CA1191391A

CA1191391A - Drop-on-demand method and apparatus using converging nozzles and high viscosity fluids

Info

Publication number: CA1191391A
Application number: CA000399064A
Authority: CA
Inventors: Francis C. Lee; Ross N. Mills; Frank E. Talke
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1981-06-18
Filing date: 1982-03-23
Publication date: 1985-08-06
Also published as: JPS57208262A; EP0067948A1; DE3265382D1; EP0067948B1

Abstract

DROP-ON-DEMAND METHOD AND APPARATUS
USING CONVERGING NOZZLES AND HIGH VISCOSITY FLUIDS

Abstract A drop-on-demand ink jet printing method and apparatus in which the print head has an ink cavity which is filled with ink, and which has a nozzle designed so that ink does not flow out under static conditions. An electromechnical transducer is selectively energized in response to print data signals so that, when energized by an electrical signal, the transducer produces a pressure wave in the ink cavity sufficient to eject one ink drop from the nozzle for each signal above a threshold value. The nozzle is a strongly convergent nozzle and the ink has a viscosity up to 100 centi-poise. In the preferred embodiment, the nozzle is formed by anisotropic etching in a silicon substrate.
An array of print heads produces a line of high-resolution printing as the print head array is moved across a print medium.

Description

SA~-`051 ~ 3~

DROP-ON-DEMAND METHOD AND ~PP~RATUS
USING CONVERGING NOZZLES AN~ HIGH VISCOSITY FI.UIDS

Description Field of Invention This invention relates to an ink jet print head and, more particularly, to an ink jet print head and method for generating ink drops on demand under control of a suitable electrical signal.

Description of Prior Art Ink jet printing has been known in the prior art, including systems which use a pressure genarated con-tinuous stream of ink, which is broken into individual drops by a continuously energized transducer. The individual drops are selectively charged and deflected either to the print medium for printing or to a sump where the drops are collected and recirculated.
Examples o~ these pressurized systems include U. S.
Patents 3,596,275 to Sweet, and 3,373,437 to Sweet et al. There have also been known in the prior art ink jet printing systems in which a transducer is used to generate ink drops on demand. One example of such a system is commonly assigned U. S. Patent 3,787,884 to Demer. In this system, the ink is supplied to a cavity by gravity flow and a transducer mounted in the back of the cavity produces motion when energized by an appro-priate voltage pulse, which results in the generation of an ink drop so that only those ink drops required for printing are generated. A different embodiment of a drop-on-demand printing system in which the transducer is radially arranged is shown in U. S.
Patent 3,683,212 to Zoltan.

The prior art drop~on-demand printlng systems have been limited by low drop procluction rates, low resolution, and low efficiency. Typical prior art drop-on-demand printing systems have utilized a constant cross-section nozzle and ink having a viscosity during operation lower than 10 centipoise. Attempts to increase the drop production rates have led to stream instability as a result oE the low viscosity ink used. Attempts to increase the ink viscosity to improve stream stability have led to choking of the nozzles and termination of ink flow due to the increased internal friction in the nozzle. A decrease in the length of the nozzle in an effort to decrease the friction resulted in unreliable nozzle operation due to air intake caused by meniscus dyn2mics.
SU~IARY OF THE INVENTION
Briefly, accordinq to one a.s~ect of the invention, there is provided a drop-on-demand ink jet printing method and a~paratus comprisiny a print head having a fluid chamber supplied with a suitable high viscosity marking fluid. An orifice comprising a strongly converging nozzle is in fluid communication with the fluid chamber, and an electromechanical transducer is mounted in mechanical communication with the fluid chamber. The transducer is selectively energized with a series of signals so that one drop of the marking fluid is ejected from the orifice for each of the signals having at least a predetermined amplitude.

Brief Description of ~he Drawings FIG. 1 is a schematic view shcwins a conversing nozzle;

FIG. 2 is a droo-on~demand ink jet printer embodying a converging nozzle;

3~3~
SA ~051 FIG. 3 is a section view taken along line 3-3 of FIG. 2 of the drop-on-demand ink jet print head.

