Abstract:
An inkjet printhead having a plurality of pressure chambers each of which is fluidly connected on the one hand, via an ink supply path, to a common ink reservoir and on the other hand to a nozzle, wherein an actuator is provided for each pressure chamber for pressurizing the ink contained therein so as to eject an ink droplet through the nozzle in accordance with a print signal, and an acoustic wave attenuator is arranged to control the acoustic reflection and transmission properties of the ink supply path.

Description:
[0001]    This non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 03076047.4 filed in Europe on Apr. 8, 2003, which is herein incorporated by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to an inkjet printhead having a plurality of pressure chambers each of which is fluidly connected on the one hand, via an ink supply path, to a common ink reservoir and on the other hand to a nozzle, wherein an actuator is provided for each pressure chamber for pressurizing the ink contained therein, so as to eject an ink droplet through the nozzle in response to a print signal.  
           [0003]    EP-A-1 022 140 describes a drop-on-demand inkjet printhead of the type indicated above, wherein the nozzles are arranged in two parallel linear arrays, so that a plurality of pixel lines of an image can be printed simultaneously. The pressure chambers associated with the nozzles of both arrays are configured as elongated ink channels that are formed in opposite surfaces of a common substrate and extend in parallel to one another. The downstream ends of the ink channels each converge into an associated nozzle, whereas the upstream ends of the ink channels of both arrays are connected to the common ink reservoir through their respective ink supply paths. The actuators are formed by piezoelectric elements that are arranged along each ink channel. When an ink droplet is to be expelled from a specific nozzle, the associated actuator is energized such that the piezoelectric element will first contract, so that ink is sucked-in through the ink supply path, and the piezoelectric element will then expend again, so that the liquid ink contained in the ink channel is pressurized and an acoustic pressure wave will propagate towards the nozzle.  
           [0004]    A problem encountered with printheads of this type is the occurrence of cross-talk among the various nozzles. A major reason for this cross-talk phenomenon is the propagation of acoustic waves in the solid material of the piezoelectric actuators and in the common substrate in which the ink channels are formed. As is known in the art, this kind of cross-talk can be suppressed, for example, by selecting an appropriate design for the substrate and the ink channels and by providing a suitable support structure for the piezoelectric actuators.  
           [0005]    Another source of cross-talk may be the propagation of acoustic waves through the liquid ink in the ink supply system. In order to avoid cross-talk of this kind, EP-A-0 726 151 proposes a printhead in which the ink supply paths connecting the pressure chambers to the common ink reservoir comprise acoustically matched sets of inlet filters, inlet ports, and inlet channels, which are designed to avoid, through acoustic matching, the propagation of acoustic waves from the various pressure chambers into the ink reservoir. In the printhead described in this document, the ink reservoir is formed by a closed chamber which is bounded on one side by a compliant wall. The purpose of this compliant wall is to further minimize pressure fluctuations in the ink reservoir during the “start up” of the printhead, until a steady ink flow is established.  
           [0006]    However, it has been found that the printed images obtained with an inkjet printer of the type described above may, under certain conditions, still show some undesired artifacts which degrade the image quality.  
         SUMMARY OF THE INVENTION  
         [0007]    It is accordingly on object of the present invention to provide a multi-nozzle inkjet printhead which provides an improved image quality.  
           [0008]    According to the present invention, this object is achieved by an inkjet printhead provided with an acoustic wave attenuator disposed to control the acoustic wave transmission and reflection properties of the ink supply path.  
           [0009]    The inventors have found that the artifacts mentioned above can be traced back to a new type of cross-talk phenomenon which has not yet been addressed in the prior art and which can be explained as follows: Ideally, the ink supply path which connects the pressure chamber to the ink reservoir and hence to the other pressure chambers of the array(s) should behave like an open end of the pressure chamber, so that acoustic waves propagating towards the ink reservoir are reflected almost completely with phase inversion. Then, for example, when the piezoelectric actuator performs its suction stroke and a negative pressure wave propagates towards the ink reservoir, this pressure wave will be reflected and will return as a positive pressure wave propagating towards the nozzle. This positive pressure wave will then be boosted further when the actuator performs its compression stroke.  
