JP3677223B2 - Development method in image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a developing method in an image forming apparatus such as a copying machine, a printer, a facsimile, or a composite machine having a photoconductor that forms an image by electrophotography, and more particularly, a non-magnetic toner using a magnetic carrier. A two-component developer that charges the toner is used, and only the charged toner is held on the developing roll to fly to the electrostatic latent image on the electrostatic latent image carrier (photoreceptor) to develop the latent image. The present invention relates to a developing method in an image forming apparatus that uses a non-contact developing method, suppresses development history and fluctuations in image density during continuous printing, and can maintain a stable image over a long period of time.
[0002]
[Prior art]
Conventionally, a two-component developer that uses a magnetic carrier to charge non-magnetic toner is used, and only the charged toner is held on the developing roll to hold the electrostatic latent image on the electrostatic latent image carrier (photoconductor). The non-contact development method for developing a latent image by flying on an image has been studied as a non-contact one-component development means. However, as a development method capable of forming a high-speed image in recent years, in particular, an electrostatic latent image carrier. A one-drum color superposition method for sequentially forming a plurality of color images on a (photosensitive member) has also been studied. In this method, it is possible to form a color image with little color misregistration by accurately superimposing toner on an electrostatic latent image carrier (photosensitive member), and is attracting attention as a technique for improving the color image quality.
[0003]
In recent years, a tandem system that uses a plurality of photoconductors corresponding to each of a plurality of colors, forms a color image in synchronization with feeding of a recording member, and superimposes colors on the recording member has been attracting attention for speeding up. ing. Although this method has an advantage of being excellent in high speed performance, it has a drawback of increasing the size because the electrophotographic process members of the respective colors have to be arranged side by side. As a countermeasure, there has been proposed a small tandem type image forming apparatus in which the interval between the photoconductors is narrowed and a downsized image forming unit is arranged.
[0004]
In the small tandem type image forming apparatus configured 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 terms of layout that the developing device is arranged in the upper direction of the photosensitive member. However, in the conventional two-component development system, when the developing device is arranged vertically as described above, the reflux of the developer, that is, the supply from the developer stirring unit to the developing member adjacent to the photosensitive member becomes complicated, and There has been a problem that there is a limit to downsizing, and carrier scattering on the photoreceptor and toner scattering due to a decrease in toner charge amount cannot be avoided.
[0005]
As another method, a one-component developing method that does not use a carrier has been proposed, but the method in which the developing roll is in contact with the photosensitive member causes torque fluctuations of the photosensitive member, and promotes color misregistration that is a weak point of the tandem type. There was a drawback. In the non-contact system, the toner is charged by the charge roll and the layer thickness on the developing roll is regulated by the elastic regulation blade, so that the toner additive adheres to the charge roll and the charging ability is lowered. In some cases, the toner adheres to the regulating blade and the layer formation becomes non-uniform, resulting in image defects.
[0006]
As one means for solving these problems, in the tandem type image forming apparatus, the toner is charged using the two-component developer as described above, and then the toner is ejected in a non-contact manner with respect to the photoreceptor. A method of making it happen has been proposed. As such prior art, US Pat. No. 3,866,574 discloses a non-magnetic toner on a donor roll (developing roll) placed in a non-contact manner with respect to an electrostatic latent image carrier (photoconductor). Proposals have been made to form a thin layer and cause the toner to fly to a latent image on an electrostatic latent image carrier (photoconductor) by an alternating electric field. U.S. Pat. No. 3,929,098 discloses a developing device that uses a magnetic roll to advance a developer to a donor roll and transfers toner onto the donor roll to form a toner layer. .
[0007]
In terms of theory, a report on the formation of a toner layer on a developing roll using a two-component developer was made by Toshiba Corp. in Journal of Electrophotographic Society, Vol. 19, No. 2 (1981). No.-121077 has a patent application.
[0008]
However, in these proposals, it is possible to form a thin layer on the donor roll using a two-component developer, but when the charge of the toner becomes high, it becomes difficult to separate the toner on the donor roll, and a strong AC An electric field is required. Since this electric field disturbs the toner layer on the electrostatic latent image carrier (photosensitive member), there is a problem with color superposition and the like. For this reason, Japanese Patent Laid-Open No. 3-113474 discloses a toner developed by providing an auxiliary electrode made of a wire between a donor roll and an electrostatic latent image carrier (photoconductor), and applying a weak alternating electric field to the auxiliary electrode. A so-called powder cloud development method has been proposed in which the above is not disturbed.
