Patent Publication Number: US-2007104514-A1

Title: Image forming method and printing method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application claims the benefit of Korean Patent Application No. 2005-105578, filed on Nov. 4, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      An aspect of the present invention relates to an image forming apparatus, and, more particularly, to an image forming apparatus that prints a mono-chromatic image regardless of a state of installation of a color developing unit and has an improved printing speed when printing a color image and a printing method thereof.  
      2. Description of the Related Art  
      Generally, a common image forming apparatus has a charging process in which the surface of a photoconductor is charged to have a predetermined electric potential, an exposing process in which an electrostatic latent image is formed by illuminating light onto the surface of the photoconductor by the use of a light scanning unit such as a laser scanning unit (LSU), a developing process in which the electrostatic latent image formed on the surface of the photoconductor is developed into a toner image by supplying a toner, or a development material, onto the electrostatic latent image formed on the surface of the photoconductor, a feeding process in which a printing medium (i.e., paper, transparency, etc.) loaded on a feeding cassette is fed along a transfer path, a transfer process in which the toner image formed on the photoconductor is transferred onto the printing medium, a fusing process in which the toner image that is transferred onto the printing medium is fused onto the printing medium by an application of heat and pressure, and a discharging process in which the printing medium, having been subjected to the fusing process, is discharged from the image forming apparatus.  
      In the image forming apparatus, which forms an image onto a printing medium by the aforementioned process, toner of a developing unit is transferred onto a photoconductor by a potential difference between the developing unit and the photoconductor. According to this method, either a selected developing unit is operated or four developing units are sequentially operated.  
       FIG. 1  is a schematic sectional view of a conventional image forming apparatus. As shown in  FIG. 1 , the image forming apparatus includes a photoconductive drum  10 , a charger  11  which charges the photoconductive drum  10 , a laser scanning unit (LSU)  12  as a light scanning unit which forms an electrostatic latent image by illuminating light on the photoconductive drum  10 , a developing unit  13  which develops the electrostatic latent image into four toner images of black, yellow, magenta, and cyan, a transfer belt  14  which sequentially superimposes the toner images of the four colors developed onto the photoconductive drum  10 , a first transfer roller  14   a  which transfers the toner images developed on the photoconductive drum  10  to the transfer belt  14 , a second transfer roller  14   b  which transfers an image, in which the four colors are superimposed, on the transfer belt  14  onto a printing medium, and a fusing unit  15  which fuses the image onto the printing medium by applying heat and pressure thereon.  
      In addition, developing rollers  13   a -K,  13   a -Y,  13   a -M, and  13   a -C of four developing units  13 -K,  13 -Y,  13 -M, and  13 -C provided in the developing unit  13  are separated from the photoconductive drum  10  by a predetermined gap, and developing bias voltages are sequentially applied thereto to form the electrostatic latent image on the photoconductive drum  10 . The reference numeral  16  denotes a feeding cassette,  17  denotes a photoconductor cleaning blade,  18  denotes a charge removing unit, and  19  denotes a transfer path that the printing medium is transferred along.  
      In the aforementioned configuration, image forming operations are carried out as follows. First, the charger  11  charges the photoconductive drum  10 . Then, the LSU  12  illuminates light thereto to form an electrostatic latent image of a first color image to be developed. For example, if black corresponds to the first color image to be developed, a predetermined bias voltage is applied to the black developing roller  13   a -K, the developing roller  13   a -K is operated by a developing unit driving motor (not shown), and a toner T attached to the outer circumferential surface thereof is transferred to an electrostatic latent image portion of the photoconductive drum  10  which is in contact therewith. The black image developed, as mentioned above, is then transferred onto the transfer belt  14  through a first transfer nip N 1 . Next, an electrostatic latent image of a second color is formed by the charging and exposing processes of the photoconductive drum  10 . For example, if yellow corresponds to the second color image to be developed, a predetermined bias voltage is applied to the yellow developing roller  13   a -Y, and the developing roller  13   a -Y is operated to form an electrostatic latent image of the photoconductive drum  10 . The yellow image developed, as mentioned above, is transferred onto the transfer belt  14  superimposed on the black image. In the same manner, magenta and cyan images are developed and transferred, and, finally, an image having a desired color is formed onto the transfer belt  14 . Next, the color image is transferred onto a printing medium supplied to a second transfer nip N 2  located between the transfer belt  14  and the second transfer belt  14   b  and is completely fused on the printing medium by an application of heat and pressure while passing the fusing unit  15 .  
