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Printheads Rule
The printhead is the core of every inkjet system. There are two primary technologies—continuous inkjet (CIJ) and drop-on-demand (DOD), with the latter employing three different techniques for putting ink on paper. CIJ heads fire a continuous stream of electrically charged ink droplets at the paper. It is used primarily by Kodak for addressing applications, hybrid offset/inkjet systems, and the company's high-speed VX and VT l four-color printers. While capable of high speeds, CIJ has traditionally been unable to achieve the image quality of other technologies.
That changes with Kodak's Stream technology used in the company's new Prosper S10 printheads and the 5000XL Prosper press. With smaller drop sizes and superior drop control, Stream can deliver near-offset quality printing (up to 175 lpi), even some coated, glossy stocks. This, along with print speeds up to 650 fpm, a 241⁄2˝ print width, and monthly print volumes of up to 120 million A4 impressions, sets it apart from other inkjet systems.
DOD heads fire ink droplets as needed, targeting specific spots on the media. Drop-on-demand heads are flexible enough to accommodate three different print technologies:
Thermal inkjet (TIJ) is the most common, thanks to countless desktop printers in homes, offices and schools, and wide-format printers and addressing systems. Thermal heads are less expensive than CIJ or piezoelectric heads and have to be replaced more often, typically when changing ink. TIJ works by sending a pulse of electrical current through a small ink chamber. The pulse causes an explosion of steam, creating a bubble that displaces ink in the chamber and fires a droplet of ink through a nozzle onto the surface of the media.
HP, TIJ's leading advocate, uses the technology in most of its desktop and wide-format printers, and has extended it into commercial printing with the T300 and the new "duplex-in-a-box" T200 inkjet web presses. Both use the same kind of heads as some of HP's wide- format systems, and HP reports that heads at customer sites are proving very reliable.
Print quality is not quite as good as the best samples from the Kodak Prosper, but are more than satisfactory for most types of direct mail, transactional/transpromo documents and many types of publishing, including books with numerous color photos and illustrations such as textbooks, cookbooks, how-to guides, etc. The only real downside is that TIJ cannot print on a coated stock. While a limitation for some applications, it is not a show stopper (and may only be a temporary shortcoming.) For now, HP requires either specially treated papers, or use of a "bonding agent" that limits ink penetration into the page, minimizes dot gain and helps to ensure ink adhesion.
Piezoelectric heads have been a mainstay of inkjet printing for a quarter century. Piezo heads can accommodate a wider variety of inks than TIJ or CIJ heads and, although more expensive, are much more durable. They are capable of thousands of hours of use, versus hundreds of hours with thermal heads. The word piezo comes from a Greek term meaning to squeeze. When voltage is applied to piezoelectric material in an ink-filled chamber behind each nozzle the material changes shape, squeezing a droplet through the nozzle.
Océ, Ricoh/InfoPrint and Screen all use piezo heads, as does Kodak in its VL series. Print quality varies by vendor, but is perfectly acceptable for most transactional/transpromo jobs, much direct mail and even publishing applications like textbooks, cookbooks and others requiring color photos and illustrations. Both Océ and Ricoh/InfoPrint also offer MICR inks for security and check printing.
Fujifilm's approach is the JetPress 720, a 28.3x20.4˝ sheetfed press that can purportedly crank through 2,700 B2 (27.8x19.7˝) sph. The machine uses Fuji's piezoelectric SAMBA print heads that deliver 1,200x1,200 dpi resolution.
Taking a different approach than vendors of the big iron is RISO. Intended for office and light production use, RISO's ComColor machines are compact, inexpensive, quiet and use sheets of paper rather than large rolls. Capable of up to 150 ppm, they are finding homes in small print shops, some transactional service bureaus and in-plants, where low-cost color printing is required. MICR capability is available through a third party.
Phase Change is something completely different. Solid ink—a polymeric resin—is rapidly melted and fired through a nozzle. The sudden heating and melting causes a molecular change in the resin, which leaves the nozzle as a liquid, but returns almost instantly to a solid as it hits the printing media.
The key difference is that the inks are not water-based. This addresses a major problem with aqueous inks—absorption by the paper. Ink that dries instantly also eliminates cockle and curl, allowing pages to lie flat, and eliminates the heat of dryers. It also enables use of inexpensive plain papers.
The technology is based on that developed by Tektronix, which Xerox has morphed into its ColorQube office printer. Fast forward to IPEX, where Xerox had a technology demo of a production solid ink press. The system was running a 20˝ roll of paper at up to 500 fpm.
Targeting direct mail and transactional/transpromo markets, the print quality is similar to some of the liquid ink machines, but still lags a bit on photographic images. But it won't be available until mid-2011, and Xerox has a history of significantly ramping up image quality as a release date nears.
So, back to where we started. None of these machines highlighted are true game-changers. It will be a machine generation or two, when cutsheet models come to market that compete directly with EP systems, and offer inkjet economics at lower print volumes (under 2 million/month) before the market truly changes. Even then, as I see it, inkjet will ultimately coexist rather than replace or dominate. PI
About the Author
Noel Ward is managing director of Brimstone Hill Associates and the Real World Print channel on YouTube, and is a frequent blogger on www.piworld.com. He can be reached at noelw@brimstonehill.com.
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