JP2000221807A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make an excellent image being free from transfer defect such as a transfer omission in a dot shape obtainable by transferring toner images laminated on an intermediate transfer body to transfer material in preventing the abnormal discharge or the like from occurring, even under such condition that the charge amount of the toner becomes very large under the low humidity environment. SOLUTION: This image forming device is provided with a negative charge roller 76 and the counter roller 77, in front of a secondary transfer part setting the transfer roller 72. Then, the device is allowed to increase the holdable maximum seeming value of the area static charge density on the transfer material up to approximately the double, by applying the electric charge in polarity opposite to the charge charged by the transfer roller 72 at the time of secondary transfer, on the transfer material P that is fed in a secondary transfer part for secondary transferring the color image on the intermediate transfer belt 82, by means of the negative charge roller 76 and the counter roller 77 prior to secondary transferring.

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 employing an electrophotographic method or an electrostatic recording method, and more particularly to an image forming apparatus which forms a color image using an intermediate transfer member.

[0002]

2. Description of the Related Art Conventionally, various image forming apparatuses using an electrophotographic system have been proposed or implemented. For example, a method in which a toner image formed on a photosensitive drum is sequentially superimposed on an intermediate transfer belt or an intermediate transfer drum and primary-transferred, and a toner image on the intermediate transfer belt or the intermediate transfer drum is secondary-transferred onto a transfer material. There is something. Hereinafter, an image forming apparatus using this method will be described.

The image forming process of an image forming apparatus using an intermediate transfer drum will be briefly described. The photosensitive drum is driven to rotate at a predetermined peripheral speed, and the surface of the photosensitive drum is uniformly charged by a charger. A laser beam is scanned and exposed by the device to form a first-color electrostatic latent image on the photosensitive drum, and the latent image is developed by a developing device. The developing device is provided with four developing devices each containing a yellow toner, a magenta toner, a cyan toner, and a black toner. 1 on the photosensitive drum
The electrostatic latent image of the color is developed by a developing unit for yellow, and is visualized as a yellow toner image.

[0004] The formed yellow toner image is transferred to a primary transfer section where the intermediate transfer drum and the photosensitive drum come into contact with each other.
The image is electrostatically transferred to the intermediate transfer drum (primary transfer). The photosensitive drum on which the primary transfer has been completed is subjected to the next color image formation after the toner remaining on the surface thereof is removed by a cleaner.

Similarly, the photosensitive drum is charged by a charger and exposed to a laser beam to form a second-color electrostatic latent image, and the latent image on the photosensitive drum is developed by a magenta developing device. Thus, a magenta toner image is formed on the photosensitive drum. The magenta toner image is transferred onto the intermediate transfer drum from above the yellow toner image.

The above process is repeated for cyan and black, and the images are sequentially superimposed and transferred onto the intermediate transfer drum. As a result, a color image in which four toner images of yellow, magenta, cyan, and black are stacked on the intermediate transfer drum is formed.

Thereafter, the secondary transfer charger, which has been separated, comes into contact with the surface of the intermediate transfer drum, and
In the secondary transfer section where the secondary transfer charger contacts, the four color toner images on the intermediate transfer drum are collectively transferred to the surface of the transfer material conveyed there at a predetermined timing (secondary transfer). Transcription).

The transfer material onto which the four color toner images have been transferred is conveyed from the intermediate transfer drum to a fixing device, where it is fixed by a heat roller or the like to form a full-color permanent image. Is discharged.

[0009]

In an image forming apparatus using the above-described method, in a secondary transfer step, a large amount of toners of four colors stacked on an intermediate transfer drum are collectively transferred onto a transfer material. Will be transcribed. At this time, if the absolute value of the charge amount of the toner to be transferred is relatively small, the secondary transfer is performed well.

However, depending on the atmosphere of the environment in which the image forming apparatus is used and the type of transfer material, particularly in a low-humidity environment, it is remarkably recognized, but in a state where the absolute value of the charge amount of the toner is extremely large. When a sufficient amount of toner is transferred and held from the intermediate transfer drum to the transfer material, there is a problem that an image defect (point-like transfer omission) such as a trace of abnormal discharge occurs. This phenomenon is considered to be because the amount of charge that can be held per unit area of the transfer material varies depending on the type of the transfer material.

