Patent Publication Number: US-8983340-B2

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(a) to Korean Patent Application No. 10-2011-0003618, filed on Jan. 13, 2011, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     The present disclosure relates generally to an image forming apparatus, and more particularly, to a development apparatus that is used in an electro-photographic image forming apparatus. 
     2. Description of the Related Art 
     In general, an electro-photographic image forming apparatus, such as a laser printer, a facsimile machine, a copy machine, or the like, is a printing apparatus which forms an electrostatic latent image on an image carrier, develops the electrostatic latent image with a developing agent, and transfers a developer image onto a printing medium. 
     A developer that is used in the image forming apparatus includes an image carrier on which an electrostatic latent image is formed by an exposure unit and a developing member supplying a developing agent to the image carrier and developing the electrostatic latent image as a developer image. A method of developing the electrostatic latent image on the image carrier through the developing member may be classified into a contact type in which the developing member comes in contact with the image carrier and a non-contact type in which the developing member does not come in contact with the image carrier. 
     The contact type developer is so configured that a developing member  101  is separated from an image carrier  100  for a predetermined distance as illustrated in  FIG. 1A  before developing, and the developing member  101  moves in a direction B and comes in contact with the image carrier as illustrated in  FIG. 1B  during developing. Here, a reference numeral  103  denotes light that is emitted from an exposure unit. When the developing is finished, the developing member  101  is separated from the image carrier  100  for a predetermined distance as illustrated in  FIG. 1A . Accordingly, when the developer is driven, a charging voltage is applied to a charging member  102 , and the developing member  101  comes in contact with the image carrier  100  to be rotated. At this time, an outer circumference A of the image carrier  100  between the charging member  102  and the developing member  101  comes in contact with the developing member  101  in a non-charging state. When the non-charging section A of the image carrier  100  comes in contact with the developing member  101 , the developing agent moves to the image carrier  100 . Because of this, image pollution occurs due to the developing agent that has moved to the non-charging section A, and unnecessary consumption of the developing agent occurs to increase the waste developing agent. 
     In order to remove the developing agent that is attached to the non-charging section in the related art, cleaning blades are installed on the image carrier and a transfer belt. However, according to this method, it is required to prepare waste developing agent chambers having a space of a predetermined size for accommodating the waste developing agent therein on the sides of the image carrier and the transfer belt, and this causes the sizes of the developer and the image forming apparatus to be increased. Also, since the developing agent is attached to the non-charging section, the amount of consumption of the developing agent becomes larger. Accordingly, the maintenance cost is increased and the design of the developer is limited. 
     Also, according to the image forming apparatus in the related art, a zener diode is installed on the ground of the image carrier to heighten the electric potential of the non-charging section from 0V to −100 to −150V, and thus the non-charging section does not occur. However, this method has the problem that the material cost is increased due to the installation cost of the zener diode. Also, the deviation of the electric potential of the electrostatic latent image on the image carrier for each developer is increased due to the characteristic of the zener diode, and thus it is difficult to obtain a uniform image quality. 
     SUMMARY 
     The present disclosure has been made to address at least the above problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present disclosure provides an image forming apparatus and a method for controlling the same, which can remove the non-charging section of the image carrier of the developer. 
     According to one aspect of the present disclosure, an image forming apparatus includes a main body of the image forming apparatus; at least one developer including an image carrier unit having an image carrier and a charged body charging the image carrier, a developing unit installed to swing at a predetermined angle with respect to the image carrier unit and having a developing roller, and a pressing member pressing the developing unit so that the developing roller comes in contact with the image carrier, and separably installed in the main body; a nip separation unit installed in the main body on one side of the developer to swing the developing unit so that the developing unit is in a position that is separated from the image carrier; and a control unit to control the developer and the nip separation unit according to a print command; wherein the control unit makes the nip separation unit swing the developing unit so that the developing roller is in the position that is separated from the image carrier during a standby state, and if the print command is received, the control unit rotates the image carrier and the developing roller and then controls the nip separation unit so that the image carrier is rotated as much as an angle between the charged body and the developing roller, and then the developing roller in a rotating state comes in contact with the image carrier. 
     Here, the at least one developer may include a first developer, a second developer, a third developer, and a fourth developer, and the control unit controls the nip separation unit, so that respective developing rollers of the first to fourth developers are separated from corresponding image carriers in a preparatory mode. 
     The control unit may control the nip separation unit, so that the respective developing rollers of the first to fourth developers come in contact with the corresponding image carriers in a color image mode, and may control the nip separation unit, so that the respective developing rollers of the first to third developers are separated from the corresponding image carriers and the developing roller of the fourth developer comes in contact with the corresponding image carrier in a mono image mode. 
     The nip separation unit may include a guide plate installed in the main body of the image forming apparatus; a first sliding member slidably installed on the guide plate to swing the fourth developer; a second sliding member slidably installed with respect to the guide plate and the first sliding member to swing the first to third developer; and a separation cam installed between the first and second sliding members to move the first and second sliding members. 
     The separation cam may include a first cam unit pushing the first and second sliding member in one direction; a second cam unit pushing only the second sliding member in the one direction; and a third cam unit that does not push the first and second sliding members. 
