JP2011027916A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus for inversion development, which reduces toner dust around an image part and ensures satisfactory transferability over the entire surface of a body to be transferred. <P>SOLUTION: A PTL exposure is carried out on an image part F in units of the number of pixels with intensity higher than on a non-image part NF. Thereby, the potential of the non-image part NF is decreased, and positive electric charges are efficiently generated in the image part F at the same time. Accordingly, generation of toner dust is stably and reliably suppressed. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a copying machine, a facsimile machine, and a printer, and more particularly to a pre-transfer exposure process in which an exposure process is performed before a toner image formed on an image carrier is transferred to a transfer target. It is.

In general, 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, image exposure corresponding to image information is performed on an image carrier whose surface is uniformly charged by a charging means such as a charging roller or a charging charger, using an exposure means such as a laser beam or LED (light emitting diode). In this method, an electrostatic latent image is formed, and toner charged to the same polarity as the charged polarity of the image carrier is attached to a portion where the potential is lowered by exposure at that time, thereby forming a toner image.
In the reversal development method, the charged potential of the image portion to which the toner adheres is lowered by exposure, and the non-image portion to which the toner is not attached remains high after the toner image is formed. In the transfer process, when a charge having a polarity opposite to that of the toner is applied to the back surface of the transfer body to transfer the toner to the surface of the transfer body, the potential difference between the non-image portion of the image carrier and the transfer body is extremely high. As a result, discharge occurs, resulting in lower transfer efficiency and image degradation. In order to solve this problem, between the development process and the transfer process, the image bearing member on which the toner image is formed is uniformly exposed to the image bearing member by an exposure means such as an LED, and the charged potential of the image bearing member And pre-transfer lighting (Pre-Transfer Lighting, hereinafter abbreviated as “PTL”) process that efficiently applies the transfer current in the transfer process to the transfer target to obtain a high transfer efficiency with a low transfer current is often used. It is done.

  However, when such pre-transfer exposure processing is performed, toner dust tends to occur. The generation of dust in the toner will be described with reference to FIG. FIG. 16 shows a potential diagram of the reversal development process, where (a) is a potential diagram during PTL exposure, (b) is a potential diagram after PTL exposure, and (c) is when toner dust occurs after PTL exposure. FIG. As shown in FIG. 16A, the state of the potential on the image carrier immediately after the toner image is formed by the developing means is the presence of the toner T from the image portion F of the toner image to which the toner T is adhered. Since the non-image portion NF of the toner image that has not been transferred is at a high potential, the potential difference between the non-image portion NF of the image carrier and the transferred body is very large when the toner image is transferred onto the transferred body. And the discharge is generated, resulting in a decrease in transfer efficiency and image deterioration. Therefore, as shown in FIG. 16A, when PTL exposure is performed from above the image carrier, the charged potential of the non-image portion NF is lowered as shown in FIG. And good transfer becomes possible. However, the charged potential of the non-image area NF thus lowered attenuates and becomes lower than the charged potential of the image area F (see FIG. 16C), and the toner T is not allowed to move from the image area F to the periphery thereof. This causes problems such as scattering and image blurring. This is because, since the charge polarity of the image carrier and the toner is the same, if the non-image portion charge is neutralized, the toner is likely to be scattered due to electrostatic repulsion.

In order to solve these problems, for example, as disclosed in Patent Documents 1 and 2, etc., it is known that the exposure amount before transfer is set to a predetermined light amount at which the non-image portion potential does not become lower than the image portion potential. . However, the charging potential of the image area and the charging of the non-image area where the pre-transfer exposure is performed due to the fluctuation of the exposure amount due to the change in the charging potential of the image carrier with time and the deterioration of the exposure element, or the fluctuation of the toner charging amount due to the environmental change. Since the potential itself fluctuates, it is difficult to control the non-image portion potential, the image portion potential and their potential difference after the pre-transfer exposure. Arise.
In order to solve these problems, as disclosed in Patent Document 3 and the like, the non-image portion potential after exposure and the image portion potential are detected, and the non-image portion potential after exposure (absolute value) is the image portion after exposure. The exposure amount of the exposure unit is controlled by the control unit so that the potential (absolute value) is equal to or greater than the potential (absolute value), and the non-image portion potential is reduced to help separation of the transfer material. By providing a predetermined potential difference ΔV, toner scattering is prevented. However, even with this method, control is difficult when the image portion potential is high, and in order to provide a potential difference, complicated configuration and control such as using pre-transfer charging must be performed.

