Patent Publication Number: US-9429902-B2

Title: Developing cartridge and image forming apparatus employing the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of Korean Patent Application No. 10-2014-0183300, filed on Dec. 18, 2014, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     1. Field 
     The present disclosure relates to developing cartridges and image forming apparatuses adopting the developing cartridges for printing images on a recording medium by using an electrophotographic method. 
     2. Description of the Related Art 
     An electrophographic image forming apparatus prints images on a recording medium by forming a visible toner image on a photosensitive body by supplying toner to an electrostatic latent image formed on the photosensitive body, transferring the toner image onto the recording medium, and fusing the toner image onto the recording medium. 
     A developing cartridge is an assembly including components for forming a visible toner image, and may be attached to/detached from a main body of an image forming apparatus. In addition, the developing cartridge is an expendable item that may be replaced when a lifespan thereof expires. In a developing cartridge that adopts a contact developing method, a developing roller and a photosensitive material contact each other to form a developing nip. 
     A developing cartridge may be distributed in a state of being attached to a main body of an image forming apparatus or in a state of being separately packaged from the image forming apparatus. Also, a developing cartridge may be in a standby state for a long time while in a state of being attached to the main body of the image forming apparatus, while the image forming apparatus is being used. 
     As described above, if a long time has elapsed in a state since a developing nip has been formed, a developing roller may be deformed or a photosensitive body may be damaged. The deformation of the developing roller and the damage to the photosensitive body may cause deformation of the developing nip, thereby adversely affecting image quality. 
     SUMMARY 
     Provided are developing cartridges and image forming apparatuses capable of separating/forming a developing nip by a manual manipulation and switching a developing nip from an isolation state to a formed state by an operation of the image forming apparatus. 
     According to an aspect of an exemplary embodiment, a developing cartridge includes: a photosensitive unit including a photosensitive drum; a developing unit including a developing roller, and being connected to the photosensitive unit to rotate to a developing location, where the photosensitive drum and the developing roller contact each other to form a developing nip, and to rotate to a non-developing location, where the developing roller and the photosensitive drum are separate from each other to remove the developing nip; an elastic member configured to apply an elastic force to the developing unit and the photosensitive unit in a direction of rotating toward the developing location; and a nip control unit including a nip separating portion switched to a nip forming location where the developing unit is located at the developing location and to a nip separating location where the developing unit is located at the non-developing location, a manipulation portion configured to switch the nip separating portion to the nip forming location and to the nip separating location by a manual manipulation, and a nip forming unit configured to switch the nip separating portion from the nip separating location to the nip forming location as the photosensitive unit and the developing unit start to operate. 
     The manipulation portion may be exposed to outside of the developing cartridge when the nip separation unit is located on the nip forming location. 
     The nip forming unit may switch the nip separating portion from the nip separating location to the nip forming location as the developing roller rotates. 
     The nip separating portion may interfere with the photosensitive unit when switched from the nip forming location to the nip separating location so that the developing unit rotates with respect to the photosensitive unit in a direction opposite to the direction in which the elastic force is applied. 
     The nip forming location may include a first nip forming location and a second nip forming location, the manipulation portion may switch the nip separating portion to the first nip forming location, and the second nip forming location, and the nip separating location, and the nip forming unit may switch the nip separating portion from the nip separating location to the second nip forming location. 
     The developing cartridge may further include a nip separation member including the nip separating portion and the manipulation portion, and being provided on the developing unit to be switched to the first nip forming location and second nip forming location and the nip separating location, wherein the nip forming unit may include: a gear to rotate with the developing roller; and a nip forming member including a cam gear portion to engage with the gear at the nip separating location and to separate from the gear at the first nip forming location and the second nip forming location, and the nip forming member being connected to the nip separation member. 
     The nip separation member and the nip forming member may be provided in the developing unit so as to rotate about a rotary shaft. The nip forming member may include a protrusion and the nip separation member may include an insertion portion into which the protrusion is inserted. The nip separation member and the nip forming member may be rotated with each other by using the protrusion of the nip forming member and the insertion portion of the nip separation member into which the protrusion is inserted. 
     The nip separating portion may interfere with the developing unit while switching from the nip forming location to the nip separation location to rotate the developing unit with respect to the photosensitive unit in an opposite direction to a direction in which the elastic force is applied. 
     The developing cartridge may further include: a nip control member including the nip separating portion and the nip control member the manipulation portion and being provided on the photosensitive unit to move to the nip forming location and the nip separating location; a lock unit configured to lock the nip control member at the nip separating location; and a return spring configured to apply an elastic force to the nip control member in a direction of locating at the nip forming location, wherein the nip forming unit may include: a gear to rotate with the developing roller and the gear including a releasing cam; and a releasing portion configured to interfere with the releasing cam when the gear rotates in a state where the nip control member is located at the nip separating location to unlock the nip control member. 
     The lock unit may include a first hook arranged on the photosensitive unit and a second hook arranged on the nip control member to be hooked by the first hook when the nip control member is located at the nip separating location. 
     The second hook and the nip separation unit may be arranged at an elastic arm extending from a body of the nip control member, and the releasing cam may interfere with the nip separating portion to push the elastic arm so that the second hook unhooks from the first hook. 
     The photosensitive unit may include a manual manipulation recess configured to push the elastic arm so that the second hook unhooks from the first hook when the nip control member is located at the nip separating location. 
     The nip separating portion may interfere with the photosensitive drum while switching from the nip forming location to the nip separating location so that the developing unit rotates with respect to the photosensitive unit in an opposite direction to the direction in which the elastic force is applied. 
     The nip forming unit may switch the nip separating portion from the nip separating location to the nip forming location as the photosensitive drum rotates. 
     The nip forming location may include a first nip forming location and a second nip forming location, the manipulation portion may switch the nip separating portion to the first and second nip forming locations and the nip separating location, and the nip forming unit may switch the nip separating portion from the nip separating location to the second nip forming location. 
     The developing cartridge may further include a nip control member including the manipulation portion and the nip separating portion, and the nip control member being provided on the developing unit to rotate to the nip separating location and to the nip forming location. 
     According to an aspect of an exemplary embodiment, an image forming apparatus includes: a main body; and the developing cartridge described above, attachable to and detachable from the main body. 
     The image forming apparatus may further include a first switching unit arranged in the main body and configured to interfere with the manipulation portion while the developing cartridge is installed in the main body so as to switch the nip separating portion from the nip forming location to the nip separating location. 
     The image forming apparatus may further include: a door configured to open and close an opening formed in the main body in order to attach the developing cartridge to the main body and detach the developing cartridge from the main body; and a first switching unit formed in the main body and configured to interfere with the manipulation portion in association with a closing operation of the door in order to switch the nip separating portion from the nip forming location to the nip separating location. 
     The nip forming location may include a first nip forming location and a second nip forming location, the nip forming unit may switch the nip separating portion from the nip separating location to the second nip forming location, and the main body may include a second switching unit for switching the nip separating portion from the second nip forming location to the nip separating location. 
     The main body may include a detection unit configured to detect a location of the nip separating portion. 
     According to an aspect of an exemplary embodiment, a developing cartridge includes a photosensitive unit comprising a photosensitive drum; a developing unit comprising a developing roller, where the photosensitive drum and the developing roller contact each other to form a developing nip, and separate from each other to remove the developing nip; an elastic member configured to apply an elastic force to the developing unit and the photosensitive unit in a direction of rotating toward contacting each other; and a nip control unit including: a nip separation member including a nip separating portion and a manipulation portion configured to move the nip separating portion to a nip forming location to form the developing nip and to a nip separating location to remove the developing nip by a manual manipulation, and a nip forming unit configured to move the nip separating portion from the nip separating location to the nip forming location as the photosensitive unit and the developing unit start to operate, wherein the nip forming unit including: a gear to rotate with the developing roller; and a nip forming member comprising a cam gear portion to engage with the gear at the nip separating location and to separate from the gear at the nip forming location, wherein the nip forming member is connected to the nip separation member. 
     According to an aspect of an exemplary embodiment, a developing cartridge includes a photosensitive unit comprising a photosensitive drum; a developing unit including a developing roller, where the photosensitive drum and the developing roller contact each other to form a developing nip, and separate from each other to remove the developing nip; an elastic member configured to apply an elastic force to the developing unit and the photosensitive unit in a direction of rotating toward contacting each other; and a nip control unit including: a nip control member including a body supported by the developing unit to be rotatable, nip separating portion protruding from the body, and a manipulation portion extending from the body to an outer portion of the development cartridge, wherein the nip control member is manually rotatable to a first nip forming location to form the developing nip and is manually rotatable to a nip separating location to remove the developing nip, and wherein the nip control unit is configured to rotate the nip control member from the nip separating location to a second nip forming location when the photosensitive unit and the developing unit start to operate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects will become apparent and more readily appreciated from the following description of exemplary embodiments, taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a schematic diagram of an image forming apparatus according to an exemplary embodiment; 
         FIG. 2  is a diagram showing an arrangement of a photosensitive drum and a developing roller in a contact developing method; 
         FIG. 3  is a side view of a developing cartridge according to an exemplary embodiment; 
         FIG. 4  is a diagram showing a state in which a nip separation member is located at a second position; 
         FIG. 5  is a partial side view of a developing cartridge according to an exemplary embodiment; 
         FIG. 6  is a side view of the developing cartridge of  FIG. 5 , showing a connecting relation between a nip forming member and a nip separation member; 
         FIG. 7A  is a diagram showing a state in which a nip separation member is located at a first position and a nip separating portion is located on a first nip forming location; 
         FIG. 7B  is a diagram showing a state in which a nip separation member is located at a second position and a nip separating portion is located at a nip separating location; 
         FIG. 7C  is a diagram showing a state in which a nip separation member is located at a third position and a nip separating portion is located at a second nip forming location; 
         FIGS. 8 and 9  are diagrams showing examples of a first switching unit that operates a nip separation member when a developing cartridge is attached to a main body to release a developing nip; 
         FIGS. 10 and 11  are diagrams showing an example of a first switching unit for switching a nip separation member from a first position to a second position in association with a closing operation of a door; 
         FIGS. 12, 13, and 14  are diagrams of a second switching unit for switching a nip separation member from a second nip forming location to a nip separating location according to an exemplary embodiment; 
         FIG. 15  is a diagram of a detection unit according to an exemplary embodiment; 
         FIG. 16  is a schematic diagram of an image forming apparatus according to an exemplary embodiment; 
         FIG. 17  is a perspective view showing an operation of attaching a developing cartridge to a main body of an image forming apparatus; 
         FIGS. 18 and 19  are a side view and a plan view of a first switching unit for releasing a developing nip by operating a nip separation member when a developing cartridge is attached to a main body, according to an exemplary embodiment; 
         FIGS. 20 and 21  are diagrams of a first switching unit for switching a nip separation member of a developing cartridge from a first position to a second position in association with a closing operation of a door, according to an exemplary embodiment; 
         FIGS. 22, 23, and 24  are diagrams of a second switching unit for switching a nip separation member from a third position to a second position, according to an exemplary embodiment; 
         FIG. 25  is a side view of a developing cartridge according to an exemplary embodiment; 
         FIG. 26  is a diagram showing an arrangement of a nip control member in the developing cartridge of  FIG. 25 ; 
         FIG. 27  is a perspective view of a nip control member according to an exemplary embodiment; 
         FIG. 28  is a diagram showing a state in which a nip separation portion is located at a nip forming location, according to an exemplary embodiment; 
         FIG. 29  is a diagram showing a state in which a nip separation portion is located at a nip separating location, according to an exemplary embodiment; 
         FIG. 30  is a diagram showing an operation of a releasing cam; 
         FIG. 31  is a diagram of a first switching unit according to an exemplary embodiment; 
         FIG. 32  is a schematic diagram of an image forming apparatus according to an exemplary embodiment; 
         FIG. 33  is a schematic diagram of a developing cartridge according to an exemplary embodiment; 
         FIGS. 34 and 35  are block diagrams of a nip control unit according to an exemplary embodiment; 
         FIG. 36  is a diagram showing a state in which a nip control member is located at a third position, according to an exemplary embodiment; 
         FIG. 37  is a diagram showing an example of a structure for locking a nip control member to a second position, a first position, and a third position, according to an exemplary embodiment; 
         FIGS. 38 and 39  are diagrams of a first switching unit operating a nip control member to release a developing nip when a developing cartridge is attached to a main body, according to an exemplary embodiment; and 
         FIGS. 40 and 41  are diagrams of a second switching unit that switches a nip control member from a third position to a second position, according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. In this regard, an exemplary embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, exemplary embodiments are merely described below, by referring to the figures, to explain aspects of exemplary embodiments. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. 
     (Image Forming Apparatus) 
       FIG. 1  is a schematic diagram of an image forming apparatus  1 - 1 . The image forming apparatus  1 - 1  according to an exemplary embodiment prints an image in a single color, e.g., black, on a recording medium P by using an electrophotographic method. 
     Referring to  FIG. 1 , the image forming apparatus  1 - 1  may include a developing device  10 , an exposure unit  20 - 1 , a transfer unit  30 - 1 , a fuser  40 - 1 , and a feed unit  50 - 1 . 
     The developing device  10  includes a photosensitive drum  11 . The photosensitive drum  11  according to an exemplary embodiment is formed as a cylinder having a surface on which a photosensitive layer is formed, but is not limited thereto. A charging roller  12  charges the photosensitive drum  11  to have a uniform surface potential. A charging brush or a corona charger may be used instead of the charging roller  12 . A developing roller  13  supplies toner in the developing device  10  onto the photosensitive drum  11 . 
     The developing device  10  may further include a charging roller cleaner (not shown) for removing a developing agent or dust from the charging roller  12 , a cleaning member  16  for removing a developing agent remaining on the photosensitive drum  11  after a transfer process that will be described later, and a regulating member (not shown) for regulating an amount of the toner supplied to a developing region, in which the photosensitive drum  11  and the developing roller  13  face each other, the toner being supplied by the developing roller  13 . 
