JP2000267472A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To always obtain an optimum full color transferred image by detecting the resistance value of a recording sheet on which a toner image is actually transferred, and controlling a transfer bias voltage applied on each transfer roll based on the detection value. SOLUTION: When the leading end of the recording sheet P is inserted in between the photoreceptor drum 9 of an image forming unit 8Y and a transfer roll 16Y, a prescribed bias voltage is applied on the transfer roll 16Y by a transfer bias applying source 20Y. The current value is detected by a current detection circuit 22. The resistance value between the photoreceptor drum 9 and the transfer roll 16Y is derived from the detected current value by the control means 21. The optimum transfer current to be applied on each transfer roll 16Y, 16M, 16C and 16Bk is selected based on the derived synthesized resistance by the control means 21, and also, the value is set in each transfer bias applying source 20Y, 20M, 20C and 20Bk. And by controlling the applied voltage, the set transfer current is made to flow in the transfer roll.

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 such as an electrophotographic copying machine or a laser printer, and more particularly, to transferring a toner image formed in accordance with image information to a recording sheet or an intermediate transfer member. The present invention relates to an improvement for more accurately controlling the transfer conditions at the time.

[0002]

2. Description of the Related Art JP-A-10-207257 discloses that
A color image forming apparatus in which a plurality of image forming units are arranged side by side along a recording sheet conveyance path is disclosed. In this color image forming apparatus, black, yellow, cyan, and magenta color toner images based on image information are formed in each image forming unit, and these toner images are multiplex-transferred to a conveyed recording sheet. By inserting the sheet through the fixing device, a full-color recorded image is formed on the recording sheet.

The transfer of a toner image from each image forming unit to a recording sheet is performed by disposing a transfer roll to which a transfer bias voltage is applied on the back side of the recording sheet. During this period, a transfer current of a predetermined magnitude flows, so that the charged toner image is transferred onto the recording sheet by Coulomb force. The transfer roll also serves as a transport roll for the recording sheet. The recording sheet is sandwiched between the transfer roll and the photosensitive drum of each image forming unit, and the recording sheet is rotated by rotating the photosensitive drum. Are transported while receiving the transfer of the toner image.

In transferring a toner image, it is necessary for a sufficient transfer current to flow between the transfer roll and the image forming unit. However, the resistance of the transfer roll depends on the temperature and humidity in the image forming apparatus. For example, since the resistance value increases in a low-temperature and low-humidity environment, if a constant transfer bias voltage is always applied, a decrease in the transfer current causes transfer failure such as image omission.

[0005] For this reason, Japanese Patent Laid-Open No.
In the color image forming apparatus disclosed in Japanese Patent Application Laid-Open No. 7-107, while the recording sheet has not yet been conveyed to the transfer position of the toner image, the resistance value of the transfer roll is actually detected, and the first stage Before the transfer of the toner image starts, an optimal transfer bias voltage in each image forming unit is determined, and an optimal control of a transfer current flowing between each image forming unit and a transfer roll is performed.

[0006]

However, it is not only the transfer roll that is affected by fluctuations in temperature and humidity. That is, the resistance value of the recording sheet on which the toner image is actually transferred is also easily affected by the temperature and the humidity, and the resistance value greatly varies depending on the type of the recording sheet. In particular,
When a large number of recording sheets are stored in a paper feed tray of a copier or the like in a stacked state, except for the uppermost recording sheet, only the periphery of the recording sheet absorbs a lot of moisture, and the toner image is removed. The central part where the transfer is mainly performed is in a dry state compared to the peripheral part.

For this reason, when the transfer bias voltage is controlled by taking only the fluctuation of the resistance value of the transfer roll into account, the transfer current becomes insufficient depending on the state of the recording sheet, and transfer defects such as image omission also occur. It is expected that such a problem will occur.

In recent years, a recording sheet is attracted to an endless sheet conveying belt, and toner images are transferred from a plurality of image forming units to the recording sheet conveyed by the sheet conveying belt in a multiplex manner. Although a color image forming apparatus configured as described above is also known, in this type of image forming apparatus, the resistance value of the sheet conveying belt is also affected by the temperature and humidity, and the transfer bias voltage is determined in consideration of this. Otherwise, there is a concern that transfer failure may occur.

SUMMARY OF THE INVENTION The present invention has been made in view of such a problem, and an object of the present invention is to always obtain an optimum full-color transfer image without being affected by environmental factors such as temperature and humidity. To provide a simple image forming apparatus.

