JPH07295373A - Image forming device - Google Patents

Abstract

PURPOSE:To prevent the fluctuation of image density in accordance with that of toner concentration ana to obtain a stable high image quality over a full density range by deciding a developing bias electric filed with a developer characteristic. CONSTITUTION:A developer is a two-component one, when it is composed of toner including carbon black and carriers, the developing bias electric field is obtained by superimposing an alternating electric filed of a single period on a DC component with a prescribed period and when the developer composed of the toner not including carbon black and the carriers, the developing bias electric field is obtained by superimposing an alternating electric field of plural periods on the DC component with the prescribed period. In the single period, 6-16Hz is obtained and in plural periods, several single periods of 8-16Hz are preferable. Moreover, for instance, when the developer is composed of the toner not including carbon black and the carriers, a charge possessed by the toner is high and the toner stuck to the carrier participates in development as well, while the positive and negative high alternating electric fields (a)-(b) are applied by several times as shown by the figure.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic type or electrostatic recording type image forming apparatus such as a printer or a copying machine, and more particularly to a latent image formed on an image carrier. To a toner image with a two-component developer having a toner and a carrier.

[0002]

2. Description of the Related Art FIG. 7 shows an example of a conventional electrophotographic color laser beam printer. This color printer is provided with first, second, third and seventh image forming stations P _M , P _C , P _Y and P _K in the printer body, and on one side thereof, that is, on the right side in FIG. Paper feed cassette 30
On the opposite side, that is, on the left side in FIG. 4, fixing units (not shown) are arranged. Further, below the path from the paper feeding unit to the fixing unit in the printer body, an endless transfer material conveying means for conveying the transfer material, for example, the transfer belt 16 is provided.
Are stretched between a plurality of rollers in a known manner. The transfer belt 16 is driven in the direction shown by the arrow in the figure, carries the transfer material fed through the paper feed section, and is sequentially conveyed to the above-mentioned image forming stations P _M , P _C , P _Y and P _K. To do.

Image forming stations P _M , P _C , P
_Y and P _K have substantially the same structure and include photosensitive drums 4M, 4C, 4Y, and 4K, which are image carriers that are normally driven to rotate in the direction of the arrow in the figure, and are provided around each photosensitive drum. Is a primary charger 22M, 2M that uniformly charges the photosensitive drum.
2C, 22Y, 22K, developing devices 9M, 9C, 9Y, 9K for developing the electrostatic latent image formed on the photosensitive drum, transfer chargers 17M, 1M for transferring the developed visible image to a transfer material.
7C, 17Y, 17K, drum cleaners 25M, 25C, 25Y for removing toner remaining on the photosensitive drum, 2
5K are sequentially arranged in the drum rotation direction. Image exposure devices 23M, 23C, 23Y and 23K are provided above the photosensitive drums 4M, 4C, 4Y and 4K, respectively.

The developing device 9M contains magenta toner, the developing device 9C contains cyan toner, the developing device 9Y contains yellow toner, and the developing device 9K contains black toner. . Image exposure devices 23M, 23C, 23
Each of Y and 23K includes a semiconductor laser, a polygon mirror, an fθ lens, etc., and receives an electric digital pixel signal input, and develops the laser beam modulated corresponding to this signal with the primary chargers 22M, 22C, 22Y and 22K. Bowl 9M, 9
C, 9Y, and 9K are scanned in the drum generatrix direction to expose the drum surface to form an electrostatic latent image of a corresponding color. The image exposure device 23M receives a pixel signal corresponding to a magenta component image of a color image, and the image exposure device 23C receives a pixel signal corresponding to a cyan component image.
To the image exposure device 23K, and a pixel signal corresponding to the black component image is input to the image exposure device 23K.

