JP2013006345A - Image forming method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming method capable of forming an image of good density at high speed by an inkjet recorder with no special mechanism without using any liquid other than ink while suppressing an ink ejection failure when the image is formed after stop of ink ejection from a recording head for some time and contamination of a recording medium by offsetting.SOLUTION: In the image forming method by a line head system inkjet recorder provided with a four or more of recording heads for ejecting black ink and three or more of color ink and a discharge part for discharging the recording medium, weight average molecular weight of a resin included in the ink and the value of resin mass/pigment mass are made within a specific range and black ink of specific constitution is ejected from a recording head of first color and further color ink is ejected from recording heads of second and subsequent color. Color ink having largest sum of the content of the pigment and the resin among color ink is ejected from the recording head of the second color.

Description

  The present invention relates to an image forming method using a line head type ink jet recording apparatus having a plurality of recording heads using black ink and three or more multi-color inks.

  In recent years, rapid progress in recording technology has made it possible to obtain high-definition image quality comparable to silver halide photography, so inkjet recording apparatuses that form images by the inkjet recording method have been widely used as image forming apparatuses. ing.

  For such an ink jet recording apparatus, it is strongly desired to improve the image forming speed while maintaining the image quality. In order to increase the image forming speed, it is effective to use a line head type ink jet recording apparatus. However, when image formation is performed at high speed in a line head type ink jet recording apparatus, the recording medium is transported and discharged by the discharge roller before the ink penetrates the recording medium such as paper, and the ink is discharged. It may adhere (offset) to the roller, and in this case, image defects are likely to occur. In order to suppress the occurrence of offset, it is conceivable to reduce the amount of ink discharged or to increase the penetrability and dryness of the ink on the recording medium. In the latter case, when the image is formed after the ink has not been ejected from the recording head for a while, ink ejection failure may occur due to a change in the viscosity of the ink.

  Under such circumstances, as a method of forming a high-density image at high speed, it is difficult to cause contamination of the recording medium such as paper due to offset, and the ink is applied to the recording medium on which the hydrophobic polymer is coated. An ink jet recording method for adhering both a treatment liquid and an ink capable of aggregating contained colorant components, the same kind as a hydrophobic polymer coated on a recording medium to the ink and / or the treatment liquid Inkjet recording methods have been proposed in which the colorant components are aggregated and fixed on the recording medium by containing fine particles of the polymer, and the solvent absorption roller provided in the image forming apparatus absorbs excess solvent (Patent Document). 1).

JP 2006-263984 A

  However, in the ink jet recording method described in Patent Document 1, since a processing liquid is used in addition to ink, a mechanism for attaching the processing liquid to a recording medium and a solvent absorbing roller are added to a normal ink jet recording apparatus. There is a need. For this reason, the method described in Patent Document 1 has problems such as an increase in the size of the ink jet recording apparatus, an increase in the manufacturing cost of the recording apparatus, and an increase in the cost for image formation per image. In addition, in the ink jet recording method described in Patent Document 1, it is necessary to contain a large amount of hydrophobic polymer in the ink and / or treatment liquid. When the ink contains a hydrophobic polymer, the solid content concentration of the ink increases. For this reason, when an image is formed after ink has not been ejected from the recording head for a while, ink ejection failure may occur due to a change in ink viscosity or the like.

  The present invention has been made in view of such circumstances, while suppressing ink ejection failure when forming an image after ink has not been ejected from the recording head for a while and contamination of the recording medium due to offset. An object of the present invention is to provide an image forming method capable of forming a high-density image at high speed by an inkjet recording apparatus that does not have a special mechanism such as a solvent absorption roller without using other liquids such as processing liquids. And

  The present inventors have formed an image by an ink jet recording apparatus of a line head type comprising a plurality of four or more recording heads that discharge black ink, three or more color inks, and a discharge unit that discharges a recording medium. In the method, the weight average molecular weight of the resin contained in the ink and the mass of the resin / the mass of the pigment are in a specific range, black ink having a specific configuration is ejected from the recording head of the first color, and the second and subsequent colors are discharged. The present invention has found that the above problems can be solved by ejecting color ink from a recording head and ejecting ink having the largest total content of pigment and resin in the color ink from the recording head of the second color. It came to complete. Specifically, the present invention provides the following.

(1) An image forming method using a line head type ink jet recording apparatus including four or more recording heads that discharge black ink, three or more color inks, and a discharge unit that discharges a recording medium. And
The black ink and the color ink include a pigment, a resin, water, and an organic solvent,
The pigment and the resin are contained in the ink as a pigment dispersion,
In the black ink and the color ink, the resin has a weight average molecular weight of 30,000 to 200,000, and a resin mass / pigment mass value of 0.3 to 1.0,
The ink ejected from the first color recording head is black ink, and the ink ejected from the second and subsequent recording heads is color ink,
The black ink has the largest resin weight average molecular weight and the smallest resin mass / pigment mass value among all inks,
The ink ejected from the recording head for the second color is an image forming method in which the total content of the pigment and the resin is the largest among the color inks.

(2) The lightness L ^* is the highest when comparing the lightness L ^* in the L ^* a ^* b ^* color system of the image formed with the same amount of single color ink as the last ink ejected among the color inks. The image forming method according to (1), which is a color ink having a high ^* .

  According to the present invention, it is possible to use other liquids of ink while suppressing ink ejection failure when forming an image after ink has not been ejected from the recording head for a while and contamination of the recording medium due to offset. In addition, an image having a good density can be formed at high speed by an ink jet recording apparatus that does not include a special mechanism.

FIG. 1 is a side sectional view showing a schematic configuration of a line head type ink jet recording apparatus. FIG. 2 is a plan view of the conveyance belt of the ink jet recording apparatus shown in FIG. 1 as viewed from above. FIG. 3 is a block diagram showing the configuration of a line head type ink jet recording apparatus. FIG. 4 is an enlarged plan view showing a part of a dot head formed on a line head and recording paper used in an ink jet recording apparatus of a line head type recording system.

  Hereinafter, embodiments of the present invention will be described in detail, but the present invention is not limited to the following embodiments, and can be implemented with appropriate modifications within the scope of the object of the present invention. In addition, although description may be abbreviate | omitted suitably about the location where description overlaps, the summary of invention is not limited.

  The image forming method of the present invention is based on a line head type ink jet recording apparatus that includes a plurality of recording heads that discharge black ink, three or more color inks, and a discharge unit that discharges a recording medium. An image forming method, using four or more colors of ink containing pigment, resin, water and organic solvent, ejecting black ink from the first color recording head, ejecting color ink from the second and subsequent recording heads, According to the order of ink ejection, the pigment content and the resin content satisfy a predetermined relationship. Hereinafter, the ink, the ink preparation method, and the image forming method will be described in order with respect to the present invention.

