JP3599190B2 - Image forming device - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile, and a multifunction machine using an electrophotographic method. In particular, the present invention uses a two-component developer that charges a non-magnetic toner using a magnetic carrier. More specifically, the present invention relates to an image forming apparatus having a hybrid type developing device which holds only charged toner on a developing roll, flies the electrostatic latent image, and develops the latent image.
[0002]
[Prior art]
Electrophotographic image forming apparatuses such as copiers, printers, facsimiles, and multifunction machines using the electrophotographic method include a two-component developing method using a toner and a magnetic carrier, and a one-component developing method using an insulating toner or a conductive toner. Developing method, using a two-component developer that charges a non-magnetic toner using a magnetic carrier, holding only the charged toner on a developing roll and flying to an electrostatic latent image to develop the latent image There is a hybrid developing method as described above.
[0003]
The two-component developing method has an advantage in that the toner is excellent in chargeability of the carrier and can have a long life, and has advantages such as uniform solid image. However, the developing device becomes large and complicated, and toner scattering and carrier pulling occur. And the image quality changes depending on the durability of the carrier. In the one-component developing method, the developing device is compact and has excellent dot reproducibility. However, the durability of the developing roller and the charge roller is low, and the cost of consumables is high because the developing device is replaced. And the disadvantage that selective development occurs. The hybrid development system is a system that is excellent in dot reproducibility, can extend the life, and can form images at high speed. However, in the past, there were problems such as generation of development ghosts and toner scattering.
[0004]
Although this hybrid development method has been studied as a means of non-contact one-component development, in recent years, as a development method capable of high-speed image formation, in particular, a method of forming a plurality of color images on a latent image carrier (photoconductor) has been studied. It has also been studied for use in a one-drum color superimposition system which is formed sequentially. In this method, a color image with little color shift can be formed by accurately superimposing toner on the latent image carrier, and has been attracting attention as a technique for improving color image quality.
[0005]
However, in the above-described 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, which hinders downsizing of the image forming apparatus. Become. For this reason, a tandem system in which a plurality of electrophotographic process members corresponding to the colors of toners to be used are arranged side by side, and a color image is formed in synchronism with the transfer of the transfer member and a color is superimposed on the transfer member has attracted attention. Was. However, although this method has an advantage of high speed, it has a drawback that the electrophotographic process members of each color must be arranged side by side to increase the size. As a countermeasure, there has been proposed a small tandem image forming apparatus in which the distance between latent image carriers is reduced and a miniaturized image forming unit is arranged.
[0006]
In the small-sized tandem-type image forming apparatus configured as described above, it is advantageous to use a vertical developing device in order to minimize the size of the image forming unit in the width direction. That is, it is desirable on the 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 arranged in the vertical type in this way, the reflux of the developer, that is, the supply from the developer stirring section to the developing member close to the latent image carrier becomes complicated, There is a problem in that there is a limit to the size reduction of the apparatus, and it is inevitable that the carrier adheres to the latent image carrier and the toner scatters.
[0007]
As another method, a one-component developing method without using a carrier has been proposed. However, in a method in which a developing roll is brought into contact with a latent image carrier, a torque fluctuation of the latent image carrier is caused, and a color shift which is a weak point of a tandem type is caused. However, there is a drawback that this is promoted. Further, in the method in which the latent image carrier is not contacted, the toner is charged by the charge roll and the layer thickness on the developing roll is regulated by the elastic regulating blade, so that the toner additive adheres to the charge roll and is charged. In some cases, the performance was reduced or toner adhered to the regulating blade, resulting in non-uniform layer formation, resulting in image defects.
[0008]
Therefore, as one of means for solving these problems, the above-described hybrid developing method has been attracting attention. That is, in the hybrid developing method, there are problems such as generation of a developing ghost and scattering of toner as described above. However, since the developing roll is not in contact with the latent image carrier, there is no torque fluctuation, and dot reproducibility is reduced. It is possible to provide a high-speed image forming apparatus capable of prolonging the service life.
[0009]
As a prior art relating to such a technique, there is disclosed in US Pat. No. 3,866,574 a thin layer formed of a non-magnetic toner on a donor roll (developing roll) placed in non-contact with a latent image carrier. A proposal has been made to fly the toner onto a latent image on a latent image carrier by an AC electric field. U.S. Pat. No. 3,929,098 discloses a developing device in which a developer is advanced to a donor roll using a magnetic roll, and toner is transferred onto the donor roll to form a toner layer. .
