Patent Publication Number: US-2022219475-A1

Title: Image forming apparatus, ink set, and image forming method

Description:
TECHNICAL FIELD 
     The present disclosure relates to an image forming apparatus, an ink set, and an image forming method. 
     BACKGROUND ART 
     A technique for forming an image by ejecting a discharge printing ink and a color ink on a fabric such as clothes is known by reference to Patent Literature 1 and the like. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP 2004-232121 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in the above-mentioned technique, there is a problem in that the color ink is easily peeled off at the time of washing, and the washing fastness of the image-formed area is impaired. 
     With the foregoing in mind, it is an object of the present disclosure to provide an image forming apparatus, an ink set, and an image forming method that improve washing fastness of an image forming portion of a fabric on which a discharge printing ink and a color ink are ejected. 
     Solution to Problem 
     In order to achieve the aforementioned object, the present disclosure provides an image forming apparatus, including: a color ink ejector; a discharge printing ink ejector; and a controller, wherein the color ink ejector is configured to eject a color ink containing an aqueous resin emulsion on a fabric, the discharge printing ink ejector is configured to eject a discharge printing ink containing a reducing agent on the fabric, and the controller is configured to control the color ink ejector and the discharge printing ink ejector to eject the color ink on the fabric in a first step, and after the first step, to eject the discharge printing ink on the fabric in a second step. 
     The present disclosure also provides an image forming method, including: a first step of ejecting a color ink containing an aqueous resin emulsion on a fabric; and a second step of ejecting a discharge printing ink containing a reducing agent on the fabric, wherein the second step is performed after performing the first step. 
     Advantageous Effects of Invention 
     The image forming apparatus, the ink set, and the image forming method of the present disclosure improve the washing fastness of the image forming portion of the fabric on which the discharge printing ink and the color ink are ejected. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic perspective view illustrating the configuration of an example of an image forming apparatus according to the present disclosure. 
         FIG. 2  is a plan view perspectively illustrating the internal structures of an ink ejector and a heat treatment section of the image forming apparatus shown in  FIG. 1 . 
         FIG. 3  is a side view perspectively illustrating the internal structures of an ink ejector and a heat treatment section of the image forming apparatus shown in  FIG. 1 . 
         FIG. 4  is a block diagram illustrating an example configuration of a controller of the image forming apparatus shown in  FIG. 1 . 
         FIGS. 5A and 5B  are photographs of fabrics before and after washing in Example 1. 
         FIGS. 6A and 6B  are graphs showing the results of a washing fastness evaluation in Example 1. 
         FIGS. 7A and 7B  are graphs showing the results of a washing fastness evaluation in Example 1. 
         FIGS. 8A and 8B  are graphs showing the results of a washing fastness evaluation in Example 2. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The image forming apparatus of the present disclosure includes: a color ink ejector; a discharge printing ink ejector; and a controller, wherein the color ink ejector is configured to eject a color ink containing an aqueous resin emulsion on a fabric, the discharge printing ink ejector is configured to eject a discharge printing ink containing a reducing agent on the fabric, and the controller is configured to control the color ink ejector and the discharge printing ink ejector to eject the color ink on the fabric in a first step, and after the first step, to eject the discharge printing ink on the fabric in a second step. 
     The color ink includes an aqueous resin emulsion. The other configuration of the color ink is not particularly limited. The aqueous resin emulsion is composed of, for example, resin fine particles and a dispersion medium such as water, and the resin fine particles are dispersed with a specific particle diameter rather than in a dissolved state in the dispersion medium. Examples of the aqueous resin emulsion include an acrylic resin emulsion, a urethane resin emulsion, a polyester resin emulsion, and a polystyrene resin emulsion. One kind of the aqueous resin emulsions may be used alone, or two or more of them may be used in combination. The aqueous resin emulsion is preferably a urethane resin emulsion, for example. 
     Examples of the urethane resin emulsion include a polycarbonate-based urethane resin emulsion, a polyester-based urethane resin emulsion, and a polyether-based urethane resin emulsion. Since the urethane resin emulsion is a polyester-based urethane resin emulsion, for example, washing fastness of an image forming portion is good. Since the urethane resin emulsion is a polycarbonate-based urethane resin emulsion, for example, washing fastness of an image forming portion is even better and storage stability is good. Since the urethane resin emulsion is a polyether-based urethane resin emulsion, for example, by performing the second step after the first step, the effect of improving washing fastness is large, and storage stability is good. 
