JP2000122359A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device capable of obtaining a high quality image having excellent image quality not only in a solid black part but also in a halftone part by satisfactorily developing the halftone part of an electrostatic latent image without causing an irregularity and a density change. SOLUTION: This image forming device has a first developing unit 3 and a second developing unit 4 of two-component developing unit. As the toner of two-component developer, the first developing unit 3 uses a low concentration toner (Ta) for a solid image whose highest reflection density is low when the solid image of an electrostatic latent image is developed, and the second developing unit 4 uses a high concentration toner (Tb) for a solid image whose highest reflection density is high. A dot distribution electrostatic latent image formed on a photoreceptor drum 1 is developed by the first developing unit 3, and the latent image is then re-developed by the second developing unit 4.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic image forming apparatus which forms an electrostatic latent image on an image carrier and develops the latent image by developing means to visualize the latent image.

[0002]

2. Description of the Related Art Conventionally, in an image forming apparatus using an electrophotographic system, an electrostatic latent image corresponding to a document is formed on an image carrier, and the latent image is developed by a developing device using a developer. The image is visualized as an image, and the toner image is transferred onto a transfer material to obtain a copy image of a document.

In particular, in the case of a dot distribution electrostatic latent image formed by performing writing by light irradiation using an LED, a laser, or the like, the obtained copied image may be rough. This does not occur very much in the case of a manuscript or the like, and the density is low when a photograph manuscript or the like is output (0.
It is often found in the halftone region (around 3 or less).

FIG. 11 shows a general relationship between the density of a document image and the density of a copied image (all densities are reflection densities) by an electrophotographic apparatus. A characteristic phenomenon of electrophotographic devices is that the density of the copied image is lower than the density of the original in the low-density area, and increases sharply from low to medium density, exceeding the density of the original. The characteristic is that after reaching almost the maximum density, the density reaches the maximum density.

This is because the latent image potential is shallow in the low-density region, and only a small developing electric field is applied between the latent image potential and the DC component of the developing bias. However, the toner does not easily transfer to the latent image portion on the image carrier, and the amount of toner adhered to the latent image portion is reduced. As a result, the density of the low-density area of the copied image is lower than the density of the original image. When this reaches the region where the density shifts from low density to medium density, the latent image potential becomes deep and a relatively large developing electric field is applied, so that the transferability of the toner on the developer carrier rapidly increases, The toner easily transfers to the latent image on the image carrier, and the amount of toner attached to the latent image portion increases rapidly. When the density of the document image further increases, the density of the middle and high density areas of the copy image exceeds the density of the document image.

[0006] Roughness occurs in the above-mentioned area from low density to medium density or lower, that is, in a halftone portion. Investigation into the cause of the roughness revealed that the main cause was that the density of each toner particle was large. In other words, as described above, the vicinity where the toner density is low to medium is an area where the amount of toner attached sharply increases and the developability is unstable, and the toner is not uniformly attached to the latent image portion. When the toner is dark, uneven adhesion of the toner is conspicuous and it seems that the toner is rough.

[0007]

The fine adjustment technique is still difficult today in the region below the range from low to medium density.

If the density of each toner particle is low, even if the amount of toner attached suddenly increases as described above,
Halftone roughness should be less likely to occur. However, in that case, the maximum density of the copied image also decreases.

Accordingly, an object of the present invention is to develop a halftone portion of an electrostatic latent image well without causing roughness or density fluctuation, and to provide a high quality image which is excellent not only in a solid black portion but also in a halftone portion. An object of the present invention is to provide an image forming apparatus capable of obtaining a proper image.

[0010]

The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides an image carrier and a dot distribution electrostatic latent image formed on the image carrier by a one-component developer comprising a toner as a developer or a two-component developer comprising a toner and a magnetic carrier. And an image forming apparatus for developing an image by using a developing agent under a developing bias to form an image. The image forming apparatus includes two types of toner, a first toner Ta and a second toner Tb. The toner Ta is an image forming apparatus which gives a solid image having a maximum reflection density of 50% or less of the toner Tb when the solid image is developed.