      In a conventional image forming apparatus having the aforementioned configuration, when printing a color image, a toner image on the photoconductive drum  10  is transferred as many as four superimposing times and is then again transferred onto the printing medium. Thus, a printing speed of a color image is approximately a quarter of a printing speed of a mono-chromatic (black and white) image.  
      An image forming apparatus to print an image using two photoconductors to improve this problem is disclosed in Japanese Unexamined Patent Application Publication No. 1992-204871. According to this publication, two photoconductors and a plurality of developing units for respective photoconductors are used when printing an image. However, the aforementioned apparatuses are selectively operated only when all developing cartridges are installed. That is, when a part of the development units are not installed, a system error is indicated and an image cannot be printed.  
     SUMMARY OF THE INVENTION  
      An aspect of the present invention provides an image forming apparatus in which a first image forming unit may be selectively installed therein, so that an image forming operation may be performed by a second image forming unit even if the first image forming unit is not installed. As such, a user may selectively purchase a mono-chromatic image forming unit or a color image forming unit.  
      According to an aspect of the present invention, there is provided an image forming apparatus, including a first image forming unit, including a first photoconductor and a first developing device that develops an image by supplying toner onto an electrostatic latent image formed on the first photoconductor, a second image forming unit, including a second photoconductor and a second developing device that develops an image by supplying toner onto an electrostatic latent image formed on the second photoconductor, and a control unit to control operations of the second image forming unit, so that, when the first image forming unit is not installed in the image forming apparatus, the image is printed in response to desired image data by an operation of the second image forming unit.  
      In the aforementioned aspect of the image forming apparatus, the first developing device may comprise a plurality of developing units in which toners to develop a color image are stored.  
      In addition, the second developing device may comprise a developing unit in which a black toner to develop a mono-chromatic image is stored.  
      In addition, the first developing device may comprise three developing units to store cyan, magenta, and yellow toner, and the second developing device may comprise a developing unit to store black toner, the second developing device being detachably disposed in the second image forming unit, irrespective of an installation state of the first developing device.  
      In addition, the image forming apparatus may further comprise first and second transfer units which face the first and second photoconductors and which are disposed at the opposite side of the surface where the first and second photoconductor face.  
      In addition, the image forming apparatus may further comprise a light scanning unit which forms the electrostatic latent image on an outer circumferential surface of the first and second photoconductors by illuminating light corresponding to image data in response to a computer signal.  
      In addition, the image forming apparatus may further comprise an intermediate transfer device which faces the first and second photoconductors and to which a toner image developed onto the first and second photoconductors is transferred.  
      In addition, the first and second image forming units may be disposed such that the toner image developed onto the first and second photoconductors is transferred onto a printing medium that is transferred along a predetermined path.  
      According to another aspect of the present invention, there is provided an image forming apparatus comprising a first image forming unit, including a first photoconductor and a first developing device that develops an image by supplying a toner onto an electrostatic latent image formed on the first photoconductor, a second image forming unit, including a second photoconductor and a second developing device that develops an image by supplying a toner onto an electrostatic latent image formed on the second photoconductor, and a control unit to control operations of the first and second image forming units, so that a process speed ratio of the first and second image forming units is approximately 3:1.  
      In the aforementioned aspect of the image forming apparatus, the control unit may control the operations of the first and second image forming units, so that a printing speed of a mono image is approximately three times faster than a printing speed of a color image.  
      In addition, the first developing device may comprise three developing units to store cyan, magenta, and yellow toner, and the second developing device comprises a developing unit to store black toner, the second developing device being detachably disposed in the second image forming unit, irrespective of an installation state of the first developing device.  
      In addition, the control unit may control the operations of the first and second image forming units, so that a black toner image is transferred onto an intermediate transfer device, while any one of toner images of cyan, magenta, and yellow are transferred onto the intermediate transfer device.  
      In addition, the first image forming unit and the second image forming unit may have a structure in which the second photoconductor rotates once to develop the black image, while the first photoconductor rotates three times to superimposedly develop the cyan, magenta, and yellow images.  
