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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...


By Mark A. Coudray

The amount of emulsion-over-mesh (EOM) is critical for assuring a fully formed halftone dot. If the EOM is less than 8 microns, it is not possible to form a stencil edge that properly gaskets the image area during printing, so the resulting printed dot will be improperly formed. Additionally, thread eclipsing will be exaggerated and moiré will be prevalent. With EOM greater than 12 microns, capillary action will prevent finer dots from transferring to the substrate--the ink will hang in the mesh.

Higher EOMs require higher mesh tension to promote complete ink transfer. EOM measurement devices are usually magnetic thickness testers and are available from stencil suppliers.

9. Use pin registration to align positives to screens.

When transferring your halftone image from the positive to the mesh, you must ensure that the halftone angle is accurate relative to the orientation of mesh threads. This is done by maintaining physical stops for the screen and using pin registration to accurately position the positive relative to the screen. If you are simply eyeballing the positive's position, you sacrifice accuracy. By utilizing stops and pin-register systems, you are always sure of accurate image position on the screen.

10. Maintain full vacuum and correct integrated exposure.

With full vacuum, you see the formation of Newton's Rings on the positive surface. This rainbow-like texture is clearly visible when the screen and positive are under vacuum, and it indicates that the positive is in complete contact with the emulsion. Full vacuum is necessary to avoid undercutting during exposure.

The biggest danger during exposure is overexposure. This can result in halation or the undesirable scatter of light, which also leads to undercutting of the positive, as well as ragged formation of halftone dots. Additionally, overexposure can close up fine dots or change the size of the dots as they are exposed on the screen. This makes it impossible to determine at which dot or tonal percentage moiré is occurring.

Control exposure with a good light integrator. The integrator sensor should be placed on the vacuum frame, not on the lamp head. This allows for accurate compensation if the lamp-to-frame distance changes.

Conclusion

I could easily add another 10 or 15 pointers to this list, but these are the ones that address the most common and frequent sources of moiré. As a final thought, I'd like to reemphasize the importance of being consistent, systematic, and organized in your work. There are simply too many changing variables to approach halftone screen printing with a hit-or-miss attitude.
 


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