Abstract:
An image forming apparatus is provided with a plurality of image forming units that include a plurality of image carriers for forming developer images of different colors on an image transfer medium. The image forming apparatus also includes a transfer unit for conveying an image transfer medium to the plurality of image carriers, and an identification unit for identifying developer image forming units necessary for forming a predetermined image. The image forming apparatus further includes a switch unit for switching between a first contact state where the transfer means is brought into contact with a part of the plurality of image forming units and a second contact state where the transfer unit is brought into contact with all of the plurality of image forming units. The image forming apparatus still further includes a mode selection unit for selecting either one of a first transfer mode for transferring an image on only one side of the image transfer medium and a second transfer mode for transferring an image on both sides of the image transfer medium, and a control unit for controlling to set the contact state between the transfer unit and the plurality of developer image forming units to the second contact state.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-275538, filed September 29, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an image forming apparatus applied as a color printer apparatus or a color copying apparatus, as well as an image forming method. 
     An example of the image forming apparatus of the above-described type is a 4-series tandem type apparatus in which a plurality of electronic photographic process units are arranged to oppose to the same transfer belt. 
     These electronic photographic process units are arranged to have predetermined intervals therebetween along the direction in which sheets are conveyed, and the transfer belt is designed to swing in a swaying direction to be brought into contact with or away from the photosensitive drums of the respective electronic photographic process units in accordance with a printing mode. These plurality of photosensitive drums are used for colors of yellow, magenta, cyan and black. 
     In an image forming operation, when, for example, a full-color printing mode is designated, the transfer belt is tilted towards the photosensitive drums and is brought into rotation-contact with all of the photosensitive drums. 
     Sheets are fed from a feeder portion, and then each of them is aligned by a pair of aligning rollers, to be output to the transfer belt. A fed sheet is conveyed to each of the photosensitive drums one after another, and thus toner images of these colors are transferred on the sheet one on another. 
     When a black and white printing mode is designated, the transfer belt is tilted in an opposite direction to the case of the full-color mode, with respect to the rotation-contact section between the transfer belt and the photosensitive drum for black, situated at the lowermost stream in the sheet conveying direction, so as to separate it from each of the photosensitive drums for yellow, magenta and cyan. Therefore, the sheet conveying power of the transfer belt decreases. Therefore, a charger is provided below a sheet introduction side of the transfer belt so as to charge the transfer belt electrostatically by the charger. By means of the electrostatic charge, each sheet is attracted to the transfer belt so that it can be conveyed surely to the photosensitive drum for black. 
     Further, a press roller which is also brought into rotation-contact with the transfer belt is provided above the sheet introduction side of the transfer belt. With this structure, in the case of the black and white printing mode, a sheet is pressed against the transfer belt by the press roller, so as to assure the sheet to be appropriately conveyed. 
     However, conventionally, in the black and white mode, when the both-side printing mode is designated, a sheet printed on one side is inversed and then guided to the aligning roller pair. During this operation, a straight sheet is given a curling shape, and thus it is easily distorted that way at the inversion site, and as a result, the sheet is not completely or entirely attached to the transfer belt, creating a portion partially floating from the belt. If a sheet partially floats from the belt, it is likely to have a problem of the sheet being stuck on the photosensitive drum, which causes the increasing of the occurrence rate of paper jam. 
     Meanwhile, an upper guide plate and a lower guide plate for guiding sheets are provided between the aligning roller pair and the press roller. Further, there is a difference between the feeding speed for sheets fed from feeder means and the conveying speed for sheets conveyed by the transfer belt, and due to this difference in speed, a sheet is deformed to have a warp. The warp deformation of the sheet is imparted within a space defined by the aligning roller pair, the press roller and the upper and lower guide plates. 
     However, with regard to a small-sized color printer or color copying machine, there is a limitation to the size of the device, and the distance between the aligning roller pair and the transfer belt cannot be designed wide. As a result, in the conventional cases, the press roller is situated always at the same position and projects in the space, regardless of the type of the printing mode, and the space is reduced in size, which easily causes the problem of paper jam. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention has been achieved in consideration of the above-described circumstances, and the object thereof is to provide an image forming apparatus capable of accurately conveying an image transfer medium material without having jamming of the image transfer medium material by varying the conveying mode of the image transfer section for the object material in accordance with the image forming state of the object material fed to the transfer section, and capable of having a design in which a large space can be prepared for the object material to warp, as well as such an image forming method. 
