JP2009168893A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of preventing voids in midsection at a transfer section by eliminating excessive toner deposited on a line part while securing proper density. <P>SOLUTION: The image forming apparatus includes: a conveyance roller 54 which conveys a two-component developer consisting of toner and a carrier whole holding it on its outer peripheral part; a developing roller 48 opposed to the conveyance roller 54 with a supply recovery area 88 interposed and opposed to a photoreceptor 12 with a development area 96 interposed; a first power supply 110 which forms a first electric field between the conveyance roller 54 and developing roller 48 to move toner to the developing roller 48; a second power supply 112 which forms a vibrating electric field between the developing roller 48 and photoreceptor 12 to move the toner from the developing roller 48 to an electrostatic latent image to form a toner image; a detection sensor 116 which detects the density of the developed toner image; and a control unit 114 which controls a conveyance bias and a development bias based upon detection results to carry out saturation development while securing proper density. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an electrophotographic image forming apparatus such as a copying machine, a printer, a facsimile, or a complex machine of these.

  2. Description of the Related Art Conventionally, in an image forming apparatus using an electrophotographic method, as a developing method of an electrostatic latent image formed on an image carrier or a photoreceptor, a one-component developing method using only toner as a developer, a toner and a carrier A two-component development system using a developer containing

  Further, for the purpose of improving the image quality required for colorization in recent image forming apparatuses, in both the one-component and two-component development systems, the developing electric field is applied to the development area between the developing roller and the photosensitive member, thereby the development. Generally, the toner is reciprocated in the region. In this case, due to the action of the oscillating electric field, most of the toner conveyed by the developing roller in the development phase moves toward the electrostatic latent image on the photoconductor, and some toner from the photoconductor in the subsequent recovery phase. Is pulled back toward the developing roller.

  Regarding the behavior of the toner in this developing area, since there is a gap between the developing roller and the photoconductor, the force that the toner moved to the edge portion of the electrostatic latent image on the photoconductor is restrained on the photoconductor is It varies greatly depending on the shape of the electrostatic latent image. In other words, a line latent image (or dot latent image) having a predetermined width compared to a solid latent image (or solid latent image) has a so-called edge effect. A binding force to the photosensitive member acts on the toner present at the edge portion of the image. As a result, the toner adhesion height in the toner image formed on the photoreceptor after the development process is higher in the line portion (or dot portion) than in the solid portion.

  As an example, the logarithmic graph of FIG. 4 shows the relationship between the data line width and the height of the toner image developed on the photosensitive member when one-component AC development is performed with a development gap of 300 μm. It can be seen that the toner image height on the photosensitive member is the highest at the dot line (line width of about 200 μm). When the process proceeds to the transfer step in this state, the toner image is compressed by the pressure received at the transfer portion and the cohesive force between the toner particles is strengthened. As a result, as shown in FIG. Image defects known as so-called “missing” remain on the surface. This hollow-out phenomenon is particularly conspicuous in a configuration in which the intermediate transfer belt is pressed against the photoconductor with its tension, or the intermediate transfer belt or paper is pressed against the photoconductor by a transfer roller.

As a measure for eliminating such a defect, a minimum amount of toner necessary for securing the toner density is conveyed to the development region so that excessive toner movement does not occur even in the development phase with respect to the line latent image. To do. In the case of a commonly used toner having a particle diameter of 6 to 10 μm, an appropriate density can be secured by adhering a toner amount of 5 to 8 g / m ² on the paper, but such an appropriate density is secured. If the ratio of the developing roller peripheral speed to the photosensitive member peripheral speed is A and the minimum toner amount necessary for securing the density is M, the toner transport amount on the developing roller is controlled to be M ÷ A. good.

  However, when such a toner conveyance amount is set in the two-component developing device, the toner at the base of the magnetic brush may vary depending on whether it is used for development due to various error factors or not. As a result, the image has a low image quality with a lot of unevenness and is not practical. Therefore, it is unavoidable to take a setting for conveying a large amount of toner to the development area as compared with the minimum necessary toner conveyance amount, and the toner height at the line portion (or dot portion) becomes high as described above. This is a cause of the occurrence of voids in the transfer portion.

