Abstract:
Disclosed is a liquid treatment agent coating device including a coating roller that coats a recording medium with a liquid treatment agent; a pressing roller that cooperates with the coating roller to nip and convey the recording medium; a first container that holds the coating roller and the liquid treatment agent, wherein the first container has an opening portion in the vicinity of a nip portion between the coating roller and the pressing roller; a second container that accommodates an amount of the liquid treatment agent held in the first container, wherein the second container has second airtightness that is greater than first airtightness of the first container; a removal channel extending from the first container to the second container; a first on-off valve disposed in the removal channel; and a return channel extending from the second container to the first container.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    Embodiments of the present invention relate to a liquid treatment agent coating device, a method of operating the liquid treatment agent coating device, and an image forming system. The liquid treatment agent coating device is for an inkjet printer that forms an image on a recording medium by discharging ink droplets. The liquid treatment agent coating device coats the recording medium with a liquid treatment agent prior to forming the image. The liquid treatment agent is, for example, a blur inhibitor that inhibits an image blur that may occur during recording by the inkjet printer. 
         [0003]    2. Description of the Related Art 
         [0004]    An inkjet image forming method has rapidly been adopted in recent years owing to an advantageous property that the method is easily enhanced for a method of forming color images, as well as properties that the method is noiseless and has a low running cost. However, when an image is formed by the inkjet image forming method on a recording medium other than dedicated paper, there may be problems in quality of the formed image at an earlier stage, such as blurring, density unevenness, color tone unevenness, and bleed-through. Additionally, there may be problems in robustness of the formed image, such as water resistance and weather resistance. Therefore, some techniques have been proposed to solve these problems. 
         [0005]    There is a method for solving these problems. In the method, quality of an image is improved by coating a sheet of paper (as a recording medium) with a liquid treatment agent prior to the ink droplets being adhered to the sheet of paper. The liquid treatment agent functions to condense the ink. As the method of coating the recording medium with the liquid treatment agent, Patent Document 1 (Japanese Patent Laid-Open Application No. H7-156538) discloses a method of coating an entire surface of a recording medium with a liquid treatment agent by using a roller. 
         [0006]      FIG. 10  is a schematic configuration diagram of a conventional liquid treatment agent coating device for coating the recording medium with the liquid treatment agent by using the roller. As shown in  FIG. 10 , the recording medium is wound around a peripheral surface of a platen roller  201  by using a pressing chuck  202 . The platen roller  201  is rotationally driven by a driving source such as a motor (not shown). 
         [0007]    On the other hand, a liquid treatment agent  205  is reserved in a liquid treatment agent tank  204 . The liquid treatment agent  205  is transferred onto a roller surface of a coating roller  208  as a thin film by an agitation-supply roller  206  and conveyance-thinning rollers  207   a  and  207   b.    
         [0008]    The coating roller  208  rotates while being pressed onto the recording medium  203 , which has been wound around the platen roller  201  being rotated, thereby coating the surface of the recording medium  203  with the liquid treatment agent  205 . When a portion of the recording medium  203 , which has been coated with the liquid treatment agent  205 , reaches a position facing an inkjet recording head  209 , the inkjet recording head  209  discharges ink droplets, thereby performing the recording. 
         [0009]    As described, according to the method of coating a recording area of the recording medium  203  with the liquid treatment agent  205  for improving image quality by using the coating roller  208 , the recording area can be coated thinly with the liquid treatment agent  205  having a relatively higher viscosity, compared to a method where a recording area is processed by spraying a liquid treatment agent by using a spraying head. Therefore, the method of coating the recording area with the liquid treatment agent  205  has such an advantageous property that an image blur can further be prevented from occurring. 
         [0010]    However, for such a conventional liquid treatment agent coating device, it has not been considered that, when the liquid treatment agent has been reserved for a long time in the liquid treatment agent coating device, viscosity of the liquid treatment agent increases due to evaporation of water or an organic solvent in the liquid treatment agent.  FIG. 11  is a characteristic diagram showing a relationship between viscosity of a liquid treatment agent and a coating amount of the liquid treatment agent per A4 size recording medium. As is clear from  FIG. 11 , the coating amount of the liquid treatment agent tends to be increased when the viscosity of the liquid treatment agent is increased. 
         [0011]    When the coating amount of the liquid treatment agent is increased, the liquid treatment agent may bleed on the recording medium. Therefore, a friction force between the recording medium and a roller that conveys the recording medium may be lowered. It can be a cause of a conveyance failure of the recording medium. Further, it is possible that the liquid treatment agent is not sufficiently dried. In such a case, it is possible that the formed image is transferred to another recording medium in a post process. Therefore, there are some problems in that the image quality is lowered and that speeding up of the image forming system is prevented. 
