JP4908774B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus that develops and visualizes an electrostatic image formed on an image carrier, and in particular, develops by applying an oscillating voltage between the image carrier and a developer carrier. The present invention relates to an image forming apparatus such as an electrophotographic method or an electrostatic recording method.

  2. Description of the Related Art Conventionally, an electrophotographic image forming apparatus charges a surface of an image carrier electrostatically by exposing the image carrier surface according to an image information signal after the image carrier is uniformly charged by a charging unit. Form an image. This electrostatic image is then visualized as a developer image by the developing means using the developer. Then, the developer image is transferred onto a recording material, or after being transferred to an intermediate transfer member, and then transferred onto the recording material, and further fixed with a fixing device, thereby obtaining a recorded image.

  This will be further described with reference to FIG. 9. FIG. 9 shows a schematic configuration of a conventional image forming apparatus using a non-magnetic one-component developing system.

  The image forming apparatus 200 has an electrophotographic photosensitive member (hereinafter referred to as “photosensitive drum”) 201 that is a regular drum shape as an image carrier, and is rotatable. A primary charger (charging roller) 202 as a charging unit uniformly charges the surface of the rotating photosensitive drum 201. Next, light exposure is performed on the photosensitive drum 201 from the exposure device 203 in accordance with image information input from an external device, and an electrostatic image is formed on the photosensitive drum 201. Next, the electrostatic image on the photosensitive drum 201 is visualized by the developing device 210 with the developer toner T having the same frictional charging polarity as the voltage applied to the charging roller 202 (that is, the charging polarity of the photosensitive drum 201). The image is developed as a toner image. The toner image formed on the photosensitive drum 201 is transferred to the recording material Q in a transfer portion M by a transfer charger (transfer roller) 204 as a transfer unit. Thereafter, the recording material Q is separated from the photosensitive drum 201 and then conveyed to the fixing device 206. Here, the unfixed toner image on the recording material Q is fixed to the recording material Q as a permanent image by heat and pressure. Next, the recording material Q is discharged out of the apparatus main body. The toner T on the photosensitive drum 201 remaining without being transferred by the transfer roller 204 is removed by a cleaning device 205 as a cleaning unit, and the photosensitive drum 201 is subjected to the next image forming process.

  The developing device 210 includes, as a developer, for example, a negatively chargeable non-magnetic one-component developer (toner) T that is negatively charged and contains a pigment of any one of yellow, magenta, cyan, and black. . The developing device 210 includes a developer stirring member (stirring member) 214 in the container 216. In the illustrated example, in the container 216, two plate-like first stirring members 214A and two second stirring members 214B are provided. The first and second agitating members 214A and 214B rotate in the direction of the arrow in the drawing, so that the toner T in the container 216 is conveyed to the developing roller 211 as a developer carrier.

  In the non-magnetic one-component development method, since toner cannot be supplied to the developing roller 211 by magnetic force, the developing roller 211 is made of an elastic material such as an elastic foam (urethane sponge or the like) as a developer supply member. The developer supply and stripping roller (hereinafter referred to as “developer supply roller”) 212 is in contact.

  A regulating blade 213 is in contact with the developing roller 211 as a developer amount regulating member, regulates the toner T on the developing roller 211 to form a thin toner layer, and develops a developing region (the photosensitive drum 201 and the developing roller 211). At the same time as defining the amount of toner conveyed to N), the toner T is charged.

  The developing roller 211 is disposed opposite to the surface of the photosensitive drum 201 in the developing region N with a predetermined interval (hereinafter referred to as “SD gap”). At least during the development process, a predetermined developing bias is applied to the developing roller 211, whereby an oscillating electric field is formed between the photosensitive drum 201 and the developing roller 211.

  In the developing device 210 having such a configuration, the toner T adhered to the surface of the developing roller 211 and transported to the developing region N with a desired charge amount and a desired layer thickness is transferred to the developing roller 211 and the photosensitive drum by the vibration electric field. Reciprocating with 201. Accordingly, the toner T is transferred from the developing roller 211 onto the photosensitive drum 201 in accordance with the electrostatic image formed on the surface of the photosensitive drum 201, and the electrostatic image is visualized as a toner image. Here, the developing device 210 forms a toner image by reversal development in which the toner T having the same polarity as the charged polarity of the photosensitive drum 201 is transferred to a portion where the potential is attenuated by exposure on the photosensitive drum 201.

  However, in the conventional technique as described above, an image defect called “streaky density unevenness” described below may occur. The streaky density unevenness will be described with reference to FIGS. 10 is a model diagram in which the photosensitive drum 201 and the developing roller 211 are observed from the longitudinal direction, and FIG. 11 is a model diagram in which the photosensitive drum 201 and the developing roller 211 are observed from the rotational direction. In the figure, shaded portions on the photosensitive drum 201 and the developing roller 211 represent the toner T. As shown in the figure, when a developing bias is applied between the photosensitive drum and the developing roller, scattered toner is generated in the vicinity of the developing region. At that time, the scattered toner attracts the surrounding toner, and as shown in the figure, it streaks and accumulates on the developing roller on the upstream side in the rotation direction of the developing roller. For this reason, the amount of toner on the developing roller increases where the scattered toner has accumulated. As a result, the image density of the portion where the toner paste amount is increased becomes high. The streaky density unevenness is a phenomenon in which the image density is uneven for the above-described reason.

