JP2008065355A - Image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent carrier from adhering to the surface of an image carrier or surface staining from occurring, by keeping the surface potential more uniform. <P>SOLUTION: This image forming device is provided with a controller 50 that functions as a voltage application control means, for applying voltage to be applied between the core bar 2a of an electrifying member 2 and a photoreceptor 1 by increasing the DC voltage stepwise as an absolute value after AC voltage rises, when the power source of the device is turned on and when an image is formed, whereby the overshoot of the voltage at ication of the voltage is restrained, and the surface potential is kept more uniform. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image forming apparatus including a charging device that charges a surface of an image carrier by applying a voltage in which an AC voltage and a DC voltage are superimposed between a charging member and the image carrier.

2. Description of the Related Art Conventionally, there is an image forming apparatus provided with a charging device that charges a surface of an image carrier by applying a voltage in which an alternating voltage and a direct current voltage are superimposed between a charging member and the image carrier (for example, Patent Documents). 1).
As described above, in an image forming apparatus in which a voltage obtained by superimposing an AC voltage and a DC voltage is applied to the charging member, the AC voltage and the DC voltage are applied simultaneously, or the alternating current (AC) rise time and the DC ( In consideration of the rise time of DC), the timing of applying the AC voltage and the DC voltage is adjusted so that the AC rise time and the DC rise time are set to the same timing.
For example, when 200 msec is required for the AC voltage to rise and 50 msec is required for the DC voltage to rise, the DC voltage is applied 150 msec after the AC voltage is applied, and the AC and DC voltage rises. The timing is the same.
Japanese Patent Laid-Open No. 5-289441

The charged potential on the surface of the image carrier charged in this way generally needs to be stable. In particular, if the background potential corresponding to the difference between the charged potential on the surface of the image carrier and the development bias becomes too high, carrier adhesion occurs on the surface of the image carrier, and the background potential becomes too low. If the charged potential on the surface of the image bearing member and the developing bias are reversed, background stains may occur.
Therefore, it is preferable to control the charging potential of the image carrier as finely as possible. The linear velocity at which the charging section does not rise to the desired potential due to voltage overshoot or the linear velocity of the rotating image carrier is too fast. It is preferable that no follow-up failure occurs.

The voltage overshoot when the applied voltage is turned on / off is more likely to occur when the applied voltage is higher, and the charged potential is likely to be unstable. When such an unstable charging potential occurs, horizontal streak-like background contamination and carrier adhesion in which carriers adhere to the surface of the image carrier are likely to occur.
When the background stain occurs, a load is applied to the cleaning device for cleaning the surface of the image carrier. In particular, the image transfer system performs transfer by bringing the transfer unit into contact with the surface of the image carrier. In the case of the system, the transfer unit is soiled with toner, and thus the transfer paper is stained on the back.
Further, when the carrier adheres to the surface of the image carrier, the carrier enters the cleaning device, so that the edge portion that is in sliding contact with the surface of the image carrier of the cleaning blade of the cleaning device is lost by the carrier. When this happens, a cleaning failure occurs.

In addition to this, when the background stain occurs, unnecessary toner is consumed every time the image is formed, so that the toner is consumed quickly, so that the running cost increases. Further, the development quality may be deteriorated due to a carrier drop.
Further, when the charging potential increases due to the voltage overshoot described above, not only the carrier but also the reversely charged toner comes to adhere to the surface of the image carrier. When such a reversely charged toner adheres to the surface of the image carrier, the cleaning device is set so as to efficiently remove the toner charged to a normal charged potential in a normal case. Since it is difficult to remove, it often affects the image.
The present invention has been made in view of the above-described problems, and by making it possible to keep the background potential more uniform at the start or stop of image formation, the surface of the image carrier is provided. The purpose is to prevent unnecessary carriers from adhering to the substrate or causing background stains.

