Inkjet printer and image forming method

Abstract

This inkjet printer comprises: a conveyance mechanism that conveys a recording medium; an inkjet head that discharges ink droplets toward the recording medium from a plurality of nozzles arranged in a direction intersecting a conveyance direction of the recording medium to record an image on the recording medium, the nozzles being capable of discharging a plurality of types of ink droplets having different diameters; and an ink mist recovery device that is disposed on a downstream side of the inkjet head in the conveyance direction of the recording medium and recovers ink mist generated from the ink droplets by suctioning the ink mist. A suction amount of the ink mist recovery device is set to a value that makes deviation amounts of landing positions of the plurality of types of ink droplets on the recording medium due to an air flow caused by suction and an air flow caused by conveyance fall within a predetermined range and allows ink mist of a predetermined amount or more to be recovered.

Description

Inkjet printer and image forming method

[0001] The technologies disclosed herein relate to inkjet printers and image forming methods.

[0002] Inkjet printers, which form images on recording media such as paper by ejecting ink droplets from nozzles in an inkjet head, are becoming widespread. However, ink mist contamination of the inkjet head is a problem with inkjet printers. Ink mist contamination of the inkjet head can cause poor ink droplet ejection, leading to a decrease in image quality. Ink mist is generated when ink droplets are ejected from the inkjet head, causing fine droplets to disperse from the ink droplets, or when ink droplets that have landed on the recording media bounce back.

[0003] Japanese Patent Publication No. 2015-134496, Japanese Patent Publication No. 2016-175402, and Japanese Patent Publication No. 2023-012723 describe inkjet printers equipped with an ink mist recovery device. The ink mist recovery devices described in Japanese Patent Publication No. 2015-134496, Japanese Patent Publication No. 2016-175402, and Japanese Patent Publication No. 2023-012723 are located downstream of the inkjet head in the transport direction of the recording medium and recover the ink mist by suction.

[0004] Ink droplets are affected by airflow caused by the suction of the ink mist recovery device and airflow caused by the transport of the recording medium. Therefore, ink droplets do not fall straight down vertically, but rather curve downstream in the direction of transport of the recording medium.

[0005] Incidentally, some inkjet printers are line-type inkjet printers that eject ink droplets from multiple nozzles arranged in a direction intersecting (perpendicular to) the transport direction of the recording medium. Some of these line-type inkjet printers eject ink droplets of different diameters, such as ink droplets with a relatively large diameter (hereinafter referred to as large droplets) and ink droplets with a smaller diameter than the large droplets (hereinafter referred to as small droplets), in order to enhance image expression. In this case, since the mass of the large droplets and the small droplets are different, they also bend differently when falling. As a result, the large droplets and small droplets, which should ideally land in the same row, may be offset in the transport direction when landing on the recording medium, and the resulting offset may appear as white streaks, potentially degrading image quality.

[0006] To reduce the difference in landing positions between large and small droplets in the transport direction, the airflow that causes the problem should be suppressed. However, airflow caused by the transport of the recording medium is unavoidable. Therefore, the only option is to suppress the airflow caused by the suction of the ink mist recovery device. However, if the suction power of the ink mist recovery device is weakened to suppress this airflow, the ink mist will not be recovered sufficiently, and there is a risk that the image quality will be degraded by the ink mist.

[0007] One embodiment of the technology of this disclosure provides an inkjet printer and an image forming method that can avoid insufficient collection of ink mist while suppressing a decrease in image quality due to misalignment of the landing positions of ink droplets of different diameters.

[0008] The inkjet printer of this disclosure comprises a transport mechanism for transporting a recording medium, an inkjet head that records an image on the recording medium by ejecting ink droplets toward the recording medium from a plurality of nozzles arranged in a direction intersecting the transport direction of the recording medium, the inkjet head being capable of ejecting multiple types of ink droplets with different diameters, and an ink mist recovery device positioned downstream of the inkjet head in the transport direction of the recording medium, which recovers ink mist generated from the ink droplets by suction, wherein the suction amount of the ink mist recovery device is set to a value that allows the amount of displacement of the landing position of multiple types of ink droplets on the recording medium due to the airflow caused by suction and the airflow caused by transport to be within a set range, and that allows the recovery of an amount of ink mist greater than or equal to the set amount.

[0009] It is preferable that the suction volume is set to a value such that the velocity of the airflow at the bottom surface of the nozzle resulting from the suction is less than or equal to a first threshold TH1 based on the set range, and greater than or equal to a second threshold TH2 based on the set volume.

[0010] Preferably, the first threshold TH1 is represented by the following equation (1), and the second threshold TH2 is represented by the following equation (3). however, Lmax (unit: μm) is the setting range, Vp (unit: m / min) is the transport speed of the recording medium, μ (unit: kg / m·s) is the viscosity coefficient of the ink droplet, Td (unit: mm) is the distance from the nozzle to the recording medium, ρ (unit: kg / m ３ ) is the density of the ink droplets, rs (unit: μm) is the radius of the smallest diameter ink droplet, rl (unit: μm) is the radius of the largest diameter ink droplet, vs (unit: m / s) is the average velocity of the smallest diameter ink droplet, vl (unit: m / s) is the average velocity of the largest diameter ink droplet, and Lg (unit: mm) is the distance from the nozzle to the ink mist suction port of the ink mist recovery device.

[0011] The setting range is preferably related to the following equation (4): {(rs + rl) ２ -R ２} １／２ (4) R is the distance between two adjacent ink droplets in a direction intersecting the transport direction of the recording medium.

[0012] The setting amount is preferably related to the following equation (5): rmis・vmis ≤ 3.63e －７ (5) rmis (unit: μm) is the radius of the ink mist, and vmis (unit: m / s) is the average velocity of the ink mist.

[0013] The ink mist recovery device preferably changes the suction amount according to the transport speed of the recording medium.

[0014] The ink mist recovery device preferably changes the suction amount according to the distance from the nozzle to the recording medium.

[0015] The present disclosure relates to an image forming method using an inkjet printer comprising: a transport mechanism for transporting a recording medium; an inkjet head for recording an image on a recording medium by ejecting ink droplets toward the recording medium from a plurality of nozzles arranged in a direction intersecting the transport direction of the recording medium, wherein the nozzles are capable of ejecting multiple types of ink droplets with different diameters; and an ink mist recovery device positioned downstream of the inkjet head in the transport direction of the recording medium, which recovers ink mist generated from ink droplets by suction, wherein the suction amount of the ink mist recovery device is set to a value that allows for the amount of displacement of the landing position of multiple types of ink droplets on the recording medium due to airflow caused by suction and airflow caused by transport to be within a set range, and that allows for the recovery of an amount of ink mist greater than or equal to the set amount.

