Inkjet printer and image forming method

Abstract

An inkjet printer comprises: a plurality of print bars including inkjet heads and sequentially arranged along the conveyance direction of a recording medium such that the longitudinal direction of the print bars intersects with the conveyance direction; a conveyance mechanism that conveys the recording medium such that a recording surface faces the print bars; and a downflow mechanism for causing a first gas to flow toward the recording surface from between the print bars arranged in the conveyance direction, wherein, in the print bars, nozzles for discharging droplets are arranged facing the recording surface, and the relationship between the area S1 between a nozzle surface having the nozzles of the print bars and the recording surface as viewed from the conveyance direction, the conveyance speed Vt of the recording medium, the area S2 between the print bars as viewed from a direction perpendicular to the recording surface, and the speed V1 of the first gas flowing toward the recording surface satisfies (S1 / S2)×Vt<V1<Vt.

Description

Inkjet printing device and image forming method

[0001] The present disclosure relates to an inkjet printing apparatus and an image forming method.

[0002] Inkjet printing devices are known that form images by ejecting liquid such as ink. In inkjet printing devices, atomized ink mist may be generated when main ink droplets are ejected. If this ink mist adheres to a nozzle surface on which ejection openings (hereinafter also referred to as nozzles) of an inkjet head (hereinafter referred to as a head) are formed, ink ejection failure may occur. Furthermore, the ink mist may adhere to other components within the printing device, causing contamination inside the device and reducing the durability of the device.

[0003] As a method for suppressing this ink mist, for example, Japanese Patent Application Laid-Open No. 2010-195008 and Japanese Patent Application Laid-Open No. 2023-5755 propose providing a mechanism for spraying a fluid such as air from the nozzle side that ejects ink toward the recording medium.

[0004] Ink mist is tiny droplets that are ejected from the head and float up without adhering to the paper, and is more likely to occur the greater the distance between the nozzle face of the head and the paper. The rising of ink mist is caused by air entrained by the paper flowing into the bottom of the head and then rising around the head.

[0005] In recent years, there has been a demand for faster printing, making the problem of ink mist even more serious. To achieve this, paper must be transported at high speed. When paper is transported at high speed, there is a high possibility that the head will be damaged if the paper collides with the head. To reduce the risk of a collision between the paper and the head, it is conceivable to increase the distance between the transport surface that transports the paper and the head. As a result, tiny droplets ejected along with the main droplets tend to rise before adhering to the paper, creating ink mist. Furthermore, as printing speeds increase, the speed of the ascending air currents generated by entrained air also increases, further promoting the generation of ink mist.

[0006] Therefore, in order to increase the printing speed, it is necessary to suppress ink mist more than ever before.

[0007] The present disclosure has been made in consideration of the above circumstances, and has an object to provide an inkjet printing apparatus and an image forming method in which the generation of ink mist is suppressed even during high-speed printing.

[0008] The inkjet printing device disclosed herein is an inkjet printing device that forms an image on the recording surface of a recording medium being transported, and includes: a plurality of print bars each equipped with an inkjet head that ejects droplets onto the recording surface of the recording medium, the plurality of print bars being arranged in order along the transport direction with their longitudinal directions intersecting the transport direction of the recording medium; a transport mechanism that transports the recording medium with its recording surface facing the print bars; and a downflow mechanism that causes a first gas to flow toward the recording surface from between the print bars arranged in the transport direction, wherein the print bars have nozzles that eject droplets facing the recording surface, and during image formation, the relationship between an area S1 between the nozzle faces of the print bars having nozzles as viewed from the transport direction and the recording surface, a transport speed Vt of the recording medium, an area S2 between the print bars as viewed from a direction perpendicular to the recording surface, and a speed V1 of the first gas flowing toward the recording surface satisfies the following: (S1 / S2) × Vt<V1<Vt.

[0009] When the recording medium is paper, the inkjet printing device of the present disclosure preferably includes a paper feed section that stores the recording medium to be transported toward the print bar, and the temperature of the first gas is preferably within ±5°C of the temperature inside the paper feed section.

[0010] When the recording medium is paper, the inkjet printing device of the present disclosure preferably includes a paper feed section that stores the recording medium to be transported toward the print bar, and the humidity of the first gas is preferably ±5% of the humidity inside the paper feed section.

[0011] The print bar preferably includes a circuit board that controls the operation of the inkjet head and a cooling fan that blows cooling air onto the circuit board, and the downflow mechanism preferably takes the cooling air out of the print bar and flows the cooling air as at least part of the first gas from between the print bars toward the recording surface.

