JPH0876532A - Image forming device - Google Patents

Abstract

PURPOSE: To obtain a high definition image with improved graininess even in the case of increasing the number of lines in order to accomplish the high definition by setting a pitch of solid image density with reference to the developing potential of a developing means to a specified value. CONSTITUTION: After uniformly and electrostatically charging the surface of a photoreceptor drum 1 at a prescribed potential by an electrostatic charging corotron 2, an electrostatic latent image is formed by a laser output scanner(ROS) 3, toner images in respective colors successively formed by developing devices 4 to 7 are superposed on top of one another and transferred to a transfer paper 9 placed on a transfer drum. At this time, by setting the beam diameter of the laser beam by the ROS 3 0.8 times as much as a pixel pitch, an exposure energy profile with high contrast is formed, and in accordance with the profile, the surface potential profile with high contrast is formed on the surface of the photoreceptor drum 1. Besides, by setting the pitch of the solid image density with reference to the developing potential >=2.0 in the developing devices 4 to 7, the high density of the image is rapidly obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus for forming an image by developing an electrostatic latent image formed on a photosensitive medium using a laser beam.

[0002]

2. Description of the Related Art Conventionally, an image forming apparatus that forms an image by developing an electrostatic latent image formed on a photosensitive medium using the above laser beam can provide an image of high speed and high print quality. As a device that adopts the possible recording method,
Various technologies relating to digital electrophotographic printers and copying machines have been proposed and are already in use.
In these image forming apparatuses such as digital electrophotographic printers and copying machines, the emission time of the semiconductor laser is controlled by the emission time control circuit, and the laser beam emitted from this semiconductor laser is converted into parallel light by a collimator lens. After that, the laser beam converted into the parallel light is applied to a polygon mirror rotating at a high speed, and the laser beam applied to the polygon mirror is reflected and deflected as the laser beam is rotated. Scanning exposure is carried out along the axial direction of the photoconductor drum via, and an electrostatic latent image according to image information is formed on the photoconductor drum. Then, the electrostatic latent image formed on the photosensitive drum is visualized by a known electrophotographic process,
It is configured to form an image.

By the way, in these digital electrophotographic printers and copying machines, a semiconductor laser is turned on / off as two-dimensional image information at a predetermined location on a photosensitive drum based on characters and image data. Binary image information is provided. As described above, when a halftone image is recorded using a method of providing binary image information for turning on / off a semiconductor laser as two-dimensional image information when forming an image on the photoconductor drum, Conventionally, the area modulation method using the halftone dot structure or the parallel line structure has been widely used.
The area modulation method using the halftone dot structure and the parallel line structure has been used in many digital electrophotographic printers, copiers, etc. because the algorithm is relatively simple and the cost is low. .

Further, as a method of forming a halftone image which does not use the area modulation method as described above, there is a method of modulating the intensity of a light beam (hereinafter referred to as an intensity modulation method). The use of this intensity modulation method has an advantage that a density-modulated image from which image noise generated when the area modulation method is selected is removed can be obtained. Specifically, the intensity modulation of the light beam is performed by adopting a method of changing the current of the semiconductor laser or a method of using an ultrasonic light modulator.

However, this intensity modulation method uses a special circuit such as an ultrasonic optical modulator or a complicated circuit that changes the current of the semiconductor laser according to image information when modulating the intensity of the light beam. Therefore, the structure becomes complicated and expensive, and the area modulation method described above is actually adopted in a printer, a copying machine, etc. of a digital electrophotographic system.

Further, as a developing device for developing a halftone electrostatic latent image formed on the photosensitive drum as described above, for example, Japanese Patent Application Laid-Open No. 5-6127 filed by the present applicant.
A two-component developing method using a developing bias in which an AC / DC voltage of a specific frequency is superposed on the developing bias disclosed in Japanese Patent No. 1 is used.

The developing method disclosed in Japanese Unexamined Patent Publication No. 5-61271 is a magnetic brush of a developer in which an insulating toner and magnetic particles are mixed on the surface of a developer carrier containing a magnetic pole whose position is fixed. And the magnetic brush is brought into contact with or close to an electrostatic latent image carrier on which an electrostatic latent image is formed by uniform charging and exposure, and the electrostatic latent image carrier and the developer carrier. In the developing method of applying a bias voltage containing an AC component between the insulating toner and the electrostatic latent image to visualize the electrostatic toner, the insulating toner has an average particle size of 6 μm or more and 8 μm or less. The magnetic particles have an electric resistivity γ (Ω · cm) of 1
Using the range of 0 ⁸ ≦ γ ≦ ¹⁰ 10, the frequency f of the AC component of the bias voltage (Hz) is, 4000 ≦ f ≦
It is configured to be set in the range of 8000.

Further, when the area modulation method is adopted in the current printers and copying machines of the digital electrophotographic system, for example, the screen frequency is 200 lines (200 lines).
Lines / inch, 8 lines / mm) are adopted, but it is desired to increase the number of screen lines for the purpose of realizing higher-definition line reproducibility and character reproducibility.

