Journal of Imaging Science and Technology · Volume 42, Number 1, January/February 1998
Progress and Trends in Ink-jet Printing Technology
Part 3
Hue P. Le*
Le Technologies, Inc., Beaverton, Oregon
Recent Developments and Trends in Technology
Printhead Design and Fabrication Processes. Today the ink-jet technologies most active in laboratories and in the market are the thermal and piezoelectric drop-on-demand ink-jet methods. In a basic configuration, a thermal ink-jet consists of an ink chamber having a heater with a nozzle nearby. With a current pulse of less than a few microseconds through the heater, heat is transferred from the surface of the heater to the ink. The ink becomes superheated to the critical temperature for bubble nucleation, for water-based ink, this temperature is29 around 300°C. When the nucleation occurs, a water vapor bubble instantaneously expand to force the ink out of the nozzle. Once all the heat stored in the ink is used, the bubble begins to collapse on the surface of the heater. Concurrently with the bubble collapse, the ink droplet breaks off and excels toward the paper. The whole process of bubble formation and collapse takes place in less than 10 µs. The ink then refills back into the chamber and the process is ready to begin again. Depending on the channel geometry and ink's physical properties, the ink refill time can be from 80 to 200 µs. This process is illustrated in Fig. 10. Figure 11 reillustrates the same process by plotting the parameters including electrical pulse, temperature, pressure, and bubble volume against time.
Figure 11. Pressure, temperature, and bubble volume changes during a drop formation cycle of thermal ink-jet.
Figure 12. A SEM photograph of a channel in the Hewlett-Packard DeskJet 850C color printhead.
Figure 12 shows a scanning electron microscope (SEM) photograph of a Hewlett-Packard 800 series thermal ink-jet channel with heater and ink barrier layer (the aper ture plate was removed). This jet was known to produce 32 pl ink droplets at the rate of 6000 drops per second. The ink channel in the SEM photograph is measured at about 0.001 of an inch in thickness and little more in width. However, the dimensional stability, accuracy, and uniformity of this channel are known to have great effects on jet performance such as drop frequency, volume, and velocity. All of these drop performances ultimately determine the quality and throughput of a printed image. The trends in the industry are in jetting smaller droplets for image quality, faster drop frequency, and a higher number of nozzles for print speed, while the cost of manufacture is
Figure 13. A light microscopic photograph of a channel in the Hewlett-Packard DeskJet 890C color printhead.
Figure 14. The basic configuration of a piezoelectric printhead.
Figure 15. The basic pressure requirement for ejecting an ink droplet.
reduced. These trends force further miniaturization of the ink-jet design. Consequently, the reliability issue becomes critical. In the latest generation of the Hewlett-Packard 800 series, the company introduced a new 192-nozzle tricolor printhead that can jet much smaller ink droplets (10 pl) at the rate of 12,000 drops per second. Figure 13 is a light microscopic photograph of an ink-jet channel from a Hewlett-Packard new tricolor printhead for the DeskJet 890C. The channel heater is measured about one mil square. Ink feeds from both sides of the heater chamber. This fluid architecture would significantly decrease the possibility of nozzle clogging that may result from particulates trapped in the printhead fabrication processes as well as in the process of making inks. A row of small openings between the ink manifold and the heater chamber was also introduced in the new design, in order to improve the reliability of the new printhead.
Another trend in the industry is market demand for lower cost per print. Printhead producers could pack in greater ink volume per cartridge to increase the print count or install a permanent or semipermanent thermal printhead to reduce the cost of new ink cartridges. Again, this trend will demand even higher reliability for thermal ink-jet printheads.
Canon is another major company that develops and produces thermal ink-jet printers. In the latest bubble-jet product BJC-7000, Canon introduced a 480-nozzle printhead. By far, this is the highest nozzle count for a single printhead module marketed to the home and small office color ink-jet printer market. In the BJC-7000 implementation, the 480-nozzle printhead consists of six colors with 80 nozzles per color. Other companies that develop and manufacture thermal ink-jet printheads are Lexmark, Olivetti, and Xerox.
