D
Deleted member 16349
Guest
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.
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.