Patent Publication Number: US-9427976-B2

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

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2014-142604 filed in Japan on Jul. 10, 2014 and Japanese Patent Application No. 2015-095040 filed in Japan on May 7, 2015. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a printing apparatus, a printing system, and a printed material manufacturing method. 
     2. Description of the Related Art 
     A process of generating plasma and making the surface of a recording medium hydrophilic has been disclosed (for example, Japanese Laid-open Patent Publication No. 2012-179747). Japanese Laid-open Patent Publication No. 2012-179747 discloses a technique to make the surface of a recording medium hydrophilic regardless of the thickness of the recording medium by moving a plasma generator in the thickness direction of the recording medium. 
     However, conventionally, it is difficult to adjust surface roughness on the surface of an ink layer formed on a processing object, such as a recording medium. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to at least partially solve the problems in the conventional technology. 
     A printing apparatus includes: a plasma processing unit that performs plasma processing on a processing target surface side of a processing object; a recording unit that ejects ink on the processing target surface side of the processing object; an acquiring unit that acquires setting information, in which an adjustment target area for adjusting surface roughness and surface roughness of the adjustment target area on a surface of an ink layer formed with the ink are set; and a plasma control unit that controls the plasma processing unit to perform plasma processing on a processing area corresponding to the adjustment target area, on the processing target surface side of the processing object, with an amount of plasma energy for obtaining the set surface roughness on the surface of the ink layer formed on the processing area. 
     A printing system includes: an image processing apparatus; and a printing apparatus capable of communicating with the image processing apparatus. The image processing apparatus includes: a receiving unit that receives setting information containing an adjustment target area for adjusting surface roughness and surface roughness of the adjustment target area on a surface of an ink layer formed on a processing target surface side of a processing object; and a generating unit that generates print data containing the setting information and image data of an image formed with ink. The printing apparatus includes: a plasma processing unit that performs plasma processing on the processing target surface side of the processing object; a recording unit that ejects ink to the processing target surface side of the processing object based on the image data; an acquiring unit that acquires the setting information; and a plasma control unit that controls the plasma processing unit to perform plasma processing on a processing area corresponding to the adjustment target area, on the processing target surface side of the processing object, with an amount of plasma energy for obtaining the set surface roughness on the surface of the ink layer formed on the processing area. 
     A printed material manufacturing method is performed by a printing apparatus including a plasma processing unit that performs plasma processing on a processing target surface side of a processing object, and a recording unit that ejects ink to the processing target surface side of the processing object. The printed material manufacturing method includes: acquiring setting information, in which an adjustment target area for adjusting surface roughness and surface roughness of the adjustment target area on a surface of an ink layer formed with the ink are set; and controlling the plasma processing unit to perform plasma processing on a processing area corresponding to the adjustment target area, on the processing target surface side of the processing object, with an amount of plasma energy for obtaining the set surface roughness on the surface of the ink layer formed on the processing area. 
     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 for explaining an outline of plasma processing according to an embodiment; 
         FIG. 2  is a diagram illustrating an example of a relationship between a pH value and viscosity of ink; 
         FIG. 3  is a graph of an evaluation result of wettability, beading, a pH value, and permeability of the surface of a processing object with respect to plasma energy; 
         FIG. 4  is a diagram illustrating a result of observation of the amount of plasma energy and the uniformity of aggregation of pigment; 
         FIG. 5  is a graph illustrating a result of measurement of a contact angle of pure water when an impermeable recording medium is subjected to plasma processing; 
         FIG. 6  is a graph illustrating diameters of dots when ink droplets with the same size were dropped on the impermeable recording medium; 
         FIG. 7  is a graph illustrating diameters of dots when ink droplets with the same size were dropped on the impermeable recording medium; 
         FIG. 8  is an image of ink dots; 
         FIG. 9  is a graph illustrating image densities; 
         FIG. 10  is a graph illustrating image densities; 
         FIG. 11  is a diagram illustrating an evaluation result of surface roughness and glossiness of ink layers; 
         FIG. 12  is a schematic diagram illustrating a schematic configuration of a printing system according to the embodiment; 
         FIG. 13  is a top view illustrating a schematic configuration of a head unit of a printing apparatus; 
         FIG. 14  is a side view illustrating the schematic configuration of the head unit along a scan direction; 
         FIG. 15  is a schematic diagram illustrating a schematic configuration of a plasma processing unit mounted on the head unit; 
         FIG. 16  is a top view illustrating a print state in printing with five scans by a multipath method; 
         FIG. 17  is a side view illustrating cross-sectional structure of the print state illustrated in  FIG. 16 ; 
         FIG. 18  is a diagram for explaining types of a printing method; 
         FIG. 19  is a block diagram of an image processing apparatus; 
         FIG. 20  is a diagram illustrating an example of an input screen; 
         FIG. 21  is a functional block diagram of the printing apparatus; 
         FIG. 22  is a diagram illustrating an example of a data structure of a first table; 
         FIG. 23  is a diagram illustrating an example of a data structure of a second table; 
         FIG. 24  is a flowchart illustrating the flow of a printing process; and 
         FIG. 25  is a hardware configuration diagram of the image processing apparatus and the printing apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Exemplary embodiments of a printing apparatus, a printing system, and a printed material manufacturing method will be described in detail below with reference to the accompanying drawings. 
     First Embodiment 
     In a first embodiment, plasma processing is performed on a processing target surface side of a processing object. 
     Processing objects used in the embodiment are, for example, an impermeable recording medium, a slowly permeable recording medium, and a permeable recording medium. 
     The impermeable recording medium is a recording medium through which droplets, such as ink, do not substantially permeate. The phrase “do not substantially permeate” means that the permeability of droplets after a lapse of one minute is equal to or lower than 5%. Examples of the impermeable recording medium include art paper, synthetic resin, rubber, coated paper, glass, metal, ceramic, and wood. For the purpose of adding a function, a base material, into which a plurality of the above-described materials are combined, may be used. Further, it may be possible to use a medium, such as plain paper provided with the above described impermeable layer (for example, a coated layer). 
     The slowly permeable recording medium is a recording medium, through which when 10 picoliters (pl) of droplets are dropped on the recording medium, it takes 100 milliseconds (ms) or longer for the entire amount of droplets to permeate, and may be art paper, for example. The permeable recording medium is a recording medium, through which when 10 pl of droplets are dropped on the recording medium, it takes 100 milliseconds (ms) or shorter for the entire amount of droplets to permeate, and may be plain paper or porous paper, for example. 
     In the embodiment, advantageous effects are obtained especially when the impermeable recording medium or the slowly permeable recording medium is applied as a processing object. 
     In the following, the processing object may be referred to as recording media or a recording medium. 
     In the embodiment, to adjust surface roughness of an ink layer formed with ink ejected to a processing area subjected to plasma processing, the plasma processing is performed on the processing area of a processing object with a certain amount of plasma energy according to desired surface roughness. 
     If the plasma processing is performed on a surface of a processing object, wettability of the surface of the processing object improves. If the wettability of the surface of the processing object improves, a dot landed on the processing object subjected to the plasma processing spreads promptly. Therefore, it becomes possible to promptly dry ink on the surface of the processing object. Consequently, it becomes possible to cause ink pigment to aggregate while preventing dispersion of the pigment. As a result, it becomes possible to prevent occurrence of beading or bleed. Further, it becomes possible to adjust surface roughness of an ink layer by aggregation of the pigment. 
     Specifically, in the plasma processing, an organic substance on the surface is oxidized by active species, such as oxygen radical, hydroxyl radical (—OH), or ozone, which is generated in plasma, and a hydrophilic functional group is formed. 
     Therefore, with use of the plasma processing, it is possible to not only control the wettability (hydrophilicity) of the surface of a processing object but also control a pH value (acidification) of the surface of the processing object. Further, with use of the plasma processing, it is possible to control aggregation of pigment contained in an ink layer formed on the processing object subjected to the plasma processing, and adjust surface roughness of the ink layer. 
     Furthermore, with use of the plasma processing, it is possible to improve circularity of an ink dot (hereinafter, simply referred to as a dot) by controlling permeability, prevent coalescence of dots, and enhance sharpness and color gamut of the dots. Consequently, it becomes possible to solve image defects, such as beading and bleed, and produce a printed material on which a high-quality image is formed. Moreover, an amount of ink droplets can be reduced by making uniform and thinning the thicknesses of aggregation of pigment on a processing object, so that it becomes possible to reduce energy for drying ink and printing costs. 
       FIG. 1  is a diagram for explaining an outline of the plasma processing employed in the embodiment. As illustrated in  FIG. 1 , in the plasma processing employed in the embodiment, a plasma processing device  10  is used, which includes a discharge electrode  11 , a counter electrode  14 , a dielectric  12 , and a high-frequency high-voltage power supply  15 . The dielectric  12  is disposed between the discharge electrode  11  and the counter electrode  14 . The high-frequency high-voltage power supply  15  applies a high-frequency high-voltage pulse voltage between the discharge electrode  11  and the counter electrode  14 . 
     The voltage value of the pulse voltage is about 10 kilovolts (kV) (peak to peak), for example. The frequency of the pulse voltage is about 20 kilohertz (kHz), for example. By supplying the above-described high-frequency high-voltage pulse voltage between the two electrodes, atmospheric pressure non-equilibrium plasma  13  is generated between the discharge electrode  11  and the dielectric  12 . A processing object  20  passes between the discharge electrode  11  and the dielectric  12  while the atmospheric pressure non-equilibrium plasma  13  is generated. Therefore, the side facing the discharge electrode  11  (that is, a processing target surface side), of the processing object  20  is subjected to the plasma processing. 
     In the plasma processing device  10  illustrated in  FIG. 1 , the rotary discharge electrode  11  and the belt-conveyor type dielectric  12  are employed as one example. The processing object  20  is conveyed while being nipped between the discharge electrode  11  being rotated and the dielectric  12 , and passes through the atmospheric pressure non-equilibrium plasma  13 . Therefore, the processing target surface side of the processing object  20  comes in contact with the atmospheric pressure non-equilibrium plasma  13  and is subjected to the plasma processing. The atmospheric pressure non-equilibrium plasma  13  is plasma using dielectric barrier discharge. 
     The plasma processing using the atmospheric pressure non-equilibrium plasma is one of preferable plasma processing methods for the processing object  20  because an electron temperature is extremely high and a gas temperature is close to a room temperature. 
     To stably generate the atmospheric pressure non-equilibrium plasma in a wide range, it is preferable to perform atmospheric pressure non-equilibrium plasma processing using dielectric barrier discharge in the manner of streamer breakdown. The dielectric barrier discharge in the manner of streamer breakdown may be generated by applying an alternating high voltage between electrodes coated with a dielectric, for example. 
