JP3884982B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus such as a copying machine, a printer, or a facsimile that forms an image by an electrophotographic method or an electrostatic recording method, and more particularly, an image without a cleaning device that removes transfer residual toner remaining on an image carrier. The present invention relates to a forming apparatus.
[0002]
[Prior art]
FIG. 5 is a schematic configuration diagram showing an example of an image forming apparatus such as a copying machine and a printer using a conventional electrophotographic system.
[0003]
This image forming apparatus rotationally drives the photosensitive drum 101 in the direction of arrow a (clockwise), and sequentially performs the following steps in the rotation process.
[0004]
First, the surface of the photosensitive drum 101 is uniformly charged by a charging roller 102 to which a bias is applied, and then an exposure device 103 performs image exposure L according to an input image signal to form an electrostatic latent image.
[0005]
The electrostatic latent image is developed as a toner image by the developing sleeve 104a of the developing device 104 carrying a thin layer of toner on the surface.
[0006]
The toner image formed on the photosensitive drum 101 is conveyed to the transfer unit Nt as the photosensitive drum 101 rotates. The transfer unit Nt includes a photosensitive drum 101 and a transfer roller 105, and a recording medium such as paper is used as a recording medium from a paper supply device (not shown) in accordance with the timing at which the toner image on the photosensitive drum reaches the transfer unit Nt. The transfer material P ′ is conveyed, and at the same time, a transfer bias having a polarity opposite to that of the toner is applied from the transfer high-voltage power source 106 to the transfer roller 105, and a charge having a polarity opposite to that of the toner is applied to the back surface of the transfer material P ′. Thus, the toner image on the photosensitive drum 101 is transferred onto the surface of the transfer material P ′.
[0007]
The transfer material P ′ onto which the toner image has been transferred is electrostatically separated from the photosensitive drum 101 by the separation charge eliminator 107 and conveyed to a fixing device (not shown).
[0008]
In this fixing device, the toner image is thermally fixed on the surface of the transfer material P ′, and then the transfer material P ′ is discharged out of the apparatus. Further, the transfer residual toner remaining on the photosensitive drum 101 after the transfer is removed and collected by the cleaning device 108, and the photosensitive drum 101 is repeatedly used for the next image forming process.
[0009]
By the way, in such an image forming apparatus, in recent years, in order to reduce the size of the entire image forming apparatus, each means and apparatus of an image forming process such as charging, exposure, development, transfer, fixing, and cleaning have been reduced in size. Has been.
[0010]
However, there is a limit to downsizing the entire image forming apparatus only by downsizing the photosensitive drum and the means / devices of each process.
[0011]
Therefore, as a new method for miniaturization, a so-called cleaner-less system (development simultaneous cleaning system) without a cleaning mechanism has been put into practical use (for example, JP-A-5-61383).
[0012]
This cleanerless system is usually used in a cleaning device for removing residual toner remaining on the photosensitive drum after the transfer process, such as a cleaning member such as a blade, a fur brush, or a roller, or a removed transfer. This is a system that does not use a waste toner container for collecting residual toner.
[0013]
For this reason, the overall size of the apparatus can be greatly reduced, and thus, when employed in a tandem type full-color image forming apparatus (inline color image forming apparatus) using a plurality of photosensitive drums, a very large effect can be obtained. Can do.
[0014]
The schematic configuration of such a cleanerless system is basically the same as that shown in FIG. 5 except that the cleaning device 108 (cleaning blade) is eliminated.
[0015]
However, if the cleaning device is simply removed, the untransferred toner adheres to the charging roller, and the adhered toner cannot be uniformly charged on the photosensitive drum, causing a partial charging failure.
[0016]
In order to solve this problem, as shown in FIG. 6, as a charging unit having a plurality of charging members, in recent years, in the rotation direction of the photosensitive drum, on the upstream side of the charging roller 102 as the first charging member, As a result, it is effective to provide at least one charging auxiliary member 109 as the upstream charging member.
