JP4836644B2 - Image forming method - Google Patents

Description

The present invention relates to an image forming method for obtaining an image on a final transfer material from a toner image formed on an image bearing member via an intermediate transfer member, and in particular, after transferring each color image to the intermediate transfer member a plurality of times and then final transfer The present invention relates to a technique for forming a high image quality by preventing toner dust in an image forming method for transferring to a material to obtain a full color image.

  In an image forming apparatus using an electrophotographic process, in recent years, a color image forming method using an intermediate transfer member has become mainstream. When an intermediate transfer member is used, the toner is transferred twice from the image carrier to the intermediate transfer member or from the intermediate transfer member to the transfer material. Therefore, to achieve high image quality, image degradation during transfer is prevented. It becomes important. As image degradation during transfer, disturbance due to dust has been a problem in the past.

This is considered to be mainly caused by transfer (pre-transfer) from the image carrier to the transfer material or intermediate transfer body before the toner image enters the transfer area. As a countermeasure, an image transfer apparatus has been proposed in which an image carrier and a member that presses the transfer body are provided on the upstream side of the transfer unit to reduce the gap before transfer and prevent scattering (see, for example, Patent Document 1). .
However, in the present invention, since the polarity opposite to the charging polarity of the toner is applied to the pressing member, the electric field formed by the pressing member is in a direction in which the toner is scattered toward the transfer member. Therefore, image dust due to pre-transfer cannot be completely prevented.

Patent Document 2 discloses an invention of an image forming apparatus with little transfer dust at the time of toner image transfer. In this invention, a potential of the same polarity as the charged polarity of the image carrier or zero is provided upstream of the transfer nip portion. It has been proposed to provide a conductive member that has a potential.
According to the present invention, since a bias having the same polarity as that of the toner used for image formation is applied, a force that actively attracts the toner hardly occurs. However, in a general image carrier, the potential of the image portion to which toner is attached is not zero, and there is a residual potential having the same polarity as the charge polarity of the image carrier. Therefore, when the potential of the conductive member is zero, dust due to toner is generated.

  In addition, in a laminated electrophotographic photoreceptor including a charge generation layer and a charge transport layer, the charge transport layer contains a photocurable resin and other resin components, and the photocurable resin is added to the charge transport layer when the charge transport layer is formed. Irradiates light in a wavelength range that can be cured in a lattice shape to form a portion in which the content of the photocurable resin is larger than the unexposed portion in a lattice shape, and the lattice-shaped exposed portion has an unexposed surface resistance or volume resistance. There has been proposed an electrophotographic photosensitive member characterized in that the area is larger than the portion (see, for example, Patent Document 3).

  Also, in an image forming apparatus, an image formed by dispersing minute regions in which the potential is not attenuated with respect to image exposure on the photosensitive layer by, for example, destroying the structure of molecules forming the charge generation layer of the photosensitive layer on the surface of the image carrier. A forming apparatus is disclosed (for example, see Patent Document 4).

  In Patent Document 5, in an electrophotographic photosensitive member in which at least a photosensitive layer is formed on a conductive substrate, the photosensitive layer includes a charge generation region and a charge transport region, and the charge generation region is formed in the charge transport region. An invention of an electrophotographic photosensitive member characterized by having a structure embedded in a provided hole is disclosed.

  Further, Patent Document 6 discloses an image forming apparatus that can suppress transfer dust and can stably suppress dot variation regardless of changes with time and developer variations, and can improve image quality by improving graininess. An invention of an image forming method is disclosed (FIGS. 3 to 11 and the like).

  Further, Patent Document 7 includes a charge generation layer having at least a charge generation material on a conductive support and a charge transport layer containing at least a charge transport material, and the charge generation layer is dispersed in the form of dots as a minute region. An invention of an electrophotographic photosensitive member and a method for manufacturing the same is disclosed. In addition, the occupation area ratio of the charge generation layer is described.

Patent Document 8 discloses an invention of an image forming apparatus characterized in that the surface potential on the image carrier is set to irradiate a predetermined multiple or more based on the light attenuation characteristics of the image carrier. The relationship between the image charge amount and its distribution formula is also described.
JP-A-8-166728 JP-A-10-186878 JP-A-11-258837 JP 11-338170 A JP 2000-231203 A JP 2004-109702 A JP 2005-099078 A Japanese Patent Laying-Open No. 2005-091882

The present invention has been made in view of the circumstances described above, and its object is to provide an image forming method for holding the potential of the image bearing member non-image portion below -200 V.

In order to solve the above-mentioned problems, the invention described in claim 1 includes a charging unit for charging an image carrier, an exposure unit for forming an electrostatic latent image on the image carrier, and the image carrier. A developing means for developing the latent image on the body with toner having the same polarity as the latent image; a first transfer means for transferring the toner image onto the intermediate transfer body; and the toner image on the intermediate transfer body as the recording body. An image forming method in which an image is formed by a second transfer means for transferring to a non-image portion, wherein the potential of the non-image portion of the image carrier is maintained at −200 V or less.

The image carrier is provided on a conductive support, and includes a charge generation layer having at least a charge generation material and a charge transport layer containing at least a charge transport material, and the charge transport layer is a charge transport material. The insulating material is provided in a lattice shape as a region surrounding the charge transport material, and the region of the insulating material and the region of the charge transport material are arranged in succession in the film thickness direction . It is characterized by.

