BACKGROUND

[0001] Print apparatus carry out print jobs to disperse print agents such as inks that include an agent, for example a dye or colorant, coating agents, thermal absorbing agents and the like. Such apparatus may comprise a printhead, which includes a set of nozzles and a mechanism for ejecting a selected print agent from the printhead as a fluid through the nozzles. To maintain printhead health, the nozzles are subjected to servicing operations when connected to the print apparatus, whereby the nozzles are cleaned by removing residual or settled print agent to avoid clogging and thus failure of the nozzles.

[0002] It is recommended to remove printheads from a print apparatus if they will not be used during a subsequent print job. This is to avoid unnecessary print agent wastage caused by servicing operations that would otherwise be carried out on the printheads if they were to remain connected to the print apparatus.

BRIEF DESCRIPTION OF DRAWINGS

[0003] Examples will now be described, by way of non-limiting example, with reference to the accompanying drawings, in which:

[0004] Figure 1 is a simplified schematic of an example print apparatus;

[0005] Figure 2 is a simplified schematic of an example print agent delivery system of a print apparatus;

[0006] Figure 3 is a simplified schematic of an example printhead of the print agent delivery system;

[0007] Figure 4 is a simplified schematic of the example print agent delivery system of Figure 2, when operating in a first mode of operation;

[0008] Figure 5 is a simplified schematic of the example print agent delivery system of Figure 2, when operating in a second mode of operation;

[0009] Figure 6 is a flowchart of an example of a method of performing a cooldown operation for the printhead; [0010] Figure 7 is a flowchart of another example of a method of performing a cooldown operation for the printhead;

[0011 ] Figure 8 is a simplified schematic of an example machine readable medium in conjunction with a processor; and

[0012] Figure 9 shows a graphical comparison of example printhead weight vs number of printhead removals from a print carriage, with and without first performing the cool-down operation of Figures 6 and 7.

DETAILED DESCRIPTION

[0013] Figure 1 shows an example of a print apparatus 100, which may, for example, be for two-dimensional printing, e.g. for applying drops of a print agent such as ink (e.g. latex ink) on to a substrate such as paper, card, plastic, metal or the like. The print apparatus 100 comprises a print agent delivery system 102 and processing circuitry 104.

[0014] The print agent delivery system 102 includes a first tank 106, a printhead 108 for receiving print agent from the first tank 106 and a pump 110 for applying a motive force to the print agent (directly or indirectly).

[0015] The first tank 106 is in the form of a housing that, e.g. is shaped so that it, defines a reservoir for storing a print agent. The first tank 106 may be a print cartridge that is a removable and/or replaceable component of the print apparatus 100. In some examples, however, and as will be described in further detail below, the first tank 106 is a permanent and integral housing of the print apparatus 100 (at least for the operable lifetime of the first tank 106), and is in addition to and separate from a removable and/or replaceable print cartridge (which may be referred to as a second tank). For example, the first tank 106 may be a so-called “intermediate tank” of the print apparatus that is fluidically connected to both a replaceable print cartridge and the printhead 108 at an intermediate position there between (in a print agent flow direction).

[0016] The first tank 106, together with the pump 110, can be used to provide an uninterrupted supply of print agent to the printhead 108, e.g. while the second tank is replaced or otherwise replenished with print agent. The pump 110 may be associated with the first tank 106 in that it operates to force print agent to exit the first tank 106 through an outlet of the first tank 106, into a first distribution line (not shown) that is fluidically connected to and thus feeds the printhead 108. Where the first tank is an intermediate tank, a minimum volume of print agent may be maintained within the first tank 106 throughout its lifetime, by refilling the intermediate tank with print agent from the print cartridge as appropriate, e.g., periodically or as and when needed.

[0017] The printhead 108 comprises a first port for receiving the print agent from the first tank 106 (along a first distribution line fluidically connecting the first tank 106 to the printhead 108) and a second port through which print agent may exit the printhead 108, or vice versa. The first port and the second port are in fluidic communication through an internal space of the printhead 108. The second port is connected to a second distribution line, which in turn may be fluidically connected to an inlet of the first tank, or an inlet of the second tank (or any other tank) of the print agent delivery system 102, if present. In an example, the second port may be connected to at least one of: a recirculation line, or an inlet of a tank (e.g. the first tank, the second tank or a waste tank).

[0018] The printhead 108 is a removable and/or replaceable component of the print apparatus 100 and is shown in dotted outline. In some examples, the printhead 108 is mounted in a printhead carriage that is movable (by suitable control signalling from the processing circuitry 104) such that it can be repositioned at different locations within the print apparatus 100. The printhead 108 comprises an internal chamber (and in some examples plural internal chambers) that defines the internal space that fluidically connects the first and second ports, and that is suitable for holding a print agent. The printhead 108 further comprises at least one print agent ejection nozzle (not shown) and a mechanism for ejecting the print agent contained within the internal chamber(s) of the printhead 108 as a fluid through the nozzle(s). In some examples the printhead 108 may be an inkjet printhead, such as a thermal inkjet printhead in which the mechanism for ejecting print agent generates and uses thermal energy to expel the print agent through the nozzle(s).