When the charge amount of at least one of the four color toners used for image formation is smaller than the charge amount of the other color toner, it is attempted to make the transfer properties of the four color toners uniform. After the end of the primary transfer and before the start of the secondary transfer, the toner image on the intermediate transfer drum is recharged by means of a corona charger or the like, and the transfer amount is similarly increased by changing the charge amounts of the four colors. Aligning is often done,
In this case, the absolute value of the charge amount of the toner becomes larger than that in the previous state, and an image defect such as a trace of abnormal discharge occurs at the time of the secondary transfer as described above.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an image forming apparatus capable of realizing stable secondary transfer and obtaining a high-quality image without transfer defects such as dot-like transfer omissions.

[0013]

The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides:
In an image forming apparatus having an image carrier for carrying a toner image and an intermediate transfer body on which the toner image on the image carrier is transferred, the toner image on the intermediate transfer body transferred from the image carrier is An image forming apparatus comprising: a charging unit that pre-charges a surface of a transfer material on which a toner image is transferred to a polarity opposite to a normal charge polarity of the toner before electrostatically transferring the transfer material to the transfer material. is there.

According to the present invention, there is provided control means for controlling the voltage applied to the charging means according to the type of the transfer material. A control unit configured to control a voltage applied to the charging unit in accordance with a length of the transfer material in a direction substantially orthogonal to a transport direction of the transfer material; There is provided humidity detecting means for detecting humidity, and the control means controls a voltage applied to the charging means according to a detection result by the humidity detecting means. There is a temperature / humidity detecting means for detecting temperature and humidity, and the control means controls a voltage applied to the charging means according to a detection result by the temperature / humidity detecting means. There is provided a determination unit for determining the type of the transfer material, and the control unit controls a voltage applied to the charging unit according to a detection result by the determination unit.

There is provided control means for controlling the amount of charge per unit area applied to the transfer material by the charging means according to the type of the transfer material. A control unit that controls an amount of charge per unit area applied to the transfer material by the charging unit according to a length of the transfer material in a direction substantially orthogonal to the transport direction. The control unit controls the amount of charge per unit area applied to the transfer material by the charging unit according to the detection result of the humidity detecting unit. There is a temperature and humidity detecting means for detecting temperature and humidity, and the control means controls an amount of electric charge per unit area applied to the transfer material by the charging means according to a detection result by the temperature and humidity detecting means. The control unit controls the amount of charge per unit area applied to the transfer material by the charging unit according to the detection result of the determination unit.

[0016] A constant current power supply is connected to the charging means, and the control means controls a value of a current flowing through the charging means. And a transfer unit for electrostatically transferring the toner image on the intermediate transfer body transferred from the image carrier to a transfer material. A plurality of color toner images are sequentially transferred from the image carrier to the intermediate transfer body, and the plurality of color toner images on the intermediate transfer body are transferred to a transfer material. The plurality of image carriers are provided to respectively carry a plurality of color toner images, and the plurality of color toner images on the intermediate transfer body sequentially transferred from the plurality of image carriers are electrostatically transferred onto a transfer material. It has a transfer means for transferring. The transfer unit is provided on a side of the intermediate transfer body to which the toner image is transferred, and when transferring the toner image on the intermediate transfer body to a transfer material, a voltage having a polarity opposite to the normal polarity of the toner is applied. You.

[0017]

Embodiments of the present invention will be described below in more detail with reference to the drawings.

Embodiment 1 FIG. 1 is a schematic structural view showing an embodiment of the image forming apparatus of the present invention. This apparatus is an electrophotographic color image forming apparatus, has four image forming units of yellow, magenta, cyan, and black, and outputs a full-color image.

This image forming apparatus uses a two-component contact developing method as a developing method, and uses a developer obtained by mixing a toner generated by a polymerization method with a magnetic carrier as a two-component developer. The developing device of each image forming unit also serves as a cleaner, and the cleaner for the photosensitive drum is omitted.

The toner images formed by the respective image forming units are transferred onto the intermediate transfer belt 82 by multiple transfer by the first transfer means, and then collectively transferred to the transfer material by the second transfer means. A full-color image is formed.

First, the image forming process will be described. First, the document G is set on the document table 10 with the side to be copied facing up. Next, copying is started by pressing the copy button. The reading of the original image is performed by the scanner unit 19, and the read red, green, and blue color image information is converted into yellow, magenta,
The color is separated into cyan and black, converted to respective signals, and sent to the printer unit.