Further , the charge generation layer is formed by being dispersed in a dot shape in the charge transport layer as a minute region.

The region of the insulating material is 10% or less.

Further , the size of the region of the charge transport material is not more than one pixel.

Moreover, occupying area ratio of the charge generating layer formed on the dots is 90% or less, the occupied area ratio (X) and the exposure beam energy J is a satisfy the following formula (1) To do.
η × J ≧ 0.01 × EXP (− (X−60) / 10) +0.002 (1)
(In the above formula (1), X is the occupied area ratio (%) in the horizontal section of the charge generation layer, η is the quantum efficiency, and J is the exposure beam energy (J / m ² ).)

Further , the image carrier and the intermediate transfer member are in contact with each other, and the intermediate transfer member corresponding to the gap on the upstream side in the rotation direction of the image carrier at the time of the first transfer by the first transfer unit, The image bearing member has means for applying a voltage having the same polarity as the charging polarity of the image carrier and whose absolute value is larger than the non-image portion potential _VL .

As described above, according to the present invention , the potential of the non-image portion of the image bearing member of the photosensitive member can be maintained at −200 V or less, and high-quality images can be formed while preventing image dust and dot variation. Can do.

This embodiment, as an image dust prevention means, the first transfer portion void of an image carrying body rotation upstream side with respect to the image carrier and the intermediate transfer member, not aspirated on the toner image bearing member to the intermediate transfer member direction It is an object of the present invention to provide an image forming method in which an electric field is applied.
In addition, by preventing discharge around the edges that may occur at this time, the image quality is stable regardless of aging fluctuations and developer variations, suppressing dot variation and improving graininess. An object of the present invention is to provide an image forming method and apparatus therefor.

Hereinafter, this embodiment will be described in detail with reference to the drawings.
FIG. 1 is a diagram showing the relationship between the non-image area potential of an image carrier and transfer dust (transfer toner dust) during transfer.
The amount of dust indicates the area per pixel of toner scattered to the non-image area. There is a correlation between the non-image portion potential and the amount of dust. As shown in FIG. 1, when the non-image portion potential is a negative potential, the toner dust is small, and the amount of dust increases as it approaches 0V, which causes image deterioration. It is thought that has become.
In the following description, the image carrier is also referred to as a photoconductor.

  Although the relationship between the image carrier non-image portion and the toner dust in FIG. 1 has been described, it is possible to reduce the toner dust by forming the non-image portion region of the image carrier at a negative potential and form a high-quality image. It was confirmed that

  In order to carry out a detailed analysis of this phenomenon, a conceptual diagram of charge carrier generation and transport in the image carrier is shown in FIG. 2, and the relationship between the image carrier surface potential of the dot image and the intermediate transfer material surface potential is shown in FIG. FIG. 3 shows the potential distribution on the surface of the image carrier when a latent dot image is formed on the image carrier.

  As shown in FIG. 2, when the image carrier is irradiated with light, carriers are generated in the charge generation layer (CGL) and diffused by the Coulomb repulsive force between the generated carriers, and in accordance with the electric field, the image carrier. It moves to the surface layer and the substrate, and forms an electrostatic latent image by neutralizing the charge on the surface of the image carrier.

In the image formation studied here, a negative positive system is used in which the image carrier is charged to a negative potential and negatively charged toner is developed. As shown in FIG. 3, the photosensitive member surface potential V is a non-image portion potential on the image carrier, _VL is an image portion potential, and toner adheres to a portion where the absolute value of the latent image is small. FIG. 3 shows the upward direction as negative. Negative toner adheres more in the lower part of the figure (in the direction in which the surface potential is closer to 0). Here, when the potential of the intermediate transfer member is GND, the intermediate transfer member is more stable than _VL for the toner, so the toner moves in the GND direction and adheres to the intermediate transfer member. This is because the potential of the image portion becomes GND when it is sufficiently exposed considering an ideal photoconductor, but in reality, for example, the _VL at the time of solid is -100 to -150V. Specifically, V _L is determined by an initial value determined by the type of the photoconductor and a change due to deterioration over time. Therefore, the toner is pre-transferred from the image carrier to the intermediate transfer member with respect to the contact portion between the image carrier and the intermediate transfer member.

It has been confirmed that when the potential of the non-image part increases in the negative direction, dust tends to decrease as shown in FIG. This is because the force from the electric field overcomes the electrostatic repulsion force between the toners, and thus the dust is considered to be suppressed.
Compared to the case where the latent image is sharp (solid line), carrier diffusion is large, and when the latent image becomes a broad latent image (dotted line), the potential in the vicinity of the image area and the non-image area is shifted to the + side. Is likely to occur.
Therefore, by keeping the non-image portion potential on the image carrier sharp and negative, dust is suppressed and a high-quality image can be formed.
In particular, it can be confirmed from FIG. 1 that the electric field strength increases as the non-image portion potential is higher on the − side, and it is confirmed from FIG. 1 that, as shown in FIG. When held, dust is suppressed and a high-quality image can be obtained.

Then, with the present embodiment will be described in more detail with reference to an example of an image forming apparatus including the intermediate transfer. The image forming apparatus according to the present embodiment includes a photosensitive member 1, a developing device 2, a cleaning device 3, a charging device 4, a writing device 5, an intermediate transfer member 6, a transfer unit 7, and a fixing unit 8. I have.

