Inkjet printing apparatus and printing method

Abstract

Regardless of transport speed, substrate type, etc., it suppresses bleeding between the first and second inks. [Solution] A first head 41a for ejecting the first ink and a second head 41b for ejecting the second ink are arranged in order from the upstream side of the transport path of the substrate 9. The surface tension difference adjustment unit 830 performs an ink characteristic changing process, such as changing the dynamic surface tension characteristics of at least one of the first ink and the second ink, as an adjustment process. The time when the first ink droplet, which is a droplet of the first ink, lands at the target position on the substrate 9 is defined as the first time, and the time when the second ink droplet, which is a droplet of the second ink, lands at the target position is defined as the second time. The absolute value of the difference between the dynamic surface tension of the first ink droplet at the second time and the dynamic surface tension of the second ink droplet at the second time is defined as the surface tension difference between the first and second inks. The surface tension difference between the first and second inks when the adjustment process is performed is smaller than the surface tension difference between the first and second inks when the adjustment process is not performed.

Description

Technical Field

[0001] The present invention relates to inkjet printing.

Background Art

[0002] Conventionally, inkjet printing apparatuses have been used. For example, in the apparatus of Patent Document 1, a first ink ejection unit and a second ink ejection unit are provided on the upstream side and the downstream side in the conveyance direction of a substrate, respectively. The first ink ejection unit ejects a first ink whose surface tension of droplets adhering to the substrate decreases with time. Further, the second ink ejection unit ejects a second ink whose surface tension of droplets when adhering to the substrate is approximately equal to the surface tension of the droplets of the first ink adhering to a position adjacent to the droplets. Thereby, the surface tensions of the droplets of the first ink and the second ink adhering to adjacent positions are approximately equalized, and bleeding and fluctuation of the boundary position at the boundary between the region of the first ink and the region of the second ink on the substrate are suppressed.

[0003] Also, in the ink set of Patent Document 2, by satisfying the relationship of -5 ≦ (γa - γb) ≦ 3 between the dynamic surface tension γa (mN / m) of ink A having a relatively high lightness at a lifetime of 500 msec and the dynamic surface tension γb (mN / m) of ink B having a relatively low lightness at a lifetime of 500 msec, bleeding is suppressed regardless of the type of the substrate. In this ink set, it is considered that ink A and ink B are ejected simultaneously. In Patent Documents 3 to 5, it is described that the flight speed of ink droplets changes by changing the shape and size of a drive waveform input to an inkjet head. Patent Document 6 describes an air turn bar that supports a printing medium without contact.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

[0005] Incidentally, in the apparatus described in Patent Document 1, the dynamic surface tension of the first and second inks is optimized at a specific transport speed of the substrate. Therefore, if the transport speed is changed, bleeding between the first and second inks may increase, potentially degrading the quality of the printed image. Furthermore, changing the type of substrate on which the image is printed may also increase bleeding between the first and second inks.

[0006] This invention has been made in view of the above problems, and aims to suppress bleeding between the first ink and the second ink, regardless of the transport speed, the type of substrate, etc. [Means for solving the problem]

[0007] One aspect of the present invention is an inkjet printing apparatus comprising: a transport mechanism for transporting a substrate; a head unit having a first head and a second head arranged in order from the upstream side with respect to the transport path of the substrate, wherein the first head ejects a first ink toward the substrate and the second head ejects a second ink toward the substrate; and an ink characteristic changing process for changing the dynamic surface tension characteristics of at least one of the first ink and the second ink, and / or a path length changing process for changing the path length of the section between the first head and the second head in the transport path of the substrate. The device includes a surface tension difference adjustment unit that is performed as an adjustment process, where the time at which a first ink droplet, which is a droplet of the first ink, lands on the target position of the substrate is defined as the first time, and the time at which a second ink droplet, which is a droplet of the second ink, lands on the target position is defined as the second time, and the absolute value of the difference between the dynamic surface tension of the first ink droplet at the second time and the dynamic surface tension of the second ink droplet at the second time is defined as the surface tension difference between the first and second inks, and the surface tension difference between the first and second inks when the adjustment process is performed is smaller than the surface tension difference between the first and second inks when the adjustment process is not performed.

[0008] A second aspect of the present invention is an inkjet printing apparatus according to aspect 1, wherein the adjustment process includes the ink characteristic modification process, in which the temperature of at least one of the inks held in the head unit and / or the temperature of the first ink droplet on the substrate is changed.

[0009] A third aspect of the present invention is an inkjet printing apparatus according to aspect 1 (or aspect 1 or 2), wherein the adjustment process includes the path length changing process, and in the path length changing process, the position of the turning portion, which is located in the section of the transport path and causes the transport path to turn back, is changed.

[0010] A fourth aspect of the present invention is an inkjet printing apparatus according to the third aspect, wherein the folded portion is an air turn bar.

[0011] Aspect 5 of the present invention is an inkjet printing apparatus according to aspect 1 (which may be any one of aspects 1 to 4), wherein the adjustment process further includes a gap changing process that changes the distance between at least one of the first head and the second head and the substrate.

[0012] Aspect 6 of the present invention is an inkjet printing apparatus according to aspect 1 (which may be any one of aspects 1 to 5), wherein the adjustment process further includes a waveform modification process that changes the shape and / or magnitude of the drive waveform input to at least one of the first head and the second head.

[0013] Aspect 7 of the present invention is an inkjet printing apparatus according to Aspect 1 (which may be any one of Aspects 1 to 6), wherein the difference in surface tension between the first and second inks when the adjustment process is performed while the substrate is being transported at a single transport speed is approximately equal to the difference in surface tension between the first and second inks when the substrate is being transported at a transport speed different from the transport speed.

[0014] Embodiment 8 of the present invention is an inkjet printing apparatus according to any one of embodiments 1 to 7, wherein the head unit is positioned downstream of the second head with respect to the transport path and further has a third head that ejects a third ink toward the substrate, and the time at which a third ink droplet, which is a droplet of the third ink, lands at the target position is defined as the third time, and the surface tension difference adjustment unit performs an adjustment process that affects the dynamic surface tension of at least one of the second ink droplet and the third ink droplet at the third time, and the absolute value of the difference between the dynamic surface tension of the second ink droplet at the third time and the dynamic surface tension of the third ink droplet at the third time is defined as the surface tension difference between the second and third inks, and the surface tension difference between the second and third inks when the adjustment process is performed is smaller than the surface tension difference between the second and third inks when the adjustment process is not performed.

[0015] Aspect 9 of the present invention is an inkjet printing apparatus according to aspect 8, wherein the head unit is positioned downstream of the third head with respect to the transport path and further has a fourth head that ejects a fourth ink toward the substrate, and the time at which a fourth ink droplet, which is a droplet of the fourth ink, lands at the target position is defined as the fourth time, and the surface tension difference adjustment unit performs an adjustment process that affects the dynamic surface tension of at least one of the third ink droplet and the fourth ink droplet at the fourth time, and the absolute value of the difference between the dynamic surface tension of the third ink droplet at the fourth time and the dynamic surface tension of the fourth ink droplet at the fourth time is defined as the surface tension difference between the third and fourth inks, and the surface tension difference between the third and fourth inks when the adjustment process is performed is smaller than the surface tension difference between the third and fourth inks when the adjustment process is not performed.

[0016] Aspect 10 of the present invention is an inkjet printing apparatus according to any one of aspects 1 to 7 (or any one of aspects 1 to 9), wherein, by performing the adjustment process, the dynamic surface tension of the second ink droplet at the second time point becomes approximately equal to the dynamic surface tension of the first ink droplet at the second time point.

[0017] Aspect 11 of the present invention is an inkjet printing apparatus according to aspect 10, wherein the head unit is positioned downstream of the second head with respect to the transport path and further has a third head that ejects a third ink toward the substrate, and the time at which the third ink droplet, which is a droplet of the third ink, lands at the target position is defined as the third time, and the surface tension difference adjustment unit performs an adjustment process that affects the dynamic surface tension of at least one of the first ink droplet and the third ink droplet at the third time, and as a result of the adjustment process, the dynamic surface tension of the third ink droplet at the third time and the dynamic surface tension of the first ink droplet at the third time become approximately equal.

[0018] Aspect 12 of the present invention is an inkjet printing apparatus according to aspect 11, wherein the head unit is positioned downstream of the third head with respect to the transport path and further has a fourth head that ejects a fourth ink toward the substrate, and the time at which the fourth ink droplet, which is a droplet of the fourth ink, lands at the target position is defined as the fourth time, and the surface tension difference adjustment unit performs an adjustment process that affects the dynamic surface tension of at least one of the first ink droplet and the fourth ink droplet at the fourth time, and the adjustment process makes the dynamic surface tension of the fourth ink droplet at the fourth time approximately equal to the dynamic surface tension of the first ink droplet at the fourth time.

