Inkjet printing system and image forming method

Abstract

Provided is an inkjet printing system including an ink and an inkjet printing device. The ink is a water-based ink having a dynamic surface tension at 50 ms of 23.0-33.0 mN / m. The inkjet printing device includes a conveyance mechanism that conveys a recording medium, an inkjet head that discharges the ink to a recording surface of the recording medium conveyed by the conveyance mechanism, and a charge control mechanism that maintains the recording medium in a state of being charged to a polarity opposite to the polarity of the charge of ink mist generated when the ink is discharged. The charge control mechanism sets the absolute value of the charge amount of the recording medium to 2.5 kV or more. Also provided is an image forming method.

Description

Inkjet Printing System and Image Forming Method

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

[0002] There is known a printing apparatus using an inkjet method for forming an image by discharging a liquid such as ink. In an inkjet printing apparatus, when a main drop of ink is discharged, a spray-like ink mist may occur. When this ink mist adheres to a nozzle surface where a discharge port (hereinafter also referred to as a nozzle) is formed in an inkjet head (hereinafter referred to as a head), ink discharge failure may occur. Further, the ink mist may adhere to other components in the printing apparatus, soiling the inside of the apparatus and leading to a reduction in the durability of the apparatus.

[0003] When conveying a recording medium, the recording medium may be charged by rubbing against a conveying roll or the like. This charging of the recording medium causes repulsion and floating of the ink mist discharged from the nozzle, which is one of the factors causing the ink mist to adhere to the nozzle surface and soiling the inside of the apparatus.

[0004] Japanese Patent Application Laid-Open No. 2018-183899 proposes an inkjet printing apparatus provided with an ion generator upstream in the conveying direction from a recording unit including an inkjet head and an electrostatic sensor in the recording unit. Then, Japanese Patent Application Laid-Open No. 2018-183899 proposes a technique for controlling the charge amount of a recording medium by reading the charge amount with an electrostatic sensor and adjusting the charge amount applied to the recording medium by the ion generator from the value.

[0005] Further, Japanese Patent Application Laid-Open No. 2007-331186 describes that the potential of a recording medium is controlled to have the opposite polarity to the charge of an ink drop. Specifically, it is described that the potential of the paper is controlled to be -200 V or -2 kV.

[0006] In recent years, there has been a growing demand for faster printing speeds. To achieve high-speed printing and produce high-quality images, it is necessary to fix and dry the ink on the paper in a relatively short time. Furthermore, ink mist generation becomes more serious during high-speed printing, and the resulting deterioration in image quality due to ejection failures caused by adhesion to the nozzle surface becomes more severe. This is because when paper is transported at high speed to increase printing speed, the upward airflow generated between the inkjet heads by the accompanying air of the recording medium during transport promotes the suspension of ink mist, making it easier for the ink mist to adhere to the nozzle surface and thus promoting ejection failures.

[0007] This disclosure is made in view of the above circumstances and aims to provide an inkjet printing system and image forming method that can form images with good image quality over a long period of time even under high-speed printing conditions.

[0008] The inkjet printing system of this disclosure is an inkjet printing system comprising ink and an inkjet printing apparatus, wherein the ink is a water-based ink having a dynamic surface tension of 23.0 mN / m to 33.0 mN / m at 50 ms, and the inkjet printing apparatus comprises a transport mechanism for transporting a recording medium, an inkjet head for ejecting ink onto the recording surface of the recording medium transported by the transport mechanism, and a charge control mechanism for maintaining the recording medium in a state where it is charged to a polarity opposite to the polarity of the charge of the ink mist generated when the ink is ejected, wherein the charge control mechanism sets the absolute value of the charge of the recording medium to 2.5 kV or more.

[0009] The recording medium is preferably a roll-shaped substrate, and the transport mechanism is preferably a transport mechanism that transports the recording medium roll-to-roll.

[0010] The charge control mechanism preferably includes a charging unit that charges the recording medium, which is located upstream of the inkjet head in the transport direction of the recording medium, and a transport mechanism that transports the recording medium without bringing it into contact with a conductive material from the charged position where the recording medium is charged by the charging unit to the ejection end position where the ink ejection by the inkjet head ends.

[0011] The transport mechanism comprises a plurality of transport members that come into contact with the recording medium, and preferably, the surface of at least the transport member installed between the charging position and the ejection end position that comes into contact with the recording medium is non-conductive and is made of a material that is relatively prone to being charged with the same polarity as the ink mist in relation to the recording medium in the triboelectric series.

[0012] Preferably, in the transport direction of the recording medium, a sensor for measuring the amount of charge on the recording medium is provided downstream of the charging position, and the charging control mechanism adjusts the amount of charge applied when charging the recording medium in the charging section based on the amount of charge measured by the sensor.

[0013] Preferably, the system further includes a drying section located downstream of the inkjet head in the transport direction of the recording medium for drying the ink, and a static elimination mechanism located between the ejection end position and the drying section to remove static charge from the recording medium.

[0014] In the transport direction of the recording medium, a sensor for measuring the amount of charge on the recording medium may be provided downstream of the charging position and upstream of the static elimination mechanism, and it is preferable that the charging control mechanism adjusts the amount of charge applied when charging the recording medium in the charging section based on the amount of charge measured by the sensor.

[0015] It is preferable that the inkjet head is an ink-recirculating type ink head.

[0016] It is preferable that the transport speed of the recording medium is 160 m / min or more.

[0017] The flight speed of the ink ejected from the inkjet head is preferably 8 m / second or higher.

[0018] The inkjet printing apparatus may be a double-sided printer that uses the first and second surfaces, which are the front and back surfaces of the recording medium, as recording surfaces, and the charge control mechanism may be applied only when ejecting ink to either the first or second surface.

[0019] The inkjet printing apparatus may be a double-sided printer that uses the first and second sides of the recording medium as recording surfaces, and the charge control mechanism may be applied to both the case where ink is ejected to the first side and the case where ink is ejected to the second side.

[0020] The present disclosure relates to an image forming method in an inkjet printing apparatus comprising a transport mechanism for transporting a recording medium and an inkjet head for ejecting ink onto the recording surface of the recording medium transported by the transport mechanism, wherein an aqueous ink having a dynamic surface tension of 23.0 mN / m to 33.0 mN / m at 50 ms is used as the ink, and the recording medium is transported with its recording surface facing the inkjet head while being charged with a polarity opposite to the polarity of the charge of the ink mist generated when the ink is ejected, and the absolute value of the charge of the recording medium is maintained at 2.5 kV or more.

