The illusion of variables in the density control problem of offset presses.
As someone who has worked with a fully closed loop inline density measurement scheme for almost 10 years, There are of course many variations, I could easily create a printed sheet with the same densities, but both looking completely different. As long as offset lithographic printing exists I don't believe we will ever remove all of the variables, all we can do is invent better ways of detecting and controlling them.
Ironically these systems designed to simplify the printing process get the best results in the hands of the best printers.
Ironically these systems designed to simplify the printing process get the best results in the hands of the best printers.
D
Deleted member 16349
Guest
Good question but also not such a good question. Even engineers have made similar errors in thinking about this issue.
The main point and one that greatly simplifies the problem of density control is that if you feed ink into the press at a constant rate (per ink key), it MUST come out onto the paper at a constant rate on average. The amount of water in the ink will not affect the average rate of ink going to the paper.
This does not mean that there are totally no variations in ink going to the paper but the variations must be on average related to the amount of ink going into the press. The variations would be about a very consistent average and would be at a level that would not be a problem.
As I have said before, you would not be able to wash the print density out if you greatly increased the water feed. And again, I am not recommending that one should run with high water settings. The demonstration that shows the one can not wash the print out is just to show how the interrelationship between ink feed and water feed has been broken.
D
Deleted member 16349
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You are right, there are many variables that cause variation. Positive ink feed does not solve everything but it does solve a very critical problem in offset. You have worked with a closed loop control control system for a long time. I am not sure what you mean by "inline". Is it in the press or next to the press and closed looped to the fountains? Makes a big difference.
Positive ink feed prevents disturbances from creating a lot of variations while closed loop only responds to variations after the disturbances have caused them.
Anyhow, I would hope that you agree that the new technologies perform better than what existed before. Positive ink feed is another benefit that helps improve performance.
And I agree, new technologies work better in the hands of more experienced operators. They can get the most out of them. That being true does not mean that one should stop developing new ideas. The process capabilities still need to be pushed forward.
turbotom1052
Well-known member
variation
variation
we all know that these variations exist and that try as we might to eliminate them there will always be some. this should not stop forward progress and historically it hasnt. i think its important that when moving forward we all keep our eyes on the ball. the inherant variations in the process should not be an excuse for press engineers to stop trying to control them.
what i think is most important when dealing with these variations as discussed is that they are isolated and dealt with individually. we all know the influence fountain solution has on color stability. we all know the influence dot gain has on color. nobody is disputing the need for ALL these issues to be recognized and addressed. but i feel that only by addressing them individually will we be capable of minimizing them. Of all the mentioned variables i too believe that the one with the biggest influences on the finished product is more precise ink feed control. the speed and efficiency of the make ready process on a sheetfed press i feel has more to do with accurate inking. any pressman worth his salt should be able to get a decent handle on dampening variations. if he cant do that he should probably be sent back to the feeder to hump paper. but to this day there still seems to be a need for more accurate inking. even with a good closed loop color system there are still fluctuations. now im fully aware of the fact that some of it may be caused by water control but there is STILL more work to be done with inking. allow me to put something out there for consideration that goes right to the heart of this......
this goes out to mostly pressman as my feeling is that they are the only ones that can really know exactly what im talking about. but if you eric can wrap yourself around this one i think it could provide a little insight as to some of mysteries we face.
lets go back for a minute to the pre cip3 and cip4 days. remember how you would toss a plate up on your console to do your initial ink settings? remember how we as pressman would strive for accuracy when it came to them initial settings? i can clearly remember looking at a form and entering what i thought was the required ink profiles for the plate being printed. there were times when my initial settings on all of the plates we pretty darn accurate. as a matter of fact i usually trust them more than i do current cip data. well now try to remember how every now and again youd get that mystery job that would require really large fountain settings in a zone with zero or close to zero ink takeoff.