      According to another aspect of the present invention, there is provided a printing method to operate an image forming apparatus having a first image forming unit which includes a first developing device developing a color image by supplying a toner onto the first photoconductor and a second image forming unit which includes a second developing device developing a mono-chromatic image by supplying a black toner onto the second photoconductor; wherein, when printing the color image, a toner image is transferred onto a recording medium by an operation of the first and second image forming units, whereas when printing the mono-chromatic image, the toner image is transferred onto the recording medium by operating the second image forming unit, even when the first image forming unit is not installed.  
      In the aforementioned aspect of the printing method of an image forming apparatus, the recording medium may be a printing medium.  
      In addition, the recording medium may be an intermediate transfer device.  
      According to another aspect of the present invention, there is provided a printing method to operate an image forming apparatus having a first image forming unit which includes a first developing device developing a color image by supplying a toner onto the first photoconductor and a second image forming unit which includes a second developing device developing a mono image by supplying a black toner onto the second photoconductor; wherein a process speed ratio of the first and second image forming units is approximately 3:1.  
      In the aforementioned aspect of the printing method of an image forming apparatus, the operations of the first and second image forming units may be controlled so that a printing speed of a mono-chromatic image is approximately three times faster than a printing speed of a color image.  
      In addition, the first developing device may comprise three developing units to store cyan, magenta, and yellow toner, and the second developing device may comprise a developing unit to store black toner, the second developing device being detachably disposed in the second image forming unit, irrespective of an installation state of the first developing device.  
      In addition, the first image forming unit and the second image forming unit may have a structure in which the second photoconductor rotates once time to develop the black image, while the first photoconductor rotates three times to superimposedly develop the cyan, magenta, and yellow images.  
      Additional and/or other aspects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:  
       FIG. 1  is a schematic sectional view of a conventional image forming apparatus;  
       FIG. 2  is a sectional view of an image forming apparatus according to an embodiment of the present invention;  
       FIG. 3  is a perspective view of a first photoconductive unit of  FIG. 2 ;  
       FIG. 4  is a perspective view of an intermediate transfer device of  FIG. 2 ; and  
       FIG. 5  is a schematic sectional view of first and second image forming units and the intermediate transfer device of  FIG. 2 . 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
      Reference will now be made in detail to the present embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures.  
       FIG. 2  is a sectional view of an image forming apparatus according to an embodiment of the present invention,  FIG. 3  is a perspective view of a first photoconductive unit of  FIG. 2 , and  FIG. 4  is a perspective view of an intermediate transfer device of  FIG. 2 . In addition,  FIG. 5  is a schematic sectional view of first and second image forming units and the intermediate transfer device of  FIG. 2 . Since configurations and operations of a second photoconductive unit are similar as in the first photoconductive unit of the  FIG. 3 , a detailed description for this will be omitted.  
      As shown in  FIG. 2 , a first image forming unit  100 , a second image forming unit  400 , a light scanning unit  103 , an intermediate transfer device  300 , and a control unit  500 , which controls operations of the respective elements, are included in a main frame  140  of the image forming apparatus. Here, the first image forming unit  100  includes a first photoconductor  101  and a first developing device  104  which performs a development of an image by supplying toner to the first photoconductor  101 . Similarly, the second image forming unit  400  includes a second photoconductor  401  and a second developing device  404  which performs a development of an image by supplying toner to the second photoconductor  401 . The second developing device  404  stores black toner which is used for printing a mono-chromatic (i.e., a black and white) image. In addition, the first image forming unit  100  and the second image forming unit  400  are disposed inside of the main frame  140  as detachable cartridges. In alternate embodiments of the invention, the first developing device  104  and the second developing device  404  are disposed inside of the main frame  140  as detachable cartridges.  
      In the first photoconductor  101 , a photoconductive material layer is coated on the outer circumferential surface of the cylindrical metal drum by various methods, such as a deposition method, and a portion of the outer circumferential surface thereof is exposed. The first photoconductor  101  rotates in a predetermined direction, and an electrostatic latent image corresponding to a desired image is formed on the outer circumferential surface thereof by the use of light illuminated by the light scanning unit  103 .  
      A charging roller  102  is an example of a charger which charges the first photoconductor  101  to have a uniform electric potential. The charging roller  102  rotates in contact or non-contact with the outer circumferential surface of the first photoconductor  101  and supplies an electric charge thereto so that the outer circumferential surface of the first photoconductor  101  is provided with a uniform electric potential. A charging bias voltage is then applied to the charging roller  102  in order to charge the outer circumferential surface of the first photoconductor  101  to have a uniform electric potential. According to an embodiment of the invention, a corona discharger (not shown) may be used instead of the charging roller  102 , although, it is understood that other charging devices may be used.  
