Patent Publication Number: US-6907209-B2

Title: Tandem-type image forming apparatus with a transfer belt

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a tandem-type image forming apparatus comprising image forming stations for respective colors which are arranged along a transfer belt and each of which is composed of an image carrier, and a charging means and a developing means which are arranged around the image carrier. The image forming apparatus forms a color image by passing the transfer belt through the respective stations. 
     As an example of such tandem-type image forming apparatuses as mentioned above, Japanese Patent Unexamined Publication No. H11-95520 describes an image forming apparatus comprising a transfer belt (paper carrying belt) which is laid around plural rollers with some tension to extend obliquely, plural image forming stations which are arranged in the tensioning direction of the transfer belt, and plural laser scanning means which are arranged to be partially superposed on each other, thus achieving the reduction in size of the entire apparatus. Disclosed in the publication is a technology for utilizing a space obliquely below the tensioned transfer belt as a space for mounting a sheet reversing path for dual-side printing or a sheet cassette or a space for allowing the operation for removing a jammed recording medium if jam occurs. 
     In addition, Japanese Patent Unexamined Publication No. 2001-249522 describes an image forming apparatus comprising a transfer belt (intermediate transfer belt) which is horizontally laid around plural rollers with some tension, plural image forming stations which are arranged to face a lower tensioned surface of the transfer belt, and writing heads of array type which are arranged below the respective image forming stations. Disclosed in the publication is a technology of shortening a recording medium carrying path by the aforementioned arrangement, whereby the area to be required to open for removing a jammed recording medium if jam occurs can be reduced, thus achieving the reduction in entire size of the image forming apparatus. 
     The reduction in entire size of the image forming apparatus is achieved by arranging the transfer belt obliquely or by employing the writing heads of array type in the aforementioned conventional technology. However, the arrangement of electrical circuit for operating an image forming apparatus is an important issue for further reducing the size of the apparatus. 
     Particularly in an tandem-type image forming apparatus just like the conventional apparatus as mentioned above, high-voltage power supply substrates for charging means, developing means, transfer means, and the like, a power supply substrate for mechanically operating image forming process, and an interface substrate for separating image data from a host computer into data for respective colors and conducting image processing should be large in order to operate the plural image forming stations at once. Therefore, as these substrates are disposed on an upper or lower portion of a side face or a back face of the image forming apparatus body, the miniaturization of apparatus turns out to be impossible. 
     Further, the aforementioned conventional methods have problems as mentioned below on the maintenance of apparatus. In the method of Japanese Patent Unexamined Publication No. H11-95520, if jam of a recording medium occurs, it is required to detach the image forming stations in the axial direction of image carriers from the apparatus before removing a jammed recording medium. Accordingly, not only it is troublesome to remove the jammed recording medium, but also there is a possibility that surfaces of the image carriers and the transfer belt may be scratched, causing image defects. The transfer belt can be evacuated by pivotal movement before removing a jammed recording medium. In this case, however, a space where the transfer belt is evacuated is necessary. This also avoids the miniaturization of apparatus. Openings for allowing the image forming stations to be inserted or removed in the axial direction of the image carriers and for allowing the removal of a jammed recording medium are required to be formed in a frame for properly positioning and supporting the respective components, thus reducing the rigidity of the frame. This may deteriorate the quality of obtained image. 
     On the other hand, the method of Japanese Patent Unexamined Publication No. 2001-249522 has a problem that the apparatus having an opening for replacing the image forming stations and an opening for removing a jammed recording medium which are separately provided along different directions and it is difficult for users to know which opening has to be handled by an operator for maintenance. In addition, since the transfer belt must be detached from the image forming apparatus body before replacing the image forming stations, an operator should prepare a space for temporally placing the transfer belt around the image forming apparatus before the replacement of the image forming stations. Therefore, there are problems that it is troublesome to replace the image forming stations and that the transfer belt may be scratched when temporally placed. 
     It is an object of the present invention to provide an image forming apparatus which can solve the aforementioned conventional problems and which is quite compact and is quite ease of maintenance. 
     SUMMERY OF THE INVENTION 
     To achieve the aforementioned object, the image forming apparatus of the present invention is an image forming apparatus comprising image forming stations for respective colors arranged along a transfer belt, each image forming station including an image carrier, a charging means and a developing means disposed around said image carrier, and is characterized in that said transfer belt is arranged obliquely, and an electrical component box in which power sources and control circuits are housed is arranged below or above said transfer belt. 
     The image forming apparatus is further characterized in that array-type writing heads are arranged around the image carriers, respectively so that a latent image is formed onto each image carrier by each array-type writing head. 
     The image forming apparatus is further characterized by further comprising a housing body in which said image forming stations and said transfer belt are arranged and a frame which is attached to said housing body such that the frame is pivotally movable, wherein a transfer belt unit and an image carrier unit, or a transfer belt unit, an image carrier unit, and the developing means are attached to said frame. 
     In addition, the image forming apparatus of the present invention is an image forming apparatus comprising image forming stations for respective colors arranged along a transfer belt, each image forming station including an image carrier, a charging means, an array-type writing head as an image writing means, and a developing means disposed around said image carrier, and is characterized in that said transfer belt is arranged obliquely, and a driving circuit for said array-type writing heads is arranged below or above said transfer belt. 
