JPH09127765A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prolong the service life of a photoreceptor by reducing the time to cause damage to the photoreceptor without impressing alternating voltage until DC voltage impressed on an electrifying roller rises to a specified value. SOLUTION: DC and alternating components impressed on the electrifying roller 2 are controlled and outputted independently, and the DC voltage is impressed on the roller 2 before starting the formation of an image. It takes 300ms to make a DC voltage value rise to -750V being a specified value. In the case only DC voltage is impressed, the surface potential of the photoreceptor is only <=20% of a desired value of electrifying the photoreceptor, so that the photoreceptor is prevented from being damaged by the electrification in the case only the DC voltage is impressed. Thereafter, the alternating voltage of 1500Vpp is impressed on the roller 2. Since it takes little time to make the alternating voltage rise, the surface of the photoreceptor 1 is electrified to -720V being a specified value for the first time. Then, DC and AC are turned off in timing that the photoreceptor 1 does not cause plastic memory after turning off the impressed voltage on a transfer roller 7.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an electrophotographic photosensitive member.
An electrophotographic process including a step of uniformly charging the surface of an image carrier with a rotating image carrier such as an electrostatic recording dielectric.
An image is formed by applying an image forming process such as an electrostatic recording process, transferring the image formed on the image carrier to a recording medium (transfer material), or forming the image on the image carrier without transfer. Is removed from the surface of the image carrier after being displayed and read while being positioned on the display unit, and the image carrier is repeatedly subjected to image formation.
The present invention relates to an image forming apparatus such as a laser beam printer / copier / image display device.

More specifically, as a charging means for uniformly charging the surface of the image carrier, a charging member applied by superimposing a DC voltage on an alternating voltage is applied to the image carrier to bring the surface of the image carrier into contact. The present invention relates to the image forming apparatus using a contact type charging device that charges.

[0003]

2. Description of the Related Art Conventionally, in an image forming apparatus, a corona discharge device has been widely used as a device for charging a surface of an image bearing member such as a photoconductor or a dielectric as a member to be charged. A corona discharge device is arranged so as to face the image carrier in a non-contact manner, and the surface of the image carrier is exposed to the corona emitted from the corona discharge device to uniformly charge the surface of the image carrier to a predetermined polarity and potential. . However, there are problems such as generation of ozone that requires a high voltage power source.

In recent years, a contact type charging device has attracted attention as a new charging processing means and has been put to practical use.

This is roller type, blade type, brush type
A charging member such as a magnetic brush type is brought into contact with the surface of an image carrier as a member to be charged, and a voltage is applied to the charging member to charge the surface of the image carrier, in comparison with a charging process by a corona discharge device. It has the advantages of lowering the voltage of the power supply and producing less ozone.

There is also a system in which a charge injection layer is provided on the surface of an image bearing member as a member to be charged, and the charge is directly injected into the charge injection layer by a contact charging member to which a voltage is applied. This is the category of formula charging.

In the contact type charging, a method of applying only a DC voltage (DC) to a charging member (DC bias applying method) and an oscillating voltage in which a DC voltage is superimposed on an alternating voltage (AC, AC voltage) are applied. Method (AC bias application method)
There is. In the AC bias application method, the alternating component smoothes unevenness of charging and the direct current component converges to a predetermined voltage, so that it is easy to obtain the uniformity of the surface potential.

As an AC bias applying method, Japanese Patent Application Laid-Open No. 63-1988
As disclosed in JP-A-149669, an oscillating voltage in which an AC component having a peak-to-peak voltage of 2 × Vth or more is superposed on a DC voltage corresponding to a desired surface-to-be-charged body surface potential Vd (the voltage value is periodic with time). The purpose of this method is to apply a voltage to the contact charging member to level the potential of the AC component, and the potential of the body to be charged is Vd, which is the center of the peak of the AC voltage. And is not affected by external disturbances such as the environment.

Vth is a discharge start voltage (charge start voltage),
That is, it is a voltage value applied to the charging member when a DC voltage is applied to the charging member to start charging of the member to be charged.

In particular, the present invention relates to an image forming apparatus in which the charging means of the image carrier is a contact type charging / AC bias applying system and the image carrier is repeatedly used.

[0011]

Problems to be solved by the invention in the image forming apparatus in which the charging means for the image carrier is the contact type charging / AC bias applying system and the image carrier is repeatedly used are as follows. There is.