[0012] The downflow mechanism is preferably configured to cause the first gas to flow toward the recording surface after passing through a dust filter.

[0013] The downflow mechanism preferably has a downstream downflow section that flows the second gas toward the recording surface downstream of the print bar that is located furthest downstream in the transport direction among the multiple print bars, and preferably satisfies (S1 / S2) × Vt < V2 < Vt, where V2 is the speed of the second gas flowing in a downstream region adjacent to the print bar located furthest downstream and having the same area as S2.

[0014] The downflow mechanism preferably has an upstream downflow section that flows a third gas toward the recording surface upstream of the print bar that is located most upstream in the transport direction among the multiple print bars, and preferably satisfies (S1 / S2) × Vt < V3 < Vt, where V3 is the speed of the third gas flowing in an upstream region adjacent to the print bar that is located most upstream and has the same area as S2.

[0015] The downflow mechanism may further include an end face side downflow section that causes gas to flow toward the recording surface along an end face that intersects with the longitudinal direction of the print bar.

[0016] The image forming method disclosed herein is an image forming method for forming an image by an inkjet method on the recording surface of a recording medium being transported, wherein when the recording medium is transported facing a plurality of print bars each having an inkjet head that ejects droplets onto the recording surface of the recording medium, the print bars being arranged in order along the transport direction of the recording medium with their longitudinal directions intersecting the transport direction of the recording medium, a first gas is caused to flow toward the recording surface from between the print bars arranged in the transport direction, and the area S1 between the nozzle faces of the print bars having nozzles that eject droplets as seen from the transport direction of the recording medium and the recording surface, the transport speed Vt of the recording medium, the area S2 between the print bars as seen from a direction perpendicular to the recording surface, and the speed V1 of the first gas flowing toward the recording surface satisfy the following relationship: (S1 / S2) × Vt<V1<Vt.

[0017] According to the inkjet printing apparatus and image forming method of the present disclosure, the generation of ink mist can be suppressed even under high-speed printing conditions.

[0018] FIG. 5 is an overall configuration diagram of an inkjet printing apparatus according to an embodiment. FIG. 6 is a perspective view showing the configuration of a print bar. FIG. 3A and FIG. 3B are diagrams showing examples of a blower. FIG. 7 is a diagram explaining the velocity V1 and areas S1 and S2 of the first gas. FIG. 5A is a diagram explaining the definitions of areas S1 and S2 for the following cases: when there is a transport surface facing the print bar and the length L of the print bar is smaller than the width of the transport surface; FIG. 5B is a diagram explaining the definitions of areas S1 and S2 for the following cases: when there is a transport surface facing the print bar and the length L of the print bar is larger than the width of the transport surface; FIG. 5C is a diagram explaining the definitions of areas S1 and S2 for the following cases: when there is no transport surface facing the print bar and the length L of the print bar is smaller than the width of the recording surface; and FIG. 5D is a diagram explaining the definitions of areas S1 and S2 for the following cases: when there is no transport surface facing the print bar and the length L of the print bar is smaller than the width of the recording surface. FIG. 7 is a diagram explaining the effect of a downflow mechanism. FIG. 8 is a diagram explaining the velocity V2 of the second gas. FIG. 9 is a diagram explaining the velocity V3 of the third gas. FIG. 10 is a diagram explaining a modified downflow mechanism. FIG. 11 is a diagram explaining a modified downflow mechanism. FIG. 12 is a diagram explaining a modified downflow mechanism. FIG. 13 is a diagram explaining the definitions of areas S1 and S2.

[0019] Hereinafter, an embodiment of an inkjet printing apparatus and an image forming method according to the present disclosure will be described with reference to the drawings. In each drawing, the same elements are designated by the same reference numerals.

[0020] 1 is a diagram showing the overall configuration of an inkjet printing apparatus 1 according to an embodiment of the present disclosure. The inkjet printing apparatus 1 implements an embodiment of the image forming method according to the present disclosure.

[0021] The inkjet printing device 1 is an inkjet color digital printing device that forms a desired image on a sheet of paper P. The inkjet printing device 1 is a device for single-sided printing that forms an image on only one side of the paper P. However, the technology of the present disclosure can also be applied to a device for double-sided printing that forms images on both sides of the paper P. The paper P is an example of a recording medium of the technology of the present disclosure.

[0022] As shown in FIG. 1, the inkjet printing apparatus 1 includes a transport mechanism 10, a paper feeder 20, an image forming unit 40, and a stacker 70.

[0023] The transport mechanism 10 transports the paper P with the recording surface (i.e., paper surface) Pa of the paper P facing a print bar (described later) of the image forming unit 40. In Figure 1, a transport path 12 along which the paper P is transported is indicated by a two-dot chain line.

