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FLEXO Magazine : June 2011
Technologies & Techniques roll (Figure 2), as well as reduce blade related wear on the anilox roll. Inker design permitting, these blades work best when they are 1/16 in. to 1/8 in.wider than the doctor blade. Plas- tic blades are the safest for an operator to handle because they are not likely to impose a cut risk. Typical plastic doctor blades range from 0.030 in. to 0.062 in. and typical contain- ment blades range from 0.014 in. to 0.020 in. EDGE PROFILES As mentioned earlier, doctor blades were originally hand shaped in house by the converter, using stones or hand files. By the time doctor blades began to be used in flexographic applications, doctor blade manufacturers were providing pre- finished doctor blades that were more precise and consistent than the hand-made blades. Typical blades in the 1960s were made with a 15-degree bevel and some had a tip that was “lapped” to help the blade seat on the cylinder faster. The footprint of the 15-degree bevel grows rapidly as it wears, which can lead to print varia- tions throughout the run. In the early 1970s, the lamella blade was introduced that offered a consistent footprint that is small and remains constant as it wears. The lamella shape doesn’t have the support of a beveled blade and can be adversely sensitive to applied force levels. Today, blades are available that have bevels in the range of two-to-five degrees. These “long” bevels combine the support and effective tip control of a beveled blade, with a nearly constant footprint comparable to the lamella blade. Doctor blades are also available without a beveled or lamella profile and are typically referred to as square and rounded edge blades. The square edge blade is essentially raw material stock that may or may not have a precision working edge. Square-edge shapes are typically plastic con- tainment blades only and are not recommended to be used as doctor blades. Rounded-edge blades are a step above square- edge blades as they offer a precision, polished working edge. Since they don’t require the addi- tional machining that beveled and lamella blades require, rounded blades can be offered at an economical price. Rounded-edge blades offer a strong working edge that is not easily damaged, but they will typically allow a little surface ink to get past the blade until it seats in fully. Rounded-edge blades are especially useful when trying to doctor rough anilox rolls. Whatever type of edge profile you use, remember the footprint of the blade must be small enough to seal against the anilox roll’s cell wall grid, without requiring application of high forces to the blade. Blades applied to the anilox roll with high force could result in excessive wear of the blade and anilox roll, as well as print defects. A general rule of thumb is that the footprint should be less than 0.010 in. for pro- cess work or fine screen anilox rolls and less than 0.025 in. for line work or coarser anilox rolls. FINAL THOUGHTS I once heard a purchasing agent report that he spent more money on supplies for his facility ’s restrooms than on doctor blades. In my mind, that statement puts the cost of doctor blades in perspective with other costs related to flexographic printing. I’m sure that same agent could not say the same thing about the expenditures on ink, substrate, and press time. With the doctor blade being a mission critical part of the printing process, purchase the best quality and most appro- priate blade you can for your application and doctor blade related problems will be reduced or eliminated. Work with your doctor blade supplier to determine the best choice of material and profile for your doctor blades. Your blade sup- plier most likely has had experience with the same printing requirements, issues, and problems that you have and will have targeted guidance in the choice of blades for your ap- plication. Regardless of the type of doctor blades you use, mate- rial quality is of utmost importance. Protect your anilox rolls and print jobs by making sure basic metal, composite, and plastic blade stock is proven in service and is purchased from a quality source and traceable by the supplier. Beyond the choice of doctor blades, your doctor blade supplier will also likely be able to help with doctor blade usage and safety training, as well as diagnostic worn blade analysis. n About the Author: Bill Warner is a design engineer with Alli- son Systems Corp. He has 23 years of experience with doctor blades and custom retrofit doctor blade systems for flexo- graphic and other printing processes. He can be reached at 856-461-9111 or firstname.lastname@example.org. 68 FLeXO June 2011 www.flexography.org
Sustainable Spring 2011