Patent Publication Number: US-6985687-B2

Title: Image forming apparatus and image forming system including the same

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming apparatus of the type capable of forming images on both sides of a sheet or recording medium, and an image forming system including the same. 
   2. Description of the Background Art 
   An image forming apparatus of the type described is implemented as, e.g., an electronic copier, a printer, a facsimile apparatus or a multifunction machine having at least two of such functions. This type of apparatus is generally constructed to transfer a first image from an image carrier to one side of a sheet, fix the image on the sheet, reverse the sheet to thereby again deliver it to the image carrier, transfer a second image from the image carrier to the other side of the sheet, and then fix the second image to thereby produce a duplex print. However, the problem with this type of apparatus is that two times of delivery of a single sheet to the image carrier increases the overall image forming time and thereby lowers productivity. 
   In light of the above, Japanese Patent Laid-Open Publication No. 11-295937, for example, discloses an image forming apparatus including an intermediate image transfer belt and a first and a second image carrier. An image formed on the first image carrier is transferred to the intermediate image transfer belt while an image formed on the second image carrier is directly transferred to one side of a sheet. The image transferred to the intermediate image transfer belt is transferred to the other side of the same sheet. This successfully reduces image forming time in a duplex print mode for thereby enhancing productivity. 
   In the image forming apparatus disclosed in the above document, the intermediate image transfer belt is elongate in the up-and-down direction, preventing the apparatus from increasing in size in the horizontal direction. However, a sheet feeder loaded with a stack of sheets is positioned below the intermediate image transfer belt. This brings about a problem that when a plurality of sheet feeders are arranged one above the other, the overall height of the apparatus noticeably increases, making it difficult for the user to operate the apparatus. Moreover, such a height of the apparatus makes the apparatus unstable when installed at the user&#39;s station. This problem becomes more serious when a plurality of first image carriers and a plurality of second image carriers are arranged at both sides of the intermediate image transfer belt. 
   Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 8-97962, 10-39550, 11-24330, 11-202565 and 11-295937. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide an image forming apparatus free from the problem described above, and an image forming system including the same. 
   An image forming apparatus of the present invention includes at least one first image carrier on which an image is to be formed, an endless, intermediate image transfer belt to which the image is to be transferred from the first image carrier, and at least one second image carrier on which an image is to be formed. The image formed on the second image carrier is transferred to the first side of a recording medium while the image transferred to the intermediate image transfer belt is transferred to the second side of the recording medium. The intermediate image transfer belt is elongate in the up-and-down direction. A plurality of medium feeders are arranged one above the other at one side of the intermediate image transfer belt, and each is loaded with a stack of recording media to be fed toward an image transfer position. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which: 
       FIG. 1  is a section showing an image forming apparatus embodying the present invention; 
       FIG. 2  is an enlarged view showing a first image carrier included in the illustrative embodiment together with process units arranged therearound; 
       FIG. 3  is an enlarged view showing a second image carrier included in the illustrative embodiment together with process units arranged therearound; 
       FIG. 4  shows a position of an intermediate image transfer belt included in the illustrative embodiment; 
       FIG. 5  is an external view showing the image forming apparatus of the illustrative embodiment and a host computer interconnected to each other; 
       FIG. 6  is a front view showing a first and a second image forming unit included in the illustrative embodiment and appearing when a front door is opened; 
       FIG. 7  is a front view showing a belt unit angularly moved from the position shown in  FIG. 6 ; 
       FIG. 8  is a sectional side elevation showing the belt unit set in the casing of the image forming apparatus; 
       FIG. 9  is a section showing the belt in a position released from the first and second image carriers; and 
       FIG. 10  is an fragmentary enlarged view showing a specific configuration of an operation panel included in the illustrative embodiment. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1  of the drawings, an image forming apparatus embodying the present invention is shown and implemented as a printer capable of forming color images or a combined printer/facsimile apparatus by way of example. As shown, the image forming apparatus includes a casing  1  accommodating a group of first image carriers  2 Y (yellow),  2 M (magenta),  2 C (cyan) and  2  BK (black) and a group of second image carriers  2 YA,  2 MA,  2 CA and  2 BKA. In the illustrative embodiment, the image carriers  2 Y through  2 BK and  2 YA through  2 BKA are implemented as photoconductive drums and will be referred to as drums in this sense hereinafter. An endless, intermediate image transfer belt (simply belt hereinafter)  3  is passed over a plurality of support members including rollers  4  and  5  and a back roller  24 , which will be described specifically later. The belt  3  runs in a direction indicated by an arrow A in FIG.  1 . 
   The first drums  2 Y through  2 BK and second drums  2 YA through  2 BKA each are arranged side by side in the direction of movement of the belt  3  and held in contact with the outer periphery of the belt  3 . The belt  3  is positioned between the drums  2 Y through  2 BK and the drums  2 YA through  2 BKA. 
     FIG. 2  shows the first drum  2 Y and arrangements around it. As shown, a discharger  6 , a charger  7 , an optical writing unit  12 , a developing unit  8  and a cleaning unit  9  are arranged around the drum  2 Y and cooperate to form a yellow toner image on the drum  2 Y, as will be described in detail later. Such process units are arranged around each of the other first drums  2 M,  2 C and  2 BK also. The difference is that a magenta toner image, a cyan toner image and a black toner image are formed on the drums  2 M,  2 C and  2 BK, respectively. 