     In a case of employing a two-component developing method, the toner and a magnetic carrier are accommodated in the developing device  10 , and the developing roller  13  is spaced away from the photosensitive drum  11  by a distance of tens to hundreds of microns. Although not illustrated, the developing roller  13  may be a magnetic roller, in which a magnetic roller is disposed in a developing sleeve. The toner is attached to a surface of the magnetic carrier. The magnetic carrier is attached to a surface of the developing roller  13  and transported to the developing area in which the photosensitive drum  11  and the developing roller  13  face each other. Only the toner is supplied to the photosensitive drum  11  due to a developing bias voltage applied between the developing roller  13  and the photosensitive drum  11  so that an electrostatic latent image formed on the surface of the photosensitive drum  11  may be developed into a visible toner image. The developing roller  13  may rotate away from the photosensitive drum  11  by a distance of tens to hundreds of microns. In the case of employing the two-component developing method, the developing device  10  may further include an agitating/transporting member for mixing and agitating the toner with the carrier in the developing device  10  and transporting the mixed and agitated toner and the carrier to the developing roller  13 . 
     In a case of employing a mono-component developing method in which the magnetic carrier is not used, the toner is accommodated in the developing device  10 , and the developing roller  13  may be spaced away from the photosensitive drum  11  by a distance of tens to hundreds microns (mono-component non-contact developing method) or may contact the photosensitive drum  11  (mono-component contact developing method). In an exemplary embodiment, the mono-component contact developing method is used. 
       FIG. 2  is a diagram showing arrangements of the photosensitive drum  11  and the developing roller  13  in the contact developing method. Referring to  FIG. 2 , the developing roller  13  contacts the photosensitive drum  11  to form a developing nip  15 . The developing roller  13  may include a rotary shaft  13   a , and an elastic layer  13   b  formed around the rotary shaft  13   a . The elastic layer  13   b  is elastically deformed when contacting the photosensitive drum  11 , thereby forming the developing nip  15 . In order to regulate a contact amount T between the developing roller  13  and the photosensitive drum  11 , gap maintenance members  13   c  having a diameter that is less than that of the developing roller  13  may be provided at opposite ends of the rotary shaft  13   a . The gap maintenance members  13   c  contact the surface of the photosensitive drum  11  to regulate the contact amount T of the developing roller  13  with respect to the photosensitive drum  11 . The developing device  10  may further include a supply roller  14  for attaching toner to the surface of the developing roller  13 . A supply bias voltage may be applied to the supply roller  14 . The developing device  10  may further include an agitator (not shown) that agitates the toner to supply the toner toward the developing roller  13 . The agitator may agitate and triboelectrically charge the toner. The toner is attached to the surface of the developing roller  13  by an electrostatic force and transported to the developing nip  15  where the photosensitive drum  11  and the developing roller  13  face each other. The toner is supplied to the photosensitive drum  11  by a developing bias voltage applied between the developing roller  13  and the photosensitive drum  11 , and then develops an electrostatic latent image formed on the surface of the photosensitive drum  11  into a visible toner image. 
     The exposure unit  20 - 1  emits light that is modulated in correspondence with image information, to the photosensitive drum  11  to form an electrostatic latent image on the photosensitive drum  11 . A laser scanning unit (LSU) that uses a laser diode as a light source or an exposure unit that uses a light emitting diode (LED) as a light source may be used as the exposure unit  20 - 1 . 
     The transfer unit  30 - 1  transfers the toner image developed on the photosensitive drum  11  onto the recording medium P. A transfer roller that is disposed to face the photosensitive drum  11  may be used as the transfer unit  30 - 1 . A transfer bias voltage is applied to the transfer roller. A transfer device such as a corona transfer device or a pin scorotron type transfer device may be used, instead of the transfer roller. 
     The fuser  40 - 1  applies heat and pressure to the toner image transferred to the recording medium P to fix the toner image to the recording medium P. The fuser  40 - 1  may include a heating member  41  and a compressing member  42 . Referring to  FIG. 1 , the heating member  41  and the compressing member  42  are roller type members that compress and contact each other to form a fusing nip. The shape of the fuser  40 - 1  is not limited to the example shown in  FIG. 1 . When the recording medium P passes through the fusing nip, the toner image is fused and fixed to the recording medium P due to the heat and pressure. 
     When a printing command is transmitted from a host (not shown), a controller (not shown) charges the surface of the photosensitive drum  11  to a constant potential by using the charging roller  12 . The exposure unit  20 - 1  irradiates a light beam that is modulated corresponding to image information to be printed to the photosensitive drum  11  so as to form an electrostatic latent image on the photosensitive drum  11 . The developing roller  13  supplies the toner to the photosensitive drum  11  to develop the electrostatic latent image into a visible toner image. The feed unit  50 - 1  transports the recording medium P to the transfer nip formed by the transfer roller and the photosensitive drum  11  along with a paper feeding path  51 . The toner image on the photosensitive drum  11  is transferred onto the recording medium P due to the transfer bias voltage applied to the transfer roller. When the recording medium P passes through the fuser  40 - 1 , the toner image is fixed on the recording medium P due to heat and pressure. The recording medium P on which the toner image has fixed is discharged to outside by a discharge roller  60 . 
     The developing device  10  is replaced when all the toner contained therein is consumed. The image forming apparatus  1 - 1  includes a main body  2 - 1  including an opening  3 - 1 , and a door  4 - 1  for opening and closing the opening  3 - 1 . The door  4 - 1  may pivot about, for example, a hinge  4   a , to open and close the opening  3 - 1 . The developing device  10  may be attached to or detached from the main body  2 - 1  via the opening  3 - 1 . Hereinafter, the developing device  10  that may be replaceable is referred to as a developing cartridge  10 . 
     In the contact developing method, image quality is largely dependent on a variation of the developing nip  15 . When the image forming apparatus  1 - 1  is provided to a user, the developing cartridge  10  may be provided in a state where the photosensitive drum  11  and the developing roller  13  contact each other. It may take a long time from when manufacturing of the image forming apparatus  1 - 1  has been completed for a user to purchase the image forming apparatus  1 - 1  and install the developing cartridge  10  in the main body  2 - 1  of the image forming apparatus  1 - 1 . During the above time period, the developing roller  13  may be deformed and the developing nip  15  may be deformed. Also, the photosensitive layer of the photosensitive drum  11  may be damaged. In addition, the toner existing on the developing nip  15  may be fixed on the developing roller  13  or the photosensitive drum  11 . 
     To address the above issues, the developing cartridge  10  may be separately packaged from the main body  2 - 1  of the image forming apparatus  1 - 1  to be provided to the user in a state where the developing roller  13  is spaced away from the photosensitive drum  11 , and the developing roller  13  and the photosensitive drum  11  may contact each other to form the developing nip  15  when the user installs the developing cartridge  10  in the main body  2 - 1 . However, in this case, the main body  2 - 1  and the developing cartridge  10  may be separately packaged and distributed, and thus, logistics costs may increase. Also, the user has to manipulate the developing cartridge  10  to form the developing nip  15  before installing the developing cartridge  10  in the main body  2 - 1 , and thus, the user may feel inconvenience when using the image forming apparatus  1 - 1 . Also, if the user installs the developing cartridge  15  in the main body  2 - 1  without forming the developing nip  15 , it is impossible to print images. Therefore, a process that the user checks whether the developing nip  15  is formed or whether the manipulation for forming the developing nip  15  is performed and a process of letting the user perform the manipulation for forming the developing nip  15  by isolating the developing cartridge  10  from the main body  2 - 1  if the developing nip  15  is not formed are necessary in control processes of the image forming apparatus  1 - 1 , and thus, the processes of controlling the image forming apparatus  1 - 1  may be complicated. Also, if the user who is not accustomed to the image forming apparatus  1 - 1  does not understand the processes of checking the developing nip  15  and forming the developing nip  15  according to the processes of controlling the image forming apparatus  1 - 1 , the user may think that the image forming apparatus  1 - 1  is broken. 
     (Developing Cartridge) 
     The developing cartridge  10  includes a nip control unit capable of isolating and forming the developing nip  15 . According to the nip control unit of an exemplary embodiment, the developing nip  15  may be separated according to a manual manipulation of the user, and when the developing cartridge  10  is installed in the main body  2 - 1  and the image forming apparatus  1 - 1  is driven, the developing nip  15  may be formed. 
       FIG. 3  is a side view of the developing cartridge  10  according to an exemplary embodiment. Referring to  FIGS. 1 and 3 , the developing cartridge  10  includes a photosensitive unit  100  and a developing unit  200 . The photosensitive unit  100  includes a first housing  101 , and the photosensitive drum  11  supported by the first housing  101 . The developing unit  200  includes a second housing  201 , and the developing roller  13  supported by the second housing  201 . The photosensitive unit  100  and the developing unit  200  may be connected to each other so as to rotate between a developing location ( FIG. 3 ) where the photosensitive drum  11  and the developing roller  13  contact each other to form the developing nip  15 , and a non-developing location ( FIG. 4 ) where the photosensitive drum  11  and the developing roller  13  are separate from each other so that the developing nip  15  is removed. For example, the photosensitive unit  100  and the developing unit  200  may be connected to rotate between the developing location and the non-developing location about a hinge shaft  301 . In the image forming apparatus  1 - 1 , the photosensitive drum  11  is related to a location of the transfer roller  30 - 1 , and thus, when the developing cartridge  10  is installed in the main body  2 - 1 , the location of the photosensitive drum  11  is fixed. Therefore, the developing unit  200  is coupled to the photosensitive unit  100  to rotate about the hinge shaft  301 . However, one or more exemplary embodiments are not limited thereto, that is, the developing unit  200  may be located at a fixed location in the main body  2 - 1  and the photosensitive unit  100  may be coupled to the developing unit  200  to be rotatable about the hinge shaft  301 . 
     An elastic member  330  provides an elastic force to the developing unit  200  and the photosensitive unit  100  so that the developing unit  200  and the photosensitive unit  100  rotate in a direction of forming the developing nip  15 , that is, in a direction toward the developing location. The developing unit  200  rotates about the hinge shaft  301  due to the elastic force of the elastic member  330  so that the developing roller  13  contacts the photosensitive drum  11 , and accordingly, the developing nip  15  is formed as shown in  FIG. 2 .  FIG. 3  shows a tensile coil spring having an end portion and the other end portion that are respectively supported by the photosensitive unit  200  and the developing unit  100  as an example of the elastic member  330 , but the elastic member  330  is not limited thereto. For example, members of various shapes such as a torsion coil spring, a plate spring, etc., may be used as the elastic member  330 . 
     Driving elements of the developing cartridge  10 , e.g., the photosensitive drum  11 , the charging roller  12 , the developing roller  13 , the supply roller  14 , and the agitator (not shown) may be driven in association with a driving connection portion (not shown) that is disposed in the main body  2 - 1  when the developing cartridge  10  is installed in the main body  2 - 1 . For example, the developing cartridge  10  may include a coupler  310  that is connected to the driving connection portion (not shown) that is disposed in the main body  2 - 1  when the developing cartridge  10  is installed in the main body  2 - 1 . The driving elements may be connected to the coupler  310  via a power connection unit that is not shown, e.g., a gear. The developing cartridge  10  may further include a coupler  320  that is connected to the driving connection portion (not shown) disposed in the main body  2 - 1  when the developing cartridge  10  is installed in the main body  2 - 1 . In this case, the driving elements of the developing unit  200 , e.g., the developing roller  13 , the supply roller  14 , the agitator (not shown), etc., may be driven in association with the coupler  310 , and the driving elements of the photosensitive unit  100 , e.g., the photosensitive drum  11  and the charging roller  12  may be driven in association with the coupler  320 . The coupler  320  may be coaxially located with, for example, the rotational axis of the photosensitive drum  11 . Although not illustrated in the drawings, the coupler  310  may be omitted, and the developing roller  13 , the supply roller  14 , and the agitator (not shown) may be connected to the coupler  320 . 
     The hinge shaft  301  may be coaxial with, for example, the rotational axis of the coupler  310 , but is not limited thereto. The hinge shaft  301  may be located at any location that the photosensitive unit  100  and the developing unit  200  rotate to separate/form the developing nip  15 . 
     The nip control unit may include a nip separating portion switching between a nip forming location and a nip separating location, where the developing unit  200  are respectively located on the developing location and the non-developing location, a manipulation portion for switching the nip separating portion to the nip forming location and the nip separating location by a manual manipulation, and a nip forming unit for switching the nip separating portion from the nip separating location to the nip forming location when the photosensitive unit  100  and the developing unit  200  operate. 
     The developing nip  15  may be formed or removed by rotating the developing unit  200  about the hinge shaft  301 . Referring to  FIG. 3 , the nip control unit includes a nip separation member  340  for removing the developing nip  15 . The nip separation member  340  may be at a first location for forming the developing nip  15  and a second location for removing the developing nip  15 .  FIG. 3  shows a state where the nip separation member  340  is located at the first location.  FIG. 4  shows a state where the separation member  340  is located at the second location. The nip separation member  340  may be provided on, for example, the second housing  201 . The nip separation member  340  may be provided on the second housing  201  so as to rotate about the rotary shaft  302 . The nip separation member  340  includes a manipulation portion  341  for providing a grip for a manual manipulation, and a nip separating portion  342  that is interfered with the photosensitive unit  100 , e.g., the first housing  101 , so as to push the photosensitive unit  100  when switching from the first location to the second location. The manipulation portion  341  is exposed to outside of the developing cartridge  10  when the nip separation member  340  is located at least on the first location, that is, when the nip separating portion  342  is located at the nip forming location (first nip forming location). As such, the user may easily access the manipulation portion  341 . 
