JP2009042631A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus for surely preventing color misregistration without providing a tension varying means and without increasing the tension of an intermediate transfer belt. <P>SOLUTION: The image forming apparatus 1 includes: the intermediate transfer belt 2 that carries and conveys a toner image; a transfer roller 7 that comes in press contact with a driving roller 3a for driving the intermediate transfer belt 2 through the intermediate transfer belt 2, for transferring the toner image on the intermediate transfer belt 2 to a recording medium. In the image forming apparatus 1, a backup roller 11 coming in press contact with a roller 10 for supporting the intermediate transfer belt 2 while facing the roller 10 through the intermediate transfer belt 2 is disposed on the upstream side of the transfer roller 7 in a paper passing direction. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image forming apparatus using an intermediate transfer belt in a copying machine, a laser beam printer, a FAX, a complex machine thereof, or the like.

  In general, in the image forming apparatus, as shown in FIG. 7, the recording medium S is conveyed by the timing roller 51 and the registration roller 52, and is conveyed to the transfer roller 54 while being in contact with the intermediate transfer belt 53. As the speed of the entire image forming system increases, the impact that the recording medium S directly enters between the intermediate transfer belt support roller 55 and the drive roller 56, which is not supported by the roller of the intermediate transfer belt 53, increases. For this reason, the leading edge of the recording medium bends the intermediate transfer belt 53, the photosensitive member 58 is shaken at that time, and the speed of the intermediate transfer belt 53 is changed, so that the image just before the primary transfer from the photosensitive member 58 is also black. There is a problem in that color misregistration occurs due to deviation from the writing pitch.

  As a means for preventing this, for example, Patent Document 1 describes an image forming apparatus that can maintain a belt tension and its speed constant by providing a tension fluctuation absorbing means, and can prevent color misregistration. Patent Document 2 describes an image forming apparatus that can maintain a belt tension constant by providing a tension roller and can prevent color overlay deviation during transfer.

  However, the image forming apparatus described in Patent Document 1 has a problem that when the thick paper is passed, the leading edge of the recording medium directly enters the belt, and thus color misregistration occurs due to an impact on the intermediate transfer belt. It was. Further, in the image forming apparatus described in Patent Document 2, when the tension of the intermediate transfer belt is increased, there is a problem that the belt can be wrinkled depending on variations in component accuracy, and quality problems such as image noise occur.

Japanese Patent Laid-Open No. 10-268595 JP-A-6-115752

  The present invention has been made in view of the above-described conventional problems, and provides an image forming apparatus capable of reliably preventing color overlay deviation without providing a tension fluctuation absorbing unit or increasing the tension of an intermediate transfer belt. The task is to do.

  As a means for solving the above-described problems, an image forming apparatus according to the present invention includes an intermediate transfer belt that conveys a toner image while carrying the toner image, and a driving roller that drives the intermediate transfer belt via the intermediate transfer belt. And a transfer roller for transferring the toner image on the intermediate transfer belt to a recording medium, and the intermediate transfer belt via the intermediate transfer belt on the upstream side of the transfer roller. A backup roller that faces the roller that supports the belt and presses against the roller is disposed. Here, the recording medium is not limited to plain paper but includes cardboard and OHP sheets.

  By arranging the backup roller, the conveyed paper does not directly enter the portion of the intermediate transfer belt that is not supported by each roller, but is once held between the backup roller and the intermediate transfer belt support roller. Prevents color misregistration due to impact when directly entering the intermediate transfer belt.

  As a specific means, the sheet conveying force of the transfer roller is smaller than the sheet conveying force of the registration roller disposed on the upstream side of the backup roller, and the sheet conveying force of the backup roller is equal to the transfer roller. It is smaller than the paper conveying force of the roller.

  Because of these paper conveyance force relationships, the paper and intermediate transfer belt are pulled by the transfer roller between the transfer roller and the backup roller, and the paper loops between the backup roller and the registration roller due to the loosening of the paper. Therefore, color misregistration is prevented.

  The paper conveying force can be changed by changing the pressure contact force, the outer diameter, and the material.

  That is, the pressure contact force of the backup roller is made smaller than the pressure contact force of the transfer roller.

