Image formation method

Abstract

An image formation method according to the present invention comprises: a treatment solution discharging step for discharging droplets of a treatment solution from a first inkjet head onto a recording medium; and an ink discharging step for discharging, from a second inkjet head, droplets of an ink containing a color material that is aggregated by the treatment solution. In the treatment solution discharging step, some pixels are randomly selected from an image formation region to which the droplets of the ink are discharged, and the droplets of the treatment solution are discharged at positions shifted from the selected pixels by a range of less than one pixel. The droplets of the treatment solution include two or more types of droplets that differ from each other in droplet size. The maximum droplet size of the treatment solution discharged in the treatment solution discharging step is smaller than the droplet size of the ink discharged in the ink discharging step.

Description

Image forming method

[0001] The present invention relates to an image forming method.

[0002]

[0003] A known image forming method involves ejecting ink droplets from an inkjet head onto a recording medium to form an image. In this type of image forming method, if the recording medium is a substrate with low ink absorbency (low-absorbency substrate), a phenomenon known as "liquid migration," in which deposited ink droplets coalesce, or color bleeding, in which different colors bleed, may occur. To prevent degradation of image quality due to these phenomena, a two-liquid image forming method is known that uses both ink and a treatment liquid that aggregates coloring materials in the ink.

[0003] In a two-liquid image formation method that uses both ink and treatment liquid, treatment liquid is conventionally ejected onto all pixels onto which ink is ejected (positions where ink dots are formed). This tends to result in a large amount of treatment liquid being applied, which can cause unevenness (cockling) on ​​the recording medium surface and, when using multiple color inks, can easily cause color mixing. For this reason, methods for reducing the amount of treatment liquid applied are being investigated.

[0004] In response to this, Japanese Patent Laid-Open No. 2007-129998 discloses a method of controlling the pixels onto which the treatment liquid is ejected (the positions onto which treatment liquid dots are formed) in accordance with the distribution state of the pixels onto which ink is ejected. Specifically, the treatment liquid dots are made larger than the ink dots, and one treatment liquid dot is formed across a plurality of positions onto which the ink dots are formed.

[0005] Furthermore, Patent Document 2 discloses a method of forming an image in which, in low-density print regions, the pixels onto which treatment liquid is ejected and the pixels onto which ink is ejected are relatively different, and in high-density print regions, the amount of treatment liquid ejected is thinned out. Specifically, in low-density print regions, the pixels onto which ink is ejected and the pixels onto which treatment liquid is ejected are shifted by one pixel, and in high-density print regions, the amount of treatment liquid ejected is thinned out.

[0006] Furthermore, Patent Document 3 discloses a method of increasing the amount of droplets of treatment liquid in areas where the area to be printed in solid is relatively large, and decreasing the amount of droplets of treatment liquid in areas where the area to be printed in solid is relatively small, i.e., a method of setting the application ratio of ink to treatment liquid at two or more levels.

[0007] Japanese Patent Application Laid-Open No. 8-52867 Japanese Patent Application Laid-Open No. 2000-127373 Japanese Patent Application Laid-Open No. 2006-264185

[0008] The image forming methods described in Patent Documents 1 to 3 can reduce the amount of treatment liquid applied because the treatment liquid can be thinned out to a certain extent. However, these methods tend to produce regular patterns on the recording medium where droplets of the treatment liquid and ink droplets come into contact and areas where the treatment liquid and ink droplets do not come into contact, which can easily result in streaky unevenness.

[0009] Furthermore, at the points where the treatment liquid droplets and the ink droplets come into contact, the contact area between the treatment liquid droplets and the ink droplets is large, so the coloring material in the ink is likely to aggregate excessively due to the treatment liquid before the ink has fully wetted and spread, which can easily result in uneven density.

[0010] Recently, there has been a need to form images by ejecting treatment liquid or ink from an inkjet head located away from the surface of a recording medium. In this case, it is desirable to increase the droplet size of the treatment liquid or ink from the perspective of improving landing accuracy and reducing mist. Increasing the droplet size makes it more likely that excessive aggregation will occur due to excessive contact between the treatment liquid and ink, making it more likely to result in uneven density.

[0011] The present invention has been made in consideration of the above circumstances, and aims to provide an image forming method that can form an image with little streak-like unevenness and little density unevenness, even when the amount of treatment liquid applied is small and the treatment liquid is applied from a position away from the surface of the recording medium.

[0012] [1] An image forming method comprising: a treatment liquid discharging step of discharging droplets of treatment liquid from a first inkjet head onto a recording medium; and an ink discharging step of discharging droplets of ink containing a colorant that aggregates in response to the treatment liquid from a second inkjet head, wherein in the treatment liquid discharging step, some pixels are irregularly selected from an image forming area onto which the ink droplets are discharged, and the droplets of the treatment liquid are discharged at positions shifted by less than one pixel from the selected pixels, the droplets of the treatment liquid comprise two or more types of droplets having different droplet sizes, and the maximum droplet size of the treatment liquid discharged in the treatment liquid discharging step is smaller than the droplet size of the ink discharged in the ink discharging step. [2] The image forming method according to [1], wherein a total amount of the treatment liquid droplets discharged onto the image forming area is 20 to 80% of a total amount of the ink droplets discharged onto the image forming area. [3] The image forming method according to [1] or [2], wherein the two or more types of droplets with different droplet sizes include first droplets having a droplet volume of 10 pL or more but less than 20 pL and second droplets having a droplet volume of less than 10 pL. [4] The image forming method according to [3], wherein in the image forming area, the total number of pixels onto which the first droplets are ejected: the total number of pixels onto which the second droplets are ejected is equal to or greater than 1:0.4 to 1:1.6. [5] The image forming method according to any one of [1] to [4], wherein the range of less than one pixel is equal to or greater than 0.4 pixels but less than 0.6 pixels. [6] The image forming method according to any one of [1] to [5], wherein the maximum droplet size of the treatment liquid is 20% or more but less than 90% of the droplet size of the ink. [7] The image forming method according to any one of [1] to [6], wherein the distance from the surface of the recording medium to the ejection surfaces of the first inkjet head and the second inkjet head is 3 mm or more. [8] The image forming method according to any one of [1] to [7], wherein the treatment liquid contains a polyvalent metal salt. [9] The image forming method according to any one of [1] to [8], wherein the recording medium is a low-absorbency substrate.

[10] The image forming method according to [9], wherein the contact angle of the treatment liquid on the surface of the low-absorbency substrate at 25°C is 40° or more and 100° or less.

