JPH09127803A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To always obtain a stable transferred image by controlling a transfer charge most suitable for a transfer which is applied onto a carrying belt, in accordance with fluctuations in transfer conditions, with a transfer current flowing from a transfer roller to the carrying belt. SOLUTION: The results of the detection by charge quantity detectors 110 are inputted to a CPU 210 as a control means and the CPU 210 executes control for making a current reference value previously set in a constant current power source 200 as a constant current supplying means very low so that the transfer current most suitable for the transfer flows from the transfer roller 35 to the carrying belt 3, in the next transfer part. Thus, even if variations in an environment, where a device is used, such as temperature and humidity or environmental fluctuations at the time of transferring occur, or moisture content, thickness, etc., are varied because the kind of recording paper 4 is different, a constant change can be always applied to the carrying belt 3 from each transfer roller 35, without being affected by the change in the transfer conditions due to the above-mentioned variations.

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 a copying machine, a facsimile, and a printer.
The present invention relates to an image forming apparatus that conveys a recording material electrostatically adsorbed on a conveyor belt by the conveyor belt and transfers a developed image formed on the surface of a latent image carrier to the recording material to form an image.

[0002]

2. Description of the Related Art Conventionally, a recording material is conveyed on an endlessly movable conveyor belt, brought into contact with the conveyor belt, and developed images formed on the surfaces of a plurality of latent image carriers are transferred to a plurality of transfer rollers using a plurality of transfer rollers. 2. Description of the Related Art An image forming apparatus is known that transfers an image to a recording material to form an image. A transfer roller used in this type of image forming apparatus is generally disposed on the opposite side of the latent image carrier via a conveyor belt, and forms a developed image between the recording material and the latent image carrier. The amount of electric charge applied to the conveyor belt to generate an electric field in the direction in which the developing agent moves from the latent image carrier to the recording material is controlled by the voltage applied to the transfer roller. The developed image above is transferred onto a recording material to form a transferred image.

[0003]

However, in forming the image by transferring the developed image formed on each of the latent image carriers by each of the transfer rollers to the recording material conveyed by the conveyor belt, , As the transfer belt is charged by the transfer and the transfer is repeated,
The charge amount on the conveyor belt becomes higher on the downstream side in the recording material conveying direction. Therefore, in order to supply the transfer belt with an appropriate transfer charge according to the amount of charge on the transfer belt at each transfer unit, the voltage applied to each transfer roller is increased toward the downstream side in the recording material transfer direction. It is necessary to apply different voltages to the transfer rollers.

However, for example, when the residual charge on the conveyor belt between the transfer portions is neutralized by using a neutralizing means or the like, when the use environment such as temperature or humidity changes, the amount of the electrical charge removed also changes. I will end up. If electric charges remain on the surface of the conveyor belt due to such a change in the amount of static elimination charges, the potential on the surface rises and it becomes difficult to give new electric charges to the conveyor belt. That is, there is a problem that transfer charges for transferring a developed image onto the recording material are less likely to be applied to the conveyor belt. Also, in the case where the conveyor belt is made of a high resistance material having a surface resistivity of, for example, 10 ¹⁴ Ω / sq. Or more,
Since electric charges remain on the back surface of the transfer belt after transfer,
The transfer charge application conditions differ for each transfer portion. That is, since the residual charge amount increases in the transfer portion on the downstream side of the recording material in the conveying direction, when the transfer charges supplied to the respective transfer portions are all set to the same condition, the transfer portion on the downstream side in the conveying direction of the recording material is set. There is a problem that the transferred image deteriorates.
As described above, even if different voltages are applied to the transfer rollers, it is difficult to maintain a constant image quality because the transfer conditions of the developed image differ depending on the use environment. It was

The present invention has been made in view of the above problems, and an object of the present invention is to transfer a transfer charge optimum for transfer given to a transfer belt from the transfer roller according to a change in transfer conditions. An object of the present invention is to provide an image forming apparatus that can always obtain a stable transferred image by controlling the transfer current flowing through the belt.

