ink flow

[pgr]

Does anyone know professional expression of ink flow, and most of the time, the color designed by clients cann't be achieved in printing due to so-called ink flow which means printer can only adjust the color in column, how do you guys solve this problem and how to express this term more professionally ?
thanks in advance.

 

[Paultheprinter]

i assume your talking about the ink density difference from grip to the tail of the sheet, ive heard this called tail off, fall off or resipricication errors. i believe a density difference of 0.05 is acceptable for a 74 sized press but there is no way of controling this prior to getting job on press unless prior experience of the form is gained. By this i mean if a standard form for a customer is printed and unacceptable standard achieved the froms could be modified to mask any press problems, this to my knowledge this is not a common practice but i have heard of this from these forums, these errors can be masked on most presses by changing the reverse point of the oscilators in relation to the plate but it on some forms this could prove imposible.If this is no help then sorry in advance if im on the right track then repost

Paul

 

[pgr]

it doesn't sound like what i'm getting at, i mean, sometimes since the layout of ink fountain, vertically from grippper to tail, we have to forgo matching color of less important image in order to match the more important part, that's what i want to know, what's it called professionally?

 

[arfamoe]

Do you mean that when you are printing a job that has different images across the sheet and gripper to tail you have to pick an image and set your ink density so that you can achieve a good colour match on that image and forgoe the rest of the images. I'm not sure it has a name but we have all encountered this at some time i am sure. Client likes an image but doesnt like the one behind it, we cannot adjust ink density to achieve this, it has to be corrected in the files. It used to be more common with advertisements that were from different film houses that were to be printed on the same sheet. Each image required a different density so it made it hard to achieve a uniform result, Am i on the right track at least?

 

[pgr]

Yes, arfamoe, you get it right, are you sure not a term to describe this issue? when you encounter this problem, how you guy describe it in a simple and clear way, and how to solve that problem besides modifying files.

 

[arfamoe]

If there is a technical name for this i dont know it, maybe someone else on this forum has and idea.
You cannot fix this problem on the press unless it is a mechanical ink laydown issue with your press, some presses print higher ink density at the front of the sheet than the back of the sheet and some the opposite.
This can sometimes be corrected with mechanical adjustements to the oscillation.
If an image on the sheet is acceptable and the image directly behind it is unacceptable the press or ink density adjustments cannot correct this. In the digital age this is something that can be corrected although you should not fix the file if the press itself has issues.
Run a fingerprint test form to establish your density/dot gain curves and make these available to anyone who will be generating files and proofs for you.
Print some dry solids on the press to see if you have a mechanical issues with one or more print units. I use cyan ink for this as density variations are seen more easily.
Hope it helps

 

[Alois Senefelder]

Mechanical Ghosting !!!!!!!!

Mechanical Ghosting !!!!!!!!

Hello everyone,

" The extent to which mechanical ghosts occur is a function of Ink Train design. Spot and runout ghosts are caused by uneven takeoff by the plate in the travel direction, and are mitigated by using multiple Form Rollers and by judicious selection of branch point locations.
Starvation Ghosts are caused by abrupt changes in Ink Coverage in the across-the-press direction. There is relatively little that the press designer or operator can do to mitigate this latter type of ghosting"

regards, Alois

 

[gordo]

This is one of the realities of the difference between presses and proofers and scanners that many color management pundits seem to (want to?) ignore. A press, unlike a proofer or a scanner, lays down color (ink) from lead to tail in such a way that, depending on press design and condition, ink usage (color) at the lead can affect ink usage (color) at the tail. The classic example is a press form like this:

The tomato will affect the color of the lady's skin tones and there is very little the press operator can do about it.

best, gordon p

my print blog here: Quality In Print

 

[SpeedKing]

This is one of the realities of the difference between presses and proofers and scanners that many color management pundits seem to (want to?) ignore. A press, unlike a proofer or a scanner, lays down color (ink) from lead to tail in such a way that, depending on press design and condition, ink usage (color) at the lead can affect ink usage (color) at the tail. The classic example is a press form like this:

The tomato will affect the color of the lady's skin tones and there is very little the press operator can do about it.

best, gordon p

my print blog here: Quality In Print

or is there?mmmm,how about adjusting the strike? doing this on the magenta could in theory make the tomato less red and the lovely lady blush? the strike determines were the ink flow starts on the plate,from the grip,the middle or the back edge.this is adjustable on most heidelbergs present and past.

