Patent Publication Number: US-10315431-B2

Title: Printing fluid circulation

Description:
BACKGROUND 
     Some printing systems have a reservoir to store printing fluid, such as ink, and a supply system to supply the printing fluid from the reservoir to a printhead, to enable the printhead to apply the printing fluid to a substrate to form an image on the substrate during a print job. The printing fluid may comprise pigment, which lends color to the printed image. The pigment may comprise particles, such as solid and/or opaque particles, that are suspended in the printing fluid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various features of the present disclosure will be apparent from the detailed description which follows, taken in conjunction with the accompanying drawings, which together illustrate, by way of example, features of the present disclosure, and wherein: 
         FIG. 1  is a schematic diagram showing a printing system, which comprises a printing fluid circulation system according to an example; 
         FIG. 2  is a schematic diagram showing a printing fluid circulation system according to another example; 
         FIG. 3  is a flow diagram showing a method of operating a printing system according to an example; and 
         FIG. 4  is a flow diagram showing a method of operating a printing system according to another example. 
     
    
    
     DETAILED DESCRIPTION 
     Some inks and other printing fluids comprise pigment or other particles, which can settle and sometimes agglomerate in a flow path or device when the fluid is at rest. Such a flow path or device can comprise, for example, a tube, a pump, a valve, a tank, or a printhead. The path or device may be part of a supply system that is to supply the printing fluid to a printhead during a print job. Over time, such settling or agglomeration can lead to partial or full blocking of the flow path or device. For example, the settled pigment or particles may make the printing fluid more viscous or form a clot. This can result in the flow of the printing fluid during a subsequent print job being hindered or prevented. 
     Certain examples as described herein provide a printing fluid circulation system for a printing system, or a method of operating a printing system. Certain examples as described herein enable printing fluid such as ink to be put into motion, such as when a print job is not being performed. This is achieved by causing the printing fluid to move from one area to another, such as from one reservoir to store printing fluid to another reservoir to store printing fluid. In some examples, this movement is via a supply system that is to supply the printing fluid to a printhead during a print job. In some examples, the supply system comprises, and the movement is through, tubes and/or pumps and/or valves and/or tanks. In some examples, the movement is through the printhead. This circulation of the printing fluid can better enable pigment or other particles in the printing fluid to remain suspended in the fluid, such as between print jobs. In some examples, this reduces the risk of the pigment or other particles settling or agglomerating. In some examples, the circulation of printing fluid is a movement of the printing fluid from one volume to another without the printing fluid moving around a complete circuit. That is, the circulation can be an end-to-end movement between the two volumes, such as between reservoirs. 
     Some examples avoid the need to mechanically vibrate components through which the printing fluid flows in use, which otherwise could be noisy and complex to implement. Some examples avoid the need to purge a supply system or printhead, which otherwise could result in the onset of a subsequent print job being delayed while the printing fluid is reintroduced to the supply system and/or printhead. Some examples avoid the need to flush a supply system or printhead with a cleaning agent other than printing fluid, which could result in printing fluid subsequently becoming contaminated with the cleaning agent when the printing fluid is reintroduced to the supply system and/or printhead. 
       FIG. 1  shows schematically a printing system  1 . The printing system  1  comprises a printing fluid circulation system  10  according to an example, and a printhead  20 . The printing fluid circulation system  10  of this example comprises a supply system  100 , a first reservoir  210 , and a second reservoir  220 . The printhead  20  has a plurality of nozzles to apply printing fluid to a substrate, such as paper. 
     The first and second reservoirs  210 ,  220  may take any form suitable to store printing fluid. For example, the first reservoir  210  may be a tank or other receptacle. The first reservoir  210  may be a closed reservoir or may be open to the atmosphere. The second reservoir  220  may be a tank or other receptacle. The second reservoir  220  may be a closed reservoir or may be open to the atmosphere. 
