JP2007086447A - Developing device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hybrid developing device capable of stably suppressing density lowering due to fine powdery toner remaining on a developing roller and the occurrence of an afterimage (ghost) phenomenon, etc., over a long period by more efficiently collecting the residual toner on the developing roller after the developing process, and also, replacing with new toner, and to provide an image forming apparatus. <P>SOLUTION: Regarding the developing device and the image forming apparatus, a counter electrode facing the developing roller is arranged on the downstream side of the developing area in the developing roller rotating direction, and also, on the upstream side of the counter position of a supply roller and the developing roller, and an electrical field is formed between the counter electrode and the developing roller, consequently, the toner on the developing roller is peeled once, and also, the toner is stuck on the developing roller again so as to form a thin toner layer on the developing roller again. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile, and a composite machine using the electrophotographic system, and a developing device used therefor. In particular, the present invention relates to a developing apparatus and an image forming apparatus that use a two-component developer composed of a carrier and toner and develop an electrostatic latent image by holding only the toner on the developing roller.

  Conventional development methods for image forming apparatuses such as copiers, printers, facsimiles, and their combined machines using electrophotographic methods are two-component development methods that use a carrier and toner, and one-component development methods that do not use a carrier. In recent years, so-called hybrid development has been developed in which a two-component developer that charges toner using a carrier is used and only the charged toner is held on the developing roller to develop an electrostatic latent image. A method is also used.

  The two-component development method is excellent in toner chargeability by the carrier and can extend the life, but has disadvantages such as a large development device and a change in image quality due to the durability of the carrier. .

  On the other hand, the one-component development method has a compact development device and excellent dot reproducibility, but the durability of the development roller and replenishment roller is generally low, and the cost of consumables changes because the development device is regularly replaced. There was a drawback of becoming expensive.

  In order to take advantage of both of these development methods, a two-component developer that charges a non-magnetic toner using a magnetic carrier is used, and a developing roller for developing an electrostatic latent image formed on a photoreceptor is used. Hybrid development systems have been introduced that only hold charged toner. This hybrid development method has been attracting attention as a development method that enables high-speed image formation and has a long life.

  However, while the hybrid development system has the above-described effects, the following problems have also arisen.

  (1) Coarse powder toner with high developability is easily selectively developed, and when continuous printing is performed, selective development occurs in which fine powder toner with high chargeability is not consumed and remains on the developing roller. There is a tendency for image density to decrease.

  In particular, when high-density images are printed continuously, the supply balance with respect to the demand for toner is poor, so the amount of fine toner that is difficult to consume increases, contaminates the carrier surface, adheres to fine particles on the developing roller, and reduces the charge amount. As a result, the developing function such as toner scattering is reduced, and image non-uniformity such as uneven image density tends to occur, which is a problem of downsizing the developing device.

  (2) Toner charge control is complicated, and it is necessary to apply a high surface potential and a large developing electric field to the photosensitive member. As a result, a toner consumption area and a non-consumption area are generated on the developing roller. The phenomenon that a part of the image appears as an afterimage (ghost) at the next development is likely to occur.

  Generally speaking, these problems are caused by the above-described hybrid development method, in which a development residual toner mainly composed of fine powder toner is generated on the development roller, but it is difficult to effectively refresh the development residual toner. Due to

  Originally, the residual toner on the developing roller should be supplied in such a way that when a new toner supply is received from the toner supply roller, it is scraped off by the magnetic brush of the supply roller, and the new toner of the magnetic brush is replaced with it. Is.

  Therefore, for example, in order to improve the above-mentioned problem by improving the scraping efficiency of residual toner by this magnetic brush, the AC component of the developing bias and the gap between the developing roller and the supply roller are adjusted, and the magnetic brush is used as the developing roller. Attempts have been made to control the conditions for supplying / recovering toner to the developing roller, such as rotating the developing roller and the supplying roller in a counter direction while making sufficient contact.

  In particular, measures have been taken to control conditions that are advantageous for toner collection by utilizing the period between the previous image formation and the next image formation.

  However, most of the residual toner on the developing roller is collected by the magnetic brush outside the development time, but a short image during continuous printing such as when a relatively large amount of fine powder in the toner adheres firmly to the surface of the developing roller. During the formation, the fine powder cannot be collected and remains on the developing roller, which may cause the above-described image loss or toner scattering.

  As measures against these residual toners, various methods for providing another means for removing the toner from the developing roller have been studied (see, for example, Patent Documents 1, 2, and 3).

  In Patent Document 1, a counter electrode is provided in a non-contact manner on the developing roller upstream of the nip portion between the developing roller and the supply roller, that is, before supplying the toner at the nip portion, and the remaining toner on the developing roller is opposed to the developing roller using an electric field Techniques for attaching to the electrode side have been proposed. The toner attached to the counter electrode is refreshed by switching the applied electric field during non-image formation.

  However, according to the above technique, in order to have an effect of removing the residual toner on the developing roller, the toner must be attached to the counter electrode side, and the removal operation is absolutely necessary. If an attempt is made to remove it, a reverse electric field is applied, which can only be done during non-development operations. Further, there is a problem that the toner adheres again to the developing roller side.