Further, between the first image forming station P _M and the sheet feeding portion, a transfer roller 1 is provided with a suction roller (not shown) which constitutes a transfer material suction means and a metal roller which is grounded.
6 are disposed opposite to each other with corona 6 interposed therebetween, and corona discharge and charge injection are performed in order to surely attract the transfer material supplied from the paper feeding unit to the transfer belt 16. On the other hand, a charge eliminating charger (not shown) is provided between the fourth image forming station P _K and the fixing unit, and the charge eliminating device is attached to the transfer belt 16 by the charge eliminating charger. An alternating voltage is applied to separate the two.

In the color printer having the above construction, when the transfer material is fed from the paper feed cassette 30 onto the transfer belt 16, the transfer material is surely attracted onto the transfer belt 16 by the action of the attraction charger and the roller. To be done. Transfer belt 1
Along with the movement of 6 in the direction of the arrow in the figure, a magenta image is formed on the photosensitive drum 4M of the first image forming station P _M.
A cyan image is formed on the photosensitive drum 4C of the second image forming station P _C, a yellow image is formed on the photosensitive drum 4Y of the third image forming station P _Y , and a photosensitive drum 4K of the fourth image forming station P _K is formed on the photosensitive drum 4K. Are formed by sharing a black image.

These images are transferred to the first to fourth image forming stations P _M to
While sequentially passing under the photosensitive drums 4M to 4K of P _K and being conveyed toward the fixing unit, the transfer chargers 17M, 17C, 17Y, and 17K of each image forming station sequentially superimpose them on the transfer material. Are transferred and composited to form a color image. After passing through the fourth image forming station P _K , the transfer material is discharged from the transfer belt 16 by the charge removing charger to which an AC voltage is applied. The transfer material separated from the transfer belt 16 is sent to the fixing section, and the transferred image is fixed in the fixing section and then discharged from the transfer material discharge port to the tray, thus completing one copying cycle.

By the way, a developing device of a color image forming apparatus such as an electrophotographic color laser beam printer or a copying machine for forming a full-color image or a multi-image is often a two-component developer from the viewpoint of the tint of the image. It is used.
As is well known, the toner concentration of this two-component developer (ie,
The ratio of the weight of toner particles to the total weight of carrier particles and toner particles, T / (T + C) ratio) is an extremely important factor in stabilizing the image quality. The toner concentration changes as toner particles of the developer are consumed during development. Therefore, the developer concentration control device (AT
It is necessary to accurately detect the toner concentration of the developer by using R), replenish the toner according to the change, control the toner concentration to be always constant, and maintain the image quality.

FIGS. 8 and 9 show an example of a developing device equipped with a conventional developer concentration control device used in each image forming station of the above-mentioned electrophotographic color laser beam printer. FIG. 8 illustrates the developing device 9M of the first image forming station for forming a magenta image.
The developing device having the same structure is used in the third image forming station. Further, FIG. 9 illustrates the developing device 9K of the fourth image forming station for forming a black image.

As shown in FIG. 8, the developing device 9M is arranged so as to face the photosensitive drum 4M, and the inside thereof is divided into a first chamber (developing chamber) 11 and a second chamber by a partition wall 10 extending in the vertical direction. It is divided into a chamber (stirring chamber) 12. First room 11
A non-magnetic developing sleeve 3 that rotates in the direction of the arrow is arranged in the opening of the above-mentioned so as to face the photosensitive drum 4M, and a magnet 13 is fixedly arranged in this developing sleeve 3. The developing sleeve 3 carries and conveys a layer of a two-component developer (including magnetic carrier and non-magnetic toner) whose layer thickness is regulated by the blade 14, and supplies the developer to the photosensitive drum 4M in a developing area facing the photosensitive drum 4M. To develop the electrostatic latent image. In order to improve the developing efficiency, that is, the rate of applying the toner to the latent image, the developing sleeve 3 is applied with a developing bias voltage having a waveform as shown in FIG. There is.

