JP3997022B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus using an electrophotographic system or an electrostatic recording system such as a copying machine or a printer.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, a color image forming apparatus that includes a plurality of developing devices (developing means) that respectively store different color developers (hereinafter referred to as “toners”) and obtains a multicolor image using an electrophotographic method is well known. ing.
[0003]
FIG. 6 shows a color image forming apparatus as described above. Here, as an example, a color image forming apparatus provided with a one-component developing device of a reversal development system using nonmagnetic negatively charged toner in contact development will be described.
[0004]
This color image forming apparatus mainly includes an image carrier (hereinafter referred to as “photosensitive drum”) 101, a charger 102 as a charging unit that uniformly charges the photosensitive drum 101, an exposure unit 103 that provides image information, and a plurality of exposure units 103. The developing unit 104 a, 104 b, 104 c, 104 d is constituted by a rotary developing device 105 and an intermediate transfer member 106.
[0005]
FIG. 7 shows an enlarged view of the rotary developing device 105. The rotary developing device 105 is a system in which one of the four developing devices 104 a to 104 d is fixed at the developing position by the rotation of the support 107. The rotary developing device 105 has a developing opening surface for each of the developing devices 104a to 104d on the same circumference around the support rotating shaft 107a.
[0006]
The developing devices 104a to 104d have developing rollers 110a, 110b, 110c, and 110d, which are rotating developer carriers for conveying the toner to the contact portion with the photosensitive drum 101, and the developing rollers 110a to 110d. Is a so-called elastic developing roller in which an elastic body is formed on a conductive metal core. Further, around the developing rollers 110a to 110d, the application rollers 111a, 111b, 111c, and 111d that rotate in the direction of the arrow for applying the non-magnetic one-component toner onto the developing rollers 110a to 110d, and the toner on the developing rollers 110a to 110d The control blades 112a, 112b, 112c, and 112d for providing a desired charge amount to the toner and for controlling the toner amount are provided, and a developing bias voltage power supply 113 that applies a developing bias voltage to the developing rollers 110a to 110d is provided.
[0007]
Further, the rotary developing device 105 rotates the developing devices 104a to 104d with the pressurizing device 114 only at the time of development to bring the developing rollers 110a to 110d in the developing devices 104a to 104d into contact with the photosensitive drum 101 and rotate. The developing roller 110 in the developing devices 104a to 104d is separated from the photosensitive drum 101 by releasing the pressurizing operation of the pressurizing device 114 while the developing device 105 is rotating or during non-development.
[0008]
In FIG. 6, the photosensitive drum 101 is rotated in the direction of the arrow and uniformly charged by the charger 102, and then an electrostatic latent image is formed on the photosensitive drum 101 by the exposure device 103. This latent image is a latent image corresponding to each color developer in the developing devices 104a to 104d, for example, yellow (Y), magenta (M), cyan (C), and black (Bk).
[0009]
More specifically, first, an electrostatic latent image corresponding to the first color, for example, a yellow developer is formed on the photosensitive drum 101, visualized by the developing device 104 a containing the yellow developer, and then transferred to the intermediate transfer member 106. . Subsequently, after the residual toner on the photosensitive drum 101 is cleaned by a cleaning device 109 such as a blade, an electrostatic latent image corresponding to the second color, for example, magenta developer is formed on the photosensitive drum 101, and the magenta developer is accommodated. After being visualized by the developing device 104b, the image is transferred onto the intermediate transfer member 106 on which the first yellow visible image is transferred.
[0010]
After the above operation is performed a plurality of times, the developer stacked in a plurality of layers on the intermediate transfer member 106 is transferred to the transfer material P and melted and fixed by the fixing device 101.
[0011]
[Problems to be solved by the invention]
However, when the image forming operation was performed with the developing device 105 shown in the conventional example, the following problems were observed. Since the developing units 104a to 104d have the same configuration, the description will be made by omitting the suffix alphabet among the reference numerals of the constituent members.
[0012]
In the conventional example, the DC bias voltage Vdc applied to the developing roller 110 has a temporal change as shown in FIG.
