JP2012163730A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus that suppresses deterioration in a primary transfer belt and an image carrier, and image transfer failure when a sheet is cardboard in a monochrome image formation mode.SOLUTION: An image forming apparatus 10 includes: a primary transfer drive roller 32; a secondary transfer belt 41; a secondary transfer roller 42; and a backup roller 46. The backup roller 46 lays over the secondary transfer belt 41 on the upstream side of the secondary transfer roller 42 in a sheet conveyance direction 91, and displaces in a direction to contact and separate with/from the primary transfer drive roller 32. When a sheet is cardboard in a monochrome image formation mode, the backup roller 46 is disposed to a position closer to the primary transfer drive roller 32 than a position in a full-color image formation mode.

Description

  The present invention relates to an endless belt-shaped primary transfer belt that rotates in one direction along a plurality of image carriers, and a secondary transfer belt that is in pressure contact with a tension roller that stretches the primary transfer belt with the primary transfer belt interposed therebetween. The present invention relates to an image forming apparatus that transfers a developer image carried on a primary transfer belt to a sheet conveyed between the two.

  In recent years, electrophotographic image forming apparatuses capable of forming full-color images have been put into practical use. Some image forming apparatuses employ an intermediate transfer system as a developer image transfer system (see, for example, Patent Document 1). The intermediate transfer type image forming apparatus includes a plurality of image carriers, an endless belt-like primary transfer belt that rotates in one direction along the plurality of image carriers, a tension roller that stretches the primary transfer belt, and a tension A secondary transfer belt that is in pressure contact with the roller across the primary transfer belt, and a secondary transfer roller that is in pressure contact with the stretching roller across the primary transfer belt and the secondary transfer belt.

  In such an image forming apparatus, in the full-color image forming mode, the primary transfer belt is pressed against all of the plurality of image carriers, and each of yellow, magenta, cyan, and black formed by the plurality of image carriers, respectively. The developer image is primary-transferred on the primary transfer belt in sequence, and the full-color developer image is conveyed from the primary transfer belt to the secondary transfer area between the primary transfer belt and the secondary transfer belt. Secondary transfer.

  On the other hand, in the monochrome image forming mode, the primary transfer belt is pressed against only the monochrome image carrier among the plurality of image carriers, and the black developer image formed by the monochrome image carrier is transferred onto the primary transfer belt. After the primary transfer, the black developer image is secondarily transferred from the primary transfer belt to the paper in the secondary transfer region.

  In the conventional image forming apparatus as described above, when the paper is thick in the monochrome image forming mode, the primary transfer belt is in pressure contact with the monochrome image carrier but is not in contact with the color image carrier. Therefore, the primary transfer belt is likely to vibrate due to the impact when the thick paper enters the secondary transfer area.

  Therefore, when the paper is thick in the monochrome image forming mode, it is conceivable to press the primary transfer belt against all the image carriers as in the full color image forming mode setting.

JP 2002-156808 A

  However, in the monochrome image forming mode, if the primary transfer belt is pressed against all the image carriers, the primary transfer belt and the image carrier are rapidly deteriorated.

  Further, in the conventional image forming apparatus, when the paper is thick paper, if the thick paper enters the secondary transfer area, the distance between the stretching roller and the secondary transfer roller is larger than that when the paper is plain paper. Since the nip width between the primary transfer belt and the secondary transfer belt is reduced by widening, transfer defects are likely to occur.

  An object of the present invention is to provide an image forming apparatus capable of suppressing deterioration of the primary transfer belt and the image carrier and transfer failure when the paper is thick in the monochrome image forming mode.