  As shown in FIG. 4, reference numeral 1 denotes a photoconductor (an example of four independent photoconductors 1a, 1b, 1c, and 1d in order from the left). Since all the image forming portions around the photoconductor are provided in the same manner for all of a, b, c, and d, only a will be described below. The photoreceptor rotates counterclockwise. Reference numeral 4 denotes a charging device, 5 denotes a writing device, 2 denotes a developing device, and 3 denotes a cleaning device. The toner image developed on the photosensitive member 1 is attracted and transferred onto the intermediate transfer member 6 by an electric field generated by a voltage applied to the transfer bias roller as a first transfer process. Further, toner is transferred to the intermediate transfer member 6 at the portions b, c, and d, and a full-color image is formed on the intermediate transfer member 6. This full-color image is transferred to a transfer material by the transfer unit 7, and the transferred transfer material is heated and fixed by the fixing unit 8.

  As the photoconductor used in the electrophotographic system, a photoconductive layer mainly composed of selenium or a selenium alloy is provided on a conductive support, and an inorganic photoconductive material such as zinc oxide or cadmium sulfide is contained in the binder. Dispersed and amorphous silicon materials are known, but organic photoconductors are widely used due to cost, productivity, high degree of freedom in photoconductor design, and non-polluting properties. It has come to be used.

  As an organic electrophotographic photoreceptor, a photoconductive resin typified by polyvinyl carbazole (PVK), typified by PVK-TNF (polyvinyl carbazole 2,4,7-trinitrofluorenone). A charge transfer complex type, a pigment dispersion type typified by a phthalocyanine-binder resin, and a function separation type photoreceptor using a combination of a charge generation material and a charge transport material are known. Among these, from the viewpoint of sensitivity, durability, freedom of design, etc., a functional separation type in which a charge generation layer containing a charge generation material and a charge transport layer containing a charge transport material are laminated on a conductive support. Photoconductors are mainly used.

  As shown in FIG. 2, the mechanism of electrostatic latent image formation in this function-separated type photoconductor is that when the photoconductor is charged and then irradiated with light, the light passes through the transparent charge transport layer, By being absorbed by the charge generating material, charge carriers (charge carriers) are generated, and the charge carriers are injected into the charge transport layer, and further move in the charge transport layer according to the electric field, thereby neutralizing the charge on the surface of the photoreceptor. An electrostatic latent image is formed. In the function separation type photoreceptor, it is known to use a combination of a charge transport material having absorption mainly in the ultraviolet region and a charge generation material having absorption mainly from the visible region to the near infrared region, and This combination is useful.

  However, in the case of the above-described function-separated type photoreceptor, since the distance from the charge generation layer to the surface of the charge transport layer is long, there is a problem that the carrier is diffused during transport and the resolution is lowered. That is, as shown in FIG. 5, the carriers are attracted and transported by the charged potential on the surface of the charge transport layer, but at that time, they are also diffused in the lateral direction, and the carriers are transported from the beam spot diameter corresponding to one dot. The diameter expands and enters an adjacent image dot region.

  In addition, since the energy distribution of the exposure beam follows a Gaussian function (normal distribution), the tail is widened, and the carrier generated by the light at the tail is further diffused and superimposed on the adjacent image dot area. As a result, the resolution is further reduced. FIG. 6 is an explanatory diagram of exposure intensity distribution by adjacent beam spot diameters, and shows exposure intensity distribution by adjacent beam diameters of 600 dpi and beam diameter of 68.9 μm in exposure beam energy distribution.

  As described above, the potential of the non-image portion tends to shift toward 0 V due to carrier diffusion and beam shape. If this phenomenon weakens the electric field on the surface of the image bearing member and the Coulomb repulsive force of the toner becomes stronger, image dust will occur.

The inventors of the present invention have a structure in which the charge transport layer in the image carrier is composed of a dielectric and an insulator, and when an image carrier having an insulating region in a lattice shape in the charge transport layer is used, carrier diffusion is reduced. It has been found that it is possible to suppress the potential drop in the non-image area and suppress the image dust.
In this way, by incorporating a material that suppresses carrier movement into the charge transport layer and combining a high-performance image carrier that suppresses carrier diffusion and the intermediate transfer belt, the potential of the non-image area is held at a negative potential. In order to maintain the high electric field strength, it is possible to suppress toner dust and obtain a high-quality image.

  FIG. 7 shows an example of a schematic view of an image carrier in which an insulating material is provided in a lattice shape in the charge transport layer. With respect to a configuration example employing this image carrier, transfer performance and image evaluation were performed, and high-quality image forming means was examined. As shown in FIG. 7, the insulating region and the CTL (carrier transport layer (charge transport layer)) region are arranged following the film thickness direction of the image carrier, and the cross-sectional shape of the grating is not particularly limited. That is, the cross-sectional shape may be ○ or ◇, and the shape is not limited.

Here, various occupancy ratios of the CTL region in the CTL were variously manufactured to produce a photoconductor, and evaluation of good reproducibility of thin lines was performed.
For the occupancy ratio of the CTL region, a photoconductor with various sizes (diameters) of the CTL region was prepared, and an experimental machine obtained by remodeling the image using a Ricoh Ipsio Color 5100 was used.

First, paying attention to the good reproducibility of fine lines, the following image output experiment was conducted.
For good reproducibility of the thin line, a 6-cycle / mm line image was reproduced, and this line image was visually evaluated. The case where the line image was reproduced was indicated by ○, and the case where the line image was not reproduced was indicated by ×.

