JP2014102303A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of reliably preventing the occurrence of image rubbing and fixing wrinkles in association with a twist of a sheet.SOLUTION: An image forming apparatus includes: a transfer member 23 that transfers a toner image onto a sheet while conveying the sheet; fixing members 30 and 32 that are provided on the downstream side of the transfer member 23 in a conveyance direction of the sheet to fix the toner image on the sheet while conveying the sheet; a loop sensor 40 that detects the deflection amount of the sheet between the transfer member 23 and the fixing members 30 and 32; and a control unit that controls the transfer speed at which the transfer member 23 conveys the sheet, and the fixing speed at which the fixing members 30 and 32 convey the sheet. The control unit increases the transfer speed or decreases the fixing speed in a period from when a tip part in the conveyance direction of the sheet passes through a detection position of the loop sensor 40 to when the tip part reaches the fixing members 30 and 32.

Description

  The present invention relates to an electrophotographic image forming apparatus, and more particularly, to an image forming apparatus provided with a loop sensor that detects a deflection amount of a sheet.

  2. Description of the Related Art Conventionally, an electrophotographic image forming apparatus includes a transfer member that is in pressure contact with an image carrier, and a fixing member that is provided on the downstream side of the transfer member in the paper transport direction. Then, after the toner image is transferred to the paper by the transfer member, the toner image is fixed on the paper by the fixing member.

  Among such image forming apparatuses, in particular, a vertical transfer image forming apparatus in which a sheet transfer path is formed in the vertical direction may use a belt transfer method. The belt transfer method is a method in which a transfer belt extending along the paper transport direction is used as a transfer member. With the belt transfer method, the paper can be transported with the paper electrostatically attracted to the transfer belt, so the paper transport can be stabilized compared to the roller transfer method that uses a transfer roller as the transfer member. It becomes.

  However, in the belt transfer method, since the transfer belt extends toward the downstream side in the transport direction, the distance at which the sheet can be bent between the transfer member and the fixing member is shorter than in the roller transfer method. Along with this, there is a drawback that when the alignment (positional alignment) between the transfer member and the fixing member is deviated, image rubbing and fixing wrinkles due to the twisting of the paper are likely to occur. Therefore, when the belt transfer system is used, the speed at which the transfer member transports the paper (hereinafter referred to as “transfer speed”) and the speed at which the fixing member transports the paper (hereinafter referred to as “roller transfer system”). , Referred to as “fixing speed”) must be accurately controlled.

  Therefore, in Patent Document 1, a loop sensor that can come into contact with a sheet is disposed between the transfer member (see “transfer belt 8”) and the fixing member, and the amount of bending of the sheet (loop amount) detected by the loop sensor. ) On the basis of the above, a method for controlling the fixing speed is disclosed. Patent Document 2 discloses a configuration in which two loop sensors are arranged between a transfer member (see “transfer roller 15”) and a fixing member.

JP 2004-61591 A JP 2007-17538 A

  However, both of the prior arts described in Patent Documents 1 and 2 are configured to control the fixing speed based on the detection result from the loop sensor after the front end of the paper conveyance direction reaches the fixing member. (See FIG. 3 of Patent Document 1 and FIG. 2 of Patent Document 2). For this reason, the fixing speed cannot be adjusted so that the paper bends appropriately before the leading edge of the paper conveyance direction reaches the fixing member, and image rubbing and fixing wrinkles due to paper twisting can be reliably prevented. It was difficult to do.

  Accordingly, an object of the present invention is to provide an image forming apparatus capable of reliably preventing the occurrence of image rubbing and fixing wrinkles accompanying sheet twisting in consideration of the above circumstances.

  An image forming apparatus of the present invention is provided with a transfer member that transfers a toner image onto a sheet while conveying the sheet, and a downstream side of the transfer member in the sheet conveying direction, and transfers the toner image onto the sheet while conveying the sheet. A fixing member to be fixed, a loop sensor for detecting a deflection amount of the sheet between the transfer member and the fixing member, a transfer speed at which the transfer member conveys the sheet, and the fixing member conveys the sheet. A control unit that controls a fixing speed, which is a speed, and the control unit is a period from when the leading end of the paper in the transport direction passes through the detection position of the loop sensor to the fixing member. The transfer speed is increased or the fixing speed is decreased.

  By adopting such a configuration, it is possible to adjust the transfer speed and the fixing speed so that the paper is appropriately bent before the front end of the paper in the conveyance direction reaches the fixing member. Therefore, compared to the case where the adjustment of the transfer speed and the fixing speed is started after the leading edge of the paper transport direction reaches the fixing member, the timing for starting the adjustment of the transfer speed and the fixing speed can be advanced. Accordingly, it is possible to reliably prevent image rubbing and fixing wrinkles due to paper twisting. In addition, since the loop sensor, which is a position sensor for detecting the amount of bending of the paper, is used as a timing sensor for detecting the front end of the paper in the conveyance direction, a new sensor is used in addition to the loop sensor. This is unnecessary, and it is possible to prevent the configuration of the apparatus from becoming complicated.