     According to the present invention, there is provided an image forming apparatus comprising a plurality of image forming means for forming developer images of different colors, transfer means for conveying an image transfer medium to the image carriers of the plurality of image forming means, and transferring the developer images formed by the plurality of developer image forming means on the image transfer medium, identification means for identifying developer image forming means necessary for forming a predetermined image, switch means for switching, in accordance with an identification result of the identification means, between a first contact state where the transfer means is brought into contact with a part of the plurality of developer image forming means and a second contact state where the transfer means is brought into contact with all of the plurality of developer image forming means, mode selection means for selecting either one of a first transfer mode for transferring an image on only one side of the image transfer medium, and a second transfer mode for transferring an image on both sides of the image transfer medium, and control means for controlling to set the contact state between the transfer means and the plurality of developer image forming means to the second contact state regardless of the identification result of the identification means in the case where the mode designated by the mode selection means is the second transfer mode. 
     Further, according to the present invention, there is provided an image forming apparatus comprising a plurality of image forming units for forming images on image carriers, a transfer unit for conveying an image transfer medium to the image carriers of the plurality of image forming units, and transferring the images on the image carriers on one surface side of the image transfer medium, a feeder unit for feeding the image transfer medium to the transfer unit, a contact-separation mechanism for bringing the transfer unit in contact with all of the plurality of image forming units in a multi-color image forming mode, and for bringing the transfer unit in contact with image carrier of some of the plurality of image forming units, in a monochrome image forming mode, a judgment unit for judging whether or not an image is formed on an other surface side of the image transfer medium fed to the feeder unit, and a control unit for controlling an operation of the contact-separation mechanism such as to bring the transfer unit into contact with all of the image carriers of the plurality of image forming units when the judgment unit judges that an image is formed on the other surface side of the image transfer medium in a single color image forming mode. 
     Furthermore, according to the present invention, there is provided an image forming method of an image forming apparatus having a plurality of image forming means for forming developer images of different colors, and transfer means for transferring the developer images formed by the plurality of developer image forming means on the image transfer medium, the method comprising a first identification step for identifying if a mode is for forming a multi-color image or a monochrome image, a second identification means for identifying a first transfer mode where an image is transferred on only one side of the image transfer medium or a second transfer mode where an image is transferred on both sides of the image transfer medium a switching step for switching, in accordance with identification results of the first and second identification means, contact states between the transfer means and the plurality of developer image forming means. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a schematic diagram showing the structure of the color electrophotographic copying device according to an embodiment of the present invention; 
     FIG. 2 is a diagram showing the state of the transfer belt in the color printing mode; 
     FIG. 3 is a diagram showing the state of the transfer belt in the black and white color printing mode; 
     FIG. 4 is a diagram showing an enlarged view of the state of conveying a sheet in the color printing mode; 
     FIG. 5 is a diagram showing an enlarged view of the state of conveying a sheet in the black and white printing mode; 
     FIG. 6 is a flowchart illustrating the printing operation; 
     FIG. 7 is a block diagram showing a drive control system which swings the transfer belt; 
     FIG. 8 is a diagram showing an enlarged view of a part of the color electrophotographic copying device according to another embodiment of the present invention; 
     FIG. 9 is a flowchart illustrating a printing operation; 
     FIG. 10 is a flowchart illustrating a printing operation; 
     FIG. 11 is a flowchart illustrating a printing operation; 
     FIG. 12 is a flowchart illustrating a printing operation; 
     FIG. 13 is a flowchart illustrating a printing operation; 
     FIG. 14 is a flowchart illustrating a printing operation; 
     FIG. 15 is a flowchart illustrating a contact-sway operation of the transfer belt, which is operated on the basis of the detection of whether or not an image is present on a rear surface of a sheet; 
     FIG. 16 is a flowchart illustrating a contact-sway operation of the transfer belt, which is operated on the basis of the detection of whether or not an image is present on a rear surface of a sheet; 
     FIG. 17 is a flowchart illustrating a contact-sway operation of the transfer belt, which is operated on the basis of the detection of whether or not an image is present on a rear surface of a sheet; 
     FIG. 18 is a diagram showing the second example of the contact-separation mechanism of the transfer belt; and 
     FIG. 19 is a diagram showing the operation of the contact-separation mechanism shown in FIG.  18 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will now be described in detail with reference to accompanying drawings. 
     FIG. 1 is a schematic diagram showing the structure of a full-color copying machine, as the color electrophotographic copying device according to an embodiment of the present invention. The full-color copying machine is of a 4-series tandem type in which a plurality of electrophotographic process units are arranged to oppose to the same transfer belt. 
