SubscribeWidely used slot coating systems for the coating industry.
Slot coaters are widely used in the coating industry and they are employed as dies that can be operated in their so-called «bead coating mode» and their «curtain coating mode», e. g. see Figs. 1 and 2. The «bead mode» is usually used for low coating velocities and the «curtain mode» for high velocities.
When both coating modes are employed sequentially, a continuous coating process can, in principle, be established that covers the entire coating velocity range from 0 to 2000 m/min. At present, there are no coating techniques available that permit the entire required range of coating velocities to be covered.
It is shown here that slot coaters permit, in principle, this coverage. It requires, however, a good understanding of the requirements for stable coatings in both of the operating modes.
coating practice, it is known that the lower and upper limits of the abovementioned velocity range, from 0 to 2000 m/min, are somewhat dependent on the rheological properties of the fluid. The use of rheologically modified coating fluids is not described in this paper.
Work is continuing in the authors' company to extend the present work to fluids with shear thinning and to shear thickening fluid properties. The corresponding computer programs are currently being written and first computations are already available to show the non-Newtonian influences on the coating velocity ranges. Furthermore, it is known that for thin layers (of the order of 5 to 10 μm), continuous coverage of the above given velocity range is not achievable.
Experiments show that, for some coating thicknesses, a high and a low velocity range for the bead coating mode exist. In addition, it is known from various expe-rimental studies that the use of low-viscosity fluids brought advantages in some parts of the range of coating velocities and disadvantages in others. It should be mentioned that practical applications of sub-pressures behind the bead coating die seem to suggest that pressures higher than about 25-30 mbar should not be applied, i. e. they do not permit any coating with slot coaters operating in their bead mode.
This phenomenon is also not clearly understood and the reasons for this subpressure limit are unknown. Hence there is a need to look at the bead coating mode of slot coaters again, to work out some more details about its stable operating ranges.
Observations of the kind mentioned above encouraged FMP Technology GmbH to extend the previous work of Durst's group (e. g. [1], [2]) to provide answers to the still open questions in the field of slot coating.
A combined analytical/numerical approach was chosen to treat this kind of coating process theoretically. The analytical part of the theoretical treatment was kept identical with that used earlier [1-3] to treat slot coaters operating in their bead coating mode. This approach has been shown to provide the major features of bead coating and, at the same time, provides the basis for a good physical understanding of what causes the limits of slot coating in the bead mode.
More detailed investigations can be carried out with existing finite volume or finite element computer codes. However, experience with such numerical studies suggests that the refined knowledge does not provide any further deeper insight into the physical reasons of the coating limits discussed in this paper. In Section 2, the basis for the conventional considerations of the coating windows is derived and example computations are given. In Section 3, some additional computations of the coating window for slot coaters, operating in its bead coating mode, are given.
Some computations are used to explain the low and high velocity ranges in which slot coaters can operate when run in their bead coating mode. The program is also capable of treating non-Newtonian fluids that show shear thinning and shear thickening rheological properties. Such computations are in progress at the authors' company.