     The printing fluid circulation system  10  of this example also comprises an outlet  310  to supply printing fluid to the printhead  20 , and an inlet  320  to receive printing fluid from the printhead  30 . In this example, the outlet  310  is fluidly connected to a fluid inlet  21  of the printhead  20 , and the inlet  320  is fluidly connected to a fluid outlet  22  of the printhead  20 . Each of the fluid inlet  21  and fluid outlet  22  may be a needle. In this example, a printhead regulator  23  is located in the printhead  20  between the fluid inlet  21  and the fluid outlet  22 . The printhead regulator  23  is described in more detail below. In some examples, the printhead  20  is omitted. Nevertheless, in those examples, the outlet  310  of the supply system  100  is to supply printing fluid to a fluid inlet of a printhead  20 , and the inlet  320  of the supply system  100  is to receive printing fluid from a fluid outlet  22  of the printhead  20 . 
     In this example, printing fluid such as liquid ink may be supplied to the outlet  310  from either one of the first and second reservoirs  210 ,  220 . Which one of the first and second reservoirs  210 ,  220  is to supply printing fluid to the outlet  310  depends on a mode in which the supply system  100  is operating. Moreover, in this example, printing fluid such as liquid ink may be supplied to either one of the first and second reservoirs  210 ,  220  from the inlet  320 , depending on the mode of operation of the supply system  100 . 
     The supply system  100  of this example is to operate in first and second modes. In the first mode, the supply system  100  is to concurrently supply printing fluid from the first reservoir  210  to the outlet  310  and from the inlet  320  to the second reservoir  220 . In the second mode, the supply system  100  is to concurrently supply printing fluid from the second reservoir  220  to the outlet  310  and from the inlet  320  to the first reservoir  210 . It will be noted that, in this example, in each of the first and second modes of operation, the supply system  100  is to supply printing fluid to the outlet  310  and from the inlet  320 . Thus, when the outlet  310  is fluidly connected to the fluid inlet  21  of the printhead  20 , and the inlet  320  is fluidly connected to the fluid outlet  22  of the printhead  20 , printing fluid may be fed from the supply system  100  and into the printhead  20  via the outlet  310  and the fluid inlet  21 , pass through the printhead  20 , and then return to the supply system  100  via the fluid outlet  22  and the inlet  320 . 
     The printing fluid circulation system  10  of this example comprises a controller  400  to control operation of the supply system  100 . In this example, the controller  400  is to determine in which of the first and second modes the supply system  100  is to operate. The controller  400  may, for example, be an integrated circuit or a microprocessor. The controller  400  may be communicatively connected to the supply system  100 . 
     The supply system  100  will now be described in more detail. In this example, the supply system  100  comprises first to fourth flow paths  101 ,  102 ,  103 ,  104 . The first flow path  101  extends from the first reservoir  210  to the outlet  310 . The first flow path  101  may fluidly connect the first reservoir  210  and the outlet  310 . The second flow path  102  extends from the inlet  320  to the second reservoir  220 . The second flow path  102  may fluidly connect the inlet  320  and the second reservoir  220 . The third flow path  103  extends from a first point  101   a  on the first flow path  101  to a second point  102   b  on the second flow path  102 . The third flow path  103  may fluidly connect the first and second points  101   a ,  102   b . The fourth flow path  104  extends from a third point  101   c  on the first flow path  101  to a fourth point  102   d  on the second flow path  102 . The fourth flow path  104  may fluidly connect the third and fourth points  101   c ,  102   d . In this example, the third point  101   c  is between the first point  101   a  and the outlet  310 , and the fourth point  102   d  is between the second point  102   b  and the second reservoir  220 . 
     In this example, the supply system  100  also comprises first to fourth valves  111 ,  112 ,  113 ,  114  to selectively block the first to fourth flow paths  101 ,  102 ,  103 ,  104 , respectively. In this example, the first valve  111  is to selectively block the first flow path  101  at a point between the first and third points  101   a ,  101   c  on the first flow path  101 , and the second valve  112  is to selectively block the second flow path  102  at a point between the second and fourth points  102   b ,  102   d  on the second flow path  102 . In this example, the third valve  113  is to selectively block the third flow path  103  at a point between the first and second points  101   a ,  102   b , and the fourth valve  114  is to selectively block the fourth flow path  104  at a point between the third and fourth points  101   c ,  102   d . In this example, each of the first to fourth valves  111 ,  112 ,  113 ,  114  is an electrovalve. However, in other examples, the valves  111 ,  112 ,  113 ,  114  may be other than electrovalves. 