[0009]
In addition, the above-described conventional technique has complicated toner charge control, and requires a high surface potential and a large development electric field to be applied to the photoreceptor. Therefore, when a toner consumption area and a non-consumption area are generated on the developing roll, the relationship between the toner adhesion state on the roll and the toner potential difference varies, so that the previous developed image as shown in FIG. There is a problem that a phenomenon that a part appears as an afterimage (ghost) at the next development, that is, a so-called history phenomenon is likely to occur. That is, in FIG. 5, 50 and 51 are solid images made up of rectangular black solids, and 52 and 53 are halftone images wider than the solid images that follow, and the toner consumption area and non-consumption on the developing roll. When the halftone images 52 and 53 are printed after the solid images 50 and 51, an afterimage (ghost) like 54 in FIG. 5B is generated.
[0010]
In order to prevent this, Japanese Patent Application Laid-Open No. 11-231652 proposes a member for scraping off the development residual toner on the developing roll and a collecting device for the scraped toner. As a method for reliably collecting the toner on the developing roll, Japanese Patent Application Laid-Open No. 2000-81788 proposes using a dedicated collecting roll. Further, in JP-A-7-72733 and JP-A-7-92804, the toner on the developing roll is collected in a magnetic roll by a potential difference by using a gap between copies or between papers to charge the toner. As a countermeasure against a hysteresis phenomenon using a magnetic brush, Japanese Patent Application Laid-Open No. 7-128983 discloses a method in which a full width at half maximum of the magnetic flux density of a magnetic roll is set as a countermeasure. Proposals have been made to collect and supply waste.
[0011]
Japanese Patent Laid-Open No. 63-249164 discloses a method for controlling a tandem type developing device, in which the operation of the developing device in the image forming unit other than the image forming unit performing the transfer process is stopped, and the developer is deteriorated. An application has been filed to prevent this. Japanese Patent Application Laid-Open No. 6-67546 discloses that the high voltage applied between the developing roll and the magnetic roll is switched at a high frequency, and the toner is peeled from the developing roll to the magnetic roll. A technology for alternately forming an electric field, and a toner peeling member is provided separately from the magnetic roll, and the toner on the developing roll is electrically peeled and collected, and then the separated and collected toner is electrically magnetically rolled. The technology to make it collect is proposed.
[0012]
[Problems to be solved by the invention]
However, the powder cloud development method described in the above-mentioned Japanese Patent Application Laid-Open No. 3-113474 is not a very general method because the wire of the auxiliary electrode is very dirty and image deterioration due to vibration occurs. Absent. Further, in JP-A-11-231652, JP-A-7-72733, JP-A-7-92804, JP-A-7-128983, JP-A-59-121077, JP-A-2000-81788, etc. The equipment shown also requires the installation of a toner scraping device and a collection roll, the toner stress increases due to the application of a special collection bias, etc., causing deterioration in the durability of the toner, and the next development. It took time to form a layer of the developing roll at the timing, and the high speed was impaired. In addition, during long-term use, the chargeability of the toner changes due to deterioration of the durability of the carrier, the charging characteristics of the toner on the developing roll change significantly, and the charge distribution of the replenishment toner and the recovered toner becomes wider. It was a cause of scattering and fogging. Furthermore, there is a troublesome replacement of a deteriorated carrier, and the actual situation is not practical.
[0013]
In addition, the apparatus disclosed in Japanese Patent Laid-Open Nos. 63-249164 and 6-67546 as a control method for a tandem type developing apparatus operates the developing apparatus other than the image forming unit performing the transfer process. Or a device that controls the high voltage applied between the developing roll and the magnetic roll at a high frequency is necessary and expensive, and the developing device must be equipped with a toner roll, a magnetic roll, and a stirring member. It has a side-by-side configuration and has the drawback of being difficult to downsize.
[0014]
In these conventional techniques, when the toner thin layer used for developing the latent image on the photosensitive member is formed on the developing roll, when the developing roll makes one round and then enters the second round of development, The amount of toner in the toner layer may differ, and after high-density development, a phenomenon occurs that the absolute amount of toner is insufficient after the second round, resulting in a decrease in image density. . In order to eliminate such a phenomenon, it is conceivable to rotate the developing roll before the development timing to secure a sufficient amount of toner or to secure a sufficient development interval. Increasing the number of rotations of the roll increases the load on the developer, causing toner charge deterioration, and if a sufficient development interval is ensured, the printing speed is reduced and the problem cannot be solved.
