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FLEXO Magazine : February 2011
Technologies & Techniques Digital UV Inline Plate Processing Flexo leaps into a new era By ian Hole Commercially available worldwide, after more than five years extensive development, this new method is proving very popu- lar. A number of factors are credited with driving this trend, namely: • Consistency: Digital UV LEDs provide complete con- sistency of exposure throughout the plate, significantly better than with UV tubes in conventional light frames. • Controlled exposure: With tube light frames, the expo- sure output to the surface unit is permanently set, ac- cording to the age of the tubes. In contrast, UV LED units can be easily set to a large extent across their full output range, regardless of the age of the light source. • Variable control: Thanks to digital technology, the LEDs can be regulated perfectly and can even be controlled in a variable and customized way. • Repeatability: UV LED systems have a significantly superior level of stability across their entire service life, compared to traditional, analog tube light frames. • Reduction of waste: Because the UV LEDs are integrated into the CTP imaging systems— known as Inline UV—the potential risk of damage to the flexo plates, when transporting them between the imager and light frame is eliminated. Pro- cessing times that unnecessarily extended the entire process are reduced as well. • Extended lifetime: The service life of the UV LED technology is better than 10 times longer than the older tube technology. Highly consistent light distribution is achieved via a special kaleidoscopic optical path that combines the light from the individual LEDs. So, in the optimum scenario, the differences in the light intensity on the plate surface fluctuate by a maximum of + 2 per- cent. This evenness of light intensity of the digital inline UV technology also represents a fundamental difference from old light frames. Three factors are responsible: 1. From few light sources in the sys- tem, kaleidoscope technology produces a seemingly unlim- ited number of light sources, thanks to mirroring. Due to the countless reflections, the light is radiated in horizontal and verti- cal directions across an angle range of almost 180 degrees. 2. Integration of the LEDs into the imager, means main UV exposure of the flexo plates are on the rotating vacuum cylinder of the imager (onto which the mask was for- merly freely lasered directly). Radiation is distributed exceptionally evenly across the plate. 3. With the inline UV process, the plate is held against the thick-walled aluminum cylinder during all processing stages and is evenly cooled. With tube light frames, air cushions between the table and the glass plate, partially impair the plate cooling during the exposure process, and can cause uneven hardening of the dots. When you take a closer look at each system, the reasons why and the significant extent to which the new inline UV tech- nology is a noteworthy advancement quickly become clear. InlIne UV UP Close A distinct advantage of the LED process is the consistent uniformity of light distribution along the entire plate. This is primarily because there is effectively no performance loss throughout the service life of the LED, meaning there is no associated uneven fading. This avoids the classic problem of tube light frames, where the UV output of some tubes fade sooner than others. This means some parts of the plate receive less energy. The result? Variable dot hardening, dis- tributed unevenly across the plate — one of the main causes of plate error. Another advantage that could be of critical importance to some users is the exceptional durability of UV LEDs — the service life is 5,000 – 10,000 hours. This has been proven using these diodes in practice, and from experience in other print industry segments. It means an extended service life with absolutely stable and consistent operation, and virtu- ally no maintenance. This is quite different from the high maintenance and shorter, 500 to 1,000 hour, life of traditional fluorescent tubes. And more important, the UV LED output is far more pow- erful than the light available from tube light frames. Flexo experts are already considering what this means: entirely different halftone dot profiles can be formed, including those familiar from flat top plates. HIgH PerformanCe / sPeCIal materIals The LED’s UV output can be tailored specifically to the customer’s requirements. It is precisely measured according to the plate format and productivity of the digital imager, mak- ing shorter overall process times possible. In other words, by being able to expose the plate with a higher intensity light, the exposure time is reduced. You get all of the benefits of excel- lent reproduction and exceptional repeatability, and a shorter exposure time. In addition to a standard program with digital UV exposure for standard plate types from well-known manufacturers, special systems with a total UV output of up to 750W are also available. With these systems, the UV output is more than 550 mW/cm2. This is about 30 times the output of a tube light frame. These high performance levels are required for the ex- posure of special materials. Another option provided by a UV LED light source is intended specifically for letterpress plates, where the typical angle distortions from traditional point light sources can be avoided. The generation of flat tops also benefits from the high UV output. These are halftone dots that do not have any round Path of the dot compensation curves (sleeve) in the highlight range with various exposure times. inline uV permits shorter exposure times than tube light frame, without changing the dot compensation curves significantly. www.flexography.org FeBruary 2011 FLeXO 43 FLX_Feb11_mech.indd 43 2/16/11 8:51 AM
Sustainable Winter 2011