JP2003043788A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent carrier from adhering to the surface of an image carrier or surface staining from occurring by keeping surface potential more uniform. SOLUTION: This image forming device is provided with a controller 50 functioning 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 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 the time of applying the voltage is restrained and the surface potential is kept more uniform.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus provided with a charging device for charging the surface of an 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. Regarding the device.

[0002]

2. Description of the Related Art Conventionally, there is an image forming apparatus provided with a charging device for charging the surface of an image carrier by applying a voltage in which an AC voltage and a DC voltage are superposed between the charging member and the image carrier. (See, for example, Japanese Patent Laid-Open No. 5-289441). As described above, in the image forming apparatus in which the voltage in which the AC voltage and the DC voltage are superimposed is applied to the charging member, the AC voltage and the DC voltage are applied at the same time, or the rising time of the AC (AC) 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 rising timing and the DC rising timing are set to the same timing. For example, it takes 200 before AC voltage rises.
msec is required, and 50 mse before the DC voltage rises
c If required, 150ms after applying AC voltage
The DC voltage is applied after ec so that the rising timings of the AC and DC voltages are the same.

[0003]

The charging potential of the surface of the image bearing member thus charged is generally required to be stable. In particular, if the background potential, which is the difference between the charged potential on the surface of the image carrier and the developing 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 carrier and the developing bias are reversed, the background stain may occur. Therefore, it is preferable that the charging potential of the image carrier can be controlled as finely as possible, and 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 tracking failure occurs.

Since the voltage overshoot when the applied voltage is turned on and off is more likely to occur when the applied voltage is higher, the charging potential is likely to be unstable. When such an instability of the charging potential occurs, horizontal stripe-shaped background stains 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 that cleans the surface of the image bearing member. Therefore, in particular, the image transfer method is a contact transfer in which the transfer unit is brought into contact with the surface of the image bearing member to perform the transfer. In the case of the method, the transfer means is soiled by the toner, so that the transfer paper is soiled on the back side. Further, when the carrier adheres to the surface of the image bearing member, the carrier comes to enter the cleaning device, so that the edge portion of the cleaning blade of the cleaning device sliding on the surface of the image bearing member is chipped by the carrier. If it happens, cleaning failure will occur.

In addition to this, when the background stain occurs, unnecessary toner is consumed every time image formation is performed, and the toner is consumed quickly, resulting in an increase in running cost. Further, the developing quality may be deteriorated due to carrier drop. Further, when the charging potential becomes high due to the above voltage overshoot, not only the carrier but also the oppositely charged toner adheres to the surface of the image carrier. When such a reverse-charged toner adheres to the surface of the image carrier, the cleaning device is usually set so as to efficiently remove the toner charged to the regular charging potential. Since it is difficult to remove, it often affects the image. This invention has been made in view of the above problems,
By making it possible to keep the background potential more uniform when starting or stopping image formation, it is possible to prevent unnecessary carriers from adhering to the surface of the image bearing member and to prevent background stains. To aim.

[0006]

In order to achieve the above-mentioned object, the present invention applies a voltage obtained by superposing an AC voltage and a DC voltage between a charging member and an image bearing member to the surface of the image bearing member. In an image forming apparatus equipped with a charging device for charging the
The application of the voltage is provided with voltage application control means for applying the DC voltage stepwise by increasing the absolute value of the DC voltage after the AC voltage rises. Then, the voltage to be applied at the beginning of the DC voltage that is gradually increased in absolute value and applied is preferably the lower limit of the absolute value of the DC stable output range of the power pack. When the power of the device is turned on and at the time of image formation, an AC voltage is applied after applying the lower limit DC voltage in the absolute value of the DC stable output range of the power pack, and the AC voltage is stably output. After that, the image forming apparatus may be configured by providing a voltage application control unit that controls the DC voltage to increase to the set value.

