I find that many of the posts and suggestions on this thread make no sense (at least to me).
I guess that the assumption is that this is a flexo application (although I don't think that was ever stated).
The notion of offsetting screen angles by 7.5° is a strategy to avoid single channel moiré. Single channel moiré can happen with some screen angle and lpi settings at various output device resolutions. What can happen is the combination of halftone frequency, angle, and device resolution can cause extraneous pixels on individual halftone dots repeating themselves with sufficient regularity that they create a visible moiré. E.g. a single color can appear to have a moiré. This can happen with both flexo and offset as well as round, Euclidean, or elliptical dot shapes. ASAIK, it has nothing to do with the anilox screening since the anilox screen frequency is typically so much higher than the frequency of the image halftone screen it is unlikely to cause moiré (i.e. anilox screen harmonically beating with the image screen).
I don't see how putting a screened spot color on the angle of the process color with the least screen % or coverage can avoid issues like moiré. Moiré results from two (or more) repeating screen patterns harmonically beating with one another. The visibility of the moiré varies with the color of the conflicting screens and their offsets. (e.g. Yellow always creates moiré because it's only 15° away from one of the other process colors - however, because Yellow is such a light color the visibility of the moiré is reduced). The 30° angle separation between process colors (C,M, and K) reduces the visibility of the resulting moiré.
Placing a 5th color on the same angle as the angle of the process color with the least screen % or coverage just means that it will probably moiré with the underlying screen as well as any other color that's less than 30° away.
Screen angles like:
C: 7.5
M: 67.5
Y: 22.5
K: 37.5
Put the Y just 15° away from both K and C with the potential for Y/K and Y/M moiré.
Screen angles like:
Y@13%
M@30%
C@50%
K@84%
Make no sense to me. I'd love an explanation as to why these angles were selected.
97.5 is indeed the same as 7.5 (just like 90 is the same as 0) even if the dots are elliptical. The individual dot shapes won't be the same but the angles will be. In any case elliptical dots are not suitable for flexo - the specification (FIRST) is round dot (non-transforming).
If the OP's initial issue is the screened trap area being screened will result in moiré - yes it will, but who cares? The moiré will be invisible because the screened area in a trap is so tiny.