[0013]
More specifically, in order to develop the latent image on the photosensitive drum 101 using the developing device 104 containing toner of a desired color, the developing device driving device is rotated by the developing device driving device at time t81 to develop the latent image. The developing device 110 is placed in contact with the photosensitive drum 101 by placing the developing device 104 at a position facing the photosensitive drum 101 and pressurizing the developing device 104 from the rear by the pressurizing device 114 at time t82. At the same time, at the time t82, the rotational driving of the photosensitive drum 1 and charging by the charger 102 are started.
[0014]
Next, at time t83, the developing bias voltage Vdc is applied to develop the latent image on the photosensitive drum 101 formed by the exposure device 103 as a toner image. After the development, the developing bias voltage Vdc is turned off at time t84, the pressurizing operation of the pressurizing device 114 is released at time t85, the rotation driving of the photosensitive drum 1 and the charging by the charger 102 are stopped, and the developing roller 110 and the photosensitive drum 101 are separated from each other.
[0015]
Here, as a specific numerical value, the developing bias voltage Vdc is −350V. Further, as a latent image potential, a portion that is not exposed by the exposure device 103, that is, a non-image portion potential Vd is set to −700V, and a portion exposed by the exposure device 103, that is, the image portion potential VL is set to −300V.
[0016]
However, during the developing process, immediately after the time t82 when the developing roller 110 in the developing device 104 is brought into contact with the photosensitive drum 101, a part of the toner adhering to the developing roller 110 is van der Waals force or mirror. In some cases, it may adhere to the photosensitive drum 101 due to the image power. Here, if the developing bias voltage Vdc is applied, the toner adhering to the photosensitive drum 101 can be made a visible image according to the latent image, but in FIG. 8, between the time t82 and the time t83. The toner once attached to the photosensitive drum 101 because the developing bias voltage Vdc is not applied cannot be collected by the developing roller 110, and the toner remains attached even in the non-exposed portion.
[0017]
Usually, immediately after time t82, a belt-like margin A corresponding to the front end of the transfer material P (see FIG. 9), that is, a belt-like portion not exposed by the exposure device 3, is developed. The problem of fogging occurs. In this specification, “fogging” means that toner adheres to a portion other than the image area on the photosensitive drum 101.
[0018]
Similarly, between time t84 and time t85, a part of the toner adhering to the photosensitive drum 101 from the developing roller 110 because the developing bias voltage Vdc is not applied is collected by the developing roller 110. The toner remains attached even in the non-exposed area. Therefore, immediately after the time t84, there arises a problem that fog occurs in the strip-shaped blank portion B (see FIG. 9) corresponding to the rear end of the transfer material.
[0019]
Even if the contact / separation operation is performed at a region that is not transferred to the transfer material P, that is, for example, before the transfer to the transfer material P at the time of contact, or after the transfer material P has passed at the time of separation, the contact / separation operation is performed. Sometimes the intermediate transfer member 106 is soiled. As a result, it is difficult to form a high-quality image.
[0020]
Further, since the developing bias voltage for biasing the photosensitive drum 101 to the developing roller 110 is not applied between the time t82 and the time t83 or between the time t84 and the time t85, the developing device is used by using the pressurizing device 114. When the abutment operation and the separation operation of the developing roller 110 and the photosensitive drum 101 in 104 are performed, an impact is generated due to the collision of the photosensitive drum 101 and the developing roller 110 or the meshing of a rotational drive gear, and the like, thereby developing roller 110. The toner adhering to the toner scatters, causing a problem of in-machine contamination.
[0021]
Accordingly, an object of the present invention is to provide an image forming apparatus capable of suppressing fogging phenomenon and preventing toner scattering in a contact development system in which an image carrier and a developer carrier are in contact with each other during development.
[0022]
[Means for Solving the Problems]
  The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention provides an image carrier on which an electrostatic latent image is formed, and a developer carrier that carries a developer, which can contact and separate from the image carrier. A developer carrier that contacts the carrier and develops the electrostatic latent image with a developer.
  At the time of the contact and separation operation of the image carrier and the developer carrier carrying the developer, an electric field that urges the developer in the direction from the image carrier to the developer carrier is formed,A contact operation between the developer carrier and the image carrier is performed after the image carrier starts charging until the image carrier makes one round,The absolute value of the DC voltage applied to the developer carrier before the development and during the contact operation between the developer carrier and the image carrier is applied to the developer carrier during development. An image forming apparatus having an absolute value smaller than an absolute value of a DC voltage.
