JP2007034098A - Developing unit and image forming apparatus equipped with the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a developing unit capable of preventing hysteresis phenomenon, and attaining high-speed image forming processing, by quickly and surely collecting residual toner on a developing roller and also quickly and surely forming a thin toner layer on a developing roller, and to provide an image forming apparatus equipped with the developing unit. <P>SOLUTION: Screws 27a and 27b function as first and second stirring screws for mixing, stirring and electrifying the toner and carrier. A magnetic brush 21 is formed on magnetic rollers 20a and 20b by the electrified developer, and then, the developer of a fixed layer thickness comes in contact with the developing roller 22, while being layer-controlled by a control blade 26; then, the thin toner layer 23 is formed on the developing roller 22 by a potential difference ΔVdc between the first and second magnetic rollers 20a and 20b and the developing roller 22. In addition, the magnetic rollers 20a and 20b respectively recovers the toner from the developing roller 22. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a hybrid developing unit that uses a two-component developer composed of a magnetic carrier and a toner, holds only the charged toner on the developing roller, and develops the electrostatic latent image on the image carrier in a non-contact manner, The present invention also relates to an image forming apparatus such as a copying machine, a facsimile, and a printer provided with the same.

  Conventionally, a two-component developer that charges a non-magnetic toner using a magnetic carrier is used, and only the charged toner is held on the developing roller 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.

  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 a space between the photosensitive members is 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 unit vertical in order to minimize the size of the image forming unit in the width direction. That is, it is desirable in terms of layout to arrange the developing unit in the upper direction of the photoreceptor. However, in the conventional two-component development method, when the development unit is arranged vertically as described above, the reflux of the developer, that is, the supply from the developer agitating unit to the development roller adjacent to the photosensitive member becomes complicated, and the apparatus 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.

  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 the charge roller, and the layer thickness on the developing roller is regulated by the elastic regulating blade, so the toner additive adheres to the charge roller 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.

  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. Has been proposed. As such a conventional technique, as shown in FIG. 7, a magnetic roller is used to advance the developer onto a developing roller placed in non-contact with the electrostatic latent image carrier (photosensitive member). Hybrid development has been proposed in which toner is transferred onto a roller to form a thin layer of non-magnetic toner, and the toner is ejected onto a latent image on an electrostatic latent image carrier (photoconductor) by an alternating electric field.

  In FIG. 7, reference numeral 20 denotes a magnetic roller that generates a magnetic brush 21 by a carrier C contained in a developer using a magnet (not shown) disposed therein, and 22 denotes toner T supplied from the magnetic brush 21. A developing roller that carries the toner thin layer 23 and develops the electrostatic latent image on the photosensitive drum 1a.

  Reference numeral 24 denotes a developing bias power source including a first DC bias power source 24a for applying a DC bias (Vdc1) to the developing roller 22 and an AC bias power source 24b for applying an AC bias (Vpp). Reference numeral 25 denotes a second DC bias power source that applies a DC bias (Vdc2) to the magnetic roller 20, and 26 denotes a regulating blade that regulates the thickness of the magnetic brush 21 on the magnetic roller 20.

  The photosensitive drum 1a is formed, for example, by laminating a photosensitive layer on an aluminum drum, and the surface is charged by a charger (not shown). Then, an electrostatic latent image in which charging is attenuated is formed on the surface that has received a laser beam from an exposure unit (not shown). As the photosensitive material for forming the photosensitive layer, an amorphous silicon photoreceptor or a positively charged organic photoreceptor (positive OPC photoreceptor) is used. When positive OPC is used, there is little generation of ozone, etc., and charging is stable. In particular, when a single layer structure of positive OPC is used for a long period of time and the film thickness changes, there is little change in photosensitive characteristics and image quality is also good. Since it is stable, it is preferably used for a long-life system.

  The developing roller 22 is disposed to face the photosensitive drum 1a with a predetermined interval. As the material of the developing roller 22, SUS, aluminum, or a uniform conductor having a conductive resin film formed thereon is used. It is done. A first DC bias power source 24a and an AC bias power source 24b are connected to the developing roller 22, and by directly applying these superimposed biases, together with the developing performance for the electrostatic latent image on the surface of the photosensitive drum 1a. Further, the performance of collecting toner on the magnetic roller 20 described later is enhanced, and the image stability during continuous printing is improved.

  As the carrier C, a magnetite carrier, a Mn-based ferrite, a Mn-Mg-based ferrite, or the like is used, and these carriers can be used as they are or after being subjected to a surface treatment within a range not increasing an appropriate resistance. If the average particle size of the carrier exceeds 50 μm, the toner concentration cannot be increased and the amount of toner supplied to the developing roller 22 decreases. Therefore, it is preferable to use a carrier having an average particle size of 50 μm or less.