     An original placement table  1  on which an original sheet is placed, is provided on an upper surface portion of the full-color copying machine, and a scanner  2  for reading image data on the original sheet placed on the original placement table  1  is provided in an lower section of the original placement table  1 . Underneath the scanner  2 , there is provided an image data processing circuit  3  for reading data from an external device such as a computer, and storing the data in an image memory, then processing the data. 
     Underneath the image data processing circuit  3 , an exposure device  4  such as a laser beam generating device for each color is provided. The exposure device  4  includes a laser beam generator, a polygon mirror  5  which reflects a beam generated by the laser generator, for scanning, an fθ lens  6  for correcting a focal point, and a reflection mirror  7  for reflecting a beam for scanning. 
     Meanwhile, a plurality of image forming units  41  which constitute an image forming section are provided along a sheet conveying direction, underneath the exposure device  4 . 
     Each of these plurality of image forming units  41  includes a respective photosensitive drum  8  serving as an image carrier, and a developer unit  9  is disposed to face the respective one of the photosensitive drums  8 . In a developer unit  9  of the image forming unit  41  which is situated at the uppermost stream side in the sheet conveying direction, a developer of yellow color (Y) is contained. Further, developer units  9  of the image forming units  41  which are arranged in the order towards the downstream side in the sheet conveying direction, toners of magenta color (M), cyan color (C) and black color (B) are respectively contained. A transfer belt  10  which constitutes a transfer unit is brought into rotation-contact with the lower sides of the photosensitive drums  8  of these image forming units  41 . The transfer belt  10  is crossed over between a driver roller  11   a  and a driven roller  11   b.    
     On the inner surface side of the transfer belt  10 , a transfer roller  13  for transferring a toner image on the photosensitive drums  8  onto a sheet, is provided. It should be noted that the photosensitive drums  8  and the transfer belt  10  move in a rotate-contact state at the same circumferential speed, and therefore a toner image can be transferred onto a sheet without being drifted or blurred. 
     An aligning mechanism unit  12  for aligning a sheet by setting its leading end to abut thereto, and then feeding the sheet to the transfer belt  10  is provided on the sheet introduction side of the transfer belt  10 . On the downstream side of the transfer belt  8  in the sheet conveying direction, a fixing unit  14  and a fed-out tray  15  for receiving a fed-out sheet are provided. 
     Underneath the transfer belt  8 , a both-side printing unit  16  is provided, and in this unit, in the case of the both-side printing mode, a sheet whose one side is printed by the fixing unit  14  is introduced via a gate, and inverted here to be guided again to the aligning mechanism unit  12 . 
     In order to form an image, for example, in the case of the color printing mode (note that in this mode, the transfer belt  10  is in a rotation-contact state with respect to all of the photosensitive drums  8  as will be described later), image data on an original sheet placed on the original placement table  1  is read by means of an image reading scanner  2 , or data is read from an external device such as a computer, and then the data is stored in the image memory. After that, the image data is processed by the image data processing circuit  3 . The image data proceed for each color in the image data processing circuit  3 , is sent to the exposure units  4  such as the laser beam generating units for those colors under the control of the image forming timing control circuit. In this manner, a beam is generated from the baser generator of each of the exposure units  4 . The beam is applied onto the polygon mirror  5 , where it is reflected, and as the polygon mirror  5  is rotated, scanning is performed. The scanning beam is focus-corrected by the fθ lens  6  and the optical path of the beam is changed by the reflection mirror  7 . In this manner, the beam is guided to each of the photosensitive drums  8 . 
     The circumferential surface of each of the photosensitive drums  8  is charged uniformly at a predetermined potential by a charger  42 , and as the beam scans on the charged circumferential surface, an electrostatic latent image is formed. The electrostatic latent images on these drums are sent to the developer units  9  as each of the photosensitive drums  8  rotates. Thus, toner is supplied from the developer units  9  to the latent images, and toner images of these colors are formed. 
     In the meantime, a sheet is fed from a paper feeder cassette  43  serving as a paper feeding unit, and the sheet is aligned by the aligning mechanism  12 , then sent out to the transfer belt  8 . The sheet sent to the transfer belt  8  is conveyed to the photosensitive drums  8 , where a toner image is transferred from each of the photosensitive drums  8  one after another, thus forming a color image on the sheet. The sheet on which the toner image has been transferred is guided to the fixing unit  14 , where the sheet is heated and pressed so as to fix the image thereon. After the fixing of the image, the sheet is fed out to the fed-out tray  15 . 
