Patent Publication Number: US-7896462-B2

Title: Cleaning method and cleaning unit of ink ejection section, and image forming apparatus

Description:
Priority is claimed to Japanese Patent Application No. 2008-042635 filed on Feb. 25, 2008, the disclosure of which is incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a cleaning method and a cleaning unit of an ink ejection section in an image forming apparatus such as an inkjet color printer, and to an image forming apparatus. 
     2. Description of Related Art 
     In image forming apparatuses, there are apparatuses of the type which forms an image on a paper by ejecting an ink onto a paper as a recording medium, such as inkjet color printers. These image forming apparatuses are provided with an image formation section capable of forming an image, a paper storage section to store a paper whose surface is used for image formation, a paper transport section to transport a paper to the image formation section, and a paper discharge section capable of discharging a paper with an image formed on the surface thereof. In these image forming apparatuses, the paper stored in the paper storage section is transported to the image formation section by the paper transport section, and the image based on image information is formed on the paper. The paper after being subjected to the image formation by the image formation section is then discharged into the paper discharge section. 
     Specifically, the image formation section has an ink ejection section capable of ejecting an ink onto a paper, and a cleaning unit to clean the ink ejection section as shown in, for example, Japanese Unexamined Patent Application Publication No. 2002-361879. During an image forming operation, the ink is ejected onto a paper from the ink ejection section, and the ink ejection section is cleaned by the cleaning unit. This cleaning unit is provided with a cleaning blade formed by an elastic material, and a wet porous elastic body. 
     In the cleaning unit as described above, after the ink of the ink ejection section is absorbed by the porous elastic body, the surface of the ink ejection section is wiped off by the cleaning blade, that is, the moisture and the like at the ink ejection section are wiped off. However, if the ink cannot be completely absorbed by the porous elastic body, cleaning efficiency becomes poor, and the water repellency of the ink ejection section is deteriorated by the adhered and dried ink at the ink ejection section. Thus, the deteriorated water repellency of the ink ejection section leads to deterioration of image quality. 
     Also in a state in which the resin composition contained in the ink is adhered to the ink ejection section, image quality might also be deteriorated by the difficulty in controlling the ink ejection. Particularly in an elongated line head, because the amount of ink adhesion is larger than that of a serial head, there is a large possibility that image quality may be deteriorated due to insufficient ink removal. That is, the ink adheres to the ink ejection section and then dries there, so that the resin composition is solidified around an ink ejection hole provided at the ink ejection section. When the ink is ejected in this state, for example, the solidified resin composition and the ejected ink contact with each other, and the normal ink ejection may be hindered to cause image quality deterioration. 
     SUMMARY OF THE INVENTION 
     An advantage of the present invention is to provide a cleaning method and a cleaning unit of an ink ejection section, and an image forming apparatus which are capable of efficiently removing the ink adhered to the ink ejection section and improving poor ink ejection. 
     The present invention is a method of cleaning an ink ejection section in an image forming apparatus that forms an image by ejecting an ink from the ink ejection section. The method includes an ink wiping step for wiping off the ink on the surface of the ink ejection section, a first cleaning step for cleaning the surface of the ink ejection section with a water-soluble organic solvent, a second cleaning step for cleaning with water the surface of the ink ejection section cleaned with the water-soluble organic solvent, and a liquid wiping step for wiping off the liquid on the surface of the ink ejection section cleaned through the second cleaning step. 
     Preferably, the water-soluble organic solvent has a solubility parameter (SP value) of 7 to 14. The term “solubility parameter” (unit: (cal/cm 3 ) 1/2 ) is a numerical value indicating the polarity of a material. A smaller SP value difference between a solute and a solvent produces a larger solubility. 
     According to the present invention, the ink solidified at the ink ejection section can be efficiently removed to enable improvement of poor ink ejection by wiping off the ink on the surface of the ink ejection section before the cleaning step, and further cleaning the surface of the ink ejection section with the water-soluble organic solvent. Additionally, the water repellency of the surface of the ink ejection section can be recovered to reduce image quality deterioration by cleaning and removing with water the water-soluble organic solvent remaining on the surface of the ink ejection section. 
     A cleaning unit of the present invention suitable for practicing the above cleaning method is shiftably mounted in an image forming apparatus provided with an ink ejection section having an ink ejection surface capable of ejecting an ink. The cleaning unit includes a first blade member for wiping off an ink contactable with the ink ejection surface, a first porous member impregnated with a water-soluble organic solvent, a second porous member impregnated with water, and a second blade member for wiping off liquid contactable with the ink ejection surface. The first blade member, the first porous member, the second porous member and the second blade member are arranged in the order named along the shift direction of the cleaning unit. 
     An image forming apparatus of the invention includes at least an image formation section provided with an ink ejection section having an ink ejection surface capable of ejecting an ink, and further includes the above cleaning unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic front view of an image forming apparatus according to the invention; 
         FIG. 2  is an explanatory drawing showing the up and down movement of a transport belt by a lift unit; 
         FIG. 3  is a sectional view of a cleaning unit according to a first preferred embodiment of the invention; 
         FIG. 4  is a plan view of the cleaning unit in  FIG. 3 ; 
         FIG. 5  is a diagram showing the configuration of the shift mechanism of the cleaning unit; 
         FIG. 6  is a sectional view of a cleaning unit according to a second preferred embodiment of the invention; and 
         FIG. 7  is a plan view of the cleaning unit in  FIG. 6 . 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     First Preferred Embodiment 
     &lt;Image Forming Apparatus&gt; 
