Patent Publication Number: US-2023136963-A1

Title: Recirculation devices with check valve

Description:
BACKGROUND 
     A printing system may include a pen or a printhead with a plurality of nozzles that deliver print agent onto a print medium so as to print an image. A print agent deliver system may supply print agent from a print agent tank to the printhead. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various example features will be apparent from the detailed description which follows, taken in conjunction with the accompanying drawings, wherein: 
         FIG.  1    schematically illustrates a printing system according to an example of the present disclosure. 
         FIG.  2   a    schematically illustrates the printhead support of the printing system of  FIG.  1   . 
         FIG.  2   b    shows the printhead support of  FIG.  2   a    in which one printhead has been replaced by a recirculating device. 
         FIG.  3    schematically represents a print agent delivery system for a printing system according to an example of the present disclosure. 
         FIGS.  4   a  and  4   b    respectively represents the print agent delivery system of  FIG.  3    in which the valve assembly connects the inlet to first path flow and the inlet to the second path flow. 
         FIG.  5    schematically represents a print agent delivery system for a printing system according to an example of the present disclosure. 
         FIGS.  6   a  and  6   b    respectively represents the print agent delivery system of  FIG.  5    in which the valve assembly connects the inlet to the first path flow and the inlet to the second path flow. 
         FIG.  7    is a block diagram of an example of a method to recirculate print agent in a printing system. 
         FIG.  8    represents a non-transitory machine-readable storage medium according to an example of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     A printing system comprises a printhead which may deliver print agent onto a print medium, e.g. a paper sheet. The printhead may be provided with a plurality of nozzles to deliver print agent, e.g. ink, onto the print medium so as to print an image. The printhead may be mounted on a printhead support. During printing, dots of print agent may be precisely delivered onto the print medium at a specific printhead-to-print medium spacing or distance. 
     In some examples, the printhead support may comprise a carriage. The printhead may thus be mounted on a carriage for moving across a scan axis. The printhead may travel repeatedly across a scan axis for delivering print agent onto a print medium which may advance along an advancing axis. The scan axis may be substantially perpendicular to the advancing axis. In some examples, several printheads may be mounted on a carriage. In some examples, four printheads may be mounted on a single carriage. In some examples, eight printheads may be mounted on a single carriage. 
     In some examples, the printhead support may be static. The printhead may extend along a width of a print medium. The plurality of nozzles may be distributed within the printhead along the width of the print medium. The width of the print may be substantially perpendicular to an advancing axis of the print medium. Such an arrangement may allow most of the width of the print medium to be printed simultaneously. These printer systems may be called as page-wide array (PWA) printer systems. 
       FIG.  1    schematically illustrates an example of a printing system  100  according to one example of the present disclosure. The printing system  100  comprises plurality of printheads  10  having a plurality of nozzles (not shown in  FIG.  1   ) to deliver print agent. Print agent may be delivered onto a print medium  200 . In this disclosure, delivering includes firing, ejecting, spitting or otherwise depositing print agent or ink. 
     In some examples, a heating element may cause a rapid vaporization of print agent in a print agent chamber or reservoir, increasing an internal pressure inside this print agent reservoir. This increase in pressure makes a drop of print agent exit from the print agent reservoir to the print medium through a nozzle. These printing systems may be called as thermal inkjet printing systems. 
     In some examples, a piezo electric may be used to force a drop of print agent to be delivered from a print agent chamber or reservoir onto the print medium through a nozzle. A voltage may be applied to the piezo electric, which may change its shape. This change of shape may force a drop of print agent to exit through the nozzle. These printing systems may be called as piezo electric printing systems. 
     The print medium  200  may advance or move along the advancing axis  210  following the direction represented by arrow A. The print medium may be moved by an advancer (not shown in  FIG.  1   ). An advancer may include a roller and/or a wheel. The print medium  200  may be of any shape or size to be used in the printing system. 
     The print medium is a material capable of receiving a print agent, e.g. ink. In some examples, the print medium may be a sheet of paper. In some examples, the print medium may be a sheet of cardboard, textile material or plastic material. In some examples, the print medium may comprise polyester based substrates and polyester coated substrates. 
     The printheads  10  of this figure are mounted on a printhead support  110 . The printhead support  110  of  FIG.  1    is a carriage. The carriage  110  supporting a plurality of printheads may travel across a scan axis  111  for delivering print agent onto a width of the print medium  200 . In this disclosure, a width of a print medium extends substantially perpendicular to the advancing axis  210  and a length of a print medium extends substantially parallel to the advancing axis  210 . In this example, the printhead support comprises eight printhead receivers for accommodating eight printheads. In some examples two printheads may be mounted on a carriage. In some examples, four printheads may be mounted on a carriage. 
