Patent Publication Number: US-9403372-B2

Title: Fluid ejection device with ACEO pump

Description:
BACKGROUND 
     Fluid ejection devices in inkjet printers provide drop-on-demand ejection of fluid drops. Inkjet printers produce images by ejecting ink drops through a plurality of nozzles onto a print medium, such as a sheet of paper. The nozzles are typically arranged in one or more arrays, such that properly sequenced ejection of ink drops from the nozzles causes characters or other images to be printed on the print medium as the printhead and the print medium move relative to each other. In a specific example, a thermal inkjet printhead ejects drops from a nozzle by passing electrical current through a heating element to generate heat and vaporize a small portion of the fluid within a firing chamber. In another example, a piezoelectric inkjet printhead uses a piezoelectric material actuator to generate pressure pulses that force ink drops out of a nozzle. 
     As nozzles sit exposed to ambient atmospheric conditions while in idle non-jetting states, evaporative water loss through the nozzle bores can alter the local composition of ink volumes within the bores, the firing chambers, and in some cases, beyond an inlet pinch toward the shelf/trench (ink slot) interface. Following periods of nozzle inactivity, the variation in properties of these localized volumes can modify drop ejection dynamics (e.g., drop trajectories, velocities, shapes and colors). This lag in nozzle renewal capabilities and the associated effects on drop ejection dynamics following non-jetting periods is referred to as decap response. Continued improvement of inkjet printers and other fluid ejection systems relies in part on mitigating decap response issues. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present embodiments will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  shows a fluid ejection system implemented as an inkjet printing system, according to an embodiment; 
         FIG. 2 a    shows a top view of a portion of an example fluid ejection device, according to an embodiment; 
         FIG. 2 b    shows a side view of a portion of an example fluid ejection device, according to an embodiment; 
         FIG. 3 a    shows a top view of a portion of an example fluid ejection device with an AC voltage being applied to ACEO electrodes, according to an embodiment; 
         FIG. 3 b    shows a side view of a portion of an example fluid ejection device with an AC voltage being applied to ACEO electrodes, according to an embodiment; 
         FIG. 4  shows an expanded side view of a section of a fluid ejection device that illustrates the 3-dimensional electrode structure of an ACEO pump within a channel, according to an embodiment; 
         FIG. 5  shows a flowchart of an example method, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Overview 
     As noted above, the decap response impacts stagnant ink volumes local to the nozzle bores, firing chambers, and other nearby areas within fluid ejection devices that interface with the surrounding environment during non-jetting idle spans. In general, decap behaviors tend to manifest in the form of Pigment Ink Vehicle Separation (PIVS) and viscous plug dependent modes. This dynamic can adversely impact drop ejection behaviors such as drop trajectories, drop velocities, drop shapes and even drop colors. Prior methods of mitigating the decap response have focused mostly on ink formulation chemistries, minor architecture adjustments, tuning nozzle firing parameters, and/or servicing algorithms. These approaches have often been directed toward specific printer/platform implementations, however, and have therefore not provided a universally suitable solution. 
     Efforts to mitigate the decap response through adjustments in ink formulation, for example, often rely upon the inclusion of key additives that offer benefits only when paired with specific dispersion chemistries. Architecture focused strategies have typically leveraged shortened shelves (i.e., the length from the center of the firing resistor to the edge of the incoming ink-feed slot) and modifications to nozzle diameters and resistor sizes. These techniques, however, usually provide only minimal performance gains. Fire pulse routines have shown some improvements in targeted architectures when exercised as sub-TOE (turn on energy) mixing protocols for stirring ink within the nozzle to combat Pigment Ink Vehicle Separation (PIVS) forms of the decap dynamic, or by delivering more energetic stimulation of in-chamber ink volumes (delivered at higher voltages or through modified precursor pulse configurations) to compete against viscous plugging forms of the decap response. Again, however, this strategy provides only marginal gains in specific non-universal contexts. Servicing algorithms have functioned as the main systems-based fix. However, servicing algorithms typically generate waste ink and associated waste ink storage issues, in-printer aerosol, and print/wipe protocols that are only feasible for implementation as pre- or post-job exercises. 