     A first cam groove to accommodate the separation cam may be formed on a surface of the first sliding member that is opposite to the second sliding member, a second cam groove to accommodate the separation cam may be formed on a surface of the second sliding member that is opposite to the first sliding member, and the separation cam may be installed in a cam space formed by the first and second cam grooves. 
     A first cam contact unit that comes in contact with the separation cam may be formed in the first cam groove of the first sliding member, and a second cam contact unit that comes in contact with the separation cam may be formed in the second cam groove of the second sliding member. 
     The first sliding member may include a first sliding body slidably installed on the guide plate; and a first projection member fixed to the first sliding body and having a pressing projection formed thereon to swing the fourth developer. 
     The second sliding member may include a second sliding body slidably installed with respect to the guide plate and the first sliding member; and a second projection member fixed to the second sliding body and having pressing projections formed thereon to swing the first to third developers. 
     A pressed projection that comes in contact with the nip separation unit may be formed at a lower end of the developing unit. 
     The developing unit may swing around a developing coupler receiving a driving power from a developing driving coupler installed in the main body, and a rotating shaft of the developing roller may be apart from a center shaft of the developing coupler. 
     The control unit may control the nip separation unit so that the developing roller becomes apart from the image carrier in a state where the developing roller and the image carrier are rotated. 
     The control unit may control the first to fourth developers in a successive circular order of a standby mode, a color image mode, and a mono image mode. 
     According to another aspect of the present disclosure, a method of controlling an image forming apparatus including a developing roller and an image carrier, which can come in contact with or can be separated from each other, and at least one developer, so that the developing roller is separated from the image carrier in a preparatory mode is provided, the method including receiving a print command; rotating the image carrier and the developing roller; making the developing roller in contact with the image carrier after the image carrier is rotated as much as an angle between the developing roller and a charged body; separating the developing roller from the image carrier if developing of an electrostatic latent image formed on the image carrier is completed; and stopping the rotation of the developing roller and the image carrier. 
     The at least one developer may include a yellow image developer, a magenta image developer, a cyan image developer, and a black image developer, and respective developing rollers of the yellow image developer, the magenta image developer, the cyan image developer, and the black image developer may come in contact with the image carrier in a color image mode. 
     The respective developing rollers of the yellow image developer, the magenta image developer, and the cyan image developer may be separated from the image carrier, and the developing roller of the black image developer may come in contact with the image carrier in a mono image mode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features and advantages of the present disclosure will be more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
         FIGS. 1A and 1B  are diagrams illustrating the operation of a contact type developer, in which  FIG. 1A  illustrates a case where the developer is in a stop state, and  FIG. 1B  illustrates a case where the developer performs developing; 
         FIG. 2  is a cross-sectional view briefly illustrating an image forming apparatus according to an embodiment of the present disclosure; 
         FIG. 3  is a view illustrating a plurality of developers and a nip separation unit in the case where the image forming apparatus of  FIG. 2  is in a preparation mode; 
         FIG. 4  is a side view of a developer that is used in the image forming apparatus of  FIG. 2 ; 
         FIG. 5  is a view illustrating a driving gear train of the developer of  FIG. 4 ; 
         FIG. 6  is a partial perspective view illustrating a developer driving coupler installed in a main body of the image forming apparatus of  FIG. 2 ; 
         FIG. 7  is a view illustrating a developing roller and an image carrier in a separated state in the developer of  FIG. 4 ; 
         FIG. 8  is a cross-sectional view illustrating a developing roller and an image carrier in a contact state in the developer of  FIG. 4 ; 
         FIG. 9  is a view illustrating a developer and a nip separation unit in a color mode of the image forming apparatus of  FIG. 2 ; 
         FIG. 10  is a view illustrating a developer and a nip separation unit in a mono mode of the image forming apparatus of  FIG. 2 ; 
         FIG. 11  is a perspective view illustrating an example of a nip separation unit that is used in the image forming apparatus of  FIG. 2 ; 
         FIG. 12  is an exploded perspective view of the nip separation unit of  FIG. 11 ; 
         FIG. 13  is a front view of the first sliding member of  FIG. 11 ; 
         FIG. 14  is an exploded perspective view illustrating a relationship between first and second sliding members of the nip separation unit of  FIG. 11  and a separation cam; 
         FIG. 15  is a view illustrating a separation cam of the nip separation unit of  FIG. 11  as seen from the direction indicated by an arrow G; 
         FIGS. 16 to 18  are views illustrating the operation of the nip separation unit of  FIG. 11 ; 
         FIG. 19  is a view illustrating another example of a nip separation unit used in an image forming apparatus according to an embodiment of the present disclosure; and 
         FIG. 20  is a flowchart illustrating a method of controlling an image forming apparatus according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The aspects and features of the disclosure and methods for achieving the aspects and features will be apparent by referring to the embodiments to be described in detail with reference to the accompanying drawings. However, the embodiments are not limited to the embodiments disclosed hereinafter, but can be implemented in diverse forms. In the following description of the present disclosure, well-known element structures and technologies are not described in detail since they would obscure the invention in unnecessary detail. Also, in the drawings, sizes and relative sizes of some constituent elements may be exaggerated for clarity in explanation. 
