EP4786189A1

EP4786189A1 - Inkjet printer and printing method

Info

Publication number: EP4786189A1
Application number: EP25225715.9A
Authority: EP
Inventors: Kae Yamashita; Masaaki Aoike; Tamio Fukui
Original assignee: Screen Holdings Co Ltd
Current assignee: Screen Holdings Co Ltd
Priority date: 2025-02-03
Filing date: 2025-12-19
Publication date: 2026-08-05

Abstract

A first head for ejecting first ink and a second head for ejecting second ink are arranged in order from an upstream side along a transport path for a base material. A surface-tensiondifference regulator executes regulation processing for changing a dynamic-surface-tension characteristic of at least one ink. In a case where the time when first ink droplets land on a target position of the base material is given as a first time, the time when second ink droplets land on the target position is given as a second time, and an absolute value of a difference between the dynamic surface tensions of the first ink droplets and the second ink droplets at the second time is given as a first-and-second ink surface tension difference, the first-and-second ink surface tension difference when the regulation processing is executed is smaller than that when the regulation processing is not executed.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority benefit of Japanese Patent Application JP2025-15975 filed in the Japan Patent Office on February 3, 2025 , the entire disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to inkjet printing.

BACKGROUND ART

Inkjet printers are used conventionally. For example, the apparatus disclosed in Japanese Patent Application Laid-Open No. 2014-69378 (Document 1) includes a first ink ejector and a second ink ejector provided on upstream and downstream sides, respectively, in a transport direction for a base material. The first ink ejector ejects first ink whose droplets when deposited on the base material have a surface tension that decreases over time. The second ink ejector ejects second ink whose droplets when deposited on the base material have a surface tension that is approximately equal to the surface tension of the first ink droplets deposited at positions adjacent to the droplets of the second ink. This nearly aligns the surface tensions of the first and second ink droplets deposited at adjacent positions and thereby suppresses bleeding at boundaries between a first ink area and a second ink area on the base material and fluctuations of the positions of the boundaries.
The ink set according to Japanese Patent Application Laid-Open No. 2008-308665 (Document 2) suppresses bleeding, irrespective of the type of a base material, by satisfying a relation of -5 ≤ (γa - γb) ≤ 3, where γa is the dynamic surface tension (mN/m) at an elapsed time of 500 milliseconds (ms) of ink A with relatively high brightness , and γb is the dynamic surface tension (mN/m) at an elapsed time of 500 ms of ink B with relatively low brightness. This ink set is considered to be based on the assumption that the ink A and the ink B are ejected simultaneously. Japanese Patent Application Laid-Open Nos. 2011-79294 (Document 3), 2011-161911 (Document 4), and 2016-36930 (Document 5) describe that the flying speed of ink droplets changes by changing the shape or magnitude of a driving waveform to be input to an inkjet head. Japanese Patent Application Laid-Open No. 2023-139840 (Document 6) describes an air turn bar that supports a printing medium without contact.
Meanwhile, the apparatus described in Document 1 optimizes the dynamic surface tensions of the first ink and the second ink at a specific transport speed of the base material. Thus, a change in transport speed may expand the range of bleeding or the like caused between the first ink and the second ink and accordingly deteriorate print image quality. The range of bleeding or the like between the first ink and the second ink may also become enlarged with a change in the type of a base material on which image printing is performed.

SUMMARY OF THE INVENTION

It is an object of the present invention to suppress bleeding or the like caused between first ink and second ink, irrespective of a transport speed, the type of a base material or the like.
Aspect 1 of the present invention is an inkjet printer that includes a transport mechanism that transports a base material, a head unit that includes a first head and a second head arranged in order from an upstream side along a transport path for the base material, the first head ejecting first ink toward the base material, the second head ejecting second ink toward the base material, and a surface-tension-difference regulator that executes ink-characteristic change processing and/or path-length change processing as regulation processing, the ink-characteristic change processing being processing for changing a dynamic surface tension characteristic of at least one ink out of the first ink and the second ink, the path-length change processing being processing for changing a path length of a section between the first head and the second head in the transport path for the base material. In a case where a time when a first ink droplet lands on a target position of the base material is given as a first time, the first ink droplet being a droplet of the first ink, a time when a second ink droplet lands on the target position is given as a second time, the second ink droplet being a droplet of the second ink, and an absolute value of a difference between a dynamic surface tension of the first ink droplet at the second time and a dynamic surface tension of the second ink droplet at the second time is given as a first-and-second ink surface tension difference, the first-and-second ink surface tension difference when the regulation processing is executed is smaller than the first-and-second ink surface tension difference when the regulation processing is not executed.
According to the present invention, it is possible to suppress bleeding or the like caused between the first ink and the second ink, irrespective of a transport speed, the type of the base material or the like.
Aspect 2 of the present invention is the inkjet printer according to Aspect 1, in which the regulation processing includes the ink-characteristic change processing, the ink-characteristic change processing involves changing a temperature of the at least one ink stored in the head unit and/or a temperature of the first ink droplet on the base material.
Aspect 3 of the present invention is the inkjet printer according to Aspect 1 or 2, in which the regulation processing includes the path-length change processing, and the path-length change processing involves changing a position of a turn-back unit that is arranged in the section of the transport path and that turns back the transport path.
Aspect 4 of the present invention is the inkjet printer according to Aspect 3, in which the turn-back unit is an air turn bar.
Aspect 5 of the present invention is the inkjet printer according to any one of Aspects 1 to 4, in which the regulation processing further includes gap change processing for changing a distance between the base material and at least one head out of the first head and the second head.
Aspect 6 of the present invention is the inkjet printer according to any one of Aspects 1 to 5, in which the regulation processing further includes waveform change processing for changing a shape and/or a magnitude of a driving waveform that is input to at least one head out of the first head and the second head.
Aspect 7 of the present invention is the inkjet printer according to any one of Aspects 1 to 6, in which the first-and-second-ink surface tension difference when the regulation processing is executed during transport of the base material at one transport speed is approximately equal to the first-and-second-ink surface tension difference during transport of the base material at a transport speed different from the one transport speed.
Aspect 8 of the present invention is the inkjet printer according to any one of Aspects 1 to 7, in which the head unit further includes a third head arranged downstream of the second head along the transport path, the third head ejecting third ink toward the base material. In a case where a time when a third ink droplet lands on the target position is given as a third time, the third ink droplet being a droplet of the third ink, the surface-tension-difference regulator executes regulation processing that affects a dynamic surface tension of at least one of the second ink droplet and the third ink droplet at the third time. In a case where an absolute value of a difference between a dynamic surface tension of the second ink droplet at the third time and a dynamic surface tension of the third ink droplet at the third time is given as a second-and-third ink surface tension difference, the second-and-third ink surface tension difference when the regulation processing is executed is smaller than the second-and-third ink surface tension difference when the regulation processing is not executed.
Aspect 9 of the present invention is the inkjet printer according to Aspect 8, in which the head unit further includes a fourth head arranged downstream of the third head along the transport path, the fourth head ejecting fourth ink toward the base material. In a case where a time when a fourth ink droplet lands on the target position is given as a fourth time, the fourth ink droplet being a droplet of the fourth ink, the surface-tension-difference regulator executes regulation processing that affects a dynamic surface tension of at least one of the third ink droplet and the fourth ink droplet at the fourth time. In a case where an absolute value of a 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 given as a third-and-fourth ink surface tension difference, the third-and-fourth ink surface tension difference when the regulation processing is executed is smaller than the third-and-fourth ink surface tension difference when the regulation processing is not executed.
Aspect 10 of the present invention is the inkjet printer according to any one of Aspects 1 to 9, in which the dynamic surface tension of the second ink droplet at the second time is made approximately equal to the dynamic surface tension of the first ink droplet at the second time by execution of the regulation processing.
Aspect 11 of the present invention is the inkjet printer according to Aspect 10, in which the head unit further includes a third head arranged downstream of the second head along the transport path, the third head ejecting third ink toward the base material. In a case where a time when a third ink droplet lands on the target position is given as a third time, the third ink droplet being a droplet of the third ink, the surface-tension-difference regulator executes regulation processing that affects a dynamic surface tension of at least one of the first ink droplet and the third ink droplet at the third time, and the regulation processing makes the dynamic surface tension of the third ink droplet at the third time approximately equal to the dynamic surface tension of the first ink droplet at the third time.
Aspect 12 of the present invention is the inkjet printer according to Aspect 11, in which the head unit further includes a fourth head arranged downstream of the third head along the transport path, the fourth head ejecting fourth ink toward the base material. In a case where a time when a fourth ink droplet lands on the target position is given as a fourth time, the fourth ink droplet being a droplet of the fourth ink, the surface-tension-difference regulator executes regulation processing that affects a dynamic surface tension of at least one of the first ink droplet and the fourth ink droplet at the fourth time. The regulation processing 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.
Aspect 13 of the present invention is a printing method using a head unit for inkjet printing. The printing method includes a) transporting a base material, b) in parallel with the operation a), ejecting first ink from a first head of the head unit toward the base material and ejecting second ink from a second head of the head unit toward the base material, the first head and the second head being arranged in order from an upstream side along a transport path for the base material in the head unit, and c) executing ink-characteristic change processing and/or path-length change processing as regulation processing, the ink-characteristic change processing being processing for changing a dynamic surface tension characteristic of at least one ink out of the first ink and the second ink, the path-length change processing being processing for changing a path length of a section between the first head and the second head in the transport path for the base material. In a case where a time when a first ink droplet lands on a target position of the base material is given as a first time, the first ink droplet being a droplet of the first ink, a time when a second ink droplet lands on the target position is given as a second time, the second ink droplet being a droplet of the second ink, and an absolute value of a difference between a dynamic surface tension of the first ink droplet at the second time and a dynamic surface tension of the second ink droplet at the second time is given as a first-and-second ink surface tension difference, the first-and-second ink surface tension difference when the regulation processing is executed is smaller than the first-and-second ink surface tension difference when the regulation processing is not executed.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