       SUMMARY OF THE INVENTION 
       [0012]    The embodiments of the present invention have been developed in view of the above-described problems of the conventional techniques. 
         [0013]    An objective of the present invention is to provide a liquid treatment agent coating device for an inkjet printer that can effectively regulate an increase of a coating amount of a liquid treatment agent due to an increase of viscosity of the liquid treatment agent; a method of operating the liquid treatment agent coating device; and an image forming system with which high quality images are obtained by performing suitable pre-processing. 
         [0014]    In one aspect, there is provided a liquid treatment agent coating device for an inkjet printer, the liquid treatment agent coating device including a coating roller configured to coat a recording medium with a liquid treatment agent prior to forming an image on the recording medium; a pressing roller configured to cooperate with the coating roller to nip and convey the recording medium, wherein the pressing roller is configured to transfer the liquid treatment agent on the coating roller by pressing the recording medium toward the coating roller; a first container configured to store the coating roller and the liquid treatment agent, wherein the first container has an opening portion in the vicinity of a nip portion between the coating roller and the pressing roller; a second container configured to accommodate an amount of the liquid treatment agent stored in the first container, wherein the second container has second airtightness that is greater than first airtightness of the first container; a removal channel extending from the first container to the second container; a first on-off valve disposed in the removal channel; and a return channel extending from the second container to the first container. 
         [0015]    In another aspect, there is provided a method of operating a liquid treatment agent coating device for an inkjet printer, the liquid treatment agent coating device including a coating roller configured to coat a recording medium with a liquid treatment agent prior to forming an image on the recording medium; a pressing roller configured to cooperate with the coating roller to nip and convey the recording medium, wherein the pressing roller is configured to transfer the liquid treatment agent on the coating roller by pressing the recording medium toward the coating roller; a first container configured to store the coating roller and the liquid treatment agent, wherein the first container has an opening portion in the vicinity of a nip portion between the coating roller and the pressing roller; a second container configured to accommodate an amount of the liquid treatment agent stored in the first container, wherein a second container has second airtightness that is greater than first airtightness of the first container; a removal channel extending from the first container to the second container; a first on-off valve disposed in the removal channel; a return channel extending from the second container to the first container; a supply channel configured to supply the liquid treatment agent to the first container, the supply channel being connected to the return channel; a cartridge configured to be detachably connected to the supply channel, wherein the cartridge is connected to the supply channel at a portion of the supply channel which is opposite to a third connection point at which the supply channel is connected to the return channel, the cartridge being filled with the liquid treatment agent; a first liquid amount detection sensor configured to detect the amount of the liquid treatment agent stored in the first container; and a second liquid amount detection sensor configured to detect a second amount of the liquid treatment agent stored in the second container, wherein, when the second liquid amount detection sensor detects that the liquid treatment agent is not stored in the second container, the liquid treatment agent stored in the cartridge is supplied to the first container through the supply channel, and when the first liquid amount detection sensor subsequently detects that a predefined amount of the liquid treatment agent has been supplied to the first container, the supply of the liquid treatment agent from the cartridge is terminated, wherein, when the second liquid amount detection sensor detects that the liquid treatment agent is stored in the second container, the liquid treatment agent stored in the second container is returned to the first container through the return channel, and when the second liquid amount detection sensor subsequently detects that the liquid treatment agent is not stored in the second container, the first liquid amount detection sensor detects whether a first stored amount of the liquid treatment agent stored in the first container reaches the predefined amount, and wherein, when the first liquid amount detection sensor subsequently detects that the first stored amount of the liquid treatment agent stored in the first container does not reach the predefined amount, a shortage amount of the liquid treatment agent is supplied from the cartridge. 
         [0016]    In another aspect, there is provided an image forming system including a liquid treatment agent coating device configured to coat a recording medium with a liquid treatment agent prior to forming an image on the recording medium; and an inkjet printer configured to form the image by discharging ink droplets onto the recording medium having been coated with the liquid treatment agent, wherein the liquid treatment agent coating device is disposed upstream in a conveyance direction of the recording medium, and the inkjet printer is disposed downstream of the liquid treatment agent coating device in the conveyance direction of the recording medium, wherein the liquid treatment agent coating device is for the inkjet printer, and wherein the liquid treatment agent coating device includes a coating roller configured to coat the recording medium with the liquid treatment agent prior to forming the image on the recording medium; a pressing roller configured to cooperate with the coating roller to nip and convey the recording medium, wherein the pressing roller is configured to transfer the liquid treatment agent on the coating roller by pressing the recording medium toward the coating roller; a first container configured to store the coating roller and the liquid treatment agent, wherein the first container has an opening portion in the vicinity of a nip portion between the coating roller and the pressing roller; a second container configured to accommodate an amount of the liquid treatment agent stored in the first container, wherein the second container has second airtightness that is greater than first airtightness of the first container; a removal channel extending from the first container to the second container; a first on-off valve disposed in the removal channel; and a return channel extending from the second container to the first container. 