  For the toner T, it is important to define the particle size distribution in order to avoid selective developability. Generally, the particle size distribution of toner is measured by a Coulter counter, and the spread of the particle size distribution is expressed by the ratio of the volume distribution average diameter to the number distribution average diameter. If the particle size distribution is wide, toner having a relatively small particle size is accumulated on the developing roller 22 in continuous printing, and developability is lowered. Therefore, in order to prevent selective development, it is necessary to reduce this ratio.

  In the developing unit configured as described above, the toner T is supplied from a toner container (not shown) to the magnetic roller 20 and is stirred together with the carrier C by a first stirring screw and a second stirring screw (both not shown). The toner is positively charged to an appropriate level. A magnetic brush 21 is formed on the magnetic roller 20 by this developer, contacts the developing roller 22 with a constant layer thickness while being regulated by the regulating blade 26, and a potential difference | Vdc2− between the magnetic roller 20 and the developing roller 22 is reached. Due to Vdc1 | (hereinafter referred to as ΔVdc), the toner T moves from the magnetic brush 21 onto the developing roller 22, and the toner thin layer 23 is formed.

  The toner T of the toner thin layer 23 flies to the electrostatic latent image formed on the photosensitive drum 1a by the AC bias Vpp applied from the AC bias power supply 24b to the photosensitive drum 1a, and development is performed. Is called. The thickness of the toner thin layer 23 on the developing roller 22 varies depending on the resistance of the developer and the difference in linear velocity between the developing roller 22 and the magnetic roller 20, but can also be controlled by the above-described potential difference ΔVdc. When ΔVdc is increased, the toner layer on the developing roller 20 is thickened, and when ΔVdc is decreased, the toner layer is thinned. The range of ΔVdc is generally about 100V to 250V.

  According to this technology, although a two-component developer can be used to form a thin toner layer on the developing roller, it becomes difficult to separate the toner on the developing roller when the charge of the toner becomes high, which is strong. An alternating 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.

  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. For this reason, when a toner consumption area and a non-consumption area are generated on the developing roller, a variation in the toner adhesion state and the toner potential difference occurs on the developing roller. There is a problem that a phenomenon that appears as an afterimage (ghost), that is, a so-called history phenomenon is likely to occur.

  This hysteresis phenomenon occurs because the toner thin layer formed in the first round of the developing roller is thinner than the saturated layer thickness. Conventionally, the toner thin layer formed by the magnetic roller during one rotation of the developing roller is about 60 to 70% of the saturation layer thickness, and the developing roller is rotated twice to supply toner for two magnetic rollers. The saturation layer thickness is reached for the first time.

  In view of this, various methods for reliably collecting the residual toner on the developing roller to prevent the hysteresis phenomenon have been proposed. For example, Patent Document 1 discloses a developing roller and a non-image forming unit such as a paper interval or before image formation. A step of peeling off the residual toner on the developing roller and collecting it on the magnetic roller by applying only an AC bias to the developing roller while rotating the magnetic roller and applying an AC bias (hereinafter referred to as a toner collecting step); , A developing method for an image forming apparatus is disclosed in which a toner thin layer on a developing roller can be refreshed by executing a step of re-forming a new toner thin layer on the developing roller (hereinafter referred to as a re-forming step). ing.

  However, in order to sufficiently refresh the toner thin layer on the developing roller by the method of Patent Document 1, the toner for one rotation of the developing roller is peeled off between papers, and further, two rotations of the developing roller (one rotation + magnetic roller) Therefore, it is necessary to set the space between the sheets more than the time required for the toner peeling and toner layer formation.

  For example, the linear speed of the photosensitive drum is 100 mm / sec, the linear speed ratio (S / D ratio) between the developing roller and the photosensitive drum is 1.5, the developing roller diameter is 16 mm, and the development from the magnetic roller to the photosensitive drum is performed. When the outer peripheral length of the roller is 0.3, the time required between the papers is about 16 × π × (1 + 1.3) / (100 × 1.5) ≈800 msec. Therefore, there has been a limit to improving the image forming efficiency during continuous printing.

  Patent Document 2 discloses a method in which a toner roller and a toner recovery magnetic roller are separately provided for a donor roller (development roller), and the developed toner is reliably recovered by the toner recovery roller. Has been. Further, in Patent Document 3, the peripheral speed of each roller is set so that the developing roller and the magnetic roller rotate once within a predetermined excess / deficiency range. A hybrid developing method is disclosed in which the remaining toner can be reliably collected.

  In the method of Patent Document 2, since the roles of the two magnetic rollers are defined, when a toner thin layer is formed on the developing roller, the image carrier (photosensitive drum) is positioned from a position facing the toner collecting roller. It is necessary to supply the toner from the toner supply roller until it reaches a position opposite to (a circumference of the developing roller). However, when the toner thin layer is formed using one toner supply roller, in order to increase the thickness of the toner thin layer to the saturation layer thickness, as described above, the toner layer for two rotations of the developing roller is newly added. It is necessary to form.

