ON-DEMAND TYPE INK JET PRINT HEAD

Abstract: A vibrating plate (10) made of a piezoelectric plate having a metal plate (16) and an electrode layer (14) provided on either surface thereof is secured by an adhesive or the like to a rectangular base member (2) having a rectangular window (4). The vibrating plate is cut into strips. Each strip is further cut at a position facing the base member window in a direction substantially orthogonal to the first cut line (20). One end of each strip is secured to the base member and the other end is left free; thus, reed pieces (23) are formed in cantilever from. A nozzle forming plate is disposed in alignment with the reed pieces to form a drive assembly. The drive assembly is immersed in ink. When an electric field is imposed on the piezoelectric plate, the reed piece (23) curves, and when it is removed, the me­ tal plate recovers by virtue of its resiliency. During this operation, a resultant dynamic pressure acts on the ink to let fly an ink drop through a nozzle opening formed in the nozzle forming plate. Since each reed piece is formed by cutting the vibrating plate from one edge to the other so that all the reed pieces are separated from each other, the vibration of and an electrical signal to one reed piece can never disturbe other adjacent reed pieces; thus, vibration is stably generated and the ink drop is allowed to fly re­ liably.

Description: This invention relates to an on-demand type ink jet print head which upon receipt of print data lets fly ink held in an ink tank in the form of liquid drops to create dots on a recording paper with this liquid ink. Ink jet print heads of the on-demand type are classified into three types: a first type which is called a bubble jet type includes a heater provided at the point of a nozzle and lets fly ink drops by means of an expansion pressure caused by vaporizing the ink using the heat of the heater; a second type includes a piezoelectric element provided in a vessel defining an ink reservoir and lets fly ink drops by means of a variation in pressure of the ink reservoir caused by the deformation of the piezoelectric element; and a third type includes a reed piece with a piezoelectric element provided in an ink reservoir having an ink drop eject orifice formed therein in confronting relation to the ink drop eject orifice and lets fly ink drops by means of a pressure generated upon deformation of the reed piece. An ink jet print head of the third type disclosed in Japanese Patent Publication No. 60-8953 (corresponds to US-A-4072959) is configured such that a vessel defining an ink tank has a plurality of nozzle openings formed in the wall surface thereof, a reed piece with a piezoelectric plate is provided in alignment with each nozzle opening, and each reed piece is actuated by means of a print signal. The foregoing print head operates in such a manner that with the reed piece previously deformed in the backward direction with respect to the nozzle opening by means of an electrical signal, the ink is allowed to fly through the nozzle opening in the form of a liquid drop by means of a dynamic pressure generated when the reed piece undergoes rapid displacement in the ink due to its resilient power of deformation exerted as the level of the electrical signal becomes zero. In the foregoing third type print head, the piezoelectric element shaped like a reed is supported in cantilever form to provide a large amount of displacement; thus, the features are that the ink can be ejected with high efficiency, the ejection operation cannot be influenced by gas, dust, etc. included in the ink, and the reliability of operation is very high. However, each reed piece is formed by machining the piezoelectric plate into the form of comb teeth; thus, all the reed pieces are connected on their root side to the same piezoelectric plate mechanically and electrically. Accordingly, adjacent reed pieces influence each other mechanically and electrically to cause mutual interference, resulting in a problem of the vibration mode of each reed piece becoming unstable. Further, the problems in manufacture are that the reed piece is easy to break and provides a bad yield since the reed piece is formed by making cut lines midway in a piezoelectric raw plate with leaving a root portion unmachined and that it is difficult to attach an electrical signal wire to an electrode since the electrode is formed on the free vibration side of the reed piece for the purpose of avoiding electrical mutual interference. It is an object of the present invention to provide a novel on-demand type ink jet print head capable of stably letting fly ink drops. It is a further object of the present invention to provide an on-demand type ink jet print head capable of decreasing electrical and mechanical influence between reed pieces as much as possible. It is another object of the present invention to provide an on-demand type ink jet print head capable of facilitating a cutting process of reed pieces to remarkably increase yield in