Further, in Patent Document 4, in an image forming apparatus that performs reversal development, in order to prevent transfer unevenness and to reduce toner dust in the periphery of a character image, and to obtain good transferability over the entire surface of a transfer object, A pre-transfer charge imparting device that imparts a charge having the same polarity as the charge polarity of the image carrier to the image carrier on which the toner image is formed, and a pre-transfer neutralization lamp that neutralizes the charged image carrier. The charge is applied to the image carrier by the pre-transfer charge applying device, and the charge amount of the toner image on the image carrier is increased to increase the mirror image force between the toner and the conductive substrate of the image carrier. However, in this method, the construction becomes complicated by adding a new charge applying device, it is difficult to secure an arrangement space due to the recent downsizing of the device, and the image carrier is further charged to charge the image. There are problems such as promoting the deterioration of the carrier.
Further, in Patent Document 5, the non-image area outside the image part and the part of the area separated by 100 μm or more from the image part are subjected to static elimination, while leaving the high potential part (unexposed part) in the peripheral part of the image part. It has been proposed to improve the separability by lowering the potential of the image area. However, it is difficult to hold the high potential portion in the peripheral region of the desired image portion in the case of alignment of the writing position of the pre-transfer exposure or a complicated image pattern such as a character or a dither pattern.

  The present invention has been made in view of the above problems, and in an image forming apparatus that performs reversal development, an image forming apparatus that can reduce toner dust in the periphery of an image portion and enables good transferability over the entire surface of a transfer target. It is to provide.

In order to solve the above problems, the invention of claim 1 is directed to an image carrier for carrying an electrostatic latent image, a charging means for uniformly charging the surface of the image carrier, and charging by the charging means. An exposure unit for forming an electrostatic latent image on the surface of the image carrier formed on the basis of image information, a developing unit for developing the electrostatic latent image into a toner image, and the developing unit A transfer unit configured to transfer a toner image formed on the image bearing member onto the transfer target; a transfer bias applying unit configured to apply a bias to the transfer unit; and the toner image is transferred onto the transfer target. In the image forming apparatus having a pre-transfer exposure unit that exposes the image carrier in advance, the pre-transfer exposure unit converts an image portion of the toner image formed on the image carrier into a non-image portion of the toner image. Control hand to expose with larger exposure Characterized in that it is controlled by.
According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the pre-transfer exposure unit can change the exposure amount in units of pixels of the toner image.
The image forming apparatus according to claim 1 or 2, wherein the potential of the non-image portion after exposure by the pre-transfer exposure unit is the image immediately before exposure by the pre-transfer exposure unit. It is characterized by being controlled by the control means so as to be equal to or less than the potential of the part.

According to a fourth aspect of the present invention, there is provided the image forming apparatus according to any one of the first to third aspects, wherein the pre-transfer exposure unit for the image portion is in accordance with a toner adhesion amount of the image portion of the toner image. The exposure amount is controlled by the control means.
According to a fifth aspect of the present invention, in the image forming apparatus according to any one of the first to fourth aspects, the image pre-exposure means exposes the image portion according to the toner color of the image portion of the toner image. The quantity is controlled by the control means.
According to a sixth aspect of the present invention, in the image forming apparatus according to the fifth aspect, the exposure amount of the pre-transfer exposure unit for the image portion is determined according to the light transmittance of the toner of the image portion of the toner image. It is controlled by the control means.

  According to the present invention, the pre-transfer exposure means is controlled by the control means so as to expose the image portion of the toner image formed on the image carrier with a larger exposure amount than the non-image portion of the toner image. As a result, in the image forming apparatus that performs reversal development, it is possible to provide an image forming apparatus that can reduce toner dust around the image portion and enables good transferability over the entire surface of the transfer target.

It is a figure which shows the relationship between the dust generation | occurrence | production evaluation of a toner, and the exposure energy (exposure amount) in the pre-transfer exposure means with a graph. (A) is a schematic diagram showing the relationship between the exposure amount of PTL, the charge potential state of the image carrier, and the charge generation state in the charge transport layer (CTL) and the charge generation layer (CGL) of the image carrier K. FIG. 2 is a schematic diagram showing the exposure amount of PTL in the state of X1 in FIG. 1, (b) in the state of X2 in FIG. 1, and (c) in the state of X3 in FIG. 4 is a schematic diagram showing the relationship between the exposure amount of the PTL according to an embodiment of the present invention, the charged potential state of the image carrier, and the charge generation state in the charge transport layer (CTL) and the charge generation layer (CGL) of the image carrier K. FIG. FIG. 6 is a graph showing the relationship between toner dust generation evaluation using the toner adhesion amount as a parameter and the exposure amount in a pre-transfer exposure unit. . FIG. 4 is a graph showing the relationship of light transmittance in each toner of black (K), magenta (M), cyan (C), and yellow (Y). . FIG. 6 is a graph showing a relationship between a toner dust evaluation rank and a PTL exposure when a toner color is used as a parameter. . 1 is a diagram illustrating a schematic configuration of a multicolor image forming apparatus according to Embodiment 1. FIG. 3 is a block diagram of a control unit that controls the exposure amount of the PTL of the multicolor image forming apparatus according to Embodiment 1. FIG. 6 is a flowchart of a control unit that controls the exposure amount of the PTL of the multicolor image forming apparatus according to the first exemplary embodiment. FIG. 10 is a diagram illustrating a schematic configuration of a multicolor image forming apparatus according to a second embodiment. FIG. 9 is a block diagram of a control unit that controls an exposure amount of a PTL in a multicolor image forming apparatus according to a second embodiment. FIG. 6 is a diagram illustrating a schematic configuration of a multicolor image forming apparatus according to a third embodiment. FIG. 10 is a block diagram of a control unit that controls an exposure amount of a PTL in a multicolor image forming apparatus according to Embodiment 3. 10 is a flowchart of a control unit that controls an exposure amount of a PTL in a multicolor image forming apparatus according to a third embodiment. FIG. 10 is a graph showing a relationship between transmittance and PTL light amount increase rate used in PTL exposure amount control of the multicolor image forming apparatus according to Embodiment 3. The potential diagram of a conventional reversal development process is shown, (a) is a potential diagram during PTL exposure, (b) is a potential diagram after PTL exposure, and (c) is a potential during dust generation of toner after PTL exposure. FIG.