1. Taking a transfer type electrophotographic apparatus as an example, as the number of times of image formation increases, that is, as the durability of the photoconductor progresses due to repeated use, the outer peripheral surface of the photoconductor (photosensitive layer) as an image bearing member becomes Gradually scraped.

On the other hand, it is known that there is a strong correlation between the amount of current flowing through the photoconductor and the amount of abrasion of the photoconductor. In the system in which the contact charging process of the photoconductor is the AC bias application system, the DC bias application system is used. As compared with the system, a large current of several hundred μA to several mA flows into the system, so that the abrasion amount of the photoreceptor is generally large as the durability of the photoreceptor advances. If the photosensitive layer becomes thinner due to the progress of the durability of the photoconductor, leakage from the contact charging member to the photoconductor substrate is more likely to occur. Is impossible.

Therefore, in order to keep the life of the photoconductor as long as possible in a system in which the contact charging of the photoconductor is an AC bias application system, the application time of the alternating voltage component of the applied voltage to the contact charging member is as much as possible. It is necessary to shorten it.

2. Conventionally, in a system in which the contact charging processing of the photoconductor is an AC bias applying method, an alternating voltage (primary AC) and a DC voltage (primary DC) for image formation applied to the contact charging member are simultaneously output from a bias power supply. (Fig. 6).

However, if an attempt is made to output a stable DC voltage from an inexpensive bias power supply, the time constant becomes large due to the circuit configuration, and as a result it takes time to rise to a predetermined voltage. On the other hand, it takes almost no time until the alternating voltage rises to a predetermined value.

Therefore, when using such a bias power source, when an alternating voltage for image formation and a direct current voltage are simultaneously output, an extra high alternating voltage is applied for the time until the direct current voltage rises to a predetermined value. Will be there.

3. The output value of the charging voltage is determined according to the desired surface potential Vd of the photoconductor, but in the system of the AC bias application system, the photoconductor surface potential (drum surface potential) substantially converges to the value of the DC component of the output voltage. To do. Therefore, the surface potential of the photoconductor is low until the DC voltage applied to the contact charging member rises to a predetermined value.

Further, in particular, in the case of reversal development in which negative toner is adhered to the exposed bright portion (low potential portion) of the electrostatic latent image formed on the photoconductor to develop the latent image, the photoconductor is charged to a predetermined potential. In this case, a predetermined voltage (developing bias is applied) is applied to the developing member (developing sleeve) carrying the developer of the developing device in order to prevent the development of the reverse toner. for that reason,
Generally, the developing bias is output in synchronization with the voltage application to the contact charging member.

Therefore, since the developing bias is applied during the time until the DC component of the charging voltage rises, that is, when the surface potential of the photoconductor is lower than a predetermined value, the toner is developed in a belt shape in the longitudinal direction of the photoconductor. (This toner image is referred to as a black band). When a black band is formed in the non-image area,
When the transfer means of the transfer type image forming apparatus is of the contact transfer type, the toner first adheres to the transfer member that comes into contact with the photoconductor, and the rear surface of the subsequent transfer material (recording medium) is stained. Further, not only the contact transfer method but also the problem that the amount of waste toner and toner consumption increases due to the generation of a black band.

Therefore, according to the present invention, the charging means of the image bearing member is of the contact type charging / AC bias applying type, and the image forming apparatus having the constitution in which the image bearing member is repeatedly used is capable of performing stable charging. To minimize the progress of shaving due to the durability of the carrier to enable image formation for a long period of time. The purpose is to prevent an increase in waste toner and toner consumption.

[0022]

SUMMARY OF THE INVENTION The present invention is an image forming apparatus having the following configuration.

(1) An image is formed on a rotating image carrier by applying an image forming process including a step of uniformly charging the surface of the image carrier, and the image carrier is repeatedly formed into an image. In the image forming apparatus to be provided, a. The charging means of the image carrier is a contact type charging device for charging the surface of the image carrier by bringing a charging member applied by superimposing a DC voltage on an alternating voltage into contact with the image carrier, b. Alternating voltage and DC voltage to the charging member are controlled independently, c. The time required from the start of the application of the DC voltage to the charging member to the charging of the portion of the surface of the image carrier that should become the image front end is the predetermined voltage value after the DC voltage is applied to the charging member. Longer than the time required for the image carrier to rotate three times, and d. The predetermined voltage value of the DC voltage is the same when applying only the DC voltage to the charging member and when applying the DC voltage by superimposing it on the alternating voltage, and the predetermined voltage value of the DC voltage is charged. An image forming apparatus characterized in that a voltage value capable of charging only 20% or less of an image carrier charging potential when an alternating voltage is simultaneously applied when only a DC voltage is applied to a member.