     In some examples, the printhead support may statically span substantially the whole width of the print medium. In these examples, the printhead support may be a print bar supporting a plurality of printheads. The printheads may be used in a page-wide array (PWA) printing system. 
     The printheads  10  of  FIG.  1    comprise a plurality of nozzles. Nozzles of the plurality of nozzles may be placed in subgroups. The subgroups may comprise nozzles grouped in rows, i.e. parallel to the scan axis  111 , and in columns, i.e. parallel to advancing axis  210 . Longer rows may lead to swaths having a greater height (in the advancing axis  210 ) if all nozzles were to deliver print agent. In this disclosure, a swath refers to an area of a print medium that can be printed by a printhead in a single pass, i.e. from one lateral side of the print medium to the opposite side along the scan axis  111 . In some examples, the printhead may deliver print agent on both on the way and the way back. 
     In the example of  FIG.  1   , the plurality of printheads  10  mounted on the printhead support  110  are misaligned relative to the scan axis  111 . A printhead of the plurality of printheads may be downstream to another printhead of the plurality of printheads. An upstream printhead may deliver printhead on a print medium and a downstream printhead may deliver print agent over a previously deposited print agent. An image on a print medium may thus comprise several layers of print agent delivered by different printheads. 
     In some examples, the plurality of printheads may be mounted aligned relative to the scan axis. 
     The printing system  100  of this example comprises a print agent delivery system (not shown in  FIG.  1   ) to supply print agent from a print agent tank or print agent supply station to a printhead. In some examples, two printheads of the plurality of printheads may deliver a print agent with a specific composition, e.g. ink having a specific color. The print agent delivery system may supply this print agent with a specific composition from a print agent tank to these two printheads. Print agent with different composition may be supplied from other print agent tanks to other printheads. 
     For example, two printheads may deliver white ink to a print medium. The print agent delivery system may provide a passage from a white ink tank to these printheads. 
     In some examples, three or more printheads may supply a print agent with a specific composition or color. The print agent delivery system may supply print agent with a specific composition to a plurality of printheads. 
     In some examples, a print agent tank or cartridge may be external from the printing system. A conduit may communicate the print agent tank to the to the printing system. In some examples, a print agent tank may be integrated with the printing system. 
     The printing system  100  of  FIG.  1    comprises a controller  130  to control the operation of the printing system. In some examples, the controller may control print agent delivered by the plurality of nozzles onto the print medium. In some examples, the controller may control the print agent delivery system. 
     In  FIG.  1   , the controller  130  includes a processor  131  and a non-transitory machine-readable storage medium  132 . The non-transitory machine-readable storage medium  132  is coupled to the processor  131 . 
     The processor  131  performs operations on data. In an example, the processor is an application specific processor, for example a processor dedicated to control a print agent delivery system. The processor  131  may also be a central processing unit for controlling the operation of the printing system. 
     The non-transitory machine-readable storage medium  132  may include any electronic, magnetic, optical, or other physical storage device that stores executable instructions. The non-transitory machine-readable storage medium  132  may be, for example, Random Access Memory (RAM), an Electrically-Erasable Programmable Read-Only Memory (EEPROM), a storage drive, an optical disk, and the like. 
       FIG.  2   a    schematically illustrates the printhead support of the printing system of  FIG.  1   . The printhead support  110  comprises eight printhead receivers  120   a - h.  Printheads  10   a - h  may be fitted in the printhead receivers  120 . A shape of a printhead receiver may substantially correspond to a shape of a printhead. 
     In this example, print agent supplied to the printheads  10   a  and  10   b  is different to the print agent delivered to the printheads  10   c - 10   h.  In  FIG.  2   a   , the print agent delivery system  20  comprises a first print agent circuit for supplying print agent to the printheads  10   a  and  10   b  accommodated in the printhead receivers  120   a  and  120  and a second print agent circuit for supplying print agent to the printheads  10   c - 10   h  accommodated in the printhead receivers  120   c - 120   h.    
     In some examples, independent print agent circuits may supply print agent to each of the printheads  120   c - 120   h . In some examples, print agent circuits may supply print agent to a single printhead or to a group of printheads. For example, a first print agent circuit may supply print agent to the printheads  10   a - 10   b , a second print agent circuit to the printheads  10   c - 10   e , a third print agent circuit to the printhead  10   f  and a fourth print agent circuit to the printheads  10   h - 10   g.  Accordingly, four different print agents, e.g. color, may be delivered to the print medium. 
     The print agent delivery system may recirculate print agent through the printheads inserted into the printhead receivers. For example, intermediate tanks, solenoid valves and print agent pumps may be used for recirculating print agent. 
     However, some types of print agents may comprise pigments that may precipitate. Particles or pigments with higher density may tend to precipitate. For example, in print agents comprising metallic particles, these metallic particles may tend to precipitate. White inks may comprise titanium oxide pigments that may precipitate. 
