JP2005055841A - Development method for image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a development method for an image forming apparatus, whereby, without making a development device complex, the thickness of a toner layer on the development roller is made constantly stabilized for a long time, and a stable image free from a density change is maintained. <P>SOLUTION: In order to detect the thickness of a toner layer on the development roller 2 on a photoreceptor drum 3, a toner layer thickness detection pattern is developed, the toner layer thickness detection pattern having a solid pattern using a toner layer 6 formed during the two rotations of the development roller 2 and a halftone pattern using a toner layer formed during the subsequent one rotation. A concentration sensor detects the toner concentration of the toner layer thickness detection pattern on the photoreceptor drum 3 or a toner layer thickness detection pattern on a transfer body obtained by the transfer of the toner layer thickness detection pattern on the photoreceptor drum 3. Then, control is performed so that the toner layer on the development roller 2 has a specific thickness according to the thus detected toner concentration. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  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, and a composite machine using an electrophotographic method, and more particularly, a two-component developer that charges a nonmagnetic toner using a magnetic carrier. In the image forming apparatus having a hybrid type developing device that holds only the charged toner on the developing roller and flies to the electrostatic latent image and develops the latent image, the toner on the developing roller The present invention relates to a developing method in an image forming apparatus in which the layer thickness can always be stabilized.

  Development methods in image forming apparatuses such as copiers, printers, facsimiles, and composite machines using electrophotography are two-component development methods using toner and a magnetic carrier, and one component using insulation toner and conductive toner. Development method, using a two-component developer that charges non-magnetic toner using a magnetic carrier, holds only the charged toner on the developing roller, and flies to the electrostatic latent image to develop the latent image There is a hybrid development method.

  The two-component development method has excellent toner chargeability with a carrier, and can have a long lifespan and has the advantage of uniform solid images, but the developing device becomes large and complicated, and toner scattering and carrier pulling occur. However, there is a drawback that the image quality changes depending on the durability of the carrier. In addition, the one-component development system has a compact developing device and excellent dot reproducibility, but has low durability due to deterioration of the developing roller and the charge roller, and the expendables are expensive because the developing device is replaced. There is a disadvantage that selective development occurs. The hybrid development method is a method that has excellent dot reproducibility and can achieve a long life and high-speed image formation. Conventionally, fine powder toner has been added to the developer due to the development ghost and the change in the toner particle size distribution. As a result, the surface of the carrier is contaminated with the fine powder toner, the charge amount is lowered, and there are problems such as scattering of the toner from the developing device and adhesion of the fine powder toner on the developing roller.

  The hybrid development method has been conventionally studied as a non-contact one-component development means. However, in recent years, as a development method capable of forming a high-speed image, a plurality of color images are formed on a photoconductor (photoconductor). It has also been examined for a one-drum color superposition method that is formed sequentially. In this method, it is possible to form a color image with little color misregistration by accurately superimposing toner on the photoconductor, and has been attracting attention as a technique for improving the color image quality.

  However, in this one-drum color superposition method, since the developing devices for the number of colors to be used must be arranged around the photoconductor, the photoconductor becomes large, which hinders downsizing of the image forming apparatus. Therefore, a tandem method in which a plurality of electrophotographic process members corresponding to the color of the toner to be used are arranged side by side, a color image is formed in synchronization with the transfer of the transfer member, and the color is superimposed on the transfer member has attracted attention. It was. However, this method has the advantage of being excellent in high-speed performance, but has the disadvantage of increasing the size because the electrophotographic process members for each color must be arranged side by side. As a countermeasure, there has been proposed a small tandem type image forming apparatus in which the intervals between the photosensitive members are narrowed and a downsized image forming unit is arranged.

  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 agitating unit to the developing member adjacent to the photosensitive member becomes complicated. There is a problem that there is a limit to downsizing and carrier adhesion to the photosensitive member and toner scattering are unavoidable.

  As another method, a one-component developing method that does not use a carrier has been proposed. However, the method in which the developing roller 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 a charge roll and the layer thickness on the developing roller is regulated by an elastic regulating blade, so that the toner additive adheres to the charge roll and the charging ability is reduced. The toner may adhere to the regulation blade and the layer formation may become non-uniform, resulting in image defects.

  For this reason, the above-described hybrid development method has attracted attention as one means for solving these problems. That is, in the hybrid developing system, since the developing roller is not in contact with the photoconductor, torque fluctuation does not occur, and a high-speed image forming apparatus that has excellent dot reproducibility and can extend the life can be provided.

  As a conventional technique related to such a technique, a thin layer is formed with a nonmagnetic toner on a donor roll (developing roller) placed in non-contact with an electrostatic latent image carrier in Patent Document 1, and static electricity is generated by an AC electric field. Proposals have been made to fly the toner onto the latent image on the electrostatic latent image carrier. Patent Document 2 discloses a developing device that forms a toner layer by using a magnetic roll to advance a developer to a donor roll and transferring toner onto the donor roll.

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

  In terms of theory, Non-Patent Document 1 reports the formation of a toner layer on a developing roller using a two-component developer from Toshiba Corporation, and Patent Document 4 has a patent application.