 

[Deleted member 16349]

Hello everyone,

" The extent to which mechanical ghosts occur is a function of Ink Train design. Spot and runout ghosts are caused by uneven takeoff by the plate in the travel direction, and are mitigated by using multiple Form Rollers and by judicious selection of branch point locations.
Starvation Ghosts are caused by abrupt changes in Ink Coverage in the across-the-press direction. There is relatively little that the press designer or operator can do to mitigate this latter type of ghosting"

regards, Alois

The press designer can improve or eliminate starvation ghosting. The Single Form roller press designs from Heidelberg and KBA eliminate this problem by inking the single form roller that is printing in register to the plate image. These single form roller presses do not have a uniform ink film on the form roller but attempt to have a uniform ink film in the areas that print an image.

Starvation ghosting on conventional presses is not caused by the abrupt changes in ink coverage of the image on the plate. I would say that it is better to say that the design of the press can not provide an even ink film to the plate under those conditions. It is a press design issue at the fundamental level.

Starvation ghosting on conventional presses is due to two factors. One is the resolution of the ink key spacing. If one could supply ink into the press with a profile that had infinite small zones of adjustment of ink across the press, one would not see starvation ghosting. At this time it is not practical to have such an ink feed system and therefore having a system of wide ink keys is a problem.

The second factor is the lateral oscillation in the press. Since the ink key zones are relatively wide, the only way to counter this problem is with the lateral movement of ink by vibratory or oscillating rollers. There is a problem when this is not designed properly.

There are three distinct modes of oscillation that can be in a press. I described these in my 1997 TAGA paper. One mode moves ink laterally from regions of high coverage to regions of low coverage. This makes starvation ghosting worse. Another mode just smooths ink but does not move it laterally. This is helpful. The third mode of oscillation will move ink laterally from low coverage regions to high coverage regions. This kind of oscillation would help greatly in reducing or even eliminating starvation ghosting. Unfortunately, most modern offset presses have predominantly the first mode of oscillation in their designs.

There is a lot that a press designer can do to eliminate mechanical and starvation ghosting beside my favorite topic, the elimination of ink/water balance and obtaining consistent density control.

Please don't give up on press design yet! It has wonderful potential for improvement.

 

[Al Ferrari]

"These single form roller presses do not have a uniform ink film on the form roller but attempt to have a uniform ink film in the areas that print an image."

Hi Erik,

Can you please expand a bit on this particular statement? I think the difference in ink uniformity between the two areas you refer to may not be obvious to all of us. In fact, am not sure I understand what you mean exactly. As you know, I have been very impressed by these single form roller presses from Heidelberg and KBA .

Thanks,

Al

 

[Al Ferrari]

Are you referring to the face in line with the tomato, or to her left hand which is anemic compared to her right hand?

Al

 

[Deleted member 16349]

"These single form roller presses do not have a uniform ink film on the form roller but attempt to have a uniform ink film in the areas that print an image."

Hi Erik,

Can you please expand a bit on this particular statement? I think the difference in ink uniformity between the two areas you refer to may not be obvious to all of us. In fact, am not sure I understand what you mean exactly. As you know, I have been very impressed by these single form roller presses from Heidelberg and KBA .

Thanks,

Al

Hi Al,

It is not obvious and is a bit difficult to explain but I will try.

In the case where a single form roller is being inked by another roller that does have a uniform ink film one has to look at how the ink films split. If one works from the substrate up to the plate, one can determine ink film thickness of the image areas.