  The image forming apparatus according to the present invention includes an endless belt-shaped primary transfer belt that rotates in one direction along a plurality of image carriers, and one tension member among a plurality of tension rollers that stretch the primary transfer belt. The developer image carried on the primary transfer belt is transferred to a sheet conveyed between an endless belt-like secondary transfer belt that is pressed against the roller with the primary transfer belt interposed therebetween. The image forming apparatus includes a nip width changing mechanism, an input unit, a first drive source, a paper feed tray, a detection unit, and a control unit. The nip width changing mechanism changes the nip width between the primary transfer belt and the secondary transfer belt between a predetermined first width and a second width that is widened to the upstream side in the paper transport direction. To do. The input unit receives an input for setting an image forming mode of either the first image forming mode or the second image forming mode. The first driving source presses the primary transfer belt to only one image carrier among the plurality of image carriers when the first image forming mode is set, and the primary transfer belt is attached to the plurality of primary transfer belts when the second image forming mode is set. The entire image carrier is pressed. Paper is placed on the paper feed tray. The detection unit detects thickness information of the paper placed on the paper feed tray. The control unit determines that the sheet is a thick sheet having a predetermined thickness or more based on the thickness information, and sets the nip width to the second width when the first image forming mode is set.

  In this configuration, when the sheet is a thick sheet in the first image forming mode, the nip width between the primary transfer belt and the secondary transfer belt is set to the second width. Therefore, the nip width between the primary transfer belt and the secondary transfer belt is wider than when the paper is plain paper or when the second image forming mode is set. Therefore, in the first image forming mode, the primary transfer belt is in contact with only one image carrier and not in contact with other image carriers, but between the primary transfer belt and the secondary transfer belt. The vibration of the primary transfer belt in the secondary transfer area when the thick paper enters the secondary transfer area is suppressed. Further, even when thick paper enters the secondary transfer region, the nip width is widened, so that the nip width between the primary transfer belt and the secondary transfer belt can be prevented from being insufficient. Further, in the first image forming mode, the primary transfer belt is in pressure contact with only one image carrier among a plurality of image carriers, and is not in contact with other image carriers, so that the image carrier and primary transfer belt are in contact with each other. The belt does not deteriorate quickly.

  In the above-described configuration, the nip width changing mechanism stretches the secondary transfer belt on the upstream side in the paper transport direction from the secondary transfer roller that presses the secondary transfer belt against the stretching roller with the primary transfer belt interposed therebetween. A backup roller that is displaceable in a direction to be in contact with and away from the stretching roller and a second drive source that displaces the backup roller can be included.

  When the sheet is thick in the first image forming mode, the backup roller is arranged closer to the tension roller than the position in the second image forming mode. For this reason, the nip width between the primary transfer belt and the secondary transfer belt is wider than when the backup roller is disposed at the position in the second image forming mode. Therefore, in the first image forming mode, the primary transfer belt is in contact with only one image carrier and is not in contact with other image carriers, but the second paper when the cardboard enters the secondary transfer region. The vibration of the primary transfer belt in the next transfer area is suppressed. In addition, even when the center distance between the tension roller and the secondary transfer roller increases due to the entry of cardboard into the secondary transfer area, the primary transfer belt and the secondary transfer belt can be moved by bringing the backup roller closer to the tension roller. It is possible to prevent a shortage of the nip width with the transfer belt. Further, in the first image forming mode, the primary transfer belt is in pressure contact with only one image carrier among a plurality of image carriers, and is not in contact with other image carriers, so that the image carrier and primary transfer belt are in contact with each other. The belt does not deteriorate quickly.

  The control unit preferably controls the second drive source so that the backup roller is arranged at a position based on the thickness information and the image forming mode before the rotation of the primary transfer belt and the secondary transfer belt is started. .

  When the backup roller approaches the stretching roller, the nip width between the primary transfer belt and the secondary transfer belt is widened, and when the backup roller is separated from the stretching roller, the primary transfer belt and the secondary transfer belt are separated from each other. Nip width is narrowed. When both the primary transfer belt and the secondary transfer belt are stopped, the nip width between the primary transfer belt and the secondary transfer belt is set. When the primary transfer belt or the secondary transfer belt is rotated, the nip width is set. Since it is not changed, damage to the primary transfer belt and the secondary transfer belt is suppressed.

  Further, the control unit controls the first drive source so that the primary transfer belt is disposed at a position based on the image forming mode input from the input unit before the rotation of the primary transfer belt and the plurality of image carriers is started. It is preferable to do.

