JP2008276095A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To stop an image forming action before a sheet winding around a fixing roller and a sheet bending in bellow shape are worsened, when a jamming occurs in a fixing apparatus. <P>SOLUTION: A printing ratio of N [mm] from a sheet tip is acquired (S4). Jamming detection time in a sheet detection sensor is set to normal, when the printing ratio is less than X[%] (S6) and set to normal-α when X[%] or more (S7) and timing is started, and at the same time, a secondary paper supplying is started by rotating a pair of registration rollers (S9). When the sheet does not reach a sheet detection sensor within jam detection time, (No in S10, No in S14), image forming action is interrupted (S15), and jamming display is carried out (S16). <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image forming apparatus that changes a jam detection time according to a printing rate.

  In a fixing device of an image forming apparatus such as a printer or a copying machine, a sheet carrying an unfixed toner image is inserted into a nip between a fixing roller and a pressure roller, thereby fixing the toner image on the sheet. A technique is known in which a sheet after image fixing is separated from the surface of the fixing roller by inserting a separation claw between the leading edge of the sheet and the fixing roller.

  However, when the margin at the leading edge of the sheet is narrow or the printing rate on the leading edge side is high, the separation of the sheet by the separation claw is not performed well, and the sheet is wound around the fixing roller or bent in a bellows shape. Paper jams. Such a jam makes it difficult to remove the sheet and causes toner to scatter.

  Patent Documents 1 and 2 disclose techniques for reducing the occurrence of such a jam. In Patent Document 1, when it is determined that the printing ratio for a certain period exceeds a predetermined ratio from the leading edge writing position of the sheet, the writing margin is set to be equal to or greater than a predetermined value.

Further, Patent Document 2 has a plurality of density area modes corresponding to the printing rate, and it is determined from the printing rate at the leading edge of the sheet that the fixed sheet may be wound around the fixing roller. In this case, a density area mode with a narrow density area is selected.
JP 2001-58450 A Japanese Patent Laying-Open No. 2005-157035

  However, according to the above-mentioned Patent Documents 1 and 2, the image on the front end side is shifted to the front end margin due to abnormal data on printing caused by a timing failure or conveyance failure (for example, slip due to paper characteristics). In this case, it is difficult to prevent jamming such as winding of the sheet around the fixing roller and bellows-like bending.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an image forming apparatus that makes it easy to remove a sheet by making the degree of sheet winding or bellows-like bending relatively small even when a jam occurs. Is.

  According to the first aspect of the present invention, a sheet printed with toner is inserted into a fixing unit formed between a fixing member and a pressure member so that the sheet is heated and pressed to fix the toner on the sheet. The present invention relates to an image forming apparatus including the apparatus. An image forming apparatus according to the present invention includes a printing rate acquisition unit that acquires a printing rate from a leading edge of a sheet to a predetermined position, and a time reference position that is set upstream of the fixing unit along the sheet conveyance direction. A sheet detection unit having a sheet detection sensor that is disposed downstream of the fixing unit along the sheet conveyance direction and detects the leading edge of the sheet; and a time after the leading edge of the sheet passes the time reference position A jam detection timer means for measuring the jam, a jam judgment means for judging a jam when a sheet does not reach the sheet detection sensor even if a predetermined jam detection time has elapsed, and the jam judgment means judges that a jam has occurred. Control means for sometimes stopping the image forming operation; and jam detection time adjusting means for changing the predetermined jam detection time based on an acquisition result of the printing rate acquisition means; Comprises, it is characterized in that.

  According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the sheet is inserted between the fixing unit and the sheet detection sensor between the leading end of the sheet on which the toner image is fixed and the fixing member. And a separation member for separating the sheet from the surface of the fixing member.

  According to a third aspect of the present invention, in the image forming apparatus according to the first or second aspect, the jam detection time adjusting unit detects the jam when the printing rate acquired by the printing rate acquisition unit is equal to or greater than a predetermined threshold. It is characterized by shortening the time.

  According to the first aspect of the present invention, when the printing rate from the leading edge of the sheet to a predetermined position is high, the jam detection time is shortened so that the jam is judged in a short time, and the degree of jam in the fixing device is deteriorated. Before the image forming operation is performed, the image forming operation can be stopped. For this reason, it is easy to remove the jammed sheet.

  According to the second aspect of the present invention, it is possible to prevent deterioration of the degree of jam that occurs when the toner-fixed sheet is not normally separated by the separation member.

