Abstract:
An image forming apparatus includes: a drum-shaped image carrier; a transfer and separation device which includes a conveyor belt for conveying a transfer material, a transfer roller for supporting the conveyor belt, and a nip regulating roller that is disposed downstream of the transfer roller in a conveyance direction of the transfer material; a transfer region changing section adapted to change a downstream end of the transfer region in a conveyance direction of the transfer material by changing a position of the nip regulating roller; and a transfer pressure changing section which changes a transfer pressure that corresponds to a pressure by the transfer roller against the image carrier, wherein the transfer pressure changing section changes the transfer pressure in correspondence with the change of the downstream end of the transfer region by the transfer region changing section.

Description:
RELATED APPLICATION 
     This application is based on Japanese Patent Application No. 2008-124525 filed on May 12, 2008 in Japanese Patent office, the entire content of which is hereby incorporated by reference. 
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to transfer and separation technique in an electrophotographic image forming apparatus. 
     2. Description of Related Art 
     In the electrophotographic image forming process, the toner image is transferred to a transfer material which is the recording paper, using an electrostatic force. 
     The main transfer methods include: the corona transfer method in which the back surface of the transfer material is corona charged and then transferring is performed; and a transfer method in which the transfer material is pressed onto an image carrier using conveyor belt or a roller to which transfer voltage has been applied. 
     The latter method is advantageous in that, transfer properties are good because transferring is done by bringing the transfer material into close contact with the image carrier and there is little image disturbance at the time of transfer. Thus, this method is now being frequently used in high quality image formation apparatuses. 
     Also, the transfer method in which the transfer material is brought into close contact with the image carrier by the conveyor belt has the advantage of stable conveyance because the transfer material is conveyed by the conveyor belt. 
     Image formation apparatuses in which transfer is done by bringing the transfer material in close contact with the image carrier using a conveyor belt are disclosed in Unexamined Japanese Patent Application Publication No. 10-10885, Unexamined Japanese Patent Application Publication No. 11-38796, and Unexamined Japanese Patent Application Publication No. 9-160324. 
     In the inventions disclosed in these publications, various measures, such as changing the contact width of the transfer material with the image carrier, have been proposed for improving transfer properties. 
     After transfer, the transfer material is separated from the image carrier, but in transfer using a conveyor belt, a conductive conveyor belt is used and the transfer material is separated from the image carrier by electrostatic suctioning of the transfer material onto the conveyor belt to which transfer voltage is being applied. 
     In the high speed process, it became clear that there was a problem with the conveyance of the transfer material in the separation section in which the transfer material is separated from the image carrier. 
     This problem is described using  FIG. 3  which is the figure for describing an embodiment of the present invention. 
     It is to be noted that the terms “upstream” and “downstream” in the present specification are used with reference to the conveyance direction of the transfer material. 
     The transfer of the toner image is carried out in a transfer region extending in the conveyance direction of the transfer material. The transfer region herein refers to the region where the transfer material is brought in close contact with the image carrier, and in  FIG. 3  it is a rectangular region in which the transfer material conveyance direction from the point AP which is the upstream contact start point to the point BP which is the downstream separation point is the short side, while the width direction which is the direction at right angles to the transfer material conveyance direction is the long side. 
     The transfer region is formed by the transfer roller  70  and the nip regulating roller  74  that is disposed downstream with respect to the transfer roller  70 . 
     The transfer material S separates from the image carrier at point BP which is the downstream end of the transfer region. 
     The conveyor belt  71  is bent at the point CP by the nip regulating roller  74 , but the transfer material S is held and conveyed by the conveyor belt  71  in the downstream section from the point CP and is conveyed smoothly on the conveyor belt  71 . 
     In the case where regular paper is used, the recording sheet S is conveyed smoothly on the conveyor belt  71  in the manner described above. 
     However, in the case where the transfer material S is thick paper, because rigidity is high, the transfer material S moves forward at point CP and separates from the conveyor belt  74 . 
     As a result, the upper surface of the transfer material that carries the toner image comes in contact with the conveyance guide and problems such as image disturbance or conveyance defects occur. 