[0010]

In order to achieve the above object, an image forming apparatus according to the present invention comprises a sheet conveying belt which is wound around a plurality of rolls, rotates, and adsorbs and conveys a recording sheet. A plurality of image forming units which are arranged side by side along a recording sheet conveyance path by the sheet conveyance belt and form a toner image according to image information;
Assuming an image forming apparatus including a plurality of transfer rolls to which a predetermined transfer bias voltage is applied so as to transfer a toner image from each image forming unit to a recording sheet, the first stage image forming unit to the recording sheet Means for detecting the resistance value of the recording sheet conveyed before the transfer of the toner image is started, and control means for controlling a transfer bias voltage applied to each transfer roll based on the detected resistance value. It is characterized by having.

According to such a technical means, the resistance value of the recording sheet conveyed toward the image forming unit, that is, the recording sheet on which the toner image is actually transferred is detected, and the detected value is used. Since the transfer bias voltage applied to each transfer roll is controlled, a transfer bias voltage suitable for the resistance value of the recording sheet, which is easily affected by environmental factors such as temperature and humidity, can be applied to the transfer roll. It is possible to obtain a transfer current of a large size.

In the present invention, the timing for detecting the resistance value of the recording sheet may be a timing before the substantial transfer of the toner image from the first-stage image forming unit to the recording sheet is started. For example, when the leading edge of the recording sheet is inserted between the first-stage image forming unit and the transfer roll, the resistance value of the recording sheet is determined by using the leading end margin where the toner image is not transferred. May be detected. In this case, since the recording sheet overlaps the sheet conveyance belt and the transfer roll, the total resistance value of the recording sheet, the sheet conveyance belt and the transfer roll is actually detected, and the transfer is performed based on the detected resistance value. Determining the bias voltage is advantageous because the resistance values of the sheet conveying belt and the transfer roll are also taken into account.

However, since the peripheral portions of the recording sheets stacked in the paper feed tray absorb moisture more than the central portion, the resistance value of the recording sheets in the central portion is larger than that in the margin at the front end. It is considered high. Therefore, from the viewpoint of more accurately detecting the resistance value of the recording sheet, the resistance value of the recording sheet is determined to be upstream of the transfer position of the toner image from the first image forming unit to the recording sheet. Preferably, it is detected. With this configuration, after detecting the resistance value over substantially the entire surface of the recording sheet, the transfer bias voltage applied to each transfer unit can be controlled based on the averaged value and the like, and each image forming unit can be controlled. The transfer current generated between the transfer means and the transfer means can be more appropriately controlled.

For example, the resistance value of the recording sheet may be detected between a pair of sheet conveying rolls that convey the recording sheet from the sheet feeding tray toward the sheet conveying belt, or the recording sheet may be detected by the sheet conveying belt. The resistance value of the recording sheet may be detected using a sheet suction member to be sucked.

However, in this case, since the resistance values of the sheet conveying belt and the transfer roll are not considered at all,
From the viewpoint of more appropriately controlling the transfer bias voltage, it is preferable to provide a means for detecting the resistance values of the sheet conveying belt and the transfer roll in addition to detecting the resistance value of the recording sheet.

Further, according to the present invention having such a configuration, not only an image forming apparatus for directly transferring a toner image from an image forming unit to a recording sheet, but also the toner image is transferred to the recording sheet via an intermediate transfer belt. The present invention can also be applied to an image forming apparatus configured as described above.

That is, the present invention provides an intermediate transfer belt which is wound around a plurality of rolls and rotates, and a toner which is disposed side by side along a rotation path of the intermediate transfer belt and which is in accordance with image information. A plurality of image forming units for forming images, a plurality of primary transfer rolls to which a predetermined transfer bias voltage is applied to transfer a toner image from each image forming unit to the intermediate transfer belt, and a recording sheet from the intermediate transfer belt. The secondary transfer of the toner image from the intermediate transfer belt to the recording sheet is started on the assumption that the image forming apparatus includes a secondary transfer roll to which a predetermined transfer bias voltage is applied to transfer the toner image to the recording sheet. Means for detecting the resistance value of the conveyed recording sheet, and a transfer bias applied to the primary transfer roll and / or the secondary transfer roll based on the detected resistance value. It can be configured as an image forming apparatus and a control means for controlling the scan voltage.

