Electrostatic ink-jet recording head

Abstract: A recording electrode 2 is formed on a base plate 1. A cover member 4 having an ink flow path 3 therein is fixed on the base plate. Color particle is used as ink. A convex portion 12 is formed at a tip end portion of a cover member 4 on an ink ejection side. The convex portion 12 is located at a position projecting beyond a tip end portion of the base plate 1 having the recording electrode 12. An ink meniscus 8 is formed along the convex portion 12 on the cover member side. Thus, a component of an electric field tangent direction in the ink meniscus is directed in the tip end direction of the convex portion 12. The convex portion is sawtoothed to increase the electric field component. As a result, it is possible to rapidly supply the charged color particle to the tip end of the meniscus. At the same time, it is possible to carry out the recording at a high speed with high precision.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an ink-jet recording head, and more particularly, to an electrostatic ink-jet recording head for ejecting ink particle by electrostatic forces to effect printing on a piece of recording paper. Description of the Prior Art A non-impact recording method is superior in that an extremely small noise is generated during the printing. In particular, the ink-jet recording method is considered a very excellent recording because a high speed recording may be attained on a normal paper with a simple mechanism. A variety of methods therefor have heretofore been proposed. The ink-jet recording methods falls into the following categories: a method in which pressure waves are generated in an ink chamber by a piezoelectric element to thereby eject ink from a nozzle for recording, another method in which ink is ejected from a nozzle by bubbles generated by heat, and an electrostatic recording method. Of these methods, the electrostatic recording method by which a size of ink droplets may readily be controlled will hereinafter be noticed. The electrostatic ink-jet recording apparatus is provided an opposite electrode disposed on a back side of the recording paper, and a ink-jet recording head for ejecting ink particle to the recording paper. The ink-jet print head has stylus electrodes confronting with an opposite electrode. A voltage is applied to the electrodes so that an electric field generated by the application causes the ink particle such as charged toner or the like to fly toward the opposite electrode. Fig. 1 is a schematic perspective view showing a conventional electrostatic ink-jet printing apparatus. In Fig. 1, a top surface of a base plate 52 of an ink-jet recording head 50 is covered by a cover 54. Slit-like ink ejection holes 55 for holding the ink are provided therein. Also, a plurality of recording electrodes 53 are disposed in parallel along the ink emission direction. They are provided on the surface of the base plate 52. These recording electrodes 53 are connected to a voltage driving section (not shown). A high voltage pulse is applied selectively to the recording electrodes 53 during the printing. On the other hand, an opposite electrode 51 is disposed through the recording paper P on a line extending from the recording electrodes 53, thereby generating an electric field together with the recording electrodes 53 during the recording. Here, since the recording electrodes 53 are formed into styli, the electric field is concentrated on a tip end of each recording electrode 53, and charge is accumulated on the ink located near to the recording electrodes. The process for accumulation of the charge depends upon the kind of the ink. In case of conductive ink, the process depends upon the electrostatic induction and in case of inductive ink, the process depends upon induction polarization. Also, in case of ink in which color agent particle (charged toner) is dispersed in solution, the color particle itself has a nominal charge due to a Zeta potential so that a floating motion of the charged color particle is biased by the electric field generated with the opposite electrode. Any type of the above-described ink may be applied to the ink used in the electrostatic ink-jet recording apparatus. However, in the recording depending upon the ejection of the ink per se, bleeding of the ink occurs on the recording paper, and feathering or color breed inherent in multi-color recording occurs, resulting in degradation in printing quality, disadvantageously. In this case, since the bleeding degree depends upon the recording paper, the printing results are different in accordance with a difference of the paper. Furthermore, since it takes a certain period of time for drying of ink, there is a disadvantage that a recording speed is limited thereby. However, if the ink in which color particle is dispersed is used and the color particle (charged toner) is ejected for recording, it is possible to carry out the recording without any bleeding of ink irrespective of the kind of the recording paper. For this reason, for the ink-jet recording apparatus shown in Fig. 1, it is preferable to use