Abstract:
An ink jet nozzle head including a channel plate defining a linear array of equidistanced nozzles and a plurality of substantially parallel ink channels communicating with an associated nozzle, a vibration plate disposed on the channel plate, at least one block member containing a comb-like array of fingers which extend toward and engage the vibration plate, some fingers functioning as actuators for exerting mechanical pressure on ink contained in the ink channels which, in turn, expels ink droplets from the nozzles, at least one actuator being provided for each nozzle, and other fingers serving as support members for supporting the actuators at the vibration plate, where the reaction forces of the actuators are received, and a backing member disposed on the block member, wherein the pitch “a” of the support members is larger than the pitch “b” of the nozzles and the fingers are evenly distributed over the length of the nozzle array.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     The present invention relates to a nozzle head for use in an ink jet printer. A nozzle head having the features specified hereinbelow is disclosed in EP-A-0 402 172. This nozzle head comprises a channel plate defining a linear array of equidistant nozzles and a plurality of parallel ink channels, each connected to a respective one of the nozzles. On one side of the channel plate there is disposed an array of elongate fingers projecting towards the nozzle plate and extending in parallel with the ink channels. The ends of these fingers facing away from the channel plate are interconnected by a plate-like backing member which is formed integrally with the fingers. The fingers and the backing plate are made of a piezoelectric ceramic material. Every second finger is provided with electrodes and serves as an actuator which, when a print signal is applied to the electrodes, compresses the ink liquid contained in the associated ink channel, so that an ink droplet is expelled from the nozzle. The other fingers intervening between the actuators serve as support members which are rigidly connected to the channel plate so that they can absorb the reaction forces generated by the actuators. 
     Since a support member is provided between each pair of consecutive actuators, each actuator is substantially shielded against the reaction forces from its neighbors, so that undesired cross-talk between the various channels is reduced. 
     In this conventional nozzle head, the pitch of the support members, i.e. the distances at which the support members are disposed in the direction of the linear nozzle array, is equal to the pitch of the nozzles. As a consequence, the total number of fingers per unit length in the direction of the linear nozzle array, i.e. the density with which the fingers have to be arranged, is twice the density of the nozzles. Since intricate manufacturing problems are involved in preparing a high-density array of fingers, it becomes difficult to reduce the pitch of the nozzles in order to improve the resolution of the printer. 
     It is accordingly an object of the present invention to provide a nozzle head for high-resolution printing which can easily be manufactured and nevertheless suppress cross-talk between the individual channels. 
     According to the present invention, the pitch of the support members is larger than that of the nozzles, so that there is no longer a one-to-one relationship between the support members on the one hand and the actuators, the ink channels and the nozzles on the other hand. The mean density of the fingers will accordingly be smaller than twice the density of the nozzles. Of course, the support members have to be arranged such that they are connected to the dam portions of the channel plate separating the individual ink channels, whereas the actuators have to be disposed adjacent to the ink channels and must not overlap with the dam portions. However, since the support members may be slightly offset from the centers of the dam portions and/or the actuators may be slightly offset from the centers of the ink channels, it is possible to distribute the fingers in such a manner that their spacings are comparatively large, so that even for a nozzle array with a reduced pitch, the array of fingers can be manufactured with conventional techniques, e.g. by cutting grooves into a block of piezoelectric material. 
     In a preferred embodiment, the ratio between the densities of the fingers and nozzles is 3:2, and every third finger is a support member. This embodiment has the advantage that each actuator has as its neighbors a support member on the one side and another actuator on the other side, so that for any pair of ink channels, the configurations of actuators and support members in the vicinity of these ink channels are either identical or mirror-symmetric. As a result, the configurations of actuators and support members will not lead to any differences in the generation of droplets. 
