It’s in their nature
Webs are very thin relative to their width and length, which means it takes very little compressive stress to buckle them out of a state of flatness. The flexibility that allows them to conform to rollers and folding boards and in wound rolls means they aren’t very stiff, also making them prone to buckle easily. T
They are often non-uniform, and variation in local properties means there are differences in stresses that lead to buckling. Their stiffness, thickness or even dimensional length can vary by cross-machine direction, machine-direction, or a combination of both.
If they are rough surfaced, they don’t slide or move easily on rollers, so if the web is buckled, it will not flatten out very easily on a roller but transfer downstream into the next span.
They interact with every web handling element
Webs touch rollers and of various types and other static elements along their web path and each of them can be a source of compressive stresses that create wrinkles. If a roller has a localized bump on it, for example due to contamination or a manufacturing error or damage, that creates a local speed difference that can lead to relative stresses in the web that will buckle it. Or if two successive rollers are not parallel to each other, shear stress that leads to diagonal wrinkles can result. Nip rollers with uneven loading pressure can create speed differences in the web that lead to wrinkles.
The tension used to transport webs has to be just right
Tension can have a stiffening effect on webs at the right level, making it less likely for wrinkles to occur. But too much tension can cause excessive “necking”, or width reduction, that can create waves oriented parallel to the machine direction. Tension also affects traction on rollers, which affects whether a wrinkle can persist on a roller or not.
They can change dimensions with thermal or moisture changes
Webs are subject to temperature and moistures changes or chemical changes that change their dimensions, either in localized areas or overall, and these dynamic changes can cause stresses leading to wrinkles. Some films shrink or expand with temperature depending on their polymer and how they were manufactured, and paper webs will swell with increased moisture of humidity.
Once a wrinkle occurs in a web, it often will remain without a countermeasure
If the stresses that created the wrinkle aren’t removed, or additional spreading action through a special roller like a concave roller isn’t used, the web will flow downstream maintaining the wrinkle and often get worse, for example in a dancer or accumulator where the source of the wrinkle was misaligned rollers, and these devices have many successive roller pairs.
Summary
So, for all the reasons above, wrinkling is the most likely failure mode observed in web handling systems. Imperfect webs and imperfect machines interact with materials prone to easily buckle under stress. Making web handling systems robust to wrinkles means paying attention to the web properties and their uniformity and striving to reduce variation, and designing and maintaining web handling equipment and process settings to reduce the wrinkle-creating stresses on the web takes diligence and knowledge, and because webs change and machines change over time, it also takes persistence to reduce the chances of wrinkles to stop a web converting machine or cause a product defect.