[0016] The technology disclosed herein provides an inkjet printer and an image forming method that can avoid insufficient ink mist recovery while suppressing the degradation of image quality due to misalignment of the landing positions of ink droplets of different diameters.

[0017] This is a diagram of an inkjet printer. This is a diagram of the nozzles of the inkjet head. This is a diagram of a large droplet. This is a diagram of a small droplet. This is a diagram of the suction port of the ink mist recovery device. This is a block diagram of the computer that constitutes the control device. This is a block diagram of the processing unit of the CPU of the control device. This is a diagram of the amount of displacement of the landing position of large and small droplets on the recording medium due to airflow. This is a diagram showing the landing positions of large and small droplets on the recording medium, where (A) is the original landing position and (B) is the actual landing position. This is a diagram of the processing of the image forming unit control section. This is a diagram of the amount of displacement of the landing position of ink droplets on the recording medium due to airflow. This is a diagram of the airflow caused by the transport of the recording medium. This is a diagram of the amount of displacement of the landing position of ink droplets on the recording medium due to airflow caused by the transport of the recording medium. This is a diagram of the airflow caused by the suction of the ink mist recovery device. This is a diagram of the amount of displacement of the landing position of ink droplets on the recording medium due to airflow caused by the suction of the ink mist recovery device. This is a diagram of the parameters related to the derivation of the second threshold. This is a diagram of the parameters related to the derivation of the setting range. This is a table showing the values ​​of each parameter in Example 1. This is a graph showing the relationship between the transport speed of the recording medium and the speed of the airflow caused by suction in Example 1. This figure shows the parameters related to the derivation of the suction volume range of the ink mist recovery device. This is a table showing the values ​​of each parameter in Example 2. This is a graph showing the relationship between the transport speed of the recording medium and the velocity of the airflow caused by suction in Example 2. This is a table showing the breakdown of the first threshold and second threshold in Example 2. This is a table showing the values ​​of each parameter in Example 3. This is a graph showing the relationship between the transport speed of the recording medium and the velocity of the airflow caused by suction in Example 3. This is a table showing the breakdown of the first threshold and second threshold in Example 3.

[0018] As an example, as shown in Figure 1, the inkjet printer 2 is a so-called continuous-feed type, and forms an image on a long recording medium RM that is transported along the transport direction CD using an inkjet method. The recording medium RM is, for example, printing paper, and the image is, for example, a poster, calendar, catalog, flyer, etc. The symbol WD next to the arrow of the transport direction CD is the width direction of the recording medium RM that intersects, in this case orthogonal to, the transport direction CD. Here, "orthogonal" refers not only to perfect orthogonality but also to orthogonality that includes errors that are generally acceptable in the art to which the technology of this disclosure belongs.

[0019] The inkjet printer 2 consists of a supply unit 10, a first buffer unit 11, an image forming unit 12, a drying unit 13, a second buffer unit 14, a recovery unit 15, and a control device 16. The supply unit 10, the first buffer unit 11, the image forming unit 12, the drying unit 13, the second buffer unit 14, and the recovery unit 15 are arranged in this order from the upstream side of the transport direction CD.

[0020] The supply unit 10 houses a supply roll 20 on which the recording medium RM is wound. The supply roll 20 is rotated counterclockwise by a motor (not shown). This supplies the recording medium RM to the image forming unit 12. The first buffer unit 11 is provided with a dancer roller (not shown). The dancer roller moves up and down to adjust the tension and transport amount of the recording medium RM supplied to the image forming unit 12.

[0021] The image forming unit 12 includes a plurality of transport rollers 21 for transporting the recording medium RM. The transport rollers 21 are an example of a "transport mechanism" according to the technology of this disclosure. In addition to the transport rollers 21, a transport belt may also be used as the transport mechanism.

[0022] The image forming unit 12 includes an inkjet head 22K for the key plate (black), an inkjet head 22C for cyan, an inkjet head 22M for magenta, and an inkjet head 22Y for yellow. These key plate inkjet heads 22K, cyan, magenta, and yellow are arranged in this order from the upstream side of the transport direction CD. In the following description, unless otherwise specified, the key plate inkjet head 22K, cyan, magenta, and yellow will be collectively referred to as the inkjet head 22.

[0023] The key plate inkjet head 22K, the cyan inkjet head 22C, the magenta inkjet head 22M, and the yellow inkjet head 22Y each eject ink droplets DP (see Figure 3, etc.) from nozzles 30 (see Figure 2) to form a full-color image on the recording medium RM. For example, the ejection method for the ink droplets DP can be a piezoelectric method utilizing the vibration pressure of a piezoelectric element, or a charge control method utilizing electrostatic attraction. Other methods such as thermal inkjet or acoustic inkjet may also be employed.

[0024] Here, around the inkjet head 22, when ink droplets DP are ejected, fine droplets are dispersed from the ink droplets DP, or ink droplets DP that have landed on the recording medium RM bounce back, generating ink mist M (see Figure 16). Ink mist M consists of extremely small particles compared to ink droplets DP. This ink mist M adheres to the ink droplet ejection port of the nozzle 30 of the inkjet head 22, contaminating the inkjet head 22 and causing poor ejection of ink droplets DP, ultimately leading to a decrease in image quality.

[0025] Therefore, the image forming unit 12 includes a key plate ink mist recovery device 23K, a cyan ink mist recovery device 23C, a magenta ink mist recovery device 23M, and a yellow ink mist recovery device 23Y. The key plate ink mist recovery device 23K is located downstream of the transport direction CD of the key plate inkjet head 22K, and the cyan ink mist recovery device 23C is located downstream of the transport direction CD of the cyan inkjet head 22C. The magenta ink mist recovery device 23M is located downstream of the transport direction CD of the magenta inkjet head 22M, and the yellow ink mist recovery device 23Y is located downstream of the transport direction CD of the yellow inkjet head 22Y. In the following description, unless otherwise specified, the key plate ink mist recovery device 23K, the cyan ink mist recovery device 23C, the magenta ink mist recovery device 23M, and the yellow ink mist recovery device 23Y will be collectively referred to as the ink mist recovery device 23.

[0026] The drying section 13 is equipped with a dryer such as a heat drum (not shown). The dryer dries the image-formed recording medium RM sent from the image forming section 12 and fixes the ink to the recording medium RM. The second buffer section 14 is equipped with a dancer roller (not shown), similar to the first buffer section 11. The dancer roller of the second buffer section 14 also moves up and down, similar to the dancer roller of the first buffer section 11, to adjust the tension and transport amount of the recording medium RM to be collected in the collection section 15. The collection section 15 houses a collection roll 24 on which the image-formed recording medium RM is wound. The collection roll 24 is rotated clockwise by a motor (not shown). This collects the image-formed recording medium RM.

