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FLEXO Magazine : February 2014
PLANTS & PROCESSES Metering Ink: The Fundamentals First in 5 Part Series on The Importance of Basic Process Element By Paul Sharkey Ask any promoter of the rotogravure or offset printing processes and they will tell you the fundamental short- coming of flexo printing is dot gain, image growth and resulting “dirty print.” Flexo detractors point to the raised image on the plate having to first collect ink from the anilox ink film and then “stamp” it onto the substrate. While a great deal has been done to minimize it, dot gain is still a persistent problem that will eventually creep into the flexo printed image over time. Over the past 25 years, improvements have been made to dramatically minimize the elements impacting dirty print. CLEAN PRINT SOURCE Precise ink metering, the kind needed to eliminate the surface ink film on an anilox, occurs when: • A doctor blade makes contact with the anilox at 30 de- grees to 35 degrees to the tangent (See Figure 1) • The contact area is kept to a minimum so as to avoid hydraulic pushback at speed While this information is not new, many flexo printers strug- gle to achieve and maintain a thin contact area at 35 degrees. Factors preventing precise metering for an entire run are: • Poor condition of the doctor blade holder and clamp • Excess blade to anilox pressure • Wrong choice of doctor blade • Lack of adequate operator training DOCTOR BLADE HOLDER It’s true, like every other press component, blade holders do wear out. Not just the clamping mechanism but the entire blade holder. Think about it: When a holder and clamp are in new condition, the entire length of the blade can make simultaneous contact with the anilox at the correct angle with minimum pressure. However, when the blade clamp is pitted, when bolts are missing and the backstop is corroded, the blade will not sit evenly and uniformly. When the holder or chamber is even slightly warped, one end of the blade will make anilox contact without the other. PRECISE INK METERING: ADDRESSING ROOT OF DOT GAIN & DIRTY PRINT Partial List of Key Process Advancements to Reduce Dot Gain Anilox Surfaces Higher Line Counts New Cell Geometry Improved Surface Finishing Plates Thinner Material Layered Technology New Dot Geometry Machine Tolerances More Robust Rolls, Cylinders and Sleeves Tighter TIR, Taper Mounting Tapes Wider Range of Density Thinner Materials Inks Higher Pigment Concentration More Stable Carriers Ink Metering Systems Stabilized “No Bounce” Chambers Lower Pressure Chambers “No Leak” Chambers Auto adjusting Doctor Blade Wider Range of New High Definition Steels Most Precise Metering Longer Lasting Lower Friction / Less Abrasive to Anilox New Containment Blade Materials 24 FLEXO FEBRUARY 2014 www.flexography.org Figure 1: A component of precise ink metering is the doctor blade making contact with the anilox at between a 30 degree and 35 degree angle to the tangent.