(2) The image forming apparatus according to (1), wherein the predetermined voltage value of the DC voltage is lower than the discharge start voltage in the state where only the DC voltage is applied to the charging member.

(3) When the bias power source that outputs a DC voltage to the charging member has a finite rising characteristic, the alternating voltage output is output to the charging member after the DC voltage output rises to a predetermined value. The image forming apparatus according to (1) or (2), characterized in that the image forming apparatus is performed.

(4) Image formation is performed by applying an image forming process including a step of charging the surface of the image carrier to a rotating image carrier, and the image carrier is repeatedly subjected to image formation. In the device: a. The charging means of the image carrier is a contact type charging device for charging the surface of the image carrier by bringing a charging member applied by superimposing a DC voltage on an alternating voltage into contact with the image carrier, b. The application of the alternating voltage is started at the same time as or after the start of the DC voltage rise to the charging member, c. The time required from the start of the application of the DC voltage to the charging member to the charging of the portion of the surface of the image carrier that should become the image front end is the predetermined voltage value after the DC voltage is applied to the charging member. Longer than the time required for the image carrier to rotate three times, and d. The alternating voltage is output at the first voltage value until the output of the DC voltage rises to the predetermined voltage value, and is larger than the first voltage value after the output of the DC voltage rises to the predetermined value. The image forming apparatus is characterized in that the voltage is output at a voltage value of.

(5) The image forming apparatus according to (4), wherein the first alternating voltage value is 50% or less of the second alternating voltage value.

(6) The latent image formed on the surface of the image carrier is developed by applying a predetermined voltage to a developing member carrying a developer to develop the latent image, and a DC voltage output to the charging member. And the voltage is applied to the developing member after the alternating voltage rises to a predetermined value (1) to (5).
The image forming apparatus according to any one of 1.

(7) The image forming apparatus according to (6), wherein the developing device is a reversal developing type device.

(8) When the time required for the output of the DC voltage to the charging member to rise to 90% or more of the predetermined voltage value is t ₀ , and the moving speed of the image carrier per unit time is p. , T ₀ × p> 1 mm, the image forming apparatus according to any one of (1) to (7).

(9) The image bearing member is an electrophotographic photosensitive member,
Imagewise exposure is performed on the uniformly charged surface of the photoconductor to form an electrostatic latent image, the electrostatic latent image is developed as a toner image by a developing device, and the toner image is transferred to a recording medium by a transfer device. The image forming apparatus according to any one of (1) to (8), wherein the image carrier is transferred and repeatedly subjected to image formation.

<Operation> That is, since the application time of the alternating voltage can be reduced by applying the alternating voltage at the voltage value for image formation after the DC voltage rises to a predetermined value, the durability of the image bearing member can be reduced. It is possible to minimize the progress of shaving (damage to the image bearing member) due to the above and to enable image formation for a long period of time.

Further, by applying the developing bias after the alternating component of the charging voltage is applied, it is possible to suppress the generation of the black band due to the delay of the rising of the direct current component and prevent the increase of the toner consumption amount and the waste toner amount. , And especially T ₀ × p
It is possible to prevent the back surface of the recording medium from being smeared in a configuration satisfying> 1 mm.

[0034]

BEST MODE FOR CARRYING OUT THE INVENTION

<Embodiment 1> (FIGS. 1 and 2) (1) Example of image forming apparatus FIG. 1 is a schematic configuration diagram of an example of an image forming apparatus. The image forming apparatus of this example is a laser beam printer of a contact type charging / AC bias applying type using a transfer type electrophotographic process.

Reference numeral 1 is a rotary drum type electrophotographic photosensitive member as an image bearing member. The photoconductor 1 of this example is made of aluminum.
A grounded conductive base layer 1b of iron or the like and a photoconductive layer 1a formed on the outer peripheral surface thereof, for example, an organic photoconductive layer are used as a basic constituent body, and a central support shaft 1c is used as a center in a clockwise direction of an arrow. It is rotationally driven at a predetermined peripheral speed (image forming speed, process speed) by the illustrated driving means. The charging polarity of the photoconductor 1 of this example is negative.