   A yellow toner image, a magenta toner image, a cyan toner image and a black toner image are also formed on the second drums  2 YA,  2 MA,  2 CA and  2 BKA, respectively.  FIG. 3  shows the second drum  2 YA by way of example together with arrangements around the drum  2 YA. As shown, a discharger  6 A, a charger  7 A, an optical writing unit  12 A, a developing unit  8 A and a cleaning unit  9 A are arranged around the drum  2 YA and cooperate to form the yellow toner image. This is also true with arrangements around the other drums  2 MA,  2 CA and  2 BKA except for the color of the toner image. 
   While the belt  3  runs in the direction A during image formation, the first drums  2 Y through  2 BK and second drums  2 YA through  2 BKA respectively rotate in directions indicated by arrows B 1  and B 2  in  FIGS. 2 and 3 , respectively. In this condition, toner images of different colors are formed on the drums, as will be described hereinafter. 
   Referring again to  FIG. 2 , the charger  7  uniformly charges the surface of the first drum  2 Y to preselected polarity, which is negative polarity in the illustrative embodiment. The optical writing unit  12  scans the charged surface of the drum  2 Y with a light beam in accordance with image data, thereby forming a latent image on the drum  2 Y. More specifically, the latent image is formed in a portion scanned by the light beam and lowered in surface potential thereby in terms of absolute value. The developing device  8  develops the latent image with yellow toner to thereby produce a yellow toner image. More specifically, the toner stored in the developing device  8  is charged to preselected polarity, i.e., negative polarity like the drum  2 Y beforehand and electrostatically deposited on the latent image to thereby develop the latent image. While the illustrative embodiment effects reversal development, as stated above, it may be configured to effect regular development. A magenta toner image, a cyan toner image and a black toner image are respectively formed on the drums  2 M,  2 C and  2 BK in exactly the same manner as the yellow toner image. 
   A first image transferring device  13 Y is positioned at the back of the belt  3  and applied with a positive voltage for image transfer, which is opposite in polarity to the toner deposited on the drum  2 Y. The image transferring device  13 Y electrostatically transfers the yellow toner image from the drum  2 Y to the outer periphery of the belt  3 , which is moving in synchronism with the drum  2 Y. After the image transfer, the cleaning unit  9  removes the toner left on the drum  2 Y. Subsequently, the discharger  6  discharges the cleaned surface of the drum  2 Y with light to thereby prepare the drum  2 Y for the next image forming cycle. 
   Likewise, first image transferring devices  13 Y,  13 C and  13 BK respectively transfer the magenta toner image, cyan toner image and black toner image from the drums  2 M,  2 C and  2 BK to the belt  3  over the yellow toner image, completing a full-color image on the belt  3 . The procedure to follow the image transfer is identical with the procedure described in relation to the yellow toner image. The belt  3  carrying the full-color image thereon continuously moves in the direction A. 
   A yellow toner image, a magenta toner image, a cyan toner image and a black toner image are respectively formed on the second drums  2 YA,  2 MA,  2 CA and  2 BKA in exactly the same manner as the images formed on the first drums  2 Y through  2 BK. More specifically, when the full-color image carried on the belt  3  arrives at a preselected position, the yellow toner image begins to be formed on the drum  2 YA. Subsequently, the magenta toner image, cyan toner image and black toner image are sequentially formed on the drums  2 MA,  2 CA and  2 BKA, respectively, over the yellow toner image. Toners forming the images on the drums  2 YA through  2 BKA are charged to the same polarity as the toners deposited on the drums  2 Y through  2 BK, i.e., negative polarity. 
   Sheet feeders or medium feeders  15 A,  15 B,  15 C and  15 E are positioned at one side of the belt  3 , and each is loaded with a stack of paper sheets, resin sheets or similar recording media P, as will be described in detail later. The sheet P paid out from any one of the sheet feeders  15 A through  15 E is conveyed to a registration roller pair  18 , which is a specific form of a registering device. The registration roller pair  18  conveys the sheet P at preselected timing toward consecutive nips between the belt  3  and the second drums  2 YA through  2 BKA, as indicated by an arrow C in FIG.  1 . The sheet P is then conveyed by the belt  3 . 
   Second image transferring devices  13 YA,  13 MA,  13 CA and  13 BKA are positioned at the back of the belt  3  in such a manner as to substantially face the drums  2 YA,  3 MA,  2 CA and  2 BKA, respectively. A voltage for image transfer opposite in polarity to the images formed on the drums  2 YA through  2 BKA, i.e., a positive voltage is applied to each of the image transferring devices  13 YA through  13 BKA. In this condition, the yellow toner image, magenta toner image, cyan toner image and black toner image are sequentially, electrostatically transferred from the drum  2 YA through  2 BKA to one side of the sheet P one above the other. As a result, a full-color image is formed on the sheet P. At the same time, the full-color image transferred from the first drums  2 Y through  2 BK to the belt  3  moves through the nips between the second drums  2 YA through  2 BKA and the belt  3 . 
   A third image transferring device  14  is positioned downstream of the second drums  2 YA through  2 BKA in the direction of movement of the belt  3  and faces, but does not contact, the belt  3 . The image transferring device  14  is implemented as a corona discharger. A voltage for image transfer opposite in polarity to the images transferred from the first drums to the belt  3 , i.e., a positive voltage is also applied to the charge wire of the image transferring device  14 . As a result, the full-color image carried on the belt  3  is transferred to the other side of the sheet P facing the outer periphery of the belt  3 . 
