DROP FLIGHT CORRECTION FOR MOVING NOZZLE INK JET

Abstract: An inkjet printhead including a plurality of nozzles each adapted to eject drops of ink through an opening (9) toward a surface to be printed; wherein the opening (9) is asymmetrical to provide a flight direction of ejected drops that differs from the flight direction provided if the opening (9) were symmetrical.

Description: DROP FLIGHT CORRECTION FOR MOVING NOZZLE INK JET Field of the Invention This invention relates to an ink jet printhead. More particularly, the invention relates to a Drop Flight Correction for Moving Nozzle Ink Jet. Background to the Invention Most ink jet printheads of the type manufactured using micro-electro mechanical systems (MEMS) technology have been proposed in a construction using nozzle chambers formed in MEMS layers on the top of a substrate with nozzle chambers formed in the layers. Each chamber is provided with a movable paddle actuated by some form of actuator to force ink in a drop through the nozzle associated with the chamber upon receipt of an electrical signal to the actuator. Such a construction is typified by the disclosure in my International Patent Application PCT/AU99/00894. Other constructions may dispense with the paddles and cause ink drops to be forced from the nozzle by decreasing the size of the nozzle chamber. It has been realised that this can be achieved by causing the actuator to move the nozzle itself downwardly in the chamber thereby avoiding the need for a paddle, simplifying construction and providing an environment which is less prone to the leakage of ink from the nozzle chamber. In both these types of construction, the actuators are typically fixed/free end beam type thermal bend actuators. During operation, these slightly rotate the paddles in the chambers in the first mentioned construction type or the nozzle openings in the second mentioned construction type. Because of the angular displacement of the paddle in the first type of nozzle, the drop may be ejected at a slight angle to the normal of the plane of the opening. Because of the angular displacement of the nozzle opening in the second type of nozzle, the drop may be ejected normal to the plane of the opening, but the normal has angularly displaced along with the opening during the ejection of the drop. This slight angular deviation of the drop is not a potential problem if all the nozzles on the printhead have the same orientation. However, to maximise the packing density of nozzles on the printhead, and thereby reduce manufacturing costs, the nozzles in adjacent rows are oriented such that their actuators extend in opposite directions. Accordingly, the angular deviations in the flight of drops from successive rows are also in opposite directions. This can ultimately have adverse effects on print quality. Summary of the Invention According to the invention there is provided an ink jet printhead including a plurality of nozzles each adapted to eject drops of ink through an opening toward a surface to be printed; wherein the opening is asymmetrical to provide a flight direction of ejected drops that differs from the flight direction provided if the opening were symmetrical. Preferably, each nozzle has an associated nozzle chamber adapted to be supplied with ink via at least one conduit in an underlying substrate, the nozzle chamber being defined by a floor portion having peripheral sidewall and a roof portion having depending peripheral sidewall, the sidewalls of the floor and roof portion arranged to overlap in a telescopic manner; said roof portion having said opening formed therein for the ejection of ink drops from said nozzle chamber; and an actuator arranged to move said roof portion upwardly and downwardly relative to said floor portion, wherein said roof portion tilts relative to said floor portion when moved downwardly by said actuator, and said opening is asymmetrically shaped so as to eject ink drops in a direction other than perpendicular to said roof portion. Preferably, the nozzle is configured to eject ink in a direction substantially perpendicular to said floor portion. Preferably, the nozzle is so configured by being provided with a relatively wider portion at the end of the opening proximate the actuator, and a relatively narrower portion at the end of the opening remote from the actuator. Preferably, the wider portion is flatter in edge configuration than the narrower portion which is relatively more pointed. Brief Description of the Drawings Notwithstanding any other forms that may fall within its scope, one preferred form of the invention will now be described by way of example only with reference to the accompanying drawings in which: Fig. 1 is a partially cutaway perspective view of a moving nozzle ink jet assembly, Fig. 2 is a similar view to Fig. 1 showing the bend actuator of the moving nozzle bent causing a drop of ink to protrude from the nozzle. Fig. 3 is a similar view