Streak Formation

Published: May 14, 2014, By Mark Miller

Streak defects are one of the most common coating defects in roll-to-roll coating. They are also very elusive! How streaks are formed and progress is a combination of the development of a particle and the presence of an area where the particle can hang-up in the process. This combination causes the fluid to move around this particle placement and form a streak on the substrate that eventually gets cured in place. The key is to eliminate or prevent the particle and eliminate or prevent the area for hang-up in the process.

For those not familiar with streaks, they are downweb lines in the coating that are not defined as flow instabilities. For those familiar with streaks, you know that they can appear in a variety of forms and for a variety of reasons. In slot die coating it is possible to have a streak occur because of agglomeration of solid particles that get trapped in the lip opening, air trapped in the process equipment, or damage in the metal flow surface of the slot die. These particles, when combined with hang-up areas such as variation in shear stress, roughened flow surfaces, or flow separation can cause coating streaks. Individually, particles and hang-up areas do not cause streaks, but the combination forms streaks due to fluid flow disturbances. The larger the particle the more pronounced the streak.

So what should you do to eliminate streaks? Preventive medicine is always best – filter the fluid to remove particles, properly mix the fluid, and remove air prior to coating. Filtration should be properly sized for the particles of interest. Mixing based off of fluid type (suspension, dispersion, or solution) should be investigated and implemented. Air entrainment and entrapment should be reviewed, including making sure that the fluid flow is always “uphill” to allow for air removal that can’t occur prior to equipment setup. Because a streak may form at any point where a particle is trapped in a hang-up area, any narrow gap is a potential issue site. Reducing dirt prior the coating head by cleaning the substrate in-line will reduce the streaks caused by trapping the dirt between the slot die and the coating roll.

You can see why the flow surfaces of the coating equipment need to be smooth. The rougher the surface, the more opportunity there is for agglomeration or fluid flow variation. This includes the pump, piping, slot die entrance, distribution manifold, flow compression zones, distribution slot onto the substrate, and all continuous substrate surfaces. Reviewing and maintaining these surfaces as flat and clean lead to flat and clean coatings. Special attention should be paid to flow surfaces that are open to the outside atmosphere, such as the external lip faces of a slot die. These lip faces can be damaged inadvertently for many reasons. If a nick in the metal is detected, the slot die may need to be refurbished to allow for improved coating performance.

In the end, the key to removing streaks is to follow the particle of interest as it flows through the coating equipment and onto the web. If there is an area of disruption that can split fluid flow along this path, it should be investigated, improved and controlled. Follow the preventative path prescribed and you will be on your way to streak free coating!