Patent Publication Number: US-10308058-B2

Title: Printing system, printing apparatus, and printed-matter production method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims the benefit of priority from U.S. Ser. No. 15/008,727, filed Jan. 28, 2016, which is a continuation of U.S. Ser. No. 14/332,777 (now U.S. Pat. No. 9,278,548), filed Jul. 16, 2014, which claims the benefit of priority from Japanese Patent Application No. 2013-159979, filed Jul. 31, 2013 and Japanese Patent Application No. 2014-117324, filed Jun. 6, 2014, the entire contents of each of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to printing systems, printing apparatuses, and printed-matter production methods. 
     2. Description of the Related Art 
     “Shuttle-head” design is presently in mainstream of inkjet recording. However, because shuttle-head printing poses difficulty in increasing printing speed, “single-pass” design using a full-page-width line head is proposed for high-speed printing. Although the single-pass design advantageously increases the printing speed, a printer of the single-pass design, ejects adjacent dots with a short interval of time. Accordingly, a second one of adjacent dots is ejected before ink of a first one, which is ejected earlier, of the adjacent dots penetrates into a print medium. Consequently, coalescence of the adjacent dots (hereinafter, sometimes referred to as “droplet interference”) can occur, which may result in degradation in image quality due to occurrence of beading or bleed. 
     Furthermore, in a situation where an inkjet printing apparatus prints an image on an impermeable medium or a low-permeable medium such as a film or coated paper, another problem can occur. That is, migration and coalescence of adjacent dots may cause an image defect such as beading or bleed. 
     Conventionally, to avoid such a problem which can occur in printing on a film or coated paper, reducing printing speed, adding a drier unit, or a like strategy is adopted. Meanwhile, existing methods for improving fixation of water-based ink onto a print medium Include a method of applying primer to the print medium in advance. 
     As another method for improving fixation of water-based ink on to a print medium, a method of applying plasma treatment onto a surface of the print medium is proposed, for example, in Japanese Laid-open Patent Publication No. 2010-058404. It is known that applying plasma treatment onto a surface of a print medium increases hydrophilicity of the surface. Accordingly, plasma treatment application can improve hydrophilicity and wettability of coated paper which is generally poor in wettability. Plasma treatment provides another advantage that, because of being a dry process, plasma treatment does not require a drying step. 
     However, the method of applying a primer can disadvantageously increase printing cost with some types of print media. The reasons therefor are the following: the primer applied as pretreatment liquid is a consumable; a device and a step for drying the primer are required. The method of applying plasma treatment can be disadvantageous in terms of safety, size of printing apparatus, and cost. This is because application of plasma treatment to some types of print media requires high-voltage plasma. 
     Accordingly, there is a need for systems, apparatuses, and printed-matter production methods configured to be capable of optimizing pretreatment according to a type of a print medium. 
     It is an object of the present invention to at least partially solve the problem in the conventional technology. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to at least partially solve the problems in the conventional technology. 
     According to the present invention, there is provided a printing apparatus comprising: a plasma treatment unit configured to acidify at least a surface of a print medium by applying plasma treatment to the surface of the print medium; a first primer applying unit configured to apply primer treatment by applying treatment liquid to the surface of the print medium having undergone the plasma treatment; and a first recording unit configured to perform recording by inkjet recording on the print medium having undergone the primer treatment. 
     The present invention also provides a printing system comprising: a plasma treatment device configured to acidify at least a surface of a print medium by applying plasma treatment to the surface of the print medium; a primer applying device configured to apply primer treatment by applying treatment liquid to the surface of the print medium having undergone the plasma treatment; and a recording device configured to perform recording by inkjet recording on the print medium having undergone the primer treatment. 
     The present invention also provides a method for producing a printed matter, the printed matter being a print medium on which an image is formed by inkjet recording, the method comprising: applying plasma treatment to a surface of the print medium to thereby acidify at least the surface of the print medium; applying primer treatment by applying treatment liquid to the surface of the print medium having undergone the plasma treatment; and performing recording by inkjet recording on the print medium having undergone the primer treatment. 
     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 graph illustrating relationship between viscosity and pH value of inks according to an embodiment of the present invention; 
         FIG. 2  is a diagram illustrating a schematic configuration of an inkjet recording apparatus according to the embodiment; 
         FIG. 3  is a diagram illustrating a schematic configuration of an acidification unit illustrated in  FIG. 2 ; 
         FIG. 4  is a schematic diagram illustrating an example of an atmospheric-pressure non-equilibrium plasma treatment unit applicable to the acidification unit illustrated in  FIG. 2 ; 
         FIG. 5  is a diagram illustrating a schematic configuration of a primer applying unit illustrated in  FIG. 2 : 
         FIG. 6  is a perspective view illustrating a pressurizing mechanism illustrated in  FIG. 5 ; 
         FIG. 7  is a schematic diagram illustrating the inkjet recording apparatus illustrated in  FIG. 2  in a more simplified manner; 
         FIG. 8  is a flowchart illustrating a procedure of inkjet recording according to the embodiment; 
         FIGS. 9( a ), 9( b ), and 9( c )  are schematic diagrams illustrating an example of a wettability detection method performed by a wettability detecting unit illustrated in  FIG. 7 ; 
         FIG. 10  is a diagram for describing a contact-angle calculation method involved in the wettability detection method illustrated in  FIGS. 9( a ) to 9( c ) ; 
         FIG. 11  is a diagram illustrating an example of an image obtained by imaging a print medium, which is poor in wettability and to which wettability test liquid is applied; 
         FIG. 12  is a diagram illustrating an example of an image obtained by imaging a print medium, which is favorable in wettability and to which the wettability test liquid is applied; 
         FIG. 13  is a graph illustrating relationship between print density (single color) and amount of ink deposited on print media to which different pretreatments are applied; 
         FIG. 14  is a diagram illustrating a schematic configuration of die overall inkjet recording apparatus according to the embodiment; 
         FIG. 15  is an enlarged view of an image obtained by imaging an image-formed surface of a printed matter obtained by performing inkjet recording on a print medium to which plasma treatment according to the embodiment is not applied; 
         FIG. 16  is a schematic diagram illustrating an example of dots formed on the image-formed surface of the printed matter illustrated in  FIG. 15 ; 
         FIG. 17  is an enlarged view of an image obtained by imaging an image-formed surface of a printed matter obtained by performing inkjet recording on a print medium to which the plasma treatment according to the embodiment is applied; 
         FIG. 18  is a schematic diagram illustrating an example of dots formed on the image-formed surface of the printed matter illustrated in  FIG. 17 : 
         FIG. 19  is a graph illustrating relationships between plasma energy density and each of wettability, beading, pH value, and permeability of a surface of a print medium according to the embodiment; 
         FIG. 20  is a graph illustrating relationship between plasma energy density and pH value according to the embodiment; 
         FIG. 21  is a graph illustrating relationship between image density and amount of ink deposited on ordinary paper, which is used as a print medium and to which combination of plasma treatment and primer treatment is applied; and 
         FIG. 22  is a graph illustrating granularity of a low-permeable print medium to which the combination of the plasma treatment and the primer treatment is applied. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings. Although the presently preferred embodiments of the present invention are described below with various technically preferred limitations, the scope of the invention should not be construed as limited by the embodiments discussed below. It should not be construed that all of elements of the embodiments discussed below are essential to the invention. 
     In an embodiment herein, appropriate one of acidification treatment, primer treatment, and combination thereof is applied to a print medium as pretreatment. Meanwhile, “acidification” in the following description denotes lowering a pH value of a surface of a print medium to a pH value at which pigments contained in ink coagulate.  FIG. 1  illustrates an example of relationship between viscosity and pH value of inks. As illustrated in  FIG. 1 , the lower the pH value of ink, the higher the viscosity of the ink. This is because the higher the acidity of the ink, the more pigments, which are negatively charged in ink vehicle, in the ink are neutralized; as a result, the pigments gradually coagulate. Accordingly, the viscosity of the ink can be increased by, for example, lowering the pH value of the surface of the print medium so that the pH value of the ink reaches a value corresponding to a desired viscosity in the graph illustrated in  FIG. 1 . This is because, when ink is deposited on an acid surface of a print medium, pigments in ink are neutralized by hydrogen ions (H+) on the surface of the print medium; as a result, the pigments coagulate. This coagulation allows preventing color mixing of adjacent dots and, simultaneously, preventing the pigments from penetrating deep to the interior (or even to the backside) of the print medium. Note that to lower pH value of the ink to a pH value corresponding to a desired viscosity, it is necessary to lower pH value of the surface of the print medium to a value lower than the pH value of the ink corresponding the desired viscosity. 
