Forces During Forward Roll Coating and Printing of Tensioned Webs

Film splitting forces, often called tack, may lead to defects while coating or printing of a web if these forces exceed the strength of the web. These defects include linting or picking: the removal of material from the surface of the web. Literature lacks an investigation regarding the effects of web tension and takeoff angle on tack development, on account that tack pressure prediction has not been a focus of past work. A novel method was developed to measure the force to pull a tensioned web from a fluid layer on a flat solid surface. The pressure developed in a nip with a tensioned web is also reported. Models are developed to predict the web deflection and pressure distribution in the fluid layer for both of these experiments under a wide range of conditions including different web takeoff angles, tensions, fluid rheologies, loadings, and speeds. Corresponding models predict different flow rates and web deflections. Two force peaks are found when measuring the force to separate a web and fluid from a surface. The first is caused by web being restrained to move due to a thin air layer between the web and the bottom support. The second is the peeling of the web from the moving surface. The model, based on viscous flow, over predicts results for larger fluid patches, but gives excellent predictions for short patches: the difference must be linked to three dimensional flows. A method was formulated to correlate tack pressure in terms of dimensionless groups. The correlation gives good results over a wide range of parameters, including data reported by other research groups. When a tensioned web exits a nip, two regions of sub-ambient pressure may exist: the first corresponds to the expanding gap at the exit of a nip, while the last corresponds to subsequent peeling of the web from the inked roll. The developed correlations and models describe the tack pressure well. Picking and tail-edge picking intensity are correlated to parameters affecting tack pressure