[0027]
DETAILED DESCRIPTION OF THE INVENTION
The image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.
[0028]
  Reference example1
  First of the present inventionReference example1 will be described with reference to FIGS.
[0029]
  First, the book according to FIG.Reference exampleThe color image forming apparatus will be described. BookReference exampleThe color image forming apparatus is a reversal development system that visualizes toner by adhering it to an image portion of an image carrier, and performs development by bringing a developer carrier carrying negatively charged toner into contact with the image carrier. This is a component image forming apparatus.
[0030]
This color image forming apparatus mainly includes a photosensitive drum 1 as an image carrier, a charger 2 as a charging unit, an exposure unit 3 that provides image information, and four developing units 4a, 4b, 4c, and 4d. Is constituted roughly by a rotary developing device 5 and an intermediate transfer member 6.
[0031]
The rotary developing device 5 is a system in which one of the four developing devices 4 a to 4 d is fixed at the developing position by the rotation of the support 7. The rotary developing device 5 has developing development surfaces of the developing devices 4a to 4d on the same circumference around the support rotating shaft 7a.
[0032]
The developing devices 4a to 4d have developing rollers 10a, 10b, 10c, and 10d, which are rotating developer carriers for transporting toner to the contact portion with the photosensitive drum 1, respectively. Is a so-called elastic developing roller in which an elastic body is formed on a conductive metal core. Further, around the developing rollers 10a to 10d, the application rollers 11a, 11b, 11c, and 11d rotating in the direction of the arrow for applying the nonmagnetic one-component toner onto the developing rollers 10a to 10d, and the toner on the developing rollers 10a to 10d are used. Restriction blades 12a, 12b, 12c, and 12d are provided that provide a desired charge amount and restrict the toner amount.
[0033]
Further, it has a DC high-voltage power supply 13 that applies a developing bias voltage to the developing rollers 10a to 10d, and a control device 14 that controls on / off of the DC high-voltage power supply 13, the value of the developing bias voltage, and temporal changes.
[0034]
In addition, the rotary developing device 5 rotates the developing devices 4a to 4d with the pressurizing device 8 only during development to bring the developing rollers 10a to 10d in the developing devices 4a to 4d into contact with the photosensitive drum 1, thereby rotating the developing devices 4a to 4d. The developing roller 10a to 10d in the developing devices 4a to 4d and the photosensitive drum 1 are separated from each other by releasing the pressing operation of the pressing device 8 while the developing device 5 is rotating or when not developing. Yes.
[0035]
The photosensitive drum 1 is rotated in the direction of the arrow and uniformly charged by the charger 2, and then an electrostatic latent image is formed on the photosensitive drum 1 by the exposure device 3. As the latent image, a latent image corresponding to each color developer in the developing devices 4a to 4d, for example, yellow (Y), magenta (M), cyan (C), and black (Bk) is formed.
[0036]
More specifically, first, an electrostatic latent image corresponding to the first color, for example, a yellow developer is formed on the photosensitive drum 1, visualized by the developing device 4 a containing the yellow developer, and then transferred to the intermediate transfer member 6. .
[0037]
Subsequently, after the residual toner on the photosensitive drum 1 is cleaned by a cleaning device 9 such as a blade, an electrostatic latent image corresponding to the second color, for example, magenta developer is formed on the photosensitive drum 1, and the magenta developer is accommodated. After being visualized by the developing device 4b, it is transferred onto the intermediate transfer body 6 onto which the first yellow visible image is transferred.
[0038]
After performing the above operation a plurality of times, the developer stacked in a plurality of layers on the intermediate transfer body 6 is transferred to the transfer material P and melted and fixed by the fixing device 10.
[0039]
  BookReference exampleIn this case, the non-image potential VD = −700 V, the image portion potential VL = −100 V, the developing bias voltage Vdc applied to the developing roller 1 is −350 V, and the relationship of | VD |> | Vdc |> | VL | Is established.
[0040]
  Where the bookReference example1 will be described in more detail with reference to the timing chart of FIG.
[0041]
First, at time t11, the rotary developing device 5 is rotated, and the developing device 4a corresponding to the latent image of the first color, for example, the developing device 4a containing yellow toner is applied to the photosensitive drum 1 uniformly charged in advance by the charger 2. Stop at the opposite position.
