Patent Publication Number: US-11027939-B2

Title: Sheet folding device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is filed under 35 U.S.C. § 371 as a PCT national phase of PCT International Application No. PCT/KR2018/002867, filed on Mar. 12, 2018, in the Korean Intellectual Property Office, which claims the priority benefit of Korean Patent Application No. 10-2017-0108849, filed on Aug. 28, 2017 in the Korean Intellectual Property Office, the contents of the PCT International Application and the Korean Patent Application are incorporated by reference herein in their entirety. 
     BACKGROUND ART 
     A sheet folding device folds a medium (hereinafter, referred to as ‘paper’) on a sheet in various forms. The sheet folding device may be included in a finisher with respect to paper discharged from a copier, a printer, or the like, and may be a stand-alone device. 
     A sheet folding device uses a folding blade to push a portion of paper, which is to be folded and between a front-end portion and a rear-end portion of the paper, into a pair of rollers rotating engaged with each other, thereby folding the paper. The sheet folding device may fold a single sheet or multiple sheets of paper. Also, the sheet folding device may have a structure for folding paper twice or more. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       These and/or other aspects will become apparent and more readily appreciated from the following description of the examples, taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a schematic structural diagram of an image forming apparatus according to an example; 
         FIG. 2  is a structural diagram of a sheet folding device according to an example and illustrates a state in which a folding blade is at a retreat location; 
         FIG. 3  illustrates a state in which a first shift member of the sheet folding device of  FIG. 2  is at a first shift location; 
         FIG. 4  illustrates a state in which a second shift member of the sheet folding device of  FIG. 2  is at a second shift location; 
         FIG. 5  is a detailed diagram illustrating pushing paper into a folding nip by using a folding blade; 
         FIG. 6  illustrates a second-folding process; 
         FIGS. 7 and 8  are schematic diagrams illustrating operations of a sheet folding device using a guide member; 
         FIG. 9  illustrates a state in which a folding blade is at a first insertion location; 
         FIG. 10  illustrates a state in which a folding blade is at a second insertion location; 
         FIG. 11  illustrates an example of a V-fold; 
         FIG. 12  illustrates an example of a C-fold; 
         FIG. 13  illustrates an example of a double gate fold; 
         FIG. 14  is a schematic structural diagram of a sheet folding device including two folding rollers, according to an example; and 
         FIG. 15  is a schematic structural diagram of a sheet folding device in which three rollers form two folding nips, according to an example. 
     
    
    
     MODE FOR INVENTION 
     Hereinafter, examples of a sheet folding device and an image forming apparatus using the same will be described with reference to the accompanying drawings. In the drawings, like reference numerals denote like elements, and a size or thickness of each component may be exaggerated for clarity of description. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Expressions such as “at least one of”, when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list. 
       FIG. 1  is a schematic structural diagram of an image forming apparatus according to an example. Referring to  FIG. 1 , the image forming apparatus includes a printer  100  and a finisher  400 . The printer  100  prints an image on a sheet-type medium (hereinafter, referred to as ‘paper’) provided from a paper feeder. The paper feeder may be, for example, a main cassette feeder  210  installed under the printer  100 , a secondary cassette feeder  220  installed under the main cassette feeder  210 , a high capacity feeder  230  installed under the main cassette feeder  210  or under the secondary cassette feeder  220 , a high capacity feeder  240  installed at a side of the printer  100 , or the like. Although not illustrated, the paper feeder may be a multi-purposetray (MPT). 
     The printer  100  may print an image on paper P by using various printing methods such as an electrophotography method, an inkjet method, a thermal transfer method, and a thermal sublimation method. For example, the image forming apparatus according to an example embodiment prints a color image on the paper P by using an electrophotography method. The above printing methods are well known in the art, and thus, a detailed description thereof will be omitted herein. 
     The image forming apparatus may further include a scanner  300  for reading an image recorded on a document. The scanner  300  may have any of various structures such as a flatbed mechanism where a document is at a fixed position and an image is read while a reading member is moved, a document feeding mechanism where a reading member is at a fixed position and a document is fed, and a combination structure thereof. The principle and structure of the scanner  300  are well known in the art, and thus, a detailed description thereof will be omitted herein. 
