Abstract:
An image forming apparatus to reverse a sheet, includes a branched conveyance path branched from a connection point between an entry and conveyance path and a reversal and conveyance path provided downstream of the entry and conveyance path. When a sheet conveyor feeds the sheet from the entry and conveyance path into the reversal and conveyance path, a controller controls a gate member to be opened on the basis of a detection result of a sensor to detect the sheet until a leading edge of the sheet passes the gate member, switches the gate member to be closed before a trailing edge of the sheet reaches the gate member, subsequently switches a sheet reversing and conveying member to rotate reversely after the trailing edge of the sheet passes the gate member, thereby feeding the sheet into the branched conveyance path.

Description:
[0001]     This application is based on Japanese Patent Application No. 2005-193680 filed on Jul. 1, 2005, which is incorporated hereinto by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a sheet reversing mechanism in an image forming apparatus. More specifically, the invention relates to a mechanism for reversing the sheet to switch the conveyance direction in an image forming apparatus such as a printer.  
         [0003]     The image forming apparatus, for example, reads a document image, writes and develops the image onto a photoreceptor applied with high voltage to form a toner image, and transfers and fixes the toner image onto a sheet as a recording paper, and then collects it on a paper ejection tray. At the time of the collection, the sheet is ejected face-up or face-down. Herein, “face-up” is referred to as the state where the surface with an image formed thereon is placed upward, and “face-down” is referred to as the state where the surface with an image formed thereon is placed downward.  
         [0004]     As a member for ejecting the sheet in the face-down state, there is provided a sheet reverse section. In Patent Document 1, as shown in  FIG. 8 , a sheet on which the toner image is fixed is guided to the underside of a feeding-path switching pawl  151  to a conveyer roller pair  121 ,  122 , and then the sheet is fed into a reverse path made up of guide plates  163 ,  164  by the conveyance roller pair  121 ,  122 . Subsequently, the sheet fed into the reverse path is kept conveyed in the feeding direction (downward) by a conveyance roller pair  131 ,  132 , until the leading edge of the sheet passes the feeding-path switching pawl  153  through the position of the conveyance roller pair  121 ,  122 . Next, when the trailing edge of the sheet passes the position of the conveyance roller pair  121 ,  122  and its passage is detected by a sheet sensor  157 , the reverse drive of the conveyance roller pair  131 ,  132  as a sheet reversing and conveying member is started, after waiting until the trailing edge of the sheet passes the feeding-path switching pawl  153 . Because of this feature, the initial trailing edge of the sheet is guided to the left side of the feeding-path switching pawl  153  in the figure. The sheet reaches a conveyance roller pair  121 ,  123 , and is kept conveyed by the conveyance roller pair  121 ,  123 . The sheet is further conveyed to an ejecting roller pair  141 ,  142  through a conveyance roller pair  111 ,  113 , and then is ejected on an ejection tray outside the apparatus in the state where the surface with the image formed thereon is placed downward.  
         [0005]     Having described an example of the sheet reverse section in the conventional technology disclosed in Patent Document 1, and the similar configuration has generally been used. The feeding-path switching pawl  153  used as the gate member always keeps to urge the sheet by a spring  176 B as also shown in the enlarged view of  FIG. 4 .  
         [0006]     However, this method is designed to set the spring pressure for urging the feeding-path switching pawl to be low, particularly considering damage that would be inflicted on a thin paper when the thin paper enters the feeding-path switching pawl. Its behavior will be described below.  
         [0007]     There is also a method that presses the sheet using a thin plate of PET serving both as the spring and the feeding-path switching pawl, but from a standpoint of abrasion, it is unsuitable for a high speed machine.  