[0009]

However, the above-mentioned prior art has the following problems. That is, in the above-mentioned conventional image forming apparatus, if the number of screen lines is simply increased for high definition, the developed line image has a blurred edge portion, and is particularly important as the image quality of a full-color image. There is a problem that the graininess deteriorates.

This is considered to be based on the following reasons. In the above conventional image forming apparatus, as described above, the emission time of the semiconductor laser is controlled by the emission time control circuit, the laser beam emitted from the semiconductor laser is converted into parallel light by the collimator lens, and then the parallel light is emitted. The polygon mirror rotating at high speed is irradiated with the laser beam converted into light, and the laser beam irradiated on the polygon mirror is reflected and deflected as the laser beam is rotated. Scanning exposure is performed along the axial direction of the body drum, and an electrostatic latent image according to image information is formed on the photoconductor drum. Then, in order to control the irradiation of the laser beam based on the image signal, a line screen or a dot screen is used.

The charge distribution of the electrostatic latent image thus formed on the photosensitive drum is shown in FIG. 13 by the exposure energy distribution of the semiconductor laser and the light attenuation characteristic (PIDC characteristic) of the photosensitive drum. The edge portion has a shape in which the potential changes gently from a medium potential to a low potential, not a complete on / off binary value. FIG. 13 shows a case where the screen has a line number of 200 and the beam diameter in the main scanning direction is 72 μm. By the way, when the edge part where the potential changes gently from the medium potential to the low potential in this way is developed, it becomes a solid image from the low density to the medium density where the toner particles are isolated and scattered, and the edge of the developed line image There is a problem that the image becomes a blurred portion, and the graininess, which is important as the image quality of a full-color image, deteriorates. Such a phenomenon is seen in the density modulation image.

Further, the electrostatic latent image formed by the density modulation as described above is developed by the two-component developing device adopting the developing method disclosed in Japanese Patent Laid-Open No. 5-61271. The density of the solid image is as shown in FIG. When this developing method is adopted, the gradient γ of the displacement of the solid image density with respect to the developing potential is a value smaller than 2.0. here,
The low-to-medium potential portions corresponding to the low-to-medium charge portions have been developed to show low-to-medium density. As shown in FIG. 14, the graininess (roughness) in the low to medium density portions is as follows.
It is larger than in the extremely low density portion and the high density portion, and this tendency becomes more remarkable as the toner particle size increases.

Next, FIG. 1 shows the charge distribution of the electrostatic latent image when the screen ruling is increased to 400 and 800 lines.
5 shows. As is clear from FIG. 15, in the case of the screen frequency of 400 lines, the width of the low to medium charge portion is 20.
It can be seen that the number of lines is larger than that in the case of the 0 line, and the edges are sparsely developed with a wider width with toner particles, the ends of the lines are blurred, and the graininess is deteriorated. Further, even when the screen frequency is 800, the charge distribution becomes flat,
The line structure is no longer formed, and the charge distribution is the same as in the case of concentration modulation.

Therefore, the present invention was made in order to solve the problems of the above-mentioned prior art, and its purpose is to achieve high definition even when the number of lines is increased for higher definition. An object of the present invention is to provide an image forming apparatus capable of obtaining an image with good graininess.

[0015]

In order to solve the above problems, the image forming apparatus of the present invention forms an electrostatic latent image on an electrostatic latent image carrier and then develops the electrostatic latent image. In the image forming apparatus for forming an image by developing by means of means, the gradient of the solid image density with respect to the developing potential of the developing means is set to 2.0 or more as shown in FIG.

As the above-mentioned electrostatic latent image carrier, for example, a photosensitive drum is used, but it is not limited to this, and a dielectric drum or photosensitive paper may be used.

As a means for forming the electrostatic latent image on the electrostatic latent image carrier, for example, a laser beam exposing means for scanning and exposing a laser beam according to image information is used, but is not limited thereto. However, other electrostatic latent image forming means such as an ion current modulating means and an LED exposing means may be used.

Further, as the developing means, for example,
A developing means using a non-magnetic two-component developer is used, but a developing means using a non-magnetic one-component toner or a developing means using a liquid developer may be used.

[0019]

In the image forming apparatus according to the present invention, in order to obtain good line reproducibility, character reproducibility and good graininess in a high-definition image, the width of the edge portion of the image in which the toner is sparsely developed is made as small as possible. Achieved by For that purpose, the gradient of the solid image density with respect to the developing potential determined by the developing method and the developing conditions of the developing means for developing the electrostatic latent image formed on the electrostatic latent image holding member is increased to a low level.
It is important to reduce the width of the medium density. By setting the gradient of the solid image density with respect to the development potential to be 2.5 or more, good line reproducibility, character reproducibility and good graininess can be obtained in a high definition image.

