- July 01, 2001, William E. Hawkins, Film Handling Solutions Ltd.
Web scratches can be a vexing problem on very clear products. Because they become visible when the webs are metallized, even very tiny scratches are objectionable on many of these products.
There are many sources of scratches, but they tend to occur in the web surface, as there is relative motion between the web and its support surface. The support surface is usually a roller. The most basic scratch solution is to be sure all roll surfaces are running at web speed.
Free-turning (low-friction) bearings are essential on all idler rolls but do not ensure that the surface speeds will match at high speeds. Usually the web's frictional driving force is reduced significantly when the web speed is increased. This is because the web and roller surfaces carry boundary air as they move. And this boundary air has the ability to float the web out of contact with the roller surface.
In a few cases, web tension may be increased to where contact is regained and the roll speed will return to the web speed, though very often the tension cannot be increased enough without damaging the product. When this is the case, the small projections that extend beyond the normal surface roughness of the roller tend to scratch the web as it slides on the roll.
A textured roller surface may be used to provide a reservoir for the boundary air to move into and allow the web to contact a significantly larger area of the roller surface. And the larger contact area will provide enough driving friction to keep the roll surface at web speed. Usually this technique will allow successful operation at normal web tension.
Normal web tension for general web processing is about 10% of the web yield strength, though in special cases, processing tension may go as high as 10% of the breaking strength.
Not all textured surfaces give equal amounts of benefits. For example, shallow depressions made by shot peening the roller surface may have a limited reservoir volume, and the boundary air will still float the web away from the roller surface at higher speeds. Scratches are likely to occur when this happens.
The diamond pattern knurled surface works well for boundary air relief. This pattern provides a route for boundary air to escape to the atmosphere and keeps the air pressure nearly equal on both sides of the web as it passes around the roll. The groove depth and width must be sufficient (about 0.007 × 0.010 in.) to allow free passage of the boundary air. Knurl grooves that are cut with 21 TPI wheels work well for this application. The roll material is also important. Aluminum is a good material to use in making textured surface idler rollers.
After the surface is knurled and smoothed, it should be black anodized. Static charge that builds on the anodized surface may aid the driving friction of the web at high speeds.
Driven rolls also may scratch the web surface, even though they are very accurately speed-controlled. This is because the web may be contracting or elongating as it passes over a particular driven roll surface. Webs that carry excessive tension may be scratched by relative motion in the creep zone of the driving roller (See figure). For example, about 0.024 in./in. of relative motion is present in the creep zone of a driven roller under the following conditions. A ½-mil PET web is running at 4 PLI on the tight side and 3 PLI on the slack side. The web is wrapped 180 deg around the roll. COF between the web and the roll is 0.35.
William E. Hawkins has 30-plus years of process and equipment development in web handling, including experience on all types of converting equipment. He specializes in thin web applications. Contact him at 740/474-5840; fax: 740/474-3148; e-mail: firstname.lastname@example.org.