Question about G7 calibration

[Prepper]

Quick question, getting ready to re-do our calibration, it's almost been a year, new papers, new inks, etc. We did this last time by getting to gray balance on press and just creating one curve for CMY, doing this manually with graphs, I don't have the curve software and won't be getting it either, and I wondered how far or what the limits are to pushing the densities around to get gray balanced on press? We achieved it last time with 170-148-135-100, kcmy and this has worked very well, when the press is ran to gray balance, but our numbers lately to get there have been more like 160-155-130-100. So what do you think the limits should be of pushing the densities around to get gray balance, and still give some latitude on press? Don't want them to have to run the density so high that it makes it more susceptible to plugging or scumming or so low as to be on the verge of washing out all the time.

Just really don't want to create separate CMY curves by hand with graphs if I don't have to! How-to says to only do it at 50% anyway for a press, so what would you do, smooth the curves from that new adjusted 50% point, if you did separate CMY curves?

Thanks for any input,
Terry

 

[Bret Hesler]

Re: Question about G7 calibration

The tolerances are 5 Delta E from the colors specified in the how to, not density. I would suspect that a 155 Cyan and a 130 Magenta are close to being out of tolerance, but it all depends on your ink set and densitometer. What Lab reading are you getting on the C-M overprint at those densities? You might have too large a tack difference between cyan and magenta.

With conventional screening, you should not have gray balance shifting throughout the tone scale, until you get into the shadow end. You really don't want to try to correct the shadow end with curves because it is more of a trap issue there. So, yes, and adjustment at the midtone with a smooth curve should do it.

Bret Hesler
Earth•Thebault, Inc.
Parsippany, NJ

 

[Prepper]

Re: Question about G7 calibration

We did stay within the Lab tolerances pushing the densities around to get gray balanced, I just wondered how much would be safe to move the densities around before causing problems on press? Maybe it would get out of Lab tolerance before you went so far as to begin causing potential problems on press?

You're right about our overprints, the CM is always the worst, sometimes slightly out of tolerance. We've always had the M too strong for some reason on this press, with different inks and papers. I guess we need to look at the strength of our ink and tack. What should the tack difference be between the 2?

 

[Bret Hesler]

Re: Question about G7 calibration

Basically whatever gets you a good overprint. Enough that you don't have backtrap mottle, but not too much that your overprints are way off target. Our ink has a 2 point spread between C-M. Our blue overprint is a touch on the red side. I'd like to see a slight reduction in the separation.

Bret

 

[meddington]

Re: Question about G7 calibration

I think most people have blue overprints a touch on the red side compared with ISO 12647-2. Fogra39 actually has a significantly higher a* value than ISO 12647-2 defines, but the Gracol dataset aimed it back toward the standard.