[0006]

In order to achieve the above-mentioned object, the invention of claim 1 provides a plurality of latent image carriers for forming latent images and a latent image of each of the plurality of latent image carriers. A plurality of developing devices for forming an image, a conveyor belt that is arranged so as to contact each of the plurality of latent image carriers, and conveys a recording material, and sandwiches the conveyor belt and faces the latent image carrier. A transfer device having a plurality of transfer rollers which are respectively disposed on the recording medium side and electrically transfer the developed image on the latent image carrier visualized by the developing device to the recording material side; A constant current supply capable of independently supplying a constant transfer current to each of the transfer rollers from the transfer rollers to generate an electric field in a direction in which the developer to be formed moves from each latent image carrier to the recording material. Means and are provided.

In particular, the invention of claim 2 is the image forming apparatus of claim 1, which is arranged upstream of the transfer rollers in the conveying direction of the recording material and moves between the latent image carriers. Static elimination means for eliminating residual charge remaining on the conveyor belt, charge amount detection means for detecting the amount of charge eliminated by the static elimination means, and the constant current supply means based on the detection result of the charge amount detection means. And a control means for controlling the.

In particular, the invention of claim 3 is the image forming apparatus of claim 1, wherein a patch latent image forming means for forming a patch latent image for image density detection is formed on each latent image carrier, and the recording material. The optical density of the patch image after visualization, which is arranged after passing the latent image carrier located on the most downstream side in the conveyance direction of the above, is transferred onto the recording material by the plurality of transfer rollers. Based on the optical detecting means for detecting and the detection result detected by the optical detecting means, the constant current is supplied so as to supply a constant transfer current to the plurality of transfer rollers at the next transfer to another recording material. A control means for controlling the supply means is provided.

Particularly, in the invention of claim 4, in the image forming apparatus of claim 1, a material having a volume resistivity of 10 ⁸ Ω · cm or less is used in a portion of the transfer roller which is in contact with the conveyor belt. It is characterized by that.

In particular, the invention of claim 5 is the image forming apparatus of claim 1, wherein the conveyor belt is 10 ^{8 to} 10
It is characterized by being constructed using a material having a surface resistivity of ¹³ Ω / sq.

In particular, the invention of claim 6 is the image forming apparatus of claim 1, wherein the conveying belt is 10 ¹⁴ Ω / sq.
It is characterized by being configured using a material having the above surface resistivity.

According to the first to sixth aspects of the present invention, since a constant transfer current is independently supplied to each transfer roller by the constant current supply means, different transfer voltages are independently applied to each transfer roller. On the other hand, even if the transfer conditions change due to environmental changes at the time of transfer such as temperature and humidity, a constant charge is always applied to the conveyor belt. Further, since the optimum transfer current flowing through the conveyor belt for transfer is substantially constant regardless of the type of recording material used, it is not necessary to supply different transfer current for each recording material used.

In particular, according to the second aspect of the invention, a part of the electric charge applied from the transfer roller to the conveyor belt remains on the conveyor belt after the transfer. By positively removing this residual charge by the charge eliminating means, fluctuations in the transfer condition at each transfer portion can be suppressed when transferring to the next recording material. Further, the charge amount detected by the charge removing unit is detected by the charge amount detecting unit, and based on the detection result, control is performed so that the transfer current flowing from each transfer roller to the conveyor belt becomes the optimum transfer current for transfer. Means controls the constant current supply means.

Particularly, in the third aspect of the invention, the image density of the patch image for image density detection formed on the recording material at the time of each development and transfer in the transfer portion is optically detected by the optical detection means. To detect. Then, based on the detection result of the patch image density, the control unit controls the constant current supply unit so that the transfer current flowing from each transfer roller to the conveyor belt at the next transfer becomes the optimum transfer current for transfer. .

