Abstract:
The invention relates to an ink jet printhead including: 
     a plurality of nozzles mounted to a rigid substrate  26 , each of the nozzles being adapted to eject drops of ink toward a surface to be printed; 
     each of the nozzles also having a bend actuator and an apertured roof portion  6 , the bend actuator adapted for anchorage to the rigid substrate  26  and connection to the apertured roof portion  6  such that in use the actuator moves the roof portion  6  away from the surface to be printed, in order to eject the ink; wherein, 
     the displacement of all points along the operative section of the bend actuator during its movement from a de-activated to an activated state, constantly increases from zero displacement at the point of anchorage  11  to the rigid substrate  12, 26 , to maximum displacement at the point of connection  13  to the apertured roof portion  6.

Description:
FIELD OF THE INVENTION 
     This invention relates to an ink jet printhead. More particularly, the invention relates to an Actuator Anchor. 
     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. 
     The present invention stems from the realisation that there are advantages to be gained by dispensing with the paddles and causing 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 thus dispensing with the paddle, simplifying construction and providing an environment which is less prone to the leakage of ink from the nozzle chamber. 
     SUMMARY OF THE INVENTION 
     According to the invention there is provided an ink jet printhead including: 
     a plurality of nozzles mounted to a rigid substrate, each of the nozzles being adapted to eject drops of ink toward a surface to be printed; 
     each of the nozzles also having a bend actuator and an apertured roof portion, the bend actuator adapted for anchorage to the rigid substrate and connection to the apertured roof portion such that in use the actuator moves the roof portion away from the surface to be printed, in order to eject the ink; wherein, 
     the displacement of all points along the operative section of the bend actuator during its movement from a de-activated to an activated state, constantly increases from zero displacement at the point of anchorage to the rigid substrate, to maximum displacement at the point of connection to the apertured roof portion. 
     Preferably, each of the nozzles further includes an associated nozzle chamber adapted to be supplied with ink via at least one conduit in the underlying substrate. 
     Preferably, the roof portion has a sidewall depending from its periphery to telescopically engage a peripheral sidewall extending from an opposing floor portion to define the nozzle chamber. 
     Preferably, the bend actuator is anchored on the substrate at a proximal end as a cantilever beam, arranged to support the roof portion of the nozzle chamber at a distal end thereof, and wherein the bend actuator comprises at least one Joule heated cantilever in parallel with a non-heated cantilever joined together at the distal end. 
     Preferably, the bend actuator is mounted on the substrate by an anchor member having an upwardly extending end wall and wherein the proximal end of the Joule heated cantilever and proximal end of the non-heated cantilever are fastened to the anchor member. It will be appreciated however, that the bend actuator may be positioned internally or externally of the nozzle chamber. Similarly, the actuator may be a thermal bend actuator or activate in some other manner such as piezo-electric means and it follows that it does not need to be a two armed cantilever. Indeed, in some forms of the invention, the bend actuator need not be in the form of a cantilever at all. 
     Preferably, the anchor member is provided with side portions extending totally from the ends of the end wall so that the anchor member is substantially U-shaped when viewed from above the substrate. 
     Preferably, the U-shaped anchor member is positioned to wrap around the proximal end of the bend actuator with the side portions of the anchor member lying alongside the bend actuator. 
     Preferably, the side portions of the anchor member are provided with recesses, and one of the Joule heated cantilever and the non-heated cantilever is provided with outwardly extending portions engagable in said recesses. 
     Preferably, the Joule heated cantilever is mounted above the non-heated cantilever. 
     According to a further form of the invention, there is provided a thermal bend actuator manufactured by MEMS techniques and mounted by way of an anchor above a substrate, the bend actuator being configured to bend toward the substrate when actuated, the bend actuator comprising a Joule heated cantilever positioned above and restrained by a non-heated cantilever, the two cantilevers being joined together at their distal ends, whereby the bending of the actuator towards the substrate is caused by expansion of the Joule heated cantilever constrained by the non-heated cantilever. 
     Preferably, the proximal end of the Joule heated cantilever and proximal end of non-heated cantilever are independently connected to a U-shaped anchor member, in turn mounted on the substrate. 
    
    
     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 prospective 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 for clarity, and 
     FIG. 11 is a similar view to FIG. 1 showing the bend actuator cut away, and the actuator anchor detached for clarity. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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 be 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 FIGS. 1 to  3  of the accompanying drawings, the nozzle is constructed on a substrate I 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 in a similar manner to a bimetallic strip 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  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 FIG.  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 FIG. 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 FIGS. 9 and 10, but in the form of invention shown in FIGS. 1 to  3 , the back pressure is formed primarily by viscous drag in the supply conduit. 
     In order to prevent ink leaking from the nozzle chamber during actuation i.e. 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 FIGS. 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” cross 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 being sprayed 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 FIGS. 5,  6  and  7 . 
     FIG. 5 is similar to FIG. 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 FIG. 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 room  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 impurities 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 FIG.  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 surface tension at the narrow part of the nozzle resulting in a net drop vector force indicated by arrow  24 A 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 inkjet with back pressure byway of a significant constriction close to the nozzle. This constriction is typically provided in the substrate layers as can be clearly seen in FIGS. 9 and 10. FIG. 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 FIG. 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. 
     Because of the relatively high mass of the roof portion  6  and sidewalls  7  in a moving nozzle ink jet (compared with the mass of a paddle in previously described ink jets) it is desirable that the proximal end  11  of the bend actuator  10  be securely anchored to the adjacent substrate  26  in order to provide accurate control over movement of the roof portion and therefore accurate dispensing of ink drops from the nozzle. 
     To this end, the proximal end  11  of the thermal bend actuator is rigidly anchored by a U-shaped anchor member  12  which can be clearly seen in FIG.  11 . 
     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 . The anchor  12  is provided in 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 defamation 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 “sliding” 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 control and accuracy of intended movement of the roof portion  6  of the moving nozzle ink jet.