  On the other hand, in the case of a one-component developing device, it is possible to set the minimum toner conveyance amount necessary for ensuring the appropriate density as described above, depending on the regulation conditions by the thin layer regulating unit that forms the toner layer on the developing roller. is there. However, there is a problem peculiar to the thin layer regulation that the toner transport amount changes more than desired or the streaky noise is generated in the toner thin layer on the developing roller by slightly changing the regulation condition. In the forming apparatus, it is extremely difficult to always maintain the same toner conveyance amount throughout printing. For this reason, as in the case of the two-component developing device, in practice, it is necessary to take a setting for conveying a large amount of toner to the developing area as compared with the minimum necessary toner conveying amount. It was.

  Patent Document 1 includes, as Embodiment 3, a magnetic brush roller that rotates while holding a developer containing toner and a carrier in a brush shape, and a developing roller that receives only toner from the magnetic brush roller. A so-called combination of two-component development and one-component development, in which only toner is carried in a thin layer and rotated to convey the toner to the development area and be used for developing the electrostatic latent image on the photosensitive drum. Although a hybrid developing device is disclosed, in this developing device, saturation development at a low developing potential is possible by reducing the toner charge amount and increasing the developing ability, thereby lowering the charging potential of the photosensitive drum. The purpose is to extend the life of the photoconductor drum, and to reduce image density reduction and uneven image density due to fluctuations in development potential. That. In the developing device of Patent Document 1, a DC voltage is used as a developing bias. However, since the DC voltage does not have a recovery phase, excessive development is not performed in the first place, and the height of the toner image attached to the line latent image. There is no technical problem in itself.

JP 2002-341633 A

  Therefore, the present invention eliminates excessive toner adhesion to the line portion (or dot portion) while ensuring an appropriate density without hindering high-quality development by applying an oscillating electric field to the development region. It is an object of the present invention to provide an image forming apparatus that can prevent a void in a transfer portion.

In order to solve the above problems, an image forming apparatus of the present invention includes a toner and a carrier, and the toner is charged to the first polarity by mutual frictional contact, and the carrier is different from the first polarity. A developer charged to a polarity of
A first transport member that is rotationally driven;
A second conveying member facing the first conveying member via a first region and facing the image carrier via a second region;
A first electric field is formed between the first conveying member and the second conveying member, and the toner in the developer held by the first conveying member is applied to the second conveying member. A first electric field forming section to be moved;
A second oscillating electric field is formed between the second conveying member and the image carrier, and the toner held by the second conveying member is turned into an electrostatic latent image on the image carrier. A second electric field forming unit that forms a toner image by moving and developing;
A detector for detecting the density of the developed toner image;
A control unit that controls the first electric field and the second oscillating electric field so that saturation development is performed while ensuring an appropriate density based on a detection result of the detection unit. To do.

  In the image forming method of the present invention, the first developer that transports the developer including the toner charged to the first polarity by mutual frictional contact and the carrier charged to the second polarity different from the first polarity. The toner is moved to the second conveying member by the action of the first electric field from the conveying member, and the second conveying member conveys the toner moved from the first conveying member to the developing region, and the developing In an image forming method, a toner image is formed by moving the toner to the electrostatic latent image and developing the electrostatic latent image formed on the image carrier in the region by the action of a second oscillating electric field. Then, the density of the developed toner image is detected, and based on the detection result, the first electric field and the second oscillating electric field are controlled so that saturation development is performed while ensuring an appropriate density. Also characterized by It is.

  In the control in the image forming apparatus and the image forming method of the present invention, first, the second oscillating electric field is adjusted so that the saturation development is achieved, and then the toner image density in the saturated development state becomes an appropriate density. The first electric field can be adjusted.

  According to the image forming apparatus and the image forming method of the present invention, based on the density detection result of the developed toner image, the first electric field and the second oscillating electric field are set so that saturation development is performed while ensuring an appropriate density. By controlling, the toner conveyance amount by the second conveyance member can be minimized. As a result, it is possible to eliminate excessive toner adhesion to the line portion (or dot portion) while ensuring an appropriate density without hindering high-quality development by applying an oscillating electric field to the development region. In addition, it is possible to prevent a void in the transfer portion.