[0010]
However, in these proposals, although a thin layer can be formed on a donor roll by employing a two-component developer, separation of the toner on the donor roll becomes difficult when the charge of the toner becomes high, and a strong AC An electric field is required. Since this electric field disturbs the toner layer on the latent image carrier, there is a problem in color superposition and the like. For this reason, Japanese Patent Application Laid-Open No. 3-113474 discloses an auxiliary electrode made of a wire provided between a donor roll and a latent image carrier, and a weak AC electric field is applied to the auxiliary electrode so that the developed toner is not disturbed. A so-called powder-cloud development method has been proposed.
[0011]
In terms of theory, Toshiba Corporation reported on the formation of a toner layer on a developing roll using a two-component developer in the Journal of the Electrophotographic Society, Vol. 19, No. 2, (1981). There is an application as a patent in JP-A-121077.
[0012]
Also, in the above-described conventional technology, the coarse powder of the toner having high developability is easily selectively developed on the latent image carrier, and when continuous printing is performed, the fine powder of the toner having high charge is deposited on the developing sleeve and selectively developed. Tends to occur and the image density tends to decrease. Further, the charging control of the toner is complicated, and it is necessary to apply a high surface potential and a large developing electric field to the latent image carrier. For this reason, if a toner consumption area and a non-consumption area occur on the developing roll, a variation occurs in the toner adhesion state and the toner potential difference on the developing roll, and as shown in FIG. There is a problem that a phenomenon in which a part of the image appears as an afterimage (ghost) at the next development, that is, a so-called hysteresis phenomenon is likely to occur. That is, in FIG. 3, reference numeral 35 denotes a solid image composed of rectangular black solids, and reference numerals 36 and 37 denote subsequent halftone images wider than the solid image. When the halftone images 36 and 37 are printed after the solid image 35 in the case where the solid image 35 is generated, an afterimage (ghost) 38 as shown in FIG.
[0013]
In order to prevent this, Japanese Patent Application Laid-Open No. 11-231652 proposes a member for scraping off the residual toner on the developing roll and a device for collecting the scraped-off toner. As a method of reliably collecting toner, Japanese Patent Application Laid-Open No. 2000-81788 proposes to use a dedicated collection 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 the hysteresis phenomenon using a magnetic brush, Japanese Patent Application Laid-Open No. 7-128983 proposes to collect and supply the toner on the developing roll by setting the half width region of the magnetic flux density of the magnetic roll wide. Has been done. Japanese Patent Application Laid-Open No. 63-249164 discloses a method for controlling a tandem type developing device. In this method, the operation of a developing device in an image forming section other than the image forming section performing the transfer process is stopped to reduce the deterioration of the developer. An application has been filed to prevent this.
[0014]
In the hybrid development method, the image density is reduced by selective development, development defects, image deterioration, development ghost, toner scattering, sleeve adhesion, etc. are caused by leaving the toner held on the development roll for a long time. No. 7,727,733 (US Pat. No. 5,420,375) discloses a magnetic roll forming a magnetic brush using a two-component developer, and a donor roll (developing roll) carrying a thin toner layer supplied from the magnetic roll. And an electrode provided between the donor roll and the latent image carrier. A bias consisting of DC and AC is applied to the electrode, a DC bias is applied to the developing roll, and a switch is applied to the magnetic roll. A high-voltage circuit configured to apply a DC bias that can be switched to a voltage having a different polarity to solve the above problem. Lid type developing apparatus is shown.
[0015]
That is, in the developing device disclosed in Japanese Patent Application Laid-Open No. 7-72733, a thin toner layer is formed on a developing roll by a magnetic brush formed on the magnetic roll by a potential difference between the magnetic roll and the developing roll. A toner cloud (cloud) is formed in the vicinity of the electrode with a bias in which the direct current and the alternating current applied between the electrodes are superimposed to develop the latent image on the latent image carrier, and when the image formation is completed, the switch is switched. Applying a DC bias in the direction of peeling the toner from the developing roll to the magnetic roll, recovering the toner on the developing roll, and switching the switch at the time of forming an image again to move the toner on the magnetic roll to the developing roll. A bias is applied to prepare for image formation, thereby solving the above problem.
