BACKGROUND

[0001] A printing system may include a pen or a printhead with a plurality of nozzles that deliver print agent onto a print medium so as to print an image. Some printheads may incorporate elements to recirculate print agent within the printheads.

BRIEF DESCRIPTION OF THE DRAWINGS

[0002] Various example features will be apparent from the detailed description which follows, taken in conjunction with the accompanying drawings, wherein:

[0003] FIG. 1 schematically illustrates a printing system according to an example of the present disclosure.

[0004] FIG. 2 schematically illustrates a portion of a printhead of the printing system of FIG. 1.

[0005] FIG. 3 shows a graph of a signal output from a drop detector according to an example of the present disclosure.

[0006] FIG. 4 shows a graph of a relationship between a drop parameter of a plurality of print agent drops and a recirculation parameter according to an example of the present disclosure.

[0007] FIG. 5 is a block diagram of a method to calibrate print agent recirculation within a printhead according to an example of the present disclosure.

[0008] FIG. 6 represents a non-transitory machine-readable storage medium according to an example of the present disclosure. DETAILED DESCRIPTION

[0009] A printing system comprises a printhead which may deliver print agent onto a print medium, e.g. a paper sheet. The printhead may be provided with a plurality of nozzles to deliver print agent, e.g. ink, onto the print medium so as to print an image. In this disclosure, delivering includes firing, ejecting, spitting or otherwise depositing print agent or ink. The printhead may be mounted on a printhead support.

[0010] In some examples, the printhead support may comprise a carriage. The printhead may thus be mounted on a carriage for moving across a scan axis. The printhead may travel repeatedly across a scan axis for delivering print agent onto a print medium which may advance along an advancing axis. The scan axis may be substantially perpendicular to the advancing axis. In some examples, several printheads may be mounted on a carriage. In some examples, four printheads may be mounted on a single carriage. In some examples, eight printheads may be mounted on a single carriage.

[0011] In some examples, the printhead support may be static. The printhead may extend along a width of a print medium. The plurality of nozzles may be distributed within the printhead along the width of the print medium. The width of the print may be substantially perpendicular to an advancing axis of the print medium. Such an arrangement may allow most of the width of the print medium to be printed simultaneously. These printer systems may be called as page-wide array (PWA) printer systems.

[0012] In some examples, a printhead may comprise a plurality of print agent ejection assemblies. A print agent ejection assembly may eject or deliver print agent from a nozzle by activating an actuator associated with the nozzle, e.g. in fluid communication with the nozzle.

[0013] In some examples, the actuator may be a heating element, e.g. a thermal resistor element. An electrical current may pass through the heating element to generate heat. This heat may cause a rapid vaporization of print agent in a print agent chamber or firing chamber, increasing an internal pressure inside this print agent chamber. This increase in pressure makes a drop of print agent exit from the print agent chamber to the print medium through a nozzle. These printing systems may be called as thermal inkjet printing systems.

[0014] In some examples, the actuator may be a piezo electric. A piezo electric may be used to force a drop of print agent to be delivered from a print agent chamber or reservoir onto the print medium through a nozzle. A voltage may be applied to the piezo electric, which may change its shape. This change of shape may force a drop of print agent to exit through the nozzle. These printing systems may be called as piezo electric printing systems.

[0015] In some examples, print agent may comprise volatile and non-volatile components. The volatile components may evaporate and the non-volatile components may remain in the print agent chamber or firing chamber. This may occur during periods of storage or non-use of the nozzles of the printhead. The non-volatile components may concentrate in a region close to the nozzles. This may impede or block print agent flow through the nozzles which can cause degradation on edges and grain performance. This defect may be called as decap. Effects of decap may alter drop ejection trajectories, velocities, shapes, and colors, all of which may negatively impact the print quality of a printing system, e.g. an inkjet printer.

[0016] In this disclosure, a decap performance quantifies the ability of the print agent ejection assembly to readily eject print agent from the printhead, upon prolonged exposure to air. A decap performance thus quantifies the ability of a nozzle of a print agent ejection assembly to be idle without producing decap defects when the nozzle delivers to next drop of print agent. A low or a bad decap performance thus implies that decap defects may occur. On the contrary, a high decap performance implies no decap defects or less decap defects than a maximum acceptable level of decap defects. A decap performance may thus indicate a level of blockage of the nozzles. Greater decap performances may thus imply that the print agent ejection assembly may remain idle for longer times.