     As the resin emulsion, for example, a commercially available product may be used. Examples of the commercially available product of a polycarbonate-based urethane resin emulsion include “SUPERFLEX® 460” produced by DKS Co., Ltd., “TAKELAC® W-6110” produced by Mitsui Chemicals, Inc., and “HYDRAN® WLS-210” produced by DIC Co., Ltd.; examples of the commercially available product of a polyester-based urethane resin emulsion include “UCOAT® UWS-145” produced by Sanyo Chemical Industries, Ltd., “TAKELAC® W-5030” produced by Mitsui Chemicals, Inc., and “HYDRAN® HW-920” produced by DIC Co., Ltd.; and examples of the commercially available product of a polyether-based urethane resin emulsion include “TAKELAC® W-6061T” produced by Mitsui Chemicals, Inc., and “HYDRAN® FCS-855” and “HYDRAN® WLS-201” produced by DIC Corporation. 
     The color ink may contain, for example, a pigment, a dye, and the like as a coloring agent. The color ink contains, for example, a pigment. The pigment is not particularly limited, and examples thereof include carbon black, an inorganic pigment, and an organic pigment. Examples of the carbon black include furnace black, lamp black, acetylene black, and channel black. Examples of the inorganic pigment include titanium oxide, an iron oxide-based inorganic pigment, and a carbon black-based inorganic pigment. Examples of the organic pigment include azo pigments such as azolakes, insoluble azo pigments, condensed azo pigments, and chelating azo pigments; polycyclic pigments such as phthalocyanine pigments, perylene and perinone pigments, anthraquinone pigments, quinacridone pigments, dioxazine pigments, thioindigo pigments, isoindolinone pigments, and quinophthalone pigments; dye lake pigments such as basic dye type lake pigments and acid dye type lake pigments; nitro pigments; nitroso pigments; and aniline black daylight fluorescent pigments. Examples of pigments other than these include C. I. Pigment Black 1, 6 and 7; C.I. Pigment Yellow 1, 2, 3, 12, 13, 14, 15, 16, 17, 55, 74, 78, 150, 151, 154, 180, 185 and 194; C.I. Pigment Orange 31 and 43; C.I. Pigment Red 2, 3, 5, 6, 7, 12, 15, 16, 48, 48:1, 53:1, 57, 57:1, 112, 122, 123, 139, 144, 146, 149, 150, 166, 168, 175, 176, 177, 178, 184, 185, 190, 202, 209, 221, 222, 224, and 238; C.I. Pigment Violet 19 and 196; C.I. Pigment Blue 1, 2, 3, 15, 15:1, 15:2, 15:3, 15:4, 16, 22, and 60; C.I. Pigment Green 7 and 36; and solid solutions of these pigments. One kind of the coloring agents may be used alone, or two or more of them may be used in combination. 
     The content of the resin emulsion and the content of the coloring agent in the total amount of the color ink is not particularly limited. The content of the resin fine particles contained in the resin emulsion in the total amount of the color ink may be, for example, 1 mass % to 15 mass %. The solid content of the pigment may be, for example, 1 mass % to 15 mass %. 
     The color ink is, for example, an ink which is aggregated by the discharge printing ink. Examples of the ink which is aggregated by the discharge printing ink include inks that use “SUPERFLEX® 460, “TAKELAC® W-6110”, “HYDRAN® WLS-210”, “UCOAT® UWS-145”, “TAKELAC® W-5030”, “TAKELAC® W-6061T”, “HYDRAN® FCS-855”, “HYDRAN® WLS-201”, and the like as the resin emulsion. Since the color ink is an ink which is aggregated by the discharge printing ink, for example, the washing fastness may be further improved, and moreover, the bleeding of the image forming portion may be reduced and the image quality may be improved. 
     The color ink may further contain, for example, water, as well as other components such as a humectant and a surfactant. 
     The discharge printing ink contains a reducing agent. Other configuration of the discharge printing ink is not particularly limited. Examples of the reducing agents include sodium hydrosulfite, thiourea dioxide, rongalite, sodium nitrite, sodium bisulfite, sodium thiosulfate, and sodium pyrosulfite. The reducing agent is, for example, thiourea dioxide. Since the reducing agent is thiourea dioxide, for example, it is good in terms of smell (for example, formaldehyde is not generated) and storage stability. 
     The concentration of the reducing agent in the total amount of the discharge printing ink is not particularly limited, and may be, for example, 0.28 mol/1 to 0.70 mol/l. 
     The discharge printing ink may further contain, for example, water, as well as other components such as a humectant and a surfactant. 
     The combination of the color ink and the discharge printing ink is not particularly limited, and may be the combination of the color ink and the discharge printing ink described above. The combination of the color ink and the discharge printing ink may be, for example, the combination in which the aqueous resin emulsion contained in the color ink is a urethane resin emulsion and the reducing agent contained in the discharge printing ink is thiourea dioxide. 