     The finisher  400  may include a sheet folding device  700  for folding, one or more times, the paper P discharged from the printer  100 . The finisher  400  may further include an alignment device  500  for aligning the paper P discharged from the printer  100 . The alignment device  500  may have a structure for stapling the paper P at an end portion thereof or punching a hole in an end portion of the paper P. The finisher  400  may further include a middle stapler  600  for stapling the paper P at a center portion thereof. Structures of the alignment device  500  and the middle stapler  600  are well known in the art, and thus, a detailed description thereof will be omitted herein. 
     Hereinafter, examples of the sheet folding device  700  will be described in detail. 
       FIG. 2  is a structural diagram of the sheet folding device  700  according to an example and illustrates a state in which a folding blade  750  is at a retreat location.  FIG. 2  illustrates a folding path  710  and a folding roller  720 . The folding roller  720  is on the folding path  710 . The folding roller  720  includes first and second rollers  721  and  722  engaging with each other to form a folding nip N and rotating. The first roller  721  is at an upstream side of the folding path  710 , and the second roller  722  is at a downstream side thereof. A second motor  723  rotates the folding roller  720 . A controller  800  controls the sheet folding device  700 . The controller  800  may rotate the folding roller  720  by driving the second motor  723 . 
     The folding blade  750 , which is moved to an insertion location (a dashed line of  FIG. 2 ) for pushing the paper P on the folding path  710  into the folding nip N, and is moved to a retreat location (a solid line of  FIG. 2 ) for escaping from the folding path  710 , is positioned at an entrance of the folding nip N. The folding blade  750  pushes a portion of the paper P, which is to be folded and between a front end and a rear end of the paper P, into the folding nip N. The folding blade  750  is moved to the insertion and retreat locations by, for example, a folding blade driver  751 . The folding blade driver  751  may have various structures. According to an example, the folding blade driver  751  may have a slider crank structure. The folding blade driver  751  includes a rotating member  751 - 2  that is rotated by a first motor  751 - 1 , a slider  751 - 3  that is linearly movable, and a crank  751 - 4  that connects the rotating member  751 - 2  to the slider  751 - 3 . The folding blade  750  is installed on the slider  751 - 3 . The controller  800  may drive the first motor  751 - 1  and thus may move the folding blade  750  to the insertion location and the retreat location. 
     Although not illustrated, the folding blade driver  751  may include the slider  751 - 3  on which the folding blade  750  is installed, and a linear motor (not illustrated) driving the slider  751 - 3 . Referring to  FIG. 2 , the folding blade  750  is linearly moved to the insertion location and the retreat location. However, the scope of the disclosure is not limited thereto. The folding blade  750  may rotate to the insertion location and the retreat location. 
     A location determination member  740  may be moved to an alignment location (a solid line of  FIG. 2 ) so as to align the paper P on the folding path  710  by supporting a front end PF of the paper P, and moved to a folding location (a dashed line of  FIG. 2 ) so as to match a folding line location of the paper P with a location corresponding to the folding nip N. The front end PF of the paper P fed along the folding path  710  is supported by the location determination member  740  that is at the alignment location. The location determination member  740  is moved to the alignment location and the folding location by an escalating member  741 . The folding location at least includes an initial folding location of the paper P. The folding location may include a location after folding is performed twice. A location of the location determination member  740  may be detected by a location detection sensor (not illustrated). The escalating member  741  may include a third motor  742 . The third motor  742  may be, for example, a linear motor. The third motor  742  may be a rotation motor. In this case, the escalating member  741  may be realized by any of various structures such as a combination of a rotation motor and a linear movement mechanism, a combination of a rotation motor and a rotary belt, and a combination of a rotation motor and a chain. 
     By the above structure, the paper P may be folded once. For example, one sheet or multiple sheets of paper P discharged from the printer  100  is fed along the folding path  710 , and as indicated by a solid line of  FIG. 2 , the front end PF of the paper P is supported by the location determination member  740  that is at the alignment location. As indicated by a dashed line of  FIG. 2 , the location determination member  740  is moved to the folding location and matches a folding line location of the paper P with the location corresponding to the folding nip N. As the folding blade  750  is moved to the insertion location, the folding blade  750  pushes the center portion of the paper P into the folding nip N. The paper P is folded once while being pushed into the folding nip N and is pushed out through an exit of the folding nip N. The folded paper P is externally discharged by a discharge roller  760 . Thus, a V-fold may be performed. The folding line location may be determined by the location determination member  740 . 
