JP5024309B2 - Tandem type color image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as an electrophotographic copying machine, a printer, a facsimile machine, or a combination machine combining two or more of these, and more particularly to a so-called tandem type color image forming apparatus.

  In order to form a color image in a color image forming apparatus, it is necessary to sequentially form toner images of a plurality of colors and transfer them to a recording medium such as recording paper. The image forming speed tends to be slow.

  Therefore, there is a so-called tandem color image forming apparatus that forms a toner image by arranging a plurality of image forming units corresponding to each color in order to improve the image forming speed in color image formation.

  In the method of transferring each color toner image to a recording medium when forming a color image in a tandem color image forming apparatus, each color toner image is sequentially superimposed on a recording medium such as recording paper conveyed by a sheet conveying belt. There are a direct transfer method for transferring, and an intermediate transfer method in which each color toner image is temporarily transferred to the intermediate transfer member for primary transfer, and then the multiple toner images on the intermediate transfer member are collectively transferred to a recording medium for secondary transfer. .

  Examples of the intermediate transfer member that can be used in the intermediate transfer method include a roller type and a belt type. In a tandem color image forming apparatus, each image forming unit that forms a toner image of the assigned color. In many cases, an intermediate transfer belt is employed because of the ease of arrangement and the like, and downsizing of the overall configuration of the apparatus.

  According to the tandem color image forming apparatus, in the color image forming mode, a plurality of image forming units that form toner images of different colors are used. In the monochrome image forming mode, an image in charge of monochrome image formation is used. Only the forming portion (typically, a black image forming portion employing black toner) is used.

  Therefore, in the monochrome image forming mode, there is not only an advantage that the image forming unit other than the image forming unit that forms the monochrome image can be used, but in the tandem color image forming apparatus using the intermediate transfer member, If such a setting is left as it is, there is a risk that the life of the image forming unit and the intermediate transfer member may be shortened.

  Therefore, in a tandem color image forming apparatus using an intermediate transfer member, in the monochrome image forming mode, in order to extend the life of the image forming unit other than the image forming unit that forms a monochrome image and the intermediate transfer member, There is also an image forming apparatus that relatively separates the image forming unit (particularly, the image carrier) and the intermediate transfer member from the mutual position for forming a toner image. This type of image forming apparatus will be further described later with reference to FIGS.

  Next, in the tandem color image forming apparatus that employs an intermediate transfer belt as an intermediate transfer member, a description will be given of the primary transfer of the toner image formed in each image forming unit to the intermediate transfer belt.

  First, an outline of the image forming unit will be described. That is, an electrostatic latent image corresponding to a responsible color is formed on an image carrier that is rotationally driven facing the intermediate transfer belt, and the electrostatic latent image is used for the responsible color toner. And developing the toner image.

  The toner image formed on the image carrier is transferred to the primary image on the intermediate transfer belt by a primary transfer member facing the image carrier with the intermediate transfer belt interposed therebetween under a transfer electric field by applying a transfer bias. Transcribed. At this time, the intermediate transfer belt is pressed against the image carrier by the primary transfer member. The toner image primarily transferred onto the belt is secondarily transferred to a recording medium by a secondary transfer member, and thereafter usually fixed to the recording medium under heat and pressure. As a typical example of the image carrier, a photosensitive member that is rotationally driven can be given. The primary transfer member is generally in the form of a rotatable roller, but there is also a member applied while allowing the intermediate transfer belt to travel without rotating.

  In any case, time for moving the toner is required for the primary transfer of the toner image on the image carrier to the intermediate transfer belt. In order to secure this time, a primary transfer nip having a width in the belt running direction between the intermediate transfer belt and the image carrier (contact between the belt for the primary transfer of the toner image and the image carrier). Nip) must be formed.

  In a tandem color image forming apparatus that employs an intermediate transfer belt, as a method for forming this primary transfer nip, (1) a primary transfer member is formed of elastic foam rubber or the like, and the belt is formed with this member. A method of forming a primary transfer nip by pressing against a rigid image carrier and elastically deforming it; or (2) arranging the nip forming members on the upstream and downstream sides of the image carrier in the belt running direction. There is known a method of forming a nip by winding an image around an image carrier.