     In some examples, printheads delivering print agent with heavy particles, e.g. white ink, may be removed from the printhead receivers. These removed printheads may be stored out of the printing systems, e.g. in an external agitation unit. Print agent contained in a print agent reservoir of these printheads may thus be stirred or agitated to prevent particles from precipitating. Printheads not in used may thus be removed from the printing system and stored in an external agitation unit. Operational life of the printhead may consequently be extended. In addition, accumulation of precipitated particles in nozzles may be reduced. 
     In  FIG.  2   a   , the printheads  10   a  and  10   b  are connected to a white ink tank through the print agent delivery system  20 . White ink may thus be contained in a print agent reservoir of the printhead (not shown in  FIG.  2   a   ). If one of these printheads is not in used, the printhead may be removed and replaced by a recirculating device. 
       FIG.  2   b    shows the printhead support of  FIG.  2   a    in which one printhead has been replaced by a recirculating device. In  FIG.  2   b    a recirculating device  30  is fitted in the printhead receiver  120   a . The recirculating device  30  comprises a first recirculation device port and a second recirculation device port in fluid communication with the first recirculation device port. Print agent may thus flow through the recirculation device. 
       FIG.  3    schematically represents a print agent delivery system for a printing system according to an example of the present disclosure. In this example, the print agent delivery system may supply print agent to a printhead and to a recirculation device connected in parallel. In some examples, the print agent delivery system may supply the print agent to a printhead or a plurality of printheads and to a recirculation device or a plurality of recirculation devices. For example, the print agent delivery system may connect in parallel a printhead and two recirculation devices. 
     The print agent delivery system  20  for a printing system of  FIG.  3    comprises an inlet  21  for inputting print agent into the print agent delivery system  20  and an outlet  22  for outputting print agent from the print agent delivery system  20 . The print agent delivery system  20  further comprises a printhead  10  to be on a printhead receiver (not shown in  FIG.  3   ) of a printhead support  110  of a printing system for delivering a print agent and a recirculation device  30  to be on a printhead receiver (not shown in  FIG.  3   ) of a printhead support  110  of a printing system. In this example, the printhead and the recirculation device are in a respective printhead receiver. 
     The printhead  10  of  FIG.  3    comprises a print agent reservoir  13 , a first printhead port  11  and a second printhead port  12  in fluid communication with the first port  11  via the print agent reservoir  13 . The recirculation device  30  of  FIG.  3    comprises a first recirculation port  31 , a second recirculation port  32  in fluid communication with the first recirculation port  31  and a check valve  33  to allow fluid flow from the second recirculation port  32  to the first recirculation port  31  and to prevent fluid flow from the first recirculation port  31  to the second recirculation port  32 . 
     The print agent delivery system of  FIG.  3    further comprises a first flow path  41  to connect the first printhead port  11  and the first recirculation port  31  when the printhead  10  and the recirculation device  30  are in respective printhead receivers, a second flow path  42  to connect the second printhead port  12  and the second recirculation port  32  when the printhead  10  and the recirculation device  30  are in respective printhead receivers and a valve assembly  50  to selectively connect the inlet  21  to the first flow path  41  to allow print agent to flow through the printhead  10  or to the second flow path  42  to allow print agent to flow through the recirculation device  30 . 
     Print agent may recirculate through the print agent delivery system  20  when a printhead  10  and a recirculation device  30  are on a respective printhead receiver of a printhead support  110 , e.g. a carriage. When the inlet  21  is connected to first flow path  41 , the check valve  33  prevents fluid flow from the first recirculation port  31  to the second recirculation port  32 , and then print agent flows through the printhead  10  from the first printhead port  11  to the second printhead port  12 . Print agent contained in the printhead reservoir  13  may thus be discharged. Accumulation of particles of print agent in the printhead and print agent clogging when a printhead and a recirculation device are mounted in a printhead support, e.g. a carriage, may thus be reduced. 
     When the inlet  21  is connected to second flow path  42 , the check valve  33  allows fluid flow from the second recirculation port  32  to the first recirculation port  31 . As the recirculation device may offer less resistance to fluid flow than the printhead, print agent is prevented from flowing through the printhead when the inlet is connected the second flow path. Print agent may thus quickly recirculate from the inlet to the outlet. For example, a print agent tank may be connected to the inlet and the print agent contained in this print agent tank may flow through the print agent delivery system. Accordingly, print agent contained in print agent tanks may recirculate. Precipitation or accumulation of particles in print agent tanks or any other parts of the print agent delivery system, e.g. titanium oxide pigments of white inks, may thus be reduced. 
     The valve assembly  50  may be separated from the printhead support  110 . Accordingly, weight of the printhead support, e.g. carriage, may be reduced. In addition, versatility of the print agent delivery system may be enhanced and independent fluid flow paths for each of the printhead receivers may be avoided. 
