JP2004045572A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent the occurrence of a density unevenness after restarting image forming operation arising by the deformation of an electrifying roller because of being left standing for a long time in a state that the electrifying roller abuts on on a photoreceptive drum. <P>SOLUTION: A bias controlling means 22 controls an electrification bias application power source 21 for applying high voltage electrification bias to a transfer roller 2 abutting on the surface of the photoreceptive drum 1. A temperature and humidity sensor 23 and an image forming stop time measuring means 24 are connected with the bias controlling means 22. The bias controlling means 22 controls electrification bias applied to the transfer roller 2 by the power source 21 based on the outputs of the temperature and humidity sensor 23 and the stop time measuring means 24 at the time of restarting the image forming operation. For instance, the electrification bias is controlled only for irreducible minimum time so as to improve the potential converging property only in the case of judging that a nip mark affects an image. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an image forming apparatus, such as a printer, a copying machine, and a facsimile, that charges the surface of an image carrier with a contact charging member.
[0002]
[Prior art]
2. Description of the Related Art In an image forming apparatus, a corona discharger has been widely used as a charging device for charging a surface of a photosensitive drum (charged member) as an image carrier. The corona discharger is disposed in a non-contact manner such that the discharge opening faces the photosensitive drum (charged member), and exposes the surface of the photosensitive drum to a predetermined polarity by exposing the surface of the photosensitive drum to a corona current from the discharge opening.・ Electrification treatment to potential. However, the corona discharger requires a high-voltage power supply and has problems such as generation of a large amount of ozone.
[0003]
On the other hand, the contact-type charging method in which a charging member to which a voltage is applied is brought into contact with the photosensitive drum to perform charging is advantageous in that the voltage of the power supply can be reduced and the amount of generated ozone is small. In recent years, it has been attracting attention and being used. A conventional image forming apparatus using a charging roller as a contact type charging member will be described with reference to FIG.
[0004]
FIG. 11 is a diagram schematically showing a cross section of a main part of a conventional image forming apparatus. In FIG. 11, a photosensitive drum 1 as an image carrier is driven to rotate in the direction of arrow R1, and the surface thereof is first exposed to an exposure lamp. After the charge potential is removed to approximately 0 V by the charging roller 13, the charging roller 2 is charged to a uniform potential (for example, −700 V). Next, an exposure light from the exposure device 3 corresponding to the document is irradiated onto the surface of the photosensitive drum 1 to form an electrostatic latent image on the photosensitive drum 1. This electrostatic latent image is developed by the developing device 4 and visualized as a toner image.
[0005]
On the other hand, a transfer material P such as paper is fed at a predetermined timing by a registration roller 10, and is carried on a transfer conveyance belt 5 stretched over a driving roller 7 and a driven roller 6. The toner image on the photosensitive drum 1 is transferred onto the transfer material P on the transfer conveyance belt 5 by a transfer bias applied to the transfer roller 8 from a transfer bias application power supply 9. The transfer material P after the transfer of the toner image is heated and pressed by passing through the fixing device 11, so that the toner image is fixed on the surface.
[0006]
The toner remaining on the photosensitive drum 1 after the transfer of the toner image is removed by the drum cleaning device 12, and the residual potential of the photosensitive drum 1 is removed again by the irradiation of light from the pre-exposure lamp 13 to be used for the next image formation. You.
[0007]
Further, toner scattered mainly during operation of the developing device 4 in the image forming apparatus or at the time of transfer or fogging toner before and after image formation adheres to the transfer conveyance belt 5, or is directly transferred due to jam during conveyance of the transfer material. The cleaning device 14 for removing the toner that has adhered may be disposed in contact with the transfer conveyance belt 5.
[0008]
The charging roller 2 may have various layer configurations. For example, a central core, an elastic conductive layer formed on the outer periphery thereof, and a high resistance layer formed on the outer periphery thereof may be used. The one having an outermost protective layer is preferably used. The charging roller 2 has both ends of a cored metal rotatably supported by bearing members (not shown), and is disposed in parallel with the photosensitive drum 1. The charging roller 2 is pressed against a surface of the photosensitive drum 1 by a pressing means (not shown). It is pressed with a pressing force. Due to this pressure contact, a band-like nip is formed between the photosensitive drum 1 and the charging roller 2 along the longitudinal direction of the charging roller 2. The charging roller 2 rotates following the driving rotation of the photosensitive drum 1.
[0009]
[Problems to be solved by the invention]
However, in the above-described conventional example, if the image forming apparatus is left unused for a long time after the image forming operation is completed, the image (especially a halftone image, a halftone image, etc.) In some cases, uneven density occurred. The density unevenness is such that traces of the nip are formed along the longitudinal direction of the charging roller 2 in the form of stripes at the same interval (circumferential pitch) as the circumference of the charging roller 2. This nip mark (pitch unevenness) occurs because the elastic layer of the charging roller 2 is deformed in the nip because the charging roller 2 is left in contact with the photosensitive drum 1 with pressing force for a long period of time. Is what you do.
[0010]
Particularly, in a low-temperature environment or the like, since the elastic modulus of the charging roller 2 is reduced, the deformed elastic layer of the charging roller 2 is hard to recover to the original shape. For this reason, nip marks continue to be generated at a relatively long pitch in the circumferential direction of the charging roller.