     As the method of generating the atmospheric pressure non-equilibrium plasma, various methods other than the above-described dielectric barrier discharge in the manner of streamer breakdown may be employed. For example, it may be possible to employ dielectric barrier discharge in which an insulating material such as a dielectric is inserted between electrodes, corona discharge in which a significantly non-uniform electric field is applied to a thin metal wire or the like, and pulse discharge in which a short pulse voltage is applied. Further, two or more of the above methods may be combined. Furthermore, while the plasma processing in the embodiment is performed in the atmosphere, it is not limited thereto. The plasma processing may be performed under a gas atmosphere, such as a nitrogen atmosphere or an oxygen atmosphere. 
     Moreover, while the discharge electrode  11  that can rotate to feed the processing object  20  in accordance with the conveying direction is employed in the plasma processing device  10  illustrated in  FIG. 1 , it is not limited thereto. For example, as will be described later, it may be possible to employ one or more discharge electrodes that can move in the vertical direction (scan direction) with respect to the conveying direction of the processing object  20 . 
     The plasma processing used in the embodiment will be described in detail below. 
     In the plasma processing, the processing object  20  is irradiated with plasma in the atmosphere, so that polymers on the surface of the processing object  20  are made to react and a hydrophilic functional group is generated. Specifically, electrons e released from a discharge electrode are accelerated in an electric field, and excite and ionize atoms and molecules in the atmosphere. The ionized atoms and molecules also release electrons, so that the number of high-energy electrons increases. Therefore, streamer discharge (plasma) is generated. The high-energy electrons generated by the streamer discharge break polymer bonds on the surface of the processing object  20  (for example, coated paper) (a coating layer of the coated paper is immobilized by calcium carbonate and starch as a binder, and the starch has a polymeric structure), and are bonded again with oxygen radical O*, hydroxyl radical (—OH), and ozone O 3  in a gas phase. Therefore, polar functional groups, such as hydroxyl groups or carboxyl groups, are formed on the surface of the processing object  20 . As a result, hydrophilicity and acidity are given to the surface of the processing object  20 . Consequently, the wettability of the surface of the processing object  20  increases, and the surface is acidified (the pH value is reduced). 
     Acidification in the embodiment means that the pH value of the surface on the processing target surface side of the processing object  20  is reduced to a pH value at which pigment contained in ink aggregates. To reduce the pH value is to increase the density of hydrogen ions H +  in an object. The pigment in the ink before coming into contact with the surface on the processing target surface side of the processing object  20  are negatively charged and dispersed in vehicle. 
       FIG. 2  is a diagram illustrating an example of a relationship between the pH value and the viscosity of ink. As illustrated in  FIG. 2 , the viscosity of ink increases as the pH value thereof decreases. This is because the negatively charged pigment in the vehicle of the ink is electrically neutralized as the acidity of the ink increases, and therefore, the pigment aggregates. Therefore, by reducing the pH value of the surface on the processing target surface side of the processing object  20  such that the pH value of the ink reaches a value corresponding to the necessary viscosity in the graph in  FIG. 2 , it is possible to increase the viscosity of the ink. This is because, when the ink adheres to the surface on the processing target surface side of the processing object  20 , the pigment is electrically neutralized by hydrogen ions H +  on the surface on the processing target surface side and the pigment aggregates. This can prevent mixture between adjacent dots and prevent the pigment from permeating deeply into the processing object  20  (or even to the back surface thereof). To reduce the pH value of the ink to the pH value corresponding to the necessary viscosity, the pH value of the surface on the processing target surface side of the processing object  20  needs to be smaller than the pH value of the ink corresponding to the necessary viscosity. 
     Further, the pH value for obtaining the necessary viscosity of the ink varies depending on the characteristics of the ink. Specifically, as illustrated in  FIG. 2 , the pigment in ink A aggregates at a pH value relatively close to the neutrality, thereby increasing the viscosity. In contrast, the pigment in ink B having a different characteristic from that of the ink A aggregates at a pH value smaller than that of the ink A. 
     The behavior of aggregation of pigment in a dot, the drying speed of the vehicle, and the permeation speed of the vehicle in the processing object  20  vary depending on a droplet amount that varies depending on a dot size (a small droplet, a medium droplet, or a large droplet), a type of the processing object  20 , a type of ink, and/or the like. Therefore, in the embodiment described below, the amount of plasma energy in the plasma processing may be controlled at an optimum value depending on the type of the processing object  20 , the amount of ink (droplet amount), or the type of ink. 
       FIG. 3  is a graph of an evaluation result of wettability, beading, a pH value, and permeability of the surface of a processing object with respect to plasma energy according to the embodiment.  FIG. 3  illustrates how surface characteristics (the wettability, the beading, the pH value, and the permeability (liquid absorption characteristics)) change depending on the plasma energy when printing is performed on coated paper serving as the processing object  20 . To obtain the evaluation illustrated in  FIG. 3 , aqueous pigment ink having characteristics, in which pigment aggregates by acid (alkaline ink in which negatively charged pigment is dispersed), was used as the ink. 
     As illustrated in  FIG. 3 , the wettability of the surface of the coated paper is sharply improved when the value of the plasma energy is low (for example, about 0.2 J/cm 2  or less), but is not much improved even when the plasma energy is increased more than that. In contrast, the pH value of the surface of the coated paper decreases to a certain extent by increasing the plasma energy. However, the pH value is saturated when the plasma energy exceeds a certain value (for example, about 4 J/cm 2 ). The permeability (liquid absorbability) is sharply improved from the point about where the decrease in pH is saturated (for example, about 4 J/cm 2 ). However, this phenomenon varies depending on polymer components included in the ink. 
     As a result, the value of beading (granularity) is extremely improved when the permeability (liquid absorption characteristics) starts to be improved (for example, about 4 J/cm 2 ). The beading (granularity) is a numerical value indicating roughness of an image and indicates variation in the density with a standard deviation of an average density. In  FIG. 3 , a plurality of densities in a solid image formed of dots of two or more colors are sampled, and a standard deviation of the densities is indicated as the beading (granularity). As described above, the ink ejected onto the coated paper subjected to the plasma processing according to the embodiment spreads into a perfect circle and permeates while aggregating. 
     The improvement in the wettability of the surface of the processing object  20  and the acidification (reduction in pH) of the surface of the processing object  20  cause the ink pigment to aggregate, improve the permeability, and cause the vehicle to permeate into the coating layer. This increases the pigment density on the surface of the processing object  20  and makes it possible to prevent movement of the pigment even if dots coalesce with one another. Consequently, it becomes possible to prevent mixture of pigments and enable the pigment to uniformly precipitate and aggregate on the surface of the processing object. 
     Further, with the improvement in the wettability of the surface of the processing object  20  and the acidification (reduction in pH) of the surface of the processing object  20 , the speed of aggregation of the pigment contained in the ink is increased and unevenness of the surface (surface roughness) of the ink layer formed with the ink is adjusted. 
     However, the effect of adjusting the surface roughness varies depending on the components of the ink (type of the ink) or an ink droplet amount (amount of the ink). For example, if the ink droplet amount corresponds to a small droplet, mixture of pigments caused by coalescence of dots is less likely to occur compared with the case of a large droplet. This is because a smaller amount of vehicle can be dried and permeate more promptly and enables the pigment to aggregate with a small pH reaction. Further, the effect of the plasma processing varies depending on the type of the processing object  20  and the environment (humidity or the like). Therefore, the amount of plasma energy in the plasma processing may be controlled to an optimum value depending on the amount of the ink, the type of the processing object  20 , the components of the ink (that is, the type of the ink), and the environment. 
       FIG. 4  is a diagram illustrating a result of observation of the amount of plasma energy and the uniformity of aggregation of pigment. The uniformity of aggregation of the pigment improves with an increase in the amount of plasma energy. 
       FIG. 5  is a graph illustrating a result of measurement of a contact angle of pure water when various impermeable recording media are subjected to the plasma processing. In  FIG. 5 , the horizontal axis indicates plasma energy. As illustrated in  FIG. 5 , even in an impermeable recording medium, the wettability is improved through the plasma processing. In the case of aqueous pigment ink, the wettability is further improved because the surface tension is lower than that of pure water. Specifically, the plasma processing causes the aqueous pigment ink to easily and thinly spread out with wetting, so that a surface state advantageous to water evaporation is obtained. In the following, vinyl chloride will be described. However, as indicated in the results described herein, the same effect of the plasma processing is obtained in an impermeable recording medium made of thermoplastic resin, such as polyester or acrylic. 
       FIG. 6  is a graph illustrating diameters of dots when ink droplets with the same size were dropped on the surface of a vinyl chloride sheet that is an impermeable recording medium.  FIG. 7  is a graph illustrating diameters of dots when ink droplets with the same size were dropped on the surface of tarpaulin that is an impermeable recording medium. Tarpaulin is a sheet composed of polyester fibers and a synthetic resin sandwiching the polyester fibers. 
     Ink used in the experiments illustrated in  FIGS. 6 and 7  was aqueous pigment ink, which was prepared by mixing about 3 wt % of pigment and about 5 wt % of styrene-acrylic resin having a particle diameter of 100 to 300 nanometers (nm) in a compound liquid of about 50 wt % of ether solvent and diol solvent and a small amount of surface active agents to disperse the pigment, and prepared to have the surface tension of 21 to 24 N/m and the viscosity of 8 to 11 mPa·s. 
     As illustrated in  FIGS. 6 and 7 , when the plasma processing was performed (5.6 J/cm 2 ), the diameters of dots were increased by 1.2 to 1.3 times as compared with the case where the plasma processing was not performed (Ref.) and where the number of heaters used to dry the ink was reduced without performing the plasma processing (0 J/cm 2 ). This result means that, when the plasma processing (5.6 J/cm 2 ) was performed, it is possible to promptly dry the ink landed on the surface of the impermeable recording medium, as described above. 
       FIG. 8  is an image of ink dots actually formed on the surface of the impermeable recording medium (vinyl chloride sheet) when ink droplets with the same size were dropped on the recording medium. In  FIG. 8 , ink dots of black ink are illustrated at the left, and ink dots of cyan ink are illustrated at the right. Further, in  FIG. 8 , four dots were formed under each condition. As illustrated in  FIG. 8 , when the plasma processing (5.6 J/cm 2 ) was performed, the diameters of the dots were increased as compared with the case where the plasma processing was not performed (Ref.) and where the number of heaters used to dry the ink was reduced without performing the plasma processing (0 J/cm 2 ). Further, when the plasma processing (5.6 J/cm 2 ) was performed, the circularity of the dots was improved as compared with the case where the plasma processing was not performed (Ref.) and where the number of heaters used to dry the ink was reduced without performing the plasma processing (0 J/cm 2 ). 
