Electrostatic ink jet recording head

Abstract: An electrostatic ink jet recording head ejects toner particles in liquid ink at a high rate operation and with a stabilized ink ejection. The head body of the ink jet recording head comprises a base plate, head substrate having a stripe protrusion and channel regions alternately arranged with a constant pitch for serving as meniscus elements, and an ejecting electrode disposed on the top surface of the protrusion. A stable ink meniscus can be formed in the channel region due to the capillary function of the channel region. The channel region can be formed by mechanical dicing or laser machining of the head substrate or a photolithography of a photosensitive layer formed on the head substrate.

Description: The present invention relates to an electrostatic ink jet recording head and, more particularly, to an electrostatic ink jet recording head for recording by ejecting ink droplets containing toner particles in an electrostatic field. The non-impact recording technique attracts special attention because of its remarkably low noise operation. Among other non-impact recording heads, an ink jet recording head has several advantages of high-speed recording, simple mechanism, direct recording onto plain paper etc. Various mechanisms for the ink jet recording head have been heretofore proposed. Examples of the proposed ink jet recording head include an electrostatic ink jet recording head, wherein a driving pulse is applied between a plurality of stylus ejecting electrodes or recording electrodes of a head body and an opposite electrode opposed to the ejecting electrodes with the recording medium disposed therebetween. Fig. 1 shows a head body of this type of the conventional electrostatic ink jet recording head described in JP-A-07-120252 (or JP-B-8-309 993) in a perspective view. The head body 100 of the ink jet recording head shown in Fig. 1 comprises a planar substrate 101 made of an insulator, on which a plurality of ejecting electrodes 102 extending along the longitudinal direction of the head body 100 are arranged in accordance with the desired resolution along the transverse direction. The ejecting electrodes 102 are formed by sputtering a metal such as Cu or Ni on the entire surface of the substrate 101 and a subsequent photolithographic etching step using a mask pattern for the ejecting electrodes 102. The ejecting electrodes 102 are connected to respective output lines of a printing driver not shown in the drawings, and selectively applied therefrom with a high voltage driving pulse during a recording operation. On the entire surface of the substrate 101 together with the ejecting electrodes 102, an insulator resin is applied by a spin-coating technique to insulate the ejecting electrodes 102 from liquid ink. Meniscus elements 103 overlap the respective ejecting electrodes 102 except for the tip portion of the ejecting electrodes 102 for forming an ink meniscus in the vicinity of each ejecting electrode 102. Referring additionally to Figs. 2A and 2B showing details of the front edge portion of the head body of Fig. 1, the meniscus elements 103 are made of a photosensitive polymer film laminated on the ejecting electrodes 102 and insulator substrate 101 and patterned by a photolithographic technology. A top cover plate 104 made of an insulator is disposed above the meniscus elements 103 to define an ink chamber between the top cover plate 104 and the insulator substrate 101. The front edge of the cover plate 104 is located slightly behind the tips of the meniscus elements 103 and the front edge of the insulator substrate 101. The top cover 104 has an ink inlet port 105 and an ink outlet port 106 for circulation of the liquid ink. The gap between the front edge portion of the insulator substrate 101 and the front edge of the top cover plate 104 constitutes an ink jet slit 107, from which the meniscus elements 103 protrude together with the tips of the ejecting electrodes 102. The meniscus elements 103 extend backwards below the top cover plate 104 for supporting the front edge of the same. The liquid ink supplied from the ink inlet port 105 through the ink chamber passes the ink jet slit 107 and is supplied in the vicinity of the tips of the meniscus elements 103 to form an ink meniscus 108 around the meniscus elements 103. One of the problems to be solved by the present invention in the conventional electrostatic ink jet recording head as mentioned above is that the supply rate of the liquid ink cannot follow the desired high- speed operation of the ink jet recording head. The inventors have found that this is partly because the meniscus elements are formed on the ejecting electrodes by a photolithographic technology, which limits the thickness of the meniscus elements below several tens of micrometers. The small thickness of the meniscus elements increases the flow resistance at the ink jet slit against the liquid ink. Another problem to be solved by the present invention is that the meniscus elements cannot provide a stable configuration of the meniscuses. This is attributable to the photolithographic technique in the process for forming the meniscus elements, which causes an unstable configuration of the ink meniscus. Location of the tips of the