Here, the gradient γ of the change in the solid image density with respect to the developing potential is defined as the magnitude of the density change with respect to the developing potential change in the low to medium potential, as shown in FIG. Here, in the case of regular development, the horizontal axis is the potential obtained by subtracting the applied voltage of the DC component applied to the developing electrode at the time of development from the surface potential obtained by initial charging of the electrostatic latent image carrier, and reversing In the case of development, the potential obtained by subtracting the applied voltage of the DC component applied to the developing electrode during development from the overall exposure potential of the electrostatic latent image carrier is set to 1.0. The vertical axis represents the maximum solid image density of 1.0.

In the image forming apparatus according to the present invention,
The electrostatic latent image formed on the electrostatic latent image holder based on the document or the image information has the surface charge profile determined as shown in FIG. 3 according to the exposure energy profile by the laser beam as follows. To be done. FIG. 4 shows an exposure energy profile on the electrostatic latent image carrier when the pixel pitch for main scanning is changed with respect to a constant laser beam diameter in the main scanning direction. When the laser beam diameter is smaller than the pixel pitch, the unevenness of the exposure energy profile increases as shown in FIG. 4 (a), and area modulation in which exposed portions and non-exposed portions are repeated is obtained. On the other hand, when the laser beam diameter increases with respect to the pixel pitch, the unevenness of the exposure energy profile decreases and the exposure intensity changes to intensity modulation. Similar results are obtained in the sub-scanning direction of the laser beam.

For example, when the laser beam diameter is 0.5 times the pixel pitch, an exposure energy profile with large unevenness is obtained as shown in FIG. 4A, and therefore the potential profile on the electrostatic latent image holding member is also obtained. Correspondingly large irregularities are obtained. On the other hand, when the laser beam diameter becomes twice the pixel pitch, an electrostatic latent image is artificially created by intensity modulation as shown in FIG. Here, for example, comparing the unevenness of the exposure energy profile with an area ratio of 20%, it is about 2.5 times larger when the laser beam diameter is 0.5 times the pixel pitch. In response to this, the latent image potential contrast also increases, the surface potential contrast also increases,
The width of low to medium potential is also narrowed. Therefore, when development is performed in which the solid image density has a large displacement with respect to the development potential, a sharply high image density is obtained at low to medium potentials. Therefore, the width of sparse development of toner is narrowed, and an image having excellent line reproducibility and graininess can be obtained. FIG. 5 shows a conceptual diagram of a developed image.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to illustrated embodiments.

FIG. 6 is a block diagram of a printer mechanism section showing an embodiment of the image forming apparatus according to the present invention.

In FIG. 6, reference numeral 1 denotes a photoconductor drum as an electrostatic latent image holding member having a photoconductor (OPC photoconductor) made of a polymer compound on its surface. The photoconductor drum 1 is shown in the figure. It is configured to be rotationally driven at a predetermined rotational speed (160 mm / sec) in the direction of arrow A by a drive device that does not. The surface of the photosensitive drum 1 is uniformly charged to a predetermined potential by the charging scorotron 2 and then the laser ROS 3 (Raster Out) is used.
The image exposure according to the image information of the first color is performed by a put scanner) to form an electrostatic latent image. The electrostatic latent image formed on the photosensitive drum 1 is developed by the developing device 4 for the first color and becomes a toner image. The first color toner image formed on the photosensitive drum 1 is
It is transferred by charging the transfer corotron 10 onto the transfer paper 9 held on the surface of the transfer drum 8 arranged adjacent to the photosensitive drum 1. The transfer paper 9 held on the surface of the transfer drum 8 is conveyed from one of the paper feed trays 11 and 12 at a predetermined timing prior to the transfer of the toner image, and is mechanically or quietly transferred onto the surface of the transfer drum 8. Held by electronic means. After that, the cleaning device 13 cleans the surface of the photosensitive drum 1 after the transfer of the toner image, such as residual toner. Then, the surface of the photosensitive drum 1 is uniformly charged again by the charging scorotron 2, and then the second color image exposure is performed by the laser ROS 3 to form an electrostatic latent image. The electrostatic latent image of the second color formed on the photosensitive drum 1 is developed by the developing device 5 of the second color to become a toner image. After that, the toner image formed on the photosensitive drum 1 is transferred onto the transfer sheet 9 held on the transfer drum 8 in a state of being superposed with the toner image already transferred.

By repeating the above operation for the images of the third and fourth colors, for example, cyan, magenta, yellow and black are transferred onto the transfer paper 9 held on the transfer drum 8. Toner images of four colors are transferred in a state of being superimposed on each other.

The transfer paper 9 on which the four color toner images have been transferred is separated from the surface of the transfer drum 8 and is then conveyed to the fixing device 15 by the conveyor belt 14 and the four color toner images on the transfer paper 9. A color toner image obtained by melting and mixing is fixed. The transfer paper 9 on which the color toner image is fixed is discharged onto a discharge tray 16 provided outside the apparatus, and the color image forming process is completed.

