The present invention relates to droplet deposition apparatus, in particular an inkjet printhead, which comprise a channel communicating with a supply of droplet liquid and an opening for ejection of droplets therefrom, at least one channel side wall being displaceable in response to electrical signals, thereby to effect ejection of droplets from the channel.
Figure 1 a is a cross-sectional view of the channels of the prior art inkjet printhead construction according to W092/22429. Piezoelectric ceramic sheet 12 is poled in its thickness direction 17 and formed in one surface with channels 11 bounded on two sides lying parallel to the channel axis by channel walls 13. By means of electrodes 23 formed on either side of each wall 13, an electric field can be applied to the piezoelectric material of the walls, causing them to deflect in shear mode in a direction transverse to the channel axis. Pressure waves are thereby generated in the ink which result in the ejection of an ink droplet. These principles are known in the art, e.g. from EP-A-O 364 136.
Channels 11 are closed along one side lying parallel to the channel axis by the surface of a cover 14 having conductive tracks 16 at the same pitch interval as the ink channels formed thereon. Solder bonds 28 are formed between tracks 16 and the channel wall electrodes 23, thereby securing the cover to the base and creating an electrical connection between the electrodes and the track in a single step. To protect them from later being corroded by the ink, electrodes and tracks are then given a passivant coating.
As shown in figure 1b, which is a sectional view taken along the longitudinal axis A of a single channel of the prior art printhead of figure 1a, a nozzle plate 20 having respective ink ejection nozzles 22 is mounted at the front of the sheet 12 whilst an ink manifold 26 is defined at the rear by a manifold structure 21. Tracks 16 are led to the rear of cover 14 for connection to a drive circuit, typically embodied in a microchip 27 which in turn is driven by signal received via input tracks 18.
In printheads of this ilk, the channel walls - and in particular the electrodes formed thereon - are often passivated so as to protect from subsequent corrosion by the ink. Reference is made in this regard to W095/07820.
Reference is also made to US 4,611,219 which discloses a base plate having piezoelectric or other liquid-jetting-pressure-generation elements disposed thereon, a spacer plate defining liquid chambers and an orifice plate with to - in an example - two orifices for each liquid chamber.
This invention provides droplet deposition apparatus comprising:
Such a construction brings to the arrangement of WO98/52763 - the advantage of reduced component count.
Corresponding method claims are also comprised in the present invention, and other aspects are as set out in other independent claims.
Further advantageous embodiments of the invention are set out in the description, drawings and dependent claims.
The disclosure of all claims is deemed incorporated here as consistory clauses, unless already set out above.
The invention will now be described by way of example by reference to the following diagrams, of which:
Figure 3 illustrates a printhead with those features that are common to figure 3 and the prior art printhead of figures 1 and 2 being designated by common reference numerals.
As in the prior art device, a piezoelectric ceramic body 12 poled in the thickness direction is formed with channels 11 separated by channel walls 13. As known from EP-A-0 364 136 referred to above, electrodes 23 are formed along each wall 13 in the ink-containing channel 11 as well as extending along a rearward groove 100 to the rear face 130 of the body. In addition, there is provided a cover 14, a surface 15 of which closes the open side of each of the channels 11, a nozzle plate 20 with nozzles 22 for droplet ejection and a manifold for supply of ink into the channel in the form of a transverse cut in the body 12. Surface 15 of cover 14 has tracks 16 formed thereon (suitable processes are well know) which in turn are connected to microchip 27 (which is illustrated figuratively in figure 3 and not to scale) which in turn receives input signals from input tracks 18.
Detail of the rear part of the printhead prior to attachment of the cover is shown in figure 4a: a passivation layer 140 (not shown in figure 3 but indicated by dashed hatching in figure 4a) is applied over the entirety of the electrodes 23 (indicated by solid hatching in both figures 3 and 4a) in the channel and part way along the rearward groove 100. In contrast to the prior art construction, passivation is carried out before attachment of the cover and advantageously according to the method described in W095/07820, belonging to the present applicant and incorporated herein by reference.
A mechanical bond between body and surface 15 of cover 14 is achieved by means of adhesive layer 160, applied to the end surfaces of the walls 13 in the region of the channels 11 prior to assembly of cover and body and preferably in accordance with the method discussed in W095/04658. Figure 4b illustrates the assembled printhead, with the adhesive bond being indicated at 220. Such a bond may indeed be tougher and have a longer fatigue life than the corresponding solder bond of the prior art construction described above.
Electrical connection between the conductive tracks 16 on the cover and that part of the electrode 23 in the rearward groove 100 is achieved by a protrusion 170 of a malleable, deformable, conductive material such as solder affixed to the end 180 of track 16. On assembly of the cover to the body, as illustrated in figure 4b, protrusion 170 comes into contact with electrode 23 and is deformed, thereby providing an effective electrical contact 200 between electrode 23 and track 16.
