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FLEXO Magazine : August 2010
Technologies & Techniques A major reason for this decision was that at speeds of above 100mpm (300ftpm) the flow of ink between the fountain and anilox roll creates a hydraulic effect, which, as the speed increases, will push the two rolls apart. Even if manual adjust- ments were made to resist this, the force would be so great that no amount of adjustment would be able to compensate for the flood of ink that would result when the gap between the rolls increased. The only option at that time was to slow the machine down, which in the competitive world of print, was really not an option. In addition, the rubber fountain roll is prone to other issues including: • Swelling. Usually heat or chemical attack, wrong com- pound. • Rapid wear. Normally from friction due to abrasive coating, misalignment, improper nip pressure, wrong compound, wrong durometer. • Shrinkage. Chemical/ozone/UV light attack, wrong com- pound. • Cracking. Chemical/ozone/UV light attack, wrong com- pound. • TIR problems. Misalignment when installed, incorrect grinding • Hardening. Oxidization, high temperatures, friction, wrong compound. • Softening. Chemical, heat, wrong compound. • Glazing. Heat, oxidization, chemicals, friction, alignment, pressure. From most people’s perspective, adding a doctor blade to the flexographic process seemed to be a relatively easy and logical step. Like most things in life, however, for every action there is a reaction, and so it proved to be with the doctor blade. The goal was clear: achieve a thinner, more uniform ink film thickness and print quality was bound to improve, not to mention ink consumption would be reduced. Initially, a trailing or positive angel blade configuration was used as in gravure print, with an angle between 55 and 60 degrees. Ironically, it was quickly realized that this configura- tion also suffered from the same hydraulic effect which in this case pushed the blade away from the face of the anilox which in turn allowed more ink to transfer to the plate. As a result, the reverse-angle or negative doctor blade was introduced to this problem once and for all, and it remains the most common format today. This blade configuration shears the ink from the surface of the anilox and is normally set at an angle of between 30 and 35 degrees. It is also less prone to allow ink particles and other contaminants to get trapped between the blade and the face of the roll. In this, case the hydraulic force works in the blades favor by forcing the blade harder to the face of the anilox as the press speed is increased. While the wiping action had improved, there were still other issues, such as ink splashing and overflow at high speeds, solvent loss for solvent-based inks, evaporation and foaming for water-based inks. This led to the next step of eliminat- ing the fountain roll. Initially, this was an open-ended setup, with ink being returned back to the ink pan. It used a reverse Tag & label all substrates Graphics screen range ( LPI) volume range (BCM) Solid color & coating applications (using rubber doctor roll or blade ) 180 - 220 11.5 - 8.5 Solid color & coating applications (with metering blade) 200-250 9.5 - 7.5 Solid color & coating applications (with metering blade) 250-300 7.5 – 6.5 Simple graphics & fonts (rubber Roll) 330-360 6.0 - 5.5 Simple graphics & fonts screens <65 (metering blade) 330 - 440 5.5 - 4.5 Process Screens < 85 450 - 700 4.0 -3.5 Process Screens < 100 750 -900 3.5 - 2.5 Process Screens < 133 900-1100 2.5 -2.0 Process Screens < 150 1100-1200 2.0 -1.75 Process Screens < 175 1200-1300 1.65 -1.26 Process Screens < 200 1300-1500 1.20- 0.95 Process Screens <275 1600-1700 0.90 - 0.85 Process Screens <300 1800-2000 0.80 - 0.75 Envelope all substrates Graphics screen range ( LPI) volume range (BCM) Solid color & coating applications (using rubber doctor roll or blade ) 180 - 220 11.5 - 8.5 Solid color & coating applications (using rubber roll or blade ) 200-220 9.5 - 8.5 Solid color & coating applications (with metering blade) 250-300 7.5 – 6.5 Simple graphics & fonts (rubber Roll) 330-360 6.0 - 5.5 Simple graphics/ fonts & screens<65 (metering blade & rubber roll) 330 - 440 5.5 - 4.5 Process Screens < 85 450 - 700 4.0 - 3.5 Process Screens < 100 750 -900 3.5 - 2.5 Process Screens < 133 900-1100 2.5 -2.0 Process Screens < 150 1100-1200 2.0 -1.75 Process Screens < 175 1200-1300 1.65 -1.25 Process Screens < 200 1300-1500 1.20 - 0.95 Process Screens <275 1600-1700 0.90 - 0.85 www.flexography.org august 2010 FLeXO 91 August2010_mech.indd 91 8/13/10 9:39 AM
Sustainable Summer 2010