Novel Technique for Measuring Pressure in the Metering NIP of a MSP Process, 2000 Coating Conference Proceedings

J. Poranen, A. Niemisto, M. Kataja, J. Gron--This paper presents a novel technique for measuring the pressure profile of a thin layer of a viscous suspension (i.e. coating color) metered at high speed in nip between a soft viscoelastic roll and a smooth metal rod. The metering process is critical to be able to achieve a uniform film thickness on the transfer roll surface. Thereby, the flow dy-namics in the nip need to be controlled and there is a need for possibilities to characterize the nip dynamics. Ever increased solids contents (up to 70 % or more) also puts more demand on the geometry of the metering geometry (rod, chamber, sealing device) to successfully apply the film uniformly onto the transfer roll surface. By utilizing this novel technique for measuring the nip pressure, it is possible to optimize conditions in the geometry to maintain a certain film thickness even at high solids contents. The technique is tested in a pilot MSP (Metered Size Press) unit were the process conditions closely resemble those of an industrial coating unit. The spatial resolution of the device was found to be less than 0.5 mm. It was also found that the measuring technique does not cause disturbances to the fluid flow. The results obtained from measurements with different rod loading and transfer roll velocity gave differences in the pressure distribution. These measure-ments were performed with suspensions with differences in solids content and composition (pigment type and binder system) to change the hydrodynamical conditions in the metering nip, which were compared against changes in pressure distributions. The results showed logical changes in the nip dynamics, which were repeatable. The ex-perimental results are also compared with numerical simulations and it was found that those agree very well.