While the SRV measures the viscosity of the ink, the controller keeps it constant. The InkSight control system compares the measured viscosity to a value set during the ColorLock process and uses a predictive tracking control method to maintain this viscosity, regardless of evaporation, temperature or addition of fresh batches of ink.
Through a system of valves, it controls the addition of solvent to the ink circuit to keep viscosity constant, despite solvent loss through evaporation. Thanks to the accuracy of the control system and the stability of the SRV, viscosity variation can routinely be held to ± 0.2-cP., or about 0.04 cup seconds. Such fine variation cannot even be measured with an efflux cup; in contrast when the sensors in a viscosity control system must be calibrated using cups, achieving this level of repeatable accuracy is out of the question.
And even the most accurate sensors calibrated against an unreliable standard cannot deliver the short setup times offered by the InkSight (see Figure 6).
The third pillar of InkSight is the ColorLock user interface. This enables the operator to match the printed color with the master, and lock on to it with a single button on the ColorLock touchscreen interface. Generally, the ink recipes for use with InkSight are prepared with less than the final amount of solvent, so that the dilution can be increased to the desired level by the control system. That way, the system recognizes that the ink is too viscous, and doses solvent according to a built-in algorithm to bring the ink to the correct working viscosity.
This takes only minutes and is done fully automatically, thanks to the “turbo” mode, which can add larger doses of solvent just after fresh ink is added, and switch into the normal fine-control mode, as soon as the ink comes to the proper working viscosity. It eliminates the danger of over-dilution, which could require remixing of the entire contents of the bucket, and saves valuable time for operators by eliminating need for exact ink dilution. This is possible only with a fast, stable, accurate viscometer like the Rheonics SRV.
InkSight in Use
InkSight has two basic modes: startup and run. During startup, the ink recipe is mixed and added to the ink bucket. The system is started and an initial viscosity set point is selected. If this is a re-run of a job, then the previous job specification is selected and loaded through the ColorLock touchscreen (see Figure 7).
The InkSight takes over viscosity control. In the screen shot from a startup process seen in Figure 8, the viscosity is about 9-cP. higher than the initial setpoint, calling for a large addition of solvent. The operator confirms switching to turbo mode, which adds larger doses of solvent to quickly bring the viscosity into the correct range. When the viscosity approaches the proper setpoint, turbo mode disengages, and the viscosity stabilizes to within about ± 1-cP. (about 0.2 cup seconds) of the new set point. The operator examines the sample print and decides (in this case) that a slightly lower viscosity is needed to match the master. The set point is bumped down by about 2-cP., the operator pushes the ColorLock button and the run proceeds.
Figure 9 shows the viscosity of an ink before and after addition of fresh ink to the bucket (taken from a log file generated by InkSight). Notice that the viscosity fluctuates between about 38.7 and 39.2, or ± 0.5-cP. before and after ink is added. This corresponds to a viscosity of about ± 0.1 cup seconds! Such small variations are impossible to measure with a cup5—and therefore, are meaningless when applied to a sensor that has been calibrated against a cup, as is necessary with many other ink viscometers.
Finally, Figure 10 is a series of measurements taken from a print job of 24,500 meters, and 13 reels. The run was interrupted several times, and so was printed on five different days, and in part at different speeds (at around 150 meters/minute). Despite the long time span and the interruptions, the Delta E 2000 value never exceeded 1.8.
Over the entire course of this print job, the SRV viscometers were never re-calibrated, nor were they cleaned. The individual runs were simply loaded from the ColorLock screen and continued from where it last left off. The ink recipe remained the same throughout the run. It should be pointed out that over three years of operation of InkSight on the press, the sensors have never needed removal for external cleaning. This has been confirmed by other customers using InkSight on their presses. InkSight was built to work in the background without requiring any maintenance or intervention. Its sophistication is purposely hidden behind a simple, intuitive, single-click interface.
The data speaks for itself. This control process is backed up by a multi-faceted data storage and reporting system. A real-time view allows monitoring the printrun and issues alerts to the operator when process parameters are outside of preset limits, as when a solvent bucket unexpectedly runs dry. It prepares a job report to keep the production supervisor informed about the progress of the run. In addition, it supplies a data stream to feed back to the factory IoT systems for full traceability of every job.
This is the InkSight approach to ink viscosity control in flexographic printing. We have shown how the Rheonics system offers both greater agility—through reduced setup times—and high color consistency over the longest runs. But perhaps the greatest advantage is the system’s ability to deliver this performance with only minimal operator intervention. This leads operators to place their trust in the system—from initial setup, to consistent runs, to re-loading of repeat jobs. It also has the side effect of encouraging operators to use greater care in the initial ink mixing process, since they know this effort will be accurately supported by the InkSight system’s reproducible accuracy.
The end effect is that higher quality color can be delivered to discriminating brand owners with less effort, lower cost and greater operator satisfaction. And pride in the finished product has always been a hallmark of the printing industry.
References
- Team Flexo. ‘Running to Delta E (dE)’ Objectively Quantify Pressroom Color. [Online]. Available from: www.spoton-color.com/pdf/sof_white_paper_11-18_final.pdf [Accessed: July 7, 2021]
- Verweel B. and Goodbread J. Viscosity Standardization: One printer’s approach to controlling a critical variable in label & flexible package production. FLEXO Magazine, February 2021, pp. 52-59.
- Hausler K. and Goodbread J. Fluid properties measurement device having a symmetric resonator. US9267872B2 (Patent) 2011.
- Rheonics. Whitepaper: Rheonics vibrational sensor technology demystified. [Online]. Available from: https://rheonics.com/whitepapers/ [Accessed July 7, 2021].
- Hebert V. Beyond the Zahn Cup. Paint & Coatings Industry Magazine. [Online] October 1, 2008. Available from: www.pcimag.com/articles/88259-beyond-the-zahn-cup [Accessed July 7, 2021].
The authors would like to acknowledge the contributions of Maybemar Hernandez and Manpreet Dash in preparing this article, as well as those early adopters who generously put their trust in early versions of InkSight.
About the Authors
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