  For this reason, the layer thickness of the formed toner thin layer is insufficient, and the hysteresis phenomenon cannot be completely prevented. Further, in order to form a toner thin layer having a sufficient layer thickness on the developing roller with the configuration of Patent Document 2, a method of increasing the linear speed of the toner supply roller is conceivable, but it is applied to the toner in the developing unit. This is not preferable because mechanical stress increases and toner deterioration is promoted.

  Further, when the time required for one rotation of the developing roller during the collection of the toner thin layer is longer than the time required for one rotation of the toner collecting roller, the toner collecting roller is developed at the second rotation by the difference of the time required for one rotation of each roller. The toner on the roller is stripped off, and the toner is further superimposed on the toner stripped off in the first round. Therefore, since toner that has already been collected is present in the second round of the toner collecting roller, the toner collecting force is weaker than that in the first round, and the toner remains on the developing roller, and the density of only that portion is low. The problem arises that the image becomes dark and a uniform image cannot be obtained.

Further, according to the method of Patent Document 3, the remaining toner can be surely removed regardless of the roller diameter ratio of each roller, and it is possible to prevent the occurrence of layer unevenness when the toner thin layer is re-formed. As in the case of No. 1, since the time required between the sheets does not change, the image forming efficiency during continuous printing cannot be improved.
JP 2003-211961 A JP 2000-81778 A Japanese Patent Laid-Open No. 2005-55840

  In view of the above problems, the present invention prevents the hysteresis phenomenon and speeds up the image forming process by quickly and reliably collecting the residual toner on the developing roller and forming the toner thin layer on the developing roller. It is another object of the present invention to provide a developing unit that can also be realized and an image forming apparatus including the developing unit.

  In order to achieve the above object, the present invention comprises a developing roller disposed opposite to an image carrier, a magnetic roller disposed opposite to the developing roller and forming a thin toner layer on the developing roller using a magnetic brush, A developing unit that applies a direct current bias to the magnetic roller and applies a direct current and alternating current bias to the developing roller to cause toner to fly on the surface of the image carrier, and a plurality of the magnetic rollers are provided. The toner is simultaneously supplied from the two or more magnetic rollers onto the developing roller.

  Further, the present invention includes a developing unit having the above-described configuration and a control unit that controls driving of the developing unit, and is applied to the magnetic roller and the developing roller after completion of image formation during continuous image formation. A toner collecting step of collecting toner from the developing roller to the magnetic roller by making a potential difference between DC biases different from that at the time of image formation, and a re-forming step of re-forming a new toner thin layer on the developing roller. The image forming apparatus can be executed by using two or more magnetic rollers.

  According to the present invention, in the image forming apparatus having the above configuration, in the re-forming step, the control unit is configured such that the linear velocity of the magnetic roller located on the downstream side in the rotation direction of the developing roller is located on the upstream side. It is characterized by making it faster.

  According to the first configuration of the present invention, it is possible to supply toner of two or more magnetic rollers during one rotation of the developing roller, and saturation can be achieved in a short time compared to the conventional method using one magnetic roller. A toner thin layer having a layer thickness can be formed. Further, since the linear speed ratio of the magnetic roller to the developing roller can be lowered as compared with the conventional one, the mechanical stress applied to the toner is reduced, and the life of the toner in the developing unit is extended.

  According to the second configuration of the present invention, the toner collection step of collecting the toner from the development roller using the development unit of the first configuration and the control means for controlling the drive of the development unit, and the development By performing a re-forming step of re-forming a new thin toner layer on the roller using a plurality of magnetic rollers, the toner is supplied and collected for two or more magnetic rollers during one rotation of the developing roller. Therefore, it is possible to provide an image forming apparatus that effectively suppresses the occurrence of a hysteresis phenomenon and increases the image forming efficiency.

  According to the third configuration of the present invention, in the image forming apparatus having the second configuration, the linear velocity of the magnetic roller positioned on the downstream side in the rotation direction of the developing roller is positioned on the upstream side when the re-forming process is executed. By making the speed faster than the magnetic roller, the toner can be sufficiently supplied from the downstream magnetic roller that forms the second toner layer, which is effective for forming the toner thin layer having the saturated layer thickness.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic cross-sectional view of an image forming apparatus equipped with a developing unit of the present invention. Here, a tandem type color image forming apparatus is shown. In the main body of the color image forming apparatus 100, four image forming portions Pa, Pb, Pc, and Pd are sequentially arranged from the upstream side in the transport direction (the right side in FIG. 1). These image forming portions Pa to Pd are provided corresponding to images of four different colors (cyan, magenta, yellow, and black), and cyan, magenta, and yellow are respectively performed by charging, exposure, development, and transfer processes. And a black image are sequentially formed.

