Abstract:
A color image forming apparatus is provided capable of easily assembling and controlling the elements relating to color registration and image quality. The apparatus includes an image forming module having a plurality of image forming units, exposure units, a transfer unit and an integrally formed image forming frame, and a main body in which the image forming module is detachably mounted by use of positioning members. Upon assembling the plurality of image forming units, exposure units and the transfer unit in the image forming frame, distances between the former elements are always maintained constant.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims benefit under 35 U.S.C. §119 from Korean Patent Application No. 2004-69975, filed on Sep. 2, 2004, the entire disclosure of which is hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates to a color image forming apparatus. More particularly, the present invention relates to a color image forming apparatus wherein spaced distances of image formation-related elements are maintained constant and easily regulated by assembling and mounting the elements.  
         [0004]     2. Description of the Related Art  
         [0005]     Referring to  FIGS. 1 and 2 , a conventional color image forming apparatus  1  comprises a plurality of image forming units  10  to form a color image, an exposure unit  20  which forms electrostatic latent images on surfaces of photosensitive members  11  constituting the image forming units  10 , a first transfer unit  30  on which each of the images formed on the surfaces of the plurality of photosensitive members  11  is transferred and superimposed, a second transfer roller  40  for transferring the color image transferred on the transfer unit  30  to a printing medium (P) such as a sheet of paper, and a medium delivery unit  50  to deliver the printing medium (P) on which the color image is transferred and fixed to an exterior of a main body of the apparatus. The above elements are mounted to a main frame (not shown). In addition, the main frame is provided with a power supply section (not shown) for supplying power to each of the elements and a controlling section (not shown) which controls the elements and thus performs a printing operation.  
         [0006]     The image forming units  10  are constructed so that, for example, yellow, magenta, cyan and black image forming units are vertically arranged in a row on one side of a transfer belt  31  of the first transfer unit  30 . Each of the image forming units  10  comprises the photosensitive member  11 , an electrification unit, a developing unit and a developing frame to store toner and support the elements. The first transfer unit  30  comprises the transfer belt  31  for supporting toner transferred from the photosensitive members  11  and a plurality of first transfer rollers  32  opposed to each of the photosensitive members  11  which generate a high voltage for transfer to the inside of the transfer belt  31 .  
         [0007]     When a printing command and printing data are received from a computer (not shown) connected to the color image forming apparatus  1 , the controlling section turns on/off the exposure unit  20  according to the printing data, forms electrostatic latent images on surfaces of each of the photosensitive members  11 , develops the electrostatic latent images with fine powdered toner, and then transfers/superimposes the developed images onto the transfer belt  31  with a high voltage of the first transfer rollers  32 . Thus, a color image is formed. Then, the controlling section delivers the printing medium (P) with the medium delivery unit  50 , transfers the color image, which is formed on the transfer belt  31 , on the printing medium (P) by a high voltage of the second transfer roller  40 , fixes the transferred image on the printing medium (P) with high temperature and pressure of a fixing member  51 , and then discharges the printing medium (P) to an exterior of a main body  60 , thereby completing one cycle.  
         [0008]     At this time, in order to obtain a relatively good color image, it is required that the images formed on the photosensitive members  11  of the image forming units  10  should be correctly transferred/superimposed to the transfer belt  31 , and the color images formed on the transfer belt  31  should be correctly transferred to the printing medium (P). This is accomplished by regularly maintaining straightness, parallelism, and spacing or pressure between the transfer belt  31  and the photosensitive members  11 , and spacing or a pressure is also maintained between the transfer belt  31  and the second transfer roller  40 . For accomplishing the aforementioned, accuracy of mounting positions of the transfer belt  31 , and the plurality of first transfer rollers  32  and the image forming units  10 , which are mounted to the main frame (not shown), is also preferred. In addition, it is required that an accuracy of a mounting position between the transfer belt  31  and the second transfer roller  40  of the main frame is also necessary.  
         [0009]     For the purpose of satisfying the above requirements, according to the conventional color image forming apparatus, each of the image forming units  10  is constructed for mounting by reference holes  12  while being guided along a guide (not shown) formed on the main frame through openings (not shown) of positions defined according to colors, and the exposure unit  20  defines an assembly position basis on the same main frame or another sub frame fixed to the main frame. Consequently, focusing distance to the photosensitive member  11  of the image forming unit  10  and straightness in the directions of scanning and sub-scanning are regulated to meet designed values and then fixed. In addition, the transfer belt  31  of the first transfer unit  30  is regulated to maintain parallelism and straightness with the photosensitive members  11  and then fixed to the main frame. The first transfer rollers  32  are fixed to the mainframe so that the corresponding photosensitive members  11  and the transfer belt  31  can maintain a predetermined spacing or pressure. In particular, after regulating a distance between the second transfer roller  40  and the transfer belt  31 , so that an image is correctly transferred to the printing medium (P), the second transfer roller  40  is fixed to the main frame. Meanwhile, a paper supply cassette of the medium delivery unit delivering the printing medium (P), which stores the printing medium (P), is mounted to a space formed in a lower part of the main frame by a guide unit, and all or a part of the medium delivery unit  50 , which picks up and delivers the printing medium (P) in the paper supply cassette is fixed to the main frame. The fixing member  55  is also structured for assembly to the mainframe. In addition, although not shown, a drive means for driving each of the elements, a power supply section for supplying and controlling power to the elements, and a controlling section are also fixed to the main frame. Additionally, the main frame functions to support the entire image forming apparatus while fixing various kinds of additional parts and exterior parts. Since the main frame has such a structure, it should have a high accuracy as a whole, a high strength and durability so that it is not varied during an extended service lifecycle.  
