JP2010152108A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus obtaining a good transfer characteristics over the whole transfer body by reducing toner scattering in an image periphery section, in the image forming apparatus carrying out reversal development. <P>SOLUTION: The image forming apparatus has: an image carrier 1Y for carrying an electrostatic latent image; a charging means 2Y for charging the image carrier 1Y; an exposure means 20 for forming the electrostatic latent image based on the image information on the image carrier 1Y; a developing means 4Y for making a toner image by developing the electrostatic latent image; a transfer means 80 for transferring the toner image to the body to be transferred; a transfer bias applying means 6Y for applying bias to the transferring means; and a transfer pre-exposure means 7Y for exposing the image carrier 1Y before transfer. The exposure amount of the transfer pre-exposure means 7Y is adjusted according to the toner charged amount. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a copying machine, a facsimile machine, or a printer that performs pre-transfer processing before transferring a toner image formed on an image carrier to a transfer body.

In an image forming apparatus using an electrophotographic method, a reversal developing method is often used as a developing method. In the reversal development method, an electrostatic latent image is formed by performing image exposure corresponding to image information with an exposure unit such as a laser beam or LED on an image carrier uniformly charged by a charging unit such as a charging roller or a charging charger. In this method, a toner image is formed by attaching a toner charged to the same polarity as the charged polarity of the image carrier to a portion where the potential is lowered due to exposure.
In the reversal development method, the charged potential of the image portion to which the toner adheres is reduced by exposure, and the background portion, that is, the non-image portion to which the toner is not attached is not exposed, so that the potential remains high after the toner image is formed.
Next, when a charge having a reverse polarity to the toner is applied to the back surface of the transfer material in the transfer step and the toner is transferred to the surface of the transfer material, the transfer material has the reverse polarity to the toner, that is, the reverse of the non-image portion of the image carrier. Since the polarity is charged, the potential difference between the image carrier and the transfer material becomes very large and adversely affects the image.

In order to solve this problem, the image bearing member on which the toner image is formed is uniformly exposed by an exposure means such as a laser beam or an LED between the developing process and the transferring process to reduce the potential of the image bearing member. The transfer pre-exposure (pre-transfer lighting, hereinafter abbreviated as PTL) process is well known in that the transfer current in the transfer process is efficiently applied to the transfer material so that stable transfer efficiency can be obtained from a low transfer current to a high transfer current. It has been.
FIG. 15 is a schematic view showing a general PTL (pre-transfer exposure) process. In FIG. 15, when the charge on the image bearing member 1 is removed from the exposure state by PTL (see FIG. 15A), the potential of the non-image portion is attenuated (see FIG. 15B). Depending on the case, a phenomenon of toner scattering may occur in the periphery of the image (see FIG. 15C).
This is because the charging polarity of the image bearing member 1 and the charging polarity of the toner T are the same, so that when the non-image portion charge is removed, the toner T is likely to be scattered as indicated by an arrow due to electrostatic repulsion. is there.
Conventionally, various techniques have been proposed in order to solve problems such as toner scattering around the image due to electrostatic repulsion (see, for example, Patent Documents 1 to 3).
In order to solve these problems, for example, as disclosed in Patent Document 1 and the like, it is known that the exposure amount before transfer is set to a predetermined light amount at which the background portion potential does not become lower than the toner image portion potential.

