Ink jet printer recording head
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an ink jet printer recording head which records an image on a recording medium by ejecting ink droplets. 
     2. Prior Art 
     Disclosed in Japanese Patent Examined Publication No. 8957/1985 is an on-demand type ink jet printer, in which piezoelectric conversion members are formed behind and slightly spaced apart from a substrate having nozzles thereby leaving a small gap between each piezoelectric conversion member and the substrate. A voltage is applied to the piezoelectric conversion members to cause them to be displaced thereby pressuring ink present between the piezoelectric conversion members and the substrate to attendantly eject the pressured ink through the nozzles in the form of ink droplets. 
     Compared to a general ink jet printer in which the ink is ejected by changing the volume of the ink chamber using the piezoelectric conversion members and by guiding the ink within the ink chamber to nozzles, the ink jet printer described above has each piezoelectric conversion member positioned adjacent to each nozzle and is displaced in the axial direction of the nozzle, so that it not only shortens the flow path of the ink and enhances the ink ejection efficiency and stability but is advantageous in that the piezoelectric conversion member can be operated without such disturbances as infiltration of air bubbles or dust in the ink. 
     In such an ink jet printer it is the gap between each piezoelectric conversion member and the substrate that plays an important role in determining the ejection speed and amount of discharged ink droplets, or the ejection response. In general, each piezoelectric conversion member, with its construction involving a laminate formed of a piezoelectric element and a metal plate, is subject to warp due to differences in thermal expansion coefficients of these two materials, thereby making it impossible to maintain a constant distance between the piezoelectric conversion member and the substrate. As a result, there exists not only the problem that the level of density fluctuates depending on the temperature, but also the extreme difficulty of making the piezoelectric conversion member thin thus losing the advantage of reducing the required drive voltage. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is, therefore, to provide an ink jet printer recording head that allows a thinner piezoelectric conversion member to be formed that can be driven at a lower voltage, and wherein a constant gap can be maintained between each piezoelectric conversion member and the substrate. 
     To achieve the above object, the present invention provides an ink jet printer recording head, in which a pair of positively and negatively polarized comb-type electrodes are formed on the surface of an ink pressuring member equals vibrating plates made of a piezoelectric material to thereby allow each ink pressuring member to be directly deformed by applying a voltage across both comb-type electrodes. 
     Another object of the present invention is to deform each ink pressuring member more efficiently. To this end, both the upper surface and the lower surface of the ink pressuring member are provided with a pair of positive and negative comb-type electrodes. Alternatively, the comb-type electrodes can be formed in either the upper surface only or the lower surface only, so long as care is taken regarding the orientation of the comb-type electrodes in the vicinity of the pressuring member facing the nozzle, as well as the differently oriented comb-type electrodes in other regions of the ink pressuring member. 
     Still another object of the present invention is to construct the ink jet printer recording head more simply. To this end, in the present invention, the surface of each ink pressuring member formed of a piezoelectric material is not only provided with a pair of positively and negatively polarized comb-type electrodes, but is also provided with an ink ejecting nozzle to cause ink droplets to be ejected directly from the deformed ink pressuring member, thus obviating the need for a nozzle plate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partially exploded view showing a recording head according to a first embodiment of the present invention; 
     FIG. 2 is a view showing an ink jet printer having the recording head of the present invention; 
     FIGS. 3a and 3b are views respectively showing the states before and after deformation at the middle region of the vibrating plate of FIG. 1. 
     FIGS. 4a and 4b are views respectively showing the states before and after deformation at both ends of the vibrating plate FIG. 1; 
     FIGS. 5a and 5b are views respectively showing the operation of ejecting ink by the vibrating plate of FIG. 1; 
     FIG. 6a is a view showing the main portion of a recording head according to a second embodiment of the present invention; and FIG. 6b is a view showing the back of a vibrating plate thereof; 
     FIG. 7 is a view showing the operation of ejecting the ink by the vibrating plate of FIG. 6a; 
     FIG. 8a is a view showing the main portion of a recording head according to a third embodiment of the present invention; and FIG. 8b is a view showing the back of a vibrating plate thereof; 
     FIG. 9a is a view showing the main portion of a recording head according to a fourth embodiment of the present invention; and FIG. 9b is a view showing the back of a vibrating plate thereof; 
     FIG. 10 is a view showing the operation of ejecting the ink by the cantilever-type vibrating plate of FIGS. 9a and 9b; 
     FIG. 11 is an exploded view showing a recording head according to a fifth embodiment of the present invention; 
     FIGS. 12a and 12b are views respectively showing electrode patterns to be formed on a vibrating plate thereof; 
     FIG. 13 is a view showing the operation of ejecting the ink by the vibrating plate; 
     FIG. 14a and 14b are views respectively showing a recording head according to a sixth embodiment of the present invention and its operation of ejecting the ink. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a partially enlarged view showing a typical recording head according to a first embodiment of the present invention. 