  The ease of occurrence of the above-described streaky density unevenness varies depending on the positional relationship between the photosensitive drum and the developing roller. 12 and 13 are diagrams showing the positional relationship between the photosensitive drum 201 and the developing roller 211. FIG. As shown in FIG. 12, when the rotation center 201A of the photosensitive drum 201 is positioned higher than the rotation center 211A of the developing roller 211 with respect to the direction of gravity, the above-described scattered toner easily accumulates on the developing roller, causing streaky density unevenness. Is likely to occur. On the other hand, as shown in FIG. 13, when the rotation center 201 </ b> A of the photosensitive drum 201 is positioned lower than the rotation center 211 </ b> A of the developing roller 211 with respect to the gravitational direction, the above-described scattered toner hardly accumulates on the developing roller. , Streaky density unevenness hardly occurs.

  By the way, it has been proposed to change the developing bias in order to reduce scattered toner. For example, in Patent Document 1, with respect to the amplitude component (Vpp) of the developing bias, the non-image forming time is set lower than the image forming time. Vpp at the time of image formation is set to 0V. Further, in the second embodiment, Vpp at the time of non-image formation is set to 300 V in consideration of the rising of high voltage.

In Patent Document 1, an image forming apparatus using a magnetic one-component developer is used, and the rotation center position of the photosensitive drum is lower than the rotation center position of the developing roller with respect to the direction of gravity.
Japanese Patent Application Laid-Open No. 2003-295566

  As described above, Patent Document 1 discloses the problem of toner scattering, but the rotation center position of the photosensitive drum is lower than the rotation center position of the developing roller with respect to the direction of gravity. There was no recognition of the problem of unevenness. That is, streaky density unevenness occurs due to the fact that the scattered toner easily accumulates on the developing roller, which is a particular problem when the rotational center of the photosensitive drum is positioned above the direction of gravity relative to the rotational center of the developing roller. There was no recognition of the problem.

  In particular, when a simple high-voltage power supply is used, an overshoot of the developing bias waveform may occur when the developing bias is switched from non-image formation to image formation, and a leak phenomenon may occur. However, Patent Document 1 has no technical idea of preventing such a leak phenomenon.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus that solves the problems caused by the scattered developer accumulated on the upstream side of the developer carrying member rotation direction with respect to the developing region.

  Another object of the present invention is to provide an image forming apparatus that suppresses the occurrence of streaky density unevenness.

  Another object of the present invention is to provide an image forming apparatus capable of stably forming a high-quality image.

  Another object of the present invention is to provide an image forming apparatus in which a leak phenomenon does not occur even when a simple power source for applying a vibration voltage between an image carrier and a developer carrier is used. .

  Another object of the present invention is to reduce the cost of a power source for applying an oscillating voltage between an image carrier and a developer carrier in an image forming apparatus provided with a plurality of image forming stations. It is an object of the present invention to provide an image forming apparatus that can prevent uneven density and the like and can form a stable image.

According to the present invention, the developer includes an image carrier and a developer carrier that carries and conveys the developer, and an oscillating voltage is applied between the image carrier and the developer carrier. In the image forming apparatus for supplying the developer from the carrier to the image carrier, the rotation center position of the image carrier is arranged upward in the gravity direction with respect to the rotation center position of the developer carrier. And a period during which a vibration voltage is applied between the image carrier and the developer carrier during non-image formation, and the peak-to-peak voltage Vpb of the vibration voltage during this period is equal to the vibration voltage during image formation. When the developer voltage is lower than the peak voltage Vpa and the developer carrier is rotating at the second speed higher than the first speed during the period than when the developer carrier is rotating at the first speed. Is the ratio of Vpb to Vpa greatly to Rukoto the b / Vpa is an image forming apparatus according to claim.

  According to the present invention, the amount of scattered developer can be reduced, and defective images such as streaky density unevenness can be suppressed. In addition, it is possible to provide an image forming apparatus using an inexpensive power source that applies an oscillating voltage between the image carrier and the developer carrier. Furthermore, it is possible to provide an image forming apparatus in which a leak phenomenon does not occur when the vibration voltage applied between the image carrier and the developer carrier is switched.

  The image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.

[Overall Configuration and Operation of Image Forming Apparatus]
First, an overall configuration and operation of an embodiment of an image forming apparatus according to the present invention will be described with reference to FIG. The image forming apparatus 100 of this embodiment is connected to a host such as a personal computer connected to an image forming apparatus main body (hereinafter referred to as “apparatus main body”) 100A, or connected to the apparatus main body. In response to an image information signal from an external device such as an original reading device to be converted into an image, a four-color full-color image such as a recording material, for example, a recording paper, an OHP sheet, or a cloth is formed using an electrophotographic method. It is a laser beam printer that can.