[0011]
Further, when a material having a relatively high resistance value is used as a material of the high resistance layer and the outermost protective layer of the charging roller 2 or a material having a relatively high hardness is used, the influence of the deformation of the elastic layer is reduced. The material of the high resistance layer is received as it is, and the same deformation occurs. In this case, even if the elastic layer recovers its shape, the high resistance layer and the outermost protective layer continue to be deformed, which may cause a trouble on the image for a long time.
[0012]
Such phenomena are similar or larger for contact charging members of other shapes and forms, but in particular roller-shaped charging members such as charging rollers, rotary fur brush rollers, and magnetic brush rollers, This is likely to occur in the charging member where the same portion comes into contact with the surface of the photosensitive drum 1 at a predetermined circumferential pitch.
[0013]
To cope with such problems, measures are taken with respect to the hardness and shape of the charging roller (for example, described in JP-A-08-211702 and JP-A-09-043935), and the circuit of the charging bias application power supply is devised. (For example, Japanese Patent Application Laid-Open No. 09-096951), or a device that detects the surface shape of a charging member and changes the image forming conditions (for example, Japanese Patent Application Laid-Open No. 8-22155). However, these methods have a problem that the effect is insufficient or a detection circuit is required, which leads to an increase in complexity and cost of the device.
[0014]
SUMMARY OF THE INVENTION It is an object of the present invention to provide an image forming apparatus in which even when image formation is not performed for a long period of time, unevenness in image density pitch (nip mark) does not occur when the image forming operation is restarted. is there.
[0015]
[Means for Solving the Problems]
According to a first aspect of the present invention, in an image forming apparatus that applies a charging bias to a charging member disposed in contact with the surface of an image carrier to charge the surface of the image carrier, a charging bias is applied to the charging member. A charging bias application power source, a control unit for controlling a charging bias of the charging bias application power source, a stop time detecting unit for detecting a time during which no image forming operation is performed, and an environmental condition for detecting an environmental condition near the charging member. Detecting means, wherein the control means controls the charging bias based on outputs from the stop time detecting means and the environmental condition detecting means during a certain time after the image forming operation is resumed or during the formation of a fixed number of images. The present invention is characterized in that a charging bias applied to the charging member by an applied power source is controlled so as to be different from that in a normal image forming operation.
[0016]
According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the control unit is configured to control the stop time detecting unit and the environmental condition during a certain time after the image forming operation is restarted or during a certain number of image formations. The charging bias applied to the charging member by the charging bias application power source is controlled based on an output from the detection unit so that the potential of the surface of the image carrier easily converges.
[0017]
According to the above configuration, even if the charging member is deformed without performing the image forming operation for a long time, the convergence of the surface potential of the image carrier is good, so that the pitch unevenness of the image density can be prevented or Can be suppressed.
[0018]
Further, since the convergence state can be controlled depending on the environmental conditions, it is possible to prevent or suppress the pitch unevenness of the image density by enhancing the convergence under the environmental conditions in which pitch unevenness is more likely to occur.
[0019]
In any case, since the charging bias is controlled only when the convergence is enhanced, it is possible to minimize the shortening of the service life of the entire image forming apparatus.
[0020]
According to a third aspect of the present invention, in the image forming apparatus according to the first aspect, the image forming apparatus further includes a usage history detection unit configured to detect a usage history of the charging member, wherein the control unit performs the predetermined time or the predetermined time after the image forming operation is restarted. During the image formation for the number of sheets, the charging bias applied by the charging bias applying power source to the charging member based on the outputs of the stop time detecting unit, the environmental condition detecting unit, and the use history detecting unit is used for the image forming. It is characterized in that the conditions are controlled so that the potential on the surface of the carrier easily converges.
[0021]
According to the above configuration, since the use history of the charging member is also taken into consideration, even if there is a difference in the recovery of the deformed state between the first use and the endurance of the charging member, it is necessary to control the charging bias only for the minimum necessary. Accordingly, it is possible to achieve both the prevention and suppression of the pitch unevenness of the image density and the suppression of the shortening of the photosensitive drum life.
[0022]
According to a fourth aspect of the present invention, in the image forming apparatus according to any one of the first to third aspects, the environmental condition detected by the environmental condition detecting means is a temperature near the charging member when the image forming operation is restarted. Humidity.
[0023]
According to a fifth aspect of the present invention, in the image forming apparatus according to any one of the first to third aspects, the environmental condition detected by the environmental condition detecting means is a temperature near the charging member when the image forming operation is restarted. It is characterized by humidity and temperature and humidity from a time that is a predetermined time after the image forming operation is restarted to a time when the image forming operation is restarted.
[0024]
According to the above configuration, even if the recovery of the deformed state of the charging member is environmentally dependent, by controlling the charging bias for a minimum necessary period, the pitch unevenness of the image density can be prevented and suppressed, and the life of the photosensitive drum can be shortened. Can be achieved at the same time.
[0025]
Considering not only the time when the image forming operation is resumed but also the environmental history before that, for example, when the work of the office or the like is started, when the use of the image forming apparatus is resumed immediately after the cooling and heating are turned on, the detection is performed. Accurate control is possible even if the environmental conditions and the actual state of the charging member differ.
