Patent Publication Number: US-8123334-B2

Title: Liquid ejection head and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid ejection head and an image forming apparatus, and more particularly to a head structure that can be used desirably for preventing drying of ink in a nozzle of an inkjet head and an image forming apparatus having such a head structure. 
     2. Description of the Related Art 
     Japanese patent application publication No. 2006-62166 discloses a so-called on-demand type of inkjet recording apparatus. In order to prevent the deterioration of recording qualities caused by increase in the viscosity of ink in nozzles in cases where the ink is not ejected over a long time, this inkjet recording apparatus has a humidification liquid supply port provided in the ejection surface of the recording head, and a moistening region in which the moistening liquid oozing from the humidification liquid supply port is evaporated. In this apparatus, the ejection surface is moistened by evaporation of the moistening liquid in the moistening region. 
     Japanese patent application publication No. 2007-261204 discloses a liquid ejection head comprising: an air flow supply port which is located near a nozzle and supplies air flow containing moisture of a volatile solvent of ink; and an air flow circulation mechanism which circulates recovered air flow and supplies it again from the air flow supply port. 
     In the inkjet recording apparatus disclosed in Japanese patent application publication No. 2006-62166, the moistening liquid is supplied to the nozzle surface and evaporation of the moistening liquid produces moistening, but the liquid merely stays in the nozzle surface and does not circulate. Therefore, paper powder, ink mist, and the like gradually accumulate in the moistening region, deteriorating the humidification performance. Further, in cases where a mixed liquid is used for the moistening liquid, a highly-volatile component first evaporates, then the composition of the moistening liquid gradually changes, and therefore, it is difficult to offer stable moistening. 
     In the liquid ejection head disclosed in Japanese patent application publication No. 2007-261204, the ink ejection direction is changed due to the air flow containing the gaseous volatile liquid, reducing the accuracy of droplet landing. 
     SUMMARY OF THE INVENTION 
     The present invention has been contrived in view of these circumstances, an object thereof being to provide a liquid ejection head that can achieve stable humidification so that the drying/blocking of a nozzle can be prevented, and an image forming apparatus comprising such a liquid ejection head. 
     In order to attain an object described above, one aspect of the present invention is directed to a liquid ejection head comprising: a nozzle plate having a nozzle surface in which at least one nozzle for ejecting droplets of a liquid are formed; an anti-drying liquid supply port which supplies an anti-drying liquid to the nozzle surface of the nozzle plate; a flow channel portion which is formed in the nozzle surface and through which the anti-drying liquid supplied to the nozzle surface from the anti-drying liquid supply port flows; and an anti-drying liquid discharge port which suctions and discharges the anti-drying liquid flowing through the flow channel portion on the nozzle surface, from the nozzle surface, wherein, while the anti-drying liquid flows through the flow channel portion, the anti-drying liquid evaporates to increase humidity. 
     According to this aspect of the invention, the surroundings of the nozzle are humidified because the anti-drying liquid flowing on the nozzle surface evaporates. By suctioning the anti-drying liquid via the anti-drying liquid discharge port while supplying the anti-drying liquid to the nozzle surface from the anti-drying liquid supply port, it is possible to move the anti-drying liquid on the nozzle surface, which can always supply and circulate a fresh anti-drying liquid and thereby realize the stable humidification. 
     Another aspect of the present invention is directed to an image forming apparatus comprising the liquid ejection head. 
     According to the present invention, stable humidification can be achieved, and drying of liquid in a nozzle can be prevented. Further, even when foreign matter such as paper powder is mixed into the anti-drying liquid flowing on the nozzle surface, the anti-drying liquid can flows without staying, and therefore beneficial effects of the humidification can be fully exerted and the nozzle surface can be kept clean. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The nature of this invention, as well as other objects and benefits thereof, will be explained in the following with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures and wherein: 
         FIG. 1  is a plan diagram illustrating a head module included in an inkjet head according to a first embodiment of the invention, viewed from the ejection surface side of a nozzle plate; 
         FIG. 2  is a cross-sectional diagram along line  2 - 2  in  FIG. 1 ; 
         FIGS. 3A to 3C  are explanatory diagrams illustrating one example of a method of forming lyophilic portions for an anti-drying liquid flowing; 
         FIG. 4  is an illustrative diagram illustrating a method of removing a lyophobic film by laser; 
         FIG. 5  is an illustrative diagram of a step of removing a lyophobic film by means of ultraviolet light or oxygen plasma; 
         FIGS. 6A to 6C  are step diagrams illustrating an example where patterning is carried out during the formation of a lyophobic film; 
         FIG. 7  is a plan diagram illustrating a head module according to another embodiment of the invention, viewed from the ejection surface side of the nozzle plate; 
         FIG. 8  is a cross-sectional diagram along line  8 - 8  in  FIG. 7 ; 
         FIG. 9  is a plan diagram illustrating a head module according to another embodiment of the invention, viewed from the ejection surface side of the nozzle plate; 
         FIGS. 10A to 10C  are diagrams illustrating examples of flow channels for an anti-drying liquid formed on a nozzle surface; 
         FIG. 11  is an explanatory diagram for specific sizes; 
         FIG. 12  is a cross-sectional diagram of an anti-drying liquid flowing on the nozzle surface; 
         FIGS. 13A and 13B  are cross-sectional diagrams illustrating examples of a groove on the nozzle surface 
         FIG. 14  is a diagram illustrating a configuration example of a long line head manufactured by connecting head modules; 
         FIG. 15  is a cross-sectional diagram illustrating a second embodiment of the present invention; 
         FIG. 16  is a general schematic drawing of an inkjet recording apparatus relating to an embodiment of the present invention; 
         FIGS. 17A and 17B  are plan view perspective diagrams illustrating an example of the composition of a print head; 
         FIG. 18  is a plan view perspective diagram illustrating a further example of the structure of a full line head; 
         FIG. 19  is a cross-sectional diagram along line  19 - 19  in  FIGS. 17A and 17B ; 
         FIG. 20  is an enlarged view illustrating a nozzle arrangement in the print head illustrated in  FIGS. 17A and 17B ; 
         FIG. 21  is a schematic drawing of an ink supply system; 
         FIG. 22  is a configuration diagram illustrating a supply system for an anti-drying liquid; and 
         FIG. 23  is a principal block diagram illustrating the system configuration of an inkjet recording apparatus according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       FIG. 1  is a plan diagram of a head module  10  which is included in an inkjet head relating to a first embodiment of the present invention, as viewed from the ejection surface (nozzle surface) side of the nozzle plate  12 . In  FIG. 1 , reference numeral  14  represents a nozzle which forms an ink ejection port and reference numeral  16  represents a lyophilic portion (which corresponds to a “flow channel portion”) through which anti-drying liquid flows. As illustrated in  FIG. 1 , a plurality of nozzles  14  are formed in the nozzle plate  12  and a so-called matrix type nozzle arrangement is adopted in which a plurality of nozzle rows are formed at a uniform interval (period) P in the x direction, each nozzle row having nozzle orifices  14  arranged on an oblique straight line which intersects at an angle of φ with the lengthwise direction of the head module  10  (the x direction: the main scanning direction in a line head). 
     On this nozzle surface, lyophilic portions  16  to create flow channels for the flow of anti-drying liquid are formed about the periphery of the nozzle rows (both the left and right-hand sides in  FIG. 1 ). A lyophobic film is formed on the portion of the nozzle surface other than the lyophilic portions  16 . The lyophilic portions  16  illustrated in  FIG. 1  are formed in a belt shape following a straight line parallel to the nozzle rows, but the form of the lyophilic portions is not limited to this and may also be a meandering shape. 
     Furthermore, supply ports  18  for supplying anti-drying liquid to the respective lyophilic portions  16  (hereinafter, this port may be called “anti-drying liquid supply port” according to requirements) and discharge ports  20  for discharging anti-drying liquid from the lyophilic portions  16  (hereinafter, this may be called “anti-drying liquid discharge port” according to requirements) are provided in the head module  10 . The anti-drying liquid supply ports  18  open in contact with one end portions of the lyophilic portions  16 , and the anti-drying liquid discharge ports  20  open in contact with the other end portions of the lyophilic portions  16 . Reference numeral  22  denotes a supply channel forming member for forming a supply channel for anti-drying liquid inside the head module  10  and reference numeral  24  denotes a discharge channel forming member for forming a discharge channel for anti-drying liquid. 
     In the present example, as illustrated in  FIG. 1 , anti-drying liquid supply and discharge channels are formed in a high-density head by means of a composition comprising the supply channel forming member  22  and a discharge channel forming member  24  in the periphery of the nozzle plate  12  (above and below same in  FIG. 1 ), but it is also possible to adopt a mode in which an anti-drying liquid supply port  18  and discharge port  20  are provided in the nozzle plate  12  (being formed so as to pass through the nozzle plate  12 ) (described below in relation to  FIG. 7 ). 
       FIG. 2  is a cross-sectional diagram along line  2 - 2  in  FIG. 1 . Incidentally,  FIG. 2  illustrates anti-drying liquid supply and discharge channels, but does not depict the ink supply channels, and the like, inside the head on the rear surface side of the nozzle plate  12 . A pressurization pump  32  for liquid supply (sending liquid) is connected to the anti-drying liquid supply channel  28  and a suction pump  34  for suctioning and discharging is connected to the discharge channel  30 . 