Fig. 1 is a diagram showing a configuration of an inkjet printer according to Embodiment 1.
Fig. 2 is a diagram showing an internal configuration of a head unit and the like.
Fig. 3 is a flowchart showing the procedure of processing performed by the ink jet printer when printing an image on a base material.
Fig. 4 is a diagram showing changes in the dynamic surface tensions of first to fourth ink droplets.
Fig. 5 is a diagram showing changes in the dynamic surface tensions of the first to fourth ink droplets.
Fig. 6A is an enlarged diagram showing part of the changes in the dynamic surface tensions of the ink droplets.
Fig. 6B is an enlarged diagram showing part of the changes in the dynamic surface tensions of the ink droplets.
Fig. 6C is an enlarged diagram showing part of the changes in the dynamic surface tensions of the ink droplets.
Fig. 7A is a diagram showing ink droplets on one type of a base material.
Fig. 7B is a diagram showing ink droplets on another type of the base material.
Fig. 8 is a diagram showing part of an inkjet printer according to Embodiment 2.

DESCRIPTION OF EMBODIMENTS

Embodiment 1

Fig. 1 is a diagram showing a configuration of an inkjet printer 1 according to Embodiment 1 of the present invention. The inkjet printer 1 is a printer that forms an image on a continuous sheet-like base material 9 such as a resin sheet formed of polyethylene terephthalate (PET) or the like or a long length of coated paper by ejecting droplets (fine droplets) of ink toward the base material 9. In Fig. 1, two horizontal directions perpendicular to each other are shown as X and Y directions, and a vertical direction perpendicular to the X and Y directions is shown as the Z direction. The X and Y directions in Fig. 1 do not necessarily have to be the horizontal directions, and the Z direction also does not necessarily have to be the vertical direction.
The inkjet printer 1 includes a main unit 11 and a control unit 80. The control unit 80 is realized by, for example, a computer that includes a CPU and the like and/or a dedicated electric circuit. The control unit 80 performs control such as ink ejection control and control of the transport of the base material 9, and also performs overall control of the inkjet printer 1. The functions of the control unit 80 will be described later in detail.
The main unit 11 includes a transport mechanism 2 that transports a sheet-like base material 9, and a head unit 4 that ejects ink droplets toward the base material 9 that is being transported by the transport mechanism 2. The transport mechanism 2 includes a plurality of rollers 21 (some of which are given reference signs 21a, 21b, and 21c), each being long in the X direction shown in Fig. 1, and an electric motor (not shown) that rotates some of the rollers 21. A feeder 31 that holds a roll of the base material 9 (feed roll) is provided in the vicinity of the roller 21 located farthest to the (-Y) side, and a rewinder 32 that holds a roll of the base material 9 (wind-up roll) is provided in the vicinity of the roller 21 located farthest to the (+Y) side. In the inkjet printer 1, the base material 9 is transported at a constant speed along a predetermined transport path from the feeder 31 to the rewinder 32 by some rollers 21 of the transport mechanism 2 rotating at a constant rotation speed.
The head unit 4 faces the base material 9 in the middle of the transport path for the base material 9. When located at a position facing the head unit 4, the base material 9 moves in the (+Y) direction from the (-Y) side. In the following description, the surface of the base material 9 that faces the head unit 4 is referred to as a "first surface", and the surface of the base material 9 that faces on the side opposite to the head unit 4 is referred to as a "second surface". The first surface is a printing surface on which the ink ejected from the head unit 4 is deposited.
Among the rollers 21 of the transport mechanism 2, three rollers 21a to 21c that face the head unit 4 serve as temperature regulating rollers capable of controlling temperature. The three temperature regulating rollers 21a to 21c are aligned in the Y direction. Hereinafter, in order to distinguish the three temperature regulating rollers 21a to 21c from one another, the temperature regulating roller 21a located farthest to the (-Y) side is referred to as a "first temperature regulating roller 21a", the temperature regulating roller 21b located adjacent to the (+Y) side of the first temperature regulating roller 21a is referred to as a "second temperature regulating roller 21b", and the temperature regulating roller 21c located adjacent to the (+Y) side of the second temperature regulating roller 21b is referred to as a "third temperature regulating roller 21c". Each of the temperature regulating rollers 21a to 21c typically includes a heater that uses electricity and heats the base material 9 in contact with the second surface of the base material 9 that is being transported. Alternatively, for example, each of the temperature regulating rollers 21a to 21c may include a Peltier element or the like and cool the base material 9. In the example shown in Fig. 1, the temperature regulating rollers 21a to 21c are cross-hatched (the same applies to Fig. 2 described later).
A meandering corrector 33 that corrects meandering of the base material 9 is provided between the feeder 31 and the temperature regulating rollers 21a to 21c in the transport path for the base material 9. A hardening part 35 that applies ultraviolet rays for curing ink on the base material 9 is provided between the temperature regulating rollers 21a to 21c and the rewinder 32. Note that the inkjet printer 1 may further include a pre-processor or the like that performs predetermined pre-treatment on the base material 9.
Fig. 2 is a diagram showing an internal configuration of the head unit 4 and the vicinity of the head unit 4, and it also shows a functional configuration of the control unit 80 in blocks. The head unit 4 includes a plurality of heads 41a, 41b, 41c, and 41d, a plurality of ink tanks 42a, 42b, 42c, and 42d, a plurality of ink temperature regulators 43a, 43b, 43c, and 43d, and a plurality of gap change mechanisms 44a, 44b, 44c, and 44d. The control unit 80 includes a transport controller 81, a drive controller 82, and a regulation processing controller 83. The transport controller 81 controls the transport mechanism 2. The drive controller 82 performs ink ejection control of the heads 41a to 41d. The regulation processing controller 83 executes regulation processing described later by controlling the ink temperature regulators 43a to 43d and the like.
In the head unit 4, the heads 41a to 41d are aligned in the Y direction. Each of the heads 41a to 41d is a member that is long in the X direction and has a plurality of ejection ports aligned in the X direction in the surface facing the base material 9 (in the example shown in Fig. 2, the surface oriented in the (-Z) direction). In the heads 41a to 41d, each ejection port is provided with a driving element (e.g., a piezoelectric element or a heating element), and ink is ejected from each ejection port in response to a drive signal that is input to the driving element. The ejection ports may be aligned in a row, or may be arranged in a staggered format. The ejection ports may be aligned at an angle relative to the X direction. The arrangement pattern of the ejection ports may be determined arbitrarily as long as the ejection ports are arranged at a fixed pitch in the X direction. Each of the heads 41a to 41d may be a group of a plurality of members.
Hereinafter, in order to distinguish among the four heads 41a to 41d from one another, the head 41a located farthest to the (-Y) side is referred to as the "first head 41a", the head 41b located adjacent to the (+Y) side of the first head 41a is referred to as the "second head 41b", the head 41c located adjacent to the (+Y) side of he second head 41b is referred to as the "third head 41c", and the head 41d located adjacent to the (+Y) side of the third head 41c is referred to as the "fourth head 41d". As described previously, when located at a position facing the head unit 4, the base material 9 moves in the (+Y) direction from the (-Y) side. Thus, the first head 41a, the second head 41b, the third head 41c, and the fourth head 41d are arranged in order from the upstream side to the downstream side along the transport path for the base material 9. Hereinafter, in order to distinguish the ink ejected from each of the first to fourth heads 41a to 41d from one another, the ink ejected from the first head 41a is referred to as "first ink", the ink ejected from the second head 42b is referred to as "second ink", the ink ejected from the third head 41c is referred to as "third ink", and the ink ejected from the fourth head 41d is referred to as "fourth head". Typically, the first ink, the second ink, the third ink, and the fourth ink are of different colors.
The ink tanks 42a to 42d are connected to the heads 41a to 41d, respectively, and store the ink to be ejected from the corresponding 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 is stored in the fourth ink. Typically, the ink tanks 42a to 42d are provided in the vicinity of the heads 41a to 41d, but may be provided at positions away from the heads 41a to 41d (e.g., at positions away from the base material 9 in plan view) depending on the design of the head unit 4. Even in this case, the ink tanks 42a to 42d are regarded as part of the head unit 4.
The ink temperature regulators 43a to 43d are mounted on the ink tanks 42a to 42d, respectively. For example, each of the ink temperature regulators 43a to 43d includes a heater that uses electricity and heats the ink in the corresponding one of the ink tanks 42a to 42d. The ink temperature regulators 43a to 43d each may include, for example, a water-cooled mechanism and cool the ink. In the head unit 4, the temperature of the ink ejected from the heads 41a to 41d (i.e., the temperature of the ink immediately after ejection) can be changed by the ink temperature regulators 43a to 43d heating or cooling the ink stored in the ink tanks 42a to 42d.