         [0008]     [Patent Document 1] Unexamined Japanese Patent Publication No. 05-51158  
         [0009]     As described above, the conventional methods have been required to set, for example, the pressure of the spring as a resilient member for urging the feeding-path switching pawl to be low, particularly considering the damage in the entrance of the thin paper into the feeding-path switching pawl, namely, the occurrences of a leading edge trace of the thin paper and a paper jam. Thus, the control force of the feeding-path switching pawl to the sheet is weak and the response of the sheet trailing edge is bad, so that it has been difficult to immediately stop the sheet in the always same position to switch the conveyance direction after the sheet trailing edge passes the feeding-path switching pawl  153 . Consequently, the reverse point at which the conveyance direction is switched has had to be located downstream the just end of the feeding-path switching pawl. As a result, the image output productivity has lowered. In order to avoid the lowering of the productivity, it is required to increase the conveyance speed of the sheet after the conveyance direction is switched. However, the motor noise grows louder as the conveyance speed is increased,, which has not been preferred.  
       SUMMERY OF THE INVENTION  
       [0010]     The invention has the following configuration.  
         [0011]     An image forming apparatus which reverses a sheet includes the following: an entry and conveyance path through which the sheet is conveyed; a reversal and conveyance path provided downstream in the sheet conveyance direction relative to the entry and conveyance path; a branched conveyance path branched from a connection point between the entry and conveyance path and the reversal and conveyance path; a sheet conveyor for conveying the sheet, provided in the entry and conveyance path; a gate member that can open and close and is provided at the connection point between the entry and conveyance path and the reversal and conveyance path; a sheet reversing and conveying member that can be rotated forwardly and reversely for conveying the sheet and is provided in the reversal and conveyance path; a sheet sensor for detecting the sheet, provided in the upstream side in the sheet conveyance direction relative to the gate member; and a controller for controlling the gate member and the sheet reversing and conveying member based on the detection result by the sensor. The controller provides control so as to keep the gate member in an open state until the leading edge of the sheet passes the gate member, when feeding the sheet into the reversal and conveyance path from the entry and conveyance path by the sheet conveyor, based on the detection result of the sensor; switch the gate member to a close state before the trailing edge of the sheet reaches the gate member; subsequently switch the sheet reversing and conveying member to the reverse rotation after the trailing edge of the sheet passes the gate member; and feed the sheet into the branched conveyance path. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a general configuration view showing an example of an image forming apparatus of the invention;  
         [0013]      FIG. 2  is a general configuration view showing a sheet reverse section incorporated in the invention;  
         [0014]      FIG. 3  is a partial enlarged view showing the vicinity of a gate member in the sheet reverse section of the image forming apparatus of the invention;  
         [0015]      FIG. 4  is a partial enlarged view showing the vicinity of a feeding-path switching pawl of the sheet reverse section which is a gate member of the conventional technology;  
         [0016]      FIG. 5  is a block circuit diagram showing a controller of the invention;  
         [0017]      FIG. 6  is a time chart showing the controller of the invention;  
         [0018]      FIG. 7  is a time chart of the controller in the conventional technology as the background art; and  
         [0019]      FIG. 8  is a general configuration view showing the sheet reverse section of the conventional technology. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0020]     Hereinafter, the preferred embodiment of an image forming apparatus of the invention will be described. It is to be understood that the description of this section does not limit the technical scope of the claim and the definitions of terms. In addition, the decisive description in the embodiment of the invention is to represent the best mode, and is not to limit the definitions of terms or the technical scope of the claim.  
         [0021]     An image forming apparatus  1  shown in  FIG. 1  is an image forming apparatus of the digital system, which is provided with an image reading section A, an image processing section B, an image forming section C, and a sheet feeding and conveying section D for feeding and conveying sheet P as recording paper. However, the image forming apparatus according to the invention is not limited to the above described one, and may be applied to inkjet printers or printing machines.  
         [0022]     Provided in an upper portion of the image reading section A is an automatic document feeder for automatically conveying the document. The documents on a document placing table  11  is separated as a single sheet of paper by a document conveyance roller  12  to carry out the image reading in a reading position  13   a . The document having been subjected to document reading is ejected onto a document ejection tray  14  by the document conveyance roller  12 .  
         [0023]     In the case when the document is placed on a platen glass  13 , the image of the document is read by the reading action of a first mirror unit  15  provided with a lighting lamp and a first mirror that make up a scanning optical system at velocity v, and by the movement of a second mirror unit  16  provided with a second mirror and a third mirror that are placed in a V-shape at velocity v/ 2  in the same direction.  