According to the present invention, as the developing means, the first developing means containing the toner Ta and the toner T
b, and a potential difference between a DC component of the developing bias and a solid portion potential of the electrostatic latent image,
At the time of development by the first developing means, it can be larger than at the time of development by the second developing means.

The present invention also provides an image carrier and a dot distribution electrostatic latent image formed on the image carrier by using a one-component developer comprising toner as a developer or a two-component developer comprising toner and a magnetic carrier. Developing means for developing an image by using a developer and applying a developing bias to form an image, wherein the developing means develops a solid image of the electrostatic latent image as a toner. Sometimes, a toner Ta having a low density that gives a solid image with a small maximum reflection density
And a toner Tb having a high density which gives a solid image having a large maximum reflection density, and wherein the average particle diameter of the toner Ta is smaller than the average particle diameter of the toner Tb by 2 μm or more. It is a forming device.

According to the present invention, the developing means is provided with a supply container containing the toners Ta and Tb specially, and is provided with a means for measuring the reflection density of the image formed on the image bearing member. Forming a small-area image including a halftone portion on the image carrier, measuring the reflection density of the halftone portion by the measuring means, and detecting an increase in the reflection density of the halftone portion; Ta from container
Can be supplied to the developing means, or the toner Tb can be supplied to the developing means.

[0014]

Embodiments of the present invention will be described below in detail with reference to the drawings.

Embodiment 1 FIG. 1 is a sectional view showing an embodiment of the image forming apparatus of the present invention. The image forming apparatus of this embodiment is a digital photographic printer.

As shown in FIG. 1, the image forming apparatus of the present embodiment has a photosensitive drum 1 rotating in the direction of an arrow as an image carrier substantially in the center of the apparatus main body, and a primary charger 2 around the photosensitive drum 1. , A first developing device 3, a second developing device 4, a transfer charger 5, a separation charger 6, a cleaner 7, and an exposure lamp 8. A laser optical system L is provided above the photosensitive drum 1.
In S, a conveyor belt 12 is installed on the lower right side of the photosensitive drum 1, and a fixing device 13 is installed on an extension of the conveyor belt 12.

In order to form an image, the surface of the photosensitive drum 1 is uniformly charged to a predetermined potential by the charger 2, and the laser optical system L
By irradiating the laser beam L with S, a dot distribution electrostatic latent image corresponding to the image information of the document is formed on the surface of the photosensitive drum 1.

The laser optical system LS has, as shown in FIG.
A laser drive circuit (light emission signal generator) 15 and the like are provided. The laser drive circuit 15 generates a light emission signal based on the image signal of the original image read by the reader unit of the printer, and drives the semiconductor laser 16 based on the light emission signal. A laser beam L corresponding to a document image is generated.
The generated laser beam L is incident on a polygon mirror 18 rotating at a high speed via a collimator 17, and is deflected by the polygon mirror 17 toward the photosensitive drum 1. The deflected laser beam L passes through the fθ lens 19 to form a spot image on the surface of the photosensitive drum 1 to perform scanning exposure, thereby forming a dot electrostatic latent image on the surface of the photosensitive drum 1 in accordance with image information of the document. Is done.

According to the present embodiment, as described above, the first and second developing units 3 and 4 are provided. The dot latent image formed on the photosensitive drum 1 is first developed by the first developing device 3 and then further developed by the second developing device 4 to be visualized as a toner image. The developing devices 3 and 4 will be described later.

The toner image formed in this manner is transferred to a transfer material supplied to the photosensitive drum 1, that is, a transfer sheet by the action of the transfer charger 5. The sheet is taken out of the sheet cassette 9, transported to the registration roller 11 via the guide 10, temporarily stopped there, and then supplied to the photosensitive drum 1 in accordance with the image formation on the photosensitive drum 1. The sheet on which the toner image has been transferred is separated from the photosensitive drum 1 by the separation charger 6 and sent to the fixing unit 13 via the conveyor belt 12, where the toner image is fixed to form a permanent image, and then discharged. It is discharged to the tray 14.

