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FLEXO Magazine : October 2012
plates work by using a second, thin photopolymer layer on the surface of the plate, molecularly bonded to the main photopolymer layer. This second layer typically has differ- ent properties than the main photopolymer layer, which can confer special capabilities to the printing plate. For instance, the use of a harder photopolymer in the cap layer (compared to the base photopolymer) can reduce dot gain by minimizing dot deformation at the print interface. Ad- ditionally, the inclusion of particles in the cap layer can be used to impart a defined surface roughness to the print surface. This engineered surface roughness in the cap significantly improves SID in many cases. It is hypothesized that the high- frequency roughness aids in the separation of the ink film from the plate after contact with the substrate, resulting in a more uniform distribution of ink compared to an uncapped plate. Typical improvements in SID and mottle for Digital Epic, the world’s first digital capped plate, made by MacDermid Printing Solutions, are shown in Figures 1 and 2. The undeniable performance benefits of capped plates have led to their widespread use in some applications, where they offer an easy and dependable increase in print perfor- mance. However, the addition of the second photopolymer layer in the plate adds to the cost of manufacture of these plates. For many, the additional value the plates deliver in print has more than justified their higher price. MEMBRANE 200 Another option for those looking to increase SID is to use the LUX exposure process and Membrane 200. This gives both a very stable LUX flat-topped dot and imparts surface roughness to an uncapped plate. The LUX dot has a number of additional print benefits, such as highlight dot stability and impression latitude, but the surface roughness induced by Membrane 200 gives a boost to SID as an added bonus. Typi- cal improvement is shown in Figures 3 & 4. The use of Membrane 200 requires the use of the LUX exposure process, which involves the simple lamination of a thin membrane over the mask of an imaged digital plate. This new step not only allows the SID improvement seen, but also gives all the additional benefits of LUX flat-top dots, including reduced impression sensitivity, more robust highlight dots, and “type-high” uniform dot height. SOLID SURFACE PATTERNING For the ultimate in SID increase and mottle reduction, it is hard to beat the use of the latest digital surface pattern- ing technologies, such as Esko’s MicroCell. For many years, platemakers put 95-98 percent tones in their solid areas because they often observed that these tones gave higher SID than a true solid. Surface patterning technology works on much the same principle, but advancements in imaging technology have led to the use of carefully designed surface patterns that can provide increases in SID and reductions in mottle that are often dramatic. See Figure 5. Although MicroCell patterns work well on plates imaged in standard digital format, the effects of MicroCell can be boosted even fur- ther by combination with the LUX exposure pro- cess. The LUX process sharpens the MicroCell pattern in the same way that it sharpens dots and reverses, yielding a more precisely defined surface pattern in the solids (Figure 6). Figure 3: Membrane 200 vs. Standard Digital. Figure 4: Membrane 200 vs. Membrane 100. Figure 5: Comparing Printed Solid. Figure 6: Printed Solid Comparison: Non-LUX at Left; LUX at Right. 76 FLEXO oCtobeR 2012 www.flexography.org 1.00 1.10 1.20 1.30 1.40 Std Digital Mem 100 Mem 200