JP2002072717A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device with which the rationalization and stabilization of a secondary transfer can be realized in the case of adopting an intermediate transfer method, without causing the increase in the size and cost of the device. SOLUTION: The measurement of combined resistance of a secondary transfer part by a resistance measuring device 50 is applied to a plurality of recording paper sheets P transported from a paper sheet housing part. While setting a proper secondary transfer bias by a bias correction means 70 on the basis of such combined resistance, the recording paper sheets P after resistance measuring are sent to a paper sheet resending path for resending to the secondary transfer part.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus typified by a copier, a printer, and the like which employ an intermediate transfer system, and a multifunction machine having a copying function and a printing function. It is possible to more appropriately set the secondary transfer bias applied in the secondary transfer device involved in the transfer method, and it is possible to separate the intermediate transfer body in the intermediate transfer method without using a dedicated release agent coating device. The present invention relates to an image forming apparatus to which a mold agent can be applied.

[0002]

2. Description of the Related Art In recent years, the number of color copying machines and color printers employing an intermediate transfer system has been increasing. However, an image forming apparatus employing this type of intermediate transfer system basically employs the following method. The formation is performed.

That is, after a toner image of each color component constituting a color image is formed on an image carrier such as a photosensitive member by an image forming apparatus using an electrophotographic technique or the like, each toner image on the image carrier is formed. Primary transfer is performed by a primary transfer device on an intermediate transfer member made of a dielectric material or the like, and then a (multiple) toner image on the intermediate transfer member is transferred onto various recording papers such as plain paper, coated paper, and OHP sheets by a secondary transfer device. The recording paper on which the toner image has been secondarily transferred is sent to a fixing device disposed downstream of the secondary transfer device in the paper transport direction, and the toner image is fixed on the recording paper. It is supposed to. Note that as an image forming apparatus for forming the color image, a type in which one image carrier is used and a plurality of toner images are sequentially formed on the one image carrier, and a plurality of image carriers are used. One color toner image is formed on each image carrier.
They are broadly divided into types that are formed by distributing colors one by one.

In an image forming apparatus employing such an intermediate transfer system, as a secondary transfer device, for example, a secondary transfer roll is disposed on the transfer surface (front surface) side of the intermediate transfer member so as to be able to press against it. At the same time, a backup roll for supporting the intermediate transfer member from the back surface side is disposed on the non-transfer surface (rear surface) side of the intermediate transfer member facing the secondary transfer roll, and further, between the secondary transfer roll and the backup roll. A secondary transfer bias is applied to the toner to form a transfer electric field (an electric field in a direction in which the toner moves from the intermediate transfer member to the recording medium), thereby electrostatically transferring the toner image on the intermediate transfer member to the recording medium. What is designed to be transferred is mainly used.

[0005]

In recent years, as the demand for color printing has increased, a wide variety of recording papers for better forming color images have been provided and used in color image forming apparatuses. It is becoming. With the use of various types of recording paper, in an image forming apparatus employing the above-described intermediate transfer method, when the toner image is secondarily transferred from the intermediate transfer body to the recording paper, the secondary transfer is performed. When image formation is performed under the same conditions, there is a problem in that the image quality may be degraded due to poor secondary transfer because physical properties such as thickness, basis weight, resistance characteristics, and smoothness of each recording paper are different. .

In view of the above, although an intermediate transfer method has not been conventionally employed, an image forming apparatus which transfers a toner image formed on a photoreceptor to a transfer material (recording paper) by a transfer roll starts its image forming operation. Before trial-flowing the transfer material, measure the volume resistance between the photoconductor and the transfer roll in a state where the transfer material is sandwiched between the photoconductor and the transfer roll, and appropriately determine the voltage applied to the transfer roll. An image forming apparatus having such a configuration has been proposed (Japanese Patent Laid-Open No. 6-161).
295).

However, the image forming apparatus for trial-flowing the transfer material does not consider the secondary transfer when the intermediate transfer method is employed. In this image forming apparatus, since the transfer material is temporarily stored in a tray after the resistance measurement, a tray for storing the transfer material is needed, and the tray and a paper transport path to the tray are required. The size and cost of the apparatus are increased due to the additional provision of such devices. Further, in this image forming apparatus, an example is shown in which one transfer material is trial-flowed for resistance measurement. However, in the image forming apparatus, usually, in a paper storage unit including a paper tray (cassette) or the like for storing the transfer material. The uppermost transfer material conveyed to the test stream is susceptible to the environment (particularly temperature and humidity), and its resistance characteristic is often changed as compared with the inner transfer material. Even if the transfer voltage is determined by performing the above-described resistance measurement on one transfer material at the top, since the transfer characteristics of the second transfer material and the subsequent transfer materials fed thereafter are different, the transfer voltage is set to two transfer materials. The transfer material may not be suitable for the transfer material subsequent to the eye, and may result in poor transfer.

Further, the present applicant has proposed that the recording paper for forming an image actually enters the transfer area of the secondary transfer unit without test-flowing the blank recording paper, so that the secondary transfer unit does not An image forming apparatus that measures a combined resistance and appropriately sets a secondary transfer bias based on information on the combined resistance has already been proposed (Japanese Patent Application Laid-Open No. H11-161053).

However, in this image forming apparatus, although there is an advantage that the productivity is not reduced because the resistance is not measured by trial-flowing the recording paper, the leading end of the recording paper for which the resistance is measured (non-image transfer) is used. Section) is only a few mm wide, and when the process speed of the image forming apparatus is high, sufficient time is set for measuring the combined resistance and then setting the secondary transfer bias and applying the secondary transfer to the recording paper. I ca n’t,
As a result, it is not possible to apply an appropriate secondary transfer bias over the entire secondary transfer of the recording sheet, which may cause transfer failure. In general, the moisture content, which greatly affects the resistance characteristics of recording paper, varies from place to place even on a single sheet of paper, and the resistance of the entire sheet cannot be measured accurately using only the leading edge of the sheet. It may not be possible to set a proper secondary transfer bias.

In addition, in an image forming apparatus adopting the intermediate transfer system, when a double-sided image forming operation for forming images on both front and rear sides of a recording sheet is performed, a toner image is formed on one side (front side). The secondary-transferred recording paper is sent to a fixing device, and is thermally fixed by being nipped and conveyed by a fixing member such as a heating member.Water contained in the recording paper is reduced by passing through the fixing device. Since the resistance is increased by the heat and changed, it is difficult to set the secondary transfer bias when the remaining one side (back side) of the toner image is secondarily transferred. In this case, for example, at the time of the secondary transfer to the remaining one side of the sheet, the resistance obtained by multiplying the information of the combined resistance on one side (the front side) of the recording sheet by a predetermined coefficient obtained in advance by testing. Although a countermeasure method of setting the secondary transfer bias as conceivable is also conceivable, since the change in the resistance characteristic due to the change in the moisture content of the recording paper after actually passing through the fixing device is not constant over the entire paper, an appropriate secondary It is not possible to set the transfer bias, which may cause secondary transfer failure.