     In the case of the black and white printing mode (note that in this mode, the transfer belt  10  is in a rotation-contact state with respect to only the photosensitive drum  8  which is situated on the most downstream side in the sheet conveying direction as will be described later), a toner image of black color is formed on the photosensitive drum  8  which is situated on the most downstream side in the sheet conveying direction, and the toner image is transferred onto the sheet. The sheet on which the toner image has been transferred is guided to the fixing unit  14 , where the sheet is heated and pressed so as to fix the image thereon. After the fixing of the image, the sheet is fed out to the fed-out tray  15 . 
     Further, in either case of the color printing and black and white printing modes, when the both-side printing mode is selected, the sheet on one side of which the image has been fixed by the fixing unit  14  is introduced to the both-side printing unit  16  via a gate. The sheet is inverted by the both-side printing unit  16 , and sent back to the aligning mechanism  12 . After while, a toner image is transferred on the opposite surface of the sheet in a similar manner to the above. Then, after the transfer of the image, the toner image is fixed and the sheet is fed out. 
     FIGS. 2 and 3 are diagram showing enlarged views of a manual paper feeding unit  30 , an aligning roller mechanism  12 , a plurality of photosensitive drums  8 Y to  8 K and a transfer belt  10 . FIG. 22 shows the state of the transfer belt  10  in the color printing mode, and FIG. 23 shows the state of the transfer belt  10  in the black and white (monochrome) printing mode. 
     The aligning mechanism  12  has a pair of aligning rollers  31  which are in rotation-contact with each other, and to one of the aligning roller pair  31 , a driving motor which is not shown is coupled. Between the aligning roller pair  31  and the manual feeder unit  30 , upper and lower guide plates  32   a  and  32   b  on the feed-in side, are provided, whereas between the aligning roller pair  31  and the transfer belt  10 , upper and lower guide plates  33   a  and  33   b  on the feed-out side, are provided. At a position close to the feed-in side upper guide plate  32   a , an OHP sheet identifying sensor  34  is disposed. With this sensor, when a medium passing through the aligning mechanism  12  is an OHP sheet, it is detected so and a signal is outputted. 
     In the image forming apparatus of this embodiment, the designation unit  53  made of a control panel and the like, which will be later explained, is operated by the user so as to designate the color printing mode or black and white printing mode with use of the designation unit. On the basis of the mode designated by the user, the CPU  51 , which will be later explained, identifies image forming means which contains a developer necessary for forming the image. That is, when the color printing mode is designated, it is judged that the image must be formed with use of developers of yellow, magenta, cyan and black supplied to the respective photosensitive drums  8 Y,  8 M,  8 C and  8 K. On the other hand, when the black and white printing mode is designated, it is judged that the image must be formed using the developer of black onto the photosensitive drum  8 K only. The CPU  51  operates to move the transfer belt  10  on the basis of the designated mode shown in FIG. 2 or  3  so that a developer image of a desired color will be formed. Needles to say, it is also possible to automatically judge which of the color printing and black and white mode should be selected from the content of an original sheet read by a scanner or the like in pre-scanning, without user&#39;s instruction. 
     In the color printing mode shown in FIG. 2, the transfer belt  10  is brought into rotation-contact with all of the photosensitive drums  8 , and with this structure, a sheet fed from the aligning mechanism  12  is guided to the photosensitive drums  8  one after another. 
     In the black and white printing mode shown in FIG. 3, the transfer belt  10  is moved obliquely downwards with respect to the rotation contact point between the black photosensitive drum  8 K and the transfer belt  10 , taken as its fulcrum, and thus it is separated from the photosensitive drums  8 Y,  8 M and  8 C. In this state, a sheet fed from the aligning mechanism  12  is guided directly to the rotation contact point between the black-color photosensitive drum  8 K situated at the most downstream side in the sheet conveying direction, and the transfer belt  10 . 
     Underneath the sheet introducing side of the transfer belt  10 , a charger  35  for charging the transfer belt  10  with static electricity in advance in the black and white printing mode, is provided. Further, above the sheet introducing side of the transfer belt  10 , a pressing sheet  36  serving as pressing member, which is smooth over its entire surface and has an elasticity, is provided. The pressing sheet  36 , in both cases shown in FIG. 2 for the color printing mode and FIG. 3 for the black and white printing mode, is brought into rotation-contact with the transfer belt  10  elastically at all times, and in this manner, a sheet is pressed forcibly thereto. 
     Therefore, even in the state where the photosensitive drums  8 Y,  8 M and  8 C, other than the photosensitive drum  8 K for black color, are separated from the transfer belt  10 , the sheet is attached electrostatically to the transfer belt  10 , and therefore it can be surely conveyed. 