     The image forming apparatus is further characterized by further comprising a housing body in which said image forming stations and said transfer belt are arranged and a frame which is attached to said housing body such that the frame is pivotally movable, wherein a transfer belt unit and an image carrier unit, or a transfer belt unit, an image carrier unit, and the developing means are attached to said frame. 
     The image forming apparatus is further characterized in that said array-type writing head and the driving circuit are detachably connected to each other via a connector. 
     The image forming apparatus is further characterized in that pivot shafts of said frame are provided at both sides of the housing body. 
     The image forming apparatus is further characterized in that pivot shafts of said frame are provided at a lateral side of the housing body. 
     The image forming apparatus is further characterized in that said transfer belt is a paper carrying belt. 
     The image forming apparatus is further characterized in that said transfer belt is an intermediate transfer belt. 
     The image forming apparatus is further characterized in that the image carriers of the respective image forming stations are arranged in contact with a belt face, of which traveling direction is downward, of said intermediate transfer belt. 
     According to the present invention, in an image forming apparatus comprising image forming stations for respective colors arranged along a transfer belt, each image forming station including an image carrier, a charging means and a developing means disposed around said image carrier, the transfer belt is arranged obliquely, and an electrical component box in which power sources and control circuits are housed is arranged below or above the transfer belt, thereby providing an image forming apparatus which is quite compact and is quite ease of maintenance. 
     According to the present invention, in an image forming apparatus comprising image forming stations for respective colors arranged along a transfer belt, each image forming station including an image carrier, a charging means and a developing means disposed around said image carrier, the transfer belt is arranged obliquely, and a driving circuit for said array-type writing heads is arranged below or above said transfer belt, thereby providing an image forming apparatus which is quite compact and is quite ease of maintenance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view showing the entire structure of an embodiment of an image forming apparatus of the present invention; 
         FIG. 2  is a front view taken from the right side of  FIG. 1 ; 
         FIG. 3  is an enlarged view of a transfer belt unit and an image forming unit shown in  FIG. 1 ; 
         FIG. 4  is a perspective view of a transfer belt and an image carrier shown in  FIG. 3 ; 
         FIG. 5  is an illustration for explaining the replacement of expendable supplies in the embodiment of  FIG. 1 ; 
         FIG. 6  is an illustration for explaining the replacement of the expendable supplies and showing the state following the state shown in  FIG. 5 ; 
         FIG. 7  is an illustration for explaining the replacement of the expendable supplies and showing the state following the state shown in  FIG. 6 ; 
         FIG. 8  is an illustration for explaining the replacement of the expendable supplies and showing the state following the state shown in  FIG. 7 ; 
         FIG. 9  is an illustration showing a variation example of the embodiment shown in  FIG. 8 ; 
         FIG. 10  is an illustration for explaining the replacement of the expendable supplies and showing the state following the state shown in  FIG. 9 ; 
         FIG. 11  is a schematic sectional view showing the entire structure of another embodiment of the image forming apparatus of the present invention; 
         FIG. 12  is a schematic illustration showing another embodiment of the image forming apparatus of the present invention; 
         FIG. 13  is a schematic illustration showing another embodiment of the image forming apparatus of the present invention; 
         FIG. 14  is a schematic illustration showing another embodiment of the image forming apparatus of the present invention; and 
         FIG. 15  is a schematic illustration showing another embodiment of the image forming apparatus of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to the attached drawings. FIG.  1  is a schematic sectional view showing the entire structure of an embodiment of an image forming apparatus of the present invention,  FIG. 2  is a front view taken from the right side of  FIG. 1 ,  FIG. 3  is an enlarged view of a transfer belt unit and an image forming unit shown in  FIG. 1 , and  FIG. 4  is a perspective view of a transfer belt and an image carrier shown in FIG.  3 . In the following description, the same components used over the drawings will be marked with the same reference numerals, thereby sometimes omitting the explanation about such components. This embodiment is of a type employing an intermediate transfer belt as a transfer belt. 
     In  FIG. 1 , the image forming apparatus  1  of this embodiment comprises a housing body  2 , a first door member  3  which is disposed on the front of the housing body  2  such that the first door member is openable and closable, and a second door member (also functioning as an outfeed tray)  4  which is disposed on the top of the housing body  2  such that the second door member is openable and closable. The first door member  3  is provided with an openable lid  3 ′ which is disposed such that the openable lid  3 ′ is openable and closable relative to the front of the housing body  2 . The openable lid  3 ′ can be opened and closed in conjunction with or independently from the first door member  3 . 
     Disposed in the housing body  2  are an electrical component box  5  in which substrates for power source circuits and substrates for control circuits are housed, an image forming unit  6 , a blower fan  7 , a transfer belt unit  9 , and a paper feeding unit  10 . Disposed in the first door member  3  are a secondary transfer unit  11 , a fixing unit  12 , and a recording medium carrying means  13 . Expendable supplies in the image forming units  6  and the paper feeding unit  10  are detachable relative to the body. In this case, by detaching the expandable supplies together with the transfer belt unit  9 , the maintenance and replacement is allowed. 
