Printing device and printing method

Abstract

To shorten time till printing start by quickly generating printing differential pressure between a supply ink storage section and a recovery ink storage section.SOLUTION: A return pump 552 is provided to feed ink from a recovery tank 531 to a supply tank 511, and differential pressure generation assistance for feeding ink from a recovery tank 531 to a return pipe 551 by a return liquid feeding section 55 is executed in parallel with execution of printing differential pressure generation (step S112). As a result, volume of an air layer Vra on an upper side of an air-liquid interface L2 is expanded and the air layer Vra is reduced in pressure in the recovery tank 531, and volume of an air layer Vfa on an upper side of an air-liquid interface L1 is compressed and the air layer Vfa is pressurized in the supply tank 511. Thus, generation of differential pressure between the recovery tank 531 and the supply tank 511 is assisted.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] This invention relates to a printing technology in which ink is ejected from an ejection head while being sent from the supply ink reservoir to the recovered ink reservoir via the ejection head by using a differential pressure generated between the supply ink reservoir, which stores ink to be supplied to the ejection head, and the recovered ink reservoir, which stores ink recovered from the ejection head. [Background technology]

[0002] Patent Document 1 describes a printing device that performs printing using an ejection head that ejects ink using an inkjet method. In particular, this printing device includes a supply subtank that stores ink to be supplied to the ejection head and a recovery subtank that stores ink recovered from the ejection head. A predetermined pressure difference is generated between the supply subtank and the recovery subtank by making the pressure in the recovery subtank lower than the pressure in the supply subtank. This pressure difference causes ink to be sent from the supply subtank to the recovery subtank via the ejection head. The ejection head then ejects the ink supplied from the supply subtank. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-146625 Summary of the Invention [Problem to be solved by the invention]

[0004] In order to supply the ink required for printing to the ejection head, a sufficient differential pressure (printing differential pressure) must be generated between the supply ink storage section (supply sub-tank) and the recovered ink storage section (recovery sub-tank). This printing differential pressure can be generated by a pressure adjustment section that adjusts the pressure inside the supply ink storage section and a pressure adjustment section that adjusts the pressure inside the recovered ink storage section. However, these pressure generation sections are unable to generate the printing differential pressure quickly, and it can sometimes take a long time before printing can begin.

[0005] The present invention has been made in consideration of the above-mentioned problems, and has an object to quickly generate a printing differential pressure between a supply ink reservoir and a recovered ink reservoir, thereby shortening the time until printing begins. [Means for solving the problem]

[0006] A printing device according to the present invention includes a discharge head having nozzles for discharging ink, a supply ink storage section for storing ink to be supplied to the discharge head, a recovered ink storage section for storing ink recovered from the discharge head, a return liquid sending section for sending ink from the recovered ink storage section to the supply ink storage section, a first pressure adjusting section for adjusting a first pressure applied to a supply gas-liquid interface which is a boundary between the ink stored in the supply ink storage section and air, a second pressure adjusting section for adjusting a second pressure applied to the recovered gas-liquid interface which is a boundary between the ink stored in the recovered ink storage section and air, and a control section for performing printing differential pressure generation which adjusts the first pressure by the first pressure adjusting section so that the first pressure becomes a supply pressure and adjusts the second pressure by the second pressure adjusting section so that the second pressure becomes a recovery pressure which is lower than the supply pressure. When the first pressure is adjusted to the supply pressure and the second pressure is adjusted to the recovery pressure and the printing differential pressure generation is completed, printing ink delivery is performed in which ink is delivered from the supply ink storage section to the recovered ink storage section via the ejection head by the printing differential pressure, which is the difference between the supply pressure and the recovery pressure, and the ejection head performs printing by ejecting ink supplied from the supply ink storage section from the nozzle in accordance with the printing ink delivery, and the control unit performs liquid level preparation in which the supply air-liquid interface is less than the first preparation liquid level and the recovery air-liquid interface is not less than the second preparation liquid level from the time printing by the ejection head is completed until the printing differential pressure generation is completed, and in parallel with the execution of the printing differential pressure generation, the return liquid delivery unit performs differential pressure generation assistance in which ink is delivered from the recovered ink storage section to the supply ink storage section.

[0007] The printing method according to the present invention includes a step of executing a printing differential pressure generation in which a first pressure adjustment unit adjusts a first pressure applied to a supply gas-liquid interface, which is a boundary between air and ink stored in a supply ink storage unit that stores ink to be supplied to a discharge head having nozzles that discharge ink, so that the first pressure becomes a supply pressure, and a second pressure adjustment unit adjusts a second pressure applied to a recovery gas-liquid interface, which is a boundary between air and ink stored in a recovery ink storage unit that stores ink recovered from the discharge head, so that the second pressure becomes a recovery pressure lower than the supply pressure; and the printing differential pressure generation is completed when the first pressure is adjusted to the supply pressure and the second pressure is adjusted to the recovery pressure. The method includes a step of performing printing ink delivery, in which ink is delivered from the supply ink reservoir to the recovered ink reservoir via the ejection head by a printing differential pressure, which is the difference between the supply pressure and the recovery pressure, and a step of causing the ejection head to perform printing by ejecting the ink supplied from the supply ink reservoir from the nozzle in conjunction with the printing ink delivery, wherein, before the generation of the printing differential pressure is completed, a liquid level preparation is performed to create a liquid level preparation state in which the supply air-liquid interface is below the first preparation liquid level and the recovery air-liquid interface is at least the second preparation liquid level, and in parallel with the execution of the printing differential pressure generation, a differential pressure generation assistance is performed to deliver ink from the recovered ink reservoir to the supply ink reservoir by a return liquid delivery unit that delivers ink from the recovered ink reservoir to the supply ink reservoir.

[0008] The present invention (printing device and printing method) configured in this manner includes a first pressure adjustment unit that adjusts a first pressure applied to the gas-liquid interface (supply gas-liquid interface) in the supply ink reservoir, and a second pressure adjustment unit that adjusts a second pressure applied to the gas-liquid interface (recovery gas-liquid interface) in the recovered ink reservoir. A printing differential pressure generation is performed in which the first pressure adjustment unit adjusts the first pressure so that the first pressure becomes the supply pressure, and the second pressure adjustment unit adjusts the second pressure so that the second pressure becomes a recovery pressure lower than the supply pressure. The difference between the supply pressure and the recovery pressure (printing differential pressure) generated by this printing differential pressure generation is used to perform printing ink delivery, in which ink is delivered from the supply ink reservoir to the recovered ink reservoir via the ejection head. The ejection head performs printing by ejecting ink supplied from the supply ink reservoir through the nozzles in conjunction with the printing ink delivery.

[0009] In particular, the present invention provides a return ink delivery unit that delivers ink from the recovered ink storage unit to the supply ink storage unit. In parallel with the execution of printing differential pressure generation, the return ink delivery unit executes differential pressure generation assistance, delivering ink from the recovered ink storage unit to the supply ink storage unit. As a result, the volume of the air layer above the recovered ink storage unit's gas-liquid interface expands, depressurizing the air layer, while in the supply ink storage unit, the volume of the air layer above the supply gas-liquid interface compresses, pressurizing the air layer. In this way, the generation of differential pressure between the recovered ink storage unit and the supply ink storage unit is assisted. During this process, a liquid level preparation is executed to create a liquid level preparation state in which the supply gas-liquid interface is below the first preparation level and the recovered gas-liquid interface is equal to or greater than the second preparation level, before the printing differential pressure generation is completed. This allows the differential pressure generation assistance to be executed after ensuring the compression width of the air layer in the supply ink storage unit and the expansion width of the air layer in the recovered ink storage unit. As a result, a printing differential pressure can be quickly generated between the supply ink reservoir and the collected ink reservoir, thereby shortening the time until printing can begin.

[0010] The control unit may be configured to terminate the differential pressure generation assistance when the supply gas-liquid interface reaches or exceeds the first final liquid level. This configuration prevents the amount of ink stored in the supply ink storage unit from becoming excessively large due to the execution of differential pressure generation assistance.