   The full-color image transferred from the first drums  2 Y through  2 BK to the other side of the sheet P by way of the belt  3  will sometimes be referred to as a first full-color image. The other full-color image directly transferred from the second drums  2 YA through  2 BKA to one side of the sheet will sometimes be referred to as a second full-color image. The timing at which the images begin to be formed on the second drums  2 YA through  2 BKA and the timing at which the sheet P begins to be paid out are controlled such that the first and second full-color images are accurately transferred to the opposite sides of the sheet P. 
   A fixing device  20  is positioned above the belt  3  and made up of a pair of rollers  21  and  21 A and a pair of heaters  23  and  23 A respectively disposed in the rollers  21  and  21 A. The rollers  21  and  21 A are rotatable in directions indicated by arrows in  FIG. 1  in pressing contact with each other. The heaters  23  and  23 A respectively heat the rollers  21  and  21 A to fixing temperature adequate for the fixation of the full-color images. The sheet P carrying the first and second full-color images on both sides thereof and left the belt  3  is passed through the nip between the rollers  21  and  21 A. The rollers  21  and  21 A respectively fix the first and second full-color images on the sheet P with heat and pressure. The sheet P with the fixed full-color images, i.e., a print is driven out to a print tray  28  by an outlet roller pair  27  while being guided by guides  25  and  26 . The print tray  28  is positioned on the top of the casing  1 . 
   A belt cleaning unit  29  removes the toner left on the belt  3  after the image transfer described above. The belt cleaning unit  29  includes a cleaning roller  30 , a blade  31 , a case  32 , and a conveyor  32   a . The cleaning roller  30  removes the toner left on the belt  3  after image transfer while the blade  31  removes the toner collected by the cleaning roller  30 . The cleaning roller  30  and blade  31  are supported by the case  32 . The conveyor  32   a  conveys the collected toner to a toner storing section not shown. The back roller  24  mentioned earlier faces the cleaning roller  30  with the intermediary of the belt  3 . 
   A cooling device  33  cools off part of the belt  3  moved away from the belt cleaning unit  29 . The cooling device  33  may be implemented as a blower for blowing atmospheric air or similar cool air against the belt  3  or a radiating member. In the illustrative embodiment, the cooling device  33  is implemented as a plurality of heat pipes  34  held in contact with the opposite surfaces of the belt  3  for absorbing heat. In this manner, the cooling device  33  lowers the temperature of the belt  3  heated by the fixing unit  20  and may be applied to any type of apparatus. This prevents the first drums  2 Y through  2 BK form being excessively heated by the belt  3  and thereby prevents the full-color image from being deteriorated. However, the cooling device  33  is not essential with the illustrative embodiment. 
   A fan  35  discharges air inside the casing  1  to the outside to thereby prevent temperature inside of the casing  1  from rising to an excessive degree. In  FIG. 1 , the reference numeral  1  designates a control unit. 
   The belt  3  is heat-resistant and provided with resistance that allows toner to be transferred thereto. For example, the belt  3  is made up of a heat-resistant base and a surface layer formed on the belt  3  and having low surface energy. The volume resistivity of the entire belt  3  is, e.g., 10 6  Ω·cm to 10 12  Ω·cm. More specifically, the base may be formed of polyimide or polyamideimide and 50 μm to 200 μm thick. The surface layer may be implemented as a coating layer having low surface energy and formed of Teflon or similar fluorocarbon resin. The surface of the belt  3  should preferably have resistivity of 10 5  Ω·cm to 10 12  Ω·cm. 
   As the image forming cycle stated earlier is repeated, a number of prints are stacked on the print tray  28 . In the illustrative embodiment, each print is driven out to the print tray  28  with the side thereof to which the second full-color image is transferred from the second drums  2 YA through  2 BKA facing downward. Therefore, to stack the prints in order of page, a second page is transferred from the first drums  2 Y through  2 BK to the other side of a sheet P by way of the belt  3  while a first page is directly transferred from the drums  2 YA through  2 BKA to one side of the same sheet P. Likewise, a fourth page is transferred from the first drums  2 Y through  2 BK to the other side of the next sheet P by way of the belt  3  while a third page is directly transferred from the second drums  2 YA through  2 BKA to one side of the same sheet. Such a procedure allows the prints to be sequentially stacked on the print tray  28  in order of page. 
   The first images formed on the first drums  2 Y through  2 BK are inverted to become mirror images when transferred to the belt  3 , and again inverted to become a non-inverted full-color image when transferred to the other side of a sheet P. The second images formed on the second drums  2 YA through  2 BKA are inverted images, but become a non-inverted full-color image when transferred to one side of the sheet P. 
   To form images with the first drums  2 Y through  2 BK and second drums  2 YA through  2 BKA in order of page, the illustrative embodiment can use a conventional method that stores image data in a memory. In addition, the illustrative embodiment can use any conventional image processing technology for forming non-inverted images and inverted images on the drums  2 Y through  2 BK and drums  2 YA through  2 BKA, respectively. 
   In the illustrative embodiment, the first image transferring devices  13 Y through  13 BK and second image transferring devices  13 YA through  13 BKA are implemented as rollers rotatable in contact with the inner periphery of the belt  3 . Alternatively, use may be made of image transferring devices using brushes, blades or brush rollers rotatable with the inner periphery of the belt  3  with a voltage being applied thereto. Further, use may be made of image transferring devices implemented as corona dischargers spaced from the inner periphery of the belt  3 . 