     By rotating the nip separation member  340  from the first location to the second location or from the second location to the first location, the nip separating portion  342  may be switched from the nip forming location (first nip forming location:  FIG. 3 ) to the nip separating location ( FIG. 4 ) or from the nip separating location to the nip forming location (first nip forming location) to control the developing nip  15 . For example, when the nip separation member  340  located at the first location is rotated in a first direction (A 1 ) to the second location as shown in  FIG. 4 , the nip separating portion  342  is switched from the nip forming location (first nip forming location) to the nip separating location and then pushes the first housing  101  so that the developing unit  200  is rotates about the hinge shaft  301  in a B 1  direction, that is, a direction opposite to the direction in which the elastic force of the elastic member  330  is applied. Then, the developing roller  13  is separate from the photosensitive drum  11 , and the developing nip  15  is removed. On the contrary, when the nip separation member  340  located at the second location is rotated in a second direction (A 2 ) that is opposite to the first direction A 1  to be switched to the first location shown in  FIG. 3 , the nip separating portion  342  is switched from the nip separating location to the nip forming location (first nip forming location) and is separate from the first housing  101  so that the developing unit  200  is rotated about the hinge shaft  301  in a B 2  direction due to the elastic force of the elastic member  330 . Then, the developing roller  13  contacts the photosensitive drum  11  to form the developing nip  15 . 
     According to the above configuration, the developing nip  15  may be formed or removed by the manual manipulation of the user. 
       FIG. 5  is a partial side view of the developing cartridge  100 . In  FIG. 5 , the nip separation member  340  is omitted. The nip control unit includes the nip forming member  350  that forms the developing nip  15  according to the operation of the image forming apparatus  1 - 1 . The nip forming member  350  of an exemplary embodiment forms the developing nip  15  by rotating the nip separation member  340  from the second location to another location (third location), for example, a location rotated further in the first direction A 1 . Accordingly, the nip separating portion  342  is switched to the second nip forming location that is different from the first nip forming location. 
     The nip forming member  350  according to an exemplary embodiment may be located at inside of a side wall  202  of the second housing  201 . The nip forming member  350  is provided to be rotated about, for example, the rotary shaft  302 . The nip forming member  350  includes a cam gear portion  351 . The cam gear portion  351  is a partial gear formed on a part of an outer circumference of the nip forming member  350 . For example, a gear  360  is coupled to a rotary shaft  13   a  of the developing roller  13  to rotate the developing roller  13 , and the cam gear portion  351  may be connected to the gear  360 . When the developing cartridge  10  is installed in the main body  2 - 1 , the driving connection portion (not shown) disposed in the main body  2 - 1  is connected to the coupler  310 , and accordingly, the developing roller  13 , the gear  360 , and the nip forming member  350  may be rotated. The nip forming unit according to an exemplary embodiment for switching the nip separating portion  342  from the nip separating location to the second nip forming location may be implemented by using the gear  360  and the cam gear portion  351 . 
     The nip forming member  350  is connected to the nip separation member  340  and then may be rotated with the nip separation member  340  about the rotary shaft  302 .  FIG. 6  is a side view of the developing cartridge  10  of  FIG. 5 , and shows an example of the connection relation between the nip forming member  350  and the nip separation member  340 . Referring to  FIGS. 5 and 6 , the nip forming member  350  includes a protrusion portion (first connection portion)  352  that protrudes to outer portion of the side wall  202  via a slot  203  formed in the side wall  202 . The nip separation member  340  includes an insertion portion (second connection portion)  343  to which the protrusion portion  352  protruding through the slot  203  is inserted. The slot  203  may be formed while taking a rotating trace of the protrusion portion  352  when the nip forming member  350  is rotated into account. According to the above configuration, the nip forming member  350  and the nip separation member  340  may be rotated together with each other about the same rotary shaft, e.g., the rotary shaft  302 . Otherwise, the protrusion portion  352  may be formed on the nip separation member  340  and the insertion portion  343  may be formed in the nip forming member  350 . 
       FIGS. 7A, 7B, and 7C  are diagrams showing relations between locations of the nip forming member  350  and the nip separation member  340 , and a state of the developing nip  15 .  FIG. 7A  shows a state where the nip separation member  340  is located at the first location and the nip separating portion  342  is located at the first nip forming location,  FIG. 7B  shows a state where the nip separation member  340  is located at the second location and the nip separating portion  342  is located at the nip separating location, and  FIG. 7C  shows a state where the nip separation member  340  is located at the third location and the nip separating portion  342  is located at the second nip forming location. 
     Referring to  FIG. 7A , the nip separation member  340  is located at the first location and the nip separating portion  342  is located at the first nip forming location. Here, the cam gear portion  351  of the nip forming member  350  is disconnected from the gear  360  and located at a releasing location. Even when the developing cartridge  10  is installed in the main body  2 - 1  and the developing roller  13  is rotated in the above state, the cam gear portion  351  and the gear  360  are disconnected from each other, and thus, a driving force of the main body  2 - 1  is not transferred to the nip forming member  350 . Therefore, the nip forming member  350  and the nip separation member  340  are not rotated. As described above, the developing roller  13  contacts the photosensitive drum  11  and the forming state of the developing nip  15  is maintained. 
     When the nip separation member  340  is rotated in the first direction A 1  to be located at the second location through the manual manipulation as shown in  FIG. 7B , the nip separating portion  342  is located at the nip separating location, but the nip separation member  340  and the nip forming member  350  are connected to each other by the insertion portion  343  and the protrusion portion  352 . Thus, when the nip separation member  340  is rotated in the first direction A 1 , the nip forming member  350  is also rotated in the first direction A 1  with the nip separation member  340 . When the nip separation member  340  reaches the second location, the cam gear portion  351  of the nip forming member  350  is located at a connection location where the cam gear portion  351  is connected to the gear  360 . As described above, the developing roller  13  is separate from the photosensitive drum  11  by the nip separating portion  342 , and the developing nip  15  is removed. 
     In a state shown in  FIG. 7B , when the developing cartridge  10  is installed in the main body  2 - 1  and the developing roller  13  is rotated in a processing direction C, the nip forming member  350  is rotated in an opposite direction to the processing direction C, that is, the first direction A 1 , because the gear  360  and the cam gear portion  351  are connected to each other. Since the nip separation member  340  and the nip forming member  350  are connected to each other via the insertion portion  343  and the protrusion portion  352 , when the nip forming member  350  is rotated in the first direction A, the nip separation member  340  is rotated in the first direction A 1  beyond the second location. Accordingly, the nip separating portion  342  is gradually separate from the first housing  101 , and the developing unit  200  is rotated in the B 2  direction, that is, a direction in which the developing roller  13  approaches to the photosensitive drum  11 , about the hinge shaft  301  by the elastic force of the elastic member  330 . 
     As shown in  FIG. 7C , when the cam gear portion  351  is located at a second releasing location on which the engagement between the cam gear portion  351  and the gear  360  is finished, the nip separating portion  342  is located at the second nip forming location. Even when the developing roller  13  rotates, the nip forming member  350  does not rotate further, and the nip separation member  340  also stops rotating and reaches the third location. The developing roller  13  contacts the photosensitive drum  11  to form the developing nip  15 . 
     According to the above described nip control unit, the nip separation member  340  may be switched to the first location and to the second location by using the manipulation portion  341  to form and separate the developing nip  15  via the manual manipulation. Also, in a state where the nip separation member  340  is located at the second location to remove the developing nip  15 , when the developing cartridge  10  is installed in the main body  2 - 1  and the image forming apparatus  1 - 1  is driven, the nip forming member  350  is rotated in association with the rotation of the developing roller  13  to make the nip separation member  340  rotate to the third location and to form the developing nip  15 . Therefore, the developing cartridge  10  may be installed in the main body  2 - 1  and packaged in a state where the developing nip  15  is not formed, and thus, packaging costs and logistics costs may be reduced. Also, since the developing nip  15  is formed when driving the image forming apparatus  1 - 1 , the user who has purchased the image forming apparatus  1 - 1  does not need to perform all the processes of separating the developing cartridge  10  from the main body  2 - 1 , manipulating the nip separation member  340  to form the developing nip  15 , and installing the developing cartridge  10  in the main body  2 - 1  again. Therefore, the user&#39;s convenience may be improved. 
     (First Switching Unit) 
     While the developing cartridge  10  in which the developing nip  15  is formed is installed in the main body  2 - 1 , the developing nip  15  may be removed. For example, when the developing cartridge  10  is installed in the main body  2 - 1 , the nip separation member  340  located at the first location may be switched to the second location. As such, the nip separating portion  342  may be switched from the first nip forming location to the nip separating location, and then, the developing nip  15  may be removed.  FIGS. 8 and 9  are diagrams showing a first switching unit that operates the nip separation member  340  when the developing cartridge  10  is installed in the main body  2 - 1  to remove the developing nip  15 , according to an exemplary embodiment. 
     Referring to  FIG. 8 , the first switching unit includes a switching member  410  disposed in the main body  2 - 1 . When the developing cartridge  10  is installed in the main body  2 - 1 , the nip separation member  340  located at the first location is interfered with the switching member  410 . For example, the manipulation portion  341  of the nip separation member  340  is interfered with the switching member  410 . Since the switching member  410  is fixed at a location, the nip separation member  340  is pushed by the switching member  410  to be rotated in the first direction A 1  as the developing cartridge  10  is inserted to the main body  2 - 1  in an installation direction denoted in  FIG. 8 . The nip separating portion  342  pushes the photosensitive unit  100 , and accordingly, the developing unit  200  is rotated in the B 1  direction and the developing roller  13  starts to be separate from the photosensitive drum  11 . 
     As shown in  FIG. 9 , when the installation of the developing cartridge  10  in the main body  2 - 1  is finished, the nip separation member  340  reaches the second location and the developing roller  13  is completely separate from the photosensitive drum  11  as shown in  FIG. 4  to remove the developing nip  15 . 
     The developing nip  15  may be removed by operations of installing the developing cartridge  10  in the main body  2 - 1  and closing the door  4 - 1 . That is, the first switching unit may switch the nip separation member  340  that has been located at the first location to the second location through a closing operation of the door  4 - 1 . 
       FIGS. 10 and 11  are diagrams showing a first switching unit for switching the nip separation member  340  from the first location to the second location in association with the closing operation of the door, according to an exemplary embodiment.  FIG. 10  shows a state where the nip separation member  340  is located at the first location, and  FIG. 11  shows a state where the nip separation member  340  is located at the second location. Referring to  FIGS. 10 and 11 , the first switching unit includes a switching member  420  that moves to an escape location where the switching member  420  is not interfered with by the nip separation member  340  and to a switching location where the switching member  420  switches the nip separation member  340  from the first location to the second location, in association with an opening operation and a closing operation of the door  4 - 1 . For example, the switching member  420  may be provided on the main body  2 - 1  to be rotated between the escape location and the switching location. The door  4 - 1  and the switching member  420  may be connected to each other via a connection member  430 . The connection member  430  may be provided on the main body  2 - 1  to be slidable. A slot  431  is formed in an end portion of the connection member  430 , and a pin  421  that is formed on the switching member  420  is inserted to the slot  431 . When the connection member  430  slides in a direction D 1  shown in  FIG. 10 , the switching member  420  is rotated to the escape location. When the connection member  430  slides in a direction D 2  shown in  FIG. 11 , the switching member  420  is rotated to the switching location. A cam portion  4   b  is formed on the door  4 - 1  for pushing the connection member  430  toward the direction D 2  when the door  4 - 1  is closed. A spring  440  applies an elastic force to the connection member  430  to be slid in a direction for rotating the switching member  420  to the escape location. 
     When the door  4 - 1  is opened, the connection member  430  slides in the direction D 1  due to the elastic force of the spring  440 , and the switching member  420  is rotated to the escape location as shown in  FIG. 10 . The switching member  420  is not interfered with the nip separation member  340  while the nip separation member  340  inserts the developing cartridge  10  located at the first location into the main body  2 - 1 . When the developing cartridge  10  is completely inserted to the main body  2 - 1  and the door  4 - 1  is closed, the connection member  430  slides in the direction D 2  due to the cam portion  4   b . Then, the switching member  420  makes the nip separation member  340  rotate in the first direction A 1  while rotating to the switching location. For example, the switching member  420  rotates the nip separation member  340  to the second location while rotating in contact with the manipulation portion  341 . When the door  4 - 1  is completely closed, the switching member  420  is located at the switching location as shown in  FIG. 11 , and the developing roller  13  is separated from the photosensitive drum  11  by the nip separation member  340  to remove the developing nip  15 . 
     While the nip separation member  340  is switched from the first location to the second location in association with the operation of closing the door  4 - 1 , the nip forming member  350  is switched from the releasing location to the connecting location as shown in  FIGS. 7A and 7B . 
     As described above, the developing nip  15  is removed during the process of installing the developing cartridge  10  in the main body  2 - 1 . Therefore, during the manufacturing processes, it is not necessary for checking whether the developing nip  15  is formed before installing the developing cartridge  10  in the main body  2 - 1  after performing tests before release, and thus, manufacturing costs may be reduced. Also, since the developing nip  15  is in an unformed state, even if the developing cartridge  10  is installed in the main body  2 - 1  for a long time after being released to the market until being sold, the developing roller  13  and the photosensitive drum  11  may not be deformed or damaged. 
     During the process of switching the nip separation member  340  from the first location to the second location in association with the operation of installing the developing cartridge  10  in the main body  2 - 1  or the operation of closing the door  4 - 1 , the cam gear portion  351  is switched from the releasing location to the connecting location as shown in  FIGS. 7A and 7B . Therefore, when the image forming apparatus  1 - 1  starts to operate, the nip separation member  340  is switched to the third location as shown in  FIG. 7C  by the nip forming unit, that is, the gear  360  and the cam gear portion  351 , and thereby forming the developing nip  15 . 
     (Second Switching Unit) 
     While the image forming apparatus  1 - 1  operates, the developing roller  13  may be separate from the photosensitive drum  11  if there is no need to perform the developing operation. For example, during a time period from a time when an end portion of the toner image has passed through a region where the developing roller  13  and the photosensitive drum  11  face each other to a time when the recording medium P on which the toner image has been transferred is completely discharged out of the main body  2 - 1  by the discharge roller  60 , the developing roller  13  does not need to contact the photosensitive drum  11 . Also, after finishing a printing operation, the developing roller  13  does not need to contact the photosensitive drum  11 . In this case, when the developing roller  13  is separate from the photosensitive drum  11  to remove the developing nip  15 , deformation or damage to the developing roller  13  and the photosensitive drum  11  may be prevented, and thus, printing images of stabilized quality may be achieved during the lifespan of the developing cartridge  10 . 