  In the above-described conventional technique, coarse powder of highly developable toner is easily developed selectively on the electrostatic latent image carrier, and fine powder of highly chargeable toner accumulates on the developing sleeve when continuous printing is performed. There was a tendency for selective development to easily occur and a decrease in image density. Further, toner charge control is complicated, and it is necessary to apply a high surface potential and a large development electric field to the electrostatic latent image carrier. For this reason, when a toner consumption area and a non-consumption area are generated on the developing roller, a part of the previous developed image is subjected to the next development because of the variation in the toner adhesion state and the toner potential difference on the developing roller. There is a problem that a phenomenon that sometimes appears as an afterimage (ghost), that is, a so-called history phenomenon is likely to occur. Furthermore, when a high density development pattern is printed continuously, the balance between the demand and supply of toner after drawing is poor, the toner layer on the development roller is not well formed, and the image density is uneven. Non-uniformity is likely to occur, which has been a problem when the developing device is downsized.

  As a method for preventing this and reliably collecting the toner on the developing roller, Patent Document 5 proposes using a dedicated collecting roll. However, these methods require a complicated mechanism and have not been put to practical use in a small-sized electrophotographic process. Further, as a countermeasure against a hysteresis phenomenon using a magnetic brush, Patent Document 6 proposes to collect and supply the toner on the developing roller by setting a full width at half maximum of the magnetic flux density of the magnetic roll. Further, as a control method for a tandem developing device, Patent Document 7 discloses an application 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 to prevent the deterioration of the developer. Has been made.

  Also, in such a hybrid development system, image density lowering due to selective development, development defects, image deterioration, development ghost, toner scattering, sleeve adhesion, etc. caused by leaving the toner on the developing roller for a long time may occur. Patent Document 8 (Patent Document 9) discloses a magnetic roll for forming a magnetic brush using a two-component developer, a donor roll (developing roller) carrying a thin layer of toner supplied from the magnetic roll, and the donor. And an electrode provided between the roll and the electrostatic latent image carrier. The electrode has a DC and AC bias, the developing roller has a DC bias, and the magnetic roll has a different polarity by a switch. The developing roller is configured to apply a DC bias that can be switched to a voltage that becomes a difference between the copies and between the sheets and between the sheets. Toners by collecting the magnetic roll, the hybrid type developing device used to stabilize the charge of the toner so as to solve the above problems is shown.

  Patent Document 10 discloses a method in which an electrode is provided between the developing roller and the electrostatic latent image carrier shown in Patent Document 8 and is not caused by vibration of an electrically biased and tensioned electrode wire. As a result of uniform development and the phenomenon that dust instantaneously adheres to the electrode and causes streaks on the developing roller, a hybrid type developing device using a developing roller with an embedded electrode was introduced as a conventional example. Even when using a developing roller with an embedded electrode, the carrier adhering to the developing roller adheres to the image, and the electrode embedded in the developing roller has a predetermined interval, so the toner supply efficiency to the electrostatic latent image carrier Phenomenon that causes poor image quality when images with a high image ratio are continuously developed, selective development occurs due to alternating bias applied to the magnetic roll and the developing roller, resulting in lower image quality and lower density Hybrid type developing apparatus such as to prevent a phenomenon causing is shown.

  However, the powder cloud development method described in Patent Document 3 is not a very general method because the wire of the auxiliary electrode is very easily contaminated and image degradation due to vibration occurs. Further, the devices shown in Patent Document 5, Patent Document 6 and the like also require the installation of a toner scraping device and a recovery roll, or the toner stress increases due to the application of a special recovery bias, etc. It becomes a factor of deterioration, and it takes time to form a layer of the developing roller at the next development timing, thereby impairing the high speed. In addition, during long-term use, the chargeability of the toner changes due to the deterioration of the durability of the carrier, the charging characteristics of the toner on the developing roller change significantly, and the distribution of the charge 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.

  The device disclosed in Patent Document 7 as a control method for a tandem developing device stops the operation of the developing device other than the image forming unit performing the transfer process, or is applied between the developing roller and the magnetic roll. It is necessary to control and switch the high pressure at a high frequency, 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. There is a drawback that it is difficult.

  Further, the devices disclosed in Patent Document 8 (Patent Document 9) and Patent Document 10 are also the above-described powder cloud developing method in which the wire of the auxiliary electrode is very dirty and the image caused by vibration. Deterioration occurs. Furthermore, the apparatus disclosed in Patent Document 10 also has a complicated configuration in which an electrode needs to be embedded in the developing roller and a brush electrode for supplying a bias in which AC and DC are superimposed on the electrode is necessary. In addition, the toner is fixed due to dirt or vibration for some reason, and the toner cannot be controlled at all.

  Further, in the apparatuses disclosed in Patent Document 8 (Patent Document 9) and Patent Document 10, toner is collected from the developing roller to the magnetic roll when image formation is completed, and the switch is switched when the image is formed again, and development is performed from the magnetic roll. A DC bias in the direction of moving the toner is applied to the roller. However, when the toner is re-supplied from the magnetic roll side to the developing roller, as shown by the line A in FIG. If there is no toner layer, the thickness of the layer formed in the first round becomes thin. Therefore, even if a layer having a certain thickness is formed in the second round of toner layer formation, when high density printing is performed in that state, the formed toner is almost used, and the next third round is used. Is printed with the toner layer being thin like the line B in FIG. 9, and the image density at the leading edge of the image may be reduced accordingly. Such a phenomenon may occur not only at the time of resupply to the developing roller but also at the start of development, and may also occur due to fluctuations in the surrounding environment such as temperature and humidity.

