JP4860300B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as an electrophotographic copying machine, a facsimile, and a printer.

Conventionally, the following image forming apparatuses are known as this type of image forming apparatus. That is, a plurality of photoconductors as image carriers are arranged in parallel between stretch rollers of a transfer belt that is a belt member that is stretched around a plurality of stretch rollers. A toner image is formed on each of the photoconductors, and the toner images on the photoconductors are superimposed on a transfer belt for primary transfer, and the toner images superimposed and transferred on the transfer belt are transferred onto a recording medium. 2 is an image forming apparatus that performs secondary transfer. In this type of image forming apparatus, each photoconductor and a primary transfer body (hereinafter referred to as a primary transfer roller) disposed so as to correspond to each photoconductor are brought into pressure-contact with each other via a transfer belt. Thus, the primary transfer nip portion is formed by bringing each photoconductor and the transfer belt into contact with each other. In this type of image forming apparatus, the primary transfer roller is disposed so that the uppermost portion of the primary transfer roller is in contact with the lowermost portion of the photoconductor via the transfer belt. Then, the primary transfer roller is constituted by a soft member such as a foamed sponge roller, and the primary transfer nip width is formed to a predetermined width by deforming the primary transfer roller.
In this image forming apparatus, in order to form a predetermined nip width, it is necessary to configure the primary transfer roller with a soft member such as a foam sponge roller that is more expensive than the metal roller.

  Therefore, the shaft center of the primary transfer roller is shifted downstream in the transfer belt movement direction with respect to the shaft center of the photoconductor so that the transfer belt upstream in the transfer belt movement direction from the primary transfer roller is wound around the photoconductor. Arranged image forming apparatuses are known. Since the transfer belt is wound around the photoconductor to obtain a predetermined nip width, the primary transfer roller can be configured with a metal roller that is less expensive than the foamed sponge roller, and the apparatus is less expensive than before. It can also be.

  Patent Document 1 discloses an image forming apparatus that forms an image in a monochrome mode that is a monochrome mode when the original image is a monochrome image that is a monochrome image, and forms an image in the color mode when the document image is a color image. Yes. In the monochrome mode, image formation is performed by separating a photoreceptor unnecessary for monochrome image formation from the transfer belt, and in the color mode, image formation is performed by bringing all the photoreceptors into contact with the transfer belt. Thereby, it is possible to suppress the deterioration of the Y, M, and C color photoreceptors. Further, the K-color photoconductor is arranged on the most downstream side in the belt moving direction with respect to the other photoconductors so as to shorten the first print time in the monochrome mode.

JP 2003-216011 A

By the way, when the transfer belt is stretched and driven by a plurality of rollers, the transfer belt may be wavy. This is because, for example, if there is a difference in circumferential length at both ends of the transfer belt due to a manufacturing error of the transfer belt, the end portion on the longer side of the belt gradually becomes longer as the belt rotates. Is wavy due to the elasticity of the belt. Further, even if the parallelism of the stretching roller is low, as described above, as the rotational driving of the belt proceeds, one end of the belt gradually remains, and that portion undulates due to the elasticity of the belt. If such belt undulations occur upstream of the transfer nip in the belt moving direction, the belt undulations reach the transfer nip. When the waviness of the belt reaches the transfer nip, there arises a problem that the transfer nip width fluctuates and transfer failure occurs. In particular, the image forming apparatus in which the transfer belt is wound around the photosensitive member by the primary transfer roller on the downstream side in the transfer belt moving direction with respect to the photosensitive member to form the above-described transfer nip, the primary transfer roller is connected to the photosensitive member via the transfer belt. The contact area is more susceptible to belt waviness than the contact area formed as a transfer nip.
In the color mode, since the transfer belt is wound around the Y, M, and C color photoreceptors with tension, the belt is undulated by the elasticity of the belt in the region upstream of each transfer nip. Is suppressed. However, in the monochrome mode, the Y, M, and C primary transfer rollers on the upstream side of the belt moving direction with respect to the K color photosensitive member are moved in the direction in which the distance from the photosensitive member is increased, and the transfer belt Y, The winding of the M and C color photoreceptors is released. For this reason, the tension of the transfer belt is weakened in a region from the tension roller on the upstream side in the belt movement direction to the K color photoreceptor from the K color transfer nip, and the belt is undulated in this region. As a result, there is a problem that this undulation reaches the K-color transfer nip, the transfer nip width fluctuates, and transfer failure occurs when the single color mode is executed.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide an image forming apparatus capable of suppressing transfer failure due to the waving of the belt member in the single color mode.

In order to achieve the above object, the invention of claim 1 is stretched between a plurality of image carriers that carry a visible image and at least two tension rollers, and each image carrier is supported between the tension rollers. A belt member that sequentially contacts the body to form a transfer nip; and a plurality of belt members that contact the back surface of the belt member at positions corresponding to the respective image carriers and apply a transfer bias to the back surface of the belt member. And transferring a visible image on the image carrier to the belt member side at each transfer nip in the process of moving the surface of the belt member from the most upstream transfer nip to the most downstream transfer nip. In the image forming apparatus for forming an image, the diameter of the image carrier is A, the diameter of the primary transfer member is B, the thickness of the belt member is C, the image carrier and the belt member are brought into contact with each other, and The belt member and the primary transfer member. When touched, on a virtual plane orthogonal to the axial direction of the image carrier, the distance L from the center of the image carrier to the axis of the primary transfer member is L> (A / 2) + The primary transfer member is arranged so that (B / 2) + C and the primary transfer member is located downstream of the corresponding image carrier in the belt movement direction, and an image is formed using all of the image carrier. And a monochrome mode in which all other image carriers except for one image carrier are relatively separated from the belt member, and an image is formed using the one image carrier. The image carrier used when executing the monochromatic mode is an image carrier that forms the most upstream transfer nip with the belt member, and the image carrier that forms the most upstream transfer nip has a position that is located closest to the belt member arrangement side. In the other image carrier, the belt member arrangement side is the most Each image carrier is arranged so as to be closer to the belt member arrangement side than the place to be placed, and in the color mode, the primary transfer member corresponding to the image carrier adjacent to the image carrier forming the most upstream transfer nip. In the primary transfer member corresponding to the image carrier that forms the most upstream transfer nip. The primary transfer member corresponding to the image carrier adjacent to the image carrier that forms the most upstream transfer nip in the single color mode so as to be closer to the image carrier placement side than the portion located on the image carrier placement side. In the primary transfer member corresponding to the image carrier that forms the most upstream transfer nip. It is characterized in that it is more on the side opposite to the image carrier arrangement side than the position located on the image carrier arrangement side .
According to a second aspect of the present invention, a plurality of image carriers that carry a visible image and at least two tension rollers are stretched, and the image carriers are sequentially brought into contact between the tension rollers. A belt member that forms a transfer nip, and a plurality of primary transfer members that contact the back surface of the belt member and apply a transfer bias to the back surface of the belt member at a position corresponding to each image carrier. In the process of moving the surface of the belt member from the most upstream transfer nip to the most downstream transfer nip, a visible image on the image carrier is transferred to the belt member side at each transfer nip to form a superimposed image. In the apparatus, the diameter of the image carrier is A, the diameter of the primary transfer member is B, the thickness of the belt member is C, the image carrier and the belt member are contacted, and the belt member and the belt member When contacted with the primary transfer body, the image On a virtual plane orthogonal to the axial direction of the holder, the distance L from the center of the image carrier axis to the axis of the primary transfer body is L> (A / 2) + (B / 2) + C. The primary transfer member is arranged in such a manner that a color mode in which an image is formed using all of the image carrier, and all other image carriers except one image carrier are moved relative to the belt member. A single-color mode for forming an image using the single image carrier, and the image carrier used when executing the single-color mode is an image carrier that forms the most downstream nip with the belt member, an image bearing member used at the time of the single-color mode is executed, between the image bearing member adjacent the image bearing member used at the time of the single-color mode is executed, the belts by applying tension to the belt member from the back surface of the belt member as suppressing member wavy suppressing waviness, the most downstream two The primary transcript which corresponds to the image bearing member to form a flop provided, most portion located in the belt member disposed side, most belt member disposed in another of the image bearing member in the image bearing member to form a downstream-transfer nip Place each image carrier so that it is closer to the belt member placement side than the location located on the side,
In the color mode, the portion of the primary transfer member corresponding to the image carrier that is adjacent to the image carrier that forms the most downstream transfer nip is located closest to the image carrier arrangement side, and the belt member is the most belt member in the adjacent image carrier. Compared with the location located on the arrangement side and the location located on the most image carrier arrangement side in the primary transfer body corresponding to the image carrier forming the most downstream transfer nip, In the single-color mode, the position of the primary transfer member corresponding to the image carrier that is adjacent to the image carrier that forms the most downstream transfer nip is located closest to the image carrier placement side in the adjacent image carrier, and the belt member is the most. Compared with the position located on the arrangement side and the position located closest to the image carrier arrangement side in the primary transfer body corresponding to the image carrier that forms the most downstream transfer nip, on the side opposite to the image carrier arrangement side Become It is characterized in that there was Unishi.
Also, the invention of claim 3, the image forming apparatus according to claim 1 or 2, the transfer nip of the image bearing member used at the time of the monochrome mode execution 3 [mm] or less, and is used when the single-color mode is executed Image carrier used when executing the monochromatic mode so that the length of the non-tensioned region of the belt member from the image carrier to the primary transfer member corresponding to the image carrier is 1 to 5 [mm]. A primary transfer member corresponding to the above is arranged.
According to a fourth aspect of the present invention, in the image forming apparatus according to any one of the first to third aspects, the axial center of the primary transfer member corresponding to the image carrier from the axial center of the image carrier used when the monochromatic mode is executed. The distance L is shorter than the distance L from the axial center of the other image carrier to the axial center of the primary transfer member corresponding to the image carrier.
According to a fifth aspect of the present invention, in the image forming apparatus according to any one of the first to fourth aspects, the image carrier used when the monochrome mode is executed on a virtual plane orthogonal to the axial direction of the image carrier. The vertical distance from the axial center to the axial center of the primary transfer member corresponding to the image carrier is from the axial center of the other image carrier to the axial center of the primary transfer member corresponding to the image carrier. It is configured to be shorter than the distance in the vertical direction.
According to a sixth aspect of the present invention, in the image forming apparatus according to any one of the first to fifth aspects, at least one of the primary transfer member and the image carrier is provided with an axial center of the primary transfer member from the axial center of the image carrier. The first abutting member that abuts against the image carrier or the primary transfer body is provided so that the distance L is a predetermined distance.
According to a seventh aspect of the present invention, there is provided the image forming apparatus according to any one of the first to sixth aspects, wherein at least one of the frame that holds the image carrier and the support member that supports the primary transfer member is attached to the image carrier. A second surface that abuts against the frame or the support member such that the position of the primary transfer member in the vertical direction relative to the corresponding image carrier on the virtual plane orthogonal to the axial direction is a predetermined position. The abutting member is provided .
Also, the invention of claim 8, in any of the image forming apparatus according to claim 1 to 7, the position of the transfer nip formed between the image bearing member and the belt member used at the time of the monochrome mode execution, other image It is characterized in that it is configured to be closer to the belt member than the position of the transfer nip formed by the carrier and the belt member.
According to a ninth aspect of the present invention, there is provided the image forming apparatus according to any one of the first to eighth aspects, wherein the belt member is disposed between the uppermost stream transfer nip and a tension roller that stretches the belt member upstream of the nip. A cleaning means for cleaning the front surface in contact with the front surface is provided.
The invention of claim 1 0, in the image forming apparatus according to claim 9, is characterized in that the cleaning means of the belt member is provided in contact with the opposing member on the rear surface of the abutting portion.

