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FLEXO Magazine : June 2011
Technologies & Techniques common type of blade material used for ink- metering doctor blades. Strip steel is the type of steel from which doctor blades are made. There are many different types of strip steels available for use in hundreds of different industrial applications. Strip steels are used for flapper valves, numerous automotive applications, razor blades and of course doctor blades. Because each application for strip steel often has different and diverse performance requirements such as durability, ductility, malleability, corrosion resistance, wear resistance, memory, and so on; strip steels can be very differ- ent from one another. When doctor blades were first introduced to flexo print- ing 40 years ago, the strip steels selected for doctor blade use were inexpensive and usually better suited for general industrial applications. However, advancements in anilox roll technology made those steels less suitable for precision metering. Today ’s advanced doctor blade strip steel is a precision- made material, both in microstructure, hardness and in ex- ecution. Requirements on steel producers are quite stringent, targeting at tensile strengths of about 2000 N/mm2, Vickers hardness of about 600, a fraction of a millimeter deviation per meter in straightness, polished surfaces, smooth shaved edges and fine-grained microstructure. Different ingredients (chemistry) and customized treat- ments, combined with specifically tailored edge treatments, mean a doctor blade can be custom engineered for maxi- mum performance in different flexo applications. Figures 6 and 7 illustrate coarser-grained and finer-grained doctor blade steel microstructure. The typically less expensive coarser-grained microstructure contains a number of rather large carbides unevenly distrib- uted, which decrease the ductility of the steel, providing sites for crack initiation and chipping. A typical blade wear surface for coarser-grained microstructure showing anilox lines and chipping is shown in Figure 8. Figure 9 depicts the debris field, as collected on press. Con- versely a finer-grained microstructure contains a larger number of smaller and more evenly-distributed carbides. The advantag- es of this high definition steel can be seen in Figure 10. Advantages of fine-grained steels are: • Uniform, clean wear and thus a lower wear rate. • Less formation of slivers. • Lower friction and scoring damage to the anilox roll surface. Collectively, those traits result in 1) lower blade consump- tion, 2) less damage to the anilox roll, 3) less production downtime and fewer stops, 4) a better print result. WHAT’S NEXT For the finest of print images, a customized and coated steel doctor blade is often preferred. These new tips ensure close wiping of the anilox roll for a clear print, while the coat- ing lubricates to decrease friction and wear on the blade. The coating is harder and less ductile than the underlying steel, increasing resistance to hard ink pigments and reducing the tendency to sliver formation and particle-sticking. There are several types of coated blades available on the market, using tribological materials of varying thicknesses. Figure 11 shows one example of a coated doctor blade tip. The development of nanotechnology will likely enable a Figure 6: Coarser-grained microstructure. Figure 7: Finer-grained microstructure. Figure 5: Resin embedded in plastic blade edge. Figure 8: Coarser grained doctor blade surface wear showing anilox lines and chipping. Figure 9: Debris from coarse strip steel collected by on-press 50 μ filter bag. 46 FLeXO June 2011 www.flexography.org
Sustainable Spring 2011