     Meanwhile, pH value at which ink has the desired viscosity depends on property of the ink. More specifically, as in the case of ink A illustrated in  FIG. 1 , pigments of ink of some types coagulate and increase viscosity of the ink at a pH value relatively close to a neutral value. However, as in the case of ink  8  which differs from the ink A in property, ink of some other types requires a pH value lower than the pH value of the ink A to cause pigments in the ink to coagulate. In an embodiment herein, appropriate one of acidification treatment, primer treatment, and combination thereof is applied according to type of a print medium with consideration given to property (e.g., type) of ink. 
     Examples of acidification treatment according to the embodiment include plasma treatment which is performed by exposing a subject to plasma in the air atmosphere. The plasma treatment as the acidification treatment is applied by exposing a subject (for example, a print medium) to plasma in the air atmosphere to cause polymers on a surface of the print medium to react, thereby forming hydrophilic functional groups. More specifically, electrons (e) emitted from discharge electrodes are accelerated in an electric field to excite and ionize atoms and molecules in She atmospheric gas. The ionized atoms and molecules also emit electrons, whereby the number of high-energy electrons is increased, and streamer discharge (plasma) is formed. The high-energy electrons produced by the streamer discharge break bonding of the polymers on the surface of the print medium (e.g., coated paper) (coating layer of the coated paper is bound with calcium carbonate and starch; the starch serving as a binder has a polymer structure) and recombine with oxygen radicals (O*), hydroxyl radicals (*OH), and ozone O 3  in the gas. This series of processing is referred to as “plasma treatment”. The plasma treatment forms polar functional groups, such as hydroxys groups and carboxyl groups, on the surface of the print medium. As a result, hydrophilicity and acidity are imparted to the surface of the print medium. Meanwhile, the surface of the print medium is acidified (i.e., the pH value of the surface is lowered) by the increase in the carboxyl groups. 
     The increased hydrophilicity makes adjacent dots on the surface of the print medium wet and spread, causing the dots to coalesce together. To prevent color mixing between dots, which can be caused by such coalescence, it is desired to coagulate, colorants (e.g., pigments or dyes) in each dot immediately, or to dry ink vehicle or cause the ink vehicle to penetrate into the print medium before the vehicle becomes wet and spread. The plasma treatment described above can accelerate coagulation of colorants in each dot; this is because the plasma treatment also acts as the acidification procedure (process) which acidifies the surface of the print medium. Also in this respect, it will be advantageous to apply the plasma treatment as pretreatment of inkjet recording. 
     Meanwhile, it is possible to apply an acidic treatment liquid referred to as a primer to a surface of a print medium, thereby imparting a greater affinity for alkaline ink. The reason for this is presumably that polymeric material contained in the treatment liquid is trapped in pore structure of the print medium and prevents excessive penetration of the ink into the print medium. Accordingly, the primer treatment is particularly effective for highly-permeable print media, examples of which include ordinary paper, coarse paper, and thin paper. However, because a certain application amount (coating thickness) of the treatment liquid is required to apply the treatment liquid uniformly, the primer treatment can lead to an increase in cost. 
     In an embodiment herein, an inkjet recording apparatus, which is employed as an example of a printing apparatus, is configured to use combination of exposing a print medium to plasma in the atmosphere and primer treatment according to type of the print medium in pretreatment. Using the combination allows reducing energy necessary for the plasma exposure and reducing an application amount of primer while maintaining quality of the print image. The printing apparatus according to the embodiment is not limited to an inkjet recording apparatus, and can be a printing apparatus, an image forming apparatus, or the like which uses ink in other fashion. 
     Meanwhile, behavior of ink in inkjet recording varies with droplet volume (small droplet, medium droplet, or large droplet) and type of a print medium. In an embodiment herein, plasma energy density for plasma exposure is adjusted to an appropriate value according to type of a print medium and a print mode (droplet volume). More specifically, wettability of the print medium and a pH of the surface of the print medium are measured, and the plasma energy density is optimized according to the measured values. Furthermore, the pretreatment is controlled differently depending on the print medium to which the pretreatment is to be applied. This configuration allows applying pretreatment optimized according to the print medium. 
     In an embodiment hereinafter, an inkjet-recording image forming apparatus is configured to switch a conveyance route of a print medium so that the print medium undergoes effective one or both of atmospheric plasma treatment and primer treatment, which applies treatment liquid to the surface of the print medium, before an image is recorded on the print medium. This configuration allows reducing load imposed on units for the respective pretreatments, thereby achieving energy saving and increasing usable lives. Aft embodiment may be configured to detect at least one of wettability and a pH of the surface of the print medium and optimize outputs of the respective treatment units based on a detected value(s). 
     An embodiment of the present invention is described in detail below with reference to the accompanying drawings. In the embodiment, as pretreatment to be applied by an inkjet recording apparatus to a print medium, combination of exposing the print medium to plasma in the atmosphere and applying a primer to the print medium is employed. The inkjet recording apparatus can reduce an amount (hereinafter, “application amount”) of the primer to be applied while reducing energy necessary for the plasma treatment regardless of whether the print medium has low permeability or high permeability by employing the combination of the plasma treatment and the primer treatment. As a result, because ink consumption can be reduced while simultaneously reducing time and energy necessary for drying the treatment liquid (the primer), tire inkjet recording apparatus is capable of producing a printed matter of high quality while achieving energy saving and low CPP (cost reduction). 
       FIG. 2  is a diagram illustrating a schematic configuration of an inkjet recording apparatus according to the embodiment. Referring to  FIG. 2 , an inkjet recording apparatus  1  includes an acidification unit  10 , a control unit  15 , a first primer applying unit  30 A, a second primer applying unit  30 B, and an inkjet recording unit  40 . 
     The inkjet recording apparatus  1  further includes, as conveyance routes of a print medium M 1 , a first route, a second route, and a third route. The first route includes conveyance paths R 1 , R 2 , and R 32 . The second route includes the conveyance path R 1 , conveyance paths R 11 , R 12 , and R 31 , and the conveyance path R 32 . The third route includes the conveyance paths R 1  and R 11 , conveyance paths R 21 , R 22 , and R 31 , and the conveyance path R 32 . The acidification unit  10  is arranged on the conveyance path R 1  included in the first to third routes. The first primer applying unit  30 A is arranged, for example, on the conveyance path R 11  included in the second and third routes. The second primer applying unit  30 B is arranged, for example, on the conveyance path R 21  included in the third route. The ink jet recording unit  40  is arranged on the conveyance path R 32  included in the first to third routes. 
     The inkjet recording apparatus  1  further includes conveyance switch units  21  and  22  for switching between routes along which the print medium M 1  is to be conveyed. The conveyance switch unit  21  switches the conveyance route of the print medium M 1  between the first route and the second route, for example. The conveyance switch unit  22  switches the conveyance route of the print medium M 1  between the second route and the third route, for example. The conveyance switch units  21  and  22  may be controlled by, for example, a control unit (not shown). More specifically, the embodiment allows selecting which one of only the plasma treatment, the plasma treatment and a single cycle of the primer treatment, and the plasma treatment and two cycles of the primer treatment, is to be applied to the print medium M 1  by switching to any one of the first to third routes according to the type of the print medium M 1 . The inkjet recording apparatus  1  may be configured to apply a single cycle or multiple cycles of the primer treatment without applying the plasma treatment. The inkjet recording apparatus  1  may be configured to, when operating as such, cut off power supply to the acidification unit  10  or cut off power supply to discharge electrodes of the acidification unit  10 . 
     The inkjet recording apparatus may be configured as follows. In a situation where the print medium M 1  is an impermeable medium, for example, the plasma treatment is applied to the print medium M 1  first. If the surface of the print medium M 1  has Been modified by the plasma treatment sufficiently, the print medium M 1  is conveyed to the inkjet recording unit  40  without passing through the primer applying units  30 A and  30 B. In a case where the plasma treatment and a single cycle of the primer treatment are insufficient to modify the surface of the print medium M 1 , the conveyance switch units  21  and  22  are controlled so as to convey the print medium M 1  to the third route along which two cycles of the primer treatment are applied by the primer applying units  30 A and  30 B. In a case where it is unnecessary to apply the plasma treatment, the print medium M 1  is conveyed along the conveyance path R 1  without receiving the plasma treatment from the acidification unit  10 . 