[0026]
According to a sixth aspect of the present invention, in the image forming apparatus according to any one of the first to fifth aspects, the control unit controls a normal image as a charging bias applied to the charging member by the charging bias application power supply. During the formation, only a DC bias is applied, and control is performed so that a bias in which AC is superimposed on DC is applied for a fixed time or a fixed number of sheets after the image forming operation is restarted.
[0027]
According to a seventh aspect of the present invention, in the image forming apparatus according to any one of the first to fifth aspects, the control unit controls a normal image as a charging bias applied to the charging member by the charging bias application power supply. During formation, a bias in which AC is superimposed on DC is applied, and AC having a higher peak-to-peak voltage than during normal image formation is superimposed for a fixed time or a fixed number of sheets after the image forming operation is restarted. And
[0028]
According to the configuration described above, charging is normally performed under a bias condition in which the influence of the discharge on the image carrier is small, and the potential convergence is improved only when it is determined that the pitch unevenness has an effect on the image. By controlling the charging bias only during the period, it is possible to achieve both the prevention and suppression of the uneven pitch of the image density and the suppression of the shortening of the life of the photosensitive drum.
[0029]
According to an eighth aspect of the present invention, in the image forming apparatus according to any one of the first to seventh aspects, a digital system having a plurality of image forming modes, a plurality of image modes, and a plurality of resolutions is adopted. A charging bias condition applied by the charging bias applying power source to the charging member after the image forming operation is resumed and a timing for returning to a normal charging bias are changed according to a forming mode, an image mode, and a resolution.
[0030]
With the above-described configuration, among image forming modes (for example, copy mode, facsimile mode, and printer mode) and image modes (for example, character mode and photo mode), for example, a facsimile mode with many character images and a character mode are actually used. When it is determined that the influence on the image is small even if the charging member is deformed, the charging bias can be controlled so that the influence on the image carrier is relatively small. Therefore, it is possible to achieve both the prevention and suppression of the pitch unevenness of the image density and the suppression of the shortening of the photosensitive drum life.
[0031]
According to a ninth aspect of the present invention, in the image forming apparatus according to any one of the first to eighth aspects, the charging member is a roller-shaped charging member.
[0032]
According to the above configuration, it is possible to further enhance the above-described effect particularly on a charging member, such as a charging roller, a rotary fur brush roller, or a magnetic brush roller, in which image unevenness tends to appear in a pitch shape.
[0033]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that, in each drawing, the components denoted by the same reference numerals have the same configuration or operation, and redundant description thereof will be omitted as appropriate.
[0034]
<Embodiment 1>
FIG. 1 shows an image forming apparatus according to Embodiment 1 as an example of an image forming apparatus according to the present invention. The image forming apparatus shown in FIG. 1 is an electrophotographic image forming apparatus, and FIG. 1 is a longitudinal sectional view schematically showing a schematic configuration thereof.
[0035]
The image forming apparatus shown in FIG. 1 includes a drum-type electrophotographic photosensitive member (hereinafter, referred to as “photosensitive drum”) as an image carrier. As the photosensitive drum 1, a photosensitive drum having an OPC (organic optical semiconductor) provided as a photosensitive layer on the surface of a cylindrical drum substrate is used. The photosensitive drum 1 is driven to rotate in a direction indicated by an arrow R1 at a predetermined process speed (peripheral speed) by a driving unit (not shown).
[0036]
Around the photosensitive drum 1, a charging roller (charging member) 2, an exposing device (exposure unit) 3, a developing device (developing unit) 4, a transfer roller (transfer unit) 8, and a cleaning roller are arranged substantially in order along the rotation direction. An apparatus (cleaning means) 12, a pre-exposure lamp (pre-exposure means) 13, and the like are provided.
[0037]
Further, a registration roller 10, a transfer / conveyance belt 5, and a fixing device (fixing unit) 11 are arranged in this order from the upstream side along the conveyance direction of the transfer material P (for example, paper, transparent film).
[0038]
The charging roller 2 is in contact with the surface of the photosensitive drum 1, and is driven to rotate in the direction of the arrow R2 with the rotation of the photosensitive drum 1 in the direction of the arrow R1. A charging bias is applied to the charging roller 2 by a charging bias application power supply 21, thereby uniformly charging the surface of the photosensitive drum 1 to a predetermined polarity and potential. The charging roller 2 will be described later in more detail.
[0039]
As the exposure unit 3, for example, a laser scanner can be used. The exposing unit 3 exposes the charged surface of the photosensitive drum 1 to an image based on the image information to remove the electric charge of the image-exposed irradiated portion, thereby forming an electrostatic latent image on the surface of the photosensitive drum 1.
[0040]
The developing device 4 includes a developing sleeve 4a. The developer is carried on the surface of the developing sleeve 4a and transported to a developing position facing the surface of the photosensitive drum 1. A developing bias is applied to the developing sleeve 4a by a developing bias applying power source (not shown). As a result, the electrostatic latent image on the photosensitive drum is developed as a toner image with toner attached.