[0016]
Japanese Patent Application Laid-Open No. 2000-250294 discloses a method in which an electrode is provided between a developing roll and a latent image carrier as disclosed in Japanese Patent Application Laid-Open No. 7-72733. As a conventional example, a hybrid type developing device using a developing roller with an embedded electrode is considered to cause uneven development due to line vibration and a phenomenon that dust adheres instantaneously to the electrode and causes streaks on the developing roll. In addition, even when a developing roll with this electrode embedded is used, the carrier attached to the developing roll adheres to the image, and since the electrodes embedded in the developing roll have a predetermined interval, the carrier on the latent image carrier Poor toner supply efficiency, resulting in image omission when continuously developing images with a high image ratio.Selected by alternating bias applied to magnetic roll and developing roll. Occurs, deterioration of image quality, and the hybrid type developing apparatus adapted to prevent a phenomenon caused a density decrease is shown.
[0017]
That is, in the apparatus disclosed in Japanese Patent Application Laid-Open No. 2000-250294, a developing roll is formed by providing a dielectric layer on a conductive sleeve and embedding electrodes at minute intervals in the dielectric layer. Improves toner supply efficiency, prevents short circuits between electrodes, and protects the surface.In addition, the dielectric constant and fringe charge are enough to dissipate the charge accumulation in a few seconds and penetrate the coating in less than about a few milliseconds An electrode relaxation permitting layer is provided. A brush electrode for supplying a bias voltage consisting of AC and DC to electrodes embedded on the developing roll is provided between the latent image carrier and the developing roll, and a brush electrode is provided between the developing roll and the magnetic roll. A magnetic brush for supplying a bias voltage consisting of AC and DC is provided to the electrodes, and the toner on the developing roll is formed into a cloud 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 supply is used in common, and the waveform is a rectangular wave. The duty ratio of the rectangular wave is set so that the time during which the toner is transported from the magnetic roll to the developing roll is shorter than the time when the toner is returned from the developing roll to the magnetic roll. Utilizing the fact that the inertia is different between the toner and the carrier, the carrier is prevented from adhering to the developing roll, and the selective conveyance of the toner is prevented.
[0018]
[Problems to be solved by the invention]
However, the powder cloud developing method described in JP-A-3-113474 is a very common method because the wire of the auxiliary electrode is very liable to be stained, and image deterioration or the like occurs due to vibration. Absent. Further, the apparatuses disclosed in JP-A-11-231652, JP-A-2000-81788, JP-A-7-128983 and the like also require installation of a toner scraping device and a collecting roll, or special collection. The application of a bias or the like increases the stress of the toner, causing a deterioration in the durability performance of the toner. Further, it takes time to form a layer of a developing roll at the next developing timing, thereby impairing the high-speed operation. Also, during long-term use, the chargeability of the toner changes due to the deterioration of the durability performance of the carrier, the charge characteristic of the toner on the developing roll changes greatly, the distribution of charge of the replenishment toner and the collected toner becomes wide, and toner It was causing scattering and fog. Furthermore, there is a troublesome replacement of the deteriorated carrier, and the fact is that it has not been practically used.
[0019]
Further, a device disclosed in JP-A-63-249164 as a control method of a tandem type developing device is configured to stop the operation of the developing devices other than the image forming unit performing the transfer process, or to control the operation of the developing roll. A device or control for switching the high voltage applied between the magnetic rolls at a high frequency is required, and it must be 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. And there is a disadvantage that miniaturization is difficult.
[0020]
Further, the apparatuses disclosed in JP-A-7-72733 (US Pat. No. 5,341,197) and JP-A-2000-250294 are also disclosed in JP-A-7-72733, which use the powder cloud developing method described above. The wire of the auxiliary electrode is very easy to become dirty, and image deterioration or the like occurs due to vibration. Further, the apparatus disclosed in Japanese Patent Application Laid-Open No. 2000-250294 needs to embed an electrode in a developing roll and apply AC and DC to this electrode. This is a complicated configuration that requires a brush electrode to supply a superimposed bias.The electrodes are intermittent in the circumferential direction. If the toner cannot be contacted, the toner cannot be controlled at all.
[0021]
In view of the above circumstances, the present invention provides a small-sized image forming apparatus that prevents development ghosts and selective development without complicating the developing device, and that supplies a charged toner to the developing roll reliably to obtain stable image quality for a long time. It is an object to provide a high-speed and inexpensive small-sized image forming apparatus having the hybrid type developing device.
[0022]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, in the present invention, as described in claim 1, a two-component developer carrier (hereinafter, referred to as a magnetic roll) that charges a developer including a carrier and a toner while magnetically holding the developer, A developing roller for transferring toner through a magnetic brush formed on the body to form a thin layer of toner only on the surface thereof; and a developing bias at a closest position (developing position) between the developing roll and the latent image carrier. In the image forming apparatus having a hybrid type developing device for developing the latent image of the latent image carrier by applying
At the developing position, the developing roller and the latent image carrier, which are entirely conductive , directly face each other via an insulating thin toner layer without passing through an electrode, and a first DC bias is superimposed on the developing roller . And a second DC bias is applied to the magnetic roll side, respectively.