Developer stirring screws 41 and 42 are arranged in the first chamber 11 and the second chamber 12, respectively. The screw 41 stirs and transports the developer in the first chamber 11, and the screw 42 and the toner supplied by the rotation of the transport screw 43 from the toner outlet of the toner replenishing tank (not shown) are already in the developing device 9M. The developer 44 and the developer are stirred and conveyed to make the toner concentration uniform. The partition wall 10 is shown in FIG.
The first chamber 11 and the second chamber
A developer passage (not shown) that communicates with the chamber 12 is formed. In the first chamber 11 in which toner is consumed by the development and the toner concentration is lowered by the conveying force of the screws 41 and 42. The developer flows from the one passage to the second chamber 12
The developer that has moved to the inside and has recovered the toner concentration in the second chamber 12 moves to the inside of the first chamber 11 from the other passage.

On the other hand, the developer concentration control device is a so-called reflected light amount detection system, and is a typical example of a developing device corresponding to the first to third stations when operated at a predetermined timing. Magenta developer 9 shown in
In M, light is emitted from the LED (light emitting diode) 1 having a light emission peak in the near-infrared region of the bidirectional light emitting type in the developer, and the reflected light is received by the photodiode 2, and the amount of the reflected light is used at this time. Toner density is detected, and this is compared with a reference level signal corresponding to the reference density to determine whether the toner is over-replenished or insufficient, and based on this determination, the developing device 9M The toner density is appropriately controlled by replenishing an appropriate amount of toner.

In the developing device 9K corresponding to the fourth station shown in FIG. 9, a predetermined patch-shaped electrostatic latent image formed on the photosensitive drum 4K is developed by the developing device 9K under a constant condition. The patch-shaped reference image 20 having the standard density formed by the above is irradiated with light from the bidirectional light emitting type LED (light emitting diode) 1 ', and the reflected light is received by the photodiode 2', The toner density at the present time is detected, this is compared with the signal of the reference level corresponding to the reference density, and the same control as the magenta developing device 9M is performed.

As described above, the reason why the ATR method is changed between the color developer such as magenta, cyan, and yellow and the black developer is that when the black toner component uses carbon black, it is mixed with the carrier in the developing device. This is because the amount of reflected light cannot be obtained.

[0015]

By the way, when an image was output by the above-mentioned color image forming apparatus, roughening occurred 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 region having a low density such as a photographic image.

Then, the cause of the roughness was examined, and the following was found. When a latent image is formed on a highlight portion by a normal dot latent image, the latent image on the photoconductor is not a broad latent image like an analog latent image but a local latent image when viewed microscopically. When attempting to reproduce even lower density, the latent image becomes dull due to the influence of the film thickness of the photoconductor. For example, when an image with a reflection density of about 0.2 is reproduced, the maximum contrast V of the latent image at that time is V
₀ is about 150 to 250V.

Further, in the case of reversal development, the surface potential of the non-image portion is set to 100 to 200 V higher than the DC component of the developing bias in order to remove the fog, so V ₀ is 1.
The potential difference V _cont with the DC component of the developing bias in the case of 50 to 250V is about 0 to 100V. This potential difference V
_{A cont} of about 0 to 100 V is a very unstable contrast whether the toner adheres to the photosensitive member side or the sleeve side. Therefore, when the latent image is developed with a two-component developer, the contact state of the magnetic brush has a great influence on the developing efficiency, and the roughness easily occurs due to dot loss or the like corresponding to the unevenness of the brush of the magnetic brush. Become.

Further, when an alternating voltage having the waveform shown in FIG. 10 is superimposed on the developing bias for the purpose of eliminating the roughness,
The toner sufficiently reciprocates between the sleeve and the photoconductor (hereinafter referred to as SD). As a result, the toner of high tribo (charge possessed by the toner) could be well developed, but the toner of low tribo was not developed, and the roughness was likely to occur.