       FIG. 9  is a graph illustrating image densities obtained when solid printing was performed on the vinyl chloride sheet, which is an impermeable recording medium, under different conditions.  FIG. 10  is a graph illustrating image densities obtained when solid printing was performed on the tarpaulin, which is an impermeable recording medium, under different conditions. As illustrated in  FIGS. 9 and 10 , when the plasma processing (5.6 J/cm 2 ) was performed, the image densities were increased as compared with the case where the plasma processing was not performed (Ref.) and where the number of heaters used to dry the ink was reduced without performing the plasma processing (0 J/cm 2 ). This result means that the plasma processing makes it possible to obtain the same density as that in the case where the plasma processing is not performed even if the ink droplet amount is reduced. 
       FIG. 11  is a diagram illustrating an evaluation result of surface roughness and glossiness of ink layers formed on areas subjected to plasma processing when the plasma processing is performed on various types of the processing objects  20 . 
     As illustrated in  FIG. 11 , when an overhead projector (OHP) sheet was used as the processing object  20 , the surface roughness of the ink layer increased and the glossiness decreased with an increase in the amount of the plasma energy applied to the surface of the processing object  20 . 
     In contrast, when LumiArt (registered trademark) was used as the processing object  20 , the surface roughness of the ink layer increased and the glossiness decreased with an increase in the amount of the plasma energy applied to the surface of the processing object  20  from the unprocessed state to 2.8 J/cm 2 . However, when LumiArt (registered trademark) was used as the processing object  20 , even if the amount of the plasma energy was increased from 2.79 J/cm 2  to 6.97 J/cm 2 , the glossiness remained approximately the same while the surface roughness increased. The glossiness is approximately the same as the glossiness of the surface of LumiArt (registered trademark). Therefore, it is considered that the glossiness is saturated, where the glossiness of the surface of the processing object  20  is the lower limit. 
     As described above, by performing the plasma processing on the processing object  20 , the surface roughness of the ink layer formed with ink on the processing object  20  increases (smoothness is reduced). This may occur because the improvement in the aggregation of the pigment due to the acidification dominantly acts over the wet spreading of the vehicle due to the hydrophilicity, so that the pigment aggregates before completion of the leveling and the surface roughness on the surface of the ink layer is increased. Further, as illustrated in  FIG. 11 , the amount of the plasma energy needed to obtain desired surface roughness on the ink layer varies depending on the type of the processing object  20 . 
     As described above, the inventors have found that surface irregularity (surface roughness) of the ink layer can be controlled by performing the plasma processing on the processing target surface side of the processing object  20  and by forming an ink layer by ejecting ink on a processing area subjected to the plasma processing. 
     Further, the inventors have found that the amount of the plasma energy needed to realize the ink layer with desired surface roughness varies depending on the type of the processing object  20 , the amount of the ink amount, and the type of the ink. 
     Specifically, as indicated in the evaluation result of the glossiness (see  FIG. 11 ), the inventors have found that the surface roughness of the ink layer can be adjusted by adjusting the amount of the plasma energy on the surface of the processing object  20 . Further, the inventors have found that the surface irregularity of the ink layer varies depending on the type of the processing object  20 . As indicated by the evaluation result, as for the surface irregularity of the ink layer, with an increase in the amount of the plasma energy, the surface roughness on the surface of the ink layer formed with ink ejected on the processing area subjected to the plasm processing is increased (roughened) and the glossiness is decreased due to diffuse reflection of light. Therefore, the inventors have found that it is preferable to reduce the amount of the plasma energy when glossy finish is to be applied to the surface of the ink layer to increase the glossiness (the wettability is improved due to plasma, and the ink layer is dried while it is thinly spread out). Furthermore, the inventors found that the increase in the amount of the plasma energy increases the acidification in an area subjected to the plasma processing, increases the speed of aggregation of the pigment, and enables the ink to be dried in a state where the surface roughness is increased. Therefore, the inventors have found that matte finish is applicable to the surface of the ink layer. 
     Therefore, in the printing system of the embodiment, the surface of the ink layer formed on the processing target surface side of the processing object  20  is subjected to the plasma processing with the amount of the plasma energy needed to obtain desired surface roughness. Consequently, the ink layer formed on the processing area subjected to the plasma processing is adjusted to have desired surface roughness. 
     Further, in the printing system of the embodiment, the processing target surface side of the processing object  20  is subjected to the plasma processing with the amount of the plasma energy needed to obtain desired surface roughness depending on the type of the processing object  20 , the amount of the ink, or the type of the ink. Therefore, the ink layer formed on the processing area subjected to the plasma processing is adjusted to have the desired surface roughness. 
     The printing system according to the embodiment will be described in detail below. 
       FIG. 12  is a schematic diagram illustrating a schematic configuration of the printing system according to the embodiment. As illustrated in  FIG. 12 , a printing system  1  includes an image processing apparatus  30  and a printing apparatus  170 . The image processing apparatus  30  and the printing apparatus  170  are connected to each other so as to be able to transmit and receive signals and data. The image processing apparatus  30  and the printing apparatus  170  are connected via a network, such as the Internet or a local area network (LAN). 
     The image processing apparatus  30  generates print data used by the printing apparatus  170  (details will be described later). The printing apparatus  170  includes a recording unit  171 , a plasma processing unit  101 , and a control unit  160 . The recording unit  171  is an inkjet recording device that forms an ink layer (that is, an image with ink) by ejecting ink droplets from nozzles. The plasma processing unit  101  has the same functions as those of the plasma processing device  10  as described above. The printing apparatus  170  sequentially conveys the processing objects  20  to a conveying path (not illustrated), performs plasma processing, and forms ink layers (images) with ink. 
     In the embodiment, a case will be described in which the image processing apparatus  30  and the printing apparatus  170  are separated. However, the image processing apparatus  30  may be mounted on the printing apparatus  170  in an integrated manner. 
     A part of the configuration of the printing apparatus  170  is schematically illustrated in  FIGS. 13 to 15 . 
     In the embodiment, as one example, a case will be described in which a multipath method is used as an inkjet recording method of the printing apparatus  170 . The inkjet recording method of the printing apparatus  170  is not limited to the multipath method, and may be a single-path method, for example. 
       FIG. 13  is a top view illustrating a schematic configuration of a head unit  173  of the printing apparatus  170 .  FIG. 14  is a side view illustrating the schematic configuration of the head unit  173  along a scan direction (a main-scanning direction or a direction of arrow X).  FIG. 15  is a schematic diagram illustrating a schematic configuration of the plasma processing unit  101  mounted on the head unit  173 . 
     As illustrated in  FIGS. 13 and 14 , the printing apparatus  170  includes the control unit  160 , the recording unit  171 , and the plasma processing unit  101 . Further, the printing apparatus  170  includes a heat-drying unit  103  and a detecting unit  102 . The detecting unit  102 , the heat-drying unit  103 , the recording unit  171 , and the plasma processing unit  101  are electrically connected to the control unit  160 . 
     The plasma processing unit  101 , the detecting unit  102 , the heat-drying unit  103 , and the recording unit  171  are mounted on a carriage  172  that runs for scanning in the main-scanning direction (in the direction of arrow X in  FIGS. 13 to 15 ). The head unit  173  includes the plasma processing unit  101 , the detecting unit  102 , the heat-drying unit  103 , the recording unit  171 , and the carriage  172 . 
     The carriage  172  is moved back and forth in the direction (referred to as the scan direction or the main-scanning direction (see the direction of arrow X)) perpendicular to the conveying direction of the processing object  20  (a sub-scanning direction or a direction of arrow Y) by a driving mechanism (not illustrated). The recording unit  171  ejects ink droplets while being conveyed in the scan direction by the carriage  172 , so that an ink layer with the ink is formed on the processing object  20 . 
     The plasma processing unit  101  includes a plurality of discharge electrodes  101   a  to  101   d  and  101   w  to  101   z . The discharge electrodes  101   a  to  101   d  and  101   w  to  101   z  discharge while being conveyed in the scan direction by the carriage  172 , so that the plasma processing is performed on the processing target surface side of the processing object  20  (a side of a surface of the processing object  20  facing the plasma processing unit  101 ). 
     The recording unit  171  includes a plurality of ejection heads (for example fives colors×four heads), for example. In the embodiment, a case will be described in which ejection heads ( 171 Y,  171 M,  171 C,  171 K, and  171 W) for five colors of black (K), cyan (C), magenta (M), yellow (Y), and white (W) are provided. However, the embodiment is not limited to these ejection heads. Specifically, it may be possible to further include ejection heads corresponding to green (G), red (R), and other colors, or include only an ejection head for black (K). In the following description, K, C, M, Y, and W correspond to black, cyan, magenta, yellow, and white, respectively. 
     The type of ink ejected by the recording unit  171  is not specifically limited. For example, ink to be used may be a substance obtained by dispersing a pigment (for example, about 3 wt %), a small amount of surface active agents, styrene-acrylic resin (for example, a particle diameter of 100 nm to 300 nm) (for example, about 5 wt %), various additive preservatives, a fungicide, a pH conditioner, a dye dissolution aid, an antioxidant, conductivity conditioner, a surface tension conditioner, or an oxygen absorber in an organic solvent (for example, ether solvent or diol solvent) (for example, about 50 wt %). 
     It may be possible to use hydrophobic resin, such as acrylic resin, vinyl acetate resin, styrene-butadiene resin, vinyl chloride resin, butadiene resin, and styrene resin, instead of the styrene-acrylic resin. The resin exemplified above preferably has a relatively low molecular weight and is formed in emulsion. 
     It is preferable to add glycols to the ink in order to effectively prevent nozzle clogging. Examples of glycols include ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, polyethylene glycol having a molecular weight of 600 or smaller, 1,3-propylene glycol, isopropylene glycol, isobutylene glycol, 1,4-butandiol, 1,3-butandiol, 1,5-pentanediol, 1,6-hexanediol, glycerine, meso-erythritol, and pentaerythritol. Furthermore, examples of glycols include other thiodiglycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, propylene glycol, dipropylene glycol, tripropylene glycol, neopentyl glycol, 2-methyl-2,4-pentanediol, trimethylolpropane, trimethylolethane, and mixtures thereof. 