     In this example, the controller  400  is to control a state of each of the first to fourth valves  111 ,  112 ,  113 ,  114 , in order to control the direction of fluid flow(s) through the supply system  100 . The controller  400  may be communicatively connected to the valves  111 ,  112 ,  113 ,  114 . In this example, the controller  400  is to cause the first and second valves  111 ,  112  to be open, and the third and fourth valves  113 ,  114  to be closed, when the supply system  100  is operating in the first mode. On the other hand, in this example, the controller  100  is to cause the first and second valves  111 ,  112  to be closed, and the third and fourth valves  113 ,  114  to be open, when the supply system  100  is operating in the second mode. 
     The supply system  100  of this example also comprises first and second pumps  121 ,  122 . The first pump  121  is between the third point  101   c  and the outlet  310  in this example. The first pump  121  is fluidly connected between the third point  101   c  and the outlet  310 , to pump printing fluid towards the outlet  310  through the first flow path  101  when the supply system  100  is operating in either the first mode or the second mode. In this example, the first pump  121  is to draw ink or other printing fluid from one of the first and second reservoirs  210 ,  220 , depending on the mode of operation of the supply system  100 , and to supply the drawn printing fluid to the fluid inlet  21  of the printhead  20 . The second pump  122  is between the inlet  320  and the second point  102   b  in this example. The second pump  122  is fluidly connected between the inlet  320  and the second point  102   b , to pump printing fluid from the inlet  320  through the second flow path  102  when the supply system  100  is operating in either the first mode or the second mode. In this example, the second pump  122  is to extract ink or other printing fluid from the printhead  20  via the fluid outlet  22  of the printhead  20 , and to supply the extracted printing fluid to one of the first and second reservoirs  210 ,  220 , depending on the mode of operation of the supply system  100 . In some examples, the second pump  122  may be omitted. 
     In this example, a damper  140  is fluidly connected between the inlet  320  and the second pump  122 . In some examples, the second pump  122  may be a diaphragm pump. In use, the diaphragm pump may move successive small volumes of fluid relatively quickly through the second flow path  102 . During this operation, the diaphragm pump  122  may first take fluid from an upstream section of the second flow path  102  into a cavity of the diaphragm pump  122  via a pump inlet of the pump  122 . Then, the fluid taken into the cavity may be ejected from the cavity to a downstream section of the second flow path  102  via a pump outlet of the pump  122 . These actions may cause pressure variation in the second flow path  102 , because the pressure of the fluid at the pump inlet and pump outlet changes during operation of the second pump  122 . The damper  140  may help provide a smoother printing fluid pressure profile in the second flow path  102 . In some examples, the second pump  122  may be a different type of pump, such as a peristaltic pump, and may not create such pressure variation. In some examples, the damper  140  may be omitted. 
     In this example, the first reservoir  210  has a first fluid port  214 , through which printing fluid may pass between the first reservoir  210  and the supply system  100 . In this example, the first reservoir  210  is fluidly connected to the supply system  100  just by the first fluid port  214 . Therefore, in this example printing fluid to pass from the first reservoir  210  to the supply system  100 , or from the supply system  100  to the first reservoir  210 , has to pass through the first fluid port  214 . In this example, the second reservoir  220  has a second fluid port  224 , through which printing fluid may pass between the second reservoir  220  and the supply system  100 . In this example, the second reservoir  220  is fluidly connected to the supply system  100  just by the second fluid port  224 . Therefore, in this example printing fluid to pass from the second reservoir  220  to the supply system  100 , or from the supply system  100  to the second reservoir  220 , has to pass through the second fluid port  224 . In this example, the supply system  100  is to concurrently supply printing fluid from the first reservoir  210  through the first fluid port  214  and to the second reservoir  220  through the second fluid port  224  when operating in the first mode, and to concurrently supply printing fluid from the second reservoir  220  through the second fluid port  224  and to the first reservoir  210  through the first fluid port  214  when operating in the second mode. This bi-directional operation through the first and second fluid ports  214 ,  224  may help to keep the first and second fluid ports  214 ,  224  clear of settled pigment or other particles. 
