Wrinkle initiation and development in heated webs on drums

Abstract

Wrinkles often occur in webs heated on drums and rollers. Anecdotal evidence from vacuum metalizers suggests that wrinkles are initiated by small grit particles or surface imperfections that lift the web off the drum. An elliptical patch of web centered on the dirt particle is lifted off the drum against the tension pressure acting to restore contact. A numerical "tent model", based on plate theory, has been formulated to predict the critical transverse direction (TD) stress at which the web spontaneously lifts off the grit, forming the wrinkle. The model iterates to find the tent shape for a given TD stress, and then uses an interval halving method to determine the critical TD stress. Results from the model will be presented. It turns out that a simpler, analytical "draped beam" model gives similar dependence on parameters but over-predicts the value.

Once a wrinkle initiates, it may grow to a size where it is more visible, but has limited length and is restrained against further growth. Further increase in compression enables it to grow by relieving the TD compressive stress in both the lifted-off region and the neighboring web still in contact with the drum. It will also extend in the machine direction (MD). In vacuum coating, wrinkling causes a loss in thermal contact with the cooled drum, the web heats up, expands and the wrinkle grows catastrophically.