Nozzle for ink jet printer

Abstract: Nozzles (20) for an ink jet printer are formed by laser ablation in a nozzle plate (24) which has previously been bonded to the body of the printer (16). The laser beam is caused to converge at a point in front of the nozzle plate (24) so that a nozzle (20) is formed which tapers toward the outlet. First and second beam masks (72,74) are established in front of a focussing lens with the masks being respectively conjugate in the lens with the nozzle inlet and outlet, which are of different shape. The nozzle has a central land (218) which controls the ink meniscus and avoids the ejected drop receiving a sideways kick from the nozzle wall.

Description: The present invention relates to a nozzle for an ink jet printer, particularly drop-on demand ink jet printheads. Since the initial proposals for drop-on-demand ink jet printers, as for example, in US-A-3,946,398 (Kyser & Sears), it has been known to employ nozzles of a generally frusto-conical shape tapering towards the nozzle outlet. The geometry of the nozzles should be accurately made with regard to their location, exit area and direction of cone axis in the nozzle plate in order to ensure uniform quality of printing by ink drops ejected from them. A single nozzle plate typically incorporates between 50-5000 nozzles which apply monochrome ink or a number of independent ink colours. Accurate placement of the nozzles in a uniform pattern within and between colourways on the nozzle plate also controls printing quality. The geometrical shape of the cone is further chosen with particular regard to the dynamic acoustic impedances of the ink in the nozzle in the ejection cycle, to minimise the energy of drop ejection and at the same time damp the residual acoustic energy in the channel in preparation for a further operating cycle. The inlet section of the nozzle is also matched as far as possible to the ink channel section of the actuator behind it in order to prevent entrapment of air bubbles behind the nozzle plate which may disrupt operation of the printer. It is further generally accepted that it is necessary for the ink to wet the nozzle bore but to be non-wetting to the material of the nozzle plate exit face. Accordingly, it is required to develop methods of nozzle manufacture whereby nozzles are formed with a well defined and precisely reproduced geometric shape and are arranged in defined locations within the nozzle plate with optical accuracy. A further requirement of the methods is that manufacture be continuous and rapid, capable of making several thousand printheads per day at extremely low cost. In patent application EP-A-0 309 146 it is proposed, inter alia, to form the nozzles in the nozzle plate of an ink jet printhead following attachment of the nozzle plate, by exposing the exit face of the nozzle plate via an apertured mask to generally parallel beams of UV or high energy radiation and at the same time rocking the nozzle plate about axes normal to the radiation direction to form nozzles tapered towards the nozzle exit. One disadvantage of the rocking method is that only a small fraction of the radiation which is exposed to the mask is usually applied to the nozzle plate for ablation, the greater part being reflected from or absorbed in the mask. Accordingly, the power of the high energy beam used for manufacture is not efficiently employed and production rates are accordingly reduced in the production process as a whole. A further disadvantage is that the incoming parallel radiation when ablating the internal work face of the nozzle with the beam entering the exit apertures, is exposed to only a fraction of the work area owing to the rocking motion. Accordingly, a greater number of energy pulses, typically by an order of magnitude or more, is required to effect ablation of a nozzle than when ablation is applied uniformly in excess of the threshold ablation energy to the entire work area. It is an object of the invention to provide an improved nozzle for an ink jet printer. It is well understood that in the formation and ejection of an ink drop, the drop remains attached for a considerable period to the residual meniscus in the nozzle by a fluid ligament. This ligament extends progressively and ultimately breaks off at the time referred to as drop break-off. If - as is likely with no special precautions - the ink meniscus is withdrawn into the nozzle at drop break-off, then the tail of the drop may tend to be drawn up the concave meniscus surface onto the wall of the nozzle. The effect of any such attachment of the drop tail to the nozzle wall at break-off will be to give the ejected drop a sideways "kick" with the consequential risk of error in the drop landing position. It is a more specific object of the invention to provide an improved nozzle which deals with this problem. Accordingly, the present invention provides a nozzle of an ink jet printer, comprising a nozzle body, a nozzle bore extending through the nozzle body and a formation provided with the bore, said formation comprising an axially located land positioned to control the location in the ink meniscus of the tail of an ejected ink drop. Preferably, said formation further comprises a support structure extending radially outwardly from said land within the nozzle bore, the