Patent Publication Number: US-11020982-B2

Title: Printhead recirculation

Description:
BACKGROUND 
     Fluid ejection devices may be implemented in various applications, such as printheads in printing systems including inkjet printers. Some fluid ejection devices may recirculate fluid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Examples will be described, by way of example only, with reference to the accompanying drawings in which corresponding reference numerals indicate corresponding parts and in which: 
         FIG. 1  illustrates an example of a printing system suitable for implementing an example method of recirculating fluid according to an example; 
         FIG. 2  illustrates an example printhead in two different views, according to an example; 
         FIG. 3  is a cross-sectional view of a printhead, according to an example; 
         FIG. 4  is a block diagram of an example method for recirculating fluid in a printhead die according to an example; 
         FIG. 5  illustrates an example method for anticipating the generation of recirculation pulses according to an example; 
         FIG. 6  illustrates an example of recirculation frequency patterns according to an example; 
         FIG. 7  illustrates an example of different recirculation lengths according to different recirculation thresholds and the corresponding application of a recirculation frequency pattern according to an example; and 
         FIG. 8  illustrates an example of a non-transitory computer readable medium encoded with instructions according to an example. 
     
    
    
     DETAILED DESCRIPTION 
     In general, a printing system, for example an inkjet printer or a 3D printing system, may include a fluid ejection device, such as a printhead, a printing fluid supply which supplies printing fluid to the fluid ejection device, and a controller which controls the fluid ejection device. Fluid ejection devices, for example printheads, may provide drop-on-demand ejection of fluid drops. In some examples, fluid ejection devices may be implemented in printing systems to facilitate on-demand ejection of printing fluid drops. In some examples, fluid ejection devices may be implemented in lab-on-a-chip devices (e.g., polymerase chain reaction devices, chemical sensors, etc.), fluid analysis devices, digital titration devices, pharmaceutical dispensation devices, fluidic diagnostic circuits, and/or other such devices in which volumes of fluids may be dispensed/ejected. 
     Printing systems may produce images or object layers by ejecting printing fluid drops, for example drops of liquid ink or drops of print agents, through a plurality of orifices or nozzles onto a print medium or a layer of print material, respectively. A print medium may be any kind of sheet-like medium, such as paper, cardboard, plastic or textile, etc., or a kind of layer of print material in case of a 3D printing system. In some example printing systems, fluid ejection devices may be implemented to print content by deposition of consumable fluids in a layer-wise additive manufacturing process. Examples of fluids, such as printing fluids, may comprise ink, toner, colorants, varnishes, finishes, gloss enhancers, binders, and/or other such materials. Images refers to any kind of depiction of signs, symbols, characters, numbers, letters, text and/or graphics which may be applied to the print medium, or to an object layer which may be applied to a layer of print material. 
     In some examples, the nozzles may be arranged in one or more columns or arrays such that properly sequenced ejection of printing fluid from the nozzles may cause images to be printed upon the print medium as the fluid ejection device and the print medium are moved relative to each other. In some examples, the arrays of nozzles may be arranged on dies or die substrates on the fluid ejection device. 
     In some examples, the fluid ejection device may eject fluid from a nozzle by activating a fluidic actuator which is in fluid communication with the nozzle. In some examples, the actuator may be a thermal resistor element and drops of fluid may be ejected from the nozzle by passing electrical current through the resistor element to generate heat and vaporize a small portion of the fluid within an ejection chamber. Some of the fluid may be displaced by the vapor bubble and may be ejected through the nozzle. In some examples, the actuator may be a piezoelectric element and drops of fluid may be ejected from the nozzle by passing electrical pulses to the piezoelectric element, causing a physical displacement which may generate pressure pulses that may force fluid out of the nozzle. 
     In some examples, a remote controller which may be located, e.g. as part of the processing electronics of a printer, controls the timing and activation of an electrical current from a power supply external to the fluid ejection device with a fire pulse. In some examples, the electrical current may be passed through a selected actuator to displace the fluid in a corresponding selected ejection chamber. In some examples, the ejection chamber may be coupled with one nozzle. In other examples, the ejection chamber may be coupled with a plurality of nozzles. 
     In some examples of printing systems, fluid ejection devices, such as printheads, may receive fire signals containing fire pulses from the controller. For example, the fire signal may be fed directly to the nozzles in the printhead. In other examples, the fire signal is latched in the printhead, and the latched version of the fire signal is fed to the nozzles to control the ejection of printing fluid drops from the nozzles. 
     In some examples, the controller of the printer may maintain control of all timing related to the fire signal. In some examples, the timing related to the fire signal primarily refers to the actual width of the fire pulse and the point in time at which the fire pulse occurs. The controller may control the timing related to the fire signal for printheads capable of printing a single column at a time. Such printheads may only need one fire signal to the printhead to control the ejection of printing fluid drops from the printhead. 
     In other examples, the printhead may have the capability of printing multiple columns of the same color or multiple columns of different colors simultaneously. 