         [0010]     According to the conventional image forming apparatus having the above-mentioned structure, firstly, since various kinds of elements are assembled or mounted to the main frame, it is very difficult to perform controls for matching focal distances, parallelism of scanning direction and straightness of the sub-scanning direction between the image forming units and the exposure unit. Therefore, color registration of the color image is very difficult. Secondly, since uniform pressure or maintenance of the distance between each of the photosensitive members and the transfer belt should be determined in the main frame, there are many reference positions in the main frame, so, dimension management is difficult. In addition, when a dimension is irregular, color realization is difficult because deviation of transfer efficiency occurs. Thirdly, when a dimension deviation occurs in every developing unit, it is difficult to maintain accuracy of the registration because it is difficult to maintain accuracy of dimension in every developing unit in the main frame supporting the entirety of the apparatus. Fourthly, deviation of an exposure position, due to a position deviation between the developing units also occurs, so, color bending results. Fifthly, since errors occur in the accuracy of dimension and straightness, which are maintained to be constant in the first assembling, due to heat distortion and a wear, results in poor color registration. Therefore, it is very difficult to match the color registration through a regulation of the first transfer unit and the image forming units which are mounted to the main frame.  
         [0011]     Accordingly, there is a need for an image forming unit with improved agreement of color registration between a transfer unit and a photosensitive member to constantly maintain straightness and parallelism between an exposure unit and the photosensitive member.  
       SUMMARY OF THE INVENTION  
       [0012]     An aspect of the present invention is to solve at least the above problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention is to prevent a disagreement of color registration between a transfer unit and a photosensitive member due to dimensional non-uniformity of a main frame to constantly maintain straightness and parallelism between an exposure unit and the photosensitive member of an image forming unit, by assembling and mounting the plurality of image forming units, the exposure unit and the transfer unit to an image forming frame formed in an integral body.  
         [0013]     Another object of the invention is to assemble and mount/dismount an image forming unit, an exposure unit and a transfer unit, which are related to a color image formation, to a separate image forming frame formed in an integral body. Thus, the dimensional management required for controlling the accuracy of color registration or exposure of a color image is limited to a dimension of the image forming frame. This facilitates the dimensional management and thus the processing of related parts, and also facilitates the control of the image forming unit, the exposure unit and the transfer unit when one of them is replaced.  
         [0014]     In addition, another object of the invention is to improve an assembling performance by sub-assembling an image forming unit, an exposure unit and a transfer unit to an image forming frame and then assembling the image forming frame to a main frame, and to easily regulate image quality during assembly, by regulating only a distance between a medium delivery unit mounted to a main body and the image forming frame. Thus, making it possible to control an image quality for transfer onto a sheet.  
         [0015]     Additionally, another object of the invention is to minimize heat distortion, deformation due to wear, and an effect due to an impact from other units, thereby maintaining uniformity and reliability of image quality.  
         [0016]     Another object of the invention is to make it possible to mount or dismount an image forming frame module assembled with an image forming unit, to or from a side of a main frame, thereby improving convenience of handling of a color image forming apparatus.  
         [0017]     The above object of the present invention is substantially realized by providing a color image forming apparatus comprising an image forming module including a plurality of elements comprising a plurality of image forming units having photosensitive members, a plurality of exposure units to expose the photosensitive members, a transfer unit to which an image formed on the photosensitive member is transferred, and an image forming frame to detachably receive and support the plurality of elements. A main body, to which the image forming module is removably mounted, includes a second transfer roller for transferring the image formed on the transfer unit to a printing medium. A medium delivery unit delivers the printing medium between the transfer unit and the second transfer roller.  
         [0018]     Preferably, the image forming frame may be integrally formed.  