   An arrangement may be made such that before the first full-color image transferred from the first drums  2 A through  2 BK to the belt  3  reaches the second drum  2 YA, a corona discharger or similar polarity inverting device inverts the polarity of the first full-color image to polarity opposite to that of the second full-color image, i.e., positive polarity. In this case, if a positive voltage is applied to each of the second image transferring devices  13 YA through  13 BKA, then the first and second full-color images can be transferred to opposite sides of a sheet P at the same time. This makes the third image transferring device  14  unnecessary. 
   While the illustrative embodiment includes a plurality of first drums  2 Y through  2 BK and a plurality of second drums  2 YA through  2 BKA, it is, of course, practicable with at least one first drum and at least one second drum. 
   Another alternative arrangement available with the illustrative embodiment is as follows. A first image carrier and a second image carrier are used as intermediate image transfer bodies. Toner images of different colors are sequentially formed on a photoconductive element, not shown, and sequentially transferred to the first image carrier one above the other to thereby form a first full-color image. Likewise, toner images of different colors are sequentially formed on another photoconductive element, not shown, and sequentially transferred to the second image carrier one above the other to thereby form a second full-color image. The first second full-color image is directly transferred to one side of a sheet while the first full-color image is transferred to the other side of the same sheet by way of an intermediate image transfer belt. The sheet with such full-color toner images is fixed and then driven out as a print. 
   It will be seen from the above that the illustrative embodiment is practicable with at least one first image carrier, an endless, intermediate image transfer belt to which an image is transferred from the first image carrier, and at least one second image carrier on which an image different from the above image is formed. The image on the second image carrier is transferred to one side of a sheet while the image transferred to the belt is transferred to the other side of the sheet. In this configuration, only if a single sheet P is conveyed to a nip between the belt  3  and the second image carrier, images can be formed on both sides of the sheet P. This successfully reduces image forming time and enhances the productivity of duplex prints. 
   Simplex prints each carrying an image on one side thereof are also achievable with the illustrative embodiment. In this case, no images are formed on the first drums  2 Y through  2 BK while images are formed on the second drums  2 YA through  2 BKA in exactly the same manner as stated earlier. Although no voltages are applied to the first drums  13 Y through  13 BKA, the drums  13 Y through  13 BKA each are rotated in the direction B 1  while the belt  3  is moved in the direction A. 
   More specifically, in a simplex print mode, while the belt  3  conveys a sheet P fed from the registration roller pair  18 , the second image transferring devices  13 YA through  13 BKA sequentially transfer images formed on the drums  2 YA through  2 BKA to one side of the sheet P one above the other, thereby completing a full-color image. The full-color image is fixed on the sheet P by the fixing device  20 . Thereafter, the sheet or print P is driven out to the print tray  28  by the outlet roller pair  27  face down while being guided by the guides  25  and  26 , as stated earlier. It follows that consecutive prints are sequentially stacked on the print tray  28  in order of page. 
   Alternatively, in the simplex print mode, images may be formed on the first drums  2 Y through  2 BK instead of the second drums  2 YA through  2 BKA, transferred to the belt  3  one above the other to form a full-color image, fixed, and then driven out to the print tray  28 . 
   As shown in  FIG. 1 , the belt  3  is elongate in the up-and-down direction. More specifically, as shown in  FIG. 4 , the belt  3  is passed over a plurality of support members  5  and  4  such that its height H1 in the vertical direction is greater than its width W1 in the horizontal direction. The belt  3  has one surface  3 A and the other surface  3 B extending in the up-and-down direction each. The plurality of sheet feeders  15 A through  15 E,  FIG. 1 , mentioned earlier are arranged one above the other along one surface  3 A of the belt  3 . 
   The belt  3  extending in the up-and-down direction reduces the overall width of the image forming apparatus in the horizontal direction and makes the entire construction compact. Further, the sheet feeders  15 A through  15 E arranged one above the other at one side of the belt  3  prevent the overall height of the apparatus from increasing to a noticeable degree. The apparatus is therefore easy to operate and is stable when installed at the user&#39;s station. By contrast, assume that a plurality of sheet feeders are arranged below an intermediate image transfer belt extending in the up-and-down direction as conventional. Then, although the overall width of this kind of apparatus in the horizontal direction decreases, the overall height of the apparatus noticeably increases and makes the apparatus uneasy to operate and unstable. 
   A specific configuration of the sheet feeders  15 A through  15 E will be described hereinafter. It is to be noted that the number and configuration of the sheet feeders are open to choice. As shown in  FIG. 1 , the second sheet feeder  15 B from the top, the bottom sheet feeder  15 E and the second sheet feeder  15 D from the bottom include cassettes  16 B,  16 E and  16 D and pickup rollers  17 B,  17 E and  17 D, respectively. The cassettes  16 B,  16 E and  16 D each are loaded with sheets P of preselected size. The pickup rollers  17 B,  17 E and  17 D rotate clockwise, as viewed in  FIG. 1 , in contact with the top sheets P of the associated cassettes  16 B,  16 E and  16 D, thereby paying them out in directions EB, EE and ED, respectively. The cassettes  16 B,  16 E and  16 D can be pulled out of the housing  1  in a direction F (see  FIG. 5 ) for the replenishment of sheets or the replacement of the sheets with another kind of sheets. 