[0021] In the image forming method disclosed herein, it is preferable to transport the recording medium roll-to-roll.

[0022] In the image forming method of the present disclosure, it is preferable to charge the recording medium upstream of the inkjet head in the transport direction of the recording medium, and to transport the recording medium without bringing it into contact with a conductive material from the charged position to the ejection completion position where the ink ejection by the inkjet head ends.

[0023] According to the inkjet printing system or image forming method of this disclosure, it is possible to form images with good image quality over a long period of time, even under high-speed printing conditions.

[0024] This is a schematic diagram showing the overall configuration of an inkjet printing system according to an embodiment of the technology disclosed herein. This is a schematic diagram showing a modified example of an inkjet printing apparatus.

[0025] Embodiments of the technology described herein will be described below with reference to the drawings. Note that the dimensional ratios in the drawings are exaggerated for illustrative purposes and may differ from actual ratios.

[0026] Figure 1 is an overall configuration diagram of an inkjet printing system 1 according to one embodiment of the present disclosure. The inkjet printing system 1 includes an inkjet printing device 2 and ink used in the inkjet printing device 2. The ink is ejected from an inkjet head, which will be described later, and is used to print on a recording medium after being ejected by the inkjet head. In this example, the inkjet printing device 2 is a single-sided printing device that transports a roll of continuous paper S in a roll-to-roll manner and forms an image on one side of the continuous paper S. The roll of continuous paper (roll paper) S is an example of a recording medium, and the roll paper is an example of a roll-shaped substrate. The substrate is not limited to paper, but may also be a resin sheet, a metal sheet, or a fiber sheet such as cloth.

[0027] The ink is stored in an ink tank (not shown) connected to the inkjet head of the inkjet printing device 2. The ink is a water-based ink with a dynamic surface tension of 23.0 mN / m to 33.0 mN / m at 50 ms. Preferably, the dynamic surface tension at 50 ms is 25.0 mN / m to 33.0 mN / m, and more preferably 26.0 mN / m to 32.0 mN / m. Water-based ink is an ink in which colorants such as pigments and dyes are dissolved or dispersed in water and / or a water-soluble solvent, and is sometimes called aqueous ink. The static surface tension of the ink is preferably 21.0 mN / m to 31.0 mN / m, and more preferably 24.0 mN / m to 30.0 mN / m. The electrical conductivity is preferably 15 mS / m to 400 mS / m, and more preferably 20.0 mS / m to 300 mS / m. Here, dynamic surface tension is assumed to have been measured using the maximum pressure method with a Kruss Bubble Pressure Tensometer. Static surface tension is assumed to have been measured with a Kyowa Interface Science Co., Ltd. Automatic Surface Tensometer CBVP-Z. Electrical conductivity is assumed to have been measured with a Toa DK Corporation EC Meter CM-41X.

[0028] The inkjet printing apparatus 2 includes a transport mechanism 10 for transporting continuous paper S, a head unit 20, a charge control mechanism 30, and a drying unit 40.

[0029] The transport mechanism 10 is a transport mechanism that transports continuous paper S in a roll-to-roll manner. The transport mechanism 10 comprises a paper feed roller 12, pass rollers 13 to 16, and a take-up roller 18. The continuous paper S is fed out from the paper feed roller 12 and taken up by the take-up roller 18. The pass rollers 13 to 16 suppress flapping of the continuous paper S and guide the transport of the continuous paper S. In Figure 1, the direction in which the continuous paper S is transported is indicated by a dashed arrow. The transport speed of the continuous paper S by the transport mechanism 10 is preferably 160 m / min or more. In the following, the unit [m / min] may be written as [mpm].

[0030] The head unit 20 is positioned on the transport path of continuous paper S transported by the transport mechanism 10. The head unit 20 includes inkjet heads 21C, 21M, 21Y, and 21K. Inkjet head 21C is a recording head that ejects droplets of cyan ink. Inkjet head 21M is a recording head that ejects droplets of magenta ink. Inkjet head 21Y is a recording head that ejects droplets of yellow ink. Inkjet head 21K is a recording head that ejects droplets of black ink. Each of the inkjet heads 21C, 21M, 21Y, and 21K is supplied with the aforementioned ink from an ink tank (not shown), which is an ink supply source for the corresponding color, via a piping route (not shown). In the following, when it is not necessary to distinguish between inkjet heads 21C, 21M, 21Y, and 21K, they may simply be referred to as inkjet head 21. The inkjet head 21 is positioned so that the surface equipped with ink ejection nozzles (hereinafter sometimes referred to as the nozzle surface) faces the continuous paper S. The inkjet head 21 ejects ink onto the recording surface of the continuous paper S, which is transported by the transport mechanism 10. An image is formed on the recording surface of the continuous paper S by the ejection of ink by the inkjet head 21. Preferably, the flight speed of the ink ejected from the inkjet head is 8 m / second or more. Here, the flight speed of the ink is the initial velocity of the ink ejected from the nozzle. Increasing the flight speed of the ink enables high-speed transport.

[0031] In this example, a configuration using four CMYK inks is illustrated, but the combination of ink colors and the number of colors is not limited to this embodiment, and light inks, dark inks, spot inks, etc. may be added as needed. For example, it is possible to add an inkjet head that ejects light inks such as light cyan and light magenta, and / or an inkjet head that ejects a spot ink such as green, orange, or white. Furthermore, there are no particular limitations on the arrangement order of the inkjet heads for each color.