  Further, the outer diameter of the backup roller is equal to or smaller than the outer diameter of the transfer roller.

  As a result, when the transfer roller is an elastic body, the sheet conveying force of the transfer roller is larger than the sheet conveying force of the backup roller regardless of the pressure contact force.

  Further, the backup roller is made of metal.

  If an elastic roller is used, the variation in the outer diameter accuracy is large, so the paper transport force relationship between the rollers may not be established. Thus, it is possible to ensure the paper conveyance force relationship.

  The backup roller is preferably capable of being forcibly driven alone.

  If the configuration is such that the backup roller is driven by branching the drive from the transfer roller, the transfer roller is an elastic body, so that the variation in outer diameter accuracy is large and the sheet conveying force tends to vary. By driving the backup roller independently, it is possible to maintain the paper conveyance force relationship between the rollers.

  Also, if the backup roller is driven by the intermediate transfer belt, an image leading edge position deviation, a sub-scanning direction magnification deviation, JAM, and the like occur due to the fact that the sheet cannot be conveyed when a thick sheet is passed. By forcibly driving, these can be prevented and the cardboard conveyance performance can be ensured.

  It is preferable that the backup roller can be pressed and separated independently from the intermediate transfer belt.

  Specifically, the front end of the paper is prevented from directly entering the intermediate transfer belt by pressing the backup roller against the intermediate transfer belt when the paper is passed. The toner remaining on the intermediate transfer belt can be prevented from adhering to the backup roller by separating the backup roller from the intermediate transfer belt except when the paper is passed.

  It is preferable that the backup roller is detachable from the intermediate transfer belt after a predetermined time after the front end of the recording medium enters the transfer portion formed by the pressure contact between the drive roller and the transfer roller.

  If the backup roller is always in pressure contact, the speed difference between the backup roller and the intermediate transfer belt may occur due to the accuracy of the transfer roller parts, and there may be a problem such as image rubbing. Since the backup roller can be separated from the intermediate transfer belt after a predetermined time, these problems can be prevented.

  The backup roller is preferably always in pressure contact with the intermediate transfer belt.

  As a result, a load is applied to the intermediate transfer belt even during the image stabilization process, and the color overlay deviation correction calculation can be performed under a condition closer to the time of sheet passing.

  The backup roller is preferably subjected to a fluororesin surface treatment with good toner releasability.

  The backup roller is preferably provided with a cleaning member that scrapes off the toner adhering to the backup roller.

  As a result, the toner remaining on the intermediate transfer belt can be prevented from adhering to the surface of the backup roller, and quality problems such as contamination of the back surface of the paper can be prevented.

  Further, it is preferable that an image reading sensor for stabilizing an image is disposed between the backup roller and the secondary transfer roller.

  If an image reading sensor is arranged on the intermediate transfer belt side between the registration roller and the backup roller, there is no escape space for the paper loop, and the image is rubbed or sub-scanned due to the paper loop being pushed toward the backup roller or the registration roller. A magnification shift or the like occurs. By disposing the image reading sensor between the backup roller and the secondary transfer roller, an escape place for the paper loop can be formed, and problems such as image rubbing and sub-scanning direction magnification deviation can be prevented.

  According to the image forming apparatus of the present invention, since the backup roller that rotates and holds the recording medium with the intermediate transfer belt support roller is provided, it is possible to prevent color misregistration without increasing the tension of the intermediate transfer belt.

