Ink Jet Printer With Ink System Filter Means

Abstract

An ink jet printing apparatus in which a pressure generated ink stream is produced from an opening through a piezoelectric transducer which produces axial vibration when energized to produce a stream of uniform ink drops. A reference electrode is provided in electrical contact with the ink and a character designating signal is coupled between a charge electrode placed adjacent the drop break-off point and the reference electrode to selectively charge individual ink drops proportional to the character designating signals. Filter means are provided for separating the chemically active part of the ink system from the remaining part of the ink system. The effects of electrolysis and corrosion within the ink system are eliminated.

Description

BACKGROUND OF INVENTION

This invention relates to an electrically operated character printer and more particularly to an improved printer of the type which utilizes selectively charged ink drops to print characters on a suitable record member.

In the prior art is has been known to utilize selectively charged ink drops to print characters; however, these printers have not been suitable for usage as an output printer for a data processing system, since they require maintenance at relatively short intervals. The usage as a computer output printer requires great reliability of operation over extended periods of time.

One factor which contributes to the reliability problem is a tendancy for the ink nozzles to clog, thereby causing a shutdown of the system. This is due in part to electrolysis of metal parts in the system as a result of electrical potential being applied across the ink and the metal parts. Another factor which contributes to the problem has been found to be variation in characteristics between jets and changes in these characteristics with time.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide an improved ink jet printer in which the effects of electrolysis are eliminated.

It is another object of this invention to provide an ink jet printer in which manufacturing costs and maintenance time are minimized so that the resulting printer is capable of operating for extended periods as a computer output printer.

It is another object of the invention to provide an ink jet print head of simplified construction so that the transducer works directly on the ink stream.

It is a further object of this invention to provide an ink jet printer in which a plurality of ink jets are produced from a single electromechanical member.

Briefly, according to the invention there is provided improved ink jet printing apparatus in which a pressure generated ink stream is passed through an opening in a transducer means having piezoelectric characteristics suitable energization of which causes the ink stream to break up into uniform drops which are individually charged proportional to a character designating input signals placed between a reference electrode in contact with the ink stream and a charging electrode placed adjacent the ink stream at the point at which drop break-off occurs so that action of a constant electric field through which the drops pass causes the ink drops to be deflected proportional to the charge placed on the drops to print characters designated by the input signals on a record member.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagrammatic schematic view of an ink jet line printer embodying the present invention;

FIG. 2 shows a section view of a single ink jet print head of an alternate embodiment of the invention.

FIG. 3 shows a specific embodiment of a multi-jet line printer embodying the present invention;

FIG. 4 shows a schematic block diagram of the electronic control circuitry for the multi-jet line printer of FIG. 3;

FIG. 5 is a voltage waveform on a time scale for the voltage applied to the charge electrode to print the last scan for characters 81 and 83 as shown in FIG. 3;

FIG. 6 is a voltage waveform on a time scale for the voltage applied to the charge electrode to print the last scan for characters 85 and 87 as shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 there is shown an ink drop printing apparatus in accordance with the invention. This apparatus has a transducer means 10 having a plurality of openings 11 which form spaced ink jet printing means. Transducer means 10 comprises a transducer member having piezoelectric characteristics and a nozzle means 12 is fastened to one side of the transducer member so that a nozzle is aligned with one end of each of the openings 11 in transducer means 10. An ink supply means 14 is provided for supplying ink under pressure to each of openings 11 so that a stream of ink 16 is produced from each of nozzle means 12. Transducer means 10 is energized by a suitable source of oscillations 24 and the application of the signal from source 24 causes transducer means 10 to expand and contract axially, thereby vibrating nozzle means 12. As a result of this vibration, each ink stream 16 breaks up into uniform drops. The spacing of the ink drops in the drawings is greatly exaggerated for illustrative purposes. However, the actual spacing is normally a few thousandths of an inch so that high printing rates can be obtained. A charging electrode means 20 is mounted at a position adjacent the point at which ink stream 16 breaks up into drops and the charging electrode is energized by a signal from character input information source 22. The character input signal is connected between a reference electrode means 18 mounted in contact with the ink stream and charging electrode means 20. As a result of the character input signal the ink drops receive a charge dependent on the amplitude of the signal applied to the charge electrode 20. After passing through charge electrode 20, the charged ink drops pass into deflection means 26 which comprises a static electric field. As a result, the charged drops required to print the characters are deflected from the stream path and reach the surface of print medium 32 to form the desired character. The drops which are not required to produce the desired character are directed to sump means 28 by the signals applied to the ink drops while passing charging electrode 20.