Reference numeral 2 is a primary charging means for uniformly and uniformly charging the surface of the photosensitive member to a predetermined polarity and potential. The charging means of this example is a contact type charging means using the charging roller 2.

The charging roller 2 includes a conductive roller (core metal) 2c at the center and a conductive layer 2b formed on the outer periphery of the conductive roller 2c.
Further, it is a composite layer roller composed of a resistance layer 2a formed on the outer periphery thereof, and both ends of the cored bar 2c are rotatably supported by bearings and are arranged in parallel with the photoconductor 1. .

The bearing members on both ends of the cored bar 2c are urged to move toward the photoconductor 1 by pressing means such as springs, whereby the charging roller 2 is provided on the surface of the photoconductor 1 in a predetermined manner. The photoconductor 1 is pressed with a contact pressure of 1, and is driven to rotate in the direction of the arrow as the photoconductor 1 rotates. The charging roller 2 may be driven and rotated by a driving means such as a motor.

A charging bias applying power source 3 is applied to the charging roller 2.
From the above, a predetermined charging bias voltage, which is a superimposed voltage of an alternating voltage (primary AC) and a negative DC voltage (primary DC) in this example, is applied through the electric contactor 3a and the cored bar 2c which is in contact with the contactor. The peripheral surface of the rotating photoconductor 1 is uniformly subjected to a primary charging process by a contact type charging / AC bias applying system to a predetermined negative surface potential.

Next, a laser beam whose intensity is modulated corresponding to the time-series electric digital pixel signal of the target image information by the laser beam scanner 4 as the image exposure means is applied to the surface of the rotary photosensitive member 1 for the primary charging. By performing the scanning exposure (image exposure) L, the surface potential of the light-exposed portion on the surface of the photosensitive member 1 is attenuated, and an electrostatic latent image corresponding to the target image information is formed on the surface of the photosensitive member 1.

Next, the formed electrostatic latent image is developed as a toner image by the developing device 5. This example is a jumping developing type reversal developing device using a negatively charged one-component magnetic toner (negative toner) as a developer.

The toner in the developing container is driven to rotate in the direction of the arrow and a predetermined thickness is applied to the surface of the developing sleeve 5a to which a predetermined developing bias voltage (DC voltage, vibration voltage) is applied from the developing bias applying power source 6. Applied on the photoconductor 1
At the developing portion, which is the opposing portion of the developing sleeve 5a and the developing sleeve 5a, toner is attached from the developing sleeve 5a side to the exposed bright portion of the electrostatic latent image on the surface of the photoconductor 1 to reverse-develo the electrostatic latent image.
The rotation of the developing sleeve 5a is controlled to be turned on / off by a clutch (not shown), and the developing sleeve 5a is rotated corresponding to the image area on the side of the photoconductor 1.

On the other hand, a transfer material P serving as a recording medium is fed from a paper feeding unit (not shown), and the transfer material P serves as the photosensitive member 1 and the transfer roller 7 serving as a contact transfer means at a predetermined control timing. The sheet is fed to the transfer nip portion N which is a contact portion. The above-mentioned predetermined control timing is such that when the front end of the toner image area on the surface of the rotary photoconductor 1 reaches the transfer nip N, the front end of the transfer material P just reaches the transfer nip N. It's timing.

The transfer roller 7 is composed of a conductive roller (core bar) and a conductive layer formed on the outer peripheral surface of the transfer roller. The both ends of the core bar are rotatably supported by bearing members so that the photosensitive member 1 is supported. In contrast, they are arranged in parallel. The bearing members at both ends of the core metal are urged to move toward the photoconductor 1 by pressing means such as springs, whereby the transfer roller 7 contacts the surface of the photoconductor 1 with a predetermined contact pressure. The photoconductor 1 is rotated by the rotation of the photoconductor 1. The transfer roller 7 may be driven to rotate in the forward direction of rotation of the photoconductor 1 by driving means such as a motor at a speed substantially equal to the peripheral speed of rotation of the photoconductor 1.