[0019] The processing circuitry 104 comprises a cool-down module to determine whether there is a cooling demand for the printhead. As will be described in further detail below, it may be that a cooling demand for the printhead is determined if a current temperature of the printhead is above a predetermined threshold temperature level. To facilitate this, the print apparatus 100 may comprise a temperature sensor (not shown) proximate to the printhead 108 to acquire a signal representing (e.g. variations in) a parameter indicative of a temperature of the printhead 108 (or rather, the print agent contained within the internal space of the printhead 108). In some examples, the temperature sensor is positioned adjacent to the at least one print agent ejection nozzle, such that it can detect the temperature of the printhead at a position that is in proximity to the print agent ejection nozzle(s). Any suitable temperature sensor may be used for this purpose.

[0020] The processing circuitry 104 may be in wired or wireless communication with various components of the print apparatus 100, including the temperature sensor (if present) and the pump 110 of the print agent delivery system 102, to control operation of those components by suitable control signalling. For example, the processing circuitry 104 is able to control operation of the pump 110 to selectively initiate and, correspondingly, terminate a flow of print agent from the first tank 106 through the internal space of the printhead 108. As will be described in further detail below, the cool-down module (processing circuitry 104) may conditionally operate (e.g. issue control signalling to turn ON and OFF) the pump 110 based on whether it determines there to be a demand for cooling the printhead 104, e.g. based on whether the temperature of the printhead is above a threshold level.

[0021 ] The processing circuitry 104 may comprise any form of processing circuitry, for example, any one or any combination of a CPU, processing unit, ASIC, logic unit, a microprocessor, programmable gate array or the like. The cool-down module may for example be implemented by a processor executing machine readable instructions stored in a memory, or a processor operating in accordance with instructions embedded in logic circuitry, or the like.

[0022] While in Figure 1 the processing circuitry 104 is shown as being local to the print agent delivery system 102 on the print apparatus 100, this may not be the case and may be remote thereto. For example, the processing circuitry 104 may communicate data to and from the temperature sensor and/or print agent delivery system 102 remotely, for example via the Internet.

[0023] Although not shown in Figure 1 , the print apparatus 100 may comprise one print agent delivery system 102 for each colour or type of print agent that is to be used by the print apparatus for printing. Each print agent delivery system may be separate to, and isolated from, other print agent delivery systems of the print apparatus 100.

[0024] Figure 2 shows an example print agent delivery system 200. In this example, the print agent delivery system 200 comprises a first print cartridge 202, a second print cartridge 204, an intermediate tank 206 (such as that described above with reference to Figure 1 ), a printhead 208 and a network 210 of distribution lines 212 for supplying print agent between those components of the print agent delivery system 200. Although the print agent delivery system 200 is shown to have one printhead, the system 200 may comprise more than one (and in examples two) printheads.

[0025] Each one of the first print cartridge 202 and the second print cartridge 204 is substantially the same as the print cartridge described above in that it is a removable and/or replaceable component of the print apparatus 100. In this example, the first and second print cartridges 202, 204 contain print agent (e.g. ink) that is used predominantly to supply the intermediate tank 206 with sufficient print agent to maintain a minimum volume within the intermediate tank 206 throughout its operational lifetime. The first cartridge 202 and the second cartridge 204 may alternately supply print agent to the intermediate tank 206. The first cartridge 202, the second cartridge 204 and the intermediate tank 206 may each comprise a single supply port for alternately ejecting and receiving print agent to and from the network 210 and thus the wider print agent delivery system 200.

[0026] The distribution lines 212 forming the network 210 are in the form of conduits, e.g. tubes, or other passages suitable for receiving a flow of print agent to distribute print agent between the components of the print agent delivery system 200. Each one of the first and second cartridges 202, 204, the intermediate tank 206 and the printhead 208 are fluidically connected to each other via the distribution lines 212. The print agent delivery system 200 further comprises a plurality of electronically actuated valves 214 operatively connected to respective distribution lines 212, to selectively allow or prevent a flow of print agent along the distribution line 212 to which the valve 214 is connected. The valves 214 may operate based on appropriate control signalling from the processing circuitry, via wired or wireless communication.

[0027] In this way, it is possible for the processing circuitry to variably control and select (i.e. open the valve of) a subset of at least one distribution line 212 (and in some examples plural distribution lines) from the larger network 210, to be used to define an unobstructed flow path along which print agent can flow between any two or more components of the distribution network 210, as may be appropriate for any given operation of the print apparatus 200. [0028] Additionally, the print agent delivery system 200 comprises a plurality of pumps 216, i.e. mechanical devices (of any type suitable) to impart a motive force upon the print agent, to drive the print agent along the selected subset of distribution lines 212 defining the flow path through the system 200. The distribution network 210 may have one pump 216 for each one of the first print cartridge 202, the second print cartridge 204, and the intermediate tank 206, for selectively pumping and/or drawing print agent to and from the first print cartridge 202, the second print cartridge 204, and the intermediate tank 206, respectively. The pumps 216 may operate based on appropriate control signalling from the processing circuitry, via wired or wireless communication.

[0029] With reference to Figure 3, the example printhead 208 comprises an outer housing 302 and an internal dividing wall 304 that extends from the housing 302 and substantially bisects the internal space defined by the outer housing 302 to form a first internal chamber 306 and a second internal chamber 308, each of which is suitable for storing a print agent 310 within the printhead 208. In the illustrated example, the first and second internal chambers 306, 308 are in fluidic communication through an opening or space 312 between a base 314 of the printhead 208 and a distal end 316 of the dividing wall 304 opposite the end that contacts the housing 302. On the exterior of the printhead 208 at the base 314, there is provided at least one print agent ejection nozzle 318 (and in examples two thousand, one hundred and twelve nozzles). Within the housing 302, there is provided a mechanism for ejecting the print agent 310 contained within both chambers 306, 308 through the nozzles 318.

