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FLEXO Magazine : June2010
Technologies & Techniques 54 FLeXO june 2010 www.flexography.org Flexo doctoring is sort of like playing a sport. The team consists of the blade actuating systems, blade holders/ chambers, and blades, as well as the press operators who set and manage these items. Each team member must be good at the game and must play by the rules. To win, a team must score real low in printed waste and press downtime. So what are the rules? Well the physics of doctoring—the main rule—starts with the hydraulic force of the ink on the blade, and the resulting blade distortion/deflection, the changing footprint of the blade as it wears, as well as blade tip-lifting, must be understood and controlled. The mechani- cal forces that must be applied to the blade system in order to make it doctor can cause some too-flexible blade holders, chambers, and entire systems to bow, resulting in blade tip banding or plate bounce lines on solid printed surfaces. And a problem chamber that bows due to lack of sufficient stiff- ness for the forces applied, or bows over time due to stress relief of the chamber material, can cause your doctoring team to lose the game. The physics of doctoring also involves the science of tribol- ogy. (“Tribos” is a Greek term for rubbing.) The interaction of different blade and roll-surface materials in motion is very important. The various types of wear that can result include abrasive wear, (grinding), and adhesive wear. Adhesive wear of some metal blade materials can cause the metal to adhere to ceramic anilox rolls, resulting in cell-wall fracture and seri- ous roll damage. Appropriate analysis of the contact angle, wear pattern, footprint(s), burr formation etc. on a worn flexo, gravure, or pad print blade tip can provide vital knowledge of the role played by each press component and each human member of the doctoring team. Figure 1 shows the basic contact angle rule. Of course, we need to find out why the blade ran too flat. Is the blade and/or its tip too flexible or misaligned? Was the chamber bowing (flexing) away from the anilox in the middle, thus causing a human member of the team to overload it so as to try to make it “ wipe better”? Of course, if the blade is running flat and the applied force is too high, the blade tip can lift and trap debris, which can sometimes score the anilox. (See Figure 1.) Figure 3 shows a microscope picture that illustrates the wear pattern of a flexo doctor blade that was cranked down. The contact angle dropped down to as low as 14.8 degrees, and a lot of dot gain appeared soon after. This was prob- ably done to try to get rid of a series of wrinkle-related print defects caused by holder screws (Figure 4). This is a common problem with too many chambers. Efficient, wrinkle-free blade clamping holders/chambers are available (Figure 5). Figure 6 shows an example of a blade analysis in which the blade is bowing away from the center of the anilox. A Doctor Blade Analysis Can Help You get a Winning score in the Doctoring game By Tom Allison • Appropriate analysis of the contact angle, wear pat- tern, footprint(s), burr formation etc. on a worn blade tip can provide vital knowledge of the role played by each component. • The physics of doctoring starts with the hydraulic force of the ink on the blade, and the resulting blade distortion/deflection. • The interaction of different blade and roll-surface materials in motion is very important. Figure 1. The basic contact angle rule. All art courtesy Allison systems Corp. Figure 2. A blade that has run too flat. FLX_June2010_mech.indd 54 6/10/10 9:39 AM