     It should be noted that the pressing sheet  36  is used in place of an attachment sheet conventionally used, and the space surrounded by the aligning roller pair  31 , the pressing sheet  36  and the upper and lower guide plates  33   a  and  33   b  becomes a space in which a sheet can warp. 
     With the above-described structure, the occupying space taken by the attachment roller protruding to the space for the warping of sheets, is reduced to about ¼ of the conventional cases by replacing the conventional attachment roller with the attachment sheet  36 . In this manner, the sheet warping space S is enlarged, and therefore the occurrence of sheet jamming can be prevented. 
     Further, the pressing sheet  36  is made of an elastic material and it elastically presses the sheet against the transfer belt  10 ; therefore the vibration of the sheet, which occurs when it separates therefrom, can be made less. Therefore, the image jitter deterioration can be effectively reduced. 
     It should be noted that the transfer belt  10  is designed to swing in the up and down direction with respect to the rotation contact portion between the transfer belt  10  itself and the photosensitive drum  8 K situated at the most downstream side in the sheet conveying direction by the driving mechanism  41  serving as a contact-separate mechanism. 
     The driving mechanism  41  has a mount frame  45  for mounting the transfer belt  10 , and one end side of the mount frame  45  is rotatably supported by a support shaft which is not shown. 
     On the other end side of the mount frame  45 , an ascending/descending mechanism  46  is provided. The ascending/descending mechanism  46  has an electromagnetic solenoid  47 , and the electromagnetic solenoid  47  has an actuator  47   a,  an upper end portion of which is connected to the mount frame  45 . The other end portion side of the mount frame  4  is urged upwards by a spring  48 . 
     When the electromagnetic solenoid  47  is demagnetized, the other end side of the mount frame  45  is lift up by the urging force of the spring  48 . Consequently, the mount frame  42  is pivoted upwards with respect to the support shaft as the center, and in this manner, the transfer belt  10  is brought into rotation-contact with all of the photosensitive drums  8 Y to  8 K. When the electromagnetic solenoid  45  is activated to be magnetized, the mount frame  42  is moved obliquely downwards with respect to the support shaft as the center against the urging force of the spring  48 . In this manner, the transfer belt  10  is separated from the other photosensitive drums  8 Y to  8 C than the photosensitive drum  8 K. 
     It should be noted here that the contact-separation operation of the transfer belt  10  is carried out with use of an electric motor  71  which rotates in clockwise and counterclockwise directions, in place of the electromagnetic solenoid  47 . 
     In this case, a screw gear  72  is mounted on the rotation shaft of the electric motor  71 , and a rack  73  is engaged with the screw gear  72 . With the rack  73 , a tooth section  75  formed in the lower end portion of a pivotal lever  74  is engaged. The pivotal lever  74  is designed to pivot with respect to the support shaft  76 . A press spring  77  is mounted to the pivotal lever  74 , and when the transfer belt  10  is pushed above with the pivotal lever  74  by means of the press spring  77 , such a state is elastically maintained. 
     When the electric motor  71  is rotated in the clockwise direction, the screw gear  72  is rotated such as to move the rack  73  in a direction indicated by arrow a. Accordingly, the pivotal lever  74  is pivoted in the clockwise direction, thus pushing the transfer belt  10  upwards. 
     When the electric motor  71  is rotated in the counter-clockwise direction, the screw gear  72  is rotated in an opposite direction such as to move the rack  73  in a direction indicated by arrow b. Accordingly, the pivotal lever  74  is pivoted in the counter-clockwise direction, thus pushing the transfer belt  10  downwards. 
     FIG. 7 is a block diagram showing the control system of the driving mechanism  41 . 
     As shown, to the electromagnetic solenoid  47  of the driving mechanism  47 , a CPU  51  serving as a control unit is connected via a control circuit, and a designation unit  53  provided in the control panel is connected to the CPU  51 . The designation unit  53  is designed to designate an image forming mode, for example, the color image forming mode or black and white image forming mode. 
     When the color image forming mode is designated with the designation unit  53 , the CPU  51  operates to demagnetize the electromagnetic solenoid  47  of the driving mechanism  41 , and when the black and white image forming mode is designated, the electromagnetic solenoid  47  is deactivated to be demagnetized. 
     When the black and white image forming mode is designated with the designation unit  53  and the both-side copy mode is designated, the CPU  51  operates to proceed the followings. That is, when an image has been copied on one side of a sheet, the electromagnetic solenoid  47  of the driving mechanism is demagnetized so as to bring the transfer belt  10  into contact with all of the photosensitive drums  8 Y to  8 K. 