  However, when the mutual pressure contact force between the image carrier and the intermediate transfer belt is high in the primary transfer nip, the pressure on the toner in the nip increases, so that the toner aggregates and between the toners or between the toner and the image carrier. As a result, the movement of the toner from the image carrier to the intermediate transfer belt may be hindered. As a result, the toner remains on the image carrier and the quality of the image decreases. In particular, since the pressure tends to increase in the central portion of the image pattern and the toner tends to remain, this is manifested as a hollow phenomenon in which the central portion of characters and the like is white.

  The hollowing out phenomenon is affected by the mutual pressure contact force between the image carrier and the belt in the primary transfer region, the temperature and humidity of the ambient atmosphere, the chargeability of the toner, the surface property of the image carrier, etc. In order to suppress the hollowing out phenomenon under unfavorable conditions such as a wet environment, it is desirable to set the mutual pressure contact force between the image carrier and the belt as low as possible.

However, in the case of the intermediate transfer belt system, tension is applied to the belt and the belt is driven. In this state, the transfer nip is formed so as to bend the belt along a predetermined path. Therefore, a pressing force exceeding the drag due to the belt tension and the degree of bending is required for forming the nip.
The tension of the belt fluctuates due to uneven dimensions, uneven belt speed during operation, deformation of the belt itself during storage, etc. In order to form an appropriate nip stably, the pressure contact force at the primary transfer nip Need to be set high to increase reliability.

  In this way, the pressure contact force between the image carrier and the intermediate transfer belt in the primary transfer nip needs to be kept low so as not to cause the hollowing out phenomenon, and in order to stably form the nip, a predetermined value is required. Need to set higher.

In order to meet such a requirement to stand up against each other, a force for bringing the image carrier and the intermediate transfer belt into contact with each other is formed directly on the transfer nip and a portion for stably forming the primary transfer nip therebetween. There is an idea of dividing it into a portion to which a pressure contact force is applied. Specifically, in order to stably form the primary transfer nip, nip forming members are arranged on both sides of the image carrier (adjacent to the upstream side and the downstream side of the image carrier in the belt traveling direction), A primary transfer nip having a desired width is formed by winding the belt around the image carrier with the nip forming members, and the belt portion forming the nip is imaged with a small pressure contact force by the primary transfer member. It has been proposed to directly press the carrier. As a result, the pressure on the toner is suppressed to suppress the void phenomenon.

  Some tandem color image forming apparatuses that employ an intermediate transfer belt adopt this concept, and use it as the image carrier of the image forming unit on the most upstream side in the belt running direction among a plurality of image forming units. An adjacent nip forming member is disposed on the upstream side, and an adjacent nip forming member is disposed on the downstream side of the image carrier of the image forming unit on the most downstream side. There is an image forming apparatus in which a belt can be pressed against a corresponding image carrier by a primary transfer member corresponding to each image carrier while being stably disposed with respect to the carrier.

  Further, in this type of image forming apparatus, in the monochrome image forming mode, the image carrier and the intermediate transfer belt are respectively connected to the upstream side and the downstream side of the image carrier of the image forming unit used for monochrome image formation. There is also an image forming apparatus in which a nip forming member for forming a primary transfer nip is disposed therebetween (see, for example, JP-A-2005-62642).

3 and 4 show an example of such an image forming apparatus.
The image forming apparatus shown in FIGS. 3 and 4 is a tandem color printer PR ′ employing an intermediate transfer belt 8.

  The printer PR 'includes a driving roller 81, a roller 82 facing the driving roller 81, and an endless intermediate transfer belt 8 wound around a tension roller 83. The transfer belt 8 is rotated counterclockwise (arrow direction in the figure) CCW in the figure by a driving roller 81 driven by a belt driving unit (not shown).

  A cleaning device 80 for cleaning secondary transfer residual toner and the like on the transfer belt 8 faces the roller 82, and a secondary transfer roller 9 faces the drive roller 81.

The secondary transfer roller 9 is pressed against a portion of the intermediate transfer belt 8 supported by the driving roller 81 by a pressing means (not shown), forms a nip portion with the intermediate transfer belt 8, and rotates the intermediate transfer belt 8. It can be rotated following the movement of the recording medium S fed to the nip portion or driven by a driving unit (not shown).
A secondary transfer bias can be applied to the secondary transfer roller 9 from a power supply (not shown).