In the piezoelectric drop-on-demand ink-jet method (Fig. 14), deformation of the piezoceramic material causes the ink volume change in the pressure chamber to generate a pressure wave that propagates toward the nozzle. This acoustic pressure wave overcomes the viscous pressure loss in a small nozzle and the surface tension force from ink meniscus so that an ink drop can begin to form at the nozzle. When the drop is formed, the pressure must be sufficient to expel the droplet toward a recording media. The basic pressure requirement is showed in Fig. 15.
Table I. A Current List of the Piezoelectric Drop-On-Demand Ink-Jet Printhead Producers
| Producer |
Piezo Deformation |
Printer Example |
| Tektronix |
Bend-mode |
Tektronix Phaser 350 & 380 |
| Sharp |
Bend-mode |
Mutoh RJ-1300 & RJ-1800 |
|
Epson
|
Bend-mode
|
Epson Color Stylus 400, 600, and 800
|
| Dataproducts |
Push mode |
Idanit 162Ad |
| Spectra |
Shear mode |
Polaroid DryJet, 3D Actua 2100 |
| Nu-Kote |
Shear mode |
Raster Graphics PiezoPrint 5000 |
| Topaz Technologies |
Bend/ Calcomp shear combination |
CrystalJet |
Figure 16. Cross section SEM photographs of a Tektronix stainless steel jet stack.
Figure 17. Cross section SEM photographs of a bond line in a Sharp stainless steel jet pack.
Figure 18. A cross section SEM photograph of a Spectra printhead.
In general, the deformation of a piezoelectric driver is on the submicron scale. To have large enough ink volume displacement for drop formation, the physical size of a piezoelectric driver is often much larger than the ink orifice. Therefore, miniaturization of the piezoelectric ink-jet printhead has been a challenging issue for many years. A list of piezoelectric drop-on-demand printhead producers is provided in Table I.
Tektronix (352 nozzle) and Sharp (48 nozzle) printheads are made with all stainless steel jet stacks. These jet stacks consist of multiple photochemical machined stainless steel plates bonded or brazed together at a high temperature. Figure 16 shows a cross section SEM photograph of a Tektronix jet stack. The thin Au intermetallic bonding layers are visible between the brazed plates. The intermetallic bond in ink-jet printhead application requires uniform thickness for design performance consistency and hermetic sealing to prevent inks from leaking externally or between two adjacent ink channels. Similar bonding characteristics are found in a Sharp jet stack. Figure 17 shows a cross section SEM photograph of the Ni intermetallic bond between the stainless steel plates of the Sharp printhead.
Besides using Au or Ni to bond metal plates together, solder and epoxy are also used to fabricate printheads. Figure 18 shows a cross section SEM photograph of a Spectra printhead where the electroformed nickel orifice plate is bonded to the jet stack by epoxy. In the same photograph, the solder bonds between multiple steel plates are also noticed. However, due to ink compatibility issues, the selection of epoxy or solder composition must be carefully considered. Given the trends to increasing the number of nozzles, decreasing their physical size, and jetting many different fluids, bond integrity and stability of the printhead become increasingly critical issue.
In 1993, Epson introduced the Stylus 800 piezoelectric ink-jet printer to compete directly with thermal ink-jet or bubble-jet technology in the low-end home and small office printer market. This product introduction was very significant in the sense that it was the first time a reliable low-cost piezoelectric ink-jet with a permanent printhead was successfully introduced in a low-end printer. This Epson printhead is based on a push-mode design with a multilayer piezoactuator.30 Based on the same printhead technology, Epson introduced Stylus Color in 1994 and Color Stylus II in 1995. A cross section SEM photograph of an Epson push-mode jet is shown in Fig. 19. Figure 20 shows a SEM photograph of a multilayer PZT actuator with 20 µm thickness per layer. Alternate electrodes are seen in both sides of each PZT layer. With this design, Epson fabricated a 64-nozzle printhead with a nozzle-to-nozzle spacing of 140 µm to achieve a nozzle density of 180 dpi.