Embodiments of the present invention will be described below with reference to the accompanying drawings.
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 photoconductor 12 that is an 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 develops the electrostatic latent image with toner to form a toner image. 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.

  Further, a density detection sensor (detection unit) for detecting the density of the toner image formed on the photoconductor 12 on the downstream side of the developing station 20 with respect to the rotation direction of the photoconductor 12 and on both upper sides of the transfer station 22. 116 is arranged to face the photoreceptor 12. As the concentration detection sensor 116, a sensor having a light emitting unit and a light receiving unit integrally can be used. The detection result by the density detection sensor 116 is output to the control unit 114.

  As shown in FIG. 2, the density detection sensor 116 is disposed to face the sheet 38 that has passed through the transfer station 22 and detects the density of the toner image transferred from the photoconductor 12 onto the sheet 38. May be.

  When the image forming apparatus 1 having the above configuration forms an image, the photoconductor 12 rotates in the clockwise direction 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 two-component developer 2 including a non-magnetic toner that is first component particles and a magnetic carrier that is second component particles, and a housing 42 that houses 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 second conveying member is provided in a space 46 formed in the vicinity of the opening 44. The developing roller 48 is a cylindrical member, and is disposed 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.

  The developing roller 48 is composed of a conductive roller made of a metal such as aluminum that has been subjected to a surface treatment. Examples of the surface treatment include polyester resin, polycarbonate resin, acrylic resin, polyethylene resin, polypropylene resin, urethane resin, polyamide resin, polyimide resin, polysulfone resin, polyether ketone resin, vinyl chloride resin, vinyl acetate resin, and silicone resin. A resin coating such as a fluororesin, or a rubber coating such as silicone rubber, urethane rubber, nitrile rubber, natural rubber, or isoprene rubber is used, but is not limited thereto. In addition, a conductive agent may be added to the inside or the surface of the coating. As the conductive agent, an electronic conductive agent or an ionic conductive agent can be used. Examples of the conductive agent include, but are not limited to, carbon black particles such as ketjen black, acetylene black, and furnace black, metal powder, and metal oxide fine particles. Examples of the ionic conductive agent include cationic compounds such as quaternary ammonium salts, amphoteric compounds, and other ionic polymer materials, but are not limited thereto.

  A separate space 52 is formed behind the developing roller 48. In the space 52, a transport roller 54 as a first transport member is arranged 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 that is supported so as to be rotatable around the magnet body 58 in the direction of arrow 80. 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 first power source (first electric field forming unit) 110 for applying a transport bias is electrically connected to the transport roller 54. In this embodiment, a DC voltage is used as the transport bias. However, the present invention is not limited to this, and an oscillating voltage may be used in the same manner as the developing bias described below. On the other hand, a second power source (second electric field forming unit) 112 for applying a developing bias is electrically connected to the developing roller 48. In this embodiment, an oscillating voltage obtained by superimposing an AC component on a DC component is used as the developing bias. A controller 114 is electrically connected to the first and second power sources 110 and 112, respectively. The first power supply 110 is configured to change the voltage value of the transport bias based on a command from the control unit 114. The second power source 112 is configured to change the potential of the DC component, the frequency, amplitude, and duty ratio of the AC component constituting the developing bias based on a command from the control unit 114.

  In this way, the transport bias is applied to the transport roller 54 and the development bias is applied to the developing roller 48, so that the supply and recovery region (first region) 88 between the rollers 54 and 46 has a transport bias and An electric field (first electric field) based on a potential difference from the developing bias is formed. Further, when a developing bias is applied to the developing roller 48, the developing region (second region) 96 where the developing roller 48 faces the photoconductor 12 is vibrated based on a potential difference between the developing bias and the surface of the photoconductor 12. An electric field (second oscillating electric field) is formed.

  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.

