JP5347230B2 - Developing device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To form a uniform toner layer on a developing roller with low energy without spoiling accumulation of charged particles having a reverse polarity in a developer in a developing tank when imparting stable toner electrification property to carrier over a long term. <P>SOLUTION: The developer includes a toner and the reverse-polarity particles. The developing device has a first conveying member, and a second conveying member opposed to a photoreceptor. An electric field forming means forms the first electric field between the first conveying member and the second conveying member so as to move the toner in the developer on the first conveying member to the second conveying member and also separate a part of the reverse-polarity particles from the toner and collect it in the developer, and forms the second electric field between the second conveying member and the photoreceptor so as to move the toner on the second conveying member to an electrostatic latent image on the photoreceptor. The first electric field comprises only DC electric field, and the second electric field is composed by superposing the DC electric field and oscillating electric field. The oscillating electric field has voltage waveform equipped with a blank part where the application of pulse voltage is stopped only in a predetermined period. <P>COPYRIGHT: (C)2008,JPO&amp;INPIT

Description

  The present invention relates to an electrophotographic developing device that can be applied to, for example, a copying machine, a facsimile machine, a printer, or a multifunction machine of these, and an image forming apparatus including such a developing device.

  As a developing method employed in an electrophotographic image forming apparatus, a one-component developing method using only toner as a main component of a developer and a two-component developing method using toner and a carrier as main components of a developer are known. ing.

  The developing device of the one-component development system includes a toner carrying member that carries and conveys toner, and a friction charging member that contacts the toner carrying surface of the toner carrying member. When the toner carried on the toner carrying member passes through the contact position of the frictional charging member, the toner is brought into frictional contact with the frictional charging member to be thinned and charged to a predetermined polarity. As described above, the one-component developing device has an advantage that the configuration is simple, small, and inexpensive because the toner is charged by frictional contact with the frictional charging member. However, since the toner is subject to strong stress at the contact position of the frictional charging member, the toner is liable to deteriorate, so that the chargeability of the toner is impaired relatively early. In addition, the contact pressure between the toner carrying member and the frictional charging member reduces the ability of the toner to adhere and charge the toner, resulting in a relatively short life of the developing device.

  In the developing device of the two-component developing method, the toner and the carrier are charged to a predetermined polarity by frictional contact between the toner and the carrier, so that the toner receives less stress than the one-component developing device. Since the surface area of the carrier is larger than that of the toner, the toner is less likely to be adhered and soiled. However, dirt (spent) adhering to the surface of the carrier increases due to long-term use, and therefore, the ability to charge the toner is reduced, causing the problem of fogging and toner scattering. In order to extend the life of the two-component developing device, it is conceivable to increase the amount of carrier accommodated in the developing device, but this leads to an increase in the size of the developing device.

  In order to solve the above-mentioned problem related to the two-component developing device, Patent Document 1 discloses that a carrier or carrier and toner are replenished to the developer little by little, and the developer whose charging performance is lowered is gradually discharged. A developing device that suppresses an increase in deteriorated carriers is disclosed. According to this technique, it is possible to extend the life of the developer without increasing the size of the developing device. However, a mechanism for collecting the discharged carrier is necessary. In addition, the consumption of the carrier is large, which includes cost and environmental problems. Further, it is necessary to repeat printing of a certain amount or more until the ratio of the undegraded carrier and the deteriorated carrier is stabilized.

  Patent Document 2 discloses a carrier in which a resin coating layer containing resin fine particles and conductive fine powder dispersed in a matrix resin is provided on a core material, and an image forming method using the carrier. When the surface of this carrier is partially scraped by contact with other particles (carrier particles, toner particles) or members (rollers, screws), new resin fine particles are exposed on the surface, which comes into contact with the toner. The toner is charged to the required level. However, the thickness of the resin coating layer is limited, and when this resin coating layer is consumed, the carrier reaches the end of its life.

  Patent Document 3 proposes a two-component developer composed of a carrier and a toner carrying charged particles on the surface, and a developing method using the developer. The charged particles are added as an abrasive for removing dirt (spent) mainly caused by toner adhering to the surface of the carrier and extending the life of the carrier. Patent Document 3 also describes that charged particles exhibit a function of polishing the surface of the electrostatic latent image carrier in the cleaning region of the electrostatic latent image carrier. However, since charged particles have a property of being charged to a polarity opposite to the charging polarity of the toner, there is a problem that the charged particles adhere to non-image portions of the electrostatic latent image and are consumed at an early stage. In particular, when an image having a small area (for example, a character image) is created, there is a problem that a large amount of charged particles is consumed, and the ability to polish and regenerate the carrier is not sufficiently exhibited.

Patent Document 4 has a developing device including a magnetic roller and a developing roller, and selectively selects only toner from the developer including toner and carrier held on the outer peripheral surface of the magnetic roller on the outer peripheral surface of the developing roller. There has been proposed an image forming apparatus that supplies and develops an electrostatic latent image (electrostatic latent image portion) on a photosensitive member using toner held on the outer peripheral surface of the developing roller. As a feature, the invention of Patent Document 4 is interposed between the toner and the carrier without being held on either surface of the toner and the carrier, and the pulverized fine powder of the toner adheres to the surface of the carrier to form a spent. The developer contains charged particles that prevent this. However, the charged particles are contained only in the developer initially introduced into the developing device. In addition, since charged particles are not held on either the surface of the toner or the carrier, a part of the charged particles is supplied to the developing roller together with the toner by electrical coupling with the toner, and then the electrostatic latent image on the photoconductor. Since it adheres to the image non-image area and is consumed gradually, for example, if a large amount of images with a small image area ratio (black and white ratio) such as a character image is printed, only a large amount of charged particles are consumed, resulting in a long-term Therefore, there is a problem that stable chargeability of toner cannot be obtained.
JP 59-1000047 A JP-A-9-269614 JP 2003-215855 A JP 2006-308687 A

  For this reason, even when printing an image with a small image area ratio, it is required to collect as many charged particles as possible in the developer. In response to this requirement, a first electric field is formed between the magnetic roller and the developing roller to move the toner in the developer held by the magnetic roller to the developing roller. A second electric field is formed between them, and the toner held by the magnetic roller is moved to the electrostatic latent image on the photosensitive member so that the electrostatic latent image is visualized as a toner image. Can be considered. In this case, in the toner supply unit that supplies the toner from the magnetic roller to the developing roller, the charged particles having the opposite polarity to the toner are separated from the toner, held on the carrier, collected in the developer, and accumulated in the developer tank. . The accumulated charged particles adhere to the carrier surface and are fixed by the stress applied to the developer in the developer tank, and can perform a charge assist function for the carrier. By collecting such charged particles as much as possible and accumulating them in the developer tank, it becomes possible to impart a stable toner charging property to the developer carrier over a long period of time.

When the developing device is configured in this way, it promotes the separation of toner from the magnetic brush formed on the magnetic roller, promotes the separation of charged particles from the toner in the toner supply region, and is formed on the developing roller. In order to make the toner layer uniform, it is desirable to apply an oscillating electric field. When applying such an oscillating electric field to the developing roller, by applying an oscillating electric field to the magnetic roller that cancels the oscillating electric field of the developing roller in addition to the DC electric field in the direction of moving the toner to the developing roller, The reciprocating motion by the oscillating electric field can be stopped to efficiently collect charged particles.
However, in this case, toner supply from the magnetic roller to the developing roller is performed only by a DC electric field, which is disadvantageous in forming a uniform toner layer on the developing roller. It also becomes difficult to reliably supply toner to the developing roller with low energy.

  In view of this, the present invention provides an accumulation of charged particles of opposite polarity in the developer in the developer tank when providing a stable toner charging property over a long period to the carrier of the two-component developer including the toner and the carrier. The purpose of this is to make it possible to form a uniform toner layer on the developing roller with low energy without impairing the toner.