  According to the invention of claim 3, when the printing rate is equal to or higher than a predetermined threshold value, that is, in a situation where a jam is likely to occur, the jam detection time is shortened. The image forming operation can be stopped before starting.

  Hereinafter, the best embodiment of the present invention will be described in detail with reference to the drawings. In addition, in each drawing, the member etc. which attached | subjected the same code | symbol are the same structures, The duplication description about these shall be abbreviate | omitted suitably. Moreover, in each drawing, members and the like that are not necessary for the description are omitted as appropriate.

<Embodiment 1>
The image forming apparatus 1 including the sheet conveying apparatus 100 according to the present invention will be described with reference to FIGS. In FIGS. 1 to 3, up, down, front, back, left, and right directions in the image forming apparatus 1 are indicated by arrows. 1 to 3, FIG. 1 is a perspective view of the entire image forming apparatus 1 as viewed from the upper left side of the front side thereof. FIG. 2 is a perspective view of the entire image forming apparatus 1 as viewed from the rear and obliquely upward on the left side. FIG. 3 is a diagram schematically showing the internal structure of the image forming apparatus 1 as viewed from the left side.

  Here, examples of the image forming apparatus 1 include a printer, a copier, a facsimile machine, and a multifunction machine of these. In the following description, a case where the image forming apparatus 1 is a printer will be described as an example. An image forming apparatus 1 shown in FIG. 1 is a four-color full-color image forming apparatus 1 that employs an electrophotographic system, an intermediate transfer system, and a tandem system.

  With reference to FIGS. 1 and 2, the configuration of the entire image forming apparatus 1 as viewed from the outside will be described. The image forming apparatus 1 includes a substantially box-shaped (cuboid) image forming apparatus main body 2 and a front cover 3 supported by the image forming apparatus main body 2 so as to be freely opened and closed.

  The image forming apparatus main body 2 has a front surface covered with a front cover 3 and a left side surface and a right side surface covered with a left exterior panel 4 and a right exterior panel 5 made of synthetic resin. On the rear surface of the image forming apparatus main body 2, a part 7 of a sheet metal constituting the main body frame 6 is exposed as a structure. The upper surface of the image forming apparatus main body 2 is covered with a front cover 3 at the front, and is covered with a paper discharge tray 10 having a sheet stacking surface 8 that is raised from the middle to the rear.

  The front cover 3 is configured by a front exterior panel 11 and a part of a sheet conveying unit 24 (see FIG. 3) described later that is incorporated inside the front cover panel 11. The front exterior panel 11 includes a front panel 12, a rear upper panel 13 that continues from the upper end, and an inverted “L” -shaped left panel 14 and right panel 15. A rectangular manual feed tray 16 is disposed on the front panel 12. The manual feed tray 16 has a swing center on the lower end side, and forms a part of the front panel 12 at the closed position of the standing posture shown in FIG. In an open position (not shown) that is rearwardly lowered, it becomes a feeding table on which the sheet is placed on the upper surface. An operation panel (operation unit) 17 is disposed on the upper panel 13. The operation panel 17 is arranged so as to be inclined substantially upward so that the rear end side is slightly higher so that an operator (for example, a user) who operates standing on the front side of the image forming apparatus 1 can easily see. A touch panel type liquid crystal display unit and various buttons are arranged on the operation panel 17, and the operator stands on the front of the image forming apparatus 1 and the entire image forming apparatus 1 is operated by the operation panel 17. It can be operated. The left panel 14 and the right panel 15 are disposed so as to cover a part of the sheet conveying unit 24 configured inside the front cover 3 from the left side and the right side, respectively.

  The front cover 3 is supported by the image forming apparatus main body 2 in a swingable manner at the lower end side, and can be opened and closed as a whole. As shown in FIG. 2, the release lever (release member) 74 that is operated when opening the front cover 3 is located at the upper end side of the left panel 14 and the inner side (right side) of the rear end side, and the upper end side of the right panel 15. And on the inner side (left side) of the rear end side. The release lever 74 has a fulcrum on the lower end side, and pushes a button 75 disposed on the upper end toward the front side to support the lock member (supported by the front cover 3 and engaged with the image forming apparatus main body 2 side). (Not shown) is released, and the front cover 3 can be opened. The front cover 3 having the above configuration removes a sheet (jam paper) that has caused a jam (paper jam) by opening and opening the front cover 3 from the front side of the image forming apparatus 1 to the front side. Is possible. Further, as will be described later, a paper feed cassette 25 (see FIG. 3) in which a sheet to be image-formed is stored can be similarly detached from the front side. As described above, the operator performs the entire operation of the image forming apparatus 1 using the operation panel 17, the jam processing when the jam occurs, the sheet replenishment to the sheet feeding cassette 25 when the sheet runs out, and the like. All can be performed from the front side of the image forming apparatus 1.