[0017]
In general, the auxiliary charging member 109 is generally a brush-like member (hereinafter referred to as an auxiliary charging brush) so that the transfer residual toner and fog toner after transfer can be temporarily collected. In addition, a bias is applied to the auxiliary charging brush 109, and the transfer residual toner is electrically attached and collected, and at the same time, the transfer residual toner having an indefinite polarity is uniformly charged to a certain polarity, which causes problems in the subsequent steps. It is made not to generate.
[0018]
The cleanerless system using this auxiliary charging brush will be described in detail below.
[0019]
When the transfer residual toner reaches the auxiliary charging brush 109, the transfer residual toner in which toners of both positive and negative polarities are mixed is collected by the auxiliary charging brush 109, and at the same time, the normal development is performed by the applied DC bias. The toner is charged with a toner (hereinafter, referred to as a strong negative toner) having a higher charge than that charged at times.
[0020]
These strong negative toners pass through the brush little by little and reach the charging portion.
[0021]
The strong negative toner that has reached the charging unit does not adhere to the charging roller 102 because of its high adhesive force on the photosensitive drum 101, and passes through the charging unit, but the AC applied to the charging roller 102 when passing through the charging unit 102. Due to the effect of the bias, the amount of charge previously charged in the strong negative toner is slightly lost, and the toner is changed to a negative toner having a normal charge amount.
[0022]
When the toner reaches the position of the developing device again, the toner is collected in the developing device by the effect of the developing bias and the rotation of the developing sleeve 104a. Here, the toner collected in the developing device is once peeled off from the developing sleeve 104a, adjusted to have a charge that can be developed again, and then used again for image formation and reused.
[0023]
The cleanerless system as described above does not discard the transfer residual toner after transfer as waste toner, but returns it to the developing device and reuses it again as a developer. It is. Further, since cleaning members such as blades, fur brushes, and rollers are not brought into contact with the surface of the photosensitive drum, mechanical wear of the photosensitive drum is reduced, so that the life of the photosensitive drum can be extended.
[0024]
[Problems to be solved by the invention]
However, in the case of the prior art as described above, the problem of adhesion of the developing carrier newly arises due to the configuration of the cleanerless system.
[0025]
This phenomenon has frequently occurred particularly when the apparatus is started or stopped.
[0026]
When the developing carrier adheres, the developing carrier adheres to the charging roller as the photosensitive drum rotates, and the resistance of the member of the adhering portion decreases.
[0027]
For this reason, the discharge centered on the carrier is concentrated on the attached portion. Therefore, the surface of the charging member in this portion gradually deteriorates and eventually breaks. If the surface of the charging member is partially damaged, it becomes impossible to charge the surface of the photosensitive drum, and a partially charged defective image is generated on the image.
[0028]
Further, as the developing carrier adheres to the photosensitive drum little by little, the carrier in the developing device is gradually lost. For this reason, the component ratio (T / C ratio) of the toner cannot be maintained properly, and an image defect such as a decrease in image density or fogging occurs.
[0029]
Further, when the development carrier attached on the image carrier is transferred onto the transfer material, the development carrier transferred onto the transfer material may damage the release layer or the like that is usually formed on the surface of the fixing device for a long time. This may cause problems such as partial fixing failure, fixing offset, and fixing paper winding, and may damage the apparatus itself.
[0030]
The cause of the adhesion of the developing carrier is that the surface of the photosensitive drum is charged by applying a bias exceeding the discharge threshold voltage to the auxiliary charging member, and a potential difference between the charged surface and the developing sleeve in the developing portion (hereinafter referred to as Vback). ) Is electrically attracted to the surface of the photosensitive drum.
[0031]
Accordingly, the present invention has been made to solve the above-described problems of the prior art, and the object of the present invention is to develop a developing sleeve on the surface of the photosensitive drum and a developing sleeve in the developing section without impairing the cleaner-less characteristics. The potential difference (Vback) is kept low, the development carrier is prevented from adhering to the photosensitive drum, the above-mentioned problems and damage to the apparatus due to this are eliminated, and a stable and stable image can be output constantly over a long period of time. An object of the present invention is to provide an image forming apparatus capable of performing the above.