Furthermore, it is preferable to gradually increase the absolute value of the voltage in steps when increasing the DC voltage from the lower limit of the DC stable output range of the power pack to the set value. After the driving of the moving image carrier rises to a stable moving speed, it is preferable to apply the first DC 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 direct current voltage are superimposed between the charging member and the image carrier, When the application of the voltage applied between the image carrier and the image carrier is stopped, a voltage application control means is provided to stop the AC voltage and the DC voltage so that the absolute values of both are gradually reduced and finally stopped. Good.
This image forming apparatus is effective when the transfer means in contact with the surface of the image carrier has a contact transfer device for transferring the visible image formed on the surface of the image carrier to a transfer material.
Further, it is also effective when a cleaning device for cleaning the surface of the image bearing member is provided by a cleaning blade slidingly contacting the surface of the image bearing member.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram showing a main configuration of an image forming apparatus according to an embodiment of the present invention, and FIG. 2 is a voltage excess when a charging device of the image forming apparatus applies an AC voltage for charging. FIG. 3 is a diagram showing a chute, and 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 arranged so as to have a predetermined gap with respect to the surface of a photoconductor 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 is applied between the core metal 2a formed of the metal of the charging member 2 and the photoconductor 1, so that 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 which rotates in the direction of arrow A as described above at the time of image formation is uniformly charged by the charging member 2, and the charged surface is a laser emitted from the exposure unit 3. It is exposed to light to form a latent image corresponding to the created image. A developing device 4 is provided downstream of the exposure position of the exposure unit 3 in the rotation direction of the photoconductor 1. In the tank 4a of the developing device 4, a two-component developer composed of toner and carrier is agitated, and the developing roller 4b and the photoconductor 1 are closest to each other in the developing portion, and the surface of the developing roller 4b rises. The magnetic brush of the carrier in the state of rubbing rubs the surface of the photoconductor 1, and the toner adhering to the carrier adheres to the latent image on the photoconductor 1, so that 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 which rotates in the direction of the arrow in FIG. 1, and is temporarily stopped by a registration roller 10 consisting of a pair of rollers, and then on the photoconductor 1. The toner image is conveyed toward the transfer position where the contact transfer device 5 is located at an accurate timing that matches the toner image. At the transfer position, the toner image on the photoconductor 1 is
By the electric field formed by the potential difference between the bias applied to the contact transfer device 5 and the photoconductor 1, the image is transferred onto the transfer paper P at the transfer portion where the photoconductor 1 and the transfer paper P are in contact with each other. The transfer paper P on which the toner image is transferred is fixed by the fixing device 13.
The toner image is fixed there and then discharged to a paper discharge tray (not shown). The residual toner remaining on the photoconductor 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, as described above, the surface of the photoconductor 1 is charged by applying a voltage in which an AC voltage and a DC voltage are superimposed between the charging member 2 and the photoconductor 1. However, if the AC voltage is set to a peak-to-peak voltage 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 photoconductor 1, the surface of the photoconductor 1 will be DC. It can be charged to substantially the same potential as the applied voltage. Here, the maximum gap position means that when the roller-shaped charging member 2 and the drum-shaped photosensitive member 1 rotate together, they rotate while being eccentric due to the straightness of the outer diameter of each of them. The mutual gaps at the closest position change depending on the rotational position, but among the changing gaps, the positions that are the largest gaps at the closest position are indicated. As described above, the charging potential on the surface of the photoconductor 1 is determined depending on whether the AC voltage is applied at a required value or more or the DC voltage is stable.

By the way, the rising of the AC voltage applied by the high-voltage AC power source 11 is, for example, 200 msec.
It takes a while. It also takes about 50 msec for the DC voltage applied by the DC power supply 12 to rise. And
The rise of these voltages (until the set voltage is reached)
The voltage does not rise linearly, and the voltage is not output for less than 10 msec after receiving the voltage application ON signal.
After 0 msec, in the case of alternating current, it rises while overshooting as shown in FIG. 2 and in the case of direct current, respectively. In FIG. Here, for example, if alternating current and direct current are applied at the same timing at the same time, as is conventionally done, the AC voltage and the direct current voltage do not rise immediately after the application of the voltage, and therefore, substantially no charging occurs. The potential becomes 0V. Then, when the DC voltage rises and the voltage at that time exceeds the discharge start voltage, some charging is performed there. Further, when an electric potential overshoot occurs, the charged electric potential becomes higher in only the overshot portion than in the surrounding area where it does not overshoot. Therefore, in the portion where the charging potential becomes high due to the overshoot, the background potential becomes high and the carrier tends to adhere to the photoreceptor 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 in which it overshoots both positive and negative sides, and as a result, they are smoothed. Therefore, the charging potential on the photoconductor 1 is as shown in FIG. In this case, the amount of overshoot of the charging potential changes depending on the DC voltage applied at that time. That is, as shown in FIG. 5, the higher the DC voltage, the higher the output voltage overshoot and the larger the charging potential overshoot. Further, as shown in FIG. 6, when the DC voltage is low, the overshoot of the output voltage is small and the overshoot of the charging potential is also small. In other words, even if the AC voltage does not rise sufficiently and is charged by only the DC voltage, if the DC voltage exceeds the discharge start voltage, the charging potential is approximately the overshoot of the DC applied voltage. Highly charged.
However, when the DC voltage does not reach the discharge starting voltage, the charging is performed higher by the amount obtained by subtracting the discharge starting voltage from the overshoot.

