Inkjet recording apparatus

Abstract

To improve the dispersion stability of a white pigment contained in ink even when the capacity of the ink is large.SOLUTION: The inkjet recording apparatus includes an ink tank 32 in which ink is stored, a recording head 21 that discharges the ink supplied from the ink tank 32, a decompression device (decompression pump 66) that decompresses the inside of the ink tank 32, a circulation flow path 47 that communicates different positions of the ink tank 32, and a circulation device (circulation pump 67) that circulates the ink through the circulation flow path 47, and the ink contains a white pigment.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an inkjet recording apparatus. [Background technology]

[0002] Inkjet recording devices eject ink droplets from the nozzles of a recording head, and if air bubbles are present in the ink, the nozzles of the recording head may become clogged. For this reason, it is desirable to reduce the amount of dissolved air in the ink, and degassing devices that remove dissolved air from the ink have been considered. For example, a known degassing device stirs the ink while the pressure inside the ink tank is reduced (see, for example, Patent Documents 1 and 2). In the degassing devices described in Patent Documents 1 and 2, a magnetic force is applied from the outside to a stirrer inside the ink tank, which rotates the stirrer and stirs the ink inside the ink tank. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 5030305 [Patent Document 2] Japanese Patent Application Publication No. 2019-142189 Summary of the Invention [Problem to be solved by the invention]

[0004] In the degassing devices described in Patent Documents 1 and 2, the ink in an ink tank is agitated to degas the ink near the liquid surface. However, because the agitator is located at the bottom of the ink tank, as the ink volume increases, it becomes difficult for the ink near the liquid surface, which has a low amount of dissolved air, to exchange with the ink near the bottom, which has a high amount of dissolved air, reducing degassing efficiency. Furthermore, when using ink containing a pigment with a high specific gravity, such as a white pigment, as the ink volume increases, the pigment is more likely to settle, reducing dispersion stability.

[0005] Therefore, an object of the present invention is to improve the dispersion stability of the white pigment contained in the ink even when the volume of the ink is large. [Means for solving the problem]

[0006] In order to solve the above problem, the inkjet recording device of the present invention comprises an ink tank in which ink is stored, a recording head that ejects ink supplied from the ink tank, a pressure reducing device that reduces the pressure inside the ink tank, a circulation flow path that connects different positions in the ink tank, and a circulation device that circulates the ink through the circulation flow path, wherein the ink contains a white pigment.

[0007] The white pigment may be titanium oxide.

[0008] The average primary particle size of the titanium oxide may be 250 [nm] or more and 350 [nm] or less.

[0009] The circulation device may be a pump that pumps liquid using a rotating body.

[0010] The circulation device may be capable of transmitting power in a non-contact manner between a pump shaft of the pump and a motor shaft via a partition wall.

[0011] The circulation device may be capable of transmitting power between the pump shaft and the motor shaft in a non-contact manner using magnetic force.

[0012] The viscosity of the ink may be 2 [mPa·s] or more and 10 [mPa·s] or less. [Effects of the Invention]

[0013] An object of the present invention is to improve the dispersion stability of the white pigment contained in the ink even when the volume of the ink is large. [Brief explanation of the drawings]

[0014] [Figure 1]1 is a schematic diagram of an inkjet recording apparatus according to an embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram of an ink supply structure according to the present embodiment. [Figure 3] FIG. 2 is a schematic diagram of a circulation pump according to the present embodiment. [Figure 4] 10 is a graph showing the relationship between reduced pressure conditions and oxygen saturation of ink. [Figure 5] 10A and 10B are diagrams illustrating an example of a degassing operation of the circulation degassing method of the present embodiment. [Figure 6] FIG. 10 is a diagram showing an example of a degassing operation of a stirring degassing method according to a comparative example. [Figure 7] 10 is a schematic diagram of an ink tank in which a circulation flow path according to Modification 1 is formed. FIG. [Figure 8] FIG. 10 is a schematic diagram of an ink tank in which a circulation flow path according to a second modification is formed. [Figure 9] FIG. 10 is a schematic diagram of an ink tank in which a circulation flow path according to a third modified example is formed. [Figure 10] FIG. 10 is a schematic diagram of an ink tank in which a circulation flow path according to a fourth modified example is formed. [Figure 11] FIG. 13 is a schematic diagram of an ink tank in which a circulation flow path according to a fifth modified example is formed. [Figure 12] FIG. 13 is a schematic diagram of an ink tank in which a circulation flow path according to a sixth modified example is formed. DETAILED DESCRIPTION OF THE INVENTION

[0015] The inkjet recording apparatus 1 of this embodiment will be described below with reference to the drawings. Fig. 1 is a schematic diagram of the inkjet recording apparatus 1 of this embodiment. For ease of explanation, the front side of the paper in Fig. 1 is the front side (front side) of the inkjet recording apparatus 1, and the left and right directions will be described based on the direction when the inkjet recording apparatus 1 is viewed from the front. Arrows L, R, U, and Lo, which are appropriately added to each figure, indicate the left side, right side, upper side, and lower side of the inkjet recording apparatus 1, respectively.

[0016] As shown in Figure 1, the inkjet recording device 1 is configured to be able to print by ejecting ink from each inkjet recording head 21 toward a sheet S as a recording medium. The inkjet recording device 1 has a box-shaped housing 10 that houses various devices. A paper feed cassette 11 in which the sheets S are set is housed in the lower part of the housing 10, and a manual feed tray 12 in which the sheets S are manually set is provided on the right side of the housing 10. A paper output tray 13 on which the recorded sheets S are stacked is provided on the upper left side of the housing 10.

