Question on screen angle

shorty83

Well-known member
Hi, I have a 4cp label that has a PMS blue screen for the background. All four process colors touch the screened PMS color. The screen angles I use for the process work are:
C: 7.5
M: 67.5
Y: 22.5
K: 37.5

My question is what angle should I use for the PMS color to avoid a moire pattern? I know screen angles should be 30 degrees apart with the exception of the yellow so can I use a 97.5º angle for the PMS? I am not sure how high you can go with screen angles.

Thanks in advance!

Molly
 
Last edited:

kyle

Well-known member
97.5 is the same as 7.5 (just like 90 is the same as 0), unless the dots are not in a regular evenly-spaced grid or they're elliptical. If they're elliptical, that won't really make much of a difference in moire, and if they're not in a regular grid, it would probably make it worse. Angles over 90 are the same as that same angle modulo 90.
 

shorty83

Well-known member
The Print Guide: Choosing the right screen angle for over-printing spot colors

What percentage is the screen? If it is only touching in trapped areas I would think you'd have to worry about moire, if it is overprinting can it be reversed out?
It is a 29% screen and it is touching trapped areas and there is a black drop shadow over the screen also. It cannot be reversed out. I think I will jet use the cyan screen since it has a value of 46% and it is the least prominent. Thanks for the info!
 

dabob

Well-known member
If they just "touch" the colors at the edges and its just a trapping issue I don't think it would really be an issue. Could you show us an example of the art????
 

jackcardc

Member
We run into this all the time. Just set your Blue spot at the angle of the 4c with the least coverage. Example: Y@13% M@30% C@50% K@84%- set the Blue at the yellow angle.
 

arkay_desai

Well-known member
Hi,
Also check with adding 7.5º angel to your screen angel. This is used for older anilox in flexo. No need to add additional angel if you are printing on flexo and anilox screen rolling is 60º. Check with color %age in design and use which color %age is lessor. But why over printing on PMS color?
 

gordo

Well-known member
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.
 

jackcardc

Member
My original post did not say anything about angles. I was referring to percent of ink coverage. My ‘Example: Y@13% M@30% C@50% K@84%- set the Blue at the yellow angle.’ is not screen angles. I don’t care what angles he is running. I just meant to use the yellow angle because of the least coverage.
 

cosmo

Well-known member
does anyone still have a problem with moire? I havent seen it since the days of film.
 

dabob

Well-known member
One other option would be to take the offending spot color plate into photoshop and change it to grayscale then to a bitmap with diffusion dither - it effectively makes it a stochastic image of it and if done at a high dpi you would hardly notice the difference - I have used this in the past and the results pleased the customer . . . .:)
 

gordo

Well-known member
My original post did not say anything about angles. I was referring to percent of ink coverage. My ‘Example: Y@13% M@30% C@50% K@84%- set the Blue at the yellow angle.’ is not screen angles. I don’t care what angles he is running. I just meant to use the yellow angle because of the least coverage.
My bad. But I still wouldn't use the Y angle just because it has the least coverage for the reasons I explained in my post.
 

gordo

Well-known member
does anyone still have a problem with moire? I havent seen it since the days of film.
Today you usually see screening moiré when the yellow printer gets contaminated and as a result gets darker. Doesn't happen often though. In my experience the most common moiré today is subject moiré.
 

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