  Next, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 shows an image forming apparatus 1 according to an embodiment of the present invention. First, a schematic configuration of the image forming apparatus 1 will be described. The image forming apparatus 1 includes an intermediate transfer belt 2 as an image carrier at a substantially central portion inside. The intermediate transfer belt 2 is supported by the outer peripheral portions of the rollers 3a and 3b and is driven to rotate in the direction of arrow A. The drive roller 3a is connected to a drive motor (not shown), and the driven roller 3b is driven to rotate as the drive roller 3a rotates. Below the lower horizontal portion of the intermediate transfer belt 2, four image forming units 4Y, 4M, 4C, and 4K respectively corresponding to yellow (Y), magenta (M), cyan (C), and black (K) colors. Are arranged side by side along the intermediate transfer belt 2. Each of the image forming units 4Y, 4M, 4C, and 4K includes photosensitive drums 5Y, 5M, 5C, and 5K, respectively. Around the photosensitive drums 5Y, 5M, 5C, and 5K, primary transfer rollers 6Y, 6M, 6C, and 6K that face the photosensitive drums 5Y, 5M, 5C, and 5K with the intermediate transfer belt 2 interposed therebetween, respectively. Has been placed. A portion of the intermediate transfer belt 2 supported by the driving roller 3a is in contact with a secondary transfer roller (transfer roller) 7, and a nip portion between the secondary transfer roller 7 and the intermediate transfer belt 2 is a transfer portion 8. It has become. The secondary transfer roller 7 detects a secondary transfer roller pressure contact / separation drive motor 7a for driving the pressure contact / separation operation of the secondary transfer roller 7 and whether the secondary transfer roller 7 is pressed against or separated from the drive roller 3a. A secondary transfer roller pressure contact / separation sensor 9 is connected. An intermediate transfer belt support roller 10 that supports the intermediate transfer belt 2 and a backup roller 11 that is in pressure contact with the intermediate transfer belt support roller 10 via the intermediate transfer belt 2 are disposed below the drive roller 3a. The backup roller 11 is installed in a pressure contact / separation mechanism 12 that performs a pressure contact or separation operation of the backup roller 11 with the intermediate transfer belt support roller 10, and the backup roller 11 is in pressure contact with the intermediate transfer belt support roller 10. Alternatively, a backup roller pressure contact / separation sensor 13 for detecting whether or not they are separated is connected. A timing roller 15 that conveys the paper S to the backup roller 11 and a registration roller 16 are disposed upstream of the backup roller 11 in the paper passage path 14 a so as to be in pressure contact with each other. A timing motor 17 is connected to the registration roller 16 to measure the timing of conveying the sheet to the transfer unit 8. A pre-fixing loop detection sensor 18 is disposed in the paper passing path 14 b on the downstream side of the transfer unit 8. Further, a fixing roller 19 and a pressure roller 20 are disposed on the downstream side. A pressure contact portion between the fixing roller 19 and the pressure roller 20 is a fixing nip region 21.

  Under the image forming apparatus 1, four types of first, second, third, and fourth types of paper feeding units 22a, 22b, 22c, and 22d are provided. The first paper feeding unit 22a and the second paper feeding unit 22b is arranged up and down, and the third sheet feeding unit 22c and the fourth sheet feeding unit 22d are arranged on the left and right below the second sheet feeding unit 22b. The sheets S stacked and accommodated in each of the sheet feeding units 22a, 22b, 22c, and 22d are sent out one by one from the top to the sheet passing path 14c. A circulation path 23 is provided on the side of the image forming apparatus 1. The single-side printed paper that is switched back by the paper discharge roller 24 is conveyed downward through the circulation path 23, and is again conveyed upward through the paper passage path 14b with the unprinted surface facing the intermediate transfer belt 2 side. It has become so. A manual paper feeding device 25 is provided below the circulation path 23. A sheet feeding path 26c from the manual sheet feeding device 25 and the sheet feeding units 22a, 22b, 22c, and 22d includes a sheet feeding roller 26 for feeding the sheet S as shown in FIG. A connected paper feed motor 26a is provided. Reference numerals 27Y, 27M, 27C, and 27K denote toner cartridges that supply toner of respective colors to the image forming units 4Y, 4M, 4C, and 4K. Reference numeral 28 denotes an exposure unit 28 that exposes the image forming units 4Y, 4M, 4C, and 4K based on image signals.

