Viscoelastic web curl due to storage in wound rolls

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 simulations 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.

Optimization of Thin-Film Winding Conditions Considering Viscoelastic Property and Thickness Variation and its Experimental Verification

Plastic films are produced using roll-to-roll systems, which allow the film to be wound into a roll and stored in a small space. Roll defects, however, can cause significant economic loss, and gage bands remain an open area of research. More recently, plastic films have become thinner, so we must now reconsider wrinkling and slippage, problems which depend on the in-roll stress condition. Therefore, predicting the stress condition is essential to preventing defects occurring in wound rolls. In addition, the in-roll stress will change over time as a result of viscoelastic properties. This study theoretically investigates and experimentally verifies winding condition optimization and in-roll stress in consideration of the viscoelastic property and web thickness profile at a constant rewinding tension. Results show that the predicted values are in agreement with the measured values.

Pressures on webs in wound rolls due to winding and contact

Paper, film, and metallic webs have designed surfaces. Process engineers design these surfaces to ensure they will coat or print correctly. In some cases, such as tissue, the manufactured surface is designed to provide softness. After the web is formed, care must be taken to maintain the web surface for the intended use or for subsequent processing. Web surfaces can be damaged by contact pressure, which is due to multiple sources. Tissues can suffer decreased loft and softness as a result of excessive pressure. Winding webs into rolls creates pressure on each web layer that varies with radial location. Almost all high speed winders must employ a nip roller in contact with the outer surface of the winding roll to prevent air entrainment. The nip roller, which may or may not be covered with an elastomer, induces local dynamic pressures where it contacts the winding roll that travel at the surface velocity of the winding roll. After rolls are wound, they can witness additional surface contact pressure. Often rolls are stored on flat surfaces and the dead weight of the roll induces contact pressure. In other cases, the roll may be moved by a clamp truck that employs hydraulic pressure to clamp and lift the wound roll. The objective of this paper is to demonstrate a method by which the total pressure in a web due to winding and to contact can be determined. Wound rolls of newsprint and polyester will be subjected to compression tests to verify the method.