[0032] The charge control mechanism 30 maintains the recording medium (in this case, continuous paper S) in a state where it is charged with a polarity opposite to that of the ink mist generated when ink is ejected from the inkjet head 21. The charge control mechanism 30 maintains the absolute value of the charge of the continuous paper S at 2.5 kV or higher. The polarity of the charge of the ink mist is determined by printing conditions, including the ink material, the material of the inkjet head, the configuration of the ejection drive, and the drive waveform. Depending on the printing conditions, the ink mist may be positively charged or negatively charged. For example, if the nozzle and the ink are insulating, friction occurs when the ink is ejected, and the inner surface of the nozzle and the ink become triboelectrically charged. In this case, if the material is more prone to negative charge than the nozzle in the triboelectric series, the ink will be negatively charged; if the material is more prone to positive charge, the ink will be positively charged. The polarity of the ink mist is determined by the printing conditions when printing on the continuous paper S, and the continuous paper S is set to be charged with a polarity opposite to that polarity. It should be noted that here, charging the continuous paper S and maintaining its charged state does not mean charging the entire surface of the continuous paper S and maintaining the charged state over that entire surface. Rather, it means charging the area to which ink is applied by the inkjet head 21 and maintaining the charged state while ink is being applied. More specifically, the charging control mechanism 30 is charged before ink ejection by the inkjet head 21C, which is located furthest upstream, begins, and the charged area is maintained in a charged state from the position where ink ejection begins until it passes the position where ink ejection by the inkjet head 21K, which is located furthest downstream, ends.

[0033] The charge control mechanism 30 includes a charging unit 32 for charging the continuous paper S. The charging unit 32 is located upstream of the inkjet head 21 in the transport direction of the continuous paper S. Examples of the charging unit 32 include a charging brush that generates an electric charge on the continuous paper S through friction with the continuous paper S, or an ionizer that generates ions to charge the continuous paper S.

[0034] The transport mechanism 10 transports the continuous paper S, which has been charged by the charging unit 32, while maintaining its charged state. For example, the transport mechanism 10 is configured to transport the continuous paper S without contacting conductive material from the charging position where it is charged by the charging unit 32 to the ejection end position where the ink ejection by the inkjet head 21 ends. In this example, the transport mechanism 10 is configured to transport the continuous paper S without contacting conductive material from the charging unit 32 until it passes through the head unit 20, i.e., the inkjet heads 21C, 21M, 21Y, and 21K. In this example, the transport mechanism 10 that transports the continuous paper S without contacting conductive material also serves as part of the charge control mechanism 30. In this example, the transport members that the continuous paper S contacts between the charging unit 32 and the head unit 20 are the pass rollers 14 and 15. At least the surfaces of these pass rollers 14 and 15 that contact the continuous paper S are made of a non-conductive material. In a configuration for transporting continuous paper S without contact with a conductive material, it is more preferable that at least the surfaces of the pass rollers 14 and 15 that contact the continuous paper S are non-conductive and are made of a material that, in relation to the recording medium (in this case, paper) in the triboelectric series, is relatively prone to being charged with the same polarity as ink mist.

[0035] The triboelectric series is a list of materials arranged in order from those that are more likely to become positively charged to those that are more likely to become negatively charged when two materials are rubbed or separated. For example, if the ink mist is positively charged, it is preferable that the surface material of the pass rollers 14 and 15 be a material that is more likely to become positively charged than paper in order to maintain the paper in a negatively charged state. Materials that are more likely to become positively charged than paper include wool, nylon, rayon, silk, cotton, linen, glass fiber, acetate, and polyester. Of these, wool is the most likely to become positively charged, and its likelihood of becoming positively charged decreases in the order of nylon, rayon, ... polyester. On the other hand, if the ink mist is negatively charged, it is preferable that the surface material of the pass rollers 14 and 15 be a material that is more likely to become negatively charged than paper in order to maintain the paper in a positively charged state. Materials that are more likely to become negatively charged than paper include fluororesin, silicone, Teflon (registered trademark), polyvinyl chloride, cellophane, polyethylene, polypropylene, acrylic, polyester, polyurethane, polystyrene, and rubber. Of the materials listed here, fluororesin is the most easily negatively charged, and its tendency to become negatively charged decreases in the order of silicon, Teflon (registered trademark), ... rubber.

[0036] Alternatively, the continuous paper S may be configured without the pass rollers 14 and 15 positioned between the charging section 32 and the head unit 20, so as not to come into contact with the transport components for transport. However, it is preferable to include the pass rollers 14 and 15 to suppress flapping of the continuous paper S. Furthermore, a static discharge section may be provided upstream of the charging section 32. By first discharging the continuous paper S, which is unevenly charged in-plane due to friction in the transport path, and then charging it in the charging section 32, a uniform surface potential can be achieved.

[0037] In the inkjet printing apparatus 2, the charge control mechanism 30 further includes a sensor 34 for measuring the amount of charge on the continuous paper S, and a static elimination mechanism 36 for removing charge from the continuous paper S.

[0038] The sensor 34 is located downstream of the charging position by the charging unit 32 in the transport direction of the continuous paper S. In this embodiment, the sensor 34 is positioned immediately after the head unit 20. The sensor 34 may also be located between the inkjet heads 21C and 21M, between the inkjet head 21M and 21Y, or between the inkjet head 21Y and 21K within the head unit 20. Furthermore, multiple sensors 34 may be provided, with each sensor 34 positioned at a different location in the transport direction.

[0039] Preferably, the charge control mechanism 30 is configured to adjust the amount of charge applied when the charging unit 32 charges the continuous paper S based on the amount of charge measured by the sensor 34. For example, it is preferable to have a control circuit that adjusts the amount of charge applied by the charging unit 32, and when the amount of charge of the continuous paper S is input to the control circuit from the sensor 34, the circuit may be configured to perform feedback control to adjust the charging unit 32 to increase the amount of charge applied by the charging unit 32 if the absolute value of the amount of charge is less than 2.5 kV.

[0040] The static elimination mechanism 36 is positioned between the head unit 20 and the drying section 40. The static elimination mechanism 36 only needs to be positioned between the ejection end position of the inkjet head 21K, located furthest downstream in the head unit 20, and the drying section 40. The static elimination mechanism 36 can be a contact member having a grounded conductive contact surface positioned in contact with the continuous paper S, or an ionizer that generates ions with polarity opposite to the charge polarity of the continuous paper S.

[0041] The drying unit 40 is located downstream of the head unit 20 in the transport direction of the continuous paper S. The drying unit 40 dries the ink ejected onto the recording surface of the continuous paper S by the head unit 20. The drying unit 40 can be a hot air blower, an infrared lamp, an ultraviolet lamp, a macrowave generator, etc., and is not particularly limited as long as it can dry the ink.