  A fixing device FX is disposed above the intermediate transfer belt 8 and the secondary transfer roller 9, a timing roller pair TR is disposed below, and a recording medium S such as recording paper is accommodated below the fixing device FX. A recording medium accommodation cassette 10 is arranged.

The fixing device FX includes a fixing heating member heated by a heat source and a pressure member pressed against the fixing heating member. Both members are in the form of rollers in the illustrated example.
The recording medium S stored in the recording medium storage cassette 10 can be pulled out one by one by the medium supply roller 10r and supplied to the timing roller pair TR.

  Between the rollers 81 and 82 around which the intermediate transfer belt 8 is wound, the yellow image forming unit Y, the magenta image forming unit M, and the cyan image forming unit C are arranged along the transfer belt 8 from the roller 82 to the roller 81. And the black image forming portion K are arranged in this order.

  Each of the image forming units Y, M, C, and K includes a drum-type photosensitive member 1 as an electrostatic latent image carrier, and a charger 2, an exposure device 3, a developing device 4 and the like around the photosensitive member. The cleaning devices 5 are arranged in this order.

  A primary transfer roller 6 is disposed opposite to the photoreceptor 1 of each image forming unit with a belt 8 therebetween. The primary transfer roller 6 is pressed in the direction of the photosensitive member 1 by a pressing unit (not shown), contacts the belt 8 and is driven to rotate, and can bring the belt 8 into contact with the photosensitive member 1.

  A primary transfer bias for primarily transferring a toner image formed on the photoreceptor 1 to the belt 8 can be applied to each primary transfer roller 6 from a power supply (not shown).

  The exposure device 3 can perform image exposure by dot (point) exposure on the photosensitive member 1 by blinking of a laser beam in accordance with image information provided from a personal computer (not shown), an image reading device, or the like.

The photoreceptor 1 in each image forming unit can be rotated in the clockwise direction in the figure by a photoreceptor drive motor (not shown).
The charger 2 in each image forming unit is a charger using corona discharge in this example, but may be a charging roller or the like.
In this example, the developing device 4 in each image forming unit can reversely develop the electrostatic latent image formed on the photoreceptor 1.

  A transfer nip forming member 84 is disposed on the upstream side of the photoreceptor 1 of the image forming portion Y on the most upstream side in the belt traveling direction among the image forming portions for Y, M, C, and K, and the image on the most downstream side. A transfer nip forming member 85 is disposed on the downstream side of the photoreceptor 1 in the forming portion K. Further, a transfer nip forming member 86 is also arranged on the upstream side of the photosensitive member 1 in the image forming portion K. In the illustrated example, these nip forming members are transfer nip forming rollers that can be driven and rotated by the belt 8.

  According to this printer, a color image can be formed using Y, M, C, and K image forming units. In the color image forming mode, as shown in FIG. 3, the upstream nip forming roller 84 is disposed on the upstream side of the photoreceptor 1 in the yellow image forming portion Y, and the downstream nip forming roller 85 is disposed in the black image forming portion K. The intermediate transfer belt 8 is disposed at the downstream side of the photosensitive member 1, and the intermediate transfer belt 8 is formed at a position where a primary transfer nip is formed with respect to each photosensitive member 1 by the both nip forming rollers 84 and 85. 8 is pressed against the corresponding photoreceptor 1.

In this state, a yellow toner image is formed in the yellow image forming portion Y, and this is primarily transferred to the transfer belt 8.
That is, in the yellow image forming portion Y, the photosensitive member 1 is rotationally driven in the clockwise direction in the drawing, the surface is uniformly charged to a predetermined potential by the charger 2, and an image for yellow image is transferred from the exposure device 3 to the charged area. Exposure is performed, and an electrostatic latent image for yellow is formed on the photoreceptor 1. This electrostatic latent image is developed by the developing device 4 having yellow toner to become a visible yellow toner image. The yellow toner image is primarily transferred onto the transfer belt 8 by the primary transfer roller 6. At this time, a primary transfer bias is applied to the primary transfer roller 6 from a power supply (not shown).

