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FLEXO Magazine : January 2009
TECHNOLOGIES & TECHNIQUES petition between two presses and three shifts. The winning crew would get the company tickets for a Red Sox game. The operators started to read the job reports and wrote down the density variation for each color as their “score.” I realized the idea was working when I started to get questions from the crew like, “How manyMulligans do we get?” (they got one per 18 col- ors—after all this wasn’t the USGA!). The end result was that the press operators started to really pay attention to the reports gen- erated during pressruns. They recognized when a run was “above or below par,” and even better, had a quantifiable way to gauge if steps they took improved their “score.” If it is important enough to measure, it should be important enough to display. There are few things in this world so useless as a bunch of numbers written in a notebook and shoved into a drawer, never to see the light of day. Display the data collected. There are several ways to apply SPC in the flexographic pressroom: ??Monitor incoming raw materials (e.g. ink) for consistency and accuracy prior to using them on press. ?? Monitor pressruns. It is possible today to read in-line roll- to-roll. Some camera based systems designed for registra- tion can be applied in this way. Multi-channel densitometric systems read the widest range of colors and give the press operator continuous real time feedback. ?? Monitor plates. With the advent of direct-to-plate computer technology it is no longer possible to read film. Measure your plates with camera-based systems to confirm dot accu- racy and consistent dot structure. FIGURE 2. A press with good process control. Let the operators take pride and ownership in it. You can in- corporate such displayed charts into plant tours for customers: “here is an example of our SPC program we use to help assure quality to our customers, etc.” It is important for the people collecting the data to understand what they are looking at and why. I once had a person checking the density of film coming out of a processor and plotting it on a chart on the wall. While I was on vacation the pH went out of control. The density plummeted and the operator continued making the marks on the chart un- til he ran out of room (scale) and continued to diligently make marks off the chart and onto the wall! I now laugh when I think back on this story, but the real lesson I learned was that the fault was really mine. I never established upper and lower con- trol limits or put in place an action plan of how to react when these limits were reached. MONITORING RAW MATERIALS A good place to start using SPC is with your incoming raw materials (ink for instance). Set up a system whereby the incom- ing ink can be checked. I had an ink supplier provide drawdowns with large volumes of ink. The drawdowns were created using agreed upon densities (or anilox rolls) and a target L*a*b* value. Upper and lower limits of acceptability were agreed upon. A QC person used a spectrophotometer to check the drawdowns of the incoming ink; a pass/fail (go or no go) check was used. Values were recorded and plotted over time (see Figure 1). The end result was that I never had a shipment of ink fail. Because the ink supplier knew the ink was being checked me- thodically at our end, it made sure they checked it themselves before sending it to us. This way, I had pushed a potential quality problem upstream to the supplier. In the end, it worked out well for both of us: the ink supplier had a grateful printer, loyal to its www. f le xography. org JANUARY 2009 FLEXO 21
End of Year 2008