Inkjet recording apparatus

Abstract: Described is an inkjet recording apparatus having an inkjet head which comprises an ink supply port, a common ink cavity (8), a filter (51) having a plurality of filter channels communicating with the ink supply port at one end and the common ink cavity (8) at the other end, a plurality of ink ejection nozzles (4) each connected to the common ink cavity by a respective ink passage (6, 7), and a corresponding plurality of pressure generating means respectively associated with said ink passages, said pressure generating means being selectively drivable to eject ink droplets through the respective nozzles (4). At least a portion (55) of the walls defining the common ink cavity (8) is flexible.

Description: The present invention relates to an inkjet recording apparatus and, more particularly, to its inkjet head. Inkjet recording apparatus having an inkjet head for selectively ejecting ink droplets from a plurality of nozzles towards a recording medium in response to electric drive pulses are well known and commonly used. Generally, the inkjet head has a common ink cavity providing an ink source for the individual nozzles and being connected to each nozzle by a separate ink passage. Each ink passage includes an ejection chamber associated with a respective pressure generating device. The pressure generating devices are responsive to the electric drive pulses for selectively and temporarily increasing the pressure in the associated ejection chamber thereby causing ejection of ink droplets. Various types of pressure generating device are known in the art such as piezoelectric devices, thermal devices and electrostatic devices. The part of the ink passage connecting the ejection chamber to the common ink cavity has a cross-sectional area substantially smaller than that of the ejection chamber itself. This part will be referred to as orifice in the following. The common ink cavity serves as an ink supply buffer and is in turn connected, via an ink supply port, to a larger volume ink supply, i.e. an ink tank etc., typically external to the inkjet head. In manufacturing such inkjet heads it is common practice to etch grooves and recessions respectively corresponding to the common ink cavity, the ink passages and the nozzles into the surface of a first substrate which is then bonded to a second substrate as disclosed in EP-A-0 580 283 and JP-B-62-8316/1987 for instance. The document JP-B-8316/1987 discloses an inkjet recording apparatus according to the precharacterizing portion of claim 1. In this prior art a filter is provided between the ink supply and the common ink cavity to prevent foreign matters from entering the ink passages and possibly clogging the nozzles. The filter comprises a plurality of filter channels provided in parallel between an ink supply opening and the common ink cavity. Grooves for the filter channels are formed simultaneously with the grooves and recesses mentioned above by etching in the vertical direction of a glass substrate using a photoetching method. To fulfill the intended function, the cross sectional area of any filter channel must be smaller than the smallest cross sectional area of the ink passages and that of the nozzles themselves. In the prior art referred to above, however, the filter channels are formed simultaneously with the common ink cavity, the nozzles and the ink passages by an isotropic etching method, and the depth of the filter channels is therefore the same as the depth of the nozzles and the other portions of the ink paths between the filter and the nozzles. As a result, the size of foreign particulate passing through the filter may be the same size as that of the nozzle and orifices. The probability of a nozzle or orifice becoming clogged is therefore high, and the filter function of the prior art not satisfactory. Inkjet heads which employ a silicon substrate allowing use of the more precise anisotropic etching are disclosed in, for example, EP-A-0 479 441, EP-A-0 580 283 and in EP-A-0 634 272, EP-A-0 629 502 and EP-A-0 629 503 (the latter three documents forming prior art according to Art. 54(3) EPC). As mentioned above, the common ink cavity supplies the ink to the ejection chambers through respective orifices, and simultaneously buffers or reduces a pressure increase caused by the backflow of ink from an ink ejection chamber when an ink droplet is ejected from the respective nozzle. The purpose of this buffering effect is to avoid or reduce an interaction, i.e. crosstalk, among the plurality of nozzles. While it would seem possible to enhance the function of the filter by reducing the cross-sectional area of the filter channels compared to that of the orifices and nozzles employing more precise manufacturing methods, it turned out that this is apt to impair the buffering effect thereby increasing