JP2008145638A - Color image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a tandem type color image forming apparatus capable of effectively preventing color shift by restraining variation in the rotating speed of an intermediate transfer belt by a simple configuration. <P>SOLUTION: The pressure contact force (a) applied to a photoreceptor drum 1d by a primary transfer roller 6d closest to a secondary transfer roller 9 among primary transfer rollers 6a to 6d disposed in pressure contact with photoreceptor drums 1a to 1d respectively via the intermediate transfer belt 8 is increased within a range that does not exceed two times the pressure contact force (b) applied to each of the photoreceptor drums 1a to 1c by the other primary transfer rollers 6a to 6c respectively. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  In the present invention, a plurality of image forming portions are arranged along an endless intermediate transfer belt, and toner images formed by sequentially laminating toner images of respective colors on the intermediate transfer belt are transferred onto a recording medium at a time. The present invention relates to an intermediate transfer type tandem color image forming apparatus.

  Conventionally, various image forming apparatuses have been proposed. Among them, there is a tandem type color image forming apparatus that forms a full color image by sequentially superimposing toner images of respective colors by a plurality of image forming units. Furthermore, the tandem type color image forming apparatus includes a direct transfer method for transferring a toner image formed by the image forming unit onto a recording medium conveyed by an endless conveying belt, and an endless type by a plurality of image forming units. There is an intermediate transfer method in which toner images are sequentially superimposed (primary transfer) on a continuous intermediate transfer belt and then transferred onto a recording medium at a time (secondary transfer).

  The intermediate transfer type tandem color image forming apparatus is a system in which one image is formed by a plurality of image forming units. Therefore, when the image forming speed is particularly increased, the image forming units are formed by the image forming units Pa to Pd. As a result, the alignment of the images of the respective colors deteriorates, and color misregistration of the toner images of the respective colors is likely to occur. This is due to periodic fluctuations in the rotation speed of the belt (rotation unevenness) due to the rotation of the rotation shaft of the drive roller that stretches the intermediate transfer belt, and the recording medium (paper) in the secondary transfer mechanism during continuous printing. For example, the belt rotation speed fluctuates due to rubbing between the intermediate transfer belt and the paper when entering.

  In view of this, various methods have been proposed for ensuring the color overlay accuracy of the toner images of the respective colors by suppressing fluctuations in the belt rotation speed. In other words, a method for suppressing fluctuations in belt speed by adding a moment of inertia of the photosensitive drum to the conveying belt by slowing down the belt is disclosed. Further, according to this method, the effect of making the surface speed of the photosensitive drum equal to the belt speed by using the play of the connecting portion between the photosensitive drum and the driving motor, or the driving motor connected to the photosensitive drum is electromagnetically braked. Is also known to have an effect of suppressing fluctuations in belt speed.

  When the surface speed of the photosensitive drum is made slower than the belt speed as in the methods of Patent Documents 1 and 2, the frictional force on the belt surface is transmitted to the photosensitive drum as a driving force. However, a cleaning unit blade, a toner sealing sponge, and the like are pressure-bonded to the surface of the photosensitive drum, and the resistance force and the driving force from the belt act in opposite directions. For this reason, the phase of the photosensitive drum varies within the range of the play amount of the connection portion with the drive motor due to the magnitude relationship between the drive force and the resistance force.

  For example, when the driving force from the belt is larger, the photosensitive drum receives a braking force from the driving motor, and the phase is delayed by the range of the play amount of the connecting portion with the driving motor. On the other hand, when the driving force from the belt is smaller, the photosensitive drum changes in a direction to obtain the driving force from the driving motor, so that the phase becomes faster by the range of the play amount of the connecting portion with the driving motor.

In other words, due to fluctuations in the driving force and resistance force from the belt, a color shift occurs by the amount of phase shift of the photosensitive drum. Further, when the photosensitive drum and the drive motor are connected via a speed reduction mechanism, the same problem occurs because mechanical play such as backlash of the speed reduction gear necessarily exists. Further, when the diameter of the photosensitive drum is small (40 mm or less), the moment of inertia of the photosensitive drum is small, and it is difficult to suppress the belt speed fluctuation by controlling the speed of the photosensitive drum.
JP 2002-268316 A JP 2003-29489 A

  In view of the above problems, the present invention provides a tandem color image forming apparatus capable of effectively preventing the occurrence of color misregistration by suppressing variations in the rotational speed of the intermediate transfer belt with a simple configuration. Objective.