[0027] The control device 16 is connected to the supply unit 10, the first buffer unit 11, the image forming unit 12, the drying unit 13, the second buffer unit 14, and the recovery unit 15, respectively. The control device 16 controls the driving of the supply unit 10, the first buffer unit 11, the image forming unit 12, the drying unit 13, the second buffer unit 14, and the recovery unit 15.

[0028] As an example, as shown in FIG. 2, the inkjet head 22 has a plurality of nozzles 30 arranged in a single row at equal intervals in the width direction WD. The nozzles 30 are provided over the entire width of the recording medium RM. Incidentally, the symbol lb is the length in the width direction WD of the inkjet head 22 (the width of the inkjet head 22). Note that the nozzles 30 may be arranged in a plurality of rows.

[0029] As an example, as shown in FIGS. 3 and 4, the nozzles 30 can eject two types of ink droplets DP having different diameters, namely large droplets LDP which are relatively large-diameter ink droplets DP and small droplets SDP which are ink droplets DP having a smaller diameter than the large droplets LDP. The large droplets LDP are an example of the "largest-diameter ink droplets" according to the technology of the present disclosure. Also, the small droplets SDP are an example of the "smallest-diameter ink droplets" according to the technology of the present disclosure. Incidentally, the symbol rl is the radius (unit: μm) of the large droplets LDP, and the symbol rs is the radius (unit: μm) of the small droplets SDP.

[0030] As an example, as shown in FIG. 5, the ink mist recovery device 23 has a suction port 32. The suction port 32 is a slit-shaped hole that opens over the entire width of the recording medium RM. The ink mist recovery device 23 recovers the ink mist M by sucking it from this suction port 32. Incidentally, the length in the width direction WD of the ink mist recovery device 23 (the width of the ink mist recovery device 23) is the same lb as that of the inkjet head 22. Note that the suction port 32 may be divided into several parts. Also, the suction port 32 may be arranged in a plurality of rows.

[0031] As an example, as shown in FIG. 6, the computer constituting the control device 16 includes a storage 35, a memory 36, a CPU (Central Processing Unit) 37, and a communication unit 38. These storage 35, memory 36, CPU 37, and communication unit 38 are interconnected via a bus line 39.

[0032] Storage 35 is a hard disk drive built into the computer that constitutes the control device 16 or connected through a cable or network. Alternatively, storage 35 is a disk array with multiple hard disk drives installed. In storage 35, control programs such as operating systems, various application programs, and various data associated with these programs are stored. Also, in storage 35, data of images formed on the recording medium RM is stored. Note that a solid state drive may be used instead of the hard disk drive.

[0033] Memory 36 is a work memory for the CPU 37 to execute processing. The CPU 37 loads the program stored in the storage 35 into the memory 36 and executes processing according to the program. Thereby, the CPU 37 comprehensively controls each part of the computer. Note that the memory 36 may be built into the CPU 37.

[0034] The communication unit 38 is an interface that controls the transmission of various information with the supply unit 10 and the like. In addition to these, the computer that constitutes the control device 16 is equipped with input devices such as a display for displaying various screens, a keyboard, a mouse, a touch panel, and a microphone for voice input that receive input of various operation instructions through various screens.

[0035] As shown in FIG. 7 as an example, an operation program 42 is stored in the storage 35. When the operation program 42 is activated, the CPU 37 functions as a supply unit control unit 45, a first buffer unit control unit 46, an image forming unit control unit 47, a drying unit control unit 48, a second buffer unit control unit 49, and a recovery unit control unit 50 in cooperation with the memory 36 and the like.

[0036] The supply unit control unit 45 rotates the motor connected to the supply roll 20 of the supply unit 10 at a specified rotational speed. The first buffer unit control unit 46 moves the dancer roller of the first buffer unit 11 up and down to set the tension and transport amount of the recording medium RM toward the image forming unit 12 to specified values. The image forming unit control unit 47 rotates the transport roller 21 of the image forming unit 12 at a specified rotational speed. The image forming unit control unit 47 also drives the inkjet head 22 of the image forming unit 12 to form an image on the recording medium RM. Furthermore, the image forming unit control unit 47 drives the ink mist recovery device 23 of the image forming unit 12 to recover the ink mist M.

[0037] The drying unit control unit 48 drives the dryer of the drying unit 13 under specified drying conditions. The second buffer unit control unit 49 moves the dancer roller of the second buffer unit 14 up and down to set the tension and transport amount of the recording medium RM toward the recovery unit 15 to specified values. The recovery unit control unit 50 rotates the motor connected to the recovery roll 24 of the recovery unit 15 at a specified rotational speed.

[0038] As an example, as shown in Figure 8, an airflow AC is generated between the inkjet head 22 and the recording medium RM. This airflow AC consists of an airflow ACC (see Figure 14) caused by the suction of the ink mist recovery device 23 and an airflow ACT (see Figure 12) caused by the transport of the recording medium RM.

[0039] The ink droplets DP, affected by the airflow AC, do not fall straight vertically, but curve downstream in the transport direction CD. As a result, the landing position is shifted downstream in the transport direction CD compared to the original landing position shown by the dashed line if the droplets fell straight vertically. As shown in Figures 3 and 4, there are two types of ink droplets DP: large droplets LDP and small droplets SDP, which have different diameters. Since these large droplets LDP and small droplets SDP have different masses, they also curve differently when falling. That is, the small droplets SDP, being lighter in mass than the large droplets LDP, are shifted further downstream in the transport direction CD. The symbol Ll represents the amount of deviation from the original landing position of the large droplet LDP due to the airflow AC (unit: μm), and the symbol Ls represents the amount of deviation from the original landing position of the small droplet SDP due to the airflow AC (unit: μm). Furthermore, the symbol dL represents the difference in the impact positions of the large droplet LDP and the small droplet SDP (difference dL = Ls - Ll).

[0040] As an example, as shown in Figure 9(A), in an ideal state unaffected by airflow AC, large LDPs and small SDPs land in the same row. However, in reality, they are affected by airflow AC, so as an example, as shown in Figure 9(B), large LDPs and small SDPs land shifted in the transport direction CD. In particular, if large LDPs and small SDPs land in completely separate rows as in Figure 9(B), the shifted marks appear as white streaks, causing a decrease in image quality. Note that the symbol R is the distance between adjacent ink droplets DP in the width direction WD, and is the so-called pixel pitch.

[0041] In the technology disclosed herein, in order to reduce the amount of displacement dL of the landing positions of the large droplet LDP and small droplet SDP in the transport direction CD, the airflow ACC, which is one of the airflow ACC caused by the suction of the ink mist recovery device 23 and the airflow ACT caused by the transport of the recording medium RM, is suppressed. However, when the suction force of the ink mist recovery device 23 is weakened in order to suppress the airflow ACC, insufficient recovery of the ink mist M is avoided.