manufacture. It is still another object of the present invention to provide an on-demand type ink jet print head capable of attachment of electrical signal wires on the fixed side to simplify the work of attaching electrodes to reed pieces. It is a still further object of the present invention to provide a novel on-demand type ink jet print head capable of arranging nozzle openings at a pitch smaller than the width of reed pieces. These objects are achieved with an ink jet print head as claimed. A novel print head according to the present invention is characterized in that a vibrating plate is made of a piezoelectric plate having an electrode layer formed on one surface thereof and a resilient metal plate provided on the other surface, this vibrating plate is bonded and secured to a rectangular base member with a through hole formed in a central portion thereof, the vibrating plate is cut into a plurality of strips, and these strips are further cut in the widthwise direction thereof to form a plurality of reed pieces. Therefore, the reed pieces are electrically and mechanically independent of each other, thus never suffer any electrical and mechanical interference. Other objects of the present invention will become apparent from the following description of specific embodiments of the invention in conjunction with drawings, wherein: Fig. 1 is an exploded perspective view showing an embodiment of a drive assembly of an ink jet print head according to the present invention; Figs. 2a through 2d are views showing the process of forming reed pieces which are significant in the present invention, in which Fig. 2a shows the relationship between a base member and a vibrating plate, Fig. 2b shows the vibrating plate secured to the base member, Fig. 2c shows strips formed by cutting the vibrating plate, and Fig. 2d shows reed pieces formed from the vibrating plate; Fig. 3 is a sectional view showing a nozzle opening formed in a nozzle forming plate; Fig. 4 is a sectional view showing an embodiment of the ink jet print head according to the present invention; Figs. 5a through 5c are views explanatory of the behavior of an ink drop adhering to the circumference of the nozzle opening; Fig. 6 is a perspective view showing an important section of a printer including the ink jet print head according to the present invention; Fig. 7 is a perspective view showing an embodiment of the structure wherein a metal plate and an electrode layer of the reed piece are connected with conductor bands formed on the base member; Figs. 8a and 8b are exploded perspective views showing another embodiment of the drive assembly used in the ink jet print head according to the present invention, in which Fig. 8a shows the relationship between the base member and the vibrating plate and Fig. 8b shows the reed pieces formed from the vibrating plate; and Fig. 9 is a sectional view showing another embodiment of the process of securing the nozzle forming plate. Fig. 1 shows an embodiment of a drive assembly which is an important section of an ink jet print head. A base member designated by reference numeral 2 is made of an electrical insulating material, such as ceramic or glass. Base member 2 has a rectangular through hole 4 formed in a central portion thereof as shown in Fig. 2a and thus defines a frame. One edge 6 of this frame has conductor bands 8 formed on the surface thereof in alignment with reed pieces 23, these conductor bands serving as terminals for electrically connecting and mechanically securing a signal supply cable and the reed pieces together. The reed pieces 23 have their respective one ends secured to the conductor bands 8 on the base member, with their respective other ends defining free ends. The process of firming the reed pieces will be described with reference to Figs. 2a through 2d. In Fig. 2a, a member designated by reference numeral 10 is a vibrating plate before being divided into the reed pieces. One surface, facing the base member 2, of a piezoelectric plate 12, which is formed by molding a piezoelectric material capable of curving upon imposition of an electric field, such as lead zirconate, into a thin plate, is provided with an electrode layer 14 which is formed by vapor deposition, nonelectrode plating, chemical plating or sputtering using a conductive material such as nickel. The other surface, facing nozzle openings 32 (Fig. 1), has a metal plate 16 of rich resiliency bonded thereto by an eutectic bonding process or using a conductive adhesive. The vibrating plate 10 is placed on the base member 2 as shown in Fig. 2b such that the conductor bands 8 are partly exposed to allow connection of a connecting cable of external units. The vibrating plate is dimensioned such that no marginal portion protrudes from the other three edges of the base member 2. With the electrode layer 14 facing the base member 2, the margin of the vibrating plate 10 is secured to the base member 2 as shown in Fig. 2b by applying a conductive adhesive to the conductor bands 8 of the base member 2 and an ordinary