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FLEXO Magazine : January 2009
TECHNOLOGIES & TECHNIQUES brand and confident in its quality. As a printer we were always confident in the quality of the ink, so when problems arose on press, we could usually eliminate the ink as a potential source of the problem and could quickly troubleshoot other areas as the possible source of problems. CONTROL ON PRESSRUNS Pioneers of SPC, Edward Deming and Joseph Juran, teach us that you can not inspect quality into a process. This concept can be applied to pressruns. You can’t look through the completed job for good samples to show the buyer. Measuring samples at spe- cific intervals (time or amount of work run) is a place to start, but it is far better to give press operators continuous real time feed- back rather than trying to inspect the finished run for good qual- ity samples. Figure 2 shows a pressrun with good process control (with a few out of tolerance occurrences). MONITOR PLATES The arrival of direct-to-plate technology means traditional transmission densitometers for reading film have been sup- planted by camera based plate-reading systems. The goal is still the same: check the plate for consistent dot structure and size. This process also lends itself to SPC since readings can be plotted, compared and monitored over time. In Figure 3 the yellow curve should get your attention. Something has changed (sharpened) the dot size at the quarter- tone and halftone. This plate could cause a potential problem when it gets on press. Better to catch the problem before it gets to the pressroom. I have heard people say: “What a bother it is to check plates and I just do not have the time to check them all.” It takes a min- ute or so to check each plate, and you will most likely find 95 percent (hopefully more) of the plates are fine. The cost of just FIGURE 3. The yellow curve shows a change in the dot size at the quartertone and halftone. This plate could cause a potential problem when it gets on press. The yellow lines represent a caution warning and the red lines are out of tolerance. All processes will show some variation (above and below the target green line). In this case, the density of the color shows good consistency, and since the color is monitored constantly, the operator gets immediate feedback on changing conditions and can react accordingly to bring the process back under control. Since you know exactly when it fell out of toler- ance you can mark the portion of the job that should not be used. This report can also serve to protect the printer. If a piece with color variation ends up on the desk of the person paying for the job, the run report can be used to show the variable piece is not indicative of the vast majority of the job. Another advantage of SPC on press can be seen during setup or makeready. Rather than run to the subjective judgment of a press operator, run to an objective target or industry specifica- tion (such as FIRST) and measure variation from that industry standard. Dramatic reductions in setup time (and variation from operator to operator) can be documented using such techniques. 22 FLEXO JANUARY 2009 one bad plate getting on press more than outweighs the time spent checking them. Bad plates can cost thousands in lost pro- ductivity on press, from trouble matching colors to spoiled work. Once during a quality audit by Ray Price of Printing Industries of America, he asked me if we checked every plate. I responded, “Most of them.”He then told me, “Well, if you can show me an unimportant plate you are making, then I guess that is one you do not have to check.” I took his advice to heart and plate prob- lems discovered on press plummeted. PARETO PRINCIPLE Also known as the 80-20 rule, or law of the significant few and trivial many, the Pareto Principle can be applied to many pro- cesses in the print shop, and its key tool is the Pareto chart. Figure 4 describes reasons for plate makeovers plotted over time. An interesting exercise is to ask the crew responsible for plate- making what are the main reasons for plate makeovers and to list them in order of what is the biggest problem. Keep track over www. f le xography. org
End of Year 2008