In order to achieve the above object, the present invention provides an image provided with a charging device that charges the surface of the image carrier by applying a voltage in which an AC voltage and a DC voltage are superimposed between the charging member and the image carrier. In the forming device,
When the power supply of the device is turned on and at the time of image formation, voltage application control means is provided for applying the above voltage by gradually increasing the DC voltage in an absolute value after the AC voltage rises. is there.
And the voltage applied at the beginning of the DC voltage to be applied by raising the voltage in absolute steps in that step may be the lower limit of the absolute value of the DC stable output range of the power pack. In addition, when the power supply of the device is turned on and at the time of imaging, an AC voltage is applied after applying a lower limit DC voltage with the absolute value of the DC stable output range of the power pack, and the AC voltage is output stably. The image forming apparatus may be configured by providing voltage application control means for controlling the DC voltage so as to increase to the set value after the above.
Furthermore, when raising the voltage from the lower limit DC voltage of the DC stable output range of the power pack to the set value, it is preferable to gradually increase the voltage by an absolute value in steps.
After the driving of the moving image carrier rises to a stable moving speed, it is preferable to apply a first direct current voltage between the charging member and the image carrier.
In addition, in an image forming apparatus including a charging device that charges the surface of the image carrier by applying a voltage in which an alternating voltage and a DC voltage are superimposed between the charging member and the image carrier.
When the application of the voltage applied between the charging member and the image carrier is stopped, the voltage is applied to the AC voltage and the DC voltage in a stepwise manner so that the absolute values of both the AC voltage and the DC voltage are reduced, and finally the voltage application is stopped. Control means may be provided.
This image forming apparatus is effective when the transfer means that is in contact with the surface of the image carrier includes a contact transfer device that transfers a visible image formed on the surface of the image carrier to a transfer material. It is also effective when a cleaning device is provided for cleaning the surface of the image carrier by bringing a cleaning blade into sliding contact with the surface of the image carrier.

According to the present invention, the following effects can be obtained.
When the power supply of the image forming apparatus is turned on and during image formation, the voltage applied between the charging member and the image carrier is applied by gradually increasing the DC voltage in an absolute value after the AC voltage rises. As a result, potential unevenness due to voltage overshoot when the charging voltage is turned on can be suppressed, and the background potential can be kept more uniform. Therefore, it is possible to prevent potential disturbance during warm-up when the voltage is raised, and to suppress background contamination and carrier adhesion to the image carrier.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a schematic configuration diagram showing the main configuration of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a voltage overload when the charging device of the image forming apparatus applies an AC voltage for charging. FIG. 3 is a diagram showing a voltage overshoot when the charging device of the image forming apparatus applies a DC voltage for charging.
In the image forming apparatus shown in FIG. 1, a roller-shaped charging member 2 is disposed so as to have a predetermined gap with respect to the surface of a photoreceptor 1 that is an image carrier that rotates in the direction of arrow A. Then, a voltage obtained by superimposing an AC voltage from the AC power supply 11 and a DC voltage from the DC power supply 12 between the metal core 2 a formed of the metal of the charging member 2 and the photoconductor 1 is applied to the photoconductor 1. A charging device 7 for charging the surface is provided.

In this image forming apparatus, the surface of the photosensitive member 1 rotating in the direction of arrow A as described above during image formation is uniformly charged by the charging member 2, and the charged surface is exposed by the laser light emitted from the exposure unit 3. Then, a latent image corresponding to the image to be created is formed there.
A developing device 4 is provided on the downstream side in the rotation direction of the photoreceptor 1 with respect to the exposure position by the exposure unit 3. In the tank 4a of the developing device 4, the two-component developer composed of toner and carrier is agitated, and the surface of the developing roller 4b is raised at the developing portion where the developing roller 4b and the photoreceptor 1 are closest to each other. The brush of the magnetic brush composed of the carrier in the state rubs against the surface of the photoconductor 1 and the toner adhering to the carrier adheres to the latent image on the photoconductor 1, whereby the latent image is developed and the toner is developed. Become a statue.

On the other hand, the transfer paper P is sent out from the tray 8 by a paper feed roller 9 that rotates in the direction of the arrow in FIG. 1, and is temporarily stopped by a registration roller 10 composed of a pair of rollers. The sheet is conveyed toward a certain transfer position of the contact transfer device 5 at an exact timing that coincides.
At the transfer position, the toner image on the photoconductor 1 is transferred so that the photoconductor 1 and the transfer paper P are in contact with each other by an electric field formed by a potential difference between the bias applied to the contact transfer device 5 and the photoconductor 1. Are transferred onto the transfer paper P. Then, the transfer paper P onto which the toner image has been transferred is conveyed to the fixing device 13 where the toner image is fixed and then discharged to a paper discharge tray (not shown).
Residual toner remaining on the photoreceptor 1 without being transferred by the contact transfer device 5 is cleaned by the cleaning blade 6 a of the cleaning device 6.