  This is the end of the description of the configuration of the entire image forming apparatus 1 viewed from the outside.

  Next, the internal structure of the image forming apparatus 1 will be described with reference to FIG. In the image forming apparatus 1, a sheet storage unit 20, a substrate storage unit 21, an image forming unit 22, a toner supply unit 23, and a paper discharge tray 10 are provided in order from the lower side to the upper side of the image forming apparatus main body 2. . A sheet conveying unit 24 is provided between the front side of the image forming apparatus main body 2 and the front cover 3.

  A sheet cassette 25 is disposed in the sheet storage unit 20. The sheet feeding cassette 25 stores a plurality of sheets in a stacked state, and biases the leading end side (the right side in the figure) of the stacked sheets upward by a lift plate 26 disposed at the bottom. Yes. As a result, the uppermost sheet in the sheet feeding cassette 25 is fed by a pickup roller 27 of a sheet transport unit 24 described later, and further, the feeding roller 28 and the retard roller 30 prevent double feeding, and only one sheet is downstream. To the side. The paper feed cassette 25 can be inserted and removed from the front side of the image forming apparatus 1. A substrate storage unit 21 is disposed above the paper feed cassette 25.

  The substrate storage unit 21 is provided with a substrate and electrical components (not shown) that control the entire image forming apparatus 1. An image forming unit 22 is disposed above the substrate storage unit 21.

  The image forming unit 22 includes an intermediate transfer belt 31 and four (four color) image forming stations having the same configuration along the rotation direction (direction of arrow R31) of the intermediate transfer belt 31, that is, yellow (Y). An image forming station 32, a magenta (M) image forming station 33, a cyan (C) image forming station 34, and a black (Bk) image forming station 35 are provided. The yellow image forming station 32 includes a photosensitive drum 36, a charging device 37, an exposure device 38, a developing device 40, and a primary transfer roller disposed around the photosensitive drum 36 along the rotation direction (arrow direction). 41, a drum cleaner 42, and the like. The photosensitive drum 36 is driven to rotate in the direction of the arrow at a predetermined process speed. The surface (outer peripheral surface) of the photosensitive drum 36 is uniformly (uniformly) charged to a predetermined polarity / potential by the charging device 37 and then exposed based on image information sent from a personal computer (not shown) or the like. By the exposure of 38, the electric charge in the exposed portion is removed and an electrostatic latent image is formed. The electrostatic latent image is developed as a toner image with the toner in the developer attached thereto by the developing device 40. In the present embodiment, a two-component developer mainly composed of toner and carrier is used as the developer.

  The toner image formed on the surface of the photosensitive drum 36 as described above is transferred to the intermediate transfer belt 31. The intermediate transfer belt 31 is stretched around a driving roller 43 and a driven roller 44, and rotates in the direction of arrow R31 by the rotation of the driving roller 43 in the direction of the arrow. The yellow toner image formed on the photosensitive drum 36 is primarily transferred onto the intermediate transfer belt 31 by the primary transfer roller 41 in the primary transfer portion T1. The toner (primary transfer residual toner) remaining on the surface of the photosensitive drum 36 after the primary transfer of the toner image is removed by the drum cleaner 42.

  The remaining three color (cyan, magenta, and black) image forming stations 33, 34, and 35 have the same configuration as the above-described yellow image forming station 32, and the photosensitive drums 36 of these image forming stations 33, 34, and 35 are provided. Similarly, cyan, magenta, and black toner images are formed on the surface of the toner image and sequentially transferred onto the intermediate transfer belt 31. Thus, the four color toner images are superimposed on the intermediate transfer belt 31. The four-color toner images on the intermediate transfer belt 31 are secondarily transferred collectively to a sheet conveyed by a sheet conveying unit 24 described later by a secondary transfer roller 45 in a secondary transfer unit T2. Toner remaining on the surface of the intermediate transfer belt 31 after the secondary transfer of the toner image (secondary transfer residual toner) is removed by a belt cleaner 46 disposed in the vicinity of the driven roller 44. A toner replenishing unit 23 is provided above the image forming unit 22.

