JP6613946B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus.

In an electrophotographic image forming apparatus such as a copying machine, a printer, and a facsimile, generally, a charging unit, an exposure unit, a developing unit, a transfer unit, a cleaning unit, and a neutralizing unit are arranged around a photosensitive member.
This is based on the idea that the basis of stable image formation is that the photoreceptor is started in the same state for each cycle of an image formation process (charging, exposure, development, transfer, cleaning, charge removal) starting from charging.
Therefore, it is necessary to have a long-life photosensitive member having stable photosensitive member characteristics and a process operation for returning the photosensitive member to the initial state. The initial state of the photoconductor is a state where the entire photoconductor is not charged and a state where there is no transfer residual toner on the surface of the photoconductor. Therefore, it is necessary to clean the surface of the photoreceptor after transfer and to remove surface charges.

Patent Document 1 proposes an image forming apparatus that does not have a charge eliminating unit. This image forming apparatus uses a photoreceptor in which an intermediate layer having a volume resistivity of 10 ¹⁰ to 10 ¹² Ωcm, a charge generation layer, and a charge transport layer are formed on a conductive support.
By defining the volume resistivity of the intermediate layer as described above, fluctuations in VL (bright part potential) peculiar to an image forming apparatus that performs contact charging only with a DC voltage can be reduced, and as a result, afterimage generation can be suppressed. Is.

However, the above-described conventional technique is not a fundamental measure for eliminating afterimages. In the image forming apparatus described in Patent Document 1, of the carriers (holes and electrons) generated in the charge generation layer in the image formation process, holes are injected into the charge transport layer, while electrons are transferred to the intermediate layer. And injected.
However, since the resistance of the intermediate layer is high, electrons are not easily injected into the intermediate layer and stay in the charge generation layer. As a result, the residual potential becomes VL variation and an afterimage occurs.

  In particular, when a high-sensitivity type photoconductor is used to reduce the size and speed of the image forming apparatus, the amount of carriers generated in the charge generation layer increases. If the volume resistivity of the intermediate layer is increased when the amount of carriers generated is large, the residual potential increases remarkably, and afterimages and uneven charging occur.

Patent Documents 2 and 3 propose a technique for specifying a half-exposure amount of an image carrier by referring to a positive ghost and a negative ghost caused by a transfer residual toner.
By using a high-sensitivity photoconductor with residual toner blocking exposure, the potential fluctuation with respect to exposure intensity is smaller than that with low-sensitivity, and the image has high reproducibility of isolated dots and gradation. Is obtained.
However, in the image forming apparatuses described in Patent Documents 2 and 3, the ghost image cannot be effectively avoided because the exposure is performed in a state where the transfer residual toner is present on the photoreceptor. By using a high-sensitivity photosensitive member that defines a half-exposure amount, the potential fluctuation can be suppressed to a small level and the reproducibility of isolated dots is increased, but ghost images due to residual charges are not effectively avoided.

As described above, when the next charging process is entered with the residual charge remaining on the photoconductor, the charging is not stable, and thus an abnormal image such as an afterimage (ghost) due to density unevenness due to charging failure and the previous image history cannot be canceled. Will occur.
Further, the photoconductor may be repeatedly exposed to strong exposure by static elimination light in order to erase the residual charge. In addition to static elimination exposure and exposure to form a latent image, the photosensitive member is exposed to a fluorescent lamp or external light during maintenance, so that the photosensitive layer is always exposed to light such as exposure. An abnormal image is generated due to a decrease in sensitivity.

  The present invention has been made in view of such a current situation, and provides an image forming apparatus that does not generate an afterimage due to a latent image potential (residual potential) and can suppress the occurrence of an abnormal image with high accuracy. Objective.

  In order to achieve the above object, an image forming apparatus of the present invention forms an electrostatic latent image on an image carrier, a charging member for charging the surface of the image carrier, and the charged surface of the image carrier. An exposure member; a developing member that visualizes the electrostatic latent image as a toner image; and a transfer member that transfers the toner image formed on the surface of the image carrier. A generation layer and a charge transport layer, wherein the charge generation layer contains an asymmetric disazo pigment and a metal-free phthalocyanine pigment, and is larger than a half-exposure amount of the image carrier by the exposure member, and a half-exposure amount The electrostatic latent image is formed with an exposure energy of 2.5 times or less, and no static elimination is performed in one printing process.

  According to the present invention, it is possible to provide an image forming apparatus that does not generate an afterimage due to a latent image potential (residual potential) and that can suppress the occurrence of an abnormal image with high accuracy.

1 is a schematic configuration diagram of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a schematic configuration diagram of a process cartridge that is detachable from an image forming apparatus main body. It is a timing chart in the conventional image formation (printing process). 6 is a timing chart in image formation (printing process) in the present embodiment. It is principal part sectional drawing which shows the layer structure of a photoreceptor. FIG. 6 is a characteristic diagram showing a difference in surface potential of a photoconductor for a solid image, a halftone image, and an afterimage. FIG. 6 is a diagram illustrating a chart for afterimage evaluation, where (a) is a diagram in a state where no afterimage is produced, (b) is a diagram in which a positive afterimage is produced, and (c) is a diagram in which a negative afterimage is produced. is there.

Embodiments of the present invention will be described below with reference to the drawings.
First, the conventional configuration and its problems will be described in detail.
In an electrophotographic image forming apparatus applied to copying machines, laser printers, facsimiles, etc., the surface of an image carrier (hereinafter also referred to as a photoreceptor) is charged in a dark place by, for example, corona discharge, and then image exposure is performed. Thus, an electrostatic latent image is formed on the photoreceptor.
The electrostatic latent image is visualized as a toner image by developing means, and the toner image is finally transferred and fixed on a recording medium.
The toner remaining on the photoconductor after the transfer is removed from the photoconductor through a cleaning process, charged on the photoconductor again, and an electrostatic latent image is formed on the photoconductor.

In recent years, a photoreceptor is expected to have high durability as well as sensitivity, electrical characteristics, and optical characteristics according to an applied electrophotographic process. In addition, maintaining a high-quality image over a long period of time is an important technical problem in the photoreceptor.
From the viewpoint of sensitivity, electrical characteristics, optical characteristics, the appearance of image forming devices that use semiconductor lasers and LEDs (Light Emitting Diodes) as light sources, and the common use of photoreceptors, the photoreceptors are visible to the near infrared region. It has been eagerly desired to have a wide spectral sensitivity characteristic.
In view of this, it has been proposed to use two or more types of pigments having spectral sensitivity characteristics in different wavelength regions as the charge generation material used for the photoreceptor (for example, Patent Document 4).
By using two or more kinds of pigments as the charge generating material, the photoreceptor can be used over a wide area.

On the other hand, the durability of photoconductors includes the durability against mechanical loads such as abrasion and wounds on the surface of the photoconductor, and the durability of electrostatic characteristics such as the accumulation of residual potential and the decrease in chargeability due to repeated light fatigue. Depends on. As described above, various systems have been developed and studied in view of the recent high durability.
Furthermore, in addition to the need for high image quality and high speed, various demands for further downsizing of image forming apparatuses and simplification of image forming components are increasing from the viewpoint of space saving and reduction of initial cost. Are being developed and studied.

  Since image forming means such as charging means, exposure means, developing means, fixing means, and cleaning means are arranged around the photoconductor, the size of the image forming apparatus largely depends on the photoconductor used. Therefore, to reduce the size of the image forming apparatus, it is necessary to reduce the occupied volume of the photosensitive member. A flexible layout is possible by making the photosensitive member into a belt shape, and a volume occupied by the photosensitive member in the apparatus can be reduced by making the photosensitive member into a small-diameter drum shape.

Advances have been made in the photoreceptor and each imaging element disposed therearound. That is, a cleaner-less system that eliminates the need for a cleaning means, a static elimination-less system that eliminates a static elimination means, and the like have been devised to reduce the initial cost.
However, most of them are limited to low-speed image forming apparatuses, and a high-speed machine with a large print volume cannot avoid a reduction in image quality.

Further, the layout of the image forming elements is reduced by the downsizing, and the interval between the elements arranged around the photosensitive member is narrowed, and the interval between the charge eliminating unit and the charging unit cannot be increased. In accordance with the linear velocity of the photosensitive member, the residual charge on the photosensitive member must be removed in a short time. Furthermore, in a high-speed machine having a high photosensitive member linear velocity, the residual charge on the photosensitive member must be removed in a shorter time.
However, in some cases, the charging process may be started with residual charge remaining on the photosensitive member, and since the charging is not stable, abnormal images such as density unevenness due to charging failure and an afterimage caused by the previous image history cannot be canceled are generated. Sometimes.

In general, the charging means in the charging step is charged only with a direct current component, and therefore, if the remaining charge remains, the charging of that portion becomes high. When copying or printing a document in which uniform writing is performed on the entire surface, the surface potential is not constant, and density unevenness occurs in development.
It is known that a technique of superimposing an AC component on a DC component is effective to avoid charging unevenness. However, it is necessary to increase the frequency of the AC component and / or the voltage difference as the nip width in charging is reduced and the linear velocity of the photosensitive member is increased as the size of the apparatus is reduced and the diameter of the photosensitive member is increased. is there.

  As a result, the repeated transfer of electrons between the photosensitive member surface and the charging member surface during charging promotes chemical deterioration of the photosensitive member surface, such as main chain breakage of the binder resin, and increases the amount of wear. End up. In that case, it becomes difficult to achieve high durability in miniaturization and high speed.

