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MEMS Goes Mainstream

Dec 1, 2007 12:00 AM


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HP has it; so do Kodak, Fujifilm Dimatix (formerly Spectra), Silverbrook and others. I'm referring to the next disruptive technology hitting inkjet head technology, the Holy Grail for manufacturers: single-pass inkjet for the masses.

While the concept and execution of single-pass inkjet printing is not new, the increasing advances in head design certainly are. There are many printers on the market that have multiple heads aligned in an array to cover the substrate width. This single-pass model uses full-size piezo heads, and has inherent issues of alignment and interlacing artifacts, not to mention the sheer number and cost of multiple heads aligned in a fixture plate.

Some years ago, many companies started to research ways of manufacturing inkjet heads using the same processes that currently are used for integrated-circuit production. This process is known as MEMS Fabrication Techniques. (See Figure 1.)

Micro-Electro Mechanical Systems (MEMS) is the technology of the very small, and merges at the nanoscale into “Nano-Electro Mechanical Systems” (NEMS) and nanotechnology. MEMS generally range in size from a micrometer (a millionth of a meter) to a millimeter (thousandth of a meter). The scale of MEMS is such that it permits larger and denser arrays of smaller ink orifices, increasing resolution and printing speed.

At these size scales, the standard constructs of classical physics do not always hold true. Some of the other benefits of silicon MEMS fabrication techniques feature sub-micron accuracy, robust material set and the ability for high-volume manufacturing. MEMS really shine in creating nozzles, holes, manifolds and channel structures in the inkjet head design.

MEMS inkjet heads can be fabricated using modified semiconductor fabrication technology, laser ablation, photolithography, molding and plating, wet etching and dry etching, electro-discharge machining (EDM) and other technologies capable of manufacturing very small devices.

A 10.79 cm (4.25 in.) HP Edgeline Technology print head is shown in Figure 2. It uses five silicon print head chips (called die), each with 2,112 nozzles placed in a staggered configuration. This arrangement is called a multi-die module. Each print head prints two colors of ink and has 10,560 nozzles — 5,280 per color.

FujiFilm Dimatix has been busy, as well. They have a different take on head construction. Look at the size difference for an equivalent nozzle configuration (Figure 3).

The definition in the internal structures is amazing. Fifty nanometers is about two millionths of an inch, and the columns are half that width (Figure 4).

The Kodak MEMS print head uses heat to control the formation and delivery of the ink droplet (Figure 5).

When the heater is pulsed on, it heats the ink and causes a vapor bubble to expand. This pushes ink out of the nozzle, where surface tension pulls it into a droplet. After the heater is pulsed off, the bubble is vented to the atmosphere and the chamber refills with ink. The chamber is now ready to be fired again.

Silverbrook, an R&D facility from Australia, has an incredibly dense array of MEMS produced heads (Figure 6). These heads are for water-based ink, and called “Suspended-heater thermal bubble” nozzle. Other head types for solvent and UV-curable are under development.

Print heads have five pairs of rows and are 20 mm in length. 20 mm sections are joined by the insertion of a specially designed, wedge-shaped piece. A 20.3 cm (8 in.) wide print head has 70,400 nozzles. Yikes!

Silverbrook has demonstrated working, single-pass, A4 (60 ppm) and photo (4 × 6) printers operating at one print every two seconds.

Another approach

The JeTrix print head, invented by Israeli researchers Moshe and Nissim Einat at the College of Judea and Samaria in Ariel, Israel, is another example of MEMS. (See Figure 7.)

In typical inkjet heads, ink is shared in a main reservoir and circulated through a cavity that has many branches, each leading to a nozzle. Getting uniform drops requires precise regulation — a difficult task that becomes greater as the number of nozzles and array size increases (Figure 8).

The JeTrix overcomes these limits in a couple of ways. First, there is no main reservoir requiring subtle pressure controls. Most importantly, it is not limited by size. By etching the printer head out of silicon wafer chips, theoretically, it could cover an entire sheet of paper in size, having one nozzle per pixel of image. The printing process would be one of almost simultaneous ejection of the entire image.

General print head design improvements

Xaar has introduced its latest Platform 3 heads, which feature 1,000 nozzles in a 70 mm (2.7 in.) width. The lightweight compact print head is designed to be truly “plug and play.”

Microprecision accuracy of nozzle plate ablation in manufacturing means that all print heads have identical nozzle placement positioning. With multiple print heads mounted on a print bar, the precision-designed mechanical interface ensures fast and accurate relocation of a new replacement print head with micron accuracy, eliminating the need for alignment or calibration procedures.

The Platform 3 heads will be an important component of a new generation of high-end inkjet color printers that will take on the fastest toner printers. The first commercial version of the Platform 3 head is in use from Canadian manufacturer PAT Technology. PAT Technology launched a range of digital coaters at the end of 2006. The Varstar sheetfed digital coater using UV-curable clear coatings comes in three widths and can coat at speeds up to 7,000 A4 sph.