   The sheet feeder  15 C is implemented as a manual sheet feeder for allowing the operator of the apparatus to insert an envelope, thick card, sheet of irregular size or similar special recording medium by hand. The manual sheet feeder  15 C includes a tray  16 C and a pickup roller  17 C for paying out a sheet P laid on the tray  16 C. More specifically, the pickup roller  17 C rotates clockwise, as viewed in  FIG. 1 , to thereby pay out the sheet P in a direction EC. The sheet feeder  15 C is arranged in a space S formed in the intermediate portion of the casing  1  in the vertical direction. The space S is so dimensioned as to accommodate most sheets although some sheets may be long and protrude from the space S. 
   The top sheet feeder  15 A is loaded with a roll of elongate recording medium RP, e.g., an elongate webbing of resin or paper. In this sense, the top sheet feeder  15 A will be referred to as a roll feeder hereinafter. The edge of the webbing paid out from the roll RP is nipped by a pullout roller pair  17 A. When the pullout roller pair  17 A is rotated, it conveys the webbing in a direction EA. A cutter  49  cuts the webbing at a preselected length. 
   The sheet P fed from any one of the sheet feeders  15 A through  15 E is conveyed to the registration roller pair  18  and then conveyed by the registration roller pair  18  at preselected timing toward the image transfer position stated previously. In this manner, various kinds of recording media can be selectively delivered to the image transfer position. 
   As shown in  FIG. 1 , the belt  3  is slightly inclined from the vertical direction in order to slightly reduce the overall height of the apparatus. The first drums  2 Y through  2 BK and second drums  2 YA through  2 BKA are positioned at both sides of the belt  3  in the horizontal direction. This, coupled with the fact that the second drums are positioned between the belt  3  and the sheet feeders  15 A through  15 E, makes the entire construction compact and enhances stability while surely balancing the height and width of the apparatus. To further reduce the overall width of the first and second drums and belt  3  in the horizontal direction, the belt  3  may be inclined such that its top is remoter from the sheet feeders, and the first drums may be positioned at a lower level than the second drums. 
   If desired, two roll feeders may be substituted for the single roll feeder  15 A shown in FIG.  1 . The crux is that at least one of the plurality of sheet feeders be implemented as a roll feeder. This is also true with the roll feeder  15 A. 
   In the illustrative embodiment, images can be formed on both sides of a sheet cut away from the webbing, which is paid out from the roll RP, only if the sheet is conveyed once. Therefore, images can be formed on both sides of a sheet having substantially any desired length. By contrast, in a conventional image forming apparatus of the type forming images on both sides of a sheet by feeding it to an image transfer position two times, the length of the sheet is limited. 
   As shown in  FIG. 1 , a position PS where the webbing is paid out from the roll RP is positioned above the center CL of the roll RP, preferably at the top of the roll PP. It is therefore easy for the operator to mount the roll RP to the sheet feeder  15 A and set the edge of the webbing between the pullout rollers  17 A. 
   The print tray  28  is positioned above the sheet feeders  15 A through  15 E that are arranged one above the other. That is, the sheet feeders  15 A through  15 E and print tray  28  all are aligned with each other in the up-and-down direction, so that the print tray  28  does not protrude from the casing  1  sideways. The casing  1  can therefore be positioned in the vicinity of, e.g., the wall of a room, occupying a minimum of space. 
   The conveyance paths extending from the sheet feeders  15 A through  15 E to the print tray  28  are configured such that prints are stacked on the print tray  28  with the image transferred from the second image carrier or carriers facing downward. Therefore, in both of the duplex print mode and simplex copy mode, prints can be stacked on the print tray  28  in order of page. Usually, the simplex print mode is predominant over the duplex copy mode, allowing the operator to perform efficient operation. 
   As shown in  FIG. 1 , the print tray  28  is implemented as a cover  28 A or part of the cover  28 A. The cover  28 A is mounted to the casing  1  via a hinge  28 B and openable about the hinge  28 B away from the casing  1  in a direction G. The operator can therefore open the cover  28 A, set the roll RP on the sheet feeder  15 A from above, and then cause the pullout roller pair  17 A to nip the edge of the webbing paid out from the roll RP. Further, the position PS where the webbing is paid out from the roll RP is positioned above the roll RP and further facilitates the above operation. At this instant, the cutter  49  does not obstruct the operator&#39;s work and therefore does not have to be retracted, so that the apparatus is simplified. Should the position PS be positioned below the roll RP, it would be uneasy for the operator to perform such work. 
   A plurality of manual sheet feeders may be substituted for the single manual sheet feeder  15 C, if desired. In the illustrative embodiment, the registration roller pair or registering device  18  is positioned at a lower level than the second drums  2 YA through  2 BKA. As shown in  FIG. 1 , the path extending from the manual sheet feeder  15 C to the registering device should preferably be linear. Should the above path be curved, it might obstruct the conveyance of an envelope, thick sheet or similar special sheet. 
   The first image carriers and second image carriers are positioned at different levels, as stated earlier. It is preferable that the second image carriers be positioned above the intermediate point of the belt  3  in the up-and-down direction, and that the registering device be positioned below the second image carriers. As shown in  FIG. 1 , a support shaft  46 , which will be described later, is positioned at the intermediate between the top and the bottom of the belt  3 . The second drums  2 YA through  2 BKA are positioned above the axis of the support shaft  46  while the registration roller pair  18  is positioned in a space beneath the second drums. Such a configuration makes the arrangement of the belt  3  and drums  2 YA through  2 BKA and therefore the entire construction compact. 
