Eliminating the Pinholing Effect in Flexo Printing
Brand owners and designers are constantly pushing the limits of printing technology—looking for ways to make packages stand out on the shelf and catch the eye of the consumer. The market for consumer products is more competitive than ever, so any advantage can be the difference between success and failure. While there are many factors that play into print quality, flexographic printing today is benefiting from technological advances that improve ink transfer and help to eliminate many common problems.
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One prominent issue in flexo is that solid print areas have historically been full of pin-holes and voids. These pin-holes are caused when the ink layer is not continuous, which results in a loss of density and color strength. To get the highest possible density with the least amount of ink, the ink needs to be printed in a thin continuous layer, with no voids. With this approach, the light from the source reflects evenly and continuously to the observer—in this case, to the consumer viewing packaging on the shelf. Inability to achieve desired density can lead to all sorts of follow-on problems in production. Trying to apply more ink to counteract the problem increases drying times and slows presses down, and applying more impression to “up” the density results in “squashed” highlights and reduced tonal reproduction. So what’s the root cause of the problem?
What Causes Pin-holing
The properties of the anilox roll in combination with the plate and substrate are typically responsible for the pin-holing effect. Anilox rolls are designed to provide flexo with a controlled mechanism to transfer ink to the plate surface in a "known" volume. Ink is transferred to the surface of the plate as a whole series of dots, with the high-speed rotating motion of the press often turning the dots into ridges of ink. These ridges of ink are then transferred from the plate to the substrate, often with elongated voids on the final substrate between them as shown below.
So while the anilox cell enables a controlled supply of ink to the plate, it also contributes to the way in which the ink is transferred from the plate to the substrate. If the ridges are not broken up, then pin-holes can be visible on the final printed result.
It should be noted that all of the ink supplied to the anilox roll is NOT transferred; in fact the common rule of thumb is that 50 percent of the volume is transferred from the anilox to the plate, and 50 percent from the plate to the substrate. This equates to approximately 25 percent of the original anilox volume being transferred to the substrate; but there are a large number of press and component parameters that can affect this in production.
Unfortunately, simply throwing more ink at the pin-holing does not solve the root cause; more ink means more cost in ink and more cost in energy to dry it. More ink also means the use of a higher volume anilox roll, but these rolls often have lower LPI value, with bigger cells and a larger gap between the centers of each ink dot resulting in potentially larger voids between the ridges of ink.
One of the ways to eliminate the pin-holing effect is to reduce the amount of gap between the anilox cells on the roll. This can also increase the ink flow, but there are limits on how much this can be done. Another option, and the one that we are going to focus on here, is to change the surface of the plate. Traditionally this approach has involved creating small cells in the surface of the plate that carry ink in an attempt to transfer more ink to the print. However, for the reasons discussed before around the use iof increased ink laydowns, this approach is just a partial workaround that often results in increased production costs, rather than solving the root cause of the issue.
Kodak Digicap NX Patterning is a standard component of the Kodak Flexcel NX System designed to address these ink challenges in flexo—specifically solid ink densities compared to gravure printing. Enabled by Kodak Squarespot Imaging Technology, unique to the Flexcel NX System, the function of Digicap NX Patterning is to image a unique pattern into the surface of the plate. Compared to traditional surface patterning technologies, Digicap NX Patterning is significantly smaller and finer. It is a true micro surface texturization of the plate surface, applied to the 1-bit .tiff in the RIPing process, and utilizing half-pixel imaging at 10.6 x 5.3 microns size, or 2400 x 4800 dpi. The following image shows the surface structure formed on the flexo plate at
The image below shows three areas of 70 percent coverage—the top left image shows no surface treatment applied, the bottom left image shows a traditional type surface patterning application with cells in the surface of the plate, and the right image shows such Patterning.
This process produces a pattern on the surface of the plate, like regular rectangular islands surrounded by a thin sea of ink that break up the cell pattern from the anilox roll. By breaking up the anilox pattern, it breaks up the pin-hole pattern, and significantly improves the ink transfer. The image below shows a before and after situation, with the printed solid from a competitive digital plate on the left, and on the right from the same press, ink and anilox, on the same substrate the print using Kodak Flexcel NX Plates with Digicap NX Patterning applied.
This breaking up of the pattern allows the transfer of the ink in a smooth layer without the large pin-holes, resulting in cleaner brighter colors, and higher densities, with the same or lower ink laydowns. Contrary to popular belief, solid densities are impacted more by the smoothness and eveness of ink laydown than they are by the actual volume of ink transferred, and increased densities with lower ink laydowns allow for better overprints for expanded gamuts, without adversely impacting drying times or press speeds.
Patterning Yields Print Quality Benefits
By leveraging the ability to create unique patterns in the surface of the plate that overcome production challenges with heavy solid areas, or the application of white ink for flexible packaging, users are seeing dramatic quality improvements. Better ink transfer allows printers to achieve their target densities with ease, generally eliminating the need to chase density with more ink, more pigment, and more pressure. These features alone often allow printers to achieve greater color gamuts, better print quality, higher print speeds, and greater consistency.
Printers also are able to reduce the volume of anilox rolls used, reducing ink use while still achieving the densities and colors required. Reducing anilox volumes reduces the risk of dirty print, allows higher LPI printing, and enables faster press speeds with lower dryer demands. Such reductions in volume are positive, but it should be stressed that too great a reduction can cause ink supply issues and non-optimal operation on press. Printers should evaluate the correct anilox LPI and volume for specific applications using a banded roll trial in partnership with the plate and ink combinations to establish optimal print conditions.
Color gamuts are improved because the solids are clean rather than being pin-holed. Also, when overprinting to build colors such as in 4- or 7-color process, the results are cleaner and brighter. Having the ability to build cleaner and brighter colors enables flexo printers to do more of their work with process printing instead of spot colors. It also improves the printer’s ability to standardize printing and ink set, whether for 4- or 7-color process printing.
Digicap NX Patterning has been one of the lead technologies in the “flexo revolution” over the last five years, truly allowing printers to achieve the densities of gravure printing, and helping to stop the chase for density caused by excessive pin-holing with traditional flexo. It has been a key enabler in brand owners increasingly accepting that flexo is no longer a second-class print process, easing the transition from offset and rotogravure to flexo.
Kodak continues to explore the mechanisms of the flexo print process and is identifying additional areas where differentiated plate imaging technology can further increase print quality and reduce or eliminate production obstacles. Proven Kodak technologies such as Digicap NX Patterning, Squarespot Imaging Technology, and Kodak Hyperflex Imaging Software will play a major role.
With more advances in screening and patterning on the horizon for flexographic printing, brand owners and package production professionals will continue to push the quality boundaries and deliver products that jump off the shelf. This is an exciting time in the package printing industry. Professionals who embrace the latest technology and understand the benefits to be gained by new thoughts and new approaches to plate imaging will be the real winners moving forward.
Kodak Commercial Imaging