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FLEXO Magazine : February 2010
40 FLEXO FEBRUARY 2010 www.flexography.org go, but given the different printing dynamics a softer cushion, not too soft, can also offer you acceptable results. If you are caught between which tape to choose the best rule of thumb is to select the tape which gives you the best reproduction with the least amount of impression. To put it simply, if both tapes offer the same print reproduction, why would I choose to use a tape that needs more impression? This additional impression only invites premature plate wear. And as a re- minder we should try to never reuse tape. Another inter- esting concept to ponder is why and what causes press bounce. Common sense says bounce is probably driven by too much impres- sion between the plate and print cyl- inder creating a de- flection. What if we use a firmer tape? We have determined that by going harder we have less im- pression. Therefore, one could derive that less impression means less chances of bounce or chatter. An interesting thought! Lastly, analog plates on the other hand change the rules. Let's remember that analog and digital plate dot shapes are very different. The "pyramid" appearance of the analog dot offers a broader foundation in comparison to the "cylinder" formation of the digital plate. So under impression the analog plate can utilize some of the softer tapes in the marketplace to minimize dot gain. BEST PRACTICES Selecting a compatible plate and tape combination doesn't finish your work! Other considerations must be given to en- sure optimal image transfer. One such procedure is cleaning or wiping the photopolymer plate prior to inking. This proven best practice removes any third party debris from the surface of the plate further inhibiting ink transfer. A once over with 100 percent alcohol removes barriers such as: • Dust and debris in the air or environment. • Oil and grease due to handling. • Remains from plate storage. • Ink or other artifacts from previously run plates. • Plate cleaners, anti-static sprays, lubricated press wipes or hand lotions. • Other third-party contaminants. On the positive side, if you choose to follow this best prac- tice you should begin seeing increases in solid ink density up to 0.15 while still maintaining excellent highlights and low dot gain. You may also find that your overall plate impression has been reduced further extending the life of the plate. As a footnote always clean your plates prior to inking once they have been loaded in the press! Please do not take any shortcuts possibly cleaning them after platemaking or mount- ing. Remember print cylinders are subjected to the environ- ment in the pressroom and are known to attract either what is in the air or oils from handling. One could argue that cleaning plates in the press increases changeover. Maybe so, but think of the time you will be saving in ink toning, and the overall time savings from setting print to run mode. In conclusion, one should always optimize their press before characterizing (fingerprinting) it! By first selecting an optimal plate and tape combination one can easily meet and possibly exceed today 's ever changing printing requirements. By considering a hard durometer plate and firmer tape solu- tion one can only anticipate the rewards of less impression (longer plate life), minimal dot gain and pleasing solid ink density. You may also experience other economic benefits such as reduced changeover time and higher press speeds. The bottom line is more available time, which then translates into greater profit! ABOUT THE AUTHOR: James Kulhanek is employed as a senior technical specialist supporting both the internal and ex- ternal customers of DuPont Packaging Graphics by improving overall operating efficiencies through plate and printing opti- mization. Kulhanek is a graduate of the University of Wisconsin Stout and has more than 28 years of experience in the printing and packaging industry. He has served as chair for the FFTA FIRST Digital Plate Subcommittee, and received the DuPont 2003 and 2005 Joe Gibson Award for Technical Leadership. FIGURE 2. Print results of a hard plate and soft tape. Dot gain charts. Density chart. TECHNOLOGIES & TECHNIQUES