support structure being located wholly behind the land in the axial direction of ink drop ejection. Suitably, with the nozzle having a nozzle inlet of greater area than the nozzle outlet, said formation divides the nozzle inlet into discrete inlet portions, the maximum dimension of each inlet portion being less than the minimum dimension of the nozzle outlet. In one form of the invention, the formation extends across the bore to inhibit ink movement radially of the bore. The invention will now be described by way of example by reference to the following diagrams, of which:- FIGURES 1(a) and (b) are respectively a plan view and a sectional view in the plane of the nozzle axes which illustrate the prior art method of nozzle manufacture by UV laser ablation using a contact mask and rocking the nozzle plate to provide a nozzle tapered towards the nozzle exit; FIGURE 2 shows in plan view a mask in which apertures are formed concentric with the axes of a fraction (shown as one in twelve) of evenly spaced nozzles in the nozzle plate; FIGURE 3 shows, according to one embodiment of the invention, two orthogonal sections X-X and Y-Y of a nozzle formed by ablation and the optical paths of the incident beam of UV excimer radiation by which the nozzle is formed; FIGURE 4 shows the nozzle and the optical radiation paths shown in Figure 3 combined in isometric projection; FIGURE 5(a) shows (not to scale) an axial section of a nozzle formed by ablation and the optical paths through an improved optical projection system of the incident beam of UV excimer laser radiation from which the nozzle is formed; FIGURE 5(b) is a scrap view to an enlarged scale of part of FIG. 5(b); FIGURE 5(c) is a view not to scale of a part of Figure 5(a) and illustrates details of the optical geometry; FIGURES 6(a) and (b) are sectional views mutually at right angles of an integrated optics lens system for parallel nozzle production in a printhead by the use of an array of individual lens systems of the form illustrated in Figure 5(a); FIGURE 7 is a plan view of a mask for use in producing a nozzle according to this invention; FIGURES 8 and 9 are, respectively sectional and plan views of a nozzle according to this invention; FIGURES 10(a) to 10(e) are diagrams illustrating the manner of drop formation and ejection from the nozzle shown in Figures 8 and 9; and FIGURES 11, 12 and 13 are respectively top plan, section, and bottom plan views of a further nozzle according to the present invention. In the drawings, like parts are designated by the same reference numerals. Figure 1 illustrates one configuration of the prior art method of nozzle ablation using a collimated high energy beam 10 such as a UV excimer laser. The collimated beam 10 is directed against the mask 12 in which apertures 18(a), (b), (c) are formed. These expose the nozzle plate 14 attached to the printhead 16 in the region of nozzles 20(a), (b), (c) etc. The collimated beam 10 is aimed in the direction of the nozzle axes and directs pulses of high energy UV radiation with energy density exceeding the ablation threshold of the nozzle plate material. Typically the material is a chemically inert polymer such as polyester or polyimide whose threshold energy density is in the range 0.2-0.5 mJcm -2 . Continued exposure to energy pulses accordingly causes progressive ablation of the nozzles 20(a), (b), (c) so that by rocking the printhead 16 and nozzle plate 14 together with the mask 12 as indicated by the arrows 13, the nozzles bores are caused to be undercut, that is to say to be made with a taper in the nozzle exit direction. By rocking the nozzle plate unequally in two orthogonal axes the nozzle bore section can be suitably adjusted to match the section of the actuation channels 17 in the printhead 16 and to have the required nozzle exit area and exit cone axis direction. As previously indicated this method of nozzle ablation employing rocking uses only a small fraction of the radiation exposed to the mask, and requires an additional number of ablation pulses to ablate each nozzle compared with direct ablation applied to the whole of the surface to be ablated. Figure 2 shows a nozzle plate 24 having a row of nozzle apertures 26(a)-(z). The area of the nozzle apertures is relatively small compared with the area of strip 27 of illumination of UV excimer radiation that would typically be exposed to the mask 12 to ablate these nozzles using a rocking motion. A lower energy density of illumination is applied to the area of the nozzle plate, such that the energy per pulse previously applied to nozzles 26(a), 26(m),26(z) is now applied to the areas 28(a), 28(m), 28(z); and the energy in these areas is focussed by a suitable lens system in each of these three nozzle axes to form the respective nozzles 26(a), 26(m) and 26(z) by direct ablation without rocking but using a suitable form of beam. In this arrangement the energy per pulse per nozzle, is reduced by a factor of typically 100. At the same time the number of pulses that is sufficient to form a nozzle such as 26(m) during ablation by rocking is able to produce not only nozzle 26(m), but also the intervening nozzles 26(n)-26(y) each formed successively by direct ablation, by successively translating the nozzle plate laterally after each successive set of nozzles is