[0030] The printhead 208 comprises a first needle interconnect 320 and a second needle interconnect 322, each of which is a conduit for receiving a flow of print agent 310 therethrough. The first needle interconnect 320 extends from a first end 324 located externally to the printhead 208 (i.e. external to housing 302) to a second end 326 located within the first chamber 306. The second needle interconnect 322 extends from a first end 328 located externally to the printhead 208 to a second end 330 located within the second chamber 308. The first ends 324, 328 of both needle interconnects 320, 322 are connected to separate distribution lines 212 of the print agent delivery system 200, to allow a flow of print agent 310 between (i.e. to and from) the network 210 and the first and second internal chambers 306, 308 of the printhead 208. In that sense, the first needle interconnect 320 and the second needle interconnect 322 form respective ports that define separate, i.e. distinct, entry and exit points for print agent within the printhead 208. However, the provision of needle interconnects is provided as an example and the printhead may comprise first and second ports of any suitable type or form.

[0031] At the second end 326, 330 of each needle interconnect 320, 322, there is provided a valve assembly, which in this example is in the fom of a “volcano valve assembly" that comprises an accumulation bag (or balloon) 332, 334 and a mechanical valve 336, 338 that is actuated by inflation and, correspondingly, deflation of the accumulation bag 332, 334. Each accumulation bag 332, 334 has an internal volume that is in fluidic communication with a respective air pump 340, 342 to inflate and deflate the bag 332, 334 to open and close the mechanical valve 336, 338 of the valve assembly, respectively. The air pumps 340, 342 may operate based on appropriate control signalling from the processing circuitry, via wired or wireless communication. When the mechanical valve 336, 338 is open, print agent 310 is able to flow between the needle interconnect and the internal chamber of the printhead into which it extends. Correspondingly, when the mechanical valve 336, 338 is closed, flow of print agent 310 between the needle interconnect and the internal chamber of the printhead into which it extends is prohibited.

[0032] Each valve assembly may, and in the illustrated example does, also comprise a solenoid valve 344, 346 and a pressure relief valve 348, 350 to control (based on control signalling from the processing circuitry) the pressure of the air within the bag 332, 334.

[0033] In use, the processing circuitry 104 will receive (e.g. from a host network or computer) a so-called “print job" representing a document to be printed. The processing circuitry will then issue control signalling to the print agent delivery system 200 to initiate a print operation for the print job. The control signalling will open the valve(s) 214 of a (or plural) distribution line(s) 212 to allow a flow of print agent from the intermediate tank 206 to the printhead 208. Further, as illustrated by arrows 352 and 354 in Figure 3, during a print operation a pump 216 of the print agent delivery system 200 is activated such that print agent 310 is supplied to both the first needle interconnect 320 and the second needle interconnect 322 of the printhead 208. In other words, positive print agent pressure is applied to the first port and second port of the printhead 208. At the same time, control signalling is sent to open both valve assemblies within the printhead 208 (by inflating the accumulation bags 332, 334), to allow print agent to flow into the printhead through the first and second needle interconnects 320, 322 and into their respective first and second internal chambers 306, 308. [0034] By providing positive print agent pressure at the two needle interconnects, the printhead 208 is supplied with print agent to be used for printing. Additionally, the pressurised print agent within the printhead 208 is forced to occupy the ejection nozzles 318 before they are ejected from the nozzles 318 by the ejection mechanism on to a substrate such as paper. The printhead 208 may be moved by the carriage to different positions in a print zone on the substrate, as may be necessary to print the document specified by the print job. The print job is complete once the entire document specified by the print job has been printed to the substrate.

[0035] The print agent delivery system 200 described above may use any type of print agent but, in examples, the print agent will comprise a colouring pigment.

[0036] The chemical composition (e.g. density) of colouring pigment in general, and white colour pigment especially, is such that the pigment tends to separate from the liquid phase of the print agent if left to remain static, e.g. when the printhead is idle between print jobs. When left untreated, sedimentation of the colouring pigment may occur, whereby the pigment settles at the bottom of components of the print agent delivery system, such as the printhead and the distribution lines etc., and thus leads to clogging of those components. To avoid such sedimentation and allow the print agent to be ready for use when needed, the print agent delivery system 200 will recirculate print agent throughout its components.

[0037] Recirculation comprises the print agent delivery system 200 operating in a mode of operation (referred to herein as a “recirculation mode of operation") in which print agent is continuously pumped through components of the print agent delivery 200 system while the printhead(s) 208 is idle, i.e. when the print agent ejection mechanism is turned OFF. The recirculation mode of operation may be, e.g., scheduled by the processing circuitry 104 to occur periodically, e.g. once for every 4 hours that the printhead has been idle, to ensure that the print agent is well mixed.

[0038] Figure 4 schematically illustrates the example print agent delivery system 200 of Figure 2, when operating in a first recirculation mode of operation to recirculate the print agent through the system.

[0039] The first mode is initiated by the processing circuitry 104 issuing control signalling to select a subset of the network 210 of distribution lines 212 from the larger network 210 of distribution lines to form a recirulation flow path for the print agent. The control signalling selects the subset by opening the valve(s) 214 of the selected subset of distribution lines 212 that are to form the recirculation flow path. Additionally, the processing circuitry will send control signalling to inflate bags 332, 334 and thus open the valves 336, 338 of the printhead 108, to allow a flow of print agent therethrough.

