Patent Publication Number: US-2007120310-A1

Title: Sheet treatment apparatus

Description:
This application claims priority under 35 USC 119 from Japanese patent application 2005-339790, the disclosure of which is incorporated by reference herein.  
     BACKGROUND  
      (i) Technical Field  
      The present invention relates to a sheet treatment apparatus that treats a sheet.  
      (ii) Related Art  
      Conventionally, in some of pieces of sheet treatment apparatus typified by a printer and a copying machine, a booklet producing unit is added besides an image forming unit which forms an image on a sheet. The booklet producing unit produces a booklet from sheets on which the images are formed. In the booklet producing unit, the sheets are laid on top of one another and folded in order to produce the booklet, and the sheets which are simply folded have a bulge. Therefore, in some of pieces of sheet treatment apparatus, a square folding process is performed by the booklet producing unit in order to produce the high-quality. During the square folding process, on the sheets in the folded state, a neighborhood of a portion corresponding to a spine of the booklet is clamped by a clamping unit, and the portion corresponding to the back is pressed against a roller. The roller is moved along the portion corresponding to the back to press the portion, which results in the booklet whose back is flattened. In some of pieces of sheet treatment apparatus, the booklet producing unit cuts and aligns edges of the folded sheets. For cutting and aligning the edge, in a piece of apparatus is disclosed a technique of shifting an area where the image is formed on the sheet toward the center side of double spread in order to prevent marginal misalignment caused by cutting and aligning the edges.  
     SUMMARY  
      According to an aspect of the invention, there is provided a sheet treatment apparatus including an image forming unit that forms images in plural sheets; a booklet producing unit that produces a booklet by laying the plural sheets on top of one another and folding the plural sheets, the folded back of the booklet being pressed to flatten a spine of the booklet; a control unit that controls the image forming unit to form the image in an image forming area of the sheet in way that, the closer the sheet is located to a booklet cover side, the farther an edge of the image forming area which is close to a center of a double pages spread is located away from the center of the double pages spread. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Embodiments of the present invention will be described in detail based on the following figures, wherein:  
       FIG. 1  shows an image forming system as a first embodiment of the invention;  
       FIG. 2  shows an outline of an image forming apparatus of the image forming system shown in  FIG. 1 ;  
       FIG. 3  shows an outline of a function of a square folding device;  
       FIG. 4  is a block diagram showing an electric configuration of the image forming system shown in  FIG. 1 ;  
       FIG. 5  schematically shows a state in which folded sheets are flattened by a square folding device  9 ;  
       FIG. 6  is a table showing a forbidden distance on each sheet;  
       FIG. 7  schematically shows a state in which the folded sheets are flattened by the square folding device  9 ;  
       FIG. 8  is a table showing the forbidden distance on each sheet;  
       FIG. 9  shows an image forming area on a sheet;  
       FIG. 10  is a flowchart showing image formation and bookbinding process in the image forming system;  
       FIG. 11  shows an image forming area on a sheet in a second embodiment of the invention;  
       FIG. 12  shows an image forming area on a sheet in a third embodiment of the invention;  
       FIG. 13  shows an image forming area on a sheet in a fourth embodiment of the invention;  
       FIG. 14  schematically shows a state in which the folded sheets are flattened by a square folding device  9  in a fifth embodiment of the invention; and  
       FIG. 15  is a table showing the forbidden distance on each sheet in the fifth embodiment of the invention. 
    
    
     DETAILED DESCRIPTION  
      Exemplary embodiments of the sheet treatment apparatus according to an aspect of the invention will be described below with reference to the drawings.  
       FIG. 1  shows an image forming system as a first embodiment of the invention.  
      An image forming system  1  includes an image forming apparatus  2  and a post-treatment apparatus  5 . The image forming apparatus  2  forms the image on the sheet, and the post-treatment apparatus  5  performs the post-treatment to the sheet on which the image is formed by the image forming apparatus  2 .  
      The image forming apparatus  2  is one that forms the image on the sheet by an electrophotographic method to convey the sheet to the post-treatment apparatus  5 . The image forming apparatus  2  includes an operation panel  21  through which a user inputs information.  
       FIG. 2  shows an outline of an image forming apparatus of the image forming system shown in  FIG. 1 .  
      The image forming apparatus  2  includes an automatic original supply device  22 , an image input device  23 , an image output device  24 , and a sheet feeder  25 .  
      The automatic original supply device  22  supplies a loaded original to the image input device  23  one by one.  
      The image input device  23  includes an imaging unit  231 . The image input device  23  reads the original supplied from the automatic original supply device  22  and the image input device  23  outputs an image signal.  
      The image output device  24  includes a scanner  241 , a photosensitive drum  242 , a charging unit  243 , a development unit  244 , a transfer unit  245 , and a cleaner  246 . The charging unit  243  evenly charges the photosensitive drum  242 . The development unit  244  develops an electrostatic latent image into a toner image. The transfer unit  245  transfers the toner image to the sheet. The cleaner  246  recovers residual toner which is not transferred to the sheet. The image signal from the image input device  23  is converted into an exposure light signal by a laser output unit  241   a  included in the scanner  241 . The exposure light signal passes through a polygon mirror  241   b,  an fθ lens  241   c,  and a reflecting objective lens  241   d  to form the electrostatic latent image on the rotating cylindrical photosensitive drum  242  based on the original image. The scanner  241  starts exposure at predetermined exposure start timing, and the electrostatic latent image is formed by scanning the surface of the photosensitive drum  242  with the exposure light at a predetermined scanning speed. The image signal can also be inputted from a computer (see  FIG. 4 ) externally connected to the image forming apparatus  2 . The scanner  241  scans the surface of the photosensitive drum  242  with the exposure light in a rotating axis direction of the photosensitive drum  242 , for forming the electrostatic latent image. The electrostatic latent image is developed to obtain the toner image. The toner image is transferred to the sheet, and the cleaner  246  removes the residual toner on the photosensitive drum  242 . Then, the charging unit  243  charges the photosensitive drum  242 . The photosensitive drum  242  corresponds to an example of the image bearing body described in the invention, and the scanner  241  corresponds to an example of the exposure device described in the invention. On the other hand, when the transfer is finished, the sheet is delivered to the fixing device  247  to fix the toner image. Thus, the image is formed in the sheet. In forming the images on the plural sheets which constitute the booklet, the images are formed in the order from the side opposite to a booklet cover, i.e., in the order from the sheet constituting the inner-most side of the booklet.  