formed. Accordingly, using a direct ablation method via an apertured mask and a lens system for each nozzle ablated, enables the number of nozzles per unit time produced with the same UV excimer laser installation to be increased by a typical factor of 100. The cost of nozzle manufacture tends, also after depreciation of the cost of the additional lens optics, to be reduced by a factor of 100. The method of direct ablation is described with reference to one nozzle by reference to Figures 3 and 4. In this method each nozzle is formed by a beam of high energy radiation, typically derived from an UV excimer laser, in which the beam is formed as a cone of high intensity radiation having the required shape to generate the nozzle profile to be ablated. Each cone of high energy radiation is formed with a separate mask and lens system per nozzle, and has an energy density in each section of the nozzle as great or greater than the ablation energy density threshold for the nozzle plate material. Typically the nozzle plate material is an inert polymer material such as polyimide or polyester, whose ablation threshold is relatively lower than that of ceramic or metals otherwise commonly used for ink jet nozzle manufacture. Figure 3 shows two orthogonal sections of the nozzle 40 and the optical axis 50 of the direct ablation system. Figure 4 shows the same sections in a combined isometric view. In these figures, incoming collimated high energy radiation is applied to a mask 52, in which an aperture 54 is formed. The aperture 54 corresponds to the apertures 28(a), 28(m) or 28(z) in Figure 2. Although drawn as a circular mask, it could be formed in other shapes such as oblate or rectangular, since it is not essential for the nozzle exit area to be circular. The lens system is indicated in Figures 3 and 4 by two cylindrical lenses 56 and 58 located at suitable heights relative to the exit plane 42 of the nozzle plate. In order to control the size of the nozzle ablated in the nozzle plate material 44, a precise optical feedback system (not shown) is utilised to control the height of the plane 42 relative to the lens system 56,58. Lens 56 is selected as regards its position, size and focal length to focus light on the axis 50 at a level 62 as indicated in Figure 3. Consequently the incident radiation through mask aperture 54 and lens 56 forms a cone corresponding to the section of nozzle 40 shown in section X-X of Figure 3. At the see time lens 58 is selected as regards its position, size and focal length to focus light on axis 50 at level 64 as indicated in section Y-Y in Figure 3. The incident radiation through mask aperture 54 and lens 58 in this section accordingly forms a cone corresponding to the section of nozzle 40 shown in section Y-Y of Figure 3. It will be evident that a wide variety of conical nozzle shapes can be formed in this manner by suitable choice of the lens positions, sizes and heights employed. It will further be apparent that if the focussing of the beam were effected by a spherical lens or lens system, a frusto-conically shaped nozzle could be formed. Also, a wide variety of lens systems may be employed to develop the optical beam profile in accordance with the art. It will be evident that lens 58 is located in the path of the focussing beam from lens 56 and consequently experiences a higher energy density of radiation than lens 56. A material capable of passing the required radiation energy density is accordingly chosen for lens 58. It will also be realised that the lenses 56 and 58 will be liable to significant spherical aberration, if they are formed as shown diagrammatically in Figures 3 and 4 in the form of a single lens each. To avoid such aberrration, a common solution is to form each lens from a series of two or more members of a compound lens system. Such lenses may be either spherical or cylindrical in accordance with the lens system design. Also a parabolic lens surface curvature may be adopted which also compensates for spherical aberration. The lenses indicated are of the simple solid curved component type that may be formed not only of glass but also of silica, sapphire, calcium fluoride or other material suitable for use in an UV optical system. Further sorts of lens such as grating, zoneplate or gradient index lens or lens systems may also be used. The high energy radiation from a UV excimer laser is usually characterised by beam divergence in the range ± 1m radians to ± 5m radians due to which the radiation into the aperture 54 of the system of Figures 3 and 4 is not in practice completely collimated. Accordingly the cone of concentrated radiation illustrated by section X-X and Y-Y in Figure 3 becomes spread over and above the size of cone that would be predicted from ray optics assuming that the incident beam is collimated. The degree of spread depends on the distance between the aperture 54 and the nozzle 40. A small degree of additional spread also arises due to beam diffraction. The spread due to beam divergence of the embodiment in Figures 3 and 4 is avoided in the conjugate arrangement disclosed in Figure 5(a)-(c). The system described with reference to Figure 3 is based on the assumption that the nozzle wall as it is ablated follows the geometrical profile of the incident radiation. In practice, however, it