[0019] Aspect 13 of the present invention is a printing method using an inkjet head unit, comprising: a) a step of transporting a substrate; b) in the head unit, a first head and a second head are arranged in order from the upstream side with respect to the transport path of the substrate, and in parallel with step a), a step of ejecting a first ink from the first head toward the substrate and a second ink from the second head toward the substrate; and c) an ink characteristic changing process that changes the dynamic surface tension characteristics of at least one of the first ink and the second ink, and / or the first head and the second head in the transport path of the substrate. The process includes a step of performing a path length changing process as an adjustment process to change the path length of the section between the first and second inks, wherein the time at which a first ink droplet, which is a droplet of the first ink, lands on the target position of the substrate is defined as the first time, and the time at which a second ink droplet, which is a droplet of the second ink, lands on the target position is defined as the first-to-second ink surface tension difference, and the first-to-second ink surface tension difference when the adjustment process is performed is smaller than the first-to-second ink surface tension difference when the adjustment process is not performed. [Effects of the Invention]

[0020] According to the present invention, bleeding between the first ink and the second ink can be suppressed regardless of the conveyance speed, the type of substrate, or the like.

Brief Description of the Drawings

[0021] [Figure 1] It is a diagram showing the configuration of an inkjet printing apparatus according to the first embodiment. [Figure 2] It is a diagram showing the internal configuration of the head unit and the like. [Figure 3] It is a diagram showing the flow of the process of the inkjet printing apparatus printing an image on a substrate. [Figure 4] It is a diagram showing the change in the dynamic surface tension of the first to fourth ink droplets. [Figure 5] It is a diagram showing the change in the dynamic surface tension of the first to fourth ink droplets. [Figure 6A] It is a diagram showing an enlarged view of a part of the change in the dynamic surface tension of the ink droplets. [Figure 6B] It is a diagram showing an enlarged view of a part of the change in the dynamic surface tension of the ink droplets. [Figure 6C] It is a diagram showing an enlarged view of a part of the change in the dynamic surface tension of the ink droplets. [Figure 7A] It is a diagram showing ink droplets on one type of substrate. [Figure 7B] It is a diagram showing ink droplets on another type of substrate. [Figure 8] It is a diagram showing a part of an inkjet printing apparatus according to the second embodiment.

Modes for Carrying Out the Invention

[0022] <First Embodiment> Figure 1 shows the configuration of an inkjet printing apparatus 1 according to the first embodiment of the present invention. The inkjet printing apparatus 1 is a printer that forms an image on a substrate 9 by ejecting ink droplets (microdroplets) toward a continuous sheet-like substrate 9, such as a resin sheet of PET (polyethylene terephthalate) or a long sheet of coated paper. In Figure 1, two horizontal directions perpendicular to each other are shown as the X direction and the Y direction, and the vertical direction perpendicular to the X direction and the Y direction is shown as the Z direction. The X direction and the Y direction in Figure 1 do not necessarily have to be horizontal, and similarly, the Z direction does not necessarily have to be vertical.

[0023] The inkjet printing apparatus 1 comprises a main unit 11 and a control unit 80. The control unit 80 is implemented, for example, by a computer having a CPU and / or a dedicated electrical circuit. The control unit 80 controls ink ejection and substrate transport, and is responsible for the overall control of the inkjet printing apparatus 1. Details of the functions of the control unit 80 will be described later.

[0024] The main unit 11 includes a transport mechanism 2 for transporting a sheet-like substrate 9, and a head unit 4 for ejecting ink droplets toward the substrate 9 as it is being transported by the transport mechanism 2. The transport mechanism 2 has a plurality of rollers 21, each long in the X direction in Figure 1 (some rollers are denoted by reference numerals 21a, 21b, and 21c), and an electric motor (not shown) for rotating some of the rollers 21. A supply unit 31 for holding a roll-shaped substrate 9 (supply roll) is provided near the roller 21 located furthest to the (-Y) side, and a winding unit 32 for holding a roll-shaped substrate 9 (winding roll) is provided near the roller 21 located furthest to the (+Y) side. In the inkjet printing apparatus 1, some of the rollers 21 of the transport mechanism 2 rotate at a constant rotational speed, so that the substrate 9 is transported at a constant speed along a predetermined transport path from the supply unit 31 to the winding unit 32.

[0025] The print head unit 4 faces the substrate 9 at an intermediate point in the substrate's transport path. At the position facing the print head unit 4, the substrate 9 moves from the (-Y) side toward the (+Y) direction. In the following description, the side of the substrate 9 facing the print head unit 4 is referred to as the "first side," and the side of the substrate 9 facing away from the print head unit 4 is referred to as the "second side." The first side is the printing surface to which ink from the print head unit 4 adheres.

[0026] Of the multiple rollers 21 of the conveying mechanism 2, the three rollers 21a to 21c facing the head unit 4 are temperature-controlled rollers. The three temperature-controlled rollers 21a to 21c are arranged along the Y direction. Hereinafter, when distinguishing between the three temperature-controlled rollers 21a to 21c, the temperature-controlled roller 21a on the (-Y) side will be called the "first temperature-controlled roller 21a", the temperature-controlled roller 21b adjacent to the (+Y) side of the first temperature-controlled roller 21a will be called the "second temperature-controlled roller 21b", and the temperature-controlled roller 21c adjacent to the (+Y) side of the second temperature-controlled roller 21b will be called the "third temperature-controlled roller 21c". Each temperature-controlled roller 21a to 21c typically includes an electric heater and heats the substrate 9 while in contact with the second surface of the substrate 9 during conveyance. The temperature-controlled rollers 21a to 21c may also include, for example, a Peltier element to cool the substrate 9. In the example shown in Figure 1, parallel diagonal lines are drawn on the temperature control rollers 21a to 21c (the same applies to Figure 2, which will be described later).

[0027] In the transport path of the substrate 9, a meandering correction unit 33 is provided between the supply unit 31 and the temperature control rollers 21a to 21c to correct the meandering of the substrate 9. Between the temperature control rollers 21a to 21c and the winding unit 32, a curing unit 35 is provided to irradiate the substrate 9 with ultraviolet light for ink curing. In addition, the inkjet printing apparatus 1 may be provided with a pre-processing unit or the like for performing predetermined pre-treatment on the substrate 9.

[0028] Figure 2 shows the internal configuration of the head unit 4 and its vicinity, and the functional configuration of the control unit 80 is also shown in blocks. The head unit 4 comprises multiple heads 41a, 41b, 41c, 41d, multiple ink tanks 42a, 42b, 42c, 42d, multiple ink temperature adjustment units 43a, 43b, 43c, 43d, and multiple gap changing mechanisms 44a, 44b, 44c, 44d. The control unit 80 comprises a transport control unit 81, a drive control unit 82, and an adjustment processing control unit 83. The transport control unit 81 controls the transport mechanism 2. The drive control unit 82 controls the ejection of ink in the multiple heads 41a to 41d. The adjustment processing control unit 83 controls the ink temperature adjustment units 43a to 43d, etc., to perform the adjustment processing described later.

[0029] In the head unit 4, multiple heads 41a to 41d are arranged in the Y direction. Each head 41a to 41d is a long member in the X direction, and multiple discharge ports are formed along the X direction on the surface facing the substrate 9 (in the example of Figure 2, the surface facing the (-Z) direction). Each discharge port in the heads 41a to 41d is provided with a drive element (e.g., a piezoelectric element or a heating element), and ink is discharged from the discharge port when a drive signal is input to the drive element. The multiple discharge ports may be arranged in a line or in a staggered pattern. The arrangement of the discharge ports may also be inclined with respect to the X direction. The arrangement pattern can be arbitrarily determined as long as the multiple discharge ports are arranged at approximately a constant pitch with respect to the X direction. Each head 41a to 41d may be a collection of multiple members.

[0030] In the following, when distinguishing between the four heads 41a to 41d, the head 41a closest to (-Y) will be called the "first head 41a," and the head 41b adjacent to the (+Y) side of the first head 41a will be called the "second head 41b." The head 41c adjacent to the (+Y) side of the second head 41b will be called the "third head 41c," and the head 41d adjacent to the (+Y) side of the third head 41c will be called the "fourth head 41d." As previously described, at the position opposite the head unit 4, the substrate 9 moves from the (-Y) side toward the (+Y) direction. Therefore, the first head 41a, the second head 41b, the third head 41c, and the fourth head 41d are arranged in order from upstream to downstream with respect to the transport path of the substrate 9. When distinguishing between the inks ejected from the first to fourth heads 41a to 41d, these inks are referred to as "first ink," "second ink," "third ink," and "fourth ink," respectively. Typically, the first to fourth inks are different colors from each other.