       FIG. 2  is a cross-sectional view briefly illustrating an image forming apparatus according to an embodiment of the present disclosure.  FIG. 3  is a view illustrating four developers installed in the image forming apparatus of  FIG. 2 , and illustrates the relationship between the four developers and the nip separation unit in a preparation mode.  FIG. 4  is a side view of a developer that is used in the image forming apparatus of  FIG. 2 , and  FIG. 5  is a view illustrating a driving gear train of the developer of  FIG. 4 . 
     Referring to  FIGS. 2 and 3 , an image forming apparatus  1  according to an embodiment of the present disclosure includes a main body  3 , a feeder unit  10 , an exposure unit  20 , a plurality of developers  30 , a nip separation unit  50 , a transfer belt unit  60 , a transfer roller  65 , a fusing unit  70 , a delivery roller  80 , and a control unit  90 . 
     The feeder unit  10  accommodates a predetermined number of sheets of printing media, and includes a pickup roller  11  that picks up and supplies the printing media sheet by sheet. In front of the pickup roller  11  in a direction in which the picked printing media P is transported, a transport roller  15  is installed to transport the picked printing media P to the transfer roller  65 . 
     The exposure unit  20  forms an electrostatic latent image on the image carrier  33  of the plurality of developers  30  through emission of light that corresponds to the received print data. 
     The plurality of developers  30  form developer images that correspond to the print data, and may include four developers that form a color image, that is, a first developer  30 Y, a second developer  30 M, a third developer  30 C, and a fourth developer  30 K. Here, the first to fourth developers  30 Y,  30 M,  30 C, and  30 K can form yellow, magenta, cyan, and black developer images, respectively. 
     The four developers  30 Y,  30 M,  30 C, and  30 K are separably installed in the main body  10  of the image forming apparatus  1 , and include image carrier units  31  and developing units  41  which can swing at a predetermined angle. Since the four developers  30 Y,  30 M,  30 C, and  30 K have the same structure, the fourth developer  30 K for forming a black image will be hereinafter described as an example. The reference numeral of the developer will be designated as “ 30 ” unless discrimination is necessary. 
     The image carrier unit  31  may include the image carrier  33  and a charged body  34  that charges the image carrier  33 . An electrostatic latent image is formed on the surface of the image carrier  33  by the light emitted from the exposure unit  20 . A photosensitive drum may be used as the image carrier  33 , and a charge roller may be used as the charged body  34 . Referring to  FIGS. 4 and 5 , the image carrier unit  31  may include a first housing  32  that rotatably support the image carrier  33  and the charged body  34 . On one side of the first housing  32 , an image carrier coupler  33 - 1 , which receives the driving power from an image carrier driving coupler  5  (see  FIG. 6 ) installed in the main body  10 , is installed. Accordingly, if the image carrier driving coupler  5  is rotated, the image carrier coupler  33 - 1  is rotated to rotate the image carrier  33 . At this time, the rotating center of the image carrier  33  is OC (see  FIG. 5 ). Also, inside the first housing  32 , a charged body cleaning member  35  that cleans the surface of the charged body  34  may be further installed. 
     Referring to  FIGS. 3 and 4 , the developing unit  41  is installed to swing at predetermined angle with respect to the image carrier unit  31 , and includes a developing roller  43 , a developer supply roller  44 , and an agitator  45 . The developing unit  41  rotatably supports the developing roller  43 , the developer supply roller  44 , and the agitator  45 , and may include a second housing  42  in which a developer space  46  for storing a predetermined developing agent is formed. The second housing  42  is formed to swing at a predetermined angle with respect to the first housing  32 . On one side of the second housing  42 , a developing coupler  43 - 1  that receives a driving power from the developing driving coupler  7  installed in the main body  3  is installed. Accordingly, the second housing  42  is formed to swing around the developing coupler  43 - 1  with respect to the first housing. As illustrated in  FIG. 5 , a plurality of gears  43 - 2 ,  43 - 3 ,  44 - 1 , and  45 - 1  delivering a driving power to the developing roller  43 , the developer supply roller  44 , and the agitator  45  is connected to the developing coupler  43 - 1  as illustrated in  FIG. 5 . Accordingly, if the developing coupler  43 - 1  is rotated by the developing driving coupler  7  installed in the main body, the developing roller  43 , the developer supply roller  44 , and the agitator  45  are rotated. Accordingly, the developing agent stored in the developer space  6  is supplied to the developing roller  43  through the developer supply roller  44 . In this case, the rotating center DC of the developing roller  43  is apart from the rotating center SC of the developing coupler  43 - 1  so that the developing roller  43  comes in contact with or is separated from the image carrier  33  according to the swing of the second housing  42 . 
     Also, at the lower end of the second housing  42  of the developing unit  41 , a pressed projection  47  that can selectively come in contact with the nip separation unit  50  is formed. The pressed projection  47  may be integrally formed with the second housing  42 . The pressed projection  47  may be installed at the lower end of the second housing  42  to be elastically supported by an elastic member  48  such as a spring as illustrated in  FIG. 8 . 
     A pressing member  40  is installed between the first housing  32  and the second housing  42 . The pressing member  40  is installed between the first housing  32  and the second housing  42  on the opposite side to the developing roller  43  around the developing coupler  43 - 1  that is the swing center of the second housing  42 , and the second housing  42  is elastically supported to rotate clockwise around the developing coupler  43 - 1 . Accordingly, the developing roller  43  installed in the second housing  42  is located in the first position, in which the developing roller  43  comes in contact with the image carrier  33  installed in the first housing  32 , by the pressing member  40 . A compression coil spring may be used as the pressing member  40 . 