[0042]
At this time, the photosensitive drum 1 and the developing roller 10a in the developing device 4a are sufficiently separated (specifically, for example, about 5 mm), and a part of the toner adhering to the developing roller 10a is a fan. It does not adhere to the photosensitive drum 1 due to the Delwars force or the like.
[0043]
Next, a developing bias voltage Vdc is applied to the developing roller 10a at time t12. At time t13 when this voltage rises, the developing device 4a is pressurized from behind by the pressure device 8, and the developing roller 10a in the developing device 4a is brought into contact with the photosensitive drum 1. Then, from time t13 to time t14, development including blank portions at the leading and trailing edges of the transfer material P, that is, development within a range indicated by C in FIG. 9 is started.
[0044]
By applying the developing bias voltage Vdc to the developing roller 10a before the contact time t13 as described above, development faithful to the latent image can be performed from the contact time t13. In particular, immediately after time t13, a belt-like blank portion (A portion shown in FIG. 9) corresponding to the leading edge of the transfer material P, that is, a non-image portion that is not exposed by the exposure device 3 on the photosensitive drum 1 is developed on the developing roller 10. Abut. Here, even if toner adheres to the photosensitive drum 1 corresponding to the blank portion due to van der Waals force or mirror force acting in the direction from the developing roller 10a toward the photosensitive drum 1, the developing roller is already at time t13. Since a developing bias voltage is applied to 10a, an electric field sufficient to overcome the above-mentioned adhesion force is formed in the direction from the photosensitive drum 1 toward the developing roller 10a, and adheres on the photosensitive drum 1 corresponding to the blank portion. The toner being drawn is pulled back to the developing roller 10a. Therefore, the fog phenomenon in which the normally negatively charged toner adheres to the band-like blank portion does not occur.
[0045]
Next, development is performed immediately before time t14 from time t13, and the pressurizing operation of the pressure device 8 is released at time t14 to separate the developing roller 10a from the photosensitive drum 1. When the separation operation between the photosensitive drum 1 and the developing roller 10a in the developing device 4a is completed and the distance is sufficiently large (specific value is about 5 mm), the developing bias voltage Vdc is turned off at time t15. At the same time, the developing device 4b corresponding to the latent image of the second color, for example, the developing device 4b storing magenta (M) toner is stopped at a position facing the photosensitive drum 1 uniformly charged by the charger 2 in advance. The subsequent operation is the same as that of the yellow developing device 4a.
[0046]
As described above, by turning off the developing bias voltage Vdc after the time t14 when the developing roller 10a is separated from the photosensitive drum 1, the latent image can be faithfully and stably developed until the time t14 when the developing roller 10a is separated. In particular, immediately before t14, a belt-like blank portion (B portion shown in FIG. 9) corresponding to the rear end of the transfer material P, that is, a non-image portion that is not exposed by the exposure device 3 on the photosensitive drum 1 is developed by the developing roller 10. Abutted. Here, similarly to immediately after time t13, even if the toner adheres to the photosensitive drum 1 corresponding to this blank portion due to the adhesion force such as van der Waals force or mirror force, the developing bias voltage is applied to the developing roller 10a until time t15. Is applied, a sufficient electric field is formed to overcome the adhesion force in the direction from the photosensitive drum 1 toward the developing roller 10a, and the toner adhering to the photosensitive drum 1 corresponding to the blank portion becomes the developing roller. It is pulled back to 10a. Therefore, the fog phenomenon in which the normally negatively charged toner adheres to the band-like blank portion does not occur.
[0047]
In addition, when a paper jam or the like occurs in the apparatus and it must be stopped urgently, the photosensitive drum 1 and the developing roller 10 may be separated even during the development. Even in such a case, if the separation operation is performed while the developing bias voltage Vdc is applied to the developing roller 10, contamination of the photosensitive drum 1 is minimized without causing toner to adhere to the non-image portion on the photosensitive drum 1. Can do.