By the way, in this image forming apparatus, the surface of the photosensitive member 1 is charged by applying a voltage in which an alternating voltage and a direct current voltage are superimposed between the charging member 2 and the photosensitive member 1 as described above. If the AC voltage is set to a peak-to-peak voltage that is twice or more the discharge start voltage at the maximum gap position between the surface of the charging member 2 and the surface of the photoconductor 1, the surface of the photoconductor 1 is set to a DC applied voltage. They can be charged to substantially the same potential.
Here, the maximum gap position means that when the roller-shaped charging member 2 and the drum-shaped photoconductor 1 rotate together, they rotate while being decentered due to the straightness of the outer diameter of each roller. The mutual gap at the closest position changes depending on the rotational position, and among the changing gaps, the gap that is the largest gap at the closest position is assumed.
As described above, the charging potential on the surface of the photoreceptor 1 is determined depending on whether an AC voltage is applied more than a necessary value or the DC voltage is stable.

Incidentally, it takes about 200 msec for the AC voltage applied by the high-voltage AC power supply 11 to rise. The rising of the DC voltage applied by the DC power supply 12 also takes about 50 msec. The rise of these voltages (until the set voltage is reached) does not rise linearly, but the voltage is not output for less than 10 msec after receiving the voltage application ON signal, and in the case of AC after 10 msec. As shown in FIG. 2 and in the case of direct current, as shown in FIG. 3, it starts to overshoot, alternating current stabilizes after 200 msec, and direct current stabilizes after 50 msec.
Here, for example, if AC and DC are simultaneously applied at the same timing, as has been done conventionally, the AC voltage and DC voltage are not rising immediately after application of the voltage, as described above. The potential is 0V. When the DC voltage rises, if the voltage at that time exceeds the discharge start voltage, the battery is charged slightly. In addition, when potential overshoot occurs, only the overshooted portion becomes higher in charging potential than the surroundings where no overshoot occurs. For this reason, the portion where the charging potential is increased due to the overshoot increases the background potential described above, so that the carrier tends to adhere to the photosensitive member side.

On the other hand, as shown in FIG. 2, the AC voltage side also has an overshoot, but the AC overshoot has a symmetrical shape that overshoots on both the positive and negative sides. Therefore, the charging potential on the photoreceptor 1 is as shown in FIG.
In this case, the overshoot amount of the charging potential varies depending on the DC voltage applied at that time. That is, as shown in FIG. 5, the higher the DC voltage, the higher the overshoot of the output voltage and the larger the overshoot of the charging potential. In addition, as shown in FIG. 6, when the DC voltage is low, the output voltage overshoot is reduced and the charging potential overshoot is also reduced.
In other words, even if the AC voltage does not rise sufficiently and is charged only with a DC voltage, if the DC voltage exceeds the discharge start voltage, the charging potential is approximately the amount of overshoot of the DC applied voltage. Highly charged. However, when the DC voltage does not reach the discharge start voltage, it is charged higher by the amount obtained by subtracting the discharge start voltage from the overshoot.

For example, when the DC voltage overshoots, the distance (gap) between the photoreceptor 1 and the charging member 2 is small, the discharge start voltage is about −600 V, and the DC applied voltage is −700 V. When a voltage of −900 V is partially applied due to overshoot, the whole is charged to −100 V by the difference between −700 V and −600 V, but only the overshooted portion is charged to −300 V.
On the other hand, when the distance between the photosensitive member 1 and the charging member 2 is wide (gap is large) and the discharge start voltage is −720 V, most portions are not charged at all and only the overshooted portion is − It will be charged by -180V which is the difference between 900V and -720V.

As another voltage application method, there is a method of delaying the application time of the DC voltage so as to match the rising time of the AC voltage. However, in this case, at the timing of applying the DC voltage, the AC voltage has reached approximately twice or more of the discharge start voltage, and the AC overshoot has already settled down. The applied voltage is substantially the same value.
That is, when the DC applied voltage is -700 V and a voltage of -900 V is partially applied due to overshoot, it is charged to approximately -700 V as it is, and only the overshoot portion is charged to -900 V. Charged potential.
In any case, if the amount of overshoot of the applied voltage is small, the smaller the amount is, the more stable the charging potential is.

