Electrostatic ink-jet recording head having a head chip provided with concave and convex portions

Abstract: The electrostatic ink-jet recording head has a head member 7 having a chamber for supplying and discharging ink, a head chip 4 in which a plurality of continuous meniscus shapes are respectively formed on concave portions 16, a cover plate 3 so as to release a part of each of corners of the concave and convex portions 15 and 16, and a protruded member 20 on which pectinate protrusions 21 are formed at the same intervals as those of the meniscus shapes formed on the head chip 4. A recording electrode 13 is formed on each concave portion 16. Each protrusion of the protruded member 20 is set on each concave portion 16 of the head chip 4 and moreover, formed so as to be protruded beyond the corner of the convex portion 15.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an electrostatic ink-jet recording head, particularly to an electrostatic ink-jet recording head for performing recording by ejecting toner to a recording medium. Description of the Prior Art A non-impact recording method is superior to impact recording method because noises generated under recording are very small. Among the non-impact recording methods, an ink-jet recording method makes it possible to record characters on plain paper by a simple mechanism, and various methods have been proposed so far about the ink-jet recording method. Conventionally, as this type of the ink-jet recording method, there is a method for performing recording by using ink obtained by dispersing toner grains into a carrier liquid, applying a voltage between a pointed recording electrode and an electrode provided behind the recording paper so as to face the pointed recording electrode, and flying color materials contained in the ink by the electrostatic force of a generated electric field as disclosed in PCT Publication No. WO 93/11866. FIG. 9 is a perspective view of a conventional ink-jet recording head obtained by modifying the ink-jet recording device disclosed in WO 93/11866, FIG. 10A is an enlarged view of a portion nearby the ink-jet port (portion a) viewed from the top of the portion when ink is supplied from the recording head, and FIG. 10B is a sectional view taken along the line b-b in FIG. 10A. In FIGS. 9 and 10A, 10B, a substrate 101 is an insulator made of plastic or the like, which supports a base film 110 on its back. The base film 110 is an insulator made of polyamide or the like, which has a thickness of approx. 50µm and on whose surface a plurality of recording electrodes 102 are formed integrally with the surface. The recording electrodes 102 are obtained by pattern-plating a conductive material such as copper on the base film 110 up to a thickness of 20 to 30µm and arranged at a pitch of 300 dpi (dots per inch), that is, at an interval of approx. 85µm. Moreover, each of the recording electrodes 102 independently protrudes beyond an end of the base film 110 up to 80 to 500µm. Furthermore, the surface of each recording electrode 102 is uniformly covered with an insulating coating member 103 at a thickness of 10µm or less. The base film 110 uses a TAB (Tape Automated Bonding) tape used for the TAB art and moreover, the insulating coating member 103 is formed through chemical vapor deposition of Parylene. A cover 104 is set on the base film 110 so as not to cover the protrusions of the recording electrodes 102. The cover 104 is an insulating member on which an ink supply port 105 and a not-illustrated ink discharge port 106 are previously formed and a space formed with the base film 110 and the cover 104 constitutes an ink chamber which is filled with the ink supplied from the ink supply port 105. Moreover, the front end of the cover 104 opens and a slit-like aperture constituted with the base film 110 and the cover 104 forms an ink jet port 107 on which an ink meniscus 108 is formed. Ink forms the ink meniscus 108 on the ink jet port 107 according to its surface tension as shown in Figs.10A and 10B. Because a negative pressure is applied to the ink in the head and moreover, the recording electrodes 102 protrude beyond the base film 110 and cover 104, the ink meniscus 108 has a diagonally downward concave shape when viewed from its side. Moreover, because the recording electrodes 102 individually protrude to the outside of the ink jet port 107, the ink meniscus 108 is formed correspondingly to each recording electrode 102 when viewed from the topside, Therefore, when applying a high-voltage pulse to an optional recording electrode 102, an electric field is concentrated on the protruded ink-meniscus front end of the recording electrode 102. The electrified toner in the ink is led by the electric field and discharged from the protruded ink-meniscus front end and flies toward a not-illustrated electrode facing the ink-jet recording head, that is, in the direction of recording paper as toner agglomerations 109. Other ink-jet recording devices using an electrostatic force are disclosed in Japanese Patent Application Laid-Open No. 58-124662 issued on July 25, 1983 and Japanese Patent Application Laid-Open No. 56-167473 issued on December 23, 1981. In the case of the ink-jet recording device disclosed in Japanese Patent Application Laid-Open No. 58-124662, the discharge point of an ink discharge port is formed on the front end of a separation wall for separating an ink channel. The separation wall is formed along a recording electrode, and the