Coudray explains how following time-honored recommendations for pairing halftone line coutns can lead to undesirable printed results.
Of course, the stated thread counts will vary differently in the warp and weft directions. The warp direction of the mesh is controlled by the beam during weaving and is generally accurate. It may vary by just a few threads per inch. The weft direction (side-to-side) is more variable. The count in this direction is controlled by the looming process as the thread moves back and forth. Depending on the manufacturer, there may as much as ;5-7% variation from the stated thread count due to weaving. This is a big number--in the worst case, it would mean that a 390-threads/in. mesh could come in anywhere between 365 and 417 threads/in. Such fluctuations are unlikely, but it's important to realize that the mesh count is probably not precisely what it is stated to be and changes over the entire bolt of mesh.
Some manufacturers will provide mesh-count values at the recommended tension level. This is ideal because it represents the real threads per inch we'll get when we use the mesh in production. Unfortunately, most manufacturers will provide mesh count in the relaxed state. This is troublesome because degrees of elongation required to achieve printing tension varies with mesh brand and type. The lower the modulus of elongation, the less change in thread count that will occur.
As an example, with 305-threads/in. mesh, the actual thread count at a tension of 25 N/cm might only be 290 threads/in. or less. The higher the tension, the lower the actual thread count at the time of imaging. The more we stretch the mesh, the more we change the thread count. Add to this the variation in weaving, and the total magnitude of variation we face becomes apparent.
To address these issues, we can use the thread counters offered by various suppliers, which allow us to determine, to the exact thread, what the mesh count is. These measuring films are refined versions of the old mesh/halftone tests that have been around for years. The older versions usually were limited to 150 threads/in., but the new versions will go all the way up to 460 threads/in. This makes them highly effective in the modern screen-print shop. Check with major suppliers for thread counters, which are generally less than $100 and well worth the investment.
One last point while we are on the subject of mesh count: It is very important to remember that the actual tension and actual mesh count are dynamic and continually change during the course of a print run. Increasing off-contact distance and peel settings can change both values because these adjustments require us to stretch the mesh down to the print surface. If the peel or off-contact is not constant, and the screen-to-print distance changes, tensions and mesh counts will be sure to vary. This is a crucial concern for those printing halftones with UV inks because it can have a huge impact on color-to-color moiré at the print surface. The altered mesh count from stretching will change the frequencies of halftone dot, causing them to interfere with previously printed dots and creating a new source of moiré.
Too many variables
So where does this all leave us? Mostly in a state of constantly changing confusion. The range of tone and detail in the image determines the size of the halftone dots as well as the coverage area. We cannot do anything about this. The rotational angle of the dots effects frequency, and we can change this. The mesh count is all over the place, and constantly changing during the run. No help there.
The bottom line is that we can only guess at the halftone dot to mesh-count ratio. We can expect moiré at some point, and we'll find a way to deal with it. Relying on an artificial rule of thumb is pointless and a waste of time. The only real defense we have is to be mindful of where the variables are coming from and counterattack.
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