  Patent Document 2 proposes a technique in which a dummy electrode that is regarded as a photoconductor is provided immediately before the nip portion between the developing roller and the photoconductor, that is, immediately before development is performed, and the toner is brought into a cloud state by applying an alternating electric field to increase the development efficiency. Has been. This directly relates to development, but it is also conceivable to take measures against residual toner by using this electrode in hybrid development. However, the development and the recovery of the remaining toner are different functions, often resulting in a complicated processing operation and insufficient cleaning effect.

  In Patent Document 3, an electrode is provided upstream of the nip portion between the developing roller and the supply roller. Originally, the position of the electrode used to make the toner in the cloud state at the time of development is brought before the toner supply by the supply roller, which is useful for cleaning the toner on the development roller.

What can be said in common with these techniques is that the remaining toner on the developing roller is to be cleaned by providing an electrode and forming an electric field with the developing roller. For cleaning, the toner adhering to the developing roller must be peeled off and collected. Simply peeling off may cause problems such as contamination by the toner peeled off during continuous image formation, and it is necessary to provide a collecting means separately or add new control for collecting. This is also true when using means other than electrodes, such as a peeling member, and there is a need for a technology that efficiently collects new toner while removing residual toner on the developing roller. It has been.
JP-A-2005-99686 US Pat. No. 6,026,264 US Pat. No. 5,523,826

  As described above, in the hybrid developing device, there are various problems in removing and collecting the fine toner remaining on the developing roller, and it is difficult to stably obtain a good developed image.

  An object of the present invention is to solve these problems and transfer charged toner to a developing roller as a toner carrier by a magnetic brush formed on a supply roller as a developer carrier using a two-component developer. A so-called hybrid development in which a toner thin layer is formed and carried on a developing roller, and the latent image on the photosensitive member is developed by an electric field formed between the photosensitive member and the electrostatic latent image carrier. In the apparatus, after developing, the toner remaining on the developing roller is more efficiently collected and replaced with new toner, thereby reducing development performance such as density reduction caused by fine toner remaining on the developing roller, or Provided are a developing device and an image forming apparatus capable of stably suppressing the occurrence of a phenomenon in which a part of a previously developed image appears as a residual image (ghost) at the time of next development for a long period of time. It is when.

  The present invention has the following features in order to solve the above problems.

  (1) A toner that is disposed to face an electrostatic latent image carrier, carries toner on the surface in a thin layer state, and develops the electrostatic latent image formed on the electrostatic latent image carrier. A carrier and a magnetic brush made of toner and carrier are formed on the surface of the carrier and are opposed to the toner carrier, and the magnetic brush is slid on the toner carrier to form a magnetic brush on the toner carrier. In a developing device including a developer carrier that collects toner and transfers a new toner to the toner carrier, the toner carrier from a position where the toner carrier and the electrostatic latent image carrier are opposed to each other A counter electrode facing the toner carrier at a position downstream of the developer carrier and the toner carrier at a position upstream of the toner carrier. Counter electrode and toner carrier By forming an oscillating electric field between the toner carrier and the toner on the toner carrier, the toner is once separated from the toner carrier and reattached to the toner carrier to re-form the toner thin layer. Development device.

  (2) The oscillating electric field is formed by applying a potential in which a direct current component and an alternating current component are superimposed on at least one of the counter electrode and the toner carrier, and the potential of the toner carrier is changed from the potential of the counter electrode. The developing device according to (1), wherein the area average value Vave of the subtracted potential waveform has the same polarity as the normal charging polarity of the toner and an absolute value of 250 V or more and 450 V or less.

  (3) The developing device according to (1) or (2), wherein a distance between the counter electrode and the toner carrier is 0.1 mm or more and 0.5 mm or less.

  (4) An image forming apparatus that forms an image on a recording material by fixing a toner image transferred from an electrostatic latent image carrier, and is described in any one of (1) to (3) An image forming apparatus comprising the developing device.

According to the present invention,
With a magnetic brush formed on a supply roller as a developer carrier using a two-component developer, the charged toner is transferred to a development roller as a toner carrier, and a thin toner layer is formed on the development roller and carried. In the developing device that develops the latent image on the photosensitive member by the electric field formed between the photosensitive member and the electrostatic latent image carrier, the counter electrode is appropriately arranged in the hybrid developing device. Then, by forming an appropriate applied electric field, before the development residual toner on the developing roller is collected by the magnetic brush, the toner thin layer adhering in advance is re-formed and remains on the developing roller. By reducing the adhesion of fine toner to the developing roller and improving the collection performance with a magnetic brush, accumulation of fine toner on the developing roller is prevented and replacement with new toner is promoted. In the long term, the development performance decline such as density reduction caused by fine toner remaining on the developing roller, and the occurrence of a phenomenon in which a part of the previous developed image appears as an afterimage (ghost) during the next development. A developing device and an image forming apparatus that can be stably suppressed can be provided.

  Exemplary embodiments of a developing device and an image forming apparatus according to the present invention will be described with reference to the drawings.

(Configuration and operation of image forming unit)
First, a configuration example of an image forming apparatus using the developing device according to the present embodiment will be described by taking a laser beam printer (hereinafter simply referred to as a printer) using an electrophotographic process as an example.