[0040] The recirculation flow path comprises a first distribution line connecting an outlet of a first tank, in this example the intermediate tank 206, to the first printhead port 320 and a second distribution line connecting the second printhead port 322 to an inlet of a second tank, in this example the second cartridge 204, of the print agent delivery system 200. Thus the recirculation flow path comprises a start point at the intermediate tank 206 and an end point at the second cartridge 204, and includes the printhead 208 as an intermediate point between the start and end points. However, the recirculation flow path may be selected to be between any two print agent tanks of the print agent delivery system 200 that are external to the printhead 208. The recirculation flow path may, for example, be between a start point at the intermediate tank 206 and an end point at any one of (or even both) the first cartridge 202 and the second cartridge 204. In other examples, if the intermediate tank comprises a separate inlet and outlet, the recirculation flow path may be between a start point at an outlet of the intermediate tank 206 and an end point at the inlet to the intermediate tank 208, via the printhead 208.

[0041] At the same time as initiating the first mode, or subsequently, the processing circuitry 104 issues control signalling to a pump 216 of the system 200, to cause the pump to force a continuous (i.e. uninterrupted) flow of print agent along the selected recirculation flow path for a first time period. As is illustrated by the arrows in Figure 4, in use the print agent flows along the recirculation flow path in a direction from the intermediate tank 206 of the print agent delivery system 200, into the printhead 208 through the first needle interconnect 320, out of the printhead 208 through the second needle interconnect 322 (and onwards to the second cartridge 204, in this example).

[0042] The flow of print agent may be achieved by the processing circuitry operating a first pump associated with the intermediate tank 206 to force print agent out of the intermediate tank 206 into a first distribution line that is in fluidic communication with (e.g. directly connected to) the first needle interconnect 320. In a further example, this is achieved by the processing circuitry operating a second pump associated with the second cartridge 204 (or a second distribution line in fluidic communication with (e.g. directly connected to) the second needle interconnect 322) to draw print agent from the second needle interconnect 322 and into the second cartridge 204. That is, during the first time period, a positive pressure is supplied to print agent at the first needle interconnect 320 while, concurrently, a negative pressure is applied to print agent at the second needle interconnect 322.

[0043] The first mode of operation may be referred to as a “forward mode" of operation in that print agent that is received within the printhead 208 from the first needle interconnect 320 is forced to move through the space 312 from the first internal chamber 306 to the second internal chamber 308 before exiting the printhead 208 through the second needle interconnect 322. However, the print agent delivery system 200 can also operate in a second recirculation mode of operation and the processing circuitry 104 may select one of the first and second modes to use for any given recirculation operation.

[0044] Figure 5 schematically illustrates the example print agent delivery system 200 of Figure 2, when operating in the second recirculation mode of operation to recirculate the print agent through the system.

[0045] The second mode of operation is similar to the first mode of operation in that it is initiated by the processing circuitry 104 issuing control signalling to select a subset of the network 210 of distribution lines 212 to form a recirulation flow path between a start point at the intermediate tank 206 and an end point at the second cartridge 204, via the printhead 208, and operate a pump(s) 216 to drive a continuous flow of print agent along the selected recirculation flow path. However, the second mode of operation differs from the first mode in that the print agent flows into the printhead 208 through the second needle interconnect 322 and out of the printhead 208 through the first needle interconnect 320.

[0046] This may be achieved by the processing circuitry operating a first pump associated with the intermediate tank 206 to force print agent out of the intermediate tank 206 into a distribution line in fluidic communication with (e.g. directly connected to) the second needle interconnect 322. That is, a positive pressure is applied at the second needle interconnect 322. In a further example, the flow is achieved by the processing circuitry also, i.e. concurrently, operating a second pump associated with the second cartridge 204 (or a distribution line in fluidic communication with (e.g. directly connected to) the first needle interconnect 320) to draw print agent from the first needle interconnect 320 and into the second cartridge 204. That is, a negative pressure is applied at the first needle interconnect 320. [0047] The second mode of operation may be referred to as a “reverse mode" of operation in that print agent that is received within the printhead 208 from the second needle interconnect 322 is forced to move through the space 312 from the second internal chamber 308 to the first internal chamber 306 before exiting the printhead 208 through the first needle interconnect 320.

[0048] For both the forward and reverse modes, the recirculation operation is terminated by the processing circuitry 104 sending control signalling to turn OFF the pump(s) 216 driving the flow of print agent (or ceasing any control signalling) and/or to close the valve(s) 214, 336, 338 to prevent a flow of print agent to and from the printhead 208.

[0049] Another way to reduce sedimentation and settling within the printhead itself is to remove the printhead from the print carriage when it is not in use and to shake the printhead so as to encourage mixing of the colouring pigment and the liquid phase. The print apparatus may also be provided with a wheel, on which a printhead may be mounted and rotated for the same purpose.

[0050] Although recirculation may help to reduce sedimentation of the print agent, print agent will also tend to solidify when exposed to atmosphere outside the print agent delivery system, e.g. through the nozzles. Therefore, the nozzles are often subjected to servicing operations, during which residual or settled print agent within the nozzles is removed and discarded to avoid clogging of the nozzles and to improve print quality.