     In other examples, there may be more than one fluid port through which printing fluid may pass between the first reservoir  210  and the supply system  100 , and/or there may be more than one fluid port through which printing fluid may pass between the second reservoir  220  and the supply system  100 . 
     The printing fluid circulation system  10  of this example also comprises a first detector  212  to detect a volume of printing fluid contained in the first reservoir  210 , and a second detector  222  to detect a volume of printing fluid contained in the second reservoir  220 . Each of the first and second detectors  212 ,  222  in this example may take any form suitable to sense how much printing fluid, such as ink, there is present in the respective reservoir  210 ,  220 . Each of the first and second detectors  212 ,  222  in this example may take any form suitable to sense when there is less than a certain predetermined volume of printing fluid, such as ink, present in the respective reservoir  210 ,  220 . Each of the first and second detectors  212 ,  222  may, for example, comprise a float that is to move with a level of liquid in the respective reservoir  210 ,  220 , and a switch that is to actuate when the float sinks in the reservoir to below a predetermined position to indicate the volume of liquid in the respective reservoir  210 ,  220 . Other forms of detector  212 ,  222  may instead be provided in other examples. In some examples, the first detector  212  and/or the second detector  222  may be omitted. 
     The controller  400  may be communicatively connected to the detectors  212 ,  222 . The controller  400  may determine the volume(s) of printing fluid in the reservoirs  210 ,  220  on the basis of signal(s) output from the detector(s)  212 ,  222 . In some examples, the controller  400  may be to determine in which one of the first and second modes the supply system  100  is to operate on the basis of a volume of printing fluid in the first reservoir  210  or the second reservoir  220  or both the first and second reservoirs  210 ,  220 . In some examples, the controller  400  may be to cause the supply system  100  to cease operating in the first mode when it is determined, such as following receipt of a signal at the controller  400  from the first detector  212 , that the first reservoir  210  contains less than a predetermined volume of printing fluid. In some examples, the controller may be to cause the supply system  100  to cease operating in the second mode when it is determined, such as following receipt of a signal at the controller  400  from the second detector  222 , that the second reservoir  220  contains less than a predetermined volume of printing fluid. This can help to avoid damage to the first and/or second pump  121 ,  122 , which could otherwise overheat if pumping too little, or no, fluid. 
     In this example, in use, and when the supply system  100  is operating in the first mode, each of the first and second valves  111 ,  112  is open, each of the third and fourth valves  113 ,  114  is closed, and each of the first and second pumps  121 ,  122  is operating to pump printing fluid through the first and second flow paths  101 ,  102 , respectively. On the other hand, when the supply system  100  is operating in the second mode in use, each of the first and second valves  111 ,  112  is closed, each of the third and fourth valves  113 ,  114  is open, and each of the first and second pumps  121 ,  122  is operating to pump printing fluid through the first and second flow paths  101 ,  102 , respectively. 
       FIG. 2  shows schematically a printing fluid circulation system  10  according to another example. The printing fluid circulation system  10  of this example comprises a first reservoir  210  to store printing fluid, a second reservoir  220  to store printing fluid, an outlet  310  to supply printing fluid to a fluid inlet of a printhead, and an inlet  320  to receive printing fluid from a fluid outlet of the printhead. The printing fluid circulation system  10  of this example also comprises a supply system  100  to operate in a first mode to concurrently supply printing fluid from the first reservoir  210  to the outlet  310  and from the inlet  320  to the second reservoir  220 , and in a second mode to concurrently supply printing fluid from the second reservoir  220  to the outlet  310  and from the inlet  320  to the first reservoir  210 . 
     In some examples, when printing fluid in the first reservoir  210  has been exhausted and passed to the second reservoir  220  as a result of operation of the supply system  100  in the first mode, the supply system  100  may subsequently be operated in the second mode so as to then supply the printing fluid from the second reservoir  220  to the first reservoir  210 . 