     The embodiment is thus configured so as to apply pretreatment differently as to whether or not to apply the plasma treatment and in the amount of the treatment liquid to be applied to the print medium M 1 . Driers (not shown) for drying the treatment liquid before printing is performed by the inkjet recording unit  40  are arranged on the corresponding conveyance paths at positions immediately downstream of the primer applying units  30 A and  30 B, respectively. 
     By switching the conveyance route of the prim medium M 1  in this way, unnecessary driving of one or more of the primer applying units can be obviated. Accordingly, load required of a system including the inkjet recording apparatus  1  to drive the primer applying units  30 A and  30 B can be reduced. As a result, energy saving and increasing usable lives of components can be achieved. Furthermore, whether or not to drive the acidification unit  10  is also selectable as necessary. Accordingly, load required of the system to drive the acidification unit  10  can be reduced, and energy saving and increasing usable lives of components can be achieved similarly. 
       FIG. 3  is a diagram illustrating a schematic configuration of the acidification unit  10  illustrated in  FIG. 2 . The acidification unit  10  according to the embodiment may be, for example, an atmospheric-pressure non-equilibrium plasma treatment device which utilizes dielectric barrier discharge. Referring to  FIG. 3 , the acidification unit  10  includes multiple discharge electrodes, denoted by  11   a  to  11   f , arranged along the conveyance path R 1 ; high-voltage high-frequency power supplies  12   a  to  12   f  configured to apply discharge voltages to the discharge electrodes  11   a  to  11   f ; a ground electrode  13 ; a dielectric  14 , which is an endless belt, interposed between the discharge electrodes  11   a  to  11   f  and the ground electrode  13 ; and rollers  17  configured to cause the dielectric  14  to revolve along the conveyance path R 1 . The print medium M 1  is plasma-heated on the way of being conveyed along a conveyance path R 1 . The discharge voltages respectively applied by the high-voltage high-frequency power supplies  12   a  to  12   f  to the corresponding discharge electrodes  11   a  to  11   f  may be controlled by, for example, the control unit  15 . 
     The control unit  15  may cause the dielectric  14  to revolve by driving the rollers  17  under control of a host device (not shown) (which can be a control unit  100  illustrated in  FIG. 7 , for example). The print medium M 1  delivered by a feeding unit IN (see  FIG. 14 ) onto the dielectric  14  is conveyed along the conveyance path R 1  by the revolving motion of the dielectric  14 . 
     The high-voltage high-frequency power supplies  12   a  to  12   f  apply high-voltage high-frequency pulse voltages respectively to the discharge electrodes  11   a  to  11   f . The pulse voltages may be applied to all of the discharge electrodes  11   a  to  11   f . Alternatively, the pulse voltage(s) may be applied to one or more of the discharge electrodes  11   a  to  11   f , the number of which depends on predetermined plasma treatment (for example, plasma treatment for lowering the pH value to a predetermined value or lower) to be applied to the surface of the print medium M 1 . The control unit  15  may control frequency and voltage values (plasma energy density) of the pulse voltages to be respectively supplied from the high-voltage high-frequency power supplies  12   a  to  12   f  to a plasma energy density necessary to apply the predetermined plasma treatment to the surface of the print medium M 1 . 
     The control unit  15  is capable of individually switching on and off the high-voltage high-frequency power supplies  12   a  to  12   f . For example, the control unit  15  may select the number of the high-voltage high-frequency power supplies  12   a  to  12   f  to be driven or adjust the intensity of plasma energy of the pulse voltages to be applied to the discharge electrodes  11   a  to  11   f  in proportion to information about a printing speed. Alternatively, the control unit  15  may adjust the number of the high-voltage high-frequency power supplies  12   a  to  12   f  to be driven and/or the plasma energy density of the pulse voltages to be applied to the discharge electrodes  11   a  to  11   f  according to type (e.g., “coated paper” or “polyethylene terephthalate (PET) film”) of the print medium M 1 . 
     Providing the multiple discharge electrodes  11   a  to  11   f  in this manner is also advantageous in uniformly acidifying the surface of the print medium M 1 . More specifically, under the same condition of conveying speed (or printing speed) of the print medium M 1 , acidification treatment using multiple discharge electrodes allows increasing duration, over which the print medium M 1  passes through plasma space, to be longer than that of acidification treatment using a single discharge electrode. Consequently, the surface of the print medium M 1  can be acidified more uniformly. 
     Meanwhile, the plasma treatment using atmospheric-pressure non-equilibrium plasma is preferable as a method for acidifying the print medium M 1 . This is because electron temperature of the atmospheric-pressure non-equilibrium plasma is extremely high, whereas gas temperature is close to room temperature. To generate atmospheric-pressure non-equilibrium plasma stably over a wide range, it will be most preferable to use dielectric barrier discharge based on streamer breakdown obtained by applying alternating high voltages across electrodes coated with a dielectric. The method for generating the atmospheric-pressure non-equilibrium plasma is not limited to the dielectric barrier discharge based on streamer breakdown, and various other methods are usable. Examples of the usable method include a method of producing dielectric barrier discharge by inserting an insulator such as a dielectric between electrodes, a method of producing corona discharge by forming a highly-non-uniform electric field around, a thin metal, wire or the like, and a method of producing pulse discharge by applying a short pulse voltage. A combination of two or more of these methods is also usable. 
       FIG. 4  is a schematic illustrating an example of an atmospheric-pressure non-equilibrium plasma treatment unit  10   a  applicable to the acidification unit  10  illustrated in  FIG. 2 . Referring to  FIG. 4 , a plasma treatment unit  10   a  includes the discharge electrodes  11 , the ground electrode  13 , the dielectric  14 , and the high-voltage high-frequency power supplies  12 . The dielectric  14  is interposed between the discharge electrodes  11  and the ground electrode  13 . Each of the discharge electrodes  11  and the ground electrode  13  may be an electrode including a bare metal portion, or may be an electrode covered with a dielectric or an electrical insulator such as electrical-insulation rubber or a ceramic. The dielectric  14  interposed between the discharge electrodes  11  and the ground electrode  13  may be an insulator such as a polyimide, silicone, or a ceramic. If corona discharge is employed as the plasma treatment, the dielectric  14  may be omitted. However, even when corona discharge is employed, it will be preferable to include (not to omit) the dielectric  14  in some configurations including a configuration which employs dielectric barrier discharge, for example. In that case where the dielectric  14  is included, the dielectric  14  is preferably located at a position near or in contact with the ground electrode  13  rather than at a position where the dielectric  14  is near or in contact with the discharge electrodes  11  so that a surface discharge area is widened and effect of the plasma treatment can be enhanced. The discharge electrodes  11  and the ground electrode  13  (or, in a configuration where the dielectric  14  is included, the dielectric  14 ) (hereinafter, sometimes referred to as the “electrode pair”) may be arranged at positions where the electrode pair is brought into contact with the print medium M 1  passing through between the electrode pair or at positions where the electrode pair is not brought into contact with the same. 
     The high-voltage high-frequency power supplies  12  apply high-voltage high-frequency pulse voltages across the discharge electrodes  11  and the ground electrode  13 . The voltage value of the pulse voltage may be approximately 10 kilovolts (kV) (peak-to-peak voltage), for example. The frequency of the pulse voltage may be approximately 20 kilohertz (kHz), for example. Applying such high-voltage high-frequency pulse voltages across the electrode pair generates atmospheric-pressure non-equilibrium plasma  16  between the discharge electrodes  11  and the dielectric  14 . The print medium M 1  passes through between the discharge electrodes  11  and the dielectric  14  during when the atmospheric-pressure non-equilibrium plasma  16  is generated. As a result, the surface of the print medium M 1  on the side of the discharge electrodes  11  side undergoes the plasma treatment. 
     The plasma treatment unit  10   a  illustrated in  FIG. 4  employs the rotary discharge electrodes  11  and the belt-conveyor type dielectric  14 . The print medium M 1  is nipped and conveyed by the rotating discharge electrodes  11  and the dielectric  14  so as to pass through the atmospheric-pressure non-equilibrium plasma  16 . The surface of the print medium M 1  is brought into contact with the atmospheric-pressure non-equilibrium plasma  16  in this way. Consequently, the surface is plasma-treated uniformly. However, the configuration of the plasma treatment device which can be employed in the embodiment is not limited to that illustrated in  FIG. 4 . The plasma treatment device may be modified in various manners. Example modifications include a configuration in which the discharge electrodes  11  are brought to vicinity of the print medium M 1  rather than into contact therewith and a configuration in which the discharge electrodes  11  are mounted on the same carriage as an inkjet head. The dielectric  14  is not limited to the belt-conveyor type; a flat-plate dielectric can be employed as the dielectric  14 . 