Further, as in Patent Document 2, the background portion potential and the toner image portion potential after exposure are detected, and the background portion potential (absolute value) after exposure is equal to or higher than the toner image portion potential (absolute value) after exposure. The exposure amount of the exposure means is controlled by the control means to lower the background portion potential to assist separation of the transfer material, and at the same time, a predetermined potential difference ΔV is provided between the background portion potential and the toner image portion potential to prevent scattering. .
Further, in Patent Document 3, in an image forming apparatus that performs reversal development, in order to prevent transfer unevenness and to reduce toner scattering at the periphery of the character image, and to obtain good transferability over the entire surface of the transfer body, the toner image and the toner A pre-transfer charge imparting device that imparts a charge of the same polarity as the charge polarity of the image carrier to the image carrier on which an image is formed, and a pre-transfer charge removal lamp that neutralizes the charged image carrier. The charge imparting device before transfer imparts charge to the image carrier and increases the amount of charge of the toner image on the image carrier to increase the mirror image force between the toner and the conductive substrate of the image carrier.
JP-A-01-191168 Japanese Patent Laid-Open No. 10-186814 JP 2002-0555544 A

However, in Patent Document 1, the toner image portion potential generated based on the variation in the exposure amount due to the change in the charging potential of the image carrier and the deterioration of the exposure element, or the variation in the toner charge amount due to the temperature and humidity change. Due to fluctuations and the like, the background portion potential and the toner image portion potential did not become the desired potential, and the toner image was scattered and the transfer material was poorly separated.
Even in Patent Document 2 for solving this problem, control is difficult when the image portion potential is high, or a complicated configuration and control using pre-transfer charging must be performed in order to provide a potential difference.
Further, in Patent Document 3, it is difficult to secure a layout space due to a complicated structure by newly adding a charge applying device, downsizing of the device which is a recent trend, and further charging the image carrier. There are problems such as promoting the deterioration of the image carrier.
SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an image forming apparatus that can reduce toner scattering at the periphery of an image and obtain good transferability over the entire surface of a transfer body in an image forming apparatus that performs reversal development in consideration of the above-described circumstances. There is to do.

In order to solve the above-mentioned problems, an invention according to claim 1 is directed to an image carrier for carrying an electrostatic latent image, a charging means for charging the image carrier, and an image on the image carrier. An exposure unit for forming an electrostatic latent image based on information, a developing unit for developing the electrostatic latent image into a toner image, and a transfer unit for transferring the toner image onto a transfer target And a transfer bias applying unit that applies a bias to the transfer unit, and a pre-transfer exposure unit that exposes the image carrier before transfer, in accordance with a toner charge amount, the pre-transfer exposure unit An image forming apparatus that adjusts the amount of exposure is characterized.
According to a second aspect of the present invention, there is provided the image forming apparatus according to the first aspect, wherein an exposure amount of the pre-transfer exposure means is equal to or greater than a necessary exposure amount for attenuating the charged potential of the image carrier to a residual potential. And
According to a third aspect of the present invention, there is provided the image forming apparatus according to the first or second aspect, wherein the exposure amount of the pre-transfer exposure unit is adjusted according to the toner charge amount and the toner color.
According to a fourth aspect of the present invention, there is provided a detecting means for detecting the toner charge amount, and the exposure amount of the pre-transfer exposure means is adjusted according to the detected toner charge amount. The image forming apparatus described above is characterized.

  According to the present invention, it is possible to reduce the fatigue of the image carrier by adjusting the PTL light amount to an optimum light amount according to the toner charge, and form a high-quality, highly stable image with less toner scattering. It becomes possible to do.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic diagram showing a first embodiment of a multicolor image forming apparatus to which the present invention is applied. Next, a first embodiment equipped with the present invention will be described.
In FIG. 1, photosensitive drums 1Y, 1M, 1C, and 1K, which are image carriers, rotate in the direction of the arrow. The subscripts Y, M, C, and K indicate yellow, magenta, cyan, and black, respectively.
Around each of the photosensitive drums 1Y, 1M, 1C, and 1K, charging members 2Y, 2M, 2C, and 2K that are charging means in the order of rotation, developing devices 4Y, 4M, 4C, and 4K, transfer charging means 6Y, 6M, 6C, 6K and cleaning means 5Y, 5M, 5C, 5K are arranged.
The charging members 2Y, 2M, 2C, and 2K constitute a charging device for uniformly charging the surfaces of the photosensitive drums 1Y, 1M, 1C, and 1K. The surface of the photosensitive drums 1Y, 1M, 1C, and 1K between the charging members 2Y, 2M, 2C, and 2K and the developing devices 4Y, 4M, 4C, and 4K is irradiated with a beam by the writing unit 20, and the photosensitive drum 1Y. An electrostatic latent image is formed on the surface of 1M, 1C, and 1K.