At the completion of development, the second DC bias is varied while maintaining greater than the voltage value of the first direct current bias, you and recovering the toner thin layer on the developing roll to the magnetic brush.
In this case, both the second DC bias and the first DC bias may be changed while maintaining the second DC bias voltage value higher than the first DC bias voltage value.
[0023]
According to the present invention, by configuring the hybrid developing device with a simple configuration for applying a developing bias to a developing roll having a conductive surface, a complicated configuration in which an auxiliary electrode is provided or an electrode is embedded in the developing roll is provided. It is not necessary, and therefore, there is no occurrence of image deterioration due to contamination or vibration of the wire electrode, and no control of toner because the brush electrode for supplying a bias to the electrode embedded in the developing roll cannot contact the electrode of the developing roll. A developing device can be obtained. The surface of the developing roll is conductive and has a continuous electrode surface.By applying a bias directly to the conductive developing roll, the developing roll, which is a rotating body, is developed at the closest part to the latent image carrier. The development is terminated as the distance increases, and the toner that causes fogging is drawn back to the developing roll side, allowing accurate development on the latent image carrier. It is possible to provide a small-sized image forming apparatus having a hybrid developing device capable of obtaining improved image quality.
[0025]
Further, according to the present invention, by applying an AC bias in which a DC bias is superimposed by making the entire developing roll conductive, it is possible to obtain good developability with the latent image carrier, Provided is an image forming apparatus having a hybrid type developing apparatus that can easily exchange toner between a developing roll and a developer conveying body at all times, suppresses selective development, and obtains stable image quality for a long time. Can be.
[0026]
The invention described in claim 3 is:
The duty ratio of the AC bias is set to 10 to 50%.
[0027]
By setting the duty ratio of the AC bias to 10% to 50%, the peak of the AC bias is sharpened, so that the toner can be effectively pulled back and the effect of pulling back from the developing roll to the developer conveying body is improved. As a result, the contamination of the developing roll with toner is eliminated. At the same time, since the toner is effectively pulled back from the latent image carrier toward the developing roll during development and fogging becomes difficult, an image forming apparatus having a hybrid developing apparatus capable of obtaining stable image quality for a long period of time is provided. An apparatus can be provided.
[0028]
Next, the invention described in claim 4 is:
The outermost surface of the developing roll is formed of a substantially uniform conductive sleeve of 10 ⁶ Ω · cm ³ or less.
[0029]
By forming the outermost surface of the developing roll with a substantially uniform conductive sleeve of 10 ⁶ Ω · cm ³ or less, the developing roll in which the bias in which the DC and AC voltages applied to the developing roll are superimposed rotates. And an image forming apparatus having a hybrid developing device capable of applying a good action between the latent image carrier and the magnetic roll without causing a decrease in image density during durability and providing stable image quality over a long period of time. Can be provided.
[0030]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be illustratively described in detail with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the components described in this embodiment are not intended to limit the scope of the present invention to them unless otherwise specified, and are merely mere descriptions. This is just an example.
[0031]
FIG. 1 is a schematic 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 diagram of a hybrid developing device according to the present invention, and FIG. 3 is a ghost image. FIG. 4 is a table showing experimental results on the relationship between the conductivity of the sleeve of the developing roll and the maintainability of density.
[0032]
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 conveyance belt of a recording medium, 6 is a developer container, and 7 is A paper feed cassette accommodating a recording medium, 8 a charger for charging the latent image carrier 3, 9 a transfer device for transferring the toner image on the latent image carrier 3 to the recording medium by a transfer bias, 10 Reference numeral denotes a fixing device for fixing the toner image transferred to the recording medium. In a tandem type image forming apparatus, a charging device 8, an exposure unit 4, and a developing device 2 installed around a latent image carrier (photoconductor) 3 It is important to design the transfer device 9, the cleaning device, etc. compactly. In the example of the present invention, the developing device 2 is arranged adjacent to the latent image carrier 3 and in a substantially vertical direction.