Focusing on only the Y direction, the first temperature regulating roller 21a located farthest to the (-Y) side is arranged between the first head 41a and the second head 41b, and the second temperature regulating roller 21b located in the center is arranged between the second head 41b and the third head 41c. The third temperature regulating roller 21c located farthest to the (+Y) side is arranged between the third head 41c and the fourth head 41d. Thus, droplets of the first ink ejected from the first head 41a on the first surface of the base material 9 on the (+Z) side move to the underside of the second head 41b while being heated or cooled by the first temperature regulating roller 21a. Droplets of the second ink ejected from the second head 41b on the base material 9 move together with the droplets of the first ink to the underside of the third head 41c while being heated or cooled by the second temperature regulating roller 21b. Droplets of the third ink ejected from the third head 41c on the base material 9 move together with the droplets of the first ink and the droplets of the second ink to the underside of the fourth head 41d while being heated or cooled by the third temperature regulating roller 21c. As described above, the inkjet printer 1 is also capable of changing, to some extent, the temperatures of the ink droplets that have landed on the base material 9.
The gap change mechanisms 44a to 44d support the heads 41a to 41d, respectively. The gap change mechanisms 44a to 44d each include, for example, a ball screw mechanism and an electric motor and are capable of moving the heads 41a to 41d in the direction of ink ejection from the heads 41a to 41d while maintaining the direction of ink ejection approximately perpendicular to the base material 9. In other words, the heads 41a to 41d are movable in a direction approximately perpendicular to their surfaces provided with the ejection ports, while maintaining their orientations so that the aforementioned surfaces become approximately parallel to the base material 9. The gap change mechanisms 44a to 44d are capable of changing the size of clearance (hereinafter referred to as a "gap") between the ejection ports of the heads 41a to 41d and the first surface of the base material 9 by moving the heads 41a to 41d. The gap change mechanisms 44a to 44d may move the heads 41a to 41d by using, for example, linear motors.
In one example of the head unit 4, the first to fourth heads 41a to 41d eject black (K) ink, cyan (C) ink, magenta (M) ink, and yellow (Y) ink, respectively, but the color of the ink ejected from each of the heads 41a to 41d may be changed as appropriate. In the present embodiment, each color ink has an ultraviolet curing property, but may be any other ink that is cured by irradiation with radiation other than ultraviolet rays (e.g., infrared rays or electron beams), or may be water-based ink. The number of heads provided in the head unit 4 is not limited to four, and may be determined to be any arbitrary number such as two, three, five, or six or more. The structures of the gap change mechanisms 44a to 44d may also be changed as appropriate.
Fig. 3 is a flowchart showing a procedure of processing performed by the inkjet printer 1 when printing an image on the base material 9. Hereinafter, basic operations of the inkjet printer 1 are described with reference to steps S11, S12, and S13 in Fig. 3. Processing performed in step S21 in Fig. 3 will be described in detail later after the description of the basic operations. Note that the gap change mechanisms 44a to 44d change the gaps from the heads 41a to 41d in a later-described processing example, and this processing is not performed in the present processing example.
In the inkjet printer 1 shown in Fig. 1, when the transport mechanism 2 starts transporting the base material 9 under the control of the transport controller 81 (step S11), a continuous portion of the base material 9 is sequentially drawn from the feeder 31. The transport mechanism 2 transports the base material 9 at almost a constant transport speed. Hereinafter, one portion of the base material 9 is regarded as a target portion, and description is given regarding processing that is performed on the target portion, but similar processing is also performed in parallel on the other portions. The target portion of the base material 9 reaches the underside of the head unit 4 via the meandering corrector 33. Under the head unit 4, the target portion moves in the (+Y) direction from the (-Y) side. In parallel with the transport of the base material 9, the drive controller 82 shown in Fig. 2 inputs drive signals to the heads 41a to 41d, so that ink is ejected from the ejection ports of the heads 41a to 41d toward the base material 9 (step S12).
Specifically, when the target portion has reached a position facing the first head 41a, droplets of the first ink are ejected toward the target portion to form an image of the first ink. After the elapse of a predetermined period of time since the landing (deposition) of the first ink, the target portion which is continuously moving reaches a position facing the second head 41b. Then, droplets of the second ink are ejected toward the target portion to form an image of the second ink. Similarly, after the elapse of a predetermined period of time since the landing of the second ink, the target portion reaches a position facing the third head 41c, and droplets of the third ink are ejected toward the target portion to form an image of the third ink. Moreover, after the elapse of a predetermined period of time since the landing of the third ink, the target portion reaches a position facing the fourth head 41d, and droplets of the fourth ink are ejected toward the target portion to form an image of the fourth ink.
In the inkjet printer 1, the target portion moves to the hardening part 35 while the first ink, the second ink, the third ink, and the fourth ink remain in the form of almost a liquid layer. Then, the first ink, the second ink, the third ink, and the fourth ink are collectively cured by irradiating the target portion with ultraviolet rays. This processing for curing the ink by the hardening part 35 is not shown in Fig. 3. The target portion is wound by the rewinder 32 (see Fig. 1). When all of the images have been formed on the base material 9, the transport of the base material 9 is stopped to end the print processing (step S13).
Each of the heads 41a to 41d of the inkjet printer 1 has their ejection ports aligned across the entire width of a printing area of the base material 9 that is orthogonal to the transport path. Therefore, image printing is completed in only one pass of the base material 9 under the head unit 4, i.e., so-called one-pass (single-pass) printing is achieved. The base material 9 with the images formed thereon is used as a variety of printed matter.
Hereinafter, description is given regarding changes in dynamic surface tensions of the droplets of the first ink, the second ink, the third ink, and the fourth ink used in the inkjet printer 1. Fig. 4 is a diagram showing changes in dynamic surface tensions (i.e., dynamic surface tension characteristics) of the droplets of the first ink, the second ink, the third ink, and the fourth ink on the base material 9 and shows the dynamic surface tensions at ordinary temperatures. In Fig. 4, the vertical axis indicates the dynamic surface tension, and the horizontal axis indicates time (the same applies to Fig. 5 described later). In Fig. 4, the changes over time in the dynamic surface tensions of the first ink, the second ink, the third ink, and the fourth ink are indicated by lines with reference signs L11, L12, L13, and L14, respectively.
As shown in Fig. 4, for each of the first ink, the second ink, the third ink, and the fourth ink, the dynamic surface tension of the droplets decreases over time from a dynamic surface tension at the time when the droplets land on the base material 9 (hereinafter, referred to as an "initial surface tension; the same applies below) to a surface tension in an equilibrium state (i.e., a static surface tension). The first ink, the second ink, the third ink, and the fourth ink having such a property are produced by, for example, controlling the amount (e.g., several percent) of a surface conditioner such as an acrylic surface conditioner added to each ink. The first ink, the second ink, the third ink, and the fourth ink having a ultraviolet curing property in the present embodiment further contain, in addition to the surface conditioner, mono-functional monomers, polyfunctional monomers, pigment dispersion (pigments), a pigment dispersant, a photopolymerization initiator, a sensitizer, and a polymerization inhibitor. Ordinarily, the dynamic surface tension characteristics vary according to the type or compounding ratio of these components, but can be controlled within a certain range by adding the surface conditioner. In the example shown in Fig. 4, the initial surface tension decreases in the order of the first ink, the second ink, the third ink, and the fourth ink.
In the following description, the droplets of the first ink, the second ink, the third ink, and the fourth ink landing adjacent to one another at one position (hereinafter, referred to as a "target position") on the first surface of the base material 9 are referred to as "first ink droplets", "second ink droplets", "third ink droplets", and "fourth ink droplets", respectively. At the target position, an interface is formed between the first ink droplets and each of the second to fourth ink droplets. An interface is also formed between the second ink droplet and each of the third and fourth ink droplets. An interface is yet also formed between the third ink droplets and the fourth ink droplets. The times when the first to fourth ink droplets land on the target position of the base material 9 are referred to as a "first time", a "second time", a "third time" and a "fourth time", respectively (the same applies to the example shown in Fig. 5 described later). The first to fourth times are shown as T11, T12, T13, and T14, respectively, in Fig. 4.
In the example shown in Fig. 4, the first ink droplets land on the target position at the first time T11, and the second ink droplets land on the target position at the second time T12 after the first time T11. The third ink droplets land on the target position at the third time T13 after the second time T12, and the fourth ink droplets land on the target position at the fourth time T14 after the third time T13. In the following description, the absolute value of a difference between the dynamic surface tension of M-th ink droplets at L-th time and the dynamic surface tension of N-th ink droplets at L-th time (e.g., the initial surface tension of the N-th ink droplets) is referred to as an "M-and-N ink surface tension difference at L-th time", where L is an integer ranging from two to four, M is an integer ranging from one to three and less than L, and N is an integer ranging from two to four and greater than M and less than or equal to L. In the case where L and N are the same value, the above difference is also simply referred to as the "M-and-N ink surface tension difference".