[0013] According to the present invention, an image forming method can be provided that can form images with little streak-like unevenness and little density unevenness, even when the amount of treatment liquid applied is reduced and the treatment liquid is applied from a position away from the surface of the recording medium.

[0014] Fig. 1A is a schematic diagram showing the main components of an image forming apparatus applicable to an image forming method according to one embodiment of the present invention, and Fig. 1B is a schematic diagram showing how ink and treatment liquid are applied to the surface of a recording medium by the apparatus of Fig. 1A. Fig. 2 is a schematic diagram showing an image formed by an image forming method according to one embodiment of the present invention. Figs. 3A to 3E are schematic diagrams showing the procedure for forming the image of Fig. 2. Fig. 4 is a schematic diagram showing an image formed by a conventional image forming method. Figs. 5A to 5E are schematic diagrams showing an image forming method according to a modified example.

[0015] If treatment liquid is ejected onto all pixels where ink is to be ejected, as in the past, the reaction between the ink and the treatment liquid is likely to be excessive. On the other hand, if the treatment liquid is thinned out regularly, streaky unevenness is likely to occur. Therefore, by thinning out the treatment liquid irregularly, streaky unevenness can be suppressed.

[0016] Furthermore, if the droplet size of the treatment liquid is larger than the droplet size of the ink, as in the past, the contact area between the ink and the treatment liquid on the recording medium becomes larger, which tends to result in excessive reaction between the ink and the treatment liquid. Excessive reaction can easily cause the colorant to aggregate before the ink has time to spread, resulting in uneven density. Furthermore, if the treatment liquid droplets are of only one type with large droplet sizes, the reaction between the ink and the treatment liquid is likely to be excessive, while if the treatment liquid droplets are of only one type with small droplet sizes, the reaction between the ink and the treatment liquid is likely to be insufficient. Therefore, by making the droplet size of the treatment liquid smaller than the droplet size of the ink and by using two or more different droplet sizes for the treatment liquid, it is possible to reduce uneven density due to excessive reaction without causing insufficient reaction.

[0017] Furthermore, as in the past, when the pixels that eject droplets of treatment liquid and the pixels that eject droplets of ink are aligned, the contact area between the ink and the treatment liquid on the recording medium is large, which still tends to result in excessive reaction between the ink and the treatment liquid. On the other hand, as in Patent Document 2, when the pixels that eject droplets of treatment liquid and the pixels that eject droplets of ink are misaligned by one pixel or more, the contact area becomes too small, which tends to result in insufficient reaction. In contrast, by misaligning the pixels that eject droplets of treatment liquid by less than one pixel relative to the pixels that eject droplets of ink, it is possible to further reduce density unevenness due to excessive reaction, without causing insufficient reaction.

[0018] That is, an image forming method according to one embodiment of the present invention includes: 1) a treatment liquid ejection step of ejecting droplets of treatment liquid from a first inkjet head onto a recording medium; and 2) an ink ejection step of ejecting droplets of ink containing a colorant that aggregates in the presence of the treatment liquid from a second inkjet head. In the treatment liquid ejection step, some pixels are irregularly selected from the image formation area onto which the ink droplets are ejected, and droplets of treatment liquid are ejected at positions shifted by less than one pixel from the selected pixels. Furthermore, the droplet size of the treatment liquid is made smaller than the droplet size of the ink, and two or more types of droplets with different droplet sizes are used as the treatment liquid droplets. This makes it possible to sufficiently reduce density unevenness while suppressing streaky unevenness.

[0019] 1. Image Forming Method An image forming method according to one embodiment of the present invention will now be described. First, an outline of the configuration of an image forming apparatus will be described.

[0020] <Image forming apparatus> Figure 1A is a schematic diagram showing the main components of an image forming apparatus applicable to an image forming method according to one embodiment of the present invention. Figure 1B is a schematic diagram showing how ink or treatment liquid is applied to the surface of a recording medium by the apparatus of Figure 1A. Figure 1A shows a total printing area P as the range in which an image can be formed on the recording medium M by scanning the droplet ejection means 2 in the X direction and transporting the recording medium M in the Y direction. The length of the total printing area P in the X direction is defined as a printing area width PW, and the length in the Y direction is defined as a printing area length PL.

[0021] As shown in FIG. 1A, the image forming apparatus 1 includes a droplet ejection unit 2 and a scanning unit 3 .

[0022] The droplet ejection means 2 includes a first inkjet head 4Pr for the treatment liquid and second inkjet heads 4Y, 4M, 4C, and 4K corresponding to each color ink (hereinafter, these are also collectively referred to as the "head unit 4"), and a carriage 5 that holds these inkjet heads.

[0023] Each inkjet head has a surface (nozzle surface) facing the surface of the recording medium M, on which a plurality of nozzles are arranged along a direction Y perpendicular to the scanning direction X, and droplets of ink or treatment liquid are ejected from the nozzles. The droplet ejection means 2 is supported in a state in which the nozzle surface of the head unit 4 is spaced a predetermined distance from the surface of the recording medium M in a direction (height direction) perpendicular to the surface of the recording medium M.

[0024] The distance between the nozzle surface (ejection surface) of each inkjet head and the surface of the recording medium M is not particularly limited and can be, for example, 1.5 mm or more, preferably 3 mm or more. The upper limit of the distance is not particularly limited, but can be 20 mm or less.

[0025] The scanning unit 3 is a mechanism for scanning the droplet discharging means 2 in the scanning direction X. In this embodiment, the scanning unit 3 has rails extending along the scanning direction X, and supports the carriage 5 movably along the rails.

[0026] In the image forming apparatus 1 configured as described above, when the droplet ejection means 2 moves once in the scanning direction X, both the inks Y, M, C, and K (hereinafter also collectively referred to as "In") and the treatment liquid Pr may be applied to an area of ​​width W in the direction Y perpendicular to the scanning direction X of the head unit 4, for a printing area width PW of the entire printing area P. In this case, the operation of simultaneously ejecting the ink In and the treatment liquid Pr by one movement of the droplet ejection means 2 in the scanning direction X is considered to be one printing pass, and multiple printing passes are performed, so that a desired image can ultimately be formed on the recording medium M (pattern 1).

[0027] Alternatively, the droplet ejection means 2 may be moved in the scanning direction X while ejecting only the treatment liquid, performing multiple printing passes across the printing area width PW of the entire printing area P, and then the recording medium M may be returned to the state at the start of printing, and the droplet ejection means 2 may be moved in the scanning direction X again while ejecting only the ink In, performing multiple printing passes across the printing area width PW of the entire printing area P. This may ultimately form a desired image on the recording medium M (Pattern 2).