         [0017]    According to the embodiments of the present invention, there are provided the liquid treatment agent coating device for the inkjet printer that can effectively regulate the increase of the coating amount of the liquid treatment agent due to the increase of the viscosity of the liquid treatment agent and the method of operating the liquid treatment agent coating device. Further, there is provided the image forming system with which the high quality images are obtained by performing the suitable pre-processing. 
         [0018]    Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a flowchart schematically showing a process flow of an image forming system according to an embodiment of the present invention; 
           [0020]      FIG. 2  is a schematic configuration diagram of a liquid treatment agent coating device utilized in the image forming system; 
           [0021]      FIG. 3  is a schematic configuration diagram of a coating unit in the liquid treatment agent coating device; 
           [0022]      FIG. 4  is a diagram illustrating a process of supplying a liquid treatment agent to a supply container of the liquid treatment agent coating device, and illustrating a circulation system; 
           [0023]      FIG. 5  is a diagram illustrating operations for supplying the liquid treatment agent from a state where a reservoir tank of the liquid treatment agent coating device does not store the liquid treatment agent; 
           [0024]      FIG. 6  is a diagram illustrating procedures to start operations for supplying the liquid treatment agent to the supply container from a state where the liquid treatment agent is stored in the reservoir tank of the liquid treatment agent coating device; 
           [0025]      FIG. 7  is a diagram illustrating procedures of a circulation operation of the liquid treatment agent stored in the supply container of the liquid treatment agent coating device; 
           [0026]      FIG. 8  is a diagram illustrating procedures of a removing operation of the liquid treatment agent stored in the supply container of the liquid treatment agent coating device; 
           [0027]      FIG. 9  is a diagram illustrating an arrangement of the supply container, the reservoir tank, a filter unit, and the like in the liquid treatment agent coating device; 
           [0028]      FIG. 10  is a schematic configuration diagram of a conventional liquid treatment agent coating device; and 
           [0029]      FIG. 11  is a characteristic diagram showing a relationship between viscosity of the liquid treatment agent and a coating amount of the liquid treatment agent per A4 size recording medium. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]    Hereinafter, an embodiment of the present invention will be explained by referring to accompanying figures.  FIG. 1  is a flowchart schematically showing a process flow of an image forming system according to the embodiment of the present invention. 
         [0031]    As shown in  FIG. 1 , a recording medium W, which is formed of, for example, a continuous paper web, is pulled out from a paper feeder  100 . The recording medium W is fed to a liquid treatment agent coating device  101 . Front and rear surfaces of the recording medium W are coated with a liquid treatment agent such as a blur inhibitor, thereby performing a pre-process. Subsequently, the recording medium W is fed to a first inkjet printer  102   a , and the first inkjet printer  102   a  forms a desired image on the front surface of the recording medium W by discharging ink droplets. After that, a paper inverter  103  inverts the front surface and the rear surface of the recording medium W. Then, the recording medium W is fed to a second inkjet printer  102   b , and the second inkjet printer  102   b  forms a desired image on the rear side of the recording medium W by discharging ink droplets. In this manner, in the image forming system, both sides of the recording medium W are printed. Subsequently the recording medium W is conveyed to a post-processing device (not shown), and a predetermined post-process is performed. 
         [0032]      FIG. 2  is a schematic configuration diagram of the liquid treatment agent coating device  101  used for the image forming system.  FIG. 2  shows a state where the recording medium W is coated with the liquid treatment agent. As shown in  FIG. 2 , plural rotatable guide rollers  1  with bearings (bearings are not shown) are disposed inside the liquid treatment agent coating device  101 , thereby forming a conveyance path of the recording medium W. 
         [0033]    In  FIG. 2 , an element indicated by a reference numeral  2  is a feed-in (FI) roller that is rotationally driven by a driving source (not shown) such as a motor. A feed-in nip roller  4  is pressed toward the FI roller  2  by a pulling force of a spring (not shown). 
         [0034]    The recording medium W is elastically nipped between the FI roller  2  and the FI-nip roller  4 . When the FI roller  2  is rotated by the driving source, the recording medium W is drawn into inside the liquid treatment agent coating device  101  from the paper feeder  100 . 
         [0035]    The recording medium W forwarded from the nip between the FI roller  2  and the FI-nip roller  4  is slightly loosened to form a loop AL. After passing through the loop AL, the recording medium W passes through a space including path shafts  5  and edge guides  6 . Though it is not shown in  FIG. 2 , the two path shafts  5  are arranged in a direction perpendicular to a conveyance direction of the recording medium W (the direction of the arrow). The recording medium W passes through the space between the two path shafts  5 , while forming an S-shape. The pair of edge guides  6  is attached to the two path shafts. Each of the edge guides  6  has a plate-like shape. A distance between the two edge guides  6  is adjusted to be substantially equal to a width of the recording medium W. 