     Preferable examples of an organic solvent include alkyl alcohols having a carbon number from 1 to 4 such as ethanol, methanol, butanol, propanol, and isopropanol; glycol ether such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, ethylene glycol monomethyl ether acetate, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol mono-n-propyl ether, ethylene glycol mono-iso-propyl ether, diethylene glycol mono-iso-propyl ether, ethylene glycol mono-n-butyl ether, ethylene glycol mono-t-butyl ether, diethylene glycol mono-t-butyl ether, 1-metyl-1-methoxybutanol, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol mono-t-butyl ether, propylene glycol mono-n-propyl ether, propylene glycol mono-iso-propyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycol mono-n-propyl ether, and dipropylene glycol mono-iso-propyl ether; formamide; acetamide; dimethyl sulfoxide; sorbit; sorbitan; acetin; diacetin; triacetin; sulfolane; pyrrolidone; and N-methyl pyrrolidone. 
     The principal component of the ink may be water. If the organic solvent, monomer, or oligomer is not used for the ink, it is not necessary to select an ink cartridge and a supply path made with a special member. Therefore, it is possible to simplify the structure of the apparatus. 
     The type of ink is determined according to the mixture ratio of the materials contained in the ink or the types of components contained in the ink. 
     In the embodiment, a case will be described in which cut paper cut in a predetermined size (for example, A4 or B4) is used as the processing object  20 ; however, it is not limited thereto. It may be possible to use continuous paper (may be referred to as roll paper). 
     The type of the processing object  20  is not specifically limited. However, when an impermeable recording medium or a slowly permeable recording medium, such as coated paper, is used as the processing object  20 , the effect of the embodiment can be enhanced. 
     In the example illustrated in  FIG. 13 , the five ejection heads ( 171 Y,  171 M,  171 C,  171 K, and  171 W) for the five colors are arranged along the main-scanning direction. Each of the ejection heads for the different colors includes a plurality of nozzles (not illustrated) arranged along the sub-scanning direction (see the direction of arrow Y in  FIGS. 13 to 15 ). Each of the nozzles ejects ink droplets corresponding to each of pixels of image data. 
     In the embodiment, the nozzles arranged on each of the ejection heads for the different colors are divided into four groups (hereinafter, referred to as nozzle groups) along the sub-scanning direction (the direction of arrow Y). Therefore, in each line in the main-scanning direction, the nozzle groups for the five colors are arranged. In this case, the recording unit  171  illustrated in  FIG. 13  includes nozzle groups (a) to (d). Further, in the following description, a belt-like area on which printing is performed by each of the nozzle groups (a) to (d) with one scan or an image printed on the belt-like area is described as a band. 
     The nozzles included in each of the nozzle groups (a) to (d) are fixed in a shifted manner so as to correct gaps in order to achieve high speed image forming with high resolution (for example, 1200 dpi). The recording unit  171  copes with a plurality of types of drive frequencies for ink dots (droplets) that are ejected from each of the nozzles, so as to cope with three types of volumes called a large droplet, a medium droplet, and a small droplet. The drive frequencies are input to the recording unit  171  from a drive circuit (not illustrated) connected to the control unit  160 . 
     The discharge electrodes  101   a  to  101   d  and  101   w  to  101   z  of the plasma processing unit  101  are mounted to both sides of the recording unit  171  so as to sandwich the recording unit  171  from the both sides in the scan direction. In  FIGS. 13 and 14 , the discharge electrodes arranged to one side of the recording unit  171  are referred to as the discharge electrodes  101   a  to  101   d  (they are collectively referred to as a discharge electrode  101 A), and the discharge electrodes arranged to the other side are referred to as the discharge electrode  101   w  to  101   z  (they are collectively referred to as a discharge electrode  101 Z). 
     The electrode length of each of the discharge electrodes  101   a  to  101   d  and  101   w  to  101   z  coincides with, for example, the length of each of the nozzle groups (a) to (d) of the recording unit  171  along the sub-scanning direction (hereinafter, referred to as a band width). For example, in a multi-scan head for four scans, the band width is one fourth of the entire length of the recording unit  171  in the sub-scanning direction. In this case, the length of each of the discharge electrodes  101   a  to  101   d  and  101   w  to  101   z  along the sub-scanning direction is also set to one fourth of the entire length of the recording unit  171  in the same manner as the band width. 
     The electrode length of each of the discharge electrodes  101   a  to  101   d  and  101   w  to  101   z  may be the length of each of the nozzles along the sub-scanning direction, and is not limited to a form that coincides with the band width. 
     As illustrated in  FIG. 15 , the plasma processing unit  101  provided with the above described discharge electrodes  101   a  to  101   d  and  101   w  to  101   z  includes high-frequency high-voltage power supplies  105   a  to  105   d  and  105   w  to  105   z  (the illustration of the high-frequency high-voltage power supplies  105   w  to  105   z  is omitted) arranged for the discharge electrodes  101   a  to  101   d  and  101   w  to  101   z , respectively, includes a dielectric  107  and a counter electrode  104  that are arranged so as to face the whole moving area of the discharge electrodes  101   a  to  101   d  and  101   w  to  101   z , and includes the control unit  160  that controls the high-frequency high-voltage power supplies  105   a  to  105   d  and  105   w  to  105   z . The dielectric  107  is disposed between the counter electrode  104  and the discharge electrodes  101   a  to  101   d  and  101   w  to  101   z , and closer to the counter electrode  104 , for example; however, it is not limited thereto. The dielectric  107  may be disposed closer to the discharge electrodes  101   a  to  101   d  and  101   w  to  101   z . In this case, the dielectric  107  may be divided into a plurality of pieces in accordance with the arrangement of the discharge electrodes  101   a  to  101   d  and  101   w  to  101   z.    
     It is preferable that each of the dielectric  107  and the counter electrode  104  illustrated in  FIG. 15  has a size that covers the whole moving range of the discharge electrodes  101   a  to  101   d  and  101   w  to  101   z , for example. A gap through which the processing object  20  can pass is provided between the counter electrode  104  and the discharge electrodes  101   a  to  101   d  and  101   w  to  101   z . The distance of the gap may be such a distance that the processing object  20  comes in contact with the discharge electrodes  101   a  to  101   d  and  101   w  to  101   z  or such a distance that it does not come in contact with them. 
     The high-frequency high-voltage power supplies  105   a  to  105   d  and  105   w  to  105   z  supply a pulse voltage of about 10 kV (peak to peak) with a frequency of about 20 kHz between the counter electrode  104  and the discharge electrodes  101   a  to  101   d  and  105   w  to  105   z  under the control of the control unit  160 , thereby generating the atmospheric pressure non-equilibrium plasma on the conveying path of the processing object  20 . The amount of the plasma energy in this case may be obtained from the voltage value and the application time of the high-frequency high-voltage pulse supplied to each of the discharge electrodes  101   a  to  101   d  and  101   w  to  101   z , and from the current flowing in the processing object  20 , for example. 
     The control unit  160  can individually turn on or off the high-frequency high-voltage power supplies  105   a  to  105   d  and  105   w  to  105   z . For example, the control unit  160  may adjust the amount of the plasma energy or an area to be subjected to the plasma processing on the processing object  20  by selectively driving a certain number of the high-frequency high-voltage power supplies  105   a  to  105   d  and  105   w  to  105   z  in proportion to printing speed information input from a higher-level device. 
     When the necessary amount of the plasma energy varies for each processing area on the processing object  20 , the control unit  160  may adjust the amount of the plasma energy by selectively driving a certain number of the high-frequency high-voltage power supplies  105   a  to  105   d  and  105   w  to  105   z  in accordance with the type of the processing object  20 . Further, it may be possible to selectively generate plasm with a desired amount of plasma energy in a specific area on the processing object  20  by combining the scanning position of the head unit  173  and on-off control of each of the high-frequency high-voltage power supplies  105   a  to  105   d  and  105   w  to  105   z.    
     In the example illustrated in  FIG. 13 , the nozzle groups (a) to (d) correspond to the respective discharge electrodes  101   a  to  101   d  or the discharge electrodes  101   w  to  101   z  on one-to-one basis. Specifically, plasma processing is performed on a band as a print target area of a certain nozzle group (for example, the nozzle group (a)) by a corresponding discharge electrode (for example, the discharge electrode  101   a  or  101   w ). In this case, plasma processing and printing are performed by one scan, so that it is possible to efficiently perform a printing process. 
     Further, nozzle groups divided more finely may be employed, and a discharge electrode may be disposed so as to correspond to each of the nozzle groups. Furthermore, a discharge electrode with the width (the length in the direction of arrow Y) corresponding to the width of the nozzle (the width of the nozzle in the sub-scanning direction (the direction of arrow Y)) may be disposed for each of the nozzles arranged in the sub-scanning direction (the direction of arrow Y). In this configuration, it becomes possible to further divide an area to be subjected to the plasma processing by the plasma processing unit  101 , and perform the plasma processing with an arbitrary amount of plasma energy for each desired area. 
     Moreover, as an image forming method using the recording unit  171  with a plurality of the nozzles arranged in the main-scanning direction, an overlap recording method may be employed. The overlap recording method is a recording method in which an image of one main-scanning line is completed by performing printing on the same main-scanning line multiple times by using different nozzles. As the image forming method using the recording unit  171 , a multipath method may be employed, in which an image is formed by repeating scanning (scans) in the main-scanning direction by using nozzles corresponding to multiple paths. 
     The image forming method using the multipath method will be described below.  FIG. 16  is a top view illustrating a print state in printing with five scans by a multipath method.  FIG. 17  is a side view illustrating cross-sectional structure of the print state illustrated in  FIG. 16 . In the print state illustrated in  FIGS. 16 and 17 , the number of paths in the sub-scanning direction is set to four, for simplicity of explanation. 
     The nozzle groups (not illustrated) of the recording unit  171  are divided into four path rows, that is, a first path row to a fourth path row (the nozzle groups (a) to (d)), for example. The nozzles arranged in each of the path rows are used to print a corresponding path. A print area formed by one scan is a belt-like band with a band width BW. From the first scan to the third scan, the nozzle groups are sequentially made to start operation from the nozzle group corresponding to the first path row in accordance with a printing start position in the sub-scanning direction. From the fourth scan to the (N−3) th  scan (the N th  scan is the last scan), all of the four path rows are printed by one scan. Therefore, from the fourth scan to the (N−3) th  scan, printing of four paths is performed by one scan. From the (N−2) th  scan to the N th  scan, the nozzle groups are sequentially made to stop operation from the nozzle group corresponding to the first path row in accordance with a printing stop position in the sub-scanning direction, in an opposite manner as that from the first scan to the third scan. On the band subjected to four scans, a complete image is formed. 