[0017] The time that nozzles can be uncovered and idle before the nozzles no longer fire properly may be called as decap time. During this idle time, print agent viscosity may increase by the evaporation of some volatile components. This increase in viscosity may difficult the ejection of the next print agent drop. A non-working time of a nozzle may refer to the time from the ejection of a drop of print agent to the consecutive ejection in a single nozzle. During this non-working time, the nozzle is not delivering print agent.

[0018] Examples of volatile components may be a print agent vehicle, e.g. water, humectants, dispersants and additives. Pigments may be an example of non-volatile components. A latex ink may be an example of print agent comprising volatile and nonvolatile components.

[0019] Recirculating print agent within the printheads may be used to increase decap performance or to increase decap time.

[0020] FIG. 1 schematically illustrates an example of a printing system 100 according to one example of the present disclosure. The printing system 100 comprises plurality of printheads 10 having a plurality of nozzles (not shown in FIG. 1) to deliver print agent. Print agent may be delivered onto a print medium 200.

[0021] The print medium 200 may advance or move along the advancing axis 210 following the direction represented by arrow A. The print medium may be moved by an advancer (not shown in FIG. 1). An advancer may include a roller and/or a wheel. The print medium 200 may be of any shape or size to be used in the printing system.

[0022] The print medium is a material capable of receiving a print agent, e.g. ink. In some examples, the print medium may be a sheet of paper. In some examples, the print medium may be a sheet of cardboard, textile material, plastic material or canvas.

[0023] The printheads 10 of this figure are mounted on a printhead support 110. The printhead support 110 of FIG. 1 is a carriage. The carriage 110 supporting a plurality of printheads may travel across a scan axis 111 for delivering print agent onto a width of the print medium 200. In this disclosure, a width of a print medium extends substantially perpendicular to the advancing axis 210 and a length of a print medium extends substantially parallel to the advancing axis 210. In this example, the printhead support 110 receives eight printheads 10. In some examples, two printheads may be mounted on a carriage. In some examples, four printheads may be mounted on a carriage. A plurality of printheads may thus be mounted on a carriage.

[0024] In some examples, the printhead support may statically span substantially the whole width of the print medium. In these examples, the printhead support may be a print bar supporting a plurality of printheads. The printheads may be used in a page- wide array (PWA) printing system.

[0025] In the example of FIG. 1 , the plurality of printheads 10 mounted on the printhead support 110 are misaligned relative to the scan axis 111. A printhead of the plurality of printheads may be downstream to another printhead of the plurality of printheads. An upstream printhead may deliver print agent on a print medium and a downstream printhead may deliver print agent over a previously deposited print agent. An image on a print medium may thus comprise several layers of print agent delivered by different printheads.

[0026] In some examples, the plurality of printheads may be mounted aligned relative to the scan axis.

[0027] The printing system 100 of FIG. 1 comprises a printhead 10 having a plurality of printhead agent ejector assemblies (not shown in FIG. 1), a drop detector 120 to determine a characteristic of the drops ejected through nozzles and a controller 130. The print agent ejector assemblies of this figure comprise a recirculation channel, a firing chamber in fluid communication with the recirculation channel, a pump to generate pump pulses for flowing print agent from the recirculation channel to the firing chamber, a nozzle associated with the firing chamber and an actuator to eject a print agent drop through the nozzle. The controller may be to obtain a number of pump pulses generated by the pump of a print agent ejector assembly, instruct the drop detector to determine a drop performance of print agent drops ejected through the nozzle of the print agent ejector assembly, analyze the determined drop performance for the number of obtained pump pulses and determine a number of pump pulses from the number of obtained pump pulses to establish a drop performance greater than a reference drop performance.

[0028] The pump may be operated to generate pump pulses that cause a fluid agent movement within the print agent ejector assembly. Print agent may be forced to flow from the recirculation channel to the firing chamber. Print agent previously contained in the firing chamber may be removed from the firing chamber and fresh print agent may be allowed to enter. Generating pump pulses may thus increase the decap time or the decap performance. [0029] In some examples, the pump may be operated by activating a recirculation mode. In a recirculation mode, the pump may be operated following an actuation profile. For example, in the recirculation mode the pump may operate at 2 to 45 kilohertz (kHz) and produce between 100 and 5000 pulses. The recirculation mode may be activated before the ejection of a print agent drop through the nozzle of the print agent ejector assembly.

[0030] In some examples, the recirculation mode may be activated between the ejection of two consecutive print agent drops. The recirculation mode may thus be activated during a period of time.

