BACKGROUND

[0001] A printing device may have a print head for depositing a printing fluid such as ink on a print medium e.g., in a two-dimensional (2D) printer or binding agents on print media in the form of build material e.g., in a three-dimensional (3D) printer. The printer may comprise a fluid distribution system for providing the printing fluid to the print head. The fluid system may also circulate the printing fluid through the print head. This may for example facilitate cooling of the print head and of the printing fluid. Circulating the printing fluid may further be used to prevent the precipitation of pigments from the printing fluid and the clog- ging of nozzles of the print head.

BRIEF DESCRIPTION OF DRAWINGS

[0002] In the following, a detailed description of various examples is given with reference to the figures. The figures show schematic illustrations of

[0003] Fig. 1: a print head with a first fluid port and a second fluid port according to an example;

[0004] Fig. 2a: a print head having a main valve and an auxiliary valve with a printing fluid being circulated in a first circulation direction in accordance with an example;

[0005] Fig. 2b: the print head of Fig. 2a with the printing fluid being circulated in a second circulation direction according to an example; [0006] Fig. 3: a printing device in accordance with an example;

[0007] Fig. 4: a printing device with two print heads according to an example;

[0008] Fig. 5: a method of circulating a printing fluid through a print head in accordance with an example; and

[0009] Fig. 6: a method of circulating a printing fluid through a print head with a selector valve according to an example. DETAILED DESCRIPTION

[0010] A print head, a printing device and a method of circulating a printing fluid are described below with reference to some examples shown in the drawings. In some instances, reference is made to ink as an example of a printing fluid. Other printing fluids instead of ink may be used, such as a conditioning fluid, pretreatment or posttreatment fluid, 3D printing fluid or binding agent and the like. Examples of inks which can be used are pigment-based inks, dye-based inks, latex inks and/or metallic inks, e.g. for printing on media such as paper, plastic, metal or textile.

[0011] If the printing fluid is circulated in a print head for cooling the print head, increasing the flow rate may provide a faster heat transfer and thus a higher cooling efficiency. If, on the other hand, the printing fluid is circulated for agitating the printing fluid such as ink to prevent the clogging of nozzles and/or to avoid precipitation of pigments, an ink flow may be generated close to the nozzles. In this case, however, increasing the flow rate may cause air ingestion through the nozzles, which may permanently damage the print head. The afore- mentioned applications may thus have contradicting requirements, which cannot be met simultaneously. Accordingly, an ink flow may be chosen that is not optimal for at least one of the applications or different print head designs may be employed depending on the application.

[0012] Fig. 1 depicts a schematic illustration of a print head 100 in accordance with an ex- ample. The print head 100 comprises a first fluid port 102 and a second fluid port 104. The first and second fluid ports 102, 104 may for example be connected to a fluid distribution system of a printing device (not shown), in which the print head 100 is employed, e.g. the printing device 300 described below with reference to Fig. 3. The fluid distribution system may for example provide a printing fluid such as ink to the print head 100 via the fluid ports 102, 104 and/or may circulate the printing fluid through the print head 100 via the fluid ports

102, 104.

[0013] The first and second fluid ports 102, 104 are in fluid communication via a main fluid path 106 through the print head 100, wherein the main fluid path 106 extends through the print head 100 from the first fluid port 102 to the second fluid port 104. In addition, the first and second fluid ports 102, 104 are also in communication via an auxiliary fluid path 108 through the print head 100, wherein the auxiliary fluid path 108 also extends through the print head too from the first fluid port 102 to the second fluid port 104. At least a portion of the auxiliary fluid path 108 is not contained in or part of the main fluid path 106 and vice versa. In some examples, a portion of the auxiliary fluid path 108 may also be part of the main fluid path 106, e.g. via common inlet/outlet tubes connected to the first and second fluid ports 102, 104.

[0014] The print head too also comprises an auxiliary valve no that is arranged in the auxiliary fluid path 108. The auxiliary valve 110 is to control a fluid flow along the auxiliary fluid path 108 independently from the main fluid path 106. The auxiliary valve 110 may for example be arranged in a portion of the auxiliary fluid path 108 that is not part of the main fluid path 106, e.g. as illustrated in Fig. 1. In some examples, the auxiliary valve 110 may be an actuated valve, e.g. a solenoid valve, a motor-actuated valve or a pneumatic valve, wherein the actuated valve may for example be controlled by a controller (not shown) of the print head 100 or of the printing device. In other examples, the auxiliary valve 110 may be a non- actuated passive valve such as a check valve or one-way valve, e.g. as described below with reference to Figs. 2a and 2b.

