The stencils you use for printing are only as good as the coating procedures you follow to produce them. Here you
Face coats are usually applied from the substrate side of the screen, but face coating from the squeegee side in addition to the substrate side has shown to improve print quality. In both cases, the Rz of the stencil is lowered because the coating trough plows over the high spots (mesh knuckles) and leaves emulsion only in the low spots (mesh openings). By combining base coating with face coating techniques, the printer has the opportunity to adjust the stencil for any substrate and for any type of printing.
After face coating, screens must be dried a final time. However, unlike after base coating, when screens are dried substrate-side down, face-coated screens are dried squeegee-side down.
Coating trough design
The shape of the lip of the coating trough has a great influence on the amount of emulsion that is deposited with each pass of the coater. As mentioned previously, it’s generally necessary to use a coating trough with a round edge (approximately 1⁄8 in. diameter) for the wet-on-wet base coats to have much influence on building EOM.
For example, consider the common emulsion buildup one will generally get with a round-edge coating trough on a 380-thread/in. mesh with 34-micron thread diameter. With two coats on the substrate side and one of the squeegee side, you can expect a 1-micron EOM. With two coats on both sides, the EOM would be 3 microns. And with two coats on the substrate side and three on the squeegee side, the resulting EOM is 6 microns.
Now let’s compare those results with the effects of coating the same mesh with a sharp-edge coating trough. The 2+1 coating regimen in this case would provide a 0-micron EOM, the 2+2 coating would give a 1 micron EOM, and the 2+3 approach would result in an EOM of 2 microns. For most applications, the number of coating passes that would be required to build a sufficient EOM make the use of a sharp-edge trough impractical.
A final guideline for coating trough use is to make sure there always is sufficient emulsion in the coater. The fill level is the driving force for pushing emulsion through the mesh. If this level varies from screen to screen, so will your results. When coating large screens, fill the coater each time you begin a new screen to keep the coating thickness consistent.
Build in consistency
Design your coating area so that your staff can more easily achieve consistent coating results. Your screens should all be at the same angle each time they are coated. Any change in angle will change the amount of emulsion that passes through the mesh. A coating stand works best to control the angle. All screens must also be coated at a consistent speed along their length. And the same coating speed should be used from screen to screen. Any increases or decreases in speed result in differences in the amount of emulsion deposited on the screen. Using an automatic coating machine is the best way to ensure coating consistency.
Let the emulsion flow
Effective screen coating requires you to carefully control EOM and Rz and understand what combination of coating techniques and tools is the best match for the mesh types you use and the jobs you print. As in all other areas of the screen printing process, establishing and adhering to standards in your screen-coating procedures will lead to improved quality, repeatability, greater efficiency, and lower costs.
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