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FLEXO Magazine : June 2013
run by adjusting the chamber ’s or blade holder’s position set- ting or loading pressure. After all of the measurements are taken and indicated on the digital display, the image is saved to a file that will later be compiled in the report. This process is then repeated for each sample. When all of the samples have been measured in the microscope, the data is compiled in a report that will contain commentary about the results of the analysis. Comments will explain what was measured, what it means, and any recommendations to help improve the doctoring efficiency. ANALYSIS & FINDINGS So, what does the analysis uncover? Many times the analysis reveals that the doctor blade was being run at a contact angle that is too flat (Figure 2) by generally accepted standards. What is too flat? Accepted contact angles for flexo- graphic printing will fall into the range of 25 to 42 degrees with a desirable angle being near 30 degrees (Figure 3). Less than 25 degrees is considered to be to flat for a flexo doctor blade. Flat angles will not provide proper doctoring or clean up of the anilox roll and will leave some surface ink on the anilox causing a shiny surface appearance. An operator may see the shiny anilox roll and apply more force to the system to try to clean up the roll (remember the multiple angles mentioned earlier). This will work initially as the contact angle changes, but success will be short lived. Too much force applied to the doctor blade, will quickly wear in a big footprint, potentially causing dot gain and inconsistent print. A flat blade angle could also trap dried ink particles, blade material, or other debris potentially causing the infamous anilox roll score lines. Conversely, a contact angle greater than 42 degrees is considered to be too sharp. An angle that is too sharp can cause the blade to chatter, resulting in lines across the web or other print defects and vibration. To illustrate the effects of flat contact angles, look at the two blade samples shown in Figure 2 and Figure 3. Notice that the flat contact angle blade shown in Figure 2 has very little wear (0.001-in.) , but a very large footprint (0.027-in.). In contrast, the good contact angle blade shown in Figure 3 has 24 times more blade wear (0.024-in.), but a footprint that is less than half the size (0.010-in.) of the flat contact angle blade. The flat angle blade had to be changed prematurely due to poor print quality. If you look closely, you can still see some rounding on the top of the blade from the initial blade tip because the flat angle is wear- ing “up” from the bottom of the blade. The good angle blade is wearing “back” from the edge of the blade, as it is designed to do and provided good blade life with consistent print quality. Containment blades found in chambered doctor blade systems are the exception to the above angles. Containment blades should generally run flatter than the doctor blade. The range of angles for the containment blade is generally accepted to be between 20 and 25 degrees. The containment blade is run flatter and is typically a flexible plastic or thinner metal blade to prevent debris trapping and back doctoring. Ideally, debris should pass easily under the containment blade and then be absorbed in the ink to ultimately be cap- tured by a filter/magnet in the ink circulation system. If your chamber is not set up to run the containment blade flatter than the doctor blade, some chamber designs will allow you to use a slightly wider containment blade, which will automati- cally flatten out the contact angle. Most chamber designs will permit a containment blade that is 1/16-in. to 1/8-in. wider than the doctor blade. Other than contact angles, a blade analysis will reveal any misalignment issues (Figure 4) that are present in the setup. Proper blade to anilox alignment is very important when using a chambered inker setup. Both blades should contact the anilox roll at the same time and evenly across the face of the anilox roll. If there is any misalignment in any axis, extra pressure will need to be applied to get the blades to seat properly and provide a Figure 3: Accepted contact angles for flexographic printing will fall into the range of 25 to 42 degrees, with a desirable angle being near 30 degrees. Figure 2: Many times, analysis reveals that the doctor blade was being run at a contact angle that is too flat. - -less than 25 degrees. Flat angles will not provide proper doctoring or clean up of the anilox roll and will leave some surface ink on the anilox causing a shiny surface appearance. Figure 4: Proper blade to anilox alignment is very important when using a chambered inker setup. Both blades should contact the anilox roll at the same time and evenly across the face of the anilox roll. If there is any misalignment in any axis, extra pressure will need to be applied to get the blades to seat properly and provide a good wipe. 76 FLEXO June 2013 www.flexography.org