  Prior to the rotation of the primary transfer belt and the plurality of image carriers, the primary transfer belt comes in contact with the image carrier. Since both the primary transfer belt and the plurality of image carriers are in contact with each other in a stopped state, compared to the case where only one of the primary transfer belt or the plurality of image carriers is rotating or the rotational speeds of both are different. Thus, damage to the primary transfer belt and the plurality of image carriers is suppressed.

  The control unit backs up to a position based on the thickness information and the image forming mode without waiting for an image forming start request to be input when the thickness information is detected and the image forming mode is set. It is preferable to control the second drive source so as to dispose the roller.

  When the thickness information is detected and the image formation mode setting is input, the backup roller is immediately placed at the position based on the thickness information and the image formation mode without waiting for the image formation start request to be input. Therefore, the time from the input of the image formation start request to the end of the image formation is shortened.

  In addition, when the setting of the image forming mode is input from the input unit, the control unit arranges the primary transfer belt at a position based on the input image forming mode without waiting for an input request to start image forming. It is preferable to control the first drive source so as to achieve this.

  When the image forming mode setting is input from the input unit, the primary transfer belt is immediately placed at a position based on the input image forming mode without waiting for an image forming start request to be input. The time from the input of the image formation start request to the end of the image formation is shortened.

  According to the present invention, it is possible to suppress the deterioration of the image carrier and the primary transfer belt and the transfer failure when the paper is thick in the monochrome image forming mode.

1 is a schematic configuration diagram of an image forming apparatus according to an embodiment of the present invention. (A) is a view showing the position of the primary transfer unit in the monochrome image formation mode, and (B) is a view showing the position of the primary transfer unit in the full-color image formation mode. It is a figure which shows the state by which the backup roller with which the secondary transfer unit was equipped was arrange | positioned in the initial position. It is a figure which shows the state by which the backup roller with which the secondary transfer unit was equipped was arrange | positioned in the approach position. 2 is a block diagram illustrating an electrical configuration of the image forming apparatus. FIG. It is a flowchart which shows an example of the process sequence of a control part.

  EMBODIMENT OF THE INVENTION Below, the form for implementing this invention is demonstrated based on drawing.

  As shown in FIG. 1, the image forming apparatus 10 operates in an image forming mode that is either a monochrome image forming mode (first image forming mode) or a full-color image forming mode (second image forming mode). Based on this, a single-color or multi-color image is formed on the paper. Examples of paper include plain paper and cardboard. The image forming apparatus 10 includes a plurality of image forming units 20A, 20B, 20C, and 20D, a primary transfer unit 30, a secondary transfer unit 40, a fixing device 51, a paper transport path 52, a paper feed cassette 53, a manual feed tray 54, and paper discharge. A tray 55 and a control unit 60 are provided. The control unit 60 comprehensively controls each unit of the image forming apparatus 10. The manual feed tray 54 constitutes a paper feed tray.

  The image forming apparatus 10 uses an image data corresponding to four hues of black, cyan, magenta, and yellow, which are three subtractive primary colors obtained by color separation of a color image. The image forming process is performed. In the image forming units 20A to 20D, toner images (developer images) of respective hues are formed. The image forming units 20 </ b> A to 20 </ b> D are arranged in a row in the horizontal direction along the primary transfer unit 30.

  Hereinafter, the image forming unit 20A will be mainly described. The image forming units 20B to 20D are configured in substantially the same manner as the image forming unit 20A. The black image forming unit 20A includes a photosensitive drum 21A, a charging device 22A, an exposure device 23A, a developing device 24A, and a cleaning unit 25A, and forms a black toner image through an electrophotographic image forming process.

  The photosensitive drum 21A and the photosensitive drums 21B, 21C, and 21D provided in the image forming units 20B to 20D rotate in one direction by receiving a driving force from the first driving motor 63 (see FIG. 5). The photosensitive drum 21A is a monochrome image carrier, and the photosensitive drums 21B to 21D are color image carriers.

  The charging device 22A is disposed so as to face the peripheral surface of the photosensitive drum 21A, and charges the peripheral surface of the photosensitive drum 21A to a predetermined potential.

  The exposure device 23A irradiates the peripheral surface of the photosensitive drum 21A with a laser beam modulated by the image data for black. As a result, an electrostatic latent image based on the image data for black is formed on the peripheral surface of the photosensitive drum 21A.