The first-and-second ink surface tension difference at the second time T12 corresponds to the magnitude indicated by an arrow A12 in Fig. 4 and is, for example, smaller than the absolute value of a difference between the dynamic surface tension of the first ink droplets at the first time T11 and the dynamic surface tension of the first ink droplets at the second time T12. The small first-and-second ink surface tension difference suppresses the mixing of the first ink droplets and the second ink droplets on the base material 9. A second-and-third ink surface tension difference at the third time T13 corresponds to the magnitude indicated by an arrow A13 in Fig. 4 and is, for example, smaller than the absolute value of a difference between the dynamic surface tension of the second ink droplets at the second time T12 and the dynamic surface tension of the second ink droplets at the third time T13. This suppresses the mixing of the second ink droplets and the third ink droplets on the base material 9. A third-and-fourth ink surface tension difference at the fourth time T14 corresponds to the magnitude indicated by an arrow A14 in Fig. 4 and is, for example, smaller than the absolute value of a difference between the dynamic surface tension of the third ink droplets at the third time T13 and the dynamic surface tension of the third ink droplets at the fourth time T14. This suppresses the mixing of the third ink droplets and the fourth ink droplets on the base material 9.
In actuality, the first-and-second ink surface tension difference and the first-third ink surface tension difference at the third time T13 and the first-and-second ink surface tension difference, the first-and-third ink surface tension difference, the first-and-fourth ink surface tension difference, the second-and-third ink surface tension difference, and the second-and-fourth ink surface tension difference at the fourth time T14 are also relatively small. This suppresses the mixing of the first to fourth ink droplets on the base material 9 and accordingly suppresses bleeding or the like caused among the first ink, the second ink, the third ink, and the fourth ink. As a result, it is possible to improve the quality of a print image formed by the inkjet printer 1.
Next description is given regarding processing for printing an image while changing the transport speed of the base material 9 by the transport mechanism 2 from the transport speed used during the basic operations. In the present processing example, the base material 9 is transported at a speed higher than the transport speed used during the basic operations. The processing in step S21 shown in Fig. 3 is also executed. Hereinafter, the transport speed during the basic operations is referred to as a "first transport speed", and the transport speed in the present processing example is referred to as a "second transport speed".
Similarly to the basic operations described above, when the transport mechanism 2 starts transporting the base material 9 (step S11), the base material 9 moves in the (+Y) direction from the (-Y) side under the head unit 4. Moreover, ink-characteristic change processing for changing the dynamic surface tension characteristics of the ink is executed as regulation processing (step S21). In the ink-characteristic change processing, the ink temperature regulators 43a to 43d heat or cool the ink stored in the ink tanks 42a to 42d under the control of the regulation processing controller 83 in Fig. 2 so as to change the temperature of each ink. Then, in parallel with the transport of the base material 9, the ink is ejected from the heads 41a to 41d toward the base material 9 (step S12). In actuality, in parallel with the ejection of the ink from the heads 41a to 41d, the ink temperature regulators 43a to 43d continue to regulate the temperature of each ink (the same applies to ink temperature regulation performed on the base material 9 by the temperature regulating rollers 21a to 21c described later).
Fig. 5 is a diagram showing the changes in the dynamic surface tensions of the first to fourth ink droplets on the base material 9. As described previously, the transport speed (second transport speed) of the base material 9 in the present processing example is higher than the first transport speed used during the basic operations. Thus, a period of time between a first time T21 when the first ink droplets land on the target position and a second time T22 when the second ink droplets land on the target position is shorter than the corresponding period of time shown in Fig. 4 (the period of time between the first time T11 and the second time T12). Similarly, a period of time between the second time T22 and a third time T23 when the third ink droplets land on the target position, and a period of time between the third time T23 and a fourth time T24 when the fourth ink droplets land on the target position are also shorter than the corresponding periods of time shown in Fig. 4.
If the regulation processing in step S21 is not executed, the dynamic surface tensions of the first to fourth ink droplets change in the same manner as those indicated by the lines L11 to L14 shown in Fig. 4, except that the start times thereof become the first time T21, the second time T22, the third time T23, and the fourth time T24, respectively. In Fig. 5, the changes in the dynamic surface tensions of the first to fourth ink droplets when the regulation processing is not executed are indicated by lines with reference signs L21 to L24 (lines L21 to L23 are thin lines).
If the regulation processing is not executed, the first-and-second ink surface tension difference at the second time T22 increases as indicated by a white arrow A22 in Fig. 5 due to short period of time between the first time T21 and the second time T22. This causes the second ink with a relatively low dynamic surface tension to easily penetrate into the area of the first ink with a relatively high dynamic surface tension. That is, the first ink droplets and the second ink droplets are easily mixed on the base material 9. Similarly, the second-and-third ink surface tension difference at the third time T23 increases as indicated by a white arrow A23 in Fig. 5, which causes the second ink droplets and the third ink droplets to be easily mixed on the base material 9. Moreover, the third-and-fourth ink surface tension difference at the fourth time T24 increases as indicated by a white arrow A24 in Fig. 5, which causes the third ink droplets and the fourth ink droplets to be easily mixed on the base material 9. As described above, if the regulation processing is not executed, print image quality will deteriorate.
In the actual inkjet printer 1, the ink-characteristic change processing is executed as the regulation processing in step S21 so as to change the dynamic surface tension characteristics of the first ink, the second ink, and the third ink. In the present processing example, the ink temperature regulators 43a to 43c heat the first ink, the second ink, and the third ink stored in the ink tanks 42a to 42c to individual set temperatures. Accordingly, as indicated by thick lines with reference sign L31 to L33 in Fig. 5, the dynamic surface tensions of the first ink, the second ink, and the third ink become lower than in the case where the regulation processing (ink-characteristic change processing) is not executed. Note that the set temperatures for the first ink, the second ink, and the third ink are determined by experiment or the like and stored in advance in the regulation processing controller 83.
Through the execution of the regulation processing, as indicated by a black arrow A32 in Fig. 5, the first-and-second ink surface tension difference at the second time T22 becomes smaller than the first-and-second ink surface tension difference when the regulation processing is not executed (see the white arrow A22). As a result, it is possible to suppress the mixing of the first ink droplets and the second ink droplets on the base material 9. In actuality, the first-and-second ink surface tension difference when the regulation processing is executed during the transport of the base material 9 at the second transport speed is approximately equal to the first-and-second ink surface tension difference during the transport of the base material 9 at the first transport speed (here, the basic operations) different from the second transport speed (the same also applies to the second-and-third ink surface tension difference and the third-and-fourth ink surface tension difference). The phrase indicating that the two ink surface tension differences are approximately the same means that a difference between the two ink surface tension differences is, for example, 10% or less and preferably 5% or less of the larger ink surface tension difference.
Similarly, as indicated by a black arrow A33, the second-and-third ink surface tension difference at the third time T23 becomes smaller than the second-and-third ink surface tension difference when the regulation processing is not executed (see the white arrow A23). Moreover, as indicated by a black arrow A34, the third-and-fourth ink surface tension difference at the fourth time T24 becomes smaller than the third-and-fourth ink surface tension difference when the regulation processing is not executed (see the white arrow A24). As a result, it is possible to suppress the mixing of the second ink droplets and the third ink droplets on the base material 9 and to also suppress the mixing of the third ink droplets and the fourth ink droplets on the base material 9. In Fig. 5, the dynamic surface tension of the fourth ink may be increased by, for example, the ink temperature regulator 43d cooling the fourth ink stored in the ink tank 42d.
As described above, in the inkjet printer 1, the regulation processing controller 83 and the ink temperature regulators 43a to 43d execute the regulation processing in cooperation with each other so as to make the first-and-second ink surface tension difference smaller than the first-and-second ink surface tension difference when the regulation processing is not executed. The same also applies to the second-and-third ink surface tension difference and the third-and-fourth ink surface tension difference. As a result, it is possible to suppress bleeding or the like caused at the boundaries of the first to fourth ink areas on the base material 9 and to improve the quality of printed matter created by the inkjet printer 1. The regulation processing described above is processing for reducing the ink surface tension difference. In the inkjet printer 1, the regulation processing controller 83 and the ink temperature regulators 43a to 43d serve as the surface-tension-difference regulator 830 that executes the regulation processing. When all of the images have been formed on the base material 9, the transport of the base material 9 is stopped to end the print processing (step S13).