[0031] Multiple ink tanks 42a to 42d are each connected to multiple heads 41a to 41d and store the ink that will be ejected from the multiple heads 41a to 41d. The ink tank 42a of the first head 41a stores the first ink, the ink tank 42b of the second head 41b stores the second ink, the ink tank 42c of the third head 41c stores the third ink, and the ink tank 42d of the fourth head 41d stores the fourth ink. Typically, the multiple ink tanks 42a to 42d are located near the multiple heads 41a to 41d, but depending on the design of the head unit 4, the multiple ink tanks 42a to 42d may be located at a distance from the multiple heads 41a to 41d (for example, at a distance from the substrate 9 in a plan view). Even in this case, the multiple ink tanks 42a to 42d are considered as part of the head unit 4.

[0032] Multiple ink temperature adjustment units 43a to 43d are attached to multiple ink tanks 42a to 42d, respectively. Each ink temperature adjustment unit 43a to 43d includes, for example, an electric heater to heat the ink in the ink tanks 42a to 42d. The ink temperature adjustment units 43a to 43d may also include, for example, a water cooling mechanism to cool the ink. In the head unit 4, the temperature of the ink ejected from the multiple heads 41a to 41d (i.e., the temperature of the ink immediately after ejection) can be changed by heating or cooling the ink in the ink tanks 42a to 42d using the ink temperature adjustment units 43a to 43d.

[0033] When considering only the Y direction, the first temperature control roller 21a on the (-Y) side is positioned between the first head 41a and the second head 41b, and the central second temperature control roller 21b is positioned between the second head 41b and the third head 41c. The third temperature control roller 21c on the (+Y) side is positioned between the third head 41c and the fourth head 41d. Therefore, droplets of the first ink ejected by the first head 41a onto the first surface on the (+Z) side of the substrate 9 move downwards towards the second head 41b while being heated or cooled by the first temperature control roller 21a. Droplets of the second ink ejected onto the substrate 9 by the second head 41b move downwards towards the third head 41c, together with the droplets of the first ink, while being heated or cooled by the second temperature control roller 21b. The droplets of the third ink ejected onto the substrate 9 by the third head 41c, along with the droplets of the first and second inks, are heated or cooled by the third temperature control roller 21c as they move downward towards the fourth head 41d. As described above, in the inkjet printing apparatus 1, the temperature of the ink droplets that land on the substrate 9 can also be changed to some extent.

[0034] Multiple gap changing mechanisms 44a to 44d each support multiple heads 41a to 41d. Each gap changing mechanism 44a to 44d has, for example, a ball screw mechanism and an electric motor, and can move the heads 41a to 41d along the ejection direction while maintaining a posture in which the ink ejection direction from the heads 41a to 41d is substantially perpendicular to the substrate 9. In other words, the heads 41a to 41d can move in a direction substantially perpendicular to the ejection surface while maintaining a posture in which the surface forming the ejection port of the heads 41a to 41d is substantially parallel to the substrate 9. By moving the heads 41a to 41d with the gap changing mechanisms 44a to 44d, the size of the gap between the ejection port of the heads 41a to 41d and the first surface of the substrate 9 (hereinafter referred to as "gap") can be changed. The gap changing mechanisms 44a to 44d may move the heads 41a to 41d using linear motors or the like.

[0035] In one example of the head unit 4, the first to fourth heads 41a to 41d eject K (black), C (cyan), M (magenta), and Y (yellow) inks, respectively, but the color of the ink ejected by each head 41a to 41d may be changed as appropriate. In this embodiment, the inks of each color are UV-curable, but inks that harden when exposed to radiation other than ultraviolet light (e.g., infrared rays or electron beams) may be used, and water-based inks may also be used. The number of heads provided in the head unit 4 is not limited to four, but may be arbitrarily determined to be two, three, five, six or more, etc. The structure of the gap changing mechanisms 44a to 44d may also be changed as appropriate.

[0036] Figure 3 shows the flow of the process by which the inkjet printing device 1 prints an image on the substrate 9. The basic operation of the inkjet printing device 1 will be explained below with reference to steps S11, S12, and S13 in Figure 3. The process of step S21 in Figure 3 will be described in detail after the explanation of the basic operation. Note that the change in the gap between the heads 41a to 41d by the gap changing mechanisms 44a to 44d is performed in the processing example described later, and is not performed in this processing example.

[0037] In the inkjet printing apparatus 1 shown in Figure 1, the transport mechanism 2 starts transporting the substrate 9 under the control of the transport control unit 81 (step S11), and continuous portions of the substrate 9 are sequentially drawn out from the supply unit 31. The transport speed of the substrate 9 by the transport mechanism 2 is almost constant. Below, we will describe the processing of a particular portion of the substrate 9, but similar processing is performed in parallel for other portions. The portion of the substrate 9 that is the focus reaches below the head unit 4 via the meandering correction unit 33. Below the head unit 4, the portion of the focus moves from the (-Y) side to the (+Y) direction. In parallel with the transport of the substrate 9, the drive control unit 82 shown in Figure 2 inputs a drive signal to the heads 41a to 41d, causing ink to be ejected from the ejection ports of the heads 41a to 41d toward the substrate 9 (step S12).

[0038] Specifically, when the area of ​​interest reaches a position opposite the first head 41a, a droplet of the first ink is ejected toward the area of ​​interest, forming an image of the first ink. The continuously moving area of ​​interest reaches a position opposite the second head 41b after a predetermined time has elapsed since the first ink landed (adhered). Then, a droplet of the second ink is ejected toward the area of ​​interest, forming an image of the second ink. Similarly, after a predetermined time has elapsed since the second ink landed, the area of ​​interest reaches a position opposite the third head 41c, and a droplet of the third ink is ejected toward the area of ​​interest, forming an image of the third ink. Furthermore, after a predetermined time has elapsed since the third ink landed, the area of ​​interest reaches a position opposite the fourth head 41d, and a droplet of the fourth ink is ejected toward the area of ​​interest, forming an image of the fourth ink.

[0039] In the inkjet printing apparatus 1, the area of ​​interest moves to the curing section 35 while the first to fourth inks on the first surface remain in a nearly liquid state. Then, the first to fourth inks are cured simultaneously by irradiation of the area of ​​interest with ultraviolet light. In Figure 3, the illustration of the ink curing process by the curing section 35 is omitted. The area of ​​interest is wound up in the winding section 32 (see Figure 1). Once all images have been formed on the substrate 9, the transport of the substrate 9 is stopped, and the printing process is completed (step S13).

[0040] In each of the inkjet printing heads 41a to 41d of the inkjet printing device 1, the ejection ports are arranged across the entire width of the printing area of ​​the substrate 9 perpendicular to the transport path. This enables so-called one-pass (single-pass) printing, in which the image is printed only once by passing the substrate 9 under the head unit 4. The substrate 9 on which the image has been formed is then used as various printed materials.

[0041] Here, we will explain the change in dynamic surface tension of droplets of the first to fourth inks used in the inkjet printing apparatus 1. Figure 4 shows the change in dynamic surface tension (i.e., dynamic surface tension characteristics) of droplets of the first to fourth inks on the substrate 9, and shows the dynamic surface tension at room temperature. In Figure 4, the vertical axis represents dynamic surface tension, and the horizontal axis represents time (similarly in Figure 5 described later). In Figure 4, the time change of the dynamic surface tension of the first to fourth inks is shown by lines labeled L11, L12, L13, and L14.

[0042] As shown in Figure 4, in each of the first to fourth inks, the dynamic surface tension of the droplet decreases over time from the dynamic surface tension upon impact with the substrate 9 (hereinafter referred to as "initial surface tension"; the same applies hereinafter) to the surface tension at equilibrium (i.e., static surface tension). Such first to fourth inks are produced, for example, by adjusting the amount (e.g., several percent) of a surface modifier, such as an acrylic surface modifier, added to each ink. The first to fourth inks with UV curability in this embodiment include, in addition to the surface modifier, monofunctional monomers, polyfunctional monomers, pigment dispersions (pigments), pigment dispersants, photopolymerization initiators, sensitizers, polymerization inhibitors, etc. Normally, the dynamic surface tension characteristics vary depending on the type and mixing ratio of these components, but it is possible to adjust the dynamic surface tension characteristics within a certain range by adding a surface modifier. In the example in Figure 4, the initial surface tension decreases in the order of the first ink, second ink, third ink, and fourth ink.