[0032]
[Means for Solving the Problems]
  In order to achieve the above object, the present invention provides:
  An image carrier;
  Charging means having a plurality of charging members for charging the image carrier;
  Exposure means for exposing the charged image carrier;
  A developing hand for developing the electrostatic latent image formed on the image carrier by the exposure unit as a toner image
Step and
  Transfer means for transferring the toner image onto a recording medium;
  Fixing means for fixing the transferred toner image on the recording medium,
  The developing means is an image forming apparatus that collects and reuses toner remaining on the image carrier,
  The developing means includes a developing sleeve for transporting a two-component developer development comprising a developing carrier and a toner as components,
  Before the start of image formation or after the end of image formation,The image charged with a voltage value equal to or higher than a discharge threshold value between the upstream charging member and the image carrier by an upstream charging member provided on the upstream side of the rotation direction of the image carrier among the plurality of charging members. While the charging surface of the carrier passes through the charging portion of the most downstream charging member provided on the most downstream side in the rotation direction of the image carrier among the plurality of charging members, an AC bias is applied to the most downstream charging member. AppliedThe DC bias is set to 0V, and the developing sleeve bias is set to 0V while the charged surface passes through the developing portion.It is characterized by that.
[0033]
  An image carrier;
  Charging means having a plurality of charging members for charging the image carrier;
  Exposure means for exposing the charged image carrier;
  Developing means for developing the electrostatic latent image formed on the image carrier by the exposure means as a toner image;
  Transfer means for transferring the toner image onto a recording medium;
  Fixing means for fixing the transferred toner image on the recording medium,
  The developing means is an image forming apparatus that collects and reuses toner remaining on the image carrier,
  The developing means includes a developing sleeve for transporting a two-component developer development comprising a developing carrier and a toner as components,
  Before the start of image formation or after the end of image formation,The image charged with a voltage value equal to or higher than a discharge threshold value between the upstream charging member and the image carrier by an upstream charging member provided on the upstream side of the rotation direction of the image carrier among the plurality of charging members. The charged surface is exposed by the exposure means while the charged surface of the carrier passes through the exposure portion of the exposure means.The bias of the developing sleeve is set to 0 V while the charged surface passes through the developing portion.It is characterized by doing.
[0034]
  While the charging surface of the image carrier passes through the charging portion of the most downstream charging member provided on the most downstream side in the rotation direction of the image carrier among the plurality of charging members, an AC bias is applied to the most downstream charging member. Is appliedNo DC bias is 0VIf not, it is preferable that the charging surface is exposed by the exposure unit while the charging surface passes through the exposure part of the exposure unit.
[0035]
A plurality of image forming units including the image carrier, a charging unit, an exposure unit, and a developing unit are arranged, and a plurality of color toner images developed on a plurality of image carriers of the plurality of image forming units are transferred by the transfer unit. The present invention can be suitably used for an image forming apparatus that superimposes and transfers images on a recording material.
[0036]
The developing means may be a developing device using a contact developing system.
[0037]
The upstream charging member may be a brush composed of conductive resin fibers.
[0038]
The upstream charging member may be a magnetic brush composed of magnetic particles.
[0039]
  SaidUpstream charging memberMay be a conductive rubber roll.
[0040]
Further, an auxiliary exposure means different from the exposure means is provided on the downstream side of the upstream charging member in the rotation direction of the image carrier,
The image carrier charged with a voltage value equal to or higher than a discharge threshold value between the charging member and the image carrier by an upstream charging member provided on the upstream side in the rotation direction of the image carrier among the plurality of charging members. The charged surface may be exposed by the auxiliary exposure unit while the charged surface of the body passes through the exposure part of the auxiliary exposure unit.
[0041]
Thereby, the potential on the surface of the image carrier charged by the bias applied to the charging auxiliary member can be set to a potential (Vback) at which the development carrier does not adhere before reaching the developing unit.
[0042]
This causes damage to the charging member surface due to the development carrier as described above, image defects such as image density reduction and fogging, and various problems such as fixing failure, fixing offset, and fixing paper winding. Can be prevented.
