JP3517621B2 - Image forming device - Google Patents

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 which employs an electrophotographic system or an electrostatic recording system, 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, the toner image formed on the photosensitive drum is sequentially superposed on the intermediate transfer belt or the intermediate transfer drum and primary-transferred, and the toner image on the intermediate transfer belt or the intermediate transfer drum is secondarily transferred to a transfer material. There is something. An image forming apparatus using this method will be described below.

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

The formed yellow toner image is transferred to the primary transfer portion where the intermediate transfer drum and the photosensitive drum are in contact with each other.
It is electrostatically transferred to the intermediate transfer drum (primary transfer). The photosensitive drum, which has completed the primary transfer, is used for image formation of the next color after removing the toner remaining on the surface thereof with 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 in an overlapping manner from the yellow toner image.

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

After that, the secondary transfer charger, which was in the separated state, is brought into contact with the surface of the intermediate transfer drum, so that
In the secondary transfer portion that comes into contact with the secondary transfer charger, the toner images of four colors on the intermediate transfer drum are collectively transferred onto the surface of the transfer material that is conveyed at a predetermined timing (secondary transfer). Transcription).

The transfer material on 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, and then outside the image forming apparatus. Is discharged to.

[0009]

In the image forming apparatus using the above-described method, a large amount of toner of four colors stacked on the intermediate transfer drum is collectively used as a transfer material in the secondary transfer process. It will be transcribed. At this time, if the absolute value of the charge amount of the transferred toner is relatively small, the secondary transfer is well performed.

However, depending on the atmosphere of the environment in which the image forming apparatus is used and the type of the transfer material, it is noticeable particularly in a low humidity environment, but in the state where the absolute value of the charge amount of the toner becomes very large. However, when an attempt is made to transfer and hold a sufficient amount of toner on the transfer material from the intermediate transfer drum, there is a problem that an image defect (dotted transfer omission) such as a mark of abnormal discharge occurs. Such a phenomenon is considered to be because the amount of charge that can be held per unit area of the transfer material differs depending on the type of the transfer material.

Among the four color toners used for image formation, when the charge amount of at least one color toner is smaller than the charge amount of the other color toners, the transferability of the four color toners is attempted. , After the completion of the primary transfer and before the start of the secondary transfer, the toner image on the intermediate transfer drum is recharged by a means such as a corona charger, and the transfer amount is similarly improved by the same amount of charge for four colors. It is often done to arrange,
By doing so, the absolute value of the charge amount of the toner becomes larger than that in the previous state, and as described above, an image defect such as a trace of abnormal discharge occurred during the secondary transfer.

An object of the present invention is to provide an image forming apparatus which can realize stable secondary transfer and can obtain a high-quality image without transfer defects such as dot-shaped transfer omissions.

[0013]

The above object can be achieved by an image forming apparatus according to the present invention. In summary, the first aspect of the present invention is directed to an image carrier that carries a toner image, an intermediate transfer member to which the toner image on the image carrier is transferred, and an intermediate transfer member.
Transfer means for transferring the toner image of
By applying a voltage to the transfer means, the toner image on the transfer material
Regular charging polarity of toner on the surface opposite to the transfer surface
An electric charge of the opposite polarity to that of the toner image is applied to the transfer material.
In an image forming apparatus in which electrostatic transfer is performed , a toner image on the intermediate transfer body transferred from the image carrier is transferred to a transfer material.
Before transcription, a contact on the surface on which the toner image of the transfer material is transferred
The region takes the toner image is transferred, the normal charging polarity of the toner is the image forming apparatus characterized by comprising a charging means for pre-charged to the opposite polarity.

According to one embodiment of 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 . In one embodiment, a humidity detecting means for detecting the degree of humidity, the control means controls the voltage applied to the charging unit according to a detection result by said humidity detecting means. In another embodiment, the temperature / humidity detecting means for detecting temperature and humidity is provided, and the control means controls the voltage applied to the charging means according to the detection result of the temperature / humidity detecting means. In still another embodiment, a discriminating unit for discriminating the type of the transfer material is provided, and the control unit controls the voltage applied to the charging unit according to the detection result of the discriminating unit.