  According to the first aspect of the present invention, the image carrier used when the monochrome mode is executed is the image carrier that forms the most upstream transfer nip with the belt member. Compared to the image carrier that forms the downstream transfer nip, the distance from the stretching roller to the image carrier for the monochrome mode when the monochrome mode is executed can be shortened. Therefore, the image carrier used when executing the monochromatic mode is an area where no tension is applied to the belt member extending from the transfer nip formed by the image carrier and the belt member in the monochrome mode to the upstream side in the belt movement direction. It can be made shorter than the image carrier that forms the belt member and the most downstream transfer nip. As a result, it is possible to suppress the occurrence of belt waviness between these areas as compared with an image carrier that forms the most downstream transfer nip with the belt member when the image carrier is used when the monochrome mode is executed. . Accordingly, when the monochrome mode is executed, the transfer nip width and the transfer nip pressure formed by the image carrier for the monochrome mode and the primary transfer member are suppressed from being fluctuated due to the waving of the belt. As a result, transfer defects in the single color mode can be suppressed.

  According to the second aspect of the present invention, the region where no tension is applied to the belt member extending upstream in the belt movement direction from the transfer nip formed by the image carrier and the belt member used when the monochrome mode is executed in the monochrome mode. It can be up to the undulation suppressing member. Therefore, it is possible to shorten the area where no tension is applied to the belt member extending upstream from the transfer nip formed by the image carrier and the belt member, and to suppress the occurrence of undulation of the belt between these areas. be able to. As a result, when the monochromatic mode is executed, the transfer nip width and the transfer nip pressure formed by the image carrier for the monochromatic mode and the primary transfer body are suppressed from fluctuating due to the waving of the belt. As a result, transfer defects in the single color mode can be suppressed.

According to the tenth aspect of the present invention, the distance L ₂ from the center of the image carrier axis to the axis of the primary transfer body is monochromatic so that L ₂ <(A / 2) + (B / 2) + C. By disposing a primary transfer member corresponding to the image carrier used when executing the mode, a portion where the primary transfer member contacts the image carrier via the belt member can be used as a transfer nip. As a result, the transfer nip formed by the image carrier for the monochrome mode and the primary transfer roller for the monochrome mode is formed by pressing the belt member from the back surface of the belt member to the image carrier by the primary transfer member. Accordingly, it is possible to suppress the influence of the corrugation of the belt on the transfer nip from the stretch roller to the image carrier for the single color mode. Accordingly, when the monochrome mode is executed, the transfer nip width and the transfer nip pressure formed by the image carrier for the monochrome mode and the primary transfer member are suppressed from being fluctuated due to the waving of the belt. As a result, transfer defects in the single color mode can be suppressed.

[Embodiment 1]
Hereinafter, as an image forming apparatus to which the present invention is applied, an embodiment of an electrophotographic printer (hereinafter simply referred to as a printer) will be described.
First, the basic configuration of the printer will be described. FIG. 1 is a schematic configuration diagram showing the printer. In the figure, this printer includes four process units 1K, Y, M, and C for forming toner images of black, yellow, magenta, and cyan (hereinafter referred to as K, Y, M, and C). Yes. These use different colors of K, Y, M, and C toners as image forming materials, but otherwise have the same configuration and are replaced when the lifetime is reached. Taking a process unit 1K for forming a K toner image as an example, as shown in FIG. 2, a drum-shaped photosensitive member 2K as a latent image carrier, a drum cleaning device 3K, a charge eliminating device (not shown), and a charging device 4K. And a developing device 5K. The process unit 1K, which is an image forming unit, can be attached to and detached from the printer body, so that consumable parts can be replaced at a time.

  The charging device 4K uniformly charges the surface of the photosensitive member 2K, which is rotated clockwise in the drawing at a peripheral speed of 150 [mm / sec], by a driving unit (not shown) to −500 [V]. The uniformly charged surface of the photosensitive member 2K is exposed and scanned by the laser beam L and carries an electrostatic latent image for K. The electrostatic latent image for K is developed into a K toner image by a developing device 5K using K toner (not shown). Then, intermediate transfer is performed on an intermediate transfer belt 16 described later. The drum cleaning device 3K removes the transfer residual toner adhering to the surface of the photoreceptor 2K after the intermediate transfer process. The static eliminator neutralizes residual charges on the photoreceptor 2K after cleaning. By this charge removal, the surface of the photoreceptor 2K is initialized and prepared for the next image formation. Similarly, in the process units (1Y, M, C) of other colors, (Y, M, C) toner images are formed on the photoconductors (2Y, M, C), and on the intermediate transfer belt 16 described later. Intermediate transfer. The cylindrical drum portion of the photoreceptor 2K is obtained by coating the organic photosensitive layer on the front surface of a hollow aluminum base tube. A flange having a drum shaft is attached to both end portions in the axial direction of the drum portion to constitute the photosensitive member 2K.

  The developing device 5K as developing means includes a vertically long hopper 6K for storing K toner (not shown) and a developing unit 7K. In the hopper portion 6K, an agitator 8K that is rotationally driven by a driving means (not shown), a stirring paddle 9K that is rotationally driven by a driving means (not shown) vertically below, and a rotational drive that is driven by a driving means (not shown) in the vertical direction thereof A toner supply roller 10K to be used is disposed. The K toner in the hopper 6K moves toward the toner supply roller 10K by its own weight while being stirred by the rotational drive of the agitator 8K and the stirring paddle 9K. The toner supply roller 10K has a metal cored bar and a roller portion made of foamed resin or the like coated on the surface of the metal core roller 10K, while adhering K toner in the hopper portion 6K to the surface of the roller portion. Rotate.

  In the developing unit 7K of the developing device 5K, a developing roller 11K that rotates while contacting the photoreceptor 2K and the toner supply roller 10K, and a thinning blade 12K that contacts the tip of the developing roller 11K are disposed. Yes. The K toner adhered to the toner supply roller 10K in the hopper 6K is supplied to the surface of the development roller 11K at the contact portion between the development roller 11K and the toner supply roller 10K. When the supplied K toner passes through the contact position between the roller and the thinning blade 12K as the developing roller 11K rotates, the layer thickness on the roller surface is regulated. Then, the K toner after the layer thickness regulation adheres to the electrostatic latent image for K on the surface of the photosensitive member 2K in the developing region which is a contact portion between the developing roller 11K and the photosensitive member 2K. By this adhesion, the electrostatic latent image for K is developed into a K toner image.

  Although the process unit for K has been described with reference to FIG. 2, the process units 1Y, M, and C for Y, M, and C also perform Y, M, and C on the surfaces of the photoreceptors 2Y, M, and C by a similar process. A C toner image is formed.

  In FIG. 1 described above, an optical writing unit 70 is disposed above the process units 1K, Y, M, and C in the vertical direction. The optical writing unit 70, which is a latent image writing device, optically scans the photoreceptors 2K, Y, M, and C in the process units 1K, Y, M, and C with laser light L emitted from a laser diode based on image information. To do. By this optical scanning, electrostatic latent images for K, Y, M, and C are formed on the photoreceptors 2K, Y, M, and C. The optical writing unit 70 is a photosensitive member that passes through a plurality of optical lenses and mirrors while polarizing a laser beam (L) emitted from a light source in a main scanning direction by a polygon mirror that is rotationally driven by a polygon motor (not shown). Is irradiated. You may employ | adopt what performs optical writing by the LED light emitted from several LED of the LED array.