  When the single-sided printing is performed on the paper or the first side of the double-sided printing is performed on the paper, the control unit is configured such that after the front end of the paper in the transport direction passes through the fixing member, the loop sensor When the detection value increases to a predetermined threshold value or more, the transfer speed may be decreased, or the fixing speed may be increased.

  By adopting such a configuration, when performing single-sided printing on paper or when performing printing on the first side of double-sided printing on paper, transfer speed and fixing speed are determined according to the detection value of the loop sensor. The amount of flexure of the paper can be optimized by changing. As a result, it is possible to more reliably prevent image rubbing and fixing wrinkles due to paper twisting.

  When printing the second side of the double-sided printing on the paper, the control unit passes at least the rear end in the paper transport direction through the detection position after the front in the paper transport direction passes through the fixing member. Until then, the transfer speed and the fixing speed may be controlled to a predetermined speed.

  By adopting such a configuration, it is possible to maintain a state in which the amount of flexure of the paper is appropriate when printing the second side of duplex printing on the paper. As a result, it is possible to more reliably prevent image rubbing and fixing wrinkles due to paper twisting.

  The control unit may increase the transfer speed or decrease the fixing speed immediately before the front end of the paper conveyance direction reaches the fixing member.

  By adopting such a configuration, it is possible to enhance the effect of optimizing the amount of bending of the paper and to more reliably prevent the occurrence of image rubbing and fixing wrinkles due to paper twisting.

  The transfer member may be a transfer belt extending along the paper transport direction.

  As described above, when the transfer belt is used as the transfer member, the conveyance of the sheet can be stabilized as compared with the case where the transfer roller is used as the transfer member, but the sheet is bent between the transfer member and the fixing member. The distance that can be formed is shortened, and image rubbing and fixing wrinkles due to twisting of the paper are likely to occur. Therefore, it is significant to prevent the occurrence of image rubbing and fixing wrinkles accompanying the twisting of the paper using the configuration of the present invention.

  The loop sensor may be a reflective optical sensor that detects the amount of bending of the paper by reflecting light on the paper.

  By adopting such a configuration, it is possible to detect the amount of deflection of the paper without bringing the loop sensor into contact with the paper. For this reason, it is possible to prevent a part of the loop sensor from coming into contact with the paper and deteriorating the paper conveyance performance.

  According to the present invention, it is possible to reliably prevent image rubbing and fixing wrinkles due to paper twisting.

1 is a schematic diagram illustrating an outline of a printer according to an embodiment of the present invention. FIG. 2 is a cross-sectional view illustrating the periphery of an image forming unit and a fixing device in a printer according to an embodiment of the present disclosure. 1 is a block diagram illustrating a configuration of a printer according to an embodiment of the present invention. In the printer according to the embodiment of the present invention, (a) is an explanatory view showing a state in which the leading end of the sheet passes through the detection position of the loop sensor, and (b) is a diagram illustrating that the leading end of the sheet is a fixing belt and It is explanatory drawing which shows the state immediately before reaching a pressurizing roller, (c) is explanatory drawing which shows the state which the loop sensor has detected the deflection amount of the paper. 6 is a flowchart illustrating control when performing single-sided printing on a sheet or performing printing on the first side of double-sided printing on a sheet in a printer according to an embodiment of the present invention. In the printer according to an embodiment of the present invention, (a) is a diagram showing a detection value of a loop sensor when printing on the first side of double-sided printing on a sheet while controlling the fixing speed in sequence. (B) is a figure which shows the detected value of a loop sensor at the time of printing the 1st surface of double-sided printing with respect to a paper, carrying out feedback control of the fixing speed. 5 is a flowchart illustrating control when printing on the second side of duplex printing on a sheet in the printer according to the embodiment of the present invention. FIG. 6 is a diagram illustrating detection values of a loop sensor when printing on the second side of double-sided printing on a sheet while sequentially controlling the transfer speed and the fixing speed in the printer according to an embodiment of the present invention.

  First, the overall configuration of a printer 1 as an image forming apparatus will be described with reference to FIG. FIG. 1 is a schematic diagram showing an outline of a printer according to an embodiment of the present invention. Hereinafter, for convenience of explanation, the front side in FIG. 1 is defined as the front side (front side) of the printer 1.

  The printer 1 includes a box-shaped printer main body 2, a paper feed cassette 3 for storing paper (not shown) is provided at the bottom of the printer main body 2, and a paper discharge tray is provided at the upper end of the printer main body 2. 4 is provided.

  An exposure unit 5 composed of a laser scanning unit (LSU) is arranged at the center of the printer main body 2, and an image forming unit 6 is provided on the right side of the printer main body 2. The image forming unit 6 is rotatably provided with a photosensitive drum 7 that is an image carrier. The photosensitive drum 7 is formed using an amorphous silicon material and has a diameter of 40 mm. Around the photosensitive drum 7, a charger 8, a developing unit 10 connected to the toner container 9, a transfer device 11, and a cleaning device 12 are arranged in the rotational direction of the photosensitive drum 7 (arrow X in FIG. 1). (See below).