In order to achieve this object, the present invention provides a developing device that visualizes an electrostatic latent image on an electrostatic latent image carrier using a developer containing at least a toner and a carrier,
A developer containing toner and carrier that are charged with different polarities by frictional contact with each other, and charged particles that are removably held on the surface of the toner and charged with a different polarity from the toner is stored. A developer reservoir,
A first conveying member that conveys the developer from the developer reservoir;
A second conveying member facing the first conveying member via a first region and facing the electrostatic latent image carrier via a second region;
A first electric field is formed between the first conveying member and the second conveying member to move the toner in the developer held by the first conveying member to the second conveying member. and, above SL to form a second electric field between the second conveying member and the electrostatic latent image bearing member, the second conveying member is the held the toner the electrostatic latent image bearing the is moved to the electrostatic latent image of the body and a electric field forming means that turn into a visible image as a toner image the electrostatic latent image,
The first electric field is an oscillating electric field in which a pulse voltage for moving the toner toward the first conveying member and a pulse voltage for moving the toner toward the second conveying member are alternately applied, and A blank portion in which the application of the pulse voltage is suspended for a predetermined period, and the electric field direction before and after the blank portion has a voltage waveform that is a direction in which the toner is moved to the second conveying member side;
It is characterized by that.

  According to such an invention, when imparting a stable toner charging property over a long period to the carrier of the two-component developer including the toner and the carrier, low energy is applied to the second conveying member by the action of the oscillating electric field. A uniform toner layer can be formed, and a blank portion in which the application of a pulse voltage is suspended for a certain period in the voltage waveform of the oscillating electric field is provided, so that the first charged particles having a polarity opposite to that of the toner are provided. The recovery efficiency to the conveying member side can be increased.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described with reference to the accompanying drawings. In the following description, terms indicating a specific direction (for example, “up”, “down”, “left”, “right”, and other terms including them, “clockwise direction”, “counterclockwise” ”) Is used to facilitate understanding of the invention with reference to the drawings, and the present invention should not be construed as being limited by the meaning of these terms. Further, in the image forming apparatus and the developing apparatus described below, the same reference numerals are used for the same or similar components.

[1. Image forming apparatus]
FIG. 1 shows a portion related to image formation of an electrophotographic image forming apparatus according to the present invention. The image forming apparatus may be any of a copier, a printer, a facsimile machine, and a multi-function machine having a combination of these functions. The image forming apparatus 1 includes a photoreceptor 12 that is an electrostatic latent image carrier. In the embodiment, the photoconductor 12 is formed of a cylindrical body, but the present invention is not limited to such a form, and an endless belt type photoconductor can be used instead. The photoreceptor 12 is drivingly connected to a motor (not shown), and is rotated in the direction of arrow 14 based on the driving of the motor. Around the photoconductor 12, a charging station 16, an exposure station 18, a developing station 20, a transfer station 22, and a cleaning station 24 are arranged along the rotation direction of the photoconductor 12.

  The charging station 16 includes a charging device 26 that charges a photosensitive layer, which is the outer peripheral surface of the photosensitive member 12, to a predetermined potential. In the embodiment, the charging device 26 is represented as a cylindrical roller. However, instead of this, other types of charging devices (for example, a rotary or fixed brush-type charging device or a wire-discharge-type charging device) may be used. Can be used. In the exposure station 18, the image light 30 emitted from the exposure device 28 disposed in the vicinity of the photosensitive member 12 or away from the photosensitive member 12 travels toward the outer peripheral surface of the charged photosensitive member 12. A passage 32 is provided. On the outer peripheral surface of the photoconductor 12 that has passed through the exposure station 18, an electrostatic latent image is formed that includes a portion where the image light is projected and the potential is attenuated and a portion where the charged potential is substantially maintained. In the embodiment, the portion where the potential is attenuated is the electrostatic latent image portion, and the portion where the charged potential is substantially maintained is the electrostatic latent image non-image portion. The developing station 20 includes a developing device 34 that visualizes the electrostatic latent image using a powder developer. Details of the developing device 34 will be described later. The transfer station 22 includes a transfer device 36 that transfers a visible image formed on the outer peripheral surface of the photoreceptor 12 to a sheet 38 such as paper or film. In the embodiment, the transfer device 36 is represented as a cylindrical roller, but other types of transfer devices (for example, wire discharge transfer devices) can also be used. The cleaning station 24 includes a cleaning device 40 that collects untransferred toner remaining on the outer peripheral surface of the photoconductor 12 without being transferred to the sheet 38 at the transfer station 22 from the outer peripheral surface of the photoconductor 12. In the embodiment, the cleaning device 40 is shown as a plate-like blade, but other types of cleaning devices (for example, a rotary type or a fixed type brush type cleaning device) may be used instead.

  When the image forming apparatus 1 having such a configuration forms an image, the photoconductor 12 rotates clockwise based on the driving of a motor (not shown). At this time, the outer peripheral portion of the photoreceptor passing through the charging station 16 is charged to a predetermined potential by the charging device 26. The charged outer periphery of the photoconductor is exposed to image light 30 at an exposure station 18 to form an electrostatic latent image. The electrostatic latent image is conveyed to the developing station 20 along with the rotation of the photosensitive member 12, where it is visualized as a developer image by the developing device 34. The visualized developer image is conveyed to the transfer station 22 along with the rotation of the photosensitive member 12, and is transferred to the sheet 38 by the transfer device 36 there. The sheet 38 to which the developer image has been transferred is conveyed to a fixing station (not shown), where the developer image is fixed to the sheet 38. The outer peripheral portion of the photosensitive member that has passed through the transfer station 22 is conveyed to the cleaning station 24 where the developer remaining on the outer peripheral surface of the photosensitive member 12 without being transferred to the sheet 38 is recovered.

[2. Development device]
The developing device 34 includes a housing 42 that houses a two-component developer including non-magnetic toner as first component particles and a magnetic carrier as second component particles, and various members described below. In order to facilitate understanding of the invention by simplifying the drawings, a part of the housing 42 is omitted. The housing 42 includes an opening 44 that is open toward the photosensitive member 12, and a developing roller 48 that is a toner conveying member (second conveying member) is provided in a space 46 formed in the vicinity of the opening 44. It is. The developing roller 48 is a cylindrical member (second rotating cylindrical body), and is arranged in parallel to the photosensitive member 12 and rotatably via the outer peripheral surface of the photosensitive member 12 and a predetermined developing gap 50. .

  A separate space 52 is formed behind the developing roller 48. In the space 52, a transport roller 54 that is a developer transport member (first transport member) is disposed in parallel with the developing roller 48 and through an outer peripheral surface of the developing roller 48 and a predetermined supply / recovery gap 56. . The conveyance roller 54 includes a magnet body 58 that is fixed so as not to rotate, and a cylindrical sleeve 60 (first rotating cylinder body) that is rotatably supported around the magnet body 58. Above the sleeve 60, a restricting plate 62 fixed to the housing 42 and extending in parallel with the central axis of the sleeve 60 is disposed so as to oppose a predetermined restricting gap 64.

  The magnet body 58 has a plurality of magnetic poles facing the inner surface of the sleeve 60 and extending in the central axis direction of the transport roller 54. In the embodiment, the plurality of magnetic poles are a magnetic pole S1 facing the upper inner peripheral surface portion of the sleeve 60 in the vicinity of the regulating plate 62, and a magnetic pole facing the left inner peripheral surface portion of the sleeve 60 in the vicinity of the supply / recovery gap 56. N1, a magnetic pole S2 facing the lower inner peripheral surface portion of the sleeve 60, and two adjacent magnetic poles N2 and N3 of the same polarity facing the right inner peripheral surface portion of the sleeve 60.

