Checker Pattern on Spot color

[generalbatzorig]

I am printing an art book with spot colors. Yesterday we started the printing, but there was a problem with the spot colors.



The Blue areas in the artwork use Spot color + Cyan(60-80%) + Black(3-10%). The application of the spot color is not flat or 100% solid, but it is continuous tone 60-90% coverage as the artwork has visible brush effects. In the areas (Blue) where it is covered with 3-10% black, a checker pattern effect is appearing on the print. This pattern is not there when I see the plates. But somehow this pattern shows up on the press somewhere from the plate to paper process. I don't know what is causing this pattern.
The images I attached here are just the examples of the artwork and do not show the pattern that I am describing. The printing machine uses LED fast curable inks. Can any one tell me what is causing this pattern effect? This pattern is quite obtrusive and easily detectable with naked eyes. We need to solve this problem very quickly.

 

[generalbatzorig]

Sample images

Here are couple of scans from the print. You can see tiny patterns on the saddle and tails of the laying horse and on the neck of the cropped image. The patterns may be more visible to the naked eye than on the scan. Please click the image for larger view.



If you have any idea or guess, please write a reply.

 

[generalbatzorig]

Today I went to the print shop and asked for the screen angles. They said C15, M75, Y90, K45 degrees. But they don't know the angles of the 2 spot colors. I used two types of blue colors, each on different images for the most part. Their system doesn't show the angles and frequencies of the spot colors. I am sure the spot colors are printed with the same screen angle as the Black. But they didn't know it until I searched the forums and told them. I asked the printer to rotate the Black angle 15 degrees, and the Blue part didn't have the moire effect anymore and looked perfect. But moire effect appeared on the CMYK part. Then I asked them to rotate all CMYK screen angles with equal amount and leave the 2 spot angles as how they were. The reason is that they didn't know how to rotate the screen angle of the spot colors as the setting for the spot color wasn't available on their system. The supervisor of the press didn't want to rotate all CMYK angles because he is afraid that it will mess up their system. Now the only option I have is to cut out all blacks from the blue area in photoshop. This is not a desirable solution for me as this causes abrupt shifting of the black contours and leaves clumsy harsh edges (Photoshop computer generated type of effect.)
There is something called Print setting Post Script Saving inside InDesign. But I don't know how it works and whether it is compatible with their system.
It is so hard to persuade these people. They are saying that they will print all pages as they are, but will change only if the Moire is bad enough. At the same time they are urging me to make the decision right away, but don't want to try my request. It is kind of funny.

With this I have another question, If I rotate the screen angle of all CMYK colors on the same direction with equal amount of 15 degrees and leave the spot color angle the same, it is not going to hurt the image quality, is it? I set the color setting of the images on Fogra 39 CMYK+Spot.

You can find a larger scan of the print from the link below.
https://www.amazon.com/clouddrive/share?s=OhKTlnLrQIAmUA-5__H_AQ

 

[printerdan]

You are correct that it certainly is a moire pattern.
I would review the articles from Gordon Pritchard here
http://the-print-guide.blogspot.ca/

Your pre-press should consider (at least for this job) Stochastic screening (review AM Screening & FM Screening articles).

The XM Screening article has a good explanation of what to consider for this non-stochastic workflow at the end of the page.

 

[turbotom1052]

Stochastic would certainly solve your moire' pattern, but I would only consider using it with a printing company that does it regularly. There is a pretty steep learning curve to stochastic, and you wouldn't want your printer "learning" on your job. Find a printer that does it as a matter of course!!!

 

[generalbatzorig]

Thank you for recommendations. I read the print guide blogspot and contacted the administrator. He gave me some good advice that might help me in the future. In the meantime I would like to learn about CMYK color profiles. The colors are turning out muddy on this job. I am guessing that it may be the result of mismatched profile. What happens if my file color profile doesn't match the printer's profile. The printing company didn't know what profile they used so they told me that the profiles on CMYK don't make much difference. So I chose Fogra39 this time. But it is not looking good.

 

[printerdan]

The use of Spot colors throws a large hurdle into this.
Why are the spot colors used?
Have you had product produced using cmyk + 2 spots for this art book previously?
I can only surmise that the spots were used to increase the color gamut of the drawings - is a cmyk equivalent without the spots an acceptable choice? You would get more predictable results but (possibly/probably) less gamut.

 

[FSA]

that a common problem with AM screening and shows ups in 5 and 6 colour jobs go to Stochastic screening problem gone

FSA