  The developing device 24A contains black toner (developer). The developing device 24A develops the electrostatic latent image into a toner image by supplying toner to the peripheral surface of the photosensitive drum 21A.

  The developing devices 24B to 24D of the image forming units 20B to 20D store toners of cyan, magenta, and yellow, and the photosensitive drums 21B to 21D of the image forming units 20B to 20D. In this case, toner images of hues of cyan, magenta, and yellow are formed.

  The primary transfer unit 30 includes a primary transfer belt 31, a primary transfer drive roller 32, a primary transfer driven roller 33, primary transfer rollers 34A to 34D, a primary transfer belt cleaning unit 35, and a primary transfer unit frame (not shown). .

  The primary transfer belt 31 has an endless belt shape, is stretched around a primary transfer driving roller 32 and a primary transfer driven roller 33, and moves around in one direction. The outer peripheral surface of the primary transfer belt 31 faces the photosensitive drums 21A to 21D of the image forming units 20A to 20D.

  The primary transfer rollers 34A to 34D are arranged so as to face the photosensitive drums 21A to 21D with the primary transfer belt 31 interposed therebetween. A region where the outer peripheral surface of the primary transfer belt 31 and the photosensitive drums 21A to 21D face each other is a primary transfer region.

  A primary transfer bias is applied to the primary transfer rollers 34 </ b> A to 34 </ b> D by constant voltage control with a toner charging polarity (for example, minus) and a reverse polarity (for example, plus). As a result, the toner images of the respective colors formed on the peripheral surfaces of the photosensitive drums 21 </ b> A to 21 </ b> D are sequentially transferred (primary transfer) on the outer peripheral surface of the primary transfer belt 31, and full color is transferred to the outer peripheral surface of the primary transfer belt 31. A toner image is formed.

  However, when image data of only a part of the hues of yellow, magenta, cyan, and black is input, a part of the four photosensitive drums 21A to 21D corresponding to the hue of the input image data. Only by this, an electrostatic latent image and a toner image are formed. For example, in the monochrome image formation mode, an electrostatic latent image and a toner image are formed only on the photosensitive drum 21A corresponding to the black hue, and only the black toner image is transferred to the outer peripheral surface of the primary transfer belt 31.

  The cleaning unit 25A collects toner remaining on the peripheral surface of the photosensitive drum 21A after development and primary transfer.

  A secondary transfer unit 40 is disposed so as to be in pressure contact with the primary transfer driving roller 32 with the primary transfer belt 31 interposed therebetween. The primary transfer driving roller 32 is one of stretching rollers that stretch the primary transfer belt 31.

  The secondary transfer unit 40 is in pressure contact with the primary transfer driving roller 32 with the primary transfer belt 31 interposed therebetween. The area where the primary transfer belt 31 and the secondary transfer unit 40 are in pressure contact is the secondary transfer area.

  The photosensitive drums 21A to 21D are arranged in the order of the photosensitive drum 21A for black, the photosensitive drum 21B for cyan, the photosensitive drum 21C for magenta, and the photosensitive drum 21D for yellow from the side closer to the secondary transfer region. Has been placed. The toner image carried on the outer peripheral surface of the primary transfer belt 31 is conveyed to the secondary transfer region by the circular movement of the primary transfer belt 31.

  The paper feed cassette 53 contains paper. On the manual feed tray 54, an irregular sheet or cardboard is placed. The paper transport path 52 is configured to guide the paper fed from the paper feed cassette 53 or the manual feed tray 54 to the paper discharge tray 55 via the secondary transfer area and the fixing device 51.

  A registration roller 56 is disposed in the vicinity of the upstream side of the secondary transfer area in the sheet conveyance direction. The sheet fed from the sheet feeding cassette 53 or the manual feed tray 54 to the sheet conveyance path 52 is corrected in its direction by being abutted against the resist roller 56 in a stopped state, and starts to rotate at a predetermined timing. Is supplied to the secondary transfer region. By supplying the sheet to the secondary transfer region, the sheet and the primary transfer belt 31 are in close contact with each other.