Further, one of the factors causing the secondary transfer failure is the surface property of the paper. In other words, if the surface property is poor due to the unevenness of the paper surface or the orientation of the fibers, the physical adhesion between the intermediate transfer body and the toner in the secondary transfer unit is higher than the physical adhesion between the recording paper and the toner. As a result, even if an appropriate secondary transfer bias is set by measuring the combined resistance in the secondary transfer section for each sheet, the static electricity due to the transfer electric field formed by the bias is obtained. Since the physical adhesion between the intermediate transfer member and the toner is superior to the force, the toner is not transferred to the recording paper side, resulting in secondary transfer failure. Therefore, as a method for solving such a secondary transfer failure, a release agent is applied to the surface of the intermediate transfer member to weaken the physical adhesion between the intermediate transfer member and the toner to reduce the surface property of the recording paper. If this means is adopted, it is necessary to separately provide a release agent coating device for applying a release agent to the intermediate transfer body, which increases the size of the device. And a new problem that causes an increase in cost occurs.

The present invention has been made in view of such various circumstances, and has as its object to reduce the size of the apparatus and the cost when the intermediate transfer method is employed without increasing the cost. An object of the present invention is to provide an image forming apparatus capable of achieving appropriate and stable next transfer.

[0013]

According to the present invention, an image forming apparatus capable of forming a toner image in accordance with image information, and an image forming apparatus formed by the image forming apparatus. An intermediate transfer member on which the toner image is transferred, a primary transfer device for transferring the toner image formed by the image forming device to the intermediate transfer member, and a toner on the intermediate transfer member transferred by the primary transfer device. A secondary transfer device for transferring an image to recording paper conveyed from a paper storage unit by a transfer electric field formed by applying a secondary transfer bias, and a fixing member for transferring the toner image transferred by the secondary transfer device A fixing device for fixing the recording paper on the recording paper by the fixing device, and a paper re-sending path for re-sending the recording paper after passing through the fixing device again to the secondary transfer portion formed between the intermediate transfer body and the secondary transfer device. ,
A resistance measuring device for measuring a combined resistance of the secondary transfer portion in a state where the recording sheet is present in the secondary transfer portion; and the secondary transfer device based on the combined resistance measured by the resistance measuring device. A bias correction means for setting and correcting the secondary transfer bias to be applied in the image forming apparatus,
The measurement of the combined resistance by the resistance measuring device is performed on a plurality of recording sheets conveyed from the sheet storage unit, and the recording sheet after the resistance measurement is set to be sent to the sheet re-feeding path. It is a feature.

In this image forming apparatus, the measurement of the combined resistance of the secondary transfer section by the resistance measuring device is performed on a blank recording sheet on which no image is formed.
It is desirable to set so as to execute resistance measurement at a plurality of locations on one recording sheet. Then, from the viewpoint of measuring the combined resistance of the secondary transfer unit more appropriately, the average value of the combined resistance at a plurality of locations on each recording sheet is calculated, and the average value of the combined resistance of all the recording sheets is calculated. Good. Further, since the recording sheet after the resistance measurement is sent to the sheet re-sending path and conveyed again to the secondary transfer unit, from the viewpoint of enabling image formation on the recording sheet, the image forming apparatus is adjusted in accordance with the re-sending timing. It is desirable to set so as to start the image forming operation according to. However, at the time of the secondary transfer to the recording paper used for the resistance measurement, considering that the recording paper has once passed through the fixing device and the resistance characteristic has changed, the secondary transfer bias at that time is previously determined. It is desirable that the change in the resistance characteristic of the recording paper after passing through the fixing device is set to a value obtained by further multiplying a correction coefficient experimentally obtained with respect to the degree. Operations such as these composite resistance measurement and paper conveyance to the paper re-sending path are as follows.
The setting is performed so as to be controlled by the control unit in the image forming apparatus.

The timing of measuring the combined resistance by the resistance measuring device is, for example, a series of image forming operations (jobs).
Before starting the image forming operation, each time when a predetermined number of image forming operations are performed, each time when the sheet storing unit is changed when there are a plurality of sheet storing units, each time when a sheet is replenished to the sheet storing unit, and the image forming apparatus. At least when the main power is turned on, and at least one of these timings may be appropriately adopted.

According to such an image forming apparatus, the measurement of the combined resistance by the resistance measuring device is performed on a plurality of recording sheets conveyed from the sheet storage unit. Accordingly, the measurement is not limited to only the sheet stored at the top of the sheet storage unit, but is also performed on other sheets. Therefore, the recording sheet stored at a specific position such as the top of the sheet storage unit is measured. It is possible to measure the combined resistance of the secondary transfer unit based on the average resistance characteristics of the recording paper stored in the paper storage unit, without being affected by the unique (variable) resistance characteristics of As a result, the secondary transfer bias (voltage or current) is set more appropriately in the bias correction means based on the combined resistance, and more appropriate and stable secondary transfer is performed.

The recording paper after the resistance measurement is sent to a paper re-feeding path. As a result, the recording sheet after the resistance measurement is again conveyed to the secondary transfer unit. By performing an image forming operation by the image forming apparatus in accordance with the re-transmission timing, image formation on the recording sheet can be performed. It becomes possible. Moreover, a predetermined time is secured between the time when the recording sheet is measured and the time when the sheet is re-transmitted to the secondary transfer section by the re-feeding conveyance path, so that the secondary transfer bias is set based on the combined resistance measured during the time. As a result, if the image forming operation is started from the recording paper after the resistance measurement, proper secondary transfer and, consequently, image formation can be reliably performed under the optimized secondary transfer bias. Will be performed. Further, since the recording paper after the resistance measurement is temporarily present on the reversing path or the like, there is no need to separately provide a storage tray or the like for storing the recording paper unlike the related art.

Further, in this image forming apparatus, when images are formed on both sides of the recording paper (that is, at the time of double-sided image formation), the resistance measurement by the resistance measuring device is completed and the recording paper is fed into the paper re-feeding path. It is preferable that the secondary transfer section again measures the combined resistance of the recording paper by the resistance measuring device.

With this configuration, the recording sheet on which the first resistance measurement is performed by the resistance measuring device passes through the fixing device, and is then conveyed again to the secondary transfer section by the sheet re-sending path. Will be used for resistance measurement. As a result, the combined resistance to (one side of) the recording sheet before passing through the fixing device and the combined resistance to (the remaining one side) of the recording sheet after passing through the fixing device once are measured. At this time, the secondary transfer bias for the first surface and the secondary transfer bias for the second surface can be individually set. In particular, at the time of image formation for the second surface, secondary transfer is performed on recording paper whose resistance characteristics have been changed due to fluctuations in water content after passing through the fixing device once. Since the combined resistance to the recording paper is measured and the secondary transfer bias for the second surface is set based on the combined resistance, the secondary transfer for the second surface is also performed under a more appropriate secondary transfer bias. Will be done.

Further, any one of the image forming apparatuses described above includes
It is preferable to use a fixing device provided with a release agent application device for applying a release agent to a fixing member.