FIG. 4 is a vlew, partially in section, of an alternate embodiment of a drop-on-demand ink jet print head;

FIG. 5 is a right side view of an array of drop-on-demand ink jet print heads;

FIG. 6 is a section view taken along lines 6-6 in FIG.
5.

Description of the Preferred Embodiments Referring to FIG. 2, the printer apparatus comprises a print head 10 to which is supplied high viscosity li~uid ink from ink supply means 12. The viscosity requirement is a function of nozzle size and maximum drop-on-demand drop production rate. The viscosity for inks for high resolution printing extends up to 100 centipoise, and the viscosity can be substantially higher for applications in which lower resolution is suitable. Control means 14 provides the voltage con-trol pulses to selectively energize print head 10 to produce one ink drop for each voltage pulse supplied to print head 10. Print head 10 comprises head body 20 having a chamber or cavity 22 formed therein. Cavity 22 is maintained filled with ink through supply line 24 from ink supply means 12. Ink from supply means 12 is not pressurized so the ink in cavity 22 is maintained at or near atmospheric pressure under static condi-tions. An exit from cavity 22 is provided by nozzle portion 26 which is designed so that the ink does not flow out of nozzle portion 26 under static conditions.
An intermediate ink reservoir 28 is formed in head body 20 and is separated from cavity 22 by internal wall portion 30. The top of cavity 22, as shown in FIG. 2, 3C~
~0051 is closed by a suitable transducer means which is fixed to the head body. Internal wall portion 30 is designed so that a narrow passageway 32 is provided for the transfer of liquid ink from intermediate ink reservoir 28 to ink cavity 22. The transducer means comprises a membrane member 34 which is fastened to an electro-mechanical transducer 36. Transducer 36 displaces radially when energized with a suitable voltage pulse and bends membrane 34 inwardly (as shown dotted in FIG.
3), and produces a pressure wave in cavity 22 s~ that liquid ink is e~pelled out through nozzle portion 26 to form a single drop. Control means 14 provides the voltage control pulses to selectively energize trans-ducer 36 to produce one ink drop for each voltage pulse applied to transducer 36.

According to the invention, nozzle portion 26 of the drop-on-demand ink jet printing apparatus comprises a converging nozzle. As shown in FIG. l, the nozzle has an entrance dimension dl, which is larger than the exit dimension d2. The nozzle shown in the drawing has a substantially linear taper in the dimension of the noæzle along its physical length e, however, other tapers s~lch as a horn configuration would also be suitable.
The flow through the nozzle is in the direction from the larger opening to the smaller opening, as shown by the arrow~

From a fluid mechanics viewpoint, the effective viscous length Rd of a converging nozzle can be calculated as R =l/3[(d /d )3 l]d R/(d d ) where dl, d2 are the dlmensions at the entrance and exi-t or the converging section, respectively, and e is the physical length of the nozzle (see FIG. 1). Thus, 53~
S~ ~051 it can be seen tha-t the conver~ing nozzle is ph~sically "long" but hydraulically "short"~ Since the converging nozzles are "short", the converging nozzles do not provide reliable drop-on-demand operation when using prior art ink formu]ations having moderate viscosities up to about 16 centipoise due to drop formation instabi-lity. ~owever, it was found that highly reliable drop-on-demand operation can be produced with converging nozzles when using marking fluids having a substan-tially higher viscosity than typical prior art systems.Although the prior art systems using constant cross-section nozzles would not even work in the drop-on-demand mode when utilizing marking ~luids of the sub-stantially higher viscosity (up to 100 centipoise for high resolution printing, for example), the combination of the converging nozzle and the high viscosity marking fluids produced not only highly reliable drop-on-demand operation, but also much higher drop-on-demand drop production rates than those obtainable by prior art drop-on-demand ink jet printers.

The operation was superior in other ways as well. For example, air ingestion into the nozzle is completely inhibited and the stream stability is improved so that a stream of drops of equal size and spacing can be produced. The stream directionality is improved, and the jet velocity is easily increased which is essential for high speed printing. The nozzle can be operated at any frequency in the frequency spectrum up to 120 kHz without jet failure, and the nozzle can be operated up to 80 k~z drop-on-demand drop production rate in high resolution printing operation.