  Below the process units 1K, Y, M, and C, there is disposed a transfer unit 15 that moves the endless intermediate transfer belt 16 endlessly in the counterclockwise direction while stretching. In addition to the intermediate transfer belt 16, the transfer unit 15 includes a driving roller 17, a driven roller 18, four primary transfer rollers 19 Y, M, C, K, a secondary transfer roller 20, a belt cleaning device 21, and a cleaning backup roller 22. Etc.

  The intermediate transfer belt 16 is stretched by a driving roller 17, a driven roller 18, a cleaning backup roller 22 and four primary transfer rollers 19Y, 19M, 19C, and 19K disposed inside the loop. Then, it is moved endlessly in the same direction by the rotational force of the driving roller 17 that is driven to rotate counterclockwise in the figure by a driving means (not shown).

  The four primary transfer rollers 19Y, 19M, 19C, and 19K sandwich the intermediate transfer belt 16 that is moved endlessly in this manner between the photoreceptors 2Y, 2M, 2C, and 2K. By this sandwiching, primary transfer nips for Y, M, C, and K where the front surface of the intermediate transfer belt 16 and the photoreceptors 2Y, M, C, and K abut are formed.

  A primary transfer bias is applied to each of the primary transfer rollers 19Y, 19M, 19C, and 19K by a transfer bias power source (not shown), whereby the electrostatic latent images on the photoreceptors 2Y, 2M, 2C, and 2K, A transfer electric field is formed between the primary transfer rollers 19Y, 19M, 19C, and 19K.

  When the K toner formed on the surface of the photoconductor 2K of the K process unit 1K enters the K primary transfer nip as the photoconductor 2K rotates, the photoconductor 2K is affected by the transfer electric field and the nip pressure. Primary transfer is performed on the intermediate transfer belt 16 from above. The intermediate transfer belt 16 on which the K toner image is primarily transferred in this way passes through the primary transfer nips for Y, C, and M along with the endless movement thereof, and the photoreceptors 2Y, 2M, and 2C. The upper Y, M, and C toner images are sequentially superimposed and primarily transferred. A four-color toner image is formed on the intermediate transfer belt 16 by the primary transfer of superposition.

  The secondary transfer roller 20 of the transfer unit 15 is disposed outside the loop of the intermediate transfer belt 16, and the intermediate transfer belt 16 is sandwiched between the drive roller 18 inside the loop. By this sandwiching, a secondary transfer nip where the front surface of the intermediate transfer belt 16 and the secondary transfer roller 20 abut is formed. A secondary transfer bias is applied to the secondary transfer roller 20 by a transfer bias power source (not shown). By this application, a secondary transfer electric field is formed between the secondary transfer roller 20 and the driven roller connected to the ground. Details of the transfer unit 15 will be described later.

  Below the transfer unit 15, a paper feed cassette 30 that stores a plurality of recording sheets P in a bundle of sheets is slidably attached to the printer housing. In the paper feed cassette 30, a paper feed roller 30a is brought into contact with the top recording paper P of the paper bundle, and the recording paper is rotated by rotating it in a counterclockwise direction in the drawing at a predetermined timing. P is sent out toward the paper feed path 31.

  A registration roller pair 32 is disposed near the end of the paper feed path 31. The registration roller pair 32 stops the rotation of both rollers as soon as the recording paper P delivered from the paper feed cassette 30 is sandwiched between the rollers. Then, rotation driving is resumed at a timing at which the sandwiched recording paper P can be synchronized with the four-color toner image on the intermediate transfer belt 16 in the above-described secondary transfer nip, and the recording paper P is directed to the secondary transfer nip. Send it out.

  The four-color toner image on the intermediate transfer belt 16 brought into close contact with the recording paper P at the secondary transfer nip is secondarily transferred onto the recording paper P under the influence of the secondary transfer electric field and the nip pressure. Combined with the white color of P, a full color toner image is obtained. The recording paper P having the full-color toner image formed on the surface in this manner is separated from the secondary transfer roller 20 and the intermediate transfer belt 16 by the curvature when passing through the secondary transfer nip. Then, the toner is fed into a fixing device 34 described later via a post-transfer conveyance path 33.

  The transfer residual toner that has not been transferred to the recording paper P adheres to the intermediate transfer belt 16 after passing through the secondary transfer nip. This is cleaned from the belt surface by a belt cleaning device 21 in contact with the front surface of the intermediate transfer belt 16. The cleaning backup roller 22 disposed inside the loop of the intermediate transfer belt 16 backs up the cleaning of the belt by the belt cleaning device 21 from the inside of the loop.

  The fixing device 34 forms a fixing nip by a fixing roller 34a containing a heat source such as a halogen lamp (not shown) and a pressure roller 34b that rotates while contacting the fixing roller 34a with a predetermined pressure. The recording paper P fed into the fixing device 34 is sandwiched between the fixing nips such that the unfixed toner image carrying surface is brought into close contact with the fixing roller 34a. Then, the toner in the toner image is softened by the influence of heating and pressurization, and the full color image is fixed.

  The recording paper P discharged from the fixing device 34 passes through the post-fixing conveyance path 35 and then reaches the branch point between the paper discharge path 36 and the pre-reversal conveyance path 41. On the side of the post-fixing conveyance path 35, a switching claw 42 that is rotationally driven around a rotation shaft 42a is disposed. By the rotation, the vicinity of the end of the post-fixing conveyance path 35 is closed. Or open it. At the timing when the recording paper P is sent out from the fixing device 34, the switching claw 42 stops at the rotational position indicated by the solid line in the drawing, and the vicinity of the end of the post-fixing conveyance path 35 is opened. Therefore, the recording paper P enters the paper discharge path 36 from the conveyance path 35 after fixing, and is sandwiched between the rollers of the paper discharge roller pair 37.

  When the single-sided print mode is set by an input operation to an operation unit including a numeric keypad (not shown) or a control signal sent from a personal computer (not shown), the recording sandwiched between the paper discharge roller pair 37 is performed. The paper P is discharged out of the machine as it is. Then, it is stacked on the stack portion that is the upper surface of the upper cover 50 of the housing.

  On the other hand, when the duplex printing mode is set, the trailing edge of the recording paper P conveyed through the paper discharge path 36 while the front end is sandwiched between the paper discharge roller pair 37 passes through the post-fixing conveyance path 35. The switching claw 42 is rotated to the position indicated by the alternate long and short dash line in FIG. At substantially the same time, the paper discharge roller pair 37 starts to rotate in the reverse direction. Then, the recording paper P is conveyed while the rear end side is directed to the top, and enters the pre-reversal conveyance path 41.

  FIG. 1 shows the printer from the front side. The front side in the direction perpendicular to the drawing is the front side of the printer, and the back side is the rear side. In the drawing, the right side of the printer is the right side and the left side is the left side. The right end portion of the printer is a reversing unit 40 that can be opened and closed with respect to the housing body by rotating about a rotating shaft 40a. When the paper discharge roller pair 37 rotates in the reverse direction, the recording paper P enters the pre-reversal conveyance path 41 of the reversing unit 40 and is conveyed from the upper side to the lower side in the vertical direction. Then, after passing between the rollers of the pair of reverse conveying rollers 43, it enters the reverse conveying path 44 that is curved in a semicircular shape. Furthermore, while the upper and lower surfaces are reversed as the sheet is conveyed along the curved shape, the traveling direction from the upper side in the vertical direction to the lower side is also reversed, and conveyed from the lower side in the vertical direction toward the upper side. Is done. Thereafter, the toner enters the secondary transfer nip again through the above-described paper feed path 31. Then, after the full color image is collectively transferred to the other surface, the post-transfer conveyance path 33, the fixing device 34, the post-fixation conveyance path 35, the paper discharge path 36, and the paper discharge roller pair 37 are sequentially passed. And discharged outside the machine.

Next, the transfer unit 15 will be described in detail.
FIG. 3 is an enlarged view showing a schematic configuration of the transfer unit 15. As shown in FIG. 3, the transfer belt 16 is mainly stretched between the driving roller 18 and the tension 17, and the winding angle of the transfer belt 16 with the driving roller 18 and the winding angle with the tension roller 17 are almost the same. The angle is set to 180 °. By adopting such a configuration, the transfer unit 15 can be shortened in the vertical direction, and the apparatus can be miniaturized. Above the transfer belt 16, photoconductors 2K, Y, M, and C are arranged in order from the upstream side in the belt moving direction. A K-color primary transfer roller 19K is disposed downstream of the K-color photoconductor 1K in the belt movement direction. Similarly, a primary transfer roller 19Y is disposed downstream of the photoreceptor 2Y, a primary transfer roller 19M is disposed downstream of the photoreceptor 2M, and a primary transfer roller 19C is disposed downstream of the photoreceptor 1C.

A secondary transfer roller 20 is in contact with the driving roller 18 via the transfer belt 16 to form a secondary transfer nip portion. In this embodiment, the diameter φ of each photosensitive member is 24 [mm], the diameter φ of the driving roller 19 is 20 [mm], the diameter φ of each primary transfer roller is 8 [mm], and the photosensitive member pitch is , 70 [mm].
As the driving roller 18, a roller made of a core metal and a surface layer of polyurethane rubber having a thickness of about 0.3 to 1 [mm] was used. Further, a thin-layer coating having a thickness of 0.05 to 0.1 [mm] is preferable because the change in diameter due to temperature is small.

  The tension roller 17 urges the transfer belt from the inner peripheral surface to the outside by an urging means such as a spring, and applies a tension of 25 [N] to 35 [N] to the transfer belt 16.