         [0036]    The two path shafts  5  and the pair of the edge guides  6  cooperate to regulate a moving position in the width direction of the recording medium W. Therefore, the recording medium W is stably conveyed. The edge guides  6  are fixed to the path shafts  5  by fixing units such as screws. The positions of the edge guides  6  can be adjusted depending on the width of the recording medium W to be used. 
         [0037]    A fixed tension shaft  7  adds tension to the recording medium W that has passed through the space including the path shafts  5  and the edge guide  6  so as to stabilize the conveyance of the recording medium W. 
         [0038]    After passing the tension shaft  7 , the recording medium W passes through a nip between an in-feed roller  8  and feed-nip rollers  9 . The in-feed roller  8  is driven by a driving source (not shown) such as a motor. Though it is not shown in  FIG. 2 , the plural feed-nip rollers  9  are disposed along a direction of an axis of the in-feed roller  8 . The feed-nip rollers  9  are pressed toward the in-feed roller  8  by a spring (not shown). 
         [0039]    After passing through a nip between the in-feed roller  8  and the feed-nip rollers  9 , the recording medium W is wound around a rotatable first dancer roller  11  from a bottom side. 
         [0040]    The first dancer roller  11  is rotatably attached to a first movable frame  12  through bearings (not shown) attached to end portions of the first dancer roller  11 , thereby forming a first dancer roller assembly  17 . Namely, the first dancer roller assembly  17  is suspended by the recording medium W. 
         [0041]    The first dancer roller assembly  17  can be moved along a direction of gravity A. A first position detection unit (not shown) is provided for detecting a position of the first dancer roller assembly  17 . The position of the first dancer roller assembly  17  can be adjusted by performing drive control of the driving source of the in-feed roller  8  depending on an output from the first position detection unit. 
         [0042]    After passing through the first dancer roller assembly  17 , both the front side and the rear side of the recording medium W are coated with the liquid treatment agent by sequentially passing through a front surface coating unit  13   f  for coating the front surface of the recording medium W with the liquid treatment agent and a rear surface coating unit  13   r  for coating the rear surface of the recording medium W with the liquid treatment agent. The front surface coating unit  13   f  and the rear surface coating unit  13   r  will be explained later. 
         [0043]    After passing through the rear surface coating unit  13   r , the recording medium W passes through a nip between an out-feed roller  14  and feed-nip rollers  19 . The out-feed roller  14  is rotationally driven by a driving source (not shown) such as a motor. Though it is not shown in  FIG. 2 , the plural feed-nip rollers  19  are arranged along a direction of an axis of the out-feed roller  14 . The feed-nip rollers  19  are pressed toward the out-feed roller  14  by a spring (not shown). 
         [0044]    After passing through the nip between the out-feed roller  14  and the feed-nip rollers  19 , the recording medium W is wound around rotatable second dancer rollers  15   a  and  15   b  and one of the guide rollers  1  disposed between the dancer rollers  15   a  and  15   b , thereby forming a W-shape. 
         [0045]    The two second dancer rollers  15   a  and  15   b  are rotatably attached to a second movable frame  16  through bearings (not shown) attached to end portions of the second dancer rollers  15   a  and  15   b , thereby forming a second dancer roller assembly  18 . Namely, the second dancer roller assembly  18  is also suspended by the recording medium W. 
         [0046]    The second dancer roller assembly  18  can also be moved along the direction of gravity A. A second position detection unit (not shown) is provided for detecting a position of the second dancer roller assembly  18 . The position of the second dancer roller assembly  18  can be adjusted by performing drive control of the driving source of the out-feed roller  14  depending on an output from the second position detection unit. 
         [0047]      FIG. 3  is a schematic configuration diagram of the coating unit  13  (any one of the front surface coating unit  13   f  and the rear surface coating unit  13   r ). As described later, the liquid treatment agent  22  is supplied to a supply container  26 . As the liquid treatment agent  22 , a solution of a water-soluble flocculant is utilized. Here, the solution is obtained by dissolving or dispersing the water-soluble flocculant, which promotes clumping or insolubilization of a water-soluble coloring material, in water or in an organic solvent. 
         [0048]    An amount of the liquid treatment agent  22  in the supply container  26  is detected by a liquid surface detection sensor  27  attached to the supply container  26 . When the liquid treatment agent  22  has been consumed by repeating printing and consequently a position of a liquid surface of the liquid treatment agent  22  in the supply container  26  becomes lower than a predefined position, the liquid treatment agent  22  is supplied to the supply container  26 . When the position of the liquid surface of the liquid treatment agent  22  in the supply container  26  reaches the predefined position, the supplying of the liquid treatment agent  22  is terminated based on a detection signal of the liquid surface detection sensor  27 . In this manner, the amount of the liquid treatment agent  22  in the supply container  26  is held to be a constant amount. 