     The fingers may be arranged equidistantly, which has the advantage that the manufacturing process can be very simple and efficient. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the present invention will now be described in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a partly broken-away perspective view of a nozzle head according to a first embodiment of the present invention; 
     FIG. 2 is a cross-sectional view in the direction of the arrow II in FIG. 1; and 
     FIG. 3 is a view similar to FIG. 2 but showing a second embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The nozzle head  10  illustrated in FIGS. 1 and 2 comprises a channel plate  12  which defines a linear array of nozzles  14  and a number of parallel ink channels  16 , only one of which is shown in FIG.  1 . The nozzles  14  and the ink channels  16  are formed by grooves cut into the top surface of the channel plate  12 . Each nozzle  14  is connected to an associated ink channel  16 . The ink channels are separated by dam portions  18 ,  18 ′. 
     The top sides of the nozzles  14  and the ink channels  16  are closed by a thin vibration plate  20 , which is securely bonded to the dam portions of the channel plate. 
     The top surface of the vibration plate  20  is formed with a series of grooves  22  which extend in parallel with the ink channels  16  and are separated by ridges  24 . The ends of the grooves  22  adjacent to the nozzles  14  are slightly offset from the edge of the vibration plate  20 . 
     An array of elongate fingers  26 ,  28  is disposed on the top surface of the vibration plate  20  such that each finger extends in parallel with the ink channels  16  and has its lower end fixedly bonded to one of the ridges  24 . The fingers are grouped in triplets, each triplet consisting of a central finger  28  and two lateral fingers  26 . The fingers of each triplet are interconnected at their top ends and are formed by a one-piece block  30  of piezoelectric material. 
     Each of the fingers  26  is associated with one of the ink channels  16  and is provided with electrodes (not shown) to which an electric voltage can be applied in accordance with a printing signal. These fingers  26  serve as actuators which expand and contract in a vertical direction in response to the applied voltage, so that a corresponding portion of the vibration plate  20  is deflected into the associated ink channel  16 . As a result, the ink liquid contained in the ink channel (e.g. a hot-melt ink) is pressurized and an ink droplet is expelled from the nozzle  14 . 
     The central fingers  28  are disposed over the dam portions  18  of the channel plate and serve as support members which absorb the reaction forces of the actuators  26 . For example, if one or both actuators  26  belonging to the same block  30  are expanded, they exert an upwardly directed force on the top portion of the block  30 . This force is largely counterbalanced by a tension force of the support member  28 , the lower end of which is rigidly connected to the channel plate  12  via the ridge  24  of the vibration plate. 
     The top ends of the blocks  30  are flush with each other and are overlaid by a backing member  32 . The backing member  32  is formed by a number of longitudinal beams  34  extending in parallel with the ink channels  16  and by transverse beams  36  which interconnect the ends of the longitudinal beams  34  (only one of the transverse beams is shown in FIG.  1 ). 
     The longitudinal beams  34  have a trapezoidal cross section and are originally interconnected with each other at their broader base portions, so that they form a continuous plate. In a subsequent manufacturing step, a comparatively thick layer of piezoelectric material which will later form the blocks  30  is bonded to the plate, i.e. the lower surface of the backing member  32  in FIG.  1 . Then, the blocks  30  and the fingers  26 ,  28  are formed by cutting grooves  38 ,  40  into the piezoelectric material. While the grooves  38  which separate the fingers  26  and  28  terminate within the piezoelectric material, the grooves  40  separating the blocks  30  are cut through into the backing member  32 , thereby separating also the longitudinal beams  34  from one another. 
     The width of the longitudinal beams  34  is substantially equal to the width of the individual blocks  30 . As a consequence, the beams  34  effectively prevent an elastic deformation of the top portions of the blocks  30  when the actuators  26  expand and contract. 