  For this reason, Patent Document 11 and Patent Document 12 describe a developing device using a one-component developer, but in an image forming apparatus that develops a latent image on a photosensitive member from a developing roller via an intermediate roller, Durability increases the amount of additives in the developing device, increases the number of toners that are charged with opposite polarity, toner that does not have a charging polarity, and toner that has a low charge. The toner density on the photoconductor, the surface potential of the developing roller and the intermediate roller, etc. are detected, and when the image density falls below the predetermined value, the developing roller is inspected. A developing device that increases the amount of toner that adheres to the intermediate roller by changing the bias of the toner and sets the potential difference between the developing roller and the intermediate roller so that the reverse polarity toner does not transfer. It is.

  In Patent Document 13, in a developing method in which a one-component developer is used and toner is transported and developed by a developing roller, toner that is not used for printing is scraped off by a reset roller. Because the amount of toner remains and is transported to the doctor blade, the charge amount of the new toner differs, resulting in an afterimage. Apply to the doctor blade, reset roller, etc. in the print area and non-print area. A developing method is shown in which the voltage to be applied is made different so that the charge amount of the toner becomes constant.

  Further, in Patent Document 14, since the sensitivity characteristics of the photosensitive drum in the image forming apparatus may change due to exposure, the surface potential and image density of the photosensitive member are periodically detected, and the toner density is a predetermined value. An image adjustment process control means for optimizing the photosensitive member charging output setting and the developing roller development voltage output setting is provided so that the image adjustment process mode is executed every time the number of printed sheets reaches a predetermined value, and the adjusted output value is set. An image forming apparatus configured to record in a non-volatile memory is shown.

U.S. Pat. No. 3,866,574 U.S. Pat. No. 3,929,098 JP-A-3-113474 JP 59-121077 JP 2000-81788 A Japanese Patent Laid-Open No. 7-128983 JP 63-249164 A Japanese Unexamined Patent Publication No. 7-72733 USP 5,420,375 publication JP 2000-250294 A JP-A-7-261468 JP-A-8-202159 JP-A-9-329958 JP 2003-84513 A The Journal of the Electrophotographic Society, Vol. 19, No. 2 (1981), pp. 44-51, “Touchdown Development”, Toshiba Corporation, Ikuo Hosaka, Yoneda, etc.

  However, the apparatuses shown in Patent Documents 11, 12, and 13 are developing apparatuses using a one-component developer different from the hybrid developing apparatus, and the process for holding the toner on the developing roller facing the photoconductor is different. When the formation is completed, the toner is collected from the developing roller to the magnetic roll, and the toner layer is not formed again on the developing roller upon image formation. The apparatus disclosed in Patent Document 14 is also a photosensitive drum. In this case, if there is no toner layer on the developing roller as described in FIG. 9, the layer thickness formed in the first round becomes thin. It is not a measure against such a phenomenon.

  SUMMARY OF THE INVENTION In view of the above circumstances, the present invention provides a developing method in an image forming apparatus for constantly stabilizing the toner layer thickness on the developing roller over a long period of time and maintaining a stable image without density change without complicating the developing apparatus. It is a problem to provide.

In order to solve the above problems, in the present invention,
A two-component developer consisting of a carrier and toner is charged and held as a magnetic brush on a magnetic roller for developer conveyance, and only toner is applied to the surface of the developing roller using a conveyance bias for conveying toner from the magnetic brush to the developing roller. The latent image on the electrostatic latent image carrier is developed by a developing bias applied between the developing roller and the electrostatic latent image carrier, and the transport bias is changed at a predetermined time. In the developing method in the image forming apparatus, the residual toner on the developing roller is collected on the magnetic roll.
In order to detect the toner layer thickness on the developing roller on the latent electrostatic image bearing member, a solid pattern using a toner layer formed in the second turn after starting to apply the developing bias of the developing roller; A toner layer thickness detection pattern on which a halftone pattern using a toner layer formed during the next rotation is arranged, and a toner layer thickness detection pattern on the electrostatic latent image carrier; Alternatively, the toner density in the toner layer thickness detection pattern on the transfer body onto which the toner layer thickness detection pattern on the electrostatic latent image carrier is transferred is detected by a density sensor, and the detected toner density is used to detect the toner density on the developing roller. The toner layer is controlled to have a predetermined layer thickness.

  Then, the control of making the toner layer thickness on the developing roller based on the detected toner density a predetermined layer thickness is performed by controlling a transport bias for transporting toner from the magnetic brush to the developing roller.

  Further, the control to make the toner layer thickness on the developing roller a predetermined layer thickness based on the detected toner concentration detects the toner concentration in the halftone pattern and conveys the toner from the magnetic brush to the developing roller. To control.

  The halftone pattern in the toner layer thickness detection pattern is a pattern with a printing rate of 10 to 40%, preferably a printing rate of approximately 25%, and the density sensor is a reflective density sensor. preferable.