Although only six character positions are shown in FIG. 1, it is usual in a computer output printer to have a much longer line comprising up to 132 characters, for example. Each of the ink jets shown in the drawing is positioned to print two adjacent character positions by deflecting the ink drops horizontally along the print medium by means of suitable voltages applied to the charge electrode. The vertical dimension of the character is provided by the vertical motion of the print medium. Alternatively, printing can be accomplished by electrically deflecting the ink jet to produce the vertical dimension while the print medium moves to produce the horizontal dimension of the characters. Sump means 28 comprises an opening for each pair of print positions so that unneeded drops enter the sump opening from both of the adjacent ink jets. Deflection means 26 comprises spaced conductive plates with one plate adjacent each side of the ink stream. In this manner, each deflection plate, except the end one, is shared between two adjacent ink jet nozzles. To accomplish proper deflection in this manner, the polarity of each of the deflection plates is alternated. This in turn means that the polarity of the voltage to the charge electrodes must be alternated along the print line.

The embodiment of the invention shown in FIG. 2 comprises a manifold having an opening which connects to each of the ink jets in the system. An electromechanical transducer 10 such an a piezoelectric ceramic is mounted in alignment with the opening from the manifold 36. An individual nozzle member 38 is fixed to the other end of the opening in the piezoelectric material and an ink pressure means 40 is provided to supply ink to the manifold under pressure so that a stream of ink 41 is produced from nozzle 38. Each of the faces of the piezoelectric material has an electrode 43 formed thereon and a source of oscillations 24 is connected to these electrodes 43 to produce a mechanical vibration in the piezoelectric material so that a vibration is produced in the axial direction of the ink jet opening to break up ink stream 41 into a series of uniformly spaced uniform ink drops 45. The charge electrode is positioned adjacent the point at which ink drops 45 break off from the ink stream 41. A voltage is applied to the charge electrode which is derived from a character generator 50 which is controlled by signals on input signal line 42. A reference electrode means such as grounding electrode 44 is positioned adjacent the ink path through manifold 36. Filter means 34 is positioned between electrode 44 and the ink path through manifold 36 so that any particles resulting from electrolysis action on electrode 44 are prevented from reaching the ink stream by filter means 34. One terminal of the character generator 50 is maintained at the reference potential such as ground and the other terminal is electrically connected to charge electrode 20. Thus, the electrical path for charging ink drops 45 is established from reference electrode 44 through filter means 34 through the ink stream 41 and to the charge electrode 20. As each ink drop is being formed, the charge electrode 20 in the vicinity of the jet stream 41 is used to induce a charge on that drop. For example, if the charge electrode 20 is driven to a positive voltage, electrons are attracted to the end of the jet stream so that the tip of the jet becomes negative. When breakoff of the drop occurs, this negative charge is trapped so that the drop is subject to the action of a subsequent electrostatic field. The charged ink drops are then directed through a static electric field produced by high-voltage supply 46 connected between deflection plates 48. In this manner, the drops forming the characters are deflected to the proper position on print medium 32 and the other drops are directed to ink sump 28. The ink from sump 28 is directed for filtering and recirculation to the ink supply system 40.

To produce reliable operation, the proper materials must be utilized in the apparatus. In addition to the electrolysis problem previously mentioned, there may be an additional chemical corrosion problem if some materials are exposed to the ink over a prolonged period of time. For this reason, the preferred material for the manifold 36 is a plastic material such as plexiglass, for example, since this material has sufficient mechanical strength yet is inert chemically when exposed to the ink over an extended period. In a specific embodiment which was successfully operated, the printer was constructed by using for transducer means 10 a piezoelectric ceramic polarized for axial vibration. A 0.040 hole was drilled through the piezoelectric material at the position for each ink jet. The opening was counter-bored to relieve each side of the hole and, in addition, to insure that the electrode surface 43 of the piezoelectric material did not extend into the hole area in which case they would be shorted by the conducting ink. A quartz nozzle was attached to one end of the hole using a suitable adhesive. It was found that a semielastic epoxy adhesive was suitable for this purpose, since this adhesive had suitable thermal expansion characteristics. One suitable material is sold under the trade name of Glasshesive by Adhesive Engineering, 1411 Industrial, San Carlos, Calif. The reference material used to attach the piezoelectric material to the manifold which was constructed from plexiglass was an epoxy adhesive sold under the trade name of Epoxylite by Epoxylite Corp., 1428 North Tyler Ave., South El Monte, Calif. The reference electrode member 44 was a stainless steel plug threaded into an opening in manifold 36 which connects with the ink stream. The filter member 34 comprises a nonconductive porous disk 52 and a nonconductive support screen 54. The filter member may suitably comprise filters sold under the trade name of Millipore Filter by Millipore Corp., Ashby Road, Bedford, Mass. The filter comprises mixtures of cellulose composed of inert esters of cellulose and the pre-filter material is a pure glass fiber with an acrylic binder. Thus, it can be seen that this construction permits an electrical circuit from the reference electrode to the charge electrode through the ink stream which is only a fraction of an inch. However, the filter prevents any particles resulting from electrolysis from entering the ink stream, thereby resulting in reliable operation.

