On the Relationship Between Curl and Cockling

Abstract

The paper concerns itself with the effect of pre-curl (induced curvature during the wet state) on cockling when paper is dried. Laboratory experiments are carried out on copy paper to identify this interplay between induced curvature and cockling. Using the uniform wavelike deformation that occurs during the intermediate stage of drying in narrow paper strips as a proxy for the extent of cockling, it is demonstrated that increased pre-curl in MD, alleviates cockling. In addition, under two contrasting drying boundary conditions of free drying on a flat table and hang drying without weights, it is seen that free drying induces more cockles than hang drying. As a corroboration of the experimental observations, numerical experiments are carried out on flat (varying tensile loads) and curved specimens (varying radii), using moisture-induced deformations together with an orthotropic elastic material model. Using the Hausdorff distance metric to compare the deformed and undeformed geometries, it is shown that, both pre-curl in MD and tensile loading in CD, alleviate cockling. The results of these experiments could be useful for aftermarket applications of paper, like printing, where pre-curl could be induced mechanically to reduce or prevent cockling.