     An inkjet printer  1  as an image forming apparatus according to the invention is shown in  FIG. 1 .  FIG. 1  is a schematic front view schematically showing the configuration of the inkjet printer  1 . The inkjet printer  1  is an apparatus which is connected to an external computer or the like (not shown), and capable of forming an image based on image information transmitted from the computer, and which is provided with an image formation section  2 , a paper storage section  3 , a paper transport section  4 , a lift unit  7  and a paper discharge section  5 . 
     The image formation section  2  forms an image based on image information, and has inkjet heads  21 . That is, the four inkjet heads  21  are arranged side by side above a later-described transport belt  42  of the paper transport section  4 , and they eject inks onto a paper based on the image information. These inkjet heads  21  store different color inks and have ink ejection sections  211  (refer to  FIG. 3 ) capable of ejecting the inks stored therein, respectively. These inkjet heads  21  are substantially rectangular parallelepiped-shaped members extending in one direction (a direction orthogonal to the paper surface in  FIG. 1 ). Each of these ink ejection sections  211  has an ink ejection surface  212  (refer to  FIG. 3 ) arranged oppositely to the paper, and is provided with a plurality of ink ejection holes arranged side by side. The ink ejection surface  212  has a rectangular shape extending long in the back-and-forth direction. The term “back-and-forth direction” means a direction perpendicular to the paper surface in  FIG. 1 , and the near side and the rear side in the direction perpendicular to the paper surface in  FIG. 1  are referred to as “the front” and “the rear,” respectively. 
     The paper storage section  3  is capable of storing papers for image formation and arranged at a lower part of the inkjet printer  1 . The paper storage section  3  has a paper supply roller  31  for supplying papers to the paper transport section  4 , above the tip end on the paper supply side. 
     The paper transport section  4  transports the paper in the paper storage section  3  to the image formation section  2 , and also transports the paper with an image formed on the surface thereof to the discharge section  5 . The paper transport section  4  is provided with a plurality of rollers  41  for transporting a paper, and the transport belt  42 . The transport belt  42  is an endless belt underlying the inkjet heads  21 , and has on both ends thereof rollers  44  and  45  for circulating the transport belt  42 . 
     The lift unit  7  underlies the transport belt  42  of the paper transport section  4  and vertically lifts the transport belt  42 . The lift unit  7  has a pair of eccentric cams  71  and  72 . Specifically, the first eccentric cam  71  positioned leftward in  FIG. 1  is provided rotatably around a shaft  73 , and rotationally driven by a motor (not shown). The first eccentric cam  71  is provided with a plurality of bearings  74 , and supports a belt support member  43  through these bearings  74 . In  FIG. 1 , the reference numeral is applied only one of these bearings  74 , with others omitted. The second eccentric cam  72  positioned rightward in  FIG. 1  has the same structure as the first eccentric cam  71 , and has a symmetrical shape with respect to the first eccentric cam  71  in the left-to-right direction in  FIG. 6 . 
       FIG. 1  shows the state in which the transport belt  42  is ascended. From this state, the transport belt  42  is descended by the inward rotation of the pair of the eccentric cams  71  and  72  (refer to  FIG. 2 ). When the image formation section  2  forms an image on a paper, the lift unit  7  provides a suitable space for printing (approximately 1 mm in the present preferred embodiment) between the inkjet heads  21  and the paper by bringing the transport belt  42  into the ascent state shown in  FIG. 1 . When removing a jam occurred on the transport belt  42 , and when later-described cleaning units  22  clean the inkjet heads  21 , the lift unit  7  provides a wide space between the inkjet heads  21  and the paper by bringing the transport belt  42  into the descent state shown in  FIG. 2 . 
     The discharge section  5  discharges the paper with the image formed thereon by the image formation section  2 , and is arranged at the upper part of the inkjet printer  1 . The discharge section  5  is provided with a plurality of rollers  51 . The paper with the image formed thereon by the image formation section  2  is transported by the transport belt  42  of the paper transport section  4  and these rollers  51  of the discharge section  5 , and then discharged outside of the apparatus from a discharge port  52 . 
     &lt;Cleaning Unit Configuration&gt; 
       FIGS. 3 and 4  are the overall schematic diagrams of the cleaning unit  22 .  FIG. 3  is a sectional view of the cleaning unit  22 , and  FIG. 4  is a plan view thereof. The cleaning units  22  for cleaning the ink ejection surfaces  212  are shiftable in the longitudinal direction of the inkjet heads  21  (refer to an arrow A 1  in  FIGS. 3 and 5 ). During image formation, the cleaning units  22  are retreated outside of the longitudinal ends of the inkjet heads  21 , so as not to hinder the ink ejection. These cleaning units  22  are provided correspondingly to the individual inkjet heads  21 , and are connected to an exhaust ink tank  226   k . Each of these cleaning units  22  has a casing  226  provided with a first blade member  221 , a cleaning liquid roller  222 , a water roller  223 , a second blade member  224  and restriction plates  225   a  and  225   b , each of which is shifted in the direction of the arrow A 1  along the ink ejection surface  212  by a shift mechanism  8  shown in  FIG. 5 . 
     The first blade member  221  wipes off the ink on the ink ejection surface  212 , which is a plate-shaped member extending in one direction (a direction orthogonal to the paper surface in  FIG. 3 , namely, the transverse direction of the ink ejection surface  212 ). The length of the first blade member  221  in the direction orthogonal to the paper surface in  FIG. 3  is substantially the same as the transverse length of the ink ejection surface  212 . The first blade member  221  is arranged on the tip end in the shift direction of the casing  226 , and the upper tip end thereof is arranged at a slightly higher position than the ink ejection surface  212 . The first blade member  221  is formed by an elastic deformable material, such as EPDM or fluoro rubber, and arranged along the transverse direction of the ink ejection surface  212 . 
