Ten Tips to Help Avoid Halftone Moiré
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...
When you screen print halftones, you can count on moiré. No matter how careful you are, the dreaded patterns will appear in your prints from time to time. You may go for months without experiencing moiré problems, then get hit with a rash of images in which moiré seems uncontrollable.
After printing halftones and process color for nearly 30 years, I have come to the conclusion that the problem of moiré cannot be entirely eliminated using current technology. But you can take steps that greatly reduce the occurrence of moiré in your prints. This month, I'd like to share ten practices that you can use to help keep moiré at bay.
1. Be systematic and organized.
This is the critical beginning point upon which everything else is built. If you are not systematic in your approach to printing, you will be doomed to experience moiré. Systematic means keeping track of everything. Eventually, you will be able to narrow the variables you have to observe. But in the beginning, there is no substitute for good records and organization.
If you do not have a data sheet where you can record screenmaking parameters, press settings, and other aspects of the job, make one. Keep copies of the sheet on clipboards posted in the art department, the screenroom, and at press side. When moiré occurs, record every detail about the current job parameters. The systematic organization of data will allow you to identify patterns of occurrence, and the written record will make it easy to spot similar circumstances that might be responsible for the moiré
2. Work from a constant set of values.
When producing halftone images, keep all production variables constant. Use the same set of halftone angles each time you do the same types of work. Use the same line count. Keep mesh tension within ±1-2 N/cm, and use mesh with the same mesh count and thread diameter from the same vendor. If you randomly substitute variables, it's like being lost in the forest when night is falling. Standardization is critical in helping to identify the problem areas that lead to moiré.
3. Never assume that what you have is correct.
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.
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.
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.
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.