Therefore, in the image forming apparatus according to this embodiment, when the apparatus is turned on and at the time of image formation, a voltage applied between the cored bar 2a of the charging member 2 and the photosensitive member 1 shown in FIG. Is provided with a control device 50 that functions as a voltage application control means for applying a DC voltage stepwise by increasing the absolute voltage after the AC voltage rises.
The control device 50 includes a central processing unit (CPU) having various determination and processing functions, a ROM storing each processing program and fixed data, a RAM serving as a data memory storing processing data, and an input / output circuit (I / O).
The control device 50 controls the AC power supply 11 and the DC power supply 12 to control the voltage applied between the charging member 2 and the photoreceptor 1.

That is, in consideration of the occurrence of overshoot when a DC voltage is applied (turned on), the control device 50 divides the applied DC voltage into two stages, and the first stage of application is the set voltage. Apply a lower DC voltage to suppress overshoot to a lesser extent, and in the next second stage (the applied voltage may be increased stepwise three times or more), the DC voltage is set to the target charging potential. The DC power supply 12 is controlled so as to increase to the set voltage that provides the above.
By the way, when the charging potential is increased stepwise in this way, the potential (developing bias) of the developing device 4 must be increased stepwise. However, the charging member 2 is not contacted with the photosensitive member 1 and is DC without contact. When only the voltage is applied, it is difficult to keep the background potential uniform because the charged potential of the photosensitive member 1 changes due to the change in the gap between the charging member 2 and the photosensitive member 1.

Therefore, in this image forming apparatus, in order to stabilize the charging potential of the photoreceptor 1, a peak that is at least twice the discharge start voltage at the maximum gap position between the surface of the charging member 2 and the surface of the photoreceptor 1 is described. The DC voltage is increased stepwise while an AC voltage that is an inter-voltage is applied between the charging member 2 and the photoreceptor 1.
The voltage applied to the first stage of the DC voltage is preferably the lower limit of the range in which the output of the DC voltage is stabilized in order to reduce overshoot. Specifically, if the DC output guaranteed range of the power pack is -100V to -1000V, by using -100V that is the lower limit in absolute value, the charging potential is made uniform and overshoot is also minimized. It can be pushed to the limit.
Note that the application of the DC voltage is not limited to two stages, and even if the voltage is gradually increased in absolute values by dividing into three or more stages, variations in background potential can be minimized.

As described above, in the image forming apparatus according to this embodiment, the DC voltage to be applied is changed stepwise in consideration of the voltage overshoot when the DC voltage is applied. After the voltage is applied and overshoot is suppressed to a small extent, the DC voltage is raised to a set voltage at which a target charging potential can be obtained, so that a stable charging potential can be obtained. Thereby, it is possible to prevent the background potential from becoming too high.
Therefore, it is possible to prevent the occurrence of horizontal streak-like background stains so that the transfer paper does not cause back stains, and it is possible to prevent the carrier from adhering to the surface of the photoreceptor 1. . Since unnecessary toner is not consumed, the running cost of toner can be suppressed.
Further, since the carrier does not adhere to the surface of the photoreceptor 1, it is possible to prevent the carrier from entering the cleaning device 6 and missing the edge portion of the cleaning blade 6a. Therefore, good cleaning properties can be obtained. Further, it is possible to prevent the development quality from being deteriorated due to a carrier drop.

Next, another embodiment (corresponding to claim 3) will be described with reference to FIG.
FIG. 7 is a schematic structural view similar to FIG. 1 showing another embodiment of the image forming apparatus according to the present invention, and portions corresponding to those in FIG.
This embodiment is different from the embodiment described with reference to FIG. 1 only in that the control content performed by the control device 50 ′ is different from the control device 50.
The control device 50 'applies an AC voltage after applying a lower limit DC voltage in the absolute value of the DC stable output range of the power pack when the power of the device is turned on and during image formation. It functions as a voltage application control means for controlling the DC voltage to be raised to a set value after it has been stably output.
The controller 50 'applies a lower charging DC voltage between the charging member 2 and the photosensitive member 1 in advance, and after the AC voltage is stably output, the voltage is reduced. Control is performed so that the DC voltage is raised to a desired voltage after rising to the set voltage.
In this way, even when the voltage overshoot when the DC voltage for charging is turned on becomes a problem, the AC voltage is not yet applied at that time, and the DC voltage has already been described above. If it is as low as about −100 V as in the example, the discharge start voltage at 1 atmosphere is at least 312 V (absolute value), so no discharge occurs even if overshoot occurs.