      The light scanning unit  103  is disposed to face downward with respect to the first photoconductor  101  and the second photoconductor  401  and forms an electrostatic latent image on the outer circumferential surface of the first photoconductor  101  and the second photoconductor  401  by illuminating light corresponding to image data onto the first photoconductor  101  and the second photoconductor  401  which are charged to have a uniform electric potential in response to a computer signal. The light scanning unit  103  includes a light source (now shown) which illuminates a laser beam and a beam deflector which deflects the laser beam. The beam deflector may be a polygon mirror (not shown) which illuminates light while being rotated by a driving source. Meanwhile, a hologram disc (not shown) which deflects light by diffracting the light using a hologram pattern may be used instead of the polygon mirror (not shown). A laser scanning unit (LSU) which normally uses a laser diode as a light source is used as the light scanning unit  103 . In the present invention, the electrostatic latent image is formed on the outer circumferential surface of the two photoconductors  101  and  401  using one light scanning unit  103 .  
      For color printing operations, in embodiments of the invention, the first developing device  104  has black toner stored in the second developing device  404  and at least one of other colored toners. Hereinafter, a case in which the first developing device  104  includes three developing units  104 C,  104 M, and  104 Y storing cyan, magenta, and yellow toner, and in which the second developing device  404  includes one developing unit  404 K storing black toner and one developing roller  425 K, will be described.  
      The first developing device  104  includes a plurality of developing units in which toners to develop a color image are stored. In the present invention, the first developing device  104  includes three developing units  104 C,  104 M, and  104 Y in which cyan, magenta, and yellow colored toners are stored. Here, the three developing units  104 C,  104 M, and  104 Y are placed in the first developing device  104 , which is a single cartridge and which is detachable from the main frame  140 . The developing units  104 C,  104 M, and  104 Y respectively include developing rollers  125 C,  125 M, and  125 Y to form a toner image by supplying the toners onto the electrostatic latent image form on the first photoconductor  101 . The developing units  104 C,  104 M, and  104 Y are replaced when the toners stored therein are used up.  
      The developing rollers  125 C,  125 M, and  125 Y cause the toners stored in the developing units  104 C,  104 M, and  104 Y to be attached onto the outer circumferential surfaces of the developing units  104 C,  104 M, and  104 Yto be positioned to be supplied to the first photoconductor  101 . The developing rollers  125 C,  125 M, and  125 Y store solid powder type toners and develop the toner image by supplying the toners onto the electrostatic latent image formed on the first photoconductor  101 . A developing bias voltage to cause the toners to be attracted to the first photoconductor  101  is applied to the developing rollers  125 C,  125 M, and  125 Y.  
      The three developing units  104 C,  104 M, and  104 Y are disposed such that respective developing rollers  125 C,  125 M, and  125 Y are separated from the outer circumferential surface of the first photoconductor  101  by a predetermined gap, or a development gap Dg. A force directed from the first photoconductor  101  to respective developing rollers  125 C,  125 M, and  125 Y is generated by the electric field, and charged toners are reciprocally vibrated in a development area formed in the development gap Dg and are transferred to be developed. A length of the development gap Dg is, in accordance with embodiments of the invention, approximately tens to hundreds of microns.  
      The second developing device  404  is disposed in a detachable manner, irrespective of a state of an installation of first developing device  104 . That is, regardless of whether the first developing device  104  is installed in the main frame  140 , the second developing device  404  may be operated. Here, it is understood that the second developing device  404  stores only black toner.  
      The intermediate transfer device  300  faces the first and second photoconductors  101  and  401  and includes an intermediate transfer device  105  to which the toner image, developed on the first and second photoconductors  101  and  401 , is transferred. In addition, first and second transfer units  304  and  305  facing the first and second photoconductors  101  and  401  are further provided at the other side of the surface facing the first and second photoconductors  101  and  401  of the intermediate transfer device  105 . A transfer bias voltage is applied to the first and second transfer units  304  and  305  so that the toner image formed on the first and second photoconductors  101  and  401  is able to be transferred to the intermediate transfer device  105 . Here, the first and second transfer units  304  and  305  include a transfer roller.  