On the other hand, prior to charging of the photoconductor, there is also an image forming apparatus provided with a charge eliminating unit that irradiates light on the surface of the photoconductor to generate carriers in the photoconductor to remove residual charges. At this time, the wavelength of the static elimination light is set to a wavelength at which the charge generation layer has sensitivity, and the surface potential is removed and canceled by exposing the entire surface of the photoreceptor.
As a result, the difference in the amount of charge remaining in the part having the image history subjected to image exposure in the photoconductor and the non-image part having no image exposure history is suppressed, and the appearance of the afterimage is suppressed.
Furthermore, at present, due to the demand for miniaturization, LED arrays are used as the light source of the static eliminator. However, there is a phenomenon that the image becomes darker at a portion where the amount of irradiation light is large and the image becomes thin at a portion where the amount of irradiation light is small. There is a case.

Therefore, in order to solve the density unevenness caused by the unevenness of the irradiation light quantity, the plurality of LED chips are arranged densely, and the illuminance is substantially uniformized with respect to the surface of the photoconductor as the irradiated surface. Light irradiation is possible.
However, as described above, the photosensitive layer is always exposed to light, and an abnormal image is generated due to a decrease in sensitivity of the photosensitive layer due to light fatigue.
Therefore, an image forming apparatus that does not include a charge eliminating unit has been proposed. However, since the charge remaining on the entire surface of the photosensitive member cannot be removed by the light removal means before the charging step, the charged potential becomes non-uniform and an afterimage is generated. I couldn't.

As shown in FIG. 7B, an afterimage is a solid image pattern 52 that is formed before a halftone image in a halftone image 50 that originally has to be a uniform and uniform image. This is a phenomenon in which the afterimage 52a emerges deeply.
An abnormal image in which such an afterimage phenomenon appears is called a positive afterimage or a positive ghost. On the other hand, as shown in FIG. 7C, a solid image pattern 52 formed in front of the halftone image may appear lightly as an afterimage 52b in the halftone image 50. Such abnormal images are called negative afterimages or negative ghosts.

In an image forming apparatus that forms a full-color image that requires high image quality, it is necessary to suppress such deterioration in image quality due to an afterimage. In particular, afterimages appear frequently in images in which solid images and halftone images appear repeatedly.
As countermeasures for afterimages (ghosts), the image forming apparatuses described in Patent Documents 1, 2, and 3 have been proposed as described above, but they are not sufficient countermeasures.

Since the structure described in Patent Document 1 suppresses charge (hole) injection from the conductive support to the intermediate layer by increasing the volume resistivity of the intermediate layer, the interface between the charge generation layer and the intermediate layer, Alternatively, no charge is accumulated in the charge generation layer.
As a result of the absence of residual charge accumulation, holes generated in the charge generation layer are injected into the charge transport layer, which promotes movement in the charge transport layer and suppresses afterimages due to apparent improvement in sensitivity. Has been.
However, actually, as described above, residual charges are accumulated, and the fundamental solution to afterimages and charging unevenness cannot be achieved.
The configurations described in Patent Documents 2 and 3 correspond to the downsizing of the apparatus and the problems peculiar to the simultaneous development cleaning or cleanerless system that does not generate waste toner. The fact that a ghost image is generated due to poor charging due to untransferred toner and light shielding is solved by using a highly sensitive photoconductor that defines a half-exposure amount.

Patent Documents 5 and 6 propose electrophotographic photoconductors that have little sensitivity fluctuation to humidity changes in the usage environment. This is because even if a specific crystal type oxytitanium phthalocyanine having high sensitivity is used as a charge generation material, the potential fluctuation in the exposure range related to the halftone image of the light attenuation curve is suppressed, and the humidity change of the use environment is suppressed. On the other hand, this is a technique for providing a stable image.
Although the oxytitanyl phthalocyanine as a charge generation material in Patent Documents 5 and 6 has high sensitivity, this high sensitivity is a function of water molecules present in the oxytitanyl phthalocyanine crystal as a sensitizer.
Therefore, by stipulating the degree of fluctuation of the light attenuation characteristics due to the half-exposure amount E1 / 2 and humidity, between an image output at normal humidity and an image output at a dry and low humidity state, This solves the problem that the obtained image density is different.

  As described above, a number of technologies have been proposed, but at the same time, the technical issues still remain in terms of solving the afterimage problem while meeting the recent demands for higher speed, smaller size, and longer life. is the current situation.

One embodiment of the present invention that can solve the afterimage problem will be described with reference to FIGS.
As shown in FIG. 1, an image forming apparatus according to this embodiment includes a photosensitive drum (hereinafter also simply referred to as “photosensitive member”) 1 as an image carrier, a charging unit 2, an exposure unit 3, and a developing unit. Means 4, transfer means 5, cleaning means 6 for cleaning the surface of the photosensitive drum 1 after transfer, fixing means 7, and the like.
A cleaning unit 6 is provided between the transfer unit 5 and the charging unit 2. In the transfer unit 5, the entire toner image on the photoconductor is not transferred, and the toner remains on the photoconductor. Such toner is removed from the photoreceptor by the cleaning means 6. The cleaning means 6 is a rubber plate-like cleaning blade 6a.

The cleaning blade 6a may be a brush such as a fur brush or a mag fur brush.
Further, a recycle unit, a control unit, and the like for collecting the toner removed by the cleaning unit 6 to the developing unit 4 may be provided.
Note that there is no neutralizing means for erasing the charge on the photosensitive drum 1 after the transfer. The reason why it is not necessary to provide the static eliminating means will be described later.

FIG. 2 is a schematic diagram illustrating an example of a process cartridge. In this embodiment, along with the photosensitive drum 1, at least one of the charging unit 2, the exposure unit 3, the developing unit 4, the transfer unit 5, and the cleaning unit 6 is detachable from the image forming apparatus main body. It is housed integrally in the process cartridge.
Each unit may be fixedly incorporated in the image forming apparatus, but can be easily replaced by being detachable in the form of a process cartridge. In FIG. 2, symbol Lb indicates a laser beam as exposure light emitted from the exposure means 3 (see FIG. 1).

The charging unit 2 is a contact charging method in which a charging roller as a charging member is brought into contact with a photosensitive member. A corona discharge system represented by Scorotron, a contact charging system in which a charging brush is brought into contact with a photoconductor, and a proximity arrangement in which the photoconductor drum 1 and the charging member have a gap (gap) of 200 μm or less in an image forming region. You may use a system suitably.
In this embodiment, the charging unit 2 employs a contact method using a charging roller that generates less ozone and consumes less power, and applies only a DC bias voltage.
It is also possible to superimpose an AC voltage on a DC voltage when a voltage is applied by the charging means. In this case, it is effective in suppressing charging unevenness, but has a drawback that dielectric breakdown of the photosensitive member is likely to occur because a high voltage is applied. For this reason, when an alternating voltage is superimposed on a direct current voltage, it is good to use in the range which does not produce a dielectric breakdown.

Although the photosensitive member is charged by such a charging unit, the electric field strength applied to the photosensitive member is less than a predetermined value because the photosensitive member of a general image forming apparatus is liable to be stained due to the photosensitive member. (For example, 40 V / μm or less).
This is because the occurrence of soiling depends on the electric field strength, and the probability of soiling increases as the electric field strength increases. However, when the electric field strength applied to the photoconductor is reduced, the electric field strength applied between the surface of the photoconductor and the conductive support is reduced.
For this reason, the straightness of the optical carrier generated in the charge generation layer is reduced, and the diffusion of the latent image due to Coulomb repulsion is increased, resulting in a reduction in resolution.

In the photosensitive drum 1 according to the present embodiment, as will be described later, since the probability of background contamination can be reduced, it is not necessary to reduce the electric field strength, and the photosensitive drum has an electric field strength of 10 to 50 V / μm or less. 1 is used.
The electric field strength is more preferably in the range of 15 to 48 V / μm, and when it exceeds 50 V / μm, soiling becomes conspicuous. If it is less than 15 V / μm, the image quality cannot be maintained due to a decrease in resolution.

The exposure means 3 uses a light source of a semiconductor laser (LD) as an exposure member. As the exposure means 3, a light source such as a light emitting diode (LED) or electroluminescence (EL) that can ensure high luminance similarly to the light emitting diode (LED) may be used. In addition, a light source incorporating a multi-beam writing head in which a plurality of semiconductor laser (LD) elements are arranged in the main scanning direction or sub-scanning direction of the photosensitive member may be used.
In order to irradiate only light in a desired wavelength range, a sharp cut filter is used. Various filters such as a band pass filter, a near infrared cut filter, a dichroic filter, an interference filter, and a color temperature conversion filter can be used instead of the sharp cut filter.
Among these light sources, the semiconductor laser and the light emitting diode have high irradiation energy and have long wavelength light of 600 to 800 nm.

Since light in this wavelength range shows high sensitivity to the charge generating material of the photoreceptor of this embodiment, a semiconductor laser having a wavelength of 650 nm or more is preferably used.
That is, the exposure means 3 exposes the surface of the photoreceptor 1 with laser light having a wavelength of 650 nm or more.