  By forming a predetermined secondary transfer electric field in the secondary transfer area, the toner image carried on the primary transfer belt 31 is transferred (secondary transfer) to the paper.

  Of the toner carried on the primary transfer belt 31, the toner remaining on the primary transfer belt 31 without being transferred to the paper is collected by the primary transfer belt cleaning unit 35. This prevents color mixing in the next step.

  The fixing device 51 includes a heating roller 511 and a pressure roller 512. The heating roller 511 and the pressure roller 512 are in pressure contact with each other. The fixing device 51 heats and presses the sheet by conveying the sheet while sandwiching the sheet at the nip portion between the heating roller 511 and the pressure roller 512, thereby firmly fixing the toner image on the sheet. The sheet on which the toner image is fixed is discharged onto the discharge tray 55 by the discharge roller pair 57.

  Next, the movement of the primary transfer unit 30 will be described. The primary transfer driving roller 32, the primary transfer driven roller 33, the primary transfer rollers 34A to 34D, and the primary transfer belt cleaning unit 35 are supported by the primary transfer unit frame.

  When the first drive source 67 (see FIG. 5) acts on the primary transfer unit frame in the vicinity of the primary transfer driven roller 33, the primary transfer unit 30 is displaced as follows. That is, as shown in FIG. 2A, the primary transfer unit 30 is arranged at the full color position where all the primary transfer rollers 34A to 34D are in pressure contact with the photosensitive drums 21A to 21D, respectively, in the full color image forming mode. As shown in FIG. 2B, in the monochrome image forming mode, only the primary transfer roller 34A for black among the plurality of primary transfer rollers 34A to 34D is disposed at the monochrome position where it presses against the photosensitive drum 21A. An example of the first drive source 67 is a solenoid. As a means for displacing the primary transfer unit 30, it is conceivable to rotate the first eccentric cam whose peripheral surface is in contact with the primary transfer unit frame by a first cam drive motor. When the first eccentric cam is used, the first cam drive motor constitutes the first drive source 67.

  Next, details of the secondary transfer unit 40 will be described. 3 and 4, the secondary transfer unit 40 includes a secondary transfer belt 41, a secondary transfer roller 42, a secondary transfer driving roller 43, a secondary transfer driven roller 44, a secondary transfer tension roller 45, And a backup roller 46.

  The secondary transfer belt 41 is configured as an endless belt. The secondary transfer belt 41 is stretched by a secondary transfer roller 42, a secondary transfer driving roller 43, a secondary transfer driven roller 44, a secondary transfer tension roller 45, and a backup roller 46.

  The secondary transfer roller 42 is in pressure contact with the primary transfer drive roller 32 with the primary transfer belt 31 and the secondary transfer belt 41 interposed therebetween. As a result, the outer peripheral surface of the secondary transfer belt 41 is in pressure contact with the outer peripheral surface of the primary transfer belt 31. More specifically, the secondary transfer region is a region where the primary transfer belt 31 and the secondary transfer belt 41 are in pressure contact with each other. The sheet is conveyed by being electrostatically attracted to the outer peripheral surface of the secondary transfer belt 41.

  The secondary transfer roller 42 is applied with a secondary transfer bias having a toner charging polarity (for example, minus) and a reverse polarity (for example, plus). As a result, the toner image carried on the primary transfer belt 31 is secondarily transferred to the paper. The sheet is conveyed toward the fixing device 51 via the secondary transfer area.

  The backup roller 46 is disposed on the upstream side of the secondary transfer roller 42 in the sheet conveyance direction 91. The backup roller 46 is configured to be freely displaceable along a direction in which the backup roller 46 is separated from or in contact with the primary transfer drive roller 32 between the initial position shown in FIG. 3 and the approach position to the primary transfer drive roller 32 shown in FIG. ing.

  The nip width between the primary transfer belt 31 and the secondary transfer belt 41 when the backup roller 46 is disposed at the initial position is the first width, and the primary transfer belt 31 when the backup roller 46 is disposed at the approach position. The nip width with the secondary transfer belt 41 is the second width.