  A belt cleaning device 21 serving as belt cleaning means is provided downstream of the driving roller 18 in the transfer belt moving direction and upstream of the K-color photosensitive member 1K. The belt cleaning device 21 includes a counter roller 22 as a counter member that contacts the back surface of the transfer belt 16, a cleaning blade 21b that contacts the counter roller 22 via the transfer belt 16, and toner scraped by the cleaning blade 21b. It is comprised with the accommodating part 21c which accommodates.

  As the transfer belt 16, PVDF (vinylidene fluoride), ETFE (ethylene-tetrafluoroethylene copolymer), PI (polyimide), PC (polycarbonate), TPE (thermoplastic elastomer), etc., conductive material such as carbon black. The material can be dispersed to form a resin film-like endless belt. The transfer belt 16 of the first embodiment has a single-layer structure in which carbon black is added to PC (polycarbonate) and the thickness thereof is adjusted to 140 [μm].

The transfer belt 16 preferably has a volume resistivity of 10 ⁸ to 10 ¹² [Ωcm] and a surface resistivity of 10 ⁹ to 10 ¹² [Ω / □]. When the volume resistivity and surface resistivity of the transfer belt 16 exceed the above-described ranges, the transfer belt 16 is charged. Therefore, it is necessary to take a measure such as setting the set voltage value higher as it goes downstream in the image forming order. For this reason, it is difficult to share the power supply to the primary transfer roller 19. This is because if the volume resistivity and the surface resistivity exceed the ranges described above, the charging potential on the surface of the transfer belt 16 is increased by the discharge generated in the primary transfer process, the secondary transfer process, etc., and self-discharge becomes difficult. It is to become. For this reason, it is necessary to provide a charge eliminating means for the transfer belt 16. If the volume resistivity and surface resistivity are below the above ranges, the charge potential decays quickly, which is advantageous for static elimination by self-discharge. However, since the current during transfer flows in the surface direction, discharge occurs at the transfer nip entrance. And toner scattering occurs. Therefore, the volume resistivity and surface resistivity of the transfer belt 16 in the present invention must be within the above ranges.

  The resistance of the transfer belt 16 is measured by connecting a probe (inner electrode diameter 50 [mm], ring electrode inner diameter 60 [mm]: JIS-K6911 compliant) to a digital ultra-high resistance microammeter (manufactured by Advantest: R8340A). A voltage of 1000 [V] (surface resistivity is 500 [V]) is applied to the front and back of the transfer belt 16 and measurement is performed at discharge 5 [sec] and charge 10 [sec]. The environment at the time of measurement is 22 [° C. ] Fixed at 55 [% RH].

  The primary transfer rollers 19K, Y, M, and C are arranged so as to be shifted from the photoreceptors 2K, Y, M, and C to the downstream side in the moving direction of the transfer belt 16, and the transfer belt 16 is pressed toward the photoreceptors. A primary transfer nip is formed by winding the transfer belt 16 upstream of the primary transfer roller 19 in the belt moving direction around the peripheral surfaces of the photosensitive members 2K, Y, M, and C. An intermediate transfer bias is applied to each of the four primary transfer rollers 19K, Y, M, and C from a power source (not shown). In each primary transfer nip, an intermediate transfer electric field is formed between the photosensitive member 2 and the primary transfer roller 19 due to the influence of the intermediate transfer bias. The toner image formed on the photoreceptor 2 is primarily transferred onto the transfer belt 16 due to the influence of the intermediate transfer electric field and nip pressure.

In the case of the first embodiment, the primary transfer rollers 19K, Y, M, and C are inexpensive metal rollers. However, when a rigid primary transfer roller 19 such as a metal roller is brought into contact with the photoreceptor 2 via the transfer belt 16, a foam sponge roller that is more expensive than the metal roller is used as the primary transfer roller 19. As compared with the case, the deterioration of the photoreceptor 2 may be accelerated. This is because when the photosensitive member 2 and the primary transfer roller 19 are decentered, the contact pressure between the photosensitive member 2 and the primary transfer roller 19 is not constant, and a portion where the contact pressure is higher than the predetermined contact pressure is generated. As a result of the contact pressure becoming higher than the predetermined value, the progress of wear of the photoconductor 2 is accelerated and the deterioration of the photoconductor 2 is accelerated.
For this reason, when a metal roller that is less expensive than the foamed sponge roller is used for the primary transfer roller 19, it is preferable to dispose the primary transfer roller 19 so as not to contact the photoreceptor 2. Specifically, as shown in FIG. 4, when the diameter of the primary transfer roller 19 is A [mm], the diameter of the photosensitive member 2 is B [mm], and the thickness of the transfer belt is C [mm], The primary transfer roller 19 is arranged so that the distance L between the axis of the transfer roller 19 and the axis of the photosensitive member 1 satisfies the following relationship.
L> (A / 2) + (B / 2) + C (Expression 1)

  By disposing the primary transfer roller 19 so as to satisfy the above formula 1, the primary transfer roller 19 and the photoreceptor 2 do not come into contact with each other, and the primary transfer roller 19 may be a metal roller that is less expensive than the foamed sponge roller. , There is no premature deterioration of the photoreceptor.

  Further, each of the photoconductors 2K, Y, M, and C and the transfer belt 16 come into contact with each other due to the tension of the transfer belt 16, and the transfer pressure is compared with the case where the metal roller is brought into contact with the transfer belt 16 via the transfer belt. Is stable. Therefore, an increase in the transfer pressure is suppressed, the cohesive force between the toners is increased at the transfer nip, and image defects such as voids are eliminated. Further, since the primary transfer rollers 19K, Y, M, and C are disposed downstream of the primary transfer nip in the transfer belt moving direction, the distance between the primary transfer nip entrance and the surface of the primary transfer roller is increased. Accordingly, the primary transfer bias does not act on the primary transfer nip entrance, and toner scattering due to gap discharge between each photoconductor 2 and the transfer belt 16 at the nip entrance can be prevented.

  As shown in FIGS. 5 and 6, a collar member 190K as a first abutting member is attached to both ends of the primary transfer roller 19K. The color member 190K is in contact with the non-image forming area of the photoreceptor 2K. The diameter of the collar member 190K is set to be larger than the diameter of the primary transfer roller 19K. Accordingly, when the color member 190K is abutted against the photosensitive member 2K, the distance L between the axial centers of the photosensitive member 2K and the primary transfer roller 19K can be made larger than the right side of the above formula 1. The primary transfer roller 19K can be disposed apart from the photoreceptor 2K. The color primary transfer rollers 19Y, 19M, and 19C have the same configuration. In addition, color members having a diameter larger than the diameter of the photoconductor are provided at both ends of the photoconductors 2K, Y, M, and C so as to be in contact with the rotation shafts of the primary transfer rollers 19K, Y, M, and C. Also good. Further, color members are provided on both the photosensitive members 2K, Y, M, and C and the primary transfer rollers 19K, Y, M, and C, and the color members of the photosensitive members 2K, Y, M, and C and the primary transfer rollers 19K, Y are provided. , M, and C may be abutted to position the primary transfer roller.

  As described above, by providing the color member 190, the primary transfer roller 19 can be arranged so as to have a predetermined center-to-axis distance L. However, the lifting amount of the transfer belt 16 of the primary transfer roller 19 (the photosensitive member) The transfer nip width and the transfer pressure vary depending on the vertical distance from the axial center of the photosensitive member to the axial center of the primary transfer roller corresponding to the photosensitive member on a virtual plane orthogonal to the axial direction. Therefore, as shown in FIGS. 5 and 7, an abutting portion 101 </ b> K that is a second abutting member that abuts the bearing portion 191 </ b> K of the primary transfer roller 19 </ b> K is provided on the frame 100 </ b> K of the process unit 1 </ b> K. The amount of lifting of the transfer belt 16 of the primary transfer roller 19K can be set to a predetermined amount by abutting the bearing portion 191K of the primary transfer roller 19K against the abutting portion 101K. As a result, the primary transfer roller 19K can be arranged so that the vertical distance and the center-to-axis distance L with respect to the photosensitive member 2K have predetermined values, and the transfer nip width and transfer nip pressure are set to predetermined values. can do. Y, M, and C have the same configuration. Further, a configuration in which an abutting member is provided on the bearing portion 191 of the primary transfer roller 19 and abuts against the frame body 100 may be employed.

The secondary transfer roller 20 is configured by covering a metal core such as SUS with an elastic body such as urethane adjusted to a resistance value of 10 ⁶ to 10 ¹⁰ Ω with a conductive material. Here, if the resistance value of the secondary transfer roller 20 exceeds the above range, it becomes difficult for the current to flow. Therefore, a higher voltage must be applied in order to obtain a required transfer property, resulting in an increase in power supply cost. . In addition, since it is necessary to apply a high voltage, discharge occurs in the gap before and after the nip of the transfer portion, and white spots are lost on the halftone image due to the discharge. Such white spot missing due to discharge becomes remarkable in a low temperature and low humidity environment (for example, 10 [° C.] 15 [% RH]). On the contrary, if the resistance value of the secondary transfer roller 20 is below the above range, the transferability between the multi-color image portion (for example, three-color superimposed image) and the single-color image portion existing on the same image cannot be achieved. This is because the resistance value of the secondary transfer roller 20 is low, so that a sufficient current flows to transfer the monochrome image portion at a relatively low voltage. However, it is optimal for the monochrome image portion to transfer the multiple color image portion. This is because a voltage value higher than the voltage is required, and setting a voltage that can transfer a plurality of color image portions causes an excessive transfer current in a single color image, resulting in a decrease in transfer efficiency.

