JP2001109238A - Electrifying device and image forming device provided with the electrifying device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent unevenness in density due to unevenness in electrification by surely electrifying a body to be electrified without generating any ozone. SOLUTION: Relating to an electrifying device 2, an electrifying roll 8 is provided in a vicinity of a surface of a photoreceptor drum 1 to form a prescribed gap in an electrifying range. DC bias that is controlled to a constant voltage and AC bias that is controlled to a constant current are fed to a core metal 11 of the electrifying roll 8 from a power source unit 12 and the surface of the photoreceptor drum 1 is uniformly electrified by them. The voltage of the AC component applied to the electrifying roll 8 is provided with a voltage value between peaks that is more than 2 times a voltage value at the start of electrifying at a maximum gap between the electrifying roll 8 and the photoreceptor drum 1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art Conventionally, an electrophotographic image forming apparatus has been provided with a charging device for charging a photosensitive member as a member to be charged. As the charging device, for example, a charging charger system is generally used as a non-contact type charging device. However, in the case of this charging system, there is an advantage that charging performance is good because uniformity of charging can be achieved, but there is a drawback that ozone (O ₃ ) which has an effect on a human body is generated. In recent years, a contact charging system in which charging is performed while a charging member is in contact with a photoconductor has become mainstream.

[0003]

However, in the case of the contact charging system, the charging member such as a charging roller is brought into direct contact with a member to be charged such as a photoreceptor. That is, in some cases, dirt is transferred from the charging member to the photoreceptor, whereby the photoreceptor is contaminated and an abnormal image is generated. In addition, there is also a risk that the photoconductor is cracked.

Further, the charging member itself is liable to be stained by toner or the like adhering to the photoreceptor, and when the charging member becomes dirty beyond its limit, the charging performance (uniformity) may be reduced. Furthermore, there is a risk that the photosensitive member may be shaved by the contacting charging member and the charged potential may be reduced. Further, there is also a case where the margin for the leak when the photoconductor has a pinhole is small.

[0005] Therefore, it is conceivable to simply arrange the charging member close to the member to be charged with a small gap, thereby charging the charging member. However, in this case, if the charging member is formed of an elastic roller so as to maintain the small gap, it is very difficult in terms of accuracy. Further, even if this is made possible, there is a case where the cost becomes extremely high. On the other hand, if the charging member is a metal roller whose dimension is relatively easy to control, when a foreign object enters between the metal roller and the charged member, the surface of the charged member is easily damaged. .

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and can reliably charge an object to be charged without generating ozone that has an effect on the human body. It is an object of the present invention to provide a charging device that does not cause density unevenness caused by the above-described charging.

[0007]

In order to achieve the above object, the present invention comprises a charging member provided close to a member to be charged so as to form a predetermined gap at least in a charging region, In the charging device, which applies a voltage obtained by superimposing an AC voltage on a DC constant voltage from a power supply to the charging member, the AC component of the voltage applied to the charging member is a voltage of the predetermined gap. It has a peak-to-peak voltage value that is at least twice the charging start voltage value at the maximum gap.

The gap between the member to be charged and the charging member is:
The position may be non-uniform and have a deviation. Further, the gap between the charged member and the charging member may be variable. Further, the charging member is preferably a rotating roller. The member to be charged may be a member that rotates or turns. The gap between the member to be charged and the charging member is preferably a gap having a charging start voltage different from the charging start voltage when the gap is zero.

Further, a charging member is provided close to the member to be charged so as to form a predetermined gap at least in a charging region, and the charging member has a DC voltage controlled by a constant voltage from a power supply and a DC voltage. The object to be charged is charged by the application of the AC voltage, and the average value of the gap at each position in the longitudinal direction and the transverse direction of the charging member in the charging region is 10 μm or more, and the variation of the gap is The charging device having an average value of 10 μm or more may be configured as follows.

That is, the charging device is configured such that the voltage applied to the charging member is such that the voltage having an AC component has a peak-to-peak voltage value that is at least twice the charging start voltage value in the maximum gap of the predetermined gap. Good.

Also, in a charging apparatus provided with a charging member provided so that a portion that contacts a charged member in a charging area and a non-contact portion are mixed, the voltage applied to the charging member is It is preferable that the voltage having the AC component has a peak-to-peak voltage value that is at least twice the charging start voltage value at the maximum gap of the gap.