     The sheet folding device  700  according to an example embodiment may perform folding two or more times by using one folding roller  720 . To this end, the sheet folding device  700  may include a guide path for guiding the paper P having passed through the folding nip N to the folding path  710  again and returning the paper P to the entrance of the folding nip N, and a shift member  730  for selectively guiding the paper P to the discharge roller  760  and the guide path, the shift member  730  being located at the exit of the folding nip N. 
     The guide path may be provided around at least one of the first roller  721  and the second roller  722 . In an example embodiment, the guide path includes a first guide path  761  and a second guide path  762  respectively provided around the first roller  721  and the second roller  722 . 
     The shift member  730  may include a first shift member  731  for selectively guiding the paper P to the first guide path  761 , and a second shift member  732  for selectively guiding the paper P to the second guide path  762 . Although not illustrated, an actuator for driving the first and second shift members  731  and  732  may be provided. The actuator may be, for example, a solenoid actuator. 
     Although not illustrated, a first sensor and a second sensor for detecting the paper P may be respectively arranged on the first guide path  761  and the second guide path  762 . The first and second sensors may provide a reference for determining second, third, or subsequent folding timing, that is, driving timing of the folding blade  750 . Although not denoted by reference numerals, driven rollers respectively arranged around the first and second rollers  721  and  722  along the first and second guide paths  761  and  762  engage with the first and second rollers  721  and  722  and feed the paper P along the first and second guide paths  761  and  762 . In an example embodiment, the first roller  721  forms a guide at one side of the first guide path  761 , and the second roller  722  forms a guide at one side of the second guide path  762 . That is, the paper P having passed through the folding nip N is fed along the first and second guide paths  761  and  762  in a direction winding around the first and second rollers  721  and  722  and is returned in a direction toward the entrance of the folding nip N. 
       FIGS. 3 and 4  are structural diagrams of the sheet folding device  700  according to an example.  FIG. 3  illustrates a state in which the first shift member  731  is at a first shift location.  FIG. 4  illustrates a state in which the second shift member  732  is at a second shift location. 
     Referring to  FIG. 2 , the first and second shift members  731  and  732  are at discharge locations for guiding the paper P to the discharge roller  760 . Referring to  FIG. 3 , the first shift member  731  is at the first shift location. The paper P discharged from the folding nip N is guided to the first guide path  761  by the first shift member  731 . Referring to  FIG. 4 , the second shift member  732  is at the second shift location. The paper P discharged from the folding nip N is guided to the second guide path  762  by the second shift member  732 . 
     Folding may be performed twice by returning the paper P folded once to the entrance of the folding nip N via the first guide path  761  or the second guide path  762  and pushing the paper P into the folding nip N by using the folding blade  750  again. A simple 4-fold folding may be performed by once again folding a center portion of the paper P that has already been folded once. Also, a C-fold or a Z-fold, which is a 3-fold folding, may be performed by performing folding twice by using the first guide path  761  or the second guide path  762 . Also, a double gate fold, a roll-fold, a W-fold, etc. may be performed by performing folding three times while allowing the paper P to sequentially pass through the first guide path  761  and the second guide path  762 . 
       FIG. 5  is a detailed diagram illustrating pushing the paper P into the folding nip N by using the folding blade  750 .  FIG. 5  illustrates only the folding roller  720 , the folding blade  750 , guide members  752 , and an elastic member  753 . Referring to  FIG. 5 , the guide members  752  are illustrated. The guide member  752  is on at least a side of the folding blade  750 . That is, the guide members  752  may be installed at a side of the folding blade  750  in a direction orthogonal to a movement direction of the folding blade  750 , or may be installed at both sides thereof. In an example embodiment, the guide members  752  are installed at the sides of the folding blade  750 , respectively. The guide members  752  may be moved along with the folding blade  750 . For example, the guide members  752  may be installed on the slider  751 - 3 . The guide members  752  may each include an inclined portion  752 - 1  inclined at an acute angle with respect to the movement direction of the folding blade  750 . The inclined portion  752 - 1  may be, for example, a plane. The inclined portion  752 - 1  may be a concave or convex surface. 
