5 Common Misconceptions About Viscosity Efflux Cups
For this exercise, we are using the Zahn cup as an example.
To determine the viscosity of a liquid, dip and fill the viscosity cup with ink or coating. After lifting the cup out of the fluid, the user measures the time until the liquid stream “breaks” (stops draining). This drain time is the corresponding efflux time.
It is not as simple as it sounds. Besides the ability to use a stopwatch, using a Zahn or efflux cup requires no specific skills. There are, however, variables involved that can contribute to an inaccurate viscosity measurement, which is hardly ideal when precise ink viscosity readings are required. These irregularities may contribute to inaccurate readings, which could affect ink consumption, cause rejections due to color issues, and contribute to anilox roller plugging, dirty print, and a host of other print quality or coating-related issues.
#1 – Zahn Cups Are Accurate
Within the printing industry, it is a well-known fact that three people using the same Zahn cup in the same ink can get (at least) three different readings, which can vary as much as two cup seconds. It is not that anyone has done anything wrong; it all has to do with how well-trained the person making the viscosity measurement is, their stopwatch reaction time, and the condition of the cup.
#2 – All Zahn Cups Are the Same
A Zahn cup purchased online is not necessarily the same as one sourced from a reputable supplier or manufacturer. There are a variety of different manufacturers across the globe—some of which adhere to strict manufacturing guidelines, and some of which are cheap knock-offs. It is not easy to tell the difference. This becomes a problem when you are comparing one cup that has the correct volume and orifice specifications to another which is different in size or volume dimensions. These disparities could result in increased ink usage, poor print quality, or pressroom housekeeping issues.
#3 – Zahn Cups Do Not Require Calibration
The fact is, Zahn cups take a beating on the shop/pressroom floor. Besides not cleaning between cup measurements, cups can be dropped, hit, or otherwise not managed with “kid gloves.” Any buildup or damage will affect the cup reading, which can impact process ink viscosity.
Since most reputable cup manufacturers offer NIST-traceable Zahn cups (cups guaranteed to comply with ASTM D4212), it is imperative that the cups used on the floor are regularly evaluated against a calibrated viscosity cup. This will ensure each cup reads as close to the same as possible and will help identify cups that are no longer usable.
#4 – A Zahn Cup Will Read the Same Value as an Automated Viscosity Control System
The principal operation of a Zahn cup is resistance to flow—a preset volume of fluid draining through a set orifice size. The only forces at work are gravity (causing the flow), the orifice size, and the gauge of the cup itself.
Automatic viscometers, employing a wide range of methodologies—such as vibrating and torsional rods, falling ball, acoustic wave, and others—create more shear stress on the fluid. As a result, automated viscosity measurement readings are much more sensitive than those from an efflux cup.
For example, using a #2 Zahn cup, it is possible to measure the ink viscosity at 22 seconds. An automated device, which has the potential to create more “shear,” may read the viscosity at a much higher—or (surprisingly) lower—value.
#5 – A Zahn Cup Is Good Enough to Control the Viscosity of a Print Job
That may have been true 20 years ago, but it is not the case anymore. Manually controlling the ink viscosity using a Zahn cup is a good start and can be helpful for benchmarking ink. However, maintaining viscosity during the print job requires a vigilant effort on the part of the operator to ensure it stays within a certain range. Due to other press concerns, this is not always possible.
Invariably, the operator may be too late in noticing a problem with the color and try to add either solvent or make-up fluid to bring the color back. By that time, hundreds of feet of printed film could be rejected due to poor print quality. Since customers are demanding a better-finished product and scrutinizing every inch of film, it is critical to ensure the viscosity is measured and maintained throughout the job.
Conclusion
In these instances, manual viscosity measurement using a Zahn cup is simply not sufficient. An automated viscosity control system provides the reliability and precision required for today’s print demands.
For more information, please contact INKSPEC. Sales@inkspec.com +14504415005 www.inkspec.com