The embodiment of the printer shown in FIG. 3 comprises a multi-jet printer in which a plurality of print heads 60 are provided with each print head operable to print two adjacent print positions. Each print head 60 is formed with an opening through a suitable piezo-ceramic material 58 polarized for axial vibration. Individual nozzles 62 are fastened to one end of the opening through the piezoceramic material. Alternatively, the nozzles may be formed by a suitably shaped orifice in a common plate as shown in FIG. 1 or by an orifice jewel for each print head. Ink under pressure is provided from ink reservoir 64 by means of ink pump 66, filter 68 and pressure regulator 70 to common manifold 72. An opening 74 is provided from manifold 72 to transmit ink under pressure to each print head 60 so that an ink stream 76 is formed. A source of oscillation such as 100KHz, for example, is provided to terminals 78, 80 of the piezo-ceramic 58 so that a continuous stream of ink drops 82 is formed due to the vibration of nozzles 62 by the piezo-ceramic transducer 58.

The individual ink drops are charged by the signal voltage applied between charge electrodes 84 and ground electrode 65. In the embodiment shown, the charge electrodes are formed from a continuous insulator material which has an opening surrounded by conductive material which leads to terminals 86. The charging voltage is generated in response to digital input signals from an associated data processing system, for example. An embodiment of the necessary components to derive the charging voltage is shown in FIG. 4. In cases where a data processing system is used to control the printer, the clock 93 and storage 94 are included in the processor system. The control means 95 may comprise either wired circuits or the functions may be performed by programmed instructions in the procesor. Control means 95 provides the electrical signals to control the operation of the remaining electrical circuits. The data from storage 94 includes the code for each of the characters to be printed in a line in the character set being used by the printer. This coded data is directed to character generator 96 which serves as an appropriate decoding device. One suitable character generator is a Read-only storage device which produces signals corresponding to dot positions within the chosen print matrix. The matrix coded data is transferred into line print buffer 97 which has one byte of storage for each print position for each scan. In the embodiment shown, seven bytes of storage are provided for each character to be printed. The timing for scans of the print matrix is derived from emitter 98 which is mounted on the same shaft as the tractor which drives print sheet 90 by power supplied from motor 92. The signals from emitter 98 are utilized in control means 95 to generate control signals which are coupled to buffer accessing control means 101. Buffer accessing control means may comprise a shift register which has a number of stages equal to the vertical matrix positions. A signal is provided to the shift register from control means 95 at the time for each scan. The signal energizes the next stage of the shift register so that all the bytes of the data in buffer 97 for that scan are transferred in parallel to a digital to analog converter 99, one of which is provided for each print position. An analog output circuit 100 is utilized to hold the voltage on terminal 86 of the charge electrodes 84 for a predetermined time as shown in FIGS. 5 and 6.

The matrix illustrated in FIG. 3 is a 5 by 7 matrix. This means that a character is formed by a selection of one or more of 5 ink drops for each horizontal scan of the print sheet 90 and a horizontal scan is made at each of 7 vertical positions as the paper 90 is driven vertically in the direction of the arrows by means of paper drive motor 92. The charge electrode voltage for scan of the print positions 81 and 83 as shown in FIG. 3 is shown in FIG. 5. This voltage applied to charge electrode 84, produces the seventh scan of the two print positions. The upper level V is the charge necessary to direct ink drops to ink sump 56 while succeeding steps of voltage direct ink drops to adjacent positions horizontally along the print axis to print the last scan of character 81. The voltage is returned to the sump level to cover the position between characters and the succeeding levels of voltage direct drops to complete the last scan of character 83.