The converging nozzle can be produced by any suitable technique. The preferred technique for producing a converging nozzle is by anisotropically etching the Sl 0051 ~ q~

nozzle in a silicon substrate. This technique will be described with reference to the embodiment of the drop-on-demand print head shown in FIG. 4. The print head comprises cylindrical transducer member 60 closed at one end ~y a nozzle plate 62, having formed therein nozzle portion 64. The other end of the transducer is fixed to body member 66. ~hen transducer 60 is actu-ated by a suitable voltage drive pulse, transducer 60 is deflected to the position shown dotted in FIG. 4 to cause a single drop of ink 78 to be expelled out through nozzle portion 64.

Nozzle plate 62 comprises a silicon substrate formed of single crystal material oriented with the (100) planes parallel to the front surface. The front surface 68 and the rear surface 70 of the nozzle plate are coated with etchant masking material. An aperture is made in the masking material on the rear surface of the no~zle plate. The nozzle plate is then sub]ected to a suit-able anisotropic etching solution such as a water, amine, pyrocatechol etchant, for example. It has been known for some time that the (111) plane is a slow etch plane in single crys~ai silicon. The nozzle is etched in the form of a truncated pyramid type opening with a square entrance aperture, tapered sides, and a smaller s~uare exit aperture. The tapered sides form an angle of 54.7 to the front surface since the etching is along the crystal planes of the silicon substrate. The etching is continued until an exit aperture of the desired size is formed.

In a particular embodiment, the silicon nozzle plate was five mils thick and the nozzle plate was etched to produce a two mil square exit aperture. In an embodi-ment similar to that shown in FIG. 4, the print head, including the above-described nozzle plate, produced reliable drop-on-demand operation up to a drop produc-SAC 051 ~ 3~

tion xate of 60 kHz at a resolution of 240 pels/inch.This resolution is considered high resolution printing since it produces print resolution approachiny that of engraved type. However, the print quality began to decline at drop production rates over 40 kHz. In this apparatus, inks havinc3 a viscosity with a range from about 15 centipoise up to 100 centipoise worked to produce inX drops in a drop-on-demand mode, and the preferred range of viscosity was from 20 to 40 centi-poise.

In a second embodiment similar to that shown in FIG. 4,a 1.2 mil square nozzle was used and this apparatus produced printing at a drop-on-demand production rate of 80 kHz at a resolution of 450 pels/inch. This apparatus worked to produce ink drops in the drop-on-demand mode with inks having a viscosity from about 10 centipoise up to about 70 centipoise. The preferred range of viscosity was from about 20 to 40 centipoise.

FIGS. 5 and 6 show a print head array 40 comprising forty print heads 42 arranged in four rows 44 with corresponding orifices 46 offset so that a line of printing can be produced at a resolution approaching engraved type as the print head moves across a print sheet. Each of the print heads 42 comprises a hollow cylindrical piezoelectric transducer 48 which forms an ink chamber 50 to which ink is supplied from common reservoir 52. A housing 54 is provided which includes a tapered channel 56 for each print head which trans~
mits ink from ink chamber 50 to the corresponding orifice 46 in nozzle plate 58. The orifices are strongly convergent nozzles, as shown in FIG. 6. In the preferred embodiment nozzle plate 58 comprises a single crystal silicon substrate and orifices are formed by anisotropic etching as described above to form square orifices in nozzle plate 58, as shown in FIG. 5-SA~ `051 ~ 3~

In a particular embodiment, a forty nozzle arraysimilar to that shown in FIGS. 5 and 6 was constructed with 2 mil square nozzles. This array can be operated to produce printing at a resolution of 240 pels/inch at a drop-on-demand drop production rate of up to 40 kHz. The array operated successfully with ink having a viscosity down to lS centipoise and up to 100 centi-poise. However, the optimum range for the viscosity was 20 to 40 centipoise.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various other changes in the form and details may be made therein without departing from the ~pirit and scope of the invention.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The method of operating a drop-on-demand ink jet head comprising:
providing an ink jet head having an ink cavity, an opening comprising a nozzle passage which converges strongly toward the exit orifice of the nozzle passage communicating with said ink cavity so that the effective viscous length of said nozzle passage is short with respect to the physical length of the nozzle passage, and an electromechanical transducer mounted in mechanical communication with said ink cavity;
filling said ink cavity with a marking fluid having any selected viscosity in the range of 15 to 100 centipoises at the normal operating temperature and selectively energizing said electromechanical transducer with a series of signals to eject one drop of said marking fluid from said opening only when the amplitude of the signal exceeds a predetermined threshold amplitude.

