Analysis of the Coating Color Runnability Considering the Effect of Dewatering Part 2: Numerical Simulation of the Complete Coating Process, 1997 Advanced Coating Fundamentals Symposium Proceedings

Paper industry has to deal with various coating defects such as bleeding, spraying or beards by increasing coating speeds. The operating conditions and the properties of the coating color have strong influence on defect formation and finally on the quality of the coated paper. Numerical simulations were carried out to investigate the influence of color properties on the formation of coating defects, and the results were compared with pilot coater runs. A major point is the modeling of the rheological behavior. The change of the coating color rheology due to the dewatering was modeled in the simulations. Furthermore, the effects of wall slip at the blade tip were considered. The complete coating process is simulated, starting with the transfer of the coating color from the application roll to the paper, followed by levelling off the excess coating color with the blade, and the formation of the final wet film downstream of the blade tip. The a priori unknown deformations of the blade and roll surface are calculated within a specially designed, custom made program. This program is linked to the CFD (Computational Fluid Dynamics) code FIDAPTM , which is used for the numerical simulations of the fluid dynamics at the blade tip and roll application. Four different coating colors were investigated. The numerical results give data on the required operating parameters such as blade loading. Calculated limits of operability agree well with runs on the pilot coater at BASF Ludwigshafen. Coating colors with strong changes of rheology due to increasing solids content and high slip velocities at the blade bevel face lead to unstable conditions at the blade tip, and therefore to a bad performance on the machine.