You would normally want to the use the highest possible line screen frequency for printing, because higher line screens increase detail resolution. However, there are factors that may restrict the upper limit of line screen frequency. The primary factor is the anilox roll. FIRST specifications recommend that the line count of the anilox roll be at least four times that of the line screen used on plate. For example, a 600-line-count anilox dictates that the line screen frequency be no higher than 150. You can use this principle to troubleshoot issues that occur when you start to use Prinergy screening.
For example, if you were using 175 lpi output with your previous workflow and start using the same specification with Prinergy, you may experience printing issues with Prinergy 175 lpi output. If Prinergy is screening at 175 lpi, and you are using a 700 lpi anilox, this is at the threshold of where possible conflicts can occur between the twoscreens, because 700/4=175 lpi. To prevent problems, it may be necessary to specify a coarser line screen in Prinergy.
If you wonder why you didn't have a problem with 175 lpi output with your previous workflow, it may be because you were not actually outputting at 175 lpi. Prinergy is very exact about frequencies. You can manually enter a target lpi in Prinergy and the screening engine will calculate a line frequency that is very close to the nominal frequency specified in the output process template. Many other RIPs do not deliver exactly what is requested, and will round down, sometimes considerably. You may believe that you have been printing at 175, but your RIPs may have been outputting screens at something like 167 lpi. The reason no moiré occurred with the previous jobs was that resulting output was well below the 4:1 ratio desirable between plate lpi and anilox lpi. The actual line frequency you used previously should be easy to confirm by using a frequency tester on your existing film.
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