[0011] The control unit may be configured to terminate the differential pressure generation assistance when the recovered gas-liquid interface falls below the second final liquid level. This configuration prevents the amount of ink stored in the recovered ink storage unit from becoming too small due to the execution of the differential pressure generation assistance.

[0012] The printing device may also be configured so that, when printing by the ejection head is completed, the control unit stops the ink return flow unit from sending ink from the recovered ink reservoir to the supply ink reservoir, and performs liquid level preparation by lowering the supply gas-liquid interface and raising the recovered ink gas-liquid interface. In this configuration, the liquid level preparation can be performed by utilizing the differential pressure between the supply ink reservoir and the recovered ink reservoir that is generated at the time when printing by the ejection head is completed.

[0013] The printing device may also be configured such that, when the control unit stops ink delivery by the return delivery unit to prepare the liquid level, the control unit adjusts the first pressure by the first pressure adjustment unit while adjusting the second pressure by the second pressure adjustment unit, thereby reducing the difference between the first pressure and the second pressure from the printing differential pressure. In this configuration, the difference between the first pressure and the second pressure is reduced in advance before the pressure adjustments by the first pressure adjustment unit and the second pressure adjustment unit are stopped. This makes it possible to mitigate the impact on the ink meniscus formed in the nozzle when the pressure adjustments are stopped.

[0014] The printing device may also be configured to further include a buffer ink reservoir that stores ink, an ink supply unit that sends ink from the buffer ink reservoir to the collected ink reservoir, and an ink recovery unit that sends ink from the supply ink reservoir to the buffer ink reservoir, and the control unit controls the ink supply unit to send ink from the buffer ink reservoir to the collected ink reservoir, while controlling the ink recovery unit to send ink from the supply ink reservoir to the buffer ink reservoir, thereby performing liquid level preparation. With this configuration, liquid level preparation can be performed by replenishing ink from the buffer tank to the collected ink reservoir and recovering ink from the supply ink reservoir to the buffer tank.

[0015] The printing device may also be configured such that the first pressure adjustment unit includes a first pressure tank connected to the supply ink reservoir and a first pressure generation unit that generates a supply pressure in the first pressure tank, and applies the supply pressure generated in the first pressure tank to the supply gas-liquid interface of the supply ink reservoir. The second pressure adjustment unit includes a second pressure tank connected to the recovered ink reservoir and a second pressure generation unit that generates a recovery pressure in the second pressure tank, and applies the recovery pressure generated in the second pressure tank to the recovered ink gas-liquid interface of the recovered ink reservoir. In this configuration, where supply and recovery pressures are generated in the first and second pressure tanks, it takes time to generate the supply and recovery pressures due to the volumes of the first and second pressure tanks. Therefore, it is preferable to apply the present invention to quickly generate a printing pressure difference between the supply ink reservoir and the recovered ink reservoir.

[0016] The printing device may also be configured such that the first pressure generating unit includes an inlet pipe that introduces externally supplied compressed air into the first pressure tank and a first speed controller attached to the inlet pipe that limits the inflow of compressed air into the first pressure tank, and the second pressure generating unit includes an exhaust pump that exhausts the second pressure tank, an exhaust pipe connecting the exhaust pump and the second pressure tank, and a second speed controller attached to the exhaust pipe that limits the outflow of air from the second pressure tank to the exhaust pump. In a configuration in which the first and second speed controllers limit the inflow of air into the first and second pressure tanks, it takes time to generate supply and recovery pressures. Therefore, it is preferable to apply the present invention to quickly generate a printing differential pressure between the supply ink reservoir and the recovered ink reservoir.

[0017] The printing device may further include a purge execution unit that executes a purge by applying purge pressure to the supply gas-liquid interface to send ink from the supply ink reservoir to the ejection head and push the ink out of the nozzles of the ejection head, the purge execution unit executes the purge after the liquid level preparation is completed and before the printing differential pressure generation starts, and the control unit may be configured to cause the purge execution unit to end the purge when the supply gas-liquid interface has become less than the first preparation liquid level as a result of the execution of the purge execution unit. With this configuration, even if the liquid level preparation state created by the liquid level preparation state is disrupted as a result of the execution of the purge, the liquid level preparation state can be restored at the end of the purge.

[0018] The printing device may further include a supply control unit that controls the transfer of ink from the supply ink reservoir to the ejection head and a recovery control unit that controls the transfer of ink from the ejection head to the recovered ink reservoir. The supply control unit prohibits the transfer of ink from the supply ink reservoir to the ejection head while the printing differential pressure is being generated, but allows the transfer of ink from the supply ink reservoir to the ejection head after the printing differential pressure is generated. The recovery control unit prohibits the transfer of ink from the ejection head to the recovered ink reservoir while the printing differential pressure is being generated, but allows the transfer of ink from the ejection head to the recovered ink reservoir after the printing differential pressure is generated. With this configuration, the outflow of ink from the supply ink reservoir and the inflow of ink into the recovered ink reservoir are prohibited while the printing differential pressure is being generated. This allows the printing differential pressure to be generated quickly.

[0019] The printing device may also be configured so that the differential pressure generation assistance is initiated after the liquid level preparation is complete.

[0020] The printing device may also be configured so that the generation of the printing differential pressure is initiated after printing by the ejection head is completed and before the differential pressure generation assistance is initiated. [Effects of the Invention]

[0021] As described above, according to the present invention, it is possible to quickly generate a printing differential pressure between the supply ink reservoir and the recovered ink reservoir, thereby shortening the time until printing starts. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a diagram schematically illustrating an example of a printing apparatus according to the present invention. [Figure 2] FIG. 2 is a diagram schematically illustrating a discharge head and an ink delivery mechanism provided for the discharge head. [Figure 3] FIG. 3 is a block diagram showing an electrical configuration of the printing apparatus for controlling the ink delivery mechanism of FIG. 2. [Figure 4] 4 is a flowchart showing an example of liquid transfer control executed by the printing apparatus of FIG. 1. [Figure 5A] FIG. 5 is a diagram schematically illustrating an example of an operation executed in accordance with the flowchart of FIG. 4. [Figure 5B] FIG. 5 is a diagram schematically illustrating an example of an operation executed in accordance with the flowchart of FIG. 4. [Figure 6A] FIG. 5 is a diagram schematically showing a modified example of the operation executed in accordance with the flowchart of FIG. 4. [Figure 6B] FIG. 5 is a diagram schematically showing a modified example of the operation executed in accordance with the flowchart of FIG. 4. [Figure 7] 10 is a flowchart showing a modified example of liquid transfer control executed by the printing apparatus of FIG. [Figure 8] 10A and 10B are diagrams schematically illustrating modified examples of a discharge head and an ink delivery mechanism provided for the discharge head. DETAILED DESCRIPTION OF THE INVENTION

[0023] 1 is a diagram schematically illustrating an example of a printing device according to the present invention. The printing device 1 includes a transport unit 2 that transports a print medium 10 roll-to-roll, and an ink ejection unit 3 that ejects ink onto the print medium 10. An image is printed on the print medium 10 by ejecting ink from the ink ejection unit 3 onto the print medium 10 in synchronization with the transport of the print medium 10 by the transport unit 2.

[0024] The transport unit 2 has a payout roller 21u and a take-up roller 21w, and transports the printing medium 10 by having the take-up roller 21w take up the printing medium 10 that has been paid out by the payout roller 21u. The transport unit 2 also has support rollers 23 and 24 that support the printing medium 10 between the payout roller 21u and the take-up roller 21w, and the ink discharge unit 3 discharges ink onto the printing medium 10 that is transported from the support roller 23 to the support roller 24. The transport unit 2 also has rollers 25 and 26 that support the printing medium 10 that is transported from the payout roller 21u to the support roller 23, and rollers 27 and 28 that support the printing medium 10 that is transported from the support roller 24 to the take-up roller 21w.

[0025] The ink ejection section 3 has a plurality of head units 31 arranged in the transport direction of the print medium 10. The plurality of head units 31 eject ink of different colors (for example, black, cyan, magenta, and yellow) using an inkjet system. Each ink ejection section 3 has an ejection head 4 that ejects ink using an inkjet system. Next, the ejection head 4 and the ink delivery mechanism that delivers ink to the ejection head 4 will be described.