[0043]
Therefore, it is possible to always obtain a good image over a long period of time.
[0044]
DETAILED DESCRIPTION OF THE INVENTION
Exemplary embodiments of the present invention will be described in detail below with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the components described in this embodiment are not intended to limit the scope of the present invention only to those unless otherwise specified. Absent. Further, the materials, shapes, etc. of the members once described in the following description are the same as those in the first description unless otherwise described.
[0045]
(First embodiment)
The first embodiment of the present invention will be described below with reference to the drawings.
[0046]
FIG. 1 is a schematic sectional view of an electrophotographic image forming apparatus using this embodiment. The details will be described in order according to the process of image formation.
[0047]
In FIG. 1, a photosensitive drum 1 is a cylindrical electrostatic latent image carrier and has a peripheral speed of 100 mm / sec. It is driven by rotation.
[0048]
The charging roller 2 is a contact-type charging roller. A conductive rubber layer and a resistance layer are formed around a conductive metal core, and the charging roller 2 rotates following the rotation of the photosensitive drum 1. The charging roller 2 is applied with a DC bias of −700 V and an AC bias of 1 kVp-p from the charging bias power source 2a. As a result, the surface of the photosensitive drum 1 as an image carrier is uniformly charged to −700V.
[0049]
The image exposure unit 3 irradiates laser light, LED light, or the like according to recorded image information, and exposes the surface of the photosensitive drum 1 to form an electrostatic latent image.
[0050]
The developing device 4 includes a developing sleeve 4a that conveys a two-component developer composed of a developing carrier and toner to the surface of the photosensitive drum 1, a developing magnet 4b that is included in the developing sleeve 4a, and a toner on the developing sleeve 4a. It is composed of a developer regulating blade 4c for regulating the coating amount.
[0051]
The developing sleeve 4 a whose surface is uniformly coated with the developer by the developer regulating blade 4 c is driven and rotated in the counter direction with respect to the photosensitive drum 1. By this rotation, the developer formed on the surface is conveyed to the developing nip portion Ng facing the photosensitive drum 1.
[0052]
When a DC voltage of -300 V and an AC bias of -1 kVp-p and frequency 2 kHz are applied to the developing sleeve 4a, the toner flies on the photosensitive drum 1 at the developing nip portion Ng, and the toner on the photosensitive drum 1 The electrostatic latent image is visualized with toner.
[0053]
The toner image on the photosensitive drum 1 visualized by the developing device 4 is conveyed to a transfer unit Nt configured by the photosensitive drum 1 and the transfer roller 5 according to the rotation of the photosensitive drum 1.
[0054]
The transfer roller 5 contacts the photosensitive drum 1 and is driven to rotate in the direction of arrow C. When the toner image on the photosensitive drum 1 reaches the transfer portion Nt, the transfer material P is conveyed to the transfer portion Nt by a paper feeding mechanism (not shown) in time with this. At the same time, a voltage having a polarity opposite to that of the toner is applied to the transfer roller 5 from the high voltage power source 6, and the toner image on the photosensitive drum 1 is transferred onto the surface of the transfer material P.
[0055]
The transfer roller 5 used in the present embodiment is an elastic roller formed by winding a single-layer rubber obtained by foaming a conductive rubber around the core metal surface, and has a volume resistance value of about 10%.⁸It is adjusted to Ω. The volume resistance value varies depending on the device, but 10⁷-10¹³The one of about Ω is generally used.
[0056]
The transfer material P onto which the toner image on the photosensitive drum 1 has been transferred is then separated from the photosensitive drum 1 by the curvature of the photosensitive drum 1 and conveyed to the fixing device 11 with the toner image remaining thereon.
[0057]
In the fixing device 11, the toner image on the surface of the transfer material P is melted and fixed by the action of heat and pressure, and then discharged outside the apparatus.
[0058]
On the other hand, the photosensitive drum 1 after transferring the toner onto the transfer material P reaches the auxiliary charging member 12 with the remaining toner that could not be transferred. In FIG. 1, a charging auxiliary brush using a conductive brush is used as the auxiliary charging member 12.