Preferably, the charging member is a rotatable elastic roller. In any one of the charging devices, the gap is formed by interposing a gap management member between the charged member and the charging member, and the maximum gap is determined by the thickness of the gap management member. It is good to Furthermore, an image forming apparatus provided with any one of the above charging devices is provided.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic configuration diagram showing an image forming unit of an image forming apparatus provided with a charging device according to the present invention.
FIG. 1 is a schematic configuration diagram showing the entire image forming apparatus.

In the image forming apparatus shown in FIG. 2, a sheet feeding unit 22 is provided in a lower portion of the apparatus main body, an image forming unit having the photosensitive drum 1 and the like is provided above the image forming unit, and a sheet discharging unit is provided above the image forming unit. Discharge rollers 26 and 27 are provided, respectively, and an image is formed on the left side surface of the transfer paper P fed from the paper feed unit 22 in FIG. To discharge to the bin tray 20 or the paper discharge tray 21. The sheet feeding section 22 is provided with trays 28 and 29 in two upper and lower stages, and a sheet feeding roller 30 is disposed in each of the sheet feeding stages.

In FIG. 2, a writing unit 23 irradiates light onto the uniformly charged surface of the photosensitive drum 1 to write an image thereon. Further, a pair of registration rollers 13 for correcting the skew of the transfer sheet and adjusting the transfer timing of the image on the photosensitive drum 1 and the transfer sheet is provided upstream of the photosensitive drum 1 in the transfer sheet transfer direction.
Is provided. Further, a fixing unit 25 is provided downstream of the photosensitive drum 1 in the transfer paper transport direction.

As shown in FIG. 1, the above-mentioned photosensitive drum 1 is provided in the image forming section so as to be rotatable in the direction of arrow A. A charging device 2 is provided around the photosensitive drum 1, and the photosensitive drum 1 is charged by the charging device 2. Developing device 4 that visualizes the electrostatic latent image on photoreceptor drum 1 written on writing surface by writing unit 23 to form a toner image, and transfer conveyance belt 5 that transfers the toner image to transfer paper P Cleaning device 6 for removing residual toner remaining on photosensitive drum 1 after transfer of the toner image
And a charge removing lamp 7 for removing unnecessary charges on the photosensitive drum 1.

In this image forming apparatus, when the image forming operation is started, the photosensitive drum 1 shown in FIG. 1 rotates in the direction of arrow A, and its surface is neutralized by a neutralization lamp 7 to be averaged to a reference potential. You. Next, the surface of the photosensitive drum 1 is uniformly charged by the charging roller 8, and the charged surface is irradiated with light La corresponding to image information from the writing unit 23, and an electrostatic latent image is formed thereon. It is formed. The latent image is moved to the position of the developing device 4 by the rotation of the photosensitive drum 1 in the direction of arrow A, where the toner is adhered by the developing sleeve 10 to become a toner image (visible image).

On the other hand, the tray 2 of the sheet feeding section 22 shown in FIG.
The transfer paper P is fed from one of the paper feed rollers 8 and 29 by the paper feed roller 30, is temporarily stopped by the registration roller pair 13, and the leading end of the transfer paper P and the leading end of the image on the photosensitive drum 1 are moved. The toner images on the photosensitive drum 1 are transferred to the transfer paper P by the transfer and conveyance belt 5 at the exact timing that matches.

The transfer paper P is conveyed by the transfer conveyance belt 5 and separated from the transfer conveyance belt 5 by the drive roller 5a by the curvature separation of the transfer paper P by the waist, and is conveyed to the fixing unit 25, where it is conveyed. The application of heat and pressure causes the toner to be fused to the transfer paper P, which is discharged to a designated discharge place, that is, either the discharge tray 21 or the bin tray 20. Then, the photosensitive drum 1
The residual toner remaining on the upper surface is rotated and moved to a cleaning position, which is the next step, and is scraped off by the cleaning blade 6a of the cleaning device 6 shown in FIG. 1, and the process proceeds to the next image forming step again.

The charging device 2 includes a charging roller 8 which is a charging member provided close to the surface of the photosensitive drum 1 so as to form a predetermined gap in a charging area, and a surface of the charging roller 8. And a charging roller cleaning member 9 made of, for example, a sponge for cleaning the charging roller 8 at all times. The charging device 2 applies a constant voltage controlled DC bias and a constant voltage controlled AC bias to the core metal 11 of the charging roller 8 from the power supply unit 12.
A bias (which may be a constant current controlled AC bias as described later) is supplied to uniformly charge the surface of the photosensitive drum 1.