     In order to remove the developing nip  15 , the nip separating portion  342  may be switched from the second nip forming location to the nip separating location by switching the nip separation member  340  from the third location to the second location.  FIGS. 12, 13, and 14  are diagrams of a second switching unit for switching the nip separating portion  342  from the second nip forming location to the nip separating location, according to an exemplary embodiment. Referring to  FIGS. 12 to 14 , the second switching unit includes a switching lever  452  that is provided on the main body  2 - 1  to move between a retreat location where the switching lever  452  is not interfered with the nip separation member  340  and an interference location for rotating the nip separation member  340 . For example, the switching lever  452  may be provided on the main body  2 - 1  to be rotated between the retreat location and the interference location. 
     Various structures may be adopted to rotate the switching lever  452 . For example, the switching lever  452  may be connected to a rotary gear  451  that is rotated by an actuator  460 . The switching lever  452  and the rotary gear  451  may be formed integrally with each other as a second switching member  450 . As an example, the actuator  460  may be connected to the rotary gear  451  by a worm gear  461  provided on a rotary shaft thereof. Although not illustrated in the drawings, the switching lever  452  may be driven by a driving unit (not shown) that drives the developing cartridge  10 . In this case, an alternative driving power connecting structure for applying a driving power alternatively, e.g., a clutch structure, may be disposed between the driving unit (not shown) and the switching lever  452 . 
     Referring to  FIG. 12 , the switching lever  452  is located at the retreat location where the switching lever  452  is not interfered with the nip separation member  340 . In this state, even if the developing cartridge  10  is attached to/detached from the main body  2 - 1 , the nip separation member  340  and the switching lever  452  do not interfere with each other. Also, even when the nip separation member  340  is switched from the first location to the second location by the first switching unit while the developing cartridge  10  is installed in the main body  2 - 1  or even when the nip separation member  340  is switched from the second location to the third location by the nip forming member  350  in a state where the developing cartridge  10  is installed in the main body  2 - 1 , the nip separation member  340  does not interfere with the switching lever  452  that is located at the retreat location. 
     In  FIG. 12 , the nip separation member  340  is switched from the second location to the third location by the nip forming member  350 . The developing nip  15  may be removed when the printing operation has finished, between printing operations (e.g., between pages), or when an end portion of the toner image has passed through the developing nip  15 . To do this, the actuator  460  may rotate the switching lever  452  in a direction E 1 . Then, the nip separation member  340  is pushed by the switching lever  452  to be rotated in the second direction A 2 . For example, the switching lever  452  pushes the manipulation portion  341  of the nip separation member  340  to rotate the nip separation member  340  in the second direction A 2 . 
     As shown in  FIG. 13 , when the switching lever  452  reaches the interference location, the nip separation member  340  is located at the second location and the operation of the actuator  460  is terminated. After that, the actuator  460  rotates the switching lever  452  in a direction E 2  that is opposite to the direction E 1  so that the switching lever  452  returns to the retreat location as shown in  FIG. 14 . 
     According to the above configuration, the developing nip  15  may be removed at necessary times during operating the image forming apparatus  1 - 1  and when the operation of the image forming apparatus  1 - 1  has finished. 
     When the nip separation member  340  is switched from the third location to the second location by the second switching unit, the cam gear portion  351  of the nip forming member  350  is switched from the second releasing location shown in  FIG. 7C  to the connecting location shown in  FIG. 7B . In the state shown in  FIG. 14 , when the developing cartridge  10  is detached from the main body  2 - 1 , the nip separation member  340  does not interfere with the switching lever  452 . Also, in the state shown in  FIG. 14 , when the image forming apparatus  1 - 1  starts to operate and the developing roller  13  rotates, the cam gear portion  351  of the nip forming member  350  is switched from the connecting location shown in  FIG. 7B  to the second releasing location shown in  FIG. 7C  since the cam gear portion  351  of the nip forming member  350  is connected to the gear  360 . Moreover, the nip separation member  340  is switched from the second location to the third location and the nip separating portion  342  is switched from the nip separating location to the second nip forming location. Therefore, the developing nip  15  is formed again, and the printing operation may be performed. 
     (Location Detection) 
     The image forming apparatus  1 - 1  may further include a detection unit for detecting whether the developing nip  15  is in a removed state. For example, the detection unit may detect the nip separation member  340  located at the second location.  FIG. 15  is a diagram of the detection unit according to an exemplary embodiment.  FIG. 15  shows that the developing cartridge  10  is installed in the main body  2 - 1 . The nip separation member  340  is located at the second location where the developing nip  15  is removed. The main body  2 - 1  includes a detection sensor  470 . The detection sensor  470  may detect the manipulation portion  341  of the nip separation member  340  located at the second location. 
     The detection sensor  470  may be, for example, a reflective optical sensor. In this case, an element denoted by the reference numeral  343  on the manipulation portion  341  may denote a reflective plate for reflecting light. The detection sensor  470  may be, for example, a photointerruptor. Otherwise, various kinds of sensors may be used as the detection sensor  470 . 
     According to the above configuration, whether the developing nip  15  is formed or removed may be determined according to whether the nip separation sensor  340  is detected by the detection sensor  470 . 
     The nip control unit may be disposed at a side portion of the developing roller  13  in a lengthwise direction. Also, the nip control unit may be respectively formed on opposite side portions of the developing roller  13  in the lengthwise direction, and in this case, the first and second switching units may be disposed at the opposite side portions of the developing roller  13  in the lengthwise direction. 
     (Image Forming Apparatus) 
       FIG. 16  is a schematic block diagram of an image forming apparatus  1 - 2  according to an exemplary embodiment. The image forming apparatus  1 - 2  of an exemplary embodiment prints color images on a recording medium P by using an electrophotographic method. Referring to  FIG. 16 , the image forming apparatus  1 - 2  may include a plurality of developing units  10 , an exposure device  20 - 2 , a transfer device  30 - 2 , a fuser  40 - 2 , and a feed unit  50 - 2 . 
     For color printing, the plurality of developing units  10  may include, for example, four developing units  10  for developing cyan (C) images, magenta (M) images, yellow (Y) images, and black (K) images. The four developing units  10  may respectively contain C, M, Y, and K toners. Although not shown in the drawings, the C, M, Y, and K toners are contained respectively in four toner supplying containers, and may be supplied from the four toner supplying containers to the four developing units  10 , respectively. The image forming apparatus  1 - 2  may further include developing units  10  for accommodating toners of various colors, e.g., light magenta and white. Hereinafter, the image forming apparatus  1 - 2  including the four developing units  10  will be described. Unless otherwise specified, references with C, M, Y, and K refer to elements for printing an image by using the C, M, Y, and K toners. 
     The developing device  10  includes the photosensitive drum  11 . The photosensitive drum  11  according to an exemplary embodiment has a cylindrical shape on which a photosensitive layer is formed, but is not limited thereto. The charging roller  12  charges the photosensitive drum  11  to have a uniform surface potential. A charging brush or a corona charger may be used, instead of the charging roller  12 . The developing roller  13  supplies a developing agent in the developing device  10  to the photosensitive drum  11 . 
     The developing device  10  may further include a charging roller cleaner (not shown) for removing impurities such as the developing agent or dust attached to the charging roller  12 , a cleaning member  16  for removing the developing agent remaining on the photosensitive drum  11  after a transfer process that will be described later, and a regulating member (not shown) for regulating an amount of the toner supplied to the developing area where the photosensitive drum  11  and the developing roller  13  face each other by the developing roller  13 . 
     The present exemplary embodiment adopts a mono-component contact type developing method. The developing device  10  adopting the mono-components contact type developing method is described above with reference to  FIGS. 1 and 2 , and thus, descriptions about detailed structure of the developing device  10  are omitted. The toner is attached to the surface of the developing roller  13  by an electrostatic force to be supplied to the developing nip  15  where the photosensitive drum  11  and the developing roller  13  face each other. The toner is supplied to the photosensitive drum  11  by a developing bias voltage applied between the developing roller  13  and the photosensitive drum  11  to develop an electrostatic latent image formed on the surface of the photosensitive drum  11  to a visible toner image. 
     The exposure device  20 - 2  emits light that has been modulated in correspondence to image information to the photosensitive drum  11  that will be described later to form the electrostatic latent image on the photosensitive drum  11 . An example of the exposure device  20 - 2  may be an LSU using a laser diode as a light source or an exposing device using an LED as a light source. 
     The transfer device  30 - 2  may include an intermediate transfer belt  31 , primary transfer rollers  32 , and a secondary transfer roller  33 . The toner image developed on the photosensitive drum  11  in each of the developing devices  10 C,  10 M,  10 Y, and  10 K is temporarily transferred to the intermediate transfer belt  31 . The intermediate transfer belt  31  circulates while being supported by support rollers  34 ,  35 , and  36 . Four primary transfer rollers  32  are disposed at locations facing the photosensitive drums  11  in the developing devices  10 C,  10 M,  10 Y, and  10 K while the intermediate transfer belt  31  is interposed between the primary transfer rollers  32  and the photosensitive drums  11 . A primary transfer bias voltage is applied to the four primary transfer rollers  32  for primary transferring the images developed on the photosensitive drums  11  onto the intermediate transfer belt  31 . A corona transfer device or a pin scorotron type transfer device may be adopted, instead of the primary transfer roller  32 . 
     The secondary transfer roller  33  is located to face the intermediate transfer belt  31 . A secondary bias voltage is applied to the secondary transfer roller  33  in order to transfer the toner image that is primarily transferred on the intermediate transfer belt  31  to the recording medium P. 
     In an exemplary embodiment, the toner image developed on the photosensitive drum  11  is primarily transferred to the intermediate transfer belt  31 , and then, secondarily transferred to the recording medium P that passes through the intermediate transfer belt  31  and the secondary transfer roller  33 , but one or more exemplary embodiments are not limited thereto. That is, the recording medium P may directly pass through between the intermediate transfer belt  31  and the photosensitive drum  11  so that the toner image that is developed on the photosensitive drum  11  may be directly transferred on the recording medium P. In this case, the secondary transfer roller  33  is not adopted. 
     The fuser  40 - 2  applies heat and pressure to the image transferred on the recording medium P to fuse the image on the recording medium P. The fuser  40 - 2  may have the same structure as that of the fuser  40 - 1  shown in  FIG. 1 . However, the shape of the fuser  40 - 2  is not limited thereto. When the recording medium P passes through the fusing nip, the toner image is melted and fixed on the recording medium P due to the heat and pressure applied thereto. 
     When receiving a printing command from a host (not shown), a controller (not shown) makes the surface of the photosensitive drum  11  charged to a uniform potential by using the charging roller  12 . The exposure device  20 - 2  emits four light beams that are modulated in correspondence with image information of each color to the photosensitive drums  11  in the four developing devices  10 C,  10 M,  10 Y, and  10 K to form electrostatic latent images on the photosensitive drums  11 . The developing rollers  13  in the developing devices  100 ,  10 M,  10 Y, and  10 K supply the C, M, Y, and K toners respectively to the photosensitive drums  11  to develop the electrostatic latent images into visible toner images. The toner images are primarily transferred to the intermediate transfer belt  31 . The feed unit  50  transports the recording medium P to the transfer nip formed by the secondary transfer roller  33  and the intermediate transfer belt  31  along with a feeding path  51 . The toner images that have been primarily transferred on the intermediate transfer belt  31  are secondarily transferred to the recording medium P by the secondary transfer bias voltage applied to the secondary transfer roller  33 . When the recording medium P has passed through the fuser  40 - 2 , the toner images are fused on the recording medium P due to the heat and pressure. The recording medium P on which the fusing operation has been finished is discharged to outside by a discharging roller  60 . 
     Hereinafter, the developing devices  10 C,  10 M,  10 Y, and  10 K that may be replaceable will be referred to as developing cartridges  10 C,  10 M,  10 Y, and  10 K. Each of the developing cartridges  10 C,  10 M,  10 Y, and  10 K is the same as the developing cartridge  10  including the nip control unit described above. That is, each of the developing cartridges  10 C,  10 M,  10 Y, and  10 K includes the nip separation member  340 , the gear  360 , and the nip forming member  350  shown in  FIGS. 3, 4, 5, 6, 7A, 7B, and 7C . 
     The developing cartridges  10 C,  10 M,  10 Y, and  10 K may be sequentially installed in a main body  2 - 2  through an opening  3 - 2  that is opened by a door  4 - 2 . Also, the developing cartridges  10 C,  10 M,  10 Y, and  10 K may be mounted in the main body  2 - 2  by using a tray method.  FIG. 17  is a perspective view showing a method of installing the developing cartridges  100 ,  10 M,  10 Y, and  10 K in the main body  2 - 2 , according to an exemplary embodiment. Referring to  FIG. 17 , the main body  2 - 2  includes a tray  5 , on which the developing cartridges  10 C,  10 M,  10 Y, and  10 K are mounted, for entering and exiting the main body  2 - 2 . For example, the door  4 - 2  is opened and the tray  5  is slid out of the main body  2 - 2  to be withdrawn, and then, the developing cartridges  100 ,  10 M,  10 Y, and  10 K are mounted on the tray  5 . After that, the tray  5  may slide into the main body  2 - 2  and the door  4 - 2  is closed. 
     (First Switching Unit) 
       FIGS. 18 and 19  are a side view and a plan view showing a first switching unit that operates the nip separation member  340  to remove the developing nip  15  when the developing cartridges  100 ,  10 M,  10 Y, and  10 K are installed in the main body  2 - 2 , according to an exemplary embodiment. 
