Presstek Dimension Moire's

[tomol]

New guy here.

Last month we purchased the Presstek Dimension 225 with Momentum RIP. We're consistently getting moire's in some of the images in our files. The Presstek guys are stumped. We considered that the files might be the issue, but we tested files that printed well using our old Ripit system, and they too have moire's.

All the moire's are in imported image files, though not all imported files are problematic.

I suspect there's some kind of conflict between the way Photoshop, InDesign (orQuark) and the Rip handle the files. Any ideas?

Thanks in advance.

Nate Baker
Perfection Press, Inc.

[Cory Smith]

What Screen Angles, Resolution and LPI are you imaging at?

[systek]

Try running flexo angles. these are 7.5 degree offset from conventional screens. I believe that machine will do stochastic screens. Moires go away with stochastic screens.

C-7.5, M-67.5, Y-82.5, K-37.5

These are one set of angles there are more that work fine...

[systek]

I would hope your images are not scans from an already printed magazine...you would certainly get moires from the rescreens....

[carlossc_73]

First you have to Really check the Images, if you have tried other rip and it does the same, stop trying to get them right the images are wrong fix them and youll have great work done, as you have tested the images are the problem, but if the images were right i would try to reinstall the Rip Software on another Pc, some times the PCs are the ones that make the moire and not the rip specialy in harlequin rip.

[Lukew]

Did you fix this problem? if so what did you do to get rid of it?

We have the exact same problem. any skin tones or colour tones just made up with yellow and another colour either magenta or cyan, we are getting moire. and no its not just from re screened files..
The tech said that there is nothing you can do to get rid of it and stated the better the ctp/press is calibrated the worse the moire will appear.

Basically there will always be a point where say the 75deg magenta dot overlaps the 0deg yellow dot, and the eye picks this up as moire..
Surley there is a way around this.. I will try the screen angles that are listed above and see what happens..
If your dot gains for cmk are in range on the press but the yellow TVI is say 12 % lower then the cmk's would this exagerate the pattern????
Ok so FM screening solves this problem but can it be used on poly plates???

[OffsetGuy@presstechnician.com]

Why did you settle on a low end machine? One CTP system they made had issues they were to have fixed with the laser or some thing. Simple solution is send the files to another printer using a better CTP system and have them output plates.

Good Luck

OG

[gordo]

7.5 degree offset angles are designed to minimize single channel moiré not inter- color screen angle moiré. Yellow always has a moiré with cyan or magenta. Normally it's not visible because yellow is very light and is usually run at 108% of the frequency of the other process colors. However, if yellow gets contaminated - and as a result is darker than it should be then the moiré that is always there will become visible.
Check your actual yellow screen frequency and also check the hue of Y to make sure that it is not contaminated by K, C, or M inks.
This assumes that moiré is not built into the original separation or the original image.
Many shops run the yellow as an FM screen (around 35 micron) to eliminate yellow caused moiré. Going to a coarse FM screen for the yellow results in a dot gain similar to 175lpi AM/XM so it's easier to implement.

best gordo

[JonMartin]

We had a job that had a moire on it, 4/4 running on a single color, problem was that the yellow ink was not clean enough/pressman didn't wash up well enough. when we ran the job the next time, same exact file we just made sure that we washed up really well, and the job came out looking great.

We are running presstek plates - we started out with a Fiery Rip, it kept crashing so we upgraded to a Xitron Xenith