Patent Publication Number: US-9403382-B2

Title: Printing apparatus, printing system, and printed material manufacturing method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2013-190736 filed in Japan on Sep. 13, 2013 and Japanese Patent Application No. 2014-180271 filed in Japan on Sep. 4, 2014. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a printing apparatus, printing system, and a printed material manufacturing method. 
     2. Description of the Related Art 
     With respect to high-speed printing, it is difficult to increase a printing speed in a shuttle system which is a mainstream of a current inkjet recording system, and therefore a single-pass system using a page-width line head has been proposed. However, although the single pass system is advantageous to the printing speed, a time interval between dropped adjacent dots is short, and an adjacent dot is dropped before previously dropped ink penetrates into a printing medium. Therefore, coalescence of the adjacent dots (called “droplet interference”) occurs, which results in occurrence of beading, bleeding, or the like, and this may cause image quality to degrade. 
     There is also a problem that when an inkjet printing apparatus prints an image on non-permeable media or on slow-permeable media, such as a film or a coated paper, adjacent ink dots move to coalesce, which causes an image defect such as beading or bleeding. 
     Conventionally, therefore, a printing speed is reduced and a drying device is installed in order to overcome the weakness of a plain paper and a coated paper. In addition, there is a method of previously applying a primer liquid to the printing medium in order to improve a setting property of aqueous ink. 
     As other method of improving the setting property of the aqueous ink, a method of performing a plasma treatment on the surface of a printing medium is proposed. It is known that the plasma treatment is performed to hydrophilize the surface of the printing medium. Therefore, the performance of the plasma treatment can improve a hydrophilic property and a permeability of a coated paper with poor wettability. Furthermore, the plasma treatment is a drying process, and therefore it is advantageous that the drying process is not necessary. 
     Therefore, there is a need to provide a printing apparatus, a printing system, and a printed material manufacturing method capable of appropriately using inks having different properties such as pH reaction. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to at least partially solve the problems in the conventional technology. 
     According to an embodiment, there is provided a printing apparatus that includes an acidification treatment unit that acidifies at least a surface of a treatment object by irradiating the treatment object with plasma; an inkjet recording unit that records to the treatment object having been acidified by the acidification pretreatment unit in an inkjet recording system; and a control unit that specifies an ink to be used by the inkjet recording unit from at least two types of inks having different properties, based on the type of the treatment object and an amount of plasma energy used in a plasma treatment performed by the acidification treatment unit or based on a pH value of the surface of the treatment object. 
     According to another embodiment, there is provided a printing system that includes an acidification treatment device that acidifies at least a surface of a treatment object by irradiating the treatment object with plasma; and an inkjet recording device that records to the treatment object having been acidified by the acidification pretreatment device in an inkjet recording system. The inkjet recording device includes a control unit that specifies an ink to be used by the inkjet recording device from at least two types of inks having different properties, based on the type of the treatment object and an amount of plasma energy used in a plasma treatment performed by the acidification treatment unit or based on a pH value of the surface of the treatment object. 
     According to still another embodiment, there is provided a printed material manufacturing method for manufacturing a printed material obtained by forming an image on a treatment object in an inkjet recording system. The method include acidifying at least a surface of the treatment object by irradiating the treatment object with plasma; specifying an ink to be used in the forming of the image from at least two types of inks having different properties, based on the type of the treatment object and an amount of plasma energy used in a plasma treatment performed at the acidifying or based on a pH value of the surface of the treatment object; and recording to the treatment object having been acidified by the acidification pretreatment unit with the ink specified. 
     The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of an example of a relationship between a pH value of ink and a viscosity of ink according to an embodiment of the present invention; 
         FIG. 2  is a schematic diagram of an example of a plasma treatment device according to the embodiment; 
         FIG. 3  is a pattern diagram of a schematic configuration of a printing apparatus (system) according to the embodiment; 
         FIG. 4  is a pattern diagram selectively illustrating a configuration from the plasma treatment device to an inkjet recording device in the printing apparatus (system) according to the embodiment; 
         FIG. 5  is a pattern diagram of a schematic configuration example of a switching unit in  FIG. 4 ; 
         FIG. 6  is a diagram of an example of a relationship between an amount of plasma energy and a pH value of the surface of a treatment object for each medium; 
         FIG. 7  is an enlarged diagram of an image obtained by capturing an image forming surface of a printed material which is obtained by performing inkjet recording processing on the treatment object that is not subjected to the plasma treatment according to the embodiment; 
         FIG. 8  is a pattern diagram of an example of dots formed on the image forming surface of the printed material in  FIG. 7 ; 
         FIG. 9  is an enlarged diagram of an image obtained by capturing an image forming surface of the printed material which is obtained by performing inkjet recording processing on the treatment object that is subjected to the plasma treatment according to the embodiment; 
         FIG. 10  is a pattern diagram of an example of dots formed on the image forming surface of the printed material in  FIG. 9 ; 
         FIG. 11  is a graph representing a relationship between an amount of plasma energy, wettability of the surface of the treatment object, beading, a pH value, and permeability according to the embodiment; 
         FIG. 12  is a graph representing a relationship between an amount of plasma energy and a pH value according to the embodiment; 
         FIG. 13  is a flowchart of an example of print processing according to the embodiment; 
         FIG. 14  is a flowchart of an example of plasma treatment as a pretest at Step S 102  of  FIG. 13 ; 
         FIG. 15  is a flowchart of an example of operations for performing the plasma treatment as a pretest according to the embodiment and recording a pH value for each number of electrodes used; 
         FIG. 16  is a flowchart of an example of operations for switching between inks to be used according to the pH value for each number of electrodes used according to the embodiment and performing inkjet recording processing; 
         FIG. 17  is a flowchart of an example of operations for performing inkjet recording processing when an ink to be used in  FIG. 16  is manually switched; 
         FIG. 18  is a diagram of a screen example used for a notification in  FIG. 17 ; 
         FIG. 19  is a graph representing measurement results of an image (dot) density of a treatment object subjected to primer treatment and of a treatment object subjected to plasma treatment with respect to an ink adhesion amount; and 
         FIG. 20  is a graph representing granularity of a treatment object which is hardly permeable when the plasma treatment and the primer treatment are combined. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Exemplary embodiments of the present invention will be explained in detail below with reference to the accompanying drawings. The embodiments explained below are the exemplary embodiments of the present invention, and therefore various technically favorable limitations are applied thereto. However, the scope of the present invention is not unreasonably limited by the following explanation, or all the components explained in the following embodiments are not necessarily essential to the present invention. 
     In the following embodiment, a pretreatment for acidifying the surface of a treatment object is performed in order to aggregate pigments while preventing dispersion of ink pigments immediately after the ink is landed on the treatment object (recording medium, printing medium, printing media, or simply called “media”). 
     The acidification in the present explanation means that the pH value of the surface of the printing medium is decreased to a pH value at which the pigments contained in the ink are aggregated. To decrease the pH value is to increase the concentration of hydrogen ion H +  in an object. The pigments in the ink before coming into contact with the surface of the treatment object are negatively charged, and are therefore dispersed in a vehicle. 
       FIG. 1  depicts an example of a relationship between the pH value of the ink and the viscosity of the ink. As illustrated in  FIG. 1 , the viscosity of the ink increases as the pH value decreases. This is because the negatively charged pigments in the vehicle of the ink are further electrically neutralized with an increase in the acidity of the ink and therefore the pigments are aggregated. Thus, in the graph illustrated in  FIG. 1 , for example, by decreasing the pH value of the surface of the printing medium so that the pH value of the ink reaches a value corresponding to the necessary viscosity, it is possible to increase the viscosity of the ink. This is because when the ink adheres to the acid surface of the printing medium, the pigments are electrically neutralized with the hydrogen ion H +  on the surface of the printing medium and the pigments are thereby aggregated. This enables to prevent color mixture between adjacent dots and to prevent the pigments from penetrating into the deep inside of the printing medium (even into its back side). To decrease the pH value of the ink so as to reach the pH value corresponding to the necessary viscosity, it is required to make the pH value of the surface of the printing medium lower than the pH value of the ink corresponding to the necessary viscosity. 
     The pH value to obtain the necessary viscosity for the ink differs depending on the properties of the ink. In other words, as illustrated in an ink A in  FIG. 1 , some inks are those in which pigments are aggregated at a pH value comparatively close to the neutral and the viscosity is thereby increased, and some other inks are those in which a pH value lower than that of the ink A is necessary in order to aggregate the pigments as illustrated in an ink B having a property different from that of the ink A. 