For example, when the DC voltage overshoots, the distance (gap) between the photoconductor 1 and the charging member 2 is small, the discharge start voltage is about -600 V, and the DC voltage is applied. Is -700V and a voltage of -900V is partially applied due to overshoot, the entire battery is charged to -100V due to the difference between -700V and -600V, but only the overshot part becomes -300V. Get charged. On the other hand, when the distance between the photoconductor 1 and the charging member 2 is large (gap is large) and the discharge start voltage is −720 V, most of the parts are not charged at all, and only the overshot part is −.
Only -180V which is the difference between 900V and -720V is charged.

As another voltage applying method, there is also 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 almost twice the discharge start voltage or more, and the AC overshoot has already settled, so the charging potential is DC. The applied voltage is almost the same as the applied voltage. That is, when the direct current applied voltage is -700V and the voltage of -900V is partially applied due to overshoot,
As it is, it is charged to about -700V, and the charging potential is such that only the overshoot portion is charged to -900V. In any case, if the amount of overshoot of the applied voltage is small, the smaller the overshoot amount of the applied voltage, the more stable the charging potential becomes and the better the result.

Therefore, in the image forming apparatus according to this embodiment, when the apparatus is powered on and at the time of image formation,
The voltage applied between the core metal 2a of the charging member 2 and the photoconductor 1 shown in FIG. 1 is applied by gradually increasing the absolute value of the DC voltage after the AC voltage rises. A control device 50 that functions as voltage application control means is provided. The control device 50 includes a central processing unit (CPU) having various judgment and processing functions, a ROM that stores each processing program and fixed data, a RAM that is a data memory that stores the processing data, and an input / output circuit (I / O) and a microcomputer. 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 photoconductor 1.

That is, the control device 50 divides the DC voltage to be applied into two stages in consideration of the occurrence of overshoot when the DC voltage is applied (turned on), and applies the first stage first. Applies a DC voltage lower than the set voltage to suppress overshoot to a small degree,
The DC power supply 12 is controlled so as to raise the DC voltage to the set voltage at which the target charging potential is obtained in the next second step (the applied voltage may be raised stepwise more than three times). By the way, when the charging potential is increased stepwise as described above, the potential (developing bias) of the developing device 4 must be increased correspondingly. However, the charging member 2 is not brought into contact with the photoconductor 1 and is contactless DC When only the voltage is applied, it is difficult to keep the background potential uniform because the charging potential of the photoconductor 1 changes due to the fluctuation of the gap between the charging member 2 and the photoconductor 1.

Therefore, in this image forming apparatus, the photoconductor 1
In order to stabilize the charging potential of the charging member 2, an AC voltage that becomes a peak-to-peak voltage that is twice or more the discharge start voltage at the above-described maximum gap position between the surface of the charging member 2 and the surface of the photosensitive member 1 is used. The DC voltage is increased stepwise while being applied to the photoconductor 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 stable in order to reduce overshoot. Specifically, the DC output guarantee range of the power pack is −100V to −100.
If it is 0V, the absolute value will be the lower limit of -100V.
By using, the charging potential can be made uniform and the overshoot can be suppressed to the minimum. The application of the DC voltage is not limited to two steps, and even if the voltage is gradually increased in absolute value by dividing into three or more steps, the variation in the background potential can be minimized.