In such a configuration, when the recording paper after the resistance measurement passes through the fixing device, the recording paper is fed to the paper transport path in a state where the release agent is applied via a fixing member, and is again returned to the secondary paper. It is transported to the transfer section. As a result, the release agent applied by passing through the fixing device is applied to the intermediate transfer member and the secondary transfer device in the secondary transfer unit via the recording paper to be re-sent. In particular, at the time of the second resistance measurement during double-sided image formation, the release agent is effectively applied from the recording paper to the intermediate transfer member or the secondary transfer device. As a result,
There is no need to provide a dedicated release agent application device for applying the release agent to the intermediate transfer member, and the release agent in the fixing device can be applied to the intermediate transfer member or the like via the recording paper.
Further, since the release agent in the fixing device can be indirectly applied to the intermediate transfer body via the recording paper, a dedicated release agent application device for applying the release agent to the intermediate transfer body is provided. Need not be provided separately.

Further, in the above-mentioned image forming apparatus using a fixing device provided with a release agent applying device, a plurality of recording sheets are conveyed from the sheet storing section to a secondary transfer section, and the resistance is measured by the resistance measuring apparatus. It is preferable that the sheet is sent to the sheet re-sending path without being measured, and is again conveyed to the secondary transfer section and passed therethrough. In this case, it is possible to form an image on a recording sheet that is re-sent to the secondary transfer unit, but it is also possible to simply pass the secondary transfer unit without forming an image. preferable. Further, if necessary, the composite resistance may be measured for the recording sheet re-transmitted to the secondary transfer unit.

In such a configuration, a plurality of recording sheets to which the release agent has been applied by the fixing device are re-sent to the secondary transfer section and pass therethrough. Thus, the release agent can be reliably and efficiently applied to the intermediate transfer body before the toner image is primarily transferred.

In each of the above image forming apparatuses using a fixing device provided with a release agent applying device, a plurality of recording sheets conveyed from the sheet storage section to the secondary transfer section are used for one cycle of the intermediate transfer body. It is preferable that the number of images to be formed is equal to or more than the number of images that can be formed therein.

With this configuration, a predetermined number or more of the recording sheets coated with the release agent by the fixing device correspond to an area corresponding to at least the entire circumference of the intermediate transfer body (an area corresponding to one cycle). Only through the secondary transfer section. This makes it possible to apply the release agent through the recording paper over the entire circumference of the intermediate transfer member.

Further, in each of the above image forming apparatuses using a fixing device provided with a release agent applying device, a plurality of recording sheets conveyed from the sheet storage section to the secondary transfer section are used in the image forming apparatus. The paper may be configured to be the maximum size paper that can be used.

With this configuration, a plurality of recording sheets of the maximum size coated with the release agent by the fixing device pass through the secondary transfer portion. Thereby, the release agent can be applied to the image formation (transfer) area of the intermediate transfer body. In particular, when the maximum size of the recording sheet is equal to or more than the number of images that can be formed in one cycle of the intermediate transfer body, the entire image forming (transferable) area of the intermediate transfer body is The release agent can be more accurately applied.

[0028]

[First Embodiment] FIG. 1 shows an intermediate transfer type color printer to which the present invention is applied. This color printer mainly consists of yellow (Y),
4 for magenta (M), cyan (C) and black (K)
Forming color toner images by electrophotography 4
Image forming units (image forming devices) 10Y, 10M, 10
C, 10K, an intermediate transfer belt (intermediate transfer body) 20 on which the toner images formed by the four image forming units 10 are transferred, and a recording sheet (toner) on which the toner image on the intermediate transfer belt 20 is transferred. The image forming apparatus includes a fixing device 40 to which a recording medium (P) is fed to fix the toner image, and various paper transport paths for transporting the recording paper P in the image forming apparatus.

The image forming units 10Y, 10M, 10M
C and 10K are arranged side by side at predetermined intervals along the horizontal direction, and each of them has the same configuration. That is, each of the image forming units 10 includes a photosensitive drum (image carrier) 12 that rotates at a predetermined speed in the direction of the arrow, a charging device 13 that uniformly charges the surface of the photosensitive drum 12, and a charging device An image writing device (ROS: Ra) for exposing a light image corresponding to image information to the surface of the photosensitive drum 12 charged at 13 to form an electrostatic latent image
(ster output scanner) 14, a developing device 15 for supplying the toner of a predetermined color to develop the electrostatic latent image formed on the photosensitive drum 12 by the ROS 14, and a developing device 15 remaining on the photosensitive drum 12 after the transfer. And a drum cleaning device 16 for removing and cleaning the untransferred toner. The ROS 14 emits a laser beam modulated from a semiconductor laser 14a in accordance with image information input from an externally connected device such as a document reading device (scanner) or a personal computer, and the ROS 14 rotates the laser beam. The beam is deflected and scanned through the polygon mirror 14b and a plurality of optical system components, and finally is exposed on the photosensitive drum 12 as a laser beam Bm.

The intermediate transfer belt 20 includes a driving roll 2
1. A driven roll 22, a tension roll 23, a backup roll (a part of the secondary transfer device 30) 24, etc. are wound around with a constant tension in a form such that the whole forms an inverted triangle, and a driving roll thereof is provided. The photosensitive drum 12 (primary transfer section) in each of the image forming units 10 is arranged between the driven roller 21a and the driven roll 22a, and rotates at a constant speed in the direction of the arrow. Reference numeral 25 in the figure denotes a belt cleaning device for removing and cleaning untransferred toner remaining on the intermediate transfer belt 20 after transfer, and 26 denotes various marks (reference position marks, control marks) formed on the intermediate transfer belt 20. For example, a detection sensor for detecting the use mark.

A primary transfer device 27 is provided at a position (primary transfer portion) where the intermediate transfer belt 20 and the photosensitive drum 12 of each image forming unit 10 face each other. In this embodiment, as a primary transfer device 27, a primary transfer roll (27) arranged so as to rotate while contacting the belt 20 with the drum 12 in a primary transfer portion, and the toner And a bias power supply device (not shown) for applying a primary transfer bias voltage having a polarity opposite to the charging polarity. In this embodiment,
The photosensitive drum 12 is uniformly charged to a negative polarity, and a toner having a negative charging polarity is used as the toner. Therefore, a positive polarity voltage is applied as the primary transfer bias voltage.

Further, at a position (secondary transfer portion) of the intermediate transfer belt 20 opposite to the backup roll 24,
Secondary transfer roll 31 constituting a part of secondary transfer device 30
Is disposed so as to be pressed against the belt 20. Further, the secondary transfer device 30 is provided on the backup roll 24 side.
Is provided. Further, as shown in FIG. 2, a secondary transfer bias power source 33 for applying a secondary transfer bias voltage is connected to the power supply roll 32. From the bias power supply 33, a secondary transfer bias voltage having the same polarity as the charge polarity of the toner transferred onto the intermediate transfer belt 20 is supplied to the power supply roll 32.
Is applied between the backup roll 24 and the secondary transfer roll 31 in the secondary transfer section. In this embodiment, since the polarity of the toner on the intermediate transfer belt 20 is negative, a voltage of negative polarity is applied as the secondary transfer bias voltage. Reference numeral 34 in FIG. 2 denotes a cleaning device for removing toner and the like adhering to the secondary transfer roll 30.

