JPH09101663A - Image forming device - Google Patents

Abstract

(57) [Summary] [Object] To provide an image forming apparatus capable of obtaining a uniform image without any toner fog. In the developing section, an alternating electric field is applied to the developer, and the developing sleeve 12 is rotated in the a direction for a character image signal and in the b direction for a gradation image signal. Further, the upper and lower developing blades 31 and 32 are arranged above and below the developing sleeve 12, respectively.
When 2 rotates in the a direction, the upper developing blade 31 is brought close to the developing sleeve 12, and the lower developing blade 32 is brought into contact with the developing sleeve 12, and vice versa when rotating in the b direction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a copying machine, a printer, a recorded image display for developing an electrostatic latent image formed on an image carrier by an electrophotographic system, an electrostatic recording system or the like to form a visible image. The present invention relates to an image forming apparatus such as an apparatus and a facsimile.

[0002]

2. Description of the Related Art Conventionally, a dry developer as a developer is carried on the surface of a developer carrier, and this developer is conveyed and supplied near the surface of an image carrier carrying an electrostatic latent image, A developing method is well known in which an electrostatic latent image is developed and visualized by applying an alternating (alternating) electric field between the carrier and the developer carrier.

A developing sleeve is generally used as the developer carrying member, so that it is referred to as a "developing sleeve" in the following description. Further, a photosensitive drum is generally used as the image carrying member in many cases. Therefore, the term "photosensitive drum" will be used in the following description.

As the above-mentioned developing method, conventionally, for example, 2
A magnetic brush was formed on the surface of a developing sleeve having a magnet inside by a developer composed of a component composition (carrier particles and toner particles), that is, a two-component developer, and they were opposed to each other with a minute developing gap maintained. This magnetic brush is rubbed or brought close to the photosensitive drum, and an alternating electric field is continuously applied between the developing sleeve and the photosensitive drum (between S and D) so that the toner particles are transferred from the developing sleeve side to the photosensitive drum side. There is known a so-called magnetic brush developing method in which development is carried out by repeatedly carrying out the transfer to and the reverse transfer in the opposite direction (for example, JP-A-55-32060 and JP-A-59).
-165082).

Further, a non-contact type alternating electric field development method using a two-component developer for the purpose of simple color development and multiple development is also known (for example, JP-A-56-14268 and JP-A-SHO). 58-68051, JP-A-56-14
4452, JP-A-59-181362 and JP-A-60-176069).

On the other hand, among the image forming apparatuses using the above-mentioned developing method, there is a laser beam printer adopting an electrophotographic method as a high speed and low noise printer. A typical application of this printer is binary recording of images such as characters and figures, and so-called binary recording in which an electrophotographic photosensitive member is scanned while emitting and extinguishing a laser beam according to an image signal to be recorded. Is done. Thus, since the recording of characters and figures does not require halftones, the printer structure is simple.

It is well known that the dither method, the density pattern method, and the like are used to represent halftones in this binary recording type laser beam printer.

However, as is well known, the dither method,
It is difficult to obtain high resolution with a printer that uses the density pattern method.

Therefore, in recent years, there has been proposed a method of forming a halftone image while obtaining a high resolution without lowering the recording density.

In this system, halftone pixels are formed by modulating the width of a pulse signal for driving a laser with an image signal. That is, the light emission time length per pixel of the laser is controlled according to the image density, and accordingly, the photoconductor irradiation time length per pixel of the laser beam for scanning the photoconductor is controlled according to the image density. It

Specifically, the pulse width of the pulse signal is shortened for a low density image portion to shorten the photoconductor irradiation time length, and the pulse width is reduced for a high density image portion. The exposure time of the photoconductor is lengthened by increasing the length. According to such a pulse width modulation (PWM) method, an image with high resolution and high gradation can be formed. Therefore, this method is indispensable especially for a color image forming apparatus that requires high resolution and high gradation. It has become a thing.

[0012]

However, when an image is output by a conventional copying machine according to the above method, roughness occurs in a halftone region having a reflection density of 0.3 or less. This roughening does not occur so much in a text original or the like, but often occurs in a low density area such as a photographic image.

Then, the cause of the roughness was examined, and the following was found.

When a latent image in a highlight portion is formed by a normal dot latent image, the latent image on the photosensitive drum is not a broad latent image like an analog latent image but a local latent image when viewed microscopically. Has become. Further, when trying to reproduce a lower density, the latent image becomes dull due to the influence of the film thickness of the photoconductor, and the maximum contrast V ₀ gradually decreases as shown in the graph of FIG. For example, when an image with a reflection density of about 0.2 is reproduced, V ₀ of the latent image at that time is 1
It will be about 50 to 250V.