     Embodiments of the present disclosure mitigate the decap response more generally through the use of an alternating current electro-osmotic (ACEO) pump mechanism that generates a net flow of fluid within a micro-fluidic environment. The ACEO pump involves the use of stepped (3-dimensional) electrodes having inter-digitated ladder topologies, where interleaved electrode “fingers” are driven with opposite polarity (i.e., 180 out of phase). The disclosed embodiments provide an effective pumping technique for flushing fresh ink from a bulk supply (e.g., the trench/ink slot) through the firing chamber to improve the quality of the ejected drop output. The pumping technique does not involve the formation of a steam bubble or depend on surrounding micro-channel asymmetries. In addition, the technique does not generate a pulsed flow, which avoids introducing additional, unwanted nozzle puddling and cross-talk between nozzles, and enables a continual pumping operation that is independent of nozzle fire sequencing (i.e., jetting events). Other advantages include less waste ink from servicing and a related reduction in the amount of servicing hardware. 
     In one example embodiment, a fluid ejection device includes a fluidic channel having first and second ends. A drop generator is disposed within the channel, and a fluid reservoir is in fluid communication with the first and second ends of the channel. An alternating-current electro-osmotic (ACEO) pump is disposed within the channel to generate net fluid flow from the reservoir at the first end, through the channel, and back to the reservoir at the second end. In one implementation, the ACEO pump includes a plurality of electrodes on the floor of the channel, where each electrode extends lengthwise across the width of the channel and is orthogonal to the direction of the net fluid flow. A first group of electrodes coupled to a first terminal of an AC power source is interleaved in an alternating manner with a second group of electrodes coupled to a second terminal of the AC power source. 
     In another example embodiment, a processor-readable medium stores code representing executable instructions. When executed by the processor, the instructions cause the processor to apply opposite electrical polarities to adjacent electrodes within a fluidic channel. The fluidic channel includes a nozzle and a chamber, and the electrodes comprise interdigitated, 3-dimensional electrodes, each having a stepped region and a non-stepped region. The instructions further cause the processor to repeatedly switch the electrical polarities applied to each electrode to generate a net fluid flow through the channel. The instructions further cause the processor to eject fluid through the nozzle as it flows through the chamber. 
     Illustrative Embodiments 
       FIG. 1  illustrates a fluid ejection system implemented as an inkjet printing system  100 , according to an embodiment of the disclosure. Inkjet printing system  100  generally includes an inkjet printhead assembly  102 , an ink supply assembly  104 , a mounting assembly  106 , a media transport assembly  108 , an electronic printer controller  110 , and at least one power supply  112  that provides power to the various electrical components of inkjet printing system  100 . In some implementations, power supply  112  can include an AC power supply to supply AC power to ACEO (alternating current electro-osmotic) pump mechanisms  126  within fluid ejection devices  114 . In this embodiment, fluid ejection devices  114  are implemented as fluid drop jetting printheads  114 . Inkjet printhead assembly  102  includes at least one fluid drop jetting printhead  114  that ejects drops of ink through a plurality of orifices or nozzles  116  toward print media  118  so as to print onto the print media  118 . Print media  118  can be any type of suitable sheet or roll material, such as paper, card stock, transparencies, Mylar, and the like. Nozzles  116  are typically arranged in one or more columns or arrays such that properly sequenced ejection of ink from nozzles  116  causes characters, symbols, and/or other graphics or images to be printed on print media  118  as inkjet printhead assembly  102  and print media  118  are moved relative to each other. 
     Ink supply assembly  104  supplies fluid ink to printhead assembly  102  and includes a reservoir  120  for storing ink. Ink flows from reservoir  120  to inkjet printhead assembly  102 . Ink supply assembly  104  and inkjet printhead assembly  102  can form either a one-way ink delivery system or a macro-recirculating ink delivery system. In a one-way ink delivery system, substantially all of the ink supplied to inkjet printhead assembly  102  is consumed during printing. In a macro-recirculating ink delivery system, however, only a portion of the ink supplied to printhead assembly  102  is consumed during printing. Ink not consumed during printing is returned to ink supply assembly  104 . 