      The sheet feeder  25  includes a tray  258  ( 258   a,    258   b,  and  258   c ), conveyance rollers  259 , and adjustment rollers  250 . The sheets are loaded on the tray  258 . The conveyance rollers  259  and the adjustment rollers  250  convey the sheet loaded on the tray  258  along a conveyance path R through the image output device  24 . The adjustment rollers  250  convey the sheet to the transfer unit  245 . The conveyance roller  259  and the adjustment roller  250  correspond to an example of the sheet conveyance unit described in the invention.  
      Returning to  FIG. 1 , the description will be continued.  
      The post-treatment apparatus  5  includes the inserter  6 , a booklet finisher  7 , a trimming machine  8 , a square folding device  9 , and a stacker  10 . The sheets are stored in the inserter  6 , and the inserter  6  additionally inserts the sheet as a booklet cover. The booklet finisher  7  binds the sheets to perform saddle stitching, and the booklet finisher  7  folds the sheets. The trimming machine  8  cuts and aligns sides of the booklet. The square folding device  9  performs a booklet finishing process. The booklets are loaded on the stacker  10 . The post-treatment apparatus  5  corresponds to an example of the booklet producing unit described in the invention.  
      The booklet finisher  7  includes a sheet tray  71 , a pair of folding rollers  72 , and a push-out unit  73 . The sheets conveyed from the image forming apparatus  2  through the inserter  6  are bound on the sheet tray  71 . The sheets are laid on top of one another on the sheet tray  71  in the order from the side opposite to the booklet cover, i.e., in the order from the sheet constituting the inner-most side of the booklet. The push-out unit  73  is movably provided between the center of the sheet on the sheet tray  71  and the pair of folding rollers  72 . When the push-out unit  73  pushes out a central portion of the laid sheets to the position located between the pair of folding roller  72 , the sheets are folded into two. The folded sheets are conveyed to the square folding device  9  through the trimming machine  8  while a portion corresponding to a spine of the booklet is in the forefront. The trimming machine  8  includes a cutter  81  that cuts and aligns the sheet. The cutter  81  is provided while being able to be vertically moved. The cutter  81  is moved to cut and align sheet portions corresponding to an edge of the booklet.  
      The square folding device  9  includes a stop plate  94 , a clamping jaw  95 , a roller  96 , a punch  97 , and a carry-out roller  98 . The stop plate  94  positions the sheets. The clamping jaw  95  clamps the sheets positioned by the stop plate  94 . The roller  96  presses and flattens the sheet portions corresponding to the spine of the booklet. The punch  97  makes a hole in the booklet whose back is flattened. The carry-out roller  98  carries out the booklet to the stacker.  
       FIG. 3  shows an outline of a function of the square folding device.  
      The square folding device  9  will be described with reference to  FIGS. 1, 2 , and  3 .  
      The stop plate  94  can be moved between a position where the stop plate  94  closes a conveyance path of the folded sheet and a position where the stop plate  94  is retracted from the conveyance path. When sheets P 1  which are folded while laid on top of one another as shown in part (a) of  FIG. 3  is delivered to the square folding device  9 , the sheets p 1  are positioned by being abutted on the stop plate  94  located at the position where the stop plate  94  closes the sheet path as shown in part (b) of  FIG. 3 . The clamping jaw  95  clamps the sheets positioned by the stop plate  94  from the cover side, and the clamping jaw  95  holds the booklet while the roller  96  presses a portion P 1   a  of the sheets corresponding to the spine of the booklet. As shown in part (c) of  FIG. 3 , the roller  96  flattens the portion P 1   a  corresponding to the spine of the booklet by pressing the portion P 1   a  along the portion P 1   a  on the sheets clamped by the clamping jaw  95 . Thus, in the post-treatment apparatus  5 , by folding the plural sheets are folded while laid on top of one another, and pressing of the sheets, which produces a booklet B 1  whose back is flattened, as shown in part (d) of  FIG. 3 . The booklet B 1  whose back is flattened is carried out and loaded onto the stacker  10  by the carry-out roller  98 . As shown in part (b) of  FIG. 3 , because the stop plate  94  located at the position where the stop plate  94  closes the sheet conveyance path is arranged with a predetermined gap to the clamping jaw  95 , the sheets P 1  are clamped while the portion P 1   a  corresponding to the back is projected from the clamping jaw  95 . Because the projected portion is pressed by the roller  96 , a contraction amount of the back P 1   a  between the state in which the sheets P 1  are folded and the state in which the back P 1   a  is pressed depends on a width d of the gap between the stop plate  94  and the clamping jaw  95  as shown in part (b) of  FIG. 3 . The square folding device  9  controls the contraction amount of the back P 1   a  is controlled by changing the width d of the gap according to setting of intensity of the square folding process. Specifically, the width d of the gap is 2 mm in a case where the intensity of the square folding process is set at a strong level, and the width d of the gap is 1 mm in a case where the intensity of the square folding process is set at a weak level.  
       FIG. 4  is a block diagram showing an electric configuration of the image forming system shown in  FIG. 1 .  
      The image forming apparatus  2  includes a control circuit  26  that controls the operation of the whole of the image forming system  1 . The control circuit  26  has a central processing unit (CPU) (not shown) that controls the operation of the whole of the image forming system based on a program, ROM (not shown) in that the program and a table are stored, RAM (not shown) that temporarily provides a storage area to CPU, and an interface circuit (not shown) that relays a signal between CPU and the outside of the control circuit  26 . A personal computer  40  of a user is externally connected to the image forming apparatus  2 .  