  The image forming apparatus 100 has a photosensitive drum 1 as an image carrier that can rotate. A primary charger (charging roller) 2 as a charging unit uniformly charges the surface of the rotating photosensitive drum 1. A predetermined charging bias is applied to the charging roller 2 from a charging bias applying power source 21 as a voltage applying unit so that the photosensitive drum 1 has a desired reference potential (dark potential Vd). Next, in response to image information input from an external device, the exposure device (laser scanner in this embodiment) 3 irradiates light onto the photosensitive drum 1 to form an electrostatic image on the photosensitive drum 1.

  Subsequently, the electrostatic latent image on the photosensitive drum 1 is reversely developed by the developing device 10 as a visible image, that is, a toner image, with the developer toner T having the same frictional charging polarity as the voltage applied to the charging roller 2. Is done. The operation of the developing device 10 will be described in detail later.

  On the other hand, in synchronism with the formation of the toner image on the photosensitive drum 1, the recording material supply unit 30 moves from the photosensitive drum 1 to a facing part (transfer unit) M between the transfer charger (transfer roller) 4 as transfer means. It is conveyed. That is, the recording material Q sent from the cassette 31 as the recording material container by the supply roller 32 as the recording material supply means synchronizes the toner image and the image transfer area on the recording material Q by the registration roller 33. The recording material Q is conveyed to the transfer unit M.

  Thus, the toner image formed on the photosensitive drum 1 is transferred to the recording material Q by a transfer charger (transfer roller) 4 as a transfer unit. A transfer bias having a polarity opposite to the normal charging polarity (negative polarity in this embodiment) of the toner T is applied to the transfer roller 4 from a transfer bias power source 41 as a voltage applying unit.

  Thereafter, the recording material Q is separated from the photosensitive drum 1 and then conveyed to the fixing device 6 by the recording material conveying means 8. Here, the unfixed toner image on the recording material Q is fixed to the recording material Q as a permanent image by heat and pressure. Next, the recording material Q is discharged out of the apparatus main body 100A.

  Further, the toner T on the photosensitive drum 1 remaining without being transferred by the transfer charger 4 is removed by a cleaning device 5 having a cleaning blade or the like as a cleaning means, and the photosensitive drum 1 is subjected to the next image forming process. Is done.

[Developer]
Next, the developing device 10 in FIG. 1 will be described in more detail.

  The basic configuration of the developing device 10 in this embodiment is the same as that described above with reference to FIG. In other words, in this embodiment, the developing device 10 adopts a non-magnetic one-component non-contact developing method, and a developing roller 11 as a developer carrying member for carrying a developer in a container (developing device main body) 16 and The developer is supplied to the developing roller 11 and the developer is peeled off from the developing roller 11. The developer supplying roller 12 as a peeling member and the amount of the developer carried on the developing roller 11 are regulated. A regulating blade 13 as a developer amount regulating member, an insulating non-magnetic one-component developer (toner) T as a developer, and a plate-like developer stirring member (stirring member) 14. .

  The container 16 for storing the developer is partially opened at the facing portion between the photosensitive drum 1 and the developing device 10, and is partially exposed outside the container 16 over substantially the entire longitudinal direction of the opening. A developing roller 11 is rotatably provided. In this embodiment, the developing roller 11 rotates so that the surface movement directions are the same in the direction of the arrow in the drawing, that is, in the facing portion (developing region) N between the photosensitive drum 1 and the developing roller 11.

  In this embodiment, the toner T is a negatively chargeable non-magnetic one-component developer (toner) that is negatively charged and contains a pigment of any color of yellow, magenta, cyan, and black. The agitating member 14 rotates in the direction of the arrow in the drawing, so that the toner T in the container 16 is conveyed to the developing roller 11.

  A developer supply roller (developer supply and stripping roller) 12 is in contact with the development roller 11, and in the counter direction at the contact portion (nip portion) with the development roller 11, that is, with the development roller 11 and developer supply. The toner T is supplied onto the developing roller 11 by rotating so that the surface movement direction of the roller 12 is the opposite direction at the contact portion. At the same time, the developer supply roller 12 peels off the toner on the developing roller 11 that has not been developed even after passing through the position opposed to the photosensitive drum 1.

  A partition plate 15 is provided in the container 16, and the height of the partition plate is optimized so that a constant amount of toner is always supplied onto the developer supply roller 12 near the developing roller 11 by the stirring member 14. Has been. The number of the agitating members 14 may be two or more as shown in FIG. 9, and the vicinity of the developing roller 11 (or the developer supply roller 12) from the end of the container 16 according to the configuration of the developing device 10. The number of toners T is not limited as long as the toner T can be transported to the end. Further, the stirring member 14 may be a plate or screw processed into various shapes.

