- 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
Meeting the Requirements of the Reverse Roll Coater
The use of a coating device known as the “reverse roll coater” is widespread in the coating of various thin films onto a variety of substrates. The metering and applicating mechanisms employed by this type of coater make it adaptable to this broad usage.
To qualify as a true reverse roll coater,
two conditions must be met.
The first is the requirement for a reverse metering nip (gap). This occurs when the coating must pass between two rolls whose surfaces are travelng in opposite directions. This is illustrated in Figure 1.
It can be seen from this illustration that the thickness of the metered film will be primarily a function of the clearance or gap between the two rolls.
Reverse metering also contributes to smoothness. The opposite approach, forward roll metering, leads to a rougher surface because the coating remains as a single stream for a short distance beyond the nip before separating and following the diverging surfaces of both rolls. This is the characteristic “film split” phenomenon. Film split almost always produces a non-uniform, visually distorted coating. Also, heavy “ribbing” of the coating is frequently the result of forward roll metering. Ribbing is a commonly used term describing the nonuniform coating condition consisting of somewhat regularly spaced concentric ridges of heavier coating on the coating roller. The reverse metering action of a reverse roll coater, therefore, provides a smoother, more uniform metered film by avoiding the film split pattern and the ribbing phenomenon.
The second requirement which must be met in order to be classified as a true reverse roll coater is that of reverse application of the coating to the substrate. In other words, the metered film must be wiped onto the moving substrate by the applicator roll as its surface moves in a direction opposite to that of the web. Again, the film split that occurs when a forward roll applies coating to a moving web is avoided by the reverse wipe.
The true reverse roll coater as we define it today requires separate metering and applicating zones, but there is a variation where these two functions are performed simultaneously to produce similar results. This design is illustrated in Figure 2 and has been referred to as “between the rolls”, “direct metering” and “Levelon” coating.
In coaters of this type, the thickness and uniformity of the substrate figures into the metering equation, whereas in the true reverse roll coater, it does not. The true reverse roll coater, in contrast to many alternative coating methods, applies a pre-metered coating of uniform thickness regardless of the variations in substrate thickness. This makes the reverse roll coater a member of a limited family of coaters known as “contour” coaters, meaning that the applied coating will follow the contours of the substrate rather than producing a coated sheet of uniform thickness.
Looking at Figure 1, it can be seen that the metering roll carries a layer of coating on its surface as it leaves the metering zone, yet the roll surface is portrayed as being clean and free of any coating residue as it returns. To accomplish this, a cleaning doctor must be employed to clean the metering roll surface as it leaves the metering zone.
Naturally, this device must be positioned so that the doctored coating can be recaptured and reused, since it represents a substantial volume of coating. It must also be positioned and generally constructed to do a very thorough job of cleaning the roll. Failure to do so results in one of the most common defects found in reverse roll coating-streaks in the coated film, and can lead to other problems as well. Some coating materials are very difficult to remove completely from the surface of the metering roll. A thin film remains and subsequentially dries and builds on itself to a point where it interacts with the surface of the metered film, spoiling the smoothness.
The materials and pressures used in doctoring must be limited to those that will not scratch the surface of the metering roll, which would also cause visual defects in the coating. Therefore, the doctoring problem is truly vexing with some types of coatings.
Occasionally, problems of this nature are circumvented by setting the metering roll speed at zero, which can cause other problems, or as a last resort, reverse the direction of the metering roll and remove the doctor completely. The latter, however, no longer qualifies as true reverse roll and brings on the problems associated with forward roll metering mentioned earlier. This limits the effectiveness of such an approach.