     By the above configuration, when the folding blade  750  is moved to the insertion location, the paper P is pushed by the guide members  752  and pressed towards the folding roller  720 . Thus, the paper P may be stably inserted into the folding nip N. The guide members  752  do not contact the folding roller  720  when the folding blade  750  reaches the insertion location. 
     The guide members  752  may be elastically biased by the elastic member  753  towards the folding roller  720 . For example, the elastic member  753  may be a compressive coil spring supported between the guide members  752  and the slider  751 - 3 . By this configuration, the elastic member  753  is pressed when multiple sheets of paper P are folded, and the guide members  752  are pushed in a direction opposite to the movement direction of the folding blade  750 . In this case, the elastic member  753  provides elasticity to push the guide members  752  towards the folding roller  720 . Thus, when a single sheet of paper P is folded, and when multiple sheets of paper P are folded, the paper P may be stably pressed towards the folding roller  720 . 
     Accordingly, the paper P may be stably inserted into the folding nip N as the guide members  752  are used, and thus a folding defect, e.g., crumpling of the paper P, which is caused by a wrong insertion of the paper P into the folding nip N, may be prevented. 
     The guide members  752  are useful during a second folding process or subsequent folding processes.  FIG. 6  illustrates the second-folding process when the guide members  752  are not used. Referring to  FIG. 6 , the paper P that has been already folded once, that is, the paper P on which a first folding line F 1  is formed, is fed again to the folding path  710  along, e.g., the first guide path  761  provided around the first roller  721 . The second folding for forming a second folding line F 2  is performed as the folding blade  750  is moved to the insertion location. In this case, two portions P 1  and P 2 , which are folded during the first folding to face each other, may not adhere to each other due to elasticity of the paper P and may be apart from each other as illustrated in  FIG. 6( a ) . Then, when the paper P is pushed into the folding nip N during the second folding, the paper P may be crumpled around the first folding line F 1 . 
     As illustrated in  FIG. 6( b ) , an unnecessary folding line F 1 ′ may be formed around the first folding line F 1 . 
       FIGS. 7 and 8  are schematic diagrams illustrating operations of the sheet folding device  700  using the guide members  752 . According to an example embodiment, as illustrated in  FIG. 7( a ) , the guide members  752  are used. When the folding blade  750  is moved to the insertion location, the portions P 1  and P 2  that are folded with respect to the first folding line F 1  are pressed towards the folding roller  720  by the guide members  752 . The portions P 1  and P 2  are adjacent to each other without being spaced apart from each other while being pushed by the guide members  752 . Thus, portions around the first folding line F 1  are not crumpled during the second folding, and as illustrated in  FIG. 7( b ) , folding may be stably performed twice without the unnecessary folding line F 1 ′. Also, by using the elastic member  753  for pressing the guide members  752  towards the folding roller  720 , the portions P 1  and P 2  folded with respect to the first folding line F 1  may be stably pressed by the guide members  752  towards the folding roller  720  even when multiple sheets of paper P are folded, as illustrated in  FIG. 8 . 
     The paper P discharged from the printer  100  is fed to the sheet folding device  700 . While being fed, the paper P may be skewed. Also, the paper P may be skewed while being fed again to the folding path  710  along a guide path after the first folding. When the skewed paper P is folded, a folding line may be crooked. 
     In an example embodiment, the controller  800  drives the third motor  742  before second or subsequent folding is performed, and moves the location determination member  740  to the alignment location as indicated by the solid line of  FIG. 2  such that an end portion of the paper P may be supported by the location determination member  740 , and skew of the paper may be corrected. The alignment location may be a location where the paper P may be completely separated from rollers for feeding the paper P. In order to perform folding, the controller  800  may drive the third motor  742  to move the location determination member  740  to the folding location as indicated by the dashed line of  FIG. 2  and may move a portion, where a folding line of the paper P is to be formed, to the location corresponding to the folding nip N. Then, the controller  800  drives the first motor  751 - 1  to move the folding blade  750  to the insertion location and pushes the portion, where the folding line of the paper P is to be formed, into the folding nip N. Creation of a crooked folding line due to the skew of the paper P may be prevented. 