meniscus elements at the position which is located behind the front edge of the substrate is another reason. EP-A-813 966 (Art 54(3)EPC) discloses an electrostatic ink jet recording head comprising a head body defining an ink chamber for receiving liquid ink, said head body having a front edge and a rear edge, and an opposing electrode opposing said front edge, said ink jet recording head ejecting the liquid ink from said head body toward said opposing electrode during a recording operation, wherein said head body includes an insulating substrate having a plurality of stripe protrusions arranged in the vicinity of said front edge, and an ejecting electrode disposed in association with said stripe protrusion. US-A-4, 504, 844 discloses an on-demand inkjet printing head with a pluralily of ink passage ways in the head. In view of the foregoing, it is an object of the present invention to provide an electrostatic ink jet recording head capable of operating in a stable manner at a high speed by improving the structure of the meniscus elements. This object is achieved with the features of the claims. In accordance with the present invention, the protrusion functions as a meniscus element which can be formed with a stable configuration and have a sufficient thickness desired for the meniscus element to thereby obtain a stable ink supply. Accordingly, a stable and high-speed operation of the electrostatic ink jet recording head can be obtained. The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings. Fig. 1 is a perspective view of a head body of a conventional ink jet recording head; Fig. 2A is a detailed top plan view of a front edge portion of the head body of Fig. 1, and Fig. 2B is a longitudinal-sectional view taken along line B-B in Fig. 2A; Fig. 3 is a partially cut-out perspective view of an electrostatic ink jet recording head according to a first embodiment of the present invention; Fig. 4 is a detailed perspective view of a front edge portion of the head body of Fig. 3 showing the details of the head body by removing the top cover plate; Figs. 5A to 5H are cross-sectional views of the head substrate in the head body of Fig. 4, consecutively showing the steps for fabrication thereof; Fig. 6 is a schematic longitudinal-sectional view of the electrostatic ink jet recording head according to the first embodiment, showing the head body of Fig. 4 and an-associated opposing electrode; Fig. 7 is a partially cut-out perspective view of an electrostatic ink jet recording head modified from the first embodiment; Fig. 8 is a detailed perspective view showing the front edge portion of a head body in an electrostatic ink jet recording head according to a second embodiment of the present invention; Figs. 9A to 9E are cross-sectional views of the head substrate in the head body of Fig. 8, consecutively showing the steps for fabrication thereof; and Fig. 10 is a schematic top plan view for showing operation of the electrostatic ink jet recording head according to the second embodiment, showing the head body of Fig. 8 and an associated opposing electrode. Now, the present invention will be more specifically described based on preferred embodiments thereof with reference to the accompanying drawings, wherein similar constituent elements are designated by the same or similar reference numerals. Referring to Figs. 3 and 4, a head body 10 of an electrostatic ink jet recording head according to a first embodiment of the present invention comprises a base plate 13, and an elongate head substrate 16 mounted on the front edge portion of the base plate 13. The elongate head substrate 16 extends in the transverse direction of the base plate 13 with the front edge of the head substrate 16 aligned with the front edge of the base plate 13. The head substrate 16 has a plurality of stripe protrusions 12 extending parallel to each other from the front edge of the head substrate 16 to the rear edge thereof in the longitudinal direction of the head body 10. The head body 10 further comprises ejecting electrodes 17 mounted on top of each stripe protrusion 12 of the head substrate 16 and having pointed tips in the vicinity of the front edge of the head substrate 16, a top cover plate 15 overlying the base plate 13 from the front edge portion of the base plate 13 to a location adjacent to the rear edge portion of the base plate 13 for defining an ink chamber 14 together with the base plate 13, and a plurality of electrode pads 18 arranged in the rear edge portion of the base plate 13 and each connected to a corresponding ejecting electrode 17 through a corresponding signal line 19 formed on the base plate 13. A channel region 32 is formed between each two of the stripe protrusions 12 for flowing the liquid ink from the ink chamber 14. By this configuration, the stripe protrusion 12 constitutes a meniscus element for forming a separate ink meniscus 11 at the front end portion of each channel region 32. The stripe protrusion 12 also supports the front edge portion of the top cover plate 15. The front edge of each ink meniscus 11 is substantially straight as viewed perpendicular to the head substrate 16, and of a U shape as viewed along the longitudinal direction of the head body 10. The channel region 32 has a small width for having a