3 and 4 are timing charts of the image forming process when two sheets of A4 size paper are passed in the horizontal direction and two sheets are continuously printed in the image forming apparatus.
FIG. 3 is a conventional image formation (image formation) timing chart, in which the QL as the charge eliminating means is synchronized with ON / OFF of the main motor. For this reason, in addition to laser (LD) exposure for forming an electrostatic latent image, the photoreceptor is irradiated with light during most of the image forming process. For this reason, the image quality has fluctuated due to a decrease in sensitivity of the photoreceptor due to light fatigue and accumulation of residual potential.

On the other hand, FIG. 4 is an image formation (image formation) timing chart of the present invention. As shown in the figure, at the end of printing, the frame gate signal (FGATE) is turned ON in synchronization with the transfer bias being turned OFF. During this time, the entire surface of the photoconductor is exposed by light irradiation with a laser (LD) (LD charge removal), and the charge remaining in the photosensitive layer is canceled to prepare for the next charging.
FGATE is a writing start signal and is a signal for controlling writing of image data in the sub-scanning direction.
When the laser (LD) is driven and the frame gate signal (FGATE) is ON, exposure of the laser (LD) is performed. With reference to the start of writing when the frame gate signal (FGATE) is turned on, for example, control of various bias timing settings, paper transport timing settings, etc. is performed from the distance between exposure-development and exposure-transfer. Is called.

Since the image forming process shown in the timing chart of FIG. 4 includes only light irradiation for forming an electrostatic latent image and a static elimination process by laser exposure by the exposure unit 3 at the end of printing, the photosensitive member is exposed to light. Opportunities decrease.
Therefore, the density unevenness caused by electrostatic deterioration of the photoconductor due to light fatigue is suppressed, so that the photoconductor replacement cycle can be extended, the running cost is low, and the frequency of downtime due to component replacement is low. Therefore, reliability can be improved.

The static elimination process by the exposure unit 3 is performed after a certain number of one printing process (one job) is completed. That is, in FIG. 4, this is performed after the continuous printing of two sheets of paper is completed, and in the static elimination performed for each conventional image forming process of one sheet of paper or throughout one printing process (image forming process). Absent. That is, the potential of the photosensitive member is initialized in preparation for the next job.
One printing step refers to a step of continuously performing a certain number of image forming operations (for example, two sheets designated by a user or the like).
FIG. 4 shows a state in which QL as the charge eliminating means is displayed in comparison with FIG. 3 and is not turned on during the image forming process, but actually the charge eliminating means is not provided as in FIG.
Note that the present invention includes a case where, in an image forming apparatus having a charge eliminating unit, the charge eliminating unit is not turned on in the image forming process.

As shown in FIG. 1, in order to visualize the electrostatic latent image formed on the photoreceptor 1, developing means 4 that is a developing device including a developing roller as a developing member is used. The development method is a two-component development method using dry toner. A one-component development method or a wet development method using a wet toner may be employed.
When the photosensitive member is positively or negatively charged and image exposure is performed, a positive or negative electrostatic latent image is formed on the surface of the photosensitive member. If this is developed with negative or positive toner (electrodetection fine particles), a positive image can be obtained. On the contrary, if development is performed with positive or negative toner, a negative image can be obtained.

The toner 8 developed on the photosensitive member by the developing unit 4 is transferred to a sheet 9 as a recording medium by the transfer unit 5. The transfer unit 5 includes a transfer roller 10 serving as a transfer member and a transfer bias power source. The transfer roller 10 is rotationally driven by a driving unit, and a transfer bias is applied to the shaft of the transfer roller 10 from the transfer bias power source.
The sheet 9 is sent from the registration roller pair 11 (see FIG. 2) to the nip portion between the transfer roller 10 and the photosensitive member 1 at a predetermined timing, and the toner image on the photosensitive member 1 is transferred to the sheet 9.
The transfer roller 10 is made of low hardness rubber to reduce the transfer pressure on the paper 9. The transfer roller 10 may be made of foam or the like in order to reduce the transfer pressure.

In this transfer process, the control method of the transfer bias applied to form the transfer electric field includes a constant current control method for supplying a transfer bias controlled to a constant current to the transfer roller, and a transfer controlled by a constant voltage to the transfer roller. There is a constant voltage control system for supplying a bias.
Although each method has a characteristic, in this embodiment, a constant current control method is adopted. According to this method, the transfer electric field formed in the transfer nip is hardly affected by the resistance fluctuation of the transfer member such as the paper or the transfer roller, and a stable transfer property can be obtained.

  On the other hand, the effective transfer current affects the dimensional width of the paper in the longitudinal direction of the photoreceptor. For example, the effective transfer current differs between when A4 size paper is passed horizontally and when it is passed vertically. When the paper is passed in the vertical direction, since the transfer roller where no paper is present and the photoconductor are in direct contact with each other, a large amount of current flows in this region, so that an effective transfer current is reduced.

Therefore, in order to equalize the effective transfer current according to the paper size, the set current value of the transfer bias is controlled according to the paper size and the direction during paper passing.
In the image forming apparatus of the present embodiment, when passing a small size paper that is in direct contact, the setting current value of the transfer bias is changed so that a larger amount of current flows than in the case of a larger size paper. To do. As a result, an effective transfer electric field that can transfer the toner image onto the sheet is secured for the image portion.

  However, the transfer bias current, which has been changed to a higher setting, flows directly into the portion of the photoconductor that is not in contact with the paper. Therefore, an excessive positive charge is injected into the area of the photoconductor that is not in contact with the paper, and the image density fluctuates in the subsequent image forming process. As a countermeasure, in this embodiment, as will be described later, an image is formed with a low exposure energy.

In the conventional image forming apparatus, in addition to securing the image density, in order to suppress the fluctuation of the image density, the fluctuation range of the potential of the light attenuation curve characteristic of the high-sensitivity photosensitive member corresponding to the high speed and miniaturization is set. Make it smaller. That is, a latent image is formed on the photosensitive member with a strong exposure power. For this reason, the bright portion potential is lowered to around 0V.
In the conventional image forming apparatus, the charging potential (dark portion potential) is set large. The reason for setting such image forming conditions is that, as described above, the straightness of photocarriers (holes) generated in the charge generation layer is improved, and the diffusion of latent images due to Coulomb repulsion in the charge transport layer is suppressed. This is because the electric field strength applied between the surface of the photoreceptor and the conductive support can be increased as much as possible.

  Under such latent image forming conditions employed in the conventional image forming apparatus, the potential difference between the exposed portion carrying the toner image and the negative charging potential (dark portion potential) which is a non-image portion is large. For this reason, the positive charge from the transfer roller corresponding to this potential difference tends to flow biased toward the negatively charged non-image portion.

Further, when a small-size sheet is passed, the transfer bias current flows directly and excessively to the portion of the photoreceptor where the sheet is not in contact, that is, the dark portion potential of the photoreceptor. In these regions where excessive positive charges are injected, uniform negative charging cannot be caught up by the charging means in the next process.
As a result, since the image is charged lower than the set charging potential, the latent image profile of the previous image history is reversed, and a negative afterimage is generated in a region where the previous image history portion and the sheet are not in contact with each other.
In other words, the image formation portion (bright portion) of the previous image history is charged to a desired potential at the time of charging in the next process, but the region where the non-image portion (dark portion potential) of the previous image history and the sheet are not in contact is positive. Charge remains in the photosensitive layer as a residual charge. For this reason, even when a latent image is formed by the exposure means at the time of the next process charging, the light portion potential of the previous image history appears as a toner image that is thinner by the minus potential difference. Since it is imaged, it appears as a negative afterimage.

  FIG. 6 is a diagram showing the potential difference between the solid image portion, the halftone image portion, and the afterimage portion at the surface potential of the photosensitive member.

In the image forming apparatus of this embodiment, a latent image is formed with low exposure energy. For this reason, the potential difference between the exposure part (bright part potential) carrying the toner image and the negative charging potential (dark part potential) which is a non-image part is small because the latent image is formed with low exposure energy.
Referring to FIG. 6, the non-image portion has a negative charging potential (dark portion potential: −900 V). The exposed portion (bright portion potential) having a higher potential than the potential of the non-image portion is a solid portion and has a negative potential in the vicinity of 0V. In the image forming apparatus according to the present embodiment, the solid portion potential is formed with low exposure energy to lower the solid portion potential, thereby reducing the potential difference between the dark portion potential and the dark portion potential.
Therefore, the positive charge from the transfer roller does not flow unevenly to the negatively charged non-image portion having the opposite polarity, so that no negative afterimage is generated.

  Further, in the image forming apparatus of the present embodiment, the transfer current value itself required for the transfer can be kept small. That is, due to the transfer electric field in the transfer process, the toner adhering to the surface of the photoconductor as the latent image is formed is attracted to the paper side with a positive charge of reverse polarity.

  However, in the image forming apparatus of the present embodiment, since the latent image is formed with low exposure energy, the bright portion potential is on the negative side. As a result, the electrostatic adhesion force with the negatively charged toner particles adhering to this portion is weakened. Therefore, the transfer current required to transfer to the paper can be kept low.

For this reason, since the transfer bias current described above can suppress an excessive flow of positive charges even at a portion where the sheet of the photosensitive member is not in contact, that is, the dark portion potential of the photosensitive member, a negative afterimage may occur. Absent.
Note that the exposure potential of the solid image portion at the time of image formation is the same as the exposure potential at the time of discharging the LD in FIG. In FIG. 4, charging and developing bias are stopped during LD neutralization, and LD neutralization is reliably performed. For this reason, the photoreceptor potential after the LD charge removal becomes a minus potential, and the charged toner (minus charge) on the developing roller flies and hardly adheres to the photoreceptor.