[0031] The recirculation mode may comprise a recirculation parameter. This recirculation parameter may define the level of recirculation. An example of recirculation parameter may be a number of pump pulses. In some examples, the number of pump pulses may comprise an average of pump pulses for a plurality of print agent ejector assemblies.

[0032] Print agent ejector assemblies of the plurality of print agent ejector assemblies may be placed in subgroups. The subgroups may comprise print agent ejector assemblies grouped in rows, i.e. parallel to the scan axis 111 , and in columns, i.e. parallel to advancing axis 210. Longer rows may lead to swaths having a greater height (in the advancing axis 210) if all nozzles were to deliver print agent. In this disclosure, a swath refers to an area of a print medium that can be printed by a printhead in a single pass, i.e. from one lateral side of the print medium to the opposite side along the scan axis 111. In some examples, the printhead may deliver print agent on both on the way and the way back. In some examples, the printing system may be an inkjet printer.

[0033] The printing system 100 of FIG. 1 comprises a drop detector 120. The drop detector 120 may determine a characteristic of the drops ejected through nozzles of the printhead or printheads of the printing system 100. The printhead carriage 110 may move the printhead 10 towards the drop detector 120 to analyze the drops ejected by the nozzles, e.g. a behavior of the plurality of nozzles. The drop detector 120 may detect drops fired, i.e. delivered, by each of the nozzles of the plurality of nozzles. [0034] The drop detector 120 may determine a drop performance of print agent drops ejected through the nozzles of the plurality of print agent ejector assemblies. A drop performance indicates a characteristic of the drops compared to a predetermined level of this characteristic. Drop performance may be an indicative of the health status of nozzles. A determined drop performance may be compared to a reference drop performance. A reference drop performance may represent an acceptable status of drops. Drop performance may thus be used to determine the decap performance. Greater drop performances imply greater decap performances. A drop performance greater than a reference drop performance may indicate an acceptable decap performance.

[0035] In some examples, a characteristic of the drops may comprise a drop parameter. A drop parameter may be a drop size, a drop volume, a drop weight or a drop mass. The drop detector may thus determine a quantity of print agent delivered by a nozzle. A quantity of print agent may be determined by measuring a drop parameter of print agent, e.g. size or a weight of drops. A drop parameter, e.g. drop size, of a print agent drop delivered by a nozzle may indicate a health status of the nozzle, e.g. a percentage of aperture of the nozzle. A drop parameter, e.g. drop size, may thus indicate the drop performance.

[0036] The drop detector 120 may comprise a transmitter to emit a signal and a receiver to receive the emitted signal to measure a drop parameter of the print agent drops passing between the transmitter and the receiver. In some examples, the drop detector may measure a size of the of print agent drops passing between the transmitter and the receiver. The emitter may be positioned spaced apart from the transmitter to allow a drop of print agent to pass therebetween. The transmitter may be a led and the receiver a light sensor, e.g. a photodetector. The transmitter may emit a signal, e.g. a light signal, towards the receiver. The drop detector may determine if a drop of print agent is passing between the receiver and the transmitter. The drop detector may also provide with information about the characteristics of the drop of print agent and about the nozzle. In some examples, the receiver may detect a shadow produced by the drop of print agent. This shadow may be measured and may be used to determine characteristics of the drop of print agent or a drop parameter. Nozzles out or being partially blocked by non-volatile particles of print agent may thus be determined. [0037] In some examples, the signal emitted by the transmitter towards the receiver may be measured by the receiver. A print agent drop between the transmitter and the receiver may cross the signal emitted by the transmitter. The signal, e.g. a light beam, emitted by the transmitter may thus be partially blocked by the print agent drop.

[0038] The printing system 100 of FIG. 1 comprises a controller 130. The controller 130 may obtain a number of pump pulses generated by the pump of a print agent ejector assembly, instruct the drop detector to determine a drop performance of print agent drops ejected through the nozzle of the print agent ejector assembly, analyze the determined drop performance for the number of obtained pump pulses and determine a number of pulses from the number of obtained pump pulses to establish a drop performance greater than a reference drop performance.

[0039] The controller may thus adjust the number of pump pulses to obtain a drop performance greater than a reference drop performance, e.g. to obtain an optimum drop performance, for the real conditions of the print agent and of the printhead. Differences between print agent batches or differences between printheads may thus be taken into account for determining an optimum number of pump pulses. Generating pump pulses may increase a temperature inside the print agent ejector assembly. Limiting the number of pump pulses allows maintaining a temperature inside the print agent ejector assembly within certain limits and enhancing the reliability of the print agent ejector assembly. Print quality may thus be enhanced.