[0024] The transport mechanism 10 includes multiple transport members arranged along the transport path 12. The multiple transport members include multiple transport roller pairs 14, a paper feed drum 16, a discharge drum 18, and a print drum 41 (described later). The transport roller pair 14 includes a drive roller 14a and a pinch roller 14b arranged opposite each other. The transport roller pair 14 sandwiches the paper P between the drive roller 14a and the pinch roller 14b and rotates to feed the paper P. The transport members may further include a transport drum, a belt conveyor, a chain gripper, a transport guide, and the like. The transport mechanism 10 also includes a drive unit, such as a motor (not shown) and a motor drive circuit (not shown) as a power source. The paper P is transported along the transport path 12 by these elements constituting the transport mechanism 10.

[0025] The paper feed device 20 is an example of a paper feed section that stores paper P to be transported toward the image forming unit 40. The paper feed device 20 includes a paper feed tray on which a large number of sheets of paper P are placed in a stacked bundle. The type of paper P is not particularly limited, but printing paper primarily made of cellulose, such as high-quality paper, coated paper, and art paper, can be used. The maximum paper size that can be used in the inkjet printing device 1 is, for example, A0 size (841 mm x 1189 mm).

[0026] The paper feeder 20 takes out the sheets P one by one from the stack set therein, starting from the top, and supplies them to the transport path 12 .

[0027] The image forming unit 40 forms an image by ejecting droplets onto the recording surface Pa of the paper P. The image forming unit 40 includes a print drum 41, a head unit 42, and a downflow mechanism 50. The print drum 41 receives the paper P from the paper feed drum 16, subjects the received paper P to image formation by the head unit 42, and discharges the paper P from the discharge drum 18. The print drum 41 includes a gripper (not shown) on its circumferential surface, and by gripping the leading edge of the paper P with the gripper and rotating, wraps the paper P around the drum circumferential surface and transports it. The print drum 41 also includes a suction mechanism (not shown), which suctions the paper P wrapped around the drum circumferential surface and transports it. Negative pressure is used for suction. The print drum 41 includes multiple suction holes on its circumferential surface, and suction is applied from the inside of the print drum 41 to suction the paper P to the circumferential surface of the print drum 41.

[0028] The head unit 42 includes print bars 43C, 43M, 43Y, and 43K. In the following, when there is no need to distinguish between the print bars 43C, 43M, 43Y, and 43K, they will be referred to as print bars 43.

[0029] FIG. 2 shows a schematic configuration of the print bar 43. The print bar 43 includes a rectangular parallelepiped housing 44, and a plurality of inkjet heads 45, a circuit board 46, a flexible cable 47, and a cooling fan 48, all of which are housed within the housing 44. The inkjet heads 45 are arranged along the longitudinal direction A of the housing 44. Each inkjet head 45 has nozzles that eject liquid, and is positioned such that a nozzle surface 45a having the nozzles faces the recording surface Pa. The circuit board 46 is a board for controlling the inkjet heads 45. The flexible cable 47 provides electrical conductivity between each inkjet head 45 and the circuit board 46. The cooling fan 48 is provided in an opening 44a on the top surface of the housing 44. The cooling fan 48 is provided to cool the circuit board 46 and has the function of drawing in air from outside the housing 44 into the housing 44 as cooling air and blowing it toward the circuit board 46.

[0030] In the image forming unit 40, the print bar 43 is disposed so that the longitudinal direction A intersects (in this example, is perpendicular to) the transport direction T of the paper P. Furthermore, as shown in FIG. 1 , the multiple print bars 43C, 43M, 43Y, and 43K are arranged in order along the transport direction T.

[0031] The print bar 43C has a plurality of inkjet heads 45 arranged in the longitudinal direction A, each of which ejects droplets of cyan ink. The print bar 43M has a plurality of inkjet heads 45 arranged in the longitudinal direction A, each of which ejects droplets of magenta ink. The print bar 43Y has a plurality of inkjet heads 45 arranged in the longitudinal direction A, each of which ejects droplets of yellow ink. The print bar 43K has a plurality of inkjet heads 45 arranged in the longitudinal direction A, each of which ejects droplets of black ink. Each of the print bars 43C, 43M, 43Y, and 43K receives ink from an ink tank (not shown), which serves as an ink supply source for the corresponding color, via a piping path (not shown). For example, a water-based ink is used as the ink for drawing. Water-based ink refers to ink containing water and a coloring material such as a pigment or dye.

[0032] The aqueous ink is not particularly limited, but for example, the inkjet ink described in WO 2023 / 047767 is suitable.

