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FLEXO Magazine : September 2009
www.flexography.org SEPTEMBER 2009 FLEXO 61 TECHNOLOGIES & TECHNIQUES 1.14 1.16 1.18 1.20 1.22 1.24 1.26 1.28 1.30 1.32 1.34 1.36 1.38 1.40 1.42 0 5 10 15 20 25 30 Solid Ink Density (SID) Sample Number Solid Ink Density Run Chart Upper Control Limit Average Upper Control Limit Customer Release Limit Customer Release Limit A sample run control chart. parameters could include ink viscosity, ink pH (for water- based inks), substrate surface tension levels, and others. Knowing how the press was set up and run and how various materials "behaved" during the run is critical in defining the same conditions for future production runs. PROCESS CONTROL PARAMETERS Process control parameters can be thought of as the outputs from the printing process when run under the defined conditions as described above. Typically these are printing characteristic attributes, but they can also be functional at- tributes such as coefficient-of-friction, gloss, rub resistance, etc. The most common printing attributes monitored during the pressrun are: • Solid Ink Density (or color of the solids) • Dot Gain (or tone reproduction) • Color of Overprint Traps • Gray Balance • Print Registration REAL-TIME CONTROL CHARTING Both mechanical and process control parameters should be monitored and charted throughout the entire pressrun. From the fingerprinting pressrun, it was determined how each process control parameter behaved. Averages and standard deviations for each should have been calculated leading to target values and upper and lower control limits. As measure- ments are taken during the production run, individual values can be plotted and compared against the targets and control limits to determine whether corrective action is necessary. PROCESS IMPROVEMENT The data generated from the fingerprinting pressrun should be used not only for setting up the process control plan, but also for identifying process improvement opportunities. Often- times the printing performance from one print deck to the next is not the same. By looking at the calculated standard devia- tions, it's possible to identify abnormal conditions. Take the following example of solid ink density from our hypothetical fingerprinting pressrun. As the table below details, the yellow, cyan, and black print decks performed similarly. However, the magenta print deck is a different story. SOLID INK DENSITY PERFORMANCE FROM FINGERPRINTING PRESSRUN Color Average Standard Deviation 3-Sigma Yellow 0.98 0.022 0.066 Magenta 1.28 0.038 0.114 Cyan 1.32 0.026 0.078 Black 1.64 0.025 0.075 The standard deviation for magenta is much higher than the others, almost 50 percent greater than the next closest value. Since standard deviation is a description of variation, what this means is that the magenta solid ink density varies much more than the other three colors. In fact, when up- per and lower control limits are calculated (+/- 3 standard deviations, or sigmas) representing about 99.7 percent of the normally distributed population, it results in upper and lower control limit values of a little more than 0.11 density units. In other words, the magenta solid ink density will vary between 1.17 and 1.39, a range of around 0.22 density units! Just imag- ine what kind of impact this amount of variation would have on a process color image, especially something like a flesh- tone. It would not be good. When our printing department manager saw this, he and his press crew started to look at the magenta print deck to try to identify the root cause of the variation. After an exhaustive