'Patterning" in CMYK Printing

[impodave]

Hi,
I've been fielding remarks from the pressroom (and from the boss ) about a 'pattern" they see from time to time on certain CMYK jobs. I have told them literally for years that there are only 3 real 30 degree angles, so yellow is angled midway between 2 of the other colors, and in certain combination of dots one will see a moire pattern of sorts, especially if you angle it at the light source you are viewing with.. I've been getting blown off about this again and again.  We are presently running 150 lpi at 2400 res, using euclidean dot. The angles were set up by Fuji installer of a new XMF workflow just 4 weeks ago, but they remarked about it with the old flow, too.
     Neither of these two have any hands-on experience with screen angles, where I did CMYK blends the sticks-and-rocks way for 20+ years, but if I'm wrong, I can take it... Fire away ...... 

[Joe]

Are they talking about an actual moire or just the rosette pattern of the screens? If the latter it's just the nature of the beast. You could try different angles, a different dot shape or even try stochastic (shudder) screening. Could be that even changing from 150 to 175 lpi might help. Might make it worse too. :laugh:

[impodave]

I think its a moire resulting from interaction by the yellow screen angle with the other two colors that it sets between.  I think the pattern peaks when the dots from the colors involved are between the 40-60 percent range, but not always....

[Joe]

We use this. Haven't heard any complaints but that means absolutely nothing. :laugh:

[impodave]

I think that's where I was before and where I am now.  Except - we were running 150 lpi at 2540 rez.  Now we're running 150 lpi at 2400 rez. Fuji tech didn't ask before he set that one up..... :rolleyes:

[Joe]

I think those are the Prinergy default numbers and that is what we use for 150 lpi @ 2400 dpi and we are using Euclidean screening as well. What kind of paper are you printing on. At 150 lpi it's only coated for us. Anywhere from 35# to 60# on the web press and using 70# or 80# on the sheetfed.

[impodave]

We run just about everything - we are strictly sheetfed here. 60# coated book to 130# coated cover.  50# uncoated book to 130# uncoated cover......

[Joe]

I think I'd give 175 lpi a shot then and see if it helps.

[Santa]

We had that when we first upgraded to CTP, the called it a "cross-hatch moire"  saw it in real defined in faces.  We upgraded our screening to Nexus's Paragon (that's our front end) and it's been fine for 4 + years @ 175 lpi.  I'm sure your rip manufacturer has something that's equivalent.  Wasn't the cheapest thing back then, 3-4 thousand dollars, but it did solve the problem.

[LoganBlade]

I am going with a stupid one. Are the pictures scans of magazine picture with a dot in it already?Just going back to crazy designers with no clue. What kind of presses? I don't see why you would be running 150 with a new system if you are doing sheet fed and running decent stock. Just saying is all. My self i would go up to the 175. More silly. Are you running linear plates? did someone create curves that may be wrong causing the problem? So many issues.  :grin:

[David]

depending on the subject, there is a chance that it could be simply press mis-register due to paper stretch or shrinkage.
We have on occasion had to run the yellow as stochastic to eliminate the moire.
We have one particular client that sell glasses, and the lenses in the glasses have subtle shades of various colors in various vignetes. These guys are notorious for making a moire pattern. We run that job at 200 line, and then wait for it to get on press and see which image will moire, then fix it and run it stochastic.

[G_Town]

You could always swap the M and K angles and see if that helps.

[DigiCorn]

You could always swap the M and K angles and see if that helps.

Yeah - that can work. I've also had pressman switch the order of the plates as to what color they lay down first as well. We use the default screen angles, at 2400dpi, 175 lpi, as shown below:

[Farabomb]

We run 175 @ 2400 with the same as Joe (kodak default) and I've never had to adjust an angle. If we see it the pressman does some sort of voodoo and it's taken care of.

[Grimace]

If it is the Crosshatch Moire, try swapping the Yellow and Back angles, it's worked for me in the past.