[0015]
SUMMARY OF THE INVENTION In view of the above circumstances, the present invention provides a developing method capable of constructing a small hybrid developing device suitable for a small tandem image forming apparatus capable of obtaining stable image quality for a long time, and at the same time complicates the developing device. In addition, the toner amount of the toner layer on the developing roll is always kept constant, and afterimages are prevented from being generated during continuous development to reliably supply the charged toner to the developing roll, resulting in different toner amounts and a decrease in image density. Therefore, it is an object to provide a control method in a developing device that can maintain stable image quality over a long period of time, and to configure a tandem image forming apparatus that stabilizes the image quality in a small size and at low cost.
[0016]
[Means for Solving the Problems]
In order to solve the above problems, in the present invention, as described in claim 1,
A developing roll that is disposed facing a photoconductor that forms a latent image by electrophotography and that develops toner by flying toner onto the latent image formed on the photoconductor with a bias in which an AC bias is superimposed on a first DC bias. , Consists of toner and magnetic carrier Image forming having a developing device in which a magnetic brush is formed with a two-component developer, and the magnetic brush is brought into contact with the developing roll and a magnetic roll for forming a thin toner layer on the developing roll by a second DC bias In the developing method in the apparatus,
Using a carrier having a small diameter of 40 μm or less as the magnetic carrier, When forming the toner thin layer used for developing the latent image on the photoreceptor on the developing roll, the potential difference between the first and second DC biases when the toner thin layer is formed on the first round of the developing roll is developed. Set larger than the potential difference when the toner thin layer is formed after the second roll. In addition, the potential difference during the toner thin layer formation after the second round of the developing roll was set from 100V to 250V. It is characterized by that.
[0017]
As described above, when forming the toner thin layer used for developing the latent image on the photoreceptor on the developing roll, the potential difference between the first and second DC biases at the time of forming the first toner thin layer is 2. Conventionally, when the potential difference between the first and second DC biases is set to be the same in both the first and second rounds, the potential difference between the first and second DC biases is set to be larger than the potential difference in forming the toner thin layer after the first round. The phenomenon that the amount of toner in the toner layer on the developing roll differs from that in the first round when the second round of development is started, and the absolute amount of toner in the second and subsequent rounds is insufficient after high density development. Therefore, a phenomenon that causes a decrease in image density can be prevented, a necessary amount of toner layer can be secured from the first turn of the developing roll during development, and the absolute amount of toner is insufficient even after high density development. The concentration is not reduced.
[0018]
For this reason, it is possible to prevent the occurrence of afterimages during continuous development, and to supply a reliably charged toner to the developing roll, so that a stable image quality can be obtained over a long period of time without causing a difference in toner amount or a decrease in image density. A control method in the developing device that can be maintained can be provided, whereby a tandem image forming apparatus that stabilizes the image quality can be configured in a small size and at a low cost.
[0019]
And the potential difference of the 1st round set larger than this 2nd round or more is as described in Claim 2,
The difference between the potential difference between the first and second DC biases in the first round and the potential difference after the second round is about 45 to 55V.
[0020]
That is, when the potential difference of the first round set larger than the second and subsequent rounds is set larger than the potential difference of the second and subsequent rounds by more than 55V, the toner amount on the developing roll increases and causes toner scattering. In addition, when the voltage is less than 45 V, no remarkable effect is seen, but by setting it to about 50 V (45 to 55 V), such a phenomenon can be prevented, and it is necessary from the first round of the developing roll in the above development. A sufficient amount of toner layer can be secured, and even after high density development, the absolute amount of toner does not become insufficient and the image density is not lowered.
[0021]
And in the present invention, as described in claim 3,
The potential difference between the first DC bias and the second DC bias is changed between the developing roll and the magnetic field before and after the image formation and during a non-image formation period such as between the image formation and the next image formation or between the recording media during the continuous image formation. An equal equipotential state is generated while the roll is rotated, and only the AC bias is applied, and the toner on the developing roll is replaced with the magnetic brush and the AC bias.