[0033]
In FIG. 2, reference numeral 21 denotes a magnetic roll serving as a developer carrier for generating a magnetic brush 30 of a two-component developer by a magnet provided therein and supplying a toner 25 to a developing roll 22; , A developing roller for developing the electrostatic latent image on the latent image carrier 3, a carrier 24, a toner 25, a thin toner layer 26 on the developing roll 22, and a developing roll 27 to the latent image carrier 3 _{Reference numeral} 27a denotes a direct current (DC) bias (V _dc1 ), and 27b denotes an alternating current (AC) bias, for causing the toner 25 of the thin toner layer 26 formed on the toner 22 to fly and perform development. 28 is a direct current (DC) bias (V _dc2 ) for conveying the toner 25 from the magnetic brush 30 on the magnetic roll 21 to the developing roll 22, and 29 is a regulating blade for regulating the thickness of the magnetic brush 30 on the magnetic roll 21 It is.
[0034]
Among these, 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. Positively charged organic photoreceptors (positive OPCs) generate less ozone and have stable charging. In particular, single-layered positively charged organic photoreceptors are photosensitive even when the film thickness changes due to long-term use. Since there is little change in characteristics and stable image quality, it is suitable for a long-life system. When the positively charged organic photoreceptor is used in a long-life system, it is preferable to set the film thickness to about 20 μm to 40 μm. In the case of 20 μm or less, black spots are noticeable due to dielectric breakdown when the film thickness is reduced to about 10 μm. On the other hand, when the thickness is 40 μm or more, the sensitivity is reduced, which causes a reduction in image density.
[0035]
The exposure unit 4 can use a semiconductor laser or an LED. When a positively charged organic photoreceptor is used, a wavelength around 770 nm is effective, and when an amorphous silicon photoreceptor is used, a wavelength around 685 nm is effective. Hereinafter, in the present invention, a case where a positively charged organic photoconductor is used as the latent image carrier 3 and an LED is used as a light source of the exposure unit 4 will be described as an example.
[0036]
The outermost surface of the developing roll 22 is formed of a sleeve made of a uniform conductive aluminum, SUS, conductive resin coating or the like whose volume resistivity is set to 10 ⁶ Ω · cm ³ or less. A direct current (DC) bias (V _dc1 ) 27 a and an alternating current (AC) bias 27 b are connected to the shaft, and the direct current (DC) bias (V _dc1 ) 27 a and the alternating current (AC) bias 27 b are connected between the rotating developing roll 22 and the latent image carrier 3 and the magnetic roll 21. And a bias in which alternating current is superimposed acts. The AC component supplied by the AC bias 27b is a rectangular wave having a duty (Duty) ratio of 50% or less. In the present invention, as an example, the DC bias ( _Vdc1 ) 27a is set to 100 V, the AC bias 27b is set to Vpp of 1.5 kV, the frequency is 3.0 kHz, and the duty (Duty) ratio is 30%. As described above, the DC bias (V _dc1 ) 27 a and the AC bias 27 _b are directly applied to the developing roll 22, and the outermost surface of the developing roll 22 is made to have a conductivity of 10 ⁶ Ω · cm ³ or less. 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 forming a toner layer at the start of development can be improved.
[0037]
The magnetic roll 21 magnetically holds the two-component developer composed of the carrier 24 and the toner 25 by a magnet provided inside as a developer transport body, generates a magnetic brush 30, and regulates the thickness of the magnetic brush 30. The toner 25 is supplied to the developing roll 22 while being regulated by the blade 29. The gap between the regulating blade 29 and the magnetic roll 21 is preferably 0.3 to 1.5 mm. The supply of the toner 25 to the developing roll 22 is based on the potential difference between the direct current (DC) bias (V _dc2 ) 28 applied to the magnetic roll 21 and the direct current bias (V _dc1 ) 27 a applied to the developing roll 22. And the AC bias 27b. In the present invention, the voltage of the DC bias (V _dc2 ) 28 is 350 V as an example, and when the toner layer on the developing roll 22 is replaced at the end of the development, the DC bias (V _dc2 ) 28 is applied with the AC bias 27 b applied. Then, the thin toner layer 26 on the developing roll 22 is collected by the magnetic brush 30. The amount of change of the DC bias (V _dc2 ) 28 is a value that produces 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 of the magnetic roll 21 is changed to the developing roll 22 side. And when the DC bias (V _dc1 ) 27a is 100 V, it is set to a higher value (that is, a value between 100 V and 350 V in this example). In the above description, the DC bias is changed by changing the DC bias (V _dc2 ) 28. However, if the DC bias voltage value on the magnetic roll 21 side is maintained larger than the DC bias voltage value on the developing roll 22 side. _{Alternatively} , both the DC bias (V _dc2 ) 28 and the DC bias (V _dc1 ) 27a may be changed.