  The toner replenishing unit 23 is provided with four toner containers 47, 48, 50, and 51 for yellow, magenta, cyan, and black, which individually store toner of each color. Each color developing device 40 is provided with a density sensor (not shown) for detecting the toner density (toner / developer weight ratio). When it is detected that the toner amount is less than a predetermined value, the toner is replenished to the developing device 40 of each color from the toner containers 47, 48, 50, 51 of each color. A paper discharge tray 10 is provided above the toner supply unit 23.

  The paper discharge tray 10 is formed so as to cover the upper surface of the image forming apparatus main body 2, and the intermediate portion in the front-rear direction is inclined so that the rear end side is positioned upward, and the rear end side is the intermediate portion. It is formed flat so as to be continuous. On the sheet stacking surface 8 on the upper surface of the sheet discharge tray 10, sheets discharged backward from a sheet discharge port 55 of a sheet conveying unit 24 described below are stacked.

  In the present embodiment, the sheet conveyance unit 24 is provided between the front side of the image forming apparatus main body 2 and the front cover 3, and a sheet conveyance path 52 that guides a sheet from below to above, and the sheet conveyance unit. It has a sheet re-conveying path 53 that is disposed on the front side of the path 52 and guides the sheet from the upper side to the lower side, and further has a manual feeding unit 54. In the present embodiment, the sheet re-conveying path 53 is a sheet conveying path that is directly opened by opening the front cover 3 described above. On the other hand, the sheet conveyance path 52 is a sheet conveyance path that is indirectly opened through a conveyance unit 73 described later when the front cover 3 is opened.

  The sheet conveyance path 52 rises upward from the vicinity of the above-described feeding roller 28, extends upward while being gently curved backward, and extends upward in the vicinity of the intermediate transfer belt 31 with the bending direction facing forward. The sheet extends obliquely upward toward the rear and reaches the sheet discharge port 55. The sheet conveyance path 52 includes a rear guide 52a and a front guide 52b facing each other, and a part of the front guide 52b is formed in a conveyance unit 73 described later. In the sheet conveyance path 52, the pickup roller 27, the feeding roller 28, the retard roller 30, the conveyance path roller pair 56, the registration roller pair 57, and the above-described intermediate transfer belt 31 are sandwiched between the driving roller 43 in order from the bottom. A secondary transfer roller 45, a fixing device 58, a conveying roller pair 60, a switching flapper 61, a discharge roller pair 62, and the like are disposed. The above-described fixing device 58 includes a fixing roller (fixing member) 63 including a heater (not shown) and a pressure roller (pressure member) that is in contact with the fixing roller 63 and forms a fixing nip portion (fixing portion) N1 therebetween. 64.

  The sheet fed from the sheet feeding cassette 25 by the pickup roller 27, the feeding roller 28, and the retard roller 30 is brought into contact with the nip of the resist roller pair 57 that is stopped by the conveying roller pair 56 and is inclined. After the line is corrected, the registration roller pair 57 is rotated in accordance with the timing at which the toner image on the intermediate transfer belt 31 is conveyed to the secondary transfer portion T2, and is supplied to the secondary transfer portion T2. As a result, the four color toner images on the intermediate transfer belt 31 are secondarily transferred collectively, and are heated and pressurized when passing through the fixing nip portion N1 to fix the toner images on the surface. The sheet on which the toner image has been fixed is guided to the lower surface of the switching flapper 61 by the conveying roller pair 60 and conveyed to the paper discharge roller pair 62, and the sheet discharge roller pair 62 moves rearward from the sheet discharge port 55 facing the back side. The sheet is discharged and stacked on the sheet stacking surface 8 of the sheet discharge tray 10. In the figure, the sensor flag 65 of the paper discharge sensor disposed immediately downstream of the paper discharge roller pair 62 is operating with the sheet P being discharged.