Here, as the transfer roller, for example,
When a roller electrode in which a conductive member is wound around a core metal is used, and the conductive member in the portion that comes into contact with the conveyor belt has a high volume resistivity, a portion between the center electrode of the transfer roller and the roller surface is used. Since the voltage drops, it is necessary to generate a high voltage by the constant current supply means in order to apply a constant current to the conveyor belt, and the load applied to the constant current supply means becomes large. The current supply means becomes large.

Therefore, in the invention of claim 4, the conductive member in the portion contacting the conveyor belt is set to 10 ⁸ Ω · c.
By using a material having a volume resistivity of m or less, the movement of charges in the conveyor belt is promoted as compared with the case of using a material having a volume resistivity of more than 10 ⁸ Ω · cm. By doing so, a desired transfer current can be supplied to the conveyor belt without generating a high voltage by the constant current supply unit.

In the invention of claim 5, 10 ⁸
By configuring the conveyor belt using a material having a surface resistivity of 10 to 10 ¹³ Ω / sq., The electric charge applied from the transfer roller to the conveyor belt passes through the conveyor belt and the recording material, It is directly associated with the charge of the developer on the latent image carrier. As a result, the amount of electric charge applied by the transfer roller directly contributes to the transfer as it is, and the transfer current can be controlled more precisely. Further, in comparison with the case where the conveyor belt is formed using a material having a surface resistivity other than the above range,
Since the residual charges remaining on the transport belt after the transfer are reduced, it is possible to make the transfer currents at the plurality of transfer parts all substantially constant.

In the invention of claim 6, 10 ¹⁴
By configuring the conveyor belt using a material having a surface resistivity of Ω / sq. Or more, a part of the electric charge applied to the conveyor belt by the transfer roller remains on the conveyor belt even after the transfer. To do. Then, the residual charge electrostatically fixes the recording material on the conveyor belt after the transfer. (Hereinafter, margin)

[0019]

DETAILED DESCRIPTION OF THE INVENTION An embodiment in which the present invention is applied to a color electrophotographic printer (hereinafter referred to as a printer) which is an image forming apparatus will be described below. FIG. 2 is a schematic configuration diagram of the printer according to the present embodiment. This printer comprises a conveyor belt 3 stretched between a driven roller 1 and a drive roller 2 and moving endlessly in the direction of arrow A, and recording paper as a recording material that is electrostatically attracted to and conveyed by the conveyor belt. Four
A black toner image forming unit B, a magenta toner image forming unit M, a yellow toner image forming unit Y, and a cyan toner image forming unit C that form a toner image to be transferred to are provided. Further, the optical writing unit 50 that forms a latent image on the photoconductor drums 30B, 30M, 30Y, and 30C as the latent image carriers in B, M, Y, and C of each of the toner image forming units is provided on each photoconductor. Drum 30B,
It is provided at a position facing 30M, 30Y, and 30C.

Since the toner image forming units B, M, Y, and C have the same configuration, the description will be given only for the black toner forming unit B, and for the other toner image forming units M, Y, and C, in the drawing. The description corresponding to the parts in the black toner image forming unit will be omitted because the parts corresponding to those in the black toner image forming unit will be marked with B, M and C after the numbers. The photoconductor drum 30B of the black toner image forming unit B is rotatably provided in the vicinity of the conveyor belt 3, and a black toner image is formed around the photoconductor drum 30B on the surface of the photoconductor drum 30B by an electrophotographic process. For this purpose, a charging device 31B, a developing device 32B, a charge eliminating device 33B, and a cleaning device 34B are provided. The developing device 32B is a wet developing device that uses a two-component developing solution in which toner is dispersed in a carrier made of a petroleum solvent or a one-component developing solution, and is a latent image formed on the surface of the photoconductor drum 30B. To develop a black toner image. Further, the developing device 32B includes a developing roller 40B that supplies the developing solution to the photosensitive drum 30B, and a squeeze roller 41B that removes an excessive amount of the developing solution supplied by the developing roller from the surface of the photosensitive drum 30B. ing.