     Referring to  FIGS. 18 and 19 , the first switching unit includes switching members  410 C,  410 M,  410 Y, and  410 K that are disposed in the main body  2 - 2 . Operations of the switching members  410 C,  410 M,  410 Y, and  410 K are the same as those of the switching member  410  shown in  FIGS. 8 and 9 . However, extension portions  343 C,  343 M,  343 Y, and  343 K extending from the manipulation portion  341  in a width direction, that is, a direction perpendicular to the direction in which the developing cartridges  10 C,  10 M,  10 Y, and  10 K are arranged, are disposed on the nip separation members  340  in the developing cartridges  10 C,  10 M,  10 Y, and  10 K. A length of the extension portion  343 K of the developing cartridge  10 K that is installed first is the shortest, and a length of the extension portion  343 C of the developing cartridge  10 C that is installed last is the longest. The switching members  410 C,  410 M,  410 Y, and  410 K are located to be stepped sequentially in a width direction, that is, a lengthwise direction of the developing roller  13 . According to the above configuration, when the developing cartridges  100 ,  10 M,  10 Y, and  10 K are sequentially installed in the main body  2 - 2  in a state where the nip separation member  340  is located at the first location, the extension portions  343 C,  343 M,  343 Y, and  343 K interfere respectively with corresponding switching members  410 C,  410 M,  410 Y, and  410 K so that the nip separation members  340  are switched from the first location to the second location. 
     The first switching unit having the above structure may be applied to a case in which the developing cartridges  10 C,  10 M,  10 Y, and  10 K mounted on the tray  5  are installed in the main body  2 - 2 . That is, when the developing cartridges  100 ,  10 M,  10 Y, and  10 K are inserted in the main body  2 - 2  in a state of being mounted on the tray  5 , the extension portions  343 C,  343 M,  343 Y, and  343 K of the developing cartridges  10 C,  10 M,  10 Y, and  10 K are respectively pushed by the switching members  410 C,  410 M,  410 Y, and  410 K disposed in the main body  2 - 2  and switched from the first location to the second location. 
     When the nip separation members  340  of the developing cartridges  10 C,  10 M,  10 Y, and  10 K are switched from the first location to the second location, the cam gear portions  351  of the nip forming members  350  are also switched from the releasing location to the connecting location. 
     The developing nip  15  may be removed by the operation of installing the developing cartridges  10 C,  10 M,  10 Y, and  10 K in the main body  2 - 2  and closing the door  4 - 2 . That is, the first switching unit may switch the nip separation members  340  located at the first location to the second location by closing the door  4 - 2 . 
       FIGS. 20 and 21  are diagrams of the first switching unit for switching the nip separation members  340  in the developing cartridges  10 C,  10 M,  10 Y, and  10 K from the first location to the second location in association with a closing operation of the door  4 - 2 , according to an exemplary embodiment.  FIG. 20  shows a state where the nip separation member  340  is located at the first location, and  FIG. 21  shows a state where the nip separation member  340  is located at the second location. 
     Referring to  FIGS. 20 and 21 , the first switching unit includes switching members  420 C,  420 M,  420 Y, and  420 K that move to a retreat location where the switching members  420 C,  420 M,  420 Y, and  420 K do not interfere with the nip separation members  340  in the developing cartridges  10 C,  10 M,  10 Y, and  10 K and to a switching location for switching the nip separation members  340  in the developing cartridges  10 C,  10 M,  10 Y, and  10 K from the first location to the second location, in association with an opening operation and a closing operation of the door  4 - 2 . For example, the switching members  420 C,  420 M,  420 Y, and  420 K may be provided in the main body  2 - 2  to be rotated to the retreat location and the switching location. The door  4 - 2  and the switching members  420 C,  420 M,  420 Y, and  420 K are connected to each other via a connection member  430   a . The connection member  430   a  is formed on the main body  2 - 2  to be slidable. The connection member  430   a  includes slots  431 C,  431 M,  431 Y, and  431 K, and pins  421 C,  421 M,  421 Y, and  421 K formed on the switching members  420 C,  420 M,  420 Y, and  420 K are inserted to the slots  431 C,  431 M,  431 Y, and  431 K. When the connection member  430   a  slide in a direction D 1  shown in  FIG. 20 , the switching members  420 C,  420 M,  420 Y, and  420 K are rotated to the retreat location. When the connection member  430   a  slides in a direction D 2  shown in  FIG. 21 , the switching members  420 C,  420 M,  420 Y, and  420 K are rotated to the switching location. The door  4 - 2  includes a cam portion  4   b  that pushes the connection member  430   a  to the direction D 2  when the door  4 - 2  is closed. A spring  440   a  applies an elastic force to the connection member  430   a  to be slid in a direction of rotating the switching members  420 C,  420 M,  420 Y, and  420 K to the retreat location. 
     When opening the door  4 - 2 , the connection member  430   a  slides in the direction D 1  due to the elastic force of the spring  440   a , and the switching members  420 C,  420 M,  420 Y, and  420 K are rotated to the retreat location as shown in  FIG. 20 . While the developing cartridges  10 C,  10 M,  10 Y, and  10 K, in which the nip separation members  340  are located at the first location, are inserted into the main body  2 - 2 , the switching members  420 C,  420 M,  420 Y, and  420 K do not interfere with the nip separation members  340 . When the developing cartridges  10 C,  10 M,  10 Y, and  10 K are completely inserted in the main body  2 - 2  and the door  4 - 2  is closed, the connection member  430   a  slides in the direction D 2  due to the cam portion  4   b . Then, the switching members  420 C,  420 M,  420 Y, and  420 K are rotated to the switching location to rotate the nip separation members  340  in the first direction A 1 . For example, the switching members  420 C,  420 M,  420 Y, and  420 K rotate the nip separation members  340  to the second location while rotating in contact with the manipulation portions  341 . When the door  4 - 2  is completely closed, the switching members  420 C,  420 M,  420 Y, and  420 K are located at the switching location as shown in  FIG. 21 , and the developing roller  13  is separate from the photosensitive drum  11  due to the nip separation members  340  to remove the developing nip  15 . 
     The first switching unit having the above structure may be applied to a case in which the developing cartridges  10 C,  10 M,  10 Y, and  10 K are installed in the main body  2 - 2  in a state of being mounted on the tray  5 . That is, when the developing cartridges  10 C,  10 M,  10 Y, and  10 K are inserted in the main body  2 - 2  in a state of being mounted on the tray  5  and the door  4 - 2  is closed, the nip separation members  340  in the developing cartridges  10 C,  10 M,  10 Y, and  10 K are pushed by the switching members  420 C,  420 M,  420 Y, and  420 K provided in the main body  2 - 2  and then switched from the first location to the second location. The switching members  420 C,  420 M,  420 Y, and  420 K may be provided on the tray  5  to rotate to the retreat location and the switching location and the connection member  430   a  may be provided on the tray  5  to be slidable. 
     While the nip separation members  340  in the developing cartridges  10 C,  10 M,  10 Y, and  10 K are switched from the first location to the second location in association with the closing operation of the door  4 - 2 , the nip forming member  350  in each of the developing cartridges  100 ,  10 M,  10 Y, and  10 K is switched from the releasing location to the connecting location as shown in  FIGS. 7A and 7B . 
     According to the above described structure, the developing nip is removed by the process of installing the developing cartridges  10 C,  10 M,  10 Y, and  10 K in the main body  2 - 2 . Therefore, it does not need to check whether to form the developing nip  15  before installing the developing cartridges  10 C,  10 M,  10 Y, and  10 K in the main body  2 - 2  after performing tests before release during the manufacturing processes, and thus, manufacturing costs may be reduced. Also, since the developing nip  15  is not formed, even if the developing cartridges  10 C,  10 M,  10 Y, and  10 K is in the installation state in the main body  2 - 2  for a long time after being released until being sold, the developing roller  13  and the photosensitive drum  11  may not be deformed or damaged. 
     During the process of switching the nip separation members  340  in the developing cartridges  10 C,  10 M,  10 Y, and  10 K from the first location to the second location in association with the installing operation of the developing cartridge  10  in the main body  2 - 2  or closing the door  4 - 2 , the cam gear portion  351  of the nip forming member  350  in each of the developing cartridges  100 ,  10 M,  10 Y, and  10 K is switched from the releasing location to the connecting location as shown in  FIGS. 7A and 7B . Therefore, when the image forming apparatus  1 - 2  starts to operate, the cam gear portion  351  in each developing cartridge  100 ,  10 M,  10 Y, or  10 K is switched from the connecting portion to the second releasing location as shown in  FIG. 7C , and accordingly, the nip separation member  340  in each of the developing cartridges  10 C,  10 M,  10 Y, and  10 K is switched to the third location to form the developing nip  15 . 
     (Second Switching Unit) 
     During the operation of the image forming apparatus  1 - 2 , the developing roller  13  may be separate from the photosensitive drum  11  in a case where the developing operation is not necessary. For example, during a time period from a time when an end portion of the toner image has passed through the region where the developing roller  13  and the photosensitive drum  11  face each other to a time when the toner image is transferred to the recording medium P and completely discharged out of the main body  2 - 2  by the discharging roller  60 , the developing roller  13  does not need to contact the photosensitive drum  11 . Also, after finishing the printing operation, the developing roller  13  does not need to contact the photosensitive drum  11 . In this case, if the developing roller  13  is separate from the photosensitive drum  11  and the developing nip  15  is removed, possibility of deforming or damaging the developing roller  13  and the photosensitive drum  11  may be reduced, and accordingly, images of stabilized image quality may be printed during a lifespan of the developing cartridge  10 . 
     In order to remove the developing nip  15 , the nip separation member  340  may be switched from the third location to the second location.  FIGS. 22, 23, and 24  are diagrams of a second switching unit for switching the nip separation member  340  from the third location to the second location, according to an exemplary embodiment. Referring to  FIGS. 22 to 24 , the second switching unit includes a switching lever  452  that is provided on the main body  2 - 2  to move to a retreat location where the switching lever  452  does not interfere with the nip separation member  340  and an interference location where the nip separation member  340  is rotated. For example, the switching lever  452  may be provided on the main body  2 - 2  to be rotated to the retreat location and the interference location. 
     Various structures for rotating the switching lever  452  may be adopted. For example, the switching lever  452  may be rotated in connection with a rotary gear  451  that is rotated by the actuator  460 . The switching lever  452  and the rotary gear  451  may be formed integrally with each other to configure the second switching member  450 . As an example, the actuator  460  may be connected to the rotary gear  451  via the worm gear  461  provided on a rotary shaft thereof. Although not illustrated in the drawings, the switching lever  452  may be driven by a driving unit (not shown) that drives the developing cartridge  10 . In this case, an alternative driving power connecting structure for connecting the driving power alternatively to the driving unit (not shown) and the switching lever  452 , for example, a clutch structure, may be disposed between the driving unit (not shown) and the switching lever  452 . 
     Referring to  FIG. 22 , the switching lever  452  is located at the retreat location where the switching lever  452  does not interfere with the nip separation member  340 . In this state, even when the developing cartridges  10 C,  10 M,  10 Y, and  10 K are attached to/detached from the main body  2 - 2 , the nip separation member  340  and the switching lever  452  do not interfere with each other. Also, even if the nip separation member  340  is switched from the first location to the second location by the first switching unit during the process of installing the developing cartridges  10 C,  10 M,  10 Y, and  10 K in the main body  2 - 2 , or even if the nip separation member  340  is switched from the second location to the third location by the nip forming member  350  in a state where the developing cartridges  10 C,  10 M,  10 Y, and  10 K are installed in the main body  2 - 2 , the nip separation member  340  does not interfere with the switching lever  452  located at the retreat location. 
     In  FIG. 22 , the nip separation member  340  is located at the third location switched from the second location by the nip forming member  350 . The developing nip  15  may be removed when the printing operation is finished, between the successive printing operations (that is, between pages), or after the end portion of the toner image has passed through the developing nip  15 . To do this, the actuator  460  rotates the switching lever  452  in a direction E 1 . Then, the nip separation member  340  is rotated in the second direction A 2  by the switching lever  452 . For example, the switching lever  452  pushes the manipulation portion  341  of the nip separation member  340  to rotate the nip separation member  340  in the second direction A 2 . 
     As shown in  FIG. 23 , when the switching lever  452  reaches the interference location, the nip separation member  340  is located at the second location and the driving of the actuator  460  is stopped. After that, the actuator  460  rotates the switching lever  452  in the direction E 2  that is opposite to the direction E 1  to make the switching lever  452  return to the retreat location as shown in  FIG. 14 . 
     According to the above configuration, the developing nip  15  may be removed when the operation of the image forming apparatus  1 - 2  is finished or at necessary times during operating the image forming apparatus  1 - 2 . 
     The second switching unit having the above structure may be applied to a case where the developing cartridges  10 C,  10 M,  10 Y, and  10 K are installed in the main body  2 - 2  in a state of being mounted on the tray  5 . That is, in a state where the developing cartridges  10 C,  10 M,  10 Y,  10 K are installed in the main body  2 - 2  in a state of being mounted on the tray  5 , the nip separation member  340  may be switched from the third location to the switching location by using the switching lever  452  during the developing operation is not formed. In this case, the second switching unit may be provided on the tray  5 . 
     When the nip separation member  340  is switched from the third location to the second location by the second switching unit, the cam gear portion  351  is switched from the second releasing location shown in  FIG. 7C  to the connecting location shown in  FIG. 7C . When the developing cartridges  10 C,  10 M,  10 Y, and  10 K are detached from the main body  2 - 2  in the state shown in  FIG. 24 , the nip separation member  340  does not interfere with the switching lever  452 . Also, when the developing roller  13  is rotated when the image forming apparatus  1 - 2  starts to operate in the state shown in  FIG. 24 , the nip forming member  350  is rotated because the cam gear portion  351  of the nip forming member  350  is connected to the gear  360 . Therefore, the nip separation member  340  is switched from the second location to the third location, and the developing nip  15  is formed again to allow the printing operation. 
     (Location Detection) 
     The image forming apparatus  1 - 2  may further include a detection unit for detecting whether the developing nip  15  is removed. For example, the detection unit may detect the nip separation member  340  located at the second location. The structure of the detection unit is the same as that of  FIG. 15 , and the image forming apparatus  1 - 2  may include four detection units corresponding respectively to the developing cartridges  10 C,  10 M,  10 Y, and  10 K. According to the above structure, it may be determined whether the developing nip  15  is removed or formed according to whether a detection sensor  470  detects the nip separation member  340 . 
