BACKGROUND

[0001] During printing, a print-zone may reach temperatures of up to 45°C or larger. A printhead and a printing fluid inside the printhead may reach similar temperatures. After finishing a print job or when a print job is paused, the printhead cools down to ambient temperature. The decreasing temperature may cause air bubbles in the printhead to shrink, and the change in air volume may result in a pressure reduction within the printhead. Due to the reduced pressure, air may be ingested through the nozzles of the printhead, which may result in a reduced printhead working life. To avoid the ingestion of air through the printhead nozzles, a printing fluid delivery system (PFDS) of printers with high print-zone temperatures can be kept pressurized during the cool-down period of the printhead.

DESCRIPTION OF DRAWINGS

[0002] The following detailed description will be best understood with reference to the drawings, wherein:

[0003] Fig. 1 shows a schematic diagram of a PFDS of a printer according to an example;

[0004] Fig. 2 shows a schematic diagram of a PFDS of a printer according to another example;

[0005] Fig. 3 shows a schematic diagram of the PFDS of Fig. 1 in combination with a printhead according to an example;

[0006] Fig. 4 shows a flow diagram of a method to control the pressure within a printhead in a cool-down period of the printhead according to an example;

[0007] Fig. 5 shows a flow diagram of another method to control the pressure within a printhead in a cool-down period of the printhead according to an example; [0008] Fig. 6 shows a flow diagram of a further method to control the pressure within a printhead in a cool-down period of the printhead according to an example; [0009] Fig. 7 shows a flow diagram of a further method to control a pressure within a printhead in a cool-down period of the printhead according to an example; [0010] Fig. 8 shows a diagram of the pressure inside a PFDS during a cool down period of a printhead according to an example.

DESCRIPTION OF EXAMPLES

[0011] The following disclosure provides many different examples, for implementing different features of the disclosed subject matter. Specific examples of components, values, operations, materials, arrangements, or the like, are described below to simplify the present disclosure. These are merely examples and are not intended to be limiting. Other components, values, operations, materials, arrangements, or the like, are contemplated. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity, respective components designated by the same reference numerals may be implemented and may operate in an identical or similar way, without being bound to this.

[0012] Fig. 1 shows a schematic diagram of a printing fluid delivery system (PFDS) 100 for a printer according to an example. In the example described, the printing fluid may be an ink, such as a color ink, including CMYK inks, and white ink. The ink may be a latex ink or another type of ink. In other examples, the printing fluid can be a type of conditioning fluid used in inkjet type printers, including 2D and 3D printers. The printing fluid further may be any type of printable liquid. The printer may be, may include, or may be part of a desktop printer, a large format printer, a plotter or the like, for example. The printer may be a 2D printer or a 3D printer. Accordingly, in the following description, the printing fluid sometimes is referred to as ink, with the understanding that other types of printing fluids may be used.

[0013] In the example of Fig.1 , the PFDS 100 comprises a supply tank 110, an intermediate tank 120, a fluid outlet 130, a controller 140, a fluid pump 150, and an air pressure pump or air pressure source 160. The supply tank 110 is shown as part of the PFDS. Alternatively, the PFDS does not comprise the supply tank 110, but the PFDS is connected to the supply tank. Whereas, in this example, the supply tank 110 is considered a part of the PFDS, the supply tank 110 may be a replaceable resource to be connected to the PFDS. The fluid outlet 130 may provide a connection to a printhead inserted in a printer. A fluid channel portion 122 connected to an output of the intermediate tank 120 merges with a fluid channel portion 112 connected to an output of the fluid pump 150 in a common fluid channel portion 132 connected to the fluid outlet 130.

[0014] A printhead may be supported in the printer by a printhead tray or a printhead carriage or the like, which may include bays to receive and connect printheads. Such bays may include power and signal ports to be connected to respective ports of a printhead. A fluid inlet of the printhead may be connectable to the fluid outlet 130 of the PFDS shown in Fig. 1 .

[0015] The fluid pump 150 may be associated with a drive motor 152 and a pressure relief valve 154. The drive motor 152 may be e.g. a DC motor controlled by a drive voltage, e.g. using a pulse width control scheme. The pressure relief valve 154 may be designed to prevent the pressure in the PFDS from rising above a cut-off pressure, which could damage components of the PFDS. Such cut-off pressure may be in a range of 35 to 50 kPa relative to the atmospheric pressure, for example.

[0016] In this example, the intermediate tank 120 can include a variable fluid volume to contain a supply of printing fluid and a variable gas volume to receive pressurized gas, such as air, to pressurize the supply of printing fluid to feed the printing fluid from the intermediate tank 120. The variable fluid volume may be contained in a collapsible fluid reservoir, such as a collapsible ink bag, for example. The variable gas volume may be contained in a fluid tank container surrounding the collapsible ink bag and may be separated from the variable fluid volume by the bag material. In another example, the variable fluid volume and the variable gas volume may be contained in a common fluid tank container and be separated by a flexible membrane. The variable fluid volume and the variable gas volume are arranged relative to each other in such a way that they are separated, but pressure applied to the gas volume can be transferred to the fluid volume and vice versa.

[0017] The supply tank 110 can include a fluid volume larger than that of the intermediate tank 120 to refill the intermediate tank during printing. For example, the supply tank 110 may have a fluid volume of several liters, such as 2 L, 3 L, or 5 L. The intermediate tank may have a smaller maximum fluid volume, e.g. a volume of less than a liter, such as a maximum fluid volume of 500 ml_, 700 ml_, or 750 ml_.

[0018] The fluid pump 150 may be designed to generate a flow rate of the printing fluid sufficient to refill the intermediate tank 120 during printing, i.e., a flow rate which is the same or larger than a maximum fluid flow rate from the intermediate tank 120 to the fluid outlet 130. In an example, the flow rate generated by the fluid pump 150 may be in the order of 30 to 300 mL/minute or 40 to 200 ml_/min, e.g. about 150 mL/min. The maximum flow rate from the intermediate tank 120 to the fluid outlet 130 may be in the order of 60 to

200 mL/min, depending on the rate at which printing fluid is ejected from the printhead. Depending on the type and size of the printer architecture, print head, and the application, tank volumes and fluid flows may vary.

[0019] A fill state of the fluid tanks, such as the intermediate tank or the supply tank, can be measured using a fluid level sensor. The fluid level sensor can be a physical sensor provided in the tank or a differential pressure sensor, for example. In the PFDS 100 of Fig. 1 , the fluid level of the intermediate tank 120 can be measured by means of the differential pressure senor 170. The differential pressure sensor 170 comprises an input connected to an air pressure supply line 162 in communication with the variable gas volume of the intermediate tank 120 and an input connected to the fluid channel portion 122 at the output of the intermediate tank 120, as shown in Fig. 1. The differential pressure sensor 170 may operate based on a pressure difference between the air pressure supplied to the intermediate tank 120 and the fluid pressure in the fluid channel portion 122.