The printer section includes yellow, magenta,
Cyan and black image forming units are provided. Each image forming unit includes a photosensitive drum 1 as an image carrier, a charging roller 31 for uniformly charging the photosensitive drum 1, and an LED solid-state scanner as an image exposure system for forming an electrostatic latent image on the charged photosensitive drum 1. 110, developing units 4Y, 4M, 4 for developing the formed electrostatic latent image with toner
C and 4K, and a transfer roller 71 of a first transfer unit that electrostatically transfers the obtained toner image to the intermediate transfer belt 82 are arranged.

When a yellow image signal is sent to the printer unit, the yellow image forming unit irradiates a light signal corresponding to the yellow image signal by the scanner 110 to the photosensitive drum 1 charged in advance to the negative polarity by the charging roller 31. Thereby, an electrostatic latent image is formed on the photosensitive drum 1. The electrostatic latent image is developed by a developing device 4Y containing a yellow toner having a normal charging polarity of negative to form a yellow toner image on the photosensitive drum 1. This yellow toner image is transferred onto the intermediate transfer belt 82 by the transfer roller 71 of the first transfer unit.

Simultaneously with this operation, the magenta image unit irradiates the photosensitive drum 1 with an optical signal corresponding to the magenta image signal, thereby forming an electrostatic latent image on the photosensitive drum 1, and this electrostatic latent image is formed. Is developed by the developing device 4M containing magenta toner, and a magenta toner image is formed on the photosensitive drum 1. The magenta toner image formed in this manner is transferred so as to be superimposed on the intermediate transfer belt 82 on which the yellow toner image has already been formed. Further, by performing the same process for cyan and black, a full-color image is formed on the intermediate transfer belt 82.

The full-color image on the intermediate transfer belt 82 obtained by the above process is transferred to the transfer material storage cassette 29.
Alternatively, the secondary transfer is performed by the transfer roller 72 to which a positive voltage is applied to the transfer material P supplied to the second transfer portion where the transfer roller 72 is provided and transferred from the transfer material 72. Transfer material P
Is taken out from the cassette 29 or 30 by the pickup roller 73 or 74, and the registration roller pair 7
5 and 75, it is supplied to the second transfer section in synchronization with the full-color image on the intermediate transfer belt 82.

According to the present invention, at this time, the transfer material P from the pair of registration rollers 75, 75 is supplied to the second transfer section via the reverse charging roller 76 and the opposing roller 77. This will be described later.

The transfer material P on which the full-color image has been transferred is
The sheet is conveyed to the fixing device 16 and thermally fixed. At this time, the toner remaining on the intermediate transfer belt 82 is cleaned by a cleaning device 51 that can be turned on / off.

As described above, in a situation where the charge amount of the toner becomes extremely large in a low humidity environment or the like, when transferring the toner image laminated on the intermediate transfer body to the transfer material, abnormal discharge or the like occurs. And image defects are likely to occur.

According to the research conducted by the present inventors, it has been clarified that this abnormal discharge is generated by the following mechanism.

First, FIG. 2 shows the relationship between the surface charge density externally applied to the surface of a transfer material sufficiently adapted to an atmosphere of a low humidity environment of 23 ° C./5% and the surface potential of the transfer material.

As can be seen from FIG. 2, in a low humidity environment, that is, in an environment in which the absolute moisture content in the air is very small, the surface potential of the transfer material does not exceed a certain value and saturates. It is clear that. By the way, 23
The absolute water content at 5 ° C./5% is 0.5 g / kg of air.

The saturation value of the surface potential of the transfer material sufficiently adapted to the atmosphere of the environment of 23 ° C./5% differs depending on the type of the transfer material, but is PB-SK paper (manufactured by Nippon Paper Co., Ltd.) In the case of 64 g / m ² ), it was about ² kV. If this is expressed in the form of the amount of charge that can be held, about 500 μC
/ M ² . This situation shows almost the same tendency regardless of the positive polarity and the negative polarity. Incidentally, even in such a low-humidity environment, in the case of a transfer material such as an OHT film which is hardly affected by the atmosphere of the environment, as shown in FIG. The amount of potential charge does not saturate.

Next, referring to FIG. 3, for example, at 23 ° C./60%
The relationship between the surface charge density externally applied to the surface of the transfer material sufficiently adapted to the atmosphere of the normal temperature and normal humidity environment and the surface potential of the transfer material is shown.