Also in this case, the timing for detecting the resistance value of the recording sheet may be any time before the substantial transfer of the toner image from the intermediate transfer belt to the recording sheet is started. When the leading end is inserted between the intermediate transfer body belt and the secondary transfer roll, the resistance value of the recording sheet may be detected by using a leading end margin where no toner image is transferred.

From the viewpoint of more accurately detecting the change in the resistance value of the recording sheet, the recording sheet is located upstream of the secondary transfer position of the toner image from the intermediate transfer belt to the recording sheet. It is preferable to detect a change in the resistance value. With this configuration, after detecting a change in the resistance value over substantially the entire surface of the recording sheet, the transfer bias voltage applied to the secondary transfer roll can be controlled based on the detected value, and the intermediate transfer belt and the transfer bias voltage can be controlled. It is possible to more appropriately control a transfer current generated between the next transfer roll.

In the image forming apparatus using the intermediate transfer belt, the primary transfer of the toner image from each image forming unit to the intermediate transfer belt is performed based on the detected change in the resistance value of the recording sheet. The bias voltage may be controlled. Although the recording sheet is not directly related to the primary transfer of the toner image, it is necessary to detect changes in the resistance value of the recording sheet to understand to some extent changes in environmental factors such as temperature and humidity in the image forming apparatus. If the transfer bias voltage of the primary transfer is controlled based on this, without providing a temperature sensor or a humidity sensor,
It becomes possible to optimally control the transfer conditions of the primary transfer.

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an image forming apparatus according to the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 shows an image forming apparatus to which the present invention is applied. The image forming apparatus is configured by arranging a plurality of image forming units along a sheet conveying belt that sucks and conveys a recording sheet.

In FIG. 1, reference numeral 1 denotes a sheet conveying belt formed in an endless shape, reference numerals 2 and 3 denote belt conveying rolls around which the sheet conveying belt 1 is wound, and reference numeral 4 denotes a paper feed tray containing recording sheets P. Reference numeral 5 denotes a feed roller for sending out the recording sheet P from the paper feed tray 4;
Reference numeral 7 denotes a registration roll for feeding the recording sheet P toward the sheet transport belt 1 at a predetermined timing, and reference numeral 7 denotes a sheet suction roll for attracting the recording sheet P to the sheet transport belt 1.

On the conveyance path of the recording sheet P by the sheet conveyance belt 1, four image forming units 8Y, 8M, 8C and 8Bk for forming respective toner images of yellow, magenta, cyan and black are arranged from below. They are arranged in order. Each image forming unit will be described using a black image forming unit 8Bk as an example. A process unit 10 having a photosensitive drum 9 and a raster for writing an electrostatic latent image on the photosensitive drum 9 according to image information are described. And a scanning unit 11. The process unit 10 further includes a charging roll 12 for charging the photosensitive drum 9 to a uniform background potential, and a photosensitive roller 9 formed on the photosensitive drum 9 by the raster scanning unit 11. A developing device 13 for developing the formed electrostatic latent image to form a toner image;
Photoconductor drum 9 after transfer of toner image to recording sheet P
A cleaning blade 14 for removing residual toner and paper dust from the surface of the printer. On the other hand, the raster scanning unit 11 includes a polygon mirror 15 that rotates at a high speed, and is configured to scan a laser beam Bm modulated according to image information in the axial direction of the photosensitive drum 9.

Each of the image forming units 8Y, 8M, 8
C, 8Bk, at positions facing the photosensitive drums 9, the transfer rollers 16 </ b> Y, 1 </ b>
6M, 16C and 16Bk are provided. By applying a predetermined transfer bias voltage to these transfer rolls 16Y, 16M, 16C and 16Bk, the photosensitive drum 9 and the transfer rolls 16Y, 16M, 16C and 16Bk are provided. An electric field is formed between the photosensitive drum 9 and the charged toner image on the photosensitive drum 9 is transferred to the recording sheet P by Coulomb force.

Further, a fixing device 17 is provided above the process units 10 arranged vertically along the sheet conveying belt 1.
The recording sheet P peeled from the sheet conveying belt 1 after the transfer of the toner image is fixed to the toner image via the fixing device 17 and then discharged to the outside of the apparatus housing 19 by the discharge roller 18. Is done.