[0059]
The auxiliary charging brush 12 has a resistance value of about 10⁶Ω fibers are made into a brush shape. In the present invention, a brush-shaped member is used, but a magnetic brush may be formed by arranging magnetic particles around a rotating magnet, or a conductive rubber roller having the same resistance value as a brush can be used. . A DC bias of −800 V is applied to the auxiliary charging brush 12 from a high voltage power source 13 in the figure.
[0060]
In order to solve the problem that the developing carrier adheres to the photosensitive drum 1 in such a process, a control sequence of each bias as shown in FIG. 2 was performed.
[0061]
The countermeasure will be described below based on the sequence of FIG.
[0062]
FIG. 2 is a high-voltage sequence chart when starting up each bias before image formation.
[0063]
First, when a signal for starting image formation is input to the apparatus, the transfer bias is turned on at the same time as the photosensitive drum 1 starts rotating. The transfer bias that is turned on at this time is a bias (transfer bias (-) in FIG. 2) having a polarity opposite to that of a normal transfer bias. This is to prevent an electric memory from being generated by the transfer bias on the photosensitive drum 1 and to prevent the undeveloped toner and fog toner on the photosensitive drum 1 from adhering to the transfer roller 5.
[0064]
Further, at the same timing, a DC bias is gradually applied to the auxiliary charging brush 12 in a slope shape. Here, the reason why the bias is ramped up is that when the bias is suddenly applied to the auxiliary charging brush 12, the toner collected in the auxiliary charging brush 12 before the image forming apparatus stops is This is because it has been found by the present inventors that they adhere to the photosensitive drum 1 at a stretch. Due to this bias, the toner on the photosensitive drum between the transfer portion and the auxiliary charging brush is charged to a toner having a strong negative charge.
[0065]
The DC bias applied to the auxiliary charging brush 12 is gradually raised from 0 V, and reaches −800 V after a predetermined time has elapsed (about 1 sec. In the present invention). When a bias of −800 V is applied in a slope like this, the discharge threshold voltage (about 500 V in the present embodiment) is exceeded during the start-up (the timing of V1 in FIG. 2). A discharge phenomenon occurs between the photosensitive drums, and the surface of the photosensitive drum is charged.
[0066]
In this embodiment, the discharge threshold voltage is set to about 500 V. However, the discharge threshold voltage is not limited to this value, and the discharge threshold is defined as a voltage at which discharge starts between the photosensitive drum and the auxiliary charging brush. Control may be performed so that the charging surface of the image carrier charged with the above voltage value is neutralized by the most downstream charging member.
[0067]
FIG. 3 is a graph showing the relationship between the bias applied to the auxiliary charging brush and the potential of the photosensitive drum surface.
[0068]
In the graph shown in FIG. 3, the horizontal axis represents the bias applied to the auxiliary charging brush, and the vertical axis represents the potential of the photosensitive drum surface. As can be seen from this graph, the potential on the surface of the photosensitive drum suddenly starts to be negatively charged around the time when the bias applied to the auxiliary charging brush becomes about −500V, and when the bias voltage finally becomes −800V, It can be seen that the drum surface potential reaches approximately -300V.
[0069]
On the other hand, as a result of examining the relationship between the potential of the surface of the photosensitive drum 1 and the potential difference between the developing sleeve 4a in the developing unit, as shown by the line Vg shown in FIG. It has been found that the development carrier adheres to the photosensitive drum due to an electric force when the potential difference (Vback) between the first and second electrodes becomes 170 V or more.
[0070]
Accordingly, if the −300 V photosensitive drum surface potential previously formed by the auxiliary charging brush bias reaches the developing portion as it is, the developing carrier adheres to the photosensitive drum surface.
[0071]
Therefore, an AC bias is applied to the charging roller 2 so that the surface potential of the charging surface of the photosensitive drum 1 charged by the auxiliary charging brush 12 reaches Vback <170 V before reaching the developing unit, and the charging surface is made to have the above-described charging surface. Static electricity was removed. Accordingly, the AC bias is applied to the charging roller 2 before the charging surface of the photosensitive drum 1 charged when the bias V1 or higher is applied to the auxiliary charging brush 12 reaches the charging portion of the charging roller 2. There is a need to.
