JP4975518B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus applied to a copying machine, a printer, a facsimile, and the like.

  2. Description of the Related Art Conventionally, in an electrophotographic image forming apparatus equipped with an intermediate transfer belt, for example, an electrostatic latent image formed on a photoreceptor is developed with toner, and a toner image obtained thereby is transferred onto, for example, an intermediate transfer belt. (Primary transfer), and further transferred (secondary transfer) from the intermediate transfer belt to the paper. The sheet is conveyed to a fixing device having a heating roller and a pressure roller, and the toner image on the sheet is melted between the two rollers and fixed on the sheet.

  On the other hand, a sheet supplied from a sheet feeding unit such as a sheet feeding cassette is transported to an intermediate transfer belt after timing is adjusted by a registration roller, and further to a fixing device. In such a conventional image forming apparatus, the sheet conveyance speed from the registration roller to the fixing device is not constant but is appropriately controlled.

For example, in the configuration described in Patent Document 1, unlike the above configuration, a toner image is directly transferred from a photosensitive member to a sheet, and the sheet is conveyed by a conveyance belt and sent to a fixing device. In the configuration described in Patent Document 1, when the leading end of the paper reaches the transport belt, the paper transport speed by the transport belt is made faster than the paper transport speed by the photoconductor. Then, before the front end of the paper enters between the rollers of the fixing device, that is, before the front end of the paper is sandwiched between the two rollers of the fixing device, the paper transport speed is reduced to the paper transport speed by the fixing device. . Note that the paper conveyance speed is reduced before the leading edge of the paper enters between the rollers of the fixing device in order to reduce the impact applied to the paper when the paper collides with the rollers of the fixing device. is there.
JP-A-8-234616 (published September 13, 1996)

  However, as described in Patent Document 1, in the configuration in which the sheet conveyance speed is reduced before the sheet enters the fixing device, the upstream of the sheet conveyance direction occurs when the sheet collides with the roller of the fixing device. It is impossible to counter the push-back action of the paper, and image disturbance (transfer deviation) is likely to occur in the toner image on the paper.

  On the other hand, if the configuration is such that the paper conveyance speed is not reduced even after the front end of the paper enters between the rollers of the fixing device, the above-mentioned transfer misalignment problem can be solved, but an image defect due to paper deflection occurs. . That is, if the sheet conveyance speed is not decelerated, deflection occurs in a portion of the sheet between the fixing device and the intermediate transfer belt (secondary transfer portion of the toner image from the intermediate transfer belt to the sheet). In this case, since a force is applied to the sheet to eliminate the deflection, the leading edge of the sheet is pushed into the fixing nip portion between both rollers of the fixing device, and the image is disturbed against the collision with the fixing device. Can be suppressed. However, when the deflection of the paper increases, the toner image transferred onto the paper by the deflection causes an image defect caused by contacting a peripheral member such as an intermediate transfer belt.

  Therefore, the present invention provides an image forming apparatus capable of preventing the image unnecessary due to the disturbance of the toner image transferred onto the paper by appropriately controlling the paper conveyance speed and obtaining a good image. It is aimed.

  In order to solve the above problems, an image forming apparatus of the present invention forms an electrostatic latent image on a photoreceptor, develops the electrostatic latent image to form a toner image, and acquires the toner image. And an operation of transferring the toner image on the paper onto the paper by a fixing portion having a pressure roller and a fixing roller that are pressed against each other and rotated in the opposite directions. In the image forming apparatus in which the sheet conveying unit, the transfer unit, and the fixing unit are arranged in this order in the sheet conveying direction in the sheet conveying path, a sheet position detecting unit that detects the position of the sheet in the sheet conveying path; Until the leading edge of the paper transported through the paper transport path reaches the first position sandwiched between the two rollers of the fixing unit, the paper transporting means uses the paper until the paper position detecting means detects it. While the conveyance speed is set to a first speed that is faster than the sheet conveyance speed in the transfer unit, when the sheet position detection means detects that the leading edge of the sheet has reached the first position, the sheet And a control means for reducing the paper transport speed by the transport means to a second speed slower than the first speed.

  According to the above configuration, until the leading end of the sheet conveyed through the sheet conveyance path reaches the first position where the sheet is sandwiched between the two rollers of the fixing unit, the sheet conveyance unit located on the upstream side of the transfer unit The sheet conveyance speed is a first speed that is faster than the sheet conveyance speed in the transfer unit. Therefore, at this time, bending occurs in the upstream portion of the transfer portion of the sheet.

  Here, when the leading end of the sheet collides with the roller of the fixing unit, a force (second force) is applied to the sheet so as to push it back upstream. However, since the first force in the direction opposite to the second force acts on the sheet due to the bending, the second force is canceled out by the first force. Therefore, the transfer deviation of the toner image on the sheet due to the second force acting can be suppressed.

  On the other hand, when the leading edge of the sheet reaches the first position, the sheet conveying speed by the sheet conveying unit is decreased to a second speed that is slower than the first speed. Accordingly, it is possible to prevent a situation where an excessively large deflection is generated in the upstream portion of the transfer portion of the sheet, and the bent portion comes into contact with a member constituting the transfer portion to disturb the toner image and cause an image defect.