[0040] In some examples, the controller may obtain a number of pump pulses from a sensor, e.g. a counter. In some examples, a clock may be used for obtaining a number of pump pulses for a given frequency of pump pulses. In some examples, the controller may instruct the pump to generate a number of pump pulses for a given time. For example, the controller may obtain the number of pump pulses generated by the pump from a memory.

[0041] The drop detector may determine a drop parameter of print agent drops ejected through the nozzle of a print agent ejector assembly according to any of the methods herein described. For example, to instruct the drop detector to determine a drop performance may comprise measuring a drop parameter of drops ejected through a nozzle of a print agent ejector assembly. A drop parameter may be, for example, a size drop or a weight drop.

[0042] In some examples, to instruct the drop detector to determine a drop performance may comprise moving the printhead towards the drop detector. In some examples, a nozzle may be positioned between a transmitter and a receiver of the drop detector. The controller may then instruct the actuator associated with this nozzle to deliver print agent.

[0043] In some examples, analyze the determined drop performance for the number of pump pulses may comprise obtaining the greatest drop performance for the drops ejected by the nozzle of the print agent ejector assembly. Obtaining the greatest drop performance of the drops ejected by a nozzle may comprise measuring a drop parameter for print agent drops ejected by the nozzle and identifying the greatest drop parameter. For example, measuring a drop size for each print agent drops ejected by the nozzle and identifying the greatest drop size. The greatest drop parameter may thus indicate the greatest drop performance.

[0044] In some examples, the greatest drop parameter may be determined by identifying the greatest signal strength between a signal valley and a consecutive signal peak of a signal detected by a receiver of the drop detector.

[0045] In some examples, determine a number of pump pulses from the number of obtained pump pulses to establish a drop performance greater than a reference drop performance may comprise identifying the accumulated number of pump pulses for each print agent drop of a nozzle. The number of accumulated pump pulses corresponding to the print agent drop having the greatest drop performance may thus be determined.

[0046] In some examples, a curve relating a drop performance of the plurality of print agent drops ejected by the nozzle and the number of accumulated pump pulses may be generated. The number of pump pulses corresponding to the print agent drop having the greatest drop performance may thus be determined from the curve. In some examples, a table may relate a drop performance of the plurality of print agent drops ejected by the nozzle and the number of accumulated pump pulses. [0047] In some examples, the number of pump pulses for obtaining the greatest drop performance may be determined for different period of times. The number of pump pulses may thus be adjusted for a given decap time.

[0048] In some examples, the controller may instruct the pump to generate the determined number of pump pulses for a given time, e.g. for an obtained decap time, to maintain the drop performance of the nozzle greater than a reference drop performance.

[0049] In some examples, the controller may determine a number of pump pulses for the pumps of a plurality of print agent assemblies by averaging the number of pulses determined for each print agent assemblies. In some examples, the determined number of pump pulses for a print agent assembly may be representative of the number of pump pulses for the pumps of the plurality of print agent assemblies.

[0050] In some examples, the controller may control the operation of the printing system. In some examples, the controller may control print agent delivered by the plurality of nozzles onto the print medium.

[0051] In some examples, the controller may detect a decap time for a nozzle of print agent assembly or for nozzles of a plurality of print agent assemblies. The controller may set a number of pump pulses to maintain a drop performance greater than a reference drop performance for the pump or pumps associated with these print agent assemblies. A decap performance greater than a reference decap performance may thus be maintained.

[0052] In FIG. 1 , the controller 130 includes a processor 131 and a non-transitory machine-readable storage medium 132. The non-transitory machine-readable storage medium 132 is coupled to the processor 131.

[0053] The processor 131 performs operations on data. In an example, the processor is an application specific processor, for example a processor dedicated to control a print agent recirculation within the printhead. The processor 131 may also be a central processing unit for controlling the operation of the printing system.

[0054] The non-transitory machine-readable storage medium 132 may include any electronic, magnetic, optical, or other physical storage device that stores executable instructions. The non-transitory machine-readable storage medium 132 may be, for example, Random Access Memory (RAM), an Electrically-Erasable Programmable Read-Only Memory (EEPROM), a storage drive, an optical disk, and the like.

[0055] The printing system 100 of FIG. 1 may be used for calibrating print agent recirculation within a printhead according to any of the examples herein disclosed.