In the image forming apparatus according to the first embodiment, the recording sheet P sent from the sheet feed tray 4 is fed between the sheet suction roll 7 and the sheet conveying belt 1 at a predetermined timing. After being attracted to the sheet conveying belt 1 at such a point, each of the image forming units 8Y, 8
M, 8C, and 8Bk pass between the photosensitive drum 9 and the transfer roll 16. Then, each image forming unit 8Y, 8
M, 8C, and 8Bk, yellow, magenta, cyan, and black toner images are transferred from the photosensitive drum 9 on the surface of the recording sheet P in a superimposed manner. By melting and pressing the sheet P, a full-color image corresponding to the image information is formed on the recording sheet P.

FIG. 2 is a block diagram showing a first embodiment of the present invention applied to this image forming apparatus. The transfer rolls 16Y, 16M, 16C, and 16Bk facing the photosensitive drums 9 of the image forming units 8Y, 8M, 8C, and 8Bk are provided with transfer bias application power supplies 20Y, 20M, 20C, and 20.
Bk are connected to each other, and a transfer bias current of a magnitude set by the control unit 21 described later is applied to each of the transfer rolls 16Y, 16M, 16C, and 16Bk. The yellow image forming unit 8Y
Transfer roller 16Y and transfer bias application power supply 2
0Y, a current detection circuit 22 is connected, and the current (transfer current) flowing through the transfer roll 16Y when a predetermined voltage is applied to the transfer roll 16Y by the transfer bias applying power supply 20Y. It is configured to detect the magnitude and output the detected value to the control means 21.

The control means comprises a microcomputer system, and each of the transfer rolls 16Y, 16M, 16C, 16Bk is controlled based on a value detected by the current detection circuit.
And the transfer bias application power supplies 20Y, 20M, 20C, and 20Bk.
Sends the setting signal to the. The transfer bias application power supplies 20Y, 20M, 20C, and 20Bk change the transfer bias voltage applied to each of the transfer rolls 16Y, 16M, 16C, and 16Bk, so that the transfer current according to the setting signal is transferred to each of the transfer rolls 16Y. , 16M, 16
C, 16Bk are energized. As shown in Table 1 below, this control means uses the current value detected by the current detection circuit 22 to determine the photosensitive drum 9 and the transfer roll 1 at that time.
6Y is stored, and as shown in Table 2, the transfer rolls 16Y and 16Y are determined based on the resistance between the photosensitive drum 9 and the transfer roll 16Y.
A table is also stored that refers to the optimum transfer bias current value to be supplied to the M, 16C, and 16Bk, and the control unit 21 selects the optimum transfer bias current value from the detected values of the current detection circuit 22 and these tables. I do.

[0029]

[Table 1]

[0030]

[Table 2]

In such a configuration, first, the recording sheet P is conveyed out of the paper feed tray 4 at a predetermined timing, and is attached to the sheet conveying belt 1 by the sheet suction roll 7, and then the leading end of the recording sheet P Is inserted between the photosensitive drum 9 of the first-stage image forming unit 8Y and the transfer roll 16Y, a bias voltage of a certain magnitude is applied to the transfer roll 16Y by the transfer bias application power supply 20Y. Applied. As a result, a current flows between the transfer roller 16Y and the photosensitive drum 9 opposed to the recording sheet P via the sheet transport belt 1, and
This current value is detected by the current detection circuit 22. Then, the control means 21 derives a resistance value between the photosensitive drum 9 and the transfer roll 16Y from the detected current value by referring to Table 1 described above.

At this time, the resistance value derived by the control means 21 is a resistance value between the photosensitive drum 9 and the transfer roll 16Y, and is a combined resistance of the recording sheet P, the sheet conveying belt 1, and the transfer roll 16Y. . Thereafter, the control means 21 refers to the above-mentioned Table 2 to determine the transfer rolls 16Y, 16M, 16C, 1C from the derived combined resistance.
While selecting the optimal transfer current to be applied to 6Bk,
The value is transferred to the transfer bias application power supply 20Y, 20M, 20M.
C, set to 20Bk. Each transfer bias power supply controls the voltage applied to the transfer roll,
The set transfer current is supplied to the transfer roll. Thereby, each image forming unit 8Y, 8M, 8C, 8B
When the toner image is transferred from k to the recording sheet P, a newly set transfer bias voltage is applied to the transfer rolls 16Y and 16Y.
The voltage is applied between the photosensitive drums 9 and 16M, 16C, and 16Bk.