  Next, a schematic operation of the image forming apparatus 1 having the above configuration will be described. The color print data obtained by reading the image by the image reading unit 29 or the image data output from a personal computer or the like is subjected to predetermined signal processing by the exposure unit 28, and then yellow (Y), magenta (M ), Cyan (C), and black (K) image signals are supplied to the image forming units 4Y, 4M, 4C, and 4K. In each of the image forming units 4Y, 4M, 4C, and 4K, a laser beam modulated by an image signal is projected onto the respective photosensitive drums 5Y, 5M, 5C, and 5K, thereby forming an image latent image. Yellow, magenta, cyan, and black toner images are formed on the respective photosensitive drums 5Y, 5M, 5C, and 5K. The formed yellow, magenta, cyan, and black toner images are primarily transferred onto the moving intermediate transfer belt 2 in sequence by the action of the primary transfer rollers 6Y, 6M, 6C, and 6K. The superimposed toner image formed on the intermediate transfer belt 2 in this way reaches the transfer unit 8 as the intermediate transfer belt 2 moves. In the transfer unit 8, the superimposed toner images are transferred from the paper supply units 22 a, 22 b, 22 c, 22 d or the manual paper supply device 25 through the registration roller 16 and the backup roller 11 by the action of the secondary transfer roller 7. Secondary transfer is performed collectively on the supplied paper S. Next, the sheet S on which the toner image is secondarily transferred passes through the pre-fixing loop detection sensor 18 and reaches the fixing nip region 21. In the fixing nip region 21, the toner image is fixed on the paper S by the action of the fixing roller 19 and the pressure roller 20. The paper S on which the toner image has been fixed is discharged to the paper discharge tray 31 via the paper discharge roller 24.

  Subsequently, the conveyance operation of the sheet S by the intermediate transfer belt 2 and each roller will be described. Referring to FIG. 2A, the sheet S conveyed from the sheet feeding units 22a, 22b, 22c, 22d or the manual sheet feeding device 25 is first nipped by the timing roller 15 and the registration roller 16, and then the intermediate transfer belt. It is conveyed toward 2. At this time, the timing at which the sheet S is conveyed to the backup roller 11 by the timing motor 17 is measured. The sheet S conveyed to the intermediate transfer belt 2 is once held between the intermediate transfer belt support roller 10 and the backup roller 11. Thus, the sheet S does not directly enter the portion of the intermediate transfer belt 2 that is not supported by the rollers 3a and 10, and color misregistration due to the impact of direct entry is prevented.

  The backup roller 11 is made of metal. If an elastic roller is used, the variation in the outer diameter accuracy is large, so the paper transport force relationship between the rollers may not be established. Thus, it is possible to ensure the paper conveyance force relationship.

  The backup roller 11 is connected to a backup roller drive motor 11a. Thereby, the backup roller 11 can be driven independently. Assuming that the backup roller 11 is driven by branching the drive from the secondary transfer roller 7, the secondary transfer roller 7 is an elastic body, so that the variation in outer diameter accuracy is large and the sheet conveying force is likely to vary. By driving the backup roller 11 independently, it is possible to maintain the paper conveyance force relationship between the rollers.

  Further, the backup roller 11 is arranged to be forcibly driven. If the backup roller 11 is driven and driven by the intermediate transfer belt 2, an image leading edge position deviation, a sub-scanning direction magnification deviation, JAM, and the like are generated due to the fact that the sheet S cannot be conveyed when a thick sheet is passed. By forcibly driving, these can be prevented and the cardboard conveyance performance can be ensured.

  The backup roller 11 can be separately pressed against and separated from the intermediate transfer belt 2 via the backup roller pressure and separation mechanism 12. Specifically, the front end of the paper is prevented from directly entering the intermediate transfer belt 2 by pressing the backup roller 11 against the intermediate transfer belt 2 when the paper is passed. The toner remaining on the intermediate transfer belt 2 can be prevented from adhering to the backup roller 11 by separating the backup roller 11 from the intermediate transfer belt 2 except when paper is passed.

  Referring to FIG. 2B, once the paper S is held between the backup roller 11 and the intermediate transfer belt support roller 10, the paper S is adjusted in order to adjust the conveyance speed of the paper S to the rotational speed of the intermediate transfer belt 2. Forms a paper loop between the intermediate transfer belt support roller 10 and the timing roller 15 toward the inside of the intermediate transfer belt 2. For this reason, if the image reading sensor 32 is disposed between the intermediate transfer belt support roller 10 and the timing roller 15 and in the vicinity of the loop locus of the paper S, the escape loop of the paper loop cannot be formed, image rubbing, sub-scanning direction magnification deviation, etc. The problem occurs. Therefore, by disposing the image reading sensor 32 between the backup roller 11 and the secondary transfer roller 7, it is possible to form a escape area for the paper loop and prevent problems such as image rubbing and sub-scanning direction magnification deviation. In this embodiment, the image reading sensor 32 detects an image pattern, and the detected sensor output signal calculates the image displacement amount of yellow, magenta, and cyan with reference to black, and controls the image writing timing from the calculation result. Used for.