In order to solve the above problems, the inventors of the present invention performed PTL exposure on the entire surface of the image carrier in an image forming apparatus having a PTL process, and examined the amount of dust around the image portion of the toner image.
The examination result will be described with reference to FIG. FIG. 1 is a graph showing the relationship between toner dust generation evaluation and exposure energy (exposure amount) in the pre-transfer exposure means. In this case, the dust generation evaluation is performed by subjectively evaluating the amount of dust around a character when a toner image is formed as a character image. Rank 1 indicates that dust generation is severe and Rank 5 indicates a good image without generation of dust. Evaluation was performed.
As is apparent from FIG. 1, when the PTL is not turned on (X1), no dust is generated, but the potential difference between the non-image portion of the image carrier and the transferred body becomes very large, and discharge occurs in this region. Therefore, particularly in an image forming apparatus controlled by constant current, there is a concern about an adverse effect on an image such as a decrease in transfer rate. As the exposure amount was increased, the Chilean rank deteriorated once (X2), but when the exposure amount was further increased, the Chilean evaluation rank tends to improve (X3) against expectations.

This phenomenon will be described with reference to FIG. FIG. 2 is a schematic diagram showing the relationship between the exposure amount of PTL, the charged potential state of the image carrier, and the charge generation state in the charge transport layer (CTL) and charge generation layer (CGL) of the image carrier K. 1A shows the XTL exposure state in FIG. 1, FIG. 1B shows the X2 state in FIG. 1, and FIG. 1C shows the X3 state in FIG. That is, in the state X1 shown in FIG. 1A, dust generation is suppressed by the potential difference between the potential (VD) of the non-image portion NF and the potential (VL) of the image portion F (the non-image portion is negatively large). Yes.
When the exposure amount of the PTL is increased to reach the X2 state, as shown in FIG. 2B, the charged charge on the image carrier K is attenuated, but the attenuation amount is the image portion F where the toner T exists. It is larger in the non-image portion NF where there is no light shielding by the toner T. Thus, when the charge of the non-image area NF is alleviated and the potential difference from the image area F is not sufficient to suppress the generation of dust, the toner T on the image carrier K becomes dust and scatters around. .
Further, when the PTL exposure amount is increased, as shown in FIG. 2C, the charge of the non-image portion NF approaches 0V, but the positive charge generated by the presence of the toner charge collects in the image portion F. Again, it is presumed that the potential difference between the image portion F and the non-image portion NF occurs and the generation of dust is suppressed.
In this way, by increasing the exposure amount in the pre-transfer exposure process, not only the non-image portion NF but also the positive charge having the opposite polarity to the toner charge is collected on the image carrier K of the image portion F, thereby suppressing the generation of toner dust. Investigated that it would be possible.

Further, as is conventionally done, the exposure amount can be limited so that the absolute value of the potential of the non-image area NF does not become less than the absolute value of the potential of the image area F. Therefore, the charge of the image carrier is reduced. In such a case, the potential difference between the potential of the non-image portion NF and the potential of the image portion F is small, and the exposure amount of the PTL is controlled so that the potential of the non-image portion NF is not made smaller than the potential of the image portion F. It has become difficult to control. In addition, in order to form a stable image even over time, there are many image forming apparatuses that use a control method for controlling the toner adhesion amount by adjusting the charging potential of the image carrier. The exposure amount must be adjusted at any time according to the fluctuation of the potential of the portion NF.
Based on such a situation, the present inventors have studied to further control the optimum exposure amount. As a result, as shown in FIG. 3, the PTL exposure stronger than the non-image portion NF is performed on the image portion F in units of the number of pixels, thereby reducing the potential of the non-image portion NF and at the same time applying positive to the image portion F. It has been found that electric charges are efficiently generated and toner dust generation can be suppressed stably and reliably, and the present invention has been completed.

In order to change the exposure amount of the PTL for the non-image portion NF and the image portion F as described above, for example, an LED array having a longitudinal direction parallel to the axial direction of the image carrier is easily used as the PTL. obtain. That is, the light emitting elements in the LED array are arranged in the longitudinal direction with a number of light emitting elements having an irradiation area substantially equal to the size of the pixel unit written on the image carrier by the writing unit. Can be easily achieved by controlling the amount of power supplied to each light emitting element so that the exposure amount of the light is larger than the exposure amount of the light emitting element with respect to the non-image portion NF.
In this case, the exposure amount to the image portion F needs to be larger than the exposure amount to the non-image portion NF, but in order to efficiently generate the positive charge to the image portion F, to the image portion F. It is desirable to vary according to the toner adhesion amount.

