JP2003270902A - Image forming method for color tandem type image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming method for a hybrid type developing device suitable for a color tandem type image forming apparatus preventing developing ghost and selective development from occurring without complicating the developing device, surely supplying electrified toner to a developing roll and also yielding stable image quality for a long time. <P>SOLUTION: In every developing device in the color tandem type image forming apparatus, toner layer peeling operation is performed from the developing roll toward a magnetic roll side when finishing development, and then the toner layer thickness of the developing device is varied when starting the development, depending on the image density of the developing device having the highest image density out of image data in respective toner colors. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming method in a color tandem type image forming apparatus used for a copying machine, a printer, a facsimile, and a composite machine thereof using an electrophotographic system, and more particularly to a magnetic carrier. A hybrid-type development in which a two-component developer for charging a non-magnetic toner is used to hold only the charged toner on a developing roll to fly to an electrostatic latent image and develop the latent image. The present invention relates to an image forming method in a color tandem type image forming apparatus having an apparatus.

[0002]

2. Description of the Related Art As a developing method in an image forming apparatus such as a copying machine, a printer, a facsimile, and a composite machine thereof using an electrophotographic method, a two-component developing method using a toner and a magnetic carrier, an insulating toner or a conductive toner is used. Using a two-component developer in which a non-magnetic toner is charged by using a magnetic carrier, and only the charged toner is held on a developing roll to fly to an electrostatic latent image. There is a hybrid developing system for developing a latent image.

The two-component developing method has an advantage that the carrier is excellent in chargeability of the toner and can have a long service life and that a solid image can be made uniform, but on the other hand, the developing device becomes large and complicated, and toner scattering and carrier occur. There are drawbacks such as the occurrence of pulling and the change in image quality due to the durability of the carrier.
Further, the one-component developing method has a compact developing device and is excellent in dot reproducibility, but has low durability due to deterioration of the developing roll and charge roller, and the cost of consumables is high because the entire developing device is replaced. However, it has the drawback that selective development occurs. The hybrid developing method is a method that is excellent in dot reproducibility, can have a long life, and can form an image at high speed, but conventionally, there have been problems such as development ghost generation and toner scattering.

This hybrid developing system has been studied as a means for non-contact one-component development.
As a developing method capable of high-speed image formation, a one-drum color superposition method for sequentially forming a plurality of color images on a latent image carrier (photoreceptor) has been studied. With this method, it is possible to form a color image with less color misregistration by accurately stacking toner on the latent image carrier, and it has attracted attention as a technique for achieving high color quality.

However, in the above-mentioned one-drum color superposition method, since the developing devices for the number of colors to be used have to be arranged around the latent image carrier, the latent image carrier becomes large and the image forming apparatus is downsized. Interfere. Therefore, a tandem system in which a plurality of electrophotographic process members corresponding to the color of the toner to be used are arranged side by side, a color image is formed in synchronization with the feeding of the transfer member, and the colors are superposed on the transfer member has attracted attention. It was However, this method has the advantage of being excellent in high speed, but has a drawback of increasing in size because the electrophotographic process members of respective colors must be arranged side by side. As a countermeasure against this, a small tandem type image forming apparatus has been proposed in which a space between the latent image carriers is narrowed and a downsized image forming unit is arranged.

In the small tandem type image forming apparatus having such a structure, it is advantageous to make the developing device a vertical type in order to minimize the size of the image forming unit in the width direction. That is, it is desirable in layout to arrange the developing device in the upper direction of the latent image carrier. However, in the conventional two-component developing method, when the developing device is vertically arranged in this manner, the flow of the developer, that is, the supply from the developer stirring section to the developing member in the vicinity of the latent image carrier becomes complicated, There has been a problem that miniaturization of the apparatus has a limit and that adhesion of the carrier to the latent image carrier and scattering of the toner cannot be avoided.

As another method, a one-component developing method which does not use a carrier has been proposed, but the method of bringing the developing roller into contact with the latent image bearing member causes torque fluctuation of the latent image bearing member, which is a weak point of the tandem type. There is a drawback that it promotes a certain color shift. In addition, in the system that does not contact the latent image carrier,
Since the toner was charged by the charger roll and the layer thickness on the developing roll was regulated by the elastic regulation blade, the toner additive adhered to the charger roll to lower the charging ability, and the toner adhered to the regulation blade. However, the layer formation becomes non-uniform, which may cause image defects.

Therefore, the hybrid developing system has been attracting attention as one of means for solving these problems. That is, in the hybrid developing method, there are problems such as development ghost generation and toner scattering as described above, but since the developing roll is not in contact with the latent image carrier, torque fluctuation does not occur and dot reproducibility is improved. It is possible to provide a high-speed image forming apparatus which is excellent in life and can be extended.

As a conventional technique relating to such a technique, US Pat. No. 3,866,574 discloses a thin non-magnetic toner on a donor roll (developing roll) placed in non-contact with a latent image carrier. It has been proposed to form a layer and cause the toner to fly to the latent image on the latent image carrier by an alternating electric field. U.S. Pat. No. 3,929,098 discloses a developing device in which a developer is advanced to a donor roll by using a magnetic roll and the toner is transferred onto the donor roll to form a toner layer. .

However, in these proposals, although a two-component developer can be used to form a thin layer on the donor roll, it becomes difficult to separate the toner on the donor roll when the toner charge becomes high. , A strong AC electric field is required. Since this electric field disturbs the toner layer on the latent image carrier, there is a problem in color superposition. Therefore, in JP-A-3-113474, an auxiliary electrode made of a wire is provided between the donor roll and the latent image carrier, and a weak AC electric field is applied to this auxiliary electrode so as not to disturb the developed toner. The so-called powder-cloud development method has been proposed.

In terms of theory, the Electrophotographic Society of Japan, No. 19
Vol. 2 (1981), Toshiba Corporation reported on the formation of a toner layer on a developing roll using a two-component developer, and there is a patent application in JP-A-59-121077. .

