Patent Publication Number: US-8113514-B2

Title: Sheet decurling device and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to and incorporates by reference the entire contents of Japanese priority document 2007-164602 filed in Japan on Jun. 22, 2007. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet decurling device and an image forming apparatus, and more particularly, to a structure that removes a curl of a sheet that occurs during a transporting process. 
     2. Description of the Related Art 
     In an image forming apparatus such as a multi function peripheral (MFP), a printer, a facsimile etc., visual images such as toner images, which are carried on a surface of photosensitive drums that are latent image carrying members, are transferred onto a sheet such as a recording sheet. The sheet is fixed by a fixing device using addition of heat and is ejected. 
     In the fixing device, a toner image carrying surface of the sheet touches a heating source member such as a heating roller and heat and pressure are used to cause the toner to melt and penetrate the sheet, thus fixing the toner on the sheet. 
     Upon receiving the heat from the heat source member, moisture of the sheet that is subjected to fixing evaporates, thus causing a change in a moisture content percentage of a sheet surface that is touching the heating source member and a back surface. Change in the moisture content percentage causes occurrence of warping in a thickness direction, in other words, a rolling tendency called a curl. 
     When the curled sheet is ejected and stacked, end edge positions of the stacked sheets do not align, thus necessitating an operation to align side end edges of the sheets when stacking is completed. 
     In one of the existing structures that is disclosed in Japanese Patent No. 3050633 and that is used to curb a curl, the sheet that has passed the fixing device is caused to touch a cooling member such as a heat pipe, thus curbing evaporation of the moisture content. Alternatively, in a structure that decurls the sheet and that is disclosed in Japanese Patent Application Laid-open No. H3-44985, among a pair of rollers that sandwich-transport the sheet, a speed of the roller opposite to a curled surface side of the sheet is enhanced at the time of sandwich-transportation of the sheet and the sheet is forcibly beaten to cause occurrence of curl such that the surface of the sheet opposite the roller with the enhanced speed is rolled around the side of the roller. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to at least partially solve the problems in the conventional technology. 
     According to an aspect of the present invention, there is provided a sheet decurling device that decurls a recording sheet, including a first transportation path and a second transportation path that can selectively link to a main transportation path of the recording sheet; a first roller arranged between the first transportation path and the second transportation path; a second roller capable of making contact with the first roller across the first transportation path; a third roller capable of making contact with the first roller across the second transportation path; a moving unit that includes the first transportation path, the second transportation path, the first roller, the second roller, the third roller, and a driving source that drives the first roller, and that moves its position such that the first transportation path and the second transportation path are selectively linked to the main transportation path; a moving position detecting unit that detects a position of the moving unit; and a mechanism that varies, upon the moving unit moving to a position at which either one of the first transportation path and the second transportation path is linked to the main transportation path, a pressing force of the second roller or the third roller with respect to the first roller. 
     Furthermore, according to another aspect of the present invention, there is provided an image forming apparatus including a sheet decurling device that decurls a recording sheet. The sheet decurling device includes a first transportation path and a second transportation path that can selectively link to a main transportation path of the recording sheet; a first roller arranged between the first transportation path and the second transportation path; a second roller capable of making contact with the first roller across the first transportation path; a third roller capable of making contact with the first roller across the second transportation path; a moving unit that includes the first transportation path, the second transportation path, the first roller, the second roller, the third roller, and a driving source that drives the first roller, and that moves its position such that the first transportation path and the second transportation path are selectively linked to the main transportation path; a moving position detecting unit that detects a position of the moving unit; and a mechanism that varies, upon the moving unit moving to a position at which either one of the first transportation path and the second transportation path is linked to the main transportation path, a pressing force of the second roller or the third roller with respect to the first roller. 
     The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic for explaining a structure of a sheet decurling device according to an embodiment of the present invention; 
         FIG. 2  is a schematic of a concrete example of the sheet decurling device shown in  FIG. 1  viewed from an ejection direction of a recording material; 
         FIG. 3  is a schematic of the sheet decurling device shown in  FIG. 2 ; 
         FIG. 4  is a schematic for explaining a principle that is related to a movement of a moving unit shown in  FIG. 2 ; 
         FIG. 5  is a schematic of a structure of a roller driving system that is used in the sheet decurling device that is shown in  FIG. 2 ; 
         FIG. 6  is a schematic for explaining a condition during a movement of the roller driving system shown in  FIG. 5 ; 
         FIG. 7  is a schematic for explaining a mode in the sheet decurling device shown in  FIG. 2 ; 
         FIG. 8  is a schematic for explaining another mode in the sheet decurling device shown in  FIG. 2 ; 
         FIG. 9  is a block diagram of a controller that is used for rotation control of the moving unit in the sheet decurling device shown in  FIG. 2 ; 
         FIG. 10  is a schematic of an image forming apparatus that incorporates the sheet decurling device according to the present invention; and 
         FIG. 11  is a schematic for explaining an example of an existing image forming apparatus that uses a sheet decurling device. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Exemplary embodiments of the present invention are explained in detail below with reference to the accompanying drawings. 