The transfer material P introduced into the transfer nip portion N is nipped and conveyed by the rotary photosensitive member 1 and the transfer roller 7. While the transfer material P is nipped and conveyed to the transfer roller 7 while passing through the transfer nip portion N, the polarity of the toner of the toner image on the surface of the photosensitive member 1 is transferred from the transfer bias applying power source 8 through the core metal. Is applied with a reverse bias, that is, a positive bias voltage,
The back surface of the rotating photosensitive member 1 is contact-charged with a polarity (plus) opposite to the polarity of the toner of the toner image by the transfer roller 7 to which the above-mentioned transfer bias voltage is applied and the back surface of the rotating photosensitive member 1 surface. The toner image on the side is transferred in the transfer nip portion N by the transfer nip portion N being nipped and conveyed.
Is sequentially electrostatically transferred to the surface side of the.

Generally, the transfer bias voltage is applied to the transfer roller 7 by applying the transfer material P fed to the transfer nip portion N.
Starts at the same time as or immediately before reaching the transfer nip portion N (about 0 to 1 cm), and at the same time or immediately after passing the transfer nip portion N at the rear end of the transfer material P (about 0 to 1 cm). ) Is controlled to end.

Further, the transfer roller 7 is partially or entirely operated from the start of image formation to the start of transfer, from the end of transfer to the next start of transfer, and from the end of transfer to the end of image formation. Applies a predetermined bias (cleaning bias) having a polarity opposite to that of the transfer bias to fly the toner adhering to the transfer roller 7 onto the photoconductor 1.
Generally, the transfer roller 7 is cleaned.

The transfer material P which has received the transfer of the toner image through the transfer nip portion N is separated from the surface of the photosensitive member 1 and conveyed to an image fixing means (not shown), where it is subjected to the fixing processing of the toner image, The final image-formed product (print, copy) is discharged outside the apparatus main body. Alternatively, in the case of the multiple copy mode or the double-sided copy mode, the sheet is fed to the transfer nip portion N again without being turned upside down or turned upside down via the re-conveying means.

After the transfer of the toner image to the transfer material P, the surface of the photosensitive member 1 is cleaned by the cleaning blade of the cleaner 9 by scraping off the adhered substances such as residual toner, and further discharged by the static eliminator (static erasing lamp) 10. After initialization, the image forming apparatus is repeatedly used for the next image forming process.

(2) Timing of Charging Voltage Output FIG. 2 shows a timing chart of the output of the DC component (primary DC) and the alternating component (primary AC) of the charging potential, which is a feature of the present invention. Process speed is 105mm / s
ec. The toner polarity of the developing device 5 is negative.

When the apparatus starts image formation, the photoconductor 1
Starts to rotate.

The DC component and the alternating component of the voltage applied to the charging roller 2 are controlled independently of each other, and the DC voltage is first applied to the charging roller 2 before the image formation is started. As shown in FIG. 2, the DC voltage value is a predetermined value of −750.
It takes about 300 ms to rise to V.

Here, the DC voltage value is constant, but is a value lower than the discharge start voltage. Therefore, when only the DC voltage is applied, the photosensitive member surface potential is 20 which is a desired value.
It is charged to less than%. Therefore, when only the DC voltage is applied, the charging of the photoconductor is not intended, and there is almost no damage to the photoconductor due to the charging.

Thereafter, the charging roller 2 is supplied with about 1500 Vpp.
The alternating voltage of is applied. Since it takes almost no time for the AC voltage to rise, the photoconductor 1
The surface is uniformly charged to a predetermined value (-720V).

After the image formation is completed, the voltage applied to the transfer roller 7 is turned off, and then the DC voltage and the alternating voltage applied to the charging roller 2 are turned off at the same time when the photosensitive member 1 does not generate a plastic memory.

In the above process, A4 size paper 1
In the case of sheet copying or printing, the application time of the DC voltage to the charging roller 2 is 4.10 sec, the application time of the alternating voltage is 3.80 sec, and the charging time due to the alternating voltage is reduced by about 7%.

As described above, according to this example, the alternating voltage is not applied during the time until the DC voltage applied to the charging roller 2 reaches a predetermined value, so that the time for damaging the photoconductor 1 can be reduced. As a result, the life of the photoconductor can be extended.

When the time from the start of application of the DC voltage to the rise to a predetermined value is t (msec),
At least t (msec) time is required from the application of the DC voltage to the charging of the portion of the photoconductor that is to be the leading edge of the image. This is, of course, because the DC voltage has risen to a predetermined value by the start of image formation. More strictly, in consideration of the stability of direct current and alternating voltage and avoidance of damage to the photoconductor 1 due to reverse charging of the transfer roller 7 (contact transfer member), the DC voltage is applied before the image tip of the photoconductor is applied. It is desirable to provide a time allowance for the photoconductor 1 to rotate three times before charging the portion to be charged.