  FIG. 1 is a configuration diagram illustrating a schematic configuration of an image forming portion of a printer including an example of a developing device according to the present embodiment as a component. The image forming portion of the printer and the operation of the developing device there will be described with reference to FIG.

  As shown in FIG. 1, the image forming portion of the printer includes the following components.

  Reference numeral 11 denotes a photosensitive drum as an electrostatic latent image carrier. The photosensitive drum 11 forms an electrostatic latent image on the surface thereof, and the electrostatic latent image is developed with toner. The photosensitive drum 11 is configured, for example, by forming an OPC photosensitive layer on the outer peripheral surface side of a conductive drum base formed of aluminum or the like in a substantially cylindrical shape. An arrow A is driven by a predetermined driving device (not shown). It is driven to rotate in the direction at a predetermined process speed (circumferential speed).

  A charger 12 charges the surface of the photosensitive drum 11 to a predetermined potential. The charger 12 has an arbitrary shape such as a roller shape, a sheet shape, or a blade shape, and is disposed so as to face the photosensitive drum 11. Charge.

  An exposure unit 13 irradiates a portion of the photosensitive drum 11 that is uniformly charged to a predetermined potential by the charger 12 with a laser beam to form an electrostatic latent image. An electrostatic latent image corresponding to the target image information is formed by scanning exposure of the laser beam corresponding to the time-series electric digital pixel signal of the target image information.

  Reference numeral 14 denotes a developing device. The developing device 14 develops the electrostatic latent image formed on the surface of the photosensitive drum 11 with toner, and visualizes it. The developing device 14 is a hybrid developing device, and the detailed configuration and functional operation thereof will be described later.

  A transfer roller 15 transfers a toner image on the photosensitive drum 11 visualized by the developing device 14 to a predetermined transfer material 19. The transfer roller 15 is disposed so as to face the photosensitive drum 11, and a transfer nip portion is formed between the two, and a transfer voltage is applied. The transfer material 19 is supplied to the transfer nip portion at a predetermined timing via a paper supply unit (not shown), and the toner image on the photosensitive drum 11 is transferred to the transfer material 19.

  Reference numeral 16 denotes a fixing unit. The transfer material 19 onto which the toner image has been transferred at the transfer nip is separated from the photosensitive drum 11 and conveyed to the fixing unit 16, and the fixing unit 16 transfers the toner image on the transfer material 19 transferred by the transfer roller 15. To settle.

  Reference numeral 17 denotes a cleaner blade that removes transfer residual toner. The transfer residual toner remaining on the surface of the photosensitive drum 11 after the transfer at the transfer nip portion is conveyed to the position where the cleaner blade 17 is disposed and collected by the cleaner blade 17 in contact with the photosensitive drum 11. . As a result, the photosensitive drum becomes ready for image formation again.

(Configuration of developing device)
Next, the configuration of the developing device in the present embodiment will be described with reference to FIG. The developing device is a hybrid developing device.

  The developing device 14 includes a developing device housing 20 that opens to face the photosensitive drum 11 and the following components located inside the developing device housing 20. The developing device housing 20 contains a two-component developer composed of toner and carrier.

  A developing roller 21 functions as a toner carrier. The developing roller 21 carries and conveys toner on the surface thereof, and develops the electrostatic latent image formed on the photosensitive drum 11.

  The developing roller 21 has an insulating surface obtained by coating a surface of a metal base material such as aluminum with alumite treatment, a resin such as phenol or acrylic, or ceramic. The surface of the developing roller 21 may not be a complete insulating layer but may be a semiconductive material having a certain degree of conductivity or a metal having conductivity.

  The developing roller 21 is spaced apart from the photosensitive drum 11 by a predetermined interval (for example, 100 μm to 250 μm), and is in the same direction as the photosensitive drum 11 (with direction) with respect to the photosensitive drum 11. It rotates in the direction of arrow B) at a peripheral speed 1.0 to 2.5 times the peripheral speed of the photoreceptor.

  A supply roller 22 functions as a developer carrying member. The supply roller 22 forms a magnetic brush composed of toner and carrier on its surface and is slid on the developing roller 21 to collect the toner on the developing roller 21 and transfer new toner.

  The supply roller 22 faces the developing roller 21 and is disposed on the side opposite to the photosensitive drum 11 with an interval of 0.4 to 0.6 mm.

  FIG. 2 shows an internal configuration of the supply roller 22. Referring to FIG. 2, the supply roller 22 includes a sleeve 22a that is rotatably arranged and a plurality of magnetic members 22b that are fixedly arranged inside the sleeve. With the plurality of magnetic members 22b, the supply roller 22 forms a plurality of magnetic poles (N1, S1, N2, S2, N3) on the sleeve 22a, and forms a repulsive magnetic field between the N1-N3 poles. .

  The sleeve 22a is rotationally driven in the direction of arrow C, and similarly conveys the developer 26 in the direction of arrow C. Developer 26 composed of non-magnetic toner and magnetic carrier is conveyed by the plurality of magnetic poles while forming a magnetic brush on the sleeve 22a.

