CD variations occur whenever the web and rollers are not square to each other. This is usually from roller misalignment, but also occurs anytime a web misaligned to well-aligned rollers.

The first cause of web position shifts between rollers is the initial threading operation. When the web line is unthreaded for maintenance or after a web break, it is difficult to place the web exactly in the running position on every roller. Even small misalignments of threading can create large CD tension profiles.

CD shifting between rollers in a moving web (a.k.a. web wander) may be due to the shifted layers of an unwinding roll, sub-optimal performance of a web guide, or the web’s interaction with air flotation ovens.

The web will often CD shift from any asymmetrical equipment error. Though most rollers are fixed in their position, some rollers will move their position in the web path (e.g., dancer roller, accumulators, turret winders). If these roller alignment is not perfect through their web engagement they will likely create a CD shift. Any asymmetry in nipping roller may also create CD shifts.

Solutions

Solutions to CD tension variations from web shifting between rollers include:

  • Avoid unthreading or loss of tension in the web line.

  • Mark the running position of the web to enable re-threading in the web’s preferred running CD position on every roller.

  • Specify acceptable side wall alignment of input rolls.

  • Use automatic guiding at unwinding to correct for unwinding roll CD shifted layers. Ensure the web guide has the range and rate to correct typical input roll errors. Ensure the unwinding roll is positioned with its side wall variations within the range of its automatic web guide.

  • Tune web guide PID to avoid undamped oscillation. Ensure web guides have proper geometry and sensor placement.

  • Optimize web guide performance to correct offset and wander of incoming web.

  • Reduce air velocity. Improve uniformity of air supply and return. Improve web uniformity.

  • Avoid contact with moving components such as opening/closing nip rollers. If unavoidable, ensure alignment in all position of these rollers. Ensure pivot axes of dancer rollers and turret winders are level and square to centerline.

  • Minimize nipping variations. See more in 5 Nip Variations.

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