Unpredictable and unwanted, moiré can devastate your halftone and process-color work. A collection of recommendations that you can use to make moiré go away...
This is a big one. Most printers work from the assumption that the materials they work with have accurate specifications. This is usually not the case. Mesh counts are rounded to the nearest 5 threads/in. from the woven metric equivalent. The halftone line-count value we print is only accurate when the angle of the halftone is 0° or 90°. For all other angles, there are fewer lines per inch because there are fewer dots per inch in an angled halftone grid.
Additionally, the RIPs that drive many imagesetters will override specified angle settings, as will many design programs. So it's important to verify all halftone angles with a protractor or angle determiner.
Be very wary of all the materials and tools you work with. I strongly suggest that you personally verify all values and tolerances for yourself before you move forward in adopting a set of parameters as your standard. This is especially important when you switch vendors.
4. Keep halftone angles 30° apart for contrasting colors.
The least damaging moiré pattern is the rosette pattern. This is a nondestructive moiré, which means it is pleasing to the eye. This rosette is formed when angles are located at 0, 30, and 60° relative to one another. As long as you have 30° between each of the three primary contrasting colors (cyan, magenta, and black) you will form a rosette. Yellow is a noncontrasting color and can be inserted at 15° between any of the other colors.
5. Rotate the full halftone set 4-8° from lithographic angles.
Litho printers commonly start with Yellow at 0°, followed by a 15° rotation for Cyan, Black at 45°, and Magenta at 75°. Alternately, Cyan is frequently set at 105° (really 15° 90°) and Magenta at 165° (75° 90°). Rotating these angles by 4-8° is an approach that has been used very successfully for years by both screen printers and flexographers.
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