[0042] Therefore, as an example shown in Figure 10, the image forming unit control unit 47 sets the suction amount Q of the ink mist recovery device 23 as follows, and drives the ink mist recovery device 23 with the set suction amount Q. That is, the suction amount Q is a value that allows the difference in the landing position dL of the large droplet LDP and the small droplet SDP in the transport direction CD to be within the set range Lmax (unit: μm) (dL ≤ Lmax), and that allows the recovery of an amount of ink mist M greater than or equal to the set amount. More specifically, the suction amount Q is a value that allows the velocity Vb (unit: m / min, see Figure 14) of the airflow ACC at the lower surface of the nozzle 30 to be less than or equal to the first threshold TH1 based on the set range Lmax, and greater than or equal to the second threshold TH2 based on the set amount (TH2 ≤ Vb ≤ TH1).

[0043] Below, we will determine the first threshold TH1 and the second threshold TH2. First, as an example, we consider a basic model to capture the phenomenon in which an ink droplet DP falls while curving downstream in the transport direction CD, as shown in Figure 11. Here, U is the velocity of the airflow (unit: m / min), and r is the radius of the ink droplet DP (unit: μm). Also, μ is the viscosity coefficient of the ink droplet DP (unit: kg / m·s), and ρ is the density of the ink droplet DP (unit: kg / m ３ )

[0044] The force F exerted on the ink droplet DP by the airflow is given by the following equation (A) according to Stokes' law of resistance: F = 6πμrU (A) The mass m of the ink droplet DP is given by the following equation (B): m = 4πr ３ ρ / 3 (B) The amount of deviation L (in μm) of the ink droplet DP from its original landing position due to the airflow AC is expressed by the following equation (C), where t (in s) is the flight time of the ink droplet DP: L = Ft ２ / m (C) Substituting equations (A) and (B) into equation (C), the displacement L is expressed by the following equation (D).

[0045] As an example, as shown in Figure 12, the airflow ACT caused by the transport of the recording medium RM is assumed to be a wet flow. Therefore, the flow velocity Ut of the airflow ACT is expressed by the following equation (E), where Vp (in m / min) is the transport speed of the recording medium RM: Ut = Vp / 2 (E)

[0046] As an example, as shown in Figure 13, the flight time t of the ink droplet DP is given by the following equation (F), where the average velocity of the ink droplet DP is v (unit: m / s), the distance from the nozzle 30 to the recording medium RM is Td (unit: mm), and the amount of deviation Ldt from the original landing position of the ink droplet DP due to the airflow ACT is very small. t = Td / v (F) Substituting equations (E) and (F) into equation (D), the amount of deviation Ldt (unit: μm) from the original landing position of the ink droplet DP due to the airflow ACT is given by the following equation (G). More precisely, the distance Td from the nozzle 30 to the recording medium RM is the shortest distance from the ejection port of the ink droplet DP of the nozzle 30 to the image forming surface of the recording medium RM.

[0047] As an example, as shown in Figure 14, the airflow ACC caused by the suction of the ink mist recovery device 23 is assumed to be a uniform flow when the distance Td from the nozzle 30 to the recording medium RM is very short. For this reason, as an example, as shown in Figure 15, the amount of deviation Ldv (in μm) of the ink droplet DP from its original landing position due to the airflow ACC can be expressed by substituting equation (F) into equation (D) and replacing U with the velocity Vb of the airflow ACC, resulting in the following equation (H).

[0048] Therefore, the total displacement Ld (in μm) of the ink droplet DP from its original landing position due to the airflow ACC caused by the suction of the ink mist recovery device 23 and the airflow ACT caused by the transport of the recording medium RM is expressed by the following equation (I) by adding equations (G) and (H) (Ldt + Ldv).

[0049] Here, the velocity Vb of the airflow ACC is expressed by the following equation (J), where Tdlb is the cross-sectional area of ​​the open area on the lower surface of the inkjet head 22 and S is the cross-sectional area of ​​the other open areas: Vb = Q / (Tdlb + S) (J)

[0050] In Figures 14 and 15, the symbol Qin represents the intake volume of the ink mist recovery device 23, and the symbol Qout represents the exhaust volume of the ink mist recovery device 23. More precisely, the intake volume Qin is the total intake volume of the image forming unit 12, including the ink mist recovery device 23, and the exhaust volume Qout is the total exhaust volume of the image forming unit 12, including the ink mist recovery device 23. The suction volume Q of the ink mist recovery device 23 is expressed by the following equation (K) using the intake volume Qin and the exhaust volume Qout: Q = Qout - Qin (K) Note that the suction volume Q, intake volume Qin, and exhaust volume Qout are the volume of air drawn in per unit time, the volume of air drawn in per unit time, and the volume of air exhausted per unit time, respectively.

[0051] First, we determine the second threshold TH2. As an example, as shown in Figure 16, we consider the conditions under which ink mist M with radius rmis (unit: μm) and average velocity vmis (unit: m / s) does not adhere to the recording medium RM. Taking into account the fluttering of the recording medium RM toward the inkjet head 22, the conditions under which ink mist M does not adhere to the recording medium RM are given by the following equation (L), where the flight distance of ink mist M toward the recording medium RM is Td / 2, and the flight time t of ink mist M is expressed by the following equation (L): t = Td / 2vmis (L) Also, the average flow velocity U of the airflow AC applied to the ink mist M during its flight distance Td / 2 is expressed by the following equation (M). U = (Vp / 4) + Vb (M) Therefore, the amount of ink mist M moved by the airflow AC, Lmis (unit: μm), can be expressed by substituting equations (L) and (M) into equation (D) as follows: (N).

[0052] The relationship between the radius rmis and the average velocity vmis of the ink mist M, which enables the ink mist recovery device 23 to recover an amount of ink mist M greater than the set amount, is defined as follows in equation (5): rmis・vmis ≤ 3.63e－７ ...(5) This equation (5) is derived through experiments or simulations.

[0053] Assume that, as shown by the dashed line in Fig. 16, the ink mist M flies in a parabolic path. In this case, if the movement amount Lmis of the ink mist M by the air current AC is half or more of the distance Lg (unit: mm) from the nozzle 30 to the suction port 32, that is, Lmis ≥ Lg / 2, it is possible to recover the ink mist M with the ink mist recovery device 23. Therefore, substituting equation (5) into equation (N) and applying the condition of Lmis ≥ Lg / 2 and arranging it with the velocity Vb of the air current ACC, it becomes the following equation (O). The second threshold value TH2 is the right side of equation (O). That is, the second threshold value TH2 is represented by the following equation (3). Note that the distance Lg from the nozzle 30 to the suction port 32 is, strictly speaking, the distance from the center of the nozzle 30 to the center of the suction port 32. When there are multiple rows of nozzles 30 and suction ports 32, the distance Lg is the distance from the center of the nozzle 30 provided on the most upstream side in the conveyance direction CD to the center of the suction port 32 provided on the most downstream side in the conveyance direction CD.