  In order to achieve the above object, the present invention provides an endless shape that is stretched over a plurality of image forming units including an image carrier and a developing device, and a plurality of rollers including a driving roller, and moves along the image forming units. An intermediate transfer belt, a plurality of primary transfer rollers pressed against each image carrier via the intermediate transfer belt, and a secondary transfer roller for transferring a toner image formed on the intermediate transfer belt onto a recording medium The primary transfer closest to the secondary transfer roller in an image forming apparatus that transfers toner images sequentially stacked on the intermediate transfer belt to a recording medium at once using the secondary transfer roller The pressure contact force of the roller is larger than the pressure contact force of the other primary transfer rollers and is in a range not exceeding twice.

  In the image forming apparatus having the above-described configuration, the pressure contact force of the primary transfer roller closest to the secondary transfer roller is 1.5 to 2 times that of the other primary transfer roller. Yes.

  According to the first configuration of the present invention, since the intermediate transfer belt is strongly sandwiched between the primary transfer roller and the image carrier positioned in the immediate vicinity of the secondary transfer roller, the intermediate in the image forming unit closest to the secondary transfer roller. The vibration of the transfer belt is suppressed, and the transmission of vibration to each upstream image forming unit is also suppressed, so that the belt speed can be stabilized. Further, there is no increase in color misregistration due to a hollowing out phenomenon or a collapsed toner image.

  According to the second configuration of the present invention, in the image forming apparatus having the first configuration, the pressing force of the primary transfer roller closest to the secondary transfer roller is 1.5 times or more that of the other primary transfer rollers. By setting it to 2 times or less, the pressure contact force of the primary transfer roller is in an optimum range, and the belt speed can be further stabilized.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram showing the configuration of a tandem color image forming apparatus of the present invention. In the main body of the image forming apparatus 100, four image forming portions Pa, Pb, Pc, and Pd are sequentially arranged from the upstream side in the transport direction (the right side in FIG. 1). These image forming portions Pa to Pd are provided corresponding to images of four different colors (cyan, magenta, yellow, and black), and cyan, magenta, and yellow are respectively performed by charging, exposure, development, and transfer processes. And a black image are sequentially formed.

  Photosensitive drums 1a, 1b, 1c, and 1d that carry visible images (toner images) of the respective colors are disposed in the image forming portions Pa to Pd, and are formed on the photosensitive drums 1a to 1d. The transferred toner image is rotated clockwise in FIG. 1 by a driving means (not shown) and sequentially transferred (primary transfer) onto the intermediate transfer belt 8 that moves adjacent to each image forming unit. The image is transferred onto the paper P at a time (secondary transfer) by the next transfer roller 9 and further fixed on the paper P by the fixing unit 7 and then discharged from the apparatus main body. An image forming process for each of the photosensitive drums 1a to 1d is executed while rotating the photosensitive drums 1a to 1d counterclockwise in FIG.

  The paper P onto which the toner image is transferred is housed in a paper cassette 16 at the lower part of the apparatus, and is conveyed to the secondary transfer roller 9 via the paper feed roller 12a and the registration roller pair 12b. A sheet made of a dielectric resin is used for the intermediate transfer belt 8, and a belt in which both ends thereof are overlapped and joined to form an endless shape, or a belt without a seam (seamless) is used. Further, a cleaning blade 19 for removing toner remaining on the surface of the intermediate transfer belt 8 is disposed on the downstream side of the secondary transfer roller 9.

  Next, the image forming units Pa to Pd will be described. There are chargers 2a, 2b, 2c, and 2d for charging the photosensitive drums 1a to 1d and image information on the photosensitive drums 1a to 1d around and below the photosensitive drums 1a to 1d that are rotatably arranged. The exposure unit 4 for exposing the toner, the developing units 3a, 3b, 3c and 3d for forming toner images on the photosensitive drums 1a to 1d, and the developer (toner) remaining on the photosensitive drums 1a to 1d are removed. Cleaning units 5a, 5b, 5c and 5d are provided.

  When the image formation start is input by the user, first, the surfaces of the photosensitive drums 1a to 1d are uniformly charged by the chargers 2a to 2d, and then light is irradiated by the exposure unit 4 to each of the photosensitive drums 1a to 1d. An electrostatic latent image corresponding to the image signal is formed on the top. Each of the developing units 3a to 3d is filled with a predetermined amount of cyan, magenta, yellow, and black toner by a replenishing device (not shown). The toner is supplied onto the photosensitive drums 1a to 1d by the developing units 3a to 3d and electrostatically attached, whereby a toner image corresponding to the electrostatic latent image formed by exposure from the exposure unit 4 is formed. It is formed.

