Just like baking a cake, drying a screen requires that you follow a specific recipe.
By Mick Orr
Before you can consider drying procedures, you must first understand how the emulsion coating itself impacts the screenmaking process. Virtually all mesh and emulsion manufacturers recommend that the emulsion thickness on the print side of a screen be approximately 10-20% of the mesh thickness to achieve a good stencil. This percentage is called the emulsion-over-mesh ratio, or simply EOM. When a stencil is too thick, it may not dry all the way through and will yield a weak stencil that is prone to premature breakdown. Another problem with emulsion that has been coated too thickly is that it has a tendency to drip, usually onto the coated screens drying below it.
Screen orientation during drying
I have seen many screen shops over the years that do a good job at coating, only to fail miserably at drying the screen. All emulsion manufacturers recommend that the coated screen be dried horizontally, squeegee side up, so that the emulsion will level out on the print side of the screen and be thicker than on the squeegee side. This is a result of gravity pulling the emulsion to the print side of the screen. Since the emulsion will be in front of the mesh during exposure, you will have less mesh interference and less chance of mesh marks, sawtoothing, and other defects in your prints.
A few screen-printing shops use screens so large that they are impossible to dry in a flat or horizontal orientation. Instead, they must be dried vertically. Since gravity can't help produce a thicker stencil on the print side of the screen, that side usually must be recoated one or more times. This will produce the desired EOM.
For drying, such large-format screens are typically leaned against a wall in a light-safe room, and a fan is used to move dry air toward the screen and accelerate the drying process. Some shops take this a step further and build separate rooms that are heated and climate controlled to dry the screens even more quickly.
Drying coated screens
When is a coated screen dry? Is it dry after 10 min, 1 hr, 1 day, a week? The answer is simple: A screen is dry when the emulsion has turned completely from a liquid into a solid. The rate at which this transformation takes place depends on environmental conditions (temperature and humidity), as well as the coating thickness.
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