[0017] A first transport path 14 is formed on the right side of the housing 10 to transport sheets S from the paper feed cassette 11 toward the recording head 21 in the center of the housing 10. A first paper feed unit 15 that takes out sheets S from the stack of sheets in the paper feed cassette 11 is provided upstream of the first transport path 14, and a pair of registration rollers 18 that adjust the timing of sending out sheets S are provided downstream of the first transport path 14. In addition, a paper feed path 16 for the manual feed tray 12 joins downstream of the first transport path 14, and a second paper feed unit 17 that takes out sheets S from the stack of sheets in the manual feed tray 12 is provided in the paper feed path 16.

[0018] A conveying device 22 and recording heads 21 for each color (for example, white, black, cyan, magenta, and yellow) are provided downstream of the pair of registration rollers 18. The pair of registration rollers 18 corrects the skew of the sheet S and sends the sheet S to the conveying device 22 in accordance with the ink ejection operation of each recording head 21. An ink container 31 and an ink tank 32 are provided for each recording head 21 in the housing 10. Ink from each ink container 31 is temporarily stored in the ink tank 32, and the ink is degassed as necessary and supplied from the ink tank 32 to the recording head 21.

[0019] The conveying device 22 is configured by a conveying belt 24 stretched over a plurality of tension rollers 23 installed below each recording head 21. A drying device 25 that dries the ink on the sheet S is provided downstream of the conveying device 22. A decurling device 26 that straightens out curls that occur in the sheet S due to the drying of the ink is provided downstream of the drying device 25. A second conveying path 27 that conveys the sheet S toward the paper discharge tray 13 is formed downstream of the second conveying path 27. A paper discharge unit 28 that discharges the recorded sheet S to the paper discharge tray 13 is provided downstream of the second conveying path 27.

[0020] Below the drying device 25, there are provided a maintenance unit 35 that cleans the recording head 21 and a cap unit 36 ​​that caps the recording head 21. The maintenance unit 35 is provided with a squeegee-like wiping blade that scrapes off ink remaining on the nozzle surface of the recording head 21. The cap unit 36 ​​is provided with a head cap that is placed over the nozzle surface of the recording head 21. The head cap prevents the ink in the nozzles from drying out. Drying of the ink in the nozzles may be further prevented by storing a liquid such as a cleaning liquid in the head cap.

[0021] The inkjet recording apparatus 1 is also provided with a control device 38 that performs overall control of the entire apparatus. The control device 38 may be configured with a processor, or may be configured with a logic circuit (hardware) formed on an integrated circuit or the like. When configured with a processor, the processor reads and executes programs stored in memory to perform various processes. As the processor, for example, a CPU (Central Processing Unit) is used. The memory is configured with one or more storage devices such as ROM (Read Only Memory) and RAM (Random Access Memory) depending on the application.

[0022] During image recording, a sheet S is taken out of the paper feed cassette 11 or the manual feed tray 12 by the first paper feed unit 15 and the second paper feed unit 17 and sent to a pair of registration rollers 18. The sheet S is sent from the pair of registration rollers 18 to a conveyor belt 24 in synchronization with the ink ejection timing, and degassed ink is ejected from each recording head 21 to record a color image on the surface of the sheet S. The sheet S is dried by a drying device 25, and any curls in the sheet S are corrected by a decurling device 26. The sheet S is conveyed to a paper discharge unit 28 via a second conveyance path 27, and the printed sheet S is discharged to the paper discharge tray 13 by the paper discharge unit 28.

[0023] However, when the ink surface comes into contact with air in the ink tank, the air dissolves, and air bubbles in the ink can clog the nozzles of the recording head 21. For this reason, it is desirable to appropriately reduce the amount of dissolved air in the ink. For example, one proposed method of degassing involves passing ink through a hollow fiber filter while reducing the pressure around the filter, causing air to move from the hollow fiber walls to the reduced pressure side. This method requires expensive hollow fiber filters and requires periodic replacement, increasing costs.

[0024] Additionally, to prevent nozzle clogging, a method has been proposed in which the ink is agitated and degassed using a stirrer while the pressure inside the ink tank is reduced to below atmospheric pressure (hereinafter referred to as the agitation degassing method). In this agitation degassing method, a magnetic force is applied from the outside to a stirrer inside the ink tank, which rotates the stirrer and agitates the ink inside the ink tank. If the ink depth or tank diameter is large, the ink becomes difficult to agitate, and degassing efficiency decreases. Increasing the rotation speed of the stirrer makes it easier to agitate, but if the rotation speed of the stirrer is too high, loss of synchronization occurs and the rotation noise of the stirrer becomes louder.

[0025] Therefore, in this embodiment, a degassing method (hereinafter referred to as the circulation degassing method) is adopted in which the ink in the ink tank 32 is circulated through the circulation flow path 47 while the pressure inside the ink tank 32 is reduced to below atmospheric pressure (see FIG. 2). In the circulation degassing method, the ink circulates through the circulation flow path 47 and the ink tank 32, swapping the ink near the liquid surface, where the amount of dissolved air is low, with the ink near the bottom, where the amount of dissolved air is high, thereby improving degassing efficiency. Unlike the agitation degassing method, this method is not affected by the ink depth or tank diameter, and the driving noise of the circulation pump 67 is suppressed more than the rotation noise of the agitator, improving quietness. In addition, the dispersion stability of ink containing a pigment with a high specific gravity is improved.

[0026] The degassing device 40 according to this embodiment will be described with reference to Figures 2 and 3. Figure 2 is a schematic diagram of the ink supply structure according to this embodiment. Figure 3 is a schematic diagram of the circulation pump 67 according to this embodiment. The inkjet recording apparatus 1 according to this embodiment is provided with an ink supply structure for each color, but one ink supply structure will be described here.