           [0010]    In a conventional printhead, the ink supply path is configured, i. e., acoustically matched, to fulfill this requirement. As a result, due to the practically complete reflection of the acoustic waves at the ink supply path, these waves should be prevented from propagating further into the ink reservoir and into the other pressure chambers. However, due to constructional constraints, the ink supply path can only have a limited cross-sectional area. In spite of this restricted cross-section, the ink supply path will act as an open end, as desired, when only a single actuator is energized. If, however, a plurality of neighboring actuators are energized simultaneously in accordance with the image information to be printed, then the restricted area where the ink supply paths of the various pressure chambers are jointly connected to the ink reservoir will form a bottleneck for the ink flowing into the pressure chambers. As a consequence, the ink supply path can no longer act as an ideal open end, and the acoustic waves propagating towards the ink reservoir will be only partially reflected, and a portion of the acoustic energy is transmitted into the ink reservoir and into the other pressure chambers which gives rise to cross-talk.  
           [0011]    According to the present invention, the acoustic wave attenuator is arranged to control the reflection and transmission behavior of the ink supply path such that, in this case, the ink supply paths will still act as almost ideal open ends in spite of the increased demand for ink. In this way, the acoustic waves can be prevented from entering into the ink reservoir and from causing cross-talk, regardless of the pixel pattern to be printed, so that the image quality is improved.  
           [0012]    The present invention is particularly useful in the case of a printhead design in which the ink supply paths leading from the ink reservoir to the various pressure chambers of one array contain a restricted inlet passage or manifold through which the plurality of ink chambers are commonly connected to the ink reservoir. The acoustic wave attenuator is arranged to attenuate acoustic waves which would otherwise be generated in this passage due to an increase demand for ink and which would then propagate into the neighboring pressure chambers and also into the ink reservoir. By suppressing pressure fluctuations in this inlet passage, the ink supply paths are all allowed to behave like open ends, and intra-array cross-talk, i. e., cross-talk among the pressure chambers belonging to the same array, can be substantially avoided.  
           [0013]    In addition, in the case of a multi-array printhead, where the pressure chambers of at least two nozzle arrays are connected to the same ink reservoir, the present invention has the further remarkable advantage that inter-array cross-talk, i. e., cross-talk between the different arrays, can also be suppressed successfully. Such inter-array cross-talk would otherwise be likely to occur, for example, in a hot-melt printhead in which an ink reservoir that is kept at atmospheric pressure and is filled with molten ink to a certain level is disposed above the pressure chambers and is connected to the pressure chambers of each array through respective inlet passages. If the pressure fluctuations in the inlet passages are not attenuated, then a pressure wave would propagate from one of the inlet passages, in which a large demand for ink occurs, into the ink reservoir, and would then be reflected at the liquid/air meniscus in the ink reservoir and would propagate into the inlet passages of the other arrays, where it would give rise to cross-talk. Thanks to the acoustic wave attenuator according to the present invention, this phenomenon can be successfully suppressed.  
           [0014]    In a preferred embodiment of the present invention, the acoustic wave attenuator is formed by a compliance element provided in each of the fluid supply paths. Preferably, the compliance element is provided in an inlet passage which forms a common part of the fluid supply paths of the same array. The compliance element may for example be formed by a flexible sheet defining a portion of the wall of the ink supply passage which is allowed to deflect in response to changes in the pressure of the liquid ink, thereby attenuating pressure fluctuations.  
           [0015]    In a frequently used printhead design, the pressure chambers are formed by an array of parallel ink channels that are covered by a common flexible sheet, and the actuators are formed as electromechanical actuators arranged to deflect the portions of the flexible sheet covering the various ink channels. Then, a sufficiently large portion of the same flexible sheet, which portion is not rigidly connected to the actuators, may serve as the acoustic wave attenuator according to the present invention. In this way, the invention may be realized with only a minor change in the conventional printhead design. The portion of the flexible sheet serving as the compliance element of the attenuator may comprise a bulge that is lifted off from the surface of the actuator to some extent, so that it is capable of being deflected not only away from the actuator in order to absorb negative pressure waves but also to deflect towards the actuator in order to absorb positive pressure waves.  