     In some examples, the valve assembly  50  may comprise a plurality of valves. The plurality of valves may be selectively actuated to connect the inlet to the first flow path or to the second flow path. Valves may control the flow of print agent by opening, closing, or partially obstructing a passageway. 
     In some examples, the plurality of valves may comprise a two-way valve. A two-way valve may comprise two ports. If the two-way valve is open, a fluid may flow between the ports and if the two-way valve is closed, a passage between the ports is blocked. 
     In some examples, the plurality of valves may comprise a three-way valve. A three-way valve may comprise three ports and may create a passageway between two of these three ports. For example, a three-way valve may comprise an inlet and two outlets. The inlet may be selectively connected to each one of the outlets. 
     In some examples, the plurality of valves may comprise a solenoid valve. A solenoid valve may be electromechanically operated. In some examples, a solenoid valve may comprise a two-way valve. In some examples, a solenoid valve may comprise a three-way valve. 
     The check valve  33  of this example is a one-way valve that allows fluid flow in one direction but prevents fluid flow in an opposite direction. In some examples, the check valve may comprise an umbrella check valve. This may simplify the control of print agent flow in the print agent delivery system. 
     An umbrella check valve may comprise a valve seat and an umbrella-shaped or diaphragm-shaped blocking element for sealing the valve seat. The umbrella-shaped blocking element may flatten out against the valve seat. The valve seat may thus be sealed by the umbrella-shaped blocking element. A pressure greater than a predetermined pressure may lift the umbrella-shaped blocking element to create a passageway through the valve seat. 
     In some examples, a predetermined pressure may be 1 psi. Accordingly, a pressure greater than 1 psi in the second recirculation port may force the umbrella valve check to lift and, thus, to allow fluid flow from the second recirculation port to the first recirculation port. In some examples, the predetermined pressure may be 3 psi. 
     In some examples, an umbrella-shaped blocking element may comprise an elastomer. The umbrella-shaped blocking element may deform upon a pressure greater than a predetermined pressure is experienced to allow fluid flow. In some examples, the umbrella check valve may comprise a spring to move the umbrella-shaped blocking element for allowing or blocking fluid flow. 
     In some examples, the recirculation device may comprise an interconnecting conduit connecting the first recirculation port to the second recirculation port. The check valve may be in the interconnecting conduit. 
     In some examples, the recirculation device may comprise a housing to fit into the printhead receivers of the printhead support. The printhead support, e.g. a carriage may comprise a plurality of printhead receivers. The recirculation device may thus be fitted in any of these printhead receivers. 
     In some examples, the housing may comprise a shape similar to a shape of printheads mounted on the printhead receivers. The housing may thus seal the printhead receiver where the recirculation device is mounted. Contamination of the print agent may thus be prevented. 
     In some examples, the printhead may comprise a suction pressure generator to generate a suction pressure in the print agent reservoir of the printhead. This suction pressure may cause print agent to enter into the print agent reservoir. 
     In some examples, the suction pressure generator may comprise a valve regulating print agent flowing from the first printhead port to the print agent reservoir. This valve may block the entry of the print agent into the print agent reservoir. While the valve is blocking the print the first printhead port, print agent contained in the print agent reservoir may be discharged, e.g. some drops of print agent may be delivered to a print medium or to the second printhead port. This may create a differential pressure between the print agent reservoir and the first printhead port. The valve may then be actuated to allow print agent to flow towards the print agent reservoir. A suction pressure may then be generated in the print agent reservoir to facilitate the entry of print agent into print agent reservoir. In some examples, the suction pressure generator may comprise an additional valve regulating print agent flow from the print agent reservoir to the second printhead port. 
     In some examples, the suction pressure generator may comprise an expandable bag inside the print agent reservoir. In an expanded state, the expandable bag may occupy a space inside the print agent reservoir. In a compressed state, the expandable bag may leave a space inside the print agent reservoir which may be occupied by the print agent. 
     In some examples, the print agent reservoir may comprise a first and a second chamber. The first chamber may be connected to the second chamber through an opening. Print agent may thus flow through the opening from the first chamber to the second chamber. The first printhead port may be connected to the first chamber and the second printhead port to the second chamber. 
     In some examples, the printhead may comprise a suction pressure generator and a print agent reservoir including a first and a second chamber. The suction pressure generator may allow print agent to flow from the first chamber to the second chamber. The suction pressure generator may be according to any of the examples herein disclosed. For example, the suction pressure generator may comprise a valve for regulating print agent flow from the first printhead port to the first chamber. An additional valve may be at the second printhead port to control print agent flow from the second chamber to the second printhead port. In some examples, an expandable bag according to any of the examples herein disclosed may be inside the first chamber and/or the second chamber to generate a suction pressure. 