[0015] The auxiliary valve 110 may for example have a closed state and an open state. The auxiliary valve 110 may block a fluid flow through the auxiliaiy fluid path 108 in the closed state while allowing a fluid flow through the auxiliary fluid path 108 in the open state. The auxiliary valve 110 may not prevent or hinder a fluid flow through the main fluid path 106 in closed state as well as in the open state, i.e. the fluid flow through the main fluid path 106 may be independent of the state of the auxiliary valve 110. In some examples, the auxiliary valve 110 may have a different number of discrete or continuous states, e.g. to control a flow rate through the auxiliary fluid path 108.

[0016] Controlling the fluid flow along the auxiliary fluid path 108 using the auxiliary valve 110 may provide increased flexibility for circulating a printing fluid through the print head 100. The auxiliary fluid path 108 may for example lie closer to nozzles (not shown) of the print head 100 than the main fluid path 106. The nozzles may e.g. be arranged in a bottom wall 100A of the print head too, for example as described below with reference to Figs. 2a, 2b. In one example, the auxiliary valve 110 may be closed to block the auxiliary fluid path 108 such that the printing fluid can be circulated through the print head too along the main fluid path 106 with a high flow rate, e.g. to cool the printing fluid, without risking damage to the nozzles or air ingestion through the nozzles into the print head. In another example, the aux- iliary valve 110 may be opened such that the printing fluid can be circulated along the main and auxiliary fluid paths 106, 108 with a low flow rate, e.g. to generate fluid flow along the nozzles to prevent clogging of the nozzles. The high flow rate may for example be a factor of 2 to 20 higher than the low flow rate.

[0017] In one example, the high flow rate may be between 25 cm' ^j /min and 100 cma/min or between 30 cm3/min and 70 cm3/min and the low flow rate may be between 3 cm3/min and 25 cm3 or between 5 cm3/min and 15 cm3/min, for example. In some examples, the auxiliary valve 110 maybe closed partially, e.g. to reduce a flow rate along the auxiliary fluid path 108.

[0018] In some examples, the print head lOO may also comprise a main valve (not shown) in the main fluid path 106, wherein the main valve is to control a fluid flow along the main fluid path 106. Similar to the auxiliary valve 110, the main valve may also be an actuated valve or a passive valve. In some examples, the main valve may be to control the fluid flow through the main fluid path 106 independently from the auxiliary fluid path. The main valve may for example be arranged in a portion of the main fluid path 106 that is not part of the auxiliary fluid path 108. In other examples, the main valve may be a global valve that is to control a fluid flow through both the main fluid path 106 and the auxiliary fluid path 108. The main valve may for example be arranged in a portion that is part of the main fluid path 106 and of the auxiliary fluid path 108, e.g. in an inlet/outlet tube.

[0019] Figs. 2a and 2b show schematic illustrations of a print head 200 according to another example. Fig. 2a depicts the print head 200 in a state in which a printing fluid is circulated through the print head 200 in a first circulation direction, whereas Fig. 2b depicts the print head 200 in a state, in which the printing fluid is circulated through the print head 200 in a second circulation direction opposite to the first circulation direction.

[0020] Similar to the print head 100, the print head 200 also comprises a printing fluid chamber 202 and a first fluid port 102 and a second fluid port 104. The fluid ports 102, 104 are in fluid communication with the printing fluid chamber 202, which may e.g. comprise plastic. The printing fluid chamber 202 may e.g. provide printing fluid to nozzles plates 204 of the print head 200 for printing and/or may store printing fluid within the print head 200. In some examples, a filter 206 may be arranged within the printing fluid chamber 202 between the nozzle plates 204 and the fluid ports 102, 104. The filter 206 may for example comprise a porous material or membrane to prevent debris and particles in the printing fluid from reaching the nozzle plates 204.

[0021] The printing fluid chamber 202 is separated into a first chamber 202A and a second chamber 202B by a wall 208. The first chamber 202A may e.g. be connected to the first fluid port 102 by a first inlet/outlet tube 210A and the second chamber 202B may e.g. be connect- ed to the second fluid port 104 by a second inlet/outlet tube 210B. The wall 208 comprises a first opening 212 and a second opening 214, through which the first chamber 202A and the second chamber 202B are in fluid communication. Accordingly, the first and second fluid ports 102, 104 are in fluid communication via a main fluid path 106 which extends from the first chamber 202A through the first opening 212 to the second chamber 202B as shown in Fig. 2a and via an auxiliary fluid part 108 which extends from the first chamber 202A through the second opening 214 to the second chamber 202B as shown in Fig. 2b. In some examples, the openings 212, 214 may have different cross-sectional areas. The cross-sectional area of the first opening 212 may for example be between two and five times as large as the cross-sectional area of the second opening 214, e.g. to allow for a larger flow rate along the main fluid path 106 than along the auxiliary fluid path 108. In another example, the first and second openings 212, 214 have the same size.