A bead 190 of a sealing paste or high viscosity glue is also applied so as to form on assembly an ink seal 210 between the end of the ink channel 11 and the electrical contact 200. Such a seal protects the electrical contact from later corrosion by ink. Preferably, the seal is positioned so as to straddle the free end 150 of the passivation layer 140, thereby preventing the seepage of ink under the passivation layer from where it might otherwise attack the electrode material 23.
In figure 5, a ceramic piezoelectric body 290 is, as in the previous embodiment, poled in the thickness direction and formed with channels 11 separated by channel walls 13 which in turn have an electrode 23 formed on each side. Ink ejection, however, takes place from a centrally located nozzle 320 formed either directly in the cover 350 or, as shown, in a nozzle plate 330 communicating with the channel via an aperture 340 formed in the cover. Body 290 is additionally formed with two manifolds 310 for supply of from both ends of the channel, as indicated by arrows 300. A further structure (not shown) will supply the manifolds with ink from a reservoir.
Such a "double-ended" printhead configuration is disclosed in W091/17051, and has advantages in terms of a lower operating voltage over the "single-ended" configuration described above. Furthermore, the configuration of base 290 is suited to manufacture by moulding - a technique that is potentially more attractive from the point of view of manufacturability than conventional sawing techniques described, in the aforementioned EP-A-0 364 136.
The connection of the channel electrode 23 to conductive tracks 370 formed on that surface of cover 350 facing body 290 is as already described with regard to figures 3, 4a and 4b, however, and is located in groove 360 formed at one side of the body 290. Similarly, in the region of the channel itself (the channel walls of which are passivated prior to assembly) and at that end 380 of the body not occupied by an electrical connection, cover 350 is attached to the piezoelectric ceramic body by a conventional adhesive bond (not shown).
In order to minimise the distance travelled by the ink from the channel proper 11 to the outlet of the nozzle 320 - thereby reducing pressure losses and consequent reductions in droplet ejection velocity - the nozzle 320 may be formed in the cover 350 itself. Advantageously the nozzle is formed by laser ablation as described, for example, in W093/15911, and to this end the cover may be made of an easily ablatable material, suitably a polymer such as polyimide, polycarbonate, polyester or polyetheretherketone, typically of 50µm thickness.
The stiffness of a cover plate formed of such an easily ablatable material may be increased by application of a coating of stiffer material to the inner and outer surfaces of the ablatable cover plate. Particularly suitable for this purpose is silicon nitride: it can also be used as a passivant coating in the process of the aforementioned W095/07820, is deposited as a smooth coating suitable for the subsequent application of a non-wetting coating, and will not short out electrodes of adjacent channels due to its non-conducting properties. Two layers of such a material placed either side of the polyimide cover and each having a thickness of around 5% of that of the cover (2.5µm in the case of a 50µm thick cover) will typically increase bending stiffness by a factor of 5-10 (based on standard compound beam theory and assuming a value of Young's Modulus for the stiffening material approximately 100 times greater than that of the polymer and good adhesion between the stiff and polymer materials). Such a thin layer has no significant effect on the ease with which the cover plate can be ablated to form a nozzle, particularly if the material of the layer itself is to some degree ablatable.
Expressed in broad terms, the cover plate for an inkjet printer comprises a layer of a first, easily ablatable, material having further layers bonded on opposite sides thereof, the further layers each being of a material having a stiffness at least an order of magnitude greater than that of the first material and being of a thickness at least an order of magnitude less than that of the first layer.
Referring now to figure 6, there is shown a printhead incorporating both first and second aspects of the present invention. Piezoelectric ceramic body 400 is formed with channels 11, channel-separating walls 13 and electrodes 23 which are supplied with actuating signals via conductive tracks 410 connected to drive circuitry (not shown). Unlike previous embodiments, however, droplet ejection takes place from a nozzle 420 communicating with an opening 430 formed in the body 400 at the closed, bottom surface 440 of the channel 11 - this is in contrast to figure 5 where the nozzle 320 is located in a cover 350 closing the open, top side of the channel 11.
Moulding is again the preferred method of manufacture of the channelled body 400, and the arrangement of figures 4a and 4b is again employed for electrical connection between the electrodes 23 and conductive tracks 410. Communication hole 430 may also be formed during the moulding process or may be formed subsequently, e.g. by means of a laser. Cover 450 no longer incorporates a nozzle but is instead formed with ink inlet ports 460. Such an arrangement has a lower component count than embodiments discussed earlier and has consequential manufacturing advantages. Alternatively, ink supply ports could be formed in the channelled component, e.g. at the channel ends.
Figure 7 illustrates an embodiment of the present invention.