Then, based on the electrostatic latent image, toner images are formed on the surfaces of the photosensitive drums 1Y, 1M, 1C, and 1K by the developing devices 4Y, 4M, 4C, and 4K. Further, a recording paper (transfer material) fed from a paper supply unit (not shown) by transfer charging means 6Y, 6M, 6C, 6K as transfer bias applying means for applying a bias to the transfer belt 80 as transfer means. The respective color toner images are sequentially transferred to a recording sheet (not shown), and finally the fixing unit 10 fixes the image on the recording paper.
Pre-transfer exposure means 7Y, 7M, 7C, and 7K are arranged upstream of the transfer charging means 6Y, 6M, 6C, and 6K, and the photoreceptor to which a toner image is adhered by the pre-transfer exposure means 7Y, 7M, 7C, and 7K. The drums 1Y, 1M, 1C, and 1K are exposed. The pre-transfer exposure means 7Y, 7M, 7C, and 7K are means for exposing the photosensitive drums 1Y, 1M, 1C, and 1K. For example, a light emitting member such as an LED is used.

FIG. 2 is a block diagram showing a control system for controlling the exposure amount of the pre-transfer exposure means. In FIG. 2, the control unit 30 determines the exposure amount of the pre-transfer exposure means 7Y (7M, 7C, 7K) based on the charge amount obtained from the detection results of the development current detection means 31 and the density sensor 32 for detecting the development current. To control.
A method of detecting the toner charge amount will be described with reference to FIGS. The image processing unit 33 that outputs image data for driving the writing unit (exposure device) 20 is controlled, and the writing unit 20 is driven with predetermined image data to drive the photosensitive drum 1Y (1M, 1C, 1K). An electrostatic latent image of a patch image consisting of a solid image is formed.
Next, the electrostatic latent image is developed by the developing device 4Y (4M, 4C, 4K), and is used in the developing device 4Y (4M, 4C, 4K) before the developed toner image is transferred to the transfer belt 80. The exposure amount of the pre-transfer exposure means 7Y (7M, 7C, 7K) is controlled according to the charge amount of the toner. In the relationship between the PTL exposure amount and the scatter rank, the scatter rank is once deteriorated by the PTL exposure, but the scatter rank is improved as the PTL exposure amount is increased.
As described above, in the image forming apparatus that adjusts the exposure amount of the pre-transfer exposure unit 7Y (7M, 7C, 7K) according to the toner charge amount, the PTL exposure amount can be adjusted to an optimum light amount according to the toner charge. As a result, it is possible to reduce the fatigue of the photosensitive drum (image carrier) 1Y (1M, 1C, 1K), and to form a high-quality and highly stable image with little toner scattering. Become.

FIG. 3 is a graph showing experimental results of the relationship between the scattering rank and the exposure energy. First, in FIG. 3, in the image forming apparatus of FIG. 1 having the PTL process, the amount of PTL exposure was varied and the amount of scattering around the image portion was confirmed. The evaluation image was a character image, and the amount of scattering around the character was subjectively evaluated. The evaluation was performed with rank 1 being a bad image with scattering and rank 5 being a good image without scattering.
When the PTL is not turned on, no scattering occurs, but the potential difference between the photosensitive drum (image carrier) 1Y (1M, 1C, 1K) and the transfer belt (transfer material) 80 in FIG. There are concerns about adverse effects on images such as a decrease in transfer rate. By increasing the amount of light, the scattering rank once deteriorated, but when the amount of light was further increased, the tendency of the scattering rank to improve was observed.