         [0019]     In addition, the image forming frame may preferably comprise mounting recesses to which the image forming units and the exposure units are respectively mounted and the apparatus may further include positioning members which determine a correct position when the units are mounted to the corresponding mounting recesses  
         [0020]     Preferably, the positioning members may comprise a reference pin and a reference hole corresponding thereto, each formed to any one of the image forming frame and the image forming unit and to any one of the image forming frame and the exposure unit. In addition, preferably, the positioning members of the exposure unit may comprise an exposure unit-supporting section and a corresponding surface provided to the image forming frame and the exposure unit, respectively.  
         [0021]     In addition, the color image forming apparatus of the invention may preferably comprise a frame-positioning assembly, which determines a correct position of the image forming module in the main body, when the image forming module is mounted to the main body. At this time, the frame-positioning assembly may preferably comprise a plurality of reference protrusions formed on the frame of the image forming module and reference grooves formed on the main body corresponding to the reference protrusions.  
         [0022]     Additionally, the image forming frame may preferably be provided with a connector allowing the image forming module to be automatically and electrically connected or disconnected to or from a power supply section and a controlling section of the main body when the image forming module is mounted or dismounted.  
         [0023]     Further, preferably, the transfer unit may comprise a transfer belt and a plurality of transfer rollers which are mounted inside of the transfer belt to correspond to the photosensitive members.  
         [0024]     In accordance with another embodiment of the invention, there is provided a color image forming apparatus comprising an image forming module having a plurality of elements comprising a photosensitive member, a plurality of electrification units to forming an image on the photosensitive member, exposure units and developing units, and an image forming frame for detachably receiving and supporting the plurality of elements, and a main body to which the image forming module is removably mounted and including a medium delivery unit delivering a printing medium so that the image is transferred from the photosensitive member. At this time, preferably, the image forming frame may be integrally formed.  
         [0025]     Preferably, the image forming frame may comprise mounting recesses to which the photosensitive member, and the plurality of electrification units, developing units and exposure units are respectively mounted, and positioning members for determining a correct position when the units are mounted to the corresponding mounting recesses.  
         [0026]     In accordance with another embodiment of the invention, there is provided a color image forming apparatus comprising an image forming module having a plurality of elements comprising a plurality of image forming units having photosensitive members, a plurality of exposure units to expose the photosensitive members, a transfer unit to transfer an image formed on the photosensitive member to a printing medium and an image forming frame to detachably receive and support the plurality of elements, and a main body to which the image forming module is removably mounted and including a medium delivery unit to deliver the printing medium to the transfer unit. At this time, preferably, the image forming frame may be integrally formed.  
         [0027]     Preferably, the transfer unit may comprise a delivery belt and a plurality of transfer rollers which are mounted inside of the delivery belt to correspond to the photosensitive members.  
         [0028]     As described above, according to the color image forming apparatus of the invention, main parts relating to the image formation, such as the image forming unit, the exposure unit, and the transfer unit, are respectively mounted to a main frame, are provided as an image forming module assembled to a separate image forming frame, and then assembled to a main frame, so that it is possible to prevent a disagreement of color registration between the exposure unit and the photosensitive member due to a dimensional error of parts.  
         [0029]     In addition, since the positions of the image forming units, the exposure units and the transfer unit are determined by each of the positioning means provided to the integral image forming frame to meet predetermined dimensions, distances between the elements are maintained constant, and the color registration is correctly maintained and an assembly performance is improved.  
         [0030]     Further, main parts relating to the image formation are structured as the image forming module formed in an integral body. Thus, it is possible to reduce heat distortion, deformation due to wear, and an effect due to an impact from other units, even when they are used for a relatively long time period. Accordingly, it is possible to uniformly maintain image quality for a long time, thereby improving reliability.  
         [0031]     In addition, the image forming module is structured to be mounted and detached to and from a side of the main body. Thus, productivity is improved and it is possible to easily perform an A/S operation after sale. In particular, when the image forming unit or exposure unit is replaced or repaired, since a regulation for agreement of the color registration is performed in the image forming module only, the regulation is very convenient in A/S.  
         [0032]     In particularly, according to conventional units, the main frame supports most of the elements of the color image forming apparatus. Thus, it is made of a material having high strength, many reinforcing members are used, and a management of many dimensions is preferable. Accordingly, processing is difficult and the number of the parts, material costs and a weight of a product are increased. However, according to the exemplary embodiments of the present invention, since a processing dimension related to an image quality is managed only for the small image forming frame compared to the main body, the material having high strength is minimized and a use of the reinforcing member is minimized. Consequently, it is possible to reduce the number of the parts, the material costs and the weight.  