[0020] Printing fluid may be transferred from the intermediate tank 120 to the fluid outlet 130 using the air pressure pump or air pressure source 160 (in the following, sometimes referred to as a pressure source), for example. Instead of air, another gas, e.g. an inert gas, can be used to generate a gas pressure on the intermediate tank 120. Accordingly, in the following description, reference to air should include also reference to another type of gas to be provided by the air pressure source 160. The intermediate tank 120 may act as a buffer of printing fluid, and during normal printing operation is pressurized with air or another gas using the pressure source 160 to supply printing fluid to the fluid outlet 130 and eventually to the printhead, see Fig. 3, for example.

[0021] During printing, the pressure source 160 operates by pressurizing the air volume inside the intermediate tank 120 and above or around the fluid volume, by applying an air pressure, which cycles between a lower_threshold_pressure and an upper_threshold_pressure. For example, the air pressure may be increased to the upper_threshold_pressure by activating the pressure source 160. The pressure source 160 is deactivated upon reaching the upper_threshold_pressure. Subsequently, the air pressure will decrease when printing fluid flows from the intermediate tank 120 towards the fluid outlet 130 and from there to a printhead. When the pressure reaches the lower_threshold_pressure, the air pressure source 160 may be activated again to increase the air pressure until it reaches the upper_threshold_pressure. This cycle may be repeated as long as the printer is operating to deliver printing fluid to the fluid outlet 130.

[0022] Further, printing fluid may be transferred from the supply tank 110 to the intermediate tank 120 using the fluid pump 150 to refill the intermediate tank 120 upon demand. A refill operation may be triggered by the differential pressure sensor 170, for example. The fluid pump 150 may be a different type of pump, such as a DC motor driven fluid pump. For example, a volumetric pump may be provided, the pump having specific flow rates at respective revolutions per minute. As another example, a centrifugal pump may provide specific flow rates at respective revolutions per minute.

[0023] For example, when the printing fluid is consumed from the intermediate tank 120, the intermediate tank 120 may be refilled from the supply tank 110 using the fluid pump 150, which pushes printing fluid into the intermediate tank 120. In one example, the refill operation is continued until the differential pressure sensor 170, measuring fluid vs. air pressure, detects an end of the refill process when the intermediate tank 120 is considered full. For example, a look up table of differential pressure values vs. fluid levels may be used to determine a fluid level based on a measured differential pressure. The end of the refill process may then be detected based on the determined fluid level. [0024] In addition, as shown in Fig. 1 , an air pressure sensor 164 can be provided for monitoring the pressure source 160 and, further, an air relief valve 166 can be provided for depressurizing the PFDS 100 and preventing the pressure in the PFDS from rising above a level, which could damage components of the PFDS.

[0025] The differential pressure sensor 170 can also be configured to measure the pressure in the fluid channel portion 132 versus the ambient pressure. For this, the air input of the pressure sensor 170 can be connected to ambient atmosphere, instead of being connected to the air pressure supply line 162. For example, the air inlet of the pressure sensor 170 can be designed to be switchable between different inputs, e.g. an input connected to air pressure supply line 162 and an input connected to atmosphere.

[0026] The PFDS 100 further comprises a controller 140. The controller 140 is to control a pressure at the fluid outlet 130 when a print job is finished or paused. Thereto, the controller 140 can be communicatively coupled to the pressure source 160. In addition, the controller 140 can be communicatively coupled to the pressure sensor 164, the relief valve 166, and/or the differential pressure sensor 170 of the PFDS 100. The controller 140 can also be communicatively coupled to other components of the PFDS 100 to be controlled or monitored. Moreover, the controller 140 can be communicatively coupled to a printhead (not shown in Fig. 1 ) to receive values indicative of a pressure within the printhead. The communication connections of the controller 140 can be wireless or wired, and the controller may be programmed to monitor and control the operation of components of the PFDS and to receive signals from sensors, for example. For ease of illustration, communication connections between the controller 140 and other components of the PFDS 100 as well as the printhead are not illustrated in the drawings.

[0027] The controller 140 can also provide other control operations such as controlling the fluid level inside the intermediate tank 120. Alternatively, the PFDS can comprise a plurality of controllers, wherein one controller 140 is to control the pressure at the fluid outlet 130, and other controllers are to control the fluid level inside the intermediate tank 120, or printing operations, for example. The controller 140 can be a single control system or a distributed control system. The controller 140 can be implemented in hardware, firmware, software and combinations thereof.

[0028] The PFDS may further comprise a check valve 180 in the fluid channel portion 122 or in common fluid channel portion 132, wherein the check valve 180 may be configured to close the respective fluid channel portion 122, 132 when the air volume of the intermediate tank 120 is depressurized. Thus, the check valve may prevent that printing fluid flows out of the fluid outlet 130 towards a printhead (not shown) when the air volume of the intermediate tank 120 is depressurized.

[0029] Fig. 2 shows a schematic diagram of another example of a PFDS 200 in a printer architecture according to an example. The same components as in Fig. 1 are designated by the same reference numbers. With regard to the type of printing fluid, volume of tanks, flow rates, pump types, controller operation and coupling, and other details of corresponding components and operation of the PFDS, reference is made to the description of Fig. 1 above.

[0030] In the example of Fig. 2, the PFDS 200 comprises a supply tank 110, a fluid outlet 130, a controller 140, a fluid pump 150, a check valve 180, and a pressure sensor 210. Alternatively, the PFDS 200 does not comprise the supply tank 110, but the supply tank 110 can be connected to the PFDS 200. The fluid outlet 130 may provide a connection to a printhead inserted in the printer. The supply tank 110 and the fluid pump 150 may be configured as described above with reference to Fig. 1 . The PFDS 200 of Fig. 2 is one which operates without intermediate tank, directly feeding printing fluid from the supply tank 110 to the fluid outlet 130 through a printing fluid channel 212. The pressure sensor 210 can measure the pressure in the printing fluid channel 212, e.g. an absolute pressure, which is measured against ambient pressure. The fluid pump 150 can be controlled to feed printing fluid to the fluid outlet 130 by controlling a voltage applied to the fluid pump 150, e.g. using a pulse width control scheme. In the example of Fig. 2, the controller 140 can be communicatively coupled to the pressure sensor 210 and the fluid pump 150 to control the pressure at the fluid outlet 130 when a print job is finished or paused. [0031] Whereas, Figs. 1 and 2 show PFDSs 100, 200 including a single supply tank 110, intermediate tank 120, and fluid outlet 130, a printing architecture may comprise a plurality of supply tanks, intermediate tanks, and/or fluid outlets, e.g. one for each color of Black, Cyan, Magenta and Yellow inks and possible additional inks and other fluid, e.g. a pre- or post-treatment fluid. The number of fluid outlets may be different from the number of printheads connected thereto, because a single printhead may be able to eject more than one color or type of ink and, hence, may be connected to more than one PFDS.