[0033] At least one of print bars 43C, 43M, 43Y, and 43K ejects ink droplets toward paper P being transported by image formation drum 41, and the ejected droplets adhere to paper P, forming an image on paper P. That is, in image formation unit 40, an image is formed by applying ink to recording surface Pa of paper P.

[0034] In this example, a configuration using four colors of ink, CMYK, is illustrated, but the combination of ink colors and the number of colors is not limited to this embodiment, and light inks, dark inks, special color inks, etc. may be added as needed. For example, a configuration is possible in which a print bar equipped with an inkjet head that ejects light-colored ink such as light cyan or light magenta is added, and / or a configuration is possible in which a print bar equipped with an inkjet head that ejects special color ink such as green, orange, or white is added. Furthermore, the arrangement order of the print bars of each color is not particularly limited.

[0035] The downflow mechanism 50 causes a first gas to flow toward the recording surface Pa of the paper P from between the print bars 43C, 43M, 43Y, and 43K arranged in the transport direction T. In this example, the downflow mechanism 50 also causes a second gas to flow toward the recording surface Pa of the paper P downstream of the print bar 43K arranged at the most downstream side, and causes a third gas to flow toward the recording surface Pa of the paper P upstream of the print bar 43C arranged at the most upstream side. In this example, the first gas, second gas, and third gas are all air. The first gas, second gas, and third gas may each contain more than 85% nitrogen. Details of the downflow mechanism 50 will be described later.

[0036] The stacking device 70 stacks the sheets P on which images have been formed. The stacking device 70 receives the sheets P discharged from the transport path 12 and stacks the sheets P in a bundle on a stacking tray (not shown).

[0037] The inkjet printing apparatus 1 includes a processor (not shown). The processor includes a CPU (Central Processing Unit). The processor functions as a processing unit and / or a control unit that performs various processes by executing instructions of a program stored in a storage device. The processor comprehensively controls the conveying mechanism 10, the paper feeder 20, the image forming unit 40, and the stacking device 70.

[0038] The downflow mechanism 50 will be described in detail below. As an example, the downflow mechanism 50 includes a plurality of blowers 51 to 55. As shown in FIG. 1, blowers 52 to 54 are arranged between the print bars. Blower 51 is arranged upstream of the most upstream print bar 43C and constitutes an upstream downflow section. Blower 55 is arranged downstream of the most downstream print bar 43K and constitutes a downstream downflow section. In this example, blowers 51 to 55 are identical.

[0039] The blower 51 may be a blower with a slit-shaped opening 51a as shown in Fig. 3A, or a blower with a nozzle-shaped opening 51b as shown in Fig. 3B. In either case, the openings 51a and 51b are installed to face the recording surface Pa of the paper P, and the openings 51a and 51b cause air to flow toward the recording surface Pa, i.e., generate a wind toward the recording surface Pa.

[0040] First, we will explain the first gas that is blown between the print bars toward the recording surface Pa. Here, as an example, we will explain the blowing of the first gas toward the recording surface Pa between print bar 43C and print bar 43M.

[0041] The area S1 [mm ２ ], conveyance speed of paper P Vt [m / s], area S2 between print bars as viewed from a direction perpendicular to recording surface Pa [mm ２The first gas is blown at a speed V1 such that the relationship between the nozzle surface S2 and the speed V1 [m / s] of the first gas flowing toward the recording surface Pa satisfies the following: (S1 / S2) × Vt<V1<Vt. The upper limit of the speed V1 means that the volumetric speed of the first gas flowing into the region S1 is greater than the volumetric speed of the entrained air that flows into the region S1 as the paper P is transported from between the nozzle surface and the recording surface Pa. The lower limit of the speed V1 means that the speed V1 of the first gas flowing into the region S1 is smaller than the transport speed Vt of the paper P.

[0042] The velocity V1 of the first gas is the average velocity of the first gas moving toward the recording surface Pa between the print bars 43, and is the average value of velocities measured using an anemometer at multiple locations. Specifically, it is the average value of measurements taken at five or more locations 100 mm apart in the vertical direction from the recording surface Pa and spaced at equal intervals along the longitudinal direction A of the print bar 43. The velocity V2 of the second gas and the velocity V3 of the third gas, which will be described later, are also values ​​measured in a similar manner.