     The cleaning liquid roller  222  cleans the ink ejection surface  212  with a water-soluble organic solvent described later, which is arranged adjacent to the first blade member  221 , namely, arranged next to the first blade member  221  in the shift direction of the cashing  226 . The length of the cleaning liquid roller  222  in the direction orthogonal to the paper surface in  FIG. 3  (namely in the axial direction of the roller  222 ) is substantially the same as the transverse length of the ink ejection surface  212 . The cleaning liquid roller  222  also has a cleaning liquid roller rotary shaft  222   a  and a first porous member  222   b . The cleaning liquid roller rotary shaft  222   a  is a cylindrical hollow shaft member and extends in a direction orthogonal to the paper surface in  FIG. 1 . The cleaning liquid roller rotary shaft  222   a  is capable of containing therein the water-soluble organic solvent, and connected to a cleaning liquid tank  222   d  containing the water-soluble organic solvent. The cleaning liquid roller rotary shaft  222   a  also has an axially extending cleaning liquid admitting slit  222   c  (except for both ends of the rotary shaft), enabling the cleaning liquid therein to be supplied to the first porous member  222   b . Instead of the cleaning liquid admitting slit  222   c , a plurality of fine holes may be provided at the cleaning liquid roller rotary shaft  222   a . The cleaning liquid roller rotary shaft  222   a  is fixed to the casing  226 . The first porous member  222   b  is a porous member, such as a sponge, arranged around the cleaning liquid roller rotary shaft  222   a . The first porous member  222   b  is arranged so that the uppermost position in the height position thereof is substantially the same as the height position of the tip end of the first blade member  221 . The first porous member  222   b  is rotatable with respect to the cleaning liquid roller rotary shaft  222   a , and rotates counterclockwise in  FIG. 1  upon the shift of the cleaning unit  22  so as to bring the first porous member  222   b  into contact with the ink ejection surface  212 . Alternatively, the cleaning liquid admitting slit  222   c  may be replaced with a plurality of through-holes. 
     The water roller  223  cleans with water the ink ejection surface  212  after being cleaned with the water-soluble organic solvent, which is arranged next to the cleaning liquid roller  222  along the shift direction of the casing  226 . The length of the water roller  223  in the direction orthogonal to the paper surface in  FIG. 3  is substantially the same as the transverse length of the ink ejection surface  212 . The water roller  223  has a water roller rotary shaft  223   a  and a second porous member  223   b . The water roller rotary shaft  223   a  is a cylindrical hollow shaft member and capable of containing therein water. The water roller rotary shaft  223   a  has a water admitting slit  223   c , enabling the water therein to be supplied to the second porous member  223   b . The water roller rotary shaft  223   a  is the member extending in the direction orthogonal to the paper surface, which is fixed to the casing  226  and connected to the water tank  223   d  shown in  FIG. 4 . The second porous member  223   b  is a porous member, such as a sponge, arranged around the water roller rotary shaft  223   a , and is rotatable with respect to the water roller rotary shaft  223   a . The second porous member  223   b  is arranged so that the uppermost position in the height position thereof is substantially the same as the height position of the upper tip end of the first blade member  221 . The second porous member  223   b  rotates counterclockwise in  FIG. 3  upon the shift of the cleaning unit  22  so as to bring the second porous member  223   b  into contact with the ink ejection surface  212 . 
     Alternatively, the water admitting slit  223   c  may be replaced with a plurality of through-holes arranged at equal intervals. 
     The second blade member  224  wipes off liquid such as an ink, water and a cleaning liquid on the ink ejection surface  212 , which is a plate-shaped member extending in one direction (a direction orthogonal to the paper surface in  FIG. 3 ). The length of the second blade member  224  in the direction orthogonal to the paper surface is substantially the same as the transverse length of the ink ejection surface  212 . The second blade member  224  is arranged at the rear end in the shift direction of the casing  226 , and the upper tip end thereof is arranged at a slightly higher position than the ink ejection surface  212 . The second blade member  224  is formed by an elastic deformable material, such as EPDM or fluoro rubber. 
     The restriction plate  225   a  is in contact with a part of the first porous member  222   b  of the cleaning liquid roller  222 , and the restriction plate  225   b  is in contact with a part of the second porous member  22   eb  of the water roller  223 . The first porous member  222   b  and the second porous member  223   b  are compressed upon contact with the restriction plates  225   a  and  225   b , respectively. This enables the squeeze of the liquid absorbed by the first porous member  222   b  and the second porous member  223   b . Thus, upon rotation of the first porous member  222   b  and the second porous member  223   b , the parts of the first porous member  222   b  and the second porous member  223   b  which are in contact with the ink ejection surface  212  are shifted and brought into contact with the restriction plates  225   a  and  225   b , so that the water and the cleaning liquid each containing the ink can be squeezed out of the first porous member  222   b  and the second porous member  223   b , respectively. As shown in  FIG. 1 , the restriction plates  225   a  and  225   b  are columnar members having a trapezoidal cross section, and the upper area is smaller than the lower area. 
     The casing  226  supports the first blade member  221 , the cleaning liquid roller  222 , the water roller  223 , the second blade member  224  and the restriction plates  225   a  and  225   b , and has an arrangement section  226   a  to arrange the first blade member  221 , the cleaning liquid roller  222  and the like, and an exhaust liquid storage section  226   b  to collect the exhaust liquid. The arrangement section  226   a  has a bottom surface  226   c , a first blade member sidewall surface  226   d , a second blade member sidewall surface  226   e , a first sidewall  226   f  and a second sidewall  226   g . The bottom surface  226   c  is a plate-shaped member underlying the first blade member  221 , the second blade member  224 , the cleaning liquid roller  222  and the water roller  223 , and has holes h 1  to h 4  to permit passage of liquid arranged at positions corresponding to the first blade member  221 , the cleaning liquid roller  222 , the water roller  223  and the second blade member  224 , respectively. 
     The first blade member sidewall surface  226   d  is arranged at the front in the shift direction of the first blade member  221 , namely on the right in  FIG. 3 , and provided with a first fixing member  226   i  for fixing the first blade member  221 . The upper height position of the first fixing member  226   i  is lower than the upper height position of the first blade member sidewall surface  226   d , making it difficult for the ink wiped off by the first blade member  221  to overflow the casing  226 . The second blade member sidewall surface  226   e  is arranged at the rear in the shift direction of the second blade member  224 , namely on the left in  FIG. 3 , and provided with a second fixing member  226   j  for fixing the second blade member  224 . The upper height position of the second fixing member  226   j  is lower than the upper height position of the second blade member sidewall surface  226   e , making it difficult for the ink wiped off by the second blade member  224  to overflow the casing  226 . 