The toner includes a binder resin, a colorant, and a charge control agent as main components, and other additives are added as necessary.
Specific examples of the binder resin include polystyrene, styrene-acrylic acid ester copolymer, polyester resin, and the like.
As the colorant (for example, yellow, cyan, magenta and black) used for the toner, those known for toners can be used. The amount of the colorant is suitably 0.1 to 15 parts by weight with respect to 100 parts by weight of the binder resin.

Specific examples of the charge control agent include a nigrosine dye, a chromium-containing complex, a quaternary ammonium salt, and the like, which are properly used depending on the polarity of the toner particles. The amount of the charge control agent is 0.1 to 10 parts by weight with respect to 100 parts by weight of the binder resin.
It is advantageous to add a fluidity imparting agent to the toner particles. Examples of the fluidity-imparting agent include fine particles of metal oxides such as silica, titania and alumina, and those obtained by surface-treating these fine particles with a silane coupling agent, titanate coupling agent, etc., polystyrene, polymethyl methacrylate, polyvinylidene fluoride. Polymer fine particles such as are used.
These fluidity imparting agents have a particle size in the range of 0.01 to 3 μm. The addition amount of these fluidity-imparting agents is preferably in the range of 0.1 to 7.0 parts by weight with respect to 100 parts by weight of the toner particles.

As a method for producing a toner for two-component developer, it can be produced by various known methods or a combination thereof. For example, in the kneading and pulverization method, a binder resin, a colorant such as carbon black, and required additives are dry mixed. Thereafter, the mixture is heated, melted and kneaded with an extruder or a two-roll, three-roll, etc., solidified by cooling, pulverized with a pulverizer such as a jet mill, and classified with an air classifier to obtain a toner.
In addition, a toner can be directly produced from a monomer, a colorant, and an additive by suspension polymerization or non-aqueous dispersion polymerization. The carrier is generally composed of the core material itself, or a carrier provided with a coating layer on the core material.

  The core material of the resin-coated carrier that can be used in the present embodiment is ferrite or magnetite. An appropriate particle size of the core material is about 20 to 60 μm. Examples of the material used for forming the carrier coating layer include vinylidene fluoride, tetrafluoroethylene, hexafluoropropylene, perfluoroalkyl vinyl ether, vinyl ether substituted with a fluorine atom, and vinyl ketone substituted with a fluorine atom. As a method for forming the coating layer, a resin may be applied to the surface of the carrier core material particles by a spraying method, a dipping method, or the like, as in the conventional case. As a developing method, not only a two-component developing method but also a one-component developing method can be used.

Hereinafter, the photoreceptor in the present exemplary embodiment will be described with reference to FIG.
As shown in FIG. 5A, the photosensitive drum 1 in this embodiment has a configuration in which a photosensitive layer 24 containing at least a charge generating material and a polymer charge transporting material is formed on a conductive support 21. have.
As another configuration of the photosensitive drum 1, as shown in FIG. 5B, a configuration in which a charge generation layer 22 and a photosensitive layer 24 including at least a charge transport layer 23 are formed on a conductive support 21. have.
As shown in FIG. 5C, an undercoat layer 25 may be formed between the conductive support 21 and the photosensitive layer 24.
Further, as shown in FIG. 5D, an undercoat layer 25 may be formed between the conductive support 21 and the photosensitive layer 24, and a protective layer 26 may be formed on the photosensitive layer 24.

That is, in the photosensitive drum 1 in the present embodiment, if the photosensitive support layer 21 has a photosensitive layer containing at least a charge generating substance and a polymer charge transporting substance, the above-described layers and the like are provided. You may combine arbitrarily.
This will be described in more detail below.
The conductive support 21 has a volume resistance of 1 × 10 ¹⁰ Ω · cm or less. For example, a metal such as aluminum, nickel, chromium, nichrome, copper, gold, silver, platinum, or a metal oxide such as tin oxide or indium oxide is coated on a film or cylindrical plastic or paper by vapor deposition or sputtering. Things can be used. Alternatively, a seamless endless belt made by a method such as electroforming of the above metal, or a plate made of aluminum, aluminum alloy, nickel, stainless steel, or the like can be used. In this case, after making these plates into raw pipes by methods such as extrusion and drawing, pipes that have been subjected to surface treatment such as cutting, superfinishing, and polishing can be used.

As the support for the endless belt-like photoreceptor, a seamless endless belt produced by a method such as the above-described conductive treatment or the above-described electroforming method of the metal can be used as the conductive support 21.
In addition, those obtained by dispersing and coating conductive powder in an appropriate binder resin can also be used as the conductive support 21 of the present invention. Examples of the conductive powder include carbon black, acetylene black, metal powder such as aluminum, nickel, iron, nichrome, copper, zinc, and silver, or metal oxide powder such as conductive tin oxide and ITO. .
The binder resin used at the same time is polystyrene, styrene-acrylonitrile copolymer, styrene-butadiene copolymer, styrene-maleic anhydride copolymer, polyester, polyvinyl chloride, vinyl chloride-vinyl acetate copolymer. , Polyvinyl acetate, polyvinylidene chloride, polyarylate resin, phenoxy resin, polycarbonate, cellulose acetate resin, ethyl cellulose resin, polyvinyl butyral, polyvinyl formal, polyvinyl toluene, poly-N-vinyl carbazole, acrylic resin, silicone resin, epoxy resin, Examples thereof include thermoplastic, thermosetting resins, and photocurable resins such as melamine resin, urethane resin, phenol resin, and alkyd resin. Such a conductive layer can be provided by dispersing and coating these conductive powder and binder resin in a suitable solvent such as tetrahydrofuran, dichloromethane, methyl ethyl ketone, toluene, and the like.

  Furthermore, it is electrically conductive by a heat-shrinkable tube in which the conductive powder is contained in a material such as polyvinyl chloride, polypropylene, polyester, polystyrene, polyvinylidene chloride, polyethylene, chlorinated rubber, Teflon (registered trademark) on a suitable cylindrical substrate. Those provided with a conductive layer can also be used favorably as the conductive support 21 of the present invention.

Next, the photosensitive layer will be described.
The image carrier used in the image forming apparatus of the present embodiment may have a laminated structure or a single layer structure, but a laminated structure is more preferable.
First, the charge generation layer will be described.
The charge generation layer contains an asymmetric disazo pigment and a metal-free phthalocyanine pigment. The metal-free phthalocyanine pigment preferably includes a τ-type metal-free phthalocyanine pigment, an X-type metal-free phthalocyanine pigment, or both.

For this reason, a mixture of an asymmetric disazo pigment having a large ionization potential (Ip: 5.82) and a metal-free phthalocyanine pigment having a small ionization potential (Ip) (Ip: 5.15 to 5.2) is used. As a result, the barrier against charge between the charge generation layer and the charge transport layer is lowered. As a result, the charging stability is maintained high by smoothly passing through the inside of the photosensitive layer without trapping the current.
The ionization potential of a charge transport material used for a general photoreceptor is 5.3 to 5.6 eV.
Further, an asymmetric disazo pigment having an ionization potential larger than that of the charge transport material is contained as a charge generation material. For this reason, the positive charge injected in the transfer process for transferring the toner from the photoconductor to a recording medium such as paper is greatly suppressed from flowing from the charge transport layer to the charge generation layer. The latent image potential (the potential of the image forming unit) is unlikely to change to the plus side.

  As a result, the potential of the photoreceptor can be kept relatively negative. As a result, the electric charge can smoothly move in the photosensitive layer without weakening the electric field strength of the latent image forming portion, and an image output that hardly causes an afterimage caused by charge trapping due to the previous image history can be realized.

As described above, in the present invention, a latent image is formed with a weak exposure energy with respect to a bright portion potential (small absolute value) in which a negatively charged photoconductor is exposed and the surface potential is small in the image forming process. .
By doing so, since the latent image potential (bright portion potential) corresponding to the image portion is on the minus side lower than 0 V, the bright portion potential becomes closer to the plus side due to the influence of the plus charge by the transfer means. Even if it changes, the final bright portion potential after the transfer can be maintained on the minus side.
As described above, the present invention has the effect of suppressing the afterimage of the latent image potential (residual potential) from both the image forming process and the photoconductor structure.

The relationship between the ionization potential of the charge generation material and the charge transport material is also related to the soil. Background smudge is a phenomenon in which an infinite number of small black spots are printed in a white background area where an image is not originally printed.
Even if it does not become a problem in the initial state, the influence is increased by repeated use, which is a major factor for determining the life of the photoreceptor.
As a mechanism for the occurrence of scumming, when the photosensitive member is charged, a positive charge having a polarity opposite to the negative charge induced on the conductive support side leaks locally, leading to the undercoat layer and the photosensitive layer. This is considered to be caused by the fact that the leaked portion is easily developed in order to cancel the surface charge of the photoreceptor.

The decrease in film thickness due to repeated use of the photoreceptor causes an increase in electric field strength (V / μm), and positive charges induced on the conductive support side are injected into the photosensitive layer, thereby significantly increasing the occurrence of scumming. .
In the present invention, a metal-free phthalocyanine pigment (Ip: Ip: 5.15 to 5.2) having a small ionization potential (Ip) is included as a charge generation material. This increases the barrier for positive charges to move from the charge generation layer to the charge transport layer due to the relationship between the ionization potentials of the two.
Therefore, even in a use situation where the electric field strength is increased to the extent that the positive charge induced on the conductive support side cannot be blocked by the undercoat layer, the positive charge transfer from the charge generation layer to the charge transport layer is blocked. In addition, it is possible to suppress background contamination that occurs due to the cancellation of the surface charge of the photoreceptor.
This is an asymmetric disazo pigment that can suppress the injection of positive charge from the transfer means, and a metal-free phthalocyanine pigment that can suppress the injection of a positive charge opposite to the charged polarity induced on the conductive support side into the photosensitive layer. It is by including both.
As a result, the effects of preventing soiling and extending the life of the photoreceptor are recognized.

