Inkjet's Industrial Limitations
R&D abounds in the functional field, but these technical elements are still holding digital back – for now.
Continued from The Challenges of Functional Inkjet Printing.
All types of inkjet technologies – CIJ, TIJ, and PIJ – present some fundamental limitations that developers must overcome for industrial work. These include the diameter of nozzle openings, fluid viscosity required for jetting, frequency of drop generation, drop volume, and fluid and substrate surface tension.
Nozzle clogging Most inkjet nozzles are very small and clog easily. Printhead manufacturers often fit nozzle plates with non-wetting coatings so that ink will not build up and dry around nozzles. As a general rule, the largest dimension of pigments and other ink particles should be less than one-fiftieth of the diameter of the nozzle they will pass through in order to avoid particle “log jamming” and nozzle blockage. Inkjet nozzles typically have diameters in the range of 10 to 50 micrometers. That means the largest dimension of pigments and other ink particles should be 0.2 to 1.0 micrometers. (Particles that are less than 0.2 micrometers are less than half the wavelength of light, by the way, and inherently transparent.)
Viscosity Most printheads require the use of fluids with very low viscosities. For example, almost all Fujifilm Dimatix PIJ printheads can function with a fluid viscosity in the range of 8 to 20 centipoise, but Dimatix recommends using fluid in the 10- to 14-centipoise range. (Water, by comparison, is 1 centipoise.) For water-based fluids with dye-based colorants, this is relatively easy to achieve, even for desktop inkjet printheads that typically require inks with less than 5-centipoise viscosity.
But polymer-forming inks without solvents, such as UV-curable inks, present a greater challenge. While screen-printing UV inks have viscosities from 1000 to 3000 centipoise, inkjet requires dramatically lower viscosities – 20 centipoise or less. Generally, the more polymeric material in an ink, the higher its viscosity. Some monomers, however, have viscosities as low as 2 centipoise and, when added to UV ink formulations, can reduce the viscosity to a level that enables jetting. Heating an ink can also reduce its viscosity. For most polymeric fluids, for every rise of 1 degree C in temperature, the viscosity will drop 2 centipoise. Too much heat, however, can polymerize UV inks.
Safety A key monomer that lowers the viscosity and also improves ink adhesion has been determined to be hazardous and can migrate out of the ink when printed onto a product. Printing that comes in contact with food, baby bottles, and other products must be safe, with very low (or no) migration of monomers or UV photoinitiators. Ink manufacturers have been reformulating their products to eliminate the questionable monomers. One solution introduced by SunJet (Sun Chemical, Parsippany, New Jersey) and Durst (Brixen, Italy) involves the use of water to reduce radiation-curable ink. Inx Digital and other ink suppliers have been reformulating their products as well. Encres Dubuit (Mitry-Mory, France) recently launched a very low-migration UV ink that is approved for use on the baby bottles printed on presses from sister company Machines Dubuit.
Print Speed Droplet size and viscosity can affect the speed of the inkjet printing system. As a general rule, the larger the drop jetted, the slower the frequency of drop generation. Higher viscosity can also lower the frequency of drop generation. These issues don’t matter in screen printing because the inks are more robust and less refined, but they present a definite barrier in the development of high-speed inkjet print lines for industrial applications.
Surface Tension Adhesion is also an inherent limitation that inkjet developers face in demanding industrial markets. Analog methods in general, and screen printing in particular, apply pressure during the printing process that promotes ink bonding to the substrate. With inkjet, only the ink touches the surface of the print. The force that causes the ink to adhere to the print surface is the surface tension of the ink in relation to the substrate. Generally, the surface tension of the ink should be lower than the wetting tension of the substrate. To ensure these conditions, printers will often use corona- or flame-treatment systems, as well as preprinted coatings to increase the wetting tension or surface area of the substrate.
So, is inkjet “ready” for a specific application you may be considering? Recently, I’ve been investigating the strengths and limitations of inkjet printers and their output along with consultants Dene Taylor (SPF, Philadelphia) and Patrice Giraud (Solstice Coopérative d’Entrepreneurs, Cergy, France).
We believe that for most two-dimensional applications, inkjet provides market-acceptable output, though typically not with the outdoor print durability of analog and certain other digital technologies. For thick coatings, including the deposition of conductive inks for printed circuit boards, inkjet does not currently compete with screen printing. More robust UV-curable inks come with a number of drawbacks, including safety. UV inks that either do not migrate or do so at acceptable levels are beginning to enter the market and find applications. With a different curing technology – electron-beam – a no-migration alternative already exists that achieves complete polymerization without the use of photoinitiators. As Taylor points out, “While e-beam has a higher initial capital cost as compared with UV, it is less expensive in the long run.” He suggests that single-pass inkjet printing with in-line electron-beam curing could offer capital and operating-cost advantages over analog and HP Indigo printing, “particularly for flexible packaging, pouch, and polyurethane substrate printing.”
As inkjet printing production speeds improve with faster computer processing, the wider availability of single-pass printing, and the formulation of new inks and functional materials that can be reliably jetted, more manufacturers will be adopting digital inkjet for its flexibility and other advantages. It has already become the dominant technology in markets such as ceramic tiles and product markings, and more will follow. Screen printing and other analog methods continue to provide characteristics that inkjet cannot yet match, but investment in research and development will continue to go toward digital technologies, bringing further breakthroughs. We’ll be covering these game-changing new technologies in Screen Printing as they emerge. Stay tuned!