  The sheet re-conveying path 53 extends obliquely from the upper side to the front side of the above-described sheet discharge port 55, and gently slopes and extends to the rear side with a steep slope, and protrudes downward at the lower end portion. Then, the sheet merges with the sheet conveyance path 52 described above. The sheet re-conveying path 53 includes a rear guide 53a and a front guide 53b that face each other. A part of the rear guide 53a is formed in a transport unit 73 described later. Further, most of the front guide 53b is incorporated inside (rear end side) of the exterior panel 11 and constitutes the front cover 3 together with the exterior panel 11. In the sheet re-conveying path 53, the reverse roller pair 66, the switching flapper 61, the first, second, third and fourth re-conveying roller pairs 67, 68, 70 and 71 are arranged.

  When images are formed on both sides of the sheet, the switching flapper 61 is switched to the position indicated by the two-dot chain line, and the sheet on which the toner image is fixed is conveyed to the conveying roller pair 60 and the switching flapper 61 It is conveyed along the upper surface and further conveyed rearward by the reverse roller pair 66. Then, the rear end of the sheet passes through the conveying roller pair 60, and the reverse roller pair 66 is reversely rotated before passing through the reverse roller pair 66, and the first to fourth re-conveying roller pairs 67, 68, 70, 71 The sheet is conveyed downward and conveyed to the sheet conveyance path 52. Then, the toner image is transferred and fixed on the rear surface of the sheet in the same manner as on the front surface, and is discharged rearward from the sheet discharge port 55 and stacked on the sheet stacking surface 8 of the discharge tray 10. The Note that a manual feed roller 72 is disposed immediately in front of the conveyance roller pair 56 in the sheet conveyance unit 24, and the manual feed tray 16 constituting a part of the front cover 3 is pulled out in the closed state. The sheet set on the manual feed tray 16 is fed toward the conveying roller pair 56 side.

  A part of the sheet conveying unit 24 described above constitutes a conveying unit 73 that can be opened and closed. The transport unit 73 is disposed between the sheet transport path 52 and the sheet re-transport path 53, and a part of the front guide 52 b in the sheet transport path 52 and a part of the rear guide 53 a in the sheet re-transport path 53. In addition, one roller 57a of the registration roller pair 57, the secondary transfer roller 45, and one of the rollers 70a and 71a of the third and fourth re-conveying roller pairs 70 and 71 are integrally configured. The transport unit 73 has a swing center on the lower end side and is supported by the image forming apparatus body 2 so as to be freely opened and closed. The transport unit 73 is configured such that when the front cover 3 is opened, the upper end side is opened forward, and a part of the sheet transport path 52 and a part of the sheet re-transport path 53 are opened.

  This completes the description of the internal structure of the image forming apparatus 1.

  In the image forming apparatus 1 described above, a jam may occur after the toner image is fixed. In this case, the jam is bent in a bellows shape when the sheet is wound around the fixing roller 63, or the trailing end is conveyed by the fixing roller 63 or the like with the leading end of the sheet covering a separation claw (separating member) 82 described later. It is jam to do. In such a jam, the longer the time from the start of the occurrence of the jam until the image forming operation is stopped, that is, the longer the time from the occurrence of the jam until the rotation of the fixing roller 63 is stopped, the fixing roller 63 increases. Since the degree of wrapping and the bellows-like bending of the sheet deteriorates, it becomes more difficult to remove the sheet.

  Therefore, in the present invention, the jam detection time is adjusted according to the printing rate in a predetermined area of the sheet. Specifically, the jam detection time is shortened when the printing rate from the leading edge of the sheet to a predetermined position exceeds a predetermined threshold. As a result, even when a jam occurs in the fixing device 58, the image forming operation is stopped while the degree of the wrapping of the sheet around the fixing roller 63 or the accordion-like bending is slight, so that the sheet can be easily removed. did.

  Hereinafter, this point will be described with reference to FIGS. 4 is an enlarged view from the vicinity of the registration roller pair 57 shown in FIG. 1 to the vicinity of the fixing device 58. FIG. 5 is a control block diagram for shortening the jam detection time. FIG. 6 is a flowchart for explaining the flow of operations when the jam detection time is shortened.

  As shown in FIG. 4, the registration roller pair 57, the drive roller 43, and the fixing roller 63 are connected to a drive motor 80 of the sheet conveying system via a clutch or the like (not shown) as necessary. 80 is connected to the control means 81. The control unit 81 controls the operation of the entire image forming apparatus 1. Particularly in relation to the present embodiment, the control unit 81 controls the drive motor 80 to control the timing of the rotation start and rotation stop of the registration roller pair 57, the drive roller 43, and the fixing roller 63. The control means 81 is connected to the above-described exposure device 38, and acquires the printing rate from the exposure device 38 as will be described later.