      The intermediate transfer device  105  includes an intermediate transfer belt (ITB) which travels in a loop and which is supported by a plurality of rollers. Cyan, magenta, and yellow toner images, which are sequentially formed on the first photoconductor  101 , and a black toner image formed on the second photoconductor  401  are sequentially transferred to the ITB. The toner images are superimposedly transferred onto the ITB and a color toner image is thereby formed. In general, the length of the ITB must be equal or greater than the length of a printing medium S (i.e., paper, a transparency, etc.) where the color toner image is finally stored.  
      Hereinafter, the intermediate transfer device  300  including a first photoconductive unit  200  and the intermediate transfer device  105  will be described in detail. Since configurations and operations of a second photoconductive unit  410  are similar as in the first photoconductive unit  200 , a detailed description or a drawing for this will be omitted.  
      As shown in FIGS.  2  to  4 , the first photoconductive unit  200  is disposed in the main frame  140  in a detachable manner. Meanwhile, the first photoconductive unit  200  may be incorporated with the first developing device  104 . The intermediate transfer device  300  is disposed to face upwards with respect to the first photoconductive unit  200  and the second photoconductive unit  410  in a detachable manner in the main frame  140 .  
      In the present embodiment, the developing units  104 C,  104 M, and  104 Y are disposed such that the cyan developing unit  104 C is followed by the magenta developing unit  104 M which is followed by the yellow developing unit  104 Y from downwards to upwards. A pre-transfer charge removing unit  110  is disposed upwards with respect to the developing unit  104 Y disposed at the top. The light scanning unit  103  and a charge removing lamp  107  are disposed downwards with respect to the first photoconductive unit  200 .  
      As shown in  FIG. 3 , the first photoconductive unit  200  includes the first photoconductor  101 , which is rotatably disposed to a shaft  201 , and a first cleaning unit  106 . The first photoconductive unit  200  may further include the charging roller  102 . The first photoconductive unit  200  may further include a light guiding member  205  which guides light illuminated by the charge removing lamp  107  to the first photoconductor  101 . In addition, the first photoconductive unit  200  may further include a handle  202  that is rotatably disposed to the shaft  201 . The handle  202  rotates towards a position  202   a  when the first photoconductive unit  200  is installed or removed from the main frame  140  and rotates towards a position  202   b  after the installation of the first photoconductive unit  200 .  
      As shown in  FIGS. 3 and 5 , the first cleaning unit  106  includes a first blade  106   a  which scrapes waste toner remaining on the surface of the first photoconductor  101  and a first transfer device  106   b  which transfers the waste toner towards a waste toner storage unit (not shown). The first transfer device  106   b  may comprise a spiral shaped wing, and an auger which rotates in a predetermined direction to transfer the waste toner.  
      Referring to  FIGS. 4 and 5 , the intermediate transfer device  300  includes the intermediate transfer device  105 , the first transfer unit  304 , a plurality of supporting rollers  301 ,  302 , and  303 , the second transfer unit  305 , and a second cleaning unit  109 .  
      The intermediate transfer device  105  faces the first and second photoconductors  101  and  401  so that the toner image developed on the outer circumferential surface of the first and second photoconductors  101  and  401  is transferable from the first and second photoconductors  101  and  401  to the intermediate transfer device  105 . Namely, the intermediate transfer device  105  is supported by the plurality of rollers and travels along a circular predetermined path, and the toner image developed on the outer circumferential surface of the first and second photoconductors  101  and  401  is transferred from the first and second photoconductors  101  and  401  to the intermediate transfer device  105 . A polyamide belt having relatively excellent charging properties may be used for the intermediate transfer device  105 , although it is understood that other charging devices may be employed.  
      The plurality of supporting rollers  301 ,  302 , and  303  are disposed on the inner circumferential surface of the intermediate transfer device  105 , support the intermediate transfer device  105 , and rotate the intermediate transfer device  105  in a predetermined direction. The supporting roller  301  is preferably a driving roller. The supporting roller  302  disposed at the opposite side with respect to the supporting roller  301  faces a transfer roller  112 .  
      The first and second transfer units  304  and  305  are disposed on the inner circumferential surface of the intermediate transfer device  105 , so that the toner image developed on the first and second photoconductors  101  and  401  is transferred to the intermediate transfer device  105 . A transfer bias voltage to transfer the toner image formed on the first photoconductor  101  to the intermediate transfer device  105  is applied to the first and second transfer units  304  and  305 .  