  The resistance value of the secondary transfer roller 20 is measured by installing the secondary transfer roller 20 on a conductive metal plate and 4.9 [N] on one side at both ends of the core metal (total of 9.8 [N] on both sides). It calculated from the value of the electric current which flows when the voltage of 1000 [V] was applied between the metal core and the metal plate in the state where the load of. The resistance of the secondary transfer roller 12 was also measured by fixing the environment at 22 [° C.] and 55 [% RH]. As the secondary transfer roller 20 of the present embodiment, a roller adjusted so as to be 7.8 LogΩ when measured by the above-described method was used.

  The image forming apparatus according to the first embodiment includes a full color mode in which all the photoreceptors 2K, Y, M, and C are brought into contact with the surface of the transfer belt 16 when the color of an image to be formed is full color, and color for a black color. A monochrome mode that is a monochrome mode for separating the photoreceptors 2Y, 2M, and 2C from the surface of the transfer belt 16 is provided.

  The mode may be selected such that the mode is automatically selected from image information to be formed, or the mode may be selected by the user.

  FIG. 8 is a perspective view of the separating means 210. As shown in FIG. 5, the separating means 210 has a roller holding member 201 provided with a bottom plate 201c. The color primary transfer rollers 19Y, 19M, and 19C and the tension roller 17 are held on both side plates 201a and 201b of the roller holding member 201. A plunger 203 is attached to the bottom plate 201 c of the roller holding member 201. Further, a spring 202 is attached to the bottom plate 201c of the roller holding member 201, and urges the roller holding member 201 to the photosensitive member side. Further, swing shafts 205 are provided on both side plates 201a and 201b of the roller holding unit 201, and are attached to side plates of an image forming apparatus (not shown).

  When the monochrome mode is executed, the plunger 203 sucks the roller holding member 201 downward in the drawing. Then, the roller holding member 201 rotates about the swing shaft 205, and the tension roller 17 and the color primary transfer rollers 19C, 19M, and 19Y move downward in the figure as shown in FIG. Then, the transfer belt 16 pressed against the color photoconductor side by the color primary transfer rollers 19C, 19M, and 19Y is stretched again by the tension roller 17 while trying to bend. As a result, the transfer belt 16 is separated from the color photoconductors 2Y, 2M, and 2C.

  On the other hand, when the color mode is executed, the suction of the plunger 203 is released. Then, the roller holding member 201 moves toward the photosensitive member centering on the swing shaft 205 by the bias of the spring 202. The color primary transfer rollers 19Y, 19M, and 19C and the tension roller 17 are moved to the positions shown in FIG. 3, and the transfer belt 16 is wound around and contacts the color photoreceptors Y, M, and C.

  The separation unit 210 described above is configured to move the tension roller 17 and the color primary transfer rollers 19Y, 19M, and 19C in a direction away from the photosensitive member in the monochrome mode, but is not limited thereto. For example, as shown in FIG. 10, only the color primary transfer rollers 19Y, 19M, and 19C may be configured to be detachable from the photoreceptor. In the case of the separation unit 210 having this configuration, only the color primary transfer rollers 19Y, 19M, and 19C are supported by the holding member 201 of the separation unit, and in the monochrome mode, the entire holding member 201 is separated from the photosensitive member. The color primary transfer rollers 19Y, 19M, and 19C are moved away from the transfer belt 16. When the color primary transfer rollers 19Y, 19M, and 19C are separated from the transfer belt 16, the transfer belt 16 that is pressed against the photosensitive member by the color primary transfer rollers 19Y, 19M, and 19C tries to bend, but the tension roller 17 Is stretched again. As a result, the transfer belt 16 is separated from the color photoconductors 2Y, 2M, and 2C.

  In addition, as shown in FIG. 11, the K-color photoconductor 2K and the color photoconductors 2Y, 2M, and 2C are arranged so as to have a step in a direction orthogonal to the photoconductor axis direction, and the K-color photoconductor 2K is arranged. The transfer nip may be positioned closer to the transfer belt than the color transfer nip. Specifically, as shown in the figure, the first imaginary line C1 passing through the center of the K-color photoconductor 2K is parallel to the first imaginary line C1 and the centers of the color photoconductors 2Y, 2M, and 2C. Each photoconductor is arranged so as to be closer to the transfer belt 16 than the second imaginary line C2 passing through. As shown in FIG. 12A, when the photoconductors are arranged in parallel and the K-color transfer nip and the color transfer nip are in the same position, a predetermined primary is applied to the K-color photoconductor. The uppermost point D1 in the drawing of the primary transfer roller 19K arranged to form the transfer nip is positioned largely above the lowermost point D2 in the drawing of the Y-color photoconductor 2Y. Therefore, the angle θ1 of the transfer belt entering the lowest point D2 of the Y color photoconductor (the imaginary line C3 passing through the lowest point of the color photoconductor, and the lowest points D2 and K of the Y color photoconductor). The angle formed by the straight line connecting the uppermost point D1 of the primary transfer roller of the color is increased. In order to move the transfer belt away from the Y-color photoconductor, the uppermost point D3 of the Y-color primary transfer roller 19 is positioned below the lowermost point D2 of the Y-color photoconductor 2Y. The angle formed by the imaginary line C3 passing through the lowest point D2, the lowest point D2 of the Y color photoreceptor and the uppermost point D3 of the Y primary transfer roller 19 is equal to or greater than the movement amount M1 at which θ1 is Y. The primary transfer roller 19Y needs to be moved away from the photoreceptor.

  On the other hand, as shown in FIG. 12B, when the primary transfer nip for K color is located below the primary transfer nip for color, a predetermined primary transfer nip is provided for the K color photoreceptor 2K. The uppermost point D1 in the figure of the primary transfer roller 19K arranged to form is not positioned greatly above the lowermost point D2 in the figure of the Y-color photoconductor 2Y. For this reason, the angle θ2 of the transfer belt entering the lowest point D2 of the Y-color photoconductor (the imaginary line C3 passing through the lowest point D2 of the Y-color photoconductor, and the lowest point D2 of the Y-color photoconductor) It is possible to suppress an increase in the angle formed by a straight line connecting the uppermost point D1 of the K primary transfer roller. Therefore, a virtual line C3 passing through the lowest point D2 of the Y color photoreceptor 2Y, and a straight line connecting the lowest point D2 of the Y color photoreceptor 2Y and the highest point D3 of the Y primary transfer roller 19Y. If the Y primary transfer roller 19Y is moved in a direction to separate the Y primary transfer roller 19Y from the photoconductor so that the angle formed by the angle is equal to or larger than θ2, the transfer belt is separated from the Y color photosensitive pair. Can do. As a result, at least when the primary transfer roller 19Y moves the movement amount M2 or more and moves the Y-color primary transfer roller 19Y away from the photoreceptor, the transfer belt 16 can be separated from the Y-color photoreceptor 2Y. it can. As a result, the transfer belt 16 can be separated from the Y-color photoconductor 2Y even when the swinging amount of the separating means 210 is smaller than when the photoconductors are arranged in parallel when the monochrome mode is executed. Therefore, the swing amount of the transfer belt 16 can be reduced, the space around the transfer belt can be reduced, the degree of freedom in layout can be increased, and the apparatus can be downsized.

  If the difference in circumferential length between both ends of the transfer belt 16 and the parallelism of the stretching rollers (drive roller 19, counter roller 22 and the like) are low, one end of the transfer belt 16 is surplus due to the driving of the belt, and the elasticity of the belt causes undulations. Arise. When the color mode is executed, the belt is stretched by the color primary transfer rollers 19Y, 19M, and 19C, thereby suppressing undulation due to the elasticity of the belt. However, when the monochrome mode is executed, the color primary transfer rollers 19Y, 19M, and 19C do not stretch the transfer belt 16, and thus the color primary transfer rollers 19Y, 19M, and 19C have no effect of suppressing waviness. When the color photoconductors 2Y, 2M, and 2C are arranged on the upstream side in the belt movement direction with respect to the K color photoconductor 2K, the color belt 2Y, M is moved by the separation means 210 when the monochrome mode is executed. , C, the transfer belt 16 is not stretched from the opposing roller 22 to the K-color photoconductor 2K. Therefore, the belt undulates in the region from the facing roller 22 to the K-color photosensitive member 2K. In this embodiment, since the primary transfer roller 19 is not in contact with the photoreceptor 2 and the nip width is formed, if there is such undulation, the width of the K color transfer nip and the nip pressure are reduced. It cannot be maintained well. As a result, transfer failure occurs when the monochrome mode is executed. However, in the image forming apparatus according to the first embodiment, the K-color photoconductor 2K is disposed upstream of the color photoconductors 2Y, 2M, and 2C in the transfer belt moving direction. As a result, when the monochrome mode is executed, the non-stretching region of the belt extending from the K-color photosensitive member 2K to the upstream side in the belt movement direction is transferred to the K-color photosensitive member 2K rather than the color photosensitive members 2Y, 2M, and 2C. It is shorter than that arranged downstream of the belt moving direction. In this way, the belt non-stretching region extending from the K-color photosensitive member 2K to the upstream side in the belt movement direction can be shortened, so that the influence of the belt waviness can be suppressed, and the K-color transfer nip can be prevented. Fluctuations in width and nip pressure can be suppressed.

  By increasing the transfer nip pressure, fluctuations in the nip width and fluctuations in the nip pressure due to the waving can be suppressed. However, when the K-color photoconductor 2K is arranged downstream of the color photoconductor, in the color mode, when the K-color toner image is transferred to the transfer belt 16, Y, There is a color toner image in which toner images of M and C are superimposed. Since the three color toner images are superimposed on the color toner image on the transfer belt, when the K color transfer nip pressure is higher than the Y, M, and C transfer nip pressures, the color toner image becomes K color. Aggregation at the transfer nip may cause sticking to the K-color photoconductor 2K and cause voids, and the nip pressure cannot be increased.

