Viscoelastic web curl due to storage in wound rolls

Abstract

While bending strains result from any web being wound at a radius of curvature into a roll, these bending strains are largest for the thicker homogeneous webs and laminates. Many webs are viscoelastic on some time scale and bending stresses will lead to creep. When the web material is unwound and cut into discrete samples, a residual curvature will remain. This curvature, called curl, is the inability for the web to lie flat at no tension. Curl is an undesirable web defect that causes loss of productivity in a subsequent web process. The goal of this research is to develop numerical and experimental tools by which process engineers can explore and mitigate machine direction curl in homogenous webs. Two numerical methods that allow the prediction of curl in a web are developed, a winding software based on bending recovery theory and the implementation of dynamic simulations of winding. One experimental method directly measures the curl online by taking advantage of the anticlastic bending resulting from the curl. All methods applied to a common isotropic LDPE web correlate well with each other and present an opportunity for process engineers to mitigate curl and its negative consequences at low time cost.