The illusion of variables in the density control problem of offset presses.

[Deleted member 16349]

It is said that there are lots of variables in offset printing and therefore the control of print density is very difficult. It is so complicated that it can not be described in a rational way that can lead to a direct solution of the problem. This has lead to the indirect solutions of operator or closed loop monitoring and adjustment.

This claim of variable complication unfortunately has provided a convenient excuse for many to not think deeply about the fundamental description. After all if something is so complicated, how can it be properly described.

The problem of density control is like a combination lock. It is very difficult to solve (open) if you do not know the combination but very easy if you do. Having the right combination is like having the rules of how to open the lock. A science for the density control problem should describe the rules of how it works.

The industry has looked at this problem for a long time. It has a lot of observations but observations are not descriptions of how it works and do not lead to what is required to solve the fundamental problem. It tends to suggest only steps to address the perceived variables.

So let's take a different view of the process. Let's just look at the simplest situation where we are printing a large solid. If one looks at the press with all its rollers, their nips and the addition of fountain solutions, trying to make sense out of ink/water balance and density variation can look like a overwhelming problem. If we look at the print, then the story is a bit different.

On the print we have a big solid area. Most people would say that the density is directly related to the amount of ink in the ink film that printed that solid. For this paper and ink combination, if the density is less, one would tend to say that there is less ink there. If the density is greater, then there is more ink there. When looking at the paper, one does not tend to say there is more or less water or that the press roller train temperature was higher or lower.

People could say that water or temperature changed and that resulted in more or less ink transfer, which would be an attempt to describe the cause but in the end, most would agree that the amount of ink on the solid determined the density level.

Changing the problem from density variation to ink variation, greatly simplifies the problem since ink is a physical substance and can be analyzed as a mass flow problem in the press.

The argument of ink transfer at the blanket paper interaction point does not explain the mass flow problem seen in printing. Changes in the ink transfer amount does not explain a steady state change in ink mass transfer or flow to the solid on the paper.

One needs to look at the total balanced mass flow of the ink going into the press and then going out onto the paper. From the principle of Conservation of Mass, we know that the ink going out must equal the ink going into the press.

Assume that the ink going into the press is constant, then the variables of water, temperature of the roller train, roller nips, etc. will have no affect on the amount of ink that goes out of the press. These variables are not really variables of the density control process because they are not variables of the mass of ink. Ink is not created or destroyed by these variables in the roller train.

But we tend see that print density and therefore the amount of ink printed on the paper varies when numerous so called variables are changed. There can be only one reason for this situation. Those variables must be affecting the amount of ink that is going into the press. It turns out that these variables are affecting the amount of ink transferred to the press roller train by the ductor.

The density control problem is not a chemical problem but a press design problem. Modern offset presses do not have positive control of ink feed going into the roller trains. Change the design.

 

[gordo]

Well put Erik.

In my experience, engineer types at their first exposure to it tend to look at printing as a deterministic process whereas most printers, usually without realizing it, look at it as a stochastic process.

If printing is looked at as a stochastic process then it can be managed somewhat by using intuition, craft, experience, gut feelings etc. Not most efficiently or effectively - but there is enough latitude in the mechanical aspects that it can be made to work to an acceptable level.

If, on the other hand, printing is looked at as a deterministic process, then one needs solid information about the fundamentals of how all the components work individually, and in combination, in order to develop solutions to make it work efficiently and effectively. Unfortunately, that information is hard to come by, or most often not available at all. It requires basic research and thinking. Resources that seem all too often to be in short supply.

best, gordon p

 

[printerpete]

why dont you change the design erik?
you seem to have a hate against press designers or manufaacturers.
the presses are built the way they sre, which is a million times better than they were 25 yrs ago, deal with it or if you dont want to deal with it design a press yourself and run it

 

[Deleted member 16349]

why dont you change the design erik?
you seem to have a hate against press designers or manufaacturers.
the presses are built the way they sre, which is a million times better than they were 25 yrs ago, deal with it or if you dont want to deal with it design a press yourself and run it

Printerpete, I would love to be involved with new press design. There are lots of opportunities for improvement.

You should actually be more interested that offset press design improves. If offset does not improve, digital presses will take over. It is in your interest, as a press operator, to have offset advance against digital presses. To be a lot better at very short makereadies, higher capacities and less running waste.

It is great that printers are proud of their skills and their craft but having so much pride that it comes out as a position against advancing the offset process will only hurt offset press operators in the end. Printing companies can always go to other printing technologies to get their product out the door. All they need is a process that meets their customer's needs.

I am not a press operator so if the industry eventually goes to digital presses, that does not affect me. I want to give offset a better fighting chance.