[0027] As shown in Fig. 2, the ink tank 32 stores ink that is supplied from the ink container 31 through a supply flow path 41. A supply pump 61 and a supply valve 51 are provided in the supply flow path 41, and the supply of ink to the ink tank 32 is controlled by the supply pump 61 and the supply valve 51. A pressure reduction flow path 42 and an air release flow path 43 are connected to an upper space 34 of the ink tank 32. A pressure reduction pump 62 (pressure reduction device) and a pressure reduction valve 52 are provided in the pressure reduction flow path 42, and the pressure inside the ink tank 32 is reduced by the pressure reduction pump 62 and the pressure reduction valve 52. An air release valve 53 is provided in the air release flow path 43, and the upper space 34 is opened to the atmosphere by the air release valve 53.

[0028] Ink is supplied from the ink tank 32 to the recording head 21 through a supply flow path 44, and the ink is recovered from the recording head 21 to the ink tank 32 through a recovery flow path 45. A supply pump 64 and a supply valve 54 are interposed in the supply flow path 44, and a recovery valve 55 is interposed in the recovery flow path 45. The supply pump 64, supply valve 54, and recovery valve 55 control the ink replacement operation and air bubble removal operation in the recording head 21. A bypass flow path 46 that bypasses the supply pump 64 is provided in the supply flow path 44, and a bypass valve 56 is provided in the bypass flow path 46. During printing, the bypass valve 56 allows ink to pass through the bypass flow path 46.

[0029] The ink in the ink tank 32 is connected near the ink surface and near the bottom surface by a circulation flow path 47. A circulation pump 67 (circulation device) is provided in the circulation flow path 47, and the ink is circulated through the circulation flow path 47 by the circulation pump 67. An inlet 72 from the circulation flow path 47 to the ink tank 32 is higher than an outlet 71 from the ink tank 32 to the circulation flow path 47. Specifically, the outlet 71 is provided on the bottom surface of the ink tank 32, and the inlet 72 is provided on the side surface of the ink tank 32 near the ink surface. The refill pump 61, the pressure reducing pump 62, the supply pump 64, the circulation pump 67, the refill valve 51, the pressure reducing valve 52, the atmosphere release valve 53, the supply valve 54, the recovery valve 55, and the bypass valve 56 are controlled by a control device 38.

[0030] The control device 38 is provided with a determination unit 39 that determines whether or not a degassing operation is necessary depending on the time the ink has been left standing. If the determination unit 39 determines that degassing of the ink is not necessary, the degassing operation is restricted. Even if air redissolves in the ink due to being left standing, the ink can be used without degassing as long as it is within the allowable time. Details of the determination process by the determination unit 39 will be described later. In this ink supply structure of the inkjet recording device 1, a degassing device 40 is formed by the ink tank 32, the pressure reduction flow path 42, the pressure reduction pump 62, the pressure reduction valve 52, the circulation flow path 47, the circulation pump 67, the determination unit 39, etc.

[0031] When the inkjet recording apparatus 1 is in a standby state, the refill valve 51, pressure reduction valve 52, and supply valve 54 are closed, and the atmosphere release valve 53, bypass valve 56, and recovery valve 55 are open. Ink is stored in the ink tank 32, and as the liquid surface comes into contact with the air in the upper space 34 that is open to the atmosphere, air dissolves in the ink over time. During pressure reduction, only the pressure reduction valve 52 is open, and the other valves 51, 53-56 are closed. The pressure reduction pump 62 is driven to remove air from the upper space 34 within the ink tank 32. When the pressure inside the ink tank 32 reaches a target pressure (for example, -50 kPa), the pressure reduction pump 62 is stopped.

[0032] During the degassing operation, all valves 51-56 are closed, and the circulation pump 67 is driven while maintaining the reduced pressure inside the ink tank 32, circulating the ink inside the ink tank 32 through the circulation flow path 47. The ink near the bottom of the ink tank 32, where the amount of dissolved air is large, flows out into the circulation flow path 47 through the outlet 71, and the ink in the circulation flow path 47 flows into the ink tank 32 near the liquid surface through the inlet 72. The ink surface is exposed to a reduced pressure atmosphere, and the air dissolved in the ink near the liquid surface is removed. The ink near the liquid surface, where the amount of dissolved air is small, is smoothly exchanged with the ink near the bottom, where the amount of dissolved air is large, improving degassing efficiency.

[0033] In this case, because the ink tank 32 is in a reduced pressure state, a reciprocating pump such as a diaphragm pump is easily affected by the reduced pressure. Therefore, it is preferable to use a pump that pumps ink using a rotating body as the circulation pump 67. For example, the circulation pump 67 may be a non-positive displacement pump such as a centrifugal pump, mixed flow pump, or axial flow pump, or a positive displacement rotary pump such as a vane pump, gear pump, or screw pump. By using these pumps, it is possible to circulate ink while reducing the effects of the reduced pressure inside the ink tank 32, unlike a reciprocating pump.

[0034] Generally, the print head 21 may be included in the circulation flow path of the circulation degassing system, but in this embodiment, the print head 21 is not included in the circulation flow path 47. In other words, the circulation flow path 47 is a dedicated degassing flow path that is provided separately from the path that supplies ink to the print head 21. By not including the print head 21 in the circulation flow path 47, it is possible to reduce the possibility that the meniscus formed in the nozzle of the print head 21 will be destroyed by the reduced pressure during degassing, and that external air will enter the print head 21.

