Successive Unit Mottling

[pworden]

We have a Heidelberg SM74 Six Color with coater and Alcolor Dampeners. When printing large areas of Pantone Colors on uncoated sheets the Pantone looks mottled with each successive unit. Such as when we print a solid in the last printing unit and don’t use the coater the Pantone color looks fine. But if we print the Solid Pantone color in the 4th unit then another color in the fifth and sixth units the Solid Pantone that was printed 4th unit seems to pull off on the fifth and sixth units and give a mottled appearance. It seems as though this began happening when we went away from film and started using Thermal Plates. Has anyone else experienced this problem and what did you do to remedy it. Any help would be appreciated.

 

[jlind]

This sounds like a back trap mottle thing. Why do you think it's related to CTP? Instead, look to your blankets and ink setting rates on that particular paper. Just a very complicated issue.

John Lind
Cranberry Township, PA
724-776-4718

 

[madjock]

This is a common problem and can be solved or disguised by moving the darkest and/or less forgiving colours as far up the press as you can, or adding opaque white to the colours sometime helps, of course sometimes you have no choice in where to assign certain colours.

 

[Sfdp2300]

you could always use multiple passes thru the press and dry trap if you run into color rotation problems. It is more time consuming so you run into a turn around time versus quality issue...

 

[tmason]

You can never really fix it unless you run the solid down last. You can try to run a higher pigment load in the ink and run the thinnest ink film possible. However, this opens you up to garbage in your solid areas.
Some colors are worse than others. You can usually get away with red and yellow solids down early but greens and purples are the worst.

 

[Beergod]

I had a similar issue on our Man Roland 700 and it turned out to be transverter grippers. Check this problem with a thicker sheet and see if it goes away.

 

[Alois Senefelder]

re - BTMottling

re - BTMottling

Hello fellow "Lithographers"


A PDF explaining "Back Trap Mottling" I hope you will find it of interest and value !!


Regards, Alois

 

[cdmaster1]

pworden,
It's all about trap and or blanket release. If you run the solid in #4, mix in a little binding varnish or get the ink company to increase the tac for you (5th and 6th must have less tac than #4 or you could experience back trapping in those colors!).
If in #6 you could use a little reducer to lay better but be careful, this is where it could back trap- especially if the previous colors are darker. Also try a quicker release blanket.