     As shown in  FIG. 2 , the housing body  2  has two pairs of stays  2   a  which are disposed on both sides of a lower front surface of the housing body  2 , respectively, to project from the front surface. The first door member  3  has pairs of pivotal portions  3   a  formed on both sides of a lower portion thereof. By inserting shafts  3   b  through the respective holes of the stays  2   a  and the pivotal portions  3   a , the first door member  3  is attached to the housing body  2  such that the first door member  3  is openable and closable relative to the housing body  2 . The first door member  3  is provided at an upper front surface thereof with a control panel  3   c . The housing body  2  is provided with an opening  3   d  for insertion of the sheet cassette  35  below the first door member  3 . In this embodiment, therefore, the respective units can be attached to and detached from the apparatus only by access from the front of the apparatus. This allows the apparatus to be placed in a narrow space. 
     In  FIG. 1 , the transfer belt unit  9  comprises a driving roller  14  which is disposed in a lower portion of the housing body  2  and is driven by a driving means (not shown) to rotate, a driven roller  15  which is disposed diagonally above the driving roller  14 , an intermediate transfer belt  16  which is laid around the two rollers  14 ,  15  with some tension and is driven to circulate in a direction indicated by an arrow, and a cleaning means  17  which abuts on the surface of the intermediate transfer belt  16 . The driven roller  15  and the intermediate transfer belt  16  are arranged obliquely to the upper left of the driving roller  14  in the drawings. Accordingly, during the operation of the intermediate transfer belt  16 , a belt face  16   a  of which traveling direction is downward takes a lower side. In this embodiment, the belt face  16   a  is a tension side (side tensioned by the driving roller  14 ) at the time of driving the intermediate transfer belt  16 . 
     The driving roller  14  and the driven roller  15  are rotatably supported by a support frame  9   a  which has a pivotal portion  9   b  formed at a lower end thereof. The pivotal portion  9   b  is fitted to a pivot shaft  2   b  disposed in the housing body  2 , whereby the support frame  9   a  is attached to the housing body  2  such that it is pivotally movable. In addition, the support frame  9   a  has a lock lever  9   c  which is rotatably disposed at an upper end thereof. The lock lever  9   c  can latch a latch pin  2   c  disposed on the housing body  2 . 
     The driving roller  14  also functions as a back-up roller for a secondary transfer roller  19  composing the secondary transfer unit  11 . As shown in  FIG. 3 , formed on the peripheral surface of the driving roller  14  is a rubber layer  14   a  which is about 3 mm in thickness and 10 5 Ω·cm or less in volume resistivity. The driving roller  14  has a metallic shaft which is grounded so as to function as a conductive path for secondary transfer bias supplied through the secondary transfer roller  19 . Since the driving roller  14  is provided with the rubber layer  14   a  having high friction and shock absorption, impact generated when a recording medium is fed into a secondary transfer section is hardly transmitted to the intermediate transfer belt  16 , thereby preventing the deterioration of image quality. 
     In this embodiment, the diameter of the driving roller  14  is set to be smaller than the diameter of the driven roller  15 . This facilitates the separation of a recording medium after secondary transfer because of the elastic force of the recording medium itself. The driven roller  15  also functions as a back-up roller for the cleaning means  17 . 
     The cleaning means  17  is located at the belt face  16   a  side, of which traveling direction is downward. As shown in  FIG. 3 , the cleaning means  17  comprises a cleaning blade  17   a  for removing toner remaining on the surface of the intermediate transfer belt  16  after the secondary transfer, and a toner carrying member  17   b  for carrying collected toner. The cleaning blade  17   a  is in contact with the intermediate transfer belt  16  at a position where is wrapped around the driven roller  15 . 
     On the back of the belt surface  16   a , of which traveling direction is downward, of the intermediate transfer belt  16 , primary transfer members  21  composed of leaf spring electrodes are disposed. The primary transfer members  21  are pressed into contact with the back of the intermediate transfer belt  16  by their elastic force at locations corresponding to image carriers  20  of respective image forming stations Y, M, C, and K, described later. A transfer bias is applied to each primary transfer member  21 . 
     In proximity to the driven roller  15 , a test pattern sensor  18  is attached to the support frame  9   a  of the transfer belt unit  9 . The test pattern sensor  18  is a sensor for positioning of toner images of respective colors on the intermediate transfer belt  16  and for compensating color registration error of images for the respective colors by detecting image density. It is preferable that a mechanism composed of screws or levers, not shown, for adjusting the position of the sensor relative to the intermediate transfer belt and is provided at a portion holding the test pattern sensor  18 . 
     Normally, the test pattern formed on the intermediate transfer belt  16  is removed by the cleaning means  17  of the intermediate transfer belt  16  without being transferred to a recording medium. A method of moving the secondary transfer roller  19  apart from the intermediate transfer belt  16  during the test pattern printing is employed in order to avoid adhesion of toner to the surface of the secondary transfer roller  19 . The movement of the secondary transfer roller  19  can avoid the adhesion of toner when the secondary transfer roller  19  is spaced apart from the intermediate transfer belt  16  only during the test pattern passes a nip portion between the second transfer roller  19  and the intermediate transfer belt  16 . However, as the image forming speed (the belt traveling speed) increases, the secondary transfer roller  19  should be moved apart from and returned to the intermediate transfer belt  16  rapidly and frequently, thus producing severe impact on the intermediate transfer belt  16  with noise and varying the tension of the intermediate transfer belt  16 . Accordingly, there is a possibility of creating a slight slip between the intermediate transfer belt  16  and the rollers supporting the intermediate transfer belt  16 . 