      Meanwhile, a first supporting unit  308  and a second supporting unit  309  which support the intermediate transfer device  300  at the time of replacement are provided at both sides of the intermediate transfer device  300 .  
      The second cleaning unit  109  removes waste toner remaining on the intermediate transfer device  105  after the toner image is transferred onto the printing medium S. As shown in  FIG. 5 , the second cleaning unit  109  includes a second blade  109   a  which scrapes the waste toner remaining on the surface of the intermediate transfer device  105  and a second transfer device  109   b  which transfers the waste toner towards a waste toner storage unit (not shown). The second transfer device  109   b  comprises a spiral shaped wing, and an auger which rotates in a predetermined direction to transfer the waste toner.  
      Referring back to  FIG. 2 , the control unit  500  prints an image corresponding to data of a desired image by controlling operations of respective elements of an image forming apparatus such as the first and second image forming units  100  and  400  and the light scanning unit  103 . The control unit  500  controls operations of the first and second image forming units  100  and  400 , so that, even when the first developing device  104  is not installed in the apparatus, an image may be printed in response to desired image data by an operation of the second image forming unit  400 . Therefore, the control unit  500  controls the operations of the first and second image forming unit  100  and  400 , so that, when only a mono-chromatic image is printed, the image may be printed regardless of the state of installation of the first image forming unit  100 .  
      For example, when printing a color image, the control unit  500  operates the first and second image forming units  100  and  400 , and, when printing a mono-chromatic image, the control unit  500  operates the second image forming unit  400 . Therefore, even when the first developing device  104  is not installed, the control unit  500  prints an image by an operation of the second image forming unit  400 .  
      According to an embodiment of the invention, the control unit  500  controls the operations of the first and second image forming units  100  and  400 , so that a process speed ratio of the first and second image forming units  100  and  400  may be approximately 3:1. For example, the control unit  500  controls the operations of the first and second image forming units  100  and  400 , so that the second photoconductor  401  rotates once to develop the black image, while the first photoconductor  101  rotates three times to superimposedly develop the cyan, magenta, and yellow images.  
      Further, the control unit  500  controls the operations of the first and second image forming units  100  and  400 , so that the black toner image is transferred onto the intermediate transfer device  105 , while any one of the cyan, magenta, and yellow toners are transferred onto the intermediate transfer device  105 . At this time, the control unit  500  controls the operations of the first and second image forming units  100  and  400 , so that the black toner image is transferred onto the intermediate transfer device  105 , while the one of the cyan, magenta, and yellow toner images that is last transferred onto the intermediate transfer device  105 , is transferred.  
      In another example, the control unit  500  controls the operations of the first and second image forming units  100  and  400 , so that the printing speed of a mono-chromatic image print is approximately three times faster than that of a color image print.  
      Conventionally, the intermediate transfer device  105  rotates four times so that the cyan, magenta, yellow, and black images may be transferred onto the intermediate transfer device  105 . However, according to aspects of the present invention, by superimposedly transferring the black toner image while transferring the last of the color toner images to the intermediate transfer device  105 , the intermediate transfer device  105  is required to rotate only three times. Therefore, by controlling the first and second image forming units  100  and  400  in this manner, a color printing speed is improved.  
      The transfer roller  112  faces the surface to which the toner image of the intermediate transfer device  105  is transferred, and a transfer bias voltage is applied thereto so that the toner image transferred onto the intermediate transfer device  105  can be transferred onto the printing medium S. Thus, the toner image is transferred onto the printing medium S by the electrostatic force acting between the intermediate transfer device  105  and the transfer roller  112 . The transfer roller  112  is separated from the intermediate transfer device  105  while the toner image is transferred onto the intermediate transfer device  105 , and comes in contact with the intermediate transfer device  105  by a predetermined pressure to transfer the toner image onto the printing medium S after the toner image is completely transferred onto the intermediate transfer device  105 . In addition, the toner image that is transferred onto the outer circumferential surface of the intermediate transfer device  105  may be transferred onto the printing medium S that passes between the transfer roller  112  and the intermediate transfer device  105  by contact pressure between the transfer roller  112  and the intermediate transfer device  105 .  
      Before transferring the toner image on the first photoconductor  101  to the intermediate transfer device  105 , a pre-transfer charge removing unit  110  removes an electric charge in a portion (non-image portion) where the toner image on the first photoconductor  101  is not formed. The pre-transfer charge removing unit  110  is disposed to improve an efficiency of the process of transferring the image from the first photoconductor  101  to the intermediate transfer device  105 .  