The resistance value of the recording sheet P is easily affected by temperature and humidity, and the recording sheet P on which the toner image is transferred.
It is expected that the resistance value will be different every time. After detecting the combined resistance including the resistance value of the recording sheet P in this manner, each of the transfer rolls 16Y, 16M is determined based on the detected resistance value. , 16C, and 16Bk, the transfer current taking into account the fluctuation of the resistance value of the recording sheet P is applied to each photoconductor 9 and the transfer rolls 16Y, 16B.
M, 16C, and 16Bk can flow, and it is possible to prevent a problem such as a transfer failure of the toner image as much as possible. Moreover, the resistance value measured in this embodiment is a combined resistance including the resistance values of the sheet conveying belt 1 and the transfer roll 16Y, and the fluctuation of these resistance values due to the fluctuation of environmental factors is taken into account at once. The transfer current can be determined, and the transfer bias voltage can be further optimized.

FIG. 3 is a block diagram showing a second embodiment of the present invention applied to this image forming apparatus. In the first embodiment shown in FIG. 2, the current detection circuit 22 is connected to the transfer roll 16Y, and when the leading end of the recording sheet P is inserted between the first-stage image forming unit 8Y and the transfer roll 16Y. Although the resistance value of the recording sheet P is detected, the recording sheet P that has been stored in the paper feed tray 4 for a long time is considered to have a lower resistance value in the peripheral part than in the central part. If the resistance value is detected only in the leading margin of P, the recording sheet P on which the toner image is mainly transferred
It is not possible to reliably grasp the resistance value at the central part of the image, and it is not possible to completely wipe out the concern that a transfer failure occurs. Therefore, in the second embodiment shown in FIG. 3, the first-stage image forming unit 8Y
The resistance value of the recording sheet P is configured to be detected by using the sheet suction roll 7 and the belt transport roll 2 provided on the upstream side. Note that the same components as those in the first embodiment are denoted by the same reference numerals in FIG. 3, and detailed description thereof will be omitted.

The sheet suction roll 7 forms a pair with the belt conveyance roll 2 around which the sheet conveyance belt 1 is wound, and the belt conveyance roll 2 has a suction bias application power supply 23 for applying a charge to the back side of the sheet conveyance belt 1. Is connected, while the sheet suction roll 7 facing the ground is grounded.
Since the recording sheet P and the sheet transport belt 1 are dielectric materials, a voltage is applied to the belt transport roll 2 from the attraction bias applying power source 23 with the recording sheet P inserted between the rolls 2 and 7. Then, charges of opposite polarities are generated on the back side of the sheet conveying belt 1 and the front side of the recording sheet P, and the recording sheet P is electrostatically attracted to the surface of the sheet conveying belt 1.

A current detection circuit 24 is connected between the belt transport roll 2 and the suction bias application power supply 23, and applies a voltage of a certain magnitude to the belt transport roll 2 by the suction bias application power supply 23. In this case, the magnitude of the current (transfer current) flowing through the belt transport roll 2 is detected, and the detected value is output to the control means 21 as in the first embodiment.

In such a configuration, first, when the recording sheet P is conveyed out of the paper feed tray 4 at a predetermined timing, the recording sheet P arrives at a position between the sheet suction roll 7 and the sheet transport belt 1 at a predetermined timing. At the same time, a bias voltage of a certain magnitude is applied to the belt transport roll 2 by the suction bias application power supply 23. As a result, while the recording sheet P is attracted to the sheet transport belt 1, a current flows between the sheet suction roll 7 and the belt transport roll 2 which are opposed to each other via the recording sheet P and the sheet transport belt 1. The value is detected by the current detection circuit 24. The detected current value is input to the control means 21. Since the detected current value is detected in a state where the recording sheet P is inserted between the sheet suction roll 7 and the belt transport roll 2, the current value will be changed from now on. This reflects the resistance value of the recording sheet P on which the transfer of the toner image is performed.

On the other hand, in the control means 21 in this embodiment, a table for deriving the resistance value of the recording sheet P from the current value detected by the current detection circuit 24 is provided in the same manner as in Tables 1 and 2 described above. A table for deriving an optimal transfer current value to be supplied to each of the transfer rolls 16Y, 16M, 16C, and 16Bk is stored in advance from the resistance value of P, and the control unit 21 performs detection by referring to these tables. From the current value obtained, each transfer roll 1
6Y, 16M, 16C, and 16Bk, the optimal transfer current to be supplied to the transfer bias application power supply 20 is determined.
Y, 20M, 20C, and 20Bk are set. Accordingly, when the toner image is transferred from each of the image forming units 8Y, 8M, 8C, 8Bk to the recording sheet P, a magnitude bias voltage corresponding to the set transfer current is applied to the transfer rolls 16Y, 16M, 16C, 16Bk and each photosensitive drum 9
Is applied between