         [0049]    The liquid treatment agent  22  held in the supply container  26  is pumped up by rotation of a squeeze roller  29  driven by a motor  28 . As the squeeze roller  29 , a roller may be utilized such that grooves have been formed on a peripheral surface of the roller. For example, an anilox roller or a wire bar may be utilized. In such a case, since the roller may not be affected by the viscosity of the liquid treatment agent  22  or a printing speed during pumping up, it is easy to control the amount of the liquid. 
         [0050]    Subsequent to the pumping up of the liquid treatment agent  22  by the squeeze roller  29 , an amount in excess of the liquid treatment agent  22  is removed by a metering blade  30 , and a predefined amount of the liquid treatment agent  22  is conveyed to a nip portion between the squeeze roller  29  and a coating roller  31 . As a material of the metering blade  30 , a metal such as a stainless steel, a plastic, and a rubber may be considered. However, from the viewpoints of wearing-out, the function to remove the redundant liquid, and a useful service life of the squeeze roller  29 , a plastic material is preferable. 
         [0051]    After the liquid treatment agent  22  has been conveyed to a nip portion between the squeeze roller  29  and the coating roller  31 , the liquid treatment agent  22  is transferred onto the coating roller  31 , while being uniformly spread in the direction of the axis of the nip portion between the squeeze roller  29  and the coating roller  31 . A peripheral surface of the coating roller  31  is covered with an elastic material such as a rubber, and the coating roller  31  is rotationally driven by a motor  32 . 
         [0052]    The recording material W being nipped and conveyed between the coating roller  31  and a pressing roller  33  is coated with the liquid treatment agent  22  that has been transferred onto the coating roller  31 . The pressing roller  33  is rotatably supported by an approximately center position of a swingable arm  34  through a bearing (not shown). The pressing roller  33  is rotated by the recording medium W being conveyed. A tension spring  36  is connected to an end portion opposite to a swing center  35  of the arm  34 . The pressing roller  33  is pressed toward the coating roller  31  by a lever force. 
         [0053]    An eccentric cam  37  is disposed between the pressing roller  33  and the tension spring  36 , and the eccentric cam  37  contacts the arm  34 . When the liquid treatment agent  22  is not being applied, the pressing roller  33  can be moved upward against the tension of the tension spring  36  in a direction in which the pressing roller  33  is separated from the coating roller  31  by the rotation of the eccentric cam  37 . 
         [0054]    An upper portion of the supply container  26  is formed to cover the coating roller  31  which is held inside the supply container  26 , and the supply container includes an opening portion  38  disposed in the vicinity of a position at which the pressing roller  33  contacts and separates from the coating roller  31 . The opening portion  38  includes an openable and closable shutter  39  so as to prevent water or an organic solvent of the liquid treatment agent  22  from evaporating. As shown in  FIG. 3 , when the pressing roller  33  is moved toward the coating roller  31 , the shutter  39  is opened, and when the pressing roller  33  is separated from the coating roller  31 , the shutter  39  is closed. After the liquid treatment agent  22  has been transferred from the coating roller  31  onto the recording medium W, the coating roller  31  is cleaned by a cleaner blade  40 , and the coating roller  31  is prepared for a next coating with the liquid treatment agent  22 . 
         [0055]    As shown in  FIG. 3 , around the pressing roller  33 , plural guide rollers  1   a - 1   e  are rotatably disposed at corresponding suitable positions. Among the plural guide rollers  1   a - 1   e , the guide roller  1   e  is disposed to face a conveyance roller  42  that is rotationally driven by a motor  41 . 
         [0056]    The continuous recording medium W supplied from the paper feeder  100  (cf.  FIG. 1 ) is nipped between the guide roller  1   e  and the conveyance roller  42 . The recording medium W is pulled by the rotationally driven conveyance roller  42  and the guide roller  1   e , and thereby the recording medium W is conveyed along the guide rollers  1   a - 1   e.    
         [0057]    After being coated with the liquid treatment agent  22  by the coating roller  31 , the recording medium W is conveyed to the inkjet printers  102   a  and  102   b  (cf.  FIG. 1 ). 
         [0058]      FIG. 4  is a diagram illustrating a process of supplying the liquid treatment agent  22  to the supply container  26  and illustrating a circulation system. As shown in  FIG. 3 , the upper portion of the supply container  26  is opened for nipping and conveying the recording medium W at the nip portion between the coating roller  31  and the pressing roller  33 , and for coating the recording medium W with the liquid treatment agent  22 . Even if the shutter  39  is provided, it is difficult to realize a completely sealed structure (a closed system). 