  As described above, in order to detect the toner layer thickness on the developing roller on the electrostatic latent image carrier, a solid pattern using the toner layer formed in the second turn from the start of applying the developing bias of the developing roller; A toner layer thickness detection pattern having a halftone pattern using a toner layer formed during the next one rotation is visualized, and a toner layer thickness detection pattern on the electrostatic latent image carrier; Alternatively, the toner density in the toner layer thickness detection pattern on the transfer body onto which the toner layer thickness detection pattern on the electrostatic latent image carrier is transferred is detected by a density sensor, and the detected toner density on the developing roller is detected. By controlling the toner layer to a predetermined layer thickness, the toner layer on the developing roller that has been consumed in the formation of the solid pattern is formed with a halftone to which layer thickness is formed during the next one rotation. Providing a developing method that can be surely known by the layer thickness detection pattern composed of turns, and thereby controlling the toner layer thickness on the developing roller, so that constant and appropriate layer thickness control can be performed at all times. it can.

  The control of making the toner layer thickness on the developing roller a predetermined layer thickness based on the detected toner density is performed by controlling the transport bias for transporting the toner from the magnetic brush to the developing roller. With this, appropriate layer thickness control can be performed.

  Further, the control to make the toner layer thickness on the developing roller a predetermined layer thickness based on the detected toner concentration detects the toner concentration in the halftone pattern and conveys the toner from the magnetic brush to the developing roller. Since the halftone pattern is optimal for density detection by the density sensor and can cope with subtle density changes, the image forming apparatus can always obtain a stable output image independent of environmental fluctuations. A development method can be provided.

  Further, the halftone pattern in the toner layer thickness detection pattern is a pattern having a printing rate of 10 to 40%, preferably a printing rate of approximately 25%, thereby making it possible to detect the density more reliably. By using a reflection type density sensor, the toner density can be easily detected.

  Hereinafter, exemplary embodiments of the present invention will be 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 unless otherwise specified, but are merely illustrative examples. Not too much.

  FIG. 1 is a schematic diagram of an embodiment of an image forming apparatus that performs the developing method according to the present invention, FIG. 2 is a schematic configuration diagram of a hybrid developing apparatus that performs the developing method according to the present invention, and FIG. FIG. 4 is a flowchart for explaining the relationship between the thickness of the toner layer formed on the developing roller of the developing device and the rotation speed of the developing roller in the image forming apparatus, and FIG. 4 is a flowchart of the developing method in the image forming apparatus according to the present invention. 5 is a graph showing the relationship between the density sensor output value and the toner layer thickness on the developing roller, FIG. 6 is a schematic configuration example of the toner density sensor, and FIG. 7 is the relationship between the printing rate and the toner density sensor output voltage. FIG. 8 shows an example of a toner layer thickness detection pattern provided on the image carrier used in the developing method according to the present invention.

In the figure, reference numeral 1 denotes a non-magnetic metal material formed in a cylindrical shape, and a plurality of fixed magnets are disposed therein so that the periphery of the fixed magnet can be rotated, and a sleeve-shaped magnetic roll 2 is formed on the magnetic roll 1 Developing roller on which a thin layer of toner is formed by a magnetic brush, 3 is a photoreceptor (electrostatic latent image carrier) drum, 4 is a carrier constituting a developer, 5 is toner, and 6 is on the developing roller 2 7a is a power source for applying a direct current (DC) bias _Vdc1 to the developing roller 2, 7b is a power source for applying an alternating current (AC) bias _Vac to the developing roller 2, and 8 is a magnetic roll 1. A power source for applying a direct current (DC) bias V _dc2 to 9, 9 a spike cutting blade for keeping the height of the magnetic brush formed on the magnetic roll 1 constant, 20 an image forming apparatus, and 50 A an image forming apparatus 20 Bra Development device 50C, also yellow development device 50B, cyan development device 50C, magenta development device 50D, 53 paper feed cassette containing recording paper, 54 endless for conveying recording paper , 56 is a charger for charging the photosensitive drum 3, 57 is an exposure device for exposing the charged photosensitive drum 3 to form a latent image, and 58 is a developed toner image on recording paper. 59, a fixing device for fixing the toner image on the recording paper to which the toner image is transferred, 60, a toner, 61, a light emitting element using an LED or the like, and 62, a light emitting element from the light emitting element 61. The light receiving element 1 for monitoring (Photo Det) for detecting light in the direction of the photosensitive drum 3 with the toner 60 and the light emission amount of the light emitting element 61 and keeping the light quantity constant by a feedback circuit. ctor) Beam splitter 1 (Beam Splitter) 64 for directing in the direction of 63, light reflected by the toner 60 on the photosensitive drum 3, light receiving element 3 (Photo Detector) 65, light receiving element 2 (Photo Detector) This is a beam splitter 2 (Beam Splitter). In the tandem type image forming apparatus, the charger 56, the exposure device 57, the developing device 50, the transfer device 58, the cleaning device and the like installed around the photosensitive drum (electrostatic latent image carrier) 3 are designed in a compact manner. In the present invention, the developing device 50 is adjacent to the photosensitive drum 3 and is disposed in a substantially vertical direction.

  Among these, as a material of the photosensitive drum (electrostatic latent image carrier) 3, an amorphous silicon (a-Si) photosensitive member, an organic photosensitive member (OPC), or the like can be used. Positively charged organic photoconductors (positive OPCs) generate less ozone and have a stable charge. In particular, positively charged organic photoconductors with a single layer structure are sensitive even when their film thickness changes over time. Since there is little change in characteristics and the image quality is stable, it is suitable for a long-life system. When the positively charged organic photoconductor is used in a long-life system, the film thickness is preferably set to about 20 μm to 40 μm. In the case of 20 μm or less, black spots are noticeably generated 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 lowered, which causes a reduction in image density.