[0056] FIG. 2 illustrates a portion of the printhead 10 of the printing system 100 of FIG. 1. The printhead 10 of FIG. 2 comprises a plurality of print agent ejector assemblies 20. A print agent ejector assembly 20 of the plurality of print agent ejector assemblies of FIG. 2 comprises a recirculation channel 30, a firing chamber 40 in fluid communication with the recirculation channel 30, a pump 50 to generate pump pulses for flowing print agent from the recirculation channel 30 to the firing chamber 40, a nozzle 70 associated with the firing chamber 40 and an actuator 60 to eject a print agent drop through the nozzle 70.

[0057] In the example of FIG. 2, the firing chamber 40 or print agent chamber comprises a nozzle 70. A nozzle may be an orifice created on the print agent chamber. In some examples, the firing chamber may comprise a plurality of nozzles to deliver print agent drops onto a print media. The actuator may be associated with a nozzle or to a plurality of nozzles. In the example of FIG. 2, the actuator 60 is at the firing chamber 40 and cause the ejection of a print agent drop through the nozzle 70.

[0058] In some examples, the actuator 60 may be a heating element, e.g. a thermal resistor. In some examples, the actuator may be a piezoelectric element. In some examples, the actuator may be at the firing chamber or adjacent to the firing chamber.

[0059] In FIG. 2 the firing chamber 40 is in fluid communication with the recirculation channel 30 through the conduit 31. In some examples, a plurality of firing chambers may be in fluid communication with a recirculation channel. In some examples, a plurality of recirculation channels may be in fluid communication with a firing chamber.

[0060] The printhead 10 of FIG. 2 comprises a print agent slot 11. The print agent slot 11 may be in fluid communication with the firing chamber 40 and with the recirculation chamber 30. Print agent may flow from the print agent slot to the firing chamber for delivering print agent through the nozzle onto a print media. A print agent delivery system may supply print agent from a print agent tank or print agent supply station to print agent slot of the printhead.

[0061] The pump may be operated to move print agent from the print agent slot to the firing chamber through the recirculation channel. Operating the pump may comprise generating pump pulses for flowing print agent from the recirculation channel to the firing chamber. In some examples, the pump may draw print agent from the print agent slot to the recirculation channel and for the recirculation to the firing chamber. This print agent flowing to the firing chamber may force the print fluid inside the firing chamber to leave. Print agent previously contained inside the print fluid may thus be replaced by print agent flowing from the recirculation channel. In some examples, print agent contained in the firing chamber may be forced to exit to the print agent slot through a conduit connecting the firing chamber to the print agent slot. In some examples, print agent removed from the firing chamber may flow to the print agent slot through the recirculation chamber. Print agent may thus recirculate through the firing chamber by operating the pump. Fresh print agent may thus enter into the firing chamber. Activating the pump may thus increase the decap time or the decap performance.

[0062] The pump may be at the recirculation channel or adjacent to the recirculation chamber. In some examples, a plurality of pumps may be at the recirculation chamber. In some examples, a pump may be associated with a plurality of recirculation channels to flow print agent from the recirculation channel to the firing chamber.

[0063] In some examples, the pump may be activated by a heating element, e.g. thermal resistor. The heating element may be heated to create a bubble that generates a drop pressure that allows print agent from the print agent slot to enter into the firing chamber through the recirculation channel.

[0064] In some examples, the pump may be activated by a piezoelectric element. The piezoelectric element may change its shape when an electric filed is applied. This change in shape forces print agent from the print agent slot to flow to the recirculation channel and to the firing chamber. In some examples, a shape memory alloy may be used to create a fluid flow from the print agent slot to the recirculation channel.

[0065] FIG. 3 shows an example of a graph of a signal 240 output from a drop detector according to the present disclosure. The signal of FIG. 3 may be the signal received by a receiver of a drop detector. The receiver may be a photodetector or an optical sensor.

[0066] This figure shows the variation of the signal 240 along the time. The x-axis 220 represents samples or time and the y-axis 230 represents the signal received by the receiver. A descend of signal received by the receiver may indicate a detection of print agent drop. The first valley 241 indicates a detection of a first print agent drop and the second valley 243 indicates a detection of a second print agent drop. A difference between y1 of the first valley 241 and y2 of the first peak 242 defines the signal strength 251 of the first print agent drop. The signal strength 252 of the second print agent drop may be defined as the difference between y3 corresponding to the second valley 243 and y4 corresponding to the second peak 244. A signal strength of a print agent drop may thus be defined as the difference between a valley and a consecutive peak of the signal.