  Reference numeral 25 denotes a regulating plate. The regulating plate 25 is disposed in the vicinity of the N1 pole with a predetermined interval from the sleeve 22a, and uniformizes the developer 26 that forms the magnetic brush conveyed together with the supply roller 22 to an optimum amount.

  Reference numeral 24 denotes a counter electrode. The counter electrode 24 forms an electric field between the developing roller 21 and the toner on the developing roller 21 is once peeled off and reattached to re-form the toner thin layer.

  The counter electrode 24 is spaced a predetermined distance from the developing roller 21 along the axial direction of the developing roller 21 on the upstream side of the developing roller 21 rotating direction (arrow B direction) from the position where the developing roller 21 and the supply roller 22 face each other. Are arranged close to each other. The shape of the counter electrode 24 is not particularly limited as long as it can form a predetermined electric field with respect to the developing roller 21.

  Reference numeral 23 denotes a stirring roller. The agitation roller 23 mixes and agitates the developer 26 composed of toner (negative polarity) and a carrier, and supplies it to the supply roller 22.

  Reference numeral 31 denotes an AC bias power source that applies an AC bias voltage Vac to the developing roller 21, and 32 denotes a DC bias power source that applies a DC bias voltage Vdc to the developing roller 21.

  Reference numeral 34 denotes a DC bias power source that applies a DC bias voltage Ve to the counter electrode 24.

  A DC bias power source 36 applies a DC bias voltage Vs to the supply roller 22.

  The relationship between the bias voltages will be described with reference to FIG. FIG. 3 shows an example of each of the above bias voltages with the horizontal axis time and the vertical axis potential.

  Reference numeral 46 denotes a DC bias voltage Vs (for example, −400 V) for the supply roller 22. Reference numeral 42 denotes a DC bias voltage Vdc (for example, −300 V) for the developing roller 21. The DC bias voltage Vdc for the developing roller 21 is set to have a smaller absolute value than the DC bias voltage Vs for the supply roller 22.

  Reference numeral 44 denotes a DC bias voltage Ve for the counter electrode 24. As Ve, a voltage (for example, −500 V) of substantially the same level as the latent image potential in the non-image area of the photosensitive drum 11 is applied. The latent image potential of the non-image area of the photosensitive drum 11 is set to, for example, −550V, and the latent image potential of the image area is set to, for example, −150V.

  Reference numeral 41 denotes an applied voltage obtained by superimposing the AC bias voltage Vac for the developing roller 21 on the DC bias voltage Vdc for the developing roller 21 of 42. For example, the AC bias voltage Vac has a frequency of 2 to 3 kHz, a peak-to-peak voltage of 1600 V, and a development phase duty of 20 to 50%.

  The relationship between these bias voltages is related to toner movement. Since the toner has a negative polarity, it is estimated that the direction in which the toner easily moves between the constituent elements is as follows.

(1) When comparing 41 (DC component is 42) and 46 in FIG. 3 between the developing roller 21 and the supply roller 22, when the value of Vac + Vdc is on the + side of Vs, the toner is supplied from the supply roller 22. The toner easily moves to the developing roller 21, and the toner easily moves from the developing roller 21 to the supply roller 22 when the value of Vac + Vdc is −side from Vs.

(2) Between the developing roller 21 and the photosensitive drum 11 In the image area of the photosensitive drum 11, the toner easily moves from the developing roller 21 to the photosensitive drum 11, and in the non-image area of the photosensitive drum 11, The toner easily moves from the photosensitive drum 11 to the developing roller 21.

(3) When comparing 41 (DC component is 42) and 44 in FIG. 3 between the developing roller 21 and the counter electrode 24, when the value of Vac + Vdc is on the + side of Ve, the toner is transferred from the counter electrode 21. The toner easily moves to the developing roller 21, and the toner easily moves from the developing roller 21 to the counter electrode 21 when the value of Vac + Vdc is on the minus side of Ve.

  As can be seen from the above (3), between the developing roller 21 and the counter electrode 24, even if the toner is once peeled off from the developing roller 21 by the AC electric field, it finally adheres to the developing roller 21 side and develops. Reform the toner layer on top.

(Developer operation)
Next, the operation of the developing device 14 will be described with reference to FIGS.

  The developer 26 in the developing device housing 20 is conveyed to the surface of the sleeve 22 a of the supply roller 22 while being mixed and stirred by the stirring roller 23. The developer 26 adheres to the surface of the sleeve 22a, that is, the surface of the supply roller 22 by the magnetic force acting between the carrier in the developer that has been conveyed and the magnetic pole N1 of the sleeve 22a.

  The developer 26 adhering to the surface of the supply roller 22 moves in the rotation direction (arrow C direction) of the sleeve 22a due to the rotation of the sleeve 22a of the supply roller 22 and the magnetic force acting between the magnetic pole N1 and the magnetic pole S1. The conveying amount of the developer is regulated by a regulating plate 25 disposed at a position facing the developing roller 21, and is conveyed to the vicinity of the magnetic pole N <b> 2 facing the developing roller 21.