However, according to the study by the present inventors, it was found that the damage to the photosensitive member 1 due to the reverse charging by the transfer roller 7 can be recovered to a practical level by at least two times of the charging by the primary charging. ing. In this example, since the primary charging portion and the transfer charging portion (transfer nip portion N) are located on the cylindrical photosensitive member 1 at positions displaced by about 180 °, the photosensitive member is provided with a period of about 1.5 rotations. . That is, the result is a balance between the life of the photoconductor and the damage to the photoconductor 1 due to transfer charging.

<Embodiment 2> (FIGS. 3 and 4) The basic structure of the image forming apparatus according to this embodiment is the same as that of Embodiment 1 described above. In the first embodiment, it has been described that by applying the alternating voltage after the DC component of the charging voltage rises to a predetermined value, the alternating voltage application time is reduced and damage to the photoconductor 1 can be reduced.

However, in such a system, the following problems occur especially when reversal development is used.

That is, in the case of reversal development, a development bias is applied to prevent development of reversal toner when the photoconductor is charged to a predetermined potential. Therefore, the developing bias is generally output in synchronization with the application of the charging voltage. FIG. 3 shows the relationship among the charging voltage, the developing bias, and the photosensitive member surface potential. The horizontal axis in FIG. 3 is an axis representing a relative position corresponding to the position of the image formed on the photoconductor. When the developing bias is applied in synchronization with the charging DC voltage as in the conventional case, a black band is likely to occur because the surface potential of the photoconductor is lower than a predetermined value until the DC voltage rises. In particular, when the alternating voltage is applied after the charging DC voltage rises as in the first embodiment, the surface potential of the photoconductor does not approach a predetermined value until the alternating voltage rises, and the development contrast increases. A black band having a high density is generated in the area A in 3.

When a high-density black band is generated in the non-image area as described above, the transfer roller 7 is soiled to cause back smearing of the transfer material P, and the toner consumption amount and the waste toner amount are increased. .

In order to prevent this, in this example, as shown in FIG. 4, the developing bias is applied after the alternating charging voltage is applied. Since the potential of the photoconductor rises to a predetermined value almost at the same time when the alternating charging voltage starts to be applied, it is desirable to apply the developing bias substantially in synchronization with the alternating charging voltage. The timing chart is described by omitting the delay time corresponding to the time required for the photosensitive member to move from the charging portion to the developing portion.

As described above, according to this example, the alternating voltage is applied after the charging DC voltage rises to a predetermined value, and the developing bias is applied in synchronism therewith or thereafter, so that the photoreceptor is damaged. It is possible to prevent the occurrence of a black band in the non-image area in the reversal developing system while reducing the time for giving the image.

<Third Embodiment> As shown in the second embodiment, if the developing bias is applied after the charging DC voltage rises to a predetermined value (and after the alternating voltage application is started), a non-image area is generated. No black belt occurs.

Strictly speaking, if the charging DC voltage value rises to 90% or more of the predetermined value, the surface potential of the photoconductor rises to the extent that a black band does not occur. The time required from the start of application of the charging DC voltage to the rise to 90% of the predetermined value is defined as t ₀ . Further, when the moving speed of the photosensitive member per unit time, that is, the process speed is p, the width of the black band generated when the timing of applying the charging DC voltage and the timing of applying the developing bias are synchronized is represented by t ₀ × p. .

Table 1 shows the width of the black band and the level of stains on the back surface of the transfer material generated therefor. From this, it can be seen that when the width of the black band generated in the non-image area is larger than 1.0 mm, the level of the backside stain becomes worse.

As described above, under the condition that t ₀ × p> 1.0 mm is satisfied, the structure of the second embodiment is more effectively utilized.

[0070]

[Table 1] ◯: Good, Δ: Between good and bad, X: Bad <Embodiment 4> (FIG. 5) The basic configuration of the image forming apparatus of this embodiment is the same as that of Embodiment 1.

FIG. 5 shows a timing chart of the output of the DC component and the alternating component of the charging voltage, which is the feature of this example.

When the apparatus starts image formation, the photoconductor 1
Starts to rotate. Before the image formation is started, the charging process is performed in the following order.