     The energy (hereinafter, sometimes referred to as “plasma energy density”) to be applied by the acidification unit  10  (see  FIG. 4 ) in the plasma treatment can be calculated from an electric current passing from the discharge electrodes  11  to the ground electrode  13  with the print medium M 1  serving as a resistor placed therebetween, an applied voltage, and pulse duration, for example. The acidification unit  10  illustrated in  FIG. 4  includes the six discharge electrodes denoted by  11   a  to  11   f . With this configuration, energy to be consumed by tire six discharge electrodes  11   a  to  11   f  in its entirety is controlled for each cycle of the plasma treatment. The control unit  15  is capable of individually switching, or and off the high-voltage high-frequency power supplies  12   a  to  12   f . The control unit  15  selects the number of the high-voltage high-frequency power supplies  12   a  to  12   f  to be driven in proportion to information about a printing speed. Necessary plasma energy density may vary with the type of the print medium M 1 . Also in such a case, the control unit  15  cause one or more of the discharge electrodes  11 , the number of which depends on the type of the print medium M 1 , to generate plasma. The print medium M 1  is caused to pass through between the discharge electrodes  11  and the dielectric  14  during when the atmospheric-pressure non-equilibrium plasma  16  is generated, to thus be plasma-treated. The plasma treatment breaks chains holding polymers in a binder resin on the surface of the print medium M 1 . The polymers recombine with oxygen radicals and ozone in the gas to form polar functional groups, whereby hydrophilicity and acidity are imparted to the surface of the print medium M 1 . Although the plasma treatment is applied in the air atmosphere, alternatively, the plasma treatment may be applied in a nitrogen gas atmosphere or the like. 
       FIG. 5  is a diagram illustrating a schematic configuration of the primer applying unit ( 30 A,  30 B) illustrated in  FIG. 2 .  FIG. 5  is a cross-sectional side view of the primer applying unit  30 A or  30 B (hereinafter, the “primer applying unit  30 ”).  FIG. 6  is a perspective view illustrating a pressurizing mechanism  31  of the primer applying unit  30 . 
     Referring to  FIG. 5 , the primer applying unit  30  includes two rollers, denoted by  35  aid  36 , configured to pinch and convey the print medium M 1  therebetween, a lift, roller  34  configured to transfer treatment liquid PL to the roller  35  so that the treatment liquid PL is applied onto the print medium M 1 , a tank  33  configured to store the treatment liquid PL in such a manner that the lift roller  34  is partially immersed m the treatment liquid PL, and the pressurizing mechanism  31  configured to control the amount of the treatment liquid PL to be transferred to the roller  35 . 
     Fine grooves are cut in the surface of the lift roller  34 . The treatment liquid carried up by the lift roller  34  is transferred onto the roller  35 . The primer treatment liquid LP contains a solvent, which is water-based and has an acidic pH, and polymer materials generally referred to as cationic polymers. The cationic polymers include amines and hydrin-based polymers (epichlorohydrin polymers). 
     In the embodiment, the plasma treatment is applied prior to the primer treatment. The reason therefor is as follows. In a case where the print medium M 1  is a low-permeable medium, the plasma treatment applied earlier increases the hydrophilicity of the surface of the medium M 1 , thereby allowing light and uniform application of the treatment liquid in the primer treatment. 
       FIG. 6  is a perspective view illustrating the pressurizing mechanism  31  illustrated in  FIG. 5 . Referring to  FIG. 6 , the pressurizing mechanism  31  includes a stepper motor  310  controlled by a control unit (not shown). A driving force of the stepper motor  310  rotating forward (direction A indicated by the double-headed arc-like arrow in  FIG. 6 ) is transmitted to a gear  313  via a gear  311 , which is arranged on a drive shaft of the stepper motor  310 , and an idler gear  312 . 
     A shaft  314 , the leading end of which is formed as a feed screw, is coupled to the gear  313 . Accordingly, the shaft  314  can pull an anchor  315  in a horizontal direction (direction C indicated the double-headed arrow in  FIG. 6 ). One end of a spring  316  is attached to the anchor  315 . The other end of the spring  316  is attached to a bracket  317  supporting a metering blade  32 . Accordingly, a pressing force exerted by the metering blade  32  varies with horizontal movement of the anchor  315 . 
     On the other hand, when the stepper motor  310  rotates backward (direction B indicated by the double-headed arc-like arrow in  FIG. 6 ), the anchor  315  is pushed back in a horizontal direction (direction D indicated by the double-headed arrow in  FIG. 6 ). As a result, the bracket  317  is pivoted in a pressure-decreasing direction, and the pressing force exerted by the metering blade  32  is reduced or eliminated. 
     A sensor for detecting a reference position may be arranged on the anchor  315 . This sensor may be embodied as a switch configured to be switched on/off by a detection piece  318  formed on a bottom portion of the anchor  315 , for example. A necessary pressing force can be applied to the metering blade  32  by adjusting a travel distance of the anchor  315  in accordance with on/off state of the sensor. It is preferable to arrange the pressurizing mechanism  31  illustrated in  FIG. 6  on each of longitudinal opposite sides of the metering blade  32 , at positions on an end of the metering blade  32  on the side opposite from the side where the metering blade  32  is in contact with the lift roller  34 . 
     In the embodiment, the pressing force exerted by the metering blade  32  is adjusted by controlling the pressurizing mechanism  31  configured as described above so that the application amount falls within a range from 0.02 to 0.2 mg/cm 2 , for example. However, the method for adjusting the application amount is not limited thereto. For example, the amount of the treatment liquid to be transferred from the lift roller  34  to the roller  35  can be adjusted by controlling the pressing force exerted from the pressurizing mechanism  31  to the metering blade  32 . In this case, one of the primer applying units  30 A and  30 B, and the conveyance path therefor may be omitted. 
     A combination of the primer applying units  30 A and  30 B which differ from each other in the application amount may be implemented by causing the depth of the fine grooves cut in the lift roller  34  to differ between the primer applying units  30 A and  30 B. In this case, it is preferable that the total application amount by the primer applying units  30 A and  30 B is adjustable within the range from 0.02 to 0.2 mg/cm 2 . 
     The inkjet recording unit  40  illustrated in  FIG. 2  includes the inkjet head to record an image by ejecting ink onto the pre-treated print medium M 1  wider control of a control unit (not shown). The inkjet recording unit  40  may include multiple heads for a same color (in the example illustrated in  FIG. 2 , four heads for each of four colors). This configuration allows increasing speed of inkjet recording. To obtain a high resolution (e.g., 1,200 dots per inch (dpi)) at a high speed, the heads of each color are held in an arrangement where nozzles, from which ink is to be ejected, are in a staggered arrangement so as to reduce gaps between the nozzles. Furthermore, the control unit feeds control signals each indicating a drive frequency corresponding to one of three droplet volumes of ink to be ejected from a nozzle, to the inkjet heads. The droplet volumes may be referred to as a large droplet, a medium droplet, and a small droplet. 
     Operation of inkjet recording, pretreatment for which can be applied by a combination of the plasma treatment and the primer treatment according to the type of a print medium, is described in detail below with reference to  FIGS. 7 and 8 .  FIG. 7  is a schematic diagram illustrating the inkjet recording apparatus  1  illustrated in  FIG. 2  in a more simplified manner.  FIG. 8  is a flowchart illustrating a procedure of inkjet recording according to the embodiment.  FIG. 8  illustrates a sequence executed by the control unit  100  which provides overall control of the inkjet recording apparatus  1 . 
     Referring to  FIG. 7 , the inkjet recording apparatus  1  includes, in addition to the elements illustrated in  FIG. 2 , a control unit  35 A configured to control the first primer applying unit  30 A, a control unit  35 B configured to control the second primer applying unit  30 B, a wettability detecting unit  51  configured to detect wettability of the print medium M 1 , a pH detecting unit  52  configured to detect a pH value of the print medium M 1 , the control unit  100  configured to provide the overall control of the inkjet recording apparatus  1 , and a storage unit  101  configured to store types of the print medium M 1 , pretreatment conditions, detection results, and the like. The wettability detecting unit  51  and the pH detecting unit  52  are arranged downstream from the acidification unit  10  and the first and second primer applying units  30 A and  30 B and upstream from the ink jet recording unit  40  to determine whether or not the plasma treatment and/or the primer treatment is appropriately applied as required. The control unit  100  controls a level the pretreatment to be applied to the print medium M 1  by controlling the control units  15 ,  35 A, and  35 B based on detection results fed from the wettability detecting unit  51  and the pH detecting unit  52 . More specifically, the control unit  100  controls the control units  15 ,  35 A, and  35 B based on the detection results fed from the wettability detecting unit  51  and the pH detecting unit  52 , thereby controlling the following: whether or not to apply the plasma treatment, whether or not to apply the primer treatment, the plasma energy density (or the voltage value or the like) of the plasma treatment, the number of cycles of the primer treatment, the treatment-liquid application amount for each cycle of the primer treatment, and the like. The inkjet recording unit  40  may be controlled by a separate control unit (not shown) or may be controlled by the control unit  100 . 