Therefore, the frequency of this developing bias is increased to
A method has been proposed in which the toner does not reciprocate with 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 _cont
<Act as DC component attracts the toner to the sleeve side in the case of 0, the toner is a bias in the sleeve side, V _cont is V _cont> in the case of 0, the DC component in response to the latent image potential, the drum side The toner works to attract the toner, and the amount of toner commensurate with the latent image potential is biased toward the photosensitive drum side.

This tendency becomes more remarkable by intermittently applying an alternating voltage as shown in the waveform diagram of FIG.
When developing under such conditions, the toner reaching the photosensitive drum repeatedly vibrates on the photosensitive drum and concentrates on the latent image portion. For this reason, the dot shape is made uniform, and it is possible to obtain a good image without unevenness.

However, this developing method has a drawback that only tribo within a certain range is used for development. Toner with high tribo is hard to fly out from the carrier or sleeve, and toner with low tribo leads to a latent image even if it jumps out. It is pulled back to the end, resulting in toner concentration,
That is, there is a drawback that the concentration greatly changes depending on the T / (T + C) ratio.

The amount of toner supplied to the developing unit is 2-3 times the amount of toner normally used for developing, but all of the toner undergoes reciprocating motion due to the latent image and bias in the developing unit for developing. If it is involved, it is considered that the developing condition is determined by the state of the latent image and the fluctuation due to the T / (T + C) ratio of the developer is reduced.

However, in the developing bias as shown in FIG. 11, the density greatly changes depending on the T / (T + C) ratio as described above. This is because Tribo only contributes to toner within a certain range, and therefore T / (T +
C) It is considered that the amount of toner involved in the development changes according to the change in the ratio, which changes the development density. Therefore, the developing bias having the waveform shown in FIG.
The density of ~ 0.3 reduces the roughness, but the toner density,
That is, there is a problem that the image density variation due to the T / (T + C) ratio becomes large especially on the low density side.

Therefore, it is an object of the present invention to provide an image forming apparatus capable of preventing a change in image density due to a change in toner density and stably obtaining a high quality image over the entire density.

[0025]

The above object can be achieved by an image forming apparatus according to the present invention. In summary, the present invention is
An image having an image bearing member on which an electrostatic latent image is formed, and a plurality of developing devices that carry the developer to a developing unit facing the electrostatic latent image and perform development using a developing bias electric field including an oscillating electric field In the image forming apparatus, the condition of the developing bias electric field is determined according to the developer characteristics.

Preferably, the developer is a two-component developer and the developer properties relate to whether or not the developer contains carbon black, wherein the developer comprises carbon black-containing toner and carrier. In this case, the developing bias electric field is an alternating electric field of a single cycle superimposed on the DC component in a predetermined cycle, and when the developer is composed of toner containing no carbon black and carrier, The electric field is obtained by superimposing an alternating electric field having a plurality of cycles on a DC component at a predetermined cycle.

It is preferable that the single cycle is 6 to 16 Hz, and the multiple cycles have a plurality of 8 to 16 Hz per single cycle.

[0028]

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.

First Embodiment A first embodiment of the image forming apparatus according to the present invention will be described with reference to FIGS. 1 to 6. The image forming apparatus and the developing device thereof of this embodiment have substantially the same structure as the conventional image forming apparatus and the developing device thereof shown in FIGS. 7 to 9, and therefore, the same members are designated by the same reference numerals. , Detailed description is omitted.

The image forming apparatus of this embodiment is described as an electrophotographic color laser beam printer, but the present invention is not limited to the electrophotographic type electrostatic image printer on an image carrier such as a photoconductor or a dielectric. A visible image is formed by developing a latent image formed by a recording method with a developing device,
It is needless to say that the present invention can be equally applied to various types of image forming apparatuses that transfer a visible image such as paper carried and conveyed by a transfer material conveying means and fix it by a fixing means to form a permanent image.