 

[Deleted member 16349]

Well put Erik.
If, on the other hand, printing is looked at as a deterministic process, then one needs solid information about the fundamentals of how all the components work individually, and in combination, in order to develop solutions to make it work efficiently and effectively. Unfortunately, that information is hard to come by, or most often not available at all. It requires basic research and thinking. Resources that seem all too often to be in short supply.

best, gordon p

That hard to find information is what I have developed. It is just waiting for the right group to come forward. It would be a fundamental change, which is hard for people to imagine.

 

[CoryKomori]

Erik,

Erik,
Once again I am super intrigued by this technology, but I do have another question. Recently Komori came out with a technology ( they call it P4 ), where it puts the principle " path of least resistance "
to work. Where they can physically duct ink back into the fountain. If the ink ball holds an ink film thickness of 3 microns and the ink train ( ductor ) holds an ink film of 6 microns, and the ductor is engaged " X " amount of times, the ink will actually travel back into the fountain, thus reducing the ink film on the roller train. Would the ITB prohibit this process ? Thank you

 

[Tron]

Wow Komori is just now implementing that concept. manroland has been able to duct ink back into the fountain for 20 years. It is called ink scraper.

 

[tmiller_iluvprinting]

Printing is no longer an art, or craft. It is a manufacturing process, profit margin will be the driving force behind the digital evolution. It may already be too late to improve the offset process/press.
Regards,
Todd

 

[Deleted member 16349]

Erik,

Erik,
Once again I am super intrigued by this technology, but I do have another question. Recently Komori came out with a technology ( they call it P4 ), where it puts the principle " path of least resistance "
to work. Where they can physically duct ink back into the fountain. If the ink ball holds an ink film thickness of 3 microns and the ink train ( ductor ) holds an ink film of 6 microns, and the ductor is engaged " X " amount of times, the ink will actually travel back into the fountain, thus reducing the ink film on the roller train. Would the ITB prohibit this process ? Thank you

CoryKomori,

First I have to say that I am not allowed to discuss the Ink Transfer Blade technology on the forum or on PrintCEO. Some readers complained and Randy Davidson has clearly told me that any discussions about it are not allowed.

The post about the illusion of variables applies to any offset press and the solution applies to any positive ink feed concept that is done properly. There are positive ink feed systems already on the market from Goss (Digirail) and Controls Group. Actually some of these systems have been on newspaper presses for a long time. One problem with these systems is that they apply the ink incorrectly and this results in a problem that was not there before which results in a very long response time and other variation conditions. I understand this problem because I made the same mistake with an experimental designed pumped ink fountain in the early 1990's.

Back to your question. The P4 technique you describe will not work with any positive ink feed concept. It will also not work with a continuous ductor, where ink is normally sheared off the ink fountain roller by means of a 0.004 to 0.006 thou gap between the ink fountain roller and the continuous high speed ductor.

I have been analyzing and developing press design concepts related to how they affect ink film management on roller trains, for a very long time. This gives me a different perspective on what should be done.

I know Komori and other press manufacturers have developed technologies and methods to get around problems but I tend to think of ways to design the press to avoid those problems.

Your P4 method is aimed at reducing ink on the roller train. Of course what you describe happens every day on presses as they run jobs. Close the keys and ink will come back.

Managing the ink films on the roller train is the real goal. One wants the ink on the roller train to be as close as possible to the running conditions of the next job. This reduces the transition and therefore reduces wasted paper.

Instead of applying all these tricks and techniques to adjust ink films on the roller train, which I think are wasteful of time and paper, I look at how the press can be designed so it is not necessary to make such adjustments.

If one looks how ink is stored on the roller train for high coverage and low coverage, one can see that there is a difference in ink quantity. If you go from a high coverage job to a low coverage job, the starting conditions will be that there is too much ink on the roller train.

There are at least two issues here. One is accurate ink feed, which is needed to control the ink storage on the roller train and the other is the design of the roller train itself.

It is critical to have accurate ink feed because if one goes into conditions where the ink feed does not match the desired ink feed for the print, the ink storage on the roller train will increase or decrease and then a transient will be required to get back to where it should be. The goal is to avoid the transients. Accurate ink feed controls the ink storage level in the press.

Roller train design can help because if you have a roller train design that stores ink in a way that there is not much difference between high coverage and low coverage, then it is easy to just go from one job to the other.

Even existing roller trains are not as bad as one thinks. From computer simulations I have had done, going from a high coverage job to a low coverage job, the density only slightly overshot its steady state conditions and slowly converged to its target. Computer simulations of this sort, are programmed assuming positive ink feed. For this problem of going to a low coverage job, positive ink feed is critical in order to avoid driving the ink storage on the roller train to a wrong level.

