Printing apparatus and ink supply method

Abstract

To suppress a large amount of ink from remaining without being stirred in a main tank.SOLUTION: A positional relationship between a low level Lml (a predetermined position) arranged corresponding to a stirring blade 873 and a liquid level of ink in a main tank 97 is detected by a main liquid level sensor S97. When it is detected that the liquid level of the ink in the main tank 97 is lower than the low level Lml (YES in the step S102), the ink is fed to a front stage buffer tank 96 from the main tank 97 (steps S106-S108, the first main liquid feeding operation). Thereby, a large amount of ink can be suppressed from remaining without being stirred in the main tank 97.SELECTED DRAWING: Figure 9

Description

Technical Field

[0001] This invention relates to a technique for supplying ink to a discharge head that discharges ink from a nozzle.

Background Art

[0002] In Patent Document 1, a printing apparatus is described that includes an intermediate tank for storing ink supplied to a discharge head and a main tank for storing ink supplied to the intermediate tank. In this printing apparatus, when the ink in the intermediate tank decreases as ink is discharged from the discharge head, ink is supplied from the main tank to the intermediate tank.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] By the way, in the printing apparatus as described above, the pigment contained in the ink stored in the main tank may settle. Such pigment settling is particularly remarkable, for example, in white ink containing titanium oxide as a pigment. Therefore, measures such as stirring the ink with a stirring member disposed in the main tank can be taken. However, as ink is supplied from the main tank to the intermediate tank as the ink in the intermediate tank decreases, the liquid level of the ink in the main tank drops below the position where it can be stirred by the stirring member. As a result, a large amount of ink in a state where it cannot be stirred by the stirring member may remain in the main tank.

[0005] This invention has been made in view of the above problems, and an object thereof is to suppress a large amount of ink from remaining in the main tank without being stirred. [Means for solving the problem]

[0006] The printing apparatus according to the present invention comprises a discharge head that discharges ink from a nozzle, a downstream buffer tank that stores the ink supplied to the discharge head, a upstream buffer tank that stores the ink supplied to the downstream buffer tank, a buffer liquid supply pipe that connects and communicates the upstream buffer tank and the downstream buffer tank, a buffer liquid supply pump attached to the buffer liquid supply pipe between the upstream buffer tank and the downstream buffer tank and sending ink from the upstream buffer tank to the downstream buffer tank along the buffer liquid supply pipe, a main tank that stores the ink supplied to the upstream buffer tank, a main liquid supply pipe that connects and communicates the main tank and the upstream buffer tank, and a main liquid supply pipe attached to the main liquid supply pipe between the main tank and the upstream buffer tank that sends main liquid from the main tank to the upstream buffer tank. The system comprises a main liquid delivery pump that delivers ink along a pipe, a main tank stirring unit which has a main tank stirring member located inside the main tank and stirs the ink stored in the main tank by rotating the main tank stirring member, a main tank liquid level detection unit which detects the positional relationship between the liquid level of the ink in the main tank and a predetermined position provided in correspondence with the main tank stirring member, and a control unit, the control unit which includes a main tank liquid level acquisition unit which acquires the detection result of the main tank liquid level detection unit, and a main liquid delivery pump control unit which controls the main liquid delivery pump so that the main liquid delivery pump performs a first main liquid delivery operation to deliver ink from the main tank to a preceding buffer tank in response to the main tank liquid level acquisition unit acquiring a detection result indicating that the liquid level of the ink in the main tank is lower than the predetermined position.

[0007] The present invention relates to an ink supply method for a printing apparatus comprising: a discharge head for discharging ink from a nozzle; a downstream buffer tank for storing ink supplied to the discharge head; a downstream buffer tank for storing ink supplied to the downstream buffer tank; a main tank for storing ink supplied to the upstream buffer tank; and a main tank stirring unit having a main tank stirring member disposed within the main tank, which stirs the ink stored in the main tank by rotating the main tank stirring member. The method comprises the steps of: detecting the positional relationship between the liquid level of ink in the main tank and a predetermined position provided corresponding to the main tank stirring member; and executing a first main liquid supply operation to send ink from the main tank to the upstream buffer tank in response to the detection that the liquid level of ink in the main tank is lower than the predetermined position.

[0008] In the present invention (printing apparatus and ink supply method) configured as described above, a main tank, a pre-buffer tank, and a post-buffer tank are provided. Ink is supplied from the main tank to the pre-buffer tank, and then from the pre-buffer tank to the post-buffer tank. The ink stored in the post-buffer tank is then supplied to the discharge head and discharged from the nozzle of the discharge head. A main tank stirring member is placed inside the main tank, and the rotation of the main tank stirring member stirs the ink stored in the main tank. The positional relationship between a predetermined position corresponding to this main tank stirring member and the ink level in the main tank is detected. When it is detected that the ink level in the main tank is lower than the predetermined position, ink is sent from the main tank to the pre-buffer tank (first main liquid supply operation). This makes it possible to suppress the retention of a large amount of ink in the main tank without being stirred.

[0009] Furthermore, the printing apparatus may be configured such that the control unit further includes a pre-stage buffer tank liquid level detection unit that detects the positional relationship between the liquid level of the ink stored in the pre-stage buffer tank and a pre-stage normal liquid level range provided for the pre-stage buffer tank, and the control unit has a pre-stage buffer tank liquid level acquisition unit that acquires the detection result of the pre-stage buffer tank liquid level detection unit, and when the pre-stage buffer tank liquid level acquisition unit acquires a detection result indicating that the liquid level of the ink in the pre-stage buffer tank is lower than the lower end of the pre-stage normal liquid level range, and the detection result acquired by the main tank liquid level acquisition unit indicates that the liquid level of the ink in the main tank is lower than a predetermined position, the main liquid supply pump control unit controls the main liquid supply pump so that the main liquid supply pump performs the first main liquid supply operation. In such a configuration, when the liquid level of the ink in the pre-stage buffer tank decreases to below the lower end of the pre-stage normal liquid level range, the ink can be replenished in the pre-stage buffer tank by the first liquid supply operation.

[0010] Furthermore, if the pre-stage buffer tank liquid level acquisition unit obtains a detection result indicating that the liquid level of ink in the pre-stage buffer tank is lower than the lower end of the pre-stage normal liquid level range, and the detection result obtained by the main tank liquid level acquisition unit indicates that the liquid level of ink in the main tank is at or above a predetermined position, the printing apparatus may be configured such that the main liquid supply pump control unit controls the main liquid supply pump to perform a second main liquid supply operation, which supplies ink from the main tank to the pre-stage buffer tank in a manner different from the first main liquid supply operation. In such a configuration, if the liquid level of ink in the pre-stage buffer tank decreases to below the lower end of the pre-stage normal liquid level range, the ink can be replenished in the pre-stage buffer tank by the second liquid supply operation.

[0011] Furthermore, in the second main liquid supply operation, the printing apparatus may be configured such that the main liquid supply pump supplies ink from the main tank to the preceding buffer tank until the ink level in the preceding buffer tank reaches the upper end of the preceding normal liquid level range. In this configuration, by performing the second main liquid supply operation, the ink level in the preceding buffer tank, which has decreased to below the lower end of the preceding normal liquid level range, can be raised to the upper end of the preceding normal liquid level range.

[0012] Furthermore, in the first main liquid supply operation, the printing apparatus may be configured such that the main liquid supply pump supplies ink from the main tank to the preceding buffer tank until the liquid level of the ink in the preceding buffer tank reaches the preceding upper limit position, which is set to a position higher than the upper limit of the preceding normal liquid level range. In such a configuration, a large amount of ink that can no longer be agitated in the main tank can be supplied from the main tank to the preceding buffer tank.

[0013] The printing apparatus may also be configured to include a pre-buffer tank stirring unit, which has a pre-buffer tank stirring member located within the pre-buffer tank, and which further stirs the ink stored in the pre-buffer tank by rotating the pre-buffer tank stirring member. In such a configuration, ink that has become unable to be stirred in the main tank can be sent to the pre-buffer tank and stirred by the pre-buffer tank stirring member.

[0014] Furthermore, the printing apparatus may be configured such that the pre-buffer tank stirring member is positioned below the normal liquid level range of the pre-buffer tank. In this configuration, the ink in the pre-buffer tank is stirred by the pre-buffer tank stirring member positioned below the normal liquid level range of the pre-buffer tank. Therefore, ink that cannot be stirred in the main tank is sent to the pre-buffer tank and reliably stirred by the pre-buffer tank stirring member.

[0015] The printing apparatus may also be configured to include a downstream buffer tank liquid level detection unit that detects the positional relationship between the liquid level of ink stored in the downstream buffer tank and a downstream normal liquid level range provided for the downstream buffer tank, and the control unit having a downstream buffer tank liquid level acquisition unit that acquires the detection result of the downstream buffer tank liquid level detection unit, and a buffer liquid pump control unit that controls the buffer liquid pump so that the buffer liquid pump performs a buffer liquid supply operation to supply ink from the upstream buffer tank to the downstream buffer tank in response to the downstream buffer tank liquid level acquisition unit acquiring a detection result indicating that the liquid level of ink in the downstream buffer tank is lower than the lower end of the downstream normal liquid level range. In such a configuration, if the liquid level of ink in the downstream buffer tank decreases to below the lower end of the downstream normal liquid level range, ink can be replenished in the downstream buffer tank by the buffer liquid supply operation.

[0016] Furthermore, in the buffer liquid delivery operation, the printing apparatus may be configured such that the buffer liquid delivery pump delivers ink from the preceding buffer tank to the subsequent buffer tank until the ink level in the subsequent buffer tank reaches the upper limit of the subsequent normal liquid level range. In such a configuration, the buffer liquid delivery operation can raise the ink level in the subsequent buffer tank, which has decreased to below the lower limit of the subsequent normal liquid level range, up to the upper limit of the subsequent normal liquid level range.

[0017] The printing apparatus may also be configured to include a downstream buffer tank stirring unit, which has a downstream buffer tank stirring member located within the downstream buffer tank, and which stirs the ink stored in the downstream buffer tank by rotating the downstream buffer tank stirring member. In such a configuration, the ink stored in the downstream buffer tank can be stirred by the downstream buffer tank stirring member.

[0018] Furthermore, the printing apparatus may be configured such that the downstream buffer tank stirring member is positioned below the normal downstream liquid level range. In this configuration, the ink stored in the downstream buffer tank can be stirred by the downstream buffer tank stirring member located below the normal downstream liquid level range.

[0019] Furthermore, the system includes a buffer return pipe connecting a branch section in the buffer liquid supply piping between the buffer liquid supply pump and the downstream buffer tank, a supply limiting solenoid valve provided in the buffer liquid supply piping between the branch section and the downstream buffer tank, and a return limiting solenoid valve provided in the buffer return pipe between the branch section and the upstream buffer tank. The control unit has a supply limiting solenoid valve control unit that controls the opening and closing of the supply limiting solenoid valve, and a return limiting solenoid valve control unit that controls the opening and closing of the return limiting solenoid valve. The supply limiting solenoid valve control unit controls the supply limiting solenoid valve so that it opens, allowing ink to be sent from the branch section to the downstream buffer tank via the supply limiting solenoid valve, while the return limiting solenoid valve control unit controls the return limiting solenoid valve so that it closes, allowing ink to be sent from the branch section to the upstream buffer tank via the return limiting solenoid valve. The printing apparatus may be configured such that, while restricting the return of ink, the buffer liquid delivery pump control unit controls the buffer liquid delivery pump to perform liquid delivery, thereby executing the buffer liquid delivery operation; while restricting the delivery of ink from the branch to the downstream buffer tank via the supply restriction solenoid valve by controlling the supply restriction solenoid valve to close the supply restriction solenoid valve, the return restriction solenoid valve control unit controls the return restriction solenoid valve to open the return restriction solenoid valve, thereby allowing ink to be returned from the branch to the upstream buffer tank via the return restriction solenoid valve; and while allowing ink to be returned from the branch to the upstream buffer tank via the return restriction solenoid valve, the buffer liquid delivery pump control unit controls the buffer liquid delivery pump to perform liquid delivery, thereby executing buffer return liquid delivery, where ink that has reached the branch of the buffer liquid delivery piping from the upstream buffer tank returns to the upstream buffer tank via the buffer return piping.

