In our last segment, “Temperature Compensated Viscosity Measurement and the Art of Selling Snake Oil!” (click here if you missed it or want to review) we brought up a couple of simple facts:
1. Coating viscosity has a major impact on performance.
2. Temperature and viscosity are intractably linked.
From this we concluded that it is important to control the coating at the desired temperature all the way to the point of application, and to measure its viscosity at that desired temperature.
Though implied, we didn’t really discuss why this is so important! And the “why” is “where”!
Well, the fact is that the real goal of this methodology is to control the viscosity at the point of application. The viscosity in the bucket doesn’t matter; neither does the viscosity in the hose. The only viscosity that matters is the viscosity of the fluid that is being applied right now.
Traditional Viscosity Control
In that last post, we talked about coatings, but for this example, let’s turn our attention to the viscosity control of inks in the printing industry. The traditional method of viscosity control for rotogravure and flexographic applications is to measure the viscosity, either in the bucket, with a cup or a viscometer, or via an in-line viscometer. Then, solvent is added to reduce the viscosity of the ink to the desired value.
This seems to make sense. The ink is circulating, so controlling the viscosity in the bucket, or even better, in the hose running to the print head, will keep the process under control, right?
Well, not exactly. In fact, not at all.
The Return to Temperature
Let’s take the startup situation. The bucket of ink is at room temperature, let’s say 25°C, and the viscosity is 30 seconds. Let’s also assume that our desired application viscosity is 26 seconds.
Whether manual or automated, we start by measuring the viscosity, determining that it is too high, and adding solvent to reduce it to our target 26 seconds. As we circulate it to the head, friction from pumping and also from the printing process, add heat to the ink, raising its temperature. Let’s say to 30°C. As the temperature increases, the viscosity of the ink falls. Let’s say at 0.8 seconds/°C, which means a 4 second drop to 22 seconds. This will definitely create a color shift. Our operator must now make adjustments to compensate.
In a sophisticated automated system, the drop in viscosity will be sensed and virgin material can be added to raise the viscosity to compensate for the increase in temperature, but this is limited by a number of factors, beginning with the volume of the source container and following all the way through the length of the run and the potential for excess “non-virgin” ink that will have to be stored afterward.
The real problem, though, is that this is a reaction. It only takes place after the problem has made it through the printing head and back to the sense point. The damage is already done. This is because there is no good way to add the solvent, or virgin material, in the hose or in the printing head – to be practical, it must be added at the source.
And we haven’t even really begun to talk about the heat generated by friction at the head itself, and the fact that it will drop the viscosity of the ink even further – just as it is being applied to our critical product.
Therefore, it is logical to conclude that the only way to effectively control the viscosity at the point of application is to stabilize the temperature there. If the temperature is stable at the point of application, the viscosity, if supplied at a stable value, will also be stable there.
Temperature and Viscosity are Intractably Linked
In short, it always comes back to temperature. If you can’t control temperature, you can’t control viscosity.
It’s just that simple.