Color migration from 1st unit to 2nd unit

[nothernlights]

We are printing a blue spot color ( about 80% coverage ) on first unit and a yellow spot color ( about 5% coverage ) on the second unit. As they are printing, the yellow print is coming out greenish. When they are printed seperately, the yellow and blue print out alright. Our press operator says that the blue from the first unit is somehow mixing with the yellow ink causing the greenish colour.

My question is what is the best way to prevent the yellow ink from being printed out "greenish". In order to get around the problem for the time being, I am printing the blue first ( 1st unit ), wait for it to dry and then print the yellow ( 2nd unit ) on the second pass.

Any suggestions would be greatly appreciated.

Background :

Normally for this job we would print the BLUE on the 2nd unit and YELLOW on the 1st unit. We switched it around because there was a visible banding ( white ) on the Blue print. The banding problem is a known problem for this machine ( 1972 SORDZ ). We are planning on selling the press this year so it may not be worth to fix the banding problem.

Note :

On our multi colour jobs I also noticed that the image from the 1st unit makes it's way to the blanket of the second unit as the machine is run over time. Is this normal?

 

[turbotom1052]

the tack of the yellow ink is pulling the blue ink off the sheet and contaminating the yellow rollers. you can reduce the tack of the yellow by mixing in some ink reducer like a gel compound, or..... you could effectively reduce the tack of the yellow by making it weaker with transparent white and then running it up a bit. My bet is that the yellow ink is too strong and that adding transparent white will do the trick. When you do this you need to add enough trans white to make a difference. i would start by adding about 4 to 5 ounces per pound. If you know how to read your rollers and know that the yellow is NOT too strong then try reducing the tack with some gel compound.
You could also achieve the same result by increasing the tack of the blue but considering the nature of todays paper quality i would tend to steer clear of the higher tack as it may result in tail picking.

 

[Deleted member 16349]

We are printing a blue spot color ( about 80% coverage ) on first unit and a yellow spot color ( about 5% coverage ) on the second unit. As they are printing, the yellow print is coming out greenish. When they are printed seperately, the yellow and blue print out alright. Our press operator says that the blue from the first unit is somehow mixing with the yellow ink causing the greenish colour.



Note :

On our multi colour jobs I also noticed that the image from the 1st unit makes it's way to the blanket of the second unit as the machine is run over time. Is this normal?

Questions.

Are you printing the yellow on top of the blue (wet trapping) in any location of the print or are the yellow and blue print totally separate?

How many units does this press have?


Note:

It is normal to see some of the image of 1st unit on the 2nd blanket.

 

[jbeniston]

You could try running the yellow plate a LITTLE wetter. This will help prevent the blue from being picked up by the yellow blanket/plate and becoming transferred into the yellow roller train.It did not happen did it when you put the yellow down first did it?
Here, in this instance, you are breaking the basic rule of multi colour press operation.i.e. Lightest coverage down before heavier coverage.
Good luck

 

[Richard]

migration!?

migration!?

running the job twice is your best quality choice for color.
but how difficult is your register? Table fed? Operator is experience?

get a tru 2 color press. it has 2 blankets, stops migration.

 

[Al Ferrari]

"get a tru 2 color press. it has 2 blankets, stops migration."

The very first post in this thread mentioned that the press is an SORDZ. In what way are these not true two color presses. These are not common blanket machines.

Al

 

[nothernlights]

We would normally run the yellow ( lighter coverage ) on the first unit followed by the blue ( heavier coverage ). If we do this there is no problem with ink migration. However, we have problem with our second unit ( visible whitish bars during printing ) which forced me to try switching the print order.

You could try running the yellow plate a LITTLE wetter. This will help prevent the blue from being picked up by the yellow blanket/plate and becoming transferred into the yellow roller train.It did not happen did it when you put the yellow down first did it?
Here, in this instance, you are breaking the basic rule of multi colour press operation.i.e. Lightest coverage down before heavier coverage.
Good luck

 

[nothernlights]

We are printing the yellow spot color on top of the blue. The area where the yellow print would go is "whited out" on the blue print so the two colours only overlap each other in the areas where there is trapping.

Our press is a two colour press.

Questions.

Are you printing the yellow on top of the blue (wet trapping) in any location of the print or are the yellow and blue print totally separate?

How many units does this press have?


Note:

It is normal to see some of the image of 1st unit on the 2nd blanket.