  The developing roller 11 is in contact with a regulating blade 13 as a developer amount regulating member, regulates the toner T on the developing roller 11 to form a thin toner layer, and controls the amount of toner conveyed to the developing region N. At the same time, the toner T is triboelectrically charged. The amount of toner conveyed to the development area N can be determined by the contact pressure or contact length of the regulating blade 13 that contacts the developing roller 11. As the regulating blade 13, a resin regulating portion is adhered or welded on a metal thin plate such as phosphor bronze or stainless steel having a thickness of several hundreds μm. The regulating blade 213 is uniformly formed by the elasticity of the metal thin plate. It is possible to use a chip blade that is in contact with the blade. The contact condition of the regulating blade 213 can be determined by the material, thickness, penetration amount, and set angle of the thin metal plate.

  The developing roller 11 is opposed to the surface of the photosensitive drum 1 at a predetermined interval (SD gap) in the developing region N, and the rotation center position of the photosensitive drum 1 is more in the gravitational direction than the rotation center position of the developing roller 11. On the other hand, it is arranged upward. At least during the development process, a oscillating electric field is formed between the photosensitive drum 201 and the developing roller 11 by applying a predetermined developing bias to the developing roller 11 as will be described in detail later.

  In the developing device 10 having such a configuration, the toner T that adheres to the surface of the developing roller 11 with a desired charge amount and a desired layer thickness and is conveyed to the developing region N is supplied from a developing bias power source 18 as a voltage applying unit. A reciprocating motion is performed between the developing roller 11 and the photosensitive drum 1 by the applied developing bias. Thereby, the toner T is transferred from the developing roller 11 onto the photosensitive drum 1 in accordance with the electrostatic image formed on the surface of the photosensitive drum, and the electrostatic image is visualized as a toner image.

More specifically with reference to FIG. 2, in this embodiment, the developing device 10 employs a non-magnetic one-component non-contact developing method (jumping developing method), and prior to development, the photosensitive drum 1 is a charging roller. 2 is uniformly charged to a predetermined potential (dark potential V _D ) having a predetermined polarity (negative polarity in this embodiment). Then, the potential is attenuated by exposure in the exposure device 3 to form a potential (bright potential V _L ) portion of the image portion of the electrostatic image. A constant gap (SD gap) is provided between the photosensitive drum 1 and the developing roller 11, and development is performed by applying an oscillating voltage in which a DC component is superimposed on an AC voltage as a developing bias to the developing roller. The developing roller 11 has a normal charging polarity (negative polarity in the present embodiment) of the toner T with respect to the light potential V _L , and a direct current that is opposite to the normal charging polarity of the toner T with respect to the dark potential V _D. The component Vdc is superimposed. The development contrast Vcont is formed by the DC component value Vdc and the potential _VL of the image portion of the electrostatic image. Of the oscillating electric field between the photosensitive drum 1 and the developing roller 11 formed in this way, the toner T is transferred from the developing roller 11 to the photosensitive drum by the peak voltage (Vmax) of the toner blow-off side voltage that urges the toner to the photosensitive drum 1. On the other hand, the toner T receives a force to pull back in the direction of the developing roller 11 by the peak voltage (Vmin) of the toner return side voltage that urges the toner to the developing roller 11. By repeating this process, the toner T adheres to the image (bright potential VL) portion of the electrostatic image. That is, an alternating electric field is formed between the photosensitive drum 1 and the developing roller 11 with respect to the dark potential and the light potential of the photosensitive drum 1.

  In this embodiment, a member constituted by applying a photosensitive material (OPC in this embodiment) to the surface of an aluminum base tube having a diameter of 30 mm is used as the photosensitive drum 1. Further, as the developing roller 11, a member obtained by spray-coating a phenol resin solution in which carbon and graphite were dispersed on the surface of a 16-mm aluminum base tube was used. At both ends of the developing roller 11 in the longitudinal (axis) direction, rollers (not shown) are installed as separating members, and abut against the surface of the photosensitive drum 1 so that the surface of the photosensitive drum 1 and the surface of the developing roller 11 Keep the SD gap. In this example, the SD gap was 300 μm. Further, as the developer supply roller 12, a member in which a 4.5 mm thick urethane foam was formed on the outer periphery of a φ5 metal core metal was used. A phosphor bronze plate having a thickness of 0.1 mm was used as the metal thin plate of the regulating blade 13.

  FIG. 3 shows a schematic control block of the image forming apparatus 100 of the present embodiment. The image forming apparatus 100 includes a control unit (CPU) 51 that is a central element of control, and includes a control unit 50 that causes the image forming apparatus 100 to perform a sequence operation using programs and data stored in a ROM 52 and a RAM 53 that are storage units. . The control unit 50 performs overall control of the operation of each part of the image forming apparatus 100 such as the charging roller 2, the exposure device 3, the developing device 10, the transfer roller 4, the fixing device 6, the recording material supply unit 30, and the power supply.