Further, in the above-mentioned conventional technique, coarse powder of toner having high developability is easily developed selectively on the latent image carrier,
When continuous printing is performed, fine powder of highly charged toner is deposited on the developing sleeve, which tends to cause a decrease in image density. Further, the toner charge control is complicated, and it is necessary to apply a high surface potential and a large developing electric field to the latent image carrier. Therefore, when a toner consumption region and a toner non-consumption region occur on the developing roll, the toner adhesion state on the developing roll and the toner potential difference vary, so that the previous developed image as shown in FIG. 3 is generated. There is a problem in that a phenomenon in which a part of the image appears as a residual image (ghost) during the next development, that is, a so-called history phenomenon easily occurs. That is, in FIG. 3, reference numeral 35 is a solid image composed of a rectangular black solid image, and 36 and 37 are halftone images wider than the solid image that follow, which are toner consumption areas and non-consumption areas on the developing roll. Occurs, the solid image 35 is followed by the halftone image 36,
When 37 is printed, an afterimage (ghost) like 38 in FIG. 3B occurs. Furthermore, when a high-density development pattern is continuously printed, image nonuniformity such as unevenness in image density is likely to occur, which is a problem in downsizing the developing device.

To prevent this, Japanese Patent Laid-Open No. 11-2316
JP-A-52-52 proposes a member for scraping off the development residual toner on the developing roll and a device for collecting the scraped toner, and as a method for surely collecting the toner on the developing roll, Japanese Patent Laid-Open No. 2000-81788 proposes to use a dedicated recovery roll.
However, these methods require a complicated mechanism and have not been put to practical use in a small electrophotographic process. Further, as a countermeasure against a hysteresis phenomenon using a magnetic brush, Japanese Patent Laid-Open No. 7-1
JP-A-28983 proposes to collect and supply the toner on the developing roll by widening the half-value width region of the magnetic flux density of the magnetic roll. Also, as a method for controlling a tandem type developing device, Japanese Patent Laid-Open No. 63-249164 has been proposed.
In the publication, there is an application for stopping the operation of the developing device in an image forming portion other than the image forming portion which is performing the transfer process to prevent the deterioration of the developer.

In such a hybrid developing system, the image density is lowered by selective development, development defects are caused by leaving the toner on the developing roll for a long time, image deterioration, development ghost, toner scattering, sleeve adhesion, etc. Therefore, Japanese Patent Laid-Open No. 7-72733 (U
SP 5,420,375), a magnetic roll forming a magnetic brush of a two-component developer, a donor roll (developing roll) carrying a thin layer of toner supplied from this magnetic roll, and this donor roll. And an electrode provided between the latent image bearing member and a bias composed of direct current and alternating current on this electrode, a direct current bias on the developing roll,
A hybrid developing device is disclosed in which a DC bias is applied to the magnetic roll so that the voltage can be switched to a voltage having a different polarity by a switch to solve the above problems.

That is, in the developing device disclosed in Japanese Unexamined Patent Publication No. 7-72733, a thin toner layer is formed on the developing roll by a magnetic brush formed on the magnetic roll due to the potential difference between the magnetic roll and the developing roll. Further, the toner cloud is formed in the vicinity of the electrode by the bias applied with the direct current and the alternating current applied between the developing roll and the electrode to develop the latent image on the latent image carrier, and when the image formation is completed, the switch is pressed. Is applied to collect the toner on the developing roll by applying a DC bias in the direction of separating the toner from the developing roll to the magnetic roll, and when the image is formed again, the toner on the magnetic roll is moved to the developing roll by switching the switch. The above problem is solved by applying a DC bias in the direction to prepare for image formation.

Further, Japanese Unexamined Patent Publication No. 2000-250294 discloses a system in which an electrode is provided between a developing roll and a latent image carrier as disclosed in Japanese Unexamined Patent Publication No. 7-72733, which is electrically biased and tension is applied. As a non-uniform development due to the vibration of the applied electrode wire or a phenomenon in which dust momentarily adheres to the electrode and causes a scratch on the developing roll, a hybrid type developing device using a developing roll with an embedded electrode is proposed. Introduced as a conventional example, even when using a developing roll with this electrode embedded, the carrier adhered to the developing roll adheres to the image, and the electrodes embedded in the developing roll have a certain interval so that a latent image bearing is carried. Phenomenon in which the toner supply efficiency to the body is low and images with a high image ratio are continuously developed, resulting in image loss, and an alternate bypass applied to the magnetic roll and the developing roll. Vinegar cause selective development, degradation of image quality,
A hybrid-type developing device is disclosed in which a phenomenon such as a decrease in density is prevented.

That is, this Japanese Patent Laid-Open No. 2000-250294
In the device disclosed in the publication, a developing layer is formed by providing a dielectric layer on a conductive sleeve and embedding electrodes at minute intervals therein, thereby improving the toner supply efficiency to the latent image carrier. Electrode relaxation tolerance that prevents short circuit between electrodes and protects the surface, and also allows the dielectric constant and fringe charge to penetrate the coating in about a few milliseconds or less to dissipate the accumulation of charge in a few seconds. Layers are provided. A brush electrode for supplying a bias voltage of AC and DC to the electrode embedded on the developing roll is embedded between the latent image carrier and the developing roll, and is embedded on the developing roll between the developing roll and the magnetic roll. A magnetic brush that supplies a bias voltage composed of AC and DC is provided to the electrode, and the toner on the developing roll is cloud-shaped between the latent image carrier and the developing roll to develop the latent image on the latent image carrier. The toner is reciprocated between the magnetic roll and the developing roll between the developing roll and the magnetic roll. The AC power source is used in common and the waveform is a rectangular wave, and the duty ratio of the rectangular wave is set so that the time for the toner to be conveyed from the magnetic roll to the developing roll is shorter than the time for the toner to be returned from the developing roll to the magnetic roll. By utilizing the difference in inertia between the toner and the carrier, the carrier is prevented from adhering to the developing roll, and the selective carrying property of the toner is prevented.