  It was confirmed that the reproducibility of the line was good by making the size of the CTL region smaller than the pixel (minimum unit capable of resolving).

  Next, a photoconductor with the size of the CTL region fixed to about 20 μm and the size of the insulating region changed was prototyped, and solid reproducibility was confirmed on each photoconductor. For image output, a solid image was output using the above-described experimental device, and solid ID measurement (density) was performed using visual observation and a Macbeth reflection densitometer. A good product was evaluated as ◯, and a defective product with missing or the like was evaluated as ×. The results are shown in Table 2. Here, the area occupancy ratio is calculated by calculating the ratio of the area of the insulating region to the pixel (the size of the smallest unit that can be resolved).

  As shown in Table 2, it was confirmed that the solid reproducibility was satisfied when the area occupation ratio of the insulating region was suppressed to 10% or less.

  As described above, in the function-separated type photoconductor (laminated photoconductor), when photocarriers pass through the charge transport layer, they are diffused in the surface direction of the charge transport layer and penetrate into the adjacent pixel region. In addition, the decrease in the charge in the non-image area occurs due to the spread of the bottom of the beam spot. In this embodiment, in order to prevent these problems, the charge generation layer is formed as a minute region dispersed in a dot shape. As a result, it is possible to use an image carrier capable of forming an electrostatic latent image having a sufficiently high contrast with a line image or a dot image while uniformly adhering toner to the entire surface of a solid image. Further, in the image forming unit that forms a color image using the intermediate transfer member, it is possible to provide a high-quality image forming unit in which toner dust is suppressed.

  In addition, since the material constituting this embodiment does not use a light-shielding material, a curable material, or the like, it can be used without changing from a conventional photoconductor, and uses a simple and accumulated photoconductor technology. As a result, it is possible to easily design a photoreceptor corresponding to high sensitivity and high image quality.

Hereinafter, the electrophotographic photosensitive member of this embodiment will be described with reference to the drawings.
FIG. 8 is a diagram showing a layer structure of the electrophotographic photosensitive member of the present embodiment. On the conductive support 31, a charge generation layer 35 containing a charge generation material as a main component and a charge containing a charge transport material. A transport layer 37 is laminated.

  FIG. 9 shows another example of the layer structure of the electrophotographic photosensitive member of the present embodiment. In FIG. 8, the intermediate layer 33 is provided between the conductive support 31 and the charge generation layer 35. Yes. Although the charge generation layer shown in these drawings is drawn as a uniform layer, it is actually configured to be dispersed as dots in a minute area.

  That is, the present embodiment includes a charge generation layer having at least a charge generation material on a conductive support as described above, and a charge transport layer containing at least a charge transport material, and the charge generation layer has a dot shape as a minute region. High-quality image formation in which toner dust is suppressed in an image forming means for forming a color image using an intermediate transfer member by using an electrophotographic photosensitive member characterized by being dispersed in It provides a means.

Regarding the effects and preferred conditions of the present embodiment, the reproducibility of isolated dots and the uniformity / density of black solid images were examined from the following simulations and experiments.
The simulation method is as follows.

The relationship between the occupied area ratio of the charge generation layer formed in the dot shape and the toner adhesion amount when the photosensitive member sensitivity and the exposure energy amount are varied is calculated by simulation.
It is known that the charge density distribution on the photoreceptor after exposure can be roughly grasped by using a PIDC (light generation discharge) curve. However, the difference in latent image distribution due to the effect that carriers generated in the CGL layer diffuse while moving through the CTL layer when the thickness of the photoconductor is changed, or in the vicinity of the CGL in the case of strong exposure. When analyzing the influence of the difference in latent image distribution caused by the recombination of positive and negative carriers with a PIDC curve, it is necessary to measure PIDC for each condition and for each film thickness. Further, it can be dealt with by actually measuring a PIDC curve at the time of a dot image, but it is impossible to measure a one-dot latent image distribution or potential distribution on a photoconductor with the current technology.

Therefore, the movement of carriers (charges) inside the photoconductor correctly takes into account the Coulomb repulsion between carriers, the recombination of carriers, and the mobility of carriers inside the photoconductor, and the latent image distribution and potential distribution are simulated. Need to predict.
The calculation method is described below. The calculation method consists of the following three steps.

1. Calculation of latent image charge distribution The latent image charge distribution on the photoreceptor after exposure is calculated. The movement of the carrier (charge) inside the photoconductor is affected by the Coulomb repulsion between the carriers, the recombination of the carriers, and the mobility (movement speed) of the carrier inside the photoconductor. Latent image formation simulation is required.
The physical model used to calculate the latent image charge distribution consists of (1) calculation of exposure amount by a Gaussian laser beam (Gaussian distribution laser beam), and (2) calculation of charge carrier generation and its transport process.

The present embodiment relates to an electrophotographic photosensitive member having an occupation area ratio (X) represented by the following formula (1) or more, while forming an electrostatic latent image having a sufficiently large contrast. In addition, an electrophotographic photosensitive member capable of uniformly adhering toner to the entire surface of a solid image can be obtained.
η × J ≧ 0.01 × EXP (− (X−60) / 10) +0.002 (1)
(In the formula (1), η represents quantum efficiency, and J represents exposure beam energy (J / m ² ).)