         [0033]     Other objects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]     The above and other objects, features, and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which:  
         [0035]      FIG. 1  is a conceptual view of a conventional color image forming apparatus;  
         [0036]      FIG. 2  is a perspective view showing elements relating to an image formation in the color image forming apparatus shown in  FIG. 1 ;  
         [0037]      FIG. 3  is a conceptual view showing a color image forming apparatus according to a first exemplary embodiment of the invention;  
         [0038]      FIG. 4  is a perspective view showing an image forming module of  FIG. 3 ;  
         [0039]      FIG. 5  is a perspective view showing the image forming module of  FIG. 3 , from an opposite view of that of  FIG. 4 ;  
         [0040]      FIG. 6  is a partial perspective view showing positioning members of an image forming unit of  FIG. 5  in detail;  
         [0041]      FIG. 7  is a perspective view showing a back of the image forming module shown in  FIG. 3 ;  
         [0042]      FIG. 8  is a side sectional view of the image forming module of  FIG. 3 ;  
         [0043]      FIG. 9  is a conceptual view showing a color image forming apparatus according to a second exemplary embodiment of the invention;  
         [0044]      FIG. 10  is a perspective view of an image forming module of  FIG. 9 ; and  
         [0045]      FIG. 11  is a conceptual view showing a color image forming apparatus according to a third exemplary embodiment of the invention. 
     
    
       [0046]     Throughout the drawings, the same drawing reference numerals will be understood to refer to the same elements, features, and structures.  
       DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0047]     The matters defined in the description such as a detailed construction and elements are provided to assist in a comprehensive understanding of the embodiments of the invention. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness.  
         [0048]     Referring to  FIG. 3 , a color image forming apparatus  100  according to an exemplary embodiment of the invention comprises an image forming module  110 , a medium delivery unit  160 , and a main body  170  in which the image forming module  110 , the medium delivery module  160  and a second transfer roller  169  are received.  
         [0049]     The image forming module  110  comprises a plurality of image forming units  120 , a plurality of exposure units  130 , a transfer unit  140  and an image forming frame  150 . An example thereof is shown in FIGS.  4  to  8 . The image forming frame  150  is provided with mounting recesses  151  to which the image forming units  120  and the exposure units  130  can be mounted. In other words, a sidewall  150   a  of the image forming frame  150  is provided with a plurality of mounting recesses  151  to insertably mount and dismount the image forming units  120 . The other side thereof is provided with separate mounting recesses to mount and dismount the exposure units  130 . The mounting recess  151  is provided with a guide  151   a  for guiding the mounting and dismounting of the image forming unit  120 , and the image forming unit  120  and the image forming frame  150  are provided with positioning members  152  for correctly fixing the image forming unit  120  to a predetermined position regarding the image forming frame  150  even when the image forming unit  120  is mounted and dismounted. The positioning members  152  are structured so that distances between photosensitive members  121  of the image forming units  120  mounted to the mounting recesses  151  are equal and straightness and parallelism therebetween are maintained constant. The positioning members  152  comprise a first reference hole  152   a  formed in a sidewall  150   b  opposed to the sidewall  150   a  in which the mounting recess  151  is formed, and a first reference pin  152   b  formed in a corresponding sidewall of the image forming unit  120  (refer to  FIG. 6 ). As shown in  FIG. 4 , the sidewall  150   a  of the image forming unit  150 , in which the mounting recess  151  is formed, is also provided with the first reference pin  152   b,  and the corresponding side of the image forming unit  120  is formed with the reference hole  152   a  into which the first reference pin  152   b  is inserted. Accordingly, the image forming units  120  are respectively fixed to prescribed positions regarding the image forming frame  150  simply by inserting the image forming units  120  in the mounting recesses  151 . At this time, the first reference pin  152   b  and the first reference hole  152   a  may be selectively formed in any one of the image forming units  120  and the image forming frame  150  corresponding to each other as needed, in addition to the above-mentioned positions. For the positioning members  152 , various other suitable arrangements and constructions may be used in addition to the reference pin/reference hole arrangement. Further, a partition wall  155  is preferably provided between the adjacent mounting recesses  151  of the image forming frame  150 . Thus, it is possible to minimize a mutual interference of the adjacent image forming units  120 . As the image forming units  120 , four units for a yellow, magenta, cyan and black are typically used to form a color image. The image forming unit  120  comprises a photosensitive drum as the photosensitive member  121 , an electrification unit for electrifying the photosensitive drum  121  and a developing unit for developing an electrostatic latent image formed on the photosensitive drum  21  by a laser beam emitted from the exposure unit  130 . The developing unit comprises a developing roller supplying toner to the photosensitive member  121 , a supply roller, and a developing frame storing the toner and supporting the photosensitive drum  121 , the electrification unit, the developing unit and the supply roller. In addition, the image forming unit  120  comprises a regulation member constantly regulating a height of the toner delivered to the developing roller.  