[0032] In a printer designed for multiple types of ink, such as BCMY inks and conditioning fluids, for example, a separate ink pump may be provided in respective separate fluid lines between respective supply tanks and respective intermediate tanks for each type of ink. Each fluid line can be connected to a respective fluid pressure sensor. Further, a single air pressure source can be connected to a plurality of intermediate tanks, or the printer can comprise a plurality of air pressure sources, for example, one for each intermediate tank. With reference to Figs. 1 and 2, the printer can comprise one differential pressure sensor 170 per type of printing fluid or one pressure sensor 210 per type of printing fluid.

[0033] Accordingly, the controller 140 may be programmed to separately control the pressure at the fluid outlet of the respective PFDS for each color or each type of printing fluid. Thus, the printer can comprise a number of PFDSs dedicated to different fluid types or colors, and the components of these PFDSs can be controlled independently. Alternatively, the controlling of a plurality of PFDSs can comprise joint operations, for example when there is a single air pressure source connected to multiple intermediate tanks.

[0034] Figure 3 shows a schematic diagram of the PFDS 100 of Fig. 1 in combination with a printhead 300, according to an example. The printhead 300 of this example includes two printing fluid chambers or reservoirs 310, 320, for ease of description also referred to as printhead chambers, each provided for a different type of printing fluid, e.g. for different color inks, wherein respective fluid levels of printing fluids are shown at 312, 322. The two printhead chambers 310, 320 may correspond to different print nozzle rows (not shown) of the printhead. The printhead 300 comprises a first fluid port 314 and a second fluid port 324. In this example, the first fluid port 314 and the first printhead chamber 310 are fluidly connected with the fluid outlet 130 of the PFDS 100, for example. The second fluid port 324 and hence the second printhead chamber 320 can be fluidly connected to a further PFDS (not shown). The printhead 300 can further comprise a nozzle plate 340 to eject printing fluid from the printhead chambers 310, 320. The printhead 300 may be received in a carriage or other printhead support in the printer (not shown).

[0035] In the following, the printhead 300 is described with reference to the first printhead chamber 310 and associated components, wherein the second printhead chamber 320 may be configured, operated, and monitored in a corresponding way.

[0036] The first fluid port 314 is in fluid communication with the interior of the printhead chamber 310 via a fluid pipe 316 and a regulator valve 318, which may be a switchable regulator valve. The regulator valve 318 comprises a stopper that is to block the outlet of the fluid pipe 316 into the printhead chamber 310 when the pressure inside the printhead chamber 310 minus the ambient pressure is greater than a predetermined threshold kBP. The threshold kBP is chosen to prevent that printing fluid leaks or drools out of the nozzles of the printhead. For example, the threshold kBP may be -0.5 kPa, -1 kPa, -1.5 kPa, or -2 kPa, depending on the type of printing fluid, amongst others. Larger or smaller thresholds kBP are possible as well. Hence, the regulator valve 318 is to prevent that the pressure within the printhead exceeds the ambient pressure plus the predetermined threshold kBP, wherein the predetermined threshold has a negative value to implement a backpressure (BP) inside the printhead.

[0037] In an example, the printhead can comprise a collapsible bag 352 inside the printhead chamber 310, wherein the collapsible bag 352 comprises an inlet connected to ambient pressure. Further, pressure plates and a spring mechanism can be arranged inside the printhead chamber 310 (not shown). The collapsible bag 352 is arranged between the pressure plates, and the spring mechanism is to bias the pressure plates against the collapsible bag 352. The regulator valve 318 is attached to a pressure plate so that, for example, the regulator valve 318 is closed when the pressure plate is in an inward position, and the regulator valve 318 is open when the pressure plate is in an outward position. In the inward position of the pressure plates, the collapsible bag encloses a small volume, and ends of the pressure plates are close to each other. In contrast, in the outward position of the pressure plates, the collapsible bag encloses a large volume, and ends of the pressure plates are more distant from each other. Hence, when the pressure within the printhead chamber 310 is larger than the ambient pressure plus the threshold kBP, the volume enclosed by the collapsible bag is small, and the spring mechanism holds the pressure plates in the inward position such that the regulator valve closes the outlet of the fluid pipe 316. On the other hand, when the pressure within the printhead chamber 310 is smaller than the ambient pressure plus the threshold kBP, the volume enclosed by the collapsible bag is large, and the spring mechanism holds the pressure plates in an outward position such that the regulator valve opens the outlet of the fluid pipe 316. Other implementations of a regulator valve, which controls the pressure inside the printhead chamber 310 for example by closing the outlet of the fluid pipe 316 into the printhead chamber 310 when the pressure inside the printhead chamber 310 minus the ambient pressure is greater than a threshold kBP are possible as well. [0038] Between the mouth of the fluid pipe 316 and the nozzle plate 340 of the printhead 300, there may be a filter element 330, which keeps any debris and particles in the printing fluid from reaching the nozzle plates. The filter element 330 may for example comprise or consist of a porous material or membrane. [0039] During printing, the printhead and the printing fluid inside the printhead may reach temperatures of 45°C or above. At the end of a print job or in a printing pause, the printhead cools down to the ambient temperature. When the printhead cools down, air bubbles within the printhead may shrink, and the pressure inside the printhead may decrease. Due to the reduced pressure inside the printhead, air may be ingested through the printhead nozzles, which may lead to image quality degradations and a reduced printhead lifetime. The ingestion of air through the printhead nozzles can occur in particular when the PFDS comprises a check valve 180, which prevents that printing fluid flows into the printhead when the PFDS is depressurized. The ingestion of air through the printhead nozzles can be prevented by keeping the PFDS pressurized during the cool-down period of the printhead, for example during a predefined time period such as for 2 hours after printing.

[0040] The regulator valve 318 can be configured to provide a backpressure so that printing fluid does not drool out of the nozzles of a printhead. However, the regulator valve 318 may not close properly in some cases, for example, when a particle obstructs the regulator valve. This is more likely to happen in dirty environments, or with certain types of ink such as white latex ink, or if printheads are frequently replaced by dummy-printheads, for example. When the regulator valve does not close properly, the printhead chamber 310 may be fluidly connected with the PFDS during the cool-down period of the printhead, so that the backpressure is lost when the PFDS is pressurized. Under these conditions, printing fluid can drool through the nozzles of the printhead. The drooling of printing fluid during the cool-down period of the printhead may soil the printhead environment, and it may cause printhead replacement, waste of printing fluid, and/or the damaging of functional parts such as encoders, motors, gears, etc. by the printing fluid.