  The image forming apparatus includes a sheet deflection amount detection unit that detects a deflection amount of the sheet on the sheet conveyance path between the transfer unit and the sheet conveyance unit, and the control unit detects the sheet deflection amount. The paper conveyance speed by the paper conveyance means may be controlled in accordance with the amount of paper deflection detected by the means.

  According to the above configuration, the sheet conveyance speed by the sheet conveyance unit is controlled in accordance with the amount of deflection of the sheet on the sheet conveyance path between the transfer unit and the sheet conveyance unit. In order to obtain this force, it can be maintained within an appropriate range.

  In the above image forming apparatus, the sheet deflection amount detecting means is a reflective sensor including a light emitting unit that projects light onto the sheet and a light receiving unit that receives reflected light from the sheet. It is good also as a structure which detects a distance.

  According to the above configuration, since the paper deflection amount detection means is a reflection type sensor using light, it is possible to appropriately detect the paper deflection amount with a simple configuration.

  In the above image forming apparatus, the control unit may control a sheet conveyance speed by the sheet conveyance unit so that a deflection amount of the sheet is a predetermined amount or less.

  According to the above configuration, the sheet conveyance speed is controlled by the sheet conveying unit so that the amount of bending of the sheet is equal to or less than the predetermined amount. Therefore, if the predetermined amount is set to an appropriate value, the bent portion of the sheet Therefore, it is possible to reliably prevent the occurrence of image defects due to contact with the members constituting the transfer portion.

  In the image forming apparatus, the control unit may reduce the sheet conveyance speed of the sheet conveyance unit from the first speed to the second speed when the amount of bending of the sheet exceeds a predetermined amount. .

  According to the above configuration, when the amount of bending of the sheet exceeds a predetermined amount, the sheet conveying speed by the sheet conveying unit decreases from the first speed to the second speed, so the amount of bending of the sheet is set to the first force. Therefore, it is possible to reliably prevent the occurrence of an image defect due to the bent portion of the sheet coming into contact with a member constituting the transfer portion.

  The image forming apparatus includes a storage unit that stores a plurality of combinations of the sheet thickness and the predetermined value corresponding to the sheet thickness, and an input unit that can input the sheet thickness. The control unit may be configured to set the predetermined value corresponding to the thickness of the sheet input from the input unit with reference to the combination information stored in the storage unit.

  According to the above configuration, for example, the predetermined value is set to be larger according to the thickness of the paper, such as the predetermined value is set to be larger as the paper is thicker. Appropriate speed control of the sheet conveying means can be performed.

  As described above, according to the configuration of the present invention, it is possible to reduce the impact when the leading end of the paper collides with the roller of the fixing unit, and to suppress the transfer deviation of the toner image on the paper. Further, it is possible to prevent a situation in which an excessively large deflection occurs in the upstream portion of the transfer portion of the sheet, and the bent portion contacts a member constituting the transfer portion to cause an image defect.

[Embodiment 1]
Embodiments of the present invention will be described below with reference to the drawings.
FIG. 2 is a longitudinal side view showing the configuration of the image forming apparatus according to the present embodiment.

  The image forming apparatus A is a recording sheet (hereinafter simply referred to as a sheet) corresponding to image data created by reading a document by a document reading device (scanner) C provided on the image forming apparatus A or image data transmitted from the outside. Multicolor and monochromatic images are formed. The external body of the image forming apparatus A is provided with a display unit (not shown) for displaying various states facing the user, and an operation panel (not shown) for inputting setting and operation instructions. .

  The image forming apparatus A includes an exposure unit 1, a developing device 2 (2a, 2b, 2c, 2d), a photosensitive drum 3 (3a, 3b, 3c, 3d), a charger 5 (5a, 5b, 5c, 5d), Cleaner unit 4 (4a, 4b, 4c, 3d), intermediate transfer belt unit 8, transfer unit (transfer unit) 11, fixing unit (fixing unit) 12, paper transport path S, paper feed tray (paper feed cassette) 10 ( 10 (1), 10 (2), 10 (3), 10 (4)) and a paper discharge tray 15. Of these means, the means provided for each color in order to constitute the image forming station for each color, such as the developing device 2, the photosensitive drum 3, the charger 5, and the cleaner unit 4 are appropriately selected. It describes. This also applies to intermediate transfer rollers 6a, 6b, 6c, and 6d described later.

  The paper discharge tray 15 is formed in a space below the document reading device C and has a substantially horizontal paper placement surface. The printed paper is discharged onto the paper discharge tray 15 face down (printing surface facing downward). A main body of the image forming apparatus A is provided below the paper placement surface of the paper discharge tray 15.

  Image data handled in the image forming apparatus A corresponds to a color image using each color of black (K), cyan (C), magenta (M), and yellow (Y). Accordingly, in the main body of the image forming apparatus A, the developing device 2 (2a, 2b, 2c, 2d), the photosensitive drum 3 (3a, 3b, 3c, 3d), and the charger 5 (5a, 5b, 5c, 5d). The cleaner units 4 (4a, 4b, 4c, 3d) are provided so as to form four types of latent images corresponding to the respective colors, and constitute image forming stations for the respective colors. In the above four means corresponding to each color, a is set to black, b is set to cyan, c is set to magenta, and d is set to yellow.