FIG. 4 is a graph showing the relationship between toner dust generation evaluation using the toner adhesion amount as a parameter and the exposure amount in the pre-transfer exposure means. As is apparent from FIG. 4, when the toner adhesion amount is relatively small at 0.2 mg / cm ² (polygonal line 1), generation of positive charges on the image portion F occurs with a small exposure amount of PTL. On the other hand, when the toner adhesion amount is relatively large as 0.5 mg / cm ² (polygonal line 2), a large amount of PTL exposure is required. As described above, when the exposure amount of the PTL is changed according to the toner adhesion amount, not only the positive charge to the image portion F is appropriately generated, but also the exposure more than necessary is suppressed and the deterioration of the image carrier is reduced. It is desirable because it becomes possible. In this way, if the amount of PTL light to the image area is controlled in accordance with the amount of image writing energy, the generation of toner dust can be suppressed with an appropriate amount of PTL exposure, and long-life control can be performed. .
With the recent colorization, the number of full-color image forming apparatuses is increasing. In general, full-color image forming devices use cyan, magenta, yellow, and black toners. However, even with the same toner adhesion amount, the relationship between the toner dust evaluation rank and the exposure amount varies depending on the toner color. Confirmed by
This is presumably because, even if the toner adhesion amount is the same, if the toner color is different, the transmittance of the toner light is different, so that the shielding effect of the image portion F by the toner is different.

FIG. 5 is a graph showing the relationship of light transmittance in each toner of black (K), magenta (M), cyan (C), and yellow (Y). Here, a solid image having an adhesion amount of about 0.5 mg / cm ² was formed for each color toner on the OHP, and the transmittance was calculated by measuring the amount of transmitted light using an optical power meter. As can be seen from FIG. 5, the transmittance varies greatly depending on the toner color, indicating that the shielding effect of the image portion of the toner image by the toner is different even with the same toner adhesion amount.
From the results of FIG. 5, it is understood that even if the toner adhesion amount is the same, it is necessary to change the PTL exposure amount according to the toner color. FIG. 6 shows the toner dust when the toner color is used as a parameter. The relationship between the evaluation rank and the exposure amount of the PTL is shown. The broken line 3 represents black toner, the broken line 4 represents magenta toner, and the broken line 5 represents cyan toner. In this case, the toner dust evaluation rank and the PTL are obtained by forming a solid image having an adhesion amount of about 0.5 mg / cm ² for each toner of black (K), magenta (M), and cyan (C) on OHP. The amount of exposure was measured.

From the result of FIG. 6, it is found that the exposure amount that can improve the toner dust evaluation rank needs to be increased in the order of black toner, cyan toner, and magenta toner. In other words, it can be understood that the toner dust evaluation rank cannot be improved unless the exposure amount is increased for a toner having a lower transmittance. Thus, by controlling and adjusting the exposure amount of the image portion of the pre-transfer exposure means according to the toner color, it is possible to form a high-quality and highly stable image with little toner dust generation.
Therefore, in order to form a high-quality and highly stable image with little toner dust when forming a color image, the toner color is detected together with the toner adhesion amount, and the exposure amount of the PTL is set according to the toner color together with the toner adhesion amount. It is necessary to control and adjust. Such toner color detection is performed when writing an image corresponding to a toner color in a later-described writing unit when an image bearing member is irradiated with writing light corresponding to the toner color corresponding to the image information when forming an electrostatic latent image. It can be easily detected by an input signal to the light source.

Next, an image forming apparatus according to an embodiment of the present invention will be described based on examples with reference to the drawings.
Example 1
FIG. 7 is a diagram illustrating a schematic configuration of the multicolor image forming apparatus according to the first embodiment. FIG. 8 is a block diagram of a control unit that controls the exposure amount of the PTL of the multicolor image forming apparatus according to the first embodiment.
In FIG. 7, in the image forming apparatus according to this embodiment, charging rollers 2Y, 2M, 2C, 2K, which are charging units, are provided around drum-shaped photoconductors 1Y, 1M, 1C, 1K, which are image carriers. Developing units 4Y, 4M, 4C, and 4K as developing means, transfer rollers 6Y, 6M, 6C, and 6K as transferring means, and cleaning means 5Y, 5M, 5C, and 5K are arranged. Here, the alphabets Y, M, C, and K attached after the reference numerals indicate yellow, magenta, cyan, and black toner colors, respectively.