As can be seen from FIG. 3, in a normal temperature and normal humidity environment, that is, in an environment in which the absolute moisture content in the air is moderate, the surface potential of the transfer material is lower than that of a normal transfer material as in the present invention. It is clear that up to about the secondary transfer voltage used for image formation in the electrophotographic process, specifically, up to about 10 kV, there is a proportional relationship with the surface charge density externally applied to the transfer material surface. Incidentally, the absolute water content at 23 ° C./60% is 10.0 g / kg of air.

FIG. 4 shows the relationship between the surface charge density that can be held by the transfer material when the absolute moisture content of the environmental atmosphere changes.

The transfer material shown in this graph is the same as the transfer material shown in FIGS.
It is SK paper (basis weight 64 g / m ² ). As can be clearly seen from FIG. 4, there is a strong correlation between the surface charge density that the transfer material can hold and the environment atmosphere, that is, the absolute water content. It has become clear that the size of the water is also getting smaller.

The mechanism for causing the problem of image failure due to abnormal discharge in consideration of the charge amount of the toner actually used for image formation and the like is re-examined as follows.

When the two-component developing method is used, the amount of charge of the toner adhering to the photosensitive drum by development is about -30 μC / g in a low humidity environment of 23 ° C./5%. The amount of the toner required for the density portion) is 0.01 kg / m ² . By the way, when performing full-color image formation, toner of four colors is superimposed. At this time, the upper limit of the amount of toner to be laid per unit area is determined by fixing conditions and an image processing method. In many cases, the value is set to almost twice that at the time of image formation. Therefore, in full-color image formation, the maximum surface charge density of the toner is -600 μC / m ² .

On the other hand, Nippon Paper Industries Co., Ltd.
When PB-SK paper (basis weight 64 g / m ² ) was used, the absolute value of the maximum surface charge density that the transfer material could hold under this low-humidity atmosphere was 500 μC / m ² . This is a calculation in which a toner charge of −100 μC / m ² cannot be held. It is considered that an abnormal discharge occurs due to the unretainable electric charge, thereby causing an image defect.

That is, in a low-humidity environment, the transfer material changes its characteristics electrically, and the transfer material does not have a surface potential of a predetermined amount or more, in other words, a charge of a predetermined amount or more is held on the surface of the transfer material. When the toner image formed on the image carrier is transferred to the transfer material, the total amount of the toner becomes large. Since it is impossible to hold an electric charge of a predetermined amount or more, it is considered that abnormal discharge is likely to occur and image defects are caused.

The above contents are represented by the following mathematical expressions.

First, the maximum surface charge Q that can be held by the transfer material
A function Qp = Qp (s, which gives p (C / m ² ) as variables the type of the transfer material and the environmental atmosphere in which the transfer material is placed.
k). Here, s is the type of the transfer material, and k is the absolute moisture content of the environmental atmosphere (g / kg of air).

Next, the charge amount Q per unit weight of the toner
t is a function that gives the environment atmosphere as a variable Qt = Qt
Assuming the value of (k), the weight per unit area of the toner transferred to the transfer material is M (kg / m ² ), and Qp (s, k) ≧ M · Qt (k) (1) When the toner amount M exceeds the toner riding amount M that satisfies the following mathematical expression (1), the toner image transferred to the transfer material cannot be held on the transfer material, and an image defect such as occurrence of abnormal discharge occurs. Therefore, depending on the maximum amount of charge that can be held by the transfer material, the maximum value of the toner riding amount M per unit area must be reduced in order to prevent image defects. That is, the concentration must be reduced.

In order to solve such a problem, the inventor of the present invention provides a transfer material in advance with a charge having a polarity opposite to that of the charge applied to the transfer material in the transfer process prior to the toner image transfer process. It has been found that the amount of charge of the toner per unit area that can be held on the material can be increased.
Hereinafter, this will be described using mathematical expressions.

Assuming that the charge density per unit area of the charge of the opposite polarity to the charge given in the transfer step which is given to the transfer material in advance is Qa (s, k) (C / m ² ), Qp (s, k) | + | Qa (s, k) | ≧ N · | Qt (k) | (2) Satisfies the above equation (2) It is possible to increase to the level that does. However, in equation (2), Qp
(S, k) and Qa, Qt (k) have opposite polarities (thus, Qa and Qt (k) have the same polarity).