[0072]
Therefore, in the high voltage timing chart when the apparatus is actually operated as shown in FIG. 2, the AC bias is applied at the timing when the charging surface reaches the charging portion of the charging roller 2 in consideration of a high voltage delay or the like. The timing to turn on is set to 100 ms earlier.
[0073]
When an AC bias is actually applied to the charging roller 2 at such timing, the potential charged by the auxiliary charging brush 12 until that time is surely eliminated to near 0 V by the AC bias, and carrier adhesion at the developing unit is prevented. I was able to do it.
[0074]
In the subsequent process until image formation, the charging DC bias and the developing DC bias are sequentially ramped up so as to maintain Vback <170 V (S1 in FIG. 2), and then the developing AC bias is turned on. The exposure corresponding to the image signal is started.
[0075]
On the other hand, when the image formation is completed, Vback <170V can be maintained even when the bias of the auxiliary charging brush is turned on by following the reverse process of the case where the bias is raised. Therefore, carrier adhesion at the developing unit is prevented.
[0076]
As described above, in the image forming apparatus of the cleanerless system shown in the present embodiment, the bias applied to the auxiliary charging member reaches a voltage value at least equal to or higher than the discharge threshold by performing the high voltage control. Since the AC bias is applied to the charging roller while the charged surface of the charged photosensitive drum passes through the charging portion of the charging roller, the charged surface is neutralized and the potential difference is reduced. A potential condition (Vback <170 V) is reached in which no carrier adhesion occurs.
[0077]
Accordingly, it is possible to prevent problems such as charging failure and development deterioration / damage of the apparatus due to adhesion of the developing carrier, and a stable and good image can be output over a long period of time.
[0078]
In addition, it is possible to reduce the size of the image forming unit without impairing the cleanerless effect.
[0079]
(Second Embodiment)
Here, a method of setting the potential of the surface of the photosensitive drum 1 formed by the auxiliary charging brush 12 to a potential of Vback <170 V that does not cause development carrier adhesion by the exposure device before reaching the developing unit will be described.
[0080]
In the description of the present embodiment, the description will be made based on a high-voltage sequence chart when starting up each bias before image formation employed in the image forming apparatus according to the present embodiment shown in FIG. Since this is the same as that of the above-described embodiment, description thereof is omitted here.
[0081]
First, as in the first embodiment, when a signal for starting image formation is input to the apparatus, the photosensitive drum 1 starts rotating and simultaneously the transfer bias is turned on.
[0082]
The transfer bias that is turned on at this time is a bias having the same polarity as that of the toner (opposite polarity at the time of printing), as in the first embodiment.
[0083]
Then, a bias is gradually applied to the auxiliary charging brush 12 in a slope shape at the same timing.
[0084]
When the bias of the auxiliary charging brush 12 gradually rises from 0V to become a V1 bias, electric discharge starts between the auxiliary charging brush 12 and the surface of the photosensitive drum 1, and the first embodiment described above. As shown in FIG. 3, the surface potential of the photosensitive drum 1 starts to rise rapidly.
[0085]
The charged photosensitive drum surface passes through the charging portion of the charging roller 2 as the drum rotates and reaches the exposure position. Immediately before the charged surface of the charged photosensitive drum 1 reaches the exposure position, the exposure device is driven to start exposure of the charged surface of the photosensitive drum 1.
[0086]
Here, in the present embodiment, taking into account the rise of the exposure apparatus and the rotation error of the photosensitive drum, 100 msec. From the time when the charged surface is actually expected to reach the exposure position. Exposure is started early (timing L1 in the figure).
[0087]
However, when a laser scanner is used as the exposure apparatus, it is necessary to start up the polygon mirror before starting the exposure.