In the case of reversal development, the surface potential of the non-image area is set to 100 to 200 V higher than the DC component of the developing bias in order to remove fog, so V ₀ is 15
In the case of 0 to 250 V, the potential difference V _cont with the DC component of the developing bias becomes about 0 to 100 V. This V
_{The cont of 0} to 100 V is a very unstable contrast depending on whether the toner is on the photosensitive member side or the developing sleeve side. Therefore, when the latent image is developed with the two-component developer, the contact state of the magnetic brush greatly contributes to the development efficiency, and the roughness due to the lack of dots corresponding to the unevenness of the ears of the magnetic brush easily occurs. is there.

Further, in order to eliminate the roughness, an alternating voltage is superimposed on the developing bias, and the bias is set to 6 kHz.
There has been proposed a method of increasing the frequency to prevent the toner from reciprocating in one pulse.

According to this method, the toner makes an oscillating motion so as not to reciprocate between SD. The potential difference V _cont between the surface potential of the photosensitive drum and the DC component of the developing bias is V
_{When cont} <0, the DC component works to attract the toner to the developing sleeve side, the toner is biased _{toward the} developing sleeve side, and when V _cont is V _cont > 0, the DC component depends on the latent image potential. Function to attract the toner to the drum side, and an amount of toner commensurate with the latent image potential is biased toward the photosensitive drum side. This tendency becomes more remarkable by intermittently applying the alternating voltage.

When developing under such conditions, the toner arriving at the photosensitive drum repeats vibration on the photosensitive drum and concentrates on the latent image portion. Therefore, it is possible to obtain a good image with uniform dot shapes and no unevenness.

However, in such an image forming method, the direction of rotation of the developing sleeve is made opposite to the direction of rotation of the photosensitive drum, and the conveying direction of the developer on the developing sleeve is changed at the portion where the developing sleeve faces the photosensitive drum. When a developing method that is the same as that for the photoconductor drum is used, and when a latent image forming a high density is on the drum, the development is performed before the latent image reaches the position facing the photoconductor drum. The supplied toner flies onto the photosensitive drum.

Further, in order to stably obtain a normal image density from a low density to a high density, the peripheral speed of the developing sleeve needs to be 1.5 times or more the peripheral speed of the photosensitive drum. In this case, when a high-density image arrives at the trailing edge of the halftone image, the toner on the developing sleeve jumps to the high-density portion before reaching the developing portion, and the trailing edge of the halftone image becomes thin. Become
There was a problem that it would come out white. Such a phenomenon becomes particularly remarkable when the above-mentioned high frequency of 6 kHz or more is used. For this reason, the highlight portion becomes smooth, but when there are dark characters in the halftone or a high-density image at the trailing edge of the halftone, the boundary portion is cut out in white.

As a countermeasure against this, the developing sleeve is rotated in the same direction as that of the photosensitive drum, and the developing sleeve on the developing sleeve is opposed to the photosensitive drum at a portion where the developing sleeve faces the photosensitive drum. By using the method, until the developer is conveyed to the position facing the photoconductor drum, the toner does not fly, the white image does not disappear even at the boundary portion, and the smooth image is obtained in the highlight portion. However, although the gradation image is uniform and smooth by this developing method, the toner in the background portion of the character image cannot be completely removed, so-called fog occurs, and a sufficiently satisfactory image is obtained in total. I couldn't do it.

Therefore, it is an object of the present invention to provide an image forming apparatus capable of obtaining an image of uniform image quality without toner fog on any image.

[0023]

The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides:
An image forming apparatus having an image carrier and a developing means having a developer carrier for carrying and carrying a developer for developing an electrostatic latent image formed on the image carrier. In the image forming apparatus, an alternating electric field is applied to the developer and the moving direction of the developer carrying member is changed according to an image signal.

It is preferable that the developer layer thickness regulating members disposed above and below the developer carrying member be changed in distance from the developer carrying member in accordance with the image signal. The image signal is preferably a character image signal or a gradation image signal. A magnet is preferably fixed inside the developer carrying member.

According to another aspect, the developer layer thickness regulating members arranged above and below the developer carrying member are fixed, and the magnets arranged inside the developer carrying member are fixed. Preferably moves according to the image signal.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.

1 and 2 schematically show an electrophotographic copying apparatus which is an embodiment of the image forming apparatus of the present invention. In the present embodiment, the latent image forming means A, the developing device B, the transfer means C, and the cleaning means are provided around the photosensitive drum 1 which is rotatably carried in the direction of the arrow as an image bearing member, as in the conventional case. D, fixing means E and the like are arranged.