  On the other hand, as in the first embodiment, the K-color photoconductor 2K is disposed upstream of the color photoconductors 2Y, 2M, and 2C, so that when the K-color toner image is transferred, the color toner is transferred onto the transfer belt. The toner images are not superimposed. Therefore, even if the K-color transfer pressure is increased, the color toner image is not lost in the K-color transfer nip, so that the K-color transfer nip pressure can be increased. Since the K-color transfer nip pressure can be increased, fluctuations in the nip width and nip pressure of the K-color transfer nip due to the waving can be further suppressed, and a K-color toner image can be transferred satisfactorily. And a good monochrome image can be obtained. On the other hand, in the Y, M, and C color transfer nips on the downstream side of the transfer belt moving direction from the K color transfer nip, toner is superimposed on the transfer belt, so that the transfer nip pressure is set higher than that of the K color transfer nip. The toner image on the transfer belt may be lost. Therefore, the Y transfer pressure of the Y, M, and C color transfer nips needs to be equal to or lower than the transfer pressure of the K color transfer nip.

  Therefore, in the image forming apparatus according to the first embodiment, the distance L between the axial centers of the K-color photosensitive member 2K and the primary transfer roller 19K is set between the color photosensitive members 2Y, M, and C and the primary transfer rollers 19Y, M, and C. It is closer than the distance L between the shaft centers. In addition, the lifting amount of the transfer belt 16 of the K primary transfer roller 19K is larger than the lifting amount of the transfer belt 16 of the primary transfer rollers 19Y, 19M, and 19C. Specifically, the diameter of the K color member 191K shown in FIGS. 5 and 6 is set to 9 [mm], and the diameters of the color members 191Y, M, and C are set to 10 [mm]. The lifting amount of the transfer belt 16 by the K primary transfer roller 19K is 1 [mm], and the lifting amount of the transfer belt 16 by the Y, M, and C primary transfer rollers 19Y, 19M, and 19C is 0.5 [mm]. ], The protrusion amounts of the K-colored abutting members 101K and Y, M, and C-colored abutting members 101Y, 101M, and 101C shown in FIGS. 6 and 7 were set. Thereby, the transfer pressure of the K color transfer nip can be made higher than the transfer nip pressure of the Y, M, and C colors.

  Further, the K-color transfer nip is 3 [mm] or less, and the length of the non-stretched region of the transfer belt 16 from the K-color photoconductor to the primary transfer row of K-color is 1 to 5 [mm]. It is preferable to arrange a K-color primary transfer roller so that When the transfer nip exceeds 3 [mm], a K-color toner image is pressed in the transfer nip for a long time. As a result, the K-color toner image may aggregate at the K-color transfer nip and stick to the K-color photoreceptor 2 </ b> K to cause a void. Also, if the length of the non-stretching region of the transfer belt 16 from the K color photoreceptor to the K primary transfer roller is less than 1 mm, the diameter of the photoreceptor and the diameter of the transfer roller vary. The variation in the transfer nip pressure is undesirably increased. On the other hand, if it exceeds 5 [mm], the primary transfer roller cannot obtain a predetermined transfer nip when the transfer nip is 3 [mm] or less, and good transfer cannot be performed. The K-color transfer nip is 3 [mm] or less, and the length of the non-tensioned region of the transfer belt 16 from the K-color photoconductor to the primary transfer row of K-color is 1 to 5 [mm]. In addition, by disposing the K primary transfer roller, variation in the transfer nip pressure is suppressed, and a predetermined nip pressure can be obtained.

[Embodiment 2]
Next, the image forming apparatus of Embodiment 2 will be described. Since the basic configuration of the image forming apparatus according to the second embodiment is the same as that of the image forming apparatus according to the first embodiment, the description thereof will be omitted, and only the characteristic points of the image forming apparatus according to the second embodiment will be described.
FIG. 13 is a main part configuration diagram of the image forming apparatus according to the second embodiment. FIG. 13A is a diagram illustrating a state in the full color mode in the image forming apparatus according to the second embodiment, and FIG. 13B is a diagram illustrating a state in the monochrome mode.
As shown in the drawing, in the image forming apparatus according to the second embodiment, a K-color photoconductor 2K is provided on the downstream side in the transfer belt moving direction from the color photoconductors 2Y, 2M, and 2C. As described above, the color primary transfer rollers 19Y, 19M, and 19C are low-cost metal rollers, and the primary transfer rollers 19Y, 19M, and 19C are arranged on the photosensitive member on the downstream side of the belt so as to satisfy the relationship of Formula 1. The transfer belt 16 is wound around the photoreceptors 2Y, 2M, and 2C without contacting the photoreceptor via the belt to form a transfer nip. On the other hand, the K primary transfer roller 19K uses a sponge roller, and the distance L between the axis of the primary transfer roller 19 and the axis of the photoreceptor 1 is
L <(A / 2) + (B / 2) + C (Equation 2)
The K primary transfer roller 19K is arranged so that That is, the primary transfer roller 19K is elastically deformed and brought into contact with the K-color photosensitive member 2K via the transfer belt 16. Therefore, in the K transfer nip, an area where the K primary transfer roller is in contact with the K photoconductor via the belt is the transfer nip. As described above, the transfer nip is formed by bringing the primary transfer roller 19K into contact with the photosensitive member 2K via the belt, so that the transfer belt 16 is wound around the photosensitive member 2 to form the transfer nip. The fluctuation of the nip width and the fluctuation of the transfer pressure due to the wave of the belt 16 can be suppressed.

  In the image forming apparatus according to the second embodiment, the K-color photoconductor 2K is disposed downstream of the color photoconductors 2Y, 2M, and 2C in the transfer belt moving direction. For this reason, when the color primary transfer rollers 19Y, 19M, and 19C are separated from the transfer belt 16 when the monochrome mode is executed, the belt non-stranding region extending from the K-color photosensitive member 2K to the upstream side in the belt moving direction becomes long. Therefore, although the belt undulates in this non-extending region, the belt is pressed against the photosensitive member by the K primary transfer roller 19K to form the transfer nip, and thus the undulation as described above occurs. Also, fluctuations in the transfer nip width and fluctuations in the transfer pressure can be suppressed. As a result, the K-color toner image during execution of the monochrome mode can be satisfactorily transferred to the transfer belt, and transfer defects can be suppressed. In addition, since the K-color photoconductor is disposed downstream of the color photoconductors 2Y, 2M, and 2C in the transfer belt moving direction, the first print when the monochrome mode is executed is made faster than the image forming apparatus of the first embodiment. Can do.

[Embodiment 3]
Next, the image forming apparatus of Embodiment 3 will be described. Since the basic configuration of the image forming apparatus according to the third embodiment is the same as that of the image forming apparatus according to the first embodiment, the description thereof is omitted, and only the feature points of the image forming apparatus according to the third embodiment are described.
FIG. 14 is a main part configuration diagram of the image forming apparatus according to the third embodiment. FIG. 14A is a diagram illustrating a state in the full color mode in the image forming apparatus according to the third embodiment, and FIG. 14B is a diagram illustrating a state in the monochrome mode.
As shown in the drawing, in the image forming apparatus according to the third embodiment, the K-color photoconductor 2K is provided on the downstream side in the transfer belt moving direction from the color photoconductors 2Y, 2M, and 2C. The primary transfer roller 19K for color K and the primary transfer rollers 19Y, 19M, and 19C for color are provided downstream of the respective photoreceptors 2K, Y, M, and C in the transfer belt moving direction. Each of the primary transfer rollers 19K, Y, M, and C is an inexpensive metal roller, and the primary transfer roller 19 is disposed without contacting the photoreceptor 2 via a belt so as to satisfy the relationship of the above formula 1. The transfer belt 16 is wound around the photoreceptor 2 to form a transfer nip. In the image forming apparatus according to the third exemplary embodiment, the K-color photoconductor 2K and the K-color photoconductor 2K are provided adjacent to each other, and are disposed upstream of the K-color photoconductor 2K in the belt moving direction. A roller-like undulation suppressing member 61 is provided between the C-color photoreceptor 2C. The undulation suppressing member 61 stretches the transfer belt 16 from the inside. Even if the primary transfer rollers 19Y, 19M, and 19C for color are separated from the transfer belt 16 when the monochrome mode shown in FIG. 5B is executed, the undulation suppressing member 61 does not separate the transfer belt 16 from the transfer belt 16. Stretch. As a result, it is possible to shorten the non-stretching region of the belt extending from the K-color photoconductor 2K to the upstream side in the belt movement direction when the monochrome mode is executed, and to reduce the influence of the wavy belt of the K-color transfer nip. be able to. As a result, the K-color toner image during execution of the monochrome mode can be satisfactorily transferred to the transfer belt 16, and transfer defects can be suppressed. In addition, since the K-color photoconductor is disposed downstream of the color photoconductors 2Y, 2M, and 2C in the transfer belt moving direction, the first print when the monochrome mode is executed is made faster than the image forming apparatus of the first embodiment. Can do.