The fixing device 15 is composed of a fixing roll 15a and a pressure roll 15b which are rotatably driven while being pressed against each other. The fixing roll 15a is
A surface of an aluminum core having a diameter of 46 mm is coated with a highly heat-conductive silicone rubber having a thickness of 2.0 mm, and further a viton rubber having a thickness of 40 μm is coated as a surface release layer.
The fixing roll 15a is supplied with a mercapto-modified silicone oil as a release agent at a rate of 10 mg per copy. The pressure roll 15b has the same configuration as the fixing roll 15b except that the thickness of the silicone rubber is 1.0 mm. The fixing roll 15a and the pressure roll 15b have a nip width of 6.0 mm and a pressure of 5 kg /
It is rotationally driven by being brought into pressure contact at a speed of cm ² and 160 mm / sec. Furthermore, the surface temperature of both the fixing roll 15a and the pressure roll 15b is controlled to 150 ° C. by a temperature controller. By passing the transfer paper through the fixing device, the toner is heated and pressure-fixed.

FIG. 7 is a schematic configuration diagram showing a laser ROS 3 used in the image forming apparatus of FIG.

The laser ROS 3 comprises a semiconductor laser 21 whose light emission is controlled by a light emission time control circuit 26, a collimator lens 22, a polygon mirror 23, and an fθ lens 24. Then, the semiconductor laser 21 uses the light emission time control circuit 26 based on the image information.
The laser beam 25 modulated by the so-called pulse width modulation method is emitted.
Laser beam 25 emitted from this semiconductor laser 21
Is a polygon mirror 23 through a collimator lens 22.
Of the fθ lens 2
Scanning exposure is performed on the photoconductor drum 1 via 4 along the main scanning direction (axial direction of the photoconductor drum 1).

The laser ROS 3 forms a halftone image by controlling on / off of the semiconductor laser 21 by the light emission time control circuit 26. The laser beam spot diameter on the photoconductor drum 1 is set to 0.8 times the pixel pitch. The resolution in the main scanning direction is 400 spi,
When the resolution in the sub scanning direction is 400 spi, the spot diameter of the laser beam is set to 51 μm which is 0.8 times the pixel pitch 63.5 μm in both the main scanning direction and the sub scanning direction. A line screen is used for the irradiation of the laser beam based on the image signal.

FIG. 8 shows the first used in the image forming apparatus.
To 4th developing devices, and the 1st to 4th developing devices are all configured in the same manner except for the developer.

In FIG. 8, reference numeral 30 denotes a housing of the developing device. The developing device housing 30 contains a developer 31, and the developing device housing 30 is a photosensitive drum. The end on the side facing 1 is open. This developing device housing 30
A non-magnetic cylindrical developing sleeve 32 as a developer carrying member is arranged at the opening end of the inside, and the developing sleeve 32 rotates counterclockwise at a rotational speed of 320 mm / sec to develop the developer. 31 is carried to the surface of the photosensitive drum 1. Inside the developing sleeve 32, a magnet roll 33 having a magnet of a predetermined polarity arranged at a predetermined position is fixedly arranged. Further, the developing sleeve 3
Reference numeral 2 is arranged close to the surface of the photoconductor drum 1, and a portion where the surfaces are close to each other is a developing area. Further, the magnet roll 33 arranged inside the developing sleeve 32 has a plurality of magnets arranged so that the S poles and the N poles are alternately located, and the magnetic field formed between the adjacent magnetic poles. To develop the magnetic brush of magnetic particles by the developing sleeve 3
The developer 31 is formed on the surface of No. 2, and the developer 31 can be conveyed as the developing sleeve 32 rotates.

Further, two screw augers 34, 35 for supplying the developer to the surface of the developing sleeve 32 are rotatably arranged inside the developing device housing 30, and these screw augers 34, 35 are provided. The toner supplied from outside by 35 is supplied to the surface of the developing sleeve 32 while being agitated with the developer 31. The developer 31 supplied to the surface of the developing sleeve 32 is regulated to have a predetermined layer thickness by the developer regulating member 36, and is transferred to a developing area facing the photosensitive drum 1 as the developing sleeve 32 rotates. It is carried and developed. One end of the developer regulating member 36 is supported and fixed to the developing device housing 30 and the other end is attached so as to project close to the surface of the developing sleeve 32, and the developer is attracted to the surface of the developing sleeve 32. Three
1 is regulated to a fixed amount. Further, a DC superimposing AC voltage is applied as a developing bias voltage from the DC power source 37 and the AC power source 38 to the developing sleeve 32, and electric charges are generated by an electric field formed at a position (developing region) close to the photosensitive drum 1. The toner having the toner adheres to the electrostatic latent image formed on the photosensitive drum 1. The applied voltage has a direct current component as a negative voltage, and the development potential contrast (difference between the direct current component voltage and the overall exposure potential of the photosensitive drum 1) is 300 V with respect to the light portion charging voltage of -200 V of the photosensitive drum 1. It is set to be a degree.