      When an image forming instruction is transmitted to the control circuit  26  from the operation panel  21  or from the externally connected user&#39;s personal computer  40 , the control circuit  26  controls the automatic original supply device  22 , the image input device  23 , the image output device  24 , and the sheet feeder  25  of the image forming apparatus  2  and the control circuit  26  forms the image on the sheet. The control circuit  26  also controls the operations of the booklet finisher  7 , the trimming machine  8 , the square folding device  9 , and the stacker  10 . For example, the control circuit  26  controls the drive of each of the clamping jaw  95 , the stop plate  94 , the roller  96 , the punch  97 , and the carry-out roller  98 . The clamping jaw  95 , the stop plate  94 , the roller  96 , the punch  97 , and the carry-out roller  98  are incorporated in the square folding device  9 . The control circuit  26  corresponds to an example of the control unit described in the invention.  
      In the image forming system  1  of the first embodiment, the edge close to the center of the double spread in the image forming area is located far away from the center as the sheet is located closer to a booklet cover side by being folded the sheet with the booklet finisher  7 . The area where the booklet is flattened with square folding device  9  and the image forming area will be described below with reference to FIGS.  5  to  8 .  
       FIG. 5  schematically shows a state in which the folded sheets are flattened by the square folding device  9 . In an example shown in part (a) of  FIG. 5 , sheets P 2  are in the state in which 20 sheets P 201  to P 220  are folded while laid on top of one another. The sheets P 203  to P 218  on the sheets P 201  to P 220  will be omitted in  FIG. 5 . When the sheets P 2  are clamped by the clamping jaw  95 , a portion P 2   a  of the sheets P 2  corresponding to the back is projected from the clamping jaw  95  by a length equal to a width d 1  of the gap between the stop plate  94  and the clamping jaw  95  as shown in part (b) of  FIG. 3 .  FIG. 5  shows the state of the case in which the intensity of the square folding process is set at the strong level. In this case, the portion P 2   a  corresponding to the back is projected by the length d 1  of 2 mm. Then, as shown in part (b) of  FIG. 5 , the roller  96  flattens the portion P 2   a  corresponding to the back by pressing the portion P 2   a.  The contraction amount of portion P 2   a  corresponding to the back from the state shown in part (a) of  FIG. 5  in which the sheets P 2  are folded to the state shown in part (b) of  FIG. 5  in which the portion P 2   a  corresponding to the back is pressed is the length d 1  by which the portion P 2   a  corresponding to the back is projected. That is, the contraction amount of portion P 2   a  corresponding to the back is controlled according to the setting of the intensity of the square folding process. In the case where the contraction amount of portion P 2   a  corresponding to the back is the length d 1  of 2 mm, as shown in part (b) of  FIG. 5 , the portion P 2   a  corresponding to the back is flattened in the whole of the thickness of the sheets P 2 . When the image is formed in the flattened area, the crease is located in the image to disturb the image, or the image in the back portion is lost because the image in the back portion is hidden behind the back of the adjacent sheet. Accordingly, the flattened area becomes an area where the image formation is forbidden. At this point, sizes of the flattened area differ from one another for the sheets P 201  to P 220 . For example, on the sheet P 201 , a distance (hereinafter referred to as forbidden distance) M 101  to the edge from the center of the area where the image formation is forbidden is substantially equal to the thickness of 20 sheets of 0.087×20=1.74 mm, in the case where the folded sheets P 2  is formed by the 20 sheets which have a thickness of 0.087 mm each. On the sheet P 202  located inside the sheet P 201 , a forbidden distance M 102  becomes the thickness of the 19 sheets of 0.087×19=1.653 mm. Thus, the forbidden distance on each sheet can be computed from the number of sheets, the position of the sheet, and the thickness of the sheets constituting the booklet.  
       FIG. 6  is a table showing the forbidden distance on each sheet.  
       FIG. 6  shows the forbidden distances M 101  to M 120  of the sheets in the case where the 20 sheets which have a thickness of 0.087 mm each are folded while the intensity of the square folding process is set at the strong level. In the table of  FIG. 6 , a value corresponding to a field of “20th sheet” indicates the forbidden distance M 101 . “19th sheet”, “18th sheet”, and “first sheet” indicate M 102 , M 103 , . . . , and M 120  respectively. The forbidden distance also depends on the setting of the intensity of the square folding process.  
       FIG. 7  schematically shows a state in which the folded sheets are flattened by the square folding device  9 . As shown in part (a) of  FIG. 7 , sheets P 3  folded while laid on top of one another are clamped by the clamping jaw  95  of the square folding device  9 . In this case, the intensity of the square folding process is set at the weak level, and a width d 2  of the gap between the stop plate  94  shown in part (b) of  FIG. 3  and the clamping jaw  95  is 1 mm. Accordingly, a portion P 3   a  corresponding to the back of the sheets P 3  is projected from the clamping jaw  95  by d 2  of 1 mm. In this case, the contraction amount of portion P 3   a  corresponding to the back from the state shown in part (a) of  FIG. 7  in which the sheets P 3  are folded to the state shown in part (b) of  FIG. 7  in which the portion P 3   a  corresponding to the back is pressed is d 2  of 1 mm. At this point, of 20 sheets P 301  to P 320 , the ten sheets P 301  to P 310  from the cover side are pressed, and the sheets P 311  to P 320  located inside the sheets P 301  to P 310  are not pressed because the sheets P 311  to P 320  are not affected by the flattening by the roller  96 . As a result, as shown in part (b) of  FIG. 7 , the portion P 3   a  corresponding to the back is flattened by a half width of the thickness of the sheets P 3 . Therefore, a forbidden distance M 201  on the sheet P 301  on the outer-most side is substantially equal to the thickness of the ten sheets of 0.087×10=0.87 mm.  
       FIG. 8  is a table showing the forbidden distance on each sheet.  
       FIG. 8  shows the forbidden distances M 201  to M 220  of the sheets in the case where the 20 sheets which have a thickness of 0.087 mm each are folded while the intensity of the square folding process is set at the weak level. In the table of  FIG. 8 , the value corresponding to the field of “20th sheet” indicates the forbidden distance M 201 , and “19th sheet”, “18th sheet”, and “first sheet” indicate M 202 , M 203 , . . . , and M 220  respectively. In the table of  FIG. 8 , the reason why the forbidden distance M 211  corresponding to “10th sheet” to the forbidden distance M 220  corresponding to “first sheet” become 0 mm is that the sheets P 311  to P 320  corresponding to the “10th sheet” to “first sheet” are not pressed by the roller  96 .  