     In some implementations, inkjet printhead assembly  102  and ink supply assembly  104  are housed together in an inkjet cartridge or pen. In other implementations, ink supply assembly  104  is separate from inkjet printhead assembly  102  and supplies ink to inkjet printhead assembly  102  through an interface connection, such as a supply tube. In either implementation, reservoir  120  of ink supply assembly  104  may be removed, replaced, and/or refilled. Where inkjet printhead assembly  102  and ink supply assembly  104  are housed together in an inkjet cartridge, reservoir  120  can include a local reservoir located within the cartridge as well as a larger reservoir located separately from the cartridge. The separate, larger reservoir serves to refill the local reservoir. Accordingly, the separate, larger reservoir and/or the local reservoir may be removed, replaced, and/or refilled. 
     Mounting assembly  106  positions inkjet printhead assembly  102  relative to media transport assembly  108 , and media transport assembly  108  positions print media  118  relative to inkjet printhead assembly  102 . Thus, a print zone  122  is defined adjacent to nozzles  116  in an area between inkjet printhead assembly  102  and print media  118 . In one implementation, inkjet printhead assembly  102  is a scanning type printhead assembly. As such, mounting assembly  106  includes a carriage for moving inkjet printhead assembly  102  relative to media transport assembly  108  to scan print media  118 . In another implementation, inkjet printhead assembly  102  is a non-scanning type printhead assembly. As such, mounting assembly  106  fixes inkjet printhead assembly  102  at a prescribed position relative to media transport assembly  108 . Thus, media transport assembly  108  positions print media  118  relative to inkjet printhead assembly  102 . 
     In one implementation, inkjet printhead assembly  102  includes one printhead  114 . In another implementation, inkjet printhead assembly  102  is a wide-array, multi-head printhead assembly. In wide-array assemblies, an inkjet printhead assembly  102  typically includes a carrier that carries printheads  114 , provides electrical communication between printheads  114  and electronic controller  110 , and provides fluidic communication between printheads  114  and ink supply assembly  104 . 
     In one embodiment, inkjet printing system  100  is a drop-on-demand thermal bubble inkjet printing system where the printhead(s)  114  is a thermal inkjet (TIJ) printhead. The TIJ printhead implements a thermal resistor ejection element in an ink chamber to vaporize ink and create bubbles that force ink or other fluid drops out of a nozzle  116 . In another embodiment, inkjet printing system  100  is a drop-on-demand piezoelectric inkjet printing system where the printhead(s)  114  is a piezoelectric inkjet (PIJ) printhead that implements a piezoelectric material actuator as an ejection element to generate pressure pulses that force ink drops out of a nozzle. 
     Electronic printer controller  110  typically includes one or more processors  111 , firmware, software, one or more computer/processor-readable memory components  113  including volatile and non-volatile memory components, and other printer electronics for communicating with and controlling inkjet printhead assembly  102 , mounting assembly  106 , and media transport assembly  108 . Electronic controller  110  receives data  124  from a host system, such as a computer, and temporarily stores data  124  in a memory  113 . Typically, data  124  is sent to inkjet printing system  100  along an electronic, infrared, optical, or other information transfer path. Data  124  represents, for example, a document and/or file to be printed. As such, data  124  forms a print job for inkjet printing system  100  and includes one or more print job commands and/or command parameters. 
     In one implementation, electronic printer controller  110  controls inkjet printhead assembly  102  for ejection of ink drops from nozzles  116 . Thus, electronic controller  110  defines a pattern of ejected ink drops that form characters, symbols, and/or other graphics or images on print media  118 . The pattern of ejected ink drops is determined by the print job commands and/or command parameters. 
     In one implementation, electronic controller  110  includes ACEO pump module  128  stored in a memory  113  of controller  110 . ACEO pump module  128  includes coded instructions executable by one or more processors  111  of controller  110  to cause the processor(s)  111  to implement various functions related to the operation of ACEO pump  126 . Thus, for example, ACEO pump module  128  executes to create AC electric fields within the fluidic micro-environment of an inkjet printhead  114  to generate a net fluid flow through micro-fluidic channels of the printhead  114 . More specifically, the ACEO pump module  128  executes to control the timing, frequency and magnitude of AC voltage applied to 3-dimensional, stepped, electrodes within the printhead channels. Application of the AC voltage polarizes the electrodes and causes charge groups within the contacting fluid (i.e., ink) to migrate toward the electrode surfaces and be swept in specified directions through their interactions with localized electrode-edge fringe fields, as discussed below with respect to  FIGS. 3 and 4 . In different implementations, ACEO pump module  128  can execute to polarize the electrodes in different ways. For example, ACEO pump module  128  can execute to generate sine waves (e.g., from an AC power source) or square waves (e.g., from a digital circuit) to polarize the electrodes. 