     How the inkjet recording is performed is described below. As illustrated in  FIG. 8 , the control unit  100  starts conveying the print medium M 1  according to a command input from an input unit (not shown) (Step S 101 ). The print medium M 1  is thus delivered onto the conveyance path R 1 . The control unit  100  then specifies the type of the print medium M 1  based on print conditions configured in advance according to an input from the input unit (Step S 102 ), and determines pretreatment and pretreatment conditions based on a print mode (color/monochrome printing, resolution, and the like), type of the ink to be used, and the like (Step S 103 ). Print conditions including the type of the print medium M 1 , the print mode, and the type of the ink to be used may be stored in the storage unit  101 , for example. Association data between the print conditions and the pretreatment may be stored in the storage unit  101 , for example. In Step S 103 , combination of the plasma treatment, first primer treatment, and second primer treatment to be applied as the pretreatment, pretreatment conditions (plasma energy density, treatment-liquid application amount, and the like) for each of the plasma treatment and the first and second primer treatments, and the like are determined. 
     The print medium M 1  delivered onto the conveyance path R 1  passes through the acidification unit  10  first. At this point, the control unit  100  determines whether or not it is determined in Step S 103  that the plasma treatment is to be applied (Step S 104 ). If it is determined in Step S 103  that the plasma treatment is to be applied (YES in Step S 104 ), the control unit  100  drives the acidification unit  10  according to the pretreatment conditions (the plasma energy density and the like) determined in Step S 103 , thereby applying the plasma treatment to the print medium M 1  (Step S 105 ). More specifically, the control unit  100  adjusts the number of the discharge electrodes  11   a  to  11   f  to be driven and/or the plasma energy density of the pulse voltages to be supplied by the high-voltage high-frequency power supplies  12   a  to  12   f  to the discharge electrodes  11   a  to  11   f  according to the pretreatment conditions determined in Step S 103 , for example. The plasma energy density can be calculated as described above from the value of the electric current passing through the print medium M 1 . If it is determined that the plasma treatment is not to be applied (NO in Step S 104 ), the control unit  100  causes processing to proceed to Step S 106 , skipping Step S 105 . 
     The control unit  100  then determines whether or not it is determined in Step S 103  that the first primer treatment is to be applied (Step S 106 ). If it is determined that the first primer treatment is not to be applied (NO in Step S 106 ), the control unit  100  controls the conveyance switch unit  21  so as to deliver the print medium M 1  to the conveyance path R 2  of the first route and causes processing to proceed to Step S 110 . 
     If it is determined that the first primer treatment is to be applied (YES in Step S 106 ), the control unit  100  controls the conveyance switch unit  21  so as to deliver the print medium M 1  to the conveyance path R 11  to cause the print medium M 1  to pass through the first primer applying unit  30 A. The control unit  100  drives the first primer applying unit  30 A according to die pretreatment conditions (the treatment-liquid application amount and the like) determined in Step S 103  when the print medium M 1  passes through the first primer applying unit  30 A, thereby applying the first primer treatment to the print medium M 1  (Step S 107 ). 
     The control unit  100  determines whether or not it is determined in Step S 103  that the second primer treatment is to be applied (Step S 108 ). If it is determined that the second primer treatment is not to be applied (NO in Step S 108 ), the control unit  100  controls the conveyance switch unit  22  so as to deliver the print medium M 1  to the conveyance path R 12  of the second route and causes processing to proceed, to Step S 110 . 
     If it is determined that the second primer treatment is to be applied (YES in Step S 108 ), the control unit  100  controls the conveyance switch unit  22  so as to deliver the print medium M 1  to the conveyance path R 21 , thereby causing the print medium M 1  to pass through the second primer applying unit  30 B. The control unit  100  drives the second primer applying unit  30 B according to the pretreatment conditions (the treatment-liquid application amount and the like) determined in Step S 103  when the print medium M 1  passes through the second primer applying unit  30 B, thereby applying the second primer treatment to the print medium M 1  (Step S 109 ), and thereafter causes processing to proceed to Step S 110 . 
     In Step S 110 , the control unit  100  obtains wettability of the print medium M 1  from a detection result output from the wettability detecting unit  51 . A method for detecting the wettability will be described later. For example, the wettability may be detected by ejecting a liquid droplet onto the print medium M 1  having undergone pretreatment and measuring a dot size and shape of the droplet. The control unit  100  obtains a pH value of the print medium M 1  from a detection result output from the pH detecting unit  52  (Step S 111 ). A method for detecting the pH value will be described later. For example, the pH value of the print medium M 1  having undergone pretreatment may be detected using a noncontact pH sensor. The wettability and the pH value detected in Steps S 110  and S 111  may be stored in the storage unit  101 , for example. When being stored, the detected wettability and the pH value may be stored as being associated with the type of the print medium M 1  specified in Step S 102 , the pretreatment conditions determined in Step S 103 , and the like. 
     Subsequently, the control unit  100  determines whether or not the wettability and the pH value detected in Steps S 110  and S 111  fall within a “printable” range (Step S 112 ). If the print medium M 1  is not determined to be printable (NO is Step S 112 ), the control unit  100  brings processing back to Step S 103  to apply pretreatment again. If the print medium M 1  is determined to be printable (YES to Step S 112 ), the control unit  100  causes processing to proceed to Step S 113 . 
     The print medium M 1  delivered to one of the first route, the second route, and the third route is thereafter conveyed through the conveyance path R 32 , which are common among the routes. The control unit  100  drives the inkjet recording unit  40  in a manner timed to passage of the print medium M 1  through the conveyance path R 32 , thereby performing inkjet recording on the print medium M 1  having undergone the pretreatment (Step S 113 ). Thereafter, the control unit  100  performs post-processing on the printed print medium M 1  as required and discharges the print medium M 1  (Step S 114 ). Then, the operation ends. 
     In the operation illustrated in  FIG. 8 , the route of the print medium M 1 , the plasma energy density (discharge voltage and frequency) of the acidification unit  10 , and the treatment-liquid application amounts of the first and second primer applying units  30 A and  30 B are automatically determined by the control unit  100  according to the print conditions and the like, but not limited thereto. Alternatively, for example, the route of the print medium M 1 , the plasma energy density (discharge voltage and frequency) of the acidification unit  10 , and the application amounts of the treatment liquid of the first and second primer applying units  30 A and  30 B may be manually set or adjusted by a user. In this case, the control unit  100  may control the units according to user-set (user-adjusted) values. 
     Basically, the plasma energy density of the plasma treatment is preferably within a range of 0.1 J/cm 2  to 10.0 J/cm 2 . Basically, the application amount of each of the first and second primer treatments is preferably within a range of 0.02 mg/cm 2  to 0.2 mg/cm 2 . Optimum conditions of the plasma energy density and the amounts of the primer to be applied (hereinafter, sometimes referred to as “primer application amount”) can be obtained by the following method, for example. Print media of various types are pre-treated with continuously-varying plasma energy density and primer application amount. Images (dots) are actually formed by inkjet recording on the pre-treated print media. The optimum conditions can be determined, by measuring the printed images (dots). Evaluation measures for the images (dots) can include print density, dot diameter, circularity, and granularity in addition to visual appearance. Other evaluation measure, such as a degree of fixation, may be measured. Because these measures are affected by ink and ink recording settings, it is preferable to measure a pH value and wettability (more specifically, a contact angle between the print medium and a purified water droplet) of each of the pre-treated print media as supplemental basic properties. The inkjet recording unit  40  may preferably be controlled according to the optimum conditions determined for each of the print media based on these results. 
     TABLE 1 below indicates results of measurements of contact angles and pH values of sheets of low-permeable paper used as the print medium M 1 , onto which the plasma treatment, the primer treatment, and the combination, of the plasma treatment and the primer treatment are respectively applied. Each of the contact angles presented in TABLE 1 indicates wettability and is obtained by measuring a contact angle of a deionized water droplet deposited on the print medium M 1 . Each of the pH values indicates acidity measured with a chemical indicator applied onto the surface of the medium. Meanwhile, each of the plasma treatment and the primer treatment acts to acidify the surface of the medium. The acidified print medium M 1  neutralizes the alkaline ink, causing pigments in the ink to coagulate and the viscosity of the ink to increase. As a result, even when coalescence of dots should occur, the pigments are less likely to migrate. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Plasma Energy 
                 Application 
                 Contact 
                   