[0022]
In this way, while the developing roll and the magnetic roll are rotated during the non-image forming period, an equipotential state in which the potential difference between the first and second DC biases is equal is generated and only the AC bias is applied. There is no special device such as a scraping blade for the toner remaining after development, or the magnetic brush on the magnetic roll comes into contact with the toner layer on the developing roll due to the difference in peripheral speed between the rolls. The developer can be collected by the brush effect, and the developer can be replaced by stirring with a mixer. Therefore, it is possible to easily collect the development residual toner that causes the residual image, and to suppress the occurrence of the residual image while avoiding the occurrence of fogging, thereby forming a clear image.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be exemplarily 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 only to those unless otherwise specified. This is just an example.
[0024]
FIG. 1 is a schematic configuration of one embodiment of a tandem type image forming apparatus for carrying out the developing method according to the present invention, FIG. 2 is a sectional view of the developing device for carrying out the developing method according to the present invention, and FIG. FIG. 4 is a graph showing the state of toner layer formation according to the number of rotations of the developing roll in the developing method of the present invention and the conventional developing method.
[0025]
In FIG. 1, 1 is a tandem image forming apparatus main body, 2 is a developing device, 3 is a photosensitive member, 4 is an exposure unit, 5 is a conveyance belt for a recording medium, 6 is a developer container, and 7 is a supply containing a recording medium. A paper cassette, 8 is a charger for charging the photoconductor 3, 9 is a transfer device for applying a transfer bias for transferring the toner image on the photoconductor 3 to the recording medium, and 10 is transferred to the recording medium. In the present invention, the developing device 2 is adjacent to the photoreceptor 3 and is arranged in a substantially vertical direction. In FIG. 2, reference numeral 20 denotes a developing roll that carries a toner thin layer and develops an electrostatic latent image on the photoreceptor 3, and 21 denotes a magnetic brush generated by a carrier contained in the developer by a magnet disposed inside. , A magnetic roll for forming a thin toner layer on the developing roll 20, and a paddle 22 for stirring the developer and scraping the magnetic brush that collects the development residual toner on the developing roll 20 to collect and stir the developer Mixer, 23 is a developer agitation mixer having a screw spirally provided on the rotation shaft, 24 is a cutting blade for regulating the layer of the magnetic brush on the magnetic roll 21, and 25 is a toner concentration sensor for detecting the toner concentration. AC 30 b is an AC bias power source, DC 30 a and DC 31 a are DC bias power sources, 26 is a developer supply hole, and 27 is a partition wall provided in the developing device 2.
[0026]
First, the operation of the tandem type image forming apparatus will be briefly described. A two-component developer composed of toner and carrier corresponding to each color such as yellow, cyan, magenta, and black is shown in FIG. The toner is replenished through the developer replenishing hole 26 of the developing device 2 shown in FIG. 2, and is agitated by the agitating mixer 23 and the paddle mixer 22, whereby the toner is charged to an appropriate level. Then, a magnetic brush is formed on the magnetic roll 21 by this developer, and is contacted with the developing roll 20 with a constant layer thickness while being regulated by the cutting blade 24, and the potential difference between the magnetic roll 21 and the developing roll 20 | DC31a -DC30a | (hereinafter referred to as Δ) forms a thin layer of toner only on the developing roll 20 from the magnetic brush. The toner thin layer on the developing roll 20 varies depending on the resistance of the developer and the rotational speed difference between the developing roll 20 and the magnetic roll 21, but can also be controlled by the potential difference Δ. When is increased, the toner layer on the developing roll becomes thicker, and when Δ is decreased, the toner layer becomes thinner. The range of Δ is generally about 100V to 250V.
[0027]
When a print start signal is received from a control circuit (not shown), the recording medium is sent out from the paper feed cassette 7 in FIG. 1 and sent to the conveying belt 5, and the recording medium reaches the photosensitive member 3 for each color. In accordance with this, the charging unit 8 is charged by the charger 8, the photosensitive member 3 is exposed by the image signal sent to the exposure unit 4, a latent image is formed, and an AC bias is applied to the developing roll 20. When the latent image is developed with the toner, and the recording medium reaches the photoreceptor 3, a transfer bias is applied by the transfer device 9 to transfer the toner image to the recording medium. This process is sequentially repeated on the photoreceptors 3 corresponding to the respective colors, and images of the respective colors are sequentially transferred onto a recording medium, fixed on the fixing device 10 and discharged.