     In some examples, a plurality of individual fluid ejection devices may be mounted on a single carrier. In some examples, a plurality of printheads may be mounted on a single carrier, where such examples may be referred to as wide-array inkjet printing systems. In such examples, fluid ejection devices thereof may have the capability of printing multiple columns of the same color or multiple columns of different colors simultaneously. In some examples, a number of nozzles and, therefore, an overall number of printing fluid drops which can be ejected per second may be increased. Since the overall number of drops which can be ejected per second may be increased, printing speed can be increased with a wide-array inkjet printing system and/or printheads having the capability of printing multiple columns simultaneously. 
     Thus, in some examples, a number of electrical pulses, i.e. fire pulses, may be generated which may be applied to one or more actuators, causing the corresponding nozzles to eject drops of printing fluid through the nozzles onto the print medium. 
     Although the present invention is described herein by way of example for a two-dimensional (2D) printing system, it is not limited thereto. The method and system according to the present invention can be readily applied to other printing systems, such as, for example, a three-dimensional (3D) printing system as well as other fluid dispensation/ejection related systems and devices. 
     In some examples, 3D printing techniques may involve the combined application of successive layers of material. For example, a printing fluid, such as a printing agent, may be applied to a layer of print material to produce an object layer. 
       FIG. 1  illustrates an example of a printing system  100  suitable for implementing an example of fluid recirculation according to an embodiment of the disclosure. Printing system  100  may include a printhead assembly  102 , a printing fluid supply assembly  104 , and an electronic controller  110 . The various electrical components of printing system  100  may be connected with at least one power supply (not shown) that provides power thereto. 
     Printhead assembly  102  includes at least one fluid ejection device  114 , such as a printhead  114 , which ejects drops of printing fluid, for example liquid ink, through a plurality of orifices or nozzles  116  toward a print medium  118 . Nozzles  116  may be arranged in one or more columns or arrays such that properly sequenced ejection of printing fluid from nozzles  116  may cause, for example characters, symbols, and/or other graphics or images to be printed on print media  118  as printhead assembly  102  and print media  118  are moved relative to each other. Printing fluid supply assembly  104  may supply printing fluid to printhead assembly  102  from a printing fluid reservoir  120  through an interface connection, such as a supply tube. The reservoir  120  may be removed, replaced, and/or refilled. 
     In some examples, as illustrated in  FIG. 1 , printing fluid supply assembly  104  and printhead assembly  102  may form a one-way printing fluid delivery system. In a one-way printing fluid delivery system, substantially all of the printing fluid supplied to printhead assembly  102  may be consumed during printing. In other examples (not shown), printing fluid supply assembly  104  and printhead assembly  102  may form a recirculating printing fluid delivery system. In a recirculating printing fluid delivery system, only a portion of the printing fluid supplied to printhead assembly  102  may be consumed during printing. Printing fluid not consumed during printing may be returned to printing fluid supply assembly  104 . 
     In some examples, electronic controller  110  may include components of a standard computing system, such as a processor, memory, firmware, software, and other electronics for controlling the general functions of system  100  and for communicating with and controlling system components such as printhead assembly  102 . In some examples, electronic controller  110  may receive data  124  from a host system, such as a computer, and may temporarily store data  124  in a memory. In some examples, data  124  may be sent to printing system  100  along an electronic, infrared, optical, or other information transfer path. Data  124  may represent, for example, a document, a file and/or a 3D object to be printed. As such, data  124  may form a print job for printing system  100  and may include one or more print job commands and/or command parameters. 
     In some examples, controller  110  may control printhead assembly  102  for ejection of printing fluid drops from nozzles  116 . Thus, electronic controller  110  may define a pattern of ejected printing fluid drops which may form, for example characters, symbols, and/or other graphics or images on print media  118 . The pattern of ejected printing fluid drops may be determined by the print job commands and/or command parameters. For example, controller  110  may generate a series or a pattern of fire pulses that may be sent to fluidic actuators to determine the pattern of ejected printing fluid drops. 
     In some examples, controller  110  may include fluid circulation module  126  stored in a memory of controller  110 . Fluid circulation module  126  may execute on controller  110 , i.e. a processor of controller  110 , to control the operation of one or more pump actuators within fluid ejection device  114 . More specifically, in some examples controller  110  may execute instructions from fluid circulation module  126  to control which pump actuators within fluid ejection device  114  are active and which are not active. Controller  110  may also control the timing of activation for the pump actuators. 
     In other examples, controller  110  may execute instructions from module  126  to control the timing and duration of forward and reverse pumping strokes, i.e. compressive and expansive fluid displacements, respectively, of the pump actuators to control, for example the direction, rate, and timing of fluid flow through fluidic channels between fluid feed slots within fluid ejection device  114 . 
     In some examples, printhead assembly  102  may include one fluid ejection device (printhead)  114 . In other examples, printhead assembly  102  may include a plurality of printheads  114 . For example, printhead assembly  102  may be a wide array or a multi-head printhead assembly. In one implementation of a wide-array assembly, printhead assembly  102  may include a carrier that carries printheads  114 , provides electrical communication between printheads  114  and controller  110 , and provides fluidic communication between printheads  114  and ink supply assembly  104 . 
