- August 22, 2011, By Mark Miller, Contributing Editor
When a fluid is being coated out of a slot die, it can be positioned in a variety of ways. This ability to position the coating head is an inherent capability of the slot die because of the closed system mode of operation.
In a typical arrangement, with the slot die coating a fluid in close proximity of a precision backing roll, we observe the fluid coming out onto the substrate and moving away from the coating head. The coating either looks clean, clear, and precise, or simply terrible.
When the coating comes out poorly, we tend to scramble for a solution; looking high and low for an answer. But have you ever looked behind the slot die? One critical location for understanding what is going on with your coating is the “coating bead” or the meniscus of fluid on the underside of the slot die.
As the web passes in between the coating head and the backing roll, the substrate carries the fluid away. However, the fluid does not always exit the coating head the same way every time.
The fluid is squeezed between the substrate and the coating head, pushing out behind the die to create a coating bead and being stretched out on the substrate as it moves away from the slot die. The coating bead can tell you a lot of what is going on in the fluid as it moves away from the coating head.
If the coating bead is too large, you will observe the fluid dripping back into an overflow pan. This phenomenon tells you that you are simply providing too much fluid to the coating head.
The opposite problem occurs when the slot die is running “dry” and the coating bead is not even present. This will show up as a skip coating without full coverage. What happens in between an over-full and an under-developed coating bead is very interesting!
Assuming the surface energy of the fluid and the surface energy of the substrate are compatible for a happy adhesion, the lack of a substantial coating bead can lead to air entrapment. The fluid/fluid interface between the liquid being coated and the air above the substrate can develop coating defects and reduce what would be a good adhesive effect. What you need to do is get up under the slot die and observe what is going on.
With a good set of eyes, a flashlight, and some patience, you will visualize some coating bead (also called a rolling bank) phenomenon that you will also recognize in long form on the coated substrate on the other side of the slot die. Stagnant areas in the coating bead may coat fine for a while, but then the liquid can degrade because of lack of movement and develop coating defects.
What do you do if the coating bead looks horrible, but you need to be at the specific set points within the coating window. Peripheral energy and equipment can help.
Similar to the ability to effect surface energy, supporting and promoting the coating bead is a balancing act between substrate, fluid, and coating head. Surface energy treatment options can help, including corona, flame, or controlling electrostatic forces.
The most common piece of equipment utilized to improve coating bead behavior is a vacuum box. This box seals against the slot die and the precision backing roll. A displacement of air and pressure differentiation cause the liquid to pin against the substrate better, and therefore reduce the effect of the air interface between the fluid and the substrate. This will improve the coating bead and reduce defects. In the opposite direction, if the coating bead is too large, then the line speed or pump needs to be altered to accommodate.
So when you setup the coating head and you are recording process conditions, make sure to take a look “under the hood” to verify that the coating bead looks solid and stable. It will be well worth your time.