FIG. 4 is a model diagram showing the amount of scattering around the image portion generated by varying the PTL exposure amount. The phenomenon of the experimental result of FIG. 3 can be explained as follows. First, the occurrence of scattering is suppressed by the potential difference between the non-image portion potential (VD) and the image portion potential (VL) (the non-image portion is negatively large). When PTL exposure is performed, the charge on the photosensitive drum (image carrier) is attenuated, but the amount of attenuation is larger in the non-image area where there is no light shielding by the toner.
However, when the PTL exposure energy is weak, electric charge remains in the non-image portion, and toner scattering does not occur (or is extremely small / see FIG. 4A). As the PTL exposure energy increases, the charge of the non-image area advances, and when the potential difference from the image area is not sufficient to suppress the occurrence of the scattering, the scattering occurs on the photosensitive drum (FIG. 2B). ).
Furthermore, when the PTL exposure energy is increased, the non-image portion charge approaches 0V, but the positive charge generated by the presence of the toner charge is collected in the image portion, so that a potential difference between the image portion and the non-image portion occurs again. And scattering is suppressed (refer to Drawing 4 (c)). In FIG. 4, CGL indicates a carrier generation layer, and CTL indicates a carrier transport layer.
In this way, by increasing the exposure amount in the pre-transfer exposure step, positive charges having a polarity opposite to that of the toner charge are gathered not only on the non-image area but also on the photosensitive drum of the image area, thereby suppressing scattering. Become.

As has been conventionally done, it is possible to limit the exposure amount so that the absolute value of the non-image portion potential does not fall below the absolute value of the image portion potential. The drum has been charged low.
In such a case, the potential difference between the non-image portion potential and the image portion potential is small, and the PTL light amount must be controlled pinpoint to control the non-image portion potential not to be smaller than the image portion potential. Control is becoming difficult.
In addition, in order to form a stable image even over time, there are many image forming apparatuses that use a control method for adjusting the charging potential of the photosensitive drum and controlling the toner adhesion amount. The amount of light must be adjusted from time to time according to potential fluctuations.
In the present invention, by increasing the exposure amount in the pre-transfer exposure step, not only the potential attenuation of the non-image portion on the photosensitive drum but also the positive charge having the opposite polarity to the toner charge is collected in the image portion and the non-image portion and the image. By generating a partial potential difference, it is possible to stably suppress scattering. However, strong exposure contributes to a reduction in the life of the photosensitive drum.
Therefore, in order to perform optimum exposure amount control, the toner scattering amount was evaluated by distributing the charge amount of the toner. At this time, the developing potential of the developing process is adjusted so that the non-image area and the image area have the same potential and the toner adhesion amount is constant regardless of the toner charge amount.

FIG. 5 is a graph showing experimental results of the relationship between the scattering rank and the PTL light amount. As shown in the experimental results of FIG. 5, it was confirmed that when the absolute value of the toner charge amount is high (in the case of -charging, it is large on the-side), the scattering rank improves quickly as the amount of light increases.
This phenomenon can be explained as follows. When the PTL exposure energy is increased, it is considered that positive charges that oppose the toner charges are collected on the surface of the photosensitive drum due to the presence of toner charges in the image area. Therefore, it is considered that if the toner charge is large, the collected positive charge is large, and scattering is easily suppressed.
Therefore, in order to obtain the same scattering rank, when the absolute value of the toner charge amount is high, the same result can be obtained with a small amount of light, and the durability of the photosensitive drum can be improved. Therefore, it is possible to reduce the fatigue of the photosensitive drum by adjusting the PTL light amount to an optimum light amount according to the toner charge.
As described above, in the image forming method having the pre-transfer exposure unit that exposes the photosensitive drum before the transfer, by adjusting the exposure amount of the pre-transfer exposure unit according to the toner charge amount, toner scattering is small, high image quality, A highly stable image can be formed.

