There is no section for flexography, and even if the issue i'm dealing with is not directly related to color management, I'd post it here seeking either some ideas on how to deal with the problem, or a suggestion for another forum to ask a question.
The application is wide format flexo, printing on corrugated boards (B, C, E flute). We got two 1628 martins and one 924 martin, with anilox rolls of 300-360 lpis. All units are mobile, with the exception of the feed unit.
Starting at 8 inches of the lead edge of every box, we have alternate areas of heavy and light coverage. The circumference from the chamber blade to the nip between the anilox roll and the plate cylinder is also 8 inches, so I know that the print comes out smooth over that area because nothing is causing the chamber or chamber blade to bounce till then. After that, we are getting these banding stripes at a frequency related to the speed of the press. The higher the speed, the less the severity of the stripes, and the greater the distance between them. The stripes end anywhere from 14 to 25 inches on the box, so it is not related with the revelations or total circumference of the anilox roll. By the way, this happens in all presses and all units.
We've tried pretty much everything we were able to think of, and even more. We have been able to alleviate the problem by using surfactants on the ink, or turning the underpack of the plate around, or adjusting the pressure, but the problem is still there... masked. We've measured the rotation frequency of the rolls for vibrations and we couldn't find a pattern that matches the banding frequency on the printed box. This would say that there is no mechanical problem. We printed sheets isolated from the delivery, so the gear of the delivery isn't causing this either. We've actually tried more stuff than I could recount...
I am about to give up and attribute this to the age of the equipment or demons. We've been having the same issues two years ago... even the frequency of the banding was identical, only not as severe.
Any ideas would be more than welcome!
A forum you could try
Sounds like you have looked at a lot of different culprits. However, have you investigated the pulsing of the diaphram ink pumps? Seems like the banding would get closer together as the press speed increases (like you mentioned) since the pumps run at a constant speed.
Your issue is definitely bouncing. Some of all the tricks to reduce bouncing: larger cylinders if possible, impose in a staggered manner to fill gaps between designs, print a thin belt all around cylinder on the edges of web (if possible in rubber 1 mil thicker than printing plate) and maybe consider a softer plate with a harder tape. Hope this helps.
This may be it
Dimitri hi here is something that you may want to look at closer the band that you are speaking of may be caused by the starvation of the amount of ink being used, I am only guessing that you are printing a large solid peice and as it rotates you se a pattern on the printed material my suggestion is to change to a different anilox with a different bcm .. if you have not looked at this yet ...
Originally Posted by dimitri
Thank you all,
These certainly are ideas worth considering. Just as an update, I'm looking more in the hard truth of the presses getting old, and possibly having backlash of the gear train... or the anilox roll not wetting enough... I do not know what would be more expensive to fix, if we were to address the root cause and not try masking the problem with softer/harder plates/stickybags/foam, pumps, doctor blades...
Any other ideas still welcome!
Originally Posted by Luc St-Pierre
You have given a good inputs. But can you please explain what do you mean by (if possible in rubber 1 mil thicker than printing plate)????
If, as an example, you are printing with .067” polymer plates mounted with a 20 mil tape: on the edges of the web, if substrate width permits, have a 1/8” wide rubber plate gauging .068-.069” and mount around the cylinder with the same tape. Since rubber is compressible as opposed to polymer, this will create a cushion. Do not forget that if you want to use this method (commonly called “tiretracks”), remember that you will have to use it on all decks. If one deck is allowed to bang on the central drum, then the banding will still happen. Other stuff I have seen causing banding is a pulsating ink pump.
What condition is your plate exposure unit in? In other words, have you checked your lamps to see if the section in the middle outputs at the same intensity as on the ends?
Back exposure? How long?
How about the brush in the washout tank? Is it even (or evenly worn)?
Are you drying the plates long enough?
Just some thoughts.
Originally Posted by Luc St-Pierre
I think this a is very useful tip. Infact one of my customer has just installed W& H miraflex and facing this bouncing issue in many solid jobs. On your suggestion i have a small question that if i use rubber plate around plate sleeve it will get in contact with anilox. will it damage anilox?? and second if i use this rubber plate on all deck (all colors) and printing film material this tyretrack line creates a pick on the edge of winded roll and creates problems for winding.
Can you also explain how does ink pump created banding.
And what should be the standard DOT gain for flexo. Meaning if my input file has 50% dot what should come out of print.