2. The method of claim 1 wherein said series of signals for selectively energizing said electromechanical transducer comprises signals repeatable at a rate up to 120 kHz.

3. The method of claim 2 wherein high resolution printing is produced and said signals are produced at a rate up to 80 kHz.

4. The method of claim 1 wherein said marking fluid viscosity is within the range of from approximately 20 to approximately 40 centipoise.

5. The method of claim 3 wherein said marking fluid viscosity is within a range of from approximately 20 to approximately 40 centipoise.

6. Drop-on-demand ink jet printing apparatus comprising a print head having a fluid chamber supplied with a marking fluid, an orifice in fluid communication with the fluid chamber, an electromechanical transducer mounted in mechanical communication with the fluid chamber, and a series of signals to selectively energize the transducer to eject one drop of the marking fluid from the orifice only when the amplitude of the signal exceeds a predetermined threshold amplitude, characterized in that said orifice comprises a nozzle passage which converges strongly toward the exit orifice of the nozzle so that the effective viscous length of said nozzle passage is short with respect to the physical length of said nozzle passage, said marking fluid has any selected viscosity in the range of 15 to 100 centipoises at the normal operating temperature, and said apparatus produces reliable drop-on-demand operation throughout the stated viscosity range.

7. The apparatus of claim 6 further characterized in that said series of signals for selectively energizing said electromechanical transducer comprises signals repeatable at a rate up to 120 kHz.

8. The apparatus of claim 7 further characterized in that the viscosity of said marking fluid is within the range of from approximately 20 to approximately 40 centipoise.

9. The apparatus of claim 8 further characterized in that high resolution printing is produced and said signals are produced at a rate up to 80 kHz.

10. The apparatus of claims 6 further characterized in that the viscosity of said marking fluid is within the range of from approximately 20 to approximately 40 centipoise.

11. The apparatus of claim 6 further characterized in that a plurality of print heads are arranged in an array comprising offset columns and rows so that a line of high resolution printing can be produced as the array is moved relative to a print medium.

12. The apparatus of claim 11 further characterized in that said signals are produced at a rate up to 40 kHz.

13. The apparatus of claim 12 further characterized in that the viscosity of said marking fluid is within the range of from approximately 20 to approximately 40 centipoise.

14. The apparatus of claim 6 wherein said nozzle passage has an included or apex angle of about 70 degrees.

15. The appratus of claim 14 further characterized in that said series of signals for selectively energizing said electromechanical transducer comprises signals repeatable at a rate up to 120 kHz.

16. The apparatus of claim 15 further characterized in that the viscosity of said marking fluid is within the range of from approximately 20 to approximately 40 centipoise.

17. The apparatus of claim 16 further characterized in that high resolution printing is produced and said signals are produced at a rate up to 80 kHz.

18. The apparatus of claim 14 further characterized in that the viscosity of said marking fluid is within the range of from approximately 20 to approximately 40 centipoise.

19. The apparatus of claim 14 further characterized in that a plurality of print heads are arranged in an array comprising offset columns and rows so that a line of high resolution printing can be produced as the array is moved relative to a print medium.

20. The apparatus of claim 19 further characterized in that said signals are produced at a rate up to 40 kHz.

21. The apparatus of claim 20 further characterized in that the viscosity of said marking fluid is within the range of from approximately 20 to approximately 40 centipoise.

22. The method of producing single marking liquid drops-on-demand, comprising ejecting the volume of marking liquid required to form a single drop from a nozzle as and when a drop is demanded, said nozzle comprising an exit orifice and a nozzle passage leading to the exit orifice, the improvement comprising that the marking liquid used may have a viscosity of any value in the range of 15 to 100 centipoises at the normal operating temperature and that the nozzle passage converges strongly towards the exit orifice of the nozzle, so that the effective viscous length of said nozzle passage is short with respect to the physical length of the nozzle passage.