FIG. 6 is a graph showing an example of the transmittance measurement result for each color. Moreover, the control method for further controllability improvement and long life was examined. With recent colorization, the number of full-color image forming apparatuses is increasing.
In general, full-color image forming apparatuses use toners of four colors, cyan, magenta, yellow, and black. These toners have different toner charge amounts for each color, and transmit light that transmits light with the same adhesion amount. It is understood that the rates are different.
As shown in FIG. 6, here, a solid image having an adhesion amount of about 0.5 mg / cm ² is formed for each toner on an OHP (overhead projector), and the amount of transmitted light is measured using an optical power meter (not shown). Measured and calculated transmittance. As can be seen from FIG. 6, the transmittance varies greatly depending on the color.
FIG. 7 is a graph showing the toner scattering evaluation in the toner color. Evaluation of toner scattering in the toner color was performed. At this time, adjustment is performed so that the toner adhesion amount and the toner charge amount become the same.
As a result, as shown in the experimental results of FIG. 7, the tendency of the scattering rank with respect to the amount of light to be improved once becomes worse, but when the toner color, that is, the light transmittance of the toner increases, the scattering becomes larger. It was confirmed that the rank was quickly improved and the required amount of PTL light was reduced.

FIG. 8 is a graph showing an example of a toner scattering evaluation result (toner charge amount) in a toner color having a light transmittance of 5%. FIG. 9 is a graph showing an example of a toner scattering evaluation result (toner charge amount) in a toner color having a light transmittance of 20%.
8 and 9 show the tendency of the scattering rank depending on the toner charge amount in the case of each light transmittance. As described above, it can be confirmed that when the absolute value of the toner charge amount is high (in the case of -charging, it is large on the-side), the scattering rank is quickly improved as the amount of light increases.
By adjusting the PTL light amount to an optimum light amount according to the toner charge and toner color, for example, in the developing means using toner having a high light transmittance and a high charge amount, the same scattering can be suppressed with a small PTL light amount. And the fatigue of the photosensitive drum can be reduced.
Therefore, it is possible to reduce the fatigue of the photosensitive drum by adjusting the PTL light amount to the optimum light amount according to the toner charge and the toner color, and with high image quality, less toner scattering over time and environment. A highly stable image can be formed.
Further, the toner charge changes with time. When the developer is used over time, the toner charge amount fluctuates. Therefore, if there is a detecting means for detecting the toner charge amount, and the exposure amount of the pre-transfer exposure means is adjusted according to the detected toner charge amount, the PTL light amount can be adjusted according to the toner charge corresponding to the environment over time. By adjusting to the optimum light amount, it is possible to reduce the fatigue of the photosensitive drum, and it is possible to form a high-quality and highly stable image with little toner scattering.

FIG. 10 is a graph showing the relationship between the PTL light quantity and the toner scattering amount (scattering rank) as an example. FIG. 11 is a table showing the relationship between the toner charge amount and the PTL light amount.
As shown in FIG. 10, the relationship between the PTL light amount and the toner scattering amount (scattering rank) in the toner charge amount to be used is obtained separately by experiment, and the PTL light amount that achieves the required scattering rank or higher is derived for each toner charge amount. . The derived example is the table shown in FIG. The PTL light amount is controlled in accordance with the relationship between the toner charge amount and the PTL light amount that achieve the required scattering rank or higher.
Further, this PTL light amount is a light amount necessary for reducing the potential of the non-image area and collecting the charge of the opposite polarity to the toner charge on the photosensitive drum in the toner adhesion region. It is desirable that the exposure amount be more than the required exposure amount to be attenuated. For example, in the example of the toner charge amount and the PTL light amount shown in FIG. 11 described above, a light amount of about 1 to 10 times the necessary exposure amount for attenuation to the residual potential of the photosensitive drum is required.