   The first drums  2 Y through  2 BK are positioned below the axis of the support shaft  46  while the cleaning device  29  and cooling device  33  are positioned above the drums  2 Y through  2 BK. In this manner, the first image carriers are positioned below the intermediate between the top and the bottom of the belt  3  while at least one of the cleaning device  29  and cooling device  33  is positioned above the first image carriers. This makes the arrangement of the belt  3 , cleaning device  29  and cooling device  33  and therefore the entire construction compact. 
   As shown in  FIG. 2 , the first drum or image carrier  2 Y, discharger  6 , charger  7 , developing device  8  and cleaning device  9  are constructed into a single process unit PU. This is also true with the other first drums  2 M,  2 C and  2 BK and process units arranged therearound. As shown in  FIG. 6 , such process units are mounted on a single unit case  10  to thereby constitute a first image forming unit  11 . Likewise, as shown in  FIG. 3 , the second drum or image carrier  2 YA, discharger  6 A, charger  7 A, developing device  8 A and cleaning device  9 A are constructed into a single process unit PUA. As shown in  FIG. 6 , such process units are mounted on a single unit case  10 A to thereby constitute a second image forming unit  11 A. In such a configuration, the optical writing unit  12  or  12 A is separate from the process unit PU or PUA or the image forming unit  11  or  11 A. 
   When the sheet P jams the path during image formation, the apparatus interrupts image formation, causes the first and second image carriers and belt  3  to stop rotating, and urges the operator to remove the jamming sheet P. Further, the image carriers and belt  3 , as well as other structural elements, need inspection or repaired from time to time. Moreover, each of such structural elements must be replaced when its service life ends. In the illustrative embodiment, when any one of the process units PU and PUA, first and second image forming units  11  and  11 A and belt  3  reaches its life, it can be replaced with new one. 
   As shown in  FIG. 5 , for the above various kinds of maintenance work, the casing  1  includes a front door  37  openable in a direction H. Also, as shown in  FIG. 1 , the upper portion of the casing  1  is implemented as a top door  39  hinged to the casing  1  via a shaft  38 . The top door  39  uncovers the upper portion of the casing  1  when opened in a direction I. When either one of the front door  37  and top door  39  is opened during image formation, a switch, not shown, associated therewith is turned off to automatically interrupt image formation. 
   By opening the top door  39  or the front door  37 , the operator can remove a jamming sheet or remove any one of the belt  3  and image forming units  11  and  11 A. At this instant, if such maintenance work is performed with the belt  3  and first and second drums remaining in contact with each other, as shown in  FIG. 1 , then the jamming sheet is likely to hit against the belt  3  or the first and second drums, scratching the surface of the belt  3  or the surfaces of the drums. 
   In light of the above, the belt  3  is configured to be movable between a first position shown in  FIG. 1  where it contacts the first and second drums and a second position shown in  FIG. 9  where the former is spaced from the latter in a direction J. This allows the operator to release the belt  3  from the first and second drums after opening the top door  39  or the front door  37 , and then perform the maintenance work without scratching the belt  3  or the drums. After the maintenance work, the operator should only bring the belt  3  into contact with the first and second drums and then close the top door  39  or the front door  37 . 
   As stated above, the belt  3  is moved away from the first and second drums to thereby promote easy, rapid operation, compared to a case where the first and second drums are moved away from the belt  3 . Further, moving the belt  3  away from the first and second drums makes the entire construction simpler and the cost lower than moving the latter away from the former. Moreover, the operator should only move the belt  3  by a small angle in the direction J. 
   As shown in  FIGS. 6 and 8 , the belt  3  is constructed into a belt unit  41  together with a frame  40  supporting the belt  3 . The frame  40  is made up of a front side wall  42 , a rear side wall  43 , and a plurality of tie bars  44  (only one is shown in  FIG. 8 ) connecting the side walls  42  and  43 . The rollers  4 ,  5  and  24  supporting the belt  3  are rotatably mounted on the side walls  42  and  43 . Further, the first image transferring devices  13 Y through  13 Y, second image transferring devices  13 YA through  13 BKA and heat pipes  34  shown in  FIG. 1  are rotatably mounted on the side walls  42  and  43 . In addition, the case  32  of the cleaning device  29  and third image transferring device  14  are mounted on the frame  40 . 
   As shown in  FIG. 8 , the support shaft  46  is affixed at one end to a rear wall  45  included in the casing  1  and extends horizontally toward the front end of the casing  1 . The front end of the support shaft  46  is free, so that the support shaft  46  is cantilevered by the casing  1 . The support shaft  46  extends through holes  47  and  48  formed in the side walls  42  and  43 , respectively. Therefore, as shown in  FIGS. 1 and 9 , the support shaft  46  supports the entire belt unit  41  such that the belt unit  41  is angularly movable about the support shaft  46 . The support shaft  46  is positioned between the opposite runs of the belt  3 . 
   As shown in  FIG. 8 , a guide tube  70  may be affixed to the side walls  42  and  43  of the frame  40  coaxially with the holes  47  and  48 , in which case the support shaft  46  will be passed through the guide tube  70 . 