     In some examples, printing system  100  may be a drop-on-demand thermal inkjet printing system wherein the fluid ejection device  114  is a thermal inkjet (TIJ) printhead. The thermal inkjet printhead implements a thermal resistor ejection element in an ink chamber to vaporize ink and create bubbles that force ink or other printing fluid drops out of a nozzle  116 . In other examples, printing system  100  is a drop-on-demand piezoelectric inkjet printing system wherein the fluid ejection device  114  is a piezoelectric inkjet (PIJ) printhead that implements a piezoelectric material actuator as an ejection element to generate pressure pulses that force printing fluid drops out of a nozzle. 
       FIG. 2  illustrates an example printhead  114  in two different views, a top view (left) and a bottom view (right). In some examples, the printhead  114  may comprise a plurality of thermal inkjet chips, referred to as dies  206   a ,  206   b . For example, the printhead  114  comprises six dies  206   a ,  206   b . In some examples, the number of dies  206   a ,  206   b  may be fewer, such as two or four dies  206   a ,  206   b , or greater than six, such as eight or ten dies  206   a ,  206   b . The dies  206   a ,  206   b  may be precision-aligned and placed on a dimensionally stable substrate. In some examples, the substrate may provide, for example mechanical alignment, printing fluid supply channels and electrical interconnection (not shown). 
     In some examples, the dies  206   a ,  206   b  may be arranged in two rows, a row of even dies  206   a  and a row of odd dies  206   b  at the bottom of the printhead  114 . Each die  206   a ,  206   b  may comprise at least one array of nozzles (not shown). In some examples, each die  206   a ,  206   b  comprises one array of nozzles for each color. For example, each die  206   a ,  206   b  may comprise four arrays of nozzles for each of four colors to be printed. 
     In some examples, the printhead  114  has no moving parts. The printhead  114  may eject drops of printing fluid through the nozzles. The ejection of printing fluid may be triggered by the controller  110 . In some examples, each drop has to emerge at a consistent weight, speed and direction to place a dot of correct size in the correct location. In some examples, the distance between the printheads  114  and the print medium  118  may be controlled accurately. 
       FIG. 3  is a cross-sectional view of a printhead  114  according to an example of the disclosure. Printhead  114  may include a die substrate  300 , for example a silicon die substrate, with a first fluid supply slot  302  and a second fluid supply slot  304  formed therein. Fluid slots  302  and  304  may be elongated slots that may be in fluid communication with a fluid supply (not shown), such as a fluid reservoir  120  (cf.  FIG. 1 ). Although in this example the concept of slot-to-slot fluid circulation is described, the method and system disclosed herein are not limited thereto. Other fluid circulations, such as fire chamber to fire chamber, slot to fire chamber, pump chamber to slot, pump chamber to fire chamber, etc. may be realized. 
     While slot-to-slot fluid circulation is described with respect to fluid ejection devices having two fluid slots, such concepts are not limited in their application to devices with two fluid slots. Rather, fluid devices having more than two fluid slots, such as four, six or eight slots, for example, are also contemplated as being suitable devices for implementing slot-to-slot fluid circulation. In addition, in other embodiments the configuration of the fluid slots may vary. For example, the fluid slots in other embodiments may be of varying shapes and sizes such as round holes, square holes, square trenches, and so on. 
     In some examples, printhead  114  may include a chamber layer  306  having walls  308  that define fluid chambers  310 ,  312 , and that separate the substrate  300  from a nozzle layer  314  having nozzles  116 . Fluid chambers  310  and  312  may comprise, respectively, fluid ejection chambers  310  and fluid pump chambers  312 . Fluid chambers  310  and  312  may be in fluid communication with a fluid slot. Fluid ejection chambers  310  have nozzles  116  through which fluid is ejected by actuation of a fluid displacement actuator  316  (i.e., a fluid ejection actuator  316   a ). 
     In some examples, fluid pump chambers  312  may be closed chambers in that they do not have nozzles through which fluid is ejected. Actuation of fluid displacement actuators  316  (i.e., fluid pump actuators  216   b ) within pump chambers  312  may generate fluid flow between slot  302  and  304 . In other examples, fluid displacement actuators  316  may be provided in a channel without having a pump chamber. 
     In some examples, chambers  310  and  312  may form columns of chambers along the inner and outer sides of slots  302  and  304 . For example, chambers  310  and  312  may form external columns and internal columns. External columns may be adjacent to a fluid slot  302  or  304  and located between the slot  302 ,  304  and an edge of the substrate  300 . Internal columns may be adjacent to fluid slots  302  and  304  and located between the slot  302 ,  304  and the center of the substrate  300 . 
     In some examples, chambers in the external columns are fluid ejection chambers  310 , while chambers in the internal columns  320  are fluid pump chambers  312 . In other examples, however, the external and internal columns can include both fluid ejection chambers  310  and fluid pump chambers  312 . 
     Fluid displacement actuators  316  are described generally throughout the disclosure as being elements capable of displacing fluid in a fluid ejection chamber  310  for the purpose of ejecting fluid drops through a nozzle  116 , and/or for generating fluid displacements in a fluid pump chamber  312  for the purpose of creating fluid flow between slots  302  and  304 . 
     One example of a fluid displacement actuator  316  is a thermal resistor element. When activated, heat from the thermal resistor element vaporizes fluid in the chamber  310 ,  312 , causing a growing vapor bubble to displace fluid. Another example of a fluid displacement actuator  316  is a piezoelectric element. The piezoelectric element may include a piezoelectric material adhered to a moveable membrane formed at the bottom of the chamber  310 ,  312 . When activated, the piezoelectric material causes deflection of the membrane into the chamber  310 ,  312 , generating a pressure pulse that displaces fluid. 