[0018]

However, in the powder cloud developing method described in Japanese Patent Laid-Open No. 3-113474, the wires of the auxiliary electrodes are very easily soiled, and the image deterioration due to vibration occurs. It's not a very common method. Further, JP-A-11-231
The devices shown in Japanese Patent Laid-Open No. 652, JP-A-2000-81788, JP-A-7-128983, etc. also require installation of a toner scraping device and a recovery roll,
The toner stress increases due to the application of a special recovery bias, which causes deterioration of the toner durability performance, and it takes time to form the layer of the developing roll at the next developing timing, impairing the high speed performance. . Also, for long-term use,
The chargeability of the toner changes due to the deterioration of the durability performance of the carrier, the charge characteristics of the toner on the developing roll change significantly, and the charge distribution of the replenishment toner and the collected toner becomes wide, which causes toner scattering and fogging due to poor charging. It was.
In reality, it is difficult to replace the deteriorated carrier, and it has not been put into practical use.

Further, the apparatus disclosed in Japanese Patent Laid-Open No. 63-249164 as a method for controlling a tandem type developing device is to stop the operation of the developing device other than the image forming section which is performing the transfer process, It is necessary to have a device and control for switching the high voltage applied between the developing roll and the magnetic roll at a high frequency, which is inevitably expensive, and the developing device has a configuration in which the donor roll, the magnetic roll, and the stirring member are arranged side by side. However, there is a drawback that it is difficult to reduce the size.

Further, Japanese Patent Laid-Open No. 7-72733 (US
P5, 341, 197), JP-A-2000-250294.
The device disclosed in Japanese Patent Laid-Open No. 7-72733 is also the powder cloud developing method described above, and the wires of the auxiliary electrodes are very easily soiled, and image deterioration due to vibration occurs. The device shown in 2000-250294 also has a complicated structure in which it is necessary to embed an electrode in the developing roll and a brush electrode for supplying a bias in which AC and DC are superposed on this electrode. When the brush electrode is discontinuous in some direction and the toner is fixed due to dirt or vibration for some reason and cannot contact the electrode of the developing roll, the toner cannot be controlled at all.

In view of the above circumstances, the present invention prevents the development ghost and the selective development from occurring without complicating the developing device, reliably supplies the charged toner to the developing roll, and provides stable image quality for a long time. It is an object to provide an image forming method in a hybrid type developing device suitable for the obtained color tandem type image forming device.

[0022]

In order to solve the above problems, according to the present invention, as described in claim 1, a two-component developer composed of a carrier and a toner is held on a developer transporting magnetic roll as a magnetic brush. After being charged, the toner is transferred by a conveying bias to form a thin layer of only toner on the surface of the developing roll, and then the developing bias applied to the closest position (developing position) between the developing roll and the latent image carrier. A color tandem in which a plurality of hybrid type developing devices for developing a latent image on a latent image bearing member to form a toner image are provided for different toner colors along the transfer medium moving direction of the toner image. In the image forming method in the image forming apparatus, the toner layer peeling operation from the developing roll toward the magnetic roll is performed at the end of development in each of the developing devices, and then each toner is removed. Among the image data of the color, characterized by variably controlling the toner layer thickness of the developing device at the developing start in accordance with the image density of the most image density high developing device. The image density described here refers to a numerical value corrected and measured by a Macbeth densitometer with a filter corresponding to each color.

In this way, after the toner layer peeling operation from the developing roll toward the magnetic roll is performed at the end of development, the image data amount of each toner color to be printed is calculated, and the developing device with the highest image density is used. By variably controlling the toner layer thickness of the developing device at the start of development according to the image density, it is possible to always obtain the toner layer thickness according to the image density. Therefore, for example, it is possible to prevent image non-uniformity such as non-uniformity of the toner layer and non-uniformity of image density, which occurs when high-density printing of 50% or more is continuously performed. In addition, when high density printing is continued, toner developability may decrease due to poor charging due to selective development of toner or insufficient toner amount on the developing roll. A color tandem image that can always supply an appropriate amount of toner and can prevent ghosts, developing roll contamination, and poor solid-following properties while maintaining a sufficient printing speed to obtain good images for a long period of time. An image forming method in a forming apparatus can be provided.

In an embodiment of the toner layer thickness that is variably controlled, as described in claims 2 to 4, the toner layer thickness at the start of the development is the gap between the developing roll and the latent image carrier. It is characterized by being 1/10 to 1/4. The toner amount of the toner layer thickness on the developing roll, which is variably controlled at the start of the development, is controlled within a range of 0.5 to 1.5 mg / cm ² . The charge amount of the toner on the developing roll at the start of the development is 5 to 20 μC /
It is characterized by controlling in the range of g.

Thus, the toner layer thickness at the start of development is set to 1/10 to 1/4 of the gap between the developing roll and the latent image carrier.
By controlling the toner amount in the range of 0.5 to 1.5 mg / cm ² and the toner charge amount in the range of 5 to 20 μC / g,
A uniform image can be obtained even if the amount of image data changes. For example, in normal printing, the amount of toner on the developing roll is set to a relatively small amount to reduce scattering, and for high density printing, increase the amount when necessary. By supplying the toner of No. 2 to the developing roll, an optimum toner layer can be always formed on the developing roll at the start of development, and thus a uniform image can always be obtained. By doing so, selective development is prevented, the layer thickness of the toner formed on the developing roll is controlled to an optimum value, toner scattering and deterioration in developability can be prevented, and a good image can be obtained for a long time. It is possible to provide an image forming method in a color tandem type image forming apparatus that can be obtained for a period of time.

According to a fifth aspect of the invention, the rotation ratio between the latent image carrier and the developing roller in the developing device is 1.
It is characterized in that it is set in the range of 5 to 3.

As described above, the number of rotations of the developing roller in the developing device is set to be 1.5 to 3 times the number of rotations of the latent image carrier so that the space between sheets (paper interval) is not increased. Can supply enough toner to the latent image above,
High-speed printing can be performed.

Next, the invention according to claim 6 is characterized in that the developing bias is constituted by superimposing an alternating current on a direct current and the developing bias is set to a volume specific resistance of 10 ⁶ Ω · cm ³ or less. It is characterized in that it is applied to a conductive sleeve on the surface, and when the development is completed, the toner layer on the developing roll is recovered to a magnetic roll.