     An installation site of a sheet decurling device is explained first.  FIG. 11  is a schematic of an overview of an image forming apparatus P that uses a common electronographic method and the sheet decurling device. 
     Based on a principle of a common electronographic printer, after a charger  1  has uniformly charged a photosensitive drum  2 , an optical unit  3  emits light on a surface of the photosensitive drum  2 . 
     Due to emission of light, an electric charge on the surface of the photosensitive drum  2  partially escapes, and an electrostatic latent image is formed on the surface of the photosensitive drum  2 . Next, a developer  4  adheres a toner, which is reversely charged with respect to the photosensitive drum  2 , on the electrostatic latent image to form a visual image. Next, a toner image, which is formed on the surface of the photosensitive drum  2 , is transferred by a transfer unit  5  onto a sheet that is a recording member and that is transported via a string of transportation paths from a sheet feeder  6 . A fixing unit  7 , which includes a heating and pressurizing roller, melts the toner that is transferred onto the recording member to form an image on the sheet. Next, the sheet is transported to a discharging unit  8 . 
     During a flow of the string mentioned earlier, when using the fixing unit  7  to carry out fusion of the toner on the sheet that includes the transferred toner image, addition of heat by the fixing unit  7  results in evaporation of water vapor included in the sheet. Due to this, a curl occurs in a transfer surface side or an opposite side of the sheet. 
     To overcome the drawback of occurrence of the curl, the sheet decurling device is necessitated in the vicinity of a G portion on a transportation path from the fixing unit  7  of the printer towards the discharging unit  8 . Due to this, when the sheet, which is curled at the time of passing the fixing unit  7  and moving towards the discharging unit  8 , passes the sheet decurling device, the curl is corrected by the sheet decurling device. 
     The sheet decurling device according to the present invention is explained next. 
     As shown in  FIG. 10 , a sheet decurling device  1000  according to the present invention includes a moving unit  100  such that a first transportation path  101  and a second transportation path  102 , which are internally included, can be selectively linked to a main path MP. The sheet decurling device  1000  shown in  FIG. 10  is internally embedded into the image forming apparatus. However, the sheet decurling device according to the present invention can also be arranged separately from the image forming apparatus. For example, the sheet decurling device can be arranged at an entrance of a sorter, a collator, or a stacking device that are used in a post process of a recording sheet. 
       FIG. 1  is a schematic for explaining an internal structure of the moving unit  100 . As shown in  FIG. 1 , the moving unit  100  includes a first roller  103 , a second roller  104 , and a third roller  105 . The first roller  103  is arranged between the first transportation path  101  and the second transportation path  102  such that a portion of a peripheral surface of the first roller  103  is exposed to the first transportation path  101  and the second transportation path  102 . The second roller  104  can touch the first roller  103  across the first transportation path  101 . The third roller  105  can touch the first roller  103  across the second transportation path  102 . 
     As shown in  FIG. 10 , the first transportation path  101  and the second transportation path  102  include an angle, indicated by arrows A and B respectively, that can link with an entering direction of the recording sheet that is set by transportation rollers  9  shown in  FIG. 10 , and an angle, indicated by arrows A′ and B′ respectively, that can set an ejection direction that can link to the discharging unit  8  shown in  FIG. 10 . 
     The first roller  103  is formed of an elastic body such as sponge rubber. The second roller  104  and the third roller  105  are metal rollers formed of stainless steel. The second roller  104  is used as an upper pressure roller that is positioned on the upper side of the first roller  103  and the third roller  105  is used as a lower pressure roller that is positioned on the lower side of the first roller  103 . The second roller  104  and the third roller  105  are pressed against the first roller  103 , thus enabling the second roller  104  and the third roller  105  to function as decurling rollers that correct the curl by causing the recording sheet to roll in a reverse direction of a curl direction. 