  Similarly, a magenta toner image is formed in the magenta image forming unit M and transferred to the transfer belt 8. A cyan toner image is formed in the cyan image forming unit C and transferred to the transfer belt 8. In the black image forming unit K. A black toner image is formed and transferred to the transfer belt 8.

Yellow, magenta, cyan, and black toner images are formed at the timing when these toner images are transferred onto the intermediate transfer belt 8.
Thus, the multiple toner image formed on the transfer belt 8 moves toward the secondary transfer roller 9 by the rotation of the transfer belt 8.

  On the other hand, the recording medium S is pulled out from the recording medium accommodation cassette 10 by the medium supply roller 10r, supplied to the timing roller pair TR, and is on standby.

  Thus, the recording medium S waiting at the timing roller pair TR is supplied to the nip portion between the transfer belt 8 and the secondary transfer roller 9 in accordance with the multiple toner image sent by the intermediate transfer belt 8. . This multiple toner image is secondarily transferred onto the recording medium S by a secondary transfer roller 9 to which a secondary transfer bias is applied from a power supply (not shown).

  Thereafter, the recording medium S is passed through the fixing device FX, where the multiple toner images are fixed on the recording medium S under heat and pressure. The recording medium S is continuously discharged to the discharge tray DT by the discharge roller pair DR.

In the primary transfer of the toner image to the belt 8, transfer residual toner or the like remaining on the photoreceptor 1 is cleaned by the cleaning device 5, and secondary transfer residual toner or the like remaining on the belt 8 by the secondary transfer is cleaned by the cleaning device 83. It is cleaned with. Each of these cleaned and removed toners is sent to a waste container by a conveying means (not shown).
In the color image forming mode, the nip forming roller 86 may be omitted.

  The printer PR ′ can form a color image in this way, but can also form a monochrome image.

The printer PR ′ can form a monochrome image using the black image forming unit K. In the monochrome image forming mode, as shown in FIG. 4, the primary transfer roller 6 corresponding to each photoconductor 1 of the image forming portions Y, M, and C is lifted and separated from the belt pressure contact position to the photoconductor 1. Further, the upstream transfer nip forming roller 84 is also raised. However, the primary transfer nip is formed between the belt 8 and the photosensitive member 1 of the image forming unit K by the transfer nip forming rollers 85 and 86 that are still positioned on the downstream side and the upstream side of the photosensitive member 1 of the monochrome image forming unit K. Secured, the primary transfer roller 6 presses the belt 8 against the photoreceptor.
In this state, a monochrome image can be formed.

  The most downstream nip forming roller 85 is used in both the color image forming mode and the monochrome image forming mode. The nip forming roller 86 may not be in the color image forming mode. However, in the monochrome image forming mode, the nip forming roller 86 is required to secure a primary transfer nip between the photosensitive member 1 and the belt 8 of the image forming portion K. Is. That is, it can be said that the nip forming roller 86 is a roller dedicated to nip formation dedicated to the monochrome image forming mode.

JP 2005-62642 A

  However, the image forming apparatus shown in FIGS. 3 and 4 is a tandem color image forming apparatus that employs an intermediate transfer belt. In the monochrome image forming mode, the image forming unit used for forming a monochrome image has an image carrier. An image in which a nip forming member that forms a primary transfer nip is disposed between the image carrier and the intermediate transfer belt on both sides of the body (on the upstream side and the downstream side of the image carrier in the belt traveling direction). In the forming apparatus, if both nip forming members adjacent to the image carrier are exclusively used for nip formation, the number of members increases accordingly, the structure becomes complicated, and the cost increases.

  Therefore, the present invention includes an endless intermediate transfer belt that is rotationally driven, and a plurality of image forming units that are sequentially arranged from the upstream side to the downstream side in the belt running direction along the intermediate transfer belt. The forming unit forms an electrostatic latent image corresponding to the assigned color on the image carrier that is rotationally driven facing the intermediate transfer belt, and develops the electrostatic latent image using the assigned color toner to form a toner image. The toner image can be primarily transferred onto the intermediate transfer belt by a primary transfer member facing the image carrier with the intermediate transfer belt interposed therebetween, and the primary transfer toner image Can be secondarily transferred to a recording medium by a secondary transfer member. In the monochrome image forming mode, the upstream side and the downstream side of the image carrier of the image forming unit used for monochrome image formation among the plurality of image forming units. Between the image carrier and the intermediate transfer belt A tandem color image forming apparatus in which the nip forming members for forming the next transfer nip are respectively arranged. The image forming apparatus can be simplified in structure and less expensive than a conventional image forming apparatus of the same type. The task is to do.