Also, the magnitude of Qa is Qp
Since the same restriction as (s, k) is imposed, | Qp (s, k) | ≧ | Qa | must be satisfied.

Therefore, the maximum value N of the transferable toner ride amount obtained when the present invention is carried out is obtained when | Qp (s, k) | = | Qa | (3) The value of the toner riding amount N at this time is N = 2M from equations (1), (2), and (3), and is twice as large as that of the related art without causing an image defect such as abnormal discharge. The toner can be transferred up to.

When the transfer material is placed in the low humidity environment described above, the charge on the transfer material is opposite to that of the secondary transfer, that is, the side opposite to the side on which the toner image of the transfer material is transferred. On the back surface of the transfer material in advance, or on the front surface of the transfer material onto which the toner image is transferred, a charge having a polarity opposite to the normal charge polarity of the toner. , The maximum value of the surface charge density that the transfer material can hold can be apparently increased to about twice, and the amount of charge per unit area of the toner image becomes the maximum charge per unit area that the transfer material can hold. It is possible to prevent the occurrence of an image defect due to abnormal discharge and obtain a good image.

Therefore, in the present invention, as described above, a reverse charging roller 76 connected to a constant current power supply and an opposing roller 77 electrically grounded are provided before the secondary transfer section where the transfer roller 72 is installed. The charge having the polarity opposite to the charge given by the transfer roller 72 during the secondary transfer
The transfer material P was applied to the transfer material P by the reverse charging roller 76 and the opposing roller 77 before the next transfer. In this embodiment, a charge of the opposite polarity (charge of the same polarity as the toner), that is, a negative charge, is applied from the side of the reverse charging roller 76 to the back surface of the transfer material P during the secondary transfer.

The specific amount of charge of the polarity opposite to that of the secondary transfer is given to the transfer material in advance based on the above-described formulas (1) to (3), but the following procedure is preferable.

As an example, when the above-mentioned PB-SK paper (basis weight 64 g / m ² ) manufactured by Nippon Paper Industries Co., Ltd. is used as the transfer material P, the maximum surface charge density of the transfer material that can be held is The minimum value of the absolute value | Qp | is when the temperature and humidity of the atmosphere of the environment where the transfer material is left is 23 ° C./5%, and the value is 500 μC / m ² .

When a toner having a negative charge characteristic is used, the polarity of the electric charge applied to the rear surface of the transfer material during the secondary transfer is positive, and the charge is applied to all the environment in which the apparatus is used (that is, under the lowest humidity environment). However, Qp can be up to +500 μC / m ² . From now on Qa is -500μC
/ M ² , the present invention uses the reverse charging roller 76 to achieve a surface charge density of −500 μC / m ² from the back surface of the transfer material (the surface opposite to the transfer surface of the toner image). The transfer material was reversely charged.

When a toner having a charge amount of about −30 μC / g is transferred to a transfer material, the maximum value of the toner riding amount per unit area of the transfer material which can be transferred is calculated according to the above equation (2). Is (| 500 | + | -500 |) /|-30|=0.03
It becomes 33 kg / m ² . Therefore, the amount of toner applied per color required to obtain a sufficient image density is approximately 0.01 kg / m ^2.
However, according to the present invention, 3.
Up to 33 colors, an image can be transferred to a transfer material without causing an image defect due to abnormal discharge. That is, even in the formation of an image of a secondary color or more, it is possible to prevent image defects without adverse effects such as a decrease in image density and a change in color.

In the present embodiment, prior to the secondary transfer, a charge having a polarity opposite to that of the secondary transfer to the transfer material is applied to the back surface of the transfer material opposite to the toner image transfer surface by the reverse charging roller 76. The configuration is such that a charge having the same polarity as that of the toner is provided. Conversely, a configuration may be employed in which a power supply is connected to the opposing roller 77 so that a charge having the same polarity as the toner is provided on the transfer material surface. The effect can be obtained.

Although the roller pair of the reverse charging roller 76 and the opposing roller 77 is used as the reverse charging member, the present invention is not limited to this, and the transfer material such as a corona charger or a brush-like charging member is charged. It is needless to say that any of them can be used as long as they can supply.