〔ink〕
The ink used in the present invention contains a pigment, a resin, water, and an organic solvent. In addition, the ink used in the present invention includes a dissolution stabilizer that stabilizes the dissolution state of the components contained in the ink, and a humectant that stabilizes the viscosity of the ink by suppressing the volatilization of the liquid component from the ink, if necessary. May be included. In the ink used in the present invention, the pigment and the resin are contained as a pigment dispersion. Hereinafter, regarding the ink used in the present invention, the pigment dispersion, water, organic solvent, dissolution stabilizer, and humectant will be described in order.

  In the present invention, black ink and three or more color inks are used. The number of inks is not particularly limited as long as the object of the present invention is not impaired. Four colors of ink consisting of black ink, cyan ink, yellow ink, and magenta ink are used because images of various hues can be satisfactorily formed, the number of recording heads can be reduced, and the inkjet recording apparatus can be easily downsized. Is preferred.

(Pigment dispersion)
The pigment that can be contained in the pigment dispersion is not particularly limited as long as it does not impair the object of the present invention, and can be appropriately selected from pigments that have been conventionally used as colorants for inks for inkjet recording apparatuses. . Specific examples of pigments used for cyan ink include C.I. I. And blue pigments such as CI Pigment Blue 15. Specific examples of pigments used in yellow ink include C.I. I. Pigment Yellow 74, 93, 95, 109, 110, 120, 128, 138, 139, 151, 154, 155, 173, 180, 185, 193 and the like are listed. Examples of pigments used in magenta ink C. I. And red pigments such as CI Pigment Red 122 and 202. Examples of pigments used for black ink include C.I. I. And black pigments such as CI Pigment Black 7 (BK-7, carbon black).

  Specific examples of the other hue pigments include C.I. I. Pigment Orange 34, 36, 43, 61, 63, 71 and other orange pigments, C.I. I. And purple pigments such as CI Pigment Violet 19, 23 and 33. Two or more of these pigments can be used in combination to adjust the hue of the ink to a desired hue.

  For magenta ink, a good hue can be reproduced in a wide color gamut. I. More preferably, CI Pigment Red 122 (PR 122) is used.

  The pigment content in the ink is not particularly limited as long as the object of the present invention is not impaired. The content of a typical pigment in the ink is preferably 3 to 10% by mass and more preferably 4 to 9% by mass with respect to the mass of the ink. The pigment is C.I. I. In the case of CI Pigment Red 122, since it is difficult to form an image having a good density, the content of the pigment in the ink is preferably 7 to 10% by mass. If the pigment content in the ink is too small, it is difficult to form an image with a good density. If the pigment content is excessive, the ink used for forming the image after the ink has not been ejected for a while from the recording head This is likely to cause a discharge failure.

  The resin contained in the pigment dispersion is not particularly limited as long as it does not impair the object of the present invention, and can be appropriately selected from various resins conventionally used in the production of pigment dispersions. Specific examples of suitable resins include styrene-acrylic acid-acrylic acid alkyl ester copolymers, styrene-acrylic acid copolymers, styrene-maleic acid copolymers, and styrene-maleic acid-acrylic acid alkyl ester copolymers. Styrene-methacrylic acid copolymer, styrene-methacrylic acid alkyl ester copolymer, styrene-maleic acid half ester copolymer, vinyl naphthalene-acrylic acid copolymer, vinyl naphthalene-maleic acid copolymer, and the like. . Among these resins, since they are easy to prepare and have excellent pigment dispersion effects, styrene-acrylic acid-alkyl acrylate ester copolymers, styrene-acrylic acid copolymers, styrene-maleic acid-alkyl acrylates. Units derived from styrene, such as ester copolymers, styrene-methacrylic acid copolymers, styrene-methacrylic acid alkyl ester copolymers, and units derived from acrylic acid, methacrylic acid, acrylic esters, or methacrylic esters A styrene-acrylic resin containing The above resin is obtained by radical polymerization.

  The weight average molecular weight (Mw) of the resin used for preparing the pigment dispersion is 30,000 to 200,000. The weight average molecular weight (Mw) of the resin contained in the pigment dispersion can be measured by gel filtration chromatography. If the molecular weight of the resin is too small, the recording medium is likely to be contaminated by offset and it is difficult to form an image with a good density. If the molecular weight of the resin is excessive, ink ejection failure is likely to occur when an image is formed after the recording head has not ejected ink for a while.

  In addition, there exists a tendency which can form an image of a favorable density | concentration, so that the weight average molecular weight (Mw) of resin is large. In order to form an image with a good density, the higher the density of the image formed with the black ink, the better. For this reason, the weight average molecular weight (Mw) of the resin contained in the black ink is preferably 70000 or more.

  The amount of resin used in the ink used in the present invention is such that the resin mass / pigment mass value is 0.3 to 1.0. By setting the masses of the resin and the pigment within such ranges, it is possible to form an image with a good density while suppressing ink ejection failure when forming an image after the ink has not been ejected from the recording head for a while.

  A method for producing a pigment dispersion containing a pigment and a resin is not particularly limited as long as the object of the present invention is not impaired, and can be appropriately selected from conventionally known methods. As a suitable method, for example, using a media type wet disperser such as Nano Glen Mill (manufactured by Asada Tekko Co., Ltd.), MSC Mill (manufactured by Mitsui Mining Co., Ltd.), Dino Mill (manufactured by Shinmaru Enterprises Co., Ltd.), water, etc. In a suitable liquid medium, a pigment and a resin are kneaded to form a pigment dispersion. In the processing by the media type wet disperser, beads having a small particle diameter are used. The particle diameter of the beads is not particularly limited and is typically a particle diameter of 0.5 to 1.0 mm. The material of the beads is not particularly limited, and a hard material such as zirconia is used.

(water)
The ink used in the present invention is a water-based ink and essentially contains water. The water contained in the ink is not particularly limited as long as it does not impair the object of the present invention, and water having a desired purity can be appropriately selected from water conventionally used for producing water-based inks. The water content in the ink is not particularly limited as long as the object of the present invention is not impaired. The content of water is appropriately changed according to the amount of other components to be described later. The typical water content in the ink is preferably 20 to 70% by mass, more preferably 30 to 60% by mass with respect to the total mass of the ink.