     Therefore, in the present embodiment, wettability of a surface of the treatment object, or aggregability or permeability of the ink pigments due to decrease in the pH value is controlled by an acidification treatment, and the inks are selectively used according to changes in pH of the surface of the treatment object due to the acidification treatment. It is thereby possible to prevent color mixture between adjacent ink dots (hereinafter, “dots”) and to prevent the pigments from penetrating into deep inside of the treatment object (even into the back side). It is also possible to improve the circularity of a dot and to prevent coalescence of dots so as to enhance sharpness of the dots and increase a color gamut. Consequently, an image defect such as beading or bleeding is improved, and a printed material on which a high quality image is formed can be obtained. By making thin and uniform the thickness of aggregation of the pigments on the treatment object, it is also possible to reduce an amount of ink droplet and achieve reduction in drying energy of the ink and reduction in printing costs. 
     In the following embodiment, a plasma treatment for irradiating the surface of the treatment object with plasma will be exemplified as a pretreatment for acidifying the surface of the treatment object. However, the embodiment is not limited thereto. For example, it is possible to arbitrarily substitute some other acidification treatment such as a primer treatment for applying a treatment liquid called an acid primer liquid to the surface of the treatment object, or to combine the primer treatment and the plasma treatment. 
     Atmospheric-pressure non-equilibrium plasma treatment for performing plasma irradiation in the atmosphere can be used as a plasma treatment which is an acidification treatment. In the atmospheric-pressure non-equilibrium plasma treatment, the plasma irradiation in the atmosphere is performed on the treatment object to cause polymer on the surface of the treatment object to react, and hydrophilic functional group is formed. 
     More specifically, electrons emitted from a discharge electrode are accelerated in an electric field to excite and ionize atoms and particles in the atmosphere. Electrons are also emitted from the ionized atoms and particles, high-energy electrons are increased, and streamer discharge (plasma) is thereby generated. The high-energy electrons generated by the streamer discharge break polymer bounds on the surface of a treatment object (e.g., coated paper) (a coated layer of the coated paper is solidified with calcium carbonate and starch as a binder, and the starch has a polymer structure), and are re-combined with oxygen radical O*, hydroxyl radical (—OH), or ozone O 3  in gas phase. These treatments are called plasma treatment. Thereby, polar functional groups such as hydroxyl groups or carboxyl groups are formed on the surface of the treatment object  20 . Accordingly, hydrophilicity or acidity is given to the surface of the treatment object  20 . By increasing the carboxyl groups, the surface of the treatment object  20  is acidified (the pH value is lowered). 
     To prevent occurrence of color mixture between dots on the treatment object, caused by wet spreading and coalescence of the dots due to an increase in the hydrophilicity, it is found to be important to aggregate colorants (e.g., pigments or dyes) in a dot, to dry the vehicles before wet spreading of the vehicles, or to cause the vehicles to penetrate into the treatment object. Therefore, in the following embodiment, to aggregate the colorants or to cause the vehicles to penetrate into the treatment object, an acidification treatment for acidifying the surface of the treatment object is executed as a pretreatment of the inkjet recording processing. 
     The atmospheric-pressure non-equilibrium plasma treatment is one of preferable methods because an electron temperature is extremely high and a gas temperature is around normal temperature. To stably generate atmospheric-pressure non-equilibrium plasma over a wide range, it is most preferable to use dielectric barrier discharge based on streamer dielectric breakdown obtained by applying alternate high-voltage between electrodes coated with a dielectric body.  FIG. 2  depicts an example of an atmospheric-pressure non-equilibrium plasma treatment device using dielectric barrier discharge. 
     As illustrated in  FIG. 2 , an atmospheric-pressure non-equilibrium plasma treatment device  10  includes a discharge electrode  11 , a counter electrode  14 , a dielectric body (belt)  12  disposed between these electrodes, and a high-frequency high-voltage power supply  15 . The discharge electrode  11  and the counter electrode  14  may be an electrode of which metal portion is exposed, or may be an electrode which is coated with a dielectric body or an insulating body such as insulating rubber or ceramics. The dielectric body  12  disposed between the discharge electrode  11  and the counter electrode  14  may be an insulating body such as polyimide, silicone, or ceramics. When corona discharge is used as the plasma treatment, the dielectric body  12  may be omitted. However, for example, when the dielectric barrier discharge is used, it may be preferable that the dielectric body  12  is disposed. In this case, for the position of the dielectric body  12 , an area of creeping discharge is more widened in a case where the dielectric body  12  is disposed near or in contact with the counter electrode  14  than in a case where it is disposed near or in contact with the discharge electrode  11 . Therefore, it is possible to further enhance the effect of the plasma treatment. The discharge electrode  11  and the counter electrode  14  (or an electrode in the side where the dielectric body  12  is disposed is the relevant dielectric body  12 ) may be arranged at a position in contact with the treatment object  20  passing through between the two electrodes, or may be arranged at a position not in contact therewith. 
     The high-frequency high-voltage power supply  15  applies a high-frequency and high-voltage pulse voltage between the discharge electrode  11  and the counter electrode  14 . A voltage value of the high-frequency and high-voltage pulse is, for example, about 10 kV (p-p). The frequency can be, for example, about 20 kHz. By supplying the high-frequency and high-voltage pulse between the two electrodes, atmospheric-pressure non-equilibrium plasma  13  is generated between the discharge electrode  11  and the dielectric body  12 . The treatment object  20  passes through between the discharge electrode  11  and the dielectric body  12  during generation of the atmospheric-pressure non-equilibrium plasma  13 . Thereby the surface of the treatment object  20  on the discharge electrode  11  side is subjected to plasma treatment. 
     The atmospheric-pressure non-equilibrium plasma treatment device  10  illustrated in  FIG. 2  adopts the discharge electrode  11  of a rotary type and the dielectric body  12  of a belt conveyor type. The treatment object  20  held and conveyed between the discharge electrode  11  and the dielectric body  12  passes through the atmospheric-pressure non-equilibrium plasma  13 . Therefore, the surface of the treatment object  20  comes in contact with the atmospheric-pressure non-equilibrium plasma  13 , and the plasma treatment is uniformly performed on the treatment object  20 . However, the plasma treatment device adopted in the embodiment is not limited to the configuration of  FIG. 2 . For example, the configuration can be variously deformed into those such as a configuration in which the discharge electrode  11  is disposed not in contact with but near the treatment object  20  or a configuration in which the discharge electrode  11  is mounted on a carriage on which an inkjet head is mounted. In addition, a flat-plate dielectric body  12  can also be adopted instead of a belt-conveyor type dielectric body  12 . 
     As a method of generating the atmospheric-pressure non-equilibrium plasma, various methods can be used in addition to the dielectric barrier discharge based on streamer dielectric breakdown. For example, dielectric barrier discharge for inserting an insulating body such as a dielectric body into between electrodes, corona discharge for forming a non-uniform electric field in a thin metal wire or the like, and pulse discharge for applying a short pulse voltage can be adopted. Moreover, a combination of two or more of these methods is also possible. 
     To prevent color mixture between dots due to wet spreading and coalescence of adjacent dots on the treatment object, it is found to be important to uniform colorants (e.g., pigments or dyes) in a dot, to dry the vehicles before wet spreading of the vehicles, or to cause the vehicles to penetrate into the treatment object. 
     The acidification treatment for acidifying the surface of the treatment object is one of effective means to dry the vehicles before wet spreading of the vehicles or to cause the vehicles to penetrate into the treatment object. The acidification treatment for acidifying the surface of the treatment object is particularly effective for a treatment object with low permeability such as a coated paper or a polymer film. However, when the plasma treatment is used as the acidification treatment, a high amount of energy is necessary for plasma irradiation in the atmosphere in order to obtain a sufficient effect for the treatment object with low permeability. 
     A behavior such that colorants are aggregated in a dot, a dry speed of the vehicles, and a penetration speed thereof into the treatment object differ depending on the size of a droplet (mj) varying depending on the size (a small droplet, a medium droplet, and a large droplet) of dots, the type of a treatment object, and the type of ink. Therefore, in the following embodiment, for example, a level of the pretreatment such as an energy of the plasma treatment (hereinafter, “amount of plasma energy”) and a coating amount of a primer liquid is set to an appropriate value according to the type of a treatment object and a print mode (droplet amount etc.), and ink to be used is changed according to the pH value of the surface of the treatment object after the pretreatment. Therefore, in the following embodiment, by changing treatment control according to the type of the treatment object, the level of the pretreatment (for example, the amount of plasma energy) is optimized, and by measuring the wettability of the treatment object after the pretreatment or the pH value of the surface and switching the ink to be used, optimal inkjet recording for all the treatment objects can be achieved. At this time, by reducing an operating load of the plasma treatment or the coating amount of the primer liquid, energy saving can be achieved and a life of the device can be prolonged. 
     The printing apparatus, the printing system, and the printed material manufacturing method according to the embodiment of the present invention will be explained in detail below with reference to the accompanying drawings. The present embodiment will explain an image forming device including discharge heads (recording heads, ink heads) for four colors of black (K), cyan (C), magenta (M), and yellow (Y); however, the embodiment is not limited to the discharge heads. In other words, the image forming device may further include discharge heads corresponding to green (G), red (R), and some other colors, or may include a discharge head for only black (K). In the following explanation, K, C, M, and Y correspond to black, cyan, magenta, and yellow, respectively. 
