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FLEXO Magazine : June 2013
tion difficult. As a result, automated ink control and optimized cleaning methods are favored because of their speed and reliability. Key components that maintain a stable flow are the supply system and dual doctor ink and coating chamber, also known as a chambered doctor blade. CHAMBERED DOCTOR BLADES A closed chambered doctor blade is standard on modern flexo presses, especially on central impression machines. This inking system enables correct, uniform transfer to the anilox cells without air trapped while backblading at higher speeds. The chambered system and anilox roll must also en- sure no residual ink is left on the anilox sleeve, and must also include a fully automatic means of cleaning the chamber 's hoses and anilox roll on the press. The chambered doctor blade features a sealed, airtight chamber at the ends and two doctor blades at the top and bottom sides of the chamber wall, into which ink is pumped and circulated. The wall formed by the two blades creates the closed environment, thus allowing controlled, consistent ink metering, prevention against leakage and easier cleaning or job changes. Two thin doctor blades are in contact with the surface of the anilox roll. The trailing doctor blade seals the chamber and is positioned at the entrance of the anilox roll into the chamber. The doctor blade lays on the anilox roll in the direction of the roller rotation. The reverse angle doctor blade cuts the ink film on the surface and removes excess ink on the cells and the ink film on the bridges between the cells. A light pressure is needed because the excess ink and coating pushes the blade against the anilox surface. A constant pressure is thus maintained regardless of speed, without risk of material wear. To keep volume and pressure stable, the chamber depth is maintained. The rotation of the anilox roll can create a suck- ing effect on the ink. Aided by the stream from the ink pumps, a perpetual state of turbulence is created. The positive doctor blade may need a higher pressure because the ink film on the anilox roll surface pushes against the doctor blade. As the blade is dragging on the anilox roll, the residual ink creates pressure in the gap between the blade and the roll surface. One blade prevents the ink from escaping the chamber, the other meters the ink from the non-cell areas. The dual doctor blade's ability to meter the ink makes the fountain roller re- dundant. Minimal manual intervention is needed because the anilox cell volume determines ink / coating laydown, ensuring consistent results. In the case of coatings, this allows increased gloss values for better reflection and more efficient drying because the whole substrate area needs the same energy to dry. Ability to achieve consistent ink laydowns allows improvement of ink yields by up to a fifth compared with double-roller alterna- tives. A rubber seal system is necessary to form a perfect, airtight closure and stop ink leakage from between the anilox roll and the chamber. Another advantage of the airtight environment made possible by the seals is that leftover ink can be stored for long periods, without evaporation or perishing. The design also enables the pressure-controlled foam-free flow of ink to be maintained. www.flexography.org JUNE 2013 FLEXO 87