Question on setting Keys on offset press and running consistent color

[shorty83]

Hi, I am trying to help out a coworker and don't have much info on offset printing or what specific press is being used so bear with me. They are running a 60" wide offset press and have to adjust the color keys across the press manually. They are having problems with color consistency throughout runs. They are not able to put density/color blocks across the press when they run jobs because they are continuous print jobs and the customer does not want color blocks on their finished product. The way they are setting their densities, etc are as follows:

Make a set of plates with color blocks/density blocks
Adjust ink keys, etc. and run the press to correct densities
unmount the color block/density plates
Mount the plates to run the job and hope the color stays consistent throughout the run.

This is not the most economical way to run jobs and they have no way of checking the color consistency throughout the run, does anyone have any suggestions/ideas on helping this situation or is this process normal for offset?

 

[Deleted member 16349]

Hi, I am trying to help out a coworker and don't have much info on offset printing or what specific press is being used so bear with me. They are running a 60" wide offset press and have to adjust the color keys across the press manually. They are having problems with color consistency throughout runs. They are not able to put density/color blocks across the press when they run jobs because they are continuous print jobs and the customer does not want color blocks on their finished product. The way they are setting their densities, etc are as follows:

Make a set of plates with color blocks/density blocks
Adjust ink keys, etc. and run the press to correct densities
unmount the color block/density plates
Mount the plates to run the job and hope the color stays consistent throughout the run.

This is not the most economical way to run jobs and they have no way of checking the color consistency throughout the run, does anyone have any suggestions/ideas on helping this situation or is this process normal for offset?

There are a couple of problems that exist and could be corrected but the industry does not want to go there.

First, at this time, there is no accurate presetting algorithm to set ink keys. On press, the ink consumption is NOT directly related to image area so most algorithms give the wrong result.

Their method of using plates with blocks to adjust the ink keys and then change to plates with no blocks is in principle a good idea. The problem, as they have found is that they can not maintain consistency. The major problem is in the consistency of the ink feed.

So the second issue is that just about all offset presses do not feed ink consistently. This is a press design issue. Presently the amount of ink feed is affected by numerous other variables. If the press could feed ink consistently and independently of all those variables, then the method your coworker uses above would work quite well.

As a side note, if one had an accurate ink key algorithm and a press with a consistent ink feed and one that also distributed the ink so there were no ghosting issues, than one would not need colour bars.

Goss has digital inkers on some of their newspaper presses and these are positive displacement ink pump feed systems. One of Goss's marketing statements is "set and forget". This basically means that once the system is set to the correct ink feed, there is no reason to adjust.

Until offset presses obtain positive ink feeds that provide the consistent volumetric ink that is required, your coworker will have to do what the industry has done for decades to deal with this issue and that is to try and make all the other variables on press as consistent as possible.

 

[gordo]

Hi, I am trying to help out a coworker and don't have much info on offset printing or what specific press is being used so bear with me. They are running a 60" wide offset press and have to adjust the color keys across the press manually. They are having problems with color consistency throughout runs. They are not able to put density/color blocks across the press when they run jobs because they are continuous print jobs and the customer does not want color blocks on their finished product. The way they are setting their densities, etc are as follows:

Make a set of plates with color blocks/density blocks
Adjust ink keys, etc. and run the press to correct densities
unmount the color block/density plates
Mount the plates to run the job and hope the color stays consistent throughout the run.

This is not the most economical way to run jobs and they have no way of checking the color consistency throughout the run, does anyone have any suggestions/ideas on helping this situation or is this process normal for offset?

I think you raise two separate issues.
1- initial press ink key setting
2- consistency through the run once set up.

1- Ink key settings are related to ink consumption requirements of the image area of the plates for the different process colors - not solid ink density. Initial ink key settings can be done either by the press operator eyeballing the plates and guessing what the initial settings should be - which may be what you're doing now. Or it can be done with software that analyses the ink needs for the different ink zones and presets the press accordingly. Using software to do that is faster and more accurate than eyeballing and guessing.