[0043] In the following explanation, the first to fourth ink droplets that land adjacent to each other at a single position on the first surface of the substrate 9 (hereinafter referred to as the "target position") are referred to as the "first ink droplet," the "second ink droplet," the "third ink droplet," and the "fourth ink droplet," respectively. At the target position, interfaces are formed between the first ink droplet and each of the second to fourth ink droplets. Interfaces are also formed between the second ink droplet and each of the third and fourth ink droplets. Interfaces are also formed between the third ink droplet and the fourth ink droplet. Furthermore, the times at which the first to fourth ink droplets land at the target position of the substrate 9 are referred to as the "first time," the "second time," the "third time," and the "fourth time," respectively (the same applies to the example in Figure 5 described later). In Figure 4, the first to fourth times are T11, T12, T13, and T14, respectively.

[0044] In the example in Figure 4, the first ink droplet lands at the target position at the first time T11, and the second ink droplet lands at the target position at the second time T12, following the first time T11. Furthermore, the third ink droplet lands at the target position at the third time T13, following the second time T12, and the fourth ink droplet lands at the target position at the fourth time T14, following the third time T13. In the following explanation, let L be an integer between 2 and 4, M be an integer between 1 and 3 less than L, and N be an integer between 2 and 4 greater than M and less than or equal to L. The absolute value of the difference between the dynamic surface tension of the M-N ink droplet at time L and the dynamic surface tension of the N-th ink droplet at time L (i.e., the initial surface tension of the N-th ink droplet) is called the "M-N ink surface tension difference at time L". Also, when L and N are the same, it is simply called the "M-N ink surface tension difference".

[0045] The difference in surface tension between the first and second ink droplets at the second time point T12 is the magnitude indicated by arrow A12 in Figure 4, and is smaller than, for example, the absolute value of the difference between the dynamic surface tension of the first ink droplet at the first time point T11 and the dynamic surface tension of the first ink droplet at the second time point T12. This small difference in surface tension between the first and second ink droplets suppresses mixing of the first and second ink droplets on the substrate 9. The difference in surface tension between the second and third ink droplets at the third time point T13 is the magnitude indicated by arrow A13 in Figure 4, and is smaller than, for example, the absolute value of the difference between the dynamic surface tension of the second ink droplet at the second time point T12 and the dynamic surface tension of the second ink droplet at the third time point T13. This suppresses mixing of the second and third ink droplets on the substrate 9. The difference in surface tension between the third and fourth ink droplets at the fourth time step T14 is the magnitude indicated by arrow A14 in Figure 4, and is smaller than, for example, the absolute value of the difference between the dynamic surface tension of the third ink droplet at the third time step T13 and the dynamic surface tension of the third ink droplet at the fourth time step T14. This suppresses the mixing of the third and fourth ink droplets on the substrate 9.

[0046] In reality, the surface tension differences between the first and second inks, and between the first and third inks at the third time T13, as well as the surface tension differences between the first and second inks, the first and third inks, the first and fourth inks, the second and third inks, and the second and fourth inks at the fourth time T13, are also relatively small. Therefore, mixing of the first to fourth ink droplets on the substrate 9 is suppressed, and bleeding between the first to fourth inks can be suppressed. As a result, the quality of the printed image formed by the inkjet printing apparatus 1 can be improved.

[0047] Next, we will describe the process of printing an image while changing the transport speed of the substrate 9 by the transport mechanism 2 from the transport speed in the basic operation. In this example, the substrate 9 is transported at a speed faster than the transport speed in the basic operation. The process of step S21 in Figure 3 is also executed. Hereinafter, the transport speed in the basic operation will be referred to as the "first transport speed," and the transport speed in this example will be referred to as the "second transport speed."

[0048] Similar to the basic operation described above, when the transport mechanism 2 starts transporting the substrate 9 (step S11), the substrate 9 moves from the (-Y) side to the (+Y) direction below the head unit 4. In addition, an ink characteristic modification process that changes the dynamic surface tension characteristics of the ink is performed as an adjustment process (step S21). In the ink characteristic modification process, the ink temperature adjustment units 43a to 43d are controlled by the adjustment process control unit 83 in Figure 2 to heat or cool the ink in the ink tanks 42a to 42d, thereby changing the temperature of the ink. Then, in parallel with the transport of the substrate 9, ink is ejected from the heads 41a to 41d toward the substrate 9 (step S12). In practice, the ink temperature adjustment by the ink temperature adjustment units 43a to 43d continues in parallel with the ejection of ink from the heads 41a to 41d (similar to the adjustment of the ink temperature on the substrate 9 by the temperature control rollers 21a to 21c described later).

[0049] Figure 5 shows the change in dynamic surface tension of the first to fourth ink droplets on the substrate 9. As previously described, the transport speed of the substrate 9 in this processing example (second transport speed) is faster than the first transport speed in the basic operation. Therefore, the period between the first time T21 when the first ink droplet lands at the target position and the second time T22 when the second ink droplet lands at the target position is shorter than the corresponding period in Figure 4 (the period between the first time T11 and the second time T12). Similarly, the period between the second time T22 and the third time T23 when the third ink droplet lands at the target position, and the period between the third time T23 and the fourth time T24 when the fourth ink droplet lands at the target position are also shorter than the corresponding periods in Figure 4.

[0050] If the adjustment process in step S21 is not performed, the change in the dynamic surface tension of the first to fourth ink droplets will be the same as lines L11 to L14 in Figure 4, except that the starting times are the first time T21, the second time T22, the third time T23, and the fourth time T24, respectively. In Figure 5, the change in the dynamic surface tension of the first to fourth ink droplets when the adjustment process is not performed is shown by lines labeled L21 to L24 (lines L21 to L23 are thin lines).

[0051] If the adjustment process is not performed, the short time interval between the first time point T21 and the second time point T22 results in a large surface tension difference between the first and second inks at the second time point T22, as indicated by the white arrow A22 in Figure 5. This makes the second ink, which has a relatively lower dynamic surface tension, more likely to erode into the area of ​​the first ink, which has a relatively higher dynamic surface tension. In other words, the first and second ink droplets are more likely to mix on the substrate 9. Similarly, as indicated by the white arrow A23 in Figure 5, the surface tension difference between the second and third inks increases at the third time point T23, making it easier for the second and third ink droplets to mix on the substrate 9. Furthermore, as indicated by the white arrow A24 in Figure 5, the surface tension difference between the third and fourth inks increases at the fourth time point T24, making it easier for the third and fourth ink droplets to mix on the substrate 9. As described above, if the adjustment process is not performed, the quality of the printed image deteriorates.

[0052] In the actual inkjet printing apparatus 1, the ink characteristic modification process is performed as an adjustment process in step S21, and the dynamic surface tension characteristics of the first to third inks are changed. In this example, the ink temperature adjustment units 43a to 43c heat the first to third inks in the ink tanks 42a to 42c to their respective set temperatures. As a result, as shown by the thick lines labeled L31 to L33 in Figure 5, the dynamic surface tension of the first to third inks becomes lower compared to when the adjustment process (ink characteristic modification process) is not performed. The respective set temperatures of the first to third inks are determined by experimentation or other means and are stored in advance in the adjustment process control unit 83.

[0053] As a result of the adjustment process, the surface tension difference between the first and second inks at the second time point T22 becomes smaller than the surface tension difference between the first and second inks when the adjustment process is not performed (see white arrow A22), as shown by the black arrow A32 in Figure 5. As a result, mixing of the first and second ink droplets on the substrate 9 is suppressed. In practice, when the substrate 9 is transported at the second transport speed, the surface tension difference between the first and second inks when the adjustment process is performed is approximately equal to the surface tension difference between the first and second inks when the substrate 9 is transported at a first transport speed different from the second transport speed (here, the basic operation) (the same applies to the surface tension difference between the second and third inks, and the surface tension difference between the third and fourth inks). When the two surface tension differences between inks are approximately equal, the difference between the two surface tension differences between inks is, for example, 10% or less, preferably 5% or less, of the larger surface tension difference between inks.

[0054] Similarly, as indicated by the black arrow A33, the surface tension difference between the second and third inks at the third time T23 is smaller than the surface tension difference between the second and third inks when the adjustment process is not performed (see white arrow A23). Also, as indicated by the black arrow A34, the surface tension difference between the third and fourth inks at the fourth time T24 is smaller than the surface tension difference between the third and fourth inks when the adjustment process is not performed (see white arrow A24). As a result, mixing of the second and third ink droplets on the substrate 9 is suppressed, and mixing of the third and fourth ink droplets is also suppressed. In Figure 5, for example, the dynamic surface tension of the fourth ink may be increased by cooling the fourth ink in the ink tank 42d with the ink temperature adjustment unit 43d.