      The charge removing lamp  107  is an example of a charge eliminating unit which removes an electric charge remaining on the outer circumferential surface of the first photoconductor  101  prior to a charging operation. The charge removing lamp  107  removes the electric charge remaining on the outer circumferential surface of the first photoconductor  101  by illuminating a constant amount of light onto the outer circumferential surface of the first photoconductor  101 .  
      A power supply  108  provides a developing bias voltage to develop toner from the first and second developing devices  104  and  404  to the first and second photoconductors  101  and  401 , an anti-developing bias voltage to prevent the toner from being attached from the first and second developing devices  104  and  404  to the first and second photoconductors  101  and  401 , a transfer bias voltage to transfer the toner from the first and second photoconductors  101  and  401  to the intermediate transfer device  105 , a transfer bias voltage to transfer the toner image from the intermediate transfer device  105  to the printing medium S, and a charging bias voltage provided to the charging roller  102 .  
      A fusing unit  111  includes a heat roller  123  and a pressure roller  124  facing each other. The fusing unit  111  fuses the toner image onto the printing medium S by an application of heat and pressure. The heat roller  123  is a heat source to permanently fix the toner image and faces the pressure roller with respect to an axial direction thereof.  
      A discharge roller  117  discharges the printing medium S out of the image forming apparatus after printing is completed. The printing medium S, having been discharged, is loaded on an out-feed tray  180 .  
      Reference numeral  113   a  denotes a feeding cassette, which is an example of a loading device, on which the printing medium S may be loaded. The loading device may further include a second feeding cassette  113   b  and a multi-purpose feeder (MPF)  113   c . The MPF  113   c  is mainly used to allow for a transferring of a non-standard printing medium S.  
      A feed roller  116  feeds the printing medium S loaded out from feeding cassettes  113   a ,  113   b , and  113   c  by the use of pickup rollers  115   a ,  115   b , and  115   c  towards a feeding unit  120 .  
      The feeding unit  120  provides a feeding path  121  placed between the feed roller  116  and the fusing unit  111  to guide the printing medium S and further provides for a duplex path  122  to allow for duplex printing operations. An arranging unit  118  is provided to the feeding unit  120 . Before the printing medium S fed from the feed roller  116 , the arranging unit  118  arranges a position of the printing medium S that passes between the intermediate transfer device  105  and the transfer roller  112 , so that the toner image may be formed on a desired portion of the printing medium S. The printing medium S fed, as mentioned above, passes between the intermediate transfer device  105  and the transfer roller  112  and is then transferred onto the printing medium S. The printing medium S having been is subjected to the heat and pressure of the fusing unit  111 , is then discharged out of the image forming apparatus  110  by the discharged roller  117 .  
      During duplex printing operations, the discharge roller  117  reversely rotates, and the printing medium S is transferred along the duplex path  122 . Here, the printing medium S is flipped over so that an image is printed on the surface where the image is not yet printed on. The flipped printing medium S is again fed through the feeding path  121  by the feed roller  116 , and the image is printed on the other side thereof.  
      Hereinafter, operations of the image forming apparatus according to aspects of the present embodiment will be described.  
      Image data corresponding to cyan, magenta, yellow, and black is combined in color image data. When printing a color image, toner images of respective colors are superimposed to the intermediate transfer device  105  in the order of cyan C, magenta M, and yellow Y. When the last color is transferred to the intermediate transfer device  105 , the black toner image is superimposed to the intermediate transfer device  105 , and is then transferred to the printing medium S, thereby forming a color image.  
      On the other hand, when a light signal corresponding to cyan image data is illuminated onto the rotating first photoconductor  101  by the light scanning unit  03 , the resistance of a light illuminated portion is reduced and an electric charge attached on the outer circumferential surface of the first photoconductor  101  is separated therefrom. As a result, an electric potential difference occurs between the light illuminated portion and the other portion, and an electrostatic latent image is thereby formed on the outer circumferential surface of the first photoconductor  101 .  