The ionization potential was measured using an atmospheric photoelectron spectrometer AC-2 manufactured by Riken Keiki Co., Ltd. As the ionization potential of the charge generation layer and the charge transport layer to be measured, those formed on a PET film were used.
As the τ-type metal-free phthalocyanine pigment, in the X-ray diffraction spectrum using Cu-Kα characteristic X-ray (wavelength 1.541 mm), the main peak with Bragg angle 2θ is 7.6 °, 9.2 °, 16.8 °. 17.4 °, 20.4 °, 20.9 °, 21.7 °, and 27.6 ° (each ± 0.2 °) can be used. It can be obtained by the methods described in JP-A-182639 and JP-A-60-19154.

As the X-type metal-free phthalocyanine pigment, in the X-ray diffraction spectrum using Cu-Kα characteristic X-ray (wavelength 1.541 mm), the main peaks with Bragg angle 2θ are 7.5 °, 9.1 ° and 16.7 °. , 17.3 °, 22.3 °, 23.8 ° (each ± 0.2 °) can be used without metal phthalocyanine pigments, US Pat. No. 3,357,989, US Pat. No. 3,594,163, JP 49-4338, It can be obtained by the method described in JP-A-60-243089.
For the purpose of generating and separating latent image charges by image exposure, a layer mainly composed of a charge generating substance can be used in combination with a binder resin as necessary.

In the present invention, an asymmetric disazo pigment represented by the following general formula (1) is used as the charge generating substance.
Cp1-N = NAN = N-Cp2 General formula (1)
In formula, A shows the bivalent residue couple | bonded with the nitrogen atom of the azo group with the carbon atom. Cp1 and Cp2 represent coupler residues having different structures.
These asymmetric disazo pigments are obtained by reacting the corresponding diazonium salt compound and the coupler corresponding to Cp1 or Cp2 sequentially in two stages, or by reacting the coupler obtained by the first coupling reaction with Cp1 or Cp2. be able to.
Examples of A, Cp1, and Cp2 of these asymmetric disazo pigments are shown in Tables 1 to 10.

The binder resin used for the charge generation layer includes polyamide, polyurethane, epoxy resin, polyketone, polycarbonate, silicone resin, acrylic resin, polyvinyl butyral, polyvinyl formal, polyvinyl ketone, polystyrene, polysulfone, poly-N-vinylcarbazole, poly Examples include acrylamide, polyvinyl benzal, polyester, phenoxy resin, vinyl chloride-vinyl acetate copolymer, polyvinyl acetate, polyphenylene oxide, polyamide, polyvinyl pyridine, cellulosic resin, casein, polyvinyl alcohol, and polyvinyl pyrrolidone.
These binder resins can be used alone or as a mixture of two or more.
The amount of the binder resin is suitably 0 to 500 parts by weight, preferably 10 to 300 parts by weight with respect to 100 parts by weight of the charge generating material.
In addition, the above-described polymer charge transport material can be used as the binder resin for the charge generation layer.
Furthermore, you may add a low molecular charge transport material as needed. Low molecular charge transport materials that can be used in the charge generation layer include electron transport materials and hole transport materials.

Examples of the electron transporting material include chloroanil, bromoanil, tetracyanoethylene, tetracyanoquinodimethane, 2,4,7-trinitro-9-fluorenone, 2,4,5,7-tetranitro-9-fluorenone, 2,4 , 5,7-tetranitroxanthone, 2,4,8-trinitrothioxanthone, 2,6,8-trinitro-4H-indeno [1,2-b] thiophen-4-one, 1,3,7-tri Examples thereof include electron-accepting substances such as nitrodibenzothiophene-5,5-dioxide and benzoquinone derivatives. These electron transport materials can be used alone or as a mixture of two or more.
Examples of hole transport materials include poly-N-vinylcarbazole and derivatives thereof, poly-γ-carbazolylethyl glutamate and derivatives thereof, pyrene-formaldehyde condensates and derivatives thereof, polyvinylpyrene, polyvinylphenanthrene, polysilane, oxazole derivatives, Oxadiazole derivatives, imidazole derivatives, monoarylamine derivatives, diarylamine derivatives, triarylamine derivatives, stilbene derivatives, α-phenylstilbene derivatives, benzidine derivatives, diarylmethane derivatives, triarylmethane derivatives, 9-styrylanthracene derivatives, pyrazolines Derivatives, divinylbenzene derivatives, hydrazone derivatives, indene derivatives, butadiene derivatives, pyrene derivatives, etc., bisstilbene derivatives, enamine derivatives, etc. Other known materials may be mentioned. These hole transport materials can be used alone or as a mixture of two or more.

Methods for forming the charge generation layer can be broadly classified into a vacuum thin film preparation method and a casting method from a solution dispersion system.
As the former method, a vacuum vapor deposition method, a glow discharge decomposition method, an ion plating method, a sputtering method, a reactive sputtering method, a CVD method, or the like is used, and the above-described inorganic materials and organic materials can be satisfactorily formed.
In order to provide a charge generation layer by the latter casting method, a ball mill, an attritor, a solvent such as tetrahydrofuran, cyclohexanone, dioxane, dichloroethane, and butanone together with a binder resin, if necessary, the above inorganic or organic charge generation materials are used. It can be formed by dispersing with a sand mill or the like and applying the solution after diluting the dispersion appropriately. The coating can be performed using a dip coating method, a spray coating method, a bead coating method, or the like.

  The thickness of the charge generation layer provided as described above is suitably about 0.01 to 5 μm, preferably 0.1 to 2 μm.

Next, the charge transport layer will be described.
Binder resins that can be used for the charge transport layer include polycarbonate (bisphenol A type, bisphenol Z type), polyester, methacrylic resin, acrylic resin, vinyl chloride, vinyl acetate, polystyrene, phenol resin, epoxy resin, polyurethane, polyvinylidene chloride, Alkyd resin, silicone resin, polyvinyl carbazole, polyvinyl butyral, polyvinyl formal, polyacrylate, polyacrylamide, phenoxy resin and the like are used.
These binders can be used alone or as a mixture of two or more, and the amount used is suitably about 0 to 30 parts by weight with respect to 100 parts by weight of the polymeric charge transport material.
The low molecular charge transport material that can be used in the charge transport layer can be the same as described in the description of the charge generation layer, and the amount used is about 0 to 30 parts by weight with respect to 100 parts by weight of the binder resin. Is appropriate.

In this invention, the addition amount of low molecular compounds, such as antioxidant, a plasticizer, and an ultraviolet absorber, is 0-150 weight part with respect to 100 weight part of binder resin of this invention in a charge transport layer. Preferably, about 0 to 30 parts by weight is appropriate. The addition amount of the leveling agent is suitably about 0 to 5 parts by weight with respect to 100 parts by weight of the binder resin of the present invention.
Furthermore, when the charge transport layer 23 is the outermost layer of the photoreceptor, it is preferable to contain a filler.

In general, organic photoreceptors have a major drawback of low durability. Although it is roughly divided into durability against mechanical load such as wear and scratches on the surface of the photoreceptor and electrostatic property durability such as accumulation of residual potential and deterioration of chargeability due to repeated use, it is particularly inferior to mechanical durability. This is a factor that determines the life of the photoreceptor.
In addition to these durability factors, the surface layer of the photoreceptor has factors that cause image deterioration such as adhesion of a low-resistance material generated by ozone generated during corona charging, filming due to poor cleaning of the toner, and fusing.
For this reason, in addition to the durability, particularly mechanical durability, releasability of various deposits on the surface of the photoreceptor that affects the life of the photoreceptor is also required.

In order to satisfy these requirements, it is very effective to contain a filler in the outermost layer of the photoreceptor. Fillers include organic fillers and inorganic fillers. Examples of the organic filler material include fluororesin powder such as polytetrafluoroethylene, silicone resin powder, a-carbon powder, and the like. Examples of inorganic filler materials include silica, tin oxide, zinc oxide, titanium oxide, alumina, zirconium oxide, indium oxide, antimony oxide, bismuth oxide, calcium oxide, antimony-doped tin oxide, tin-doped indium oxide, and the like. Examples thereof include metal oxides, metal fluorides such as tin fluoride, calcium fluoride, and aluminum fluoride, potassium titanate, and boron nitride.
In addition to these materials, known materials can be used, and the above-described fillers can be used alone or in combination of two or more.

Furthermore, these fillers are preferably surface-treated with at least one inorganic substance or organic substance for reasons such as improving dispersibility and modifying surface properties.
Decreasing the dispersibility of the filler not only increases the residual potential, but also decreases the transparency of the coating, causes defects in the coating, and decreases the wear resistance. It can develop into a big problem.
As the surface treatment agent, all conventionally used surface treatment agents can be used, but a surface treatment agent capable of maintaining the insulating properties of the filler is preferable. For example, a titanate coupling agent, an aluminum coupling agent, a zircoaluminate coupling agent, a higher fatty acid, etc., or a mixing treatment of these with a silane coupling agent, Al ₂ O ₃ , TiO ₂ , ZrO ₂ , Silicone, aluminum stearate, or the like, or a mixed treatment thereof is preferable in terms of filler dispersibility.