  An image processing unit 60 is connected to the control unit 50, and the image processing unit 60 receives an image signal from a host device such as a personal computer or a document reading device connected to the apparatus main body 100A in a communicable manner. In addition, a signal related to image formation is transmitted to the control unit 50. The control unit 50 controls the operation of each unit of the image forming apparatus 100 according to the image forming signal. Further, the image forming apparatus main body 100A is provided with an operation unit 70 including an input unit such as a display unit and a key, and is connected to the CPU 51 of the control unit 50.

[Development bias]
Next, the developing bias applied between the photosensitive drum 1 and the developing roller 11 used in this embodiment will be described with reference to FIG. FIG. 4A shows a development bias waveform during image formation (development), and FIG. 4B shows a model diagram of the development bias waveform during non-image formation (non-development). Here, the time of image formation is when the electrostatic latent image forming area is at the development position. In other words, an area where an electrostatic latent image can be formed on the photosensitive drum 1 with respect to arbitrary image information is developed. When it is in position. On the other hand, the time of non-image formation is when an area other than an area where an electrostatic latent image can be formed on the photosensitive drum 1 is at the development position. In this embodiment, there is a period during which an oscillating voltage is applied between the photosensitive drum 1 and the developing roller 11 during non-image formation. In the photosensitive drum 1Vmax, it is −1200V at the time of image formation and −840V at the time of non-image formation. Vmin is +600 V for both image formation and non-image formation. In other words, the peak-to-peak voltage of the developing bias oscillation voltage is set smaller during non-image formation than during image formation. Further, the absolute value of Vmax is set smaller during non-image formation than during image formation.

[Optimum value of Vmax during non-image formation]
Next, the Vmax value during non-image formation will be described. As described in [Background Art], streaky density unevenness occurs when scattered toner generated in the developing region accumulates on the upstream portion in the rotation direction of the developing roller. In order to reduce this, it is effective to make the absolute value of Vmax of the developing bias, which is the electric field for toner removal, as small as possible to prevent the toner on the developing roller from flying to the photosensitive drum. However, the larger the potential difference between the Vmax value during image formation and the Vmax value during non-image formation, such as setting the Vmax value to 0 V, the greater the amount of overshoot of the high-voltage power supply that occurs during bias switching. FIG. 5 shows an overshoot model diagram. A solid line in the figure is a bias waveform when an overshoot occurs, and a broken line is a set waveform. Due to this overshoot, the actual maximum peak-to-peak voltage value (Vpp) becomes larger than the preset peak-to-peak voltage Vpp value, which causes problems such as a leak phenomenon.

  According to the experiments by the present inventors, when the peak-to-peak voltage Vpp value at the time of image formation is Vpa and the peak-to-peak voltage Vpp value at the time of non-image formation is Vpb, streaky density unevenness It was found that the effect is obtained. Further, by setting Vpb to Vpa × 0.9 or less (90% or less), it is possible to reduce the above-described streak-like density unevenness and to an extent that does not cause a problem on an image. On the other hand, if the AC component at the time of non-image formation is turned off and Vpb is set to 0V, an overshoot occurs. Therefore, the AC component at the time of non-image formation is preferably turned on, and further, Vpb is set to Vpa × 0.7 or more It has been found that by maintaining (70% or more), a leak phenomenon due to overshoot can be prevented even in a low-cost high-voltage power supply using a simple high-voltage circuit.

  In this embodiment, as described above, the peak-to-peak voltage Vpp at the time of image formation is 1800 V, and the peak-to-peak voltage Vpp at the time of non-image formation is 1440 V = Vpa × 0.8.

  It is not always necessary to apply the oscillating voltage between the photosensitive drum and the developing roller during non-image formation. It is not always necessary to apply the vibration voltage during non-image formation. good. For example, the vibration voltage may be applied only during non-image formation immediately before image formation. Further, the peak-to-peak voltage of the oscillating voltage may be gradually increased in a plurality of steps from the time of non-image formation to the time of image formation. As described above, a period in which an oscillating voltage is applied between the photosensitive drum 1 and the developing roller 11 during non-image formation is provided, and the peak-to-peak voltage of the oscillating voltage during this period is greater than the peak-to-peak voltage of the oscillating voltage during image formation. May be set to be smaller.

  Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of the present embodiment are substantially the same as those of the first embodiment. Accordingly, elements having substantially the same or corresponding configurations and functions as those of the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted here.

  In the second embodiment, Vmax during image formation is −1200 V, Vmin is +600 V, Vmax during non-image formation is −1020 V, and Vmin is +420 V.

  The basic configuration and operation of the image forming apparatus of the present embodiment are substantially the same as those of the first embodiment. Accordingly, elements having substantially the same or corresponding configurations and functions as those of the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted here.

  In the third embodiment, Vmax during image formation is −1200 V and Vmin is +600 V, while Vmin during non-image formation is +240, and V and Vmax are equal to −1200 V during image formation.