high-molecular adhesive 18 free of electrical conductivity to the other three edges not provided with the conductor bands 8 (Fig. 2a). Then, as shown in Fig. 2c, the vibrating plate is cut by a diamond cutter to a depth greater than the thickness of the vibrating plate to form cut lines whose spacing is, for example, 0.2 mm, corresponding to the desired width of the reed piece, so that slits 20 are formed which pass through outward at least on the side of the conductor bands 8. As a result, strips 21 are formed from the vibrating plate 10, each strip comprising integrally the metal plate 16, piezoelectric plate 12 and electrode layer 14. In the foregoing process of slit forming, since the vibrating plate 10 is secured to the base member 2, the strips 21 can never break nor split even under the vibration of the diamond cutter. In the course of the foregoing strip cutting process, grooves are also formed between the adjacent conductor bands, so that the conductor bands 8 of the base member 2 are separated from each other and become independent terminal portions. Then, as shown in Fig. 2d, a similar process to the above is performed in the widthwise direction of the strips 21 to form a slit 22; as a result, each of the strips 21 is changed to a vibrating piece in the form of a cantilever whose one end on the side of the conductor bands 8 serves as a fixed end and the other end as a free vibrating end, or, reed pieces 23 are finished. Referring again to Fig. 1, a band-shaped member designated by reference numeral 24 is a spacer which is made of a conductive material and provided to set the spacing between the free ends of the reed pieces 23 and nozzle openings 32 in the quiescent state to no more than 200 µm (to this distance the ink can rise by virtue of surface tension), normally within the range of 5 to 30 µm. The spacer 24 is extended parallel to the arrangement direction of the reed pieces 23 and secured by a conductive adhesive at a position where it gives no influence to the vibration of the reed pieces 23, or in the vicinity of the position facing the conductor bands 8 of the base member 2. A member designated by reference numeral 30 is a nozzle forming plate which has one row of nozzle openings 32 formed therein in alignment with the points or free ends of the reed pieces 23. As shown in Fig. 3, each nozzle opening 32 is shaped like a funnel or progressively widened downward on the one side facing the reed piece 23 and on the other side, is protruded from the surface of the nozzle forming plate 30 by a height L. It is desirable that the protrusion height L from the surface of the nozzle forming plate 30 be set to 10 to 150 µm and the thickness D of the nozzle point be set to no more than 150 µm. Such a nozzle opening can be formed concurrently with a substrate forming step by an electro forming process. The nozzle forming plate 30 is secured to the surface of the spacer 24 after the distances between the nozzle openings 32 and the free ends of the reed pieces 23 are made uniform. Consequently, the spacing between the reed piece 23 and the nozzle opening 32 is kept to a distance determined by the thickness of the spacer 24, and the respective metal plates 16 of the reed pieces 23 are electrically connected in common by the spacer 24. After the completion of attaching of the nozzle forming plate 30, the individual signal wires of a flexible cable for connection with a drive circuit not shown are connected and secured to the corresponding conductor bands 8 of the base member 2, and a common earth wire is connected and secured to the spacer 24. This connecting process is carried out to connect the lead wires to the conductor bands 8 of the base member 2, thus is very easy as compared with the process of connecting the signal wires to the reed pieces. Fig. 4 shows an embodiment of the ink jet print head including the foregoing drive assembly. A casing designated by reference numeral 40 is divided into two chambers 46 and 48 by a partition plate 44 having a through hole 42 formed in a lower section thereof, one chamber 46 serving as an ink tank and the other chamber 48 as a drive assembly accommodating room. A wall 50 forming part of the drive assembly accommodating room 48 has a through hole 52 formed therein, the nozzle forming plate 30 of the drive assembly is disposed in tight contact with the through hole 52, and in this condition, the base member 2 is secured to the partition plate 44. Reference numeral 55 designates a heater accommodated in the ink tank 46, which is included to maintain the ink temperature at a level best suited for printing. In the thus configured print head, when an ink 53 having no electrical conductivity, such as oil ink, is charged in the ink tank 46 of the casing 40, it flows through the through hole 42 of the partition plate 44 into the drive assembly accommodating room 48, rises up the gap between the nozzle forming plate 30 and the reed pieces 23 by virtue of surface tension, and reaches a liquid level enough to immerse the point of