Although the treatment with the silane coupling agent is somewhat reduced in resistance, the influence may be suppressed by applying a mixing treatment of the surface treatment agent and the silane coupling agent.
The surface treatment amount varies depending on the average primary particle size of the filler used, but is preferably 3 to 30% by weight, and more preferably 5 to 20% by weight.
Dispersing solvents include methyl ethyl ketone, acetone, methyl isobutyl ketone, cyclohexanone ketones, ethers such as dioxane, tetrahydrofuran and ethyl cellosolve, aromatics such as toluene and xylene, halogens such as chlorobenzene and dichloromethane, ethyl acetate and butyl acetate Esters such as are used. As the dispersing means, known dispersing means such as a ball mill, a sand mill, and a vibration mill can be used.

When the charge transport layer is the outermost layer, the content of the filler is preferably 0 to 40% by weight, more preferably 0.1 to 30% by weight with respect to the total solid content. When the filler amount is 0.1% by weight or less, it is not preferable from the viewpoint of wear resistance.
On the other hand, if it is 40% by weight or more, image deterioration such as reduction in resolution due to film opacification and reduction in image density due to sensitivity reduction occurs. The volume average particle diameter of the filler is preferably 0.05 to 1.0 [mu] m, preferably 0.05 to 0.8 [mu] m.
If the particle size is smaller than 0.05 μm, uniform dispersion is difficult to perform, and if the particle size is larger than 1.0 μm, the filler may cue up on the surface of the photoconductor, which may damage the cleaning blade and cause poor cleaning.

As the coating method, a dipping method, a spray coating method, a ring coating method, a roll coater method, a gravure coating method, a nozzle coating method, a screen printing method, or the like is employed. During coating, when the coating solution dissolves the lower photosensitive layer, it is easy to control the contact time between the lower layer and the coating solution, the contact amount between the lower layer and the solvent in the coating solution, It is preferable to use a spray coating method, a ring coating method, or the like. The thickness of the charge transport layer is suitably about 5 to 100 μm, preferably 10 to 40 μm.
The degree is appropriate.

As described above, the photoreceptor used in the present invention can be provided with an undercoat layer between the conductive support and the charge generation layer. The undercoat layer is provided for the purpose of improving adhesiveness, preventing the occurrence of interference fringes, improving the coatability of the upper layer, and reducing the residual potential. In general, the undercoat layer is mainly composed of a resin. However, considering that the photosensitive layer is applied with a solvent on these resins, the resin may be a resin having a high resistance to a general organic solvent. desirable.
Examples of such resins include water-soluble resins such as polyvinyl alcohol, casein, and sodium polyacrylate, alcohol-soluble resins such as copolymer nylon and methoxymethylated nylon, polyurethane, melamine resins, alkyd-melamine resins, and epoxy resins. Examples thereof include curable resins that form a three-dimensional network structure.

Further, fine powders such as metal oxides exemplified by titanium oxide, silica, alumina, zirconium oxide, tin oxide, indium oxide and the like, or metal sulfides and metal nitrides may be added. These undercoat layers 25 can be formed using an appropriate solvent and coating method, as in the case of the photosensitive layer described above.
Furthermore, a metal oxide layer formed by, for example, a sol-gel method using a silane coupling agent, a titanium coupling agent, a chromium coupling agent or the like as the undercoat layer is also useful. In addition, the undercoat layer of the present invention is provided with Al ₂ O ₃ by anodic oxidation, organic substances such as polyparaxylylene (parylene), SiO, SnO ₂ , TiO ₂ , ITO, CeO _2. A material provided with an inorganic material such as a vacuum thin film can also be used favorably. The thickness of the undercoat layer is suitably 0.5 to 15 μm.

  In the photoreceptor of the present invention, for the purpose of protecting the photosensitive layer, a protective layer may be provided on the photosensitive layer (a charge transport layer in the case of a laminated structure). Materials used for this include ABS resin, ACS resin, olefin / vinyl monomer copolymer, chlorinated polyether, allyl resin, phenol resin, polyacetal, polyamide, polyamideimide, polyacrylate, polyallylsulfone, polybutylene, Polybutylene terephthalate, polycarbonate, polyethersulfone, polyethylene, polyethylene terephthalate, polyimide, acrylic resin, polymethylpentene, polypropylene, polyphenylene oxide, polysulfone, AS resin, AB resin, BS resin, polyurethane, polyvinyl chloride, polyvinylidene chloride, Resins such as epoxy resins can be used.

  In addition, the filler described in the description of the charge transport layer can be added to the protective layer for the purpose of improving wear resistance. As a method for forming the protective layer, a normal coating method is employed. In addition, about 0.5-10 micrometers is suitable for the thickness of a protective layer. In addition to the above, known materials such as i-C and a-SiC formed by a vacuum thin film manufacturing method can also be used as the protective layer. If necessary, the above-described charge transport material, antioxidant, plasticizer, lubricant, ultraviolet absorber and leveling agent may be added.

  In the present invention, an antioxidant may be added for the purpose of preventing the decrease in sensitivity and the increase in residual potential, in order to improve environmental resistance. The antioxidant may be added to any layer containing an organic substance, but good results are obtained when it is added to a layer containing a charge transport material. In the present invention, known antioxidants can be used.

EXAMPLES Next, although an Example demonstrates this invention further in detail, this invention is not limited to a following example. In addition, all the parts used in an Example represent a weight part.
Moreover, Mw represents the weight average molecular weight of polystyrene conversion measured by the gel permeation chromatography (GPC) method.

(Image carrier production example 1)
An aluminum cylinder having a diameter of 30 mm was prepared as a conductive cylindrical support. On this aluminum cylinder, an undercoat layer coating solution, a charge generation layer coating solution, and a charge transport layer coating solution having the following composition are sequentially applied and dried to form a 4.5 μm undercoat layer, 0 A 2 μm charge generation layer and a 40 μm charge transport layer were formed to obtain the image carrier 1 of the present invention.

[Coating liquid for undercoat layer]
Alkyd resin 6 parts (Beccosol 1307-60-EL, manufactured by Dainippon Ink & Chemicals, Inc.)
Melamine resin 4 parts (Super Becamine G-821-60, manufactured by Dainippon Ink & Chemicals, Inc.)
Titanium oxide 40 parts (CR-EL, manufactured by Ishihara Sangyo Co., Ltd.)
200 parts of methyl ethyl ketone

[Coating liquid for charge generation layer]
24 parts of bisazo pigment (P-1) represented by the following structural formula

τ-type metal-free phthalocyanine pigment 12 parts Polyvinyl butyral 12 parts (ESREC BL-1, manufactured by Sekisui Chemical Co., Ltd.)
Cyclohexanone 1680 parts Methyl ethyl ketone 390 parts

[Coating liquid for charge transport layer]
Bisphenol Z-type polycarbonate (Mw: 112,000) 10 parts (Panlite TS-2050, manufactured by Teijin Chemicals)
7 parts of charge transport material represented by the following structural formula (D-1) Ionization potential of charge transport material (D-1) (Ip: 5.4)

0.002 part of silicone oil (KF-50: manufactured by Shin-Etsu Chemical Co., Ltd.)
70 parts of tetrahydrofuran

(Image carrier production example 2)
An image carrier 2 was produced in the same manner as in the image carrier production example 1 except that the charge generation layer coating solution in the image carrier production example 1 was changed to one having the following composition.
Bisazo pigment (P-1) 24 parts τ-type metal-free phthalocyanine pigment 12 parts Polyvinyl butyral 24 parts (ESREC BL-1, manufactured by Sekisui Chemical Co., Ltd.)
Cyclohexanone 1680 parts Methyl ethyl ketone 390 parts

(Image carrier production example 3)
An image carrier 3 was produced in the same manner as in the image carrier production example 1 except that the charge generation layer coating solution in the image carrier production example 1 was changed to one having the following composition.
Bisazo pigment (P-1) 24 parts X-type metal-free phthalocyanine pigment 12 parts Polyvinyl butyral 24 parts (Eslec BL-1, manufactured by Sekisui Chemical Co., Ltd.)
Cyclohexanone 1680 parts Methyl ethyl ketone 390 parts

(Image carrier production example 4)
An image carrier 4 was produced in the same manner as in the image carrier production example 1 except that the thickness of the charge generation layer was changed to 0.2 μm in the carrier production example 1.

