Coudray identifies environmental factors and processing concerns that you need to control in order to produce reliable, high-quality printing screens.
Heat and humidity
Certain environmental conditions in your shop can affect the useful life of a coated screen. Heat and humidity have the greatest influence on this time frame. But when you combine these factors with the amount of fine detail you want to maintain in an image, you may find that you only have a short time in which to put the screen to use. At the lower end of what can be printed, which is determined by the mesh threads blocking the openings, the time can be measured in hours. But with halftones above 100 lpi, these conditions can greatly shorten the life of the screen.
The addition of heat can speed up most chemical reactions in a screen-printing shop. This is certainly the case with photo emulsions. When the temperature rises above about 85°F (29.4°C), the sensitizer becomes very active and looks for something to grab onto. This could be either a water molecule or an oxygen-hydrogen (OH) group in the emulsion resin. In either case, that sensitizer molecule no longer is available for the photo-imaging process, because it has reacted with something that interferes with image resolution. And if the sensitizer has reacted with a resin molecule, it no longer will wash out; it becomes insoluble. Essentially, detail is lost when the emulsion prematurely exposes itself.
A lack of proper air flow across coated screens usually is to blame for excess humidity. It is crucial to keep large volumes of air moving across the emulsion’s surface, because water evaporates from the emulsion as it dries. The water vapor is heavier than air, so the evaporating moisture quickly forms a saturated laminar layer on the top side of the screen that effectively blocks further evaporation. On the bottom side of the screen, the heavier water vapor sinks into the laminar layer of the screen below and makes it even harder for evaporating moisture to escape.
The remedy is simple. Circulating air over the screen surface can easily be accomplished with some 20-in. box fans. They cost less than $20.00 each, and they’re highly effective. Equip each fan with a 20-in. furnace filter on the intake side to minimize the amount of particulates in the large volumes of air you create. This method allows your screens to dry quickly and completely, but I have a word of caution: The coated screens will only dry to the room’s level of ambient humidity. Keep the relative humidity between 40-60% for best results. You can measure both temperature and humidity with an inexpensive instrument from Radio Shack. Models that measure both minimum and maximum temperature and humidity over a 24-hr period are available for less than $30.00.
Did you enjoy this article? Click here to subscribe to the magazine.