The photosensitive drum 1 after the transfer of the toner image removes the transfer residual toner remaining on the surface by the cleaner 7 and removes the residual charge on the surface by the pre-exposure lamp 8.
It is prepared for the next image formation.

The developing units 3 and 4 will be described. According to the present embodiment, the developing units 3 and 4 are two-component developing units, and have substantially the same configuration except that the type of non-magnetic toner used for the developer is different. Therefore,
The developing device 3 will be described.

As shown in FIG. 3, the developing device 3 includes a developing container 20 containing a two-component developer 25.
A substantially lower half of the inside is divided into a developing chamber R1 and a stirring chamber R2 by a partition wall 26 having openings (not shown) at both ends on the near side and the far side. A developing sleeve 21 as a developer carrying member is rotatably provided at an opening of the developing container 20 on the side of the developing chamber R1, and a regulating blade 23 is provided above the developing sleeve 21. In the developing sleeve 21, a roller-shaped magnet 22 having a plurality of magnetic poles S1, N1, S2, N2, and N3 is non-rotatably arranged.

In the stirring chamber R1 and the developing chamber R2, developer conveying screws 27 and 28 are provided.
A toner hopper 29 (not shown in FIG. 1) containing replenishment toner 30 is provided above and protruding from the developing container 20. The hopper 29 is in the shape of a bulge toward the bottom, and has an opening at the lower end thereof.
A replenishing roller 31 is provided. The toner 30 in the hopper 29 is dropped into the stirring chamber R2 by the replenishing roller 31 and is replenished as necessary as the toner of the developer 25 is consumed by the development.

The conveying screws 27 and 28 of the developing chamber R1 and the stirring chamber R2 are both formed to have an outer diameter of 32 mm, and the conveying screw 27 in the developing chamber R1 is rotated to rotate the developer 25 in the developing chamber R1. Is transported along one longitudinal direction of the developing sleeve 21 while stirring.
The developer is fed into the stirring chamber R2 through the opening at one end. In this process, a part of the developer 25 is
It is sucked up. Conveying screw 2 in stirring chamber R2
8 rotates in the same direction as the conveying screw 27, and conveys the developer 25 and the toner supplied from the hopper 29 in the stirring chamber R <b> 2 in one direction in the opposite direction to the above while stirring. It is returned into the developing chamber R1 through the opening at the other end.

As described above, the developer 25 is supplied to the developing chamber R1.
And the stirring chamber R2, the toner in the developer is friction-charged to a polarity suitable for development by friction with the magnetic carrier in the process.

The developing sleeve 21 is made of a non-magnetic member such as aluminum or non-magnetic stainless steel, and is arranged with a predetermined gap of about 0.2 to 1 mm from the photosensitive drum 1. The developing sleeve 21 rotates in the direction of the arrow b in which the portion facing the photosensitive drum 1 moves in the same direction, and in the process, the magnetic force of the pumping pole N2 of the magnet 22 causes
The developer 25 in the developing chamber R1 is sucked up and carried on the surface of the developing sleeve 21. The developer carried on the developing sleeve 21 is conveyed toward the developing section A facing the photosensitive drum 1 with the rotation of the developing sleeve 21, and in the course of the conveyance, the layer thickness is controlled by the regulating section where the regulating blade 23 is arranged. Subject to regulations.

The regulating blade 23 is made of a magnetic member, and is disposed above the developing sleeve 21 with a predetermined gap therebetween so as to substantially oppose the regulating pole S2 located substantially at the top of the magnet 22. The layer thickness of the developer carried on the developing sleeve 21 is magnetically regulated by the regulating blade 23 and the regulating pole S2. The layer thickness of the developer after the regulation is such that when the developer is formed on the magnetic brush in the developing section A, the height of the magnetic brush is 1.2 to less than the minimum gap between the photosensitive drum 1 and the developing sleeve 21. The amount is preferably about three times.