     Specifically, as illustrated in  FIGS. 16 and 17 , upon completion of the first scan, an image ( 1 ) is formed by the first scan on a band  201  that corresponds to the printing start position in the sub-scanning direction. Subsequently, with the movement of the recording unit  171  or the processing object  20  in the sub-scanning direction, a scan position of the recording unit  171  is moved in the sub-scanning direction by the band width BW with respect to the processing object  20 , and images ( 2 ) are formed on the band  201  and a band  202  by the second scan. Thereafter, the scan position of the recording unit  171  is moved in the sub-scanning direction by the band width BW with respect to the processing object  20  by each scan, and images ( 3 ) and subsequent images are overlapped on each band. Then, four images are overlapped by four scans, and an image of each band is completed. For example, as illustrated in  FIGS. 16 and 17 , upon completion of the fifth scan, images of the bands  201  and  202  are completed. 
     Referring back to  FIGS. 13 and 14 , the heat-drying unit  103  dries the ink ejected by the recording unit  171 . In the embodiment, a case will be described in which the heat-drying unit  103  is a heating device that applies heat. However, it is sufficient that the heat-drying unit  103  is a device that dries or cures an ink layer, and may be appropriately adjusted depending on the type of the ink. 
     In the embodiment, the heat-drying unit  103  is arranged so as to sandwich the recording unit  171  and the detecting unit  102  from both sides in the main-scanning direction (the direction of arrow X). The heat-drying unit  103  includes a heat-drying unit  103 Z arranged on a side adjacent to the plasma processing unit  101 A of the recording unit  171 , and a heat-drying unit  103 A arranged on a side adjacent to the plasma processing unit  1012  of the recording unit  171 . 
     The detecting unit  102  detects a plasma processing state subjected to the plasma processing by the plasma processing unit  101 . As the detecting unit  102 , a known pH meter for solid substances is used, for example. The detecting unit  102  is not limited to the pH meter, and a known measuring device capable of detecting the plasma processing state is applicable. Further, the head unit  173  may not include the detecting unit  102 . In the embodiment, the detecting unit  102  is arranged so as to sandwich the recording unit  171 , the detecting unit  102 , and the plasma processing unit  101  from both sides in the scan direction (the direction of arrow X). 
     Therefore, when the head unit  173  performs scanning toward one side (for example, in a direction of arrow XA, see  FIG. 14 ) in the main-scanning direction (the direction of arrow X), a detecting unit  102 A detects an area subjected to the plasma processing by the plasma processing unit  101 A, and the recording unit  171  ejects ink droplets. Further, when the head unit  173  performs scanning toward the other end (for example, in a direction of arrow XB, see  FIG. 14 ) in the main-scanning direction (the direction of arrow X), a detecting unit  102 Z detects an area subjected to the plasma processing by the plasma processing unit  1012 , and the recording unit  171  ejects ink droplets. 
     To form a plurality of ink layers in an overlapping manner, the control unit  160  causes the head unit  173  (the recording unit  171 , the plasma processing unit  101 , and the heat-drying unit  103 ) to repeat a series of scanning including ejection of ink droplets for one layer and heating by the heat-drying unit  103 , the same number of times as the number of ink layers. 
     In this case, the control unit  160  may control printing by changing an ink ejection area of each of the ejection heads ( 171 Y,  171 M,  171 C,  171 K, and  171 W) for the different colors. For example, it is assumed that a printed material is obtained by laminating a white ink layer with white ink and a color ink layer with color ink (CMYK) in this order on the processing object  20 . 
     In this case, the control unit  160  causes the nozzle groups (a) and (b) of the ejection head  171 W, which are on the upstream side in the sub-scanning direction (the direction of arrow Y) for ejecting white ink, to eject white ink droplets, and causes the nozzle groups (c) and (d) of the ejection heads ( 171 Y,  171 M,  171 C, and  171 K), which are on the downstream side in the sub-scanning direction (the direction of arrow Y) for ejecting color ink, to eject CMYK ink droplets. In this case, the control unit  160  also controls drive of the head unit  173  in the main-scanning direction. Therefore, the color ink layer is laminated on the white ink layer. 
     Further, it is assumed that a printed material is obtained by laminating a color ink layer and a white ink layer in this order on the processing object  20 . 
     In this case, the control unit  160  causes the nozzle groups (c) and (d) of the ejection head  171 W, which are on the downstream side in the sub-scanning direction (the direction of arrow Y) for ejecting white ink, to eject white ink droplets, and causes the nozzle groups (a) and (b) of the ejection heads ( 171 Y,  171 M,  171 C, and  171 K), which are on the upstream side in the sub-scanning direction (the direction of arrow Y) for ejecting color ink, to eject CMYK ink droplets. In this case, the control unit  160  also controls drive of the head unit  173  in the main-scanning direction and conveyance of the processing object  20  in the sub-scanning direction for each band width. Therefore, the white ink layer is laminated on the color ink layer. 
     Furthermore, it is assumed that a printed material is obtained by laminating a color ink layer, a white ink layer, and a color ink layer in this order on the processing object  20 . 
     In this case, the control unit  160  controls, for each color, nozzle groups for ejecting ink with each scan in the main-scanning direction (the direction of arrow X), with respect to each nozzle group that is obtained by dividing the nozzles of the multiple colors in the recording head  171  into three groups in the sub-scanning direction (the direction of arrow Y). Consequently, a printed material with three ink layers is obtained. 
     Incidentally, there are multiple printing methods as a method of obtaining a printed material by forming ink layers on the processing object  20 . 
       FIG. 18  is a diagram for explaining types of the printing method. 
     As illustrated in  FIG. 18 , examples of the printing method include normal printing, underlay printing, overlay printing, three layer printing, and white ink printing. 
     For example, it is assumed that a transparent medium is used as the processing object  20 . 
       FIG. 18  illustrates normal printing at (a).  FIG. 18  illustrates underlay printing at (b).  FIG. 18  illustrates overlay printing at (c).  FIG. 18  illustrates three layer printing at (d).  FIG. 18  illustrates white ink printing at (e). 
     As illustrated at (a) in  FIG. 18 , the normal printing is a method to form a color ink layer  22  with color ink on the processing object  20 . As illustrated at (b) in  FIG. 18 , the underlay printing is a printing method to laminate a white ink layer  24  with white ink and the color ink layer  22  with color ink in this order on the processing object  20  when a transparent medium is used as the processing object  20 . 
     As illustrated at (c) in  FIG. 18 , the overlay printing is a printing method to form the color ink layer  22  of a color image subjected to a mirroring process (symmetrical process) on the transparent processing object  20 , and further form the white ink layer  24  with white ink. The overlay printing is a printing method to enable the color ink layer  22  to be viewed from the transparent processing object  20  side, where the transparent processing object  20  provides surface glossiness and protects the color ink layer  22 . 
     As illustrated at (d) in  FIG. 18 , the three layer printing is a printing method to laminate the color ink layer  22 , the white ink layer  24 , and the color ink layer  22  in this order on the transparent processing object  20 . The three layer printing is used when a printed material is attached to a transparent material based on the assumption that the printed material is to be viewed from both sides of the processing object  20 . 
     As illustrated at (e) in  FIG. 18 , the white ink printing is a printing method to form the white ink layer  24  with white ink on the processing object  20 . 
     Conventionally, in some cases, there is a need to apply glossy finish with the increased glossiness or matte finish with a delustering effect by providing a specific area of the ink layer formed on the processing object  20  with certain surface roughness that is different from surface roughness on other areas. However, conventionally, to adjust the surface roughness of a specific area on the surface of the ink layer or to adjust the surface of the ink layer to have multiple different types of surface roughness, it is necessary to separately apply transparent toner or the like and it is difficult to perform adjustment easily. 
     Further, in the case where a printed material is the transparent processing object  20  on which an ink layer is formed, a light source is disposed on a side adjacent to one surface of the printed material such that the printed material can be viewed from a side adjacent to the other surface. Examples of this case include a case where the printed material is used for an electric sign board. If the printed material is used for an electric sign board, ejection unevenness of ink ejected on the processing object  20  is intensified by light, and may be visually recognized as density unevenness. 
     In this case, for example, it is necessary to reduce density unevenness, which may be visually recognized, by adjusting surface roughness that may cause light scattering on the surface of an ink layer such as a white ink layer. 
     Therefore, the printing apparatus  170  of the embodiment controls the plasma processing unit  101  to perform plasma processing on a processing area corresponding to an adjustment target area for adjusting surface roughness of an ink layer on the processing target surface side of the processing object  20 , with the amount of plasma energy for obtaining set surface roughness on the surface of the ink layer formed on the processing area. 
     The image processing apparatus  30  generates print data containing setting information, in which an adjustment target area for adjusting surface roughness and surface roughness of the adjustment target area on the surface of the ink layer are set. The printing apparatus  170  adjusts the amount of plasma energy for obtaining the surface roughness contained in the setting information in accordance with the setting information contained in the print data. 
     The image processing apparatus  30  will be described below. 
       FIG. 19  is a block diagram of the image processing apparatus  30 . 
     The image processing apparatus  30  includes a control unit  32 , an input unit  34 , a display unit  36 , and a storage unit  38 . The control unit  32 , the input unit  34 , the display unit  36 , and the storage unit  38  are connected to one another so as to be able to transmit and receive data. The input unit  34  receives an operation instruction from a user. The input unit  34  is, for example, a keyboard, a mouse, a microphone, or the like. The display unit  36  is a known display device that displays various images. A touch panel in which the input unit  34  and the display unit  36  are integrated may be employed. The storage unit  38  stores therein various kinds of data. 
     The control unit  32  controls the entire image processing apparatus  30 . The control unit  32  includes a communication unit  32 A, a receiving unit  32 B, and a generating unit  32 C. A part or all of the communication unit  32 A, the receiving unit  32 B, and the generating unit  32 C may be realized by causing a processing device, such as a central processing unit (CPU), to execute a program, that is, by software, may be realized by hardware, such as an integrated circuit (IC), or may be realized by a combination of software and hardware, for example. 
     The communication unit  32 A communicates with external apparatuses (not illustrated) and the printing apparatus  170 . The receiving unit  32 B receives image data of an image formed with ink from an external apparatus or the like. 
     The receiving unit  32 B also receives input of setting information from the input unit  34 . The setting information is data containing an adjustment target area for adjusting surface roughness and surface roughness of the adjustment target area on the surface of an ink layer formed on the processing target surface side of the processing object  20 . 
     In the embodiment, a case will be described in which the setting information contains the intensity of surface roughness of the adjustment target area as the surface roughness of the adjustment target area. Further, as one example, the setting information indicates three types of intensities of “high intensity”, “normal intensity”, and “low intensity” as the intensities of the surface roughness of the adjustment target area. The intensities of the surface roughness are not limited to the three intensities as described above, and may be four or more intensities indicating subdivided intensities of the surface roughness. Furthermore, the setting information may contain a value of the surface roughness of the adjustment target area. 