  The developer 26 on the supply roller 22 that has been conveyed receives the magnetic force of the magnetic pole N2 to become a magnetic brush, contacts the surface of the developing roller 21, and the magnetic brush rubs the surface of the developing roller 21 as both rollers rotate. .

  Moreover, since the electric field from the supply roller 22 toward the development roller 21 is formed by the bias voltages (Vs and Vac + Vdc) respectively applied to the supply roller 22 and the development roller 21, the magnetic brush formed on the supply roller 22 is formed. The toner inside moves to the developing roller 21 side and adheres to the surface of the developing roller 21 to form a toner layer having a certain thickness.

  At this time, most of the movement of the toner is performed on the upstream side in the rotation direction of the developing roller 21 from the position where the supply roller 22 and the developing roller 21 are closest to each other.

  In addition, the toner adhering to the surface of the developing roller 21 is conveyed to a developing area facing the photosensitive drum 11 as the developing roller 21 rotates.

  In this developing region, a developing bias voltage (Vac + Vdc) in which an AC bias is superimposed on a DC bias is applied to the developing roller 21, so that an oscillating electric field formed between the developing roller 21 and the photosensitive drum 11 is applied. The toner moves to the photosensitive drum side and develops the electrostatic latent image formed on the surface of the photosensitive drum.

  At this developing position, the toner adhering to the surface of the developing roller 21 is divided into toner that moves to the photosensitive drum side and toner that remains on the developing roller 21. Of these, the toner remaining on the developing roller 21 is conveyed again toward the supply roller 22 as the developing roller 21 rotates.

  The undeveloped toner on the developing roller 21 is conveyed along with the rotation of the developing roller 21, and reaches the position facing the supply roller 22, and the counter electrode 24 disposed at a predetermined interval facing the developing roller 21. Pass through the gap.

  Bias voltages (Vac + Vdc and Ve) are applied to the developing roller 21 and the counter electrode 24, respectively, and an oscillating electric field is formed between the two. The development residual toner on the developing roller 21 is in contact with the developing roller 21 and the counter electrode. Separation / adhesion is repeated between 24 and 24, but finally a toner thin layer is again formed on the developing roller 21 again.

  In this way, once the peeling / adhesion oscillating electric field is applied to the thin layer of the development residual toner, the adhesion force of the fine powder toner firmly adhered to the surface of the developing roller 21 is reduced, and the toner thin layer is made uniform. The

  The residual toner that has been vibrated by the action of the counter electrode 24 is conveyed to a position facing the supply roller 22, where the rubbing force of the magnetic brush and the bias voltage (each applied to the supply roller 22 and the developing roller 21). Vs and Vac + Vdc) are taken into the magnetic brush on the supply roller 22 and collected by the potential difference.

  Thereafter, the developer 26 on the supply roller 22 that supplies new toner to the developing roller 21 and collects the undeveloped toner is conveyed to the vicinity of the magnetic pole N3 as the sleeve 22a rotates, and further toward the magnetic pole N1. Try to move. However, since a repulsive magnetic field is formed in this region on the sleeve 22a, the developer 26 cannot move, and is peeled off from the surface of the supply roller 22 and returned to the stirring roller 23.

  New toner is supplied to the stirring roller 23 from a toner hopper (not shown), and the supplied new toner and the developer 26 peeled off from the supply roller 22 are stirred by the stirring roller 23 to have a predetermined toner concentration. A developer is formed again. Thereafter, the developer is supplied again to the supply roller 22 by the stirring roller 23.

  As described above, in this embodiment, the adhesion force of the development residual toner to the developing roller 21 is reduced and made uniform, in addition to the conventional rubbing force of the magnetic brush and the collection by the supply roller 22 due to the electric field. Since the oscillating electric field is applied before recovery, the recovery performance of the development residual toner is improved, and the occurrence of development history such as density reduction and ghost can be suppressed.

Further, since the counter electrode 24 can be applied with a potential substantially equal to that of the non-image area of the photoconductive drum 11, it is possible to prevent adhesion of toner and to clean the counter electrode 24 when no image is formed. unnecessary. That is, the counter electrode 24 only improves the recoverability of the development residual toner by re-forming the toner layer, and the recovery itself is left to the supply roller 22, so that the functional operation related to the adhesion and recovery of the toner is not necessary.
[Example 1]
Using the developing device having the configuration shown in FIGS. 1 and 2, continuous printing was actually performed, and changes in the characteristics of the residual toner on the developing roller were evaluated. In addition, the occurrence of ghosts and density reduction in printed images was evaluated.

  The setting conditions of the used developing device are as follows.

  As the developing roller, a roller provided with an anodized film surface layer (10 μm) on an aluminum substrate was used. The diameter of the developing roller was 18 mm, and the gap with the opposing photosensitive drum was set to 150 μm. The rotation direction of the developing roller was the same direction as the photosensitive drum in the vicinity of the photosensitive drum (see arrows A and B in FIG. 1), and the peripheral speed was 150 mm / s.

  The developing bias applied to the developing roller was a DC bias voltage Vdc of −310 V, and an AC bias voltage Vac having a frequency of 2 kHz (peak-to-peak voltage of 1600 V, development phase duty of 40%) was superimposed and applied.