     FIGS. 4 and 5 are diagrams showing enlarged views of the aligning mechanism  12  and the most upstream portion of the transfer belt  10  in the sheet conveying direction. 
     Further, FIG. 4 shows the state in which a sheet P is conveyed in the case where the color printing mode is designated, whereas FIG. 5 shows the state in which a sheet P is conveyed in the case where the black and white printing mode is designated. 
     In the color printing mode shown in FIG. 4, a sheet P fed out from the aligning roller pair  31  is elastically pressed onto the transfer belt  10  by the pressing sheet  35 . During this period, the charger  35  does not operates, and therefore the transfer belt  10  is not electrostatically charged. 
     Further, in the color printing mode, the transfer belt  10  is moved obliquely upwards to be brought into rotation-contact with all of the photosensitive drums  8 Y to  8 K. With this structure, the distance from the pressing site of the pressing sheet  36  to the rotation-contact point between the photosensitive drum  8 Y located at the most upstream side, and the transfer belt  10  is very small. Therefore, even if the elastic force of the pressing sheet  36  is weak and the pressing function becomes insufficient, the top end of the sheet P is not excessively curled and therefore the sheet is appropriately interposed smoothly at the rotation contact point, thus preventing the jamming of the paper. 
     In the black and white printing mode shown in FIG. 5, a sheet P fed out from the aligning roller pair  31  is elastically pressed onto the transfer belt  10  by the pressing sheet  35 . During this period, the charger  35  for attracting sheets, operates, and therefore the transfer belt  10  is electrostatically charged. Thus, the sheet P is conveyed by the transfer belt  10  while being tightly attached thereto, and an image is formed directly onto the sheet by the photosensitive drum  8 K located at the most downstream side, thus suppressing the possibility of the paper jamming. 
     Here, it should be noted that the both-side printing may be designated in the black and white printing mode. In this case, the sheet P on one side of which an image is printed as described above, is guided to the both-side printing unit  16  via the fixing device  14 . The sheet guided to the printing unit  16  is inverted and conveyed, then fed again to the transfer belt  10  via the aligning mechanism  12 . The sheet fed to the transfer belt  10  is conveyed to the black-color photosensitive drum  8 K, where an image is transferred onto the other surface side, thus finishing the both-side printing. In this both-side printing mode, each sheet is forcibly inverted in the both-side printing unit  16 , and therefore a curling deformation is imparted to each sheet. This curly deformation of the sheet is not straightened completely by the pressing force of the pressing sheet  36  while the sheet is passing through the aligning mechanism  12 , and therefore each sheet is guided to the transfer belt  10  and conveyed in such a state that upward wavy deformation partially remains in the sheet. When the sheet is conveyed in such a state that upward wavy deformation partially remains in the sheet, it is likely to be stuck between the third photosensitive drum  8 C and the transfer belt  10 , where the gap is narrowest, thus creating the problem of paper jamming. 
     Therefore, according to the present invention, in the case of the black and white printing mode, when the both-side printing mode is designated with the designation unit  53 , the transfer belt  10  is moved obliquely upwards to be brought into rotation-contact with all of the photosensitive drums  8 Y to  8 K as shown in FIG.  4 . With this structure, the distance from the pressing site of the pressing sheet  36  to the rotation-contact point between the photosensitive drum  8 Y located at the most upstream side, and the transfer belt  10  is very small. Therefore, even if the sheet is curly deformed and the pressing function of the pressing sheet  36  becomes insufficient, the sheet P is appropriately interposed smoothly at the rotation contact point between the drums and belt and conveyed by the photosensitive drums  8 Y to  8 K and the transfer belt  10 , thus preventing the jamming of the paper. 
     FIG. 6 shows the contact and separating motion of the transfer belt  10  with respect to the photosensitive drums  8 Y to  8 C, which is operated in compliance with the designation of a printing mode. 
     When a printing mode is designated by the designation unit  53  (step S 1 ), and then a start button (not shown) is turned on to start a printing (step S 2 ), it is detected by the CPU  51  whether or not the printing is in the black and white printing mode (step S 3 ). When it is judged to be a black and white printing, the electromagnetic solenoid  47  is excited, and therefore the transfer belt  10  is moved obliquely downwards to be separated from the photosensitive drums  8 Y to  8 C (step S 4 ). Subsequently, it is judged whether or not the printing is in the automatic both-side copy mode (step S 5 ), and when it is judged to be an automatic both-side copy, a sheet is conveyed to the photosensitive drum  8 K (step S 6 ), and a black-color image is printed on one side of the sheet (step S 7 ). After a while, the sheet is conveyed to the automatic both-side printing unit  16  (step S 8 ). Then, it is detected whether or not the printing is in the black and white printing mode (step S 9 ). When it is judged to be a black and white printing, the CPU  51  instructs that the electromagnetic solenoid  47  is excited, and therefore the transfer belt  10  is moved obliquely upwards to be brought into rotation-contact with all of the photosensitive drums  8 Y to  8 C (step S 10 ). The inverted sheet is sent to the photosensitive drum  8 K, with which a black and white image is printed (step  11 ). 
