Effect of thickness and viscoelastic properties of roll cover on deformable roll coating flows

Roll coating is distinguished by the use of one or more gaps between rotating cylinders to meter a continuous liquid layer and to apply it to a flexible substrate. Of the two rolls that make a forward-roll coating gap, one is often covered by a layer of deformable elastomer. Liquid carried into the gap develops high enough pressure to deform the resilient roll cover. The complete understanding of the coupling between the liquid flow and roll cover deformation is vital to the optimization of this widely used and simple coating method. Most of the earlier Works on deformable roll coating analyzed the action with both the lubrication approximation and the full Navier-Stokes solution of the liquid flow, and one-dimensional elastic models of the roll cover deformation. The effect of the roll cover thickness was not explored and can explain some of the discrepancy observed from the available theoretical predictions and experimental measurements from different researchers. Moreover, rubber and rubber-like materials used as roll covers do not behave purely elastically. Their responses depend to a great extent on the stress history and the temperature of the roll cover. In this work, the flow between a rigid and a deformable rotating roll was examined by solving the complete Navier-Stokes system coupled with a plane-strain elastic and viscoelastic model of the roll cover deformation. The stress at each location of the roll cover was evaluated by an integral of the deformation along the material path of the point being analyzed. The equation system was solved by the Galerkin/finite element method. Results show how thickness of the roll cover and its viscoelastic properties affect the performance of deformable roll nips. (C) 2003 Elsevier Ltd. All rights reserved.