In general, the discharge start voltage depends on the film thickness and relative dielectric constant of the photosensitive member 1 but is usually 400 V or more. Therefore, the minimum value at which the power pack DC can be stably output is about -200 V. If so, it has almost no effect.
As described above, after the DC voltage is applied between the charging member 2 and the photoreceptor 1, the AC voltage is applied after waiting for a time for the DC voltage to rise in order to allow for overshoot. Here, when the AC voltage rises, the charging potential on the photoconductor 1 rises to a DC applied voltage value.
Since the charging potential at this time depends on the rising characteristics of the AC voltage and the gap between the charging member 2 and the surface of the photosensitive member 1 at that time, even if it takes 200 msec for the AC voltage to stabilize, the charging potential is actually charged. It is very unstable how many milliseconds it takes for the potential to reach the same level as the DC voltage.

Therefore, the DC voltage to be applied first needs to be a voltage within a range where no discharge occurs and a voltage that does not cause carrier adhesion to the photosensitive member 1 even if the developing bias is zero. The margin of carrier adhesion varies depending on the type of the developing device 4.
Then, the control device 50 ′ shown in FIG. 7 controls the DC voltage so that the charging voltage is raised to the target voltage after the AC voltage has risen until it is stably output. Even at this time, increasing the voltage to be applied stepwise enables fine potential control, so that the margin for background contamination and carrier adhesion increases.
As described above, in this embodiment, a DC voltage lower than the set value is first applied. However, since the voltage is low, there is no particular problem even when the photosensitive member 1 is in a stopped state. There is no such thing as breakdown caused by the chute.

In the case of this embodiment, the DC voltage to be applied is first raised at a low voltage, then the AC voltage is raised, and finally the DC voltage is raised to the set value, so that there are at least three stages. In addition, when the rise to the set value of the last DC voltage is performed stepwise by changing the voltage, it takes more time for the voltage to rise.
Therefore, in the case of this embodiment, the first DC voltage (first stage voltage) is applied before the timing at which the photosensitive member 1 starts to rotate, or at the same time as the photosensitive member 1 rotates. The initial DC voltage may be applied at a stage where the motor for driving the photoreceptor 1 has not yet risen to the stable rotation range.
However, when foreign matter adheres to the opposing surface of the charging member 2 or the photosensitive member 1, current may flow through the portion where the foreign matter is attached to cause leakage, so the photosensitive member 1 is preferably removed. It is preferable to apply the first DC voltage after the motor to be driven rises to a stable rotation range (stable moving speed).

Next, still another embodiment (corresponding to claim 6) will be described with reference to FIG.
FIG. 8 is a schematic structural view similar to FIG. 1 showing still another embodiment of the image forming apparatus according to the present invention, and portions corresponding to those in FIG. 1 are denoted by the same reference numerals.
This embodiment is different from the embodiment described with reference to FIG. 1 only in that the control content performed by the control device 50 ″ is different from that of the control device 50.
When stopping the application of the voltage applied between the charging member 2 and the photoreceptor 1, the control device 50 ″ reduces the voltage stepwise so that the absolute value of both the AC voltage and the DC voltage is reduced. It functions as a voltage application control means for stopping.
In general, in the case of a high voltage system, the above-described voltage overshoot occurs not only when the voltage is turned on, but also when the voltage is turned off. Therefore, if unnecessary charges remain on the surface of the photosensitive member 1 when the photosensitive member 1 is stopped due to the overshoot, there is a possibility that background contamination or carrier adhesion may occur at the start of the next image formation.

The amount of overshoot increases as the voltage output when the off signal is output increases.
Therefore, for example, before turning off the DC voltage applied between the charging member 2 and the photosensitive member 1, the voltage is reduced to -100V or less, or the AC voltage is set to the lowest level. Turn off.
For example, regarding the AC voltage, if the DC voltage is −500 V when the AC voltage is turned off, the discharge start voltage at the position where the gap between the charging member 2 and the photosensitive member 1 is minimized (the closest position). Assuming that the voltage is −600 V, if the AC peak-to-peak voltage is 200 V, there is a possibility that the overcharge will be charged.
Therefore, it is preferable not to turn off (cut) the AC voltage until the DC voltage drops to some extent. For example, if the DC voltage drops to −100V, even if the peak-to-peak voltage is about 200V, it only takes a maximum of −200V. Therefore, there is a margin of 400V with respect to the minimum discharge start voltage, so an overshoot has occurred. However, there is no concern that unnecessary charges remain on the photosensitive member 1 after the charging voltage is turned off.

