Questions for you.
If I increase the press speed, thereby shortening the press interval, thereby decreasing the tack of the first down ink with respect to the second down ink, thereby decreasing the trapping of the second down ink by the first down ink. How will your proposed, more consistent ink film thickness at the uppermost inking rollers affect the ink film transfer between the second down ink film between the blanket and the first down ink film already applied to the sheet?
Being that your Patent indicates modifications at the uppermost range of rollers, how can that influence ink emulsification, to a greater degree than any other press?
How is it that you pronounce that having control of, ink stroke, ink keys and dampening rate do not constitute independent control?
Attached is your patent PDF.
Otherthoughts,
Good questions and I will try to answer them as well as I can.
I will start with the last question about non independent control first before addressing the others.
The lack of independence is due to the inconsistent ink transfer by the ductor roller. This is also true for continuous metering roller ductors as well. If we look at the ink feed system, there is the gap between the ink key and ink fountain roller that meters ink onto the ink fountain roller surface. As the ink goes around, the intermittent ductor roller comes into contact with the ink fountain roller and picks up ink. The assumption might be that while the ductor is in contact with the ink fountain roller, the ink film between these two rollers split consistently and then transfers ink to the roller train as the ductor moves to those high speed rollers.
This is the simple explanation but it does not explain what really happens. It implies that the ink transfer is in only one direction when in fact it is transfering ink in both directions at the same time. This can be shown when the press has backtrapping problems. The ink that was backtrapping will go all the way up the roller train and transfer across to the ink fountain. The fact that backtrapped ink will find its way into the ink fountain is proof of the two direction ink transfer conditions.
Also before the ink film splits, there is some mixing of the two ink films together in the nip. It is not a simple splitting of ink films. This means that in normal running conditions, emusified ink will be transfered to the ink fountain as part of this two way transfer condition. Ink is always emuslified. That is normal.
What the press rollers get is the net ink transfer amount, which is the difference between the amount of ink going from the ink fountain to the press minus the ink from the roller train going back to the ink fountain. It is the net transfer of ink that eventually affects the printed density.
The net ink transfer amount will be affected by the amount of water in the ink on the roller train, the temperature of the ink and the speed of the press. This explanation should be enough to show how the ink feed is not independent of other variables and that there is no mechanical device that can be set on even modern offset presses that is directly related to the ink feed rate.
Next thing I will discuss is the concept of Conservation of Mass. This is not a theory but is considered a Law. If one has an explanation of a process and it violates the C of M law, then one can be sure the explanation is in error.
Now if we take a simple mind experiment, the importance of the C of M law can be seen as it relates to density control on offset presses.
Let's say we are printing a 20% solid across the sheet or web. It prints at a consistent density of say 1.3. The press is running along and everything is stable. A steady state conditions exists.
At a steady state condition, the amount of ink stored on the rollers is constant and the amount of ink printed on the paper must equal the amount of ink being fed into the roller train. This agrees with C of M. The ink being the Mass in this example.
In a conventional press, one could increase the water and normally this will result in a decrease in print density. Let's say this is done and the press eventually stabalizes at a new steady state condition and the density is now 1.2.
Most people will say that there is less ink on the print. If this is true and there is less ink being printed, then based on C of M, there has to be less ink going into the press. This change of ink feed can be explained by the description of the transfer conditions of the ductor above.
For a second mind experiment, we change the inconsistent ductor transfer method to a positive ink transfer method which is not affected by water, temperture or press speed.
We start at the original steady state conditions printing a density of 1.3. Now we increase the water setting and wait. Since the original ink feed rate is not affected by the change in water, the ink must come out in the print at the same rate. So what is printed is the same amount of ink but with a bit more water in it. The print density will be 1.3 as it was before the addition of water. And this is indeed what it shows in tests.
The Ink Transfer Blade invention.
The purpose of the ITB is NOT to provide consistent ink films but to transfer consistent ink volumes (mass). The ink films on the roller train will change but that is not what is important. It is the ink feed rate that is important and this fact makes the problem of density control much easier. The C of M approach makes the control of the problem independent of all the complicated actions taking place in the roller train.
Another mind experiment. This will be for the positive ink transfer conditions. Let's say that all the nips on the roller train were splitting the ink films 50%-50%. We are at steady state and printing a solid density of 1.3 .
Then magically, we change all the split ratios to 60%- 40%. We have disturbed the equilibrium conditions and the pres will now have to go through a transient to reestablish steady state conditons.
So at the moment we make the change, 40% of the ink film is printed to the paper instead of 50%. This results in an immediate drop in density. So less ink is going out of the system but at the same time, the ink going into the system has not changed. This results in a transient where the ink stored on the rollers increases by increasing ink film thicknesses. This transient continues until the ink film at the nip between the paper and blanket reaches a point where the new amount of ink at the 40% split value equals the old amount of ink at the 50% split value. The new steady state condition exists after the density of the print recovers to 1.3 due to the fact that finally the ink going out is equal to the ink going in.
Now this is a drastic change but small variations will be happening all the time but they are also correcting themselves due to the C of M principle.
Hopefully you will also notice that the amount of water is not important with respect to ink printed. The consistency of printed density is not related to emulsification. The use of the term emusification of ink is a catch phrase to cover the lack of understanding of what is actually happening in the press.
Your first questions is a bit more complicated since it is dealing with wet trapping. So again let's look at a mind experiment.
First the more simple one. First down ink is the 20% solid bar across the sheet. The second down ink is the same 20% printed right on top of the first. In this case, as long as there is no backtrapping, the ink film on the second down ink will be consistent and independent of press speed with a positive ink transfer condition.
The second experiment is more complicated. Same 20% bar for the first down but this time the second 20% bar is not directly over the first. Let's say 10% overprinting and 10% directly on the paper. With the positive ink transfer condition, we know that the same amount of ink will print to the paper, but now there is the issue of where it goes.
If there is a drop in the wet trapping of the second down ink onto the first down ink in the 10% overprint area as the press speed increases, this will mean that some of the constant fed ink will have to go to the 10% region where the second down ink prints directly onto the paper. This is a possibility. But now we may have some tools to correct this. If we increase the water setting slightly for the second down ink, this may increase the wet trapping performance and resulting in more ink printing in the over print area and then less in the direct to paper area, restoring the original print conditions.
I am not saying that this would be a problem but there is more of a chance of improving any possible condition with a system that corrects the first order problem of consistent and independent ink feed than one would expect with what exists now.
You were discussing tack in the context of time and sequence. Tack on press is much different than tack measured in the lab. Water in the ink greatly reduces tack. You can see this by running some ink on a tackometer. It will give you a value but add a few drops of water and the tack drops like a rock. On press as long as there is no back trapping problem, the issue about tack has little to do with the ultimate solution to the density control problem. By the way, viscosity is also not an issue. Controlling volume (mass) of ink into the ink train is.
I hope this has answered your questions and given you something to think about.