         [0024]     The read image is formed on a receiving surface of an imaging element CCD which is a line sensor through a projection lens  17 . The linear optical image that is formed on the imaging element CCD is photoelectrically converted into electrical signals (luminance signals) in order. The electrical signals are A/D converted, and subjected to processes such as density conversion and filtering in the image processing section B. The image data is temporarily stored in a memory.  
         [0025]     In the image forming section C, a drum-shaped photoreceptor  21  which is an image carrier is provided on a periphery thereof with a charger  22  for charging the photoreceptor  21 , a potential detector  220  for detecting the surface potential of the charged photoreceptor, a developing device  23 , a transfer electrode  24  and separation electrode  25  which are a transfer and separation device, a cleaning device  26  of the photoreceptor  21 , and a PCL (pre-charge lamp)  27  as a light discharger, which are arranged in the order of the actions respectively as an image forming unit. Further, provided in the downstream side of the developing device  23  is a reflectance density detector  222  for measuring the reflectance density of a patch image developed on the photoreceptor  21 . The photoreceptor  21  is formed such that the photoconductive compound is coated on a drum base, and for example, an organic photoconductor (OPC) is preferably used therein. The photoreceptor  21  is driven and rotated in the clockwise direction shown in the figure.  
         [0026]     The rotating photoreceptor  21  is uniformly charged by the charger  22 . Then, the imagewise exposure is carried out based on the image signal called from the memory of the image processing section B, by an exposure optical system  30  as an imagewise exposure device. The exposure optical system  30  as the imagewise exposure device which is a writing device serves as the main scanning in which the optical path is bended by a reflection mirror  32  through a rotating polygon mirror  31 , an fθ lens  34 , and a cylindrical lens  35 , with a laser diode not shown as an emission source. The imagewise exposure is applied to the photoreceptor  21  in the position Ao and a latent image is formed by the rotation (sub-scanning) of the photoreceptor  21 . In an example of the embodiment, the exposure is applied to the character part to form a latent image.  
         [0027]     The latent image on the photoreceptor  21  is reversed and developed by the developing device  23 , and a visible toner image is formed on a surface of the photoreceptor  21 . The sheet feeding and conveying section D is provided below the image forming unit with paper feeding units  41 (A),  41 (B),  41 (C) as sheet storing devices in which the sheet P as the recording paper of different size is accommodated respectively, and with a conveyance roller  43 D. It is further provided laterally with a hand feeding unit  42  for manually feeding. The sheet P selected from any of the feeding units is fed along a conveyance path  40  by a guide roller  43 , and temporarily stopped by a registration roller pair  44  that corrects the skew and shift of the sheet P to be fed, and then is fed again. The sheet P is guided to the conveyance path  40 , a pre-transfer roller  43   a , a paper feeding path  46  and an entrance guide plate  47 , in which the toner image on the photoreceptor  21  is transferred onto the sheet P by the transfer electrode  24  and the separation electrode  25  in the transfer position Bo. Then, the sheet P is separated from the surface of the photoreceptor  21 , while being set on and conveyed by a conveyance belt  454  of a conveyance belt device  45 , and conveyed to a fixing device  50  by the conveyance belt device  45 .  
         [0028]     The fixing device  50  has a heat roller  51  as a rotation body drive member with a heat source as a heater, and a pressure roller  59  without a heat source as a heater. The sheet P is passed through between the heat roller  51  and pressure roller  59  to fix the toner by heating and pressure. The sheet P on which the toner image has been fixed is ejected to a paper ejecting section  81  through a paper ejecting roller pair  81 A,  81 B, in the state of face-down in which the sheet is reversed by a sheet reverse section  170 .  
         [0029]     The configuration and function of the sheet reverse section  170  will be described also using the configuration view of  FIG. 2 .  