     In some examples, the print agent delivery system of  FIG.  3    may comprise pumps, intermediate tanks and/or print agent valves. 
     In some examples, the print agent delivery system may comprise a controller. The controller may be a dedicated controller to control the operation of the print agent delivery system or a printing system controller to control the operation of the printing system. The controller may comprise a processor and a non-transitory machine-readable storage medium according to any of the examples herein disclosed. 
     The controller may detect if a printhead and/or a recirculation device is on printhead receivers of the printhead support and control the valve assembly to selectively connect the inlet to the first flow path or to the second flow path in function of the printheads and recirculation devices detected. 
     The controller may detect the printheads and the recirculation devices fitted in each of the printhead receivers of the printhead support, e.g. of the carriage. The controller may thus distinguish the recirculation devices accommodated in the printhead receivers from the printheads accommodated in the printhead receivers. Accordingly, the controller may detect a printhead support configuration. 
     In some examples, a printhead and/or a recirculation device may comprise an identifiable feature. The controller may identify the identifiable feature. A detector or a sensor may be used for identifying the identifiable feature of the printhead and of the recirculation device. 
     For example, if the printhead receivers accommodate printheads, the valve assembly may connect the inlet to the first flow path. Similarly, if the printhead receivers accommodate recirculation devices, the valve assembly may connect the inlet to the second flow path. If the controller detects that the printhead receivers accommodate a printhead and a recirculating device, the valve assembly may be activated to selectively connect the inlet to the first flow path to allow print agent to flow through the printhead or the inlet to the second flow path to allow print agent to flow through the recirculation device. 
     In some examples, the controller may activate a pressure suction generator according to any of the examples herein disclosed. 
       FIGS.  4   a  and  4   b    respectively represents the print agent delivery system of  FIG.  3    in which the valve assembly connects the inlet to the first path flow and the inlet to the second path flow. 
     In  FIG.  4   a   , the valve assembly  50  connects the inlet  21  to first flow path  41  and the outlet  22  to the second flow path  42 . In  FIG.  4   b   , the valve assembly  50  connects the inlet to the second flow path  42  and the outlet  22  to the first flow path  41 . 
     In these figures, print agent flow direction is indicated by arrows. The check valve  33  of the recirculation device  30  of these figures prevents fluid flow from the first recirculation port  31  to the second recirculation port  32 , e.g. a fluid blocking direction, but allows fluid flow from the second recirculation port  32  to the first recirculation port  31 , e.g. fluid passing direction. 
     In  FIG.  4   a   , the check valve  33  blocks print agent flow from the first recirculation port  31  to the second recirculation port  32 . Print agent may thus enter into the printhead reservoir  13  through the first printhead port  11  and exit through the second printhead port  12 . 
     Print agent may flow from the inlet  21  to the first flow path  41  through the valve assembly  50 , then print agent may flow from the first flow path  41  to the second flow path  42  through the printhead  10  and finally, from the second flow path  42  to the outlet  22  through the valve assembly  50 . 
     As print agent may flow through the printhead  10 , the print agent delivery system  20  may be in a printhead circulation mode. Print agent contained in the printhead reservoir  13  may be discharged. Print agent may be supplied from the inlet and may be forced to recirculate through the printhead. For example, the inlet may be connected to a print agent tank. 
     In some examples, a suction pressure generator device may facilitate circulation of print agent through the printhead. 
     In  FIG.  4   b   , the check valve  33  allows print agent flow from the second recirculation port  32  to the first recirculation port  31 . In this print agent direction, the recirculation device  30  offers less resistance to fluid flow than the printhead. Accordingly, print agent may be forced to circulate through the recirculation device  30 . 
     Print agent may flow from the inlet  21  to the second flow path  42  through the valve assembly  50 , then print agent may flow from the second flow path  42  to the first flow path  41  through the recirculation device  30  and finally, from the first flow path  41  to the outlet  22  through the valve assembly  50 . 
     As print agent may flow through the recirculation device  30 , the print agent delivery system  20  may be in a recirculation device circulation mode. Print agent may recirculate from the inlet to the outlet. Print agent may be supplied from the inlet and may be forced to recirculate through the recirculation device. 
       FIG.  5    schematically represents a print agent delivery system for a printing system according to an example of the present disclosure. 
     The print agent delivery system  20  for a printing system of  FIG.  5    comprises an inlet  21  for inputting print agent into the print agent delivery system  20  and an outlet  22  for outputting print agent from the print agent delivery system  20 . The print agent delivery system  20  further comprises a printhead  10  on a printhead receiver (not shown in  FIG.  5   ) of a printhead support  110  of a printing system for delivering a print agent and a recirculation device  30  on a printhead receiver (not shown in  FIG.  5   ) of a printhead support  110  of a printing system. The printhead and the recirculation device may be according to any of the examples herein disclosed. 