For analysis, we assume all ink films split 50%-50%. May not be true but it helps see how things might work.

If the ink film on the substrate has a value of 1T this had to be the result of the ink film on the blanket being 2T before the split at the nip to the paper and after the nip the ink film on the blanket must be 1T.
(The actual value of T is not important. It can be more or less depending on the density being printed. All the ink films are proportional in this model.)

To get 2T on the blanket before the impression nip requires that the ink film on the plate after the plate/blanket nip is also 2T because of the 50-50 split. 2T + 2T = 4T. A total of 4T must be going into the plate/blanket nip. Since the return ink film on the blanket is 1T this means that the ink film on the plate before the plate/blanket nip must be 3T.

At this point, the ink film on the image areas is the same for all offset presses. One will see 2T before the inking rollers and 3T after the inking rollers. This is independent of how many form rollers one has.

Now let's look at a single form roller that is printing in register with the plate. This form roller needs to add 1T of ink film to the plate in the image area. We start with the image area in this example.

If the plate has 3T after the form/plate nip, then the form roller must have 3T in the related image area on the form roller. Since the total of the two ink films is 6T, then the form roller must have 4T in the image area to add to the 2T on the plate before the form/plate nip.

Now in this example, we are inking the form roller with an uniformly inked roller. This could be the anilox roller used in the Heidelberg or KBA presses or even some other uniformly inked roller.

If we keep the logic going, the ink film on the supply roller in the image area must also be 4T after the supply/form roller nip due to the 50-50 split. The total is 8T in that nip and therefore, the ink on the uniformly inked supply roller must be 5T to combine with the 3T on the form roller in the image area.

I hope you have been sketching the roller diagram and labeling the ink films.

The above description describes the image area but what about the non image area. If no ink is being taken by the plate from the form roller in the non image area, then the ink film must be exactly the same as the supply roller. So the ink film in the non image area is 5T.

This description is for a form roller that prints on every rotation. With the Karat press, the form roller rotates twice for each print and this complicates the calculation and I will not try to describe it here but I hope you get the idea.

So we have on the example single form roller that prints in register, supplied by a uniform ink film roller, 5T on the non image area of the form roller and 4T on the image area of the roller. This is fine since we are mainly interested in how the image area of the plate gets inked. No mechanical or starvation ghosting is possible.

It is nice to be able to analyze the ink films to know that they are different in the image area and non image area but one can also understand that it a different way. If the form roller did not print in register, one would see mechanical ghosting which is a result of nonuniform ink films on that form roller.

Some interesting issues.

In the screen area we have a mixture of image area (dots) and non image area. In a simple theory the dots should get the 4T and the non image area between the dots would have 5T on the form roller. But I suspect in reality, or should I say a more complicated theory, the ink films on the form roller will mix in these fine detail areas. That does not mean that the print is going to be bad but I suspect from this analysis that these presses tend to have higher dot gains in the low percentage range of screens due the the availability of this extra ink which could have more mechanical dot gain.

Another interesting issue. I work at a company that operates dry offset presses for printing on plastic food containers. All of these presses have a 1st form roller that prints in register with the plate. One interesting observation has been that these presses require form rollers that are quite hard. Durometers of 60 are the norm. A 1st form roller with a durometer of 30 would not print properly. I think this is because the fact that the form roller is printing in register, it needs a hard roller to squash uneven ink that looks like mottling. A mottled ink film will print in register and continually reinforce the patern. A form roller that is not printing in register would not produce such a mottle since the peaks in the ink are continually landing in different locations.

There are other interesting concepts that have potential. Anilox supply rollers is not so great because there is no way that I know to make it a positive ink feed device. Therefore it inherently can vary.

 

[Al Ferrari]

Thanks for the detailed answer Erik. I need to spend some quite time with it using the pencil and paper. Great analysis.

Al

 

[Tarun Chopra]

Hi Erik,

Great explanation, it explains a lot of things but I am confused as to what you need to imply when you say in para 5:

Since the return ink film on the blanket is 1T this means that the ink film on the plate before the plate/blanket nip must be 3T.