The transfer of the yellow toner image to the recording sheet P is started because the sheet suction roll 7 is located on the upstream side of the first-stage image forming unit Y with respect to the conveyance path of the recording sheet P. By this time, such a large area of the recording sheet P has passed through the sheet suction roll 7, and the control means 21 has detected not only the margin at the front end of the recording sheet P but also the resistance value near the center. The transfer bias voltage can be determined. Since the peripheral portion of the recording sheet P is considered to have a higher moisture content and a lower electrical resistance value than the central portion, the transfer bias voltage is determined based on the resistance value of only the margin at the leading end of the recording sheet P. In this case, a sufficient transfer current cannot be obtained at the central portion of the recording sheet P, and there is a concern that transfer failure of the toner image may occur. However, when the transfer bias voltage is determined in consideration of the resistance value of the central portion of the recording sheet P as in the second embodiment, such a concern can be eliminated, and a sufficient transfer current can always be obtained. To each of the image forming units 8Y, 8M, 8
C, 8Bk and transfer rolls 16Y, 16M, 16C, 16
Bk, and a good transfer image can be obtained on the recording sheet.

When the resistance value of the recording sheet P is determined by the control means 21 as described above, as shown in FIG.
The bias voltage applied to the belt transport roll 2 may be changed from 3. When the resistance value of the recording sheet P changes, the energy required for electrostatically attracting the recording sheet P to the sheet conveying belt 1 also changes. Therefore, the bias voltage is changed according to the resistance value of the recording sheet P. By doing so, it is possible to prevent poor adhesion of the recording sheet P to the sheet conveying belt 1, and it is possible to prevent the transfer position of the toner image from shifting and the occurrence of jams and the like.

When environmental factors such as temperature and humidity in the image forming apparatus change, not only the resistance value of the recording sheet P but also the resistance values of the transfer rolls 16Y, 16M, 16C, 16Bk and the sheet conveying belt 1 change. Therefore, in the second embodiment shown in FIG. 3, if it is desired to more strictly control the transfer bias voltage to obtain a transfer current of a desired magnitude, a state in which the recording sheet P is not conveyed at all. Therefore, it is preferable to detect the combined resistance of the transfer roll 16Y and the sheet conveying belt 1 and determine the optimum transfer bias voltage by taking this into consideration. Further, when the resistance value of the recording sheet is detected between the sheet suction roll 7 and the belt transporting roll 2, since the fluctuation of the resistance value of these rolls 2 and 7 is included, the recording is more strictly performed. If the resistance value of the sheet P is to be detected, the resistance value between the sheet suction roll 7 and the belt transport roll 2 is detected in a state where the recording sheet P is not inserted, and the recording sheet P is determined from the difference between the sheet suction roll 7 and the belt transport roll 2. It is preferable to grasp the resistance value of.

Further, the portion for measuring the resistance value of the recording sheet P is not limited to between the sheet suction roll 7 and the belt transport roll 2 as long as it is located upstream of the transfer position of the first-stage toner image. For example, it may be between a pair of registration rolls 6 shown in FIG. In this case, a power supply for applying a constant voltage to the registration roll 6 and a current detection circuit are provided.

FIG. 4 shows a third embodiment in which the present invention is applied to an image forming apparatus of the type which transfers a toner image to a recording sheet using an intermediate transfer belt. In the figure, reference numeral 30 denotes an intermediate transfer belt, and reference numerals 31 and 32 denote belt transport rolls around which the intermediate transfer belt 30 is wound. One side of the rotating intermediate transfer belt 30 is yellow, magenta, The cyan and black image forming units 8Y, 8M, 8C, and 8Bk are arranged. Primary transfer rolls 33Y, 33M, 33C, and 33Bk are disposed at positions facing the photosensitive drums 9 of the image forming units 8Y, 8M, 8C, and 8Bk via an intermediate transfer belt 30. The toner images of the respective colors formed on the intermediate transfer belt 9 are primarily transferred onto the intermediate transfer belt 30 in a superimposed manner. A secondary transfer roll 34 is provided at a position facing the belt transport roll 32 with the intermediate transfer body belt 30 interposed therebetween.
The full-color toner image multi-transferred from Y, 8M, 8C, 8Bk to the intermediate transfer belt 30 is secondarily transferred to the recording sheet P inserted between the secondary transfer roll 34 and the belt transport roll 32. It has become. FIG.
Reference numeral 35 denotes a registration roll that feeds the recording sheet P at a predetermined timing toward the secondary transfer position. Further, the image forming units 8Y, 8
Since M, 8C, and 8Bk have exactly the same configuration as that described in FIG. 1, only the photosensitive drum 9 of each image forming unit is shown in FIG. 4 for simplification.