[0028]
In the present invention, the value of the bias DC31a of the magnetic roll in FIG. 2 is varied so that the potential difference Δ in the first development start of the developing roll 20 is changed to the potential difference in the second and subsequent rounds as shown at 30 in FIG. Set larger. As described above, the value of the potential difference Δ can be set in the range of 100 V to 250 V. However, when the reference potential difference is set to 150 V, the potential difference Δ in the first development start of the developing roll 20 is about +50 V. (+45 to 55V) is appropriate. If a very large potential difference is set, the amount of toner on the developing roll increases, causing toner scattering. Moreover, if it is less than 45V, a remarkable effect may not be seen.
[0029]
That is, as shown in 31 of FIG. 3, when the potential difference Δ in the first development start of the developing roll 20 is the same as the potential difference in the second and subsequent rounds, the rotation of the developing roll 20 as shown in 41 of FIG. Even if the number is two rotations, the toner amount 42 necessary to obtain uniform developability cannot be formed on the developing roll 20, resulting in afterimages as shown in FIG. 5 and density reduction in continuous printing. End up. This phenomenon occurs because the amount of toner on the developing roll is not uniform after development. However, as in the present invention, the potential difference Δ at the first round of development is greater than the potential difference after the second round. As a result, as indicated by reference numeral 40 in FIG. 4, the toner layer thickness can be made constant from the first rotation of the developing roll 20.
[0030]
Further, in the present invention, any non-image forming period from the development of one image when continuously forming a plurality of images of each process unit to the start of the development of the next image or an arbitrary image before the start of image formation. An equipotential state in which the surface potentials DC31a and DC30a of the developing roll 20 and the magnetic roll 21 are made equal is generated while the developing roll 20 and the magnetic roll 21 and the paddle mixer 22 are rotated during the set time, and this equipotential state Then, the development residual toner on the developing roll 20 is collected with a magnetic brush. Note that this non-image forming period is not only for continuously forming a plurality of images, but also before the start of image formation, for example, a print start signal comes from a control circuit (not shown), and the recording medium is fed from the paper feed cassette 7 to the transport belt It may be a period until it is sent to 5. Further, the non-image forming period may be detected based on a print start signal from a control circuit (not shown), print image data sent to the exposure unit 4, etc. It may be detected by the leading edge or the trailing edge of.
[0031]
The equipotential state of the surface potentials of the developing roll 20 and the magnetic roll 21 may be such that both surface potential differences | DC31a-DC30a | (Δ) are 0 V. For example, both potentials are set to 0 V, or any potential such as 50 V is set. You may do it. In addition, this equipotential state may be realized by controlling both surface potentials of the developing roll 20 and the magnetic roll 21, or may be controlled so that the surface potential on one side is matched with the other. Instead of setting all the periods to the equipotential state, only a part may be set to the equipotential state.
[0032]
In this way, before the start of image formation, or before the start of image formation, toner is attached to the developing roll by generating an equipotential state in which the surface potential is made equal while the developing roll, the magnetic roll, and the paddle mixer are rotated. As a result, the development residual toner can be efficiently collected in the magnetic roll without imposing a burden on the toner by the magnetic brush effect due to the difference in peripheral speed between the developing roll and the magnetic roll. Therefore, it is possible to easily collect the development residual toner that causes the residual image, and to suppress the occurrence of the residual image while avoiding the occurrence of fogging, thereby forming a clear image.
[0033]
Further, in the present invention, of the thin layer of only the toner on the developing roll 20, the toner that has not been used for developing the latent image on the photoreceptor 3, that is, the development residual toner, is formed on the magnetic roll 21. The brush was brought into contact with the developing roll 20 so that it could be recovered by the brush effect caused by the peripheral speed difference of each roll. Further, the toner collected by the magnetic brush of the magnetic roll 21 can be mixed with a new developer by scraping the magnetic brush with the paddle of the paddle mixer 22, and the mixed developer is further supplied to the developing roll 20. Thus, the toner can be easily collected and replaced without providing a special device such as a scraping blade.
