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FLEXO Magazine : October 2011
Control the Atmosphere for Controlled Platemaking Pushing the Envelope in Digital Platemaking By Ray Bodwell Introduction of the digital dot at drupa 1995 enabled flexography to achieve new levels of quality, consistency and productivity. Tone range expanded, highlights became cleaner and reverses opened up. Elimination of film simplified the platemaking process and the repeatability of the digital process reduced press makeready times and improved consistency throughout the run; from run-to-run . Today, it is estimated that more than 90 percent of the solid sheet photo- polymer sold in North America is digital. The only performance area where digital plates did not show a discernible improvement over their analog coun- terparts was in solids. Compared to gravure, flexo often measured lower solid ink density and showed reticulation or “pinholing” in solid colors. In the mid 1980s, several flexo plate manufacturers intro- duced capped analog plates in order to improve ink density. The capping layer created a fine texture on the surface of the plate to carry more ink. Flexo tradeshops also attempted to im- prove ink density through a technique called solid screening to create a fine screen across solid areas. This process created small wells in the surface of the plate in order to carry more ink. Although both of these techniques worked well with analog plates, they did not prove to be as effective in the digital work- flow. Capped digital plates did not show the consistency and high resolution of uncapped digital plates. Although ink density improved, it frequently came with higher dot gain. Solid screen- ing programs also proved less effective in the standard digital workflow, due to the well known “oxygen effect. ” the Oxygen’s effect did a wonderful job of creating a natural cut back that compensated for flexo’s high press gain, but it also tended to diminish the impact of the solid screening programs in use. CONTROLLED ATMOSPHERE In 2007, DuPont introduced an optimized digital plate expo- sure process to the corrugated industry. The objective was to create a digital dot with a profile similar to that of the analog dots that had proved so effective in minimizing the flute lines frequently found in corrugated printing. The workflow used a carefully controlled atmosphere that eliminated oxygen during the plate main exposure, resulting in 1:1 reproduction of the LAMs layer mask elements, and a dot profile that was optimized for printing on corrugated board. The process was simple and the consumables were very inexpensive. Resulting plates demonstrated the desired performance on press, and the workflow has been widely adopted by corrugated tradeshops. Success of the controlled atmosphere exposure process for thick corrugated plates led to testing of the same approach for the thinner plates commonly used in the flexible packaging and tag and label segments. Here, the objective was to determine if there was a way to reintroduce the benefits of solid screening to digital SIMPLE, FLEXIBLE PROCESS • New workflow is focused specifically on improving the solid ink coverage without trading off the benefits of the digital dot. • Plates made in the controlled atmosphere show well- formed features, good shoulder angles and clean profiles, whether processed in solvent or thermal systems. • Text and linework elements have sharp edges. • Solid screening programs produce clearly defined cell patterns. • Minimum highlight dots long associated with the digital workflow are retained. www.flexography.org octoBER 2011 FLEXO 15