By thus forming the outermost surface of the developing roll with a substantially uniform conductive sleeve of 10 ⁶ Ω · cm ³ or less, the bias applied with the DC and AC voltages applied to the developing roll is rotated. An image having a hybrid-type developing device capable of being applied between the developing roll, the latent image carrier, and the magnetic roll so that the image density is not deteriorated during durability, and stable image quality can be obtained for a long period of time. A forming device can be provided.

[0030]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be exemplarily described in detail below with reference to the drawings. However, the dimensions, materials, shapes, relative positions, and the like of the constituent parts described in this embodiment are not intended to limit the scope of the present invention thereto, unless otherwise specified, and are merely It is only an example.

FIG. 1 is a schematic configuration diagram of an embodiment of a color tandem type image forming apparatus for carrying out the image forming method according to the present invention, and FIG. 2 is a hybrid type developing apparatus for carrying out the image forming method according to the present invention. FIG. 3 is a diagram for explaining the generation of an afterimage (ghost), FIG. 4 is a table showing the relationship between the conductivity of the sleeve of the developing roll and the maintainability of the density, and FIG. FIG. 6 is a graph showing the relationship between the number of revolutions and the weight of the toner layer formed on the developing roll, and FIG. 6 is for explaining the conditions of the film thickness of the thin toner layer on the developing roll and ghost, scattering, and solid density followability. FIG.

In FIG. 1, 1 is a tandem type image forming apparatus main body, 2 is a developing device, 3 is a latent image carrier (photoreceptor), 4
Is an exposure unit, 5 is a recording medium conveyance belt, 6 is a developer container, 7 is a paper feed cassette containing a recording medium, 8 is a charger for charging the latent image carrier 3, and 9 is a latent image carrier. 3 is a transfer device for transferring the toner image on the recording medium to the recording medium by a transfer bias, and 10 is a fixing device for fixing the toner image transferred to the recording medium. In the color tandem type image forming apparatus, a latent image carrier is provided. It is important to design the charger 8, the exposure unit 4, the developing device 2, the transfer device 9 and the cleaning device, which are installed around the (photoreceptor) 3, in a compact size. It is adjacent to the image carrier 3 and arranged in a substantially vertical direction.

In FIG. 2, reference numeral 21 denotes a magnetic brush 30 of a two-component developer generated by a magnet disposed inside,
A magnetic roll as a developer carrier for supplying the toner 25 to the developing roll 22, a developing roll 22 for carrying a thin toner layer 26 to develop an electrostatic latent image on the latent image carrier 3, 24 a carrier, 25 Is a toner, 26 is a thin toner layer on the developing roll 22, 27 is a developing bias for causing the toner 25 of the thin toner layer 26 formed on the developing roll 22 to fly to the latent image carrier 3 to perform development. , 27a is a direct current (D
C) bias (V _dc1 ), 27b is an alternating current (AC) bias. 28 is a magnetic brush 30 on the magnetic roll 21
Direct current (DC) bias (V _dc2 ) for transporting the toner 25 from the toner to the developing roll 22, 29 is the magnetic roll 21
It is a regulation blade that regulates the thickness of the upper magnetic brush 30.

Among them, as a material of the latent image carrier (photoconductor) 3, an amorphous silicon (a-Si) photoconductor, an organic photoconductor (OPC) or the like can be used. The positively charged organic photoconductor (positive OPC) is stable in charging due to less generation of ozone and the like. Especially, the positively charged organic photoconductor having a single-layer structure is sensitive even when the film thickness is changed due to long-term use. It is suitable for a long-life system because the characteristics do not change much and the image quality is stable. When the positively charged organic photoreceptor is used in a system with a long life, the film thickness is 20 μm.
It is preferable to set the thickness from m to about 40 μm. 20μ
When the thickness is less than m, black spots are noticeable due to dielectric breakdown when the film thickness decreases to about 10 μm. On the other hand, when the thickness is 40 μm or more, the sensitivity decreases, which causes a decrease in image density.

As the exposure unit 4, a semiconductor laser or LED can be used. The wavelength around 770 nm is effective when the positively charged organic photoconductor is used, and the wavelength around 685 nm is effective when the amorphous silicon photoconductor is used. In the present invention, the case where a positively charged organic photoreceptor is used as the latent image carrier 3 and an LED is used as the light source of the exposure unit 4 will be described below as an example.

The outermost surface of the developing roll 22 is composed of a sleeve made of uniform conductive aluminum, SUS, conductive resin coating, etc., whose volume resistivity is set to 10 ⁶ Ω · cm ³ or less. And, the shaft part has a direct current (D
C) The bias (V _dc1 ) 27a and the alternating current (AC) bias 27b are connected, and the bias in which the direct current and the alternating current are superposed acts between the rotating developing roll 22, the latent image carrier 3, and the magnetic roll 21. To do so. The AC component supplied by the AC bias 27b is a rectangular wave having a duty ratio of 50% or less.
In the present invention, the DC bias (V _dc1 ) 27a is set to 100 V, the AC bias 27b is set to Vpp of 1.5 kV, the frequency of 3.0 kHz, and the duty ratio of 30%, as an example. As described above, the direct current bias (V _dc1 ) 27a and the alternating current bias 27b are directly applied to the developing roll 22, and the outermost surface of the developing roll 22 is made to have conductivity of 10 ⁶ Ω · cm ³ or less, so that development is performed at the time of development. A sharp bias component can be applied between the roll 22, the latent image carrier 3, and the magnetic roll 21, and the reaction of toner layer formation at the start of development can be improved.