As described above, in the image forming apparatus according to this embodiment, in consideration of the voltage overshoot when the DC voltage is applied, the DC voltage to be applied is changed stepwise, and the DC voltage is first changed from the set voltage. Since a low direct current voltage is applied to suppress the overshoot to a small degree, and then the direct current voltage is raised to a set voltage at which a target charging potential is obtained, a stable charging potential can be obtained. This can prevent the background potential from becoming too high. Therefore, it is possible to prevent the occurrence of horizontal streak-like background stains and prevent backside stains from occurring on the transfer paper, and it is also possible to prevent the carrier from being attached to the surface of the photoreceptor 1 by the carriers. . Further, since the unnecessary toner is not consumed, the running cost of the toner can be suppressed. Further, since the carrier does not adhere to the surface of the photoconductor 1, the carrier enters the cleaning device 6 and the cleaning blade 6a.
It is also possible to prevent the edge portion of the chip from missing. Therefore, good cleaning properties can be obtained. Further, it is possible to prevent deterioration of development quality due to carrier drop.

Next, another embodiment (corresponding to claim 3)
Will be described with reference to FIG. 7. 7 is a schematic configuration diagram similar to FIG. 1 showing another embodiment of the image forming apparatus according to the present invention, and the portions corresponding to those in FIG. 1 are designated by the same reference numerals. This embodiment differs from the embodiment described with reference to FIG. 1 only in that the control contents performed by the control device 50 ′ are different from those of the control device 50. The controller 50 'applies an AC voltage after applying a DC voltage at the lower limit of the absolute value of the DC stable output range of the power pack when the apparatus is powered on and at the time of image formation. It functions as a voltage application control unit that controls the DC voltage to increase to a set value after stable output. The control device 50 'applies a low charging DC voltage between the charging member 2 and the photoconductor 1 in advance so that the AC voltage is stably output, that is, the voltage is Control is performed so that the DC voltage rises to a desired voltage after rising to the set voltage. By doing this, 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 explained. As in the example, if the voltage is set as low as -100V, the discharge start voltage at 1 atm is at least 312V.
Since it is an absolute value, no discharge occurs even if it overshoots.

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

Therefore, the DC voltage applied first is within a range in which no discharge occurs, and the developing bias is 0.
Even so, it is necessary to set the voltage to such a level that carrier adhesion does not occur on the photoconductor 1. It should be noted that the margin of carrier attachment differs depending on the type of the developing device 4. The control device 50 'shown in FIG. 7 controls the DC voltage so that the charging voltage rises to a target voltage after the AC voltage rises until stable output. Also in this case, it is possible to control the potential more finely by increasing the applied voltage stepwise, so that the margin for the background stain and the carrier adhesion is increased. 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 photoconductor 1 is in the stopped state, and the voltage is overvoltage. The shoot does not cause dielectric breakdown.

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. At least three stages of time are required, and when the last rise of the DC voltage to the set value is made to be different in different voltages,
It takes more time for the voltage to rise.
Therefore, in the case of this embodiment, the first DC voltage (1
The voltage of the step) is applied, or the first DC voltage is applied at the same time as the photoconductor 1 rotates (the motor driving the photoconductor 1 has not yet risen to the stable rotation range). Good. However, the charging member 2
When a foreign substance is attached to the opposite surface of the photoconductor 1 or the photoconductor 1, a current may flow through the portion to which the foreign substance is attached to cause a leak. Therefore, the motor that drives the photoconductor 1 is preferably stable. It is preferable to apply the first DC voltage after rising to the rotation range (stable moving speed).

Next, another different embodiment (corresponding to claim 6) will be described with reference to FIG. FIG. 8 is a schematic configuration diagram similar to FIG. 1 showing still another embodiment of the image forming apparatus according to the present invention, and the portions corresponding to those in FIG. 1 are designated by the same reference numerals. This embodiment is different from the embodiment described with reference to FIG. 1 only in that the control content of the control device 50 ″ is different from that of the control device 50. The control device 50 ″ includes the charging member 2 and the photoconductor. When stopping the application of the voltage applied between 1 and
It functions as a voltage application control unit that gradually lowers the AC voltage and the DC voltage so that the absolute values thereof become smaller, and finally stops. Generally, in the case of a high voltage system, the voltage overshoot described above occurs not only when the voltage is turned on but also when the voltage is turned off. Therefore, due to the overshoot, if unnecessary charges remain on the surface of the photoconductor 1 while the photoconductor 1 is stopped, there is a possibility that background stains or carrier adhesion may occur at the start of the next image formation.