is found that the nozzle wall is formed at an angle relative to the light rays at the edges of the incident beam such that the wall is insufficiently undercut to provide the desired taper. As is well known, a parallel high energy beam directed at a plate produces a nozzle which tapers in the direction of the beam. The nozzle taper angle, moreover, is not constant. It depends both on the nozzle plate material and the density of the radiation. Thus, the angle between the wall and the rays is relatively large at low energy density near the ablation threshold and is small at high energy densities. One reason for this formation of the taper angle during ablation is that at a grazing incidence a significant proportion of the light, instead of being absorbed, is reflected, so that the edges are etched at a slower rate as the depth of the hole providing the nozzle increases. Even at the extreme of high fluence energy density the reduced undercut (which equates with the taper in the case of a parallel beam) is about 2-3 degrees. A further reason for the formation of the reduced undercut taper angle is that the etch depth per radiation pulse is found to reduce with increasing depth of the nozzle hole particularly at low and medium fluence. If the reduction in material removed is associated with a lowering in the photon density arriving at the bottom of the etched hole, reduced undercut occurs at the same time. The main reason for the reduction per pulse of photons arriving at the bottom of the etched hole is believed to be absorption or refraction in the plume of ablation products in the hole. Further, in a tapered nozzle formed by a parallel beam, the plume is concentrated, which further increases the absorption in the plume of the incident high energy with consequent decrease in the taper angle. In the case of the apparatus of Figure 3(a) and (b), the taper angle of the nozzle compared with the geometrical angle of the incident beam will be reduced by 2 to 3 degrees for high incident energy and by anything up to 10 to 15 degrees for lower energy, depending on the nozzle plate material and the aspect ratio of the nozzle. In Figure 5(a), the beam 10 of radiation is transmitted through a circular aperture 75 in a mask 74 by way of a convergent lens 88 which occupies the aperture and is shown as a plano-convex lens. In practice the lens 88 would be a compound convergent lens designed to avoid spherical aberration and formed by material which would transmit the radiation without appreciable absorption. The beam 10 converged by the lens 88 passes first through an aperture 71 in a mask 72, the aperture being centred on the optical axis 80 of lens 88, and thereafter through a convergent lens 79 disposed coaxially with the lens 88, which is shown here as a plano-convex lens but which in practice would be a compound convergent lens for the same reason as given in respect of lens 88 and would also be made of low radiation absorption material. The focussed beam is incident upon the surface 84 of the nozzle plate 70. The incident beam delivered by a UV excimer laser has a divergence but this is generally much less than the required angle α. Accordingly a layer 86 is placed against the lens 88 to increase to the angle α the divergence of the beam incident into the aperture. This layer may be a ground or etched surface or a thin film containing a medium having suitable light scattering properties, such as a colloid or opalescent material. One objective of a direct ablation system providing projection optics for ablation is to employ the incident photon energy as efficiently as possible. Accordingly, it is desirable for the beam energy transmitted by the mask aperture 75 and focussed by lens 88 to be evenly distributed over the mask aperture 71. To this end a degree of divergence α is desirable which can be defined as the angle subtended by the mask aperture 71 at the mask 74. Rays of the beam of incident radiation are shown at the periphery and centre of the beam as having a divergence angle α at the lens 88 created by the layer 86. The rays at the centre of the beam diverge at angle α from the lens to occupy fully the aperture 71 in the case where the latter is circular whilst the rays at the periphery of the beam are bent by the lens through the focussing angle  (see Figure 5(c), the value of which, as is plain from Figure 5(b), is given by:  = β 1 - 1/2 α = β 1 + β 2 2 where β 1 is the angle which the outer and β 2 is the angle which the inner extremity of the diverged peripheral ray (after diverging by the layer 86 and focussing by the lens 88) makes with the optical axis. In this way, the diverged peripheral ray passes through and fully occupies the aperture 71 in the mask 72. After passage through the aperture 71, the rays of the radiation beam 10 are converged by the lens 79. As is seen in Figure 5(a), the rays at the outer extremity of the diverged peripheral rays of the beam of radiation incident on the lens 88 pass, after focussing by lens 79, through a point 100 (see Fig. 5(c)) on the optical axis. From the point 100, they diverge to intersect surface 82 of nozzle plate 70 to define the inlet of the nozzle formed in the plate. Prior to intersection at the location 100, these rays intersect surface 84 of the nozzle plate to define the nozzle