The latent image formed on the surface of the photosensitive drum 1 by the latent image forming means A is developed by the developing device B to be visualized. This visible image is transferred to the transfer material fed from a paper feeding device (not shown) by the transfer means C at the transfer portion, and the visible image transferred by the fixing means E is fixed on the transfer material, and is transferred to the outside of the machine. Is ejected. The residual toner on the photosensitive drum 1 after the transfer is removed by the cleaning unit D.

Since these structures and operations are means well known to those skilled in the art, detailed description thereof will be omitted, and here, the developing device B, which is a feature of the present invention, will be described.

In this embodiment, the photosensitive drum 1 has an outer diameter of 80 mm, and in order to visualize the electrostatic latent image formed on the photosensitive drum 1 by the latent image forming means A. A developing device B is arranged so as to face the photosensitive drum 1.

The developing device B includes a developer container 16 as shown in FIG. 1, and the inside of the developer container 16 is divided into a developing chamber R1 and a stirring chamber R2 by a partition wall 17, and the developer container 16 is also provided. A toner storage chamber R3 that accommodates a replenishment toner (non-magnetic toner) 18 is provided above the stirring chamber R2. A replenishment port 20 is provided at the bottom of the toner storage room R3.
Is provided, and through the replenishment port 20, the replenishment toner 18 in an amount commensurate with the consumed toner is dropped and replenished into the stirring chamber R2.

On the other hand, the developer 19 is contained in the developing chamber R1 and the stirring chamber R2. The developer 19 is a two-component developer including a non-magnetic toner and magnetic particles (carrier). The mixing ratio was such that the nonmagnetic toner was about 5% by weight.

In this embodiment, the non-magnetic toner has a volume average particle diameter of about 8 μm, and the magnetic particles are resin-coated ferrite particles (maximum magnetization 60 emu /
g), its weight average particle diameter was 50 μm, and its resistance value was 10 ⁸ Ωcm or more. The magnetic permeability of the magnetic particles was about 5.0.

An opening is provided in a portion of the developer container 16 which is close to the photosensitive drum 1, and a developing sleeve 11 as a developer carrying member projects halfway to the outside from the opening. The developing sleeve 11 is incorporated in the developer container 16 so as to be rotatable in both directions. In this embodiment, the developing sleeve 11 has an outer diameter of 32 mm and a peripheral speed of 280.
mm / sec. The developing sleeve 11 is arranged so that the distance between the developing sleeve 11 and the photosensitive drum 1 is 500 μm. The developing sleeve 11 is made of a non-magnetic material, and a magnet 12 as a magnetic field generating means is fixed inside the developing sleeve 11.

The magnet 12 includes a developing magnetic pole S1 and a developer 19
Magnetic poles N3, N2, S2, and N1 for transporting The magnet 12 is arranged in the developing sleeve 11 so that the developing magnetic pole S1 faces the photosensitive drum 1. The developing magnetic pole S1 forms a magnetic field in the vicinity of the developing portion between the developing sleeve 11 and the photosensitive drum 1, and the magnetic field forms a magnetic brush.

An upper developing blade 31 and a lower developing blade 32 are arranged above and below the developing sleeve 11, and the distance between each developing blade and the developing sleeve 11 can be varied at a predetermined timing by means not shown. Is configured. The closest spacing is set to 800 μm, and the furthest spacing is set to 2 mm.

The upper and lower developing blades 31, 32 are made of a non-magnetic material such as aluminum or SUS316, and regulate the layer thickness of the developer 19 on the developing sleeve 11.

Here, the operation of the developing device for developing a character image will be described with reference to FIG. In this case, the developing sleeve 11 rotates in the direction of arrow a. The upper developing blade 31 is installed at the closest position by means (not shown), and the distance from the developing sleeve 11 is 800 μm. Further, the lower developing blade 32 has a distance of 2 mm from the developing sleeve by means not shown.

By moving the developing sleeve 11 in the direction a opposite to the rotation of the photosensitive drum 1, the developing blade 3
1 supplies a predetermined amount (about 50 mg / cm ² ) of developer to the developing section. In this case, the image is centered on the character image, and the boundary between the halftone portion and the solid portion is almost absent in the image, so that the above-mentioned white void does not occur at all. Further, the determination as to whether or not it is a character image may be manually set before image formation, or means for automatically recognizing that it is a character image if the ratio of white characters in the image data is a predetermined value or more. May be used.

Since the developer was conveyed in the developing section in the same direction as the photosensitive drum, fog did not occur and good character images were obtained.

After the development is completed, the developer on the developing sleeve 11 passes through the lower developing blade 32 and is taken into the developing chamber R1.
There is a distance of 2 mm between the rollers 1 and they are smoothly taken into the developing chamber R1.

