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Using Manual Presses for Four-Color Graphics, Part 4

(June 2006) posted on Sun Jun 18, 2006

MacDougall explains how to print a process-color graphic using low-tech, manual equipment,

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By Andy MacDougall

He went on to say that matching a SWOP proof is almost impossible without adding gain to the highlights through the three-quarter tones and pulling back the gain in the shadow areas on the film. "This lack of understanding of dot gain and its relationship to a standard target proof is probably the single biggest contributing factor to printers pulling process inks out of the press and altering the densities on almost every job, which leads to tremendous amounts of down time," he says. "I think linear film is the beginning of the end on most screen-printing jobs."

Whatever Taylor did seemed to work. I used the TW 5000 series process inks straight from the can and, other than the yellow being brighter than the proof yellow, the inks ended up printing almost the same as the graduated color bars on the proof. Imagine if I had tested first!

I used the positives from Chromalogic to expose 380-thread/in. mesh tensioned to 21-24 Newtons and coated with Murakami GFX emulsion (double coat on both sides). The screen had been dried to 38% relative humidity for three hours prior to exposure. I used my solar exposure system (described in my article "Tools and Techniques for Economical, Environmentally Friendly Screen Printing," Screen Printing, Apr. 2004, p. 51) to expose three of the four screens. The fourth was exposed on a conventional exposure system. The sky was overcast when I used the solar exposure unit, and I wondered if this would affect my ability to reproduce fine process dots. Chris had some ideas on that, too.

"A big problem we see with regards to dot gain and linear (uncorrected) film is that when a screen printer exposes the stencil correctly in order to achieve optimal curing, they lose too much detail to accurately reproduce their target image," he says. "In an effort to deal with this situation, many screen printers try to compensate by underexposing their screens. This, in turn, leads to weak stencils that are full of pinholes and can break down on press or become susceptible to ink and solvent penetration, crosslinking with the emulsion, making reclaiming very difficult, and leaving nasty stains behind. I believe it's easier to compensate the film in the first place."


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