[0022] The print head 200 comprises an auxiliary valve 110, which is arranged in the second opening 214 in the wall 204, i.e. in the auxiliary fluid path 108. The print head 200 may further comprise a main valve 216, which is arranged in the first opening 212 in the wall 204, i.e. in the main path 106. In the example of Figs. 2a, 2b, the auxiliary valve 110 and the main valve 204 are check valves with opposite orientations. The auxiliary valve 110 blocks a fluid flow along the auxiliary fluid path 108 from the first fluid port 102 to the second fluid port 104 while allowing a fluid flow along the auxiliary fluid path 108 from the second fluid port 104 to the first fluid port 102. The main valve 204 blocks a fluid flow along the main fluid path 106 from the second fluid port 104 to the first fluid port 102 while allowing a fluid flow along the main fluid path 106 from the first fluid port 102 to the second fluid port 104.

[0023] In the example of Figs. 2a, 2b, the auxiliary valve no and the main valve 204 are umbrella valves comprising a T-shaped or umbrella-shaped stopper. The stopper may e.g. comprise a flexible material such as an elastomer material or rubber. The stoppers of the auxiliary valve 110 and of the main valve 216 have opposite orientations. If the printing fluid is circulated through the print head 200 in a first circulation direction from the first fluid port 102 the second fluid port 104, as shown in Fig. 2a, the stopper of the auxiliary valve no is pressed against the wall 208, thereby closing the second opening 214 and blocking a fluid flow along the auxiliary fluid path 108. The stopper of the main valve 216 is pressed away from the wall 208, thereby clearing the first opening 212 and allowing a fluid flow along the main fluid path

106 as illustrated in Fig. 2a. If the printing fluid is circulated through the print head 200 in a second, opposite circulation direction from the second fluid port 104 to the first fluid port 102, as shown in Fig. 2b, the stopper of the main valve 216 is pressed against the wall 208, thereby closing the first opening 212 and blocking the main fluid path 106. The stopper of the auxiliary valve 110 is pressed away from the wall 208, thereby clearing the second opening 214 and allowing a fluid flow along the auxiliary fluid path 108.

[0024] By using check valves with opposite orientations, one of the flow paths 106, 108 may be selected by setting the circulation direction of the printing fluid accordingly. The respective check valve is opened automatically due to the flow pressure generated by circulation of the printing fluid, thereby allowing the printing fluid to flow along the selected flow path. The other check valve is closed automatically by the flow pressure, thereby preventing the print- ing fluid from flowing along the other flow path. A flow along the other flow path may be generated by reversing the circulation direction. Accordingly, the flow path may be selected without relying on actuated components such as solenoid valves within the print head 200. This may increase the robustness and reliability of the print head 200. [0025] In some examples, the first and second openings 214 may be arranged in the wall 208 such that the auxiliary fluid path 108 passes through the filter 206, whereas the main fluid path 106 does not pass through the filter 206. The first opening may for example be arranged on a side of the filter 206 opposite to the nozzle plates 204 and the second opening may be arranged between the filter 206 and the nozzle plates 204, as illustrated in Figs. 2a, 2b. [0026] In some examples, the first opening 212 may comprise a plurality of openings, e.g. to provide a larger cross-sectional area for the main fluid path 106 than for the auxiliary fluid path 108. Accordingly, the main valve 216 may comprise a plurality of valves, e.g. one valve arranged in each of the plurality of openings.

[0027] The print head 200 may further comprises a selector valve that is to control circula- tion of the printing fluid through the print head 200, e.g. by controlling the fluid flow along the main and auxiliary fluid paths 106, 108. The selector valve may for example be arranged in one of the inlet/outlet tubes 210A, 210B to block or allow a fluid flow through the print head 200. The selector valve may e.g. be an actuated bidirectional valve or a switchable check valve, i.e. a check valve that may be forced open or “switched off”. In normal operation, the check valve may e.g. allow a fluid flow into the printing fluid chamber 202 while blocking a fluid flow out of the printing fluid chamber 202.