         [0050]     Referring to  FIG. 7 , the exposure unit  170 , such as, a laser scanning unit is mounted to a side of the image forming unit  120  to scan the laser beam to the photosensitive member  121  of the image forming unit  120 . A position thereof is determined by two second reference pins  153  provided proximal to a middle wall  150   c  connecting both of the sidewalls  150   a ,  150   b  of the image forming frame  150  and an exposure unit-supporting section  154  provided to a rear end of the image forming frame  150 . Thus, a scanning line of the laser beam emitted from the exposure unit  130  is parallel to a rotational axis of the photosensitive member  121 . At this time, second reference holes  131  corresponding to the second reference pins  153  are provided to a front surface of the exposure unit  130  and a corresponding surface  132  corresponding to the exposure unit-supporting section  154  is provided to a back surface thereof. Accordingly, it can be structured so that the laser beam emitted from the exposure unit  130  can be scanned parallel with the corresponding rotational axis of the photosensitive member  121  of the image forming unit  120 . This is done by simply inserting the second reference pin  153  into the second reference hole  131  of the exposure unit  130  with. Then, the exposure unit is placed on the exposure unit-supporting section  153 . The positioning members  131  and  153 ,  132  and  154  of the exposure unit  130 , shown in  FIG. 7 , can be used to fix the exposure unit  130  to a predetermined position of the image forming unit  154 .  
         [0051]     As shown in  FIG. 8 , the transfer unit  140  comprises a transfer belt  141  that is assembled to a front surface of the image forming frame  150  and to which the images formed on the photosensitive members  121  are transferred/superimposed. A plurality of first transfer rollers  142  are located inside of the transfer belt  141  and allow the image formed on the corresponding photosensitive member  121  to be transferred to the transfer belt  141 . The image forming frame  150  is formed with third reference holes  156  ( FIG. 4 ), to which a driving roller axis  143  and a driven roller axis  144  of the transfer belt  141  are respectively assembled, so that the photosensitive members  121  of the image forming units  120  and the transfer belt  141  are assembled at a predetermined distance. Accordingly, the transfer belt  141  is assembled for location at the predetermined distance from the photosensitive member  121  of the image forming unit  120  just by assembling the driving roller axis  143  and the driven roller axis  144  of the transfer belt  141  to the third reference holes  156  formed in the image forming frame  150 . In  FIGS. 4 and 5 , the first transfer roller  142  is omitted for clarity and conciseness. The first transfer roller  142  is preferably structured so that it can regulate a distance between the photosensitive member  121  of the image forming unit  120  and the transfer belt  141  according to kinds of the colors, as needed. In addition, the mounting position of the first transfer roller  142  is preferably located at a predetermined distance on the basis of the third reference hole  156  of the image forming frame  150 .  
         [0052]     As described above, the image forming frame  150  comprises the positioning members at positions to which the elements  120 ,  130 ,  140  are assembled and mounted so that the image forming units  120 , the exposure units  130 , and the transfer unit  140  can be mounted at a predetermined distance from each other.  
         [0053]     In addition, it is formed in an integral body so as not to be deformed even when it is processed and used for a long time, and made of a material having good durability.  
         [0054]     Further, preferably, the image forming frame  150  is structured to be mounted and dismounted to and from one side of the main body  170 , and comprises frame-positioning assembly so that a position thereof is automatically determined when it is mounted to the main body  170 . The image forming frame  150  and the main body  170  are provided with a guide rail  111  and a frame guide  171 , respectively, so that the image forming frame  150  can be mounted and dismounted by sliding or reciprocal linear movement relative to the main body  170 . In addition, the frame-positioning assembly comprises a reference groove (not shown) provided for a wall of the main body  170  to which the image forming frame  150  is mounted and a reference protrusion  157  provided to a corresponding surface of the image forming frame  150 , as shown in  FIG. 5 , and allows a position of the image forming module  110  to be correctly determined automatically when the image forming frame  150  is mounted to the main body  170 . At this time, the reference groove and the reference protrusion  157  are formed so that the transfer belt  141  mounted to the image forming frame  150  maintains a predetermined distance with the second transfer roller  169  mounted to the main body  170 . Accordingly, when the image forming frame  150  is mounted to the main body  170  along the guide rail  111  and the frame guide  171 , the reference protrusion  157  of the image forming frame  150  is inserted into the reference groove of the main body  170 . Thus, the transfer belt  141  of the image forming frame  150  is fixed while maintaining the predetermined distance with the second transfer roller  169  of the main body  170 . Then, the image is transferred from the transfer belt  141  which is assembled with the image forming frame  150 , to a printing medium (P) delivered between the transfer belt  141  and the second transfer roller  169 . Even if a distance between the transfer belt  141  and the second transfer roller  169  deviates from a standard due to an error of an part or other causes and thus a poor image results, it is possible to correct the distance therebetween by regulating a position of the image forming frame  150  or the second transfer roller  169 . Consequently, an image quality can be relatively easily controlled.  