  A developer stirring chamber 66 is formed behind the transport roller 54. The stirring chamber 66 includes a front chamber 68 formed in the vicinity of the transport roller 54 and a rear chamber 70 separated from the transport roller 54. A front screw 72 that is a pre-stirring and conveying member that conveys the developer while stirring the developer from the front surface to the back surface of the drawing is rotatably disposed in the front chamber 68, and the rear chamber 70 is rotated from the back surface to the front surface of the drawing. A rear screw 74 that is a rear stirring member transporting member that transports the developer while stirring is disposed rotatably. As shown in the figure, the front chamber 68 and the rear chamber 70 may be separated by a partition wall 76 provided therebetween. In this case, the partition portions near both ends of the front chamber 68 and the rear chamber 70 are removed to form a communication passage, and the developer that has reached the downstream end of the front chamber 68 passes through the communication passage. The developer that has been fed to 70 and reaches the downstream end of the rear chamber 70 is fed to the front chamber 68 via a communication passage.

  The operation of the developing device 34 configured as described above will be described. During image formation, the developing roller 48 and the sleeve 60 rotate in the directions of arrows 78 and 80, respectively, based on driving of a motor (not shown). The front screw 72 rotates in the direction of arrow 82 and the rear screw 74 rotates in the direction of arrow 84. Thereby, the developer 2 accommodated in the developer stirring chamber 66 is stirred while being circulated and conveyed through the front chamber 68 and the rear chamber 70. As a result, the toner and the carrier contained in the developer come into frictional contact with each other and are charged with opposite polarities. In the embodiment, it is assumed that the carrier is positively charged and the toner is negatively charged. As shown in FIG. 2, the carrier 4 is considerably larger than the toner 6. Therefore, as shown in FIG. 3, the negatively charged toner 6 adheres around the positively charged carrier 4 mainly based on the electrical attraction force of both.

  Returning to FIG. 1, the charged developer 2 is supplied to the transport roller 54 in the process of being transported through the front chamber 68 by the front screw 72. The developer 2 supplied from the front screw 72 to the conveyance roller 54 is held on the outer peripheral surface of the sleeve 60 by the magnetic force of the magnetic pole N3 in the vicinity of the magnetic pole N3. The developer 2 held by the sleeve 60 constitutes a magnetic brush along the magnetic field lines formed by the magnet body 58, and is conveyed in the counterclockwise direction based on the rotation of the sleeve 60. The amount of developer 2 held by the magnetic pole S <b> 1 in the area facing the restriction plate 62 (restriction area 86) is regulated by the restriction plate 62 to a predetermined amount. The developer 2 that has passed through the regulation gap 64 is conveyed to a region (supply / recovery region) 88 where the developing roller 48 and the conveying roller 54 are opposed to each other, where the magnetic pole N1 is opposed. As will be described in detail later, in the supply / recovery region 88, an upstream region (supply region) 90 mainly in the rotation direction of the sleeve 60, the presence of an electric field formed between the developing roller 48 and the sleeve 60. Thus, the toner 6 adhering to the carrier 4 is electrically supplied to the developing roller 48. Further, in the supply / recovery area 88, a region (collection area) 92 on the downstream side mainly in the rotation direction of the sleeve 60, as will be described later, the development sent back to the supply / recovery area 88 without contributing to the development. The toner on the roller 48 is scraped off by a magnetic brush formed along the magnetic field lines of the magnetic pole N1 and collected in the sleeve 60. The carrier 4 is held on the outer peripheral surface of the sleeve 60 by the magnetic force of the magnet body 58 and does not move from the sleeve 60 to the developing roller 48. When the developer 2 that has passed through the supply / recovery region 88 is held by the magnetic force of the magnet body 58 and passes through the opposing portion of the magnetic pole S2 along with the rotation of the sleeve 60, the developer 2 reaches the opposing region (discharge region 94) of the magnetic poles N2 and N3. The repulsive magnetic field formed by the magnetic poles N2 and N3 is discharged from the outer peripheral surface of the sleeve 60 to the front chamber 68 and mixed with the developer 2 being conveyed through the front chamber 68.

The toner 6 held on the developing roller 48 in the supply area 90 is conveyed in the counterclockwise direction along with the rotation of the developing roller 48, and is an area (developing area) 96 where the photosensitive body 12 and the developing roller 48 face each other. It adheres to the electrostatic latent image portion formed on the outer peripheral surface. In the image forming apparatus according to the embodiment, a predetermined negative potential V _H is applied to the outer peripheral surface of the photoreceptor 12 by the charging device 26, and the electrostatic latent image image portion on which the image light 30 is projected by the exposure device 28 is predetermined. attenuated until the potential V _L, an electrostatic latent image non-image portion of the image light 30 is not projected by the exposing device 28 maintains a substantially charge potential V _H. Accordingly, in the developing region 96, the negatively charged toner 6 adheres to the electrostatic latent image portion due to the action of the electric field formed between the photosensitive member 12 and the developing roller 48, and the electrostatic latent image portion. The latent image is visualized as a developer image.

  When the toner 6 is consumed from the developer 2 in this way, it is preferable to supply the developer 2 with an amount of toner corresponding to the consumed amount. For this purpose, the developing device 34 includes means for measuring the mixing ratio of the toner and the carrier accommodated in the housing 42. In addition, a toner replenishment section 98 is provided above the rear chamber 70. The toner supply unit 98 includes a container 100 for storing toner. An opening 102 is formed at the bottom of the container 100, and a supply roller 104 is disposed in the opening 102. The replenishing roller 104 is drivingly connected to a motor (not shown), and the motor is driven based on the output of the means for measuring the mixing ratio of toner and carrier so that the toner drops and replenishes the rear chamber 70.

[3. Electric field forming means]
In order to efficiently move the toner 6 from the sleeve 60 to the developing roller 48 in the supply region 90, the developing roller 48 and the sleeve 60 are electrically connected to the electric field forming device 110. Various embodiments can be considered as specific examples of the electric field forming apparatus. As will be described later, FIGS. 5A to 9 show reference examples for collecting more charged particles into the developer when supplying toner to the developing roller.

Field forming device 110 of Example 1 shown in FIG. 5A, a second power supply 11 4 connected to the first power supply 11 2 and the sleeve 60 connected to the developing roller 48. The first power supply 112 includes a DC power supply 118 connected between the developing roller 48 and the ground 116, and a first DC voltage V _DC1 (for example, −200 volts) having the same polarity as the charging polarity of the toner 6. Is applied to the developing roller 48. The second power supply 114 has a DC power supply 120 connected between the sleeve 60 and the ground 116, and has the same polarity as the charging polarity of the toner 6 and a second DC voltage that is higher than the first DC voltage. V _DC2 (eg, −400 volts) is applied to the sleeve 60. As a result, in the supply region 90, the negatively charged toner 6 is electrically attracted from the sleeve 60 to the developing roller 48 under the action of a DC electric field formed between the developing roller 48 and the sleeve 60. Is done. At this time, the positively charged carrier 4 is not attracted to the developing roller 48 from the sleeve 60. In the developing region 96, the negative polarity toner held on the developing roller 48, as shown in FIG. 5B, is developed between the developing roller 48 (V _DC1 : for example, −200 volts) and the electrostatic latent image portion (V _L : For example, −80 volts) and adheres to the electrostatic latent image portion. At this time, the negative polarity toner is not electrostatic latent image non-imaged due to a potential difference between the developing roller 48 (V _DC1 : for example −200 volts) and the electrostatic latent image non-image part (V _H : for example −600 volts). It does not adhere to the part.

In the electric field forming apparatus 122 of FIG. 6A according to Reference Example 2, the first power supply 124 has a DC power supply 128 connected between the developing roller 48 and the ground 126, similarly to the power supply of the first embodiment. A first DC voltage V _DC1 (for example, −200 volts) having the same polarity as the charging polarity of the toner 6 is applied to the developing roller 48. The second power source 130 includes a DC power source 132 and an AC power source 134 between the sleeve 60 and the ground 126. The DC power supply 132 applies a second DC voltage V _DC2 (for example, −400 volts) having the same polarity as the charging polarity of the toner 6 and higher than the first DC voltage to the sleeve 60. As shown in FIG. 6B, the AC power supply 134 applies an _AC voltage VAC having a peak-to-peak voltage V _PP of, for example, 300 volts between the sleeve 60 and the ground 126. As a result, in the supply region 90, the negatively charged toner 6 is electrically transferred from the sleeve 60 to the developing roller 48 under the action of a pulsating electric field formed between the developing roller 48 and the sleeve 60. Sucked. At this time, the positively charged carrier 4 is held by the sleeve 60 by the magnetic force of the fixed magnet inside the sleeve 60 and is not supplied to the developing roller 48. In the developing region 96, the negative polarity toner held on the developing roller 48 includes the developing roller 48 (V _DC1 : for example, −200 volts) and the electrostatic latent image portion (V _L : for example, −80 volts). And adhere to the electrostatic latent image portion.