Each image forming unit 8Y, 8M, 8C, 8
Transfer rolls 33Y and 3 facing the photosensitive drum 9 of Bk
Primary transfer bias application power supplies 36Y, 36M, 36C, and 36Bk are connected to 3M, 33C, and 33Bk, respectively, and transfer currents of a magnitude set by the control unit 40 are applied to the primary transfer rolls 33Y, 33M, 33C, and 33Bk.
Is to be energized. Further, a secondary transfer bias applying power source 37 is connected to the secondary transfer roll 34, and a transfer current having a magnitude set by the control means 40 is also supplied to the secondary transfer roll 34. I have. In addition, a current detection circuit 38 is connected between the secondary transfer roll 34 and the secondary transfer bias application power supply 37, and the secondary transfer roll 34 is fixed to a predetermined size by the secondary transfer bias application power supply 37. Is applied, the magnitude of the current (transfer current) flowing through the secondary transfer roll 34Y is detected, and the detected value is output to the control means 40.

The control means 40 is constituted by a microcomputer system, and each of the primary transfer rolls 33Y, 33M, 33M based on the value detected by the current detection circuit 38.
C, 33Bk and the value of the optimum transfer current to be supplied to the secondary transfer roll 34 are calculated, and the primary transfer bias application power source 3
The setting signal is sent to 6Y, 36M, 36C, 36Bk and the secondary transfer bias applying power source 37. The control means 40 stores a table for referring to the resistance value between the belt transport roll 32 and the secondary transfer roll 34 at that time from the current value detected by the current detection circuit 38. , 34, the primary transfer rolls 33Y, 33M, 33C, 33Bk
And a table for referencing the optimum value of the transfer current to be supplied to the secondary transfer roll 34.
Selects the optimum transfer current value from the detected values of the current detection circuit 38 and these tables.

In such a configuration, first, the image margin at the leading end of the recording sheet P conveyed from a paper feed tray (not shown) is formed by the belt conveying roll 32 and the secondary transfer roll 3.
4, the secondary transfer bias applying power source 3
7 applies a bias voltage of a certain magnitude to the secondary transfer roll 34. As a result, a current flows between the belt transfer roll 32 and the secondary transfer roll 34 which face each other via the recording sheet P and the intermediate transfer belt 30, and the current value is detected by the current detection circuit 38. The detected current value is input to the control means 40. Since the detected current value is detected in a state where the recording sheet P is inserted between the secondary transfer roll 34 and the belt transport roll 32, This reflects the resistance value of the recording sheet P on which the secondary transfer of the toner image is performed.

Thereafter, the control means 21 determines the optimum transfer current value to be supplied to the secondary transfer roll 34 from the detected current value by referring to the stored table, and transfers the value to the secondary transfer. The bias application power supply 37 is set. As a result, when the full-color toner image is transferred from the intermediate transfer belt to the recording sheet, the transfer bias voltage required to obtain the set transfer current value is applied between the secondary transfer roll 34 and the belt transport roll 32. Applied between
Regardless of the fluctuation of the resistance of the recording sheet P, a sufficient transfer current for transferring the full-color toner image onto the recording sheet P can be applied between the secondary transfer roll 34 and the belt transport roll 32.

In this embodiment, since the resistance value of the recording sheet P is detected, the fluctuation of environmental factors such as temperature and humidity in the image forming apparatus is indirectly detected. Therefore, the temperature and humidity in the image forming apparatus are estimated from the detected resistance value of the recording sheet P, and the primary transfer bias voltage based on the estimated values is set in each of the primary transfer bias application power supplies 36Y, 36M, 36C, and 36Bk. You can also. With this configuration, the bias voltage applied to each of the primary transfer rolls 33Y, 33M, 33C, and 33Bk can be appropriately controlled without providing a temperature sensor or a humidity sensor in the image forming apparatus, and the photosensitive drum can be controlled. 9, the primary transfer of the toner image to the intermediate transfer belt 30 can be favorably performed.