     The first sidewall  226   f  is arranged so as to connect one end of the first blade member sidewall surface  226   d  and one end of the second blade member sidewall surface  226   e , and provided with a hole to permit connection of a pipe for supplying the cleaning liquid from the cleaning liquid tank  222   d  to the cleaning liquid roller  222 , and a hole to permit connection of a pipe for supplying the water from the water tank  223   d  to the water roller  223 . The second sidewall  226   g  is opposed to the first sidewall  226   f , and cooperates with the first sidewall  226   f  to support the water roller rotary shaft  223   a  and the cleaning liquid roller rotary shaft  222   a . The exhaust liquid storage section  226   b  underlies the arrangement section  226   a  and temporarily stores the water, the ink, the cleaning liquid and the like after passing through the holes h 1  to h 4  arranged at the bottom surface  216   c . The exhaust liquid storage section  226   b  is connected to the exhaust ink tank  226   k . The water tank  223   d , the cleaning liquid tank  222   d  and the exhaust ink tank  226   k  are detachably mounted on the surface of a later-described shift support member  85  opposite the cleaning unit  22 . 
     The shift mechanism  8  shown in  FIG. 5  shifts the cleaning unit  22  in the longitudinal direction of the inkjet heads  21 .  FIG. 5  is a schematic plan view schematically showing the configuration of the shift mechanism  8 . The shift mechanism  8  is provided with a pair of rail members  81  and  82 , a pair of sliders  83  and  84 , the shift support member  85 , a plurality of pulleys  86   a  to  86   e , a drive motor  87  and a frame body (not shown) to support these members. The pair of the rail members  81  and  82  have a slender shape extending back and forth, and are spaced laterally and arranged parallel to each other. The pair of the sliders  83  and  84  are slidably provided along the rail members  81  and  82 , respectively. 
     The shift support member  85  is formed by bending a plate material, and extends between the pair of the rail members  81  and  82 . The shift support member  85  is fixed to the pair of the sliders  83  and  84 , and shifts back and forth (in the direction indicated by the arrow A 1  or A 2 ) by the sliders  83  and  84  that shift along the rail members  81  and  82 . A plurality of the cleaning units  22  are fixed to the front portion of the upper surface of the shift support member  85 . A plurality of cap members  89  corresponding to the ink ejection surfaces  212  of the inkjet heads  21  are provided at the rear of the cleaning unit  22 . In  FIG. 5 , the reference numeral is applied only one of these cap members  89 , with others omitted. In the standby state, these cap members  89  cover the ink ejection surfaces  212 , thereby preventing the ink from being dried and deteriorated. 
     Specifically, the plurality of pulleys  86   a  to  86   e  are the first pulley  86   a  to the fifth pulley  86   e , each of which is provided rotatably. The first pulley  86   a  and the second pulley  86   b  are arranged on the front and the rear of the rail member  81  with the rail member  81  interposed therebetween, respectively. A belt  88   a , one end of which is fixed to the front end of the slider  83  and the other end is fixed to the rear end of the slider  83 , is entrained around the first pulley  86   a  and the second pulley  86   b . The third pulley  86   c  and the fourth pulley  86   d  are arranged on the front and the rear of the rail member  82  with the rail member  82  interposed therebetween, respectively. A belt  88   b , one end of which is fixed to the front end of the slider  84  and the other end is fixed to the rear end of the slider  84 , is entrained around the third pulley  86   c  and the fourth pulley  86   d . The third pulley  86   c  is rotationally driven by the drive motor  87 . The fifth pulley  86   e  rotates in the opposite direction to the third pulley  86   c  upon the rotation transmission from the drive motor  87  through a gear (not shown) rotationally driven by the drive motor  87 . An endless belt  88   c  is entrained around the fifth pulley  86   e  and the first pulley  86   a.    
     In the shift mechanism  8 , when the third pulley  86   c  is rotationally driven counterclockwise in  FIG. 5  by the drive motor  87 , the belt  88   b  is driven and the slider  84  is shifted forward. At the same time, the fifth pulley  86   e  is driven clockwise, so that the endless belt  88   c  and the belt  88   a  are driven and the slider  83  is shifted forward. Thus, the cleaning unit  22  shifts forward together with the shift support member  85  (refer to the arrow A 1 ). When the drive motor  87  is driven in a direction opposite to the above-mentioned direction, the shift support member  85  and the cleaning unit  22  shift rearward (refer to the arrow A 2 ). 
     &lt;Ink&gt; 
     Any one of conventionally used inkjet printer inks is applicable to the inkjet printer  1 . These inks are composed mainly of a coloring agent (mostly a pigment), a resin and a solvent, and also contain various types of additives such as dehydrating agent and antioxidant when necessary. Although no limit is imposed on the resin, there are, for example, polymers and copolymers such as polystyrene, acryl resin, polyester, polyethylene and polyamide, low molecular weight polyethylene and polypropylene. The solubility parameter (hereinafter referred to as an “SP value”) of these resins is normally in the range of 7 to 14, without being limited thereto. In general, 1 to 20% by weight of a resin is contained with respect to the total amount of an ink. 
     Usually, water alone or an aqueous medium obtained by adding an aqueous organic solvent into water is used as a solvent. Alternatively, surfactant, antiseptic and fungicide may be contained in the ink. 
     &lt;Water-Soluble Organic Solvent&gt; 
     The water-soluble organic solvent used as a cleaning liquid is preferably compatible with a resin. Examples thereof include alcohols such as ethanol, isopropyl alcohol, n-hexanol, 1,3-butandiol, hexylene glycol, ethylene glycol, triethylene glycol monobutyl ether, 2-pyrrolidone and glycerin, and ethers. 
     It is particularly preferable to use a water-soluble organic solvent having an SP value that approximates the SP value of the resin contained in an ink, in order to dissolve the solidified resin. Specifically, because the resin having an SP value of 7 to 14 is generally used in an ink, the SP value of the water-soluble organic solvent is preferably not more than 14, more preferably not more than 12. Further, the water-soluble organic solvent having a smaller SP value difference from the resin exhibits better dissolution. Therefore, the SP value difference between the resin and the water-soluble organic solvent is preferably not exceeding 0.5. Examples of suitable water-soluble organic solvents include ethanol (12.4), isopropyl alcohol (11.0), n-hexanol (10.2), 1,3-butandiol (13.9), hexylene glycol (11.8), triethylene glycol monobutyl ether (8.5) and 2-pyrrolidone (13.8), where the values in parentheses are the SP values of these organic solvents, respectively. 