crosstalk. The buffering effect of the common ink cavity depends on the compliance of the ink volume contained in it and any contribution by the ink supply system upstream of the common ink cavity. As will be shown later, the compliance is proportional to the square of the ink volume. Because of the general demand for small sized inkjet heads the volume of the common ink cavity should be as small as possible resulting in a correspondingly small buffering effect of the ink within the common ink cavity itself. The smaller the filter channels, the less is the contribution that the supply system upstream of the common ink cavity may have to the total buffering effect. The object of the present invention is to provide an inkjet recording apparatus having an inkjet head with multiple nozzles and a filter, wherein a good filtering function and substantially no crosstalk are obtained at the same time. This object is achieved with an inkjet recording apparatus as claimed in claim 1. Preferred embodiments of the invention are subject-matter of dependent claims. It has been found out that the buffering effect of the common ink cavity may be increased beyond that of the ink itself by means of a flexible wall or wall portion of the common ink cavity. In this way the cross-sectional area of the filter channels can be made relatively small to enhance the filtering function without sacrificing the advantage of no or substantially no crosstalk. The invention will be described in more detail below with reference to the drawings which illustrate preferred embodiments only and in which: Fig. 1 is a partial exploded perspective view of an inkjet head according to the preferred embodiment of the present invention; Fig. 2 is a perspective view of an inkjet head according to the preferred embodiment of the present invention; Fig. 3 is a lateral cross section of an inkjet head according to the preferred embodiment of the present invention; Fig. 4 is an enlarged partial plan view of the substrate of an inkjet head according to the preferred embodiment of the present invention; Fig. 5(a) to (c) are lateral cross sections showing the ink ejection operation of an inkjet head according to the preferred embodiment of the present invention; Fig. 6(a) to (c) are simplified illustrations of what occurs when a voltage is applied between the diaphragm and electrode of the inkjet head shown in Fig. 5; Fig. 7 is a plan view of an inkjet head according to the preferred embodiment of the invention; and Fig. 8 is a cross section taken along line D-D in Fig. 7. The embodiment of the invention described below is an edge type inkjet head wherein ink droplets are ejected from nozzles provided at the edge of a substrate. It is to be noted that the invention may also be applied to a face type inkjet head wherein the ink is ejected from nozzles provided on the top surface of the substrate. The inkjet head 10 of this embodiment is made up of three substrates 1, 2, 3 one stacked upon the other and structured as described in detail below. A first substrate 1 is sandwiched between second and third substrates 2 and 3, and is made from a silicon wafer. Multiple nozzles 4 are formed between the first and the third substrate by means of corresponding nozzle grooves 11 provided in the top surface of the first substrate 1 such as to extend substantially in parallel at equal intervals from one edge of the substrate. The end of each nozzle groove opposite said one edge opens into a respective recess 12. Each recess in turn is connected via a respective narrow groove 13 to a recess 14. In the assembled state the recess 14 constitutes a common ink cavity 8 communicating with the nozzles 4 via orifices 7 formed by the narrow grooves 13 and ejection chambers 6 formed by the recesses 12. A filter 51 is formed by a plurality of grooves 13a disposed at the back of recess 14, i.e. the ink supply side. In the assembled state of the substrates the grooves 13a form filter channels (in the following the same reference numeral 13a will be used for the grooves and the channels) The cross sectional area of each filter channel 13a is smaller than that of a nozzle 4, i.e. the filter channels provide an effective filtering function preventing the introduction of foreign matter into the ink in the common ink cavity 8, the ink passage (6,7) and the nozzles 4. The bottom of each ejection chamber 6 comprises a diaphragm 5 formed integrally with the substrate 1. As will be understood, the grooves and recesses referred to above can be easily and precisely formed by photolithographic etching of the semiconductor substrate. Diaphragms 5 are preferably formed by first doping substrate 1 with boron to provide for etch stopping followed by etching to form the diaphragms with a thin, uniform thickness. Electrostatic actuators each comprising a diaphragm and an