[0028] In FIG. 1A , the area to which ink and treatment liquid are applied in one printing pass of the droplet ejection means 2 is an area (hereinafter also referred to as the "printing area A") having an area obtained by multiplying the printing area width PW in the scanning direction X by the width W of the head unit 4 in the direction Y perpendicular to the scanning direction X (hereinafter also referred to as the "width W of the head unit 4").

[0029] The total printing area P is a collection of these printing areas A. The number of printing areas A that make up the total printing area P is indicated by the value obtained by dividing the printing area length PL by the width W of the head unit 4. For example, in the case of the recording medium M shown in FIG. 1A, the total printing area P is made up of six printing areas A, with the total printing area P being made up of printing areas A1, A2, A3, A4, A5, and A6 arranged in order from the front to the back of the recording medium M.

[0030] In the image forming apparatus 1, the control unit determines, for each pixel, whether or not to apply treatment liquid or ink and the amount to be applied (discharge amount) in accordance with image data. Then, based on this determination, the droplet discharge means 2 discharges droplets of treatment liquid or ink onto the surface of the recording medium M, thereby forming an image. Then, under the control of the control unit, each step of the image forming method described below is carried out.

[0031] The type of recording medium M is not particularly limited, but a low-absorbency substrate is preferred. A low-absorbency substrate is a substrate in which a 0.5 μL water droplet is dropped on the recording surface of the substrate and the contact angle reduction rate (comparing the contact angle 0.5 milliseconds after impact with the contact angle 5 seconds after impact) is 1.0% or more and less than 30%. The contact angle can be measured at 25°C by the liquid drop method, for example, using a contact angle meter manufactured by Kyowa Interface Science Co., Ltd. The contact angle of the treatment liquid on the recording surface of the low-absorbency substrate at 25°C is, for example, 10° or more and 105° or less, preferably 20° or more and 90° or less.

[0032] Examples of low absorbency substrates include uncoated paper such as wood-burning paper, medium-grade paper, and fine paper; coated paper such as coated paper, art paper, and cast paper; Yupo synthetic paper; leather substrates; rubber substrates; and resin film substrates.

[0033] Next, an image forming method according to the present embodiment will be described.

[0034] <Image Forming Method> Fig. 2 is a schematic diagram showing an image 10 formed by the image forming method according to the present embodiment. Figs. 3A to 3E are schematic diagrams showing the procedure for forming the image 10 shown in Fig. 2. In these figures, the smallest unit of a square partitioned into a grid pattern represents one pixel.

[0035] The image forming method according to the present embodiment includes the steps of 1) ejecting droplets of a treatment liquid from a first inkjet head (treatment liquid ejection step), and 2) ejecting droplets of ink containing a coloring material from a second inkjet head (ink ejection step). The order of steps 1) and 2) is not particularly limited, and steps 1) and 2) may be performed in that order, or steps 2) and 1). In the present embodiment, steps 1) and 2) are preferably performed in that order.

[0036] In this embodiment, as described above, the head unit 4 may be moved in the scanning direction X while performing the steps 1) and 2) simultaneously (pattern 1), or the head unit 4 may be moved in the scanning direction X while performing the step 1) for the entire printing area P, and then the head unit 4 may be returned to its original position, and the head unit 4 may be moved in the scanning direction X while performing the step 2) for the entire printing area P (pattern 2).

[0037] 1) Treatment Liquid Discharge Step First, based on the image data to be formed, pixels onto which ink droplets are discharged are determined, and an image formation area a is determined (see FIG. 3A). The image formation area a constitutes part of the image formation area Im (see FIG. 1A). In FIG. 3A, filled-in pixels are pixels onto which ink droplets are discharged, and form the image formation area a.

[0038] Next, some pixels are randomly selected from the pixels that make up the image formation area a (see FIG. 3B). The method for randomly selecting pixels is not particularly limited, and may be a dither method or an error diffusion method.

[0039] Then, a droplet 12 of treatment liquid is ejected from the selected pixel at a position shifted by less than one pixel in the nozzle row direction (Y direction) (see FIG. 3C) (see FIG. 3D). In this embodiment, the shift is in the nozzle row direction (Y direction), but this is not limiting and the shift may also be in a direction perpendicular to the nozzle row (X direction) (see FIG. 5C, which will be described later).

[0040] The shift width W1 for the selected pixel needs only to be smaller than the width W of one pixel, and is preferably 0.3 pixels or more and 0.7 pixels or less, more preferably 0.4 pixels or more and 0.6 pixels or less, and even more preferably 0.5 pixels (half a pixel) (see Figure 3C).

[0041] The method for shifting the position at which the treatment liquid droplets are ejected relative to the position at which the ink droplets are ejected is not particularly limited, and for example, in the image forming device 1, this may be done by shifting the nozzle position of the first inkjet head 4Pr by less than one pixel relative to the nozzle position of the second inkjet head 4Y, 4M, 4C or 4K, or by changing the feed amount of the recording medium M during multiple passes.

[0042] In the image formation region a, the ratio (pixel ratio) of the total number of pixels onto which treatment liquid droplets 12 are ejected in the treatment liquid ejection step to the total number of pixels onto which ink droplets 14 are ejected in the ink ejection step is preferably 30% or more and 65% or less, and more preferably 30% or more and 50% or less. When the pixel ratio is 30% or more, the reaction between the ink and the treatment liquid is more likely to be sufficient, and image density can be further increased. When the pixel ratio is 65% or less, the reaction between the treatment liquid and the ink is less likely to be excessive, and density unevenness can be further suppressed.

[0043] The size of the treatment liquid droplets 12 ejected in the treatment liquid ejection step is smaller than the size of the ink droplets 14 ejected in the ink ejection step. This reduces the contact area between the treatment liquid droplets and the ink droplets on the recording medium M, thereby suppressing overreaction. Furthermore, the treatment liquid droplets 12 include two or more types of droplets with different droplet sizes (see FIG. 3D ). In FIG. 3D , the treatment liquid droplets 12 include first droplets 12a, which are large in size, and second droplets 12b, which are small in size.

[0044] The maximum droplet size (droplet volume) of the treatment liquid is preferably 20% to 90% of the droplet size (droplet volume) of the ink. When the droplet size of the treatment liquid is 20% or more, insufficient reaction between the ink and the treatment liquid can be more effectively suppressed, and when it is 90% or less, excessive reaction can be more effectively suppressed.