(Image carrier preparation example 5)
An image carrier 5 was produced in the same manner as in the image carrier production example 1 except that the charge generation layer coating solution in the image carrier production example 1 was changed to one having the following composition.
Bisazo pigment (P-1) 12 parts τ-type metal-free phthalocyanine pigment 24 parts Polyvinyl butyral 24 parts (ESREC BL-1, manufactured by Sekisui Chemical Co., Ltd.)
Cyclohexanone 1680 parts Methyl ethyl ketone 390 parts

(Image carrier preparation example 6)
An image carrier 6 was produced in the same manner as in the image carrier production example 1 except that the charge generation layer coating solution in the image carrier production example 1 was changed to one having the following composition.
Bisazo pigment (P-1) 12 parts X-type metal-free phthalocyanine pigment 24 parts Polyvinyl butyral 36 parts (Eslec BL-1, manufactured by Sekisui Chemical Co., Ltd.)
Cyclohexanone 1680 parts Methyl ethyl ketone 390 parts

(Image carrier preparation example 7)
An image carrier 7 was produced in the same manner as in the image carrier production example 1 except that the charge generation layer coating solution in the image carrier production example 1 was changed to one having the following composition.
Bisazo pigment (P-1) 12 parts τ-type metal-free phthalocyanine pigment 12 parts Polyvinyl butyral 24 parts (ESREC BL-1, manufactured by Sekisui Chemical Co., Ltd.)
Cyclohexanone 1680 parts Methyl ethyl ketone 390 parts

(Image carrier production example 8)
An image carrier 8 was produced in the same manner as in the image carrier production example 1 except that the charge generation layer coating solution in the image carrier production example 1 was changed to one having the following composition.
Bisazo pigment (P-1) 12 parts τ-type metal-free phthalocyanine pigment 12 parts Polyvinyl butyral 16 parts (ESREC BL-1, manufactured by Sekisui Chemical Co., Ltd.)
Cyclohexanone 1680 parts Methyl ethyl ketone 390 parts

(Image carrier production example 9)
An image carrier 9 was produced in the same manner as in the image carrier production example 1 except that the charge generation layer coating solution in the image carrier production example 1 was changed to one having the following composition.
Bisazo pigment (P-1) 24 parts Polyvinyl butyral 12 parts (ESREC BL-1, manufactured by Sekisui Chemical Co., Ltd.)
Cyclohexanone 1680 parts Methyl ethyl ketone 390 parts

(Image Carrier Production Example 10)
An image carrier 10 was produced in the same manner as in the image carrier production example 1 except that the charge generation layer coating solution in the image carrier production example 1 was changed to one having the following composition.
τ-type metal-free phthalocyanine pigment 24 parts Polyvinyl butyral 12 parts (ESREC BL-1, manufactured by Sekisui Chemical Co., Ltd.)
Cyclohexanone 1680 parts Methyl ethyl ketone 390 parts

(Image carrier preparation example 11)
An image carrier 11 was produced in the same manner as in the image carrier production example 1 except that the charge generation layer coating solution in the image carrier production example 1 was changed to one having the following composition.
12 parts of bisazo pigment (P-1) 12 parts of titanyl phthalocyanine pigment (major peaks of Bragg angle 2θ are 9.6 ° ± 0.2 °, 24.0 ° ± 0.2 °, 27.
Crystal form at 2 ° ± 0.2 °)
12 parts of polyvinyl butyral (ESREC BX-1, manufactured by Sekisui Chemical Co., Ltd.)
Cyclohexanone 1680 parts Methyl ethyl ketone 390 parts

(Image carrier preparation example 12)
An image carrier 12 was produced in the same manner as in the image carrier production example 1 except that the charge generation layer coating liquid in the image carrier production example 1 was changed to one having the following composition.
X-type metal-free phthalocyanine pigment 12 parts Titanyl phthalocyanine pigment 24 parts (major peaks with Bragg angle 2θ are 9.6 ° ± 0.2 °, 24.0 ° ± 0.2 °, 27.
Crystal form at 2 ° ± 0.2 °)
12 parts of polyvinyl butyral (ESREC BX-1, manufactured by Sekisui Chemical Co., Ltd.)
Cyclohexanone 1680 parts Methyl ethyl ketone 390 parts

(Image carrier preparation example 13)
An image carrier 13 was produced in the same manner as in the image carrier production example 1 except that the charge generation layer coating solution in the image carrier production example 1 was changed to one having the following composition.
24 parts of titanyl phthalocyanine pigment (major peaks with Bragg angle 2θ are 9.6 ° ± 0.2 °, 24.0 ° ± 0.2 °, 27.
Crystal form at 2 ° ± 0.2 °)
16 parts of polyvinyl butyral (ESREC BX-1, manufactured by Sekisui Chemical Co., Ltd.)
480 parts of methyl ethyl ketone

Table 11 shows the structure and film thickness of the charge generation layer of the image carrier produced as described above.
The film thickness of the charge generation layer is measured using an eddy current film thickness meter (Fischerscope MMS, manufactured by Fischer Instruments) before and after coating the charge generation layer formed on the undercoat layer. These differences were taken as the film thickness of the charge generation layer.

<Examples 1-8 and Comparative Examples 1-5>
The obtained image carrier was set in an actual sensitivity simulator equipped with a charger, LD, surface potential meter, etc., and the drum rotation speed and light quantity were variable, and the half exposure (μJ / cm ² ) was measured.
In this actual sensitivity simulator, an exposure device including a charging device and an LD is arranged around a photosensitive member, and a surface potential meter is arranged between the charging device and the exposure device. Each device arranged around the photosensitive member can be arbitrarily moved in the circumferential direction, radial direction, and longitudinal direction with respect to the photosensitive member, and the characteristics of the photosensitive members of various sizes can be measured.
The actual machine sensitivity simulator is manufactured in-house and disclosed in Japanese Patent Application Laid-Open No. 2000-275872.

By changing the number of light attenuating filters according to the required exposure power, or changing the type of light attenuating filter, and measuring the surface potential of the photoconductor while varying the LD drive current of the exposure apparatus, exposure is performed. Energy and surface potential data can be obtained.
The half-exposure amount means an exposure amount that becomes Vd / 2 when the surface potential of the photoreceptor is Vd. In the examples and comparative examples, the image carrier is charged to -900 V, and light is irradiated by the exposure means. The exposure amount required to attenuate to -450V was obtained.
The half exposure amount is a value indicating the sensitivity of the image carrier, and the smaller the half exposure value, the higher the sensitivity.

[Paper test]
The image carriers 1 to 13 produced as described above are mounted on an image forming apparatus [image MP3353 manufactured by Ricoh Co., Ltd.], which is mounted on a process cartridge having the structure shown in FIG. evaluated.
The exposure means was set so that the semiconductor laser was 780 nm and the exposure energy was 0.42 μJ / cm ² .
Table 12 shows the results of the bright portion potential (solid image portion potential) and (exposure energy) / (half exposure amount) when a latent image was formed at an exposure energy of 0.42 μJ / cm ² .

A contact charging roller was used as the charging member of the image forming apparatus. A DC voltage is applied to the charging member, the developing bias is adjusted so that the linear velocity of the photosensitive member is 150 mm / sec, the charging potential of the photosensitive member is −900 V, and the density of the solid image portion is 1.4 or more. The current value applied to the means was set to 15 μA. The electric field strength applied to the photoreceptor is 22.5 V / μm.
The developer for developing the electrostatic latent image includes a pulverized toner (manufactured by Ricoh) having a volume average particle size of about 9.5 μm and a Ricoh product coated with a silicone resin having a volume average particle size of about 50 μm. A carrier mixed to have a toner concentration of 5% by weight was used.

  As an evaluation method, an A4 document having a pixel density of 600 dpi × 600 dpi and an image area ratio of 5% in an environment of normal temperature and humidity (23 ° C., 55% RH) was used. Durability test of 50,000 sheets in total, which is copied and printed on MyPaper made by NBS Ricoh Co. under the condition of printing continuously 5 sheets at a time, fluctuation amount of in-machine potential (dark part potential: VD, bright part potential: VL) A comprehensive evaluation of background stains, wear characteristics, and images was performed. The results are shown in Table 13.

[Evaluation of afterimage by potential]
Evaluation of the afterimage based on the potential is performed by measuring the potential of the surface of the photoconductor while outputting the chart shown in FIG. 7A. When the potential of the halftone portion is V1 and the potential of the afterimage portion is V2, the afterimage potential ( Va) can be obtained by Va = V2-V1.
When Va is a positive value, it is a positive afterimage, and when Va is a negative value, it is a negative afterimage. As the value increases, the afterimage appearing in the halftone portion becomes more prominent.

[Measurement of transfer rate]
The measurement of the transfer rate was stopped in a state in which A4 size plain paper in which a plurality of rectangular solid black images of 1 cm × 3 cm were arranged was passed in the vertical direction and the toner was transferred to the paper surface. The transfer rate was calculated based on the following equation by measuring the toner weight M0 on the photoconductor after development and before transfer: M0 and the toner weight M1 on the photoconductor not transferred.
Transfer rate (%) = (M0−M1) / M0 × 100

[Evaluation of wear amount]
About the amount of wear, the film thickness before and after the paper passing test was measured using an eddy current film thickness meter (Fischerscope MMS, manufactured by Fischer Instruments Co., Ltd.), and the difference between these was taken as the amount of wear.

[Evaluation of ΔVD and ΔVL]
For the bright portion potential, an electrometer probe was attached to the developing portion, and the surface potential of the image carrier when moving to the developing portion after charging and image exposure was measured.
Note that ΔVD and ΔVL represent the amount of potential change in the dark part potential and the bright part potential at the initial time and after the 50,000-sheet passing test, respectively.
ΔVD = (Dark part potential at initial time: −900 V) − (Dark part potential after passing 50,000 sheets)
ΔVL = (light portion potential at initial time) − (light portion potential after passing 50,000 sheets)

[Image evaluation]
Image characteristics were comprehensively evaluated for halftone, resolution, and abnormal images.
The evaluation results are shown in Table 13.