     For example, the receiving unit  32 B displays an input screen for inputting an adjustment target area for adjusting surface roughness and the intensity of the surface roughness on the display unit  36 . 
       FIG. 20  is a diagram illustrating an example of an input screen  25 . For example, the receiving unit  32 B displays, on the input screen  25 , an image  27  of the received image data, and character information for requesting input of an adjustment target area and the intensity of surface roughness. A user sets an adjustment target area P for adjusting surface roughness on the image  27  (ink layer) by operating the input unit  34 . The user may set a single or a plurality of adjustment target areas P. 
     For example, it is assumed that a user sets adjustment target areas P 1  to P 3  for adjusting surface roughness by operating the input unit  34 . The user also inputs desired surface roughness for each of the adjustment target areas P 1  to P 3 . In the embodiment, as one example, a case will be described in which the intensity of the surface roughness is input by setting the intensity of the surface roughness (“high intensity”, “normal intensity”, or “low intensity”) in each of the adjustment target areas P 1  to P 3 , as described above. 
     In the embodiment, the intensity of the surface roughness indicates a rate of the intensity of the surface roughness with respect to reference energy to be described later. In the example illustrated in  FIG. 20 , the user sets higher (stronger) surface roughness in the adjustment target area P 1 , the adjustment target area P 2 , and the adjustment target area P 3  in this order (P 1 &lt;P 2 &lt;P 3 ). 
     The user may input a value of desired surface roughness by the input unit  34 , instead of the intensity of the surface roughness. Further, the user may set an arbitrary position, range, shape of the adjustment target area P by providing operation instructions through the input unit  34 . Furthermore, the user may set a different intensity of the surface roughness in each of the adjustment target areas. 
     Referring back to  FIG. 19 , the receiving unit  32 B receives, from the input unit  34 , the setting information containing an adjustment target area for adjusting surface roughness and surface roughness of the adjustment target area (in the embodiment, the intensity of the surface roughness), which are set by the user. For example, the receiving unit  32 B receives setting information, in which the adjustment target area set by the user is indicated in units of objects each representing an adjustment target area and in which the intensity of the surface roughness of the adjustment target area is indicated by a pixel value (for example, a density value). 
     The generating unit  32 C generates print data containing the setting information and image data. 
     Specifically, the generating unit  32 C converts image data received by the receiving unit  32 B to a data format that can be processed by the printing apparatus  170 . For example, the generating unit  32 C performs a conversion process of converting vector data to raster data, a color conversion process of converting colors to CMYKW, or gamma correction, thereby converting the received image data to a data format that can be processed by the printing apparatus  170 . 
     Further, the generating unit  32 C converts the surface roughness of each of the adjustment target areas (in the embodiment, the intensity of the surface roughness), which is set in the setting information received by the receiving unit  32 B, to setting information that is set in units of pixels. Specifically, setting information in the raster format is generated by setting a pixel value indicating the set surface roughness (in the embodiment, the intensity of the surface roughness) as a pixel value of each of pixels of the adjustment target area represented in the vector format. Each of the pixel positions in the setting information in the raster format corresponds to each of the pixel positions in the image data in the raster format. 
     The generating unit  32 C generates print data containing the image data converted to the raster format and the setting information converted to the raster format. The communication unit  32 A outputs the generated print data to the printing apparatus  170 . The data format is not limited to these formats. 
       FIG. 21  is a functional block diagram of the printing apparatus  170 . 
     The printing apparatus  170  includes the control unit  160 , a storage unit  162 , the plasma processing unit  101 , the recording unit  171 , the detecting unit  102 , and the heat-drying unit  103 . The control unit  160 , the storage unit  162 , the plasma processing unit  101 , the recording unit  171 , the detecting unit  102 , and the heat-drying unit  103  are connected to one another so as to be able to transmit and receive data and signals. As described above, the plasma processing unit  101 , the recording unit  171 , the detecting unit  102 , and the heat-drying unit  103  form the head unit  173 . The storage unit  162  stores therein various kinds of data. 
     The control unit  160  is a computer including a CPU and the like, and controls the entire printing apparatus  170 . The control unit  160  may be configured by a circuit other than the CPU. 
     The control unit  160  includes a communication unit  160 A, an acquiring unit  160 B, a calculating unit  160 C, a plasma control unit  160 D, and a recording control unit  160 E. A part or all of the communication unit  160 A, the acquiring unit  160 B, the calculating unit  160 C, the plasma control unit  160 D, and the recording control unit  160 E may be realized by causing a processing device, such as a CPU, to execute a program, that is, by software, may be realized by hardware, such as an IC, or may be realized by a combination of software and hardware, for example. 
     The communication unit  160 A communicates with the image processing apparatus  30  and external apparatuses (not illustrated). In the embodiment, the communication unit  160 A receives print data from the image processing apparatus  30 . 
     The acquiring unit  160 B acquires setting information contained in the received print data. Specifically, the acquiring unit  160 B acquires setting information, in which an adjustment target area for adjusting surface roughness and surface roughness (the intensity of the surface roughness) of the adjustment target area on the surface of an ink layer formed with ink are set. If a plurality of adjustment target areas are set, the acquiring unit  160 B acquires setting information, in which the adjustment target areas and surface roughness of each of the adjustment target areas on the surface of the ink layer are set. 
     The calculating unit  160 C calculates the amount of plasma energy for obtaining the set surface roughness on the surface of the ink layer formed on the processing area corresponding to the adjustment target area set in the setting information, on the processing target surface side of the processing object  20 . 
     In the embodiment, a case will be described in which the calculating unit  160 C calculates the amount of plasma energy to be applied to the surface on the processing target surface side of the processing object  20  (that is, the surface of the processing object  20 ). In the following descriptions, the surface on the processing target surface side of the processing object  20  may simply be described as the surface of the processing object  20 . 
     For example, the storage unit  162  stores therein, in advance, surface roughness on the surface of the ink layer and the amount of plasma energy to be applied to the surface of the processing object  20  to realize the surface roughness, in an associated manner. The calculating unit  160 C calculates the amount of plasma energy by reading, from the storage unit  162 , the amount of the plasma energy corresponding to the surface roughness of the adjustment target area set in the setting information. 
     It is preferable that the calculating unit  160 C calculates the amount of the plasma energy to be applied to the processing area corresponding to the adjustment target area, in accordance with at least one of the type of the processing object  20 , the amount of ink applied to the processing area on the surface of the processing object  20 , and the type of the ink applied to the processing area. 
     In the embodiment, as one example, a case will be described in which the calculating unit  160 C calculates the amount of the plasma energy to be applied to the processing area corresponding to the adjustment target area, on the surface of the processing object  20 , in accordance with the type of the processing object  20  (hereinafter, referred to as a paper type), the amount of ink applied to the processing area, and the type of the ink applied to the processing area. 
     For example, the control unit  160  stores a first table and a second table in the storage unit  162  in advance. 
     The first table is a table indicating a relationship between resolution and a droplet amount.  FIG. 22  is a diagram illustrating an example of a data structure of the first table. As illustrated in  FIG. 22 , the first table is a table, in which droplet amounts (pl) corresponding to a small droplet, a medium droplet, and a large droplet, as the amounts of droplets ejected from the nozzles, are associated with each resolution of an image to be recorded. 
     The recording control unit  160 E calculates a droplet amount corresponding to the pixel value of each of the pixels of the image data. The recording control unit  160 E controls the recording unit  171  such that the calculated amounts of ink droplets are ejected from the corresponding nozzles. Therefore, the recording control unit  160 E controls the recording unit  171  such that ink droplets with the droplet amount corresponding to the resolution and the density at each pixel position indicated in the image data are ejected from a corresponding nozzle at a scanning position corresponding to a pixel at each pixel position. 
     Therefore, the amount of ink ejected in an area corresponding to each of the pixels on the processing object  20  is determined by the resolution of a print image and the pixel value of each of the pixels defined in the image data. 
     The storage unit  162  stores therein the second table corresponding to each type of ink in advance. The second table is data, in which the type of ink and the amount of reference energy corresponding to a paper type are associated with each other. The amount of the reference energy is the amount of plasma energy to be applied to the surface of the processing object  20  in order to realize reference surface roughness determined in advance. The reference surface roughness is surface roughness of an ink layer and serves as a reference determined in advance. Arbitrary surface roughness may be set as the reference surface roughness. 
     Specifically, each of the amounts of the reference energy registered in the second table is the amount of the reference energy corresponding to a type of ink, an amount of ink, and a paper type. 
       FIG. 23  is a diagram illustrating an example of a data structure of the second table.  FIG. 23  illustrates the second table corresponding to a single type of ink (a relationship between the amount of the ink and the amount of the reference energy corresponding to a paper type). In reality, the storage unit  162  stores therein, in advance, the second table for each of the types of ink (a table in which the amount of ink and the amount of reference energy corresponding to a paper type are registered). 
     It is preferable for a user to measure, in advance by using the printing apparatus  170 , the amount of the plasma energy (the amount of the reference energy) to be applied to the surface of the processing object  20  in order to obtain the reference surface roughness on the surface of the ink layer, by using a plurality of paper types, a plurality of types of ink, and a plurality of different amounts of ink in advance. The control unit  160  registers, in the second table corresponding to each type of ink, the amount of the plasma energy corresponding to each of measured conditions, as the reference energy corresponding to measurement conditions (a paper type, a type of ink, and an amount of ink). 
     The calculating unit  160 C calculates the amount of the plasma energy applied to the processing area corresponding to the adjustment target area by using the print data, the first table, and the second table corresponding to the type of ink to be used. 
     The calculating unit  160 C extracts pixels at pixel positions overlapping the adjustment target area set in the setting information acquired by the acquiring unit  160 B from among pixels of the image data contained in the print data received by the communication unit  160 A. The calculating unit  160 C determines an ejection amount of ink droplets (a large droplet, a medium droplet, or a small droplet) corresponding to each of the extracted pixels from the pixel value of each of the pixels. Specifically, the calculating unit  160 C determines that the amount corresponds to a small droplet when the pixel value of each of the extracted pixels is smaller than a first threshold set in advance, corresponds to a medium droplet when the pixel value is equal to or greater than the first threshold and smaller than a second threshold that is greater than the first threshold, and corresponds to a large droplet when the pixel value is equal to or greater than the second threshold. 
     The calculating unit  160 C acquires resolution for printing. The resolution may be contained in the print data and acquired by being read from the print data. The calculating unit  160 C may acquire, from an input unit (not illustrated) provided in the printing apparatus  170 , resolution for printing specified by the user. 