The amount of overshoot is
The higher the voltage output when the off signal is output, the higher the voltage. Therefore, for example, before turning off the DC voltage applied between the charging member 2 and the photosensitive member 1, the voltage is dropped to −100 V or less and then turned off, or the AC voltage is also set to the lowest level. Turn it off. For example, with respect to the AC voltage, if the DC voltage is -500 V when the AC voltage is turned off, the position where the gap between the charging member 2 and the photoconductor 1 is the minimum (the closest position)
If the discharge start voltage at −600V is −600V, AC
If the peak-to-peak voltage is 200 V, there is a possibility of being charged by the amount of overshoot. Therefore, it is advisable not to turn off (disconnect) 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, only a maximum of -200V is applied, so there is a margin of 400V with respect to the minimum discharge start voltage, and therefore overshoot occurs. However, there is no concern that unnecessary charges will remain on the photoconductor 1 after the charging voltage is turned off.

[0026]

Embodiment 1 (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 is started in 50 msec, the photoconductor 1
AC voltage between the charging member 2 and the charging member 2 is started, and when the AC voltage rises 200 msec after the AC voltage for charging is applied, the DC voltage for charging is set to −100.
V (lower limit of absolute value of DC stable output range of power pack)
From −700V to the developing roller 4 of the developing device 4 at the position on the photoconductor 1 to which −700V is applied.
When reaching a certain position b, a developing bias of -500V is applied to the developing device 4 (charge potential is -100V).
In that case, even if the developing bias is 0 V, neither background stain nor carrier adhesion occurs). As a result, the transferred transfer paper did not have a back stain and a good image without carrier adhesion was obtained, and there was no problem with time. However, when the surface of the drum of the photoconductor 1 was observed, a large amount of toner slightly on the background was seen. This is the
The rising curve of the voltage when pulling up from 100V to -700V and the voltage of the developing device 4 from 0V to -500
There is a slight difference in the rising curve of the voltage when the voltage rises to V, and it is considered that the background potential is partially varied due to this.

Next, the second embodiment (claims 1, 2, 7, 8)
Will be described). In this second embodiment, FIG.
After the motor for driving the photoconductor 1 rises to a stable rotation in 50 msec, an AC voltage is started to be applied between the photoconductor 1 and the charging member 2, and the AC voltage for charging rises 200 msec after the AC application. , DC voltage for charging is -100V, -300V, -5 every 20msec.
When the position on the photoconductor 1 to which a voltage of −300V is applied reaches a certain position of the developing roller 4b of the developing device 4, a voltage of −100V is gradually applied.
The image was output by gradually increasing the developing bias to V, -300V, -500V. As a result, the transfer paper was free from stains on the back side, and a good image without carrier adhesion was obtained, and there was no problem over time. Further, the photoconductor 1
The surface of the drum was observed, but there was no problem.

Next, the result of confirmation performed as Comparative Example 1 will be described. In Comparative Example 1, a signal for applying an AC voltage between the photoconductor 1 and the charging member 2 after the motor for driving the photoconductor 1 started up to a stable rotation, and then applying a −700V DC voltage for charging. And send -70
The developing bias is applied when the position on the photosensitive member 1 to which 0 V is applied reaches the developing position, that is, the certain position of the developing roller 4b of the developing device 4. As a result, horizontal streak-like stains having a width of about 1 to 2 mm were generated on the back side of the transfer paper, which is considered to be due to the stain on the contact transfer member.
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, the cleaning device 6
When opened and checked, the cleaning device 6 contained about 2 g of the carrier. When the cleaning failure portion was seen, the edge of the cleaning blade 6a was chipped.

Next, the third embodiment of the image forming apparatus according to the present invention will be described.
An embodiment (corresponding to claims 3, 7 and 8) will be described. In the third embodiment, a DC voltage for charging of -100V (lower limit of absolute value of DC stable output range of power pack) lower than the set value is set at the same time as the driving of the motor for driving the photoconductor 1 and the charging member 2. It is applied between the photoconductor 1 and its 50 mse
After c, an AC voltage for charging is applied between the charging member 2 and the photoconductor 1. Then, after 200 msec, the charging DC voltage is raised to -700 V, and the position on the photoconductor 1 to which the -700 V is applied is rotationally moved, so that the developing device 4 is moved to the developing unit 4 in accordance with the timing of reaching the developing unit. 5
A developing bias of 00V was applied, and an image was output and confirmed. According to the confirmation result, a good image was obtained in which the transfer paper was not stained on the back and was not attached to the carrier. However, on the drum surface of the photoconductor 1, as in the case of Example 1 described above, toner adhesion, which is considered to be due to the variation in the background potential, was slightly observed in the background portion.