         [0059]    Therefore, in the embodiment, a reservoir tank  43  having a substantially sealed structure is provided concurrently with the supply container  26 . The reservoir tank  43  includes a container portion that can store the liquid treatment agent  22  held in the supply container  26 . As shown in  FIG. 4 , the reservoir tank  43  communicates with the supply container  26  through a removal channel  44 . An electromagnetic valve  45  is disposed in the removal channel  44  in front of the reservoir tank  43 . 
         [0060]    The reservoir tank  43  is disposed at a lower position with respect to the position of the supply tank  26 , so that a hydraulic head difference is generated between the supply container  26  and the reservoir tank  43 . When the liquid treatment agent  22  is not utilized for a long time, the liquid treatment agent  22  is moved from the supply container  26  to the reservoir tank  43  having high airtightness by opening the electromagnetic value  45  and by using the hydraulic head difference. In this manner, there is prevented an increase of the viscosity of the liquid treatment agent  22  due to drying. 
         [0061]    It may be a trigger for opening the electromagnetic valve  45  when the printing is stopped for a longer time, such as for one hour, compared to a processing time for a normal print job. For example, when the printing is stopped for a long time for replacing the recording medium W in the liquid treatment agent coating device  101  or for changing a printing pattern, the electromagnetic valve  45  may be opened. In this manner, a waiting time for filling the supply container  26  with the liquid treatment agent  22  is avoided each time the printing is stopped. 
         [0062]    Further, for the electromagnetic valve  45 , a normal open type valve is utilized. In a normal state where electricity is turned off, the electromagnetic valve  45  is opened. When the electricity of the liquid treatment agent coating device  101  is turned off, the liquid treatment agent  22  in the supply container  26  is automatically removed and stored in the reservoir tank  43 , thereby preventing the liquid treatment agent  22  from being held for a long time in the supply container  26 , which has low airtightness. 
         [0063]    As shown in  FIG. 4 , a circulation channel  46  branches off from the removal channel  44  in the middle. A tip portion of the circulation channel  46  is connected to the supply container  26 . A filter unit  47  is disposed in between the portions of the circulation channel  46 . An electromagnetic valve  48  is disposed between the filter unit  47  and a connection point at which the circulation channel  46  is connected to the removal channel  44 . On the other hand, a circulation pump  49  is disposed at a side of an exit of the filter unit  47 . 
         [0064]    A supply channel  50  is connected in between the circulation pump  49  and a connection point near which the circulation channel  46  is connected to the supply container  26 . A cartridge  51  having a sealed structure is connected to a free end side of the supply channel  50 . The cartridge  51  is replaceable and filled with the liquid treatment agent  22 . Further, a supply pump  52  and an electromagnetic valve  53  are disposed in the middle of the supply channel  50 . 
         [0065]    A circulation supply channel  54  is provided for communicating between the reservoir tank  43  and the filter unit  47 . An electromagnetic valve  55  is disposed in the middle of the circulation supply channel  54 . Further, the reservoir tank  43  is provided with a liquid surface detection sensor  56 . The liquid surface detection sensor  56  monitors an amount of the liquid treatment agent  22  in the reservoir tank  43 . The removal channel  44 , the circulation channel  46 , the supply channel  50 , and the circulation supply channel  54  are formed of corresponding tubes. 
         [0066]    Hereinafter, there will be explained operations of systems included in the liquid treatment agent coating device  101  by referring to  FIGS. 5-8 . 
         [0067]    &lt;Operations to Supply the Liquid Treatment Agent  22 &gt; 
         [0068]      FIG. 5  is a diagram illustrating operations for supplying the liquid treatment solution  22  from a state where the liquid treatment solution  22  is not stored in the reservoir tank  43 .  FIG. 5  shows a half-way state where the liquid treatment agent  22  is being supplied to the supply container  26 . 
         [0069]    In this case, as shown in  FIG. 5 , the electromagnetic valves  45 ,  48 , and  55  are closed (in the figure, the electromagnetic valves are colored in black) and the circulation pump  49  is stopped (in the figure, the circulation pump  49  is colored in black). The liquid treatment agent  22  in the cartridge  51  is being supplied to the supply container  26  through the supply channel  50  and a portion of the circulation channel  46  by opening the electromagnetic valve  53  (in the figure, the electromagnetic valve  53  is colored in white) and driving the supply pump  52  (in the figure, the supply pump  52  is colored in white). 
         [0070]    The liquid surface detection sensor  27  (cf.  FIG. 3 ) monitors whether the amount of the liquid treatment agent  22  that has been supplied to the supply container  26  reaches a predefined value. When the amount of the liquid treatment agent  22  reaches the predefined value, the supply pump  52  is stopped based on a detection output from the liquid surface detection sensor  27 , and the electromagnetic valve  53  is closed. 