[3. Developer)
The developer 2 used in the image forming apparatus 1 of the present embodiment is a non-magnetic toner and frictionally contacts with the non-magnetic toner to charge the toner to a negative polarity (first polarity), while itself has the opposite polarity. It consists of a positively charged magnetic carrier. The charging characteristics of the toner and carrier constituting the two-component developer used in the present invention may be opposite to those described above, that is, the toner is charged positively and the carrier is negatively charged. May be.

  The toner is not particularly limited, and publicly known toners can be used, and a colorant, a charge control material, a release material, and the like are contained in a binder resin, and an external additive Can be added. The toner particle size is preferably about 3 to 15 μm, but is not limited thereto. Such a toner can be manufactured by a publicly known method, for example, using a pulverization method, an emulsion polymerization method, a suspension polymerization method, or the like.

  Moreover, it does not specifically limit as a carrier, The well-known carrier generally used can be used, A binder type carrier, a coat type carrier, etc. can be used. The carrier particle size is preferably 15 to 100 μm, but is not limited thereto.

  The mixing ratio of the toner and the carrier may be adjusted so as to obtain a desired toner charge amount (for example, 10 to 50 μC / g). The toner ratio is 3 to 50% by weight based on the total amount of the toner and the carrier, Preferably, 6 to 30% by weight is suitable.

[4. (Image forming operation)
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. The carrier is considerably larger than the toner. Therefore, the negatively charged toner adheres around the positively charged carrier mainly based on the electrical attraction force of both.

  The charged developer 2 is supplied to the transport roller 54 while 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 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 developer 2 held by the magnetic pole S <b> 1 in the restriction region 86 between the restriction plate 62 and the amount passing through the restriction gap 64 is restricted to a predetermined amount by the restriction plate 62. The developer 2 that has passed through the regulation gap 64 is transported to a supply / recovery area 88 where the developing roller 48 and the transporting roller 54 are opposed, where the magnetic pole N1 is opposed. In the supply / recovery region 88, the toner attached to the carrier is developed by the action of an electric field formed between the developing roller 48 and the sleeve 60, mainly in the upstream supply region 90 with respect to the rotation direction of the sleeve 60. Electrically moved to the roller 48 and supplied. Further, in the supply / recovery area 88, the toner on the developing roller 48 fed back to the supply / recovery area 88 without contributing to the development mainly in the recovery area 92 on the downstream side with respect to the rotation direction of the sleeve 60 is the magnetic pole N1. It is scraped off by a magnetic brush formed along the magnetic field lines and collected in the sleeve 60. The carrier 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. The developer 2 that has passed through the supply / recovery region 88 is held by the magnetic force of the magnet body 58, passes through the opposing portion of the magnetic pole S2 along with the rotation of the sleeve 60, and reaches the discharge region 94 that is the opposing region of the magnetic poles N2 and N3. Then, 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 as the developing roller 48 rotates, and the outer peripheral surface of the photoreceptor 12 is in the developing area 96 where the photoreceptor 12 and the developing roller 48 face each other. The toner image is moved and attached to the electrostatic latent image portion formed on the toner image, thereby developing and forming a toner image. In the image forming apparatus 1 of the present embodiment, the electrostatic latent image portion where the outer peripheral surface of the photoreceptor 12 is given a predetermined negative potential V _H by the charging device 26 and the image light 30 is projected by the exposure device 28. Is attenuated to a predetermined potential V _L, and the non-image portion of the electrostatic latent image on which the image light 30 is not projected by the exposure device 28 substantially maintains the charged 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.

[5. (Toner transport amount control)
Next, the toner conveyance amount control in the control unit 114 of the image forming apparatus 1 will be described with reference to FIG. Here, the toner conveyance amount means the amount of toner that is held on the outer peripheral surface of the developing roller 48 of the developing device 34 and rotated and conveyed. The toner conveyance amount control can be performed at an appropriate timing. For example, when image stabilization control is performed when the image forming apparatus 1 is turned on and started up, a non-image is formed every time a predetermined number of images are formed. It is executed during the forming operation.