outlet. Also rays at the inner extremity of the diverged peripheral rays incident on lens 88 pass after focussing by lens 79 through a point 102 on the optical axis from which they diverge and intersect the surfaces 84 and 82 of the nozzle plate 70 also to define respectively the outlet and inlet of the nozzle. These rays further define in their passage through the plate 70, the bore of the nozzle. The rays which are incident on the lens 88 at the optical axis, diverge from the lens 88 and thereafter from the aperture 71, and are incident upon and focussed by the lens 79 to locations on the nozzle outlet side of the point 100 and therefore pass pass through the plate 70 at points within the nozzle bore. Thus it is apparent that the nozzle in the plate 70 can be ablated by rays which lie within the conical frustum extending between the apertures 75 and 71 of the masks 74 and 72. The conical frustum is therefore by way of lens 79 conjugate with the nozzle and apertures 75 and 71 are accordingly respectively conjugate with the nozzle outlet and inlet in the surfaces 84 and 82. In practice the nozzle inlet is preferably made rectangular to match the shape of the channel with which the nozzle communicates whilst the nozzle outlet is preferably circular. This means that whilst aperture 75 is circular, aperture 71 is rectangular. As has been stated, one objective of a direct ablation system providing projection optics for ablation is to employ the incident photon energy as efficiently as possible. Accordingly it is desirable for layer 86 to have minimum back scatter, and as far as practical spread the incident radiation energy as uniformly as possible into the range of divergence defined by the angle α and at the same time to avoid spreading radiation into angles of divergence outside the angle α which will not be focussed into the focussing lens 79. It will be evident that the angle α is determined by the nozzle cone shape and also by the focal length of lens 78. The distance between the apertures 75 and 71 is equal to the focal length of the lens 88. The angle of divergence α will vary in the orthogonal sections X-X and Y-Y in the nozzle, if the nozzle, as is usually the case to match the section of the channels 17, has unequal dimensions in the two sections. It will normally only be possible for the divergence of the beam to differ in the two sections if the layer 86 is suitably anistropic. However, in practice, the layer 88 will usually have uniform properties in the different orientations. Due to the divergence characteristics of practical materials used in the layer 86, compared with its ideal divergence properties, together with the other sources of loss indicated above, there will generally be a loss of incident energy in return for the improved geometrical accuracy of the nozzle profile provided by ablation using the embodiment of Figures 5(a) to (c). Figures 6(a) and 6(b) illustrate an integrated optic lens system 90 for forming five nozzles which are uniformly spaced over the length of a serial (i.e. shuttle type) printhead 92 of an ink jet printer. After forming five nozzles on the nozzle axes as indicated the printhead 92 is indexed through one nozzle pitch and a further five nozzles are then formed. This operation is then repeated until all nozzles of the printhead are formed. The optical axes of the lens system are thus spaced at an integral multiple of the spacing of the nozzles required to be formed in the nozzle plate and the nozzles are formed in a number of stages equal to that multiple and between which the indexing of the printhead through one nozzle pitch takes place. It will be evident that if the printhead is an array type of construction spanning a relatively greater width than serial printhead 92, then the integrated optics array 90 may be correspondingly wider and incorporate a correspondingly greater number of nozzle axes. Alternatively if the production system is one where a large number of serial printheads 90 are traversed end to end through the nozzle ablation system, then there may be a substantial number of integrated optics arrays side by side each with five (or another suitable number) of nozzle axes so that each integrated optics array is used to ablate a corresponding printhead, after which the printheads are removed and a further batch presented. In such arrangements high energy radiation can be presented to large numbers of nozzles in parallel. In WO91/17051, assigned to the applicants, there is disclosed a nozzle forming mask. The integrated optics array 90 of Figures 6(a) and (b) is one possible form of the aforementioned mask. The base of the array 90 is therefore made up of a material, such as silicon or quartz, whose thermal expansion coefficient is low, being substantially matched to that of piezoelectric material (suitably, PZT), from which the printhead 92 is formed. The base is also of a material whereby an alignment mask for assembly of the component parts of an array printhead can be made from the same material and in register. The integrated optics array 90 of Figures 6(a) and 6(b) is shown with UV laser beam 10 exposed to the layer 86, which causes the divergence of the input beam to be increased. The input radiation is then transmitted through each of the individual optical systems by way