According to one embodiment of the present invention, there is provided control means for controlling the amount of charge per unit area applied to the transfer material by the charging means in accordance with the type of the transfer material . Real
A embodiments with has a humidity detecting means for detecting the degree of humidity, the control means controls the amount of charge per unit area imparted to the transfer material by the charging means according to a detection result by said humidity detecting means . In another embodiment, the temperature / humidity detection means for detecting temperature and humidity is provided, and the control means is
The charge amount per unit area applied to the transfer material by the charging unit is controlled according to the detection result of the temperature and humidity detecting unit. In still another embodiment, the control unit has a discriminating unit for discriminating the type of the transfer material, and the control unit determines the amount of charge per unit area given to the transfer material by the charging unit according to the detection result of the discriminating unit. To control.

According to one embodiment of the present invention, a constant current power source is connected to the charging means, and the control means controls a current value flowing through the charging means . In one embodiment,
Are toner images are sequentially transferred in a plurality of colors from the previous Kizo bearing member onto said intermediate transfer member, toner images of plural colors on the intermediate transfer body is transferred to the transfer material. According to the second aspect of the present invention, the toner
-An image bearing member carrying an image and a toner image on the image bearing member
In an image forming apparatus having an intermediate transfer member for transferring
The toner on the intermediate transfer member transferred from the image carrier.
Toner on the transfer material before electrostatically transferring the toner image to the transfer material.
-The surface on which the image is transferred is opposite to the normal charging polarity of the toner.
Means for pre-charging the transfer material; the transfer material conveying direction and the transfer material
Applied to the charging means according to the length in the direction orthogonal to each other.
Control means for controlling the voltage depending on the
An image forming apparatus is provided. Furthermore, the third aspect of the present invention
According to the above, an image carrier that carries a toner image and the image carrier
An image having an intermediate transfer member to which the toner image on the holding member is transferred.
In the image forming apparatus, the image transferred from the image carrier is
Before electrostatically transferring the toner image on the intermediate transfer body to the transfer material
The surface of the transfer material onto which the toner image is transferred
Charging means for pre-charging the opposite polarity to the charging polarity; transfer material
According to the length in the direction substantially orthogonal to the transport direction of
The charge per unit area applied to the transfer material by the charging means.
Control means for controlling the load amount;
An image forming apparatus is provided.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will now be described in more detail with reference to the drawings.

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

The image forming apparatus uses a two-component contact developing method as a developing method, and uses a developer in which a toner produced by a polymerization method is mixed 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 of the photosensitive drum is omitted.

The toner images formed by the respective image forming units are transferred to the transfer material in a batch by the second transfer means after being multiple-transferred onto the intermediate transfer belt 82 by the first 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. Then, copying is started by pressing the copy button. The original image is read by the scanner unit 19, and the read red, green, and blue color image information is used for yellow, magenta, and
The color is separated into cyan and black, converted into respective signals and sent to the printer section.

The printer section includes yellow, magenta,
Image forming units for cyan and black are installed. Each image forming unit includes a photosensitive drum 1 which is an image carrier, a charging roller 31 which uniformly charges the photosensitive drum 1, and an LED solid-state scanner as an image exposure system which forms an electrostatic latent image on the charged photosensitive drum 1. 110 and developing devices 4Y, 4M, 4 for developing the formed electrostatic latent image with toner.
C, 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 the yellow image signal is sent to the printer section, in the yellow image forming unit, the photosensitive drum 1 which is negatively charged in advance by the charging roller 31 is irradiated with the optical signal corresponding to the yellow image signal by the scanner 110. By doing so, an electrostatic latent image is formed on the photosensitive drum 1. This electrostatic latent image is developed by the developing device 4Y containing a yellow toner having a negative charging polarity and a yellow toner image is formed 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 means.

