Roll-to-roll metering systems were once the standard metering method for inks, starches, and glues. Many of the higher quality printing machines now use blade metering. And we are now see the replacement of roll-to-roll metering in starch and adhesive applications.
What causes the variation in metering in roll-to-roll systems? First, we have to look at the concept of gap, which is the distance between the two rolls. Usually the gap is measured when there is no fluid between the rolls and the rolls are stopped. This is called a static gap. Although static gap has some importance, you are more interested in the dynamic gap, which is the gap measured with rolls turning and a fluid such as starch between the rolls. Most roll to roll machinery manufacturers tell you only the static gap, not the dynamic gap. And they don’t tell you about the variations in the dynamic gap as the rolls turn with a fluid between them.
Next, let’s examine the static gap from the context of machining. Normal machining of rolls produces a small variation in its diameter, which is called runout. All rolls have it, and typical rolls in our industry have a runout of 0.001 inch at manufacture. If you have two rolls turning close to one another, the gap gets larger and smaller by 0.002 inches. If you are attempting to maintain a static gap of 0.015, then the 0.002 variation is probably not too noticeable. But if you are trying to keep the gap at 0.005 or less, then 0.002 variation becomes a major problem.
As bearings wear, the dynamic gap will increase. An extra 0.001 increase in bearing tolerance will mean that the rolls will be separated another 0.002 inches. As bearings wear, the dynamic gap slowly increases, and the amount of fluid that is metered increases with the same machine setting. The wear can also cause the dynamic gap to vary, depending upon how the fluid pushes the rolls apart.
Next, let’s examine the dynamics of fluid metering in a roll-to-roll system. As you just read above, the fluid will attempt to push the two rolls apart. But the two rolls have a characteristic called deflection, which is the amount that they will bow when subjected to a force such as a fluid. The fluid causes the rolls to bow apart more in the middle than at the end. So the gap becomes wider in the middle, which results in more fluid metered through the gap in the middle than at the ends. Worse yet, the amount of the deflection changes with the viscosity of the fluid.
Furthermore, the physics of fluid dynamics says that the amount of fluid that is pulled through the gap changes with the speed of the rolls and the viscosity of the fluid. So the faster the machine speed, the more fluid is metered through.
And lastly, there is that ugly situation called wrap-ups. This will cause a permanent deflection in the two rolls, causing yet another change in the fluid metering characteristics.
So, let’s add all of this up – roll-to-roll dynamic fluid metering varies:
- With roll runout, causing a through-machine variation in metering.
- With bearing wear, causing a slow increase in metering.
- With roll deflection or bow, causing more fluid to be metered in the center.
- With machine speed, causing more fluid to be metered as speed increases.
- With viscosity, causing metering to change as viscosity changes.
So, how do you compensate for all of this? It is difficult, but there is a way. On a roll-to-roll system, you can increase the static gap to a ridiculously high number, which will tend to minimize the effect of the variations. On corrugators, this means more starch, which means more energy, less speed, more washboarding, and lower bond strength. On inking systems, this means flooding the paper with ink, which can result in poor trapping and ink smear. On adhesive systems, this means applying so much adhesive that it takes a long time to tack and even longer to set up.
What is the best solution? With the laws of physics and fluid dynamics not favorable to even and consistent metering on a roll to roll system, look at other metering systems, such as the rod metering system that is used on Kohler glue machines and adhesive applicators. All of the variations discussed here don’t exist on rod metering systems because there isn’t a metering roll or a gap.
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