The magnetic roll 21 includes a carrier 24 and a toner 25 by means of a magnet provided inside as a developer transport body.
The two-component developer consisting of is magnetically held to generate the magnetic brush 30, and the thickness of the magnetic brush 30 is regulated by the blade 2.
The toner 25 is supplied to the developing roll 22 while being regulated by 9. The gap between the regulation blade 29 and the magnetic roll 21 is preferably 0.3 to 1.5 mm. The toner 25 is supplied to the developing roll 22 by applying a direct current (DC) bias (V _dc2 ) 28 applied to the magnetic roll 21 and a direct current bias (V _dc1 ) applied to the developing roll 22.
This is performed by the potential difference with respect to 27a and the AC bias 27b. In the present invention, the voltage of the DC bias (V _dc2 ) 28 is set to 350 V, and when the toner layer on the developing roll 22 is replaced at the end of development, the DC bias (V _dc2 ) is applied with the AC bias 27 b applied.
28 is changed and the thin toner layer 26 on the developing roll 22 is collected by the magnetic brush 30. This DC bias (V
The change amount of _dc2 ) 28 is a value that causes a voltage difference that always causes the toner of the magnetic brush 30 to be transferred to the developing roll 22, that is, the DC bias voltage value on the magnetic roll 21 side is the DC bias voltage value on the developing roll 22 side. When the DC bias (V _dc1 ) 27a is maintained at 100 V, a value higher than that is maintained (ie, 100 V to 35 V in this example).
(Value between 0V). It should be noted that this change in the DC bias changes the DC bias (V _dc2 ) 28 in the above description, but if the DC bias voltage value on the magnetic roll 21 side is kept larger than the DC bias voltage value on the developing roll 22 side. _{Alternatively} , both the DC bias (V _{dc 2} ) 28 and the DC bias (V _dc1 ) 27a may be changed.

The conductivity of the sleeve on the outermost surface of the developing roll 22 is related to the maintenance of the print density. This is shown in the table of FIG. 4, in which the conductivity of the sleeve of the developing roll 22 is changed and the change in the image density (initially 1.43) after printing 10,000 sheets is examined. The DC bias (V _dc1 ) 27a applied to the developing roll 22 is 100V, and the Vpp in the AC bias 27b is 1.5V.
kV, frequency 3.0 kHz, duty
The ratio was 30%, and the DC bias (V _dc2 ) 28 applied to the magnetic roll 21 was 350V. As can be seen from these results, when the volume resistivity is 10 ⁷ Ω · cm ³ or more, the density decrease after printing 10000 sheets is large, and when the volume resistivity is 10 ⁶ Ω · cm ³ or less, there is almost no problem with the image density. I understand.

The distance between the latent image carrier 3 and the developing roller 22 is, for example, about 250 μm, and no wire electrode or the like is used between them. Normally, the latent image carrier 3 and the developing roll 22 are
The distance between and is 150 to 400 μm, preferably 200
To 300 μm, if this interval is narrower than 150 μm, it causes fog, and if it is wider than 400 μm, the toner 2
It becomes difficult to fly 5 onto the latent image carrier 3 and sufficient image density cannot be obtained. In addition, it becomes a factor that causes selective development. Magnetic roll 21 and developing roll 2
The distance from 2 is about 0.3 to 1.5 mm.

The developer is composed of toner 25 and carrier 24, and it is important that the toner 25 defines a particle size distribution in order to avoid selective developability. Generally, the spread of the particle size distribution of the toner is measured by a Coal counter and expressed by the ratio of the volume distribution average particle size to the number distribution average particle size.
In order to prevent selective development, it is important to reduce the ratio. When the distribution is wide, toner having a relatively small particle size is deposited on the developing roll during continuous printing, which lowers the developability. Further, in the present invention, the charge amount was controlled within the range of 5 to 20 μC / g in order to prevent selective development. The toner charge amount is related to the layer thickness of the toner thin layer 26 formed on the developing roll 22, and the toner charge amount is 10 μC / g.
Below, especially, when it is as low as 5 μC / g or less, the thickness of the toner layer becomes thick and the scattering increases. On the other hand, the toner charge amount is 20 μC /
When it is more than g, the toner layer becomes thin, the charge is increased and the developability of the toner is lowered. The amount of charge of the toner 25 is determined by the toner thin layer 26 on the developing roll 22 by the Trek Company Q.
It was measured by suctioning with an M meter. In the present invention, the case where positively charged toner is used will be described as an example, but it is obvious that the same configuration can be achieved even when negatively charged toner is used by reversing the relationship with the bias described above.

As the carrier 24, a magnetite carrier, Mn-based ferrite, Mn-Mg-based ferrite, or the like can be used, and it is also possible to use it by surface-treating it within a range that does not raise an appropriate resistance value. In the present invention, as an example, the volume resistivity is 10 ⁸ Ωcm ³ coated with a silicone resin, the saturation magnetization is 70 emu / g, and the average particle size is 35 μ.
m ferrite carrier was used. Average particle size is 50 μm
If it exceeds, the stress of the carrier increases and the toner concentration cannot be increased, and the amount of toner supplied to the developing roll 22 decreases. By setting the average particle size of the carrier to 50 μm or less, sufficient charge can be imparted even when the concentration of the toner in the developer is set to 5 to 20%, and stable development can be performed.

The mixing ratio of the toner 25 and the carrier 24 is 5 to 20% by weight, preferably 5 to 15% by weight of the total amount of the carrier 24 and the toner 25. If the mixing ratio of the toner 25 is less than 5% by weight, the charge amount of the toner becomes high and a sufficient image density cannot be obtained, and if it exceeds 20% by weight, a sufficient charge amount cannot be obtained this time. Are scattered from the developing device to contaminate the inside of the image forming apparatus, or toner fogging occurs on the image.

The thin layer 26 of toner 25 at the start of development on the developing roll is 10 to 100 μm, preferably 3
When the toner 25 has a thickness of 0 to 70 μm and the average particle diameter of the toner 25 is 7 μm, it corresponds to about 5 to 10 layers of the toner 25, and the gap (15) between the developing roll and the latent image carrier is
0-400 μm, preferably 200-300 μm) 1
It becomes / 10 to 1/4. Therefore, in the present invention, in a series of image forming processes, the layer thickness of the toner 25 formed on the conductive sleeve of the developing roll 22 is 50 μm or less, and the toner amount is in the range of 0.5 to 1.5 mg / cm ² . By the control, a clear latent image is formed on the latent image carrier 3, and at the same time, the toner on the developing roll 22 is easily replaced, and the developing ghost is controlled.