  For the supply roller, an aluminum sleeve was used, and the surface was blasted to 8 μmRz. The internal fixed magnetic pole has the arrangement shown in FIG. The diameter of the supply roller was φ18 mm, and the gap with the opposing developing roller was set to 450 μm. The rotation direction of the supply roller, that is, the sleeve was the opposite direction to the developing roller (see arrows B and C in FIG. 1) in the vicinity of the developing roller, and the peripheral speed was 220 mm / s.

  The bias voltage Vs applied to the supply roller was a DC bias voltage of −400V.

  For the counter electrode, a stainless counter electrode was used, and the gap Dg with the opposing developing roller was set in four ways: 0.1 mm, 0.3 mm, 0.5 mm, and 0.7 mm.

  The DC bias voltage Ve to be applied to the counter electrode was set in five ways in increments of 100V from -300V to -700V.

  The setting conditions of the photosensitive drum and the developer are as follows.

  As the photosensitive drum, an OPC photosensitive member having a drum diameter of 30 mm was used. The peripheral density of the photoconductor was 100 mm / s, and the charging and exposure conditions were set so that the surface potential of the photoconductor was −550 V in the non-image forming portion and −150 V in the image forming portion.

  As the developer, a two-component developer, a negative polarity polymerized toner having a volume average particle diameter of 6.3 μm and a ferrite carrier coated with a silicone resin having a volume average particle diameter of 33 μm were used.

  Under the above condition settings, 1000 images with an image density ratio of 2% were printed, the residual toner layer on the developing roller at the initial stage and after 1000 sheets was sampled, and the toner charge amount and toner particle size were measured. In addition, with respect to the output image at that time, the occurrence state of image ghost and the decrease in density were visually evaluated.

  As described above, in the embodiment, the DC bias voltage Ve applied to the counter electrode is five, the gap Dg between the counter electrode and the developing roller is four, and 20 conditions (Examples 1-1 to 1-20). It carried out in. Further, as Comparative Example 1, the combination of the DC bias voltage Ve = -100V and the gap Dg = 0.3 mm, that is, the absolute value of the DC bias voltage Ve is small, and the potential of the counter electrode is averaged with respect to the potential of the developing roller. Since it is on the negative side, the toner layer was peeled off on the developing roller, and the toner layer was not re-formed. Moreover, it implemented also on the conditions which do not provide counter electrode itself as the comparative example 2. FIG.

  The results from Example 1-1 to Example 1-20 are shown in Tables 1 to 4. The results for Comparative Examples 1 and 2 are shown in Table 5.

  Table 1 shows the results when the gap Dg = 0.1 mm, Table 2 shows the results when Dg = 0.3 mm, Table 3 shows Dg = 0.5 mm, and Table 4 shows the results when Dg = 0.7 mm. .

  In each table, Ve is a DC bias voltage applied to the counter electrode, and Vave is an area average value of the developing bias Vdc + Vac applied to the developing roller.

  FIG. 4 shows the concept of area average values for AC developing bias. In FIG. 4, the horizontal axis represents time, and the vertical axis represents potential. A line 61 indicates a developing bias Vdc + Vac in which a DC bias and an AC bias are superimposed. Line 62 is the area average value of the developing bias potential. That is, in the relationship between the line 62 and the line 61 in the figure, the area where the line 61 exceeds the line 62, that is, the area of the area 63, and the area where the line 61 falls below the line 62, that is, the area of the area 64 are equal. The potential level is an area average value.

  In each table, the potential difference is the difference between Vave and Ve, and represents an average electric field between the developing roller and the counter electrode. That is, as will be described later, the evaluation result depends on the strength of the electric field.

  The change in charge indicates how much the charge amount of the toner on the developing roller has increased after printing 1000 sheets compared to the initial state. In all of the examples and comparative examples, the charge amount tends to increase, but as described above, this is due to the increase in fine toner, and the larger the increase, the easier the decrease in density and ghosting occur.

  Similarly, the change in particle size indicates how much the average particle size of the toner on the developing roller has decreased after printing 1000 sheets compared to the initial value. This also shows that the average particle diameter is decreasing in all Examples and Comparative Examples, which supports the increase in fine toner.

  Ghosts in image evaluation were evaluated using images as shown in FIG. FIG. 5 is a diagram illustrating a case where an image used for ghost evaluation is reproduced well and a case where a ghost is generated.

  In FIG. 5, reference numeral 50a denotes an image that is well reproduced without ghosting. Reference numeral 51 denotes a maximum density region provided for density measurement. The surrounding area is a minimum density, that is, a white area to which toner does not adhere. Reference numeral 52 denotes a light gray area, which is developed after the highest density area 51 at the time of image formation.

  50b is an example when a ghost occurs. The region with the highest density of 51 is not particularly problematic. However, 53 white areas are generated in 52 gray areas. This is a ghost. The occurrence is due to the following process.

  During image formation, 51 areas are developed in advance. That is, the area on the photoreceptor corresponding to 51 is developed with a larger amount of toner than the surrounding area. If this is seen from the developing roller side, a part of the toner is consumed in a particularly large amount. That is, a state where the balance of the toner adhesion amount is lost on the developing roller occurs.