   As stated above, the belt unit  41  is angularly movable about the support shaft  46 , causing the belt  3  to move between the first position and the second position stated earlier. The operator can therefore easily, stably move the belt  3  away from the first and second drums substantially at the same time. At this instant, the movable range of the belt unit  41  is limited such that when the belt  3  is brought to the second position shown in  FIG. 9 , the belt unit  41  does not interfere with members constituting the conveyance paths. More specifically, the belt  3  at the second position adjoins, e.g., the registration roller pair  18 , but does not interfere with it or damage it. Moreover, the belt unit  41  can be easily moved to locate the belt  3  at the second position because the first drums  2 Y through  2 BK are positioned below the intermediate between the top and the bottom of the belt  3  and because the second drums  2 YA through  2 BKA are positioned above the same, as sated earlier. 
   A locking device, not shown, should preferably be used to hold the belt  3  stably at the first position by inhibiting the movement of the belt unit  41 . By opening the front door  37  and unlocking the locking device, the operator can freely move the belt unit  41  to the second position by hand. 
   The belt unit  41  is mounted on the support shaft  46  in such a manner as to be slidable in the axial direction of the shaft  46 . This allows the operator to remove the belt unit  41  from the casing  1  by opening the front door  37 , moving the belt unit  41  from the first position to the second position, and then pulling out the belt unit  41  toward the operator along the support shaft  46 , as indicated by an arrow M in FIG.  8 . To mount the belt unit  41  to the casing  1 , the operator performs the above procedure in the reverse order; the belt unit  41  is pushed into the casing  1  in a direction indicated by an arrow N. 
   In the illustrative embodiment, the first and second image forming units  11  and  11 A, like the belt unit  41 , can be mounted and dismounted from the casing  1  at the front of the casing  1 . More specifically, as shown in  FIGS. 1 ,  6 ,  7  and  9 , a fist and a second guide stay  52  and  52 A, respectively, are disposed in the casing  1  and extend from the rear toward the front of the casing  1 . The unit cases  10  and  10 A of the image forming units  11  and  11 A are respectively supported by the guide stays  52  and  52 A in such a manner as to be slidable in the front-and-rear direction of the casing  1 . To dismount the image forming units  11  and  11 A from the casing  1 , the operator opens the front door  37 , moves the belt unit  41  to the second position shown in  FIGS. 7 and 9 , and then pulls the image forming units  11  and  11 A toward the operator one by one. To mount the image forming units  11  and  11 A to the casing  1 , the operator performs the above procedure in the reverse order. 
   A locking device should preferably be provided for locking the image forming units  11  and  11 A at preselected positions inside the casing  1 , in which case the operator will operate the locking device to unlock the image forming units  11  and  11 A before pulling them out. 
   In any case, the operator can pull out the image forming units  11  and  11 A and then replace them with new ones or replace only the process units PU and PUA by removing them from the unit cases  10  and  10 A. In this manner, the image forming units  11  and  11 A and belt unit  41  can be pulled out via the front end of the casing  1 . Should such units be configured to be pulled out of the casing  1  sideways, peripheral units around the casing  1  would have to be positioned below the casing  1 , further increasing the overall height of the apparatus. 
   The optical writing units  12  and  12 A assigned to the process units PU and PUA, respectively, are implemented as members separate from the first and second image forming units  11  and  11 A, as stated earlier. This is because the writing units  12  and  12 A are generally longer in service life than the drums and the other process units; if the writing units  12  and  12 A are included in the image forming units  11  and  11 A and replaced together, then the writing units  12  and  12 A are simply wasted despite that they are still usable. 
   The writing units  12  and  12 A can be removed from the casing  1  alone when they should be replaced or cleaned. Also, the image forming units  11  and  11 A can be removed from the casing  1  while leaving the writing units  12  and  12 A in the casing  1 . That is, the writing units  12  and  12 A are removably mounted on the casing  1  independently of the image forming units  11  and  11 A. 
   More specifically, as shown in  FIGS. 7 and 9 , a first and a second guide rail  53  and  53 A, respectively, are respectively affixed to the first and second guide stays  52  and  52 A in correspondence to the writing units  12  and  12 A. The guide rails  53  and  53 A extend in the front-and-rear direction of the casing  1 , i.e., substantially in parallel to the support shaft  46 , and guide the writing units  12  and  12 A, respectively. The operator can therefore pull out the writing units  12  and  12 A toward the operator along the guide rails  53  and  53 A or push them into the casing  1  along the guide rails  53  and  53 A. 
   The writing units  12  and  12 A may be implemented as laser units for scanning the associated drums in the main scanning direction with laser beams in accordance with image data. In the illustrative embodiment, the writing units  12  and  12 A are implemented as light source arrays, e.g., LED (Light Emitting Diode) arrays. Light beams issuing from the individual light sources are focused on the charged surface of the associated drum via lenses, not shown, forming a latent image thereon. This kind of writing unit is disclosed in, e.g., Japanese Utility Model Publication No. 2-42454. The LED array, for example, is far smaller in size than a laser unit and therefore occupies a minimum of space in the casing  1 , thereby reducing the size of the casing  1 . 
   The belt unit  41  is supported such that it can be pulled out of the casing  1  toward the front, as stated previously. The operator may inadvertently pull the belt unit  41  held in the first position, i.e., held in contact with the first and second drums toward the operator. This would cause the belt  3  and first and second drums to rub and scratch each other. In light of this, the illustrative embodiment includes belt unit checking means for inhibiting, when the belt  3  is held in the first position, the belt unit  41  from being pulled out of the casing  1 . 