     The use of the cleaning agent, namely, the above water-soluble organic solvents enables the removal of the resin composition of the ink adhering to the ink ejection sections  211  of the inkjet heads  21 , thereby improving poor ink ejection. 
     &lt;Cleaning Method&gt; 
     When each of the ink ejection sections  211  of the inkjet heads  21  is cleaned, the cleaning unit  22  shifts in the longitudinal direction of the ink ejection surface  212  by the shift mechanism  8 , from the state where the first blade member  221  is in contact with one end of the longitudinal direction of the ink ejection surface  212 . Before starting a cleaning operation, the cleaning unit  22  is retracted rearwardly of the inkjet head  21  (refer to  FIG. 5 ), and shifts forward upon starting the cleaning operation. Thus, the casing  226  is shifted to the other end in the longitudinal direction of the ink ejection surface  212 . This shift brings the first blade member  221 , the cleaning liquid roller  222 , the water roller  223 , the second blade member  224  into contact with the ink ejection section  211  in the order named. 
     At this time, the first blade member  221  wipes off the ink at the ink ejection section  211  (the ink wiping step). The ink wiped off by the first blade member  221  is admitted into the exhaust liquid storage section  226   b  through the hole h 1 , and then recovered into the exhaust ink tank  226  through the pipe. 
     Subsequently, the cleaning roller  222  slidingly contacts the portion from which the ink has been wiped off by the first blade member  221 , and this portion is then cleaned with the water-soluble organic solvent (the first cleaning step). The cleaning liquid is supplied from the cleaning liquid tank  226   k  through the pipe, the hollow portion of the cleaning liquid roller rotary shaft  222   a  and the cleaning liquid admitting slit  222   c  to the cleaning roller  222 . The portion of the second porous member  222   b  of the cleaning roller  222  which is brought into contact with the ink ejection surface  212  causes a rotational shift to contact the restriction plate  225   a , so that the cleaning liquid containing the ink recovered from the ink ejection surface  212  can be squeezed from the first porous member  222   b . The squeezed cleaning liquid is admitted into the exhaust liquid storage section  226   b  through the hole h 2 , and then recovered into the exhaust ink tank  226   k  through the pipe. 
     Next, the water roller  223  slidingly contacts the portion cleaned with the water-soluble organic solvent (the cleaning liquid), and this portion is then cleaned with water (the second cleaning step). The water is supplied from the water tank  223   d  through the pipe, the hollow portion of the water roller rotary shaft  222   a  and the water admitting slit  223   c  to the water roller  223 . The portion of the second porous member  223   b  of the water roller  223  which is brought into contact with the ink ejection surface  212  causes a rotational shift to contact the restriction plate  225   b , so that the water containing the ink recovered from the ink ejection surface  212  can be squeezed from the second porous member  223   b . The squeezed cleaning liquid is admitted into the exhaust liquid storage section  226   b  through the hole h 3 , and then recovered into the exhaust ink tank  226   k  through the pipe. 
     Thereafter, the second blade member  224  wipes off the ink, the cleaning liquid and the water on the ink ejection surface  212  of the ink ejection section  211  (the liquid wiping step). The liquid thus wiped off by the second blade member  224  is admitted into the exhaust liquid storage section  226   b  through the hole h 4 , and then recovered into the exhaust ink tank  226   k  through the pipe. 
     Second Preferred Embodiment 
     A second preferred embodiment of the invention will be described below and, in some cases, the description of the identical parts to those of the first preferred embodiment will be omitted. 
       FIGS. 6 and 7  are overall schematic diagrams of a cleaning unit according to the second preferred embodiment (hereinafter referred to as “second cleaning units  6 ”).  FIG. 6  is a sectional view of each of the second cleaning units  6 , and  FIG. 7  is a plan view thereof. 
     Each of the second cleaning units  6  cleans an ink ejection surface  212 , and the width W 2  of the second cleaning unit  6 , namely the length in a direction perpendicular to the paper surface in  FIG. 6  (refer to  FIG. 7 ) is larger than the width in the transverse direction of the ink ejection surface  212 . These second cleaning units  6  are provided correspondingly to individual inkjet heads  21 , and are connected to an exhaust ink tank  66   n . The second cleaning units  6  are shifted in the direction of an arrow A 1  in  FIG. 5  along the ink ejection surfaces  212  by the same shift mechanism as the first preferred embodiment. Each of the second cleaning units  6  is provided with a third blade member  61 , a first elastic gear  62 , a second elastic gear  63 , a fourth blade member  64 , restriction plates  65   a  and  65   b , and a second casing  66 . 
     The third blade member  61  is a plate-shaped member for wiping off the ink on the ink ejection surface  212 , and extends in one direction (a direction orthogonal to the paper surface). The length of the third blade member  61  in the direction orthogonal to the paper surface is substantially the same as the transverse length of the ink ejection surface  212 . The third blade member  61  is arranged at one end of the second casing  66 , and the upper tip end thereof is arranged at a slightly higher position than the ink ejection surface  212 . The third blade member  61  is formed by an elastic deformable material, such as EPDM or fluoro rubber. 
     The first elastic gear  62  cleans the ink ejection surface  212  with a water-soluble organic solvent, and is arranged next to the third blade member  61  along the shift direction A 1  of the second casing  66 . The length of the first elastic gear  62  in a direction orthogonal to the paper surface is substantially the same as the transverse length of the ink ejection surface  212 . The first elastic gear  62  also has a first rotary shaft  62   a  and a first elastic body member  62   b . The first rotary shaft  62   a  extends in a direction orthogonal to the paper surface in  FIG. 6 , and is fixed to the second casing  66 . The first elastic body member  62   b  has a tubular part  62   c  arranged around the first rotary shaft  62   a , and blade parts  62   d  arranged radially from the tubular part  62   c . At least the surface portions of the blade parts  62   d  are composed of a porous material, such as a sponge, and thus capable of absorbing a cleaning liquid. The first elastic body member  62   b  is arranged so that the uppermost position in the height position thereof is substantially the same as the height position of the upper tip end of the third blade member  61 . The first elastic body member  62   b  is rotatable with respect to the first rotary shaft  62   a , and rotates counterclockwise upon the shift of the second cleaning unit  6  so as to bring the blade parts  62   d  into contact with the ink ejection surface  212 . 