[0067] The signal strength may be used to compare a drop parameter between different print agent drops. In this figure, the signal strength 252 is greater than the signal strength 251 . This may be indicative that the drop parameter of the second print agent drop is greater than the drop parameter of the second print agent drop.

[0068] In some examples, the drop parameter may be a drop size or a drop weight. For example, the signal strength may indicate a drop size of a print agent drop. In this figure, as the signal strength 252 is greater than the signal strength 251 , the drop size of the second print agent drop is greater than the drop size of the first print agent drop.

[0069] Analyzing the signal received by a receiver of the drop detector may be used for identifying a signal strength of a plurality of print agent drops. In some examples, by analyzing these signal strengths a drop characteristic or a drop parameter, e.g. a drop size, of the plurality of print agent drops may thus be determined. [0070] In some examples, the signal strengths of a plurality or print agent drops may be used for determining a drop performance of each print agent drop of the plurality of print agent drops. In some examples, the signal strengths of a plurality of print agent drops may be used for determining a decap performance of a printhead.

[0071] FIG. 4 shows an example of a graph of a relationship between a drop parameter of a plurality of print agent drops and a recirculation parameter.

[0072] In this figure, x-axis 260 represents the recirculation parameter of the recirculation mode. In some examples, the recirculation parameter may be a number of pump pulses of a print agent ejector assembly according to any of the examples herein disclosed. The number of pulses of the pump pulses may be obtained by any of the examples herein disclosed. In some examples, time may be proportional to the number of pump pulses.

[0073] The y-axis 270 of this figure represents a drop parameter. For example, the drop parameter may be a drop size. In some examples, the drop size may be obtained by analyzing the signal strength between a valley and a consecutive peak of signal outputted by a drop detector according to any of the examples herein disclosed.

[0074] In FIG. 4, the relationship between the drop parameter and the recirculation parameter, e.g. the number of pump pulses, results in a drop performance 280. As can be seen from this graph, a low recirculation parameter, e.g. a lower number of pump pulses, results in lower drop performances.

[0075] At a first region I, the drop performance 280 increases with the recirculation parameter. This may indicate that the print agent ejector assembly is under recirculated. The drop performance 280 decreases with the recirculation parameter at the third region III. The print agent ejector assembly may be over recirculated in the region III. A high value of the recirculation parameter, e.g. a high number of pump pulses, may cause an increase of the temperature inside the print agent ejector assembly which may reduce the print drop parameter, e.g. drop size.

[0076] The second region II represents an optimum drop performance 281. The optimum drop performance may be greater than a reference drop performance. The optimum drop performance 281 may indicate the greatest drop parameter or the greatest drop size. The optimum drop performance 281 correspond to a value P of the recirculation parameter, e.g. a P number of pump pulses. Setting the P for the recirculation parameter allows optimizing the drop performance, and consequently, the decap performance. An optimum recirculation parameter value, e.g. an optimum number of pump pulses, may thus be obtained taking into account the real conditions of the print agent and of the printhead.

[0077] In some examples, the graph of FIG. 4 may be used for determining a value for a recirculation parameter during a period of time for establishing a decap performance greater than a reference decap performance. In some examples, the graph of FIG. 4 may be used for determining a number of pump pulses to establish a drop performance greater than a reference drop performance. In some examples, the graph of FIG. 4 may be used for determining a number of pump pulses corresponding to an optimum drop characteristic.

[0078] In some examples, a table may be used to relate the drop parameter and the recirculation parameter. For example, a greatest drop parameter may be identified and then the corresponding recirculation parameter may be determined.

[0079] FIG. 5 is a block diagram of a method to calibrate print agent recirculation within a printhead according to an example of the present disclosure. The method 500 to calibrate print agent recirculation within a printhead comprises activating 510 a recirculation mode during a period of time to recirculate print agent through a firing chamber, wherein the recirculation comprises a recirculation parameter; ejecting 520 during the period of time a plurality of print agent drops through a nozzle associated with the firing chamber; determining 530 a decap performance of the printhead based on the ejected plurality of print agent drops; and determining 540 a value for the recirculation parameter during the period of time based on the determined decap performance for establishing a decap performance greater than a reference decap performance.

[0080] The recirculation parameter may thus be adjusted for obtaining a decap performance greater than a reference decap performance. As the decap performance is determined based on the drops ejected by a printhead, printhead and print agent health status are taken into account to determine an optimum value for the recirculation parameter. Differences between print agent batches and between health status of several printheads are thus taken into account. The recirculation parameter may thus be precisely adjusted and decap performance may thus be increased.