The quantum efficiency η in the above formula (1) is calculated by the following formula (2).
In the formula (2), C is a capacitance per unit area of the photoreceptor, e is an elementary charge of electrons, J is monochromatic light energy, V is a surface potential, and t is time. . The quantum efficiency η is a physical quantity independent of the film thickness and represents carrier generation efficiency (quantum efficiency of photoelectric conversion). The quantum efficiency η is obtained from the transient attenuation characteristics when exposed to monochromatic light (energy J per unit time) and the photosensitive member capacitance C, and is calculated by fitting to an equation of electric field dependency, η = αEn. To do.

  The exposure amount calculation is calculated by approximating the exposure amount distribution of the single stationary beam with the following formula (3) and integrating in the X direction by the moving distance (Vx * lighting time) in the lighting time.

When this amount of exposure is applied to the dot-like charge generation layer, only the portion where the charge generation layer exists is exposed. Therefore, the exposure distribution in the region without the charge generation layer is deleted, and the charge generation phase region is removed. Only the exposure distribution was taken out and used as the exposure distribution.
Next, FIG. 3 shows a conceptual diagram of charge carrier generation (2) and its transport in OPC. The process is governed by the following positive and negative carrier continuity equation and Poisson equation.

  Here, in the above formula, n, μ, E, Γ, R, ε, and e are the number density of carriers, mobility, electric field strength, amount of carriers generated per unit time, and number of carriers per unit time, respectively. The coupling coefficient, dielectric constant, and elementary charge are shown. Subscripts p and n indicate positive and negative carriers, respectively.

Charge carriers were assumed to be generated uniformly in the layer, taking into account the thin CGL layer. For this reason, the carrier generation amount Γ is connected to the incident light intensity F, the quantum efficiency η, and the thickness d of the CGL layer in the following relationship.
Γ = β · η · F / (d · hυ) (7)
In the above formula (7), β and hυ are the light absorption efficiency in the CGL layer and the energy per photon of the laser beam, respectively.
Further, the carrier recombination term of the second item on the right side of the above formulas (4) and (5) is for explaining that the amount of generated carriers decreases experimentally when positive and negative carriers coexist in the same vicinity. It has been introduced.

Among the physical quantities described above, the light absorption coefficient β and the recombination coefficient R are calculated by experimental fitting from the surface potential during black solid exposure.
Further, the quantum efficiency η is a value obtained from the equation (2) and is as described above.
Before the exposure, it is assumed that the photoreceptor is uniformly charged, and the charge amount on the surface of the photoreceptor is calculated. Thereafter, by calculating the above formula on the assumption that carriers are generated in the exposure region, the charge density distribution on the photoreceptor after exposure is calculated.

  As a calculation method, a difference method regarding np, nn, and φ was adopted. Here, φ represents a potential. The time forward difference is used for the time differential terms of the charge carrier continuity equations (4) and (5), and the upstream difference method is used for the advection term of the two items on the left side. Except for the time derivative term, a complete implicit method that evaluates implicitly is used.

  Here, δt, n, and n + 1 are a time step, a step number indicating the calculated current step, and a step number indicating the next future step, respectively.

The nonlinear algebraic equation thus obtained is calculated using a double iteration method. First, the whole three equations are calculated using a successive approximation method. In this iteration, equations (8) and (9) are evaluated for variables other than those appearing in the time derivative term, for example, quantities other than equation np, using the values of known iteration results. First, each formula is calculated based on Successive Over Relaxation. The Poisson equation (10) performs the same calculation for φ. Iterates until the residual by the successive approximation method for the whole expression reaches a specified value. Here, the designated values are designated for φ, nn, and np, respectively, 5 × 10 ⁻³ , 1 × 10 ⁻² , and 1 × 10 ⁻² .

2. Calculation of development electric field strength distribution The development electric field strength distribution is calculated from the latent image charge distribution on the photoconductor obtained in step 1 and the development conditions. It is assumed that the developing sleeve and the photoconductor are approximated by parallel plates, and the developer (carrier and toner) is uniformly filled between them. That is, the developer is treated as a uniform dielectric layer having an average dielectric constant. Here, the average dielectric constant is the product of the average relative dielectric constant and the vacuum dielectric constant, and the average relative dielectric constant ε ′ is ε ′ = a · ε1 + (1−a) (11)
It is obtained by. Here, ε1 is the relative dielectric constant of the carrier, and a is the proportion of the volume occupied by the carrier in the development nip.

The potential distribution can be obtained by solving the Poisson equation with the sleeve surface as the developing bias potential, the lower surface of the photoconductor as 0 V constant boundary conditions, and the left and right boundaries as periodic boundary conditions. E = -gradφ on the surface of the photoconductor
Is obtained, the development electric field strength distribution on the surface of the photoreceptor is obtained.

3. Calculation of toner adhesion amount 2. The toner adhesion amount is calculated from the electric field strength distribution obtained in step 1 and the development conditions.

Here, k is a toner adhesion coefficient, vr / vp is a sleeve linear velocity ratio, l is a developing nip width, R is a carrier radius, ε0 is a dielectric constant, and q is a toner charge amount. .
FIG. 10 shows the relationship between the charge generation layer area ratio and the toner adhesion amount obtained by the simulation.