     The second elastic gear  63  cleans the ink ejection surface  212  with water, and is arranged next to the first elastic gear  62  along the shift direction A 1  of the second casing  66 . The length of the second elastic gear  63  in a direction orthogonal to the paper surface is substantially the same as the transverse length of the ink ejection surface  212 . The second elastic gear  63  has a second rotary shaft  63   a  and a second elastic body member  63   b . The second rotary shaft  63   a  extends in a direction orthogonal to the paper surface and is fixed to the second casing  66 . The second elastic body member  63   b  has a tubular part  63   c  arranged around the second rotary shaft  63   a , and blade parts  63   d  arranged radially from the tubular part  63   c . These blade parts  63  are provided with a porous material such as a sponge, and thus capable of absorbing water. The second elastic body member  63   b  is arranged so that the uppermost position in the height position thereof is substantially the same as the height position of the upper tip end of the fourth blade member  64 . The second elastic body member  63   b  is rotatable with respect to the second rotary shaft  63   a , and rotates counterclockwise upon the shift of the second cleaning unit  6  so as to bring the blade parts  63   d  into contact with the ink ejection surface  212 . 
     The fourth blade member  64  wipes off the liquid, such as an ink, water and a cleaning liquid, on the ink ejection surface  212 , which is a plate-shaped member extending in one direction (a direction orthogonal to the paper surface in  FIG. 6 ). The fourth blade member  64  is arranged at the side end opposite the side end at which the third blade member  61  of the second casing  66  is arranged, and the upper tip end thereof is arranged at a slightly higher position than the ink ejection surface  212 . The fourth blade member  64  is formed by an elastic deformable material, such as EPDM or fluoro rubber. 
     The restriction plates  65   a  and  65   b  are plate-shaped members projecting upward from a bottom surface  66   c  of the second casing  66 , and arranged so as to contact the blade parts  62   d  and  63   d  of the first and second elastic gears  62  and  63 , respectively. At the time of rotation of the first elastic gear  62  and the second elastic gear  63 , the restriction plates  65   a  and  65   b  contact the blade parts  62   d  and  63   d , and also press the blade parts  62   d  and  63   d , thereby enabling the squeeze of the ink, the water and the cleaning liquid absorbed by the blade parts  62   d  and  63   d.    
     The second casing  66  supports the third blade member  61 , the first elastic gear  62 , the second elastic gear  63 , the fourth blade member  64  and the restriction plates  65   a  and  65   b , and has an arrangement section  66   a  to arrange the third blade member  61 , the first elastic gear  62  and the like, and an exhaust liquid storage section  66   b  to collect the exhaust liquid. The arrangement section  66   a  has the bottom surface  66   c , a third blade member sidewall surface  66   d , a fourth blade member sidewall surface  66   e , a first sidewall  66   f  and a second sidewall  66   g.    
     The bottom surface  66   c  is a plate-shaped member underlying the third and fourth bade members  61  and  64 , and the first and second elastic gears  62  and  63 , and has holes h 5  to h 8  to permit passage of liquid arranged at positions corresponding to the third blade member  61 , the first elastic gear  62 , the second elastic gear  63  and the fourth blade member  64 , respectively. The bottom surface  66   c  is further provided with partition projections  66   i  to partition the space for the third blade member  61 , the space for the first elastic gear  62 , the space for the second elastic gear  63 , and the space for the fourth blade member  64 . 
     The third blade member sidewall surface  66   d  is arranged at the front in the shift direction of the third blade member  61 , namely on the right in  FIG. 6 , and provided with a third fixing member  66   j  for fixing the third blade member  61 . The upper height position of the third fixing member  66   j  is lower than the upper height position of the third blade member sidewall surface  66   d , making it difficult for the ink wiped off by the third blade member  61  to overflow the second casing  66 . The fourth blade member sidewall surface  66   e  is arranged at the rear in the shift direction of the fourth blade member  64 , namely on the left in  FIG. 6 , and provided with a fourth fixing member  66   k  for fixing the fourth blade member  64 . The upper height position of the fourth fixing member  66   k  is lower than the upper height position of the fourth blade member sidewall surface  66   e , making it difficult for the ink wiped off by the fourth blade member  64  to overflow the second casing  66 . 
     The first sidewall  66   f  is arranged so as to connect one end of the third blade member sidewall surface  66   d  and one end of the fourth blade member sidewall surface  66   e , and provided with a hole  91  to permit connection of a pipe for supplying the water-soluble organic solvent from a cleaning liquid tank  66   q  to the first elastic gear  62 , and a hole  92  to permit connection of a pipe for supplying water from a water tank  66   p  to the second elastic gear  63 . The second sidewall  66   g  is opposed to the first sidewall  66   f , and cooperates with the first sidewall  66   f  to support the first and second elastic gears  62  and  63 . The exhaust liquid storage section  66   b  underlies the arrangement section  66   a , and temporarily stores the water, the ink, the cleaning liquid and the like after passing through the holes h 5  to h 8  provided at the bottom surface  66   c . The exhaust storage section  66   b  is connected to the exhaust ink tank  66   n.    
     &lt;Cleaning Method&gt; 
     When each of the ink ejection sections  211  of the inkjet heads  21  is cleaned, the second casing  66  is shifted in the direction indicated by the arrow A 1  from the state in which the third blade member  61  is in contact with one end of the longitudinal direction of the ink ejection section  211 , to the other end in the longitudinal direction of the ink ejection section  211 . This shift brings the third blade member  61 , the first elastic gear  62 , the second elastic gear  63  and the fourth blade member  64  into contact with the ink ejection section  211  in the order named. 
     At this time, the third blade member  61  wipes off the ink at the ink ejection section  211  (the ink wiping step). The ink wiped off by the third blade member  61  is admitted into the exhaust liquid storage section  66   b  through the hole h 5 , and then recovered into the exhaust ink tank  66   n  through the pipe. 