Below the black toner image forming unit B, a paper feeding unit 5 for feeding the recording paper 4 to the conveyor belt 3.
Is provided. On the other hand, downstream of the cyan toner image forming unit C in the rotation direction of the conveyor belt 3, a separating device 6 for separating the recording paper 4 on which the toner image is transferred from the conveyor belt 3 is provided. The separating device 6 is composed of a separating charger that separates the recording paper 4 from the conveying belt 3 due to its rigidity by weakening the electrostatic attraction force between the conveying belt 3 and the recording paper 4 at the separating position 7. Further, a transport roller 9 for transporting the recording paper 4 to the tray 8 is provided downstream of the separating device 6 in the transportation direction of the recording paper 4.

Next, the operation of this printer will be described. In forming an image, the paper feeding unit 5 supplies the recording paper 4 to the conveyor belt 3, and the transfer roller 35B applies an electric charge to the conveyor belt 3 at the electric charge applying position 10. Then, the conveyor belt 3 in the state in which this electric charge is applied
However, the recording paper 4 supplied by the paper supply unit 5 is electrostatically adsorbed. Here, the drive roller 2 driven by a drive mechanism (not shown) transmits a rotational drive force to the conveyor belt 3 to rotate the belt in the direction of arrow A. In addition, a drive system (not shown) is used for each of the photosensitive drums 30B, 30M, 30
B and 30C are rotated so that the peripheral speed thereof is the same as the moving speed of the conveyor belt 3.

Each device forms a black toner image on the surface of the photosensitive drum 30B which rotates at the same speed as the conveying belt 3. That is, the charging device 31B is the photosensitive drum 30.
The surface of B is uniformly charged, and the optical writing unit 50 forms an electrostatic latent image on the uniformly charged surface of the developing device 32B.
Supplies a black developer to the electrostatic latent image to form a black toner image. Then, the transfer roller 35B is
The black toner image is transferred onto the recording paper 4 that has been conveyed to the nip portion formed by the conveyor belt 3 and the photosensitive drum 30B. That is, the transfer roller 35B applies an electric charge to the conveyor belt 3 to generate an electric field in the vicinity of the nip portion, and the charged black toner particles forming the toner image formed on the surface of the photosensitive drum 30B are transferred to the recording paper 4. The black toner image is transferred to the recording sheet 4 by moving the recording sheet to the recording sheet 4. Further, the static eliminator 33B removes the residual charge on the surface of the photoconductor drum 30B on which the toner image is transferred, and the cleaning device 34B removes the black toner remaining on the surface of the photoconductor drum 30B.

By the above-described operation, the recording paper 4 in the state where the black toner image is formed on the surface is transferred to the conveying belt 3
Are sequentially conveyed to a nip portion formed by the conveyor belt 3 and the photosensitive drum 30M, a nip portion formed by the conveyor belt 3 and the photosensitive drum 30Y, and a nip portion formed by the conveyor belt 3 and the photosensitive drum 30C. . Then, in each of the nip portions, the transfer roller 35M forms a magenta toner image, the transfer roller 35Y forms a yellow toner image, and the transfer roller 35
C transfers the cyan toner image to the recording paper 4,
The toner image is superimposed on the surface of the recording paper 4. The photosensitive drums 30M and 30 of the toner image forming units M, Y, and C, respectively.
The operation of forming the toner images on Y and 30C, and the transfer rollers 35M, 35Y, and 35C form the toner images on the recording paper 4 respectively.
Since the operation of transferring to the same as that of the black toner image forming unit B and the transfer roller 35B, the description thereof will be omitted.