           [0016]    Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:  
         [0018]    [0018]FIG. 1 is an exploded perspective view, partly broken away, of an inkjet printhead according to the present invention;  
         [0019]    [0019]FIG. 2 is a cross-sectional view taken along the line II-II in FIG. 1; and  
         [0020]    [0020]FIG. 3 is an enlarged detail of the sectional view shown in FIG. 2.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]    [0021]FIG. 1 shows the essential parts of a hot-melt inkjet printhead which has a symmetric structure and includes a substrate  10  made of graphite, for example, which defines an upwardly open ink reservoir  12  in its upper part. A lower portion of the substrate  10  is configured as a channel plate  14  which has opposite side surfaces only one of which is visible in FIG. 1. Each of these side surfaces is formed with an array  16  of parallel ink channels  18  which have only been shown schematically in FIG. 1. The ink channels  18  are cut into the surface of the channel plate  14 , and the lower ends thereof are converged so as to form nozzles  20  through which ink droplets are to be expelled. In this way, a linear array of nozzles  20  is formed on either side of the channel plate  14 . The symmetric arrangement of arrays  16  of ink channels  18  and nozzles  20  on both sides of the channel plate  14  can be seen in FIG. 2. Each of the arrays  16  of ink channels  18  is covered by a flexible sheet  22  that is bonded to the ridges of the channel plate  14  separating the individual ink channels  18 . Thus, the open outwardly facing sides of all the ink channels  18  and of the nozzles  20  are closed-off by the sheets  22 .  
         [0022]    An actuator block  24  is bonded to the outer surface of each sheet  22 . The actuator block  24  is made of a piezoelectric ceramic material and has a comb-like structure forming a plurality of parallel, vertically extending piezoelectric fingers  26  and is provided with electrodes (not shown) associated with each of the fingers  26 . A flexible lead foil  28  is attached to the outer surface of each of the actuator blocks  24  and is formed with electric leads for individually energizing the piezoelectric fingers  26 .  
         [0023]    The actuator blocks  24  are protected by a cap  30  fitted over the lower end of the channel plate  14  and bonded to the lower edges of the sheets  22  and the lower end face of the channel plate  14 .  
         [0024]    In FIG. 2, the sectional plane passes to the piezoelectric fingers  26  of the actuator blocks  24 . It can be seen that these fingers  26  project towards the flexible sheet  22  and each engage a portion of the sheet covering one of the ink channels  18 . The top end of the ink channels  18  of each array  16  are connected to the ink reservoir  12  through an inclined inlet passage  32 . The top ends of the inlet passages  32 , in the plane of the bottom of the ink reservoir  12 , may be covered by a filter element  34  which prevents solid particles from entering into the ink channels  18  and clogging the nozzles  20 .  
         [0025]    As is shown in FIG. 1, a receptacle  36  for accommodating another (coarser) filter element is defined in the walls of the ink reservoir  12 . Although not shown in the drawing, the ink reservoir  12  further accommodates a heating element for heating the hot-melt ink so as to maintain the ink in the liquid state. The meniscus of the liquid ink in the ink reservoir  12  is shown at  38  in FIG. 2.  
         [0026]    When the printhead is operating, electric signals are supplied to the individual piezoelectric fingers  26  via the lead foil  28 , so that the piezoelectric fingers perform expansion and retraction strokes towards and away form the associated ink channel  18 , so that the sheet  22  covering this ink channel is flexed, and the liquid ink contained in the ink channel is pressurized and an ink droplet is jetted-out through the nozzle  20 . Thus, the ink channels  18  serve as pressure chambers for pressurizing the ink. More precisely, when an ink droplet is to be expelled, the associated piezoelectric finger  26  will at first be retracted, so that ink is sucked-in through the inlet passage  32 .  