(Organic solvent)
The ink used in the present invention contains an organic solvent for the purpose of promoting the penetration of the ink into the recording medium. Specific examples of suitable organic solvents include ethylene glycol monobutyl ether, triethylene glycol monomethyl ether, triethylene glycol monobutyl ether, diethylene glycol monomethyl ether, alkylene glycol monoalkyl ether such as ethylene glycol monomethyl ether, and 1,2-hexylene. Examples include alkanediols having 6 to 9 carbon atoms such as glycol. These organic solvents may be used alone or in combination of two or more. 10-60 mass% is preferable with respect to the total mass of an ink, and, as for content of the organic solvent in an ink, 20-50 mass% is more preferable.

(Dissolution stabilizer)
The dissolution stabilizer is a component that compatibilizes the components contained in the ink and stabilizes the dissolved state of the ink. Specific examples of the dissolution stabilizer include 2-pyrrolidone, N-methyl-2-pyrrolidone, and γ-butyrolactone. These dissolution stabilizers can be used in combination of two or more. When the ink contains a dissolution stabilizer, the content of the dissolution stabilizer is preferably 1 to 20% by mass and more preferably 3 to 15% by mass with respect to the total mass of the ink.

(Humectant)
The humectant is a component that stabilizes the viscosity of the ink by suppressing the volatilization of the liquid component from the ink. Specific examples of the humectant include polyethylene glycol, polypropylene glycol, ethylene glycol, propylene glycol, 1,3-propanediol, butylene glycol, diethylene glycol, dipropylene glycol, triethylene glycol, tripropylene glycol, 1,2,6-hexane. Examples include triol, thiodiglycol, 1,3-butanediol, 1,5-pentanediol, and glycerin. Among these humectants, glycerin is more preferable because of its excellent effect of suppressing volatilization of liquid components such as water. Two or more kinds of humectants can be used in combination. When the ink contains a humectant, the content of the humectant is preferably 5 to 50% by mass and more preferably 10 to 40% by mass with respect to the total mass of the ink.

[Ink production method]
The ink production method is not particularly limited as long as ink components such as a pigment dispersion, water, and an organic solvent can be mixed uniformly. As a specific example of the method for producing the ink for an ink jet recording apparatus, there is a method in which each component of the ink is uniformly mixed by a mixer and then foreign matters and coarse particles are removed by a filter having a pore diameter of 10 μm or less. In addition, when manufacturing ink, components such as a dissolution stabilizer and a humectant, surfactants, antioxidants, viscosity modifiers, pH adjusters, antiseptic and antifungal agents, etc. Various additives that have been added to inks for inkjet recording devices can be added.

(Image forming method)
Hereinafter, referring to the drawings, as a preferred example of the image forming method of the present invention, a line head type ink jet recording apparatus having four color recording heads is used, and a recording paper as a recording medium is used. A case where an image is formed using black ink, cyan ink, magenta ink, and yellow ink will be described. FIG. 1 is a side sectional view showing a schematic configuration of an ink jet recording apparatus of a line head type recording system, and FIG. 2 is a plan view of a transport belt of the ink jet recording apparatus shown in FIG.

  As shown in FIG. 1, a paper feed tray 2 that stores recording paper P is provided on the left side of the inkjet recording apparatus 100, and the recording paper P that is stored at one end of the paper feed tray 2 is A feeding roller 3 for feeding and feeding one sheet at a time in order from the uppermost recording sheet P and a driven roller 4 that is in pressure contact with the feeding roller 3 and driven to rotate are provided.

  A transport belt 5 is rotatably disposed on the downstream side (right side in FIG. 1) of the paper feed roller 3 and the driven roller 4 in the paper transport direction. The conveying belt 5 is stretched between a belt driving roller 6 disposed on the downstream side in the sheet conveying direction and a belt roller 7 disposed on the upstream side and rotated by the belt driving roller 6 via the conveying belt 5. As the belt driving roller 6 is driven to rotate clockwise, the recording paper P is conveyed in the arrow X direction.

  Here, since the belt driving roller 6 is arranged on the downstream side in the paper transport direction X, the paper feeding side (upper side in FIG. 1) of the transport belt 5 is pulled by the belt driving roller 6, so that belt tension is applied. Therefore, the recording paper P can be stably conveyed. In addition, a sheet made of dielectric resin is used for the conveyance belt 5, and a (seamless) belt or the like having no seam is preferably used.

  Further, on the downstream side of the conveyance belt 5 in the sheet conveyance direction, the recording sheet P on which an image is recorded by being driven clockwise in the drawing is discharged to the outside of the apparatus main body, and is pressed against the upper portion of the discharge roller 8a. A driven roller 8b that is driven to rotate is provided, and a discharge tray 10 on which the recording paper P discharged outside the apparatus main body is stacked is provided on the downstream side of the discharge roller 8a and the driven roller 8b.

  Since the driven roller 8b directly touches the printing surface, the material forming the surface of the driven roller 8b is preferably a water repellent material. By forming the surface of the driven roller 8b with a water-repellent material, it is possible to suppress the ink that has not penetrated the recording paper P from adhering to the roller and easily suppress the occurrence of offset. Suitable water repellent materials include polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, tetrafluoroethylene-ethylene copolymer, tetrafluoro Ethylene-vinylidene fluoride copolymer, tetrafluoroethylene-hexafluoropropylene-perfluoroalkyl vinyl ether copolymer, polychlorotrifluoroethylene, chlorotrifluoroethylene-ethylene copolymer, chlorotrifluoroethylene-vinylidene fluoride copolymer Examples thereof include fluororesins such as polymers, polyvinylidene fluoride, and polyvinyl fluoride. Similar to the driven roller 8b, the surface of the member in contact with the printing surface is preferably formed of a water repellent material.

  An image is recorded on the recording paper P that is supported above the conveying belt 5 at a height that forms a predetermined interval with respect to the upper surface of the conveying belt 5. Line heads 11a, 11b, 11c and 11d to be performed are arranged. About these line heads 11a to 11d, the line head 11a was prepared such that the resin having the largest weight average molecular weight was included in all the inks, and the resin mass / pigment mass value was the smallest. Filled with black ink. The line heads 11b to 11d are filled with three different color inks (cyan, magenta, and yellow), and the line head 11b has the most pigment and resin content among the color inks. Is filled with ink.

