There are two takeaways from this experiment. First, Delta E 2000 isn’t everything! Despite the densitometer readings indicating that the color is well within normal tolerance limits, there is a tremendous range of visual appearances that can make the difference between an acceptable print and a truly great print. Secondly, the SRV’s stability and accuracy, when backed up by the InkSight control system and ColorLock software, can make this kind of color accuracy part of the routine workflow in any flexographic system. It is a solid, easy-to-use tool; not a laboratory curiosity.
When setting up a run, ink viscosity can be measured and adjusted during the mixing phase, where relative quantities of ink, varnish and solvent can be added to give the ink the desired body and color density. Once the proper viscosity has been reached, it is necessary to check the performance of the ink during the setup process, in which a sample is printed and the color compared with a master for the print. At this point, adjustments to viscosity can be made by changing the setpoint and letting the system add solvent in order to reach the desired color.
Once the proper viscosity has been established by test prints, maintaining this viscosity throughout the print job ensures constant color quality, despite the fact that ink concentration is varying as a result of evaporation. Maintaining constant ink viscosity throughout a run is the key to color consistency. Since it is measurable and controllable with high accuracy, it offers a simple and direct method for maintaining high and consistent color quality throughout even the longest printruns.
Although the Delta E value can put the strength of each color in the right ballpark, the operator’s experienced eye is the last word on overall color quality. The operator makes the decision, “Good to print.” Having mixed an ink recipe, the operator makes test prints to check against the print master, so that final adjustments can be made before the run is started. Once these adjustments have been made, the operator pushes a single button and InkSight takes control of color quality by adjusting the viscosities of each ink color, despite variations caused by evaporation or addition of a fresh batch of ink.
This process is called ColorLock, a feature baked into the predictive tracking control system and the operator interface of the InkSight. ColorLock locks in the initial match of the printed color to the master; InkSight ensures this match is maintained over the entire printrun, no matter how long, and no matter how many times fresh ink must be added. Figure 3 offers a look at a ColorLock interface screenshot.
Let’s take a look inside the InkSight to see how all the parts work together to give truly great color quality.
Under the Hood
InkSight is built on three pillars: Rheonics’ SRV viscosity sensor, the predictive tracking control system, and the ColorLock display and user interface. These three subsystems work in concert to give the finetuned viscosity control that is essential for accurate and consistent color.
The first pillar on which InkSight rests is the Rheonics SRV viscometer. Based on patented3 symmetric resonator4 technology, the SRV unites the essential characteristics of accuracy, reproducibility, and robustness in a small and easily installed viscometer probe (see Figure 4).
Sensor accuracy enables drastically shortened setup times. Once an ink recipe has been established for a given job, it can be filed for re-use, whenever the customer order is repeated. The SRV’s accuracy ensures InkSight will set precisely the same viscosity, from the beginning of a run, through reel changes and ink additions, through any number of re-orders of the same job. This means that after the initial setup the first time a job is run, the operator can check and lock the color from a simple and intuitive user interface, each time the job is loaded. Instead of hours, setup time is reduced to minutes, with exact matches to the job master.
The ability of the sensor to deliver the same measurement for the same ink, no matter how long the run, removes the necessity for re-calibrating the viscosity sensors each time a job is set up and run.
The SRV is self-checking; it automatically detects any deposits of dried-on inks that may have accumulated during downtime of the machine. If deposits are detected, the sensor can be easily removed and cleaned with a solvent-soaked wiper, without any danger of damaging the sensor or changing its calibration. However, in normal use, the sensor seldom needs cleaning. The SRV is only 5-in. long and 1-in. in diameter, and weighs less than one pound. Despite its compact size, it is stable, with no need for calibration, no matter how and where it is mounted on the press. It installs directly in the ink hose eliminating the need for additional space on press (see Figure 5).