     Since the support members  28  inevitably have a certain elasticity, expansion of one or both actuators  26  of one of the blocks  30  will also cause a minor expansion of the support members  28  and will tend to cause a slight deflection of the backing member  32 . If the backing member were a non-profiled flat plate, this deflective force would be transmitted to the neighboring blocks  30  and would lead to the generation of parasitic acoustic waves in the neighboring ink channels, known as cross-talk. Such long-range cross-talk may cause problems, especially when a large number of actuators in neighboring blocks  30  are simultaneously energized. However, since the backing member  32  is formed by separate beams  34  which are only interconnected at their opposite ends by the transverse beams  36 , and these transverse beams are additionally weakened by the grooves  40 , the deflective forces are effectively confined to the blocks  30  from which they originate. Thus, the long-range cross-talk phenomenon can be successfully suppressed. 
     It is not always necessary to cut the grooves  40  through into the backing member  32 . Good results were obtained by cutting grooves  40  only into the piezoelectric material, the depth of the grooves  40  being equal to or slightly deeper than the grooves  38 . Although, in this situation, the piezoelectric material is not explicitly divided into separate groups, the cross-talk is acceptable and the spacing of the fingers is comparatively large. It is further possible to omit the separate backing member  32 . In this situation, the piezoelectric material is chosen to be thicker compared to the thickness of the piezoelectric block from FIG.  1 . When the grooves are cut with the same depth as in FIG. 1, the uncut upper portion of the piezoelectric material fulfills the function of the backing member  32 . 
     The subdivision of the array of fingers  26 ,  28  into separate blocks  30 , each consisting of only three fingers, also facilitates the further suppression of short range cross-talk, i.e. cross-talk between the ink channels associated with the same block  30 . To this end, it is sufficient to make a distinction between two cases: (a) only one of the two actuators  26  is energized; (b) both actuators are energized. In the (b) case, the support member  28  will be subject to a larger elastic deformation than in the (a) case. This effect can easily be compensated by slightly increasing the voltage applied to the actuators in the (b) case. It should be noted that this measure will not lead to an increased long-range cross talk, because the blocks  30  are separated from each other. 
     Conversely, in the (a) case, the top portion of the block  30  and the beam  34  will be caused to slightly tilt about the top end of the support member  28 , thereby compressing the ink in the neighboring channel. However, this effect will be very small, due to the stabilizing effect of the transverse beams  36 . If necessary, this minor effect can also be compensated for by applying a small compensation voltage with appropriate polarity to the actuator associated with the non-firing channel. 
     Since the support members  28  are made of piezoelectric material, it is also possible to provide additional electrodes for the support members  28  in order to actively counterbalance the reaction forces of the actuators  26 . 
     In the embodiment shown, the width of the grooves  40  is identical to the width of the grooves  38 , and the fingers  26 ,  28  are arranged equidistantly with respect to each other. The pitch “a” of the support members  28  is larger than the pitch “b” of the nozzles  14  by a factor 2. Since every third finger is an actuating member  26 , the pitch of the fingers  26 ,  28  is 2b/3, in comparison to a pitch of b/2 for the conventional case in which a support member is provided between each pair of adjacent ink channels. As a result, the pitch “b” of the nozzles and hence the resolution of the print head can be made small without exceeding the limits imposed by the manufacturing process for the piezoelectric actuators and support members. 
     In a practical embodiment the pitch “b” of the nozzles  14  may be as small as 250 m (i.e. four nozzles per millimeter). The pitch of the support members  28  will accordingly be 500 m, and the pitch of all fingers (including the actuators  26 ) will be 167 m. In this case, the width of each individual finger  26  or  28  may, for example, be 87 m, and the grooves  38 ,  40  will have a width of 80 m and a depth in the order of 0.5 mm. 