[0041]
The transfer roller 8 transfers the toner image on the photosensitive drum 1 onto the transfer material P. The transfer material P is stored in a paper feed cassette (not shown), and is transported to the registration roller 10 by a feed roller, a transport roller (neither is shown), or the like. After the transfer material P is temporarily stopped by the registration roller 10, the transfer material P is carried on the transfer conveyance belt 5 so as to match the timing with the toner image on the photosensitive drum 1. The transfer conveyance belt 5 is stretched around a driving roller 7 and a driven roller 6, and is rotated in the direction of arrow R5 by the rotation of the driving roller 7 in the direction of arrow R7. The transfer conveyance belt 5 is pressed against the surface of the photosensitive drum 1 by the transfer roller 8, thereby forming a transfer nip between the surface of the photosensitive drum 1 and the transfer roller 8. The transfer material P carried on the surface of the transfer conveyance belt 5 is conveyed to the transfer nip by rotating the transfer conveyance belt 5 in the direction of arrow R5. At this time, a transfer bias is applied to the transfer roller 8 by the transfer bias application power supply 9, whereby the toner image on the photosensitive drum 1 is transferred onto the transfer material P.
[0042]
After the transfer of the toner image, the toner remaining on the surface of the photosensitive drum 1 that has not been transferred to the transfer material P (transfer residual toner) is removed by the cleaning blade 12 a of the cleaning device 12, and further the charge is removed by the pre-exposure lamp 13. Then, it is subjected to the next image formation.
[0043]
On the other hand, the transfer material P after the transfer of the toner image is transported to the fixing device 11 by the rotation of the transfer transport belt 5 in the direction of arrow R5.
[0044]
The fixing device 11 has a fixing roller 11a and a pressing roller 11b pressed by the fixing roller 11a, and heats and presses the transferred transfer material P. Thus, the toner image is fixed on the surface of the transfer material P, and the image formation is completed.
[0045]
Note that the toner adhered to the transfer conveyance belt 5 is removed by the cleaning device 12.
[0046]
The above-mentioned charging roller 2 is composed of, for example, a central metal core, an elastic conductive layer formed on the outer periphery thereof, a high resistance layer formed on the outer periphery thereof, and an outermost protective layer (all are not shown). ing. In the charging roller 2, both ends of the core are rotatably supported by bearing members (not shown). The charging roller 2 is arranged in parallel with the photosensitive drum 1 and pressed against the surface of the photosensitive drum 1 with a predetermined pressing force by a pressing means (not shown). Due to this pressure contact, a belt-shaped charging nip C is formed between the photosensitive drum 1 and the charging roller 2 along the longitudinal direction of the charging roller 2. The charging roller 2 is driven to rotate in the direction of the arrow R2 with the driving rotation of the photosensitive drum 1 in the direction of the arrow R1.
[0047]
In the present embodiment, as shown in FIG. 1, the transfer roller 2 is connected to a charging bias applying power supply 21 for applying a high charging bias to the transfer roller 2. (Control means) 22 is connected. Further, the bias control unit 22 includes a temperature / humidity sensor (environment detection unit) 23 disposed near the charging roller 2 and an image formation stop time measurement unit (stop time) for detecting the time during which image formation is stopped. (Detection means) 24 is connected.
[0048]
An output signal from a temperature / humidity sensor 23 that detects temperature and humidity near the charging roller 2 and an output signal from an image formation stop time measuring unit 24 that detects an image forming stop time are sent to a bias control unit 22. The charging bias applied from the charging bias applying power source 21 to the charging roller 2 is controlled.
[0049]
Here, differences in the charging status depending on whether the charging method is the DC charging method or the AC charging method will be described below with reference to the drawings.
[0050]
FIG. 2 shows a DC charging method, that is, a DC bias Vdc and a surface potential Vs of the photosensitive drum when the charging bias applied to the charging member (the charging roller 2 in the present embodiment) is only a DC (direct current) bias. FIG. As shown in the figure, Vs increases linearly from the point where the DC bias Vdc exceeds a certain threshold value Vth. Also, its features are:
{Circle around (1)} The thickness of the dielectric layer (photosensitive layer) on the surface of the photosensitive drum due to durability is relatively small with respect to AC charging.
(2) There is no vibration between the photosensitive drum and a charging member typified by a charging roller at the time of charging, so that the toner as a developer is thinly coated on the surface of the photosensitive drum. The phenomenon does not easily occur, no charging noise occurs,
There is such a merit.
[0051]
On the other hand, since the convergence of the charged potential is poor, there are the following disadvantages.
(3) The charging potential varies due to partial resistance unevenness of the charging member and surface dirt.
{Circle around (4)} When the surface of the photosensitive drum is scraped due to durability and the thickness of the dielectric layer is reduced, the charging potential changes. In order to maintain a constant potential, it is necessary to control a DC bias value. In addition to the uniform scraping, the unevenness of the film thickness becomes remarkable depending on the position within one photosensitive drum, so that the charging potential also changes.