ever wonder where all that ink was going???? now i know full well that at times you might have been compensating for some poor contact in your roller train but what about the times when you knew for a fact that all your roller settings were up to snuff. ive seen this phenomenon happen when the questionable printing unit was with new rollers freshly set to spec. to this day i still dont know where all that ink was going. ive at times had to print
5 times the amount of ink i originally set up the fountain for just to print a damn color bar up to density.
perhaps someone here can find all that missing ink cuz i know ive looked for it and not been able to figure it out. just an example of a part of the inking process that needs to at least be given some thought.
variation
we all know that these variations exist and that try as we might to eliminate them there will always be some. this should not stop forward progress and historically it hasnt. i think its important that when moving forward we all keep our eyes on the ball. the inherant variations in the process should not be an excuse for press engineers to stop trying to control them.
what i think is most important when dealing with these variations as discussed is that they are isolated and dealt with individually. we all know the influence fountain solution has on color stability. we all know the influence dot gain has on color. nobody is disputing the need for ALL these issues to be recognized and addressed. but i feel that only by addressing them individually will we be capable of minimizing them. Of all the mentioned variables i too believe that the one with the biggest influences on the finished product is more precise ink feed control. the speed and efficiency of the make ready process on a sheetfed press i feel has more to do with accurate inking. any pressman worth his salt should be able to get a decent handle on dampening variations. if he cant do that he should probably be sent back to the feeder to hump paper. but to this day there still seems to be a need for more accurate inking. even with a good closed loop color system there are still fluctuations. now im fully aware of the fact that some of it may be caused by water control but there is STILL more work to be done with inking. allow me to put something out there for consideration that goes right to the heart of this......
this goes out to mostly pressman as my feeling is that they are the only ones that can really know exactly what im talking about. but if you eric can wrap yourself around this one i think it could provide a little insight as to some of mysteries we face.
lets go back for a minute to the pre cip3 and cip4 days. remember how you would toss a plate up on your console to do your initial ink settings? remember how we as pressman would strive for accuracy when it came to them initial settings? i can clearly remember looking at a form and entering what i thought was the required ink profiles for the plate being printed. there were times when my initial settings on all of the plates we pretty darn accurate. as a matter of fact i usually trust them more than i do current cip data. well now try to remember how every now and again youd get that mystery job that would require really large fountain settings in a zone with zero or close to zero ink takeoff.
ever wonder where all that ink was going???? now i know full well that at times you might have been compensating for some poor contact in your roller train but what about the times when you knew for a fact that all your roller settings were up to snuff. ive seen this phenomenon happen when the questionable printing unit was with new rollers freshly set to spec. to this day i still dont know where all that ink was going. ive at times had to print
5 times the amount of ink i originally set up the fountain for just to print a damn color bar up to density.
perhaps someone here can find all that missing ink cuz i know ive looked for it and not been able to figure it out. just an example of a part of the inking process that needs to at least be given some thought.
D
Deleted member 16349
Guest
Tom, as usual you have very good comments.
The unfortunate fact about CIP3/4 is that a lot of effort has been made to get a standard method to get ink key presetting data to the press but very little effort was made to make that data accurate. The ink consumption requirements of a job are NOT directly related to the image coverage area on the plate. If you use the simple model for ink consumption based on the ink key zone image area alone, the errors can be as high as 40%. Ink consumption per ink key is job related and also press related. I covered this issue in my 1997 TAGA paper.
It is no surprise to me that on some jobs, your manual methods resulted in good results and at other times they did not. I had developed an experimental positive ink feed system in the early 1990's and it was run in production. We got image coverage data from a plate scanner and this was then used to set the outputs to the pumps used for each ink key. Some jobs were preset well and others were not.
Of course this was a problem because I also did not understand the cause but after a few months I realized where the error was. There error was in the knowledge base of the industry. I had assumed that what the industry had been saying about the relationship of ink consumption to image area was true. Actually it was grossly wrong. That misinformation wasted a lot of money and time because this experimental technology was partially dependent on accurate ink key presetting data.