First embodiment (corresponding to claims 1, 2, 7 and 8)
In the image forming apparatus described with reference to FIG. 1, after the motor for driving the photoconductor 1 has started up in 50 msec, an AC voltage is applied between the photoconductor 1 and the charging member 2, and the AC voltage for charging is applied. When the AC voltage rises 200 msec after application, the DC voltage for charging is gradually increased from −100 V (lower limit in the absolute value of the DC stable output range of the power pack) to −700 V, and the −700 V The developing bias of −500 V is applied to the developing device 4 when the position on the photoconductor 1 to which the voltage is applied reaches the position where the developing roller 4 b of the developing device 4 is located (when the charging potential is −100 V, the developing bias is applied). Even if the voltage is 0 V, no soiling or carrier adhesion occurs), and confirmation was made.
As a result, the output transfer paper was free from back stains and a good image without carrier adhesion was obtained, and there was no problem over time.
However, when the drum surface of the photosensitive member 1 was observed, a slightly larger amount of toner was found on the background. This is because there is a slight difference between the voltage rising curve when the charge is pulled up from -100V to -700V and the voltage rising curve when the voltage of the developing device 4 is increased from 0V to -500V. This seems to be due to variations in the surface potential.

Next, a second embodiment (corresponding to claims 1, 2, 7, and 8) will be described.
In the second embodiment, after the motor for driving the photosensitive member 1 of FIG. 1 has risen to a stable rotation in 50 msec, an AC voltage is started to be applied between the photosensitive member 1 and the charging member 2 to charge the photosensitive member. The position on the photosensitive member 1 where the DC voltage for charging is gradually increased to −100 V, −300 V, −500 V, and −700 V every 20 msec after the AC voltage is applied 200 msec after the AC voltage rises, and a voltage of −300 V is applied As the toner reaches a certain position of the developing roller 4b of the developing device 4, the developing bias is gradually increased to -100V, -300V, and -500V to perform image output.
As a result, the transfer paper was free from back stains and a good image without carrier adhesion was obtained, and there was no problem over time. Further, the drum surface of the photoreceptor 1 was observed, but there was no problem at all.

Next, the confirmation result performed as Comparative Example 1 will be described.
In Comparative Example 1, a signal for applying an AC voltage between the photosensitive member 1 and the charging member 2 after the motor that drives the photosensitive member 1 has risen to a stable rotation, and then applying a DC voltage for charging of −700 V is used. The developing bias is applied when the position on the photosensitive member 1 to which −700 V is applied reaches the developing position, that is, the position where the developing roller 4 b of the developing device 4 is located.
As a result, a horizontal streak-like stain having a width of about 1 to 2 mm, which seems to be affected by the stain on the contact transfer member, occurred on the back side of the transfer paper. Further, when the image forming operation was continued as it was, a cleaning failure by the cleaning device 6 occurred at the stage of passing 20,000 sheets. Therefore, when the cleaning device 6 was opened and confirmed, about 2 g of carrier was contained in the cleaning device 6. When the poorly cleaned portion was seen, the edge of the cleaning blade 6a was missing.

Next, a description will be given of a third embodiment (corresponding to claims 3, 7 and 8) of an image forming apparatus according to the present invention.
In this third embodiment, a charging DC voltage of −100 V (lower limit in absolute value of the DC stable output range of the power pack), which is lower than the set value at the same time as the driving of the motor for driving the photoreceptor 1, is applied to the charging member 2. An AC voltage for charging is applied between the charging member 2 and the photosensitive member 1 after 50 msec. Then, after 200 msec, the charging DC voltage is raised to −700 V, and the position on the photosensitive member 1 to which −700 V is applied is rotated and moved to the developing device 4 in accordance with the timing of reaching the developing portion. A development bias of 500 V was applied and an image was output for confirmation.
According to the confirmation result, a good image was obtained in which there was no stain on the back of the transfer paper and no carrier adhesion. However, on the drum surface of the photoreceptor 1, as in the case of Example 1 described above, toner adhesion that seems to be due to variations in the background potential was slightly observed on the background portion.