[0054] Next, the first threshold value TH1 is obtained. The deviation amount Ll of the large droplet LDP from its original landing position due to the air current AC is represented by the following equation (P). Note that the symbol vl is the average velocity (unit: m / s) of the large droplet LDP. Similarly, the deviation amount Ls of the small droplet SDP from its original landing position due to the air current AC is represented by the following equation (Q). Note that the symbol vs is the average velocity (unit: m / s) of the small droplet SDP. As described above, in order to suppress the deterioration of the image quality caused by the deviation of the large droplet LDP and the small droplet SDP from their original landing positions due to the air current AC, it is necessary that the deviation amount dL of the landing positions of the large droplet LDP and the small droplet SDP, which is the difference between the deviation amounts Ll and Ls, is within the set range Lmax. Therefore, substituting equations (P) and (Q) into dL = Ls - Ll and applying the condition of dL ≤ Lmax and arranging it with the velocity Vb of the air current ACC, it becomes the following equation (R). The first threshold value TH1 is the right side of equation (R). That is, the first threshold value TH1 is represented by the following equation (1). However, in equations (R) and (1), A is given by equation (2).

[0055] Let's consider the setting range Lmax further. For the misalignment of the large droplet (LDP) and the small droplet (SDP) to not appear as white streaks, as shown in Figure 17 as an example, the large droplet (LDP) and the small droplet (SDP) must be in contact without any gaps. In this case, the amount of misalignment dL between the landing positions of the large droplet (LDP) and the small droplet (SDP) is the opposite side of the right-angled triangle RT shown by the dashed line. The adjacent side of this right-angled triangle is the distance R between adjacent ink droplets (DP) in the width direction WD, and the hypotenuse is the radius rl of the large droplet (LDP) and the radius rs of the small droplet (SDP). Therefore, the setting range Lmax is expressed by the following equation (4): Lmax = {(rs + rl)} ２ -R ２} １／２ (4) Therefore, equation (R) ultimately becomes equation (S). In other words, the first threshold TH1 is expressed by the following equation (T).

[0056] The setting range Lmax is half the print resolution. For example, if the print resolution is 600 dpi (= 42.3 μm / pixel), the setting range Lmax is approximately 21 μm. If the print resolution is 1200 dpi (= 21.1 μm / pixel), the setting range Lmax is approximately 10 μm.

[0057] Next, the operation of the above configuration will be explained. Under the control of the supply unit control 45, the recording medium RM is supplied from the supply roll 20 to the first buffer unit 11. In the first buffer unit 11, the dancer roller is moved up and down under the control of the first buffer unit control 46, thereby adjusting the tension and transport amount of the recording medium RM. The recording medium RM is sent from the first buffer unit 11 to the image forming unit 12.

[0058] In the image forming unit 12, under the control of the image forming unit control unit 47, ink droplets DP are ejected from the nozzle 30 of the inkjet head 22 toward the recording medium RM being transported by the transport roller 21, and an image is formed on the recording medium RM. Also, as shown in Figure 10, the image forming unit control unit 47 sets the suction amount Q for the ink mist recovery device 23, and the ink mist M is sucked up by the ink mist recovery device 23. The suction amount Q is set to a value that allows the displacement dL of the landing positions of large droplets LDP and small droplets SDP on the recording medium RM, caused by the airflow ACC due to the suction of the ink mist recovery device 23 and the airflow ACT due to the transport of the recording medium RM, to be within the set range Lmax, and that allows the recovery of an amount of ink mist M equal to or greater than the set amount. Furthermore, the suction amount Q is set to a value that allows the velocity Vb of the airflow ACC at the lower surface of the nozzle 30 to be less than or equal to the first threshold TH1 based on the set range Lmax, and greater than or equal to the second threshold TH2 based on the set amount. The image-formed recording medium RM is sent from the image forming unit 12 to the drying unit 13.

[0059] In the drying section 13, the image-formed recording medium RM is dried by a dryer under the control of the drying section control unit 48. The recording medium RM is sent from the drying section 13 to the second buffer section 14. In the second buffer section 14, the dancer roller is moved up and down under the control of the second buffer section control unit 49, thereby adjusting the tension and transport amount of the recording medium RM. The recording medium RM is sent from the second buffer section 14 to the recovery section 15. In the recovery section 15, the recording medium RM is recovered onto the recovery roll 24 under the control of the recovery section control unit 50.

[0060] As explained above, the suction amount Q of the ink mist recovery device 23 is set to a value that allows the amount dL of the displacement of the landing positions of large droplets LDP and small droplets SDP on the recording medium RM, caused by the airflow ACC due to the suction of the ink mist recovery device 23 and the airflow ACT due to the transport of the recording medium RM, to be within the set range Lmax, and that allows the recovery of an amount of ink mist M greater than or equal to the set amount. Therefore, it is possible to avoid insufficient recovery of ink mist M while suppressing the deterioration of image quality due to the displacement of the landing positions of large droplets LDP and small droplets SDP.

[0061] As shown in Figure 10, the suction amount Q is set to a value such that the velocity Vb of the airflow ACC caused by the suction of the ink mist recovery device 23 at the lower surface of the nozzle 30 is less than or equal to the first threshold TH1 based on the set range Lmax, and greater than or equal to the second threshold TH2 based on the set amount. Therefore, it is possible to avoid insufficient recovery of ink mist M while suppressing the deterioration of image quality due to the difference in the landing positions of large droplets LDP and small droplets SDP.

[0062] The first threshold TH1 is expressed by equation (1), and the second threshold TH2 is expressed by equation (3). Therefore, the first threshold TH1 and the second threshold TH2 can be calculated.

[0063] The setting range Lmax is related to equation (4). Therefore, it is possible to suppress the deterioration of image quality due to the difference in the impact positions of large droplet LDP and small droplet SDP.

[0064] The set amount is related to equation (5). This makes it possible to avoid insufficient recovery of ink mist M.

[0065] The following describes an example. In this example, the inkjet printer 2 is capable of transporting the recording medium RM at six different transport speeds Vp: 50 m / min, 100 m / min, 150 m / min, 200 m / min, 250 m / min, and 300 m / min.

[0066] (Example 1) The values ​​of each parameter in Example 1 are shown in Table 60 of Figure 18. As an example, Graph 62 shown in Figure 19 shows the first threshold TH1 and second threshold TH2 for six different transport speeds Vp when the values ​​in Table 60 shown in Figure 18 are applied to equations (1), (equation (T)), and (3). According to Graph 62, it can be seen that the final first threshold TH1 should be set to the minimum value (≒2.2) when the transport speed Vp is the maximum of 300 m / min. It can also be seen that the final second threshold TH2 should be set to the maximum value (≒1.5) when the transport speed Vp is the minimum of 50 m / min. In other words, the velocity Vb of the airflow ACC caused by the suction of the ink mist recovery device 23 should be between 1.5 m / s and 2.2 m / s (1.5 ≤ Vb ≤ 2.2).