     If the developer  30  as constructed above is mounted on the main body  3 , the image carrier coupler  33 - 1  and the developing coupler  43 - 1  of the developer  30  are engaged with the image carrier driving coupler  5  and the developing driving coupler  7 , respectively. The image carrier coupler  33 - 1  receives the driving power from the image carrier driving coupler  5 , and the developing coupler  43 - 1  receives the driving power from the developing driving coupler  7 . The image carrier driving coupler  5  and the developing driving coupler  7  are independently driven. Also, the positions of the image carrier coupler  33 - 1  and the developing coupler  43 - 1  are completely restricted and fixed by the image carrier driving coupler  5  and the development driving coupler  7  in the main body  3 . Although the image carrier  33  is restricted and the position thereof is fixed, the development roller  43  can swing at a predetermined angle around the developing coupler  43 - 1  as illustrated in  FIG. 7 . 
     The nip separation unit  50  is installed in the main body  3  of the image forming apparatus  1 , and is formed to swing the developing unit  41  by selectively pressing the pressed projection  47  of the developer  30 . Accordingly, the nip separation unit  50  is installed on the lower side of the developer  30  inside the main body  3 . If the nip separation unit  50  presses the pressed projection  47 , the developing unit  41  is rotated counterclockwise around the developing coupler  43 - 1 . If the developing unit  41  is rotated counterclockwise, the developing roller  43  is located in the second position that is separated from the image carrier  33  as shown in  FIG. 9 . 
     Referring to  FIGS. 11 and 12 , the nip separation unit  50  may include a guide plate  51 , a first sliding member  52 , a second sliding member  54 , and the separation cam  56 . 
     The guide plate  51  is installed below the developer  30  in the main body  3  of the image forming apparatus  1 , and supports the sliding movement of the first and second sliding members  52  and  54 . 
     The first sliding member  52  is slidably installed on the guide plate  51 , and is formed to selectively come in contact with the pressed projection  47  of the fourth developer  30 K. If the first sliding member  52  presses the pressed projection  47  of the fourth developer  30 K, the developing unit  43  of the fourth developer  30 K swings counterclockwise around the developing coupler  43 - 1 . The first sliding member  52  may include a first sliding body  52 - 1 , a first projection member  52 - 2 , and a first guide pin  53 . The first sliding body  52 - 1  is slidably installed on the guide plate  51 , and forms a first elongated hole  52 - 3  into which the first guide pin  53  is inserted. The first projection member  52 - 2  is fixed to the first sliding body  52 - 1 , and includes a pressing projection  52   a  that comes in contact with the pressed projection  47  of the fourth developer  30 K to swing the developing unit  41 . Accordingly, the first sliding member  52  slides along the guide plate  51  by the first guide pin  53  fixed to the guide plate  51  and the first elongated hole  52 - 3 . If the first sliding member  52  slides, the pressing projection  52   a  of the first projection member  52 - 2  comes in contact with or is separated from the pressed projection  47  of the fourth developer  30 K. 
     The second sliding member  54  is slidably installed on the upper side of the first sliding member  52  with respect to the first sliding member  52 , and is formed to selectively come in contact with the respective pressed projections  47  of the first to third developers  30 Y,  30 M, and  30 C. In this case, a portion of the second sliding member  54  may be directly slidably installed with respect to the guide plate  51 . Accordingly, even in the case where the first sliding member  52  does not move, the second sliding member  54  can move with respect to the guide plate  51  and the first sliding member  52 . Also, if the second sliding member  54  presses the respective pressed projections  47  of the first to third developers  30 Y,  30 M, and  30 C, the respective developing units  41  of the first to third developers  30 Y,  30 M, and  30 C swing counterclockwise around the developing coupler  43 - 1 . 
     The second sliding member  54  may include a second sliding body  54 - 1 , a second projection member  54 - 2 , and a second guide pin  55 . The second sliding body  54 - 1  is slidably installed on the guide plate  51  and the first sliding member  52 , and forms a second elongated hole  54 - 3  into which the second guide pin  55  is inserted. The second projection member  54 - 2  is fixed to the second sliding body  54 - 1 , and includes three pressing projections  54   a ,  54   b , and  54   c  that come in contact with the respective pressed projections  47  of the first to third developers  30 Y,  30 M, and  30 C to swing the developing unit  41 . The three pressing projections  54   a ,  54   b,  and  54   c , as illustrated in  FIG. 12 , are formed to be apart for a distance that corresponds to the first to third developers  30 Y,  30 M, and  30 C to the upper side of the second projection member  54 - 2 . Accordingly, the second sliding member  54  slides along the guide plate  51  by the second guide pin  55  fixed to the guide plate  51  and the second elongated hole  54 - 3 . If the second sliding member  54  slides, the three pressing projections  54   a ,  54   b , and  54   c  of the second projection member  54 - 2  simultaneously come in contact with or are separated from the respective pressed projections  47  of the first to third developers  30 Y,  30 M, and  30 C. 