[0081] The method 500 may be applied to any of the examples of print systems herein disclosed.

[0082] Recirculation mode is an operational mode of a printhead wherein print agent is recirculating within the firing chambers. Recirculation mode may be defined by a recirculation parameter. An example of a recirculation parameter may be the pump pulses. In some examples, a pump may generate pump pulses to generate a print agent flow within each print agent ejector assemblies to force fresh print agent to enter into the firing chambers. Activating a recirculation mode may comprise generating a plurality of pump pulses during the period of time for pumping and extracting print agent to and from the firing chamber. In some examples, the recirculation parameter may comprise pump pulses for recirculating print agent within the firing chamber and determining the value for the recirculation may comprise determining a number of pump pulses during the period of time for establishing a decap performance greater than a reference decap performance. Different number of pump pulses may be generated for different period of times. Number of pulses may thus be adjusted to a specific decap time.

[0083] In some examples, the method 500 may comprise storing the determined value for a recirculation parameter for a period of time. In some examples, the method 500 may comprise determining a value for the recirculation parameter during a different period of time and storing the determined value for a recirculation parameter for the different period time. This value may be determined according to any of the examples herein disclosed.

[0084] The recirculation mode may a adopt a value for the recirculation parameter from a plurality of recirculation parameters determined for different periods of time. Accordingly, depending on the estimated time of operation of the printhead in the recirculation mode, different values of recirculation parameters may be set. In this sense, the value of recirculation parameter may be adapted to different decap times.

[0085] In some examples, determining a decap performance of the printhead may include estimating the decap performance of the printhead from decap performances of a plurality of nozzles of the printhead. For example, an average from a plurality of decap performances for different nozzles may be calculated. In some examples, a decap performance of a nozzle may be representative of the decap performance of the printhead.

[0086] In some examples, the decap performance of the printhead may comprise a first decap performance for a first plurality of print agent ejector assemblies of a printhead and a second decap performance for a second plurality of print agent ejector assemblies of the printhead. Different values of recirculation parameters may be determined for the first plurality of print agent ejector assemblies and for the second plurality of print agent ejector assemblies.

[0087] In some examples, determining 530 a decap performance may comprise determining a drop performance. In some examples, determining a decap performance may comprise determining a drop performance for a plurality of print agent ejector assemblies and averaging these drop performances. Drop performance may be determined according to any of the examples herein disclosed.

[0088] In some examples, determining 530 a decap performance of the printhead may comprise determining a drop parameter of the ejected plurality of print agent drops during the period of time. In some examples, a drop parameter may comprise a drop size. In some examples, a drop parameter may comprise a drop volume or a drop weight.

[0089] In some examples, determining a decap performance may comprise identifying a print agent drop having the greatest drop parameter from the determined drop parameter of the ejected plurality of print agent drops. A print agent drop having the greatest drop parameter may be determined according to any of the examples herein disclosed. For example, a drop detector may be used to measure the plurality of ejected print agent drops and identify the greatest drop parameter of these print agent drops.

[0090] A drop parameter may be determined according to any of the examples herein disclosed. For example, determining a drop parameter of the ejected plurality of print agent drops may comprise positioning the nozzle delivering the plurality of print agent drops between an emitter and a transmitter, emitting a signal from a transmitter towards a receiver, measuring the signal received by the receiver and analyzing the received signal by identifying signal strengths between a valley and a consecutive peal of the received signal. Signal strengths corresponding to each print agent drop may thus obtained. Consequently, a drop parameter, e.g. a drop size, for each print agent drop delivered by a nozzle may be determined.

[0091] The emitter and the transmitter may be comprised in a drop detector according to any of the examples herein disclosed. Signal strengths for each print agent drops may be obtained as explained in connection with FIG. 3.

[0092] In some examples, determining 540 a value for a recirculation parameter may comprise identifying a value of the recirculation parameter corresponding to the print agent drop having the greatest drop parameter. For example, a number of pump pulses corresponding to the print agent drop having the greatest drop parameter, e.g. a drop size, may be identified.

[0093] In some examples, identifying a value of the recirculation parameter corresponding to the print agent drop having the greatest drop parameter may comprise creating a curve relating the determined drop parameter of the ejected plurality of print agent drops to the recirculation parameter. For example, the curve may be created as explained in connection to FIG. 4.