[0052]
On the other hand, in an AC charging system, that is, a charging system in which an AC bias and a DC bias are superimposed, as shown in JP-A-3-52058 as an example, a charging member is a member to be charged (in this embodiment, a photosensitive member). A voltage having a DC voltage component and a peak-to-peak voltage value that is at least twice the charging start voltage value for the member to be charged, including a region where the distance to the surface of the member to be charged is increased on the downstream side in the moving direction of the drum 1). Is applied between the member to be charged and the charging member to form an oscillating electric field between the surface of the member to be charged and the region of the charging member, whereby the member to be charged can be uniformly charged without uneven charging. It is possible.
[0053]
As the charging characteristics, since the AC component smoothes the unevenness of the charging and converges to a predetermined potential by the DC component, as shown in FIG. 3, the surface potential of the photosensitive drum is almost equal to the applied DC bias Vdc. It becomes.
[0054]
As a feature of the AC charging system, contrary to the DC charging,
{Circle around (1)} The amount of scraping of the film thickness of the dielectric layer on the photosensitive drum surface due to the durability is relatively large as compared with DC charging. The ratio varies depending on the type of developer used, the prescription of the photosensitive drum, the configuration of the cleaning means for the photosensitive drum surface, the rotation speed, and the like. However, as shown in FIG. is there. Therefore, the life of the photosensitive drum is shortened accordingly. Further, when the peak voltage of the AC component is further increased, the convergence of the potential is increased, but the shaving amount is further increased.
{Circle over (2)} Due to the occurrence of vibration between the charging member and the photosensitive drum during charging, problems such as vibration noise and the above-described filming and fusion are relatively likely to occur. However, this can be prevented by optimizing the settings of the AC current value and various settings such as the developer, the material and prescription of the photosensitive drum surface, and the cleaning means of the photosensitive drum.
Although there are disadvantages such as the above, there are the following advantages due to good convergence of the charged potential.
(3) Small variations in charging potential due to the influence of the charging member itself such as partial resistance unevenness of the charging roller and its surface deposits,
(4) Even if the thickness of the dielectric layer on the surface of the photosensitive drum decreases with the endurance or the film thickness becomes uneven, relatively uniform charging is possible. Therefore, the change in the charging potential and the increase in the charging unevenness due to the durability are small, and relatively uniform and stable charging is possible. That is, the image density becomes relatively uniform.
[0055]
FIG. 4 shows how the thickness of the dielectric layer of the photosensitive drum 1 changes with the use of each of the AC charging system and the DC charging system, that is, how the film thickness changes with an increase in the number of durable sheets (image formation sheets). . Although the value varies depending on the prescription of the photosensitive drum 1 to be used, the configuration of the cleaning device, and the conditions such as bias, the amount of scraping is usually 1.5 to 2.5 times larger in the AC charging system. Further, as described above, when the peak voltage of the AC component is further increased (AC reinforcement in FIG. 4), the convergence of the potential is increased, but the shaving amount is further increased.
[0056]
Next, FIG. 5 shows that the charging roller 2 is left for 3 days in a state where the charging roller 2 is pressed against the photosensitive drum 1, and then the charging is performed. Shows the transition. In the figure, the portion where the potential has dropped indicates that the charging property has dropped due to the deformation of the charging roller 2. The solid line A indicates the case of DC charging, the broken line B indicates the case where an AC component is superimposed, and the broken line C indicates the case where a larger AC component is superimposed. Compared with DC charging, AC superimposition has better potential convergence even if the shape of the charging roller 2 is deformed. Further improvement is achieved by increasing the peak voltage of AC. As described above, the fact that the surface potential at the time of charging before the image exposure irradiation is uneven means that the potential unevenness occurs when a normal image is formed, which leads to the unevenness of the density and the occurrence of an image defect. That is. However, it also suggests that image defects can be prevented or suppressed by making the conditions of the bias applied to the charging roller 2 better convergence.
[0057]
However, if the bias is always set as described above, damage to the photosensitive drum 1 will be large and the life will be shortened.
[0058]
Therefore, in the present embodiment, as shown in FIG. 6, the bias applied to the charging roller 2 is controlled based on the time during which no image formation is performed and the environmental conditions when the image formation operation is restarted (when image formation is restarted). is there. That is, the bias control unit 22 controls the charging bias applied by the charging bias applying power supply 21 to the transfer roller 2 based on the output of the image formation stop time measuring unit 24 and the output of the temperature and humidity sensor 23 in FIG. I did it.
[0059]
In the environmental conditions shown in the figure, N / L indicates a normal temperature and low humidity environment, N / N indicates a normal temperature and normal humidity environment, and H / H indicates a high temperature and high humidity environment. In the same figure, a DC bias is applied at the time of normal image formation, and a predetermined number (control number) after the image forming operation is resumed after leaving for a predetermined time is to apply a bias in which AC is superimposed on DC. The numbers in the columns indicate the numbers. During the formation of the predetermined number of images, the rotation of the charging roller 2 is repeated so that the pitch unevenness is alleviated, and the non-uniformity of the potential and the image defect are eliminated or reduced to a level at which there is no practical problem. Therefore, after that, the DC bias may be returned to the state where the influence on the photosensitive drum 1 is small.
[0060]
If the leaving time is 5 hours or less, no particular control is required because the nip mark is not at a problematic level. Further, as described above, since the nip mark is more likely to occur in a low humidity environment, the number of sheets to be controlled after resuming the image forming operation is increased.