 

[nothernlights]

Thanks for all the suggestions.

We are currently printing on the blue on the first unit ( we are using a two colour press ) with no ink / impression on the second unit. I do see the blue ink building up on the blanket of the second unit.

 

[turbotom1052]

the issues you describe happens every day on offset presses all around the world. The peanut butter and jelly analogy is the best way to describe what your experiencing. If you make a peanut butter and jelly sandwich its the peanut butter that goes down on the bread first. What do you suppose would happen if you were to try to spread your peanut butter after the jelly was already down on the bread? The same principles apply here when laying ink down on paper. You are right to try laying down the darker color first and then printing the lighter color on the second unit. This would be the rule a majority of the times. It all has to do with the level of transparency of the inks. The yellow being more transparent will allow the blue to come through!!!
Your issue is most likely a tack issue. If the yellow ink (the peanut butter in this case) has more tack than the blue ink (the jelly) it will pull the blue ink off the sheet. If the blue ink is pulled off the sheet by the yellow blanket it has to go somewhere. Each and every press revolution pulls off more blue ink. It then builds up on the yellow blanket and eventually finds its way up through the rollers and if run long enough will even contaminate the ink in your fountain.
So how do we make the yellow ink more like jelly so it will not pull the blue ink off the blanket? The first thing you will need to determine is if the high tack levels of the yellow are coming from the ink simply being too tacky or if the high tack levels are coming from running a too thin ink film. As the ink film thickness decreases the tack will increase!!! If your yellow ink is too tacky as a result of it being too strong (as i suspect it is) you will need to find a way to to make the yellow ink weaker so that you can effectively run a "thicker" ink film. The way to make it weaker is the dilute the pigment strength with either some transparent white or with some varnish. In many cases i would prefer varnish as transparent white often has a tendency to be tacked pretty high. Once you are able to run a thicker (less tacky) ink film on your rollers you will be making the yellow ink more like jelly and it will then adhere better to the blue ink thats already down without lifting it off the sheet. Always remember..... when you set your ink feed through either your press console or your ink keys on the fountain be sure to allow a little bit of ink to flow into the rollers even in areas that are not inline with any image.This will further aid in preventing the ink film from thinning out and thus help prevent the migration of the blue ink up your rollers and into your fountain. in this particular case it would be better to start up your press with a little too much yellow ink on the rollers as opposed to too little. What i just described are generally accepted principles of multicolor wet trap printing. Good luck and let us all know how you make out.

 

[Alois Senefelder]

Adulteration !

Adulteration !

Northernlights,

Forget about "Adulteration" of the Ink" -- when printing a single colour on # 1 unit of a 2 colour press,

on the Second Unit have a Blank Plate and just run enough Damp to keep the plate Clean also

run the Unit on Impression.


Regards, Alois

 

[Deleted member 16349]

We are printing the yellow spot color on top of the blue. The area where the yellow print would go is "whited out" on the blue print so the two colours only overlap each other in the areas where there is trapping.

.

OK. So you are not wet trapping over any large area but just a bit at the edges where prepress has applied trapping.

So the next questions would be:

Is the yellow ink getting contaminated with blue right away or does it take time for this to happen when printing?

Is the contamination of the yellow happening only in line with the yellow and blue overlapped print or is it happening all across the press?

In general, for the yellow ink on the rollers to get contaminated, the blue must somehow go from the 2nd blanket to the plate and then to the roller train.

This can happen in different ways.

If the problem is wet trapping of the yellow on the blue, then one would tend to see the contamination in line with those areas of overlapping print. One would not see it all across the press.

If it is a wet trapping problem, one might try to run the blue as dry as possible and the yellow with more water to help the yellow wet trap on the blue.

If the contamination is all across the press, you might want to start the print sequence with the yellow printing first so the plate gets water first before starting the blue. If you started printing the blue first, the dry yellow plate would easily pick up the blue from the 2nd blanket if the impression is on. This may be press related and I am not sure of how your press functions.

There is also the possibility that there is not a nice wet trap condition at the 2nd impression nip but one where there is mixing of the blue and yellow inks. Possibly the squeeze is not correct.

Hope this helps.

 

[Alois Senefelder]

Question

Question

Northernlights,

1) Does the Yellow Image have to overlap onto the Blue Image area by X amount, so forming a Green Border ??

2) If not reduced the Yellow Image in size and print in Hairline Register


Regards, Alois