     In addition to thermal resistive elements and piezoelectric elements, other types of fluid displacement actuators  316  may also be suitable for implementation in a fluid ejection device  114  to generate, e.g. slot-to-slot fluid circulation. For example, printheads  114  may implement electrostatic (MEMS) actuators, mechanical/impact driven actuators, voice coil actuators, magneto-strictive drive actuators, and so on. 
     In some examples, as shown in  FIG. 3 , a fluid ejection device  114  may include fluidic channels  322 . Fluidic channels  322  extend from the first fluid slot  302  across the center of the die substrate  300  to the second fluid slot  304 . In some examples, fluidic channels  322  may couple the fluid pump chambers  [MJ1][CvP2]   312  of the first internal column with respective fluid pump chambers  312  of the second internal column. The fluid pump chambers  312  may be in the fluidic channels  322  and may be considered to be part of the channels  322 . Thus, each fluid pump chamber  312  may be located asymmetrically (i.e., off-centered) within a fluidic channel  322 , toward an end of the channel. 
     As indicated in  FIG. 3 , some fluid pump actuators  316   b  are active and some are inactive. Inactive pump actuators  316   b  are designated with an “X”. The pattern of active and inactive pump actuators  316   b  may be controlled by controller  110  executing fluid circulation module  126  (cf.  FIG. 1 ) to generate fluid flow through channels  322  that circulates fluid between the first slot  302  and the second slot  304 . Direction arrows show which direction fluid flows through channels  322  between slots  302  and  304 . The direction of fluid flow through a channel  322  may be controlled by activating one or the other of the fluid pump actuators  316   b  at the ends of the channel  322 . Thus, various fluid circulation patterns can be established between slots  302  and  304  by controlling which pump actuators  316   b  are active and which are not active. For example, controlling groups of pump actuators  316   b  to be active and inactive generates fluid flowing from the first slot  302  to the second slot  304  through some channels  322 , and from the second slot  304  back to the first slot  302  through other channels  322 . Channels  322  in which no pump actuator  316   b  is active have little or no fluid flow. 
     An example method  400  for recirculating fluid in a printhead die is depicted in  FIG. 4 . It is to be understood that the method  400  shown in  FIG. 4  will be discussed in detail herein, and in some instances,  FIGS. 5 through 7  will be discussed in conjunction with  FIG. 4 . 
     As described above, an image may be printed onto a print medium by generating a series or pattern of fire pulses that activate fluidic actuators, which are in communication with corresponding nozzles. The fluidic actuators which are activated cause drops of printing fluid to be ejected through the corresponding nozzle onto the print medium, thereby printing characters or images to be printed. 
     The generation of fire pulses may be controlled by a controller. The controller may receive print data, which represents, for example, the image to be printed. The print data may be processed by the controller, e.g. by a processor of the controller, and forms a print job for the printing system including print job commands and/or command parameters. Thus, by processing the print data, the controller can control the timing of drop ejection and which nozzle(s) have to eject drops of printing fluid onto the print medium at what printing column. 
     In some examples, inkjet printheads used in inkjet printing systems may have problems with printing fluid blockage and/or clogging. For example, a cause of printing fluid blockage may be an excess of air that accumulates as air bubbles in the printhead. For example, when printing fluid is exposed to air, such as while the printing fluid is stored in a reservoir, additional air may dissolve into the printing fluid. The subsequent action of ejecting printing fluid drops from the ejection chamber of the printhead may release excess air from the printing fluid, which then may accumulate as air bubbles. The bubbles may move from the ejection chamber to other areas of the printhead where they may block the flow of printing fluid to the printhead and within the printhead. Bubbles in the ejection chamber absorb pressure, reducing the force on the fluid pushed through the nozzle, which may reduce drop speed or prevent ejection. 
     In some examples, inkjet printing systems may use pigment-based inks or dye-based inks as printing fluids. Pigment-based inks may also cause printing fluid blockage or clogging in printheads due to pigment-ink vehicle separation (PIVS). PIVS may be a result of water evaporation from ink in the nozzle area and pigment concentration depletion in ink near the nozzle area due to a higher affinity of pigment to water. During periods of storage or non-use, pigment particles may settle or crash out of the ink vehicle, which may impede or block ink flow to the ejection chambers and nozzles in the printhead. 
     In some examples, other factors related to “decap”, such as evaporation of water or solvent may cause PIVS and viscous ink plug formation. Decap is the amount of time inkjet nozzles can remain uncapped and exposed to ambient environments without causing degradation in the ejected ink drops. 
     Some examples of the present disclosure may reduce printing fluid blockage and/or clogging in printing systems by recirculating fluid between fluid supply slots (i.e. from slot-to-slot) or by recirculating fluid from ejection chamber to ejection chamber, slot to ejection chamber, pump chamber to slot, pump chamber to ejection chamber, etc. 
     In some examples, fluid may circulate between the slots and/or chambers through fluidic channels that may or may not include pump chambers having pump actuators to pump the fluid. 