       FIG. 2  shows a top view ( FIG. 2 a   ) and a side view ( FIG. 2 b   ) of a portion of an example fluid ejection device  114  (i.e., printhead  114 ), according to an embodiment of the disclosure. Printhead  114  includes a substrate  200  (e.g., glass, silicon) with a fluid slot  202  or trench formed therein. In general, fluid slot  202  and other features of printhead  114  are formed using various precision microfabrication techniques such as electroforming, laser ablation, anisotropic etching, sputtering, spin coating, dry etching, photolithography, casting, molding, stamping, machining, and the like. Referring again to  FIG. 2 , printhead  114  further includes a fluidic channel  204  that extends from the fluid slot  202  at a first end  206  of the channel, and back to the fluid slot  202  at a second end  208  of the channel  204 . The first and second channel ends ( 206 ,  208 ) can be referred to as the channel inlet  206  and channel outlet  208 , respectively, depending on the direction of fluid flow through the channel  204 . In some implementations, printhead  114  also includes particle tolerant architectures  210 . As used herein, particle tolerant architectures (PTA) refer to barrier objects placed in the fluid/ink path (e.g., channel inlet  206  and outlet  208 ) to prevent particles such as dust and air bubbles from interrupting ink or printing fluid flow. The PTAs  210  help prevent particles from blocking ejection chambers and/or nozzles  116 . 
     Each channel  204  of printhead  114  includes a drop generator  212  to eject fluid drops out of the printhead. Each drop generator  212  includes a fluid ejection chamber  214  and associated nozzle  116 . On the floor of each ejection chamber  214  is an ejection element  216  that activates to eject fluid from the chamber  214  through nozzle  116 . In one implementation, ejection element  216  comprises a thermal resistor heating element. Activation of the thermal resistor to eject a fluid drop includes passing electrical current through the element, which heats the element and vaporizes a small portion of the fluid within the chamber  214 . The formation of the vapor bubble forces a fluid drop through the nozzle  116 . In another implementation, ejection element  216  comprises a piezoelectric material actuator. Activation of the piezoelectric material actuator to eject a fluid drop includes applying a voltage across a piezoelectric membrane which deforms the actuator, generating pressure pulses within the chamber  214  that force fluid drops out of the nozzle  116 . 
     Each channel  204  of printhead  114  additionally includes an ACEO pump mechanism  126  that comprises a plurality of ACEO electrodes  218 . The electrodes  218  are disposed on the floor of the channel  204  such that the electrode lengths extend across the channel width, between the sides of the channel  204 . The electrode lengths (i.e., electrode “fingers”) extend across the channel width such that the electrodes are orthogonal both to the length of the channel  204  and to the eventual net flow of fluid through the channel  204 . As discussed further below with respect to  FIG. 4 , each electrode  218  comprises a 3-dimensional structure that includes a stepped electrode region  220  and a flat, or non-stepped electrode region  222 . 