[0048]
Further, even when there is an impact generated by the contact operation and separation operation when the contact operation and separation operation of the photosensitive drum 1 and the development roller 10 are performed, the development bias voltage Vdc is applied to the development roller 10 during the contact operation. By being applied, an electric field is constantly formed in the direction from the photosensitive drum 1 toward the developing roller 10a, so that the toner adhering to the photosensitive drum 1 and the developing roller 10 is collected by the developing roller 10, In-machine contamination can be prevented without toner scattering.
[0049]
Actually, when multi-color development is performed with a development bias voltage having a temporal change as shown in FIG. 1, no fogging phenomenon occurs at the leading and trailing margins of the transfer material, and image formation that is always faithful to the latent image is formed. Was able to do. In addition, it was possible to prevent in-machine contamination without toner scattering in the machine.
[0050]
  As mentioned above, the bookReference exampleIn this image forming apparatus, the DC developing bias voltage is applied before the contact operation between the photosensitive drum and the developing roller, and the DC developing bias voltage is turned off after the separating operation between the photosensitive drum and the developing roller, thereby suppressing the occurrence of the fog phenomenon. In addition, toner scattering can be prevented.
[0051]
  Reference example2
  Next, the second of the present invention.Reference exampleWill be described with reference to FIG.
[0052]
  FirstReference exampleHas described an image forming apparatus in which the direct current developing bias voltage Vdc is applied before the contact operation between the photosensitive drum 1 and the developing rollers 10a to 10d, thereby suppressing the occurrence of fogging immediately after the start of development and preventing toner scattering. However, when a large amount of image information is transmitted to the image processing circuit, image processing takes time, and development may not be started immediately after the DC development bias voltage Vdc is applied. In this case, if the photosensitive drum 1 and the developing rollers 10a to 10d are rotated in contact with each other from the time t13 until the image processing is completed, the photosensitive drum 1 is accelerated and the developing rollers 10a to 10d and the toner are deteriorated. There is a possibility that the life of the photosensitive drum 1 and the developing devices 4a to 4d may be shortened by acceleration.
[0053]
  So bookReference exampleThen, by controlling the timing of the rotational driving and charging operation of the photosensitive drum 1 and the timing at the time of applying the DC developing bias voltage, a color image of contact development that can solve the above problems and prevent the occurrence of fog phenomenon and toner scattering. A forming apparatus is provided.
[0054]
  Figure 3 shows the bookReference example1 is a timing chart for explaining operations related to the development process in FIG.
[0055]
  BookReference exampleIn this case, the potential relationship is such that the non-image potential VD = −700 V, the image portion potential VL = −100 V, the developing bias voltage Vdc = −300 V applied to the developing roller 1 as the latent image potential, and | VD |> | Vdc | > | VL | is established.
[0056]
First, at time t21, the rotation of the rotary developing device 5 and the photosensitive drum 1 and the operation of the charger 2 are started, and the developing device 4a corresponding to the first color latent image, for example, yellow (Y) toner is stored. The developing device 4a is stopped at a position facing the photosensitive drum 1. At this time t22, the photosensitive drum 1 and the developing roller 10a in the developing device 4a are sufficiently separated from each other (specific value is about 5 mm), and a part of the toner adhering to the developing roller 10a is van der. It does not adhere to the photosensitive drum 1 due to the Waals force or the mirroring force.
[0057]
  First hereReference exampleThen, development is started immediately, but if a large amount of image information is transmitted to the image processing circuit at time t20 before time t21, it takes time for image processing, and development cannot be started immediately after time t23. There is. In that case, if the photosensitive drum 1 and the developing roller 10a are rotated in contact from the time t23 to the time t25 when the image processing ends, as described above, the photosensitive drum 1 is scraped or a load is applied to the developing roller 10a. A leak may occur. Further, since the toner passes through the regulating blade 12a many times without being developed on the photosensitive drum 1 and attached to the circumference of the developing roller 10a, the deteriorated toner is increased in the developing device 4a, and the toner life is shortened. The life as the developing device 4a may be shortened.
[0058]
Therefore, when development cannot be started immediately after time t23, in order to avoid these problems, at time t24, the development bias voltage Vdc is turned off and the photosensitive drum 1 is stopped, and at the same time, the photosensitive drum 1 is charged by the charger 2. To stop. At this time, the developing roller 10a remains in contact with the photosensitive drum 1 by the pressure device 8, so that development can be started as soon as image processing is completed.
