moire in fleshtones MY

[mlarmour]

Hi all,

I'm hoping someone can give me a bit of insite into moire. We had to pull a job off press today that had moire in fleshtones, obviously moire between the Y and M plates. We're going to try putting the Y plate at a finer screen than the CMK plates which I have read helps. Other ideas to help? We're currently running Y at 0 degrees, C at 15, M at 75, K at 45 degrees. Swap the angles of M and K?

My larger question though is what, if any press variables can produce moire? This job was in good register, so that doesn't appear to be it. Are there blanket/packing/roller/sequence/trapping issues which can cause or make moire worse?

Thanks,

Matthew Larmour

 

[gordo]

Re: moire in fleshtones MY

Hi Matthew,

From a fellow Victorian...Yellow always moirés. Register, or lack thereof has virtually no effect since moiré is a screen angle issue. You don't usually see the yellow moiré because it is so light in comparison to the other process colors. What usually makes the moiré visible is contamination (dirtying) of the yellow printer caused by ink travellng from unit to unit, or poor washup e.g. converting from a unit using a pms color. Virtually all modern screens run yellow at about 108% of the frequency of the other process colors to minimize moiré. Switching the M angle to K will help. You may also consider running the yellow FM at about 35 micron.

Contact me off-line if you want more detail.

best, gordo

 

[APS]

Re: moire in fleshtones MY

Hi Mathew

Most RIPs have internal setups to reduce moire by optimising angle and frequency for a specific screen set. In some cases you have an option to overide the RIP settings and use the application settings. Maybe your RIP is honouring the application angles and not applying it's own optimisation.

 

[marktonk]

Re: moire in fleshtones MY

Hi Matthew,

As Gordon stated, swapping the M and K should solve the fleshtone moire. Check also the Screen Systems your rip offers, you may have the choice of the finer yellow. Paul also brings up a good point, check that your rip settings are being utilized

Here is a link to a Screening book that provides theory on screening and suggestions for Heidelberg Screening. The theory is applicable to screening in general though:

[Heidelberg Screening Guide|http://www.heidelberg.com/wwwbinaries/bin/files/dotcom/en/prinect/expert_guide_screening_tech.pdf]


Regards,

Mark Tonkovich
Heidelberg USA
Product Manager, CtP & Proofing

 

[WharfRat]

Re: moire in fleshtones MY

You might try placing your yelo at your Cyan angle along with the cyan.

MSD

 

[imageworks]

Re: moire in fleshtones MY

If you get fleshtone moire on all jobs, you've probably got a rip issue. If it's only one job, the yellow ink is probably contaminated with magenta ink - or the hue of the yellow is too warm. Some yellow inks will get warmer as the densities are run higher on press - don't run above D 1.05, and if the job needs more yellow, add it to the plate. Try switching the yellow ink to a green-shade yellow, or a brighter yellow.

 

[mlarmour]

Re: moire in fleshtones MY

Most RIPs have internal setups to reduce moire by optimising angle and frequency for a specific screen set. In some cases you have an option to overide the RIP settings and use the application settings. Maybe your RIP is honouring the application angles and not applying it's own optimisation.

The RIP is ignoring application angles, but it's got all seps at the same ruling by default.

 

[mlarmour]

Re: moire in fleshtones MY

Here is a link to a Screening book that provides theory on screening and suggestions for Heidelberg Screening.

That's a great read, thanks for posting it.

 

[mlarmour]

Re: moire in fleshtones MY

Thanks for your comments everyone. Regarding contamination of the yellow - a YMCK sequence would be best to deal with this?

In terms of getting rid of the moiré in fleshtones, is a finer yellow screen as effective as swapping the M and K angles? Swapping the M and K angles would presumably then produce moiré between the yellow and black, which in this case might be ok, but in other situations could be another problem.

Thanks,

Matthew Larmour

 

[gordo]

Re: moire in fleshtones MY

A YMCK sequence is not suitable for sheetfed offset application for several reasons. Apart from the obvious one of messing up your on-press color response - most images carry a great deal of yellow in the separations, laying down yellow first would therefore create a severe trapping problem for the next colors in the sequence. Yellow down last, because of the ink coverage also acts a bit like a poor man's varnish. Better to simply clean out the unit completely - if a dirtied yellow is the cause. That being said, yellow down first is often used in coldset newspaper printing where it acts a bit like a paper sealant allowing the next colors down to not absorb as much into the very porous paper. You can increase the fineness of the yellow printer - but then you get higher dot gain which then means a separate curve for the yellow plate and added gray balance issues. Better to go with a coarse FM. A 35 micron FM will have about the same dot gain as a 175 lpi screen. Putting the yellow close to the black angle can work because that combination is fairly rare in images.
Typically screen angles where skin colors predominate are:
C = 15 degrees
M = 45 degrees
Y = 90 degrees
K = 75 degrees

Typically screen angles where light green colors predominate are:
C = 45 degrees
M = 75 degrees
Y = 90 degrees
K = 15 degrees

Typically screen angles where neutral grays predominate are:
C = 45 degrees
M = 15 degrees
Y = 0 degrees
K = 75 degrees

Besides checking your screen angles with a screen angle finder, I would still start by checking the hue/density of your yellow printer if you have not seen this problem before. Misregistration caused by a slightly rotated plate may affect the Y/M moiré you'd see but wouldn't cause it.

best, gordo

 

[meddington]

Re: moire in fleshtones MY

We typically ran the yellow at 60 degrees tom reduce moires in fleshtones.

 

[rich apollo]

Re: moire in fleshtones MY

What I've done in the past is run:
K = 45º
C = 75º
M = 105º
Y = 60º
We combined these angles with an increased yellow frequency (line screen). If you're running a 175lpi then the yellow would be at 200 lpi. If you're running a 150lpi the yellow would be at 175lpi.

This moves the potential for moire into the greens.

I avoid anything running at 0º or 90º - this just always seemed to be problematic. If I'm running conventional AM screening I like the K to be at 45º. This lessens the edge effects on screened type.

 

[FKarlone]

Re: moire in fleshtones MY

Matt,


Red on 45 should solve Your problem!

 

[vikram]

Re: moire in fleshtones MY

The screen angels of Black and Magenta should be swapped - Magenta at 45 and Black at 90. Finer Yellow won't work much. Higher screen ruling on all colors may help and for that matter - less gloss on paper can also help by refracting moire>.vikram