[0043] FIG. 4 is a schematic diagram illustrating the above-mentioned areas S1 and S2. For ease of explanation, the curved transport surface of the image forming drum 41 is shown in a planar form. As shown in FIG. 4, the distance between the nozzle surface 45a of the print bar 43C and the transport surface 41a of the image forming drum 41 is G, and the distance between the print bar 43C and the print bar 43M is D. The example shown in FIG. 4 is a case where a transport member (here, the image forming drum 41) is provided with the transport surface 41a facing the print bar 43C, as shown in FIG. 5A, and the length L of the print bar 43 in the longitudinal direction A is smaller than the width Wt of the image forming drum 41. In this case, S1 = L × G and S2 = L × D are defined.

[0044] 5B, in the case of a transport member (here, image forming drum 41) having a transport surface 41a facing print bar 43C, and the length L of print bar 43C in the longitudinal direction A is greater than the width Wt of image forming drum 41, then S1 = Wt × G and S2 = Wt × D are defined. Here, as in the case of FIG. 4, G is the distance between nozzle surface 45a of print bar 43C and transport surface 41a, and D is the distance between print bars 43C and 43M.

[0045] As described above, in a configuration in which the recording medium is paper P and the transport surface 41a that transports the paper P is disposed opposite the nozzle surface 45a, the "area between the nozzle surface of the print bar and the recording surface" is read as the "area between the nozzle surface of the print bar and the transport surface." Because the paper P is sufficiently thin compared to the distance between the nozzle surface 45a and the transport surface 41a, the thickness of the paper P is ignored.

[0046] On the other hand, if the transport member does not have a transport surface facing the print bar 43C, the distance between the nozzle surface 45a of the print bar 43C and the recording surface Pa of the paper P being transported facing the nozzle surface 45a is defined as G. Then, as shown in Figure 5C, if the length L of the print bar 43C in the longitudinal direction A is smaller than the width Wp of the paper P, then S1 = L × G and S2 = L × D are defined.

[0047] Furthermore, when the transport member does not have a transport surface facing the print bar 43C, and the length of the longitudinal direction A of the print bar 43C is greater than the width Wp of the paper P, as shown in Figure 5D, S1 = Wp x G and S2 = Wp x D are defined.

[0048] An example of a case in which the transport member does not have a transport surface facing the print bar 43C is when the paper P is configured to be sandwiched between a pair of transport rollers and sent to a position facing the print bar 43.

[0049] The distance D between the print bars is, for example, 50 mm to 100 mm, and the distance G between the nozzle surface 45a and the transport surface is, for example, 0.5 mm to 1.5 mm. The length L of the print bar 43 in the longitudinal direction is, for example, about 1 m. The transport speed Vt of the paper P is, for example, 1 to 5 m / s.

[0050] The above describes the first gas that flows toward the recording surface Pa between print bar 43C and print bar 43M, but the same applies between print bar 43M and print bar 43Y, and between print bar 43Y and print bar 43K.

[0051] In a conventional device not equipped with a downflow mechanism 50, as shown in Figure 12, as the paper P is transported by the rotation of the image forming drum 41, entrained air flows from the gap between the nozzle surface 45a of the print bar 43 and the transport surface 41a, as indicated by the arrows. In this case, for example, if entrained air that has flowed in from the gap between print bar 43C and the transport surface 41a flows out between print bars 43C and 43M, at least a portion of the air will create an upward flow of air. As mentioned above, faster printing requires faster paper transport, which makes ink mist more likely to occur.

[0052] In contrast, the inkjet printing apparatus 1 of this embodiment is provided with a downflow mechanism 50 that causes the first gas to flow toward the recording surface Pa between the print bars, and the speed V1 of the first gas flowing toward the recording surface Pa between the print bars satisfies (S1 / S2)×Vt<V1<Vt. This makes it possible to prevent air entrained by the transport of paper from rising between the print bars, as shown in Fig. 6, and therefore makes it possible to effectively suppress ink mist even during high-speed transport.

[0053] It is also preferable that the second gas flowing toward the recording surface Pa downstream of the print bar 43K arranged at the most downstream side flows at a similar velocity. That is, as shown in Fig. 7, when the velocity of the second gas flowing in an area downstream of the print bar 43K arranged at the most downstream side and having the same area as the area S2 is defined as V2, it is preferable that the following relationship be satisfied: (S1 / S2) x Vt<V2<Vt.

[0054] If the second gas is caused to flow toward the recording surface Pa at V2 that satisfies the above, ink mist can be effectively suppressed even on the most downstream side of the print bar.

[0055] Furthermore, as shown in FIG. 8, when the velocity of the third gas flowing in an upstream region adjacent to the print bar 43C arranged most upstream and having the same area as the above-mentioned area S2 is defined as V3, it is preferable that the following relationship be satisfied: (S1 / S2)×Vt<V3<Vt.