The thickness of the thin toner layer 26 is a DC bias.
(V_dc2) 28 and DC bias (V _dc1) 27a
Difference, and the amount of toner charge as described above
Be done. Toner charge is less than 10μC / g, especially 5μC / g
If it is as low as g or less, the toner layer becomes thick and scattering increases.
It On the other hand, if the toner charge amount exceeds 20 μC / g,
The thickness of the toner layer becomes thin, the charge increases, and the developability of the toner increases.
descend. As described above, the DC bias (V_dc1)
27a is 100V, DC bias (V_dc2) 28 values
Is set to 350 V, a toner layer of about 40 μm is obtained.
Be done. At this time, the amount of toner per unit area is about 1.0 m
g / cm^TwoIs. Thin toner layer 26 is 0.5 mg / cm
^TwoIf the density is too thin, the density of high density images
Tracking performance is reduced, image unevenness is more likely to occur, and toner
Layer is 1.5mg / cm^TwoIf it is too thick,
Development ghosts like the one shown are conspicuous, and toner scattering
Tends to stand out. That is, as described above, in FIG.
Numeral 35 is a solid image composed of rectangular black solid
And 36 and 37 are from this solid image that follows.
With a wide halftone image, the toner on the developing roll 22 is
If a consumed area and a non-consumed area occur, this solid image
Print halftone images 36 and 37 after image 35
Then, an afterimage (ghost image such as 38 in FIG. 3B)
) Occurs.

To replace the toner thin layer 26 on the developing roll 22, the DC bias (V _dc2 ) 28 is changed with the AC bias 27b applied at the end of the development, and the toner thin layer 26 on the developing roll 22 is magnetically changed. Collect in the brush 30. When the rotation speed of the magnetic roll 21 is set to 1 to 2 times the rotation speed of the developing roll 22, replacement of the toner on the developing roll 22 is promoted. At this time, it is preferable that the magnetic roll 21 rotates in the same direction as the developing roll 22, for example, when the developing roll 22 rotates in the counterclockwise direction, the magnetic roll 21 also rotates in the counterclockwise direction. In order to supply sufficient toner to the latent image on the latent image carrier 3,
When the peripheral speed of the developing roll 22 is set to 1.5 to 3 times that of the latent image carrier 3, the toner can be taken in and out in a short time. If the speed difference is 1.5 times or less, it takes too much time to start development, and a sufficient paper interval is required.
High speed printing is not achieved. On the other hand, if it is 3 times or more, the rotation speed of the developing roll 22 increases, which causes vibration and heat generation.

In a small developing device, for example, 50
When high density printing of more than 100% is continuously performed, the developing roll 2
The developability of the toner on No. 2 may be deteriorated. One of the possible reasons for this is the selective development of toner, and the other is that the amount of toner on the developing roll 22 is insufficient. As a countermeasure against the selective development, it is proposed that the toner 26 is pulled back from the developing roll 22 to the magnetic roll 21 by the reverse electric field after the completion of the development, but in this case, another reason is to start the next development. Since the amount of toner on the roll 22 is insufficient, there is a high possibility that the image density becomes uneven.

Therefore, in order to stabilize the image density, when the print data is sent, the image data amount of the print data is calculated by the dot counter provided in the control device (not shown) in the image forming apparatus, and the image data is calculated. The layer thickness of the toner thin layer 26 on the developing roll 22 may be controlled according to the amount of data. FIG. 5 is a graph showing the relationship between the rotation speed of the developing roll 22 and the weight of the thin toner layer 26 formed on the developing roll 22. In the graph of FIG. 5, the horizontal axis represents the number of rotations of the developing roll 22, the vertical axis represents the weight of the toner layer 26 formed on the developing roll 22, and the solid line represents a toner layer suitable for image data of a normal printing rate. The weight represents the weight, and the dotted line represents the weight of the toner layer suitable for the image data having a high printing rate. Further, a direct current (DC) bias (V _dc1 ) 27a applied to the developing roll 22 and a direct current (DC) bias (V _dc2 ) 2 applied to the magnetic roll 21.
8 is a direct current (DC) bias (V _dc1 ) 27a
0 V and direct current (DC) bias (V _dc2 ) 28 were set to 350 V in the solid line in the image data of the normal printing rate and 400 V in the dotted line in the image data of the high printing rate.

As can be seen from this graph, the minimum required toner amount on the developing roll 22 is about 0.7 to 0.8 mg /
Direct current (DC) bias (V _dc1 ) 27 for cm ^2.
a is 100 V, direct current (DC) bias (V _dc2 ) 28
In the case of a solid line corresponding to image data of a normal printing rate of 350 V, the toner amount is about 0.4 to 0.5 mg / cm ² at the first rotation of the developing roll 22, and the minimum required toner at the second rotation. The amount exceeds about 0.7 to 0.8 mg / cm ² . Also in the case of the dotted line corresponding to the image data having a high printing rate, about 0.6 to 0.
The toner amount is 7 mg / cm ² , and it is about 0.9 at the second rotation.
To 1.0 mg / cm ² , which also exceeds the minimum required toner amount in the second rotation, and by controlling the voltage value of the direct current (DC) bias (V _{d c2} ) 28 in this way, the developing roll 22 The amount of toner above can be varied.

That is, as described above, the magnetic roll 21
Is set to be 1 to 2 times the rotation speed of the developing roll 22 and the rotating direction of the magnetic roll 21 is the same as that of the developing roll 22.
C) By controlling the voltage value of the bias (V _dc2 ) 28, the developing roll 22 is adjusted according to the image data amount of the print data.
The layer thickness of the upper toner thin layer 26 may be controlled to 1/10 to 1/4 of the gap (150 to 400 μm, preferably 200 to 300 μm) between the developing roll 22 and the latent image carrier 3. Therefore, when performing continuous printing, a printing rate of print data is calculated by a dot counter provided in a control device of the image forming apparatus, and a DC bias (V _dc2 ) 28 is changed according to the printing rate to obtain a uniform image. Can be obtained. This is important also in terms of preventing the toner from scattering from the developing roll 22, and in normal printing, the amount of toner on the developing roll is set to be relatively small to reduce the scattering, which is necessary for high density printing and the like. By adjusting the amount of toner on the developing roll only at this time, an optimum toner layer can be formed on the developing roll at the start of development.