[0051] Printheads may be removed from the print carriage to avoid unnecessary print agent wastage that would be caused by servicing operations carried out on the printheads if they are left to remain in the print carriage during and/or between print jobs. For example, it is possible to replace a printhead with a dummy, i.e. non-functional, printhead. Removal from the print carriage is often done for printheads containing print agent in the form of an ink having a white colour pigment, which are used less often than coloured printheads because many print jobs specify that a coloured print agent is deposited on an already white substrate such as paper. Additionally, white pigments are larger than pigments for other colours and have a greater tendency to settle and cause related issues with printhead health. Therefore, servicing operations for white printheads tend to result in more print agent wastage compared to their colored (non-white) print agent counterparts. [0052] A printhead may be removed from the print carriage shortly after a print job that used the printhead is complete. However, the printhead and print agent inside the printhead will often be hot when the printhead is to be removed from the print carriage at this point. This is because the printhead will tend to increase in temperature during operation of the print apparatus (e.g. during a print job), e.g. as a result of print zone heating, operation of the print agent ejection mechanism, or even intentional heating of the circuitry to maintain consistent temperatures.

[0053] The Applicant has recognised that this increase in temperature causes existing air within the printhead to expand. It has been further recognised that, in cases where the printhead is removed from the print carriage whilst it is hot (e.g. shortly after printing), subsequent cooling of the printhead due to natural heat dissipation will create negative pressure within the printhead (due to contraction of the expanded air within the printhead) and cause air ingress into the printhead, e.g. through its nozzles or the needle interconnects. An accumulation of air within the printhead over time displaces print agent and can eventually lead to starvation of (i.e. a shortage of print agent in) the ejection mechanism (amongst other things) and thus failure of the printhead.

[0054] Although air ingress to the printhead could be reduced by refilling the printhead with print agent as it cools naturally when mounted to the print carriage, this is not possible in circumstances where the printhead has been removed. Further, natural cooling of the printhead in the print carriage can take over 2 hours of time that could otherwise be used more effectively, e.g. to carry out subsequent print jobs. Thus there is provided a method for actively cooling the printhead, e.g. after a print job is completed but before removal of the printhead from the print apparatus, to lower the amount of air ingested once removed and prolong the working life of the printhead.

[0055] Figure 6 illustrates an example method of cooling the printhead, in particular a method of using the print agent delivery system described above to perform a cool-down operation.

[0056] At block 602 the method comprises determining, using processing circuitry, that there is a cooling demand for the printhead.

[0057] Determining that there is a cooling demand for the printhead may comprise the processing circuitry determining that a predetermined (i.e. pre-characterised) trigger event (or events) has occured. The trigger event may be an event that has been predetermined (and characterized) as being associated with a temperature of the printhead being above an acceptable threshold temperature level.

[0058] The predetermined trigger event may be that a current temperature of the printhead is above a threshold temperature level. Accordingly, the method at block 602 may comprise processing circuitry comparing a temperature of the printhead to a threshold temperature level and determining that there is a cooling demand for the printhead if the temperature of the printhead is above the threshold temperature level. As described above, the temperature of the printhead may be determined based on sensor data acquired by a temperature sensor received from a sensor via a wired or wirleless connection therewith.

[0059] The threshold temperature level may be a predetermined, e.g. selected, temperature, at which the print agent within the printhead will be at an acceptable temperature, e.g. a temperature at which air ingress will be reduced or minimised after removal of the printhead from the print carriage. For example, the threshold temperature level may be set to an ambient temperature of a room in which the printhead is to be stored when removed from the print apparatus, e.g. a temperature in the range 18-26 degrees Celsius. Thus the processing circuitry may determine, at block 602, whether the printhead is at an elevated temperature as compared to the ambient environment within which the printhead is to be stored on removal from the print carriage. In some examples, the processing circuitry may be in communication with an ambient temperature sensor and will set the threshold temperature level to equal the ambient temperature in real time during operation of the print apparatus.

[0060] According to some examples, the processing circuitry may determine that there is a cooling demand regardless of (and without determining) a current temperature of the printhead, or comparing the current printhead temperature with a threshold level. That is, the method may comprise the processing circuitry determining that there is a cooling demand for the printhead without confirming the actual, current temperature of the printhead.

[0061] For example, the trigger event may be a predetermined (i.e precharacterized or identified) event, the occurence of which indicates that there is an increased risk of air ingestion to the printhead and that cooling the printhead will reduce that risk (or rather the extent of air ingestion). Such events may be shortly after a print job is completed (at which point the temperature of the printhead is likely to be at an unnaceptable level) or when the user has indicated that the printhead is to be removed from the print carriage (such that air ingress will occur if the temperature of the printhead is in fact above an acceptable threshold level). Accordingly, determining that a predetermined trigger event has occurred may comprise the processing circuitry: determining that a print job is completed; and/or receiving an input signal, from a user of the print apparatus to which the printhead is connected, indicating that the printhead is to be removed from the print apparatus.

[0062] Block 604 comprises processing circuitry, in response to determining a cooling demand for the printhead, forcing, for a first time period, a continuous flow of print agent (e.g. a print agent comprising a white colour pigment) along a flow path from the first tank of the print agent delivery system, into the printhead through a first printhead port and out of the printhead through a second printhead port.