In the electric field forming device 136 shown in FIG. 7A according to Reference Example 3, the first power source 138 includes a DC power source 142 and an AC power source 144 between the developing roller 48 and the ground 140. The DC power supply 142 applies a first DC voltage V _DC1 (for example, −200 volts) having the same polarity as the charging polarity of the toner 6 to the developing roller 48. The AC power supply 144 applies an _AC voltage VAC having an amplitude (peak-to-peak voltage) _VP-P of, for example, 1,600 volts between the developing roller 48 and the ground 140. The second power source 146 includes a DC power source 150 connected between the terminal 148 between the developing roller 48 and the AC power source 144 and the sleeve 60. The DC power supply 150 can output a predetermined DC voltage V _DC2 , and the anode is connected to the terminal 148 and the cathode is connected to the sleeve 60, whereby the sleeve 60 is biased to the negative polarity with respect to the developing roller 48. (See FIG. 7B). Accordingly, in the supply region 90, the negatively charged toner 6 is electrically attracted from the sleeve 60 to the developing roller 48 due to the action of the pulsating electric field formed between the developing roller 48 and the sleeve 60. Is done. In the developing area 96, the negative polarity toner on the developing roller 48 is caused by a potential difference between the developing roller 48 (V _DC1 : for example −200 volts) and the electrostatic latent image portion (V _L : for example −80 volts). Is attached to the electrostatic latent image portion.

The electric field forming apparatus 152 shown in FIG. 8 according to the reference example 4 is different from the electric field forming apparatus shown in FIG. 5A in that AC electric field forming apparatuses 154 and 156 are added to the first power supply 112 and the second power supply 114, respectively. Is. The output voltages of the AC electric field forming devices 154 and 156 are V _AC1 and V _AC2 . The voltage values and periods of the voltages V _AC1 and V _AC2 may be the same or different. An electric field forming device 158 shown in FIG. 9 according to Reference Example 5 is obtained by adding an AC power supply 160 to the first power supply 112 in the power supply of the reference example shown in FIG. 5A. The output voltage of the AC power supply 160 is _{V AC.} Similarly to the electric field forming devices 110, 122, and 136, the electric field forming devices 152 and 158 of these forms also receive the action of the pulsating electric field formed between the developing roller 48 and the sleeve 60, and in the supply region 90. The negatively charged toner 6 is supplied from the sleeve 60 to the developing roller 48, and in the developing region 96, the negatively charged toner 6 is supplied from the developing roller 48 to the electrostatic latent image portion (V _L : For example, Based on a potential difference of −80 volts). The basic configuration of the electric field forming device 158 of Reference Example 5 shown in FIG. 9 is the same as the basic configuration of the electric field forming device according to the embodiment of the present invention described later, except that the voltage waveform of the AC power supply 160 is different. It is common.

[4. Developer)
In general, in a two-component developer containing toner and carrier as main components, dirt (spent) is generated by the toner adhering to the surface of the carrier, and this reduces the life of the carrier. In order to solve this problem, in the present invention, charged particles (implant particles) are added as a third component to the two-component developer.

  2 to 4, the image forming apparatus and the developing apparatus according to the present invention make the toner 6 have a normal polarity (not shown) by frictional contact with the toner 6 in addition to the toner 6 and the carrier 4. In this case, charged particles 8 having a diameter smaller than that of the toner 6 are included. In the embodiment, the charged particles 8 are detachably held on the outer peripheral surface of the toner 6 and are replenished together with the toner 6 from the toner replenishing unit 98.

  At the time of image formation, the charged particles 8 are transported through the housing 42 together with the toner 6 and the carrier 4, and then are held by the sleeve 60 and move in the regulation region 86, the supply / recovery region 88, and the discharge region 94. In this conveyance process, the charged particles 8 held on the surface of the toner 6 and charged to the positive polarity are placed in the electric field of the supply / recovery region 88, which is opposite to the electric force acting on the toner 6. The toner 6 is detached from the outer peripheral surface of the toner 6 by receiving the electric force in the direction. The separated charged particles 8 are held or driven on the outer peripheral surface of the carrier 4 by stress acting between the separated charged particles 8 and the carrier 4. As shown in FIG. 4, when a part or the whole of the outer peripheral surface of the carrier 4 is covered with the spent 10, the charged particles 8 are driven into the spent 10. The charged particles 8 held or driven on the outer peripheral surface of the carrier 4 are charged to a polarity opposite to that of the toner 6 by frictional contact with the toner 6. In the embodiment, since the toner 6 is charged to a negative polarity, the charged particles 8 are charged to a positive polarity. As a result, the carrier 4 into which the charged particles 8 are implanted maintains the same chargeability as the state without the spent 10 even if at least a part of the outer peripheral surface thereof is covered with the spent 10, and the toner 6 is preliminarily provided. Charged to the polarity.

  As described above, the charged particles 8 are charged with a polarity opposite to that of the toner 6. Therefore, as shown in FIG. 10, in the supply / recovery area 88, the toner 6 moves from the sleeve 60 to the developing roller 48 based on the electric field formed between the developing roller 48 and the sleeve 60. Further, the charged particles 8 separated from the toner 6 are quickly held on the carrier surface of the developer that is relatively carrier-rich by the toner 6 being taken away in the supply region 90 and supplied to the developing roller 48 together with the toner 6. Even if not supplied or supplied to the developing roller 6, the amount is extremely small.

  By the way, as shown in FIG. 11, when similar charged particles are used in the developing device 34 ′ in which the developing roller is removed from the developing device shown in FIG. 1, different results are brought about. Specifically, an electrostatic latent image is formed on the outer peripheral surface of the photoconductor 12 facing the conveying roller 54 of the developing device 34 '. The electrostatic latent image includes, for example, a high potential electrostatic latent image non-image portion that maintains a substantially charged potential and a low potential electrostatic latent image in which the light 30 is projected by the exposure device 28 and the potential is attenuated. An image image portion is provided, and the high potential electrostatic latent image non-image portion and the low potential electrostatic latent image image portion pass through a facing portion of the conveying roller 54. At the time of image formation, in the development area, for example, the negatively charged toner 6 adheres to the low-potential electrostatic latent image portion and does not adhere to the electrostatic latent image non-image portion. However, the charged particles 8 for charging the toner 6 to the negative polarity are themselves charged to the positive polarity. Accordingly, the charged particles 8 in a free state in the development region adhere to the electrostatic latent image non-image portion as shown in FIG. As described above, according to the developing device 34 ′, the charged particles 8 separated from the toner 6 are consumed in a large amount in the electrostatic latent image non-image portion of the photosensitive member 12 in the developing region. As a result, the number of charged particles 8 that are driven into the outer peripheral surface of the carrier 4 is extremely small compared to the developing device 34 described above, and the carrier 4 to which the spent adheres cannot have sufficient toner charging performance.

  By the way, in the developing device described in the above-mentioned Patent Document 4, charged particles exist in a relatively free state between the toner and the carrier without being held on either surface. The charged particles initially introduced into the developing device are charged with a polarity opposite to the charging polarity of the toner. For this reason, after being electrically coupled to the toner and supplied to the developing roller together with the toner, it adheres to the non-image portion of the electrostatic latent image on the photosensitive member and gradually disappears, and at the same time, the chargeability of the toner decreases. However, in the developing device according to the present invention, as described above, the charged particles 8 separated from the toner 6 in the supply / recovery region 88 are then quickly held by the carrier 4 and remain on the outer peripheral surface of the sleeve 60. As described above, since the toner is not supplied to the photoconductor 12 via the developing roller 48 and consumed, stable toner chargeability can be obtained over a long period of time. However, in this embodiment, some of the charged particles 8 are supplied to the developing roller 48 together with the toner 6. However, since the charged particles 8 are newly supplied from the supply unit 98 together with the toner, they are not lost. Thus, stable toner chargeability can be obtained.