Dimatix

The company offers new, shaped piezo, silicone M-class grayscale print heads. By varying the size of the drop fired rather than the number of drops fired or passes by the print head to create each pixel, VersaDrop technology makes it possible for inkjet systems to satisfy a wide range of commercial print requirements, including applications demanding fine tonal gradations and crisp four-pt. text printed within the same job. Each head has 304 nozzles and a native resolution of true 1,080 dpi.

New VersaDrop jetting technology, on the M-class print heads, produces favorably shaped variable size drops. This is accomplished by two-pulse and three-pulse burst mode firing, which forms a bigger drop right at the nozzle plate, not during the drop flight — as is the case with most gray scale print heads, resulting in a satellite-free spot. The silicon nozzle plate used in these print heads is not only resistant to scratching but also provides precise directionality of ink drops, resulting in sharp, clean edges while producing vector art or text. These heads are currently used in the RasterPrinter T600UV flatbed printer.

While you wait for these single-pass wonders…

Go with a massive head array printer. Inca Digital has built the Inca Onset. Carrying a staggering array of 576 Spectra print heads and addressing 73,728 inkjet nozzles, the printer boats a print speed of 500 sq. ms (5,382 sq. ft.) per hour. The heads are situated in “plug-in print bars,” utilizing an innovative alignment system. Beta testing is currently in progress, with an estimated price tag of $3 million (estimate subject to change; check with manufacturer for more details).

FastJet

SunChemical and Inca Digital jointly developed the FastJet. (See Figure 9.) Aimed at the corrugated industry to replace flexographic presses, this fixed-array print head system can print a 121.9 × 203.2 cm (48 × 80 in.) board in four colors with UV-curable ink, with a throughput potential of up to 6,000 sq. m (7,176 sq. yds) per hour and a 300 × 200-dpi resolution. The Alpha model is currently testing in England.

Spectra heads are utilized with 24 heads integrated into a “module.” Four modules make up an array (that's 96 heads per color). There are five colors (CMYK and varnish) with a total of 20 modules in the machine (480 heads, some 60,000 nozzles).

Variable printing single-pass solutions

Truepress Jet520

Dainippon Screen has incorporated Epson DOD piezo heads into a web-fed press that runs to a maximum rate of 64 m (210 ft.) per minute. This uses water-based inks, running roll stock with a maximum paper width of 51.82 cm (20.4 in.). That's 420 full-color, letter-sized pages per minute. You can tie two of these devices together for full duplexing capabilities. The workflow uses JDF-based job tickets as well as other standard file formats.

Agfa Dotrix

This web-fed, UV-curable, single-pass inkjet device has a maximum printing width of 63 cm (24.8 in.), running at speeds between 5.6 m to 23.7 m (17 ft. to 78 ft.) per minute and capable of a 300-dpi resolution.

Agfa Transcolor

This newer UV-inkjet model has been developed for transactional printing using variable data. It incorporates dual printing units, capable of producing two-sided documents at rates up to 484 pages per minute. Maximum print width is 45.5 cm (17.92 in.), with extremely fast impressions per minute numbers that vary with the particular configuration set.

Kodak Versamark

Kodak still is one of the leaders in inkjet printing speed for transactional print jobs. The Versamark series is a modular one, where heads can be added to make up any type of ink scenario necessary (black only, black with spot, add four-color process and add duplexing if need be). Resolution ranges from 300 × 300 dpi to 300 × 600 dpi or 300 × 1,200 dpi.

Inkjet press hybrids

Inov-Media of France has the unique sheetfed/inkjet press hybrid. The Jet7Pro is basically a B1 sheetfed press with the inking apparatus replaced with traversing Xaar heads. The machine can be supplied with a various number of heads, printing bidirectionally at resolutions between 360 dpi and 720dpi. Speed varies based on the number of heads supplied. Print speeds are approximately 160 B1 sheets at 720 dpi or 400 sheets per hour at 360 dpi. B1 sheet size is 70.61 × 99.8 cm (27.8 × 39.3 in.).

Inkjet arrays

Most inkjet printer manufacturers have moved to integrate multiple heads in their inkjet printers to improve productivity. Gandinnovations has 24 head models: Durst Quadro, Mimaki JV5, ColorSpan and Roland just to name a few.

Unanswered questions

In the last few decades, printers have become familiar with the idea of adapting to new technology to survive. Inkjet has held out the promise of huge benefits over conventional print processes, for a long time. The improvements to traditional offset — together with high levels of automation — have boosted analog print productivity but haven't been able to diversify print in the way that digital can, with its ability to personalize or create versioned print runs. My prediction is that the next five years will produce some of the most amazing digital printing devices ever seen.


Reprinted with permission from the Specialty Graphic Imaging Assn. (SGIA). This article by Jeff Burton, digital specialist for SGIA, first appeared in the third quarter edition of the SGIA Journal. For more information, visit www.sgia.org, keyword: comprt.