     The separation cam  56  is rotatably installed between the first and second sliding members  52  and  54 , and is formed to move the first and second sliding members  52  and  54 . The separation cam  56  is formed to be rotated by a cam shaft  57 , and a cam gear  58  is installed at one end of the cam shaft  57  to receive the driving power from a driving source (not illustrated) of the main body  3 . Referring to  FIGS. 14 and 15 , the separation cam  56  includes a first cam unit  56 - 1  pushing both the first and second sliding members  52  and  54  in one direction, a second cam unit  56 - 2  pushing only the second sliding member  54  in the same direction, and a third cam unit  56 - 3  that does not push the first and second sliding members  52  and  54 . The first cam unit  56 - 1  is formed in a circular arc shape having a radius that can simultaneously press the first and second sliding members  52  and  54 . The second cam unit  56 - 2  can press the second sliding member  54  from the first cam unit  56 - 1 , and the first sliding member  52  is extended in a circular arc shape for a predetermined length with a thickness to the extent of non pressing. That is, the second cam unit  56 - 2  is formed in a stepped circular arc shape from the first cam unit  56 - 1 . Accordingly, the second cam unit  56 - 2  presses the second sliding member  54 , but does not press the first sliding member  52 . The third cam unit  56 - 3  is formed in a circular arc shape having a radius that does not press the first and second sliding members  52  and  54 . 
     As shown in  FIGS. 13 and 14 , on a surface that is opposite to the surface that is opposite to the guide plate  51  of the first sliding member  52 , that is, on a surface that is opposite to the second sliding member  54 , a first cam groove  52 - 4  for accommodating the separation cam  56  is formed, and on a surface that is opposite to the first sliding member  52  of the second sliding member  54 , a second cam groove  54 - 4  for accommodating the separation cam  56  is formed. Accordingly, if the second sliding member  54  is located on the upper side of the first sliding member  52 , a cam space is formed by the first and second cam grooves  52 - 4  and  54 - 4 . The separation cam  56  is rotatably installed in the cam space. 
     In the first cam groove  52 - 4  of the first sliding member  52 , a first hole  52 - 6  through which the cam shaft  57  passes and the separation cam  56 , that is, a first cam contact unit  52 - 5  that comes in contact with the first cam unit  56 - 1  of the separation cam  56 , are formed. In the second cam groove  54 - 4  of the second sliding member  54 , a second hole  54 - 6  through which the cam shaft  57  passes and the separation cam  56 , that is, a second cam contact unit  54 - 5  that comes in contact with the first and second cam units  56 - 1  and  56 - 2  of the separation cam  56 , are formed. Accordingly, if the first cam unit  56 - 1  of the separation cam  56  comes in contact with the first and second cam contact units  52 - 5  and  54 - 5  of the first and second sliding members  52  and  54 , as illustrated in  FIG. 16 , the first and second sliding members  52  and  55  are pushed by the separation cam  56  and move in one direction (a direction indicated by an arrow F). The second cam unit  56 - 2  of the separation cam  56  does not come in contact with the first cam contact unit  52 - 5  of the first sliding member  52 , but comes in contact with only the second cam contact unit  54 - 5  of the second sliding member  54  to press the second sliding member  54  in one direction. If the third cam unit  56 - 3  reaches a position that is opposite to the first and second cam contact units  52 - 5  and  54 - 5  of the first and second sliding members  52  and  54 , the separation cam  56  does not press the first and second sliding members  52  and  54 . 
     Accordingly, if the separation cam  56  is rotated clockwise in a state where the first cam unit  56 - 1  of the separation cam  56  comes in contact with the first and second cam contact units  52 - 5  and  54 - 5 , the third cam unit of the separation cam  56  is opposite to the first and second cam contact units  52 - 5  and  54 - 5  as illustrated in  FIG. 17 . In this state, the first and second sliding members  52  and  54  do not receive force in a direction indicated by an arrow F by the separation cam  56 . If the separation cam  56  continues rotation in a state as illustrated in  FIG. 17 , the second cam unit  56 - 2  of the separation cam  56  reaches a position in which the second cam unit  56 - 2  comes in contact with the first and second cam contact units  52 - 5  and  54 - 5  of the first and second sliding members  52  and  54 . Accordingly, the second cam unit  56 - 2  of the separation cam  56  pushes only the second sliding member  54  in a direction indicated by an arrow F as illustrated in  FIG. 18 , but does not push the first sliding member  52 . Accordingly, the first sliding member  52  keeps its position. 
     The cam gear  58  is connected to the driving source (not illustrated) of the main body  3  through a gear train  59 , and the rotation of the cam gear  58  is controlled by a stop member  58 - 1  that can stop the rotation of the gear train  59 . The stop member  58 - 1  may use a solenoid, and if the shaft of the solenoid  58 - 1  is inserted into the groove  59 - 1  a formed on the first gear  59 - 1  of the gear train  59 , the rotation of the cam gear  58  is stopped. The control unit  90  controls the stop member  58 - 1  to control the rotating angle of the cam gear  58 , and by this, the rotating angle of the separation cam  56  can be controlled. 