  Photosensitive drums 1a, 1b, 1c, and 1d that carry visible images (toner images) of the respective colors are disposed in the image forming portions Pa to Pd, and are formed on the photosensitive drums 1a to 1d. The transferred toner image is rotated clockwise in FIG. 1 by a driving means (not shown), and sequentially transferred onto the intermediate transfer belt 8 that moves adjacent to each image forming unit, and then transferred by the transfer roller 9. The image is transferred onto the paper P at a time, and further fixed on the transfer paper P in the fixing unit 7 and then discharged from the apparatus main body. An image forming process for each of the photosensitive drums 1a to 1d is executed while rotating the photosensitive drums 1a to 1d counterclockwise in FIG.

  The transfer paper P onto which the toner image is transferred is accommodated in a paper cassette 16 at the lower part of the apparatus, and is conveyed to the transfer roller 9 via the paper supply roller 12a and the registration roller 12b. A sheet made of a dielectric resin is used for the intermediate transfer belt 8, and a belt in which both ends thereof are overlapped and joined to form an endless shape, or a belt without a seam (seamless) is used.

  Next, the image forming units Pa to Pd will be described. There are chargers 2a, 2b, 2c, and 2d for charging the photosensitive drums 1a to 1d and image information on the photosensitive drums 1a to 1d around and below the photosensitive drums 1a to 1d that are rotatably arranged. The exposure unit 4 for exposing the toner, the developing units 3a, 3b, 3c and 3d for forming toner images on the photosensitive drums 1a to 1d, and the developer (toner) remaining on the photosensitive drums 1a to 1d are removed. Cleaning units 5a, 5b, 5c and 5d are provided. The detailed configuration of the developing units 3a to 3d will be described later.

  When the image formation start is input by the user, first, the surfaces of the photosensitive drums 1a to 1d are uniformly charged by the chargers 2a to 2d, and then light is irradiated by the exposure unit 4 to each of the photosensitive drums 1a to 1d. An electrostatic latent image corresponding to the image signal is formed on the top. Each of the developing units 3a to 3d is filled with a predetermined amount of cyan, magenta, yellow, and black toner by a replenishing device (not shown). The toner is supplied onto the photosensitive drums 1a to 1d by the developing units 3a to 3d and electrostatically attached, whereby a toner image corresponding to the electrostatic latent image formed by exposure from the exposure unit 4 is formed. It is formed.

  After an electric field is applied to the intermediate transfer belt 8 at a predetermined transfer voltage, cyan, magenta, yellow, and black toner images on the photosensitive drums 1a to 1d are transferred to the intermediate transfer belt 8 by the intermediate transfer rollers 6a to 6d. Transcribed above. These four color images are formed with a predetermined positional relationship predetermined for forming a predetermined full-color image. Thereafter, the toner remaining on the surfaces of the photosensitive drums 1a to 1d is removed by the cleaning units 5a to 5d in preparation for the subsequent formation of a new electrostatic latent image.

  The intermediate transfer belt 8 is stretched between an upstream conveyance roller 10 and a downstream drive roller 11, and the intermediate transfer belt 8 rotates clockwise as the drive roller 11 is rotated by a drive motor (not shown). When the rotation starts, the transfer paper P is conveyed from the registration roller 12b to a transfer roller 9 provided adjacent to the intermediate transfer belt 8 at a predetermined timing, and a full color image is transferred. The transfer paper P onto which the toner image is transferred is conveyed to the fixing unit 7.

  The transfer paper P conveyed to the fixing unit 7 is heated and pressurized by the fixing roller pair 13 so that the toner image is fixed on the surface of the transfer paper P, and a predetermined full color image is formed. The transfer paper P on which the full-color image is formed is distributed in the transport direction by the branching portion 14 that branches in a plurality of directions. When an image is formed only on one side of the transfer paper P, it is discharged as it is onto the discharge tray 17 by the discharge roller 15.

  On the other hand, when images are to be formed on both sides of the transfer paper P, the transfer paper P that has passed through the fixing unit 7 is distributed to the paper transport path 18 by the branching unit 14, and is transferred again to the transfer roller 9 with the image surface reversed. Be transported. Then, the next image formed on the intermediate transfer belt 5 is transferred to the surface of the transfer paper P on which the image is not formed by the transfer roller 9, conveyed to the fixing unit 7, the toner image is fixed, and then discharged. It is discharged to the tray 17.

  FIG. 2 is a side sectional view showing the configuration of the developing unit of the present invention. Portions common to FIG. 7 of the conventional example are denoted by the same reference numerals and description thereof is omitted. Here, the developing unit 3a disposed in the image forming portion Pa will be described, but the configuration of the developing units 3b to 3d is exactly the same. In the present invention, two magnetic rollers including a first magnetic roller 20a and a second magnetic roller 20b are provided, and the toner collecting step and the re-forming step can be performed on each magnetic roller.

