The Flexo Process: Be Careful

Flexo 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 a day-to-day basis on flexographic presses. There is no absolute recipe for this work, but below are some typical specifications:

Ink Densities Anilox Plate
Yellow 0.95-1.05 Type Laser Engraved Ceramic Type Digital Photoploymer
Magenta 1.25-1.35 Screen Angle 60 deg Caliper (in.) 0.045, 0.067
Cyan 1.25-1.35 Line Count (lpi) 800-1400 Screen Count (lpi) 120, 133
Black 1.45-1.55 Volume (bcm) 2-1.2 Finished Durometer (Shore) 70-75

The ability to print and hold accurate process dots over a wide tonal range is dependent on ink/anilox/plate interactions. The basic idea is to print the thinnest ink film possible so the ink has less tendency to be impressed over the shoulders of the dots. In the real world there are limits to all concepts, and examination of the components and how they work together will provide some insights. The schematic above traces the path of ink from the enclosed ink chamber to the substrate.

An anilox roller with a volume of 2 bcm/in.2 has the equivalent of a continuous peripheral ink film thickness of just 3 microns. About 50% of this ink is transferred from the anilox roller to the plate. On nonabsorbent film substrates, there is a transfer rate of about 50% from plate to substrate, giving a printed wet ink thickness of around 0.75 microns.

Thinner plates have many advantages. They conform well to the plate cylinder, lift less, and hold more rigid and higher durometer dots. Digital photopolymer plates produce dots with squarer shoulders, allowing for more precise impression settings, almost to the point of an on-off condition. There are probably no technical downsides to moving to thinner digital photopolymer plates.

Laser-engraved ceramic rollers are available in accurate and reproducible volume down to 1.2 bcm/in.2 and less. The quest for thinner ink films drives the industry to lower-volume anilox rollers with higher cell counts. Here we start to reach the limits of the system. Smaller, shallower cells are inherently more prone to plugging, and loss of volume may make ink density targets unattainable.

In the process ink formulation, pigment loading can be increased, but at some point the functional and end-use properties will be unacceptable. It should also be noted that thinner films of stronger inks require the use of slower evaporating solvents to avoid drying in the anilox and on the plate after impression.

Roto and litho print from a plane surface, whereas in flexo printing the image is raised. When viewed under magnification, flexo dots are not as precise, but in most packaging jobs this is not an issue. Working back from the print, it is evident that smooth surfaces, such as film, allow for less impression of plate to substrate and less dot gain. However, the raised image is an advantage in flexo when printing rough substrates.

The flexographic print station is now a precision ink metering system. There may be more developments to come, but most printers are refining their current setups to produce consistent quality at high speeds, with quick changeovers and reduced waste.

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.


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