(I) First, a DC voltage is applied, and in synchronism therewith, or thereafter, a first alternating voltage is applied.

(Ii) After the DC voltage rises to -750V, the second alternating voltage is applied to the charging roller 2.

Here, the second AC voltage is an alternating voltage value for image formation. The first alternating voltage value is 50% or less of the second alternating voltage value. In this embodiment, the first alternating voltage is 700 Vpp and the second alternating voltage value is 1500 Vpp.

At the time of (i), the DC voltage value is lower than the discharge start voltage, and the first alternating voltage value is sufficiently low so as not to contribute to the discharge, so that the photoconductor is hardly charged and charged. The damage to the photoconductor due to is very small.

At the time of (ii), the surface of the photosensitive member 1 is uniformly charged to a predetermined value of -720 V for the first time when the second alternating voltage is applied.

After the image formation is completed, the voltage applied to the transfer roller 7 is turned off, and then the direct current voltage and the alternating voltage to the charging roller 2 are turned off at the same timing when the photosensitive member 1 does not generate a positive memory.

As described above, according to the present embodiment, since the alternating voltage is suppressed to a sufficiently low value until the DC voltage applied to the charging roller 2 rises to a predetermined value, the photoconductor is damaged. The time can be reduced and the life of the photoconductor can be extended.

The timing when the application of the DC voltage is started is the same as in the first embodiment.

According to the means of this embodiment, the damage to the photoreceptor due to the alternating voltage until the DC voltage rises to the predetermined value is reduced while applying the AC voltage before the DC voltage rises to the predetermined value. It becomes possible to do.

<Other Embodiments> 1) In the image forming apparatus according to the embodiments, the contact charging member 2 is not limited to the roller type, but may be any other type such as a blade type, a rod type, a brush type, a magnetic brush type. It may be a contact charging member. Even if the charging member is not necessarily in contact with the surface of the charged body with a pressure contact force, if the dischargeable area determined by the gap voltage and the Paschen curve is securely guaranteed between the charging member and the surface of the charged body, it is in close proximity without contact. However, in the present invention, the contact charging is also included in this case.

2) As the waveform of the alternating voltage, a sine wave, a rectangular wave, a triangular wave or the like can be appropriately used. Alternatively, it may be a rectangular wave formed by periodically turning the DC power supply on and off. At this time, controlling the alternating voltage means controlling the peak-to-peak voltage. In this way, the alternating voltage can be biased so that the voltage changes periodically.

3) The image exposing means may be other appropriate exposing means / mechanism such as slit exposing means for the original image.

4) The electrophotographic photosensitive member as the image bearing member is not limited to the drum type, but may be a rotating member of other forms such as a rotating belt type. It can also be of the injection charging type. Not limited to the electrophotographic photosensitive member, it may be an electrostatic recording dielectric or the like.

5) The development method may be normal development.

6) The transfer roller 7 as the contact charging type transfer means is a single-layer transfer roller having a conductive roller and a conductive layer in the embodiment, but may be a multi-layered transfer roller. The rotating body is not limited to the roller type and may be another rotating body such as a belt type. It may be a corona discharge device for transfer.

7) The image forming apparatus removes the toner image formed on the image bearing member from the surface of the image bearing member after the toner image formed on the image bearing member is not displayed on the recording medium but is displayed and read on the image display section to be displayed and read. It may be an image display device that is repeatedly used for image formation.

[0089]

As described above, according to the present invention, the charging method of the image carrier is a contact type charging / AC bias applying method in which a voltage obtained by superposing an alternating voltage with a DC voltage is applied to the contact charging member. In an image forming apparatus configured to repeatedly use an image carrier, by applying an alternating voltage at a voltage value for image formation after a DC voltage rises to a predetermined value, the application time of the alternating voltage is reduced, It is possible to minimize the progress of abrasion (damage to the image bearing member) due to the durability of the bearing member and enable image formation for a long period of time.

Further, by applying the developing bias after the alternating component of the charging voltage is applied, it is possible to suppress the generation of the black band due to the delay of the rising of the direct current component and prevent the increase of the toner consumption amount and the waste toner amount. , And especially T ₀ × p
In a structure satisfying> 1 mm, backside smearing of the recording medium can be prevented, and the intended purpose is often achieved.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an image forming apparatus according to a first exemplary embodiment.