to Fig. 1 showing the nozzle returned to the original position and a drop of ink ejected from the nozzle. Fig. 4 is cross-sectional view through the mid line of the apparatus as shown in Fig. 2. Fig. 5 is a similar view to Fig. 1 showing the use of an optional nozzle poker. Fig. 6 is a similar view to Fig. 5 showing the bend actuator bent and a drop of ink protruding from the nozzle. Fig. 7 is a similar view to Fig. 5 showing the bend actuator straightened and the drop of ink being ejected from the nozzle. Fig. 8 is a similar view to Fig. 1 without the portions cut away. Fig. 9 is a similar view to Fig. 8 with the nozzle and bend actuator removed and showing an optional constriction in the nozzle chamber. Fig. 10 is a similar view to Fig. 9 with the upper layers removed, and Fig. 11 is a similar view to Fig. 1 showing the bend actuator cut away, and the actuator anchor detached for clarity. It will be appreciated that a large number of similar nozzles are simultaneously manufactured using MEMS and CMOS technology as described in our co-pending patent applications referred to at the beginning of this specification. For the purposes of clarity, the construction of an individual ink jet nozzle alone will now be described. Whereas in conventional ink jet construction of the type described in our above referenced co-pending patent applications, ink is ejected from a nozzle chamber by the movement of a paddle within the chamber, according to the present invention the paddle is dispensed with and ink is ejected through an opening (nozzle) in the upper surface of the chamber which is moved downwardly by a bend actuator, decreasing the chamber volume and causing ink to be ejected through the nozzle. Throughout this specification, the term "nozzle" is to be understood as an element defining an opening and not the opening itself. Furthermore, the relative terms "upper" and "lower" and similar terms are used with reference to the accompanying drawings and are to be understood to be not in any way restrictive on the orientation of the ink jet nozzle in use. Referring now to figures 1 to 3 of the accompanying drawings, the nozzle is constructed on a substrate 1 by way of MEMS technology defining an ink supply aperture 2 opening through a hexagonal opening 3 (which could be of any other suitable configuration) into a chamber 4 defined by floor portion 5, roof portion 6 and peripheral sidewalls 7 and 8 which overlap in a telescopic manner. The sidewalls 7, depending downwardly from roof portion 6, are sized to be able to move upwardly and downwardly within sidewalls 8 which depend upwardly from floor portion 5. The ejection nozzle is formed by rim 9 located in the roof portion 6 so as to define an opening for the ejection of ink from the nozzle chamber as will be described further below. The roof portion 6 and downwardly depending sidewalls 7 are supported by a bend actuator 10 typically made up of layers forming a Joule heated cantilever which is constrained by a non-heated cantilever, so that heating of the Joule heated cantilever causes a differential expansion between the Joule heated cantilever and the non-heated cantilever causing the bend actuator 10 to bend. The proximal end 11 of the bend actuator is fastened to the substrate 1, and prevented from moving backwards by an anchor member 12 which will be described further below, and the distal end 13 is secured to, and supports, the roof portion 6 and sidewalls 7 of the ink jet nozzle. In use, ink is supplied into the nozzle chamber through passage 2 and opening 3 in any suitable manner, but typically as described in our previously referenced co-pending patent applications. When it is desired to eject a drop of ink from the nozzle chamber, an electric current is supplied to the bend actuator 10 causing the actuator to bend to the position shown in figure 2 and move the roof portion 6 downwardly toward the floor portion 5. This relative movement decreases the volume of the nozzle chamber, causing ink to bulge upwardly through the nozzle rim 9 as shown at 14 (Fig. 2) where it is formed to a droplet by the surface tension in the ink. As the electric current is withdrawn from the bend actuator 10, the actuator reverts to the straight configuration as shown in figure 3 moving the roof portion 6 of the nozzle chamber upwardly to the original location. The momentum of the partially formed ink droplet 14 causes the droplet to continue to move upwardly forming an ink drop 15 as shown in Fig. 3 which is projected on to the adjacent paper surface or other article to be printed. In one form of the invention, the opening 3 in floor portion 5 is relatively large compared with the cross-section of the nozzle chamber and the ink droplet is caused to be ejected through the nozzle rim 9 upon downward movement of the roof portion 6 by viscous drag in the sidewalls of the aperture 2, and in the supply conduits leading from the ink reservoir (not shown) to