Cup Reliability vs High Shear Viscometers

In the previous article we discussed non-Newtonian fluids and how they perform when measured with efflux cups.

Efflux cups are employed in most applications as a benchmark measurement for the fluid in process; they are by no means accurate. Why, you ask? Human nature! You may have 2 or 3 operators measuring the fluids and each may get three disparate readings of the same fluid. This may have to do with operator training required to take cup readings. It may be due to the operator’s reaction time on the stopwatch. It may be because the operator actuating the stopwatch when pulling the efflux cup from the fluid. Whatever the reason, there is less precision when taking a cup measurement.

Differences in shear stress and how it relates to viscosity measurement

To do so, it is important to remember that there are several existing methods to measure viscosity. Efflux cups, capillary tubes, plate and cone devices, vibration, torsional and falling piston/ ball and so on.

Each employing a different principle of measurement, each creating shear on the fluid, either by two surface physical contact with the ink (falling piston, plate & cone), high or low frequency dampening (torsional, vibration), and efflux cup drain time (Zahn cup, Ford cup). 

Since viscosity is a measurement of how a fluid reacts to shear applied, it is safe to assume that the high shear devices such as the vibrating rod, falling ball, and torsional type of viscometers will be the most sensitive, i.e., see smaller changes in viscosity.

Case Study 1:

Let us assume we have three different viscometer types on a recirculating print line: a falling piston, a vibrating rod and a torsional. Now take a Zahn #2 cup measurement of the ink, which, after averaging, reads 22 seconds.

Now actuate the sensors and notice the measurements, the falling piston reads, for example, 30 seconds, the vibrating reed displays 18 seconds, the torsional indicates 24 seconds. 

You may think there is something wrong with the viscometers. The fact is that they are all reading correctly! Each method of measurement creates a different shear stress on the ink resulting in disparate readings. The non-Newtonian physical properties and chemical makeup of inks have different reactions to the shear stress applied.

Case Study 2:

As another example you may have three coatings in process, all of which read 22 Zahn cup seconds. You may put a falling piston sensor in line with each. The first sensor reads 22 seconds, the second reads 32 seconds, and the third reads 18 seconds.

Once again, they are correct. The non-Newtonian properties of the fluid react, or perform, differently to the shear stress applied.

Based on this fact, it is reasonable to assume that lower shear devices are less sensitive to small viscosity changes.

Conclusion 

Whereas the higher shear devices will, by virtue of the more precise nature of their measuring method, will sense smaller changes in viscosity. That sensitivity, when introduced to an ink or coating system employing a viscosity control system, can ensure tighter process control. That tighter control can result in more consistent coating or color, leading to less ink consumption, a reduced amount of substrate waste due to rejections and less press down time for cleaning related issues, ensuring less press down time, maintenance issues and increased profits.

For more information, please contact INKSPEC.  Sales@inkspec.com  +14504415005  www.inkspec.com