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FLEXO Magazine : November 2013
TeChNOLOGIES & TeChNIQUES The Journey to Closed Loop Color Control Spectrophotometers & Steps to Implement Automation by Dan Dupuis Ashashappenedinsomanyotherindustriesinthe past, the flexographic printing market is making the journey from human judgment being “good enough,” to “process monitoring,” to “process control,” all the way to full automation for closed loop color control. As stakeholders have become more and more aware of color’s importance and sought to minimize waste, the industry evolved from subjective human judgment to objective device measurement. Eventually this evolution led to the handheld spectrophotometers we see today in the general print industry. Certain web processes, such as gravure, are currently using online real-time measurements, but so far the costs have been too high to penetrate the flexographic market. With improve- ments in computing power and the reduction of processing cost, there now exists the ability to measure at acceptable speeds and cost-effective price points. While full automation is still a bit out of reach, there is hope for the future. MEANINGFUL MEASUREMENTS • Ambient Temperature • Media Temperature • Ink Viscosity • Ambient Humidity •InkpH • Dryer Temperatures THe SPeCTroPH0TomeTer Measuring the results of the flexo process is achievable through the use of inline spectrophotometers. These devices have the ability to measure the media at nearly unlimited rates of speed, allowing print providers to monitor process output in real time. To drive the full value of monitoring color on the press, the inline spectrophotometer must be capable of making spectral measurements, calculating color computations and reporting results to the press operator. At a minimum, it should have the following performance characteristics: • Report 31 spectral bands from 400-nm . to 700-nm . • Compute L* a* b*, L*C*h̊, and status T&E densities while press is running at full speed • Avoid media contact and allow for height variability of 0.5-mm. minimum Mounted on a traverse arm the measurement instrument is able to measure at any point across the web. This motion con- trol makes it possible to measure and ensure the uniformity of color across the width of the web. Typically this would require a color bar printed on the media along the direction of the pressrun, likely on either side of the web, so the results of the plate pressure can be monitored. The bar consists of solid and tint patches of each process color, as well as overprints and gray balances. There may also be one, several, or possibly all spot colors included in the color bar to ensure that special colors, such as those used for company logos, are reproduced accurately. The color bar, though, is only an approximation of the actual colors in the field and sometimes sheet and image size restriction don’t leave any room for it. A significant benefit of horizontal motion control of the spectrophotometer is that the operator can periodically measure, monitor and, if necessary, correct spot colors and special process colors in the work. It can be compatible with on-press visual inspection system cameras to allow press operators to identify the places of critical color locations to be monitored in the work. Likewise, with a press synchronization system, there ex- ists the ability to position and time the measurement in such a way to measure in a field of print as long as the area is large and consistent enough for a valid measurement. Just like coordination with a camera-based system, this enables the measurement of specific elements of the printed page, such as solid spot colors in a logo or critical solids printed by means of process color. uSing meASuremenTS Using an in-line spectrophotometer is only the beginning of closing the control loop. A system of instruments networked to a centralized computer and a sophisticated process monitoring software are needed to display the information in a useable manner. 54 FLeXO NOVEMbEr 2013 www.flexography.org www.flexography.org NOVEMbEr 2013 FLeXO 55