  The photosensitive drum 3 is disposed (mounted) on the upper part of the image forming apparatus. The photosensitive drums 3 (3a, 3b, 3c, 3d) corresponding to the four image stations are arranged in the order of 3d, 3c, 3b, 3a toward the transfer unit 11, and these photosensitive drums 3 ( An intermediate transfer belt 7 is disposed on 3a, 3b, 3c, 3d). In the intermediate transfer belt unit 8, the intermediate transfer belt 7 provided on each photosensitive drum 3 (3a, 3b, 3c, 3d) is in contact with the surface of each photosensitive drum 3 (3a, 3b, 3c, 3d). It is a configuration that circulates.

  A partition wall 8a is disposed above the intermediate transfer belt unit 8 in the horizontal direction, and a plurality of toner boxes 2X are disposed in the space between the partition wall 8a and the paper discharge tray 15. These toner boxes 2X contain toners of four types of developer supplied to the developing devices 2 (2a, 2b, 2c, 2d), respectively.

  The charger 5 uniformly charges the surface of the photosensitive drum 3 to a predetermined potential, and a brush type or a charger type may be used in addition to the contact type roller type shown in FIG.

  The exposure unit 1 uses a laser scanning unit (LSU) including a laser irradiation unit and a reflection mirror. In addition to this method, the exposure unit 1 may be a method using, for example, an EL or LED writing head in which light emitting elements are arranged in an array. The exposure unit 1 exposes the charged photosensitive drum 3 in accordance with the input image data, thereby forming an electrostatic latent image corresponding to the image data on the surface thereof.

  The developing device 2 visualizes the electrostatic latent image formed on the corresponding photosensitive drum 3 with (K, C, M, Y) toner. The cleaner unit 4 removes and collects toner remaining on the surface of the photosensitive drum 3 after development and image transfer.

  The intermediate transfer belt unit 8 includes an intermediate transfer belt 7, an intermediate transfer belt drive roller 71, an intermediate transfer belt tension mechanism 73, an intermediate transfer belt driven roller 72, an intermediate transfer roller 6 (6a, 6b, 6c, 6d), and an intermediate transfer. A belt cleaning unit 9 is provided.

  The intermediate transfer belt driving roller 71, the intermediate transfer belt tension roller 73, the intermediate transfer roller 6 and the intermediate transfer belt driven roller 72 stretch the intermediate transfer belt 7 and rotate it in the direction of arrow B.

  The intermediate transfer roller 6 provides a transfer bias for transferring the toner image on the photosensitive drum 3 onto the intermediate transfer belt 7. The intermediate transfer roller 6 is rotatably supported by the intermediate transfer roller mounting portion of the intermediate transfer belt tension mechanism 73 of the intermediate transfer belt unit 8.

  The intermediate transfer belt 7 is provided so as to come into contact with the respective photosensitive drums 3, and the toner images of the respective colors formed on the respective photosensitive drums 3 are sequentially superimposed and transferred. Transfer of the toner image from the photosensitive drum 3 to the intermediate transfer belt 7 is performed by the intermediate transfer roller 6 in contact with the back side of the intermediate transfer belt 7. As a result, a single color toner image (multicolor toner image) is formed on the intermediate transfer belt 7. The intermediate transfer belt 7 is formed in an endless shape using a film having a thickness of about 100 μm to 150 μm.

  A high voltage transfer bias (a high voltage having a polarity (+) opposite to the toner charging polarity (−)) is applied to the intermediate transfer roller 6 in order to transfer the toner image. The intermediate transfer roller 6 is based on a metal (for example, stainless steel) shaft having a diameter of 8 to 10 mm, and the surface thereof is covered with a conductive elastic material (for example, EPDM, urethane foam, or the like). Due to the presence of the conductive elastic material, a high voltage can be uniformly applied to the intermediate transfer belt. In this embodiment, a roller shape is used as the transfer electrode, but a brush shape may also be used.

  As described above, an electrostatic latent image is formed on each photosensitive drum 3 in accordance with each hue of the image data, and these electrostatic latent images are visualized with the corresponding color toner, and the obtained toner The image is laminated on the intermediate transfer belt 7 and becomes one toner image corresponding to the image data. The toner image is transferred onto the sheet by the transfer unit 11 disposed at the contact position between the sheet and the intermediate transfer belt 7 by the rotation of the intermediate transfer belt 7. The transfer unit 11 constitutes a secondary transfer portion and has a transfer belt unit structure.

  The transfer unit 11 is a belt-type transfer unit in which the transfer belt 11 a circulates on a plurality of rollers, and a transfer roller 11 b is provided at a position facing the intermediate transfer belt driving roller 71. Accordingly, the intermediate transfer belt 7 and the transfer belt 11a move while being in contact with each other between the intermediate transfer belt driving roller 71 and the transfer roller 11b, and the color on the intermediate transfer belt 7 is transferred to a sheet passing between the belts 7 and 11a. The toner image is transferred.

  At this time, the intermediate transfer belt 7 and the transfer belt 11a are pressed against each other with a predetermined nip width, and a voltage for transferring the toner image onto the sheet is applied to the transfer belt 11a. This voltage is a high voltage having a polarity (+) opposite to the charging polarity (−) of the toner.