     This recording head is to be applied to an ink jet printer shown in FIG. 2. The ink jet printer recording head 10 is constructed so that it travels in the axial direction of a platen 4 and records a desired image on the surface of a recording sheet 3 that is forwarded by rotation of the platen 4 in the direction indicated by the arrow. 
     This recording head 10 comprises a nozzle plate 11 and vibrating plates 14 made of a piezoelectric material. The nozzle plate 11 is provided with a plurality of nozzles 12 arrayed from the upper left to the lower right in FIG. 1. On the nozzle plate 11 are 10-20 μm thick gap plates 13 that are bonded so as to interpose the nozzle 12 therebetween. These gap plates 13 may be unitized with the nozzle plate 11. 
     Each vibrating plate 14 serves as an ink pressuring member that pressures that ink introduced into a gap formed between the nozzle plate 11 and the vibrating plate itself, and ejects the pressured ink on a recording sheet from its nozzle 12. The vibrating plate 14 is stretched over the gap plates 13 such that a predetermined gap is formed with the nozzle plate 11. Each vibrating plate 14 is constructed having a thickness of 100 μm, and a width of only 0.34 mm. The narrower width of each vibrating plate 11 corresponds to the pitch between the nozzles 12 so as to allow each nozzle 12 to eject ink independently of the others. Each vibrating plate is bonded on the gap plates 13. It may be arranged by bonding a large width vibrating plate on the gap plates 13 and by cutting this vibrating plate with a dicing saw or photoetching it separately into a plurality of narrow width vibrating plates 14 corresponding to their respective nozzles. 
     Each vibrating plate 14 has electrodes integrally patterned on an upper surface 14a opposite to the nozzle plate 11 so that a positive comb-type electrode 17 and a negative comb-type electrode 18 can be meshed with each other. These electrodes 17, 18 are connected to a power supply. 
     The comb-type electrodes 17, 18 serve to deform the vibrating plate 14 by a voltage applied therebetween and it is desirable to set the pitch between their teeth to about half the thickness of the vibrating plate 14. The comb-type electrodes 17, 18 are formed so that their teeth extend in a longitudinal direction along the vibrating plate 14 at the middle region Lc right above the nozzle 12, but extend in a horizontal direction across the vibrating plate 14 at both end regions Ls. 
     Upon application of a unidirectionally pulsed voltage between the comb-type electrodes 17, 18, an electric field as shown in FIG. 3a is produced in directions of arrow E between both electrodes 17, 18 at the middle region Lc, and strains are produced both in the direction of arrow y which is parallel to the electric field and in the directions of arrows x and z which are perpendicular thereto, respectively. With respect to the strains produced in the directions along (x direction) and across (z direction) the vibrating plate 14, the electric field intensity is larger on the upper surface 14a where the electrode pattern is formed. This causes the upper surface 14a to contract both lengthwise and widthwise at the middle region Lc of the vibrating plate 14, thereby producing larger strains lengthwise. As a result, the vibrating plate is bent in such a way that the upper surface, i.e., the surface 14a opposite to the nozzle plate 11, concaves as shown in FIG. 3b. 
     On the other hand, at the end regions Ls, when an electric field is produced in the direction of arrow E between both electrodes 17, 18 as shown in FIG. 4a, strains are likewise produced both in the direction of arrow y which is parallel to the electric field and in the directions of arrows x and z which are perpendicular thereto, respectively. And with respect to the strains produced in the direction of arrow y, the electric field intensity is larger on the surface 14a where the electrodes 17, 18 are formed, and this causes the surface 14a to be elongated both lengthwise and widthwise, thereby producing larger strains lengthwise thereon. As a result, the vibrating plate 14 is bent downward with each gap plate 13 as a fulcrum; i.e., the surface 14a opposite to the nozzle plate 11 convexes as shown in FIG. 4b. 