As the charge generation layer area ratio increases, the toner adhesion amount increases. Further, the amount of toner adhesion increases by increasing the exposure energy.
Here, the relationship between the area ratio x of the charge generation layer and the toner adhesion amount M is:
M = ax ² + bx + c
Can be approximated by

Here, the coefficients a, b, and c can be approximated by a linear expression of Power.
Therefore, the relationship between the area ratio x of the charge generation layer and the toner adhesion amount M can be expressed by the following formula (13).
M = a (p) X ² + b (p) X + c (13)

Here, with respect to the reproducibility of the black solid image and the image density required for the black solid image, the toner adhesion amount per unit area is required to be 0.45 mg / cm ² or more, so the toner at 2 * 2 dots The required amount of adhesion can be calculated by the following formula.
Toner adhesion amount M ≧ 0.45 mg / cm ² = 0.45 / 1000000 * (2.54 / 1200) 2 * 4 = 8.065E-12 (kg)

Next, FIG. 11 shows the relationship between the charge generation layer area ratio (DM ratio) satisfying the toner adhesion amount M ≧ 0.45 mg / cm ² and the quantum efficiency * Power.
It can be seen that the quantum efficiency * Power and the charge generation layer area ratio have a certain relationship regardless of the OPC type. This can be approximated as shown below by an approximate expression represented by the following formula (14).

  Here, two types of OPC were calculated with different resolutions, but a = 0.01, b = 60, and c = 0.002 were obtained.

  Next, in the above, the area occupancy of the CGL dot portion is shaken in the range of 100% (photosensitive body that does not form a CGL in a normal dot shape) to 60% (area where a solid ID can be secured from the above simulation results). A photoconductor drum was prototyped, and an image was output using the obtained photoconductor to evaluate the image quality.

  As for image output, the following image output experiments were conducted by paying attention to two items: good reproduction of fine lines and stable density change in the highlight portion. First, with respect to good reproduction of fine lines, visual evaluation was performed using an image pattern with 400 lines described in FIG. 12 using an image output experimental machine with a resolution of 1200 dpi described below.

  The case where a 400-line line image was reproduced was marked as ◯, and the case where a 400-line line image was not reproduced was marked as x. The 400-line line image that is set as a criterion for determination here is whether or not the low-contrast character / color character is printed with a printer, the outline of the character part (not to become a rattling outline), and the color reproduction of the character This is set as a substitute characteristic for reproducing both items such as the property (reproducible target color).

Next, with respect to the stable density change in the highlight portion, an 8-bit Tiff image data including patches of 0 to 256 gradations (15 mm square) is used by using an image output experimental machine with a resolution of 1200 dpi described below. Output image data (post-pseudo halftone processed data) that has been subjected to dither processing (screen line number of about 170 lines, screen angle of 45 degrees, line screen type, quantization number of 2 bits, resolution of 1200 dpi) is prepared in advance. Then, image output is performed using the output image data. Evaluation of the output image was performed using the result of measuring the reflection density (ID value) of the highlight portion (data 0 to 32) with a spectral reflection densitometer (model 938 manufactured by X-Rite). A linear equation is approximated with respect to the relationship between the reflection density value and the input data value, and a case where the value of R ⁻² is 0.95 or more is given as ◯, and the value of R ⁻² is less than 0.95. The case was marked with x. The condition that the value of R ^ 2 of the linear expression approximation in the highlight portion (data 0 to 32) set as a criterion here is 0.95 or more is an abnormality called a pseudo contour in a gradation image This is set as a substitute characteristic for good reproduction without generating an image.

  Next, a description will be given of the experimental machine used for image output. This experimental machine is an experimental machine modified on the basis of Ricoh's Ipsio Color 5100, and is mainly a modified writing unit. In this experimental machine, a four-channel LD array in which four light emitting points (laser diodes) are formed in a line on one chip is used as an LD element. Thereby, a resolution of 1200 dpi was realized. In this experimental machine, the optical element and the aperture are adjusted so that the beam diameter is 40 (main scanning direction) × 40 (sub-scanning direction) μm on the photosensitive member position. Further, the measurement of the amount of static beam J of the laser beam (exposure beam energy is hereinafter referred to as the amount of static beam) was performed as follows. An optical power meter (model 3292) manufactured by Yokogawa Electric Corporation was used for measurement of the amount of static beam light. The probe (light receiving portion) of this optical power meter was placed at the laser beam imaging position (corresponding to the position where the photosensitive drum is placed) and set so that the laser beam was incident on the probe center. In this state, the output value of the optical power meter was recorded while continuously turning on the LD (laser diode), and the light amount was taken as a stationary beam. The light quantity was adjusted and set by these measurements.

Next, an image output experiment and results thereof will be described. Prototype photosensitive drums were produced in which the area occupied by the charge generation layer was varied in the range of 100 to 60%.
Using the photoconductor drum produced in this way, image output is performed while changing the amount of laser stationary beam, and the above two items (reproduction of 400-line image, highlight part is approximated by a linear expression, Tables 3 to 4 below show the results of evaluation on the value of R ^ 2 of 0.95 or more.

  When the occupied area ratio of the charge generation layer formed in the dot shape is larger than 90%, an electrostatic latent image having a sufficiently large contrast cannot be formed, and a highlight portion (reflection density is 0 to 0). In the region of about 0.2), the concentration changes suddenly, and an effect unique to this embodiment cannot be obtained.