[0026] FIG. 2 is a diagram schematically illustrating the ejection head 4 and an ink delivery mechanism provided for the ejection head 4, and FIG. 3 is a block diagram illustrating the electrical configuration of the printing device for controlling the ink delivery mechanism of FIG. 2. As shown in FIG. 3, the printing device 1 includes a control unit 100. The control unit 100 is configured with a processor such as a CPU (Central Processing Unit) or an FPGA (Field Programmable Gate Array), a memory, etc. The printing device 1 also includes a UI (User Interface) 110. The UI 110 is configured, for example, with a touch panel display, and can accept input operations by an operator and transmit them to the control unit 100, and display information to the operator based on commands from the control unit 100.

[0027] As shown in FIG. 2, the ejection head 4 has a housing 41, and a plurality of nozzles 42 are opened in a horizontally staggered arrangement on the bottom surface of the housing 41. Inside the housing 41, a plurality of cavities 43 are provided, each communicating with the plurality of nozzles 42, and an ink supply chamber 44 is provided, each communicating with the plurality of cavities 43. Each cavity 43 stores ink supplied from the ink supply chamber 44. Each cavity 43 is provided with a piezoelectric element 45, and the piezoelectric element 45 is displaced in response to a drive signal (electrical signal), thereby applying a pressure fluctuation to the ink in the cavity 43. This pressure fluctuation pushes the ink out of the cavity 43, and the ink is ejected from the nozzle 42 communicating with the cavity 43. An ink inlet 46 and an ink outlet 47 are opened on the top of the housing 41. As will be described next, ink that flows from the ink delivery mechanism 5 into the ink supply chamber 44 through the ink inlet 46 flows out from the ink supply chamber 44 to the ink delivery mechanism 5 through the ink outlet 47 .

[0028] The printing device 1 includes an ink delivery mechanism 5 shown in Fig. 2. The ink delivery mechanism 5 includes an ink supply unit 5A that supplies ink to the ejection head 4, an ink recovery unit 5B that recovers ink from the ejection head 4, an ink return unit 5C that delivers ink from the ink recovery unit 5B to the ink supply unit 5A, and an ink delivery amount adjustment unit 5D that adjusts the amount of ink delivered to the ejection head 4.

[0029] The ink supply unit 5A has a supply liquid delivery unit 51 that supplies ink to the ejection head 4. The supply liquid delivery unit 51 has a supply tank 511 that stores ink to be supplied to the ejection head 4, and a pipe 512 that connects the supply tank 511 to the ink inlet 46 of the ejection head 4, and the ink is delivered from the supply tank 511 to the ejection head 4 via the pipe 512. The supply liquid delivery unit 51 also has a head valve 513 provided on the pipe 512. When the control unit 100 opens the head valve 513, the delivery of ink from the supply tank 511 to the ejection head 4 via the pipe 512 is permitted. When the control unit 100 closes the head valve 513, the delivery of ink from the supply tank 511 to the ejection head 4 via the pipe 512 is prohibited.

[0030] The ink supply unit 5A also has a pressure adjustment unit 52 that adjusts the pressure P1 inside the supply tank 511. Specifically, air accumulates in the supply tank 511 above the gas-liquid interface L1, which is the liquid level of the ink, forming an air layer, and the pressure adjustment unit 52 adjusts the pressure P1 applied to the gas-liquid interface L1. The pressure adjustment unit 52 has a pressure tank 521 that stores air, a pipe 522 that connects the air layers of the pressure tank 521 and the supply tank 511, and a tank valve 523 attached to the pipe 522. When the control unit 100 opens the tank valve 523, the pressure tank 521 and the supply tank 511 communicate with each other via the pipe 522, and the pressures of the air layers in the pressure tank 521 and the supply tank 511 become equal. When the control unit 100 closes the tank valve 523, the pressure tank 521 and the supply tank 511 are disconnected. In this embodiment, the tank valve 523 is basically always open.

[0031] The pressure adjustment unit 52 has an inlet pipe 524 that introduces compressed air into the pressure tank 521, and a pressurization valve 525 attached to the inlet pipe 524. When the control unit 100 opens the pressurization valve 525, compressed air is introduced from the inlet pipe 524 into the pressure tank 521, and the air layer in the pressure tank 521 is pressurized. When the control unit 100 closes the pressurization valve 525, the introduction of compressed air from the inlet pipe 524 into the pressure tank 521 is prohibited. Furthermore, the pressure adjustment unit 52 has an inlet pipe 526 that introduces atmospheric pressure into the pressure tank 521, and a release valve 527 attached to the inlet pipe 526. When the control unit 100 opens the release valve 527, the pressure tank 521 is released to atmospheric pressure via the inlet pipe 526. When the control unit 100 closes the release valve 527, the pressure tank 521 is isolated from atmospheric pressure.

[0032] Furthermore, pressure adjustment unit 52 has a pressure detector 528 attached to pipe 522 between pressure tank 521 and tank valve 523, and this pressure detector 528 detects the pressure inside pipe 522, i.e., the pressure inside pressure tank 521, and outputs the detected pressure to control unit 100. Therefore, control unit 100 can adjust pressure P1 applied to gas-liquid interface L1 of supply tank 511 by controlling the opening and closing of pressurizing valve 525 and release valve 527 based on the pressure detected by pressure detector 528 while tank valve 523 is opened to connect supply tank 511 and pressure tank 521.

[0033] The ink recovery unit 5B has a recovery liquid transfer unit 53 that recovers ink from the ejection head 4. The recovery liquid transfer unit 53 has a recovery tank 531 that stores ink recovered from the ejection head 4, and a pipe 532 that connects the recovery tank 531 to the ink outlet 47 of the ejection head 4, and transfers ink from the ejection head 4 to the recovery tank 531 via the pipe 532. The recovery liquid transfer unit 53 also has a head valve 533 provided on the pipe 532. When the control unit 100 opens the head valve 533, the transfer of ink from the ejection head 4 to the recovery tank 531 via the pipe 532 is permitted. When the control unit 100 closes the head valve 533, the transfer of ink from the ejection head 4 to the recovery tank 531 via the pipe 532 is prohibited.

[0034] The ink recovery unit 5B also has a pressure adjustment unit 54 that adjusts the pressure P2 inside the recovery tank 531. Specifically, air accumulates in the recovery tank 531 above the gas-liquid interface L2, which is the ink liquid level, forming an air layer, and the pressure adjustment unit 54 adjusts the pressure P2 applied to the gas-liquid interface L2. The pressure adjustment unit 54 has a pressure tank 541 that stores air, a pipe 542 that connects the air layers of the pressure tank 541 and the recovery tank 531, and a tank valve 543 attached to the pipe 542. When the control unit 100 opens the tank valve 543, the pressure tank 541 and the recovery tank 531 communicate with each other via the pipe 542, and the pressures of the air layers in the pressure tank 541 and the recovery tank 531 become equal. When the control unit 100 closes the tank valve 543, the pressure tank 541 and the recovery tank 531 are disconnected. In this embodiment, the tank valve 543 is basically always open.

[0035] The pressure adjustment unit 54 has an exhaust pipe 544 connecting the exhaust pump 549 and the pressure tank 541, and an exhaust valve 545 attached to the exhaust pipe 544. When the control unit 100 opens the exhaust valve 545, the exhaust pump 549 exhausts air from the collection tank 531 through the exhaust pipe 544, thereby reducing the pressure of the air layer in the collection tank 531. When the control unit 100 closes the exhaust valve 545, the exhaust pump 549 is prohibited from exhausting air from the collection tank 531 through the exhaust pipe 544. Furthermore, the pressure adjustment unit 54 has an inlet pipe 546 that introduces atmospheric pressure into the pressure tank 541, and a release valve 547 attached to the inlet pipe 546. When the control unit 100 opens the release valve 547, the pressure tank 541 is released to atmospheric pressure through the inlet pipe 546. When the control unit 100 closes the release valve 547, the pressure tank 541 is isolated from atmospheric pressure.