[0045] The allocation method of the first droplets 12a and the second droplets 12b is not particularly limited, but can be based on, for example, a general halftone. The ratio of the total number of pixels onto which the first droplets 12a are ejected to the total number of pixels onto which the second droplets 12b are ejected in the image formation area a is preferably 1:0.4 to 1:1.6. Increasing the proportion of the first droplets 12a can further reduce insufficient reaction between the ink and the treatment liquid, thereby further suppressing a decrease in image density. Reducing the proportion of the first droplets 12a can further suppress excessive reaction between the ink and the treatment liquid, thereby further suppressing density unevenness. In particular, from the perspective of more easily suppressing a decrease in image density and stabilizing impact even from positions distant from the recording medium, it is preferable that the total number of pixels onto which the first droplets 12a are ejected be greater than the total number of pixels onto which the second droplets 12b are ejected.

[0046] In the treatment liquid ejection step, the first droplet 12a and the second droplet 12b may be ejected simultaneously or sequentially.

[0047] The droplet size of the first droplets 12a is preferably 10 pL or more and less than 20 pL, and more preferably 14 pL or more and 18 pL or less, and the droplet size of the second droplets 12b is preferably less than 10 pL, and more preferably 5 pL or more and 9 pL or less.

[0048] The difference in droplet size between the first droplet 12a and the second droplet 12b is preferably 4 pL or more and 10 pL or less.

[0049] In the image formation region a, the ratio of the total droplet volume of treatment liquid ejected in the treatment liquid ejection step to the total droplet volume of ink ejected in the ink ejection step is preferably 20% or more and 80% or less. This ratio is expressed by formula (1). Formula (1): Ratio (%) of total droplet volume of treatment liquid = [(droplet size of treatment liquid x number of pixels ejected) / (droplet size of ink x number of pixels ejected)] x 100

[0050] If the ratio is 20% or more, the contact area between the ink and the treatment liquid on the recording medium M becomes appropriately large, making it easier to cause a sufficient reaction and further increasing the image density. If the ratio is 80% or less, the contact area between the ink and the treatment liquid on the recording medium M does not become too large, making it less likely that the reaction will be excessive and making it possible to further reduce density unevenness. From the same perspective, it is more preferable that the ratio be 50% or more and 80% or less.

[0051] 2) Ink Discharge Process Ink droplets 14 are discharged onto the image formation area a (see FIG. 3E). Specifically, ink droplets 14 are discharged onto each of the pixels that make up the image formation area a.

[0052] As described above, the size of the ink droplets 14 is larger than the maximum droplet size of the treatment liquid droplets 12 (the size of the first droplets 12a in FIG. 3D ). This makes it less likely that the wetting and spreading of the ink that has landed on the recording medium M will be hindered by reaction with the treatment liquid, making it possible to suppress density unevenness.

[0053] Although the size of the ink droplets 14 is not particularly limited, it is preferable that the volume of each droplet be 15 pL or more and 45 pL or less. In particular, from the viewpoint of stabilizing the landing of the droplets and further reducing mist even when the droplets are ejected from an ejection surface of an inkjet head located away from the surface of the recording medium M, a larger droplet volume is preferable, and a volume of 20 pL or more and 45 pL or less is more preferable.

[0054] (Function) The function of the image forming method according to the present embodiment will be described below in comparison with a comparative image forming method. Figure 4 is a schematic diagram showing the comparative image forming method.

[0055] In the comparative image forming method, in the treatment liquid ejection step, droplets 22 of treatment liquid are ejected onto all pixels onto which ink droplets 24 are ejected (see FIG. 4). As a result, (even if the size of the treatment liquid droplets 22 is smaller than the size of the ink droplets 24) the reaction between the treatment liquid and the ink tends to be excessive, which tends to result in density unevenness. Furthermore, because the treatment liquid droplets are of only one size, streaky unevenness is also likely to form.

[0056] In contrast, in the image forming method according to the above embodiment, in the treatment liquid ejection step, some pixels are randomly selected from the pixels onto which ink droplets are ejected, and droplets of treatment liquid are ejected at positions shifted by less than one pixel from the selected pixels. Furthermore, the droplet size of the treatment liquid is made smaller than the ink droplet size, and two or more types of droplets with different droplet sizes are combined. This makes it possible to suppress excessive reaction between the ink and treatment liquid while suppressing streaky unevenness, thereby sufficiently reducing density unevenness.

[0057] (Variant) In the above embodiment, the pixels from which droplets of treatment liquid are ejected in the treatment liquid ejection process are shifted by less than one pixel in the nozzle row direction relative to the pixels from which droplets of ink are ejected in the ink ejection process. However, this is not limited to this, and the pixels may be shifted by less than one pixel in a direction perpendicular to the nozzle row direction, or may be shifted by less than one pixel in both the nozzle row direction and the direction perpendicular to the nozzle row direction.

[0058] 5A to 5E are schematic diagrams showing the procedure for forming an image according to a modified example. As shown in FIG. 5C, the pixels may be shifted by less than one pixel not only in the nozzle row direction but also in a direction perpendicular to the nozzle row direction. This makes it possible to further reduce the contact area between the treatment liquid and the ink on the recording medium. Therefore, excessive reaction between the treatment liquid and the ink can be further suppressed to the extent that a decrease in density does not occur, and density unevenness can be further suppressed. Note that the range of the shift width W2 of the pixels onto which droplets of treatment liquid are ejected in the direction perpendicular to the nozzle row can also be the same as the range of the shift width W1.

[0059] In the above embodiment, two types of droplets 12a and 12b having different droplet sizes are combined as the droplets 12 of the treatment liquid, but this is not limiting and three or more types of droplets having different droplet sizes may be combined. For example, in addition to the first droplet 12a and the second droplet 12b, a third droplet having a droplet volume of 20 pL to 45 pL may be further combined.

[0060] Furthermore, in the above embodiment, an example was shown in which a scan-type inkjet head was used as the droplet ejection means 2 of the image forming apparatus 1, but the present invention is not limited to this, and a line-type inkjet head may also be used.

[0061] 2. Ink and Treatment Liquid The ink and treatment liquid used in the above image forming method will be described.

[0062] 2-1. Ink The ink contains at least a coloring material. The coloring material is preferably a pigment. The ink preferably contains, for example, the pigment as the coloring material, a polymer dispersant for dispersing the pigment, resin fine particles, water, and an organic solvent.

[0063] (Colorant) The colorant is preferably a pigment. Examples of the pigment include anionic dispersed pigments, such as anionic self-dispersing pigments and pigments dispersed with an anionic polymer dispersant. In particular, it is preferable to use a pigment dispersion liquid in which the pigment is dispersed with an anionic polymer dispersant.

[0064] The pigment is not particularly limited and may be an inorganic pigment or an organic pigment. Examples of inorganic pigments include titanium oxide. Examples of organic pigments include insoluble pigments and lake pigments.