ink in which the color particle is dispersed. The color particle contained in the ink solution is subjected to a strong attractive force toward the opposite electrode 51 side in Fig. 1, i.e., in a direction toward the recording paper P by a Coulomb force generated by the charge accumulated by the recording electrodes 53. If the Coulomb force overcomes the surface tension of the ink, the ink is ejected to stick to the recording paper P. In this way, in response to an image to be recorded, a high voltage pulse is applied to the recording electrodes 53 to thereby carry out a desired recording operation. However, since the conductivity or inductivity of the ink to be used for recording is larger than the conductivity or inductivity of air, a position where the electric field is concentrated is not only determined by the arrangement of the recording electrodes, but the shape of ink meniscus formed on the ink ejection port is affected. In the ink-jet recording head 50 shown in Fig. 1, it is preferable that the ink meniscus is uniform in the longitudinal direction of the ink ejection ports. However, actually, fine irregularity is formed on the surface of the ink meniscus due to the machining allowance of the opening portions, the vibration of the meniscus after the ink ejection, the natural swing of the meniscus, and the like. In this case, the electric field is concentrated on the convex portions of the fine corrugation of the meniscus generated in the vicinity of the recording electrode by the relationship of the conductivity and inductivity of the ink. Then, when the shape of the meniscus begins to be deformed by the Coulomb force, the concentration of the electric field is further promoted. As a result, the position where the ink flies is minutely displaced due to the irregularity of the fine meniscus in the initial stage. If this occurs, even if a high voltage pulse is applied to a certain recording electrode, the ink would fly from a position displaced from the recording electrode. As a result, there are cases where it would be impossible to carry out the recording on a desired position. This leads to degradation in printing quality. On the other hand, Japanese Patent Application Laid-Open No. Sho 60-228162 laid open on November 13, 1986 discloses an example of countermeasure for the above described problem. In this publication, the electrostatic ink-jet recording head has convex portions 66 at a tip end position of the recording electrode 63 at a tip end portion of the base plate 62 as shown in Fig. 2. Thus, the convex and concave shapes corresponding to the shape of the convex portion 66 are applied to the ink meniscus formed in the vicinity of the tip ends of the recording electrodes 63. Also, the electric field at the tip ends of the recording electrodes 63 is concentrated onto the convex portions 66. As a result, in the case where the ink containing color particle is used, the charged color particle in the ink migrates toward the tip ends (tip ends of the convex portions 66) of the meniscus in accordance with the electric field in the ink to thereby first fly. However, in the ink-jet recording head shown in Fig. 2, for example, in the case where the resolving power of the printer is set at 11,8 dots/mm (300 dpi), it is necessary to set up the arrangement pitch of the recording electrodes 63 to about 85 µm. Therefore, the fine convex and concave portions must be formed at the tip end of the base plate 62 corresponding to the recording electrodes having such a fine pitch. For this reason, the base plate material and the machining process are largely limited, causing an increased cost, disadvantageously. Also, in order to meet a sufficient mechanical strength of the base plate 62, the latter needs a thickness of 100 µm or more, whereas the pitch of the convex and concave portions to be machined must be one fifth or sixth of this thickness or less. Accordingly, the machining is extremely difficult, and also a cost for manufacture is increased, disadvantageously. In Fig. 2, it is possible to form the base plate 62 of thin and soft material by providing a reinforcement plate 67 at a position where the recording electrodes 63 are partitioned. However, this degrades the mechanical strength of the convex portions of the tip end of the base plate. Accordingly, when the head tip portion is to be cleaned, or the recording paper is jammed, it is disadvantageous that it is likely that the convex portions of the tip end of the base plate is damaged. Furthermore, since the direction of the electric field in the vicinity of the recording electrodes diverges (in directions indicated by arrows) from the recording electrodes as shown in Fig. 3, the migration of color particle does not occur toward the meniscus end on the axes of the recording electrodes or does not smoothly occur. As a result, the performance for the high speed and continuous ejection of the ink has to be improved to some extent. SUMMARY OF THE INVENTION In order to overcome defects inherent in the conventional technology, an object of the present invention is to provide an electrostatic ink-jet