     An example method of operating a printing system  1  will now be described with reference to  FIGS. 1 and 3 . At block  501 , it is determined, such as by the controller  400 , whether the printing system  1  has been in an idle state for more than a predetermined period of time. In this example, the printing system  1  is considered to be in an idle state when the printing system  1  is not performing a print job by applying printing fluid to a substrate, and when printing fluid is not being circulated. The predetermined period of time may be, for example, ten minutes, thirty minutes, an hour, two hours, three hours, or four hours. In some examples, the predetermined period of time may be a period of time other than one of the examples listed. When the printing system  1  is in an idle state, the printing fluid may not be in motion. Accordingly, pigment particles or other particles within the printing fluid may be at risk of settling or agglomerating. In this example, if it is determined at block  501  that the printing system  1  has been in an idle state for more than a predetermined period of time, then it is determined that circulation of printing fluid is to be performed and the method moves to block  502 . If the determination at  501  is that the printing system  1  has not been in an idle state for more than the predetermined period of time, then the method returns to block  501 . 
     In some examples, block  501  may be omitted. In some examples, the method may be performed for some or all of the time that the printing system  1  is not performing a print job. In some examples, the method may be performed periodically when the printing system  1  is not performing a print job, such as every X minutes. X may be, for example, ten minutes, thirty minutes, sixty minutes, one hundred twenty minutes, one hundred eighty minutes, or two hundred forty minutes. 
     In this example of the method, at block  502  it is then determined which one of the first and second printing fluid tanks  210 ,  220  of the printing system  1  is to supply printing fluid to the fluid inlet  21  of the printhead  20  of the printing system  1 . This determination could, for example, be made on the basis of the following factor(s): (a) which one of first and second reservoirs  210 ,  220  of the printing system  1  last supplied printing fluid to the fluid inlet  21  of the printhead  20 , and/or (b) a volume of printing fluid in the first reservoir  210 , and/or (c) a volume of printing fluid in the second reservoir  220 . In some examples, when it is determined that one of first and second reservoirs  210 ,  220  of the printing system  1  last supplied printing fluid to the fluid inlet  21  of the printhead  20 , then it may be determined that the same one of the first and second reservoirs  210 ,  220  is to supply printing fluid to the fluid inlet  21 . This may be to reduce the number of valves that need to be actuated to suitably set the supply system  100  to enable circulation of printing fluid. 
     In some examples, when it is determined that a volume of printing fluid in one of the first and second reservoirs  210 ,  220  is less than a predetermined volume, and the volume of printing fluid in the other of the first and second reservoirs  210 ,  220  is not less than a predetermined volume, then it may be determined that the other of the first and second reservoirs  210 ,  220  is to supply printing fluid to the fluid inlet  21 . This would enable circulation of printing fluid to be performed with a greater volume of printing fluid, because the source of printing fluid for the circulation process would be the reservoir  210 ,  220  containing the greater volume of printing fluid. 
     In this example the method then moves to block  503 , at which the printhead regulator  23  is opened. Ordinarily, a bag of the regulator  23  is fluidly connected to the atmosphere by a regulator valve  25  being in an open state, so as to be at atmospheric pressure. The regulator  23  is opened by the regulator valve  25  being closed and a regulator pump  24  blowing air into the bag of the regulator  23 . Opening of the regulator  23  causes the fluid outlet  22  of the printhead  20  to be opened. In some examples, such as examples in which the printhead is of a different type to the printhead  20  described herein, the regulator pump  24  and/or the regulator valve  25  and/or the printhead regulator  23  may be omitted. 
     In this example, the method then moves to block  504 , at which it is provided that the one of the first and second reservoirs  210 ,  220  is fluidly connected to the fluid inlet  21 , and that the fluid outlet  22  is fluidly connected to the other of the first and second reservoirs  210 ,  220 . In some examples, the valves  111 - 114  of the supply system  100  may already be suitably set so that these fluid connections are already present. In other examples, one or some or all of the valves  111 - 114  may be opened or closed, for example under the control of the controller  400 , so as to provide the fluid connections. In some examples, block  503  may be performed before block  504 , or after block  504 , or simultaneously with block  504 . 