[0028] In the example of Figs. 2a, 2b, the selector valve comprises a first switchable check valve with a stopper 218A that is to block an outlet of the first inlet/outlet tube 210A into the printing fluid chamber 202 if printing fluid is extracted through the first inlet/outlet tube 210A and a second switchable check valve with a stopper 218B that is to block an outlet of the second inlet/outlet tube 210B into the printing fluid chamber 202 if printing fluid is extracted through the second inlet/outlet tube 210 B. The first and second check valves may be forced open by removing the respective stopper 218A, 218B from the corresponding outlet, e.g. as described below. [0029] In some examples, the selector valve may be coupled to a pressure regulator of the print head 200 such that the selector valve can be actuated or switched using the pressure regulator. The pressure regulator may for example comprise an inflatable bag in the printing fluid chamber 202, which may be inflated or deflated to adjust a pressure inside the printing fluid chamber 202. In the example of Figs. 2a, 2b, the pressure regulator comprises a first inflatable bag 220A in the first chamber 202 and a second inflatable bag 220B in the second chamber 202B. The first inflatable bag 220A and the second inflatable bag 220B are coupled to the stoppers 218A and 218B, respectively, such that the stoppers 218 A, 218B may be moved in front of or removed from the respective opening by inflating or deflating the corresponding inflatable bag 220A, 220B. [0030] In this way, the check valves of the selector valve may be opened to allow for a circulation of the printing fluid through the print head 200. If the printing fluid is for example to be circulated along the first circulation direction, the stopper 218B may be removed from the outlet of the second inlet/outlet tube 210B e.g. by deflating the inflatable bag 220B such that the printing fluid can flow from the first fluid port 102 to the second fluid port 104 by pushing the stopper 218A away from the opening of the first inlet/outlet tube 210A as illustrated in Fig. 2a. In Fig. 2b, the stopper 218A is removed from the outlet of the first inlet/outlet tube 210A by deflating the inflatable bag 220A such that the printing fluid can flow from the second fluid port 104 to the first fluid port 102, thereby pushing the stopper 218B away from the opening of the second inlet/outlet tube 210B. [0031] Fig. 3 shows a schematic illustration of a printing device 300 in accordance with an example. The printing device 300 may for example be an ink-jet printer, e.g. a large-format printer and/or a latex printer. The printing device 300 comprises a print head 302 for depositing a printing fluid such as ink on a print medium (not shown). The print head 302 may e.g. be movable across the print medium along a print head path 304. In some examples, the printing device 300 may be a 3D printer and the print head 302 may be to deposit a printing fluid such as a binding agent onto a bed of build material.

[0032] The print head 302 may be similar to one of the print heads 100 and 200. In particular, the print head 302 has a first fluid port 102 and a second fluid port 104. The first and second fluid ports 102, 104 are connected by a first fluid path 108 through the print head 302 and by a second fluid path 106 through the print head 302. The print head 302 further comprises a first internal valve 110 that is to control the first fluid path 108. The first internal valve 110 is arranged inside the print head 302 along the first fluid path 108. The first internal valve 110 may in particular be to control the first fluid path 108 independently from the second fluid path 108, e.g. as described above. [0033] The printing device 300 further comprises a fluid distribution system 306 for circulating a printing fluid through the print head 302. The fluid distribution system 306 may for example be connected to the first and second fluid ports 102, 104 by a pair of tubes to provide printing fluid to the print head 302 and/or to extract printing fluid from the print head 302. For this, the fluid distribution system 306 may comprise a pump and a reservoir, e.g. as de- tailed below for the printing device 400. [0034] The printing device 300 also comprises a controller that is to control a state of the first valve 110 between an open state and a closed state. The controller 308 may be implemented in hardware, software or a combination thereof. The controller 308 may for example comprise a microcontroller with a processor and a storage medium storing instructions to be executed by the processor to provide the functionality described in the following. The controller 308 may in particular be to execute one of the methods 500 and 600 described below at least in part. In some examples, the controller 308 may be integrated into a main controller of the printing device 300, which for example also executes print jobs, e.g. by controlling movement of the print head 302 along the print head path 304 and/or by controlling the ejection of the printing fluid from the print head 302.

[0035] In the open state, the first valve 110 may allow a fluid flow along the first fluid path 108. In the closed state, the first valve 110 may block a fluid flow along the first fluid path 108. The first internal valve 110 may for example be an actuated valve and the controller 308 may control the state of the first valve 110 by controlling an actuator of the first valve 110. In other examples, the first internal valve 110 may be a passive valve such as a check valve. The controller may e.g. control the state of the first valve 110 by controlling the fluid distribution system 306 to set a circulation direction of the printing fluid through the print head 302, e.g. as detailed below for the method 500.

[0036] Fig. 4 illustrates a printing device 400 according to another example. The printing device 400 comprises two print heads 302A and 302B, which may for example be mounted in a print head carriage 401. The print head carriage 401 may be movable along a print head path 304 to distribute printing fluid on a print medium (not shown). In some examples, the print heads 302A, 302B may be used to print with different printing fluids, e.g. inks of two different colors. In some examples, the printing device 400 may comprise a different number of print heads, e.g. more than two print heads.