     The calculating unit  160 C reads, from the first table (see  FIG. 22 ), a droplet amount corresponding to the resolution and the ejection amount (a large droplet, a medium droplet, or a small droplet) of a pixel at each of the pixel positions overlapping the adjustment target area in the image data. 
     The calculating unit  160 C calculates the amount of ink applied to the processing area corresponding to the adjustment target area, on the surface of the processing object  20 . For example, the calculating unit  160 C calculates, as the amount of ink applied to each of the pixel positions in the processing area, an additional value of the droplet amount to be applied to each of the pixel positions in the thickness direction (the lamination direction of the ink layer), for each of the pixel positions overlapping the adjustment target area set in the setting information in the image of the image data. Accordingly, the calculating unit  160 C calculates the amount of ink applied to the processing area corresponding to the adjustment target area, on the surface of the processing object  20 . 
     The calculating unit  160 C reads the type of ink used for the printing. The calculating unit  160 C reads the type of ink by receiving a signal indicating the type of ink from a sensor (not illustrated) provided in the recording unit  171 , for example. The calculating unit  160 C may acquire the type of ink from an input unit (not illustrated) provided in the printing apparatus  170 , for example. For example, the user inputs the type of ink used for the printing by operating the input unit (not illustrated). The calculating unit  160 C acquires the type of ink by receiving the type of ink from the input unit. The calculating unit  160 C may read the type of ink from the print data. In this case, the print data may be configured to contain the type of ink. 
     The calculating unit  160 C also reads the type of the processing object  20  (paper type) used for the printing. For example, the print data may be configured to contain information indicating the paper type, and the calculating unit  160 C may read the paper type from the print data. In this case, the image processing apparatus  30  may generate the print data containing the paper type of a printing object in accordance with an operation of the input unit  34  by the user, for example. The calculating unit  160 C may receive a signal indicating the paper type from a sensor (not illustrated) provided in a storage (not illustrated), which is provided in the printing apparatus  170  and stores therein the processing object  20 . In this case, the calculating unit  160 C may acquire the paper type by reading the signal indicating the paper type received from the sensor. 
     The calculating unit  160 C reads the amount of reference energy corresponding to the acquired paper type and the calculated amount of ink from the second table (see  FIG. 23 ) corresponding to the acquired type of ink, for each of the pixel positions. Therefore, the calculating unit  160 C calculates the amount of the reference energy to be applied to the processing area corresponding to the adjustment target area, on the surface of the processing object  20 . 
     Then, the calculating unit  160 C reads information indicating the intensity of the surface roughness corresponding to the adjustment target area indicated by the setting information. For example, the intensity of the surface roughness of “low intensity” indicates 50% (a half) of the reference energy, “normal intensity” indicates the reference energy (that is, 100% (the same magnification)), and “high intensity” indicates 150% (one and a half) of the reference energy. These values are arbitrary, and may be set appropriately or changed appropriately according to an operation instruction by the user. 
     The calculating unit  160 C calculates, as the amount of the plasma energy to be applied to each of the pixel positions of the processing area, a value obtained by multiplying the amount of the reference energy calculated for each processing target area (that is, a pixel position of each of the pixels in the processing target area) by a value (50% (a half), 100% (the same magnification), or 150% (one and a half)) corresponding to the intensity of the surface roughness set in the corresponding adjustment target area. 
     Therefore, for example, in the processing area corresponding to the adjustment target area in which the intensity of the surface roughness of “low intensity” is set, the amount of plasma energy corresponding to a half of the calculated amount of the reference energy is set. Further, for example, in the processing area corresponding to the adjustment target area in which the intensity of the surface roughness of “normal intensity” is set, the amount of plasma energy corresponding the calculated amount of the reference energy is set. Furthermore, for example, in the processing area corresponding to the adjustment target area in which the intensity of the surface roughness of “high intensity” is set, the amount of plasma energy corresponding to twice of the calculated amount of the reference energy is set. 
     As described above, the calculating unit  160 C calculates the amount of the plasma energy for obtaining the set surface roughness on the surface of the ink layer formed on a processing area corresponding to the adjustment target area indicated by the setting information on the surface of the processing object  20 , for each adjustment target area (each processing area). 
     The plasma control unit  160 D controls the plasma processing unit  101  to perform the plasma processing on the processing area corresponding to the adjustment target area of the ink layer set in the setting information on the surface of the processing object  20 , with a corresponding amount of the plasma energy calculated by the calculating unit  160 C. 
     In the embodiment, a case will be described in which the plasma control unit  160 D controls the plasma processing unit  101  to perform the plasma processing on the processing area corresponding to the adjustment target area of the ink layer on the surface of the processing object  20  with the corresponding amount of the plasma energy calculated by the calculating unit  160 C. 
     The amount of the plasma energy is, as described above, the amount of energy of plasma to cause pigment contained in an adjustment target ink layer to aggregate such that the surface roughness set in the setting information is obtained. 
     The plasma control unit  160 D controls the plasma processing unit  101  to perform the plasma processing on a corresponding processing area with the amount of the plasma energy that is calculated for each of the processing areas corresponding to the adjustment target area. For example, the plasma control unit  160 D controls selection of a discharge electrode to which a voltage is applied among the discharge electrodes  101   a  to  101   d  and  101   w  to  101   z  provided in the plasma processing unit  101 , controls a voltage value of the voltage applied to the discharge electrode, controls a voltage application time, controls a speed of the carriage  172  in the sub-scanning direction, and controls a feed timing of the processing object  20  in the main-scanning direction in a combined manner, thereby causing the plasma processing to be performed on the processing area corresponding to the adjustment target area, on the surface of the processing object  20  with a calculated corresponding amount of plasma energy. 
     Further, when the setting information contains a plurality of adjustment target areas, the plasma control unit  160 D performs plasma processing on each of the processing areas on the processing object  20  corresponding to the adjustment target areas, with the amount of the plasma energy for obtaining the surface roughness on the surfaces of ink layers formed on the respective processing areas. 
     Therefore, the surface of the ink layer formed with ink on the processing area subjected to the plasma processing can be adjusted to have desired surface roughness. 
     The flow of a printing process performed by the printing apparatus  170  will be described below.  FIG. 24  is a flowchart illustrating the flow of the printing process performed by the printing apparatus  170 . 
     First, the communication unit  160 A receives print data from the image processing apparatus  30  (Step S 100 ). The communication unit  160 A stores the received print data in the storage unit  162  (Step S 102 ). 
     The acquiring unit  160 B acquires setting information and image data from the print data (Step S 104 ). 
     The calculating unit  160 C acquires a paper type used for printing (the type of the processing object  20 ) (Step S 106 ). The calculating unit  160 C acquires a type of ink used for printing (Step S 108 ). 
     The calculating unit  160 C reads the first table (see  FIG. 22 ) stored in the storage unit  162  and the second table (see  FIG. 23 ) corresponding to the acquired type of the ink (Step S 110 ). 
     The calculating unit  160 C calculates the amount of ink applied to a processing area corresponding to the adjustment target area, on the surface of the processing object  20  by using the image data and the setting information acquired at Step S 104  and by using the first table read at Step S 110  (Step S 112 ). 
     The calculating unit  160 C reads, from the second table (see  FIG. 23 ) corresponding to the type of ink acquired at Step S 108 , the amount of reference energy corresponding to the paper type acquired at Step S 106  and the amount of the ink calculated at Step S 112 . Through the process, the calculating unit  160 C calculates the amount of the reference energy to be applied to the processing area corresponding to each of the adjustment target areas (Step S 114 ). 
     The calculating unit  160 C reads information indicating the intensity of the surface roughness corresponding to the adjustment target area indicated in the setting information (Step S 116 ). The calculating unit  160 C calculates, for each processing area, the amount of the plasma energy for obtaining the surface roughness set in the setting information on the surface of the ink layer formed on the processing area corresponding to the adjustment target area (Step S 118 ). Specifically, as described above, the calculating unit  160 C calculates, as the amount of the plasma energy to be applied to the processing area, a value obtained by multiplying the amount of the reference energy of each processing area calculated at Step S 114  by a value indicating the intensity of the surface roughness set for the corresponding adjustment target area indicated in the setting information (the value is 1.5 for “high intensity”,  1  for “normal intensity”, or 0.5 for “low intensity” as described above). 
     The plasma control unit  160 D controls the plasma processing unit  101  to perform the plasma processing on each of the corresponding processing areas on the processing target surface side of the processing object  20 , with the amount of the plasma energy calculated at Step S 118  (Step S 120 ). 
     The recording control unit  160 E causes the recording unit  171  to eject ink droplets to a corresponding position in accordance with the density value of each of the pixels indicated by the image data (Step S 122 ). 
     In the processes at Step S 120  to Step S 122 , the control unit  160  controls scanning of the head unit  173  and conveyance of the processing object  20 . 
     The control unit  160  repeats the processes from Step S 120  to Step S 122  (NO at Step S 124 ) until the image of the image data contained in the print data is formed (YES at Step S 124 ). If a determination result is positive at Step S 124  (YES at Step S 124 ), the routine is finished. 
     As described above, the printing apparatus  170  according to the embodiment includes the plasma processing unit  101 , the recording unit  171 , the acquiring unit  160 B, and the plasma control unit  160 D. The plasma processing unit  101  performs plasma processing on the processing target surface side of the processing object  20 . The recording unit  171  ejects ink. The acquiring unit  160 B acquires setting information, in which an adjustment target area for adjusting surface roughness and surface roughness of the adjustment target area on the surface of the ink layer are set. The plasma control unit  160 D controls the plasma processing unit  101  to perform the plasma processing on the processing area corresponding to the adjustment target area, on the processing target surface side of the processing object  20 , with the amount of the plasma energy for obtaining the set surface roughness on the surface of the ink layer formed on the processing area. 
     Therefore, the printing apparatus  170  of the embodiment can easily adjust the surface roughness on the surface of the ink layer formed on the processing object  20  to desired surface roughness. 
     Further, the printing apparatus  170  can easily adjust the surface roughness on the surface of the ink layer to desired surface roughness, so that it is possible to easily adjust the surface roughness of an arbitrary area on the surface of the ink layer or to adjust the glossiness of a white ink layer. 
     Specifically, with an increase in the surface roughness of the ink layer, more light is diffusely reflected. Therefore, it is possible to apply matte effect, such as a delustering effect, to the adjustment target area desired by a user on the surface of the ink layer. Further, by adjusting the amount of the plasma energy, it is possible to apply gloss finish with the increased glossiness on the adjustment target area desired by a user on the surface of the ink layer. 