  In FIG. 2, reference numerals 27a and 27b denote first and second agitating screws for mixing and agitating a toner supplied from a toner container (not shown) with a carrier for agitation. A magnetic brush 21 (see FIG. 7) is formed on the magnetic rollers 20a and 20b by the charged developer, and comes into contact with the developing roller 22 with a constant layer thickness while being regulated by the regulating blade 26. A toner thin layer 23 is formed on the developing roller 22 due to the potential difference ΔVdc between 20 b and the developing roller 22.

The saturated toner amount of the toner thin layer 23 is basically determined by the potential difference ΔVdc between the DC bias Vdc1 applied to the developing roller 22 and the DC bias Vdc2 applied to the magnetic rollers 20a and 20b. For example, when Vdc1 = 100V and Vdc2 = 400V are set, ΔVdc = 300V, and the toner thin layer 23 of about 1.0 mg / cm ² is obtained on the second turn of the developing roller 22. When the toner thin layer 23 is 0.5 mg / cm ² or less, the density followability when a high density image is continuous decreases, and image unevenness is likely to occur. On the other hand, when the toner thin layer 23 is 1.5 mg / cm ² or more, development ghost and toner scattering tend to be noticeable.

  Further, when the toner charge amount is as low as 10 μC / g or less, particularly 5 μC / g or less, the thickness of the toner thin layer 23 increases and scattering increases. On the other hand, when the toner charge amount is 20 μC / g or more, the thickness of the toner thin layer 23 is reduced, the toner charge is increased, and the developability is lowered. Further, since the image density (printing rate) to be printed also contributes to the optimum adjustment of the toner thin layer 23, a uniform image can be obtained regardless of the printing rate if the value of Vdc2 is varied according to the image data. Can be obtained.

  FIG. 3 is a block diagram showing the configuration of the image forming apparatus of the present invention. Portions common to FIGS. 1 and 2 are denoted by the same reference numerals and description thereof is omitted. The image forming apparatus 100 includes an image forming unit Pa to Pd, a fixing unit 7, an image input unit 30, an AD conversion unit 31, a control unit 32, a storage unit 33, an operation panel 34, and the like.

  When the image forming apparatus 100 is a copying machine, the image input unit 30 is a scanning optical system equipped with a scanner lamp that illuminates an original during copying and a mirror that changes the optical path of reflected light from the original, and reflected light from the original. If the image forming apparatus 100 is a printer, a personal computer or the like is a condensing lens for condensing and forming an image and a CCD for converting the imaged image light into an electric signal. It is a receiving part which receives the image data transmitted from. The image signal input from the image input unit 30 is sent to the control unit 32, appropriately performs image processing such as gradation processing, and after being converted into a digital signal by the AD conversion unit 31, an image in the storage unit 33 to be described later It is sent to the memory 40.

  The image forming portions Pa to Pd are composed of the photosensitive drums 1a to 1d, the chargers 2a to 2d, the developing units 3a to 3d, the exposure unit 4, the intermediate transfer rollers 6a to 6d, and the like. Based on the image data stored in the memory 60, electrostatic latent images are formed on the photosensitive drums 1a to 1d. The storage unit 33 includes an image memory 40, a RAM 41, and a ROM 42. The image memory 40 stores an image signal read by the image input unit 30 and digitally converted by the AD conversion unit 31, and is stored in the control unit 32. Send it out. The RAM 41 and the ROM 42 store a processing program, processing content, and the like of the control unit 32. In addition, the values of the direct current and alternating current bias voltage applied to the magnetic rollers 20a and 20b or the developing roller 22 in the toner collecting process or the re-forming process are also stored.

  The operation panel 34 includes an operation unit composed of a plurality of operation keys, and a display unit (none of which is shown) that displays setting conditions, the state of the apparatus, and the like, and the user sets printing conditions and the like. In addition, for example, when the image forming apparatus 100 has a facsimile function, it is also used for various settings such as registering a facsimile transmission destination in the storage unit 33 and reading or rewriting the registered transmission destination. The main motor 35 includes various parts of the apparatus including the photosensitive drums 1a to 1d of the image forming units Pa to Pd, the developing units 3a to 3d, the intermediate transfer rollers 6a to 6d, and the fixing unit 7 in accordance with a control signal from the control unit 32. Drive.

  The control unit 32 generally controls the image input unit 30, the image forming units Pa to Pd, the fixing unit 7 and the like according to the set program, and the image signal input from the image input unit 30 as necessary. The image data is converted into image data by scaling processing or gradation processing. The exposure unit 4 irradiates a laser beam based on the processed image data, and forms latent images on the photosensitive drums 1a to 1d.