[0059]
At the same time as the image processing is completed at time t25, the developing bias voltage Vdc is applied to the developing roller 10 again. Shortly thereafter, at time t26, the rotation of the photosensitive drum 1 is started again, and at the same time, charging of the photosensitive drum 1 by the charger 2 is resumed. Since charging is performed in advance when the photosensitive drum 1 is driven at time t21, the surface potential VD1 (for example, VD1 = −695 V) of the photosensitive drum 1 at time t26 may be slightly lower than VD due to dark decay. However, considering the processing speed of the current image processing circuit, the image processing time is about several minutes at the longest, and | VD1 | >> | Vdc | holds true.
[0060]
When the development bias voltage Vdc is applied before the photosensitive drum 1 and the developing roller 10 are in contact with each other and the photosensitive drum 1 is charged in advance, the photosensitive drum 1 is started to rotate. At t26, since an electric field in the direction in which the photosensitive drum 1 is urged to the developing roller 10 is already formed, the toner does not adhere to the non-image portion on the photosensitive drum 1 corresponding to the front end of the transfer material P, and the fog phenomenon occurs. Can be prevented.
[0061]
Then, development is performed from immediately after time t26 to immediately before time t27. By releasing the pressurization operation of the pressure device 8 at time t27, development is performed faithfully and stably with respect to the latent image without fogging phenomenon until time t27 when the pressure device 8 is separated. be able to. Immediately before time t27, the non-image portion on the photosensitive drum 1 corresponding to the rear end of the transfer material P is in contact with the developing roller 10, and at this time, the developing bias voltage is applied to the developing roller 10, and the photosensitive drum Since the electric field in the direction of urging from 1 to the developing roller 10 is formed, the fog phenomenon does not occur at the rear end of the transfer material P. When the separation between the photosensitive drum 1 and the developing roller 10a in the developing device 4a is completed and the distance is sufficiently large, the developing bias voltage Vdc is canceled at time t28.
[0062]
Further, the developing bias voltage Vdc is applied to the developing roller 10 when the contact operation and separation operation of the developing roller 10 are performed at times t23 and t27, and the photosensitive drum 1 is urged to the developing roller 10 during the contact / separation operation. Since the electric field in the direction is formed, it is possible to prevent toner scattering due to impact caused by contact / separation operation or gear meshing.
[0063]
When the multi-color development is actually performed with the development bias voltage having a time change as shown in FIG. 3, the development process can be started immediately after the image processing is completed, and the leading edge and trailing edge of the transfer material P are transferred. Thus, the fogging phenomenon did not occur in the blank area, and it was possible to form an image faithful to the latent image, and to prevent toner contamination in the machine.
[0064]
As described above, in a state where the photosensitive drum and the developing roller are in contact with each other, the fogging phenomenon is suppressed and toner scattering is prevented by rotating and driving the photosensitive drum that is uniformly charged in advance after the development bias voltage is applied. Is possible. Further, the ability to control the start of development without performing the contact / separation operation between the photosensitive drum and the developing roller can easily cope with the length of the image processing time, and can shorten the waiting time from the image processing to the development time. it can.
[0065]
  Example1
  Next, the present inventionExamples ofWill be described with reference to FIGS.
[0066]
  When the photosensitive drum 1 is charged by the charger 2, the firstReference exampleAs described above, the surface potential of the photosensitive drum 1 is generally the non-image potential VD. However, the non-image potential does not always become VD due to the optical characteristics and deterioration of the photosensitive drum 1. Specifically, when the photosensitive drum having a surface potential of 0V is charged by the same charger and when the photosensitive drum having a surface potential of −100V is charged, the surface of the photosensitive drum after charging is charged. The potential may not be the same.
[0067]
Therefore, in this embodiment, in the color image forming apparatus for contact development, the surface potential variation of the photosensitive drum at the start of charging is controlled by controlling a plurality of different DC development bias voltages and a plurality of different DC development bias voltages applied. In addition to suppressing the fogging and image density unevenness caused, toner scattering is prevented.