capillary function, and has a thickness larger than the width to allow a sufficient ink flow at a high rate. The top cover plate 15 has a front edge for defining an ink jet slit 36, which is located slightly behind the front edge of the head substrate 16, the front ends of the stripe protrusions 12 and the tips of the ejecting electrodes 17. The top cover plate 15 has an ink inlet port 20 and an ink outlet port not shown in Fig. 3, similarly to the conventional ink jet recording head. The head substrate 16 is made of an insulator such as glass, and the ejecting electrodes 17 are made of a metal such as Ni or Cu. The head substrate 16 is formed to define the stripe protrusions 12 and channel regions 32 alternately arranged in the transverse direction of the head body 10 with a constant pitch, which corresponds to a desired resolution of the printing. Each channel region 32 supplies the liquid ink 11 from the ink chamber 14 to the front edge of the head substrate 16, and provides a sufficient space for holding an ink meniscus 11 at the front end of the channel region 32. The base plate 13 is made of an insulator, on which the electrode pads 18 together with the signal lines 19 are formed by a photolithographic technique to supply high positive voltage driving voltages to the ejecting electrodes 17. The front end of each signal line 19 is connected to the rear end of a corresponding ejecting electrode 17 by wire bonding, and the rear end of a signal line 19 is connected to a corresponding electrode pad 18. The rear ends of the ejecting electrodes 17, bonding wires and signal lines 19 are coated by an insulator resin. The top cover plate 15 is made from an insulating resin by using an injection molding technique. The ink inlet port 20 and the ink outlet port are connected to an ink reservoir with tubes (not shown) for circulation of the liquid ink which is effected by a pump. Figs. 5A to 5H consecutively show the steps of fabricating the head substrate 16, shown in Fig. 4, mounting thereon ejecting electrodes 17. A metal such as Cu or Ni is sputter-deposited on the entire flat surface of a sheet-shaped-substrate 16 to form a conductor layer 21 thereon, as shown in Fig. 5A. A photoresist layer 22 is then formed on the conductor layer 21 by a spin-coating or laminating technique, as shown in Fig. 5B. The photoresist layer 22 is then exposed to exposure light, as shown in Fig. 5C, through a mask 23 which defines a pattern of the ejecting electrodes 17, followed by development of the photoresist layer 22 to selectively remove the photoresist layer 22 to form a photoresist pattern on the conductor layer 21, as shown in Fig. 5D. Then, the resultant head substrate is immersed into an etching solution, which selectively etches the conductor layer 21, thereby leaving the ejecting electrodes 17 underlying the photoresist pattern 22, as shown in Fig. 5E. Thereafter, the resultant head substrate is immersed into another etching solution, which etches the photoresist pattern 22 selectively from the conductor pattern, to thereby obtain the ejecting electrodes 17 arranged on the head substrate 16 with a constant pitch, which is equivalent to the desired resolution of the printing, as shown in Fig. 5F. In the above steps, a plurality of head substrates 16 maybe formed according to the fabrication steps mentioned above on a single wafer, followed by cutting the wafer into segments to provide a plurality of head substrates 16. Subsequently, the head substrate 16 is subjected to a dicing step for forming the stripe protrusions 12 and channel regions 32 by using a dicing blade 24, as shown in Fig 5G. The entire surface of the head substrate 16 is then coated by a coating resin 25, as shown in Fig 5H. The depth of the channel regions 32 can be easily determined by the feed of the dicing blade 24 to a desired depth. Alternatively, the stripe protrusions 12 and channel regions of the head substrate 16 may be formed by laser beam machining by using an excimer laser and a mask. In this case, the depth of the channel regions 32 can be easily determined by adjusting the length of time for laser radiation. Alternatively, the stripe protrusions 12 of the head substrate 16 may be made of a photosensitive resin film patterned by a photolithographic technique using ultraviolet rays. The depth of the channel regions 32 can be easily adjusted by the length of time for irradiation by the ultraviolet rays. Fig. 6 shows a schematic illustration of the electrostatic ink jet recording head according to the present embodiment including the head body 10 of Fig. 3 and an associated opposing electrode 28. The head body 10 is supported by a support member 37 so that the head body 10 is inclined at an angle of about 45° with respect to a horizontal plane to thereby direct the top corner of the front edge of the stripe protrusion 12 toward the opposing electrode 28. The pointed tips of the ejecting electrodes 17 are directed toward the opposing electrode 28, which is maintained at a ground potential as shown in Fig. 6 or a negative potential. The opposing electrode 28 serves as a platen for carrying a recording paper 29. The support member 37 is slidably mounted on a horizontal shaft 38 extending parallel to the front edge of the head body 10. The ejecting