  FIG. 4 is a timing chart showing an example of operation control of the developing unit during continuous image formation in the image forming apparatus of the present invention. With reference to FIGS. 1 to 3, an image forming process in the case where continuous image formation is instructed by the user will be described with reference to FIG. 4.

  First, the main motor 35 for driving the photosensitive drums 1a to 1d and the intermediate transfer rollers 6a to 6d and the charge eliminating device are turned on, and then charging of the surfaces of the photosensitive drums 1a to 1d is started by the chargers 2a to 2d. At the same time, the warm-up of the developing units 3a to 3d and the exposure unit 4 is also started, and a predetermined DC bias Vdc1 and an AC bias Vpp are applied to the developing roller 22.

  This initial charging and warm-up are continuously performed for a predetermined time (arrow A in FIG. 4). After the warm-up is completed, a predetermined DC bias Vdc2 is applied to the magnetic rollers 20a and 20b, and the frequency (kHz) and Duty (%) of the AC bias Vpp applied to the developing roller 22 are lowered to develop A thin toner layer having a predetermined layer thickness is formed on the roller 22 (arrow B in FIG. 4).

  Thereafter, the latent images formed on the photosensitive drums 1a to 1d by the exposure unit 4 are developed by the toner thin layer, primarily transferred onto the intermediate transfer belt 8, and further transferred onto the transfer paper P (see FIG. 4). 4 arrow C). A predetermined interval (between sheets) is secured until the next transfer sheet P is conveyed (arrow D in FIG. 4). The image formation timing on the photosensitive drums 1a to 1d and the transfer timing of the transfer paper P differ depending on the time lag from image formation to primary transfer and from primary transfer to secondary transfer, but the interval between the images The intervals at which the transfer paper P is conveyed are equal, and both are times represented by arrows D.

  In the gap D between the sheets, the residual toner and the residual charges on the photosensitive drums 1a to 1d are removed by the cleaning units 5a to 5d and the static eliminator. That is, the time obtained by combining the arrows C and D is the output interval per recording sheet, and thereafter, the above process is repeated until the image formation for a preset number of sheets is completed.

  Further, in the paper gap D, the application of the DC bias Vdc2 to the magnetic rollers 20a and 20b is stopped, and the frequency (kHz) and duty ratio (%) of the AC bias Vpp applied to the developing roller 22 are increased, A toner recovery step (arrow T1 in FIG. 4) for recovering the toner on the developing roller 22, the DC bias Vdc2 is applied to the magnetic rollers 20a and 20b again, and the frequency (kHz) of the AC bias Vpp applied to the developing roller 22 ) And the duty ratio (%), and a re-forming step (arrow T2 in FIG. 4) for forming a toner thin layer is performed.

  Next, the toner collection process and the re-formation process executed using the first magnetic roller 20a and the second magnetic roller 20b will be described in detail. FIG. 5 is a schematic diagram showing the state of the developing unit when the toner recovery step is executed by converting the DC bias Vdc2 applied to the magnetic rollers 20a and 20b into the toner recovery bias. Portions common to FIG. 2 are denoted by the same reference numerals and description thereof is omitted.

  Now, assuming that the position where the developing roller 22 and the first magnetic roller 20a face each other is A, and the position where the developing roller 22 and the second magnetic roller 20b face each other is B, the developing of the photosensitive drum 1a is performed as shown in FIG. The toner thin layer 23 that is not used in the above is collected on the magnetic rollers 20a and 20b at A and B while the developing roller 22 makes one round. This toner collecting step is continued until the point on the developing roller 22 that has passed the position A at the time of bias conversion passes A again.

  As a result, all points on the developing roller 22 pass through A and B once in the toner collecting step, and the toner for two rounds of the magnetic roller can be collected while the developing roller makes one round. Therefore, compared to the conventional method of collecting using a single magnetic roller, the toner can be collected quickly and reliably. Further, since the time required for the toner recovery process (T1 in FIG. 4) is shortened, the interval between sheets (arrow D in FIG. 4) can be shortened, which contributes to the improvement of image forming efficiency. Further, even if the time required for one round of the developing roller is longer than the time required for one round of the magnetic roller, the toner collecting power is not reduced so much because the toner is collected by two magnetic rollers. The thickness unevenness of the thin layer can be reduced.

  FIG. 6 is a schematic diagram showing a state of the developing unit when the re-forming process is executed. At the time when the toner collecting process is completed, the DC bias Vdc2 applied to the magnetic rollers 20a and 20b is converted to a toner supply bias, and toner supply onto the developing roller 22 is started as shown in FIG. At this time, the timing of image writing on the photosensitive drum 1a and the transfer sheet so that development starts when the point on the developing roller 22 that has passed the position A at the time of bias conversion reaches the position of the photosensitive drum 1a. The transport timing is set in advance.