discharging point is formed at the end of the recording electrode. In the case of the ink-jet recording device disclosed in Japanese Patent Application Laid-Open No. 56-167473, a division plate for dividing an ink channel is formed in the ink channel. The division plate as a plurality of recording electrodes on both surface. The front end of the division plate is formed so as to protrude beyond an ink discharge port. A first problem of these conventional electrostatic ink-jet recording heads lies in the fact that toner grains for forming a desired dot becomes insufficient. This is because a discharge point is formed by a recording electrode, an electrostatic force in the direction bound for the discharge point is not generated for toner grains nearby the discharge point when a recording voltage is applied and thereby, toner grains sufficient to form a desired dot cannot be concentrated on the discharge point. A second problem lies in the fact that ink droplets are unstably discharged. This is because convex ink meniscuses using a discharge point as an apex are continuously connected to each other, vibrations of the liquid surface nearby a discharge point discharging ink influence the ink meniscus of other discharge points, and thereby ink meniscuses cannot be always stably obtained. A third problem lies in the fact that ink droplets are irregularly discharged due to concentration of excessive toner grains. This problem is caused by the prior art shown in FIGS. 9 and 10A, 10B and Japanese Patent Application Laid-Open No. 56-167473. This is because a discharge aperture for supplying ink to a discharge point is formed on a part of an ink chamber as a slit for preventing ink from overflowing and thereby, the ink does not flow in the discharge aperture to cause the ink viscosity to increase due to concentration of excessive toner grains in the discharge aperture. The present invention is made to solve the above problems and its object is to provide an electrostatic ink-jet recording head for discharging ink droplets containing toner grains in ink in an electrostatic field, capable of preventing toner grains for forming a desired dot from becoming insufficient, stably discharging ink droplets, and moreover preventing ink droplets from being irregularly discharged due to concentration of excessive toner grains in a discharge aperture. SUMMARY OF THE INVENTION An electrostatic ink-jet recording head of the present invention is used for recording characters by applying an electric field to ink in which electrified toner grains are dispersed and discharging said toner grains by an electrostatic force generated by said electric field. The ink-jet recording head has a head member having a chamber for supplying ink; a head chip fixed to the head member and having convex portions and concave portions arranged alternately in a direction perpendicular to ink ejecting direction, a plurality of continuous meniscus shapes being respectively formed on the concave portions; a recording electrode formed on each of the concave portions; and protrusions which are provided with the head chip at the same intervals as those of the meniscus shapes formed on the concave portions. Each of the protrusions is located on each of the concave portions and protrudes beyond ends of the convex portions in a direction of the ink ejecting direction. According to the above present invention, discharge points are formed on the protrusions protruding beyond the ends of the convex portions. An equipotential line nearby the discharge point, while a recording voltage is applied to the recording electrode, is formed so as to be almost perpendicular to a discharge or ejecting direction, and an electrostatic force in the direction toward the discharge point is generated in the electrified toner grains nearby the discharge point. Therefore, even while the recording voltage is selectively applied to the recording electrode on each of the concave portions, toner grains are stably supplied onto the discharge point. Moreover, because an ink bank due to a convex meniscus is present before the recording electrode, it is possible to obtain toner grains for forming a desired dot. BRIEF DESCRIPTION OF THE DRAWINGS This above-mentioned and other objects, features and advantages of this invention will become more apparent by reference to the following detailed description of the invention taken in conjunction with the accompanying drawings, wherein: FIG. 1 is a schematic diagram of an embodiment of an electrostatic ink-jet recording head of the present invention; FIG. 2 is a sectional view taken along the line A-A of FIGs. 1; FIG. 3 is an enlarged front view of a head chip of the electrostatic ink-jet recording head of FIGS. 1 and 2; FIG. 4 is an enlarged view of a portion P of the electrostatic ink-jet recording head of the embodiment in FIG. 1; FIG. 5 is a sectional view taken along the line B-B of FIG. 4; FIG. 6 is a sectional view taken along the line C-C of FIG. 4; FIG. 7 is a schematic view of equipotential lines generated when the electrostatic ink-jet recording head of the embodiment in FIG. 4 records characters; FIG. 8 is a schematic view of equipotential lines generated when a conventional electrostatic ink-jet recording head records characters; FIG. 9 is a perspective view of a conventional electrostatic ink-jet print head; FIG. 10A is a top view of the front end of the conventional electrostatic ink-jet recording head in FIG. 9; and FIG. 10B is a sectional view taken along the line b-b in FIG. 10A. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic diagram of an embodiment of an electrostatic ink-jet recording head of the present invention, and FIG. 2 is a sectional view taken along the line A-A in FIG. 1. In FIGS. 1 and 2, a ink-jet recording head 1 has a head member 7, a cover plate 3, a head chip 4, a contact substrate 2 having electric contact lines (not shown). At the front of the recording head 1, a sheet of recording paper 6 is located spaced by a predetermined gap, and an opposite electrode 5 is located at an opposite side of the recording paper 6. An inner chamber 8 (FIG. 2) and an outer chamber 10 are independently formed in the head member 7. An ink supply port 9 communicating with a not-illustrated ink tank is formed on a part of the inner chamber 8 and an ink discharge port 11 communicating with a not-illustrated ink tank is formed on a part of the outer chamber 10. The head chip 4 is coupled to a front end of the head member 7. The head chip 4 has a shape of a quadrilateral solid made of an insulating material having a low permittivity such as ceramics or macromolecular material as a base material. The contact substrate 2 is fixed on the head member 7 and a front end of the contact substrate 2 reaches the head chip 4. The contact substrate 2 has a plurality of contact electrodes 2a as shown in FIG. 1 to connect them to a driver 100. The cover plate 3 is fixed on a forward portion of the head member 7 and on the head chip 4 at the side of the opposite electrode 5. An opening is formed at the cover plate 3 so as to permit a corner end of the head chip 4 to protrude from the opening in a direction of the ink ejecting direction. As shown in FIG. 3, the head chip 4 is provided with convex portions 15 and concave portions 16 at the side facing to the opposite electrode 5. A tip 15a of each of the convex portions 15 protrudes in the direction of the opposite electrode 5 of FIG. 2 (in a direction of ink ejecting direction). The convex portions 15 are arranged at the same interval as a desired dot pitch. In FIG. 3, each of recording electrodes 13-1 to 13-m ("m" is a integer) is formed on two surfaces of each concave portion 16 and side surfaces of two convex portions 15 facing with each other. But, the recording electrodes are not formed on front surfaces of the convex portions 15 facing to the opposite electrode 5. As shown in FIG. 5, each of the recording electrodes 13-1 to 13-m is covered with an insulating film 17. The recording electrodes 13-1 to 13-m are connected with the electric contact lines (not shown) formed in the contact substrate 2 of FIGS. 1 and 2. The contact substrate 2 is a TAB tape. As shown in FIG. 1, the contact substrate 2 has the contact electrodes 2a and they are connected to the recording electrodes 13-1 to 13-m (FIG. 3) through the electric contact lines (not shown). The contact electrodes 2a are connected to the external driver 100 to be supplied with ejection signals for ejecting ink. As shown in FIGS. 4 to 6, the convex portions 15 and the concave portions 16 are arranged alternately in a direction perpendicular to ink ejecting direction. A protruded member 20 is fixed on rear portions of the convex portions 15. The protruded member 20 covers the rear portions of the convex portions 15 but does not cover forward portions (end portions) of the convex portions 15. The protrude member 20 is made of an insulating material having a low permittivity such as ceramics or macromolecular material. Protrusions 21 are pectinately formed at one end of the protruded member 20 at the same interval as a desired dot pitch. The protrusions 21 and the convex portions 15 are alternately positioned as shown in FIG. 4. Each protrusion 21 is located at the topside of each concave portion 16 and does not contact each of the recording electrodes 13-1 to 13-m. The protrusions 21 protrude beyond the convex portions 15 to form discharging points 18. The front end of each protrusion 21 is set on one meniscus forming plane on the head chip 4 so as to protrude beyond the corner of the convex portion 15 formed on each of two planes on the head chip 4. In FIG. 2, the inner chamber 8 and the outer chamber 10 respectively are connected through the concave portions 16 of the head chip 4, and the ink 14 in the inner chamber 8 and that in the outer chamber 10 communicated with each other through the concave portion 16. Thereby, the ink 14 supplied to the inner chamber 8 from a not-illustrated ink tank through the ink supply port 9 is discharged into the outer chamber 10 by using the concave portion 16 formed on the head chip 4 as a channel and collected in a not-illustrated ink tank through the ink discharge port 11. To prevent bubbles coming through the head chip 4 from remaining in the recording head 1, it is preferable that the outer chamber 10 is set to the upper side of the inner chamber 8. The forward portions of the convex portions 15, the concave portions 16 and the protrusions 21 protrude from the opening of the cover plate 3 to form the meniscus on the head chip 4. The meniscus shapes on the head chip 4 serve as the discharge point 18 (FIGS. 4 to 6) of an ink droplet 19 in order to prevent the excessive ink 14 supplied from the concave portion 16 of the head chip 4 from overflowing. A migration electrode 12 is made of a conductive material such as a metal and connected with a not-illustrated external power supply and set in the inner chamber 8 at the side to which