FIG. 15 is a first modification of the image forming apparatus according to the third embodiment. FIG. 15A is a diagram illustrating a state in the full color mode of the image forming apparatus according to the first modification, and FIG. 15B is a diagram illustrating a state in the monochrome mode.
The first modification of the third embodiment shown in the drawing is a C-color photoconductor 2C provided adjacent to the K-color photoconductor 2K and disposed upstream of the K-color photoconductor 2K in the belt moving direction. The C-color primary transfer roller 19C for transferring the toner image is also used as a undulation suppressing member. Therefore, as shown in FIG. 4B, when the monochrome mode is executed, the M and Y primary transfer rollers 19M and Y upstream of the C primary transfer roller 19C in the belt moving direction are moved from the transfer belt 16. The C-color primary transfer roller 19 </ b> C is configured so as to keep the transfer belt 16 stretched as it is. As a result, the unstretched belt region extending upstream of the belt moving direction from the K-color photosensitive member 2K when the monochrome mode is executed can be set to the position of the C-color primary transfer roller 19C. Can be shortened. Further, there is an advantage that the number of parts can be reduced by using the C-color primary transfer roller 19C as a undulation suppressing member.

FIG. 16 is a second modification of the image forming apparatus according to the third embodiment. FIG. 16A is a diagram illustrating a state in the full color mode of the image forming apparatus according to the second modified example, and FIG. 16B is a diagram illustrating a state in the monochrome mode.
In the second modification shown in the drawing, the K primary transfer roller 19K is disposed on the upstream side in the belt movement direction from the K photoconductor 2K, and the K primary transfer roller 19K is also used as a undulation suppressing member. Is. Therefore, the non-stretchable region of the belt extending from the K-color photosensitive member 2K upstream in the belt movement direction when the monochrome mode is executed can be made to the position of the K-color primary transfer roller 19K, and the non-strandable region can be shortened. can do. Further, there is an advantage that the number of parts can be reduced by using the K primary transfer roller 19K as the undulation suppressing member.

(1)
As described above, according to the image forming apparatus of the first embodiment, the K color photoconductor as the image carrier used when the monochrome mode as the monochromatic mode is executed is the photoconductor that forms the most upstream transfer nip with the transfer belt as the belt member. . Therefore, the distance from the tension roller to the K color photosensitive member when the monochrome mode is executed can be shortened as compared with the case where the K color photosensitive member is a photosensitive member that forms the most downstream transfer nip with the transfer belt. it can. Therefore, in the monochrome mode, the area where no tension is applied to the transfer belt extending upstream from the transfer nip formed by the K color photoreceptor and the transfer belt, the K color photoreceptor is transferred to the transfer belt and the most downstream side. It can be shortened compared to a photoconductor that forms a nip. As a result, it is possible to suppress the waviness of the belt between these regions as compared with the case where the K-color photoconductor is a photoconductor that forms the most downstream transfer nip with the transfer belt. Accordingly, when the monochrome mode is executed, the transfer nip width and the transfer nip pressure formed by the K-color photosensitive member and the primary transfer roller are suppressed from being fluctuated due to the waving of the belt. As a result, transfer defects in the monochrome mode can be suppressed.
(2)
Further, according to the image forming apparatus of the third embodiment, a region where no tension is applied to the transfer belt extending from the K color transfer nip in the monochrome mode to the upstream side in the belt moving direction can be made up to the undulation suppressing member. Therefore, it is possible to shorten a region where no tension is applied to the transfer belt extending from the K-color transfer nip to the upstream side in the belt movement direction, and it is possible to suppress the occurrence of belt waviness between these regions. As a result, when the monochrome mode is executed, the K transfer nip width and the transfer nip pressure are prevented from fluctuating due to the waving of the belt. As a result, transfer defects in the monochrome mode can be suppressed.
(3)
Further, according to the image forming apparatus of the first modification of the third embodiment, since the undulation suppressing member is a C-color primary transfer body adjacent to the K-color photoreceptor, the number of parts can be reduced. it can.
(4)
Further, according to the image forming apparatus of the second modification of the third embodiment, since the undulation suppressing member is a K-color primary transfer roller, the number of parts can be reduced.
(5)
Further, according to the image forming apparatus of the present embodiment, the K-color transfer nip is 3 [mm] or less, and the non-strandable region of the transfer belt 16 from the K-color photosensitive member to the K-color primary transfer rollers. It is preferable to arrange the K-color primary transfer roller so that the length is 1 to 5 [mm]. When the transfer nip exceeds 3 [mm], a K-color toner image is pressed in the transfer nip for a long time. As a result, the K-color toner image may aggregate at the K-color transfer nip and stick to the K-color photoreceptor 2 </ b> K to cause a void. Also, if the length of the non-stretching region of the transfer belt 16 from the K color photoreceptor to the K primary transfer roller is less than 1 mm, the diameter of the photoreceptor and the diameter of the transfer roller vary. The variation in the transfer nip pressure is undesirably increased. On the other hand, if it exceeds 5 [mm], the primary transfer roller cannot obtain a predetermined transfer nip when the transfer nip is 3 [mm] or less, and good transfer cannot be performed. The K-color transfer nip is 3 [mm] or less, and the length of the non-tensioned region of the transfer belt 16 from the K-color photoconductor to the primary transfer row of K-color is 1 to 5 [mm]. In addition, by disposing the K primary transfer roller, variation in the transfer nip pressure is suppressed, and a predetermined nip pressure can be obtained.
(6)
Further, according to the image forming apparatuses of the first and third embodiments, the distance L from the axial center of the K-color photosensitive member to the axial center of the K-color primary transfer roller is from the axial center of the color photosensitive member. It is configured to be shorter than the distance L to the axis center of the primary transfer roller. Thereby, when the primary transfer roller for color and the primary transfer roller for K color have the same diameter and the vertical position of the primary transfer roller for K color on the virtual plane orthogonal to the axial direction of the photosensitive member is the same, The force (transfer pressure) with which the K primary transfer roller is brought into contact with the K color photoreceptor can be made higher than the color transfer pressure. The K-color primary transfer roller can strongly press the belt against the K-color photosensitive member, and the K-color transfer nip can be made less susceptible to the undulation of the belt.
(7)
Further, according to the image forming apparatuses of Embodiments 1 and 3, the position of the primary transfer roller for K color is higher than the position of the primary transfer roller for color on a virtual plane orthogonal to the axial direction of the photosensitive member. It is comprised so that it may be located in. Therefore, when the color primary transfer roller and the K-color primary transfer roller have the same diameter and the distance L from the photosensitive member shaft center to the primary transfer roller shaft center is the same, the K-color primary transfer roller is the K-color photosensitive member. The force to be brought into contact with the toner (transfer pressure) can be made higher than the transfer pressure for color. The K-color primary transfer roller can strongly press the belt against the K-color photosensitive member, and the K-color transfer nip can be made less susceptible to the undulation of the belt.
(8)
Further, according to the image forming apparatuses of the first and third embodiments, the distance L from the center of the photoreceptor axis to the axis center of the primary transfer roller is set to a predetermined distance on at least one of the primary transfer roller and the photoreceptor. A color member is provided as a first abutting member that abuts against the photoreceptor or the primary transfer roller. As a result, the primary transfer roller can be positioned so that the distance L from the photosensitive member shaft center to the primary transfer roller shaft center is a predetermined distance. Accordingly, the primary transfer roller can be disposed at a predetermined position, and the transfer nip can be set to a predetermined nip width and a predetermined pressure.
(9)
In addition, according to the image forming apparatus of the first and third embodiments, the second abutting member that abuts against the frame of the process unit that is the frame that holds the image carrier or the bearing that is the support member that supports the primary transfer roller. A contact member is provided. By abutting the abutting member against the frame or the bearing portion, the primary transfer roller can be positioned in the vertical direction with respect to the photoreceptor on a virtual plane orthogonal to the axial direction of the photoreceptor. Accordingly, the primary transfer roller can be disposed at a predetermined position, and the transfer nip can be set to a predetermined nip width and a predetermined pressure.
(10)
Further, according to the image forming apparatus of the second embodiment, the distance L ₂ from the center of the K-color photosensitive member axis to the center of the K-color primary transfer roller is L ₂ <(A / 2) + (B / 2). ) A primary transfer roller of K color was arranged so as to be + C. Accordingly, the K-color transfer nip is a portion where the K-color primary transfer roller is in contact with the K-color photoconductor via the transfer belt. Thus, the transfer nip formed by the K-color photoconductor and the K-color primary transfer roller is formed by pressing the transfer belt against the photoconductor from the back surface of the belt member by the primary transfer roller. Therefore, the influence of the undulation of the belt does not reach the transfer nip from the stretch roller to the K color photoreceptor. Accordingly, when the monochrome mode is executed, the transfer nip width and the transfer nip pressure formed by the K-color photosensitive member and the primary transfer roller are suppressed from being fluctuated due to the waving of the belt. As a result, transfer defects in the single color mode can be suppressed.
(11)
Further, according to the image forming apparatus of the second embodiment, the primary transfer roller is elastically deformed by using a member having an elastic layer as the K-color primary transfer roller, and is brought into contact with the photoreceptor via the belt. be able to. Therefore, the transfer nip can be brought into contact with a predetermined nip width and a predetermined pressure.
(12)
Further, according to the image forming apparatus of the present embodiment, the position of the K color transfer nip is configured to be closer to the transfer belt member than the position of the color transfer nip. Therefore, when the color primary transfer roller is separated from the transfer belt when the monochrome mode is executed, the position of the transfer belt from the K color transfer nip to the stretching roller is parallel to the color transfer nip. It can be positioned on the transfer belt side as compared with the arrangement. In this way, the position of the transfer belt from the K-color transfer nip to the stretching roller can be positioned on the transfer belt side, so that the transfer belt can be moved to the color photoreceptor only by slightly swinging the stretching roller. Can be separated from Thereby, the rocking space of the stretching roller can be reduced, and the size of the apparatus can be reduced and the degree of freedom in the layout of the apparatus can be increased.