[0088]
  The charged surface that has reached the exposure position has a low surface potential due to exposure, and when it reaches the development section, the carrier adheres to the development section.do not doIt becomes a potential (Vback <170V). As a result of actual examination in the present embodiment, the potential on the photosensitive drum when the bias of −800 V is applied to the auxiliary charging brush is about −300 V, and the potential on the photosensitive drum after exposure is about −50 V. It was. Therefore, since the potential difference with the developing sleeve is Vback = 50 V, carrier adhesion does not occur in the developing portion.
[0089]
Thereafter, the neutralization of the drum potential by the exposure device is continued until the charging AC bias is turned on before image exposure, and is temporarily turned off after the charging AC bias is turned on (L2 in FIG. 4). This is because, as described in the first embodiment, when the AC bias is applied to the charging roller, the potential on the drum surface is neutralized by the AC bias. Therefore, it is necessary to make Vback <170 V by exposure. This is because it disappears.
[0090]
After the charging AC bias rises, the charging DC bias and the development DC bias are sequentially raised in a slope shape so as to maintain Vback <170 V (S1 in FIG. 4), and then developed, as in the first embodiment. The AC bias is turned on to enter the final image exposure process.
[0091]
On the other hand, when the image formation is completed, Vback <170V can be maintained even when the bias of the auxiliary charging brush is turned on by following the reverse process of the case where the bias is raised. Accordingly, it is possible to prevent carrier adhesion at the developing portion.
[0092]
In this embodiment, the photosensitive drum is exposed using an exposure output device corresponding to the image signal. For example, when the exposure device cannot be used due to a problem such as a laser emission life, charging assist is performed. The same effect can be obtained by separately arranging an auxiliary exposure device at a position that does not affect image exposure on the downstream side of the member. In this case, since the auxiliary exposure apparatus does not need to form an image and only needs to be able to eliminate static electricity, in addition to an expensive and precise laser, an inexpensive lamp or light emitter can be used.
[0093]
As described above, in the image forming apparatus of the cleanerless system shown in the present embodiment, the bias applied to the charging auxiliary member (charging auxiliary brush) is at least discharged by performing the high pressure / exposure control. While the voltage value is equal to or higher than the threshold value and no AC bias is applied to the charging member (charging roller) located at the most downstream side, the carrier on the developing unit is adhered to the image carrier by performing exposure static elimination. A potential condition (Vback <170V) that does not occur is achieved.
[0094]
Accordingly, it is possible to prevent problems such as charging failure and development deterioration / damage of the apparatus due to adhesion of the developing carrier, and a stable and good image can be output over a long period of time.
[0095]
【The invention's effect】
As described above, according to the present invention, in the image forming apparatus employing the cleanerless system, the auxiliary charging member is installed on the upstream side of the charging member, and the bias applied to the auxiliary charging member is charged at a voltage higher than the discharge threshold voltage. By applying an AC bias to the charging member while the charged surface of the image carrier passes through the charging portion of the charging member, or by exposing the surface of the image carrier with an exposure device while passing through the exposure portion, The potential charged on the carrier by the bias applied to the auxiliary charging member can be neutralized by the AC bias or exposure applied to the downstream charging member.
[0096]
For this reason, since the potential difference with the image carrier becomes small in the developing unit, it is possible to prevent the development carrier from being attached due to the potential difference.
[0097]
As a result, damage to the surface of the charging member caused by the development carrier adhering to the charger as described above, image defects such as image density reduction and fogging, and fixing failure, fixing offset, fixing paper winding, etc. It is possible to prevent the occurrence of various problems.
[0098]
Therefore, it is possible to always obtain a good image over a long period of time.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view of an image forming apparatus according to a first embodiment.
FIG. 2 is a high-pressure sequence chart before image formation employed in the image forming apparatus according to the first embodiment.
FIG. 3 is a graph showing the relationship between the auxiliary charging brush bias and the photosensitive drum surface potential in the image forming apparatus according to the present invention.
FIG. 4 is a high-pressure sequence chart before image formation employed in the image forming apparatus according to the second embodiment.
FIG. 5 is a schematic sectional view showing an example of a conventional image forming apparatus.
FIG. 6 is a schematic cross-sectional view illustrating an example of an image forming apparatus employing a conventional cleanerless system.