[0061]
The control of the charging bias in accordance with the change in the temperature and humidity may be performed based on the value of the moisture in a certain volume in the air obtained from the value of the temperature and humidity.
[0062]
As described above, in order to improve the potential convergence only when charging is normally performed under a bias condition that has a small effect on the photosensitive drum 1 and it is determined that the nip mark has an effect on the image, By controlling the charging bias for a limited time, it is possible to achieve both the prevention and suppression of the pitch unevenness of the image density and the suppression of the shortening of the photosensitive drum life.
[0063]
<Embodiment 2>
The image forming apparatus according to the second embodiment is different from the first embodiment in that a method of controlling the charging bias to the charging roller 2 based on the time during which no image formation is performed and the environmental conditions at the time of restarting the image forming operation. Are different. Other points are the same as in the first embodiment. Therefore, the image forming apparatus according to the second embodiment can also be applied to the above-described image forming apparatus shown in FIG.
[0064]
In this embodiment, as shown in FIG. 7, during normal image formation, a DC bias is applied as a charging bias, and control is performed for 30 sheets after being left for a predetermined time, as shown in FIG. Depending on the leaving time and environmental conditions, there are a case where only the same DC bias is applied as usual, and a case where a bias in which AC is superimposed on DC is applied for improving the potential convergence. When DC + AC is applied as a charging bias for 30 sheets in a case where the influence of the nip mark is liable to occur, when the standing time is long, in a low humidity environment, or a combination thereof, the influence of the nip mark can be prevented from appearing on the image.
[0065]
Also in the present embodiment, in order to improve the potential convergence only when charging is normally performed under a bias condition that has a small influence of the discharge to the photosensitive drum 1 and it is determined that the nip mark has an effect on the image. By controlling the charging bias only for the minimum necessary period, it is possible to achieve both the prevention and suppression of the uneven pitch of the image density and the suppression of the shortening of the photosensitive drum life.
[0066]
<Embodiment 3>
The image forming apparatus according to the third embodiment is different from the first and second embodiments in that the charging bias to the charging roller 2 is determined by the time during which the image formation is not performed and the environmental conditions when the image forming operation is restarted. The control method is different. Other points are the same as those of the first and second embodiments. Therefore, the image forming apparatus according to the second embodiment can also be applied to the above-described image forming apparatus shown in FIG.
[0067]
In the present embodiment, a DC bias is applied during normal image formation, and control is performed as shown in FIG. 8 for a predetermined number of sheets after being left for a predetermined time. Depending on the leaving time and environmental conditions, only the same DC bias is applied as usual, a bias in which AC is superimposed on DC for improving the potential convergence, and a bias in which the peak voltage of the AC component is further increased is applied. There are three types. DC + AC is applied in cases where the influence of the nip mark is likely to occur, when the standing time is long, in a low-humidity environment, or in a combination thereof, and when the conditions are more severe, an AC bias having a higher peak voltage with better potential convergence is superimposed. By doing so, the influence of the nip mark can be prevented from appearing in the image.
[0068]
In this embodiment, three types of bias have been described. However, the control may be performed by setting the AC current value more finely.
[0069]
Also in the present embodiment, in order to improve the potential convergence only when charging is normally performed under a bias condition that has a small influence of the discharge to the photosensitive drum 1 and it is determined that the nip mark has an effect on the image. By controlling the charging bias only for the minimum necessary period, it is possible to achieve both the prevention and suppression of the uneven pitch of the image density and the suppression of the shortening of the photosensitive drum life.
[0070]
<Embodiment 4>
The image forming apparatus according to the fourth embodiment is different from the above-described first to third embodiments in that the charging bias to the charging roller 2 depends on the time during which no image formation is performed and the environmental conditions at the time of restarting the image forming operation. The control method is different, and the control method is changed depending on the image forming mode and the difference in resolution. Other points are the same as those of the first and second embodiments. Therefore, the image forming apparatus according to the second embodiment can also be applied to the above-described image forming apparatus shown in FIG.
[0071]
In the present embodiment, a DC bias is applied during normal image formation, and control is performed as shown in FIGS. 9 and 10 for a predetermined number of sheets after leaving for a predetermined time. FIG. 9 shows a case where the mode after the image forming operation is resumed is the facsimile (Fax) mode or the character mode. FIG. 10 shows a case of a photograph mode or a high resolution mode in which uneven charging is more likely to appear in an image.
[0072]
As shown in FIGS. 9 and 10, in the case of the Fax / character mode, the influence of uneven charging is less likely to appear on the image. Are superimposed. On the other hand, in the case of the photograph / high-resolution mode, even if the charging roller 2 is deformed similarly to the fax / character mode, the influence on the image is relatively large. Good potential convergence such as superposition and AC component enhancement is used. Further, the predetermined number of bias enhancement control after the image forming operation is restarted is also increased.
[0073]
Note that, in the present embodiment, three types of bias have been described in FIG. 10, but control may be performed by setting the AC current value more finely.
[0074]
Also in the present embodiment, in order to improve the potential convergence only when charging is normally performed under a bias condition that has a small influence of the discharge to the photosensitive drum 1 and it is determined that the nip mark has an effect on the image. By controlling the charging bias only for the minimum necessary period, it is possible to achieve both the prevention and suppression of the uneven pitch of the image density and the suppression of the shortening of the photosensitive drum life.