     In the present embodiment, a continuous roll sheet (hereinafter, “roll sheet”) is used as the treatment object; however, the embodiment is not limited thereto. For example, any medium, such as a cut sheet, capable of forming an image may be employed. Therefore, an overhead projector (OHP) sheet, a synthetic resin film, a metallic thin film, or others capable of forming an image on their surfaces with ink or the like can be used as the treatment object. When the treatment object is a non-permeable sheet or a slow permeable sheet as coated paper, the present embodiment can be more effective than the other cases. A continuous sheet used as a roll sheet may have a line of perforations, along which the roll sheet can be cut off, formed at a predetermined interval. In this case, a page in the roll sheet is an area between lines of perforations formed at a predetermined interval. When a sheet such as a roll sheet or a cut sheet is used as the treatment object, a type used for the sheet includes plain paper, high-quality paper, recycled paper, thin paper, cardboard, coated paper, and the like. 
       FIG. 3  is a pattern diagram of a schematic configuration of a printing apparatus (system)  1  according to the present embodiment. As illustrated in  FIG. 3 , the printing apparatus (system)  1  includes a feeding unit  30  that feeds (conveys) the treatment object  20  (roll sheet) along a conveying path D 1 , a plasma treatment device  100  that performs a plasma treatment as a pretreatment on the fed treatment object  20 , and an image forming device  40  that forms an image on the surface of the treatment object  20  subjected to the plasma treatment. These devices may be provided as separate housings, and all of which may be configured as the system, or may be installed in the same housing as a printing apparatus. When these devices are configured as a printing system, a control unit that controls the whole or part of the system may be included in any one of the devices or may be provided in any other independent housing. 
     The image forming device  40  includes an inkjet recording device  170  that forms an image, through inkjet processing, on the treatment object  20  subjected to the plasma treatment. The image forming device  40  may further include a post-processing unit  70  that performs post-processing on the treatment object  20  on which the image is formed. A pH detecting unit  180  used to detect a pH value of the surface of the treatment object  20  after the pretreatment performed by the plasma treatment device  100 , is disposed between the plasma treatment device  100  and the image forming device  40 . The printing apparatus (system)  1  may include a drying unit  50  that dries the post-processed treatment object  20  and a discharging unit  60  that discharges the treatment object  20  on which the image is formed (in some cases, the post-processing is further performed). As a pretreatment unit that performs a pretreatment on the treatment object  20 , the printing apparatus (system)  1  may further include a primer treatment unit (not illustrated) that applies a treatment liquid, called a primer liquid including polymer materials, to the surface of the treatment object  20 , in addition to the plasma treatment device  100 . Furthermore, the printing apparatus (system)  1  includes a control unit (not illustrated) that controls the operations of the units. The control unit may be connected to a print control device that generates raster data from, for example, the image data as an object to be printed. The print control device may be provided inside the printing apparatus (system)  1  or may be provided outside the printing apparatus (system)  1  via a network such as the Internet or a local area network (LAN). 
     A configuration from the plasma treatment device  100  to the inkjet recording device  170  in the printing apparatus (system)  1  illustrated in  FIG. 3  is selectively illustrated in  FIG. 4 . As illustrated in  FIG. 4 , the printing apparatus (system)  1  includes the plasma treatment device  100  that performs a plasma treatment on the surface of the treatment object  20 , the pH detecting unit  180  that measures a pH value of the surface of the treatment object  20 , the inkjet recording device  170  that forms an image on the treatment object  20  through inkjet recording, and a control unit  160  that controls the entire printing apparatus (system)  1 . The printing apparatus (system)  1  includes conveying rollers  190  for conveying the treatment object  20  along the conveying path D 1 . The conveying rollers  190  are rotationally driven according to the control from, for example, the control unit  160 , to convey the treatment object  20  along the conveying path D 1 . 
     Similarly to the atmospheric-pressure non-equilibrium plasma treatment device  10  illustrated in  FIG. 2 , the plasma treatment device  100  includes a discharge electrode  110 , a ground electrode  141 , a high-frequency high-voltage power supply  150 , and a dielectric belt  121  disposed between these electrodes. As illustrated in  FIG. 4 , the discharge electrode  110  includes five discharge electrodes  111  to  115 , and the ground electrode  141  is disposed along the area facing the discharge electrodes  111  to  115  across the dielectric belt  121 . The high-frequency high-voltage power supply  150  includes five high-frequency high-voltage power supplies  151  to  155  according to the number of discharge electrodes  111  to  115 , respectively. 
     An endless belt is preferably used as the dielectric belt  121  because it is also used for the purpose of conveying the treatment object  20 . Therefore, the plasma treatment device  100  further includes rotating rollers  122  used to circulate the dielectric belt  121  and convey the treatment object  20  thereon. The rotating rollers  122  are rotationally driven based on an instruction from the control unit  160  to thereby circulate the dielectric belt  121 . Accordingly, the treatment object  20  is conveyed along the conveying path D 1 . 
     The pH detecting unit  180  is disposed at a downstream side of the plasma treatment device  100 , detects a pH value of the surface of the treatment object  20  subjected to the pretreatment (acidification treatment) performed by the plasma treatment device  100 , and inputs the detected value to the control unit  160 . 
     The control unit  160  can individually turn on/off the high-frequency high-voltage power supplies  151  to  155 . The control unit  160  can also adjust a pulse intensity of a high-frequency and high-voltage pulse supplied from the high-frequency high-voltage power supplies  151  to  155  to the discharge electrodes  111  to  115 , respectively. 
     The treatment object  20  passes through between the discharge electrode  110  and the dielectric belt  121  in the middle of generation of plasma in the plasma treatment device  100 , so that the plasma treatment is performed on the treatment object  20 . Thereby chains of a binder resin of the surface of the treatment object  20  are broken, and oxygen radical or ozone in the gas phase is re-combined with the polymer, so that the polar functional groups are generated on the surface of the treatment object  20 . Consequently, hydrophilic property and acidification are given to the surface of the treatment object  20 . In the present example, although the plasma treatment is performed in the atmosphere, it may be performed in a gas atmosphere such as nitrogen or rare gas. 
     The inkjet recording device  170  includes an inkjet head  171 , a plurality of ink tanks  172  and  173  (two units in  FIG. 4 ), and a switching unit  174  provided on ink channels between the inkjet head  171  and the two ink tanks  172  and  173 . The ink tank  172  stores, for example, the ink A. The ink tank  173  stores, for example, the ink B having a property (see  FIG. 1 ) different from that of the ink A. 
     The inkjet head  171  includes a plurality of heads for one color (for example, 4 colors×4 heads) in order to, for example, speed up a printing speed. To achieve high-resolution (e.g., 1200 dpi) image formation at a high speed, ink discharge nozzles of the heads in the colors are shifted and fixed so as to correct each interval. Moreover, the inkjet head  171  can be driven at a plurality of drive frequencies so that ink dots (droplets) discharged from the respective nozzles correspond to three sizes called a large droplet/a middle droplet/a small droplet. 
     The inkjet head  171  is disposed at the downstream side of the plasma treatment device  100  on the conveying path of the treatment object  20 . The inkjet recording device  170  forms an image by discharging the inks to the treatment object  20  on which the plasma treatment device  100  performs the pretreatment (acidification treatment) based on the control from the control unit  160 . 
     In the present embodiment, the control unit  160  switches the ink to be used according to the pH value detected by the pH detecting unit  180 . In other words, the control unit  160  controls the switching unit  174  according to the pH value detected by the pH detecting unit  180  and thereby switches the ink discharged from the inkjet head  171  to either one of the ink A and the ink B. Specifically, as illustrated in  FIG. 5 , the switching unit  174  includes a switch control unit  174 A that independently drives a plurality of individual heads  171   a ,  171   b , . . . provided in the inkjet head  171 . The individual heads  171   a ,  171   b , . . . are connected with the ink tanks  172 ,  173 , . . . via individual ink channels, respectively. For example, when the ink A is to be used, the control unit  160  drives the individual head  171   a  connected to the ink tank  172  for the ink A. When the ink B is to be used, the control unit  160  drives the individual head  171   b  connected to the ink tank  173  for the ink B. 
     The present embodiment is not limited to the configuration in which the ink to be used is automatically switched by causing the control unit  160  to control the switching unit  174 . For example, it may be configured that an individual inkjet head  171  is provided for each type of inks to be used and these inkjet heads are automatically switched to one another by the control unit  160  according to the ink to be used. Alternatively, it may be configured that when it is determined that an ink different from the currently set ink is to be used, the control unit  160  notifies the user to replace the ink to be used or an inkjet head containing the ink to be used. Alternatively, it may be configured that a switching valve is provided on the ink channels connecting the inkjet head and the multiple ink tanks to switch the ink to be supplied to the inkjet head under control of the switching valve when the ink to be used is switched. 
     The configuration as above allows the inks to be selectively used according to the treatment object  20  after the pretreatment, thus stably manufacturing high-quality printed materials while further suppressing the costs. In the explanation, it is assumed that the ink A, in which aggregation due to a pH reaction begins early, is stored in the ink tank  172  and the ink B, in which aggregation due to a pH reaction is slow, is stored in the ink tank  173 . 
       FIG. 6  and Table 1 depict an example of a relationship between an amount of plasma energy (the number of the discharge electrodes  110  (hereinafter, “number of electrodes used”)) used in the plasma treatment performed on two types of media a and media b each of which is the treatment object  20  and a pH value of the surface of the treatment object  20  after the plasma treatment. Table 1 lists the relationship illustrated in  FIG. 6  as a table. As illustrated in  FIG. 6  and Table 1, the amount of plasma energy per discharge electrode is set to 0.14 J/cm 2 . 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Measured Value of pH Value 
               