  A paper transport path 13 is provided on the right side of the printer body 2 from below to above. That is, the printer 1 of this embodiment is a vertical conveyance. A paper feed unit 14 is provided at the upstream end of the conveyance path 13, the transfer device 11 is provided at a middle portion of the conveyance path 13, and a fixing device 16 is provided at a downstream part of the conveyance path 13. . On the right side of the transport path 13, a reverse path 17 for duplex printing is provided.

  Next, an image forming operation of the printer 1 having such a configuration will be described.

  When the printer 1 is turned on, various parameters are initialized, and initial setting such as temperature setting of the fixing device 16 is executed. Then, when image data is input from a computer or the like connected to the printer 1 and an instruction to start printing is given, an image forming operation is executed as follows.

  First, after the surface of the photosensitive drum 7 is charged by the charger 8, the exposure corresponding to the image data is performed on the photosensitive drum 7 by the laser light from the exposure device 5 (see the one-dot chain line P in FIG. 1). As a result, an electrostatic latent image is formed on the surface of the photosensitive drum 7. Next, the developing unit 10 develops the electrostatic latent image into a toner image with the toner supplied from the toner container 9.

  On the other hand, the sheet taken out from the sheet feeding cassette 3 by the sheet feeding unit 14 is conveyed to the image forming unit 6 in synchronization with the above-described image forming operation, and the toner image on the photosensitive drum 7 is transferred by the transfer device 11. Transferred to paper. The sheet on which the toner image has been transferred is conveyed downstream in the conveyance path 13 and enters the fixing device 16 where the toner image is fixed on the sheet. The sheet on which the toner image is fixed is discharged from the downstream end of the transport path 13 to the discharge tray 4 or switched back to the reverse path 17 for duplex printing. The toner remaining on the photosensitive drum 7 is collected by the cleaning device 12.

  Next, the transfer device 11 will be described with reference to FIG. The transfer device 11 is a driving roller 21 provided at the lower right of the photosensitive drum 7, a driven roller 22 provided at the upper right of the photosensitive drum 7, and a transfer member wound around the driving roller 21 and the driven roller 22. A transfer belt 23 and a transfer roller 24 provided between the driving roller 21 and the driven roller 22 and in contact with the inner surface of the transfer belt 23 are provided.

  The driving roller 21 and the driven roller 22 have a long shape in the front-rear direction (the depth direction in FIG. 2). The driving roller 21 and the driven roller 22 are formed with a diameter of 20 mm, for example.

  The transfer belt 23 is provided so as to be inclined rightward toward the upper side. The transfer belt 23 extends along the paper conveyance direction, and is configured to transfer the toner image onto the paper while conveying the paper. The transfer belt 23 is in pressure contact with the photosensitive drum 7 above the drive roller 21 and below the driven roller 22, thereby forming a transfer nip 25.

The transfer belt 23 has an endless shape. For example, the surface of a belt main body made of a rubber sheet material such as acrylonitrile butadiene rubber (NBR) is coated with a fluorine resin (for example, PTFE). It is formed by that. For example, the transfer belt 23 may be formed by joining both end portions of a rubber sheet material, or may be formed using a seamless belt material. The transfer belt 23 is formed with a thickness of 600 μm and a volume resistivity of 10 ⁹ Ω · cm, for example.

  The transfer roller 24 has a shape that is long in the front-rear direction. The transfer roller 24 includes, for example, a metal base layer and a foamed rubber layer provided around the base layer. The transfer roller 24 is formed with a diameter of 14 mm, for example.

  Next, the fixing device 16 will be described with reference to FIG. The fixing device 16 includes a fixing roller 26 provided immediately above the transfer device 11, a heating roller 27 provided on the left side of the fixing roller 26, a tension roller 28 provided on the upper left side of the fixing roller 26, and the fixing roller 26. A fixing belt 30 as a fixing member wound around the heating roller 27 and the tension roller 28, an IH fixing unit 31 provided on the left side of the fixing belt 30, and an addition as a fixing member provided on the right side of the fixing belt 30. And a pressure roller 32.

  The fixing belt 30 is provided downstream of the transfer belt 23 in the sheet conveyance direction. The fixing belt 30 has an endless shape, for example, a base material layer made of a ferromagnetic material such as Ni electroforming, an elastic layer made of silicon rubber or the like provided around the base material layer, and the elasticity And a release layer made of PFA or the like that covers the layer.

  The IH fixing unit 31 is housed in the case member 33, the case member 33, the induction heating coil 34 provided in an arc shape along the outer periphery of the fixing belt 30, and the case member 33 is housed in the induction heating coil. 34, and a core 35 provided along the outer periphery of 34. A high-frequency current flows through the induction heating coil 34 to generate a high-frequency magnetic field in the induction heating coil 34, and an eddy current is generated in the heating roller 27 by the high-frequency magnetic field, thereby heating the heating roller 27 and the fixing belt 30. It has become so.

