R&D abounds in the functional field, but these technical elements are still holding digital back – for now.
By Vince Cahill
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.
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