of convergent lens 88, apertures 75 and 71 of masks 74 and 72 and converging system 79. Preferably, this system 79 takes zone plate form, having masks 78 deposited on opposing quartz surfaces. The converging system 79 thereby focuses the radiation into the plane of the nozzle plate 70 of the printhead. Suitably, the system 79 and mask 78 can be made in a form where they are common to all the parallel optical systems forming the array. In order to ensure that the focussed radiation is at the correct height and attitude, the integrated array 90 is mounted on position control actuators 94 and 96 for vertical and horizontal as well as rotational movement. Typically these control systems maintain the nozzle tolerances required to ensure that in operation the ink drop velocity and volume are within defined limits. There will now be described a novel nozzle structure which can be produced using the above-mentioned methods, or in other ways. Referring to Figure 7, there is shown a modified mask 200 that can be substituted - for example - for the mask 72 in the arrangement of Figure 5(a). The mask 200 has a circular aperture 202 with three radial vanes 204 and a central disc 206. When light passes a mask of this form and is used to generate a cone of radiant energy suitable for nozzle ablation, a structure is produced as shown in Figures 8 and 9. The nozzle bore 208 contains three radial webs 210 which support an axial pin 212. The webs 210 and the pin 212 taper in transverse dimension generally with the taper in the nozzle bore from the nozzle inlet 214 to the nozzle outlet 216, though as can be seen from Figure 5(a), the angle of taper will vary across the mask as B◊1◊ and B◊2◊. It must also be recognised that the shape of the nozzle which is ablated is not identical with the projection of the masks, as a result - among other things - of diffraction. It will be seen that the tapering pin 212 and webs 210 do not extend fully to the nozzle exit plane. As shown in Figure 8, the pin 212 terminates short of the nozzle outlet 216 providing a central land 218 which is less than a nozzle outlet diameter in distance from the nozzle outlet plane. The webs 210 are similarly cut back and it is important to note that the land 218 is the foremost part of the formation, in the direction of drop ejection arrowed at 220. The land 218 serves an important function in controlling the tail of an ink drop as it is expelled from the nozzle. It is well understood that in the formation and ejection of an ink drop, the drop remains attached for a considerable period to the residual meniscus in the nozzle by a fluid ligament. This extends progressively and ultimately breaks off at the time referred to as drop break-off. If - as is likely with no special precautions - the ink meniscus is withdrawn into the nozzle at drop break-off, then the tail of the drop may tend to be drawn up the concave meniscus surface onto the wall of the nozzle. The effect of any such attachment of the drop tail to the nozzle wall at break-off will be to give the ejected drop a sideways "kick" with the consequential risk of error in the drop landing position. This problem is illustrated in Figure 10 (e). In accordance with this feature of the present invention, the land 218 controls the ink meniscus in such a way that the drop tail is centralised in the nozzle at break-off. Referring to the sequence of Figures 9(a) to 9(d), it will seem that the ink meniscus is extended until a drop is formed which is connected to the body of ink remaining in the nozzle by a tail. As the periphery of the ink meniscus retracts into the nozzle bore, the tail is held captive on the land 218. With continuing extension and ultimate break-off of the drop, the risk of the drop receiving a sideways kick from contact with the nozzle plate is very much reduced. A further advantage which the present invention provides in controlling the tail of an ejected drop, is that there is a reduced likelihood of small droplets of ink being left on the front surface of the nozzle plate. It will be seen that if the tail of the drop is in contact with the nozzle wall at break off, any small amount of ink which is not drawn foward with the tail to join the ejected drop, may remain on the front surface of the nozzle plate and dry. Such accumulations of dried ink on the nozzle plate can lead to drop placement errors and eventually to nozzle blockage. Since, in accordance with the present invention, the tail of the drop is centralised at break-off, any small amount of ink which is not drawn forward with the tail will remain on the central land to rejoin the body of ink within the nozzle on the next forward movement of the ink meniscus. In a separate effect, the webs 210 are believed to serve the useful function of inhibiting ink movements in the radial direction. Unwanted transverse oscillations of the ink meniscus within the nozzle, have been observed under certain operating conditions. These waves travelling across the nozzle can trap air pockets leading to problems with bubbles in the ink. By inhibiting radial or transverse ink movements, the present invention reduces the likelihood of problems arising with bubbles. The described arrangement of a pin providing a central land which is set back a nozzle diameter or less from the nozzle