  The pressure roller 32 is provided downstream of the transfer belt 23 in the paper transport direction. The pressure roller 32 is in pressure contact with the fixing belt 30, and a fixing nip 36 is formed between the fixing belt 30 and the pressure roller 32. In the fixing nip 36, the toner image is fixed on the sheet while the sheet is conveyed.

  Between the transfer device 11 and the fixing device 16 configured as described above, as shown in FIG. 2, a static eliminator 37, a fixing approach guide 38, and a loop sensor 40 are provided. Hereinafter, these will be described in order.

  First, the static eliminator 37 will be described. The static eliminator 37 is provided downstream of the transfer device 11 in the paper transport direction. The static eliminator 37 is provided on the right side of the transport path 13 so as to face the transport path 13. The static eliminator 37 has an elongated shape in the front-rear direction. The static eliminator 37 is electrically grounded.

  The static eliminator 37 is provided with a static elimination plate 41. The neutralization plate 41 is formed of, for example, a metal such as SUS or a conductive member such as a conductive resin, and extends along the sheet conveyance direction. A number of static elimination needles 42 are arranged in a row at a predetermined interval (for example, 1 mm) in the front-rear direction at the upper end portion (downstream end portion in the transport direction) of the neutralization plate 41.

  Next, the fixing approach guide 38 will be described. The fixing approach guide 38 is provided downstream of the static eliminator 37 in the paper transport direction. The fixing approach guide 38 is provided on the right side of the transport path 13 so as to face the transport path 13. The fixing approach guide 38 is formed of, for example, a metal such as SUS or a conductive member such as a conductive resin. The fixing approach guide 38 includes a guide portion 43 that extends along the sheet conveyance path 13 and a bent portion 44 that is bent from the lower end portion of the guide portion 43 toward the upper right, and has a substantially L-shaped cross section. Is made.

  Next, the loop sensor 40 will be described. The loop sensor 40 is provided on the right side of the static eliminator 37. The loop sensor 40 is a reflective optical sensor and includes a light emitting unit 45 and a light receiving unit 46. The light emitted from the light emitting unit 45 (see the lead-out line Y in FIG. 2) is reflected by the sheet passing through the transport path 13 and received by the light receiving unit 46.

  The loop sensor 40 is configured to detect the amount of paper deflection (loop amount) based on the amount of light received by the light receiving unit 46. Specifically, when the sheet is conveyed in a state where the sheet is sufficiently bent to the right (see arrow A in FIG. 2), the distance between the loop sensor 40 and the sheet is shortened, and therefore the amount of light received by the light receiving unit 46 is reduced. As a result, the detection value of the loop sensor 40 increases. On the other hand, when the sheet is conveyed with insufficient bending (see arrow B in FIG. 2), the distance between the loop sensor 40 and the sheet becomes long, so the amount of light received by the light receiving unit 46 decreases, and the loop The detection value of the sensor 40 becomes small. 2 indicates a position where the loop sensor 40 detects the amount of deflection of the paper (hereinafter referred to as “detection position of the loop sensor 40”).

  Next, the control system of the printer 1 will be described with reference to FIG.

The printer 1 is provided with a control unit 51. The control unit 51 is connected to a storage unit 52 configured by a storage device such as a ROM and a RAM, and the control unit 51 controls each unit of the printer 1 based on a control program and control data stored in the storage unit 52. It is comprised so that control may be performed. The storage unit 52 stores a threshold value V _th (1.85 V) of the detection value of the loop sensor 40.

  The control unit 51 is connected to the loop sensor 40, and the detection value of the loop sensor 40 is output as a voltage to the control unit 51.

  The control unit 51 is connected to the transfer roller 24 via the voltage application unit 53, and a transfer voltage is applied from the voltage application unit 53 to the transfer roller 24 under the control of the control unit 51.

  The control unit 51 is connected to the drum motor 50, and the drum motor 50 is connected to the photosensitive drum 7. The drum motor 50 rotates the photosensitive drum 7 based on a signal from the control unit 51.

  The controller 51 is connected to the first drive motor 54, and the first drive motor 54 is connected to the drive roller 21. Then, when the first drive motor 54 rotates the drive roller 21 based on the signal from the control unit 51, the transfer belt 23 rotates accordingly. That is, the control unit 51 can control the speed at which the transfer belt 23 transports the paper (hereinafter referred to as “transfer speed”).

  The controller 51 is connected to the second drive motor 55, and the second drive motor 55 is connected to the pressure roller 32. When the second drive motor 55 rotates the pressure roller 32 based on a signal from the control unit 51, the fixing belt 30 is rotated by being driven by the pressure roller 32. That is, the control unit 51 can control the speed at which the fixing belt 30 and the pressure roller 32 convey the paper (hereinafter referred to as “fixing speed”).