What I have understood of the entire process is that (presuming a 50-50 inksplit and 1 micron of inkfilm is required on paper) before plate banket nip inkfilm thickness is 4 (microns) --- After plate blanket nip is 2 (microns) THEN before blanket to impression nip inkfilm thickness is 2 (microns) --- After blanket to impression nip is 1 (microns).

This 1 (microns) remains on the blanket and moves back to the plate blanket nip ie before the plate blanket nip there is 5 (microns) of inkfilm thickness ie 4 (microns) from plate and 1 (micron) from blanket, what I understand from the above quote is that its (3 microns).

Have I understood the point you are making correctly or I am totally of track???

Regards
Tarun Chopra

 

[Deleted member 16349]

Hi Erik,

Great explanation, it explains a lot of things but I am confused as to what you need to imply when you say in para 5:



What I have understood of the entire process is that (presuming a 50-50 inksplit and 1 micron of inkfilm is required on paper) before plate banket nip inkfilm thickness is 4 (microns) --- After plate blanket nip is 2 (microns) THEN before blanket to impression nip inkfilm thickness is 2 (microns) --- After blanket to impression nip is 1 (microns).

This 1 (microns) remains on the blanket and moves back to the plate blanket nip ie before the plate blanket nip there is 5 (microns) of inkfilm thickness ie 4 (microns) from plate and 1 (micron) from blanket, what I understand from the above quote is that its (3 microns).

Have I understood the point you are making correctly or I am totally of track???

Regards
Tarun Chopra

Tarun,

Hopefully I understand your question correctly and can clear up the confusion.

When analyzing ink distributions in roller trains one must look at the equation at the nips. The law of Conservation of Mass governs the equation. Therefore the amount of ink going into the nip must be equal to the amount of ink going out of the nip.

Since the goal is to have 1T ( what ever that ink film thickness is ) on the paper, we start from there and work up. One can not arbitrarily select an ink film on any surface of the roller train from the paper up. It is a result of the splitting. (Note, ink films are not integer values. They are often fractions.)

Each nip has an equation relating the ink films on the surfaces before the nip and on the same surfaces after the nip.

ink film on surface 1 plus ink film on surface 2 = ink film on surface 1 plus ink film on surface 2

and


After the nip ink film on surface 1 = ink film on surface 2 due to the 50-50 split.

Diagrams are great help to understand the conditions and your diagram clearly shows a problem.
You have an equation that is:

4T + 1T = 2T + 2T 5T = 4T This can not be true.

When I said a total of 4T must be going to the plate/blanket nip this total is from the 3T on the plate and the 1T on the blanket.

When calculating these ink films from the paper up to the plate, the method is quite easy and is the same for every offset press. Ink films calculations in the roller train are much more complicated because each nip represents an equation and the solution is to solve all the equations simultaneously. Fortunately there are math programs that do this fairly easily. Each different roller train design has to have a different calculation. Changing the roller train by adding or removing a roller or nip condition might have surprisingly large effects on the ink film calculations.

In the density control problem, the average consistency of the SID is only related to the ink feed and not the roller train. If the ink feed is made positive, in that it feeds ink at a consistent rate, density consistency is independent of roller train design. **This is very good news.** It means that any conventional and legacy offset press can be made to print consistent density that is independent of variables such as water, press speed, press temperature.

Roller train design is important to provide consistent ink film at all locations on the plate which would avoid mechanical and starvation ghosting and also a consistent print that can be profiled for colour management. If one can not ink the plate consistently in all locations then the press can not have a reliable profile. There is a problem even with modern offset presses with respect to this.

Managing the ink feed and the ink distribution on the surfaces of the rollers in the roller train are where the opportunities are for advancing the design of the offset press to make newer versions that make the older ones obsolete.

I hope my response helped.

 

[Tarun Chopra]

Hi Erik,

Please confirm if the correct explanation can be done with this attached jpg file.