[0034]
The carrier used in the developing apparatus for carrying out the developing method of the present invention has a volume resistivity of 10 ⁷ A ferrite carrier with Ωcm, saturation magnetization of 70 emu / g, and average particle size of 35 μm was used. Examples of high magnetic force and low resistance carriers include magnetite carriers, Mn ferrites, Mn-Mg ferrites, etc. These carriers may be used as they are, but they may be used after being surface-treated within a range where resistance is not increased. Is possible. This carrier has the role of collecting the development residual toner on the developing roll 20 and the subsequent toner supply, and has a resistance value of 10 ⁶ Ωcm to 10 ⁹ If the carrier is in the range of Ωcm, the toner that is strongly and electrostatically attached at the nip between the developing roll 20 and the magnetic roll 21 can be peeled off with a magnetic brush, and the toner necessary for development can be supplied.
[0035]
At this time, in order to increase the contact point with the toner, it is preferable to use a carrier having a small diameter of 40 μm or less and to increase the surface area of the carrier. The resistance value of this carrier is 10 ⁶ A low resistance of Ωcm or less is effective as a countermeasure against development ghosts by collecting the development residual toner on the development roll 20, but it is difficult to maintain the development without causing fogging with accurate charging. In addition, when the toner is further operated for a long time, the toner is scattered from the surface of the developing roll, which causes a problem of contaminating the charger and the exposure unit 4. 10 ⁹ With a resistance of Ωcm or more, charging performance can be imparted, but there is a problem that charging tends to increase. By making the resistance value of the carrier appropriate, it is possible to recharge the charged toner to the developing roller 20 while collecting the toner on the developing roller 20.
[0036]
On the other hand, by controlling the charge amount of toner to 5 to 20 μC / g, toner scattering and fogging can be prevented, and by developing at a low electric field, no development history phenomenon is left on the developing roll 21 and the toner is recovered. A development system with excellent properties can be provided. The charged toner forms a magnetic brush on the magnetic roll 21 together with the carrier, forms a thin layer on the developing roll 20 with a potential difference | DC31a−DC30a | (Δ) between the magnetic roll 21 and the developing roll 20, and is photosensitive. The latent image is developed by flying to the photosensitive member 3 by a bias in which direct current and alternating current are superimposed on the body 3. At this time, the development residual toner on the developing roll 20 is collected by the brush effect generated by the peripheral speed difference of each roll when the magnetic brush formed on the magnetic roll 21 comes into contact with the toner layer on the developing roll 20, It is possible to easily collect and replace the toner without providing a special device such as a scraping blade by stirring with the paddle mixer 22.
[0037]
The mixing ratio of the toner in the present invention is 2 to 40% by weight, preferably 3 to 30% by weight, and more preferably 4 to 25% by weight with respect to the total amount of the carrier and the toner. When the mixing ratio of the toner is less than 2% by weight, the toner charge amount becomes high and a sufficient image density cannot be obtained, and when it exceeds 40% by weight, a sufficient charge amount cannot be obtained. Scattered and contaminated the inside of the image forming apparatus, or toner fog occurred on the surface image.
[0038]
In order to promote replacement of the developer, in the present invention, the rotation speed of the magnetic roll 21 is set to be higher than the speed of the development roll 20 and less than twice the rotation speed of the development roll 20. It is possible to form a uniform toner layer by collecting the toner and supplying the developer set to an appropriate toner concentration to the developing roll 20.
[0039]
When an a-Si photosensitive member is used as the photosensitive material of the photosensitive member 3, the surface has a very low post-exposure potential of 10 V or less. However, when the film thickness is reduced, the saturation charging potential is lowered. In addition, the withstand voltage leading to dielectric breakdown decreases. On the other hand, when the latent image is formed, the charge density on the surface of the photoreceptor 3 is improved, and the development performance tends to be improved. This characteristic is particularly noticeable when the film thickness is 25 μm or less, more preferably 20 μm or less, for an a-Si photoreceptor having a high dielectric constant of about 10. In this case, the DC developing bias DC30a is set to 150V or less, more preferably 100V or less, and AC30b is set to V as the AC component. _p-p It is possible to develop with 500 to 2000 V and a frequency of 1 to 3 kHz.