  In the embodiment, the toner 6 is charged to a negative polarity and the carrier 4 is charged to a positive polarity by frictional contact between the toner 6 and the carrier 4. Further, the charged particles 8 are charged negatively by contact with the toner 6, and the charged particles 8 are charged positively. The chargeability of the toner, carrier, and charged particles used in the present invention is not limited to such a combination. Specifically, the toner 6 is charged positively by the frictional contact between the toner 6 and the carrier 4, the carrier 4 is charged negatively, and the charged particles 8 are charged positively by the contact with the toner 6. A combination in which the charged particles 8 are negatively charged is also conceivable.

[5. Specific materials)
Specific materials of the toner, carrier, charged particles, and other particles contained in the developer will be described.

[Charged particles]
The charged particles preferably used are appropriately selected according to the charging polarity of the toner. The number average particle diameter of the charged particles is, for example, 0.5 to 1.5 μm. When using toner that is negatively charged by frictional contact with the carrier, charged particles are fine particles that are positively charged by contact with the toner. Such fine particles include, for example, inorganic fine particles such as strontium titanate, barium titanate, calcium titanate, beryllium titanate, magnesium titanate, alumina, acrylic resin, benzoguanamine resin, nylon resin, polyimide resin, polyamide resin, etc. It can be composed of a thermoplastic resin or a thermosetting resin. The resin constituting the fine particles may contain a positive charge control agent that is positively charged by contact with the toner. As the positive charge control agent, for example, nigrosine dye, quaternary ammonium salt and the like can be used. The charged particles may be composed of nitrogen-containing monomers. Examples of the material constituting the nitrogen-containing monomer include 2-dimethylaminoethyl acrylate, 2-diethylaminoethyl acrylate, 2-dimethylaminoethyl methacrylate, 2-diethylaminoethyl methacrylate, vinylpyridine, N-vinylcarbazole, There is vinylimidazole.

  In the case of a toner that is positively charged by frictional contact with the carrier, fine particles that are negatively charged by contact with the toner are used as the charged particles. Examples of such fine particles include inorganic fine particles such as silica and titanium oxide, and fine particles made of thermoplastic resin or thermosetting resin such as fluororesin, polyolefin resin, silicone resin, and polyester resin. A negative charge control agent that is negatively charged by contact with the toner may be contained in the resin constituting the charged particles. Examples of negative charge control agents include salicylic acid-based and naphthol-based chromium complexes, aluminum complexes, iron complexes, and zinc complexes. The charged particles may be a copolymer of a fluorine-containing acrylic monomer or a fluorine-containing methacrylic monomer.

  In order to control the chargeability and hydrophobicity of the charged particles, the surface of the inorganic fine particles may be surface-treated with a silane coupling agent, a titanium coupling agent, silicone oil, or the like. In particular, when imparting positive electrode chargeability to inorganic fine particles, it is preferable to surface-treat with an amino group-containing coupling agent. When imparting negative chargeability to the fine particles, it is preferable to surface-treat with a fluorine group-containing coupling agent.

〔toner〕
As the toner, a known toner that has been conventionally used in an image forming apparatus can be used. The toner particle size is, for example, about 3 to 15 μm. A toner containing a colorant in a binder resin, a toner containing a charge control agent or a release agent, and a toner holding an additive on the surface can also be used.
The toner can be produced by a known method such as a pulverization method, an emulsion polymerization method, or a suspension polymerization method.

[Binder resin]
The binder resin used for the toner is not limited. For example, styrene resin (monopolymer or copolymer containing styrene or styrene-substituted product), polyester resin, epoxy resin, vinyl chloride resin, phenol resin. , Polyethylene resin, polypropylene resin, polyurethane resin, silicone resin, or any mixture of these resins. The binder resin preferably has a softening temperature in the range of about 80 to 160 ° C and a glass transition point in the range of about 50 to 75 ° C.

[Colorant]
For the colorant, a known material such as carbon black, aniline black, activated carbon, magnetite, benzine yellow, permanent yellow, naphthol yellow, phthalocyanine blue, first sky blue, ultramarine blue, rose bengal, lake red, etc. should be used. Can do. In general, the addition amount of the colorant is preferably 2 to 20 parts by weight with respect to 100 parts by weight of the binder resin.

[Charge control agent]
As the charge control agent, materials conventionally known as charge control agents can be used. Specifically, for the positively charged toner, for example, nigrosine dyes, quaternary ammonium salt compounds, triphenylmethane compounds, imidazole compounds, and polyamine resins can be used as charge control agents. For the negatively charged toner, metal-containing azo dyes such as Cr, Co, Al, and Fe, salicylic acid metal compounds, alkyl salicylic acid metal compounds, and curixarene compounds can be used as charge control agents. The charge control agent is preferably used at a ratio of 0.1 to 10 parts by weight with respect to 100 parts by weight of the binder resin.

〔Release agent〕
As the release agent, a known release agent conventionally used as a release agent can be used. As the material for the release agent, for example, polyethylene, polypropylene, carnauba wax, sazol wax, or a mixture of them as appropriate is used. The release agent is preferably used at a ratio of 0.1 to 10 parts by weight with respect to 100 parts by weight of the binder resin.

[Other additives]
In addition, a fluidizing agent that promotes fluidization of the developer may be added. As the fluidizing agent, for example, inorganic fine particles such as silica, titanium oxide, and aluminum oxide, and resin fine particles such as acrylic resin, styrene resin, silicone resin, and fluorine resin can be used. In particular, it is preferable to use a material hydrophobized with a silane coupling agent, a titanium coupling agent, silicon oil or the like. The fluidizing agent is preferably added at a ratio of 0.1 to 5 parts by weight with respect to 100 parts by weight of the toner. The number average primary particle size of these additives is preferably 9 to 100 nm.

[Carrier]
As the carrier, a known carrier that has been generally used can be used. Either a binder type carrier or a coat type carrier may be used. The carrier particle size is not limited, but is preferably about 15 to 100 μm.

  The binder type carrier is obtained by dispersing magnetic fine particles in a binder resin, and those having fine particles or a coating layer charged positively or negatively on the surface can be used. The charging characteristics such as the polarity of the binder type carrier can be controlled by the material of the binder resin, the chargeable fine particles, and the type of the surface coating layer.

  Examples of the binder resin used for the binder-type carrier include thermoplastic resins such as vinyl resins, polyester resins, nylon resins, polyolefin resins, and the like typified by polystyrene resins, and curable resins such as phenol resins. .

  Magnetic fine particles of the binder type carrier include spinel ferrite such as magnetite and gamma iron oxide, and magnets such as spinel ferrite and barium ferrite containing one or more metals other than iron (Mn, Ni, Mg, Cu, etc.). Plumbite type ferrite, iron or alloy particles having an oxide layer on the surface can be used. The shape of the carrier may be granular, spherical, or needle-shaped. In particular, when high magnetization is required, it is preferable to use iron-based ferromagnetic fine particles. In consideration of chemical stability, it is preferable to use ferromagnetic fine particles of magnetoplumbite type ferrite such as spinel ferrite and barium ferrite containing magnetite and gamma iron oxide. A magnetic resin carrier having a desired magnetization can be obtained by appropriately selecting the type and content of the ferromagnetic fine particles. The magnetic fine particles are suitably added in an amount of 50 to 90% by weight in the magnetic resin carrier.

  Silicone resin, acrylic resin, epoxy resin, fluorine resin, etc. are used as the surface coating material for the binder type carrier. The charge imparting ability of the carrier can be improved by coating and curing these resins on the carrier surface to form a coat layer.