Simultaneously with this operation, the magenta image unit irradiates the photosensitive drum 1 with an optical signal corresponding to the magenta image signal to form an electrostatic latent image on the photosensitive drum 1, and the electrostatic latent image is formed. Is developed by a developing device 4M containing magenta toner, and a magenta toner image is formed on the photosensitive drum 1. The magenta toner image thus formed is transferred so as to be superposed on the intermediate transfer belt 82 on which the yellow toner image has already been formed. 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 from the transfer roller 72 to which a positive voltage is applied to the transfer material P that is conveyed from 30 and is supplied to the second transfer portion in which the transfer roller 72 is installed. 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
It is supplied to the second transfer portion 5 and 75 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 portion 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 is transferred is
The toner 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 the cleaning device 51 that can be turned ON / OFF.

As described above, in a low humidity environment or the like where the charge amount of the toner is extremely large, an abnormal discharge or the like may occur when the toner image laminated on the intermediate transfer member is transferred to the transfer material. It is likely that image defects will occur.

According to the research conducted by the present inventors, it has been elucidated that this abnormal discharge occurs by the mechanism as described below.

First, FIG. 2 shows the relation between the surface charge density of the transfer material and the surface charge density given from the outside to the surface of the transfer material which has been sufficiently adapted to the low humidity environment of 23 ° C./5%.

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

The saturation value of the surface potential of the transfer material that has been sufficiently adapted to the environment of 23 ° C./5% varies depending on the type of the transfer material, but it is PB-SK paper manufactured by Nippon Paper Industries 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 retained, it is approximately 500 μC.
/ M ² . This situation shows almost the same tendency regardless of whether the polarity is positive or negative. By the way, even in such a low humidity environment, in the case of a transfer material such as an OHT film, which is not easily affected by the atmosphere of the environment, as shown in FIG. There is no saturation of the possible charge.

Next, referring to FIG. 3, for example, 23 ° C./60%
The relation between the surface potential of the transfer material and the surface charge density given from the outside to the surface of the transfer material, which is sufficiently adapted to the atmosphere of normal temperature and normal humidity, is shown.

As can be seen from FIG. 3, the surface potential of the transfer material is the same as that of the present invention under the normal temperature and normal humidity environment, that is, under the environment where the absolute water content in the air is moderate. It is clear that the secondary transfer voltage used in image formation in the electrophotographic process, specifically up to about 10 kV, is proportional to the surface charge density externally given to the transfer material surface. By the way, the absolute water content at 23 ° C./60% is 10.0 g / 1 kg of air.

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

The transfer material shown in this graph is the same as the transfer material shown in FIGS. 2 and 3 and is made by PB- manufactured by Nippon Paper Industries Co., Ltd.
It is SK paper (basis weight 64 g / m ² ). As is clear from FIG. 4, there is a strong correlation between the surface charge density that the transfer material can hold and the atmosphere in the environment, that is, the absolute water content, and the surface charge density that the transfer material can hold as the absolute water content becomes smaller. It became clear that it became smaller.

The following is a re-verification of the mechanism of the problem of the image defect due to the abnormal discharge in consideration of the charge amount of the toner actually used for image formation.

When the two-component developing method is used, the charge amount of the toner adhered to the photosensitive drum by development is about -30 μC / g in a low humidity environment of 23 ° C./5%, and the solid image portion (that is, the maximum image). The amount of toner required for the density portion) is 0.01 kg / m ² . By the way, in the case of forming a full-color image, four color toners are piled up. At this time, the upper limit of the toner amount per unit area is determined by the fixing condition and the image processing method. It is often set to almost twice as much as when the image is formed. Therefore, in the full-color image formation, the maximum surface charge density of the toner is −600 μC / m ² .

On the other hand, as a transfer material, the above-mentioned Nippon Paper Industries Co., Ltd.
When using PB-SK paper (64 g / m ² basis weight), the absolute value of the maximum surface charge density that can be held by the transfer material was 500 μC / m ² under the atmosphere of this low humidity environment. It is calculated that the toner charge of −100 μC / m ² cannot be held. It is conceivable that abnormal electric discharge occurs due to the electric charges that cannot be held, which causes an image defect.

