart with a small gap. The gap is typically as small as the machine controls can safely hold, perhaps 1/8 in., but the benefits are still there with much larger values. The principle of operation is this: on the roller flat, on the roll flat. In other words, IF you can keep the web flat on the roller, THEN it has too little room and too little time for a wrinkle to develop. However, if you send a wrinkle toward the winder you will store that wrinkle in the wound roll for all to see. Also, if you inadvertently close the gap you may kick off a wrinkle.

There are tricks to keeping the web flat. The first is to use our best spreading efforts before the windup. A concave roller or a bowed roller are two possibilities. In either case, traction is required.

The second trick is to use the flattening principle. Here, the roller that forms the gap would be lightly wrapped and slippery to the onset of slippage. Also it is important that this roller is as large in diameter as practical. Thus, we perhaps should call it a drum rather than a roller so we get in the right frame of mind. This principle of flattening is not as widely known as it should be, given that its benefits are nearly as useful as spreading.

Obviously, as the roll diameter builds, the machine will have to move either the winding roll or the roller to maintain that gap. The diameter of the roll must be accurately known. The most sophisticated and accurate means is the ratio of roller RPM to the winding spindle RPM, times the diameter of the roller. With high count encoders, the diameter can be measured to the nearest wrap. Ultrasonics is another means of diameter measurement. The machine would typically move the roll(er) by means of electric motor and gearbox.

The gap winder is a pure center winder. However, our machinery need not be so limited. A full-featured turret winder could be configured to run in a gap, centerwind, centerwind with layon, and center-surface wind mode. It takes very little to convert many turrets to include a gap-winding mode. Thus, you could use a layon roller for high speeds or thick products, and gap winding for wrinkle-prone materials.

Now the downside. Without a nip, you have no way to keep air out of the wound roll. Air brought in by the web gets between the layers. The problem is not so much air going in. It is how the air goes out. The edges are not sealed, so that air weeps out over a period of minutes to hours. As the air is removed, it is like pulling bricks out of a wall. Remove enough air/bricks and the roll/wall collapses. This is known as an air buckle. The danger of air outgassing on thin smooth webs begins at about 100 ft./min. (100 mpm). Without a nip you may have to slow the machine down or increase tension. In the first case, you'll experience reduced productivity. In the second case, increasing tension can cause defects.

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