     Alternatively, when it is judged that the printing is not in a black and white printing mode, the electromagnetic solenoid  47  is demagnetized, and therefore the transfer belt  10  is moved obliquely upwards by the urging force of the spring  48  to be brought into rotation-contact with all of the photosensitive drums  8 Y to  8 C (step S 12 ). After a while, the operations from the step S 5  onwards are carried out. 
     Further, when it is judged in the step S 5  that the printing is not of an automatic both-side copying operation, the conveying path for the one-side printing is taken (step S 13 ), and a color image is printed on one side of the sheet (step S 14 ). 
     Further, when it is judged in the step S 9  that the it is not a black and white printing, the transfer belt  10  is brought into rotation-contact with all of the photosensitive drums (step S 15 ), and the operation of the step S 11  is carried out. 
     It should be noted here that the above-described switching operation for the transfer belt  10  is applied not only in the designation of the both-side printing in the black and white printing mode, but also in the case where it is forcibly selected manually. 
     As described above, in the case of a both-side copying mode in a single color, and when copying on one side is finished and the sheet is turned over to carry out copying on the other side, the transfer belt  10  is moved obliquely upwards to be brought into rotation-contact with all of the photosensitive drums  8 Y to  8 K. With this structure, the distance from the pressing site of the pressing sheet  36  to the rotation-contact point between the photosensitive drum  8 Y and the transfer belt  10  can be made very small. Therefore, the top end of the sheet P is not excessively curled and therefore the sheet is appropriately interposed smoothly at the rotation contact point, thus preventing the jamming of the paper. 
     Further, the pressing member for pressing a fed paper sheet P against the transfer belt  10  is made to have a sheet-like structure, and therefore the pressing member will not protrude into the space between the aligning roller pair  31  and the sheet introduction side of the transfer belt  10 . Consequently, the space is not narrowed by the pressing member, and therefore the warping of a sheet within the space is never disturbed, thus making it possible to certainly prevent the paper jam. 
     FIG. 8 shows another embodiment of the present invention. 
     Similar structural elements to those already described in the above embodiment will be designated by the same reference numerals and the explanations therefor will not be repeated. 
     In this embodiment, a detection sensor  61  which constitutes a judgment unit is provided at substantially a center position of the object material introducing side of the aligning roller pair  31  in its width direction. The detection sensor  61  is situated underneath an OHP sensor  34 , and is designed to detect an image formed on a rear surface side of an image-transfer object material. As the detection sensor  61 , for example, a reflection-type optical sensor for outputting an analog signal as to whether or not there is an image, or an imaging element (C-MOS type) for notifying with illumination of an LED element is used. 
     The detection sensor  61  monitors the image-transfer object material as to whether or not an image is formed on its rear surface side until the object material reaches the aligning roller  31 . 
     According to this embodiment, in the case of the one-side black and white printing mode, when it is detected by the detection sensor  61  that an image is formed on the rear surface side of the object material, the rotation of the aligning roller  31  is stopped, and at the same time, the entire paper feeding system is stopped, thus setting a re-start stand-by state. Further, the transfer belt  10  is moved obliquely upwards from this state, to be brought into contact with all of the photosensitive drums  8 Y to  8 K. After making contacts, the drive of the aligning roller  31  and the entire paper feeding system are re-started, and paper sheets are thus fed. 
     It should be noted here that unless it is confirmed that there is no image detected on the rear surfaces of fed sheets by a predetermined number of sheets in a row (for example, continuously 5 sheets), the transfer belt  10  is maintained in contact with all of the photosensitive drums  8 Y to  8 K, so as to continue the black and white printing. In this manner, the wasteful switching time for the contact and separation motion of the transfer belt  10 , which may occur, can be omitted for saving time. 
     FIGS. 9 to  14  are flowchart illustrating printing operations of this embodiment. 
