Just like baking a cake, drying a screen requires that you follow a specific recipe.
By Mick Orr
The key to drying is that all the water contained in the emulsion must be completely evacuated from the coating. This is generally accomplished by using heat, a dehumidifier, or both. A typical coated screen generally will be dry to the touch after one hour at 70°F (21°C) and relative humidity of 50%. When the drying environment is 100°F with 20% humidity, however, a coated screen can be completely dry in as little as 15-20 min.
To take the guesswork out of drying screens, you can use an instrument--such as Saati's TQM-Aqua-Check--to measure the moisture content of the emulsion coating. But what an instrument like this can't tell is what sort of stencil quality you can expect from the coating. The quality of the final stencil is largely determined by both the drying environment and the methods you employ in screen drying.
Proper conditions for drying
Maintaining the proper drying environment means paying attention to several factors, including temperature, airflow, and lighting.
Temperature Emulsion manufacturers typically recommend drying coated screens at a temperature around 110°F (40°C) because higher temperatures can have an adverse effect on stencil performance. All diazo-based emulsions are sensitive to heat. In fact, any temperature above freezing will start the decay of the diazo sensitizer in the emulsion (this sensitizer allows the coating to cure properly during exposure). The higher the temperature, the faster the diazo sensitizer decays and the greater the likelihood of exposure problems later.
When using either direct or indirect stencil systems, it's best to work in cool or ambient conditions to minimize this decay. It's also important to keep the temperature consistent, since abrupt temperature changes can alter the dimensions of the coating and lead to such problems as edge lifting and reduced adhesion in the final stencil. The safest strategy is to always dry screens as quickly as possible after coating and avoid high temperatures.
Airflow A few years back I talked to a screenmaker who was having a great deal of trouble because his screens were taking more than three days to dry. After coating, he stacked the screens, squeegee side up, in a "drying box." This protective box was absolutely light safe and free of dust. The problem was that it was also airtight. Very little dry air could get in and very little wet air could escape, which caused the drying process to drag on for days.
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