         [0027]    As can be seen in FIG. 1, the ink passage  32  extends transversely of the ink channels  18 , and its cross-section is significantly larger than that of the ink channels  18 . Thus, when a negative pressure wave propagates in the liquid ink from the ink channel  18  towards the inlet passage  32 , the transition between the ink channel and the inlet passage will act like an open end at which the acoustic wave is reflected almost completely, with phase reversal. As a result, a positive pressure wave will then propagate through the ink channel  18  toward the nozzle  20 . At appropriate timing, the piezoelectric finger  26  is expanded again, so that the positive pressure wave is boosted. Positive pressure waves propagating towards the inlet passage  32  will also be reflected at the transition, so that no substantial pressure fluctuations should occur in the inlet passage  32 .  
         [0028]    However, when a plurality adjacent ink channels  18  are energized simultaneously, the demand for ink in the associated portion of the inlet passage  32  may become so large that the ink flow is restricted by the limited cross-section of the inlet passage  32 . As a result, the transitions between the ink channels  18  and the ink passage  32  would no longer act as ideal open ends, and the acoustic waves arriving from the ink channels  18  would no longer be reflected completely, but would be partially transmitted through the inlet passage  32  into the ink reservoir  12 . A ridge  40  (FIG. 2) formed centrally on the bottom wall of the ink reservoir  12  prevents the direct propagation of the transmitted wave from one inlet passage  32  to the other. However, the pressure waves propagating through the liquid ink in the ink reservoir  12  would be reflected at the meniscus  38  and could then enter into the other inlet passage  32 , as is indicated by a dot-dashed line in FIG. 2. If no countermeasures are taken, this propagation of acoustic waves from one inlet passage  32  to the other could give rise to inter-array cross-talk.  
         [0029]    In order to avoid this type of cross-talk, the present invention provides an acoustic wave attenuator  42  for controlling the acoustic wave transmission and reflection properties of the ink supply paths connecting the ink reservoir  12  to the ink channels  18  of the two arrays  16 . In the present embodiment, as is shown in FIG. 3, such an attenuator  42  is formed by a portion of the flexible sheet  22  which closes off the downstream end of the inlet passage  32  and the top (upstream) end portions of the ink channels  18 . In this portion, the sheet  22  is not rigidly connected to the piezoelectric fingers  26  but instead forms a small bulge  44  which slightly projects into the inlet passage  32  and extends transversely of the ink channels  18  throughout the length of the inlet passage  32 . Thus, in the bulge  44 , the sheet  22  is separated from the piezoelectric finger  26  by a small gap, so that it is free to flex inwardly and outwardly of the inlet passage  32 . The rest of the sheet  22  is adhered to the piezoelectric fingers  26  by means of a layer of adhesive  46  which, however, is interrupted in the vicinity of the bulge  44 . Only a very small strip of adhesive  48  is applied at the very top end of the actuator block  24 . Thus, any pressure waves that might be created in the inlet passage  32  can be attenuated by the flexing movement of the portion of the sheet  22  forming the attenuator  42 . This portion of the sheet serves as a compliance element which smoothens out any pressure fluctuations in the inlet passage  32  and assures that the transition between the ink channel  18  and the inlet passage  32  will always act as an open end, with complete reflection of acoustic waves in the ink, even in the case of an increased demand for ink in the inlet passage  32 . As a result, no pressure waves will propagate through the inlet passage  32  into the ink reservoir  12  and into the ink passage  32  of the other array, and inter-array cross-talk is eliminated. Similarly the attenuator  42  also helps to reduce cross-talk among adjacent ink passages of the same array.  
         [0030]    In a modified embodiment, the length of the actuator block  24  may be reduced so that it covers only the ink channels  18  but not the end of the inlet passage  32 . Then, the sheet  22  would freely span the downstream end of the ink passage  32  and would thus be free to act as a compliance element.  
         [0031]    In yet another embodiment, the downstream end of the ink supply passage  32  may be closed-off by a rigid member, and the attenuator  42  may be formed in the top ends of the ink channels  18  adjacent to the inlet passage  32 . The attenuator  42  may also be formed by other means, for example by a piece of sponge-like material arranged in or close to the inlet passage  32 , a trap formed on purpose for capturing an air bubble in the inlet passage  32 , and the like.  
         [0032]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.