         [0030]     In the downstream side next to the fixing device  50 , there are provided an entrance detection sensor  62  for detecting a sheet at an entrance gate  171  of the sheet reverse section  170 , a conveyance roller pair as a sheet conveyor  61  for feeding a sheet, the entrance gate  171 , a reversal timing sensor  63 , a gate member  175  which is closed by the pressing force of a spring  176 A as a resilient member allowing the sheet to pass through and is opened by a solenoid  176  as a forced opening and closing device, and a sheet reversing and conveying member  178  which can be rotated forwardly and reversely, which are arranged in the description order along the conveyance path. Of the conveyance path, the upstream from the above described gate member  175  is an entry and conveyance path  172  and the downstream is a reverse conveyance path  177 . The conveyance path is further provided with a branched conveyance path  179  from the switched gate member  175  to the paper ejecting section  81 .  
         [0031]     The action of the sheet reverse section  170  having such a configuration and a controller  200  will be described using the block circuit diagram of  FIG. 5  and the time chart of  FIG. 6 .  
         [0032]     As shown in the block circuit diagram of  FIG. 5 , there are provided an entrance sensor  62  made up of an actuator  62 B and a photoelectric switch  62 A and a reversal timing sensor made up of an actuator  63 B and a photoelectric switch  63 A. The information of set timer values T 1 , T 2 , Ta for the detection values of the above sensors is input to the controller  200 . The powers of the sheet conveyor  61  and solenoid  176  are respectively turned on and off based on an instruction of the controller  200  in response to the information corresponding to each of the set timer values. Thereby, the gate member  175  and the spring  176 A as the resilient member are activated, and the sheet reversing and conveying member  178  which can be rotated forwardly and reversely is activated as well.  
         [0033]     Further, as also shown in the time chart of  FIG. 6 , the sheet P on which the toner image has been fixed is detected by the entrance sensor  62 . The sheet P is just driven to the fixing device  50 , passes the conveyor roller pair  61  as the sheet conveyor, and is conveyed through the opened entrance gate  171  and the entry and conveyance path  172  of the sheet reverse section  170  in the state where the gate member  175  is opened, without any load imposed by the gate member  175 . Incidentally, the gate member  175  is opened when the timer works to reach the timer value T 1  after the entrance sensor  62  has detected the sheet P. At the time when the timer reaches the timer value T 1 , the leading edge of the sheet P does not reach the gate  175  yet.  
         [0034]     Incidentally, instead of the sheet conveyor  61  to the entrance gate  171  of the sheet reverse section  170 , the roller pair by the fixing device  50  may be used as the sheet conveyor.  
         [0035]     After the leading edge of the sheet P passes the gate member  175  and when the timer reaches the timer value T 2  following the timer value T 1 , the gate member  175  which has been forced opened by the solenoid  176  is closed, and thereby a guide surface  175 A of the gate member  175  urges the sheet surface with the pressing force F of the spring  176 A as the resilient member.  
         [0036]     It is further preferable that the guide surface  175 A of the gate member  175  is designed to keep the state of urging the sheet surface with the pressing force F of the spring  176 A as the resilient member, by closing the gate member  175  which has been forced opened by the solenoid  176 , after the leading edge of the sheet P passes the gate member  175  and reaches the conveyance roller pair which is the sheet reversing and conveying member  178  which can be rotated forwardly and reversely, as well as after the timer value T 2  passes following the timer value T 1 , because the conveyance resistance of the sheet P can be avoided.  
         [0037]     Subsequently, the reversal timing sensor  63  detects the sheet trailing edge and the timer works as well. The timer value Ta can be set to smaller than the timer value Tb as shown in the time chart of the conventional example of  FIG. 7 . Incidentally, it can be seen that 8 ms of the timer value Ta in the invention is smaller than 24 ms of the conventional timer value Tb. The former corresponds to a distance of 5 mm when the sheet is conveyed at high speed with a linear velocity of 700 mm, and the latter corresponds to a distance of 15 mm. Thus, the invention makes it possible to switch the conveyance direction at a position further upstream than in the conventional technology, and it is possible to substantially raise the sheet conveyance efficiency in the image forming apparatus required to realize a high speed output. In addition, as an error of about +10 mm may occur in the distance due to the mechanical assembling accuracy or controlled accuracy, a difference of at most about 20 mm may occur between in the conventional example and the invention.  