     The pressure pump  32  is operated and anti-drying liquid  40  is caused to seep out from a supply port  18  (the liquid wets and spreads without dropping off from a supply port  18 ). The anti-drying liquid  40  wets and spreads along the lyophilic portions  16  due to the wetting properties of the lyophilic portions  16  in the nozzle plate  12 , and eventually arrives at the discharge ports  20 . By driving the suction pump  34  and suctioning the anti-drying liquid  40  from each discharge port  20 , it is possible to promote the flow of anti-drying liquid  40  into the lyophilic portions  16  between the supply ports  18  and the discharge ports  20 , and it is possible thereby to circulate the anti-drying liquid  40 . 
     It is also possible to adopt a composition which omits the pressure pump  32  for supplying liquid, and it is possible to create a flow of anti-drying liquid in the nozzle surface by means of the suction pump  34  only. 
       FIGS. 3A to 3C  are illustrative diagrams illustrating a manufacturing process for forming lyophilic portions on the nozzle surface of a nozzle plate. 
     Step 1: Step of Forming Nozzles and Lyophobic Film 
     Firstly, as illustrated in  FIG. 3A , a lyophobic film  44  is formed on the ejection side surface of a nozzle plate  12  which comprises nozzles  14 . Various methods can be chosen as the concrete method of obtaining the nozzle plate  12  having nozzles  14  and the lyophobic film  44 . For example, the nozzles can be formed by etching a silicon substrate, whereupon a lyophobic film  44  is formed by coating (application) or vapor deposition. As a further method, it is also possible to adopt a mode in which a nozzle plate  12  having nozzles  14  is manufactured by electroforming, and a lyophobic film  44  is formed on this plate by coating (application) or eutectic (eutectoid) plating. Since various other methods can also be chosen, the appropriate method should be employed in view of the required accuracy, costs and other factors. 
     As an example of the specific dimensions of the nozzle plate  12  used in the inkjet recording apparatus, the nozzle diameter r is 10 to 50 μm, the nozzle length L is 10 to 100 μm, the thickness t of the lyophobic film (created by a film deposition method) is several nm to 5 μm, and the nozzle row pitch (see  FIG. 1 ) is 100 to 1000 μm. 
     Step 2: Step of Removing a Portion of the Lyophobic Film at the Periphery of the Nozzles in Order to Improve Wetting Properties in that Portion 
     Next, as illustrated in  FIG. 3B , portions of the lyophobic film  44  about the periphery of each nozzle  12  (the portions which are to be lyophilic portions via a later stage) are removed. This removed portions  46  (lyophilic portions  16 ) have better wetting properties than the portions where the lyophobic film  44  is still present. 
     As a method of removing a portion of the lyophobic film  44 , for example, there is a mode in which the film is removed with laser light (see  FIG. 4 ), or a mode where the area other than the portion for removal is masked, and then a portion of the film is removed by plasma processing (using an oxygen plasma, or the like), or by irradiation of ultraviolet light ( FIG. 5 ). 
     As illustrated in  FIG. 4 , by irradiating laser light  52  from a laser processing head  50  onto portions of the nozzle plate  12  where the lyophobic film  44  formed thereon is to be removed, it is possible to remove the portions of the lyophobic film  44 . For the laser light source, it is possible to select one of various types of the laser light source, such as an excimer laser, a carbon dioxide (CO 2 ) laser, a YAG laser, or the like. 
       FIG. 5  illustrates an example of a mode of carrying out plasma processing or irradiation of ultraviolet light. As illustrated in  FIG. 5 , by irradiating an oxygen plasma or ultraviolet light via a mask member  56  which has openings  54  in positions corresponding to portions for removal, portions of the lyophobic film  44  which are exposed via the openings  54  are removed. 
     If the modes in  FIG. 4  and  FIG. 5  are compared, then if a portion of the film is removed by laser, a merit is obtained in that no masking member is required. On the other hand, if an oxygen plasma or ultraviolet light is used, then a mask member  56  must be fabricated and aligned with the nozzles  14 , but a merit is obtained in that batch processing can be carried out over a surface. The most efficient method should be selected in view of the size of the nozzle plate, the production volume, or other factors. 
     Furthermore, as a method of improving the wetting properties of one portion (rendering a lyophilic characteristic to one portion) other than a mode which removes a portion of the lyophobic film  44 , there is a mode in which the lyophobic film  44  is modified partially ( FIG. 3C ) or a method where, in locations where it is wished to apply the resin, an intermediate film (not illustrated) that enables the resin to be provided is provided. 
     Reference numeral  47  in  FIG. 3C  represents a modified portion of the lyophobic film  44 . As a means of selectively modifying a portion of the lyophobic film  44 , for example, it is possible to employ oxygen plasma processing using a mask member  56 , similarly to  FIG. 5 . 
     Another Manufacturing Method 
     Concerning the method of manufacture described in  FIGS. 3A to 3C , it is stated that after forming a lyophobic film  44  uniformly on the ejection surface side of the nozzle plate  12 , a portion of that film is removed or modified ( FIGS. 3B and 3C ), but it is also possible to adopt a mode in which portions where lyophobic film is formed and portions where lyophobic film is not formed are patterned when the lyophobic film  44  is formed, rather than carrying out a staged process as described above. In other words, when forming a lyophobic film, the lyophobic film is deposited in such a manner that the lyophobic film is not present in the portions where a flow channel in which an anti-drying liquid flows is formed in a later stage. 
       FIGS. 6A to 6C  are step diagrams illustrating an example where patterning is carried out during the formation of the lyophobic film. 
     Firstly, as illustrated in  FIG. 6A , on the ejection surface side of a nozzle plate  12  in which nozzles  14  are formed, a member (sacrifice layer) which can be removed in a later stage, such as a resist (photosensitive resin)  60 , is formed onto the portions (lyophobic film removal portions) where it is wished to form lyophilic portions in a later step. 
     Thereupon, a lyophobic film  44  is formed by eutectic plating (eutectoid plating), vapor deposition, or the like ( FIG. 6B ), and the resist  60  (sacrifice layer) is then removed ( FIG. 6C ). 
     According to this method of manufacture, although the step of patterning the resist  60  is added, the step of removing the lyophobic film  44  is eliminated. Furthermore, when the lyophobic film  44  is removed subsequently, if the film is not removed satisfactorily, then there is a possibility that it becomes difficult to flow the anti-drying liquid stably, but according to the method of manufacture described in  FIGS. 6A to 6C , it is possible reliably to form a portion where there is no lyophobic film. However, in order to form a lyophobic film  44  on a substrate where resist  60  has been formed ( FIG. 6B ), it is necessary to match the resist material with the method of forming the lyophobic film, and hence there is a possibility that methods which can be employed to form the lyophobic film can be restricted. 
     Anti-Drying Liquid 
     The anti-drying liquid is composed of a solution containing either the component having the highest composition ratio, of the ink components apart from the coloring material and the anti-drying agent, or taking this as a main component and also including other components. In other words, it is suitable to use for the anti-drying liquid, either water, which is the main component of the liquid, or an aqueous solution including components such as a permeation agent which forms part of the ink, a pH adjusting agent, an antiseptic and antibacterial agent, or the like. In particular, in the case of a pigment-based ink, it is desirable for the pH of the aqueous solution to be substantially the same as that of the ink, in order to prevent decline in dispersibility due to change in the pH of the ink at the ejection port. Moreover, in order to improve wetting with respect to the lyophilic portion, it is also possible to adjust the surface tension of the anti-drying liquid by means of a surfactant, alcohol, or the like. 
     The anti-drying liquid may also use the liquid used in cleaning the nozzle surface. In this case, it is possible to clean simply by wiping in a state where the anti-drying liquid is present, without especially applying cleaning liquid during wiping. The following substance is used as a combined anti-drying liquid and cleaning liquid. 
     Combined Anti-Drying Liquid and Cleaning Liquid 
     The combined anti-drying liquid and cleaning liquid has a characteristic feature in that the entire solvent contains 50 percent by mass or more of a solvent having an SP value (solubility parameter) of 27.5 or less. By containing 50 percent by mass or more of a solvent having an SP value of 27.5 or less in the entire solvent, it is possible to improve the maintenance properties. 
     Desirably, apart from using the solvent described above, it is also desirable to use water, but besides this there are no particular restrictions. From the viewpoint of improving the performance in removing solidified ink adhering material attached to the inkjet head, it is more desirable to include an adjusting agent which adjusts the pH or surfactant, and furthermore, it is also possible to use other additives, such as an antibacterial agent, an anti-rusting agent, an antiseptic agent, or a viscosity adjusting agent, as necessary. 
     Solvent Having SP Value of 27.5 or Less 
     The solvent having an SP value of 27.5 or less which is used in the present embodiment (hereinafter, “solvent”) is contained at a ratio of 50 percent by mass or more in the entire solvent, but from the viewpoint of improving performance in removing solidified ink adhering material attached to the inkjet head, more desirably, this content ratio is 60% or above, even more desirably, 70% or above, and yet more desirably, 80% or above. If the content ratio is less than 50 percent by mass, then the performance in removing the solidified ink adhering material is insufficient. The solubility parameter value (SP value) of the solvent described in the present embodiment is a value expressed as the square root of the molecular aggregation energy. This value can be calculated by the method described in R. F. Fedors in Polymer Engineering Science, 14, p. 147 (1974), and is the value used in the present embodiment. 