[0065] Titanium oxide has three crystalline forms: anatase, rutile, and brookite. The most commonly used titanium oxides are roughly classified into anatase and rutile. Among these, rutile is preferred because of its high refractive index and high opacity. Specific examples include the TR series from Fuji Titanium Industries Co., Ltd., the JR series from Teika Corporation, and Typepaque from Ishihara Sangyo Kaisha, Ltd.

[0066] Examples of insoluble pigments include azo, azomethine, methine, diphenylmethane, triphenylmethane, quinacridone, anthraquinone, perylene, indigo, quinophthalone, isoindolinone, isoindoline, azine, oxazine, thiazine, dioxazine, thiazole, phthalocyanine, diketopyrrolopyrrole, and the like.

[0067] Examples of organic pigments include the following pigments: Examples of pigments for magenta or red include C.I. Pigment Red 2, C.I. Pigment Red 3, C.I. Pigment Red 5, C.I. Pigment Red 6, C.I. Pigment Red 7, C.I. Pigment Red 15, C.I. Pigment Red 16, C.I. Pigment Red 48:1, C.I. Pigment Red 53:1, C.I. Pigment Red 57:1, C.I. Pigment Red 122, C.I. Pigment Red 123, C.I. Pigment Red 139, C.I. Pigment Red 144, C.I. Pigment Red 149, C.I. Pigment Red 166, C.I. Examples of pigments for orange or yellow include C.I. Pigment Red 177, C.I. Pigment Red 178, C.I. Pigment Red 202, C.I. Pigment Red 222, C.I. Pigment Violet 19, and the like. Examples of pigments for orange or yellow include C.I. Pigment Orange 31, C.I. Pigment Orange 43, C.I. Pigment Yellow 12, C.I. Pigment Yellow 13, C.I. Pigment Yellow 14, C.I. Pigment Yellow 15, C.I. Pigment Yellow 15:3, C.I. Pigment Yellow 17, C.I. Pigment Yellow 74, C.I. Pigment Yellow 93, C.I. Pigment Yellow 128, C.I. Pigment Yellow 94, C.I. Examples of pigments for green or cyan include C.I. Pigment Blue 15, C.I. Pigment Blue 15:2, C.I. Pigment Blue 15:3, C.I. Pigment Blue 16, C.I. Pigment Blue 60, and C.I. Pigment Green 7. Examples of pigments for black include C.I. Pigment Black 1, C.I. Pigment Black 6, and C.I. Pigment Black 7.

[0068] The content of the pigment is not particularly limited, but in the case of titanium oxide, it is preferably 7% by mass to 18% by mass of the ink, and the content of the organic pigment is preferably 0.5% by mass to 7% by mass of the ink.

[0069] (Polymer Dispersant) The polymer dispersant is not particularly limited, but a polymer dispersant having an anionic group is preferred.

[0070] Examples of polymer dispersants include block copolymers and random copolymers having a structure derived from two or more monomers selected from styrene, styrene derivatives, vinylnaphthalene derivatives, acrylic acid, acrylic acid derivatives, maleic acid, maleic acid derivatives, itaconic acid, itaconic acid derivatives, fumaric acid, and fumaric acid derivatives, as well as salts thereof, polyoxyalkylenes, and polyoxyalkylene alkyl ethers.

[0071] The polymer dispersant preferably has an acryloyl group, and is preferably neutralized with a neutralizing agent (neutralizing base) before addition. The neutralizing base is preferably an organic base such as ammonia, monoethanolamine, diethanolamine, N-methylethanolamine, N-methyldiethanolamine, triethanolamine, or morpholine. In particular, when the pigment is titanium oxide, the titanium oxide is preferably dispersed with a polymer dispersant having an acryloyl group. Commercially available polymer dispersants may be used. Examples of commercially available polymer dispersants include Joncryl 819 manufactured by BASF.

[0072] The molecular weight of the polymer dispersant may be, for example, from 5,000 to 200,000. The molecular weight can be measured as a polystyrene-equivalent value by a GPC method.

[0073] The content of the polymer dispersant is not particularly limited, but is preferably from 10% by mass to 100% by mass, and more preferably from 10% by mass to 40% by mass, based on the pigment.

[0074] The average particle size of the pigment dispersed in the ink is preferably 50 nm or more and less than 200 nm. This further improves the dispersion stability of the pigment and the storage stability of the ink. The particle size of the pigment can be measured by dynamic light scattering.

[0075] (Resin Particles) The resin particles are preferably water-insoluble resin particles that have solubility or affinity for the ink.

[0076] As mentioned above, water-insoluble resin microparticles are essentially water-insoluble, but have the form of resin dispersed in aqueous medium as micro-particles, and are water-insoluble resins that are forcibly emulsified and dispersed in water using emulsifiers etc., or water-insoluble resins that are self-emulsifiable and form stable aqueous dispersions by themselves without using emulsifiers or dispersion stabilizers by introducing hydrophilic functional groups into the molecule.These resins are usually used in the state of being emulsified and dispersed in water or water / alcohol mixed solvent.

[0077] The resin particles are preferably at least polyester resin, polyurethane resin, polyacrylic resin, or composite resin particles of polyurethane resin and polyacrylic resin.

[0078] The resin fine particles are preferably anionic from the viewpoint of affinity with the pigment and the pigment dispersant. In particular, the resin fine particles preferably contain an acidic group. This makes it possible to disperse the resin fine particles in water even with a small amount of surfactant added, thereby improving the water resistance of the resulting image. Such resin fine particles are also called self-emulsifying resin fine particles, and can be dispersed and stabilized in water only by molecular ionicity without using a surfactant. Examples of the acidic group include a carboxy group (-COOH), a sulfonic acid group (-SO ３ H) and the like.

[0079] It is preferable that at least a portion of the acidic groups are neutralized. Neutralizing the acidic groups can further improve the water dispersibility of the resin. As a neutralizing agent for neutralizing the acidic groups, organic amines are preferred, and it is preferable to use organic amines such as trimethylamine, triethylamine, tripropylamine, tributylamine, N-methyldiethanolamine, and triethanolamine.