The developer whose layer thickness is regulated by the regulating portion reaches the developing portion A via the transport pole N1 of the magnet 22 as the developing sleeve 21 rotates. Magnet 2 in developing section A
The second developing pole S1 is located, and a developing magnetic field is formed in the developing section A. The magnetic force of the developing pole S1 is
The peak value of the magnetic flux density in the vertical direction on the surface of
An extent of 2000 Gauss is preferred.

The developer conveyed to the developing section A is a developing pole S
The magnetic carrier is raised by the developing magnetic field of 1 and is formed on the magnetic brush of the developer. The magnetic brush of the developer comes into contact with the surface of the photosensitive drum 1, and the toner attached to the carrier is transferred to a dot latent image formed on the surface of the photosensitive drum 1, and adheres to the exposed portion of the latent image. And develop.
Not only the toner adhering to the carrier of the magnetic brush but also the toner adhering to the surface of the developing sleeve 21 are transferred to the latent image and contribute to development.

During development, an oscillating bias voltage in which a DC voltage is superimposed on an AC voltage is applied to the developing sleeve 21 between the photosensitive drum 1 and a developing power source (not shown). The latent image potential (non-exposed area potential) and light area potential (exposed area potential) of the latent image
Is located between the maximum value and the minimum value of the oscillation bias potential. As a result, an alternating electric field in which the direction alternates is formed in the developing section A, and the toner and the carrier of the developer violently vibrate in the alternating electric field, and the toner shakes off the electrostatic restraining force on the carrier and the developing sleeve 21. Transfer to the latent image.

The difference (peak-to-peak voltage) between the maximum value and the minimum value of the AC voltage component of the vibration bias voltage is preferably about 1 to 5 kV, and the frequency is preferably 1 to 10 kHz. As the waveform of the oscillation bias voltage, a rectangular wave, a sine wave, a triangular wave, or the like can be used. Further, the DC voltage component takes a value between the dark portion potential and the bright portion potential of the latent image, and it is determined that the absolute value of the DC voltage component is closer to the dark portion potential than to the minimum bright portion potential. It is preferable for preventing fog toner from adhering.

The developer used for the development in the developing section A is returned into the developing container 20 according to the rotation of the developing sleeve 21, and is separated from the surface of the developing sleeve 21 by the repulsive magnetic field between the magnetic poles N3 and N2 of the magnet 22. Then, it falls into the developing chamber R1. As described above, the developer that has fallen into the developing chamber R1 is sent from the developing chamber R1 to the stirring chamber R2, and is supplied with toner.

In the present invention, as the non-magnetic toner used for the two-component developer 25, a known non-magnetic toner obtained by adding a colorant, a charge control agent and the like to a binder resin can be used. The volume average particle diameter of the toner is preferably about 6 μm. The volume average particle diameter of the toner is measured, for example, by the following method.

Call counter TA-II as measuring device
An interface (manufactured by Nikkaki) for outputting a number average distribution and a volume average distribution, and a personal computer CX-i (manufactured by Canon) were connected using (Coulter). The electrolyte is about 1% using primary grade sodium chloride.
% NaCl aqueous solution was prepared.

To 100 to 150 ml of the above electrolyte solution, 0.1 to 5 ml of a surfactant, preferably an alkylbenzene sulfonate, is added as a dispersant, and 0.5 to 50 mg of a toner as a measurement sample is further added. The electrolytic solution in which the sample was suspended was subjected to a dispersion treatment for about 1 to 3 minutes with an ultrasonic disperser, and was subjected to 100 μm
Is used to measure the particle size distribution of the toner particles of 2 to 40 μm to determine the volume distribution. From this volume distribution, the volume average particle size of the toner is determined.

In the present invention, as the magnetic carrier used for the two-component developer 25, a carrier in which the surface of magnetic particles is coated with a resin very thinly is preferably used. The average particle size of the carrier is preferably about 5 to 70 μm. The average particle size of the carrier is indicated by the maximum chord length in the horizontal direction, and the measurement was performed by microscopically selecting 300 or more carriers at random, measuring the diameter and arithmetically averaging the diameters.