As described above, by adjusting the exposure amount of the pre-transfer exposure unit according to the toner charge amount, it is possible to reduce the fatigue of the photosensitive drum by adjusting the PTL light amount to the optimum light amount according to the toner charge. Therefore, it is possible to form a high-quality and highly stable image with little toner scattering.
Although the image forming apparatus has been described as an embodiment using a full color forming apparatus here, a single color image forming apparatus may be used. Also, as a transfer material, direct transfer in which a recording material such as paper is transported and transferred by a transfer belt is used, and a toner image is once carried on the transfer belt and transferred to the recording material by using a second transfer means (not shown). It is also possible to use an intermediate transfer method.
As described above, when the toner color, that is, the light transmittance of the toner increases, the scattering rank improves faster and the required light amount decreases. Further, it has been confirmed that when the absolute value of the toner charge amount is high (in the case of -charge, it is large on the-side), the scattering rank is improved quickly as the PTL light quantity increases.

FIG. 12 is a table showing the relationship among toner transmittance, toner charge amount, and PTL light amount. The relationship between the light transmittance of the toner that achieves the required scattering rank or higher, the toner charge amount, the PTL light amount, and the toner scattering amount (scattering rank) is obtained by a separate experiment. FIG. 12 shows an example derived by experiment. Based on these relationships, the PTL light amount that achieves a required scattering rank or higher according to the exposure amount and charge amount of the toner used is controlled.
By adjusting the PTL light amount to an optimum light amount according to the toner charge and toner color, for example, in a developing device using toner having a high light transmittance and a high charge amount, the same scattering suppression is achieved with a small PTL light amount. And fatigue of the photosensitive drum can be reduced.
Therefore, it is possible to reduce the fatigue of the photosensitive drum by adjusting the PTL light amount to the optimum light amount according to the toner charge and toner color, and with high image quality with little toner scattering over time and in accordance with the environment. A highly stable image can be formed.

FIG. 13 is a schematic diagram showing a second embodiment of the multicolor image forming apparatus to which the present invention is applied. Next, a second embodiment equipped with the present invention will be described. In FIG. 13, the photosensitive drums 1Y, 1M, 1C, and 1K, which are image carriers, rotate in the direction of the arrow. The subscripts Y, M, C, and K indicate yellow, magenta, cyan, and black, respectively.
As in the first embodiment, around each photosensitive drum 1Y, 1M, 1C, and 1K, chargers 2Y, 2M, 2C, and 2K, developing devices 4Y, 4M, 4C, 4K, Transfer charging means 6Y, 6M, 6C, 6K and cleaning means 5Y, 5M, 5C, 5K are arranged.
The charging members 2Y, 2M, 2C, and 2K constitute a charging device for uniformly charging the surfaces of the photosensitive drums 1Y, 1M, 1C, and 1K. The surface of the photosensitive drums 1Y, 1M, 1C, and 1K between the charging members 2Y, 2M, 2C, and 2K and the developing devices 4Y, 4M, 4C, and 4K is irradiated with a beam by the writing unit 20, and the photosensitive drum 1Y, Electrostatic latent images are formed at 1M, 1C, and 1K.

Based on the electrostatic latent image, toner images are formed on the surfaces of the photosensitive drums 1Y, 1M, 1C, and 1K by the developing devices 4Y, 4M, 4C, and 4K. Further, a color toner image is formed on the transfer belt 80 as a transfer material by the transfer charging units 6Y, 6M, 6C, and 6K.
The transfer toner image is sequentially transferred to the transfer belt 80, and the color toner image is transferred to the recording material by the secondary transfer unit 100 and finally fixed by the fixing unit 10. The respective color toner images are sequentially transferred, and finally the fixing unit 10 fixes the image on the recording paper.
Pre-transfer exposure means 7Y, 7M, 7C, and 7K are arranged upstream of the transfer charging means 6Y, 6M, 6C, and 6K, and the photoreceptor to which a toner image is adhered by the pre-transfer exposure means 7Y, 7M, 7C, and 7K. The drums 1Y, 1M, 1C, and 1K are exposed. The pre-transfer exposure means 7Y, 7M, 7C, and 7K are means for exposing the photosensitive drums 1Y, 1M, 1C, and 1K. For example, a light emitting member such as an LED is used.
The exposure amounts of the pre-transfer exposure means 7Y, 7M, 7C, and 7K are controlled by exposure control means (not shown) according to the charge amount of toner used in the developing devices 4Y, 4M, 4C, and 4K.