[0020] In addition, the main tank stirring member is a stirring blade that spreads due to the centrifugal force generated during rotation, and the printing apparatus may be configured such that the predetermined position is provided corresponding to the position of the stirring blade spread by the centrifugal force. When stirring is performed using such a stirring blade, the lower end of the rotating stirring blade rises from the lower end of the stationary stirring blade. Therefore, in the main tank, the ink that accumulates at the bottom below the lower end of the rotating stirring blade cannot be stirred. On the other hand, by providing the above-described predetermined position corresponding to the position of the stirring blade spread by the centrifugal force and configuring it to execute the first main liquid feeding operation, it is possible to suppress a large amount of ink from remaining in the bottom of the main tank without being stirred.

[0021] Note that various specific positional relationships between the stirring blade and the predetermined position can be assumed. Therefore, the predetermined position may be provided at the same height as the lower end of the stirring blade spread by the centrifugal force, may be provided at the same height as the upper end of the stirring blade spread by the centrifugal force, or may be provided at a position higher than the stirring blade spread by the centrifugal force.

[0022] Also, when the ink is white ink, the present invention may be applied. Thereby, it becomes possible to suppress the pigment of the white ink from settling due to a large amount of white ink remaining in the main tank without being stirred.

Effect of the Invention

[0023] As described above, according to the present invention, it is possible to suppress a large amount of ink from remaining in the main tank without being stirred.

Brief Description of the Drawings

[0024] [Figure 1] A front view schematically showing an example of a printing system including a printing apparatus according to the present invention. [Figure 2] A front view schematically showing the printing apparatus included in the printing system of FIG. 1. [Figure 3] A view schematically showing the bottom surface of the discharge head included in the head unit. [Figure 4] A diagram schematically showing the configuration of a discharge head and an ink supply mechanism for supplying ink to the discharge head. [Figure 5] A diagram schematically showing a mechanism for supplying ink to a buffer tank. [Figure 6] A block diagram showing the electrical configuration of a printing apparatus. [Figure 7A] A diagram schematically showing the detection position of a rear-stage liquid level sensor provided for a rear-stage buffer tank in order to execute liquid feeding control. [Figure 7B] A diagram schematically showing the detection position of a front-stage liquid level sensor provided for a front-stage buffer tank in order to execute liquid feeding control. [Figure 8] A diagram schematically showing the detection position of a main liquid level sensor provided for a main tank in order to execute liquid feeding control. [Figure 9] A flowchart showing an example of ink liquid feeding control from a main tank executed by a printing apparatus. [Figure 10A] A diagram schematically showing the operations executed in the ink liquid feeding control of FIG. 9. [Figure 10B] A diagram schematically showing the operations executed in the ink liquid feeding control of FIG. 9. [Figure 11] A flowchart showing an example of ink liquid feeding control from a front-stage buffer tank executed by a printing apparatus. [Figure 12] A diagram schematically showing the operations executed in the ink liquid feeding control of FIG. 11. [Figure 13] A flowchart showing an example of liquid feeding mode switching control executed by a printing apparatus. [[ID=3G]] S [Figure 14A] A diagram schematically showing an example of a change in a low level provided corresponding to a stirring blade. [Figure 14B] A diagram schematically showing an example of a change in a low level provided corresponding to a stirring blade.

Embodiments for Carrying Out the Invention

[0025] Figure 1 is a schematic front view showing an example of a printing system equipped with a printing apparatus according to the present invention. In Figure 1 and the following figures, the horizontal direction X and the vertical direction Z are indicated as appropriate. As shown in Figure 1, the printing system 1 comprises a printing apparatus 3 and a drying apparatus 6 arranged in the horizontal direction X. This printing system 1 conveys a long, strip-shaped printing medium M from a feed roll 11 to a take-up roll 12 in a roll-to-roll manner. The material of the printing medium M is a film such as OPP (oriented polypropylene) or PET (polyethylene terephthalate). However, the material of the printing medium M is not limited to film and may be paper or the like. Such a printing medium M is flexible. Furthermore, in the following, of the two sides of the printing medium M, the side on which the image is printed will be appropriately referred to as the front surface M1, and the side opposite to the front surface M1 will be appropriately referred to as the back surface M2.

[0026] The printing device 3 prints an image onto the surface M1 of the printing medium M by ejecting water-based ink using an inkjet method onto the surface M1 of the printing medium M, which is transported from the feed roll 11 to the take-up roll 12. The detailed configuration of the printing device 3 will be described later. The printing medium M, on which the image has been printed, is then transported horizontally X from the printing device 3 to the drying device 6.

[0027] The drying apparatus 6 is equipped with a drying oven 60 and dries the printing medium M that has been discharged from the printing apparatus 3 as it is transported from the feed roll 11 to the winding roll 12. Inside the drying oven 60 are two upper air blowing units 61u arranged horizontally in the X direction, two middle air blowing units 61m arranged horizontally in the X direction below these upper air blowing units 61u, and two lower air blowing units 61l arranged horizontally in the X direction below these middle air blowing units 61m.

[0028] The printing medium M discharged from the outlet 312 of the printing device 3 passes through two upper air blowing units 61u in the horizontal direction X, and is then turned back towards two middle air blowing units 61m by a pair of rollers 62. Subsequently, the printing medium M passes through the two middle air blowing units 61m in the horizontal direction X, and is then turned back towards two lower air blowing units 61l by a pair of air turn bars 63. Furthermore, the printing medium M passes through two lower air blowing units 61l in the horizontal direction X, and is then discharged to the outside of the drying device 6.

[0029] The upper air blowing unit 61u has two air blowing chambers 64 positioned to sandwich the printing medium M passing horizontally in the X direction from the vertical Z direction. Each air blowing chamber 64 has a plurality of nozzles 65 arranged horizontally in the X direction, and each nozzle 65 sprays warm air (gas at 60 degrees Celsius or higher) onto the printing medium M. In this way, the printing medium M is dried by the warm air sprayed from the nozzles 65 of the air blowing chambers 64 as it passes between the two air blowing chambers 64 located above and below each other. The middle air blowing unit 61m and the lower air blowing unit 61l also each have two air blowing chambers 64 that sandwich the printing medium M from the vertical Z direction, similar to the upper air blowing unit 61u.

[0030] Incidentally, the specific configuration of the upper air blower unit 61u is not limited to this example. For example, instead of the lower air blower chamber 64 of the upper air blower unit 61u, a plurality of rollers arranged in the horizontal direction X may be provided. In such a configuration, the back surface M2 of the printing medium M is supported from below by the plurality of rollers, while hot air can be sprayed from the upper air blower chamber 64 onto the front surface M1 of the printing medium M.

[0031] Figure 2 is a schematic front view showing the printing apparatus of the printing system of Figure 1. In Figure 2, one side X1 and the other side X2 in the horizontal direction X are shown as appropriate. Here, one side X1 is the side from the printing apparatus 3 toward the drying apparatus 6, and the other side X2 is the opposite side of one side X1. The printing apparatus 3 comprises a housing 31, a color printing unit 32 located inside the housing 31, a white printing unit 33 located above the color printing unit 32 inside the housing 31, and a transport unit 4 that transports the printing medium M using a plurality of rollers located inside the housing 31.

[0032] The color printing unit 32 has a plurality (6) of head units 321 arranged above the printing medium M transported by the transport unit 4 in the direction of travel of the printing medium M (from the other side X2 to the one side X1). Each of the plurality of head units 321 has nozzles that face the surface M1 of the printing medium M passing below them from above, and ejects color inks of different colors from the nozzles using an inkjet method. Here, color ink means ink other than white, and includes inks such as cyan, magenta, yellow, and black. In this way, the plurality of head units 321 of the color printing unit 32 eject color inks from above onto the surface M1 of the printing medium M passing below them, thereby printing a color image on the surface M1 of the printing medium M.

[0033] Furthermore, the white printing unit 33 has a single head unit 331 positioned above the printing medium M that is transported by the transport unit 4. The head unit 331 has nozzles that face the surface M1 of the printing medium M passing below it from above, and ejects white ink from the nozzles in an inkjet manner. In this way, the head unit 331 of the white printing unit 33 prints a white image on the surface M1 of the printing medium M by ejecting white ink from above onto the surface M1 of the printing medium M passing below it.

[0034] An entrance 311 opens in the side wall X2 of the housing 31, while an exit 312 opens in the side wall X1 of the housing 31. The transport unit 4 then transports the printing medium M from the entrance 311 to the exit 312, passing through the color printing unit 32 and the white printing unit 33.

[0035] This transport unit 4 includes a loading unit 41 located below the color printing unit 32, an upward transport unit 42 located on one side X1 of the color printing unit 32, an upward transport unit 43 located above the color printing unit 32, and a downward transport unit 44 located on the other side X2 of the color printing unit 32. The loading unit 41 transports the printing medium M loaded from the loading entrance 311 to one side X1 using rollers 411, the upward transport unit 42 transports the printing medium M transported by the loading unit 41 upward using rollers 421, the upward transport unit 43 transports the printing medium M transported by the upward transport unit 42 to the other side X2 using rollers 431, and the downward transport unit 44 transports the printing medium M transported by the upward transport unit 43 downward using rollers 441.

[0036] Furthermore, the transport unit 4 has a color transport unit 45 that supports the printing medium M facing the color printing unit 32 from below, and the printing medium M that has passed through the descending transport unit 44 enters the color transport unit 45. This color transport unit 45 has a plurality of rollers 451 arranged from the other side X2 to the one side X1, and each roller 451 contacts the back surface M2 of the printing medium M from below. In this way, the surface M1 of the printing medium M supported by the color transport unit 45 faces upward, and each head unit 321 of the color printing unit 32 ejects color ink while facing this surface M1 from above.

[0037] Furthermore, the transport unit 4 has rollers 461, 462, and 463 positioned between the color transport unit 45 and the lower transport unit 44 in the direction of travel of the printing medium M. Roller 461 is a drive roller that drives the printing medium M. Rollers 462 and 463 are driven rollers that rotate in accordance with the printing medium M.

[0038] Furthermore, the transport unit 4 has an inversion transport unit 47 that inverts the printing medium M, which has been transported from the color transport unit 45 to one side X1, twice. This inversion transport unit 47 has a plurality of rollers 471 to 477, including a drive roller 471, and these rollers 471 to 477 contact the back surface M2 of the printing medium M while inverting the printing medium M twice. In other words, the inversion transport unit 47 transports the printing medium M transported from the color transport unit 45 downwards by rollers 471 and 472, and then changes the direction of travel of the printing medium M to the other side X2 by roller 472, thereby inverting the front surface M1 and back surface M2 of the printing medium M. Subsequently, the inversion transport unit 47 transports the printing medium M from one side X1 to the other side X2 by a plurality of rollers 473, and then transports the printing medium M upwards by rollers 474 to 476. Furthermore, the reversal transport unit 47 changes the direction of travel of the printing medium M to one side X1 using the roller 476, thereby reversing the front surface M1 and back surface M2 of the printing medium M again, and transports the printing medium M from the other side X2 to the one side X1 using the roller 477.