associated nozzle electrode are formed between the first and the second substrate. A common electrode 17 of the actuators is provided on the first substrate 1. A thin oxide film (not shown in figures), approximately 1 µm thick, is formed on the entire surface of first substrate 1 except for the common electrode 17. This creates an insulation layer for preventing dielectric breakdown and shorting during inkjet head drive. Borosilicate glass is used for the second substrate 2 bonded to the bottom surface of first substrate 1. A recess 15 for accommodating a respective nozzle electrode 21 is formed in the top of second substrate 2 below each diaphragm 5. When the second substrate 2 is bonded to the first substrate 1 vibration chambers 9 are formed at the positions of recesses 15 between each diaphragm 5 and the opposing nozzle electrode 21. In this embodiment, recesses 15 formed in the top surface of the second substrate 2 provide for gaps between the diaphragms and the respective electrodes 21. The length G (see Fig. 3; hereinafter the "gap length") of each gap is equal to the difference between the depth of recess 15 and the thickness of the electrode 21. It is to be noted that this recess can alternatively be formed in the bottom surface of the first substrate 1. In this preferred embodiment, the depth of recess 15 is 0.3 µm, and the pitch and width of nozzle grooves 11 are 0.2 mm and 80 µm, respectively. As shown in Fig. 1, the wiring formed in the top surface of second substrate 2 comprises the nozzle electrodes 21 and lead members 22 connecting each nozzle electrode to a respective terminal member 23. As shown, the lead members are located in grooves 22a connecting to the recesses 15. The terminal members 23 are located in a corresponding recess formed at one edge of second substrate 2. Borosilicate glass is also used for the third substrate 3. Nozzles 4, ejection chambers 6, orifices 7, and ink cavity 8 are formed by bonding third substrate 3 to the top surface of first substrate 1. Support member 36 in ink cavity 8 adds reinforcement to prevent collapsing recess 14 when first substrate 1 and third substrate 3 are bonded together. First substrate 1 and second substrate 2 are anodically bonded at 270 to 400°C by applying a voltage 500 to 800 V, and first substrate 1 and third substrate 3 are then bonded under the same conditions to assemble the inkjet head as shown in Fig. 3. After anodic bonding, the gap length G formed between diaphragm 5 and nozzle electrode 21 on second substrate 2 is 0.2 µm in this embodiment. After the inkjet head is thus assembled, drive circuit 102 is connected by connecting flexible printed circuit (FPC) 101 between common electrode 17 and terminal members 23 of nozzle electrodes 21 as shown in Figs. 3 and 4. An anisotropic conductive film is used in this embodiment to bond leads 101 with electrodes 17 and 23. Ink supply tube 33 and ink supply vessel 32 are fit externally to the back of the inkjet head. Ink 103 is supplied from an ink tank (not shown in the figures) into first substrate 1 via ink supply tube 33, vessel 32, an ink supply port (not shown) and the filter channels 13a at the rear of ink cavity 8 to fill ink cavity 8 and ejection chambers 6. The ink in ejection chambers 6 becomes ink droplets 104 ejected from nozzles 4 and printed to recording paper 105 when inkjet head 10 is driven as shown in Fig. 3. Fig. 4 is an enlarged partial plan view of substrate 1. Substrate 1 of an inkjet head according to the present embodiment is manufactured by anisotropic etching of a single crystal silicon substrate. Anisotropic etching is an etching processing in which the etching speed varies according to the etching direction. The etching speed of crystal face (100) in single crystal silicon is approximately forty times that of crystal face (111), and this is used to form nozzle grooves 11, recesses 12, narrow grooves 13, recess 14, and filter grooves 13a in the present embodiment. Nozzle grooves 11, narrow grooves 13, and filter grooves 13a are formed as V-shaped grooves from crystal faces (111) where the etching speed is slower, resulting in the nozzle grooves 11, narrow grooves 13, and filter grooves 13a having a triangular cross section. Nozzle grooves 11 are 60 µm wide at the base of the triangle. Narrow grooves 13 form three parallel flow channels, each having a base width of 55 µm. Filter grooves 13a are 50 µm wide at the base of the triangle, and 54 parallel filter grooves 13a are formed continuous to recess 14. Recesses 12 and 14 have a trapezoidal cross-sectional shape of which the bottom is crystal face (100) and the sides are crystal face (111). The depth of recesses 12 and 14 is controlled by adjusting the etching time. The V-shaped nozzle grooves 11, narrow grooves 13, and filter grooves 13a are shaped only by crystal face (111) , which has