I started to understand that the industry does not have the specific kind of knowledge that can be used to develop future technologies. That is when I started to develop a science myself based on a more in depth analysis of how things work. The good news is that many things can be explained and better still there is a great potential to make significant improvements that do not require large costs.
My quest is not just to make presses more consistent and predictable but to lead the industry in a direction that develops better theory that can actually be use to develop technologies, both hardware and software, that will have consistent and predictable performance at low cost.
Even today, the science and technical community in the printing industry use experimental results and a collection of confusing and faulty ideas of how the process works to develop technologies that are based as much on "I hope this will work" thinking as on engineering skills.
This goes for the engineering schools in Germany that specialize in the printing process and technology. I have seen recent PHD work that took many years of effort and which has been presented at graphic technology conferences, that will have basically no chance of resulting in any successful conclusion. It is so sad because it is quite easy for me to see that some basic questions were never asked that would have stopped the effort a long time ago. But it seemed clear that that engineering school had no idea of the implications of their concept.
Industry does this all the time too. Remember the single fluid ink from Flint. Promoted strongly at a previous Drupa as a potential new technology. Nothing wrong with the idea of a single fluid ink but it needs to pass a simple test. If it can't pass the simple test then it will not work. Having some theoretical understanding of what is required very easily tells what that critical test needs to be. Instead, Flint just thought that a bit more effort in the ink lab would make it successful. They were fooled into thinking that if one was at a 90% success point, that the additional 10% would not be a problem. Many times the last 10% is impossible to obtain because of some factors that were not understood. You don't hear about Flint talking about single fluid ink anymore. That must have hurt.
It happens to press manufacturers too. Goss had a Positive Keyless Inker press concept and it was a total failure. Not hard to understand why. I analyzed it in 1996. If they would have had the ability to analyze their own press concept, they would have avoided wasting a lot of money. They installed a few and had to modify many of them back to normal ink fountain inkers.
Good science not only leads to better technologies but it also helps prevent wasting money and time doing the wrong thing. Or buying the wrong thing.
gordo
Well-known member
I'm probably completely wrong on this (be gentle Erik) - I've seen the same thing happen usually with Annual Reports which often have very thin lines of ink - often a PMS ink - in the financial section.
I used to think that, because the lines were so thin that they would be over powered by the fountain solution. So the fountain solution would wash out the ink on those fine lines and you wouldn't get the density you thought you should based on the amount of ink you were pouring into the press.
However, if you put a large enough ink take off bar inline with those thin lines their density would increase to what you would have expected.
So, and this is complete speculation, I'm guessing that there is an ink push-pull relationship. You can push ink into the system, however, it also has to be pulled onto the sheet. The thin lines just didn't have a large enough area to pull the ink off the blanket. However, by putting an inline take-off bar on the sheet that increased the "ink pull" area in that zone and allowed to ink to flow which restored density to the thin lines.
gordon p
turbotom1052
Well-known member
the biggest reason for a take off bar gordo besides to control ghosting would be to be able to offer some control to the fountain. this is one of the reasons why its so much easier to get color on a heavy coverage form than a light coverage form. ive often been given jobs by the production dept in which a client has been scheduled for a press ok where the assumption was that because the form consisted of such small amounts of coverage that the makeready should be quicker. in reality the OPPOSITE is true. its not only alot easier to get color on a full coverage sheet like say a poster but you can also do it with less wasted sheets. adjustments to the ink fountain find their level much quicker when theres lots of "peel" off the ink fountain ball. another factor is that at higher ink levels like that, hitting on an acceptable ink level winds up being exponentially less critical. thats not to say that the ink feed doesnt need to be right but that theres a greater operating window to an acceptable fountain setting for any given ink zone. what often happens with those thin low coverage lines gordo is that the ink tends to "stagnate" on the rollers in them particular zones. so in effect the "push pull" anology you make is kind of true. the addition of a large take off bar allows the replenishment of fresh ink into the rollers. the unresolved mystery to me is that at times even after making a new plate with the addition of a take off bar that particular zone will not always require a corresponding addition to the ink feed. Again where on earth was all that ink going???
no doubt the addition of a large ink takeoff will probably result in a more stable ink /water emulsion but i think it has less to do with water control as it does with ink control.
no doubt the addition of a large ink takeoff will probably result in a more stable ink /water emulsion but i think it has less to do with water control as it does with ink control.