Then, the conveyor belt 3 conveys the recording paper 4 on which the four color toners are superposed to the separating position 7. Then, at the separating position 7, the separating device 6 separates the recording paper 4 from the surface of the conveyor belt 3. That is, the separating device 6 removes a part of the electric charges adhering to the surface of the conveyor belt 3 and weakens the electrostatic attraction force between the conveyor belt 3 and the recording paper 4. Then, the recording paper 4 is separated from the conveyor belt 3 due to the weakened electrostatic attraction to the conveyor belt 3 and its rigidity, and is conveyed to the tray 8 by the conveyor rollers 9. Further, the surface portion of the conveyor belt 3 from which the recording paper 4 is separated moves endlessly and reaches the vicinity of the belt cleaning device 11. Then, the belt cleaning device 11 removes foreign matters such as toner adhering to the conveyor belt 3. The surface portion of the conveyor belt 3 from which the foreign matter is removed by the belt cleaning device 11 continues to move endlessly and reaches the charge applying position 10. Then, the above operation is repeated thereafter.

FIG. 1 is an explanatory diagram showing a schematic structure for controlling a transfer current according to the printer of this embodiment. A transfer roller 35 that applies a constant current to the conveyor belt 3 is provided on the opposite side of the conveyor belt 3 from the photosensitive drum 30. In addition, from each transfer roller 35 to the conveyor belt 3
Is provided with a constant current power supply 200 as a constant current supply means for supplying a preset constant current,
The constant current supply from the constant current power source 200 is controlled by the CPU 210 as a control means. Further, there are provided a static eliminator 33 for neutralizing the residual charges remaining on the transfer belt 3 after transfer, and a charge amount detector 110 as a charge amount detecting means capable of measuring the amount of charge eliminated in each transfer section. The reference value of the constant current power source 200 can be finely adjusted by the control of the CPU 210 as a control means. As a specific mode of static elimination of the conveyor belt 3, the conveyor belt 3 is electrically grounded, a pair of conductive brushes to which a DC voltage or an AC voltage is applied, or a conductive belt wound around a conductive substrate. It is desirable that the non-woven fabric or the like is brought into contact with the conveyor belt 3.

Here, FIG. 3 is a graph showing the relationship between the image density and the transfer current in this embodiment. According to FIG. 3, the maximum density is reached around 30 μA and the ID value is almost constant, so the reference current value is set to 30 μA in the constant current power supply 200 of the present embodiment, and variations in the transfer conditions are handled in this vicinity. The constant current power supply 200 is controlled so that the constant transfer current flows from the transfer roller 35B to the conveyor belt 3. According to experiments conducted by the present inventors, the transfer current at which the image density becomes maximum and the ID value becomes almost constant is
It was confirmed that it was almost constant regardless of the type of the recording paper 4.

In the above embodiment, a constant electric current is passed from the transfer roller 35 to the conveyor belt 3 to generate a transfer electric field between the photosensitive drum 30 and the recording paper 4 in proportion to the electric current. The toner image formed on the photosensitive drum 30 is transferred onto the recording paper 4. The direction of the electric field generated by the transfer roller 35 is the direction in which the toner particles forming the toner image on the surface of the photoconductor drum 30 are moved from the photoconductor drum 30 to the recording paper 4. Also,
Based on the charge amount detected by the charge amount detection device 110, the reference value of the transfer current is finely adjusted for each transfer portion. That is, the detection result detected by the charge amount detection device 110 is input to the CPU 210 as a control unit, and the C
The PU 210 transfers the transfer roller 35 at the next transfer portion.
Control is performed so that a current reference value preset in the constant current power supply 200 as a constant current supply means is slightly changed so that a transfer current most suitable for transfer from the transfer belt 3 to the conveyor belt 3 flows.