[0035] As shown in Fig. 3, a pump shaft 73 and a motor shaft 75 of the circulation pump 67 are arranged with a partition wall 77 sandwiched therebetween, allowing power to be transmitted without contact. A pump casing 76 is formed midway through the circulation flow path 47, and a pump shaft 73 with an impeller 74 attached is housed inside the pump casing 76. A motor (not shown) is installed outside the circulation flow path 47. Disks 78 and 79 are provided at the ends of the pump shaft 73 and the motor shaft 75, and the disks 78 and 79 face each other with the partition wall 77 of the pump casing 76 in between. Magnets (not shown) with S poles and N poles arranged alternately in the circumferential direction are installed on the opposing surfaces of the disks 78 and 79, respectively.

[0036] The pump shaft 73 and motor shaft 75 are magnetically coupled (magnetically coupled), and power is transmitted from the motor shaft 75 to the pump shaft 73 using magnetic force. The impeller 74 inside the pump casing 76 can be rotated without the motor shaft 75 having to penetrate the pump casing 76, while the pump casing 76 remains liquid-tight. Because the pump shaft 73 and the disks 78, 79 of the motor shaft 75 are separated by a partition wall 77 of the pump casing 76, even if a pressure difference occurs between the inside and outside of the pump casing 76 when the ink tank 32 is decompressed, ink leakage caused by this pressure difference is reliably prevented.

[0037] 2, during the ink replacement operation in the recording head 21, the refill valve 51, pressure reducing valve 52, and bypass valve 56 are closed, and the atmosphere release valve 53, supply valve 54, and recovery valve 55 are open. The supply pump 64 is driven to supply ink from the ink tank 32 to the recording head 21 through the supply flow path 44, and the ink is recovered from the recording head 21 to the ink tank 32 through the recovery flow path 45. By circulating ink between the recording head 21 and the ink tank 32, ink whose viscosity has increased in the recording head 21 is replaced and air bubbles are removed from the recording head 21.

[0038] During a printing operation using the recording head 21, the refill valve 51, pressure reduction valve 52, and supply valve 54 are closed, and the atmosphere release valve 53, bypass valve 56, and recovery valve 55 are open. In other words, during a printing operation, the ink tank 32 is open to the atmosphere and at atmospheric pressure. During a printing operation, the ink tank 32 is not depressurized to the extent that substantial degassing occurs. Every time ink is ejected from the recording head 21, ink is supplied from the ink tank 32 to the recording head 21 through the bypass flow path 46 and recovery flow path 45. Ink may be replenished during an ink replacement operation, a printing operation, or the like. During this ink replenishment operation, the refill valve 51 is opened and the refill pump 61 is driven. Driving the refill pump 61 replenishes ink from the ink container 31 to the ink tank 32 through the refill flow path 41.

[0039] 1 and other figures are drawn schematically, and in reality, the recording head 21 is disposed above the ink tank 32. Negative pressure is applied to the ink in the recording head 21 due to the difference in head height with the ink in the ink tank 32, and this negative pressure forms a meniscus in the nozzles of the recording head 21. After ink is ejected from the recording head 21, the surface tension of the ink acts to reduce the surface area of ​​the meniscus, and the resulting negative pressure draws the reduced amount of ink from the ink tank 32 into the recording head 21. Note that the recovery valve 55 may be closed and ink may be supplied to the recording head 21 only through the bypass flow path 46.

[0040] Furthermore, if the ink tank 32 is depressurized to a degree that substantially degassing occurs while the recording head 21 and ink tank 32 are connected, the meniscus in the nozzle may be destroyed. Even if the meniscus is not destroyed, the shape of the meniscus in the nozzle may change from when the ink tank 32 is open to the atmosphere, which may result in a change in ink ejection characteristics. In this embodiment, the ink tank 32 is not depressurized during printing operations to a degree that substantially degassing occurs, so the meniscus in the nozzle of the recording head 21 is not destroyed, and its shape does not change, resulting in a change in ejection characteristics.

[0041] The degassing performance of the circulation degassing method and the agitation degassing method will be described below with reference to FIGS. 4-6 and Tables 1-8. FIG. 4 is a graph showing the relationship between the reduced pressure conditions of ink and oxygen saturation. FIG. 5 is a diagram showing an example of the degassing operation of the circulation degassing method of this embodiment. FIG. 6 is a diagram showing an example of the degassing operation of the agitation degassing method of a comparative example. Note that the oxygen saturation measurement methods and image quality evaluation methods in Tables 1-8 are similar unless otherwise specified.

[0042] The degassing device 40 described above uses a vacuum degassing method, which reduces the pressure inside the ink tank 32 to remove air. In this vacuum degassing method, the amount of dissolved air in the liquid converges to a saturated dissolved amount that corresponds to environmental conditions such as air pressure and liquid temperature, and the amount of dissolved air decreases when the air pressure drops or the temperature rises. The amount of dissolved oxygen is often used instead of the amount of dissolved air, and in this embodiment, degassing performance is explained using the oxygen saturation calculated from the following equation (1). As shown in Figure 4, oxygen saturation decreases with pressure reduction, but degassing occurs at the liquid surface, so degassing performance is related to the ability to transport ink with a high oxygen saturation level to the liquid surface. Oxygen saturation = Amount of dissolved oxygen / Amount of saturated dissolved oxygen at atmospheric pressure × 100 Equation (1)

[0043] As shown in Figure 5, in the degassing device 40 using the circulation degassing method, air dissolved in the ink is removed near the liquid surface of the decompressed ink tank 32, resulting in a low oxygen saturation level in the ink near the liquid surface and a high oxygen saturation level in the ink near the bottom. By driving the circulation pump 67, the ink near the bottom is sent to the liquid surface through the circulation flow path 47, creating an ink flow that extends from the liquid surface to the bottom, thereby effectively degassing the ink. With the circulation degassing method, even if the ink depth or tank diameter increases and the ink volume increases, the ink near the bottom is sent to the liquid surface, thereby maintaining degassing efficiency.