     The fabric includes both knitted and woven fabrics. The material of the fabric may be natural fibers. Examples of the natural fiber include cotton and silk. 
     Next, an image forming apparatus of the present disclosure will be described. The image forming apparatus of the present disclosure may be of an integral-type including a color ink ejector, a discharge printing ink ejector, and a controller inside one casing or a system independently including the color ink ejector, the discharge printing ink ejector, and the controller. 
       FIG. 1  shows an example of the configuration of the integral-type image forming apparatus. As shown in  FIG. 1 , the image forming apparatus  1  includes an ink set storage  2 , an ink ejector  3 , a heat treatment section  4 , and a controller  5 . The heat treatment section  4  may have any configuration and may not be included in the apparatus  1 . 
     The ink set storage  2  is configured to store an ink set. The ink set includes the color ink and the discharge printing ink. Note that, the ink set storage  2  may include, for example, a color ink set storage configured to store an ink set of the color ink and a discharge printing ink set storage configured to store an ink set of the discharge printing ink, separately. 
     The ink ejector  3  also serves as the color ink ejector and the discharge printing ink ejector. Inside the ink ejector  3 , an ink ejection unit is provided as described below. However, the image forming apparatus  1  is not limited to the one in which the ink ejector  3  serves as both the color ink ejector and the discharge printing ink ejector. That is, the image forming apparatus  1  may include the color ink ejector and the discharge printing ink ejector as separate configurations, and a color ink ejection unit may be provided inside the color ink ejector, and a discharge printing ink ejection unit may be provided inside the discharge printing ink ejector. 
     Inside the heat treatment section  4 , the contact heating unit is provided as described below. 
     The right end in front of the ink ejector  3  is provided with an operation panel  6  having a display device  7  and a button  8 . The display device  7  is, for example, a liquid crystal display. The button  8  inputs an image formation start signal to the controller  5 . In addition to or instead of the button  8 , for example, the display device  7 , which is a touch panel, may accept a user input and input an image formation start signal to the controller  5 . An opening  10  through which a platen  31  and a fabric held on the platen  31  are taken in or out is provided on the front wall of the casing  9  of the ink ejector  3  and the heat treatment section  4 . Hereinafter, the front, rear, left and right directions are as indicated by arrows in  FIG. 1 . 
       FIG. 2  is a plan view and  FIG. 3  is a side view perspectively illustrating the internal structure of the image forming apparatus  1  shown in  FIG. 1 . As shown in  FIGS. 2 and 3 , the inside of the ink ejector  3  is provided with an ink ejection unit (liquid ejection head)  12  having an ink-jet nozzle line and a carriage  13  including this liquid ejection head  12 . A pair of upper and lower guide rods  14  and  15  for guiding the carriage  13  are respectively disposed in the left-right direction. 
     A carriage motor  18  composed of a stepping motor that reciprocally conveys the carriage  13  in the left-right direction is provided at a right end of the guide rods  14  and  15 . The carriage  13  is linked to a belt (not shown) set to be parallel with the guide rods  14  and  15 . The belt is set to a pulley (not shown) linked to the carriage motor  18  and a pulley (not shown) provided at a left end of the guide rods  14  and  15 . With the driving of the carriage motor  18 , the liquid ejection head  12  and the carriage  13  are conveyed along the guide rods  14  and  15  in the left-right direction (main scanning direction). The carriage  13 , the guide rods  14  and  15 , the carriage motor  18 , the pulleys, and the belt configure a liquid ejection head conveying mechanism. 
     An ink storage container (ink cartridge) stored in the ink set storage  2  is connected to the liquid ejection head  12  via a supply tube. While  FIGS. 1 to 3  show an example in which each of the ink ejection units is a liquid ejection head  12  having an ink-jet nozzle line, the ink ejection unit may be any unit as long as it ejects an ink on a fabric. Examples of the ejection unit include a spray, a stamp for applying an ink to a fabric, a brush, and a roller. Note that the ink ejection unit for ejecting the color ink and the ink ejection unit for ejecting the discharge printing ink may be the same or different from each other. 
     A pair of left and right guide rails (not shown) for guiding a moving member  60  including the platen  31  is arranged in a row in the front-rear direction at the center in the left-right direction of the ink ejector  3  and the heat treatment section  4 . A tray  63  is fixed to the moving member  60  below the platen  31 . 