In 1997, Epson introduced Color Stylus 400, 600, and 800 with a bend-mode design piezoelectric printhead. Color Stylus 800 employs two printheads: 128-nozzle for black and 192-nozzle for color (CMY). The technological breakthrough in this new bend-mode piezo printhead introduction is in the unique fabrication method for the thick film PZT sintered on top of the zirconia diaphragm to make piezoelectric drivers. A SEM photograph of the PZT/diaphragm drivers is shown in Fig. 21. These PZT/diaphragm structures measure less than 1 mils in total thickness. In contrast to the PZT/diaphragm structures in a Tektronix bend-mode printhead, PZT thickness is about 6 mils and stainless steel diaphragm thickness is about 3 mils. Significant reduction in the thickness of driver structures allows Epson to miniaturize the 192-nozzle printhead to about 18 ¥ 34.8 mm with a nozzle density of 180 dpi. Note that, as compared to the push mode with a long PZT structure design, the new Epson thick film PZT bend-mode device has a planar structure. The fabrication process for the new design is simple and less costly. Furthermore, with a small, flat and thin printhead structure, any addition of heaters to control the operating temperature of the printhead is much easier to design. The trends here are to increase the number of nozzles and add more flexibility in ink formulations, as was potentially realized with Epson's new printhead technology.
Figure 19. A cross section SEM photograph of an Epson Stylus 800 printhead.
Figure 20. A SEM photograph of a multilayer piezoceramic driver in the Epson Stylus 800 printhead.
Figure 21. A SEM photograph of the thick film PZT on the zirconia diaphragm in the Epson Color Stylus 800 printhead.
Nu-Kote 128-nozzle and Topaz Technologies 256-nozzle piezoelectric drop-on-demand printheads are the two newest additions to the ink-jet market. The Nu-Kote printhead is based on the development of a Xaar shear-mode shared wall design.31 Raster Graphics uses three 128-nozzle printheads per color in their newly introduced PiezoPrint 5000 large-format color ink-jet printer. The technology is about 10 years old, but the field experience is new. A key challenge for the Nu-Kote printhead is its reliability in the market.
The Topaz 256-nozzle printhead is also new to the industry. It is known to combine both the bend and shear modes to jet ink droplets at a relatively high-drop-ejection frequency. The technology was introduced in the Calcomp CrystalJet large-format ink-jet printer. Thus far, no product has been shipped yet.
Independently from the thermal or piezo ink-jet method, bend or shear mode, one of the most critical components in a printhead design is its nozzle. Nozzle geometry such as diameter and thickness directly effects drop volume, velocity, and trajectory angle. Variations in the manufacturing process of a nozzle plate can significantly reduce the resulting print quality. Image banding is a common result from an out-of-specification nozzle plate. Various nozzle geometries designed for the ink-jet printheads are summarized in Fig. 22. The two most widely used methods for making the orifice plates are electroformed nickel (Fig. 23) and laser ablation on the polyimide (Fig. 24). Other known methods for making ink-jet nozzles are electro-discharged machining (Fig. 25), micropunching, and micropressing.
Because smaller ink drop volume is required to achieve higher resolution printing, the nozzle diameter of printheads has become increasingly small. For jetting an ink droplet of 14 pl, Epson Color Stylus 800 printhead has nozzle diameter of less than 30 µm. For 10 pL droplets, Hewlett-Packard DeskJet 890C color printhead has nozzle diameter of around 20 µm. With the trends towards smaller diameters and lower cost, the laser ablation method has become popular for making ink-jet nozzles.
Figure 22. Various types of ink-jet printhead nozzle designs
Figure 23. A SEM photograph from the entrance side of an electroformed Ni nozzle.
Figure 24. SEM photographs from the entrance side of a laser ablation polyimide nozzle plate.
Figure 25. A SEM photograph of an EDM stainless steel nozzle.
©COPYRIGHT -- The Society for Imaging Science and Technology
To report problems on this website, please contact webmaster@imaging.org
To report a website error, please contact webmaster@imaging.org
For all other questions e-mail: info@imaging.org
IS&T, 7003 Kilworth Lane, Springfield, VA 22151
Phone: 703-642-9090; Fax: 703-642-9094;
©
The Society for Imaging Science and Technology