[0055] As described above, in the inkjet printing apparatus 1, the adjustment processing control unit 83 and the ink temperature adjustment units 43a to 43d work together to perform the adjustment process, thereby reducing the surface tension difference between the first and second inks to less than the surface tension difference between the first and second inks when the adjustment process is not performed. The same applies to the surface tension difference between the second and third inks, and between the third and fourth inks. As a result, bleeding at the boundaries between the first to fourth ink regions on the substrate 9 can be suppressed, and the quality of the printed material produced by the inkjet printing apparatus 1 can be improved. The above adjustment process is a process to reduce the surface tension difference between inks. In the inkjet printing apparatus 1, the adjustment processing control unit 83 and the ink temperature adjustment units 43a to 43d become a surface tension difference adjustment unit 830 that performs the adjustment process. When all images are formed on the substrate 9, the transport of the substrate 9 is stopped, and the printing process ends (step S13).

[0056] In the example shown in Figure 5, the initial surface tension of the first ink is lowered by heating the first ink in ink tank 42a, but the initial surface tension of the second ink is increased by cooling the second ink in ink tank 42b, thereby reducing the surface tension difference between the first and second inks. Furthermore, at room temperature, the initial surface tension of the first ink does not necessarily need to be higher than that of the second ink; the initial surface tension of the first ink may be lower than that of the second ink. In this case, the surface tension difference between the first and second inks may be reduced by cooling the first ink in ink tank 42a and / or heating the second ink in ink tank 42b. Thus, the adjustment process that reduces the surface tension difference between the first and second inks when the adjustment process is performed to less than the surface tension difference when the adjustment process is not performed is achieved by an ink characteristic modification process that heats and / or cools the temperature of at least one of the first and second inks.

[0057] The same applies to the surface tension difference between the second and third inks, and between the third and fourth inks. That is, an adjustment process that makes the surface tension difference between the second and third inks smaller when the adjustment process is performed than the surface tension difference between the second and third inks when the adjustment process is not performed is achieved by an ink characteristic modification process that heats and / or cools the temperature of at least one of the second and third inks. An adjustment process that makes the surface tension difference between the third and fourth inks smaller when the adjustment process is performed than the surface tension difference between the third and fourth inks when the adjustment process is not performed is achieved by an ink characteristic modification process that heats and / or cools the temperature of at least one of the third and fourth inks.

[0058] The above ink characteristic modification process may be achieved by the temperature control rollers 21a to 21c in Figure 2 changing the temperature of the ink on the substrate 9 under the control of the adjustment process control unit 83. Specifically, an ink characteristic modification process is performed as an adjustment process in which the first ink droplet ejected onto the substrate 9 is heated or cooled by the first temperature control roller 21a. As a result, the surface tension difference between the first and second inks is made smaller than the surface tension difference between the first and second inks when the adjustment process is not performed. Similarly, an ink characteristic modification process is performed as an adjustment process in which the second ink droplet ejected onto the substrate 9 is heated or cooled by the second temperature control roller 21b, and the surface tension difference between the second and third inks is made smaller than the surface tension difference between the second and third inks when the adjustment process is not performed. Furthermore, an ink characteristic modification process is performed as an adjustment process in which the third ink droplet discharged onto the substrate 9 is heated or cooled by the third temperature control roller 21c, so that the surface tension difference between the third and fourth inks is smaller than the surface tension difference between the third and fourth inks when the adjustment process is not performed. Of course, the ink characteristic modification process may be achieved by both the ink temperature adjustment units 43a to 43d and the temperature control rollers 21a to 21c, and the surface tension difference adjustment unit 830 may include the ink temperature adjustment units 43a to 43d and / or the temperature control rollers 21a to 21c.

[0059] As described above, the inkjet printing apparatus 1 comprises a transport mechanism 2, a head unit 4, and a surface tension difference adjustment unit 830. The transport mechanism 2 transports the substrate 9. The head unit 4 has a first head 41a and a second head 41b arranged in order from the upstream side with respect to the transport path of the substrate 9. The first head 41a ejects the first ink toward the substrate 9, and the second head 41b ejects the second ink toward the substrate 9. The surface tension difference adjustment unit 830 performs an ink characteristic modification process as an adjustment process, which modifies the dynamic surface tension characteristics of at least one of the first ink and the second ink. The time at which a first ink droplet, which is a droplet of the first ink, lands on the target position on the substrate 9 is defined as the first time, and the time at which a second ink droplet, which is a droplet of the second ink, lands on the target position is defined as the second time. The absolute value of the difference between the dynamic surface tension of the first ink droplet at the second time and the dynamic surface tension of the second ink droplet at the second time is defined as the surface tension difference between the first and second inks. When the adjustment process is performed, the surface tension difference between the first and second inks is smaller than when the adjustment process is not performed. In the inkjet printing apparatus 1, bleeding between the first and second inks can be suppressed regardless of the transport speed, etc. As a result, the quality of the printed image formed by the inkjet printing apparatus 1 can be improved.

[0060] Preferably, in the ink characteristic modification process, the temperature of at least one of the inks held in the head unit 4 and / or the temperature of the first ink droplet on the substrate 9 is changed. This makes it possible to change the dynamic surface tension characteristics of the ink with a simple configuration and easily realize an adjustment process that affects the dynamic surface tension of at least one of the first and second ink droplets at a second time point.

[0061] Preferably, the head unit 4 is positioned downstream of the second head 41b with respect to the transport path of the substrate 9 and further includes a third head 41c that ejects the third ink toward the substrate 9. Taking the time when the third ink droplet, which is a droplet of the third ink, lands at the target position as the third time, the surface tension difference adjustment unit 830 performs an adjustment process that affects the dynamic surface tension of at least one of the second ink droplet and the third ink droplet at the third time. The absolute value of the difference between the dynamic surface tension of the second ink droplet at the third time and the dynamic surface tension of the third ink droplet at the third time is defined as the surface tension difference between the second and third inks, and the surface tension difference between the second and third inks when the adjustment process is performed is smaller than the surface tension difference between the second and third inks when the adjustment process is not performed. This makes it possible to suppress bleeding between the second ink and the third ink regardless of the transport speed, etc.

[0062] More preferably, the head unit 4 is positioned downstream of the third head with respect to the transport path of the substrate 9 and further includes a fourth head that ejects the fourth ink toward the substrate 9. The surface tension difference adjustment unit 830 performs an adjustment process that affects the dynamic surface tension of at least one of the third and fourth ink droplets at the fourth time, with the time at which the fourth ink droplet lands at the target position being defined as the fourth time. The absolute value of the difference between the dynamic surface tension of the third ink droplet at the fourth time and the dynamic surface tension of the fourth ink droplet at the fourth time is defined as the surface tension difference between the third and fourth inks, and the surface tension difference between the third and fourth inks when the adjustment process is performed is smaller than the surface tension difference between the third and fourth inks when the adjustment process is not performed. This makes it possible to suppress bleeding between the third and fourth inks regardless of the transport speed, etc.

[0063] Figures 6A, 6B, and 6C are magnified views showing a portion of the changes in the dynamic surface tension of ink droplets on the substrate 9. Figure 6A shows the vicinity of the second time point T22, Figure 6B shows the vicinity of the third time point T23, and Figure 6C shows the vicinity of the fourth time point T24. As previously described, lines L31 to L33 show the changes in the dynamic surface tension of the first to third ink droplets, respectively, and line L24 shows the change in the dynamic surface tension of the fourth ink droplet.

[0064] In the inkjet printing apparatus 1, by performing an adjustment process that affects the dynamic surface tension of at least one of the first and second ink droplets at the second time T22, the dynamic surface tension of the second ink droplet at the second time T22 may be approximately equal to that of the first ink droplet at the second time T22, as shown in Figure 6A. This makes it possible to more reliably suppress bleeding between the first and second inks. When the surface tensions between the two inks are approximately equal, it means that the difference between the two ink surface tensions is, for example, 10% or less, preferably 5% or less, of the larger of the two ink surface tensions.

[0065] By performing an adjustment process that affects the dynamic surface tension of at least one of the first and third ink droplets at the third time T23, the dynamic surface tension of the third ink droplet at the third time T23 may be approximately equal to that of the first ink droplet at the third time T23, as shown in Figure 6B. The same applies to the relationship between the first and second ink droplets, and between the second and third ink droplets. This makes it possible to more reliably suppress bleeding between the first to third inks.

[0066] By performing an adjustment process that affects the dynamic surface tension of at least one of the first and fourth ink droplets at the fourth time T24, the dynamic surface tension of the fourth ink droplet at the fourth time T24 may be approximately equal to that of the first ink droplet at the fourth time T24, as shown in Figure 6C. The same applies to the relationship between the first and second ink droplets, the first and third ink droplets, the second and third ink droplets, the second and fourth ink droplets, and the third and fourth ink droplets. This makes it possible to more reliably suppress bleeding between the first to fourth ink droplets.