      Thus, in the state in which the portions P 2   a  and P 3   a  corresponding to the booklet back are pressed, the contraction amount of portions P 2   a  and P 3   a  depend on the width of the gap between the stop plate  12  and the clamping jaw  95 , and the forbidden distances M 101  to M 120  and M 201  to M 220  depend on the contraction amount of portions P 2   a  and P 3   a  corresponding to the booklet back.  
      As described above, the forbidden distance on each sheet is computed from the number of the sheets constituting the booklet, the position of the sheet, the thickness of the sheets, and the setting of the intensity of the square folding process.  
      A procedure of determining a distance from the center to the edge close to the center of the double spread in the image forming area, where the image is formed, according to the forbidden distance will be described below.  
       FIG. 9  shows the image forming area on the sheet.  
      Part (a) of  FIG. 9  shows the sheet P 220  located on the inner-most side as an example of the sheet. The sheet P 220  is located on the side opposite to the booklet cover side. Part (b) of  FIG. 9  shows the sheet P 201  located on the side closest to the booklet cover. The image output device  24  forms the image in a direction shown by an arrow Z on the sheets P 220  and P 201 . The conveyance roller  259  and adjustment roller  250  shown in  FIG. 2  convey the sheets P 220  and P 201  in the direction opposite to the direction shown by the arrow Z. The surface of each of the sheets P 220  and P 201  corresponds to two pages of the booklet because the surface is folded. Therefore, each two image forming areas PV 1  and PV 2  corresponding to the two pages of the booklet are arranged on each of the sheets P 220  and P 201 . A center C of the sheet becomes the center of the double spread of the booklet.  
      As shown in  FIG. 9 , in the image forming areas PV 1  and PV 2  where the images are formed on the sheet, the distance between the center C and edges PV 1   a  and PV 2   a  close to the center becomes a distance in which a predetermined marginal width m is added to the width between the center of the area where the image formation is forbidden and the center C. For example, as shown in part (a) of  FIG. 9 , on the sheet P 220  located inside on the side opposite to the booklet cover side, the distance between the center C of the sheet P 220  and the edges PV 1   a  and PV 2   a  becomes the distance in which the marginal width m is added to the forbidden distance M 120 . As shown in part (b) of  FIG. 9 , on the sheet P 201  located on the booklet cover side, the distance between the center C of the sheet P 201  and the edges PV 1   a  and PV 2   a  becomes the distance in which the marginal width m is added to the forbidden distance M 101 .  
      As described above, the forbidden distance is increased as the sheet is located closer to the booklet cover side when the sheet is folded. Therefore, the edges PV 1   a  and PV 2   a  close to the center of the double spread are located farther away from the center C of the sheet as the sheet is located closer to the booklet cover side.  
      The forbidden distance depends on the setting of the intensity of the square folding process. The contraction amount of the back also depends on the setting of the intensity of the square folding process. Accordingly, the edges PV 1   a  and PV 2   a  close to the center of the double spread of the image forming areas PV 1  and PV 2  is located according to the contraction amount of the back, because the edges PV 1   a  and PV 2   a  are determined according to the forbidden distance.  
      In the first embodiment, the positions of the edges PV 1   a  and PV 2   a  of the image forming areas PV 1  and PV 2  on the sheet are controlled by exposure start timing at which the scanner  241  scans the surface of the photosensitive drum  242  in the rotating axis direction of the photosensitive drum  242  with exposure light.  FIG. 9  shows the image forming area on the sheet. At this point, the image on the sheet is one in which the toner image obtained in the surface of the photosensitive drum  242  is transferred and fixed. The toner image is obtained when the electrostatic latent image that is formed by scanning the surface of the rotating photosensitive drum  242  with the exposure light of the scanner  241  is developed. Therefore,  FIG. 9  shows both the image forming area on the sheet and the state in which the electrostatic latent image formed by scanning the surface of the rotating photosensitive drum  242  with the exposure light using the scanner  241  is spread out in a plane. In this case, the direction shown by an arrow Y corresponds to the direction in which the scanner  241  scans the surface of the rotating photosensitive drum  242  in the rotating axis direction of the photosensitive drum  242  with the exposure light. On the other hand, as time advances, the electrostatic latent image is formed in the direction shown by the arrow Z by the rotation of the photosensitive drum  242 . That is, the arrow Z of  FIG. 9  also indicates a time axis direction when the scanner  241  forms the electrostatic latent image on the surface of the photosensitive drum  242 .  
      The control circuit  26  (see  FIG. 4 ) sets the exposure start timing in the scanner  241  such that the distance from the center C of the sheet P 220  to the edges PV 1   a  and PV 2   a  close to the sheet center of the image forming areas PV 1  and PV 2  becomes the distance in which the predetermined marginal width m is added to the forbidden distance. In the case where the direction shown by the arrow Z of  FIG. 9  is the time axis direction, exposure start timing ta 1  of the electrostatic latent image for forming the image in the image forming area PV 1  of the sheet P 220  shown in part (a) of  FIG. 9  is set earlier than exposure start timing ta 0  by a time period while the surface of the photosensitive drum  242  is moved by the distance of the forbidden distance M 120 . The exposure start timing ta 0  is a reference in the case where the flattening is not performed by the square folding device  9 . The exposure start timing ta 1  is determined from the computation of a circumferential velocity of the photosensitive drum  242 . The start of the exposure at the exposure start timing ta 1  earlier than the exposure start timing ta 0  makes it earlier that the exposure of the electrostatic latent image is ended with respect to the image in the image forming area PV 1 . As a result, the distance from the center C of the sheet P 220  to the edge PV 1   a  close to the center of the image forming area PV 1  becomes the distance in which the marginal width m is added to the forbidden distance M 120 , after the electrostatic latent image formed on the surface of the photosensitive drum  242  is developed to be transferred to the sheet P 220 . On the other hand, the exposure start timing tb 1  of the electrostatic latent image for forming the image in the image forming area PV 2  of the sheet P 220  is set later than the exposure start timing ta 0  by the time period while the surface of the photosensitive drum  242  is moved by the distance of the forbidden distance M 120 , and the exposure is started at the later exposure start timing tb 1 . The exposure start timing ta 0  is the reference in the case where the flattening is not performed by the square folding device  9 . Therefore, the distance from the center C of the sheet P 220  to the edge PV 2   a  close to the center of the image forming area PV 2  becomes the distance in which the marginal width m is added to the forbidden distance M 120 , after the electrostatic latent image formed in the surface of the photosensitive drum  242  is developed to be transferred the toner image to the sheet P 220 . For other sheets P 202  to P 219 , the exposure start timing ta 1  or tb 1  is made earlier or later than the exposure start timing ta 0  or tb 0  according to the forbidden distances M 102  to M 119  respectively. Therefore, the distance from the center C to the edge PV 1   a  close to the center of the image forming area PV 1  and the distance from the center C to the edge PV 2   a  close to the center of the image forming area PV 2  become the distance in which the marginal width m is added to each of the forbidden distances M 102  to M 119 . Accordingly, the start of the formation of the electrostatic latent image is controlled by the exposure start timing depending on the sheet position in the booklet. Thus, the position of the edge close to the center of the image forming area can be precisely controlled.  