[0036] Furthermore, pressure adjustment unit 54 has a pressure detector 548 attached to pipe 542 between pressure tank 541 and tank valve 543, and this pressure detector 548 detects the pressure inside pipe 542, i.e., the pressure inside pressure tank 541, and outputs the detected pressure to control unit 100. Therefore, control unit 100 can adjust pressure P2 applied to gas-liquid interface L2 of recovery tank 531 by controlling the opening and closing of exhaust valve 545 and release valve 547 based on the pressure detected by pressure detector 548 while opening tank valve 543 to connect recovery tank 531 and pressure tank 541 and causing exhaust pump 549 to perform exhaust.

[0037] In the above configuration, the control unit 100 controls the pressure adjustment unit 52 to adjust the pressure P1 in the supply tank 511 to a supply pressure Pf, and also controls the pressure adjustment unit 54 to adjust the pressure P2 in the recovery tank 531 to a recovery pressure Pr that is lower than the supply pressure Pf. Here, the supply pressure Pf is a positive pressure higher than atmospheric pressure, and the recovery pressure Pr is a negative pressure lower than atmospheric pressure. In this way, a printing differential pressure ΔPp=(Pf-Pr) is generated between the supply tank 511 and the recovery tank 531. When this printing differential pressure ΔPp is generated with the head valve 513 and the head valve 533 open, ink is sent along the liquid sending path Ca that runs from the supply tank 511 to the recovery tank 531 via the ejection head 4.

[0038] The ink return unit 5C has a return liquid sending unit 55 that sends ink from the recovery tank 531 to the supply tank 511. The return liquid sending unit 55 has a return pipe 551 that connects the recovery tank 531 and the supply tank 511, and a return pump 552 that is provided on the return pipe 551 between the recovery tank 531 and the supply tank 511. The return pump 552 sends ink from the recovery tank 531 to the supply tank 511. Therefore, by causing the return pump 552 to send ink, the control unit can send ink along the liquid sending path Cb that leads from the recovery tank 531 to the supply tank 511 via the return pipe 551. The return liquid sending unit 55 also has a return valve 553 that is attached to the return pipe 551 between the return pump 552 and the supply tank 511. When the control unit 100 opens the return valve 553, the return pump 552 is allowed to send ink along the liquid sending path Cb, and when the control unit 100 closes the return valve 553, the return pump 552 is prohibited from sending ink along the liquid sending path Cb.

[0039] The ink feed rate adjustment unit 5D has a buffer tank 56, a recovery and delivery unit 57 that recovers ink from the supply tank 511 to the buffer tank 56, and a replenishment and delivery unit 58 that replenishes ink from the buffer tank 56 to the recovery tank 531. The buffer tank 56 stores ink in a capacity larger than both the supply tank 511 and the recovery tank 531. The recovery and delivery unit 57 has a recovery pipe 571 that connects the supply tank 511 and the buffer tank 56, and a recovery pump 572 attached to the recovery pipe 571 between the supply tank 511 and the buffer tank 56. The recovery pump 572 delivers ink from the supply tank 511 to the buffer tank 56 along the recovery pipe 571. The replenishment and delivery unit 58 has a refill pipe 581 that connects the buffer tank 56 and the recovery tank 531, and a refill pump 582 attached to the refill pipe 581 between the buffer tank 56 and the recovery tank 531. The supply pump 582 sends ink from the buffer tank 56 toward the recovery tank 531 along the supply pipe 581. Therefore, by causing the recovery pump 572 and the supply pump 582 to send ink, the control unit 100 can send ink along the liquid sending path Cc from the supply tank 511 to the recovery tank 531 via the buffer tank 56.

[0040] The ink delivery mechanism 5 also has a liquid level detector 591 that detects the gas-liquid interface L1 of the ink stored in the supply tank 511, and a liquid level detector 592 that detects the gas-liquid interface L2 of the ink stored in the recovery tank 531. The liquid level detected by the liquid level detector 591 is transmitted from the liquid level detector 591 to the control unit 100, and the liquid level detected by the liquid level detector 592 is transmitted from the liquid level detector 592 to the control unit 100.

[0041] As shown in FIG. 2, the printing apparatus 1 also has a purge mechanism 6. The purge mechanism 6 has an inlet pipe 61 that introduces compressed air into the air layer of the supply tank 511, and a purge valve 62 attached to the inlet pipe 61. When the control unit 100 opens the purge valve 62, compressed air is introduced from the inlet pipe 61 to the gas-liquid interface L1 in the supply tank 511, and this compressed air pushes down the gas-liquid interface L1. As a result, ink flows from the supply tank 511 through the pipe 512 into the ejection head 4, causing the ink to flow out of the nozzles 42 of the ejection head 4 (purging). The control unit 100 can open the head valve 513 to allow ink to flow from the supply tank 511 into the ejection head 4, thereby permitting purging, or close the head valve 513 to prohibit purging.

[0042] Fig. 4 is a flowchart showing an example of liquid delivery control executed by the printing apparatus of Fig. 1, and Figs. 5A and 5B are diagrams schematically showing an example of the operation executed in accordance with the flowchart of Fig. 4. The flowchart of Fig. 4 is executed by the control unit 100. Figs. 5A and 5B show the levels Le, Lm, and Lf (heights) at which ink detection is executed. Here, the middle level Lm is higher than the empty level Le, and the full level Lf is higher than the middle level Lm.

[0043] The liquid level detector 591, which detects ink in the supply tank 511, detects the presence or absence of ink at the empty level Le and the middle level Lm. If the liquid level detector 591 detects ink at the middle level Lm, the control unit 100 can determine that the gas-liquid interface L1 in the supply tank 511 is equal to or higher than the middle level Lm. If the liquid level detector 591 does not detect ink at the middle level Lm but does detect ink at the empty level Le, the control unit 100 can determine that the gas-liquid interface L1 in the supply tank 511 is equal to or higher than the empty level Le but lower than the middle level Lm. If the liquid level detector 591 does not detect ink at the empty level Le, the control unit 100 can determine that the gas-liquid interface L1 in the supply tank 511 is lower than the empty level Le.

[0044] The liquid level detector 592, which detects ink in the collection tank 531, detects the presence or absence of ink at the empty level Le, the middle level Lm, and the full level Lf. If the liquid level detector 592 detects ink at the full level Lf, the control unit 100 can determine that the gas-liquid interface L2 in the collection tank 531 is equal to or higher than the full level Lf. If the liquid level detector 592 does not detect ink at the full level Lf but detects ink at the middle level Lm, the control unit 100 can determine that the gas-liquid interface L2 in the collection tank 531 is equal to or higher than the middle level Lm but lower than the full level Lf. If the liquid level detector 592 does not detect ink at the middle level Lm but detects ink at the empty level Le, the control unit 100 can determine that the gas-liquid interface L2 in the collection tank 531 is equal to or higher than the empty level Le but lower than the middle level Lm. When the liquid level detector 592 does not detect ink at the empty level Le, the control unit 100 can determine that the gas-liquid interface L2 in the recovery tank 531 is below the empty level Le.

[0045] Such liquid level detectors 591 and 592 can be configured with a plurality of float sensors or the like provided at different heights. Note that the levels Le and Lm set for the supply tank 511 do not necessarily have to be equal to the levels Le and Lm set for the recovery tank 531, respectively.

[0046] 4, ink is sent along the liquid sending path Ca by the printing differential pressure ΔPp generated between the supply tank 511 and the recovery tank 531, and the ejection head 4 performs printing by ejecting ink from the nozzles 42. During this printing, the control unit 100 monitors whether the gas-liquid interface L1 in the supply tank 511 is equal to or higher than the middle level Lm based on the detection result of the liquid level detector 591 (step S101). If the gas-liquid interface L1 is not equal to or higher than the middle level Lm ("NO" in step S101), the control unit 100 starts the return pump 552 to send ink from the recovery tank 531 to the supply tank 511 along the liquid sending path Cb (step S103). This causes the gas-liquid interface L1 in the supply tank 511 to rise. On the other hand, if the gas-liquid interface L1 is equal to or higher than the middle level Lm (YES in step S101), the control unit 100 stops the return pump 552 to stop the ink transfer from the recovery tank 531 to the supply tank 511 along the liquid transfer path Cb (step S103). This stops the rise of the gas-liquid interface L1 in the supply tank 511.

