A theoretical and experimental investigation of tri-helical gravure roll coating

• Published in: Chemical engineering science 2006-08, Vol.61 (16), p.5487-5499
• Main Authors: Hewson, R.W., Kapur, N., Gaskell, P.H.
• Format: Article
• Language: English
• Subjects: Applied sciences; Chemical engineering; Coating processes; Exact sciences and technology; Fluid mechanics; Hydrodynamics of contact apparatus; Lubrication theory; Mathematical; Modelling; Thin films
• ISSN: 0009-2509; 1873-4405
• DOI: 10.1016/j.ces.2006.04.021

Tri-helical, gravure roll coating, operated in reverse mode, is investigated via a complementary experimental and theoretical approach. Flow visualisation reveals the underlying flow structure within the roll-to-web transfer region and highlights when loss of stability, that is a propensity to streaking, occurs. The latter is found to be influenced in turn 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 pick-out from the grooves, although the coated film thickness may decrease. Extensive data are provided for coated film thickness, pick-out and meniscus location as a function of speed ratio, for different operating parameters. A key feature of the present investigation is the formulation of a novel complementary mathematical model for the process. The base flow within the grooves is modelled as a Poisson equation in conjunction with simplified approximations for the attendant free-surface boundary conditions. The resultant equation set is written in terms of finite Fourier sine transforms and unique solutions obtained by varying the flux until a consistent pressure distribution throughout the coating bead is obtained. Results from the model are found to be in generally good agreement with their experimental counter-parts, and are seen to predict and capture all the major features of the process over the operating range explored.