2- Once the proper solid ink densities are achieved then the issue is to keep them consistent through the run. SIDs will naturally vary through the run. The variation is typically cyclical i.e. goes up then down then up again from the target density once the press stabilizes. On a properly maintained press and if the run is long enough for the press to achieve stability then theoretically the press operator shouldn't have to adjust the SIDs - i.e. keeps his hands of the controls.

To enhance press color consistency through the run by doing the separations with heavy GCR (or using so-called ink savings software). You can also enhance consistency by using FM screening.

Usually the color "blocks"/patches are cut off of the final presswork so having them on the live job is not an issue. If you can't have those patches on the live press sheet then it might be possible to use some areas of the live image area and measure those. Running plates to set up a press then switching plates to run the actual job is problematic to say the least.

Oops - Erik posted at the same time that I did.
I would argue - to one of his points - that ink key presetting software is always going to be better than a press operator eyeballing and guessing.

 

[Deleted member 16349]

I would argue - to one of his points - that ink key presetting software is always going to be better than a press operator eyeballing and guessing.

Totally agree. Poor ink key presetting software is always better than the operator guessing but it is not good enough to provide the final setting values. Final ink key setting values are almost always different from the preset values.

With a properly calculated algorithm, the preset values and the final values could be the same. It is not required for the presetting values to be exactly correct due to the +/- tolerances. The results can be within tolerance even if the algorithm has some small errors. The fact that even with modern presetting algorithms require adjustments even with Goss digital inkers means the errors in those algorithms were large enough to put the results outside the tolerances.

For printing, the tolerances are not tight. They are quite large with respect to ink volume. For commercial printing it is probably on the order of +/- 6% of ink film. With newspaper inks, this is much higher because their inks are much weaker.

The existing tolerances in printing are like a barn door which they still can't hit.

 

[aqazi81]

What we do here on manual presses is we set all the ink keys for a light coverage on the fountain roller, run press and engage the ductor roller for 12 to 15 times to transfer even ink film coverage on all rollers, disengage the ductor and then set the keys according to the image on plate. Engage the ductor again for a about 20 times and then do make ready on waste sheets to register all plates. get some waste sheets of the same size at least 100 and run through the press and print few clean sheets. Judge the color if you do not have any device to measure and then adjust keys accordingly.
Once the keys are properly set then you don't have to play much with the keys, just adjust the fountain roller sweep.
One more suggestion, adjust the ink keys so that your fountain roller sweep setting is in middle, like if you have 100 steps in fountain roller sweep adjustment the best setting would be at 50 so you have room for adjustment both ways.

Hope this helps.

 

[D Ink Man]

Hi, I am trying to help out a coworker and don't have much info on offset printing or what specific press is being used so bear with me. They are running a 60" wide offset press and have to adjust the color keys across the press manually. They are having problems with color consistency throughout runs. They are not able to put density/color blocks across the press when they run jobs because they are continuous print jobs and the customer does not want color blocks on their finished product. The way they are setting their densities, etc are as follows:

Make a set of plates with color blocks/density blocks
Adjust ink keys, etc. and run the press to correct densities
unmount the color block/density plates
Mount the plates to run the job and hope the color stays consistent throughout the run.

This is not the most economical way to run jobs and they have no way of checking the color consistency throughout the run, does anyone have any suggestions/ideas on helping this situation or is this process normal for offset?

Your coworker 100% positively sure needs an Ink Doctor and perhaps a fountain solution TECHNICAL person working in an honest and integral manner to absolutely put himself into a perfected operating mode with minimized troulbe free runnabilty and printabilty realization.

D Ink Man

 

[Cornishpastythighs]

Sounds like your friend is doing the best he can under the conditions he has been given. You dont say whether its a 60" web press or a sheetfed press. With no pre-seeting, remote inking or measurable elements on the sheet your friend (and the boss) is going to have to live with colour variation. I would suggest however that someone convince the boss man that having a good maintenance program even on an older press is still worthwhile. Make a list of all the things that could contribute to variation, Poor roller condition and settings, Fountain solution condition and dampening system settings, Ink key condition(I know you said its a manual key system) Roller temperatures, Ink fountain blade condition, Just to start you off, I'm sure others have their list to contribute. Your friend can a least try and improve the press stability by reducing the influence of things he can control.
Good luck