  A smoothing member 33 that is in contact with the intermediate transfer belt 2 and smoothes the surface of the image reading sensor 32 is disposed facing the image reading sensor 32. Thereby, it is possible to reduce the reading error of the image stabilization pattern due to the shaking of the intermediate transfer belt 2.

  Subsequently, the sheet S that is nipped by the backup roller 11 and the intermediate transfer belt support roller 10 and conveyed downstream along the intermediate transfer belt 2 is nipped by the driving roller 3 a and the secondary transfer roller 7. The At this time, the sheet conveying force of the registration roller 16 is larger than the sheet conveying force of the secondary transfer roller 7, and the sheet conveying force of the secondary transfer roller 7 is larger than the sheet conveying force of the backup roller 11.

  The pressure contact force of the backup roller 11 is smaller than the pressure contact force of the secondary transfer roller 7. For example, the pressure contact force of the backup roller 11 is about 10 N, and the pressure contact force of the secondary transfer roller 7 is about 50 N.

  Further, the outer diameter of the backup roller 11 is equal to or smaller than the outer diameter of the secondary transfer roller 7. For example, the outer diameter of the backup roller 11 is 21.8 mm, and the outer diameter of the secondary transfer roller 7 is 24 mm. Accordingly, when the secondary transfer roller 7 is an elastic body, the paper conveyance force of the secondary transfer roller 7 is larger than the paper conveyance force of the backup roller 11 under any conditions of the pressure contact force.

  Due to the above-described sheet conveyance force relationship, the sheet S and the intermediate transfer belt 2 are pulled by the secondary transfer roller 7 between the secondary transfer roller 7 and the backup roller 11, and the backup roller 11 and the registration roller 16 are in contact with each other. In the meantime, the sheet S is slackened, so that a sheet loop is formed, thereby preventing color misregistration. However, the pressure contact force and the outer diameter of the backup roller 11 and the secondary transfer roller 7 are not particularly limited as long as the conveyance force relationship of the paper S between the rollers 11 and 7 can be ensured.

  The pressure contact / separation mechanism 12 of the backup roller 11 will be described with reference to FIG. The backup roller 11 is fitted and supported at both ends so as to be rotatable to the tip of a lever 34 that moves in the front-rear direction. The lever 34 has a protruding piece 35 directed inward at the rear end. A compression coil spring 36 abuts on the rear surface of the projecting piece 35 to urge the lever 34 forward. A rotatable cam 37 is in contact with the front surface of the projecting piece 35. The cam 37 is fixedly connected to both ends of the shaft 38. The shaft 38 has an idle gear 39 in the vicinity of the center, and the idle gear 39 meshes with a drive gear 42 of a pressure contact / separation drive motor 41. A backup roller pressure contact / separation sensor 13 for detecting whether the backup roller 11 is pressed against or separated from the intermediate transfer belt support roller 10 between the idle gear 39 and the cam 37 is attached to the shaft 38. The backup roller pressure contact / separation sensor 13 includes an irradiation unit 13a that emits sensor light and a light shielding plate 13b that blocks the sensor optical path. The light shielding plate 13b that rotates together with the shaft 38 blocks the sensor optical path irradiated between the grooves of the concave portion toward the shaft 38 of the irradiation unit 13a, thereby detecting whether the backup roller 11 is pressed or separated.

  When the pressure contact / separation drive motor 41 rotates in the forward direction, the drive is transmitted from the drive gear 42 to the idle gear 39, and the cam 37 rotates at the same time as the shaft 38 rotates. At this time, since the projecting piece 35 in contact with the cam 37 and the compression coil spring 36 moves forward, the backup roller 11 also moves forward via the lever 34 and presses against the intermediate transfer belt support roller 10. When the pressure contact / separation drive motor 41 rotates in the reverse direction, the protrusion 35 moves rearward due to the rotation of the cam 37, the backup roller 11 also moves rearward via the lever 34, and is separated from the intermediate transfer belt support roller 10.