  As shown in FIG. 4, a separation claw 82 and a sheet detection sensor 83 are disposed on the downstream side of the fixing nip portion N1 along the sheet conveyance direction (arrow K direction). The separation claw 82 is disposed on the fixing roller 63 side immediately downstream of the fixing nip portion N1. The separation claw 82 is arranged in the counter direction with respect to the rotation direction of the fixing roller 63 (arrow R63 direction), and the claw portion 82a at the tip is brought close to the surface of the fixing roller 63. A gap G between the claw portion 82a and the surface of the fixing roller is set to about 0.2 to 0.5 mm, for example. The separation claw 82 fixes the sheet P by inserting the claw portion 82a between the front end P1 of the sheet P on which the toner image is fixed at the fixing nip portion N1 and the surface of the rotating fixing roller 63. The roller 63 is separated from the surface. The sheet P normally separated from the surface of the fixing roller 63 by the separation claw 82 is conveyed along the sheet discharge path 84 and is discharged onto the above-described discharge tray 10 (see FIG. 1). .

  The sheet detection sensor 83 is disposed immediately downstream of the separation claw 82 in the sheet discharge path 84. The sheet detection sensor 83 includes, for example, a light transmission type optical sensor having a light emitting unit 83a and a light receiving unit 83b arranged to face each other across the sheet discharge path 84, and the light emitting unit 83a. A sheet presence detection means 85 of the control means 81 shown in FIG. 5 detects that a sheet has been conveyed by blocking the optical path formed between the light receiving portion 83b and the conveyed sheet P. It is. In other words, the sheet detecting sensor 83 and the sheet presence / absence detecting means 85 constitute a sheet detecting means.

  As shown in the block diagram of FIG. 5, the control unit 81 includes an image forming unit 86, a printing rate acquisition unit 87, a jam detection time adjustment unit 88, a sheet presence / absence detection unit 85, a jam detection timer unit 90, and a secondary paper feed start. The RAM 94 stores a printing rate 95 of N [mm] from the leading edge of the sheet, and a jam detection time 96.

  The image forming unit 86 controls the entire image forming apparatus 1. For example, the operation timing of each member in each process of charging, exposure, development, transfer, cleaning, and fixing, and a high-voltage power supply (not shown). The voltage value, application timing, etc. are controlled.

  The print rate acquisition unit 87 detects the print rate based on the image information input to the exposure device 38 described above. More specifically, in the present embodiment, the exposure device 38 has a laser oscillator that emits laser light based on image information, and the printing rate acquisition means 87 counts the number of dots of the laser light and totals it. The printing rate is calculated from the area. At this time, regarding the printing rate on the rear end side in the sheet conveyance direction on the printing surface of the sheet P, the ratio that contributes to the quality of separation of the sheet P after fixing the toner image is small. Therefore, in the present embodiment, the printing rate of the area from the leading edge of the sheet P to N [mm] is detected. However, this N is shorter than the length in the sheet conveyance direction. As an example, in the case of A4 longitudinal, N is set to 10 mm. The RAM 94 stores a printing rate X [%] of N [mm] from the leading edge of the sheet as a threshold value as a reference value.

  The jam detection time adjusting unit 88 is configured to detect a jam, which will be described later, when the printing rate acquired by the printing rate acquisition unit 87 is X [%] or more (for example, 10% or more) stored in the RAM 94. Time has come to be shortened.

  As described above, the sheet presence / absence detection means 85 detects that the sheet P has reached the sheet detection sensor 83 based on the detection signal from the sheet detection sensor 83.

  The jam detection timer means 90 starts measuring time from a reference time (time) T0. Here, in this embodiment, as the reference time T0, the time when the registration roller pair 57 starts to rotate, that is, the rotation of the registration roller pair 57 causes the sheet P to be secondary fed toward the secondary transfer nip T2. A time during which the sheet P is fed by the pickup roller 27 corresponds to primary feeding is employed. As described above, when the sheet P is conveyed from the upstream side, the registration roller pair 57 stops rotating, and the leading end P1 of the sheet P is brought into contact with the registration nip portion (time reference position) N2. Thus, the skew of the sheet P is corrected. Thereafter, the registration roller pair 57 starts to rotate so that the toner image formed on the intermediate transfer belt 31 reaches the secondary transfer nip T2, and the sheet P is transferred to the secondary transfer nip T2. Is supplied. The time at which the rotation of the registration roller pair 57 is started at this time is set as a reference time T0.