According to this embodiment, as described above, the two-component developer 2 is used in the first developing device 3 and the second developing device 4.
5, the type of non-magnetic toner used is different. Specifically, the first developing device 3
The toner Ta used as the developer is a toner whose particle density is low, and the toner Tb used as the developer of the second developing device 4 is a toner whose particle density is higher than the toner Ta. Therefore,
In the present embodiment, the dot latent image formed on the photosensitive drum 1 is first developed by the first developing device 3 with the developer using the thin toner Ta, and then developed by the second developing device 4 using the dark toner Tb. Develop with agent.

Here, whether the density of the toner is high or low is numerically determined by a solid image obtained when a solid image is developed with the same toner adhesion amount using a two-component developer using the toner. It is indicated by the maximum reflection density of the image.

In this embodiment, as the thin toner Ta of the first developing device 3, as shown in FIG. 4, a toner having a maximum reflection density of a solid black image of about 0.6 is used.
The maximum reflection density is also about 1.6 as a dark toner Tb.
The first and second developing devices 3 and 4 are subjected to development using the toner to be used, and an image including a halftone portion is formed by the image forming apparatus of FIG. 1 to investigate the image quality of the obtained image. did.

For comparison, as shown in FIG. 5, the order of the first developing unit 3 and the second developing unit 4 is changed and the developing unit 3 is subjected to development.
The same applies except that the latent image on the photosensitive drum 1 is first developed by the second developing device 4 with the developer using the dark toner Tb, and then developed by the first developing device 3 with the developer using the thin toner Ta. An image was formed and the image quality of the image was investigated.

As a result, in this embodiment, a high-quality image was obtained in which there was no roughness in the halftone portion and the density of the solid black portion was sufficient, whereas in the comparative example, the density of the solid black portion of the image was sufficient. However, roughening occurred in the halftone portion. According to the present inventors' consideration of this result,
The following was found.

That is, since the potential difference between the latent image potential in the halftone portion and the DC component of the developing bias is small, the second developing device 4 using the dark toner
When the second development is performed, the latent image portion is buried with the toner Tb, and if the toner Tb is unevenly attached to the latent image portion, the toner Tb has a high density and is conspicuous. Appear as. Then, the thin toner T
a in the first developing device 3 using a
Since the halftone portion potential is filled by the second accumulation of the charge amount of the toner Tb, the toner Ta hardly adheres to the halftone portion, and the roughness of the halftone portion is not eliminated.

On the other hand, in the present embodiment, the latent image portion is filled with the toner Ta by the first development in the first developing device 3 using the toner Ta having a low density, and the toner Ta is applied to the latent image portion. Is not noticeable because of the low density of the toner Ta, and does not appear as roughness in the halftone portion. In the solid black portion, the latent image portion is not filled with the toner Ta in the first development in the first developing device 3, but the potential difference between the latent image potential and the DC component of the developing bias is large. In the second development at No. 4, a sufficient amount of the dark toner Tb adheres to the solid black portion, and the density of the solid black portion becomes sufficient.

In this embodiment, in order to check the range of the density of the thin toner Ta used in the first developing device 3, the toner T
a, the maximum reflection density of the solid black image is about 0.4, 0.
6, 0.8, 1.0, 1.2, and 1.4, respectively.
For b, a toner having a maximum reflection density of about 1.6 was used, and the first developing device 3 and the second developing device 4 were used for development, and an image having a halftone portion was formed.

The charging potential (dark portion potential) Vd of the photosensitive drum 1 by the charger 2 is 650 V, and the halftone portion potential is 3
50 V, the solid black portion potential was 650 V, and the DC component of the developing bias was 500 V.

The degree of elimination of halftone roughness in the obtained image was examined. Table 1 shows the results.

[0048]

[Table 1]

As shown in Table 1, the maximum reflection density of the toner Ta used in the first developing unit 3 is 0.8 or less, which is half of the maximum reflection density 1.6 of the toner Tb of the second developing unit 4. ,
The effect of eliminating half-tone roughness has appeared,
The roughness was most eliminated at the maximum reflection density of the toner Ta of 0.6.