[0037] Each of the print heads 302A, 302B comprises first and second fluid ports 102, 104, which are connected by first and second fluid paths 108, 106 through the respective print head 302A, 302B. In some examples, the print heads 302A, 302B may have an identical design or structure, e.g. as depicted in Fig. 4 and described in the following. A first internal valve 110 is arranged in the first fluid path 108 to control a fluid flow along the first fluid path 108. A second internal valve 214 is arranged in the second fluid path 106 to control a fluid flow along the second fluid path 106. In some examples, one or both of the valves 110, 214 may be to control the fluid flow along the respective fluid path independently from the other fluid path. The valves 110, 214 may for example be check valves with opposite orientations as shown in Fig. 4. The valves may e.g. be similar to the valves of the print head 200 in Figs. 2a, 2b. The first check valve may e.g. block a fluid flow along the first fluid path 108 if the print- ing fluid circulates from the first fluid port 102 to the second fluid port 104 Accordingly, the second check valve 214 may e.g. block a fluid flow along the second fluid path 106 if the printing fluid circulates in the opposite direction from the second fluid port 104 to the first fluid port 102. [0038] Similar to the printing device 300, the printing device 400 also comprises a fluid distribution system 306 for circulating a printing fluid through the print heads 302A, 302B. The fluid distribution system 306 may comprise a reservoir 402 for storing the printing fluid, e.g. an ink tank. In some examples, the fluid distribution system 306 may comprise a plurality of reservoirs. The fluid distribution system 306 may in particular comprise an intermediate res- ervoir 402A for providing printing fluid to the print heads 302A, 302B and a main reservoir 402B for receiving printing fluid from the print heads 302A, 302B and/or for refilling the intermediate reservoir 402A. Additionally or alternatively, the fluid distribution system 306 may comprise reservoirs for different types of ink (not shown), e.g. a first reservoir with a first ink connected to the print head 302A and a second reservoir with a second ink connect- ed to the print head 302B.

[0039] The fluid distribution system 306 may further comprise a pump 404 that is to pump printing fluid into and/or out of the reservoir 402, e.g. to provide printing fluid to the print heads 302A, 302B for printing. The pump 404 may further be to circulate the printing fluid through the print heads 302A, 302B, e.g. from the intermediate reservoir 402A to the main reservoir 402B. The pump 404 may for example be connected to an outlet of the reservoir 402 by a tube, e.g. to an outlet of the intermediate reservoir 402A as shown in Fig. 4. In other words, the pump 404 may be arranged upstream of the print heads 302A, 302B. In other examples, the pump 404 may be connected to an inlet of the reservoir 402 by a tube, e.g. to an inlet of the main reservoir 402B. In other words, the pump 404 may be arranged down- stream of the print heads 302A, 302B.

[0040] The pump 404 may be a unidirectional pump or a bidirectional pump. The pump 404 may for example be to generate a flow of the printing fluid with a flow rate between o cm ³ /min and 150 cm ³ /min. In some examples, the fluid distribution system 306 may comprise multiple pumps, e.g. an additional pump for transferring printing fluid from the main reservoir 402B to the intermediate reservoir 402A and/or an additional pump for transferring printing fluid from the print heads 302A, 302B to the main reservoir 402B.

[0041] The fluid distribution system 306 may also comprise a plurality of valves 406A, 406B, 406C, and 406D, which may e.g. be actuated valves such as solenoid valves. The valves 406A-406D may for example be arranged such that printing fluid may be circulated through the print heads 302A, 302B in opposite circulation directions with a unidirectional pump 404, e.g. in a configuration as shown in Fig. 4. A first valve 406A may be arranged in a tube connecting the pump 404 and the first fluid ports 102 of the print heads 302A, 302B. A second valve 406B may be arranged in a tube connecting the pump 404 and the second fluid ports 104 of the print heads 302A, 302B. A third valve 406C may be arranged in a tube con- necting the second fluid ports 104 and an inlet of the reservoir 402. A fourth valve 406D may be arranged in a tube connecting the first fluid ports 102 and the inlet of the reservoir 402. In this way, the printing fluid may be circulated through the print heads 302 A, 302B in opposite circulation directions with a unidirectional pump 404, namely in a first circulation direction by opening the valves 406 A and 406C and in a second, opposite circulation direction by opening the valves 406B and 406D. Furthermore, printing fluid may be provided to the print heads 302A, 302B through both fluid ports 102, 104, e.g. for printing, by opening the valves 406A, 406B and closing the valves 406C, 406D.

[0042] In the example of Fig. 4, the fluid distribution 306 is to provide printing fluid from the same reservoir to both of the print heads 302A, 302B. In other examples, the fluid distri- bution 306 may be modified so as to provide a first printing fluid from a first reservoir to the print head 302A and a second printing fluid from a second reservoir to the print head 302B. For this, the fluid distribution system 306 may e.g. comprise two copies of the structure shown in Fig. 4, one of which is in fluid communication with the print head 302A and the other one of which is in fluid communication with the print head 302B. [0043] The printing device 400 also comprises a controller 308, which may e.g. be similar to the controller of the printing device 300 described above. The controller 308 is to control a state of the first valve no between an open state and a closed state as well as a state of the second valve 214 between an open state and a closed state. The controller 308 may in particular be to control the states of the first and second valves no, 214 by controlling the valves 406A-406D to set a circulation direction of the printing fluid through the print heads 302A,

302B, e.g. by opening and/or closing the valves 406A-406D as described above. The controller 308 may e.g. control the valves 406A-406D by generating corresponding analog or digital control signals. The controller 308 may further be to circulate printing fluid through the print heads 302A, 302B by controlling the pump 404. [0044] In some examples, the controller 308 may also be to adjust flow parameters such as a flow rate or ink pressure by controlling the pump 404. The controller 308 may for example set the pump 404 to a high flow rate if the first valve 110 is in the closed state, i.e. for the first circulation direction, and to a low flow rate if the first valve 110 is in the open state, i.e. for the second circulation direction. The high flow rate may for example be a factor of 2 to 20 higher than the low flow rate. In one example, the high flow rate may be between 30 cm3/min and 70 cms/min per print head and the low flow rate between 5 cm3/min and 15 cm3/min per print head.