     The folding blade  750  is in a form of a thin plate. In order to push the paper P into the folding nip N, the folding blade  750  may be inserted into the folding nip N at the insertion location. In this case, when multiple sheets of paper P are folded at the same time, sharpness of the folding line is slightly affected by a thickness of the folding blade  750 , but when a single sheet of paper P is folded, the sharpness of the folding line is more affected by the thickness of the folding blade  750 . 
     In order to form a sharp folding line, the folding blade  750  may not be inserted into the folding nip N at the insertion location. For example, as illustrated in  FIG. 9 , the folding blade  750  may not be inserted into the folding nip N and may be moved to a location (a first insertion location) where the paper P, which has been pushed by the folding blade  750  towards the folding nip N, may be naturally pushed into the folding nip N due to friction with the folding roller  720 . By a structure for moving the folding blade  750  to the first insertion location, when a single sheet of paper P is folded, the thickness of the folding blade  750  does not affect the sharpness of the folding line, and a sharp folding line may be formed. 
     In the structure for moving the folding blade  750  to the first insertion location, when multiple sheets of paper P are folded at the same time, separation in which a sheet close to the folding roller  720  from among the sheets of paper P is inserted into the folding nip N first may occur, and the other sheets of paper P may be sequentially inserted into the folding nip N. When multiple sheets of paper P are folded at the same time, the sharpness of the folding line is slightly affected by the thickness of the folding blade  750 . Accordingly, when multiple sheets of paper P are folded at the same time, the folding blade  750  may be moved to a location (a second insertion location) where the folding blade  750  is inserted into the folding nip N, as illustrated in  FIG. 10 . 
     The controller  800  may control the folding blade driver  751  such that the folding blade  750  is moved to the first insertion location when a single sheet of paper P is folded and the folding blade  750  is moved to the second insertion location when the multiple sheets of paper P are folded at the same time. For example, the controller  800  controls the first motor  751 - 1  such that the folding blade  750  is moved to the first insertion location when a single sheet of paper P is folded and the folding blade  750  is moved to the second insertion location when the multiple sheets of paper P are folded at the same time. 
     As another exemplary method of forming a sharp folding line, the folding blade  750  is inserted into the folding nip N at the insertion location, but when a single sheet of paper P is folded, before the folding blade  750  is inserted into the folding nip N, the paper P having pushed towards the folding nip N may be allowed to be naturally pushed into the folding nip N due to friction with the folding roller  720 . Since the folding blade  750  is inserted into the folding nip N after the paper P is inserted into the folding nip N first and then the folding line is formed, a sharp folding line may be formed without being affected by the thickness of the folding blade  750 . When the guide members  752  are used, the paper P is pushed by the guide members  752  towards the folding roller  720  as the folding blade  750  gets close to the folding nip N. Accordingly, the paper P may be stably inserted into the folding nip N before the folding blade  750  reaches the folding nip N. 
     The above exemplary method may be realized by controlling at least one of movement linear velocity of the folding blade  750  and rotation linear velocity of the folding roller  720 . For example, when the movement linear velocity of the folding blade  750  is V 1  and the rotation linear velocity of the folding roller  720  is V 2 , the controller  800  controls at least one of the folding roller  720  and the folding blade driver  751  so as to satisfy V 1 &lt;V 2  when a single sheet of paper P is folded. The above condition may be realized by fixing V 1  and changing V 2  or by changing V 1  and fixing V 2 . Of course, both V 1  and V 2  may be changed. 
     When multiple sheets of paper P are folded at the same time, under a condition of V 1 &lt;V 2 , separation in which a sheet close to the folding roller  720  from among the sheets of paper P is inserted into the folding nip N first may occur, and the other sheets of paper P may be sequentially inserted into the folding nip N. The separation may occur under a condition of V 1 =V 2 . In order to solve such a problem, when multiple sheets of paper P are folded at the same time, the controller  800  may control at least one of the folding roller  720  and the folding blade driver  751  to satisfy a condition of V 1 &gt;V 2 . Such a condition may be realized by fixing V 1  and changing V 2  or by fixing V 2  and changing V 1 . Of course, both V 1  and V 2  may be changed. Since the movement linear velocity of the folding blade  750  is higher than transfer velocity of the paper P by the folding roller  720 , the folding blade  750  may be inserted into the folding nip N in a state in which the sheets of paper P are not separated. Accordingly, when multiple sheets of paper P are folded at the same time, separation may be prevented. 