[0039] Furthermore, the transport unit 4 has a white transport unit 48 that supports the printing medium M facing the white printing unit 33 from below, and the printing medium M, which has been inverted twice by the inversion transport unit 47, enters the white transport unit 48. This white transport unit 48 has a roller 481 that contacts the back surface M2 of the printing medium M from below. In this way, the surface M1 of the printing medium M supported by the white transport unit 48 faces upward, and the head unit 331 of the white printing unit 33 ejects white ink while facing this surface M1 from above.

[0040] Furthermore, the transport unit 4 has an unloading unit 49 located above the upper transport unit 43. The unloading unit 49 has a plurality of rollers 491 arranged from the other side X2 to the one side X1 in the horizontal direction X. This unloading unit 49 transports the printing medium M, which has been transported by the white transport unit 48, to the one side X1 using the plurality of rollers 491, thereby unloading it from the unloading outlet 312 of the housing 31 to the drying device 6.

[0041] As described above, the color printing section 32 and the white printing section 33 of the printing device 3 have head units 321 and 331. Next, we will describe the ejection heads H of the head units 321 and 331 and the ink supply mechanism that supplies ink to the ejection heads H. Note that the head units 321 and 331 have a common basic configuration, and the basic configuration of the ink supply mechanism is also common to the head units 321 and 331. Therefore, we will describe the configuration of the head unit 331 that ejects white ink.

[0042] Figure 3 schematically shows the bottom surface of the ejection head of the head unit, and Figure 4 schematically shows the configuration of the ejection head and the ink supply mechanism that supplies ink to the ejection head. In Figure 3, in addition to the horizontal direction X and the vertical direction Z, a horizontal direction Y perpendicular to the horizontal direction X is shown. As shown in Figure 3, in the head unit 331, multiple ejection heads H that eject ink of the same color (white ink) are arranged in a line in the horizontal direction Y, and each ejection head H has a rectangle when viewed from below. Note that the arrangement of the multiple ejection heads H is not limited to the example in Figure 3, and the multiple ejection heads H may be arranged in a staggered pattern.

[0043] As shown in Figure 4, the discharge head H has a housing Ha, and at the bottom surface of the housing Ha, multiple nozzles Hn are arranged in a staggered pattern horizontally in the direction Y and open. Inside the housing Ha, there are multiple cavities Hb, each communicating with one of the multiple nozzles Hn, and an ink supply chamber Hc, which communicates with the multiple cavities Hb. Ink supplied from the ink supply chamber Hc is stored in the cavities Hb. In addition, a piezoelectric element D is provided in each cavity Hb, and the piezoelectric element D is displaced in response to a drive signal (electrical signal), causing pressure fluctuations in the ink in the cavity Hb. Due to these pressure fluctuations, the ink is pushed out of the cavity Hb and discharged from the nozzle Hn that communicates with that cavity Hb. Furthermore, at the top of the ejection head H, an ink inlet Hd and an ink outlet He are open, and ink flows from the ink supply mechanism 9a (described later) through the ink inlet Hd into the ink supply chamber Hc, and flows out from the ink supply chamber Hc through the ink outlet He towards the ink recovery mechanism 9b (described later).

[0044] Next, the ink supply, recovery, and circulation mechanism 9, which supplies, recovers, and circulates ink, will be described. The ink supply, recovery, and circulation mechanism 9 comprises an ink supply mechanism 9a that supplies ink to the discharge head H, an ink recovery mechanism 9b that recovers ink from the discharge head H, and an ink return mechanism 9c that returns the recovered ink back to the ink supply mechanism 9a. The ink supply mechanism 9a includes an ink supply unit 91 that supplies ink toward the discharge head H and a pressure generating unit 93 that generates the supply pressure applied to the ink supply unit 91. The ink supply unit 91 includes a supply tank 91a that stores the ink to be supplied to the discharge head H and a supply pipe 91b that delivers the ink supplied from the supply tank 91a to the discharge head H. The ink recovery mechanism 9b includes an ink recovery unit 92 that recovers ink from the discharge head H and a pressure generating unit 94 that generates the pressure applied to the ink recovery unit 92. The ink recovery unit 92 includes a recovery tank 92a for storing the ink recovered from the discharge head H, and a recovery pipe 92b for sending the ink recovered from the discharge head H to the recovery tank 92a.

[0045] The supply tank 91a and the recovery tank 92a are each positioned above the discharge head H. The ink return mechanism 9c includes a return pipe 902 that connects the recovery tank 92a and the supply tank 91a, a circulation pump 90 interposed in the middle of the return pipe 902, and a degassing filter 901 interposed in the middle of the return pipe 902 and positioned between the circulation pump 90 and the supply tank 91. The circulation pump 90 sends ink from the recovery tank 92a to the supply tank 91a, and the degassing filter 901 removes gas from the ink before it flows out of the circulation pump 90 and into the supply tank 91. With this ink supply mechanism 9, the circulation pump 90 can send ink along a first path Ca from the recovery tank 92 to the supply tank 91.

[0046] Furthermore, the ink supply mechanism 9a includes a supply-side pressure generating unit 93 (hereinafter referred to as "pressure generating unit 93" as appropriate) that applies pressure P1 (negative pressure) to the supply tank 91a. This pressure generating unit 93 includes a pressure tank 931, an exhaust pump 932 that exhausts the pressure tank 931 and generates pressure P1 in the pressure tank 931, a flexible tube 933 with one end connected to the pressure tank 931, and a pressure transmission pipe 934 with one end connected to the other end of the tube 933 so as to be able to communicate, with the other end facing the atmosphere inside the supply tank 91a, and which transmits the pressure generated inside the pressure tank 931 to the supply tank 91a. The pressure P1 generated inside the pressure tank 931 by the exhaust pump 932 is then supplied to the supply tank 91a via the tube 933 and the pressure transmission pipe 964.

[0047] In contrast, in the supply tank 91a, gas (air) is accumulated above the ink level. In other words, in the supply tank 91a, ink is stored below the gas-liquid interface, and gas is present above the gas-liquid interface. Therefore, the pressure generating unit 93 applies pressure P1 (negative pressure) to the gas-liquid interface in the supply tank 91a.

[0048] Furthermore, the ink recovery mechanism 9b includes a recovery-side pressure generating unit 94 (hereinafter appropriately referred to as "pressure generating unit 94") that applies pressure P2 (negative pressure) to the recovery tank 92a. This pressure generating unit 94 includes a pressure tank 941, an exhaust pump 942 that exhausts the pressure tank 941 and generates pressure P2 in the pressure tank 941, a flexible tube 943 with one end connected to the pressure tank 941, and a pressure transmission pipe 944 with one end connected to the other end of the tube 973 so as to be able to communicate, with the other end facing the atmosphere inside the recovery tank 92, and which transmits the pressure generated in the pressure tank 941 to the recovery tank 92a. The pressure P2 generated in the pressure tank 941 by the exhaust pump 942 is then supplied to the recovery tank 92a via the tube 943 and the pressure transmission pipe 944.

[0049] In contrast, in the recovery tank 92a, gas (air) accumulates above the ink level. In other words, in the recovery tank 92a, ink is stored below the gas-liquid interface, and gas exists above the gas-liquid interface. Therefore, the pressure generation unit 94 applies pressure P2 (negative pressure) to the gas-liquid interface in the recovery tank 92a.

[0050] In the state shown in Figure 4, the pressure P2 applied by the pressure generating unit 94 to the recovery tank 92a is lower than the pressure P1 applied by the pressure generating unit 93 to the supply tank 91a. Due to this difference between pressure P2 and pressure P1, ink flows along the second path Cb from the supply tank 91a through the ink supply chamber Hc of the discharge head H to the recovery tank 92a. The ink that has flowed into the recovery tank 92a along the second path Cb is returned to the supply tank 91a by the circulation pump 90 along the first path Ca. In this way, the ink circulates along a circulation path (second path Cb + first path Ca) that goes from the supply tank 91a to the recovery tank 92a via the discharge head H and then back to the supply tank 91a.

[0051] The ink supply, recovery, and circulation mechanism 9 includes a buffer tank 95, which can store a larger amount of ink than the supply tank 91a and the recovery tank 92a. The ink supply, recovery, and circulation mechanism 9 also includes a pipe 951 connecting the buffer tank 95 and the supply tank 91a, and a pipe 952 connecting the buffer tank 95 and the recovery tank 92a. In other words, the buffer tank 95 is in communication with the supply tank 91a via pipe 951 and with the recovery tank 92a via pipe 952.

[0052] Furthermore, the ink supply, recovery, and circulation mechanism 9 includes a recovery pump 953 attached to piping 951 between the buffer tank 95 and the supply tank 91a, a supply pump 954 attached to piping 952 between the buffer tank 95 and the recovery tank 92a, a solenoid valve 955 attached to piping 951 between the recovery pump 953 and the supply tank 91a, and a solenoid valve 956 attached to piping 952 between the supply pump 954 and the recovery tank 92a. Therefore, with the solenoid valve 955 open, the recovery pump 953 can recover ink from the supply tank 91a to the buffer tank 95 via the solenoid valve 955, and with the solenoid valve 956 open, the supply pump 962 can supply ink from the buffer tank 95 to the recovery tank 92a via the solenoid valve 956.

[0053] In the ink supply, recovery, and circulation mechanism 9 configured in this way, the recovery pump 961 and the supply pump 962 operate, causing ink to flow along a third path Cc from the supply tank 91a through the buffer tank 95 to the recovery tank 92a. The ink that flows into the recovery tank 92a along the third path Cc is returned to the supply tank 91a along the first path Ca by the circulation pump 90. In this way, the ink circulates along a circulation path (third path Cc + first path Ca) that goes from the supply tank 91a to the recovery tank 92a via the buffer tank 95 and then back to the supply tank 91a.

[0054] Furthermore, the ink supply, recovery, and circulation mechanism 9 includes a mechanism for supplying ink to the buffer tank 95. This will be explained using Figure 5. Figure 5 is a schematic diagram showing the mechanism for supplying ink to the buffer tank. As shown in Figure 5, the ink supply, recovery, and circulation mechanism 9 includes a buffer tank 96 that stores the ink supplied to the buffer tank 95, and a main tank 97 that stores the ink supplied to the buffer tank 96. In other words, ink is supplied from the main tank 97 to the buffer tank 96, and then from the buffer tank 96 to the buffer tank 95. The ink stored in the buffer tank 95 is then supplied to the discharge head H and discharged from the nozzle Hn. The buffer tank 96 and the main tank 97 have volumes that can store a larger amount of ink than the buffer tank 95, and the buffer tank 96 has a volume that can store a larger amount of ink than the main tank 97. For example, the buffer tank 95 has a volume of 60 liters, the buffer tank 96 has a volume of 250 liters, and the main tank 97 has a volume of 200 liters. However, the relative sizes of these volumes are not limited to this example. In the following, buffer tank 95 will be referred to as "downstream buffer tank 95" and buffer tank 96 as "upstream buffer tank 96" as appropriate.