In this embodiment, Nippon Paper Industries Co., Ltd.
Although the case where PB-SK paper (basis weight 64 g / m ² ) is used has been described, it is possible to prevent image defects from occurring in other transfer materials by applying the same reverse charge. However, since the surface charge density per unit area that can be held varies depending on the transfer material, it is preferable to be able to change the amount of reverse charge when reverse charging to this value according to the type of transfer material.

That is, according to the type of the transfer material, the value of the current flowing from the constant current power supply to the reverse charging member is controlled by the control means (CP
It is preferred to control by U). When performing reverse charging of a transfer material using a constant voltage power supply, the voltage applied from the constant voltage power supply to the reverse charging member may be controlled by the CPU.

As a further preferred embodiment of this embodiment, a temperature and humidity sensor 20 capable of measuring temperature and humidity is provided in the main body of the image forming apparatus, and as close to the transfer material storage cassettes 29 and 30 as possible. Sensor 2
Using the temperature and humidity in the apparatus detected as 0, the absolute moisture content is calculated. On the other hand, the relational expression between the absolute water content and the surface charge density that can hold the transfer material as shown in FIG. 4 is stored in the apparatus main body (a storage means such as a ROM) by the number corresponding to the type of the transfer material. When the user inputs the type of transfer material to the apparatus main body in advance when using the image forming apparatus, the input transfer material information and the absolute moisture content of the atmosphere in the apparatus main body are stored in the apparatus main body in advance. It is clear that more preferable results can be obtained by determining the maximum surface charge density that can be held for each transfer material.

The current (applied voltage) flowing to the opposite charging member is controlled by the CPU according to the length of the transfer material in a direction substantially orthogonal to the transport direction, so that the toner image on the transfer material is The transfer current required for transfer can be made appropriate.

Further, the surface charge density per unit area capable of holding the transfer material is automatically detected by the following method.
The reverse charge amount may be determined based on this detection value.

As a method of automatic detection, apart from normal image formation, a transfer material is transferred to a secondary transfer portion without transferring a toner image onto an intermediate transfer belt 82, and is fixed to a transfer roller 72. A transfer voltage was applied by current control, and the transfer voltage at this time was detected.

That is, the transfer voltages for the secondary transfer current value I0 and the secondary transfer current value I1 larger by ΔI are measured, and the difference ΔV1 between these transfer voltages is calculated. Further, the transfer voltage for the secondary transfer current value I2 which is larger than I1 by ΔI is measured, and the difference ΔV2 from the transfer voltage at the transfer current I1 is calculated. The transfer voltage differences ΔV1 and ΔV2 are usually constant, but when the amount of charge supplied to the transfer material exceeds the amount of charge that the transfer material can hold, ΔVi
(I = 1, 2) starts to decrease. Therefore, the secondary transfer current immediately before ΔVi starts to decrease is determined from the above detection flow, the maximum retained charge amount of the transfer material is estimated based on the secondary transfer current value, and the reverse charge amount during reverse charging (the electric pole value) Or applied voltage).

With this configuration, the amount of reverse charge can be automatically determined without inputting the type of transfer material.

However, the above-mentioned means is merely a means for automatically detecting the maximum amount of charge held on the transfer material, and may be other means. For example, the output of the secondary transfer high-voltage power supply may be controlled at a constant voltage to detect the current at that time, or the moisture content of the transfer material may be detected by an infrared moisture meter to detect the moisture content of the transfer material. From the relationship between the amount and the maximum retained charge amount, the maximum retained charge amount of the transfer material may be automatically detected.

[0065]

As described above, according to the present invention,
Before secondary transfer of a plurality of color toner images from the intermediate transfer member to the transfer material, whether a charge of the same polarity as that of the toner is previously applied to the back surface of the transfer material opposite to the side on which the toner image is transferred. Alternatively, a charge having a polarity opposite to that of the toner is preliminarily applied to the surface of the transfer material onto which the toner image is transferred, so the apparent maximum surface charge density of the transfer material is increased to about twice as high as apparently. Then, the amount of charge per unit area of the toner image can be made not to exceed the maximum amount of charge per unit area that the transfer material can hold, and the toner image stacked on the intermediate transfer body, particularly in a low humidity environment. It is possible to obtain a high-quality color image by transferring onto a transfer material without causing abnormal discharge and free from transfer defects such as dot-like transfer omissions.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing one embodiment of an image forming apparatus of the present invention.

