Finite element investigation of lamination-induced curl due to residual stresses

Abstract

Lamination is commonly used in roll-to-roll manufacturing to create a product with specific properties. During the lamination process, two or more webs are bonded in the nip contact zone between two rollers. A common defect of laminated webs is curl, that is the inability of the web to lie flat under no tension. Curl originates from residual stresses in the laminate, often resulting from strain incompatibilities between the laminae. Complex nip mechanics induce shear and normal strains in the web laminae. Strain incompatibilities and consequent curl can be avoided when the strain in the laminae are identical, which results in a complex inverse problem for operators. A robust predictive tool can avoid using a costly trial-and-error approach. We developed a finite element model of the lamination process to determine the influence of the process parameters on the resulting curl. The model consists of two web laminae and an adhesive layer laminated by a rigid roller and a rubber-covered roller and considers the influence of web tensions, angular velocity of the roller, nip load and torque applied to the rubber-covered roller, adhesive viscoelasticity, and wrap angle. Results show that the lamination model can help predict curl but needs to include a soft adhesive layer to properly represent the physics of the nip mechanics. Curl highly depends on web tensions, as expected, but also on rubber-covered roller torque and entry wrap angle. Curl moderately depends on the nip load as a secondary factor. The properties of the adhesive may impact the laminate strain depending on the lamination speed, which controls the strain rate in the nip contact zone.