     The second roller  104  and the third roller  105  are opposite to each other across the first roller  103  and are positioned such that a line L, which joins touching positions of the second roller  104  and the third roller  105  with respect to the first roller  103 , is perpendicular to progressing directions of the sheet that enters from the first transportation path  101  and the second transportation path  102  (directions indicated by the arrows A and B respectively). Due to this, the recording sheet, which enters inside the sheet decurling device  1000 , can enter in a tangential direction with respect to the first roller  103 , the second roller  104 , and the third roller  105 , and can receive pressure from the first roller  103 , the second roller  104 , and the third roller  105  most efficiently. 
     As shown in  FIG. 1 , for selectively linking the first transportation path  101  and the second transportation path  102  to the main transportation path MP, the moving unit  100  can move in a perpendicular direction. 
       FIGS. 2 and 3  are schematics of the moving unit  100  and a moving mechanism of the moving unit  100 . As shown in  FIG. 2 , an elevating unit  200 , which includes a motor  201  and a motor  202 , is arranged in an upper portion of the moving unit  100  that is indicated by a dotted line. The motor  201  is an elevatably driving source of the moving unit  100  and the motor  202  is a rotatably driving source of the first roller  103 . 
     As shown in  FIG. 3 , a chassis portion of the elevating unit  200  surrounds a periphery of the moving unit  100 . Due to this, the moving unit  100  can elevate inside the elevating unit  200  that is arranged separately from the moving unit  100 . 
     A driving pulley  201 A is arranged on the motor  201  that is arranged as the elevatably driving source on the elevating unit  200 . An elevating screw  205 , which is integrated with a driven pulley  204 , can link with the driving pulley  201 A via a belt  203 . 
     The elevating screw  205  is fitted to a not shown nut inside a side wall  100 A that is integrated with the upper portion of the moving unit  100 . The moving unit  100  is elevated according to a rotation direction of the elevating screw  205 . 
     An elevation position of the moving unit  100  is detected by a position detecting sensor PS (see  FIG. 2 ) that uses an optical sensor that can detect light interception upon entry of a sector member that is arranged on a side of the moving unit  100 . The moving unit  100  stops at a predetermined position. 
     An elevation amount of the moving unit  100  is equivalent to a distance till a position that enables any one of the first transportation path  101  and the second transportation path  102  to link to the main transportation path MP. 
     A plurality of idle gears IG 1  to IG 3 , which can link with the motor  202 , are arranged on the motor  202  that is arranged on the elevating unit  200  as the rotatably driving source. The idle gears IG 1  to IG 3  treat as a first level, a driving gear  202 A that is fixed to an output shaft of the motor  202 . A decurler roller gear  103 A, which is fixed to a rotating shaft  103 B of the first roller  103 , meshes with the idle gear IG 3  of the last level. 
     The sheet decurling device includes a structure that changes a touching force of any one of the second roller  104  and the third roller  105  with respect to the first roller  103  when any one of the first transportation path  101  and the second transportation path  102  moves to a position that links to the main transportation path MP. The structure and working of the structure are explained next with reference to  FIG. 4  onwards. 
       FIG. 4  is a schematic for explaining a principle that is used for causing the moving unit  100  at the elevation position to change a pressurizing force of the second roller  104  and the third roller  105  with respect to the first roller  103 . 
     As shown in  FIG. 4 , rotating shafts  103 B,  104 A, and  105 A, of the first roller  103 , the second roller  104 , and the third roller  105  respectively, are inserted into the side wall  100 A of the moving unit  100 . 
     Among the rotating shafts  103 B,  104 A, and  105 A, an axial end of the rotating shaft  103 B of the first roller  103  is inserted into a long hole  200 A 1  that is formed in a side wall  200 A of the elevating unit  200  that is arranged separately from the moving unit  100 . The long hole  200 A 1 , which is formed in the side wall  200 A, is arranged along a moving direction of the moving unit  100 . 
     In the side wall  101 A of the moving unit  100 , the rotating shaft  103 B of the first roller  103  is inserted into a freely-moving supporting unit  100 A 1  that is formed as a long hole along the moving direction of the moving unit  100 . The rotating shafts  104 A and  105 A of the second roller  104  and the third roller  105  respectively are rotatably supported via a shaft bearing SB. Unlike the supporting unit  100 A 1 , the shaft bearing SB does not cause the rotating shafts  104 A and  105 A to move in the moving direction of the moving unit  100 . 
     By adding a longitudinal direction length of the supporting unit  100 A 1  that is arranged in the side wall  100 A of the moving unit  100  to a longitudinal direction length of the long hole  200 A 1  that is formed in the side wall  200 A on the side of the elevating unit  200 , a stroke is set that enables the second roller  104  and the third roller  105  to increase the pressurizing force with respect to the first roller  103 . 