The fixing device 40 is provided with a heating roll 41 and a nip belt 42 which is wound around a plurality of rolls so as to abut on a part of the heating roll 41 with a predetermined width and freely rotates. The recording paper P after the secondary transfer is passed between the heating roll 41 and the nip belt 42 (belt nip portion). In the fixing device 40, a release agent coating device 45 for uniformly applying a release agent such as silicone oil or amine oil for preventing toner offset to the heating roll 41 is provided.

The above-mentioned paper transport system transfers the recording paper P, which is fed by a feed roll 61a from a paper feed tray 61 serving as a paper storage section 60 in which the recording paper P is stored, to the intermediate transfer belt 20 and the secondary transfer device 30. Secondary transfer roll 31)
, A transfer path 63 that conveys the recording paper P after the transfer from the secondary transfer section to the fixing device 40, and a recording path that passes through the fixing device 40. And a discharge path 64 for discharging the paper P to a discharge tray 65. Of these, the paper feed path 62 and the discharge path 64 are constituted by a plurality of roll pairs and a plurality of transport guide members disposed to face each other with a vertical interval so as to be interposed between the roll pairs. . A roll pair 61a immediately before the secondary transfer unit among a plurality of roll pairs in the paper feed path 62
Is a registration roll for sending the recording paper P to the secondary transfer section at a predetermined timing. The transfer path 63 includes two suction-type conveyance belt devices 63a and 63b that convey the recording paper P after the secondary transfer by suction and suction on the surface of the conveyance belt. Note that the paper feed tray 6
One may be one, but usually a plurality are provided.

The paper transport system may be either for reversing the front and back sides of the fixed recording paper P and discharging it to the paper discharge tray 65, or for reversing the recording paper P when performing double-sided image formation. An inversion path 66 for re-sending to the paper path 62 is provided. A switching claw 67 for switching the path of the sheet conveyance path is provided in the middle of the discharge path 64. The switching operation of the conveyance path switching claw 67 causes the recording sheet P after fixing to be discharged to the discharge path 64 and the reversing path. 66 to one of them. A re-sending path 68 for re-sending the reversed recording sheet P to the sheet feeding path 62 is provided in the course of the reversing path 66. Each of these transport paths 6
Each of the reference numerals 6 and 68 is constituted by a plurality of transport roll pairs and sheet guide members disposed vertically or horizontally facing each other so as to be interposed between the respective transport roll pairs.

The intermediate transfer bell and the intermediate transfer belt 20 are made of a synthetic resin such as polyimide or polyamide to which a conductive agent such as carbon black is applied. The endless belt having a thickness of about 0.1 mm is used. The one molded into a shape is used. The volume resistivity of the intermediate transfer belt 20 is adjusted to 10 ⁶ to 10 ¹⁴ Ω · cm.

As the transfer roll of the primary transfer device 27, a urethane foam rubber layer in which carbon black is dispersed (volume resistance of 10 ⁶ to 10 ¹⁰ Ω,
A roll having a roll diameter of 28 mm and an Asuka C hardness of 35 ° is used.

Further, as the backup roll 24 in the secondary transfer device 30, an EPDM
An (ethylene propylene diene) rubber layer and a tube layer (surface resistivity: 10 ⁷ to 10 ¹⁰ Ω / □, Asuka C hardness: 70 °) made of a blend rubber of EPDM and NBR having carbon dispersed on the surface are laminated in this order. Roll diameter 2 formed
An 8 mm one is used. Also, the secondary transfer roll 30
Examples are: a metal shaft, a foamed urethane rubber layer in which carbon is dispersed, a tube layer made of urethane rubber in which carbon is dispersed on the surface, and a fluororesin coating layer (volume resistance of 10 ³ to 10 ¹⁰ Ω, Asuka C hardness). 30 °) in this order and a roll diameter of 28 mm is used.

Further, in this color printer, formation of a full-color image (color print mode) and formation of a single-color image (single-color print mode) can be selectively performed. Further, it is also possible to selectively perform image formation on one side of the recording paper P (one-sided print mode) and image formation on the front and back surfaces (double-sided print mode).

The single-sided printing of a full-color image by such a color printer is performed as follows.

First, image information for printing is input from an original reading device or a connected device connected to the printer to an image processing device (not shown), and predetermined image processing is performed. (Y, M, C, and B gradation data) have four image forming units 10Y, 10M, and 10M.
C and 10K, respectively.

Subsequently, the image forming units 10Y, 10M, 1
After the electrostatic latent images are formed on the surfaces of the photosensitive drums 12 uniformly charged by the charging device 13 by operating the ROSs 14 at 0C and 10K according to the gradation data of the four colors. Each of the electrostatic latent images is developed by a developer (toner) of a predetermined color by each developing unit 15, so that four color toner images are individually formed on each photosensitive drum 12. The four color toner images are primarily transferred so as to be sequentially superimposed on the intermediate transfer belt 20 rotating so as to pass through the primary transfer portion of each photosensitive drum 12, and then conveyed to the secondary transfer portion. The primary transfer of the toner image is performed electrostatically by a transfer electric field formed between the transfer roll 27 and the photosensitive drum 12 by a primary transfer bias voltage applied to the primary transfer roll 27. The untransferred toner is removed from the photosensitive drum 12 after the primary transfer by the cleaning device 16 to be cleaned.

On the other hand, in accordance with the formation of the toner image, a predetermined size and type of recording paper P
The intermediate transfer belt 2 serving as a secondary transfer unit is supplied to the registration roll 62a by the registration roll 62a at the time of the secondary transfer timing.
When the toner image is fed between 0 and the secondary transfer roll 31, the multiple toner images on the intermediate transfer belt 20 are collectively and secondarily transferred to the recording paper P. The secondary transfer of the toner image is electrostatically performed by a transfer electric field formed between the secondary transfer roll 31 and the secondary transfer roll 31 by the secondary transfer bias voltage applied to the backup roll 24 from the bias power supply 33 via the power supply roll 32. Done in

The recording paper P after the secondary transfer is
After being separated from the intermediate transfer belt 20, it is sent to the fixing device 40 via the transfer path 63,
And the nip belt 42, the toner image is fixed by heating and pressing. At this time, since the release agent (oil) is applied to the heating roll 41 by the release agent applying device 45, the recording paper P is surely peeled off from the heating roll 41 without causing toner offset. become. In the single-sided image formation (single-sided print) mode, the recording paper P after fixing is discharged from the fixing device 40 and then discharged and stored in the discharge tray 65 via the discharge path 64. The untransferred toner and the like are removed from the intermediate transfer belt 20 after the secondary transfer by the cleaning device 25 to be cleaned. As described above, a full-color image (for the front surface) is formed on (the front surface of) the recording paper P, and the one-sided printing is completed.

The two-sided printing of a full-color image by this printer is performed as follows.