               
               
                   
                   
                 Density 
                 Amount 
                 Angle θ 
                   
               
               
                   
                 Treatment 
                 (J/cm 2 ) 
                 (mg/cm 2 ) 
                 (deg.) 
                 pH 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 None 
                 — 
                 — 
                 71 
                 6.4 
               
               
                   
                 Plasma 
                 0.14 
                 — 
                 26 
                 6.2 
               
               
                   
                 Treatment 
                 2.78 
                 — 
                 23 
                 4.8 
               
               
                   
                 Primer 
                 — 
                 0.06 
                 62 
                 5.8 
               
               
                   
                 Application 
                 — 
                 0.10 
                 57 
                 5.6 
               
               
                   
                 Combination 
                 0.14 
                 0.05 
                 37 
                 5.6 
               
               
                   
                   
                 0.14 
                 0.06 
                 40 
                 5.6 
               
               
                   
                   
                 0.14 
                 0.11 
                 37 
                 5.6 
               
               
                   
                   
               
            
           
         
       
     
     Referring to TABLE 1, when none of the plasma treatment and the primer treatment is applied, the contact angle is large. This large contact angle indicates that coated paper, which is the print medium, is repelling deionized water. In contrast, the smaller contact angle of the plasma-treated coated paper indicates that wettability is improved by the plasma treatment. Furthermore, the plasma treatment acidifies the pH of the coated paper. This is presumably because polar functional groups generated by the plasma treatment on the surface of the coated paper acidify the coated paper. Furthermore, coating layer of the coated paper is broken and pores are formed by discharge; as a result, hydrophilcity is imparted to the surface of the coated paper. Although not presented in TABLE 1, the pH value changed little when the plasma energy density was increased to approximately 2.8 J/cm 2  or higher. The coated paper, to which the pretreatment was applied, presented in TABLE 1 exhibited favorable property. However, the same treatment undesirably enhanced wettability or permeability excessively when applied to some types of ordinary paper and coarse paper which are more porous. 
     Referring to TABLE 1, although the primer treatment lowers pH to acid pH, the primer treatment does not change the contact angles so greatly as the plasma treatment does. Because it is difficult to apply the water-based primer lightly and uniformly to the hydrophilic coated paper, a certain amount of the primer is necessary to apply the primer lightly and uniformly. 
     Referring to TABLE 1, the combination of the plasma treatment and the primer treatment yields improvement in wettability and moderate acidification or, in short, results between those of the plasma treatment and those of the primer treatment. Meanwhile, the plasma treatment not only acidifies the surface of coated paper hut also roughens the surface. Accordingly, the plasma treatment also yields an effect of making the surface more primer-wettable, thereby allowing light and uniform application of the primer. 
     It has thus been indicated that the combination of the plasma treatment and the primer treatment is considerably effective for both of print media for which the plasma treatment is effective and print media for which the primer treatment is effective. 
     Methods for detecting the wettability and the pH value of the print medium M 1  are described below.  FIGS. 9A to 9C  are schematic diagrams illustrating an example of wettability detection method performed by the wettability detecting unit  51  illustrated in  FIG. 7 .  FIG. 10  is a diagram for describing a contact-angle calculation method involved in the wettability detection method illustrated in  FIGS. 9( a ) to 9( c ) . 
     As illustrated in  FIGS. 9( a ) to 9( c ) , a dot D is formed by actually ejecting a liquid droplet onto the pre-treated print medium M 1 . The wettability detecting unit  51  performs imaging of the dot D from a lateral direction, (which is a direction flush with and parallel to a printed surface of the print medium M 1 ) using a light source  511  and a camera  512 , and determines a shape of the dot D from the obtained image. The obtained image of the dot D may be transmitted to the control unit  100 , for example. The control unit  100  determines a contact angle θ by analyzing the received image of the dot D, and obtains the wettability of the print medium M 1  from the determined contact angle θ. 
     More specifically, as illustrated in  FIG. 10 , the control unit  100  assumes a portion near an endpoint where the dot D contacts the surface of the print medium M 1  as a part of an imaginary circle (or sphere) O. Center M of the circle O is determined from three points, denoted by A 1 , A 2 , and A 3 , on a circular arc of the dot D. A tangent line m at the point A 1  is obtained. The contact angle θ on the left side of the dot D is obtained as an angle between the tangent line m and a surface M 10  of the print medium M 1 . Similarly, the contact angle θ on t he right side of the do t D can be obtained from points B 1 , B 2 , and B 3  on a circular arc of the dot D. 
     Various methods other than the above-described method are usable as the wettability detection method. Examples of the usable method include a method of applying a wettability test liquid onto the print medium M 1 , obtaining an image indicating how the print medium M 1  is wet with a camera, and determining wettability based on the obtained image.  FIG. 11  illustrates an example of an image obtained by imaging a print medium, which is poor in wettability and onto which the wettability test liquid is applied.  FIG. 12  illustrates an example of an image obtained by imaging a print medium, which is favorable in wettability and onto which the wettability test liquid is applied. As will be apparent from comparison between  FIGS. 11 and 12 , the wettability test liquid is repelled from the print medium ( FIG. 11 ) having poor wettability, whereas the wettability test liquid spreads over the print medium ( FIG. 12 ) having favorable wettability. Wettability of a print medium can be determined from such an extent of spread of wettability test liquid. 
     As described above, a pH sensor with a noncontact probe can be used as the pH detecting unit  52 . 
       FIG. 13  is a graph illustrating relationship between print density (single color) and amount of ink deposited cat print media to which different pretreatments are applied. In  FIG. 13 , the solid line indicates a result of no pretreatment. The long dashed short dashed line indicates a result of the primer treatment with an application amount of 0.1 mg/cm 2 . The dashed line indicates a result of the plasma treatment with a plasma energy density of 2.78 J/cm 2 . The long dashed double-short dashed line indicates a result of the combination of the plasma treatment with a plasma energy density of 0.14 J/cm 2  and the primer treatment with an application amount of 0.06 mg/cm 2 .  FIG. 13  presents results obtained using water-based pigment ink (i.e., ink in which pigments are dispersed in alkaline solution) having a property that the pigments in the ink coagulate in acid. The results illustrated m  FIG. 13  are obtained using coated paper, which is a low-permeable medium, as the print medium. 
     Referring to  FIG. 13 , any one of the results of the pretreatments yields a higher print density than the result of no pretreatment (solid line). The result of only the primer treatment and the result of only the plasma treatment are substantially identical in print density. However, when comparison is made between actually-printed images, an image obtained with the primer treatment contains more image portions (dots) where two colors are overlaid and is also inferior in dot sharpness to an image obtained with the plasma treatment. In contrast, the image obtained with the plasma treatment has no color-mixture and exhibits favorable dot sharpness. An image obtained with the combination treatment yields highest print density among the four series presented in  FIG. 13 . The image obtained with only the plasma treatment is highest in granularity of dots. 
     The inkjet recording apparatus  1  and a method for producing a printed matter are described in detail below with reference to the drawings. In the description below, ejection heads (recording heads or ink heads) for four colors of black (K), cyan (C), magenta (M), and yellow (Y) are used as the inkjet head of the inkjet recording unit  40 . However, the inkjet head of the inkjet recording apparatus  1  is not limited thereto. More specifically, the inkjet head may additionally include ejection heads for other colors such as green (G) and red (R). The inkjet head may be an ejection head only for black (K). In the description below, K, C, M, and Y represent black, cyan, magenta, and yellow, respectively. 
     Although a roll of continuous paper (hereinafter, “roll paper”) is used as the print medium M 1  in the embodiment, the print medium M 1  is not limited thereto. Any print medium on which an image can be formed, such as cut paper, may be used as the print medium M 1 . The roll paper may be continuous paper (continuous stationary or continuous form paper) perforated transversely at regular intervals to allow tear-off at the perforation. When such continuous paper is used, a page of the roll paper corresponds to an area between adjacent perforation lines. 
     Example types of paper usable as the print medium be lode ordinary paper, woodfree paper, recycled paper, thin paper, thick paper, and coated paper. An overhead projector sheet, a synthetic resin film, a metal thin film, or other medium on which an image can be formed with ink or the like may be used as the print medium M 1  as well. 
       FIG. 14  illustrates a schematic configuration of the overall inkjet recording apparatus  1  according to the embodiment. Note that in the configuration illustrated in  FIG. 14 , only one of the primer treatment units  30  is depicted. Referring to  FIG. 14 , the inkjet recording apparatus  1  includes the feeding unit IN configured to feed (convey) the print medium M 1  (roll paper) along the conveyance path D 1 , the acidification unit  10  configured to apply the plasma treatment as pretreatment to the fed print medium M 1 , the primer applying unit  30  configured to apply the primer treatment to the print medium M 1  as pretreatment, and an image forming apparatus  120  configured to form an image on the surface of the print medium M 1  having undergone the pretreatment. These units and apparatus may be provided in separate casings to configure a printing system. Alternatively, these units and apparatus may be provided in a single casing to serve as a printing apparatus. When configured as the printing system, a control unit winch, controls the whole or a part of the system may he either included in any one of the units and apparatus or provided in a separate casing. 