  The backup roller pressure contact / separation mechanism 12 is not limited to the above embodiment, and various modifications are possible. As long as the performance of performing the pressing / separating operation of the backup roller 11 is ensured, the structure of the backup roller pressing / separating mechanism 12 is not limited to that of the embodiment.

  Next, the operation of pressing and separating the backup roller 11 will be described with reference to FIG. The pressing operation of the backup roller 11 and the secondary transfer roller 7 starts and ends at the same time, but the backup roller 11 starts the separation operation at an earlier timing than the secondary transfer roller 7 and ends. Specifically, when the user presses a print button (not shown), the paper feed motor 26a is turned on at time t1 shown in FIG. 4A, the leading edge of the paper S reaches the timing roller 15, In order to adjust the inclination of the leading edge of the paper S (paper loop creation) and the image writing timing, the paper feed motor 26a is fixed to the ON state until a predetermined time. Thereafter, at timing t4, the timing motor 17 is turned on, and the sheet S is conveyed to the transfer unit 8. While the paper feed motor 26a is in an ON state and before t3 when an image print request from a controller (not shown) is output, the backup roller pressure contact / separation drive motor 41 is rotated at time t2 to start pressure contact. The pressing operation is completed at time t3 when the backup roller pressing / separating sensor 13 detects the OFF edge. At this time, the backup roller 11 and the secondary transfer roller 7 simultaneously start the pressure contact operation and end simultaneously.

  Regarding the separation operation, the backup roller 11 detects the ON edge of the timing motor 17 that drives the registration roller 16 at the time t4, and then the time t5 after a predetermined time when the leading edge of the paper S passes the transfer unit 8. Since the sheet S can be conveyed by the secondary transfer roller 7 and the registration roller 16, the backup roller pressure contact / separation drive motor 41 is rotated and the backup roller 11 starts the separation operation. At time t6, the backup roller After detecting the ON edge of the pressure contact / separation sensor 13, the separation operation is terminated. Thereafter, the secondary transfer roller 7 starts the separation operation at time t7 and ends at time t8. If the backup roller 11 is always in pressure contact, the speed difference between the backup roller 11 and the intermediate transfer belt 2 may occur due to the component accuracy of the secondary transfer roller 7, which may cause problems such as image rubbing. These disadvantages can be prevented by separating the backup roller 11 from the intermediate transfer belt 2 at time t7. Further, by separating the backup roller 11 ahead of the secondary transfer roller 7, it is possible to prevent the toner remaining on the intermediate transfer belt 2 from adhering in addition to image rubbing and the like. For this reason, the surface treatment of the backup roller 11 and the installation of a cleaning member become unnecessary, and the cost of the backup roller 11 can be reduced.

  In another embodiment, as shown in FIG. 4B, the backup roller 11 and the secondary transfer roller 7 may be operated simultaneously. In this embodiment, the pressure contact / separation mechanism is used in combination by the backup roller 11 and the secondary transfer roller 7. Regarding the separation operation of the backup roller 11 and the secondary transfer roller 7, ON / OFF of the pre-fixing loop detection sensor 18 in which the leading edge of the sheet S conveyed from the secondary transfer roller 7 is arranged before the fixing nip region 21 enters. After t9 detecting the state and detecting the OFF edge of the pre-fixing loop detection sensor 18, the pressure contact / separation drive motor is rotated to enter the separation operation at the time t10 after a predetermined time has elapsed, and the secondary transfer roller pressure contact The separation operation is terminated at time t11 when the ON edge of the separation sensor 9 and the backup roller pressure contact separation sensor 13 is detected. At this time, the backup roller 11 and the secondary transfer roller 7 simultaneously start the separation operation and end simultaneously. In this way, by using the pressure contact / separation mechanism in combination with the backup roller 11 and the secondary transfer roller 7, it is possible to reduce the cost compared to operating each independently.