     In the image formation apparatuses disclosed in Unexamined Japanese Patent Application Publication No. 10-10885, Unexamined Japanese Patent Application Publication No. 11-38796, and Unexamined Japanese Patent Application Publication No. 9-160324, transfer property has been improved but the aforementioned problem with separation has not been solved. 
     The object of the present invention is to supply an image forming apparatus which: solves the aforementioned problem which occurs in the separation section in which the transfer material separates from the image carrier; performs favorable transfer for various types of transfer material; and carries out smooth transfer of the transfer material in the separation section. 
     SUMMARY 
     The aforementioned objective is achieved by the following invention. 
     An image forming apparatus comprising: a drum-shaped image carrier; a transfer and separation device which comprises: a conveyor belt for conveying a transfer material; a transfer roller for supporting the conveyor belt; and a nip regulating roller that is disposed downstream of the transfer roller in a conveyance direction of the transfer material, wherein the transfer and separation device, nips the transfer material with the image carrier and the conveyor belt by pressing the conveyor belt and the transfer material against the image carrier with the transfer roller, and regulates a transfer region in which the transfer member is nipped by the image carrier and the conveyor belt with the nip regulating roller; and a transfer region changing section adapted to change a downstream end of the transfer region in a conveyance direction of the transfer material by changing a position of the nip regulating roller; wherein the transfer region changing section sets the nip regulating roller, at a first position at which the conveyor belt is supported such that a direction of separation of the conveyor belt from the image carrier at the downstream end of the transfer region coincides with a tangential direction with respect to the image carrier, or at a second position at which the downstream end of the transfer region is located upstream of the downstream end of the transfer region at a time when the nip regulating roller is set at the first position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the overall structure of the image forming apparatus according to an embodiment of the present invention. 
         FIG. 2  shows the transfer and separation device. 
         FIG. 3  is for describing the transfer and separation operation. 
         FIGS. 4   a  and  4   b  show the operation of the nip regulating roller. 
         FIGS. 5   a  and  5   b  show the mechanism which moves the nip regulating roller and changes the points. 
         FIG. 6  is a block diagram showing the control system of the image forming apparatus according to an embodiment of the present invention. 
         FIG. 7  is a timing chart for the operation of the transfer and separation device. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following is a description of the present invention based on an embodiment of the present invention, but the present invention is not limited to this embodiment. 
       FIG. 1  shows the overall structure of the image forming apparatus according to an embodiment of the present invention. The image forming apparatus of this embodiment comprises an automatic document feeding device  1  and inside the image forming apparatus main body, there are; an image reading device  2 ; an image forming section  3 ; a transfer material storage section  4 ; a sheet supply section  5 ; a reversal and sheet re-feeding section  6 , and a circulation and conveyance section  8 . The automatic document feeding device  1  feeds a document one sheet at a time and conveys the document to the image reading position and the document whose image has been read is subjected to discharge processing at a predetermined location. 
     The automatic document feeding device  1  comprises: a document mounting table  11 ; a document separating section  12  for separating the document mounted on the document mounting table  11 ; a document conveyance section  13  which includes multiple rollers for conveying the documents separated at the document separating section  12 ; a document discharge section  14  for discharging documents conveyed by the document conveying section  13 ; a document discharge table  15  for loading the document discharged by the document discharge section  14 ; and a document reversing section  16  made up of roller pairs for flipping the front and back of the document when reading images on both sides of the document. 
     The multiple sheets of document (not shown) loaded on the document loading table  11  are separated one by one, by the document separating section  12  and then conveyed towards the image reading position by the document conveying section  13 . 
     The document reading device is provided under the document conveying section  13  and slit  21  of image reading device  2  is used to read images on the document. 
     The document whose images have been read is discharged onto the document discharge table  15  by the document discharge section  14 . 
     The automatic document feeding device  1  is capable of document conveyance for reading both back and front of the document. 
     When the images on both back and front are to be read, the document for which images on one surface have been read is led to the document reversal section and the front and back of the document are reversed using reverse rotation control of the rollers when the rear end of the document is applied to the roller. Next, the image on the other surface is read at the document reading position by once again conveying the document using the document conveying section  13 . 