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D
Deleted member 16349
Guest
Oh come on Gordon, I don't try to be mean.
These are good points. One does supply the ink into the system which is a push like action, but the pull idea is very important and true. The plate pulls the ink from the form rollers but this only happens when the ink film is greater on the form rollers than on the plate.
In your example, the line on the plate pulls ink from the form rollers. So far so good. The problem you are describing IMO has to do with a lack of adequate oscillation capacity on the form rollers. In this example the critical issue is the inking of the plate and not so much the blanket or paper.
Think of this. The ink needed for that line comes from an ink key that is about 30mm wide. The ink from that ink key is to supply the ink needed for the line but it comes down the roller train in a wide band. The only thing that can supply the ink on the form roller, in line with the line is by the lateral movement of ink by the oscillators.
The ink that inked the line comes from the form roller surface and there is then a depression in the ink film on the form roller after the nip, where the line got its ink. That depression has only one rotation of the form roller to recover that original ink film. Since the ink volume is supplied in that band of ink coming down the roller train, there is some problem of getting it to that line at the desired rate. This causes a slight starvation of the ink going to the line.
By adding the take off bar, this increases the overall amount of ink going down the roller train and this then helps the lateral movement of ink to the line. What you would probably see is that in that take off bar, you might see a ghost of a lighter density of that line.
This is a little tricky for me to explain so I can understand if you don't quite get the idea. You need to be able to visualize how ink is moving vertically and laterally at the same time. How it fills depressions etc.
Anyhow there is an explanation and it is also something that can be designed out of the process with better roller train design so there would be no need to add take off bars.
The art of press design is to know how to manage ink films on form rollers that ink the plate, even under these extreme situations. There is a lot that can be done to improve press designs.
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Alex Arriaza
Well-known member
i wanted to test Erik´s ITB on a komori 40" but before that, i needed to prove to my boss that i was right and that Erik was right so i started on small presses, and started studying the way ink is transfered and i noticed that the problem we have is that we cant really control the volume of control needed for the press to run without variation in density, of course the ink going in is the ink that needs to go out and the storage needs to be constant no matter what, better yet we need to maintain a "constant flow" of ink during a run but this so called constant flow varies depending the job, imagine a 10 sq in image and a 20 sq in image, both at the same speed, you will then need a higher mass flow to provide the same results, so if you have this limitation that the presses do have "constant mass flow" you then will have to reduce the speed on the 20" to have the same results as in the 10", so i believe that the device that should be on the press doesn't need to be on the blade, but on the nearest point of the plate, then the plate will take what it needs, but, i do agree in something very clearly with Erik and that is that the oscillator near the ductor has to go.
I believe that the whole problem is because we need constant speed, (speed during a run) and the plate is the one that should demand the amount of ink needed for a job, so speed in the fountain rollers need to be constant to, is the plate picks only the amount of ink needed for the job and you have a volume of control so the plate can do this then you will solve the problem, its al about fluid dynamics and van der wals interaction nothing more than that, i haven't been able to make more investigations on the subject because its kind of busy in here but as soon as i make something ill post the results.
I believe that the whole problem is because we need constant speed, (speed during a run) and the plate is the one that should demand the amount of ink needed for a job, so speed in the fountain rollers need to be constant to, is the plate picks only the amount of ink needed for the job and you have a volume of control so the plate can do this then you will solve the problem, its al about fluid dynamics and van der wals interaction nothing more than that, i haven't been able to make more investigations on the subject because its kind of busy in here but as soon as i make something ill post the results.