  After an electric field is applied to the intermediate transfer belt 8 at a predetermined transfer voltage, the cyan, magenta, yellow, and black toner images on the photosensitive drums 1a to 1d are transferred to the intermediate transfer belt 8 by the primary transfer rollers 6a to 6d. Primary transferred onto. These four color images are formed with a predetermined positional relationship predetermined for forming a predetermined full-color image. Thereafter, the toner remaining on the surfaces of the photosensitive drums 1a to 1d is removed by the cleaning units 5a to 5d in preparation for the subsequent formation of a new electrostatic latent image.

  The intermediate transfer belt 8 is stretched over a driven roller 10, a drive roller 11, and a tension roller 20, and the intermediate transfer belt 8 starts to rotate clockwise as the drive roller 11 is rotated by a drive motor (not shown). Then, the sheet P is conveyed from the registration roller 12b to the secondary transfer roller 9 provided adjacent to the intermediate transfer belt 8 at a predetermined timing, and the sheet P is conveyed at a nip portion (secondary transfer nip portion) with the intermediate transfer belt 8. A full color image is secondarily transferred onto P. The paper P on which the toner image is transferred is conveyed to the fixing unit 7.

  The sheet P conveyed to the fixing unit 7 is heated and pressurized when passing through the nip portion (fixing nip portion) of the pair of fixing rollers 13 to fix the toner image on the surface of the sheet P, and a predetermined full-color image is formed. It is formed. The paper P on which the full-color image is formed is distributed in the transport direction by the branching section 14 that branches in a plurality of directions. When an image is formed on only one side of the paper P, it is discharged as it is onto the discharge tray 17 by the discharge roller 15.

  On the other hand, when forming images on both sides of the paper P, a part of the paper P that has passed through the fixing unit 7 is once projected from the discharge roller 15 to the outside of the apparatus. Thereafter, the paper P is distributed to the paper transport path 18 by the branching section 14 by rotating the discharge roller 15 in the reverse direction, and is transported again to the secondary transfer roller 9 with the image surface reversed. Then, after the next image formed on the intermediate transfer belt 8 is transferred to the surface of the paper P on which the image is not formed by the secondary transfer roller 9 and conveyed to the fixing unit 7 to fix the toner image, It is discharged to the discharge tray 17.

  FIG. 2 is a schematic diagram showing a configuration around the image forming unit in the image forming apparatus of the present invention. Portions common to FIG. 1 are denoted by the same reference numerals and description thereof is omitted. Here, for convenience of explanation, the chargers 2a to 2d, the developing units 3a to 3d, the exposure unit 4, and the cleaning units 5a to 5d are not shown. In the present embodiment, a driving roller 11 that drives the intermediate transfer belt 8 is disposed on the downstream side in the transport direction (left side in the drawing) with respect to the respective photosensitive drums 1a to 1d, and among the primary transfer rollers 6a to 6d, The pressure contact force a of the primary transfer roller 6d closest to the secondary transfer roller 9 to the photosensitive drum 1d is larger than the pressure contact force b of the other primary transfer rollers 6a to 6c to the photosensitive drums 1a to 1c. Has been.

  With this configuration, the intermediate transfer belt 8 is strongly sandwiched between the primary transfer roller 6d and the photosensitive drum 1d located immediately upstream of the secondary transfer roller 9, so that the intermediate transfer belt 8 in the image forming unit Pd Vibration is suppressed, and transmission of vibration to the upstream image forming units Pa to Pc is also suppressed. Therefore, it is possible to stabilize the belt speed by suppressing the vibration of the rotation shaft of the drive roller 11 and the vibration of the intermediate transfer belt 8 due to the paper entering the nip portion between the intermediate transfer belt 8 and the secondary transfer roller 9. Thus, it is possible to effectively suppress the color misregistration of the toner images of the respective colors formed in the image forming units Pa to Pd.

  Here, if the pressure contact force is too strong than a predetermined value, a so-called hollow-out phenomenon occurs in which the toner in the center portion is lost with respect to the contour portion of the image. Further, the intermediate transfer belt 8 is strongly pressed against the photosensitive drum 1d to widen the nip, and the toner image formed on the drum is transferred onto the belt in a crushed state, thereby causing a large amount of color misregistration. Become. On the contrary, if the pressure contact force of the primary transfer roller 6d is weaker than a predetermined value, the effect of stabilizing the belt speed cannot be sufficiently obtained. Therefore, in order to stabilize the belt speed while suppressing an increase in color misregistration due to a hollowing out phenomenon or a collapse of the toner image, the pressure contact force a of the primary transfer roller 6d is the pressure contact of the other primary transfer rollers 6a to 6c. It is necessary to set it within a range that does not exceed twice the force b, and it is particularly preferably 1.5 to 2 times.