     The controller  800  controls at least one of the first motor  751 - 1  and the second motor  723  so as to control a relation between V 1  and V 2  as described above. That is, when a single sheet of paper P is folded, the controller  800  may control at least one of the first motor  751 - 1  and the second motor  723  to satisfy the condition V 1 &lt;V 2 , and when multiple sheets of paper P are folded at the same time, the controller  800  may control at least one of the first motor  751 - 1  and the second motor  723  to satisfy the condition V 1 &gt;V 2 . 
     A structure for controlling the movement linear velocity of the folding blade  750  and the rotation linear velocity of the folding roller  720  may be rather simple compared to the above structure for controlling the insertion location of the folding blade  750 . For example, in a case where the folding blade driver  751  having a slider-crank structure is used as illustrated in  FIG. 2 , in the structure for controlling the insertion location of the folding blade  750 , when a single sheet of paper P is folded, the first motor  751 - 1  stops working at the first insertion location before the rotating member  751 - 2  rotates once and then rotates again in a reverse direction. However, in the structure for controlling the movement linear velocity of the folding blade  750  and the rotation linear velocity of the folding roller  720 , the folding blade  750  may be returned to the retreat location from the insertion location as the rotating member  751 - 2  rotates once. According to necessity, rotation velocity of the first motor  751 - 1  and/or the second motor  723  may be adjusted. 
     Hereinafter, various examples of a paper folding method according to the above examples will be described. A V-fold will be described as an example of a 1-fold folding, a C-fold will be described as an example of a 2-fold folding, and a double gate fold will be described as an example of a 3-fold folding. 
     [V-Fold] 
       FIG. 11  illustrates an example of a V-fold.  FIG. 11  illustrates the folding roller  720 , the location determination member  740 , the folding blade  750 , and the first and second shift members  731  and  732 . 
     As indicated by a solid line of  FIG. 11( a ) , the location determination member  740  is at the alignment location. The paper P discharged from the printer  100  is fed along the folding path  710 . The front end PF of one or multiple sheets of paper P is supported by the location determination member  740  that is at the alignment location, and skew is corrected. 
     As indicated by a dashed line of  FIG. 11( a ) , the location determination member  740  is moved to the folding location. The folding location is a location where the center portion of the paper P is at the entrance of the folding nip N. That is, a distance from the folding nip N to the location determination member  740  that is at the folding location is ½ L when a length of the paper P is L. Then, the folding blade  750  is moved to the insertion location to push a ½ L point of the paper P into the folding nip N ( FIG. 11( b ) ). As the paper P passes through the folding nip N, the ½ L point of the paper P is folded. The first and second shift members  731  and  732  are at the discharge locations. Therefore, the paper P folded in a form of a V is discharged by the discharge roller  760  ( FIG. 11( c ) ). 
     [C-Fold] 
       FIG. 12  illustrates an example of a C-fold.  FIG. 12  illustrates the folding roller  720 , the location determination member  740 , the folding blade  750 , and the first and second shift members  731  and  732 . 
     Although not illustrated, the location determination member  740  is at the alignment location. The paper P discharged from the printer  100  is fed along the folding path  710 . The front end PF of one sheet or multiple sheets of paper P is supported by the location determination member  740  that is at the alignment location, and thus skew may be corrected. 
     As illustrated in  FIG. 12( a ) , the location determination member  740  is moved to a folding location where a ⅓ L point of the paper P is located at the entrance of the folding nip N. Then, the folding blade  750  is moved to the insertion location to push the ⅓ L point of the paper P into the folding nip N ( FIG. 12( b ) ). As the paper P passes through the folding nip N, the ⅓ L point of the paper P is folded. The folding blade  750  then returns to the retreat location. 
     The paper P having passed through the folding nip N is fed along a roller on an opposite side of the front end PF of the paper P from among the first and second rollers  721  and  722 , that is, the first roller  721 , and is returned to the entrance of the folding nip N. To this end, the first shift member  731  is at the first shift location, and the paper P having passed through the folding nip N is returned to the entrance of the folding nip N along the first guide path  761  ( FIGS. 12( c ) and 12( d ) ). 
     Although not illustrated, the paper P fed along the folding path  710  in a forward direction is supported by the location determination member  740  that is at the alignment location, and skew is corrected. 
