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FLEXO Magazine : August 2010
Technologies & Techniques are of equal importance. Most printers agree that an angle of between 30 to 40 degrees is ideal for the wiper blade and although the containment blade can be run at a similar angle for higher speed presses, some printers are recommending a shallower angle. If the containment blade angle gets too shallow, it can result in back doctoring and ink slinging. Extreme loading pressures can result in burrs forming on the blade edge, which will result in metal particle contamination of your ink and will ultimately result in scoring of the anilox. As this will happen from time to time and other metal contamination and hardened particles may get into the ink you are using it is strongly recommended that rare earth magnets and stainless steel mesh filters are used on all presses. Blade angle can be affected by the loading pressure, which if too great, can lower the contact angle to the point that you will not get clean metering; and, as the blade wears, the contact angle will increase. The combination of blade wear and load will move the blade contact point away from its original center line as well. Anilox Selection Like with doctor blades, it is essential to choose the cor- rect anilox for the application. This includes surface coating, screen count and volume. Again, there are many factors that come in to play before making the selection: • Ink system: solvent, water, UV. • Metering system: rubber roll, doctor blade, chamber system. • Substrate: absorbent or non-absorbent. • Speed of press. • Type of press and age. • Graphics to be printed: solid, solid and line, tints, process screen. • Line count to be used on printing plate, e .g. <65lpi, <85lpi, <100lpi, <133lpi, >150lpi. • Printing plate: rubber or photopolymer (yes, this is still relevant). • Operator skill level. There is no doubt that if you intend to run any configuration of doctor blade against the surface of your anilox roll, then it needs to be laser-engraved ceramic. With a hardness of 1,000 and 1,350 vickers, or 73 Rockwell, plasma-coated ceramic that is diamond ground, diamond polished and then laser engraved is, presently, the most durable surface to run a doctor blade against in the marketplace. Traditional chrome-plated anilox are just too prone to wear, if run with a doctor blade, no matter how careful you are or what blade material you run with. Before we can go any further, let’s go over some of the ba- sic terminology so that we are all thinking the same thing. Screen count/line count/lines per inch all refer to the number of cells per linear inch or linear cm of engraving, and are measured at the angle of engraving. Screen angles of traditional mechanically engraved rolls were mostly 45 degrees, although 30 degrees and 60 degrees were some- times offered. Laser engraved anilox started with 30 degrees and progressed to 60 degrees, which has become a global standard, although alternative angles are available and used mostly for coating applications. Cell volume is the theoretical carrying capacity of each individual cell per given calculated area. Notice, I have used the word theoretical, as the final amount of ink that is trans- 94 FLeXO august 2010 www.flexography.org Doctor BlADe Q&A DoeS the ink formulAtion hAve to Be conSiDereD? The ink system or formulation that you choose to use for a particular job is one of the most critical compo- nents of the print station that has a huge impact on your blade. In the case of flexography, we have three ink systems to consider: solvent, water or UV ink. Each of these ink systems has different flow properties, pigment concentration, pigment particle size, viscosities, abrasiveness, corrosiveness, alkalinity, acidity and so on. Generally, the thicker and more viscous your ink or coat- ing, the thicker and more rigid your blade needs to be. DoeS the Anilox mAteriAl or engrAving Affect the choice of BlADe? The anilox surface has a major affect on the longev- ity of your doctor blade, and the lower/courser the line count and higher the volume the more quickly the blade will wear as well. The doctor blade must be rigid enough to cope with thicker coat weights, and have ad- equate abrasion resistance when run against a ceramic or coarse line count WhAt effect Do preSS SpeeDS hAve? As you run faster greater friction and vibration can occur. The blade must have good rigidity and high abra- sion resistance. Do you hAve to conSiDer grAphic QuAlity AnD proceSS printing? If you are printing very high graphics, with fine screens on your plates and high line counts on your anilox, you will need to transfer a thinner ink film. The doctor blade needs to have the thinnest contact thick- ness possible and therefore high-quality Swedish steel or micro-alloy materials with 0.008in. or less are gener- ally recommended. WhAt Affect Do WAter-BASeD or other corroSive SolutionS hAve on the BlADe? Water-based, acidic or alkaline solutions can and will attack all steel carbon materials. Only stainless steel or composite materials can be considered for corrosive environments. Do uv inkS hAve Any BeAring for Doctor BlADe/Anilox Selection? UV inks present special challenges to the printer be- cause of the heavy pigment concentration and thickness that they are generally run at, in particular, when print- ing at higher speeds. Again, thicker more rigid blades are recommended, in particular stepped or lamella blades, have been found to be very good with UV inks. WhAt ABout high ABrASive inkS AnD coAtingS? By their very nature, abrasive inks and coatings present unique challenges for the printer in terms of longevity of the blade and premature wear to the anilox roll. New micro-alloy coated and ceramic coated steel blades are recommended for these conditions. August2010_mech.indd 94 8/13/10 7:47 AM
Sustainable Summer 2010