[0038]
The conductivity of the outermost sleeve in 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 was changed and the change in the image density after printing 10,000 sheets (initial value: 1.43) was examined. The DC bias (V _dc1 ) 27 a applied to the developing roll 22 is 100 V, the Vpp in the AC bias 27 b is 1.5 kV, the frequency is 3.0 kHz, the duty (Duty) ratio is 30%, and the DC bias (V dc) applied to the magnetic roll 21 is _dc2 ) 28 was set to 350V. As can be seen from the results, when the volume resistivity is 10 ⁷ Ω · cm ³ or more, the density decrease after printing 10,000 sheets becomes large, and when the volume resistivity is 10 ⁶ Ω · cm ³ or less, there is almost no problem in image density. You can see that.
[0039]
The distance between the latent image carrier 3 and the developing roll 22 is, for example, about 250 μm, and a wire electrode or the like is not used between them. Usually, the distance between the latent image carrier 3 and the developing roll 22 is 150 to 400 μm, preferably 200 to 300 μm. If the distance is smaller than 150 μm, it causes fogging. It becomes difficult to fly on the carrier 3, and a sufficient image density cannot be obtained. In addition, this is a factor that causes selective development. The distance between the magnetic roll 21 and the developing roll 22 is about 0.3 to 1.5 mm.
[0040]
The developer is composed of a toner 25 and a carrier 24, and it is important to define a particle size distribution of the toner 25 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 size to the number distribution average particle size. In order to prevent selective development, the ratio was found to be 1.25 or less. When the ratio is 1.25 or more, toner having a relatively small particle size is deposited on the developing roll during continuous printing, thereby deteriorating developability. In the present invention, in order to prevent selective development, the ratio of volume average particle diameter / number average particle diameter is 1.25 or less, and the charge amount is controlled in the range of 5 to 20 μC / g. The charge amount of the toner is also related to the thickness of the thin toner layer 26 formed on the developing roll 22, and if the charge amount of the toner is as low as 10 μC / g or less, particularly 5 μC / g or less, the toner layer 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 toner developability decreases. The charge amount of the toner 25 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, a case where a positively charged toner is used will be described as an example. However, it is obvious that the same configuration can be made even when a negatively charged toner is used by reversing the relationship with the bias described above.
[0041]
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 the carrier after surface treatment within a range that does not increase an appropriate resistance value. In the present invention, as an example, a ferrite carrier having a volume resistivity of 10 ⁸ Ωcm ³ coated with a silicone resin, a saturation magnetization of 40 emu / g, and an average particle diameter of 35 μm is used. If the average particle size exceeds 50 μm, the stress of the carrier increases and the toner concentration cannot be increased, so that the toner supply amount to the developing roll 22 decreases. By setting the average particle size of the carrier to 50 μm or less, even when the toner concentration in the developer is set to 5 to 20%, sufficient charging can be provided, and stable development can be performed.
[0042]
The mixing ratio of the toner 25 and the carrier 24 is 5 to 20% by weight, preferably 5 to 15% by weight, based on 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. Are scattered from the developing device and contaminate the inside of the image forming apparatus, and toner fog occurs on the image.
[0043]
The thin layer 26 of the toner 25 on the developing roll has a thickness of 10 to 100 μm, preferably 30 to 70 μm, and when the average particle diameter of the toner 25 is 7 μm, the thickness of the toner 25 is reduced to about 5 to 10 layers. The corresponding value. 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 set to 50 μm or less, and the toner amount is set in the range of 0.5 to 1.7 mg / cm ² . By performing the control, a clear latent image is formed on the latent image carrier 3, and at the same time, the replacement of the toner on the developing roll 22 is facilitated to control the developing ghost.
[0044]
The thickness of the thin toner layer 26 is determined by the difference between the DC bias (V _dc2 ) 28 and the DC bias (V _dc1 ) 27 a and the toner charge amount as described above. When the toner charge amount is as low as 10 μC / g or less, particularly as low as 5 μC / g or less, the thickness of the toner layer is increased and scattering is increased. 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 toner developability decreases. As described above, when the DC bias (V _dc1 ) 27 a is set to 100 V and the DC bias (V _dc2 ) 28 is set to 350 V, a toner layer of about 40 μm is obtained. At this time, the amount of toner per unit area is about 1.5 mg / cm ² . Toner thin layer 26 is reduced followability concentration in the case of continuous high-density image with 0.5 mg / cm ² or less and too thin, it image unevenness tends to occur, the toner layer is a 1.7 mg / cm ² If the thickness is too large, the development ghost as shown in the example in FIG. 3 is conspicuous, and toner scattering tends to be conspicuous. That is, as described above, in FIG. 3, 35 is a solid image composed of rectangular black solids, and 36 and 37 are subsequent halftone images wider than this solid image. In the case where the non-consumption area occurs, when the halftone images 36 and 37 are printed after the solid image 35, an afterimage (ghost) 38 occurs as shown in FIG. 3B.