  That is, in both Examples 2 and 3, the peak-to-peak voltage of the oscillating voltage applied between the photosensitive drum and the developing roller is smaller during non-image formation than during image formation. In Example 2, the absolute value of Vmax is The value is smaller in non-image formation than in image formation.

  Below, the effect of above-mentioned Examples 1-3 is demonstrated using a comparative example.

(Comparative Example 1)
The basic configuration and operation of the image forming apparatus of the comparative example are substantially the same as those of the first embodiment. Accordingly, elements having substantially the same or corresponding configurations and functions as those of the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted here.

  In Comparative Example 1, a developing bias having the same value as the peak-to-peak voltage value of the developing bias oscillation voltage was applied during image formation and during non-image formation.

(Comparative Example 2)
The basic configuration and operation of the image forming apparatus of the comparative example are substantially the same as those of the first embodiment. Accordingly, elements having substantially the same or corresponding configurations and functions as those of the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted here.

  In Comparative Example 2, the voltage value between the oscillating voltage peaks of the developing bias is 1800 Vpp which is the same during image formation and non-image formation, but the DC component Vdc during non-image formation is lower than that during image formation. It is a configuration. Vdc at the time of non-image formation was set to −120 V, whereas Vdc at the time of image formation was −300 V.

(Comparative Example 3)
The basic configuration and operation of the image forming apparatus of the comparative example are substantially the same as those of the first embodiment. Accordingly, elements having substantially the same or corresponding configurations and functions as those of the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted here.

  In Comparative Example 3, the peak-to-peak voltage value of the developing bias oscillation voltage is equal to 1800 Vpp during image formation and non-image formation, but the DC component Vdc during non-image formation is higher than that during image formation. It is a configuration. Vdc at the time of non-image formation was set to −480 V, whereas Vdc at the time of image formation was −300 V.

  The occurrence state of streaky density unevenness in Examples 1 to 3 and Comparative Examples 1 to 3 is shown in Table 1 below.

  The level of occurrence of streaky density unevenness was evaluated by visual evaluation in a halftone image and a solid black image.

  O in the table indicates that there is no occurrence, OΔ is a level at which it can be judged that it is slightly visible but does not cause an image problem, Δ is a level at which it can be visually observed and causes an image problem, and X is very bad.

  As can be seen from Table 1, in Examples 1 to 3, it is ◯ and ◯ Δ, and streaky density unevenness can be improved. In particular, among the AC components of the developing bias, the configuration of the first embodiment in which the peak voltage Vmax that urges the toner in the direction from the developing roller toward the photosensitive drum is the most effective, and is indicated by “◎”. On the other hand, in Comparative Examples 1 to 3, streaky density unevenness is reduced, but at a level that causes an image problem.

  Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of the present embodiment are substantially the same as those of the first embodiment. Accordingly, elements having substantially the same or corresponding configurations and functions as those of the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted here.

  In the second embodiment, the image forming apparatus includes a temperature / humidity sensor that detects the use atmosphere environment of the apparatus, and changes the Vmax value during non-image formation depending on the use environment. The smaller the absolute value of Vmax, the smaller is the peak-to-peak voltage during non-image formation. According to the study by the present inventors, a phenomenon has been found in which the amount of scattered toner increases in a high temperature and high humidity usage environment, and the amount of scattered toner decreases as the temperature decreases and the humidity decreases. The relationship between the peak-to-peak voltage Vpp (Vpb) at the time of non-image formation and the peak-to-peak voltage Vpp (Vpa) at the time of image formation is shown in Table 2 below according to the use environment.

  As described above, by changing the Vpb value according to the use environment, it is possible to reliably prevent the stripe-like density unevenness that changes depending on the use environment.

  Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of the present embodiment are substantially the same as those of the first embodiment. Accordingly, elements having substantially the same or corresponding configurations and functions as those of the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted here.

  In the fifth embodiment, the image forming apparatus capable of switching the surface rotation speed (circumferential speed) of the photosensitive drum changes the Vmax value during non-image formation. The smaller the absolute value of Vmax, the smaller is the peak-to-peak voltage during non-image formation. According to the study by the present inventors, the level of streaky density unevenness changed depending on the surface rotation speed of the photosensitive drum (generally called process speed). Table 3 below shows the relationship between the process speed, Vpp (Vpb) during non-image formation, and Vpp (Vpa) during image formation in this example.

  In the image forming apparatus in this embodiment, the surface rotation speed of the developing roller is 150% of the surface rotation speed of the photosensitive drum. That is, as the process speed increases, the developing roller also rotates faster. As the process speed increases, the amount of scattered toner increases, but the rotation of the developing roller also increases, so the scattered toner rotates before it accumulates upstream, so the level of streak density unevenness increases as the process speed increases. Gave good results. For this reason, the configuration of this embodiment can surely prevent streaky density unevenness that varies depending on the process speed.