            
           
           
               
               
               
            
               
                   
                 Number of Electrodes Used 
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 1 
                 2 
                 3 
                 4 
                 5 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Type 
                 Media a 
                 6.2 
                 5.2 
                 4.6 
                 4.3 
                 4.2 
               
               
                   
                 Media b 
                 6.5 
                 6.2 
                 6.1 
                 5.9 
                 5.7 
               
               
                   
               
               
                 0.14 J/cm 2  per electrode 
               
            
           
         
       
     
     As illustrated in  FIG. 6  and Table 1, the pH value of the surface of the treatment object  20  tends to become lower with an increase in the number of electrodes used. In other words, the greater the total amount of plasma energy in one plasma treatment is made, the acidification of the surface of the treatment object tends to become higher. However, the change amount of the acidification of the surface of the treatment object with respect to the amount of plasma energy differs depending on the type of the treatment object  20 . In the examples of  FIG. 6  and Table 1, the pH value of the media a tends to become lower than that of the media b. 
     The data illustrated in  FIG. 6  and Table 1 can be obtained by forming five types of areas with a different number of electrodes used in the plasma treatment by performing plasma discharge while sequentially selecting the number of electrodes used in the plasma treatment from one unit to five units, and by measuring the pH value of each area in the pH detecting unit  180 . These data is preferably stored on, for example, a memory (not illustrated) connected to the control unit  160 . In this case, the stored data can be used as data for calibration when an optimal parameter for the pretreatment is calculated from the pH value detected by the pH detecting unit  180  at the time of actual operation of the printing apparatus (system)  1 . In the example, the number of the discharge electrodes  110  is set to five; however, the embodiment is not limited thereto. Therefore, it is preferable that any number of discharge electrodes  110  necessary to obtain a required pH value is installed. In that case, the data for calibration may be measured for each number of units to be installed, or the data may be measured at each interval of predetermined units (for example, intervals of every two electrodes). Furthermore, when the primer treatment is used as the pretreatment, the amount of plasma energy (number of electrodes used) is only replaced with the coating amount of the primer liquid. When the primer treatment and the plasma treatment are combined, a combination of the amount of plasma energy (number of electrodes used) and the coating amount of the primer liquid is simply set as a parameter. In other words, the parameter indicating the level of the treatment executed as the pretreatment is simply managed in association with the acidification. 
     As explained with reference to  FIG. 1 , the pH value required for aggregating pigments contained in an ink differs depending on the properties of the ink. Therefore, a required number of electrodes when the treatment object and the ink are combined is represented in Table 2. The required number of electrodes is the number of the discharge electrodes  110  necessary to obtain the pH value required for aggregation of the pigments contained in the ink. As illustrated in Table 2, also, the amount of plasma energy per discharge electrode is set to 0.14 J/cm 2 . 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                   
                   
                 Required Number 
               
               
                   
                 Type 
                 Ink A 
                 Ink B 
                 of Electrodes 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Media a 
                 ◯ 
                   
                 2 
               
               
                   
                 Media a 
                   
                 ◯ 
                 3 
               
               
                   
                 Media b 
                 ◯ 
                   
                 4 
               
               
                   
                 Media b 
                   
                 ◯ 
                 Unusable 
               
               
                   
                   
               
            
           
         
       