  First, the transport bias applied to the transport roller 54 by the first power supply 110 is set to a default value (initial value) (step S1), n = 1 is set, and the second power supply 112 is applied to the developing roller 48. The development bias to be set is set to a default value (initial value) (step S2).

  Then, the image forming apparatus 1 is operated to form a solid image (patch image) on the photoconductor 12, and the density S (n) is detected by the density detection sensor 116 (step S3). The solid image passes through the transfer station 22 and is collected by the cleaning device 40.

  A detection result by the concentration detection sensor 116 is input to the control unit 114. Based on this, the control unit 114 determines whether n = 1 or whether the detected concentration S (n) is higher than the previous detected concentration S (n−1) (step S4). Here, since n is 1 because it is the first density detection, the development bias is increased by 1 step in the direction of increasing the density of the toner image in the next step S5 (for example, the DC component included in the development bias is reduced by 25 volts to the negative side). To change). As a result, the oscillating electric field acting on the developing region works to move the toner on the developing roller 48 toward the photoreceptor 12 with a stronger electrostatic force.

  Then, the process returns to step S3 to form a solid image and detect its density, and determine whether the detected density is equal to or higher than the previously detected density (step S4). In this manner, the saturated development state in which the toner on the developing roller 48 is completely or substantially all (for example, about 95% or more) consumed for forming a solid image while the routines of steps S3, 4, and 5 are repeatedly executed. become. In this case, the determination in step S4 is NO because the concentration does not increase any more. As a result, the development bias for realizing the saturated development at that time is specified.

Next, it is determined whether or not the density S (n) of the solid image formed in the saturated development state is appropriate (step S6). An example of the proper density here is a toner adhesion amount of 5 g / m ² ± 5%. If it is determined that the density S (n) of the solid image is appropriate, the bias adjustment routine ends (step S10), and finally, the transport bias and the development bias that can realize the saturation development while ensuring the appropriate density are finally obtained. It will be set (step S10).

  On the other hand, if it is determined in step S6 that the concentration S (n) is not appropriate, it is subsequently determined whether or not the concentration S (n) is excessive (step S7). If it is determined that the density S (n) is excessive, the transport bias is changed by one step (for example, 25 volts) in the direction of decreasing the toner supply amount from the transport roller 54 to the developing roller 48 (step S8). On the other hand, if it is determined that the density S (n) is not excessive (that is, the density is insufficient), the conveyance bias is increased by one step (for example, 25 volts) in the direction of increasing the toner supply amount from the conveyance roller 54 to the developing roller 48. Only the change is made (step S9). Then, under the condition of the transport bias set in step S8 or S9, the process returns to step S3 again, and steps S3 to S6 are repeatedly executed to specify the saturated development bias.

  By repeating Steps S3 to S9 as described above, a development bias and a transport bias that can realize saturated development while ensuring an appropriate density are finally set (Step S10).

  As described above, according to the image forming apparatus 1 of the present embodiment, the conveyance bias and the development bias are controlled based on the density detection result of the developed toner image so that saturation development is performed while ensuring an appropriate density. Thus, the toner conveyance amount by the developing roller 48 can be minimized. This eliminates excessive toner adhesion to the line portion (or dot portion) of the electrostatic latent image while ensuring an appropriate density without hindering high-quality development by applying an oscillating electric field to the development region. As a result, it is possible to prevent the transfer station 22 from being lost.

  Next, an experimental example performed using the image forming apparatus 1 of the present embodiment will be described. In these experiments, the following toner and carrier were used.

  0.2 parts by weight of the first hydrophobic silica and 0.5 parts by weight of the second hydrophobic silica with respect to 100 parts by weight of the toner base material having a volume average particle diameter of about 6.5 μm prepared by the wet granulation method. And a negatively chargeable toner obtained by subjecting 0.5 parts by weight of hydrophobic titanium oxide to a surface treatment for 3 minutes at a speed of 40 m / s using a Henschel mixer (manufactured by Mitsui Metal Mining Co., Ltd.). Was used. The first hydrophobic silica used here is a silica (#) 130 (manufactured by Nippon Aerosil Co., Ltd.) having a number average primary particle size of 16 nm and surface treated with hexamethyldisilazane (HMDS) as a hydrophobizing agent. It is. Hydrophobic titanium oxide is obtained by surface-treating anatase-type titanium oxide having a number average primary particle size of about 30 nm with isobutyltrimethoxysilane which is a hydrophobizing agent in an aqueous wet process.