  It is sufficient that the developer is sufficiently refreshed until the developing roller rotates and the next development of the photosensitive member. However, if a non-uniform state of the remaining toner remains at the next development, the gray in 50b In an area where toner adhesion is small and noticeable, such as the area 52, a deviation in the balance of the developed toner amount appears as an afterimage (ghost) as shown in the figure.

  For the evaluation of the occurrence of ghost, the presence or absence of occurrence was evaluated by visually observing an image after printing 1000 sheets with the image as shown in FIG. It was recognized that no ghost was generated, ◯, a slight ghost was generated, but Δ if it was within the allowable range, and X if it was clearly recognized that a ghost was generated. Similarly, the decrease in density was evaluated by ◯ Δ × based on visual observation.

  As can be seen from the evaluation results in Tables 1 to 5, in the examples, both ghost generation and density decrease are ◯ or Δ, but in the comparative examples all are X.

  When the potential difference is 50 V or less as in Comparative Example 1 in Table 5, toner is deposited on the counter electrode, and ghosting and density reduction are seen in quality. In addition, the accumulated toner overflowed to the outside of the machine, causing a problem of contaminating the surrounding area. In addition, as in Comparative Example 2 in Table 5, when the counter electrode itself was not provided, the charge change and the particle size change were large, resulting in the occurrence of ghosts and significant decrease in concentration.

  In Examples 1-1 to 1-20, good images were obtained, but there are also examples in which some evaluations are Δ. This relates to the influence of the electric field between the counter electrode and the developing roller, and as shown in Tables 1 to 4, the embodiment in which the potential difference is about 250V to 450V has the most stable effect.

  Accordingly, it is desirable that the bias voltage of the developing roller and the bias voltage of the counter electrode are set so that the potential difference between them is about 250V to 450V. The direction of the electric field is a direction in which the toner is guided to the developing roller side.

  Further, the width of the gap Dg between the counter electrode and the developing roller also affects the electric field strength, so it is better to examine the setting so that a better effect can be obtained. In an example in which Dg is 0.7 mm, there is some concern about image quality. In addition, it is practically difficult to set the gap Dg to be smaller than 0.1 mm, and the risk of leakage increases.

Therefore, the gap Dg between the counter electrode and the developing roller is preferably set to about 0.1 mm to 0.5 mm.
[Example 2]
Using a developing device with a different bias voltage applied from Example 1, continuous printing was actually performed, and changes in the characteristics of the residual toner on the developing roller were evaluated. In addition, the occurrence of ghosts and density reduction in printed images was evaluated.

  FIG. 6 shows a configuration in the second embodiment. What is different from the configuration in the first embodiment (FIG. 2) is a bias voltage applied to the developing roller 21, the supply roller 22, and the counter electrode 24. In FIG. 2 (Example 1), the DC bias Vdc and the AC bias Vac are superimposed on the developing roller 21 by the DC bias power supply 32 and the AC bias power supply 31, but in FIG. Only the bias Vdc is provided. Instead, in the configuration of FIG. 2 (Embodiment 1), the DC bias Ve applied by the DC bias power supply 34 is applied to the counter electrode 24, and in FIG. 6 (Embodiment 2), the DC bias power supply 34 and the AC bias power supply 33 provide direct current. A bias Vedc and an AC bias Veac are applied in a superimposed manner. 2 (Embodiment 1), the DC bias Vs is applied to the supply roller 22 to which the DC bias Vs by the DC bias power supply 36 is applied. In FIG. 6 (Embodiment 2), the DC bias power supply 36 and the AC bias power supply 35 are used. And an AC bias Vac are superimposed and applied.

  Differences from the first embodiment regarding the setting conditions of the developing device used, including specific bias conditions, are shown below.

  The difference between the developing roller and the first embodiment is that the gap with the opposing photosensitive drum is set to 100 μm, and the developing bias applied to the developing roller is a DC bias voltage Vdc (−650 V).

  The difference between the supply roller and the first embodiment is that, as the bias voltage Vs applied to the supply roller, an AC bias voltage Vac (peak-to-peak voltage 1600 V, duty 50%) having a frequency of 2 kHz is superimposed on a DC bias voltage of −400 V. It is applied.

  The difference between the counter electrode and the first embodiment is that the gap Dg between the opposite developing rollers is fixed at 0.3 mm, and the DC bias voltage Vedc is −800 V to −1200 V as a bias applied to the counter electrode. In other words, the AC bias voltage Veac having a frequency of 2 kHz (a peak-to-peak voltage of 1400 V and a development phase duty of 50%) is applied in a superimposed manner.

  The setting conditions of the photosensitive drum and the developer are the same as those in the first embodiment.

  Under the above condition settings, as in Example 1, 1000 images with an image density ratio of 2% were printed, the toner charge amount and toner particle size after the initial and 1000 sheets were measured, and the output image at that time On the other hand, the occurrence of image ghost and the decrease in density were visually evaluated.

  As described above, in the example, the DC bias voltage Vedc applied to the counter electrode was performed under five conditions (Examples 2-1 to 2-5).

  Table 6 shows the results for Examples 2-1 to 2-5.