     The process is repeated for the number of document sheets loaded on the document loading table  11 . Further, the automatic document feeding device  1  is structured so as to be tiltable and when the automatic document feeding device  1  is raised and an upper side of the platen glass  22  is opened, the document can be loaded on the platen glass  22  and copying can be done. 
     The image reading device  2  is the section for reading images on the document and obtaining image data and comprises a first mirror unit  23  in which the slit  21 , the lamp  231  which is a light source for irradiating the document and a first mirror  232  which reflects light reflected from the document are integrally formed; a second mirror unit  24  in which a second mirror  241  and a third mirror  242  which reflects light from the first mirror  232  are integrally formed; an imaging lens  25  which forms images using light reflected from the second mirror unit  24  on the image capturing element (CCD hereinafter)  26  which will be described below; and a linear CCD  26  which photoelectrically converts the light image formed by the imaging lens  25  and generates image signals. 
     The image information that has been subjected to appropriate image processing for the image signals is temporarily stored in memory (not shown). 
     In the aspect where the document that is being sent by the automatic document feeding device  1  is read by the image reading device  2 , the first mirror unit  23  and the second mirror unit  24  are fixed at the positions shown in the figure. 
     On the other hand, in the aspect in which the images on the document loaded in the platen glass  22  are read, the first mirror unit  23  and the second mirror unit  24  are moved along the platen glass  22  while maintaining the light path length, to thereby read and scan the document. 
     The image forming section  3  forms images on the transfer material S based on image data obtained at the image reading device  2 . 
     The image forming section  3  is the section which forms images using electrophotographic processes, and it comprises: a photoreceptor drum  31  which is the drum-shaped image carrier; a charger  32  which uniformly charges the surface of the photoreceptor drum  31 , a laser writing system  33  which is an exposure section which operates based on the image data that has been subjected to image processing and is for forming latent images by exposing the surface of the photoreceptor drum  31 ; developing unit  34  which performs reverse development of the latent images formed on the photoreceptor drum  31  and thereby forms toner images; a transfer roller  70  for transferring the toner images onto the transfer material S; a cleaning section  37  for cleaning the photoreceptor drum  31  after the transfer process; a heat roller type fixing device  38  for fixing the toner images on the transfer material S; and a reversal discharge and re-feeding section  6  and circulation and conveyance section  8  which are described above. 
     In image formation done by the image formation section  3  having the structure described above, the photoreceptor drum  31  which is rotated in the direction of the arrow by an appropriate driving section (not shown) is uniformly charged in sequence by the charger  32  and then electrostatic latent images are formed by dot exposure using the laser writing system  33  and the toner images are formed using the developing device  34 . 
     Subsequently, images are formed on the transfer material S by transferring the toner images onto the transfer material S using the transfer electric field from the roller  70 . The images are formed so as to be superposed on the toner image region by synchronizing toner image formation with the timing of the sheet feeding operation associated with the start of rotation of the registration roller  56  which is the second sheet feeding section. 
     After this, the transfer material S to which the toner image has been transferred is separated from the photoreceptor drum  31  and the fixing process is carried out by the heating and pressing action of the fixing device  38  and the toner image is thereby fixed to the transfer material S. 
     On the other hand, the photoreceptor drum  31  which passes through the transfer region continues to rotate and is prepared for the next image formation after the residual toner is cleaned by the cleaning section  37 . 
     The transfer material storage section  4  comprises feeding trays  400 ,  410  and  420  which are vertically arranged and which comprise storage sections  405 ,  415 , and  425  which in turn are formed from storage containers for storing the transfer materials S in a stacked state. The storage sections  405 ,  415 , and  425  are integrally formed with the corresponding sheet feeding units  51 ,  52  and  53 . The feeding trays  400 ,  410  and  420  respectively store transfer materials S of different sizes. 
     For example, the setting may be such that, the sheet feeding tray  400  stores the standard size transfer materials S (letter size);  410  stores the A4 size transfer material S; and  420  stores the wide size transfer material S. All the transfer materials S are stored such that the short side is fed, or in other words such that the transfer material short side is aligned in the and conveyance direction. 
     The transfer materials S that are stored in these sheet feeding trays  400 ,  410  and  420  are not limited to regular paper, and recycled paper, coated paper, OHP sheets and the like may also be used. In addition, transfer material S of thickness varying from thin to thick may be stored. 