  As shown in the results of Tables 3 and 4, when the occupation area ratio of the charge generation layer is 90% or less, the problem to be solved by the invention as described in the prior art is improved. It turns out that it is possible. Further, when the occupation area ratio of the charge generation layer is equal to or greater than a certain relationship (described separately), solid ID = 1.4 or more can be ensured, and good image quality can be realized.

  Next, a comprehensive test chart including image quality elements such as a highlight portion, a low-contrast character portion, and a solid portion was output using some of the conditions evaluated above. The results are shown in Table 5.

In Tables 3 and 4, it can be seen that high-quality images can be obtained even in the output results of the comprehensive test chart under the conditions where good results can be obtained.
Therefore, the occupying area ratio of the charge generation layer formed in the dot shape is 90% or less and the occupying area ratio (X) represented by the following formula (1) or more. The present invention relates to a body, and an electrophotographic photosensitive member capable of uniformly adhering toner to the entire surface of a solid image while forming an electrostatic latent image having a sufficiently large contrast can be obtained.

η × J ≧ 0.01 × EXP (− (X−60) / 10) +0.002 (1)
In the above formula (1), η represents quantum efficiency, and J represents exposure beam energy (J / m ² ).

By maintaining the non-image portion at a negative potential in Examples 1 and 2, it was possible to obtain an image forming unit that suppresses image dust and forms a high-quality image.
In an experimental machine equipped with this method, when an actual image was confirmed in detail, there were cases where the dot area varied and the image was poor in graininess.

In order to consider this phenomenon, FIG. 13 shows a relationship diagram between the potential on the image carrier of the dot image and the surface potential of the intermediate transfer member upstream from the contact portion with the image carrier.
As can be seen from the diagram shown in FIG. 13, the potential difference between the edge portion and the intermediate transfer member is larger than the potential difference between the dot central portion and the intermediate transfer member, and when this potential difference becomes large, discharge tends to occur. As a result, the polarity of the toner on the image carrier adhering to the periphery of the edge is changed, making it difficult to transfer, and it is considered that the dot area varies and deteriorates the graininess.

  Therefore, in order to prevent discharge around the edge, the following configuration was examined. FIG. 14 is an enlarged configuration diagram showing the periphery of the transfer configuration in an enlarged manner. The image forming apparatus of the present embodiment includes a photoreceptor 1, a developing device 2, a cleaning device 3, a charging device 4, a writing device 5, an intermediate transfer unit (intermediate transfer member) 6, a transfer unit 7, And a fixing unit 8. The intermediate transfer unit 6 includes an intermediate transfer belt 61, stretching rollers 62, 63, and 64, a counter bias blade 65, an intermediate transfer blade 66, and a belt cleaning device 67.

  Reference numeral 1 denotes a photoconductor, which is 1a, 1b, 1c, and 1d in order from the left. Since all the image forming portions around the photosensitive member are similarly provided for a, b, c, and d, only a will be described. The photoreceptor rotates counterclockwise. 4 is a charging device, 5 is laser writing light from a writing device, 2 is a developing device, and 3 is a cleaning device. As a first transfer process, the toner image developed on the photosensitive member is attracted and transferred onto the intermediate transfer member 6 by an electric field generated by a voltage applied to the transfer bias roller. Further, toner is transferred to the intermediate transfer member 6 at the portions b, c, and d, and a full-color image is formed on the intermediate transfer member 6. This full-color image is transferred to a transfer material by the transfer unit 7, and the transfer material is fixed by being heated by the fixing unit 8.

  In FIG. 14, the tandem image forming unit has four process units. Reference numeral 1 denotes a photoconductor, which is 1a, 1b, 1c, and 1d in order from the left, and a, b, c, and d are different in toner color, but the other configurations are almost the same. Therefore, only the configuration of “a” will be described in detail, and description of other process units will be omitted. The process unit includes a charging device 4, a writing device 5, a developing device 2, a cleaning device 3, and the like. While the image carrier 1a is rotationally driven counterclockwise in the figure by a driving unit (not shown), the surface of the image carrier 1a is uniformly charged by the charger 4 and becomes a non-image portion potential V. Then, the surface after uniform charging is irradiated with the laser writing light 5 to become the image portion potential VL, whereby an electrostatic latent image is formed. This electrostatic latent image is developed by a developing device 2 using toner as an image forming substance. The toner image on the photosensitive drum 1 a is intermediately transferred onto the intermediate transfer belt 61. The surface of the photosensitive drum 1a that has undergone the intermediate transfer process is cleaned of the transfer residual toner on the surface by the cleaning device 3 of the photosensitive drum. Similar processes are performed in other process units, and Bk, Y, M, and C toner images are formed.

  On the other hand, as described above, the intermediate transfer unit 6 includes the intermediate transfer belt 61, the three stretching rollers 62, 63, 64, the belt cleaning device 67, and the like. Also provided are four counter bias blades 65, four intermediate transfer blades 66, a belt cleaning device 67, and the like. The intermediate transfer belt 61, which is an intermediate transfer member, is stretched clockwise by the rotation of any one of the stretching rollers by driving means (not shown) while being stretched by the three stretching rollers 62, 63, 64. Move to. The four counter bias blades 65a, 65b, 65c, and 65d and the four intermediate transfer blades 66a, 66b, 66c, and 66d are arranged so as to contact the back surface (inner peripheral surface) of the intermediate transfer belt 61, respectively. . A counter bias blade and an intermediate transfer blade with the same symbol form one pair. For example, the counter bias blade 66 and the intermediate transfer blade 66 form a pair. The intermediate transfer blades 66a, 66b, 66c, and 66d are disposed between the tandem image forming unit and the intermediate transfer unit 6 by disposing the intermediate transfer belt 61 in the direction of the photosensitive drums 1a, 1b, 1c, and 1d, respectively. The four intermediate transfer nips for Bk, Y, M, and C are formed.