     Thus, each vibrating plate 14 is deformed in such a way that the surface 14a concaves at the middle region Lc and convexes at the end regions Ls, respectively. As a result, the vibrating plate 14 pressures the ink present between the nozzle plate 11 and itself and ejects the pressured ink in the form of ink droplets on a recording sheet (not shown) from its nozzle 12. 
     The maximum vibrating frequency of each vibrating plate 14 is determined by the Young&#39;s modulus of a piezoelectric material and the dimensions of the vibrating plate itself. According to an experiment, it was possible to eject the ink at a frequency of about 6 KHz. Although the comb-type electrodes 17, 18 in this embodiment are formed so as to be oriented differently at the middle region Lc and at the end regions Ls of each vibrating plate 14, they may be formed only at the middle region Lc to cause the vibrating plate 14 to be bent in a desired direction. 
     FIGS. 6, 8, and 9 show embodiments of the present invention in which the comb-type electrodes are formed on both surface of each vibrating plate so that the vibrating plate can be bent more efficiently. 
     In a second embodiment shown in FIG. 6, each of vibrating plates 24 is fixed while stretched over gap plates 23 that are arranged on both sides of a nozzle plate 21 in a manner similar to that in the first embodiment shown in FIG. 1. 
     On an upper surface 24a opposite to the nozzle plate 21 of the vibrating plate 24, as shown in FIG. 6a, a pair of comb-type electrodes 27a, 28a are formed so as to be meshed with each other along the vibrating plate 24 at the middle region which is right above a nozzle 22. On the lower surface 24b facing the nozzle plate 21, as shown in FIG. 6b, are a pair of comb-type electrodes 27b, 28b connected to the electrodes, 27a, 28a arranged on the upper surface through an end surface 24c. In contrast to the electrode pattern formed on the upper surface 24a, these comb-type electrodes 27b, 28b are patterned so that they are meshed with each other in the longitudinal direction at both ends of the vibrating plate 24. 
     Upon application of a voltage across both electrodes 27, 28 formed on the upper and lower surfaces 24a and 24b, the comb-type electrodes 27a, 28a at the middle region cause that region to contract, while on the lower surface 24b, the comb-type electrodes 27b, 28b disposed at both ends cause these ends to contract, as previously described with reference to FIG. 3. As a result, each vibrating plate 24 is bent with the middle region toward the nozzle 22 as shown in FIG. 7 thereby pressuring the ink in that region and ejecting it in the form of ink droplets through the nozzle 22. 
     A third embodiment shown in FIG. 8 has positively and negatively polarized comb-type electrodes 37, 38 formed on both upper and lower surfaces so as to face each other across a vibrating plate 34. That is, on an upper surface 34a opposite to a nozzle plate 31 of the vibrating plate 34 are both comb-type electrodes 37a, 38a formed at both ends, whereas on the lower surface 34b facing the nozzle plate 31 are both comb-type electrodes 37b, 38b patterned at the middle region, as shown in FIG. 8b. These electrodes 37b, 38b are connected to the electrodes 37a, 38a through an end surface 34c of the vibrating plate 34. 
     Also in this embodiment, similar to FIG. 7, on the upper surface 34a of the vibrating plate 34, the comb-type electrodes 37a, 38a at both end regions cause such regions of the upper surface 34a to be elongated while the electrodes 37b, 38b at the middle region cause such region of the lower surface 34b to be elongated. As a result, the vibrating plate 34 is bent toward the nozzle 32. 
     FIG. 9 shows a fourth embodiment in which each of vibrating plates is formed as a cantilever and has comb-type electrodes arranged on both surfaces thereof. 
     On a gap plate 43 fixed on one surface of a nozzle plate 41 is the base end of each vibrating plate 44 that extends so that its free end covers a nozzle 42. On the vibrating plate 44 are a pair of positive and negative comb-type electrodes 47a, 48a on an upper surface 44a opposite to the nozzle plate 41 so that these electrodes face each other across the vibrating plate. Further, on the lower surface 44b facing the nozzle plate are comb-type electrodes 47b, 48b formed so that their teeth are meshed with each other along the vibrating plate 44 as shown in FIG. 9b. These electrodes 47b, 48b are connected to the electrodes 47a, 48a through an end surface 44c of vibrating plate 44. 