In the example shown in Fig. 5, the initial surface tension of the first ink is lowered by heating the first ink stored in the ink tank 42a, but the initial surface tension of the second ink may be increased by cooling the second ink stored in the ink tank 42b in order to reduce the first-and-second ink surface tension difference. At ordinary temperatures, the initial surface tension of the first ink does not necessarily have to be higher than the initial surface tension of the second ink, and the initial surface tension of the first ink may be lower than the initial surface tension of the second ink. In this case or the like, the first-and-second ink surface tension difference may be reduced by cooling the first ink stored in the ink tank 42a and/or heating the second ink stored in the ink tank 42b. In this way, the regulation processing for making the first-and-second ink surface tension difference when the regulation processing is executed smaller than the first-and-second ink surface tension difference when the regulation processing is not executed is realized by the ink-characteristic change processing for heating and/or cooling the temperature of at least one of the first ink and the second ink.
The same described above applies to the second-and-third ink surface tension difference and the third-and-fourth ink surface tension difference. That is, regulation processing that makes the second-and-third ink surface tension difference when the regulation processing is executed smaller than the second-and-third ink surface tension difference when the regulation processing is not exerted is realized by the ink-characteristic change processing for heating and/or cooling the temperature of at least one of the second ink and the third ink. Regulation processing that makes the third-and-fourth ink surface tension difference when the regulation processing is executed smaller than the third-and-fourth ink surface tension difference when the regulation processing is not executed is realized by ink-characteristic change processing for heating and/or cooling the temperature of at least one of the third ink and the fourth ink.
The ink-characteristic change processing described above may also be realized by the temperature regulating rollers 21a to 21c in Fig. 2 changing the temperature of the ink on the base material 9 under the control of the regulation processing controller 83. Specifically, ink-characteristic change processing in which the first temperature regulating roller 21a heats or cools the first ink droplets ejected on the base material 9 is executed as the regulation processing. This makes the first-and-second ink surface tension difference smaller than the first-and-second ink surface tension difference when the regulation processing is not executed. Similarly, ink-characteristic change processing in which the second temperature regulating roller 21b heats or cools the second ink droplets ejected on the base material 9 is executed as the regulation processing in order to make the second-and-third ink surface tension difference smaller than the second-and-third ink surface tension difference when the regulation processing is not executed. Moreover, ink-characteristic change processing in which the third temperature regulating roller 21c heats or cools the third ink droplets ejected on the base material 9 is executed as the regulation processing in order to make the third-and-fourth ink surface tension difference smaller than the third-and-fourth ink surface tension difference when the regulation processing is not executed. It goes without saying that the ink-characteristic change processing may be realized by both of the ink temperature regulators 43a to 43d and the temperature regulating rollers 21a to 21c, and the surface-tension-difference regulator 830 may include the ink temperature regulators 43a to 43d and/or the temperature regulating rollers 21a to 21c.
As described above, the inkjet printer 1 includes the transport mechanism 2, the head unit 4, and the surface-tension-difference regulator 830. The transport mechanism 2 transports the base material 9. The head unit 4 includes the first head 41a and the second head 41b that are arranged in order from the upstream side along the transport path for the base material 9, the first head 41a ejecting the first ink toward the base material 9, the second head 41b ejecting the second ink toward the base material 9. The surface-tension-difference regulator 830 executes, as the regulation processing, the ink-characteristic change processing for changing the dynamic surface tension characteristics of at least one of the first ink and the second ink. In the case where the time when the droplets of the first ink, i.e., the first ink droplets, land on the target position of the base material 9 is given as the first time, the time when the droplets of the second ink, i.e., the second ink droplets, land on the target position is given as the second time, and the absolute value of the difference between the dynamic surface tension of the first ink droplets at the second time and the dynamic surface tension of the second ink droplets at the second time is given as the first-and-second ink surface tension difference, the first-and-second ink surface tension difference when the regulation processing is executed is smaller than the first-and-second ink surface tension difference when the regulation processing is not executed. The inkjet printer 1 is capable of suppressing bleeding or the like caused between the first ink and the second ink, irrespective of the transport speed or the like. As a result, it is possible to improve the quality of a print image created by the inkjet printer 1.
Preferably, the ink-characteristic change processing may involve changing the temperature of the aforementioned at least one ink stored in the head unit 4 and/or the temperature of the first ink droplets on the base material 9. This allows the dynamic surface tension characteristics of the ink to be changed by a simple configuration and to easily realize the regulation processing that affects the dynamic surface tension of at least one of the first ink droplets and the second ink droplets at the second time.
Preferably, the head unit 4 may further include the third head 41c arranged downstream of the second head 41b along the transport path for the base material 9, the third head ejecting the third ink toward the base material 9. In the case where the time when the droplets of the third ink, i.e., the third ink droplets, land on the target position is given as the third time, the surface-tension-difference regulator 830 executes the regulation processing that affects the dynamic surface tension of at least one of the second ink droplets and the third ink droplets at the third time. In the case where the absolute value of the difference between the dynamic surface tension of the second ink droplets at the third time and the dynamic surface tension of the third ink droplets at the third time is given as the second-and-third ink surface tension difference, the second-and-third ink surface tension difference when the regulation processing is executed is smaller than the second-and-third ink surface tension difference when the regulation processing is not executed. This suppresses bleeding or the like caused between the second ink and the third ink, irrespective of the transport speed or the like.
More preferably, the head unit 4 may further include the fourth head 41d arranged downstream of the third head 41c along the transport path for the base material 9, the fourth head ejecting the fourth ink toward the base material 9. In the case where the time when the droplets of the fourth ink, i.e., the fourth ink droplets, land on the target position is given as the fourth time, the surface-tension-difference regulator 830 executes the regulation processing that affects the dynamic surface tension of at least one of the third ink droplet and the fourth ink droplet at the fourth time. In the case where the absolute value of the difference between the dynamic surface tension of the third ink droplets at the fourth time and the dynamic surface tension of the fourth ink droplets at the fourth time is given as the third-and-fourth ink surface tension difference, the third-and-fourth ink surface tension difference when the regulation processing is executed is smaller than the third-and-fourth ink surface tension difference when the regulation processing is not executed. This suppresses bleeding or the like caused between the third ink and the fourth ink, irrespective of the transport speed or the like.
Figs. 6A, 6B, and 6C are enlarged diagrams showing part of the changes in the dynamic surface tensions of the ink droplets on the base material 9. Fig. 6A shows changes around the second time T22, Fig. 6B shows changes around the third time T23, and Fig. 6C shows changes around the fourth time T24. As described previously, the lines L31 to L33 indicate the changes in the dynamic surface tensions of the first to third ink droplets, respectively, and the line L24 indicates the changes in the dynamic surface tension of the fourth ink droplets.
In the inkjet printer 1, the dynamic surface tension of the second ink droplets at the second time T22 and the dynamic surface tension of the first ink droplets at the second time T22 may be made approximately the same as shown in Fig. 6A by executing regulation processing that affects the dynamic surface tension of at least one of the first ink droplets and the second ink droplets at the second time T22. This more reliably suppresses bleeding or the like caused between the first ink and the second ink. The phrase indicating that the two dynamic surface tensions are approximately the same means that a difference between the two dynamic surface tensions is, for example, 10% or less and preferably 5% or less of the larger dynamic surface tension.
The dynamic surface tension of the third ink droplets at the third time T23 and the dynamic surface tension of the first ink droplets at the third time T23 may be made approximately the same as shown in Fig. 6B by executing regulation processing that affects the dynamic surface tension of at least one of the first ink droplets and the third ink droplets at the third time T23. The same also applies to the relation between the first ink droplets and the second ink droplets and the relation between the second ink droplets and the third ink droplets at the third time T23. This more reliably suppresses bleeding or the like caused among the first ink, the second ink, and the third ink.