     Referring to  FIG. 4 , an example method  400  of recirculating fluid in a printhead die comprises, at box  402 , processing of print data  124  (cf.  FIG. 1 ). The print data may be processed by controller  110  (cf.  FIG. 1 ), e.g. by a processor of controller  110 . In some examples, print data  124  may represent a document, a file, an image or an object to be printed. In some examples, print data  124  may include one or more print job commands and/or command parameters. Thus, as explained above, by processing the print data  124 , the controller can control the timing of drop ejection and which nozzle(s) have to eject drops. 
     In some examples, the recirculation is performed for at least one nozzle  116 , such that the recirculation finishes before complete ejection of a drop by the corresponding nozzle(s)  116 . 
     In some examples, recirculation for nozzle(s)  116  is performed by activating the corresponding recirculation pumps. In some examples, recirculation is performed for each nozzle separately and each nozzle is in communication with a corresponding pump. In some examples, on pump is provided for each nozzle. In other examples, one pump is provided for more than one nozzle. For example, on pump may be provided for two nozzles or for a group of nozzles comprising a plurality of nozzles, such as three or five nozzles, or in the order of 50 nozzles. 
     In some examples, the timing of recirculation is controlled such that the recirculation finishes before the corresponding nozzle(s)  116  eject a drop. For example, the timing may be controlled such that the recirculation finishes shortly before drop ejection, e.g. in the order of milliseconds before drop ejection. In other examples, the recirculation may finish during a start phase of drop ejection, but before complete ejection of the drop. For example, the recirculation may finish while the corresponding thermal actuator heats up printing fluid and/or while a bubble forms within the ejection chamber which starts to force printing liquid out of the nozzle. 
     In some examples, the print data  124  is processed in advance to determine which nozzle(s)  116  will eject a drop and/or when the corresponding nozzle(s)  116  will eject the drop. 
     Referring to  FIG. 4 , the example method  400  comprises, at box  404 , determining a nozzle score. 
     In some examples, a nozzle score is determined for each nozzle. In other examples, a nozzle score may be determined for a selected number of nozzles on a pre nozzle basis. In some examples, a nozzle score may be determined for a group of nozzles comprising a number of nozzles. 
     In some examples, the nozzle score is determined to estimate the nozzle status, e.g. when it is about to print a drop. The nozzle status may be indicative of the decap situation of the corresponding nozzle. If, for example, the nozzle is in a bad decap situation when it is about to print a drop, it may need recirculation in order to perform the drop ejection correctly. 
     In some examples, the nozzle score accumulates the number of blank columns since last drop. As explained above, inkjet printheads may have problems with printing fluid blockage and/or clogging due to an excess of air that accumulates as air bubbles in the printhead. In some examples, pigment-based inks may also cause printing fluid blockage or clogging in printheads due to pigment-ink vehicle separation (PIVS). During periods of storage or non-use, pigment particles may settle or crash out of the ink vehicle, which may impede or block ink flow to the ejection chambers and nozzles in the printhead. 
     In some examples, the nozzles  116  are capped while the printer is not printing. While printing, however, the nozzles  116  remain uncapped, whether they are about to eject a drop or not. Thus, in some examples, a nozzle or a number of nozzles may remain uncapped and exposed to ambient environments while they are not ejecting drops for a number of columns, i.e. for a number of blank columns. Thus, the nozzle score may be converted in time of non-use of the corresponding nozzle(s) using the printing firing frequency. Accordingly, in some examples, a nozzle score for a particular nozzle may be based at least in part on a time between drop ejections performed with the particular nozzle. In other words, a nozzle score for a particular nozzle may be based at least in part on use and/or non-use of the particular nozzle, where such use and/or non-use for the particular nozzle may be determined based at least in part on the print data. 
     In some examples, the nozzles score may accumulate up to several seconds. In some examples, the nozzle score may be a print resolution. In other examples, the nozzle score may not be a print resolution. 
     Referring to  FIG. 4 , the example method  400  comprises, at box  406 , comparing the nozzle score with a recirculation threshold. 
     In some examples, at least one recirculation threshold is provided. For example, one recirculation threshold may be provided. In other examples, more than one recirculation threshold, such as, e.g. four recirculation thresholds may be provided. For example, having more than one recirculation threshold may provide flexibility to address different nozzle situations. 
     In some examples, for each drop to be ejected it has to be determined whether the corresponding nozzle needs recirculation or not. In other examples, it may be determined whether the corresponding nozzle needs recirculation or not for a selected number of drops to be ejected, for example for every second drop or for one drop out of, e.g. ten drops to be ejected. In order to determine whether a nozzle or a group of nozzles needs recirculation or not, the recirculation threshold may be utilized. Therefore, in some examples, whether to perform recirculation for a particular nozzle may be determined immediately prior to drop ejection with the particular nozzle. 
     If the nozzle score exceeds the recirculation threshold shortly before ejecting a drop, a recirculation process is produced, otherwise no recirculation needs to be performed, as shown at box  406  in  FIG. 4 . 
     In the case of more than one recirculation threshold, the nozzle score is compared to all or at least some of the recirculation thresholds. For example, if the nozzle score exceeds one of the recirculation thresholds, it may be determined whether another one of the recirculation thresholds is above the nozzle score or not. 