         [0038]     The difference of the reverse point at which the conveyance direction is switched is schematically shown in the partial enlarged view in the vicinity of the gate member of the invention shown in  FIG. 3  and the partial enlarged view in the vicinity of the feeding-path switching pawl which is the gate member of the conventional example shown in  FIG. 4 .  
         [0039]     As described in the background of the invention, conventional switching of the conveyance path is carried out by the method that the feeding-path switching pawl is in the state of being pressed against the sheet conveyance path by the spring or the thin plate of PET so that the sheet passes through the spring or the thin plate counter to the urging force thereof. Thus, the increased pressing force may cause the leading edge trail left when the sheet leading edge passes through or the occurrence of a paper jam, so that the pressing force F is set to a lower value ranging from 49 to 59 mN. When the sheet trailing edge passes through and moves to the reverse and conveyance path, the timer value Tb has had to be set so that the stop position of the sheet trailing edge is located in the downstream side, because the control force to the sheet is weak and the response of the sheet trailing edge is bad.  
         [0040]     On the other hand, the gate member used in the invention, when the sheet leading edge passes through, is forced opened enough to allow the sheet to pass through without load, for example, by the solenoid, so that the leading edge trail is not left and the paper jam can be prevented. Further, the gate surface is urged by substantially strong pressing force F ranging from 490 to 590 mN when the leading edge does not passes through, and an appropriate control force is applied to the sheet, so that the response of the sheet trailing edge is good, and the stop position of the sheet trailing edge can be located further upstream than in the conventional technology. This makes it possible to avoid the event involving the occurrence of unpleasant motor noise when the reverse speed of the sheet is substantially increased, as well as to increase the image output productivity.  
         [0041]     As described above, at the time when the sheet trailing edge reaches the position downstream the gate member  175  and in the vicinity of the gate member  175 , the conveyance before reversal is completed. Then the sheet reversing and conveying member which can be rotated forwardly and reversely starts the reverse drive. The sheet P is conveyed through the branched conveyance path  179  to the paper ejecting section  81  with a guide surface  175 B opposite to the guide surface  175 A of the switched gate member  175  as a guide. Thereby, the sheet P reaches the paper ejecting section  81 .  
         [0042]     In this embodiment, the opening and closing operation of the gate member  175  is controlled based on the detection of the entrance sensor  62  placed upstream the reversal timing sensor  63 . Because of this feature, the spacing between the reversal sensor  63  and the gate member  175  is narrow, so that the opening and closing timing of the gate member  175  can be precisely performed in the apparatus that conveys the sheet P at high speed. However, the invention is not limited to this embodiment, and can be designed that the opening operation of the gate member  175  is carried out based on the detection information by the entrance sensor  62  and the closing operation is carried out based on the detection information of the reversal timing sensor  63 . It can be also designed that a common sensor serving both as the entrance sensor  62  and the reversal timing sensor  63  is provided upstream the gate  175  to control both of the gate member  175  and the sheet reversing and conveying member  178 , based on the detection information of the relevant sensor and the timer value.  
         [0043]     The image forming apparatus of the invention is not limited to the above context, and can be applied to inkjet printers, printer machines and other related machines.  
         [0044]     According to the embodiment, the occurrences of the leading edge trail and paper jam are suppressed even with the thin sheet, the control force to the sheet trailing edge works, and the response of the sheet trailing edge becomes good, so that it is possible to locate the reverse point at which the sheet conveyance direction is switched in the upstream side. This makes it possible to raise the sheet conveyance efficiency after image forming as well as to increase the image output productivity.  
         [0045]     Further, the present invention can be applied not only to the reversing mechanism of the sheet which is the recording paper on which the image to be recorded, but also to the reversing mechanism to the sheet of the document. Particularly, when it is applied to an apparatus for reversing the document before reading the image of the document, the document conveyance efficiency rises with the document reading speed improved.