[0075]
<Embodiment 5>
In the first to fourth embodiments, the temperature and the humidity at the time when the image forming operation is restarted are used as the environmental conditions used by the image forming apparatus left for a predetermined time for the control determination after the image forming operation restarts. It is preferable to take into account data on temperature and humidity at a certain time before the restart of the forming operation.
[0076]
As described above, for example, in an office environment where the image forming apparatus is installed, for example, when the power of the image forming apparatus is turned on after a certain period of time when the cooling / heating switch is turned on when work in the office is started, or When the image formation is restarted, there may be a difference between the temperature and the humidity around and inside the image forming apparatus. Even if there is no difference in temperature and humidity between the inside and outside of the image forming apparatus, if there is a sudden change, the characteristics of the photosensitive drum 1 and the charging roller 2 may have the original environmental conditions.
[0077]
Therefore, not only the temperature and humidity at the time of resuming the image forming operation, but also always monitor the temperature and humidity at regular time intervals, store the monitoring results, and make a determination using data that has been retroactive for a certain period of time since the resuming of the image forming operation. good.
[0078]
As an example, a method is used in which temperature and humidity data is collected every hour, and the data during a period of 10 hours from when the image forming operation is restarted is simply averaged. Alternatively, the data may be weighted and processed for a time close to the time when the image forming operation is restarted. Alternatively, only data from three hours before the restart of the image forming operation to the restart of the image forming operation may be used, and these may be appropriately selected.
[0079]
In the description of each of the above-described embodiments, the time left without forming an image, the number of charging bias enhancement control after resuming the image forming operation, the environmental conditions, and the like are shown as examples in FIGS. Although shown, it is not limited to the numerical values in these figures, and can be changed as appropriate. Further, the control may be performed not only by the number of sheets but also by the time after the restart of the image forming operation or a combination of both. For example, the shape of the charging roller 2 is relaxed after a certain period of time even if the number of images formed after the image forming operation is restarted is small, so that the normal charging bias may be returned.
[0080]
Further, the number of sheets for the charging bias enhancement control after the image forming operation is restarted may be changed in the continuous mode or the intermittent mode. This is because the total number of rotations of the charging roller 2 is different between, for example, performing image formation on 30 continuous 30 sheets and performing 30 times for each sheet.
[0081]
Further, the situation differs depending on the outer shape and hardness of the charging roller 2, the prescription, the outer diameter of the photosensitive drum 1, the pressure contact force between them, and the like.
[0082]
Further, the present invention is not limited to the configuration of the image forming apparatus shown in FIG. 1, but is applicable to various image forming apparatuses such as black and white and color printers, copiers, and facsimile machines.
[0083]
As a charging member for charging the photosensitive drum 1, a foaming sponge having an appropriate resistance value around a conductive metal core (shaft) such as metal, or a charging roller 2 provided with rubber is preferably used. However, other charging members, for example, a charging roller having a layer configuration different from that described above, or a charging member such as a rotary fur brush roller or a magnetic brush roller may be used.
[0084]
The time left without forming an image can be measured by a timer. However, if a battery is used as a power supply for supplying power to the timer and the temperature / humidity sensor 23, a power supply of the image forming apparatus main body is used. Temperature and humidity can be constantly monitored over time regardless of ON / OFF, and data can be stored sequentially. Further, by replacing data after a certain period of time with new data, the memory capacity can be minimized.
[0085]
In the above embodiment, the recovery state of the charging roller 2 as the charging member differs depending on the usage history of the charging roller 2. For example, the charging roller 2 in the initial stage of use recovers quickly, whereas the charging roller 2 with advanced durability takes a long time to recover, so that the nip mark does not easily disappear. Further, depending on the charging roller, the hardness may change due to the durability, and conversely, the charging roller with the advanced durability may have less nip marks. Therefore, a use history detecting unit (for example, a timer for integrating the time from the start of use of the charge roller 2: not shown) for detecting the use history of the charge roller 2 is provided to check the use history (use time) of the charge roller 2. The charging bias control is changed according to the detection result. Thus, even if there is a difference in the recovery of the deformed state between the first use and the endurance of the charging roller 2, by controlling the charging bias only for a necessary minimum, the pitch unevenness of the image density can be prevented. It is possible to achieve both suppression and suppression of shortening of the photosensitive drum life.
[0086]
【The invention's effect】
As described above, according to the present invention, even if the charging member is deformed without performing the image forming operation for a long time, even if the charging member is deformed for a certain period of time after the image forming operation is restarted, The charging bias applied to the charging member by the charging bias applying power source is set to be different from that during the normal image forming operation based on the outputs of the stop time detecting unit and the environmental condition detecting unit (for example, the surface of the image carrier). Is controlled so that the potential of the image is easily converged), the pitch unevenness of the image density can be prevented or suppressed. In addition, since the convergence state can be controlled depending on the environmental conditions, the convergence is enhanced under the environmental conditions in which the pitch unevenness is more likely to occur, whereby the pitch unevenness of the image density can be prevented or suppressed. In any case, since the charging bias is controlled only when the convergence is enhanced, it is possible to minimize the shortening of the service life of the entire image forming apparatus.