     For example, in the case of four recirculation thresholds, e.g. a first recirculation threshold t 0  of 4000, a second recirculation threshold t 1  of 3000, a third recirculation threshold t 2  of 2000, and a fourth recirculation threshold t 3  of 1000, it may be determined whether the nozzle score exceeds the first recirculation threshold. If it does, a recirculation process may be produced. If it does not exceed the first threshold, it may be determined whether it exceeds the second threshold, and so forth. As long as the nozzle score does not exceed a recirculation threshold, no recirculation process is produced. As soon as the nozzle score exceeded a particular threshold, a recirculation process is initiated and no further comparison of the nozzle score to remaining thresholds may be necessary. 
     In some examples, a recirculation length is provided. For example, one recirculation length may be provided. In other examples, more than one recirculation lengths may be provided, such as two or more recirculation lengths. In some examples, a recirculation length is provided for each recirculation threshold that is provided. 
     The recirculation length may indicate the length of the recirculation process, i.e. its duration. In the case that a recirculation length is provided for each recirculation threshold, it may be possible to produce recirculation processes with different lengths (durations) depending on the nozzle score value, i.e. on the particular threshold that the nozzle score exceeds. 
     In some examples, a recirculation may be performed for each nozzle for which the nozzle score exceeds a corresponding recirculation threshold. In some examples, a recirculation may be performed a group of nozzles for which the nozzle score exceeds the corresponding recirculation threshold, or which comprises a number of nozzles for which the nozzle score exceeds the corresponding recirculation threshold. 
     For example, a group of nozzles may comprise a number of nozzles, e.g. 20 nozzles. A recirculation may be performed for this group of nozzles, e.g. for the particular 20 nozzles, if the nozzle score for a fraction of these nozzles, e.g. one half of the group of nozzles, i.e. 10 nozzles, exceeds the corresponding threshold. A recirculation may be performed for a group of nozzles when some or all nozzles of that group of nozzles are about to eject a drop. For example, a recirculation may be performed if a fraction of the nozzles of a group of nozzles is about to eject a drop and if the nozzle score for that group of nozzles or for some or all nozzles of that group exceeds a corresponding recirculation threshold. 
     A recirculation is initiated by generating a number of recirculation pulses, as indicated at box  408 . 
     In some examples, the number of recirculation pulses is applied on one pump or pump actuator for each nozzle, as shown at box  410  in  FIG. 4 . For example, one pump may be provided for each nozzle and may be in communication with that nozzle. In other examples, one pump may be provided for a group of nozzles, such as two or more nozzles and that pump may be in communication with each nozzle of that group of nozzles. In some examples, the number of recirculation pulses is applied on more than one pump for each nozzle. 
     In some examples, the recirculation pulses activate the pumps, such that flow of fluid may be generated. In some examples, the number of recirculation pulses is generated based on the processed print data. In some examples, the print data is processed in advance, such that the number of generated before complete ejection of a drop by the corresponding nozzle(s). 
     In some examples, the print data is processed in advance such that the controller knows in advance which nozzle(s) are about to eject a drop. In some examples, the controller may control the generation and application of recirculation pulses, such that in particular for those nozzle(s) which are about to eject a drop and which are in a bad decap situation, i.e. for which the nozzle score exceeds a corresponding threshold, a recirculation is performed. Furthermore, in some examples, the pattern of recirculation pulses generated may be based at least in part on a respective recirculation threshold for which a respective nozzle has exceeded. For example, a first pattern of recirculation pulses may be generated for a respective nozzle if the respective nozzle exceeds a first recirculation threshold. Continuing the example, a second pattern of recirculation pulses may be generated for the respective nozzle if the respective nozzle exceeds a second recirculation threshold. It may be appreciated that patterns of recirculation pulses may differ in total duration, pulse duration, frequency, etc. 
     Referring to  FIG. 5 , an exemplary concept of how to process print data in advance for the recirculation is shown. In some examples, it may be determined in advance whether a nozzle needs recirculation, so that the recirculation process may be performed before the drop is ejected. For example, some printheads contain several trenches, with a different color in each trench. A leading trench starts printing first and the others may be delayed so that the resulting image has all colors properly aligned on print media. In other words, leading trench must process the image, i.e. the print data, in advance. 
     A similar approach may be implemented for the recirculation. In some examples, each trench may be split in two different virtual trenches. One virtual trench may contain some or all the nozzles and the other virtual trench may contain some or all the pumps. The two virtual trenches my process the same exact input image with the same exact configuration, such that they generate the same exact drops. However, the virtual trench with the pumps may be processed in advance, as many columns as necessary to fit the train of recirculation pulses when required. 
     A printhead with recirculation support may be controlled as a very large printhead with its trenches set very far away. This concept is shown in  FIG. 5 . In some examples, the virtual trench including the pumps is processed a number of columns, e.g. 1000 columns, before the virtual trench that contains the nozzles. They process the same input image, i.e. the same print data. When a nozzle that is about to eject a drop requires recirculation, the virtual trench containing pumps sees this drop, e.g. 1000 columns in advance and generates the train of recirculation pulses such that the recirculation finishes just before, or a configurable time earlier than the trench with nozzles ejects the drop. 