  The secondary paper feed start means 91 is for starting the rotation of the registration roller pair 57 described above. Note that the secondary paper feed start unit 91 is actually included in the image forming unit 86 described above.

  The jam judging means 92 is a jam detecting means in which a jam occurs when the sheet P does not reach the sheet detecting sensor 83 even after a predetermined time t0 has elapsed since the above-described jam detecting timer means 90 starts measuring time. (Judge). Here, the predetermined time t0 will be described. The leading edge of the sheet P reaches the sheet detection sensor 83 after the registration roller pair 57 starts rotating and the secondary feeding of the sheet P starts, that is, after the jam detection timer means 90 starts counting. The time to do this can be calculated (theoretically). Assuming that the calculated time is t1, a time obtained by adding the jam detection time t2 to the time t1 is set as a predetermined time t0. The jam detection time t2 is set to a time (for example, 0.05 sec) that can be regarded as a jam with a high probability when the sheet detection sensor 83 does not detect the sheet P within this time. The jam determination unit 92 determines that a jam has occurred when the sheet detection sensor 83 does not detect the sheet P even after the time t0 has elapsed since the jam detection timer unit 90 started timing. If it is determined that the jam has occurred, the image forming unit 86 stops the image forming operation. Note that the jam detection time t2 when the printing rate acquired by the above-described printing rate acquisition unit 87 is less than X [%] is defined as “normal detection time”.

  When the above-described jam determination unit 92 determines that a jam has occurred, the jam display unit 93 displays that fact on the operation panel 17 (see FIGS. 1 and 2).

  The flow of the operation of this embodiment will be described along the flowchart of FIG. 6 with reference to FIGS. 3, 4 and 5 as appropriate.

  Image formation is started, and the sheet P in the paper feeding cassette 25 is primarily fed by the pickup roller 27 (step S1, hereinafter abbreviated as “S1”). The primary fed sheet P is brought into contact with the registration nip portion N2 of the resist roller pair 57 whose front end P1 is stopped, and the skew is corrected and temporarily stopped (S2). A toner image is formed on the intermediate transfer belt 31 in the image forming unit 22 (S3). The printing rate acquisition unit 87 acquires a printing rate of N [mm] from the leading edge P1 of the sheet P (S4). The acquired printing rate is compared with the printing rate X [%] stored in the RAM 94 (S5). As a result, if it is less than X [%] (No in S5), the jam detection time t2 in the sheet detection sensor 83 is set to “normal” (S6). On the other hand, when the printing rate is equal to or greater than X [%] in S5 (Yes in S5), the jam detection time t2 in the sheet detection sensor 83 is set to “normal-α” (S7). Here, α is 0 <α <t2. After setting the jam detection time t2, the registration roller pair 57 is rotated to start the secondary feeding of the sheet P (S8), and at the same time, the jam detection timer means 90 is started (S9). It is checked whether or not the sheet P has reached the sheet detection sensor 83 (S10). If it has arrived (Yes in S10) and within the jam detection time (Yes in S11), the jam detection timer means 90 is cleared (S12), and the image formation is terminated (S13).

  On the other hand, when the sheet P does not reach the sheet detection sensor 83 in S10 (No in S10), it is checked whether or not it is within the jam detection time (S14). As a result, when it is within the jam detection time (Yes in S14), the process returns to S10. On the other hand, if it is not within the jam detection time (No in S14), the image forming operation is interrupted (S15), and the fixing roller 63, the drive roller 43, the registration roller pair 57, etc. are stopped, and a jam occurs on the operation panel 17. The occurrence is displayed (S16), and the user is notified.

  As described above, according to the present embodiment, when the printing rate of N [mm] from the front end of the sheet, which greatly contributes to the separation performance of the sheet P from the fixing roller 63, is equal to or greater than the threshold value, the jam occurrence rate Is expected to increase. In such a case, the jam detection time is shortened by α from the normal jam detection time t2. That is, the time until it is determined that a jam has occurred is shortened. As a result, even when a jam actually occurs, the damage of the jam can be minimized. That is, since the time until the image forming operation stops after the occurrence of a jam is shortened, the degree of wrapping of the sheet P around the fixing roller 63 and the bellows-like bending is relatively small. For this reason, even when a jam occurs due to a conveyance failure of the sheet P, the jammed sheet P can be easily removed.

