Incremental printing of symbolic information – Ink jet – Fluid or fluid source handling means
Reexamination Certificate
2002-06-20
2003-08-19
Barlow, John (Department: 2853)
Incremental printing of symbolic information
Ink jet
Fluid or fluid source handling means
C106S031950
Reexamination Certificate
active
06607268
ABSTRACT:
FIELD OF INVENTION
The present invention relates to ink compositions suitable for thermal inkjet printing, and, more particularly, to ink compositions that are film forming and provide improved drop-velocity stability and prolonging resistor life in inkjet pens.
BACKGROUND OF INVENTION
The use of inkjet printing systems has grown dramatically in recent years. This growth may be attributed to substantial improvements in print resolution and overall print quality coupled with appreciable reduction in cost. Today's inkjet printers offer acceptable print quality for many commercial, business, and household applications at costs fully an order of magnitude lower than comparable products available just a few years ago. Notwithstanding their recent success, intensive research and development efforts continue toward improving inkjet print quality, while further lowering cost to the consumer.
An inkjet image is formed when a precise pattern of dots is ejected from a drop-generating device known as a “printhead” onto a printing medium. The typical inkjet printhead has an array of precisely formed nozzles located on a nozzle plate and attached to an inkjet printhead substrate. The substrate incorporates an array of firing chambers that receive liquid ink (colorants dissolved or dispersed in a solvent) through fluid communication with one or more ink reservoirs. Each chamber has a thin-film resistor, known as a “firing resistor,” located opposite the nozzle so ink can collect between the firing resistor and the nozzle. In particular, each resistor element, which is typically a pad of a resistive material, measures about 35 &mgr;m×35 &mgr;m. The printhead is held and protected by an outer packaging referred to as a print cartridge, i.e., inkjet pen.
Upon energizing of a particular resistor element, a droplet of ink is expelled through the nozzle toward the print medium, whether paper, transparent film or the like. The firing of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the resistor elements, thereby forming alphanumeric and other characters on the print medium.
The small length scale of the nozzles, typically 10 to 40 &mgr;m in diameter, require that the ink not clog the nozzles. Further, repeated firings of the resistor elements that must withstand many millions of firings over the life of the ink cartridge to be commercially practical, can result in fouling of the resistor elements and degrading pen performance. This build up of residue on the resistor elements is unique to thermal inkjet printers and is known as kogation and defined as the build-up of residue (koga) on the resistor surface.
Besides the problem of kogation, firing resistor surfaces are susceptible to passivation layer damage by cavitation, contamination and many other sources. Such passivation layer damage literally results in microscopic holes on the resistor surface which significantly shorten resistor life. Energizing of the firing resistor after hundreds of millions or even tens of billions of times can erode away the top passivation layer, which is typically tantalum. This erosion may be from a combination of oxidation, chemical attack by the ink at high temperatures, and cavitation.
Erosion of the top passivation layer can lead to the failure of the underlying electrically insulating layers, causing the circuit which provides power to the resistor to short out. If the electrically insulating layers are not compromised, erosion can degrade drop velocity stability by adversely affecting the heat conduction properties of the resistor.
Minimizing drop-velocity variations between nozzles and within nozzles is critical for accurate drop placement on paper. Drop placement errors degrade both text and image quality. The magnitudes of the placement errors caused by velocity variations are dependent on pen-to-paper spacing and pen scanning speed relative to the paper. Therefore, as thermal inkjet printers become faster and print on a greater variety of media, greater pen-to-paper distances will be needed and it will become more important to decrease drop velocity variations. Furthermore, drop placement errors are more noticeable with small drop-volume pens; the smaller drops cannot mask the errors.
Drop velocity variations are thought to be due to a combination of erratic drive bubble nucleation and variations in energies delivered to each resistor. The former may be more important for velocity variations within a given nozzle. The latter may be more important for velocity variations between nozzles and can be due to different resistances through the electrical traces between the power supply and each resistor. These parasitic resistances result in slightly different amounts of power being delivered to each resistor. Erratic drive-bubble nucleation can be due to surface roughness or pits on the resistor surface that provide low energy nucleation sites. Koga, a carbonaceous film formed from thermal decomposition of organic components in the ink, can especially contribute to surface roughness. Also, erratic bubble nucleation may be caused by sharp temperature gradients on the resistor surface that may cause nucleation to occur first over the center hot spot of the surface of resistor as opposed to a uniform nucleation over a greater fraction of the resistor surface area. The problem of sharp temperature gradients is worse in small drop volume pens. In addition, sharp temperature gradients can lead to local high temperatures on the resistor. Higher resistor temperatures worsen kogation build up. This rough carbonaceous deposit provides many nucleation sites, leading to early, erratic vapor-drive bubble formation, low drop velocity and drop weights.
Customer and profit demands require smaller drop volumes, color-laser-like ink permanence, and “permanent” print heads. Smaller drop volumes give better spatial and chroma resolutions. However, passivation layer damage appears to be worse in smaller drop volume pens. In small drop volume pens each resistor must fire a greater number of times to transfer the same amount of ink to the page. The greater number of firings required of the resistor results in more passivation layer damage.
Reducing passivation layer damage by increasing the passivation layer thickness is typically not practical in high throughput printers. Resistors with thicker passivation layers require more energy to eject an ink drop. However, most of this excess energy is retained as heat within the passivation layer and is not effectively transferred to the ink. Therefore the power requirements are greater and more expensive printer components may be needed. Furthermore, this retained heat can build up in the thermal inkjet pens that would cause the pens to overheat. Printing speeds would need to be reduced or elaborate cooling schemes employed to avoid the overheating.
The trend is towards longer print-head life, using pens with replaceable ink supplies such as (but not limited to) off-axis ink reservoirs that are connected to the pens by hoses and ink reservoirs that snap onto the print head. Infrequent need for replacement of the print heads with prolonged resistor life reduces the cost and servicing required of the customer. High-speed, high-throughput photocopier-like products that may be envisioned for the future will greatly increase ink usage and will most likely greatly push further the demands on print-head life. With higher pen-to-paper relative speeds, high-throughput products will be more sensitive to passivation layer damage induced drop velocity variations.
Even though some kogation and/or passivation layer damage control methods in inkjet ink pens are known, all of them are either limited in their effectiveness, are not economically feasible or have undesirable side effects for pens needing long resistor life. Thus, there is even more of a need to find a way to effectively deal with the problem of passivation layer damage on inkjet resistors.
Currently, tantalum is typically used as the material in the top coa
Bruinsma Paul J.
Lassar Noah C.
Barlow John
Brooke Michael S
Haymond W. Bradley
Hewlett--Packard Development Company, L.P.
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