In this embodiment, as the developer 31, a two-component developer prepared by mixing magnetic particles and non-magnetic insulating toner particles is used. The magnetic particles have an electric resistivity (volume resistance) of 10 ⁷ to
A ferrite carrier having an average particle size of 10 ⁹ Ω · cm and a mean particle size of 50 μm is used. As each color toner of magenta, cyan, and yellow, a toner particle having a mean particle size of 5 to 6 μm made of a color pigment and a polyester resin is a black toner. As the toner particles, toner particles having an average particle diameter of 7 to 8 μm made of carbon black pigment and polyester resin are used.

The electric resistivity γ (Ω ·
cm) is the voltage applied per length of 1 (V / μm) in which magnetic particles are placed in a cell of fixed volume sandwiched between two electrodes.
Is the resistance value.

Further, the gap between the developing sleeve 32 and the photosensitive drum 1 is arranged to be 500 μm. In this state, the AC power source 38 and the DC power source 37 provide a frequency of 8 KHz and a peak-to-peak voltage of 1.5 kV. , A DC superimposed AC voltage having a DC component of −500 V is applied to the developing sleeve 32. Further, in this embodiment, the dark potential of the electrostatic latent image is set to -650V and the bright potential of the electrostatic latent image is set to -200V.

The gradient γ of the solid image density with respect to the developing potential of the solid image formed by the area modulation method by the developing devices 4 to 7 constructed as described above is measured by the inventors, and as a result, the developing characteristics shown in FIG. Among them, the gradient γ was 2.5.

With the above-mentioned structure, the image forming apparatus according to this embodiment realizes good line reproducibility, character reproducibility, and good graininess in a high-definition image as follows. . In the image forming apparatus according to the above-described embodiment, the beam diameter of the laser beam from the laser ROS 3 is set to 0.8 times the pixel pitch, and the beam diameter of this laser beam is set to 0.8 times the pixel pitch. As shown in FIG. 4, the area-modulated latent image obtained in this case has exposure energy with a higher contrast than the pseudo-intensity-modulated latent image obtained when the laser beam diameter is 1.5 times the pixel pitch or more. A profile is formed. Then, according to this exposure energy profile, a surface potential profile with a large contrast is formed on the surface of the photoconductor drum 1.

Further, the developing devices 4 to 7 according to this embodiment.
Then, as shown in FIG. 1, the gradient γ of the displacement of the solid image density with respect to the developing potential is set to a large value of 2.5, and as shown below, a steeply high image density is obtained at low to medium potentials. To be Therefore, the width of sparse development of toner is narrowed, and an image having excellent line reproducibility and graininess can be obtained.

That is, in this embodiment, since the beam diameter of the laser beam is set to 0.8 times the pixel pitch as described above, an exposure energy profile with large unevenness can be obtained as shown in FIG. Accordingly, a potential profile with correspondingly large irregularities is obtained, the surface potential contrast is also increased, and the width of the low to medium potential is narrowed. Therefore, when development is performed in which the solid image density has a large displacement with respect to the development potential, a sharply high image density is obtained at low to medium potentials. Therefore, the width of sparse development of toner is narrowed, and an image having excellent line reproducibility and graininess can be obtained. FIG. 5 shows a conceptual diagram of a developed image.

FIG. 9 shows the relationship between the ratio of the beam diameter of the laser beam to the pixel pitch, the gradient γ of the image density with respect to the developing potential, and the line reproducibility and graininess. From Table 1, sufficient image quality is obtained when the beam spot diameter in the main scanning direction on the photosensitive drum 1 is 1.0 times or less the pixel pitch in the main scanning direction and the image density gradient with respect to the developing potential is 2.5 or more. You can see that you can get it. Similar results are obtained in the sub-scanning direction of the laser beam.

FIG. 10 is a graph showing the relationship between the image density gradient γ with respect to the developing potential, the line reproducibility and the graininess, which the inventors of the present invention obtained as a result of an experiment. As is clear from this figure, good line reproducibility and graininess can be obtained if the gradient γ of the image density with respect to the development potential is 2.0 or more.

Second Embodiment FIG. 11 shows a second embodiment of the present invention. The same parts as those of the above-mentioned embodiment are designated by the same reference numerals and explained.
The second embodiment is configured to form an image by forming an electrostatic latent image formed on the electrostatic latent image carrier by a developing device using a non-magnetic one-component toner.

That is, the printer mechanism portion of the image forming apparatus according to the second embodiment is the same as that of the embodiment shown in FIG. The spot diameter of the laser beam 25 scanning-exposed on the photosensitive drum 1 is 0.8 pixel pixel pitch.
It is set to double. When the resolution on the photosensitive drum 1 in the main scanning direction is 600 dpi, the spot diameter of the laser beam 25 is set to 34 μm in both the main scanning direction and the sub scanning direction. A line screen is used for image exposure based on an image signal for driving the semiconductor laser 21 of the laser ROS 3.

FIG. 11 shows a developing device using the non-magnetic one-component toner according to the second embodiment.