[Overall Configuration and Operation of Image Forming Apparatus]
With reference to FIG. 6, the overall configuration and operation of another embodiment of the image forming apparatus according to the present invention will be described. The image forming apparatus 110 according to the present exemplary embodiment is a tandem color image forming apparatus in which four developing devices of the image forming apparatus 100 according to the first exemplary embodiment are vertically arranged, and is substantially the same as that according to the first exemplary embodiment. Alternatively, elements having corresponding configurations and functions are denoted by the same reference numerals, and detailed description thereof is omitted here.

  In the image forming apparatus of Embodiment 6, magenta M is used for the first image forming station (referred to as 1st), cyan C is used for the second image forming station (2st), and yellow Y is used for the third image forming station (3st). The fourth image forming station (4st) is provided with black K toner T. Then, the transfer material is sequentially conveyed to the image carrier of each image forming station, and the toner image formed at each image forming station is sequentially superimposed and transferred onto the transfer material to obtain a color image. . Each image forming station has the same configuration except for the color of the accommodated toner, and the reference numerals are omitted in FIG. 6 except for the fourth black image forming station.

[Development bias timing]
In this apparatus, in order to simplify the developing bias high-voltage power supply 111 that applies an oscillating voltage between the photosensitive drum 1 and the developing roller 11, the developing bias Vmin for each color in each image forming station is shared. And Vmin applying means 113 for applying the same value, and Vmax applying means 112 for taking an arbitrary value for each color with respect to Vmax. By using the development bias high voltage power supply 111, the cost of the power supply of the image forming apparatus 110 can be reduced.

  The developing bias for each color is applied simultaneously by the developing bias power supply 111, but the Vmax (Va) value of the oscillating voltage at the time of image formation of each developing device is −1200 V, and the oscillating voltage Vmax (at the time of non-image formation). Vb) The value is -840V. FIG. 7 shows a timing chart of the developing bias of this embodiment. As shown in FIG. 7, in each image forming station, the switching timings of ON and OFF of the oscillation voltage Vmax and Vmin as the developing bias are the same, and from the start of the 1st image formation to the end of the 4st image formation Vmax and Vmin are continuously turned on. However, the application timing of the Va value and the Vb value while Vmax is ON is different for each image forming station. For example, in 1st, Vmax is a Va value from the start of 1st image formation to the end of 1st image formation, and Vb from the end of 1st image formation to the end of 4st image formation. Value. As described above, Vmax is the Va value in any image forming station when the image forming station itself forms an image, and the image forming station itself does not form an image and the other. In the image forming station, the Vb value is used when an image is formed. That is, when viewed at an image forming station, the absolute value of Vmax (= Vb) during non-image formation is set to be smaller than the absolute value of Vmax (= Va) during image formation. As described above, the peak-to-peak voltage of the oscillating voltage at the time of non-image formation is set smaller than that at the time of image formation.

  As a result, in a color image forming apparatus using a high-voltage power supply whose cost has been reduced, streaky density unevenness of each color can be prevented, and a stable image can be provided without causing a leak phenomenon.

  In FIG. 7, the start of the 1st image formation and the switching from OFF to ON of Vmax and Vmin are made simultaneous. However, the switching from OFF to ON of Vmax and Vmin is performed at the time of 1st image formation. It is also possible to be a little before the start of. In this case, it is preferable that Vmax is set to Vb, which is a small value, from the time when Vmax is switched from OFF to ON at the first time until the start of image formation. Similarly, in FIG. 7, Vmax and Vmin may be switched from ON to OFF slightly after the end of the 4st image formation.

  The basic configuration and operation of the image forming apparatus of this embodiment are substantially the same as those of the embodiment 6 (FIG. 6). Accordingly, elements having substantially the same or corresponding configurations and functions as those of the sixth embodiment are denoted by the same reference numerals, and detailed description thereof is omitted here.

  In the seventh embodiment, the Vmax (Vb) value at the time of non-image formation of only the image forming station including 4st black toner is set to −840V. FIG. 8 shows a timing chart of the developing bias of this embodiment.

  As shown in FIG. 8, in each image forming station, the switching timings of the vibration voltages Vmax and Vmin are ON and OFF are the same, and continue from the start of the 1st image formation to the end of the 4st image formation. Vmax and Vmin are turned ON. However, only the 4st black image forming station switches between the Va value and the Vb value while Vmax is ON. That is, in the 4st black image forming station, Vmax is the value of Vb from the start of the 1st image formation to the start of the 4st image formation, and from the start of the 4st image formation to the 4st image formation time. The value of Va was used until the end of.