[0041] To prevent the leakage or drooling of printing fluid out of the printhead, the controller 140 of the PFDS 100, 200 may perform the method as illustrated in Fig. 4. This figure shows a flow diagram of an example method for controlling the pressure inside a printhead when the printhead cools down. The method of Fig. 4 may be performed under the control of the controller 140 of the PFDS 100 depicted in Fig. 3, for example.

[0042] At 405, it may be detected that a print job is finished or paused. Alternatively, it may be detected that the printhead moved into a capping position. More generally, it may be detected that a cool-down period of the printhead started.

[0043] Upon detecting that a print job is finished or paused, or that the printhead moved into the capping position, the PFDS is controlled at 410 to be in a first state S1 . The first state S1 of the PFDS is to be distinguished from a second state S2 of the PFDS, wherein the PFDS is to provide a higher pressure at its fluid outlet in the second state S2 as compared to the first state S1 . In the first state S1 , the PFDS may be in an inactive or stand-by mode, for example, and the pressure within the printhead may decrease until air is ingested through the printhead nozzles. In contrast, in the second state S2, the PFDS may be active or turned- on, and the PFDS may be controlled to switch from the first state S1 to the second state S2 to raise the pressure within the printhead.

[0044] For example, considering the PFDS 100 of Figs. 1 and 3, the air pressure source 160 may be deactivated and the relief valve 166 may be opened in the first state S1 . In contrast, in the second state S2, the air pressure source 160 may be activated and the relief valve 166 may be closed to provide a higher pressure at the fluid outlet 130 as compared to the first state S1. For example, the PFDS 100 may be depressurized in the first state S1 and pressurized in the second state S2. It is to be noted that the air pressure within the intermediate tank 120 may be different from the pressure at the fluid outlet 130 of the PFDS 100, depending on, amongst others, the height of the top surface of the printing fluid inside the intermediate tank 120 relative to the height of the fluid outlet 130. Flowever, based on input provided by the differential pressure sensor 170, the controller 140 may compensate for that difference and control the air pressure source 160 in the second state S2 to provide a desired pressure at the fluid outlet 130 of the PFDS 100. Similarly, the controller 140 may control the air pressure source 160 to provide a desired pressure within the printhead in the second state S2, using measurement results provided by the differential pressure sensor 170, and assuming that the regulator valve 318 is open.

[0045] In another example, considering the PFDS 200 of Fig. 2, the pump 150 may be deactivated by the controller 140 in the first state S1. In contrast, in the second state S2, the pump 150 may be activated to provide the higher pressure at the fluid outlet 130 as compared to the first state S1 .

[0046] In a further example not depicted in the figures, the intermediate tank of a PFDS may be arranged at a certain height above the fluid outlet of the PFDS. Such a PFDS may not comprise a pressure source 160 and a relief valve 166, and the air pressure inside the intermediate tank may be equal to the ambient atmospheric pressure. The controller 140 may be communicatively coupled with a solenoid valve, which is arranged at a height between the fluid outlet of the intermediate tank and the fluid outlet of the PFDS. Thus, the solenoid valve is arranged below the fluid outlet of the intermediate tank, and closing the solenoid valve may cause the fluid column weighing on the fluid outlet of the PFDS to be reduced. Thus, the PFDS may be in the first state S1 when the solenoid valve is closed, and the PFDS may be in the second state S2 when the solenoid valve is open.

[0047] Further, at 410, an indicator TpH(ti) of the printhead (PH) temperature at an initial time ti is determined. For example, the controller 140 may be communicatively coupled to the printhead, and the printhead may comprise a temperature sensor such as a thermal sensing resistor (TSR). Thus, the controller 140 may receive the indicator TpH(ti) from the printhead, wherein the indicator TpH(ti) may be equal to or may be based on a measurement result provided by the TSR of the printhead. The initial time ti may be equal to the time at which the PFDS is controlled to be in the first state S1 in response to determining that the cool-down period of the printhead started.

[0048] At 415, an indicator TpH(ti) of the printhead temperature at a first time ti is determined, wherein the first time ti is later than the initial time ti. The temperature indicator TpH(ti) may be determined similarly as explained for TpH(ti) at 410. The pressure within the printhead at the time ti can be estimated based on the printhead temperature at the initial time ti, the pressure within the printhead at the initial time ti, and the printhead temperature at the first time ti, e.g. based on a model relating pressures inside the printhead with printhead temperatures during a cool-down period of the printhead, assuming that the PFDS remains in the first state S1. Such a model can be determined beforehand, for example experimentally, and may be stored in a look-up table, for example. The model may depend on the printhead, amongst others. The printhead temperature at the initial time ti may be determined based on the temperature indicator TRH(ΪΪ). Similarly, the printhead temperature at the first time ti may be determined based on the temperature indicator TRH(ΪI ). The pressure within the printhead at the initial time ti relative to the ambient atmospheric pressure can be assumed to be equal or close to the threshold kBP of the regulator valve 318. Thus, the pressure within the printhead at the time ti can be determined based on the temperature indicator TpH(t-i), so the temperature indicator TpH(ti) is an indicator for the pressure within the printhead at the first time ti. The time difference between ti and ti may be five minutes, 10 minutes, 20 minutes, 30 minutes, or 60 minutes, for example. Other time periods are possible as well, in particular shorter time periods.

[0049] At 420, it is determined based on the indicator TpH(ti) of the temperature of the printhead at the first time ti if a condition for increasing the pressure within the printhead is satisfied. For example, based on the information about the printhead temperature at the initial time ti, the information about the pressure within the printhead at the initial time ti, and the model relating pressures inside the printhead with printhead temperatures during the cool-down period, it may be determined that a certain temperature reduction corresponds to a certain pressure reduction within the printhead. More specifically, it may be determined that the pressure inside the printhead is below a certain value if the reduction of the printhead temperature is larger than a threshold kn. In this case, the pressure inside the printhead may need to be increased in order to prevent the ingestion of air through the printhead nozzles. Thus, if the indicator TpH(ti) of the printhead temperature at the first time ti indicates that the printhead temperature at the time ti minus the printhead temperature at the time ti is greater than the threshold kn, it may be determined that the condition for increasing the pressure inside the printhead is satisfied, indicating that the pressure inside the printhead is to be increased in order to prevent the ingestion of air through the printhead nozzles. A fixed value of the threshold kn may be determined based on an assumed printhead temperature at the initial time ti, the assumed pressure within the printhead at the initial time ti, and the model relating pressures inside the printhead with printhead temperatures during the cool-down period of the printhead. The threshold kn may be 5°C, 7°C, 10°C, 12°C, or 15°C, for example. Alternatively, the threshold kn may be chosen adaptively, depending on the temperature indicator TRH(Ϊ _Ϊ ), for example.