     The slip causes a problem that color registration error occurs even when the timing compensation for image formation of the respective image forming stations is conducted by, for example, detecting resist marks. The formation of resist marks or patch images may be repeated until the positioning relation among respective color images and the density of toner image reaches a desired range, so the second transfer roller  19  is repeatedly moved apart from or returned to abut on the intermediate transfer belt  16 , leading to failure of a shifting mechanism of moving the secondary transfer roller  19  and thus damaging the reliability of the image forming apparatus. 
     To avoid the aforementioned problems, in this embodiment, the secondary transfer roller  19  is moved apart from the intermediate transfer belt  16  before starting the formation of a series of test patterns and the secondary transfer roller  19  is returned to abut on the intermediate transfer belt  16  at such a slow speed not to create a slight slip between the intermediate transfer belt  16  and the rollers supporting and driving the belt after the series of test patterns repeatedly formed pass a position where the secondary transfer roller  19  faces the intermediate transfer roller  16 . 
     The image forming unit  6  comprises the image forming stations Y (for yellow), M (for magenta), C (for cyan), and K (for black) for forming multi-color images (in this embodiment, four-color images). As clearly shown in  FIG. 3 , each image forming station Y, M, C, K has an image carrier  20  composed of a photosensitive drum, a charging means  22 , image writing means  23 , and developing means  24  which are arranged around the image carrier  20 . Only reference numerals for the charging means  22 , the image writing means  23 , and the developing means  24  of the image forming station Y are indicated on the drawings and the indication of the reference numerals for the other image forming stations is omitted because the image forming stations have the same structure. It should be understood that the image forming stations Y, M, C, K may be arranged in any order. 
     The image forming stations Y, M, C, K are disposed such that the respective image carriers  20  are in contact with the belt face  16   a , of which traveling direction is downward, of the intermediate transfer belt  16 . As a result of this, the image forming stations Y, M, C, K are arranged in an obliquely leftward direction relative to the driving roller  14  in the drawings. Each image carrier  20  is driven to rotate in the traveling direction of the intermediate transfer belt  16  as indicated by arrows. 
     The charging means  22  is a conductive brush roller which is connected to a high-voltage source and rotates at a peripheral speed about twice to triple the speed of the image carrier  20  as a photoreceptor in opposite direction with being in contact with the surface of the image carrier  20  so as to uniformly charge the surface of the image carrier  20 . The conductive brush roller comprises a well-conductive shaft member (for example, metal shaft) of 5-8 mm in diameter and a fabric wound around the shaft member in spiral form, wherein the fabric is filled with semi-conductive fibers of 2-6 deniers in thickness and of 10 7-10   9  Ω in electrical resistance of yarns to have 150,000 to 430,000 piles per a square inch. The brush roller is rotatably held such a manner as to have contact depth of 0.3-0.5 mm relative to the image carrier  20 . 
     In case of using, a photoreceptor of a type to be negatively charged as the image carrier  20 , preferable voltage to be applied to the brash roller is a voltage composed of a direct current component in a range from −300 V to −500 V and an alternating current component in a range from 800 V to 1300 V of a frequency on the order of 1 kHz is superimposed on the direct current component. In case of an image forming apparatus of a cleaner-less type just like this embodiment, it is preferable that a bias of a polarity opposite to the polarity of charged toner is applied to the brush roller during non image forming, whereby residual toner adhering to the brush roller is emitted to the image carrier  20 , is transferred to the intermediate transfer belt  16  at the primary transfer portion, and is collected by the cleaning means  17  of the intermediate transfer belt  16 . 
     Since the charging means  22  enables charging of the surface of the image carrier with extremely small amount of electric current, the charging means never pollute inside and outside of the apparatus with large amount of ozone like in case of using a corona charging method. In addition, since the charging means  22  softly touch the image carrier  20 , adhesion of toner remaining after transfer onto a charging roller and an image carrier which easily occurs in case of using a roller charging method hardly occurs, thereby ensuring the stability of the image quality and the reliability of the apparatus. 
     The image writing means  23  employs an array-type writing head in which elements such as liquid crystal shutters having a light-emitting diode and a backlight are aligned in line(s) in the axial direction of the image carrier  20 . The array-type writing head is more compact than a laser scanning optical system because of its short optical path length so that the array-type writing head can be arranged in proximity to the image carrier  20 , thereby miniaturizing the entire apparatus. In this embodiment, the image carrier  20 , the charging means  22 , and the image writing means  23  of each image forming station Y, M, C, K are united together into an image carrier unit  25  (FIG.  3 ), thereby keeping the position of the array-type writing head. When the image carrier unit  25  is replaced, the array-type writing head is also replaced together. After that, the light volume adjustment and positioning are conducted relative to a new image carrier unit and then is put into service again. 