D
Deleted member 16349
Guest
Alex,
It is quite clear you do not understand the concept.
Ink feed is adjustable for different jobs. Ink is adjustable across the press to what ever ink feed rate is required by the image across the press. It is very clear. You adjust the ink feed rate with the ink keys in the normal way.
Why have you not bothered to ask me any questions for all these months? I suspect that your judgment is not so good and I would prefer that you do not contact me again.
I am not allowed to discuss this concept here and I won't.
Alois Senefelder
Well-known member
Roller Trains - the "True Story" of their design !
Roller Trains - the "True Story" of their design !
Hello fellow Lithographers, learners also Mr. E. Nikkanen,
Salient Points -
#1 Lithographic Printing Ink is a "Non- Newtonian Fluid, #2 The fundamental principle of Ink Transfer is the "Walker & Fetsko" equation, proposed 50 years ago. #3 The primary funcation of the Vibrating Rollers is to diminish the formation of "Ribbing"
#4 What happens in the "Roller Nips" ? - 1 Fluid behavior? 2 This is governed by properties of the elements of the roller train which are 1a - roller dia. 1b hardness 1c thickness of the compliant roller cover 1d the amount of ink delivered to the Nip 1e Viscosity, pressure and roller stripe setting.
The printer receives his Ink as a "Flocculated Thixotropic Gel"
Regards, Alois
Roller Trains - the "True Story" of their design !
Hello fellow Lithographers, learners also Mr. E. Nikkanen,
Salient Points -
#1 Lithographic Printing Ink is a "Non- Newtonian Fluid, #2 The fundamental principle of Ink Transfer is the "Walker & Fetsko" equation, proposed 50 years ago. #3 The primary funcation of the Vibrating Rollers is to diminish the formation of "Ribbing"
#4 What happens in the "Roller Nips" ? - 1 Fluid behavior? 2 This is governed by properties of the elements of the roller train which are 1a - roller dia. 1b hardness 1c thickness of the compliant roller cover 1d the amount of ink delivered to the Nip 1e Viscosity, pressure and roller stripe setting.
The printer receives his Ink as a "Flocculated Thixotropic Gel"
Regards, Alois
Alois Senefelder
Well-known member
Part 2 the "True Story" of roller trains !
Part 2 the "True Story" of roller trains !
Gentlemen, I continue with more PDFs to whet your thirst for --- well ??????
Regards, Alois
Part 2 the "True Story" of roller trains !
Gentlemen, I continue with more PDFs to whet your thirst for --- well ??????
Regards, Alois
Alois Senefelder
Well-known member
Part 3 - the "Roller Train " story
Part 3 - the "Roller Train " story
Gentlemen - I hope I'm not boring you with this Saga !! so I shall continue !!!!!!!!!
yet more PDFs
Regards, Alois
Part 3 - the "Roller Train " story
Gentlemen - I hope I'm not boring you with this Saga !! so I shall continue !!!!!!!!!
yet more PDFs
Regards, Alois
D
Deleted member 16349
Guest
Alois, thanks for all the reference material. It is always interesting to see how people were thinking about problems.
I would say that it confirms my view that what has been presented in the industry as science is not good enough or specific enough to be used to develop any future technology that will have consistent and predictable results.
The statement that the "Walker & Fetsko" equation describes ink transfer is also questionable in the sense that it is probably difficult to use it in a practical way.
In the case of positive ink feed, the "Walker & Fetsko" equation is irrelevant. That's the great benefit of the positive ink feed approach.
Alex Arriaza
Well-known member
Erik, with all respect, i found very childish your attitude towards my reply, i just wanted to post my results, and my concerns to a wider audience, it has nothing to do with the ITB, the reply was about my doubts about how the process really works, thanks to alois, for posting all that info it is great, and im sorry Erik cant take critics very well
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