[0045]
To replace the thin toner layer 26 on the developing roll 22, the DC bias (V _dc2 ) 28 is changed with the AC bias 27 b applied at the end of development, and the thin toner layer 26 on the developing roll 22 is transferred to the magnetic brush 30. to recover. In order to maintain a good printing speed when replacing the toner, it is only necessary to adjust the interval between sheets and adjust the time for putting the toner layer on the developing roll 22 in and out in a certain period. When the rotation speed of the magnetic roll 21 is set to be 1 to 2 times the rotation speed of the developing roll 22, replacement of toner on the developing roll 22 is promoted. At this time, when 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, it is preferable that the magnetic roll 21 is also counterclockwise. If the rotation ratio between the developing roll 22 and the magnetic roll 21 does not exist, the recovery of the toner from the developing roll 22 becomes insufficient. On the other hand, if it is twice or more, the rotation speed of the magnetic roll 21 increases, causing vibration and heat generation, and increasing the stress on the toner.
[0046]
In order to supply a sufficient amount of toner to the latent image on the latent image carrier 3 without increasing the paper interval, the peripheral speed of the developing roll 22 is set to 1.1 times or more with respect to the latent image carrier 3. Then, the toner can be taken in and out in a short time. If the speed difference is 1.1 times or less, it takes too much time to start development, and a sufficient paper interval is required, so that high-speed printing cannot be achieved.
[0047]
In the tandem image forming apparatus of the present invention having the hybrid type developing device configured as described above, the two-component developer including the toner 25 and the carrier 24 corresponding to each color such as yellow, cyan, magenta, and black is developed. The magnetic brush 30 is supplied from the developer container 6 to the developing device 2 and is formed on the magnetic roll 21 shown in FIG. 2, and the toner 25 is charged by stirring. Then, the layer of the magnetic brush 30 on the magnetic roll 21 is regulated by the regulating blade 29, and 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. A thin layer 26 of only the toner 25 is formed on the developing roll 2 by the potential difference and the AC bias 27b.
[0048]
When a print start signal is received from a control circuit (not shown), first, the charger 8 charges the latent image carrier 3 composed of a positively charged organic photoconductor (positive OPC) to, for example, 400 V, and thereafter, Exposure by the LED having a wavelength of, for example, 770 nm constituting the exposure unit 4 causes the potential of the latent image carrier 3 after exposure to about 70 V to form a latent image. The latent image is developed with the toner flying from the thin toner layer 26 on the developing roller 22 to the latent image carrier 3 by the DC bias (V _dc1 ) 27 a and the AC bias 27 b applied to the developing roller 22. An image is formed.
[0049]
When the toner image is formed on the latent image carrier 3 in this manner, the recording medium is taken out of the paper supply cassette 7 and conveyed by the conveyance belt 5 at the timing when the toner image reaches the transfer position. A transfer bias is applied by a transfer device 9 installed at each transfer position to transfer a toner image to a recording medium. When the toner images of the respective colors are sequentially transferred to the recording medium and reach the fixing device 10, the toner images are fixed by the fixing device 10 and discharged.
[0050]
The AC bias 27b applied to the developing roll 22 is a rectangular wave having a duty (Duty) ratio of 50% or less as described above. The time for pulling the toner 25 back from the magnetic roll 22 to the magnetic roll 21 is shortened. The direct current bias (V _dc1 ) 27 a and the alternating current bias 27 b are applied directly to the developing roll 22, and the outermost surface of the developing roll 22 has a conductivity of 10 ⁶ Ω · cm ³ or less. The peak of the alternating current component when the upper thin toner layer 26 is pulled back to the magnetic roll 21 becomes sharp, and the pulling back of the toner 25 is effectively performed. Accordingly, the effect of pulling back the toner from the developing roll 22 to the magnetic roll 21 as the developer conveying body is increased, so that the toner is prevented from being contaminated on the developing roll 22, and the toner is developed from the latent image carrier 3 during development. Pulling back toward the roll 22 is also effectively performed, and it is possible to provide a developing device that can prevent fogging and obtain stable image quality for a long period of time.