outlet and three angularly spaced supporting webs, is of course only one example of a formation within the nozzle bore which can serve to control the nozzle ink meniscus. In relatively minor modifications, the number, thickness and angular positioning of the webs can be varied to optimise the performance of a particular nozzle or nozzle array. By varying the projection geometry, the distance which the central land is set back from the nozzle may be adjusted. It is believed to be preferable for the amount of set-back to be not greater than about two nozzle diameters. Whilst the described nozzle structure, with any of the above variations, can conveniently be produced using methods as hereinbefore described and discussed, a range of known nozzle manufacture techniques can alternatively be employed. Thus the prior art laser ablation and rocking technique of EP-A-0 309 146 may be adapted with the use of a mask as shown in Figure 7, to produce a nozzle in accordance with this aspect of the invention. In this approach, the mask would be employed as a contact rather than a projection mask. The advantage would remain that nozzle formation was carried out from the nozzle outlet side of the nozzle plate, so that nozzles can be formed after bonding of the nozzle plate to the body of the printer. If this is not a requirement, other laser ablation or photoresist-based manufacturing techniques can be employed with suitable masking. A further alternative nozzle manufacturing technique resulting in a rather different formation within the nozzle bore, will now be described with reference to Figures 11 and 12. Using any appropriate technique, for example laser ablation or ion beam drilling, six inclined bores 300 are formed in the nozzle plate 302. At the inlet face 304 of the nozzle plate, the bores 300 are equivalently spaced on a common radius. At the outlet face 306, the bores coalesce to form a nozzle outlet 308 which can be seen to be the envelope of six ellipses. Within the body of the nozzle plate, a central land 310 is defined with supporting webs 312 of complex form. In operation of the so-formed nozzle, the Iand 310 and webs 312 function in the same manner as land 218 and webs 210 of the previously described embodiment. It will be recognised that the combined nozzle inlet area, being the sum of the areas of the bores 300 as projected onto the nozzle inlet face, is greater than the nozzle outlet area, meeting the functional requirement of a tapering nozzle. However, each bore has an inlet which is smaller than the common outlet. This means that any particles in the ink which are large enough to block the nozzle outlet are prevented form entering the nozzle. The nozzle structure can thus be seen to combine the features of a tapering nozzle and a particle filter. In fact, this feature of filtering out particles in the ink which would block the nozzle outlet, can be provided in a nozzle such as that shown in Figures 8 and 9, by appropriate choice of the mask dimensions. The described multi-bore nozzle can be produced using a high energy beam in six passes with the angle and position of the nozzle plate being incremented between passes. There will in certain cases be advantage in combining a method such as described above in relation to Figure 3 or Figure 5 with such an approach. An alternative is to employ in place of the projection mask shown in Figure 7, a projection mask having a series of, say, six holes equiangularly spaced on a common radius. Still further alternatives will occur to the skilled man.

Claims: A nozzle for an ink jet printer, comprising a nozzle body; a nozzle bore extending through the nozzle body and a formation provided within the bore, said formation comprising an axially located land positioned to control the location in the ink meniscus of the tail of an ejected ink drop. A nozzle according to Claim 1, wherein the land is positioned behind the nozzle outlet in the axial direction of ink drop ejection by a distance which is not substantially greater than twice the maximum dimension of the nozzle outlet. A nozzle according to Claim 2, wherein the land is positioned behind the nozzle outlet in the axial direction of ink drop ejection by a distance which is not substantially greater than the maximum dimension of the nozzle outlet. A nozzle according to Claim 1, Claim 2, or Claim 3, wherein said formation further comprises a support structure extending radially outwardly from said land within the nozzle bore, the support structure being located wholly behind the land in the axial direction of ink drop ejection. A nozzle according to Claim 4, wherein said support structure comprises one or more radially extending webs. A nozzle according to Claim 5, having a nozzle inlet of greater area than the nozzle outlet, wherein said formation divides the nozzle inlet into discrete inlet portions, the maximum dimension of each inlet portion being less than the minimum dimension of the nozzle outlet. A nozzle according to any one of the Claims 1 to 6, wherein the formation extends across the bore to inhibit ink movement radially of the bore.

Inventors: SHEPHERD MARK, TEMPLE STEPHEN, SHEPHERD, MARK, TEMPLE, STEPHEN
Applicants: XAAR TECH LTD, XAAR TECHNOLOGY LIMITED
Application year: 1993
Main classification: B41J2
IPC codes: B41J2, B23K26