     The location determination member  740  is moved to the folding location such that a ⅔ L point of the paper P is located at the entrance of the folding nip N ( FIG. 12( e ) ). The folding blade  750  is moved to the insertion location, and thus the ⅔ L point of the paper P is pushed into the folding nip N ( FIG. 12( f ) ). The first and second shift members  731  and  732  are located at discharge locations. Therefore, the paper P folded in a form of a C is discharged by the discharge roller  760  ( FIG. 12( g ) ). The folding blade  750  then returns to the retreat location. 
     [Double Gate Fold] 
       FIG. 13  illustrates an example of a double gate fold.  FIG. 13  illustrates only the folding roller  720 , the location determination member  740 , the folding blade  750 , and the first and second shift members  731  and  732 . 
     Although not illustrated, the location determination member  740  is located at the alignment location. The paper P discharged from the printer  100  is fed along the folding path  710 . The front end PF of one sheet or multiple sheets of paper P is supported by the location determination member  740  that is at the alignment location, and thus skew is corrected. 
     As illustrated in  FIG. 13( a ) , the location determination member  740  is moved to the folding location such that a ¼ L point of the paper P is located at the entrance of the folding nip N. Then, the folding blade  750  is moved to the insertion location to push the ¼ L point of the paper P into the folding nip N ( FIG. 13( b ) ). As the paper P passes through the folding nip N, the ¼ L point of the paper P is folded. The folding blade  750  then returns to the retreat location. 
     The paper P having passed through the folding nip N is fed along a roller on an opposite side of the front end PF of the paper P from among the first and second rollers  721  and  722 , that is, the first roller  721 , and is returned to the entrance of the folding nip N. To this end, the first shift member  731  is at the first shift location, and the paper P having passed through the folding nip N is returned to the entrance of the folding nip N along the first guide path  761  ( FIGS. 13( c ) and 13( d ) ). 
     Although not illustrated, the paper P fed along the folding path  710  in a forward direction is supported by the location determination member  740 , and thus skew is corrected. 
     Next, the location determination member  740  is moved to the folding location such that a ¾ L point of the paper P is located at the entrance of the folding nip N ( FIG. 13( e ) ). The folding blade  750  is moved to the insertion location, and the ¾ L point of the paper P is pushed into the folding nip N ( FIG. 13( f ) ). The folding blade  750  then returns to the retreat location. 
     The paper P having passed through the folding nip N is fed along a roller on an opposite side of the front end PF of the paper P from among the first and second rollers  721  and  722 , that is, the second roller  722 , and is returned to the entrance of the folding nip N. To this end, the second shift member  732  is at the second shift location, and the paper P having passed through the folding nip N is returned to the entrance of the folding nip N along the second guide path  762  ( FIG. 13( g ) ). 
     Although not illustrated, the paper P fed along the folding path  710  in a reverse direction is supported by the location determination member  740  that is at the alignment location, and skew is corrected. 
     The location determination member  740  is moved to the folding location such that a ½ L point of the paper P is located at the entrance of the folding nip N ( FIG. 13( h ) ). The folding blade  750  is moved to the insertion location, and thus the ½ L point of the paper P is pushed into the folding nip N (see  FIG. 13( i ) . The first and second shift members  731  and  732  are located at discharge locations. Therefore, the paper P folded in a form of a double gate is discharged by the discharge roller  760  ( FIG. 13( j ) ). The folding blade  750  then returns to the retreat location. 
     Other than the above-described folding methods, a 4-fold folding, a Z-fold, a W-fold, a roll fold may be available. 
     The examples of  FIGS. 2 to 10  may be applied to a sheet folding device including two or more folding nips. 