[0047] During the execution of steps S101 to S103, if a decrease in the liquid level at the gas-liquid interface L2 in the recovery tank 531 is confirmed due to the ejection of ink from the ejection head 4 that is performing printing, the control unit 100 causes the supply pump 582 to send ink from the buffer tank 56 to the recovery tank 531, thereby replenishing the ink to the recovery tank 531. In addition, the recovery pump 572 may basically be stopped.

[0048] The control unit 100 continues to control the gas-liquid interface L1 (steps S101 to S103) in this manner until it is determined in step S104 that the circulation should be stopped. For example, it is determined to stop the circulation when the printing performed in parallel with steps S101 to S103 is completed and no further printing is scheduled, or when the operator inputs a command to stop the circulation into the UI 110. Here, circulation refers to the operation of sending ink along the liquid sending path Ca by the printing differential pressure ΔPp and sending ink along the liquid sending path Cb by the return pump 552. In other words, it refers to the operation of sending ink by the control of steps S101 to S103.

[0049] If it is determined in step S104 that the circulation should be stopped (YES), the control unit 100 stops the return pump 552 (step S105). The control unit 100 also reduces the differential pressure between the pressures P1 and P2 from the printing differential pressure ΔPp (step S106). This reduction in the differential pressure can be achieved by intermittently opening the release valves 527 and 547 to intermittently open the supply tank 511 and the recovery tank 531 to the atmosphere.

[0050] Since the return pump 552 was stopped in step S105, the transfer of ink from the recovery tank 531 via the liquid transfer path Cb is prohibited. Meanwhile, the differential pressure between the pressures P1 and P2 is smaller than the printing differential pressure ΔPp but larger than zero, and the pressure P2 is lower than the pressure P1. Therefore, the ink is transferred from the supply tank 511 to the recovery tank 531 via the liquid transfer path Ca. As a result, the gas-liquid interface L1 in the supply tank 511 decreases over time, and the gas-liquid interface L2 in the recovery tank 531 increases over time. By executing steps S105 and S106, the control unit 100 adjusts the gas-liquid interface L1 and the gas-liquid interface L2 to the liquid level state (liquid level preparation state) shown in FIG. 5A (liquid level preparation).

[0051] 5A, the gas-liquid interface L1 in the supply tank 511 is below the middle level Lm, and the gas-liquid interface L2 in the recovery tank 531 is above the full level Lf (liquid level preparation state). In other words, when the gas-liquid interface L1 becomes below the middle level Lm and the gas-liquid interface L2 becomes above the full level Lf, the control unit 100 determines that the liquid level preparation is complete and stops the ink feed along the liquid feed path Ca due to the differential pressure between the pressures P1 and P2 (differential pressure liquid feed) (step S108). Specifically, the differential pressure liquid feed can be stopped by closing the pressurizing valve 525 and opening the release valve 527 to set the pressure P1 to atmospheric pressure, and by closing the exhaust valve 545 and opening the release valve 547 to set the pressure P2 to atmospheric pressure. Then, the control unit 100 closes the head valve 513 and the head valve 533 to prohibit the transfer of ink from the supply tank 511 to the ejection head 4 and from the ejection head 4 to the recovery tank 531 (step S109).

[0052] In step S110, the control unit 100 determines whether to start circulation. For example, when a command indicating printing is input to the UI 110, the control unit 100 determines to start circulation ("YES" in step S110) and starts generating a differential pressure (step S111). That is, the control unit 100 closes the release valve 527 and opens the pressure valve 525 to start pressurizing the supply tank 511. The control unit 100 also closes the release valve 547 and opens the exhaust valve 545 to start depressurizing the collection tank 531. As a result, the pressure P1 in the supply tank 511 increases and the pressure P2 in the collection tank 531 decreases, increasing the difference (differential pressure) between the pressure P1 and the pressure P2. At this time, the head valves 513 and 533 are closed, and ink delivery along the liquid delivery path Ca from the supply tank 511 to the collection tank 531 via the ejection head 4 is prohibited.

[0053] Furthermore, the control unit 100 starts the return pump 552 (step S112). The ink is sent from the recovery tank 531 to the supply tank 511 via the liquid sending path Cb. As a result, as shown in FIG. 5B, in the supply tank 511, the gas-liquid interface L1 rises and the air layer Vfa above the gas-liquid interface L1 is compressed. Therefore, the air layer Vfa is pressurized and the pressure P1 increases. Furthermore, in the recovery tank 531, the gas-liquid interface L2 drops and the air layer Vra above the gas-liquid interface L2 expands. Therefore, the air layer Vra is decompressed and the pressure P2 decreases. In other words, the liquid sending by the return pump 552 assists in generating a pressure difference between the pressures P1 and P2.

[0054] In step S113, the control unit 100 determines whether the conditions for ending the differential pressure generation assistance by the ink supply from the return pump 552 are met. Specifically, the following conditions are met: The gas-liquid interface L1 in the supply tank 511 is equal to or higher than the middle level Lm. The gas-liquid interface L2 in the recovery tank 531 is below the empty level Le. If at least one of the above is satisfied, it is determined that the termination condition is satisfied (YES in step S113).

[0055] When the termination condition is thus satisfied, the control unit 100 stops the return pump 552 (step S114), thereby stopping the transfer of ink from the recovery tank 531 to the supply tank 511 via the liquid transfer path Cb.

[0056] The control unit 100 also determines whether generation of the differential pressure has been completed. Specifically, the control unit 100 determines whether the differential pressure between the pressure P1 in the supply tank 511 and the pressure P2 in the collection tank 531 has reached the printing differential pressure ΔPp based on the pressures detected by the pressure detectors 528 and 548 (step S115). When the differential pressure between the pressures P1 and P2 reaches the printing differential pressure ΔPp ("YES" in step S115), the control unit 100 stops pressurizing the supply tank 511 and stops depressurizing the collection tank 531 (step S116). That is, the control unit 100 closes the pressurizing valve 525 and the exhaust valve 545. Next, the control unit 100 opens the head valves 513 and 533 (step S117). This starts the ink delivery along the liquid delivery path Ca from the supply tank 511 to the collection tank 531 via the ejection head 4. Then, the process returns to step S101, and ink delivery control for printing is started (steps S101 to S103).

[0057] In the embodiment described above, the system includes a pressure adjustment unit 52 (first pressure adjustment unit) that adjusts a pressure P1 (first pressure) applied to a gas-liquid interface L1 (supply gas-liquid interface) in the supply tank 511 (supply ink storage unit), and a pressure adjustment unit 54 (second pressure adjustment unit) that adjusts a pressure P2 (second pressure) applied to a gas-liquid interface L2 (recovery gas-liquid interface) in the recovery tank 531 (recovery ink storage unit). Then, a printing differential pressure generation is performed in which the pressure adjustment unit 52 adjusts the pressure P1 so that the pressure P1 becomes the supply pressure Pf, and the pressure adjustment unit 54 adjusts the pressure P2 so that the pressure P2 becomes a recovery pressure Pr that is lower than the supply pressure Pf (step S111). A printing differential pressure ΔPp, which is the difference between the supply pressure Pf and the recovery pressure Pr generated by this printing differential pressure generation, sends ink from the supply tank 511 to the recovery tank 531 via the ejection head 4. Furthermore, the ejection head 4 executes printing by ejecting ink supplied from the supply tank 511 from the nozzles 42 in accordance with the printing ink delivery.