[0063] Forcing a continuous flow of print agent along the flow path may comprise processing circuitry sending control signalling to turn ON the pump(s) of the print agent delivery system that is associated with the flow path to drive the flow of print agent, and/or to open the valve(s) associated with the flow path to direct the flow of print agent along the flow path. The flow path may be from the first tank to the same tank or a second tank of the print agent delivery system via the printhead.

[0064] The first time period may be a predetermined, e.g. predefined or preset, time period, such as 60 seconds. Further, the cool-down operation may be divided into a set of plural successive time periods, and the method may comprise, in response to the processing circuitry determining there is a cooling demand for the printhead: pumping, for respective time periods of the set, a continuous flow of print agent along a flow path from the first tank of a print agent delivery system, into the printhead through the first printhead port and out of the printhead through the second printhead port. That is, the first time period may be a respective one of plural time periods during which a continuous flow of print agent is pumped along the flow path.

[0065] The first tank is a separate component of the print agent delivery system to the printhead, such that print agent within the first tank will be cooler than that within the printhead. Therefore, by pumping a continuous flow of print agent from the first tank, into and out of the printhead through the internal space of the printhead, the printhead is continuously supplied with fresh, colder print agent. When fresh print agent passes through the printhead, the temperature of the printhead itself is reduced due to the energy exchange between the printhead and the colder print agent, and also due to the hot print agent (e.g. print agent that has gained temperature by the heat exchange from the hotter printhead) inside the printhead being replaced with fresh, cooler print agent throughout the cool-down operation.

[0066] Actively reducing the temperature of the printhead and its contents in this manner may increase the rate of cooling of the printhead, as compared to arrangements in which the printhead is left to cool by natural heat dissipation. This in turn may reduce the amount of time that a user is expected to wait before the printhead can be removed from the carriage, i.e. the time it takes for the temperature of the printhead to reach the threshold temperature level at which air ingestion post-removal is minimised. Further, reducing the temperature of the printhead reduces the extent of air ingestion post-removal, and so the life of the printhead can be extended by a significant amount. Reducing the waiting time may also cause the user to remove the printheads more often to minimise the number of servicing operations that the printhead will be subjected to when idle between print jobs, and thus the amount of print agent that is unnecessarily wasted.

[0067] To increase the extent of heat transfer, the flow path may include a nontrivial location within the printhead, e.g. a region within the printhead that is prone to excessive heating or is adjacent to a heat source. The internal space through which the continuous flow of print agent passes may be adjacent to the ejection nozzles of the printhead and/or the ejection mechanism of the printhead. The first tank itself may also be cooled to increase a temperature difference between the print agent and the printhead and thus the extent of heat transfer between the printhead and the print agent.

[0068] In some examples, the flow path for the cool-down operation is a recirculation flow path of the print agent delivery system, i.e. a flow path that would be selected and used during a recirculating mode of operation to prevent settling of the print agent within the components of the print agent delivery system (as described above with respect to Figures 4 and 5). The processing circuitry may cause performance of the cooldown operation by operating the print agent delivery system in the first and/or second recirculation modes of operation described above with respect to Figures 4 and 5. That is, in response to determining there is a cooling demand for the printhead, the processing circuitry may issue appropriate control signalling to cause the print agent delivery system to perform a recirculation operation, as described above, for the first time period.

[0069] Re-using the print agent delivery system and recirculation flow paths for the cool-down operation may be particularly useful and appropriate in that it obviates the need to provide additional components, e.g. distribution lines and corresponding pumps etc, to cool the printhead. Further, cooling the printhead by recirculating the print agent before removal from the print carriage encourages good mixing between the colouring pigment and liquid phase of the print agent, to reduce the extent of sedimentation after removal.

[0070] Figure 7 illustrates another example method of using the print agent delivery system 200 to perform a cool-down operation. In this example method, the processing circuitry determines whether there is a cooling demand based on whether a set of plural predetermined trigger events occur.

[0071] The method begins at block 702, which comprises the processing circuitry determining that a first predetermined trigger event has occured. The first trigger event may be a trigger event substantially as described above with respect to Figure 6, i.e. an event, the occurence of which indicates that there is a risk of air ingestion to the printhead and that cooling the printhead will likely reduce that risk (or the extent of air ingestion).

[0072] The first trigger event may comprise processing circuitry determining that a print job is completed. In this example, however, the first trigger event comprises processing circuitry receiving an input signal from a user of a print apparatus to which the printhead is connected, wherein (receipt of) the input signal indicates that the printhead is to be removed from the print apparatus. The print apparatus may comprise a human- machine interface, e.g. a touch sensitive display screen, that allows the user to provide such an input. For example, the interface presents the user with a display option or prompt to remove the printhead from the print carriage, so that it may be replaced with a dummy printhead for a subsequent print job, for example, and the user will provide an input to confirm selection of that option (or otherwise authorise removal of the printhead).

[0073] In response to the processing circuitry receiving such an input from a user of the print apparatus, and thus determining that a first trigger event has occured, the method proceeds to block 704 at which the processing circuitry communicates with the temperature sensor to determine whether a second trigger event has occured, and in particular whether the temperature of the printhead is above the threshold temperature level, by comparing the current temperature of the printhead with the threshold temperature level.

[0074] In this way the determination at block 704 is performed conditionally, and the condition for performing the determination is that the first trigger event has occured. In some cases, the condition for performing the determination at block 704 may be that more than one predetermined trigger event has occured. Conditional performance of the determination at block 704 may avoid using unnecessary processing power or communications bandwidth, e.g. in circumstances where the printhead is not to be removed from the print apparatus.