     In this example, a printhead and a recirculation device connected in parallel are illustrated. In some examples, the print agent delivery system may comprise additional printheads and/or recirculation devices. The printhead support may thus comprise a plurality of printhead receivers. 
     The printhead  10  of  FIG.  5    comprises a print agent reservoir  13 , a first printhead port  11  and a second printhead port  12  in fluid communication with the first port  11  via the print agent reservoir  13 . The recirculation device  30  of  FIG.  5    comprises a first recirculation port  31 , a second recirculation port  32  in fluid communication with the first recirculation port  31  and a check valve  33  to allow fluid flow from the second recirculation port  32  to the first recirculation port  31 , e.g. in a fluid passing direction and to prevent fluid flow from the first recirculation port  31  to the second recirculation port  32 , e.g. in a fluid blocking direction. 
     In this figure, a first flow path  41  connects the first printhead port  11  and the first recirculation port  31  and a second flow path  42  connects the second printhead port  12  and the second recirculation port  32 . 
     The print agent delivery system of  FIG.  5    comprises an input flow path  43  connected to the inlet  21  and output flow path  44  connected to the outlet  22 . In this figure, an input branch flow path  45  connects the input flow path  43  to the second flow path  42  and an output branch flow path  46  connects the first flow path  41  to the output flow path  44 . 
     In this figure, the valve assembly  50  comprises a first valve  51  selectively connecting the input flow path  43  to the first flow path  41  and second valve  52  selectively connecting the output flow path  44  to the second flow path  42 . Furthermore, the valve assembly  50  of  FIG.  5    comprises a third valve  53  at the input branch flow path  45  and a fourth valve  54  at the output branch flow path  46 . 
     These valves may be opened to allow a fluid flow or closed to block a fluid flow. The first to fourth valves of  FIG.  5    are two-way valves. These valves may comprise two-way solenoid valves. 
     Different circulation modes may be provided by selectively opening and closing these valves, e.g. a printhead circulation mode and/or a recirculation device circulation mode. The valve assembly may selectively connect the inlet to the first flow path or to the second flow path by controlling these four valves. 
     The print agent delivery system  20  of  FIG.  5    comprises a controller  130 . The controller  130  is coupled to the printhead support  110  and to the valve assembly  50 . The controller may be according to any of the examples herein disclosed. 
     The controller may detect if a printhead and/or a recirculation device is on printhead receivers of the printhead support. The controller may thus identify a position of the printheads and/or recirculation devices mounted in the printhead receivers of the printhead support. For example, the controller may detect an identifiable feature of a printhead and/or of a recirculation device. A printhead support configuration may thus be determined. 
     The controller  130  may control the valve assembly to selectively connect the inlet to the first flow path or to the second flow path in function of the printheads and recirculation devices detected. In some examples, the controller may activate the opening and closing of the first valve, the second valve, the third valve and the fourth. 
     The controller  130  of  FIG.  5    is a controller to control the operation of the print agent delivery system. In some examples, the controller may also control other operations of a printing system. 
     In this figure, the controller  130  comprises a processor  131  coupled to a non-transitory machine-readable storage medium  132 . The processor  131  and/or the non-transitory machine-readable storage medium  132  may be according to any of the examples herein disclosed. 
       FIGS.  6   a  and  6   b    respectively represents the print agent delivery system of  FIG.  5    in which the valve assembly connects the inlet to the first path flow and the inlet to the second path flow. As in  FIGS.  4   a  and  4   b   , in  FIGS.  6   a  and  6   b    the arrows indicate the print agent flow direction. 
     In  FIG.  6   a   , a printhead circulation mode is illustrated. In the printhead circulation mode print agent circulates through the printhead  10 . Print agent may flow along the input flow path  43  from the inlet  21 . In  FIG.  6   a   , the third valve  53  is closed and the first valve  51  is open. Therefore, print agent may pass through the first valve  51  towards the first flow path  41 . Print agent flows in a blocking direction relative to the check valve  33  so that the fluid flow from the first recirculation port  31  to the second recirculation port  32  is prevented. Consequently, print agent may enter into the printhead reservoir  13  through the first printhead port  11 . Print agent contained into the printhead reservoir may exit to the second flow path  42  through the second printhead port  12 . 
     The second valve  52  is open and the fourth valve  54  is closed. Print agent may thus pass through the second valve  52  from the second flow path  42  to output flow path  44 . 
     In  FIG.  6   b   , a recirculation device circulation mode is illustrated. In the recirculation device circulation mode print agent circulates through the recirculation device  30  as the check valve  33  allows fluid flow from the second recirculation port  32  to the first recirculation port  31 . In this figure, the first valve  51  and the second valve  52  are closed and the third valve  53  and the fourth valve  54  are open. 