     Referring again to  FIG. 2 , the transfer belt unit  60  includes a transfer belt  61 , a driving roller  62 , and a driven roller  63 . The transfer belt  61  repeatedly receives the developer images from the image carriers  33  of the four developers  30 Y,  30 M,  30 C, and  30 K, and moves the developer images toward the transfer roller  65 . The driving roller  62  and the driven roller  63  support the transfer belt  61 , and the transfer belt  61  performs a caterpillar operation. 
     At one end of the transfer belt unit  60 , the transfer roller  65  is installed. The transfer roller  65  transfers the developer image formed on the transfer belt  61  to a printing medium that is supplied from the feeder unit  10  between the transfer roller  65  and the transfer belt  61 . 
     The fusing unit  70  includes a pressing roller  71  and a heating roller  72  that are opposite to each other. The pressing roller  71  and the heating roller  72  apply predetermined heat and pressure to the printing medium P to which the developer image is transferred by the transfer roller  65  to fuse the image. 
     A delivery roller  80  is formed to discharge a printing medium P on which the image is fused by the transfer roller  65  and the printing is completed to the outside of the main body  3  of the image forming apparatus  1 . 
     The control unit  90  forms the image that corresponds to the received print data on the printing medium by controlling the feeder unit  10 , the exposure unit  20 , the plurality of developers  30 , the nip separation unit  50 , the transfer belt unit  60 , the transfer roller  65 , the fusing unit  70 , and the delivery roller  80 . During a print standby state, that is, in the preparation mode, the control unit  90  makes the nip separation unit  50  swing the respective developing units  41  of the plurality of developers  30  so that the developing roller  43  is located in the second position that is apart from the image carrier  33 . Then, if a print command is received, the control unit  90  rotates the image carrier  33  and the developing roller  43  of at least one developer  30  according to a control mode, and controls the nip separation unit  50  so that the developing roller  43  in a rotating state reaches the first position in which the developing roller  43  comes in contact with the image carrier  33  after the image carrier  33  is rotated at least as much as the angle between the charged body  34  and the developing roller  43 . 
     Hereinafter, the operation of the image forming apparatus  1  having the above-described construction according to the present disclosure will be described in detail with reference to the accompanying drawings. 
     First, a color image mode in which the image forming apparatus  1  prints a color image will be described. 
     In the preparation mode before the print start, as illustrated in  FIG. 3 , the pressing projections  54   a ,  54   b ,  54   c , and  52   a  of the nip separation unit  50  press the pressed projections  47  of the four developers  30 Y,  30 M,  30 C, and  30 K. Accordingly, the respective developing units  41  of the developers  30 Y,  30 M,  30 C, and  30 K swing at a predetermined angle around the developing coupler  43 - 1  that is a swing center, and a portion of the developing unit  41  on the upper side of the developing coupler  43 - 1  becomes apart from the image carrier  33 , and a portion of the developing unit  41  on the lower side of the developing coupler  43 - 1  becomes close to the image carrier unit  31 . Accordingly, in the preparation mode before the printing starts, the respective developing rollers  43  of the developing units  41  of the four developers  30 Y,  30 M,  30 C, and  30 K are separated from the image carrier  33 , and the pressing member  40  is in compressed state. 
     If the print command is received (S 10 ), the control unit  90  controls the exposure unit  20  to emit light that corresponds to the print data, and thus electrostatic latent images are formed on surfaces of the image carriers  33  of the four developers  30 Y,  30 M,  30 C, and  30 K. 
     Almost at the same time, a high voltage is applied to the charged body  34  to charge the image carrier  33 . Also, the control unit  90  makes the image carriers  33  and the developing rollers  43  of the four developers  30 Y,  30 M,  30 C, and  30 K be rotated in a separated state from each other as illustrated in  FIG. 2  (S 20 ). The control unit  90  may first drive the image carrier  33  earlier than the developing roller  43  for about 200 msec. At this time, the image carrier  33  receives the driving power from the image carrier driving coupler  5 , and the developing roller  43  is rotated by the developing roller gear  43 - 3  (see  FIG. 5 ) that receives the driving power from the developing driving coupler  7 . At this time, since the image carrier  33  and the developing roller  43  are rotated in a state where they are apart from each other to form a gap between them, and thus the developing agent of the developing roller  43  is not attached to the non-charging section on the image carrier  33 . 
     Since the charging is performed in all sections of the surface of the image carrier  33  after the image carrier  33  is rotated as much as the non-charging section between the charged body  34  and the developing roller  43 , no further non-charging section exists on the image carrier  33 . 