  Further, as the carrier, a carrier for bizhub C350 manufactured by Konica Minolta Technologies, Inc., which is a coated carrier in which an acrylic resin coat is formed on carrier core particles made of a magnetic material, has an average particle diameter of about 33 μm.

[First Experimental Example]
The developer used in the first experimental example has a toner ratio of 8%. The toner ratio is the ratio of the total amount of toner and external additives to the total amount of developer (the same applies hereinafter).

In this experimental example, a DC conveyance bias is applied to the conveyance roller 54, and a rectangular wave in which an AC component having a frequency of 2 kHz, an amplitude of 1.8 kV, and a negative duty ratio of 40% is superimposed on the DC component −400V is applied to the developing roller 48. A development bias of. As the developing roller 48, an aluminum roller having an alumite treatment on the surface was used, and the gap 56 at the closest part between the transport roller 54 and the developing roller 48 was set to 0.3 mm. The interval 64 between the regulating member 62 and the transport roller 54 is set to 0.4 mm so that the magnetic brush on the transport roller 54 slides on the developing roller 48. The surface movement direction of the conveying roller 54 and the surface movement direction of the developing roller 48 in the supply / recovery area 88 are opposite to each other. Under such conditions, a developer conveyance amount of 200 g / m ² by the conveyance roller 54 was obtained. Further, the non-image portion potential of the electrostatic latent image formed on the photoconductor 12 is −550 V, the image portion potential is −60 V, and the gap 50 at the closest portion between the photoconductor 12 and the developing roller 48 is set to 0. The peripheral speed of the developing roller 48 was set to 1.2 times the peripheral speed of the photosensitive member 12 of 15 mm.

With such a configuration, the toner transport amount on the developing roller 48 was examined by varying the transport bias to -300 V, -500 V, and -700 V, and found to be 2.5 g / m ² and 4.2 g / respectively. m ² , 6.1 g / m ² was obtained.

  Under each condition of the toner conveyance amount, each electrostatic latent image corresponding to 600 dpi 1, 3, 5, 10, 20, 100 dots line and solid is developed so that the toner image height on the photosensitive member 12 is not contacted. In addition to the measurement using a type optical measuring device, the number of lines in which the hollows occurred in the six types of line images each having 20 line widths finally transferred to the paper was examined. For the solid image, as shown in FIG. 2, the output voltage of the density detection sensor 116 disposed opposite to the paper is monitored, and the density determination is performed by comparing with the voltage assumed at the proper density. The results are shown in Table 1.

As shown in Table 1, when the amount of toner transported on the developing roller 48 is large (in the case of 6.1 g / m ² ), the 3-10 dot line image is higher than the solid image in the toner image height. Is significantly higher, which corresponds to the fact that 3 to 5 omissions have been confirmed. On the other hand, it is considered that the toner image height is low in the 1 dot line image because the potential attenuation of the latent image is not so large, so that the toner movement itself does not easily occur during the developing phase of the oscillating electric field. In this case, the solid image density is within an appropriate range, and the toner moved to the vicinity of the latent image during the development phase of the oscillating electric field is pulled back during the recovery phase and remains on the developing roller 48 as a development residual toner. I understand that.

On the other hand, when the toner conveyance amount was small (in the case of 2.5 g / m ² ), although the excessive development did not occur in the line latent image portion, the density itself was low and it could not be used in practice. On the other hand, when the toner conveyance amount is appropriate (in the case of 4.2 g / m ² ), it is possible to obtain a condition in which excessive development does not occur at any line width while ensuring an appropriate density. did it. At this time, the development residual toner on the developing roller 48 was hardly recognized. In addition, the confirmation of the toner remaining after development is performed by visually checking whether or not the toner remains, and when it remains, the toner is removed from the developing roller, and the weight is measured and converted into a toner conveyance amount.