     In  FIG.  6   b   , print agent may flow from the inlet  21  to the input flow path  43 . As the first valve  51  is closed and the third valve  53  is open, print agent flows from the input flow path  43  to the second fluid flow  42  through the input branch flow path  45  and the third valve  53 . Then, print agent may circulate through the recirculation device  30  and exit to the first flow path  41 . As the first valve  51  is closed and the fourth valve  54  is open, print agent may flow along the output branch flow path  46 . Then, print agent may be guided to the outlet  22  by the output flow path  44 . 
     The valves may be operated to opening and closing so as to switch between a printhead circulation mode ( FIG.  6   a   ) and a recirculation device circulation mode ( FIG.  6   b   ). 
     In some examples, three-way valves, e.g. three-way solenoid valves, may be used to switch between a printhead circulation mode and a recirculation device circulation mode. For example, four three-way valves may be used to connect or disconnect the flow paths of  FIG.  5   . 
       FIG.  7    is a block diagram of an example of a method to recirculate print agent in a printing system. The method  500  to recirculate print agent in a printing system comprises detecting  510  a recirculation device comprising a check valve to allow fluid flow in a passing direction but to prevent fluid flow in a fluid blocking direction, the fluid blocking direction being opposite to the fluid blocking direction. At block  520 , detecting a printhead connected in parallel to the detected recirculation device. The method  500  further comprises selecting  530  a printhead circulation mode to circulate print agent through the detected printhead and selecting  540  a recirculation mode to circulate print agent through the detected recirculation device. 
     Print agent may be recirculated through a print agent delivery system comprising a printhead and a recirculation device. Accumulation of pigments of print agent along the print agent delivery system may be prevented. In addition, as a printhead has been replaced by a recirculation device, this printhead may be stored in an external agitation unit to extend the operational life. 
     The method  500  may be applied to any of the examples of print agent delivery systems and/or printing systems herein disclosed. 
     In some examples, detecting  510  a recirculation device with a check valve may identify if a recirculation device is mounted in a printhead receiver of the plurality of printhead receivers of a printhead support. Accordingly, a recirculation device mounted in a printhead receiver of a carriage may be detected. This may indicate that a printhead has been replaced by the detected recirculation device. A printhead support configuration may thus be determined. 
     In some examples, detecting a recirculation device may comprise detecting a position of the recirculation device relative to a position of a printhead. Detecting a recirculation device may comprise detecting a position of the recirculation relative to the printhead receivers of the printhead support. 
     In some examples, detecting a recirculation device may comprise identifying an identifiable feature of the recirculation device. A chip may be an example of an identifiable feature. The identifiable feature may provide data about the recirculation device, e.g. time in use and/or serial number. 
     Similar to detecting a recirculation device, detecting a printhead connected in parallel to the detected recirculation device may comprise identifying a printhead mounted in a printhead receiver of a printhead support. In some examples, a position of the detected printhead may be detected. In some examples, an identifiable feature, e.g. indicating time in use and/or serial number, of the printhead may be identified. 
     In some examples, the method may comprise determining if a printhead or a recirculation device is mounted in each of the printhead receivers of a printhead support. For example, a sensor associated with each of the printhead receivers may identify an identifiable feature. 
     In a printhead circulation mode, print agent flows through the detected printhead.  FIGS.  4   a  and  6   a    are examples of a printhead circulation mode. A printhead circulation mode may be selected by operating a valve assembly, e.g. a plurality of valves, of the print agent delivery system to force print agent to flow in the blocking direction. The valve assembly may according to any of the examples herein disclosed. 
     In some examples, selecting a printhead circulation mode may comprise generating a suction pressure in a print agent reservoir of the detected printhead. A suction pressure generator according to any of the examples herein may be activated to generate such a suction pressure. 
     In some examples, selecting a printhead circulation mode may comprise activating a print agent flow path between an inlet and an outlet of a print agent delivery system to flow print agent through the detected printhead along the fluid blocking direction. For example, a valve assembly may be actuated to connect the inlet to a first printhead port and the second printhead port to the outlet.  FIGS.  4   a  and  6   a    are examples wherein the valve assembly is actuated to activate a print agent flow path between the inlet and the outlet through the detected printhead in a fluid blocking direction, i.e. in a direction that the check valve of the recirculation prevents fluid flow. 
     In a recirculation device circulation mode, print agent flows through the detected recirculation device.  FIGS.  4   b  and  6   b    are examples of a recirculation device circulation mode. A recirculation device circulation mode may be selected by operating a valve assembly, e.g. a plurality of valves, of the print agent delivery system to force print agent to flow in the passing direction. The valve assembly may be according to any of the examples herein disclosed. 