[0075] If it is determined at block 704 that the printhead is at or below the threshold temperature level, this is taken as an indication that there is not a cooling demand for the printhead and that removal of the printhead will not cause air ingress. Accordingly, in that case the method proceeds to block 706, at which the method comprises the processing circuitry holding, using the moveable printhead carriage, the printhead at a position that is accessible to a user, to allow the printhead to be removed from the print apparatus by the user. This may comprise the processing circuitry issuing suitable control signalling to the print carriage. The position at which the print carriage and printhead is held may be at a parking zone separate to the print zone, although any position accessible to the user may be used. At block 706, the print apparatus, and in particular the human-machine interface may prompt the user to remove the printhead from its carriage.

[0076] If, however, it is determined at block 704 that the printhead is above the threshold temperature level and is thus at an elevated temperature, the processing circuitry may take this as an indication that there is a cooling demand for the printhead because removal of the printhead will result in air ingress (due to natural cooling of the printhead). In that case the method proceeds to block 708, which comprises pumping or forcing, for a first time period, a continuous flow of print agent along a flow path from the first tank of the print agent delivery system, into the printhead through the first printhead port and out of the printhead through the second printhead port.

[0077] As mentioned above with respect to block 604 of Figure 6, the flow path may be a recirculation flow path of the print agent delivery system, i.e. a flow path that would normally be selected and used during a recirculating mode of operation to prevent settling of the print agent within the components of the print agent delivery system (as described above with respect to Figures 4 and 5). Correspondingly, the processing circuitry may cause performance of the cool-down operation by operating the print agent delivery system in the first and/or second recirculation modes of operation described above with respect to Figures 4 and 5.

[0078] At the end of the first time period, the continuous flow of print agent is terminated. Terminating the continuous flow of print agent may comprise the processing circuitry issuing appropriate control signalling to turn OFF the pump(s) driving the flow of print agent (or ceasing control signalling) and/or to close the valve(s) of the printhead and/or distirbution lines of the print agent distribution system, to prevent a flow of print agent to and from the printhead.

[0079] The example method of Figure 7 further comprises repeatedly looping between blocks 704 and 708 until the temperature of the printhead drops to a value that is equal to or less than the threshold temperature level, in response to which the method ends at block 706. However, in other examples the method ends (at block 706) after the method at block 708 has been performed a predetermined number of times, i.e. after pumping a continuous flow of print agent along the flow path for a predetermined number of successive (first) time periods.

[0080] Although the method of Figure 7 has been described above with respect to determining a first trigger event at block 702, this determination need not be performed. Instead, the example method of Figure 7 may begin at block 704, regardless of whether the first trigger event has occured prior to that.

[0081] Figure 8 shows an example of a tangible machine-readable medium 802 associated with a processor 804 (acting as any part of the processing circuitry 104 of Figure 1). The machine-readable medium 802 comprises instructions 806 which, when executed by the processor 804, cause the processor 804 to carry out tasks.

[0082] In this example, the instructions 806 comprise a cool-down module 808 to cause the processor 804 to determine if there is a cooling demand for the printhead, e.g. whether a temperature of a printhead is above a threshold temperature level; and in response to determining that there is a cooling demand for the printhead, e.g. that the temperature of the printhead is above the threshold temperature level: operating a pump, during a first time period, to force a continuous flow of print agent along a flow path from the first tank of the print agent delivery system, into the printhead through the first printhead port and out of the printhead through the second printhead port.

[0083] The machine readable medium 802 may comprise instructions to cause the processor 804 to carry out any one or any combination of the blocks of Figures 6 and 7.

[0084] In the manner described above, it can be seen that the technology described herein may actively reduce the temperature of the printhead and its contents. In that regard, preliminary testing has shown that the technology described herein can singificantly increase, and in some cases double, the rate of cooling of the printhead as compared to arrangements in which the printhead is allowed to cool by natural dissipation (in particular natural convection). For example, in a room having an ambient temperature of 26 degrees Celsius, the temperature of the internal chamber of the printhead has been found to be reduced by seven degrees Celsius when performing a cool-down operation over a period of sixty-five minutes. In contrast to this, when the printhead is allowed to cool naturally in the same conditions, the temperature of printhead is reduced by three-and-a- half degrees Celsius over the same time period.

[0085] Figure 9 shows a graphical comparison between an example of printhead weight (or “pen weight”, colloquially) over time with and without performing the cool-down operation. In particular, Figure 9 shows two plots of printhead weight vs the number of times the printhead is removed from the print carriage, for four different printheads. The first plot, on the left-hand side of Figure 9, is with respect to printheads that are removed substantially immediately after completing a print job (without performing a cool-down operation), whereas the second plot, on the right-hand side of Figure 9, is with respect to printheads that are removed from the print carriage substantially immediately after completing a cool-down operation.

[0086] As shown in the first plot of Figure 9, when the printhead is removed without performing a cool-down operation, the weight of the printhead is reduced significantly with increasing number of removals, which is indicative of significant air ingress into the printhead as the print agent within the printhead is replaced with comparatively lighter air. For example, the printhead weight is reduced to at most 63 grams after 15 removals. In contrast to this, when the printhead is subjected to a cool-down mode of operation as described above, the weight of the printhead is maintained at or above 65 grams after the same number of removals, which is indicative of less air ingress into the printhead. [0087] Although specific examples in the present disclosure have been described with respect to arrangements in which the printhead and print agent delivery system contain white-coloured print agent, this is provided as one suitable example. Indeed, the teachings herein may be implemented in any print apparatus having a print agent delivery system for any type of print agent, where the delivery system comprises a first tank and a printhead for receiving print agent from the first tank, and processing circuitry to operate a pump to force a continuous flow of print agent along a flow path from the first tank, into the printhead through a first printhead port and out of the printhead through a second printhead port.