  In order to form an image with a good density, it is preferable that the density of the image formed with the black ink is higher than that of the color ink. When the weight average molecular weight of the resin contained in the ink is increased and the resin mass / pigment mass value is decreased, the density of the formed image tends to increase. On the other hand, in such a case, the recording medium tends to be contaminated by the offset. Here, the distance between the line head 11a and the driven roller 8b is the longest, and when ink is ejected from the line head 11a, the problem of offset hardly occurs. For this reason, the black ink prepared from the line head 11a as the first color ink so as to contain the resin having the largest weight average molecular weight among all the inks and having the smallest resin mass / pigment mass value. By discharging the ink, it is possible to form an image with a good density while suppressing the contamination of the recording medium due to the offset.

  Note that the black ink realizes suppression of ink ejection failure when forming an image after the recording head has not ejected ink for a while, suppression of contamination of the recording medium due to offset, and good print density. Since it is easy, the resin mass / pigment mass value is preferably 0.45 to 0.55, and the resin weight average molecular weight (Mw) is preferably 70000 to 200000.

  In addition, for color inks, if the pigment content is high, the resin content will inevitably increase, and therefore, ink ejection failure is likely to occur when an image is formed after the ink has not been ejected from the recording head for a while. . For this reason, in inks with a high pigment content, even if it is effective to increase the resin mass / pigment mass value for suppressing contamination of the recording medium due to offset, the ink resin content is increased. It is hard to let you.

  Therefore, in the present invention, among the color inks, the color ink having the largest total content of pigment and resin is ejected from the line head 11b as the second color ink. In such a case, the line head 11b has the longest distance from the driven roller 8b among the line heads 11b to 11d. Therefore, even if the color ink having the largest total content of pigment and resin is ejected, the line head 11b is offset. The recording medium is hardly contaminated. In such a case, the value of resin mass / pigment mass is reduced within a range that does not cause an offset problem, and ink ejection failure is caused when an image is formed after the recording head has not ejected ink for a while. Easy to suppress.

  As the color ink ejected to the second color, C.I. I. Magenta ink using Pigment Red 122 is preferable. C. I. Since the pigment red 122 has low coloring power, it is necessary to increase the content of the pigment in the ink in order to form an image with a good density. However, C.I. I. By ejecting magenta ink using Pigment Red 122 to the second color, it is possible to prevent ink ejection failure and offset problems when forming an image after the recording head has not ejected ink for a while. I. A good image can be formed by taking advantage of the excellent properties of CI Pigment Red 122.

  Of the three color inks, when the total content of the resin and the pigment of two or more color inks is equal, any color ink may be ejected from the line head 11b as the second color ink. The line heads 11b to 11d have a content of the pigment and the resin because the ink ejection failure and the offset problem when forming an image after the ink has not been ejected from the recording head for a while are more easily suppressed. It is more preferable that the color inks are ejected in the descending order of the total.

  For color inks, it is easy to achieve poor ink ejection when forming an image after the ink has not been ejected from the recording head for a while, suppression of recording medium contamination due to offset, and good print density. Therefore, in the cyan ink, the resin mass / pigment mass value is preferably 0.7 to 0.8 and the weight average molecular weight (Mw) of the resin is preferably 70000 to 200000. The mass / pigment mass value is preferably 0.3 to 0.8, and the weight average molecular weight (Mw) of the resin is preferably 30000 to 200000. In magenta ink, P.I. When R-122 is used, the resin mass / pigment mass value is 0.6 to 0.7, and the resin weight average molecular weight (Mw) is 40000 to 70000, or the resin mass / pigment mass. The mass value is preferably 0.55 to 0.6, and the weight average molecular weight (Mw) of the resin is preferably 70000 to 140000.

Further, the color ink discharged last from the color ink, that is, the color ink discharged from the line head 11d is the brightness (L in the L ^* a ^* b ^* color system of the image formed with the same amount of single color ink. ^* ), It is preferable that the color ink has the highest lightness (L ^* ). When forming an image by ejecting the highest color ink lightness (L ^*) at the end, as compared with the other to form a high excellent image lightness (L ^*).

  Among the line heads 11a to 11d, after the ink droplets discharged from the line head 11d have landed on the recording paper P, the ink landing location on the recording paper P consists of the discharge roller 8a and the driven roller 8b. The time required to reach the discharge unit 8 for discharging the recording paper P is preferably within 1 second in order to reduce the size of the apparatus. Even when the time is within 1 second, according to the image forming method of the present invention, it is easy to suppress the occurrence of contamination of the recording medium due to the offset at the time of image formation at high speed.

Also, ejected from the line head 11a~11d the recording paper P, the total amount of write ink hit in the recording paper P is preferably 20 g / ^{m 2} or less, more preferably 15 g / ^{m 2} or less, especially 10 g / ^{m 2} or less preferable. By setting the ink discharge amount to such an amount, it is easy to form an image at high speed while suppressing the occurrence of offset.

  As shown in FIG. 2, these line heads 11 a to 11 d include a nozzle row in which a plurality of nozzles are arranged in a direction (vertical direction in FIG. 2) orthogonal to the transport direction, and the width of the recording paper P being transported Having the above recording area, the image of one line can be collectively recorded on the recording paper P conveyed on the conveying belt 5.

  In the line head type ink jet recording apparatus, a plurality of nozzles are arranged in the longitudinal direction of a long head body formed to have a width dimension larger than that of the transport belt 5, thereby increasing the width of the recording paper P. However, the recording paper is conveyed by arranging a plurality of short head units each having a plurality of nozzles in the width direction of the conveying belt 5, for example. A line head that can record an image over the entire width in the P width direction may be used.

  Further, as the ink ejection method of the line heads 11a to 11d, for example, ink droplets are ejected using pressure generated in the liquid chambers of the line heads 11a to 11d using a piezoelectric element (piezo element) (not shown). Various methods such as a piezoelectric element method and a thermal ink jet method in which bubbles are generated by a heating element and ink is ejected under pressure can be applied. The ink ejection method is preferably a piezoelectric element method because the ejection amount can be easily controlled.

  FIG. 3 is a block diagram showing a configuration of a line head type ink jet recording apparatus. Portions common to FIGS. 1 and 2 are denoted by the same reference numerals and description thereof is omitted. The inkjet recording apparatus 100 includes a control unit 20. An interface 21, ROM 22, RAM 23, encoder 24, motor control circuit 25, line head control circuit 26, voltage control circuit 27, and the like are connected to the control unit 20. Has been.

  The interface 21 transmits / receives data to / from a host device such as a personal computer (not shown). The control unit 20 converts the image signal received via the interface 21 into image data by performing scaling processing or gradation processing as necessary. And a control signal is output to the various control circuits mentioned later.

  The ROM 22 stores a control program or the like for recording the image by driving the line heads 11a to 11d. The RAM 23 stores the image data subjected to scaling processing or gradation processing by the control unit 20 in a predetermined area.