     The sheet feeding units  51 ,  52  and  53  are fixed in a loaded state in which the feeding trays are pushed into a predetermined location inside the device. The sheet feeding units  51 ,  52  and  53  comprise sheet feeding rollers  505 ,  535  and  555  which feed the transfer materials S one sheet at a time and separation rollers  506 ,  536  and  556  which are for preventing sheet overlap. The conveyance roller  55  is provided at the junction of the conveyance path of the transfer material fed from the transfer material storage section  4  and the circulation and conveyance section  8 . 
     The sheet feeding section  5  is a conveyance section for conveying the transfer material S from each of the sheet feeding trays to the image forming section  3 , and as shown in the figure, the sheet feeding section comprises multiple conveyance rollers such as  506 ,  536 ,  556  and the like, as well as conveyance guides. 
     The registration roller  56  of the sheet feeding section  5  conveys the transfer material S so as to synchronize with toner image formation on the photoreceptor drum  31 . 
     The reversal discharge and re-feeding section  6  re-feeds the transfer sheet on which images have been formed at the time of reversal paper discharge or image formation on the back surface. The reversal discharge and re-feeding section  6  has a switching section which switches the conveyance path according to whether the transfer material S that has been discharged by the fixing and discharging roller  61  is discharged as it is to the paper discharge tray  64 ; whether the transfer material S discharged after front and hack have been flipped; or whether the transfer material S is to be re-fed for image formation on its back surface. 
     In the case where the transfer material S on which image formation has been carried out is to be discharged as it is, or in other words, with the image surface of the transfer material S being at the upper side, the switching section  62  is positioned at the position shown by the broken line in the figure. Further, in the case where the front and back surfaces of the transfer material S on which images have been formed are flipped, the switching section  62  is set at the position shown by the solid line in the figure. In this case, the transfer material S that is to be conveyed by the fixing and discharge roller  61  is first conveyed to the circulation and conveying section  8  side and after the transfer material S passes the switching section  62 , it passes to the left of the switching section  62  and is discharged to the discharge tray  64  which is outside of the device. 
     In the case where image formation is carried out on the back surface of the transfer material S, the transfer material S that is conveyed by the fixing and discharge roller  61  is conveyed to the circulation and conveying section  8  by the conveyance rollers of the reversal discharge and re-feeding section  6  which is driven by the sheet discharge roller. Subsequently, a switchback is done and the front and back are flipped and then the transfer material S is circulated and conveyed to the registration roller  56  via the conveyance roller  55  at the junction. 
       FIG. 2  shows the transfer and separation device  7 . 
     The transfer and separation device  7  comprises: a transfer roller  70 ; a conveyor belt  71 ; rollers  72  and  73  which support the conveyor belt  71 ; a nip regulating roller  74 ; a tension roller  75  and a support frame  80  which supports all these rollers. 
     The transfer roller  70  and the conveyor belt  71  are made of high resistance rubber having a resistance approximately between 10 5  Ω·cm and 10 12  Ω·cm and a transfer voltage is applied to the transfer roller  70  by the power source E (refer to  FIG. 3 ). 
     The support frame  80  can rotate, as shown by the arrow V 3 , about the rotational axis of the roller  73 . 
     The support frame  80  is supported by the support frame  81  and is set at the position shown in the figure. 
     The support frame  81  can rotate as shown by the arrow V 2  about the shaft  81 A. 
     The left end of the support frame  81  is supported by a slidable operation lever  82 . The operation lever  82  can be rotated as shown by the arrow V 1  by being operated by an operator. 
     When the operation lever  82  is rotated by an operator as shown by the arrow V 1 , the support frame  81  rotates as shown by arrow V 2  and the support frame  80  rotates as shown by arrow V 3  due to this rotation of the support frame  81 . 
     The transfer roller  70  and the conveyor belt  71  separate from the photoreceptor drum  31  due to the rotation of the support frame  80 . 
     The operation, namely the operation in which the operation lever  82  is rotated as shown by the arrow V 1  opens up the transfer region vicinity and is performed for clearing paper jams and the like. 
     The transfer and separation operation in the transfer and separation device  7  will be described with reference to  FIG. 3 . 
     The transfer material S is pressed by the transfer roller  70 , via the conveyor belt  71  and brought into close contact with the photoreceptor drum  31 . A transfer electric field is formed in the transfer region where the transfer material S is in close contact with the photoreceptor drum  31 , and the toner image is transferred from the photoreceptor drum  31  to the transfer material S. The transfer region is the range from the point AP which is the point for start of contact at the upstream side for the transfer material conveyance direction, to the point BP which is point where the transfer material S separates from the photoreceptor drum  31 . 