[0088] Further, although the cool-down operation has been described above with respect to avoiding air ingestion during and after printhead removal, there may be other temperature-driven motivations for performing cool-down operation for the printhead. As such, the cool-down mode of operation is applicable more widely to any arrangement in which there is a demand or motivation (at any time) to cool a printhead having a first printhead port and a second printhead port. In such cases, the processing circuitry may determine there is a cooling demand in any suitable manner for the purpose in question. For example, a designer may predetermine and characterize, in advance of operation of the cool-down mode, a trigger event that is indicative of a temperature of the printhead being above an acceptable threshold level for the purpose in question. In those cases, the print apparatus and method will be substantially as described above, i.e. by forcing a continuous flow of print agent along a flow path from the first tank, into the printhead through a first printhead port and out of the printhead through a second printhead port, in response to determining that the predetermined trigger event has occured.

[0089] Examples in the present disclosure can be provided as methods, systems or machine readable instructions, such as any combination of software, hardware, firmware or the like. Such machine readable instructions may be included on a computer readable storage medium (including but is not limited to disc storage, CD-ROM, optical storage, etc.) having computer readable program codes therein or thereon.

[0090] The present disclosure is described with reference to flow charts and/or block diagrams of the method, devices and systems according to examples of the present disclosure. Although the flow diagrams described above show a specific order of execution, the order of execution may differ from that which is depicted. Blocks described in relation to one flow chart may be combined with those of another flow chart. It shall be understood that each flow and/or block in the flow charts and/or block diagrams, as well as combinations of the flows and/or diagrams in the flow charts and/or block diagrams can be realized by machine readable instructions.

[0091] The machine readable instructions may, for example, be executed by a general purpose computer, a special purpose computer, an embedded processor or processors of other programmable data processing devices to realize the functions described in the description and diagrams. In particular, a processor or processing apparatus may execute the machine readable instructions. Thus functional modules of the apparatus and devices may be implemented by a processor executing machine readable instructions stored in a memory, or a processor operating in accordance with instructions embedded in logic circuitry. The term ‘processor’ is to be interpreted broadly to include a CPU, processing unit, ASIC, logic unit, or programmable gate array etc. The methods and functional modules may all be performed by a single processor or divided amongst several processors.

[0092] Such machine readable instructions may also be stored in a computer readable storage that can guide the computer or other programmable data processing devices to operate in a specific mode.

[0093] Such machine readable instructions may also be loaded onto a computer or other programmable data processing devices, so that the computer or other programmable data processing devices perform a series of operations to produce computer-implemented processing, thus the instructions executed on the computer or other programmable devices realize functions specified by flow(s) in the flow charts and/or block(s) in the block diagrams.

[0094] Further, the teachings herein may be implemented in the form of a computer software product, the computer software product being stored in a storage medium and comprising a plurality of instructions for making a computer device implement the methods recited in the examples of the present disclosure.

[0095] Although the print apparatus has been described above with respect to two- dimensional printing (for example for applying drops of a print agent such as ink on to a substrate such as paper, card, plastic, metal or the like), the print apparatus may instead be for three-dimensional printing (also referred to as “additive manufacturing”). Such a print apparatus may be substantially the same as for two-dimensional manufacturing, and thus comprise the same features described above, except that the apparatus applies drops of print agents which cause selective fusing or colouring of a build material, for example a powdered build material such as a plastic powder. Such print agents include, for example, a colouring agent, for example comprising a dye or colorant, coating agents, thermal absorbing agents and the like.

[0096] Additive manufacturing techniques may generate a three-dimensional object through the solidification of a build material. The build material may be powder- based and the properties of generated objects may depend on the type of build material and the type of solidification mechanism used. In a number of examples of such techniques including sintering techniques, build material is supplied in a layer-wise manner and the solidification method includes heating the layers of build material to cause melting in selected regions. In other techniques, chemical solidification methods may be used.

[0097] Additive manufacturing systems may generate objects based on structural design data. This may involve a designer generating a three-dimensional model of an object to be generated, for example using a computer aided design (CAD) application. The model may define the solid portions of the object. To generate a three-dimensional object from the model using an additive manufacturing system, the model data can be processed to generate slices of parallel planes of the model. Each slice may define a portion of a respective layer of build material that is to be solidified or caused to coalesce by the additive manufacturing system.

[0098] While the method, apparatus and related aspects have been described with reference to certain examples, various modifications, changes, omissions, and substitutions can be made without departing from the spirit of the present disclosure. It is intended, therefore, that the method, apparatus and related aspects be limited by the scope of the following claims and their equivalents. It should be noted that the above- mentioned examples illustrate rather than limit what is described herein, and that those skilled in the art will be able to design many alternative implementations without departing from the scope of the appended claims.

[0099] The word “comprising” does not exclude the presence of elements other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the claims. [00100] The features of any dependent claim may be combined with the features of any of the independent claims or other dependent claims.