[0080] The resin microparticles may be commercially available products. Examples of commercially available resin microparticles are listed below by resin type. <Polyester-based> PESRESIN A-110F, A-520, A-613D, A-615GE, A-640, A-645GH, A-647GEX manufactured by Takamatsu Oil & Fat Co., Ltd., EliteL KA-5034, KA-5071S, KA-1449, KA-0134, KA-3556, KA-6137, KZA-6034, KT-8803, KT-8701, KT-9204, KT-8904, KT-0507, KT-9511, etc. manufactured by Unitika Ltd. <Urethane-based> NeoRez R-967, R-600, R-9671 manufactured by Kusumoto Chemicals, Inc., W-6061, W-5661, WS-4000, etc. manufactured by Mitsui Chemicals, Inc. <Acrylic-based> NeoCryl A-1127 manufactured by Kusumoto Chemicals, Inc., Mowinyl 6899D, 6969D, 6800, 6810 manufactured by Japan Coating Resins, TOCRYL W-7146, W-7150, W-7152, etc. manufactured by Toyochem Co., Ltd.

[0081] The content of the resin particles is not particularly limited, but is preferably from 2% to 10% by mass, and more preferably from 2% to 5% by mass, based on the ink.

[0082] (Organic Solvent) As the organic solvent, a water-soluble organic solvent can be suitably used. Examples of the water-soluble organic solvent include alcohols, polyhydric alcohols, amines, amides, glycol ethers, and 1,2-alkanediols having 4 or more carbon atoms.

[0083] <Alcohols> Examples of alcohols include methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 2-methyl-1-propanol, t-butanol, 3-methoxy-1-butanol, 3-methoxy-3-methylbutanol, 1-octanol, 2-octanol, n-nonyl alcohol, tridecyl alcohol, n-undecyl alcohol, stearyl alcohol, oleyl alcohol, and benzyl alcohol.

[0084] <Polyhydric Alcohols> Examples of polyhydric alcohols include ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycols having 5 or more ethylene oxide groups, propylene glycol, dipropylene glycol, tripropylene glycol, polypropylene glycols having 4 or more propylene oxide groups, butylene glycol, hexanediol, pentanediol, glycerin, hexanetriol, and thiodiglycol.

[0085] <Amines> Examples of amines include ethanolamine, diethanolamine, triethanolamine, N-methyldiethanolamine, N-ethyldiethanolamine, morpholine, N-ethylmorpholine, ethylenediamine, diethylenediamine, triethylenetetramine, tetraethylenepentamine, polyethyleneimine, pentamethyldiethylenetriamine, and tetramethylpropylenediamine.

[0086] <Amides> Examples of amides include formamide, N,N-dimethylformamide, and N,N-dimethylacetamide.

[0087] <Glycol Ethers> Examples of glycol ethers include ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, propylene glycol monopropyl ether, dipropylene glycol monomethyl ether, and tripropylene glycol monomethyl ether.

[0088] <1,2-Alkanediols Having 4 or More Carbon Atoms> Examples of 1,2-alkanediols having 4 or more carbon atoms include 1,2-butanediol, 1,2-pentanediol, 1,2-hexanediol, and 1,2-heptanediol.

[0089] Among these, polyhydric alcohols are preferred from the viewpoint of suitably suppressing bleeding during high-speed printing, and for example, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propylene glycol, dipropylene glycol, and tripropylene glycol are preferred.

[0090] The content of the organic solvent is preferably, for example, 10% by mass or more and 60% by mass or less relative to the ink.

[0091] (Water) The content of water is preferably, for example, 45% by mass or more and 5% by mass or less relative to the ink.

[0092] (Other Additives) The ink may contain various known additives as needed to improve various performances, such as surfactants, ejection stability, storage stability, image storage stability, and the like. Among these, it is preferable for the ink to contain a surfactant. This can further improve the ejection stability of the ink from the nozzles and make it easier to control the spread of ink droplets that have landed on a recording medium (increase in dot diameter).

[0093] The surfactant is not particularly limited, but when the ink contains an anionic compound as another component, the ionicity of the surfactant is preferably anionic, nonionic, or betaine type.

[0094] For example, in this embodiment, fluorine-based or silicone-based surfactants having a high static surface tension reducing ability, anionic surfactants such as dioctyl sulfosuccinate having a high dynamic surface tension reducing ability, and nonionic surfactants such as relatively low molecular weight polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, acetylene glycols, Pluronic surfactants (Pluronic is a registered trademark), and sorbitan derivatives are preferably used. It is also preferable to use a fluorine-based or silicone-based surfactant in combination with a surfactant having a high dynamic surface tension reducing ability.

[0095] Adding a silicone-based or fluorine-based surfactant as the surfactant is preferred in that it can further suppress ink mixing (beading) on ​​recording media made of various hydrophobic resins, including vinyl chloride sheets, and recording media with low ink absorption capacity, such as printing paper, thereby enabling high-quality printed images to be obtained.

[0096] The silicone surfactant is preferably a polyether-modified polysiloxane compound, and examples thereof include KF-351A and KF-642 manufactured by Shin-Etsu Chemical Co., Ltd., BYK345, BYK347, and BYK348 manufactured by BYK-Chemie, and Tegowet 260 manufactured by Evonik.

[0097] Fluorine-based surfactants refer to surfactants in which some or all of the hydrogen atoms bonded to the carbon atoms of the hydrophobic groups of ordinary surfactants have been replaced with fluorine. Among these, those having a perfluoroalkyl group in the molecule are preferred.

[0098] Some fluorine-based surfactants are commercially available from Dainippon Ink and Chemicals, Inc. under the trade name Megafac F, from Asahi Glass Co., Ltd. under the trade name Surflon, from Minnesota Mining and Manufacturing Company, Inc. under the trade name Fluorad FC, from Imperial Chemical Industries, Inc. under the trade name Monflor, from E.I. duPont Nemelas and Company, Inc. under the trade name Zonyls, and from Falbewerke-Hoechst under the trade name Licowet VPF.

[0099] The content of the surfactant is not particularly limited, but is preferably 0.1% by mass or more and 5.0% by mass or less relative to the ink.

[0100] The viscosity of the ink is, for example, preferably 1 mPa·s or more and 40 mPa·s or less, and more preferably 2 mPa·s or more and 10 mPa·s or less, at 25°C. The viscosity of the ink can be measured using a rotational viscometer. Unless otherwise specified, the viscosity in this specification is the viscosity at 25°C.

[0101] The static surface tension of the ink is preferably 25 mN / m or more and 33 mN / m or less, and more preferably 25 mN / m or more and 29 mN / m or less, at 25° C. The static surface tension of the ink can be measured using a surface tensiometer that employs the platinum plate method (Wilhelmy method) or the like. Unless otherwise specified, the static surface tension in this specification is the static surface tension at 25° C.

[0102] 2-2. Treatment Liquid The treatment liquid contains at least a flocculant. The treatment liquid preferably contains, for example, a flocculant, a surfactant, water, and an organic solvent. It is preferable that the treatment liquid does not contain resin particles.