In order to solve the above problems, the present invention provides the following image forming apparatus. That is,
An endless intermediate transfer belt that is driven to rotate, and a plurality of image forming units that are sequentially arranged from the upstream side to the downstream side in the belt running direction along the intermediate transfer belt, and each image forming unit includes the intermediate transfer belt An electrostatic latent image corresponding to the assigned color is formed on the image carrier that is rotationally driven facing the transfer belt, and the electrostatic latent image is developed using the assigned color toner to form a toner image. The toner image can be primarily transferred onto the intermediate transfer belt by a primary transfer member facing the image carrier with the intermediate transfer belt interposed therebetween, and the primary transfer toner image is transferred to the secondary transfer member. In the monochrome image forming mode, the image carrier and the image carrier on the upstream side and the downstream side of the image carrier used for monochrome image formation among the plurality of image forming units. Primary transfer nip between the intermediate transfer belt Nip forming member for forming is a tandem type color image forming apparatus are disposed, respectively.

The image forming unit used in the image forming apparatus according to the present invention in the monochrome image forming mode, in the intermediate transfer belt running direction is an image forming unit located at the most downstream side. The primary transfer member corresponding to the image carrier of the image forming unit adjacent to the most downstream image forming unit on the upstream side is an image carrier of the image forming unit that forms the monochrome image in the monochrome image forming mode. Of the nip forming members arranged on the upstream side and the downstream side, this also serves as the nip forming member arranged on the upstream side .
As described above, the nip forming member on the upstream side of the image carrier of the image forming unit used for forming the monochrome image is used as the primary transfer member corresponding to the image carrier of the image forming unit adjacent to the image forming unit that forms the monochrome image. Therefore, the nip forming member on the upstream side of the image carrier of the monochrome image forming unit does not have to be provided exclusively for nip formation, and the number of members can be reduced, the structure can be simplified, and the cost can be reduced. Can be done.

In the image forming apparatus according to the present invention, a transfer member pressure contact / separation device is further provided. In the color image forming mode, the transfer member pressure contact / separation device arranges a primary transfer member corresponding to the image carrier of each image forming unit at a color image formation mode position where the intermediate transfer belt is pressed against the image carrier. In the monochrome image forming mode, the primary transfer member corresponding to the image carrier in the image forming unit other than the image forming unit located on the most downstream side is separated from the color image forming mode position and on the most downstream side. The primary transfer member corresponding to the image carrier of the image forming unit adjacent to the image forming unit located is located on the upstream side of the image carrier of the image forming unit located on the most downstream side of the nip forming member. Move to the position .

  In any case, a typical example of the image forming unit used in the monochrome image forming mode is a black image forming unit that forms a toner image using black toner.

  As described above, according to the present invention, an endless intermediate transfer belt that is rotationally driven and a plurality of image forming units that are sequentially arranged from the upstream side to the downstream side in the belt running direction along the intermediate transfer belt are provided. Each of the image forming units forms an electrostatic latent image corresponding to the assigned color on the image carrier that is rotationally driven facing the intermediate transfer belt, and develops the electrostatic latent image using the assigned color toner. A toner image can be formed, and the toner image can be primarily transferred onto the intermediate transfer belt by a primary transfer member facing the image carrier with the intermediate transfer belt interposed therebetween, A primary transfer toner image can be secondarily transferred to a recording medium by a secondary transfer member. In the monochrome image formation mode, the upstream side of the image carrier of the image formation unit used for monochrome image formation among the plurality of image formation units and On the downstream side, the image carrier and the intermediate A tandem color image forming apparatus in which a nip forming member for forming a primary transfer nip is disposed between the image forming belt and the structure can be simplified and less expensive than a conventional similar type image forming apparatus. An image forming apparatus obtained in the above can be provided.