FIG. 14 is a block diagram showing a control system for performing light quantity control of the pre-transfer exposure means used in the second embodiment of FIG. In FIG. 14, the control unit 40 is based on the charge amount obtained from the detection results of the development current detection means 41 and the density sensor (toner adhesion amount measurement means) 90 for detecting the development current, and the pre-transfer exposure means 7Y (7M, 7M, 7C, 7K) is controlled.
The detection of the toner charge amount will be described below with reference to FIGS. The image processing unit 43 that outputs image data for driving the writing unit (exposure device) 20 is controlled, and the writing unit 20 is driven with predetermined image data to drive the photosensitive drum 1Y (1M, 1C, 1K). An electrostatic latent image of a patch image consisting of a solid image is formed.
The toner adhesion amount measuring unit 90 is disposed on the downstream side of the secondary transfer of the transfer belt 80 and on the upstream side of the transfer material cleaning unit 101, and appropriately measures the toner adhesion amount. In the present description, the toner adhesion amount measuring unit 90 is disposed on the upstream side of the secondary transfer of the transfer belt 80. However, the toner adhesion amount measuring unit 90 may be disposed at the pre-transfer position of the photosensitive drum 1Y (1M, 1C, 1K).

Next, the toner adhesion amount measuring unit 90 will be described. The toner adhesion amount measuring means 90 detects the density of the toner image transferred from the photosensitive drum 1Y (1M, 1C, 1K) to the transfer belt 80 as an intermediate transfer material by a density sensor constituting the toner adhesion amount measuring means 90. Thus, the toner adhesion amount is detected.
The toner adhesion amount measuring means (density sensor) 90 detects the density of the color toner image on the transfer belt 80, and receives the light emitting element that irradiates the transfer belt 80 with light and the reflected light from the transfer belt 80. And a reflection type density sensor composed of a light receiving element.
Next, the toner charge amount is detected as follows. First, the image processing unit 43 that outputs image data for driving the writing unit 20 that is an exposure apparatus is controlled, and the writing unit 20 is driven with predetermined image data, so that the photosensitive drums 1Y (1M, 1C, 1K) are driven. ) An electrostatic latent image of a patch image consisting of a solid image is formed on the top.
Next, the electrostatic latent image is developed by the developing device 4Y (4M, 4C, 4K), the developed toner image is transferred to the transfer belt 80, and the density of the toner image on the transfer belt 80 is measured by the toner adhesion amount measuring means. Detect at 90.

The detection result of the toner adhesion amount measuring means (density sensor) 90 is fed back to the developing bias current of the developing device 4Y (4M, 4C, 4K) so that the output of the toner adhesion amount measuring means 90 becomes constant. The development bias current is controlled from the value measured by the detection means 41 and the value stored in the memory 42, and the development current when a toner image of a predetermined density is formed is measured by the development current detection means (ammeter) 41. To do.
The toner adhesion amount is already determined by the toner adhesion amount measuring means 90 because the toner adhesion amount is measured at a constant value, and the controller 40 calculates the toner charge amount from the toner adhesion amount and the development current.
In the control unit 40, the toner charge amount q is a predetermined area S, and is calculated from the following equation by detecting the development current I when a toner image having a toner adhesion amount M per unit area is formed. I = dQ / dt
Q = q × M × S