[0044] In contrast, as shown in Figure 6, in a degassing device 81 using the agitation degassing method, a stirrer 83 is placed on the bottom of an ink tank 82 whose pressure has been reduced. A magnet 84 is provided below the ink tank 82, and the magnetic force of the magnet 84 rotates the stirrer 83, stirring the ink. As the ink is stirred, the ink near the bottom is carried to the liquid surface, where it is degassed. However, in the agitation degassing method, the stirrer 83 is placed in the center of the bottom of the ink tank 82, so the stirring effect is strong at the center of the bottom but weaker near the liquid surface and the periphery. For this reason, degassing efficiency decreases as the ink depth or tank diameter increases, resulting in an increased ink volume.

[0045] Table 1 shows the effect of ink depth on oxygen saturation and image quality. Here, we investigated the effects of ink depth and tank diameter on oxygen saturation and image quality using the circulation degassing method and the agitation degassing method. The ink viscosity was set to 7 mPa·s, the ink temperature to 25°C, the tank diameter of ink tanks 32 and 82 to 60 mm, and the ink depth was varied in stages from 14.1 mm to 70.7 mm. For the circulation degassing method, the ink circulation flow rate was set to 770 ml / min. For the agitation degassing method, a capsule-shaped agitator 83 with a diameter of 8 mm and a length of 35 mm was used, and the rotation speed of the agitator 83 (magnet 84) was set to 560 rpm.

[0046] The oxygen saturation and image quality were then checked 10 minutes after the ink tanks 32 and 82 were depressurized to -50 kPa. The oxygen saturation was measured using an existing measuring device, and the image quality was evaluated using the following criteria based on the number of pins with ejection defects such as missing or misaligned pins, after drawing a single line with a line head of approximately 2,000 pins. [Image quality evaluation criteria] 〇: No loosening or twisting △: The number of pins that are missing or twisted is 1 or more and less than 10 ×: The number of pins that are missing or twisted is 10 or more. Note that this evaluation standard is just one index, and an "X" does not mean that the product is not suitable for practical use. [Table 1]

[0047] Table 2 shows the effect of tank diameter on oxygen saturation and image quality. The ink viscosity was set to 7 mPa·s, the ink temperature to 25°C, and the ink depth to 28 mm. The tank diameter of ink tanks 32 and 82 was changed in stages between 45 mm and 120 mm. [Table 2]

[0048] As shown in Tables 1 and 2, with the circulation degassing method, low oxygen saturation levels are maintained even when the ink depth or tank diameter is changed. On the other hand, as shown in Table 1, with the agitation degassing method, degassing performance declines once the ink depth exceeds 40 mm, and degassing stops once the ink depth exceeds 70 mm. Furthermore, as shown in Table 2, with the agitation degassing method, degassing performance is low even when the tank diameter is 45 mm, and degassing stops once the tank diameter exceeds 60 mm. In this way, the circulation degassing method achieves better degassing performance than the agitation degassing method.

[0049] As mentioned above, because degassing performance is related to the ability to transport ink with high oxygen saturation to the liquid surface, the ink circulation flow rate is important in the circulation degassing method, and the rotation speed of the stirrer 83 is important in the agitation degassing method. Therefore, we confirmed the effect of the ink circulation flow rate on oxygen saturation and image quality in the circulation degassing method, and the effect of the rotation speed of the magnet 84 on oxygen saturation and image quality in the agitation degassing method. Note that the agitation degassing method rotates the stirrer 83 inside the ink tank 82 using the magnetic force of the external magnet 84, and because it is difficult to control the rotation speed of the stirrer 83 itself, the rotation speed of the magnet 84 is used instead of the rotation speed of the stirrer 83.

[0050] Table 3 shows the effect of ink circulation flow rate on oxygen saturation and image quality. For the circulation degassing method, the ink viscosity was set to 7 mPa·s, the ink temperature to 25°C, the ink tank diameter to 60 mm, and the ink depth to 28 mm. The ink circulation flow rate was varied in stages from 41 ml / min to 2018 ml / min. As shown in Table 3, in the circulation degassing method, increasing the ink circulation flow rate improved degassing performance. Degassing performance was achieved when the ink circulation flow rate was 53 ml / min or higher, and an ink circulation flow rate of 440 ml / min or higher was desirable. [Table 3]

[0051] Table 3 also shows the effect of circulation flow rate and particle size on the whiteness of white pigment ink. When using ink containing a pigment with a high specific gravity, as the ink volume increases, the pigment tends to settle, reducing dispersion stability. Here, a pigment with a high specific gravity refers to a white pigment with a higher specific gravity than the black, cyan, magenta, and yellow pigments commonly used in inkjet printing. For example, titanium oxide is used as a white pigment, which tends to settle.

[0052] White pigment ink is used when forming a white base on a transparent film or when forming white letters on a dark film. Since film does not absorb liquid, the ink must be quick-drying. When using quick-drying ink, it is desirable to use a circulating recording head 21 as in this embodiment to prevent nozzle clogging.