[0056] If the third gas is flowed toward the recording surface Pa at V3 that satisfies the above, ink mist can be effectively suppressed even on the most downstream side of the print bar.

[0057] In the above embodiment, the downflow mechanism 50 is equipped with blowers 51 to 55 and is configured to blow air toward the recording surface Pa, but there are no particular restrictions on the downflow mechanism 50 as long as it can blow gas under the above conditions.

[0058] 9 and 10 , the downflow mechanism 50 may be incorporated into the print bar 43. As described above, the print bar 43 includes a circuit board 46 that drives and controls the inkjet heads, and a cooling fan 48 that blows cooling air onto the circuit board 46. The downflow mechanism 50 may be configured to extract the cooling air from the cooling fan 48 to the outside of the print bar 43, and to flow at least a portion of the cooling air as at least a part of the first gas through the gaps in the print bar 43 toward the recording surface Pa. Specifically, an opening 44b is provided in at least one of the faces of the housing 44 of the print bar 43 that are perpendicular to the direction of travel.

[0059] Although the internal structure of the print bar 43 is omitted in Figure 9, the internal structure is the same as that shown in Figure 2. Figure 10 is a side view of the print bars 43C, 43M, 43Y, and 43K arranged along the transport direction, and is a schematic diagram showing the main parts of the print bar 43. The circuit board 46 is disposed within the housing 44 with its plane extending perpendicular to the recording surface Pa of the print bar 43. The cooling fan 48 is provided in an opening 44a on the top surface of the housing 44 (the surface facing the nozzle surface), and draws air into the housing 44 through this opening 44a and blows it toward the circuit board 46. A portion of the gas blown toward the circuit board 46 by the cooling fan 48 is exhausted to the outside through an opening 44b on the side surface of the housing 44, forming a downflow that flows toward the recording surface Pa of the recording medium.

[0060] By using the cooling air from the cooling fan 48 in this way, it is possible to effectively utilize the existing configuration and realize a downflow mechanism without providing a separate air blower. It is preferable that a dust filter 49 is provided at the air intake of the cooling fan 48. By providing the dust filter 49, it is possible to prevent dust from adhering to the nozzles and the recording surface Pa due to the gas containing dust being blown in. In this embodiment, the dust filter 49 is provided at the intake of the cooling fan 48, but the position of the dust filter 49 is not limited to the embodiment shown in FIG. 9 as long as the gas flowing toward the recording surface Pa passes through the dust filter 49 before reaching the recording surface Pa.

[0061] 11 , the inkjet printing apparatus 1 may further include an end face side downflow section in which the downflow mechanism 50 flows gas toward the recording surface Pa along the end face that intersects with the longitudinal direction A of the print bar 43 (in this example, perpendicular to the longitudinal direction A). In the example shown in FIG. 11 , the end face side downflow section includes air blowers 56 and 57. By providing such an end face side downflow section, it is possible to suppress the generation of ink mist even on the end face side of the print bar 43 that intersects with the longitudinal direction A.

[0062] When the recording medium is paper P as in the above embodiment, it is preferable that the temperature of the gas (including the first gas, second gas, and third gas) flowing toward the recording surface Pa is ±5°C of the temperature inside the paper feeder 20. By setting the temperature of the gas flowing toward the recording surface Pa to ±5°C of the temperature of the paper feeder 20, image formation in the image forming unit 40 can be performed in a state where the temperature of the paper P does not change significantly from the state when it is stored in the paper feeder 20.

[0063] Furthermore, when the recording medium is paper P, it is preferable that the humidity of the gas (including the first gas, second gas, and third gas) flowing toward the recording surface Pa is ±5% of the humidity inside the paper feeder 20. By setting the humidity of the gas flowing toward the recording surface Pa to the temperature of the paper feeder 20 ±5°C, image formation in the image forming unit 40 can be performed in a state where the humidity of the paper P does not change significantly from the state when it is stored in the paper feeder 20.

[0064] It is particularly preferable that the temperature of the gas (including the first gas, second gas, and third gas) flowing toward the recording surface Pa is within ±5°C of the temperature inside the paper feeder 20, and that the humidity is within ±5% of the humidity inside the paper feeder 20. This allows image formation on the paper P in an environment substantially equivalent to the environment in which the paper P is stored in the paper feeder 20.

[0065] It is preferable for image formation that the amount of moisture contained in the paper P does not change significantly before and after paper feeding. Note that adjusting the gas to an appropriate temperature and humidity can be achieved, for example, by passing the gas through a blower with temperature and humidity control functions.

[0066] When ultraviolet-curable ink is used, it is preferable to provide an ultraviolet light source between the print bars. In this case, it is preferable to use a gas containing more nitrogen than air, specifically a gas containing 85% or more nitrogen, as the gas flowing toward the recording surface Pa, as this can promote ultraviolet curing.