  In order to constantly obtain the nip at the secondary transfer portion where the intermediate transfer belt 7 and the transfer belt 11a are in contact, either the transfer roller 11b or the intermediate transfer belt driving roller 71 is formed of a hard material (metal or the like). The other is made of a soft material (such as an elastic rubber roller or a foaming resin roller).

  The toner adhering to the intermediate transfer belt 7 due to contact with the photosensitive drum 3 or the toner remaining on the intermediate transfer belt 7 without being transferred onto the sheet by the transfer unit 11 causes a color mixture of toner in the next process. Therefore, it is removed and collected by the intermediate transfer belt cleaning unit 9.

  The intermediate transfer belt cleaning unit 9 includes a cleaning blade as a cleaning member that contacts the intermediate transfer belt 7, for example. The intermediate transfer belt 7 in contact with the cleaning blade is supported by an intermediate transfer belt driven roller 72 from the back side.

  The paper feed tray 10 stores paper used for image formation, and is provided below the exposure unit 1. The paper feed tray 10 includes a plurality of paper feed cassettes 10 (1) to 10 (4), and sheets of different sizes are stored in the paper feed cassettes 10 (1) to 10 (4).

  The paper transport path S is for sending the paper in the paper feed tray 10 to the paper discharge tray 15 via the transfer unit 11 and the fixing unit (fixing unit) 12. In the vicinity of the paper transport path S from the paper feed tray 10 to the paper discharge tray 15, there are a pickup roller 16, a PS roller 14 (paper transport means), a transfer unit 11, a fixing unit 12, a transport roller 25 for transporting paper, and the like. It is arranged. In addition, a waste toner box 34 that stores waste toner collected by the cleaner unit 4 or the like is disposed on the side of the paper feed tray 10.

  The transport rollers 25 are small rollers for promoting and assisting the transport of paper, and a plurality of transport rollers 25 are provided along the paper transport path S. The pickup roller 16 is a pull-in roller that is provided at the end of the paper feed tray 10 and feeds paper one by one from the paper feed tray 10 to the paper transport path S.

  The PS roller 14 is a paper stop roller that temporarily holds the paper conveyed in the paper conveyance path S, and then transfers the paper to the transfer unit at the timing when the leading edge of the toner image on the photosensitive drum 3 is aligned with the leading edge of the paper. It is for carrying.

  The fixing unit 12 includes a fixing roller 31 and a pressure roller 32. The fixing roller 31 is set so that the heat generation amount of a built-in heater is controlled by a control unit based on a signal from a temperature detector (not shown), and reaches a predetermined fixing temperature. The fixing roller 31 and the pressure roller 32 melt and mix the multi-color toner image transferred to the paper and thermally fix the paper on the paper by thermocompression while rotating with the paper sandwiched therebetween.

  The sheet after the fixing of the multicolor toner image is transported to the reverse paper discharge path of the paper transport path S by the transport roller 25, and is discharged in a reversed state (with the multicolor toner image facing downward). The paper is discharged onto the tray 15.

  In the image forming apparatus A, paper can be supplied from both the paper feed tray 10 and the manual paper feed tray 20. The manual paper feed tray 20 is used when printing a small number of sheets.

  A sheet (sheet) conveyed from the sheet feeding tray 10 is conveyed to the PS roller 14 by a conveying roller 25 in the conveying path. Next, the sheet is conveyed from the PS roller 14 to the transfer roller 11b at a timing when the leading edge of the sheet and the leading edge of the image information on the intermediate transfer belt 7 are aligned, and the toner image is transferred from the intermediate transfer belt 7 onto the sheet. Next, as the paper passes through the fixing unit 12, unfixed toner on the paper is melted by heat and fixed on the paper. Thereafter, the paper is discharged from the paper discharge roller (not shown) onto the paper discharge tray 15 through the transport roller 25 (in the case of single-sided printing request). This operation is the same when supplying paper from the manual paper feed tray 20.

  If the print request content is double-sided printing, the trailing edge of the paper that has finished single-sided printing and passed through the fixing unit 12 is held by the paper discharge roller, and the paper is conveyed by the reverse rotation of the paper discharge roller. After being guided to the roller 25, the back side printing is performed through the PS roller 14 and discharged to the paper discharge tray 15.

  FIG. 1 is a schematic diagram illustrating the configuration of the image forming apparatus A in the vicinity of the sheet conveyance path from the PS roller 14 to the fixing unit 12.

  As shown in FIG. 1, the transfer unit 11 includes a transfer belt 11a, a transfer roller 11b, and a plurality of guide rollers 11c. The transfer belt 11a is installed on the transfer roller 11b and the plurality of guide rollers 11c and is rotatably supported. It is.

  The transfer roller 11b is disposed opposite to the intermediate transfer belt drive roller 71 of the intermediate transfer belt unit 8 via the intermediate transfer belt 7 and the transfer belt 11a, and the toner image on the intermediate transfer belt 7 is between the rollers 71 and 11b. Is a secondary transfer section for transferring the image to the paper.

  The fixing roller 31 of the fixing unit 12 and the intermediate transfer belt drive roller 71 of the intermediate transfer belt unit 8 shown in FIG. 1 are driven by a drive motor 31a and a drive motor 71a, respectively, and the PS roller 14 is driven by a PS motor 14b. . A PS clutch 14 a is provided in the driving force transmission path from the PS motor 14 b to the PS roller 14. Further, a paper sensor (paper position detecting means) 17 for detecting the presence or absence of paper is provided at a position upstream of the PS roller 14 in the paper transport path.