[0042] As described above, the inkjet printing system 1 of this embodiment includes ink and an inkjet printing apparatus 2, wherein the ink is a water-based ink having a dynamic surface tension of 23.0 mN / m to 33.0 mN / m at 50 ms. By using a water-based ink having a dynamic surface tension of 23.0 to 33.0 mN / m at 50 ms, the ink's fixation and drying properties during high-speed transport can be improved. Furthermore, the inkjet printing apparatus 2 of this embodiment includes a transport mechanism 10 for transporting a recording medium (continuous paper S in this case), an inkjet head 21 for ejecting ink onto the recording surface of the recording medium transported by the transport mechanism 10, and a charge control mechanism 30 for maintaining the recording medium in a state where it is charged with a polarity opposite to the polarity of the charge of the ink mist generated when the ink is ejected. The charge control mechanism 30 sets the absolute value of the charge of the recording medium to 2.5 kV or more. When using inks with relatively low dynamic surface tension, such as those with a dynamic surface tension of 33.0 mN / m or less as described above, the ligaments of the ink ejected from the nozzle tend to become longer, making it difficult for the ink to form droplets, thus easily generating ink mist. By charging the recording medium with a polarity opposite to that of the ink mist, the floating of the ink mist can be suppressed, and the ink mist can be made to adhere to the recording medium. In particular, by setting the absolute value of the charge to 2.5 kV or more, the force that attracts the ink mist to the recording medium can be strengthened, resulting in a high level of effectiveness in suppressing mist floating. As a result, the adhesion of ink mist to the nozzle surface can be effectively suppressed, and the problem of nozzle blockage and ejection failure due to ink mist adhesion can be suppressed. As a result, image formation with good image quality can be achieved even under high-speed printing conditions.

[0043] In particular, when performing high-speed printing where the transport speed is 160 mph or more and the flight speed of the ink ejected from the inkjet head 21 is 8 m / s or more, ink mist tends to fly up, so the above configuration is highly effective in suppressing the adhesion of ink mist to the nozzle surface.

[0044] In the above inkjet printing apparatus 2, the conveyance mechanism 10 is a conveyance mechanism that conveys the continuous paper S in a roll-to-roll manner. However, the technology of the present disclosure is not limited to this mode. As the recording medium, instead of the continuous paper S, a sheet-fed paper may be applied. When using a sheet-fed paper, the conveyance mechanism may have a configuration that conveys the sheet-fed paper.

[0045] When it is the conveyance mechanism 10 that conveys in a roll-to-roll manner as in the present embodiment, the recording medium becomes easily charged. If the recording medium is charged with the same polarity as the ink mist, the floating of the ink mist becomes remarkable. Therefore, the effect of the charge control mechanism 30 that charges the recording medium with the opposite polarity to the ink mist is remarkable.

[0046] When, as in the present embodiment, the surface of the conveyance member (here, the pass rollers 14 and 15) that contacts the recording medium and is installed between the charging position and the discharge end position is non-conductive and is made of a material that is relatively easily charged with the same polarity as the ink mist in relation to the recording medium in the charging row, it becomes easier to maintain the recording medium in a charged state with the opposite polarity to the ink mist.

[0047] When, as in the present embodiment, the charge control mechanism 30 includes a sensor 34 that measures the charge amount of the recording medium downstream of the charging position of the recording medium, and has a configuration that adjusts the amount of charge imparted to the recording medium in the charging unit 32 based on the measured charge amount, the charge amount of the recording medium can be adjusted to an appropriate charge amount for attracting the ink mist.

[0048] Furthermore, by providing a drying unit 40 that dries the ink and is arranged downstream of the inkjet head in the conveyance direction of the recording medium as in the present embodiment, the ink can be dried quickly.

[0049] Furthermore, by providing a static elimination mechanism 36 downstream of the ink ejection end position by the inkjet head 21 to remove static charge from the recording medium, the adhesion of dust to the recording medium can be suppressed. In the case of a drying section 40 as in this embodiment, it is preferable to position the static elimination mechanism 36 between the ejection end position and the drying section 40. This is because removing static charge at an early stage after ejection can further reduce the risk of dust adhesion.

[0050] Furthermore, in the case where the charge control mechanism 30 is equipped with a static elimination mechanism 36, as in this embodiment, the sensor 34 for measuring the amount of charge can be positioned upstream of the static elimination mechanism 36 to detect the charge state of the recording medium in the head unit 20.

[0051] In the inkjet printing apparatus 2 of this embodiment, the configuration of the inkjet head 21 is not particularly limited, but it is preferable that it be an ink-circulating head. An ink-circulating head is a head that has a structure that recovers and circulates ink that is not ejected from the ink held near the nozzle during ejection. For information on circulating heads, refer to, for example, Japanese Patent Application Publication No. 2024-93608, and other conventionally known technologies can be applied. When the charge of the recording medium becomes large, weak discharges are more likely to occur in the head nozzle. By providing an ink-circulating head, ejection failures caused by bubbles generated in the ink due to weak discharges can be suppressed. In particular, when the absolute value is 2.5 kV or more, bubbles are more likely to occur, so the effect of suppressing ejection failures by using an ink-circulating head is significant.

[0052] In this embodiment, the inkjet printing system 1 is equipped with an inkjet printing device 2 for single-sided printing, but as shown in Figure 2, it may also be equipped with an inkjet printing device 102 for double-sided printing.

[0053] The inkjet printing apparatus 102 in Figure 2 is a double-sided printing machine that uses both the front and back surfaces of a continuous paper S as recording surfaces to form images on both sides. In Figure 2, the same reference numerals are used for components that are the same as those in Figure 1.

[0054] The inkjet printing apparatus 102 comprises a transport mechanism 110, a first printing section 104, and a second printing section 105.

[0055] The transport mechanism 110 is substantially the same as the transport mechanism 10, but in order to transport continuous paper S to two printing units 104 and 105, it includes pass rollers 13A to 16A and 13B to 16B, and a reversing unit 112 positioned between the two printing units 104 and 105. The reversing unit 112 performs a reversal process that swaps the front and back sides of the first side SA and the second side SB of the continuous paper S transported from the first printing unit 104. With the front and back sides reversed, the continuous paper S is transported to the second printing unit 105. The reversing unit 112 can be equipped with any known reversing mechanism, for example, the reversing device described in Japanese Patent Application Publication No. 2017-035870 can be applied.

[0056] The first printing unit 104 and the second printing unit 105 are arranged on a transport path and are equipped with a head unit 20, a charge control mechanism 30, and a drying unit 40, respectively. The first printing unit 104 forms an image on the first surface SA of the continuous paper S that is the surface when it is fed from the paper feed roller 12. For convenience, the surface on which printing is first performed on the continuous paper S fed from the paper feed roller 12 is referred to as the first surface SA, and the surface opposite the first surface SA is referred to as the second surface SB. The second printing unit 105 forms an image on the second surface SB, which has been inverted by the inversion unit 112 to become the surface.