     Then, details of the developing means  24  will be described by taking the image forming station K of  FIG. 3  as an example. In this embodiment, since the image forming stations Y, M, C, K are obliquely arranged and the image carriers  20  are disposed to be in contact with the belt face  16   a , of which traveling direction is downward, of the intermediate transfer belt  16 , toner storage containers  26  are arranged obliquely downward to the lower left of the image carriers  20 . For this, special structure is employed in the developing means  24 . 
     That is, the developing means  24  each comprises the toner storage container  26  storing toner (indicating by hatching), a toner storage area  27  formed in the toner storage container  26 , a toner agitating member  29  disposed inside the toner storage area  27 , a partition  30  defined in an upper portion of the toner storage area  27 , a toner supply roller  31  disposed above the partition  30 , a blade  32  attached to the partition  30  to abut the toner supply roller  31 , the development roller  33  arranged to abut both the toner supply roller  31  and the image carrier  20 , and a regulating blade  34  arranged to abut the development roller  33 . 
     The image carrier  20  is rotated in the traveling direction of the intermediate transfer belt  16 . The development roller  33  and the supply roller  31  are rotated in a direction opposite to the rotational direction of the image carrier  20  as shown by arrows. On the other hand, the agitating member  29  is rotated in a direction opposite to the rotational direction of the supply roller  31 . Toner agitated and scooped up by the agitating member  29  in the toner storage area  27  is supplied to the toner supply roller  31  along the upper surface of the partition  30 . Friction is caused between the toner and the flexible blade  32  so that mechanical adhesive force and adhesive force by triboelectric charging are created relative to the rough surface of the supply roller  31 . By these adhesive forces, the toner is supplied to the surface of the development roller  33 . The toner supplied to the development roller  33  is regulated into a coating layer having a predetermined thickness by the regulating blade  34 . The toner layer as a thin layer is carried to the image carrier  20  so as to develop a latent image on the image carrier  20  at and near a nip portion which is a contact portion between the development roller  33  and the image carrier  20 . 
     In this embodiment, the development roller  33  disposed facing the image carrier  20 , the toner supply roller  31 , and the contact portion of the regulating blade  34  relative to the development roller  33  are not submerged in the toner in the toner storage area  27 . This arrangement can prevent the contact pressure of the regulating blade  34  relative to the development roller  33  from being varied due to the decrease of the stored toner. In addition, since excess toner scraped from the development roller  33  by the regulating blade  34  spills onto the toner storage area  27 , thereby preventing filming of the development roller  33 . 
     The contact portion between the development roller  33  and the regulating blade  34  is positioned below the contact portion between the supply roller  31  and the development roller  33 . There is a passage for returning excess toner, which was supplied to the development roller  33  by the supply roller  31  but not transmitted to the development roller  33 , and excess toner, which was removed from the development roller  33  by the regulating operation of the regulating blade  34 , to the toner storage area  27  at the lower portion of the developing means. The toner returned to the toner storage area  27  is agitated with toner in the toner storage area  27  by the agitating member  29 , and is supplied to a toner inlet near the supply roller  31  again. Therefore, the excess toner is let down to the lower portion without clogging the friction portion between the supply roller  31  and the development roller  33  and the contact portion between the development roller  33  and the regulating blade  34  and is then agitated with toner in the toner storage area  27 , whereby the toner in the developing means deteriorates slowly so that portentous changes in image quality just after the replacement of the developing means is prevented. 
     Further details will be described by taking the image forming station M of  FIG. 3  as an example. The developing means  24  has a development roller aperture  33   a  disposed adjacent to the development roller  33 . The image writing means  23  has an upward opening  23   a  which opens upwardly to the image carrier  20 . If the upward opening  23   a  of the image writing means  23  is positioned below the development roller aperture  33   a , toner spills from the development roller aperture  33   a  because of the gravity and thus enters into the image writing means  23  through the upward opening  23   a  so as to undesirably stain the image writing means  23 . 
     In this embodiment, the upward opening  23   a  of the image writing means  23  is offset toward the intermediate transfer belt  16  from the development roller aperture  33   a  of the developing means  24  such that the upward opening  23   a  does not overlap relative to the development roller aperture  33   a . This can solve the possible problem that toner spills from the development roller aperture  33   a  because of the gravity and thus enters into the image writing means  23  through the upward opening  23   a  so as to undesirably stain the image writing means  23 . 
     Returning to  FIG. 1 , the sheet supply unit  10  comprises a sheet cassette  35  in which a pile of recording media P are held, and a pick-up roller  36  for feeding the recording media P from the sheet cassette  35  one by one. 
     Arranged inside the first door member  3  are a pair of resist rollers  37  for regulating the feeding of a receiving medium P to the secondary transfer portion at the right time, a secondary transfer unit  11  as a secondary transfer means abutting on and pressed against the driving roller  14  and the intermediate transfer belt  16 , a fixing unit  12 , the recording medium carrying means  13 , a pair of outfeed rollers  39 , and a dual-side printing passage  40 . 
     The secondary transfer unit  11  comprises a pivot lever  42  pivotally supported to a fixed shaft  41 , the secondary transfer roller  19  rotatably mounted to an end of the pivot lever  42 , and a spring  43  disposed between the other end of the pivot lever  42  and the first door member  3 . Normally, the secondary transfer roller  19  moves in a direction of arrow by the biasing force of the spring  43  and thus is pressed against the intermediate transfer belt  16  and the driving roller  14 . An eccentric cam  44  is arranged on the spring side of the pivot lever  42 . The pivot lever  42 , the spring  43 , and the eccentric cam  44  cooperate together to compose the shifting means for the secondary transfer roller  19 . By the rotation of the eccentric cam  44 , the pivot lever  42  pivots against the biasing force of the spring  43  so that the secondary transfer roller  19  is moved away from the intermediate transfer belt  16 . 