         [0055]     In addition, the sidewall of the image forming frame  150 , to which the reference protrusion  157  is formed, is provided with a connector  158  for allowing the elements  120 ,  130 ,  140  to make an electrical connection with a power supply section and a controlling section of the main body  170 . A counter connector (not shown), which is connected to the power supply section and the controlling section and coupled to the connector  158 , is provided to a corresponding surface of the main body  170 . At this time, the connector  158  and the counter connector is structured for electrical connection and disconnection automatically when the image forming module  110  is mounted and dismounted to and from the main body  170 . Accordingly, when the image forming module  110  is mounted along the frame guide  171  provided to the main body  170 , the electrical connection between the image forming module  110  and the power supply section and the controlling section of the main body  170  is automatically made. Thus, a color image is formed in the image forming module  110  and transferred to the printing medium (P) according to a signal of the controlling section.  
         [0056]     The medium delivery unit  160  allows the image formed in the image forming module  110  to be transferred to the printing medium (P) such as a printing sheet, and discharges the printing medium (P) having the image transferred thereto to an exterior of the main body  170 . The medium delivery unit  160  comprises a paper supply cassette  161  in which the printing medium (P) is loaded, a pick-up roller  162  for picking up the loaded printing medium (P) one by one, a delivery unit  163  for moving the picked-up printing medium (P) between the transfer belt  141  and the second transfer roller  169  and allowing the image to be transferred to the printing medium (P), and a paper discharge unit  164  for discharging the printing medium (P) having the image transferred thereto into the exterior of the main body  170 . The medium delivery unit  160  may be further provided with an MP unit  165  manually supplying various kinds of printing medium (P).  
         [0057]     The main body  170  comprises a frame (not shown), to which the image forming module  110  and the medium delivery unit  160  are fixed, a cover (not shown) covering the frame, a power supply section (not shown) for supplying a required power and a controlling section (not shown) for controlling the image forming module  110  and the medium delivery unit  160 . In addition, a fixing member  175  for fixing the image, which is transferred to the printing medium (P), to the printing medium (P) is further mounted between the delivery unit  163  and the paper discharge unit  164 .  
         [0058]     Hereinafter, an assembly process of the color image forming apparatus having the above-described structure according to a first embodiment of the invention will be explained regarding the invention.  
         [0059]     Firstly, the image forming module  110  is completed by assembling the plurality of image forming units  120 , the exposure units  130  and the first transfer unit  140  to the image forming frame  150 . At this time, the image forming units  120  are inserted into the mounting recesses  151  formed in the image forming frame  150 , so that the positioning members  152  are matched and the exposure units  130  are matched with the second reference pins  153  and the exposure unit-supporting section  154 , thereby making the distances, the parallelism and the straightness of the elements  120 ,  130 ,  140  be within the prescribed dimensional ranges.  
         [0060]     Next, the main body  170  is completed by assembling the medium delivery unit  160 , the fixing member  175 , the power supply section and the controlling section to the main frame.  
         [0061]     Finally, the assembly of the color image forming apparatus  100  is completed by mounting the guide rail  111  of the image forming module  110  to one side of the main body  170  along the frame guide  171 . At this time, the distance between the transfer belt  141  of the image forming module  110  and the second transfer roller  169  is made to be within the prescribed range just by mounting the image forming module  110  along the frame guide  171  and thus fitting the reference protrusion  157  of the image forming module  110  into the reference groove of the main body  170 .  
         [0062]     When a printing command is received from a computer (not shown) connected to the controlling section, the controlling section operates the electrification unit of the image forming unit  120  to electrify the surface of the photosensitive drum  121 , and controls the exposure unit  130  to scan the laser beam according to the printing data, thereby forming an electrostatic latent image on the photosensitive drum  121 . The electrostatic latent image of the photosensitive drum  121  is developed into a specific color of an image by the developing unit of the image forming unit  120 . The images formed on each of the photosensitive drums  121  of the plurality of image forming units  120  are transferred and superimposed to the transfer belt  141 , thereby forming a color image. The color image formed on the transfer belt  141  is transferred to the printing medium (P) which is picked-up from the paper supply cassette  161  and then delivered between the second transfer roller  169  and the transfer belt  141  according to the signal of the controlling section. The printing medium (P) having the image transferred thereto is fixed with the image while passing through the fixing member  175  by the delivery unit  163 , and then discharged into the exterior of the main body  170  via the paper discharge unit  164 . The operations like this are repeated according to the printing command of the computer.  
         [0063]     As described above, the image forming frame  150  comprising the image forming module  110  is structured to be mounted and detached to and from the side of the image forming frame  150 . However, the image forming unit  120  may be structured to be mounted and dismounted to and from a front or rear surface of the image forming frame  150 . In this case, likewise the above example, the positioning members is also provided to the image forming unit and the image forming frame so that positions of the image forming units can be determined just by mounting the image forming units to the image forming frame. Accordingly, a detailed description thereof based on the drawings is omitted for clarity and conciseness.  
