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FLEXO Magazine : June 2014
From this gathered data and the press operator’s feedback on print quality, the supplier can then determine if it has made the correct doctor blade choice for your pressroom. If needed, adjustments should be made to blade thickness, tip configuration or whatever is needed to achieve the set goal of being as effective as possible in your pressroom with the proper optimized doctor blades for the new press. Keep in mind that not every press in the pressroom can or should run the same doctor blade. If something changes, get to work with all sup- pliers to make sure their current products are still viable for the pressroom preforming at the top of its potential. USED BLADE ANALYSIS There is a huge benefit in analyzing used doctor blades from a press. Used blades show a little history and how the pressmen have been running the press. The first step in the analysis is a visual inspection of the used blades to note any odd or heavy wear patterns (See Image 1). Then, measure the width across the blade in four or five places (depending on length) and compare it to the new blade width for wear measurement (See Image 2). This can also be done by weight, with scales. Place a clean new blade of exact dimensions on a scale and then weigh the used blade for comparison of steel removed. The third step is measuring wear angles of the blade. This will show if the blade was set properly, if the chamber is aligned square to the anilox roller and if the ideal angles were achieved for metering. The ideal angle for newer presses is 35 degrees to 45 degrees for metering (chambered and single blade systems) and 10 degrees to 15 degrees for containment. Presses around 10 years old or older are looking for 30 degrees to 35 degrees for top and bottom for ideal wear angles. All of these angles are determined on the chamber design. Then, measure the contact area (See Image 3). This is the part of the blade that actu- ally comes into contact with the anilox roller. This footprint should be as small as possible. On a typical setup with a 0.004-in. tip blade at a 40 degree contact angle, you should be at around 170-μm. (0.0067 -mm .) of contact area. With normal wear and pressure added, the blade should be close to 220-μm. (0.0087 -mm.) for an ideal contact area after use. The larger the contact area/footprint on the doctor blade, the less ink you will remove from the anilox. This will result in leaving a thicker ink film than desired on the anilox roller and thus transferring too much ink, leading to dot gain, density shifts and color variation. METALLURGICAL REPORT The makeup of doctor blade steel is very specific and is held to ex- tremely high standards. This is done for a product that less than 1 per- cent of—on average—is consumed by the printer. Imagine buying a bag of chips and eating only one. This is why the doctor blade supplier should be able to provide metallurgical product specification docu- mentation. The information should include the chemical composition of the steel. This is the amount of carbon, silicon, sulfur phosphorous and manganese of your steel. With today’s fine grain structure of steel, almost everyone has the ability to provide this data, along with photos of the dispersion of the particles and sizing. Now with all of those metallurgical terms out of the way, let’s look at what the average printer should look for in quality doctor blades. Ask about the mechanical properties such as tensile strength and hardness of the steel. What is the straightness or bow tolerance along with the flatness? What are the thickness and width specifications and tolerances? There is also a roughness specification for doctor blade steel—find out what it is. There are answers for all of these questions, so ask for the documentation from the doctor blade supplier. “ The most critical part of doctor blade steel is consistency from batch to batch,” said Dr. Sibylle Stiltz, ISO-Certified metallurgist for Max Daetwyler Corp. “Metallographic analysis is performed daily in our lab (See Image 4) and documentation to ISO standards so the data is always correct and readily available.” ISO certification has been commonplace in the manufacturing industries for years and printers should know if their suppliers are ISO certified. This will insure the printer is getting the same prod- uct, made to the same ISO standards, that is being manufactured to those standards consistently. You should see no difference in quality or performance from your ISO certified doctor blades in your day to day pressruns. This helps eliminate one variable that may arise in the printing process. n About the Author: Johnny Stamey began his printing career in 1987 at RR Donnelley as a student in the apprenticeship program. After 4 years and 8,000 hours of combined education and practical experience, Johnny received his Journeyman Printer Certification from the North Carolina Department of Labor. He worked at RR Donnelley as a press operator for 11 years before being employed at Keating Gravure as a proof press technician. In August 2000, Johnny joined Max Daetwyler Corp. as a technical sales representative. In 2004, Johnny was awarded FTA’s Author of Excellence award for his article on doctor blade trouble- shooting. He is currently the technical product manager for Daetwyler and covers the U.S ., Canada and Mexico. He is actively involved in FTA, TLMI and GAA. Image 1 Image 2 Image 3 56 FLEXO | JUNE 2014