  As a result, toner is supplied from the magnetic rollers 20a and 20b at A and B while the developing roller 22 rotates from the position A via B to the drum position. The toner for two rollers can be supplied, and a toner thin layer having a saturated layer thickness can be formed in a short time compared to the conventional method in which the toner is supplied using one magnetic roller. Further, since the time required for the re-forming process (T2 in FIG. 4) is shortened, the gap between sheets (arrow D in FIG. 4) can be shortened, which contributes to the improvement of the image forming efficiency.

  In addition, since toner is supplied using two magnetic rollers, a sufficient amount of toner is supplied even when the linear speed ratio (M / S) of each of the magnetic rollers 20a and 20b with respect to the developing roller 22 is smaller than the conventional one. it can. Therefore, by reducing the linear velocity ratio, it is possible to reduce the mechanical stress applied to the toner in the regulating blade 26 and to extend the life of the toner.

  The linear speeds of the magnetic rollers 20a and 20b may be the same, but the upstream first magnetic roller 20a that forms the first toner thin layer on the developing roller 22 can form only a thin toner layer. For forming a toner thin layer having a saturated layer thickness, the second toner thin layer formed by the second magnetic roller 20b located on the downstream side is effective. Therefore, in the re-forming step, it is preferable that the linear velocity of the second magnetic roller 20b located on the downstream side is made faster than that of the first magnetic roller 20a.

  In addition, this invention is not limited to the said embodiment, A various change is possible in the range which does not deviate from the meaning of this invention. For example, in the above embodiment, the two magnetic rollers 20a and 20b are provided, and the toner is collected on the developing roller 22 and supplied to the developing roller 22 using the magnetic rollers 20a and 20b. Any one of the magnetic rollers may be dedicated to supply. Further, three or more magnetic rollers may be provided, and in that case, any one of them may be dedicated for collection. That is, if toner is supplied using at least two magnetic rollers, a toner thin layer having a saturated layer thickness can be formed in a short time. It is possible to increase the efficiency and suppress the history phenomenon.

  In the above-described embodiment, for example, control for changing the linear velocity of one or both of the developing roller and the magnetic roller may be performed. For example, by increasing the linear velocity of the developing roller or decreasing the linear velocity of the magnetic roller in the toner recovery process, the time difference for each rotation of the roller is reduced, and the toner stripped on the first round of the magnetic roller is reduced. This reduces the amount of toner that is collected and improves the toner collection capability, and therefore, it is possible to further reduce the uneven thickness of the toner thin layer in the re-forming process by suppressing the remaining toner on the developing roller. Become.

  Further, although a tandem type color image forming apparatus has been described here as an example, the present invention can be applied to a digital multi-function peripheral or an analog type monochrome image as long as the image forming apparatus includes a hybrid developing type developing unit. The present invention can also be applied to the image forming apparatus or other image forming apparatus such as a facsimile or a printer.

  Using the tandem color image forming apparatus of FIG. 1 equipped with a developing unit having two magnetic rollers as shown in FIG. 2, the toner thin layer forming performance and toner stripping performance (recovery) during continuous image printing Performance). Further, as a comparative example, the case where a conventional developing unit having one magnetic roller was mounted was similarly investigated.

  As test conditions, in both the present invention and the comparative example, the surface potential of the OPC photosensitive drums 1a to 1d is 300 V, the drum bright potential is 90 V, the drum diameter is 30 mm, and the AC roller bias of the developing roller 22 when the toner thin layer is formed. Vpp was 1.6 kV, frequency was 2.7 kHz, duty ratio was 27%, DC bias Vdc1 was 100V, and DC bias Vdc2 of the magnetic rollers 20a and 20b was 400V. Further, the AC bias Vpp of the developing roller 22 at the time of toner collection was 1.6 kV, the frequency was 6.0 kHz, the duty ratio was 50%, the DC bias Vdc1 was 100 V, and the DC bias Vdc2 of the magnetic rollers 20a and 20b was 0 V.

  As other conditions, the distance between the drum and the developing roller is 235 μm, the linear speed ratio S / D of the developing roller to the photosensitive drum is 1.5, the carrier has a particle diameter of 35 to 45 μm, and a saturation magnetization of 40 emu / g, A two-component developer composed of a toner having a particle diameter of 9 μm, a toner mixing ratio of 8.0% by weight and a toner charge amount of 15 μC / g was used. The test results are shown in Table 1.

  As is apparent from Table 1, the image forming apparatus of the present invention having two magnetic rollers can collect the toner for two rotations of the magnetic roller while the developing roller makes one rotation at the time of toner collection. Compared to the comparative example in which the magnetic roller is used for collection, the toner can be collected quickly and reliably. Further, since the time required for the toner recovery process is shortened, the time between sheets can be shortened.