That is, in a low-humidity environment, the transfer material electrically changes its characteristics so that the transfer material does not have a surface potential higher than a predetermined amount. In other words, the surface of the transfer material retains an electric charge of a predetermined amount or more. When it is impossible to transfer the toner image formed on the image carrier to the transfer material, the total charge amount of the toner becomes large, and if a transfer electric field corresponding to it is applied to the transfer material, the transfer material is transferred. Is unable to hold a predetermined amount of charge or more, abnormal discharge is likely to occur, which is considered to cause image defects.

The above contents are expressed by the following mathematical expressions.

First, the maximum surface charge amount Q that can be held by the transfer material
A function Qp = Qp (s, which gives p (C / m ² ) as variables for the type of transfer material and the environment in which the transfer material is placed.
k). Here, s is the type of transfer material, and k is the absolute water content (g / air 1 kg) in the environmental atmosphere.

Next, the charge amount Q per unit weight of toner
t is a function that gives the environmental atmosphere as a variable Qt = Qt
If the value of (k) is taken, 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) The toner image transferred onto the transfer material beyond the toner carrying amount M satisfying the following mathematical expression (1) cannot be held on the transfer material, resulting in an image defect such as abnormal discharge. Therefore, depending on the maximum charge amount of the transfer material that can be held, the maximum value of the toner carrying amount M per unit area must be reduced in order to prevent image defects. That is, the concentration is forced to decrease.

In order to solve such a problem, the present inventor applies a charge having a polarity opposite to that of the charge applied to the transfer material in the transfer step in advance to the transfer material before the transfer step of the toner image. 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 electric charge having the opposite polarity to the electric charge given in advance in the transfer step is Qa (s, k) (C / m ² ), the transferable unit area is The maximum value N of the amount of applied toner per unit is | Qp (s, k) | + | Qa (s, k) | ≧ N · | Qt (k) | (2) The above formula (2) is satisfied. It is possible to increase to a 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).

The size of Qa is Qp.
Since it is subject to the same limitation as (s, k), | Qp (s, k) | ≧ | Qa | needs to be satisfied.

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

When the transfer material is in a state where it is placed in the above-mentioned low humidity environment, the charge opposite to that at the time of the secondary transfer, that is, the side opposite to the side on which the toner image of the transfer material is transferred is transferred to the transfer material. Make sure to give a charge of the same polarity as the regular charge polarity of the toner to the back surface of the toner in advance, or give a charge of the opposite polarity to the regular charge polarity of the toner to the surface of the transfer material onto which the toner image is transferred. By this, the maximum value of the surface charge density that can be held by the transfer material is apparently increased to about twice, and the charge amount per unit area of the toner image is increased by the maximum charge per unit area that can be held by the transfer material. It is possible to prevent the occurrence of image defects due to abnormal discharge, and it is possible to obtain good images.

Therefore, in the present invention, as described above, the reverse charging roller 76 connected to the constant current power source and the electrically grounded counter roller 77 are provided in front of the secondary transfer portion in which the transfer roller 72 is installed. The charge having a polarity opposite to the charging charge given by the transfer roller 72 at the time of the secondary transfer is provided.
Prior to the next transfer, it is applied to the transfer material P by the reverse charging roller 76 and the counter roller 77. In this embodiment, a charge having the opposite polarity (charge having the same polarity as the toner), that is, a negative charge, is applied to the back surface of the transfer material P from the side of the reverse charging roller 76.

To what extent the charge having the opposite polarity to the secondary transfer is given to the transfer material in advance is based on the above-mentioned formulas (1) to (3), but the following procedure is preferable.

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

When a toner having a negative charging characteristic is used, the polarity of the charge given to the back surface of the transfer material at the time of the secondary transfer is positive, and it is suitable for all the environment in which the apparatus is used (that is, in the environment with the lowest humidity). However, Qp can be up to +500 μC / m ² . From now on Qa is -500 μC
Since it is possible to / m ^2, the present in invention, so that the (opposite side surface to the transfer surface of the toner image) from the surface charge density of -500μC / m ² back surface of the transfer material by the reverse charging roller 76, The transfer material was reversely charged.

