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FLEXO Magazine : June 2012
Technologies & Techniques Uniform Doctor Blade Metering How to Achieve Desired Results By Bill Warner Flexographic printing requires the doctor blade to provide uniform metering, so that the ink volume carried by the anilox to the plate is determined only by the anilox vol- ume. If the doctor blade is not working correctly, the ink volume carried to the plate will include the anilox volume plus some amount of surface ink. Any surface ink remaining on the anilox will be variable leading to variation in the printed product. In order to achieve uniform metering, the correct doctor blade needs to be used for the job and it needs to be installed and set properly. This article will provide guidelines on how to achieve uniform metering throughout the run. A significant number of modern flexographic presses are running dual blade chambered inkers, so the information be- low is oriented to a chambered inker being used. If you aren’t using a chambered inker, the same basic principles will also apply to single blade reverse angle blade holders. BLADE SELECTION There are many options in doctor blades, and the blade material you choose will have an effect on the metering qual- ity and life of the blade. Typically, metal doctor blades will provide the best metering, followed by composites, and then plastic blades. Within the category of metal blades, there is carbon, stain- less, and long life tool steel materials. Any of them can have an extra coating applied to help them perform better. The shape of the tip will also have an effect on the metering qual- ity, whether it is a rounded tip, lamella, or beveled edge. Typi- cally you will want to run a thinner blade with a small polished working tip to be able to maintain a small blade footprint on the anilox roll. All of these variables can be confusing and I suggest you work with your blade supplier to find the right blade combina- tion for the type of jobs you run. BLADE ANGLE The blade contact angle (Figure 1) is fixed on a chambered inker system, so there isn’t much you can do to control the angle, other than running the chamber at the minimum ap- plied pressure required to maintain a clean, uniform wipe. A generally acceptable flexo doctor blade contact angle range is 25 to 42 degrees, depending on the system. A doctor blade contact angle of 30 degrees is a nominal good angle for preventing print defects. Flat angles, or angles less than 25 degrees, will not provide proper doctoring of anilox rolls, since they will require more force to doctor cleanly. Too much force applied will quickly wear in a big flat, cause dot gain and inconsistent print, and possibly load dry ink and/ or blade material into the anilox and damage it. Too sharp con- tact angles, angles greater than 42 degrees, can sometimes cause catching of the blade in the anilox cells, resulting in lines across the web or vibration and other related print defects. Flexo containment blades should generally run flatter, about 20 degrees, and be thinner metal or flexible, but em- bedment resistant plastic should be used to prevent debris trapping and anilox wear or damage. Ideally, debris should pass easily under the containment blade and then be cap- tured by a filter/magnet in the ink circulation system. BLADE INSTALLATION The method of installing the blade in the chamber is ex- tremely important. If your blade clamping device uses bolts to secure the blade, be sure to tighten all the bolts the same by using a torque wrench set to an appropriate specification for the size of bolt being used. Also, start in the center of the chamber and work out to the ends alternating from side to side. Before installing the doctor METERING METRICS • Blade material will have an effect on metering quality and life of the blade • Metal blades typically provide the best metering, followed by composite and then plastic • Acceptable contact angle range is 25-degrees to 42 degrees • The correct doctor blade needs to be set and installed properly 78 FLeXO june 2012 www.flexography.org