[0103] (Aggregating Agent) The aggregating agent may be any material that generates aggregates when it combines with ink containing a coloring material. This increases the interaction between the treatment liquid and the ink when they combine, solidifying the ink dots and suppressing the increase in dot diameter. The aggregating agent can be selected depending on the type of coloring material contained in the ink.

[0104] The flocculant preferably contains any of a cationic polymer, an organic acid, or a polyvalent metal salt, and more preferably contains a cationic polymer or a polyvalent metal salt.

[0105] The cationic polymer and polyvalent metal salt can aggregate anionic components in the ink (usually coloring materials or pigments) by salting out, while the organic acid has the function of aggregating anionic components in the ink by changing the pH.

[0106] Examples of the cationic polymer include polyallylamine, polyvinylamine, polyethyleneimine, polydiallyldimethylammonium chloride, etc. Commercially available cationic polymers include KHE100L and FPA100L manufactured by Senka Corporation, and PAS-92A, PAS-M-1A, and PAS-21CL manufactured by Nittobo Medical Co., Ltd.

[0107] Examples of polyvalent metal salts include water-soluble salts such as calcium salts, magnesium salts, aluminum salts, zinc salts, etc. Compounds that form salts with polyvalent metals include hydrochloric acid, hydrobromic acid, hydroiodic acid, sulfuric acid, nitric acid, phosphoric acid, thiocyanic acid, organic carboxylic acids such as acetic acid, oxalic acid, lactic acid, fumaric acid, citric acid, salicylic acid, and benzoic acid, and organic sulfonic acids.

[0108] The organic acid is capable of aggregating pigments that may be contained in the ink, and preferably has a first dissociation constant of 3.5 or less, more preferably 1.5 or more and 3.5 or less.

[0109] Examples of organic acids include formic acid, acetic acid, propionic acid, isobutyric acid, oxalic acid, fumaric acid, malic acid, citric acid, malonic acid, succinic acid, maleic acid, benzoic acid, 2-pyrrolidone-5-carboxylic acid, lactic acid, acrylic acid and derivatives thereof, methacrylic acid and derivatives thereof, compounds having a carboxy group including acrylamide and derivatives thereof, sulfonic acid derivatives, and phosphoric acid and derivatives thereof.

[0110] The content of the aggregating agent is not particularly limited, but is preferably 5% by mass or less, and more preferably 1% by mass or more and 4% by mass or less, relative to the treatment liquid. When the content of the aggregating agent is within the above range, the anionic components in the ink can be more effectively aggregated, thereby further increasing the image density. The content of the organic acid may be an amount that adjusts the pH of the treatment liquid to less than the first dissociation constant of the organic acid.

[0111] (Other Components) The content of water is not particularly limited, but is preferably 45% by mass or more and 80% by mass or less relative to the treatment liquid.

[0112] The treatment liquid may further contain an organic solvent in addition to water. As the organic solvent, the same organic solvents as those exemplified for the ink may be used. The content of the organic solvent is preferably 10% by mass or more and 50% by mass or less of the treatment liquid.

[0113] The treatment liquid may contain a surfactant. The surfactant may be the same as those exemplified for the ink. The surfactant content is preferably, for example, 0.08% by mass or more and 3% by mass or less relative to the treatment liquid.

[0114] The treatment liquid may contain other components such as a crosslinking agent, an antifungal agent, a bactericide, etc., as required.

[0115] The viscosity of the treatment liquid is adjusted by adding a solvent, etc., so that the treatment liquid can be ejected from the nozzles of the inkjet head. The viscosity of the treatment liquid is, for example, preferably 1 mPa·s or more and 40 mPa·s or less at 25° C., and more preferably 1 mPa·s or more and 10 mPa·s or less.

[0116] The static surface tension of the treatment liquid at 25°C is preferably 22 mN / m or more and 30 mN / m or less, and more preferably 22 mN / m or more and 26 mN / m or less.

[0117] Furthermore, the dynamic surface tension of the treatment liquid at 25°C and 50 ms is preferably 40 mN / m or less, more preferably 36 mN / m or less, and even more preferably 25 mN / m to 35 mN / m. The dynamic surface tension of the treatment liquid can be determined at 25°C using a Dynamic Surface Tension Meter BP-D4 type manufactured by Kyowa Interface Science Co., Ltd., and is the value at a lifetime of 50 ms when bubbles are continuously generated. Unless otherwise specified, the dynamic surface tension in this specification is the dynamic surface tension measured at 25°C and 50 ms using the maximum bubble pressure method.

[0118] The present invention will be specifically explained below with reference to examples, but the present invention is not limited to these examples.

[0119] 1. Preparation of Treatment Liquid and Ink <Preparation of Treatment Liquid> Treatment liquid a was prepared by mixing the following components: Polyvalent metal salt: calcium acetate 3% by mass Organic solvent: propylene glycol 30% by mass Surfactant: KF-351A (manufactured by Shin-Etsu Chemical Co., Ltd.) 0.5% by mass Water: balance

[0120] <Ink Preparation> Ink A was prepared by mixing the following components: Pigment: Pigment Blue 15:3 5% by mass Polymer dispersant: Joncryl 819 (manufactured by BASF) 2% by mass Neutralizer: N-methyldiethanolamine 0.4% by mass Resin particles: NeoRez R-650 (manufactured by DSM Coating Resin, polyether-based urethane resin emulsion) 5% by mass Organic solvent: propylene glycol 30% by mass Surfactant: KF-351A (manufactured by Shin-Etsu Chemical Co., Ltd.) 0.5% by mass Water: balance

[0121] 2. Image Formation Test and Evaluation Example 1 (Image Formation Test) An acrylic rubber substrate was prepared as a recording medium. The contact angle of the treatment liquid measured by the sessile drop method at 25°C was 60°. Next, an image formation test was carried out using the treatment liquid and ink prepared above.

[0122] First, an image forming apparatus having an independently driven inkjet head (360 dpi, droplet volume: small droplets 5 pL to 7 pL, medium droplets 14 pL to 16 pL, large droplets 20 pL to 25 pL) manufactured by Konica Minolta, Inc., as shown in Figure 1 was prepared. The treatment liquid a and ink A prepared above were each set.

[0123] Next, an area consisting of 3,000 rows and 3,000 columns, totaling 9 million pixels, was designated as image formation area a. Within this image formation area a, pixels onto which treatment liquid was to be ejected were selected to satisfy the method and pixel ratio shown in Table 1, and positions shifted by half a pixel in the nozzle row direction from the selected pixels were set as positions onto which treatment liquid was to be ejected. The shift width W1 of the pixels onto which treatment liquid was to be ejected from the selected pixels was achieved by shifting the distance between the nozzle position of the inkjet head 4Pr for the treatment liquid and the nozzle position of the inkjet head for the ink by half a pixel (0.5 times the pixel) in the nozzle row direction (Y direction).