  In the printer 1 configured as described above, when performing single-sided printing on a sheet or printing on the first side of double-sided printing on a sheet, the moisture content of the sheet is relatively high. Is less likely to be charged and the sheet is less likely to be electrostatically attracted to the static eliminator 37 and the fixing approach guide 38. Therefore, as indicated by an arrow A in FIG. 2, there is a high possibility that the sheet is conveyed with an insufficient amount of bending. If the sheet is conveyed in such a state where the amount of bending is insufficient, the conveyance of the sheet becomes unstable, and image rubbing and fixing wrinkles due to the twisting of the sheet easily occur.

  On the other hand, when printing on the second side of the double-sided printing on the paper, the moisture in the paper evaporates when printing the first side of the paper (particularly fixing the toner image on the paper). Therefore, the moisture content of the paper is relatively high. Therefore, the sheet is easily charged, and the sheet is easily electrostatically attracted to the static eliminator 37 and the fixing approach guide 38. Therefore, as indicated by an arrow A in FIG. 2, the possibility that the sheet is conveyed in a state of being sufficiently bent to the right side is increased. When the paper is conveyed in a sufficiently bent state in this way, the conveyance of the paper is stabilized, and image rubbing and fixing wrinkles due to paper twisting are less likely to occur.

  As described above, when performing single-sided printing on a sheet or when performing printing on the first side of double-sided printing on paper and when performing printing on the second side of double-sided printing on paper The rate of occurrence of image rubbing and fixing wrinkles due to twisting differs. Therefore, in the present embodiment, as described below, when performing single-sided printing on a sheet or when performing printing on the first side of duplex printing on a sheet and printing on the second side of duplex printing on a sheet. Depending on the case, different control methods are used.

  First, an example of controlling the transfer speed and the fixing speed when performing single-sided printing on a sheet or performing printing on the first side of double-sided printing on a sheet will be described mainly with reference to FIGS. . A lead line S in FIG. 4 indicates a sheet.

  First, when image data is input from a computer or the like connected to the printer 1 and an instruction to start printing is given, the control unit 51 sets the transfer speed to a predetermined initial value (for example, 294.7 mm / s). In addition, the transfer belt 23 is rotated, and the fixing belt 30 and the pressure roller 32 are rotated so that the fixing speed becomes a predetermined initial value (for example, 294.1 mm / s). In addition, under the control of the control unit 51, the voltage application unit 53 applies a transfer voltage to the transfer roller 24. In this state, when the sheet is conveyed from the upstream side (lower side) of the conveyance path 13, the toner image is transferred to the sheet at the transfer nip 25.

  The sheet on which the toner image has been transferred in this way is transported to the downstream side of the transport path 13 by the transfer belt 23 and is transported to the downstream side of the transport path 13 along the static eliminator 37. When the paper is thus transported to the downstream side of the transport path 13, as shown in FIG. 4A, the leading end of the paper in the transport direction (hereinafter abbreviated as “paper front end”) is looped. Passing the detection position of the sensor 40, the loop sensor 40 detects the leading edge of the paper (step S101). The time at this time is assumed to be t0. At time t0, the transfer speed and the fixing speed are maintained at their initial values.

  The leading edge of the sheet that has passed the detection position of the loop sensor 40 is conveyed to the downstream side of the conveyance path 13 along the fixing approach guide 38. When a predetermined time (for example, 180 msec) elapses from time t0, the state immediately before the leading end of the sheet reaches the fixing belt 30 and the pressure roller 32 is obtained as shown in FIG. The control unit 51 recognizes from the elapsed time from the time t0 that the front end of the sheet has just reached the fixing belt 30 and the pressure roller 32, and sets the fixing speed to an initial value (294.294). 1 mm / s) to a deceleration value (292.3 mm / s) (step S102). The time at this time is assumed to be t1.

Next, the leading end of the sheet reaches the fixing belt 30 and the pressure roller 32, and the leading end of the sheet passes through the fixing nip 36 (step S103). In this way, when the leading edge of the sheet passes through the fixing nip 36 and a predetermined time (for example, 260 msec) has elapsed from time t0, the fixing speed based on the detection value of the loop sensor 40 is obtained as shown in FIG. Feedback control is started (step S104). The time at this time is assumed to be t2. At this time t2, it is assumed that the amount of paper deflection is smaller than the threshold value V _th (1.85 V) stored in the storage unit 52.

In the above feedback control, first, the loop sensor 40 detects the amount of deflection of the paper, and outputs the detected value as a voltage to the control unit 51 (step S105). The control unit 51 determines whether or not the detection value of the loop sensor 40 has risen to a threshold value V _th (1.85 V) or more stored in the storage unit 52 (step S106). When the determination in step S106 is NO, the control unit 51 determines that the amount of bending of the sheet is insufficient, and the detected value of the loop sensor 40 is stored in the storage unit 52 while the fixing speed is maintained at the deceleration value. Until the threshold value _Vth stored in step S105 is reached, the detection of the amount of deflection of the sheet in step S105 and the determination in step S106 are repeated.