recording head which may readily be manufactured at low cost to thereby rapidly eject color particle in ink in a stable manner. According to the present invention, there is provided an electrostatic ink-jet recording head which uses ink containing color particle charged with a predetermined polarity, comprising a base plate, at least one recording electrode formed on a surface thereof for generating an electric field for ink ejection; a cover member for forming an ink flow path in an ink ejection direction on the surface of said base plate where said recording electrode is formed; and an ink ejection port formed at an end portion of the ink flow path on the ink ejection side. At least one convex portion is formed at a tip end portion of said cover member on the ink ejection side, and a tip end portion of the convex portion is set to project in the ink ejection direction beyond the tip end portion of the base plate on the ink ejection side. According to the present invention, when the ink containing the color particle is filled in the ink flow path, the ink meniscus is formed along the shape of the convex portion of the cover member. When the electric field is diverged from the recording electrode, it is concentrated on the tip end portion of the cover member along the shape of the surface of the ink meniscus due to the relationship of the conductivity and inductivity of the ink and air. By the concentration of the electric field, the color particle (charged toner) in the ink migrates toward the tip end of the meniscus and the color particle flies from the tip end of the cover member together with the ink solution. According to the present invention, since the convex portion of the cover member on the ink ejection side projects beyond the tip end portion of the base plate, the electric field from the tip end of the recording electrode concentrates to the meniscus on the tip end portion of the convex portion. It is therefore possible to rapidly supply color particle to the meniscus end portion, and to carry out the recording at a high speed with high precision. According to another aspect of the present invention, a plurality of stylus recording electrodes are arranged in parallel in a longitudinal direction along the ink ejection direction and a sawtoothed portion is formed on the ink ejection side of the cover member. The convex portions form convex portions of said sawtoothed portion, and the convex portions are arranged so as to overlap lines extending from the stylus recording electrodes. Thus, the convex ink meniscus is formed corresponding to the sawtoothed shape, the electric field diverged from the recording electrode concentrates on the tip end of the convex portion of the ink meniscus, and the color particle is effectively ejected from the tip end of the convex portion. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: Fig. 1 is a schematic perspective view showing a conventional electrostatic ink-jet recording apparatus; Fig. 2 is a perspective view showing another conventional electrostatic ink-jet recording apparatus; Fig. 3 is a schematic cross-sectional view showing a state of an electric field diverging from the recording electrode and a state of an ink meniscus formed at an ink ejection port in an electrostatic ink-jet recording head shown in Fig. 2; Fig. 4 is a perspective view showing an embodiment of an electrostatic ink-jet recording head according to the present invention; Fig. 5 is a partially fragmentary plan view of the ink-jet recording head shown in Fig. 4; Fig. 6 is a cross-sectional view taken along line C-C of Fig. 5; Fig. 7 is a cross-sectional view taken along line D-D of Fig. 5; and Fig. 8 is an enlarged cross-sectional view showing a part indicated by character B in Fig. 6. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In Fig. 4 and Fig. 5, an electrostatic ink-jet recording head includes a base plate 1, a plurality of recording electrodes 2 formed on a too surface of the base plate 1 for generating an electric field for the ink ejection, a cover member 4 for forming slit-like ink flow paths 3 in an ink ejection direction (indicated by arrow A) along the recording electrodes 2, and ink ejection ports 5 formed at a tip end portion on the ink ejection side of the slit-like ink flow paths 3. The tip end portion of the cover member 4 on the ink ejection side has a shape which projects beyond the tip end portion of the base plate 1 on the ink ejection side in the ink ejection direction. As shown in Fig. 5, the recording electrodes 2 are formed into styli in the longitudinal direction along the ink ejection direction. Also, the tip end portion of the cover member 4 on the ink ejection side are formed into sawteeth. A plurality of convex portions 12 form the sawteeth having sharp-pointed ends, and they are arranged so as to overlap on a line extending from each stylus recording electrode 2. More specifically, the base plate 1 is formed of insulating material such as glass. The recording electrodes 2 one formed by patterning through a photo-lithographic method after nickel, chromium or the like has been sputtered