PFFC - May 2008 - (Page 14)

PROCESS MANAGEMENT Ink Transfer from 2 bcm Anilox Roller The Flexo Process: Be Careful �� F By David Argent Contributing Editor Yellow lexo process printing now is competitive in quality with other printing processes and can have advantages of cost and the ability to print a large number of different substrates. One trip down the supermarket aisles will display superb flexo �� process print. However, in manufacturing, flexo process �� printing is something that often “just barely works.” All input elements of the process are interdependent, so �� they must be selected carefully and controlled to form a �� functional and robust system. In wide web flexible package printing on film substrates, exceptional process print is produced on Laser-engraved ceramic rollers are available in aca day-to-day basis on flexographic presses. There is curate and reproducible volume down to 1.2 bcm/ no absolute recipe for this work, but below are some in.2 and less. The quest for thinner ink films drives typical specifications: the industry to lower-volume anilox rollers with Ink Densities 0.95–1.05 1.25–1.35 1.25–1.35 1.45–1.55 Type Screen Angle Line Count (lpi) Volume (bcm) Magenta Cyan Black Anilox Laser Engraved Ceramic Type 60 deg 800–1400 2–1.2 Caliper (in.) Screen Count (lpi) Finished Durometer (Shore) Plate Digital Photoploymer 0.045, 0.067 120, 133 70–75 The ability to print and hold accurate process higher cell counts. Here we start to reach the limits dots over a wide tonal range is dependent on ink/ of the system. Smaller, shallower cells are inheranilox/plate interactions. The basic idea is to print ently more prone to plugging, and loss of volume the thinnest ink film possible so the ink has less may make ink density targets unattainable. tendency to be impressed over the shoulders of the In the process ink formulation, pigment loading can be increased, but at some point the functional and dots. In the real world there are limits to all concepts, and examination of the components and how end-use properties will be unacceptable. It should also they work together will provide some insights. The be noted that thinner films of stronger inks require schematic above traces the path of ink from the en- the use of slower evaporating solvents to avoid drying in the anilox and on the plate after impression. closed ink chamber to the substrate. An anilox roller with a volume of 2 bcm/in.2 has Roto and litho print from a plane surface, whereas the equivalent of a continuous peripheral ink film in flexo printing the image is raised. When viewed thickness of just 3 microns. About 50% of this ink under magnification, flexo dots are not as precise, is transferred from the anilox roller to the plate. On but in most packaging jobs this is not an issue. nonabsorbent film substrates, there is a transfer rate Working back from the print, it is evident that of about 50% from plate to substrate, giving a print- smooth surfaces, such as film, allow for less impresed wet ink thickness of around 0.75 microns. sion of plate to substrate and less dot gain. Thinner plates have many advantages. However, the raised image is an advantage They conform well to the plate cylinder, in flexo when printing rough substrates. lift less, and hold more rigid and higher The flexographic print station is durometer dots. Digital photopolymer now a precision ink metering sysplates produce dots with squarer shoultem. There may be more developders, allowing for more precise impresments to come, but most printers DYNE sion settings, almost to the point of an are refining their current setups to LEVELS on-off condition. There are probably no produce consistent quality at high Part 1 of 2 technical downsides to moving to thinner speeds, with quick changeovers and digital photopolymer plates. reduced waste. COMING NEXT MONTH Process improvement expert David Argent has 30+ years of experience in process analysis with particular emphasis on ink and coating design and performance. Contact him at 636-391-8180; djvargent@sbcglobal.net. 14 | MAY 2008 WWW.PFFC-ONLINE.COM http://WWW.PFFC-ONLINE.COM

Table of Contents Feed for the Digital Edition of PFFC - May 2008

PFFC - May 2008 Contents First Glance Calendar of Events From the Editor Web Lines Process Management News Clips Narrow Web & Label Reporter Narrow Web & Label What's New Products Labels/Sleeves Cover - European Converting Drupa New Product Digest What's New Products Services Directory Classified Marketplace Advertisers Index Experience Speaks

PFFC - May 2008

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