The charging rollers 2Y, 2M, 2C, and 2K are in contact with or not in contact with the surfaces of the photoreceptors 1Y, 1M, 1C, and 1K, and rotate with the rotation of the photoreceptors 1Y, 1M, 1C, and 1K in the direction of the arrows. It is a charging member constituting charging means for uniformly charging the surfaces of 1Y, 1M, 1C, and 1K.
The developing devices 4Y, 4M, 4C, and 4K store yellow, magenta, cyan, and black toners in the case, and these toners correspond to the colors formed on the surfaces of the photoreceptors 1Y, 1M, 1C, and 1K. The electrostatic latent image is supplied by a developing roller to form a toner image.
Transfer rollers 6Y, 6M, 6C, and 6K transfer bias from transfer bias means 15Y, 15M, 15C, and 15K to transfer paper P that is a transfer target conveyed by an endless conveyance belt 14, as will be described later. Is a transfer means for transferring the toner images formed on the photoreceptors 1Y, 1M, 1C, and 1K. Here, the conveying belt 14 is stretched between the driving roller 13a and the driven roller 13b and is transferred in the direction of arrow A by the rotation of the driving roller 13 in the direction of the arrow. By transferring the transfer paper P, the transfer paper P is supplied to the transport belt.

A method for forming an image on the transfer paper P using such an image forming apparatus will be described. First, the surfaces of the photoreceptors 1Y, 1M, 1C, and 1K are uniformly charged by the charging rollers 2Y, 2M, 2C, and 2K on the rotating photoreceptors 1Y, 1M, 1C, and 1K. The surfaces of the photoreceptors 1Y, 1M, 1C, and 1K thus charged are irradiated with light LY, LM, LC, and LK corresponding to the image information of yellow, magenta, cyan, and black from the writing unit 20, An electrostatic latent image corresponding to each color is formed. The toners of the respective colors are supplied from the developing devices 4Y, 4M, 4C, and 4K to the electrostatic latent images of the photoreceptors 1Y, 1M, 1C, and 1K on which the electrostatic latent images corresponding to the respective colors are formed in this way. Thus, a toner image of each color is formed.
The toner images formed on the photoconductors 1Y, 1M, 1C, and 1K in this way are conveyed to the conveyance belt 14 by the registration roller 12 at the same timing, and are further conveyed by the conveyance belt 14 to the photoconductors 1Y, 1M, and 1C. A transfer bias is supplied by the transfer rollers 6Y, 6M, 6C, and 6K onto the transfer sheet P conveyed to 1K, and transferred. In this case, the toner images formed on the photoreceptors 1Y, 1M, 1C, and 1K are sequentially stacked on the transfer paper P and transferred to form a color toner image. The transfer paper P on which the color toner image is thus formed is heated and pressed by the heating roller 10a and the pressure roller 10b of the fixing device 10, and the color toner image is fixed on the transfer paper P.

On the other hand, the photoreceptors 1Y, 1M, 1C, and 1K on which the toner images are transferred onto the transfer paper P are removed and cleaned by the cleaning means 5Y, 5M, 5C, and 5K after the transfer, so that the next image can be formed. The charging rollers 2Y, 2M, 2C, and 2K are charged. In this way, the same operation is repeated to form an image on the transfer paper P.
In such image formation, as described above, when the toner images are transferred onto the transfer paper P from the photoconductors 1Y, 1M, 1C, and 1K, the photoconductors 1Y, 1M, 1C, and 1K, There may be a case where a discharge occurs between the transfer efficiency and the transfer efficiency and image deterioration.
In the image forming apparatus according to the first embodiment, in order to suppress such discharge between the photoreceptors 1Y, 1M, 1C, and 1K and the transfer paper P, the developing device developers 4Y, 4M, Pre-transfer exposure means 7Y, 7M, 7C, and 7K are disposed around the photoreceptors 1Y, 1M, 1C, and 1K between the 4C and 4K and the transfer rollers 6Y, 6M, 6C, and 6K. Then, the control means 80Y, 80M, 80C, and 80K make the exposure amount for the image portion of the toner image stronger than the exposure amount for the non-image portion with respect to the pre-transfer exposure means 7Y, 7M, 7C, and 7K. The photoconductors 1Y, 1M, 1C, and 1K are exposed by being controlled.
As the pre-transfer exposure means 7Y, 7M, 7C, 7K, for example, an LED array having a longitudinal direction parallel to the axial direction of the photoconductor is used, and the light emitting elements in the longitudinal direction are the size of pixels written by the writing unit. The exposure position and exposure intensity in the longitudinal direction can be set for each element by a control means described later. In this embodiment, the LED array has been described, but a scanning laser diode (LD) can also be used.

Next, this control method will be described with reference to FIG. Since each control means 80Y, 80M, 80C, 80K is basically the same configuration, for example, the control means 80Y will be described as a representative (hereinafter the same). As shown in FIG. 8, the control unit 80 includes a control unit 80a and a ROM 80b. In the ROM 80b, data such as the rotation speed of the photosensitive member 1, the distance from the developing device 4 to the pre-transfer exposure unit 7, Table of exposure amount in toner image portion and non-image portion in toner color, formula for calculating time for toner image to reach pre-transfer exposure means 7 from developing device 4, and position of image portion and non-image portion Programs to be stored are stored.
As shown in FIG. 8, when the input signal from the image formation start time input means 21 fed from the optical writing unit 20 is input to the control unit 80a, the time for the image unit to reach the PTL 7 is set as the PTL position. And the rotational speed of the photosensitive member 1, and control is performed so that the PTL 7 is performed on the non-image portion at the timing when the same time has elapsed after the exposure. In addition, the control unit 80a performs control so that the area determined to be the image part from the image information input unit 22 fed from the optical writing unit 20 is subjected to exposure that is greater than the exposure amount of the non-image part.