[0051]
Note that the voltage values, Vpp, frequencies, and the like of the DC bias (V _dc1 ) 27 a, the AC bias 27 b, and the DC bias (V _dc2 ) 28 described above are examples, and it is obvious that they can be changed according to the situation. .
[0052]
【The invention's effect】
According to the onset light as described above, without requiring a complicated structure or embed the electrode to the developing roller or an auxiliary electrode, a hybrid with a simple structure for applying a developing bias to the developing roller having a conductive surface This makes it possible to construct a mold development device, and thus image deterioration due to contamination and vibration of the wire electrode, and also makes it impossible to control toner because the brush electrode that supplies bias to the electrode embedded in the development roll cannot contact the electrode of the development roll. Does not occur. The surface of the developing roll is conductive and has a continuous electrode surface. By applying a bias directly to the surface of the conductive developing roll, the developing roll, which is a rotating body, is positioned closest to the latent image carrier. As the distance increases, the development is completed, and the development is terminated, and the toner, which causes fogging, is drawn back to the developing roll side, and development on the latent image carrier can be accurately performed. It is possible to provide a small-sized image forming apparatus having a hybrid developing device capable of obtaining stable image quality for a long period.
[0053]
According element to the present invention, by applying an AC bias overlapped with the DC bias across the developer roll as the conductive, it becomes possible to obtain good developability between the latent image bearing member, Provided is an image forming apparatus having a hybrid type developing apparatus that can easily exchange toner between a developing roll and a developer conveying body at all times, suppresses selective development, and obtains stable image quality for a long time. Can be.
[0054]
According to the element present invention, by setting the duty ratio of the AC bias to 10-50%, the toner pullback becomes support efficient sharpened peaks of the AC bias, the developer conveying member from the developing roll The effect of pulling back the toner from the latent image carrier to the developing roll at the time of development is effectively performed, and the fogging becomes difficult. It is possible to provide an image forming apparatus having a hybrid type developing device capable of obtaining stable image quality for a period.
[0055]
According element to the present invention, bias the outermost surface of the developing roll, by forming a substantially uniform 10 ⁶ Ω · cm ³ or less conductive sleeve, superimposed of the applied direct current, alternating voltage to the developing roller There is a hybrid developing device that can apply a good action between the rotating developing roll, latent image carrier and magnetic roll, does not cause a decrease in image density during durability, and provides stable image quality over a long period of time. Image forming apparatus can be provided.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of one 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 occurrence of an afterimage (ghost).
FIG. 4 is a table showing experimental results on the relationship between the conductivity of a sleeve of a developing roll and the maintainability of density.
[Explanation of symbols]
2 Developing device 3 Latent image carrier (photoconductor)
4 Exposure Unit 21 Magnetic Roll 22 Developing Roll 24 Carrier 25 Toner 26 Thin Toner Layer 27 Developing Bias 27a Direct Current (DC) Bias ( _Vdc1 )
27b alternating current (AC) bias 28 direct current (DC) bias (V _dc2 )
29 Regulator blade 30 Magnetic brush

Claims (4)

A two-component developer carrier (hereinafter referred to as a magnetic roll) that charges a developer composed of a carrier and toner while magnetically holding the toner, and transfers the toner via a magnetic brush formed on the carrier to transfer the toner to the surface thereof. A hybrid developing device that applies a developing bias to the closest position (developing position) between the developing roll and the latent image carrier to develop the latent image on the latent image carrier. In the image forming apparatus having
At the developing position, the developing roller and the latent image carrier, which are entirely conductive , directly face each other via an insulating thin toner layer without passing through an electrode, and a first DC bias is superimposed on the developing roller . AC bias, while a second DC bias is applied to the magnetic roll side, respectively.
At the completion of development, the second DC bias is varied while maintaining greater than the voltage value of the first direct current bias, images the toner thin layer on the developing roll you and recovering the magnetic brush Forming equipment. 2. The device according to claim 1, wherein both the second DC bias and the first DC bias are changed while the second DC bias voltage is maintained higher than the first DC bias voltage. 3. the images forming apparatus. The image forming apparatus according to claim 1, wherein the duty ratio of the AC bias is set to 10 to 50%. The image forming apparatus according to claim 1, wherein the outermost surface of the developing roll is formed of a substantially uniform conductive sleeve of 10 ⁶ Ω · cm ³ or less.

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