  For example, the charging fine particles or the conductive fine particles can be fixed to the surface of the binder type carrier by, for example, mixing the magnetic resin carrier and the fine particles uniformly and adhering the fine particles to the surface of the magnetic resin carrier. This is done by driving fine particles into the magnetic resin carrier by applying a strong impact force. In this case, the fine particles are not completely embedded in the magnetic resin carrier, but are fixed so that a part thereof protrudes from the surface of the magnetic resin carrier. Organic and inorganic insulating materials are used for the chargeable fine particles. Specifically, organic insulating materials include polystyrene, styrene-based copolymers, acrylic resins, various acrylic copolymers, nylon, polyethylene, polypropylene, fluororesin, and cross-linked products thereof such as organic insulating fine particles. is there. The charge imparting ability and the charge polarity can be adjusted to the material of the chargeable fine particles, the polymerization catalyst, the surface treatment and the like. As the inorganic insulating material, negatively charged inorganic fine particles such as silica and titanium dioxide, and positively charged inorganic fine particles such as strontium titanate and alumina are used.

  The coat type carrier is a carrier in which carrier core particles made of a magnetic material are coated with a resin, and like the binder type carrier, chargeable fine particles that are charged positively or negatively can be fixed to the surface of the carrier. The charging characteristics such as the polarity of the coated carrier can be adjusted by selecting the type of the surface coating layer and the electrifying fine particles. As the coating resin, the same resin as the binder resin of the binder type carrier can be used.

  The mixing ratio of the toner and the carrier may be adjusted so as to obtain a desired toner charge amount, and the toner ratio is preferably 3 to 50% by weight, preferably 6 to 30% by weight based on the total amount of the toner and the carrier. .

[Reference test]
The effect of charged particles was examined using an image forming apparatus having the developing device of FIG. 1 and an image forming apparatus having the developing device shown in FIG. For the experiment, toner A not carrying charged particles and toner B carrying charged particles were prepared.

[Toner A]
The manufacturing method of the toner A is as follows. To 100 parts by weight of a toner base material having a volume average particle diameter of about 6.5 μm prepared by a wet granulation method, 0.2 parts by weight of a plurality of additives-first hydrophobic silica and 0.2% of second hydrophobic silica are added. 5 parts by weight and 0.5 parts by weight of hydrophobic titanium oxide were added. Next, using a Henschel mixer manufactured by Mitsui Mining Co., Ltd., the toner base material to which the additive was added was agitated to adhere the additive to the surface of the toner base material, and negatively charged toner A was obtained. The rotating speed of the mixer was 40 m / second, and the stirring time was 3 minutes. The first hydrophobic silica is obtained by surface-treating silica (# 130: manufactured by Nippon Aerosil Co., Ltd.) having a number average primary particle diameter of 16 nm with a hydrophobizing agent hexamethyldilazan (HMDS). The second hydrophobic silica is obtained by surface-treating silica (# 90: manufactured by Nippon Aerosil Co., Ltd.) having a solid average primary particle size of 20 nm with HMDS. The hydrophobic titanium oxide is obtained by surface-treating anatase-type titanium oxide having a number average primary particle size of 30 nm with a hydrophobizing agent, isobutyltrimethoxysilane, in an aqueous wet environment.

[Toner B]
The manufacturing method of the toner B is as follows. To toner A, strontium titanate having a number average particle diameter of 350 nm was added as charged particles. The amount of charged particles added was 2 parts by weight with respect to 100 parts by weight of toner base material particles contained in toner A. Next, the toner A to which charged particles were added was stirred with a Henschel mixer manufactured by Mitsui Mining Co., Ltd., and the charged particles were adhered to the surface of the toner to obtain toner B. The rotating speed of the mixer was 40 m / second, and the stirring time was 3 minutes.

[Carrier]
The carrier used in the experiment is a bizhub C350 carrier manufactured by Konica Minolta Business Technologies. This carrier is a coated carrier in which carrier core particles made of a magnetic material are coated with an acrylic resin.

[Test Example 1]
As the developing device, a developing device having the form shown in FIG. 1 was used. As the developer, the carrier and toner B described above were used. The toner ratio in the developer was adjusted to 8%. The toner ratio is a ratio of the total weight of the toner and the additive containing charged particles to the total weight of the developer. The electric field forming apparatus employs the form shown in FIG. 9, and a DC voltage V _{DC2 of} −500 volts is applied to the conveying roller, and an AC voltage of DC voltage V _{DC1 of} −300 volts is applied to the developing roller. AC voltage frequency: 2 kHz, the amplitude _V P-P: 1,600 volts, minus the duty ratio (toner collecting duty ratio): 40%, plus a duty ratio (toner supply duty ratio): was 60% of the rectangular wave (See FIG. 13). Therefore, the supply potential difference (toner supply voltage) for biasing the negatively charged toner from the sleeve to the developing roller is 1,000 volts, and the recovery voltage difference (toner recovery voltage) for biasing the toner from the developing roller to the sleeve is 600 volts. is there.

As the developing roller, an aluminum roller whose surface was anodized was used. The supply / recovery gap between the developing roller and the sleeve was set to 0.3 mm. As a result, the electric field supplied to the toner formed between the developing roller and the sleeve was 3.3 × 10 ⁶ V / m (= 1,000 V / 0.3 mm). The regulation gap between the regulation plate and the sleeve was 0.4 mm so that the magnetic brush on the sleeve was in contact with the outer peripheral surface of the developing roller. The developing roller and the sleeve rotate in the same direction so that the developer transport direction on the sleeve and the toner on the developing roller move in the opposite directions in the supply and recovery area. The charged potential of the photosensitive member was −550 volts, and the potential of the electrostatic latent image non-image portion image portion formed on the photosensitive member was −60 volts. The developing gap between the photoconductor and the developing roller was set to 0.15 mm.

[Test Example 2]
The same developer as in Test Example 1 was used. The developing device shown in FIG. 11 was used as the developing device. A rectangular wave having an amplitude of 1,400 volts, a DC voltage of -300 volts, a negative duty ratio of 50%, and a frequency of 4 kHz was applied to the sleeve. The development gap between the sleeve and the photosensitive member was set to 0.3 mm. Other conditions are the same as in Test Example 1.

[Test Example 3]
Instead of toner B having charged particles, toner A having no charged particles was used. Other conditions are the same as in Test Example 1.

[Evaluation]
Using an image forming apparatus modified from Konica Minolta Business Technologies Co., Ltd. bizhub C350, 50,000 original images with an image area ratio of 5% were printed under a plurality of conditions. The developer in the developing device was sampled every 10,000 printed sheets, and the toner charge amount was measured. The results are shown in FIG. As is apparent from this figure, in Test Example 1, the toner charge amount maintained a substantially constant value regardless of the increase in the number of printed sheets. In contrast, in Test Examples 2 and 3, the toner charge amount decreased as the number of printed sheets increased.

  After printing 50,000 sheets, the carrier was separated from the developer, and the surface of the carrier was observed with a scanning electron microscope. FIG. 15A is an enlarged photograph of the surface of a carrier separated from a developer containing toner B having charged particles. When small particles appearing in the photograph were analyzed by X-ray photoelectron spectroscopy (Electron Spectroscopy for Chemical Analysis), strontium, the main component of the charged particles, was detected. This detection result confirmed that charged particles were implanted into the surface of the carrier. 15B and 15C are enlarged photographs of the surface of the carrier separated from the developer containing toner B without charged particles. As is clear from comparison with FIG. 15A, there were almost no microparticles on the surface of the carrier.

