- Part 1: Conditions of the Reverse Roll Coater
- Part 2: Two Basic Approaches to Reverse Roll Coating
- Part 3: Factors that Influence Defects in Metered Roll Coating
- Part 4: Position of the Wetting Line Affects Roll Coating Thickness
- Part 5: Controlling the Position of the Wetting Line in Reverse Roll Coating
- Part 6: Affects of Non Newtonian Flow and Viscoelasticity on Reverse Roll Coating
- Part 7: Using Hydraulic Force to control the Capillary Number and Metering Roll Ratio
- Part 8: Mechanical Design of the Reverse Roll Coater
Nip Fed Reverse Roll Coating vs. Pan Fed Reverse Roll Coating
Reverse roll coating appears in two basic forms – the nip fed and the pan fed. There are various configurations of each that have been deployed over the years, but the current popularity of two basic approaches really stands out today. The first of these is commonly described as the 3-roll nip fed reverse roll coating. The metering and applicator rolls have already been described in Part 1 of this Series. The third roll in the system is the back-up roll, used to bring the moving web into contact with the applicator roll. Technically, this roll is not required to satisfy the definition of a reverse roll coater, as the applicator could be “kiss” wiped by the web, but almost all reverse roll coaters for thin, flexible substrates do employ the back-up roll.
In the nip fed design, the metering nip is flooded with coating. A reservoir is constructed to create a pond of coating material at the inlet to the metering zone. The most accepted approach places the metering and applicator rolls at an angle as illustrated in Figure 3.
This approach allows the nip to be flooded utilizing only a minimal amount of coating and virtually all of the coating can be used up by the process at the end of the run.
The width of the coating pond is set by repositionable dams contoured to fit into the roll set. This also serves to limit the amount of coating required to properly charge the system in the event the substrate is narrower than the coating rolls.
While the advantages of the reservoir have made the nip fed a very popular approach, one negative lies with the difficulty in adequately sealing off the coating reservoir so that it does not leak excessively. As illustrated in Figure 3, the dams must seal to three surfaces, one of which may move about to establish the proper metering gap. Then, too, the dam would ideally include an extremely thin, feathered edge to extend down into center of the metering gap, yet must be soft enough not to mar the surface of the coating rolls ~ a very difficult set of requirements. Obviously, there is no such thing as the perfect dam. It is realistic to anticipate having to deal with this problem of leaks, and to recognize that low viscosity coatings will present more problems, perhaps best dealt with by going to the pan fed design. The second approach in current demand is 4-roll pan fed reverse roll coating. This is illustrated in Figure 4.
This concept employs a feed pan instead of a coating pond and associated leakage potential. This is an advantage with low viscosity coatings. In order to avoid having the applicator roll running directly in the liquid, a fourth roll, running at a reduced speed, is utilized to pick up the coating and supply the applicator roll. Coaters not utilizing this fourth roll are called 3-roll pan fed reverse roll coaters. They have generally proven to be limited to low viscosity/speed applications because of the difficulty in getting the high speed applicator roll to wet properly as it rotates through the liquid bath. The fourth roll, as well as the large volume of coating in the pan, can be avoided by supplying a slot feeding device located close enough to the applicator roll that the coating being pumped through the slot is forced onto the applicator. Note that both the fourth roll and the slot feeder produce an additional shear point, which may not be desirable for certain coatings.