[0045] Each of the print heads 302A, 302B may further comprise a selector valve 218 that is to control circulation of the printing fluid through the respective print head 302A, 302B, e.g. as described above with reference to Figs. 2a, 2b. The controller 308 may be to select one or both of the print heads 302A, 302B by controlling the respective selector valve 218. The controller 308 may e.g. close the selector valve 218 of one of the print heads 302A, 302B to prevent the printing fluid from circulating through the respective print head and may open the selector valve 218 of the other one of the print heads 302A, 302B to allow the printing fluid to circulate through the respective print head. In some examples, the controller 308 may control the selector valves 218 by controlling a pressure regulator of the respective print head, e.g. by controlling an air pump to inflate or deflate an inflatable bag of the pressure regulator.

[0046] Fig. 5 shows a flowchart of a method 500 of circulating a printing fluid through a print head in accordance with an example. The print head comprises a first fluid port and a second fluid port that are connected by a first fluid path and by a second fluid path. In the following, the method 500 is described using the printing device 300 and the print head 200 as an example. This is, however, not intended to be limiting in any way and the method 500 may be executed with other print heads and/or printing devices, e.g. the print head too and/or the printing device 400. [0047] The method 500 may for example be executed as part of a maintenance or servicing routine of the printing device 300 and/or during execution of a print job by the printing device 300. The method 500 may be executed in regular intervals, e.g. every 5 min to 60 min, and/or in response to a triggering event, e.g. a triggering event based on sensor data such as a temperature of the printing fluid or of the print head 200 or a triggering event associated with a user action, e.g. the replacement of a print head.

[0048] The method 500 comprises, at block 502, receiving a path selection identifying a path along which the printing fluid is to be circulated. The path selection may for example have been generated by a main controller of the printing device 300 and may be received by the controller 308. In other examples, the path selection may have been generated by a first module of the controller 308 and may be received by a second module of the controller 308. The path selection may for example indicate that the printing fluid is to be circulated along the auxiliary or first fluid path 108, but not along the main or second fluid path 106, e.g. to prevent clogging of the nozzles in the nozzle plates 204. In another example, the path selection may indicate that the printing fluid is to be circulated along the main fluid path 106, but not along the auxiliary fluid path 108, e.g. to cool the printing fluid and/or the print head 200 or to mix the printing fluid to prevent precipitation of pigments. In some examples, the path selection may also indicate that the printing fluid is to be circulated along both the main and auxiliary fluid paths 106, 108.

[0049] In some examples, the method 500 may be executed more frequently for one fluid path, e.g. the auxiliary fluid path 108, than for another fluid path, e.g. the main fluid path 106. By generating corresponding path selections, the method 500 may for example be executed for the auxiliary fluid path 108 at a first repetition rate and for the main fluid path 106 at a second repetition rate different from the first repetition rate. The first repetition rate may e.g. be every 5 min to 15 min to prevent the clogging of nozzles in the nozzles plates 204, whereas the second repetition rate may e.g. be every 4 hours to 8 hours to mix the ink for avoiding precipitation. In other examples, the main fluid path 106 may be selected to cool the print head 200 from an operating temperature, which may e.g. be between 40°C and 70°C, to room temperature, i.e. 25°C, if a user wants to access the printing device 300, e.g. to replace the print head 200. [0050] The method 500 further comprises, at block 504, determining a circulation direction of the printing fluid between the first and second fluid ports 102, 104 based on the path selection. The circulation direction determines which fluid port is used as an inlet for providing printing fluid to the print head 200 and which fluid port is used as an outlet for extracting printing fluid from the print head 200. In a first circulation direction, the printing fluid is circulated from the first fluid port 102 to the second fluid port 104 as shown in Fig. 2a. In a second circulation direction, the printing fluid is circulated from the second fluid port 104 to the first fluid port 102 as shown in Fig. 2b. In the following, the first circulation direction may also be referred to as the clockwise or forward circulation direction. The second circulation direction may also be referred to as the counter-clockwise or reverse circulation direction, respectively.