[Charge polarity of charged particles]
The charged polarity of the charged particles was confirmed using an experimental apparatus 170 shown in FIG. The experimental apparatus 170 includes a fixed cylindrical body 172, a magnet roller 174 rotatably disposed inside the cylindrical body 172, and an outer cylinder 176 that surrounds the magnet roller 174. Between the cylindrical body 172 and the outer cylinder 176, an electric field that does not electrically bias the negative polarity toner from the cylindrical body 172 toward the outer cylinder 176, that is, charged particles having a polarity opposite to that of the toner from the cylindrical body 172. An electric field urging toward the outer cylinder 176 was applied. Then, after the developer composed of toner B containing charged particles and the carrier was stirred, the developer after stirring was held on the outer peripheral surface of the cylindrical body 172, and the magnet roller 174 was rotated. As a result, strontium titanate of charged particles that received an electric field urging force from the cylindrical body 172 toward the outer cylinder 176 was separated from the developer and separated, and was confirmed on the inner surface of the outer cylinder 176. This shows that strontium titanate is charged with the opposite polarity (positive polarity) to that of the toner.

[Conclusion]
As described above, in the developing device, strontium titanate charged to the opposite polarity to the toner adheres to the surface of the carrier, thereby compensating for the decrease in the ability of the carrier to charge the toner, It was found that the charge amount was kept at the required value.

In the present embodiment, a uniform toner layer can be formed on the developing roller 48 with low energy without impairing the accumulation of the charged particles 8 of opposite polarity in the developer 2 in the developer stirring chamber 66. Therefore, an electric field for moving the toner 4 in the developer 2 held by the sleeve 60 to the developing roller 48 between the sleeve 60 of the conveying roller 54 and the developing roller 48 is constituted only by a DC electric field. Yes. On the other hand, the toner 4 held by the developing roller 48 is moved to the electrostatic latent image on the photosensitive member 12 between the developing roller 48 and the photosensitive member 12, and the electrostatic latent image is visualized as a toner image. The electric field to be formed was configured by superimposing a DC electric field and an oscillating electric field.
The basic configuration of such an electric field forming device is basically the same as that shown as Reference Example 5 in FIG. 9 except that the output voltage waveform of the AC power supply is different. Therefore, in the following description of the embodiment of the present invention, the same reference numerals are used for the components corresponding to those shown in FIG.

That is, in the electric field forming apparatus according to the present embodiment, as shown in FIG. 9, the first power supply 112 connected to the developing roller 48 has a pulse voltage V _AC (peak) of the oscillating electric field generated by the AC power supply 160 as shown in FIG. The two-peak voltage V _PP is 1000 volts, for example, and is superimposed on the DC voltage V _DC1 (for example, −500 volts) of the direct current electric field generated by the first DC power supply 118. On the other hand, the second power source 114 connected to the sleeve 60 outputs a DC voltage V _DC2 (for example, = −800 volts) of a DC electric field generated by the second DC power source 120.
In the present embodiment, as shown in FIG. 17, the oscillating electric field connected to the developing roller 48 side has a voltage waveform including a blank portion BP in which application of a pulse voltage is suspended for a predetermined period. The blank part BP is provided for every fixed period of the pulse voltage of the oscillating electric field. More preferably, in the blank portion BP, the electric field direction before and after the blank portion BP moves the toner 4 to the developing roller 48 side (in other words, charged particles 8 having a polarity opposite to that of the toner 4 are moved to the sleeve 60 side. It is set to be the direction of collection).

In this configuration, in the supply / recovery area 88, as shown in FIG. 17, the toner supply voltage is applied in the direction indicated by the arrow Y, and in the blank portion BP provided for every fixed period of the pulse voltage of the oscillating electric field, Application of the pulse voltage is suspended for a period of time. During this suspension period, a potential difference based on the DC voltage V _DC1 of the DC electric field generated by the first DC power supply 118 and the DC voltage V _DC2 of the DC electric field generated by the second DC power supply 120 acts on the supply and recovery region 88. become.
Further, an electric field in the application direction of the toner supply voltage (arrow Y direction) acts before and after the blank portion BP, that is, immediately before the application of the pulse voltage is stopped and immediately after the application is restarted. As a result, the toner 4 moves toward the developing roller 48 immediately before the application of the pulse voltage is stopped, and immediately after the application of the pulse voltage is stopped, the toner 4 that is moving is not moved. The inertial force ensures that the developing roller 48 is reached and held. Further, the reverse polarity charged particles 8 released from the toner 4 are more reliably collected on the sleeve 60 side. In addition, by applying a pulse in the toner supply direction (arrow Y direction) immediately after the blank portion BP, the charged particles 8 having the opposite polarity that remain in the blank portion BP without being collected in the sleeve 60 and are floating are reliably obtained. Suction can be performed on the sleeve 60 side.
On the other hand, during application of the pulse voltage, the toner 4 forms a uniform layer on the developing roller 48 while vibrating. Further, the charged particles 8 are relatively easily separated from the toner 4 by vibration and are always subjected to the force of the electric field in the collecting direction to the sleeve 60, so that the charged particles 8 are reliably collected and enter the developer stirring chamber 66. Accumulated.

  As described above, according to the present embodiment, a uniform toner layer with low energy can be formed on the developing roller 48 by the action of the oscillating electric field, and a pulse voltage is applied to the voltage waveform of the oscillating electric field only for a certain period. By providing the blank portion BP that is stopped, the charged particles 8 can be more reliably collected on the sleeve 60 side.

[Experiment I]
In order to confirm the effect of adopting the developing device according to the present embodiment, the following experiment I was performed.
[Example 1]
A developing device incorporating the electric field forming device shown in FIG. 17 was used.
The imaging conditions were as follows.
Photoconductor potential: -600 volts Development DC bias (first DC voltage V _DC1 ): _-500 volts Development AC bias (first AC voltage V _AC1 ): Peak-to-peak voltage (V _PP ) is 1000 V, frequency is 3 kHz, and a blank portion for stopping application of the pulse voltage for 5 ms after application in the 20th cycle in the toner movement direction every 20 cycles is set. Sleeve DC bias (second DC voltage V _DC2 ): -800 volts

The following developers were used.
Toner: Toner particle size of 6 μm and 1.5% external addition treatment of strontium titanate with a particle size of 0.5-1.5 μm as a charged particle Carrier: Ferrite coated carrier with a particle size of 30 μm Ratio: 10%

Under the above conditions, after printing 100,000 sheets of an image with an image area ratio of 5%, the presence or absence of printing defects was examined.
Even after printing 100,000 sheets, the image density was 1.5, there was no density unevenness, and no so-called background fogging was seen, and good printing results were obtained.

[Comparative Example 1]
Under the conditions of Example 1, the voltage waveform of the development AC bias was a continuous wave without a blank portion. Except for this point, all other conditions were the same as in the example, and the same durability printing test as in Example 1 was performed.
The image density was as good as 1.5, but background fogging occurred at about 70,000 sheets. Further, density unevenness due to non-uniformity of the toner layer occurred.
This is because there is no blank after application in the developing AC bias in the toner moving direction (that is, the direction in which the charged particles are collected on the sleeve side), and the charged particles repeat only the reciprocating motion, so that they also adhere to the developing roller. This is considered to be due to the deterioration of the charge collection performance of the charged particles on the sleeve due to the deterioration of the charged particle accumulation effect in the developing tank and the deterioration of the charging performance of the carrier.

[Comparative Example 2]
Under the conditions of Example 1, the development AC bias was not applied, and the development bias was only DC. Except for this point, all other conditions were the same as in the example, and the same durability printing test as in Example 1 was performed.
The image density was as low as 1.1, and background fogging occurred at about 80,000 sheets. Further, density unevenness due to non-uniformity of the toner layer also occurred.
This is presumably because the toner does not easily move away from the magnetic brush when the development AC bias is not applied, so that the toner movement amount is reduced and the toner layer is non-uniform.

[Example 2]
Under the conditions of Example 1, the peak-to-peak voltage of the developing AC bias was 2000 volts, the frequency was 3 kHz, the period for providing the blank portion was 30 periods, and the blank period was 10 ms. The sleeve DC bias was 400 volts.
Except for the above points, all other conditions were the same as in the example, and the same durability printing test as in Example 1 was performed.
Even after printing 100,000 sheets, the image density was 1.5, there was no uneven density, and no toner scattering or background fogging was observed, and good printing results were obtained.