the reed pieces 23 in the ink. In this condition, when a print signal is received through a signal cable 54, it is supplied through the conductor band 8 of the base member 2 to the electrode layer 14 of a respective reed piece 23, so that an electric field is generated between the electrode layer and the metal plate 16 connected through the spacer 24 to the other polarity, which is imposed on the piezoelectric plate 12. As a result, the piezoelectric plate 12 forming part of the reed piece 23 curves in unison with the metal plate 16 and electrode layer 14 (as illustrated by the dotted line in Fig. 4) so that the free end shifts backward from the nozzle opening 32 with the one end on the side of the conductor band 8 acting as a supporting point. In this condition, when the voltage level becomes zero, the deformation retaining power of the piezoelectric plate 12 instantly disappears; thus, a mechanical energy accumulated in the metal plate 16 is released, so that the reed piece 23 rapidly returns toward the nozzle opening 32. In the course of the foregoing operation, the reed piece 23 exerts a dynamic pressure on the ink 53 held in contact therewith, so that the ink 53 is allowed to fly out through the nozzle opening 32 in the form of a liquid drop. Here, since the individual reed pieces 23 are completely separated from each other by the slits 20 and secured to the base member 2 in the form of a rigid body, the vibration of one reed piece 23 can never influence other adjacent reed pieces 23. Further, since the individual piezoelectric plates 12 forming part of the corresponding reed pieces 23 are completely separated from each other by the slits 20, any signal to the electrode layer 14 or metal plate 16 of one reed piece 23 can never act on the piezoelectric plate forming part of a different reed piece; thus, no electrical and similar interference is caused. On the other hand, as shown in Fig. 5a, a portion of the ink drop ejected through the nozzle opening 32 adheres to the point of the nozzle opening 32 by virtue of surface tension, inevitably resulting in liquid drops 33, and these liquid drops gradually expand on the surface of the nozzle forming plate 30 by virtue of gravity and/or surface tension (Fig. 5b). However, since the point of the nozzle opening 32 protrudes from the surface of the nozzle forming plate 30 in the flying direction of the ink drop, ink 35 adhering to the surface of the nozzle forming plate 30 can never stop up the nozzle opening 32; thus, the ink drop is allowed to fly in the direction defined by the nozzle opening 32. Also, the ink 35 adhering to the plate surface can be collected through a flow path 37 (Fig. 5c). As shown in Fig. 6, the thus configured print head 66 is mounted on a carriage 68 which in turn is seated movably on guide members 62 and 64 disposed parallel to a platen 60. Therefore, while moving in the widthwise direction of a printing paper 65 stretched on the platen surface by a paper bail 67 and a paper feed roller 69, the print head 66 can create dots corresponding to print data by letting fly the ink drops toward the printing paper. Fig. 7 shows an embodiment of the structure wherein the electrode layer and metal plate of the reed piece are connected with external signal wires. A member designated by reference numeral 70 is configured, similarly to the abovementioned reed piece 23 (Fig. 2d), such that its one surface has a metal plate 74 secured thereto by an eutectic bonding process or using a conductive adhesive and the other surface has an electrode layer 76 formed thereon by an electro forming process or a vapor deposition process. A member designated by reference numeral 78 is a base member similar to the abovementioned base member. This member 78 is formed, on its one edge to which the reed piece 70 is secured, with electrically-separated conductor bands 80 and 82 dimensioned so as to divide the width of the reed piece 70 into two. On the other hand, a portion, facing the conductor band 82, of the electrode layer 76 of the reed piece is formed with an L-shaped notch 84, this leaving an island-like terminal portion 86 on the end side of the reed piece 70. This terminal portion 86 is connected to the metal plate 74 by a conductor member 88 formed so as to cover the end of the reed piece. By applying a conductive adhesive to the conductor bands 80 and 82 of the base member 78 and securing the reed piece 70 to these bands, the conductor layer 76 is electrically and mechanically secured to the one conductor band 80, and the metal plate 74 to the other conductor band 82 via the conductor member 88 and the terminal portion 86. With the lead wires of a cable 90 connected to the opposite ends of the conductor bands 80 and 82, when signals are supplied through the two conductor bands 80 and 82 to the electrode layer 76 and the metal plate 74, the reed piece 70 can curve and recover. Although each reed piece of the above embodiment has been described as a unit member for convenience of explanation, the same result can be obtained