I see two stages of press development potential. Stage one is to modify the existing designs so that they have positive ink feed and accurate presetting data. With that level of modification, I am expecting that a normal press will get to the target densities in about 50 impressions without ink adjustments from an operator or closed loop control.

Stage two would be the redesign of the roller train itself plus other things. In that case, I would expect to be at target densities in about 20 impressions without any ink adjustments by the operator or closed loop control. Also the manufacturing cost of that press would be less than existing presses. In both stages, density would be consistent throughout the run. It could also be applied to sheetfed or web presses.

I hope that gives you an idea of what is potentially possible. Now all I need is a press manufacturer who wants to go there.

 

[Deleted member 16349]

Printing is no longer an art, or craft. It is a manufacturing process, profit margin will be the driving force behind the digital evolution. It may already be too late to improve the offset process/press.
Regards,
Todd

You maybe very right about it being too late. The longer the press manufacturers delay in rethinking the process, the harder it will be to fight back.

It is like a war and the digital presses have landed on the beach and are moving inland.

If the press manufacturers have no will for a counter attack, they will just get pushed back to a smaller market territory.

 

[Ian Mackenzie]

You maybe very right about it being too late. The longer the press manufacturers delay in rethinking the process, the harder it will be to fight back.

It is like a war and the digital presses have landed on the beach and are moving inland.

If the press manufacturers have no will for a counter attack, they will just get pushed back to a smaller market territory.

Erik -

You are referencing military activities from last century. The landing took place around 1995. By 2000, the beach had been captured along with several small towns and an airstrip.

By 2005, the captured indigenous tribes people were already speaking the dialect of the invading army. By 2008 (DRUPA), the displaced refugees had finally ceded to the inevitable and had lost the will to fight back. After a significant financial and morale hemorage, they joined forces with the conquerors for the betterment of (print) society.

Today, there is no "us vs. them" in print. It is "print vs. everything not print". Now the manufacturers are seeing the errors of their arrogance and have rushed to swim downstream with the new spawning innovators.

You know what happens to salmon who swim upstream, right?

Mixed metaphors aside, howling at the moon gets you a perpetually sore throat. Today, there are not enough new offset presses sold to justify changes in core engineering. However, all that said, if your ITB system can be engineered as a 3rd party retrofit to existing iron (like 3rd party closed loop color), then it sounds like a winner!!

otherwise, I recommend Halls throat lozenges and a new helmet.

Cheers,
Ian

 

[Deleted member 16349]

Erik -

You are referencing military activities from last century. The landing took place around 1995. By 2000, the beach had been captured along with several small towns and an airstrip.

By 2005, the captured indigenous tribes people were already speaking the dialect of the invading army. By 2008 (DRUPA), the displaced refugees had finally ceded to the inevitable and had lost the will to fight back. After a significant financial and morale hemorage, they joined forces with the conquerors for the betterment of (print) society.

Today, there is no "us vs. them" in print. It is "print vs. everything not print". Now the manufacturers are seeing the errors of their arrogance and have rushed to swim downstream with the new spawning innovators.

You know what happens to salmon who swim upstream, right?

Mixed metaphors aside, howling at the moon gets you a perpetually sore throat. Today, there are not enough new offset presses sold to justify changes in core engineering. However, all that said, if your ITB system can be engineered as a 3rd party retrofit to existing iron (like 3rd party closed loop color), then it sounds like a winner!!

otherwise, I recommend Halls throat lozenges and a new helmet.

Cheers,
Ian

Very good Ian.

Probably the fight is between the remaining press manufacturers for that bit of territory that remains. A counter attack by offset against digital presses is not the only path. One press manufacturer can grow by beating the others into the dirt. Last man (press manufacturer) standing, wins even though the total market shrinks.

 

[Cornishpastythighs]

Erik -

if your ITB system can be engineered as a 3rd party retrofit to existing iron (like 3rd party closed loop color), then it sounds like a winner!!
Cheers,
Ian

I thought the ITB could be retrofit to existing press equipment.

 

[gordo]

I thought the ITB could be retrofit to existing press equipment.

I don't think that Erik can answer that question: "First I have to say that I am not allowed to discuss the Ink Transfer Blade technology on the forum or on PrintCEO. Some readers complained and Randy Davidson has clearly told me that any discussions about it are not allowed. "

I wonder what the complaints were about.

best, gordon p

 

[Deleted member 16349]

I don't think that Erik can answer that question: "First I have to say that I am not allowed to discuss the Ink Transfer Blade technology on the forum or on PrintCEO. Some readers complained and Randy Davidson has clearly told me that any discussions about it are not allowed. "

I wonder what the complaints were about.

best, gordon p

I brought the topic up in too many places where it did not belong. A valid complaint. I am going to follow that restriction in the present and into the future, even if the topic is demonstrated to be true. I will not discuss it on WTT.