  On the other hand, since the toner can be supplied from the two magnetic rollers even when the toner layer is formed, the toner for two rotations of the magnetic roller can be supplied during one rotation of the developing roller, and one magnetic roller is used. A toner layer having a uniform saturation layer thickness could be formed as compared with the comparative example in which the supply was performed. In addition, a thin toner layer can be formed around the circumference of the developing roller, and the time between sheets can be shortened. Further, even when the linear speed ratio (M / S) of each magnetic roller to the developing roller is made smaller than before, a sufficient amount of toner can be supplied or recovered, so that the mechanical stress on the toner is reduced compared to the comparative example. It was confirmed that it was possible.

  The developing conditions in the above embodiment are merely examples, and can be appropriately set according to the specifications of the image forming apparatus, such as the number of processed sheets, process speed, developing roller diameter, and magnetic roller diameter.

  The present invention includes a developing roller disposed opposite to an image carrier, and a magnetic roller disposed opposite to the developing roller to form a toner thin layer on the developing roller using a magnetic brush, and a DC bias is applied to the magnetic roller. In a developing unit that applies and applies a direct current and an alternating current bias to the developing roller to cause the toner to fly on the surface of the image carrier for development, a plurality of magnetic rollers are provided, and two or more magnetic rollers are provided on the developing roller. Toner is supplied simultaneously.

  As a result, more than two magnetic rollers of toner can be supplied during one rotation of the developing roller, and a toner thin layer with a saturated layer thickness can be formed in a short time compared to the conventional method using one magnetic roller. Therefore, it is possible to provide a developing unit that reduces the hysteresis phenomenon due to the uneven thickness of the toner thin layer, and achieves both high image quality and improved image formation efficiency. Further, since the mechanical stress applied to the toner can be reduced by lowering the linear speed ratio of the magnetic roller to the developing roller than in the conventional case, it contributes to the extension of the life of the toner in the developing unit.

  In addition, the developing unit of the present invention and a control unit for controlling the driving of the developing unit are mounted on the image forming apparatus, a toner collecting step for collecting toner from the developing roller, and a new toner thin layer on the developing roller. If the re-forming step for re-forming is performed using a plurality of magnetic rollers, the toner can be collected and supplied quickly and reliably, and the occurrence of the hysteresis phenomenon can be more effectively suppressed and the image can be reduced. An image forming apparatus with further improved formation efficiency is obtained.

  Further, when the re-forming process is executed, if the linear velocity of the magnetic roller located downstream in the rotation direction of the developing roller is made higher than that of the magnetic roller located upstream, the downstream magnet forming the second toner layer is formed. The toner can be sufficiently supplied from the roller, which is advantageous for forming a toner thin layer having a saturated layer thickness.

FIG. 1 is a schematic diagram showing an overall configuration of an image forming apparatus of the present invention. These are side sectional views showing the structure of the developing unit of the present invention. 1 is a block diagram showing a configuration of an image forming apparatus of the present invention. These are timing charts showing an example of operation control of the developing unit during continuous image formation in the image forming apparatus of the present invention. FIG. 4 is a schematic diagram illustrating a developing unit when a toner recovery process is performed in the image forming apparatus of the present invention. FIG. 3 is a schematic diagram showing a developing unit when a re-forming process is executed in the image forming apparatus of the present invention. FIG. 3 is a schematic diagram showing a configuration of a conventional hybrid developing unit.

Explanation of symbols

Pa to Pd Image forming section 1a to 1d Photosensitive drum (image carrier)
3a to 3d Developing unit 20a First magnetic roller 20b Second magnetic roller 21 Magnetic brush 22 Developing roller 23 Toner thin layer 27a First stirring screw 27b Second stirring screw 32 Control unit (control means)
33 Storage Unit 35 Main Motor 100 Image Forming Apparatus T1 Toner Collection Process T2 Re-formation Process

Claims (3)

A developing roller disposed opposite to the image carrier, and a magnetic roller disposed opposite to the developing roller to form a thin toner layer on the developing roller using a magnetic brush,
In a developing unit that applies a direct current bias to the magnetic roller, and applies a direct current and an alternating current bias to the developing roller to cause toner to fly on the surface of the image carrier and develop,
A developing unit comprising a plurality of the magnetic rollers, and supplying toner from the two or more magnetic rollers simultaneously onto the developing roller.   A direct current bias applied to the magnetic roller and the developing roller after completion of image formation during continuous image formation, comprising the developing unit according to claim 1 and a control unit that controls driving of the developing unit. A toner collecting step for collecting toner from the developing roller to the magnetic roller by making a difference in potential between the developing roller and a re-forming step for re-forming a new toner thin layer on the developing roller. An image forming apparatus capable of being executed using at least the magnetic roller.   3. The control unit according to claim 2, wherein, in the re-forming step, the linear speed of the magnetic roller located downstream in the rotation direction of the developing roller is made faster than the magnetic roller located upstream. The image forming apparatus described.

Images