         [0064]      FIG. 9  is a conceptual view showing a color image forming apparatus according to a second embodiment of the invention. Referring to  FIG. 9 , a color image forming apparatus  200  according to a second embodiment of the invention comprises an image forming module  210 , a medium delivery unit  260  and a main body  270  in which the image forming module  210  and the medium delivery unit  260 , are received.  
         [0065]     The image forming module  210  comprises a photosensitive member  220 , a plurality of electrification units  225 , a plurality of developing units  240 , a plurality of exposure units  230  and an image forming frame  250  and an example thereof is shown in  FIG. 10 . The image forming frame  250  is provided with mounting recesses to which the plurality of developing units  240 , the exposure units  230 , the electrification units  225  and the photosensitive member  220  can be mounted, respectively. A front wall of the image forming frame  250  is formed with a plurality of mounting recesses  251  in two rows to insertably mount and dismount the developing units  240 . At this time, the mounting recess  251  is provided with a guide  251   a  for guiding the mounting and dismounting of the developing units  240 , and the developing unit  240  and the image forming frame  250  are provided with positioning members  252  so that the developing unit  240  is always fixed at a predetermined location regarding the image forming frame  250  when the developing unit  240  is mounted. The positioning members  252  is structured so that a distance between axes of developing rollers  241  of the developing units  240  respectively mounted to the mounting recesses  251  is equal and straightness and parallelism thereof are constantly maintained. The positioning members  252  comprises a first reference hole (not shown) formed in a sidewall opposed to a sidewall  250   a  of the image forming frame  250  in which the mounting recesses  251  are formed and a first reference pin (not shown) formed in a side of the corresponding developing unit  240 . In addition, the sidewall  250   a  of the image forming frame  250 , in which the mounting recesses  251  are formed, is also provided with the first reference pin  252   a , and a side of the corresponding developing unit  240  is formed with the first reference hole  252   b  into which the first reference pin  252   a  is inserted. Accordingly, the plurality of developing units  240  are fixed at predetermined locations regarding the image forming frame  250  simply by inserting the developing units  240  into the mounting recesses  251 . At this time, the first reference pin  252   a  and the first reference hole  252   b  may be selectively formed in any one of the developing unit  240  and the image forming frame  250  corresponding to each other, as needed, in addition to the above-mentioned locations. As for the positioning members  252 , various suitable arrangements and constructions may be used in addition to the reference pin/reference hole ways. Additionally, a partition wall may be formed between the mounting recesses  251  vertically located of the image forming frame  250 . Thereby, it is possible to minimize an interference occurring between the adjacent developing units  240 . As the image forming units  240 , four units for yellow, magenta, cyan and black are typically used to form a color image. The developing unit  240  develops the electrostatic latent image formed on a surface of the photosensitive member  220 , and comprises a developing roller  241  supplying toner to the photosensitive member  220 , a supply roller and a developing frame storing the toner and supporting the developing roller  241  and the supply roller. In addition, the developing unit  240  further comprises a regulation member constantly regulating a height of the toner delivered to the developing roller  241 .  
         [0066]     The photosensitive member  220  is assembled to the image forming frame  250  so that it is located between the mounting recesses  251  formed in two rows. The photosensitive member  220  uses a photosensitive belt to correspond to the developing units  240  which are located two by two at both sides. Two electrification units  225  are provided with the image forming frame  250  so that each of the electrification units  225  can electrify one and the other surfaces of the photosensitive belt  220 , respectively. At this time, it is possible to maintain the distances between the photosensitive belt  220  and the plurality of developing units  240  constant by managing a dimension between an assembly position of a roller driving the photosensitive belt  220  and the positioning members  252  of the developing unit  240 . In addition, a mounting position of the electrification unit  225  is determined on the basis of the photosensitive belt  220 .  
         [0067]     An exposure unit  230  such as a laser scanning unit is mounted one by one to a side of the image forming units  240  located in two rows to scan the laser beam to the photosensitive belt  220  as the photosensitive member, and the position thereof is determined by two second reference pins  253  provided to a upper ceiling  250   c  connecting front and rear walls of the image forming frame  250  and two reference pins  253  provided to the sidewall  250   b  of the image forming frame and two reference pins  253  provided to the sidewall  250   b  of the image forming frame  250  so that scanning lines of the laser beam emitted from the exposure unit  230  are parallel to each other on the photosensitive belt  220 . At this time, second reference holes  231 , corresponding to the second reference pins  253 , are provided to a corresponding surface of the exposure unit  230 . Accordingly, it can be structured such that the laser beam emitted from the exposure unit  230  can be scanned to be parallel with each other on the photosensitive belt  220  just by fitting the second reference hole  231  of the exposure unit  230  with the second reference pin  253 . The positioning members  231  and  253  of the exposure unit  230  shown in  FIG. 10  are provided as an example, any one of the suitable positioning members can be used to fix the exposure unit  230  to a predetermined position of the image forming unit  250 .  
