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Taking Control of Screen Coating

(May 2009) posted on Wed May 20, 2009

The stencils you use for printing are only as good as the coating procedures you follow to produce them. Here you

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By KIWO Technical Support Department

The process of screen coating includes a base-coating procedure, a drying stage, and, for some applications, an additional face-coating procedure.

Base coating Creating the base of emulsion on the screen is typically referred to as wet-on-wet or base coating (Figure 3). (Figure Reference: Base coating fills a clean screen mesh with emulsion and is responsible for building up stencil thickness. With a round-edged coating trough, apply coats of emulsion to the substrate side of the screen until the emulsion flows completely to the squeegee side. Then turn the screen around and coat as many passes on the squeegee side as it takes to build up the required thickness on the substrate side. Finally, dry the screen with the substrate side down.) Each successive pass with the coating trough adds more emulsion to the wet emulsion already on the mesh. The important thing to be aware of is that base coating accumulates emulsion on the side of the screen opposite the coating trough.

For most mesh counts you will do all your base coating with a round-edge coater. Always start coating from the substrate side of the screen, and coat as many times from the substrate side as is necessary to cause the emulsion to visibly gloss on the squeegee side. The purpose is to push out all the air bubbles that may be trapped in the mesh openings. If left in the mesh, this air can cause print-quality defects or pinholing on press.

Note that finer mesh counts may require more coats than coarser mesh counts. Mesh regulates emulsion flow in the same way that it meters, or regulates, the amount of ink going to the substrate. As the percent open area of a mesh decreases, it restricts the flow of emulsion. For example, a 380-thread/in. mesh with a 34-micron thread diameter may require three coats from the substrate side followed by two coats from the squeegee side to build a 3-micron EOM. In contrast, a 380-thread/in. mesh with 27-micron thread diameter might have a 5-micron EOM after only one coat on the substrate side and one coat on the squeegee side. Although the mesh count is the same, the mesh with the 27-micron thread diameter has much more open area and less resistance to flow because the emulsion doesn’t have to push against so much thread.



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