     As is shown in FIG. 2, the grooves  22  and ridges  24  of the vibration plate  20  and the nozzles  14 , and the ink channels  16  are not evenly distributed over the length of the nozzle array. Instead, the ink channels  16  are grouped in pairs separated by comparatively broad dam portions  18 , whereas the ink channels of each pair are separated by a comparatively narrow dam portion  18 ′. The broad dam portions  18  coincide with the ridges  24  of the vibration plate and with the support members  28 , whereas the smaller dam portions  18 ′ coincide with the grooves  22  of the vibration plate and the grooves  40  between the blocks  30 . The width of the ink channels  16  at the top surface of the channel plate  12  is larger than the width of the fingers  26 ,  28 , and the ink channels are offset relative to the nozzles  14  to such an extent that none of the actuators  26  overlap with the dam portions  18 ,  18 ′. 
     The portions of the vibration plate  20  on both sides of the ridges  24 , which are held in contact with the actuators  26 , are weakened by the grooves  22 , and at least a major part of these weakened portions is still within the area of the ink channels  16 . Thus, the vibration plate  20  can readily be flexed into the ink channel  16  in response to expansion strokes of the actuators  26 . The width of the ridges  24  is slightly smaller than that of the fingers  26 ,  28 . 
     With the above configuration, an excessive bending or shearing stress in the vibration plate  20  near the edges of the dam portions  18 ,  18 ′ is avoided, so that a high durability of the vibration plate  20  can be achieved. 
     In general, the flexibility of the vibration plate  20  is a critical parameter, so that thickness tolerances of the vibration plate may influence the process of droplet generation. Since, in the above embodiment, the ink channels  16  have a rather large width in comparison to the fingers  26 ,  28  and are offset relative to the nozzles  14 , the spacing between the actuators  26  and the edges of the dam portions  18 ,  18 ′ remains so large that a sufficient flexibility can be achieved with a relatively thick vibration plate, so that the tolerances are less critical. 
     The flexibility of the vibration plate should be matched to the modulus of elasticity of the channel plate  12 . If the vibration plate  20  is rather stiff and the channel plate is comparatively soft, then the dam portions adjacent to an active channel may be slightly compressed, so that the volume of the neighboring channels is also reduced the some extent. The result is a positive coupling between the neighboring channels. Conversely, if the channel plate  12  is rather stiff, the portions of the vibration plate  20  on both sides of a dam portion  18  or  18 ′ may behave like a balance, which results in a negative coupling between adjacent channels. Thus, by appropriately matching the stiffness of the vibration plate and the channel plate, these effects can be caused to cancel each other so that cross-talk is reduced to a minimum. 
     The vibration plate  20  may be formed by a relatively soft resin foil, e.g. a polyimide resin foil, which is welded to the channel plate  12  and the ends of the fingers  26 ,  28 . Alternatively, the vibration plate may be formed by a thin film of glass or metal, e.g. aluminum, which is soldered to the channel plate and to the fingers. 
     While a specific embodiment of the present invention has been described above, it will be obvious to a person skilled in the art that various modifications can be made which would fall within the scope of the present invention. 
     For example, the pitch “a” of the support members  28  may be another integral or even non-integral multiple of the pitch “b” of the nozzles. The width of the actuators  26  may be different from that of the support members  28 . Likewise, the width of the grooves  40  may be different from that of the grooves  38 , resulting in an uneven distribution of the fingers  26 ,  28 . 
     Some other modifications are illustrated in FIG. 3, where the ink channels  16  are arranged equidistantly, without being offset relative to the corresponding nozzles  14 . 
     Instead of using a profiled vibration plate having grooves  22  and ridges  24 , a vibration plate  20  with a uniform thickness is used in FIG.  3 . In this case, the vibration plate is in contact with the actuators  26  via the ridges  24 ′ formed at the bottom ends of the actuators  26  and appropriately offset from the respective centers of the latter. 
     As is further shown in FIG. 3, not only the grooves  40  but also the grooves  38  are cut through into the backing member  32 , so that one obtains a configuration in which all fingers are completely separated from each other. Alternatively, the depth of the grooves  28 ,  40  may be reduced such that all fingers  26 ,  28  are formed by a one-piece member which is not separated into blocks. 
     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.