[0050] Note that texts inside the figure boxes can be simplified for the sake of conciseness, assuming for example that temperature indicators TpH(t) are actual temperatures and not just indicators thereof, see for example box 420 in Fig. 4. [0051] If it is determined at 420 that the condition for increasing the pressure within the printhead is satisfied for the indicator TpH(ti) of the printhead temperature at the time ti, the PFDS is controlled at 440 to be in the second state S2, so that the pressure at the fluid outlet of the PFDS is increased.

[0052] The condition for increasing the pressure inside the printhead can be such that the regulator valve 318 is open when the condition is satisfied. Then, controlling the PFDS to transit from the first state S1 to the second state S2 may cause the check valve to open and the pressure inside the printhead to increase, since the printhead chamber is fluidly connected with the fluid outlet of the PFDS. [0053] In the second state S2, the PFDS 100, 200 can be controlled to provide a pressure at the fluid outlet 130 that remains below the ambient atmospheric pressure. In particular, the PFDS 100, 200 may be controlled to provide a target pressure within the printhead, wherein the target pressure lies between a maximum pressure and a minimum pressure, wherein the maximum pressure is the pressure at which printing fluid starts to drool out of the printhead nozzles, and the minimum pressure is the pressure at which air starts to be ingested through the nozzles of the printhead. To prevent that printing fluid drools out of the printhead nozzles, the pressure within the printhead should be less than -0.5 kPa, -1 kPa, or -1.5 kPa relative to the ambient pressure, for example. The pressure at which air starts to be ingested through the nozzles of the printhead may lie between -4 kPa and -7 kPa relative to the ambient pressure, for example. Both the maximum and the minimum pressures may depend on the printhead and/or the printing fluid, amongst others. Thus, the target pressure to be provided by the PFDS inside the printhead may lie between -2 kPa and -3 kPa relative to the ambient pressure, for example.

[0054] In other examples, the PFDS 100, 200 can be controlled to provide in the second state S2 a pressure at the fluid outlet 130 that is larger than the ambient atmospheric pressure. In particular, the pressure provided by the PFDS 100, 200 in the second state S2 may cause printing fluid to drool out of the nozzles of the printhead 300 when the regulator valve 318 does not close properly. Flowever, as explained below, the PFDS 100, 200 may remain within the second state S2 for just a short period of time to prevent that large amounts of printing fluid drool out of the nozzles of the printhead 300.

[0055] At 445, the PFDS may be controlled to return to the first state S1 if a condition for returning to the first state is satisfied. The operations for controlling the PFDS to return to the first state S1 may depend on the PFDS as described above for 410. The condition for returning to the first state S1 may be that the PFDS remained in the second state S2 for a first time-interval. The duration of the first time-interval can be short as compared to the duration of the cool-down period of the printhead. For example, the duration of the first time-interval can be 1 second, 2 seconds, 5 seconds, 10 seconds, 30 seconds, or 60 seconds, for example. In other examples, the duration of the first time-interval may be determined adaptively, for example depending on the printhead temperature reduction from the initial time ti to the first time ti.

[0056] At 450, an indicator TRH(Ϊ3) of the printhead temperature at a third time t3 may be determined, wherein the third time t3 is later than the first time ti. The determination of the temperature indicator TRH(Ϊ3) may be similar to the determination of the temperature indicator TpH(ti) at 410 described above. The time difference t3- ti may be equal to or different from the time difference ti - ti. In particular, the time difference t3 - ti may be longer than the time difference ti - ti, since the rate of reduction of the printhead temperature may decrease over time. [0057] At 455, it may be determined, based on the indicator TRH(Ϊ3) of the temperature of the printhead at the third time t3, if a condition for increasing the pressure inside the printhead is satisfied. For example, based on the printhead temperature when the PFDS returns to the first state S1, the pressure within the printhead when the PFDS returns to the first state S1, and the model relating pressures inside the printhead with printhead temperatures during the cool-down period, it may be determined that a certain temperature reduction corresponds to a certain pressure reduction within the printhead. More specifically, it may be determined that the pressure inside the printhead is below a certain value if the reduction of the printhead temperature is larger than a threshold kj2, indicating that the pressure within the printhead is to be increased to prevent the ingestion of air through the printhead nozzles. The temperature reduction during the first time-interval may be considered negligible, since the first time-interval may be short. Thus, the printhead temperature when the PFDS returns to the first state S1 may be determined based on TpH(ti). Consequently, if the indicator TRH(Ϊ3) of the printhead temperature at the third time t3 indicates that the temperature of the printhead at the time ti minus the temperature of the printhead at the time t3 is greater than the threshold kj2, the condition for increasing the pressure inside the printhead may be satisfied. The threshold Wj2 may be different from the threshold kn. Alternatively, a fixed threshold kTi = kT2 may be used, as noted above.

[0058] If it is determined at 455 that the condition for increasing the pressure inside the printhead is satisfied for the temperature indicator TRH(Ϊ3), then the PFDS can be controlled at 460 to transit again to the second state S2 in order to increase the pressure inside the printhead, similar as explained for 440.

[0059] If it is determined at 420 that the condition for increasing the pressure inside the printhead is not satisfied for the temperature indicator TpH(ti), then another indicator TRH(Ϊ2) of the printhead temperature at a second time t2 may be determined at 425, wherein t2 is later than ti. The temperature indicator TRH(Ϊ2) may be determined similarly as TpH(ti) at 410 described above. The time difference t2 - ti may be equal to or different from the time difference ti - ti. For example, the second time t2 may be determined adaptively, depending on, for example, the indicator TpH(ti) of the printhead temperature at the first time ti. In particular, if TpH(ti) indicates that the temperature at the initial time ti minus the temperature at the first time ti is close to the threshold kn, then t2 may be determined such that t2 - ti is small.

[0060] At 430, it may be determined, based on the temperature indicator TRH(Ϊ2), if a condition for increasing the pressure inside the printhead is satisfied. In particular, the determination if the condition for increasing the pressure inside the printhead is satisfied at 430 may be implemented similarly as described for 420 with T pH (ti ) being replaced by TRH(Ϊ2).

[0061] If the condition for increasing the pressure inside the printhead is satisfied for the indicator TRH(Ϊ2) of the printhead temperature at the second time t2, then the PFDS may be controlled at 435 to be in the second state S2. [0062] The method depicted in Fig. 4 may be continued similarly as described above until an exit condition is satisfied. The exit condition may be, for example, that the difference between the temperature of the printhead and the ambient temperature is smaller than a predetermined threshold kexit. Alternatively, the exit criterion may be that the difference between a current time and the initial time ti is greater than some threshold. Other exit criteria are also possible.