First, the recording paper P on which single-sided printing has been completed
After the fixing, the sheet is sent to the reversing path 65 side by the switching operation of the conveyance path switching claw 67, and its front and back surfaces are reversed. Thereafter, the sheet is re-sent to the sheet feeding path 62 via the re-sending path 68. Next, the re-transmitted recording paper P is again fed into the secondary transfer portion by the registration roll 62a at the secondary transfer timing, and the toner image for the back surface formed by the image forming unit 10 is transferred from the intermediate transfer belt 20 to the secondary transfer portion. After the secondary transfer, the sheet is fixed by passing through a fixing device 40 in the same manner as in the case of the single-sided printing. by this,
A full-color image for the back surface is formed on the back surface of the recording paper P, and the duplex printing is completed.

In this color printer,
As shown in FIG. 2, in a state where the recording paper P is present in the secondary transfer portion, a resistance measuring device 50 for measuring a composite resistance of the secondary transfer portion, and a synthetic resistance measured by the resistance measuring device 50. A bias correcting unit 70 is provided for setting and correcting a secondary transfer bias applied by the secondary transfer device 30 based on the resistance.

The resistance measuring device 50 includes a measuring unit 53 including a voltmeter 51 and an ammeter 52 appropriately disposed between the bias power supply 33 and the secondary transfer device 30, and a voltage measured by the measuring unit 53. And a combined resistance calculating unit 54 for calculating the combined resistance of the secondary transfer unit from the current, and a predetermined measuring current (for example, −60 μA) or voltage from the bias power supply 33 during the combined resistance measurement. It is configured to apply to the transfer unit. Further, in this embodiment, the voltage or current at 10 points (10 locations) is measured for one sheet of the recording paper P passing through the secondary transfer unit by the resistance measuring device 50, and the calculation is performed based on the measurement data. The unit 54 calculates the electrical resistance (combined resistance) from Ohm's law.

On the other hand, the bias correcting means 70 performs processing for setting a secondary transfer bias (voltage or current) based on the combined resistance measured by the resistance measuring device 50. In this embodiment, as shown in FIG. 3, reference data is prepared for the combined resistance of the secondary transfer portion obtained by a test in advance and the appropriate secondary transfer bias corresponding to each combined resistance (storage means). And an appropriate secondary transfer bias voltage (BV) is determined based on the control data. A plurality of types of the control data are prepared so as to be selectively applied in accordance with each condition such as temperature and humidity and elapsed time. And
The obtained appropriate secondary transfer bias voltage is applied at the time of subsequent secondary transfer. When an image is formed on the recording paper P used for the measurement of the combined resistance, the secondary transfer bias voltage applied at the time of the secondary transfer is assumed to pass through the fixing device 40 once and change the resistance characteristic. In consideration of the above, fine adjustment is performed by multiplying the determined appropriate secondary transfer bias voltage (BV) by a predetermined correction coefficient. The correction coefficient is prepared by examining a change in the resistance characteristic when the sheet passes through the fixing device 40 by a test in advance. Reference numeral 35 in FIG. 2 is a control device for the secondary transfer bias voltage. The bias voltage correction means 70
It can be configured as a part of the control device 35.

In this color printer, the measurement of the combined resistance by the resistance measuring device 50 is performed by the sheet feed tray 61.
The control is performed such that the control is performed on a plurality of recording papers P conveyed from the printer and the recording paper P after the resistance measurement is sent to the retransmission path 68. Further, the measurement of the combined resistance by the resistance measuring device 50 and the conveyance of the plurality of recording papers P used for the measurement employ a slightly different configuration between single-sided printing and double-sided printing, as described later. Such control operations may be configured to be executed by, for example, a system controller of the image forming apparatus.

The measurement of the combined resistance in the secondary transfer section is performed at each time when each print job is started.
The setting is made so as to be executed at a preset time, such as every time when the number 1 is changed, every time paper is supplied to the paper feed tray 61, or when the power of the printer is turned on. It is also possible to configure such that the resistance measurement timing can be arbitrarily selected by, for example, an operation on an operation panel.

Next, the operation related to the resistance measuring device 50 and the bias voltage correcting means 6 will be described.

First, the operation in the case of performing one-sided printing will be described with reference to FIG.

First, when the start button on the operation panel (not shown) is pressed after the conditions for single-sided printing have been set, as shown in FIG. -1 and P-2 are transported between the intermediate transfer belt 20 of the secondary transfer unit and the (secondary transfer roll 31 of) the secondary transfer device 30 via the paper feed path 62. At this time, the image forming operation by each image forming unit 10 has not been started. The two recording sheets P-1 and P-2 are:
Paper of the type and size used for the single-sided printing.

While the two recording sheets P-1 and P-2 pass through the secondary transfer section, a measuring current is applied to the secondary transfer section from the bias power supply 33 via the power supply roll 32. At this time, in the resistance measuring device 50, a voltage value of 10 points is measured for one recording sheet by the voltmeter 51 of the measuring unit 51, and the measured voltage and the test current value are transmitted to the combined resistance calculating unit 54. According to Ohm's law, the combined resistance of the secondary transfer portion including the sheet is calculated. Since two recording sheets P-1 and P-2 are conveyed for the resistance measurement, a total of 20 points of combined resistance data can be obtained. In this embodiment, the two recording sheets P-1 and P-2
Of all data obtained by excluding the maximum and minimum values of each sheet, finally calculate the average value,
The average value of the whole is defined as a combined resistance.

FIG. 4 shows the data results of the combined resistance obtained at this time. In FIG. 4, low temperature and low humidity (1
0 ° C, 15% RH) and high temperature and high humidity (28 ° C, 85% RH)
Data of each combined resistance of the first recording paper P-1 and the second recording paper P-2 under the two environments (10 each)
). Also, the dotted lines a to d in the figure are the average values of all data of each sheet, and the dashed line is the average value of the first sheet and the second sheet in each environment described above.
As is clear from the results shown in FIG. 4, the first recording paper (the paper stored at the uppermost position of the paper feed tray 61) P-1 and the second recording paper (the paper There is a difference in the average value of the combined resistance between the sheet P-2 and the sheet P-2 which is stored second from the uppermost position. For example, in the case of high temperature and high humidity, the combined resistance of the first recording paper P-1 is lower than that of the second recording paper P-2. This is because the first recording sheet P-2 is located at the uppermost position of the sheet feed tray 61 and is affected by high temperature and high humidity, and its moisture content is high, so that the combined resistance is low. Can be Therefore, by measuring the combined resistance of a plurality of recording sheets and calculating the average value of the combined resistance, a more appropriate combined resistance of the entire sheet contained in the sheet feed tray 61 is measured. As a result, a more appropriate setting of the secondary transfer bias voltage can be performed.

The information on the combined resistance value of the secondary transfer section measured using the two recording sheets P-1 and P-2 is sent to the bias correction means 6. Then, the bias correction means 6 sets (determines) an appropriate secondary transfer bias voltage based on the information of the combined resistance value. That is, the bias correction means 6 determines the secondary transfer bias voltage for the combined resistance measured by the resistance measuring device 50 in light of the reference data as shown in FIG.

