Coating Matters: Defects | Finding the Cause

In the world of coating equipment, the coating station is considered the heart of the beast. For better or worse, when the fluid comes in contact with the substrate, the coating station is where the action is. A manufacturing engineer spends a large chunk of time looking at, adjusting, measuring, and hovering around the coating station to determine how to make a coated product as fast as possible with the fewest defects.

As you learn more about coating — the interactions and the limitations of the equipment and materials — the more you will be able to directly influence defect reduction and product yield. Let's take an example:

  • A flexible web coating company has been slot die coating a polyester substrate with a 50% solids fluid at room temperature. Unfortunately, a third of the coated web is scrap due to a consistent streak being coated in the machine direction. What can we blame for the defect?

The first step is to make sure the coating head itself is in proper operating condition. This includes making sure that the front edges of the slot die are not damaged.

This is a great starting point, but where do you go from here? This is where you have to think about all the interactions.

Typically, a down-web streak is contamination of a trapped particle, so you have to go after the contamination and the gap problem. The particle can be dirt from the fluid or substrate or simply a particle that is too large for the gaps provided in the equipment setup.

Analyzing the “head” of a streak on the coated web may provide you with a particle that, under a microscope, will reveal whether it is dirt or a large fluidized particle. If it is a useful particle, the next step is to understand why the particle is not being filtered properly or why the particle gets trapped. If the particle is dirt, make sure the fluid is filtered and the substrate is cleaned.

Once particles have been eliminated as a defect causing agent, you need to review the equipment setup. Is the slot opening narrower at the site of the defect? Is the web quality or tension poor in that area?

To analyze the interaction with the web, flip the substrate by replacing the existing roll with one from a different lot. If the streak remains in the same location, then the web is not to blame.

Another quick technique is to “shim” the slot die by running some plastic shim stock into the slot opening to clean out the exit of the coating head. Once the coating defect origin is found, you can start working on the solution.

As you can see, adjustments to the coating head itself may not solve the problem. That is why this column is going to focus not just on the coating station itself but everything that influences the coating process.

Because knowing what you have to work with (the equipment, fluid, and substrate limitations) is only half the battle. Knowing how to recognize common defects, understanding the mechanical challenges, and even the manufacturing management philosophy can affect the product success.

My goal is to help manufacturing engineers and operators, the product engineers who provide the new challenge, and the process engineer who is scaling up this new product to production. “Coating Matters” is dedicated to understanding the coating station, the interactions that occur with upstream and downstream equipment, common defects and cures, and especially how to utilize this information to improve the bottom line of the business.

Editor's Note | Starting this month, you will find Mark Miller's new column, “Coating Matters,” in every issue of PFFC. Mark is a coating industry consultant and holds a Master's degree in Polymer Science and Engineering and a Bachelor's in Chemical Engineering. He's eager to hear from you about your coating matters at This email address is being protected from spambots. You need JavaScript enabled to view it..

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; This email address is being protected from spambots. You need JavaScript enabled to view it.;

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