  In FIG. 1, when the sheet fed from the PS roller 14 reaches the secondary transfer portion that is a position where the intermediate transfer belt drive roller 71 of the intermediate transfer belt unit 8 and the transfer roller 11 b of the transfer unit 11 face each other, the intermediate transfer is performed. A toner image is transferred from the belt 7 onto the paper. The sheet on which the toner image has been transferred is conveyed along the transfer belt 11a. Thereafter, the sheet is peeled off from the transfer belt 11 a at the guide roller 11 c portion at the end (upper end) of the transfer unit 11 and conveyed to the fixing unit 12.

  The sheet conveyed to the fixing unit 12 is guided to the sheet guide portion 12 a of the fixing unit 12 and is sent to the pressure contact portion between the fixing roller 31 and the pressure roller 32. Thereafter, as described above, the toner image is fixed on the paper at the press contact portion.

  FIG. 3 is a block diagram showing a configuration for controlling the driving units (motors) of the rollers shown in FIG. As shown in FIG. 3, the image forming apparatus A includes a control unit (control means, paper position detection means) 35 for controlling the PS clutch 14a, the PS motor 14b, the drive motor 31a, and the drive motor 71a. The control unit 35 is connected to the PS clutch 14a, the PS motor 14b, the drive motor 31a, and the drive motor 71a via drivers 41, 42, 43, and 44, and further to the paper sensor 17. Therefore, the control unit 35 controls the rotation and rotation speed (circumferential speed) of the drive motor 31a, the drive motor 71a, and the PS motor 14b. Further, the operation of the PS clutch 14a, that is, whether the driving force of the PS motor 14b is transmitted to the PS roller 14 is controlled.

  Specifically, the peripheral speed of the PS roller 14 is controlled to the first speed V1 or the second speed V2 (V1> V2). In the present embodiment, the first speed is 67.4 mm / sec and the second speed is 66.7 mm / sec. The peripheral speed of the fixing roller 31 is 65.2 mm / sec, the process speed (the peripheral speed of the photosensitive drum 3) is 66.5 mm / sec, and the peripheral speed of the transfer belt 11a is 66.8 mm / sec.

  As described above, in the image forming apparatus A, the peripheral speed of the fixing roller 31 ≈ the process speed ≈ the peripheral speed of the transfer belt 11 a is set. The process speed (the peripheral speed of the photosensitive drum 3) and the peripheral speed of the transfer belt 11a are set so that the process speed is slower by 0.3%.

  In the above configuration, the sheet conveying operation in the image forming apparatus A will be described below with reference to FIGS. FIG. 4 is an explanatory diagram showing a state in which the sheet is fed by the PS roller 14 in the configuration shown in FIG. FIG. 5 is an explanatory diagram illustrating a state in which the sheet is bent between the PS roller 14 and the transfer unit 11 in the configuration illustrated in FIG. 1. FIG. 6 illustrates a state in which the leading end of the sheet is sandwiched between the fixing roller 31 and the pressure roller 32 and the trailing end of the sheet is sandwiched between the PS rollers 14 in the configuration illustrated in FIG. It is explanatory drawing. FIG. 7 is an explanatory diagram showing a state in which the deflection of the paper between the transfer region (transfer portion) G and the PS roller 14 becomes excessive in the configuration shown in FIG. In this embodiment, it is assumed that the fixing unit (fixing unit) 12 is disposed substantially vertically above the transfer unit (transfer unit) 11.

  For example, the paper P sent out from the paper feed tray 10 is temporarily stopped by the PS roller 14 via the paper sensor 17. By this stop operation, the sheet is obliquely fed by the PS roller 14.

  Thereafter, as shown in FIG. 4, the sheet is sent out toward the transfer region G (secondary transfer portion) by the PS roller 14. At this time, the peripheral speed of the PS roller 14 is set to the first speed V1. Accordingly, the sheet moves toward the transfer region G at the first speed V1. The first speed V1 is a normal speed of the PS roller 14 and is faster than the peripheral speed of the transfer belt 11a.

  When the sheet is conveyed at the first speed V1, the sheet is bent between the PS roller 14 and the transfer unit 11 due to the difference in speed between the transfer belt 11a and the PS roller 14 as shown in FIG. This flexure causes the paper to swell in a direction orthogonal to the paper transport direction. The bending occurs because the conveyance speed (first speed V1) of the paper P by the PS roller 14 is faster than the conveyance speed of the paper P by the transfer belt 11a (transfer area G). In this way, the bending of the paper P generated at the upstream position of the transfer region G causes a force (first force) to push the leading end of the paper P into the transfer region G. That is, the paper P is forcibly pushed into the transfer area G by the PS roller 14.

  The toner image on the intermediate transfer belt 7 is transferred onto the surface of the paper P that passes through the transfer region G. Further, the sheet P that has passed through the transfer region G moves together with the transfer belt 11a of the transfer unit 11, and sequentially peels from the transfer belt 11a from the leading end portion and proceeds toward the fixing unit 12.