     The fixing unit  12  comprises a fuser roller  45  which has a built-in heating element such as a halogen heater and which is freely rotatable, a pressure roller  46  pressing the fuser roller  45 , a belt tensioning member  47  which is disposed to freely swing relative to the pressure roller  46 , and a heat resistant belt  49  which is lied around the pressure roller  45  and the belt tensioning member  47 . A color image secondarily transferred to a recording medium is fixed to the recording medium at the nip portion formed between the fuser roller  45  and the heat resistant belt  49  at a predetermined temperature. In this embodiment, the fixing unit  12  can be arranged in a space formed obliquely upward the intermediate transfer belt  16 , that is, a space formed on the opposite side of the image forming units  6  relative to the intermediate transfer belt  16 . This arrangement enables the reduction in heat transfer to the electrical component box  5 , the image forming unit  6 , and the intermediate transfer belt  16 , and lessens the frequency of taking the action for correcting color registration error. 
       FIG. 4  shows the transfer belt unit  9 , the intermediate transfer belt  16 , the cleaning means  17 , and the image carriers  20  of the respective image forming stations Y, M, C, and K as described with reference to FIG.  1 . The transfer belt unit  9  comprises a pair of right and left side frames  9   a . The driving roller  14  and the driven roller  15  are attached to the frames  9   a . The intermediate transfer belt  16  is laid around the driving roller  14  and the driven roller  15  with some tension. One end of the driving roller  14  is connected to a belt driving gear  51  and one end of each image carrier  20  is connected to image carrier driving gear  52 . Idle gears  53  are provided such that each idle gear  53  is meshed with each pair of the adjacent driving gears  52 . The driving motor  54  is located near the uppermost position of the belt face  16   a  of which traveling direction is downward. A transmission gear  55  which is a combination gear is meshed with a pinion gear  54   a  fixed to the rotational shaft of the driving motor  54  and meshed with the driving gear  52  of the uppermost image carrier  20 . Numeral  56  designates a toner collecting container into which waste toner removed by the cleaning means  17  is collected. The toner collecting container  56  is shaped to extend along the side of the frame  9   a , thereby also miniaturizing the entire apparatus. 
     In the above arrangement, the respective image carriers  20  are rotatably supported by a pair of frames (not shown) and are driven by the single driving motor  54  disposed near the upper portion of the intermediate transfer belt  16  because the rotation of the driving motor  54  are sequentially transmitted by the transmission gear  55 , the image carrier driving gears  52 , and the idle gears  53 . The rotation is further transmitted from the lowermost image carrier driving gear  52  to the driving roller  14  of the intermediate transfer belt  16  via the belt driving gear  51  so as to drive the driving roller  14 . The number of teeth of the idle gear  53  is set to be the same as the number of teeth of the belt driving gear  51  so as to synchronize their rotational periods to make one period of the driving roller  14  substantially equal to the interval of the primary transferring portion of each image forming means. Therefore, the rotational phases among the respective image carriers  20  and the gears can be set in the manufacturing process. Even after the image carriers  20  are replaced, there is a minimized possibility of occurrence of color registration error due to the periodic error among the respective image carriers  20 . 
     In this case, the feeding speed of the transfer belt  16  is set to be faster than the peripheral velocity of the image carriers  20  by approximately 1-3%, thereby preventing the slack of the transfer belt  16  and thus achieving the stable driving of the transfer belt. It can also prevents the occurrence of color registration error so as to improve the image quality and can avoid the necessity of providing the cleaning means because the transfer efficiency is improved by setting a velocity differential. 
     In the image forming apparatus of this embodiment, as shown in  FIG. 1 , the intermediate transfer belt  16  and the image forming stations Y, M, C, and K are obliquely arranged in the housing body  2  and the electrical component box  5  is disposed below the image forming stations Y, M, C, and K. The wirings (shown by two-dot chain line of  FIG. 1 ) from electrical circuits such as a power source circuit and a control circuit in the electrical component box  5  are connected to the primary transfer members  21 , the charging means  22 , the image writing means  23 , and the test pattern sensor  18  via a connector  57 . The wirings may be connected to the secondary transfer unit  11 , the fixing unit  12 , and the like in the first door member  3  via the connector  57  or by passing near the shafts  3   b  of the first door member  3 . 
     The electrical circuits are an interface circuit for converting image data from an outside host controller into data recordable in the image forming stations, a control circuit for controlling the image forming apparatus, high-voltage power supply for supplying high voltage to charging means, developing means, transfer means, and the like, low-voltage power supply for operating a driving motor, clutch, and a control circuit of the apparatus. 
     By arranging the electrical circuits below the image forming stations and above the sheet cassette  35 , the necessity of increasing the width, the depth, and the height of the entire apparatus because of the installation of the electrical circuits can be eliminated, thereby achieving the compact image forming apparatus. 