  In Table 6, Vdc is a DC bias voltage applied to the developing roller, and Veave is an area average value of the developing bias Vedc + Veac applied to the counter electrode. The area average value is as described above.

  The potential difference is the difference between Vdc and Veave and represents an average electric field between the developing roller and the counter electrode. That is, as in Example 1, the evaluation result depends on the strength of the electric field.

  In all the examples, as in Example 1, the charge change is in the direction of increasing the charge amount, and the particle size change is also in the direction of decreasing the average particle size.

  The evaluation method of the output image, that is, the evaluation method of occurrence of ghost and density reduction is the same as that in the first embodiment.

  As can be seen from the evaluation results in Table 6, in the examples, both ghost generation and density decrease are ◯ or Δ.

  Regarding the influence of the electric field between the counter electrode and the developing roller, the most stable effect was obtained when the potential difference was about 250 V to 450 V, as in Example 1.

  When the result of Example 1 and the result of Example 2 are combined, the appropriate relationship between the bias voltage of the developing roller and the bias voltage of the counter electrode is the average area potential of the bias in which an AC bias is superimposed on one DC bias, and The potential difference of the other DC bias is preferably about 250 V to 450 V, and an electric field is preferably formed on the counter electrode side so that toner does not adhere.

  According to the embodiment described above, in the hybrid developing device, the counter electrode is appropriately arranged and an appropriate applied electric field is formed, so that the development residual toner on the developing roller is collected by the magnetic brush. In addition, the toner thin layer adhering in advance is re-formed, the adhesion force of the fine toner remaining on the developing roller to the developing roller is reduced, and the recoverability by the magnetic brush is improved, so that By preventing the accumulation of fine toner and promoting the replacement with new toner, the development performance declines due to the fine powder toner remaining on the developing roller, and a part of the previous developed image is Thus, it is possible to provide a developing device and an image forming apparatus that can stably suppress the occurrence of a phenomenon that appears as an afterimage (ghost) during the development.

  In addition, the above-mentioned embodiment example is an illustration and restrictive at no points. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

1 is a configuration diagram illustrating a schematic configuration of an image forming unit of an image forming apparatus including an example of a developing device according to the present embodiment as a component. 1 is a configuration diagram illustrating a detailed configuration of a developing device used in Example 1. FIG. It is a figure for demonstrating the relationship of each bias voltage regarding a developing device. It is a figure for demonstrating the view of the area average value about an alternating development bias. It is a figure for demonstrating the occurrence condition of a ghost in the evaluation image of a ghost. FIG. 6 is a configuration diagram illustrating a detailed configuration of a developing device used in Example 2.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Photosensitive drum 12 Charger 13 Exposure part 14 Developing apparatus 15 Transfer roller 16 Fixing part 17 Cleaner blade 19 Transfer material 20 Developer housing 21 Developing roller 22 Supply roller 24 Counter electrode 25 Restriction plate 26 Developer 31 AC power supply (Developing roller) for)
32 DC power supply (for developing roller)
33 AC power supply (for counter electrode)
34 DC power supply (for counter electrode)
35 AC power supply (for supply roller)
36 DC power supply (for supply roller)
41 Developing roller bias Vac + Vdc
42 Developing roller bias Vdc
44 Counter electrode bias Ve
46 Supply roller bias Vs
50a Image without ghost generation 50b Image with ghost generation 51 Maximum density region 52 Gray region 53 Ghost generation region 61 Bias potential where DC and AC are superimposed 62 Area average value of bias potential 63 Region where bias potential exceeds area average value 63 The area where the bias potential turns around the area average value

Claims (4)

A toner carrier disposed opposite to the electrostatic latent image carrier, carrying and transporting toner in a thin layer state on the surface thereof, and developing the electrostatic latent image formed on the electrostatic latent image carrier; ,
A magnetic brush composed of toner and a carrier is formed on the surface of the toner carrier, and the toner is rubbed against the toner carrier to collect the toner on the toner carrier. And a developing device comprising a developer carrier for transferring new toner to the toner carrier,
Rotation of the toner carrier from a position where the toner carrier and the electrostatic latent image carrier are opposite to each other in the rotation direction of the toner carrier, and from a position where the developer carrier and the toner carrier are opposed. A counter electrode facing the toner carrier at a position upstream in the direction;
By forming an oscillating electric field between the counter electrode and the toner carrier, the toner on the toner carrier is once separated from the toner carrier and reattached to the toner carrier to form a thin toner layer. A developing device characterized by re-forming. The oscillating electric field is formed by applying a potential in which a DC component and an AC component are superimposed on at least one of the counter electrode and the toner carrier, and a potential obtained by subtracting the potential of the toner carrier from the potential of the counter electrode. 2. The developing device according to claim 1, wherein the average area value Vave of the waveform has the same polarity as the normal charging polarity of the toner, and an absolute value of 250V to 450V. The developing device according to claim 1, wherein a distance between the counter electrode and the toner carrier is 0.1 mm or more and 0.5 mm or less. An image forming apparatus for forming an image on a recording material by fixing a toner image transferred from an electrostatic latent image carrier,
An image forming apparatus comprising the developing device according to claim 1.

Images