[0058] In particular, in this embodiment, a return pump 552 is provided that sends ink from the recovery tank 531 to the supply tank 511, and in parallel with the execution of printing differential pressure generation, the return liquid sending unit 55 executes differential pressure generation assistance by sending ink from the recovery tank 531 to the return piping 551 (step S112). As a result, in the recovery tank 531, the volume of the air layer Vra above the gas-liquid interface L2 expands and the air layer Vra is depressurized, and in the supply tank 511, the volume of the air layer Vfa above the gas-liquid interface L1 compresses and the air layer Vfa is pressurized. In this way, the generation of the differential pressure between the recovery tank 531 and the supply tank 511 is assisted. At this time, before starting printing differential pressure generation (step S111), liquid level preparation is performed (steps S105 to S107) to create a liquid level preparation state (FIG. 5A) in which the gas-liquid interface L1 in the supply tank 511 is at middle level Lm (less than the first preparation liquid level) and the gas-liquid interface L2 in the collection tank 531 is at full level Lf (not less than the second preparation liquid level). This allows differential pressure generation assistance (step S112) to be performed after ensuring the compression range of the air layer Vfa in the supply tank 511 and the expansion range of the air layer Vra in the collection tank 531. As a result, printing differential pressure ΔPp can be quickly generated between the supply tank 511 and the collection tank 531, shortening the time until printing begins.

[0059] Furthermore, when the gas-liquid interface L1 in the supply tank 511 reaches or exceeds the middle level Lm (first final liquid level), the control unit 100 ends the differential pressure generation assistance (step S113). With this configuration, it is possible to prevent the amount of ink stored in the supply tank 511 from becoming excessive due to the execution of the differential pressure generation assistance.

[0060] Furthermore, the control unit 100 ends the differential pressure generation assistance when the gas-liquid interface L2 in the collection tank 531 falls below the empty level Le (second final liquid level) (step S113). With this configuration, it is possible to prevent the amount of ink stored in the collection tank 531 from becoming too small due to the execution of the differential pressure generation assistance.

[0061] Furthermore, when printing by the ejection head 4, which is performed in parallel with steps S101 to S103, is completed, the control unit 100 stops the return liquid sending unit 55 from sending ink from the recovery tank 531 to the supply tank 511 (step S105), and performs liquid level preparation by lowering the gas-liquid interface L1 in the supply tank 511 and raising the gas-liquid interface L2 in the recovery tank 531 (steps S105 to S107). With this configuration, the liquid level preparation can be performed by utilizing the differential pressure between the supply tank 511 and the recovery tank 531 that is generated at the time when printing by the ejection head 4 is completed.

[0062] Furthermore, when the control unit 100 stops the ink delivery by the return delivery unit 55 to prepare the liquid level, it reduces the difference between the pressures P1 and P2 from the printing differential pressure ΔPp by adjusting the pressure P1 by the pressure adjustment unit 52 and adjusting the pressure P2 by the pressure adjustment unit 54 (step S106). In this configuration, the difference between the pressures P1 and P2 is reduced in advance before step S108, in which the pressure adjustment by the pressure adjustment units 52 and 54 is stopped. Therefore, it is possible to mitigate the impact applied to the ink meniscus formed in the nozzle 42 when the pressure adjustment is stopped.

[0063] The pressure adjusting unit 52 also has a pressure tank 521 (first pressure tank) connected to the supply tank 511, and an inlet pipe 524 and a pressurizing valve 525 (first pressure generating unit) that generate a supply pressure Pf by introducing compressed air into the pressure tank 521. The supply pressure Pf generated in the pressure tank 521 is applied to the gas-liquid interface L1 of the supply tank 511. The pressure adjusting unit 54 also has a pressure tank 541 (second pressure tank) connected to the recovery tank 531, and an exhaust pipe 544, an exhaust valve 545, and an exhaust pump 549 (second pressure generating unit) that generate a recovery pressure Pr by exhausting the pressure tank 541. The recovery pressure Pr generated in the pressure tank 541 is applied to the gas-liquid interface L2 of the recovery tank 531. In this configuration in which supply pressure Pf and recovery pressure Pr are generated in pressure tanks 521 and 541, fluctuations in the compressed air introduced through inlet pipe 524 and in the exhaust air from exhaust pump 549 are absorbed by the volume of pressure tanks 521 and 541, thereby suppressing the influence on the ink meniscus formed in nozzle 42. However, the volume of pressure tanks 521 and 541 is a factor in the time required to generate supply pressure Pf and recovery pressure Pr. Therefore, it is preferable to perform differential pressure generation assistance as described above so that the printing differential pressure ΔPp can be quickly generated between supply tank 511 and recovery tank 531.

[0064] The system also includes a head valve 513 (supply control unit) that controls the transfer of ink from the supply tank 511 to the ejection head 4, and a head valve 533 (recovery control unit) that controls the transfer of ink from the ejection head 4 to the recovery tank 531. The head valve 513 prohibits the transfer of ink from the supply tank 511 to the ejection head 4 while the printing differential pressure is being generated (steps S111 to S115), but allows the transfer of ink from the supply tank 511 to the ejection head 4 after the printing differential pressure generation is complete ("YES" in step S115) (step S116). The head valve 533 also prohibits the transfer of ink from the ejection head 4 to the recovery tank 531 while the printing differential pressure is being generated (steps S111 to S115), but allows the transfer of ink from the ejection head 4 to the recovery tank 531 after the printing differential pressure generation is complete ("YES" in step S115). With this configuration, while the printing differential pressure is being generated (steps S111 to S115), the outflow of ink from the supply tank 511 and the inflow of ink into the recovery tank 531 are prohibited. In other words, fluctuations in the air layers Vra and Vfa caused by the outflow and inflow of ink are prevented. As a result, the printing differential pressure ΔPp can be generated quickly.

[0065] Figures 6A and 6B are diagrams schematically showing a modified example of the operation executed in accordance with the flowchart of Figure 4. In this modified example, the control unit 100 executes steps S105 and S106 to adjust the gas-liquid interface L1 and the gas-liquid interface L2 to the liquid level state (liquid level preparation state) shown in Figure 6A (liquid level preparation).

[0066] 6A, the gas-liquid interface L1 in the supply tank 511 is below the empty level Le, and the gas-liquid interface L2 in the recovery tank 531 is above the middle level Lm (liquid level preparation state). In other words, when the gas-liquid interface L1 becomes below the empty level Le and the gas-liquid interface L2 becomes above the middle level Lm, the control unit 100 determines that the liquid level preparation is complete and stops the ink feed along the liquid feed path Ca due to the differential pressure between pressures P1 and P2 (differential pressure liquid feed) (step S108).

[0067] Furthermore, after starting generation of a pressure difference in step S111, the control unit 100 starts the return pump 552 in step S112. As a result, ink is sent from the recovery tank 531 to the supply tank 511 via the liquid sending path Cb. Therefore, as shown in FIG. 6B, in the supply tank 511, the gas-liquid interface L1 rises and the air layer Vfa above the gas-liquid interface L1 is compressed. As a result, the air layer Vfa is pressurized and the pressure P1 increases. Furthermore, in the recovery tank 531, the gas-liquid interface L2 drops and the air layer Vra above the gas-liquid interface L2 expands. As a result, the air layer Vra is decompressed and the pressure P2 decreases. In other words, the liquid sending by the return pump 552 assists in generating a pressure difference between the pressures P1 and P2.

[0068] That is, in this modified example, before starting printing differential pressure generation (step S111), liquid level preparation is performed (steps S105 to S107) to create a liquid level preparation state (FIG. 6A) in which the gas-liquid interface L1 in the supply tank 511 is at empty level Le (less than first preparation liquid level) and the middle level Lm (recovery gas-liquid interface) in the recovery tank 531 is at or above middle level Lm (second preparation liquid level). This allows differential pressure generation to be performed after ensuring the compression range of the air layer Vfa in the supply tank 511 and the expansion range of the air layer Vra in the recovery tank 531 (steps S111 to S115). As a result, printing differential pressure ΔPp can be quickly generated between the supply tank 511 and the recovery tank 531, shortening the time until printing starts.

[0069] Fig. 7 is a flowchart showing a modified example of the liquid feed control executed by the printing apparatus of Fig. 1. The flowchart of Fig. 7 is executed between steps S109 and S110 in the flowchart of Fig. 4. That is, after the liquid level preparation in steps S105 to S107 is completed and head valves 513 and 533 are closed in step S109, the flowchart of Fig. 7 is executed during the period waiting for the start of circulation in step S110. Also, it is assumed here that the liquid level preparation state of Fig. 5A has been achieved by steps S105 to S107.