[0068]
FIG. 4 is a timing chart for explaining operations related to the development process in this embodiment. Here, Vdc1: a first developing bias voltage that is first applied to the developing roller 10, Vdc2: a second developing bias voltage that is applied to the developing roller 10 after applying the first developing bias voltage Vdc1, and VD: a latent image. When non-image potential as potential, VL: image portion potential, | VD |> | Vdc2 |> | Vdc1 |> | VL |, and specific numerical values are VD = −700V, Vdc2 = −350V, It is assumed that Vdc1 = −300V and VL = −100V.
[0069]
First, the operation in the developing process of this embodiment will be described. At time t31, the rotary developing device 5 is rotated, and the developing device 4a corresponding to the first color latent image, for example, the developing device 4a storing yellow (Y) toner is stopped at a position facing the photosensitive drum 1. At this time t32, the photosensitive drum 1 and the developing roller 10a in the developing device 4a are sufficiently separated from each other (specific values are about 5 mm), and a part of the toner adhering to the developing roller 10a is mirrored. It does not adhere to the photosensitive drum 1 due to the image power or the like.
[0070]
At the time t32, the charging of the photosensitive drum 1 is started using the charger 2 at the same time as the development bias voltage Vdc1 is applied, and at the time t33, the photosensitive drum 1 and the developing roller 10a are brought into contact with each other using the pressure device 8. Then, from time t33 to time t35, development including blank portions at the leading and trailing edges of the transfer material, that is, development within a range indicated by C in FIG. 9 is started.
[0071]
Next, the developing bias voltage is switched to Vdc2 at a time t34 when several seconds have elapsed after the developing start time t33. The value of | t34−t32 | is the time from the start of charging of the photosensitive drum 1 until the photosensitive drum 1 makes one full turn.
[0072]
Development is performed until time t35, and the pressure operation of the pressure device 8 is released at time t35 to separate the developing roller 10a from the photosensitive drum 1. After the separation between the photosensitive drum 1 and the developing roller 10a in the developing device 4a is completed, the developing bias voltage Vdc is canceled at time t36. The same applies to the second, third, and fourth color developing devices 4b to 4d.
[0073]
Here, the surface potential VD of the photosensitive drum 1 does not rise completely at t32 immediately after the charging of the photosensitive drum 1 where the surface potential was 0 V due to the optical characteristics and deterioration of the photosensitive drum 1, and therefore, for a while after the charging starts. In some cases, the surface potential may be slightly lower than that (for example, time t3) (see VD "in FIG. 5). When the image portion potential VL exposed by the exposure device 3 also passes for a while after the start of charging (see FIG. 5). For example, it may be slightly lower than time t3) (see VL ″ in FIG. 5). As specific numerical values, VD ″ = − 650V and VL ″ = − 50V.
[0074]
When the second developing bias voltage Vdc2 that is always constant in this situation is applied to the developing roller 10, as shown in FIG. 5, at t32 immediately after the start of charging, | VD−Vdc2 |> | VD ″ −Vdc2 | Therefore, there is a problem that the fog phenomenon is likely to occur, and a problem that | VL−Vdc2 | <| VL ″ −Vdc2 | and the image density is not always constant.
[0075]
Therefore, from the start of charging of the photosensitive drum 1 until one rotation, that is, from time t32 to t34, | VD−Vdc2 | ≈ | VD ″ −Vdc1 | or | VL−Vdc2 | ≈ | VL ″ −Vdc1 By applying the first developing bias voltage Vdc1 that becomes |, and applying the second developing bias voltage Vdc2 to the developing roller 10 after the second round, the fog phenomenon can be prevented and the image density can be made constant at all times. it can.
[0076]
Further, by applying the developing bias voltage Vdc1 to the developing roller 10a before the time t33 when the photosensitive drum 1 and the developing roller 10 are brought into contact with each other, development can be started from the contact time t33, and a margin corresponding to the leading edge of the transfer material. The fogging phenomenon of the part can be prevented.
[0077]
Further, when the photosensitive drum 1 and the developing roller 10 are contacted and separated at time t33 and time t34, the developing bias voltage Vdc is applied to the developing roller 10 to prevent toner scattering.
[0078]
Actually, when multi-color development was performed using a development bias voltage having a time change as shown in FIG. 4, the development can be started from time t33, and fogging is applied to the leading and trailing margins of the transfer material. No phenomenon occurred and image formation faithful to the latent image could be performed.