Example 2 As described in Example 1, the first development of the latent image
When the development is performed in the first developing device 3 using the thin toner Ta as the toner of the component developer, the halftone latent image portion is changed to the thin toner Ta.
Is embedded, even if there is uneven adhesion of the toner Ta, it is inconspicuous, and a halftone portion without roughness is obtained.

Therefore, at the time of development by the first developing device 3 using the thin toner Ta, the potential difference between the latent image potential of the halftone portion and the DC component of the developing bias is increased, so that the halftone latent image portion is developed. If the adhesion of the thin toner Ta is promoted, it is considered that the effect of eliminating the roughness in the halftone portion becomes larger.

In this embodiment, in order to examine the range of the DC component of the developing bias at the time of development by the first developing device 3 using the thin toner Ta, the DC component is set to 200, 30.
0, 400, 500, 600, 650V, while
The DC component of the developing bias at the time of development by the second developing device 4 using the dark toner Tb is fixed to 500 V, the first developing device 3 and the second developing device 4 are used for development, and an image having a halftone portion is used. Was formed.

The charged potential (dark portion potential) Vd of the photosensitive drum 1
Is 650 V, the halftone portion potential is 350 V, the solid black portion potential is 650 V, and the DC component of the developing bias at the time of development by the second developing device 4 is 500 V. Table 2 shows the results of examining the degree of elimination of halftone roughness in the obtained image.

[0054]

[Table 2]

As shown in Table 2, at the time of development by the first developing device 3 using the thin toner Ta, the DC component of the developing bias was increased to increase the potential difference from the latent image potential of the halftone portion. As the amount increases, the amount of toner Ta adhering to the halftone portion increases, so that the effect of eliminating the roughness of the halftone portion increases, and the roughness disappears when the DC component is 600 V or more.

In this case, if the DC component of the developing bias at the time of development by the first developing device 3 is too large, the solid black portion is also filled with the thin toner Ta, and the dark toner Tb during the second development by the second developing device 4. It is necessary to pay attention to this point, since the amount of the toner adhered to the solid black part is greatly reduced, and the density of the solid black part may be reduced.

Embodiment 3 FIG. 6 is a sectional view showing still another embodiment of the image forming apparatus of the present invention.

In this embodiment, the image forming apparatus is provided with one two-component developing unit 40, and a mixture of a thin toner Ta and a dark toner Tb is used as a two-component developer toner of the developing unit 40. Toner T having a large average particle diameter of toner Ta
It was made smaller by 1 to 2 μm than b. The hopper 29 of the developing device 40 contains the mixed toner of the toner Ta and Tb for replenishment.

The other structure of the image forming apparatus of this embodiment is the same as that of the first embodiment. In FIG. 6, the same reference numerals as those shown in FIG. 1 denote the same members. When the inventor observed a halftone area having a density of around 0.3 or less on the photosensitive drum 1 immediately after development with the two-component developer, the halftone toner area had an average particle diameter of the initial value. 1-2 than
It was found that the toner was formed by toner having a size of μm smaller.

Accordingly, if the thin toner Ta is formed into a toner smaller by 1 to 2 mm than the toner Tb having a large average particle diameter, even if the thin toner Ta and the dark toner Tb are mixed and used in the developing device 40, In one development by the developing device 40,
An image in which the thin toner Ta is preferentially attached to the halftone portion and the dark toner Tb as well as the thin toner Ta can be adhered to the solid portion, thereby eliminating the roughness of the halftone portion and obtaining a sufficient density of the solid portion. Can be obtained.

In this embodiment, as the toner of the two-component developer of the developing device 40, the thin toner Ta is used with the volume average particle diameter changed to 4, 5, 6, 7, or 8 μm, while the dark toner Tb is used. Is used by fixing to a volume average particle size of 6 μm,
The developing device 40 was subjected to development to form an image. Table 3 shows the results.

