Weeping in Blade Coating, Part III Effects Revealed by Flow Visualization, 1995 Coating Conference Proceedings

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This is the third in a series of papers describing a detailed examination of the blade coating defect known as weeping. Parts I and II reported investigations by high-speed video and still macro-photography of the process as well as scanning electron microscopy and x-ray microanalysis of the material. Weeping is a complex phenomenon. This paper reports further studies, but concentrates on the results of improved flow visualization techniques. More informative viewing angles, clearer time resolutions, and substantially greater spatial resolution were achieved. These results reveal unanticipated features of weeping and bear out the previous visualizations of large, fluid, bulbous “crowns” attached to the blade by drier appearing, finer “trunks.” The weeps acquire coating color by flow up their sides and by repeated contact with the liquid moving rapidly past on the web. This work shows that weeping is preceded at somewhat lower coating speeds by microwhiskering and often microweeping which are not visible to the naked eye, but may produce tiny “spits.” Deliberately grooved blade bevels in some cases produce worm-like, pasty extrudates. As speed is raised, crowns can be seen erupting from drier whiskers, which become the trunks of weeps, and other crowns can be seen erupting through or budding from already drying crowns. The effects of coating color composition on the speed at onset and the nature of weeping were explored systematically, and are fully described in Part IV.