[0053] In the experiment, four types of white pigment with different particle sizes were used to form an image with 100% image density and evaluate the whiteness. The white pigment was titanium oxide. The particle sizes were 200, 280, 350, and 400 nm. The ink depth was 70.4 mm, the tank diameter was 90 mm, and the circulation flow rate was 1052 ml / min. After degassing for 10 minutes using the circulation degassing method, the image printed was used as a reference. The change in whiteness, ΔE, was measured using a colorimeter, and the whiteness was evaluated using the following evaluation criteria. [Whiteness evaluation criteria] ○:ΔE<3 ×: ΔE≧3 Note that this evaluation standard is just one index, and an "X" does not mean that the product is not suitable for practical use.

[0054] As shown in Table 3, in the circulation degassing method, whiteness improves by increasing the ink circulation flow rate. When the particle size is 200, 280, or 350 nm, the whiteness evaluation criteria are met at circulation flow rates of 315, 440, and 497 ml / min or higher, respectively. When the particle size is 400 nm, the evaluation criteria are not met even at the maximum circulation flow rate. However, in practice, it has been found that sufficient whiteness cannot be achieved unless the particle size is 250 nm or higher. Therefore, based on the results of this experiment, the recommended particle size (average primary particle size) is between 250 nm and 350 nm.

[0055] Table 4 shows the effect of the rotation speed of the magnet 84 on oxygen saturation and image quality. For the agitation degassing method, the ink viscosity was set to 7 mPa·s, the ink temperature to 25°C, the ink tank 82 diameter to 60 mm, and the ink depth to 28 mm. The rotation speed of the magnet 84 was changed in stages between 218 rpm and 825 rpm. As shown in Table 4, for the agitation degassing method, increasing the rotation speed of the magnet 84 improved degassing performance. While degassing performance was achieved at a rotation speed of the magnet 84 of 560 rpm or higher, decoupling occurred once the rotation speed of the magnet 84 exceeded 600 rpm, making it difficult to use in actual equipment. [Table 4]

[0056] Table 5 shows the effects of reduced pressure conditions on oxygen saturation and image quality. Because the reduced pressure conditions in the ink tank 32 affect degassing performance, we investigated the effects of varying the reduced pressure conditions on oxygen saturation and image quality using the circulation degassing method. The ink viscosity was set to 7 mPa·s, the ink temperature to 25°C, the ink tank 32 diameter to 60 mm, the ink depth to 28 mm, and the ink circulation flow rate to 770 ml / min. The reduced pressure conditions were gradually changed between 0 kPa and -90 kPa. The degassing performance was confirmed 30 minutes after the degassing process began to slow. As shown in Table 5, degassing performance was achieved when the pressure was reduced to -30 kPa or below, and it was desirable to reduce the pressure to -50 kPa or below. [Table 5]

[0057] Table 6 shows the effect of ink viscosity during degassing on oxygen saturation and image quality when using the circulation degassing method. The ink temperature was set to 25°C, the ink tank 32 diameter was 60 mm, and the ink depth was 28 mm. Several inks with different viscosities ranging from 1 mPa·s to 13.7 mPa·s were prepared. The ink was degassed after sufficient air was dissolved, and the degassing performance was confirmed for degassing times of 0, 10, 20, 30, and 60 minutes. The ink viscosity was measured using a falling-ball viscometer (with an error of ±0.5%) in accordance with JIS Z 8803.

[0058] As shown in Table 6, ink with low viscosity is more easily degassed, while ink with high viscosity is more difficult to degas. However, if the ink viscosity is less than 2 mPa·s, the line head's ejection performance will not be stable, and even ink that has been sufficiently degassed will be prone to image distortion. If the ink viscosity exceeds 10 mPa·s, a degassing time of 20 minutes or more will be required to obtain sufficient image quality. To prevent deterioration in ejection performance due to low ink viscosity and deterioration in image quality due to high ink viscosity, it is desirable for the ink viscosity to be between 2 mPa·s and 10 mPa·s. [Table 6]

[0059] Table 7 shows the effect of ink viscosity on oxygen saturation and image quality when using the circulation degassing method. The ink temperature was set to 25°C, the ink tank 32 diameter was 60 mm, and the ink depth was 28 mm. Several inks with different viscosities ranging from 1 mPa·s to 13.7 mPa·s were prepared. After thoroughly degassing the ink tank 32, the ink tank 32 was returned to atmospheric pressure and left to stand. The degassing performance was confirmed after 0 hours, 4 hours, 8 hours, 24 hours, and 32 hours. This confirmed the ease of redissolving air depending on the ink viscosity.

[0060] As shown in Table 7, air redissolves more easily when the ink viscosity is low, and less easily when the ink viscosity is high. If the ink viscosity is less than 2 mPa·s, even if the ink is left standing for a short time, the oxygen saturation level increases due to the redissolution of air into the ink, and image quality deteriorates. As the ink viscosity increases, the redissolution of air into the ink is suppressed, so degassing of the ink tank 32 may not be necessary depending on the time the ink is left standing. For example, if the ink viscosity is 3 mPa·s, degassing is not required if the ink is left standing for less than four hours. [Table 7]

[0061] In this case, the control device 38 (see FIG. 2) is provided with a timer that measures the time the ink is left standing. The viscosity of the ink can be estimated from one or more parameters, such as atmospheric pressure, ink temperature, and the time elapsed since the previous printing. For this reason, the determination unit 39 stores conversion information indicating the correspondence between each parameter and the viscosity of the ink, and estimates the viscosity of the ink based on each parameter. The determination unit 39 also stores conversion information indicating the correspondence between the viscosity of the ink and the permissible time, and sets the permissible time based on the viscosity of the ink. The permissible time is the time during which printing is permitted without degassing, even if the ink is left standing.

