- November 01, 2001, William E. Hawkins, Film Handling Solutions Ltd.
MD (machine-direction) wrinkle problems (sometimes called “tin canning”) still plague many converters and/or film suppliers. The two main contributing parameters are: (1) at least two persistent MD gauge bands must be present in the winding roll; and (2) the winder must be equipped with a lay-on or rider roll. A third parameter that is not necessary for the formation of MD wrinkles — but does mask them when films are wound open to the atmosphere — is entrapped boundary air. Rolls wound in a vacuum display MD wrinkles as they are formed.
Based on years of winding development and observation, my theory for MD wrinkle formation in light-caliper film webs is as follows:
The gauge band areas wind at higher radial pressure and carry higher winding tension than the balance of the web. These areas also carry most of the lay-on contact loading force. And the film wraps essentially are locked in position relative to the roll core at these points. The lay-on roll elastomer covering is deformed more at these contact locations than it is in the balance of the lay-on roll contact line. When the roll-covering deformation at these gauge band areas becomes sufficient in magnitude, the film web is elongated or stretched over the gauge band areas in the TD (trans-direction) as it passes under the lay-on roll (see figure).
Elongation over the gauge bands produces a slightly greater film width between the gauge band areas than is necessary to span the distance. This increased width between the gauge band areas usually is masked by boundary air that is entrapped when the film is winding open to atmosphere. Boundary air is entrapped between the gauge band areas because of the lower contact pressure from the lay-on roll.
There is normally sufficient MD tension, and subsequent radial compression toward the core, in the film wraps to force the entrapped boundary air slowly through the wrap interfaces toward the roll ends. Boundary air begins to escape immediately during roll formation and continues long after the roll has been doffed. When enough of the supporting air leaves the area between the gauge bands, the wraps begin to fold into the undulating pattern of transverse compression failure of a thin web material. This pattern is known in the trade as MD wrinkles or “tin canning.” Following are the observations that led to this theory:
- Normal MD wrinkles do not occur when there is no lay-on roll.
- Soft lay-on roll coverings are more prone to generate MD wrinkles with the same gauge bands than more firm ones. Very soft lay-on roll covers are the worst offenders on films with large gauge bands.
- Increasing lay-on roll contact pressure does not eliminate MD wrinkles. Usually this action makes them more severe.
- There are usually no MD wrinkles or evidence of MD wrinkles found in the first wraps near the core unless there is a problem with core diameter uniformity. MD wrinkles can be generated near the core by building artificial gauge bands on the core with tape.
- Two or more gauge bands must be present for MD wrinkles to occur.
- Films that have a rough surface will wind with fewer MD wrinkles than smooth films with the same gauge variation and the same winder operating settings. This is because the rougher surface films can absorb some of the gauge variation in the surface asperity.
MD wrinkles may be reduced during winding by applying a significant spreading action with the lay-on roll during roll buildup. One method that has been successful is to use a flexible bowed contact roll in conjunction with a rigid backup roll, but there are other methods in use today.
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: email@example.com.