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view illustrating a schematic configuration of an image forming apparatus according to the present invention.
FIG. 2 is a diagram illustrating a relationship between a DC bias by a DC charging bias and a photosensitive drum surface potential.
FIG. 3 is a diagram illustrating a relationship between a DC bias by an AC + DC charging bias and a photosensitive drum surface potential.
FIG. 4 is a diagram showing the relationship between the number of image formations and the amount of photosensitive drum scraping for DC charging, AC + DC charging bias, and AC (enhanced) + DC charging bias.
FIG. 5 is a schematic diagram showing a state of surface potential unevenness due to a nip mark at each charging bias.
FIG. 6 is a diagram illustrating an example of a charging bias control determined based on an idle time of the image forming apparatus and an environmental condition when the image forming operation is restarted in the first embodiment.
FIG. 7 is a diagram illustrating an example of charging bias control determined based on an idle time of an image forming apparatus and environmental conditions at the time of restarting an image forming operation in the second embodiment.
FIG. 8 is a diagram illustrating an example of a charging bias control determined based on an idle time of an image forming apparatus and an environmental condition when an image forming operation is restarted in a third embodiment.
FIG. 9 is a diagram illustrating an example of charging bias control determined from an idle time of an image forming apparatus and environmental conditions at the time of resuming an image forming operation when an image forming mode is a Fax mode or a character mode in the fourth embodiment. It is.
FIG. 10 illustrates an example of charging bias control determined from an idle time of an image forming apparatus and an environmental condition when image forming operation is resumed when an image forming mode is a photograph mode / high resolution mode in the fourth embodiment. FIG.
FIG. 11 is a longitudinal sectional view illustrating a schematic configuration of a conventional image forming apparatus.
[Explanation of symbols]
1 Image carrier (photosensitive drum)
2 Charging member (charging roller)
3 Exposure means (exposure equipment)
4 Developing means (developing device)
5 Transfer conveyor belt
8 Transfer roller (transfer means)
11 Fixing means (fixing device)
12 Cleaning means (cleaning device)
21 Power supply for charging bias
22 control means (bias control means)
23 Environmental detection means (temperature and humidity sensor)
24 Stop time detecting means (image forming stop time measuring means)
P transfer material

Claims (9)

In an image forming apparatus for charging the surface of the image carrier by applying a charging bias to a charging member disposed in contact with the surface of the image carrier,
A charging bias application power source for applying a charging bias to the charging member,
Control means for controlling the charging bias of the charging bias application power supply,
A stop time detecting means for detecting a time when the image forming operation is not performed;
Environmental condition detecting means for detecting an environmental condition near the charging member,
The control unit controls the charging bias application power supply to the charging member based on an output of the stop time detecting unit and the environmental condition detecting unit during a predetermined time after the image forming operation is restarted or during a predetermined number of images formed. The charging bias to be applied is controlled so as to be different from that during a normal image forming operation.
An image forming apparatus comprising: The control unit controls the charging bias application power supply to the charging member based on an output of the stop time detecting unit and the environmental condition detecting unit during a predetermined time after the image forming operation is restarted or during a predetermined number of images formed. Controlling the charging bias to be applied to a condition under which the potential of the surface of the image carrier easily converges,
The image forming apparatus according to claim 1, wherein: A use history detection unit for detecting a use history of the charging member,
The controller controls the charging bias based on outputs from the stop time detector, the environmental condition detector, and the usage history detector during a certain period of time or after a certain number of images are formed after the image forming operation is restarted. Controlling the charging bias applied to the charging member by the applied power supply to a condition under which the potential of the surface of the image carrier is easily converged;
The image forming apparatus according to claim 1, wherein: The environmental condition detected by the environmental condition detecting means is a temperature and a humidity near the charging member when the image forming operation is restarted.
The image forming apparatus according to claim 1, wherein: The environmental conditions detected by the environmental condition detecting means are a temperature and a humidity near the charging member when the image forming operation is restarted, and a temperature and a humidity from when the image forming operation is restarted to when the image forming operation is restarted. ,
The image forming apparatus according to claim 1, wherein: The control unit applies only a DC bias during normal image formation as a charging bias applied to the charging member by the charging bias application power supply, and applies a DC bias for a certain time after the image forming operation is restarted or for a certain number of sheets. Control to apply a bias in which AC is superimposed,
The image forming apparatus according to claim 1, wherein: The control means applies a bias in which AC is superimposed on DC during normal image formation as a charging bias applied to the charging member by the charging bias application power source, and applies a predetermined time or a fixed number of sheets for a predetermined time after the image forming operation is restarted. During the period, AC having a larger peak-to-peak voltage than during normal image formation is superimposed,
The image forming apparatus according to claim 1, wherein: Adopting a digital method with multiple image forming modes, multiple image modes, and multiple resolutions,
According to these image forming modes, image modes, and resolutions, after the image forming operation is restarted, the charging bias application power supply changes the charging bias conditions applied to the charging member and the timing of returning to the normal charging bias.
The image forming apparatus according to claim 1, wherein: The charging member is a roller-shaped charging member,
The image forming apparatus according to any one of claims 1 to 8, wherein:

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