[0040]
As the photoreceptor 3 used in the image forming apparatus, an organic photoreceptor (OPC) has been conventionally known. However, this OPC photoreceptor has a problem that the surface of the photosensitive layer is soft and the photosensitive layer is easily scraped by rubbing with a cleaning blade. was there. Therefore, since the surface is harder than the OPC photoconductor and is excellent in durability and function retention (maintenance-free), an amorphous silicon (a-Si) photoconductor having a photosensitive layer thickness of 25 μm or more. Has been used in recent years. However, since the a-Si photosensitive member is formed by using a glow discharge decomposition method or the like, if the photosensitive layer is thick as described above, manufacturing time and manufacturing cost are increased, which is economically disadvantageous. Such a problem can be solved by setting the thickness to 25 μm or less, more preferably 20 μm or less.
[0041]
When a positively charged organic photoconductor (OPC) is used as the photoconductor 3, in order to reduce the residual potential to 100 V or less, the film thickness of the photosensitive layer is set to 25 μm or more, and the amount of charge generation material added can be increased. Of particular importance. In particular, OPC having a single-layer structure is advantageous because a charge generating material is added to the photosensitive layer, and therefore the sensitivity change is small even when the photosensitive layer is reduced. Even in this case, the development bias DC30a is preferably set to 500 V or less, more preferably 400 V or less, from the viewpoint of preventing a strong electric field from being applied to the toner.
[0042]
Setting the developing bias to be low in this manner is effective in suppressing the dielectric breakdown of the thin film a-Si photoreceptor, preventing the toner from being overcharged, and suppressing the development history phenomenon. Further, a toner layer of 10 to 100 μm, preferably 30 to 70 μm, is formed on the developing roll 20, and the gap between the developing roll 20 and the photoreceptor 3 is set to 150 to 400 μm, preferably 200 to 300 μm. A sharp image can be obtained by causing the toner to fly on the photoreceptor 3 by an alternating electric field.
[0043]
The gap between the ear cutting blade 24 and the magnetic roll 20 is set to 0.3 to 1.5 mm, and the gap between the magnetic roll 21 and the developing roll 20 is similarly set to about 0.3 to 1.5 mm. The thin toner layer was 6 to 100 μm, preferably 30 to 70 μm thick. This thickness is a value corresponding to about 5 to 10 layers of toner when the average particle size of the toner is 7 μm. Further, the gap between the developing roll 20 and the photoreceptor 3 was set to 150 to 400 μm, preferably 200 to 300 μm. If the gap is smaller than 150 μm, it causes fogging. If the gap is larger than 400 μm, it becomes difficult to cause the toner to fly to the photoreceptor 3, and a sufficient image density cannot be obtained. Further, it becomes a factor that causes selective development.
[0044]
Such a developing method according to the present invention was evaluated by performing printing at a process speed of 84 mm / sec and a speed of 14 sheets / min using the developing apparatus in the evaluation machine using the tandem type image forming apparatus shown in FIG. 3 in FIG. 3 shows a model of the potential difference Δ between the magnetic roll 21 and the developing roll 20 in this case, and 40 in FIG. 4 shows the toner weight on the developing roll 20 in this case. In order to suppress the development ghost and maintain a stable density even when high density printing is continuously performed, when forming a toner thin layer used for developing a latent image on the photoreceptor on the developing roll 20, As indicated by 42 in FIG. Omg / cm ² Although it is necessary to secure the above toner, in the present invention, 1. Omg / cm ² The above toner weight can be secured and stable printing can be performed.
[0045]
That is, when the developing method according to the present invention is used, a necessary amount of toner is obtained from the first rotation of the developing roll 20, and accordingly, an afterimage as shown in FIG. 5 is prevented. That is, in the development method of the present invention, unlike the conventional development method, it is not necessary to idle the development roll by the development timing or to secure a sufficient development interval, and the load on the developer is increased. There is no concern about deterioration of toner charging or a decrease in printing speed, and there is no concern about deterioration in developability due to selective development. As a result, even when printing in which a color image and a monochrome image are mixed, the development amount of each color can be kept constant and good developability can be maintained.
[0046]
【The invention's effect】
As is apparent from the above description, according to the present invention, in spite of a small tandem color machine, by controlling the potential difference between the developing roll and the magnetic roll in the hybrid type developing device, It becomes possible to make the toner layer uniform. Further, by incorporating the elimination of the potential difference at the timing outside development of each color, the development history can be suppressed, the charging of the toner can be stabilized, and a always clear image can be obtained. As a result, even when printing with a mixture of a color image and a monochrome image is performed, the development amount of each color can be kept constant and good developability can be maintained.