     A platen motor  26  is provided near the front end of the pair of left and right guide rails. A pulley  16  is fixed to the output shaft of the platen motor  26 . A pulley  27  is supported at the bottom of the applying section on the front side of the platen motor  26 . A belt  17  is set over the pulley  16  and pulley  27 . The pulley  27  has a pulley  24  coaxially above the pulley  27 . A pulley  28  is rotatably supported at the lower part of the heat treatment section  4 . A belt  25  is set over the pulley  24  and pulley  28 . A moving member  60  is linked to the belt  25 . The pulley  16  rotates by the rotational drive of the platen motor  26 . With the rotation of the pulley  16 , the pulley  24  is rotated via the belt  17 . The rotation of the pulley  24  is transferred to the belt  25 , and the moving member  60  is conveyed in the front-rear direction (subscanning direction) along the pair of left and right guide rails. The platen motor  26 , the pair of left and right guide rails, the pulleys  16 ,  24 ,  27 , and  28 , and the belt  25  configure a platen conveying mechanism. 
     As shown in  FIG. 3 , the inside of the heat treatment section  4  is provided with a contact heating unit  30  configured to subject a discharge printing ink (containing reducing agent)-applied portion to contact heating to promote a reduction reaction. The contact heating unit  30  includes a heat press plate  34 , an elastic support mechanism  43  for elastically supporting the heat press plate  34  so as to be slidable in the vertical direction, and a lifting drive mechanism (not shown) for vertically driving the heat press plate  34  so as to be able to move up and down detachably with respect to the surface of the fabric held by the platen  31 . The heat press plate  34  may be composed of, for example, a heat insulating plate made of silicon rubber foam and a heat press section fixed to the bottom surface of the heat insulating plate. The heat press section may be composed of, for example, a heat sheet (heating section) including a nichrome wire arranged therein and a heat storage material made of iron, arranged so as to sandwich the heat sheet. 
     The heat treatment section  4  may further include a non-contact heating unit in the inside thereof. Examples of the non-contact heating unit include an oven and a belt conveyor oven. The non-contact heating unit may not be provided inside the heat treatment section  4  and may be provided independently from the image forming apparatus  1 . Instead of the contact heating unit  30 , the non-contact heating unit may heat the discharge printing ink-applied portion. 
     Next, the controller  5  will be described with reference to  FIG. 4 . In the controller  5 , a central processing unit (CPU)  87 , a read only memory (ROM)  88 , a random access memory (RAM)  89 , an input interface  85 , and an output interface  90  are electrically connected via a bus  81 . The button  8  and the like are electrically connected to the input interface  85 . The liquid ejection head  12 , the heat press plate  34 , the carriage motor  18 , the platen motor  26 , the display device (display)  7 , and the like are electrically connected to the output interface  90  via drive circuits  91  to  95 , respectively. 
     The CPU  87  performs various operations and processes based on signals input with the button  8  and various programs and data stored in the ROM  88  and the RAM  89 . Then, data and the like are sent to each component of the ink ejector  3  and the heat treatment section  4  via the output interface  90 . The RAM  89  is a volatile storage device which is readable and writable and stores results of the operations and the like obtained in the CPU  87 . 
     The controller  5  is configured to control the color ink ejector and the discharge printing ink ejector to eject the color ink on the fabric in the first step, and after the first step, to eject the discharge printing ink on the fabric in the second step. The first step and the second step may be repeatedly performed. 
     The ejection of the ink on the fabric using the image forming apparatus of the present embodiment is performed, for example, as follows. First, as the first step, a color ink containing the aqueous resin emulsion is applied (ejected) to a fabric. 
     More specifically, while the platen  31  and the liquid ejection head  12  are conveyed by the liquid ejection head conveying mechanism and the platen conveying mechanism, an image is printed on an image forming area of the fabric using the color ink. At least one of the ROM  88  and the RAM  89  in the controller (control unit)  5  of the image forming apparatus  1  stores various pieces of image data created by software and various pieces of data for every kind of fabric such as a T-shirt. In response to the instruction of printing by an operator, the image data is sent to the liquid ejection head  12  via the output interface  90 , inks are ejected from the liquid ejection head  12  based on the image data, and an image is printed on the fabric held by the platen  31 . 
     The amount of the color ink applied to the fabric is not particularly limited and may be, for example, 5 mg/cm 2  to 32 mg/cm 2 . 
     Next, as the second step, the discharge printing ink containing the reducing agent is applied (ejected) to the fabric in the same manner as in the first step. 