[0063] Figure 5 shows a flow diagram of another example method for controlling the pressure inside the printhead in a cool-down period of the printhead. This method may be performed, for example, in the PFDS of Fig. 3 under the control of the controller 140.

[0064] The method starts with 405 and 410 explained above. Then, at 570, it may be determined if an exit condition is satisfied. In particular, it may be determined if the indicator TpH(ti) of the printhead temperature at the initial time ti indicates that the temperature of the printhead at the initial time ti minus the ambient temperature Tamb is smaller than a threshold kexit. If that condition is satisfied, the method may be stopped at 575, since the temperature of the printhead may hardly decrease further in a significant amount. The threshold kexit may be 3°C, 5°C, or 7°C, for example. The PFDS 100, 200 may comprise a temperature sensor for determining the ambient temperature Tamb.

[0065] As long as the exit condition is not yet satisfied, the following may be performed iteratively as depicted in Fig. 5.

[0066] At 515 and 520, indicators TpH(tn) of the printhead temperature at time instants t _n , n = 1 , 2, ... are determined iteratively up until an indicator TpH(tn) for the printhead temperature at the n-th time indicates that a condition for increasing the pressure inside the printhead is satisfied, similarly as described for 415 and 420 above. The index n may have been initialized to n = 1 before the method started (not shown in the figure). The index n is incremented in each iteration, and t _n is later than t _n -i. The time intervals t _n - t _n -i may be constant for all n, or the time intervals may be different for different n. Further, the times t _n may be predetermined, or the times t _n may be determined adaptively, depending on previous indicators of the printhead temperature, for example. If the indicator TpH(tn) of the temperature of the printhead at the n-th time t _n indicates that the temperature of the printhead at the time ti minus the temperature of the printhead at the time t _n is greater than a threshold kTm, it may be determined based on an assumed pressure within the printhead at the time ti, the printhead temperature at the time ti, and a model relating pressures inside the printhead with printhead temperatures during the cool-down period of the printhead that the pressure inside the printhead fell below a certain value, indicating that the pressure inside the printhead is to be increased in order to prevent the ingestion of air through the printhead nozzles. It is to be noted that different thresholds kTm may be utilized in different iterations of the outer loop depicted in Fig. 5, and the index m may have been initialized to m = 1 before the method started (not shown in the figure). [0067] If the condition for increasing the pressure within the printhead is satisfied for the indicator TpH(t _n ), then the PFDS can be controlled at 540 to be in the second state S2 to increase the pressure inside the printhead, similar as described above for 440.

[0068] If a condition for returning to the first state S1 is satisfied, the PFDS may be controlled to return to the first state S1 , similar as described above for 445. Moreover, the indicator TpH(ti) for the printhead temperature at the initial time ti may be updated, i.e. , TpH(ti) may be set equal to the indicator TpH(tn) of the printhead temperature at the n-th time t _n , so that the updated indicator is used in case of further iterations of the outer loop depicted in Fig. 5. In addition, the index n and/or the index m may be incremented.

[0069] Figure 6 shows a flow diagram of a further example method for controlling the pressure inside the printhead in a cool-down period of the printhead. This method may be performed, for example, in the PFDS of Fig. 3 under the control of the controller 140.

[0070] The method starts with 405 explained above. Then, if it is detected that the cool-down period of the printhead started, the PFDS is controlled at 610 to be in the first state S1 , similar as explained for 410.

[0071] At 615, an indicator PpH(ti) of the pressure within the printhead at the time ti is determined, wherein ti is later than the time at which the PFDS is controlled to be in the first state S1 . For example, the printhead may comprise a pressure sensor, and the controller 140 may receive the indicator PpH(ti) of the pressure within the printhead at the time ti from the printhead, wherein the pressure indicator PpH(ti) may be based on or equal to a measurement result provided by the pressure sensor within the printhead.

[0072] At 620, it is determined if a condition for increasing the pressure within the printhead is satisfied for the indicator PpH(ti) of the pressure within the printhead at the time ti. In particular, it may be determined based on the indicator PpH(ti) if the pressure inside the printhead is smaller than a threshold kp. The threshold kp can be larger than the pressure at which air starts to be ingested through the nozzles of the printhead. There may be a security margin between the threshold kp and the pressure at which air starts to be ingested through the nozzles of the printhead to ensure that no air is ingested. The threshold kp may be determined beforehand, and the threshold kp may depend on the printhead and/or the type of printing fluid, amongst others. Again, texts inside boxes of the figures can be simplified for the sake of conciseness, assuming for example that pressure indicators PpH(t) are actual pressures within the printhead and not just indicators thereof, see for example 620 in Fig. 6.

[0073] If it is determined at 620 that the condition for increasing the pressure within the printhead is satisfied, then the PFDS is controlled at 640 to transit from the first state S1 to the second state S2, i.e. , the PFDS may be pressurized to provide a higher pressure at the fluid outlet, similar as described above.

[0074] Then, if a condition for returning to the first state S1 is satisfied, the PFDS may be controlled at 645 to return to the first state. The condition may be that the PFDS was in the second state S2 for a first time-interval as described for 445 above. Alternatively, when the printhead comprises a pressure sensor providing pressure indicators PpH(t) to the controller 140 of the PFDS, the condition may be that another indicator PpH(t-ia) of the pressure within the printhead at a time tia > ti indicates that the pressure within the printhead is greater than a threshold kp2, wherein kp2 > kp. Moreover, the threshold kp2 may be smaller than the atmospheric pressure to ensure that printing fluid does not drool out of the nozzles of the printhead even when the regulator valve 318 of the printhead does not close properly. The threshold kp2 may be -0.5 kPa, -1 kPa, -1.5 kPa, or -2 kPa, for example. The threshold kp2 may depend on the printhead and/or the type of printing fluid, amongst others. Larger or smaller thresholds kp2 are possible as well.

[0075] Subsequently, an indicator RRH(Ϊ3) of the pressure within the printhead at a third time t3 may be determined at 650, wherein the pressure indicator RRH(Ϊ3) may be determined similarly as PpH(ti) described above. At 655, it may be determined if the condition for increasing the pressure within the printhead is satisfied for the indicator RRH(Ϊ3) of the pressure within the printhead at the third time t3. If this condition is satisfied, the PFDS may be controlled again to change from the first state S1 to the second state S2 at 660.

[0076] If it is determined at 620 that the condition for increasing the pressure within the printhead is not satisfied for PpH(ti), an indicator RRH(Ϊ2) of the pressure within the printhead at a second time t2 may be determined at 625, wherein the pressure indicator RRH(Ϊ2) may be determined similarly as PpH(ti) described above. At 630, it may be determined if the condition for increasing the pressure within the printhead is satisfied for the indicator RRH(Ϊ2) of the pressure within the printhead at the second time t2. If this condition is satisfied, the PFDS may be controlled to change from the first state S1 to the second state S2 at 635. The first, second, and third times ti, t2, and t3 may be determined as described for the method illustrated by Fig. 4.