     In some examples, heat may be built up in the printhead due to recirculation. In some examples, this built up heat, or at least part of it, is dissipated by the subsequent ejection of a drop. 
     In some examples, the recirculation pulses may actuate the pumps based directly on to-be-printed image areas without determining which particular nozzles are about to eject a drop. 
     In some examples, various flush “points” in a single die where heat is dissipated may be determined to initiate circulation in adjacent channels. In other examples, flush “areas” within a single die where heat is dissipated may be determined to initiate circulation in adjacent channels. 
     In some examples, recirculation during drop ejection may be avoided by timing the recirculation such that it finishes before drop ejection. In other examples, recirculation may finish during an initial start phase of drop ejection. 
     In some examples, recirculation is only performed for nozzles which are about to print. For example, it may be avoided to perform recirculation for a nozzle that is not about to print for a while, even if the corresponding nozzle score exceeded a recirculation threshold, since the heat that may be built up during recirculation may not be dissipated by subsequent ejection of a drop. 
     In some examples, a recirculation frequency pattern is determined. In some examples, one recirculation frequency pattern is determined. In other examples, more than one recirculation frequency pattern may be determined. 
     A recirculation frequency is a frequency with which the recirculation pulses are sent to the pump actuators. The maximum recirculation frequency may be the same as the printing frequency for the fire pulses. The recirculation frequency may be a fraction of the fire frequency, e.g. one half the fire frequency. This may be achieved by not generating pulses for all the columns. 
     In some examples, pumps are digitally driven as the rest of nozzles, pulses may only be generated in regular printing columns. Therefore, the maximum possible recirculation frequency may be the same as the print firing frequency. In this case, the train of pulses will contain a pulse in every single firing column. Lower frequencies can be achieved not generating pulses in all the columns. In order to configure the recirculation frequency, a frequency pattern of a particular length, e.g. a 16-bit frequency pattern, may be provided. The train of pulses may be constructed by repeating this pattern all over the columns. 
       FIG. 6  illustrates 32 columns of train of pulses for different frequency patterns. As it can be seen, the maximum frequency is achieved with pattern 0xFFFF. It is possible to reduce frequency to half or one forth with patterns 0x5555 or 0x1111, respectively. The recirculation frequency is determined by the number of bits asserted in the frequency pattern, so 16 different fractions of the print firing frequency may be possible. 
     In the exemplary case shown in  FIG. 7 , the frequency pattern is 0x5555, so the recirculation generates a pulse every second column. The programmed shift between the recirculation slot and the regular slot may be, e.g. 66 columns, so the left part of the figure shows the moment when a drop is seen by the recirculation virtual trench and the right of the figure when the same drop is seen by the regular virtual trench 66 columns later. These columns are enough to fit all the trains of pulses. 
     The bottom part of the figure shows the definition for four recirculation thresholds, namely a first recirculation threshold t 0  of 4000 (topmost pattern), a second recirculation threshold t 1  of 3000 (second pattern from the top), a third recirculation threshold t 2  of 2000 (third pattern from the top), and a fourth recirculation threshold t 3  of 1000 (penultimate pattern). The final pattern at the bottom shows the situation in the case that the nozzle score s does not exceed any of the four different thresholds. 
     Each unit in the number of cycles or blank cycles represents 16 printing columns, a full frequency pattern. The central part of the figure shows the five possible actions that are possible when the drop is seen by the recirculation trench depending on the nozzle score s at that moment. 
     For example, in the case that the nozzle score s may be below the lowest threshold t 3 , which may be e.g. 1000, no recirculation is performed at all (lowest pattern). In the exemplary case that the nozzle score s may exceed t 3 , but still be lower than a further threshold t 2 , e.g. of 2000, three initial blank cycles may be performed before one recirculation pattern is initiated (second lowest pattern). In the exemplary case that the nozzle score s may exceed t 2 , but still be lower than a third threshold t 1 , e.g. of 3000, two initial blank cycles may be performed before two recirculation patterns are initiated (middle pattern). In the exemplary case that the nozzle score s may exceed t 1 , but still be lower than a fourth threshold t 0 , e.g. of 4000, one initial blank cycle may be performed before three recirculation patterns are initiated (second pattern from the top). If, however, the nozzle score s may exceed the fourth and largest threshold t 0 , no initial blank cycles are preformed and the recirculation pattern is initiated four times (topmost pattern). 
     In any of these exemplary cases, the distance between the end of the recirculation process and the drop ejection is two columns, this is implicitly defined with the difference between the shift between virtual trenches (66 in this case) and the number of columns required for the train of pulses (64 in this case). 
       FIG. 8  illustrates a non-transitory computer readable medium  800 . The medium  800  may be any kind of non-transitory computer readable medium, such as, e.g. a CD-ROM or the like. In some examples, the medium  800  may be encoded with instructions  802 .  804 ,  806 ,  808 ,  810 . In some examples, the instructions may be executable by, e.g. a processor, for example a computer processor. In some examples, the medium  800  may be encoded with instructions  802  that, when executed by a processor, cause the processor to process print data. In some examples, the medium  800  may be encoded with instructions  804  that, when executed by a processor, cause the processor to determine a nozzle score. In some examples, the medium  800  may be encoded with instructions  806  that, when executed by a processor, cause the processor to compare the nozzle score with a recirculation threshold. In the exemplary case that the nozzle score does not exceed the threshold, the instructions may cause the processor to continue to process print data. Otherwise, in the exemplary case that the nozzle score does exceed the threshold, the instructions may cause the processor to generate a recirculation pattern, i.e. a pattern of recirculation pulses. For this reason, in some examples, the medium  800  may be encoded with instructions  808  that, when executed by a processor, cause the processor to generate a pattern of recirculation pulses. Moreover, in some examples, the medium  800  may be encoded with instructions  810  that, when executed by a processor, cause the processor to apply the recirculation pulses to pumps. 