[0057] The charge control mechanism 30 is applied in both cases: when ink is ejected by the inkjet head 21 to the first surface SA, and when ink is ejected by the inkjet head 21 to the second surface SB. That is, in the first printing section 104, the continuous paper S is maintained in a state where it is charged with an absolute value of 2.5 kV or more with a polarity opposite to that of the charge of the ink mist while the ink is being ejected. Similarly, in the second printing section 105, the continuous paper S is maintained in a state where it is charged with an absolute value of 2.5 kV or more with a polarity opposite to that of the charge of the ink mist while the ink is being ejected.

[0058] In the case of an inkjet printing apparatus 102, which has two printing units 104 and 105, the adhesion of mist to the nozzle surface of the inkjet heads 21 of each of the printing units 104 and 105 can be suppressed by providing a charge control mechanism 30 in each of the printing units 104 and 105.

[0059] Furthermore, in the case of an inkjet printing apparatus 102, which has two printing units 104 and 105, the charge control mechanism 30 may be provided in only one of the first printing unit 104 and the second printing unit 105.

[0060] An example of a specific ink preparation method used in the inkjet printing system of this embodiment is shown below.

[0061] Using the following aqueous dispersion of resin particles, black ink, cyan ink, magenta ink, and yellow ink can be obtained.

[0062] <Preparation of aqueous dispersion of resin particles> In a three-necked flask equipped with a stirrer, thermometer, reflux condenser, and nitrogen gas inlet tube, water (250 g), 12-methacrylamide dodecanoic acid (6.7 g), potassium bicarbonate (2.46 g), and isopropanol (20 g) were charged and the temperature was raised to 85°C under a nitrogen stream. A mixed solution consisting of 4,4'-azobis(4-cyanovaleric acid) (radical polymerization initiator, product name "V-501", manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) (0.11 g), potassium bicarbonate (0.08 g), and water (9 g) was added and stirred for 10 minutes. Next, a monomer solution consisting of styrene (14 g), benzyl methacrylate (14 g), methyl methacrylate (48 g), butyl methacrylate (3.3 g), and hydroxyethyl methacrylate (14 g) was added dropwise to the three-necked flask at a constant rate so that the addition would be completed in 3 hours. Furthermore, a mixed solution consisting of V-501 (0.06 g), potassium bicarbonate (0.04 g), and water (6 g) was added in two portions: immediately after the start of the addition of the monomer solution and 1.5 hours after the start of the addition of the monomer solution. After the addition of the monomer solution was completed, the mixture was stirred for 1 hour. Subsequently, a mixed solution consisting of V-501 (0.06 g), potassium bicarbonate (0.04 g), and water (6 g) was added to the resulting reaction mixture, and the mixture was stirred for a further 3 hours. The resulting reaction mixture was filtered through a 50 μm mesh to obtain an aqueous dispersion of resin particles.

[0063] <Preparation of Black Ink> Mix each component according to the composition below to obtain a mixture. Then, remove coarse particles from the mixture using a 1 μm filter to obtain black ink.

[0064] -Composition- (Pigments) ・Black pigment (Product name "Pro-Jet Black APD1000"): Pigment dispersion containing carbon black (pigment concentration 14.3%, manufactured by FUJIFILM Imaging Colorants) ... Amount such that the pigment content in the ink is 3.5 parts by mass ・Cyan pigment (Product name "Pro-Jet Cyan APD1000"): Pigment dispersion containing pigment blue 15:3 (pigment concentration 14.1%, manufactured by FUJIFILM Imaging Colorants) ... Amount such that the pigment content in the ink is 1.0 part by mass ・Magenta pigment (Product name "Pro-Jet Magenta APD1000"): Pigment dispersion containing pigment red 122 (pigment concentration 14.0%, manufactured by FUJIFILM Imaging (Manufactured by Colorants) ... Amount such that the pigment content in the ink is 1.0 part by mass (Organic solvent) ・Propylene glycol (PG) (Manufactured by ADEKA) ... 20.0 parts by mass ・Diethylene glycol monoethyl ether (DEGmEE) (Manufactured by Tokyo Chemical Industry Co., Ltd.) ... 8.0 parts by mass (Wax) ・"AQUACER 531" (Solid content 45% by mass) manufactured by BIC Chemie Japan ... Amount such that the wax content is 1.0 part by mass (Resin particles) ・Dispersion of the above-prepared resin particles (Solid content concentration 25% by mass) ... Amount such that the resin particle content is 1.0 part by mass (Surfactants) ・"Dynol 604" (Manufactured by Evonik) ... 1.30 parts by mass ・"Emulgen 103" (Manufactured by Kao Corporation) ... 0.9 parts by mass ・"BYK-345" (Manufactured by BYK) ... 0.1 parts by mass (Other additives) • Urea (manufactured by Nissan Chemical Corporation) ... 3% by mass • Water ... Amount that makes a total of 100 parts by mass

[0065] <Preparation of Cyan Ink> Mix each component according to the composition below to obtain a mixture. Then, remove coarse particles from the mixture using a 1 μm filter to obtain cyan ink.

[0066] -Composition- (Pigment) ・Cyan pigment (product name "Pro-Jet Cyan APD1000"): Pigment dispersion containing pigment blue 15:3 (pigment concentration 14.1%, manufactured by FUJIFILM Imaging Colorants) ... Amount such that the pigment content in the ink is 3.0 parts by mass (Organic solvent) ・Propylene glycol (PG) (manufactured by ADEKA) ... 25.0 parts by mass ・Diethylene glycol monoethyl ether (DEGmEE) (manufactured by Tokyo Chemical Industry Co., Ltd.) ... 13.0 parts by mass (Wax) ・"AQUACER 531" manufactured by BIC CHEMMIE Japan (solids content 45% by mass) ... Amount such that the wax content is 1.0 part by mass (Resin particles) ・Dispersion of the above-prepared resin particles (solids content concentration 25% by mass) ... Amount such that the resin particle content is 1.0 part by mass (Surfactant) - Evonik's "Dynol 604" ... 1.3 parts by mass - Kao's "Emulgen 103" ... 0.9 parts by mass - BYK's "BYK-345" ... 0.5 parts by mass (Other additives) - Urea (Nissan Chemical Corporation) ... 3% by mass - Water ... Amount that brings the total to 100 parts by mass

[0067] <Preparation of Magenta Ink> Mix each component according to the composition below to obtain a mixture. Then, remove coarse particles from the mixture using a 1 μm filter to obtain magenta ink.