The dynamic surface tension of the fourth ink droplets at the fourth time T24 and the dynamic surface tension of the first ink droplets at the fourth time T24 may be made approximately the same as shown in Fig. 6C by executing regulation processing that affects the dynamic surface tension of at least one of the first ink droplets and the fourth ink droplets at the fourth time T24. The same also applies to the relation between the first ink droplets and the second ink droplets, the relation between the first ink droplets and the third ink droplets, the relation between the second ink droplets and the third ink droplets, the relation between the second ink droplets and the fourth ink droplets, and the relation between the third ink droplets and the fourth ink droplets at the fourth time T24. This more reliably suppresses bleeding or the like caused among the first ink, the second ink, the third ink, and the fourth ink.
Meanwhile, the dynamic surface tension of each ink may vary depending on the type of the base material 9. Fig. 7A is a diagram showing ink droplets on one type of base material 9, and Fig. 7B is a diagram showing ink droplets on a different type of base material 9a. The left side in Figs. 7A and 7B shows a first ink droplet D1 at the first time, and the right side shows the first ink droplet D1 and a second ink droplet D2 at the second time. Symbols "A" and "B" within the ink droplets D1 and D2 indicate components A and B, respectively, contained in the ink droplets D1 and D2.
In general, inkjet printers use ink that contains various components (e.g., pigment dispersion, glycerin, or propylene glycol). The ease of penetration into the base material varies depending on the component. For example, solid contents such as pigments basically remain on the first surface without penetrating into the base material, but solvents, surface-active agents and the like easily penetrate into the base material. Thus, the ratio of the components in the ink droplets changes after the ink droplets have landed on the base material. Besides, the presence or absence and degree of penetration of each component into the base material may become noticeable depending on the type of the base material (e.g., whether the base material is film-based or paper-based).
Here, in the case where the component that particularly easily penetrates into the base material is given as a component A and the component that does not easily penetrate into the base material is given as a component B, the ratio of the components A and B contained in the ink droplets after landing varies depending on the type of the base material. For example, the ratio of the components A and B contained at the second time in the first ink droplets D1 that have landed on the base material 9 at the first time as shown in Fig. 7A is different from the ratio of the components A and B contained at the second time in the first ink droplets D1 that have landed on the base material 9a at the first time as shown in Fig. 7B. Therefore, the first-and-second ink surface tension difference at the second time (i.e., the difference in dynamic surface tension between the first ink droplets D1 and the second ink droplets D2) differs between the base materials 9 and 9a. In the examples shown in Figs. 7A and 7B, the first-and-second ink surface tension difference on the base material 9a is greater than the first-and-second ink surface tension difference on the base material 9.
Even in this case, in the inkjet printer 1, the first-and-second ink surface tension difference can be made smaller than the first-and-second ink surface tension difference when the regulation processing is not executed, by the surface-tension-difference regulator 830 executing the regulation processing (here, the ink-characteristic change processing). As a result, it is possible to suppress bleeding or the like caused between the first ink and the second ink, irrespective of the type of the base material or the like. The same also applies to the other ink.
In the inkjet printer 1, the surface-tension-difference regulator 830 may include the gap change mechanisms 44a to 44d (see Fig. 2) and may execute gap change processing for changing the size of clearance between the base material 9 and the ejection ports of the heads 41a to 41d (i.e., the gap from the heads 41a to 41d). For example, the flying time of the first ink droplets can be lengthened by the gap change mechanism 44a increasing the gap from the first head 41a. This increases the amount of time from immediately after the ejection at which the dynamic surface tension starts decreasing until the landing on the target position of the base material 9. Accordingly, the dynamic surface tension of the first ink droplets at the second time becomes slightly lower than the dynamic surface tension thereof when no change is made to the gap. The flying time of the first ink droplets can also be shortened by the gap change mechanism 44a reducing the gap from the first head 41a. This reduces the amount of time from immediately after the ejection to the landing on the target position of the base material 9. Accordingly, the dynamic surface tension of the first ink droplets at the second time becomes slightly higher than the dynamic surface tension thereof when no change is made to the gap.
Moreover, the flying time of the second ink droplets can be lengthened by the gap change mechanism 44b increasing the gap from the second head 41b. This increases the amount of time from immediately after the ejection at which the dynamic surface tension starts decreasing until the landing on the target position of the base material 9. Accordingly, the dynamic surface tension of the second ink droplets at the second time becomes slightly lower than the dynamic surface tension thereof when no change is made to the gap. The flying time of the second ink droplets can also be shortened by the gap change mechanism 44b reducing the gap from the second head 41b. This reduces the amount of time from immediately after the ejection to the landing on the target position of the base material 9. Accordingly, the dynamic surface tension of the second ink droplets at the second time becomes slightly higher than the dynamic surface tension thereof when no change is made to the gap. As described above, fine adjustments of the first-and-second ink surface tension difference become possible if the regulation processing performed by the surface-tension-difference regulator 830 includes the gap change processing for changing the distance between the base material 9 and at least one of the first head 41a and the second head 41b.
Similarly, fine adjustments of the second-and-third ink surface tension difference become possible if the regulation processing includes the gap change processing for changing the distance between the base material 9 and at least one of the second head 41b and the third head 41c. Also, fine adjustments of the third-and-fourth ink surface tension difference become possible if the regulation processing includes the gap change processing for changing the distance between the base material 9 and at least one of the third head 41c and the fourth head 41d.
The first-and-second ink surface tension difference, the second-and-third ink surface tension difference, and the third-and-fourth ink surface tension difference may be adjusted by changing the flying speed of each ink. In this case, the type of the drive signals input from the drive controller 82 to the heads 41a to 41d, i.e., the shape and/or magnitude of the driving waveform, is changed under the control of the regulation processing controller 83. The magnitude of the driving waveform includes a peak value or a pulse width. For example, the techniques described in Japanese Patent Application Laid-Open Nos. 2011-79294 , 2011-161911 , and 2016-36930 (Documents 3 to 5 described above) are applicable to changing the driving waveform.
The flying time of the first ink droplets can be lengthened by changing the driving waveform so as to reduce the flying speed of the first ink from the first head 41a. This increases the amount of time from immediately after the ejection at which the dynamic surface tension starts decreasing until the landing on the target position of the base material 9. Accordingly, the dynamic surface tension of the first ink droplets at the second time becomes slightly lower than the dynamic surface tension thereof when no change is made to the driving waveform. The flying time of the first ink droplets can be also shortened by increasing the flying speed of the first ink from the first head 41a. This reduces the amount of time from immediately after the ejection until the landing on the target position of the base material 9. Accordingly, the dynamic surface tension of the first ink droplets at the second time becomes slightly higher than the dynamic surface tension thereof when no change is made to the driving waveform.
The flying time of the second ink droplets can be lengthened by changing the driving waveform so as to reduce the flying speed of the second ink from the second head 41b. This increases the amount of time from immediately after the ejection at which the dynamic surface tension starts decreasing until the landing on the target position of the base material 9. Accordingly, the dynamic surface tension of the second ink droplets at the second time becomes slightly lower than the dynamic surface tension thereof when no change is made to the driving waveform. The flying time of the second ink droplets can also be shortened by increasing the flying speed of the second ink from the second head 41b. This reduces the amount of time from immediately after the ejection to the landing on the target position of the base material 9. Accordingly, the dynamic surface tension of the second ink droplets at the second time becomes slightly higher than the dynamic surface tension thereof when no change is made to the driving waveform. As described above, the regulation processing performed by the surface-tension-difference regulator 830 includes waveform change processing for changing the shape and/or magnitude of the driving waveform that is input to at least one of the first head 41a and the second head 41b. This changes the flying speed of the ink and thereby allows fine adjustments of the first-and-second ink surface tension difference.
Similarly, fine adjustments of the second-and-third ink surface tension difference become possible if the regulation processing includes waveform change processing for changing the shape and/or magnitude of the driving waveform that is input to at least one of the second head 41b and the third head 41c. Moreover, fine adjustments of the third-and-fourth ink surface tension difference become possible if the regulation processing includes waveform change processing for changing the shape and/or magnitude of the driving waveform that is input to at least one of the third head 41c and the fourth head 41d.

