Standard ink densities?

[Bloodsaler]

In G7 how to book 2009 it states,contolling a press by CIELab measurement is theoretically more accurate than conventional densitometry,but also more difficult.And the density maybe different between substrate,especially the yellow ink.so in ISO standard,only define the CIELab value of each ink,not define the standard density~

 

[gordo]

In G7 how to book 2009 it states,contolling a press by CIELab measurement is theoretically more accurate than conventional densitometry,but also more difficult.And the density maybe different between substrate,especially the yellow ink.so in ISO standard,only define the CIELab value of each ink,not define the standard density~

"Theoretically". It also says:
"The good news is that densitometric press control is still quite valid (even recommended) once the optimum density values for a particular combination of ink, paper and measuring instrument have been determined. Printing to these solid values while also measuring the CMY densities of the mid-tone gray patch (HR) will produce equally good results as CIELab measurements, but is more intuitive to experienced press operators."

Best, gordon p

 

[Chief_1975]

In the case of running spot colours what do people considered to a be a maximum safe ink density for offset printing ? Would it be reasonable to assume that the black density should be followed as a guide ?

 

[gordo]

In the case of running spot colours what do people considered to a be a maximum safe ink density for offset printing ? Would it be reasonable to assume that the black density should be followed as a guide ?

No. The black density should not be followed as a guide for maximum safe spot color ink density for offset printing.
Try printing Pantone 372 at anywhere near black density. ;-)
If you have an ink draw-down measure that to get the approximate density.
Otherwise use the closest process color or process color overprints as a guide.

best, gordo

 

[BeauchampT]

When it comes to 'standard' ink density, it is important to remember what you are actually trying to control - ink film thickness (IFT).

If I understand the engineering concept, most roller trains are designed on the concept of 1.4g/m2 of ink on the rollers. If you start to go above that, you get less stable emulsification of your printing ink. So, if you can't acheive proper Lab values for your targets, it isn't a matter of more ink; rather it is a matter of better ink with higher colour strength, etc.

I know in my plant, we are running an improved MFC paper (ISO class 3), and our ink densities float around 1 to 1.3 (Status E). But we really see the stability difference when we push it to the top end. I've checked with an IFT guage and seen the differences.

To me, it seems wise to find what is the right IFT for your press running (according to manufacturer design) and then guage from there whether your current ink supply is adequate. Better supplies = better print.

 

[Deleted member 16349]

If I understand the engineering concept, most roller trains are designed on the concept of 1.4g/m2 of ink on the rollers. If you start to go above that, you get less stable emulsification of your printing ink. So, if you can't acheive proper Lab values for your targets, it isn't a matter of more ink; rather it is a matter of better ink with higher colour strength, etc.

.

BeauchampT,

The printed ink film thickness (ink only) on the paper is about 1.0 to 1.5 g/m2 (about 1 micron) . The ink film on the plate is about 3 times this amount. As you go up the press towards the the ink fountain, the ink film increases from the 3 times the printed ink film to higher amounts that can be 7 or 8 times. The increase in ink film as you go up the roller train depends on the design of the roller train itself and the coverage percentage. The higher the percentage the more the ink film increases from the plate to the ink ductor.

This is a well know condition and can be seen if one does an ink film distribution calculation on the roller train. If I do such a calculation I do two calculations. One is for 100% coverage and the other is for almost 0%. This gives me the range of average ink films on the rollers of a specific press.

These calculations are for ink without water content. Add the water content and those total ink film values would be greater. These calculations are not expected to be highly accurate but they have a practical value because one can see how ink is stored on rollers under different conditions. Do a few of these calculations on different roller train concepts and one quickly gets an idea of how a roller train will distribute ink.

 

[BeauchampT]

Could you direct me to where I could find these approximate calculations?? I'd like to do the same on my press.

 

[Kaoticor]

RE: Densities

RE: Densities

Your densities are actually not super critical as long as you can achieve a reasonable color gamut. The dot gains are of much more importance to note - ISO training guidlines states "The human eye can pick up as little as a 3% dot size change" (more so in lighter screens and lower contrast colors).

You will find you actually can run a wide range of densities for both coated and uncoated stock which will produce an acceptable product(assuming, of course, your ink is good enough to provide a decent gamut), however, having a consistent dot gain will produce your best proof matching.

 

[Deleted member 16349]

Could you direct me to where I could find these approximate calculations?? I'd like to do the same on my press.

The method I used is based on the solving of the simultaneous equations of the nip equations. The idea is that one makes an equation for each nip. The ink films going into the nip from the surfaces of each roller are added in the nip and then split with a 50%-50% split. So a general nip equation would look like :
a + b = 2c where a and b are the ink films going into the nip and c values of the ink going out of the nip.