A conveying screw 13 is housed in the developing chamber R1. By the rotational driving of the conveying screw 13 in the c direction, the developer in the developing chamber R1 is removed by the developing sleeve 11
Is conveyed in the longitudinal direction of the. By rotating the conveying screw 13 in the c direction, the repulsive poles N2 and N
The developer dropped between 3 and the developer being stirred can be smoothly supplied to the N3 pole.

A conveying screw 14 is housed in the stirring chamber R2. The conveyance screw 14 conveys the developer along the longitudinal direction of the developing sleeve 11 by the rotation in the d direction, and the replenishment toner 18 that has fallen freely from the replenishment port 20 into the stirring chamber R2 is stirred with the developer.

Next, the case of outputting a gradation image is shown in FIG.
This will be described with reference to FIG. In this case, the developing sleeve 11 is (b)
Rotate in the direction. The upper developing blade 31 is separated from the developing sleeve 11 by means (not shown) and is set at an interval of 2 mm. Further, the lower developing blade 32 has a distance of 800 μm from the developing sleeve 11 by means not shown.

Here, the developing sleeve 11 is the photosensitive drum 1.
By moving in the direction (b) which is the same direction as the rotation of, the predetermined amount (about 50 mg / cm ² ) of the developer is supplied to the developing section. Further, since the developer is conveyed in the developing section in the opposite direction to the photosensitive drum 1, a particularly uniform halftone image can be stably obtained as described above. Also, unlike character images, fogging did not pose a problem at all.

After completion of development, the developer 1 on the development sleeve 11
9 passes through the upper developing blade 31 and is taken into the developing chamber R1.
The distance is 1 to 2 mm, and the developer is taken into the developing chamber R1. The screw 13 in the developing chamber R1 rotates in the e direction to form a smooth flow of the developer around the developing sleeve 11. The rotation direction of the carrying screw 13 is opposite to that in the above-described character formation, and the carrying direction of the developer is also opposite, but stable circulation can be obtained by the construction of the developing device. The conveying screw 1 in the stirring chamber R2
4 rotates in the same direction as the conveying screw 13 in the developing chamber R1 to stir and convey the developer.

As described above, by changing the rotating direction of the developing sleeve in accordance with the image, it is possible to always obtain a stable high quality image.

Second Embodiment Next, a second embodiment of the image forming apparatus according to the present invention will be described with reference to FIG. The difference from the first embodiment is that the upper and lower developing blades 31 and 32 are fixed at a position where the distance from the developing sleeve 11 is 80 μm, and the magnet 12 is rotatable.

At the time of forming a character image, the magnetic pole S2 is positioned directly below the upper developing blade 31, and the lower developing blade 32 rotates the magnet 12 so as to be located between the magnetic poles S1 and N2. The developer is stably conveyed when rotating in the direction.

Further, when the gradation image is formed, the magnetic pole N2 is made to face the lower developing blade 32, and the upper developing blade 31 rotates the magnet 12 so as to be located between the magnetic poles N1 and S2. The developer is stably conveyed when rotating in the b direction.

A stable image was obtained by performing the same steps as in Example 1 except for the above points.

[0052]

As is apparent from the above description, by applying an alternating electric field to the developer in the developing section and changing the moving direction of the developer carrying member according to the image signal, it is possible to obtain what kind of image However, uniform image quality can be obtained without fogging.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a character image output in a first embodiment of an image forming apparatus according to the present invention.

FIG. 2 is an explanatory diagram showing a gradation image output in the image forming apparatus of FIG.

FIG. 3 is an explanatory diagram showing Embodiment 2 of the image forming apparatus according to the present invention.

FIG. 4 is a graph for explaining the surface potential of a photoconductor.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Image carrier (photosensitive drum) 11 Developer carrier (developing sleeve) 12 Magnet 19 Developer 31, 32 Developer layer thickness regulating member (developing blade) B Developing device

Claims (6)

[Claims] 1. An image forming apparatus comprising: an image bearing member; and a developing means having a developer bearing member that carries and conveys a developer for developing an electrostatic latent image formed on the image bearing member. In the image forming apparatus, an alternating electric field is applied to the developer in the developing unit, and the moving direction of the developer carrying member is changed according to an image signal. 2. The developer layer thickness regulating member disposed above and below the developer carrying member has a gap between the developer carrying member and the developer carrying member changed according to the image signal. The image forming apparatus according to claim 1. 3. The image forming apparatus according to claim 1, wherein the image signal is a character image signal or a gradation image signal. 4. The image forming apparatus according to claim 1, wherein a magnet is fixed inside the developer carrying member. 5. A developer layer thickness regulating member arranged above and below the developer carrying member is fixed, and a magnet arranged inside the developer carrying member is responsive to the image signal. The image forming apparatus according to claim 1, wherein the image forming apparatus moves. 6. The image forming apparatus according to claim 5, wherein the image signal is a character image signal or a gradation image signal.