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Ten Tips to Help Avoid Halftone Moiré

(February 2002) posted on Sun Feb 03, 2002

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...

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By Mark A. Coudray

To determine how far you should rotate the angle set, you must consult with your prepress supplier, service bureau, or RIP software manufacturer, who can tell what angles are supported at the halftone line count you are using. The two most common ones are Y 5°, C 20°, K 50°, and M 80° or Y 7.5°, C 22.5°, K 52.5°, and M 82.5°. Contrasting colors can be printed at any of these angles. When your separations are output, remember to verify the rotated angle positions with a protractor or angle determiner.

6. Use the highest mesh-to-dot ratio.

Mesh-dot-ratio is the ratio of mesh threads to halftone dots. Over the years, I have heard all kinds of theories about what this ratio should be, most commonly 4:1 or 3.5:1. In truth, this is just another example of something you shouldn't believe.

Relationships like mesh-to-dot ratio only serve as a guide because there are so many factors that influence them. For instance, the thread count is continuously variable based on tension. The higher the tension, the fewer threads per inch that there are.

Also, the line count of a halftone is only accurate at the 50% tonal value, where one dot is fully formed and a fully formed space is present. For all other values, the area covered by the halftone dot is much less than the actual line count specified. In fact, the relationship of dot-to-mesh is continuously changing.

Your best course of action is always to select the highest possible mesh count and lowest possible halftone line count in an effort to maximize the ratio. The higher the ratio, the less chance of moiré.

7. Choose mesh with a high percentage open area and thin threads.

Here you seek to minimize interference between the halftone dot opening in the stencil and the mesh thread. The finer the thread, the less chance of interference. However, as the thread gets thinner, the ability of that mesh count to sustain higher printing tensions is reduced. Generally speaking, it is most desirable to seek finer threads (sometimes referred to as "S" thread) for any given mesh count. Some of my favorite mesh-count/thread-diameter combinations are 305 threads/in. with a 34-micron thread diameter; 380 threads/in., 30 micron thread diameter; and 420 threads/in., 27 micron thread diameter. It's also important that all of these meshes be specified as plain weave.

8. Maintain 8-12 microns emulsion-over-mesh.


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