     The nip control unit may be disposed at a side portion of the developing roller  13  in a length direction of the developing roller  13 . Also, the nip control unit may be disposed at opposite side portions of the developing roller  13  in the length direction of the developing roller, and in this case, the first and second switching units may be also disposed at the opposite side portions of the developing roller  13  in the length direction of the developing roller  13 . 
     (Developing Cartridge) 
     The structure of the nip control unit is not limited to the examples shown in  FIGS. 3, 4, 5, 6, 7A, 7B, and 7C . 
       FIG. 25  is a side view of a developing cartridge  10 - 1 .  FIG. 26  is a diagram illustrating an example of arranging a nip control member  370  in the developing cartridge  10 - 1  of  FIG. 25 .  FIG. 27  is a perspective view of the nip control member  370  according to an exemplary embodiment.  FIG. 28  is a diagram showing a state where a nip separating portion  372  is located at the nip forming location, and  FIG. 29  is a diagram showing a state where the nip separating portion  372  is located at the nip separating location. 
     The developing cartridge  10 - 1  of an exemplary embodiment may be applied to the image forming apparatuses  1 - 1  and  1 - 2  shown in  FIG. 1  and  FIG. 16 . Hereinafter, a case in which the developing cartridge  10 - 1  is applied to the image forming apparatus  1 - 1  will be described below as an example. 
       FIG. 25  is a side view of the developing cartridge  10 - 1  according to an exemplary embodiment. Referring to  FIG. 25 , the developing cartridge  10 - 1  includes a photosensitive unit  100 - 1  and a developing unit  200 - 1 . The photosensitive unit  100 - 1  includes a first housing  101 - 1 , and the photosensitive drum  11  supported by the first housing  101 - 1 . The developing unit  200 - 1  includes a second housing  200 - 1 , and the developing roller  13  supported by the second housing  200 - 1 . The photosensitive unit  100 - 1  and the developing unit  200 - 1  are connected to each other so as to rotate to a developing location ( FIG. 25 ) where the photosensitive drum  11  and the developing roller  13  contact each other to form the developing nip  15 , and to a non-developing location ( FIG. 30 ) where the photosensitive drum  11  and the developing roller  13  are separate from each other to remove the developing nip. For example, the photosensitive unit  100 - 1  and the developing unit  200 - 1  may be connected to each other to rotate to the developing location and the non-developing location about a hinge shaft  301 . In the image forming apparatus  1 - 1 , since the photosensitive drum  11  is related to the location of the transfer roller  30 - 1 , the photosensitive drum  11  is fixed at a location when the developing cartridge  10 - 1  is installed in the main body  2 - 1 . Therefore, the developing unit  200 - 1  is coupled to the photosensitive unit  100 - 1  so as to rotate about the hinge shaft  301 . However, one or more exemplary embodiments are not limited thereto, that is, the developing unit  200 - 1  is fixed at a location in the main body  2 - 1  and the photosensitive unit  100 - 1  may be coupled to the developing unit  200 - 1  so as to rotate about the hinge shaft  301 . 
     The elastic member  330  provides an elastic force to the developing unit  200 - 1  and the photosensitive unit  100 - 1  to rotate in a direction of forming the developing nip  15 , that is, in a direction toward the developing location. The developing unit  200 - 1  is rotated about the hinge shaft  301  by the elastic force of the elastic member  330  so that the developing roller  13  contacts the photosensitive drum  11 , and accordingly, the developing nip  15  may be formed as shown in  FIG. 25 . In  FIG. 25 , a tensile coil spring having opposite end portions that are respectively supported by the photosensitive unit  200 - 1  and the developing unit  100 - 1  is shown as an example of the elastic member  330 , but the elastic member  300  is not limited thereto. For example, various members such as a torsion coil spring, a plate spring, etc., may be adopted as the elastic member  330 . 
     The hinge shaft  301  may be coaxial with a rotational axis of the coupler  310  that is connected to a driving connection unit (not shown) provided on the main body  2 - 1  when the developing cartridge  10 - 1  is installed in the main body  2 - 1 . The developing roller  13 , the supply roller  14 , and the agitator (not shown) may be power connected to the coupler  310  by the power connection unit (not shown), e.g., a gear. The coupler  320  may be provided on the rotary shaft of the photosensitive drum  11 . The coupler  320  may be connected to a driving connection unit (not shown) provided on the main body  2 - 1  when the developing cartridge  10 - 1  is installed in the main body  2 - 1 . The photosensitive drum  11  and the charging roller  12  may be connected to the coupler  320  via a power connection unit such as a gear. Although not shown in the drawings, the coupler  320  may be omitted and the photosensitive drum  11  and the charging roller  12  may be connected to the coupler  310 . Otherwise, the coupler  310  may be omitted, and the developing roller  13 , the supply roller  14 , and the agitator (not shown) may be connected to the coupler  320 . 
     The nip control unit may include a nip separating portion switched to the nip forming location and the nip separating location where the developing unit  200 - 1  are located respectively at the developing location and the non-developing location, a manipulation portion for switching the nip separating portion to the nip forming location and the nip separating location through manual manipulation, and a nip forming unit for witching the nip separating portion from the nip separating location to the nip forming location according to the operations of the photosensitive unit  100 - 1  and the developing unit  200 - 1 . 
     The nip control unit may form the developing nip  15  or removing the developing nip by rotating the developing unit  200 - 1  about the hinge shaft  301 . As an example, the nip separating portion and the manipulation portion may be realized by the nip control member  370 . The nip control member  370  is provided on the photosensitive unit  100 - 1  so as to move to the first location for forming the developing nip  15  and the second location for separating the developing nip  15 . For example, the nip control member  370  is provided on the side wall  102  of the first housing  101 - 1  so as to move to the first location and the second location. A returning member  380  applies an elastic force to the nip control member  370  to a direction toward the first location. The returning member  380  may include, for example, a compression coil spring, but is not limited thereto. That is, various types of elastic members such as a plate spring, a torsion coil spring, etc. may be used as the returning member  380 . 
     Referring to  FIGS. 26 and 27 , the nip control member  370  includes a body  374  supported by the photosensitive unit  100 - 1  to be moveable to the first location and the second location, a manipulation portion  371  for providing a grip to a user to perform manual manipulation, and a nip separating portion  372  switched to the nip separating location and the nip forming location where the nip separating portion  372  contacts and is separate from the developing unit  200 - 1  according to the location of the nip control member  370  to remove and form the developing nip  15 . 
     The manipulation portion  371  extends from the body  374 . As shown in  FIG. 28 , the manipulation portion  371  may protrude out of the developing cartridge  10 - 1  to be exposed when the nip control member  370  is located at the first location (e.g., when the nip separating portion  372  is located at the nip forming location). As such, the user may easily access the nip control member  370  to manually manipulate the nip control member  370  and to switch the nip control member  370  to the second location. The nip separating portion  372  may extend from the body  374  to an opposite direction to the manipulation portion  371 . The nip separating portion  372  pushes the developing unit  200 - 1 , for example, the interference portion  203 , to be rotated to a direction opposite to the direction in which the elastic force of the elastic member  330  is applied, as the nip control member  370  is moved from the first location ( FIG. 28 ) to the second location ( FIG. 29 ). The interference portion  203  may be provided on, for example, the second housing  201 - 1 . 
     The nip control unit may further include a lock unit for locking the nip control member  370  at the second location, that is, the nip separating portion  372  at the nip separating location. The lock unit may include a first hook  103  formed on the photosensitive unit  100 - 1 , e.g., the first housing  101 - 1 , and a second hook  373  formed on the nip control member  370  to be hooked by the first hook  103  when the nip control member  370  is located at the second location (e.g., when the nip separating portion  372  is located at the nip separating location). The second hook  373  may be formed on a locking arm  375  that extends from the body  374  in a direction opposite to the manipulation portion  371 . The locking arm  375  may be elastically curved. The first housing  101 - 1  may include a support portion  104  supporting the nip control member  370  located at the first location to maintain the nip control member  370  at the first location. The nip control member  370  is moved from the second location to the first location by the elastic force of the returning member  380 , and is supported by the support portion  104  to be maintained at the first location. 
     As shown in  FIG. 28 , when the user pushes the manipulation unit  371  in a state where the nip control member  370  is located at the first location in order to move the nip control member  370  toward the second location, the nip separating portion  372  pushes the interference portion  203  of the developing unit  200 - 1  while moving toward the nip separating location. Then, the developing unit  200 - 1  rotates about the hinge shaft  301  in the direction opposite to the direction, in which the elastic force of the elastic member  330  is applied, that is, a direction F 1  ( FIG. 29 ), and the developing roller  13  is separate from the photosensitive drum  11  and the developing nip  15  is removed. When the nip control member  370  reaches the second location (e.g., when the nip separating portion  372  reaches the nip separating location), the second hook  373  is hooked by the first hook  103  as shown in  FIG. 29 , and even when the force applied to the manipulation portion  371  is eliminated, the nip control member  370  is maintained at the second location. 
     The nip control member  370  may be returned from the second location to the first location (e.g., the nip separating portion  372  is returned from the nip separating location to the nip forming location) through the manual manipulation, that is, by releasing the second hook  373  from the first hook  103  in a state where the nip control member  370  is located at the second location. To do this, the photosensitive unit  100 - 1  may include a manipulation recess  105  so as to push the locking arm  375 . When the user pushes the locking arm  375  via the manipulation recess  105  in a state where the nip control member  370  is located at the second location as shown in  FIG. 29 , the second hook  373  unhooks from the first hook  103  and the locking of the nip control member  370  is released, and then, the nip control member  370  is moved to the first location by the elastic force of the returning member  380 . When the contact state between the nip separating portion  372  and the interference unit  203  is terminated, the developing unit  200 - 1  is rotated about the hinge shaft  301  in a direction F 2  ( FIG. 28 ) by the elastic force of the elastic member  330 , and accordingly, the developing roller  13  approaches the photosensitive drum  11 . When the nip control member  370  reaches the second location, the nip separating portion  372  returns to the nip forming location, and the developing roller  13  contacts the photosensitive drum  11  due to the elastic force of the elastic member  330  to form the developing nip  15 . 
     According to the above configuration, the developing nip  15  may be removed or formed by the manual manipulation. 
     The nip forming unit forms the developing nip  15  according to the operation of the image forming apparatus  1 - 1 . To do this, the nip forming unit may return the nip control member  370  from the second location to the first location (e.g., returns the nip separating portion  372  from the nip separating location to the nip forming location) according to the operation of the image forming apparatus  1 - 1 . 
     Referring to  FIG. 26 , a gear  390  is illustrated. The gear  390  is rotated when the developing cartridge  10 - 1  is installed in the main body  2 - 1  and is driven in connection with the driving connection unit (not shown) disposed in the main body  2 - 1 . For example, the gear  390  may be coupled to the rotary shaft  13   a  of the developing roller  13 . As such, when the developing roller  13  rotates, the gear  390  also rotates. The gear  390  includes a releasing cam  391 . 
     Referring to  FIG. 27 , the nip control member  370  includes a releasing portion  376 . For example, the releasing portion  376  is disposed on the locking arm  375 . The releasing portion  376  may have a shape that may interfere with the releasing cam  391  when the image forming apparatus  1 - 1  is driven and the gear  390  is rotated in a state where the nip control member  370  is located at the second location. 
     An example of the nip forming unit may be implemented by using the gear  390  including the releasing cam  391  and the releasing portion  376 . 
     As shown in  FIG. 26 , when the nip control member  370  is located at the first location, the releasing portion  376  is spaced away from the releasing cam  391 . Therefore, in this state, even if the gear  390  is rotated with the developing roller  13 , the nip control member  370  is maintained at the first location. 
       FIG. 30  is a diagram illustrating operations of the releasing cam  391 . As shown in  FIG. 30 , when the developing cartridge  10 - 1  is driven in a state where the nip control member  370  is located at the second location, the gear  390  is rotated. As the gear  390  is rotated, the releasing cam  391  pushes the releasing portion  376  while contacting the releasing portion  376 . Then, the locking arm  375  is curved, and the second hook  373  is separate from the first hook  103 . When the second hook  373  is completely unhooked from the first hook  103 , the nip control member  370  is moved to the first location by the elastic force of the returning member  380 . When the contact state between the nip separating portion  372  and the interference portion  203  is terminated, the developing unit  200 - 1  is rotated about the hinge shaft  301  in the direction F 2  ( FIG. 28 ) by the elastic force of the elastic member  330  and the developing roller  13  approaches toward the photosensitive drum  11 . When the nip control member  370  reaches the first location, the developing roller  13  is in contact with the photosensitive drum  11  by the elastic force of the elastic member  330  and the developing nip  15  is formed. The nip control member  370  is supported by the support portion  104  to be maintained at the first location. 
     According to the above described nip control unit, the nip separating portion  372  is switched to the nip forming location and to the nip separating location by switching the nip control member  370  to the first location and the second location, and thus, the developing nip  15  may be formed and removed by the manual manipulation. Also, when the developing cartridge  10 - 1  is installed in the main body  2 - 1  and the image forming apparatus  1 - 1  is driven in a state where the developing nip  15  is removed by locating the nip control member  370  at the second location, the gear  390  is rotated and the nip control member  370  returns to the first location. Therefore, the developing cartridge  10 - 1  may be installed in the main body  2 - 1  and packaged in a state where the developing nip  15  is removed, and thus, packaging costs may be reduced and logistics costs may be reduced. Also, when the image forming apparatus  1 - 1  is driven, the developing nip  15  is formed. Thus, the user who has purchased the image forming apparatus  1 - 1  does not need to perform processes of separating the developing cartridge  10 - 1  from the main body  2 - 1 , manipulating the nip control member  370  to form the developing nip  15 , and installing the developing cartridge  10 - 1  in the main body  2 - 1  again. Therefore, user&#39;s convenience may be improved. 
