Yellow ink contamination from Magenta ink (back trapping)

[dschell]

We are having an issue with our yellow ink being contaminated with magenta. We have been told this is from "back trapping" and have tried a few things to correct the problem but have not been successful. We have changed the ink sequence from KCMY to YCMK and this stops the yellow from being affected but our vanish, 5th unit, is still getting contaminated. This shows up much more with a job that has heavy magenta coverage. This issue came to light when we went up on an exact reprint and some solid yellow bars kept printing with an orangish tint. We have been told that the ink tack is the most likely cause and we have tried various different inks and are currently having our inks tested.

Is this the same problem that I've seen listed as "Back Trap Mottle"?

Any suggestions on possible causes and solutions would be greatly appreciated.

 

[Paultheprinter]

Re: Yellow ink contamination from Magenta ink (back trapping)

Hi,

ive been waiting for this problem to occur at other printer for some time now, firstly running ycmk must make your work look sh*t, every time i put anything onto yellow the dot spreads and looks poor. The backtrap problem is something which we have had for about a year now, ive talked to some realy knowledgeable people and had visits from four experts from four different companies and still no cure. Ive tried K&E Novavit ink, van son v5, stelin inkrediable and stelin resista higher tack set, combifix fount and varn supreme. most of the combinations give the same results and the only variable which makes a noticable difference is the blankets, accura backtrap and david M QL blues don't do it so much, why? i don't know yet. Ink weights seem to play a part too but to what extent its difficult to say, uncoated backtraps more than coated and nobody knows any way of sorting it as yet. My only tip would be reduce plate to blanket squeeze to absolute minimum to reduce any contamination and use a good quick release blanket.

I would call this issue back trapping not back trap mottle unless the non transfered ink affects the printed layer causing the mottle. My only concern is that i get the previous color transferring back into the duct in areas were there is only color bars. My guess is that the ink will be fine as its your press what is changing its physical characteristics, unless they check your tack at point of transfer on your press then the results are useless. If they were to do this then they could see where and why your and my press fall out of tack sequence.

Paul

Please keep us all updated as i feel this could be the tip of the iceberg on this issue.

Edited by: Paul Green on Apr 16, 2008 5:52 PM

 

[RTD]

Re: Yellow ink contamination from Magenta ink (back trapping)

Does this occur immediately upon start up or does it appear after running for while?

 

[dschell]

Re: Yellow ink contamination from Magenta ink (back trapping)

This happens within the first 100 to 200 sheets.

We did try Van Son inks last night and it appears that the problem has gone away. We had tried a couple other inks prior to that but we still had the back trapping issue. We will run the Van Son inks for a few days to see if we continue to have success and to see how the printing matches to previous printed material but things look promising right now.

Also, we found out that our ink supplier had changed the tack on the yellow ink we were using. They dropped the tack slightly but we only found out about that because we had the ink tested and through their ink tech. I don't know if this was the issue but...

Edited by: dschell on Apr 17, 2008 3:32 PM

 

[Deleted member 16349]

Re: Yellow ink contamination from Magenta ink (back trapping)

Ink tack on the Tackometer is determined with ink that has no fountain solution in it. When you are printing, the tack of an ink will drop with increased water. One can see this happen on the Tackometer by dropping a little bit of water on the rollers as they are running. The tack drops and slowly recovers as the water evaporates.

Therefore, on press the tack values are quite a bit lower than the ink tack values measured on the ink testing equipment.

One normally wants a lower tack on the later inks being printed but one can also help things along. In your case, what you might try is to run the water low on Magenta and a bit higher on Yellow. This should in principle decrease the tack of the yellow even more and resist any tendency of the yellow pulling the magenta up into the press.

Of course don't over do this. Moderation is a good guide. There are lots of factors but this might help.

 

[akiyama_king]

Re: Yellow ink contamination from Magenta ink (back trapping)

was that on a press with a common imp cylinder?

Edited by: heidelberg king on Apr 20, 2008 2:42 PM