Viscoelastic web curl due to storage in wound rolls, TAPPI Journal July 2020

Application: When polymeric films are stored in the form of rolls, they adopt a cylindrical shape. At unwinding, this shape remains in the form of curl and creates issues with further processing. This paper proposes and compares an analytical, a numerical, and an experimental method to predict and measure curl in any viscoelastic material. 

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Author: Sheng Pan, Aurelie Azoug, and James K. Good
Viscoelastic web curl due to storage in wound rolls, TAPPI J
ABSTRACT: Winding is often the final operation in a roll-to-roll manufacturing process. Web materials, i.e., materials that are thin compared to their length, are wound into rolls because this form is the only practical means to store them. The resulting bending strains and associated stresses are large for thick webs and laminates. As many webs are viscoelastic on some time scale, bending stresses lead to creep and inhomogeneous changes in length. When the web material is unwound and cut into discrete samples, a residual curvature remains. 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. This paper describes the development and implementation of modeling and experimental tools to explore and mitigate curl in homogenous webs. Two theoretical and 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 simula-tions of winding. One experimental method is developed that directly measures the curl online by taking advantage of the anticlastic bending resulting from the curl. These methods are demonstrated for a low-density polyethylene web.

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