     In this example, the method then moves to block  505 , at which printing fluid is supplied from the one of the first and second reservoirs  210 ,  220  to the fluid inlet  21 , and from the fluid outlet  22  to the other of the first and second reservoirs  210 ,  220 . In this example, this comprises the first pump  121  being operated, for example under the control of the controller  400 , so as to pump printing fluid towards the outlet  310  through the first flow path  101 . In some examples, this also comprises the second pump  122  being operated, for example under the control of the controller  400 , so as to pump printing fluid from the inlet  320  through the second flow path  102 . This circulation of printing fluid helps to prevent pigment or particles within the printing fluid from settling or agglomerating in the supply system  100  or in the printhead  20 , which could otherwise result in flow of the printing fluid during a subsequent print job being hindered or prevented. The printing fluid may be supplied from the one of the first and second reservoirs  210 ,  220  to the fluid inlet  21 , through the printhead  20 , and then from the fluid outlet  22  to the other of the first and second reservoirs  210 ,  220 . In some examples, block  503  may be performed before block  505 , or simultaneously with block  505 , or after block  505 . 
     In this example, the method then moves to block  506 , at which it is determined, such as by the controller  400  for example on the basis of a signal received from a print driver, whether a print job on a substrate is to be performed. If it is determined at block  506  that a print job is to be performed, then the method moves to block  509 , discussed below. If it is determined at block  506  that a print job is not to be performed, then the method moves to block  507 . 
     In this example, at block  507  it is determined, such as by the controller  400  on the basis of a signal output from one of the detectors  212 ,  222 , whether the one of the reservoirs  210 ,  220  from which printing fluid is being sourced contains less than a predetermined volume of printing fluid. If the determination at block  507  is “no”, then the method of this example returns to block  505 , so that the circulation of printing fluid continues. However, if the determination at block  507  is “yes”, then the method of this example moves to block  508 , at which the supply system  100  ceases operating in the current one of the first and second modes, for example under the control of the controller  400 . For example, the controller  400  may cause the first and second pumps  121 ,  122  to cease pumping. Accordingly, circulation of printing fluid stops. The method then returns to block  501 . 
     In a subsequent performance of the method, at block  504  it may be provided that the other of the first and second reservoirs  210 ,  220  is fluidly connected to the fluid inlet  21 , and that the fluid outlet  22  is fluidly connected to the one of the first and second reservoirs  210 ,  200 . Correspondingly, at block  505  printing fluid may be supplied from the other of the first and second reservoirs  210 ,  220  to the fluid inlet  21 , and from the fluid outlet  22  to the one of the first and second reservoirs  210 ,  220 . Accordingly, the printing fluid may flow in different flow paths of the supply system  100  as compared to the flow paths of the supply system  100  used in the first instance of block  505  discussed above. Accordingly, this can enable printing fluid to be moved through more of the supply system  100 , which can further help avoid or reduce agglomeration of pigment or particles in the supply system  100 . 
     In this example, at block  509  it is provided that a particular one of the first and second reservoirs  210 ,  220  is fluidly connected to the fluid inlet  21  of the printhead  20 . The particular one of the first and second reservoirs  210 ,  220  may be the one of the first and second reservoirs  210 ,  220  that was supplying printing fluid to the fluid inlet  21  at block  505 . Alternatively, the particular one of the first and second reservoirs  210 ,  220  may be the other of the first and second reservoirs  210 ,  220 , in which case one or some or all of the valves  111 - 114  may be opened or closed, for example under the control of the controller  400 , so as to provide the fluid connection. In this example, at block  509  valve  112  is closed, for example under the control of the controller  400 , so as to prevent fluid flow through the second flow path  102 . The method then moves to block  510 . 