[0055] Furthermore, the ink supply, recovery, and circulation mechanism 9 includes a pre-stage liquid delivery unit 81 that delivers ink from the main tank 97 to the pre-stage buffer tank 96. This pre-stage liquid delivery unit 81 has a pre-stage liquid delivery pipe 811 that connects the main tank 97 and the pre-stage buffer tank 96, and the main tank 97 and the pre-stage buffer tank 96 are in communication via the pre-stage liquid delivery pipe 811. The pre-stage liquid delivery unit 81 also has a pre-stage liquid delivery pump 812 attached to the pre-stage liquid delivery pipe 811, which drives the ink in the pre-stage liquid delivery pipe 811 from the main tank 97 to the pre-stage buffer tank 96. In addition, the pre-stage liquid delivery unit 81 has a pre-stage solenoid valve 813 attached to the pre-stage liquid delivery pipe 811 between the pre-stage liquid delivery pump 812 and the main tank 97. This front-stage solenoid valve 813 allows ink to flow between the main tank 97 and the front-stage liquid transfer pump 812 when opened, and blocks ink flow between the main tank 97 and the front-stage liquid transfer pump 812 when closed. In this front-stage liquid transfer unit 81, with the front-stage solenoid valve 813 open, the front-stage liquid transfer pump 812 operates to send ink from the main tank 97 to the front-stage buffer tank 96.

[0056] Furthermore, the ink supply, recovery, and circulation mechanism 9 includes a downstream liquid delivery unit 82 that delivers ink from the upstream buffer tank 96 to the downstream buffer tank 95. This downstream liquid delivery unit 82 has a downstream liquid delivery pipe 821 that connects the upstream buffer tank 96 and the downstream buffer tank 95, and the upstream buffer tank 96 and the downstream buffer tank 95 are in communication via the downstream liquid delivery pipe 821. The downstream liquid delivery unit 82 also has a downstream liquid delivery pump 822 attached to the downstream liquid delivery pipe 821, which delivers ink from the upstream buffer tank 96 to the downstream buffer tank 95. Furthermore, the downstream liquid delivery unit 82 has a downstream solenoid valve 823 attached to the downstream liquid delivery pipe 821 between the downstream liquid delivery pump 822 and the upstream buffer tank 96. The downstream solenoid valve 823, when opened, allows ink to flow between the upstream buffer tank 96 and the downstream liquid transfer pump 822, and when closed, blocks ink flow between the upstream buffer tank 96 and the downstream liquid transfer pump 822. The downstream liquid transfer unit 82 also has a downstream solenoid valve 824 attached to the downstream liquid transfer piping 821 between the downstream liquid transfer pump 822 and the downstream buffer tank 95. This downstream solenoid valve 824, when opened, allows ink to flow between the downstream liquid transfer pump 822 and the downstream buffer tank 95, and when closed, blocks ink flow between the downstream liquid transfer pump 822 and the downstream buffer tank 95. In this downstream liquid transfer unit 82, with the downstream solenoid valves 823 and 824 open, the downstream liquid transfer pump 822 operates to send ink from the upstream buffer tank 96 to the downstream buffer tank 95.

[0057] Furthermore, the ink supply, recovery, and circulation mechanism 9 includes an ink return unit 83 that returns ink that has flowed from the upstream buffer tank 96 to the downstream liquid delivery unit 82 back to the upstream buffer tank 96. This ink return unit 83 has an ink return pipe 831 that connects the upstream buffer tank 96 to a branch section 825 provided in the downstream liquid delivery piping 821 between the downstream liquid delivery pump 822 and the downstream solenoid valve 824, and the branch section 825 of the downstream liquid delivery piping 821 and the upstream buffer tank 96 are in communication via the ink return pipe 831. The ink return unit 83 also has an ink return solenoid valve 832 provided in the ink return pipe 831 between the branch section 825 and the upstream buffer tank 96. When the ink return solenoid valve 832 is opened, it allows the flow of ink between the branch section 825 and the upstream buffer tank 96, and when it is closed, it blocks the flow of ink between the branch section 825 and the upstream buffer tank 96. When ink is supplied from the upstream buffer tank 96 to the downstream buffer tank 95, the ink return solenoid valve 832 of the ink return unit 83 is closed, and the flow of ink from the branching section 825 to the upstream buffer tank 96 is blocked.

[0058] On the other hand, during print standby periods when ink ejection from the nozzle Hn of the ejection head H is stopped, the following ink circulation operation is performed by the downstream liquid supply unit 82 and the ink return unit 83 to return the ink that has flowed from the upstream buffer tank 96 to the downstream liquid supply piping 821 back to the upstream buffer tank 96 via the ink return unit 83. Specifically, the downstream solenoid valve 823 opens, the downstream solenoid valve 824 closes, and the ink return solenoid valve 832 opens. Therefore, the flow of ink along the downstream liquid supply piping 821 from the upstream buffer tank 96 to the downstream liquid supply pump 822 is enabled by the downstream solenoid valve 823, the flow of ink from the branch 825 of the downstream liquid supply piping 821 to the downstream buffer tank 95 is blocked by the downstream solenoid valve 824, and the flow of ink along the ink return piping 831 from the branch 825 of the downstream liquid supply piping 821 to the upstream buffer tank 96 is enabled by the ink return solenoid valve 832. In this state, the subsequent liquid transfer pump 822 operates, executing the ink circulation operation described above.

[0059] Furthermore, the ink supply, recovery, and circulation mechanism 9 includes a downstream stirring unit 85 that stirs the ink stored in the downstream buffer tank 95, a upstream stirring unit 86 that stirs the ink stored in the upstream buffer tank 96, and a main stirring unit 87 that stirs the ink stored in the main tank 97. Of the stirring units 85, 86, and 87, the stirring unit 87 is provided only in the ink supply, recovery, and circulation mechanism 9 that supplies white ink, and is not provided in the ink supply mechanism 9 that supplies color ink.

[0060] The downstream stirring unit 85 includes a shaft 851 extending parallel to the vertical Z, a stirring motor 852 that rotates the shaft 851, and a stirring blade 853 attached to the lower end of the shaft 851. The stirring motor 852 is located outside the downstream buffer tank 95. On the other hand, the stirring blade 853 is located inside the downstream buffer tank 95 and is immersed in the ink stored in the downstream buffer tank 95. When the stirring motor 852 rotates the shaft 851, the stirring blade 853 rotates around a rotation axis parallel to the vertical Z. The ink in the downstream buffer tank 95 is stirred by the rotating stirring blade 853.

[0061] The pre-stage stirring unit 86 includes a shaft 861 extending parallel to the vertical Z, a stirring motor 862 that rotates the shaft 861, and a stirring blade 863 attached to the lower end of the shaft 861. The stirring motor 862 is located outside the pre-stage buffer tank 96. On the other hand, the stirring blade 863 is located inside the pre-stage buffer tank 96 and is immersed in the ink stored in the pre-stage buffer tank 96. When the stirring motor 862 rotates the shaft 861, the stirring blade 863 rotates around a rotation axis parallel to the vertical Z. The ink in the pre-stage buffer tank 96 is stirred by the rotating stirring blade 863.

[0062] The main stirring unit 87 includes a shaft 871 extending parallel to the vertical Z, a stirring motor 872 that rotates the shaft 871, and a stirring blade 873 attached to the lower end of the shaft 871. The stirring motor 872 is located outside the main tank 97. The stirring blade 873, on the other hand, is located inside the main tank 97 and is immersed in the ink stored in the main tank 97. When the stirring motor 872 rotates the shaft 871, the stirring blade 873 rotates around a rotation axis parallel to the vertical Z. The ink in the main tank 97 is stirred by the rotating stirring blade 873.

[0063] The stirring blade 873 is a centrifugal-type stirring blade that opens due to centrifugal force. The top surface of the main tank 97 is provided with an insertion port 971 into which the stirring blade 873 can be inserted. In other words, the stirring blade 873 can be placed inside the main tank 97 by inserting it through the insertion port 971 of the main tank 97 in its closed state. Then, when the stirring motor 872 rotates the stirring blade 873, the stirring blade 873 opens due to centrifugal force and stirs the ink.

[0064] By agitating the ink with these agitator blades 853, 863, and 873, the physical properties of the ink stored in the downstream buffer tank 95, the upstream buffer tank 96, and the main tank 97 can be made uniform. In particular, white ink contains a large amount of titanium dioxide as a pigment to ensure a high opacity. As a result, white ink has low viscosity and a high specific gravity of pigment relative to the specific gravity of solvent, making the pigment prone to sedimentation. Such pigment sedimentation can lead to problems such as insufficient opacity or deterioration of the white ink ejection performance from the ejection head H. In contrast, agitating the white ink with the agitator blades 853, 863, and 873 can suppress pigment sedimentation.

[0065] Furthermore, the downstream buffer tank 95 is provided with a downstream liquid level sensor S95 for detecting the liquid level of the ink stored in the downstream buffer tank 95, the upstream buffer tank 96 is provided with an upstream liquid level sensor S96 for detecting the liquid level of the ink stored in the upstream buffer tank 96, and the main tank 97 is provided with a main liquid level sensor S97 for detecting the liquid level of the ink stored in the main tank 97.

[0066] Figure 6 is a block diagram showing the electrical configuration of a printing device. As shown in Figure 6, the printing device 3 includes a control unit 5 that comprehensively controls the entire device. This control unit 5 is a processor, for example, composed of a CPU (Central Processing Unit). The printing device 3 also includes a UI (User Interface) 301 having input devices such as a keyboard and mouse, and output devices such as a display, and the control unit 5 has a UI control unit 501 that controls the UI 39. The UI control unit 501 performs control in response to the operator's input operations on the input devices of the UI 39, while also controlling the UI 39 so that the output devices of the UI 39 display information to the operator. Note that the input devices and output devices of the UI 39 may be integrated using, for example, a touch panel display.

[0067] The control unit 5 includes a stirring motor control unit 502 that controls the stirring motor 872 of the main stirring unit 87. By controlling the stirring motor 872 so that it rotates, the stirring blades 873 rotate, and the ink in the main tank 97 is stirred. The control unit 5 also includes a main sensor output acquisition unit 503 that acquires the detection result of the main liquid level sensor S97.

[0068] The control unit 5 has a pre-stage solenoid valve control unit 504 that controls the opening and closing of the pre-stage solenoid valve 813 of the pre-stage liquid delivery unit 81, and the pre-stage solenoid valve 813 opens and closes according to the control of the pre-stage solenoid valve control unit 504. The control unit 5 also has a pre-stage pump control unit 505 that controls the pre-stage liquid delivery pump 812 of the pre-stage liquid delivery unit 81, and the pre-stage liquid delivery pump 812 starts or stops the ink delivery according to the control of the pre-stage pump control unit 505.

[0069] The control unit 5 includes a stirring motor control unit 506 that controls the stirring motor 862 of the upstream stirring unit 86. By controlling the stirring motor 862 so that it rotates, the stirring blades 863 rotate, and the ink in the upstream buffer tank 96 is stirred. The control unit 5 also includes an upstream sensor output acquisition unit 507 that acquires the detection result of the upstream liquid level sensor S96.

[0070] The control unit 5 includes a downstream solenoid valve control unit 508 that controls the opening and closing of the downstream solenoid valve 823 of the downstream liquid supply unit 82, and the downstream solenoid valve 823 opens and closes according to the control of the downstream solenoid valve control unit 508. The control unit 5 also includes a downstream pump control unit 509 that controls the downstream liquid supply pump 822 of the downstream liquid supply unit 82, and the downstream liquid supply pump 822 starts or stops supplying ink according to the control of the downstream pump control unit 509.

[0071] The control unit 5 includes a downstream solenoid valve control unit 510 that controls the opening and closing of the downstream solenoid valve 824 of the downstream liquid supply unit 82, and the downstream solenoid valve 824 opens and closes according to the control of the downstream solenoid valve control unit 510. The control unit 5 also includes a return solenoid valve control unit 511 that controls the opening and closing of the ink return solenoid valve 832 of the ink return unit 83, and the ink return solenoid valve 832 opens and closes according to the control of the return solenoid valve control unit 511.

