Coudray identifies environmental factors and processing concerns that you need to control in order to produce reliable, high-quality printing screens.
Once you have environmental and lighting conditions figured out, the next step is screen coating. Unless you’re coating large quantities of screens, or are producing fineline halftones, you do not need an automatic coater. As long as you dry the screens horizontally, you can hand-coat screens for halftones up to 100 lpi.
When I consider the coating sequence, I look for evenness of the coating and sufficient emulsion over mesh (EOM) to form a good edge profile. EOM varies depending on level of detail, thread diameter, and ink type. I normally consider 8 microns the minimum. As you increase the EOM, you can run into problems printing highlight dots. Regardless, it is much more common to have an EOM that is too thin than too thick. An electronic thickness gauge is very helpful in measuring emulsion thickness.
To maximize your coating, I recommend using a dull (.5-1-mm radius) coater-edge profile. Use a slow stroke with even pressure. Coat the print side first with two passes, wet-on-wet. Immediately turn the screen and coat with two more wet-on-wet passes from the squeegee side. Dry with the print side down. Measure the EOM. If it is too thin, increase the number of passes on the squeegee side until you reach the level desired. The EOM is determined by the viscosity and solids content of the emulsion. Each brand is different, so test accordingly. Most of the time, you will need to apply between two and four coats to the inside of the screen. I know this sounds incredibly basic, but you would not believe how many different techniques are out there. They may work, but I know this method to be reliable. The important thing is to measure. When you take a bunch of measurements, you begin to see patterns. These patterns help you settle on the best practices necessary to obtain the ideal stencil thickness for your shop.
The last step is exposure. While calculating exposure time is relatively easy, it’s hard to figure out why the practice of using metal-halide bulbs until they fail to light is so common. This is a big mistake. Metal-halide bulbs generally are good for about 1000 hr of operation. This is about 6 mo in a busy shop. Lighting engineers have told me that a bulb loses 2-4 hr of life every time it’s lit. The more times you light the bulb, the faster it dies.
A bulb’s UV output drops quickly as it ages. The dropoff is invisible to the human eye, so it must be measured--or approximated. If you use an integrator, check the length of the exposure. When the output reaches 2/3-1/2 the exposure of a new bulb, it is time for replacement. I can’t tell you how many times I’ve been in shops where they have never changed the bulb. They figure that if it is still giving off light, it must be working. For coarse work, this may be acceptable. But replace the bulb every 6 mo if you plan to print any kind of halftone work. As you should with the emulsion and coated screens, also date the bulb when you install it.
Focus on the front end
The solutions I’ve described here are big, easy, and quick. They should help you overcome a majority of the quality problems in your screen room. You can get as specific as you want or need with these adjustments, but you just can’t ignore the theme that’s common to the all tips I’ve presented: measure and document. I know I say it a lot, but paying attention to the process, as well as the product, is the key to improving your work.
Mark A. Coudray is president of Coudray Graphic Technologies, San Luis Obispo, CA. He has served as a director of the Screenprinting and Graphic Imaging Association Int'l (SGIA) and as chairman of the Academy of Screen Printing Technology. Coudray has authored more than 70 papers and articles over the last 20 years, and he received the SGIA Swormstedt Award in 1992 and 1994. He covers electronic prepress issues monthly in Screen Printing magazine.
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