[0047]
That is, according to the first aspect of the present invention, when the toner thin layer used for developing the latent image on the photosensitive member is formed on the developing roll, the first and second toner thin layers are formed. Conventionally, by setting the potential difference between the second DC bias to be larger than the potential difference at the time of forming the toner thin layer in the second and subsequent rounds, the potential difference between the first and second DC biases has been conventionally reduced in the first and second rounds. After that, the toner amount in the toner layer on the developing roll is different from that in the first round when two days of development, which had occurred in the same case, and after high density development, two rounds are performed. The phenomenon that the absolute amount of toner after the first eye is insufficient and the image density is lowered can be prevented, and a necessary amount of toner layer can be secured from the first turn of the developing roll during development, and after high density development is performed. But the absolute amount of toner is not enough It does not lead to a decrease in image density.
[0048]
For this reason, it is possible to prevent the occurrence of afterimages during continuous development, and to supply a reliably charged toner to the developing roll, so that a stable image quality can be obtained over a long period of time without causing a difference in toner amount or a decrease in image density. A control method in the developing device that can be maintained can be provided, whereby a tandem image forming apparatus that stabilizes the image quality can be configured in a small size and at a low cost.
[0049]
According to the second aspect of the present invention, when the potential difference of the first round set larger than the second and subsequent rounds is set larger than the potential difference of the second and subsequent rounds by more than about 50V, The toner amount increases and causes toner scattering, and when the voltage is less than 50V, no remarkable effect is observed. However, by setting the voltage to about 50V, such a phenomenon can be prevented. In developing, a necessary amount of toner layer can be secured from the first turn of the developing roll, and the absolute amount of toner is not insufficient even after high density development, and image density is not lowered.
[0050]
According to the third aspect of the present invention, while the developing roll and the magnetic roll are rotated during the non-image forming period, an equipotential state in which the potential difference between the first and second DC biases is made equal is generated. By applying only the AC bias, the magnetic brush on the magnetic roll is applied to the toner layer on the developing roll without providing a special device such as a scraping blade or applying a burden on the toner. The developer can be recovered by the brush effect due to the difference in peripheral speed between the rolls, and the developer can be replaced by stirring with a mixer. Therefore, it is possible to easily collect the development residual toner that causes the residual image, and to suppress the occurrence of the residual image while avoiding the occurrence of fogging, thereby forming a clear image.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of an embodiment of a tandem type image forming apparatus that performs a developing method according to the present invention.
FIG. 2 is a cross-sectional view of a developing device that performs the developing method according to the present invention.
FIG. 3 is a diagram for explaining a developing method in the present invention.
FIG. 4 is a graph showing a toner layer formation state according to the number of rotations of the developing roll in the developing method according to the present invention and the conventional developing method.
FIG. 5 is a diagram for explaining the occurrence of an afterimage (ghost).
[Explanation of symbols]
30 Potential application method according to the present invention
31 Conventional method of applying potential

Claims (3)

A developing roll that faces the photosensitive member that forms a latent image by electrophotography and that develops toner by flying toner onto the latent image formed on the photosensitive member by a bias in which an alternating current bias is superimposed on a first direct current bias; A magnetic brush is formed with a two-component developer comprising toner and a magnetic carrier , the magnetic brush is brought into contact with the developing roll, and a magnetic roll is formed on the developing roll by a second DC bias. In the developing method in the image forming apparatus having the developing device,
When forming a toner thin layer used for developing a latent image on a photoreceptor on the developing roll using a carrier having a small diameter of 40 μm or less as the magnetic carrier, The potential difference between the first and second DC biases is set larger than the potential difference when the toner thin layer is formed after the second rotation of the developing roll, and the potential difference when the toner thin layer is formed after the second rotation of the developing roll is 100V. A developing method in an image forming apparatus, wherein the developing method is set to 250V .   2. The developing method for an image forming apparatus according to claim 1, wherein a difference between a potential difference between the first and second DC biases in the first round and a potential difference after the second round is set to 45 to 55V.   The potential difference between the first DC bias and the second DC bias is changed between the developing roll and the magnetic field before and after the image formation and during the non-image formation period such as between the image formation and the next image formation or between the recording media during the continuous image formation. 3. The equal potential state is generated while the roll is rotated, only the AC bias is applied, and the toner on the developing roll is replaced with the magnetic brush by the AC bias. Development method in the image forming apparatus.

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