[0072] Furthermore, the control unit 5 includes a stirring motor control unit 512 that controls the stirring motor 852 of the downstream stirring unit 85. The stirring motor control unit 512 controls the stirring motor 852 so that it rotates, causing the stirring blades 853 to rotate and the ink in the downstream buffer tank 95 to be stirred. The control unit 5 also includes a downstream sensor output acquisition unit 513 that acquires the detection result of the downstream liquid level sensor S95.

[0073] The control unit 5 includes a recovery pump control unit 514 that controls the recovery pump 953, and the recovery pump 953 starts or stops supplying ink according to the control of the recovery pump control unit 514. The control unit 5 also includes a supply pump control unit 515 that controls the supply pump 954, and the supply pump 954 starts or stops supplying ink according to the control of the supply pump control unit 515.

[0074] Furthermore, the control unit 5 has a solenoid valve control unit 516 that controls the opening and closing of the solenoid valve 955, and the solenoid valve 955 opens and closes according to the control of the solenoid valve control unit 516. In addition, the control unit 5 has a solenoid valve control unit 517 that controls the opening and closing of the solenoid valve 956, and the solenoid valve 956 opens and closes according to the control of the solenoid valve control unit 517.

[0075] In the printing apparatus 3 having the above configuration, ink supply is controlled based on the liquid levels detected by the liquid level sensors S95, S96, and S97. Next, this liquid supply control will be explained. Figure 7A is a schematic diagram showing the detection position of the downstream liquid level sensor provided on the downstream buffer tank for executing liquid supply control, Figure 7B is a schematic diagram showing the detection position of the upstream liquid level sensor provided on the upstream buffer tank for executing liquid supply control, and Figure 8 is a schematic diagram showing the detection position of the main liquid level sensor provided on the main tank for executing liquid supply control.

[0076] The downstream liquid level sensor S95, which detects the liquid level of the ink stored in the downstream buffer tank 95, detects the presence or absence of ink in the downstream buffer tank 95 at a low level Lll, a high level Llh (higher than the low level Lll), and a limit level Llt (higher than the high level Llh), as shown in Figure 7A. This downstream liquid level sensor S95 can be constructed by combining liquid detection sensors arranged to correspond to each level Lll, Llh, and Llt. Various sensors such as capacitive sensors or float sensors can be used as liquid detection sensors. In addition, depending on the type of sensor used, the sensor can be placed inside or outside the downstream buffer tank 95.

[0077] In this configuration, if the downstream liquid level sensor S95 does not detect ink at the low level Lll, the ink level in the downstream buffer tank 95 is below the low level Lll. If the downstream liquid level sensor S95 detects ink at the low level Lll but does not detect ink at the high level Llh, the ink level in the downstream buffer tank 95 is above the low level Lll and below the high level Llh. If the downstream liquid level sensor S95 detects ink at the high level Llh but does not detect ink at the limit level Llt, the ink level in the downstream buffer tank 95 is above the high level Llh and below the limit level Llt. If the downstream liquid level sensor S95 detects ink at the limit level Llt, the ink level in the downstream buffer tank 95 is above the limit level Llt.

[0078] In the control unit 5, based on the detection result of the downstream liquid level sensor S95 acquired by the downstream sensor output acquisition unit 513, the downstream pump control unit 509 controls the supply of ink by the downstream liquid supply pump 822, thereby adjusting the liquid level of the ink in the downstream buffer tank 95 to the normal liquid level range R95, from low level Lll to high level Llh. In contrast, the stirring blade 853 is positioned below the normal liquid level range R95 (in other words, below the low level Lll). Therefore, the stirring blade 853 is positioned below the liquid level adjusted to the normal liquid level range R95 and is immersed in the ink in the downstream buffer tank 95.

[0079] Furthermore, the lower end of the pipe 952 that sends ink from the downstream buffer tank 95 to the discharge head H is positioned slightly higher than the bottom of the downstream buffer tank 95 and below the low level Lll. Therefore, the pipe 952 can draw in ink from the downstream buffer tank 95, whose liquid level is normally adjusted to the liquid level range R95, through the opening at the lower end of the pipe 952 and send it to the discharge head H.

[0080] The pre-stage liquid level sensor S96, which detects the liquid level of the ink stored in the pre-stage buffer tank 96, detects the presence or absence of ink in the pre-stage buffer tank 96 at a low level Lfl, a high level Lfh (higher than the low level Lfl), and a limit level Lft (higher than the high level Lfh), as shown in Figure 7B. This pre-stage liquid level sensor S96 can be constructed by combining liquid detection sensors arranged to correspond to each level Lfl, Lfh, and Lft. Various sensors such as capacitive sensors or float sensors can be used as liquid detection sensors. Furthermore, depending on the type of sensor used, the sensor can be placed inside or outside the pre-stage buffer tank 96.

[0081] In this configuration, if the pre-stage liquid level sensor S96 does not detect ink at the low level Lfl, the liquid level of ink in the pre-stage buffer tank 96 is below the low level Lfl. If the pre-stage liquid level sensor S96 detects ink at the low level Lfl but does not detect ink at the high level Lfh, the liquid level of ink in the pre-stage buffer tank 96 is above the low level Lfl and below the high level Lfh. If the pre-stage liquid level sensor S96 detects ink at the high level Lfh but does not detect ink at the limit level Lft, the liquid level of ink in the pre-stage buffer tank 96 is above the high level Lfh and below the limit level Lft. If the pre-stage liquid level sensor S96 detects ink at the limit level Lft, the liquid level of ink in the pre-stage buffer tank 96 is above the limit level Lft.

[0082] In the control unit 5, based on the detection result of the pre-stage liquid level sensor S96 acquired by the pre-stage sensor output acquisition unit 507, the pre-stage pump control unit 505 controls the supply of ink by the pre-stage liquid supply pump 812, thereby adjusting the liquid level of the ink in the pre-stage buffer tank 96 to the normal liquid level range R96, from low level Lfl to high level Lfh. In contrast, the stirring blade 863 is positioned below the normal liquid level range R96 (in other words, below the low level Lfl). Therefore, the stirring blade 863 is positioned below the liquid level adjusted to the normal liquid level range R96 and is immersed in the ink in the pre-stage buffer tank 96.

[0083] Furthermore, the lower end of the downstream liquid transfer pipe 821, which sends the ink from the upstream buffer tank 96 to the downstream buffer tank 95, is positioned slightly higher than the bottom of the upstream buffer tank 96 and below the low level Lfl. Therefore, the downstream liquid transfer pipe 821 can draw the ink from the upstream buffer tank 96, whose liquid level is normally adjusted to the liquid level range R96, through the opening at the lower end of the downstream liquid transfer pipe 821 and send it to the downstream buffer tank 95.

[0084] The main liquid level sensor S97, which detects the liquid level of the ink stored in the main tank 97, detects the presence or absence of ink in the main tank 97 at the low level Lml, as shown in Figure 8. The main liquid level sensor S97 can be composed of liquid detection sensors positioned corresponding to the low level Lml. Various sensors such as capacitive sensors or float sensors can be used as liquid detection sensors. Depending on the type of sensor used, the sensor can be positioned inside or outside the main tank 97. Note that the main liquid level sensor S97 for the main tank 97 is provided only in the ink supply, recovery, and circulation mechanism 9 that supplies white ink, and is not provided in the ink supply, recovery, and circulation mechanism 9 that supplies color ink.

[0085] As shown in Figure 8, the low level Lml is provided at a height corresponding to the stirring blade 873 located inside the main tank 97. This stirring blade 873 has multiple rotating blades 873a that rotate around a pivot axis 871a provided at the lower end of the shaft 871. As shown in the "Static State" column of Figure 8, when the stirring blade 873 is stationary, the rotating blades 873a hang down from the pivot axis 871a due to gravity, and the stirring blade 873 closes. On the other hand, as shown in the "Rotating State" column of Figure 8, when the stirring blade 873 is rotating, the rotating blades 873a are subjected to centrifugal force and rotate upward around the pivot axis 871a, causing the stirring blade 873 to open. The low level Lml is provided at the same height as the lower end of the stirring blade 873 that has opened due to rotation.

[0086] In this configuration, if the main liquid level sensor S97 does not detect ink, the ink level in the main tank 97 is below the low level Lml. On the other hand, if the main liquid level sensor S97 does detect ink, the ink level in the main tank 97 is above the low level Lml.

[0087] Furthermore, the lower end of the pre-stage liquid transfer pipe 811, which sends ink from the main tank 97 to the pre-stage buffer tank 96, is positioned slightly above the bottom of the main tank 97 and below the low level Lml. Therefore, the pre-stage liquid transfer pipe 811 can draw in ink from the main tank 97 when the liquid level has dropped below the low level Lml through the opening at the lower end of the pre-stage liquid transfer pipe 811 and send it to the pre-stage buffer tank 96.

[0088] Figure 9 is a flowchart showing an example of ink supply control from the main tank performed in the printing device, and Figures 10A and 10B schematically show the operations performed in the ink supply control in Figure 9. The ink supply control in Figure 9 controls the supply of white ink from the main tank 97 to the pre-buffer tank 96 and is performed by the control unit 5. Note that the ink supply control in Figure 9 does not apply to the supply of color ink.

[0089] At the start of the ink supply control shown in Figure 9, the agitator blade 863 rotates to agitate the ink in the buffer tank 96, and the agitator blade 873 rotates to agitate the ink in the main tank 97. The solenoid valve 813 is closed, and the supply of ink from the main tank 97 to the ink supply pump 812 via the solenoid valve 813 is blocked. The ink supply pump 812 is also stopped.

[0090] In step S101, the pre-stage sensor output acquisition unit 507 confirms whether the ink level in the pre-stage buffer tank 96 is below the low level Lfl, based on the detection result of the pre-stage liquid level sensor S96 installed in the pre-stage buffer tank 96. As shown in the "S101" column of Figure 10A, if the ink level in the pre-stage buffer tank 96 is below the low level Lfl (if the result in step S101 is "YES"), the process proceeds to step S102.

[0091] In step S102, the main sensor output acquisition unit 503 confirms whether the ink level in the main tank 97 is below the low level Lml based on the detection result of the main liquid level sensor S97 installed in the main tank 97. As shown in column "S102" in Figure 10A, if the ink level in the main tank 97 is above the low level Lml (if "NO" is found in step S102), the liquid supply operation (ink replenishment liquid supply operation) in steps S103 to S105 is executed, and the ink level in the upstream buffer tank 96 rises as shown in columns "S103" and "S105" in Figure 10A.

[0092] In step S103, the pre-stage solenoid valve control unit 504 controls the pre-stage solenoid valve 813 so that it opens, and the pre-stage pump control unit 505 controls the pre-stage liquid transfer pump 812 so that it starts supplying ink. As a result, the pre-stage liquid transfer pump 812 starts supplying ink from the main tank 97 to the pre-stage buffer tank 96. Along with this ink supply, the ink level in the main tank 97 decreases, while the ink level in the pre-stage buffer tank 96 increases. Then, when the pre-stage sensor output acquisition unit 507 confirms that the ink level in the pre-stage buffer tank 96 has exceeded the low level Lfl and reached the high level Lfh based on the detection result of the pre-stage liquid level sensor S96 (if "YES" is obtained in step S104), the pre-stage pump control unit 505 controls the pre-stage liquid transfer pump 812 to stop supplying ink, and the pre-stage solenoid valve control unit 504 controls the pre-stage solenoid valve 813 to close (step S105). As a result, the supply of ink from the main tank 97 to the pre-stage buffer tank 96 by the pre-stage liquid transfer pump 812 is stopped. Then, the process returns to step S101.