  Since the backup roller 46 is disposed at the approach position, the nip width between the primary transfer belt 31 and the secondary transfer belt 41 is higher in the sheet conveyance direction than when the backup roller 46 is disposed at the initial position. It is spread to the side.

  The plurality of photosensitive drums 21 </ b> A to 21 </ b> D are arranged on the upstream side of the secondary transfer region in the moving direction of the primary transfer belt 31.

  As shown in FIG. 5, in addition to the control unit 60, the image forming apparatus 10 includes a display unit 61, an input unit 62, a first drive motor 63, a second drive motor 64, a third drive motor 65, a detection unit 66, A first drive source 67 and a second drive source 68 are provided.

  The display unit 61 displays various displays such as a display for prompting input of an image forming mode setting of either a monochrome image forming mode or a full color image forming mode. The input unit 62 accepts an input of an image formation mode setting of either a monochrome image formation mode or a full-color image formation mode, or an input of an image formation start request by pressing a start key (not shown).

  The first drive motor 63 rotates the photosensitive drums 21 </ b> A to 21 </ b> D based on the first drive data output from the control unit 60. The second drive motor 64 rotates the primary transfer drive roller 32 based on the second drive data output from the control unit 60. The third drive motor 65 rotates the secondary transfer drive roller 43 based on the third drive data output from the control unit 60.

The detection unit 66 acquires the thickness information of the paper placed on the manual feed tray 54. The control unit 60 determines whether the paper is plain paper or thick paper based on the thickness information acquired by the detection unit 66. For example, an optical sensor is used as the detection unit 66. The detection unit 66 measures the reflectance of light by the paper. Note that the detection unit 66 can also determine whether the paper is plain paper or thick paper by measuring the light transmittance of the paper. Further, by measuring the electrical resistance value of the paper by the detection unit 66, it can be determined whether the paper is plain paper or thick paper. As an example, the control unit 60 determines a sheet having a basis weight of 200 g / m ² or more as a thick sheet.

  The first drive source 67 displaces the primary transfer unit 30 between the full color position and the monochrome position based on the fourth drive data output from the control unit 60.

  The second drive source 68 displaces the backup roller 46 between the initial position and the approach position based on the fifth drive data output from the control unit 60. The backup roller 46 and the second drive source 68 are included in the nip width changing mechanism.

  An example of the second drive source 68 is a solenoid. As a means for displacing the backup roller 46, it is conceivable to rotate a second eccentric cam whose peripheral surface is in contact with the rotation shaft of the backup roller 46 by a second cam drive motor. When the second eccentric cam is used, the second cam drive motor constitutes the second drive source 68.

  As shown in FIG. 6, when the image forming mode is input from the input unit 62 (S1) and the full color image forming mode is set (S2), the control unit 60 determines whether the primary transfer unit 30 is at the full color position. It is determined whether or not (S3). When the primary transfer unit 30 is not arranged at the full color position, the control unit 60 controls the first drive source 67 without waiting for the start key to be pressed after the setting of the image forming mode is input. The primary transfer unit 30 is displaced to the full color position (S4). When the primary transfer unit 30 is placed at the full color position, the control unit 60 determines whether or not the start key has been pressed (S5). If the start key has been pressed, the image in the full color image forming mode is determined. A formation process is executed (S6).

  If it is determined in S2 that the monochrome image forming mode has been set, the control unit 60 determines whether or not the primary transfer unit 30 is in the monochrome position (S7). When the primary transfer unit 30 is not arranged at the monochrome position, the control unit 60 controls the first drive source 67 without waiting for the start key to be pressed after the setting of the image forming mode is input. The primary transfer unit 30 is displaced to the monochrome position (S8).

  As described above, when the setting of the image forming mode is input, the primary transfer unit 30 is immediately placed at a position based on the input image forming mode without waiting for the start key to be pressed. The time from the pressing to the end of the image forming process is shortened.