On the other hand, the recording papers P-1, P-
5 is sent to the re-sending path 68 through the secondary transfer section as shown in FIGS. 5B and 5C. At this time, two recording papers P
After passing through the fixing device 40, both -1 and P-2
The sheet is sent to the reversing path 64 and sent to the re-sending path 68 with its front and back surfaces reversed.

Therefore, the recording paper P after the resistance measurement is performed.
-1 and P-2 are conveyed again to the secondary transfer section via the re-sending path 68 during a series of processes up to the setting of the secondary transfer bias voltage. Conversely, the process of measuring the combined resistance and setting the secondary transfer bias is performed until the recording sheets P-1 and P-2 after the resistance measurement are retransmitted to the secondary transfer unit.

Then, the recording papers P-1, P
An image forming operation for single-sided printing is started and executed in the image forming unit 10 in synchronization with the timing at which -2 reaches the secondary transfer portion. Thereby, as shown in FIG. 5D, the toner image T formed by the image forming unit 10 is primarily transferred to the intermediate transfer belt 20 and the intermediate transfer belt 2
The toner image T transferred to 0 is sequentially secondary-transferred to the re-transmitted recording sheets P-1 and P-2 after the resistance measurement.

At the time of this secondary transfer, an appropriate secondary transfer bias voltage (B
V) is applied from the bias power supply 33 to the secondary transfer section.
However, the secondary transfer bias voltage at this time is set in advance in consideration of the fact that the two recording sheets are heated after passing through the fixing device 40 after the measurement of the combined resistance and the moisture content thereof changes as described above. A voltage value obtained after multiplying the prepared correction coefficient is applied. As a result, appropriate secondary transfer is performed based on the actually used recording paper and the current resistance state of the secondary transfer unit. Toner image T
After the secondary transfer of the recording paper P-1 and P-2, the fixing processing by the fixing device 40 is performed, and then the recording paper P-1 and P-2 are stored in the paper discharge tray 65 via the discharge path 64 (FIG. 5E). . The recording papers P-1 and P-
2 is used for image formation immediately after being resent to the secondary transfer unit via the resending path 68 without being stored in a storage tray or the like after the resistance measurement.

Subsequently, the recording paper P-
Image formation for 1 and P-2 (secondary transfer of toner image, etc.)
5e, an image (toner image T) is formed on a new recording sheet P in the image forming unit 10 and a new recording sheet P- 3, P-4, etc. are conveyed to the secondary transfer unit via the paper feed path 62, and the intermediate transfer belt 20 for such new recording papers P-3, P-4, etc.
Transfer of the toner image T is performed. At this time, the secondary transfer is performed by applying the set appropriate secondary transfer bias voltage (BV) from the bias power supply 33 to the secondary transfer unit.

Therefore, in this image forming apparatus, as described above, the secondary transfer to which the optimum secondary transfer bias voltage set based on the combined resistance of the secondary transfer portion is performed is performed with certainty. The storage state of the recording paper P (the state of being stored in the paper supply tray, the state before being stored in the paper supply tray, etc.), the variation in the resistance characteristics of the recording paper P, the environmental fluctuation, and the transfer performance in the secondary transfer unit. The secondary transfer is always performed under stable and optimal conditions even when the conditions are changed, so that it is possible to appropriately prevent occurrence of secondary transfer failure such as transfer omission. In this embodiment, two sheets of recording paper P are used to measure the combined resistance of the secondary transfer section. However, the combined resistance is measured more accurately, and a more appropriate secondary transfer bias voltage is measured. From the viewpoint of setting, the recording paper P used when measuring the combined resistance
Can be set to three or more.

Next, the contents of the operation when performing double-sided printing will be described with reference to FIG.

First, when the start button on the operation panel (not shown) is pressed after the conditions for the two-sided printing are set, two sheets are initially recorded from the paper feed tray 61 of the sheet storage unit 60 as in the case of the one-sided printing. The sheets P-1 and P-2 are conveyed between the intermediate transfer belt 20 of the secondary transfer unit and the (secondary transfer roll 31 of) the secondary transfer device 30 via the paper feed path 62, and the two sheets While the combined resistance of the secondary transfer portion with respect to the recording sheets P-1 and P-2 is measured by the resistance measuring device 50 (FIGS. 5A to 5B), the recording sheet after the resistance measurement is sent out to the re-sending path 68. (FIG. 5c). And
Bias correction means 6 based on the measured combined resistance
In this case, an appropriate secondary transfer bias voltage is determined in the same manner as in the case of single-sided printing. The secondary transfer bias voltage at this time is used as a bias voltage for the first surface (for the front side) of the recording paper P. You.

Subsequently, in the case of double-sided printing, unlike the case of single-sided printing, the recording paper after the resistance measurement, which is re-sent to the secondary transfer unit, is again synthesized by the resistance measuring device 50 into the secondary transfer unit. A resistance measurement is performed. That is,
As shown in FIG. 5D, instead of forming an image (secondary transfer) on the recording sheets P-1 and P-2 after the resistance measurement at the time of single-sided printing, the image is displayed on the secondary transfer unit as shown in FIGS. Recording paper P- re-sent with its back side reversed
The second measurement of the combined resistance for P-1 and P-2 is performed in the same manner as the first measurement. The recording sheets P-1 and P-2 after the completion of the second resistance measurement are sent out again to the re-sending path 68 (FIG. 6h). During this time, the secondary transfer bias voltage is determined by the bias correction unit 70, and the secondary transfer bias voltage at this time is used as a bias voltage for the second surface (for the back surface) of the recording paper P.

The recording paper P- on which the second resistance measurement was performed
The image forming unit 10 starts and executes the image forming operation for the first-side printing in accordance with the timing at which the first and P-2 arrive at the secondary transfer unit. Thus, as in the case of single-sided printing, the toner image T formed by the image forming unit 10 is primarily transferred to the intermediate transfer belt 20, and the toner image T transferred to the intermediate transfer belt 20 is retransmitted. The secondary transfer is successively performed on the first surfaces of the recording sheets P-1 and P-2 after the resistance measurement (FIG. 5D). At the time of the secondary transfer, the set secondary transfer bias voltage for the second surface (for the back surface) is applied in consideration of the fact that the recording paper once passes through the fixing device 40 and the resistance characteristic fluctuates. Is done.
Then, in addition to the toner image for the first surface printing on the two recording sheets P-1 and P-2, the intermediate transfer belt 20
When another toner image for printing on the first surface is formed on the first surface, a new recording sheet P-3 or the like is conveyed from the paper feed tray 61 to the secondary transfer unit, and the toner image for the first surface is reprinted. Next transfer is performed. At the time of the secondary transfer to the new recording paper P-3 or the like, since the recording paper P-3 or the like does not pass through the fixing device 40, the secondary transfer of the set first surface (for the front surface) is performed. A bias voltage is applied.