  The encoder 24 is connected to a belt driving roller 6 on the paper discharge side that drives the conveyance belt 5, and outputs a pulse train according to the rotational displacement amount of the rotating shaft of the belt driving roller 6. The control unit 20 calculates the rotation amount by counting the number of pulses transmitted from the encoder 24, and grasps the paper feed amount (paper position). Then, the control unit 20 outputs a control signal to the motor control circuit 25 and the line head control circuit 26 based on the signal from the encoder 24.

  The motor control circuit 25 drives the recording medium transport motor 28 by an output signal from the control unit 20. The recording medium conveying motor 28 is driven to rotate the belt driving roller 6 and rotate the conveying belt 5 clockwise in FIG. 1 to convey the sheet in the direction of the arrow X.

  The line head control circuit 26 transfers the image data stored in the RAM 23 to the line heads 11a to 11d based on the output signal from the control unit 20, and the line heads 11a to 11d from the line heads 11a to 11d based on the transferred image data. Controls ink ejection. By this control and the control of the conveyance of the sheet by the conveyance belt 5 driven by the recording medium conveyance motor 28, the recording process on the sheet is performed.

  The voltage control circuit 27 generates an alternating electric field by applying a voltage to the belt roller 7 on the paper feeding side based on an output signal from the control unit 20, and electrostatically attracts the paper P to the transport belt 5. The electrostatic adsorption is released by grounding the belt roller 7 or the belt driving roller 6 based on an output signal from the control unit 20. Here, the voltage is applied to the belt roller 7 on the paper feed side, but the voltage may be applied to the belt drive roller 6 on the paper discharge side.

  A method of forming dots using a line head type ink jet recording apparatus will be described in detail with reference to FIG. In FIG. 4, the line head 11 a is described as an example among the line heads 11 a to 11 d illustrated in FIGS. 1 and 2, but the other line heads 11 b to 11 d are also described in the same manner.

  As shown in FIG. 4, the line head 11a is provided with nozzle rows N1 and N2 each including a plurality of nozzles arranged in parallel in the conveyance direction (arrow X direction) of the recording paper P. That is, as nozzles for forming each dot row in the conveyance direction of the recording paper P, each nozzle row N1 and N2 is provided with a total of two nozzles (for example, nozzles 12a and 12a ′ in the dot row L1). . For convenience of explanation, only 16 nozzles of 12a to 12p and 12a ′ to 12p ′ corresponding to the dot rows L1 to L16 are shown in the nozzles constituting the nozzle rows N1 and N2. In reality, however, it is assumed that a larger number of nozzles are arranged in a direction perpendicular to the conveyance direction of the recording paper P.

  Then, an image is formed on the recording paper P by sequentially using the nozzle rows N1 and N2. For example, after forming the dot row D1 for one row in the width direction (left and right direction in the figure) of the recording paper P by ink ejection from the nozzle row N1 (solid line arrow in the figure) while moving the recording paper P in the transport direction. The next dot row D2 for one row is formed by ink ejection from the nozzle row N2 (broken arrows in the figure), and the next dot row D3 for the next row is again formed by ink ejection from the nozzle row N1. . Hereinafter, the dot rows D4 and thereafter are similarly formed by alternately using the nozzle rows N1 and N2.

  According to the image forming method described above, while suppressing ink ejection failure when forming an image after the ink has not been ejected from the recording head for a while and contamination of the recording medium due to offset, An image having a good density can be formed at high speed by an ink jet recording apparatus that does not have a special mechanism without using a liquid. For this reason, the image forming method of the present invention is suitably used in various ink jet recording apparatuses that employ a line head system.

  Hereinafter, the present invention will be described more specifically with reference to examples. In addition, this invention is not limited at all by the Example.

[Preparation Example 1]
(Manufacture of styrene-acrylic resin)
Resins 1 to 8 (R1 to R8), which are styrene-acrylic resins used for the preparation of the pigment dispersion, were produced by a macromonomer synthesis method. Specifically, an oligomer (AS-6, manufactured by Toa Gosei Co., Ltd., number average molecular weight (Mn) 6,000) having a (meth) acryloyl group bonded to one of polystyrene molecular ends, and a predetermined amount of methacrylic acid. , Methyl methacrylate, and butyl acrylate in methyl ethyl ketone in the presence of a polymerization initiator (2,2′-azobis (2,4-dimethylvaleronitrile)), the amount of polymerization initiator used, the polymerization temperature, Resin 1 to Resin 8 (R1 to R8), which are styrene-acrylic resins having different molecular weights, were produced by changing the polymerization time. The weight average molecular weight (Mw) of the obtained resin was confirmed under the following conditions using gel filtration chromatography (HLC-8020GPC (manufactured by Tosoh Corporation)). Table 1 shows the weight average molecular weights (Mw) of Resin 1 to Resin 8 (R1 to R8). Moreover, when the acid value (mgKOH / g) of the obtained resin was confirmed by titration, in any case, the acid value was 300 or less.

<Weight average molecular weight measurement conditions>
Column: TSKgel, Super Multipore HZ-H (manufactured by Tosoh Corporation, 4.6 mm ID × 15 cm)
Number of columns: 3 Eluent: Tetrahydrofuran Flow rate: 0.35 ml / min Sample injection volume: 10 μl
Measurement temperature: 40 ° C
Detector: IR detector calibration curves are F-40, F-20, F-4, F-1, A-5000, A-2500, A from standard samples (TSK standard, polystyrene, manufactured by Tosoh Corporation). -1000 and 8 types of n-propylbenzene were selected and prepared.

[Reference Example 1]
(Preparation of pigment dispersion)
Carbon black, which is a black pigment, and P.I., which is a cyan pigment. B-15: 3, a magenta pigment, P.I. R-122 and yellow pigment P.I. Black, cyan, magenta, and yellow pigment dispersions were prepared using Y-74.

  Using resin 1 to resin 8, the resin mass / pigment mass values were 0.2, 0.3, 0.4, 0.45, 0.5, 0.55, 0.6, 0.00. 96 types of pigment dispersions were prepared for each color so as to be 7, 0.8, 0.9, 1.0, and 1.1.

  Specifically, a pigment, a resin, and ion-exchanged water are charged into Dynomill (manufactured by Shinmaru Enterprises Co., Ltd.) at the ratio (mass%) shown in Table 2, and zirconia having a particle diameter of 0.5 mm. After filling the beads into a Dynomill vessel, a pigment dispersion was prepared by kneading the pigment and resin while cooling with water. After preparing the pigment dispersion, it was confirmed that the average particle diameter of the pigment was in the range of 70 to 150 nm. Measurement of the average particle diameter of the pigment was performed with Zeta Sizer Nano (manufactured by Sysmex Corporation) using a liquid obtained by diluting the pigment dispersion 300 times with ion-exchanged water as a sample.