I will discuss general science related to the fundamental problems.

 

[turbotom1052]

my feeling on this is not so much a need to control high storage vs low storage in the ink train as it is to more accurately control the ink feed from the fountain. precise replenishment of ONLY the amount of ink stripped off the roller train in any given ink zone within the requirements of the particular plate form needs to perfected. im not real familair with the above mentioned systems that would actually strip off excessive ink film thicknesses zonally but its my feelings that any efforts to better control this process needs to be codependant on an equal effort to better bring prepress curves into line with running consistent ink films from job to job!!!

 

[Deleted member 16349]

my feeling on this is not so much a need to control high storage vs low storage in the ink train as it is to more accurately control the ink feed from the fountain. precise replenishment of ONLY the amount of ink stripped off the roller train in any given ink zone within the requirements of the particular plate form needs to perfected.

This is correct in my view also. The amount of ink stored on the rollers is a by product of getting the right ink "infeed" condition for the print. The right amount of ink that goes to the plate and then to the blanket and paper, is very much related to the ink films on the form rollers and roller train.

One does not control the ink films on the roller train directly but the various ink film thicknesses on all rollers are generated by splitting of ink in the nips. When one gets to steady state conditions where the ink being fed into the roller train is exactly the amount of ink being printed, then the ink distribution on the roller train also has settled into a relatively consistent condition.

Going from one density to another requires going through a transient from one ink storage distribution on the roller train to another. That is one reason why it takes such a long time for low coverage to respond . It takes a lot of time to change the ink distribution on the rollers when feeding only a small amount of ink.

In general the ink distribution on the roller train is not so important because it is just a result of matching the ink feed requirements of the image. One can not force a specific distribution and one can not be sure what it really is. Fortunately it is not important for control at steady state.

 

[turbotom1052]

i did a test years ago on a 4/c sheetfed press to prove a point. ive always been a firm believer in the visual ink film thickness on the rollers. after hangin 4 plates and visually setting the fountains on a non console press i proceded to register 4 colors without paying any attention at all to the printed images. i then went on to do a full color makeready without ever looking at a single sheet. i would run a hundred or so waste sheets then visually observe and adjust as needed the ink film visable on the form roller right above the plate. after a series of about 4 or 5 pulls i then comapred my sheet to the provided matchprint. my color was very close to the matchprint and was able to get a color ok with very little adjustment. after proving my point i handed the densitometer to the pressroom supervisor and asked him to keep it in his office if he needed it.

 

[Deleted member 16349]

i did a test years ago on a 4/c sheetfed press to prove a point. ive always been a firm believer in the visual ink film thickness on the rollers. after hangin 4 plates and visually setting the fountains on a non console press i proceded to register 4 colors without paying any attention at all to the printed images. i then went on to do a full color makeready without ever looking at a single sheet. i would run a hundred or so waste sheets then visually observe and adjust as needed the ink film visable on the form roller right above the plate. after a series of about 4 or 5 pulls i then comapred my sheet to the provided matchprint. my color was very close to the matchprint and was able to get a color ok with very little adjustment. after proving my point i handed the densitometer to the pressroom supervisor and asked him to keep it in his office if he needed it.

Very interesting test. This is a bit related to your test. If one does a calculation of the ink film thickness on the last form roller of a press with say 4 form rollers, the ink film thickness is not very much different between high coverage and low coverage image areas. I would say that your test and experience and the calculation of ink film on the last form roller would tend to agree. I hope I was understanding your test properly.

Viewing ink film on a rotating roller takes a lot of skill. That would be something I could never do. I can hardly see my monitor clearly.

 

[turbotom1052]

my visual of the roller was not while it was spinning. id inch the press around a couple of revs and look to read the "nap" of the ink film before the split film settled and smoothed over. by consistently doing it in this way ive learned with experience what the proper ink film should look like under them repeatable conditions. your absolutely right in saying that the differences in ink film thickness between high take off areas and low take off areas should be pretty minimal. its all about repenishment of stripped off ink. ive always tried to pass on these pearls of wisdom to press assistants that i was training. ive found it beyond the grasp of many and even suprised to find that some accomplished pressman dont understand the concept as we are discussing it. this is one of the very reasons that im not that fond of densitometers in many instances. dont get me wrong there is a place for densitometers but i feel many in the industry use them as a crutch.