  Further, since the driving roller 11 is disposed on the downstream side with respect to the photosensitive drums 1a to 1d, the photosensitive drums 1a to 1d do not pull the intermediate transfer belt 8 with respect to the driving roller 11, and the driving roller 11 Variation in the speed of the intermediate transfer belt 8 due to uneven rotation can be suppressed. That is, even when the rotation of the driving roller 11 is slightly faster or slower, the tension of the intermediate transfer belt 8 is slightly changed between the photosensitive drum 1d and the driving roller 11, and each photosensitive drum. Since the tension of the intermediate transfer belt 8 between 1a and 1d hardly changes, there is no possibility of affecting the color overlay accuracy in the image forming portions Pa to Pd.

  FIG. 3 is a schematic side view showing a pressing mechanism of the primary transfer roller. Here, the primary transfer roller 6d will be described as an example, but the same applies to the other primary transfer rollers 6a to 6c, and the description thereof will be omitted. As shown in FIG. 3, the primary transfer roller 6 d has a structure in which the outer peripheral surface of the rotating shaft 21 is covered with a roller body 23 made of an elastic material, and both ends of the rotating shaft 21 are rotatably supported by bearing members 25. ing.

  A lower end of a compression spring 27 that urges the primary transfer roller 6 d toward the photosensitive drum 1 d is pressed against the upper surface of the bearing member 25, and an upper end of the compression spring 27 is pressed against a guide frame 29. The bearing member 25 is slidably supported along the guide frame 29, and the primary transfer roller 6d is only allowed to move in the vertical direction. The primary transfer roller 6d is connected to a high-voltage power source (not shown) via a bearing member 25 and a compression spring 27, and a toner image formed on the photosensitive drum 1d is primarily transferred onto the intermediate transfer belt 8. A predetermined transfer voltage is applied during transfer.

  As a method of increasing the pressure contact force of the primary transfer roller 6d, the compression spring 27 having a stronger urging force than the primary transfer rollers 6a to 6c may be used, or a spring receiving member that receives the upper end of the compression spring is used as a guide frame. 29, and the compression length of the compression spring 27 may be varied by selecting the thickness of the spring receiving member. Further, for example, a spring receiving member is provided so as to be slidable, and the spring receiving member is moved up and down by a driving mechanism including a driving motor and a cam or a gear to change the compression length of the compression spring 27 to thereby apply the pressure contact force. It is good also as a structure which can be adjusted.

  FIG. 4 is a schematic diagram illustrating another configuration around the image forming unit. Portions common to FIG. 1 are denoted by the same reference numerals and description thereof is omitted. In this example, a driving roller 11 for driving the intermediate transfer belt 8 is arranged on the upstream side (right side in the drawing) in the transport direction with respect to the photosensitive drums 1a to 1d. Also in this configuration, the pressure contact force a of the primary transfer roller 6d closest to the secondary transfer roller 9 is less than twice the pressure contact force b of the primary transfer rollers 6a to 6c, particularly 1.5 times as in the case of FIG. By setting it to be 2 times or less, it is possible to effectively suppress the color shift of the toner image without causing a void phenomenon.

  In the case where the driving roller 11 is arranged on the upstream side with respect to each of the photosensitive drums 1a to 1d as shown in FIG. 4, uneven rotation of the driving roller 11 causes the tension of the intermediate transfer belt 8 between the photosensitive drums 1a to 1d. It is easy to influence the condition. Therefore, the configuration in which the driving roller 11 is disposed on the downstream side of each of the photosensitive drums 1a to 1d as shown in FIG. 2 makes the tension of the intermediate transfer belt 8 more stable, and the intermediate state from the photosensitive drums 1a to 1d. Since the transfer of the toner image onto the transfer belt 8 is further stabilized, it is preferable.

  In addition, this invention is not limited to the said embodiment, A various change is possible in the range which does not deviate from the meaning of this invention. For example, the arrangement order of the image forming units Pa to Pd corresponding to each color of cyan, magenta, yellow, and black can be arbitrarily set. Although a color printer has been described as an example here, the present invention can be applied to various image forming apparatuses using an intermediate transfer belt such as a color copying machine and a facsimile.

When the image was formed using the intermediate transfer type color image forming apparatus shown in FIG. 1, the relationship between the pressure contact force of the primary transfer roller and the color shift was investigated. As test conditions, the pressure contact force b of the primary transfer rollers 6a to 6c was fixed to 150 g / cm ² , which is a minimum value in consideration of hollowing out. Then, the pressure contact force a of the primary transfer roller 6d closest to the secondary transfer roller is changed in six stages of 150, 180, 225, 270, 330, and 375 g / cm ² to form a test image, and whether or not there is a color shift. evaluated. As an evaluation method, the formed test image was visually observed, and a color shift amount of 100 μm or less was set as an appropriate range. The results are shown in FIG.