[0077] The method depicted in Fig. 6 may be continued similarly as described above until an exit condition is satisfied, wherein the exit condition may depend on, for example, the time period between the last and the last but one transition of the PFDS from the first state S1 to the second state S2. Alternatively, the exit criterion may be that the difference between a current time and the initial time ti is greater than some threshold. Other exit criteria are possible as well.

[0078] Figure 7 shows a flow diagram of a further example method for controlling the pressure inside the printhead in a cool-down period of the printhead. This method may be performed, for example, in the PFDS of Fig. 3 under the control of the controller 140. In the method of Fig. 7, the printhead may not comprise a temperature or pressure sensor, and the controller 140 may not receive an indicator of the printhead temperature or the pressure within the printhead from the printhead. [0079] The method starts with 405 explained above.

[0080] Then, if it is detected that the cool-down period of the printhead started, the PFDS is controlled at 710 to be in the first state S1 , similar as explained for 410. Further, an initial time ti is determined. The initial time ti may be determined by querying an internal clock of the controller 140 to provide the current time, for example. The initial time ti may be the time at which the PFDS is controlled to be in the first state S1 in response to detecting that the cool-down period of the printhead started.

[0081] At 715, a first time ti > ti is determined, for example by querying again the internal clock of the controller to provide the current time. For example, if the printhead temperature at the initial time ti and the ambient temperature are known, then the printhead temperature at the first time ti may be determined based on a model of the printhead temperature as a function of time in the cool-down period. The printhead temperature at the initial time ti may be assumed to be an expected maximum temperature of the printhead during printing, for example. Alternatively, the printhead temperature at the initial time ti may be estimated based on parameters of the preceding print job. The ambient temperature may be determined by means of a temperature sensor of the PFDS, for example. Alternatively, the ambient temperature may be a parameter configurable by a user by means of a graphical user interface communicatively coupled to the controller 140, for example. The model of the printhead temperature during the cool-down period may be determined experimentally and may be stored in a look-up table of temperatures versus time. Flence, based on an assumed printhead temperature at the initial time ti, the ambient temperature, and the model of the printhead temperature in the cool-down period, temperatures of the printhead for different times may be determined, allowing to apply the method of Fig. 4 even if the printhead does not comprise a temperature sensor.

[0082] Further, as explained for 415, if the pressure within the printhead at the initial time ti, the printhead temperature at the initial time ti, and the printhead temperature at the first time ti are known, then the pressure within the printhead at the first time ti may be determined based on a model relating the pressure inside the printhead with the printhead temperature during the cool-down period, assuming that the PFDS is in the first state S1. Thus, the first time ti is itself an indicator of the pressure within the printhead at the first time ti. Thereby, the pressure within the printhead at the initial time ti may be assumed to be the backpressure kBP provided by the regulator valve, for example.

[0083] At 720, it is determined if a condition for increasing the pressure within the printhead is satisfied for the first time ti. As explained above, based on information about the printhead temperature at the initial time ti, the ambient temperature, and the model of the printhead temperature in the cool-down period, the printhead temperature at the first time ti can be determined. Further, based on information about the pressure within the printhead at the initial time ti, the printhead temperature at the initial time ti, the printhead temperature at the first time ti, and the model relating pressures inside the printhead with the printhead temperature, the pressure inside the printhead at the first time ti may be determined. Thus, it may also be determined if the pressure within the printhead fell below the threshold kp, in which case the PFDS should be controlled to change from the first state S1 to the second state S2 to prevent the ingestion of air through the printhead nozzles. The condition for increasing the pressure within the printhead may be satisfied when ti - ti is greater than a threshold kn, wherein the expected pressure within the printhead at the time ti + kn may be equal to kp, and/or wherein the expected temperature reduction from the initial time ti to the time ti + kn may be equal to kn. The threshold ku may be determined beforehand, for example experimentally, and the threshold kn may depend on the printhead and/or the type of printing fluid, amongst others.

[0084] If it is determined at 720 that the condition for increasing the pressure within the printhead is satisfied for the first time ti, then the PFDS is controlled at 740 to transit from the first state S1 to the second state S2, i.e. , the PFDS may be pressurized to provide a higher pressure at the fluid outlet, as described above. Then, if a condition for returning to the first state is satisfied, the PFDS may be controlled at 745 to return to the first state. The condition for returning to the first state S1 may be that the PFDS remained in the second state S2 for a first time- interval as described for 445 above. [0085] Subsequently, at 750, a third time t3 may be determined. Based on the assumed printhead temperature at the initial time ti, information about the ambient temperature, and the model of the printhead temperature as a function of time in the cool-down period, the printhead temperature at the third time t3 may be determined. Further, based on an assumed pressure within the printhead when the PFDS returns to the first state S1 , the temperature of the printhead when the PFDS returns to the first state S1 , the temperature of the printhead at the third time t3, and the model relating the pressure inside the printhead with the printhead temperature, the pressure inside the printhead at the third time t3 may be determined. Thus, the third time t3 may be itself an indicator for the pressure within the printhead at the third time t3. The pressure within the printhead when the PFDS returns to the first state S1 may be assumed, for example, to be equal to the backpressure kBP provided by the regulator valve 318. Further, temperature changes of the printhead during the first time-interval may be considered negligible, since the first time-interval may be short. Thus, the printhead temperature when the PFDS returns to the first state S1 may be assumed to be equal to the expected printhead temperature at the first time ti.

[0086] At 755, it may be determined if a condition for increasing the pressure within the printhead is satisfied for the third time t3. As explained above, based on the assumed printhead temperature at the initial time ti, information about the ambient temperature, and the model of the printhead temperature as a function of time in the cool-down period, the printhead temperature at the third time t3 may be determined. Further, based on the assumed pressure within the printhead when the PFDS returns to the first state S1 , the temperature of the printhead when the PFDS returns to the first state S1 , the temperature of the printhead at the third time t3, and the model relating the pressure inside the printhead with the printhead temperature, the pressure inside the printhead at the third time t3 may be determined. The condition for increasing the pressure within the printhead may be satisfied when the expected pressure within the printhead at the third time t3 is smaller than the threshold kp. Alternatively or equivalently, the condition for increasing the pressure within the printhead may be satisfied when t3 - ti is greater than a threshold kt2, wherein the expected pressure within the printhead at the time ti + kt2 may be equal to kp, and/or wherein the expected temperature reduction from the first time ti to the time ti + kt2 may be equal to kT2. The threshold kt2 may be determined beforehand, for example experimentally, and the threshold kt2 may be different from the threshold kn. In particular, the threshold kt2 may be larger than the threshold kn, owing to a decreasing rate of reduction of the printhead temperature in the cool-down period.