[0067] The inkjet printing apparatus 1 may include a pretreatment liquid application unit and a pretreatment liquid drying unit upstream of the image forming unit 40. The inkjet printing apparatus 1 may also include a drying unit downstream of the image forming unit 40.

[0068] The term "recording medium" is a general term for various terms such as paper, recording paper, printing paper, printing medium, print medium, print-receiving medium, image-forming medium, image-receiving medium, image-receiving medium, and ejection-receiving medium.

[0069] The configurations described in the above embodiments and the features described in the modified examples can be used in appropriate combinations, and some features can also be replaced.

[0070] The above-described embodiments of the present invention may be modified, added, or deleted as appropriate within the scope of the spirit of the present invention. The present invention is not limited to the above-described embodiments, and many modifications are possible within the technical concept of the present invention by those having ordinary skill in the relevant field.

[0071] The following supplementary notes are further disclosed regarding the above embodiment: <Supplementary Note 1> An inkjet printing device that forms an image on the recording surface of a recording medium being transported, comprising: a plurality of print bars each equipped with an inkjet head that ejects droplets onto the recording surface of the recording medium, the plurality of print bars being arranged in order along the transport direction with their longitudinal directions intersecting the transport direction of the recording medium, a transport mechanism that transports the recording medium with the recording surface facing the print bars, and a downflow mechanism that causes a first gas to flow toward the recording surface from between the print bars arranged in the transport direction, wherein the print bars have nozzles that eject droplets facing the recording surface, and wherein, during image formation, the relationship between an area S1 between a nozzle face having the nozzles of the print bar as viewed from the transport direction and the recording surface, a transport speed Vt of the recording medium, an area S2 between the print bars as viewed from a direction perpendicular to the recording surface, and a speed V1 of the first gas flowing toward the recording surface satisfies the following: (S1 / S2) × Vt<V1<Vt. <Appendix 2> The inkjet printing device of Appendix 1, wherein the recording medium is paper, and wherein the inkjet printing device comprises a paper feed unit that accommodates the recording medium transported toward the print bar, and wherein the temperature of the first gas is within ±5°C of the temperature inside the paper feed unit. <Appendix 3> The inkjet printing device of Appendix 1 or Appendix 2, wherein the recording medium is paper, and wherein the inkjet printing device comprises a paper feed unit that accommodates the recording medium transported toward the print bar, and wherein the humidity of the first gas is within ±5% of the humidity inside the paper feed unit. <Appendix 4> The inkjet printing device of any one of Appendix 1 to Appendix 3, wherein the print bar comprises a circuit board that drives and controls the inkjet head, and a cooling fan that blows cooling air to the circuit board, and wherein the downflow mechanism extracts the cooling air to the outside of the print bar and flows the cooling air as at least a part of the first gas from between the print bars toward the recording surface. <Appendix 5> The inkjet printing device of any one of Appendix 1 to Appendix 4, wherein the downflow mechanism flows the first gas toward the recording surface after passing it through a dust filter.<Supplementary Note 6> The inkjet printing device according to any one of Supplementary Notes 1 to 5, wherein the downflow mechanism has a downstream downflow section downstream of the print bar that is located furthest downstream in the transport direction and that causes a second gas to flow toward the recording surface, and wherein, when V2 is the velocity of the second gas flowing in an area that is a downstream region in contact with the furthest downstream print bar and has the same area as S2, the following relationship is satisfied: (S1 / S2) × Vt < V2 < Vt. <Supplementary Note 7> The inkjet printing device according to any one of Supplementary Notes 1 to 6, wherein the downflow mechanism has an upstream downflow section upstream of the print bar that is located furthest upstream in the transport direction and that causes a third gas to flow toward the recording surface, and wherein, when V3 is the velocity of the third gas flowing in an upstream region in contact with the furthest upstream print bar and has the same area as S2, the following relationship is satisfied: (S1 / S2) × Vt < V3 < Vt. <Appendix 8> The inkjet printing device according to any one of Appendices 1 to 7, wherein the downflow mechanism further comprises an end face side downflow section that flows gas toward the recording surface along an end face that intersects the longitudinal direction of the print bar. <Appendix 9> An image forming method for forming an image by an inkjet method on the recording surface of a conveyed recording medium, wherein when the recording medium is conveyed facing a plurality of print bars each having an inkjet head that ejects droplets onto the recording surface of the recording medium, the plurality of print bars being arranged in order along the conveyance direction with their longitudinal directions intersecting the conveyance direction of the recording medium, a first gas is caused to flow toward the recording surface from between the print bars arranged in the conveyance direction, and the area S1 between the nozzle faces of the print bars having nozzles that eject droplets, as seen from the conveyance direction of the recording medium, and the recording surface, the conveyance speed Vt of the recording medium, the area S2 between the print bars as seen from a direction perpendicular to the recording surface, and the speed V1 of the first gas flowing toward the recording surface satisfy the following relationship: (S1 / S2) × Vt<V1<Vt.