A method of controlling the exposure of the pre-transfer exposure means 7 using this control means will be described based on the flowchart shown in FIG.
First, when the image forming operation is started, a time t until the toner image reaches the PTL is calculated (step S1). In this case, the time t is determined by calculating the PTL arrival time from the rotational speed of the photoreceptor 1 and the position of the PTL, using the time information mounted on the apparatus main body, and the time from the start of image formation. This is done by comparing the PTL arrival times. Similarly, the image end determination method is determined by comparing the time obtained by adding the image size passage time to the PTL arrival time and the time. When the time t becomes equal to or longer than the PTL arrival time, PTL exposure is started (step S2). Further, the image portion is determined (step S3), and when it is determined that the image portion is an image portion, the PTL exposure amount is increased in units of pixels (step S4). As a method for determining the image portion, image information used for image exposure by the writing unit 20 is used, and if writing has been performed, the image portion is determined. Then, it is determined whether or not the image formation is completed (step S5). If YES, the process ends. If NO, the process returns to the image part determination operation S3. Increase the amount.

ＰＴＬ光量とチリランクとの関係表
表１の結果から明らかなように、画像部と非画像部の転写前露光量を制御して、画像部の転写前露光量が非画像部への転写前露光量以上にすることによって最適な光量に調整、制御することが可能となり、トナーチリの少ない、高画質、高安定な画像を形成することが可能となる。また、非画像部の感光体電位は低いほど非画像部と被転写材との電位差が小さくなり、放電発生を回避できるため、非画像部には画像部電位以下の露光量を与えることが望ましい。 By performing the above operation, the potential of the non-image portion is lowered and no discharge with the transfer paper P occurs, and the photosensitive member 1 in the image portion is given a stronger exposure than the non-image portion. Positive charges generated due to the presence gather, and a potential difference occurs between the image portion and the non-image portion, so that dust can be suppressed. In this case, the exposure amount to the image portion is set to be equal to or greater than the exposure amount to the non-image portion. For example, Table 1 shows examples of the PTL light amount (image portion, non-image portion) and dust rank. It was confirmed that the amount of exposure to the image area was suppressed by setting the light amount to three times or more that of the non-image area.
[Table 1]

Table of relationship between PTL light quantity and dust rank As is apparent from the results in Table 1, the pre-transfer exposure amount of the image portion and the non-image portion is controlled so that the pre-transfer exposure amount of the image portion is the pre-transfer exposure to the non-image portion. By adjusting the amount to be greater than or equal to the amount, it is possible to adjust and control to an optimum light amount, and it is possible to form a high-quality and highly stable image with less toner dust. Further, since the potential difference between the non-image area and the material to be transferred becomes smaller as the photoreceptor potential of the non-image area is lower, discharge can be avoided. .

画像部のＰＴＬ光量増加率を変え、各色画像を確認したところ、チリの発生を防止できていることがわかった。トナーの透過率が高いものほどＰＴＬ光量増加量は少なくてよく、各色の透過率にしたがってＰＴＬ光量を調整することにより、過剰な露光を行うことなくチリを抑制することが可能となる。 (Example 2)
In the image forming apparatus shown in Embodiment 1, the exposure amount of the image portion in the pre-transfer exposure means 7Y, 7M, 7C, and 7K is changed as shown in Table 2 according to the toner color detected from the image information input means. And controlled.
[Table 2]
PTL increase for each color

When the PTL light quantity increase rate of the image portion was changed and each color image was confirmed, it was found that generation of dust was prevented. The higher the toner transmittance, the smaller the amount of increase in the PTL light amount. By adjusting the PTL light amount according to the transmittance of each color, it is possible to suppress dust without excessive exposure.