     Desirable practical examples of compounds forming a solvent having an SP value of 27.5 or less according to the present embodiment and their corresponding SP values (indicated in brackets) are stated below, but the present invention is not limited to these. 
     Practical Examples 
     
         
         
           
             Diethylene glycol monoethyl ether (22.4); 
             Diethylene glycol monobutyl ether (21.5), 
             Triethylene glycol monobutyl ether (21.1), 
             Dipropylene glycol monomethyl ether (21.3), 
             Dipropylene glycol (27.2) 
           
         
       
    
     
       
         
         
             
             
         
       
         
         
           
             nC 4 H 9 O(AO) 4 —H (AO=EO or PO, ratio—EO:PO=1:1) (20.1) 
             nC 4 H 9 O(AO) 10 —H (AO=EO or PO, ratio—EO:PO=1:1) (18.8) 
             HO(A′O) 40 —H (A′O=EO or PO, ratio—EO:PO=1:3) (18.7) 
             HO(A″O) 55 —H (A″O=EO or PO, ratio—EO:PO=5:6) (18.8) 
             HO(PO) 3 —H (24.7) 
             HO(PO) 7 —H (21.2) 
             1,2-hexane diol (27.4) 
           
         
       
    
     In the present specification, EO and PO represent an ethylene oxy group and a propylene oxy group. 
     These may be used independently or two or more types may be used in combination. 
     In the present embodiment, a solvent having an SP value of 27.5 or less is contained at a ratio of 50 percent by mass or more in the whole of the solvent, but from the viewpoint of improving the solubility and swelling characteristics of the solidified ink adhering material, desirably, the solvent is one having an SP value of 24 or less, and more desirably, a solvent having an SP value of 22 or less. 
     Furthermore, in the present embodiment, it is possible to combine the use of other solvents, within a scope that does not impair the beneficial effects of the present embodiment. 
     Ink 
     The ink composition used in the present embodiment is a pigment-based ink composition which contains pigment. Other than the pigment, the ink contains water but apart from this, there are no particular restrictions; however, it is desirable that the ink should contain a solvent having an SP value of 27.5 or less and should contain polymer particles. 
     The ink composition according to the present embodiment contains at least one type of pigment. There are no particular restrictions on the pigment used in the present embodiment and it may be selected appropriately according to the objectives. For example, the pigment may be an organic pigment or an inorganic pigment. 
     Desirably, the ink composition according to the present embodiment contains at least one type of polymer particles. By this means, the wear resistance of the image formed is improved effectively. 
     Specific examples of the polymer particles in the present embodiment include: a thermoplastic, thermocurable or denaturable acrylic, epoxy, polyurethane, polyether, polyamide, unsaturated polyester, phenol, silicone or fluorine resin, a polyvinyl resin such as vinyl chloride, vinyl acetate, polyvinyl alcohol, polyvinyl butylal, or the like, a polyester resin such as an alkyd resin, phthalic acid resin, or the like, an amino material such as melamine resin, melamine formaldehyde resin, amino-alkyd co-condensated resin, urea resin, or the like, or particles of a resin having an anionic group, such as copolymers or mixtures of these. Of these, an anionic acrylic resin can be obtained, for example, by polymerising an acryl monomer having an anionic group (an anionic group-containing acryl monomer) and other monomers which can be copolymerised with the anionic group-containing acryl monomer, as necessary, in a solvent. Examples of the anionic group-containing acryl monomer include: acryl monomers having at least one or more group selected from a carboxyl group, a sulfonate group, and a phosphone group, and of these, an acryl monomer having a carboxyl group (for example, acrylic acid, methacrylic acid, crotonic acid, ethacrylic acid, propyl acrylic acid, isopropyl acrylic acid, itaconic acid, fumaric acid, or the like) is desirable, and acrylic acid or methacrylic acid are especially desirable. 
     As the polymer particles in the present embodiment, from the viewpoint of ejection stability and solution stability (particularly dispersion stability) when using a pigment as described hereinafter, self-dispersing polymer particles are desirable, and self-dispersing polymer particles having a carboxyl group are more desirable. Self-dispersing polymer particles are particles of a water-insoluble polymer which can be obtained in a dispersed state in an aqueous medium by means of a functional group contained in the polymer itself (in particular, an acidic group or salt thereof) in the absence of a further surfactant, and which does not contain a free emulsifier. 
     Desirably, the ink composition in the present embodiment contains water. 
     There are no particular restrictions on the added amount of water used in the present embodiment, but from the viewpoint of maintaining stability and ejection reliability, desirably, the added amount of water is 10 percent by mass or more and 99 percent by mass or less, more desirably, 30 percent by mass or more and 80 percent by mass or less, and yet more desirably, 50 percent by mass or more and 70 percent by mass or less, in the whole ink composition. 
     Desirably, the ink composition in the present embodiment contains a solvent. 
     For the solvent, it is possible to use the solvents described above for the cleaning liquid, and of these, a solvent of which 70 percent by mass or more has an SP value of 27.5 or less is desirable from the viewpoint of suppressing curl and performance in dissolving solidified adhering material originating from the ink composition, and an SP value of 26 or less is more desirable, and 24 or less is even more desirable. 
     For the solvent having an SP value of 27.5 or less in the present embodiment, it is possible to cite the solvents described above for the cleaning liquid, and the desirable examples are the same. 
     The solvent may be used independently, or two or more types of solvents may be used in a combined fashion. 
     There are no particular restrictions on the content ratio of the solvent in the ink composition, but from the viewpoint of maintaining stability and ejection reliability, the content ratio is desirably 1 to 60 percent by mass, more desirably, 5 to 40 percent by mass, and particularly desirably, 5 to 30 percent by mass, of the whole ink composition. 
     Furthermore, desirably, the solvent having an SP value of 27.5 or less is contained at a ratio of 70 percent by mass or more, and more desirably, 80 percent by mass or more, and particularly desirably, 90 percent by mass or more, or the whole solvent. 
     The ink composition according to the present embodiment may also include other components, as necessary, in addition to the valuable components described above. The other components may be, commonly known additives, for example, a surfactant, an ultraviolet light absorber, an anti-fading agent, an antibacterial agent, a pH adjuster, an anti-rusting agent, an antioxidant, an emulsion stabilizer, an antiseptic agent, an antifoaming agent, a viscosity adjusting agent, a dispersion stabilizer, a chelating agent, or the like. 
     Action and Beneficial Effects 
     According to the present embodiment, since a composition is adopted in which an anti-drying liquid is caused to flow on the nozzle surface and this liquid is then circulated, it is possible to prevent drying of the ink inside the nozzles because the peripheral area of the nozzles is humidified by evaporation of the liquid during its flow over the nozzle surface. Furthermore, since fresh anti-drying liquid flows in a continuous fashion, then the composition of the anti-drying liquid is stabilized and a valuable effect in preventing drying is obtained. 
     Another beneficial effect apart from this is that even if paper powder or ink mist becomes mixed into the anti-drying liquid due to the flow of the liquid, since the liquid flows without stagnating, it is possible to display a sufficient humidifying effect and furthermore, it is also possible to keep the nozzle surface clean. 
     Modification Example 1 
       FIG. 7  is a plan diagram illustrating a head module  70  relating to a further embodiment as viewed from the ejection surface side of the nozzle plate  72 . Furthermore,  FIG. 8  illustrates a cross-sectional diagram along line  8 - 8  in  FIG. 7 . In  FIG. 7  and  FIG. 8 , elements which are the same as or similar to the composition illustrated in  FIGS. 1 and 2  are labelled with the same reference numerals and description thereof is omitted here. 
     The example illustrated in  FIG. 7  and  FIG. 8  is a mode where a supply port  18  and a discharge port  20  for the anti-drying liquid are provided in the nozzle plate  72 . Although not illustrated in  FIG. 8 , flow channels such as flow channels of a kind which form ink passage channels to the nozzles  14 , pressure chambers, common flow channels, and the like, are formed in the rear side portion of the nozzle plate  72  in the head module  70 . In particular, in the case of a high-density head in which a lot of nozzles are formed at high density, there are cases where it is difficult to form a supply channel  28  and a discharge channel  30  for the anti-drying liquid which passes through the structure  77  of the flow channels, without interfering with the flow channels of the ink. In such cases, as illustrated in  FIG. 1  and  FIG. 2 , a desirable mode is one where members (reference numerals  22  and  24 ) which form a supply port  18  and a discharge port  20  for the anti-drying liquid are formed to the outside of the nozzle plate. 
     Modification Example 2 
       FIG. 9  is a plan diagram illustrating a head module  80  relating to yet a further embodiment as viewed from the ejection surface side of the nozzle plate  82 . In  FIG. 9 , elements which are the same as or similar to the composition illustrated in  FIGS. 1 and 7  are labelled with the same reference numerals and description thereof is omitted here. 
