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FLEXO Magazine : July 2008
TECHNOLOGIES & TECHNIQUES FIGURE 5. SCREENSHOT OF 1-PERCENT DOT AREA. PRINTING 1% DOTS If we assume that 1 and 2 percent have the same amount of dot gain on press, no one will likely see any difference between these on the final impression. Finally, we have to consider a phenomenon that can be regularly observed on prints. As the dot structure and dot shape is thinner and fragile, there is on some occasions more defor- mation on the I-percent dots, during the impression, with a conse- quence of a higher dot gain on the 1 percent than on 2 percent. This is unfortunately not a unique case (Figure 6). This negative effect is also indicated by ISO in a corresponding standard. In fact, there are different factors that have influences on a final print-substrate structure, flexo plate, technical condition of a press, etc. Assuming that all technical factors are in proper order, reproduction capability could be very different from substrate to substrate. Considering ISO 12647-6 (2006) for flexo printing, we can see that only printing fùrns and foils are able to provide the widest range of reproducibility from 2 percent through 90 percent (due to the results represented by ISO). It would be more or less impossible to get the same values on very rough corrugated substrate and on a high speed machine. Table 3. Tone value ranges (film or data)* 1 Corru!ate 2 Uncoated 3 Coated Paper 4 Film/foil boar paper 8 to 75% 5 to 75 % 3 to 85% 2 to 90% *ISO 12647-6 Restrictions are also tight within screen rulings (frequencies) and dot shapes. ISO recommends (requires) using explicit parameters of the rul- ing for different substrates (Table 4). As we can see, there are also some limitations. As soon as we go above the mentioned ranges, which are obviously possible to reach, we face some technical re- strictions. The main limitation will be in very precise workflow from prepress to press control. All steps of reproduction have to be tuned and use the smallest product phase fluctuation as possible. Secondly all raw and half-finished products have to be suited to each other. This means for instance, that a thick flexo plate has to be suited to the surface structure of corrugate board. The ideal reproduction process allows reaching 10 microns on films with ruling of 60 lines/ cm, but as mentioned above, it is extremely hard to get 20 microns. Table 4. Screen frequency range* 1 corru!ate 2 Uncoated 3 Coated 4 Film/foil boar paper Paper 14 to 33 18 to 40 45 to 54 36 to 60 lines / em lines / em lines / em lines / em *ISO 12647-6 FIGURE 6. THE 1 PERCENT HIGHLIGHT IN MAGENTA AND BLACK ARE PRINTING HEAVIER THAN 3 PERCENT OR 5 PERCENT DOTS. CONCLUSION Through this work we have demonstrated the advantages and disadvantages of holding the dot size variance to 1 percent on conventional flexographic plates (I-percent dots). Furthermore, we provided a few suggestions for improving quality in this process. As we have stated, holding this degree of detail is often viewed as a means of competing or matching the appearance of the offset printing process. Additionally, we have demonstrated that in the vast major- ity of cases, the drawbacks far outweigh the benefits of holding a I-percent dot at both the plate development stage and again at the final print process. Finally, we have shown that it is very important to have proper process control on each plate at each of the production stages. . REFERENCES 1. FIRST 3rd Edition 2. ANSI CGATS.9-2007 Graphic technology - Graphic arts trans- mission densitometry measurements - Terminology, equations, image elements and procedures 3. ISO 12647-6 Graphic technology- Process control for the pro- duction of halftone color separations, proofs and production prints - Part 6: Flexographic printing. ABOUT THE AUTHORS: Pierre Paul Moyson is graphic arts manager for Asahi Photoproducts (Europe) sa/nv. After getting his degree in graphics and communication, Moyson worked in a prepress studio as graphic designer, and then moved to an offset printing company as the head of the prepress and platemaking department. Afterward, he joined Asahi Photoproducts (Europe), where he is in charge of the European graphic arts center. Maxim Siniak, Ph.D. is application expert at X-Rite Europe AG, Switzerland. He received his doctorate in technical science at Moscow State University before work- ing as senior teacher at the University as well as a freelance engineer for GretagMacbeth. Recently he hasjoined the GretagMacbeth-X-Rite team. Siniak has published nearly 60 articles. J U L Y 2008 - www.f I exog ra p hy.o rg FLEXO
Flexo Sustainable Fall 2008