[0067] Here, as an example shown in Figure 20, the cross-sectional areas S of the other open parts in equation (J) are specifically the cross-sectional areas S1, S2, and S3. Cross-sectional area S1 is the cross-sectional area of ​​the space 65 between the inkjet head 22U, which is located upstream of the inkjet head 22U in the transport direction CD, and the inkjet head 23. Cross-sectional area S2 is the cross-sectional area of ​​the space 66 between the inkjet head 22D, which is located downstream of the inkjet head 23 in the transport direction CD, and the inkjet head 23. Cross-sectional area S3 is the cross-sectional area of ​​the space 67 between the inkjet head 23 and the recording medium RM. The cross-sectional area S2 is expressed by the following equation (U), where Ts is the distance from the suction port 32 to the recording medium RM. S2 = Tslb (U) Therefore, equation (J) becomes the following equation (V). Vb = Q / (Tdlb + Tslb + S1 + 2S3) (V) From equations (K) and (V), the exhaust volume Qout required to satisfy 1.5 ≤ Vb ≤ 2.2 must satisfy the following inequality (W): 1.5(Tdlb + Tslb + S1) + Qin ≤ Qout ≤ 2.2(Tdlb + Tslb + S1) + Qin (W) More precisely, the distance Ts from the suction port 32 to the recording medium RM is the shortest distance from the suction port 32 to the image forming surface of the recording medium RM.

[0068] (Example 2) In Example 2, the inkjet printer 2 forms an image on the recording medium RM in either print mode A or print mode B. Print mode A has a maximum transport speed of 150 m / min, and print mode B has a maximum transport speed of 300 m / min (see Figure 21). Because print mode A has a slow transport speed, it is possible to increase the amount of large droplet LDP ejected and increase the size difference between large droplet LDP and small droplet SDP, resulting in high image quality. In other words, print mode A can be said to be a print mode that prioritizes image quality. On the other hand, print mode B can be said to be a print mode that prioritizes the speed of image formation over image quality.

[0069] The values ​​of each parameter in Example 2 are shown in Table 70 of Figure 21. As mentioned above, the mass and radius rl of the large droplet LDP in printing mode A are greater than the mass and radius rl of the large droplet LDP in printing mode B.

[0070] As an example, Graph 72 shown in Figure 22 shows the first threshold TH1 and second threshold TH2 for six different transport speeds Vp when the values ​​from Table 70 shown in Figure 21 are applied to equations (1), (equation (T)), and (3). According to Graph 72, in print mode A, the final first threshold TH1A should be set to the minimum value (≒3.1) when the transport speed Vp is the maximum of 150 m / min. In contrast, in print mode B, the final first threshold TH1B should be set to the minimum value (≒2.2) when the transport speed Vp is the maximum of 300 m / min. Furthermore, in both print mode A and print mode B, the final second threshold TH2 should be set to the maximum value (≒1.5) when the transport speed Vp is the minimum of 50 m / min. In other words, in printing mode A, the velocity Vb of the airflow ACC caused by the suction of the ink mist recovery device 23 should be between 1.5 m / s and 3.1 m / s (1.5 ≤ Vb ≤ 3.1). In contrast, in printing mode B, the velocity Vb of the airflow ACC caused by the suction of the ink mist recovery device 23 should be between 1.5 m / s and 2.2 m / s (1.5 ≤ Vb ≤ 2.2). These can be summarized as shown in Table 75 in Figure 23.

[0071] Thus, the ink mist recovery device 23 may change the speed Vb of the airflow ACC caused by suction, and consequently the suction amount Q, according to the transport speed Vp of the recording medium RM. In this way, the ink mist recovery device 23 can be set to an appropriate suction amount Q for each of the multiple printing modes with different transport speeds. Even if the transport speed Vp of the recording medium RM changes, it is possible to avoid insufficient recovery of ink mist M while suppressing the deterioration of image quality due to the misalignment of the landing positions of large droplets LDP and small droplets SDP. As a means of changing the suction amount Q of the ink mist recovery device 23, a pressure adjustment mechanism such as a regulator can be mentioned.

[0072] Note that TH1B may be used as the first threshold for both print mode A and print mode B. However, in that case, print mode A, which can be set to TH1A, which is higher than TH1B, will suffer a loss. For this reason, it is preferable to change the first threshold for print mode A and print mode B as described above.

[0073] (Example 3) In Example 3, the inkjet printer 2 forms an image on the recording medium RM in either print mode A or print mode B, as in Example 2. In Example 3, the inkjet printer 2 handles two types of recording medium RM with thicknesses of 0.1 mm or 0.3 mm. A thickness of 0.1 mm is, for example, the thickness of general copy paper. A thickness of 0.3 mm is, for example, the thickness of certificates, official postcards, etc.

[0074] The values ​​of each parameter for Example 3 are shown in Table 80 of Figure 24. As mentioned above, there are two types of recording media RM: one with a thickness of 0.1 mm and another with a thickness of 0.3 mm. In the case of a recording media RM with a thickness of 0.1 mm, the distance Td from the nozzle 30 to the recording media RM is 1.2 mm. In contrast, in the case of a recording media RM with a thickness of 0.3 mm, the distance Td from the nozzle 30 to the recording media RM is 1.0 mm. As the thickness of the recording media RM decreases, the distance Td from the nozzle 30 to the recording media RM increases, and therefore the amount of deviation dL between the landing positions of the large droplet LDP and the small droplet SDP increases. Consequently, when the thickness of the recording media RM is thin, the first threshold value TH1 becomes a stricter value.

[0075] As an example, Graph 82 shown in Figure 25 shows the first threshold TH1 and second threshold TH2 for six different transport speeds Vp when the values ​​from Table 80 shown in Figure 24 are applied to equations (1) (equation (T)) and (3). According to Graph 82, when handling a recording medium RM with a thickness of 0.1 mm in print mode A, the final first threshold TH1AX should be set to the minimum value (≒1.7) when the transport speed Vp is the maximum of 150 m / min. Also, when handling a recording medium RM with a thickness of 0.3 mm in print mode A, the final first threshold TH1AY should be set to the minimum value (≒3.0) when the transport speed Vp is the maximum of 150 m / min.

[0076] In contrast, when handling a recording medium RM with a thickness of 0.1 mm in print mode B, it can be seen that the final first threshold TH1BX should be set to the minimum value (≒1.1) when the transport speed Vp is at its maximum of 300 m / min. Similarly, when handling a recording medium RM with a thickness of 0.3 mm in print mode B, it can be seen that the final first threshold TH1BY should be set to the minimum value (≒2.7) when the transport speed Vp is at its maximum of 300 m / min.