       FIG. 3  shows a top view ( FIG. 3 a   ) and a side view ( FIG. 3 b   ) of a portion of an example fluid ejection device  114  (i.e., printhead  114 ) with an AC voltage being applied to ACEO electrodes  218 , according to an embodiment of the disclosure. The AC voltage used to actuate the ACEO electrodes  218  is typically a low voltage on the order of 1-3 Vpp, although other voltages are possible and contemplated by this disclosure. Application of the AC voltage polarizes the electrodes and generates a net fluid flow (i.e., ACEO Flow  300 ) through the printhead channel  204 . More specifically, when AC voltage is applied to the electrodes  218  as shown in  FIG. 3 b   , adjacent, interdigitated, electrode “fingers” are driven to opposite electrical polarities (i.e., 180° out of phase with one another). The opposite electrical polarities of the electrode fingers are switched repeatedly at the frequency of the applied AC voltage. Application of the AC voltage is achieved in part by coupling alternate “fingers” of the electrodes to different output terminals of the AC power source  302 , as shown in  FIG. 3 . Thus, a first group of the electrodes  218  is coupled to a first output terminal  304  of the AC power supply  302 , while another group of electrodes  218  that alternate with, or are interleaved between, the first group of electrodes  218 , is coupled to a second output terminal  306  of the AC power supply  302 . In addition, application of the AC voltage includes controlling the AC power supply  302  by executing coded instructions of ACEO pump module  128  with a processor(s) of controller  110 . Such control includes, for example, controlling the frequency and magnitude of the AC voltage applied to electrodes  218 . 
       FIG. 4  shows an expanded side view of a section of a printhead  114  that illustrates the 3-dimensional electrode structure of the ACEO pump  126  within a channel  204 , according to an embodiment of the disclosure. The side view shown in  FIG. 4  is generally the left portion of the side view of  FIG. 3 b   . Thus, the channel side wall at the left of  FIG. 4  corresponds with the channel side wall at the left of  FIG. 3 b   , and the right side of the channel  204  in  FIG. 4  continues on to the chamber  214  and fluid slot  202 , as shown in  FIG. 3 b   . When polarized, the electrodes  218  cause charge groups within the contacting fluid (i.e., ink) to migrate toward the electrode surfaces and be swept in specified directions through their interactions with localized electrode-edge fringe fields. In order for the charge groups within the ink to migrate and cause a net fluid flow (i.e., ACEO Flow  300 ) through the channel  204  in a common, prescribed, direction, the implementation of the electrodes  218  involves using 3-dimensionally stepped electrodes having interdigitated ladder topologies, where electrode “fingers” of opposite electrical polarity (i.e., 180° out of phase) interleave with one another. Each electrode finger in this interleaved pattern is comprised of two distinct height regions. A first height region in each electrode  218  is a stepped electrode region  220  having a first height. The stepped region  220  extends part way across the width of an electrode finger. A second height region in each electrode  218  is a non-stepped region  222 , or flat region, having a second height. The non-stepped region  222  extends the remainder of the way across the width of the electrode finger. 
     The regions of different heights in the electrodes  218  (i.e., stepped region  220  and non-stepped region  222 ) in combination with the applied time dependent, polarity shifting signaling (e.g., the AC voltage from AC power source  302 ) form small fluid recirculation zones  400  (represented in  FIG. 4  as elliptical dotted lines) along each step of each electrode  218  within the interdigitated ACEO ladder topology. As illustrated in  FIG. 4 , the top edge of each recirculation zone  400  rotates in a forward direction that is compatible with and contributes to the slip flow  402  (represented in  FIG. 4  as a straight dotted line) which is native to the elevated, stepped region  220  of each electrode  218 . The recirculation zone  400  is recessed below the stepped region  220  such that the stepped region  220  provides a physical shelter that prevents the bottom edge of each recirculation zone  400 , which flows in a reverse direction, from competing against the slip flow  402  and the overall net ACEO fluid flow  300 . As such, the slip flow  402  across the tops of the stepped regions  220  and the flow in the top edges of the recirculation zones  400  cooperate to collaboratively push fluid in a common direction. This cooperation in flows generates a net ACEO fluid flow  300  that is orthogonal to the orientation of the electrode fingers stationed within the channel  204 . In addition, controlled variations in the AC voltage magnitude and frequency (i.e., by execution of ACEO pump module  128  in controller  110 ) can alter the slip flow  402  and the rotational flow in recirculation zones  400  to enhance the net ACEO fluid flow  300 . The ACEO fluid flow  300  through the channel  204  and chamber  214  provides fresh ink to the fluid ejection nozzles that helps to offset decap behaviors noted above. 