  An intermediate transfer electric field acts on these intermediate transfer nips by applying an intermediate transfer bias to the intermediate transfer blades 66a, 66b, 66c, and 66d by a power source (not shown). Here, a counter bias blade 65 is positioned upstream of the intermediate transfer nip in the rotation direction of the intermediate transfer member. A bias is applied to the counter bias blade 65 from a power source (not shown). By applying an electric field in the direction in which the toner on the image bearing member is not attracted to the intermediate transfer member to the counter bias blade 65, the discharge around the edge is prevented, and stable regardless of the variation with time and the variation of the developer. It became possible to improve the graininess by suppressing the variation of dots.

  As described above, according to the photosensitive member and the image forming apparatus of the present embodiment, the charging unit for charging the image carrier, the exposure unit for forming an electrostatic latent image on the image carrier, and the latent image on the image carrier. Developing means for developing the image with toner having the same polarity as the latent image; first transfer means for transferring the toner image onto the intermediate transfer member; and a second transfer means for transferring the toner image on the intermediate transfer member to the recording member. In the image forming means for forming an image by the transfer means, the image forming means in which the potential of the unexposed portion corresponding to the background portion of the electrostatic latent image is held at ½ or more of the maximum value in absolute value, In a color image forming method using an intermediate transfer member, it is possible to suppress image deterioration in a transfer process and provide an image with high image quality.

In addition, by setting the occupation area ratio of the charge generation layer formed in a dot shape to 90% or less and making the relationship between the occupation area ratio and the exposure energy within the range of the above-described formula (1), “the fine line is good It is possible to obtain an image that achieves both “reproducibility” and “stable density change in the highlight portion”, and to obtain a sufficient image density even when placed on a black solid image.
Furthermore, pre-transfer of the toner on the image carrier before transfer and discharge to the toner on the photoconductor before transfer are prevented, image dust is prevented, and a stable image is formed regardless of changes with time and variations in developer. can do.

FIG. 6 is an explanatory diagram illustrating a relationship between an image carrier non-image portion potential and transfer dust during toner transfer according to the present exemplary embodiment. It is a conceptual diagram which shows the model of the charge carrier production | generation in the image carrier of this embodiment, and its transport. FIG. 6 is an explanatory diagram illustrating a relationship between an image carrier surface potential of a dot image and an intermediate transfer material surface potential of the present embodiment. 1 is a schematic cross-sectional view showing a configuration of an image forming apparatus mounted with an intermediate transfer member according to the present embodiment. It is a conceptual diagram which shows the carrier movement process in the image carrier which concerns on this embodiment. It is exposure explanatory drawing by the adjacent beam spot diameter, and is the figure which showed the exposure intensity distribution by the adjacent beam diameter of 600 dpi and the beam diameter of 68.9 micrometers in the energy distribution of the exposure beam. FIG. 3 is a perspective view of an image carrier in which insulating regions are provided in a lattice shape in the charge transport layer according to the present embodiment. It is sectional drawing which shows the structure of an example of the image carrier used for this embodiment. It is sectional drawing which shows the structure of another example of the image carrier used for this embodiment. FIG. 6 is an explanatory diagram showing a relationship between a charge generation layer area ratio of an image carrier used in the exemplary embodiment and toner adhesion. It is explanatory drawing which shows the relationship of toner adhesion in this embodiment, a charge generation layer area rate, and quantum efficiency. It is a top view for description of a 400 line image pattern used for this embodiment. FIG. 6 is an explanatory diagram illustrating a relationship between an image carrier potential of a dot image and an upstream intermediate transfer member surface potential in the present embodiment. It is a schematic structure sectional view expanding and showing the transfer composition periphery of this embodiment.

Explanation of symbols

1a, 1b, 1c, 1d Photoconductor 2 Developing device 3 Cleaning device 4 Charging device 5 Writing device (laser writing light)
6 Intermediate transfer body 7 Transfer section 8 Fixing section 31 Conductive support 33 Intermediate layer 35 Charge generation layer 37 Charge transport layer 61 Intermediate transfer belt 62, 63, 64 Tension roller 65 Counter bias blade 66 Intermediate transfer blade 67 Belt cleaning device

Claims (1)

Charging means for charging the image carrier, exposure means for forming an electrostatic latent image on the image carrier, and development for developing the latent image on the image carrier with toner having the same polarity as the latent image An image forming method in which an image is formed by a first transfer unit that transfers a toner image onto an intermediate transfer member, and a second transfer unit that transfers the toner image on the intermediate transfer member to a recording member. The image carrier is provided on a conductive support and comprises a charge generation layer having at least a charge generation material and a charge transport layer containing at least a charge transport material, wherein the charge transport layer is a charge transport material. And the insulating material is provided in a lattice shape as a region surrounding the charge transport material, and the region of the insulating material and the region of the charge transport material are arranged in succession in the film thickness direction, and The area of insulating material is less than 10% Ri, the image forming method characterized in that the potential of the non-image portion of said image bearing member was kept below -200 V.

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