The ability to quickly and effectively dry screens impacts productivity and print quality. Of course, it can often create a bottleneck in the process if not completed quickly enough to meet screen demand in production. Shortcuts taken to rush the screen to press often lead to improperly dried screens being exposed, ultimately leading to a whole host of problems on press including pinholes, stencil breakdown, difficulty reclaiming, poor edge definition.
Consequently, the so-called shortcut actually becomes a costly long delay.
So what are the optimal conditions for drying screens? There are three issues that interact to create the ability of an environment to dry screens: temperature, percent relative humidity and air flow.
Progressively warmer air will absorb and carry more moisture than cooler air. Relative humidity is the ratio of the actual amount of moisture in the atmosphere to the amount of moisture the atmosphere can hold. Air that is drier, i.e. lower percent relative humidity, can absorb a greater amount of moisture before becoming saturated because it has less moisture to begin with. Temperature and relative humidity are linked. As temperature increases, its ability to hold more water (humidity) increases. So as the temperature in a room goes up, the relative humidity will go down (with all else being equal).
Once these two conditions are in place, good airflow can speed things along by replacing the moisture-saturated air with drier air that can pull more moisture from the material being dried. Effective airflow will exchange the air that is loaded with water from the drying/evaporating process with fresh dry air across the drying surfaces.
Based on these principles, here are some suggestions to improve drying conditions for screens.
· Increase air temperature with a heater. Evenness of the heat is also important. The heater must be directed properly inside the cabinet or drying space to minimize hot spots and wide temperature swings. Do not exceed temperatures of 105°F (40°C).
· Install a dehumidifier in the drying area. At a relative humidity of 100%, the air can hold no more water. Once the maximum drying temperature is reached, the only way to reduce relative humidity is condense it out with a dehumidifier. This small investment should be standard equipment in every screen room.
· Don't use a dirt-caked fan directed at the screen to create airflow. In this case, instead of airflow, you create a dirt blower that will create pinholes in great quantities. In addition, unless a fan is set up to move new air in to replace the 'wet' air, it will just blow the same humid air around which will do little to improve drying times. Creating the proper airflow conditions can be much more effective than just increasing temperature or reducing humidity to dry the screens.
Controlling these three factors is best done with an enclosed drying cabinet, whether purchased or home made. A good cabinet will have an accurate thermostat, with tolerances of no more than plus or minus 5° F (2.8° C) to control the heat. Additionally, enclosed cabinets need a good airflow system. If airflow is not sufficient, the moisture-laden air is simply trapped inside the enclosure and the screens will not dry. The intake on such cabinets should draw dry air in and an exhaust should move the wet air out of the space. Drying cabinets can also be made to filter the air circulating over the wet emulsion, reducing dust contamination.
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