Proven advice for industrial applications and other situations where delivering less-than-flawless prints isn’t an option.
By Mike Young
Some printing operations find it beneficial to allow two days before proceeding, particularly with larger screens (anything with images larger than 20 x 30 inches). If someone objects that there are not enough screens to implement this practice, then buy more. It’s very cost-effective when compared to constantly throwing money down the drain dumping nonshippable prints, to say nothing about the expensive materials and production time lost forever.
Secret #4: Don’t Assume You Have the Right Emulsion Over Mesh Ratio
As you would expect, suppliers of automatic coating machines talk highly about their products, but I am going to take a different route on the subject – discussing the coating trough rather than the machine itself and its role in determining the emulsion over mesh ratio (EOM). A coating trough is supposed to be a no-brainer – so why is it number four in our top five? It has to do with one specific aspect of the print, an issue commonly encountered with critical applications: controlling the ink thickness for lines, fine detail, halftones/gradations, and solid areas. When using capillary film, both the EOM and Rz value (the surface roughness) are uniformly maintained, but what about screens coated with direct emulsion? How can one be sure they’re uniform, too?
To achieve a consistent ink deposition for superb image resolution, the emulsion thickness over the whole screen should be similarly uniform. But is it? Use a thickness gauge to check screens, especially the center in comparison to the outer edges and corners within the image area to see if there are unacceptable differences. Alternatively, lay coating troughs carefully on a clean light table (with a glass surface, not plastic) facing downward at the correct coating angle, then slide sheets of paper between the edges and the surface of the glass to see how much space appears.
I have seen many trough edges that are badly worn (curved in a concave fashion), and even new ones in the 10-micron-plus range, and once a whopping 19 microns. It doesn’t take imagination to realize how this can potentially jinx any kind of challenging application from the start. Be mindful, too, that screens are generally coated several times – a great way to compound unevenness with any worn edges.
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