[0067] Incidentally, the dynamic surface tension of the ink may vary depending on the type of substrate 9. Figure 7A shows an ink droplet on one type of substrate 9, and Figure 7B shows an ink droplet on another type of substrate 9a. The left side of Figures 7A and 7B shows the first ink droplet D1 at the first time step, and the right side shows the first ink droplet D1 and the second ink droplet D2 at the second time step. The "A" and "B" written inside the ink droplets D1 and D2 indicate component A and component B contained in the ink droplets D1 and D2, respectively.

[0068] Generally, inks used in inkjet printing machines contain various components (e.g., pigment dispersions, glycerin, propylene glycol, etc.). The ease with which each component penetrates the substrate varies. For example, solid components such as pigments generally do not penetrate the substrate and remain on the surface, while solvents and surfactants penetrate easily. Therefore, after an ink droplet lands on the substrate, the ratio of components within the ink droplet changes. Furthermore, the presence or absence and degree of penetration of each ink component into the substrate can become more pronounced when the type of substrate changes (e.g., film-based substrate, paper-based substrate, etc.).

[0069] Here, if we define component A as the component that penetrates the substrate particularly easily, and component B as the component that does not penetrate easily, the ratio of component A to component B in the ink droplet after impact will vary depending on the type of substrate. For example, as shown in Figure 7A, the ratio of component A to component B at time 2 of the first ink droplet D1 that landed on substrate 9 at time 1 is different from the ratio at time 2 of the first ink droplet D1 that landed on substrate 9a at time 1, as shown in Figure 7B. Therefore, the difference in surface tension between the first and second inks at time 2 (i.e., the difference in dynamic surface tension between the first ink droplet D1 and the second ink droplet D2) will not be the same for substrate 9 and substrate 9a. In the examples of Figures 7A and 7B, the difference in surface tension between the first and second inks in substrate 9a is greater than the difference in surface tension between the first and second inks in substrate 9.

[0070] Even in such cases, the inkjet printing apparatus 1 can reduce the surface tension difference between the first and second inks to a smaller value than the surface tension difference between the first and second inks when the adjustment process is not performed, by having the surface tension difference adjustment unit 830 perform an adjustment process (in this case, an ink characteristic modification process). As a result, bleeding between the first and second inks can be suppressed regardless of the type of substrate, etc. The same applies to other inks.

[0071] In the inkjet printing apparatus 1, the surface tension difference adjustment unit 830 may include gap changing mechanisms 44a to 44d (see Figure 2), and a gap changing process may be performed to change the size of the gap between the ejection ports of the heads 41a to 41d and the substrate 9 (i.e., the gap between the heads 41a to 41d). For example, by increasing the gap of the first head 41a, the flight time of the first ink droplet can be increased. This increases the time from immediately after ejection, when the decrease in dynamic surface tension begins, to impact on the target position on the substrate 9, and the dynamic surface tension of the first ink droplet at the second time point becomes slightly lower than the dynamic surface tension when the gap is not changed. Alternatively, by decreasing the gap of the first head 41a, the flight time of the first ink droplet can be shortened. This shortens the time from immediately after ejection to impact on the target position on the substrate 9, and the dynamic surface tension of the first ink droplet at the second time point becomes slightly higher than the dynamic surface tension when the gap is not changed.

[0072] Furthermore, by increasing the gap of the second head 41b, the gap changing mechanism 44b can lengthen the flight time of the second ink droplet. This increases the time from immediately after ejection, when the dynamic surface tension begins to decrease, to impact on the target position on the substrate 9, resulting in the dynamic surface tension of the second ink droplet at the second time point being slightly lower than the dynamic surface tension when the gap is not changed. By decreasing the gap of the second head 41b, the gap changing mechanism 44b can shorten the flight time of the second ink droplet. This shortens the time from immediately after ejection to impact on the target position on the substrate 9, resulting in the dynamic surface tension of the second ink droplet at the second time point being slightly higher than the dynamic surface tension when the gap is not changed. As described above, by including a gap changing process that changes the distance between at least one of the first head 41a and the second head 41b and the substrate 9 in the adjustment process by the surface tension difference adjustment unit 830, it becomes possible to fine-tune the surface tension difference between the first and second inks.

[0073] Similarly, by including a gap-changing process in the adjustment process, which modifies the distance between at least one of the second head 41b and the third head 41c and the substrate 9, it becomes possible to fine-tune the surface tension difference between the second and third inks. Furthermore, by including a gap-changing process in the adjustment process, which modifies the distance between at least one of the third head 41c and the fourth head 41d and the substrate 9, it becomes possible to fine-tune the surface tension difference between the third and fourth inks.

[0074] The surface tension difference between the first and second inks, the surface tension difference between the second and third inks, and the surface tension difference between the third and fourth inks may be adjusted by changing the flight speed of the ink. In this case, the type of drive signal input from the drive control unit 82 to the heads 41a to 41d, i.e., the shape and / or magnitude of the drive waveform, is changed by the control of the adjustment processing control unit 83. The magnitude of the drive waveform includes the peak value and pulse width. For changing the drive waveform, for example, the methods described in Japanese Patent Publication No. 2011-79294, Japanese Patent Publication No. 2011-161911, and Japanese Patent Publication No. 2016-36930 (Patent Documents 3 to 5 above) can be used.

[0075] By changing the drive waveform, the flight speed of the first ink droplet from the first head 41a can be reduced, thereby increasing the flight time of the first ink droplet. This increases the time from immediately after ejection, when the dynamic surface tension begins to decrease, until the droplet lands on the target position on the substrate 9, resulting in the dynamic surface tension of the first ink droplet at the second time step being slightly lower than the dynamic surface tension when the drive waveform is not changed. Conversely, by increasing the flight speed of the first ink droplet from the first head 41a, the flight time of the first ink droplet can be reduced. This reduces the time from immediately after ejection until the droplet lands on the target position on the substrate 9, resulting in the dynamic surface tension of the first ink droplet at the second time step being slightly higher than the dynamic surface tension when the drive waveform is not changed.

[0076] Furthermore, by changing the drive waveform, the flight speed of the second ink droplet from the second head 41b is reduced, making it possible to increase the flight time of the second ink droplet. This increases the time from immediately after ejection, when the decrease in dynamic surface tension begins, to impact on the target position on the substrate 9, resulting in the dynamic surface tension of the second ink droplet at the second time point being slightly lower than the dynamic surface tension when the drive waveform is not changed. Conversely, by increasing the flight speed of the second ink droplet from the second head 41b, the flight time of the second ink droplet is reduced. This shortens the time from immediately after ejection to impact on the target position on the substrate 9, resulting in the dynamic surface tension of the second ink droplet at the second time point being slightly higher than the dynamic surface tension when the drive waveform is not changed. As described above, the adjustment process by the surface tension difference adjustment unit 830 includes a waveform modification process that changes the shape and / or magnitude of the drive waveform input to at least one of the first head 41a and the second head 41b. This makes it possible to fine-tune the surface tension difference between the first and second inks by changing the ink flight speed.

[0077] Similarly, by including a waveform modification process that changes the shape and / or magnitude of the drive waveform input to at least one of the second head 41b and the third head 41c, the adjustment process can be used to fine-tune the surface tension difference between the second and third inks. Furthermore, by including a waveform modification process that changes the shape and / or magnitude of the drive waveform input to at least one of the third head 41c and the fourth head 41d, the adjustment process can be used to fine-tune the surface tension difference between the third and fourth inks.

[0078] <Second Embodiment> Figure 8 is a diagram showing a part of an inkjet printing apparatus 1a according to a second embodiment of the present invention, showing the internal configuration of the head unit 4 and the vicinity of the head unit 4. The inkjet printing apparatus 1a is equipped with a plurality of opposing rollers 22 and a plurality of (three in the example of Figure 8) path length changing units 26a to 26c instead of the temperature control rollers 21a to 21c of Figure 2. The other configurations are the same as those of the inkjet printing apparatus 1 described above, and the same reference numerals are used for the same components.

[0079] Multiple opposing rollers 22 are part of the conveying mechanism 2 and are arranged along the Y direction at a position facing the head unit 4. Specifically, the opposing rollers 22 are positioned on the (-Y) and (+Y) sides of each head 41a to 41c so that the substrate 9 faces directly toward the multiple discharge ports of each head 41a to 41c. The opposing rollers 22 contact the second surface of the substrate 9. In the example of Figure 8, the discharge port forming surfaces of each head 41a to 41c are approximately parallel to the XY plane, but as in Figure 2, the heads 41a to 41c may be arranged in an inclined state so that the discharge port forming surfaces are aligned with a cylindrical surface centered on a central axis parallel to the X direction. In this case, the multiple opposing rollers 22 are also arranged along the cylindrical surface.