[0072] The disclosure of Japanese Patent Application No. 2024-154271, filed on September 6, 2024, is incorporated herein by reference in its entirety. All publications, patent applications, and technical standards mentioned herein are incorporated herein by reference to the same extent as if each individual publication, patent application, and technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. An inkjet printing device that forms an image on the recording surface of a recording medium being transported, comprising: a plurality of print bars equipped with inkjet heads that eject droplets onto the recording surface of the recording medium, the plurality of print bars being arranged in order along the transport direction of the recording medium with their longitudinal directions intersecting the transport direction; a transport mechanism that transports the recording medium with the recording surface facing the print bars; and a downflow mechanism that flows a first gas from between the print bars arranged in the transport direction toward the recording surface, wherein the print bars have nozzles that eject the droplets arranged opposite the recording surface, and wherein the relationship between an area S1 between a nozzle face of the print bar having the nozzles as seen from the transport direction and the recording surface during image formation, a transport speed Vt of the recording medium, an area S2 between the print bars as seen from a direction perpendicular to the recording surface, and a speed V1 of the first gas flowing toward the recording surface satisfies the following: (S1 / S2) x Vt<V1<Vt.

2. An inkjet printing device as described in claim 1, wherein the recording medium is paper, the inkjet printing device is provided with a paper feed unit that stores the recording medium transported toward the print bar, and the temperature of the first gas is within ±5°C of the temperature inside the paper feed unit.

3. An inkjet printing device as described in claim 1, wherein the recording medium is paper, the inkjet printing device is provided with a paper feed unit that stores the recording medium transported toward the print bar, and the humidity of the first gas is ±5% of the humidity inside the paper feed unit.

4. An inkjet printing device according to any one of claims 1 to 3, wherein the print bar comprises a circuit board that drives and controls the inkjet head, and a cooling fan that blows cooling air onto the circuit board, and the downflow mechanism extracts the cooling air to the outside of the print bar and causes the cooling air, as at least part of the first gas, to flow from between the print bars toward the recording surface.

5. The inkjet printing device according to any one of claims 1 to 3, wherein the downflow mechanism causes the first gas to flow toward the recording surface after passing through a dust filter.

6. An inkjet printing device according to any one of claims 1 to 3, wherein the downflow mechanism has a downstream downflow section that causes a second gas to flow toward the recording surface downstream of the print bar that is located furthest downstream in the transport direction among the plurality of print bars, and wherein, when the speed of the second gas flowing in an area that is downstream and in contact with the print bar located furthest downstream and has the same area as the area S2 is taken as V2, the following relationship is satisfied: (S1 / S2) x Vt < V2 < Vt.

7. An inkjet printing device according to any one of claims 1 to 3, wherein the downflow mechanism has an upstream downflow section that flows a third gas toward the recording surface upstream of the print bar that is located furthest upstream in the transport direction among the plurality of print bars, and when the speed of the third gas flowing in an upstream area that is in contact with the print bar located furthest upstream and has the same area as the area S2 is taken as V3, the following relationship is satisfied: (S1 / S2) x Vt < V3 < Vt.

8. An inkjet printing device according to any one of claims 1 to 3, wherein the downflow mechanism further has an end face side downflow section that flows gas toward the recording surface along an end face of the print bar that intersects with the longitudinal direction.

9. An image forming method for forming an image by an inkjet method on the recording surface of a recording medium being transported, wherein the recording medium is transported facing a plurality of print bars equipped with inkjet heads that eject droplets onto the recording surface of the recording medium, the plurality of print bars being arranged in order along the transport direction of the recording medium with their longitudinal directions intersecting the transport direction of the recording medium, and a first gas is caused to flow from between the print bars arranged in the transport direction towards the recording surface, and the relationship between the area S1 between the nozzle faces of the print bars having nozzles that eject the droplets, as seen from the transport direction of the recording medium, and the recording surface, the transport speed Vt of the recording medium, the area S2 between the print bars as seen from a direction perpendicular to the recording surface, and the speed V1 of the first gas flowing towards the recording surface satisfies the following: (S1 / S2) x Vt<V1<Vt.

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