In the image forming apparatus shown in FIG. 1, each of the magenta, cyan, and yellow image forming stations P _M ,
The electrostatic latent image formed by P _C and P _Y is sequentially developed by the developing bias power source 15a to be a toner image. At this time, the developer is a two-component developer including a non-magnetic toner charged to (-) and magnetic carrier particles charged to (+). The mixing ratio of this developer was such that the non-magnetic toner was about 5% by weight. The non-magnetic toner has a volume average particle diameter of about 8 μm. The magnetic carrier particles are resin-coated ferrite particles (maximum magnetization 60 emu / g), and the weight average particle diameter is 50.
μm, and its resistance value is 10 ⁸ Ωcm or more. The magnetic permeability of the magnetic carrier particles is about 5.0.

An opening is provided at a portion of each of the developing units 9M, 9C, 9Y and 9K in the vicinity of the photosensitive drums 4M, 4C, 4Y and 4K. As shown in FIGS. 8 and 9, the developing sleeve 3 is opened from the opening. Is exposed to the outside. The developing sleeve 3 is rotatably incorporated in the developing devices 9M, 9C, 9Y and 9K. The outer diameter of the developing sleeve 3 is 32 mm, and the peripheral speed thereof is 280 mm / sec. The developing sleeve 3 has a distance of 5 between it and the photosensitive drums 4M, 4C, 4Y, and 4K.
It is arranged so as to be 00 μm. Development sleeve 3
Is made of a non-magnetic material, and a magnet 13, which is a magnetic field generating means, is fixed inside thereof.

The magnet 13 includes a developing magnetic pole N1, a magnetic pole S3 located downstream thereof, and magnetic poles S2 and N for carrying the developer.
2 and S1. The magnet 13 is arranged in the developing sleeve 3 so that the developing magnetic pole N1 faces the photosensitive drums 4M, 4C, 4Y, and 4K. The developing magnetic pole N1 forms a magnetic field in the vicinity of the developing section between the developing sleeve 3 and each of the photosensitive drums 4M, 4C, 4Y, and 4K, and the magnetic field forms a magnetic brush.

A blade 14 is arranged above the developing sleeve 3 at a predetermined distance from the developing sleeve.
The distance between the developing sleeve 3 and the blade is 800 μm.
The blade 14 is fixed to the developing devices 9M, 9C, 9Y, 9K. Blade 14 is aluminum, SUS316
It is made of a non-magnetic material such as and regulates the layer thickness of the developer on the developing sleeve 3.

A conveying screw 41 is housed in the developing chamber 11. The conveying screw 41 is rotated in the direction of the arrow in the figure, and the developer in the developing chamber 41 is conveyed in the longitudinal direction of the developing sleeve 3 by the rotational driving of the conveying screw 41.

A conveying screw 42 is housed in the stirring chamber 12. The conveying screw 42 conveys the toner along the longitudinal direction of the developing sleeve 3 by its rotation, and the toner is conveyed from the toner replenishing tank (not shown) by the conveying screw 43 and freely falls from the replenishing port into the stirring chamber 12.

The developing sleeve 3 carries the developer at a position near the magnetic pole S2, and conveys the developer toward the developing section as the developer rotates. When the developer reaches the vicinity of the developing portion, the magnetic carrier particles of the developer rise from the developing sleeve 3 while being connected by the magnetic force of the magnetic pole N1, and a magnetic brush of the developer is formed. The tip of the magnetic brush is on each photosensitive drum 4M,
The surfaces of 4C, 4Y, and 4K are rubbed, and a DC voltage of 500 is applied between the developing sleeve 3 and the photosensitive drums 4M, 4C, 4Y, and 4K.
By applying a developing bias described below to V, it is possible to obtain an image in which the highlight portion is smooth and is less affected by the T / (T + C) ratio fluctuation.