[0077] Furthermore, in both printing mode A and printing mode B, when handling a recording medium RM with a thickness of 0.1 mm, it can be seen that the final second threshold TH2X should be set to the maximum value (≒1.0) when the transport speed Vp is the minimum of 50 m / min. Also, in both printing mode A and printing mode B, when handling a recording medium RM with a thickness of 0.3 mm, it can be seen that the final second threshold TH2Y should be set to the maximum value (≒1.5) when the transport speed Vp is the minimum of 50 m / min.

[0078] In other words, when handling a recording medium RM with a thickness of 0.1 mm in printing mode A, the velocity Vb of the airflow ACC caused by the suction of the ink mist recovery device 23 should be between 1.0 m / s and 1.7 m / s (1.0 ≤ Vb ≤ 1.7). Also, when handling a recording medium RM with a thickness of 0.3 mm in printing mode A, the velocity Vb of the airflow ACC caused by the suction of the ink mist recovery device 23 should be between 1.5 m / s and 3.0 m / s (1.5 ≤ Vb ≤ 3.0). In contrast, when handling a recording medium RM with a thickness of 0.1 mm in printing mode B, the velocity Vb of the airflow ACC caused by the suction of the ink mist recovery device 23 should be between 1.0 m / s and 1.1 m / s (1.0 ≤ Vb ≤ 1.1). Furthermore, when handling a recording medium RM with a thickness of 0.3 mm in print mode B, the velocity Vb of the airflow ACC caused by the suction of the ink mist recovery device 23 should be between 1.5 m / s and 2.7 m / s (1.5 ≤ Vb ≤ 2.7). This can be summarized as shown in Table 85 in Figure 26.

[0079] Thus, the ink mist recovery device 23 may change the velocity Vb of the airflow ACC caused by suction, and consequently the suction amount Q, according to the distance Td (thickness of the recording medium RM) from the nozzle 30 to the recording medium RM. In this way, the ink mist recovery device 23 can be set to an appropriate suction amount Q for each of several types of recording mediums RM with different thicknesses. Even if the thickness of the recording medium RM changes, it is possible to avoid insufficient recovery of ink mist M while suppressing the deterioration of image quality due to the difference in the landing positions of large droplets LDP and small droplets SDP.

[0080] While two types of ink droplets (DP) with different diameters, LDP (large droplet) and SDP (small droplet), have been given as examples, the system is not limited to these. There may be three types, such as large, medium, and small droplets, or four or more types.

[0081] The printing modes shown in Example 2 are not limited to the two examples provided. There may be three or more printing modes. Also, the thickness of the recording medium RM shown in Example 3 is not limited to the two examples provided. Recording medium RM of three or more different thicknesses may be used.

[0082] The setting range Lmax is {(rs + rl)} in example equation (4) ２ -R ２} １／２ It's not limited to that. If you can tolerate some white streaks, then {(rs+rl) ２ -R ２} １／２ You may set a looser value. Conversely, if you want to further reduce the difference in the impact locations of large LDP and small SDP droplets, you can use {(rs + rl) ２ -R ２} １／２ You may set a stricter value. Similarly, the right-hand side of equation (5) relating to the setting amount is 3.63e as shown in the example. －７ Not limited to that. 3.63e －７ You can set it to a more lenient value or a stricter value.

[0083] The recording medium RM is not limited to printing paper; it may also be a resin film. Furthermore, the inkjet printer is not limited to the full-color type shown in the example; it may also be a monochrome type.

[0084] In the embodiments described above, a so-called continuous-feed inkjet printer 2 has been used as an example, but the invention is not limited to this. The technology of this disclosure can also be applied to sheet-fed inkjet printers.

[0085] In each of the above embodiments, the processing of each processing unit, such as the supply unit control unit 45, the first buffer unit control unit 46, the image forming unit control unit 47, the drying unit control unit 48, the second buffer unit control unit 49, and the recovery unit control unit 50, is executed on any computer. Furthermore, any computer may execute these processes using a processor as hardware, a program as software, or a combination thereof. In this case, the processor is configured to work in cooperation with the program to execute the various processes in each of the above embodiments, and can function as each unit or means in each of the above embodiments. Also, the execution order of the processes by the processor is not limited to the order described and may be changed as appropriate. Any computer may be a general-purpose computer, a computer designed for a specific application, a workstation, or any other system capable of executing each of the processes.

[0086] A processor may consist of one or more hardware components, and the type of hardware is not limited. For example, a processor may consist of the example CPU 37, or programmable logic devices such as an MPU (Micro Processing Unit), FPGA (Field Programmable Gate Array), dedicated circuits for executing specific processes such as an ASIC (Application Specific Integrated Circuit), a GPU (Graphic Processing Unit), or an NPU (Neural Processing Unit). Furthermore, the type of hardware may be a combination of different types of hardware. When multiple hardware components are configured to execute one or more processes of a processor, these multiple hardware components may reside in physically separate devices or in the same device. Furthermore, in any embodiment, the order of each process performed by the processor is not limited to the order described above and may be changed as appropriate. The hardware is composed of an electrical circuit (circuitry) or the like, which is a combination of circuit elements such as semiconductor elements.

[0087] Furthermore, the program may be firmware or software such as microcode. Alternatively, the program may be, for example, a set of program modules, each function of which may be implemented by a processor configured to perform its respective function. The program may be program code or multiple code segments stored on one or more non-temporary computer-readable media (e.g., storage media or other storage devices). The program may be divided and stored on multiple non-temporary computer-readable media located in physically separate devices. Program code or code segments may represent any combination of procedures, functions, subprograms, routines, subroutines, modules, software packages, classes, or instructions, data structures, or program statements. Program code or code segments may be connected to other code segments or hardware circuits by sending and receiving information, data, arguments, parameters, or memory contents.

[0088] From the above description, the technology described in the following supplementary information can be understood.