[0068] -Composition- (Pigment) ・Magenta pigment (product name "Pro-Jet Magenta APD1000": pigment dispersion containing pigment red 122 (pigment concentration 14.0%, manufactured by FUJIFILM Imaging Colorants) ... amount such that the pigment content in the ink is 5.0 parts by mass (Organic solvent) ・Propylene glycol (PG) (manufactured by ADEKA) ... 25.0 parts by mass ・Diethylene glycol monoethyl ether (DEGmEE) (manufactured by Tokyo Chemical Industry Co., Ltd.) ... 8.0 parts by mass (Wax) ・"AQUACER 531" manufactured by BIC CHEMMIE Japan (solids content 45% by mass) ... amount such that the wax content is 1.0 part by mass (Resin particles) ・Dispersion of the above-prepared resin particles (solids content concentration 25% by mass) ... amount such that the resin particle content is 1.0 part by mass (Surfactants) - Dynol 604 manufactured by Evonik: 1.3 parts by mass - Emulgen 103 manufactured by Kao Corporation: 0.9 parts by mass - BYK-345 manufactured by BYK Corporation: 0.4 parts by mass (Antifoaming agent) - BYK-094 manufactured by BYK Corporation: 0.05 parts by mass (Other additives) - Urea (manufactured by Nissan Chemical Corporation): 3% by mass - Water: an amount that brings the total volume to 100 parts by mass

[0069] <Preparation of Yellow Ink> Mix each component according to the composition below to obtain a mixture. Then, remove coarse particles from the mixture using a 1 μm filter to obtain yellow ink.

[0070] -Composition- (Pigment) ・Yellow pigment (product name "Pro-Jet Yellow APD4000": pigment dispersion containing pigment yellow 74 (pigment concentration 19.9%, manufactured by FUJIFILM Imaging Colorants) ... amount such that the pigment content in the ink is 4.0 parts by mass (Organic solvent) ・Propylene glycol (PG) (manufactured by ADEKA) ... 25.0 parts by mass ・Diethylene glycol monoethyl ether (DEGmEE) (manufactured by Tokyo Chemical Industry Co., Ltd.) ... 8.0 parts by mass (Wax) ・"AQUACER 531" manufactured by BIC CHEMMIE Japan (solids content 45% by mass) ... amount such that the wax content is 1.0 part by mass (Resin particles) ・Dispersion of the above-prepared resin particles (solids content concentration 25% by mass) ... amount such that the resin particle content is 1.0 part by mass (Surfactant) - Dynol 604 (manufactured by Evonik) ... 1.3 parts by mass - Emulgen 103 (manufactured by Kao Corporation) ... 0.9 parts by mass - BYK-345 (manufactured by BYK Corporation) ... 0.4 parts by mass (defoaming agent) - BYK-094 (manufactured by BYK Corporation) ... 0.05 parts by mass (other additives) - Urea (manufactured by Nissan Chemical Corporation) ... 3% by mass - Water ... amount equal to 100 parts by mass in total

[0071] Furthermore, the dynamic surface tensions at 50 ms measured using the maximum pressure method for black, cyan, magenta, and yellow prepared according to the above formulation were 29.7 mN / m, 29.0 mN / m, 29.6 mN / m, and 28.1 mN / m, respectively. The static surface tensions were 27.0 mN / m, 26.2 mN / m, 27.0 mN / m, and 26.5 mN / m, respectively. The electrical conductivity was 50.3 mS / m, 26.6 mS / m, 41.1 mS / m, and 49.9 mS / m, respectively.

[0072] [Evaluation Test] A printing test was conducted using the inkjet printing system according to the embodiment, and the printability and mist adhesion were evaluated.

[0073] The printing conditions were as follows: transport speed 160 mpm, transport resolution 600 dpi. The inkjet head 21 was a 1200 dpi piezo-circulating head. The nozzle diameter was 15-25 μm, and the ink droplet volume was controlled to a range of 2-3 pL (picoliters) for small droplets and 3-4 pL for medium droplets. Under the transport speed condition of 160 mpm, the maximum printable ink droplet volume was 4 pL. As continuous paper S, Nippon Paper Industries Ltd.'s "NPi Form NEXT-IJ", 540 mm wide and approximately 7000 m long was used. Inks A, B, and C with different 50 ms dynamic surface tensions were prepared. The 50 ms dynamic surface tensions of each ink were as follows: Ink A: 20 mN / m Ink B: 40 mN / m Ink C: 29.7 mN / m Ink C was a black ink with the formulation described in the above embodiment.

[0074] The charging unit 32 was equipped with a charging brush to negatively charge the continuous paper S. The amount of charge was adjusted by changing the time the charging brush was in contact with the paper. From the time the paper was charged in the charging unit 32 until it passed through the head unit 20, the paper was transported without coming into contact with any conductive material, thereby maintaining the charged state while printing was performed by ejecting ink from the inkjet head 21.

[0075] "Mist Polarity" When using ink B and when using ink C, the amount of mist adhering to the nozzle surface was visually evaluated after printing for a certain period of time with the charging unit 32 inactive (the charge level at this time was approximately -200V) and after printing for a certain period of time with the charging unit 32 activated and charged to -1.0kV. Since the amount of mist adhering decreased when the charging unit 32 was activated and negatively charged compared to when the charging unit 32 was not activated, it was determined that the mist polarity is positive (+).

[0076] "Print Performance at 160 mmm" Prints were made using ink density in 10% increments from 0% to 100% and evaluated according to the following criteria: OK: No streaks are visible in any gradation. NG: Streaks appear in some density ranges (especially on the higher density side) (the streaks are easily visible due to the small size of the dots).

[0077] "Mist Adhesion Amount" The amount of mist adhering to the nozzle surface of the inkjet head was visually inspected after continuously printing an image with a print duty cycle of approximately 20% for 7000m and evaluated according to the following criteria: OK: No ink buildup on the nozzle surface after 7000m printing. NG: Ink buildup present.