     At the point BP, the transfer material S and the conveyor belt  71  move in a tangential direction of a tangential line which contacts with the surface of the photoreceptor drum  31  at point BP and then they separate from the photoreceptor drum  31 . 
     Transfer voltage is being applied to the transfer roller  70  by the power source E and thus the toner image on the photoreceptor drum  31  is transferred to the transfer material S in the transfer region. 
     The toner image is transferred from the photoreceptor drum  31  to the transfer material S in a transfer region having a predetermined width (Length AP−BP in the transfer material conveyance direction). 
     The nip regulating roller  74  regulates the width of the transfer region in which the transfer material S is brought in close contact with the photoreceptor drum  31  by adjusting the conveyor belt  71  to the photoreceptor drum  31 . 
     The transfer material S that has passed through the transfer region formed by the nip of the photoreceptor drum  31  and the transfer roller  70  separates from the receptor drum  31  at the point BP where the conveyor belt  71  separates from the receptor drum  31 . 
     At the point BP, resistance is high, but the transfer material S is electrostatically suctioned to the conductive conveyor belt  71  and so when the conveyor belt  71  separates from the photoreceptor drum  31 , the transfer material S also separates from the photoreceptor drum  31 . 
     The transfer material S that has been separated from the photoreceptor drum  31  is conveyed in a state where it is suctioned to the conveyor belt  71 , but the conveyor belt  71  is bent at the position of the nip regulating roller  74 . 
     The transfer material S is conveyed along the conveyor belt  71  which bends at the point CP. 
     A toner image is transferred to the transfer material S in the transfer and separation device using the operation described above, and the transfer material S to which an image has been transferred is separated from the photoreceptor drum  31  and conveyed to the fixing device  38  by the conveyor belt  71 . 
     The nip regulating roller  74  is displaced vertically in accordance with the type of paper. 
       FIG. 4   a  and  FIG. 4   b  show the operation of the nip regulating roller  74  and  FIG. 4   a  shows the first position of the nip regulating roller  74  in the first mode in which regular paper or thin paper is passed through and image formation is done, while  FIG. 4   b  shows the second position of the nip regulating roller  74  in the second mode in which thick paper is passed through and image formation is done. 
     In  FIG. 4   a , the nip regulating roller  74  is set at the raised first position and is at a position close to the surface of the photoreceptor drum  31  and the extent of the transfer region is large and the point BP is set at a downstream side position. 
     In the case where transfer material S formed from regular paper or thin paper is used, at the point BP where the conveyor belt  71  separates from the photoreceptor drum  31 , it is important that at point BP, the conveyor belt  71  moves in parallel with the tangential line drawn on the surface of the photoreceptor drum  31  and separates from the photoreceptor drum  31 . 
     When the conveyor belt  71  does not move parallel to the tangential line and separates from the photoreceptor drum  31 , in some cases the transfer material S is not suctioned to the conveyor belt  71  but rather is suctioned to the photoreceptor drum  31  and conveyed. 
     That is to say, in some cases, separation defects occur. 
     In order to prevent this type of separation defect, in the case where the transfer material S is regular paper or thin paper, the nip regulating roller  74  is set such that at point BP, the conveyor belt  71  moves parallel to the tangential line and separates from the photoreceptor drum  31 . 
     That is to say, the nip regulating roller  74  is set at the first position. 
     By setting the nip regulating roller  74  in this manner, separation of the transfer material S from the photoreceptor drum  31  can be ensured. 
     In the case where the images are formed on a transfer material S made from thick paper, the nip regulating roller  74  is set at a position that is further from the photoreceptor drum  31  than the first position, namely the second position in  FIG. 4   b.    
     In the state shown in  FIG. 4   b , there is little or no bending of the conveyor belt  71  by the nip regulating roller  74 . 
     In the state shown in  FIG. 4   a , when a transfer material S made from thick paper is conveyed, the transfer material S separates from the conveyor belt  71  without bending at the position of the nip regulating roller  74  due to its strong rigidity and is conveyed in the upper left direction. 
     As a result, the unfixed toner on the transfer material S contacts the conveyance guide and the like and is disturbed and this causes image disturbance. Furthermore, conveyance defects are generated. 
     By setting the nip regulating roller  74  at the second position shown in  FIG. 4   b , these types of problems are solved. 
     It is to be noted that in the example shown in the figure, in the case where thick paper is used (in the case of  FIG. 4   b ), there is little or no bending of the conveyor belt  71  at the position of the nip regulating roller  74 . However, in the case of thick paper, by changing the position of the nip regulating roller  74  with respect to the photoreceptor drum  31 , if the nip regulating roller  74  that is pressed to the conveyor belt  71  moves point BP in  FIG. 3  further to the upstream side than in the case of regular paper or thin paper, the image and conveyance defects generated in the case of thick paper can basically be prevented. 