Recording paper P on which printing on the first side has been completed
-1 and the like are conveyed again to the secondary transfer unit via the reversing path 64 and the re-sending path 68. Then, the image for the second surface printing (the toner image T) is formed in the image forming unit 10 at the timing of re-sending to the secondary transfer unit, and the toner image T is recorded in the secondary transfer unit. Secondary transfer is performed on the second surface of the sheet P-1 or the like. During this secondary transfer, the set appropriate secondary transfer bias voltage for the second surface (for the back surface) is applied.

Therefore, in this double-sided printing, the combined resistance of the secondary transfer portion is two sheets of recording paper P-1, P
-2 is performed on the front and back surfaces (that is, the paper that has not passed through the fixing device 40 and the paper that has passed), and based on the respective combined resistances, the appropriate two for the first and second surfaces of the recording paper After the respective secondary transfer bias voltages are set, the secondary transfer for the first surface and the secondary transfer for the second surface are performed. Accordingly, even during double-sided printing, the storage state of the recording paper P (the state of being stored in the paper supply tray, the state before being stored in the paper supply tray, etc.), the variation in the resistance characteristics of the recording paper P, and the environmental fluctuation Since the secondary transfer (especially, the secondary transfer on the second surface) is always performed under stable and optimal conditions even when the transfer performance in the secondary transfer section is changed, the transfer is performed. It is possible to appropriately prevent occurrence of secondary transfer failure such as omission.

In this image forming apparatus,
As shown in FIG. 7, fixing is performed via the recording papers P-1 and P-2 which are transported and used before the print job in order to measure the combined resistance of the secondary transfer portion particularly during the above-described double-sided printing. The release agent 46 applied to the heating roll 41 by the release agent coating device 45 of the device 40 is
Etc. are indirectly applied.

That is, the recording paper P-1 and the like after the first measurement of the combined resistance passes through the fixing device 45 before being sent to the re-sending path 68. By contacting the applied heating roll 41 and the nip belt 42 to which the release agent is indirectly applied, the release agent 46 is transferred to and adheres to the front and back surfaces (entire surface) of the sheet.
Then, the recording paper P on which the release agent 46 adheres to the front and back surfaces
When −1 or the like is re-sent to the secondary transfer unit to measure the second combined resistance, the image forming operation is not performed and the intermediate transfer belt 20 that does not carry the toner image T is contacted. The release agent 46 on the recording paper is transferred to the intermediate transfer belt 20.
And is applied indirectly. FIG. 8 shows the results of measuring the contact angle of water on the surface of the intermediate transfer belt 20 before the recording paper for resistance measurement passes (contacts) and after the recording paper for resistance measurement passes for the second time. Show. As is evident from the result, the contact angle after the recording paper for the second resistance measurement has passed is increased, which indicates that the releasability of the intermediate transfer belt 20 is improved. .

Thus, needless to say, there is no need to provide a dedicated release agent applying device for applying the release agent to the intermediate transfer belt 20, but the release agent 46 is applied to the intermediate transfer belt 20. This makes it possible to perform the secondary transfer satisfactorily and stably irrespective of the quality of the surface properties of the recording paper.

[Embodiment 2] In this embodiment, one cycle of the intermediate transfer belt 20 (approximately equivalent to the entire circumference of the belt) is used as recording paper for measuring the combined resistance of the secondary transfer portion during double-sided printing. ), Except that it is set to use the same number of recording papers as the number of images that can be formed in
The configuration is the same as that of the image forming apparatus according to the first embodiment.

That is, in this embodiment, as shown in FIG. 9, when the recording paper P used for double-sided printing is of a certain size (for example, A4 size paper), an image corresponding to that paper size is set to an intermediate size. Assuming that the number of images that can be formed in one cycle of the transfer belt 20 is “8”, a recording sheet P (A4 size sheet) for measuring the combined resistance of the secondary transfer portion
Is set to “8” according to the number of images, and the measurement of the combined resistance and the setting of the secondary transfer bias voltage are performed in the first embodiment.
This was performed in the same manner as in the case of. The area indicated by the dotted line in the figure is the maximum image-formable area for each recording sheet P.

The eight recording sheets P-1 to P-
At the time of the second combined resistance measurement for No. 8, as shown in FIG. 9, each of the eight recording sheets comes into contact with almost the entire area of the intermediate transfer belt 20 in the secondary transfer portion. Thereby, the intermediate transfer belt 20 is formed by the eight recording sheets.
The release agent 46 in the fixing device 40 is indirectly applied to almost the entire area (see FIG. 7).
As a result, when two-sided printing is continuously performed (over eight sheets) on the recording paper of the above size, the ninth and subsequent sheets newly conveyed from the sheet feed tray 61 not involved in the resistance measurement are printed. Since the secondary transfer to the sheet P-9 is performed between the intermediate transfer belt 20 and the release agent 46 applied to almost the entire area, the secondary transfer is performed in the same manner as the application of the appropriately set secondary transfer bias voltage. It is also better done.

From the viewpoint of applying the release agent to the intermediate transfer belt 20 via the recording paper after the resistance measurement most efficiently in relation to the entire belt, the recording paper used for the measurement of the combined resistance is used. As P, it is preferable to use a sheet of the maximum size that can be used in the image forming apparatus and at least as many sheets as the number of images that can be formed in one cycle of the intermediate transfer belt 20.

For example, the maximum size recording paper P usable in the image forming apparatus according to this embodiment is A3 size paper, and the number of images that can be formed in one cycle of the intermediate transfer belt 20 is “4”. In this case, four sheets of A3-size recording paper are conveyed to the secondary transfer unit for measuring the combined resistance. As a result, when the A4 three-plate size recording papers P-1 to P-4 are retransmitted to the secondary transfer portion and passed through at the time of the second combined resistance measurement, as shown in FIG.
The four recording papers P-1 to P-4 are transferred to the intermediate transfer belt 2.
0 comes into contact with the entire image-formable area,
Release agent 4 is applied to the entire area of intermediate transfer belt 20 where an image can be formed.
6 will be applied. Therefore, the secondary transfer in this case is also performed between the intermediate transfer belt 20 coated with the release agent 46 over the entire area where the image can be formed, and accordingly, the secondary transfer bias voltage appropriately set is set. In combination with the application of

It is preferable to use a sheet of the maximum size that can be used in the image forming apparatus as the recording sheet P used for the measurement of the combined resistance when performing double-sided printing at that size. This maximum size paper is used as image quality setup for the image forming apparatus at a time that is not related to the printing operation (for example, when the power is turned on, when the paper feed tray is changed,
When used at the time of paper replenishment, the maximum size paper is finally discharged to a discharge tray.

[Embodiment 3] In Embodiments 1 and 2,
The indirect application of the release agent 46 to the intermediate transfer belt 20 is performed via a recording sheet used for measuring the combined resistance. The resistance measurement device 50 does not measure the combined resistance of the secondary transfer unit for the plurality of recording sheets P conveyed to the next transfer unit, but simply passes through the secondary transfer unit and sends it to the re-sending path 68, and again The sheet was conveyed so as to pass through the next transfer section.