  In the above-described embodiment, the backup roller 11 has the pressure contact / separation mechanism 12. However, the backup roller 11 may not always have the pressure contact / separation mechanism 12 and may be urged so as to be always in pressure contact with the intermediate transfer belt support roller 10. . In this case, the metal backup roller 11 is provided with a cleaning member (not shown) that performs fluororesin-based surface treatment with good toner releasability or scrapes off the toner adhering to the backup roller 11. ing. Thereby, it is possible to prevent the toner remaining on the intermediate transfer belt 2 from adhering to the surface of the backup roller 11 and to prevent the occurrence of quality problems such as the backside contamination of the paper S.

  In order to test the effect of the present invention, the inventors used black in the sub-scanning direction on A3 size paper with a paper feed speed of 240 mm / sec and a weighing of 220 g when using the conventional technique and the present invention. An experiment was conducted to measure the shift amount of each color (C, M, Y). As shown in FIG. 5, in the case of the conventional technique in which the backup roller 11 is not installed, the color image significantly precedes the black image from the position of 130 mm from the front end of the paper S, and the color shift is up to a visible amount of 0.15 mm. Occurred. As shown in FIG. 6, when the backup roller 11 according to the present invention is installed, the leading of the color image from the 130 mm position is remarkably reduced, and the amount is not noticeable as color misregistration (0.1 mm or less). It became possible.

1 is a schematic diagram showing an image forming apparatus according to the present invention. (A) Side view showing a sheet being conveyed before being nipped by a backup roller. (B) Side view showing a sheet being nipped and conveyed by a backup roller. The perspective view of a backup roller press-contact separation mechanism. (A) Graph showing control timing related to the pressure contact and separation operation of the backup roller in a form in which the backup roller and the secondary transfer roller are separately separated. (B) Pressure backup and separation in the form in which the backup roller and the secondary transfer roller are simultaneously separated. The graph which shows the control timing regarding operation | movement. 6 is a graph showing color misregistration in a conventional image forming apparatus. 3 is a graph showing color misregistration in the image forming apparatus of the present invention. FIG. 6 is a side view of an intermediate transfer belt that conveys paper in a conventional image forming apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 2 Intermediate transfer belt 3a Drive roller 7 Secondary transfer roller (transfer roller)
DESCRIPTION OF SYMBOLS 8 Transfer part 10 Intermediate transfer belt support roller 11 Backup roller 12 Backup roller press-contact separation mechanism 16 Registration roller 32 Image reading sensor

Claims (12)

An intermediate transfer belt that conveys the toner image while being carried, and a transfer roller that is pressed against the driving roller that drives the intermediate transfer belt via the intermediate transfer belt and transfers the toner image on the intermediate transfer belt to a recording medium In an image forming apparatus comprising:
An image forming apparatus, wherein a backup roller that faces and presses a roller that supports the intermediate transfer belt is disposed on the upstream side of the transfer roller through the intermediate transfer belt.   A sheet conveying force of the transfer roller is smaller than a sheet conveying force of a registration roller arranged on the upstream side of the backup roller, and a sheet conveying force of the backup roller is smaller than a sheet conveying force of the transfer roller. The image forming apparatus according to claim 1.   The image forming apparatus according to claim 2, wherein a pressure contact force of the backup roller is smaller than a pressure contact force of the transfer roller.   The image forming apparatus according to claim 2, wherein an outer diameter of the backup roller is equal to or smaller than an outer diameter of the transfer roller.   The image forming apparatus according to claim 2, wherein the backup roller is made of metal.   The image forming apparatus according to claim 1, wherein the backup roller can be forcibly driven independently.   The image forming apparatus according to claim 1, wherein the backup roller can be pressed and separated independently from the intermediate transfer belt.   The backup roller can be separated from the intermediate transfer belt after a predetermined time after a leading edge of the recording medium enters a transfer portion formed by pressure contact between the drive roller and the transfer roller. 8. The image forming apparatus according to 7.   The image forming apparatus according to claim 1, wherein the backup roller is always in pressure contact with the intermediate transfer belt.   The image forming apparatus according to claim 9, wherein the backup roller is subjected to a fluororesin-based surface treatment with good toner releasability.   The image forming apparatus according to claim 9, wherein the backup roller is provided with a cleaning member that scrapes off toner adhering to the backup roller.   The image forming apparatus according to claim 1, wherein an image reading sensor for stabilizing an image is disposed between the backup roller and the transfer roller.

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