As is apparent from FIG. 5, when the pressure contact force a is 225 g / cm ² (a / b = 1.5) and 270 g / cm ² (a / b = 1.8), that is, 1.5 ≦ a / When b ≦ 2 was satisfied, the color misregistration amounts were 80 μm and 90 μm, both of which were within an appropriate range. On the other hand, when the pressing force a was 150 g / cm ² (a / b = 1), a color shift (140 μm) exceeding the proper range was observed. Further, when the pressure contact force a is 180 g / cm ² (a / b = 1.2), a color shift (120 μm) exceeding the appropriate range was observed, but when the pressure contact force a is equal to the pressure contact force b. Compared with the improvement of color shift, it was recognized.

Further, even when the pressure contact force a is 330 g / cm ² (a / b = 2.2) and 375 g / cm ² (a / b = 2.5), the color shift exceeding the appropriate range (160 μm and 180 μm). Was observed. This is presumably because the nip width of the intermediate transfer belt is increased, the toner image formed on the photosensitive drum is transferred to the belt in a crushed state, and the deviation amount is increased.

  From the above results, the ratio a / b between the pressure contact force a of the primary transfer roller closest to the secondary transfer roller and the pressure contact force b of the other primary transfer roller does not exceed 2, particularly 1.5 ≦ a / b. It was confirmed that color misregistration can be effectively prevented by setting the range to ≦ 2.

  The present invention includes a plurality of image forming units including an image carrier and a developing device, an endless intermediate transfer belt which is stretched around a plurality of rollers including a driving roller and moves along each image forming unit, and the intermediate transfer belt An intermediate transfer belt, comprising: a plurality of primary transfer rollers arranged to face each image carrier via a transfer belt; and a secondary transfer roller for transferring a toner image formed on the intermediate transfer belt onto a recording medium. In an image forming apparatus for transferring toner images sequentially stacked on a recording medium at once using a secondary transfer roller, the pressure contact force of the primary transfer roller closest to the secondary transfer roller is applied to the other primary transfer roller. The range is larger than the pressing force and does not exceed twice.

  As a result, it is possible to stabilize the belt speed by suppressing the vibration of the rotating shaft of the driving roller and the vibration of the intermediate transfer belt due to the paper entering the secondary transfer roller, and preventing the occurrence of the hollowing out phenomenon. Thus, it is possible to provide a tandem color image forming apparatus capable of effectively suppressing the color shift of the toner image.

  Further, by setting the pressure of the primary transfer roller closest to the secondary transfer roller to 1.5 to 2 times that of the other primary transfer roller, which is in the optimum range, a higher quality color image can be formed. An image forming apparatus is obtained.

1 is a schematic side view showing the configuration of a tandem color image forming apparatus of the present invention. FIG. 3 is a schematic side view showing a configuration around an intermediate transfer belt in the image forming apparatus of the present invention. These are schematic side views which show the pressurization mechanism of a primary transfer roller. FIG. 10 is a schematic side view showing another configuration example around the intermediate transfer belt. These are graphs showing the relationship with the amount of color misregistration when the ratio a / b of the pressure contact force of the primary transfer roller is changed in the embodiment.

Explanation of symbols

Pa to Pd Image forming section 1a to 1d Photosensitive drum (image carrier)
6a to 6d Primary transfer roller 8 Intermediate transfer belt 9 Secondary transfer roller 10 Driven roller 11 Drive roller 20 Tension roller 21 Rotating shaft 23 Roller body 25 Bearing member 27 Compression spring 29 Guide frame 100 Image forming apparatus

Claims (2)

A plurality of image forming units including an image carrier and a developing device;
An endless intermediate transfer belt that is stretched around a plurality of rollers including a driving roller and moves along each of the image forming units;
A plurality of primary transfer rollers pressed against each image carrier via the intermediate transfer belt;
A secondary transfer roller for transferring the toner image formed on the intermediate transfer belt onto a recording medium,
In the image forming apparatus for transferring the toner images sequentially laminated on the intermediate transfer belt to a recording medium at once using the secondary transfer roller,
The image forming apparatus, wherein a pressure contact force of the primary transfer roller closest to the secondary transfer roller is larger than a pressure contact force of other primary transfer rollers and does not exceed twice.   The image forming apparatus according to claim 1, wherein a pressure contact force of the primary transfer roller closest to the secondary transfer roller is 1.5 to 2 times that of the other primary transfer roller.

Images