Next, the developing bias of this embodiment will be described while explaining the movement of the toner. FIG. 2 is a waveform diagram showing a developing bias waveform used in each of the magenta, cyan, and yellow developing devices in this embodiment. In the portion (a), first, a high electric field of (+) is applied to the developing sleeve 3 at a high frequency (28 kHz) with respect to the photosensitive drums 4M, 4C and 4Y.
At this time, the toner charged with (-) does not separate from the carrier and remains as it is. Next, in the portion (b), a high electric field of (-) is applied to the developing sleeve 3 at a high frequency (28 kH).
z). At that time, the toner charged in (-) moves away from the carrier toward the photosensitive drum surface.

Next, in the portion (c), the toner is regenerated (a).
It is returned to the developing sleeve side by the same returning electric field as
The distance is shorter than the flying electric field of (b) by the amount of the C electric field, and does not completely return to the developing sleeve 3. Then (d)
In the portion of (1), the flying electric field similar to that of (b) again moves toward the photosensitive drum surface. After that, in the portion (e), the DC electric field causes the toner to move at a constant velocity, and reaches the latent image on the photosensitive drum. That is, while the alternating electric field is applied a plurality of times, the toner having high tribo and adhering to the carrier is also affected by the electric field and participates in the development.

From the results of the experiment, the high electric field high frequency was effective not only at 28 kHz but also at 16 to 32 kHz.

As a result, the image density variation due to the T / (T + C) ratio variation is small, and an image with excellent highlight reproducibility can be obtained. Further, since the magenta, cyan, and yellow developing devices using this developing bias have little image density variation due to T / (T + C) ratio variation, the description has been made with reference to FIG.
A developer concentration control (ATR) method using a toner concentration sensor provided in the developing device is used.

On the other hand, the electrostatic latent image formed at the black image forming station P _K is sequentially developed by the developing bias power source 15b to become a toner image.

The black developer is a two-component developer consisting of a non-magnetic toner charged with (-) charged with carbon black and a carrier which is a magnetic particle charged with (+). Other basic materials and developing device configurations conform to those for magenta, cyan, and yellow. The developing bias used in the black developing is of a conventional type in which an alternating electric field of one cycle as shown in FIG. 11 is superimposed on a DC electric field at a predetermined interval. The black developing device using this developing bias has many image density fluctuations due to T / (T + C) ratio fluctuations.
A method of creating a patch-like reference density pattern on the photosensitive drum 4K described with reference to FIG. 9 is used. There is no problem because the black image density fluctuation does not relate to the gray balance.

The developing bias power source 15a for magenta, cyan, and yellow uses an alternating current (AC) component of 7 KHz,
A rectangular wave of 2 KVpp is output, and a developing bias in which a direct current (DC) component is superimposed on the AC component is used. The development characteristics under such conditions are shown in FIGS.

FIG. 3 is a graph showing the output image density with respect to the input image density of the color developer, and FIG. 4 is a graph showing the output of the density detecting sensor 50 at that time. As can be seen from the graph of FIG. 4, the sensitivity for detecting the developer concentration is about 200 mV / wt%.

The developing bias power source 15b for the black developer is
As shown in FIG. 11, a bias waveform in which a single-cycle alternating electric field is intermittently formed is output. That is, in this embodiment,
The development bias waveform was as follows: overall frequency 2.66 KHz rectangular portion 8 KHz blank portion 250 μSec peak-to-peak 2 KVpp.

With this developing bias waveform, the developing efficiency was increased, and the developing with low contrast became possible. That is, with a normal rectangular wave (2 KHz, 2 KVpp), the maximum density is 1.5
It was possible to obtain the maximum density of 1.5 at a value about 100 V lower than the contrast potential required for obtaining.

From the results of the experiment, the above rectangular portion is 8 kHz.
It was effective at 6 to 16 kHz as well as z.