     The image forming apparatus  120  includes the inkjet recording unit  40  configured to form an image on the plasma-treated print medium M 1  by inkjet recording. The image forming apparatus  120  may further include a post-processing unit  121  configured to perform post-processing on the print medium M 1  on which the image is formed. The inkjet recording apparatus  1  may further include a drier unit  130  configured to dry the post-processed print medium M 1  and an output unit OUT configured to convey out the print medium M 1  on which the image is formed (or on which post-processing is additionally performed). The inkjet recording apparatus  1  includes the control unit  100  (see  FIG. 7 ) configured to provide control of operations of the units. 
     According to the embodiment, the inkjet recording apparatus  1  illustrated in  FIG. 14  performs the plasma treatment of acidifying the surface of the print medium M 1  and the primer treatment of applying treatment liquid onto the print medium M 1  as described above prior to inkjet recording as appropriate. As the plasma treatment, atmospheric-pressure non-equilibrium plasma treatment which utilizes dielectric barrier discharge can be used as described above. Meanwhile, the plasma treatment utilizing atmospheric-pressure non-equilibrium plasma is preferable as a plasma treatment method. This is because the electron temperature of the atmospheric-pressure non-equilibrium plasma is extremely high, whereas the gas temperature is close to room temperature. 
     It will be preferable to use dielectric barrier discharge based on streamer breakdown to generate atmospheric-pressure non-equilibrium, plasma stably over a wide range. The dielectric barrier discharge based on the streamer breakdown can be produced by applying alternating high voltages across electrodes coated with a dielectric, for example. 
     The method for generating the atmospheric-pressure non-equilibrium plasma is not limited to the dielectric barrier discharge based on streamer breakdown, and various other methods are usable. Examples of the usable method include a method of producing dielectric barrier discharge by inserting an insulator such as a dielectric between electrodes, a method of producing corona discharge by forming a highly-non-uniform electric field around a thin metal wire or the like, and a method of producing pulse discharge by applying a short pulse voltage. A combination of two or more of these methods is also usable. 
     Difference between a printed matter obtained without application of the plasma treatment according to the embodiment and a printed matter obtained with the same is described below with reference to  FIGS. 15 to 18 .  FIG. 15  is an enlarged view of an image obtained by imaging an image-formed surface of a printed matter obtained by performing inkjet recording on a print medium to which the plasma treatment according to the embodiment is not applied.  FIG. 16  is a schematic diagram illustrating an example of dots formed on the image-formed surface of the printed matter illustrated in  FIG. 15 .  FIG. 17  is an enlarged view of an image obtained by imaging an image-formed surface of a printed matter obtained by performing inkjet recording on a print medium to which the plasma treatment according to the embodiment is applied.  FIG. 18  is a schematic diagram illustrating an example of dots formed on the image-formed surface of the printed matter illustrated in  FIG. 17 . The printed matters illustrated in  FIGS. 15 and 17  were obtained using a desktop-type inkjet recording apparatus. General coated paper  60  having a coating layer  61  was used as the print medium M 1 . 
     The coated paper  60  to which the plasma treatment according to the embodiment is not applied is poor in wettability at the coating layer  61  on the surface of the coated paper  60 . Therefore, as illustrated in  FIGS. 15 and 16  for example, shape (shape of a vehicle CT 1 ) of a dot in the image formed by performing inkjet recording on the not-plasma-treated coated paper  60  is deformed when the dot is deposited on the surface (the coating layer  61 ) of the coated paper  60 . When, before a dot becomes sufficiently dried, an adjacent dot is formed, as illustrated in  FIGS. 15 and 16 , the vehicle CT 1  and a vehicle CT 2  of the adjacent dot coagulate at deposition of the adjacent dot on the coated paper  60 . As a result, migration (color mixture) of pigments P 1  and pigments P 2  can occur between the dots, which can undesirably result in inconsistencies in density caused by beading or the like. 
     In contrast, the coating layer  60   p  on the surface of the coated paper  60  to which the plasma treatment according to the embodiment is applied is improved in wettability. Accordingly, as illustrated in  FIG. 17  for example, the vehicle CT 1  of a dot in the image formed by inkjet recording on the plasma-treated coated paper  60  spreads in a shape close to a relatively-flat perfect circle on the surface of the coating layer  60   p  of the coated paper  60 . As a result, as illustrated in  FIG. 18 , the dot has a flat shape. Furthermore, because polar functional groups formed by the plasma treatment acidifies the surface of the coating layer  60   p  of the coated paper  60 , ink pigments are neutralized and the pigments P 1  coagulate, causing viscosity of the ink to increase. As a result, even when the vehicles CT 1  and CT 2  coagulate as illustrated in  FIG. 18 , migration (color mixture) of the pigments P 1  and P 2  between the dots can be reduced. Moreover, because the polar functional groups are generated also inside the coating layer  60   p , the permeability of the vehicle CT 1  increases. As a result, drying in a relatively short period of time can be achieved. Because the dots, which are spread in shapes close to a perfect circle by virtue of the improved wettability, coagulate while penetrating into the coated paper, the pigments P 1  coagulate uniformly in height direction. As a result, occurrence of inconsistency in density which can otherwise be caused by beading or the like can be reduced. Note that  FIGS. 16 and 18  are schematic diagrams and, in practice, the pigments coagulate in a layer also on the coated paper illustrated  FIG. 18 . 
     As described above, the print medium M 1  to which the plasma treatment according to the embodiment is applied is improved in wettability because hydrophilic functional groups are generated on the surface of the print medium M 1  by the plasma treatment. Furthermore, because the plasma treatment increases surface roughness of the print medium M 1 , the wettability of the surface of the print medium M 1  is further improved. Furthermore, the polar functional groups generated by the plasma treatment acidify the print medium M 1 . These phenomena cause deposited ink to spread uniformly on the surface of the print medium M 1  and neutralize negatively-charged pigments. The neutralized pigments coagulate on the surface of the print medium M 1  and increase the viscosity. As a result, even when dots coalescence should occur, migration of pigments can be reduced. Moreover, because the polar functional groups are generated also inside the coating layer formed, on the surface of the print medium M 1 , vehicle permeates into the print medium M 1  quickly, which leads to reduction in drying time. In other words, a dot which has spread in a shape close to a perfect circle by virtue of the increase in wettability permeates into the print medium M 1  in a state where migration of pigments is reduced by pigment coagulation. Accordingly, the dot can retain its shape close to a perfect circle. 
       FIG. 19  is a graph illustrating relationships between plasma energy density and each of wettability, beading, pH value, and permeability of a surface of a print medium according to the embodiment.  FIG. 19  illustrates how surface properties (wettability, beading, pH value, and permeability (liquid absorption characteristics)) of coated paper, which is used as the print medium M 1  and on which an image is printed, change with the plasma energy density. The evaluation result illustrated in  FIG. 19  was obtained using water-based pigment ink (i.e., ink in which pigments are dispersed in alkaline solution) having a property that the pigments in the ink coagulate in acid. 
     As illustrated in  FIG. 19 , the wettability of the surface of the coated paper is improved sharply in a range where the plasma energy density is low (e.g., approximately 0.2 J/cm 2  or lower) but not improved greatly even when the plasma energy density is increased to be higher than the range. By contrast, as the plasma energy density increases, the pH value of the surface of the coated paper decreases to near a certain value. However, this decrease in the pH value saturates at the certain value (e.g., approximately 4 J/cm 2 ) of the plasma energy density. The permeability (liquid absorption characteristics) sharply improves from near a value corresponding to the plasma energy density (e.g., approximately 4 J/cm 2 ) at which the decrease in the pH value saturates. However, this phenomenon varies with polymers contained in the ink. 
     As described above, in terms of the relationship between surface properties of the print medium M 1  and image quality, the dot circularity is improved as the wettability of the surface is improved. This is presumably because the surface roughness increased by the plasma treatment and the hydrophilic polar functional groups generated by the same not only improve the wettability of the surface of the print medium M 1  but also make the wettability more uniform. Another possible cause may be that the plasma treatment removes water-repellent factors such as dusts, oil, and calcium carbonate from the surface of the print medium M 1 . More specifically, the plasma treatment improves the wettability of the surface of the print medium M 1  and removes the water-repellent factors from the surface of the print medium M 1 . As a result, a droplet is evenly spread toward its circumference, and therefore dot circularity is improved. 
     Furthermore, acidifying (lowering the pH of) the surface of the print medium M 1  results in coagulation of ink pigments, improvement in permeability, and penetration of vehicle to inside the coating layer. Because pigment density on the surface of the print medium. M 1  is increased by these phenomena, even, when dot coalescence should occur, migration of pigments can be reduced. As a result, mixture of the pigments is reduced, making H possible to cause the pigments to settle and coagulate uniformly on the surface of the print medium M 1 . However, the effect of reducing pigment mixture depends on components of the ink and/or the volume of the ink droplet. For example, the pigment mixture is less likely to occur in a small ink droplet than in a large ink droplet. This is because, the smaller the vehicle, the faster the vehicle dries and penetrates and, accordingly, the smaller the vehicle, the smaller the necessary change in pH to cause the pigments in the vehicle to coagulate. Meanwhile, the effect yielded by the plasma treatment varies with the type of the print medium M 1 , the environment (humidity or the like), and the like. For this reason, the plasma energy density of the plasma treatment may be controlled to an optimum value according to the type of the print medium M 1 , the environment, and the like. Such control can possibly improve surface modification efficiency of the print medium M 1 , thereby achieving further energy saving. 