[0062] If the ink is left standing for a period of time within the allowable time, the oxygen saturation level is low, so the determination unit 39 determines that degassing is not necessary, and the degassing operation is restricted. If the ink is left standing for a period of time beyond the allowable time, the oxygen saturation level is high, so the determination unit 39 determines that degassing is necessary, and the degassing operation is carried out. Note that map data, lookup tables, conversion formulas, etc. are used for the conversion information showing the correspondence between each parameter and ink viscosity, and the conversion information showing the correspondence between ink viscosity and allowable time. These map data, lookup tables, and conversion formulas are obtained in advance experimentally, empirically, and theoretically.

[0063] Table 8 shows the change in circulation flow rate when the viscosity is changed in the circulation degassing method. Here, a centrifugal pump was used as the circulation pump 67. In this embodiment, power is transmitted from the motor to the impeller 74 via a magnetic coupling. If the ink viscosity becomes too high, the magnetic coupling will lose synchronization. On the other hand, if the ink viscosity becomes too low, a secondary problem occurs in which the torque acting on the motor shaft 75 becomes lighter, causing the motor to heat up. If the ink viscosity is between 2 [mPa·s] and 10 [mPa·s], such problems occurring in the circulation pump 67 can be prevented. [Table 8]

[0064] The inkjet recording apparatus 1 according to the present embodiment described above includes an ink tank 32 containing ink, a recording head 21 that ejects ink supplied from the ink tank 32, a pressure-reducing device (pressure-reducing pump 62) that reduces the pressure inside the ink tank 32, a circulation channel 47 that connects different positions in the ink tank 32, and a circulation device (circulation pump 67) that circulates the ink through the circulation channel 47. The ink contains a white pigment. This configuration removes air dissolved in the ink near the liquid surface, which is exposed to a reduced-pressure atmosphere. The ink in the ink tank 32 is circulated through the circulation channel 47, replacing the ink near the liquid surface, which has a low dissolved air content, with the ink near the bottom, which has a high dissolved air content. This improves degassing efficiency even when the ink volume is large. Furthermore, by using this degassing device 40 in the inkjet recording apparatus 1, clogging of the recording head 21 due to air bubbles in the ink is effectively suppressed, improving image quality.

[0065] Furthermore, the inkjet recording apparatus 1 according to this embodiment can improve the dispersion stability of the white pigment contained in the ink, even when the ink volume is large. Ink containing a white pigment is highly useful when used, for example, to form a white base on a transparent film or to form white text on a dark film. Therefore, the present invention is suitable for inkjet recording apparatuses 1 that use ink containing a white pigment, and can improve the dispersion stability of the white pigment contained in the ink, even when the ink volume is large. Note that, although titanium oxide is used as an example of a white pigment in this embodiment, dispersion stability can also be improved when zinc oxide, white lead (basic lead carbonate), lithopone (barium sulfate, zinc sulfide), or the like is used as the white pigment.

[0066] Furthermore, in the inkjet recording apparatus 1 according to this embodiment, the white pigment is titanium oxide, and therefore the dispersion stability of the titanium oxide contained in the ink can be improved.

[0067] Furthermore, according to the inkjet recording apparatus 1 of this embodiment, the average primary particle diameter of the titanium oxide is 250 [nm] or more and 350 [nm] or less, so that high whiteness can be obtained.

[0068] Furthermore, according to the inkjet recording device 1 of this embodiment, the circulation device is a pump (circulation pump 67) that pumps liquid using a rotating body (impeller 74), so the ink can be circulated while suppressing the effects of reduced pressure within the ink tank 32.

[0069] Furthermore, according to the inkjet recording apparatus 1 of this embodiment, the circulation device is capable of transmitting power without contact between the pump shaft 73 of the pump and the motor shaft 75 via the partition wall 77. With this configuration, power can be transmitted while the circulation pump 67 remains liquid-tight, so ink leakage is reliably prevented even if a pressure difference occurs between the inside and outside of the circulation pump 67 due to reduced pressure in the ink tank 32.

[0070] Furthermore, according to the inkjet recording apparatus 1 of this embodiment, the circulation device can transmit power without contact using magnetic force between the pump shaft 73 and the motor shaft 75. With this configuration, power can be transmitted while the circulation pump 67 remains liquid-tight, so ink leakage is reliably prevented even if a pressure difference occurs inside and outside the circulation pump 67 due to reduced pressure in the ink tank 32.

[0071] Furthermore, according to the inkjet recording apparatus 1 of this embodiment, the viscosity of the ink is 2 [mPa·s] or more and 10 [mPa·s] or less, so it is possible to suppress deterioration of ejection performance due to low ink viscosity and deterioration of image quality due to high ink viscosity.

[0072] [Variations] The above embodiment may be modified as follows.

[0073] 7 to 12 are schematic diagrams showing Modifications 1 to 6. In the above embodiment, the circulation flow path 47 is formed so as to connect the side surface of the ink tank 32 near the liquid level with the bottom surface of the ink tank 32, but the circulation flow path 47 may be formed so as to connect different positions of the ink tank 32.

[0074] 7, the bottom of the ink tank 86 is connected to the liquid surface by a circulation flow path 87. An outlet 88 from the ink tank 86 to the circulation flow path 87 opens to the bottom of the ink tank 86, and an inlet 89 from the circulation flow path 87 to the ink tank 86 is in contact with the ink liquid surface. Even with this configuration, the ink near the liquid surface, which has a low amount of dissolved air, and the ink near the bottom, which has a high amount of dissolved air, are smoothly exchanged, improving degassing efficiency.