  Thereafter, the leading end portion Pa of the sheet P collides with the rotating fixing roller 31 or the pressure roller 32 of the fixing unit 12 and then enters between the two rollers, that is, the pressure contact portion of both rollers, as shown in FIG. It is sandwiched between the press contact portions. At this time, the rear end portion of the paper P is not passing through the PS roller 14. Next, as described above, at the timing when the leading end portion Pa of the paper P is sandwiched between the pressure contact portions between the fixing roller 31 and the pressure roller 32, the transport speed of the paper P by the PS roller 14 is the second speed V2. Will be slowed down.

  As described above, when the leading end portion Pa of the paper P collides with the roller of the fixing unit 12, a force (second force) is applied to the paper P to push it back upstream. However, since the first force in the direction opposite to the second force acts on the paper P due to the bending, the second force is canceled out by the first force. Therefore, the transfer deviation of the toner image on the paper P due to the second force acting is suppressed.

  In particular, in the case of a hard paper such as a thick paper, since the flexibility of the paper is low, the influence when the leading end collides with the roller of the fixing unit 12 is easily transmitted directly to the upstream portion of the paper. Therefore, if the above configuration is applied when using a hard sheet such as a thick sheet, the function of suppressing the transfer deviation of the toner image is remarkably performed.

  Further, even when the leading end portion Pa of the paper P is sandwiched between the pressure contact portions between the fixing roller 31 and the pressure roller 32, the transport speed of the paper P by the PS roller 14 is maintained at the first speed V1. 7, the deflection of the paper between the transfer region G and the PS roller 14 becomes too large. In this case, as shown in the figure, since the paper P contacts the intermediate transfer belt 7 on the upstream side of the transfer region G, the toner image on the intermediate transfer belt 7 is disturbed and an image defect occurs. In this case, the printing magnification increases. The increase in the printing magnification is due to the fact that the conveyance speed of the paper P is governed by the conveyance speed V1 by the PS roller 14 and becomes faster than the conveyance speed of the toner image by the intermediate transfer belt 7, so that the toner transferred onto the paper P This is because the image is stretched. Such a problem is that, as described above, at the timing when the leading end portion Pa of the paper P is sandwiched between the pressure contact portions between the fixing roller 31 and the pressure roller 32, the transport speed of the paper P by the PS roller 14 is the first. It is prevented by decelerating to 2 speed V2.

FIG. 8 is a timing chart showing the operation of the PS roller 14 based on the control of the control unit 35.
In FIG. 8, first, the paper P conveyed toward the PS roller 14 is stopped when the leading edge reaches the PS roller 14 after the leading edge is detected by the paper sensor 17. At this time, the PS clutch 14a is OFF and the PS roller 14 is stopped. The oblique feeding of the paper P that has reached the PS roller 14 is corrected by the PS roller 14.

  Next, the PS clutch 14a is turned on after time t1 after the leading edge of the paper P is detected by the paper sensor 17. As a result, the PS roller 14 starts to rotate. At this time, the sheet conveyance speed by the PS roller 14 is the first speed V1. When the PS roller 14 starts to rotate, the paper P is conveyed toward the transfer region G and further toward the fixing unit 12. During this time, the toner image is transferred from the intermediate transfer belt 7 to the paper P.

  Next, after a time t2 after the leading edge of the paper P is detected by the paper sensor 17, the paper transport speed by the PS roller 14 is switched from the first speed V1 to the second speed V2. This switching timing corresponds to the timing at which the leading edge of the paper P is sandwiched between the pressure contact portion between the fixing roller 31 and the pressure roller 32.

  Next, the PS clutch 14a is turned off at time t3 after the trailing edge of the paper P is detected by the paper sensor 17. As a result, the PS roller 14 stops.

FIG. 9 is a flowchart showing the control operation of the control unit 35.
In FIG. 9, first, when the print button of the image forming apparatus A is operated and a print request is input to the image forming apparatus A (S10), the conveyance speed of the paper P by the PS roller 14 is set to the first speed V1. Set (S11).

  Next, the paper P in the paper tray 10 is picked up by the pickup roller 16 (S12), and the apparatus waits until the time t1 elapses after the leading edge of the paper P is detected by the paper sensor 17 (S13). Thereafter, when the time t1 elapses, the PS clutch 14a is switched from the off state to the on state (S14). As a result, the PS roller 14 starts to rotate.

  Next, the process waits until the time t2 elapses after the paper sensor 17 detects the leading edge of the paper P (S15). Thereafter, when the time t2 elapses, the conveyance speed of the paper P by the PS roller 14 is switched from the first speed V1 to the second speed V2 (S16).

  Next, the process waits until the time t3 elapses after the paper sensor 17 detects the trailing edge of the paper P (S17). Thereafter, when the time t3 elapses, the PS clutch 14a is switched from ON to OFF (S18). As a result, the PS roller 14 stops.

  With the above control, printing for one page is completed. Thereafter, if there is printing of the next page, the process returns to S11 and the following processing is repeated, and if there is no printing of the next page, the processing is terminated (S19).

[Embodiment 2]
Another embodiment of the present invention will be described below with reference to the drawings.
FIG. 10 is a schematic diagram showing a configuration in the vicinity of the conveyance path of the paper P from the PS roller 14 to the fixing unit 12 of the image forming apparatus A in the present embodiment.

