Proven advice for industrial applications and other situations where delivering less-than-flawless prints isn’t an option.
By Mike Young
I have seen cheap, awful-looking troughs that were machined – no, strike that, whacked – into shape by sheet metal cowboys down the street. I was immensely puzzled once when troubleshooting three particular jobs at a high-profile research company that turned sour for no apparent reason – or so I thought. After going through each step of the way meticulously, I noticed the coating trough’s end caps protruded a little beyond the coating edge itself, just enough to allow it to coat far more emulsion than it should, thereby rendering the trough useless. Perhaps the issue might have been detected earlier if they’d had an operable thickness gauge!
Screen makers have also been known to deliberately coat their screens with far too much emulsion, claiming it lasts longer and prevents on-press breakdowns – often at the request of press operators themselves. This waste is good news for suppliers, but adds no value to the printing process. Just as the mesh tension and mesh itself should be of a uniform thickness, the emulsion must be, too; otherwise, it will be all but impossible to achieve the required results. If the EOM is too thick in certain areas, particularly with finer meshes, not only will resolution suffer, but edges around the image will be hideously thick and may spread. For a smoother edge profile with fine-line printing, it is better to reduce the EOM down to perhaps 10 percent of mesh thickness as a starting point. Taking the two mesh examples from above (the 355/34 and 355/31 screen fabrics), the EOM would be around 5.6 and 4.8 microns, respectively. At this EOM, printing will be less painful and will yield a more pleasing line plateau while improving conductivity, using less emulsion and ink.
To achieve a lower EOM, it is better to consider the “wet-on-dry” coating principle, rather than the more traditional “wet-on-wet” approach for superior results. With intermediate drying times between coats, the emulsion is built up in thinner, firmer layers that produce a uniform, controlled thickness. This also improves mesh bridging to promote sharper image detail and overall resolution. Additionally, it is better to use short-length coating troughs that cover the intended image size rather than coating the whole screen needlessly for every job. The results will be more consistent, and of course, blockout is considerably cheaper than emulsion.
Secret #5: Control the Client’s Expectations
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