Sign & Digital Graphics

September '14

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56 • September 2014 • S I G N & D I G I T A L G R A P H I C S remain uncapped for a much longer time between the purging cycles that remove the more viscous, thicker, ink from the nozzle that forms when water evapo- rates. This is called "decap time." The trick here is to be sure that each nozzle is fired before the ink viscosity increases to a point where the drop is either not ejected, is the wrong size or is ejected at the wrong velocity. So HP disclosed that they have devel- oped an additive that forms a very thin layer on the meniscus of the ink where the ink in the nozzle interfaces with the air. The meniscus is the curve in the upper surface of a liquid that is close to the surface of its container, an effect that is caused by surface tension. HP's ink additive retards the evapo- ration but doesn't otherwise interfere with the inks performance either in the ejection process or on the media. HP has extended their "decap time" by 10 times more than with previous inks. Both Memjet and HP have disclosed that they use a technique that HP calls "Spit-on-Page." When the printer deter- mines that it needs to clear a nozzle that has been decapped too long, it spits a drop onto the print. Since both com- pany's products produce drops too small to see without magnification, they are essentially invisible and thus the reliabil- ity is maintained without stopping the printing process or causing degradation of the print quality. The Media All of these products are designed for technical printing applications—not high-end graphics. The media for these printers are all porous paper or coated film products like those used in other water-based inkjet printers. The ink must absorb quickly since the drops are printed nearly simultaneously. One of the challenges with single-pass printing has been to achieve high-density black print- ing. Usually, when black is needed in a scanning printer, other colors like cyan are printed on the same spot to aid in achieving a high-density black spot. HP has disclosed that they developed a black ink that provides higher-density black on HP heavyweight paper stock. This is accomplished both by formulating the black ink with a higher opacity and by surface-sizing the paper to hold out the pigment and avoid losing pigment down into the paper where it cannot contribute to the density. Conclusion We are now finally reaching a point where high-resolution fixed array printing has arrived with print speeds up to 50 feet per minute, with good quality and reli- ability on porous substrates. I believe that this is only the beginning. As the technol- ogy continues to be developed you can expect low-cost high-speed inkjet print- ing for many more applications, further displacing traditional analog printing. SDG The 40"-wide print bar from HP delivers ink across the entire width of the print as the media moves under it. (Illustration courtesy of Hewlett-Packard) HP PageWide Pigment Inks resist the effects of extended decap time. After a drop is ejected, film-forming agents in the ink rapidly migrate to the meniscus to produce a vapor barrier that reduces the rate of water loss. (Illustration courtesy of Hewlett- Packard) Memjet's 42"-wide "Waterfall" printhead has been integrated in high-speed water- based wide-format printers from Océ, Xante, Own-X and now Fuji Xerox.

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