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

In this embodiment, prior to the secondary transfer, the charge having the opposite polarity to that at the time of the secondary transfer is applied to the transfer material by the reverse charging roller 76 so that the charge is applied to the back surface of the transfer material opposite to the toner image transfer surface. Although the configuration is such that an electric charge having the same polarity as that of the toner is applied, conversely, a configuration may be adopted in which a power source is connected to the opposing roller 77 to apply an electric charge having the same polarity as the toner to the surface of the transfer material. The effect can be obtained.

Although a roller pair of the reverse charging roller 76 and the counter 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-shaped charging member is charged. It goes without saying that any of the above can be used as long as it can supply.

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

That is, the control means (CP) controls the value of the current flowing from the constant current power source to the reverse charging member according to the type of transfer material.
It is preferable to control by U). When the transfer material is reversely charged by using the 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 more preferable embodiment of this embodiment, a temperature / humidity sensor 20 capable of measuring temperature and humidity is provided in the image forming apparatus main body, and as close as possible to the transfer material storage cassettes 29 and 30, and the temperature and humidity are measured. Sensor 2
The absolute water content is calculated by using the temperature and humidity in the device detected by 0. On the other hand, as many relational expressions as shown in FIG. 4 between the absolute water content and the surface charge density that can be held by the transfer material are stored in the main body of the apparatus (storage means such as ROM) according to the type of the transfer material. When the user uses the image forming apparatus to input the type of transfer material into the apparatus body in advance, it is stored in the apparatus body in advance based on the input transfer material information and the absolute moisture content of the atmosphere in the apparatus body. It is obvious that more preferable results can be obtained by calculating the maximum surface charge density amount that can be held for each transfer material.

Further, by controlling the current (voltage applied) flowing through the reverse charging member by the CPU in accordance with the length of the transfer material in the direction substantially orthogonal to the conveying direction, the toner image on the transfer material is controlled. The transfer current required for transfer can be made appropriate.

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

As a method of automatically detecting, in addition to the normal image formation, the transfer material is transferred to the secondary transfer portion without transferring the toner image onto the intermediate transfer belt 82, and is fixed on the transfer roller 72. The transfer voltage was applied by current control, and the transfer voltage at this time was detected.

That is, the transfer voltage for the secondary transfer current value I0 and the secondary transfer current value I1 larger by ΔI than that is 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 differences ΔV1 and ΔV2 between the transfer voltages are usually constant, but when the amount of charge supplied to the transfer material exceeds the amount of charge that can be held by the transfer material, ΔVi
(I = 1, 2) begins to decrease. Therefore, the secondary transfer current immediately before ΔVi starts to decrease is obtained from the above-described detection flow, the maximum retained charge amount of the transfer material is estimated from this secondary transfer current value, and the reverse charge amount at the time of reverse charging (electric pole value Alternatively, the applied voltage) is determined.

With this configuration, the amount of reversely charged charges can be automatically determined without inputting the type of transfer material.

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

[0065]

As described above, according to the present invention,
An image carrier that carries a toner image and a toner image on the image carrier
And the toner image on the intermediate transfer body
And a transfer means for transferring the
By applying pressure, the toner image on the transfer material is transferred
The opposite polarity to the regular charging polarity of toner on the surface opposite to the surface
Charge is applied, electrostatic transfer of the toner image to the transfer material
In the image forming apparatus that is performed, the image is transferred from the image carrier.
Before transferring the toner image on the intermediate transfer body to the transfer material,
The toner image on the surface of the material to which the toner image is transferred is transferred
Area of the toner is opposite to the normal charging polarity of the toner.
Since it is configured to have charging means for charging,
The charge retention capacity in the
For example , the maximum value of the surface charge density that can be held by the transfer material is apparently increased to about twice, and the charge amount per unit area of the toner image is The toner image stacked on the intermediate transfer body can be transferred to the transfer material without causing abnormal discharge, especially in a low humidity environment, so that the maximum charge amount is not exceeded to prevent transfer defects such as dot-shaped transfer omissions. It is possible to obtain a high quality color image that does not exist.

[Brief description of drawings]

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

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

FIG. 3 is a diagram showing 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 environment atmosphere and a surface charge density that can be held by a transfer material.

