A parametric study of offset gravure coating

A combined theoretical and experimental investigation of an offset gravure coater with a subsequent roll-to-web fluid transfer (operated at web-to-roll speed ratio, S, greater than one) is presented. Under typical operating conditions at the offset nip (applicator and gravure roll contra-rotating with the same peripheral speed) the magnitude of the speed has little effect on the metered film thickness. The fractional pick-out of fluid from different gravure cell patterns, on the other hand, is shown to be dependent on a complex set of parameters, one of which is the cell volume. An experimental study of the roll-to-web transfer region confirms that the whole of the metered film is transferred from the applicator roll to the web at S > 1 and that at high S the transfer of fluid becomes unstable with a resultant loss of coat quality, leading to a corresponding lower limit of the film thickness for the production of uniform coatings. Lubrication theory is employed to (i) model the associated equilibrium flow and (ii) to perform a stability analysis and map out operability plots. In particular, it is found that as the speed of the applicator roll is increased, the critical speed ratio for the onset of instability decreases.