[0093] On the other hand, as shown in column "S102" of Figure 10B, if the ink level in the main tank 97 is less than the low level Lml (if "YES" is answered in step S102), the liquid supply operation (disposable ink supply operation) in steps S106 to S108 is performed, and the ink level in the buffer tank 95 rises as shown in columns "S106" and "S107" of Figure 10B.

[0094] In step S106, the pre-stage solenoid valve control unit 504 controls the pre-stage solenoid valve 813 so that it opens, and the pre-stage pump control unit 505 controls the pre-stage liquid transfer pump 812 so that it starts supplying ink. As a result, the pre-stage liquid transfer pump 812 starts supplying ink from the main tank 97 to the pre-stage buffer tank 96. Along with this ink supply, the ink level in the main tank 97 decreases, while the ink level in the buffer tank 95 increases. Then, when the pre-stage sensor output acquisition unit 507 confirms that the ink level in the pre-stage buffer tank 96 has exceeded the low level Lfl and the high level Lfh and reached the limit level Lft, based on the detection result of the pre-stage liquid level sensor S96 (YES in step S107), the pre-stage pump control unit 505 controls the pre-stage liquid transfer pump 812 to stop supplying ink, and the pre-stage solenoid valve control unit 504 controls the pre-stage solenoid valve 813 to close (step S108). As a result, the supply of ink from the main tank 97 to the pre-stage buffer tank 96 by the pre-stage liquid transfer pump 812 is stopped.

[0095] The ink supply operation in steps S106-S108 (disposable ink supply operation) sends a larger amount of ink from the main tank 97 to the buffer tank 96 than the ink supply operation in steps S103-S105 (ink replenishment supply operation). By performing the ink supply operation in steps S106-S108, a small amount of ink within a predetermined range (for example, about 1-5% of the volume of the main tank 97) remains at the bottom of the main tank 97, and the rest of the ink is sent from the main tank 97 to the buffer tank 96. As a result, the amount of ink remaining in the main tank 97 becomes virtually zero. Therefore, the UI control unit 501 controls the UI 39 so that it notifies the operator of a low ink level warning indicating that the main tank 97 has run out of ink (step S109).

[0096] Figure 11 is a flowchart showing an example of ink supply control from the front buffer tank performed in the printing device, and Figure 12 is a schematic diagram showing the operations performed in the ink supply control in Figure 11. The ink supply control in Figure 11 controls the supply of ink from the front buffer tank 96 to the rear buffer tank 95, and is performed by the control unit 5.

[0097] At the start of the ink supply control shown in Figure 9, the agitator blade 853 rotates to agitate the ink in the downstream buffer tank 95, and the agitator blade 863 rotates to agitate the ink in the upstream buffer tank 96. The downstream solenoid valve 824 is open, allowing ink to be supplied from the branching section 825 to the downstream buffer tank 95 via the downstream solenoid valve 824, while the ink return solenoid valve 832 is closed, blocking ink supply from the branching section 825 to the upstream buffer tank 96 via the ink return solenoid valve 832. Furthermore, the downstream solenoid valve 823 is closed, blocking ink supply from the upstream buffer tank 96 to the downstream buffer tank 95 via the downstream solenoid valve 823. The downstream liquid supply pump 822 is also stopped.

[0098] In step S201, the downstream sensor output acquisition unit 513 confirms whether the ink level in the downstream buffer tank 95 is below the low level Lll, based on the detection result of the downstream liquid level sensor S95 installed in the downstream buffer tank 95. As shown in the "S201" column of Figure 12, if the ink level in the downstream buffer tank 95 is below the low level Lfl (if "YES" is found in step S201), the liquid supply operation (ink replenishment liquid supply operation) in steps S202 to S204 is executed, and the ink level in the downstream buffer tank 95 rises, as shown in the "S204" column of Figure 12.

[0099] In step S202, the downstream solenoid valve control unit 508 controls the downstream solenoid valve 823 so that it opens, and the downstream pump control unit 509 controls the downstream liquid transfer pump 822 so that it starts transferring ink. As a result, the downstream liquid transfer pump 822 starts transferring ink from the upstream buffer tank 96 to the downstream buffer tank 95. Along with this ink transfer, the ink level in the upstream buffer tank 96 decreases, while the ink level in the downstream buffer tank 95 increases. Then, when the downstream sensor output acquisition unit 513 confirms that the ink level in the downstream buffer tank 95 has exceeded the low level Lll and reached the high level Llh based on the detection result of the downstream liquid level sensor S95 (if "YES" is obtained in step S203), the downstream pump control unit 509 controls the downstream liquid transfer pump 822 to stop supplying ink, and the downstream solenoid valve control unit 508 controls the downstream solenoid valve 823 to close (step S204). As a result, the supply of ink from the upstream buffer tank 96 to the downstream buffer tank 95 by the downstream liquid transfer pump 822 is stopped. Then, the process returns to step S201.

[0100] Figure 13 is a flowchart showing an example of liquid delivery mode switching control performed in a printing device. The liquid delivery mode switching control in Figure 13 switches the liquid delivery mode for delivering ink from the pre-load buffer tank 96, and is performed by the control unit 5.

[0101] In step S301, ink is supplied from the upstream buffer tank 96 to the downstream buffer tank 95 by the ink supply control shown in Figure 11. In step S302, the control unit 5 determines whether the printing operation, which prints an image on the printing medium M using the ink ejected from the nozzle Hn of the ejection head H, has been stopped. If the printing operation is not stopped (if the answer in step S302 is "YES"), the ink supply control shown in Figure 11 continues (step S301).

[0102] On the other hand, if the printing operation stops (if "YES" is answered in step S302), the ink supply operation (ink circulation operation) in steps S303 to S306 is executed. In step S303, the return solenoid valve control unit 511 controls the ink return solenoid valve 832 so that it opens. This allows ink to be supplied from the branching section 825 to the upstream buffer tank 96 via the ink return solenoid valve 832. In step S304, the downstream solenoid valve control unit 510 controls the downstream solenoid valve 824 so that it closes. This shuts off the supply of ink from the branching section 825 to the downstream buffer tank 95 via the downstream solenoid valve 824.

[0103] In step S305, the downstream solenoid valve control unit 508 controls the downstream solenoid valve 823 so that it opens, and in step S306, the downstream pump control unit 509 controls the downstream liquid transfer pump 822 so that it starts transferring ink. As a result, the downstream liquid transfer pump 822 starts transferring ink from the upstream buffer tank 96 to the branching section 825. Also, as described above, since the downstream solenoid valve 824 is closed and the ink return solenoid valve 832 is open, the ink that reaches the branching section 825 flows into the ink return pipe 831 and is returned to the upstream buffer tank 96.

[0104] In the embodiment described above, a main tank 97, a pre-buffer tank 96, and a post-buffer tank 95 are provided. Ink is supplied from the main tank 97 to the pre-buffer tank 96, and then from the pre-buffer tank 96 to the post-buffer tank 95. The ink stored in the post-buffer tank 95 is then supplied to the discharge head H and discharged from the nozzle Hn of the discharge head H. A stirring blade 873 (main tank stirring member) is also placed inside the main tank 97, and the rotation of the stirring blade 873 stirs the ink stored in the main tank 97. The positional relationship between the low level Lml (a predetermined position) provided in conjunction with the stirring blade 873 and the liquid level of the ink in the main tank 97 is detected by the main liquid level sensor S97. Furthermore, if it is detected that the ink level in the main tank 97 is lower than the low level Lml (YES in step S102), ink is sent from the main tank 97 to the preceding buffer tank 96 (steps S106-S108, first main ink supply operation). This makes it possible to prevent a large amount of ink from remaining in the main tank 97 without being agitated.

[0105] Furthermore, the system is equipped with a pre-stage liquid level sensor S96 (pre-stage buffer tank liquid level detection unit) that detects the positional relationship between the liquid level of the ink stored in the pre-stage buffer tank 96 and the normal liquid level range R96 (pre-stage normal liquid level range) provided for the pre-stage buffer tank 96. In response, the control unit 5 has a pre-stage sensor output acquisition unit 507 (pre-stage buffer tank liquid level acquisition unit) that acquires the detection result of the pre-stage liquid level sensor S96. Then, if the pre-stage sensor output acquisition unit 507 obtains a detection result indicating that the liquid level of the ink in the pre-stage buffer tank 96 is lower than the lower end (low level Lfl) of the normal liquid level range R96 (YES in step S101), and if the detection result obtained by the main sensor output acquisition unit 503 (main tank liquid level acquisition unit) indicates that the liquid level of the ink in the main tank 97 is lower than the low level Lml (YES in step S102), then the pre-stage pump control unit 505 (main liquid supply pump control unit) controls the pre-stage liquid supply pump 812 (main liquid supply pump) to execute the liquid supply operation (first main liquid supply operation) in steps S106 to S108. In this configuration, if the liquid level of the ink in the pre-stage buffer tank 96 decreases to below the lower end (low level Lfl) of the normal liquid level range R96, ink can be replenished in the pre-stage buffer tank 96 by the liquid supply operation in steps S106 to S108.

[0106] Furthermore, if the pre-stage sensor output acquisition unit 507 obtains a detection result indicating that the ink level in the pre-stage buffer tank 96 is lower than the lower end (low level Lfl) of the normal liquid level range R96 (if "YES" is obtained in step S101), and the detection result obtained by the main sensor output acquisition unit 503 indicates that the ink level in the main tank 97 is at or above the low level Lml, then the pre-stage pump control unit 505 controls the pre-stage liquid transfer pump 812 to transfer ink from the main tank 97 to the pre-stage buffer tank 96 in a manner different from the liquid transfer operation in steps S106 to S108 (steps S103 to S105). In this configuration, if the ink level in the pre-stage buffer tank 96 decreases to below the low level Lfl of the normal liquid level range R96, ink can be replenished in the pre-stage buffer tank 96 by the liquid transfer operation in steps S103 to S105.

[0107] Furthermore, in the liquid supply operation of steps S103 to S105, the pre-stage liquid supply pump 812 sends ink from the main tank 97 to the pre-stage buffer tank 96 until the liquid level of the ink in the pre-stage buffer tank 96 reaches the upper end (high level Lfh) of the normal liquid level range R96. In this configuration, by performing the liquid supply operation of steps S103 to S105, the liquid level of the ink in the pre-stage buffer tank 96, which has decreased to below the lower end (low level Lfl) of the normal liquid level range R96, can be raised to the upper end (high level Lfh) of the normal liquid level range R96.

[0108] Furthermore, in the liquid transfer operation of steps S106 to S108, the pre-stage liquid transfer pump 812 sends ink from the main tank 97 to the pre-stage buffer tank 96 until the liquid level of the ink in the pre-stage buffer tank 96 reaches the limit level Lft (pre-stage upper limit position), which is set at a position higher than the upper end (high level Lfh) of the normal liquid level range R96. With this configuration, a large amount of ink that can no longer be agitated in the main tank 97 can be sent from the main tank 97 to the pre-stage buffer tank 96.

[0109] Furthermore, the system is equipped with a pre-stage stirring unit 86 (pre-stage buffer tank stirring section) which has stirring blades 863 (pre-stage buffer tank stirring member) positioned inside the pre-stage buffer tank 96, and which stirs the ink stored in the pre-stage buffer tank 96 by rotating the stirring blades 863. In this configuration, ink that has become unable to be stirred in the main tank 97 can be sent to the pre-stage buffer tank 96 and stirred by the stirring blades 863.