on a surface, on the cover member side, of the base plate 1. The recording electrodes 2 are arranged at 11,8 dots/mm (300 dpi), i.e., at an interval of about 85 µm. Also, as shown in Figs. 5 and 6, the tip end portion of each recording electrode 2 is covered by an insulating layer 6. The cover member 4 is formed by molding resin such as polysulphone or the like. The cover member 4 is mounted to face the base plate 1. As shown in Fig. 6, slit-like ink flow paths 3 are formed inside of the cover member 4 and an ink 9 is contained in the ink flow path 3. The ink ejection ports 5 are formed at the tip end portion of the cover member 4. The ink ejection ports 5 are formed by thickening the cover member 4. In other words, the ink ejection ports 5 are narrowed portions of the ink flow paths 3. Three side walls of the cover member 4 form the side surface of the ink flow paths 3 and they are intimately contacted with the base plate 1. With this arrangement, the ink flow paths 3 are closed except for the ink ejection ports 5. Also, two ink recirculation ports 7 are provided on the top surface of the cover member 4 and are connected to an ink cartridge (not shown) via a pair of tubes (not shown). Then, under the condition that vacuum pressure at about 1 cmH 2 0 is applied to the ink 9 in the ink flow paths 3, the ink recirculation is forcibly effected. Here it is preferable that the recording ink is of a type such that color fine particles (toner) made of thermoplastic resin color are dispersed in liquid of petroleum type organic solution (isoparaffin hydrocarbon) together with charge controlling agent. In this embodiment, the color particle is a charged toner which has been nominally charged with a positive polarity by Zeta potential. As shown in Fig. 5 and Fig. 7, in the cover member 4, a plurality of path separators 4a of the cover member 4 are formed at positions where the adjacent recording electrodes 2 are partitioned. The path separators 4a are arranged so as to be in contact with the insulating layer 6 provided on the base plate 1 between the convex portions 12 of the cover member 4. The overall operation of the recording head according to the embodiment will now be described. When the ink 9 is filled in the ink flow paths 3 from the ink recirculation port 7, as shown in Fig. 6, an ink meniscus 8 is formed on the ink 9 at each ink ejection port 5 by the surface tension. Here, the negative pressure is applied to the ink 9 within the ink flow path 3, and also, the tip end portions of the convex portions 12 at the tip end of the cover member 4 project beyond the tip end of the base plate 1 in the ink ejection direction. For this reason, the ink meniscus 8 takes a concave surface shape slanted upwardly with an apex at the convex portion 12 as viewed from the side in Fig. 6. Furthermore, the ink meniscus 8 projects along the convex portion 12 as viewed from above as shown in Fig. 5. When a voltage pulse is applied to a desired electrode 2 from a drive circuit (not shown), the electric field is formed from the recording electrode 2 to the opposite electrode (not shown) (corresponding to the opposite electrode 51 in Fig. 1), i.e., toward the recording paper. The direction of the electric field in the vicinity of the tip end of the recording electrode 2 is such that it diverges from the recording electrode 2. The charged toner in the ink 9 migrates in the ink along the electric field. As shown by the arrow in Fig. 8, a part of charged toner migrates directly toward the tip end portion of the meniscus 8. Also, another part of charged toner first migrates toward the ink meniscus surface and thereafter migrates to the meniscus end portion along the surface shape of the meniscus 8. The charged toner which has been concentrated on the tip end portion of the meniscus 8 is drawn by the strong electric field concentrated on the tip end portion of the meniscus and, as shown in Fig. 5, becomes a charged toner group 10 containing the ink solution to fly toward the opposite electrode (not shown), i.e., the recording paper to thereby perform the recording. The toner sticking on the recording paper to form the recording dots is heated by a heater (not shown) and fixed. On the other hand, the charge corresponding to the amount of charge held in the charged toner group 10 which has flown is supplied to the ink 9 in the vicinity of the ink ejection part 5 from the portion which is not covered by the insulating layer 6 of the recording electrode 2. Thus, the electric balance is always kept. As described above, according to the embodiment, since the convex portion 12 of the cover member 4 on the ink ejection side is located at a position projecting beyond the tip end portion of the base plate 1 in the ink ejection direction, the ink meniscus 8 projects along the convex portion 12 as shown in Fig. 5. Thus, the component in the tangent direction of the electric field with the meniscus 8 is toward the tip end of the meniscus 8 and its component becomes large. Accordingly, it is possible to rapidly supply the meniscus tip end portion with charged toner and to carry out high