     
     As illustrated in Table 2, when the ink A is used, the pH value at which the aggregation reaction starts is 6.1. Therefore, when the media a is used, the required number of electrodes is two units, and when the media b is used, the required number of electrodes is four units. Meanwhile, when the ink B is used, the pH value at which the aggregation reaction starts is 5.2. Therefore, when the media a is used, the required number of electrodes is three units, while when the media b is used, the necessary pH value cannot be obtained even if all (five units) the installed discharge electrodes  110  are used. In this case, for a combination of the ink B and the media b, the required number of electrodes may be represented as Unusable as illustrate in Table 2, or the number of discharge electrodes  110  to be installed in the plasma treatment device  100  may be increased, or the amount of plasma energy per each discharge electrode may be increased, or a plasma treatment may be divided into a plurality of treatments to be performed by reciprocally conveying the treatment object  20 . 
     As a method for decreasing the pH value of the surface of the treatment object  20  to a required one, there is a method of increasing a time for plasma treatment in addition to a method of switching the number of electrodes to be driven and increasing or decreasing the amount of plasma energy. The method of increasing the time for plasma treatment can be implemented by using, for example, a method of delaying the conveying speed of the treatment object  20  and a method of turning back the treatment object  20  along the conveying path D 1  and performing the plasma treatment thereon a plurality of times. However, if image recording is performed on the treatment object  20  at a high speed, the time for plasma treatment is desired to be reduced. The method of reducing the time for plasma treatment may include, as explained above, a method for providing a plurality of discharge electrodes  111  to  115  and driving a required number of the discharge electrodes  111  to  115  according to the printing speed and the required pH value and a method for adjusting the intensity of the amount of plasma energy to be given to each of the discharge electrodes  111  to  115 . However, the embodiment is not limited thereto, and therefore a method of combining the methods and some other methods can be changed accordingly. 
     Therefore, the control unit  160  according to the present embodiment may select the number of units to be driven among the high-frequency high-voltage power supplies  151  to  155  in a proportion to, for example, printing speed information, may adjust a pulse intensity of a high-frequency high-voltage pulse supplied from each of the high-frequency high-voltage power supplies  151  to  155  to each of the discharge electrodes  111  to  115 , may adjust a conveying speed of the treatment object  20  by controlling the rotational speed of the conveying rollers  190 , may turn back the treatment object  20  by reversely rotating the conveying rollers  190 , or may execute these controls in combination. The printing speed information may be information such as print mode (color printing, black and white printing, or resolution etc.) in the inkjet recording device  170  or may be information such as a rotational speed of the conveying rollers  190  and a throughput derived from the information. The pulse intensity corresponds to the amount of plasma energy, and may be a frequency and a voltage value (amplitude) of a high-frequency high-voltage pulse, or may be a control value calculated from these parameters. 
     The amount of plasma energy required for the plasma treatment may differ depending on a type of media. In this case, the control unit  160  may select the number of units to be driven of the high-frequency high-voltage power supplies  151  to  155 , according to the type of media, may adjust a pulse intensity of a high-frequency high-voltage pulse supplied from each of the high-frequency high-voltage power supplies  151  to  155  to each of the discharge electrodes  111  to  115 , may adjust a conveying speed of the treatment object  20  by controlling the rotational speed of the conveying rollers  190 , may turn back the treatment object  20  by reversely rotating the conveying rollers  190 , or may execute these controls in combination. 
     The behavior of a pigment contained in an ink differs depending on the properties of the ink, as explained above. Therefore, the control unit  160  may select the number of units to be driven of the high-frequency high-voltage power supplies  151  to  155  according to the type (property etc.) of an ink to be used, may adjust a pulse intensity of a high-frequency high-voltage pulse supplied from each of the high-frequency high-voltage power supplies  151  to  155  to each of the discharge electrodes  111  to  115 , may adjust a conveying speed of the treatment object  20  by controlling the rotational speed of the conveying rollers  190 , may turn back the treatment object  20  by reversely rotating the conveying rollers  190 , or may execute these controls in combination. 
     Furthermore, the control unit  160  may adjust the pH value of the surface of the treatment object  20  after the pretreatment by feedback-controlling the plasma treatment device  100  based on the pH value received from the pH detecting unit  180 . 
     The amount of plasma energy required for the plasma treatment can be calculated from a voltage value and an application time of the high-frequency high-voltage pulse supplied from each of the high-frequency high-voltage power supplies  151  to  155  to each of the discharge electrodes  111  to  115 , and from a current passing through the treatment object  20  at that time. The amount of plasma energy required for the plasma treatment may be controlled as a whole amount of energy of the discharge electrodes  110  not as each of the discharge electrodes  111  to  115 . 
     If the discharge electrodes  111  to  115  are provided, it is effective in uniform acidification of the surface of the treatment object  20 . In other words, for example, when conveying speeds (or printing speeds) are the same as each other, the time during which the treatment object  20  passes through a plasma space can be made longer in a case where the acidification treatment is performed by a plurality of discharge electrodes than in a case where the acidification treatment is performed by a single discharge electrode. Accordingly, the acidification treatment can be more uniformly performed on the surface of the treatment object  20 . 
     A difference in printed materials between a case of subjecting the plasma treatment to the treatment object and a case of not subjecting the plasma treatment to the treatment object will be explained below with reference to  FIG. 7  to  FIG. 10 .  FIG. 7  is an enlarged diagram of an image obtained by capturing an image forming surface of a printed material which is obtained by performing inkjet recording processing on the treatment object that is not subjected to the plasma treatment according to the embodiment.  FIG. 8  is a pattern diagram of an example of dots formed on the image forming surface of the printed material in  FIG. 7 .  FIG. 9  is an enlarged diagram of an image obtained by capturing an image forming surface of the printed material which is obtained by performing inkjet recording processing on the treatment object that is subjected to the plasma treatment according to the embodiment.  FIG. 10  is a pattern diagram of an example of dots formed on the image forming surface of the printed material in  FIG. 9 . A desk top type inkjet recording device is used to obtain the printed material illustrated in  FIG. 7  and  FIG. 9 . An ordinary coated paper with a coated layer is used for the treatment object  20 . 
     The coated paper not subjected to the plasma treatment according to the embodiment has less wettability in the coated layer on the surface of the coated paper. Therefore, in the image formed through inkjet recording processing performed on the coated paper that is not subjected to the plasma treatment, the shape of a dot (shape of a vehicle CT 1 ) attached to the surface of the coated paper when the dot is landed is deformed as illustrated in  FIG. 7  and  FIG. 8 . When neighboring dots are formed before the dots are dried sufficiently, as illustrated in  FIG. 7  and  FIG. 8 , vehicles CT 1  and CT 2  coalesce when the neighboring dots are landed on the coated paper, and this causes movement (color mixture) of pigments P 1  and P 2  between the dots, which may result in uneven density due to beading or so. 
     On the other hand, for the coated paper subjected to the plasma treatment according to the embodiment, the wettability of the coated layer on the surface of the coated paper is improved. Therefore, in the image formed through the inkjet recording processing performed on the coated paper that is subjected to the plasma treatment, as illustrated in  FIG. 9 , for example, the vehicles CT 1  spread in a comparatively flat circle over the surface of the coated paper. Accordingly, the dots are made flat as illustrated in  FIG. 10 . In addition, the surface of the coated paper becomes acidity caused by the polar functional groups formed by the plasma treatment, and therefore the ink pigments are electrically neutralized, the pigments P 1  are aggregated, and the viscosity of the ink is increased. Therefore, even when the vehicles CT 1  and CT 2  coalesce as illustrated in  FIG. 10 , the movement (color mixture) of pigments P 1  and P 2  between the dots can be suppressed. Furthermore, because the polar functional groups are generated inside the coated layer, the permeability of the vehicle CT 1  is increased. Therefore, the pigments can be dried in a comparatively short time. The dots spreading in a circular shape due to improvement in the wettability are aggregated while penetrating, and the pigments P 1  are thereby uniformly aggregated in their height direction, thus suppressing occurrence of uneven density due to beading or so.  FIG. 8  and  FIG. 10  are only pattern diagrams, and in actual cases, the pigments are aggregated to form a layer also in a case of  FIG. 10 . 