     The amount of the discharge printing ink applied to the fabric is not particularly limited and may be, for example, 5 mg/cm 2  to 32 mg/cm 2 . 
     The controller  5  may execute the second step after the first step, and the interval time between the steps is not particularly limited, and is, for example, 1 to 60 seconds. 
     For example, in the second step, the controller  5  may control the discharge printing ink ejector to eject the discharge printing ink on an area of the fabric on which the color ink is ejected. 
     Further, for example, after the first step and the second step, a heat treatment step of thermally fixing the ink to the fabric may be performed. The heat treatment step, for example, is carried out as follows. After the fabric and the platen  31  are moved to the heat treatment section  4  by the driving of the platen motor  26 , the contact heating unit  30  is lowered by the lifting drive mechanism to press the fabric by the heat press plate  34  at a predetermined temperature for a predetermined time, whereby subjecting the color ink/discharge printing ink-applied portion to contact heating to promote a reduction reaction. Heating temperature and heating time in the reduction reaction promotion is not particularly limited, may be set appropriately. 
     The mechanism for improving the washing fastness of the fabric on which the discharge printing ink and the color ink are ejected according to the present disclosure is presumed as follows, for example. That is, a discharge printing ink is firstly ejected on a fabric, and then a color ink is ejected in the conventional method. In this method, when the color ink comes into contact with the discharge printing ink previously ejected on the fabric, aggregation of the color ink and the discharge printing ink tends to occur. Then, entanglement between the color ink and the fabric fibers becomes weak due to the occurrence of the aggregation or the like before the color ink penetrates into the fabric. Therefore, it is presumed that the color ink is washed out at the time of washing, and the washing fastness is deteriorated. In contrast, since a color ink is firstly ejected on a fiber and then a discharge printing ink is ejected in the present disclosure, it is presumed that the color ink penetrates into the fabric and prone to be entangled with the fabric fibers, thereby improving washing fastness. However, the present disclosure is not limited to the above presumption. 
     The present disclosure also provides an image forming method. The image forming method of the present disclosure includes a first step of ejecting a color ink containing an aqueous resin emulsion on a fabric, and a second step of ejecting a discharge printing ink containing a reducing agent on the fabric, wherein the second step is performed after performing the first step. 
     The image forming method of the present disclosure is performed using, for example, the image forming apparatus shown in  FIGS. 1 to 4  described above. The first step and the second step are as described above. 
     The image forming method of the present disclosure may further include, for example, a heat treatment step of thermally fixing the ink to the fabric after the first step and the second step. The heat treatment step is as described above. 
     The present disclosure also provides a program for forming an image. The program of the present disclosure is a program for causing an image forming apparatus, which is a computer, to execute each step of the image forming method of the present disclosure as a procedure. In the present disclosure, the “procedure” may be read as “processing”. The program of the present embodiment may be recorded on, for example, a computer-readable recording medium. The recording medium is not particularly limited, and examples thereof include a read-only memory (ROM), a hard disk (HD), and an optical disk. 
     The present disclosure further provides a method for producing a fabric having an image. The fabric producing method the present disclosure is a method for producing a fabric having an image, and characterized in that it includes a step of forming an image on the fabric by the image forming method of the present disclosure. 
     The image forming method and the fabric producing method of the present disclosure improve the washing fastness of the image forming portion of the fabric on which the discharge printing ink and the color ink are ejected. 
     EXAMPLES 
     Next, the examples are described below together with comparative examples. The present disclosure, however, is by no means limited thereto. 
     Example 1 
     (Color Ink) 
     As a color ink, inks (color: yellow, magenta, cyan, and black) included in commercially available ink sets A to C were used. Note that, the magenta color inks included in the ink sets A to C are hereinafter also referred to as color inks A to C, respectively. 
     The color inks A to C were analyzed to examine that color inks A and C contain a urethane resin and that color ink B contains an acrylic resin. 
     (Discharge Printing Ink) 
     As the discharge printing ink 1, the discharge printing ink having the following composition was prepared. A 0.6 mol/l rongalite aqueous solution was prepared as the discharge printing ink 2. 
     (Discharge Printing Ink 1) 
       