According to the above-described embodiment, in each transfer portion, the constant current power supply 200 supplies a constant transfer current, which is independently supplied from the transfer roller 35 to the conveyor belt 3, and which is optimum for transfer. Thus, for example,
If there is a change in the operating environment of the device such as temperature or humidity or an environmental change during transfer, or if the type of recording paper 4 is different,
Even if the water content or thickness changes, it does not depend on the changes in transfer conditions due to these changes.
It is possible to always apply a constant charge from each transfer roller 35 to the conveyor belt 3. Therefore, as compared with the case where different transfer voltages are independently applied to the transfer rollers 35,
It is possible to easily and stably obtain a transferred image.

In particular, in the present embodiment, even when the transfer condition changes, for example, when the charge removal amount of the residual charge remaining on the conveyor belt 3 changes due to an environmental change such as temperature and humidity when the apparatus is used. The transfer from the constant current power supply 200 is performed based on the detection result detected by the charge amount detection device 110 so that an appropriate amount of current is supplied so that the transfer current at the next transfer portion is not affected by this change. Since the current can be finely adjusted, a more stable transferred image can be obtained.

When only the charge less than the allowable range of the standard value of the charge removal amount is removed, a desired current is supplied to the constant current power supply 200 at the next transfer portion unless a high voltage is applied. Because it can no longer flow to
The allowable value of the constant current power supply 200 may be exceeded. Therefore, in order to prevent such a fear, the CPU 210 is provided with a constant current power supply control function of lowering the current below the reference value or stopping the constant current supply to the transfer roller 35B. There is. This ensures the safety of the device.

As another embodiment, FIG.
Color patches 300 for detecting the image density as shown in (a) and (b) are formed on the recording paper 4 at each transfer section,
The optical density sensor 150 as an optical detecting means for reading the density of the color patch 300 may be provided immediately after passing through the final transfer portion. According to this, for example, based on the detection result of the image density read by the optical density sensor 150, when the image density is out of the preset allowable range, it is supplied to each transfer roller 35 at present. It is considered that the current amount that is present has deviated from an appropriate value for some reason, the transfer current is slightly lowered within the allowable range at the time of the next transfer of the recording paper 4, and the output color patch 3
When the image density of 00 is detected and the density is still decreased, the transfer current is slightly increased within the allowable range,
It can be adjusted to obtain an optimum current value. Therefore, a more stable transferred image can be obtained.

Here, the transfer roller 35 is made of a flexible material having a volume resistivity of 10 ⁸ Ω · cm or less. Examples of the material having such a volume resistivity include hydrin rubber and silicon rubber to which a conductive material such as carbon is added, and conductive plastic. The transfer roller 3
When the volume resistivity of 5 itself is larger than 10 ⁸ Ω · cm, it is necessary to generate a high voltage by the constant current power supply 200 in order to apply a constant current to the conveyor belt 3, and the load applied to the constant current power supply 200 is increased. It is not preferable because it becomes large. For comparison, a metal core of a mild steel roller having a diameter of 7 mm and a total length of 310 mm is used as an electrode, and a roller having a conductive material having a thickness of 3 mm wound around it is pressed against a metal conductive material to apply a voltage,
The flowing current was measured by the current detection device 100. According to this, in the case of a conductive material having a volume resistivity of 10 ⁹ Ω · cm, only 0.1 μA flows at an applied voltage of 20 V, whereas the conductivity having a volume resistivity of 10 ⁸ Ω · cm. In the case of a conductive material, the result obtained was that 4 μA of current was applied at an applied voltage of 20V.

As described above, if a material having a volume resistivity of 10 ⁸ Ω · cm or less is used as the conductive material used for the transfer roller 35, a material having a volume resistivity of 10 ⁸ Ω · cm or more is used. On the other hand, since the electric charge easily moves inside the transfer belt 3, even if the same voltage is generated by the constant current power source 200, a large current can be passed through the conveyor belt 3 and the load applied to the constant current power source 200 can be reduced. It can be reduced. If a flexible material is used as the conductive material, damage to the conveyor belt 3 due to use over time can be minimized.