[Evaluation]
As described above, in both the first and second embodiments in which the blanks that are stopped for a predetermined period after being applied to the developing AC bias in the toner moving direction are provided at a constant period, high image density can be ensured, and density unevenness and background fogging can be ensured. In contrast, in Comparative Example 1 in which the development AC bias was a continuous wave with no blank, the image density was secured, but uneven density and background fogging occurred. Further, in Comparative Example 2 in which the development AC bias was not applied and the development bias was only DC, the image density was low, and density unevenness and background fogging occurred from the beginning or early of the test.
From these test results, it is possible to improve the recovery of charged particles in the developing tank and make the toner layer uniform by providing a blank that is paused for a predetermined period after being applied to the developing AC bias in the toner movement direction. As a result, it was confirmed that a high image density can be secured, and the occurrence of density unevenness and background fogging can be effectively prevented.

[Experiment II]
Using the same developing device and developer as those used in Example 1, changing the electric field forming conditions in various ways, and after printing 100,000 sheets of an image with an image area ratio of 5%, whether print defects occurred A test was conducted to investigate.
The test results under each condition were evaluated by the following four ranks.
-: Unacceptable printing state-△: Inferior printing state but acceptable printing state-○: Good printing state that is lower than the initial state-A: Good printing state that is not different from the initial state Table 1 shows the test results .

Based on the test results in Table 1, a more effective range for each condition of the development AC bias was examined.
As for the peak-to-peak voltage (V _PP ) of the developing AC bias, when V _PP = 800 volts, density unevenness is likely to occur on the low frequency side, and insufficient density is likely to occur on the high frequency side. When V _PP = 1000, 2000, 2500 volts, uneven density tends to occur at a low frequency of 800 Hz. Further, when V _PP = 2800, discharge occurred in the development nip and the toner supply / recovery nip, and white spots were easily generated in so-called solid images and black spots were easily generated in white paper portions. Therefore, by setting the peak-to-peak voltage (V _PP ) of the developing AC bias within the range of 1000 volts to 2500 volts, it is possible to obtain a better image that does not cause uneven density or discharge. Further, when the frequency is as high as 6000 Hz, the concentration is likely to be insufficient.

As for the setting period of the blank part, if the frequency of the blank part is lowered by extending it to the voltage pulse 60 period, the recoverability of charged particles to the sleeve side deteriorates, and the carrier due to the charged particle accumulation effect in the developing tank is deteriorated. The effect of improving the charging property is reduced, and the background fog is likely to occur after printing 100,000 sheets. This tendency was the same for both V _PP = 1000 volts and 2500 volts. On the other hand, when the frequency of the blank part is increased by shortening the set period of the blank part to 8 voltage pulses, uneven density tends to occur. Therefore, it is preferable that the blank is provided every 10 to 50 periods of the voltage pulse.
In this case, with respect to the frequency A (kHz) of the voltage pulse, by setting the blank period in the range of (5/1000 A) seconds to (20/1000 A) seconds, the image density can be secured. It was found that a better printing state without unevenness or background fogging can be obtained.

1 is a diagram illustrating a schematic configuration of an image forming apparatus according to the present invention and a cross section of a developing device according to the present invention. The figure which illustrates the composition of a developer typically. The figure which shows typically the state in which the toner which hold | maintained the charged particle has adhered to the surface of a carrier. The figure which shows typically the state by which the charged particle was bombarded on the surface of the carrier to which the spent adhered. The figure which shows the reference example of an electric field formation apparatus. FIG. 5B is a diagram showing the relationship between the voltage supplied from the electric field forming device shown in FIG. The figure which shows the other reference example of an electric field formation apparatus. FIG. 6B is a diagram showing the relationship between the voltage supplied from the electric field forming device shown in FIG. 6A to the sleeve of the conveying roller and the developing roller. The figure which shows the other reference example of an electric field formation apparatus. The figure which shows the relationship between the voltage supplied to the sleeve of a conveyance roller, and the developing roller from the electric field formation apparatus shown to FIG. 7A. The figure which shows the other reference example of an electric field formation apparatus. The figure which shows the other reference example of an electric field formation apparatus. The figure which shows typically the motion of the toner and charged particle in a supply collection area | region. FIG. 2 is a cross-sectional view of another form of developing apparatus in which a developing roller is deleted from the developing apparatus of FIG. FIG. 12 is a diagram schematically showing the movement of toner and charged particles in the developing region of the developing device shown in FIG. 11. FIG. 3 is a diagram illustrating a waveform of a voltage supplied to a conveying roller sleeve and a developing roller in the first exemplary embodiment. 6 is a graph showing the relationship between the number of printed sheets and the toner charge amount. Enlarged photo of a carrier with charged particles on the surface. Enlarged photo of carriers with no charged particles on the surface. Enlarged photo of carriers with no charged particles on the surface. Sectional drawing of the apparatus which tests the charged state of a charged particle. The figure which shows typically the voltage waveform of the oscillating electric field applied between the developing roller which concerns on embodiment of this invention, and the sleeve of a conveyance roller.

Explanation of symbols

1: image forming apparatus, 2: developer, 4: carrier, 6: toner, 8: charged particles, 10: spent, 12: photoconductor, 16: charging station, 18: exposure station, 20: developing station, 22: Transfer station 24: Cleaning station 26: Charging device 28: Exposure device 30: Image light 32: Passage 34: Development device 36: Transfer device 38: Sheet 40: Cleaning device 42: Housing 44: opening, 46: second space, 48: developing roller, 50: developing gap, 52: second space, 54: transport roller, 56: supply / recovery gap, 58: magnet body, 60: sleeve, 63 : Restriction plate, 64: restriction gap, 66: developer stirring chamber, 68: front chamber, 70: rear chamber, 72: front screw, 74: rear screw, 76: partition wall, 8 : Restriction area, 88: supply / recovery area, 90: supply area, 92: collection area, 94: discharge area, 96: development area, 98: toner replenishment section, 100: container, 102: opening, 104: replenishment roller, 112: 1st power supply, 114: 2nd power supply, 118: 1st DC power supply, 120: 2nd DC power supply, 160: AC power supply, BP: Blank part.

Claims (5)

A developing device that visualizes an electrostatic latent image on an electrostatic latent image carrier using a developer including at least a toner and a carrier,
A developer containing toner and carrier that are charged with different polarities by frictional contact with each other, and charged particles that are removably held on the surface of the toner and charged with a different polarity from the toner is stored. A developer reservoir,
A first conveying member that conveys the developer from the developer reservoir;
A second conveying member facing the first conveying member via a first region and facing the electrostatic latent image carrier via a second region;
A first electric field is formed between the first conveying member and the second conveying member to move the toner in the developer held by the first conveying member to the second conveying member. and, above SL to form a second electric field between the second conveying member and the electrostatic image bearing member, the second conveying member is the held the toner the electrostatic latent image bearing the is moved to the electrostatic latent image of the body and a electric field forming means that turn into a visible image as a toner image the electrostatic latent image,
The first electric field is an oscillating electric field in which a pulse voltage for moving the toner toward the first conveying member and a pulse voltage for moving the toner toward the second conveying member are alternately applied, and A blank portion in which the application of the pulse voltage is suspended for a predetermined period, and the electric field direction before and after the blank portion has a voltage waveform that is a direction in which the toner is moved to the second conveying member side;
A developing device. The period of the blank part is set in a range of (5/1000 A) seconds to (20/1000 A) seconds with respect to the frequency A (kHz) of the pulse voltage application part of the oscillating electric field, and the blank part has 10 The developing device according to claim 1, wherein the developing device is provided every 50 cycles. It said frequency A (kHz), the developing device according to claim 1 or 2, characterized in that it is set to a range of 5kHz from 1 kHz. It said peak-to-peak voltage of the oscillating electric field, developing device according to any one of claims 1 to 3, characterized in that it is set to a range of 2500 volts 1000 volts. Image forming apparatus characterized by claim 1 comprising any of the developing device 4.