[0110] Furthermore, the stirring blades 863 are normally positioned below the liquid level range R96. In this configuration, the ink in the upstream buffer tank 96 is stirred by the stirring blades 863, which are normally positioned below the liquid level range R96. Therefore, ink that cannot be stirred in the main tank 97 is sent to the upstream stirring unit 86 and can be reliably stirred by the stirring blades 863.

[0111] Furthermore, the system is equipped with a downstream liquid level sensor S95 (downstream buffer tank liquid level detection unit) that detects the positional relationship between the liquid level of the ink stored in the downstream buffer tank 95 and the normal liquid level range R95 (downstream normal liquid level range) provided for the downstream buffer tank 95. In response, the control unit 5 has a downstream sensor output acquisition unit 513 (downstream buffer tank liquid level acquisition unit) that acquires the detection result of the downstream liquid level sensor S95. Moreover, the control unit 5 has a downstream pump control unit 509 that controls the downstream liquid transfer pump 822. The downstream pump control unit 509 controls the downstream liquid transfer pump 822 (buffer liquid transfer pump) to perform the liquid transfer operation (buffer liquid transfer operation) of steps S202 to S204, in response to the downstream sensor output acquisition unit 513 obtaining a detection result indicating that the liquid level of ink in the downstream buffer tank 95 is lower than the lower end (low level Lll) of the normal liquid level range R95 ("YES" in step S201). In this configuration, if the liquid level of ink in the downstream buffer tank 95 decreases below the lower end (low level Lll) of the normal liquid level range R95, the downstream buffer tank 95 can be replenished with ink by the liquid transfer operation of steps S202 to S204.

[0112] Furthermore, in the liquid transfer operation of steps S202 to S204, the downstream liquid transfer pump 822 sends ink from the upstream buffer tank 96 to the downstream buffer tank 95 until the ink level in the downstream buffer tank 95 reaches the upper limit (high level Llh) of the normal liquid level range R95. In this configuration, by performing the liquid transfer operation of steps S202 to S204, the ink level in the downstream buffer tank 95, which has decreased to below the lower limit (low level Lll) of the normal liquid level range R95, can be raised to the upper limit (high level Llh) of the normal liquid level range R95.

[0113] Furthermore, the system is equipped with a downstream stirring unit 85 (downstream buffer tank stirring section) which has stirring blades 853 (downstream buffer tank stirring member) positioned inside the downstream buffer tank 95, and which stirs the ink stored in the downstream buffer tank 95 by rotating the stirring blades 853. In this configuration, the ink stored in the downstream buffer tank 95 can be stirred by the stirring blades 853.

[0114] Furthermore, the stirring blade 853 is normally positioned below the liquid level range R95. In this configuration, the ink stored in the downstream buffer tank 95 can be stirred by the stirring blade 853, which is positioned below the liquid level range R95.

[0115] Furthermore, an ink return pipe 831 (buffer return pipe) is provided between the downstream liquid supply pump 822 (buffer liquid supply pump) and the downstream buffer tank 95, connecting a branch section 825 provided in the downstream liquid supply piping 821 (buffer liquid supply piping) to the upstream buffer tank 96. Additionally, a downstream solenoid valve 824 (feed limiting solenoid valve) is provided in the downstream liquid supply piping 821 between the branch section 825 and the downstream buffer tank 95, and an ink return solenoid valve 832 (return limiting solenoid valve) is provided in the ink return pipe 831 between the branch section 825 and the upstream buffer tank 96. The control unit 5 includes a downstream solenoid valve control unit 510 (feed limiting solenoid valve control unit) that controls the opening and closing of the downstream solenoid valve 824, and a return solenoid valve control unit 511 (return limiting solenoid valve control unit) that controls the opening and closing of the ink return solenoid valve 832.

[0116] Furthermore, the liquid supply operation in steps S202 to S204 is performed as follows: The downstream solenoid valve control unit 510 controls the downstream solenoid valve 824 so that it opens, thereby enabling ink to be sent from the branching section 825 to the downstream buffer tank 95 via the downstream solenoid valve 824. In addition, the return solenoid valve control unit 511 controls the ink return solenoid valve 832 so that it closes, thereby restricting the return of ink from the branching section 825 to the downstream buffer tank 95 via the ink return solenoid valve 832. In this state, the downstream pump control unit 509 controls the downstream liquid supply pump 822 so that it performs liquid supply, thereby executing the liquid supply operation in steps S202 to S204 (step S301).

[0117] Furthermore, the ink circulation operation in steps S303 to S306 is performed as follows: The downstream solenoid valve control unit 510 controls the downstream solenoid valve 824 so that it closes, thereby restricting the supply of ink from the branch section 825 to the downstream buffer tank 95 via the downstream solenoid valve 824. In addition, the return solenoid valve control unit 511 controls the ink return solenoid valve 832 so that it opens, thereby enabling the return of ink from the branch section 825 to the upstream buffer tank 96 via the ink return solenoid valve 832. In this state, the downstream pump control unit 509 controls the downstream liquid supply pump 822 so that it performs liquid supply, thereby performing buffer return liquid supply, in which ink that has reached the branch section 825 of the downstream liquid supply piping 821 from the upstream buffer tank 96 returns to the upstream buffer tank 96 via the ink return piping 831. In this configuration, for example, during the print standby period when the ejection head H stops ejecting ink from the nozzle Hn, the ink can be circulated and agitated by performing a buffer return liquid delivery.

[0118] Furthermore, the stirring blade 873 (main tank stirring member) is a centrifugal-type stirring blade that expands due to the centrifugal force generated as it rotates, and the low level Lml (predetermined position) is provided corresponding to the position of the stirring blade 873 that has expanded due to centrifugal force. When stirring is performed using such a stirring blade 873, the lower end of the rotating stirring blade 873 rises above the lower end of the stationary stirring blade 873. Therefore, in the main tank 97, the ink that accumulates at the bottom below the lower end of the rotating stirring blade 873 cannot be stirred. In contrast, by providing the low level Lml corresponding to the position of the stirring blade 873 that has expanded due to centrifugal force, and configuring the system to perform the liquid supply operation (first main liquid supply operation) in steps S106 to S108, it is possible to suppress the retention of a large amount of ink at the bottom of the main tank 97 that cannot be stirred.

[0119] Furthermore, the above liquid delivery control is applied to the ink supply mechanism 9 that supplies the white ink, which is preferable. This makes it possible to suppress the settling of the white ink pigment by preventing a large amount of white ink from remaining in the main tank 97 without being agitated.

[0120] In the embodiments described above, the printing device 3 corresponds to an example of the "printing device" of the present invention, the control unit 5 corresponds to an example of the "control unit" of the present invention, the main sensor output acquisition unit 503 corresponds to an example of the "main tank liquid level acquisition unit" of the present invention, the pre-stage pump control unit 505 corresponds to an example of the "main liquid supply pump control unit" of the present invention, the pre-stage sensor output acquisition unit 507 corresponds to an example of the "pre-stage buffer tank liquid level acquisition unit" of the present invention, the post-stage pump control unit 509 corresponds to an example of the "buffer liquid supply pump control unit" of the present invention, and the post-stage solenoid valve control unit 510 corresponds to an example of the "feed limiting solenoid valve control unit" of the present invention. This corresponds to an example, the return solenoid valve control unit 511 corresponds to an example of the "return limiting solenoid valve control unit" of the present invention, the downstream sensor output acquisition unit 513 corresponds to an example of the "downstream buffer tank liquid level acquisition unit" of the present invention, the upstream liquid supply piping 811 corresponds to an example of the "main liquid supply piping" of the present invention, the upstream liquid supply pump 812 corresponds to an example of the "main liquid supply pump" of the present invention, the downstream liquid supply piping 821 corresponds to an example of the "buffer liquid supply piping" of the present invention, the downstream liquid supply pump 822 corresponds to an example of the "buffer liquid supply pump" of the present invention, and the downstream solenoid valve 824 corresponds to an example of the "feed limiting solenoid valve" of the present invention. The branch section 825 corresponds to an example of the "branch section" of the present invention, the ink return pipe 831 corresponds to an example of the "buffer return pipe" of the present invention, the ink return solenoid valve 832 corresponds to an example of the "return limiting solenoid valve" of the present invention, the downstream stirring unit 85 corresponds to an example of the "downstream buffer tank stirring section" of the present invention, the stirring blade 853 corresponds to an example of the "downstream buffer tank stirring member" of the present invention, the upstream stirring unit 86 corresponds to an example of the "upstream buffer tank stirring section" of the present invention, the stirring blade 863 corresponds to an example of the "upstream buffer tank stirring member" of the present invention, and the main stirring unit 8 7 corresponds to an example of the "main tank stirring section" of the present invention, stirring blade 873 corresponds to an example of the "main tank stirring member" of the present invention, downstream buffer tank 95 corresponds to an example of the "downstream buffer tank" of the present invention, upstream buffer tank 96 corresponds to an example of the "upstream buffer tank" of the present invention, main tank 97 corresponds to an example of the "main tank" of the present invention, low level Lml corresponds to an example of the "determined position" of the present invention, limit level Lft corresponds to an example of the "upstream upper limit position" of the present invention, normal liquid level range R95 corresponds to an example of the "downstream normal liquid level range" of the present invention,The normal liquid level range R96 corresponds to an example of the "pre-stage normal liquid level range" of the present invention, the subsequent liquid level sensor S95 corresponds to an example of the "sub-stage buffer tank liquid level detection unit" of the present invention, the pre-stage liquid level sensor S96 corresponds to an example of the "pre-stage buffer tank liquid level detection unit" of the present invention, the main liquid level sensor S97 corresponds to an example of the "main tank liquid level detection unit" of the present invention, the discharge head H corresponds to an example of the "discharge head" of the present invention, the nozzle Hn corresponds to an example of the "nozzle" of the present invention, steps S103 to S105 correspond to an example of the "second main liquid supply operation" of the present invention, steps S106 to S108 correspond to an example of the "first main liquid supply operation" of the present invention, and steps S202 to S204 correspond to an example of the "buffer liquid supply operation" of the present invention.

[0121] It should be noted that the present invention is not limited to the embodiments described above, and various modifications can be made without departing from the spirit of the invention. For example, the height of the low level Lml provided in correspondence with the stirring blade 873 may be appropriately changed. Figures 14A and 14B schematically show examples of changes to the low level provided in correspondence with the stirring blade.

[0122] In the example shown in Figure 14A, a low-level Lml is provided at the same height as the upper end of the stirring blade 873 which is spread out by centrifugal force. The main liquid level sensor S97, which detects the ink in the main tank 97, detects the presence or absence of ink in this low-level Lml.

[0123] In the example shown in Figure 14B, a low level Lml is positioned higher than the stirring blades 873 which are spread out by centrifugal force. The main liquid level sensor S97, which detects the ink in the main tank 97, detects the presence or absence of ink at this low level Lml. In the example shown in Figure 14B, for example, if the liquid level in the main tank 97 is above the stirring blades 873 which are spread out by centrifugal force and below the low level Lm, and the liquid supply operation in steps S106 to S108 is performed, the liquid level in the main tank 97 will drop to below the stirring blades 873 which are spread out by centrifugal force. In this way, a large amount of ink is sent from the main tank 97 to the buffer tank 95.

[0124] Furthermore, the amount of ink remaining in the main tank 97 after the completion of the liquid supply operation in steps S106 to S108 can be changed as appropriate, and is not limited to the above 1-5%, but may be more than 5%. Alternatively, all the ink in the main tank 97 may be sent to the preceding buffer tank 96 without leaving any ink in the main tank 97.