FIG. 2 is a diagram illustrating a relationship between a surface charge density given to a transfer material and a surface potential of the transfer material in a low humidity environment.

FIG. 3 is a diagram illustrating a relationship between a surface charge density given to a transfer material and a surface potential of the transfer material under a normal humidity environment.

FIG. 4 is a diagram showing a relationship between an absolute moisture content of an environmental atmosphere and a surface charge density capable of holding a transfer material.

FIG. 5 is a schematic diagram illustrating a conventional image forming apparatus.

[Explanation of symbols]

 Reference Signs List 1 photosensitive drum 20 temperature and humidity sensor 71 primary charging roller 72 secondary transfer roller 75 registration roller 76 reverse charging roller 77 opposing roller 82 intermediate transfer belt P transfer material

Claims (16)

[Claims] 1. An image forming apparatus comprising: an image carrier that carries a toner image; and an intermediate transfer body onto which the toner image on the image carrier is transferred, wherein the intermediate transfer body transferred from the image carrier is provided. Before electrostatically transferring the upper toner image onto the transfer material, a charging unit is provided which pre-charges the surface of the transfer material on which the toner image is transferred to a polarity opposite to the normal charging polarity of the toner. Image forming apparatus. 2. The image forming apparatus according to claim 1, further comprising control means for controlling a voltage applied to said charging means in accordance with a type of a transfer material. 3. The image forming apparatus according to claim 1, further comprising control means for controlling a voltage applied to said charging means in accordance with a length of the transfer material in a direction substantially orthogonal to a conveying direction. 4. The image forming apparatus according to claim 2, further comprising humidity detecting means for detecting humidity, wherein said control means controls a voltage applied to said charging means in accordance with a result of detection by said humidity detecting means. . 5. A temperature and humidity detecting means for detecting temperature and humidity, wherein said control means controls a voltage applied to said charging means in accordance with a result of detection by said temperature and humidity detecting means. Image forming apparatus. 6. The image forming apparatus according to claim 2, further comprising a determination unit configured to determine a type of the transfer material, wherein the control unit controls a voltage applied to the charging unit in accordance with a detection result of the determination unit. apparatus. 7. The image forming apparatus according to claim 1, further comprising a control unit that controls an amount of charge per unit area applied to the transfer material by the charging unit according to a type of the transfer material. 8. A control means for controlling a charge amount per unit area applied to the transfer material by the charging means in accordance with a length of the transfer material in a direction substantially orthogonal to the transport direction. Image forming device. 9. A humidity detecting unit for detecting humidity, wherein the control unit controls an amount of electric charge per unit area applied to the transfer material by the charging unit according to a detection result by the humidity detecting unit. The image forming apparatus according to claim 7. 10. A temperature / humidity detecting means for detecting a temperature and a humidity, wherein said control means controls a charge per unit area applied to a transfer material by said charging means according to a detection result by said temperature / humidity detecting means. 9. The image forming apparatus according to claim 7, wherein the amount is controlled. 11. A discriminating means for discriminating a type of a transfer material, wherein the control means controls a charge per unit area applied to the transfer material by the charging means in accordance with a detection result by the discriminating means. 9. The image forming apparatus according to claim 7, wherein 12. The image forming apparatus according to claim 2, further comprising a constant current power supply connected to said charging unit, wherein said control unit controls a value of a current flowing through said charging unit. 13. The image forming apparatus according to claim 1, further comprising a transfer unit configured to electrostatically transfer the toner image on the intermediate transfer body transferred from the image carrier to a transfer material. apparatus. 14. The image forming apparatus according to claim 13, wherein a plurality of color toner images are sequentially transferred from the image carrier to the intermediate transfer body, and the plurality of color toner images on the intermediate transfer body are transferred to a transfer material. . 15. A plurality of image carriers are provided to respectively carry a plurality of color toner images, and a plurality of color toner images on the intermediate transfer body sequentially transferred from the plurality of image carriers are transferred to a transfer material. The image forming apparatus according to claim 1, further comprising a transfer unit that electrostatically transfers the image to the image forming apparatus. 16. The transfer means is provided on a side of the intermediate transfer member to which the toner image is transferred, and has a polarity opposite to a normal polarity of the toner when transferring the toner image on the intermediate transfer member to a transfer material. The image forming apparatus according to claim 13, wherein the voltage is applied.