the opening 2. This is a distinction from many previous forms of ink jet nozzles where the "back pressure" in the nozzle chamber which causes the ink to be ejected through the nozzle rim upon actuation, is caused by one or more baffles in the immediate location of the nozzle chamber. This type of construction can be used with a moving nozzle ink jet of the type described above, and will be further described below with specific reference to figures 9 and 10, but in the form of invention shown in figures 1 to 3, the back pressure is formed primarily by viscous drag and ink inertia in the supply conduit. In order to prevent ink leaking from the nozzle chamber during actuation ie. during bending of the bend actuator 10, a fluidic seal is formed between sidewalls 7 and 8 as will now be further described with specific reference to figures 3 and 4. The ink is retained in the nozzle chamber during relative movement of the roof portion 6 and floor portion 5 by the geometric features of the sidewalls 7 and 8 which ensure that ink is retained within the nozzle chamber by surface tension. To this end, there is provided a very fine gap between downwardly depending sidewall 7 and the mutually facing surface 16 of the upwardly depending sidewall 8. As can be clearly seen in Fig. 4 the ink (shown as a dark shaded area) is restrained within the small aperture between the downwardly depending sidewall 7 and inward faces 16 of the upwardly extending sidewall by the proximity of the two sidewalls which ensures that the ink "self seals" across free opening 17 by surface tension, due to the close proximity of the sidewalls. In order to make provision for any ink which may escape the surface tension restraint due to impurities or other factors which may break the surface tension, the upwardly depending sidewall 8 is provided in the form of an upwardly facing channel having not only the inner surface 16 but a spaced apart parallel outer surface 18 forming a U-shaped channel 19 between the two surfaces. Any ink drops escaping from the surface tension between the surfaces 7 and 16, overflows into the U-shaped channel where it is retained rather than "wicking" across the surface of the nozzle strata. In this manner, a dual wall fluidic seal is formed which is effective in retaining the ink within the moving nozzle mechanism. As has been previously described in some of our co-pending applications, it is desirable in some situations to provide a "nozzle poker" to clear any impurities which may build up within the nozzle opening and ensure clean and clear ejection of a droplet from the nozzle under actuation. A configuration of the present invention using a poker in combination with a moving nozzle ink jet is shown in the accompanying figures 5, 6 and 7. Figure 5 is similar to figure 1 with the addition of a bridge 20 across the opening 3 in the floor of the nozzle chamber, on which is mounted an upwardly extending poker 21 sized to protrude into and or through the plane of the nozzle during actuation. As can be seen in figure 6, when the roof portion 6 is moved downwardly by bending of the bend actuator 10, the poker 21 is caused to poke up through the opening of the nozzle rim 9 and part way into the bulging ink drop 14. As the roof portion 6 returns to its original position upon straightening of the bend actuator 10 as shown in Fig. 7 the ink droplet is formed and ejected as previously described and the poker 21 is effective in dislodging or breaking any dried ink which may form across the nozzle rim and which would otherwise block the nozzle. It will be appreciated, that as the bend actuator 10 is bent causing the roof portion to move downwardly to the position shown in Fig. 2, the roof portion tilts relative to the floor portion 5 causing the nozzle to move into an orientation which is not parallel to the surface to be printed, at the point of formation of the ink droplet. This orientation, if not corrected, would cause the ink droplet 15 to be ejected from the nozzle in a direction which is not quite perpendicular to the plane of the floor portion 5 and to the strata of nozzles in general. This would result in inaccuracies in printing, particularly as some nozzles may be oriented in one direction and other nozzles in a different, typically opposite, direction. The correction of this non-perpendicular movement can be achieved by providing the nozzle rim 9 with an asymmetrical shape as can be clearly seen in figure 8. The nozzle is typically wider and flatter across the end 22 which is closer to the bend actuator 10, and is narrower and more pointed at end 23 which is further away from the bend actuator. This narrowing of the nozzle rim at end 23, increases the force of the surface tension at the narrow part of the nozzle, resulting in a net drop vector force indicated by arrow 24A in the direction toward the bend actuator, as the drop is ejected from the nozzle. This net force propels the ink drop in a