In Comparative Examples 1 to 4, a negative afterimage appeared in the halftone image.
In Comparative Example 1, it is considered that this is due to the use of only the τ-type metal-free phthalocyanine pigment as the charge generation material, and the positive charge injected in the transfer process remains in the photosensitive layer. It seems that a negative afterimage was generated by the residual charge.
About Comparative Examples 2-4, it is thought that it originates in having used highly sensitive titanyl phthalocyanine for a charge generation substance, The positive charge inject | poured at the transfer process remains in the photosensitive layer, This residual charge seems to have caused a negative afterimage.
In Comparative Examples 2 to 4, since highly sensitive titanyl phthalocyanine is used as the charge generating material, the fluctuation of the bright part potential is large, and when multiple copies of the same image are printed, there is a density unevenness for each print and it is stable. There was a problem with the image output.

In Comparative Examples 2, 3, and 4, since the latent image is formed with exposure energy strong against the half-exposure amount, the bright portion potential is near zero (−100 V or less). For this reason, it is considered that the electrostatic adhesion force with the negatively charged toner particles adhering to the portion where the electrostatic latent image is formed is strong and the transfer rate is lowered.
On the other hand, in Examples 1 to 8, since the electrostatic latent image is formed with low exposure energy, the exposed portion potential is close to minus, and the electrostatic adhesion force between the photosensitive member and the toner particles is weak. In addition, the transfer rate to the paper side is improved, and a sufficient toner density on the paper can be maintained.

  In the photoreceptor of the present invention in which the charge generation layer contains an asymmetric disazo pigment and a metal-free phthalocyanine pigment, good results were obtained.

<Examples 9 to 12 and Comparative Examples 5 to 12>
Next, a paper passing test was conducted in the same manner as in Example 1 while changing the exposure energy so that the exposure means had a semiconductor laser of 655 nm and a desired bright portion potential.
Table 14 shows the results of the bright part potential (solid image part potential) and (exposure energy) / (half exposure amount), and Table 15 shows the result of the paper passing test.

In Comparative Examples 5, 6, 7, and 10, since the potential difference between the dark portion potential (−900 V) and the light portion potential (VL) is large, an excessive transfer current flows into the non-image portion. Since a sufficient transfer current to the image portion cannot be secured, the transfer rate is lowered and the resolution of the character portion is lowered. In a region having a large image area, density unevenness occurred due to insufficient transferability.
In Comparative Examples 7, 8, and 9, since only the asymmetric disazo pigment is used as the charge generation material, the electric field strength is increased due to the abrasion of the photoreceptor by the paper passing test due to the ionization potential relationship with the charge transport material. At the same time, the positive charge injected from the conductive support passes through the charge generation layer and reaches the charge transport layer, so that the surface charge is canceled and scumming occurs.

  In Comparative Examples 11 and 12, only the highly sensitive titanyl phthalocyanine is used as the charge generation material. For this reason, when a latent image is formed by lowering the exposure energy, the fluctuation of the bright part potential (VL) becomes even larger, so that the density fluctuation between pages is large in the output of a plurality of images, and stable image quality can be maintained. There wasn't.

<Examples 13 to 15 and Comparative Examples 13 to 15>
Next, the exposure means is adjusted so that the semiconductor laser is 780 nm and the charging potential (VD) is −1000, −800, and −700 V, the half-exposure amount is measured under each condition, and the exposure energy is further changed. And set to the desired VL. The developing bias was determined so that the density of the solid image portion was 1.4 or more, and a paper passing test was performed.
As a paper passing test, an A4, manuscript with a pixel density of 600 dpi × 600 dpi and an image area ratio of 5% in an environment of normal temperature and normal humidity (23 ° C., 55% RH) was applied to MyPaper manufactured by NBS Ricoh. 500 sheets were output continuously, and the density fluctuation was visually observed for 500 images. The results are shown in Table 16.

In Examples 13 to 14, the half-exposure amount was in the range of 0.12 to 0.28 μJ / cm ² , and the density between pages did not change even when the image was output while changing the exposure energy.
On the other hand, Comparative Examples 13 to 14 are extremely high-sensitivity photoconductors having a small half exposure value. When an image is continuously output using such a high-sensitivity photoconductor, the bright part potential (VL) easily fluctuates with time, and the density of the solid image part fluctuates accordingly. In Comparative Examples 13 and 14 having a high light portion potential (VL), the density variation was large due to the density becoming darker or darker among the 500 sheets continuously output due to the fluctuation in VL.
In Comparative Example 15, VL was set low as -150 V, but it was estimated that the sensitivity of the photoconductor further increased during the 500 sheet passing test, and the density gradually increased.

On the other hand, the charge generation layer uses the photoreceptor of the present invention containing an asymmetric disazo pigment and a metal-free phthalocyanine pigment, and is larger than the half-exposure amount of the photoreceptor and 2.5 times the half-exposure amount. When an electrostatic latent image was formed with the exposure energy described below (see Example 15 in Table 16), good results were obtained.
In this embodiment, the half-exposure amount of the photoreceptor 1 is 0.17 to 0.3 μJ / cm ² as is clear from Example 15 in Table 16 and Example 7 in Table 12.
The difference between the bright portion potential when forming the electrostatic latent image on the photoreceptor 1 and the bright portion potential before charging the surface of the image carrier after transfer is preferably 50 to 350V.

  As described above, an image carrier containing an asymmetric disazo pigment and a metal-free phthalocyanine pigment and having a charge-stable photosensitive layer having no charge retention at the photosensitive layer interface and having a half-exposure amount of 2. A latent image is formed with a low exposure energy of 5 times or less. For this reason, there are few potential changes in the dark part potential and the bright part potential, and injection of positive current in the transfer process is prevented or suppressed, so that no afterimage due to the latent image potential (residual potential) occurs.

In addition, since an image carrier having a photosensitive layer having a wide spectral sensitivity characteristic from the visible region to the near infrared region and having a high charge stability is used, high speed and small space can be achieved. Since the static elimination process by the static elimination means becomes unnecessary, it is possible to reduce the size and cost of the static elimination member, and to suppress the photodegradation (deterioration of sensitivity characteristics) of the image carrier due to excessive light energy, thereby further improving the potential stability.
Further, it is possible to prevent the occurrence of a positive afterimage that is generated when the charged potential of the image portion on which the electrostatic latent image is formed by the exposure unit changes in the positive direction by the transfer current through the development and transfer processes.
At the same time, since the potential contrast difference between the image portion and the non-image portion is reduced, the transfer current that has flowed excessively into the non-image portion can be suppressed, so that the occurrence of a negative afterimage can be prevented.

  Therefore, according to the present invention, high-speed printing, space saving, and miniaturization can be realized, and the occurrence of abnormal images due to the latent image potential (residual potential) can be suppressed with high accuracy.

The preferred embodiments of the present invention have been described above. However, the present invention is not limited to such specific embodiments, and unless specifically limited by the above description, the present invention described in the claims is not limited. Various modifications and changes can be made within the scope of the gist.
The effects described in the embodiments of the present invention are merely examples of the most preferable effects resulting from the present invention, and the effects of the present invention are limited to those described in the embodiments of the present invention. is not.

DESCRIPTION OF SYMBOLS 1 Photoconductor as image carrier 2 Charging means 3 Exposure means 4 Developing means 5 Transfer means 21 Conductive support 22 Charge generation layer 23 Charge transport layer 24 Photosensitive layer

JP 2002-107983 A Japanese Patent No. 3586011 Japanese Patent No. 3416444 Japanese Patent No. 3350834 Japanese Patent No. 5668733 JP 2014-197237 A

Claims (8)

An image carrier, and a charging member for charging the surface of the image carrier;
An exposure member that forms an electrostatic latent image on the surface of the charged image carrier;
A developing member for visualizing the electrostatic latent image as a toner image;
A transfer member for transferring the toner image formed on the surface of the image carrier,
The image carrier has a charge generation layer and a charge transport layer,
The charge generation layer comprises an asymmetric disazo pigment and a metal-free phthalocyanine pigment;
The exposure member forms the electrostatic latent image with an exposure energy that is greater than the half-exposure amount of the image carrier and less than 2.5 times the half-exposure amount, and performs static elimination within one printing process. No image forming device. The image forming apparatus according to claim 1.
The image forming apparatus that forms an electrostatic latent image by exposing the surface of the image carrier with a laser beam having a wavelength of 650 nm or more. The image forming apparatus according to claim 1, wherein
An image forming apparatus in which the image carrier has a half-exposure amount of 0.17 to 0.3 μJ / cm ² . The image forming apparatus according to any one of claims 1 to 3,
An image forming apparatus that exposes the image carrier with the exposure member after the one printing step is completed. The image forming apparatus according to any one of claims 1 to 3,
The image forming apparatus, wherein the metal-free phthalocyanine pigment is a τ-type metal-free phthalocyanine pigment or an X-type metal-free phthalocyanine pigment. The image forming apparatus according to any one of claims 1 to 3,
An image forming apparatus in which an electric field intensity applied to the photosensitive layer having the charge generation layer and the charge transport layer when the image carrier is charged by the charging member is 10 to 50 V / μm. The image forming apparatus according to any one of claims 1 to 3,
The image forming apparatus, wherein the charging member is a charging member in contact with the image carrier to which only a DC bias voltage is applied. The image forming apparatus according to any one of claims 1 to 3,
A transfer bias is applied to the transfer member, and a bright portion potential when the electrostatic latent image is formed on the image carrier and a bright portion potential before charging the surface of the image carrier after the transfer. An image forming apparatus having a difference of 50 to 350V.

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