     After the image carrier  33  performs one revolution, the control unit  90  controls the nip separation unit  50  so that the developing roller  43  comes in contact with the image carrier  33  (S 30 ) by separating the pressing projections  54   a ,  54   b ,  54   c , and  52   a  from the pressed projections  47 . That is, the control unit  90  rotates the separation cam  56  of the nip separation unit  50  so that the first cam unit  56 - 1  gets out of the first cam contact unit  52 - 5  of the first sliding member  52  and the second cam contact unit  54 - 5  of the second sliding member  54  and the third cam unit  56 - 3  stands opposite to the first and second cam contact unit  52 - 5  and  54 - 5  of the first and second sliding member  52  and  54 . In this case, the pressing forces, which are applied from the four pressing projections  54   a ,  54   b ,  54   c , and  52   a  of the nip separation unit  50  to the pressed projections  47  of the four developers  30 Y,  30 M,  30 C, and  30 K, respectively, are removed, and thus the developing units  41  of the respective developers  30  are rotated by the pressing members  40  at a predetermined angle clockwise around the developing coupler  43 - 1 . If the developing units  41  are rotated at the predetermined angle, the rotating developing rollers  43  come in contact with the rotating image carrier  33  (see  FIG. 9 ). In this case, the pressing projections  54   a ,  54   b ,  54   c , and  52   a  of the first and second sliding members  52  and  54  of the nip separation unit  50 , as illustrated in  FIG. 9 , are located in places that are apart from the pressed projections  47  of the four developers  30 Y,  30 M,  30 C, and  30 K by the separation cam  56 . The control unit  90  controls the stop member  58 - 1  to control the rotation of the cam gear  58 , and thus the rotating angle of the separation cam  56  can be controlled. Here, it is exemplified that the control unit  90  controls the nip separation unit  50  so that the developing roller  43  comes in contact with the image carrier  33  after the image carrier  33  performs one revolution. However, by controlling the nip separation unit  50  so that the developing roller  43  comes in contact with the image carrier  33  after the image carrier  33  is rotated at least as much as the non-charging section A (see  FIGS. 1A and 1B ) of the image carrier  33 , the developing agent is prevented from being attached to the non-charging section. 
     If the developing of the electrostatic image formed on the image carrier  33  is completed, the control unit makes the developing rollers  43  of the four developers  30 Y,  30 M,  30 C, and  30 K be apart from the image carrier  33  (S 40 ). That is, the control unit  90  makes the first cam unit  56 - 1  come in contact with the first and second cam contact units  52 - 5  and  54 - 5  of the first and second sliding members  52  and  54  by rotating the separation cam  56  clockwise. If the first cam unit  56 - 1  of the separation cam  56  come in contact with the first and second cam contact units  52 - 5  and  54 - 5 , the first and second sliding members  52  and  54  move in a direction indicated by an arrow F in  FIG. 9 . Specifically, since the second cam unit  56 - 2  exists between the third cam unit  56 - 3  and the first cam unit  56 - 1  of the separation cam  56 , the separation cam  56  is rotated clockwise, so that the second cam unit  56 - 2  first comes in contact with the second cam contact unit  54 - 5  of the second sliding member  54  to move in the direction indicated by the arrow F. If the separation cam  56  continues rotation, the first cam unit  56 - 1  comes in contact with the first and second cam contact units  52 - 5  and  54 - 5  of the first and second sliding members  52  and  54 , and thus the first sliding member  52  also moves in the direction indicated by the arrow F. If the first and second sliding members  52  and  54  move in the direction indicated by the arrow F, the four developers  30 Y,  30 M,  30 C, and  30 K that correspond to the four pressing projections  54   a ,  54   b ,  54   c , and  52   a  press the pressed projections  47 . If the pressed projections  47  are pressed, the developing units  41  are rotated counterclockwise around the developing coupler  43 - 1 . Accordingly, the pressing members  40  below the developing coupler  43 - 1  are compressed, and the developing rollers  43  on the upper side of the developing coupler  43 - 1  are separated from the image carrier  33  and are located in the second position. Thereafter, the control unit  90  stops the rotation of the developing rollers  43  and the image carrier  33  (S 50 ). 
     The developer images formed by the four developers  30 Y,  30 M,  30 C, and  30 K are repeatedly transferred to the transfer belt  61  to form a color image. The color image formed on the transfer belt  61  is transferred to the printing medium P supplied from the feeder unit  10  by the transfer roller  65 . 
     If the printing medium P onto which the color image is transferred passes through the fusing unit  70 , the color image is fused on the printing medium P by the heat and pressure that is applied by the fusing unit  70 . The printing medium P on which the printing is completed is discharged to the outside of the main body  3  through the delivery roller  80 . 
     Next, a mono image mode in which the image forming apparatus  1  prints a black/white image will be described. 
     Since a process of operating only one developer  30 K that forms a black image among the four developers  30 Y,  30 M,  30 C, and  30 K is different from the above-described color image forming process, a process of operating only one developer  30 K using the nip separation unit  50  will be described hereinafter. 