  When the determination in step S106 is YES, the control unit 51 determines that the amount of deflection of the sheet is sufficient, and increases the fixing speed from the deceleration value (292.3 mm / s) to the initial value (294.1 mm / s). (Step S107). The time at this time is t3.

  When the sheet is further conveyed to the downstream side of the conveyance path 13, the trailing end of the sheet in the conveyance direction (hereinafter abbreviated as “sheet trailing end”) passes the detection position of the loop sensor 40. The time at this time is t4. Table 1 shows the transfer speed and fixing speed from time t0 to time t4 in this control example.

  In the present embodiment, as described above, the fixing speed is reduced between the time when the leading end portion of the paper in the conveyance direction passes through the detection position of the loop sensor 40 and reaches the fixing belt 30 and the pressure roller 32. Therefore, it is possible to adjust the fixing speed so that the sheet is appropriately bent before the leading end of the sheet in the conveyance direction reaches the fixing belt 30 and the pressure roller 32. Therefore, compared with the case where the adjustment of the fixing speed is started after the front end portion of the paper conveyance direction reaches the fixing belt 30 and the pressure roller 32, the timing for starting the adjustment of the fixing speed can be advanced. Accordingly, it is possible to reliably prevent image rubbing and fixing wrinkles due to paper twisting. Further, since the loop sensor 40 which is a position sensor for detecting the amount of bending of the paper is used as a timing sensor for detecting the front end of the paper in the conveyance direction, a new sensor is added in addition to the loop sensor 40. It is possible to prevent complication of the configuration of the apparatus.

  In this embodiment, in particular, since the fixing speed is reduced immediately before the front end of the paper conveyance direction reaches the fixing belt 30 and the pressure roller 32, the effect of optimizing the amount of paper deflection is enhanced, It is possible to more reliably prevent image rubbing and fixing wrinkles due to twisting.

  In addition, since the transfer belt 23 is used as the transfer member, the conveyance of the paper can be stabilized as compared with the case where the transfer roller is used as the transfer member, but the paper is bent between the transfer member and the fixing member. The distance that can be formed is shortened, and image rubbing and fixing wrinkles due to twisting of the paper are likely to occur. Therefore, it is significant to prevent the occurrence of image rubbing and fixing wrinkles accompanying the twisting of the paper using the configuration of the present invention.

  Further, since the reflective optical sensor is used as the loop sensor 40, the amount of bending of the paper can be detected without bringing the loop sensor 40 into contact with the paper. Therefore, it is possible to prevent a part of the loop sensor 40 from coming into contact with the paper and reducing the paper conveyance performance.

Further, in the present embodiment, after the leading end portion in the paper transport direction passes through the fixing belt 30 and the pressure roller 32, when the detection value of the loop sensor 40 increases to a predetermined threshold value _Vth or more, the fixing speed is increased. Yes. Thus, by controlling the fixing speed based on the detection value of the loop sensor 40, the amount of deflection of the paper can be optimized, and accompanying this, the occurrence of image rubbing and fixing wrinkles due to paper twisting occurs. Can be more reliably prevented. This effect will be further described with reference to FIG.

  FIG. 6A is a diagram illustrating a detection value of the loop sensor 40 when the first side of the double-sided printing is performed on the sheet while the fixing speed is controlled in sequence. FIG. It is a figure which shows the detected value of the loop sensor 40 at the time of printing the 1st surface of double-sided printing with respect to a paper, carrying out feedback control of the fixing speed.

Further, L in FIGS. 6A and 6B indicates a time period in which the detection value of the loop sensor 40 is lower than the threshold value _Vth . This time zone L is a time zone in which the amount of deflection of the paper is insufficient and the conveyance of the paper is unstable. Therefore, the longer the time zone L is, the more easily image rubbing and fixing wrinkles are caused by the twisting of the paper.

When feedback control of the fixing speed is performed (see FIG. 6B), the amount of flexure of the sheet is increased while the fixing speed is decreased until the detection value of the loop sensor 40 becomes equal to or higher than the threshold value _Vth . The time period L in which the detection value of the loop sensor 40 is lower than the threshold value _Vth is shorter than when the fixing speed feedback control is not performed (see FIG. 6A). Therefore, when performing single-sided printing on a sheet or when performing printing on the first side of double-sided printing on a sheet, feedback control of the fixing speed is more likely to cause image rubbing due to sheet twisting. It can be seen that fixing wrinkles hardly occur.

  Next, a control example of the transfer speed and the fixing speed when printing on the second side of the double-sided printing on the paper will be described mainly using FIG. The control until the toner image is transferred to the paper is the same as when performing single-sided printing on the paper or when printing the first side of double-sided printing on the paper, and thus the description thereof is omitted. .