     Therefore, even in the case where the transfer material S is formed from thick paper, the nip regulating roller  74  may be brought in contact with the conveyor belt  71 . That is to say, in the case where the transfer material is thick paper, by moving point BP further to the upstream side than in the case of regular paper or thin paper, the tangential direction of the photoreceptor drum  31  at the point BP becomes more horizontal and thus the conveyance direction of the transfer material S also becomes more horizontal. 
     The extent to which the BP in the case of thick paper is moved further to the upstream side than in the case of regular paper, can be suitably determined based on the structure of the transfer and separation device, the properties of the transfer material to be transferred and the like. 
       FIG. 5   a  and  FIG. 5   b  show the mechanism for moving the nip regulating roller  74  and changing the point BP. 
     The transfer roller  70 , the roller  72  and the nip regulating roller  74  are supported by the support frame  80 . 
     It is to be noted that although this is not shown in  FIG. 5   a  and  FIG. 5   b , as shown in  FIG. 2 , the roller  73  and the tension roller  75  are also supported by the support frame  80 . 
     The support frame  81  has a cam  84  which is rotatable. The support frame  81  has at its left end portion, a lever  85  that is rotatable about the bearing  85 A. The raise portion at the right end of the lever  85  has a roller  85 D and the roller  85 B is on top of the cam  84 . 
     The lower end of a coil spring  86  is on top of the lever  85  while the upper end presses the roller  87  that is provided on the support frame  80 . 
     The bearing  85 A on the support frame  81  has an operation member  88  which rotates integrally with the lever  85  and the operation member  88  presses a rotor  89  and thereby rotates. 
     The protrusion portion that protrudes in the horizontal direction of the rotor  89  displaces the nip regulating roller  74  vertically. The rotor  89  is urged in the anticlockwise direction by a spring which is not shown and the nip regulating roller  74  is urged upwards by the rotor  89  and pushes up the conveyor belt  71 . 
       83  is a hook that anchors the support frame  80  at a fixed upper limit position. 
     The transfer roller  70  is pressure-contacted with the photoreceptor drum  31  using the compression force of the coil spring  86 , but the upper limit is controlled by the hook  83 . 
       FIG. 5   a  shows the state of the transfer and separation device  7  in the case where the transfer material is regular paper or thin paper (first mode). 
     In  FIG. 5   a , the operation point of the cam  84  is at a low position. Therefore, the lever  85  is at a position where it rotates in the clockwise direction about the bearing  85 A, and the coil spring  86  is in a state of low compression. 
     The force with which the coil spring  86  presses the support frame  80  is therefore weak and the contact pressure of the transfer roller  70  on the photoreceptor drum  31  is small. That is to say, transfer pressure is low. 
     Also, the operation member  88  and the lever  85  are at a position where they have rotated in the clockwise direction. Therefore, the rotor  89  is not pressed by the operation member  88  and the nip regulating roller  74  is at the upper position as shown in  FIG. 4   a.    
     The driving section which is not shown may, for example, be placed in the state of  FIG. 5   b  (second mode) by the cam  84  being rotated in the clockwise direction by the motor. 
     In  FIG. 5   b , the lever  85  is rotated in the anticlockwise direction the roller  85   b  being pressed up. On the other hand, the support frame  80  is anchored by the hook  83  and thus the coil spring  86  is compressed. 
     As a result, the contact pressure of transfer roller  70  on the photoreceptor drum  31  will be increased. That is to say the transfer pressure becomes high. 
     In other words, the transfer of the toner image to the thick paper is performed with high transfer pressure. 
     The operation member  88  which is integral with the lever  85  is rotated in the anticlockwise direction, and when the rotor  89  is pressed, it rotates in the clockwise direction. 
     The nip regulating roller  74  is lowered by the rotation of the rotor  89  and is in the state shown in  FIG. 5   b  and thus the point BP moves to the upstream side. 
     The operation lever  82  is rotated when the conveyance path in the vicinity of the transfer position is to be opened for clearing jams and the like. 
     The support frame  81  is rotated as shown by the arrow V 2  about the bearing  81 A by operating the operation lever  82  which is rotated in the clockwise direction, or in other words, in the V 1  direction in  FIG. 2 . The support frame  80  rotates as shown by the arrow V 3  and the right end portion of the support frame  80  is lowered and the transfer roller  70  and the conveyor belt  71  are separated from the photoreceptor drum  31  by a large distance and thus the conveyance path in the vicinity of the transfer region is opened. 
     The position of the nip regulating roller  74  described above can be switched in accordance with the thickness of the transfer material. For example, switching such as that shown in Table 1 can be carried out. 
     