the ink 14 is supplied so as to contact with the ink 14. The opposite electrode 5 is made of a conductive material such as a metal and grounded or connected with a not-illustrated external power supply and set so that the discharge point 18 in the recording head 1 most closely approaches the opposite electrode 5 while securing a recording gap with the recording head 1. Moreover, the recording paper 6 is transferred through the recording gap between the opposite electrode 5 and the recording head 1 so as to contact with the opposite electrode 5 and electrified up to a potential equal to that of the opposite electrode 5. A positive constant bias voltage is continuously applied to the migration electrode 12 and a ground-level voltage or negative constant bias voltage is continuously applied to the opposite electrode 5. The ink 14 in the recording head 1 is constantly circulated by a not-illustrated external pump between a not-illustrated ink tank and the recording head 1 through the ink supply port 9 and ink discharge port 11. Moreover, the ink 14 is supplied by using each concave portion 16 in the head chip 4 as a channel and an independent convex meniscus is formed every concave portion 16 by using the front end of each protrusion 21 of the protruded member 20 as the discharge point 18. The ink 14 supplied to the inner chamber 8 is raised up to a potential at which the ink 14 is not discharged by the migration electrode 12 contacting the ink 14 and supplied to the discharge point 18 by using each concave portion 16 on the head chip 4 as a channel. Under printing, a driving pulse voltage from the driver 100 is selectively applied to the recording electrodes 13-1 to 13-m, and then an electrostatic force works on the toner in the ink 14 supplied to the discharge point 18 by an electric field generated between the recording electrodes 13-1 to 13-m and the facing electrode 5. When the electrostatic force of the toner overcomes the surface tension of the meniscus on the discharge point 18, the ink droplet 19 containing electrified toner agglomerations on the discharge point 18 is discharged toward the facing electrode 5 and characters are printed on the recording paper 6. Thereafter, because an ink flow in the direction from the inner chamber 8 toward the outer chamber 10 continuously occurs in the ink 14 supplied to the discharge point 18, toner not discharged are directly ejected toward the outer chamber 10 independently of the discharge of the ink droplet 19. FIG. 8 is a schematic view of equipotential lines generated when a conventional electrostatic ink-jet recording head prints characters. In FIG. 8, an equipotential line 22 nearby the discharge point 18, when the recording voltage is applied, is formed along the shape of each of the recording electrodes 102. The equipotential line 22 almost parallel with a discharge direction is generated on the ink 14 nearby each of the discharge points 18. Therefore, if toners in the ink 14 are insufficiently supplied onto the discharge point 18 while a recording voltage is applied to the recording electrodes, it is impossible to obtain toner agglomerations for forming a desired dot. FIG. 7 is a schematic view of equipotential lines generated when the electrostatic ink-jet recording head of this embodiment prints characters. In FIG. 7, the equipotential line 22 nearby the discharge point 18 while a recording voltage is applied to the recording electrodes 13-1 to 13-m is formed so as to be almost perpendicular to a discharge direction and an electrostatic force in the direction toward the discharge point 18 is generated in the electrified toner grains nearby the discharge point 18. Therefore, even while the recording voltage is selectively applied to the recording electrode 13-1 to 13-m, toner grains are stably supplied onto the discharge point 18. Moreover, because an ink bank due to a convex meniscus is present before the recording electrode 13, it is possible to obtain toner grains for forming a desired dot. As described above, the first advantage of the present invention lies in the fact that toner grains (agglomerations) enough to form a desired dot can be obtained. This is because discharge points are formed before recording electrodes, moreover each recording electrode is formed so as to surround each discharge point and thereby, it is possible to generate an electrostatic force in the direction toward the discharge points for the toner grains nearby the discharge points when a recording voltage is applied and thus, it is possible to collect toner grains enough to form a desired dot on the discharge points. The second advantage lies in the fact that ink droplets can be stably discharged. This is because each convex ink meniscus using a discharge point as an apex is independent in each concave portion on a head chip and vibrations of the liquid surface nearby a discharge point discharging ink droplets do not influence the ink meniscus of other discharge point and thus, it is possible to always obtain a stable ink meniscus. The third advantage lies in the fact that it is possible to prevent ink droplets from being irregularly discharged due to concentration of excessive toner grains. This is because a forced flow in the direction from an inner chamber toward an outer chamber is made to occur in each concave portion formed nearby a discharge point and thereby, it is possible to prevent toner grains from accumulating nearby a discharge point closest to a facing electrode.