[0094] In some examples, the recirculation parameter may be a number of pump pulses. A number of pump pulses corresponding to the print agent drop having the greatest drop parameter may be identified by creating a curve relating the drop parameters of the print agent drops to the number of pump pulses. An optimum number of pump pulses for the greatest drop parameter may thus be determined.

[0095] In some examples, a table may define a relationship between recirculation parameters and print agent drops. The greatest print agent drop may be detected and then the corresponding recirculation parameter may be identified. For example, a number of pump pulses for each drop size may be defined in a table.

[0096] In some examples, determining a decap performance of the printhead may comprise determining a drop performance of a plurality of print agent assemblies. This may involve identifying the greatest drop parameter for the print agent drops delivered by this plurality of print agent assemblies. A recirculation parameter value may be determined for each of the plurality of print agent assemblies. In some examples, the recirculation parameter determined for each print agent assembly may be averaged to a value for a recirculation parameter for a group of print agent assemblies.

[0097] FIG. 6 represents a non-transitory machine-readable storage medium according to an example of the present disclosure. The non-transitory machine- readable storage medium 132 is encoded with instructions which, when executed by a processor, cause the processor to obtain drop characteristic from a plurality drops delivered by a printhead during a predetermined period of time as represented at block 710, obtain a number of pump pulses generated by a pump associated with a recirculation channel of the printhead during the predetermined period of time as represented at block 720, identify an optimum drop characteristic from the obtained drop characteristic as represented at block 730 and determine a number of pump pulses corresponding to the determined optimum drop characteristic as represented at block 740.

[0098] The processor may be according to any of the examples herein disclosed. For example, the processor may be coupled to the non-transitory machine-readable storage medium. For example, a controller according to any of the examples herein disclosed may comprise the processor and the non-transitory machine-readable storage medium.

[0099] The instructions encoded in the non-transitory machine-readable storage medium for the processor represented at blocks 710, 720, 730 and 740 may participate in calibrating print agent recirculation within a printhead according to any of the examples herein disclosed.

[00100] The processor may obtain a drop characteristic from a plurality drops delivered by a printhead during a predetermined period of time. A drop characteristic or a drop parameter may comprise drop speed, drop size, drop mass, drop volume or drop weight. A drop characteristic or a drop parameter may be obtained according to any of the examples herein disclosed. For example, the processor may instruct a drop detector to measure print agent drops delivered during this predetermined period of time. In some examples, the processor may instruct the printhead to move towards the drop detector. [00101] The number of pump pulses generated by a pump associated with a recirculation channel of the printhead may be obtained for a predetermined period of time according to any of the examples herein disclosed. In some examples, the processor may receive a number of pump pulses from a counter associated with the pump.

[00102] In some examples, the number of pump pulses may be obtained from a timer. The processor may instruct the pump to operate at a predetermined frequency. The number of pump pulses may thus be obtained from the timer and the predetermined frequency of the pump pulses. This predetermined frequency may be set for a recirculation mode. Data related to the recirculation mode may be obtained from a storage medium coupled to the processor.

[00103] Identifying an optimum drop characteristic from the obtained drop characteristic may be according to any of the examples herein disclosed. In some examples, identifying an optimum drop characteristic may comprise identifying a greatest drop parameter from the plurality of print agent drops. In some examples, the processor may instruct a drop detector to identify the optimum drop characteristic or optimum drop parameter.

[00104] The processor may identify a number of pump pulses corresponding to the determined optimum drop characteristic according to any of the examples herein disclosed. In some examples, the processor may create a curve or a table relating a drop characteristic to the number of pump pulses.

[00105] In some examples, the non-transitory machine-readable storage medium may cause the processor to store the determined number of pump pulses for the predetermined period of time and associate the predetermined period of time to a nonworking time of the nozzle. A number of pump pulses to maintain decap performance in an acceptable level may thus be set for a non-working time of the nozzle.

[00106] In some examples, the non-transitory machine-readable storage medium may cause the processor to detect a non-working time of nozzle and to set a number of pump pulses corresponding to the detected non-working time. Several number of pump pulses may thus be set for several non-working times or decap times. Number of pump pulses may thus be precisely adjusted. An overall decap performance may consequently be increased.

[00107] The preceding description has been presented to illustrate and describe certain examples. Different sets of examples have been described; these may be applied individually or in combination, sometimes with a synergetic effect. This description is not intended to be exhaustive or to limit these principles to any precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is to be understood that any feature described in relation to any one example may be used alone, or in combination with other features described, and may also be used in combination with any features of any other of the examples, or any combination of any.