For a roller train with a lot of nips, there might be more than 15 equations to solve. One can do this manually but that can be quite frustrating since it is easy to make a mistake. It is the same method that is used in high school math for solving a few simultaneous equations. I have used a computer with a math program to do my solutions. This method is mainly used for solving the 100% coverage condition.

Press simulation programs can be used to see how non 100% coverage conditions will run. From these one can see that patterns in the ink film thickness are constantly changing on rollers as the press runs. These programs work by making incremental rotations of the roller train and calculating the ink film splits. As the program runs the ink films are established.

These kinds of programs are very good for investigating transients. The only problem with these programs is that they assume a positive ink feed. If the press would have a positive ink feed, then their performance would be more in line with these simulators.

Not sure where you can get specific info on these methods. I know there was a TAGA paper that was specifically aimed at the first method above and was written in 1984 I think.

If you want to see how the ink film increases as one goes up the roller train, make a simple roller train that consists of only a chain of rollers. Blanket cylinder, plate cylinder, one form roller, then a stack of rollers on that.

Assume 100% coverage and a 50-50 split.

Start from the paper with 1t on the paper and 1t on the blanket returning to the plate after the blanket paper nip.

On the blanket going to the paper blanket nip there would be a ink film of 2t. 2t = t + t

At the plate blanket nip one would have t + 3t = 2t + 2t . At the form roller plate nip one gets
2t + 4t = 3t + 3t

At the form roller to distributor roller nip one gets 3t + 5t = 4t + 4t

Keep going up and you can see that the ink film increases at each step but the ink transfer at each nip is the same. In this case at each nip only 1t of ink is transferred to the next roller.

 

[gordo]

This thread has somewhat spiraled off the OP's original question. However, if you are interested in the scientific minutiae that Erik is writing about there is a very comprehensive book that covers this topic:

Fundamentals of Lithographic Printing by John MacPhee

It's currently out of print but can be purchased second hand online. Just Google the title. Here's one listing that has a good description of the contents:

Fundamentals of Lithographic Printing, Vol. 1: Mechanics of Printing

best, gordo

 

[Alois Senefelder]

Practical - SID

Practical - SID

Gentlemen and fellow Lithographers,

Densitometer: for reading Non-Process Colors (Spot Colors)

1) Read the pass sheet through the MAGENTA CYAN YELLOW filters and using the ONE that gives the highest density reading for color control.

IFT - Roller Trains and Evaluation of Ink Transfer Theory

Fig - A single Form Roller System, the # in Parentheses are average IFT in Microns.

I hope the PDFs will be of interest.

Regards, Alois

 

[Alois Senefelder]

More on I F T

More on I F T

Beauchamp T

I suggest you read the PDFs I've posted on this forum

#1 Thread -Ink Quality, posted 09/19/2010

#2 Postive Ink Feed Simulation Test, posted 08/12/1010


Regards, Alois

 

[otherthoughts]

Link

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[D Ink Man]

Ahh hah..... Just what I have been trying to suggest all alone. Coverage, the amount of ink take off from one print form to another, is the single most important factor in lithographic success or failure; depending on how you intend to control that variation is extremely emminent. Think about that. D

 

[BeauchampT]

Thanks very much for the info...very interesting.
T

 

[discountprinting]

Hi Gregg,

IMHO, your standard densities can be can be of any value. As long as it gives you a color gamut that is big (hopefully bigger than ISO 12647-2) and that it is gray balanced, and that your CMYK solids are within Delta E of 5.

Once you get your Delta E within the limit, measure now your solids and check your density value per color (CMYK).

These new values are now your personalized standard densities for that particular set of ink and paper. To be on the safe side, you may want to cross check whether the same density values can also be applied when you change any of your materials (ink, paper weight, etc.).

Hope this helps.

Larry

I agree with Larry in that you should be running to gray balance, in other words calibrate your plates to linear (25%=25, 50%=50 etc) then run test strip on press and measure your dot gain, create a curve in your rip with your recorded values, retest on press and tweak until your cmy gray patches match the black at your 1/4 1/2 and 3/4 tones...when you achieve this you are printing to gray balance, and you run to standard ink densities on every job...as long as you can measure and control your dot gain on press then you will print to gray balance...I recommend these densities for coated stock C130 M140 Y105 and K170...running to gray balance give you attainable consistency from job to job, G7 in my opinion is over rated, to costly and unrealistic as the tolerances are to narrow to hold consistently by which I mean if you densities and dot gain are not dead on you will cast...

 

[gordo]

I agree with Larry in that you should be running to gray balance, in other words calibrate your plates to linear (25%=25, 50%=50 etc) then run test strip on press [snip]

In a direct to plate environment there is generally no need to linearize the plates first. ( this explains why: The Print Guide: To linearize your CtP plates or not? )

 

[dazenmonkey]

Wow

Wow

Gentlemen,
Outstanding work! This thread is the bomb. Very helpful and a testament to some very wise lithographic minds.
Thank you all!