      The image formation and bookbinding process in the image forming system  1  will be described below.  
       FIG. 10  is a flowchart showing the image formation and bookbinding process in the image forming system.  
      On the basis of a program, the control circuit  26  shown in  FIG. 4  controls the operation of each unit to realize the image formation and bookbinding process shown in  FIG. 10 . The flow chart of  FIG. 10  will be described with reference to  FIG. 4 .  
      In the image formation and bookbinding process, first the control circuit  26  performs a sheet information obtaining process (Step S 11 ). The control circuit  26  causes the operation panel  21  to display a message for encouraging a user to input information on the thickness for each size of the sheets having different sizes stored in the tray  258  and the inserter  6 . When the user operates the operation panel  21  to input the information on the thickness, the inputted information is supplied to the control circuit  26 .  
      Then, the control circuit  26  performs a bookbinding condition setting process (Step S 12 ). The control circuit  26  causes the operation panel  21  to display the message that encourages the user to input the pieces of information on the number of sheets constituting the booklet, the size of the sheet used, and the intensity of the square folding process. When the user operates the operation panel  21  to input the pieces of information on the number of sheets constituting the booklet, the size of the sheet used, and the intensity of the square folding process, the inputted pieces of information are supplied to the control circuit  26  and a job for producing the booklet is instructed. At this point, the control circuit  26  obtains information on the number of sheets constituting the booklet from the user. The control circuit  26  also obtains sheet thickness information on the sheet thickness corresponding to the sheet size used. The tray in that the corresponding sheets are stored is selected according to the information on the sheet size.  
      The control circuit  26  performs an exposure start timing computing process (Step S 13 ). The exposure start timing is timing at which the scanner  241  starts the scanning of the surface of the photosensitive drum  242  with the exposure light, and the exposure start timing depends on the sheet constituting the booklet. The control circuit  26  computes the forbidden distance for each sheet position in the booklet by using the number of sheets indicated by the information on the number of sheets, the sheet thickness indicated by the information on the sheet thickness, and the setting of the intensity of the square folding process that has an influence on the contraction amount of back. Then, the control unit  26  computes the exposure start timing according to the computed forbidden distance. For each sheet, the two pieces of exposure start timing ta 1  and tb 1  are computed corresponding to the image forming areas PV 1  and PV 2  arranged on each sheet.  
      The control circuit  26  performs an exposure timing setting process (Step S 14 ). In the exposure timing setting process, according to the sheet position in the booklet where the next image is formed, i.e., according to the order of the sheet from the booklet cover, the control circuit  26  selects and sets the exposure start timing from the pieces of exposure start timing determined for each sheet in Step S 13 .  
      Then, the control circuit  26  performs an image forming process (Step S 15 ). The control circuit controls each of the devices of the image forming apparatus  2  to form the image on one sheet. For example, while the control circuit  26  causes the photosensitive drum  242  to rotate at a constant speed, the control circuit  26  causes the scanner  241  to start the formation of the electrostatic latent image on the photosensitive drum  242  at the timing set in Step S 14 . The control circuit  26  also causes the conveyance roller  259  and adjustment roller  250  to convey the sheet at predetermined conveyance start timing.  
      When the image formation for a sheet is finished to one sheet, the control circuit  26  determines whether or not a counter value of the number of sheets becomes the number of sheets constituting the booklet (Step S 16 ). When the control circuit  26  determines that the counter value becomes the number of sheets constituting the booklet, the control circuit performs the bookbinding process in Step S 17 . When the control circuit  26  determines that the counter value does not reach the number of sheets constituting the booklet, the control circuit forms the images in the image forming areas according to the positions of the sheet edges in the booklet for all the sheets constituting the booklet by repeating the processes from Step S 14 .  
      In Step S 17 , the bookbinding process is performed. Specifically, the booklet finisher  7  binds the sheets. Then, the booklet finisher  7  performs saddle stitching while the sheets are laid on top of one another, and the booklet finisher  7  folds the sheets. The trimming machine  8  cuts and aligns the side of the folded sheets corresponding to the edge of the booklet, and the square folding device  9  flattens the portion corresponding to the spine of the booklet according to the setting of the intensity of the square folding process. This enables the production of the booklet whose back is flattened.  
      Then, the control circuit  26  performs determination of the number of booklets (Step S 18 ). The control circuit  26  determines whether or not the number of bound booklets reaches the specified number of booklets. When the control circuit  26  determines that the number of bound booklets does not reach the specified number of booklets, the control circuit continues the production of the booklet by repeating the processes from Step S 14 . When the control circuit  26  determines that the number of bound booklets reaches the specified number of booklets, the image formation and bookbinding process is ended.  
