DI inks

We Use Saffira inks from heidelberg as well as toyo on our DI.
In the solid color bars the ink does not lay down 100% if you look at it under a glass you will see it looks motley (kind of like a course mezzotint look) thus we are not able to get nice rich colors on our solids.
An example is a nice Xmas Red (4-color tint of 100% magenta and 100% yellow) on our press using the inks above we get a lighter red not close to PMS 185.
I thinks it has something to do with the solids not laying down smoothly. We are on a #1 gloss sheet so paper is not an issue.
Is it possible the ink does not adhere to the silicon-based plate properly?

 

Ron,

I am going to assume you have a QMDI and not an SM74DI, mainly because I have never heard of anyone complaining about an SM74DI having motled solids.

However, I have heard that complaint with the QMDI and it lies in the way the the ink is transfered to the blanket from the plate.

On the QMDI the ink is stored in wells (Lowest part of the plate) that were cut from the silicon layer by the diodes. Since the ink has to be pulled from within the "wells", the solids seem to have a slight motling to them. On a conventionally imaged plate, such as the ones on a the SM74DI or any plate done on a CTP machine the, ink sits on top of the image area (Highest point on the plate) and is pushed onto the blanket creating a more smooth solid.

To me it has always been more noticeable when comparing text from a DI with silicone plates to text from a plate made conventionally. The text from the DI does not look as smooth under a loupe, it looks like it has sort of a dimpling effect. It is barely noticeable to the eye unless you know what you are looking for.

It seemed to be less conspicuous in a large solid area as the blanket is able to receive the ink better due to there being less of a wall to pull the ink from. However, if you look closely at the edges of a large solid area you will see the dimpling effect more so that you do farther out to the middle of the solid. It is very slight and suble.

If you took a drop of Elmer's glue and put it on your thumb and then pinched it with your pointer finger and kept doing so, opening and closing your finger and thumb tip, after a while the glue would start to dry and you would start to notice glue threads stretching between your finger tips. As the threads break you notice dimples of glue where they came from on each finger. This is an example of what happens on a much smaller scale when ink is being pulled from a silicone DI plate and explains why solids do not seem as smooth.

As for it changing the colors of PMS screen builds, I imagine it is possible, but I have never actually seen it happen.

 

Look at the printing plate, if the ink is not being picked up correctly you will see the same effect on the plate areas of white mixed in with the colour.

If this is the case you can wipe the area mentioned over with isopropyl alcohol.


If the image mentioned looks 100% solid and flawless, are your blankets it good condition? if you have low spots on your blanket you could find it effecting the print in this way.

have you checked your impression cylinder to blanket pressure? to little pressure can cause this.

I believe the way to calculate this is 0 = .11mm
So if you measure the sheet with a mocrometer and its thinkness is equal to .17 you would subtrack .11 from your .17 value set your pressure in the black area of the scale to .06

 

I found my roller settings to be crucial in improving this problems and very satisfactory after changing them with new ones.

 

I agree with you there Salam, you are right on the money !