  The control unit 60 also arranges the first drive source 67 so that the primary transfer unit 30 is arranged at a position based on the input image forming mode before the rotation of the primary transfer belt 31 and the photosensitive drums 21A to 21D is started. To control. For this reason, the primary transfer belt 31 comes in contact with the photosensitive drums 21A to 21D before the rotation of the primary transfer belt 31 and the photosensitive drums 21A to 21D starts. Since both the primary transfer belt 31 and the photosensitive drums 21A to 21D are in contact with each other in a stopped state, only one of the primary transfer belt 31 or the photosensitive drums 21A to 21D is rotating or the rotational speed of both is high. Compared to the case where the difference is made, damage to the primary transfer belt 31 and the photosensitive drums 21A to 21D is suppressed.

  Next, the control unit 60 determines whether the paper supplied to the secondary transfer area is plain paper or thick paper (S9). Since it is preferable to pass the cardboard as straight as possible, the cardboard is placed on the manual feed tray 54 when image forming processing is performed on the cardboard. For this reason, when the paper is fed from the paper feed cassette 53, the control unit 60 determines that the paper is plain paper. When paper is fed from the manual feed tray 54, the control unit 60 determines whether the paper is plain paper or thick paper based on the thickness information input from the detection unit 66.

  If the paper is plain paper even in the monochrome image formation mode, the control unit 60 determines whether or not the backup roller 46 is placed at the initial position (S10), and when the paper is not placed at the initial position. Controls the second drive source 68 to displace the backup roller 46 to the initial position (S11).

  When the backup roller 46 is disposed at the initial position, the control unit 60 determines whether or not the start key has been pressed (S12). If the start key has been pressed, image formation in the monochrome image forming mode is performed. Processing is executed (S13).

  The control unit 60 determines whether or not the backup roller 46 is disposed at the approach position when the sheet is thick in the monochrome image forming mode (S14). If the sheet is not disposed at the approach position, the backup roller 46 is determined. The second drive source 68 is controlled so as to be displaced to the approach position (S15).

  When the paper is thick in the monochrome image forming mode, the backup roller 46 is disposed closer to the primary transfer driving roller 32 than the position in the full color image forming mode. Therefore, the nip width between the primary transfer belt 31 and the secondary transfer belt 41 is wider than when the backup roller 46 is disposed at the initial position. Therefore, in the monochrome image forming mode, the primary transfer belt 31 is not in contact with the color photosensitive drums 21B to 21D, but the primary transfer in the secondary transfer area when the thick paper enters the secondary transfer area. The vibration of the belt 31 is suppressed.

  Further, since the nip width is widened to the upstream side in the paper transport direction, the distance from the black photosensitive drum 21A is shortened, and the vibration of the primary transfer belt is further suppressed.

  Furthermore, even when the distance between the axes of the primary transfer driving roller 32 and the secondary transfer roller 42 increases due to the cardboard entering the secondary transfer region, the primary transfer belt can be provided by arranging the backup roller 46 at the approach position. It is possible to prevent the nip width between the belt 31 and the secondary transfer belt 41 from being insufficient. Therefore, occurrence of transfer failure is suppressed.

  In the monochrome image forming mode, the primary transfer belt 31 is pressed against only the black photosensitive drum 21A among the plurality of photosensitive drums 21A to 21D, and is not in contact with the color photosensitive drums 21B to 21D. Therefore, the primary transfer belt 31 and the photosensitive drums 21B to 21D are not deteriorated quickly. Therefore, the deterioration of the primary transfer belt 31 and the photosensitive drums 21B to 21D when the paper is thick in the monochrome image forming mode is suppressed.

  In addition, when the thickness information is detected and the image forming mode is set, the control unit 60 does not wait for the start key to be pressed and arranges the backup roller 46 at a position based on the thickness information and the image forming mode. . Therefore, when the thickness information is detected and the image forming mode is input, the backup roller 46 is immediately placed at a position based on the thickness information and the image forming mode without waiting for the start key to be pressed. Therefore, the time from the pressing of the start key to the end of the image forming process is shortened.

  Further, the controller 60 places the backup roller 46 at a position based on the thickness information and the image forming mode before the rotation of the primary transfer belt 31 and the secondary transfer belt 41 is started. Therefore, the nip width between the primary transfer belt 31 and the secondary transfer belt 41 is set in a state where both the primary transfer belt 31 and the secondary transfer belt 41 are stopped, and the primary transfer belt 31 or the secondary transfer belt 41 is set. Since the nip width is not changed in a state where at least one of them is rotated, damage to the primary transfer belt 31 and the secondary transfer belt 41 is suppressed.