  The exposure device 57 can use a semiconductor laser or an LED. When a positively charged organic photoconductor is used, a wavelength around 770 nm is effective, and when an amorphous silicon photoconductor is used, a wavelength around 685 nm is effective. Hereinafter, in the present invention, a case where a positively charged organic photoreceptor is used as the photoreceptor drum 3 and an LED is used as a light source of the exposure device 57 will be described as an example.

The outermost surface of the developing roller 2 is composed of a sleeve made of uniform conductive aluminum, SUS, conductive resin coating, or the like. A direct current (DC) bias (V _dc1 ) 7 a and an alternating current (AC) bias 7 b are connected to the shaft portion, and this direct current is connected between the rotating developing roller 2, the photosensitive drum 3, and the magnetic roll 1. A bias with superimposed alternating current is applied. The AC component supplied by the AC bias 7b is a rectangular wave having a duty ratio of 50% or less. In the present invention, as an example, DC bias (V _dc1 ) 7a is 100V, AC bias 7b is Vpp 1.5kV, frequency 3.0kHz, duty (Duty).
The ratio was set to 30%. In this way, a direct bias (V _dc1 ) 7 a and an AC bias 7 b are directly applied to the developing roller 2 to apply a sharp bias component between the developing roller 2, the photosensitive drum 3, and the magnetic roll 1 during development. It is possible to improve the reaction of forming the toner layer at the start of development. In addition to good developability for the latent image on the photosensitive drum 3, the recoverability of the toner thin layer 6 with respect to the magnetic roll 1 is enhanced, and the stability of continuous printing is improved. As an example, the interval between the photosensitive drum 3 and the developing roller 2 is about 250 μm, and no wire electrode or the like is used between them.

  It is important to define the particle size distribution of the toner 5 in order to avoid selective developability. In general, the spread of the particle size distribution of the toner is measured by a coal counter, and the spread of the particle size distribution is expressed by a ratio between the volume distribution average particle size and the number distribution average particle size. In order to prevent selective development, it is important to reduce the ratio. When the distribution is wide, toner having a relatively small particle size accumulates on the developing roller 2 during continuous printing, and developability is deteriorated. In the present invention, the case where positively charged toner is used will be described as an example. However, it is obvious that the same configuration can be obtained even when negatively charged toner is used by reversing the relationship with the bias described above.

As the carrier 4, a magnetite carrier, a Mn-based ferrite, a Mn-Mg-based ferrite, or the like can be used, and the carrier 4 can be used after being surface-treated 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 and a saturation magnetization of 40 emu / g and an average particle size of 35 μm was used. If the average particle size exceeds 50 μm, the carrier stress increases and the toner density cannot be increased, and the amount of toner supplied to the developing roller 2 decreases. These carriers may be used as they are, but they can also be used after being surface-treated within a range in which an appropriate resistance value is not increased.

The saturated toner amount of the toner thin layer 6 on the developing roller is determined by the difference between the DC bias (V _dc2 ) 8 and the DC bias (V _dc1 ) 7a. When the DC bias (V _dc1 ) 7a is set to 150 V and the value of the DC bias (V _dc2 ) 8 is set to 400 V, a toner layer of about 1.0 mg / cm ² is obtained on the second turn of the developing roller. The adjustment of the thickness of the toner layer 6 is basically obtained by a potential difference of | (V _dc2 ) − (V _dc1 ) |, but factors such as the toner charge amount and the magnetic pole magnetic pole strength also contribute. There is a case. _Regarding the thickness of the toner layer 6, a uniform image can be obtained by changing the value of the DC bias (V _dc2 ) 8 according to the image data.

If the toner layer 6 is too thin at 0.5 mg / cm ² or less, the followability of the density when a high-density image continues is reduced, and image unevenness is likely to occur, and the toner layer exceeds 1.5 mg / cm ² . If it is too thick, the development ghost is noticeable and the toner scattering tends to be noticeable. The thickness of the toner thin layer 6 depends on the charge amount of the toner. When the toner charge amount is as low as 10 μC / g or less, particularly 5 μC / g or less, the toner layer thickness increases and scattering increases. On the other hand, when the toner charge amount is 20 μC / g or more, the thickness of the toner layer becomes thin, the charge increases, and the developability of the toner decreases.

In order to stabilize the image density in continuous printing, it is necessary to periodically remove the toner 6 from the developing roller 2 according to the print data and refresh it. This is to collect the toner thin layer 6 on the developing roller on the magnetic brush 10 by changing the direct current (DC) bias (V _dc2 ) 8 while applying the alternating current (AC) bias (V _ac ) 7 b at the end of development. . If the toner is peeled off from the developing roller 2 every time the development is completed, the toner is always refreshed. However, it takes time to form a stable toner layer again, and a sufficient printing speed cannot be achieved. In order to maintain a good printing speed, it is only necessary to adjust the time between loading and unloading the toner thin layer 6 on the developing roller 2 in a certain period by adjusting the sheet interval. In order to supply sufficient toner to the latent image on the photosensitive drum 3 without increasing the paper interval, the peripheral speed of the developing roller 2 is set to 1.5 times or more with respect to the photosensitive drum 3 for a short time. The toner can be taken in and out. Further, when the magnetic roll 1 is set to a speed exceeding 1 and 2 times or less than that of the developing roller 2, the replacement of the toner thin layer 6 is promoted. At this time, it is preferable that the rotating direction of the magnetic roll 1 is opposite to the developing roller 2.