[Explanation of symbols]
1 Photoconductor
2 Charging roller (the most downstream charging member)
3 Exposure equipment
4 Two-component developer (developer)
5 Transfer roller (transfer member)
6 Transfer high-voltage power supply
11 Fixing device
12 Charging auxiliary brush (upstream charging member)
Ng development section
Nt transcription part

Claims (9)

An image carrier;
Charging means having a plurality of charging members for charging the image carrier;
Exposure means for exposing the charged image carrier;
Developing means for developing the electrostatic latent image formed on the image carrier by the exposure means as a toner image;
Transfer means for transferring the toner image onto a recording medium;
Fixing means for fixing the transferred toner image on the recording medium,
The developing means is an image forming apparatus that collects and reuses toner remaining on the image carrier,
The developing means includes a developing sleeve for transporting a two-component developer development comprising a developing carrier and a toner as components,
Before the start of image formation or after the end of image formation, the upstream charging member provided on the upstream side in the rotation direction of the image carrier among the plurality of charging members is disposed between the upstream charging member and the image carrier. The charging surface of the image carrier charged with a voltage value equal to or higher than the discharge threshold passes through the charging portion of the most downstream charging member provided on the most downstream side in the rotation direction of the image carrier among the plurality of charging members. An AC bias is applied to the most downstream charging member during the interval , the DC bias is set to 0 V, and the bias of the developing sleeve is set to 0 V while the charged surface passes through the developing unit. . An image carrier;
Charging means having a plurality of charging members for charging the image carrier;
Exposure means for exposing the charged image carrier;
Developing means for developing the electrostatic latent image formed on the image carrier by the exposure means as a toner image;
Transfer means for transferring the toner image onto a recording medium;
Fixing means for fixing the transferred toner image on the recording medium,
The developing means is an image forming apparatus that collects and reuses toner remaining on the image carrier,
The developing means includes a developing sleeve for transporting a two-component developer development comprising a developing carrier and a toner as components ,
Before the start of image formation or after the end of image formation, the upstream charging member provided on the upstream side in the rotation direction of the image carrier among the plurality of charging members is disposed between the upstream charging member and the image carrier. While the charged surface of the image carrier charged with a voltage value equal to or higher than the discharge threshold passes through the exposed portion of the exposing means, the exposed surface is exposed by the exposing means, and the charged surface serves as the developing portion. An image forming apparatus characterized in that the bias of the developing sleeve is set to 0 V during the passage . While the charging surface of the image carrier passes through the charging portion of the most downstream charging member provided on the most downstream side in the rotation direction of the image carrier among the plurality of charging members, an AC bias is applied to the most downstream charging member. When the direct current bias is not 0 V, the charged surface is exposed by the exposure means while the charged surface passes through the exposure portion of the exposure means. The image forming apparatus according to 2.   A plurality of image forming units including the image carrier, charging means, exposure means and developing means are arranged, and a plurality of color toner images developed on a plurality of image carriers of the plurality of image forming units are transferred by the transfer means. The image forming apparatus according to claim 1, wherein the image forming apparatus superimposes and transfers the image on a recording material.   The image forming apparatus according to claim 1, wherein the developing unit includes a developing unit using a contact developing method.   The image forming apparatus according to claim 1, wherein the upstream charging member is a brush made of conductive resin fibers.   The image forming apparatus according to claim 1, wherein the upstream charging member is a magnetic brush made of magnetic particles. The image forming apparatus according to claim 1, wherein the upstream charging member is a conductive rubber roll. Auxiliary exposure means different from the exposure means is provided downstream of the upstream charging member in the rotation direction of the image carrier,
The image carrier charged with a voltage value equal to or higher than a discharge threshold value between the charging member and the image carrier by an upstream charging member provided on the upstream side in the rotation direction of the image carrier among the plurality of charging members. 9. The image forming apparatus according to claim 2, wherein the charging surface is exposed by the auxiliary exposure unit while the charged surface of the body passes through the exposure unit of the auxiliary exposure unit.

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