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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


By KIWO Technical Support Department

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Direct emulsions offer screen printers mechanical endurance, solvent and water resistance, potentially high print quality, and affordability, all in one bucket. But to realize these benefits, you must have a good understanding about how to properly coat screens prior to exposure. The guidelines presented on the following pages will help you perfect your coating procedures and ensure that you get reliable, high-performance stencils every time.

 

Start with a clean screen

Proper reclaiming and degreasing are always first steps in coating a screen with emulsion. Anything left behind on the screen is a contaminant, including degreaser. Contaminants getting between the fabric and the emulsion create the potential for emulsion breakdown when the squeegee and floodbar shear across the screen during printing. You should examine your screenroom procedures carefully to make sure your reclaiming and degreasing efforts always result in pristine screens.

One of the most common places to find a printer adding contamination is during degreasing. If you use a power washer during degreasing, chances are high that the spray is not only hitting the screen, but also striking the walls of washout booth, where the water picks up contaminants you just removed before bouncing back onto the screen and re-depositing them. If you de-ink, reclaim, and degrease in the same washout booth, all the things you want to remove from the screen have the potential to end up back on the mesh after degreasing.

To avoid this blow-back from re-contaminating your screens, use a quality degreaser with foaming action and flood the screen as a final rinse with a high volume of water under low pressure (for example, use a garden hose without a nozzle). Rinse until the screen runs free of bubbles and water sheets from the screen smoothly without sagging or showing voids.

If water does not sheet from the fabric, degrease again. Should the water flow remain unimproved when you rinse again, you will need to apply a combination of haze remover and screen wash to remove printing residues still on the screen.


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