1 is a diagram illustrating an outline of a configuration of an example of a tandem color image forming apparatus according to the present invention in a state in a color image forming mode. FIG. 2 is a diagram illustrating the image forming apparatus illustrated in FIG. 1 in a state in a monochrome image forming mode. It is a figure which shows the outline of a structure of one example of the conventional tandem type color image forming apparatus in the state at the time of a color image formation mode. It is a figure which shows the image forming apparatus shown in FIG. 3 in the state at the time of a monochrome image formation mode.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 schematically shows the configuration of an example PR of an image forming apparatus according to the present invention. The image forming apparatus PR is a tandem type full color printer.

  This printer PR is an improvement of the printer PR 'shown in FIGS. 3 and 4, and the same reference numerals as those of the printer PR' are assigned to the same parts and portions as the printer PR '. The difference between the printer PR and the printer PR ′ is that the roller 86 dedicated for forming the transfer nip disposed on the upstream side of the photosensitive member 1 of the black image forming portion K used in the monochrome image forming mode in the printer PR ′ is omitted. The primary transfer roller 6 corresponding to the photoreceptor 1 of the image forming unit C adjacent to the upstream side of the forming unit K is made to exhibit the function of the roller 86.

This eliminates the need for the nip forming roller 86, simplifies the configuration of the apparatus, and makes it possible to reduce the cost.
Others are substantially the same in configuration and operation as the printer PR ′.

  The difference from the printer PR 'will be further described. In the printer PR, the primary transfer roller 6 corresponding to the photoreceptor 1 of the image forming unit Y is rotatably supported by the frame F1. A nip forming roller 84 on the upstream side of the photoreceptor 1 is also rotatably supported by the frame F1.

The primary transfer roller 6 corresponding to the photoreceptor 1 of the image forming unit M is rotatably supported by the frame F2.
The primary transfer roller 6 corresponding to the photoreceptor 1 of the image forming unit C is rotatably supported by the frame F3.
The primary transfer roller 6 corresponding to the photoreceptor 1 of the image forming unit K is rotatably supported by the frame F4.

  The frame F1 is attached to an image forming apparatus member (not shown) so as to be reciprocally swingable clockwise and counterclockwise in the drawing, and is given a force that rotates counterclockwise in the drawing by a spring s1. As a result, in the color image forming mode, as shown in FIG. 1, the transfer nip forming roller 84 is pressed against the belt 8 on the upstream side of the photoconductor 1 of the image forming portion Y, and 1 corresponding to the photoconductor 1 is used. The next transfer roller 6 presses the belt 8 against the photoreceptor 1. In this state, a part of the frame F1 comes into contact with a stopper provided on the drive frame 101, and further counterclockwise swinging is restricted.

  The frame F2 is also attached to an image forming apparatus member (not shown) so as to be reciprocally swingable in the clockwise and counterclockwise directions in the drawing, and is given a force that rotates counterclockwise in the drawing by a spring s2. Thereby, in the color image forming mode, as shown in FIG. 1, the primary transfer roller 6 corresponding to the photosensitive member 1 of the image forming unit M presses the belt 8 against the photosensitive member 1. In this state, a part of the frame F2 comes into contact with a stopper provided on the drive frame 101, and further counterclockwise swinging is restricted.

The frame F3 is supported by the drive frame 101. The frame 101 extends in the lateral direction inside the endless transfer belt 8 , in other words, in the arrangement direction of the image forming units Y, M, C, and K, and can be reciprocated in the lateral direction by a driving unit (not shown). The transfer roller 6 is supported by the frame F3 so as to be movable up and down and is pressed downward by a spring s3. In the color image forming mode, the transfer roller 6 is pressed by the spring s3 to press the belt 8 against the photoreceptor 1 of the image forming portion C.

  The frame F4 is attached to an image forming apparatus member (not shown), and the transfer roller 6 on the frame F4 is supported by the frame F4 so as to be movable up and down and pressed downward by a spring s4. The transfer roller 6 is pressed by the spring s4 to press the belt 8 against the photosensitive member 1 of the image forming portion K in both the color image forming mode and the monochrome image forming mode.

  The transfer nip forming roller 85 located on the downstream side of the photoconductor 1 in the image forming portion K is pressed downward by a spring s5. The roller 85 is pressed against the belt 8 by being pressed by the spring s5 in both the color image forming mode and the monochrome image forming mode.