     In the mode illustrated in  FIG. 1  and  FIG. 7 , lyophilic portions  16  through which the anti-drying liquid flows are formed following the shorter edges of the nozzle plate  12 ,  72 , but the mode of the portions through which the anti-drying liquid flows (lyophilic portions  16 ) is not limited to this and as illustrated in  FIG. 9 , these portions may also be parallel to the longer edges of the nozzle plate  82  or have a meandering shape (not illustrated). The pattern of lyophilic portions  16  is designed on the basis of the dimensions, such as the nozzle pitch, in such a manner that the anti-drying liquid can flow readily and flows to the portions where a humidifying effect is obtained. 
     Modification Example 3 
     In the modes illustrated in  FIG. 1 ,  FIG. 7  and  FIG. 9 , a supply port  18  and a discharge port  20  are provided in a one-to-one correspondence with respect to the portions (lyophilic portions  16 ) where the anti-drying liquid flows, but as illustrated in  FIGS. 10A to 10C , it is also possible to provide one set of a supply port  18  and a discharge port  20  for a plurality of portions where the anti-drying liquid flows, and it is also possible for the number of supply ports  18  and discharge ports  20  to be different.  FIG. 10A  is an example where a common set comprising a supply port  18  and a discharge port  20  are provided in respect of two lyophilic portions  16 - 1  and  16 - 2  through which the anti-drying liquid flows respectively. In  FIG. 10B , supply ports  18 - 1  and  18 - 2  are provided respectively for the two lyophilic portions  16 - 1  and  16 - 2 , and one discharge port  20  is formed for these two lyophilic portions  16 - 1  and  16 - 2 . It is also possible to adopt a mode where the relationship of supply ports and discharge ports can be interchanged. 
       FIG. 10C  illustrates an example where a common set comprising a supply port  18  and a discharge port  20  are provided in respect of three lyophilic portions  16 - 1 ,  16 - 2  and  16 - 3  through which the anti-drying liquid flows respectively. 
     Furthermore, if channels along which the anti-drying liquid flows (lyophilic portion  16 ) are provided in all the regions between the nozzle rows as illustrated in  FIG. 1 , then the humidifying effect is high, but the used amount of anti-drying liquid becomes large, and therefore it is possible to form the flow channels for the anti-drying liquid on the nozzle surface at appropriate intervals, for instance, by providing one anti-drying liquid flow channel for two nozzle rows, within a range whereby the desired humidifying effect is obtained. 
     Examples of Dimensions 
       FIG. 11  illustrates examples of dimensions. For example, the distance d from the edge of the nozzle  14  to the lyophilic portion  16  are designed appropriately in the ranges of 10 to 200 μm and the width W of the lyophilic portion  16  (the width of the flow channel through which the anti-drying liquid  40  flows) are designed appropriately in the ranges of 80 to 800 μm. The height h of the anti-drying liquid is determined by the width W, the angle of contact with the nozzle surface, and the supply volume of anti-drying liquid. 
     The flow rate of the anti-drying liquid depends on the use environment of the head (temperature, humidity), but to give an example, if the width W of the flow channels for the anti-drying liquid is 300 μm, the length of the flow channel (the length L from the supply port  18  to the discharge port  20 , see  FIG. 1 ) is 20 mm, and the angle of contact with respect to the nozzle surface is 30 degrees, then the flow rate is 1 to 5 μL/min in each channel. For example, if the liquid flows in 5 seconds along a flow channel having a length of 20 mm, then the flow rate is 2 μL/min. 
     Shape of Portion where Anti-Drying Liquid Flows 
     Rather than simply rendering lyophilic the portion where the anti-drying liquid flows on the nozzle surface, it is also possible to form a groove and then render this groove portion lyophilic. Examples of groove shapes are illustrated in  FIGS. 13A and 13B .  FIGS. 13A and 13B  are cross-sectional diagrams illustrating the shape of such a groove in a cross-section perpendicular to the direction of flow of the anti-drying liquid  40  (the cross-sectional shape of the flow channels formed by the grooves). There is a mode which adopts a square-shaped groove  84  as illustrated in  FIG. 13A  and a mode which adopts an inverse-trapezoid-shaped groove  86  as illustrated in  FIG. 13B . By rendering the groove portions lyophilic in this way, the holding force of the anti-drying liquid is further improved. Of course, the cross-sectional shape of the grooves is not limited to the examples illustrated in the drawings. 
     Temperature Adjustment of Anti-Drying Liquid 
     Desirably, the anti-drying liquid  40  supplied to the nozzle surface is heated and controlled to a prescribed temperature. In general, an inkjet head is adjusted to a specified temperature in order to stabilize ejection performance, and the like, but desirably, the anti-drying liquid flowing on the nozzle surface is heated and adjusted to a higher temperature than the head temperature. By this means, it is possible to promote the evaporation of the anti-drying liquid on the nozzle surface. 
     Combined Use of Other Viscosity Increase Prevention Methods 
     Known methods of preventing increase in viscosity in the ink inside the nozzles include a method which circulates the ink inside the nozzles, and a method which causes the meniscus inside a nozzle to vibrate slightly without being ejected (meniscus shaking). By combining the use of these viscosity increase prevention methods and the anti-drying liquid circulating technology according to embodiments of the present invention, an even greater effect in preventing increase in viscosity is obtained. 
     Increasing the Length of the Head 
     It is possible to adopt a mode in which main scanning direction nozzle rows corresponding to the maximum paper width are achieved using singly the head module  10  explained with reference to  FIG. 1 , and it is also possible to adopt a mode in which a long line head  90  achieving main scanning direction nozzle rows corresponding to the maximum paper width is formed by aligning in one row and joining together a plurality of head modules  10  each having a substantially parallelogram-shaped planar faun as illustrated, for example, in  FIG. 14 . 
     Second Embodiment 
     As illustrated in  FIG. 15 , a desirable mode is one in which the nozzle surface  12 A of the head module  10  is disposed at an inclination of angle θ with respect to the horizontal plane HL and the anti-drying liquid  40  is made to flow over the nozzle surface so as to travel in a vertical direction following this inclination. In  FIG. 15 , elements which are the same as or similar to the composition illustrated in  FIGS. 1 and 2  are labelled with the same reference numerals and description thereof is omitted here. 
     A case where a head is disposed about the periphery of a drum, for example, (see  FIG. 16 ) is one mode of an apparatus where the nozzle surface  12 A is disposed at an inclination as illustrated in  FIG. 15 . 
     According to the composition in  FIG. 15 , the flow of anti-drying liquid  40  is made smooth by the additional effect of gravity. Furthermore, the anti-drying liquid  40  never drips off from the nozzle surface  12 A. 
     As regards the angle of inclination θ of the nozzle surface  12 A with respect to the horizontal plane HL, in the case of a two-dimensional matrix arrangement in particular, the larger the angle θ, the difference in back pressure between nozzles becomes greater and ejection becomes less stable. Therefore it is desirable to set θ to the range of 3° to 30° with respect to the horizontal direction. 
     Example of Application to Inkjet Recording Apparatus 
     Next, an example of an image forming apparatus which uses the inkjet head comprising the nozzle plate described above will be explained. 
       FIG. 16  is a general configuration diagram of an inkjet recording apparatus including an image forming apparatus according to an embodiment of the present invention. As illustrated in  FIG. 16 , an inkjet recording apparatus  110  according to the present embodiment adopts a pressure drum direct describing system which forms an image directly onto a recording medium held on the circumferential surface of a pressure drum  112 . 
     The inkjet recording apparatus  110  principally comprises: a pressure drum  112  which holds and conveys a recording medium on the circumferential surface thereof; a paper supply unit  116  which supplies a recording medium  114 ; a print unit  118  which performs image formation by depositing colored inks onto a recording medium  114  held by the pressure drum  112 ; a solvent drying unit  120  which dries the solvent of the ink; a fixing processing unit  122  which makes the image permanent; an output unit  124  which conveys and outputs the recording medium  114  onto which an image has been formed; and a maintenance processing unit  126  which carries out maintenance processing of the inkjet heads  118 K,  118 C,  118 M and  118 Y of the print unit  118 . 
     A paper supply tray  128  which supplies recording media  114  in the form of cut sheet is provided in the paper supply unit  116 . A recording medium  114  fed out from the paper supply tray  128  by the paper supply roller  130  is supplied onto the circumferential surface of the pressure drum  112  via a guide roller  132  and is held on the circumferential surface of the pressure drum  112 . 
     It is also possible to use a recording medium of a continuous format which is wound in a roll shape, instead of a recording medium  114  in a cut paper format. If using recording medium in a continuous paper format, a device for holding the paper roll and a cutter for cutting a long recording medium to a prescribed size are provided. 
     Although not illustrated in the drawings, a plurality of suction holes are disposed according to a prescribed arrangement pattern on the circumferential surface of the pressure drum  112 , and the region where the plurality of suction holes are disposed functions as a recording medium holding region which suctions and holds a recording medium. The suction holes are connected to the suction flow channels provided inside the pressure drum  112  as well as being connected to an external suctioning apparatus (pump) via the suction flow channels. Instead of a negative pressure suctioning method described above, it is also possible to employ an electrostatic attraction method which holds a recording medium  114  on the recording medium holding region of the pressure drum  112  by means of static electricity. Since the conveyance of the recording medium is stable, then it is possible to reduce conveyance errors. 
