JP2003280357A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-speed, stable, and small-sized hybrid type image forming apparatus having a small-sized hybrid type developing device which can obtain stable image quality for a long period of time by preventing the occurrence of a development ghost and a selection phenomenon without making the developing device complicated and securely supplying an electrostatically charged toner to a developing roll. <P>SOLUTION: At least the surface of the developing roll is made conductive and a continuous electrode surface which electrifies a development position with a developing bias in a circumferential direction is formed of the conductive surface; and the developing roll and a latent image carrier are arranged at the closest positions (development position) between the developing roll and latent image carrier to directly face each other not across the electrode, but across an insulating toner thin layer; and an AC bias of 10 to 50% in duty ratio on which a DC bias is superposed is applied to the developing roll to prevent the occurrence of the development ghost and the selection phenomenon. <P>COPYRIGHT: (C)2004,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 apparatus such as a copying machine, a printer, a facsimile, and a composite machine thereof using an electrophotographic method, and more particularly to charging a non-magnetic toner by using a magnetic carrier. The present invention relates to an image forming apparatus having a hybrid-type developing device that uses a two-component developer and holds only a charged toner on a developing roll to fly to an electrostatic latent image and develop the latent image. is there.

[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 the durability is low due to deterioration of the developing roller and the charge roller, and the cost of consumables is high because the developing device is replaced. However, there is a defect that selective development occurs. And the hybrid development method is
Although this method has excellent dot reproducibility, a long life, and high-speed image formation, conventionally, there have been problems such as development ghost generation and toner scattering.

This hybrid developing system is a non-contact type 1
Although it has been studied as a means for developing a component, in recent years, as a developing method capable of high-speed image formation, particularly for a one-drum color superposing method for sequentially forming a plurality of color images on a latent image carrier (photoreceptor). Has also been considered. 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-described one-drum color superposition method, since the developing devices for the number of colors to be used must be arranged around the latent image carrier, the latent image carrier becomes large and the image forming apparatus becomes compact. Interfere with 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-sized tandem type image forming apparatus constructed as described above, 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 charge roll and the layer thickness on the developing roll was regulated by the elastic regulation blade, the toner additive adhered to the charge roll to lower the charging ability, or the toner adhered to the regulation blade. The layer formation becomes non-uniform, which sometimes causes 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, and selective development is apt to occur, so that the image density tends to decrease. 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 rectangular black solids, and 36 and 37.
Is a halftone image wider than the solid image that follows, and when toner consumption areas and non-consumption areas are generated on the developing roll, the solid image 35 and the halftone images 36 and 37 are printed. An afterimage (ghost) such as 38 in 3 (b) occurs.

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.

Further, in the hybrid developing system, the image density is lowered by selective development, and development defects, image deterioration, development ghost, toner scattering, sleeve adhesion, etc. occur due to leaving the toner on the developing roll for a long time. JP, 7-72733, A (USP 5,4
No. 20,375), a magnetic roll forming a magnetic brush of a two-component developer, a donor roll (developing roll) carrying a thin toner layer supplied from the magnetic roll, the donor roll and a latent image. An electrode provided between the carrier and the carrier, a bias composed of direct current and alternating current can be switched to this electrode, a direct current bias can be switched to the developing roll, and a voltage of different polarity can be switched to a magnetic roll by a switch. A hybrid-type developing device configured to apply the DC bias as described above and solving the above problems is shown.

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, A device and a control for switching the high voltage applied between the developing roll and the magnetic roll at a high frequency are required, which is inevitably expensive, and the developing device has a configuration in which a toner roll, a magnetic roll, and a 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, and reliably supplies the charged toner to the developing roll to obtain stable image quality for a long time. An object of the present invention is to provide a high-speed and inexpensive small-sized image forming apparatus having the obtained small-sized hybrid type developing apparatus.

[0022]

In order to solve the above-mentioned problems, according to the present invention, as described in claim 1, a two-component developer carrier for charging a developer consisting of a carrier and a toner while magnetically holding it. , A developing roll that transfers toner from the carrier to form a thin layer of only toner on the surface, and a latent image by applying a developing bias to the closest position (developing position) between the developing roll and the latent image carrier. In an image forming apparatus having a hybrid-type developing device for developing a latent image on a carrier, the developing roll and the latent image carrier are directly connected to each other at the developing position via an insulating toner thin layer without an electrode. While facing each other, at least the surface of the developing roll is electrically conductive, and the conductive surface forms a continuous electrode surface in which a developing bias is applied in the circumferential direction at the developing position.