     In the structure mentioned earlier, when the first roller  103  is pressed against an end of the long hole  200 A 1  due to a movement of the moving unit  100  and the moving unit  100  continues to move, the second roller  104  and the third roller  105  move independently with respect to the first roller  103 , and depending on the moving direction, the pressurizing force of any one of the second roller  104  and the third roller  105  with respect to the first roller  103  increases. 
     For example, when the moving unit  100  is elevated, the rotating shaft  103 B of the first roller  103  is pressed against an end, in other words, the upper end, of the long hole  200 A 1 , and the moving unit  100  continues to be elevated further, the supporting unit  100 A 1  can cause the moving unit  100  to move without interfering with the first roller  103 . Due to this, the third roller  105 , which is positioned in a peripherally downward direction with respect to the first roller  103 , is pressed against the first roller  103  and the pressurizing force of the third roller  105  with respect to the first roller  103  increases. 
     When the moving unit  100  moves downward, the rotating shaft  103 B of the first roller  103  is pressed against the lower end of the long hole  200 A 1 , and the moving unit  100  continues to move downward, the supporting unit  100 A 1  can cause the moving unit  100  to move without interfering with the rotating shaft  103 B of the first roller  103 . Due to this, the second roller  104 , which is positioned in a peripherally upward direction with respect to the first roller  103 , is pressed against the first roller  103 . Thus, the pressurizing force of the second roller  104  with respect to the first roller  103  increases. 
       FIGS. 5 and 6  are schematics for explaining a driving path towards the first roller  103  according to a movement position of the moving unit  100 . A side surface shown in  FIGS. 5 and 6  indicates a side surface that is seen from the opposite side of the side surface shown in  FIG. 1 . 
       FIG. 5  is a schematic of the driving path when the moving unit  100  is elevated and the second roller  104  is touching the first roller  103 .  FIG. 6  is a schematic of the driving path when the moving unit  100  has moved downward and the third roller  105  is touching the first roller  103 . 
     As shown in  FIG. 5 , when the moving unit  100  is being elevated, a torque from the driving gear  202 A of the motor  202 , which is set to rotate in one direction, is transmitted to the idle gears IG 1  to IG 3 . Further, the torque is also transmitted to the decurler roller gear.  103 A that meshes with the idle gear IG 3 . 
     As shown in  FIG. 6 , when the moving unit  100  is moving down, the torque from the driving gear  202 A on the side of the motor  202  is transmitted to the idle gears IG 1  and IG 2 . Further, the torque is also transmitted to the decurler roller gear  103 A that meshes with the idle gear IG 2 . 
     Thus, simply setting a rotation direction of the motor  202  to one direction enables to establish a meshing relation between the idle gears IG 1  to IG 3  and the decurler roller gear  103 A that are opposite to each other when the moving unit  100  is elevated, thus enabling to set the rotation direction of the first roller  103  such that the first roller  103  can eject the sheet using the first transportation path  101  and the second transportation path  102 . 
     Thus, in the present embodiment, using the structure mentioned earlier enables to set a position such that any one of the first transportation path  101  and the second transportation path  102  links to the main transportation path MP according to the moving direction of the moving unit  100 . 
       FIGS. 7 and 8  are schematics for explaining a relation among the moving unit  100 , the main transportation path MP, and an ejection path (for the sake of convenience, the ejection path is indicated by a symbol EX) that is turned towards the discharging unit  8 . The ejection path when the moving unit  100  is elevated is shown in  FIG. 7 . The ejection path when the moving unit  100  has moved downwards is shown in  FIG. 8 . 
     As shown in  FIG. 7 , if the curl of the recording sheet, which is transported via the main transportation path MP that is arranged on a main body side of the image forming apparatus, is concave shaped, the moving unit  100  is elevated and a condition shown in  FIG. 7  is set. Thus, the second transportation path  102  in the moving unit  100  links to the main transportation path MP and based on the operation explained with reference to  FIG. 4 , the pressurizing force of the third roller  105  with respect to the first roller  103  increases. Due to this, the recording sheet, which is sandwich-transported by the first roller  103  and the third roller  105 , is subjected to decurling in an opposite direction of the curl direction of the curl that occurs in the recording sheet itself and the curl is corrected. 