     The print unit  118  has inkjet heads (hereinafter, simply called “heads”)  118 K,  118 C,  118 M and  118 Y corresponding to the four colors of black (K), cyan (C), magenta (M) and yellow (Y) which are provided at positions opposing the circumferential surface of the pressure drum  112 , and carries out image recording by ejecting inks of respective colors in accordance with image data onto a recording medium  114  held on the circumferential surface of the pressure drum  112 . 
     As illustrated in  FIG. 16 , the heads  118 K,  118 C,  118 M and  118 Y are disposed at an oblique inclination with respect to the horizontal plane, following the circumferential surface of the pressure drum  112 . In other words, the heads  118 K,  118 C,  118 M and  118 Y are disposed in such a manner that the perpendicular direction to the nozzle surfaces (ink ejection surface) of the respective heads  118 K,  118 C,  118 M and  118 Y coincides with the normal direction of the circumferential surface of the pressure drum  112 , and the distance between the ink ejection surfaces of the heads  118 K,  118 C,  118 M and  118 Y and the droplet ejection position on the pressure drum  112  (namely, on the recording medium  114 ) is the same in each of the heads  118 K,  118 C,  118 M and  118 Y. In particular, by disposing the heads in a circular arc shape about the periphery of the pressure drum  112 , the depositing position accuracy dependent on the droplet ejection distance is guaranteed and it becomes possible to form images of high quality. 
     Although a configuration with the four standard colors of K, C, M and Y is depicted in  FIG. 16 , the combinations of the ink colors and the number of colors are not limited to those. Light and/or dark inks, and special color inks can be added as required. For example, a configuration is possible in which ink heads for ejecting light-colored inks, such as light cyan and light magenta, red, blue, gold and silver, are added, and there is no particular restriction on the arrangement sequence of the heads of the respective colors. 
     A solvent drying unit  120  is provided at a downstream stage from the print unit  118 . A recording medium  114  on which image recording has been carried out is supplied to the solvent drying unit  120  via a guide roller  134  and a solvent drying process is carried out. In the solvent drying unit  120 , a hot wind of 50° C. to 130° C. is blown onto the image recording surface of the recording medium  114 , and the solvent, such as water, remaining on the image recording surface of the recording medium  114  is evaporated off. As a further mode of the solvent drying unit  120 , it is also possible to use, instead of or in combination with the hot air drying method, heating by a radiation method using an infrared heater, or a contact drying method in which a heated roller with an in-built heater is brought into contact with the recording medium  114  from the surface on the opposite side to the image forming surface of the medium. In other words, desirably, the solvent is dried without making contact with the image recording surface, and contact soiling inside the apparatus due to incomplete drying, or rear surface soiling due to stacking of the output recording media, and the like, is prevented. 
     A fixing processing unit  122  which carries out a fixing process on the recording medium  114  after the drying process is provided at a downstream stage after the solvent drying unit  120 . The fixing processing unit  122  illustrated in  FIG. 16  comprises a heating roller  138  having an in-built heater  136  and a supporting roller  140  which is disposed on the opposite side of the heating roller  138  via the conveyance path of the recording medium. 
     A recording medium  114  which has undergone a drying process is sandwiched between the heating roller  138  and the supporting roller  140  while the image recording surface is toward the heating roller  138  side, the image recording surface of the recording medium  114  is heated via the heating roller  138  by the heat radiated from the heater  136 , and the recording medium  114  is pressurized by the pressing force of the heating roller  138  and the supporting roller  140 . By this means, the wear resistance of the image portion of the recording medium is improved. 
     A recording medium  114  which has undergone a fixing process by the fixing process unit  122  is output to the exterior of the apparatus via the output unit  124 . A desirable mode of the output unit  124  is one in which a sorter is provided in such a manner that media are distinguished and output separately according to each image (or according to order). 
     The maintenance processing unit  126  has maintenance units  126 K,  126 C,  126 M and  126 Y corresponding respectively to the heads  118 K,  118 C,  118 M and  118 Y. As illustrated in  FIG. 16 , the maintenance units  126 K,  126 C,  126 M and  126 Y are disposed in an obliquely inclined fashion with respect to the horizontal plane, so as to be parallel with the heads  118 K,  118 C,  118 M and  118 Y. 
     The maintenance processing unit  126  is disposed in a maintenance position which is separated in the direction perpendicular to the plane of the drawing in  FIG. 16 , from the print position where the pressure drum  112  is disposed. In  FIG. 16 , by moving the heads  118 K,  118 C,  118 M and  118 Y in parallel in the perpendicular direction with respect to the plane of the drawings, it is possible to move the heads  118 K,  118 C,  118 M and  118 Y between a print position directly above the pressure drum  112  and a maintenance position. 
     Structure of Head 
     Next, the structure of a head will be described. The heads  118 K,  118 C,  118 M and  118 Y of the respective ink colors have the same structure, and a reference numeral  150  is hereinafter designated to any of the heads  118 K,  118 C,  118 M and  118 Y. 
       FIG. 17A  is a perspective plan view illustrating an example of the configuration of the head  150 , and  FIG. 17B  is an enlarged view of a portion thereof. Further,  FIG. 18  is a perspective plan view illustrating another example of the configuration of the head  150 , and  FIG. 19  is a cross-sectional view taken along line  19 - 19  in  FIGS. 17A and 17B , illustrating the inner structure of a droplet ejection element of one flow channel constituting a recording element unit (an ink chamber unit for one nozzle  151 ). For simplified explanation, flow channels for the anti-drying liquid are omitted from  FIGS. 17A ,  17 B,  18  and  19 . 
     The nozzle pitch in the head  150  should be minimized in order to maximize the density of the dots printed on the surface of the recording medium  114 . As illustrated in  FIGS. 17A  and  17 B, the head  150  according to the present embodiment has a structure in which a plurality of ink chamber units (droplet ejection elements)  153 , each comprising a nozzle  151  (equivalent to nozzles  14  in  FIG. 1 ) forming an ink ejection port, a pressure chamber  152  corresponding to the nozzle  151 , and the like, are disposed two-dimensionally in the form of a staggered matrix, and hence the effective nozzle interval (the projected nozzle pitch) as projected (orthogonal projection) in the lengthwise direction of the head (the direction perpendicular to the paper conveyance direction) is reduced and high nozzle density is achieved. 
     The mode of forming nozzle rows of a length greater than the length corresponding to the entire width Wm of the recording medium  114  in a direction (the direction indicated by arrow M; the main-scanning direction) substantially perpendicular to the conveyance direction of the recording medium  114  (the direction indicated by arrow S; the sub-scanning direction) is not limited to the example described above. For example, instead of the configuration in  FIG. 17A , as illustrated in  FIG. 18 , a line head having nozzle rows of a length corresponding to the entire width of the recording medium  114  can be formed by arranging and combining, in a staggered matrix, short head modules  150 ′ having a plurality of nozzles  151  arrayed in a two-dimensional fashion. 
     As illustrated in  FIGS. 17A and 17B , the planar shape of the pressure chamber  152  provided for each nozzle  151  is substantially a square, and an outlet to the nozzle  151  is provided in one of corners on a diagonal line of the square, and an inlet of supplied ink (supply port)  154  is provided in the other corner. The shape of the pressure chamber  152  is not limited to that of the present example and various modes are possible in which the planar shape is a quadrilateral shape (diamond shape, rectangular shape, or the like), a pentagonal shape, a hexagonal shape, or other polygonal shape, or a circular shape, elliptical shape, or the like. 
     As illustrated in  FIG. 19 , the head  150  is formed by a structure in which a nozzle plate  12 , a flow channel plate  78 , a diaphragm  156 , and the like, are laminated and bonded together. 
     The nozzle plate  12  is manufacture according to the manufacturing method illustrated in  FIGS. 3A to 3C  and  6 A to  6 C, and the like. This nozzle plate  12  forms the nozzle surface (ink ejection surface)  150 A of the head  150 , and a plurality of nozzles  151  which are respectively connected to the pressure chambers  152  are formed in a two-dimensional configuration in the nozzle plate  12 . 
     The flow channel plate  78  is a flow channel forming member which constitutes the side wall sections of the pressure chambers  152 , and forms a supply port  154  constituting a restrictor section (narrowest section) of the independent supply channel that guides ink from the common flow channel  155  into the pressure chamber  152 . For the purpose of the description,  FIG. 18  illustrates a simplified depiction, but the flow channel plate  78  in fact has a structure in which one or a plurality of substrates are laminated together. 
     As well as forming one side surface of the pressure chambers  152  (the upper surface in  FIG. 19 ), the diaphragm  156  is made of a conductive material such as stainless steel (SUS) or silicon (Si) with a nickel (Ni) conductive layer, or the like, and therefore also serves as a common electrode for the plurality of actuators (here, the piezoelectric elements)  158  which are disposed so as to correspond to the respective pressure chambers  152 . A mode is also possible in which a diaphragm is formed by a non-conductive material, such as resin, and in this case, a common electrode layer made of a conductive material, such as metal, is formed on the surface of the diaphragm member. 