(Example 3)
Next, a PTL exposure amount control method according to the adhesion amount of each color toner will be described with reference to FIG. The image forming apparatus of this embodiment is basically the same as the image forming apparatus shown in FIG. 7 of Embodiment 1 described above, and the same components are denoted by the same reference numerals and description thereof is omitted. In the image forming apparatus according to the third embodiment, the toner image is transferred between the developing devices 4Y, 4M, 4C, and 4K and the pre-transfer exposure units 7Y, 7M, 7C, and 7K as compared with the first embodiment. Toner adhesion amount detecting means 9Y, 9M, 9C and 9K for detecting the toner adhesion amount are provided. These toner adhesion amount detection means 9Y, 9M, 9C, and 9K detect the toner adhesion amount, thereby performing PTL exposure amount control according to the toner adhesion amount.
As the toner adhesion amount detection means 9Y, 9M, 9C, 9K, for example, a means capable of measuring the toner adhesion amount on the photoreceptors 1Y, 1M, 1C, 1K such as a density sensor can be used. The density sensor detects the density of the toner images on the photoreceptors 1Y, 1M, 1C, and 1K. The density sensor irradiates light to the photoreceptors 1Y, 1M, 1C, and 1K, and the photoreceptors 1Y, 1M, and 1K. It comprises a reflection type density sensor composed of a light receiving element that receives reflected light from 1C and 1K. The density sensor is arranged so that the infrared light LED and the phototransistor can detect specular reflection light and the like on the detection surface, and the output voltage of the phototransistor is measured by the light emission of the LED.
When there is no toner, the output voltage of the phototransistor is large, and the output voltage decreases according to the toner concentration, that is, the toner adhesion amount. Therefore, the toner adhesion amount can be detected from the output voltage value. For example, an image pattern capable of measuring the target adhesion amount is created, and the output voltage of the phototransistor is measured.

In this way, the output voltage detected according to the toner adhesion amount by the toner adhesion amount detection means 9 is input to the control unit 81a of the control means 81Y, 81M, 81C, 81K shown in FIG. 11, and the ROM 81b and NVRAM (nonvolatile) Random access memory) 81c is used to control the pre-transfer exposure means 7Y, 7M, 7C and 7K. The output voltage detected here is converted into an adhesion amount by an output voltage-toner adhesion amount conversion formula stored in advance in the ROM 81b, and then stored in the NVRAM 81c.
The detection of the toner adhesion amount is performed when the power is turned on, when a certain number of sheets have elapsed, and the like, and can cope with the toner adhesion amount variation due to environmental variation and temporal variation. The ROM 81b also stores data on the toner adhesion amount and the PTL light amount that does not generate dust, and the PTL light amount is corrected according to the measured toner adhesion amount by referring to this data. . This makes it possible to properly determine the amount of PTL increase in the image area even when the amount of toner adhering to the photoconductor changes due to environmental changes and changes over time, and to form a high-quality, highly stable image with less toner dust. Is possible.

Example 4
In the third embodiment, the method for calculating the PTL light amount increase amount of the image portion according to the measured toner adhesion amount has been described, but further, PTL light amount correction according to the toner adhesion amount for each pixel was performed according to the image information. .
In the fourth embodiment, as shown in FIG. 12, an endless intermediate transfer belt 25 is used as a transfer target, and toner images formed on the photoreceptors 1Y, 1M, 1C, and 1K are subjected to primary transfer. By using the rollers 27Y, 27M, 27C, and 27K and the primary transfer bias means 28Y, 28M, 28C, and 28K, a color toner image is formed by sequentially laminating and transferring onto the intermediate transfer belt 25. The color toner image formed on the intermediate transfer belt 25 receives a transfer bias from the secondary transfer bias means 30 by the secondary transfer roller 29 and is transferred from the registration roller 12 at the same timing. The color toner image is fixed by being conveyed to the fixing device 10 by a conveying belt 31 which is transferred onto the driving roller 32a and the driven roller 32b.
In FIG. 12, reference numerals 26a, 26b, and 26c denote a driving roller and a driven roller that stretch the intermediate transfer belt 25, and the intermediate transfer belt 25 is moved in the arrow B direction by the rotation of the driving roller 26a in the arrow direction. It is supposed to let you.

  In this case, in the control means 82Y, 82M, 82C, and 82K of the pre-transfer exposure means 7Y, 7M, 7C, and 7K of the image forming apparatus of Embodiment 4, for example, power modulation (exposure power is set to pixels (minimum unit of image) ) Method for changing each)) Using image information in toner density control writing for each pixel such as pulse width modulation, detecting a difference in toner amount adhering to each pixel, and performing PTL control according to this toner amount Is possible. Then, the photoreceptors 1Y, 1M are detected by the toner adhesion amount detection means 9Y, 9M, 9C, 9K disposed between the developing devices 4Y, 4M, 4C, 4K and the pre-transfer exposure means 7Y, 7M, 7C, 7K. By detecting the toner adhesion amount of the toner images formed on 1C and 1K, an image pattern capable of measuring the target adhesion amount is created, and the output voltage of the phototransistor is measured. The measured output voltage is calculated by the controller 82a based on the output voltage-toner adhesion amount conversion formula stored in advance in the ROM 82b of the control means 82 shown in FIG.

Further, the writing light amount information for each pixel is read from the image information, and the adhesion amount for each pixel is calculated by the LD light amount-toner adhesion amount conversion formula at the time of electrostatic latent image writing stored in advance in the ROM 82b. In accordance with this adhesion amount, the PTL light amount is corrected from the toner adhesion amount recorded in the ROM 82b-PTL light amount data in which no dust is generated.
From the above, predicting the toner adhesion amount for each pixel and adjusting the light amount of the pre-transfer exposure unit accordingly, the PTL light amount is adjusted to a more optimal light amount, thereby producing a high-quality, highly stable image with less toner dust. It becomes possible to form.
Furthermore, if these controls are performed for each toner color, it is possible to perform control with a more optimal light amount. The PTL control by the control means 82 will be described based on the flowchart shown in FIG. For each toner color, the toner adhesion amount detection means 9Y, 9M, 9C, 9K detects the toner adhesion amount (step S6), and at the time of writing stored in advance in the ROM 82b according to the image information for each color (step S7) The amount of adhesion for each pixel is predicted by the LD light quantity-toner adhesion amount conversion formula (step S8).