[0051] The auxiliary valve 110 may for example be a check valve, e.g. as shown in Figs. 2a, 2b. If the path selection specifies that the printing fluid is to be circulated along the auxiliaiy fluid path 108, the controller 308 may select the counter-clockwise circulation direction such that the check valve 110 is opened by the printing fluid flow. If the path selection specifies that the printing fluid is not to be circulated along the auxiliary fluid path 108, the controller 308 may select the clockwise circulation direction such that the check valve 110 is closed by the printing fluid flow.

[0052] In some examples, the print head 200 may also comprise the main valve 214, which may e.g. be a check valve that is oriented opposite to the auxiliary valve 110 as shown in Figs. 2a, 2b. The controller 308 may select the clockwise circulation direction if the path selection specifies that the printing fluid is to be circulated along the main fluid path 106 and the counter-clockwise circulation direction if the path selection specifies that the printing fluid is not to be circulated along the main fluid path 106.

[0053] At block 506, the printing fluid is circulated through the print head 200 in the circu- lation direction that was determined in block 504. For this, the controller 308 may e.g. set the pumping direction of a bidirectional pump of the fluid distribution system 306. In other examples, the controller 308 may open or close a valve in the fluid distribution system 306 to set the circulation direction, e.g. as described below for the method 600. Subsequently, the controller 308 may switch the pump on to circulate the printing fluid through the print head 200.

[0054] By setting the circulation direction according to the path selection, the flow pressure generated by the pump automatically opens the check valve corresponding to the flow path identified by the path selection, whereas the other check valve closes automatically due to the flow pressure. In this way, a flow along the respective fluid path may be generated selectively by choosing the circulation direction accordingly, i.e. without using actuated components within the print head 200. The other one of the flow paths 106, 108 may be selected by reversing the circulation direction.

[0055] The printing fluid may for example be circulated for a predetermined circulation time, e.g. between 5 s and 30 s. In some examples, the controller 308 may determine the cir- dilation time based on the path selection. The circulation time may e.g. be longer if the printing fluid is circulated along the main fluid path 106 than if the printing fluid is circulated along the auxiliary fluid path 108. The printing fluid may for example be circulated with a predetermined flow rate, e.g. between 5 cm3/min and 70 cma/min. In some examples, the controller 308 may determine the flow rate based on the path selection. The flow rate may e.g. be higher if the printing fluid is circulated along the main fluid path 106 than if the printing fluid is circulated along the auxiliary fluid path 108. The flow rate may e.g. be between 30 cm ³ /min and 70 cm ³ /min for the main fluid path 106 and between 5 cm ³ /min and 15 cm ³ /min for the auxiliary fluid path 108.

[0056] In the example of Figs. 2a, 2b, the printing fluid circulates along the main fluid path 106, but not along the auxiliary fluid path 108 if the determined circulation direction is the forward circulation direction. On the other hand, if the determined circulation direction is the reverse circulation direction, the printing fluid circulates along the auxiliary fluid path 108, but not along the main fluid path 106.

[0057] Fig. 6 shows a flowchart of a method 600 of circulating a printing fluid through a print head according to another example. In the following, the method 500 is described using the printing device 400 with the print heads 302A, 302B as an example. This is, however, not intended to be limiting in any way and the method 500 may be executed with other print heads and/or printing devices, e.g. the print head 100 or 200 and/or the printing device 300. Furthermore, the method 600 is not limited to the order of execution indicated by the flow chart of Fig. 6. As far as technically feasible, the method 600 may be executed in an arbitrary order and parts thereof may also be executed simultaneously at least in part.

[0058] Similar to the method 500, the method 600 also comprises receiving a path selection identifying a path along which the printing fluid is to be circulated at block 602. In some examples, e.g. for printing devices with multiples print heads such as the printing device 400, block 602 may also comprise receiving a print head selection identifying a print head through which the printing fluid is to be circulated. The print head selection may for example specify that the printing fluid is to be circulated through the print head 302A, but not through the print head 302B. In other examples, the print head selection may specify that the printing fluid is to be circulated through both print heads 302A, 302B. In some examples, the print head selection may specify that printing fluid is to be circulated through all print heads used with a certain type of printing fluid, e.g. ink of a certain color. The print head selection may for example have been generated by a module of the controller 308 or by a main controller of the printing device 400.

[0059] In some examples, the method 600 may be executed more frequently for one print head, e.g. the print head 302A, than for another print head, e.g. the print head 302B. The print head 302A may for example be used for depositing a printing fluid with heavier pigments such as white ink, whereas the print head 302B may be used for depositing a printing fluid with lighter pigments. Executing the method 600 may prevent precipitation of the pigments and/or clogging of the nozzles of the print head 302A. [0060] In block 604, the circulation direction of the printing fluid is determined based on the path selection, e.g. as described above for block 504.