by previously forming the terminal portion 86 and the conductor member 88 on the abovementioned vibrating plate (Fig. 2a), securing the base member 78 to the above, and forming slits to divide into the reed pieces. Figs. 8a and 8b show a second embodiment of the present invention. A member designated by reference numeral 100 is a vibrating plate similar to that shown in Fig. 2a, which is constructed such that an electrode layer 104 is formed on one surface of a thin piezoelectric plate 102, and a metal plate 106 serving as a spring element and an electrode is secured to the other surface so as to ensure conductive relation. This vibrating plate 100 has an L-shaped notch, similar to that shown in Fig. 7, formed in its portion facing one, e.g. 118, of paired conductor bands 118 and 120 formed on a base member 110 hereinafter described, this leaving a terminal portion which is connected through a conductor member 103 to the metal plate. The base member 110 is made of a rigid and electrical-insulating material, such as glass or ceramic, and formed in its central portion with a rectangular through hole 112. The surfaces of two opposing edges 114 and 116 of the base member have two groups of conductor bands 118 and 120 electrically separated from each other, the two occupying the width of each reed piece being formed, that are formed by vapor deposition of metal or printing with conductive adhesive. As shown in Fig. 8b, the vibrating plate 100 is disposed such that its one surface where the electrode layer 104 is formed faces the conductor bands 118 and 120 of the base member 110, and then secured to the conductor bands 118 and 120 using a conductive adhesive and to the other two edges using an ordinary adhesive. After the base member 110 and the vibrating plate 100 are secured integrally together as described above, slits 122 are formed at an angle to the direction orthogonal to the arrangement lines A-A and B-B of the nozzle openings, and a second slit 124 is formed so as to pass midway between the two nozzle arrangement lines A-A and B-B. Consequently, the vibrating plate 100 is changed to two groups of separated reed pieces 126 and 128 each having one end secured to either edge 114 or 116 of the base member 110 with the other end being made free. The metal plate 106 of each reed piece 126, 128 is connected through a connecting member, a terminal portion, and a conductive adhesive to the conductor band 118, and the electrode layer 104 through an adhesive to the conductor band 120. A member designated by reference numeral 130 is a nozzle forming plate which has nozzle openings 132 and 134 formed therein in zigzag form in alignment with the free ends of the reed pieces 126 and 128, and is formed, at its portions coming to contact with the fixed side of the reed pieces, with thick portions 136 to define the spacing between the nozzle openings 132 and 134 and the free ends of the reed pieces. With the thick portions 136 of the nozzle forming plate 130 arranged so as to face the fixed side of the reed pieces 126 and 128, the nozzle openings 132 lying on the first arrangement line A-A come to face the free ends of the reed pieces 126 secured to the one edge 114 of the base member 110, and the nozzle openings 134 lying on the second arrangement line B-B come to face the free ends of the reed pieces 128 secured to the other edge 116 of the base member. In the thus configured print head, without decreasing the widthwise size of each reed piece 126 and 128, the distance W between the nozzle openings can be cut down, shortening the inter-spacing of the dots; thus, the number of dots creatable per length on the printing paper can be increased. That is, the nozzle openings can be formed or arranged at a pitch smaller than the width of the reed piece. Of course, the second slit may be formed orthogonally to the first slits. However, it is preferable that the angle of intersection between the second slit and the first slits be decreased and the nozzle openings of both rows be formed in the nozzle forming plate at a one-half pitch shift. Although the above embodiment includes, instead of using a spacer, the thick portions formed on both edges of the nozzle forming plate to adjust the spacing between the nozzle forming plate and the reed pieces, the same effect can also be obtained by making flat the nozzle forming plate and securing it using a spacer of given thickness. Although the foregoing embodiments regulate the spacing between the nozzle openings and the reed pieces by the use of the spacer or the thick portions formed on the nozzle forming member, a nozzle forming plate 150 as shown in Fig. 9 may be secured to reed pieces 154 by the use of a binding material which is prepared by mixing an adhesive 142 with beads 140 having an outer diameter substantially equal to a desired spacing. In this case, the spacing between the nozzle openings 152 and the free ends of the reed pieces 154 is defined by the outer diameter of the beads 140, this avoiding the work of interposing a spacer.

