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FLEXO Magazine : June 2013
WEAR RESISTANCE An important quality of doctor blade steel that differs from other strip steel applications is wear resistance. Wear resis- tance is the ability of a material to keep its original surface. The wear of a doctor blade’s edge is mainly abrasive – the result of scratching. The wear resistance of steel is affected by: • Hardness of the steel matrix • Presence of carbides • Size and distribution of carbides Carbide = Compound of carbon with one or more metallic elements e.g. Iron, Chromium, Tungsten, Molybdenum, Vanadium Carbides are relatively hard and increase wear resistance measured in VICKERS (HV) 1200..........................................1800.......................................2500 HV (200g) At the top of the list of developments is the anilox roll moving from a high line count of 360 lpi on an engraved and chrome plated surface, up to 1,200 lpi on laser engraved ceramic. This move allowed a significant reduction in anilox surface ink film. In turn, the raised image or dot on the plate can now collect ink from the anilox without being overwhelmed; thereby avoid- ing dot gain or dirty print even on longer runs. Each of the many advances made to the process elements of anilox, plates, inks, ink delivery, substrates and so on, were matched by prepress and press technology advances. In most instances, these advances were evolutionary in that they happened over time. Often an advance in one area enabled another area to improve. HD FLEXO PRESS In a recent interview, a major wide web flexo printer in Pennsylvania said it expanded capacity with a new high speed 10-color CI press from Bobst, not so much because of speed, but because of the press’s ability to achieve and maintain the highest quality print from the moment the press is running until the job is finished. While speed is important, it follows concerns for quality and an ability to change 10-color jobs in as little as 15 minutes; a feat made possible as a result of the press’ Graphic Position- ing System. Printers can efficiently and profitably run 360,000 ft. over eight jobs, ranging from 10,000 ft. to 50,000 ft., all in a single eight-hour shift. Being able to run top quality short run and long run jobs with little waste and downtime enables them to be more competitive in more markets. This example of new HD flexo technology is the result of the combined advances of many small and not-so -small process element improvements made over many years. HD DOCTOR BLADE Like all flexo process components, the doctor blade has evolved to the point that many blade materials, commonly used in 1980, are no longer available. To understand the driving forces in this evolution, it’s important to first review the fundamental function of a doctor blade. The core purpose of a doctor blade in the flexo process is to remove excess ink from the anilox surface; to leave behind a perfectly smooth ink film for transfer to the image on the plate and to do this without damaging the anilox. The most precise and even metering takes place when the blade makes contact with the anilox in the reverse or sheering posi- tion at an angle between 30° - 35°, with the least amount of contact area. MoC CrC FeC VC www.flexography.org June 2013 FLEXO 67