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Tiourea dioxide 
                 6.5 wt % 
               
               
                   
                 Acrylamide 
                 8.4 wt % 
               
               
                   
                 2-amino-2-methyl-aminopropanol 
                 17.5 wt %  
               
               
                   
                 Water, others 
                 Balance 
               
               
                   
                   
               
            
           
         
       
     
     (Image Formation) 
     Commercially available black T-shirts A and B (material: 100% cotton) were used as fabrics. First, color inks included in the ink sets A to C were applied to the fabrics so that each fabric has yellow, magenta, cyan, black, red, green, and blue on it. Note that, the red, green, and blue were prepared by mixing the above color inks. Thereafter, the discharge printing ink (discharge printing ink 1 or 2) was applied to a color ink-applied portion. Further, the color ink/discharge printing ink-applied portion was subjected to contact heating, thereby performing thermal fixing. The contact heating was performed using a heat press machine at 180° C. for 90 seconds. Further, as a comparative example, an image was formed on a fabric in the same manner as described above except that the discharge printing ink was applied first and then the color ink was applied. 
     (Washing) 
     A portion including the color inks-applied portion was cut out from the T-shirt and subjected to a total of five cycles of washing and drying according to AATCC 61IIA method. 
     (Evaluation of Washing Fastness) 
     The color of the color inks A to C-applied portions of the washed fabric and unwashed fabric was measured using a CIE1976L*a*b* color space scale colorimeter X-Rite939 produced by X-Rite Corporation, thereby calculating the optical density OD. The ΔOD value was calculated by the formula: ΔOD value=unwashed OD value−washed OD value. If the ΔOD value is a smaller value, it may be determined that the change in numerical value before and after washing is smaller, the peel-off of the ink is little, and the washing fastness is good. 
       FIGS. 5A and 5B  show photographs of the fabric when ink set A was used.  FIG. 5A  is a photograph of the fabrics of before and after washing in which inks were applied in the order of Comparative Example and  FIG. 5B  is a photograph of the fabrics of before and after washing in which inks were applied in the order of Example. In each of  FIGS. 5A and 5B , the upper photograph shows the fabric before washing and the lower photograph shows the fabric after washing, showing the results of yellow, magenta, cyan, black, red, green, and blue from the left. In addition, the rightmost of the fabric shows the result of using only the discharge printing ink 1. 
     As shown in  FIGS. 5A and 5B , when the fabrics of the Examples and Comparative Examples were visually examined before washing, there was no apparent difference in hue. When the optical density (OD value) before washing was compared, the values were almost the same. From this, even if the color ink was applied earlier than the discharge printing ink, there was no influence on the coloring ability of the color ink and the discharge printing ability of the discharge printing ink in the present disclosure. Further, as shown in  FIGS. 5A and 5B , when the fabrics of Examples and Comparative Examples were visually examined after washing, the peel-off of the ink was observed as compared to before washing in all colors in Comparative Example. On the other hand, the peel-off of the ink was hardly observed as compared to before washing in Example. 
     Next, the results of the evaluation of the washing fastness are shown in  FIGS. 6A and 6B .  FIGS. 6A and 6B  are graphs showing the ΔOD value when the color ink A and the discharge printing inks 1 and 2 are used.  FIG. 6A  shows the result when the T-shirt A is used, and  FIG. 6B  shows the result when the T-shirt B is used. In each of  FIGS. 6A and 6B , the horizontal axis indicates the discharge printing inks 1 and 2, in each of which the left bar indicates the result of Comparative Example and the right bar indicates the result of Example, and the vertical axis indicates the ΔOD value. 
     As shown in  FIGS. 6A and 6B , the ΔOD value was reduced in Example as compared to in Comparative Example under both conditions. This shows that even in the case of using a discharge printing ink containing either of thiourea dioxide and rongalite as the reducing agent, the washing fastness is improved by setting the order of applying the ink as in the present disclosure. 
     Further, as shown in  FIGS. 6A and 6B , in particular, when the discharge printing ink 1 containing thiourea dioxide as the reducing agent was used, the Δ OD value was greatly reduced in Example as compared to in Comparative Example in both of the T-shirts A and B. This shows that in the case of using a discharge printing ink containing thiourea dioxide as the reducing agent, the washing fastness is further improved by setting the order of applying the ink as in the present disclosure. 
     Next, the ΔOD value was calculated in the same manner as described above except that the discharge printing ink 1 was used as the discharge printing ink and color inks A and C containing a urethane resin and color ink B containing an acrylic resin were used as the color ink. 
     The results are shown in  FIGS. 7A and 7B .  FIGS. 7A and 7B  are graphs showing the ΔOD values when color inks A to C are used.  FIG. 7A  shows the result when T-shirt A is used, and  FIG. 7B  shows the result when T-shirt B is used. In each of  FIGS. 7A and 7B , the horizontal axis indicates the color inks A to C, in which the left bar indicates the result of Comparative Example and the right bar indicates the result of Example, and the vertical axis indicates the ΔOD value. 
     As shown in  FIGS. 7A and 7B , the ΔOD value was reduced in Example as compared to in Comparative Example under both conditions. This shows that, even in the case of using a color ink containing either of the urethane aqueous resin emulsion and the acrylic aqueous resin emulsion, the washing fastness is improved by setting the order of applying the ink as in the present disclosure. 
     Further, as shown in  FIGS. 7A and 7B , in particular, when the color inks A and C containing urethane resin as the aqueous resin emulsion were used, the ΔOD value was greatly reduced in Example as compared to in Comparative Example in both of the T-shirts A and B. This shows that in the case of using a color ink containing a urethane resin as the aqueous resin emulsion, the washing fastness is further improved as compared to a case of using a color ink containing an acrylic resin by setting the order of applying the ink as in the present disclosure. 
     Example 2 
     (Preparation of Color Ink) 
     As a color ink, an aqueous resin solution having the composition shown in Table 1 below was prepared. First, using urethane resins (UC001 to UC009) shown in Table 2 below as the resin, an ink solvent containing an aqueous resin emulsion was obtained by uniformly mixing the resin and pure water so as to have the above composition ratio. The types of the urethane resin are as follows. That is UC001 to UC003 are polycarbonate-based urethane resins, UC004 to UC006 are polyester-based urethane resins, and UC007 to UC009 are polyether-based urethane resins. Next, using a coloring agent (color: magenta) as a pigment, color inks U1 to U9 corresponding to the urethane resins (UC001 to UC009) were obtained by adding the ink solvent to the coloring agent and mixing uniformly so as to have the above composition ratio. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Component 
                 Solid component ratio 
               