      According to the first embodiment, as shown in  FIG. 9 , in image forming areas PV 1  and PV 2  where the images are formed, the edges PV 1   a  and PV 1   b  close to the center of the double spread are separated away from the center, and the images are formed in the areas separated away from the flattened portion P 2   a  corresponding to the spine of the booklet. The flattened area indicated by each of the forbidden distances M 101  to M 120  is enlarged as each of the sheets P 201  to P 220  is located closer to the booklet cover side. However, according to the first embodiment, the edges PV 1   a  and PV 1   b  of the image forming areas are located farther away from the center as the sheet is located closer to the booklet cover side, so that the image is properly formed while separated away from the flattened portion according to the sheet position in the booklet. Accordingly, the image disturbance by the flattening is prevented in the back portion of the booklet and the lack of the image is also prevented in the crease, so that the booklet quality can be improved.  
      Because the edges PV 1   a  and PV 1   b  of the image forming areas are located farther away from the center as the sheet is located closer to the booklet cover side, the image positions are aligned with respect to the center of the double spread when the booklet is opened. Therefore, the booklet has the high quality.  
      The width of the flattened portion depends on the contraction amount of the pressed back. The edge close to the center of the image forming area is located at the distance according to the setting of the intensity of the square folding process. Therefore, the edge is located at the distance according to the contraction amount of back, which enables the image to be properly formed in the area according to a degree in which the spine of the booklet is flattened.  
      The edges PV 1   a  and PV 1   b  of the image forming areas where the images are formed are located at the distance according to the number of sheets constituting the booklet and the sheet thickness, accordingly the image can be properly formed in the area according to the booklet thickness which depends on the number of sheets.  
      In the first embodiment, the photosensitive drum is described as an example of the image bearing body. However, a photosensitive belt may be used as image bearing body. Even in the case where the image forming apparatus includes the exposure devices of at least four colors of Y (yellow), M (magenta), C (cyan), and K (black), the plural exposure devices may similarly be controlled.  
     Second Embodiment  
      In the first embodiment, the timing at which the formation of the electrostatic latent image is started is controlled at the computed exposure start timing by the scanner  241 . However, the change in position of the edge PV 1   a  of the image forming area is not limited only to the control of the start timing of the scanner  241 . Then, a second embodiment of the invention will be described. In the second embodiment, the sheet conveyance start timing is simultaneously controlled by an adjustment roller  250 . In the following description of the second embodiment, the same component is designated by the same numeral as the first embodiment, and a difference between the first embodiment and the second embodiment will be described below.  
       FIG. 11  shows an image forming area on the sheet in the second embodiment of the invention.  
      Part (a) of  FIG. 11  shows the sheet P 220  located on the inner-most side as an example of the sheet. The sheet P 220  is located on the side opposite to the booklet cover side. Part (b) of  FIG. 9  shows the sheet P 201  located on the side closest to the booklet cover. The sheets P 220  and P 201  are conveyed in the direction opposite to the direction shown by the arrow Z with the conveyance roller  259  and the adjustment roller  250  as shown in  FIG. 2 .  
      In the second embodiment, the edge close to the center of the double spread is located farther away from the center C as the sheet is located closer to the booklet cover side. Therefore, in the adjustment roller  250 , the conveyance start timing is adjusted for each sheet.  
      In the second embodiment, the control circuit  26  (see  FIG. 4 ) sets the conveyance start timing in the adjustment roller  250  such that the distance from the center C of the sheet P 220  to the edge PV 1   a  close to the sheet center of the image forming area PV 1  becomes the distance in which the predetermined marginal width m is added to the forbidden distance. That is, in the case where the direction shown by the arrow Z of  FIG. 11  is set to the time axis, conveyance start timing tc 1  at which the adjustment roller  250  starts the conveyance of the sheet P 220  shown in part (a) of  FIG. 11  is set later than the conveyance start timing tc 0  by a time period while the surface of the photosensitive drum  242  is moved by the distance of the forbidden distance M 120 . The conveyance start timing tc 0  is the timing in the case where the flattening is not performed by the square folding device  9 . The conveyance start timing tc 1  is determined by the computation from the circumferential speed of the photosensitive drum  242 . The sheet conveyance is started at the later conveyance start timing tc 1 , which relatively makes it earlier that the transfer of the toner image to the sheet is completed in the image forming area PV 1 . Therefore, with the image which is transferred and fixed to the sheet P 220 , the distance from the center C of the sheet P 220  to the edge PV 1   a  close to the center of the image forming area PV 1  becomes the distance in which the marginal width m is added to the forbidden distance M 120 .  
      In the second embodiment, through the processes in Steps S 13  and S 14  of  FIG. 10 , the control circuit  26  computes and sets the conveyance start timing along with the exposure start timing.  
      Thus, because the start of the sheet conveyance is controlled by the conveyance start timing determined based on the sheet position in the booklet, similarly by the start timing control of the electrostatic latent image formation in the first embodiment, the position of the edge close to the center of the image forming area can precisely be controlled. In the second embodiment, because the sheet conveyance start timing is changed, similarly to the image forming area PV 1 , the position movement occurs in the image forming area PV 2 . For the image forming area PV 2 , in consideration of the delay of the sheet conveyance start timing, the exposure start timing tb 1  for forming the electrostatic latent image is further delayed.  
      In the second embodiment, the conveyance start timing is adjusted by the adjustment roller. Alternatively, a timing adjustment sensor is provided at a position of the adjustment roller, the sheet is not temporarily stopped at the adjustment roller, and the sheet is accelerated or decelerated in a period during which the image forming area PV 1  reaches the photosensitive drum since the sensor detects a front end of the sheet. This may enable the control of the time when the image forming area PV 1  reaches the photosensitive drum to adjust the exposure start timing. Similarly in the image forming area PV 1 , the position movement occurs in the image forming area PV 2  because the sheet conveyance start timing is changed. However, the time when the image forming area PV 2  reaches the photosensitive drum may be controlled and then the exposure start timing from the center C is adjusted through accelerating or decelerating the sheet in the period from when the image forming area PV 1  passes through the photosensitive drum to when the image forming area PV 2  reaches the photosensitive drum.  