Next, a fourth image forming apparatus according to the present invention will be described.
Examples (corresponding to claims 3, 4, 7, and 8) will be described. In the fourth embodiment, a DC voltage for charging of -100V is applied between the charging member 2 and the photosensitive member 1 at the same time as the driving of the motor for driving the photosensitive member 1, and 50 msec after that, an AC voltage for charging is applied. The voltage is applied, and 200 msec after that, the DC voltage for charging is gradually increased to a set value of -300V, -500V, and -700V. Then, in accordance with the timing when the position on the photoconductor 1 to which the DC voltage of −300 V is applied is rotationally moved and reaches the developing unit, −1 is set.
The developing bias of the developing device was gradually increased to 00V, -300V, and -500V. As a result, a good image was obtained with no stain on the back of the transfer paper and no carrier adhesion.
Further, although the image formation was continued as it was, the problems such as the back stain of the transfer paper and the carrier adhesion did not occur with the passage of time. No toner adhered to the drum surface of the photoconductor 1 due to the variation in the background potential.

Next, the result of confirmation performed as Comparative Example 2 will be described. In Comparative Example 2, several particles of metal having a size of about 50 μm were scattered between the photoconductor 1 and the charging member 2 to form an image with the same configuration as in the first and second embodiments. As a result, black spots of about φ1 were generated at a pitch corresponding to the peripheral length of the surface of the photoconductor 1. Then, when the surface of the photoconductor 1 was checked, a round hole was opened on the surface. When the hole was enlarged, the edge of the hole was melted like a leak had occurred. Next, a fifth embodiment (corresponding to claims 3, 4, 5, 7, and 8) of the image forming apparatus according to the present invention will be described. In the fifth embodiment, a DC voltage for charging of -100V is applied between the charging member 2 and the photosensitive member 1 after the motor for driving the photosensitive member 1 has risen to a predetermined rotation speed, that is, a stable moving speed. , 50m
AC voltage for charging is applied after sec, and 200 ms
DC voltage for charging after ec is -300V, -500V,
It gradually increased to the set value with -700V.

Then, in accordance with the timing when the position on the photosensitive member 1 to which the DC voltage of −300V is applied is rotationally moved and reaches the developing portion, −100V, −300.
The developing bias of the developing device 4 was gradually increased to V, -500V. As a result, a good image was obtained with no stain on the back of the transfer paper and no carrier adhesion. Further, although the image formation was continued as it was, the problems such as the back stain of the transfer paper and the carrier adhesion did not occur with the passage of time. And
No toner adhered to the drum surface of the photoconductor 1 due to variations in the background potential.

Next, the sixth embodiment of the image forming apparatus according to the present invention will be described.
An example (corresponding to claims 6, 7, and 8) will be described.
In the sixth embodiment, in the image forming apparatus described with reference to FIG. 1, a signal is sent so as to reduce the charging DC voltage to -100 V at the end of image formation, and 50 msec thereafter, the DC voltage is turned off (voltage application). Was stopped), and then the AC voltage for charging was dropped to a peak-to-peak voltage of 200 V and then turned off. As a result, horizontal streak-like back stain did not occur at the start of the next image formation. Next, the confirmation result performed as Comparative Example 3 will be described. In Comparative Example 3, the charging DC voltage is -70 with the configuration of the sixth embodiment described above.
The voltage was turned off at once from 0 V, and the AC voltage for charging was turned off at once from the peak-to-peak voltage of 2 kV. As a result, when the next image was formed, horizontal streak-like back stains were always generated on the transfer paper at the same position from the leading edge.