In the developing device housing 40, toner 4 is supplied from a toner storage box (not shown) disposed outside the housing 40.
The toner 41 that has been conveyed to the inside of the developing device housing 40 is supplied to the surface of the toner carrier 43 by the toner supply member 42. The toner carrier 43 is usually formed to have a diameter of 5 to 40 mm. After cutting a cylindrical member or a cylindrical member made of aluminum or stainless steel, the circumferential surface is subjected to sandblasting, liquid honing, emery polishing, or other mechanical processing. Surface roughness Ra = 0.1
Use a product with irregularities of about 1.0 μm, or after cutting a cylindrical or cylindrical member made of aluminum or stainless steel, provide a semiconductive layer of phenol resin or the like on the circumferential surface, and perform emery polishing or the like. After mechanical polishing, Ra = 0.
A surface roughness of about 1 to 1.0 μm is used,
Desirably, a surface roughness of Ra = 0.2 to 0.4 μm is used.

As the toner carrier 43, there may be used a roll of aluminum which is mechanically polished and then anodized. When the semiconductive layer is provided on the toner carrier 43, the volume resistance value of the surface layer of the toner carrier in the thickness direction is set to about 10 ⁵ to 10 ¹² Ωcm. Further, the rotation number of the toner carrier 43 is 1
It is set to 00 to 300 rpm. Further, a predetermined developing bias is applied to the toner carrier 43 by a developing bias power source 44. The developing bias applied to the toner carrier 43 is usually -5.
1000 with a DC bias voltage of 0 to -400V superimposed
The AC bias voltage is ˜4000 Vpp, the frequency of the AC bias is 1 to 5 kHz, and the value obtained by dividing the V bias of the AC bias voltage by the distance between the toner carrier and the photosensitive drum 1 is in the range of 4 to 7 V / μm. Then, the frequency is used as 2.5 to 4.0 kHz. The voltage applied to the toner carrier has a DC component as a negative voltage, and the development potential contrast (the DC component voltage and the overall exposure potential of the photosensitive drum 1) with respect to -200 V which is the light portion charging voltage of the photosensitive drum 1. The difference) is set to about 300V.

The toner amount held on the surface of the toner carrier 43 is regulated to a predetermined amount by the toner regulating member 45. As the toner regulating member 45, a plate spring made of stainless steel having a thickness of about 0.03 to 0.3 mm and vulcanized with Si or EPDM rubber is used, and the contact pressure to the toner carrier 43 is 20 to 200 g /
The toner regulating member 45 forms a thin layer having a thickness of about 5 to 30 μm and imparts a charge of about 2 to 20 μC / g.

Further, the toner supply member 42 for supplying the toner to the toner carrier 43 has a cylindrical member made of aluminum or stainless steel having a diameter of 10 to 20 mm and a thickness of 1 to 4 mm, and has several openings on the peripheral surface thereof. It is provided, faces the toner carrier 43 with a gap of about 0.5 to 2.0 mm, and is rotationally driven at a peripheral surface speed of about 1 to 5 times that of the toner carrier 43. Further, a predetermined bias voltage is applied to the toner supply member 42 by a supply member bias power supply 46. Bias power supply 46 for the supply member applied to the toner supply member 42
Supplies a DC bias voltage of about 200 to 1000 V between the toner supply member 42 and the toner carrier 43, and the polarity of the toner supply member 42 is set to the same polarity as the toner. Further, in order to prevent the toner 41 from filming on the surface of the toner supply member 42, the Mylar 4 having a thickness of about 50 to 500 μm is formed on the surface of the toner supply member 42.
You may make 7 etc. contact.

Then, the toner supply member 42 is formed by the bias power supply 46 for the toner supply member, and the toner supply member 42 is formed by the electric field between the toner supply member 42 and the toner carrier 43.
The toner is supplied from the toner carrier 43 to the toner carrier 43. The toner 41 supplied onto the toner carrier 43 is sufficiently charged by the frictional charging of the toner regulating member 45, and a thin layer of the toner 41 is formed. The toner 41 formed in a thin layer on the toner carrier 43 develops the electrostatic latent image formed on the photosensitive drum 1 by the developing bias.

The toner 41 used in the developing device configured as described above is a non-magnetic one-component toner, and is a pigment such as carbon black in various thermoplastic resins such as styrene resin, acrylic resin and polyester resin. Disperse a polarity control agent such as a metal-containing azo dye or the like and grind and classify
The size is about 20 μm, and a charge control agent is externally added. As the charge control agent, fine particles of 0.1 μm or less such as silica, alumina, and titanium which have been hydrophobized are used, and hydrophobic silica is most preferable.

As the photosensitive drum 1, Se is used.
System photoconductors and organic photoconductors are usually used, and the photoconductor drum 1
And the toner carrier 43 of the developing devices 4 to 7 is usually 100 to
Opposed with a gap of about 400 μm, preferably 150-
They are opposed to each other with a gap of about 300 μm.

The gradient γ of the displacement of the solid image density with respect to the developing potential of the developing devices 4 to 7 thus constructed has a large value of 4.

According to the image forming apparatus using the developing device of this embodiment, the beam diameter of the laser beam is set to 0.8 times the pixel pitch.
The area-modulated latent image obtained by this laser beam has an exposure energy profile with greater unevenness than the pseudo-intensity-modulated latent image obtained when the beam diameter is 1.5 times the pixel pitch or more. . A surface potential profile having a large contrast is formed according to the exposure energy profile.

Further, the main developing system has a large gradient γ of the displacement of the solid image density with respect to the developing potential, which is as large as 4.
A sharply high image density can be obtained at a medium potential. Therefore, the width of sparse development of toner is narrowed, and an image having excellent line reproducibility and graininess can be obtained.

The other structure and operation are the same as those in the first embodiment, and therefore the description thereof is omitted.

Third Embodiment FIG. 12 shows a third embodiment of the present invention. The same parts as those of the above-mentioned embodiment are designated by the same reference numerals and will be described.
The third embodiment is configured to form an image by forming an electrostatic latent image formed on the electrostatic latent image carrier by a developing device using a liquid developing system.

That is, in the image forming apparatus according to this embodiment, the spot diameter of the laser beam on the photosensitive drum 1 is set to 0.8 times the pixel pitch. When the resolution of the laser beam in the main scanning direction is 600 spi and the resolution in the sub scanning direction is 600 spi, the spot diameter of the laser beam is 42 μm in both the main scanning direction and the sub scanning direction.
m. A line screen is used for the irradiation of the laser beam based on the image signal.

FIG. 12 shows an image forming apparatus using a wet type developing device according to the third embodiment of the present invention.

In FIG. 12, reference numeral 1 denotes a photosensitive sheet carrying drum capable of carrying a photosensitive sheet 51 on its surface. The photosensitive sheet carrying drum 1 is a cylindrical conductive member made of aluminum or the like. It is composed of a base body, and is rotationally driven at a predetermined speed in the direction of arrow A by a driving means (not shown). Further, the photosensitive sheet 51 is supplied to the surface of the photosensitive sheet carrying drum 1 from a supply cassette (not shown), and the conveying roller 50
Thus, the photosensitive sheet 51 is conveyed at a predetermined timing. Then, on the surface of the photoconductor sheet supporting drum 1, the photoconductor sheet 5 is subjected to electrostatic force by a charger or mechanical force by a gripper.
1 is carried in a wound state.

The photosensitive sheet 51 is, for example,
A thin film conductive support made of aluminum or the like coated with zinc oxide, titanium oxide, or the like, which is an inorganic photoconductor, is used. The photoconductor sheet 51 is held on the surface of the photoconductor sheet carrying drum 1,
The thin-film conductive support is brought into contact with the drum 1 and is connected to the ground via the photosensitive sheet carrying drum 1.

The surface of the photoconductor sheet 51 carried on the surface of the photoconductor sheet carrying drum 1 is uniformly charged to a predetermined potential by the primary charger 2 composed of a scorotron or the like. The image is scanned and exposed by the laser ROS 3 to form an electrostatic latent image.

Next, the pre-wetting liquid is applied by the pre-wetting means 54 to the surface of the photosensitive sheet 51 on which the electrostatic latent image corresponding to the image information is formed as described above.
The pre-wetting means 54 is rotatably arranged in the pre-wet liquid reservoir 54a containing the pre-wetting liquid and in the pre-wet liquid reservoir 54a, and is disposed close to the photoconductor sheet 51 via a minute gap. Alternatively, it is provided with a roll-shaped member 54b arranged so as to be in contact with each other.
The prewetting means 54 holds the prewetting liquid on the surface of the rotationally driven roll-shaped member 54b due to the viscosity of the liquid, and the roll-shaped member 54b makes the prewetting liquid uniform on the surface of the photoreceptor sheet 51. It is designed to be applied. In the non-contact type in which the roll-shaped member 54b of the pre-wetting means 54 does not contact the surface of the photoconductor sheet 51, the amount of the pre-wetting liquid supplied between the gaps can be adjusted by the rotation speed of the roll-shaped member 54b. Further, as the prewetting liquid,
The carrier liquid used in the developing device described later can be used as it is.

By carrying out the pre-wetting step in this manner, it is possible to prevent the background stain which is caused by the unnecessary toner which is electrically or physically adsorbed to the non-image portion during the development.

As the above-mentioned pre-wetting means, a blade, a sprayer or the like can be used in addition to the above. In general, the non-contact type is more effective because it does not disturb the electrostatic latent image. Is.

Next, the electrostatic latent image on the photoconductor sheet 51 coated with the prewetting liquid is developed by any one of the developing devices 4 to 7 of the wet developing devices arranged below the photoconductor sheet carrying drum 1. Is visualized by. These four wet developing devices 4 to 7 are arranged so as to be movable along the horizontal direction, and one selected developing device is vertically moved to a developing position close to the photosensitive sheet carrying drum 1. It is possible to move along.

The four wet developing devices 4 to 7 are for color developing using four color toners, and each of cyan (C), magenta (M), yellow (Y) and black (Br). It is composed of a liquid developing device.

The liquid developer supplied to the wet developing devices 4 to 7 contains Isopar L (Exxon Co.) as a dispersant.
A liquid developer containing is used. The liquid developer is supplied from a supply pump (not shown) to the developing electrode 4a curved with the same curvature as that of the photosensitive sheet carrying drum 1, circulates in the developing device, and then returns to the supply pump. It is like this.

Immediately after the development, the excess toner in the developing solution on the photoconductor sheet 51 is removed by the sleeve 4b which rotates in a direction opposite to the conveying direction while maintaining a constant gap with the photoconductor sheet carrying drum 1. To be done.

Further, the drum 1 for carrying the photoconductor sheet.
The upper photoconductor sheet 51 is the drum 1 for carrying the photoconductor sheet.
And a developing electrode 4 that holds a constant distance, for example, 200 μm
It is conveyed to a and developed. The surface potential of the electrostatic latent image is -2
The voltage is 50 V, and a bias voltage of -10 to -100 V is applied to the developing electrode during development in order to prevent electric fog in the non-image area.
Is applied.

The developing electrode 41a is heated to 40.degree. As a result, the developing solution around the developing electrode is heated and the viscosity of the developing solution is reduced to 2/3 of that at 20 ° C., and the developing efficiency is increased. At this time, the gradient γ of the displacement of the solid image density with respect to the developing potential was 2.5.

Further, the photosensitive sheet 51 after the developing process is dried by the drying and fixing means 57 which blows warm air or dry air supplied from a blower (not shown), and the developing toner is fixed on the photosensitive sheet 51. To be done.

The photoconductor sheet 51 is exposed by the erase lamp 58 arranged so as to face the photoconductor sheet 51, and the residual charge on the photoconductor sheet 51 is neutralized.

These processes are repeated for the required number of colors, after which the photoconductor sheet 1 is peeled off by the peeling claws 59, and is conveyed by the conveyance belt 60 to the discharge tray provided outside to carry the photoconductor sheet. The surface of the drum 1 is cleaned by a cleaning device 61 driven by a driving means (not shown), and the whole process is completed.

In the above-mentioned structure, in this embodiment, the image development is steeply high at low to medium potentials by the main developing method in which the variation γ of the solid image density with respect to the developing potential is as large as 2.5. Therefore, the width of sparse development of toner is narrowed, and an image having excellent line reproducibility and graininess can be obtained.

In this embodiment, in order to set the gradient of the solid image density with respect to the developing potential of the developing means to 2.0 or more, any means for increasing the gradient of the solid image density can be applied. For example, control of the charge amount of the developer, control of the color material concentration, control of the distance between the developing electrode and the electrostatic latent image carrier, supply of the developer between the developing electrode and the electrostatic latent image carrier. For example, control of quantity.

The other structure and operation are the same as those of the above-mentioned embodiment, and the explanation thereof will be omitted.

[0079]

EFFECT OF THE INVENTION The present invention has the above-described structure and operation. Even when a high-definition image is reproduced, toner particles are sparsely developed at the edge portion of a line image or dot image forming the image. The area to be covered is reduced, and an image having excellent line reproducibility and graininess can be obtained.

[Brief description of drawings]

FIG. 1 is a graph showing developing characteristics of an image forming apparatus according to the present invention.

FIG. 2 is a graph showing developing characteristics of the image forming apparatus according to the present invention.

FIG. 3 is a graph showing a potential profile of an electrostatic latent image of the image forming apparatus according to the present invention.

FIG. 4A to FIG. 4D are graphs showing exposure profiles of the image forming apparatus according to the present invention.

5A and 5B are schematic diagrams respectively showing graininess of a developed image of the image forming apparatus according to the present invention.

FIG. 6 is a block diagram showing an embodiment of an image forming apparatus according to the present invention.

FIG. 7 is a configuration diagram showing a laser ROS.

FIG. 8 is a configuration diagram showing a developing device.

FIG. 9 is a chart showing image quality with respect to a ratio of a beam spot diameter and a pixel pit.

FIG. 10 is a graph showing image quality with respect to potential gradient.

FIG. 11 is a configuration diagram showing a developing device of an image forming apparatus according to a second embodiment of the present invention.

FIG. 12 is a configuration diagram showing an image forming apparatus according to a third embodiment of the present invention.

FIG. 13 is a graph showing a potential profile of an electrostatic latent image in a conventional image forming apparatus.

FIG. 14 is a graph showing the relationship between image density and graininess.

15A and 15B are graphs showing potential profiles of electrostatic latent images in a conventional image forming apparatus, respectively.

[Explanation of symbols]

1 photoconductor drum, 4 to 7 developing device, 32 developing sleeve, γ gradient of solid image density with respect to image potential.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazuhiko Arai 2274 Hongo, Ebina City, Kanagawa Prefecture Fuji Xerox Co., Ltd.

Claims (1)

[Claims] 1. An image forming apparatus for forming an image by forming an electrostatic latent image on an electrostatic latent image bearing member and then developing the electrostatic latent image by a developing means. The image forming apparatus is characterized in that the gradient of the solid image density with respect to is set to 2.0 or more.