FIG. 6 shows a thin toner layer 2 on the developing roll 22.
The film thickness (mg / cm ² ) of No. 6 was changed from 0.4 to 2.0, and the relationship between ghost, scattering, and solid density followability was shown. When ○ is good in each item, Δ is When a little is observed, x is a defect. This Figure 6
As can be seen from the above, if the toner layer is too thin, such as 0.4 mg / cm ² , the followability of density when the high-density image is continuous decreases, and if the toner layer exceeds 1.7 mg / cm ² , it is too thick. Development ghost and toner scattering tend to be noticeable. Therefore, the film thickness of the toner thin layer 26 (mg / cm ² )
It can be seen that is preferably about 0.5 to 1.5 (mg / cm ² ).

In the tandem type image forming apparatus of the present invention having the hybrid type developing device thus constructed,
The two-component developer including the toner 25 and the carrier 24 corresponding to each color such as yellow, cyan, magenta, and black is supplied from the developer container 6 to the developing device 2,
The magnetic brush 30 is formed on the magnetic roll 21 shown in FIG. 2, and the toner 25 is charged by stirring. The magnetic brush 30 on the magnetic roll 21 is layer-regulated by the regulating blade 29, and is between the DC bias (V _dc2 ) 28 applied to the magnetic roll 21 and the DC bias (V _dc1 ) 27 a applied to the developing roll 22. The thin layer 26 of only the toner 25 is formed on the developing roll 2 by the potential difference and the AC bias 27b.

When a print start signal comes from a control circuit (not shown), the latent image carrier 3 composed of a positively charged organic photoconductor (positive OPC) is first charged to 400 V by the charger 8. After that, by exposure with an LED having a wavelength of 770 nm, for example, which constitutes the exposure unit 4, the potential after exposure of the latent image carrier 3 becomes about 70 V and a latent image is formed. Then, the latent image is formed on the developing roller 22 by the DC bias (V _dc1 ) 27a and the AC bias 27b applied to the developing roller 22.
Is developed with the toner flying from the latent image carrier 3 to form a toner image.

When the toner image is formed on the latent image carrier 3 in this way, the recording medium is taken out of the paper feeding cassette 7 and conveyed by the conveying belt 5 at the timing when the toner image reaches the transfer position. Then, a transfer bias is applied by the transfer device 9 installed at the transfer position for each color, and the toner image is transferred to the recording medium. When the toner images of the respective colors are sequentially transferred to the recording medium and reach the fixing device 10, the fixing device 10 fixes the toner images and discharges the paper.

The AC bias 27b applied to the developing roll 22 is a rectangular wave having a duty ratio of 50% or less as described above, and is longer than the time when the toner 25 is conveyed from the magnetic roll 21 to the developing roll 22. The time for pulling back the toner 25 from the developing roll 22 to the magnetic roll 21 is short. In addition, DC bias (V _dc1 ) 2
7a and AC bias 27b are directly applied to the developing roll 22, and the outermost surface of the developing roll 22 is 10 ⁶ Ω · c.
By having a conductivity of m ³ or less, the developing roll 22
The AC component when the upper toner thin layer 26 is pulled back to the magnetic roll 21 has a sharp peak, and the toner 25 is effectively pulled back. Therefore, the effect of pulling back the toner from the developing roll 22 to the magnetic roll 21 as a developer transporting body is increased, the toner contamination on the developing roll 22 is prevented, and the toner is developed from the latent image carrier 3 at the time of development. The pulling back toward the roll 22 is also effectively performed, and it is possible to provide a developing device capable of preventing fogging and obtaining stable image quality for a long period of time.

In the present invention, when the control circuit (not shown) in the image forming apparatus receives the print data, the dot counter calculates the printing rate of the image data of each toner color used in the color tandem type image forming apparatus. Then, the thin toner layer 26 is peeled off from the developing roll 22, refreshed, and the toner layer thickness is variably controlled at the start of development in accordance with the image density of the developing device having the highest image density. That is, this refresh changes the DC bias (V _dc2 ) 28 while applying the AC bias 27b between the printing papers after the development, and collects the toner thin layer 26 on the developing roll 22 on the magnetic brush 30. At the start of development, the voltage is returned to a value corresponding to the image density to change the toner layer thickness. In this way, the layer thickness of the toner thin layer 26 on the developing roll 22 is
Gap between developing roll and latent image carrier (150-400μ
m, preferably 200 to 300 μm) 1/10 to 1 /
Controlling to 4 prevents development of ghosts, contamination of the developing rolls, and poor solid-following property to obtain a good image, and development in a small color tandem type image forming apparatus capable of obtaining stable image quality for a long period of time. A method can be provided.

The DC bias (V
The voltage values, Vpp, frequency, etc. of the _dc1 ) 27a, the AC bias 27b, and the DC bias (V _dc2 ) 28 are examples, and it is obvious that they can be changed according to the situation.

[0057]

As described above, according to the invention described in claim 1 of the present invention, each toner color to be printed after the toner layer peeling operation from the developing roll toward the magnetic roll is performed at the end of development. The amount of image data is calculated, and the toner layer thickness of the developing device is variably controlled at the start of development according to the image density of the developing device having the highest image density, so that the toner layer thickness according to the image density is always obtained. be able to. Therefore, for example, it is possible to prevent image non-uniformity such as non-uniformity of the toner layer and non-uniformity of image density, which occurs when high-density printing of 50% or more is continuously performed. In addition, when high density printing is continued, toner developability may decrease due to poor charging due to selective development of toner or insufficient toner amount on the developing roll. A color tandem image that can always supply an appropriate amount of toner and can prevent ghosts, developing roll contamination, and poor solid-following properties while maintaining a sufficient printing speed to obtain good images for a long period of time. An image forming method in a forming apparatus can be provided.