[Second Experimental Example]
The same experiment as the first experimental example was performed by changing the developing bias. At this time, the toner conveyance amount on the developing roller 48 was 4.2 g / m ² which was an appropriate value in the first experimental example, and the toner image height depending on the line width although an appropriate value can be obtained in terms of density. The conveyance bias to be applied to the conveyance roller 54 is adjusted so as to be two levels of 6.1 g / m ² where the difference has occurred, and at the same time, the potential of the DC component and the frequency of the AC component that are factors of the development bias The experiment was conducted by changing the amplitude and the negative duty ratio. The experimental results are shown in Table 2.

  As shown in Table 2, even if the amount of toner transported on the developing roller 48 is appropriate, in the case of a developing bias condition in which toner remains on the developing roller 48 in a solid image, it depends on the line width. It was found that there was a difference in toner image height. Further, when the amount of toner transported on the developing roller 48 is excessive, even if the toner amount detection determination (that is, toner density detection determination) is appropriate, the toner image height varies depending on the line width regardless of the development bias condition. Was found to have occurred.

  From the above experimental results, by setting the amount of toner conveyed onto the developing roller 48 to an appropriate level and developing bias that enables saturated development so that no toner remains on the developing roller 48 at the solid portion, It was confirmed that a good image with no voids could be obtained by optimizing the image density and suppressing variations in the toner image height due to the line width.

1 is an overall schematic configuration diagram of an image forming apparatus. The figure which shows the modification of arrangement | positioning of a density | concentration detection sensor. 6 is a flowchart of toner conveyance amount control. 6 is a graph showing the relationship between the line width in dots and the toner height on the photoreceptor. An enlarged photograph showing a hollow state of a line image.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus 12 ... Photoconductor (image carrier)
34. Developing device 46. Developing roller (second conveying member)
54 ... Conveying roller (first conveying member)
110... First power source (first electric field forming unit)
112 ... Second power source (second electric field forming unit)
114: Control unit 116: Concentration detection sensor (concentration detection unit)

Claims (4)

A developer comprising a toner and a carrier, wherein the toner is charged to a first polarity by mutual frictional contact and the carrier is charged to a second polarity different from the first polarity;
A first transport member that is rotationally driven;
A second conveying member facing the first conveying member via a first region and facing the image carrier via a second region;
A first electric field is formed between the first conveying member and the second conveying member, and the toner in the developer held by the first conveying member is applied to the second conveying member. A first electric field forming section to be moved;
A second oscillating electric field is formed between the second conveying member and the image carrier, and the toner held by the second conveying member is moved to the electrostatic latent image on the image carrier. A second electric field forming unit that forms a toner image by developing the toner image;
A detector for detecting the density of the developed toner image;
A control unit that controls the first electric field and the second oscillating electric field so that saturation development is performed while ensuring an appropriate density based on the detection result of the detection unit;
An image forming apparatus comprising:   In the control by the control unit, first, the second oscillating electric field is adjusted so that saturated development is achieved, and then the first electric field is adjusted so that the toner image density in the saturated developing state becomes an appropriate density. The image forming apparatus according to claim 1. The action of the first electric field from the first conveying member that conveys the developer including the toner charged to the first polarity by the mutual frictional contact and the carrier charged to the second polarity different from the first polarity. The toner is moved to the second conveying member by the second conveying member, and the second conveying member conveys the toner moved from the first conveying member to the developing area, and is formed on the image carrier in the developing area. An image forming method for forming a toner image by moving the toner to the electrostatic latent image and developing the electrostatic latent image on the electrostatic latent image by the action of a second oscillating electric field,
The density of the developed toner image is detected, and based on the detection result, the first electric field and the second oscillating electric field are controlled so that saturation development is performed while ensuring an appropriate density. An image forming method. In the control, first, the second oscillating electric field is adjusted so that saturation development is achieved, and then the first electric field is adjusted so that the toner image density in the saturated development state becomes an appropriate density. The image forming method according to claim 3.

Images