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Evolutions in food packaging printing

You are here: Conferences & Events * 2010 Montreal * Abstracts * 3. Improving printing process

3. Improving printing process

3.04 The effect of surface properties on the printability of flexographic printing plates

David Galton, David Bould and Tim Claypole

Abstract
The move to environmentally friendly, water-washable flexographic printing plates has resulted in a variation in the surface properties of the plate, compared to plates processed traditionally, using solvent. In order to assess the impact on printability, a trial has been performed, to compare the two plate chemistries. The objective of the investigation was to compare the printing performance of the two types of material and to quantify the results in terms of tone value increase, physical dot area and solid density. Two plate materials were selected, one conventionally processed, using solvents to wash the plate, and a water washable plate. Large differences were observed in the printing characteristics of the two plates and it was concluded that the higher contact angle of the water washable plate resulted in the ink being pinned on the surface of the halftone dot, during ink transfer, rather than being squeezed down the dot shoulder.

Keywords: environmental, flexography, surface energy
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3.05 Ink mist formation in roller trains

Tim C. Claypole, Georgios Vlachopoulos, David C. Bould

Abstract
Misting occurs during ink film splitting when filaments are ruptured in multiple points giving rise to droplets of ink that detach from the roller train. A laboratory based physical simulation uses trapping techniques on a tack-tester was developed to enable the investigation of misting mechanisms under controlled conditions. The trapped mist was analysed both in terms of the location combined with the size distribution of the droplets. This provides unique information on the origin of the misting droplets which can be related to local dynamic nip and roller conditions.

The origin of misting can be traced to the ink film splitting mechanisms at the exit to the printing nip. The film splits into ribs and then ligaments. These extend at increasing speed as the rollers separate. Under the action of viscoelastic forces these ligaments of ink can split to form both satellite droplets and larger droplets attached by a ligament of ink to one roller. The interaction between the centripetal forces and the visco-elastic forces can cause either the ink to collapse back onto the roller, leading to a rib or for the ink to accumulate at the outer end eventually leading to a large breakaway droplet.

Keywords: offset, ink transfer, film splitting, misting
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3.06 A laboratory method to characterize and predict ink misting

Saeid Savarmand, Douglas Bousfiel, Richard R. Durand Jr., Bob Warren

Abstract
Misting is a common issue for high speed offset press operation. A laboratory device is used to characterize misting. Computer controlled drives turn rolls at known speeds. A flush mounted pressure transducer is used to record the pressure distribution during operation. Plastic strips, placed under the nip, capture mist generated. Several inks were characterized with a stress-controlled rheometer. Proposed dimensionless numbers consolidate the results for the inks studies in this work. A model of fluid slinging from a roll is proposed. The amount of misting inversely correlates with the pressure peak generated in the nip and directly increases with increasing the Reynolds number. The slinging model predicts that centrifugal forces can pull fluid into defects to enlarge filaments that can lead to slinging, but this mechanism only explains the results at high speeds. Misting at low speeds must come from a filament breakup pattern driven by surface tension.

Keywords: misting, filaments, rheology
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