The charged drops go through a static electric field set up between deflection plates 71, 73 by a suitable high voltage supply from high voltage source 79. The positive terminal of source 79 is connected to support means 75 and to each of deflection plates 71 due to their rigid mounting upon support means 75. The negative terminal of source 79 is connected to support means 77 and to each of deflection plates 73 due to their rigid mounting upon support means 77. This arrangement means that alternately polarized voltages must be provided to adjacent print heads. For example, adjacent print heads in FIG. 3 print the same data, but the voltage to the charge electrode is different as shown in FIGS. 5 and 6. This difference can be easily accommodated on a one-time basis when setting up the character patterns in the Read-only Storage device. There is an advantage from a mechanical standpoint, however, since this arrangement permits one sump means 56 to service four print positions. This construction requires less vacuum from vacuum source 88 to insure that ink does not collect in the sump, but is returned to the in reservoir for filtering for recirculation in the system. The deflection plates are mounted so that electrical shorts are prevented in the event that ink accumulates around these plates in operation. In addition, the electrolysis problem is solved since there is no metal in the ink path between filters 67 and 68.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in the form and details may be made therein without departing from the spirit and scope of the invention. For example, a multi-jet array can be constructed of individual nozzles similar to those shown in FIG. 2. This has the advantage that individual nozzles can be replaced and in this case the mounting must provide for alignment of individual jets. One suitable construction is to provide a spherical surface on the manifold end of the nozzle so that the newly installed nozzle can be accurately adjusted and then fixed into position with suitable mounting screws. In addition, the nozzle structure shown in FIG. 3 may be attached with screws if desirable so that the nozzles can be removed for ease in cleaning the nozzles and the manifold, if necessary.

Claims

What is claimed is:

1. In an ink drop character printer of the type wherein individual ink drops are selectively charged according to character input information signals the improvement comprising:

an ink system having an ink reservoir composed solely of material that is chemically inert when exposed to ink;

transducer means having piezoelectric characteristics having an opening through said transducer means, said transducer means forming at least part of said reservoir;

means in said ink system for supplying ink under pressure to said reservoir to produce a stream of ink;

means for energizing said transducer means to produce a succession of uniform ink drops from said stream of ink;

filter means in said ink system for separating that part of said ink system which is chemically active to ink from the remaining part of said ink system;

reference electrode means and means for mounting said reference electrode for electrically contacting said ink under pressure;

charging electrode means mounted closely adjacent to said stream of ink drops; and

means for coupling character information signals between said reference electrode means and said charging electrode means so that said ink drops are selectively charged proportional to said information signals.

2. The apparatus according to claim 1 wherein said transducer means comprises a single element having piezoelectric characteristics which produces a plurality of ink jets.

3. The apparatus according to claim 1 additionally comprising means for deflecting said ink drops proportional to the charge on said drops.

4. The apparatus according to claim 3 wherein said deflecting means comprises a plurality of deflecting electrodes mounted between alternate print positions and means to energize said deflecting electrodes with alternate fixed potentials.

5. The apparatus according to claim 3 additionally comprising means for intercepting ink drops charged with a predetermined charge, the remainder of said ink drops being directed to a record member to produce the selected characters.

6. The apparatus according to claim 5 wherein said intercepting means comprises one sump member between alternate ink jets.

7. In an ink drop character printer of the type wherein individual ink drops are selectively charged according to character input information signals the improvement comprising:

transducer means having piezoelectric characterisitics having an opening through said transducer, said transducer means directly contacting said ink and forming an ink reservoir;

nozzle means coupled at one end of said opening through said transducer means;

an ink supply means;

electrically isolated means connected to said ink supply means for delivering ink under pressure to the other end of said opening through said transducer means to produce a stream of ink from said nozzle means;

means for energizing said transducer means to produce a succession of uniform ink drops from said stream of ink;

a first filter means having two spaced surfaces with one surface contacting said ink supplied to said other end of the opening through said transducer means;

reference electrode means and means for mounting said reference electrode for contacting the other surface of said first filter means;

a second filter means positioned between said ink supply means and said reservoir; whereby said first and second filter means remove the effects of electrolysis and corrosion from said ink;

charging electrode means mounted closely adjacent to said stream of ink drops; and

means for coupling character input information signals between said reference electrode means and said charging electrode means so that said ink drops are selectively charged proportional to said information signals.

8. The apparatus according to claim 7 additionally comprising means for deflecting said ink drops proportional to the charge on said drops.

9. The apparatus according to claim 8 wherein said deflecting means comprises a plurality of deflecting electrodes mounted between alternate print positions and means to energize said deflecting electrodes with alternate fixed potentials.

10. The apparatus according to claim 8 additionally comprising means for intercepting ink drops charged with a predetermined charge, the remainder of said ink drops being directed to a record member to produce the selected characters.

11. The apparatus according to claim 10 wherein said intercepting means comprises one sump member between alternate ink jets.