  Therefore, in the color image forming mode, as in the printer PR ′ shown in FIG. 3, the upstream nip forming roller 84 is disposed on the upstream side of the photoreceptor 1 in the yellow image forming portion Y, and the downstream nip forming roller 85 is set to black. The image forming portion K is disposed on the downstream side of the photosensitive member 1, and the intermediate transfer belt 8 is disposed at a position where a primary transfer nip is formed with respect to each photosensitive member 1 by the nip forming rollers 84 and 85. The transfer roller 6 presses the belt 8 against the corresponding photoreceptor 1. Thus, a color image can be formed using the Y, M, C, and K image forming units.

  On the other hand, in the monochrome image forming mode, the primary transfer member corresponding to the image carrier of the image forming unit is pressed against the image carrier for the image forming unit other than the image forming unit used for monochrome image formation. The monochrome image can be formed by using the monochrome image forming unit away from the position where the image is formed. In this example, the monochrome image forming unit is the black image forming unit K.

  In the monochrome image forming mode, as shown in FIG. 2, the drive frame 101 is driven in the right direction in the drawing until the stopper pin p fitted in the frame long hole a comes into contact with the inner wall of the end of the long hole a. As a result, the frames F1 and F2 are swung clockwise against the springs s1 and s2, and the primary transfer roller 6 corresponding to the transfer nip forming roller 84 and the image forming portions Y and M, respectively, is pressed against the belt 8. Separate from above.

  Further, the primary transfer roller 6 corresponding to the image forming unit C is moved in the same direction as the drive frame 101, so that it is positioned upstream of the photosensitive member 1 of the monochrome image forming unit K and should function as a transfer nip roller. Move to.

Here, the drive frame 101, the frames F1, F2, and F3, the springs s1 , s2, and s3 are arranged at positions where the primary transfer roller 6 is pressed against the corresponding photoreceptor 1 in the color image forming mode. In the monochrome image forming mode, the transfer roller pressure contact / separation device 100 is configured to separate the primary transfer roller from the belt pressure contact position of the image forming unit other than the image forming unit that forms a monochrome image with respect to the photoreceptor 1.

Thus, the transfer roller 6 corresponding to the cyan image forming portion C, together with the transfer nip forming roller 85 located at the same position as in the color image forming mode, on the downstream side of the photosensitive member 1 of the monochrome image forming portion K. A transfer nip for primary transfer of a monochrome toner image is formed between the photosensitive member 1 and the belt 8.
In this state, a monochrome image can be formed using the image forming unit K.

  The nip forming rollers 84 and 85 can be constituted by, for example, a metal roller or a roller using an elastic material such as rubber. Although the required pressing force for the nip forming roller depends on the tension of the belt 8 and its path, an example in which the required pressing force is set to about half of the additional pressing force of the tension roller 83 can be given.

As long as there is no problem, the nip forming roller may be constituted by a roller or a fixed sliding member whose position is fixed, instead of being biased by a spring or the like.
Note that the primary transfer roller 6 corresponding to the cyan image forming portion C has different belt pressing force when forming a color image and when forming a monochrome image. The position of the roller may be regulated and the belt 8 may be disposed as a fixed roller. It is also possible to adopt a configuration in which the amount of spring contraction is increased to increase the pressure contact force.

  As can be seen from the above description, in the image forming apparatus PR, the nip forming roller 86 as used in the printer PR ′ shown in FIGS. 3 and 4 is unnecessary, and the apparatus configuration can be simplified and the cost can be reduced accordingly. it can.

  The image forming apparatus described above is a printer. However, the present invention is not limited to a printer as long as it is a tandem color image forming apparatus that employs an intermediate transfer belt. It can also be applied to multi-function machines that combine the above.

  The present invention is a tandem color image forming apparatus, as compared with an image forming apparatus provided with a transfer nip forming member dedicated to transfer nip formation on both the upstream side and the downstream side of the image carrier of the monochrome image forming unit. The present invention can be used to provide an image forming apparatus that has a simplified structure and is inexpensive.

