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FLEXO Magazine : March 2009
TECHNOLOGIES & TECHNIQUES FIGURE 2. Banded roll layout. Once the results of the banded roll test are reviewed, you will uncover the optimum cell count and volume values for SID and reduced dot gain. volume delivered the optimum results in relationship to plate screen values and SID When designing your banded roll and cal- culating the proper ratio of cpi to support your minimum desired polymer dot, remember the 6:1 ratio. This means multiplying the plate screen by six to determine the minimum cell count re- quired. For example, a lsolpi plate would equal a minimum anilox of 900cpi anilox engraving. A 2-percent dot requires 900cpi to maintain lsolpi printing. This ratio minimizes overloading the smallest dot with too much ink, which would result in dot gain, dot bridging and dirty print, especially in the highlight areas. A lower cell count would de- crease the amount of support for the dot and allow over inking due to the larger cell openings. Once the results of the banded roll test are reviewed, you will uncover the optimum cell count and volume values for SID and reduced dot gain. With this information, the proper cell count and volumes can be selected and ordered. You will now have a standardized selection of rollers for your four-color process print- ing. The next step would be to fingerprint the press with these engravings to determine the final performance characterization. N ow you will have all the documentation needed to print by the numbers. Banded roll tests can also be done for line colors and coatings. This is a very efficient way to select and standardize your anilox inventory. Many converters are now choosing this approach be- cause it helps to standardize the anilox inventory. In many facili- ties you will find anilox cell counts and volumes are all over the place. They could have 10 400cpi rollers that have 10 different volumes. The operators only remember running a 400cpi roller. If they do not choose the exact roller they used before, they have limited chance of matching the color properly. This is where we start color matching on-press to overcome a simple concern. Performing a banded roll test will determine the proper volumes needed to match color consistently. Once that is determined, then all 400cpi rollers in the future can be ordered with the same volume. Now it will not matter what 400cpi roller is chosen. This will allow you to reduce your anilox inventory and manage it much more efficiently. Many converters are standard- izing their inventories by selecting, for example, five cell count values along with five volume values and setting them as the new standard. Anytime future rolls are ordered, they must fall within this profile. Now, regardless of what operator, or on what press, the results will be consistent and repeatable. ANI LOX STANDARDIZATION EXAMPLES 360cpi @ 6.3 bcm soocpi @ s.obcm 660cpi @ 4.obcm 7 00c pi @ 3.S bcm 900cpi @ 2.sbcm 600cpi @ 4.3bcm 7 00c pi @ 3.S bcm 900cpi @ 2.8bcm 1,000cpi @ 2.2bcm 1,400cpi @ 1.7 bcm COLOR MATCHING Another great benefit of the banded roll trial is the opportunity to test and adopt thinner ink films. The objective is to use the thinnest ink film possible and still achieve acceptable color. This will result in sharper graphics, lower dot gain and better tone distribution. It will also improve press efficiency with increased drying speeds and reduced downtime in cleaning plates. Now that we have a standardized anilox inventory, determin- ing the proper ink formulations for color matching will be much MARCH 2009 - www.flexography.org FLEXO
Sustainable Winter 2009