Does positive inkfeed technology reduce starvation or ghosting related issues on press? Have you implemented positive inkfeed at commercial printing locations? Positive inkfeed makes lot of sense.....

Thanks
Tarun Chopra

 

[Deleted member 16349]

Hi Erik,

Please confirm if the correct explanation can be done with this attached jpg file.

Does positive inkfeed technology reduce starvation or ghosting related issues on press? Have you implemented positive inkfeed at commercial printing locations? Positive inkfeed makes lot of sense.....

Thanks
Tarun Chopra

Tarun,

I would say that now your diagram is correct.

Positive ink feed in general has no affect on reducing mechanical or starvation ghosting.

Mechanical ghosting is closely related to how the image on the plate affects the ink film on the form rollers.

Starvation ghosting is related to the resolution of the ink profile of the ink fountain. (width of ink keys) and the application of oscillation by the vibratory rollers in the roller train. The wrong application of oscillation will make starvation ghosting worse and the correct application of oscillation will reduce or eliminate starvation ghosting. In designing a roller train, consideration to the placement, size of rollers and lateral speed of oscillation are required to have a press that inherently does not have mechanical or starvation ghosting.

I have tested my version of positive ink feed (Ink Transfer Blade) on three different presses and it confirms the theory. I am not a manufacturer and it really is up to press manufacturers to move in this direction.

In the market right now, the closest thing to positive feed is with this the Digital inkers by Goss and others. These systems feed ink in a positive volumetric way but they are applied incorrectly in the presses they used in and there for do not get the full potential of the concept. This weakness in application was referred to in my 1997 TAGA paper.

 

[Al Ferrari]

"One interesting observation has been that these presses require form rollers that are quite hard. Durometers of 60 are the norm."

So is this the approximate durometer of the rubber blanked covering on the form rollers on the KBA and Heidelberg versions of these Anilox supplied presses? Or does the substrate they print on make a difference? I am also thinking here of the dedicated Metronic presses that print directly on CDs. Are these comparable to these dry offset presses for printing on plastic food containers?

"Anilox supply rollers is not so great because there is no way that I know to make it a positive ink feed device. Therefore it inherently can vary."

I would seem that the Anicolor press would be more subject to this with it's use of a water fount. Any one know of reports comparing the Anilcolor and the Gravuflow presses based on actual field observations?

Al

 

[Deleted member 16349]

"One interesting observation has been that these presses require form rollers that are quite hard. Durometers of 60 are the norm."

So is this the approximate durometer of the rubber blanked covering on the form rollers on the KBA and Heidelberg versions of these Anilox supplied presses? Or does the substrate they print on make a difference? I am also thinking here of the dedicated Metronic presses that print directly on CDs. Are these comparable to these dry offset presses for printing on plastic food containers?

"Anilox supply rollers is not so great because there is no way that I know to make it a positive ink feed device. Therefore it inherently can vary."

I would seem that the Anicolor press would be more subject to this with it's use of a water fount. Any one know of reports comparing the Anilcolor and the Gravuflow presses based on actual field observations?

Al

Al,

I would not know the comparable durometer of the blanket covered form rollers but I would not be surprised if it was higher than conventional form rollers. Also Dry Offset uses a raised plate similar to flexo. This might also have an effect on the need for such a durometer. I am not sure how the substrate might affect this printing condition, although we print on plastic and that would tend to highlight mottle like conditions since the substrate does not absorb ink.

I would expect the Anicolor press to have more problems than the Gravuflow and I would also like to hear about actual comparisons in running conditions. I am curious.

Al, there have been press designs that are potentially very good at reducing or eliminating mechanical and starvation ghosting. Diddie's MVP and Heidelberg's TOK press seem like they would do the job and with the ITB on them, they could potentially compete well with the Anicolor. They would be able to run any special inks and also have very consistent density control. With the right presetting software, they would basically perform like a keyless concept press. That then would result in very short makereadies.