FIG. 2 is a timing chart of charging voltage application.

FIG. 3 is a timing chart explaining the cause of the occurrence of a black band.

FIG. 4 is a timing chart of charging voltage and developing bias application in the image forming apparatus according to the second exemplary embodiment in which the occurrence of black bands is prevented.

FIG. 5 is a timing chart of charging voltage application in the image forming apparatus of the fourth embodiment.

FIG. 6 is a timing chart of charging voltage application in a conventional device.

[Explanation of symbols]

 1 Photosensitive drum as image carrier 2 Charging roller as contact charging member 3 Charging bias applying power source 4 Laser beam scanner as image exposing means 5 Reversal developing device 6 Developing bias applying power source 7 Transfer roller 8 Transfer bias applying power source 9 Cleaner 10 Static eliminator L Laser beam P Recording medium (transfer material) N Transfer nip part

Claims (9)

[Claims] 1. An image forming process including a step of uniformly charging the surface of the image carrier is applied to a rotating image carrier to perform image formation, and the image carrier is repeatedly subjected to image formation. In the image forming apparatus, a. The charging means of the image carrier is a contact type charging device for charging the surface of the image carrier by bringing a charging member applied by superimposing a DC voltage on an alternating voltage into contact with the image carrier, b. Alternating voltage and DC voltage to the charging member are controlled independently, c. The time required from the start of the application of the DC voltage to the charging member to the charging of the portion of the surface of the image carrier that should become the image front end is the predetermined voltage value after the DC voltage is applied to the charging member. Longer than the time required for the image carrier to rotate three times, and d. The predetermined voltage value of the DC voltage is the same when applying only the DC voltage to the charging member and when applying the DC voltage by superimposing it on the alternating voltage, and the predetermined voltage value of the DC voltage is charged. An image forming apparatus, wherein a voltage value capable of charging only 20% or less of a charging potential of an image carrier when an alternating voltage is simultaneously applied to a member is applied. 2. The image forming apparatus according to claim 1, wherein the predetermined voltage value of the DC voltage is lower than the discharge start voltage in the state where only the DC voltage is applied to the charging member. 3. When a bias power source for outputting a DC voltage to the charging member has a finite rising characteristic, an alternating voltage is output to the charging member after the DC voltage output rises to a predetermined value. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus. 4. An image forming apparatus in which an image forming process including a step of charging the surface of the image bearing member is applied to a rotating image bearing member to execute image formation, and the image bearing member is repeatedly subjected to image forming. In: a. The charging means of the image carrier is a contact type charging device for charging the surface of the image carrier by bringing a charging member applied by superimposing a DC voltage on an alternating voltage into contact with the image carrier, b. The application of the alternating voltage is started at the same time as or after the start of the DC voltage rise to the charging member, c. The time required from the start of the application of the DC voltage to the charging member to the charging of the portion of the surface of the image carrier that should become the image front end is the predetermined voltage value after the DC voltage is applied to the charging member. Longer than the time required for the image carrier to rotate three times, and d. The alternating voltage is output at the first voltage value until the output of the DC voltage rises to the predetermined voltage value, and is larger than the first voltage value after the output of the DC voltage rises to the predetermined value. The image forming apparatus is characterized by outputting at the voltage value of. 5. The image forming apparatus according to claim 4, wherein the first alternating voltage value is 50% or less of the second alternating voltage value. 6. A developing device for developing a latent image formed on the surface of an image bearing member by applying a predetermined voltage to a developing member carrying a developer, and a DC voltage output to a charging member and The image forming apparatus according to claim 1, wherein the voltage is applied to the developing member after the alternating voltage rises to a predetermined value. 7. The image forming apparatus according to claim 6, wherein the developing device is a reversal developing type device. 8. The time required for the output of the DC voltage to the charging member to rise to 90% or more of a predetermined voltage value is t ₀ , and the moving speed of the image carrier per unit time is p.
The image forming apparatus according to claim 1, wherein t ₀ × p> 1 mm. 9. The image carrier is an electrophotographic photosensitive member, and an electrostatic latent image is formed by imagewise exposing the uniformly charged surface of the photosensitive member, and the electrostatic latent image is developed by a developing device. 9. The image according to claim 1, wherein the image is developed as a toner image, the toner image is transferred to a recording medium by a transfer device, and the image carrier is repeatedly used for image formation. Forming equipment.