         [0071]    When the printing is continued and the liquid surface detection sensor  27  detects that the amount of the liquid treatment agent  22  in the supply container  26  has been decreased, the electromagnetic valve  53  is opened again, and the liquid treatment agent  22  in the cartridge  51  is supplied to the supply container  26  by driving the supply pump  52 . 
         [0072]      FIG. 6  is a diagram illustrating procedures to start operations for supplying the liquid treatment agent  22  to the supply container  26  from a state where the liquid treatment agent  22  has been stored in the reservoir tank  43 .  FIG. 6  shows a half-way state where the liquid treatment agent  22  in the reservoir tank  43  is being supplied to the supply container  26 . 
         [0073]    In this case, as shown in  FIG. 6 , the electromagnetic valves  45 ,  48 , and  53  are closed (in the figure, the electromagnetic values  45 ,  48 , and  53  are colored in black), and the supply pump  52  is stopped (in the figure, the supply pump  52  is colored in black). The liquid treatment agent  22  in the reservoir tank  43  is supplied to the supply container  26  through the circulation supply channel  54  and a portion of the circulation channel  46  by opening the electromagnetic valve  55  (in the figure, the electromagnetic valve  55  is colored in white) and driving the circulation pump  49  (in the figure, the circulation pump  49  is colored in white). 
         [0074]    When the liquid treatment agent  22  is being supplied to the supply container  26 , the liquid treatment agent  22  passes through the filter unit  47 , and thereby foreign materials, such as paper powder, included in the liquid treatment agent  22  are removed. The thus filtered liquid treatment agent  22  is supplied to the supply container  26 . If the paper powder is mixed in the liquid treatment agent  22 , the liquid treatment agent  22  becomes pasty and the viscosity of the liquid treatment agent  22  increases. Therefore, the foreign materials such as the paper powder are removed from the liquid treatment agent  22  at a certain frequency. 
         [0075]    The liquid surface detection sensor  56  monitors whether discharging of the liquid treatment agent  22  in the reservoir tank  43  is completed. When the discharging of the liquid treatment agent  22  is completed, the electromagnetic valve  55  is closed and the circulation pump  49  is stopped. 
         [0076]    At the time at which the removal of the liquid treatment agent  22  from the reservoir tank  43  has been completed, the liquid surface detection sensor  27  determines whether a predefined amount of the liquid treatment agent  22  has been supplied to the supply container  26 . When the liquid surface detection sensor  27  determines that the amount of the liquid treatment agent  22  supplied to the supply container  26  has reached the predefined amount, the operations to supply the liquid treatment agent  22  are terminated. On the other hand, when the liquid surface detection sensor  27  determines that the amount of the liquid treatment agent  22  supplied to the supply container  26  has not reached the predefined amount, the liquid treatment agent  22  is supplied from the cartridge  51  through the procedures which have been explained by referring to  FIG. 5 . 
         [0077]    An order of supplying the liquid treatment agent  22  is as follows. Namely, first, when liquid surface detection sensor  56  detects that the liquid treatment agent  22  is stored in the reservoir tank  43 , the liquid treatment agent  22  stored in the reservoir tank  43  is moved to the supply container  26 . Subsequently, when the liquid surface detection sensor  27  detects that the amount of the liquid treatment agent  22  held in the supply container  26  is insufficient, a deficient amount of the liquid treatment agent  22  is supplied from the cartridge  51 . 
         [0078]    It is not preferable to adopt a method of supplying the liquid treatment agent  22  in which the liquid treatment agent is supplied from the cartridge  51  to the supply container  26  while the liquid treatment agent  22  is still stored in the reservoir tank  43 . In such a case, since an amount of the in-excess liquid treatment agent  22  is increased in the system, it is possible that the liquid treatment agent  22  overflows from the reservoir tank  43 . 
         [0079]    &lt;Circulation Operation of the Liquid Treatment Agent  22 &gt; 
         [0080]      FIG. 7  is a diagram illustrating a circulation operation of the liquid treatment agent  22 .  FIG. 7  shows a state where the liquid treatment agent  22  is being circulated through a portion of the removal channel  44  and the circulation channel  46 . 
         [0081]    In this case, as shown in  FIG. 7 , the electromagnetic valves  45 ,  53 , and  55  are closed (in the figure, the electromagnetic valves  45 ,  53 , and  55  are colored in black), and the supply pump  52  is stopped (in the figure, the supply pump  52  is colored in black). The liquid treatment agent  22  is circulated through the portion of the removal channel  44  and the circulation channel  46  by opening the electromagnetic valve  48  (in the figure, the electromagnetic valve  48  is colored in white) and driving the circulation pump  49  (in the figure, the circulation pump  49  is colored in white). At that time, the liquid treatment agent  22  passes through the filter unit  47 , and thereby the foreign materials, such as the paper powder, are removed. The thus filtered liquid treatment agent  22  is returned to the supply container  26 . During printing, the circulation operation of the liquid treatment agent  22  is suitably performed at every predetermined time interval. 