Claims: An electrostatic ink-jet recording head for recording characters by applying an electric field to ink in which electrified toner grains are dispersed and discharging said toner grains by an electrostatic force generated by said electric field, comprising: a head member (7) having a chamber for supplying ink; a head chip (4) fixed to said head member and characterized in that said head chip has convex portions (15) and concave portions (16) arranged alternately in a direction perpendicular to ink ejecting direction, a plurality of continuous meniscus shapes being respectively formed on said concave portions; a recording electrode (each of 13-1 to 13-m) formed on each of said concave portions; and protrusions (21) which are provided with said head chip at the same intervals as those of the meniscus shapes formed on said concave portions, each of said protrusions being located on each of said concave portions (16) and protruding beyond ends of the convex portions (15) in a direction of the ink ejecting direction. The electrostatic ink-jet recording head according to claim 1, further comprises a cover member (3) which covers said head chip and is provided with an opening so as to allow ends of said concave and convex portions on which the meniscus shapes are formed to protrude. The electrostatic ink-jet recording head according to claim 1, wherein said recording electrode is further formed on side surfaces of each convex portions (15) of said head chip. The electrostatic ink-jet recording head according to claims 3, wherein said convex portions and said concave portions have corner ends protruding in the direction of the ink ejecting direction. The electrostatic ink-jet recording head according to claims 4, wherein said head chip (4) has a shape of a quadrilateral solid, and said corner ends of said convex portions and said concave portions correspond to one corner of said quadrilateral solid. The electrostatic ink-jet recording head according to claim 1, wherein said chamber has an inner chamber (8) for supplying the ink and an outer chamber (10) for outputing the ink, and said inner chamber communicates with said outer chamber by using each of said concave portions (16) as a channel. The electrostatic ink-jet recording head according to claim 6, wherein a migration electrode (12) contacting with ink is set in said inner chamber. The electrostatic ink-jet recording head according to claim 3, wherein said recording electrode is covered with an insulating film (17). The electrostatic ink-jet recording head according to claim 1, 2 or 3, further comprises a contact substrate (2) having a contact electrode connected to said recording electrode.

Inventors: HAGIWARA YOSHIHIRO, MINEMOTO HITOSHI, MIZOGUCHI TADASHI, SAEKI TOMOYA, SHIMA KAZUO, SUETSUGU JUNICHI, TAKEMOTO HITOSHI, YAKUSHIJI TORU, HAGIWARA, YOSHIHIRO, MINEMOTO, HITOSHI, MIZOGUCHI, TADASHI, SAEKI, TOMOYA, SHIMA, KAZUO, SUETSUGU, JUNICHI, TAKEMOTO, HITOSHI, YAKUSHIJI, TORU, Hagiwara, Yoshihiro, c/o NEC Niigato, Ltd., Minemoto, Hitoshi, c/o NEC Niigato, Ltd., Mizoguchi, Tadashi, c/o NEC Niigato, Ltd., Saeki, Tomoya, c/o NEC Niigato, Ltd., Shima, Kazuo, c/o Nec Niigata, Ltd., Suetsugu, Junichi, c/o NEC Niigato, Ltd., Takemoto, Hitoshi, c/o NEC Niigato, Ltd., Yakushiji, Toru, c/o NEC Niigato, Ltd.
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1998
Main classification: B41J2
IPC codes: B41J2