1 is a schematic configuration diagram illustrating an image forming apparatus according to a first embodiment. FIG. 3 is an enlarged configuration diagram illustrating a process unit for K of the image forming apparatus. FIG. 2 is a schematic configuration diagram illustrating a transfer unit of the image forming apparatus. FIG. 3 is a diagram illustrating an arrangement relationship between a photoconductor and a primary transfer roller. FIG. 4 is a diagram illustrating a state in which a primary transfer roller is positioned with a color member abutting against a photosensitive member. The figure seen from the A direction of FIG. The figure which shows a mode that the butting member has contact | abutted to the bearing part of the primary transfer roller. FIG. 3 is a view showing contact / separation means of the image forming apparatus. The figure which shows the transfer unit at the time of monochrome mode execution. The figure which shows the other structural example of a separation means. FIG. 5 is a diagram illustrating the positions of the axial center of the photosensitive member and the axial center of the color photosensitive member when the K-color transfer nip is disposed closer to the transfer belt than the color transfer nip. (A) is a Y-color primary transfer roller when the transfer belt is separated from the Y-color photosensitive member when the axial center of the K-color photosensitive member and the axial center of the color photosensitive member are on the same line. FIG. (B) shows the primary transfer of the Y color when the transfer belt is separated from the Y color photosensitive member when the axial center of the K color photosensitive member is closer to the transfer belt side than the axial center of the color photosensitive member. The figure which shows the moving amount | distance of a roller. (A) is a figure which shows the mode at the time of color mode execution in the image forming apparatus of Embodiment 2, (b) is a figure which shows the mode at the time of monochrome mode execution. (A) is a figure which shows the mode at the time of color mode execution in the image forming apparatus of Embodiment 3, (b) is a figure which shows the mode at the time of monochrome mode execution. (A) is a figure which shows the mode at the time of color mode execution in the 1st modification of Embodiment 3, (b) is a figure which shows the mode at the time of monochrome mode execution. (A) is a figure which shows the mode at the time of color mode execution in the 2nd modification of Embodiment 3, (b) is a figure which shows the mode at the time of monochrome mode execution.

Explanation of symbols

1Y, M, C, K: Process unit 2Y, M, C, K: Photoconductor 15: Transfer unit 16: Transfer belt 17: Tension roller 18: Drive roller 19Y, M, C, K: Primary transfer roller 20: 2 Next transfer roller 34: Fixing device 70: Optical writing unit 210: Separating means

Claims (10)

A plurality of image carriers that carry a visible image, and belt members that are stretched by at least two or more stretching rollers and that sequentially contact each image bearing member between the stretching rollers to form a transfer nip. A plurality of primary transfer members that contact the back surface of the belt member at a position corresponding to each image carrier and apply a transfer bias to the back surface of the belt member. In an image forming apparatus that forms a superimposed image by transferring a visible image on an image carrier to the belt member side at each transfer nip in the process of moving from the transfer nip to the most downstream transfer nip.
The diameter of the image carrier is A, the diameter of the primary transfer member is B, the thickness of the belt member is C, the image carrier and the belt member are brought into contact, and the belt member and the primary transfer member Is a distance L from the center of the image carrier axis to the axis of the primary transfer body on a virtual plane orthogonal to the axial direction of the image carrier,
L> (A / 2) + (B / 2) + C
And the primary transfer member is arranged so that the primary transfer member is downstream of the corresponding image carrier in the belt movement direction,
A color mode in which an image is formed using all of the image carrier and all other image carriers except one image carrier are relatively separated from the belt member, and the single image carrier is used. A monochrome mode for forming an image, and an image carrier used for executing the monochrome mode is an image carrier that forms the most upstream transfer nip with the belt member ,
In the image carrier that forms the most upstream transfer nip, the position closest to the belt member placement side is closer to the belt member placement side than the position that is located closest to the belt member placement side in other image carriers. Arrange each image carrier,
In the color mode, the position of the primary transfer member corresponding to the image carrier adjacent to the image carrier forming the most upstream transfer nip is located closest to the image carrier arrangement side, and the belt member is the most adjacent to the image carrier. Compared to the location located on the arrangement side and the location located on the image carrier arrangement side most in the primary transfer body corresponding to the image carrier forming the most upstream transfer nip,
In the monochromatic mode, the position of the primary transfer body corresponding to the image carrier adjacent to the image carrier forming the most upstream transfer nip is located closest to the image carrier arrangement side, and the belt member is the most belt member in the adjacent image carrier. Compared to the position located on the arrangement side and the position located closest to the image carrier arrangement side in the primary transfer body corresponding to the image carrier forming the most upstream transfer nip, on the side opposite to the image carrier arrangement side. an image forming apparatus comprising such a manner things become. A plurality of image carriers that carry a visible image, and belt members that are stretched by at least two or more stretching rollers and that sequentially contact each image bearing member between the stretching rollers to form a transfer nip. A plurality of primary transfer members that contact the back surface of the belt member at a position corresponding to each image carrier and apply a transfer bias to the back surface of the belt member. In an image forming apparatus that forms a superimposed image by transferring a visible image on an image carrier to the belt member side at each transfer nip in the process of moving from the transfer nip to the most downstream transfer nip.
The diameter of the image carrier is A, the diameter of the primary transfer member is B, the thickness of the belt member is C, the image carrier and the belt member are brought into contact, and the belt member and the primary transfer member Is a distance L from the center of the image carrier axis to the axis of the primary transfer body on a virtual plane orthogonal to the axial direction of the image carrier,
L> (A / 2) + (B / 2) + C
The primary transfer member is arranged so that
A color mode in which an image is formed using all of the image carrier and all other image carriers except one image carrier are relatively separated from the belt member, and the single image carrier is used. With a monochrome mode to form an image,
The image carrier used when the monochrome mode is executed is the image carrier that forms the most downstream nip with the belt member, the image carrier used when the monochrome mode is executed, and the image carrier used when the monochrome mode is executed. between the adjacent image carrier, as suppressing member wavy suppress waving of belts to impart tension to the belt member from the back surface of the belt member, which corresponds to the image bearing member to form the most downstream nip Providing a primary transfer member ,
The position of the image carrier that forms the most downstream transfer nip is closest to the belt member arrangement side compared to the position of the other image carrier that is closest to the belt member arrangement side. Arrange each image carrier,
In the color mode, the portion of the primary transfer member corresponding to the image carrier that is adjacent to the image carrier that forms the most downstream transfer nip is located closest to the image carrier arrangement side, and the belt member is the most belt member in the adjacent image carrier. Compared with the location located on the arrangement side and the location located on the most image carrier arrangement side in the primary transfer body corresponding to the image carrier forming the most downstream transfer nip,
In the single-color mode, the position of the primary transfer member corresponding to the image carrier that is adjacent to the image carrier that forms the most downstream transfer nip is located closest to the image carrier placement side in the adjacent image carrier, and the belt member is the most. Compared with the position located on the arrangement side and the position located closest to the image carrier arrangement side in the primary transfer body corresponding to the image carrier that forms the most downstream transfer nip, on the side opposite to the image carrier arrangement side an image forming apparatus comprising such a manner things become. The image forming apparatus 請 Motomeko 1 or 2,
The transfer nip of the image carrier used at the time of executing the monochrome mode is 3 mm or less, and there is no belt member from the image carrier used at the time of executing the monochrome mode to the primary transfer member corresponding to the image carrier. An image forming apparatus, wherein a primary transfer member corresponding to an image carrier used in executing the monochromatic mode is arranged so that a length of a stretch region is 1 to 5 [mm]. The image forming apparatus according to any one of claims 1 to 3 ,
A distance L from the axial center of the image carrier used for executing the monochromatic mode to the axial center of the primary transfer body corresponding to the image carrier corresponds to the image carrier from the axial center of another image carrier. An image forming apparatus configured to be shorter than a distance L to an axis center of a primary transfer body. The image forming apparatus according to claim 1 to 4,
On the virtual plane orthogonal to the axial direction of the image carrier, the vertical direction from the axial center of the image carrier used when executing the monochromatic mode to the axial center of the primary transfer body corresponding to the image carrier. An image forming apparatus characterized in that the distance is shorter than the vertical distance from the axial center of another image carrier to the axial center of a primary transfer member corresponding to the image carrier. The image forming apparatus according to claim 1 to 5,
At least one of the primary transfer body or the image carrier, the image carrier or the primary transfer body so that a distance L from the axis center of the image carrier to the axis center of the primary transfer body is a predetermined distance. An image forming apparatus comprising a first abutting member that abuts against the image forming apparatus. The image forming apparatus according to claim 1 to 6,
An image carrier that corresponds to the primary transfer member on a virtual plane orthogonal to the axial direction of the image carrier on at least one of a frame that holds the image carrier or a support member that supports the primary transfer member. An image forming apparatus comprising a second abutting member that abuts against the frame or the support member so that a vertical position of the primary transfer member with respect to the frame is a predetermined position . 請 Motomeko 1 to 7 in any of the image forming apparatus,
The position of the transfer nip formed by the image carrier and the belt member used at the time of executing the monochromatic mode is closer to the belt member than the position of the transfer nip formed by the other image carrier and the belt member. An image forming apparatus characterized by comprising. The image forming apparatus according to claim 1 to 8,
Cleaning means for cleaning the front surface by contacting the front surface of the belt member between the uppermost stream transfer nip and a stretching roller that stretches the belt member upstream of the nip is provided. An image forming apparatus characterized by that. The image forming apparatus according to claim 9 .
An image forming apparatus, comprising: a counter member that contacts a back surface of a portion of the belt member that contacts the cleaning unit.

Images