[0062]

[Table 3]

As shown in Table 3, the dark toner Tb having an average particle size of 6 μm was added to the thin toner Ta having an average particle size of 4 to 5 μm.
By combining the toners, the toners Ta and Tb can be mixed and used to eliminate the halftone roughness.

Therefore, according to the present embodiment, it is possible to obtain an image having a sufficient density of the solid black portion without roughening of the halftone portion with a simple configuration of development by the single developing device 40. The effects of the present invention can be achieved.

Embodiment 4 FIG. 7 is a sectional view showing still another embodiment of the image forming apparatus of the present invention.

In this embodiment, as in the third embodiment, one 2
The toner of the two-component developer 25 includes a toner Ta having a small average particle diameter and a dark toner Tb having a large average particle diameter.

As shown in FIG. 8, the inside of the hopper 29 provided in the developing container 20 of the developing device 40 is divided into two parts by a vertical partition wall 29a, one of which is provided with toner Ta for replenishment and the other is provided with toner Ta. The toner Tb is stored in the developing container 20 by the supply rollers 31a and 31b at the lower end of the hopper 29.

In the case of the third embodiment shown in FIG. 6, when only images having a large number of halftone portions are continuously formed, thin toner Ta is greatly consumed, and Ta and Tb from the hopper 29 of the developing device 40 are greatly consumed. It has been found that the replenishment of the mixed toner has a problem that the amount of the toner Ta in the developer becomes relatively smaller than the toner Tb, and the roughness of the halftone portion gradually deteriorates.

At this time, as shown in FIG. 7, an image reflection density sensor 42 is installed on the photosensitive drum 1, and as shown in FIG. When the reflection density of the image section 44 is measured by irradiating light to the image section 44 after the development and receiving the reflected light by the light receiving element 42b, the image quality of the image section 44 starts to deteriorate. The density of the area where the reflection density was 0.7 at the beginning of the portion 44 began to increase.

Therefore, in the present embodiment, at the time of image formation, a small-area image having a halftone portion is formed on the photosensitive drum 1 between sheets, and the reflection density of the halftone portion is sequentially measured by the sensor 42. As shown in FIG. 10, when the density of the area where the reflection density of the halftone portion was 0.7 began to increase, the supply of the thin toner Ta was repeated.

As a result, the density of the area where the density began to increase rapidly decreases and converges to the initial reflection density of 0.7.
It was confirmed that the rough tone in the halftone part was settled. The frequency of measurement of the reflection density is increased, and the toner T
It was confirmed that the fineness of the replenishment of “a” can stably eliminate the roughness.

In each of the above embodiments, the developing device
Although a two-component developer using a two-component developer composed of a non-magnetic toner and a magnetic carrier is used, a one-component developer using a magnetic toner or a non-magnetic toner alone may be used.

[0073]

As described above, according to the present invention,
A first developing unit and a second developing unit, each using a one-component developer composed of a toner or a two-component developer composed of a toner and a magnetic carrier, are installed. When the solid image of the electrostatic latent image is developed, the dot distribution electrostatic latent image is developed using a toner Ta having a low density which gives a solid image having a small maximum reflection density, and then the maximum reflection is performed by the second developing means. Using a dark toner Tb that gives a solid image with a large density,
Since the latent image is developed again, the halftone part of the latent image can be developed well without any roughness or density fluctuation, and high quality images with excellent image quality not only in solid black areas but also in halftone areas Can be obtained.

Further, as a toner, when developing a solid image of an electrostatic latent image, a toner Ta having a low density which gives a solid image having a small maximum reflection density, and a dark toner giving a solid image having a large maximum reflection density are used. When the toner Tb is mixed and used, and the average particle diameter of the toner Ta is smaller than the average particle diameter of the toner Tb by 1 to 2 μm, the latent image is similarly generated by one development by one developing device. Can be developed well without causing roughness or density fluctuation, and a high-quality image excellent not only in solid black areas but also in halftone areas can be obtained.