     In this example, at block  510  printing fluid is supplied from the particular one of the first and second reservoirs  210 ,  220  to the fluid inlet  21 . In this example, this comprises the first pump  121  being operated, for example under the control of the controller  400 , so as to pump printing fluid towards the outlet  310  through the first flow path  101 . In this example, the second pump  122  is not operated, since the second flow path  102  is blocked by the second valve  112 . The method then moves to block  511 . 
     In this example, at block  511  printing fluid is fed from the fluid inlet  21  of the printhead  20  to the nozzle of the printhead  20 , to apply printing fluid to the substrate during the print job. When the print job is completed, the method moves to block  512 , at which the supply system  100  ceases supplying printing fluid to the fluid inlet  21 , for example under the control of the controller  400 . For example, the controller  400  may cause the first pump  121  to cease pumping. The method then returns to block  501 . 
     Another example method of operating a printing system will now be described with reference to  FIG. 4 . The printing system of this example comprises a first reservoir to store printing fluid, a second reservoir to store printing fluid, a printhead having a fluid inlet and a fluid outlet, and a supply system to supply printing fluid from either one of the first and second reservoirs to the fluid inlet, and to supply printing fluid from the fluid outlet to either one of the first and second reservoirs. At block  601  of the method, it is provided that one of the first and second reservoirs is fluidly connected to the fluid inlet, and that the fluid outlet is fluidly connected to the other of the first and second reservoirs. In some examples, the supply system  100  may already be suitably prepared so that these fluid connections are already present. In other examples, the supply system  100  may undergo some change or reconfiguration so as to provide the fluid connections. 
     The method then moves to block  602 , at which printing fluid is supplied from the one of the first and second reservoirs to the fluid inlet, and from the fluid outlet to the other of the first and second reservoirs. In some examples, the supply system may comprise a pump, and this process may comprise the pump being operated so as to pump printing fluid from the one of the first and second reservoirs towards the fluid inlet. In some examples, the supply system may comprise a second pump, and this process may comprise the second pump being operated so as to pump printing fluid from the fluid outlet to the other of the first and second reservoirs. This circulation of printing fluid helps to prevent pigment particles or other particles within the printing fluid from settling or agglomerating in the supply system or in the printhead, which could otherwise result in flow of the printing fluid during a print job being hindered or prevented. 
     Some examples provide a non-transitory computer-readable storage medium comprising a set of computer-readable instructions stored thereon, which, when executed by a processor of a printing system, cause the processor to determine which one of first and second printing fluid tanks of the printing system is to supply printing fluid to a fluid inlet of a printhead of the printing system, and to cause a supply system of the printing system to supply printing fluid from the one of the first and second printing fluid tanks to the fluid inlet concurrently with supplying printing fluid from a fluid outlet of the printhead to the other of the first and second printing fluid tanks. In some examples, the printing system may be that shown in  FIG. 1 , in which case the first and second printing fluid tanks may be the first and second reservoirs  210 ,  220 , the printhead and fluid inlet and fluid outlet thereof may be the printhead  20 , fluid inlet  21  and fluid outlet  22 , and the supply system may be the supply system  100 . In some such examples, the controller  400  of the printing system  1  may comprise the processor. 
     In some examples, the non-transitory computer-readable storage medium may comprise any one of many physical media such as, for example, electronic, magnetic, optical, electromagnetic, or semiconductor media. More specific examples of suitable non-transitory computer-readable storage media include, but are not limited to, a portable magnetic computer diskette such as floppy diskettes or hard drives, a read-only memory (“ROM”), an erasable programmable read-only memory, a portable compact disc or other storage devices that may be coupled to the processor directly or indirectly. Alternatively, the non-transitory computer-readable storage medium may be a random access memory (“RAM”) device. The non-transitory computer-readable storage medium may comprise any combination of one or more of the foregoing and/or other devices as well. In some examples, the processor may comprise a microprocessor. 
     Certain examples described herein provide a system or method to help avoid the settling or agglomeration of pigment or particles of printing fluid, which could otherwise result in flow of the printing fluid during a print job being hindered or prevented. An advantage of some examples is a reduced wastage of printing fluid. 
     The preceding description has been presented to illustrate and describe examples of the principles described. 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.