     If the transparent processing object  20  is used and a printed material is applied to an electric sign board irradiated with light from a surface opposite to the surface on which the ink layer is formed, the ink layer on the printed material is viewed through the transparent processing object  20 . Therefore, by adjusting the surface roughness on the surface of the ink layer by adjusting the amount of the plasma energy, it is possible to adjust the transmission amount of light that transmits through the printed material. Consequently, it is possible to realize gradation expression by adjusting the transmission amount of light. Specifically, by causing the transmission light of a back light to be diffusely reflected, the transmission amount of light is adjusted and thus gradation can be adjusted. In particular, by adjusting the surface roughness on the surface of a white ink layer, gradation can be applied easily. 
     Further, density unevenness, which is viewed when ink ejection unevenness (in particular, white ink) is intensified by light and which is disadvantageous for application to an electric sign board, can be reduced by the effect of light scattering by intensifying (increasing) the surface roughness of a white ink layer. 
     Further, the printing apparatus  170  of the embodiment adjusts the surface roughness on the surface of the ink layer formed on the processing object  20  by performing the plasma processing on the processing object  20 , rather than by adjusting the surface roughness of the processing object  20  through the plasma processing. Therefore, even if the smoothness of the surface of the processing object  20  is not changed by the plasma processing, it is possible to easily adjust the surface roughness of the ink layer by improving the aggregation of ink by the plasma processing. 
     Incidentally, the plasma processing unit  101  may detect the processing area subjected to the plasma processing by the plasma processing unit  101  during scanning by the head unit  173 , and output a detection result to the control unit  160 . The control unit  160  may correct the amount of the plasma energy of the plasma processing unit  101  so that a desired plasma processing result can be obtained. 
     In the embodiment, a case has been described in which the amount of the reference energy is registered in the second table (see  FIG. 23 ). However, it may be possible to register conditions to realize plasma processing with the amount of the reference energy, instead of registering the amount of the reference energy. For example, it may be possible to register, in the second table, a value in which a drive frequency of the discharge electrode of the plasma processing unit  101 , a voltage value of the voltage to be applied to a discharge electrode, a voltage application time, the speed of the carriage  172  in the sub-scanning direction, and a feed timing of the processing object  20  in the main-scanning direction are combined, instead of the amount of the reference energy. 
     Second Embodiment 
     In the above described embodiment, a case has been described in which the calculating unit  160 C calculates the amount of plasma energy of plasma applied to the surface of the processing object  20 . In the above described embodiment, a case has been described in which the plasma control unit  160 D performs plasma processing on a processing area on the surface of the processing object  20 . 
     However, it is sufficient that the plasma control unit  160 D performs plasma processing on the processing target surface side of the processing object  20 , and a layer to be subjected to the plasma processing is not limited to the surface of the processing object  20 . 
     Specifically, it is sufficient that the plasma control unit  160 D performs the plasma processing on a surface of a layer located closer to the processing object  20  than an ink layer that is a target of surface roughness adjustment. 
     As described in the above embodiment, the inventors have found that, by performing the plasma processing on the surface of the processing object  20 , the speed of aggregation of the pigment contained in the ink ejected on the processing area subjected to the plasma processing on the processing object  20  is increased. Further, the inventors have found that, by performing the plasma processing on the ink layer formed on the surface of the processing object  20 , resin (for example, siloxane or polyether) contained in the ink reacts, and the speed of aggregation of the pigment contained in the ink ejected on the ink layer is also increased. 
     Therefore, when the recording unit  171  laminates a plurality of ink layers on the processing target surface side of the processing object  20 , the plasma control unit  160 D may control the plasma processing unit  101  to perform plasma processing on a processing area corresponding to an adjustment target area on at least one of the surface of the processing object  20  and one or more layers located closer to the processing object  20  than an ink layer that is a target of surface roughness adjustment (hereinafter, this ink layer is referred to as an adjustment target layer) among the ink layers, by using a certain amount of plasma energy for obtaining the set surface roughness. 
     In this case, the print data may be configured to include print condition information indicating the number of ink layers to be formed and an ink layer to be adjusted. 
     For example, the control unit  32  of the image processing apparatus  30  (see  FIG. 19 ) displays an input screen of a printing method and an adjustment target layer that is an ink layer to be adjusted on the display unit  36 , and receives input of the printing method from a user. The image processing apparatus  30  stores therein the number of ink layers corresponding to each printing method in advance. 
     For example, the control unit  32  of the image processing apparatus  30  displays, on the display unit  36 , a list of printing methods such as normal printing, underlay printing, overlay printing, three layer printing, and white ink printing as described in the first embodiment, and displays, on the display unit  36 , character information to request input of an adjustment target layer. A user selects a printing method and an adjustment target layer by operating the input unit  34 . Further, similarly to the first embodiment, the user inputs an adjustment target area for adjusting surface roughness of the adjustment target layer by operating the input unit  34 . 
     The receiving unit  32 B of the image processing apparatus  30  receives, from the input unit  34 , setting information containing the printing method, the adjustment target layer, the adjustment target area on the adjustment target layer, and surface roughness of the adjustment target area. 
     The generating unit  32 C of the control unit  32  generates print data containing image data in the raster format and setting information in the raster format, which are generated in the same manner as in the first embodiment. 
     When the printing method contained in the setting information on the print data indicates a printing method for forming a plurality of ink layers (underlay printing, overlay printing, or three layer printing (see  FIG. 18 )), the plasma control unit  160 D of the printing apparatus  170  (see  FIG. 21 ) determines that an image with a plurality of laminated ink layers is to be recorded. 
     When determining that the image with a plurality of laminated ink layers is to be recorded, the plasma control unit  160 D controls the plasma processing unit  101  to perform plasma processing on a processing area corresponding to an adjustment target area on at least one of the surface of the processing object  20  and one or more layers located closer to the processing object  20  than an ink layer that is a target of surface roughness adjustment among the ink layers, by using a certain amount of plasma energy for obtaining the set surface roughness on the surface of the adjustment target layer formed on the processing area. 
     In this case, the storage unit  162  stores therein, in advance, a corresponding second table (a table in which the amount of the reference energy corresponding to the amount of ink and a paper type is registered) for each combination of a printing method, a layer as an adjustment target layer to be subjected to plasma processing (including the surface of the processing object  20 ), and a type of ink, instead of the second table corresponding to the type of ink as illustrated in  FIG. 23  (a table in which the amount of reference energy corresponding to the amount of ink and a paper type is registered). The layer to be subjected to the plasma processing (hereinafter, referred to as a plasma processing target layer) may be the surface of the processing object  20  or the surface of an ink layer located closer to the processing object  20  than the adjustment target layer. 
     The amount of the reference energy that meets the above described conditions is measured and registered in a corresponding second table in advance. 
     The calculating unit  160 C calculates, for each plasma processing target layer, the amount of ink applied to the processing area corresponding to the adjustment target area in the plasma processing target layer, from the resolution, the image data, and the first table (see  FIG. 22 ). The amount of ink is calculated in the same manner as in the first embodiment. 
     Further, the calculating unit  160 C reads the printing method, the plasma processing target layer corresponding to the adjustment target layer, the type of ink, and a corresponding second table, and reads the amount of the reference energy corresponding to the amount of ink and the paper type in the second table. Through this process, the calculating unit  160 C calculates the amount of the reference energy of plasma to be applied to the processing area corresponding to the adjustment target area in the plasma processing target layer. 
     The calculating unit  160 C calculates the amount of the plasma energy to be applied to the processing area in the plasma processing target layer by using the calculated reference energy and the intensity of the surface roughness corresponding to the adjustment target area indicated in the setting information, in the same manner as in the first embodiment. 
     The plasma control unit  160 D controls the plasma processing unit  101  to perform plasma processing on a processing area corresponding to an adjustment target area on the plasma processing target layer from among the surface of the processing object  20  and at least one of layers located closer to the processing object  20  than the ink layer as a target of surface roughness adjustment among the ink layers, with the amount of the plasma energy corresponding to each plasma processing target layer and each processing area calculated by the calculating unit  160 C. 
     In this case, the plasma control unit  160 D controls a timing such that the plasma processing is performed on the surface of the set plasma processing target layer on any of the surface of the processing object  20  and one or more ink layers formed on the processing object  20 , with the amount of the plasma energy calculated by the calculating unit  160 C, in accordance with a timing at which the recording unit  171  ejects ink droplets to form the ink layers. 
     As described above, when a plurality of ink layers are laminated, the plasma control unit  160 D may control the plasma processing unit  101  to perform plasma processing on a processing area corresponding to an adjustment target area on at least one of the surface of the processing object  20  and one or more ink layers located closer to the processing object  20  than a layer that is a target of surface roughness adjustment among the ink layers, with the amount of the plasma energy for obtaining the set surface roughness on the surface of the adjustment target layer formed on the processing area. 
     Third Embodiment 
     Hardware configurations of the image processing apparatus  30  and the printing apparatus  170  will be described below. 
       FIG. 25  is a hardware configuration diagram of the image processing apparatus  30  and the printing apparatus  170 . The image processing apparatus  30  and the printing apparatus  170  mainly includes, as a hardware configuration, a CPU  2901  that controls the entire apparatus, a ROM  2902  that stores therein various kinds of data and various programs, a RAM  2903  that stores therein various kinds of data and various programs, an input device  2905  such as a keyboard or a mouse, a display device  2904  such as a display, and a communication device  2906 , and has a hardware configuration using a normal computer. 
     A program executed by the image processing apparatus  30  and the printing apparatus  170  of the above described embodiments is provided as a computer program product by being recorded in a computer-readable recording medium, such as a compact disc (CD)-ROM, a flexible disk (FD), a compact disc-recordable (CD-R), or a digital versatile disk (DVD), in a computer-installable or a computer-executable file. 
     Further, the program executed by the image processing apparatus  30  and the printing apparatus  170  of the above described embodiments may be stored in a computer connected to a network, such as the Internet, and provided by being downloaded via the network. Furthermore, the program executed by the image processing apparatus  30  and the printing apparatus  170  of the above described embodiments may be provided or distributed via a network, such as the Internet. 
     Moreover, the program executed by the image processing apparatus  30  and the printing apparatus  170  of the above described embodiments may be provided by being incorporated in a ROM or the like in advance. 
     The program executed by the image processing apparatus  30  and the printing apparatus  170  of the above described embodiments has a module structure including the above described units. As actual hardware, a CPU (processor) reads the program from the above described storage medium and executes the program, so that the units are loaded on a main storage device and generated on the main storage device. 
     According to an embodiment, it is possible to easily adjust surface roughness on the surface of an ink layer formed on a processing object to desired surface roughness. 
     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.