     A piezoelectric body  159  is provided on the surface of the diaphragm  156  on the side opposite to the pressure chambers  152  (the upper side in  FIG. 19 ) at each position corresponding to the pressure chambers  152 , and an individual electrode  157  is formed on the upper surface of the piezoelectric body  159  (the surface of the piezoelectric body  159  on the side opposite to the surface in contact with the diaphragm  156  which also serves as a common electrode). A piezoelectric element which functions as an actuator  158  is constituted by the individual electrode  157 , the common electrode opposing same (in the present embodiment, this also doubles as the diaphragm  156 ), and the piezoelectric body  159  which is interposed between these two electrodes. As the material of the piezoelectric body  159 , it is desirable to use a piezoelectric material, such as lead titanate zirconate, barium titanate, or the like. 
     Each pressure chamber  152  is connected to a common flow channel  155  through the supply port  154 . The common flow channel  155  is connected to an ink tank (not illustrated), which is a base tank that supplies ink, and the ink supplied from the ink tank is delivered through the common flow channel  155  to the pressure chambers  152 . 
     When a drive voltage is applied to the individual electrode  157  of the actuator  158  and the common electrode, the actuator  158  deforms, thereby changing the volume of the pressure chamber  152 . This causes a pressure change which results in ink being ejected from the nozzle  151 . When the displacement of the actuator  158  returns to its original position after ejecting ink, the pressure chamber  152  is supplied with new ink from the common flow channel  155 , via the supply port  154 . 
     As illustrated in  FIG. 20 , the high-density nozzle head according to the present embodiment is achieved by arranging a plurality of ink chamber units  153  having the above-described structure in a lattice fashion based on a fixed arrangement pattern, in a row direction which coincides with the main scanning direction, and a column direction which is inclined at a fixed angle of w with respect to the main scanning direction, rather than being perpendicular to the main scanning direction. 
     More specifically, by adopting a structure in which a plurality of ink chamber units  153  are arranged at a uniform pitch d in line with a direction forming an angle of w with respect to the main scanning direction, the pitch PN of the nozzles projected so as to align in the main scanning direction is d×cos ψ, and hence the nozzles  151  can be regarded to be substantially equivalent to those arranged linearly at a fixed pitch PN along the main scanning direction. 
     In a full-line head comprising rows of nozzles that have a length corresponding to the entire width of the image recordable width, the “main scanning” is defined as printing one line (a line formed of a row of dots, or a line formed of a plurality of rows of dots) in the width direction of the recording paper (the direction perpendicular to the conveyance direction of the recording paper) by driving the nozzles in one of the following ways: (1) simultaneously driving all the nozzles; (2) sequentially driving the nozzles from one side toward the other; and (3) dividing the nozzles into blocks and sequentially driving the nozzles from one side toward the other in each of the blocks. 
     In particular, when the nozzles  151  arranged in a matrix such as that illustrated in  FIG. 20  are driven, the main scanning according to the above-described (3) is preferred. More specifically, the nozzles  151 - 11 ,  151 - 12 ,  151 - 13 ,  151 - 14 ,  151 - 15  and  151 - 16  are treated as a block (additionally; the nozzles  151 - 21 ,  151 - 22 , . . . ,  151 - 26  are treated as another block; the nozzles  151 - 31 ,  151 - 32 , . . . ,  151 - 36  are treated as another block; . . . ); and one line is printed in the width direction of the recording medium  114  by sequentially driving the nozzles  151 - 11 ,  151 - 12 , . . . ,  151 - 16  in accordance with the conveyance velocity of the recording medium  114 . 
     On the other hand, “sub-scanning” is defined as to repeatedly perform printing of one line (a line formed of a row of dots, or a line formed of a plurality of rows of dots) formed by the main scanning, while moving the full-line head and the recording paper relatively to each other. 
     The direction indicated by one line (or the lengthwise direction of a band-shaped region) recorded by the main scanning as described above is called the “main scanning direction”, and the direction in which sub-scanning is performed, is called the “sub-scanning direction”. In other words, in the present embodiment, the conveyance direction of the recording medium  114  is called the sub-scanning direction and the direction perpendicular to same is called the main scanning direction. 
     In implementing the present invention, the arrangement of the nozzles is not limited to that of the example illustrated. Moreover, a method is employed in the present embodiment where an ink droplet is ejected by means of the deformation of the actuator, which is typically a piezoelectric element; however, in implementing the present invention, the method used for discharging ink is not limited in particular, and instead of the piezo jet method, it is also possible to apply various types of methods, such as a thermal jet method where the ink is heated and bubbles are caused to form therein by means of a heat generating body such as a heater, ink droplets being ejected by means of the pressure applied by these bubbles. 
     Configuration of Ink Supply System 
       FIG. 21  is a schematic drawing illustrating the configuration of the ink supply system in the inkjet recording apparatus  110 . The ink tank  160  is a base tank that supplies ink to the head  150  and is set in the ink storing and loading unit  114  described with reference to  FIGS. 10A to 10C . In other words, the ink tank  160  in  FIG. 15  is equivalent to the ink storage and loading unit  114  in  FIGS. 10A to 10C . The aspects of the ink tank  160  include a refillable type and a cartridge type: when the remaining amount of ink is low, the ink tank  160  of the refillable type is filled with ink through a filling port (not illustrated) and the ink tank  160  of the cartridge type is replaced with a new one. In order to change the ink type in accordance with the intended application, the cartridge type is suitable, and it is desirable to represent the ink type information with a bar code or the like on the cartridge, and to perform ejection control in accordance with the ink type. 
     A filter  162  for removing foreign matters and bubbles is disposed between the ink tank  160  and the head  150  as illustrated in  FIG. 21 . The filter mesh size of the filter  162  is desirably equivalent to or less than the diameter of a nozzle. Although not illustrated in  FIG. 21 , it is desirable to provide a sub-tank integrally to the print head  150  or nearby the head  150 . The sub-tank has a damper function for preventing variation in the internal pressure of the head and a function for improving refilling of the print head. 
     The inkjet recording apparatus  110  is also provided with a cap  164  as a device to prevent the nozzles  151  from drying out or to prevent an increase in the ink viscosity in the vicinity of the nozzles  151 , and a cleaning blade  166  as a device to clean the nozzle face  150 A. A maintenance unit (restoration device) including the cap  164  and the cleaning blade  166  can be relatively moved with respect to the head  150  by a movement mechanism (not illustrated), and is moved from a predetermined holding position to a maintenance position below the head  150  as required. 
     The maintenance unit including the cap  164  and the cleaning blade  166  is equivalent to the maintenance unit  126 K,  126 C,  126 M and  126 Y of the maintenance processing unit  126  illustrated in  FIG. 16 . 
     The cap  164  illustrated in  FIG. 21  is displaced up and down relatively with respect to the head  150  by an elevator mechanism (not illustrated). When the power of the inkjet recording apparatus  110  is turned OFF or when in a print standby state, the cap  164  is raised to a predetermined elevated position so as to come into close contact with the head  150 , and the nozzle face  150 A is thereby covered with the cap  164 . 
     The cleaning blade  166  is composed of rubber or another elastic member, and can slide on the nozzle surface  150 A (surface of the nozzle plate) of the head  150  by means of a blade movement mechanism (not illustrated). When ink droplets or foreign matter has adhered to the surface of the nozzle plate, the nozzle surface is wiped by sliding the cleaning blade  166  on the nozzle plate. Alternatively, the following is also possible: the position of the cleaning blade  166  is fixed, and the wiping is performed by moving the head  150  to the maintenance position. 
     During printing or standby, a preliminary discharge (dummy ejection operation) is made to eject the degraded ink toward the cap  164  (which also serves as an ink receptacle) in order to discharge ink in nozzles, as appropriate. 
     After the nozzle surface is cleaned by a wiper such as the cleaning blade  166  provided as the cleaning device for the nozzle face  150 A, a preliminary discharge is also carried out in order to prevent the foreign matter from becoming mixed inside the nozzles  151  by the wiper sliding operation. 
     On the other hand, if air bubbles become intermixed into a nozzle  151  or a pressure chamber  152 , or if the rise in the viscosity of the ink inside a nozzle  151  exceeds a certain level, then it may not be possible to eject ink in the dummy ejection operation described above. In cases of this kind, the cap  164  forming a suction device is pressed against the nozzle surface  150 A of the print head  150 , and the ink inside the pressure chambers  152  (namely, the ink containing air bubbles of the ink of increased viscosity) is suctioned by a suction pump  167 . The ink suctioned and removed by means of this suction operation is sent to a recovery tank  168 . The ink collected in the recovery tank  168  may be used, or if reuse is not possible, it may be discarded. 
     Since the suctioning operation is performed with respect to all of the ink in the pressure chambers  152 , it consumes a large amount of ink, and therefore, desirably, restoration by preliminary ejection is carried out while the increase in the viscosity of the ink is still minor. The suction operation is also carried out when ink is loaded into the print head  150  for the first time, and when the head starts to be used after being idle for a long period of time. 