  In addition to the configuration shown in the first embodiment, the image forming apparatus A according to the present embodiment includes a paper deflection amount sensor (paper deflection amount detection unit) that detects the amount of paper deflection between the PS roller 14 and the transfer unit 11. 40). In the present embodiment, the sheet deflection amount sensor 40 is composed of a reflection type sensor (light receiving / emitting sensor) provided in the sheet conveyance path between the PS roller 14 and the transfer unit 11. This reflection type sensor detects the distance from the reflection type sensor to the sheet P as the measurement object by reflecting the light emitted from the light emitting unit with the measurement object and receiving the reflected light with the light receiving unit. is there.

  Specifically, as shown in FIG. 10, when the amount of paper deflection increases to the amount indicated by the broken line, the paper deflection amount sensor 40 detects the amount of paper compared to the ideal paper deflection amount indicated by the solid line. The distance to P becomes closer. Therefore, the amount of deflection of the paper P can be obtained by calculating the difference between the distances.

  Further, in the present embodiment, the control unit 35 has a control function for making the paper deflection amount constant based on the detection result of the paper deflection amount sensor 40. As a result, an image defect prevention function can be enhanced.

  That is, when the leading edge of the sheet reaches the transfer region G, the amount of sheet deflection gradually increases as the sheet conveyance speed by the PS roller 14 continues to be higher than the sheet conveyance speed by the transfer unit 11. If the amount of sheet deflection increases excessively, as described with reference to FIG. 7, the sheet P contacts the intermediate transfer belt 7 on the upstream side of the transfer region G, and the toner image on the intermediate transfer belt 7 is disturbed, resulting in an image defect. Will occur. Therefore, the sheet deflection amount sensor 40 detects the sheet deflection amount at the above position, and the control unit 35 controls the sheet conveyance speed by the PS roller 14 so that the sheet deflection amount is within a predetermined value. Specifically, when the sheet deflection amount exceeds a predetermined value range, the control unit 35 changes the sheet conveyance speed by the PS roller 14 from the first speed V1 to the second speed V2, that is, the transfer belt 11a. Is reduced to a normal speed substantially equal to the paper transport speed (paper transport speed by the fixing unit 12).

  FIG. 11 is a block diagram showing a configuration for controlling the driving units (motors) of the rollers shown in FIG. FIG. 12A is a flowchart showing the control operation of the control unit 35 of the present embodiment, and FIG. 12B is a flowchart showing details of the operation of S20 shown in FIG. The control operation of the control unit 35 will be described below based on FIG. Since S10 to S19 are the same as those shown in FIG. 9, detailed description thereof will be omitted.

  In FIG. 12A, the conveyance speed of the paper P by the PS roller 14 is set to the first speed V1 (S11), and when the time t1 elapses after the leading edge of the paper P is detected by the paper sensor 17 (S13). ), The PS clutch 14a is switched from the off state to the on state (S14). As a result, the PS roller 14 starts to rotate.

  In this state, parallel processing (subroutine 1) for controlling the amount of sheet deflection is started (S20). This parallel processing is executed by the time sharing processing of the control unit 35.

  In subroutine 1, as shown in FIG. 12B, it is determined whether or not the sheet deflection amount exceeds a predetermined value by the sheet deflection amount sensor 40 (S22). The sheet conveyance speed is decreased from the first speed V1 to the second speed V2 (S23). If the paper deflection amount does not exceed the predetermined value, the process of determining whether or not the paper deflection amount exceeds the predetermined value (S22) is continued while the subroutine 1 is being executed.

  On the other hand, when the PS clutch 14a is turned off, the main routine (FIG. 12A) is instructed to the subroutine 1 to end the subroutine 1 (parallel processing) (S210 in FIG. 12A). In subroutine 1, when an end command is received from the main routine (S24), the process is terminated.

  The predetermined value of the paper deflection amount can be set according to the thickness of the paper P. If the paper P is thicker, the predetermined value is larger, that is, if the deflection amount is set larger, the collision with the roller of the fixing unit 12 will occur. The impact of time can be reduced. However, even in this case, the predetermined value is premised on the range where the sheet P does not contact the intermediate transfer belt 7 on the upstream side of the transfer region G.

  The image forming apparatus A is configured such that the user can input the thickness of the paper P so that the predetermined value of the amount of paper deflection can be changed according to the thickness of the paper. An input means for this purpose is, for example, the operation panel 37 (see FIG. 11; input means). The memory (see FIG. 11; storage means) 36 stores information indicating the relationship between the sheet thickness and the predetermined value corresponding thereto. Accordingly, the control unit 35 is information that the memory 36 stores a corresponding predetermined value based on the thickness of the sheet input from the operation panel 37, for example, the thickness of the sheet loaded in each sheet feeding tray 10. Can be set with reference to.

  As described above, in the image forming apparatus A of the present embodiment, the sheet conveyance speed by the PS roller 14 is controlled in accordance with the amount of deflection of the sheet on the sheet conveyance path between the transfer region G and the PS roller 14. . Therefore, the amount of bending of the sheet can be maintained in an appropriate range for obtaining a force (first force) for pushing the leading end of the sheet P into the transfer region G. In addition, the amount of paper deflection between the transfer area G and the PS roller 14 becomes excessive, and the bent portion of the paper comes into contact with a member constituting the transfer area G (for example, the intermediate transfer belt 7), resulting in an image defect. Can be reliably prevented.