[Reference Example 2]
(Evaluation of black ink)
Using 96 types of black pigment dispersions prepared in Reference Example 1, the pigment dispersion and surfactant (Orphine E1010, ethylene oxide adduct of acetylenic diol (Nisshin Chemical Co., Ltd.) were used at the ratios shown in Table 3. Made by Kogyo Co., Ltd.), 1,2-hexylene glycol (organic solvent), triethylene glycol monobutyl ether (organic solvent), 2-pyrrolidone (dissolution stabilizer), and glycerin (humectant) using a stirrer. And mixed with a filter having a pore size of 5 μm to obtain 96 types of black ink. With respect to the obtained 96 types of ink, according to the following method, intermittent ejection property, image smear due to offset, re-dissolvability, and print density were measured. Table 4 shows the evaluation results of intermittent ejection performance, Table 5 and Table 6 show the evaluation results of image smearing due to offset, Table 7 shows the evaluation results of redissolvability, and Table 8 shows the evaluation results of printing density. Show.

<Evaluation method of intermittent ejection property>
The intermittent ejection property is an index of the probability of ink ejection failure when an image is formed after the recording head has not ejected ink for a while. Evaluation of intermittent ejection performance is performed by setting the temperature of the head to 25 ° C. using an image forming apparatus having a recording head capable of detecting the temperature inside the head and having a heater capable of maintaining the temperature inside the head. The test was performed in a 15 ° C. RH environment. Glossy paper (photographic glossy paper, KA4100PGP (manufactured by Seiko Epson Corporation)) was used as the recording medium. Using the head corresponding to 11a of the apparatus shown in FIG. 1, after printing a line image in the longitudinal direction of the head, after passing through any non-printing section, the line image is printed again, and the printing state of the line image is checked with a microscope. Observation and evaluation of intermittent discharge were performed. The evaluation criteria for intermittent discharge are described below.
○: The line image is not disturbed even if it exceeds the vertical non-printing section of A3 size paper.
Δ: The line image is not disturbed up to the non-printing section corresponding to the length of A3 size paper.
X: Disturbance occurs in the line image in a non-printing section corresponding to the length of A3 size paper.

<Evaluation method of image smudge due to offset>
Evaluation of image smear due to offset is an image formation including a recording head having a heater capable of keeping heat inside the head and capable of detecting the temperature inside the head, and a discharge unit 8 having a driven roller 8b whose surface material is PTFE. Using the apparatus, the temperature of the head was set to 25 ° C., and the test was performed in an environment of 32.5 ° C. and 85% RH. As a recording medium, paper (IJW (manufactured by Oji Paper Co., Ltd.)) cut into A4 size was used. Using the above-described image forming apparatus, ink was ejected so that the amount of ink applied to the recording medium was 15 g / m ² , three solid images of 10 cm × 10 cm were printed, and contacted with the driven roller The recording medium on which the subsequent image was formed was observed to evaluate the occurrence of image contamination due to offset. Note that the evaluation of image contamination due to offset was performed by discharging ink from the recording head 11a and the recording head 11d in FIG. Further, the conveyance speed of the recording medium from the paper feed to the discharge unit is 846.7 mm / second, and the time required for the movement of the recording medium between the recording head 11a and the driven roller is 0.8 seconds, The time required for the recording medium to move between the recording head 11d and the driven roller is 0.3 seconds. The evaluation criteria for image contamination due to offset are described below.
(Double-circle): A stain | pollution | contamination cannot be confirmed in all the 3 sheets in the non-printing part of the image rear end side.
◯: One sheet of smudge can be confirmed in the non-printing area on the rear end side of the image.
(Triangle | delta): In the non-printing part by the side of the rear end of an image, a stain cannot be confirmed only for the first printed sheet.
X: Dirt can be confirmed on all three sheets in the non-printing area on the rear end side of the image.

<Resolubility evaluation method>
Weigh 5 g of ink in a petri dish (φ50 mm) whose mass was measured in advance, and dry the ink with a 60 ° C. incubator until the mass corresponding to the water contained in 5 g of ink was reduced. The ink mass W ¹ was measured. Next, 5 g of undried ink was added to the dried petri dish, and then the petri dish was allowed to stand for 30 minutes on a horizontal table. Thereafter, the petri dish was tilted so that the angle between the bottom surface of the petri dish and the horizontal surface on the table was 135 °, and then the petri dish was lifted vertically upward, and the petri dish was allowed to stand still for 20 seconds to drop ink from the petri dish. The mass was measured W ² of the ink in the petri dish after dropping the ink. The re-dissolution rate of the ink was measured from W ¹ and W ² according to the following formula. The value of 0.3 (g) in the following calculation formula is the average of the ink adhering to the petri dish when 5 g of undried ink was dropped until no ink droplets dropped from the petri dish. Mass.
(Redissolution rate calculation formula)
Re-dissolution rate (%) = (1− (W ² −0.3) / W ¹ ) × 100

  The re-dissolution rate is an indicator of the likelihood of ink ejection failure when an image is formed after the recording head has not ejected ink for a while. For example, if the re-dissolution rate is 50% or more, the ink is purged at a pressure of 100 to 200 kPa even if the recording head is left in a low-temperature and low-humidity environment or a high-temperature and low-humidity environment without capping the recording head for about two weeks. A normal maintenance operation in which the thickened ink is pushed out from the nozzle of the recording head and the pushed ink is scraped off with a rubber wipe enables normal ink ejection. For this reason, the re-dissolution rate was evaluated as ◯ when the re-dissolution rate was 50% or more, and evaluated as x when less than 50%.

<Print density evaluation method>
The print density is evaluated by setting the temperature of the head to 25 ° C. using an image forming apparatus having a recording head capable of detecting the temperature inside the head and having a heater capable of maintaining the temperature inside the head. The test was performed in a 60% RH environment. As a recording medium, paper (IJW (manufactured by Oji Paper Co., Ltd.)) cut into A4 size was used. A solid image of 10 cm × 10 cm is obtained by ejecting ink from the recording head corresponding to 11a in FIG. 1 so that the amount of ink applied to the recording medium is 15 g / m ² using the image forming apparatus described above. Is printed. The recording medium on which the solid image was printed was allowed to stand for a whole day and night, and the image density was measured with an image densitometer (manufactured by Gretag Macbeth) at a D50 light source and a viewing angle of 2 °. Note that the conveyance speed of the recording medium from the paper supply to the discharge portion was 846.7 mm / sec. An image density of 1.10 or more was judged as ◯, and an image density of less than 1.10 was judged as x.