     Third Embodiment  
      In the above embodiments, the photosensitive drum  242  forms the electrostatic latent image at a constant speed. That is, the speed at which the scanner  241  scans the surface of the photosensitive drum  242  in the rotating axis direction of the photosensitive drum  242  with the exposure light is kept constant. However, the position of the edge PV 1   a  of the image forming area can also be changed through changing the scanning speed of the scanner  241 . Then, a third embodiment of the invention will be described. In the third embodiment, the scanner  241  controls the scanning speed. In the following description of the third embodiment, the same component is designated by the same numeral as the above embodiments, and the difference between the third embodiment and the above embodiments will be described below.  
       FIG. 12  shows the image forming area on the sheet in the third embodiment of the invention.  
      Part (a) of  FIG. 12  shows the sheet P 220  located on the inner-most side as an example of the sheet. The sheet P 220  is located on the side opposite to the booklet cover side. Part (b) of  FIG. 12  shows the sheet P 201  located on the side closest to the booklet cover. The image output device  24  forms the image in the direction shown by the arrow Z on the sheets P 220  and P 201 .  
      In the third embodiment, the speed at which the scanner  241  scans the surface of the photosensitive drum  242  in the rotating axis direction of the photosensitive drum  242  with the exposure light is increased as the sheet is located closer to the booklet cover side.  
      In the third embodiment, the control circuit  26  (see  FIG. 4 ) sets the scanning speed of the scanner  241  such that the distance from the center C of the sheet P 220  to the edge PV 1   a  close to the sheet center of the image forming area PV 1  becomes the distance in which the predetermined marginal width m is added to the forbidden distance. That is, in the case where the direction shown by the arrow Z of  FIG. 12  is set to the time axis, the scanning speed of the scanner  241  is increased such that the scanning is ended earlier by the time period while the surface of the photosensitive drum  242  moves over the distance of the forbidden distance M 120 . The accelerated speed is determined by the computation from the circumferential speed of the photosensitive drum  242 . The scanner  241  scans the surface of the photosensitive drum  242  at the high scanning speed, which makes it earlier that the exposure of the electrostatic latent image is ended for the image of the image forming area PV 1 , while the exposure start timing or the sheet conveyance start timing is kept constant. That is, the width in the direction of the end of the double spread from the center of the double spread of the to be formed becomes shortened. Therefore, in the image which is transferred and fixed to the sheet P 220 , the distance from the center C of the sheet P 220  to the edge PV 1   a  close to the center of the image forming area PV 1  becomes the distance in which the marginal width m is added to the forbidden distance M 120 .  
      Thus, because the scanner  241  is controlled by the scanning speed determined based on the sheet position in the booklet, similarly by the start timing control of the electrostatic latent image formation in the first embodiment, the position of the edge close to the center of the image forming area can precisely be controlled. Further, in accordance with the third embodiment, because the width in the direction of the end of the double spread from the center of the double spread of the to be formed becomes shortened according to the change in position of the edge close to the center, the position of the edge close to the double spread end of the image forming area is kept constant for all the sheets.  
      In the third embodiment, because the scanning speed of the scanner  241  is changed, not only the width in the direction of the end of the double spread from the center of the double spread of the formed image but also the width in the direction of the spine of the booklet become shortened. In the third embodiment, the scanning speed of the scanner  241  is changed. Therefore, for the image forming area PV 2 , in consideration of shortening the image width, it is necessary to delay the start timing for forming the electrostatic latent image.  
      In the third embodiment, through the processes in Steps S 13  and S 14  of  FIG. 10 , the control circuit  26  computes and sets the scanning speed of the scanner  241  along with the exposure start timing.  
     Fourth Embodiment  
      In the third embodiment, the scanning speed of the scanner  241  is changed. However, the position of the edge PV 1   a  of the image forming area can also be changed through changing the conveyance speed of the adjustment roller  250  while the scanning speed of the scanner  241  is kept constant. Then, a fourth embodiment of the invention will be described. In the fourth embodiment, the conveyance speed of the adjustment roller  250  is controlled. In the following description of the fourth embodiment, the same component is designated by the same numeral as the above embodiments, and the difference between the fourth embodiment and the above embodiments will be described below.  
       FIG. 13  shows the image forming area on the sheet in the fourth embodiment of the invention.  
      Part (a) of  FIG. 13  shows the sheet P 220  located on the inner-most side as an example of the sheet. The sheet P 220  is located on the side opposite to the booklet cover side. Part (b) of  FIG. 13  shows the sheet P 201  located on the side closest to the booklet cover. The image output device  24  forms the image in the direction shown by the arrow Z on the sheets P 220  and P 201 .  
      In the fourth embodiment, the conveyance speed of the adjustment roller  250  is decreased as the sheet is located closer to the booklet cover side.  
      In the fourth embodiment, the control circuit  26  (see  FIG. 4 ) sets the conveyance speed of the adjustment roller  250  such that the distance from the center C of the sheet P 220  to the edge PV 1   a  close to the sheet center of the image forming area PV 1  becomes the distance in which the predetermined marginal width m is added to the forbidden distance. That is, in the case where the direction shown by the arrow Z of  FIG. 13  is set to the time axis, the conveyance speed of the adjustment roller  250  is further decreased such that the sheet is delayed by the distance of the forbidden distance M 120  at the time when the transfer of the electrostatic latent image corresponding to the edge PV 1   a  close to the sheet center of the image forming area PV 1  to the sheet is finished. The decelerated speed is determined by the computation from the circumferential speed of the photosensitive drum  242 . The adjustment roller  250  conveys the sheet at the slow conveyance speed, which allows the width in the direction of the end of the double spread from the center of the double spread of the to be formed to be shortened while the exposure start timing is kept constant. Therefore, in the image which is transferred and fixed to the sheet P 220 , the distance from the center C of the sheet P 220  to the edge PV 1   a  close to the center of the image forming area PV 1  becomes the distance in which the marginal width m is added to the forbidden distance M 120 .  
