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.