Claims: An on-demand type ink jet print head including a drive assembly comprising a vibrating plate (10; 100) made of a piezoelectric plate (12; 102) which deforms upon imposition of an electric field and has an electrode layer (14; 104) formed on one surface thereof and a metal plate (16; 106) of rich resiliency secured on the other surface, a base member (2; 110) formed, on the surface of one or both of two opposite edges thereof, with a plurality of conductor bands (8; 118, 120) identical in width to reed pieces (23; 126, 128) to be formed, said vibrating plate being secured at its margin to said base member with leaving edge portions of said conductor bands exposed, and being formed with a plurality of first slits (20; 122) which run from one edge of said vibrating plate to the other with each spacing between said first slits set equal to the width of said conductor bands and a second slit (22; 124) which intersects said first slits and runs in a portion of said vibrating plate opposite to said conductor bands of said base member, thereby forming a plurality of reed pieces (23; 126, 128), a nozzle forming plate (30; 130) having a plurality of nozzle openings (32; 132, 134) formed therein in alignment with the free ends of said reed pieces, said nozzle forming plate being secured to the fixed ends of said reed pieces with leaving a given spacing between said reed pieces and said nozzle forming plate to thereby form said drive assembly, and said drive assembly being accommodated in an ink tank (46, 48) so that ink is supplied between said reed pieces and said nozzle forming plate. A print head acording to claim 1, wherein said vibrating plate (10; 100) is secured at its portion facing said conductor bands (8; 118, 120) of said base member (2; 110) using a conductive adhesive. A print head according to claim 1, wherein a spacer (24) made of a conductive material is interposed between said reed pieces (23) and said nozzle forming plate (30), which serves as a common electrode for said reed pieces. A print head according to claim 1, wherein the spacing between said reed pieces (23; 126, 128) and said nozzle openings (32; 132, 134) is set within the range of 5 µm to 200 µm. A print head according to claim 1, wherein said vibrating plate (10) is dimensioned so that its edge facing said conductor bands (8) is so positioned as to allow said conductor bands to be exposed and that the other edge is positioned inside of the outer edge of said base member (2). A print head according to claim 1, wherein said ink tank (46, 48) is divided by a partition plate (44) with a through hole (42) formed therein into an ink storage chamber (46) and a drive assembly accommodating chamber (48), and said base member (2; 110) of said drive assembly is secured to said partition plate. A print head according to claim 1, wherein the point of each of said nozzle openings (32; 132, 134) protrudes a height of 10 µm to 150 µm from the surface of said nozzle forming plate (30; 130). A print head according to any of the preceding claims, wherein said conductor bands (118, 120) are formed on the surfaces of both of said opposite edges of the base member (110), and wherein said nozzle forming plate (130) has two rows of nozzle openings (132, 134) formed therein in alignment with the free ends of said reed pieces. A print head according to claim 8, wherein said first slits (122) are inclined with respect to said second slit (124), and said nozzle openings (132) of one row are staggered from those (134) of the other row. A print head according to claim 8, wherein said vibrating plate (100) is dimensioned so that its two edges facing said conductor bands (118, 120) are so positioned as to allow said conductor bands to be exposed and that the other two edges are positioned inside of the outer edge of said base member (110).

Inventors: KATAKURA, TAKAHIRO C/O SEIKO EPSON CORPORATION, KURASHIMA, NORIHIKO C/O SEIKO EPSON CORPORATION, MATSUZAWA, MASANAO C/O SEIKO EPSON CORPORATION, MIYAZAWA, YOSHINORI C/O SEIKO EPSON CORPORATION, NAKAMURA, OSAMU C/O SEIKO EPSON CORPORATION, NIYAZAWA, HISASHI C/O SEIKO EPSON CORPORATION, OMAE, HIDENORI C/O SEIKO EPSON CORPORATION
Applicants: SEIKO EPSON CORPORATION
Application year: 1989
Main classification: B41J2
IPC codes: B41J2