[0078] "Inkjet head ejection failure" Ejection failures caused by discharge or the generation of air bubbles in the ink due to discharge were evaluated according to the following criteria: OK: No non-ejecting nozzles occurred. NG: Non-ejecting nozzles occurred.

[0079] Table 1 summarizes the ink type, charge amount, and evaluation results for comparative examples and examples in which the ink and the amount of charge applied to the charged section were varied.

[0080] In Comparative Example 1, printing was performed using ink A under the above printing conditions. In Comparative Example 2, printing was performed using ink B under the above printing conditions. In Comparative Example 3 and Examples 1 and 2, printing was performed using ink C under the same printing conditions. Ink A could not be ejected stably even after adjusting the waveform applied to the drive unit of the inkjet head. Here, the state of not being able to eject stably refers to a state in which the ejection direction is poor and streaks appear in the printed image. As previously described, when the dynamic surface tension of ink decreases, the ink droplets become less likely to stick together. With ink A, which has a dynamic surface tension of 20 mN / m, the ink droplets were less likely to stick together, making it impossible to adjust the amount of ink dispensed, and thus stable ejection was not possible. Because stable ink ejection could not be achieved, charging and other evaluations were not performed for Comparative Example 1.

[0081] Comparative Examples 2 and 3, and Examples 1 and 2, all demonstrated stable discharge. Furthermore, the mist polarity was positive in all cases. In Comparative Examples 2 and 3, the charge applied by the charging unit was -1 kV, while in Example 1 it was -2.5 kV, and in Example 2 it was -5.6 kV.

[0082] In Comparative Example 2, there was little mist adhesion on the nozzle surface of the inkjet head and no ejection failures, but the printing performance at high-speed transport of 160 mpm was insufficient. The ink B used in Comparative Example 2 had a dynamic surface tension of 40 mN / m, which was significantly higher than 33 mNm.

[0083] Comparative Example 3 showed sufficient printing performance during high-speed transport, but mist adhesion occurred on the nozzle surface.

[0084] Examples 1 and 2, which satisfy the conditions that the ink has a dynamic surface tension of 23.0 mN / m to 33.0 mN / m at 50 ms and an absolute value of charge of 2.5 kV or more, exhibited sufficient 160 mpm printing performance and suppressed mist adhesion.

[0085] Furthermore, in Comparative Examples 2 and 3, and Examples 1 and 2, no discharge failures occurred due to discharge or bubble generation caused by discharge.

[0086] The following additional information is disclosed regarding the above embodiments. (Addendum 1) An inkjet printing system comprising ink and an inkjet printing apparatus, wherein the ink is a water-based ink having a dynamic surface tension of 23.0 mN / m to 33.0 mN / m at 50 ms, and the inkjet printing apparatus comprises a transport mechanism for transporting a recording medium, an inkjet head for ejecting ink onto the recording surface of the recording medium transported by the transport mechanism, and a charge control mechanism for maintaining the recording medium in a state where it is charged to a polarity opposite to the polarity of the charge of the ink mist generated when the ink is ejected, wherein the charge control mechanism sets the absolute value of the charge of the recording medium to 2.5 kV or more. (Addendum 2) The inkjet printing system according to Addendum 1, wherein the recording medium is a roll-shaped substrate, and the transport mechanism is a transport mechanism for transporting the recording medium roll to roll. (Note 3) The inkjet printing system according to Note 1 or Note 2, wherein the charge control mechanism includes a charging unit that charges the recording medium, which is located upstream of the inkjet head in the transport direction of the recording medium, and a transport mechanism that transports the recording medium without contacting the recording medium with a conductive material from the charging position where the recording medium is charged by the charging unit to the ejection end position where the ink ejection by the inkjet head ends. (Note 4) The inkjet printing system according to Note 3, wherein the transport mechanism comprises a plurality of transport members that contact the recording medium, and of the plurality of transport members, at least the surface of the transport member installed between the charging position and the ejection end position that contacts the recording medium is nonconductive and is made of a material that is relatively prone to being charged with the same polarity as the ink mist in relation to the recording medium in the triboelectric series. (Note 5) The inkjet printing system according to Note 3 or 4, wherein a sensor for measuring the amount of charge of the recording medium is provided downstream of the charging position in the transport direction of the recording medium, and the charge control mechanism adjusts the amount of charge applied when charging the recording medium in the charging unit based on the amount of charge measured by the sensor.(Note 6) An inkjet printing system according to Note 3 or 4, further comprising a drying section for drying ink located downstream of the inkjet head in the transport direction of the recording medium, and a static elimination mechanism located between the ejection end position and the drying section to remove static charge from the recording medium. (Note 7) An inkjet printing system according to Note 6, comprising a sensor for measuring the amount of charge on the recording medium located downstream of the charging position and upstream of the static elimination mechanism in the transport direction of the recording medium, wherein the charge control mechanism adjusts the amount of charge applied when charging the recording medium in the charging section based on the amount of charge measured by the sensor. (Note 8) An inkjet printing system according to any one of Notes 1 to 7, wherein the inkjet head is an ink-circulating type ink head. (Note 9) An inkjet printing system according to any one of Notes 1 to 8, wherein the transport speed of the recording medium is 160 m / min or more. (Note 10) An inkjet printing system according to any one of Notes 1 to 9, wherein the flight speed of the ink ejected from the inkjet head is 8 m / sec or more. (Note 11) An inkjet printing system according to any one of Notes 1 to 10, wherein the inkjet printing device is a double-sided printing machine that uses the first and second surfaces, which are the front and back surfaces of the recording medium, as recording surfaces, and the charge control mechanism is applied only when ink is ejected to either the first or second surface. (Note 12) An inkjet printing system according to any one of Notes 1 to 10, wherein the inkjet printing device is a double-sided printing machine that uses the first and second surfaces, which are the front and back surfaces of the recording medium, as recording surfaces, and the charge control mechanism is applied both when ink is ejected to the first surface and when ink is ejected to the second surface.(Note 13) An image forming method in an inkjet printing apparatus comprising a transport mechanism for transporting a recording medium and an inkjet head for ejecting ink onto the recording surface of the recording medium transported by the transport mechanism, wherein the ink used is an aqueous ink having a dynamic surface tension of 23.0 mN / m to 33.0 mN / m at 50 ms, and the recording medium is transported with its recording surface facing the inkjet head while being charged to a polarity opposite to the polarity of the charge of the ink mist generated when the ink is ejected, and the absolute value of the charge of the recording medium is maintained at 2.5 kV or more. (Note 14) The image forming method according to Note 13, wherein the recording medium is transported roll-to-roll. (Note 15) The image forming method according to Note 13 or Note 14, wherein the recording medium is charged upstream of the inkjet head in the transport direction of the recording medium, and the recording medium is transported from the charged position to the ejection end position where the ink ejection by the inkjet head ends without contacting the recording medium with a conductive material.