     In the case of the mono image mode, the control unit  90  rotates the developing roller  43  and the image carrier  33 , and then controls the nip separation unit  50  so that the three developers  30 Y,  30 M, and  30 C that form yellow, magenta, and cyan images maintain the second position in which the developing roller  43  and the image carrier  33  are apart from each other, and only the developing roller  43  of the developer  30 K that forms a black image comes in contact with the image carrier  33 . That is, the control unit  90  rotates the separation cam  56  clockwise so that the first cam unit  56 - 1  gets out of the first and second cam contact units  52 - 5  and  54 - 5  of the first and second sliding members  52  and  54  and the third cam unit  56 - 3  stands opposite to the first and second cam contact unit  52 - 5  and  54 - 5 , In this case, the developing units  41  are rotated clockwise at a predetermined angle by the pressing members  40  of the four developers  30 Y,  30 M,  30 C, and  30 K, and thus the developing rollers  43  come in contact with the image carrier  33 . If the separation cam  56  continues rotation, the second cam unit  56 - 2  comes in contact with the second cam contact unit  54 - 5  of the second sliding member  54 . Accordingly, the second sliding member  54  moves in the direction indicated by the arrow F, and the first sliding member  52  maintains its current position. If the second sliding member  54  move in the direction indicated by the arrow F, the pressed projections  47  of the three developers  30 Y,  30 M, and  30 C are pressed by the pressing projections  54   a ,  54   b , and  54   c . If the pressed projections  47  are pressed, the developing unit  41  of the developer  30  is rotated counterclockwise around the developing coupler  43 - 1 . If the developing unit  41  is rotated counterclockwise around the developing coupler  43 - 1 , the pressing member  40  below the developing coupler  43 - 1  is compressed, and the developing roller  43  on the upper side of the developing coupler  43 - 1  is separated from the image carrier  33 . If the second cam unit  56 - 2  of the separation cam  56  comes in contact with the second cam contact unit  54 - 5  of the second sliding member  54 , the control unit  90  stops the separation cam  56 . Accordingly, the three developers  30 Y,  30 M, and  30 C that form yellow, magenta, and cyan images maintain the position in which the developing roller  43  and the image carrier  33  are apart from each other, and only the developer  30 K that forms a black image maintains the position in which the developing roller  43  and the image carrier  33  comes in contact with each other. Accordingly, the developer  30 K can form the black/white image. 
     If the print of the black/white image is completed, the control unit  90  rotates the separation cam  56  clock wise. Accordingly, the second cam unit  56 - 2  of the separation cam  56  gets out of the first and second cam contact units  52 - 5  and  54 - 5  of the first and second sliding members  52  and  54 , and the first cam unit  56 - 1  is located in the position. Accordingly, the first sliding member  52  is also pushed in the direction indicated by the arrow F by the first cam unit  56 - 1  of the separation cam  56 . In this case, the pressed projection  47  of the fourth developer  30 K is pressed by the pressing projection  52   a  of the first sliding member  52 . If the pressed projection  47  is pressed, the developing unit  41  is rotated counterclockwise around the developing coupler  43 - 1 , and the developing roller  43  is separated from the image carrier  33 . Thereafter, the control unit  90  stops the rotation of the developing roller  43  and the image carrier  33 . 
     As described above, it is exemplified that the image forming apparatus  1  performs a control operation using the nip separation unit  50  that controls contact and separation of the developing rollers  43  of the four developers  30 Y,  30 M,  30 C, and  30 K and the image carrier  33  by one driving source. However, the method of controlling the contact and separation of the developing roller  43  is not limited thereto. 
     As another example, the contact and the separation of the developing rollers of the four developers  30 Y,  30 M,  30 C, and  30 K may be controlled using separate nip separate members. An example of the nip separation member and the developers is illustrated in  FIG. 19 . 
     Referring to  FIG. 19 , on one side of the pressed projections  47  of the four developers  30 Y,  30 M,  30 C, and  30 K, four nip separation members  95   a ,  95   b ,  95   c , and  95   d  are installed. Accordingly, the respective pressed projections  47  can be pressed by the nip separation members  95   a ,  95   b ,  95   c , and  95   d . In a preparation step before the printing, the four nip separation members  95   a ,  95   b ,  95   c , and  95   d  press the corresponding pressed projections  47 , and the developing roller  43  is apart from the image carrier  33 . 
     In case of forming a color image, the control unit  90  rotates the developing rollers  43  of the developers  30 Y,  30 M,  30 C, and  30 K and the image carrier  33 , and then controls the first to four nip separation members  95   a ,  95   b ,  95   c , and  95   d  so that the pressed projections  47  are not pressed. Accordingly, the developing unit  41  swings at a predetermined angle by the pressing member  40 , and the rotating developing roller  43  comes in contact with the rotating image carrier  33 . 
     In the case of forming a black/white image, the control unit  90  rotates the developing rollers  43  of the developers  30 Y,  30 M,  30 C, and  30 K and the image carrier  33 , and then controls the first to four nip separation members  95   a ,  95   b ,  95   c , and  95   d  so that the first to third nip separation member  95   a ,  95   b , and  95   c  maintain their current state, and only the fourth nip separation member  95   d  is controlled not to press the pressed projections  47  of the fourth developer  30 K. Accordingly, the rotating developing roller  43  of the fourth developer  30 K comes in contact with the rotating image carrier  33  to form the black/white image. 
     As described above, it is exemplified that the image forming apparatus  1  includes fourth developers  30 Y,  30 M,  30 C, and  30 K and forms a color image. However, the present disclosure can be applied to a mono image forming apparatus including only one developer. 
     As described above, according to the present disclosure, when the developer forms an image, the developing roller and the image carrier are first rotated, and after the image carrier is rotated so that the non-charging section of the image carrier passes the contact point with the developing roller, the rotating developing roller comes in contact with the rotating image carrier to prevent the developing agent from being attached to the non-charging section. Accordingly, it is not necessary to prepare a waste developer chamber that accommodates the waste developing agent that is removed from the image carrier and the transfer belt, and thus the developer and the image forming apparatus can be miniaturized. Also, since there is no developing agent that is attached to the non-charging section, the amount of consumption of the developing agent is reduced, and thus the maintenance cost can be reduced. Also, a uniform image quality can be obtained. 
     While the present disclosure has been shown and described with reference to certain embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention, as defined by the appended claims.