In the third embodiment, the resistance value of the recording sheet P is detected by utilizing the margin of the leading end of the recording sheet P inserted between the secondary transfer roll 34 and the belt transport roll 32. However, from the viewpoint of detecting the resistance value at the center of the recording sheet P and further optimizing the transfer bias voltage, as in the second embodiment, the conveyance of the recording sheet P is considered. The configuration may be such that the resistance value of the recording sheet P is detected using the registration roll 35 located on the upstream side of the secondary transfer position of the toner image with respect to the path.

[0050]

As described above, according to the image forming apparatus of the present invention, the resistance value of the recording sheet on which the toner image is actually transferred is detected, and the detected value is applied to each transfer roll. By controlling the transfer bias voltage to be applied, a transfer bias voltage suitable for the resistance value of the recording sheet, which is easily affected by environmental factors such as temperature and humidity, can be applied to the transfer roll. Current can be obtained, and an optimal full-color transfer image can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a schematic configuration of an image forming apparatus to which the present invention is applied.

FIG. 2 is a block diagram showing a first embodiment of the present invention.

FIG. 3 is a block diagram showing a second embodiment of the present invention.

FIG. 4 is a block diagram showing an embodiment in which the present invention is applied to an image forming apparatus using an intermediate transfer member belt.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Sheet conveyance belt, 8Y, 8M, 8C, 8Bk ... Image forming unit, 21 ... Control means, 30 ... Intermediate transfer belt, P ... Recording sheet

Claims (7)

[Claims] 1. A sheet conveying belt which is wound around a plurality of rolls, rotates, and adsorbs and conveys a recording sheet, and is arranged and arranged along a conveying path of the recording sheet by the sheet conveying belt. An image forming apparatus comprising: a plurality of image forming units for forming toner images corresponding to information; and a plurality of transfer rolls to which a predetermined transfer bias voltage is applied to transfer a toner image from each image forming unit to a recording sheet. A means for detecting a resistance value of the recording sheet conveyed before the transfer of the toner image from the first-stage image forming unit to the recording sheet is started; A control unit for controlling a transfer bias voltage applied to the transfer roll. 2. The image forming apparatus according to claim 1, wherein
The resistance value of the recording sheet is detected in a state where the leading end margin of the recording sheet is inserted between the first-stage image forming unit and the first-stage transfer roll opposed thereto. Image forming device. 3. The image forming apparatus according to claim 1, wherein
An image forming apparatus for detecting a resistance value of a recording sheet at a position upstream of a transfer position of a toner image from a first-stage image forming unit to the recording sheet in a recording sheet conveyance path. 4. The image forming apparatus according to claim 3, wherein
Before the recording sheet is inserted between the first-stage image forming unit and the transfer roll facing the first-stage image forming unit, the first
Means for detecting the resistance values of the transfer roll and the sheet transport belt of the stage are provided, and the control means applies the transfer to each transfer roll based on the detected resistance values of the transfer roll, the sheet transport belt and the recording sheet. An image forming apparatus that controls a bias voltage. 5. An intermediate transfer belt which is wound around a plurality of rolls and rotates, and is arranged along the rotation path of the intermediate transfer belt and forms a toner image according to image information. A plurality of image forming units, a plurality of primary transfer rolls to which a predetermined transfer bias voltage is applied to transfer a toner image from each image forming unit to the intermediate transfer belt, and a toner image from the intermediate transfer belt to a recording sheet. An image forming apparatus including a secondary transfer roll to which a predetermined transfer bias voltage is applied for transfer, wherein the toner image is conveyed before the secondary transfer of the toner image from the intermediate transfer belt to the recording sheet is started. Means for detecting the resistance value of the received recording sheet, and a system for controlling a transfer bias voltage applied to the primary transfer roll and / or the secondary transfer roll based on the detected resistance value. An image forming apparatus comprising the means. 6. The image forming apparatus according to claim 5, wherein
An image forming apparatus for detecting a resistance value of a recording sheet at a position upstream of a secondary transfer position of a toner image from the intermediate transfer belt to the recording sheet in a recording sheet conveyance path. 7. The image forming apparatus according to claim 6, wherein
Before the recording sheet is inserted between the intermediate transfer belt and the secondary transfer roll opposed thereto, means for detecting the resistance values of the intermediate transfer belt and the secondary transfer roll are provided. The image forming apparatus controls the transfer bias voltage applied to the secondary transfer roll based on the detected resistance values of the secondary transfer roll, the intermediate transfer belt, and the recording sheet.