               
                   
                   
               
             
            
               
                   
                 Resin 
                 8% 
               
               
                   
                 Coloring agent 
                 3% 
               
               
                   
                 Pure water 
                 89%  
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Urethane resin 
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Sample name 
                 Product name 
                 Type of resin 
               
               
                   
                   
               
               
                   
                 UC001 
                 SUPERFLEX 460 
                 Carbonate 
               
               
                   
                 UC002 
                 TAKELAC W-6110 
                 Carbonate 
               
               
                   
                 UC003 
                 HYDRAN WLS-210 
                 Carbonate 
               
               
                   
                 UC004 
                 UCOAT UWS-145 
                 Ester 
               
               
                   
                 UC005 
                 TAKELAC W-5030 
                 Ester 
               
               
                   
                 UC006 
                 HYDRAN HW-920 
                 Ester 
               
               
                   
                 UC007 
                 TAKELAC W-6061T 
                 Ether 
               
               
                   
                 UC008 
                 HYDRAN FCS-855 
                 Ether 
               
               
                   
                 UC009 
                 HYDRAN WLS-201 
                 Ether 
               
               
                   
                   
               
            
           
         
       
     
     As the discharge printing ink, the discharge printing inks 1 and 2 of Example 1 were used. A portion including the ink-applied portion of each color was cut out and subjected to a total of five cycles of washing and drying using a household washing machine and a drying machine. 
     The washing fastness was evaluated in the same manner as in Example 1 in the case of using the color inks U1 to U9. 
     Next, the color inks U1 to U9 were divided into a group of color inks U1 to U3 (carbonate) containing a polycarbonate-based urethane resin, a group of color inks U4 to U6 (ester) containing a polyester-based urethane resin, and a group of color inks U7 to U9 (ether) containing a polyether-based urethane resin, and an average value of ΔOD values was calculated for each of the groups. 
     The results are shown in  FIGS. 8A and 8B .  FIGS. 8A and 8B  are graphs showing an average value of ΔOD values in the case of using the color inks of each of the groups.  FIG. 8A  shows the result for each type of T-shirt. In  FIG. 8A , the horizontal axis indicates groups of the carbonate, ester, and ether, each group showing the result of Comparative Example using the T-shirt A, the result of Example using the T-shirt A, the result of Comparative Example using the T-shirt B, and the result of Example using the T-shirt B from the left. In  FIG. 8A , the vertical axis indicates the average value of ΔOD values.  FIG. 8B  shows the average value of the results of T-shirts A and B. In  FIG. 8B , the horizontal axis indicates groups of the carbonate, ester, and ether, each group showing the result of Comparative Example and the result of Example from the left. In  FIG. 8B , the vertical axis indicates the average value of ΔOD values. 
     As shown in  FIG. 8A , the ΔOD value was reduced in Example as compared to in Comparative Example under both conditions. This shows that, even in the case of using a color ink containing any of a polycarbonate-based urethane resin, a polyester-based urethane resin, and a polyether-based urethane resin, the washing fastness is improved by setting the order of applying the ink as in the present disclosure. 
     Further, as shown in  FIG. 8B , when a color ink containing a polycarbonate-based urethane resin was used, the ΔOD value in Example was low and the washing fastness was good as compared to a case where a color ink containing a polyester-based urethane resin or a polyether-based urethane resin was used. When a color ink containing a polyester-based urethane resin was used, the Δ OD value in Examples was low and the washing fastness was good as compared to a case where a color ink containing a polyether-based urethane resin was used. In addition, when a color ink containing a polyether-based urethane resin was used, as compared to a case where a color ink containing a polycarbonate-based urethane resin or a polyester-based urethane resin was used, the amount of decrease in Δ OD value was larger by setting the order of application of the ink as in the present disclosure, and the washing fastness was further improved by setting the order of application of the ink as in the present disclosure. 