  According to Tables 4 and 6, in the case of the black ink, the resin mass / pigment mass value is 0.45 to 0.8, and the resin weight average molecular weight (Mw) is 70000 to 200000. It can be seen that it is particularly easy to suppress ink ejection failure when forming an image after ink has not been ejected from the recording head for a while and contamination of the recording medium due to offset.

  Further, according to Tables 5 and 6, when ink is ejected from the recording head of 11a that is far from the discharge unit, it is due to offset rather than from the recording head of 11d that is close to the discharge unit. It can be seen that the recording medium is hardly contaminated.

  According to Table 7, as for the black ink, if the weight average molecular weight of the resin is in the range of 30000-200000 and the resin mass / pigment mass value is 0.3-1.0, any ink is good. It can be seen that re-dissolvability is exhibited.

  According to Table 8, it can be seen that for black ink, the value of the image density increases as the molecular weight of the resin increases. It can also be seen that the smaller the resin mass / pigment mass value, the greater the image density value. Also, with black ink, an image with a good density is formed when the resin mass / pigment mass value is 0.3 to 0.55 and the weight average molecular weight (Mw) of the resin is 70000 to 200000. It turns out that it is easy to do.

  According to the results in Table 8 and Tables 4 and 6, it is possible to suppress the ejection failure of the ink when forming an image after the ink is not ejected from the recording head for a while, and the recording medium due to the offset. In order to realize both suppression of contamination and good printing density, the resin mass / pigment mass value is 0.45 to 0.55, and the resin weight average molecular weight (Mw) is 70000 to It can be seen that 200000 is preferable.

[Reference Example 3]
(Evaluation of cyan ink)
96 kinds of cyan inks having a pigment concentration of 5 mass% were obtained in the same manner as in Reference Example 2, except that 96 kinds of cyan pigment dispersions prepared in Reference Example 1 were used. For the obtained 96 types of inks, as in Reference Example 2, intermittent ejection properties, image smear due to offset, re-dissolvability, and print density were measured. Table 9 shows the evaluation results of the intermittent ejection performance, Table 10 and Table 11 show the evaluation results of the image stain due to offset, Table 12 shows the evaluation results of the re-dissolution property, and Table 13 shows the evaluation results of the print density. Show. As for the print density, 1.00 or more was judged as ◯, and less than 1.00 was judged as x.

  According to Tables 9 and 11, in the case of cyan ink, the resin mass / pigment mass value is 0.7 to 0.8, and the resin weight average molecular weight (Mw) is 70000 to 200000. It can be seen that it is particularly easy to suppress ink ejection failure when forming an image after ink has not been ejected from the recording head for a while and contamination of the recording medium due to offset.

  According to Table 12, for cyan ink, if the weight average molecular weight of the resin is in the range of 30,000 to 200,000, and the resin mass / pigment mass value is 0.3 to 1.0, any ink is good. It can be seen that re-dissolvability is exhibited.

  According to Table 13, for the cyan ink, any ink is acceptable as long as the weight average molecular weight of the resin is in the range of 30,000 to 200,000 and the resin mass / pigment mass value is 0.3 to 1.0. It can be seen that an image can be formed with a high print density.

[Reference Example 4]
(Evaluation of yellow ink)
96 kinds of yellow inks having a pigment concentration of 5 mass% were obtained in the same manner as in Reference Example 2, except that 96 kinds of yellow pigment dispersions prepared in Reference Example 1 were used. For the obtained 96 kinds of inks, the intermittent ejection property, the re-dissolvability, and the print density were measured in the same manner as in Reference Example 2. For yellow ink, since it was difficult to visually determine the presence or absence of image stains, image stains due to offset were not evaluated. Table 14 shows the evaluation results of intermittent ejection performance, Table 15 shows the evaluation results of re-dissolvability, and Table 16 shows the evaluation results of printing density. Regarding the print density, 1.00 or more was judged as ◯, and less than 1.00 was judged as x.

  According to Table 14, in the yellow ink, when the resin mass / pigment mass value is 0.3 to 0.8 and the weight average molecular weight (Mw) of the resin is 30,000 to 200,000, It can be seen that it is particularly easy to suppress ink ejection failure when forming an image after the ink has not been ejected for a while.

  According to Table 15, for the yellow ink, if the weight average molecular weight of the resin is in the range of 30000-200000 and the value of the resin mass / pigment mass is 0.3 to 1.0, any ink is good. It can be seen that re-dissolvability is exhibited.

  According to Table 16, if the weight average molecular weight of the resin is in the range of 30,000 to 200,000 and the value of the resin mass / pigment mass is 0.3 to 1.0, the printing density is good. It can be seen that an image can be formed.

[Reference Example 5]
(Evaluation of magenta ink)
96 types of magenta pigment dispersions prepared in Reference Example 1 and 96 types of inks were changed in the same manner as in Reference Example 2 except that the ink composition was changed to the composition shown in Table 17. A magenta ink was obtained. In magenta ink, P.I. Since R-122 was used, the content of the pigment dispersion in the ink was increased as compared with other color inks. For the obtained 96 types of inks, as in Reference Example 2, intermittent ejection properties, image smear due to offset, re-dissolvability, and print density were measured. The image smear due to the offset was evaluated in the case where ink was ejected from the recording head corresponding to 11b in addition to the recording head corresponding to 11a and 11d in FIG. Table 18 shows the evaluation results of intermittent ejection performance, Tables 19 to 21 show the results of image stain due to offset, Table 22 shows the results of evaluation of re-dissolution, and Table 23 shows the evaluation results of printing density. . As for the print density, 1.00 or more was judged as ◯, and less than 1.00 was judged as x.

  The magenta ink of Reference Example 5 is a P.I. Since R-122 is used, the pigment content is set higher than that of the other color inks. Therefore, in the image forming method of the present invention, the magenta ink of Reference Example 5 is ejected as the second color ink from the head 11b. According to Table 18 and Table 20, when the magenta ink of Reference Example 5 is ejected from the head 11b, the resin mass / pigment mass value is 0.6 to 0.7, and the resin weight average molecular weight ( Mw) is 40,000 to 70,000, or the resin mass / pigment mass value is 0.55 to 0.6, and the weight average molecular weight (Mw) of the resin is 70,000 to 140,000. It can be seen that it is particularly easy to suppress ink ejection failure when forming an image after ink has not been ejected for a while and contamination of the recording medium due to offset.