[0061] The method 600 may further comprise, at block 606, determining a target value for a flow parameter for circulating the printing fluid. The flow parameter may for example be a flow rate or an ink pressure. The target value may be determined based on the path selection and/or the print head selection received in block 602 and/or based on the circulation direction determined in block 604. In some examples, a first target value or first set of target values may be used if the determined circulation direction is the forward circulation direction, e.g. if the printing fluid is to circulate along the main fluid path 106, and a second target value or second set of target values may be used if the determined circulation direction is the re- verse circulation direction, e.g. if the printing fluid is to circulate along the auxiliary fluid path 108. For example, the flow rate may be higher if the printing fluid is to circulate along the main fluid path 106 than if the printing fluid is to circulate along the auxiliary fluid path 108. In one example, a flow rate between 30 cm ³ /min and 70 cm ³ /min may be used for the main fluid path 106, e.g. to provide efficient cooling, and a flow rate between 5 cm ³ /min and 15 cm ³ /min may be used for the auxiliary fluid path 108, e.g. to prevent air ingestion through the nozzles of the nozzle plates 204 and/or to avoid clogging or damaging the filter 206.

[0062] The method 600 may also comprise, at block 608, opening an internal selector valve of a print head that is to control circulation of the printing fluid through the respective print head, e.g. one or both of the selector valves 218 of the print heads 302A, 302B. The selector valve may in particular be opened based on the print head selection received in block 602. The print head selection may for example specify that the printing fluid is to be circulated through the print head 302A, but not through the print head 302B. Accordingly, at block 608, the selector valve 218 of the print head 302A may be opened, whereas the selector valve 218 of the print head 302B may remain closed such that the printing fluid can circulate through the print head 302 A, but not through the print head 302B. Opening the internal selector valve may comprise controlling a pressure regulator of the print head using the controller 308, e.g. to inflate or deflate an inflatable bags to open the selector valve.

[0063] In some examples, the selector valve 218 of the print head 302A may comprise a first check valve associated with the first fluid port 102 and a second check valve associated with the second fluid port. As in the example of Figs. 2a, 2b, the first check valve may e.g. be arranged at an outlet of a first inlet/outlet tube 210A connected to the first fluid port 102 and the second check valve may be arranged at an outlet of a second inlet/outlet tube 210B connected to the second fluid port 104. In other examples, the first and second check valves may be arranged in the first and second inlet/outlet tube 210A, 210B, respectively. The first check valve may for example close if the printing fluid circulates in the second circulation direction, e.g. since a stopper 218A of the first check valve is pressed against the outlet of the second inlet/outlet tube 210A as in the example of Figs. 2a, 2b. The second check valve may close if the printing fluid circulates in the first circulation direction, e.g. since a stopper 218B of the second check valve is pressed against the outlet of the second inlet/outlet tube 210B as in the example of Figs. 2a, 2b.

[0064] Opening the selector valve 218 may comprise opening the first check valve 218A or the second check valve 218B to allow circulation of the printing fluid in the determined circulation direction. In other words, the respective check valve may be forced open or deactivated to prevent the check valve from closing, which would normally prevent circulation of the printing fluid through the print head 302A. Accordingly, if the determined circulation direction is the first circulation direction, the selector valve associated with the second fluid port 102 may be opened, e.g. by deflating the inflatable back 220B to remove the stopper 218B from the outlet of the second inlet/outlet tube 210B. If the determined circulation direction is the second circulation direction, the selector valve associated with the first fluid port 102 may be opened, e.g. by deflating the inflatable bag 220A to remove the stopper 218B from the out- let of the first tube 210A.

[0065] In block 610, the printing fluid is circulated in the determined circulation direction through the print heads identified by the print head selection. For this, the controller 308 may control the fluid distribution system 306 to set the circulation direction of the printing fluid and to start the circulation of the printing fluid. If the pump 404 of the fluid distribution system 306 is a bidirectional pump, the controller 308 may e.g. set the pumping direction of the pump 404 to the determined circulation direction and may switch the pump 404 on. If the pump 404 is a unidirectional pump, the controller 308 may adjust the valves 406A-406D based on the determined circulation direction and may switch the pump 404 on. The controller 308 may e.g. open the valves 406A , 406C and close the valves 406B, 406D if the circula- tion direction is the forward circulation direction. If the circulation direction is the reverse circulation direction, the controller may e.g. open the valves 406B, 406 D and close the valves 406A, 406C. Block 610 may also comprise controlling a flow parameter such as the flow rate of the printing fluid based on the path selection, e.g. by adjusting the flow parameter to the target value determined in block 606. For this, the controller 308 may e.g. adjust a pumping speed of the pump 404 and/or may adjust a valve of the fluid distribution system 306, e.g. by partially closing or opening the valve 406A or 406B.

[0066] The description is not intended to be exhaustive or limiting to any of the examples described above. The print head, the printing device and the method of circulating a printing fluid through a print head disclosed herein can be implemented in various ways and with many modifications without altering the underlying basic properties.