     Varying the aspect ratios of the electrode  218  footprint within channel  204  impacts the degree of ACEO net flow through the channel  204 . In some implementations, the aspect ratio of the electrodes  218  and their spacing within the channel  204  for the given dimensions a, b, c, d and e, as shown in  FIG. 4 , is approximately 1:1:1:1:1. The dimensions shown in  FIG. 4  include; a, the width of the stepped region  220  of electrode  218 ; b, the width of the non-stepped region  222  of electrode  218 ; c, the space between adjacent electrodes in channel  204 ; d, the height of the stepped region  220  of electrode  218  from the floor of the channel  204  to the top edge of the stepped region  220 ; and e, the distance from the top edge of the stepped region  220  to the roof of the channel  204 . In a particular example, where the height of the channel  204  is on the order of 10 microns, each of the dimensions a, b, c, d and e is approximately 5 microns. The 1:1:1:1:1 aspect ratio applied to these electrode dimensions and their spacing within channel  204  have been found to provide enhanced ACEO fluid flow  300  through the channel. However, the electrode dimensions and spacing within the channel  204  are not limited in this regard, and other aspect ratios that provide beneficial net flow are also contemplated by this disclosure. 
     As noted above, the features of printhead  114  can be formed using various precision microfabrication techniques such as electroforming, laser ablation, anisotropic etching, sputtering, spin coating, dry etching, photolithography, casting, molding, stamping, machining, and the like. Thus, the height of the stepped region  220  in electrode  218  can be formed by the deposition and processing of an SU8 material, for example, followed by the deposition and processing of a metal layer that covers the SU8 and forms the electrode metal of the non-stepped region  222  and the top, sides and bottom of the stepped region  220 . In some implementations, the metal layer of electrodes  218  is formed of platinum and/or platinum family materials that provide beneficial protection of the electrode  218  against the corrosive effects of various ink chemistries. While platinum and platinum family materials are mentioned as candidates for the formation of electrodes  218 , other suitable metal materials are also possible and are contemplated by this disclosure. 
       FIG. 5  shows a flowchart of an example method  500 , according to an embodiment of the disclosure. Method  500  is related to a fluid ejection device  114  with an ACEO pump mechanism as discussed herein, and is associated with embodiments discussed above with respect to  FIGS. 1-4 . Details of the steps shown in method  500  can be found in the related discussion of such embodiments. The steps of method  500  may be embodied as programming instructions stored on a computer/processor-readable medium, such as a memory  113  of controller  110  as shown in  FIG. 1 . In an embodiment, the implementation of the steps of method  500  may be achieved by the reading and execution of such programming instructions by a processor, such as processor  111  as shown in  FIG. 1 . While the steps of method  500  are illustrated in a particular order, the disclosure is not limited in this regard. Rather, it is contemplated that various steps may occur in different orders than shown, and/or simultaneously with other steps. 
     Method  500  begins at block  502  where the first step shown is to apply opposite electrical polarities to adjacent electrodes in a fluidic channel. The channel includes a nozzle and a chamber, and the electrodes comprise interdigitated, 3-dimensional electrodes, each having a stepped region and a non-stepped region. At block  504 , the next step of method  500  is to repeatedly switch the electrical polarities applied to each electrode to generate a net fluid flow through the channel. Repeatedly switching the electrical polarities comprises applying an AC voltage to the electrodes. In different implementations, a processor executing instructions from ACEO pump module  128  controls the switching of electrical polarities by controlling the generation of sine waves (e.g., from an AC power source) or square waves (e.g., from a digital circuit) to polarize the electrodes. In other implementations, the electrodes can be driven by a simple waveform generator coupled to the electrodes without processor control. Repeatedly switching the electrical polarities of the interleaved/interdigitated electrodes generates a slip fluid flow over the stepped region of the electrode and a fluid recirculation zone over the non-stepped region of the electrode. The recirculation zone has a top edge flowing in a forward direction to contribute to the slip fluid flow, and a bottom edge flowing in a reverse direction. 
     At block  506 , the next step of method  500  is to vary the AC voltage magnitude and frequency to alter the slip fluid flow and the fluid recirculation zone to enhance the net fluid flow through the channel. At block  508  of method  500 , the next step is to eject fluid through the nozzle as it flows through the chamber. Ejecting fluid through the nozzle comprises activating an ejection element within the chamber by applying a voltage to the ejection element. In different implementations the ejection element is selected from the group consisting of a thermal resistor and a piezoelectric membrane.