Next, a description will be given of a fourth embodiment (corresponding to claims 3, 4, 7 and 8) of an image forming apparatus according to the present invention.
In this fourth embodiment, a DC voltage for charging of −100 V is applied between the charging member 2 and the photosensitive member 1 simultaneously with the driving of the motor for driving the photosensitive member 1, and after 50 msec, an AC voltage for charging is applied. After 200 msec, the charging DC voltage is raised to the set value in steps of -300 V, -500 V, and -700 V.
The developing bias of the developing device is stepwise in accordance with the timing at which the position on the photosensitive member 1 to which the DC voltage of −300 V is applied is rotated to reach the developing unit. It was raised.
As a result, a good image was obtained in which there was no stain on the back of the transfer paper and no carrier adhesion. Further, the image formation was continued as it was, but the problems of back transfer stains and carrier adhesion did not occur over time. Further, no toner adhered to the drum surface of the photoreceptor 1 due to variations in the background potential.

Next, the confirmation result performed as Comparative Example 2 will be described.
In Comparative Example 2, several particles of metal of about 50 μm were scattered between the photosensitive member 1 and the charging member 2 to form an image with the same configuration as in the first and second examples.
As a result, black spots of about φ1 occurred at a pitch corresponding to the circumferential length of the surface of the photoreceptor 1. Therefore, when the surface of the photoreceptor 1 was confirmed, a round hole was formed on the surface. When the hole was enlarged, the edge of the hole melted like a leak.
Next, a description will be given of a fifth embodiment (corresponding to claims 3, 4, 5, 7 and 8) of an image forming apparatus according to the present invention.
In this fifth embodiment, a DC voltage for charging of −100 V is applied between the charging member 2 and the photosensitive member 1 after the motor for driving the photosensitive member 1 rises to a predetermined rotational speed, that is, a stable moving speed. Then, an AC voltage for charging was applied 50 msec later, and the DC voltage for charging was increased stepwise to −300 V, −500 V, and −700 V to a set value after 200 msec.

The developing bias of the developing device 4 is stepwise in accordance with the timing at which the position on the photosensitive member 1 to which the DC voltage of −300 V is applied rotates and reaches the developing unit. It was raised to.
As a result, a good image was obtained in which there was no stain on the back of the transfer paper and no carrier adhesion. Further, the image formation was continued as it was, but the problems of back transfer stains and carrier adhesion did not occur over time. No toner adheres to the drum surface of the photoreceptor 1 due to variations in the background potential.

Next, a description will be given of a sixth embodiment (corresponding to claims 6, 7 and 8) of the image forming apparatus according to the present invention.
In the sixth embodiment, in the image forming apparatus described with reference to FIG. 1, a signal is sent so that the DC voltage for charging is dropped to −100 V at the end of image formation, and then the DC voltage is turned off (application of voltage) after 50 msec. After that, the charging AC voltage was lowered to a peak-to-peak voltage of 200 V and then turned off.
As a result, no horizontal streak-like back stain occurred at the start of the next image formation.
Next, the confirmation result performed as Comparative Example 3 will be described.
In Comparative Example 3, in the configuration of the above-described sixth embodiment, the charging DC voltage was turned off at a stroke from −700 V, and the charging AC voltage was turned off at a stroke from the peak-to-peak voltage of 2 kV.
As a result, when the next image was formed, horizontal streak-like back contamination was always generated at the same position from the leading edge of the transfer paper.

〔The invention's effect〕
As described above, according to the present invention, the following effects can be obtained.
According to the image forming apparatus of any one of claims 1 to 5, when the apparatus is turned on and at the time of image formation, the voltage applied between the charging member and the image carrier is a DC voltage after the AC voltage rises. Since the voltage is gradually increased and applied in steps, potential unevenness due to voltage overshoot when the charging voltage is turned on can be suppressed and the background potential can be kept more uniform. Therefore, it is possible to prevent potential disturbance during warm-up when the voltage is raised, and to suppress background contamination and carrier adhesion to the image carrier.
According to the image forming apparatus of the sixth aspect, when the application of the voltage between the charging member and the image carrier is stopped, the voltage is stepwise so that the absolute value of both the AC voltage and the DC voltage becomes small. Since the voltage application is finally stopped after the voltage is dropped, potential unevenness due to voltage overshoot can be suppressed even when the application of the voltage is stopped, such as when image formation is stopped or warm-up is completed. Therefore, it is possible to suppress background contamination and carrier adhesion to the image carrier when the voltage is cut.