       
         
               
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                   
                 Basis weight 
               
             
          
           
               
                   
                   
                 100 g/m 2 - 
                 200 g/m 2 - 
               
               
                   
                 −100 g/m 2   
                 200 g/m 2   
                 300 g/m 2   
               
               
                   
               
             
          
           
               
                 Transfer 
                 Regular paper 
                 Nip regulating 
                 Nip 
               
               
                 material 
                 High quality 
                 roller First 
                 regulating 
               
               
                   
                 paper 
                 position 
                 roller 
               
               
                   
                 Coated paper 
                   
                 Second 
               
               
                   
                   
                   
                 position 
               
               
                   
               
             
          
         
       
     
     In Table 1, “Up” is the position shown in  FIG. 4   a  and “Down” is the position shown in  FIG. 4   b.    
       FIG. 6  is a block diagram showing the control system of the image forming apparatus according to an embodiment of the present invention. 
       FIG. 7  is a timing chart for the operation of the transfer and separation device. 
     The control section  100  is a control section for controlling the image forming step as well as transfer region changing section for changing the position of the nip regulating roller  74 . 
     The control section  100  is provided at the upstream portion of the registration roller  56  (refer to  FIG. 1 ) and it controls the operations shown in  FIG. 7  by controlling the motor MT 1  which drives the registration roller and the motor MT 2  which drives the cam  84  (refer to  FIG. 5 ) based on the detection signal from the transfer sensor SE. 
       101  is the paper supply tray and the information for the transfer materials used for image formation that are loaded in the sheet supply tray is output to the control section  100 . 
     In  FIG. 7 , A (registration roller) shows the operation of the registration roller  56  (refer to  FIG. 1 ), and the high portion indicates the rotation of the registration roller. In addition, the information inside the parentheses indicates the thickness of the transfer material conveyed by the registration roller  56 , and (thin paper) indicates regular paper or thin paper, while (thick paper) indicates thick paper. 
     B (transfer section passage) shows passage of the transfer material between the photoreceptor drum  31  and the transfer roller  70  and the high portion indicates transfer material passage. 
     D (nip regulating roller) indicates ON/OFF state of the motor MT 2  which drives the cam  84  in  FIG. 5  and the position of the nip regulating roller  74  is switched by turning ON the motor MT 2  once. “Nip roller up” indicates the state shown in  FIG. 5   a  and “Nip roller down” indicates the state shown in  FIG. 5   b.    
     E (point passage) indicates that the transfer material passes the point BP in  FIG. 3  and the high portions indicate passage of the transfer material. 
     As shown in the figure, the transfer material passes through the transfer section with the same timing that the transfer current turns ON and thus transfer is carried out. 
     In the case where the information from the sheet feeding tray  101  indicates that the transfer material has been changed from regular paper to thick paper, the timing Q 1  for the passage of the regular paper through the transfer section is used as a trigger and the motor MT 2  operates and the nip regulating roller  74  is switched from the upper to the lower (Q 2 ). 
     Next, startup (Q 3 ) of the registration roller  56  which feeds thick paper is carried out using as a trigger, the signal which indicates completion of the setting of the conversion operation which starts at Q 2 , namely, the thick paper conveyance state (state shown in  FIG. 4   b ) where the nip regulating roller  74  is lowered downward. 
     By controlling the position of the nip regulating roller  74  based on the transfer material detection signal from the sensor SE for each individual sheet of the transfer material that is continuously fed, the transfer and separation device  7  can be set to an ideal state without stopping the image formation process in order to change the point BP. 
     Thus, image formation can be performed in an ideal state without reducing image formation productivity. 
     As described above, in the embodiment of the present invention, the downstream end of the transfer region in which the transfer material separates from the image carrier is changed in accordance with the type of transfer material, and thus the conveyance direction of the transfer material at the point is changed in accordance with the type of transfer material. 
     Therefore, conveyance of the transfer material after separation is carried out smoothly and problems such as that of the unfixed toner on the transfer material contacting the conveyance guide and the like and generating conveyance defects are sufficiently prevented.