Flexo Screen Angles

[meddington]

Need some input from the Flexo heads out there.

We're having reports of a moire pattern in some press work. The screening is 120 LPI round dot with the following angles: C22, M52, Y7, K82 with a 60 anilox.

A green banner ...C&Y with a K vignette (C93%, Y95%, K16-53%) is apparently showing "lines" on press. The printer is thinking its a conflict between the K and anilox angles and is recommending to swap the cyan and black angles to C82 and K22. Just wondering if there's an opinion on whether this would adequately address the issue based on nothing but the data above. That's what we have to work with as we are not on site, so I have no image to post unfortunately.

 

[mazengh]

can you switch to FM screening? I tried harlequin's HXM medium screen VERY successfully to print flexo jobs....

 

[FlexoGrunt]

Curious what is the material they are printing on? (gloss paper, matte paper, synthetic, kraft)
What is ink type? (solvent, UV, water)
What kind of press? (gear or servo driven).

Flexo presses can exhibit gear chatter which looks like banding in gradients (some people mistake it for moire). Sometimes a screen angle change can hide it a little but only a little.

What we try when we see excessive chatter:
1. Different gear on the station (some gears are attached to cyls so it would be a different cyl.)
2. Different sticky back to hold the plate on. This can make a large improvement depending on the plate material.
3. Different plate material (harder materials can be less forgiving. You sacrifice dot gain but get smoother gradients).

 

[madflexo]

1) You could try using offset angles.
2) Using a Chainlink dot shape that runs with the web direction. (this will help with a banding issue)
3) Converting the C and Y to a spot green thus preventing any moire with K.

 

[gordo]

The screening is 120 LPI round dot with the following angles: C22, M52, Y7, K82 with a 60 anilox.

• Round dot is the specification (any other dot shape and you are no longer printing to FIRST)

• 120 lpi is correct

• 60° anilox is pretty much standard for flexo

• Normally the screen angle offset is 7.5° rather than 7° - this shouldn't make a difference

• C22, M52, Y7, K82 - They've swapped the M and K angles. Typically the angles would be C22, M82, Y7, K52. The angle set they're using has the K 30° away from the anilox so I doubt they're getting any kind of moiré from a K angle anilox conflict. So changing the K to 22° shouldn't make any difference.

I don't think the problem is caused by the screening, therefore changing the screening will likely not fix it.

best gordon p

 

[TerryWyse]

Need some input from the Flexo heads out there.

We're having reports of a moire pattern in some press work. The screening is 120 LPI round dot with the following angles: C22, M52, Y7, K82 with a 60 anilox.

A green banner ...C&Y with a K vignette (C93%, Y95%, K16-53%) is apparently showing "lines" on press. The printer is thinking its a conflict between the K and anilox angles and is recommending to swap the cyan and black angles to C82 and K22. Just wondering if there's an opinion on whether this would adequately address the issue based on nothing but the data above. That's what we have to work with as we are not on site, so I have no image to post unfortunately.

What's the resolution of the anilox? Shouldn't it be something like 4x LPI or better?

 

[meddington]

I should get some print samples to day, but upon further testing on the printers end, changing the screen angles (from C22, K82 to C82, K22) indeed didn't help. Not sure what the anilox res is, but will find out. Substrate is clear poly.

Its now being described as horizontal banding occurring every 1/8" or less and being described as "gear pitch chatter". So FlexoGrunt, I'll be following up with them with your suggestions to see if they can troubleshoot. They seem to be concentrating on the screen angle of 82 still and are looking into if there is an issue with their CDI with this angle.

 

[meddington]

plot thickens:

According to the printer, the issue of banding is apparent on the plate, which rules out any mechanical issues on press. They have narrowed the issue down to their new CDI imager. They imaged plates using an older CDI and the banding was alleviated on the plate. Thing is they use the new imager with other separations with no banding issues, so they are now looking into incompatibility with 1-bits generated from our Rip, or with specific angles in general. Out 1 bits are generated on a Xitron Navigator v8.0 Rip (Harlequin). Anyone ever hear of compatibility issues with newer CDIs from Esko?

 

[mazengh]

I tried HXM screening on a Harlequin rip to generate 1bit tiff files for a CDI and the results were amazing.