direction which is not perpendicular to the roof portion 6 and can therefore be tailored to compensate for the tilted orientation of the roof portion at the point of ink drop ejection. By carefully tailoring the shape and characteristics of the nozzle rim 9, it is possible to completely compensate for the tilting of the roof portion 6 during actuation and to propel the ink drop from the nozzle in a direction perpendicular to the floor portion 5. Although, as described above, the back pressure to the ink held within the nozzle chamber may be provided by viscous drag in the supply conduits, it is also possible to provide a moving nozzle ink jet with back pressure by way of a significant constriction close to the nozzle. This constriction is typically provided in the substrate layers as can be clearly seen in figures 9 and 10. Figure 9 shows the sidewall 8 from which depend inwardly one or more baffle members 24 resulting in an opening 25 of restricted cross-section immediately below the nozzle chamber. The formation of this opening can be seen in figure 10 which has the upper layers (shown in Fig. 9) removed for clarity. This form of the invention can permit the adjacent location of ancillary components such as power traces and signal traces which is desirable in some configurations and intended use of the moving nozzle ink jet. Although the use of a restricted baffle in this manner has these advantages, it also results in a longer refill time for the nozzle chamber, which may unduly restrict the speed of operation of the printer in some uses. The bend actuator which is formed from a Joule heated cantilever 28 positioned above a non-heated cantilever 29 joined at the distal end 13 needs to be securely anchored to prevent relative movement between the Joule heated cantilever 28 and the non-heated cantilever 29 at the proximal end 11, while making provision for the supply of electric current into the Joule heated cantilever 28. Figure 11 shows the anchor 12 which is provided in a U-shaped configuration having a base portion 30 and side portions 31 each having their lower ends formed into, or embedded in the substrate 26. The formation of the bend actuator in a U-shape, gives great rigidity to the end wall 30 preventing any bending or deformation of the end wall 30 relative to the substrate 26 on movement of the bend actuator. The non-heated cantilever 29 is provided with outwardly extending tabs 32 which are located within recesses 33 in the sidewall 31, giving further rigidity , and preventing relative movement between the non-heated cantilever 29 and the Joule heated cantilever 28 in the vicinity of the anchor 27. In this manner, the proximal end of the bend actuator is securely and firmly anchored and any relative movement between the Joule heated cantilever and the non-heated cantilever prevented in the vicinity of the anchor. This results in enhanced efficiency of movement of the roof portion 6 of the moving nozzle ink jet.

Claims: Claims 1. An ink jet printhead including a plurality of nozzles each adapted to eject drops of ink through an opening toward a surface to be printed; wherein the opening is asymmetrical to provide a flight direction of ejected drops that differs from the flight direction provided if the opening were symmetrical. 2. An ink jet printhead as claimed in claim 1, wherein each nozzle has an associated nozzle chamber adapted to be supplied with ink via at least one conduit in an underlying substrate, the nozzle chamber being defined by a floor portion having peripheral sidewall and the roof portion having depending peripheral sidewall, the sidewalls of the floor and roof portion arranged to overlap in a telescopic manner; said roof portion having said opening formed therein for the ejection of ink drops from said nozzle chamber; and an actuator arranged to move said roof portion upwardly and downwardly relative to said floor portion, wherein said roof portion tilts relative to said floor portion when moved downwardly by said actuator, and said opening is asymmetrically shaped so as to eject ink drops in a direction other than perpendicular to said roof portion. 3. An ink jet printhead as claimed in claim 2, wherein the nozzle is configured to eject ink in a direction substantially perpendicular to said floor portion. 4. An ink jet printhead as claimed in claim 3, wherein the nozzle is so configured by being provided with a relatively wider portion at the end of the opening proximate the actuator, and a relatively narrower portion at the end of the opening remote from the actuator. 5. An ink jet printhead as claimed in claim 4, wherein the wider portion is flatter in edge configuration than the narrower portion which is relatively more pointed.

Inventors: SILVERBROOK KIA, SILVERBROOK, KIA
Applicants: SILVERBROOK KIA, SILVERBROOK RES PTY LTD, SILVERBROOK RESEARCH PTY. LTD., SILVERBROOK, KIA
Application year: 2001
Main classification: B41J2
IPC codes: B41J2