the slower etching speed, and the depth is therefore controlled by the groove base width independent of the etching time. These nozzle grooves 11, narrow grooves 13, and filter grooves 13a greatly contribute to the ink ejection volume and speed characteristics of the inkjet head, and require the highest processing precision. In the present embodiment, those parts requiring the highest processing precision are made using the crystal faces with the slowest etching speed by means of anisotropic etching, making it possible to obtain channels of different dimensions with high precision. As described above, the cross sectional area of the filter channels 13a is the smallest cross sectional area of any part of the total ink path. As a result, foreign particulate that could clog the nozzles 4 or orifices 7 is reliably blocked by the filter channels 13a from entering the common ink cavity and the ink passage. A major reason for dropped pixels and other printing defects is thus eliminated, and the reliability of the inkjet head can be assured. Figs. 5 (a) to (c) are lateral cross sections of an inkjet head according to the preferred embodiment of the invention, and are used below to describe the process of deforming the diaphragm from a standby position to cause ink to be ejected from the respective nozzle. Figs. 6 (a) to (c) are simplified diagrams illustrating what happens when a voltage is applied between a diaphragm 5 and nozzle the corresponding electrode 21 in the corresponding states shown in Figs. 5 (a) to (c). An example of the inkjet head operation according to the present invention is described below with reference to Figs. 5 and 6. Fig. 5 (a) shows the inkjet head in the initial state, and Fig. 6 (a) shows the capacitor formed by diaphragm 5 and nozzle electrode 21 at that time is discharged due to the short circuit via resistor 46. In this initial state the ink passage is filled with ink, and the inkjet head is ready to eject ink. When a voltage is applied to an actuator, the capacitor comprising diaphragm 5 and nozzle electrode 21 is charged, and the diaphragm 5 is attracted to electrode 21 by electrostatic force and distorted as shown in Fig. 6 (b). The attraction of diaphragm 5 to nozzle electrode 21 at this time causes the pressure inside ejection chamber 6 to drop as shown in Fig. 5 (b), and ink is supplied in the direction of arrow B from ink cavity 8 to ejection chamber 6. The meniscus 102 formed at nozzle 4 at this time is pulled toward ejection chamber 6. When the drive voltage is removed and the capacitor is discharged, diaphragm 5 returns to its initial state in a short time as shown in Fig. 6 (c). The return of diaphragm 5 increases the pressure in ejection chamber 6, thus causing an ink droplet 104 to be ejected from nozzle 4 while some ink from the ejection chamber 6 is returned in the direction of arrow C through orifice 7 into ink cavity 8 at the same time as shown in Fig 5 (c). The oscillation of ink in the ink path is damped by the orifice 7 having a high flow resistance, and diaphragm 5 returns to the standby position shown in Fig. 5 (a) and is ready for the next eject operation. In the above drive method, the diaphragm is not deformed in the standby state but only deformed when driven. The force applied to the diaphragm is released immediately after the pressure inside the ejection chamber is reduced, which causes the pressure inside the ejection chamber to rise again and eject an ink droplet from the nozzle (a so-called "pull-push-ejection" method). It is to be noted that a so-called "push-ejection" method wherein the diaphragm is constantly deformed in the standby state and released only during inkjet head drive to eject ink may be alternatively used. The "pull-push-ejection" method described in the present embodiment provides a greater ink ejection volume and improved frequency characteristics. It is to be further noted that the action and effect of the present invention are the same even if the drive force and drive method differ. Figs. 7 and 8 show details of the preferred embodiment of the invention not shown in the figures explained so far. Fig. 7 is a plan view and Fig. 8 is the cross section at line D-D in Fig. 7. The embodiment shown in Fig. 7 comprises plural parallel ink passages of which a few are shown. As shown in Figs. 7 and 8, this embodiment comprises a pressure buffer chamber 53, which is a hollow space formed below the common ink cavity 8. As shown in the figures, the pressure buffer chamber is formed in the same way as the vibration chambers 9 from a recess in the surface of substrate 2 and the bottom of the common ink cavity 8. A transparent oxide conductive film 54 is formed on the bottom of pressure buffer chamber 53 from the same ITO material as nozzle electrodes 21. The bottom of the common ink cavity 8 has substantially the same