  The sheet on which the toner image has been transferred is transported along the transfer belt 23 to the downstream side (upper side) of the transport path 13 and is transported along the static eliminator 37 to the downstream side of the transport path 13. Thus, when the paper is transported to the downstream side of the transport path 13, the leading edge of the paper passes the detection position of the loop sensor 40, and the loop sensor 40 detects the leading edge of the paper (step S201). The time at this time is assumed to be t0. At time t0, the transfer speed and the fixing speed are maintained at their initial values (transfer speed is 294.7 mm / s and fixing speed is 294.1 mm / s).

  The leading edge of the sheet that has passed the detection position of the loop sensor 40 is conveyed to the downstream side of the conveyance path 13 along the fixing approach guide 38. When a predetermined time (for example, 180 msec) elapses from time t0, the state immediately before the leading end of the sheet reaches the fixing belt 30 and the pressure roller 32 is obtained. The control unit 51 recognizes from the elapsed time from the time t0 that the front end of the sheet has just reached the fixing belt 30 and the pressure roller 32, and sets the fixing speed to an initial value (294.294). 1 mm / s) to a deceleration value (293.5 mm / s) (step S202). The time at this time is assumed to be t1. The above steps S201 and S202 are the same as steps S101 and S102 in the case of performing single-sided printing on a sheet or the printing of the first side of duplex printing on a sheet.

  Next, the leading end of the sheet reaches the fixing belt 30 and the pressure roller 32, and the leading end of the sheet passes through the fixing nip 36 (step S203). Thereafter, without performing feedback control of the fixing speed based on the detection value of the loop sensor 40, the transfer speed and the fixing speed are set to predetermined speeds (the transfer speed is 294.7 mm / s and the fixing speed is 293.5 mm). / S).

  When the sheet is further conveyed to the downstream side of the conveyance path 13, the rear end portion of the sheet passes the detection position of the loop sensor 40. Accordingly, the control unit 51 increases the fixing speed from a predetermined speed (293.5 mm / s) set in advance to an initial value (294.1 mm / s) (step S204). The time at this time is assumed to be t2. Table 2 shows the transfer speed and fixing speed from time t0 to time t2 in this control example.

  In the present embodiment, as described above, when printing on the second surface of the double-sided printing on the paper, at least the paper transport direction after the front end of the paper transport direction passes through the fixing belt 30 and the pressure roller 32. The transfer speed and the fixing speed are controlled to predetermined speeds until the rear end portion passes the detection position of the loop sensor 40. That is, the transfer speed and the fixing speed are controlled in sequence. For this reason, it is possible to maintain an appropriate amount of deflection of the paper, and accordingly, it is possible to more reliably prevent image rubbing and fixing wrinkles due to paper twisting. This effect will be further described with reference to FIG.

  FIG. 8 shows the detection value of the loop sensor 40 when the second side printing is performed on the sheet while the transfer speed and the fixing speed are controlled in sequence. As shown in FIG. 8, when printing on the second side of the double-sided printing on the paper, the output value of the loop sensor 40 is equal to the threshold value Vth (1) without feedback control of the transfer speed and the fixing speed. .85V) or higher. On the other hand, when the feedback control is performed in spite of the fact that the output value of the loop sensor 40 is maintained at the threshold value Vth or more in this way, the transfer speed is unnecessarily transferred when the loop sensor 40 is erroneously detected. Or the fixing speed is changed, and on the contrary, image rubbing and fixing wrinkles due to the twisting of the paper are likely to occur. In particular, the erroneous detection of the loop sensor 40 as described above is likely to occur in a high-temperature and high-humidity environment where the moisture content of the paper is high. For this reason, when printing on the second side of the double-sided printing on the paper, image rubbing and fixing wrinkles due to paper twisting are less likely to occur when the transfer speed and the fixing speed are controlled in sequence.

  In this embodiment, when performing duplex printing on a sheet, feedback control is performed on the fixing speed when printing on the first side of the sheet, and sequence control is performed on the fixing speed when printing on the second side of the sheet. Yes. Thus, the effect by combining feedback control and sequence control will be described mainly using Table 3.

Table 3 shows the relationship between the fixing speed control method and the sensor output NG rate. In Table 3, the sensor output NG rate means that when printing is performed using the printer 1 having the above-described configuration, the output value of the loop sensor 40 is lower than the NG value (for example, 1.6 V). The percentage is shown as a percentage. Specifically, ten continuous printings are performed three times (30 times in total) in a high-temperature and high-humidity environment, and the number of times that the output value of the loop sensor 40 has fallen below the NG value among the 30 times is N.
Sensor output NG rate = (N / 30) × 100
It is said.

  In Table 3, “No misalignment” indicates that the distance from the fixing approach guide 38 to the fixing nip 36 is as designed. On the other hand, “maximum misalignment” indicates a state in which the distance from the fixing approach guide 38 to the fixing nip 36 is maximally deviated from the design value due to component tolerance. In the state of “maximum misalignment”, the conveyance of the sheet is likely to be unstable compared to the state of “no misalignment”.

