Tri-helical direct gravure coating

Abstract

Tri-helical, gravure roll coating, operated in reverse mode, is investigated via a combined experimental, theoretical and computational approach. New experiments were conducted on a pilot coating rig, designed to simulate the industrial process. Flow visualisations reveal the underlying flow structure within the roll-to-web transfer region and highlight when loss of coating stability (streaking) occurs. The latter is found to be influenced by the depth of the tri-helical grooves and the capillary number. Experiments show that as the web-to-roll speed ratio is increased, so too is fluid pickout from the grooves, although the coated film thickness may decrease. A key feature of the present investigation is the formulation of a novel complementary mathematical model. By starting with a simplified form of the coating process and progressively adding complexity a set of models are developed, first for simple zero pitch angled rectangular grooves then for grooves of arbitrary shape and groove pitch. A further extension to the model is the inclusion of a non-Newtonian model for the fluid (specifically a shear thinning power law formulation). Analysis of the application of shear thinning fluids to the moving substrate is also conducted. The base model developed is that for rectangular grooves of zero pitch, which takes the form of an analytical solution of the flow equation (a Poisson equation) along a groove. An extension to this model is made by solving the Poisson equation for non-rectangular grooves using the finite element method. Simple meniscus models were applied to make the problem tractable. Agreement between experimental data and predictions from the model is seen to be good for the range of operating conditions considered up to the onset of streaking. A final extension to the model considers grooves at non-zero pitch angles, representative of the industrial coating process. The limitations of this model, when compared to experimental data are examined and a physical explanation is postulated for the breakdown of the model at steep pitch angles. Under conditions of breakdown, the high pressures within the groove are consistent with the idea that elastohydrodynamics is an important mechanism in the transfer of fluid within the coating bead for discrete cell gravure coating