[0125] Furthermore, the rotation of the stirring blades 853, 863, or 873 may be controlled in conjunction with the execution of the ink supply control shown in Figure 9. For example, in step S102, if it is determined that the ink level in the main tank 97 is less than the low level Lml (if "YES"), the rotation of the stirring blade 873 may be stopped.

[0126] Furthermore, the details of the control for supplying color ink from the main tank 97 to the buffer tank 95 are not specifically described above. However, the supply of color ink can be performed, for example, by a flowchart that sequentially executes steps S101, S103, S104, and S105 in Figure 9. Such supply of color ink can be appropriately performed by conventional control methods. [Industrial applicability]

[0127] The present invention is applicable to all technologies for supplying ink to an ejection head that ejects ink from a nozzle. [Explanation of Symbols]

[0128] 3...Printing device 5…Control Unit 503...Main sensor output acquisition unit (main tank liquid level acquisition unit) 505...Pre-stage pump control unit (main liquid delivery pump control unit) 507...Pre-stage sensor output acquisition unit (Pre-stage buffer tank liquid level acquisition unit) 509... Downstream pump control unit (buffer liquid transfer pump control unit) 510... Subsequent solenoid valve control unit (feed limiting solenoid valve control unit) 511...Return Solenoid Valve Control Unit (Return Restriction Solenoid Valve Control Unit) 513... Subsequent sensor output acquisition unit (subsequent buffer tank liquid level acquisition unit) 811...Pre-stage liquid supply piping (main liquid supply piping) 812...Pre-stage liquid transfer pump (main liquid transfer pump) 821... downstream liquid supply piping (buffer liquid supply piping) 822... downstream liquid transfer pump (buffer liquid transfer pump) 824... Subsequent solenoid valve (feed limiting solenoid valve) 825... Branching point 831... Ink return piping (buffer return piping) 832... Ink return solenoid valve (return limiting solenoid valve) 85...Retro-stage stirring unit (retro-stage buffer tank stirring section) 853... Stirring blade (stirring component for downstream buffer tank) 86…Pre-stage stirring unit (pre-stage buffer tank stirring section) 863... Stirring blade (stirring component for the pre-stage buffer tank) 87…Main stirring unit (main tank stirring section) 873...Agitation blade (main tank agitation component) 95... Rear buffer tank 96... Pre-stage buffer tank 97... Main Tank Lml…Low level (designated position) Lft... Limit level (upper limit position of the previous stage) R95…Normal liquid level range (second stage normal liquid level range) R96…Normal liquid level range (first stage normal liquid level range) S95... downstream liquid level sensor (downstream buffer tank liquid level detection unit) S96...Pre-stage liquid level sensor (pre-stage buffer tank liquid level detection unit) S97...Main liquid level sensor (main tank liquid level detection unit) H...Discharge head Hn... Nozzle S103~S105...Step (Second main fluid delivery operation) S106~S108...Step (First main fluid delivery operation) S202~S204...Step (buffer fluid delivery operation)

Claims

1. The ejection head that ejects ink from the nozzle, A downstream buffer tank for storing the ink supplied to the aforementioned discharge head, A front buffer tank that stores the ink supplied to the aforementioned downstream buffer tank, A buffer liquid supply pipe connects and communicates the aforementioned upstream buffer tank and the aforementioned downstream buffer tank, A buffer liquid delivery pump is attached to the buffer liquid delivery piping between the preceding buffer tank and the preceding buffer tank and delivers ink from the preceding buffer tank to the preceding buffer tank along the buffer liquid delivery piping, A main tank for storing ink supplied to the aforementioned buffer tank, A main liquid supply pipe connects and communicates the main tank and the preceding buffer tank, A main liquid supply pump is attached to the main liquid supply piping between the main tank and the preceding buffer tank to supply ink from the main tank to the preceding buffer tank along the main liquid supply piping, A main tank stirring unit having a main tank stirring member positioned inside the main tank, which stirs the ink stored in the main tank by rotating the main tank stirring member, A main tank liquid level detection unit detects the positional relationship between the liquid level of ink in the main tank and a predetermined position provided corresponding to the main tank stirring member. Control unit and Equipped with, The control unit, A main tank liquid level acquisition unit that acquires the detection result of the main tank liquid level detection unit, A main liquid supply pump control unit controls the main liquid supply pump so that, in response to the main tank liquid level acquisition unit obtaining a detection result indicating that the liquid level of ink in the main tank is lower than the predetermined position, the main liquid supply pump performs a first main liquid supply operation to supply ink from the main tank to the upstream buffer tank, Having, Printing device.

2. The system further includes a pre-buffer tank liquid level detection unit that detects the positional relationship between the liquid level of the ink stored in the pre-buffer tank and the pre-buffer tank's normal liquid level range. The control unit, The system includes a pre-buffer tank liquid level acquisition unit that acquires the detection result of the pre-buffer tank liquid level detection unit, If the preceding buffer tank liquid level acquisition unit obtains a detection result indicating that the liquid level of the ink in the preceding buffer tank is lower than the lower end of the preceding normal liquid level range, and the detection result obtained by the main tank liquid level acquisition unit indicates that the liquid level of the ink in the main tank is lower than the predetermined position, then the main liquid supply pump control unit controls the main liquid supply pump so that the main liquid supply pump performs the first main liquid supply operation. The printing apparatus according to claim 1.

3. If the preceding buffer tank liquid level acquisition unit obtains a detection result indicating that the liquid level of ink in the preceding buffer tank is lower than the lower end of the preceding normal liquid level range, and the detection result obtained by the main tank liquid level acquisition unit indicates that the liquid level of ink in the main tank is at or above the predetermined position, the main liquid supply pump control unit controls the main liquid supply pump so that the main liquid supply pump performs a second main liquid supply operation, which supplies ink from the main tank to the preceding buffer tank in a manner different from the first main liquid supply operation. The printing apparatus according to claim 2.

4. In the second main liquid supply operation, the main liquid supply pump supplies ink from the main tank to the preceding buffer tank until the liquid level of the ink in the preceding buffer tank reaches the upper end of the preceding normal liquid level range, as described in claim 3.

5. The printing apparatus according to claim 2, wherein in the first main liquid supply operation, the main liquid supply pump supplies ink from the main tank to the preceding buffer tank until the liquid level of the ink in the preceding buffer tank reaches the preceding upper limit position, which is set to a position higher than the upper end of the preceding normal liquid level range.

6. The printing apparatus according to claim 2, further comprising a pre-buffer tank stirring unit having a pre-buffer tank stirring member disposed within the pre-buffer tank, which stirs the ink stored in the pre-buffer tank by rotating the pre-buffer tank stirring member.

7. The printing apparatus according to claim 6, wherein the preceding buffer tank stirring member is positioned below the normal liquid level range of the preceding stage.

8. The system further includes a downstream buffer tank liquid level detection unit that detects the positional relationship between the liquid level of the ink stored in the downstream buffer tank and the downstream normal liquid level range provided for the downstream buffer tank. The control unit, A downstream buffer tank liquid level acquisition unit acquires the detection result of the downstream buffer tank liquid level detection unit, A buffer liquid supply pump control unit controls the buffer liquid supply pump so that, in response to the downstream buffer tank liquid level acquisition unit obtaining a detection result indicating that the liquid level of ink in the downstream buffer tank is lower than the lower end of the downstream normal liquid level range, the buffer liquid supply pump performs a buffer liquid supply operation to supply ink from the upstream buffer tank to the downstream buffer tank. Having, The printing apparatus according to claim 1.

9. In the buffer liquid delivery operation, the buffer liquid delivery pump delivers ink from the preceding buffer tank to the subsequent buffer tank until the ink level in the subsequent buffer tank reaches the upper end of the subsequent normal liquid level range. The printing apparatus according to claim 8.

10. The printing apparatus according to claim 8, further comprising a downstream buffer tank stirring unit having a downstream buffer tank stirring member disposed within the downstream buffer tank, which stirs the ink stored in the downstream buffer tank by rotating the downstream buffer tank stirring member.

11. The printing apparatus according to claim 10, wherein the downstream buffer tank stirring member is positioned below the downstream normal liquid level range.

12. A buffer return pipe is provided between the buffer liquid delivery pump and the downstream buffer tank, connecting a branch in the buffer liquid delivery piping to the upstream buffer tank, Between the branching section and the downstream buffer tank, a feed limiting solenoid valve is provided in the buffer liquid supply piping, Between the branch section and the preceding buffer tank, a return limiting solenoid valve is provided in the buffer return piping, Furthermore, The control unit, A feed limiting solenoid valve control unit that controls the opening and closing of the feed limiting solenoid valve, A return limiting solenoid valve control unit that controls the opening and closing of the aforementioned return limiting solenoid valve, It has, The feed limiting solenoid valve control unit controls the feed limiting solenoid valve so that the feed limiting solenoid valve opens, thereby allowing ink to be sent from the branching section to the downstream buffer tank via the feed limiting solenoid valve, while the return limiting solenoid valve control unit controls the return limiting solenoid valve so that the return limiting solenoid valve closes, thereby restricting the return of ink from the branching section to the upstream buffer tank via the return limiting solenoid valve, and the buffer liquid delivery pump control unit controls the buffer liquid delivery pump so that the buffer liquid delivery pump performs liquid delivery, thereby executing the buffer liquid delivery operation. The feed limiting solenoid valve control unit controls the feed limiting solenoid valve so that the feed limiting solenoid valve is closed, thereby limiting the supply of ink from the branch section to the downstream buffer tank via the feed limiting solenoid valve. At the same time, the return limiting solenoid valve control unit controls the return limiting solenoid valve so that the return limiting solenoid valve is open, thereby allowing ink to be returned from the branch section to the upstream buffer tank via the return limiting solenoid valve. The buffer liquid supply pump control unit controls the buffer liquid supply pump so that the buffer liquid supply pump performs liquid supply, thereby performing buffer return liquid supply, in which ink that has reached the branch section of the buffer liquid supply piping from the upstream buffer tank is returned to the upstream buffer tank via the buffer return piping. The printing apparatus according to claim 8.

13. The main tank stirring member is a stirring blade that expands due to the centrifugal force generated as it rotates. The printing apparatus according to claim 1, wherein the predetermined position is provided in correspondence with the position of the stirring blades that have spread out due to centrifugal force.

14. The printing apparatus according to claim 13, wherein the predetermined position is provided at the same height as the lower end of the stirring blade which is spread out by centrifugal force.

15. The printing apparatus according to claim 13, wherein the predetermined position is provided at the same height as the upper end of the stirring blade that has spread out due to centrifugal force.

16. The printing apparatus according to claim 13, wherein the predetermined position is provided at a position higher than the stirring blades which are spread out by centrifugal force.

17. The printing apparatus according to claim 1, wherein the ink is white ink.

18. An ink supply method in a printing apparatus comprising: an ejection head for ejecting ink from a nozzle; a downstream buffer tank for storing ink supplied to the ejection head; a upstream buffer tank for storing ink supplied to the downstream buffer tank; a main tank for storing ink supplied to the upstream buffer tank; and a main tank stirring unit having a main tank stirring member disposed within the main tank, which stirs the ink stored in the main tank by rotating the main tank stirring member, wherein A step of detecting the positional relationship between the liquid level of the ink in the main tank and a predetermined position provided in correspondence with the main tank stirring member, The process includes:

1. Executing a first main ink supply operation in which ink is supplied from the main tank to the preceding buffer tank in response to the detection that the ink level in the main tank is lower than the predetermined position; Equipped with, Ink supply method.

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