electrodes 17 are selectively applied with driving pulses to generate an electrostatic field for ejecting of ink droplets 30 from the ink meniscus 11, which is formed at the front end of the channel regions adjacent to the ejecting electrodes 17 applied with the driving pulses, toward the opposing electrode 28. The recording paper 29 is fed along the opposing electrode 28 through the gap between the head body 10 and the opposing electrode 28 to receive the ejected ink droplets 30. The ink droplets 30 are ejected at the corner edge of the stripe protrusion 12 defined by the front surface of the head substrate 16, top surface of the stripe protrusion 12 and the side surface of the stripe protrusion 12, i.e., side surface of the channel region 32. In this embodiment, the angle of 45° of the inclined head body 10 allows the ink meniscus 11 to be positioned due to gravity at the top corner of the front edge of the protrusion 12 on which the ejecting electrode 17 is disposed. Accordingly, the ejection of the ink droplets 30 occurs accurately at the top corner of the front edge of the protrusion 12, which allows a stable ejection of the ink droplets. Fig. 7 shows a modification from the head body 10 of the electrostatic ink jet recording head of the first embodiment. The head body 10A of the ink jet recording head of Fig. 7 is additionally provided with an electrophoretic electrode 33 at the inner rear wall of the top cover plate 15. The electrophoretic electrode 33 is applied with a static voltage having the same polarity as that of the toner particles to move the toner particles in the ink chamber 14 toward the ink jet slit 36 of the head body 10A by an electrophoretic force. Fig. 8 shows, similarly to Fig. 4, the front edge portion of a head body of an electrostatic ink jet recording head according to a second embodiment of the present invention. The head body 10B of this embodiment is similar to the head body 10 of Fig. 4 except for the location of the ejecting electrodes 17. Specifically, each ejecting electrode 17 in this embodiment is disposed on the bottom surface and both side surfaces of the channel region 32. Alternatively, the ejecting electrode may be formed only on the side surfaces of the channel region 32. The ejecting electrodes 17 are coated with an insulating resin for insulation. Figs. 9A to 9E show consecutive steps in the fabrication of the head substrate 16 of the present embodiment. An insulator plate 16A is first subjected to a mechanical dicing step using a dicing blade 24, as shown in Fig. 9A, to form a head substrate 16 having a plurality of stripe protrusions 12 and channel regions 32 arranged alternately at a constant pitch corresponding to a desired resolution of the printing, as shown in Fig. 9B. The width of the channel region 32 is determined by the thickness of the dicing blade 24 and the depth of the channel region 32 is determined by the amount of the feed of the dicing blade 24. Alternatively, the head substrate 16 may be formed by laser beam machining of an insulator plate 16A by using excimer laser or by photolithography of a photosensitive layer by using ultraviolet rays. The depth of the channel region 32 is controlled by the time length of the irradiation by the laser beam or by the thickness of the photosensitive layer formed on the head substrate 16. Thereafter, a metallic film 21 such as Cu, Ni etc. is formed on the entire surface of the head substrate 16 by sputter-deposition, for example, as shown in Fig. 9C. The metallic film 21 on the top surface of the stripe protrusions 12 is then selectively etched away to leave the ejecting electrodes 17 remaining on the bottom surface and side surfaces of the channel regions 32 by using a photoresist pattern, as shown in Fig. 9D. An overcoat 25 is then formed on the entire surface of the head substrate 16 and the ejecting electrodes 17 by a spin-coating technique for insulating the ejecting electrodes 17 from the liquid ink, as shown in Fig. 9E. The ink jet recording head of the present embodiment operates in combination of the head body 10B and an associated opposing electrode, similarly to the case of the first embodiment as described with reference to Fig. 6. Fig. 10 shows a top plan view of the ink jet recording head of the present embodiment in operation. When a driving pulse is selectively applied to desired ejecting electrodes 17 during a printing operation, electric lines 34 of force are generated between the ejecting electrode 17 applied with the driving pulse and the opposing electrode 28. An ink meniscus 11 is formed in the channel region 32, having a concave front edge as viewed perpendicular to the head substrate 16 at the front end of the channel region 32. The liquid ink is ejected as ink droplets 30 from a corner edge 35 of the stripe protrusion 12, at which the ink meniscus 11 has a maximum change in curvature, along the electric lines 34 of force, as shown in Fig. 10. The corner edge 35 is defined by the side surface of the stripe protrusion 12, top surface of the stripe protrusion 12 and the front surface of the head substrate 14. A modification similar to the modification 10A from the first embodiment may be made in which an electrophoretic electrode is provided at the rear edge of the ink chamber.