     If the curl of the recording sheet is convex shaped, the moving unit  100  moves downward and a condition shown in  FIG. 8  is set. Thus, the first transportation path  101  in the moving unit  100  continues to the main transportation path MP and the pressurizing force of the second roller  104  with respect to the first roller  103  increases. Due to this, the recording sheet, which is sandwich-transported by the first roller  103  and the second roller  104 , is subjected to decurling in the opposite direction of the curl direction of the curl that occurs in the recording sheet itself and the curl is corrected. 
     Thus, in any one of the operations explained with reference to  FIGS. 7 and 8 , a configuration of the first transportation path  101  and the second transportation path  102  is linked to the ejection path EX. Due to this, the recording sheet, which is sandwich-transported among the first roller  103 , the second roller  104 , and the third roller  105 , is ejected normally without occurrence of an ejection defect. 
     If the curl direction is prior recognized, an elevation setting of the moving unit  100  can be carried out to set the condition that can carry out curl correction. However, the elevation setting of the moving unit  100  can also be set by prior estimating the curl direction. A structure which prior estimates the curl direction is explained below. 
     Because the curl direction and a curl amount change according to a sheet size, environmental temperature and humidity, an area ratio of the toner image etc., in the present embodiment, rotation control of the motor  201 , which is a movably driving source of the moving unit  100 , can also be carried out. 
       FIG. 9  is a functional block diagram of a controller  300  that is used for rotation control of the motor  201 . As shown in  FIG. 9 , a controller, which executes an image forming sequence, is used as the controller  300 . An operation panel  301 , which enables specification of the sheet size, a temperature and humidity sensor  302 , and an image area ratio-input unit  303  are connected to an input side of the controller  300 . The driving pulley  201 A of the motor  201  is connected to an output side of the controller  300 . 
     Based on pixel signals from a scanner or received image signals, the image area ratio-input unit  303 , which is connected to the controller  300 , calculates an image area ratio with respect to the sheet size. 
     Based on input data such as the sheet size, the temperature and humidity, or the image area ratio, the controller  300  prior estimates the curl direction of the curl that occurs in the recording sheet and sets a selection of the first transportation path  101  and the second transportation path  102  and the rotation direction of the motor  201  such that the recording sheet can be subjected to decurling in the opposite direction of the curl direction. 
     The curl direction and the curl amount are affected by a moisture content percentage of the recording sheet. Testing an evaporation rate of the moisture content due to heating during fixing enables to prior estimate the curl direction and the curl amount to some extent. Apart from a sheet area, the sheet size can also include a sheet thickness. 
     A relation between the sheet size and the image area ratio that are affected by the evaporation rate of the moisture is mapped. Further, a relation between the temperature and humidity and the evaporation rate of the moisture content is also mapped. A correspondence is established between each input data and the evaporation rate of the moisture content to prior estimate the curl direction and the curl amount and to set the condition that enables to correct the curl, in other words, to set a movement amount of the moving unit  100  that enables to regulate the selection of a transportation path and a pressurizing force for correction. When prior estimating the curl direction and the curl amount, if occurrence of the curl is negligible, the controller  300  can also decide that movement of the moving unit  100  is not necessary. 
     In the structure mentioned earlier, based on each data of the sheet size, the temperature and humidity, and the image area ratio, the evaporation rate of the moisture content is determined. Further, based on a map of a relation between the evaporation amount and the curl amount, a curl correction amount is set and the pressurizing force of the second roller  104  or the third roller  105  with respect to the first roller  103  in the moving unit  100  is regulated such that the curl correction amount is obtained. The curl direction is affected by a difference between moisture evaporation rates of an image carrying surface and the opposite surface of the recording sheet. Thus, based on the difference between the moisture evaporation rate of the image carrying surface that is based on the image area ratio and the moisture evaporation rate of the non image carrying surface that is not based on the image area ratio, the curl direction can be decided. 
     According to an embodiment of the present invention, selectively linking a first roller, a second and a third rollers, and a first and a second transportation paths to a main transportation path enables to change a touching force of the second and the third rollers with respect to the first roller, thus enabling to subject a recording sheet to decurling for curl correction. Due to this, the curl can be corrected regardless of a curl direction by using a single unit and removing a necessity to arrange a plurality of curl correcting mechanisms according to the curl direction. Thus, an increase in a size of the sheet decurling device can be prevented. 
     A curl amount and a curl direction of the curl that occurs in the sheet change according to a sheet size, environmental temperature and humidity, an area ratio of the toner image etc. 
     The curl in different directions can be corrected by including a mechanism that corrects the respective curl according to the curl direction. However, using such a structure increases a space occupied by a curl correcting mechanism inside the image forming apparatus. 
     Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.