     In some examples, a non-transitory computer readable medium encoded with instructions is provided, that, when executed by a processor, cause the processor to perform a method of recirculating fluid in a printhead die, wherein the recirculation is performed for at least one nozzle, such that the recirculation finishes before complete ejection of a drop by the corresponding at least one nozzle. 
     In some examples, the non-transitory computer readable medium is further encoded with instructions, that, when executed by a processor, cause the processor to determine a nozzle score for each nozzle. In some examples, the nozzle score accumulates the number of blank columns since the last drop for each nozzle. 
     In some examples, the non-transitory computer readable medium is further encoded with instructions, that, when executed by a processor, cause the processor to provide at least one recirculation threshold. 
     In some examples, the non-transitory computer readable medium is further encoded with instructions, that, when executed by a processor, cause the processor to determine a recirculation length for each recirculation threshold. 
     In some examples, the non-transitory computer readable medium is further encoded with instructions, that, when executed by a processor, cause the processor to perform the recirculation for each nozzle for which the nozzle score exceeds the corresponding recirculation threshold or for each group of nozzles comprising a number of nozzles for which the nozzle score exceeds the corresponding recirculation threshold. 
     In some examples, the non-transitory computer readable medium is further encoded with instructions, that, when executed by a processor, cause the processor to apply a number of recirculation pulses on at least one pump for each nozzle or for each group of nozzles. 
     In some examples, the non-transitory computer readable medium is further encoded with instructions, that, when executed by a processor, cause the processor to process print data in advance and to generate the number of recirculation pulses based on the processed print data, such that the number of recirculation pulses finishes before complete ejection of a drop by the corresponding at least one nozzle. 
     In some examples, the non-transitory computer readable medium is further encoded with instructions, that, when executed by a processor, cause the processor to process the print data in advance to determine which at least one nozzle will eject a drop and when the corresponding at least one nozzle will eject the drop. 
     In some examples, the non-transitory computer readable medium is further encoded with instructions, that, when executed by a processor, cause the processor to determine at least one recirculation frequency pattern. 
     In some examples, the non-transitory computer readable medium is further encoded with instructions, that, when executed by a processor, cause the processor to perform the recirculation for each nozzle or group of nozzles by generating a number of recirculation pulses according to the recirculation frequency pattern and the corresponding recirculation length.# 
     In some examples, a non-transitory computer readable medium encoded with instructions is provided, that, when executed by a processor, cause the processor to perform a method of recirculating fluid in a printhead die, wherein heat that has built up during the recirculation is dissipated by the ejection of a drop. 
     In some examples, a printing system is provided, comprising a printhead assembly including at least one printhead having a printhead die and at least one nozzle, a printing fluid supply assembly which is in fluid communication with the printhead assembly, and a controller, wherein the controller is to control a method of recirculating fluid in a printhead die, wherein the recirculation is performed for at least one nozzle, such that the recirculation finishes before complete ejection of a drop by the corresponding at least one nozzle. 
     In some examples, the controller of the printing system is further to determine a nozzle score for each nozzle. In some examples, the nozzle score accumulates the number of blank columns since the last drop for each nozzle. 
     In some examples, the controller of the printing system is further to provide at least one recirculation threshold. 
     In some examples, the controller of the printing system is further to determine a recirculation length for each recirculation threshold. 
     In some examples, the controller of the printing system is further to perform the recirculation for each nozzle for which the nozzle score exceeds the corresponding recirculation threshold or for each group of nozzles comprising a number of nozzles for which the nozzle score exceeds the corresponding recirculation threshold. 
     In some examples, the controller of the printing system is further to apply a number of recirculation pulses on at least one pump for each nozzle or for each group of nozzles. 
     In some examples, the controller of the printing system is further to process print data in advance and to generate the number of recirculation pulses based on the processed print data, such that the number of recirculation pulses finishes before complete ejection of a drop by the corresponding at least one nozzle. 
     In some examples, the controller of the printing system is further to process the print data in advance to determine which at least one nozzle will eject a drop and when the corresponding at least one nozzle will eject the drop. 
     In some examples, the controller of the printing system is further to determine at least one recirculation frequency pattern. 
     In some examples, the controller of the printing system is further to perform the recirculation for each nozzle or group of nozzles by generating a number of recirculation pulses according to the recirculation frequency pattern and the corresponding recirculation length. 
     In some examples, the controller of the printing system is further to perform a method of recirculating fluid in a printhead die, wherein heat that has built up during the recirculation is dissipated by the ejection of a drop. 
     While several examples have been described in detail, it is to be understood that the disclosed examples may be modified. Therefore, the foregoing description is to be considered non-limiting.