  When the backup roller 46 is disposed at the approach position, the control unit 60 determines whether or not the start key has been pressed (S12). If the start key has been pressed, the image in the monochrome image forming mode is determined. A formation process is executed (S13).

  If the image forming apparatus 10 includes at least one photosensitive drum 21A for black and one or more photosensitive drums 21B to 21D for color, the primary transfer in the case where the paper is thick in the monochrome image forming mode. It is possible to achieve an effect that the deterioration of the belt 31 and the photosensitive drums 21B to 21D and the transfer failure can be suppressed.

  The above description of the embodiment is to be considered in all respects as illustrative and not restrictive. The scope of the present invention is shown not by the above embodiments but by the claims. Furthermore, the scope of the present invention is intended to include all modifications within the meaning and scope equivalent to the scope of the claims.

DESCRIPTION OF SYMBOLS 10 Image forming apparatus 20A-20D Image forming part 21A Photoconductor drum for black 21B-21D Photoconductor drum for color 30 Primary transfer unit 31 Primary transfer belt 32 Primary transfer drive roller (stretching roller)
34A to 34D Primary transfer roller 40 Secondary transfer unit 41 Secondary transfer belt 42 Secondary transfer roller 46 Backup roller 54 Manual feed tray (paper feed tray)
60 Control Unit 62 Input Unit 66 Detection Unit 67 First Drive Source 68 Second Drive Source

Claims (6)

An endless belt-like primary transfer belt that rotates in one direction along a plurality of image carriers, and the primary transfer belt on one of the stretching rollers that stretch the primary transfer belt. An image forming apparatus for transferring a developer image carried on the primary transfer belt to a sheet transported between an endless belt-shaped secondary transfer belt that is sandwiched and in contact with the secondary transfer belt,
The nip width that changes the nip width between the primary transfer belt and the secondary transfer belt between a predetermined first width and a second width that is widened to the upstream side in the paper transport direction. A change mechanism;
An input unit for receiving an input of setting of an image forming mode of either the first image forming mode or the second image forming mode;
When the first image forming mode is set, the primary transfer belt is brought into pressure contact with only one of the plurality of image carriers, and when the second image forming mode is set, the primary transfer belt is moved to the plurality of images. A first drive source that is pressed against all of the carrier;
A paper feed tray on which paper is placed;
A detection unit for detecting thickness information of the paper placed on the paper feed tray;
A control unit that determines that the sheet is a thick sheet having a predetermined thickness or more based on the thickness information and sets the nip width to the second width when the first image forming mode is set; And an image forming apparatus. The nip width changing mechanism is
The secondary transfer belt is stretched upstream of the secondary transfer roller that presses the secondary transfer belt against the stretching roller across the primary transfer belt in the paper transport direction, and is separated from the stretching roller. A backup roller that is freely displaceable in the direction of contact;
The image forming apparatus according to claim 1, further comprising: a second drive source that displaces the backup roller.   The controller controls the second drive source so that the backup roller is disposed at a position based on the thickness information and the image forming mode before the rotation of the primary transfer belt and the secondary transfer belt is started. The image forming apparatus according to claim 2, wherein the image forming apparatus is controlled.   The control unit is configured to arrange the primary transfer belt at a position based on an image forming mode input from the input unit before the rotation of the primary transfer belt and the plurality of image carriers is started. The image forming apparatus according to claim 3, wherein the image forming apparatus controls a drive source.   The controller, based on the thickness information and the image formation mode, does not wait for an image formation start request to be input when the thickness information is detected and the image formation mode is set. The image forming apparatus according to claim 2, wherein the second drive source is controlled so that the backup roller is disposed at a position.   When the image forming mode setting is input from the input unit, the control unit does not wait for an image forming start request to be input, and moves the primary transfer belt to a position based on the input image forming mode. The image forming apparatus according to claim 5, wherein the first drive source is controlled to be arranged.

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