[0080] Multiple path length changing sections 26a to 26c are arranged along the Y direction at a position facing the head unit 4. Hereinafter, when distinguishing between the three path length changing sections 26a to 26c, the path length changing section 26a on the (-Y) side will be called the "first path length changing section 26a", the path length changing section 26b adjacent to the (+Y) side of the first path length changing section 26a will be called the "second path length changing section 26b", and the path length changing section 26c adjacent to the (+Y) side of the second path length changing section 26b will be called the "third path length changing section 26c". Focusing only on the Y direction, the first path length changing section 26a is located between the first head 41a and the second head 41b, the second path length changing section 26b is located between the second head 41b and the third head 41c, and the third path length changing section 26c is located between the third head 41c and the fourth head 41d.

[0081] Each path length changing section 26a to 26c is equipped with a folding section 261 and a folding position changing mechanism 266. The folding section 261 can be, for example, similar to the air turn bar described in Japanese Patent Application Publication No. 2023-139840 (Patent Document 6 above). In one example, the folding section 261 has a first support circumferential surface 263 and a second support circumferential surface 264. The first support circumferential surface 263 is a part of a cylindrical surface centered on a central axis parallel to the X direction, and is the part of the cylindrical surface in which the outward-facing normal changes from the (-Y) direction to the (-Z) direction. The second support circumferential surface 264 is a part of a cylindrical surface centered on a central axis parallel to the X direction, and is the part of the cylindrical surface in which the outward-facing normal changes from the (-Z) direction to the (+Y) direction.

[0082] The substrate 9 is placed on the first and second support surfaces 263 and 264. In practice, the first and second support surfaces 263 and 264 face the first surface of the substrate 9 onto which ink from the heads 41a to 41c lands. Each of the first and second support surfaces 263 and 264 is provided with multiple nozzles for injecting a predetermined gas (e.g., air), and by injecting this gas, the first and second support surfaces 263 and 264 substantially support the substrate 9 with a small gap between them and the first surface (i.e., in a non-contact state). The first support surface 263 changes the direction of the transport path of the substrate 9 from the (-Z) direction to the (+Y) direction. The second support surface 264 changes the direction of the transport path of the substrate 9 from the (+Y) direction to the (+Z) direction. In this way, the transport path of the substrate 9 is reversed at the folded section 261. The folded portion 261 may have other structures; for example, a folded portion having a U-shaped groove may be used. In this folded portion, a suction port is provided on the inner surface of the groove, and the first surface of the base material 9 is gently sucked in so that it follows the inner surface of the groove. Even with such a folded portion, it is possible to reverse the transport path of the base material 9 without contact with the base material 9.

[0083] The folding position changing mechanism 266 includes, for example, a ball screw mechanism and an electric motor, and is capable of moving the folding section 261 along the Z direction. The folding position changing mechanism 266 may also move the folding section 261 using a linear motor or the like. By moving the folding section 261 with the folding position changing mechanism 266 of the first path length changing section 26a, the path length of the section between the first head 41a and the second head 41b in the transport path of the base material 9 (hereinafter referred to as the "path length between the first and second heads") is changed.

[0084] For example, the folding position changing mechanism 266 of the first path length changing section 26a lowers the folding section 261, thereby increasing the path length between the first and second heads. This makes it possible to lengthen the period from when the first ink droplet ejected from the first head 41a adheres to the first surface of the substrate 9 until it reaches the position facing the second head 41b, that is, the period between the first time point and the second time point. As a result, the dynamic surface tension of the first ink droplet at the second time point becomes lower than the dynamic surface tension when the path length between the first and second heads is not changed. Also, the folding position changing mechanism 266 of the first path length changing section 26a raises the folding section 261, thereby shortening the path length between the first and second heads. This makes it possible to shorten the period between the first time point and the second time point. As a result, the dynamic surface tension of the first ink droplet at the second time point becomes higher than the dynamic surface tension when the path length between the first and second heads is not changed.

[0085] Similar to the first path length changing section 26a, the folding position changing mechanism 266 of the second path length changing section 26b moves the folding section 261, thereby changing the path length of the section between the second head 41b and the third head 41c in the transport path of the substrate 9 (hereinafter referred to as the "second-to-third head path length"). This makes it possible to make the dynamic surface tension of the second ink droplet at the third time step higher or lower than the dynamic surface tension when the second-to-third head path length is not changed. Also, the folding position changing mechanism 266 of the third path length changing section 26c moves the folding section 261, thereby changing the path length of the section between the third head 41c and the fourth head 41d in the transport path of the substrate 9 (hereinafter referred to as the "third-to-fourth head path length"). This makes it possible to make the dynamic surface tension of the third ink droplet at the fourth time step higher or lower than the dynamic surface tension when the third-to-fourth head path length is not changed.

[0086] In the printing process of the inkjet printing apparatus 1a shown in Figure 8, a path length change process is performed as an adjustment process in step S21 of Figure 3 to change the path length between the heads. The path lengths between the first and second heads, the second and third heads, and the third and fourth heads are predetermined according to the transport speed of the substrate 9. The dynamic surface tension characteristics of the first to fourth inks in this processing example are the same as in Figure 5. In this processing example, the substrate 9 is transported at a second transport speed that is faster than the first transport speed in the basic operation.

[0087] Here, if the adjustment process in step S21 is not performed (i.e., the above-mentioned inter-head path length is not changed), the period between the first time point and the second time point becomes shorter compared to the basic operation, so the surface tension difference between the first and second inks becomes larger (see white arrow A22 in Figure 5). Similarly, the period between the second time point and the third time point becomes shorter, so the surface tension difference between the second and third inks becomes larger (see white arrow A23 in Figure 5), and the period between the third time point and the fourth time point becomes shorter, so the surface tension difference between the third and fourth inks becomes larger (see white arrow A24 in Figure 5).

[0088] In the actual inkjet printing apparatus 1a, the adjustment processing control unit 83 and the path length changing units 26a to 26c work together to perform adjustment processing (in this case, path length changing processing), so that the path length between the first and second heads, the path length between the second and third heads, and the path length between the third and fourth heads are longer than during basic operation. Here, the period between the first and second time points, the period between the second and third time points, and the period between the third and fourth time points become approximately the same as during basic operation, and the surface tension difference between the first and second inks, the surface tension difference between the second and third inks, and the surface tension difference between the third and fourth inks also become approximately the same as during basic operation. In other words, the surface tension difference between the first and second inks is made smaller than the surface tension difference between the first and second inks when the adjustment processing is not performed, and the surface tension difference between the second and third inks is made smaller than the surface tension difference between the second and third inks when the adjustment processing is not performed. Furthermore, the surface tension difference between the third and fourth inks is made smaller than the surface tension difference between the third and fourth inks when the adjustment process is not performed.

[0089] As a result, bleeding at the boundaries between the first to fourth ink regions on the substrate 9 can be suppressed, and the quality of the printed material produced by the inkjet printing apparatus 1a can be improved. In the inkjet printing apparatus 1a, the adjustment processing control unit 83 and the path length changing units 26a to 26c become the surface tension difference adjustment unit 830 that performs the adjustment processing. The second transport speed may be slower than the first transport speed, in which case the path length changing process shortens the path length between the first and second heads, the path length between the second and third heads, and the path length between the third and fourth heads. Furthermore, the path length changing process may be performed when changing the type of substrate.

[0090] As explained above, in the inkjet printing apparatus 1a shown in Figure 8, the surface tension difference adjustment unit 830 performs a path length changing process as an adjustment process, which changes the path length of the section between the first head 41a and the second head 41b in the transport path of the substrate 9. When the adjustment process is performed, the surface tension difference between the first and second inks becomes smaller than when the adjustment process is not performed. In the inkjet printing apparatus 1a, bleeding between the first and second inks can be suppressed regardless of the transport speed, the type of substrate, etc. The same applies to the surface tension difference between the second and third inks, and the surface tension difference between the third and fourth inks.

[0091] Preferably, in the path length changing process, the position of the folding section 261, which is positioned in the above section of the transport path of the substrate 9 and folds back the transport path, is changed. This allows the path length to be changed with a simple configuration, and an adjustment process that affects the dynamic surface tension of the first ink droplet at the second time can be easily realized. Furthermore, if the folding section 261 is an air turn bar, path length changing sections 26a to 26c that change the path length without contacting the first surface of the substrate 9 can be easily realized.

[0092] In the inkjet printing apparatus 1a, as with the inkjet printing apparatus 1 in Figure 1, the dynamic surface tension of the first ink droplet at the second time step may be made approximately equal to the dynamic surface tension of the second ink droplet at the second time step by performing an adjustment process (in this case, a path length change process). The same applies to the relationship between the first to third ink droplets at the third time step, and the relationship between the first to fourth ink droplets at the fourth time step. Furthermore, the adjustment process may include a gap change process and a waveform change process.