Claims: An electrostatic ink jet recording head comprising a head body (10) defining an ink chamber (14) for receiving liquid ink, said head body (10) having a front edge and a rear edge, and an opposing electrode (28) opposing said front edge, said ink jet recording head ejecting the liquid ink from said head body (10) toward said opposing electrode (28) during a recording operation, wherein said head body (10) includes an insulating substrate (16) having a plurality of stripe protrusions (12) arranged in the vicinity of said front edge, an ejecting electrode (17) disposed in association with said stripe protrusion (12), wherein said head body (10A) further comprises an electrophoretic electrode (33) in the vicinity of a rear edge of said ink chamber. An electrostatic ink jet recording head comprising a head body (10) defining an ink chamber (14) for receiving liquid ink, said head body (10) having a front edge and a rear edge, and an opposing electrode (28) opposing said front edge, said ink jet recording head ejecting the liquid ink from said head body (10) toward said opposing electrode (28) during a recording operation, wherein said head body (10) includes an insulating substrate (16) having a plurality of stripe protrusions (12) arranged in the vicinity of said front edge, an ejecting electrode (17) disposed in association with said stripe protrusion (12), further comprising a support member (37) for supporting said head body (10) at an angle of approximately 45° with respect to a horizontal plane. An electrostatic ink jet recording head as defined in claim 1 or 2, wherein said stripe protrusions (12) extend parallel to each other from said front edge toward said rear edge. An electrostatic ink jet recording head as defined in claim 1, 2 or 3, wherein said head body (10) further comprises a base plate (13) mounting said insulating substrate (16) at a front edge portion of said base plate (13). An electrostatic ink jet recording head as defined in claim 1, 2, 3 or 4 wherein a space between two of said stripe protrusions (12) constitutes a channel region (32) for the passage of liquid ink. An electrostatic ink jet recording head as defined in claim 5 wherein said channel region (32) has a capillary function and has a depth larger than a width of said channel region. An electrostatic ink jet recording head as defined in claim 5 or 6 wherein said channel region (32) has a surface formed by machining. An electrostatic ink jet recording head as defined in claim 5, 6 or 7 wherein each said ejecting electrode (17) is formed on side surfaces and a bottom surface of said channel region (32). An electrostatic ink jet recording head as defined in any one of claims 1 to 8 wherein said ejecting electrode (17) is covered by an insulating film (25). An electrostatic ink jet recording head as defined in any one of claims 1 to 9 wherein said stripe protrusion (12) is made of a photosensitive material. An electrostatic ink jet recording head as defined in any one of claims 1 to 10 wherein said ejecting electrode (17) is disposed on top of said stripe protrusion (12). The electrostatic ink jet recording head as defined in any one of claims 1 to 11, wherein a top cover plate overlies the protrusions for defining an ink jet slit in the vicinity of the front edge of the head body and the ink chamber adjacent to the ink jet slit.

Inventors: HAGIWARA YOSHIHIRO, MINEMOTO HITOSHI, MIZOGUCHI TADASHI, SHIMA KAZUO, SUETSUGU JUNICHI, TAKEMOTO HITOSHI, UEMATSU RYOSUKE, YAKUSHIJI TORU, HAGIWARA, YOSHIHIRO, MINEMOTO, HITOSHI, MIZOGUCHI, TADASHI, SHIMA, KAZUO, SUETSUGU, JUNICHI, TAKEMOTO, HITOSHI, UEMATSU, RYOSUKE, YAKUSHIJI, TORU
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1997
Main classification: B41J2
IPC codes: B41J2