       FIG. 14  is a schematic structural diagram of a sheet folding device including two folding rollers  720 , according to an example.  FIG. 14  illustrates a first folding roller  720 - 1  and a second folding roller  720 - 2 . The first folding roller  720 - 1  includes the first and second rollers  721  and  722  engaging with each other to form a first folding nip N 1 . The second folding roller  720 - 2  includes third and fourth rollers  724  and  725  engaging with each other to form a second folding nip N 2 . The second folding nip N 2  is formed in a direction that is different from a direction in which the first folding nip N 1  is formed. A first folding blade  750 - 1  is moved to an insertion location at which the paper P fed along the folding path  710  is pushed into the first folding nip N 1  and to a retreat location retreating from the insertion location. A second folding blade  750 - 2  is moved to an insertion location at which the paper P having passed through the first folding nip N 1  is pushed into the second folding nip N 2  and to a retreat location retreating from the insertion location. The location determination member  740  may be moved to an alignment location at which the paper P fed along the folding path  710  is supported and aligned, and to a folding location at which a portion, where a folding line of the paper P is to be formed, is located at an entrance of the first folding nip N 1 . A structure for moving the first folding blade  750 - 1  and the second folding blade  750 - 2  to the insertion location and the retreat location may be the same as the structure described above with reference to  FIG. 2 . A structure for moving the location determination member  740  to the alignment location and the folding location may be the same as the structure described with reference to  FIG. 2 . 
     The guide members  752  and the elastic member  753  which are described above may be applied to at least one of the first folding blade  750 - 1  and the second folding blade  750 - 2 . The guide members  752  and the elastic member  753  are useful after folding is performed twice, and thus, if the guide members  752  and the elastic member  753  are applied to at least one of the first folding blade  750 - 1  and the second folding blade  750 - 2 , it may be effective to apply the guide members  752  and the elastic member  753  to the second folding blade  750 - 2 . Also, at least one of rotation linear velocity of the first and second folding rollers  720 - 1  and  720 - 2  and movement linear velocity of the first and second folding blades  750 - 1  and  750 - 2  may be controlled as described above, depending on the number of sheets of paper P which are folded at the same time. In addition, the insertion locations of the first folding blade  750 - 1  and the second folding blade  750 - 2  may be controlled as first and second insertion locations as described above, depending on the number of sheets of paper P which are folded at the same time. 
       FIG. 15  is a schematic structural diagram of a sheet folding device in which three rollers form two folding nips, according to an example.  FIG. 15  illustrates the first folding roller  720 - 1  and the second folding roller  720 - 2 . The first folding roller  720 - 1  includes the first and second rollers  721  and  722  engaging with each other to form a first folding nip N 1 . The second folding roller  720 - 2  includes a third roller  724  engaging with the second roller  722  to form the second folding nip N 2 . The second folding nip N 2  is formed in a direction that is different from a direction in which the first folding nip N 1  is formed. The first folding blade  750 - 1  is moved to the insertion location at which the paper P fed along the folding path  710  is pushed into the first folding nip N 1  and to the retreat location retreating from the insertion location. The second folding blade  750 - 2  is moved to an insertion location at which the paper P having passed through the first folding nip N 1  is pushed into the second folding nip N 2  and to the retreat location retreating from the insertion location. The location determination member  740  may be moved to the alignment location at which the paper P fed along the folding path  710  is supported and aligned, and to the folding location at which the portion, where a folding line of the paper P is to be formed, is located at the entrance of the first folding nip N 1 . The structure for moving the first folding blade  750 - 1  and the second folding blade  750 - 2  to the insertion location and the retreat location may be the same as the structure described with reference to  FIG. 2 . The structure for moving the location determination member  740  to the alignment location and the folding location may be the same as the structure described with reference to  FIG. 2 . 
     The guide members  752  and the elastic member  753  which are described above may be applied to at least one of the first folding blade  750 - 1  and the second folding blade  750 - 2 . The guide members  752  and the elastic member  753  are useful after folding is performed twice, and thus, if the guide members  752  and the elastic member  753  are applied to at least one of the first folding blade  750 - 1  and the second folding blade  750 - 2 , it may be effective to apply the guide members  752  and the elastic member  753  to the second folding blade  750 - 2 . Also, at least one of the rotation linear velocity of the first and second folding rollers  720 - 1  and  720 - 2  and the movement linear velocity of the first and second folding blades  750 - 1  and  750 - 2  may be controlled as described above, depending on the number of sheets of paper P which are folded at the same time. In addition, the insertion locations of the first folding blade  750 - 1  and the second folding blade  750 - 2  may be controlled as first and second insertion locations as described above, depending on the number of sheets of paper P which are folded at the same time. 
     It should be understood that examples described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each example should typically be considered as available for other similar features or aspects in other examples. 
     While one or more examples have been described with reference to the drawings, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the inventive concept as defined by the following claims.