  In the present embodiment, when the paper deflection amount detected by the paper deflection sensor 40 exceeds a predetermined value, the control unit 35 changes the paper conveyance speed by the PS roller 14 from the first speed V1 to the second speed. The speed is switched to V2 (normal speed substantially equal to the paper transport speed by the transfer belt 11a). However, in this case, the speed to be switched from the first speed V1 is not limited to the second speed V2, and is slower than the first speed V1 and faster than the second speed V2, and is the first speed V1 and the second speed V2. It may be a speed between.

  The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope shown in the claims, and embodiments obtained by appropriately combining technical means disclosed in different embodiments. Is also included in the technical scope of the present invention.

FIG. 3 is a schematic diagram illustrating a configuration in the vicinity of a sheet conveyance path from a PS roller illustrated in FIG. 2 to a fixing unit in the image forming apparatus according to the embodiment of the present invention. 1 is a longitudinal side view illustrating a configuration of an image forming apparatus according to an embodiment of the present invention. It is a block diagram which shows the structure for controlling the drive part of each roller shown in FIG. FIG. 2 is an explanatory diagram illustrating a state in which a sheet is fed by a PS roller in the configuration illustrated in FIG. 1. FIG. 2 is an explanatory diagram illustrating a state in which a sheet is bent between a PS roller and a transfer unit in the configuration illustrated in FIG. 1. FIG. 2 is an explanatory diagram illustrating a state in which the leading end of the sheet is sandwiched between a fixing roller and a pressure roller and the trailing end of the sheet is sandwiched between PS rollers in the configuration illustrated in FIG. 1. In the configuration shown in FIG. 1, an explanation is given showing a state in which the deflection of the sheet between the transfer region G and the PS roller becomes excessively large. It is a timing chart which shows operation | movement of the PS roller based on control of the control unit shown in FIG. It is a flowchart which shows the control operation of the control unit shown in FIG. FIG. 6 is a schematic diagram illustrating a configuration in the vicinity of a sheet conveyance path from a PS roller illustrated in FIG. 2 to a fixing unit in an image forming apparatus according to another embodiment of the present invention. It is a block diagram which shows the structure for controlling the drive part of each roller shown in FIG. FIG. 12A is a flowchart showing the control operation of the control unit shown in FIG. 11, and FIG. 12B is a flowchart showing details of the operation of S20 shown in FIG.

Explanation of symbols

3a-3d photoconductor drum
2a to 2d Developing device 7 Intermediate transfer belt 8 Intermediate transfer belt unit 11 Transfer unit 11a Transfer belt 11b Transfer roller 12 Fixing unit (fixing unit)
14 PS roller (paper transport means)
14a PS clutch 14b PS motor 17 Paper sensor (paper position detecting means)
31 Fixing roller 31a Drive motor 32 Pressure roller 35 Control unit (control means, paper position detection means)
36 Memory (memory means)
37 Operation panel (input means)
40 Paper deflection sensor (paper deflection detection means)
71 Intermediate transfer belt drive roller 71a Drive motor A Image forming apparatus G Transfer area (transfer section)

Claims (6)

The electrostatic latent image is formed on the photoconductor, the electrostatic latent image is developed to form a toner image, and the toner image is acquired and transferred onto the paper by the transfer unit. A fixing unit having a pressure roller and a fixing roller rotating in a direction performs an operation of fixing the toner image on the sheet to the sheet, and a sheet conveying unit, the transfer unit, and the In the image forming apparatus in which the fixing units are arranged in this order,
Paper position detecting means for detecting the position of the paper in the paper transport path;
Until the paper position detecting means detects that the leading edge of the paper conveyed along the paper conveyance path has reached the first position sandwiched between the two rollers of the fixing unit, the paper conveying means While the paper transport speed is set to a first speed that is faster than the paper transport speed in the transfer section, when the paper position detecting means detects that the leading edge of the paper has reached the first position, An image forming apparatus comprising: a control unit that reduces a sheet conveying speed by the sheet conveying unit to a second speed that is slower than the first speed. A paper deflection amount detecting means for detecting a deflection amount of the paper on the paper conveyance path between the transfer section and the paper conveyance means;
The image forming apparatus according to claim 1, wherein the control unit controls a sheet conveyance speed by the sheet conveyance unit in accordance with a sheet deflection amount detected by the sheet deflection amount detection unit.   The paper deflection amount detecting means is a reflection type sensor having a light emitting unit for projecting light onto the paper and a light receiving unit for receiving reflected light from the paper, and detects the distance from the sensor to the paper. The image forming apparatus according to claim 2.   The image forming apparatus according to claim 2, wherein the control unit controls a sheet conveyance speed by the sheet conveyance unit so that a deflection amount of the sheet is a predetermined amount or less.   3. The control unit according to claim 2, wherein the control unit reduces a sheet conveyance speed of the sheet conveyance unit from the first speed to the second speed when a deflection amount of the sheet exceeds a predetermined amount. Image forming apparatus. Storage means for storing a plurality of combinations of the sheet thickness and the predetermined value corresponding to the sheet thickness;
Input means capable of inputting the thickness of the paper,
6. The control unit according to claim 5, wherein the control unit sets the predetermined value corresponding to the sheet thickness input from the input unit with reference to the combination information stored in the storage unit. The image forming apparatus described.

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