         [0082]    &lt;Removal Operation of the Liquid Treatment Agent  22 &gt; 
         [0083]      FIG. 8  is a diagram illustrating a removal operation of the liquid treatment agent  22 .  FIG. 8  shows a half-way state of the removal operation. 
         [0084]    In this case, as shown in  FIG. 8 , the electromagnetic values  48 ,  53 , and  55  are closed (in the figure, the electromagnetic values  48 ,  53 , and  55  are colored in black), and the circulation pump  49  and the supply pump  52  are stopped (in the figure, the circulation pump  49  and the supply pump  52  are colored in black). The liquid treatment agent  22  held in the supply container  26  is removed through the removal channel  44  by opening the electromagnetic valve  45  and by using the hydraulic head difference between the supply container  26  and the reservoir tank  43 . In this manner, there is prevented an increase of the viscosity of the liquid treatment agent  22  due to drying. 
         [0085]    The operations for returning the liquid treatment agent  22  in response to a request for printing which has been generated after the liquid treatment agent  22  has been removed from the supply container  26  is the same as the operations for supplying the liquid treatment agent  22  from the reservoir tank  43  to the supply container  26 , which have been explained by referring to  FIG. 6 . 
         [0086]      FIG. 9  is a diagram illustrating a state where the components such as the supply container  26 , the reservoir tank  43 , and the filter unit  47  are mounted in the liquid treatment agent coating device  101 . 
         [0087]    According to the embodiment, the liquid treatment agent coating device  101  is divided into a device main body  57  and an attached housing  58 . The device main body  57  is disposed at a front side and the attached housing  58  is disposed at a rear side, when the liquid treatment agent coating device  101  is observed by an operator of the image forming system. 
         [0088]    As shown in  FIG. 9 , the supply container  26  is mounted in the device main body  57  together with, for example, the dancer roller assemblies  17  and  18 . On the other hand, the components such as the reservoir tank  43 , the filter unit  47 , the cartridge  51 , and the pumps  49  and  52  are mounted in the attached housing  58 . The device main body  57  and the attached housing  58  are connected through the removal channel  44  and the circulation channel  46 . 
         [0089]    In  FIG. 9  the reference numeral  59  indicates a waste liquid electromagnetic valve, and the reference numeral  60  indicates a waste liquid tank. Since quality of the liquid treatment agent  22  tends to be lowered as the liquid treatment agent  22  is used, the liquid treatment agent  22  is discarded regularly or as required, and the liquid treatment agent  22  is renewed. Therefore, the waste liquid electromagnetic valve  59  and the waste liquid tank  60  are provided. Incidentally, in  FIGS. 4-8 , for the sake of simplicity of the figures, indications of the waste liquid electromagnetic valve  59  and the waste liquid tank  60  are omitted. 
         [0090]    There has been explained the case of forming images on both sides of the recording medium W by referring to  FIG. 1 . However, single side printing is possible with this image forming system as it is. Specifically, when an image is formed only on one side of the recording medium W, the pressing roller  33  of the rear surface coating unit  13   r  of the liquid treatment agent coating device  101  is released from contacting the coating roller  31 , and the paper inverter  103  and the second inkjet printer  102   b  are not used. In this manner, the single side printing may be performed. 
         [0091]    Alternatively, the single printing may be performed by providing only one surface coating unit in a liquid treatment agent coating device and by coating one of the surfaces of the recording medium W with the liquid treatment agent  22 . Further, the single printing may be performed by successively arranging two surface coating units in a liquid treatment agent coating device and by coating one of the surfaces of the recording medium W twice with the liquid treatment agent  22 . 
         [0092]    In the above description, the liquid treatment agent coating device for the inkjet printer, the method of operating the liquid treatment agent coating device, and the image forming system have been explained by the embodiments. However, the present invention is not limited to the above-described embodiments, and various modifications and improvements may be made within the scope of the present invention. 
         [0093]    In the embodiment, the conveyance roller  42  is utilized as a conveyor of the recording medium W. However, the embodiment of the present invention is not limited to this. For example, another conveyance unit such as a conveyance tractor may be utilized. 
         [0094]    In the embodiment, as shown in  FIG. 4 , the filter unit  47  is provided in between the portions of the circulation channel  46 . However, the embodiment of the present invention is not limited to this. For example, the filter unit  47  may be disposed at a position between the supply container  26  and a branch point where the circulation channel  46  is separated from the removal channel  44 . 
         [0095]    The present application is based on Japanese Priority Application No. 2011-235088 filed on Oct. 26, 2011, the entire contents of which are hereby incorporated herein by reference.