[Brief description of the drawings]

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

FIG. 2 is a schematic diagram showing a laser beam scanner installed in the image forming apparatus of FIG.

FIG. 3 is a cross-sectional view illustrating first and second developing units installed in the image forming apparatus of FIG. 1;

FIG. 4 is a graph showing a reflection density characteristic of a document versus a copy image with toner Ta and toner Tb used in first and second developing devices.

5 is a cross-sectional view showing that the order of development of first and second developing units of the image forming apparatus of FIG. 1 has been changed.

FIG. 6 is a sectional view showing another embodiment of the image forming apparatus of the present invention.

FIG. 7 is a sectional view showing still another embodiment of the image forming apparatus of the present invention.

FIG. 8 is a sectional view showing a developing device installed in the image forming apparatus of FIG. 7;

9 is an explanatory diagram showing how a reflection density sensor installed in the image forming apparatus of FIG. 7 measures a reflection density of a halftone portion of an image after development of a dot latent image.

10 is an explanatory diagram showing a transition in image quality of a halftone portion in image formation performed while repeating reflection density measurement of a small area image and replenishment of toner Ta to a developing unit in the image forming apparatus of FIG. 7;

FIG. 11 is an explanatory diagram illustrating density characteristics of a document versus a copy image of the electrophotographic apparatus.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photosensitive drum 3 1st developing device 4 2nd developing device 20 Developing container 21 Developing sleeve 22 Magnet 25 2 component developer 29 Hopper 40 Developing device 42 Image density sensor Ta Thin toner Tb Dark toner

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) G03G 15/08 507 G03G 15/08 507L

Claims (5)

[Claims] 1. An image carrier and a dot distribution electrostatic latent image formed on the image carrier using a one-component developer composed of a toner or a two-component developer composed of a toner and a magnetic carrier as a developer. And an image forming apparatus including a developing unit for developing an image by applying a developing bias to form an image, wherein the developing unit applies two kinds of toners, a first toner Ta and a second toner Tb. Use the toner Ta
Means that the maximum reflection density when a solid image is developed is the toner T
An image forming apparatus which gives a solid image which is 50% or less of b. 2. The image forming apparatus according to claim 1, further comprising a first developing unit containing the toner Ta and a second developing unit containing the toner Tb, wherein the developing unit includes a DC component of the developing bias and a DC component of the electrostatic latent image. The image forming apparatus according to claim 1, wherein a potential difference from a solid portion potential is larger at the time of development by the first developing unit than at the time of development by the second developing unit. 3. An image carrier and a dot distribution electrostatic latent image formed on the image carrier using a one-component developer composed of a toner or a two-component developer composed of a toner and a magnetic carrier as a developer. And developing means for developing an image by applying a developing bias to form an image, wherein the developing means has a maximum reflection when a solid image of the electrostatic latent image is developed as a toner. A toner Ta having a low density which gives a solid image having a low density and a toner Tb having a high density which gives a solid image having a high maximum reflection density are mixed and used, and the average particle diameter of the toner Ta is the toner Tb.
An image forming apparatus characterized in that the average particle size is at least 2 μm smaller than the average particle size. 4. The image forming apparatus according to claim 1, further comprising: a replenishing container for storing the toners Ta and Tb in the developing unit; and a unit for measuring a reflection density of an image formed on the image carrier. A small area image including a halftone portion is formed on the body, and the reflection density of the halftone portion is measured by the measuring means. When an increase in the reflection density of the halftone portion is detected, the toner Ta is supplied from the replenishing device. 4. The image forming apparatus according to claim 3, wherein the developer is supplied to the developing unit. 5. The image forming apparatus according to claim 1, further comprising: a replenishing container for storing the toners Ta and Tb in the developing unit; and a unit for measuring a reflection density of an image formed on the image carrier. A small area image including a halftone portion is formed on the body, and the reflection density of the halftone portion is measured by the measuring means. When an increase in the reflection density of the halftone portion is detected, the toner Tb is supplied from the replenishing device. 4. The image forming apparatus according to claim 3, wherein the developer is supplied to the developing unit.