All four developing devices 4Y, 4M, 4C, and 4K are operated to form the patch image, detect the toner charge amount, derive the toner charge amount for each color, and change the PTL light amount according to the toner charge amount. adjust.
It is also possible to detect and control not only all of the toner charge amounts of the four colors, but only K toner that tends to show toner scattering and K and C toners with low transmittance.
As described above, the toner adhesion amount measuring means (detection means) 90 for detecting the toner charge amount is provided, and the exposure amount of the pre-transfer exposure means 7Y (7M, 7C, 7K) is adjusted according to the detected toner charge amount. By adjusting the PTL light amount to the optimum light amount according to the toner charge, it is possible to reduce the fatigue of the photosensitive drum 1Y (1M, 1C, 1K). It is possible to form a high-quality and highly stable image with little toner scattering corresponding to the environment.

1 is a schematic diagram illustrating a first embodiment of a multicolor image forming apparatus to which the present invention is applied. It is a block diagram which shows the control system which performs exposure amount control of the pre-transfer exposure means. It is a figure which shows the experimental result of the relationship between a scattering rank and exposure energy with a graph. It is a model figure which shows the amount of scattering of the periphery of an image part which generate | occur | produces by changing PTL exposure amount. It is a figure which shows the experimental result of the relationship between a scattering rank and a PTL light quantity with a graph. It is a figure which shows an example of the transmittance | permeability measurement result of each color with a graph. It is a figure which shows the toner scattering evaluation in a toner color with a graph. FIG. 10 is a graph showing an example of a toner scattering evaluation result (toner charge amount) in a toner color having a light transmittance of 20%. FIG. 7 is a graph showing an example of a toner scattering evaluation result (toner charge amount) in a toner color having a light transmittance of 5%. FIG. 6 is a graph illustrating an example of a relationship between a PTL light amount and a toner scattering amount (scattering rank). It is a figure which shows the relationship between a toner charge amount and a PTL light quantity as a table | surface. FIG. 6 is a table showing a relationship among toner transmittance, toner charge amount, and PTL light amount. It is the schematic which shows 2nd Embodiment of the multi-color corresponding | compatible image forming apparatus to which this invention is applied. It is a block diagram which shows the control system which performs exposure amount control of the pre-transfer exposure means used for 2nd Embodiment of FIG. It is the schematic which shows the process of general PTL (exposure before transfer).

Explanation of symbols

  A image forming apparatus (first embodiment), B image forming apparatus (second embodiment), 1Y image carrier (photosensitive drum), 2Y charging means (charging member), 4Y developing means (developing apparatus) ) 5Y cleaning means, 6Y transfer bias applying means (transfer charging means), 7Y pre-transfer exposure means, 10 fixing means, 20 exposure means (exposure device, writing unit), 80 transfer material (intermediate transfer material, transfer belt) 90 detecting means (toner adhesion amount measuring means, density sensor), 100 secondary transfer means, 101 transfer material cleaning means

Claims (4)

An image carrier for carrying an electrostatic latent image; a charging means for charging the image carrier; an exposure means for forming an electrostatic latent image on the image carrier based on image information; Development means for developing the electrostatic latent image into a toner image, transfer means for transferring the toner image onto the transfer target, transfer bias applying means for applying a bias to the transfer means, and before transfer In an image forming apparatus comprising: a pre-transfer exposure unit that exposes the image carrier.
An image forming apparatus comprising: adjusting an exposure amount of the pre-transfer exposure unit in accordance with a toner charge amount.   2. The image forming apparatus according to claim 1, wherein an exposure amount of the pre-transfer exposure unit is equal to or greater than a necessary exposure amount for attenuating a charging potential of the image carrier to a residual potential.   3. The image forming apparatus according to claim 1, wherein an exposure amount of the pre-transfer exposure unit is adjusted according to the toner charge amount and a toner color.   3. The image forming apparatus according to claim 1, further comprising a detection unit configured to detect the toner charge amount, and adjusting an exposure amount of the pre-transfer exposure unit according to the detected toner charge amount.

Images