     In addition, by arranging the interface circuit and the control circuit as mentioned above and connecting the wirings from the positions near the respective connected portions of the circuit substrates to the image writing means  23  and the test pattern sensor  18  via the connector  57 , the length of the wirings to the image writing means  23  can be shortened, thereby reducing the wiring cost and also reducing noise produced during the transmission of image signals of high frequency such as several hundred MHz to the image writing means  23 . Moreover, by arranging the high-voltage power supply and the low-voltage power supply as mentioned above and connecting the wirings from the positions near the respective connected portions of the circuit substrates to the charging means and the transfer means via the connector  57 , the length of the wirings can be shortened, thereby not only reducing the wiring cost and but also preventing the waveform distortion of a bias composed of an alternating current voltage superimposed on a direct current voltage and preventing the power loss. 
     Further, there is no necessary to arrange a lot of wirings via the shafts of the door member  3  which are complex, thereby avoiding the possibility that wirings are nipped and thus broken during the rotation of the frame  9   a  on which the transfer belt and the image carriers are mounted for the maintenance of the apparatus. 
     The actions of the image forming apparatus as a whole will be summarized as follows: 
     (1) As a printing command (image forming signal) is inputted into the control unit in the electric component box  5  from a host computer (personal computer) (not shown) or the like, the image carriers  20  and the respective rollers of the developing means  24  of the respective image forming stations Y, M, C, K, and the intermediate transfer belt  16  are driven to rotate. 
     (2) The outer surfaces of the image carriers  20  are uniformly charged by the charging means  22 . 
     (3) In the respective image forming stations Y, M, C, K, the outer surfaces of the image carriers  20  are exposed to selective light corresponding to image information for respective colors by the image writing means  23 , thereby forming electrostatic latent images for the respective colors. 
     (4) The electrostatic latent images formed on the image carriers  20  are developed by the developing means  24  to form toner images. 
     (5) The primary transfer voltage of the polarity opposite to the polarity of the toner is applied to the primary transfer members  21  of the intermediate transfer belt  16 , thereby transferring the toner images formed on the image carriers  20  onto the intermediate transfer belt  16  one by one. According to the movement of the intermediate transfer belt  16 , the toner images are superposed on the intermediate transfer belt  16 . 
     (6) In synchronization with the movement of the intermediate transfer belt  16  on which primary images are transferred, a receiving medium P accommodated in the sheet cassette  35  is fed to the secondary transfer roller  19  through the pair of resist rollers  37 . 
     (7) The primary-transferred image meets with the receiving medium at the secondary transfer portion. A bias of the polarity opposite to the polarity of the primary transferred image is applied by the secondary transfer roller  19  which is pressed against the driving roller  14  for the intermediate transfer belt  16  by the pressing mechanism, whereby the primary-transferred image is secondarily transferred to the receiving medium fed in the synchronization manner. 
     (8) Residual toner after the secondary transfer is carried toward the driven roller  15  and is scraped by the cleaning means  17  disposed opposite to the roller  15  so as to refresh the intermediate transfer belt  16  to allow the above cycle to be repeated. 
     (9) The receiving medium passes through the fixing means  12 , whereby the toner image on the receiving medium is fixed. After that, the receiving medium is carried toward a predetermined position (toward the outfeed tray  4  in case of single-side printing, or toward the dual-side printing passage  40  in case of dual-side printing). 
     Now, with reference to FIG.  5  through  FIG. 10 , the removal of a jammed recording medium and the replacement of the expendable supplies will be described.  FIG. 5  is a state that only the openable lid  3 ′ of the first door member  3  is pivotally moved downwards to expose the dual-side printing passage  40 . In this state, it is possible to remove recording media jammed at the dual-side printing passage  40 . 
       FIG. 6  shows a state that the first door member  3  is pivotally moved downwards together with the openable lid  3 ′ to expose the fixing unit  12  and the secondary transfer unit  11 . In this state, it is possible to remove recording media jammed at the fixing unit  12  and the secondary transfer unit  11  and it is also possible to easily repair or replace the fixing unit  12  and the secondary transfer unit  11 . In addition, the second door member  4  is pivotally moved upwards to expose the transfer belt unit  9 ′, whereby it is possible to remove recording media jammed at the transfer belt unit  9 . 
       FIG. 7  shows a state that the lock lever  9   c  provided on the top of the frame  9   a  of the transfer belt unit  9  is pivotally moved to disengage itself from the latch pin  2   c  and the frame  9   a  is pivotally moved to the right about the pivot shaft  2   b  so as to expose the image forming unit  6 . During this, the connection of the wirings at the connector  57  is cancelled. 
     Then, as shown in  FIG. 8 , the respective developing means  24  composing the image forming unit  6 , the image carrier unit  25 , and the transfer belt unit  9  are detached from the frame  9   a , thereby allowing the repair and replacement of them. In this embodiment, since it is easy to ensure the positional relations among the developing means  24 , the image carriers  20 , and the intermediate transfer belt  16 , thus facilitating the replacement operation. 
     FIG.  9  and  FIG. 10  show a variation example of the embodiment shown in FIG.  8 . In this variation example, the transfer belt unit  9  and the image carrier unit  25  are supported by the frame  9   a  and the developing means  24  are supported at the housing body  2  side. Therefore, the image carrier unit  25  and the developing means  24  can be selectively replaced independently from each other. It should be noted that the image carrier unit  25  may be supported at the housing body  2  side. 