[0070] In step S201, the control unit 100 determines whether to start purging. If purging is to be started (if "YES" in step S201), the control unit 100 opens the head valve 513 provided for the ejection head 4 to be purged (step S202). This allows ink to be sent from the supply tank 511 to the ejection head 4.

[0071] In step S203, the control unit 100 operates the return pump 552 and the refill pump 582 to send ink from the buffer tank 56 to the supply tank 511 via the recovery tank 531. For example, the control unit 100 executes the ink sending by controlling the amount of ink sent by the return pump 552 and the amount of ink sent by the refill pump 582 to be equal. Alternatively, when the return pump 552 sends ink from the recovery tank 531 to the supply tank 511 and the gas-liquid interface L2 in the recovery tank 531 falls below the full level Lf determined in the liquid level preparation state ( FIG. 5A ), the control unit 100 sends ink from the buffer tank 56 to the recovery tank 531 so that the gas-liquid interface L2 becomes equal to or higher than the full level Lf.

[0072] In step S204, the control unit 100 determines whether the amount of ink required for purging has been secured in the supply tank 511. Specifically, when the gas-liquid interface L1 in the supply tank 511 reaches or exceeds the middle level Lm, it is determined that ink has been secured ("YES" in step S204). When the purged ink has been secured in this manner, the control unit 100 stops sending ink to the supply tank 511 (step S205). Note that when the ink sending from the buffer tank 56 to the recovery tank 531 in step S203 to the supply tank 511 is completed, the gas-liquid interface L2 in the recovery tank 531 is at or above the full level Lf determined in the liquid level preparation state (FIG. 5A).

[0073] In step S206, the control unit 100 closes the tank valve 523 and opens the purge valve 62 to introduce compressed air into the supply tank 511, thereby starting to pressurize the supply tank 511. The compressed air introduced into the supply tank 511 causes ink to flow from the supply tank 511 into the ejection head 4, and causes ink to flow out of the nozzles 42 of the ejection head 4 (purging).

[0074] As this purging is performed, the gas-liquid interface L2 in the supply tank 511 drops. Then, when the gas-liquid interface L1 in the supply tank 511 falls below the middle level Lm determined in the liquid level preparation state (FIG. 5A), the control unit 100 determines that the purging should be ended ("YES" in step S207) and stops pressurizing the supply tank 511 (step S208). Specifically, the control unit 100 closes the purge valve 62 and opens the tank valve 523. As a result, the pressure P1 in the supply tank 511 becomes atmospheric pressure. Then, the control unit 100 closes the head valve 513 (step S209).

[0075] In this modified example, the purge mechanism 6 (purge execution unit) applies purge pressure to the gas-liquid interface L1 in the supply tank 511 to send ink from the supply tank 511 to the ejection head 4 and perform a purge to push the ink out of the nozzles 42 of the ejection head 4. The purge mechanism 6 performs a purge from the completion of the liquid level preparation in step S107 until the start of generating a printing differential pressure in step S111 (FIG. 7). In response to this, the control unit 100 causes the purge mechanism 6 to end the purge when the gas-liquid interface L1 in the supply tank 511 falls below the middle level Lm (first preparation liquid level) as a result of the execution of the purge by the purge mechanism 6 (step S207). With this configuration, even if the liquid level preparation state (FIG. 5A) created by the liquid level preparation in step S107 is disrupted as the purge is performed (particularly as the purged ink is secured), the liquid level preparation state (FIG. 5A) can be restored at the end of the purge.

[0076] As described above, in this embodiment, the printing device 1 corresponds to an example of the "printing device" of the present invention, the control unit 100 corresponds to an example of the "control unit" of the present invention, the ejection head 4 corresponds to an example of the "ejection head" of the present invention, the nozzle 42 corresponds to an example of the "nozzle" of the present invention, the supply tank 511 corresponds to an example of the "supply ink storage unit" of the present invention, the pressure adjustment unit 52 corresponds to an example of the "first pressure adjustment unit" of the present invention, the recovery tank 531 corresponds to an example of the "recovered ink storage unit" of the present invention, the pressure adjustment unit 54 corresponds to an example of the "second pressure adjustment unit" of the present invention, and the return liquid sending unit 55 corresponds to an example of the "return liquid sending unit" of the present invention. the gas-liquid interface L1 corresponds to an example of a "supply gas-liquid interface" of the present invention, the gas-liquid interface L2 corresponds to an example of a "recovery gas-liquid interface" of the present invention, the pressure P1 corresponds to an example of a "first pressure" of the present invention, the pressure P2 corresponds to an example of a "second pressure" of the present invention, the supply pressure Pf corresponds to an example of a "supply pressure" of the present invention, the recovery pressure Pr corresponds to an example of a "recovery pressure" of the present invention, the printing differential pressure ΔPp corresponds to an example of a "printing differential pressure" of the present invention, steps S105 to S107 correspond to an example of a "liquid level preparation" of the present invention, and steps S112 and S113 correspond to an example of a "differential pressure generation assistance" of the present invention.

[0077] The present invention is not limited to the above-described embodiment, and various modifications other than those described above are possible without departing from the spirit of the present invention. For example, the generation of the liquid level preparation state shown in Fig. 5A or 6A may be performed by the ink liquid delivery rate adjustment unit 5D. That is, the ink liquid delivery rate adjustment unit 5D has a buffer tank 56 (buffer ink storage unit) that stores ink, a replenishment liquid delivery unit 58 (ink replenishment unit) that delivers ink from the buffer tank 56 to the recovery tank 531, and a recovery liquid delivery unit 57 (ink recovery unit) that delivers ink from the supply tank 511 to the buffer tank 56.

[0078] In response to this, the control unit 100 controls the supply of ink from the buffer tank 56 to the recovery tank 531 by the replenishing liquid supply unit 58 based on the detection result of the liquid level detector 592, and also controls the supply of ink from the supply tank 511 to the buffer tank 56 by the recovery liquid supply unit 57 based on the detection result of the liquid level detector 591. This generates the liquid level preparation state of Fig. 5A or 6A (liquid level preparation). With this configuration, liquid level preparation can be performed by supplying ink from the buffer tank 56 to the recovery tank 531 and recovering ink from the supply tank 511 to the buffer tank 56.

[0079] In the above embodiment, the pressure P1 inside the ink supply tank 511 was a positive pressure. However, as long as the magnitude relationship is ensured such that the pressure P1 inside the ink supply tank 511 is equal to or greater than the pressure P2 inside the recovery tank 531, the pressure P1 inside the ink supply tank 511 may be a negative pressure. In this case, the pressure inside the pressure tank 521 will also be a negative pressure, so it is desirable that the introduction pipe 524 communicate with an exhaust pump rather than with compressed air.

[0080] Fig. 8 is a diagram schematically illustrating a modified example of a discharge head and an ink delivery mechanism provided for the discharge head. Fig. 8 differs from Fig. 2 in that speed controllers SC1 and SC2 are provided. Specifically, pressure adjustment unit 52 has speed controller SC1 attached to inlet pipe 524 that introduces compressed air into pressure tank 521. This speed controller SC1 limits the inflow of compressed air into pressure tank 521. In addition, pressure adjustment unit 54 has speed controller SC2 attached to exhaust pipe 544 that connects exhaust pump 549 and pressure tank 541. This speed controller SC2 limits the outflow of air from pressure tank 541 to exhaust pump 549.