     When a printing operation is started, the feeding of paper sheets is started (step S 21 ). Counting from the start of the printing operation, when the timer operates for a time of T 1  (step S 22 ), it is checked if the OHP sensor  34  detects an image-transfer object material or not (step S 23 ). When an object material is detected, an identification sequence which identifies if the object sheet is a paper sheet or a transparent sheet is activated (step S 24 ). After a while, it is judged whether it is black and white printing or not (step S 25 ). When it is judged to be the black and white printing, the rear surface confirmation sequence is activated (step S 26 ) as shown in FIG.  10 . Subsequently, it is detected as to whether or not there is an operation of a detection switch  62  for detecting a paper sheet immediately before the aligning roller  31  (step S 27 ). When the detection switch  62  is turned on, the timer operates for a time of T 2  (step S 28 ). After while, it is judged whether or not the timer activation time T 2  is larger than a predetermined aligning time (step S 29 ). When it is judged to be larger, the rotation of the resist aligning roller  31  is started (step S 30 ). Further, when the timer operates for a time of T 3  (step S 31 ), the image formation sequence is started (step S 32 ) as shown in FIG.  11 . Subsequently, it is detected as to whether or not the detection switch  62  for detecting a paper sheet immediately before the aligning roller  31  (step S 27 ) is turned on. When the detection switch  62  is turned off, the operation stands by for the next paper feeding sequence to be started (step S 34 ). 
     It the step S 23 , when the sensor output is judged to be off, then as shown in FIG. 12, it is judged whether or not the timer activation time T 1  is larger than a predetermined paper jam detection time (step S 35 ). When judged to be larger, it is considered that the feeding of paper sheets has been unsuccessful and the driving of the system is stopped (step S 36 ). Then, the jamming is treated (step S 37 ). In the step S 35 , the timer activation time T 1  is judged to be smaller than the predetermined paper jam detection time, the current status is maintained and continued without any treatment (step S 38 ). 
     It the step S 27 , when the sensor output is judged to be off, then as shown in FIG. 13, it is judged whether or not the timer activation time T 1  is larger than a predetermined paper jam detection time (step S 39 ). When judged to be larger, it is considered that the feeding of paper sheets has been unsuccessful and the driving of the system is stopped (step S 40 ). Then, the jamming is treated (step S 41 ). In the step S 39 , the timer activation time T 1  is judged to be smaller than the predetermined paper jam detection time, the current status is maintained and continued without any treatment (step S 42 ). 
     It the step S 33 , when the sensor output is judged to be on, then as shown in FIG. 14, it is judged whether or not the timer activation time T 3  is larger than a predetermined paper jam detection time (step S 43 ). When judged to be larger, it is considered that the feeding of paper sheets has been unsuccessful and the driving of the system is stopped (step S 44 ). Then, the jamming is treated (step S 45 ). In the step S 43 , the timer activation time T 3  is judged to be smaller than the predetermined paper jam detection time, the current status is maintained and continued without any treatment (step S 46 ). 
     FIG. 15 is a flowchart illustrating the operation (rear surface confirmation sequence) in accordance with the presence/absence of the image on the rear surface of a sheet. 
     When the rear surface confirmation sequence is started, it is judged whether or not there is an output from the detection sensor  61  (step S 51 ). When it is judged that there is a detection output from the detection sensor  61 , whether or not the transfer belt  10  is separated from the photosensitive drums is judged (step S 52 ). When it is judged that the transfer belt  10  is separated from the photosensitive drums, the driving of the paper feeding unit is stopped, and at the same time, the data on the state is preliminary stored (step S 53 ). Then, when the transfer belt  10  is moved obliquely upwards (step S 54 ), so as to be brought into contact with all of the photosensitive drums (step S 55 ), the paper feeding unit is re-started from the preliminary storage state (step S 56 ), the rear surface confirmation sequence is completed. 
     In the step S 51 , if the detection output from the detection sensor  61  is not confirmed, it is judged whether or not the transfer belt  10  is separated from the photosensitive drums (step S 56 ). When judged to be separated, the current status is maintained and continued (step S 57 ). When judged to be not separated, a sheet number counter counts up the number of sheet thus detected (step S 58 ). Then, it is judged whether or not the counted number N 1  is larger than 5 (step S 59 ). When the counted number N 1  is judged to be larger than  5 , as shown in FIG. 17, the driving of the entire paper feeding system is stopped, and the data on the state is preliminary stored (step S 60 ). Then, the sequence for separating the transfer belt  10  is started (step S 61 ). When the sequence for separating the transfer belt  10  from the photosensitive drums is finished (step S 62 ) the paper feeding unit is re-started from the preliminary storage state (step S 56 ), and the rear surface confirmation sequence is thus completed. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.