The printhead of figure 7 also employs a cover component 500 having ink inlet ports 520, 522 and 524 located at either end and in the middle of a channel 11 formed in a piezoelectric body 530. Channel walls are separated by a gap 540 into two sections 550,560 supplied by ports 520,522 and 522,524 respectively, with each section being independently actuable by means of respective electrodes 570, 580 driven by drive circuits (not shown) via conductive tracks 650,660. For each section there is provided a respective nozzle 610,620 formed in a nozzle plate 615 and communicating with a section of the channel 11 via communication holes 630,640 formed in the bottom surface of the channel at points located midway between the respective inlet ports for that section.
Such a configuration results in a printhead having two parallel rows of independently actuable printing elements that is compact and which has a reduced actuating voltage per unit droplet ejection velocity due to the double-ended ink supply to each channel section.
Unlike earlier embodiments, the conductive tracks 650,660 that electrically connect the channel electrodes to the drive chips are formed on the piezoelectric body itself, advantageously in the same step in which the electrodes 570,580 are deposited on the channel walls. Such an arrangement is known from EP-A-0 397 441. Connection between track 650,660 and drive chip 590,600 may be achieved by any conventional method, including wire bonding or gold ball connection.
Piezoelectric body 530 may be moulded: in addition to having clear manufacturing advantages, such a process permits the end of the channel 11 to be formed as illustrated in figure 8, namely with a smooth, continuous transition 700 from the top surface 720 of the body to both the channel wall 730 and the bottom, longitudinal surface 710 of the channel. This in turn avoids discontinuities in the subsequently-deposited electrode material and the associated heating effects which might have a defeterious effect on the operational life of the printhead as a whole.
Alternatively, channels may be formed in the piezoelectric component by sawing using a disc cutter - as described e.g. in EP-A-0 309 148 - and illustrated in the sectional and detail sectional views of figures 9 and 10. It follows that the depth of the channel 11 will run out more gradually at each end, as shown at 800, and that the piezoelectric channel wall defined between adjacent sawn channels 11 will run continuously between the two active sections 550,560. However, a break 810 in the electrodes on the channel walls at a location between the two sections ensures that each the wall in active section can be actuated independently by signals supplied via electrical input 820. Such a break may be achieved e.g. by masking during deposition of the metal plating or by removal of the plating by a laser.
Connection between the electrodes on the channel walls and the electrical input 820, whilst not shown in detail, may be achieved by any of the known techniques including wire bond between tracks formed in shallow "run-out" grooves formed in the area 900 rearward of the channel 11 (described in the aforementioned EP-A-0 364 136) or conductive adhesive (e.g. anisotropic conductive adhesive) between conductive tracks formed in area 900 on the surface of the piezoelectric sheet itself and (described in EP-A-0 397 441).
As in the embodiment of figure 7, each channel 11 is closed along its two active sections 550,560 by appropriate lengths 820,830 of a cover component 500 which is also formed with ports 520,522,540 that allow ink to be supplied to each channel active section and, optionally, allow ink to be circulated through each channel section for cleaning purposes, as is generally known. Ports may be positioned so as to define the edge of an active section, as in the case of port 522, in which case manufacture is simplified. In the example shown, the width of cover port 552 and the cover closing lengths 820, 803 are of the same order of magnitude, typically 2mm.
Ink ejection from each active section is again via openings that communicate the channel with the opposite surface of the piezoelectric component (sheet 860) to that in which the channel is formed. In the present embodiment, these openings take the form of slots 840,850 which extend some distance - typically 200µm - in the longitudinal direction of the channel so as to allow some leeway in the placing of the respective nozzles 870,880 in nozzle plate 890. Offsetting of nozzles is generally necessary whenever simultaneous droplet ejection from adjacent channels is not possible e.g. in "shared wall" printheads of the kind illustrated, is generally known e.g. from EP-A-0 376, and will not therefore be discussed in any greater detail.
Printheads according to the present invention may also be made in a modular format as described in the aforementioned W091/17051, each module being formed in opposite end surfaces thereof with respective channel parts so that, upon butting together of modules, further channels are formed between respective pairs of butted modules. In such arrangements, the respective channel parts may include at least part of a slot formed in the channel base and of sufficient length that, even if a pair of butted modules and their respective slot parts are not perfectly aligned, there remains an overlap between the two slot halves sufficient to accommodate a nozzle.
As in the previous embodiment, nozzles 870,880 are formed in a nozzle plate 890 which, as illustrated, may extend over the substantially the entire length of piezoelectric sheet 860 so as to provide a suitably large area for engagement e.g. of a capping and/or wiping mechanism.
The piezoelectric channel walls, for example, can be polarised in opposite directions normal to the plane of the channel axes as known, for example, from EP-A-0 277 703. Alternatively, polarisation of the channel walls can be parallel to the plane of the channel axes with electrodes formed in the channel walls themselves as known, for example, from EP-A-0 528 647.
Nor is every channel in a printhead required to be capable of droplet ejection: active channels capable of droplet ejection may be alternated in the printhead with inactive - so-called "dummy" channels - as described, for example, in the aforementioned EP-A-0 277 703.