As a result, the plurality of recording sheets coated with the release agent 46 after passing through the fixing device 40 are re-sent to the secondary transfer portion and passed therethrough, so that the release agent 46 is transferred to the intermediate transfer belt 20. It will be applied indirectly. Therefore, it is not necessary to set the secondary transfer bias voltage based on the combined resistance, and this is effective when it is necessary to apply a release agent to the intermediate transfer belt 20. Even in such a case, the recording paper used to apply the release agent may be subjected to image formation after twice passing through the secondary transfer portion. Further, from the viewpoint of efficiently applying the release agent to almost the entire area of the intermediate transfer belt 20, the number of images that can be formed in one cycle of the intermediate transfer belt is increased. It is preferable to use the same number of sheets or more, or to use sheets of the maximum size usable in the image forming apparatus.

[Other Embodiments] In the first to third embodiments, an image forming apparatus (printer) for forming a plurality of toner images of different colors by a plurality of image forming units 10 has been described. A plurality of toners having different colors are provided by one image forming apparatus in which four developing devices (15) for individually storing the four color developers (toners) are provided for one photosensitive drum 12. The present invention is also applicable to an image forming apparatus capable of forming an image in four times,
Similar effects can be obtained. Further, each component constituting the image forming apparatus can be appropriately changed as long as it exhibits the same function and the like. For example, a belt-shaped photosensitive member may be used instead of the photosensitive drum 12 or the like. Instead of the intermediate transfer belt 20, a drum-shaped intermediate transfer member can be used, or a pair of rolls can be used as a fixing rotating body of a fixing device.

[0082]

As described above, according to the image forming apparatus of the present invention, the setting of the secondary transfer bias based on the combined resistance of the secondary transfer section measured using a plurality of recording sheets is performed. In order to optimize and stabilize the secondary transfer when the intermediate transfer method is adopted, regardless of the storage state of the recording paper, the variation in the resistance characteristics, and the change in the transfer performance in the secondary transfer section, etc. Can be. Further, the recording paper after the resistance measurement is sent to the re-sending path and transported again to the secondary transfer unit without being stored in the storage tray, so that it is not necessary to separately provide such a storage tray or the like. It does not lead to cost increase. Furthermore, when a fixing device provided with a release agent application device is used, the release agent is indirectly applied to the intermediate transfer body via a recording sheet or the like used for resistance measurement. This also makes it possible to optimize and stabilize the secondary transfer when the intermediate transfer method is adopted. Further, since there is no need to provide a dedicated release agent coating device on the intermediate transfer member, there is no increase in size and cost of the device.

Therefore, when an image is formed using such an image forming apparatus, the secondary transfer bias can be adjusted more appropriately even if factors that affect the secondary transferability may fluctuate. Since the release agent is applied to the intermediate transfer member and the release agent is applied to the intermediate transfer member, good secondary transfer can be stably performed, and an image with stable image quality can be formed.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a color printer according to a first embodiment.

FIG. 2 is a main part schematic diagram showing the configuration of a secondary transfer device, a resistance measuring device, and a bias setting unit.

FIG. 3 is a graph showing a relationship (comparison data) between a combined resistance of a secondary transfer portion and a secondary transfer bias voltage.

FIG. 4 is a graph showing a measurement result of a combined resistance of two recording sheets.

FIG. 5 is an explanatory diagram showing a combined resistance measurement at the time of single-sided printing and states of a plurality of recording sheets related to the resistance measurement.

FIG. 6 is an explanatory diagram showing a main state of a plurality of recording sheets related to a combined resistance measurement during double-sided printing and the resistance measurement.

FIG. 7 is an explanatory diagram illustrating a state in which a release agent is indirectly applied to an intermediate transfer belt via a recording sheet.

FIG. 8 is a graph showing the contact angle of the surface of the intermediate transfer belt with water before and after the contact of the recording paper.

FIG. 9 is a development explanatory diagram showing a relationship between an image formable area of the intermediate transfer belt and the number of recording sheets.

FIG. 10 is a development explanatory view showing the relationship between the maximum usable recording paper and the area of the intermediate transfer belt.

[Explanation of symbols]

10 image forming unit (image forming device), 20 intermediate transfer belt (intermediate transfer member), 27 primary transfer device, 30 secondary transfer device, 40 fixing device, 41 heating roll (fixing member), 45: release agent coating device, 50: resistance measuring device, 6
0: paper storage unit, 68: paper re-sending path, 70: bias correction means, P: recording paper.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yukio Hayashi 2274 Hongo, Ebina City, Kanagawa Prefecture, within Fuji Xerox Co., Ltd. (72) Inventor Masao Okubo 2274 Hongo, Ebina City, Kanagawa Prefecture, within Fuji Xerox Co., Ltd. 72) Inventor Yoko Miyamoto 2274 Hongo, Ebina-shi, Kanagawa Prefecture, F-term in Fuji Xerox Co., Ltd. AA09 AA46 BA41

Claims (6)

[Claims] An image forming apparatus capable of forming a toner image according to image information, an intermediate transfer member to which a toner image formed by the image forming device is transferred, A primary transfer device for transferring the toner image formed by the image device, and a secondary transfer bias is applied to a recording sheet conveyed from the sheet storage unit to the toner image on the intermediate transfer body transferred by the primary transfer device. Secondary transfer device for transferring by a transfer electric field formed by the above, a fixing device for fixing the toner image transferred by the secondary transfer device to recording paper by a fixing member, and recording after passing through the fixing device A sheet re-sending path for re-sending the sheet again to a secondary transfer section formed between the intermediate transfer member and the secondary transfer device; Measuring the combined resistance of the transfer section An image forming apparatus comprising: a resistance measurement device; and a bias correction unit configured to set and correct a secondary transfer bias applied by the secondary transfer device based on a combined resistance measured by the resistance measurement device. The measurement of the combined resistance by the resistance measuring device is performed on a plurality of recording sheets conveyed from the sheet storage unit, and the recording sheet after the resistance measurement is set to be sent to the sheet re-feeding path. Image forming apparatus. 2. The image forming apparatus according to claim 1, wherein when forming an image on both sides of the recording paper, the resistance measurement by the resistance measuring device is completed and the recording paper sent to the paper re-feeding path is formed on the recording paper. On the other hand, the image forming apparatus is set so that the combined resistance is measured again by the resistance measuring device in the secondary transfer section. 3. An image forming apparatus according to claim 1, wherein said fixing device is provided with a release agent applying device for applying a release agent to a fixing member. Forming equipment. 4. The image forming apparatus according to claim 3, wherein a plurality of recording sheets are conveyed from the sheet storage section to a secondary transfer section, and the sheet re-sending path is performed without performing resistance measurement by the resistance measurement device. An image forming apparatus which is set so as to be fed to a secondary transfer unit and conveyed again to the secondary transfer unit. 5. The image forming apparatus according to claim 3, wherein a plurality of recording sheets conveyed from the sheet storage unit to a secondary transfer unit can be formed in one cycle of the intermediate transfer body. An image forming apparatus, wherein the number is equal to or more than the number. 6. The image forming apparatus according to claim 3, wherein a plurality of recording sheets conveyed from the sheet storage section to a secondary transfer section have a maximum size usable by the image forming apparatus. An image forming apparatus comprising: