JP2002023469A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make resolution compatible with gradation by using the proper diameter of a light beam spot in accordance with recording density. SOLUTION: When the diameter of the light beam spot on a photoreceptor drum in the case of exposure by an exposure device is Db and a pixel pitch decided according to the recording density of an image is Dp, it is controlled so that the relation of Db<3Dp is satisfied. Thus the high-definition image excellent in the gradation is obtained without impairing the density reproducibility of a highlight part even when the recording density of the image is increased.

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 such as a printer, a digital copying machine, a facsimile apparatus, etc., which forms an image by an electrophotographic method.

[0002]

2. Description of the Related Art In an image forming apparatus using an electrophotographic method in recent years, the demand for resolution and gradation is increasing more and more. Is relatively small, so that sufficient gradation cannot be obtained.

Here, the beam spot diameter Db is defined as follows. As shown in FIG. 6, the spot diameter of the light beam is represented by a portion (B) until it decreases to 1 / e ² of the peak intensity (A). As the light intensity distribution, there are a Gaussian distribution, a Lorentz distribution, and the like, and a portion ab up to 1 / e ² of each peak intensity is defined as a spot diameter Db.

Since the light spot generally has an elliptical shape as shown in FIG. 6, the spot diameter cd in the longitudinal direction of the photoconductor (image carrier) is set to the main scanning spot diameter Dbh, and the rotation of the photoconductor is performed. The spot diameter ef in the direction to the sub-scanning spot diameter D
bv, the beam spot diameter Db is Dbh, Dbv unless the direction is specified in the present invention.
Is considered to include both.

It is generally known that the gradation characteristic changes depending on the beam spot diameter. When the beam spot diameter is larger than the pixel density, the reproducibility of a low density portion (highlight portion) in an image deteriorates. This is because when writing an isolated dot, the latent image becomes shallow due to the low exposure energy density, and the reproducibility of the developed dot becomes unstable. On the other hand, in the high-density portion of the image, the adjacent pixels are exposed so as to overlap, so that the image density is quickly saturated with respect to the image area ratio,
A so-called gamma phenomenon occurs. That is, the gradation is deteriorated. In addition, by increasing the writing exposure amount per dot, it is possible to reproduce an isolated dot, but the formed dot becomes large and the gradation is further deteriorated.

In order to improve the resolution while maintaining the gradation, it is necessary to reduce the beam spot diameter according to the pixel density. In order to reduce the beam spot diameter, for example, in a scanning optical system such as a laser, the wavelength of a laser beam is reduced, or the NA of an fθ lens is reduced.
(Aperture ratio). on the other hand,
In a solid-state optical system such as an LED array, there are a method of using a selfoc lens array (SLA) having a small aperture angle and a method of reducing the shape of a light emitting element.

In image forming apparatuses, high resolution and high gradation, which were conventionally difficult to achieve due to problems such as mechanical accuracy and cost, are now being put to practical use in many products due to technological progress.

[0008]

However, even in the case of exposure using a beam having a beam spot diameter equal to the size of a pixel, sufficient gradation can be obtained in recent high recording density images. It is hard to say that there is. In particular, the reproducibility in a highlight portion tends to deteriorate as the recording density increases. The reason is that, in the formation of isolated dots, although a charge distribution corresponding to small-diameter dots is obtained in the formation of a latent image, a dot image larger than the target dot diameter is formed by the edge effect in development, and The toner in the image area is returned toward the developing roller by the rubbing of the toner image by the magnetic brush having the counter charge after development, so that the dot area becomes large and the image density reproducibility in the highlight area is increased. Is worse.

Further, in forming a high recording density image of 1200 dpi, since the diameter of the isolated dot becomes smaller, it becomes difficult to form an isolated dot image in development, and the image density reproducibility of a highlight portion deteriorates. .

Accordingly, the present invention provides an optical beam spot having an appropriate diameter corresponding to the recording density in order to obtain a high quality image which can achieve both resolution and gradation even when the image has a high recording density. It is an object to provide an image forming apparatus using the same.

[0011]

According to the present invention, there is provided an image carrier, an exposure apparatus for exposing the image carrier to form an electrostatic latent image, and an image forming apparatus formed on the image carrier. And a developing device for developing the electrostatic latent image, wherein the developing device is configured such that a magnet roller included in a developer carrier has a main magnetic pole adjacent to a main magnetic pole close to the image carrier. When an auxiliary magnetic pole for adjusting the magnetic force and half-value width is provided, the light beam spot diameter on the image carrier at the time of exposure by the exposure device is Db, and the pixel pitch determined by the image recording density is Dp. , Db <3Dp.

Further, in order to solve the above-mentioned problem, the present invention proposes that the relationship of Db> 0.8 Dp is established. According to another aspect of the present invention, there is provided an image carrier, an exposure apparatus configured to expose the image carrier to form an electrostatic latent image, and an electrostatic latent image formed on the image carrier. An image forming apparatus including a developing device for developing an image, wherein the developing device is configured such that a magnet roller included in a developer carrying member has a magnetic force of a main magnetic pole adjacent to the main magnetic pole close to the image carrying member. When an auxiliary magnetic pole for adjusting a value width is provided, a light beam spot diameter on the image carrier when exposing by the exposure device is Db, and a pixel pitch determined by an image recording density is Dp, Db> 0 It is proposed that the relationship 0.8Dp is established.

According to another aspect of the present invention, there is provided a laser scanning system in which the exposure apparatus forms an electrostatic latent image on the image carrier by scanning a laser beam in a main scanning direction. Suggest that there is.

According to another aspect of the present invention, a light beam spot diameter in the sub-scanning direction on the image carrier at the time of exposure by the exposure device is determined by Dbv, and the sub-scanning direction is determined by an image recording density. It is proposed that, when the pixel pitch in the direction is Dpv, the relationship of Dbv> 0.8 Dpv holds.

According to another aspect of the present invention, there is provided an exposure apparatus, wherein the exposure apparatus comprises: a light emitting element array in which a plurality of LED elements are arranged on a line in a main scanning direction; It is an LED array head system including a lens array that forms an image on a body, and it is proposed that the LED array head be arranged to face the image carrier.

In order to solve the above-mentioned problems, the present invention provides a main scanning device which determines a light beam spot diameter in the main scanning direction on the image carrier in exposure in the main scanning direction by Dbh and an image recording density. It is proposed that when the pixel pitch in the direction is Dph, the relationship of Dbh> 0.8 Dph holds.

Further, in order to solve the above-mentioned problems, the present invention proposes that the image forming apparatus is a color image forming apparatus.

[0018]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a cross-sectional view illustrating a configuration of an image forming unit in an example of an image forming apparatus according to the present invention.

In FIG. 1, a charging device 2 such as a scorotron for uniformly charging the surface of a photosensitive drum 1, which is an electrostatic latent image carrier, and an exposure device 3 to be described later A developing device 4 for forming a toner image by attaching a charged toner to a latent image on the drum surface formed by light, an intermediate transfer device 5 for carrying the formed toner image on the drum, and removing residual toner on the drum Cleaning devices 6 and the like are arranged in order.

In the present embodiment, the developing device 4 stores cyan, magenta, yellow, and black toners.
This is a so-called revolver-type developing device in which two developing devices rotate sequentially and move to a developing position.

In such a configuration, a toner image of each color is formed on the photoreceptor 1 and sequentially transferred onto a belt of the intermediate transfer device 5 to obtain a toner image in which colors are superimposed on the belt. This is transferred to a recording paper conveyed from a paper tray (not shown),
The recording paper is fixed on the recording paper by a fixing device (not shown). On the other hand, the photosensitive drum 1 is initialized by the charge removing lamp, and is used for the next image forming process. The toner remaining on the photosensitive drum 1 without being transferred is collected by the cleaning device 6.

FIG. 2 is a perspective view showing the configuration of the exposure apparatus according to the present embodiment. As shown in this figure, the exposure device 3 is a so-called laser optical system including a laser light emitting element 31, a collimator lens 32, an aperture 33, a cylindrical lens 34, a polygon mirror 35, an fθ lens 36, and the like. The light beam emitted from the laser light emitting element 31 is converted into a parallel light beam by the collimator lens 32 and is incident on the cylindrical lens 34. The light beam is condensed in the sub-scanning direction by the cylindrical lens 34 and is incident on the polygon mirror 35. The light beam is scanned by the polygon mirror 35 in the main scanning direction parallel to the rotation axis direction of the photosensitive drum. The light beam scanned in the main scanning direction is adjusted by the fθ lens 36 so that the scanning angle and the scanning distance are proportional to each other, and is converged in the sub-scanning direction to form an image on the photosensitive drum 1.

When a laser optical system is used, the recording density of an image can be easily changed by changing the rotation speed of the polygon mirror and changing the clock of laser irradiation in the main scanning direction. Further, instead of changing the rotation speed of the polygon mirror, the recording density can be similarly changed by changing the linear speed of the photoconductor.

As shown in FIG. 1, the developing device 4 is of a revolver developing system in which developing units for each color are integrated to constitute a developing unit. The developing unit can be driven to rotate around the revolver shaft in the direction of the arrow (counterclockwise in the figure), and the developing unit rotates so that the developing roller of the developing device comes to a position facing the photoconductor for each color. Development is performed in the order of K (black) → Y → C → M. The developer is a two-component developer composed of a toner and a carrier. The average particle size of each color toner is 6.8 μm, and the average particle size of the carrier is 50 μm.

Although the structure and operation of the developing device 4 will be described later, in the present embodiment, the magnet roller included in the developing roller is arranged so that the magnetic force of the main magnetic pole and the half-value width are adjacent to the main magnetic pole close to the photosensitive member. , The thinning of the horizontal line image and the white spot phenomenon in the halftone image are improved, and the developing ability can be kept high. Further, the edge effect can be reduced due to the developing conditions (the developing gap is small), and the reproducibility in a low density portion (highlight portion) in an image which is difficult to be reproduced in a conventional image forming apparatus (normal magnet roller) is obtained. It has been found that the gradation is improved because of the improvement.

FIG. 3 shows the results of examining the tone reproducibility by performing image output under the following conditions in the image forming apparatus of this embodiment. Beam spot diameter Db
Is 30, 50, 70, 90, 110, 130 [μm] 6
The types and recording densities of images are 400, 600, 1200 [d
pi] (Dp is 63.5, 42.3, respectively)
21.2 [μm]), a 256-step patch by binary error diffusion was output. The beam spot diameter was changed by changing the size of the opening of the aperture 33 in the exposure apparatus 3, and the pixel pitch was changed by changing the recording density described above. The horizontal axis of FIG. 3 represents the ratio of the beam spot diameter to the pixel pitch: Db / Dp, and the vertical axis represents the linearity of the area ratio gamma, the density reproducibility of the highlight portion, and the maximum density (solid black) as the gradation rank. Part)
Are comprehensively evaluated, and the larger the value, the better.

It has been found that the gradation rank satisfies the specified value when 3.5 or more, and Db / Dp at that time is 0.8 or more and 3 or less. In a region where Db / Dp is larger than 3, the area ratio gamma rises, and the density reproducibility of the highlight portion is poor. In this case, the number of gradations that can be expressed is reduced, so that a photographic image or the like is an image having a remarkably lack of smoothness. When Db / Dp is 0.
In the region smaller than 8, the image density of the solid black portion was not sufficiently obtained. This may be because the latent image is not completely buried for the input signal having the image area ratio of 100%, even though the exposure is fully lit.

In the case where a laser optical system is used as the exposure device as in this embodiment, the laser beam is irradiated while scanning in the main scanning direction (longitudinal direction of the photosensitive member). When the laser is fully turned on to form a solid black image, the laser is scanned in the main scanning direction for the irradiation time, so the latent images in adjacent pixels in the main scanning direction have overlap. . However, the overlap of the latent images in the pixels in the sub-scanning direction (the rotation direction of the photoconductor) is determined only by the beam spot diameter Dbv in the sub-scanning direction of the laser. Accordingly, in order to sufficiently obtain the image density of the solid black portion, the relationship between the beam spot diameter Dbv in the laser sub-scanning direction and the pixel pitch Dpv in the sub-scanning direction determined by the recording density needs to satisfy Dbv> 0.8 Dpv. There is. This is a unique condition when the exposure apparatus is a laser optical system.

As described above, the light beam spot diameter Db on the photosensitive drum 1 at the time of exposure by the exposure device
And the pixel pitch Dp determined by the image recording density is set to Db <3Dp, so that even when the image recording density increases, the density reproducibility of the highlight portion is not impaired, and high-definition gradation production is achieved. An excellent image can be obtained.

By setting Db> 0.8 Dp, the relationship between the image area ratio and the image density at the time of gradation expression is maintained linearly regardless of the image recording density, and
A sufficient black solid density can be obtained.

Then, Db <3Dp and Db> 0.8D
By setting it to p, even if the recording density of the image is increased, sufficient black solid density can be obtained without deteriorating the density reproducibility of the highlight portion, and high definition and excellent gradation can be obtained. An image having a stable image density can be obtained.

Further, when the exposure apparatus is of a laser type, exposure is performed using the same beam in the main scanning direction (L
In the case of the ED array, exposure is performed by the beams of the light emitting element arrays arranged in the main scanning direction), so that a stable exposure beam spot diameter can be obtained regardless of the position (position) in the main scanning direction.

In the case of the laser system, since the relationship between the beam spot diameter Dbv in the sub-scanning direction of the laser and the pixel pitch Dpv in the sub-scanning direction is Dbv> 0.8 Dpv, even if the laser type exposure apparatus is used, It is possible to define the conditions of the exposure beam spot diameter in the sub-scanning direction so that the solid image density can be sufficiently obtained.

Next, a second embodiment of the present invention will be described. In the color image forming apparatus according to the second embodiment shown in FIG. 4, a photosensitive drum 1, a charging device 2, an exposure device 3
b, a developing device 4 b, a cleaning device 6, and four image forming units.
Each image forming unit corresponds to each color of cyan, magenta, yellow, and black, and the image forming units are arranged in series. The color image is obtained by superimposing and transferring the toner images formed in the respective image forming units on the recording paper held on the transfer belt 5b sequentially, and fixing them by the fixing device 7.

The developing device 4b according to the present embodiment includes a first
The structure is the same as that of each color developing unit of the developing device 4 in the embodiment, and will be described later. FIG. 5 is a cross-sectional view illustrating a configuration of the exposure apparatus 3b according to the present embodiment.

As the exposure device 3b, an LED array head which is a solid-state optical writing device is used. The LED array head has a large number of LED elements arranged in the main scanning direction. By controlling lighting of each LED element based on an image signal, optical writing is performed on the photoconductor 1 and an electrostatic latent image is formed. You.

FIG. 5 shows a cross section of the LED array head and the photoconductor on a plane perpendicular to the central axis of the photoconductor 1. The light emitting element array substrate 30 has a light emitting element array 31 in which a plurality of light emitting elements are arranged linearly along the generatrix of the photosensitive drum 1. Reference numeral 32 denotes an image forming lens, which includes the light emitting element array substrate 30 and the photosensitive drum 1
And passes the light emitted from the light emitting element array 31 to form an image on the surface of the photosensitive drum.

The light emitting element array 31, the light emitting element array substrate 30, and the image forming lens 32 are main components constituting an LED array unit 33 as an exposure device. As the imaging lens 32, a selfoc lens array (hereinafter, referred to as SLA) having good light collecting properties is widely used. In this embodiment, SLA12D having an opening angle of 12 degrees is used.

A similar experiment was conducted on the tone reproducibility performed in the above embodiment using the image forming apparatus of the present embodiment having the above-described configuration. FIG. 3 shows a case where a laser optical system was used as the exposure apparatus. The same data could be reproduced.

The means for changing the beam diameter and changing the pixel pitch determined by the recording density is based on the former (change of the beam diameter) by changing the distance between the LED array and the photosensitive member and defocusing the beam. The latter (change of the pixel pitch) was performed by changing the arrangement pitch of the light emitting element array 31 of the LED array head.

As in this embodiment, the exposure apparatus is an LE
When the D array head is used, the light emitting elements are arranged in the main scanning direction (longitudinal direction of the photoconductor) and are fixed and driven to be lit, so that scanning is performed in the rotation direction of the photoconductor (sub scanning). Is the same as When the light-emitting elements are fully lit to form a solid black image, light scanning is performed in the sub-scanning direction for the lighting time, so that latent images in adjacent pixels in the sub-scanning direction overlap. However, the overlap of the latent images in the pixels in the main scanning direction (longitudinal direction of the photoconductor) is caused by the beam spot diameter D in the main scanning direction on the photoconductor.
It depends only on bh. Therefore, in order to sufficiently obtain the image density of the solid black portion, the pixel pitch D in the main scanning direction determined by the beam spot diameter Dbh in the main scanning direction and the recording density.
The relationship of ph needs to be Dbh> 0.8 Dph. This is a unique condition when the exposure device is an LED array head.

As described above, also in the present embodiment using the LED array head, by setting the relationship between the light beam spot diameter Db and the pixel pitch Dp to Db <3Dp, even when the image recording density increases. It is possible to obtain an image with high definition and excellent gradation production without impairing the density reproducibility of the highlight portion.

By setting Db> 0.8 Dp, the relationship between the image area ratio and the image density at the time of gradation expression is maintained linearly regardless of the image recording density, and
A sufficient black solid density can be obtained.

Then, Db <3Dp and Db> 0.8D
By setting it to p, even if the recording density of the image is increased, sufficient black solid density can be obtained without deteriorating the density reproducibility of the highlight portion, and high definition and excellent gradation can be obtained. An image having a stable image density can be obtained.

Further, by using an LED array head as the exposure device, it is possible to increase the image recording density without increasing the size of the device, and even in such a case, without deteriorating the reproducibility of the density of the highlight portion. In addition, a sufficient black solid density can be obtained, and an image having high definition, excellent gradation, and always having a stable image density can be obtained.

In the case of the LED array head system, the relationship between the beam spot diameter Dbh in the main scanning direction and the pixel pitch Dph in the main scanning direction determined by the recording density is Dbh> 0.8.
By setting Dph, it is possible to define the condition of the exposure beam spot diameter in the main scanning direction such that a sufficient image density of a solid black portion can be obtained even in an LED array head type exposure apparatus.

In the color image forming apparatuses such as the first and second embodiments, the tone reproduction of a color photographic image or the like is strictly required. According to the present invention, the condition of the exposure beam spot diameter can be defined so that a sufficient black solid image can be obtained without impairing the density reproducibility of the highlight portion when the recording density of the image is increased. It is possible to obtain an image which is excellent in gradation and always has a stable image density.

Hereinafter, the developing device used in the image forming apparatuses of the first and second embodiments will be described. FIG. 7 is a cross-sectional view showing an example of a configuration of a developing device for each color in the developing device 4 of the first embodiment or a developing device 4b in the second embodiment. In FIG. 7, they are collectively represented as a developing device 4.

As shown in FIG. 7, in the developing device 4, a developing roller 41, which is a developer carrying member, is disposed so as to be close to the photosensitive drum 1. A development area where the drum and the magnetic brush are in contact is formed. The developing roller 41 is configured such that a developing sleeve 43 formed of a non-magnetic material such as aluminum, brass, stainless steel, or conductive resin in a cylindrical shape is rotated clockwise in the figure by a rotation driving mechanism (not shown). I have. As an example, the drum diameter of the photosensitive drum 1 is set to 60 mm, the linear velocity of the drum is set to 240 mm / sec, the sleeve diameter of the developing sleeve 43 is set to 20 mm, and the linear velocity of the sleeve is set to 600 mm / sec. Therefore, the ratio of the sleeve linear speed to the drum linear speed is 2.5. The developing gap, which is the distance between the photosensitive drum 1 and the developing sleeve 43, is 0.4 m.
m.

In FIG. 7, an upstream portion of the developing area in the developer conveying direction (clockwise direction in the figure) includes:
A doctor blade 45 that regulates the height of the developer chain, that is, the amount of the developer on the developing sleeve, is provided. The doctor gap, which is the distance between the doctor blade 45 and the developing sleeve 43, is set to 0.4 mm. Further, a screw 47 for pumping the developer in the developing device casing to the developing roller 41 while stirring the developer in the developing device casing is provided in a region of the developing roller opposite to the photosensitive drum.

In the sleeve 43 of the developing roller, a magnet roller body (magnet roller) 44 for forming a magnetic field so as to cause the developer to spike on the peripheral surface of the developing sleeve 43 is fixedly provided. Along with the normal magnetic field lines emitted from the magnet roller body, a carrier of the developer is spiked in a chain on the developing sleeve 43, and the charged toner is attached to the carrier sprung in the chain, A magnetic brush is configured. The magnetic brush is transferred in the same direction as the developing sleeve 43 (clockwise as viewed in the figure) by the rotation of the developing sleeve 43. The magnet roller body 44 has a plurality of magnetic poles (magnets). More specifically, the developing main magnet P1b for causing the developer to spike in the developing area, the main magnetic pole magnetic force forming auxiliary magnets P1a and P1c for assisting the formation of the developing main magnetic pole magnetic force, and the developing sleeve 4
A magnet P4 for pumping up the developer, magnets P5 and P6 for transporting the pumped developer to the development area, and magnetic poles P2 and P3 for transporting the developer in the area after development are provided on the surface 3. These magnets P1b, P1a, P1c, P4, P
5, P2 and P3 are arranged radially of the developing sleeve 43. In this example, the magnet roller body 44 is
Although it is constituted by pole magnets, the number of magnets (magnetic poles) is further increased between the P3 pole and the doctor blade 45 in order to improve the pumping property and the black solid image followability, thereby increasing the number of poles to 10 poles or 12 poles.
It may be composed of poles.

As shown in FIG. 7 in particular, the main developing pole group P1 is composed of magnets having small cross sections arranged in order from the upstream side in the order of P1a, P1b and P1c. These magnets having a small cross section are made of a rare earth metal alloy, but a samarium alloy magnet, particularly a samarium cobalt alloy magnet, can also be used. Among the rare earth metal alloy magnets, a typical iron neodymium boron alloy magnet has a maximum energy product of 358 kJ / m3, and a iron neodymium boron alloy bonded magnet has a maximum energy product of 80 kJ / m3.
It is around 3. With such a magnet, unlike the conventional magnet, the required magnetic force on the surface of the developing roller can be secured even if the size is considerably reduced. The maximum energy product of conventional ordinary ferrite magnets and ferrite bonded magnets is 36 kJ / m
3 and 20 kJ / m3. If it is permissible to increase the sleeve diameter, the half-width can be reduced by using ferrite magnets or ferrite bonded magnets to make the shape larger, or by forming the magnet end facing the sleeve narrower. It is.

In this embodiment, the developing main magnet P1b, the magnet P4 for pumping the developer onto the developing sleeve 43, and the magnet P6 for transporting the pumped developer to the developing area.
And magnetic poles P2 and P3 for transporting the developer in the region after development form N poles, and main magnetic pole magnetic force forming auxiliary magnets P1a and P1c for assisting the formation of the developing main magnetic pole magnetic force, and transport the pumped developer. The magnet P5 forms the south pole. As can be understood from FIG. 8 showing the details of the magnetic force, a magnet having a normal direction magnetic force of 85 mT or more on the developing roller was used as the main magnet P1b. It has been confirmed that, if the auxiliary magnet having the magnetic pole forming magnetic force on the downstream side of the main magnet P1b has a magnetic force of, for example, 60 mT or more, no abnormal image such as carrier adhesion occurs. When the magnetic force was smaller than this, carrier adhesion occurred. The magnetic force related to carrier adhesion is a tangential magnetic force. To increase this tangential magnetic force, it is necessary to increase the magnetic force of P1b or P1c. Can be. The magnet width of the magnets P1a, P1b, P1c was 2 mm. At this time, the half width of P1b is 16 °.
Met. In this case, the main magnetic pole magnetic force forming auxiliary magnets P1a,
Even if the developing main pole group P1 is formed with P1c (the configuration in FIG. 8), the main magnetic pole magnetic force forming auxiliary magnet P1c is arranged only on the downstream side of the main magnet P1b (the configuration in FIG. 9). Magnet P1b
The width at half maximum at the same time does not change, and the magnetic force of the main magnetic pole (P1b portion) is reduced only by a few percent. Since there was no main magnetic pole magnetic force forming auxiliary magnet (P1a) on the upstream side, it was confirmed that the magnetic force of the P1a portion was reduced to about 30 mT. However, this portion was a portion to be covered by the inlet seal. Since it is not exposed to the image portion, it is possible to provide the developer to the main magnetic pole without affecting the image. Further, it was confirmed that the half width was further reduced by reducing the width of the magnet. 1.
The half width of the main pole when using a 6 mm width was 12 °. From the graph values in FIG. 8, the maximum normal magnetic force of P1b is 90
It can be read as mT. The half width at this time is 45m
At T, its angular width is 25 °. It was confirmed that an abnormal image was generated when the half width of the main magnetic pole was set to be larger than 25 °. For comparison, FIG. 10 schematically shows details of the magnetic force of the conventional magnet roller.

The half value width of the main magnetic pole magnetic force forming auxiliary magnets P1a and P1c is set to 35 ° or less. The half width at this portion cannot be set relatively small as at the main pole because the half width of P2 and P6 located outside is large.

FIG. 11 shows the positional relationship between the main magnet P1b and the auxiliary magnets P1a and P1c for forming the main magnetic pole magnetic force. Main magnet P
1b, P1a, P1 on both sides of main pole 1b
The included angle by c is set to 30 ° or less. In the example above,
The included angle was set to 25 ° in order to set the half width at the main pole to 16 °. Further, the main magnetic pole magnetic force forming auxiliary magnets P1a, P1
The included angle of the inflection point (0 mT: the point at which the magnetic force changes from N pole to S pole and from S pole to N pole) by 1c and the magnets P2 and P6 outside the auxiliary magnet is set to 120 ° or less.

By satisfying the above conditions, the method in which the magnetic brush at the main magnetic pole contacts the photoreceptor and develops it is as follows.
The developing nip is 2 mm or less when the developer particle size is larger than the developer particle diameter, and there is no trailing edge white spot, and a small image such as a horizontal thin line or 1 dot can be sufficiently formed. The image is shown in FIG. 12 and compared with the conventional example of FIG.

The magnetic brush on the sleeve surface formed by the magnetic force of the main magnet P1b has a developing nip at a portion where the photoreceptor comes into contact with the magnetic brush by setting the portion where the brush starts to stand (width of the root portion) to 2 mm or less. The width can be formed to 2 mm or less.

When the magnetic brush of this embodiment is used for developing a toner image (a toner image having a small amount of adhesion) on a photoreceptor having a low image density, the developing nip width is small. The contact amount (time) of the magnetic brush that rubs the magnetic brush decreases, and the amount of counter charge generated at the tip of the magnetic brush decreases. As a result, it is possible to suppress the phenomenon that the trailing edge of the image becomes white due to the carrier having the counter charge attracting the toner image. Therefore, it has become possible to improve the reproducibility of a toner image (a toner image having a small amount of adhesion) on a photoconductor having a low image density. The reason why the image density is increased is that by using the magnet roller of FIG. 7, the length of the magnetic brush of P1b can be reduced, and the developing nip width can be reduced. , P1b, the shortened magnetic brush starts to rise, and the time required to pass between the development nips is shortened (the phenomenon that the linear velocity ratio with respect to the photoconductor is shortened by this portion occurs).
The image density increases because the amount of developer rubbing against the photoreceptor increases. Further, since the developing nip width is small, the amount of the developer in the developer stagnation portion in front of the developing nip area is small, and it is possible to suppress the occurrence of the counter charge. It is possible to provide a developing device having improved image ability without edge blanking.

FIG. 14 shows the developing device 4 of the first embodiment.
FIG. 9 is a cross-sectional view illustrating a configuration of another example of a developing device for each color in FIG. 7 or a developing device 4b in the second embodiment.
In FIG. 14, they are collectively represented as a developing device 4.

In FIG. 14, a developing roller 41, which is a developer carrier, is arranged in the developing device 4 so as to be close to the photosensitive drum 1, and a developing region is formed at both opposing portions. . In the developing roller 41, a developing sleeve 43 formed of a non-magnetic material such as aluminum, brass, stainless steel, or conductive resin in a cylindrical shape is rotated clockwise by a rotation drive mechanism (not shown). . As an example, the drum diameter of the photosensitive drum 1 is 60 m
m, the drum linear velocity is set to 240 mm / sec, the sleeve diameter of the developing sleeve 43 is set to 20 mm, and the sleeve linear velocity is set to 600 mm / sec. Therefore, the ratio of the sleeve linear speed to the drum linear speed is 2.5. The developing gap, which is the distance between the photosensitive drum 1 and the developing sleeve 43, is set to 0.4 mm.

A doctor blade 45 for regulating the height of the developer chain, ie, the amount of the developer on the developing sleeve, is provided on the upstream side of the developing area in the developer conveying direction (clockwise direction in the drawing). Is installed. The doctor gap, which is the distance between the doctor blade 45 and the developing sleeve 43, is set to 0.4 mm. Further, a screw 47 for pumping the developer in the developing casing 46 to the developing roller 41 while agitating the developer in the developing casing 46 is provided in a region of the developing roller opposite to the photosensitive drum.

In the developing sleeve 43, there is fixedly provided a magnet roller body 44 for forming a magnetic field so as to make the developer stand on the peripheral surface of the developing sleeve 43. Along with the normal magnetic field lines emitted from the magnet roller body, a carrier of the developer is spiked in a chain on the developing sleeve 43, and the charged toner is attached to the carrier sprung in the chain, A magnetic brush is configured. The magnetic brush is transferred in the same direction as the developing sleeve 43 (clockwise as viewed in the figure) by the rotation of the developing sleeve 43. The magnet roller body 44
Has a plurality of magnetic poles (magnets). More specifically, a developing main magnet P1 for raising the developer in the developing area portion, and a magnet P for pumping the developer onto the developing sleeve 43
4. Magnets P5 and P6 for transporting the pumped developer to the development area, and magnetic poles P for transporting the developer in the area after development
2, P3. These magnets P1, P4, P
5, P2 and P3 are arranged radially of the developing sleeve 43. In this example, the magnet roller body 44 is
Although the magnets are formed by pole magnets, the magnets (magnetic poles) may be further increased from P3 pole to the doctor blade 45 to increase the number of poles to 8 or more in order to improve the pumping property and the solid black image followability.

In particular, as shown in FIG. 14, the main magnet P1 forming the developing main pole is formed of a magnet having a small cross section. Although the magnet having a small cross section is made of a rare earth metal alloy, a samarium alloy magnet, in particular, a samarium cobalt alloy magnet can be used. Among the rare earth metal alloy magnets, a typical iron neodymium boron alloy magnet has a maximum energy product of 358 kJ / m3, and a iron neodymium boron alloy bonded magnet has a maximum energy product of about 80 kJ / m3. With such a magnet, unlike the conventional magnet, the required magnetic force on the surface of the developing roller can be secured even if the size is considerably reduced. The maximum energy products of conventional ordinary ferrite magnets and ferrite bonded magnets are around 36 kJ / m3 and around 20 kJ / m3. If it is permissible to increase the sleeve diameter, the half-width can be reduced by using ferrite magnets or ferrite bonded magnets to make the shape larger, or by forming the magnet end facing the sleeve narrower. It is.

In this embodiment, a magnet P4 for pumping the developer onto the developing sleeve 43, a magnet P6 for transporting the pumped developer to the development area, and a magnet P2 for transporting the developer in the area after development. P3 forms the N pole, and the developing main magnet P1 and the magnet P5 for transporting the pumped developer are S
It is pole. As can be understood from FIG. 15 showing the details of the magnetic force, a magnet having a normal magnetic force of 85 mT or more on the developing roller was used as the main magnet P1. For example, 60
It has been confirmed that if the magnetic force is at least mT, no abnormal image such as carrier adhesion will occur. The downstream magnet P2, which forms the developer carrying magnetic pole, seems to have a function of assisting the formation of the main magnetic pole. When the magnet P2 is at least 60 mT, carrier adhesion does not occur. It was also confirmed that adhesion occurred. Magnet P1
Was 2 mm in width. At this time, the half width of P1 is 2
2 °. Further, it was confirmed that the half width was further reduced by reducing the width of the magnet. When the 1.6 mm width was used, the half width of the main pole was formed at 16 °. It was confirmed that an abnormal image was generated when the half-width of the main pole (P1) was larger than 25 °. For control,
FIG. 10 schematically shows the details of the magnetic force of the conventional magnet roller.

Further, as shown in FIG. 16, the inflection point (0) caused by the main magnet P1 and the magnets P2 and P6 outside the main magnet P1.
mT: Magnetic force changes from N pole to S pole and S pole to N pole)
May be 60 ° or less.

By satisfying the above conditions, the method in which the magnetic brush at the main magnetic pole contacts the photoreceptor and develops it is as follows.
The developing nip is 2 mm or less when the developer particle size is larger than the developer particle diameter, and there is no trailing edge white spot, and a small image such as a horizontal thin line or 1 dot can be sufficiently formed. The image is shown in FIG. 17 and compared with the conventional case of FIG.

The magnetic brush on the sleeve surface formed by the magnetic force of the main magnet P1b has a developing nip at a portion where the photoreceptor comes into contact with the magnetic brush by setting the portion where the ears start to stand (width of the root portion) to 2 mm or less. The width can be formed to 2 mm or less.

When the magnetic brush of the present embodiment is used to develop a toner image (a toner image with a small amount of adhesion) on a photoreceptor having a low image density, the developing nip width is small.
The contact amount (time) of the magnetic brush rubbing the photoreceptor decreases, and the amount of counter charge generated at the tip of the magnetic brush decreases. As a result, it is possible to suppress the phenomenon that the trailing edge of the image becomes white due to the carrier having the counter charge attracting the toner image. Therefore, it has become possible to improve the reproducibility of a toner image (a toner image having a small amount of adhesion) on a photoconductor having a low image density. The reason why the image density is increased is that by using the magnet roller of FIG. 14, the length of the magnetic brush P1 is reduced, and the width of the developing nip can be reduced. , P1, the shortened magnetic brush starts to rise, and the time required to pass between the developing nips is shortened (the phenomenon that the linear velocity ratio to the photosensitive member is increased by this portion occurs), and the photosensitive member is rubbed. The image density increases because the amount of the developing agent increases. Further, since the developing nip width is small, the amount of the developer in the developer stagnation portion in front of the developing nip area is small, and it is possible to suppress the occurrence of the counter charge. It is possible to provide a developing device having an improved image ability without edge blanking.

Finally, an entire electrophotographic color copying machine (hereinafter referred to as a color copying machine) which is an image forming apparatus to which the present invention is applied will be described. The schematic configuration and operation of the color copying machine will be described with reference to FIG. The color copying machine includes a color image reading device (hereinafter, referred to as a color scanner) 11, a color image recording device (hereinafter, referred to as a color printer) 12, a paper feed bank 13, and the like.

The photosensitive drum 20 in the color printer 12 corresponds to the photosensitive drum 1 shown in FIG. The writing optical unit 22 of the laser system corresponds to the exposure device 3 in FIG. The revolver developing unit 23 corresponds to the developing device 4 in FIG. Further, the intermediate transfer device 26 corresponds to the intermediate transfer device 5 in FIG.

The intermediate transfer device 26 includes an intermediate transfer belt 261, a belt cleaning device 262, a paper transfer corona discharger (hereinafter, referred to as a paper transfer device) 263, and the like. The intermediate transfer belt 261 includes a driving roller 264a,
Transfer opposed roller 264b, cleaning opposed roller 26
4c and a group of driven rollers, and is driven and controlled by a drive motor (not shown). The belt cleaning device 262 includes an entrance seal, a rubber blade, a discharge coil, an entrance seal, a mechanism for contacting and releasing the rubber blade, and the like. During the belt transfer of the fourth color image, the entrance seal and the blade are separated from the surface of the intermediate transfer belt 261 by the contact / separation mechanism. The paper transfer unit 263 is an AC voltage + DC voltage using a corona discharge method.
Alternatively, a DC voltage is applied to collectively transfer the superimposed toner image on the intermediate transfer belt 261 to recording paper.

The recording paper cassette 207 in the color printer 12 and the recording paper cassette 30 in the paper supply bank 13 are also provided.
Recording papers of various sizes are stored in a, 30b, and 30c.
Paper is fed and conveyed in the direction of the registration roller pair 29 by the paper feed rollers 28, 31a, 31b, 31c. Further, a manual tray 21 for manually feeding OHP paper or thick paper is provided on the right side of the printer 12 in the drawing.

[0073]

As described above, according to the image forming apparatus of the present invention, the light beam spot diameter on the image carrier upon exposure by the exposure device is Db, and the pixel pitch determined by the recording density of the image is Dp. , Db <3
Since the relationship of Dp is established, even when the recording density of the image is increased, it is possible to obtain an image with high definition and excellent in gradation production without impairing the density reproducibility of the highlight portion.

According to the second aspect of the present invention, since the relationship of Db <3Dp and Db> 0.8Dp is satisfied, even if the recording density of the image is increased, the good reproducibility of the density of the highlight portion is not impaired. A sufficient black solid density can be obtained, and an image having high definition, excellent in gradation, and always having a stable image density can be obtained.

According to the third aspect of the present invention, Db> 0.8 Dp
By satisfying the following relationship, the relationship between the image area ratio and the image density at the time of gradation expression can be kept linear regardless of the image recording density, and a sufficient solid black density can be obtained.

According to the fourth aspect of the present invention, the exposure apparatus is a laser scanning system that forms an electrostatic latent image on an image carrier by scanning a laser beam in the main scanning direction.
Since exposure is performed using the same beam in the main scanning direction, a stable exposure beam spot diameter can be obtained regardless of the location (position) in the main scanning direction.

According to a fifth aspect of the present invention, when the light beam spot diameter in the sub-scanning direction on the image carrier when exposing by the exposure device is Dbv, and the pixel pitch in the sub-scanning direction determined by the recording density of the image is Dpv. , Dbv> 0.8 Dpv, the exposure beam spot diameter condition in the sub-scanning direction is sufficient to obtain sufficient image density of the solid black portion even with a laser type exposure apparatus. Can be defined. Therefore, a sufficient black solid density can be obtained without deteriorating the density reproducibility of the highlight portion, and an image having high definition, excellent gradation, and always having a stable image density can be obtained.

According to a sixth aspect of the present invention, there is provided a light emitting element array in which a plurality of LED elements are arranged on a line in a main scanning direction, and a lens array for forming an image of light from the light emitting element array on a photosensitive member. And the LED array head is disposed so as to face the image carrier, so that the image recording density can be increased without increasing the size of the apparatus.

