An article we featured in July about an experiment to determine the best color order for process-color screen printing drew an insightful response from master printer Michel Caza. Learn what he sees as the key factors for getting the most accurate and intense colors out of a four-color inkset.
By Michel Caza
For printing, we used 250-g, matte, coated paper that measured 22 x 28 in. The ink we selected was a very thixotropic formulation from Encres Dubuit called UV Mat 8 MK. Our squeegee was a triple-durometer Fimor Serilor blade with 90-65-90 shore hardness, and it was set to print at an angle of 75°. The images were printed on a semi-automatic Grafica clamshell press, model GF-3040, with a 30 x 40-in. bed and with 0.078 in. of offcontact between screen and substrate. Squeegee speed was set at approximately 13 in./sec, while the speed of the 500- micron-thick floodbar was set at half the squeegee speed. The prints were cured on a Grafica curing unit under one 120- watt/in. lamp and with a belt speed of 148 ft/min.
One of the most important parameters to note in this experiment was the single coat of emulsion. Low emulsion over mesh (EOM) is absolutely critical in four-color-process printing with UV inks. The greater the EOM, the greater the potential for dot build-up that can lead to gain and cause dots from the fourth color to fail to transfer to the substrate. Of course, this single coating, even on the 450-thread/in. mesh, resulted in a relatively high Rz. But with 150-line/in. halftones, the dots are so small that even if their shape is not very good, the imperfections are invisible to the eye. Also, the tonal curves were compensated to produce the correct results on these screens, which made Rz a less critical issue.
Another option would be to use a capillary-film stencil system, which can provide EOMs as low as 6 microns with low Rz values. The primary drawback of capillary films, however, is that they are substantially more expensive than direct emulsions. For a company like mine, where we regularly produced 40-50 screens per day measuring from 20 x 28 to 85 x 56 in., the cost of the stencil system was a very significant concern.
Seeing the big picture
Church’s article provided an objective and informative look at the influence of color order in process-color printing and its effect on the appearance of the final print. My objective here was to emphasize that color order must be viewed as just one of the many parameters involved in halftone screen printing. If any of these parameters is missing or neglected, it becomes impossible to achieve accurate, high-quality halftones. Graphics screen printing is a giant world of variables, and you must hold all those variables closely in check to be assured of a controlled, predictable workflow.
Michel Caza is the former owner of Grafi-Caza, an acclaimed printing company based in Cergy, France. A specialist in process-color printing with UV ink, Caza has won numerous awards in international printing competitions and trained scores of other award-winning printers. He is a board member of FESPA and a former president of the organization, as well as a member of the Academy of Screen Printing Technology, which he previously chaired. Caza frequently speaks at trade events around the world and has authored many technical articles and books about screen printing.
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