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An experimental investigation of roll coating phenomena

By Barry Malone


This thesis gives an account of an extensive experimental investigation of the operation of a twin roll coater. Two distinct modes of operation are identified: Classical (fullyflooded inlet) and Meniscus (ultra-starved inlet). The former has been the subject of investigation for a number of years; the latter is less well known and would appear to have escaped the attention of the Coating Community at large prior to the research work reported here being carried out.\ud \ud A detailed description of how the key features of an industrial roll coater can be reproduced using a piece of simple but well designed experimental apparatus, which\ud encapsulates all the necessary elements for an in-depth study of the flow, is presented. Various methods are used to visualise the flow. These include dye injection and novel\ud computerised particle tracking techniques, coupled with state-of-the-art image processing and High Speed Video photography.\ud \ud Experiments reveal that the flow associated with the Classical mode of operation is essentially one-dimensional throughout the nip; Meniscus coating flow, on the other\ud hand, is uniquely two-dimensional, containing large vortical structures. Also the pressure distributions are found to be quite distinct. A fully-flooded nip results in a pressure profile which exhibits a characteristic maximum and minimum, while an ultra-starved nip produces one which is linear and entirely sub-ambient.\ud \ud The transformation of the flow from one mode of operation to the other is then considered, a key feature of which is the behaviour of the upstream free surface which movesin to a minimum point and then out again as the flux is reduced, giving a non-singular result, that is, there are two non-dimensional values of the flux for each free surface position. \ud \ud Finally, the subject of instability in roll coating is addressed, for both the Classical and Meniscus regimes. A number of new instabilities were observed using High Speed\ud Video photography and tentative explanations for their occurrence are given

Publisher: School of Mechanical Engineering (Leeds)
Year: 1992
OAI identifier: oai:etheses.whiterose.ac.uk:692

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