     In this embodiment, upon application of a voltage across these electrodes 47, 48, the comb-type electrodes 47a, 48a on the upper surface 44a cause this upper surface to elongate while the comb-type electrodes 47b, 48b on the lower surface 44b cause the lower surface to contract, as shown in FIG. 10. As a result, the vibrating plate 44 is bent with its free end bowed toward the nozzle 42 to thereby pressure the ink present between the nozzle plate 41 and the vibrating plate 44 and eject the pressured ink in the form of ink droplets through the nozzle 42. 
     FIGS. 9a and 9b show the comb-type electrodes 47, 48 formed on both upper and lower surfaces of each cantilever-type vibrating plate 44. However, the advantage similar to that described above may be provided by forming the comb-type electrodes 47, 48 only on the upper surface 44a opposite to the nozzle plate 41. 
     In contrast thereto, a fifth embodiment which is shown in FIG. 11 et. seq. has each nozzle formed on each vibrating plate itself to make the recording head simpler and thinner in design. 
     FIG. 11 shows the general construction of this fifth embodiment. The recording head 50 comprises a frame 51 and an ink pressuring member 55 fixed on the frame. The frame 51 is formed as a plate-like block that is E-shaped in cross section extending in the direction of arraying the nozzles 52. On both sides of the frame are projections 51a supporting the ink pressuring member 55. Between the ink containing grooves 51b in the middle region facing the nozzles 52 of the ink pressuring member 55, there is formed integrally with frame 51 a gap forming projection 51c that creates a gap of about 10 μm together with the vibrating plate 54. 
     The ink pressuring member 55 comprises a plurality of vibrating plates 54 separated from each other by slits 56. On each vibrating plate 54 is a nozzle 52 arranged at the middle in the longitudinal direction thereof. 
     As shown in FIG. 12a, each vibrating plate 54 has a positive comb-type electrode 57 connected to an individual signal electrode 57c and a negative comb-type electrode 58 connected to the common electrode 58c on a surface 54a that does not come in contact with the ink. These comb-type electrodes 57, 58 are formed so that one or more comb tooth-like electrodes extending inward from both ends of the vibrating plate 54 can be meshed at the middle region Lc where a nozzle 52 is formed. 
     In FIG. 11, reference numeral 59 designates a seal body made of a soft resin material bonded on the upper surface of the ink pressuring member 55 to prevent leakage of the ink from the slits 56. This seal body 59 is provided with holes 59a not to hinder the ejection of ink from the nozzles 52. 
     In this embodiment, upon application of a voltage across the common electrode 58c and one or more selected signal electrodes 57c, the respective comb-type electrode 57, 58 on the vibrating plates 54 connected to these common and signal electrodes cause these selected vibrating plates 54 to be bent toward the gap forming projection 51c as shown in FIG. 13 thereby to increase the pressure on the ink on the periphery of the gap forming projection 51c and eject the pressured ink toward a recording sheet from the nozzles of these selected vibrating plates 54. 
     FIG. 12b shows another embodiment of the electrode pattern to be formed on each vibrating plate. 
     In this embodiment, a positive comb-type electrode 67 and a negative, comb-like electrode 68 are formed so as to face each other on both ends except for the middle region Lc of each vibrating plate 64 where a nozzle 62 is formed. 
     It is noted that the fifth embodiment shown in FIG. 11 is an example in which each vibrating plate 54 having the nozzle 52 is separated by the slits 56 so as to allow the vibrating plates to operate independently of each other. 
     FIG. 14 shows a sixth embodiment so constructed that only selected nozzle forming portions can be deformed using a single ink pressuring member. 
     An ink pressuring member 75 fixed on supporting projected groove portions 71a of a frame 71 is made up of a plate body formed of a single piezoelectric material. On ink pressuring member 75 are a plurality of nozzles 72 arrayed in a direction along a gap forming projected groove portion 71c located at the middle of the frame 71. On an upper surface 75a that does not come in contact with the ink pressuring member 53 are a positive annular comb-type electrode 77 connected to an individual signal electrode 77c and a negative electrode 78c formed concentrically with each nozzle 72 so as to surround the nozzle 72. 
     Upon application of a voltage across the selected one or more signal electrodes 77c and the common electrode 78c, both annular comb-type electrodes 77, 78 cause a middle region Lc surrounding the corresponding nozzles 72 to be bent toward the gap forming projected groove portion 71c, thereby pressuring the ink in that region and ejecting the pressured ink in the form of ink droplets through the corresponding nozzles 72.