Despite the highly sophisticated quality control and automation systems available on the current generation of printing machines, one of the most crucial variables in the printing process—ink viscosity—is still frequently measured with inexact and cumbersome methods, such as efflux cups and falling ball viscometers.
Besides the negative effects these methods have on printing quality, they perpetuate an attitude toward ink viscosity measurement that is reflected in the conventional unit of “cup seconds”—the time required for ink to flow out of a measurement funnel as documented with a stopwatch!
Viscosity is a very important parameter in the final quality of the printed matter. It requires close attention, given:
- If the viscosity is not correct, the flow behavior and ink layer thickness will vary, leading to deterioration of print quality
- Poorly adjusted ink viscosity may cause excessive ink consumption and unnecessarily high costs
- Viscosity automation and predictive tracking control results in waste reduction and efficiency improvements
Although a wide range of more modern technologies is now available for inline and continuous monitoring of ink viscosity, sensitivity to contamination, installation variables and baseline shifts have led to some suspicion of these methods as long-term solutions to ink viscosity measurement and standardization. This article addresses some of the advantages and shortcomings of available technologies, and the benefits from using stable, easily cleanable and repeatable viscosity sensors.
Using sensors that measure up to the accuracy operators expect from their color control and press adjustment systems enables online, automatic, dynamic control of viscosity within previously unattainable narrow limits, affording start-to-finish maintenance of print quality in even the longest printruns.
Goals of Viscosity Control
Above all, print quality matters!
In a highly competitive industry with often tight margins, each job rejected by a customer for unacceptable print quality can be a major blow to both the reputation and the profit of the operator. “Reject” here can mean a truck returning with several tons of wasted material! The main purpose of viscosity control is to maintain print quality from start to finish of a job, no matter how long or complex.
A second goal is improvement of operating efficiency. Efficiency starts with setting up a job. Being able to nail the proper viscosity for all stations without cut-and-dry tinkering means rapid job turnover, keeping the machines printing instead of idling.
Achieving these goals requires a system: on the one hand, an accurate and repeatable sensor is needed that can provide the viscosity resolution necessary for producing accurate and consistent color. Equally important is an automatic control system that continually and smoothly adjusts the ink viscosity, considering variables like temperature and rate of evaporation of solvents.
Every operator is used to working with some sort of viscosity measuring cup. Cup measurements have never been totally standardized and are only “reliable” over a relatively narrow measuring range with a large margin (5 percent to 10 percent) of error. Some of these errors are caused by the cup itself, others are a function of operator skill:
- Measurements are not repeatable
- Temperature, which has a strong influence on viscosity, is difficult to control
- Contamination of the cup and different densities of inks influence the run-out speed
- All add up to poor repeatability and accuracy of viscosity cup measurements
In practice, error margins can be as high as 10 percent, which is a big bandwidth of viscosity. For example, for an ink that has a viscosity of 20 seconds, an error margin between 5 percent and 10 percent means a bandwidth between 1 second and 2 seconds!
Having years of experience using a great variety of viscosity sensors in a flexographic plant, I’m familiar with several that are based on electromechanical resonators—elements that vibrate in various ways, and whose vibration is damped by the viscosity of the ink. Electronics units connected to these sensors evaluate the damping of the vibrations and translate it into a viscosity measurement.
Sensors utilize various forms of resonators, including quartz crystals, surface acoustic wave resonators, vibrating metal rods, and torsional resonators that execute microscopic, high-speed twisting vibrations. The latter are particularly robust, and are relatively insensitive to both contamination by ink residues and to the influence of mechanical vibrations from the printing machine, two influences that have been particular challenges for vibrational viscometers.
One particularly compact, robust and accurate sensor is based on a so-called “symmetric torsional resonator” (US patents 10,502,670 and 9,267,872) SRV. It offers high accuracy—better than 5 percent of the actual reading—and reproducibility better than 1 percent of its reading, making it particularly suitable for high-accuracy, reproducible job-to-job color matching. Because its resonator is completely balanced, its accuracy is immune to its mechanical environment. It is also extremely robust, meaning it can be cleaned, when necessary, by wiping it with a solvent-soaked rag.
What do its broad measuring range and high accuracy mean in practice? We tested its accuracy by adding 20-g. of solvent to 25-kg. of ink. The sensor (installed in the press ink line) registered a viscosity change of 0.1-mPa.s, which is the equivalent of a cup measurement difference of 0.02 seconds! This is a previously unknown accuracy of the measurement of viscosity in this industry. And because the SRV incorporates an accurate temperature measurement into the sensing element, it is possible to accurately compensate for the effects of temperature.
We have found that working with the cup is not only outdated, but actually counterproductive. After a few months, we stopped converting to cup seconds altogether, finally elevating viscosity measurement, the last crucial variable in printing, to the same technological level as the rest of the process. We finally brought our printing press fully to the 21st century.
Due to the accuracy and repeatability of the sensor, we have gained a lot of insight into the behavior of inks—sometimes more than we expected. Ink is a rheologically complex medium, and the sensor gives us some insight into that complexity that is not observable with the DIN cup.
Solvent-based inks exhibit non-Newtonian behavior. Under the influence of shearing force, their viscosity changes. Ink is also thixotropic, a stationary ink having an appreciably different viscosity than an ink that is in motion. The viscosity of a stationary ink can differ from that of a moving ink by as much as 20 percent!
In addition, ink viscosity is strongly temperature dependent. On printing presses on which the temperature of the inks is not conditioned, ink temperature—and therefore viscosity—can vary greatly due to changes in the ambient temperature, but also due to the heat generation in the press itself. One of the first things that we have explored with the sensor is the temperature dependency of ink viscosity.
We built a test setup consisting of a closed flow loop in which the ink is continuously pumped in a circuit, at a speed comparable to that of the ink circuit in our press, and slowly heated up. Every second the temperature and viscosity are measured, giving more than a thousand measurement points in a typical test run.
The graph in Figure 1 shows the temperature dependence of the viscosity of a number of different inks (modified nitrocellulose ink yellow, magenta, silver and a polyurethane white) over a temperature range of 20 degrees Celsius. Over this range, the viscosity can differ by up to 60 percent.
One of the most important uses of viscosity measurement is to determine when and by how much ink must be diluted in order to compensate for loss of solvent during the printing process. Solvent evaporation increases the pigment loading of the ink, resulting in poor print quality and excess ink consumption. This loss of solvent also increases the viscosity of the ink. However, since viscosity is also a strong function of temperature, it is necessary to distinguish between the effects of temperature and evaporation in order to determine the amount and timing of solvent addition.
Without temperature compensation, an ink at a low temperature would give a high viscosity reading, suggesting that dilution is necessary. However, diluting the ink would give a lower color density, since the higher viscosity was due to lower temperature, not due to higher pigment loading.
Using graphs such as those in Figure 1, we developed an algorithm that allows us to compensate for the effect of temperature on the viscosity, resulting in a “temperature-compensated viscosity” that is a true measure of the pigment loading. Therefore, it can be used directly to control solvent addition to make up for evaporation, because it removes temperature as a variable affecting the measurement.
Using our compensation algorithm, we reduce the error deviation to 1 percent over the entire temperature range. In the automatic viscosity control, the temperature compensation can be selected for each type of ink. We have determined this curve for almost every ink we use, and have determined the temperature-compensation parameters using our special algorithm, enabling us to finally achieve tighter control over pigment loading and its effect on printing quality.