As described above, by constructing a hybrid type developing device with a simple structure for applying a developing bias to a developing roll having a conductive surface, an auxiliary electrode is provided or an electrode is embedded in the developing roll. Therefore, the image deterioration due to dirt and vibration of the wire electrode, and the brush electrode that supplies a bias to the electrode embedded in the developing roll cannot contact the electrode of the developing roll, so that the toner cannot be controlled. No developing device can be obtained. In addition, the surface of the developing roll is electrically conductive and a continuous electrode surface is formed.By directly applying a bias to the electrically conductive developing roll, the developing roll, which is a rotating body, develops at the closest portion to the latent image carrier. Is maximized, and as the distance increases thereafter, development ends and fogging is caused.Toner is pulled back to the developing roll side and development on the latent image carrier can be performed accurately, so fogging is prevented and stable for a long period of time. It is possible to provide a small-sized image forming apparatus having a hybrid-type developing device that can achieve the above image quality.

The invention described in claim 2 is characterized in that the developing roll as a whole is conductive, and an AC bias in which a DC bias is superimposed is applied to the developing roll.

As described above, by applying an AC bias in which the entire developing roll is made conductive and a DC bias is superimposed, it becomes possible to obtain good developability between the latent image carrier and the developing roller and To provide an image forming apparatus having a hybrid-type developing device capable of constantly exchanging toner between a roll and a developer transport body, suppressing selective development, and obtaining stable image quality for a long period of time. it can.

The invention described in claim 3 is characterized in that the duty ratio of the AC bias is set to 10 to 50%.

As described above, by setting the duty ratio of the AC bias to 10 to 50%, the peak of the AC bias becomes sharp and the toner can be effectively pulled back, and at the same time from the developing roll to the developer carrier. The pullback effect is increased and the toner contamination on the developing roll is eliminated.
At the same time, the toner is effectively pulled back from the latent image carrier toward the developing roller during development, and fogging is less likely to occur. Therefore, image formation having a hybrid developing device capable of obtaining stable image quality for a long period of time. A device can be provided.

Next, the invention according to claim 4 is characterized in that the outermost surface of the developing roll is composed of a substantially uniform conductive sleeve of 10 ⁶ Ω · cm ³ or less.

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 tandem type image forming apparatus having a hybrid type developing device according to the present invention, FIG. 2 is a schematic configuration diagram of the hybrid type developing device according to the present invention, and FIG. 3 is an afterimage. FIG. 4 is a table for explaining the occurrence of (ghost), and FIG. 4 is a table showing the experimental results of the relationship between the conductivity of the sleeve of the developing roll and the maintainability of the density.

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. A transfer device 10 for transferring the toner image on the recording medium 3 to the recording medium by a transfer bias is a fixing device for fixing the toner image transferred to the recording medium. In the tandem type image forming apparatus, a latent image carrier ( A charger 8, an exposure unit 4, a developing device 2, a transfer device 9 installed around the photoconductor 3;
It is important to design the cleaning device and the like compactly, and in the example of the present invention, the developing device 2 is disposed adjacent to the latent image carrier 3 and 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 or the like having a volume resistivity of 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 Coulter counter and is expressed by the ratio of the volume distribution average particle diameter to the number distribution average particle diameter. It has been found that the ratio is 1.25 or less in order to prevent selective development. If it is 1.25 or more, toner having a relatively small particle size is accumulated on the developing roll during continuous printing,
Developability is reduced. In the present invention, in order to prevent selective development, the ratio of volume average particle diameter / number average particle diameter is 1.25.
Below, the charge amount was controlled within the range of 5 to 20 μC / g. This toner charge amount is related to the layer thickness of the toner thin layer 26 formed on the developing roll 22, and if the toner charge amount is 10 μC / g or less, particularly 5 μC / g or less, the toner layer thickness becomes thick. , Scattering increases. On the other hand, when the toner charge amount is 20 μC / g or more, the toner layer thickness becomes thin, the charge increases, and the developability of the toner decreases. This toner 2
The charge amount of 5 was measured by sucking the thin toner layer 26 on the developing roll 22 with a QM meter manufactured by Trek. In the present invention, the case of using positively charged toner will be described as an example, but by reversing the relationship with the bias,
It is obvious that the same configuration can be achieved even when negatively charged toner is used.

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 40 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 on the developing roll is
The thickness is 10 to 100 μm, preferably 30 to 70 μm, and is a value corresponding to about 5 to 10 layers of the toner 25 when the average particle diameter of the toner 25 is 7 μm. 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 1.7 mg / cm ² . By controlling, 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. The amount of toner per unit area at this time is about 1.5 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 1.7 mg / 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.
Wide halftone image, toner consumption on developer roll
When an area and a non-consumed area occur, this solid image 3
Print halftone images 36 and 37 following step 5.
And an afterimage (ghost) like 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. In order to maintain a good printing speed when the toner is replaced, the paper interval may be adjusted to adjust the time for taking in and out the toner layer on the developing roll 22 for a certain period. 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,
The rotation direction of the magnetic roll 21 is the same as that of the developing roll 22,
For example, when the rotation direction of the developing roll 22 is counterclockwise, the magnetic roll 21 is also preferably counterclockwise. If the rotation ratio between the developing roll 22 and the magnetic roll 21 does not exist, the recovery of toner from the developing roll 22 will be insufficient. Further, if it is doubled or more, the rotation speed of the magnetic roll 21 increases, which causes vibration and heat generation and increases stress on the toner.