     Subsequently, the first elastic gear  62  slidingly contacts the portion from which the ink has been wiped off by the third blade member  61 , and this portion is then cleaned with the water-soluble organic solvent (the first cleaning step). Hereat, the cleaning liquid supplied from the hole  91  is absorbed by the first elastic gear  62 , and the portion of the first elastic gear  62  containing the cleaning liquid causes a rotational shift and contacts the ink ejection surface  212 , thereby cleaning the ink ejection surface  212 . Upon a further rotation of the first elastic gear  62  so as to contact the restriction plate  65   a , the water-soluble organic solvent that contains the ink by cleaning the ink ejection surface  212  is squeezed from the first elastic gear  62  and admitted into the exhaust liquid storage section  66   b  through the hole h 6 , and then recovered into the exhaust ink tank  66   n  through the pipe. The water-soluble organic solvent excessively supplied from the hole  91  is also exhausted from the hole h 6 . In order to reduce a waste of the water-soluble organic solvent, a sensor to detect the liquid surface level of the stored water-soluble organic solvent may be provided to control the supply amount of the water-soluble organic solvent based on the liquid surface level detected by the sensor. 
     Next, the second elastic gear  63  slidingly contacts the portion cleaned with the water-soluble organic solvent, and this portion is then cleaned with water (the second cleaning step). Hereat, the water supplied from the hole  92  is absorbed by the second elastic gear  63 , and the portion of the second elastic gear  63  containing water causes a rotational shift and contacts the ink ejection surface  212 , thereby cleaning the ink ejection surface  212 . Upon a further rotation of the second elastic gear  63  so as to contact the restriction plate  65   b , the water that contains the ink by cleaning the ink ejection surface  212  is squeezed from the second elastic gear  63  and admitted into the exhaust liquid storage section  66   b  through the hole h 7 , and then recovered into the exhaust ink tank  66   n  through the pipe. The water excessively supplied from the hole  92  is also exhausted from the hole h 7 . In order to reduce a waste of water, a sensor to detect the water level of the stored water may be provided to control the supply amount of water based on the water level detected by the sensor. 
     Thereafter, the fourth blade member  64  wipes off the ink, the cleaning liquid and the water on the ink ejection surface  212  of the ink ejection section  211  (the liquid wiping step). The liquid thus wiped off by the fourth blade member  64  is discharged through the hole h 8 . Otherwise, the cleaning method is identical to that of the first preferred embodiment, and therefore the description thereof is omitted here. 
     Other Preferred Embodiments 
     The shift mechanism for shifting the cleaning units are not limited to those of the foregoing preferred embodiments, and a different mechanism may be employed. Alternatively, the cleaning units may be fixed, and a shift mechanism for shifting the inkjet heads may be employed. 
     In the foregoing preferred embodiments, the cleaning units are applied to the image forming apparatus that forms an image on a paper. Alternatively, the cleaning units may be applied to an image forming apparatus that forms an image on an image formation object other than papers. 
     Examples of the present invention will be described below. It is understood, however, that the examples are for the purpose of illustration and the invention is not to be regarded as limited to any of the specific materials or condition therein. 
     EXAMPLES 
     Example 1 
     &lt;Preparation of Pigment Dispersion Liquid&gt; 
     A pigment dispersion liquid was prepared by mixing 30% by weight of C.I. pigment red 122 as a pigment, 30% by weight of styrene-acrylic resin (“JONCRYL61” manufactured by Johnson Polymer Corporation), 10% by weight of glycerin and 35% by weight of an ion exchanged water, and dispersing this mixture with 0.5 mm zirconia beads by using a ball mill until the mean particle size became 100 nm. The SP value of the used styrene-acrylic resin was 8 to 12. The mean particle size was measured by a dynamic light scattering particle size distribution measuring apparatus (“LB-550” manufactured by HORIBA Ltd.) after the pigment dispersion liquid was diluted five times with the ion exchanged water. 
     &lt;Preparation of Ink&gt; 
     An ink was prepared by mixing 0.5% by weight of ethylene oxide addition product of acetylene diol as being surfactant (“Olfine E1010” manufactured by Nisshin Chemical Industry Co., Ltd.), 5% by weight of triethylene glycol monobuthyl ether, 5% by weight of 2-pyroridone, 20% by weight of the above-mentioned pigment dispersion liquid and 69.5% by weight of water, and sufficiently stirring the mixture, followed by filtering with a filter having a hole diameter of 5 μm. 
     &lt;Cleaning&gt; 
     Under the environment of 25° C. and 50% RH, the ink ejection surfaces of the inkjet heads filled with the ink were exposed to the air and left for a week. Thereafter, the cleaning of the ink ejection surfaces was carried out by the cleaning method of the first preferred embodiment by using triethylene glycol monobuthyl ether (8.5 in SP value) as a water-soluble organic solvent. After cleaning, the ink ejection surfaces were pressed and purged, and recleaned by the same method as the above cleaning method. 
     Example 2 
     Cleaning was carried out in the same manner as in Example 1, except that instead of triethylene glycol monobuthyl ether, 2-pyroridone (13.8 in SP value) was used as a cleaning agent. 
     Example 3 
     Cleaning was carried out in the same manner as in Example 1, except that instead of triethylene glycol monobuthyl ether, glycerin (18.1 in SP value) was used as a cleaning agent. 
     Examples 4 and 5 
     An ink was prepared in the same manner as in Example 1, except that a pigment dispersion liquid was prepared by mixing 30% by weight of C.I. pigment red 122 as a pigment, 30% by weight of acrylic resin (“Acrylpolymer T540” manufactured by To a Gosei Kagaku Co., Ltd.), 10% by weight of glycerin and 35% by weight of an ion exchanged water. Subsequently, cleaning was carried out in the same manner as in Example 1, except that triethylene glycol monobuthyl ether and 2-pyroridone were used as a water-soluble organic solvent for cleaning. The SP value of the used acrylic resin was 8 to 12. 
     Example 6 
     Cleaning was carried out in the same manner as in Example 4 or 5, except that glycerin (18.1 in SP value) was used as a cleaning agent. 
     Comparative Example 1 
     Cleaning was carried out in the same manner as in Example 1, except that water (23.4 in SP value) was used as a cleaning liquid. 
     Comparative Example 2 
     Cleaning was carried out in the same manner as in Example 4 or 5, except that water (23.4 in SP value) was used as a cleaning liquid. 
     &lt;Evaluation Test and Evaluation Method&gt; 
     After the cleaning was terminated, the non-ejection of the ink was confirmed by printing a 1 (one)-dot and 1 (one)-space vertical line on a glazed paper under the conditions that the drive frequency of the inkjet heads was 20 kHz, the distance between the nozzle ejection surface and a recording medium was 1.0 mm, and the transfer speed of the recording medium was 847 mm/sec. Further, the line width was measured to confirm whether the line width error was less than ±20 μm. Table 1 shows these results. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Resin used for 
                 SP value of 
                   