     In this way, in the treatment object  20  subjected to the plasma treatment according to the embodiment, hydrophilic functional groups are generated on the surface of the treatment object  20  due to the plasma treatment and the wettability is improved. As a result of formation of the polar functional groups due to the plasma treatment, the surface of the treatment object  20  is acidified. Thereby the landed ink is uniformly spread over the surface of the treatment object  20 , the negatively charged pigments are neutralized on the surface of the treatment object  20  to be aggregated, so that the viscosity thereof increases. Consequently, the movement of the pigments can be suppressed even if the dots coalesce. The polar functional groups are generated inside the coated layer formed on the surface of the treatment object  20 , and the vehicles are thereby quickly penetrated into the inside of the treatment object  20 . Therefore, the drying time can be reduced. In other words, the dots spreading in a circular shape due to improvement in the wettability are penetrated into the inside thereof while the movement of the pigments is suppressed due to the aggregation, thus maintaining the dot in a shape near an exact circle. 
       FIG. 11  is a graph representing a relationship between an amount of plasma energy according to the embodiment, a wettability of the surface of the treatment object, beading, a pH value, and permeability.  FIG. 11  represents how the surface properties (the wettability, the beading, the pH value, and the permeability (liquid absorption property)) when an image is printed on the coated paper as the treatment object  20  are dependent on the amount of plasma energy and how the surface properties are changed thereby. To obtain the evaluation illustrated in  FIG. 11 , an aqueous pigment ink (alkaline ink in which negatively charged pigments are dispersed) having a property in which pigments are aggregated due to acid was used for the ink. 
     As illustrated in  FIG. 11 , the wettability of the surface of the coated paper is sharply increased when the amount of plasma energy is low (e.g., about 0.2 J/cm 2  or lower), and is not improved much even if the energy is increased more than that. On the other hand, the pH value of the surface of the coated paper is decreasing to a certain extent with an increase in the amount of plasma energy. However, saturation occurs when the amount of plasma energy exceeds a certain value (e.g., about 4 J/cm 2 ). The permeability (liquid absorption property) is rapidly improved around a point where the decrease in the pH value is saturated (e.g., about 4 J/cm 2 ). However, the phenomenon varies depending on the polymer component contained in the ink. 
     As a result, the value of beading (granularity) is in very good condition after the permeability (liquid absorption property) begins to improve (e.g., about 4 J/cm 2 ). The beading (granularity) in this case represents the degree of roughness of the image by values, and represents the density unevenness by standard deviation of an average density. As illustrated in  FIG. 11 , densities of a color solid image formed from dots of two colors or more are sampled a plurality of times, and the standard deviation of the densities is represented as the beading (granularity). In this way, the ink discharged to the coated paper subjected to the plasma treatment according to the embodiment spreads in the exact circle and penetrates into the coated paper while being aggregated, thus improving the beading (granularity) of the image. 
     As explained above, in the relationship between the property of the surface of the treatment object  20  and the image quality, the dot circularity improves with improvement of the wettability of the surface. This is because the wettability of the surface of the treatment object  20  is improved and uniformed due to an increase in the surface roughness and hydrophilic polar functional groups generated through the plasma treatment. Furthermore, water repellent factors such as foreign particles, oil, and calcium carbonate on the surface of the treatment object  20  are removed by the plasma treatment, which is thought one of factors of the improvement. In other words, as a result of improving the wettability of the surface of the treatment object  20  and removing the destabilizing factors from the surface of the treatment object  20 , the droplet is evenly spread in its circumferential direction, thus the dot circularity is improved. 
     Moreover, by acidifying the surface of the treatment object  20  (reducing pH), the ink pigments are aggregated, the permeability is improved, and the vehicle penetrates into the inside of the coated layer. Therefore, the pigment density on the surface of the treatment object  20  increases, so that even if the dots coalesce, the movement of the pigments can be suppressed. Accordingly, it is possible to suppress pigment mixture and to evenly settle the pigments on the surface of the treatment object  20  and aggregate them. However, the suppressing effect on pigment mixture varies depending on the components of the ink and the size of the ink droplet. For example, when the size of the ink droplet is a small droplet, the pigment mixture due to coalescence of dots is hard to occur as compared with a large droplet. This is because when the vehicle size is a small droplet, the vehicle more quickly dries and penetrates and because the pigments can be aggregated at a low pH reaction. The effect of the plasma treatment fluctuates depending on the type of the treatment object  20  and environments (humidity, etc.). Therefore, the amount of plasma energy in the plasma treatment may be controlled to an optimal value according to the size of the droplet, the type of the treatment object  20 , the environments, and the like. Consequently, the efficiency of surface modification of the treatment object  20  is improved, and further energy saving may be achieved. 
       FIG. 12  is a graph representing a relationship between an amount of plasma energy and a pH value according to the embodiment. The pH is generally measured in a solution, but, recently, the pH of the solid surface can be measured. A measuring instrument is, for example, a pH meter B-211 manufactured by HORIBA Ltd. 
     In  FIG. 12 , a solid line represents plasma energy dependence of the pH value of the coated paper, and a dotted line represents plasma energy dependence of the pH value of PET film. As illustrated in  FIG. 12 , the PET film is acidified by less amount of plasma energy as compared with that of the coated paper. However, even in the coated paper, the amount of plasma energy at the time of acidification is about 3 J/cm 2  or less. When an image is recorded by an inkjet processing device that discharges alkaline aqueous pigment ink to the treatment object  20  of which pH value becomes 5 or less, the dot of the formed image was formed as a shape close to an exact circle. A satisfactory image without pigment mixture due to coalescence of dots and without blurring was obtained (see  FIG. 9 ). 
     Therefore, in the embodiment, as illustrated in  FIG. 4 , the pH detecting unit  180  for solid is disposed at the downstream side of the plasma treatment device  100  which is an acidification treatment unit, and information on the pH of the surface of the treatment object  20  is read by the pH detecting unit  180 . Feedback control or feedforward control is performed on the plasma treatment device  100  based on the information on the read pH, so that the pH value of the surface of the treatment object  20  is reduced to a predetermined value (e.g., pH=5) or less. 
     The print processing according to the embodiment will be explained in detail below with reference to the accompanying drawings.  FIG. 13  is a flowchart of an example of the print processing according to the embodiment. In the example in  FIG. 13 , the printing apparatus (system)  1  illustrated in  FIG. 4  is used, the two types of the ink A and the ink B illustrated in  FIG. 1  are used as the ink, and the cut sheet (printing medium cut to a predetermined size) is used as the treatment object  20 . The same print processing is applicable to the roll sheet instead of the cut sheet. 
     As illustrated in  FIG. 13 , in the print processing, first of all, the control unit  160  drives the feeding unit (see  FIG. 3 ) to feed the treatment object  20  to the conveying path D 1  (Step S 101 ). The treatment object  20  is the one for a test used for the plasma treatment as a pretest. 
     Then, when the treatment object  20  is conveyed up to the plasma treatment device  100 , the control unit  160  drives the plasma treatment device  100  to perform the plasma treatment as the pretest on the treatment object  20  (Step S 102 ). In the plasma treatment as the pretest, the number of the electrodes used in a plurality of patterns is used to perform the plasma treatment. The details of the plasma treatment will be explained later with reference to  FIG. 14 . 
     Subsequently, the control unit  160  measures the pH value of the surface of the treatment object  20  subjected to the plasma treatment as the pretest for each number of the discharge electrodes  110  (number of electrodes used) used for the plasma treatment, and records the measured pH value associated with each number of electrodes used in a predetermined memory (not illustrated) (Step S 103 ). 
     The control unit  160  determines whether there is a pH value which is lower than the pH reaction value (the acidification is higher) required for the surface of the treatment object  20  when the ink B is used, among the pH values recorded at Step S 103  (Step S 104 ). When there is a pH value lower than the pH reaction value in the case of the ink B (YES at Step S 104 ), the control unit  160  identifies the number of electrodes used corresponding to the pH value lower than the pH reaction value in the case of the ink B, drives the plasma treatment device  100  with a least number of electrodes used among the identified number, and thereby starts discharging (Step S 105 ). The control unit  160  switches the switching unit  174  illustrated in  FIG. 4  to the ink tank  173  for the ink B (Step S 106 ), and proceeds to Step S 110 . Thereby, the ink B stored in the ink tank  173  is supplied to the inkjet head  171 . 
     Meanwhile, when there is no pH value lower than the pH reaction value in the case of the ink B (NO at Step S 104 ), the control unit  160  further determines whether there is a pH value lower than the pH reaction value required for the surface of the treatment object  20  when the ink A is used (Step S 107 ). When there is a pH value lower than the pH reaction value in the case of the ink A (YES at Step S 107 ), then the control unit  160  identifies the number of electrodes used corresponding to the pH value lower than the pH reaction value in the case of the ink A, drives the plasma treatment device  100  with a least number of electrodes used among the identified number, and thereby starts discharging (Step S 108 ). The control unit  160  switches the switching unit  174  illustrated in  FIG. 4  to the ink tank  172  for the ink A (Step S 109 ), and proceeds to Step S 110 . Thereby, the ink A stored in the ink tank  172  is supplied to the inkjet head  171 . However, when there is no pH value lower than the pH reaction value in the both cases of the ink A and the ink B (NO at Step S 104 , NO at Step S 107 ), the control unit  160  notifies the user of an error (Step S 116 ), and ends the present operation. 