 

[meddington]

I tried HXM screening on a Harlequin rip to generate 1bit tiff files for a CDI and the results were amazing.

I'd be willing to give that a try, but we currently don't have access to that screen set.

 

[mazengh]

if you send me a file, i an rip it and send you 1 bit tiffs... i can also take a screenshot of the rip curve and settings i used...

 

[gordo]

I tried HXM screening on a Harlequin rip to generate 1bit tiff files for a CDI and the results were amazing.

HXM is Harlequin's flavor of Cross-Modulated Screening. It is functionally the same as Agfa :Sublima and Kodak's equivalent and is used for exactly the same purpose. Between ~9% and ~91% it is identical to a conventional AM screen.

best, gordon p

 

[gordo]

plot thickens:

According to the printer, the issue of banding is apparent on the plate, which rules out any mechanical issues on press. They have narrowed the issue down to their new CDI imager. They imaged plates using an older CDI and the banding was alleviated on the plate. Thing is they use the new imager with other separations with no banding issues, so they are now looking into incompatibility with 1-bits generated from our Rip, or with specific angles in general.

Is it possible that the banding is there but because the other colors are so light (or in this case near solid values) that they're just not visible?

Although I doubt it in this case, it's certainly possible that the screening needs to be tweaked for the output device. That usually is done on capstan film imagesetters and some digital laser printers though. It is done when the film/paper advance mechanism or the plotter itself, has a harmonic beat due to the gearing mechanisms. If the imaging/plotter harmonically beats with the gearing it can cause an interference pattern - a moiré if you will. This can usually be compensated for by a screening design adjustment which is much cheaper than a mechanical fix. Again, I doubt that's the case in this instance. I.e. I doubt that it's your 1-bit tiffs causing the problem.

Another possible cause of artifacts is if they are doing any circumferencial dot distortion to compensate for the stretching of the "rubber" plates around the cylinder. That would be exacerbated by the screen angle.

Would it be possible to image the different screen angles (round dot 120 lpi) at a tone value that was the worst for banding, all on the same plate and print that plate using black ink? That might show you if one of the angles is causing the problem.

best, gordon p

 

[meddington]

Well, we are doing the distortion for plates prior to creation of the 1-bits, and since these image fine on the older CDI, I more suspect an issue with the newer CDI...maybe a laser power issue, but I'd just be speculating.

I'll see if they're open to a test of different screen angles per your suggestion.

 

[mazengh]

distortion

distortion

I used to do my distortions in the page setup of my harlequin rip... there is a horizontal and vertical scaling...

 

[CumminsLabel]

Flexo angles

Flexo angles

We use:

C-97.5
M-67.5
Y-82.5
K-37.5

ECRM Harlequin RIP v8.3

133 lpi x 2400 dpi

I wish that ECRM's version if the Harlequin RIP had implemented HXM screening.

 

[FlexoArt]

I use:

C=22.5
M=82.5
Y=7.5
K=52.5

and occasionally:

C=7.5
M=67.5
Y=82.5
K=37.5

I'm outputting tiff files for a CDI from a Nexus RIP. My resolution is 2400 dpi @ 150 lpi for most jobs. Sometimes I'll use 175 lpi, but that's not often needed for most of our work.

 

[michaelejahn]

which model ?

which model ?

I'd be willing to give that a try, but we currently don't have access to that screen set.

Sure sounds like a resolution incompatibility issue - which model ? (link of models below)

Flexo CTP: Models of CDI Imagers

 

[meddington]

Sure sounds like a resolution incompatibility issue - which model ?

Hi Michael,

50x80 spark , 40 beam, optics 80

 

[[email protected]]

Savvy

Savvy

I'd be willing to give that a try, but we currently don't have access to that screen set.

1. There would be no conflict between cyan and yellow since they are at 93 and 95 which probably gains to about 98 99 percent so there would be no angle conflict.
2. The angle of 82.5 (82)j is the moire angle which you want to avoid when running 1,2, 3 colour process. This angle is ok for fulll 4 col processs because you have more coverage of ink on the print (max 300%).
3. The black gradient should not be pulled back for gain as some prepress facilities still do this...The black should have a bump of about 1 to 5 dependant on the plate material you are using.


Have you contacted Esko about the problem, sounds like you may have a calibration issue with the cdi..

Have you performed a stain test on the plate material?