Metering film hydrodynamics in film coating

The Film Coating technique is used for the surface treatment of fragile webs, as very little stress is applied on the paper.; This thesis deals with the understanding of the hydrodynamics of the metering nip flow, the hydrodynamic instabilities generated and the rheological behavior of coating colors in the nip. To carry out this investigation, a laboratory coater featuring industrial operating conditions and the CFD commercial software POLY2D™ were used. Industrial paper coating colors were tested, as well as Newtonian reference fluids. The instabilities that appear in the metering nip were evaluated in terms of operating conditions and coating color rheology. Moreover, three models of process viscosity were proposed, which were functions of either the maximum pressure, the torque, or the combination of the pressure gradient and the torque. Each process viscosity model was assessed by means of Computer Fluid Dynamic simulations of the metering nip flow.; Among the main findings, we observed that the instabilities are related to the process speed, coating color rheology, and mixing makedown procedure. The ribbing pattern is practically insensible to the metering and transfer speeds (at very high transfer speeds), whereas increasing the metering speed can eliminate the spitting phenomenon. Very viscous fluids generate wide ribs and the spitting appears at higher transfer speeds. In terms of the nip gap, three hydrodynamic configurations were observed, depending on the capillary, inertia, or air entrapment effects. The spitting phenomenon was found to be independent of the hydrodynamic configuration. Moreover, metering nip pressure measurements were found useful in assessing the stability of the coating flow in the nip in terms of ribbing and spitting. And finally, from temperature measurements, viscous dissipation in the nip was found to be negligible.; In summary, this investigation have shown new insights about the metering nip flow in terms of behavior, control and elimination of the instabilities generated, flow stability assessment, and the manner coating colors behave, from a rheological viewpoint, when flowing through the metering nip. (Abstract shortened by UMI.)...