     At Step S 110 , the control unit  160  drives the feeding unit  30  (see  FIG. 3 ) to actually feed the treatment object  20  for printing an image to the conveying path D 1 . Subsequently, the control unit  160  drives a conveying roller (not illustrated) and the rotating rollers  122  in the plasma treatment device  100  to perform the plasma treatment on the treatment object  20  when the treatment object  20  is passing through the plasma treatment device  100  while being conveyed along the conveying path D 1  (Step S 111 ). Then, the control unit  160  drives the inkjet recording device  170  to perform the inkjet recording processing on the treatment object  20  subjected to the plasma treatment (Step S 112 ), and discharges the treatment object  20  on which the image is thereby formed from the discharging unit  60  (Step S 113 ). The control unit  160  may perform post-processing as necessary before discharging the treatment object  20 . 
     The control unit  160  determines whether the print processing is to be terminated (Step S 114 ). When it is not to be terminated (NO at Step S 114 ), for example, when there still remains some print data, the control unit  160  returns to Step S 110 , and, thereafter, repeatedly performs the same operation until the inkjet recording processing on all the print data is completed. Meanwhile, when the print processing is to be terminated (YES at Step S 114 ), the control unit  160  stops discharging in the plasma treatment device  100  (Step S 115 ), and ends the present operation. 
       FIG. 14  represents an example of the plasma treatment as a pretest performed at Step S 102  in  FIG. 13 . In  FIG. 14 , the number of the discharge electrodes  110  in the plasma treatment device  100  is set to 5. 
     As illustrated in  FIG. 14 , in the plasma treatment as the pretest, the control unit  160  drives the conveying roller (not illustrated) and the rotating rollers  122  in the plasma treatment device  100  to convey the treatment object  20  fed to the conveying path D 1  from the feeding unit  30  up to a predetermined position in the plasma treatment device  100  (Step S 121 ). The predetermined position may be a position between at least one of the discharge electrodes  111  to  114  and the counter electrode  141 . 
     Subsequently, the control unit  160  resets a counter value N (not illustrated) to 0 (Step S 122 ), and then adds 1 to the counter value N (Step S 123 ). Therefore, the counter value N at this stage is 1. 
     The control unit  160  drives an arbitrary number of discharge electrodes  110  corresponding to the value of the counter value N to start discharging (Step S 124 ). Subsequently, the control unit  160  drives the rotating rollers  122  (also the conveying rollers  190  if necessary) to convey the treatment object  20  for a predetermined distance (Step S 125 ). 
     The control unit  160  determines whether the counter value N reaches 5 (Step S 126 ). When it reaches 5 (YES at Step S 126 ), the control unit  160  causes the plasma treatment device  100  to stop discharging (Step S 127 ), and, thereafter, returns to the operation illustrated in  FIG. 13 . Meanwhile, when the counter value N does not reach 5 (NO at Step S 126 ), then the control unit  160  returns to Step S 123 , and repeatedly performs subsequent operations until the counter value N reaches 5. 
     With these operations, a plurality of areas subjected to the plasma treatment with different amounts of plasma energies (number of electrodes used) are formed on the surface of the treatment object  20 . At Step S 103  in  FIG. 13 , the control unit  160  measures a pH value in each of the areas, and records the measured pH value for each of the areas associated with the number of discharge electrodes  110  used in the plasma treatment performed on each of the areas. 
     In the operation as illustrated in  FIG. 13 , when a roll sheet is used as the treatment object  20 , the control unit  160  may perform the plasma treatment as a pretest by using a leading portion of the treatment object  20  fed by the feeding unit  30  and acquire a pH value for each number of electrodes used at Steps S 101  to S 109 . When the roll sheet is used, the property of a single roll hardly changes, and therefore after the amount of plasma energy is adjusted by using the leading portion, stable and continuous printing becomes possible without changing the setting. However, if the device is stopped for a long time without using all the length of the roll sheet, then the property of the sheet may change. Therefore, the plasma treatment as the pretest may be performed again by using the leading portion in the same manner as that before restart of the printing, to acquire a pH value for each number of electrodes used. 
       FIG. 13  exemplifies the case where the processing from the plasma treatment as the pretest to the actual inkjet recording processing is performed through a series of flow; however, the embodiment is not limited thereto. In other words, the operation for performing the plasma treatment as the pretest and recording a pH value for each number of electrodes used can be performed separately from the operation for switching between inks to be used according to the pH value for each number of electrodes used and performing inkjet recording processing. 
       FIG. 15  is a flowchart of an example of operations for performing the plasma treatment as a pretest and recording the pH value for each number of electrodes used.  FIG. 16  is a flowchart of an example of operations for switching between inks to be used according to the pH value for each number of electrodes used and performing inkjet recording processing. 
     First of all, as illustrated in  FIG. 15 , in the operation for performing the plasma treatment as a pretest and recording the pH value for each number of electrodes used, the control unit  160  performs the operations the same as these at Steps S 101  to S 103  in  FIG. 13 , that is, performs the plasma treatment as the pretest on the fed treatment object  20 , and records the measured pH value associated with the number of electrodes used and the type of the fed treatment object  20  in a predetermined memory (not illustrated). 
     Then, similarly to Step S 104  in  FIG. 13 , the control unit  160  determines whether there is a pH value which is lower than the pH reaction value (the acidification is higher) required for the surface of the treatment object  20  when the ink B is used, among the pH values recorded at Step S 103  (Step S 104 ). When there is a pH value lower than the pH reaction value in the case of the ink B (YES at Step S 104 ), the control unit  160  identifies the number of electrodes used corresponding to the pH value lower than the pH reaction value in the case of the ink B, records a least number of electrodes used, among the identified number, associated with information for specifying the ink B as the ink to be used and the type of the fed treatment object for test in the predetermined memory (not illustrated), and ends the present operation. 
     Meanwhile, when there is no pH value lower than the pH reaction value in the case of the ink B (NO at Step S 104 ), similarly to Step S 107  in  FIG. 13 , the control unit  160  further determines whether there is a pH value lower than the pH reaction value required for the surface of the treatment object  20  when the ink A is used (Step S 107 ). When there is a pH value lower than the pH reaction value in the case of the ink A (YES at Step S 107 ), then the control unit  160  identifies the number of electrodes used corresponding to the pH value lower than the pH reaction value in the case of the ink A, records a least number of electrodes used, among the identified number, associated with information for specifying the ink A as the ink to be used and the type of the fed treatment object for test in the predetermined memory (not illustrated) (Step S 202 ), and ends the present operation. However, when there is no pH value lower than the pH reaction value in the both cases of the ink A and the ink B (NO at Step S 104 , NO at Step S 107 ), the control unit  160  notifies the user of an error (Step S 116 ), and ends the present operation. 
     As illustrated in  FIG. 16 , in the operation for the inkjet recording processing by switching between the inks to be used according to the pH value for each number of electrodes used, a treatment object detecting unit (not illustrated) included in the printing apparatus (system)  1  identifies the type of the treatment object  20  set (type of paper) (Step S 210 ). The treatment object detecting unit is, for example, a mechanism that identifies the type of paper in such a manner that the surface of the treatment object  20  is irradiated with a laser beam and the interference spectra of the resultant reflected light are analyzed, or in a such a manner that a barcode attached to a package of the treatment object  20  is read by a reader. Next, the control unit  160  reads the information for the ink to be used and the number of electrodes used from the predetermined memory used at Step S 201  or S 202  in  FIG. 15 , based on the identified type of the treatment object  20  (Step S 211 ), and identifies the ink to be used and the number of electrodes used. The identification of the type of the treatment object  20 , information of the ink to be used, and the number of electrodes used is not limited to the above manner, and may be achieved by using all tables read from the predetermined memory used at Step S 201  or Step S 202  in  FIG. 15 . 
     Subsequently, the control unit  160  determines whether the ink to be used is the ink B (Step S 212 ). When the ink to be used is the ink B (YES at Step S 212 ), similarly to Step S 106  in  FIG. 13 , the control unit  160  switches the switching unit  174  illustrated in  FIG. 4  to the ink tank  173  for the ink B. Meanwhile, when the ink to be used is not the ink B (NO at Step S 212 ), then the control unit  160  determines whether the ink to be used is the ink A (Step S 213 ). When the ink to be used is the ink A (YES at Step S 213 ), similarly to Step S 109  in  FIG. 13 , the control unit  160  switches the switching unit  174  illustrated in  FIG. 4  to the ink tank  172  for the ink A. However, when the ink to be used is neither the ink A nor the ink B (NO at Step S 212 , NO at Step S 213 ), the control unit  160  notifies the user of an error or of switching to the ink tank  172  or to the ink tank  173  (Step S 215 ), and ends the present operation. 