     (First Switching Unit) 
     The image forming apparatus  1 - 1  may include a first switching unit for switching the nip control member  370  located at the first location to the second location (e.g., the nip separating portion  372  located at the nip forming location to the nip separating location) when the developing cartridge  10 - 1  is installed in the main body  2 - 1 .  FIG. 31  is a block diagram of the first switching unit according to an exemplary embodiment. Referring to  FIG. 31 , the first switching unit includes a first inclination portion  471  that is upwardly inclined in a direction of installing the developing cartridge  10 - 1 . The first inclination portion  471  may interfere with the nip control member  370  located at the first location when the developing cartridge  10 - 1  is installed in the main body  2 - 1 . According to an exemplary embodiment, the first inclination portion  371  interferes with the manipulation portion  371  of the nip control member  370  located at the first location. When the developing cartridge  10 - 1  is installed in the main body  2 - 1 , the manipulation portion  371  interferes with the first inclination portion  471 , the nip control member  370  is pushed by the first inclination portion  471  to a direction opposite to the direction, in which the elastic force of the returning member  380  is applied, and switched to the second location, and the second hook  373  is hooked by the first hook  103  to be maintained at the second location. As such, the nip separation unit  372  may be switched from the nip forming location to the nip separating location in association with the installing operation of the developing cartridge  10 - 1 . 
     In this state, when the image forming apparatus  1 - 1  starts to operate, the engagement between the second hook  373  and the first hook  103  is released by the interference between the releasing cam  391  formed on the gear  390  and the releasing portion  376  as described above, and then, the nip control member  370  is switched to the first location by the elastic force of the returning member  380 . The nip separating portion  372  is switched from the nip separating location to the nip forming location, and then, the developing nip  15  may be formed. 
     In a state where the nip control member  370  is located at the first location (e.g., the nip separation unit  372  is located at the nip forming location), the manipulation portion  371  protrudes out of the developing cartridge  10 - 1 . In this state, when the developing cartridge  10 - 1  is detached from the main body  2 - 1 , the manipulation portion  371  may interfere with the first inclination portion  471 . To address the above, the first switching unit may further include a second inclination portion  472  that interferes with the manipulation portion  371  of the nip control member  370  located at the first location when the developing cartridge  10 - 1  is detached from the main body  2 - 1 . The second inclination portion  372  is located at a downstream portion of the first inclination portion  471  in the installation direction of the developing cartridge, and is inclined upward in a reverse direction of the installation direction. According to the above configuration, when the developing cartridge  10 - 1  is detached from the main body  2 - 1  in a state where the nip control member  370  is located at the first location, the manipulation portion  371  is pushed by the second inclination portion  372  so that the nip control member  370  is switched to the second location. Therefore, the developing cartridge  10 - 1  may be detached in a state where the developing nip  15  is removed. 
     (Location Detection) 
     The image forming apparatus  1 - 1  may further include a detection unit for detecting whether the developing nip  15  is in the removed state. For example, the detection unit may detect the nip control member  370  located at the second location. For example, the structure of the detection unit according to an exemplary embodiment is the same as that illustrated with reference to  FIG. 15 . That is, the nip separation member  340  of  FIG. 15  may be considered as the nip control member  370  in an exemplary embodiment, and the manipulation portion  341  of  FIG. 15  may be considered as the manipulation portion  371  of an exemplary embodiment. A detection sensor  470  may detect the manipulation portion  371  of the nip control member  370  located at the nip separating location. 
     According to the above configuration, whether the developing nip  15  is removed or formed may be determined according to whether the detection sensor  470  detects the nip control member  370 . 
     The nip control unit may be disposed at a side portion of the developing roller  13  in a length direction of the developing unit. Otherwise, the nip control unit may be disposed at opposite side portions of the developing roller  13  in the length direction of the developing roller  13 , and in this case, the first switching unit may be disposed at the opposite side portions of the developing roller  13  in the length direction. 
     In the above exemplary embodiment, it is assumed that the developing cartridge  10 - 1  and the first switching unit are applied to the image forming apparatus  1 - 1  shown in  FIG. 1 , but are not limited thereto, that is, the developing cartridge  10 - 1  an the first switching unit may be applied to the image forming apparatus  1 - 2  shown in  FIG. 16 . 
     (Image Forming Apparatus) 
       FIG. 32  is a schematic block diagram of an image forming apparatus  1 - 3  according to an exemplary embodiment. The image forming apparatus  1 - 3  of an exemplary embodiment prints mono-color images. Referring to  FIG. 32 , a main body  2 - 3  and a developing cartridge  10 - 2  are illustrated. The main body  2 - 3  includes an opening  3 - 3  for providing a path through which the developing cartridge  10 - 2  is installed and detached. A door  4 - 3  opens/closes the opening  3 - 3 . The main body  2 - 3  includes an exposure unit  20 - 3 , a transfer roller  30 - 3 , and a fuser  40 - 3 . Also, the main body  2 - 3  includes a recording medium conveying unit  50 - 3  for conveying recording media P, on which images will be formed, loaded thereon. The developing cartridge  10 - 2  includes a photosensitive unit  100 - 3  and a developing unit  200 - 3 . 
     The photosensitive unit  100 - 3  includes the photosensitive drum  11 . The photosensitive drum  11  may include, for example, a conductive metal pipe and a photosensitive layer formed on an outer circumference of the conductive metal pipe. The charging roller  12  is an example of a charger that charges the photosensitive drum  11  to a uniform surface potential. A charging brush, a corona charger, etc. may be adopted instead of the charging roller  12 . A cleaning roller  17  removes impurities on the surface of the charging roller  12 . A cleaning blade  16  is an example of a cleaning unit for removing toner and impurities remaining on the surface of the photosensitive drum  11  after performing a transfer process that will be described later. Another type of cleaning device such as a rotating brush may be adopted instead of the cleaning blade  16 . 
     The developing unit  200 - 3  includes the developing roller  13 . The developing unit  200 - 3  supplies toner contained therein to an electrostatic latent image formed on the photosensitive drum  11  to develop the electrostatic latent image into a visible toner image. In an exemplary embodiment, a mono-component contact type developing method is adopted. The developing device  10 - 1  adopting the mono-component contact type developing method is described above with reference to  FIGS. 1 and 2 , and thus, detailed descriptions about the structure of the developing cartridge  10 - 2  are omitted. The toner is attached to the surface of the developing roller  13  due to an electrostatic force and conveyed to the developing nip  15  on which the photosensitive drum  11  and the developing roller  13  face each other. The toner is supplied to the photosensitive drum  11  due to a developing bias voltage applied between the developing roller  13  and the photosensitive drum  11  to develop the electrostatic latent image formed on the surface of the photosensitive drum  11  into a visible toner image. 
     The exposure unit  20 - 3  emits light modulated in correspondence with image information to the photosensitive drum  11  to form the electrostatic latent image on the photosensitive drum  11 , and an example of the exposure unit  20 - 3  may include an LSU using a laser diode as a light source and an exposure device using an LED as a light source. 
     The transfer roller  30 - 3  is an example of a transfer device for transferring the toner image from the photosensitive drum  11  to the recording medium P. A transfer bias voltage is applied to the transfer roller  30 - 3  to transfer the toner image onto the recording medium P. A transfer device such as a corona transfer device or a pin scorotron type transfer device may be adopted, instead of the transfer roller  30 - 3 . 
     The fuser  40 - 3  applies heat and pressure to the image transferred on the recording medium P to fix the image on the recording medium P. The fuser  40 - 3  may have the same structure as the fuser  40 - 1  shown in  FIG. 1 , but is not limited thereto. When the recording medium P has passed through a fusing nip, the toner image is melted and fixed on the recording medium P due to the heat and pressure. 
     When receiving a printing command from a host (not shown), a controller (not shown) charges the surface of the photosensitive drum  11  to a uniform potential by using the charging roller  12 . The exposure unit  20 - 3  emits a light beam modulated in correspondence with image information to be printed to the photosensitive drum  11  to form an electrostatic latent image on the photosensitive drum  11 . The developing roller  13  supplies toner to the photosensitive drum  11  to develop the electrostatic latent image into a visible toner image. The recording medium conveying unit  50 - 3  transports the recording medium P to a transfer nip formed by the transfer roller  30 - 3  and the photosensitive drum  11  through a feeding path  51 . The toner image on the photosensitive drum  11  is transferred to the recording medium P by a transfer bias voltage applied to the transfer roller  30 - 3 . When the recording medium P passes through the fuser  40 - 3 , the toner image is fixed on the recording medium P by the heat and pressure. The recording medium P on which the fusing operation has finished is discharged out of the image forming apparatus  1 - 3  by the discharge roller  60 . 
     (Developing Cartridge) 
       FIG. 33  is a schematic diagram of the developing cartridge  10 - 2  according to an exemplary embodiment. Referring to  FIG. 33 , the developing cartridge  10 - 2  includes the photosensitive unit  100 - 3  and the developing unit  200 - 3 . The photosensitive unit  100 - 3  includes a first housing  101 - 3 , and the photosensitive drum  11  supported by the first housing  101 - 3 . The developing unit  200 - 3  includes a second housing  201 - 3 , and the developing roller  13  supported by the second housing  201 - 3 . The photosensitive unit  100 - 3  and the developing unit  200 - 3  are connected to each other to be rotated to a developing location (see  FIGS. 35 and 36 ) on which the photosensitive drum  11  and the developing roller  13  contact each other to form the developing nip  15 , and a non-developing location (see  FIG. 34 ) on which the photosensitive drum  11  and the developing roller  13  are separate from each other to remove the developing nip  15 . For example, the photosensitive unit  100 - 3  and the developing unit  200 - 3  are connected to each other to rotate to the developing location and the non-developing location about the hinge shaft  301 . In the image forming apparatus  1 - 3 , since the photosensitive drum  11  is related to a location of the transfer roller  30 - 3 , the photosensitive drum  11  is fixed at a location when the developing cartridge  10 - 2  is installed in the main body  2 - 3 . Therefore, the developing unit  200 - 3  is coupled to the photosensitive unit  100 - 3  to be rotatable about the hinge shaft  301 . However, one or more exemplary embodiments are not limited thereto, and the developing unit  200 - 3  may be fixed at a location in the main body  2 - 3  and the photosensitive unit  100 - 3  may be coupled to the developing unit  200 - 3  to be rotatable about the hinge shaft  301 . 
     The elastic member  330  provides the developing unit  200 - 3  and the photosensitive unit  100 - 3  with the elastic force in a direction of forming the developing nip  15 , that is, to be rotated to the developing location. The developing unit  200 - 3  is rotated about the hinge shaft  301  by the elastic force of the elastic member  330 , and accordingly, the developing roller  13  contacts the photosensitive drum  11 . Then, the developing nip  15  may be formed as shown in  FIGS. 35 and 36 .  FIG. 33  illustrates a tensile coil spring having opposite end portions respectively supported by the photosensitive unit  100 - 3  and the developing unit  200 - 3  as an example of the elastic member  330 , but the elastic member  330  is not limited thereto. For example, various types of members such as a torsion coil spring, a plate spring, etc. may be adopted as the elastic member  330 . 
     Although not illustrated in the drawings, the developing cartridge  10 - 2  may include a coupler (not shown) connected to a driving connection unit (not shown) formed on the main body  2 - 3  when the developing cartridge  10 - 2  is installed in the main body  2 - 3 . The photosensitive drum  11 , the charging roller  12 , the developing roller  13 , the supply roller  14 , and the agitator (not shown) may be power connected to the coupler via a power connection unit (not shown). The power connection structure between the developing cartridge  10 - 2  and the main body  2 - 3  is not limited thereto. For example, the developing cartridge  10 - 2  may include a coupler (not shown) for driving the photosensitive drum  11  and the charging roller  12 , and a coupler (not shown) for driving the developing roller  13 , the supply roller  14 , and the agitator (not shown). 
     The developing cartridge  10 - 2  includes a nip control unit for forming and removing the developing nip  15 . The nip control unit may include a nip separating portion switched to a nip forming location and a nip separating location where the developing unit  200 - 3  are respectively located at the developing location and the non-developing location, a manipulation portion for switching the nip separation unit to the nip forming location and the nip separating location by a manual manipulation, and a nip forming unit for switching the nip separating portion from the nip separating location to the nip forming location when the photosensitive unit  100 - 3  and the developing unit  200 - 3  start to operate. 
       FIGS. 34 and 35  are diagrams of the nip control unit according to an exemplary embodiment. Referring to  FIGS. 34 and 35 , the nip control unit includes a nip control member  510 . The nip control member  510  includes a body  511  supported by the developing unit  200 - 3  to be rotatable, a nip separating portion  512  protruding from the body  511 , and a manipulation portion  513  extending from the body  511  to an outer portion of the developing cartridge  10 - 2 . 
     The nip control member  510  may be switched to the second location ( FIG. 34 ) for separating the developing nip  15 , and the first location ( FIG. 35 ) for forming the developing nip  15 . For example, the nip control member  510  may be provided on the developing unit  200 - 3  to be rotatable to the first location and the second location. As the nip control member  510  is switched to the first location and the second location, the nip separating portion  512  may be switched to the first nip forming location where the nip separation unit  512  is separate from the photosensitive drum  11  to form the developing nip  15  and a nip separating location where the nip separation unit  512  is supported by the photosensitive drum  11  to remove the developing nip  15 . The manipulation portion  513  provides a grip by which the nip control member  510  is rotated to the first location and to the second location. 
     Referring to  FIG. 34 , the nip control member  510  is located at the second location. The nip separating portion  512  is supported by the photosensitive drum  11 . The developing unit  200 - 3  and the photosensitive unit  100 - 3  are rotated about the hinge shaft  301  in a direction away from each other, and thus, the developing roller  13  and the photosensitive drum  11  are separate from each other. Accordingly, the developing nip  15  is removed. 
     When the nip control member  510  is rotated in the first direction A 1  at the second location, the nip control member  510  is switched to the first location as shown in  FIG. 35 . In this process, the nip separating portion  512  is switched to the first nip forming location, and separate from the photosensitive drum  11 . The developing unit  200 - 3  and the photosensitive unit  100 - 3  are rotated about the hinge shaft  301  by the elastic force of the elastic member  330  in a direction of approaching each other, and the developing roller  13  and the photosensitive drum  11  contact each other to form the developing nip  15 . 