According to the present invention described in claims 2 to 4, the toner layer thickness at the start of development is 1/10 to 1/4 of the gap between the developing roll and the latent image bearing member, and the toner amount is 0. 5
To 1.5 mg / cm ² , and the toner charge amount is 5 to 20 μm.
By controlling in the range of C / g, it is possible to obtain a uniform image even if the amount of image data changes. For example, in normal printing, the amount of toner on the developing roll is set to a relatively small amount to reduce scattering. However, by supplying a large amount of toner to the developing roll when necessary for high density printing, an optimum toner layer can always be formed on the developing roll at the start of development, so that a uniform image can always be obtained. Become. Further, by doing so, selective development is prevented, the layer thickness of the toner formed on the developing roll is controlled to an optimum value, and toner scattering and deterioration of developability can be prevented,
It is possible to provide an image forming method in a color tandem type image forming apparatus capable of obtaining a good image for a long period of time.

According to the fifth aspect of the present invention, the number of rotations of the developing roller in the developing device is set to 1.5 to 3 times the number of rotations of the latent image carrier, so that the sheet interval (paper interval) is increased. ) Is not increased, sufficient toner can be supplied to the latent image on the latent image carrier, and high-speed printing can be performed.

According to the present invention described in claim 6, the outermost surface of the developing roll has a substantially uniform 10 ⁶ Ω · cm ³
By configuring with the following conductive sleeve, it is possible to apply the bias applied to the developing roll so that the bias with the superimposed DC and AC voltages works well between the rotating developing roll, latent image carrier and magnetic roll. Therefore, it is possible to provide an image forming apparatus having a hybrid type developing device that can obtain stable image quality for a long period of time without causing a decrease in image density during durability.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an embodiment of a color tandem type image forming apparatus for carrying out an image forming method according to the present invention.

FIG. 2 is a schematic configuration diagram of a hybrid developing device for carrying out the image forming method according to the present invention.

FIG. 3 is a diagram for explaining the occurrence of an afterimage (ghost).

FIG. 4 is a table showing the relationship between the conductivity of the sleeve of the developing roll and the maintainability of the density.

FIG. 5 is a graph showing the relationship between the number of rotations of the developing roll and the weight of the toner layer formed on the developing roll.

FIG. 6 is a diagram for explaining the film thickness of the thin toner layer on the developing roll and the conditions of ghost, scattering, and solid density followability.

[Explanation of symbols]

2 developing device 3 latent image carrier (photoreceptor) 4 exposure unit 21 magnetic roll 22 developing roll 24 carrier 25 toner 26 toner thin layer 27 developing bias 27a direct current (DC) bias (V _dc1 ) 27b alternating current (AC) bias 28 direct current (DC) bias (V _dc2 ) 29 regulating blade 30 magnetic brush

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Shoichi Sakata             704 No. Matabei, Noshino, Tamaki Town, Tokai District, Mie Prefecture             19 Kyocera Corporation Mie Factory Tamaki Block             Within (72) Inventor Toshinori Nishimura             704 No. Matabei, Noshino, Tamaki Town, Tokai District, Mie Prefecture             19 Kyocera Corporation Mie Factory Tamaki Block             Within F-term (reference) 2H073 AA01 BA04 BA13 BA41 BA45                       CA03 CA22                 2H077 AC04 AC13 AD02 AD06 AD13                       AD17 AD36 BA03 DB01 DB08                       DB12 DB14 EA01 FA19 FA25                       FA29 GA13                 2H300 EA06 EA08 EB04 EB12 EB23                       EF02 EF06 EF09 EJ09 EJ19                       EJ22 EJ23 EJ25 EJ32 EJ39                       EJ42 EJ51 EJ53 FF05 GG01                       GG02 GG03 GG04 GG11 GG40                       GG45 GG49 HH21 MM15 MM22                       MM25 PP06 QQ02 QQ17 RR18

Claims (6)

[Claims] 1. A two-component developer comprising a carrier and a toner is held on a developer-conveying magnetic roll as a magnetic brush to be charged, and the toner is transferred by a conveying bias to form a thin layer of only the toner on the surface of the developing roll. After that, a hybrid type developing device for developing a latent image on the latent image bearing member by a developing bias applied to the closest position (developing position) between the developing roll and the latent image bearing member to form a toner image is provided. In an image forming method in a color tandem type image forming apparatus in which a plurality of different toner colors are arranged along the transfer medium moving direction of the toner image, in all of the developing devices, from the developing roll to the magnetic roll side. Toner layer peeling operation is performed at the end of development, and then at the start of development according to the image density of the developing device with the highest image density among the image data of each toner color. The image forming method in a color tandem-type image forming apparatus characterized by serial variably controlling the toner layer thickness of the developing device. 2. The color tandem image according to claim 1, wherein the toner layer thickness at the start of development is 1/10 to 1/4 of the gap between the developing roll and the latent image carrier. Image forming method in forming apparatus. 3. The toner amount of the toner layer thickness on the developing roll, which is variably controlled at the start of the development, is controlled within a range of 0.5 to 1.5 mg / cm ^2. An image forming method in the described color tandem type image forming apparatus. 4. The image formation in the color tandem image forming apparatus according to claim 1, wherein the charge amount of the toner on the developing roll at the start of the development is controlled within a range of 5 to 20 μC / g. Method. 5. The color tandem type image forming apparatus according to claim 1, wherein the rotation ratio between the latent image carrier and the developing roller in the developing device is set in the range of 1.5 to 3. Image forming method. 6. The developing bias is constituted by superimposing a direct current on an alternating current, and the developing bias is constituted by a volume resistivity 10.
The toner layer on the developing roll is recovered to a magnetic roll at the end of development by applying to a conductive sleeve on the outermost surface of the developing roll set to ⁶ Ω · cm ³ or less. Forming method in the color tandem type image forming apparatus described above.