               
               
                   
                 Water-soluble 
                 pigment 
                 water-soluble 
                 Evaluation 
               
               
                   
                 organic solvent 
                 dispersion liquid 
                 organic solvent 
                 results 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Example 1 
                 Triethylene glycol 
                 JONCRYL61 
                 8.5 
                 ⊚ 
               
               
                   
                 monobuthyl ether 
               
               
                 Example 2 
                 2-pyroridone 
                 JONCRYL61 
                 13.8 
                 ◯ 
               
               
                 Example 3 
                 glycerin 
                 JONCRYL61 
                 18.12 
                 Δ 
               
               
                 Example 4 
                 Triethylene glycol 
                 Acrylpolymer T540 
                 8.5 
                 ⊚ 
               
               
                   
                 monobuthyl ether 
               
               
                 Example 5 
                 2-pyroridone 
                 Acrylpolymer T540 
                 13.8 
                 ◯ 
               
               
                 Example 6 
                 glycerin 
                 Acrylpolymer T540 
                 18.12 
                 Δ 
               
               
                 Comparative 
                 water 
                 JONCRYL61 
                 23.4 
                 X 
               
               
                 Example 1 
               
               
                 Comparative 
                 water 
                 Acrylpolymer T540 
                 23.4 
                 X 
               
               
                 Example 2 
               
               
                   
               
               
                 “⊚” denotes the absence of the non-ejection and the line width error of less than ±20 μm. 
               
               
                 “◯” denotes the absence of the non-ejection and the line width error of not less than ±20 μm. 
               
               
                 “Δ” denotes a slight non-ejection at such a level as not to cause any problem, and the line width error of not less than ±20 μm. 
               
               
                 “X” denotes the presence of the non-ejection. 
               
            
           
         
       
     
     As shown in Table 1, both Comparative Example 1 and Comparative Example 2 caused the non-ejection of the ink. On the other hand, none of Examples 1 to 4 caused the non-ejection of the ink.