  FIG. 1 is a schematic view of an embodiment of an image forming apparatus for carrying out the developing method according to the present invention. In this image forming apparatus 20, an endless belt 54 fixes a recording sheet from a paper feed cassette 53 to a fixing device 59. The black developing device 50A, the yellow developing device 50B, the cyan developing device 50C, and the magenta developing device 50D are disposed above the belt 54 that conveys the recording paper. Has been. These developing devices 50 (A, B, C, D) are developed in close proximity to the magnetic roll 1 (A, B, C, D) and the magnetic roll 1 (A, B, C, D), respectively. Rollers 2 (A, B, C, D) are arranged, and the photosensitive drum 3 (A, B, C, D) faces the developing roller 2, and further around the photosensitive drum 3, A charger 56 (A, B, C, D) and an exposure unit 57 (A, B, C, D) are arranged.

In the tandem type image forming apparatus of the present invention having the hybrid type developing apparatus configured as described above, the two-component developer including the toner 5 and the carrier 4 corresponding to each color such as yellow, cyan, magenta, and black is developed. A magnetic brush 10 is formed on the magnetic roll 1 shown in FIG. 2 and supplied to each developing device 50 from the agent container, and the toner 5 is charged by stirring. Then, the magnetic brush 10 on the magnetic roll 1 is layer-regulated by the regulating blade 9, and between the DC bias (V _dc2 ) 8 applied to the magnetic roll 1 and the DC bias (V _dc1 ) 7 a applied to the developing roller 2. A thin layer 6 of only toner 5 is formed on the developing roller 2 by the potential difference and the AC bias 7b.

When a print start signal is received from a control circuit (not shown), first, the photosensitive drum 3 composed of a positively charged organic photoreceptor (positive OPC) is charged by the charger 56 to 400 V, for example, and then, for example, 770 nm. As a result of the exposure by the exposure device 57 using the LED of the wavelength, the post-exposure potential of the photosensitive drum 3 becomes about 70 V and a latent image is formed. This latent image is developed with the toner that has jumped from the toner thin layer 6 on the developing roller 2 to the photosensitive drum 3 by the DC bias (V _dc1 ) 7a and the AC bias 7b applied to the developing roller 2, and the toner image Is formed. Then, the recording paper is fed from the paper feed cassette 53 and sent by the belt 54. When the recording paper reaches the photosensitive drum 3, the recording paper is transferred by the transfer device 58 (A, B, C, D). A toner image is transferred to the recording paper by applying a bias, and is fixed by the fixing device 59 and discharged. Thereafter, as described above, the toner thin layer 6 on the developing roller is changed by periodically changing the direct current (DC) bias (V _dc2 ) 8 while applying the alternating current (AC) bias (V _ac ) 7 b according to the print data. Is recovered in the magnetic brush 10.

  In the image forming apparatus configured as described above, in the present invention, the thickness of the toner layer 6 on the developing roller 2 is controlled in accordance with the flowchart shown in FIG. That is, first, in step S2, the power is turned on. In step S3, it is determined whether the toner layer thickness on the developing roller 2 needs to be controlled. If not necessary, the process proceeds to step S9, and if necessary, the process proceeds to step S4.

  Then, the controller of the image forming apparatus not shown in step S4 charges the photosensitive drum 3 by the charger 56 as described above, and develops after forming the layer thickness control pattern latent image by the exposure unit 57. However, in order to make each layer thickness control pattern formation position coincide with the rotation circumference of the developing roller, it is known per se in consideration of the peripheral length and peripheral speed of the photosensitive drum 3 and the distance from the charging start position to the developing position. The timing is controlled by this method. The number of rotations of the developing roller 2 is counted from the time when the developing bias is applied. This is because the application of the developing bias is started, the toner layer thickness is secured in the first round, and a layer thickness control pattern described later is developed in the second round and thereafter. Specifically, first, a developing bias of the developing roller 2 is applied to a toner layer thickness detection pattern formed as a solid pattern as shown in FIG. Writing is performed at the timing of development by the toner layer formed in the second round from the beginning.

Next, the developing roller 2 performs the next rotation (that is, the third rotation) of the toner layer thickness detection pattern formed as a halftone pattern as shown in FIG. ) Is written at the timing of development by the toner layer formed during (). These solid patterns and halftone patterns are developed by the toner layer formed while the developing roller 2 rotates twice, and the halftone patterns are developed by the next rotation of the developing roller 2 (that is, the third rotation). Development is performed with the toner layer formed between the two, and each is visualized. The solid pattern and the halftone pattern are formed for each color of black indicated by B, yellow indicated by Y, magenta indicated by M, and cyan indicated by C in FIG.