Embodiment 2

Fig. 8 is a diagram showing part of an inkjet printer 1a according to Embodiment 2 of the present invention and shows an internal configuration of the head unit 4 and the vicinity of the head unit 4. The inkjet printer 1a includes a plurality of counter rollers 22, instead of the temperature regulating rollers 21a to 21c shown in Fig. 2, and a plurality of (three in the example shown in Fig. 8) path-length change units 26a to 26c. The other constituent elements are identical to those of the inkjet printer 1 described above, and the identical constituent elements are given the same reference signs.
The counter rollers 22 configure part of the transport mechanism 2 and are aligned in the Y direction at positions facing the head unit 4. To be more specific, the counter rollers 22 are arranged on the (-Y) and (+Y) sides of respective heads 41a to 41c such that the base material 9 directly face the ejection ports of the heads 41a to 41c. The counter rollers 22 are in contact with the second surface of the base material 9. In the example shown in Fig. 8, the surface of each of the heads 41a to 41c where the ejection ports are formed is approximately parallel to an XY plane, but as in the case shown in Fig. 2, the heads 41a to 41c may be arranged at an angle such that the surface where the ejection ports are formed is along a cylindrical surface having a central axis parallel to the X direction as its center. In this case, the counter rollers 22 are arranged also along the cylindrical surface.
The path-length change units 26a to 26c are aligned in the Y direction at positions directly facing the head unit 4. Hereinafter, in order to distinguish the three path-length change units 26a to 26c from one another, the path-length change unit 26a located farthest to the (-Y) side is referred to as a "first path-length change unit 26a", the path-length change unit 26b located adjacent to the (+Y) side of the first path-length change unit 26a is referred to as a "second path-length change unit 26b", and the path-length change unit 26c located adjacent to the (+Y) side of the second path-length change unit 26b is referred to as a "third path-length change unit 26c". Focusing on only the Y direction, the first path-length change unit 26a is arranged between the first head 41a and the second head 41b, the second path-length change unit 26b is arranged between the second head 41b and the third head 41c, and the third path-length change unit 26c is arranged between the third head 41c and the fourth head 41d.
Each of the path-length change units 26a to 26c includes a turn-back unit 261 and a turn-back-position change mechanism 266. For example, the turn-back unit 261 used may be a structural component similar to the air turn bar described in Japanese Patent Application Laid-Open No. 2023-139840 (Document 6 described above). In one example, the turn-back unit 261 has a first support peripheral face 263 and a second support peripheral face 264. The first support peripheral face 263 is part of the cylindrical surface having a central axis parallel to the X direction as its center and corresponds to a portion of the cylindrical surface at which the outward-facing normal changes from the (-Y) direction into the (-Z) direction. The second support peripheral face 264 is part of the cylindrical surface having a central axis parallel to the X direction as its center and corresponds to a portion of the cylindrical surface at which the outward-facing normal changes from the (-Z) direction into the (+Y) direction.
The base material 9 is put on the first and second support peripheral faces 263 and 264. In actuality, the first and second support peripheral faces 263 and 264 face the first surface of the base material 9 on which the ink ejected from the heads 41a to 41c land. Each of the first and second support peripheral faces 263 and 264 has a plurality of jet openings from which a predetermined gas (e.g., air) is jetted out. The jetting of the gas allows the first and second support peripheral faces 263 and 264 to substantially support the base material 9 with slight clearance from the first surface (i.e., without contact). The first support peripheral face 263 changes the direction of the transport path for the base material 9 from the (-Z) direction to the (+Y) direction. The second support peripheral face 264 changes the direction of the transport path for the base material 9 from the (+Y) direction to the (+Z) direction. In this way, the turn-back unit 261 turns back the transport path for the base material 9. The turn-back unit 261 may have any other structure and may, for example, have a U-shaped groove. Such a turn-back unit has a suction port in the inner surface of the groove, and the first surface is sucked up slowly so that the base material 9 curves along the inner surface of the groove. Even such a turn-back unit is capable of turning back the transport path for the base material 9 without any contact with the base material 9.
The turn-back-position change mechanism 266 includes, for example, a ball screw mechanism and an electric motor and is capable of moving the turn-back unit 261 in the Z direction. Alternatively, the turn-back-position change mechanism 266 may use a linear motor or the like to move the turn-back unit 261. When the turn-back unit 261 is moved by the turn-back-position change mechanism 266 of the first path-length change unit 26a, the path length of a section between the first head 41a and the second head 41b (hereinafter, referred to as a "first-and-second head path length") is changed in the transport path for the base material 9.
For example, the first-and-second head path length is increased when the turn-back unit 261 is moved downward by the turn-back-position change mechanism 266 of the first path-length change unit 26a. This lengthens the period of time from after the first ink droplets ejected from the first head 41a are deposited on the first surface of the base material 9 until the first ink droplets reach the position facing the second head 41b, i.e., the period of time between the first time and the second time. As a result, the dynamic surface tension of the first ink droplets at the second time becomes lower than the dynamic surface tension thereof when no change is made to the first-and-second head path length. The first-and-second head path length can also be shortened when the turn-back unit 261 is moved upward by the turn-back-position change mechanism 266 of the first path-length change unit 26a. This shortens the period of time between the first time and the second time. As a result, the dynamic surface tension of the first ink droplets at the second time becomes higher than the dynamic surface tension thereof when no change is made to the first-and-second head path length.
Like the first path-length change unit 26a, the turn-back-position change mechanism 266 of the second path-length change unit 26b moves the turn-back unit 261 so as to change the path length of a section between the second head 41b and the third head 41c (hereinafter, referred to as a "second-and-third head path length") in the transport path for the base material 9. Accordingly, the dynamic surface tension of the second ink droplets at the third time can be made higher or lower than the dynamic surface tension thereof when no change is made to the second-and-third head path length. Moreover, the turn-back-position change mechanism 266 of the third path-length change unit 26c moves the turn-back unit 261 so as to change the path length of a section between the third head 41c and the fourth head 41d (hereinafter, referred to as a "third-and-fourth head path length") in the transport path for the base material 9. Accordingly, the dynamic surface tension of the third ink droplets at the fourth time can be made higher or lower than the dynamic surface tension thereof when no change is made to the third-and-fourth head path length.
In the print processing performed by the inkjet printer 1a shown in Fig. 8, path-length change processing for changing the above-described head path length is executed as the regulation processing in step S21 shown in Fig. 3. The first-and-second head path length, the second-and-third head path length, and the third-and-fourth head path length are determined in advance according to the transport speed of the base material 9. The dynamic surface tension characteristics of the first to fourth ink droplets in the present processing example are the same as those in Fig. 5. In the present processing example, the base material 9 is transported at the second transport speed higher than the first transport speed used during the basic operations.
Here, if the regulation processing in step S21 is not executed (i.e., no change is made to the above-described head path length), the first-and-second ink surface tension difference will increase (see the white arrow A22 in Fig. 5) because the period of time between the first time and the second time is shorter than that during the basic operations. Similarly, the second-and-third ink surface tension difference will increase (see the white arrow A23 in Fig. 5) because the period of time between the second time and the third time is shortened, and the third-and-fourth ink surface tension difference will increase (see the white arrow A24 in Fig. 5) because the period of time between the third period and the fourth period is shortened.
In the actual inkjet printer 1a, the regulation processing controller 83 and the path-length change units 26a to 26c execute the regulation processing (here, the path-length change processing) in cooperation with each other so as to make the first-and-second head path length, the second-and-third head path length, and the third-and-fourth head path length longer than those during the basic operations. Here, the period of time between the first time and the second time, the period of time between the second time and the third time, and the period of time between the third time and the fourth time are approximately the same as those during the basis operations, and the first-and-second ink surface tension difference, the second-and-third ink surface tension difference, and the third-and-fourth ink surface tension difference are also approximately the same as those during the basic operations. In other words, the first-and-second ink surface tension difference is smaller than the first-and-second ink surface tension difference when the regulation processing is not executed, and the second-and-third ink surface tension difference is smaller than the second-and-third ink surface tension difference when the regulation processing is not executed. Moreover, the third-and-fourth ink surface tension difference is smaller than the third-and-fourth ink surface tension difference when the regulation processing is not executed.
As a result, it is possible to suppress bleeding or the like caused at the boundaries of the areas of the first ink, the second ink, the third ink, and the fourth ink on the base material 9 and to improve the quality of printed matter created by the inkjet printer 1a. In the inkjet printer 1a, the regulation processing controller 83 and the path-length change units 26a to 26c serve as the surface-tension-difference regulator 830 that executes the regulation processing. Note that the second transport speed may be lower than the first transport speed. In this case, the first-and-second head path length, the second-and-third head path length, and the third-and-fourth head path length are shortened by the path-length change processing. Alternatively, the path-length change processing may be performed in order to change the type of the base material.
As described above, in the inkjet printer 1a shown in Fig. 8, the surface-tension-difference regulator 830 executes, as the regulation processing, the path-length change processing for changing the path length of the section between the first head 41a and the second head 41b in the transport path for the base material 9. Thus, the first-and-second ink surface tension difference when the regulation processing is executed becomes smaller than the first-and-second ink surface tension difference when the regulation processing is not executed. The inkjet printer 1a is capable of suppressing bleeding or the like caused between the first ink and the second ink, irrespective of the transport speed, the type of the base material, or the like. The same also applies to the second-and-third ink surface tension difference and the third-and-fourth ink surface tension difference.
Preferably, the path-length change processing involves changing the position of the turn-back unit 261 that is arranged in the aforementioned section of the transport path for the base material 9 and that turns back the transport path. Accordingly, it is possible to change the path length by a simple configuration and to easily realize the regulation processing that affects the dynamic surface tension of the first ink droplets at the second time. In the case where an air turn bar serves as the turn-back unit 261, it is possible to easily realize the path-length change units 26a to 26c that change the path length without any contact with the first surface of the base material 9.
In the inkjet printer 1a, as in the inkjet printer 1 shown in Fig. 1, the dynamic surface tension of the first ink droplets at the second time may be made approximately the same as the dynamic surface tension of the second ink droplets at the second time by execution of the regulation processing (here, the path-length change processing). The same also applies to the relation of the first to third ink droplets at the third time and the relation of the first to fourth droplets at the fourth time. Besides, the regulation processing may include the gap change processing, or may include the waveform change processing.
In the inkjet printer 1a shown in Fig. 8, the ink temperature regulators 43a to 43d may change the temperature of each ink stored in the ink tanks 42a to 42d. That is, the regulation processing that is executed by the surface-tension-difference regulator 830 may include the ink-characteristic change processing in addition to the path-length change processing. In this case, the temperature of each ink on the base material 9 may be changed by, for example, a heater 27 indicated by the broken line in Fig. 8. As described above, the surface-tension-difference regulator 830 of the inkjet printer may execute, as the regulation processing, the ink-characteristic change processing and/or the path-length change processing, the ink-characteristic change processing being processing for changing the dynamic surface tension characteristics of at least one of the first ink and the second ink, the path-length change processing being processing for changing the path length of the section between the first head 41a and the second head 41b in the transport path for the base material 9.
The inkjet printers 1 and 1a and the printing method described above may be modified in various ways.
The transport speed of the base material 9 does not necessarily have to be constant, and may be changed in succession or in stages. Even in this case, the occurrence of bleeding or the like between the first ink and the second ink on the base material 9 can be suppressed by changing, according to a change in transport speed, the regulation processing to be performed by the surface-tension-difference regulator 830 (e.g., by changing the temperature of each ink or the path length).
Depending on the designs of the inkjet printers 1 and 1a, for example, the gap change processing for changing the distance between the base material 9 and each of the heads 41a to 41d may be implemented by moving the base material 9 in the Z direction under the head unit 4.
The base material 9 does not necessarily have to be a continuous sheet-like printing medium, and may be an individually cut sheet-like printing medium. In this case, for example, the transport mechanism 2 may, for example, be a mechanism for moving a stage that holds the individually cut sheet-like printing medium.
The base material 9 may be formed of a material other than paper or a resin. Ink bleeding or the like caused by a difference in dynamic surface tension is likely to occur in particular with an ink-resistant base material (a base material impermeable to ink). Therefore, the inkjet printers 1 and 1a described above are in particular suitable for image printing on an ink-resistant base material. Examples of such a base material include base materials formed of plastic such as PET, glass, or metal and base materials having a surface layer that is impermeable to ink (e.g., coated paper).
The configurations of the above-described preferred embodiments and variations may be appropriately combined as long as there are no mutual inconsistencies.
While the invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations can be devised without departing from the scope of the invention.