With respect to these toner adhesion amounts, the PTL transmittances corresponding to the respective color toner adhesion amounts (step S9) are predicted by the relational expression of the toner adhesion amount of each color stored in the ROM 82b in advance (step S10). The PTL light quantity increase rate is calculated by the equation of transmittance-PTL light quantity increase rate (step S11). In this case, the PTL light quantity increase rate is calculated by calculating the PTL light quantity condition of the image area where the amount of toner adhesion is not changed and generating dust, converting the toner adhesion quantity into the transmittance, and calculating the transmittance-PTL light quantity from these information. The increase rate relationship is obtained and set in the ROM 82b in advance.
Incidentally, the relationship between the transmittance and the PTL light amount increase rate is as follows: y (PTL light amount increase rate) = 523.94x (transmittance) ^−0.3542 when R ² = 0.9956 as shown in FIG. There is a curve as shown in FIG. The points K, C, M, and Y in the figure indicate the toner colors of black, cyan, magenta, and yellow, respectively.
As described above, the toner adhesion amount for each color is calculated, the adhesion amount for each pixel is predicted from the image information, the transmittance is calculated from the adhesion amount in consideration of the toner color, and the PTL exposure increases according to the transmittance. By calculating the amount, it is possible to give an optimum PTL light amount for each image of each color, and it is possible to form a high-quality and highly stable image with little toner dust.

  1Y, 1M, 1C, 1K photoconductor, 2Y, 2M, 2C, 2K charging roller, 4Y, 4M, 4C, 4K developer, 5Y, 5M, 5C, 5K cleaning means, 6Y, 6M, 6C, 6K transfer roller, 7Y, 7M, 7C, 7K Pre-transfer exposure means (PTL), 9Y, 9M, 9C, 9K Toner adhesion amount detection means, 10 fixing device, 12 registration roller, 14 transport belt, 15Y, 15M, 15C, 15K transfer bias means , 20 Writing unit, 21 Image formation start time input means, 22 Image information input means, 25 Intermediate transfer belt, 27Y, 27M, 27C, 27K Primary transfer roller, 28Y, 28M, 28C, 28K Primary transfer bias means, 29 Secondary Transfer roller, 30 Secondary transfer bias means, 31 Conveyor belt, 80Y, 80M, 80C, 80 K PTL exposure control means, 80a, 81a, 82a control section, 80b, 81b, 82b ROM, 81Y, 81M, 81C, 81K PTL exposure control means, 82Y, 82M, 82C, 82K PTL exposure control means

JP-A-1-191168 JP 10-340014 A JP-A-10-186814 JP 2002-55544 A JP 11-174857 A

Claims (6)

An image carrier for carrying an electrostatic latent image, a charging means for uniformly charging the surface of the image carrier, and an electrostatic charge on the surface of the image carrier charged by the charging means based on image information. An exposure means for forming a latent image, a developing means for developing the electrostatic latent image into a toner image, and a toner image formed on the image carrier by the developing means A transfer means for transferring the toner image to the transfer means, a transfer bias applying means for applying a bias to the transfer means, and a pre-transfer exposure means for exposing the image carrier before transferring the toner image onto the transfer body. In the image forming apparatus,
The pre-transfer exposure unit is controlled by a control unit so as to expose an image portion of a toner image formed on the image carrier with a larger exposure amount than a non-image portion of the toner image. Image forming apparatus. The image forming apparatus according to claim 1.
The image forming apparatus according to claim 1, wherein the pre-transfer exposure unit is capable of changing the exposure amount in pixel units of the toner image. The image forming apparatus according to claim 1 or 2,
The non-image area potential after exposure by the pre-transfer exposure means is controlled by the control means so as to be equal to or less than the potential of the image area immediately before exposure by the pre-transfer exposure means. Image forming apparatus. The image forming apparatus according to any one of claims 1 to 3,
An image forming apparatus, wherein an exposure amount of the pre-transfer exposure unit for the image portion is controlled by the control unit in accordance with a toner adhesion amount of the image portion of the toner image. The image forming apparatus according to any one of claims 1 to 4, wherein:
An image forming apparatus, wherein an exposure amount of the pre-transfer exposure unit for the image portion is controlled by the control unit in accordance with a toner color of an image portion of the toner image. The image forming apparatus according to claim 5.
An image forming apparatus, wherein an exposure amount of the pre-transfer exposure unit for the image portion is controlled by the control unit in accordance with the light transmittance of toner in the image portion of the toner image.

Images