PR, PR 'Printer 8 Intermediate transfer belt 81 Drive roller 82 Opposing roller 83 Tension roller 80 Cleaning device 9 Secondary transfer roller 10 Recording medium storage cassette 10r Medium supply roller S Recording medium FX Fixing device TR Timing roller pair Y Yellow image forming section M Magenta image forming part C Cyan image forming part K Black image forming part 1 Photoconductor 2 Charger 3 Exposure device 4 Developing device 5 Cleaning device 6 Primary transfer rollers 84, 85, 86 Transfer nip forming member 100 in the form of a roller Transfer roller Pressure contact / separation device 101 Drive frame a Long hole p Stopper pins F1, F2, F3, F4 Frames s1, s2, s3, s4, s5 Spring

Claims (3)

An endless intermediate transfer belt that is driven to rotate, and a plurality of image forming units that are sequentially arranged from the upstream side to the downstream side in the belt running direction along the intermediate transfer belt, and each image forming unit includes the intermediate transfer belt An electrostatic latent image corresponding to the assigned color is formed on the image carrier that is rotationally driven facing the transfer belt, and the electrostatic latent image is developed using the assigned color toner to form a toner image. The toner image can be primarily transferred onto the intermediate transfer belt by a primary transfer member facing the image carrier with the intermediate transfer belt interposed therebetween, and the primary transfer toner image is transferred to the secondary transfer member. In the monochrome image forming mode, the image carrier and the image carrier on the upstream side and the downstream side of the image carrier used for monochrome image formation among the plurality of image forming units. Primary transfer nip between the intermediate transfer belt A tandem type color image forming apparatus nip forming member for forming are arranged,
The image forming unit used for the monochrome image formation is an image forming unit located on the most downstream side in the traveling direction of the intermediate transfer belt, and an image carrier of an image forming unit adjacent to the image forming unit located on the most downstream side And the nip forming member to be arranged on the upstream side of the image carrier of the image forming unit located on the most downstream side in the monochrome image forming mode,
A transfer member pressure contact / separation device is further provided, and in the color image forming mode, the transfer member pressure contact / separation device transfers a primary transfer member corresponding to the image carrier of each image forming portion to the image carrier. In the monochrome image forming mode, the primary transfer member corresponding to the image carrier in the image forming unit other than the image forming unit located on the most downstream side is disposed in the color image forming mode position where the transfer belt is pressed against. A primary transfer member that is spaced apart from the formation mode position and that corresponds to the image carrier of the image forming unit adjacent to the image forming unit located on the most downstream side of the image carrier of the image forming unit located on the most downstream side. A tandem type color image forming apparatus, wherein the tandem type color image forming apparatus is arranged to move to a position of the nip forming member to be arranged on the upstream side . The transfer member pressure contact / separation device includes:
The primary transfer member corresponding to the image carrier of the image forming unit other than the image forming unit adjacent to the image forming unit located on the most downstream side among the image forming units other than the image forming unit located on the most downstream side. A first frame that supports each of the color image formation mode position and a position in the monochrome image formation mode spaced apart from the color image formation mode position so as to be reciprocally swingable.
The primary transfer member corresponding to the image carrier of the image forming unit adjacent to the image forming unit located on the most downstream side is separated from the color image forming mode position and the color image forming mode position and on the most downstream side. A second frame that supports the nip forming member to be disposed on the upstream side of the image carrier of the image forming unit positioned so as to reciprocate along the arrangement direction of the image forming unit;
A drive frame that drives the first frame and the second frame by reciprocation along the arrangement direction of the image forming units,
In the color image forming mode, the driving frame is arranged such that the first transfer member supported by the first frame and the second frame is disposed at the color image forming mode position, and in the monochrome image forming mode, the driving frame is located on the most downstream side. Each of the primary transfer members corresponding to the image carrier of the image forming unit other than the image forming unit adjacent to the image forming unit located on the most downstream side among the image forming units other than the positioned image forming unit is the color. The primary transfer member disposed at a position in the monochrome image forming mode separated from the image forming mode position and corresponding to the image carrier of the image forming unit adjacent to the image forming unit located on the most downstream side is a color image. It is moved and arranged at the position of the nip forming member to be arranged on the upstream side of the image carrier of the image forming unit located on the most downstream side, which is separated from the formation mode position. Wherein the first frame and the image forming apparatus according to claim 1, wherein the second frame is drivable frame. The tandem color image forming apparatus according to claim 1 or 2 , wherein the image forming unit used in the monochrome image forming mode is a black image forming unit that forms a toner image using black toner .

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