The Role of Exposure to Air
Briefly, rollers rotate at very high revolutions per minute and are exposed to the press room atmosphere. The centers of the rollers are sheltered from ambient air, while the ends of the rollers are exposed to this ambient air from outside of the roller train. If you drew a curve of exposure to atmosphere it would show high atmospheric exposure on the roller ends and low exposure in the center. One must realize that this exposure has a drying up effect on the rollers. The higher the exposure the more we dry up. One example is the recent innovation which uses air bars to blow air into the ink train to reduce excessive water. This does work - air flow dries up the roller trains. Ambient air on the roller ends also dries up rollers.
The Resulting Problem
The result of this imbalance of air drying is that the pressman is constantly trying to balance his water to ink ratio. Typically, he must overfeed water to the center of his product to provide adequate dampening to the ends. In critical configurations he is constantly struggling between emulsification in the middle and scumming on the ends. in a four-high configuration this constant yo-yo of up and down those ladders, can be a killer and in spite of all of that struggle the job never does look right.
|Brush dampening system with water stops.|
Now, let's look at the dampening configuration. The current most commonly used damper is the spiral brush. This brush delivers a consistent edge-to-edge fog of water into the roller train. Some deliver water directly to the plate, but the roller train configuration is most common. This design does not in any way adjust the amount of water to demand. It functions on the premise that one size fits all. That is patently false. What this system does is take advantage of the forgiving rate of the process which allows a wider range of water to be applied without significant changes to the printed product. The process can and does tolerate over and under dampening, up to a point. However, in critical areas, such as on the roller ends, the deviation from the optimum pushes the forgiving rate over the edge of acceptable printing. That explains the pressman's struggle. He needs zone control and the dampener is incapable of delivering it. So what now?
There are several actions the pressman can take. Let's examine them one at a time. I would guess that as a permanent solution the pressman will use each of these together rather than only one.
1. The Water Stop
Most water stops, as supplied by press manufacturers, are not really effective in providing zone control. They are mostly designed for narrow web restrictions. Effective water stops can be homemade. The best homemade water stops I have seen are cut out of heavy Mylar and are saw-toothed in varying designed to cut off anywhere from 10% to 70% of the surface water on the dampening rollers. A pressman can be very innovative in these designs. They closely resemble the sophisticated rubber diamond designs that are available from industry sources. The Mylar (or rubber diamond) can be very effective in reducing the water feed in the center of the press. Unfortunately, this Mylar system is a patch job and requires some innovative skills of design and application, which may be in short supply. Given time and skill, this can work effectively.
2. Dampener Brush Roller Configuration
Many press manufacturers and roller suppliers are trying to cope with the edge drying problem by increasing the bristles on the ends of the brush rollers and reducing the bristles in the centers. This is a logical move and in some cases has been successful. I would certainly endorse such design changes.
3. Unit Design
I would recommend an add on to side frames of the unit to restrict the flow of ambient air. These would of course have to be removable.
4. Ink Design
The first two corrective actions have merit, but I would not record them as new information. Ink design is a new concept. This contribution was made by a pressman in Australia. Some background is necessary to fully understand it.
|Modify your ink for high water pickup.|
Recognizing this, how can this be useful? Here's how - if we want to deliver a more consistent water coverage to our plate, we should use a high water pickup ink. That's quite simple. Ink can be designed with a wide range of water pickups. We tend to favor low water pick up ink to improve trapping, for instance. This may be counter-productive from the standpoint of edge-to-edge dampening. I would tend to favor low water pick up ink to improve trapping, for instance. This may be counter-productive from the standpoint of edge-to-edge dampening. I would heartily endorse the switch to high water pick up inks. Obviously, there are limitations. But a series of ink tests should provide your answers.
Let's discuss this concept for a moment. We need more water on roller ends. We cannot pour water on across the roller because of the limited forgiving rate of the process. By using a high water pick up ink, the ink carries enough water to extend that forgiving rate. One additional possibility would be to use high water pick up inks on the ends of our fountains and normal pick up inks in the center. Doing this would negate that ambient air curve mentioned earlier.
The most innovative solution of all is to use all of this input to design the "perfect" water balancing system. I may be prejudiced here, because it's my idea. Let's explore the givens:
- Air blows dampener water away or dries it up.
- Imbalanced air produced unbalanced dampening.
- Air curtains now in use reduce excessive water.
- The current problem is too much in the center and starvation on the ends.
Install an air bar that covers only the center 60-70% of the roller train. This air bar will act as an external water stop to balance water across the rollers. The volume of air can be regulated to needs. The air can be directionally adjusted by zones. The air must be clean, high volume, low pressure air, possibly fed from the angle bar blower. A simple valve can regulate volume. This simple add on air system will make it possible to establish a consistent flow of dampening solution into the roller train and eliminate the almost impossible conditions that exist today. I highly recommend a trial on your presses.
The combination of these concepts should improve and help troubleshoot the edge-to-edge dampening on your presses. It should improve the quality and prolong your pressman's life.