  As shown in Table 3, when the fixing speed is sequence-controlled on both the first and second sides of the sheet, when printing is performed on the first side of the sheet in a state of “maximum misalignment”, The sensor output value NG rate is 67%. When the fixing speed is feedback controlled for both the first side and the second side of the sheet, the sensor output value NG ratio is set when printing is performed on the second side of the sheet in the “maximum misalignment” state. 37%. On the other hand, when printing is performed on the first side of the paper as in the present embodiment, the fixing speed is feedback-controlled, and when printing is performed on the second side of the paper, the fixing speed is sequence-controlled. In both the “maximum misalignment” and “no misalignment” states, the sensor output NG rate was 0% when printing was performed on either the first side or the second side of the paper. Therefore, as in this embodiment, when printing is performed on the first side of the paper, the fixing speed is feedback-controlled, and when printing is performed on the second side of the paper, the fixing speed is sequence-controlled. It can be seen that it is possible to prevent image rubbing and fixing wrinkles due to paper twisting.

  In the present embodiment, a case has been described in which the fixing speed is decreased from the time when the leading end portion in the sheet conveyance direction passes the detection position of the loop sensor 40 to the time when it reaches the fixing belt 30 and the pressure roller 32. On the other hand, in another different embodiment, the transfer speed is increased between the time when the leading end portion of the paper in the conveyance direction passes the detection position of the loop sensor 40 and reaches the fixing belt 30 and the pressure roller 32. Also good.

  In the present embodiment, when performing single-sided printing on a sheet or printing on the first side of double-sided printing on a sheet, the front end of the sheet in the conveyance direction passes through the fixing belt 30 and the pressure roller 32. In the above description, the fixing speed is increased when the detection value of the loop sensor 40 increases to a predetermined threshold value or more. On the other hand, in another different embodiment, when performing single-sided printing on a sheet or when performing printing on the first side of double-sided printing on a sheet, the leading end of the sheet in the conveyance direction is the fixing belt 30 and the pressure roller. If the detection value of the loop sensor 40 rises above a predetermined threshold after passing through 32, the transfer speed may be lowered.

  In the present embodiment, the loop sensor 40 is arranged on the side (right side) where the paper in the transport path 13 bends. However, in another different embodiment, the loop sensor 40 on the side opposite to the side where the paper is bent in the transport path 13. May be arranged.

  In the present embodiment, the case where the transfer belt 23 is used as the transfer member has been described. However, in another different embodiment, a transfer roller may be used as the transfer member.

  In the present embodiment, the case where the fixing belt 30 and the pressure roller 32 are used as the fixing member has been described. However, in another different embodiment, a fixing roller and a pressure roller may be used as the fixing member.

  In the present embodiment, the induction heating coil 34 is used as a heat source of the fixing device 16, but in other different embodiments, other heat sources such as a halogen heater and a ceramic heater may be used.

  In this embodiment, the case where the configuration of the present invention is applied to the printer 1 has been described. However, in another different embodiment, the configuration of the present invention is applied to other image forming apparatuses such as a copying machine, a facsimile machine, and a multifunction machine. It is also possible.

23 Transfer belt (transfer member)
30 Fixing belt (fixing member)
32 Pressure roller (fixing member)
40 Loop sensor 51 Control unit

Claims (6)

A transfer member that transfers a toner image onto the paper while transporting the paper;
A fixing member provided downstream of the transfer member in the paper conveyance direction and fixing the toner image onto the paper while conveying the paper;
A loop sensor for detecting the amount of deflection of the paper between the transfer member and the fixing member;
A controller that controls a transfer speed, which is a speed at which the transfer member transports paper, and a fixing speed, which is a speed at which the fixing member transports paper,
The control unit increases the transfer speed or decreases the fixing speed after the leading end of the sheet in the conveyance direction passes through the detection position of the loop sensor and reaches the fixing member. An image forming apparatus.   When the single-sided printing is performed on the paper or the first side of the double-sided printing is performed on the paper, the control unit is configured such that after the front end of the paper in the transport direction passes through the fixing member, the loop sensor 2. The image forming apparatus according to claim 1, wherein when the detection value increases to a predetermined threshold value or more, the transfer speed is decreased or the fixing speed is increased.   When printing the second side of the double-sided printing on the paper, the control unit passes at least the rear end in the paper transport direction through the detection position after the front in the paper transport direction passes through the fixing member. 3. The image forming apparatus according to claim 1, wherein the transfer speed and the fixing speed are controlled to a predetermined speed set in advance.   4. The control unit according to claim 1, wherein the control unit increases the transfer speed or decreases the fixing speed immediately before the front end of the sheet in the conveyance direction reaches the fixing member. 2. The image forming apparatus according to item 1.   5. The image forming apparatus according to claim 1, wherein the transfer member is a transfer belt extending along a sheet conveyance direction.   The image forming apparatus according to claim 1, wherein the loop sensor is a reflective optical sensor that detects the amount of bending of the sheet by reflecting light on the sheet.

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