  As described above, according to the conventional technology, the image writing margin for the sheet P is set to a predetermined value or more according to the printing ratio, or the narrow density area mode is selected to prevent the fixing device from jamming. However, all of these are based on the assumption that the sheet is normally conveyed. That is, when a conveyance failure occurs in the sheet, for example, when the sheet slips and enters the secondary transfer nip portion later than normal, the transfer position of the toner image with respect to the sheet is the leading end side of the sheet. Therefore, jamming is likely to occur in the fixing device. And if a jam has occurred, no countermeasure is taken. On the other hand, according to the present invention, even when a conveyance failure occurs in the sheet and a jam occurs in the fixing device 57, the damage can be minimized as described above. This point is greatly different from the conventional example.

  As described above, in the embodiment, the rotation start of the registration roller pair 57 is set as a reference for time measurement in the jam detection timer means 90, but the present invention is not limited to this. For example, if the jam detection time t2 can be changed according to the printing rate of the sheet P before the leading end P1 of the sheet P enters the fixing nip portion N1, any arbitrary upstream side of the fixing nip portion N1 can be changed. A reference position can be set for the position. Further, for this reference position, it is not necessary to stop the sheet P with the leading edge P1 aligned, and the sheet P being conveyed may pass through.

  In the above description, the case where the jam detection time is changed in accordance with the difference in the printing rate has been described. However, the present invention is not limited to the sheet material (plain paper, art paper, recycled paper, etc.) or thickness (whether thick or thin). The present invention can also be applied to cases where the probability of occurrence of a jam in the fixing device differs depending on the size (large or small). For example, even if the printing rate is the same, it is presumed that a thin and large art paper is likely to cause a jam in the fixing device. Therefore, it is effective to shorten the jam detection time.

FIG. 2 is a perspective view of the entire image forming apparatus as viewed from the upper left oblique side of the front side. FIG. 2 is a perspective view of the entire image forming apparatus as viewed from the rear and obliquely upward on the left side thereof. FIG. 2 is a diagram schematically showing an internal structure of the image forming apparatus as viewed from the left side. FIG. 2 is an enlarged view from the vicinity of a registration roller pair shown in FIG. 1 to the vicinity of a fixing device. It is a control block diagram for shortening jam detection time. It is a flowchart explaining the flow of operation | movement at the time of shortening jam detection time.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 2 ... Image forming apparatus main body, 58 ... Fixing apparatus, 63 ... Fixing roller (fixing member), 64 ... Pressure roller (pressing member), 81 ... Control means, 82 ... Separation claw (separation member) 83. Sheet detection sensor (sheet detection means) 85. Sheet presence / absence detection means (sheet detection means) 87... Print rate acquisition means 88. , 90... Jam detection timer means, 92... Jam judgment means, N1... Fixing nip portion (fixing portion), N2... Registration nip portion (time reference position) P .. Sheet, P1. ... Jam detection time

Claims (3)

In an image forming apparatus provided with a fixing device that heats and presses a sheet on which printing has been performed with toner through a fixing unit formed between a fixing member and a pressure member to fix the toner on the sheet ,
A printing rate acquisition means for acquiring a printing rate from the leading edge of the sheet to a predetermined position;
A time reference position set on the upstream side of the fixing unit along the sheet conveying direction;
A sheet detection unit having a sheet detection sensor disposed on the downstream side of the fixing unit along the sheet conveyance direction to detect the leading edge of the sheet;
Jam detection timer means for measuring the time from when the leading edge of the sheet passes through the time reference position;
Jam determination means for determining a jam when a sheet does not reach the sheet detection sensor even if a predetermined jam detection time has elapsed;
Control means for stopping the image forming operation when the jam judging means judges that a jam;
Jam detection time adjusting means for shortening the predetermined jam detection time when the printing rate acquired by the printing rate acquisition means is high,
An image forming apparatus. A separation member is disposed between the fixing unit and the sheet detection sensor to separate the sheet from the surface of the fixing member inserted between the leading end of the sheet on which the toner image is fixed and the fixing member. ing,
The image forming apparatus according to claim 1. The jam detection time adjusting means shortens the jam detection time when the printing rate acquired by the printing rate acquisition unit is a predetermined threshold or more.
The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.

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