[0093] In the inkjet printing apparatus 1a shown in Figure 8, the ink temperature adjustment units 43a to 43d may change the temperature of the ink in the ink tanks 42a to 42d. That is, the adjustment process by the surface tension difference adjustment unit 830 may include an ink characteristic change process in addition to the path length change process. In this case, the temperature of the ink on the substrate 9 may be changed by a heater 27 or the like, shown by a dashed line in Figure 8. As described above, the surface tension difference adjustment unit 830 in the inkjet printing apparatus may perform an ink characteristic change process to change the dynamic surface tension characteristics of at least one of the first ink and the second ink, and / or a path length change process to change the path length of the section between the first head 41a and the second head 41b in the transport path of the substrate 9 as an adjustment process.

[0094] Various modifications are possible with the above-described inkjet printing apparatus 1,1a and printing method.

[0095] The transport speed of the substrate 9 does not necessarily have to be constant; it may change continuously or in steps. Even in this case, by changing the adjustment process by the surface tension difference adjustment unit 830 in accordance with the change in transport speed (for example, by changing the ink temperature or path length), it is possible to suppress bleeding between the first ink and the second ink on the substrate 9.

[0096] Depending on the design of the inkjet printing apparatus 1,1a, a gap changing process may be performed to change the distance between the heads 41a to 41d and the substrate 9 by moving the substrate 9 in the Z direction below the head unit 4, for example.

[0097] The substrate 9 does not necessarily have to be a continuous sheet of printing media; it may be a single-fed printing medium. In this case, the transport mechanism 2 may be, for example, a mechanism that moves a stage that holds a single-fed printing medium.

[0098] The substrate 9 may be formed from a material other than paper or resin. Ink bleeding due to differences in dynamic surface tension is particularly likely to occur on substrates that do not easily absorb ink (substrates that are impermeable to ink). Therefore, the above inkjet printing apparatus 1,1a is particularly suitable for image printing on substrates that do not easily absorb ink. Examples of such substrates include plastics such as PET, substrates formed from glass or metal, and substrates with a surface layer that is impermeable to ink (such as coated paper).

[0099] The configurations in the above embodiments and each modified example may be combined as appropriate, as long as they do not contradict each other. [Explanation of Symbols]

[0100] 1,1a Inkjet printing device 2. Conveying mechanism 4 Head Units 9,9a Base material 41a~41d Head 261 Turning section 830 Surface tension difference adjustment section D1, D2 Ink Drops S11~S13, S21 Step

Claims

1. An inkjet printing device, A transport mechanism for transporting the base material, A head unit having a first head and a second head arranged in order from the upstream side with respect to the transport path of the substrate, wherein the first head discharges a first ink toward the substrate and the second head discharges a second ink toward the substrate, A surface tension difference adjustment unit performs, as an adjustment process, an ink characteristic modification process that changes the dynamic surface tension characteristics of at least one of the first ink and the second ink, and / or a path length modification process that changes the path length of the section between the first head and the second head in the transport path of the substrate. Equipped with, An inkjet printing apparatus in which the surface tension difference between the first and second inks is defined as the absolute value of the difference between the dynamic surface tension of the first ink droplet at the second time and the dynamic surface tension of the second ink droplet at the second time, and the surface tension difference between the first and second inks when the adjustment process is performed is smaller than the surface tension difference between the first and second inks when the adjustment process is not performed.

2. An inkjet printing apparatus according to claim 1, The adjustment process includes the ink characteristic modification process, An inkjet printing apparatus in which, in the ink characteristic modification process, the temperature of at least one of the inks held in the head unit and / or the temperature of the first ink droplet on the substrate are changed.

3. An inkjet printing apparatus according to claim 1, The adjustment process includes the path length change process, In the aforementioned path length modification process, the position of the turning section, which is located in the section of the transport path and causes the transport path to be turned back, is changed in an inkjet printing apparatus.

4. An inkjet printing apparatus according to claim 3, An inkjet printing apparatus in which the aforementioned folded portion is an air turn bar.

5. An inkjet printing apparatus according to claim 1, An inkjet printing apparatus wherein the adjustment process further includes a gap changing process that changes the distance between at least one of the first head and the second head and the substrate.

6. An inkjet printing apparatus according to claim 1, An inkjet printing apparatus wherein the adjustment process further includes a waveform modification process that changes the shape and / or magnitude of the drive waveform input to at least one of the first head and the second head.

7. An inkjet printing apparatus according to claim 1, An inkjet printing apparatus in which, when the adjustment process is performed while the substrate is being transported at a single transport speed, the surface tension difference between the first and second inks is approximately equal to the surface tension difference between the first and second inks when the substrate is being transported at a transport speed different from the transport speed.

8. An inkjet printing apparatus according to any one of claims 1 to 7, The head unit is positioned downstream of the second head with respect to the transport path and further includes a third head that ejects the third ink toward the substrate. The third time is defined as the time when the third ink droplet, which is a droplet of the third ink, lands at the target position. The surface tension difference adjustment unit then performs an adjustment process that affects the dynamic surface tension of at least one of the second ink droplet and the third ink droplet at the third time. An inkjet printing apparatus in which the absolute value of the difference between the dynamic surface tension of the second ink droplet at the third time and the dynamic surface tension of the third ink droplet at the third time is defined as the surface tension difference between the second and third inks, and the surface tension difference between the second and third inks when the adjustment process is performed is smaller than the surface tension difference between the second and third inks when the adjustment process is not performed.

9. An inkjet printing apparatus according to claim 8, The head unit is positioned downstream of the third head with respect to the transport path and further includes a fourth head that ejects the fourth ink toward the substrate. The fourth time is defined as the time when the fourth ink droplet, which is a droplet of the fourth ink, lands at the target position. The surface tension difference adjustment unit then performs an adjustment process that affects the dynamic surface tension of at least one of the third ink droplet and the fourth ink droplet at the fourth time. An inkjet printing apparatus in which the absolute value of the difference between the dynamic surface tension of the third ink droplet at the fourth time and the dynamic surface tension of the fourth ink droplet at the fourth time is defined as the surface tension difference between the third and fourth inks, and the surface tension difference between the third and fourth inks when the adjustment process is performed is smaller than the surface tension difference between the third and fourth inks when the adjustment process is not performed.

10. An inkjet printing apparatus according to any one of claims 1 to 7, An inkjet printing apparatus in which, by performing the adjustment process, the dynamic surface tension of the second ink droplet at the second time point is approximately equal to the dynamic surface tension of the first ink droplet at the second time point.

11. An inkjet printing apparatus according to claim 10, The head unit is positioned downstream of the second head with respect to the transport path and further includes a third head that ejects the third ink toward the substrate. An inkjet printing apparatus in which, with the time at which the third ink droplet, which is a droplet of the third ink, lands at the target position being defined as the third time, the surface tension difference adjustment unit performs an adjustment process that affects the dynamic surface tension of at least one of the first ink droplet and the third ink droplet at the third time, and as a result of the adjustment process, the dynamic surface tension of the third ink droplet at the third time and the dynamic surface tension of the first ink droplet at the third time are approximately equal.

12. An inkjet printing apparatus according to claim 11, The head unit is positioned downstream of the third head with respect to the transport path and further includes a fourth head that ejects the fourth ink toward the substrate. An inkjet printing apparatus in which, with the time at which the fourth ink droplet, which is a droplet of the fourth ink, lands at the target position being defined as the fourth time, the surface tension difference adjustment unit performs an adjustment process that affects the dynamic surface tension of at least one of the first ink droplet and the fourth ink droplet at the fourth time, and as a result of the adjustment process, the dynamic surface tension of the fourth ink droplet at the fourth time and the dynamic surface tension of the first ink droplet at the fourth time are approximately equal.

13. A printing method using an inkjet head unit, a) A process for transporting the substrate, b) In the head unit, a first head and a second head are arranged in order from the upstream side with respect to the transport path of the substrate, and in parallel with step a), a step is performed in which a first ink is ejected from the first head toward the substrate and a second ink is ejected from the second head toward the substrate, c) A step of performing an ink characteristic modification process as an adjustment process, which modifies the dynamic surface tension characteristics of at least one of the first ink and the second ink, and / or a path length modification process, which modifies the path length of the section between the first head and the second head in the transport path of the substrate. Equipped with, A printing method in which the first ink droplet, which is a droplet of the first ink, lands on a target position on the substrate, is defined as the first time, the second ink droplet, which is a droplet of the second ink, lands on the target position, and the absolute value of the difference between the dynamic surface tension of the first ink droplet at the second time and the dynamic surface tension of the second ink droplet at the second time is defined as the surface tension difference between the first and second inks, wherein the surface tension difference between the first and second inks when the adjustment process is performed is smaller than the surface tension difference between the first and second inks when the adjustment process is not performed.

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