[0079]
In addition, if an electrometer that can detect the surface potential of the photosensitive drum 1 is installed between the charger 2 and the contact position between the photosensitive drum 1 and the developing roller 10 in accordance with the rotation direction of the photosensitive drum 1, Even when the non-image portion potential VD or the image portion potential VL of the photosensitive drum 1 changes suddenly during development, the developing bias voltage is changed according to the detected potential, so that there is no fog phenomenon or the image density is constant. Image formation can be performed.
[0080]
As described above, after applying the first developing bias voltage Vdc1 to the developing roller 10 and simultaneously starting charging and rotating the photosensitive drum, the photosensitive drum and the developing roller are brought into contact until the photosensitive drum makes one turn. Then, the development is started, and the second development bias voltage Vdc2 is applied while developing after the second round, so that it is possible to form a color image with no fogging phenomenon and a constant image density.
[0081]
In the above embodiment, the negative reversal development system has been described as an example. However, in the regular development system and the positive reversal development system, the present invention can be applied to eliminate the fog phenomenon and the toner scattering. It is possible to perform no image formation. In the above embodiment, the multicolor development using the non-magnetic toner has been described. However, the same effect can be obtained by using the magnetic toner.
[0082]
Further, in the rotary developing device shown in the above embodiment, the direction of the developing device is always constant (the same direction as when facing the photosensitive drum), but the direction of the developing device at a position not facing the photosensitive drum is as follows. There is no particular limitation.
[0083]
【The invention's effect】
  As is apparent from the above description, according to the image forming apparatus of the present invention, during the contact / separation operation between the image carrier and the developer carrier carrying the developer, the developer is transferred from the image carrier to the developer. An electric field is formed that urges in the direction toward the carrier,A contact operation between the developer carrier and the image carrier is performed after the image carrier starts charging until the image carrier makes one round,The absolute value of the DC voltage applied to the developer carrier before the development and during the contact operation between the developer carrier and the image carrier is applied to the developer carrier during development. By being smaller than the absolute value of the DC voltage, the fog phenomenon can be suppressed and toner scattering can be prevented, so that a high-quality image can be obtained and contamination in the apparatus can be prevented.
[Brief description of the drawings]
FIG. 1 shows the first of the present invention.Reference exampleFIG. 10 is a timing diagram for explaining the operation of the developing bias voltage according to FIG.
FIG. 2 shows the first of the present invention.Reference example1 is a schematic configuration diagram showing a color image forming apparatus according to FIG.
FIG. 3 shows the second of the present invention.Reference exampleFIG. 10 is a timing diagram for explaining the operation of the developing bias voltage according to FIG.
FIG. 4 The present inventionExamples ofFIG. 10 is a timing diagram for explaining the operation of the developing bias voltage according to FIG.
FIG. 5 shows the present invention.Examples ofFIG. 6 is a potential diagram illustrating a potential of the photosensitive drum according to the embodiment.
FIG. 6 is a schematic configuration diagram illustrating an example of a conventional color image forming apparatus.
7 is an enlarged view showing a rotary developing device of the color image forming apparatus of FIG. 6. FIG.
FIG. 8 is a timing chart for explaining the operation of a conventional developing bias voltage.
FIG. 9 is an explanatory diagram showing leading and trailing margin portions of a transfer material.
[Explanation of symbols]
  1 Photosensitive drum (image carrier)
  2 Charger (charging means)
  4 (4a, 4b, 4c, 4d) Developing device (developing means)
  5 Rotating developer
  12 (12a, 12b, 12c, 12d) Development sleeve (developer carrier)

Claims (1)

An image bearing member on which an electrostatic latent image is formed and a developer bearing member for bearing a developer, which can be brought into contact with and separated from the image bearing member. In an image forming apparatus comprising a developer carrier for developing an electrostatic latent image with a developer,
During the contact and separation operation of the image carrier and the developer carrier carrying the developer, an electric field is formed to urge the developer in a direction from the image carrier to the developer carrier. The developer carrier and the image carrier are brought into contact with each other between the start of charging and the rotation of the image carrier, and before the development , the developer carrier The absolute value of the DC voltage applied to the developer carrier during the contact operation between the image carrier and the image carrier is smaller than the absolute value of the DC voltage applied to the developer carrier during development. Image forming apparatus.

Images