  In the next step S5, of the formed toner layer thickness detection pattern, the density of the halftone pattern is measured by a toner density sensor as shown in FIG. The toner density sensor shown in FIG. 6 is a reflection type density sensor, and the light emitted from the light emitting element 61 using an LED or the like is applied to the photosensitive drum 3 (with the toner 60) by the beam splitter 1 (Beam Splitter) 62. Or the direction of the intermediate transfer member to which the toner layer thickness detection pattern formed on the photosensitive drum 3 is transferred, and the light emission amount of the light emitting element 61, and for monitoring to keep the light amount constant by the feedback circuit. The light receiving element 1 (Photo Detector) 63 is divided. Then, the light reflected by the toner 60 on the photosensitive drum 3 is divided into two polarization components of a light receiving element 2 (Photo Detector) 66 and a light receiving element 3 (Photo Detector) 65 by a beam splitter 2 (Beam Splitter) 64. The toner amount is detected at the ratio.

  The relationship between the output voltage of the toner density sensor and the printing rate is as shown in the graph in FIG. In FIG. 7, the horizontal axis represents the printing rate expressed in%, the vertical axis represents the output voltage (V), and the intermediate transfer in which the photosensitive drum 3 (or the toner layer thickness detection pattern formed on the photosensitive drum 3 is transferred) is transferred. The reflected light is maximized and the output voltage is maximized when the toner 60 is not present in the body (printing rate is 0). As the amount of toner increases, the amount of reflection is reduced because light is absorbed. However, the output voltage does not decrease so much until the printing rate is about 10%, and when the printing rate is between 10% and 40%, When the printing rate exceeds 40%, the change in the output voltage becomes very small.

  That is, as apparent from the graph of FIG. 7, the reflection type toner density sensor can sensitively detect a change in density when the printing rate is between about 10 and 40%, and the printing rate is 10%. The change in density cannot be sensitively detected below and above 40%. Therefore, in the present invention, as shown in FIG. 8 and described above, in order to accurately detect the toner density, the halftone pattern in the toner layer thickness detection pattern has a printing rate of 10 to 40%, preferably approximately. 25%.

  That is, as described above, the toner layer thickness detection pattern formed as a solid pattern as shown in FIG. 8 as “development roller second round, solid 100%” and “development roller third round, half 25%”. The solid layer pattern at the timing when the toner layer thickness detection pattern formed as a halftone pattern as shown in FIG. By writing the halftone pattern at the timing when the developing roller 2 is developed by the toner layer formed during the next rotation (that is, the third rotation), as shown by the line A in FIG. Even if a layer having a certain thickness is formed in the second round, if the layer thickness threshold is not reached, high density printing (that is, a solid pattern) is performed in that state. The toner formed on the developing roller 2 is almost used, and the printing on the third round (that is, the halftone pattern) is performed with the toner layer being thin as shown by the line B in FIG. As a result, the image density decreases accordingly.

  For this reason, in the present invention, the solid layer is written at the timing of development by the toner layer formed in the second turn after the development bias of the development roller 2 is started, so that the toner layer on the development roller 2 is consumed and developed. When the roller 2 is developed by the toner layer formed during the next rotation (that is, the third rotation), a halftone pattern is written and its density is detected, so that the developing bias of the developing roller 2 starts to be applied. It is possible to detect whether or not the toner layer formed on the second round from the toner thickness has reached the layer thickness shown as the layer thickness threshold in FIG. That is, as shown in FIG. 3, the toner layer formed on the developing roller 2 is formed with a considerable thickness on the first round of the developing roller 2 as shown by A ′, and this layer thickness threshold is formed on the second round. 3 in the same manner, even if the solid pattern is written at the timing of development by the toner layer formed while the developing roller 2 rotates twice, the toner is consumed by the dotted line in FIG. As shown in B, the toner layer thickness on the developing roller 2 does not extremely decrease, and the toner layer formed during the next rotation (that is, the third rotation) of the developing roller 2 already has this layer thickness threshold. Therefore, even if a halftone pattern is written, the density does not become insufficient.

Therefore, in the next step S6 in FIG. 4, it is determined whether or not the density of the halftone pattern in the toner layer thickness detection pattern is appropriate based on the signal from the toner density sensor. If it is appropriate, the process goes to step S9. Proceed to S7. The adjustment of the thickness of the toner thin layer 6 on the developing roller 2 can be performed by the potential difference of | (V _dc2 ) − (V _dc1 ) | as described above, and the density sensor output and the direct current applied to the developing roller 2 can be adjusted. Since the relationship with the (DC) bias (V _dc1 ) 7a is as shown in the graph of FIG. 5, the “developing roller toner layer thickness” is shown in FIG. The values of the direct current (DC) bias (V _dc1 ) 7a and the direct current (DC) bias (V _dc2 ) 8 are set in step S7 so that the density sensor output value shown as the “specified range” is obtained, and in the next step S8. Adjust the density. By controlling the toner layer thickness on the developing roller 2 in this way, a constant print density can always be obtained if the print density is controlled, and a print ready signal is output in the next step S9, and the process ends. To do. This control is preferably performed when the integrated value of the number of printed sheets and the printing rate, that is, when a certain printing rate is reached.

  The above is the developing method in the image forming apparatus according to the present invention. According to the present invention, the developing bias of the developing roller 2 is applied to the photosensitive drum 3 in order to detect the toner layer thickness on the developing roller 2. The toner layer thickness detection pattern in which the solid pattern using the six toner layers formed in the second round from the beginning and the halftone pattern using the toner layer formed during the next rotation is revealed. The toner density in the toner layer thickness detection pattern on the transfer body onto which the toner layer thickness detection pattern on the photosensitive drum 3 or the toner layer thickness detection pattern on the photosensitive drum 3 has been transferred is imaged. By detecting the sensor and controlling the toner layer on the developing roller 2 to have a predetermined layer thickness based on the detected toner density, The thickness of the toner layer formed during the next rotation can be surely known by the layer thickness detection pattern constituted by the halftone pattern, and thereby the toner on the developing roller 2 can be recognized. By controlling the layer thickness, it is possible to provide a developing method capable of always performing appropriate and appropriate layer thickness control.