[0087] If it is determined at 755 that the condition for increasing the pressure inside the printhead is satisfied for the third time t3, then the PFDS may be controlled again to change from the first state S1 to the second state S2 at 760. [0088] If it is determined at 720 that the condition for increasing the pressure within the printhead is not satisfied for the first time ti, then a second time t2 > ti may be determined at 725. At 730, it may be determined if the condition for increasing the pressure within the printhead is satisfied for the second time t2, wherein this determination may be similar as described for the first time ti at 720. If the condition for increasing the pressure inside the printhead is satisfied for the second time t2, then the PFDS may be controlled to change from the first state S1 to the second state S2 at 735.

[0089] The method depicted in Fig. 7 may be continued similarly as described above until an exit condition is satisfied. The exit condition may be, for example, that the difference between the expected temperature of the printhead and the ambient temperature is smaller than the predetermined threshold kexit. Alternatively, the exit condition may be that the difference between the current time and the initial time ti is greater than some threshold.

[0090] It is to be understood that the methods illustrated by Figs. 4 to 7 have many similarities. Some aspects explained for one of these methods may be applicable similarly for another method, even if not mentioned for this method. [0091] Figure 8 illustrates the pressure inside a PFDS during the cool-down period of a printhead according to an example. In particular, Fig. 8 may illustrate the air pressure inside the intermediate tank 120 of the PFDS 100 shown in Figs. 1 and 3 relative to the ambient atmospheric pressure. Flence, Fig. 8 may depict the air pressure provided by the air pressure source 160 and the relief valve 166 of the PFDS 100 under the control of the controller 140. The pressure at the fluid outlet 130 of the PFDS 100 and the pressure within the printhead 300 may differ from the pressure as illustrated by Fig. 8, for example due to a difference in height between the top surface of the printing fluid inside the intermediate tank 120 on the one hand and the fluid outlet 130 or the printhead 300 on the other hand. [0092] Figure 8 shows the initial time ti at which the controller 140 of the PFDS 100 may detect that a print job is finished or paused. At the initial time ti, the PFDS 100 may be controlled to change from the second state S2 to the first state S1. As explained above, the PFDS 100 may be pressurized in the second state S2 and depressurized in the first state S1.

[0093] In the first state S1 , the air pressure inside the intermediate tank may therefore be equal to the ambient atmospheric pressure. In this state, the controller 140 may deactivate the air pressure source 160 and open the relief valve 166.

[0094] Fig. 8 also shows three short time-intervals after the initial time ti, wherein the PFDS 100 is in the second state S2. In this state, the air pressure source 160 may provide an air pressure of for example 30 kPa above the ambient atmospheric pressure, in order to raise the pressure within the printhead and to prevent that air is ingested through the printhead nozzles. The three time- intervals, in which the PFDS is in the second state S2, are short to prevent that large amounts of printing fluid drool out of the printhead nozzles when the regulator valve 318 does not close properly. The time difference between subsequent transitions from the first state S1 to the second state S2 may increase due to a logarithmically decaying printhead temperature.

[0095] At the time tend, an exit criterion may be satisfied. For example, it may be determined that the temperature of the printhead is close to the ambient temperature so that it can be expected that further reductions of the printhead temperature are small and do not lead to an ingestion of air through the printhead nozzles. The time difference between ti and tend may be two hours, for example. [0096] In one example, a method for controlling a pressure within a printhead in a cool-down period of the printhead is provided. A fluid inlet of the printhead is fluidly connected to a fluid outlet of a PFDS, wherein the PFDS comprises first and second states, and a pressure provided by the PFDS at the fluid outlet is higher in the second state as compared to the first state. The method comprises: controlling the PFDS to be in the first state. After controlling the PFDS to be in the first state, determining a first value, wherein the first value is indicative of the pressure within the printhead at a first time. Based on the first value, it is then determined if a condition for increasing the pressure within the printhead is satisfied. If the condition for increasing the pressure within the printhead is satisfied for the first value, the PFDS is controlled to change from the first state to the second state.

[0097] In another example, a PFDS is provided, which comprises a fluid outlet fluidly connectable to a fluid inlet of a printhead, first and second states, wherein a pressure provided by the PFDS at the fluid outlet is higher in the second state as compared to the first state, and a controller. The controller is to determine that a print job is finished or paused, and, in response to determining that a print job is finished or paused, control the PFDS to be in the first state. Moreover, the controller is to, after controlling the PFDS to be in the first state, determine a value indicative of the pressure within the printhead. In addition, the controller is to determine if a condition for increasing the pressure within the printhead is satisfied for the value, and if the condition for increasing the pressure within the printhead is satisfied for the value, the controller is to control the PFDS to change from the first state to the second state.

[0098] In a further example, a printing system comprising a PFDS and a printhead is provided, wherein a fluid outlet of the PFDS is fluidly connected to a fluid inlet of the printhead. The PFDS comprises a controller, wherein the controller is to determine that a print job is finished or paused. In response to determining that a print job is finished or paused, the controller is further to control the PFDS to be in a first state. Moreover, the controller is to, after controlling the PFDS to be in the first state, iteratively determine a value indicative of the pressure within the printhead, and control the PFDS to change from the first state to a second state for a predetermined time interval when the value indicates that a condition for increasing the pressure within the printhead is satisfied, wherein the PFDS is to provide a higher pressure in the second state as compared to the first state. [0099] In a further example, a computer program is provided. The computer program comprises machine readable instruction to cause a PFDS to control a pressure within a printhead in a cool-down period of the printhead, when a fluid inlet of the printhead is fluidly connected to a fluid outlet of the PFDS. The PFDS comprises first and second states, and a pressure provided by the PFDS at the fluid outlet of the PFDS is higher in the second state as compared to the first state. The computer program is to cause the PFDS to perform the following: controlling the PFDS to be in the first state. After controlling the PFDS to be in the first state, determining a first value, wherein the first value is indicative of the pressure within the printhead at a first time. Based on the first value, determining if a condition for increasing the pressure within the printhead is satisfied. If the condition for increasing the pressure within the printhead is satisfied for the first value, the PFDS is controlled to change from the first state to the second state. [0100] In a further example, a computer-readable medium is provided. The computer-readable medium has stored thereon the computer program to cause the PFDS to control the pressure within the printhead in the cool-down period of the printhead. The computer-readable medium can be a non-transitory computer- readable medium.

[0101] The foregoing outlines features of several examples to better understand the aspects of the present disclosure. The present disclosure may be used as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same or similar effects as the examples introduced herein.