- Part 1: Seven Common Web Spreading Systems
- Part 2: Concave Web Spreading Rollers
- Part 3: Bowed Web Spreading Rollers
- Part 4: Bent Pipe and D-Bar Web Spreading Rollers
- Part 5: Dual Spreader Rollers
- Part 6: Expander Web Spreading Rollers
- Part 7: Edge Pull Web Spreading Rollers
- Part 8: Benefits of Web Flattening without Web Spreading Rollers
As we’ve seen, Web Flattening with Web Spreading Rollers is an active process which temporarily forces a web to be wider than it would like to be. However, Web Spreading Rollers are expensive and are not always required if Web Flattening would suffice. Web flattening is a passive process which allows the web to be as wide as it would prefer to be.
Web Flattening works on the principle that the web would avoid compound curvature because it is a high energy state. The web which must first follow the arc of the roller or bar, would not like to hold a second arc as a trough or wrinkle crossing the roller or bar. As seen in Figure 10, the wrinkle crossing the roller induces compressive CD forces that would tend to push the edges outward. These forces are very weak and thus can seldom overcome the friction or traction between the web and the roller. If the friction can be reduced sufficiently, however, the trough will push itself outward until the web is flat.
Traction can be reduced by increasing the roller diameter, wrapping it lightly and decreasing the coefficient of web/roller friction. Ideally this device would be a bar, however, a roller could also be used if traction could be reduced enough to allow some but not wholesale slippage. While this technique is very effective for thicker webs, webs less than a few mils may not have enough push at a wrinkle to precipitate flattening.
In a perfect world we might not ever need spreaders. However, most machinery, products and process could use the benefits provided by spreaders which include wrinkle reduction, web flattening or opening up die gaps between slit webs. The keys to achieving these benefits are to understand why web spreading is needed, where it is needed, and how to best accomplish it.