CHROMiX

Profile Ink Limit and Green Rich Black

We have just finished a press calibration to G7 and have made a press profile. Everything looks good except for the Rich Blacks and dark grays that have a green cast do them. Any thoughts on that and also, what is a good profile ink limit for a calibrated offset press.

First with the ink limit. 320 is the age old industry norm. If you want to save some money on ink, crank up the GCR and drop it down to 300 or 280. If you want more shadow detail, and are printing on good paper and aqueous coating, there is no reason you can’t go up to 340-350.

As for the greenish shadows, it starts with your trapping. You will probably see that your 300% CMY patch has the cast you are seeing in rich blacks. Were your inks dead on target or could you have run the Magenta a little higher. Or if the magenta is good, could you run the Cyan down a little. If you like the solid primaries, how are your overprints? The blue overprint is most likely where the problem is occurring. In the original ISO 12647-2 standard, the a* of the blue was 25. It was then adjusted to the 17 a* in the Gracol characterization data. When the Fogra39 data set was released, the blue was changed to 22 a* (just within tolerance). What I have been seeing when doing G7 calibrations is that when the blue overprint is 19-22a*, the 300% overprint is more likely to be neutral.

If the solids are really good, and the blue overprint is 17a* or less, you might have to look at reformulating the inks to increase the trapping.

Bret