     In this embodiment, since the first door member  3  as the apparatus cover, the fixing unit  12 , and the secondary transfer unit  11  are opened together into a space for inserting and drawing the sheet cassette  35  for supply of recording media, the apparatus has good operationality for maintenance of the fixing unit  12  and a high level of visibility and good operationality when a recording medium is jammed. In addition, since at least one of the transfer belt unit  9 , the image carrier unit  25 , and the developing means  24  can be opened in the space above the opened first door member  3  to allow the replacement of the image carriers  20  and/or the developing means  24 , the apparatus also has good operationality for replacement of the expendable supplies. 
     Unlike the conventional technique, since it is not necessary to form a large access opening for replacing the expendable supplies in the frame supporting the image forming means, the rigidity of the frame is increased, thereby stably obtaining high-quality images. 
     It can prevent an operator from touching the transfer belt when the frame  9   a  is opened and thus prevent the occurrence of image defect due to fingerprints putted by the touching. In addition, even if toner spills during the replacement of expendable supplies, the toner can be received by the frame  9   a , thereby preventing the recording medium carrying means  13 , the fixing unit  12 , and the secondary transfer unit  11  from being stained. 
     Moreover, since the secondary transfer roller  19  is moved apart from the intermediate transfer belt  16  according to the opening of the first door member  3 , recording media jammed around the transfer belt  16  during printing plural recording media can be easily removed. 
       FIG. 11  is a schematic sectional view showing the entire structure of another embodiment of the image forming apparatus of the present invention. It should be noted that the same components as those of the embodiment of  FIG. 1  are marked with the same numerals, so description of such components will be omitted. This embodiment is of a type employing a paper carrying belt as a transfer belt. That is, instead of the intermediate transfer belt  16  of  FIG. 1 , a paper carrying belt  59  is employed. 
     In this embodiment, a transfer belt unit  9  and a fixing unit  12  are arranged in a first door member  3 . The transfer belt unit  9  comprises a driving roller  14  which is disposed in an upper portion of a housing body  2  and is driven by a driving means (not shown) to rotate, a driven roller  15  and a backup roller  60  which are disposed diagonally below the driving roller  14 , a paper carrying belt  59  which is laid around the three rollers with some tension and is driven to circulate in a direction indicated by an arrow, and a cleaning means  17  which abuts on the surface of the paper carrying belt  59  to oppose the backup roller  60 . The driving roller  14  and the paper carrying belt  59  are arranged obliquely to the upper left of the driven roller  15 . Accordingly, a belt tension side  59   a  at the time of driving the paper carrying belt  59  is on the lower side and a belt slack side is on the upper side. 
     On the back of the paper carrying belt  59 , transfer members  61  composed of leaf spring electrodes are disposed. The transfer members  61  are pressed into contact with the back of the paper carrying belt  59  by their elastic force at locations corresponding to image carriers  20  of respective image forming stations Y, M, C, and K. A transfer bias is applied to each transfer member  61 . The image carriers  20  of the image forming stations Y, M, C, K are in contact with the belt tension side  59   a  of the paper carrying belt  59 . As a result of this, the image forming stations Y, M, C, K are arranged in an obliquely leftward direction relative to the driven roller  15  in FIG.  11 . 
     FIG.  12  through  FIG. 15  are schematic illustrations each showing another embodiment of the image forming apparatus of the present invention. The embodiment of  FIG. 12  is an example in which an electrical component box  5  is arranged above the paper carrying belt  59  and image forming stations Y, M, C, K are arranged below the paper carrying belt  59 . The embodiment of  FIG. 13  is an example in which image forming stations Y, M, C, K are arranged above the paper carrying belt  59  and an electrical component box  5  is arranged below the paper carrying belt  59 . The embodiment of  FIG. 14  is an example in which image forming stations Y, M, C, K and an electrical component box  5  are arranged above the intermediate transfer belt  16 . The embodiment of  FIG. 15  is an example in which toner storage containers  26  of image forming stations Y, M, C, K are arranged above the intermediate transfer belt  16  as compared to the embodiment of FIG.  1 . 
     Though the present invention has been described with reference to the embodiments disclosed herein, the present invention is not limited thereto and the components of the present invention may be replaced with or include conventionally known or well known techniques. 
     For example, though the array-type writing heads are used as the image writing means  23  in the aforementioned embodiments, laser exposure device may be employed. In this case, the electrical component box  5  may be placed on the side of the laser exposure device (the depth side or the front side of FIG.  1 ). 
     Further, though the driving roller  14  is located at the lower side and the driven roller  15  is located at the upper side in the embodiment of  FIG. 1 , the driven roller  15  may be located at the lower side and the driving roller  14  may be located at the upper side. 
     In addition, though the pivot shafts of the first door member  3  and the frame  9   a  are located at the both sides of the housing body  2  so that they are movable vertically in the above embodiments, the pivot shafts may be located at a lateral side of the housing body  2  so that they may be movable horizontally. 
     It should be noted that the intermediate transfer belt and the paper carrying belt are generally defined as transfer belt.