thickness as diaphragm 5 and constitutes a flexible membrane or buffer wall 55. The pressure increase in ink cavity 8 created when diaphragm(s) 5 in ejection chamber(s) 6 is (are) driven is absorbed, buffered, and effectively cancelled by buffer wall 55, thereby preventing the pressure interference or the crosstalk. The primary reason for providing transparent oxide conductive film 54 is to prevent buffer wall 55 from adhering to second substrate 2 and becoming nonfunctional when substrate 1 and second substrate 2 are anodically bonded. Any other material serving this purpose could be used instead. With regard to the manufacturing, however, use of the same material as that of the nozzle electrodes is preferred since then film 54 can be formed simultaneously with the nozzles electrodes by the same manufacturing step. When the ink capacity (compliance) of ink cavity 8 is sufficiently great, the pressure created by the "driven" nozzles and transferred to ink cavity 8 can be buffered by the ink compliance alone. By additionally disposing buffer wall 55 sufficient compliance can be obtained even with a small capacity ink cavity 8. Furthermore, with the flexible buffer wall 55 and the chamber 53 below it, crosstalk can even be avoided without caring for the ratio of inertances, provided a sufficiently great total compliance is achieved to suppress any pressure increase in the common ink cavity 8 below that causing the crosstalk. While the invention has been described so far with reference to embodiments using an electrostatic actuator as pressure generating device it will be understood that, as far as crosstalk suppression is concerned, it makes no difference whether the pressure is generated electrostatically, thermally (by means of resistance heating elements provided in each ejection chamber 6) or by means of a piezoelectric element (provided on the side of diaphragm 5 opposite each ejection chamber 6). All kinds of pressure generating device resulting in the same basic function of the inkjet head as that explained above can therefore be employed in the context of the invention. Since such alternative pressure generating devices are known in the art, no further description will be given here. Yet, since the pressure in inkjet heads using electrostatic actuators tends to be higher than with other types of pressure generator, in combination with electrostatic actuators the invention may be particularly useful. In this case it offers the additional advantage that manufacturing steps required for forming the actuators may at the same time be used to provide characteristics of the invention.

Claims: An inkjet recording apparatus having an inkjet head (10) which comprises: an ink supply port, a common ink cavity (8), a filter (51) having a plurality of filter channels (13a) communicating with the ink supply port at one end and the common ink cavity (8) at the other end, a plurality of ink ejection nozzles (4) each connected to the common ink cavity by a respective ink passage (6, 7), and a corresponding plurality of pressure generating means (5, 21) respectively associated with said ink passages, said pressure generating means (5, 21) being selectively drivable to eject ink droplets through the respective nozzles (4), characterized in that at least a portion (55) of the walls defining the common ink cavity (8) is flexible. The apparatus according to Claim 1, wherein said wall portion (55) separates the common ink cavity (8) from a hollow chamber (53). The apparatus according to any one of the preceding Claims, wherein said nozzles (4), said ink passages (6, 7), said common ink cavity (8) and said filter (52) are disposed on an anisotropic crystalline substrate (1). The apparatus according to Claim 3 wherein said anisotropic crystalline substrate (1) is made of single crystalline silicon. The apparatus according to any one of the preceding Claims, wherein each pressure generating means (5, 21) is an electrostatic actuator comprising a diaphragm (5) forming a wall portion of the ink passage (6,7) and a nozzle electrode (21) provided opposite to the diaphragm (5) via a gap (G). The apparatus according to any one of Claims 1 to 4, wherein each of said ink passages (6, 7) comprises a wall portion forming a diaphragm and each pressure generating means comprises a piezoelectric element attached to the respective diaphragm. The apparatus according to any one of Claims 1 to 4, wherein said pressure generating means comprises an electrically drivable heating element disposed in the respective ink passage.

Inventors: FUJII MASAHIRO, HAGATA TADAAKI, MARUYAMA HIROYUKI, MIYASHITA IKUHIRO, MUKAIYAMA KEIICHI, FUJII, MASAHIRO, HAGATA, TADAAKI, MARUYAMA, HIROYUKI, MIYASHITA, IKUHIRO, MUKAIYAMA, KEIICHI
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1995
Main classification: B41J2
IPC codes: B41J2