[0075] 8, the bottom of an ink tank 91 is connected to a position above the liquid surface by a circulation flow path 92. An outlet 93 from the ink tank 91 to the circulation flow path 92 opens at the bottom of the ink tank 91, and an inlet 94 from the circulation flow path 92 to the ink tank 91 is exposed above the liquid surface. With this configuration, the ink is exposed to a reduced pressure atmosphere while falling from the inlet 94, making it easier to remove air dissolved in the ink and improving degassing efficiency.

[0076] 9, the bottom of the ink tank 96 is connected to two liquid surface positions by a circulation flow path 97. An outlet 98 from the ink tank 96 to the circulation flow path 97 opens to the bottom of the ink tank 96, and two inlets 99 from the circulation flow path 97 to the ink tank 96 are in contact with the liquid surface. With this configuration, ink spreads from the two inlets 99 over the entire liquid surface, making it easier to remove air dissolved in the ink and improving degassing efficiency.

[0077] In the fourth modification shown in Figure 10, a side position near the bottom of the ink tank 101 and a submerged position slightly below the liquid surface are connected by a circulation channel 102. An inlet 104 from the circulation channel 102 to the ink tank 101 is located higher than an outlet 103 from the ink tank 101 to the circulation channel 102. Even with this configuration, a flow of ink occurs within the ink tank 101, improving degassing efficiency compared to when the ink is left standing.

[0078] In the fifth modification shown in Figure 11, a side position of the ink tank 106 and a submerged position deep in the liquid surface are connected by a circulation channel 107. An outlet 108 from the ink tank 106 to the circulation channel 107 and an inlet 109 from the circulation channel 107 to the ink tank 106 are located at approximately the same height. Even with this configuration, a flow of ink occurs within the ink tank 106, improving degassing efficiency compared to when the ink is left as is.

[0079] 12, a side position near the liquid surface of the ink tank 111 and a submerged position deeper below the liquid surface are connected by a circulation channel 112. An inlet 114 from the circulation channel 112 to the ink tank 111 is located lower than an outlet 113 from the ink tank 111 to the circulation channel 112. Even with this configuration, a flow of ink occurs within the ink tank 111, and ink near the liquid surface, which has a low amount of dissolved air, is sent, improving degassing efficiency.

[0080] Although the degassing device 40 provided in the inkjet recording apparatus 1 has been exemplified in this embodiment, the degassing device 40 can also be applied to devices used in other fields such as the semiconductor manufacturing field and the display manufacturing field. That is, the degassing device 40 can also be applied to degassing chemical solutions, electrolytes, liquid resins, adhesives, solvents, lubricants, liquid foods, beauty serums, and the like other than ink.

[0081] Furthermore, in this embodiment, the decompression pump 62 is exemplified as the decompression device, but the decompression device may be any device that can decompress the ink tank 32, and for example, the decompression device may be an ejector.

[0082] Furthermore, in this embodiment, the circulation pump 67 is exemplified as the circulation device, but the circulation device may be any device that can circulate ink through the circulation flow path 47, and the circulation device may be, for example, an ejector.

[0083] In addition, in this embodiment, the pump shaft 73 of the circulation pump 67 and the motor shaft 75 are magnetically connected, but the pump shaft 73 of the circulation pump 67 and the motor shaft 75 may be mechanically connected as long as ink leakage is suppressed.

[0084] In this embodiment, the determination unit 39 calculates the viscosity of the ink and the allowable time to determine whether or not a degassing operation is required, but in an environment where the viscosity of the ink does not change significantly, the calculation process of the viscosity of the ink and the allowable time may be omitted by appropriately setting the allowable time. Also, the control device 38 does not have to be provided with the determination unit 39.

[0085] In this embodiment, the sheet may be any sheet-like object on which an image can be formed, such as plain paper, coated paper, tracing paper, or an OHP (Over Head Projector) sheet.

[0086] Although the present embodiment has been described, other embodiments may be obtained by combining the above-described embodiments and modifications in whole or in part.

[0087] Furthermore, the technology of the present invention is not limited to the above-described embodiments, and may be variously modified, substituted, or altered within the scope of the spirit of the technical idea. Furthermore, if the technical idea can be realized in a different way due to technological advances or other derived technologies, it may be implemented using that method. Therefore, the claims cover all embodiments that may fall within the scope of the technical idea. [Explanation of symbols]

[0088] 1. Inkjet recording device 21 Recording head 32 Ink tank 47 Circulation flow path 62 Decompression pump (decompression device) 67 Circulation pump (circulation device) 73 Pump shaft 74 Impeller (rotating body) 75 motor shaft 77 Bulkhead

Claims

1. an ink tank in which ink is stored; a recording head that ejects ink supplied from the ink tank; a pressure reducing device for reducing the pressure inside the ink tank; a circulation flow path that connects different positions of the ink tank; a circulation device that circulates ink through the circulation flow path, The inkjet recording apparatus is characterized in that the ink contains a white pigment.

2. 2. An ink jet recording apparatus according to claim 1, wherein the white pigment is titanium oxide.

3. 3. The inkjet recording apparatus according to claim 2, wherein the average primary particle diameter of the titanium oxide is 250 nm or more and 350 nm or less.

4. 4. The inkjet recording apparatus according to claim 1, wherein the circulation device is a pump that pumps out liquid using a rotating body.

5. 5. The inkjet recording apparatus according to claim 4, wherein the circulation device is capable of transmitting power in a non-contact manner between a pump shaft and a motor shaft via a partition wall.

6. 6. The inkjet recording apparatus according to claim 5, wherein the circulation device is capable of transmitting power between the pump shaft and the motor shaft in a non-contact manner using magnetic force.

7. 2. The inkjet recording apparatus according to claim 1, wherein the viscosity of the ink is 2 mPa.s or more and 10 mPa.s or less.

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