Further, in order to supply sufficient toner to the latent image on the latent image carrier 3 without increasing the sheet interval, the peripheral speed of the developing roller 22 is 1.1 times that of the latent image carrier 3. With the above setting, the toner can be taken in and out in a short time. When the speed difference is 1.1 times or less, it takes too much time to start the development, and a sufficient paper interval is required, so that high speed printing cannot be achieved.

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 is received 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.

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.

[0052]

As described above, according to the invention described in claim 1 of the present invention, it is not necessary to provide a complicated structure such as providing an auxiliary electrode or burying an electrode in a developing roll, and developing with a conductive surface is possible. A hybrid-type developing device can be configured with a simple configuration that applies a developing bias to the roll. Therefore, image deterioration due to dirt and vibration of the wire electrode, and a brush electrode that supplies a bias to the electrode embedded in the developing roll are the electrodes of the developing roll. There is no possibility that the toner cannot be controlled because the toner cannot be contacted. Further, the developing roll surface is conductive and a continuous electrode surface is formed,
By directly applying a bias to the surface of the conductive developing roll, the developing roll, which is a rotating body, maximizes the development at the position closest to the latent image carrier, and then the development ends as the distance increases, causing a fog factor. The toner that becomes is pulled back to the developing roll side, and development on the latent image carrier can be performed accurately,
It is possible to provide a small-sized image forming apparatus having a hybrid-type developing device in which fogging is prevented and stable image quality can be obtained for a long period of time.

According to the second aspect of the present invention, by applying an AC bias in which the entire developing roll is made conductive and a DC bias is superimposed, good developability is achieved between the latent image carrier and the latent image carrier. As well as being able to obtain
To provide an image forming apparatus having a hybrid-type developing device capable of constantly exchanging toner between a developing roll and a developer transport body, suppressing selective development, and obtaining stable image quality for a long period of time. You can

According to the third aspect of the present invention, by setting the duty ratio of the AC bias to 10 to 50%, the peak of the AC bias becomes sharp and the toner can be effectively returned. , The effect of pulling back from the developing roll to the developer transport body is increased and the contamination of the toner on the developing roll is eliminated, and the pulling back of the toner from the latent image carrier to the developing roll during development is also effective. It is possible to provide an image forming apparatus having a hybrid-type developing device that is hardly fogged and is capable of obtaining stable image quality for a long period of time.

According to the present invention described in claim 4, 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 tandem image forming apparatus having a hybrid developing device according to the present invention.

FIG. 2 is a schematic configuration diagram of a hybrid developing device 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 experimental results of the relationship between the conductivity of the sleeve of the developing roll and the maintainability of the density.

[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 Yuji Kamiyama             704 No. Matabei, Noshino, Tamaki Town, Tokai District, Mie Prefecture             19 Kyocera Corporation Mie Factory Tamaki Block             Within (72) Inventor Shoichi Sakata             704 No. Matabei, Noshino, Tamaki Town, Tokai District, Mie Prefecture             19 Kyocera Corporation Mie Factory Tamaki Block             Within F-term (reference) 2H073 AA01 BA04 BA06 BA13 CA02                       CA03 CA14                 2H077 AC04 AC12 AD06 AD36 EA03                       EA16 FA11 FA13 FA26 FA27                 3J103 AA02 AA69 AA85 FA12 FA18                       HA04 HA20

Claims (4)

[Claims] 1. A two-component developer carrier for charging a developer consisting of a carrier and toner while magnetically holding it, and a developing roll for transferring toner from the carrier to form a thin layer of only toner on the surface thereof. An image forming apparatus having a hybrid type developing device for developing a latent image on the latent image carrier by applying a developing bias to the closest position (developing position) between the developing roll and the latent image carrier. In addition, the developing roll and the latent image bearing member are directly opposed to each other through the insulating toner thin layer without passing through the electrode, and at least the surface of the developing roll is conductive, and the conductive surface allows the developing position to be improved. An image forming apparatus having a hybrid type developing device, characterized in that a continuous electrode surface in which a developing bias is applied in the circumferential direction is formed. 2. The hybrid developing apparatus according to claim 1, wherein the developing roll as a whole is conductive, and an AC bias in which a DC bias is superimposed is applied to the developing roll. Image forming apparatus. 3. The duty ratio of the AC bias is 10
The image forming apparatus having the hybrid developing device according to claim 1, wherein the image forming apparatus is set to 50%. 4. The outermost surface of the developing roll has a substantially uniform surface.
The image forming apparatus having the hybrid developing device according to claim 1, wherein the image forming apparatus is composed of a conductive sleeve having a resistance of 0 ⁶ Ω · cm ³ or less.