[0087] The disclosure of Japanese Patent Application No. 2024-167871, filed on 26 September 2024, is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards described herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard were specifically and individually noted to be incorporated by reference.

Claims

1. An inkjet printing system comprising ink and an inkjet printing apparatus, wherein the ink is a water-based ink having a dynamic surface tension of 23.0 mN / m to 33.0 mN / m at 50 ms, and the inkjet printing apparatus comprises a transport mechanism for transporting a recording medium, an inkjet head for ejecting the ink onto the recording surface of the recording medium transported by the transport mechanism, and a charge control mechanism for maintaining the recording medium in a state where it is charged to a polarity opposite to the polarity of the charge of the ink mist generated when the ink is ejected, wherein the charge control mechanism sets the absolute value of the charge of the recording medium to 2.5 kV or more.

2. The inkjet printing system according to claim 1, wherein the recording medium is a roll-shaped substrate, and the transport mechanism is a transport mechanism that transports the recording medium roll-to-roll.

3. The inkjet printing system according to claim 1, wherein the charge control mechanism includes a charging unit that charges the recording medium and is located upstream of the inkjet head in the transport direction of the recording medium, and a transport mechanism that transports the recording medium without bringing it into contact with a conductive material from the charged position where the recording medium is charged by the charging unit to the ejection end position where the ink ejection by the inkjet head ends.

4. The inkjet printing system according to claim 2, wherein the charge control mechanism includes a charging unit that charges the recording medium and is located upstream of the inkjet head in the transport direction of the recording medium, and a transport mechanism that transports the recording medium without bringing it into contact with a conductive material from the charged position where the recording medium is charged by the charging unit to the ejection end position where the ink ejection by the inkjet head ends.

5. The inkjet printing system according to claim 3, wherein the transport mechanism comprises a plurality of transport members that come into contact with the recording medium, and of the plurality of transport members, at least the surface of the transport member installed between the charging position and the ejection end position that comes into contact with the recording medium is nonconductive and is made of a material that is relatively prone to being charged with the same polarity as the ink mist in relation to the recording medium in the triboelectric series.

6. The inkjet printing system according to claim 4, wherein the transport mechanism comprises a plurality of transport members that come into contact with the recording medium, and at least the surface of the transport member that comes into contact with the recording medium, which is installed between the charging position and the ejection end position, is non-conductive and is made of a material that is relatively prone to being charged with the same polarity as the ink mist in relation to the recording medium in the triboelectric series.

7. An inkjet printing system according to any one of claims 3 to 6, wherein in the transport direction of the recording medium, a sensor for measuring the amount of charge on the recording medium is provided downstream of the charging position, and the charging control mechanism adjusts the amount of charge applied when charging the recording medium in the charging section based on the amount of charge measured by the sensor.

8. The inkjet printing system according to any one of claims 3 to 6, further comprising: a drying unit for drying the ink, which is located downstream of the inkjet head in the transport direction of the recording medium; and an anti-static mechanism for removing static charge from the recording medium, which is located between the ejection end position and the drying unit.

9. The inkjet printing system according to claim 7, further comprising: a drying unit for drying the ink, which is located downstream of the inkjet head in the transport direction of the recording medium; and an anti-static mechanism for removing static charge from the recording medium, which is located between the ejection end position and the drying unit.

10. The inkjet printing system according to claim 8, wherein, in the transport direction of the recording medium, a sensor for measuring the amount of charge of the recording medium is provided downstream of the charging position and upstream of the static elimination mechanism, and the charging control mechanism adjusts the amount of charge applied when charging the recording medium in the charging section based on the amount of charge measured by the sensor.

11. The inkjet printing system according to claim 9, wherein, in the transport direction of the recording medium, a sensor for measuring the amount of charge on the recording medium is provided downstream of the charging position and upstream of the static elimination mechanism, and the charging control mechanism adjusts the amount of charge applied when charging the recording medium in the charging section based on the amount of charge measured by the sensor.

12. The inkjet printing system according to any one of claims 1 to 6, wherein the inkjet head is an ink-recirculating ink head.

13. The inkjet printing system according to any one of claims 1 to 6, wherein the transport speed of the recording medium is 160 m / min or more.

14. The inkjet printing system according to any one of claims 1 to 6, wherein the flight speed of the ink ejected from the inkjet head is 8 m / second or more.

15. The inkjet printing system according to any one of claims 1 to 6, wherein the inkjet printing apparatus is a double-sided printer in which the first and second surfaces, which are the front and back surfaces of the recording medium, are the recording surfaces, and the charge control mechanism is applied only when the ink is ejected to either the first surface or the second surface.

16. The inkjet printing system according to any one of claims 1 to 6, wherein the inkjet printing apparatus is a double-sided printer in which the first and second surfaces, which are the front and back surfaces of the recording medium, are the recording surfaces, and the charge control mechanism is applied to both when the ink is ejected to the first surface and when the ink is ejected to the second surface.

17. An image forming method in an inkjet printing apparatus comprising a transport mechanism for transporting a recording medium and an inkjet head for ejecting ink onto the recording surface of the recording medium transported by the transport mechanism, wherein the ink is an aqueous ink having a dynamic surface tension of 23.0 mN / m to 33.0 mN / m at 50 ms, and the recording medium is transported with its recording surface facing the inkjet head while being charged to a polarity opposite to the polarity of the charge of the ink mist generated when the ink is ejected, and the absolute value of the charge of the recording medium is maintained at 2.5 kV or more.

18. The image forming method according to claim 17, wherein the recording medium is transported roll-to-roll.

19. The image forming method according to claim 17 or 18, wherein the recording medium is charged upstream of the inkjet head in the transport direction of the recording medium, and the recording medium is transported from the charged position to the ejection completion position where the ink ejection by the inkjet head ends without contacting the recording medium with a conductive material.

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