      Thus, because the conveyance of the adjustment roller  250  is controlled at the conveyance speed determined based on the sheet position in the booklet, similarly by the start timing control of the electrostatic latent image formation in the first embodiment, the position of the edge close to the center of the image forming area can precisely be controlled. Further, in accordance with the fourth embodiment, the width of the image in the direction of the end of the double spread from the center of the double spread becomes shortened according to the change in position of the edge close to the center, the position of the edge close to the double spread end of the image forming area is kept constant for all the sheets. In the fourth embodiment, because the scanning speed of the scanner  241  is kept constant, the width of the formed image in the direction of the spine of the booklet is not shortened. In the fourth embodiment, the conveyance speed of the adjustment roller  250  is changed. Therefore, for the image forming area PV 2 , in consideration of the image width shortened, the start timing for forming the electrostatic latent image is delayed. Alternatively, the sheet is accelerated or decelerated in the period during which the image forming area PV 2  reaches the photosensitive drum since the image forming area PV 1  passes through the photosensitive drum. This may enable the control of the time when the image forming area PV 2  reaches the photosensitive drum to adjust the exposure start timing of the electrostatic latent image.  
      In the fourth embodiment, through the processes in Steps S 13  and S 14  of  FIG. 10 , the control circuit  26  computes and sets the conveyance speed of the adjustment roller  250  along with the exposure start timing.  
     Fifth Embodiment  
      In the fourth embodiment, the area where the image formation is forbidden is limited to the portion where the back is flattened. However, the area where the image formation is forbidden can be enlarged. A fifth embodiment of the invention will be described below. In the fifth embodiment, the area where the image formation is forbidden is further enlarged. In the following description of the fifth embodiment, the same component is designated by the same numeral as the above embodiments, and the difference between the fifth embodiment and the above embodiments will be described below.  
       FIG. 14  schematically shows a state in which the folded sheets are flattened by the square folding device  9  in the fifth embodiment of the invention.  
      For example, as shown in part (a) of  FIG. 14 , sheets P 5  include 20 sheets P 501  to P 520  which are folded while laid on top of one another. The fifth embodiment differs from the above embodiments in that a clamping jaw  195  of the square folding device  9  has a projection  195   a  in the surface in which the clamping jaw  195  clamps the booklet P 5 . In this case, a recess P 5   a  is formed in the portion which is clamped by the projection  195   a  of the clamping jaw  195 . When the image is formed in the recess P 5   a,  the disturbance of image occurs in the recess P 5   a,  or the image is lost. Accordingly, in the case where the clamping jaw  195  has the projection  195   a,  the area where the image formation is forbidden also includes the recess P 5   a  formed by the projection  195   a.  Even in this case, the area where the image formation is forbidden is depends on each sheet. For example, as shown in part (b) of  FIG. 14 , on the sheet P 501  located on the side closest to the cover, the distance to the edge from the center of the area where the image formation is forbidden is set to the forbidden distance N 501 . As shown in part (a) of  FIG. 14 , before the flattening is performed by the roller  96 , it is assumed that t is a width of the projection  195   a,  w is a distance between the projection  195   a  and the surface in which the roller  96  of the clamping jaw  195  is moved, and d 3  is a width of a gap between the stop plate  94  (not shown) and the clamping jaw  195 . A distance v between the front end of the sheet P 520  located on the side opposite to the booklet cover and the front end of the portion P 5   a  corresponding to the spine of the booklet is equal to the thickness of 20 sheets. For example, in the case where the thickness of the booklet is 0.087 mm, the distance v is 0.087×20=1.74 mm. In this case, a half length u of an arc possessed by the portion P 5   a  corresponding to the spine of the booklet is a quarter of a circumference of a circle having a radius of the distance v. Therefore, the length u is u=1.74×2×π/4=2.73 mm. At this point, for example, it is assumed that d 3  is 1.5 mm, w is 4 mm, t is 0.3 mm, and the width from the sheet center in the area where the image formation is forbidden, i.e., forbidden distance is N 501 . Then, N 501 =u+(d 3 +w+t−v)=6.79 mm. Similarly to the forbidden distance N 501 , the forbidden distances N 502  to N 520  corresponding to other sheets P 502  to P 520  can be computed from the number of sheets constituting the booklet, the sheet position, and the thickness of the sheets.  
       FIG. 15  is a table showing the forbidden distance on each sheet in the fifth embodiment of the invention  
       FIG. 15  shows the forbidden distances N 501  to N 520  on each sheet in the case where the 20 sheets are folded while the intensity of the square folding process is set to the strong level. Each of the sheets has the thickness of 0.087 mm. In the table of  FIG. 15 , the value corresponding to the field of “20th sheet” indicates the forbidden distance M 501 , and “19th sheet”, “18th sheet”, . . . , and “first sheet” indicate M 502 , M 503 , and M 520  respectively.  
      In the fifth embodiment, through the exposure start timing computing process shown in Step S 13  of  FIG. 10 , in consideration of the projection  195   a  of the clamping jaw  195 , the forbidden distance is computed on each sheet position in the booklet.  
      In the above embodiments, control circuit  26  is incorporated into the image forming apparatus  2 . However, the invention is not limited to the above embodiments. For example, the control circuits may be mounted on both the image forming apparatus and the square folding device such that the functions are shared.  
      In the above embodiments, the forbidden distance is determined based on the information on the number of sheets and the sheet thickness information. However, the invention is not limited to the above embodiments. For example, the forbidden distance may be determined based on either the information on the number of sheets or the sheet thickness information.  
      In the above embodiments, through the square folding process, the back is pressed by the roller  96  while the clamping jaw  95  clamps the booklet. However, the invention is not limited to the above embodiments. Other modes may be used as the shape of the roller and the way how to move as long as the roller produces the booklet whose back is flattened. Any device which flattens the back may be used instead of the roller.  
      According to an aspect of the present invention, the edge close to the center of the double pages spread in the image forming area where the image is formed is located away from the center, and the image is formed away from the flattened portion. Also, the flattened portion is enlarged as the sheet is located closer to the booklet cover side. However, in the sheet treatment apparatus of the invention, because the edge in the image forming area is located farther away from the center as the sheet is located closer to the booklet cover side, the image is formed while properly located away from the flattened portion which depends on the sheet position in the booklet. Accordingly, the image disturbance by the flattening is prevented in the back portion of the booklet and the lack of the image is also prevented in the crease, so that the booklet quality can be improved.  
      The foregoing description of the embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.