speed recording with high precision. Also, the tip end portion of the cover member 4 is formed in the sawteeth, and the position of the convex portion 12 for forming the sawteeth is overlapped with a line extending from the stylus recording electrode 2. As shown in Fig. 5, the ink meniscus 8 has an apex corresponding to the convex portion 12 of the cover member 4. For this reason, it is possible to more effectively concentrate the electric field diverging from the recording electrode 2 onto the convex portion 12, and to rapidly carry out the supply of the charged toner to the tip end portion of the meniscus. At the same time, it is possible to carry out the high speed printing with high precision by using the plurality of recording electrodes 2. Also, since the sawtoothed portion for concentration of the electric field is formed not on the base plate 1 but on the cover member 4, it is possible to widen the variety of selection of the base plate material. Also, it is possible to widen the freedom of selection of the formation process for the recording electrode arranged on the base plate 1 with high precision. As a result, by selecting the low cost material which would not be suitable in the conventional example due to the machining problem and using an easy process, it is possible to reduce the manufacture cost. Moreover, the thickness of the plate of the cover member 4 is made to one which is suitable for machining, it is possible to facilitate the machining of the sawtoothed portion, to thereby further reduce the manufacture cost. Furthermore, since the cover member 4 is made of moldable material, it is possible to impart sufficient mechanical strength to the sawtoothed portion formed at the tip end portion on the ink ejection side. Thus, it is possible to prevent the damage of the sawtoothed irregularity portion due to the cleaning of the head tip portion and the clogging of the ports. Also, since the tip end portion of the recording electrode 2 is covered by the insulating layer 6, the dielectric strength between the recording electrode 2 and the opposite electrode may be increased, and it is possible to effectively avoid such a disadvantage that an excessive current would flow due to the generation of jamming of the recording paper or the like to damage the drive device or the like. In Fig. 4, the cover member 4 may be made of other material. Also, for the electrostatic ink-jet recording head in accordance with the embodiment, the insulating layer 6 is formed after the recording electrode 2 has been printed first on the surface of the base plate 1, and thereafter, the cover member 4 which is separately machined is mounted on the base plate 1. Such an order of the manufacture process is preferable.

Claims: An electrostatic ink-jet recording head which uses ink containing color particle charged with a predetermined polarity, comprising a base plate (1), at least one recording electrode (2) formed on a surface of said base plate for generating an electric field for ink ejection; a cover member (4) for forming an ink flow path (3) in an ink ejection direction on the surface of said base plate where said recording electrode is formed; and an ink ejection port (5), formed in a tip end portion of said ink flow path, for ejecting the ink, characterized in that, at least one convex portion (12) is formed at a tip end portion of said cover member and projects in the ink ejection direction, and said convex portion is set to project beyond a tip end portion of said base plate in the ink ejection direction. The electrostatic ink-jet recording head according to claim 1, wherein said convex portion (12) has sharp-pointed end. The electrostatic ink-jet recording head according to claim 2, wherein a plurality of recording electrodes (2) are formed on the surface of said base plate and are arranged in parallel in a longitudinal direction along the ink ejection direction, and a sawtoothed portion is formed at the tip end of said cover member to form a plurality of convex portions (12), the convex portions being arranged so as to overlap on a line extending from the recording electrodes. The electrostatic ink-jet recording head according to claim 2, wherein said cover member (4) is made of moldable material. The electrostatic ink-jet recording head according to claim 2, wherein a tip end portion of said recording electrode (2) is covered by insulating material. The electrostatic ink-jet recording head according to claim 3, wherein said cover member (4) has path separators (4a) for separating the ink flow path (3), and said path separators are formed at positions where the recording electrodes (2) are partitioned. The electrostatic ink-jet recording head according to claim 2, wherein said ink ejection port (5) is formed by thickening the cover member to narrow the ink flow path.

Inventors: HAGIWARA YOSHIHIRO, MINEMOTO HITOSHI, SHIMA KAZUO, SUETSUGU JUNICHI, UEMATSU RYOSUKE, HAGIWARA, YOSHIHIRO, MINEMOTO, HITOSHI, SHIMA, KAZUO, SUETSUGU, JUNICHI, UEMATSU, RYOSUKE
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1996
Main classification: B41J2
IPC codes: B41J2