[0034]

As described above, according to the present invention, the following effects can be obtained. According to the image forming apparatus of claims 1 to 5, the voltage applied between the charging member and the image carrier when the power of the apparatus is turned on and during image formation is
Since the DC voltage is gradually increased in absolute value and applied after the AC voltage rises, potential unevenness due to voltage overshoot when the charging voltage is turned on is suppressed and the background potential is further improved. Can be kept uniform. Therefore, it is possible to prevent the potential from being disturbed at the time of warming up the voltage and suppress the background stain and the carrier adhesion to the image carrier. According to the image forming apparatus of claim 6, when the application of the voltage between the charging member and the image carrier is stopped, the voltage is stepwise so that both the AC voltage and the DC voltage become smaller in absolute value. Since the voltage application is finally stopped after the voltage is dropped, it is possible to suppress the potential unevenness due to the voltage overshoot even when the voltage application is stopped such as when the image formation is stopped or the warm-up is finished. Therefore, it is possible to suppress the background stain when the voltage is cut off and the carrier adhesion to the image carrier.

According to the image forming apparatus of the seventh aspect, even if the transfer means in contact with the surface of the image carrier is equipped with the contact transfer device for transferring the visible image formed on the surface of the image carrier to the transfer material,
Since the carrier does not adhere to the surface of the image carrier and the background stain does not occur, it is possible to prevent the surface of the transfer unit from becoming dirty. As a result, the backside of the transfer material can be prevented from being soiled. According to the image forming apparatus of claim 8, even if a cleaning device for cleaning the surface of the image carrier by sliding the cleaning blade onto the surface of the image carrier is provided, the carrier adheres to the surface of the image carrier. Since it can be prevented that the edge of the cleaning blade is scraped by the carrier on the image carrier, it can be prevented from chipping.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing a main configuration of an image forming apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram showing a voltage overshoot when the charging device of the image forming apparatus applies a charging AC voltage.

FIG. 3 is a diagram showing a voltage overshoot when the charging device of the image forming apparatus applies a DC voltage for charging.

FIG. 4 is a diagram showing a charging potential when the charging device of the image forming apparatus applies a voltage in which alternating current and direct current are superimposed.

FIG. 5 is a diagram showing an overshoot of an output voltage waveform when a DC voltage is high.

FIG. 6 is a diagram showing overshoot of the output voltage waveform when the DC voltage is low.

FIG. 7 is a schematic configuration diagram similar to FIG. 1 showing another embodiment of the image forming apparatus according to the present invention.

FIG. 8 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)

[Claims] 1. An image forming apparatus comprising a charging device for charging 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, When the power is turned on and at the time of image formation, the voltage application control means is provided for applying the voltage by gradually increasing the absolute value of the direct current voltage and applying the voltage after the alternating current voltage rises. Image forming apparatus. 2. The voltage applied at the beginning of the DC voltage which is gradually increased in absolute value and applied is set to the lower limit of the absolute value of the DC stable output range of the power pack. The image forming apparatus according to item 1. 3. A charging device for charging the surface of the image bearing member by applying a voltage in which an AC voltage and a DC voltage are superimposed between the charging member and the image bearing member whose surface facing the charging member moves. In an image forming apparatus equipped with, when the power of the apparatus is turned on and at the time of image formation, an AC voltage is applied after applying the lower limit DC voltage with the absolute value of the DC stable output range of the power pack. The image forming apparatus is provided with a voltage application control unit that controls the DC voltage so that the DC voltage is increased to a set value after stable output. 4. The step of increasing the voltage from the lower limit DC voltage of the DC stable output range of the power pack to the set value by gradually increasing the absolute value of the voltage. Item 3. The image forming apparatus according to item 3. 5. The first DC voltage is applied between the charging member and the image carrier after the driving of the moving image carrier rises to a stable moving speed. Alternatively, the image forming apparatus according to item 4. 6. An image forming apparatus comprising a charging device for charging 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, When the application of the voltage applied between the member and the image carrier is stopped, the voltage application control means for stopping the AC voltage and the DC voltage so that the absolute values of both the AC voltage and the DC voltage are gradually reduced and finally stopped. An image forming apparatus comprising: 7. The transfer device in contact with the surface of the image carrier comprises a contact transfer device for transferring a visible image formed on the surface of the image carrier to a transfer material. The image forming apparatus according to any one of 6 above. 8. The cleaning device according to claim 1, further comprising a cleaning device that cleans the surface of the image carrier by sliding a cleaning blade onto the surface of the image carrier. The image forming apparatus described.