     Example 3 
     (Aggregation Test) 
     The color ink A of Example 1 and the color inks U1 to U9 of Example 2 were used as the color ink, and the discharge printing inks 1 and 2 of Example 1 were used as the discharge printing ink. 
     2 ml of the discharge printing ink was placed in a commercially available test tube, and 20 μl of the color ink was dropped into the test tube. After the dropping, it was allowed to stand for 1 minute, and the contents of the test tube were suction-filtered using a 10 μm membrane filter. The contents remaining on the filter surface were observed with 50× magnification using an optical microscope and an image was obtained. The presence or absence of aggregates with a size of 300 μm or more on the filter surface was examined by naked eyes, and further, the area ratio of the aggregate in the image (a ratio (%) of an area of the aggregate in an area of the image) was calculated using image analysis software. 
     The results are shown in Tables 3 and 4. Table 3 shows the presence or absence and area ratio of the aggregate when the color ink A and the discharge printing ink 1 or 2 were used. Table 4 shows the presence or absence and area ratio of the aggregate when any of the color inks U1 to U9 and the discharge printing ink 1 were used. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Area 
                 Presence or absence of aggregates 
               
               
                   
                 ratio (%) 
                 with a size of 300 μm or more 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Discharge printing ink 1 
                 70.429 
                 Present 
               
               
                 Discharge printing ink 2 
                 0 
                 Absent 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Area 
                 Presence or absence of aggregates 
               
               
                   
                 ratio (%) 
                 with a size of 300 μm or more 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Color ink U1 
                 83.494 
                 Present 
               
               
                   
                 Color ink U2 
                 90.306 
                 Present 
               
               
                   
                 Color ink U3 
                 78.18 
                 Present 
               
               
                   
                 Color ink U4 
                 58.836 
                 Present 
               
               
                   
                 Color ink U5 
                 44.534 
                 Present 
               
               
                   
                 Color ink U6 
                 1.037 
                 Absent 
               
               
                   
                 Color ink U7 
                 36.01 
                 Present 
               
               
                   
                 Color ink U8 
                 92.79 
                 Present 
               
               
                   
                 Color ink U9 
                 82.029 
                 Present 
               
               
                   
                   
               
            
           
         
       
     
     As shown in Table 3, when the color ink A and the discharge printing ink 1 were used, an aggregate was observed. On the other hand, when the color ink A and the discharge printing ink 2 were used, no aggregate was observed. 
     As shown in Table 4, when any of the color inks U1 to 5 or U7 to 9 and the discharge printing ink 1 were used, an aggregate was observed. On the other hand, when the color ink U6 and the discharge printing ink 1 were used, no aggregate was observed. The ink with which an aggregate is observed has high image quality because, for example, it reduces bleeding of an image forming portion. 
     INDUSTRIAL APPLICABILITY 
     As described above, the image forming apparatus of the present disclosure improves the washing fastness of the image forming portion of the fabric on which the discharge printing ink and the color ink are ejected. 
     REFERENCE SIGNS LIST 
     
         
           1 : image forming apparatus 
           2 : ink set storage 
           3 : ink ejector 
           4 : heat treatment section 
           5 : controller