The transfer belt 3 is made of a material such as PVDF or ETFE in which a conductive material such as carbon is added to a fluorine resin such as PVDF, ETFE, or the like, and has a surface in the range of 10 ^{8 to} 10 ¹³ Ω / sq. Even if a material having a resistivity is used, the load on the constant current power supply 200 can be reduced. In this case, further, the residual charge after the transfer is once performed is small, and the transfer currents at the plurality of transfer portions can all be set to a substantially constant value (reference value ± 5 μA). Since it is not necessary to provide a charge removing unit between the units 30, not only the quality of the transferred image is stabilized, but also the cost is reduced.

When the conveyor belt 3 is made of a high resistance material having a surface resistivity of 10 ¹⁴ Ω / sq. Or more, such as PET (polyethylene terephthalate), the conveyor belt 3 is used. Since the recording paper 4 is attracted to the conveyor belt 3 by the electrostatic force generated between the recording paper 4 and the electric charge applied to the recording paper 4, the conveying property of the recording paper 4 can be improved.

[0037]

According to the first to sixth aspects of the invention, as compared with the case where different transfer voltages are independently applied to the respective transfer rollers, for example, changes in the operating environment of the apparatus such as temperature and humidity and transfer are performed. Even if the environmental changes occur at any time, or if the type of recording material changes and the moisture content or thickness changes, the transfer conditions are not affected by the changes in the transfer conditions. Since a constant charge is always applied to the conveyor belt, there is an excellent effect that a constant good transfer image quality can be obtained.

In particular, according to the second aspect of the present invention, the charge imparted to the conveyor belt by the transfer roller remains on the conveyor belt even after the transfer. However, this residual charge is positively removed by the discharging means. It is possible to suppress variations in the transfer conditions at each transfer section. Further, the charge amount removed by the charge removing unit is detected by a charge amount detecting device,
Based on the detection result, the control unit changes the reference current value of the constant current supply unit so that the current flowing from the transfer roller to the conveyor belt in the next transfer unit has a current value more suitable for transfer. Therefore, the quality of the transferred image can be improved as compared with the case where such a control unit is not provided.

In particular, according to the third aspect of the invention, in the transfer portion of each of the transfer rollers, the patch image transferred onto the recording material together with the developed image visualized on the latent image carrier is optically transferred. Optically detected by the detection means, and based on the detection result of the patch image density, the control means controls the above so that the current flowing from the transfer roller to the conveyor belt at the next transfer may have a current value more suitable for transfer. Since the control is performed so as to change the reference current value of the constant current supply means, even if some external factor such as a change in the charge amount of the developer or a change in the transfer condition occurs, the transfer image deteriorates due to these changes. It is possible to always obtain a good transferred image.

[0040] In particular, according to the fourth aspect of the invention, by constituting a material having a conductive member of 10 ⁸ Ω · cm or less in volume resistivity of the portion in contact with the conveyor belt, 10 ⁸ Omega -Unlike when using a material having a volume resistivity larger than cm, a desired transfer current can be applied to the conveyor belt to perform good transfer even if the voltage generated by the constant current supply unit is lowered. The load on the constant current supply means can be reduced. Further, it is possible to avoid increasing the size of the constant current supply means.

In particular, according to the fifth aspect of the present invention, the charge amount applied by the transfer roller directly contributes to the transfer as it is, so that the transfer current can be controlled more precisely. Further, as compared with the case where a material having a surface resistivity other than the range of 10 ^{8 to} 10 ¹³ Ω / sq. Is used, the residual charge remaining on the transfer belt after transfer is reduced, and thus a plurality of each of It is possible to make the transfer current in the transfer portion all substantially constant. Therefore, the transfer image quality can be stabilized without providing a charge eliminating unit for eliminating the residual charge on the transfer belt for each transfer unit, and thus the cost can be reduced.