According to a seventh aspect of the present invention, the light beam spot diameter in the main scanning direction on the image carrier when exposing by the exposure device is Dbh, and the pixel pitch in the main scanning direction determined by the recording density of the image is Dph. When Dbh> 0.8Dp
Since the relationship h is established, the condition of the exposure beam spot diameter in the main scanning direction is specified so that the image density of the solid black portion can be sufficiently obtained even with an LED array head type exposure apparatus. be able to. Therefore, a sufficient black solid density can be obtained without deteriorating the high density reproducibility of the highlight portion, and high definition and excellent gradation can be obtained.
An image having a stable image density can always be obtained.

According to the structure of the eighth aspect, the tone reproducibility of a color photographic image or the like is strictly required and even in a color image forming apparatus, the density reproducibility of a highlight portion when the recording density of an image is increased is not impaired. The conditions of the exposure beam spot diameter can be defined so that a sufficient black solid image can be obtained, and an image having high definition, excellent gradation, and always having a stable image density can be obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating a configuration of an image forming unit in an example of an image forming apparatus according to the present invention.

FIG. 2 is a perspective view illustrating a configuration of an exposure device in the image forming apparatus.

FIG. 3 is a graph showing experimental results of gradation reproducibility in the image forming apparatus according to the embodiment.

FIG. 4 is a sectional view illustrating a configuration of an image forming unit in a color image forming apparatus according to a second embodiment of the present invention.

FIG. 5 is a cross-sectional view illustrating a configuration of an exposure device in the image forming apparatus.

FIG. 6 is a schematic diagram illustrating a spot diameter of an exposure beam.

FIG. 7 is a detailed configuration diagram of a developing device.

FIG. 8 is a diagram illustrating a magnetic force distribution of a developing roller and its magnitude.

FIG. 9 is a diagram showing a magnetic force distribution when there is no auxiliary magnet P1a.

FIG. 10 is a diagram showing a magnetic force distribution of a conventionally known developing roller for comparison.

FIG. 11 is a diagram illustrating a positional relationship between a main magnet and an auxiliary magnet.

FIG. 12 is a schematic diagram illustrating the size of a development gap and a nip in a development region.

FIG. 13 is a schematic diagram illustrating the size of a conventionally known developing gap and nip for comparison.

FIG. 14 is a detailed configuration diagram of a developing device in which a main developing pole is configured by a single-pole magnet.

FIG. 15 is a diagram showing a magnetic force distribution of a developing roller and its magnitude in the developing device.

FIG. 16 is a diagram showing a positional relationship between a main magnet and magnets on both sides.

FIG. 17 is a schematic diagram illustrating the size of a development gap and a nip in a development area.

FIG. 18 is a cross-sectional configuration diagram illustrating the entire image forming apparatus according to the present invention.

[Explanation of symbols]

 Reference Signs List 1 photosensitive drum (image carrier) 2 charging device 4, 4b developing device 5 intermediate transfer device 5b transfer belt 41 developing roller 43 developing sleeve (developer carrier) 44 magnet roller body Db light beam spot diameter Dp pixel pitch

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2C162 AE02 AE12 AE13 AE21 AE28 AE47 AE74 AF43 AF49 AF69 FA17 FA57 2C362 AA22 AA28 AA29 BA52 CA08 CA14 CB03 CB80 2H031 AC19 DA03 DA07 FA01 2H076 AB05 AB09 AB12 AB16 AB42 EA

Claims (8)

[Claims] An image carrier, an exposure device for exposing the image carrier to form an electrostatic latent image, and a developing device for developing the electrostatic latent image formed on the image carrier. In the image forming apparatus, the developing device includes a magnet roller included in the developer carrier, and an auxiliary magnetic pole for adjusting a magnetic force and a half-value width of the main magnetic pole adjacent to the main magnetic pole close to the image carrier. When the light beam spot diameter on the image carrier at the time of exposure by the exposure device is Db, and the pixel pitch determined by the recording density of the image is Dp, a relationship of Db <3Dp is established. Image forming device. 2. The image forming apparatus according to claim 1, wherein a relationship of Db> 0.8 Dp is satisfied. 3. An image carrier, comprising: an exposure device that exposes the image carrier to form an electrostatic latent image; and a developing device that develops the electrostatic latent image formed on the image carrier. In the image forming apparatus, the developing device includes a magnet roller included in the developer carrier, and an auxiliary magnetic pole for adjusting a magnetic force and a half-value width of the main magnetic pole adjacent to the main magnetic pole close to the image carrier. When the light beam spot diameter on the image carrier at the time of exposure by the exposure device is Db, and the pixel pitch determined by the recording density of the image is Dp, the relationship of Db> 0.8Dp is established. Image forming apparatus. 4. The apparatus according to claim 1, wherein said exposure device is of a laser scanning type for forming an electrostatic latent image on said image carrier by scanning a laser beam in a main scanning direction. The image forming apparatus according to claim 1. 5. The light beam spot diameter in the sub-scanning direction on the image carrier when exposing by the exposure device is Db.
5. The image forming apparatus according to claim 4, wherein the relationship of Dbv> 0.8 Dpv is satisfied, where Dpv is a pixel pitch in the sub-scanning direction determined by v and the recording density of the image. 6. An LED comprising: a light emitting element array in which a large number of LED elements are arranged on a line in a main scanning direction; and a lens array for imaging light from the light emitting element array on a photosensitive member. An array head type LED
The image forming apparatus according to claim 1, wherein an array head is disposed to face the image carrier. 7. A light beam spot diameter in the main scanning direction on the image carrier at the time of exposure by the exposure device is set to Db.
7. The image forming apparatus according to claim 6, wherein the relationship of Dbh> 0.8Dph is satisfied, where Dph is a pixel pitch in the main scanning direction determined by h, the recording density of the image. 8. The image forming apparatus according to claim 1, wherein said image forming apparatus is a color image forming apparatus.