- February 01, 2011, By Mark Miller Contributing Editor
Let's talk chatter. Not just because it's cold outside, but because it is a common defect in the coating industry.
What is chatter? For those who have not dealt with this issue, chatter is a variation in coating that looks like a solid crossweb line at regular intervals down the web.
The reason I want to talk chatter is because it is a common defect that people want to blame on the coating head. However, it typically aligns with another piece of equipment.
But if I see the fluid coming out of the coating head, and the defect starts right at the application of the fluid to the web, why can't I blame the coating station? Because the underlying reason for chatter is one of two things, and you can't see either of them.
- The first possibility is the pump delivering the fluid to the coating station. If the fluid is not being delivered at a constant rate, the output at the coating head will be apparent.
For the best pulse-free results, a coating station should be equipped with a gear, progressive cavity, or pressure pot pump. These constant feed-style pumps reduce down web variations in the coating fluid. If another style of pump is required, then dampening the pulsation can improve the coating appearance.
Also be aware that compressible tubes from the pump to the coating station can cause pulsations and coating fluctuations. Be aware of the fluid delivery system, and make this a starting point for process troubleshooting.
- The second possibility is a mechanical solution. The easiest check on this issue (which requires no downtime!) is to compare the chatter frequency to the mechanical variation. If the chatter matches the regular intervals of an up-stream piece of equipment, the answer may lie in replacing the equipment.
Mechanical variations can occur from rolls that are not in compliance, vibrations in the structure or framework, or drives that run the upstream equipment. The rolls that are not “true” will match up with the frequency of the defect, so it is good to know the diameters of the rolls in the process.
The vibrations can be monitored with a vibration analyzer, and the drives can be matched for frequency.
How Do You Solve the Problem of Chatter?
Well, the mechanical problems should have mechanical solutions (i.e., replace the bad rollers, drives, etc.). Likewise, the pumps should be replaced with non-pulsating alternatives.
However, if there is still some chatter after these adjustments have been made, you can consider a piece of stabilizing equipment. One option is a vacuum chamber at the site where the fluid interacts with the substrate. A vacuum box can stabilize the substrate/fluid interface and reduce variation.
But be careful, a vacuum box also can cause chatter! You need to make sure the box is sealed and operating properly.
The reason a vacuum box may help reduce the defect of chatter is simply that the fluid likes itself more than it likes the substrate. This interface problem also may be improved by encouraging the fluid to adhere to the substrate by pre-treating the substrate chemically or with an energy source.
I hope this has provided an example of how coating is an interactive process. For the coating defect of chatter, even though the fluid meets the substrate at the coating head, the main issue is not the coating station itself but the peripheral equipment.
So remember to keep your thoughts open and look outside of the coating station when analyzing defects. The root cause may not be obvious to the observer.
Roll-to-roll coating industry expert Mark Miller, owner of Coating Tech Service, has 14+ years of slot die coating experience and troubleshooting. Contact him at 715-456-9545; email@example.com; www.coatingtechservice.com.