The control for making the toner layer thickness on the developing roller 2 a predetermined layer thickness based on the detected toner density controls a direct current (DC) bias (V _dc1 ) 7a that conveys toner from the magnetic brush 10 to the developing roller 2. Thus, appropriate layer thickness control can be performed with a simple configuration.

In addition, the control to change the toner layer thickness on the developing roller 2 based on the detected toner density to a predetermined layer thickness is a direct current (DC) that detects the toner density in the halftone pattern and conveys the toner from the magnetic brush 10 to the developing roller 2. ) By controlling the bias (V _dc1 ) 7a, the halftone pattern is optimal for density detection by the density sensor and can cope with subtle density changes, so that it is always stable output images that do not depend on environmental fluctuations. A developing method in an image forming apparatus that can obtain the above can be provided.

  Further, the halftone pattern in the toner layer thickness detection pattern is a pattern having a printing rate of 10 to 40%, preferably a printing rate of approximately 25%. By using a reflection type density sensor, the toner density can be easily detected.

  According to the present invention, it is possible to provide a developing method in an image forming apparatus in which the toner layer thickness on the developing roller 2 can always be set to a value at which an appropriate density can be obtained, and a stable output image without density change can be obtained. .

1 is a schematic diagram of an embodiment of an image forming apparatus that performs a developing method according to the present invention. It is a schematic block diagram of the hybrid type developing apparatus which implements the developing method according to the present invention. FIG. 6 is a diagram for explaining a relationship between a thickness of a toner layer formed on a developing roller of a developing device and a rotation speed of the developing roller in the image forming apparatus according to the present invention. It is a flowchart of the developing method in the image forming apparatus according to the present invention. 6 is a graph showing the relationship between the density sensor output value and the toner layer thickness on the developing roller. 2 is a schematic configuration example of a toner density sensor. 6 is a graph showing a relationship between a printing rate and a toner density sensor output voltage. 2 is an example of a toner layer thickness detection pattern provided on an image carrier used in the developing method according to the present invention. FIG. 4 is a diagram for explaining a relationship between a thickness of a toner layer formed on a developing roller of a developing device in the image forming apparatus and a rotation speed of the developing roller.

Explanation of symbols

_{DESCRIPTION OF SYMBOLS} 1 Magnetic roll 2 Developing roller 3 Photoconductor 4 Carrier 5 Toner 6 Toner thin layer formed on developing roller 7a Direct current (DC) bias (V _dc1 ) power source 7b Alternating current (AC) bias (V _ac ) power source 8 Direct current (DC) ) Bias (V _dc2 ) Power supply 9 _Panning blade 20 Image forming device 50A Black developing device 50B Yellow developing device 50C Cyan developing device 50D Magenta developing device 53 Paper feed cassette 54 Endless belt 56 Charging device 57 Exposure device 58 Transfer device 59 Fixing device

Claims (5)

A two-component developer consisting of a carrier and toner is charged and held as a magnetic brush on a magnetic roller for developer conveyance, and only toner is applied to the surface of the developing roller using a conveyance bias for conveying toner from the magnetic brush to the developing roller. The latent image on the electrostatic latent image carrier is developed by a developing bias applied between the developing roller and the electrostatic latent image carrier, and the transport bias is changed at a predetermined time. In the developing method in the image forming apparatus, the residual toner on the developing roller is collected on the magnetic roll.
In order to detect the toner layer thickness on the developing roller on the latent electrostatic image bearing member, a solid pattern using a toner layer formed in the second turn after starting to apply the developing bias of the developing roller; A toner layer thickness detection pattern on which a halftone pattern using a toner layer formed during the next rotation is arranged, and a toner layer thickness detection pattern on the electrostatic latent image carrier; Alternatively, the toner density in the toner layer thickness detection pattern on the transfer body onto which the toner layer thickness detection pattern on the electrostatic latent image carrier is transferred is detected by a density sensor, and the detected toner density is used to detect the toner density on the developing roller. A developing method in an image forming apparatus, wherein the toner layer is controlled to have a predetermined layer thickness.   2. The control for adjusting the toner layer thickness on the developing roller based on the detected toner density to a predetermined layer thickness is performed by controlling a transport bias for transporting toner from the magnetic brush to the developing roller. The developing method in the image forming apparatus described in 1.   The control of adjusting the toner layer thickness on the developing roller based on the detected toner density to a predetermined layer thickness detects the toner density in the halftone pattern and controls the transport bias for transporting the toner from the magnetic brush to the developing roller. The developing method in the image forming apparatus according to claim 1, wherein the developing method is performed.   4. The halftone pattern in the toner layer thickness detection pattern is a pattern having a printing rate of 10 to 40%, preferably a printing rate of approximately 25%. A developing method in an image forming apparatus.   5. The developing method in the image forming apparatus according to claim 1, wherein the density sensor is a reflection type density sensor.

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