      When the first photoconductor  101  rotates and the electrostatic latent image approaches the cyan developing unit  104 C, the developing roller  125 C of the cyan developing unit  104 C starts to rotate. A development bias voltage is applied from the power supply  108  to the developing roller  125 C of the cyan developing unit  104 C. In addition, an anti-developing bias voltage to prevent a development is applied to the developing rollers  125 M,  125 Y, and  125 K of other developing units  104 M,  104 Y, and  104 K. Then, only the cyan toner passes across the development gap Dg and is attached on the electrostatic latent image formed on the outer circumferential surface of the first photoconductor  101 , thereby forming a cyan toner image.  
      When the first photoconductor  101  rotates and the cyan toner image approaches the intermediate transfer device  105 , the toner image is transferred to the intermediate transfer device  105  by the transfer bias voltage or contact pressure between the first photoconductor  101  and the intermediate transfer device  105 .  
      After the cyan toner image is completely transferred to the intermediate transfer device  105 , toner images of magenta and yellow are subject to the above mentioned operations and are transferred to the intermediate transfer device  105  and superimposed over the cyan image.  
      When the yellow image is transferred to the intermediate transfer device  105 , a black image is subject to the above mentioned operations and is transferred from the second image forming unit  400  to the intermediate transfer device  105 .  
      During the above-mentioned operations, the transfer roller  112  is separated from the intermediate transfer device  105 . When all of the four toner images are transferred to the intermediate transfer device  105  and a color toner image is formed on the intermediate transfer device  105 , the transfer roller  112  comes into contact with the intermediate transfer device  105  in order to transfer the color toner image onto the printing medium S.  
      By the time a front end of the color toner image formed on the intermediate transfer device  105  reaches a point where the intermediate transfer device  105  and the transfer roller  112  are in contact with each other, the printing medium S is supplied from the feeding cassette  113   a  (or  113   b ) or the MPT  113   c , so that the frond end of the printing medium S reaches the point where the intermediate transfer device  105  and the transfer roller  112  are in contact with each other. When the printing medium S passes between the intermediate transfer device  105  and the transfer roller  112 , the color toner image is transferred onto the printing medium S by the transfer bias voltage, is then fixed onto the printing medium S in the fusing unit  111  by applying heat and pressure, and is then discharged, thereby completing color image forming.  
      For the next printing operation to be undertaken, the second cleaning unit  109  removes the waste toner remaining on the intermediate transfer device  105 .  
      In a common image forming apparatus in which developing materials of cyan, magenta, yellow, and black are provided in one developing unit, a color image is transferred to an intermediate transfer device after the intermediate transfer device rotates as many as four times. However, in the aforementioned image forming apparatus according to aspects of the present invention having the first developing device  104  and the second developing device  404 , the black color in the second developing device  404  is transferred by the time when the last color in the first developing device  104  is transferred, so the color image is completely transferred to the intermediate transfer device  105  by the time the intermediate transfer device  105  has only rotated three times. Therefore, when printing a color image, the image forming apparatus according to an aspect of the present invention provides an improved printing speed than that of as in the conventional image forming apparatus. In addition, when printing a black and white image, only the second image forming unit  400  is used, so the printing speed is approximately three times faster than that of color printing.  
      According to aspects of the aforementioned configuration of the present invention, when printing a color image, the toner image is transferred onto a recording medium by operating the first and second image forming units  100  and  400 , and when printing a black and white image, the toner image is transferred onto the recording medium by operating only the second image forming unit  400 . At this time, the control unit  500  controls operations of each element. Meanwhile, the recording medium may be the intermediate transfer device  105  shown in FIGS.  2  to  5  or the printing medium S.  
      Accordingly, in an image forming apparatus and a printing method thereof according to aspects of the present invention, unlike as in the related art, an image forming unit to print a black and white image and an image forming unit to print a color image are separately configured, so that when printing a black and white image, one image forming unit is operated, whereas when printing a color image, two image forming units are operated. By this configuration in which two image forming units are separated from the image forming apparatus, compatibility between a color model apparatus and a mono-chromatic (black and white) model apparatus is improved. In addition, in comparison to a conventional multi-path type image forming apparatus in which a speed rate of color printing and black and white printing is 1:4, the present invention provides the printing speed rate of 1:3. In addition, when printing only a mono-chromatic (black and white) image, a user may purchase a model having only one image forming unit, resulting in increasing a user&#39;s convenience and saving purchase cost. In addition, the user may additionally purchase a color image forming unit, if needed, to be later incorporated into the image forming apparatus, thereby forming a color image forming apparatus. Namely, various forms of image forming apparatus may be provided with one image forming apparatus.  
      Although a few embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.