[0124] Then, the head unit 4 was moved in the scanning direction X and the recording medium M was moved in the transport direction Y, and the treatment liquid a was ejected onto the recording medium M at the set pixels in the first pass, and then the ink A was ejected onto all pixels in the second pass to form an image.

[0125] Each liquid was ejected at 25°C and 50% RH. Furthermore, each liquid was ejected at 540 dpi in the main scan direction and 720 dpi in the sub-scan direction. dpi represents the number of ink droplets (dots) per 2.54 cm. The ejection frequency was 22.4 kHz. The ink droplet size (droplet volume) and the treatment liquid droplet size (droplet volume) were the values ​​shown in Table 1.

[0126] After the treatment liquid a and ink A were ejected, they were dried in a dryer at 80° C. for 15 minutes to obtain an image-formed product.

[0127] Examples 2 to 6 and Comparative Examples 1 to 7 Image formation was carried out in the same manner as in Example 1, except that the image formation conditions were changed as shown in Table 1.

[0128] <Evaluation> The resulting image-formed product was evaluated for streaky unevenness, density unevenness, and image edge disorder by the following methods.

[0129] (1) Streaky Unevenness The central part of the image was visually observed to check for the presence or absence of streaky unevenness, and was evaluated based on the following criteria: ○: No streaks were observed △: Some streaks were observed, but this was not a problem for practical use ×: Streaks were observed throughout

[0130] (2) Density Unevenness The central part of the image was visually observed to check for density unevenness, and was evaluated based on the following criteria: ○: No density unevenness was observed △: Some density unevenness was observed but this was not a problem for practical use ×: Density unevenness was observed XX: Obvious density unevenness or density reduction was observed

[0131] (3) Image Edge Disturbance The periphery of the image was visually observed to check for edge disturbance. Evaluation was then made based on the following criteria: ○: No image edge disturbance was observed △: Some image edge disturbance was observed, but this was not a problem for practical use ×: Image edge disturbance was observed XX: Obvious image edge disturbance was observed

[0132] The evaluation results of Examples 1 to 6 and Comparative Examples 1 to 7 are shown in Table 1. In Table 1, "-" indicates that the evaluation was not performed.

[0133]

[0134] As shown in Table 1, in Comparative Example 1, in which treatment liquid was applied to all pixels in the image formation region a, the coloring material in the ink was excessively aggregated by the treatment liquid, resulting in density unevenness. Furthermore, in Comparative Examples 2, 4, and 6, in which the droplet size of the treatment liquid was smaller than the droplet size of the ink and the treatment liquid was regularly thinned out, density unevenness and streaky unevenness were somewhat reduced when the printing distance was short, but density unevenness and streaky unevenness when the printing distance was long were not suppressed. Furthermore, in Comparative Examples 3, 5, and 7, in which the application positions of the ink and treatment liquid were not shifted even when the treatment liquid was irregularly thinned out, density unevenness when the printing distance was long was not sufficiently suppressed.

[0135] In contrast to this, it can be seen that in Examples 1 to 6, density unevenness and streaky unevenness can be sufficiently suppressed not only when the printing distance is short, but also when the printing distance is long.

[0136] From these findings, it can be seen that by applying the treatment liquid irregularly, making the droplet size of the treatment liquid smaller than the droplet size of the ink, combining two or more types, and shifting the ink and treatment liquid by less than one pixel, it is possible to suppress density unevenness and streaky unevenness caused by excessive aggregation of the ink colorant, even when the printing position is far away.

[0137] This application claims priority based on Japanese Patent Application No. 2023-121883, filed on July 26, 2023. The contents of the specification and drawings of that application are incorporated herein by reference in their entirety.

[0138] According to the present invention, an image forming method can be provided that can form images with little streak-like unevenness and little density unevenness, even when the amount of treatment liquid applied is reduced and the treatment liquid is applied from a position away from the surface of the recording medium.

[0139] REFERENCE SIGNS LIST 1 Image forming apparatus 2 Droplet ejection means 3 Scanning section 4 Head unit 4Y, 4M, 4C, 4K Ink ink heads 4Pr Ink jet head for treatment liquid 10 Image 12 Treatment liquid droplet 12a First droplet 12b Second droplet 14 Ink droplet a Image forming area

Claims

1. An image forming method comprising: a treatment liquid ejection step of ejecting droplets of treatment liquid from a first inkjet head onto a recording medium; and an ink ejection step of ejecting droplets of ink containing a colorant that aggregates with the treatment liquid from a second inkjet head, wherein in the treatment liquid ejection step, some pixels are irregularly selected from an image forming area onto which the ink droplets are ejected, and droplets of the treatment liquid are ejected at positions shifted by less than one pixel from the selected pixels, the droplets of the treatment liquid include two or more types of droplets having different droplet sizes, and the maximum droplet size of the treatment liquid ejected in the treatment liquid ejection step is smaller than the droplet size of the ink ejected in the ink ejection step.

2. The image forming method according to claim 1, wherein a total amount of droplets of the treatment liquid ejected onto the image forming area is 20% or more and 80% or less of a total amount of droplets of the ink ejected onto the image forming area.

3. The image forming method according to claim 1, wherein the two or more types of droplets having different droplet sizes include first droplets having a droplet volume of 10 pL or more and less than 20 pL, and second droplets having a droplet volume of less than 10 pL.

4. The image forming method according to claim 3, wherein in said image forming area, the total number of pixels onto which said first droplets are discharged: the total number of pixels onto which said second droplets are discharged is 1:0.4 to 1:1.

6.

5. The image forming method according to claim 1, wherein the range of less than one pixel is a range of 0.4 pixels or more and 0.6 pixels or less.

6. The image forming method according to claim 1, wherein the maximum droplet size of said treatment liquid is 20% or more and 90% or less of the droplet size of said ink.

7. The image forming method according to claim 1, wherein the distance between the ejection surfaces of said first inkjet head and said second inkjet head and the surface of said recording medium is 3 mm or more.

8. The image forming method according to claim 1, wherein the treatment liquid contains a polyvalent metal salt.

9. The image forming method according to claim 1, wherein the recording medium is a low absorbent substrate.

10. The image forming method according to claim 9, wherein the contact angle of the treatment liquid on the surface of the low absorbent substrate at 25° C. is from 40° to 100°.

Images