         [0068]     The image forming frame  250  is formed in an integral body so that the elements can be assembled to a predetermined location, and formed with the mounting recesses  251  to which the developing units  240  can be mounted and dismounted. The photosensitive member  220  is assembled between the mounting recesses  251  in two rows. In addition, the image forming frame  250  is structured so that it can be mounted and detached by sliding or reciprocally linear-moving the image forming frame  250  relative to the main body  270 , as with the first embodiment. Accordingly, it is assembled so that a distance between the photosensitive member  220  of the image forming module  210  and the transfer roller  269  assembled to the main body  270  is made to be within a prescribed dimensional range just by mounting the image forming module  210  having the photosensitive member  220 , the plurality of developing units  240 , the exposure units  230 , and the electrification units  225 , which are assembled to the image forming frame  250 , to the main body  270 .  
         [0069]     Since the color image is applied directly to the printing medium (P) from the photosensitive member  220 , the medium delivery unit  260  and the main body  270  are not provided with the transfer unit  140  (refer to  FIG. 3 ). However, except for these, the elements thereof are nearly same as the first embodiment, so a detailed explanation thereof is omitted for clarity and conciseness.  
         [0070]      FIG. 11  is a conceptual view showing a color image forming apparatus according to a third exemplary embodiment of the invention. Referring to  FIG. 11 , a color image forming apparatus  300  according to a third embodiment of the invention comprises an image forming module  310 , a medium delivery unit  360  and a main body  370  in which the image forming module  310  and the medium delivery unit  360  are received.  
         [0071]     The image forming module  310  comprises a plurality of image forming units  320  vertically positioned in a row, a plurality of exposure units  330  positioned in a row at a side of the image forming unit  320 , a transfer unit  340  comprising a delivery belt  341  and transfer rollers  342 , and an image forming frame  350 . The delivery belt  341  of the transfer unit  340  is mounted to a front of photosensitive drums  321  of the image forming units  320  positioned in a row, similarly to the transfer belt  141  of the color image forming apparatus  100  according to the first exemplary embodiment of the invention, and the transfer rollers  342  are mounted inside the delivery belt  341  to correspond to the photosensitive drums  321 . In order to constantly maintain a positional relationship between the elements, even when the image forming units  320 , the exposure units  330  and the transfer unit  340  are assembled, mounted or detached, positioning members are provided in accordance with the image forming module  110  of the color image forming apparatus  100  according to the first exemplary embodiment of the invention. Accordingly, a detailed description thereof is omitted for clarity and conciseness.  
         [0072]     The medium delivery unit  360  delivers the printing medium (P) to the image forming module  310  and discharges the printing medium (P) having the image transferred in the image forming module  310  into the exterior of the main body  370 . The medium delivery unit  360  comprises a paper supply cassette  361  in which the printing medium (P) is loaded, a pick-up roller  362  picking up the loaded printing medium (P) one by one, a pick-up unit  363  moving the picked-up printing medium (P) between the transfer belt  341  and the photosensitive drum  321  of the image forming unit  320  and a paper discharge unit  364  discharging the printing medium (P) having the image transferred thereto into the exterior of the main body  370 . The medium delivery unit  360  may be further provided with a MP unit  365  for manually supplying various kinds of printing medium (P).  
         [0073]     The main body  370  comprises a frame (not shown) to which the image forming module  310  and the medium delivery unit  360  are fixed, a cover (not shown) covering the frame, a power supply section (not shown) supplying a required power and a controlling section (not shown) controlling the image forming module  310  and the medium delivery unit  360  to perform a printing operation. In addition, a fixing member  375  is further provided between the image forming module  310  and the paper discharge unit  364  to fix the image transferred to the printing medium (P).  
         [0074]     Hereinafter, an operation of the color image forming apparatus according to the third exemplary embodiment of the invention will be specifically described.  
         [0075]     When a printing command and printing data are received from a computer (not shown) connected to the controlling section, the controlling section operates the electrification unit of the image forming unit  320  to electrify the surface of the photosensitive drum  321 , and controls the exposure unit  330  to scan the laser beam according to the printing data, thereby forming an electrostatic latent image on the photosensitive drum  321 . The electrostatic latent images formed on the photosensitive drums  321  are respectively developed into specific colors of images by the developing units of the image forming units  320 . The images developed on each of the photosensitive drums  321  of the plurality of image forming units  320  are transferred/superimposed to the printing medium (P), which is delivered between the delivery belt  341  and the photosensitive drums  321 , by the transfer rollers  342 , thereby forming a color image. The printing medium (P) having the image transferred thereto is fixed with the image while passing through the fixing member  375  and then discharged into the exterior of the main body  170  via the paper discharge unit  364 . The operations like this are repeated according to the printing command of the computer.  
         [0076]     While the invention has been shown and described with reference to certain embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.