     As explained above, after the switching unit  174  is switched to the ink tank  172  or  173 , the control unit  160  drives the plasma treatment device  100  using the number of electrodes used identified at Step S 211  to start discharging (Step S 214 ). Thereafter, the control unit  160  executes the same operations as these at Steps S 110  to S 115  in  FIG. 13 , and repeatedly performs the operations from the feeding of the treatment object  20  to discharging of the treatment object on which the image is formed until the printing of all the print data is completed. 
     When the pH value of the treatment object  20  after the plasma treatment is previously known, the pH detecting unit  180  may be omitted. The inks to be used may be manually switched to one another for use, other than the method of automatically switching between the inks to be used and using the ink. In this case, the determination, or the like, on the inks to be used can be performed by manual setting or the like performed on the control unit  160 . The manual switching between the inks to be used may be a replacement for each inkjet head including an ink tank as explained above. 
       FIG. 17  is a flowchart of an example of operations for performing inkjet recording processing when an ink to be used in  FIG. 16  is manually switched. As illustrated in  FIG. 17 , the same signs are assigned to operations the same as these of  FIG. 16 , and overlapping explanation thereof is therefore omitted. 
     In the operations illustrated in  FIG. 17 , when it is determined that the ink to be used is the ink B at Step S 212  (YES at Step S 212 ), the control unit  160  determines whether the ink B is already set (Step S 301 ). When the ink B is already set (YES at Step S 301 ), the control unit  160  proceeds to Step S 214 , and performs the subsequent operations. Meanwhile, when the ink B is not set (NO at Step S 301 ), the control unit  160  notifies the user to set the ink B (Step S 302 ), thereafter, returns to Step S 301 , and waits until the ink B is set. 
     When it is determined that the ink to be used is the ink A at Step S 213  (YES at Step S 213 ), the control unit  160  determines whether the ink A is already set (Step S 303 ). When the ink A is already set (YES at Step S 303 ), the control unit  160  proceeds to Step S 214 , and performs the subsequent operations. Meanwhile, when the ink A is not set (NO at Step S 303 ), the control unit  160  notifies the user to set the ink A (Step S 304 ), thereafter, returns to Step S 303 , and waits until the ink A is set. 
     As a method of notifying the user that the ink A or the ink B is set, it is possible to use, for example, the screen as illustrated in  FIG. 18 . The screen may be displayed on an operation screen of, for example, a digital front end (DFE) or on a display of the printing apparatus (system)  1 . Notification by voice or notification by light using LED or so can be used in addition to the notification by screen. 
       FIG. 19  represents measurement results of an image (dot) density of the treatment object subjected to the primer treatment and of the treatment object subjected to the plasma treatment with respect to an ink adhesion amount. A plain paper is used as the treatment object  20  and a black ink is used as the ink in  FIG. 19 . As illustrated in  FIG. 19 , when the plain paper is used as the treatment object  20 , the dot density of the plain paper subjected to the plasma treatment is totally higher than that of the plain paper not subjected to any pretreatment (hereinafter, “untreated plain paper”); however, the saturation concentration of the plain paper is low as compared with the plain paper subjected to the primer treatment. 
     The dot density (halftone density) before it reaches a density equilibrium state more effectively increases in the plasma treatment than in the primer treatment. This indicates that when halftone dots are formed, the ink adhesion amount to obtain the same dot density that is required for the plain paper subjected to the plasma treatment is less than for the plain paper subjected to the primer treatment. Specifically, the ink adhesion amount required for the plain paper subjected to the plasma treatment can be reduced by 1% to 18% as compared with that for the untreated plain paper, and can be reduced by 15% to 29% as compared with that for the plain paper subjected to the primer treatment. 
     The reason that the saturation concentration in the plain paper subjected to the plasma treatment becomes lower than the saturation concentration in the plain paper subjected to the primer treatment is because the dot density in the plain paper subjected to the primer treatment is increased due to the effect of setting. In other words, because dots landed on the plain paper subjected to the primer treatment spread, the pigments are dispersed by an amount of the spread and a peak density drops even if the adhesion amount is the same. However, because the dots hardly spread on the plain paper subjected to the primer treatment, the saturation concentration increases accordingly. 
     From the results obtained in the above manner, different effects can be obtained between the plasma treatment and the primer treatment performed on the treatment object which is hardly permeable and on the treatment object which is easily permeable. It is found from the obtained effects that a combination of the plasma treatment and the primer treatment as the printing system allows improvement of a response capability of the treatment object  20  to image formation. The combination of the plasma treatment and the primer treatment also allows reduction of the amount of plasma energy to, for example, about 1/20 of the plasma treatment as a single treatment, and reduction of the coating amount to about ⅗ of the primer treatment as a single treatment. This means that a printed material with high image quality can be obtained with a low energy consumption and a low coating amount. Furthermore, the high dot density can be obtained, which enables reduction of the amount of ink to adhere. Thus, printing costs can further be reduced. 
     Moreover, it is found from the results in  FIG. 19  that the plasma treatment effectively acts on the treatment object which is hardly permeable and that the primer treatment effectively acts on the treatment object which is easily permeable. This indicates that an optimal pretreatment performed on a treatment object can be achieved by appropriately adjusting implementation conditions of the plasma treatment and the primer treatment according to the property of the treatment object. 
       FIG. 20  is a graph representing granularity of a treatment object which is hardly permeable when the plasma treatment and the primer treatment are combined. The graph illustrated in  FIG. 20  represents that a more satisfactory image is obtained as the granularity is lower. In  FIG. 20 , the broken line indicates a result of the treatment liquid in the primer treatment with respect to the coating amount when the amount of plasma energy is set to 0 J/cm 2  (i.e. when the plasma treatment is not performed). The solid line indicates a result of the treatment liquid in the primer treatment with respect to the coating amount when the amount of plasma energy is set to 0.14 J/cm 2  (i.e. when the plasma treatment and the primer treatment are combined). As illustrated in  FIG. 20 , to achieve a granularity of, for example, 0.5 or lower, a coating amount of about 0.2 mg/cm 2  is needed when only the primer treatment is performed, while only a coating amount of about 0.1 mg/cm 2 , which is a half of the coating amount, is needed when the primer treatment and the plasma treatment are combined. 
     The optimization control derived from  FIG. 20  is for the treatment object. Considering optimization of an image, it is more preferable to perform optimization control based on the printed material obtained through actual printing. For example, a reflection densitometer is incorporated in the printing apparatus (system)  1 , energy for the plasma treatment or a coating amount for the primer treatment performed on the treatment object is continuously changed, a printing pattern which is a reference is printed by the inkjet recording device  170 , and a printing density of the obtained printed material is measured by the reflection densitometer. A treatment condition with which the highest printing density is obtained is determined as an optimal condition, and inkjet recording is performed while executing the optimization control so as to maintain the optimal condition. Accordingly, the measurement and the treatment condition can be changed in a short period of time, thus improving a throughput of the print processing. In addition, the optimal condition identified based on density information acquired from the reflection densitometer can be stored as database. 
     When the component or the type of ink or the type of a treatment object is changed, the optimal condition may also be changed. In this case, by storing and managing the optimal condition in association with the component and the type of ink and the type of the treatment object, the optimization control can be achieved according to various conditions. 
     Moreover, for example, an electrical resistance of a treatment object is measured before the plasma treatment is performed, and the thickness and properties of the treatment object are identified to some extent, so that the study is conducted to derive an optimal condition. 
     Furthermore, when the treatment object is a cut sheet, it may be configured to provide a sensor in the discharging unit of the plasma treatment device  100  and in a discharging unit of a primer treatment device, to obtain a state of the respective treatments, and to perform re-treatment via a different conveying path as needed. In this case, the control unit  160  may perform feedback control or feedforward control on the respective treatment conditions of the plasma treatment device  100  and of the primer treatment device based on the information from the sensors. 
     As explained above, the combined treatment of the plasma treatment and the primer treatment allows downsizing of the printing apparatus (system)  1  while reducing the energy required for the plasma treatment, and allows reduction of a treatment liquid, a drying time of vehicles, and of a drying energy while reducing a coating amount required for the primer treatment. The combined treatment also allows reduction of an ink use amount. Furthermore, when the combined treatment of the plasma treatment and the primer treatment is executed to perform inkjet recording, dots can be formed close to an exact circle and pigments can be prevented from being mixed even if the dots coalesce, thus obtaining a satisfactory image with less blurring. 
     According to the embodiment, it is possible to achieve the printing apparatus, the printing system, and the printed material manufacturing method capable of providing stable quality of image. 
     Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.