       FIG. 20  is a graph illustrating relationship between plasma energy density and pH value according to the embodiment. Although the pH is generally measured in a solution, measuring a surface pH of a solid has become possible in recent years. As a measuring instrument for such solid surface pH measurement, a pH meter B-211 manufactured by HORIBA, Ltd. may be used. 
     In  FIG. 20 , the solid line illustrates dependency relationship between pH value of coated paper and plasma energy density; the dashed line illustrates dependency relationship between pH value of a PET film and plasma energy density. As illustrated in  FIG. 20 , the PET film is acidified with lower plasma energy density than the coated paper. It should be noted that the plasma energy density necessary for acidifying the coated paper is as low as 3 J/cm 2  or lower. Shapes of dots of an image formed by an inkjet recording apparatus by ejecting alkaline water-based pigment ink on the print medium M 1 , the pH value of which was lowered to 5 or lower, were close to a perfect circle. A favorable image free from mixture of pigments resulting from dot coalescence and bleeding was obtained (see  FIG. 17 ). 
     Possible methods for obtaining the plasma energy density necessary to acidify the surface of the print medium M 1  include increasing time over which the plasma treatment is applied (hereinafter, “plasma treatment time”). This can be achieved for example, decreasing the conveying speed of the print medium M 1 . However, the plasma treatment time is desirably reduced when high-speed recording of an image onto the print medium M 1  is desired. Possible methods for reducing the plasma treatment time include the above-described method of providing the multiple discharge electrodes  11   a  to  11   f  and driving one or more of the discharge electrodes  11   a  to  11   f , the number of which depends on the printing speed and the necessary plasma energy density, and a method of adjusting the intensity of plasma energy to be applied to each of the discharge electrodes  11   a  to  11   f . However, the possible methods are not limited to these, and may include combinations of these methods, other methods, and appropriate modifications. 
     Relationship between amount of ink deposited on ordinary paper, which is used as the print medium M 1  and to which the combination of the plasma treatment and the primer treatment is applied, and image density is described below with reference to  FIG. 21 . 
     Referring to  FIG. 21 , when applied to ordinary paper used as the print medium M 1 , the plasma treatment can be said to be superior to the primer treatment in a density range (halftone density) where dots of a printed image are not in density equilibrium (density saturated) yet. Density of dots on the plasma-treated print medium is slightly higher than density of dots on a print medium to which neither the plasma treatment nor the primer treatment is applied. It should be noted that saturation density of the dots on the plasma-treated print medium is lower than that on a primer-applied print medium. The dot density on the primer-applied print medium is increased by the effect of the primer treatment of improving fixation of ink onto the print medium. 
     An amount of ink to be deposited (hereinafter, “ink deposition amount”) to obtain a same gray level is smaller with a plasma-treated print medium than that with a primer-treated print medium. More specifically, the plasma treatment allows reducing an amount of ink to be deposited on a halftone image by approximately 1% to 18% than that to be deposited (to obtain the same gray level) on a print medium to which no pretreatment is applied. The plasma treatment allows reducing the ink deposition amount on a halftone image by approximately 15% to 29% than the primer treatment. Meanwhile, the reason why the plasma treatment is inferior to the primer treatment in saturation density (maximum density) is presumably that application of SDF treatment to ordinary paper causes dots to spread wider. As a result, more gaps between dots are filled under the same condition of the ink deposition amount. By contrast, the reason why the primer treatment increases the saturation density is presumably that because dots on a primer-applied, print medium spread less, the dots have higher dot density, whereby the saturation density is increased. 
     It can be said from the above that the effect of the plasma treatment and that of the primer treatment vary between a low-permeable print medium and a highly-permeable print medium. Accordingly, configuring a system to apply the combination of the plasma treatment and the primer treatment leads to improving adaptability to print media (i.e., effect of pretreatment). The combination of the plasma treatment and the primer treatment allows reducing the plasma energy density to approximately 1/20 of pretreatment of only the plasma treatment and reducing the application amount to approximately ⅗ of pretreatment of only the primer treatment. As a result, the combination allows obtaining a printed matter of a high image quality with low energy consumption and small application amount. Moreover, the high density of dots on the printed image indicates that the ink deposition amount can be reduced. Accordingly, the combination allows cost reduction by reducing the ink deposition amount. The plasma treatment is effective for a low-permeable print medium, whereas the primer treatment is effective for a highly-permeable print medium. Accordingly, optimum pretreatment can be applied by changing the combination of the plasma treatment and the primer treatment and pretreatment conditions therefor according to properties of a print medium. 
       FIG. 22  is a graph illustrating granularity of a low-permeable print medium to which the primer treatment only is applied and the combination of the plasma treatment and the primer treatment is applied.  FIG. 22  indicates that the lower the granularity, the more favorable the image on the print medium is. The legend entries indicate plasma treatment energy. In  FIG. 22 , the series “PLASMA ENERGY DENSITY: 0 J/cm 2 ” indicates a result of the application amount of the treatment liquid applied only by the primer treatment. The series “PLASMA ENERGY DENSITY: 0.139 J/cm 2 ” indicates a result of application of the combination. Referring to  FIG. 22 , the necessary application amount to achieve granularity of 0.5 or lower, for example, only by the primer treatment is approximately 0.2 mg/cm 2 . In contrast, the necessary application amount to achieve the same by the combination of the plasma treatment and the primer treatment is approximately 0.1 mg/cm 2 , which is substantially a half of that of only the primer treatment. 
     The optimizing control described above depends on the properties of the print medium. A modification may be configured to perform optimizing control based on an image to be printed. This modification may be implemented as follows, for example. The inkjet recording apparatus  1  is configured to include a reflection density meter. Reference print patterns are printed by the inkjet recording unit  40  with continuously-varying plasma energy densities and application amounts of the primer. Print densities of the printed images are measured with the reflection density meter. Pretreatment conditions which yield a highest print density are defined as optimum conditions. Inkjet recording is performed so as to maintain the optimum conditions. This modification allows quick measurement and adjustment of pretreatment conditions, thereby achieving fast inkjet recording. The modification may be further modified so as to store density data output from the reflection density meter in the storage unit  101  or the like as being associated with pretreatment conditions for the print medium M 1 , thereby forming a database of the data. 
     Meanwhile, the optimum conditions also vary with a composition of ink, type of the ink, type of a print medium, or a combination thereof. Therefore, optimum inkjet recording can be implemented by storing pretreatment conditions and density information for each of these factors in the inkjet recording apparatus  1 , so that a printed matter of high quality can be produced stably. 
     Still another possible modification may include measuring electrical resistance across the print medium M 1  to roughly determine the thickness and the properties of the print medium M 1  prior to the plasma treatment, optimizing the pretreatment conditions as described above based on the thickness and the properties, and performing the combination pretreatment with the optimized pretreatment conditions. 
     Still another possible modification may be configured such that the inkjet recording apparatus  1  further includes a sensor for checking a result of the plasma treatment downstream of the plasma treatment unit  10   a  and a sensor for checking a result of the primer treatment downstream of the primer applying unit  30  so that, when the print medium M 1  is cut paper, the pretreatment can be repeated via another conveyance route as necessary. This modification may further be modified such that data obtained using the sensors is transmitted to the control unit  100 , and the control unit  100  changes the pretreatment conditions accordingly. 
     As described above, the pretreatment by the combination of the plasma treatment and the primer treatment not only reduces energy necessary to perform the plasma treatment, which leads to reduction in the size of the plasma treatment unit  10   a , but also reduces the application amount of the primer to be applied in the primer treatment, thereby reducing time and energy necessary for drying the treatment liquid and, furthermore, reducing ink consumption. Moreover, when an image is recorded on a print medium to which the combination of the plasma treatment and the primer treatment is applied, dots of the image have shapes close to perfect circle and, even when coalescence of the dots should occur, mixture of pigments is less likely to occur. Accordingly, a favorable image with less bleeding can be obtained. 
     As described above, the embodiment is configured to be capable of applying the combination of the plasma treatment and the primer treatment as pretreatment of the inkjet recording and, accordingly, can apply pretreatment which takes advantages of both the plasma treatment and the primer treatment. The advantages include, for example, reducing the plasma energy density and reducing the application amount of the primer while maintaining high quality of print image. Furthermore, compensation for disadvantage of each of the plasma treatment and the primer treatment can be made by controlling each of the treatments differently according to the type of the print medium M 1 . Consequently, optimum pretreatment can be applied to every type of the print medium. 
     Although the invention has been described with regard to certain preferred embodiments thereof, it is to be understood that the description is not meant as a limitation. Further modifications may occur to those skilled in the art, and it is intended to cover such modifications as fall within the scope of the appended claims. 
     Aspects of the present invention provide systems, apparatuses, and printed-matter production methods configured to be capable of optimizing pretreatment according to a type of a print medium. 
     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.