  Note that the magenta ink content can be reduced by changing the magenta pigment to a pigment with good color developability. In this case, an ink having properties similar to those of the cyan ink of Reference Example 3 can be prepared.

  According to Table 22, for magenta inks, if the weight average molecular weight of the resin is in the range of 30000-200000 and the resin mass / pigment mass value is 0.3-1.0, any ink is good. It can be seen that re-dissolvability is exhibited.

  According to Table 23, for magenta ink, if the weight average molecular weight of the resin is in the range of 30,000 to 200,000, and the resin mass / pigment mass value is 0.3 to 1.0, the printing density is good. It can be seen that an image can be formed.

[Reference Example 6]
Using an image forming apparatus having a recording head capable of detecting the temperature inside the head and having a heater capable of keeping the temperature inside the head, the temperature of the head is set to 25 ° C., and in an environment of 24 ° C. and 60% RH. went. As a recording medium, paper (IJW (manufactured by Oji Paper Co., Ltd.)) cut into A4 size was used. A solid image of 10 cm × 10 cm is obtained by ejecting ink from the recording head corresponding to 11a in FIG. 1 so that the amount of ink applied to the recording medium is 10 g / m ² using the above-described image forming apparatus. Is printed. The recording medium on which the solid image was printed was left still for a whole day and night, and then the brightness (L ^* ) of the image formed with each color ink at a D50 light source and a viewing angle of 2 ° by an image densitometer (manufactured by Gretag Macbeth) ^. ) Was measured. Note that the conveyance speed of the recording medium from the paper supply to the discharge portion was 846.7 mm / sec. Table 24 shows the lightness (L ^* ) of the image formed with each color ink.

[Reference Example 7]
According to the following method, the influence of the color ink ejection order on the lightness (L ^* ) of the image was confirmed. A solid image for evaluation was formed using an image forming apparatus having a heater capable of keeping heat inside the head and having a recording head capable of detecting the temperature inside the head. In the formation of a solid image for evaluation, the temperature of the head is set to 25 ° C., and under the environment of 24 ° C. and 60% RH, two colors of ink are applied from the recording head corresponding to 11b and 11c in FIG. The discharge amount was 5 g / m ² (total 10 g / m ² ). As a recording medium, paper (IJW (manufactured by Oji Paper Co., Ltd.)) cut into A4 size was used. The size of the solid image for evaluation was 10 cm × 10 cm, and the conveyance speed of the recording medium was 846.7 mm / second. The recording medium on which the solid image was printed was allowed to stand for a whole day and night, and then the lightness (L ^* ) was measured with an image densitometer (manufactured by Gretag Macbeth) at a D50 light source and a viewing angle of 2 °.

According to Table 25, it can be seen that a clear image having a higher lightness (L ^* ) can be formed by ejecting ink having a higher lightness (L ^* ) last, even if the image has the same hue.

[Example 1, Example 2, and Comparative Examples 1-4]
The inks shown in Table 26 were discharged in the order shown in Table 26 from the recording heads corresponding to 11a to 11d in FIG. 1 so that the discharge amount of each color was 5 g / m ² (total 20 g / m ² ). An image was formed by ejection, and image stain due to offset was evaluated in the same manner as in Reference Example 2. Note that the inks used in Example 1, Example 2, and Comparative Examples 1 to 4 have no problems with respect to intermittent ejection property, re-dissolution property, and print density. There wasn't. Table 26 shows the evaluation results of the image stains due to the offsets of Example 1, Example 2, and Comparative Examples 1 to 4. In Table 26, the ink hue is indicated as K for black, C for cyan, Y for yellow, and M for magenta.

  According to Examples 1 and 2, in the black ink and the color ink, the weight average molecular weight of the resin is 30,000 to 200,000, and the resin mass / pigment mass value is 0.3 to 1.0. The ink ejected from the recording head of the first color is black ink, the ink ejected from the recording heads of the second and subsequent colors is color ink, and the black ink is the weight of the resin in all the inks. The average molecular weight is the largest, and the resin mass / pigment mass value is the smallest, and the ink ejected from the recording head of the second color is the most total of the pigment and resin content in the color ink. When the number is large, it can be seen that image smear due to offset can be satisfactorily suppressed.

  On the other hand, according to Comparative Examples 1 and 2, the color inks are ejected in descending order of the total content of the pigment and the resin, the resin has the largest weight average molecular weight, and the resin mass. / Even if the black ink having the smallest pigment mass value is used, it is found that if the black ink is not ejected first, image smearing is likely to occur due to offset.

  Further, according to Comparative Examples 3 and 4, the black ink having the largest resin weight average molecular weight and the smallest resin mass / pigment mass value among all inks may be ejected first. It can be seen that when the color ink having the largest content of the pigment and the resin is not ejected to the second color, image smearing easily occurs due to the offset.

DESCRIPTION OF SYMBOLS 2 Paper feed tray 3 Paper feed roller 4 Driven roller 5 Conveyance belt 6 Belt drive roller 7 Belt roller 8 Ejection part 8a Ejection roller 8b Follower roller 10 Ejection tray 11a, 11b, 11c, 11d Line head 12a-12p, 12a'- 12p 'nozzle 20 control unit 30 detection means 100 inkjet recording apparatus D1 to D4 dot row (row direction)
L1 to L16 dot row (transport direction)
N1, N2 Nozzle array P Recording paper

Claims (2)

An image forming method using an ink jet recording apparatus of a line head type including a plurality of recording heads for discharging black ink and three or more color inks, and a discharge unit for discharging a recording medium,
The black ink and the color ink include a pigment, a resin, water, and an organic solvent,
The pigment and the resin are contained in the ink as a pigment dispersion,
In the black ink and the color ink, the resin has a weight average molecular weight of 30,000 to 200,000, and a resin mass / pigment mass value of 0.3 to 1.0,
The ink ejected from the first color recording head is black ink, and the ink ejected from the second and subsequent recording heads is color ink,
The black ink has the largest resin weight average molecular weight and the smallest resin mass / pigment mass value among all inks,
The ink ejected from the recording head for the second color is an image forming method in which the total content of the pigment and the resin is the largest among the color inks. The color that has the highest lightness L ^* when comparing the lightness L ^* in the ^* La ^* b ^* color system of the image formed with the same amount of single color ink as the last ink ejected among the color inks. The image forming method according to claim 1, wherein the image forming method is ink.

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