REFERENCE SIGNS LIST

1, 1a: inkjet printer
2: transport mechanism
4: head unit
9, 9a: base material
41a to 41d: head
261: turn-back unit
830: surface-tension-difference regulator
D1, D2: ink droplets
S11 to S13, S21: step

Claims

An inkjet printer (1, 1a) comprising:
a transport mechanism (2) that transports a base material (9, 9a);

a head unit (4) that includes a first head (41a to 41d) and a second head (41a to 41d) arranged in order from an upstream side along a transport path for said base material, said first head ejecting first ink toward said base material, said second head ejecting second ink toward said base material; and

a surface-tension-difference regulator (830) that executes ink-characteristic change processing and/or path-length change processing as regulation processing, the ink-characteristic change processing being processing for changing a dynamic surface tension characteristic of at least one ink out of said first ink and said second ink, the path-length change processing being processing for changing a path length of a section between said first head and said second head in said transport path for said base material,

wherein, in a case where a time when a first ink droplet lands on a target position of said base material is given as a first time, the first ink droplet being a droplet of said first ink, a time when a second ink droplet lands on said target position is given as a second time, the second ink droplet being a droplet of said second ink, and an absolute value of a difference between a dynamic surface tension of said first ink droplet at said second time and a dynamic surface tension of said second ink droplet at said second time is given as a first-and-second ink surface tension difference, said first-and-second ink surface tension difference when said regulation processing is executed is smaller than said first-and-second ink surface tension difference when said regulation processing is not executed.
The inkjet printer according to claim 1, wherein
said regulation processing includes said ink-characteristic change processing,

said ink-characteristic change processing involves changing a temperature of said at least one ink stored in said head unit and/or a temperature of said first ink droplet on said base material.
The inkjet printer according to claim 1 or 2, wherein
said regulation processing includes said path-length change processing, and

said path-length change processing involves changing a position of a turn-back unit (261) that is arranged in said section of said transport path and that turns back said transport path.
The inkjet printer according to claim 3, wherein
said turn-back unit is an air turn bar.
The inkjet printer according to any one of claims 1 to 4, wherein
said regulation processing further includes gap change processing for changing a distance between said base material and at least one head out of said first head and said second head.
The inkjet printer according to any one of claims 1 to 5, wherein
said regulation processing further includes waveform change processing for changing a shape and/or a magnitude of a driving waveform that is input to at least one head out of said first head and said second head.
The inkjet printer according to any one of claims 1 to 6, wherein
said first-and-second-ink surface tension difference when said regulation processing is executed during transport of said base material at one transport speed is approximately equal to said first-and-second-ink surface tension difference during transport of said base material at a transport speed different from said one transport speed.
The inkjet printer according to any one of claims 1 to 7, wherein
said head unit further includes a third head (41a to 41d) arranged downstream of said second head along said transport path, said third head ejecting third ink toward said base material,

in a case where a time when a third ink droplet lands on said target position is given as a third time, the third ink droplet being a droplet of said third ink, said surface-tension-difference regulator executes regulation processing that affects a dynamic surface tension of at least one of said second ink droplet and said third ink droplet at said third time, and

in a case where an absolute value of a difference between a dynamic surface tension of said second ink droplet at said third time and a dynamic surface tension of said third ink droplet at said third time is given as a second-and-third ink surface tension difference, said second-and-third ink surface tension difference when said regulation processing is executed is smaller than said second-and-third ink surface tension difference when said regulation processing is not executed.
The inkjet printer according to claim 8, wherein
said head unit further includes a fourth head (41a to 41d) arranged downstream of said third head along said transport path, said fourth head ejecting fourth ink toward said base material,

in a case where a time when a fourth ink droplet lands on said target position is given as a fourth time, the fourth ink droplet being a droplet of said fourth ink, said surface-tension-difference regulator executes regulation processing that affects a dynamic surface tension of at least one of said third ink droplet and said fourth ink droplet at said fourth time, and

in a case where an absolute value of a difference between the dynamic surface tension of said third ink droplet at said fourth time and the dynamic surface tension of said fourth ink droplet at said fourth time is given as a third-and-fourth ink surface tension difference, the third-and-fourth ink surface tension difference when said regulation processing is executed is smaller than said third-and-fourth ink surface tension difference when said regulation processing is not executed.
The inkjet printer according to any one of claims 1 to 9, wherein
the dynamic surface tension of said second ink droplet at said second time is made approximately equal to the dynamic surface tension of said first ink droplet at said second time by execution of said regulation processing.
The inkjet printer according to claim 10, wherein
said head unit further includes a third head (41a to 41d) arranged downstream of said second head along said transport path, said third head ejecting third ink toward said base material,

in a case where a time when a third ink droplet lands on said target position is given as a third time, the third ink droplet being a droplet of said third ink, said surface-tension-difference regulator executes regulation processing that affects a dynamic surface tension of at least one of said first ink droplet and said third ink droplet at said third time, and

said regulation processing makes the dynamic surface tension of said third ink droplet at said third time approximately equal to the dynamic surface tension of said first ink droplet at said third time.
The inkjet printer according to claim 11, wherein
said head unit further includes a fourth head (41a to 41d) arranged downstream of said third head along said transport path, said fourth head ejecting fourth ink toward said base material,

in a case where a time when a fourth ink droplet lands on said target position is given as a fourth time, the fourth ink droplet being a droplet of said fourth ink, said surface-tension-difference regulator executes regulation processing that affects a dynamic surface tension of at least one of said first ink droplet and said fourth ink droplet at said fourth time, and

said regulation processing makes the dynamic surface tension of said fourth ink droplet at said fourth time approximately equal to the dynamic surface tension of said first ink droplet at said fourth time.
A printing method using a head unit (4) for inkjet printing, the printing method comprising:
a) transporting a base material (9, 9a) (S11);

b) in parallel with said operation a), ejecting first ink from a first head (41a to 41d) of said head unit toward said base material and ejecting second ink from a second head (41a to 41d) of said head unit toward said base material, said first head and said second head being arranged in order from an upstream side along a transport path for said base material in said head unit (S12); and

c) executing ink-characteristic change processing and/or path-length change processing as regulation processing, the ink-characteristic change processing being processing for changing a dynamic surface tension characteristic of at least one ink out of said first ink and said second ink, the path-length change processing being processing for changing a path length of a section between said first head and said second head in said transport path for said base material (S21),
wherein, in a case where a time when a first ink droplet lands on a target position of said base material is given as a first time, the first ink droplet being a droplet of said first ink, a time when a second ink droplet lands on said target position is given as a second time, the second ink droplet being a droplet of said second ink, and an absolute value of a difference between a dynamic surface tension of said first ink droplet at said second time and a dynamic surface tension of said second ink droplet at said second time is given as a first-and-second ink surface tension difference, said first-and-second ink surface tension difference when said regulation processing is executed is smaller than said first-and-second ink surface tension difference when said regulation processing is not executed.