Particularly, according to the invention of claim 6, a part of the electric charge applied to the conveyor belt by the transfer roller remains on the conveyor belt even after the transfer, and the recording material is transferred by the residual electric charge. Even after that, the recording material is electrostatically adsorbed on the conveyor belt, so that the recording material can be easily conveyed. In addition, even if the transfer condition is changed to the transfer portion located on the downstream side in the recording material conveyance direction due to the accumulation of residual electric charges on the conveyance belt due to the repetition of a plurality of transfer operations, the constant current supply means transfers each transfer Since the transfer currents optimum for transfer, which are supplied from the rollers to the conveyor belt, are independently supplied, it is possible to prevent the quality of the transferred image from being deteriorated without being affected by the residual charge.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a configuration for appropriately controlling a transfer current.

FIG. 2 is a front view showing a schematic configuration of an electrophotographic printer according to an embodiment.

FIG. 3 is an explanatory diagram showing the relationship between transfer current and image density.

4A and 4B are explanatory diagrams showing a patch image on a recording paper obtained by a printer according to another embodiment and a detection method for detecting this patch image by an optical density sensor.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Driven roller 2 Driving roller 3 Conveying belt 4 Recording paper 6 Separation device 7 Separation position 10 Charge application position 30 Photosensitive drum 31 Charging device 32 Developing device 33 Electrification device 34 Cleaning device 35 Transfer roller 100 Current detection device 110 Charge amount detection device 150 Optical Density Sensor 200 Constant Current Power Supply 210 CPU 300 Color Patch B Black Toner Image Forming Unit M Magenta Toner Image Forming Unit Y Yellow Toner Image Forming Unit C Cyan Toner Image Forming Unit

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Kozo Sudo 1-3-6 Nakamagome, Ota-ku, Tokyo Inside Ricoh Co., Ltd.

Claims (6)

[Claims] 1. A plurality of latent image carriers that form latent images, a plurality of developing devices that visualize the latent images of the plurality of latent image carriers, and a plurality of latent image carriers, respectively. Conveying belts that are arranged so as to contact each other and that conveys the recording material, and latent image bearings that are visualized by the developing device and that are respectively disposed on the side opposite to the latent image bearing body across the conveying belts. A transfer device having a plurality of transfer rollers for electrically attracting and transferring the developed image on the body to the recording material side; and a developer for forming the developed image in a direction in which the latent image carrier moves to the recording material. An image forming apparatus, comprising: a constant current supply unit capable of independently supplying a constant transfer current from each of the transfer rollers to the transfer belt to generate an electric field. 2. The image forming apparatus according to claim 1, wherein the residual toner is disposed on the upstream side of the transfer rollers in the transport direction of the recording material and remains on the transport belt that moves between the latent image carriers. A discharging means for discharging the electric charge; a charge amount detecting means for detecting the charge amount discharged by the discharging means; a control means for controlling the constant current supply means based on the detection result of the charge amount detecting means; An image forming apparatus comprising: 3. The image forming apparatus according to claim 1, wherein a patch latent image forming unit for forming a patch latent image for detecting an image density is formed on each latent image carrier, and the most downstream in a conveyance direction of the recording material. An optical detection unit that is disposed after passing the latent image carrier located on the side, and that optically detects the image density of the visualized patch image transferred onto the recording material by the plurality of transfer rollers, Control means for controlling the constant current supply means so as to supply a constant transfer current to the plurality of transfer rollers at the next transfer to another recording material based on the detection result detected by the optical detection means. An image forming apparatus comprising: 4. The image forming apparatus according to claim 1, wherein a material having a volume resistivity of 10 ⁸ Ω · cm or less is used in a portion of the transfer roller which is in contact with the conveyor belt. apparatus. 5. The image forming apparatus according to claim 1, wherein the conveyor belt is made of a material having a surface resistivity of 10 ^{8 to} 10 ¹³ Ω / sq. 6. The image forming apparatus according to claim 1, wherein the conveyor belt is made of a material having a surface resistivity of 10 ¹⁴ Ω / sq. Or more.