     In some examples, selecting a recirculation circulation mode may comprise activating a print agent flow path between an inlet and an outlet of a print agent delivery system to flow print agent through the detected recirculation device along the fluid passing direction. A valve assembly may be actuated to activate a print agent flow path between the inlet and a second recirculation port and between a first recirculation port and the outlet. 
     In some examples, the method to recirculate print agent in a printing system may comprise switching from a printhead circulation mode to a recirculation device circulation mode after a predetermined time. In some examples, this predetermined time may be between 30 and 120 seconds. In some examples, print agent may be recirculated along the print agent delivery system during this predetermined time. Similarly, the method may comprise switching from the recirculation device circulation mode to the printhead circulation mode after a predetermined time. 
     In some examples, the method to recirculate print agent in a printing system may comprise controlling a valve assembly to switch between the printhead circulation mode and the recirculation device circulation mode. Valves of the valve assembly may operated to activate a print agent flow path in a blocking direction so as to allow print agent to flow through the printhead (printhead circulation mode) or a print agent flow path in the passing direction so as to allow print to flow through the recirculation device (recirculation device circulation mode). 
     For example, the valves of  FIGS.  6   a  and  6   b    may be operated to switch between the printhead circulation mode and the recirculation device circulation mode. For example, in  FIG.  6   a    the first valve  51  and the second valve  52  may be actuated to close the fluid flow through them and the third valve  53  and the fourth valve  54  may be actuated to open to allow print agent flow through them. Accordingly, the valves may be actuated for switching from a printhead circulation mode ( FIG.  6   a   ) to a recirculation device circulation mode ( FIG.  6   b   ). 
     In some examples, the method may comprise selecting a printhead circulation mode if a recirculation device is not detected in a printhead receiver. In some examples, the method may comprise selecting a recirculation device circulation mode if a printhead is not detected in a printhead receiver. 
       FIG.  8    represents a non-transitory machine-readable storage medium according to an example of the present disclosure. The non-transitory machine-readable storage medium  132  is encoded with instructions which, when executed by a processor, cause the processor to determine a printhead support configuration by identifying recirculation devices having a check valve and printheads mounted on a printhead support of a printing system as represented at block  710 , select a circulation mode based on the determined printhead support configuration as represented at block  720  and control a valve assembly based on the selected circulation mode to activate a print agent flow path between an inlet and an outlet of a print agent delivery system through the printhead support as represented at block  730 . 
     The processor may be according to any of the examples herein disclosed. For example, the processor may be coupled to the non-transitory machine-readable storage medium. For example, a controller according to any of the examples herein disclosed may comprise the processor and the non-transitory machine-readable storage medium. 
     The recirculation devices and the printheads mounted on a printhead support may be identified according to any of the examples herein disclosed. Depending on the printhead support configuration a circulation mode may be selected. For example, if no recirculation devices mounted on the printhead receivers are determined, the processor may select a printing circulation mode. For example, if no printheads are mounted on the printhead receivers, the processor may select a recirculation device circulation mode. For example, if a printhead and a recirculation device are identified, the processor may firstly select a printhead circulation mode and then a recirculation device circulation mode, or vice versa. 
     A valve assembly may be controlled by the processor to activate a print agent flow path between an inlet and an outlet of a print agent delivery system through the printhead support. In some examples, the valve assembly may be controlled to activate a print agent flow path in a printhead circulation mode. In some examples, the valve assembly may be controlled to activate a print agent flow path in a recirculation device circulation mode. 
     In some examples, the non-transitory machine readable storage medium may cause the processor to select a recirculation device circulation mode if a recirculation device is identified to allow print agent flow through the recirculation device and to control the valve assembly to activate a print agent flow path between the inlet and the outlet of the print agent delivery system in a direction that the check valve allows fluid flow. The processor may thus control the valve assembly to activate a print agent flow path in a recirculation device circulation mode. The valve assembly may be controlled according to any of the examples herein disclosed. 
     In some examples, the non-transitory machine readable storage medium may cause the processor to select a printhead circulation mode if a printhead is identified, to generate a suction pressure in a print agent reservoir of the identified printhead and to control the valve assembly to activate the print agent flow path between the inlet and the outlet of a print agent delivery in a direction opposite to the direction that the check valve allows fluid flow. 
     In some examples, the processor may control the valve assembly for switching between the printhead circulation mode and the recirculation device circulation mode. 
     The instructions encoded in the non-transitory machine-readable storage medium for the processor represented at blocks  710 ,  720  and  730  may participate in recirculating a print agent along a print agent delivery system. 
     The preceding description has been presented to illustrate and describe certain examples. Different sets of examples have been described; these may be applied individually or in combination, sometimes with a synergetic effect. This description is not intended to be exhaustive or to limit these principles to any precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is to be understood that any feature described in relation to any one example may be used alone, or in combination with other features described, and may also be used in combination with any features of any other of the examples, or any combination of any.