   The first and second image forming units  11  and  11 A are also supported such that they can be pulled out of the casing  1  toward the front, as stated previously. The operator may inadvertently pull any one of the image forming units  11  and  11 A held in contact with the belt  3  toward the operator. This would cause the first and second drums and belt  3  to rub and scratch each other. In light of this, the illustrative embodiment includes image forming unit checking means for inhibiting, when the belt  3  is held in the first position, the image forming units  11  and  11 A from being pulled out of the casing  1 . 
   More specifically, as shown in  FIGS. 6 ,  7  and  8 , the casing  1  includes a cover  54  that, in turn, includes a front portion  55 . The front portion  55  is formed with a notch  56  for allowing the belt unit  41  to pass when the belt unit  41  is mounted to or dismounted from the casing  1 . As shown in  FIG. 6 , when the belt  3  is held in contact with the first and second drums at the first position, the belt unit  41  is located at a position deeper than the front portion  55  of the cover  55 . In this condition, part  58  of the frame  40  included in the belt unit  41  overlaps a stop  57  formed by part of the front portion  55  adjoining the notch  56 , as seen from the front of the casing  1 . Therefore, so long as the belt  3  is held in the first position, the operator cannot pull out the belt unit  41  toward the front because the stop  57  checks the part  58  of the frame  40 . 
   When the operator moves the belt unit  41  to the position shown in  FIG. 7  so as to move the belt  3  to the second position spaced from the first and second drums, the frame  40  of the belt unit  41  is shifted to a position where it does not interfere with the stop  57 . In this condition, the operator can pull the belt unit  41  toward the front out of the casing  1 . The stop  57  may, of course, be implemented by a member other than the cover  55  or may even be implemented as an independent stop affixed to the casing  1 . 
   As for the image forming unit checking means, as shown in  FIGS. 6 and 7 , the frame  40  of the belt unit  41  is formed with a first lug  59  and a second lug  59 A while the cases  10  and  10 A of the first and second image forming units  11  and  11 A are formed with a first recess  60  and a second recess  60 A, respectively. As shown in  FIG. 6 , when the stop  57  checks the belt unit  41  with the belt  3  remaining at the first position, the lugs  59  and  59 A respectively mate with the recesses  60  and  60 A for thereby preventing the image forming units  11  and  11 A from being pulled toward the front of the casing  1  away from the belt unit  41 . This prevents the operator from pulling out the image forming units  11  and  11 A out of the casing  1 . 
   When the operator moves the belt unit  41  so as to bring the belt  3  to the second position released from the first and second drums, the lugs  59  and  59 A of the belt unit  41  are released from the recesses  60  and  60 A of the image forming units  11  and  11 A. In this condition, the operator can pull the image forming units  11  and  11 A out of the casing  1 . 
   The lugs  59  and  59 A of the belt unit  41  and the recesses  60  and  60 A of the image forming units  11  and  11 A may, of course, replaced with each other so long as they can constitute locking portions. 
   Referring again to  FIG. 1 , the image forming apparatus described above may be connected to a host computer or similar host  61  to thereby construct an image forming system. In this case, the operator inputs, e.g., desired image forming conditions on the host computer  61  or an operation panel  51  mounted on the casing  1 .  FIG. 10  shows a specific configuration of part of the operation panel  51  including various keys and a display. The operator may select a duplex print mode by pressing a duplex key on the operation panel  51 , select special sheets by pressing a sheet feed button, and select a sheet feeder by pressing a set button, as desired. Alternatively, the operator may cause the host computer  61  to show a detailed picture on its display and then input desired process conditions for image formation based on the image. To transfer the characteristics of a recording medium, the operator may input them on the operation panel  51  or on the keyboard while watching a picture on the host computer  61 . Alternatively, a particular switch may be assigned to each sheet feeder, in which case the operator will operated any one of such switches matching with the kind of recording medium. Further, an exclusive tray for, e.g., envelopes or postcards may be prepared and removably mounted to the casing  1 , so that the apparatus can identify the exclusive tray. In any case, the apparatus can print information to be dealt with by the host computer  61  in accordance with the object and the kind of recording medium. 
   In the specific system in which the image forming apparatus and host computer  61  are interconnected by a network, the apparatus mainly plays the role of an output terminal (printer) or a facsimile apparatus for the host computer  61 . A scanner, not shown, may be connected to the apparatus so as to construct a copy system. Of course, the apparatus and host computer  61 , for example, may communicate with each other by radio. 
   Images can be printed on both sides of the webbing paid out from the roll RP, as stated earlier. The operator may therefore paginate on the host computer  61  and cause the webbing to be folded, stapled cut or otherwise finished to produce, e.g., a booklet. 
   Further, the operating conditions of the image forming apparatus, the remaining amounts of supplies and so forth may be displayed on the host computer  61 . 
   While the illustrative embodiment has concentrated on a printer, the present invention is similarly applicable to any other image forming apparatus, e.g., an electronic copier or facsimile apparatus or a multifunction machine or even to an image forming system including the image forming apparatus. 
   In summary, it will be seen that the present invention provides an image forming apparatus having a minimum of height despite that it includes a plurality of sheet feeders, while preserving the merits of an intermediate image transfer belt extending in the up-and-down direction. 
   Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.