  In the configuration of this embodiment, the application of the developing bias of the developing devices of all the image forming stations is started at the same time (in FIG. 8, at the same time as the start of 1st image formation), so that actual development is performed at 4st. The non-image forming time in which the developing bias is applied before the start is the longest. That is, the non-image forming time is a time from the start of the 1st image formation to the start of the 4st image formation. As the non-image forming period to which the developing bias is applied becomes longer, that is, the downstream side of the image forming station, the stripe density unevenness of the image is more likely to occur. Therefore, as described above, the downstream image forming station (for example, 4st) Therefore, it is effective to set the absolute value of Vmax (Vb) at the time of non-image formation small (−840 V). Further, according to the study by the present inventors, the black toner has a smaller charge amount per unit weight than other color toners, and scattered toner is likely to be generated. Therefore, the streaky density unevenness of the 4st black image is most noticeable. Therefore, in this embodiment, only in the 4st developing device, the absolute value of Vmax (= Vb) at the time of non-image formation is set smaller than the absolute value of Vmax (= Va) at the time of image formation. As a result, streaky density unevenness can be reduced in both full color and mono color.

  Of course, as described in the seventh embodiment, it is also possible to switch Vmax and Vmin from OFF to ON slightly before the start of the 1st image formation, and slightly after the end of the 4st image formation. Later, Vmax and Vmin may be switched from ON to OFF.

  As described above, at least one of the plurality of image forming stations includes a period in which the oscillating voltage is applied between the photosensitive drum 1 and the developing roller 11 during non-image formation, and the peak of the oscillating voltage in the period is provided. The inter-voltage may be set to be smaller than the peak-to-peak voltage of the vibration voltage at the time of image formation.

  In the sixth and seventh embodiments, an example in which a full-color image is formed with four image forming stations is shown. However, the number of image forming stations is not limited thereto. An image may be formed.

1 is a schematic configuration diagram of an embodiment of an image forming apparatus according to the present invention. It is explanatory drawing for demonstrating the oscillating electric field formed between a photosensitive drum and a developing roller. 1 is a schematic control block diagram of an embodiment of an image forming apparatus according to the present invention. It is a model figure of the bias waveform of this invention. It is a model figure of an overshoot waveform. 1 is a schematic configuration diagram of an embodiment of an image forming apparatus according to the present invention. It is a figure which shows the timing chart of this invention. It is a figure which shows the timing chart of this invention. It is a principal part schematic block diagram of an example of the image forming apparatus for demonstrating the conventional image forming apparatus. It is a figure explaining a stripe-like density nonuniformity. It is a figure explaining a stripe-like density nonuniformity. It is a figure explaining a stripe-like density nonuniformity. It is a figure explaining a stripe-like density nonuniformity.

Explanation of symbols

1 Photosensitive drum (image carrier, electrophotographic photosensitive member)
2 Charging roller (primary charger, charging means)
3 Laser scanner (exposure device, exposure means)
4 Transfer roller (transfer charger, transfer means)
5 Cleaning device (cleaning means)
8 Photosensitive drum speed changing means 10 Developing device (developing means)
11 Developing roller 18 Developing bias power supply (voltage applying means)
19 Development bias changing means 51 CPU (control means)
T Toner (Developer)
DESCRIPTION OF SYMBOLS 100 Image forming apparatus 110 Image forming apparatus 111 Developing bias power supply (voltage application means)
112 Vmax value applying means 113 Vmin value applying means

Claims (7)

An image carrier and a developer carrier that carries and conveys the developer, and applying an oscillating voltage between the image carrier and the developer carrier to apply the image carrier to the image carrier. In an image forming apparatus for supplying a developer to a body,
The rotation center position of the image carrier is disposed above the rotation center position of the developer carrier in the direction of gravity,
A period during which a vibration voltage is applied between the image carrier and the developer carrier during non-image formation, and the peak-to-peak voltage Vpb of the vibration voltage during this period is between the peaks of the vibration voltage during image formation. Less than the voltage Vpa ,
In the period, the ratio of the Vpb to the Vpa is greater when the developer carrier is rotating at a second speed higher than the first speed than when the developer carrier is rotating at the first speed. an image forming apparatus comprising large to Rukoto the Vpb / Vpa is.   The absolute value of the peak voltage that urges the developer in the direction from the developer carrier to the image carrier among the oscillating voltages of the period is the developer voltage from the developer carrier among the oscillating voltages during image formation. The image forming apparatus according to claim 1, wherein the image forming apparatus is smaller than an absolute value of a peak voltage biased in a direction toward the image carrier.   3. The image forming apparatus according to claim 1, wherein the peak-to-peak voltage of the oscillating voltage in the period is 70% to 90% of the peak-to-peak voltage of the oscillating voltage during image formation. In the period, than in the first use environment, the person in the case of the second use environment of low temperature and low humidity than the first environment of use, to increase the Vpb / Vpa is the percentage of the Vpb to said Vpa The image forming apparatus according to claim 1, wherein: The image forming apparatus according to claim 1, wherein providing a predetermined gap between the developer carrying member and said image bearing member. The developer can be any of the image forming apparatus according to claim 1 to 5, characterized in that a non-magnetic one-component developer. The image forming apparatus according to claim 1, wherein the image forming apparatus includes a plurality of the developer carriers to form a color image.

Images