     Anti-Drying Liquid Supply System 
       FIG. 22  is a schematic drawing illustrating the composition of an anti-drying liquid supply system in the inkjet recording apparatus  110 . In  FIG. 22 , anti-drying liquid to be supplied to the head  150  is stored in a supply tank  92  which is the same as or similar to the composition illustrated in  FIG. 2 . The anti-drying liquid fed out from the supply tank  92  is heated to a prescribed temperature by the heater  94  and then supplied to the head  150 . By raising the temperature of the anti-drying liquid  40  which flows over the nozzle surface, the gasification (evaporation) of the anti-drying liquid  40  is promoted. Furthermore, the anti-drying liquid that has been recovered from the anti-drying liquid discharge port  20  of the head  150  is sent to a recovery tank  96  by driving the suction pump  34 . The liquid recovered into the recovery tank  96  is subjected to processing for removing dirt by means of a filter (not illustrated) and for readjusting the composition, and so on, whereupon the liquid is returned to the supply tank  92  and can then be reused. 
     Description of Control System 
       FIG. 23  is a block diagram illustrating a system composition of the inkjet recording apparatus  110 . As illustrated in  FIG. 23 , the inkjet recording apparatus  110  comprises a communications interface  170 , a system controller  172 , an image memory  174 , a ROM  175 , a motor driver  176 , a heater driver  178 , a print controller  180 , an image buffer memory  182 , a head driver  184 , and the like. 
     The communications interface  170  is an interface unit (image data input device) for receiving image data which is transmitted by a host computer  186 . For the communications interface  170 , a serial interface, such as USB (Universal Serial Bus), IEEE 1394, an Ethernet (registered tradename), or a wireless network, or the like, or a parallel interface, such as a Centronics interface, or the like, can be used. It is also possible to install a buffer memory (not illustrated) for achieving high-speed communications. 
     Image data sent from the host computer  186  is read into the image forming apparatus  110  via the communications interface  170 , and is stored temporarily in the image memory  174 . The image memory  174  is a storage device which stores an image input via the communications interface  170 , and data is read from and written to the image memory  174  via the system controller  172 . The image memory  174  is not limited to being a memory composed of a semiconductor element, and may also use a magnetic medium, such as a hard disk. 
     The system controller  172  is constituted by a central processing unit (CPU) and peripheral circuits thereof, and the like, and functions as a control apparatus which controls the whole of the inkjet recording apparatus  110  in accordance with prescribed programs, as well as functioning as a calculation apparatus which carries out various calculations. In other words, the system controller  172  controls the various units, such as the communications interface  170 , the image memory  174 , the motor driver  176 , the heater driver  178 , and the like, and controls communications with the host computer  186  as well as controlling the reading and writing of data to the image memory  174  and the ROM  175 , and furthermore, it also generates control signals for controlling the motor  188  of the conveyance system and the heater  189 . 
     The ROM  175  stores programs which are executed by the CPU of the system controller  172  and various data required for control purposes (including data of the ejection waveform for image formation and the ejection waveform for dummy ejection), and the like. The ROM  175  may be a non-rewritable storage device, or it may be a writable storage device, such as and EEPROM. The ROM  175  according to the present embodiment is constituted by a rewritable EEPROM and also serves as a history information storage device which stores operating history information for each of the heads of the respective heads, and ejection history information for each nozzle. 
     The image memory  174  is used as a temporary storage region for the image data, and it is also used as a program development region and a calculation work region for the CPU. 
     The motor driver (drive circuit)  176  drives the motor  188  of the conveyance system in accordance with commands from the system controller  172 . In  FIG. 23 , reference numeral  188  represents motors arranged in respective parts of the apparatus. The motor  188  includes the motor driving the pressure drum  112  illustrated in  FIG. 16 , the motor driving the paper feeding roller  130 , and other motors. 
     The heater driver (drive circuit)  178  drives the heater  189  of the post-drying unit  142  or the like in accordance with commands from the system controller  172 . In  FIG. 23 , reference numeral  189  represents heaters arranged in the inkjet recording apparatus  110 . The heater  189  in  FIG. 23  includes the heater of the solvent drying unit  120 , the heater  136  of the fixing processing unit  122 , the heater  94  functioning as a heating means for the anti-drying liquid illustrated in  FIG. 22 , and other heaters. 
     The print controller  180  has a signal processing function for performing various tasks, compensations, and other types of processing for generating print control signals from the image data (original image data) stored in the image memory  174  in accordance with control commands from the system controller  172  so as to supply the generated print data (dot data) to the head driver  184 . In the print controller  180 , required signal processing is performed, and the ejection volume and ejection timing of ink droplets of the head  150  are controlled via the head driver  184  on the basis of the image data. This control can realize a desired dot size and a desired dot arrangement. 
     The image buffer memory  182  is provided with the print controller  180 , and image data, parameters, and other data are temporarily stored in the image buffer memory  182  when image data is processed in the print controller  180 .  FIG. 23  illustrates a mode in which the image buffer memory  182  is attached to the print controller  180 ; however, the image memory  174  may also serve as the image buffer memory  182 . Also possible is a mode in which the print controller  180  and the system controller  172  are integrated to form a single processor. 
     To give a general description of the sequence of processing from image input to print output, image data to be printed is input from an external source via the communications interface  170 , and is accumulated in the image memory  174 . At this stage, RGB image data is stored in the image memory  174 , for example. 
     In this inkjet recording apparatus  110 , an image which appears to have a continuous tonal graduation to the human eye is formed by changing the droplet ejection density and the dot size of fine dots created by ink (coloring material), and therefore, it is necessary to convert the input digital image into a dot pattern which reproduces the tonal gradations of the image (namely, the light and shade toning of the image) as faithfully as possible. Therefore, original image data (RGB data) stored in the image memory  174  is sent to the print controller  180  through the system controller  172 , and is converted to the dot data for each ink color by a half-toning technique, using a threshold value matrix, error diffusion, or the like, in the print controller  180 . 
     In other words, the print controller  180  performs processing for converting the input RGB image data into dot data for the four colors of K, C, M and Y. The dot data generated by the print controller  180  in this way is stored in the image buffer memory  182 . 
     The head driver  184  outputs drive signals for driving the actuators  58  corresponding to the nozzles  151  of the head  150 , on the basis of print data (in other words, dot data stored in the image buffer memory  182 ) supplied by the print controller  180 . A feedback control system for maintaining constant drive conditions in the head may be included in the head driver  184 . 
     By supplying the drive signals output by the head driver  184  to the print heads  150 , ink is ejected from the corresponding nozzles  151 . By controlling ink ejection from the print head  150  while controlling the conveyance speed of the recording medium  114  so as to be a prescribed speed, an image is formed on the recording medium  114 . 
     Furthermore, the system controller  172  functions as a device which controls the negative pressure suctioning of the recording medium  114  by the pressure drum  112  in  FIG. 16 , and when the recording medium  114  is received onto the pressure drum  112 , a command signal is sent to the suctioning apparatus so as to generate a negative pressure in the suction ports which are provided on the circumferential surface of the pressure drum  112 . 
     Moreover, the system controller  172  also functions as a device which controls the nip pressure of the fixing process unit  122 . When type information relating to a recording medium  114  is acquired, the system controller controls the clearance between the heating roller  138  of the fixing process unit  122  and the supporting roller  140  so as to achieve a nip pressure corresponding to the recording medium  114  that is being processed. 
     Furthermore, the system controller  172  sends command signals to the respective sections of the apparatus on the basis of determination signals obtained from the sensor  192 . The sensor  192  in  FIG. 23  includes a paper supply sensor which is provided in a receiving portion for recording media  114  of the pressure drum  112  in  FIG. 16 , a temperature sensor which determines the surface temperature of the pressure drum  112 , a temperature sensor which is provided in the solvent drying unit  120 , a temperature sensor which is provided in the fixing process unit  122 , and a temperature sensor which is provided in order to control the heater  94  in  FIG. 22 , and the like. 
     Data of the image capture results for the recorded image is input to the print controller  180  in  FIG. 23 , from the print determination unit (in-line sensor)  194  which is disposed in the output unit  124  in  FIG. 16 . The print determination unit  194  has an image sensor for capturing the ink droplet deposition result by the print unit  118 , and functions as a device to check for ejection abnormalities, such as blocking of nozzles from the droplet ejection image read in by the image sensor. 
     More specifically, the print determination unit  194  reads an image (test pattern) printed on the recording medium  114 , determines the print conditions (presence of the ejection, variation in the dot formation, and the like) by performing required signal processing, or the like, and provides the determination results of the print conditions to the print controller  180 . 
     The print controller  180  implements various corrections with respect to the head  150 , on the basis of the information obtained from the print determination unit  194 , according to requirements, and it implements control for carrying out cleaning operations (nozzle restoring operations), such as preliminary ejection, suctioning, or wiping, as and when necessary. 
     For example, whenever an ejection defect is detected in the head  150  by the print determination unit  194 , then the print controller  180  implements control in such a manner that preliminary ejection is carried out automatically. Alternatively, it is possible to adopt a mode in which, whenever an ejection defect of the head  150  has been determined by the print determination unit  194 , control is implemented in such a manner that preliminary ejection is carried out automatically only in the head ( 118 C,  118 M,  118 Y and  118 K) where the ejection defect has been determined, or only in the nozzle row or the particular nozzle which is suffering an ejection defect in that head. 