     As shown in  FIG. 35 , when the nip control member  510  located at the first location is rotated in the second direction A 2  to be switched to the second location shown in  FIG. 34 , the nip separating portion  512  is switched to the nip separating location and contacts the photosensitive drum  11 , the developing unit  200 - 3  and the photosensitive unit  100 - 3  are rotated about the hinge shaft  301  in a direction away from each other, and the developing nip  15  is removed again. 
     The nip separating portion  512  may be switched to the first nip forming location and the nip separating location to form and remove the developing nip  15  through the manual manipulation by switching the nip control member  510  to the first location and the second location via the manipulation portion  513 . The manipulation portion  513  is exposed to outside of the developing cartridges  10 - 2 , and thus, the user may easily access the manipulation portion  513 . 
     The nip forming unit may form the developing nip  15  as the image forming apparatus  1 - 3  starts to operate. To do this, the nip forming unit may switch the nip control member  510  from the second location to the third location (the nip separating portion  512  from the nip separating location to the second nip forming location) as the image forming apparatus  1 - 3  operates. 
     In the state of  FIG. 34 , that is, in a state where the nip control member  510  is located at the second location, when the developing cartridge  10 - 2  is installed in the main body  2 - 3  and the image forming apparatus  1 - 3  is driven, the photosensitive drum  11  rotates in a processing direction C. The nip separating portion  512  of the nip control member  510  is in contact with the photosensitive drum  11  due to the elastic force of the elastic member  330 . In this state, when the photosensitive drum  11  rotates in the processing direction C, the nip control member  510  is rotated in the second direction A 2 . 
       FIG. 36  is a diagram showing a state where the nip control member  510  is located at the third location. When the nip control member  510  rotates in the second direction A 2 , the nip separating portion  512  is switched to the second nip forming location. Then, the contact state between the nip separating portion  512  and the photosensitive drum  11  is terminated, the developing unit  200 - 3  is rotated about the hinge shaft  301  by the elastic force of the elastic member  330  in a direction approaching the photosensitive unit  100 - 3 , and the developing roller  13  contacts the photosensitive drum  11  to form the developing nip  15 . 
     Therefore, in an exemplary embodiment, the nip forming unit may be implemented by the photosensitive drum  11  that rotates in the processing direction C in contact with the nip separating portion  512  to rotate the nip control member  510  and switch the nip separating portion  512  to the second nip forming location. 
     According to the above nip control unit, the nip separating portion  512  may be switched to the first nip forming location and the nip separating location by switching the nip control member  510  to the first location and the second location, thereby forming and removing the developing nip  15 . Also, when the developing cartridge  10 - 2  is installed in the main body  2 - 3  in a state where the nip control member  510  is located at the second location (e.g., the nip separating portion  512  is located at the nip separating location) and the image forming apparatus  1 - 3  is driven, the photosensitive drum  11  switches the nip control member  510  to the third location while rotating. Accordingly, the nip separating portion  512  is switched from the nip separating location to the second nip forming location, and the developing nip  15  is formed. Therefore, the developing cartridge  10 - 2  may be installed in the main body  2 - 3  and packaged in a state where the developing nip  15  is removed, the packaging costs and the logistics costs may be reduced. Also, since the developing nip  15  is formed when the image forming apparatus  1 - 3  starts to operate, the user who has purchased the image forming apparatus  1 - 3  does not need to perform processes of detaching the developing cartridge  10 - 2  from the main body  2 - 3 , manipulating the nip control member  510  to form the developing nip  15 , and installing the developing cartridge  10 - 2  in the main body  2 - 3  again. Therefore, the user&#39;s convenience may be improved. 
       FIG. 37  is a diagram showing a structure of locking the nip control member  510  at the second location, the first location, and the third location (e.g., locking the nip separating portion  512  at the first nip forming location, the nip separating location, and the second nip forming location), according to an exemplary embodiment. Referring to  FIG. 37 , the developing unit  200 - 3  includes a frame  204  supporting the nip control member  510  to be rotatable. The frame  204  includes a rotary shaft  205 , and the nip control member  510  is coupled to the rotary shaft  205  to be rotatable. The frame  204  includes a first hook  206 - 1 , a second hook  206 - 2 , and a third hook  206 - 3  that are arranged in a circumferential direction based on the rotary shaft  205 . The first to third hooks  206 - 1 ,  206 - 2 , and  206 - 3  respectively correspond to the second location, the first location, and the third location of the nip control member  510 . The nip control member  510  includes a lock portion  514 . The lock portion  514  may be formed as, for example, a recess, and the first to third hooks  206 - 1 ,  206 - 2 , and  206 - 3  are formed as protrusions that are inserted in the lock portion  514  of the recess shape. Otherwise, the lock portion  514  may be formed as a protrusion, and the first to third hooks  206 - 1 ,  206 - 2 , and  206 - 3  may be formed as recesses in which the lock portion  514  of the protrusion shape may be inserted. 
     According to the above configuration, the nip control member  510  may be locked at the second location, the first location, and the third location. Coupling between the lock portion  514  and the first to third hooks  206 - 1 ,  206 - 2 , and  206 - 3  is sufficiently strong for the user to feel a so-called ‘clicking sensation’ when the nip control member  510  is rotated respectively to the second location, the first location, and the third location. 
     (First Switching Unit) 
     The developing nip  15  may be removed while the developing cartridge  10 - 2 , in which the developing nip  15  is formed, is installed in the main body  2 - 3 . For example, the developing nip  15  may be removed by switching the nip separating portion  512  located at the first nip forming location to the nip separating location when the developing cartridge  10 - 2  is installed in the main body  2 - 3 .  FIGS. 38 and 39  are diagrams of a first switching unit that removes the developing nip  15  by operating the nip control member  510  when the developing cartridge  10 - 2  is installed in the main body  2 - 3 , according to an exemplary embodiment. 
     Referring to  FIG. 38 , the first switching unit includes a switching member  410   b  disposed on the main body  2 - 3 . The switching member  410   b  may be formed, for example, on the frame  7  of the main body  2 - 3 . When the developing cartridge  10 - 2  is installed, the nip control member  510  located at the first location interferes with the switching member  410   b . For example, the manipulation portion  513  of the nip control member  510  interferes with the switching member  410   b . Since the switching member  410   b  is fixed at a location, the nip control member  510  is pushed by the switching member  410   b  to be rotated in the first direction A 1  as the developing cartridge  10 - 2  is inserted in the main body  2 - 3  in the installation direction shown in  FIG. 38 . Accordingly, the nip separation unit  512  contacts the photosensitive drum  11 , and the developing unit  200 - 3  rotates about the hinge shaft  301  in a direction B 1  so that the developing roller  13  starts to separate from the photosensitive drum  11 . 
     As shown in  FIG. 39 , when the installation of the developing cartridge  10 - 2  in the main body  2 - 3  is finished, the nip control member  510  reaches the second location. The nip separating portion  512  reaches the nip separating location, and the developing roller  13  is completely separate from the photosensitive drum  11  and the developing nip  15  is removed. 
     According to the above configuration, the developing nip  15  is removed by the process of installing the developing cartridge  10 - 2  in the main body  2 - 3 . Therefore, there is no need to check whether the developing nip  15  is formed before installing the developing cartridge  10 - 2  in the main body  2 - 3  after performing tests before release during the manufacturing processes, and thus, manufacturing costs may be reduced. Also, since the developing nip  15  is in the removed state, even when the developing cartridge  10 - 2  has been installed in the main body  2 - 3  for a long time until the image forming apparatus is sold after being released to market, the developing roller  13  and the photosensitive drum  11  may not be deformed or damaged. 
     When the image forming apparatus  1 - 3  starts to operate in the above state, the nip control member  510  is rotated in the first direction A 1  due to the rotation of the photosensitive drum  11  in the processing direction C, and thus, the nip control member  510  is switched to the third location as shown in  FIG. 40 . As such, the developing roller  13  and the photosensitive drum  11  contact each other and the developing nip  15  is formed. Therefore, even if the developing cartridge  10 - 2  is installed in the main body  2 - 3  in a state where the developing nip  15  is not formed, the printing operation may be performed. 
     (Second Switching Unit) 
     Even when the image forming apparatus  1 - 3  is operating, the developing roller  13  may be separate from the photosensitive drum  11  when there is no need to perform the developing operation. For example, during a time period from a time when an end portion of the toner image has passed through the region where the developing roller  13  and the photosensitive drum  11  face each other to a time when the toner image is transferred to the recording medium P and the recording medium P is discharged out of the main body  2 - 3  by the discharging roller  60 , the developing roller  13  does not need to contact the photosensitive drum  11 . Also, the developing roller  13  does not need to contact the photosensitive drum  11  when the printing operation is finished. In this case, if the developing nip  15  is removed by separating the developing roller  13  from the photosensitive drum  11 , the deformation or damage to the developing roller  13  and the photosensitive drum  11  may be reduced, and accordingly, images of high image quality may be printed stably during the lifespan of the developing cartridge  10 - 2 . 
     In order to remove the developing nip  15 , the nip separating portion  512  may be switched from the second nip forming location to the nip separating location. The above operation may be achieved by switching the nip control member  510  from the third location to the second location.  FIG. 41  is a diagram of the second switching unit for switching the nip separating portion  512  from the second nip forming location to the nip separating location, according to an exemplary embodiment. Referring to  FIGS. 38, 39, 40, and 41 , the second switching unit includes a switching lever  520  provided in the main body  2 - 3  to be moveable to a retreat location where the switching lever  520  does not interfere with the nip control member  510  and to an interference location where the switching lever  520  rotates the nip control member  510 . The switching lever  520  may be provided on the frame  7  to rotate about a rotary shaft  7   a  to the retreat location to the interference location. The switching lever  520  may rotate the nip control member  510  from the third location to the second location by interfering with the manipulation portion  513  while rotating from the retreat location to the interference location. 
     Various structures for rotating the switching lever  520  may be adopted. For example, the switching lever  520  may be rotated in connection with the rotary gear  451  that is rotated by the actuator  460  as described above with reference to  FIG. 12 . The switching lever  520  and the rotary gear  451  may be formed integrally with each other. For example, the actuator  460  may be connected to the rotary gear  451  by the worm gear  461  formed on the rotary shaft thereof. Although not illustrated in the drawings, the switching lever  520  may be driven by a driving unit (not shown) that drives the developing cartridge  10 - 2 . In this case, an alternative power connection structure, for example, a clutch structure, may be interposed between the driving unit (not shown) and the switching lever  520 . 
     Referring to  FIGS. 38, 39, and 40 , the switching lever  520  is located at the retreat location where the switching lever  520  does not interfere with the nip control member  510 . On the retreat location, the nip control member  510  and the switching lever  520  do not interfere with each other even when the developing cartridge  10 - 2  is attached to/detached from the main body  2 - 3  in a state where the nip control member  510  is located at the second location ( FIG. 38 ) or the first location ( FIG. 39 ). Also, even when the nip control member  510  is switched from the first location to the second location by the first switching unit during the process of installing the developing cartridge  10 - 2  in the main body  2 - 3  or even when the nip control member  510  is switched from the second location to the third location by the rotation of the photosensitive drum  11  in a state where the developing cartridge  10 - 2  is installed in the main body  2 - 3 , the switching lever  520  located at the retreat location does not interfere with the nip control member  510 . 
     In  FIG. 40 , the nip control member  510  is located at the third location due to the rotation of the photosensitive drum  11 . The nip separating portion  512  is located at the second nip forming location. The developing nip  15  may be removed when the printing operation has finished, between the printing operations (e.g., between pages), or after the end portion of the toner image has passed through the developing nip  15 . To do this, the actuator  460  rotates the switching lever  520  in a counter-clockwise direction in  FIG. 40 . Then, the switching lever  520  contacts the manipulation portion  513  of the nip control member  510 , and then pushes the manipulation portion  513  to rotate the nip control member  510  in the first direction A 1 . 
     As shown in  FIG. 41 , when the switching lever  520  reaches the interference location, the nip control member  510  is located at the second location (e.g., the nip separation unit  512  is located at the nip separating location), and the driving of the actuator  460  is terminated. After that, the actuator  460  rotates the switching lever  520  in the counter-clockwise direction to return the switching lever  520  to the retreat location as shown in  FIG. 39 . 
     According to the above configuration, the developing nip  15  may be removed at necessary times during operation of the image forming apparatus  1 - 3  and when the operation of the image forming apparatus  1 - 3  is finished. 
     When the image forming apparatus  1 - 3  starts to operate and the photosensitive drum  11  rotates in the state shown in  FIG. 41 , the nip control member  510  is rotated in the second direction A 2  to be switched to the third location. Therefore, the nip separation unit  512  is switched from the nip separating location to the second nip forming location, and the developing nip  15  is formed again to perform the printing operation. 
     (Location Detection) 
     The image forming apparatus  1 - 3  may further include a detection unit for detecting whether the developing nip  15  is removed. For example, the detection unit may detect the nip control member  510  located at the second location. The structure of the detection unit is the same as that shown in  FIG. 15 . That is, the nip separation member  340  of  FIG. 15  may be considered as the nip control member  510  and the manipulation portion  341  of  FIG. 15  may be considered as the manipulation portion  513 . The detection sensor  470  may detect the manipulation portion  513  of the nip control member  510  located at the separating location. 
     According to the above configuration, whether the developing nip  15  is removed or formed may be determined according to whether the nip control member  510  is detected by the detection sensor  470 . 
     The nip control unit may be disposed at a side portion of the developing roller  13  in a length direction of the developing roller  13 . Also, the nip control unit may be disposed at opposite side portions of the developing roller  13  in the length direction of the developing roller  13 , and in this case, the first and second switching units may be disposed at the opposite side portions of the developing roller  13  in the length direction. 
     It should be understood that exemplary embodiments described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each exemplary embodiment should typically be considered as available for other similar features or aspects in other exemplary embodiments. 
     While one or more exemplary embodiments have been described with reference to the figures, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope as defined by the following claims.