According to the image forming apparatus of claim 7, even if the transfer means in contact with the surface of the image carrier includes the contact transfer device for transferring the visible image formed on the surface of the image carrier to the transfer material, the image carrier Since the carrier does not adhere to the surface of the film and the background dirt does not occur, the surface of the transfer means can be prevented from becoming dirty. As a result, the back surface of the transfer material can be prevented from being stained.
According to the image forming apparatus of the present invention, even if a cleaning device is provided that cleans the surface of the image carrier by sliding the cleaning blade on the surface of the image carrier, the carrier adheres to the surface of the image carrier. Therefore, it is possible to prevent the blade edge from being chipped by the carrier on the image carrier.

  The present invention can be applied to an image forming apparatus provided with a charging device.

1 is a schematic configuration diagram illustrating a main configuration of an image forming apparatus according to an embodiment of the present invention. Similarly, the charging device of the image forming apparatus is a diagram showing a voltage overshoot when an AC voltage for charging is applied. FIG. 5 is a diagram showing voltage overshoot when the charging device of the image forming apparatus applies a DC voltage for charging. Similarly, the charging device of the image forming apparatus is a diagram showing a charging potential when a voltage in which alternating current and direct current are superimposed is applied. It is a diagram which shows the overshoot of an output voltage waveform when a DC voltage is high. Similarly, it is a diagram showing an overshoot of the output voltage waveform when the DC voltage is low. FIG. 3 is a schematic configuration diagram similar to FIG. 1 showing another embodiment of the image forming apparatus according to the present invention. FIG. 2 is a schematic configuration diagram similar to FIG. 1 showing still another embodiment of the image forming apparatus according to the present invention.

Explanation of symbols

1: Photoconductor (image carrier) 2: Charging member 4: Developing device 5: Contact transfer device 6: Cleaning device 6a: Cleaning blade 7: Charging device 50, 50 ', 50 ": Control device (voltage application control means)

Claims (8)

In an image forming apparatus including a charging device that charges a surface of the image carrier by applying a voltage in which an alternating voltage and a DC voltage are superimposed between a charging member and the image carrier,
When the power source of the apparatus is turned on and at the time of image formation, voltage application control means is provided for applying the voltage by gradually increasing the DC voltage stepwise after the AC voltage rises. An image forming apparatus.   2. The image according to claim 1, wherein the voltage applied at the beginning of the DC voltage applied by raising the voltage stepwise in absolute steps is the lower limit of the absolute value of the DC stable output range of the power pack. Forming equipment. Image formation provided with a charging device for charging the surface of the image carrier by applying a voltage in which an alternating voltage and a direct current voltage are superimposed between the charging member and the image carrier on which the surface facing the charging member moves In the device
When the power of the device is turned on and at the time of imaging, an AC voltage is applied after applying a lower limit DC voltage in the absolute value of the DC stable output range of the power pack so that the AC voltage is output stably. An image forming apparatus comprising voltage application control means for controlling the DC voltage so as to increase to a set value after becoming.   4. The image according to claim 3, wherein when the voltage is raised from the lower limit DC voltage of the DC stable output range of the power pack to the set value, the voltage is gradually increased in absolute value in steps. Forming equipment.   5. The image according to claim 3, wherein the first DC voltage is applied between the charging member and the image carrier after driving of the moving image carrier has risen to a stable movement speed. Forming equipment. In an image forming apparatus including a charging device that charges a surface of the image carrier by applying a voltage in which an alternating voltage and a DC voltage are superimposed between a charging member and the image carrier,
When the application of the voltage applied between the charging member and the image carrier is stopped, the voltage application is finally stopped by decreasing the voltage stepwise so that the absolute values of both the AC voltage and the DC voltage are reduced. An image forming apparatus comprising a control unit.   7. The contact transfer device for transferring a visible image formed on the surface of the image carrier to a transfer material, wherein the transfer means in contact with the surface of the image carrier is provided. The image forming apparatus described in the item.   8. The image forming apparatus according to claim 1, further comprising a cleaning device that cleans the surface of the image carrier by bringing a cleaning blade into sliding contact with the surface of the image carrier. 9. .

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