FIG. 5 is a schematic view showing a conventional image forming apparatus.

[Explanation of symbols]

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 (13)

(57) [Claims] And 1. A image bearing member for bearing a toner image, an intermediate transfer member the toner image on the image bearing member is transferred, between the intermediate
Equipped with transfer means for transferring the toner image on the transfer body to the transfer material.
Then, by applying a voltage to the transfer means, the toner of the transfer material is
Regular toner charging on the surface opposite to the surface on which the image is transferred
A charge of the opposite polarity to that of the polarity is applied to the toner on the transfer material.
In the image forming apparatus in which the electrostatic transfer is performed on the image, prior to transcription to a transfer material the toner image on the intermediate transfer member is transferred from the image bearing member, in the plane on which the toner image of the transfer material is transferred the area where the toner image is transferred, the image forming apparatus characterized by the normal charging polarity of the toner having a charging means for pre-charged to the opposite polarity. 2. The image forming apparatus according to claim 1, further comprising control means for controlling the voltage applied to the charging means according to the type of transfer material. 3. An image carrier that carries a toner image, and the image.
And an intermediate transfer member to which the toner image on the carrier is transferred.
In the image forming apparatus, the toner transferred from the image carrier onto the intermediate transfer member
A toner image on the transfer material before the image is electrostatically transferred to the transfer material.
The surface to which the toner is transferred has a polarity opposite to the regular charging polarity of the toner.
An image forming apparatus comprising: a charging unit that is charged in advance ; and a control unit that controls a voltage applied to the charging unit according to a length of a transfer material in a direction substantially orthogonal to a conveying direction. 4. The image forming apparatus according to claim 2, further comprising a humidity detecting unit for detecting humidity, wherein the control unit controls a voltage applied to the charging unit according to a detection result of the humidity detecting unit. . 5. A temperature / humidity detecting means for detecting temperature and humidity is provided, and the control means controls the voltage applied to the charging means according to the detection result of the temperature / humidity detecting means. Image forming device. 6. The image forming apparatus according to claim 2, further comprising a discriminating unit for discriminating a kind of the transfer material, wherein the control unit controls a voltage applied to the charging unit according to a detection result of the discriminating unit. apparatus. 7. The image forming apparatus according to claim 1, further comprising a control unit that controls the amount of charge per unit area applied to the transfer material by the charging unit according to the type of the transfer material. 8. An image bearing member carrying a toner image, and the image.
And an intermediate transfer member to which the toner image on the carrier is transferred.
In the image forming apparatus, the toner transferred from the image carrier onto the intermediate transfer member
A toner image on the transfer material before the image is electrostatically transferred to the transfer material.
The surface to which the toner is transferred has a polarity opposite to the regular charging polarity of the toner.
It has a charging unit that is charged in advance , and a control unit that controls the amount of charge per unit area applied to the transfer material by the charging unit according to the length of the transfer material in a direction substantially orthogonal to the conveying direction. An image forming apparatus characterized by the above. 9. A humidity detecting means for detecting humidity is provided, and the control means controls the amount of electric charge per unit area given to the transfer material by the charging means according to the detection result by the humidity detecting means. The image forming apparatus according to claim 7 or 8. 10. A temperature / humidity detecting means for detecting temperature and humidity, wherein the control means is a charge per unit area given to the transfer material by the charging means in accordance with a detection result by the temperature / humidity detecting means. The image forming apparatus according to claim 7, wherein the amount is controlled. 11. A discriminating means for discriminating the type of the transfer material is provided, and the control means controls the amount of electric charge per unit area given to the transfer material by the charging means according to the detection result of the discriminating means. The image forming apparatus according to claim 7 or 8. 12. The image forming apparatus according to claim 2, further comprising a constant current power source connected to the charging unit, wherein the control unit controls a current value flowing through the charging unit. 13. The toner images of a plurality of colors are sequentially transferred from the image carrier to the intermediate transfer member, and the toner images of the plurality of colors on the intermediate transfer member are transferred to a transfer material .
The image forming apparatus according to any one of items .