     Furthermore, the system controller  172  in  FIG. 23  functions as a control device which controls the operation of the pressurization pump  32  provided in the anti-drying liquid supply section and the operation of the suction pump  34  provided in the discharge section, and the system controller  172  outputs control signals to the drive circuits  197 ,  198  of the respective pumps. 
     The system controller  172  drives the pressurization pump  32  and the suction pump  34  in such a manner that anti-drying liquid flows continuously over the nozzle surface during printing. Furthermore, the pressurization pump  32  and the suction pump  34  are also operated during printing standby, as necessary, so as to perform humidification. 
     In the embodiments described above, an inkjet recording apparatus using a method of forming an image by ejecting ink droplets directly onto a recording medium  114  (direct recording method) is described, but the range of application of the present invention is not limited to this, and it is also possible to apply the present invention to an image forming apparatus of an intermediate transfer type which once forms an image (primary image) on an intermediate transfer body and then transfers that image onto a recording paper in a transfer unit, thereby forming an image finally. 
     In the embodiments described above, an inkjet recording apparatus using a page-wide full line type head having a nozzle row of a length corresponding to the entire width of the recording medium is described (an image forming apparatus with a single-path system that finishes an image by one sub-scanning action), but the scope of application of the present invention is not limited to this, and the present invention may also be applied to an inkjet recording apparatus which performs image recording by means of a plurality of head scanning actions which move a short recording head, such as a serial head (shuttle scanning head), or the like. 
     Furthermore, the meaning of the term “image forming apparatus” is not restricted to a so-called graphic printing application for printing photographic prints or posters, but rather also encompasses industrial apparatuses which are able to form patterns that may be perceived as images, such as resist printing apparatuses, wire printing apparatuses for electronic circuit substrates, ultra-fine structure forming apparatuses, or the like. 
     APPENDIX 
     As has become evident from the detailed description of the embodiments of the present invention given above, the present specification includes disclosure of various technical ideas including the invention described below. 
     One aspect of the present invention is directed to a liquid ejection head comprising: a nozzle plate having a nozzle surface in which at least one nozzle for ejecting droplets of a liquid are formed; an anti-drying liquid supply port which supplies an anti-drying liquid to the nozzle surface of the nozzle plate; a flow channel portion which is formed in the nozzle surface and through which the anti-drying liquid supplied to the nozzle surface from the anti-drying liquid supply port flows; and an anti-drying liquid discharge port which suctions and discharges the anti-drying liquid flowing through the flow channel portion on the nozzle surface, from the nozzle surface, wherein, while the anti-drying liquid flows through the flow channel portion, the anti-drying liquid evaporates to increase humidity. 
     Desirably, the flow channel portion is a lyophilic region formed in the nozzle surface. 
     According to this aspect of the invention, the lyophilic region becomes the region where the anti-drying liquid flows on the nozzle surface, which can prevent the anti-drying liquid from flowing into the nozzle. 
     Desirably, the flow channel portion is a groove formed in the nozzle surface. 
     According to this aspect of the invention, the groove becomes the region where the anti-drying liquid flows on the nozzle surface, which can prevent the anti-drying liquid from flowing into the nozzle. In cases of the groove having lyophilic properties, the holding force of the liquid can be improved further. 
     Desirably, the nozzle surface is inclined with respect to a horizontal direction in such a manner that the anti-drying liquid flows downward on the nozzle surface in accordance with an inclination of the nozzle surface. 
     According to this aspect of the invention, the liquid smoothly flows without staying on the nozzle surface. 
     Desirably, the liquid ejection head further comprises: a supply channel forming member which includes the anti-drying liquid supply port and a supply channel connected to the anti-drying liquid supply port; and a discharge channel forming member which includes the anti-drying liquid discharge port and a discharge channel connected to the anti-drying liquid discharge port, wherein the supply channel forming member and the discharge channel forming member are situated across the nozzle plate. 
     According to this aspect of the invention, it is possible to easily provide the anti-drying liquid supply port and the anti-drying liquid discharge port even in cases of high density head. 
     Desirably, the liquid ejection head further comprises a suction pump which is connected to the anti-drying liquid discharge port for suctioning the anti-drying liquid on the nozzle surface. 
     By suctioning and discharging the anti-drying liquid forcibly with the suction pump, it is possible to promote the flow of the anti-drying liquid on the nozzle surface. 
     Desirably, the liquid ejection head further comprises a pressurization pump which is connected to the anti-drying liquid supply port for sending the anti-drying liquid onto the nozzle surface. 
     In supplying the anti-drying liquid, it is desirable to adjust the pressurization so that the anti-drying liquid may not drip from the nozzle surface. 
     Another aspect of the present invention is directed to an image forming apparatus comprising one of the above-described liquid ejection heads. 
     According to image forming apparatus of embodiments of the present invention, nozzle blockages are prevented and stable image formation is possible. 
     The inkjet recording apparatus which is one mode of the image forming apparatus of the present invention comprises: a liquid ejection head (recording head) in which a plurality of liquid droplet ejection elements (ink liquid chamber units) are arranged at high density, each liquid droplet ejection element comprising a nozzle (ejection port) for ejecting an ink droplet in order to form a dot and a pressure generating device (piezoelectric element or heating element for heating and bubble generation) which generates an ejection pressure; and an ejection control device which controls the ejection of liquid droplets from the liquid ejection head on the basis of ink ejection data (dot image data) generated from the input image. An image is formed on a recording medium by means of the liquid droplets ejected from the nozzles. 
     For example, color conversion and halftone processing are carried out on the basis of the image data (print data) input via the image input device, and ink ejection data corresponding to the ink colors is generated. The driving of the pressure generating elements corresponding to the respective nozzles of the liquid ejection head is controlled on the basis of this ink ejection data, and ink droplets are ejected from the nozzles. 
     In order to achieve high-resolution image output, a desirable mode is one using a recording head in which a large number of liquid droplet ejection elements (ink chamber units) are arranged at high density, each liquid droplet ejection element comprising a nozzle (ejection port) which ejects ink liquid, a pressure chamber corresponding to the nozzle, and a pressure generating device. 
     A compositional example of a recording head based on an inkjet method of this kind is a full line type head having a nozzle row in which a plurality of ejection ports (nozzles) are arranged through a length corresponding to the full width of the recording medium. In this case, a mode may be adopted in which a plurality of relatively short ejection head modules having nozzles rows which do not reach a length corresponding to the full width of the recording medium are combined and joined together, thereby forming nozzle rows of a length that correspond to the full width of the recording medium. 
     A full line type head is usually disposed in a direction that is perpendicular to the relative feed direction (relative conveyance direction) of a recording medium, but a mode may also be adopted in which the head is disposed following an oblique direction that forms a prescribed angle with respect to the direction perpendicular to the conveyance direction. 
     The “recording medium” is a medium receiving an ink deposition ejected from an ejection opening of a recording head (which may also be called a print medium, an image forming medium, a recording medium, an image receiving medium, or an ejection receiving medium, or the like. There are no particular restrictions on the shape or material of the recording medium, which may be various types of media, irrespective of material and size, such as sheet paper (cut paper), sealed paper, continuous paper, resin sheets such as OHP sheets, film, cloth, a printed circuit substrate on which a wiring pattern, or the like, is formed, a rubber sheet, an intermediate transfer medium, a metal sheet, or the like. 
     The conveyance device for causing a recording medium and a recording head to move relative to each other may include a mode where the recording medium is conveyed with respect to a stationary (fixed) head, or a mode where a head is moved with respect to a stationary recording medium, or a mode where both the head and the recording medium are moved. When forming color images by means of an inkjet print head, it is possible to provide print heads for the respective colors of a plurality of colored inks (recording liquids), or it is possible to eject inks of a plurality of colors, from one recording head. 
     Desirably, the liquid ejected from the at least one nozzle is an ink composition containing pigment; and the anti-drying liquid is a liquid containing a solvent having a solubility parameter of 27.5 or less, the solvent being 50 percent by mass of an entire solvent. 
     According to this aspect of the invention, the anti-drying liquid also serves as a cleaning liquid, and therefore a special cleaning liquid is not required. 
     Desirably, a portion other than the flow channel portion of the nozzle surface has a liquid repellent property. 
     Desirably, the anti-drying liquid supply port and the anti-drying liquid discharge port are formed in the nozzle plate. 
     Desirably, the flow channel portion has a meandering shape in the nozzle surface. 
     Desirably, the flow channel portion is formed in a direction of a long side of the nozzle plate. 
     Desirably, the flow channel portion is formed in a direction of a short side of the nozzle plate. 
     Desirably, the flow channel portion includes a plurality of flow channels which are connected to the anti-drying liquid supply port. 
     Desirably, the flow channel portion includes a plurality of flow channels which are connected to the anti-drying liquid discharge port. 
     Desirably, a nozzle row formed by the nozzles are formed in the nozzle surface, and the flow channel portion is formed along the nozzle row. 
     Desirably, the flow channel portion has a cross section having an inverted trapezoidal shape. 
     Desirably, the anti-drying liquid has a higher temperature than the nozzle plate. 
     It should be understood that there is no intention to limit the invention to the specific forms disclosed, but on the contrary, the invention is to cover all modifications, alternate constructions and equivalents falling within the spirit and scope of the invention as expressed in the appended claims.