[0089] [Note 1] An inkjet printer comprising: a transport mechanism for transporting a recording medium; an inkjet head for recording an image on the recording medium by ejecting ink droplets toward the recording medium from a plurality of nozzles arranged in a direction intersecting the transport direction of the recording medium, wherein the nozzles are capable of ejecting a plurality of types of ink droplets with different diameters; and an ink mist recovery device positioned downstream of the inkjet head in the transport direction of the recording medium, which recovers ink mist generated from the ink droplets by suction, wherein the suction amount of the ink mist recovery device is set to a value such that the amount of displacement of the landing positions of the plurality of types of ink droplets on the recording medium due to the airflow caused by the suction and the airflow caused by the transport falls within a set range, and that an amount of ink mist greater than or equal to the set amount can be recovered. [Note 2] The inkjet printer according to Note 1, wherein the suction amount is set to a value such that the velocity of the airflow caused by the suction at the bottom surface of the nozzle is less than or equal to a first threshold TH1 based on the set range, and greater than or equal to a second threshold TH2 based on the set amount. [Note 3] The inkjet printer described in Note 2, wherein the first threshold TH1 is represented by the following formula (1) and the second threshold TH2 is represented by the following formula (3). however, Lmax is the setting range, Vp is the transport speed of the recording medium, μ is the viscosity coefficient of the ink droplet, Td is the distance from the nozzle to the recording medium, ρ is the density of the ink droplet, rs is the radius of the ink droplet with the smallest diameter, rl is the radius of the ink droplet with the largest diameter, vs is the average velocity of the ink droplet with the smallest diameter, vl is the average velocity of the ink droplet with the largest diameter, and Lg is the distance from the nozzle to the ink mist suction port of the ink mist recovery device. [Note 4] The setting range is the inkjet printer described in Note 3 relating to the following formula (4). {(rs + rl) ２ -R ２} １／２... (4) R is the distance between two adjacent ink droplets in a direction intersecting the transport direction of the recording medium. [Note 5] The setting amount is the inkjet printer described in Note 3 or Note 4 relating to the following formula (5). rmis・vmis ≤ 3.63e －７ ... (5) rmis is the radius of the ink mist, and vmis is the average velocity of the ink mist. [Note 6] The ink mist recovery device is an inkjet printer according to any one of Note 1 to Note 5, wherein the amount of suction is changed according to the transport speed of the recording medium. [Note 7] The ink mist recovery device is an inkjet printer according to any one of Note 1 to Note 6, wherein the amount of suction is changed according to the distance from the nozzle to the recording medium.

[0090] The technology of this disclosure can be appropriately combined with the various embodiments and / or variations described above. Furthermore, it is understood that various configurations can be adopted without departing from the spirit of the invention, and the invention is not limited to the embodiments described above. In addition, the technology of this disclosure extends not only to programs, but also to storage media for non-temporarily storing programs, and to computer program products containing programs.

[0091] The descriptions and illustrations presented above are detailed explanations of the technical aspects of this disclosure and are merely examples of the technical aspects. For example, the above descriptions of the structure, function, operation, and effect are examples of the structure, function, operation, and effect of the technical aspects of this disclosure. Therefore, it goes without saying that you may delete unnecessary parts, add new elements, or replace elements in the descriptions and illustrations presented above, as long as you do not deviate from the essence of the technical aspects of this disclosure. Furthermore, in order to avoid confusion and facilitate understanding of the technical aspects of this disclosure, explanations of common technical knowledge and the like that do not require special explanation to enable the implementation of the technical aspects of this disclosure have been omitted from the descriptions and illustrations presented above.

[0092] In this specification, "A and / or B" is synonymous with "at least one of A and B." That is, "A and / or B" means that it may be A alone, or B alone, or a combination of A and B. Furthermore, in this specification, the same concept as "A and / or B" applies when expressing three or more things linked by "and / or."

[0093] All documents, patent applications, and technical standards described herein are incorporated by reference to the same extent as if each individual document, patent application, and technical standard were specifically and individually noted to be incorporated by reference.

Claims

1. An inkjet printer comprising: a transport mechanism for transporting a recording medium; an inkjet head for recording an image on the recording medium by ejecting ink droplets toward the recording medium from a plurality of nozzles arranged in a direction intersecting the transport direction of the recording medium, wherein the nozzles are capable of ejecting multiple types of ink droplets with different diameters; and an ink mist recovery device positioned downstream of the inkjet head in the transport direction of the recording medium, which recovers ink mist generated from the ink droplets by suction, wherein the suction amount of the ink mist recovery device is set to a value such that the amount of displacement of the landing positions of the multiple types of ink droplets on the recording medium due to the airflow caused by the suction and the airflow caused by the transport falls within a set range, and that an amount of ink mist greater than or equal to the set amount can be recovered.

2. The inkjet printer according to claim 1, wherein the suction amount is set to a value such that the velocity of the airflow caused by the suction at the lower surface of the nozzle is less than or equal to a first threshold TH1 based on the set range and greater than or equal to a second threshold TH2 based on the set amount.

3. The inkjet printer according to claim 2, wherein the first threshold TH1 is represented by the following formula (1), and the second threshold TH2 is represented by the following formula (3). however, Lmax (unit: μm) is the setting range, Vp (unit: m / min) is the transport speed of the recording medium, μ (unit: kg / m·s) is the viscosity coefficient of the ink droplet, Td (unit: mm) is the distance from the nozzle to the recording medium, ρ (unit: kg / m ３ ) is the density of the ink droplet, rs (unit: μm) is the radius of the smallest diameter ink droplet, rl (unit: μm) is the radius of the largest diameter ink droplet, vs (unit: m / s) is the average velocity of the smallest diameter ink droplet, vl (unit: m / s) is the average velocity of the largest diameter ink droplet, and Lg (unit: mm) is the distance from the nozzle to the ink mist suction port of the ink mist recovery device.

4. The setting range is the inkjet printer according to claim 3, relating to the following formula (4): {(rs + rl) ２ -R ２ } １／２ ... (4) R is the distance between two adjacent ink droplets in a direction intersecting the transport direction of the recording medium.

5. The setting amount is the inkjet printer according to claim 3, relating to the following formula (5): rmis・vmis ≤ 3.63e －７ ... (5) rmis (unit: μm) is the radius of the ink mist, and vmis (unit: m / s) is the average velocity of the ink mist.

6. The setting amount is the inkjet printer according to claim 4, relating to the following formula (5). rmis・vmis≦3.63e －７ ... (5) rmis (unit: μm) is the radius of the ink mist, and vmis (unit: m / s) is the average velocity of the ink mist.

7. The inkjet printer according to any one of claims 1 to 6, wherein the ink mist recovery device changes the amount of suction according to the transport speed of the recording medium.

8. The inkjet printer according to any one of claims 1 to 6, wherein the ink mist recovery device changes the amount of suction according to the distance from the nozzle to the recording medium.

9. An image forming method using an inkjet printer comprising: a transport mechanism for transporting a recording medium; an inkjet head for recording an image on the recording medium by ejecting ink droplets toward the recording medium from a plurality of nozzles arranged in a direction intersecting the transport direction of the recording medium, wherein the nozzles are capable of ejecting a plurality of types of ink droplets with different diameters; and an ink mist recovery device positioned downstream of the inkjet head in the transport direction of the recording medium, which recovers ink mist generated from the ink droplets by suction, wherein the suction amount of the ink mist recovery device is set to a value such that the amount of displacement of the landing positions of the plurality of types of ink droplets on the recording medium due to the airflow caused by the suction and the airflow caused by the transport falls within a set range, and that an amount greater than or equal to the set amount of ink mist can be recovered.

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