Sign & Digital Graphics

April '18

Issue link:

Contents of this Issue


Page 40 of 88

34 • April 2018 • S I G N & D I G I T A L G R A P H I C S DIGITAL PRINTING AND FINISHING DIGITAL GRAPHICS lamp or an LED light to provide it to the photoinitiator to initiate the crosslinking reaction. Though basically the same, ink formulations are somewhat different for arc lamp and LED curing systems. So, in summary, the ink composi- tion incorporates the acrylic polymers (monomers and oligomers), sensitizers, photoinitiators, pigments and other addi- tives needed to adjust surface tension and viscosity. Advantages and Disadvantages of UV-Cure Inks Unlike water-based or solvent-based inkjet inks, U V -cure inks are much thicker and higher in viscosity. The printheads are heated to reduce the vis- Stray UV light will cause premature crosslinking which will clog nozzles, so care should be taken to avoid it. (Right) Industrial inkjet printheads are required for UV ink jet printing. (Image courtesy of Ricoh) Be sure to read carefully and adhere to the recommendations of the ink supplier for safe use and disposal. (Image courtesy of Nazdar) cosity so they can be efficiently jetted; and for this, industrial piezo printheads must be used. Thermal inkjet or low-vis- cosity piezo inkjet printheads like those sold in printers such as the HP DesignJet or Epson SureColor cannot be used. Industrial printheads for UV-cure inks are much more expensive, and the print- ers that incorporate them are as well. UV inks do offer a significant advan- tage in that the inks do not evaporate in the nozzles like water-based or solvent- based inks, so the capping and wiping are much easier to manage and clogging is much less a problem. On the down side, the inks are very susceptible to stray UV light (such as ambient light) which can cause premature crosslinking which can clog up the nozzles. Equipment features in UV-curing printers are designed to avoid stray light, and other features keep nozzles free by randomly ejecting drops from nozzles not frequently used—criti- cal features in a reliable printer design. Another UV ink challenge is the fact that oxygen in the air competes with the monomers and oligomers for the free radicals that are formed upon UV exposure. Excess photoinitiator is usu- ally incorporated into the ink to com- pensate for the oxygen degradation. If the oxygen consumes the photoinitiator before the crosslinking reaction then the crosslinking will be incomplete. To avoid this problem and allow for much higher printing speeds and more complete cur- ing, EFI employs a nitrogen "blanket" the printing area of some of its print- ers to provide an oxygen-free printing environment. To achieve a good cure and durable result, the UV light must get to all of the ink all the way down to the substrate. Since yellow and black pigments absorb UV light very effectively, care should be taken to be sure enough UV light is used when high coverage is printed. In addition, printing onto fabrics and other porous substrates poses an additional problem. If the ink penetrates the fabric or porous substrate before it is cured, unreacted ink can remain behind the fibers or deep within the porous surface.

Articles in this issue

view archives of Sign & Digital Graphics - April '18