[0081] In this configuration with the speed controllers SC1 and SC2, fluctuations in the compressed air introduced through the inlet pipe 524 and the exhaust air from the exhaust pump 549 are absorbed by the speed controllers SC1 and SC2, preventing the ink meniscus formed in the nozzle 42 from being affected. However, the restriction of airflow by the speed controllers SC1 and SC2 causes the generation of the supply pressure Pf and the recovery pressure Pr to take time. Therefore, it is preferable to perform the differential pressure generation assistance as described above so that the printing differential pressure ΔPp can be quickly generated between the supply tank 511 and the recovery tank 531. [Industrial Applicability]

[0082] The present invention can be applied to general printing technologies in which ink is ejected from an ejection head while being sent from the supply ink reservoir to the recovered ink reservoir via the ejection head by using a differential pressure generated between the supply ink reservoir, which stores the ink to be supplied to the ejection head, and the recovered ink reservoir, which stores the ink recovered from the ejection head. [Explanation of symbols]

[0083] 1...Printing device 100...Control unit 4...Discharge head 42...Nozzle 511...Supply tank (supply ink storage section) 52...Pressure adjusting unit (first pressure adjusting unit) 531...Recovery tank (recovered ink storage section) 54...Pressure adjusting section (second pressure adjusting section) 55...Return liquid delivery section L1... Gas-liquid interface (supply gas-liquid interface) L2: Gas-liquid interface (recovered gas-liquid interface) P1...Pressure (first pressure) P2...Pressure (second pressure)

Claims

1. an ejection head having nozzles for ejecting ink; a supply ink reservoir that stores ink to be supplied to the ejection head; a recovered ink storage section that stores ink recovered from the ejection head; a return ink sending section that sends the ink from the recovered ink storage section to the supply ink storage section; a first pressure adjusting unit that adjusts a first pressure applied to a supply gas-liquid interface that is a boundary between the ink stored in the supply ink storage unit and air; a second pressure adjusting unit that adjusts a second pressure applied to a recovered gas-liquid interface that is a boundary between the ink stored in the recovered ink storage unit and air; a control unit that executes printing differential pressure generation, adjusting the first pressure by the first pressure adjustment unit so that the first pressure becomes a supply pressure, and adjusting the second pressure by the second pressure adjustment unit so that the second pressure becomes a recovery pressure lower than the supply pressure; Equipped with when the first pressure is adjusted to the supply pressure and the second pressure is adjusted to the recovery pressure and the generation of the printing differential pressure is completed, a printing ink delivery is performed in which ink is delivered from the supply ink reservoir to the recovered ink reservoir via the ejection head by a printing differential pressure that is the difference between the supply pressure and the recovery pressure, the ejection head performs printing by ejecting the ink supplied from the supply ink reservoir from the nozzles in association with the printing ink delivery; The control unit performs liquid level preparation to generate a liquid level preparation state in which the supply gas-liquid interface is less than a first preparation liquid level and the recovered gas-liquid interface is equal to or greater than a second preparation liquid level from the time the printing by the ejection head is completed until the printing differential pressure generation is completed, and in parallel with the execution of the printing differential pressure generation, performs differential pressure generation assistance by using the return liquid delivery unit to deliver the ink from the recovered ink storage unit to the supply ink storage unit.

2. The printing apparatus according to claim 1 , wherein the control unit terminates the differential pressure generation assistance when the supply gas-liquid interface reaches or exceeds a first final liquid level.

3. The printing apparatus according to claim 2 , wherein the control unit terminates the differential pressure generation assistance when the recovered gas-liquid interface falls below a second final liquid level.

4. The printing device according to claim 3, wherein when the printing by the ejection head is completed, the control unit stops the return liquid delivery unit from delivering the ink from the recovered ink storage unit to the supply ink storage unit, and performs the liquid level preparation by lowering the supply gas-liquid interface while raising the recovered gas-liquid interface.

5. The printing device described in claim 4, wherein when the control unit stops the ink delivery by the return delivery unit to prepare the liquid level, the control unit adjusts the first pressure by the first pressure adjustment unit while adjusting the second pressure by the second pressure adjustment unit, thereby reducing the difference between the first pressure and the second pressure from the printing differential pressure.

6. a buffer ink reservoir that stores the ink; an ink supply unit that sends the ink from the buffer ink storage unit to the recovered ink storage unit; an ink recovery unit that transfers the ink from the supply ink storage unit to the buffer ink storage unit; Furthermore, The printing device described in claim 1, wherein the control unit performs the liquid level preparation by controlling the ink supply unit to send ink from the buffer ink storage unit to the recovered ink storage unit, while controlling the ink recovery unit to send ink from the supply ink storage unit to the buffer ink storage unit.

7. the first pressure adjusting unit has a first pressure tank connected to the supply ink reservoir and a first pressure generating unit that generates the supply pressure in the first pressure tank, and applies the supply pressure generated in the first pressure tank to the supply gas-liquid interface of the supply ink reservoir; 2. The printing device according to claim 1, wherein the second pressure adjustment unit has a second pressure tank connected to the recovered ink storage unit and a second pressure generation unit that generates the recovered pressure in the second pressure tank, and applies the recovered pressure generated in the second pressure tank to the recovered gas-liquid interface of the recovered ink storage unit.

8. the first pressure generating unit includes an inlet pipe that introduces compressed air supplied from an external source into the first pressure tank, and a first speed controller that is attached to the inlet pipe and that limits the inflow of the compressed air into the first pressure tank; 8. The printing device according to claim 7, wherein the second pressure generating unit includes an exhaust pump that exhausts the second pressure tank, an exhaust pipe that connects the exhaust pump and the second pressure tank, and a second speed controller that is attached to the exhaust pipe and limits the outflow of air from the second pressure tank to the exhaust pump.

9. a purge execution unit that executes purging by applying a purge pressure to the supply gas-liquid interface to send the ink from the supply ink reservoir to the ejection head and push the ink out of the nozzles of the ejection head, the purge execution unit executes the purge from the end of the liquid level preparation until the start of generating the printing differential pressure, The printing apparatus according to claim 1 , wherein the control unit causes the purge execution unit to end the purge when the supply gas-liquid interface becomes lower than the first preliminary liquid level as a result of the execution of the purge by the purge execution unit.

10. a supply control unit that controls the transfer of ink from the supply ink reservoir to the ejection head; a recovery control unit that controls the transfer of ink from the ejection head to the recovered ink storage unit; Furthermore, the supply control unit prohibits the ink from being sent from the ink supply storage unit to the ejection head during the execution of the printing differential pressure generation, and allows the ink to be sent from the ink supply storage unit to the ejection head after the printing differential pressure generation is completed; 2. The printing device according to claim 1, wherein the recovery control unit prohibits the transfer of ink from the ejection head to the recovered ink storage unit while the printing differential pressure generation is being performed, but allows the transfer of ink from the ejection head to the recovered ink storage unit after the printing differential pressure generation is completed.

11. The printing apparatus according to claim 1 , wherein the differential pressure generation assistance is started after the liquid level preparation is completed.

12. The printing apparatus according to claim 1 , wherein the generation of the printing differential pressure is initiated after the printing by the ejection head is completed and before the differential pressure generation assistance is initiated.

13. a step of executing a printing differential pressure generation process in which a first pressure adjustment unit adjusts a first pressure applied to a supply gas-liquid interface, which is a boundary between the ink stored in a supply ink storage unit that stores ink to be supplied to a discharge head having nozzles that discharge ink, and adjusts the first pressure so that the first pressure becomes a supply pressure, and a second pressure adjustment unit adjusts a second pressure applied to a recovery gas-liquid interface, which is a boundary between the ink stored in a recovery ink storage unit that stores ink recovered from the discharge head, and the air, and adjusts the second pressure so that the second pressure becomes a recovery pressure lower than the supply pressure; a step of performing printing ink delivery in which ink is delivered from the supply ink reservoir to the recovered ink reservoir via the ejection head by a printing differential pressure that is the difference between the supply pressure and the recovery pressure, by adjusting the first pressure to the supply pressure and adjusting the second pressure to the recovery pressure to complete the generation of the printing differential pressure; a step of causing the ejection head to perform printing by ejecting the ink supplied from the supply ink reservoir from the nozzles in accordance with the printing ink delivery; Equipped with a liquid level preparation is performed to generate a liquid level preparation state in which the supply gas-liquid interface is lower than a first preparation liquid level and the recovery gas-liquid interface is equal to or higher than a second preparation liquid level before the generation of the printing differential pressure is completed; A printing method in which, in parallel with the execution of the printing differential pressure generation, a differential pressure generation assistance is executed to send the ink from the recovered ink storage section to the supply ink storage section by a return liquid sending section that sends the ink from the recovered ink storage section to the supply ink storage section.

Images