FIG. 5 is a graph relating to the output image density characteristic of the black developer with respect to the input image density, and FIG. 6 is a graph showing the density sensor output at that time. 3 and 4
As can be seen by comparing with, the output image density has a large variation with respect to the developer density. That is, the image density fluctuation amount (sensitivity) is about 300 mV / wt with respect to the developer density fluctuation of 1 wt%.
%, This sensitivity is similar to that of ATR using reflected light from the magenta, cyan, and yellow developers.

As described above, the developing bias in which the alternating electric field of two cycles is superimposed on the DC component is applied to the color developing device, and the alternating electric field of the single cycle is superimposed on the DC component to the black developing device. By applying the developing bias, the image density does not depend on the fluctuation of the toner density, so that a high quality image can be stably obtained.

Embodiment 2 In the first embodiment, as the developing bias waveforms of magenta, cyan, and yellow, a rectangular portion as shown in FIG. 2 is provided for two cycles and a shape in which it is superimposed on a direct current (DC) component is used. The present embodiment is characterized in that the rectangular portion is provided in three cycles or more.
With this, the same effect as that of the first embodiment can be obtained. Incidentally, if the DC component exists as the developing bias waveform, basically any number of the above rectangular portions may be used.
Furthermore, the developing bias in which the rectangular portion is superimposed on the DC component for a plurality of cycles including the first embodiment detects the toner concentration in the developer other than the developer using carbon black.
It can be applied to TR type developers.

When using a magnetic or non-magnetic one-component developer, it is not necessary to use ATR. in this case,
It is preferable to use a developing bias waveform having a plurality of periods of rectangular portions and a DC component capable of suppressing image density fluctuation due to toner density (T / (T + C ratio) fluctuation.

[0053]

As is apparent from the above description, the image forming apparatus according to the present invention prevents the fluctuation of the image density due to the fluctuation of the toner density by determining the condition of the developing bias electric field according to the developer characteristics. A high-quality image can be stably obtained over all densities.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a color image forming apparatus that can embody the present invention.

FIG. 2 is a waveform diagram showing a developing bias for a color developer.

FIG. 3 is a graph showing a relationship between an input image density and an output image density of a color developer.

FIG. 4 is a graph showing a relationship between an output image density of a color developer and an output of a density detection sensor.

FIG. 5 is a graph showing a relationship between an input image density and an output image density using a developing bias of a black developer.

FIG. 6 is a graph showing the relationship between the output image density of the black developer and the output of the density detection sensor.

FIG. 7 is a configuration diagram showing a conventional image forming apparatus.

8 is a configuration diagram of a developing device for color development of the image forming apparatus of FIG.

9 is a configuration diagram of a black developing device of the image forming apparatus of FIG.

FIG. 10 is a waveform diagram showing a conventional developing bias.

FIG. 11 is a waveform diagram showing a conventional improved type developing bias.

[Explanation of symbols]

 4 Photosensitive Drum (Image Carrier) 9 Developing Device 15a Development Bias Power Supply for Color Developer 15b Development Bias Power Supply for Black Developer

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Masahiro Inoue 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc.

Claims (3)

[Claims] 1. An image bearing member on which an electrostatic latent image is formed, and a plurality of developing devices for carrying the developer to a developing unit facing the electrostatic latent image and performing development by a developing bias electric field including an oscillating electric field. An image forming apparatus having: a developing bias electric field condition determined by the characteristics of the developer. 2. The developer is a two-component developer, the developer characteristics relate to whether or not the developer contains carbon black, and the developer and the toner containing carbon black and a carrier. When the developing bias electric field is formed by superposing a single cycle alternating electric field on the DC component at a predetermined cycle, the developing bias electric field is composed of a toner containing no carbon black and a carrier. 2. The image forming apparatus according to claim 1, wherein the developing bias electric field is an alternating electric field having a plurality of cycles superimposed on a DC component at a predetermined cycle. 3. The single period is 6 to 16 Hz,
The image forming apparatus according to claim 2, wherein the plurality of cycles has a plurality of 8 to 16 Hz per single cycle.