The photoreceptor drum 1 has a multilayer structure in which an aluminum (Al) tube is coated with a UL layer, a CGL layer, and a CTL layer, respectively. Is driven to rotate. The charging roller 8 is an elastic roller in which cores 11, 11 at both ends are rotatably supported by bearings, respectively, and Teflon as a gap management member is provided at both ends of the elastic roller portion 8a as shown in FIG. (Registered trademark) tubes 14, 14
Is attached in close contact.

Then, the Teflon tubes 1 on both sides thereof
When the portions 4 and 14 come into contact with the surface of the photosensitive drum 1, a minute gap corresponding to the thickness of the Teflon tube 14 is formed between the surface of the photosensitive drum 1 and the elastic roller portion 8a of the charging roller 8 in the charging area. Is formed between them. That is, in this charging device, the maximum gap between the photosensitive drum 1 and the charging roller 8 is determined by the thickness of the Teflon tube 14 interposed between the photosensitive drum 1 and the elastic roller portion 8a of the charging roller 8. It is determined.

The maximum gap is defined as follows. That is, as described above, the surface of the photosensitive drum 1 and the elastic roller portion 8 of the charging roller 8 which oppose each other with a minute gap corresponding to the thickness of the Teflon tube 14 are provided.
The maximum gap Gmax at a certain moment in the closest part a shown in FIG.

Further, the gap between the surface of the photosensitive drum 1 and the elastic roller portion 8a of the charging roller 8 at the closest approach portion a is the same as the closest approach portion a in FIG. Since the position generally differs depending on the precision of the components of both the photosensitive drum 1 and the charging roller 8, the gap amount becomes the largest at each position in the depth direction (longitudinal direction of the charging roller 8) at the closest portion a. The gap at the position is defined as the maximum gap at the closest part a.

This will be described with reference to FIGS. 6 and 7. The gap between the surface of the photosensitive drum 1 and the elastic roller portion 8a of the charging roller 8 is shown in FIGS. 6 and 7 (for convenience of explanation). (Illustrated exaggerated), as shown in FIG.
6 becomes a position where the maximum gap Gmax is formed at a certain moment due to the rotation and the variation of the straightness, and a position c shown in FIG. 7 becomes a position where the maximum gap Gmax is formed at another certain moment. Therefore, the position where the maximum gap Gmax is formed changes in the longitudinal direction of the charging roller 8. In this manner, the maximum gap is defined, and as shown in FIG. 5, each gap Gc at the end of the area on both sides of the discharge area Adc formed between the photosensitive drum 1 and the charging roller 8 is set to the maximum. It is not a gap.

Next, the result of actually measuring the position where the maximum gap occurs to examine the change in the position where the maximum gap appears will be described with reference to Tables 1 and 2. The measurement of the gap between the surface of the photosensitive drum and the surface of the charging roller was performed for two sets. Table 1 shows the measured data of the first set, and Table 2 shows the measured data of the second set.

[0027]

[Table 1]

[0028]

[Table 2]

In measuring this gap, the diameter is 30 mm.
, A photosensitive drum having a circumference of 94 mm, and a charging roller having a diameter of 12 mm and a circumference of 37.6 mm disposed opposite to the photosensitive drum, and measuring points of the gap are spaced apart in the longitudinal direction of the photosensitive drum. Five points were set at 60 ° in the circumferential direction of the photosensitive drum in the rotation direction.

According to the measurement results shown in Table 1, the basic pattern is equivalent to a diameter of 60 mm (peripheral length: 188 mm) which is the least common multiple of the diameter of the photosensitive drum 30 mm and the diameter of the charging roller 12 mm, and the pattern appears repeatedly. It can be seen that there is a gap.

In the measurement results shown in Table 1, five gaps which are very similar to each other appear in the pattern while the photosensitive drum makes two turns (188 mm), that is, while the charging roller makes five turns. Therefore, in this case, it can be said that the straightness of the charging roller greatly affects the gap. On the other hand, in the case of Table 2, since the gap approximating with a very similar pattern appears twice while the photosensitive drum makes two rounds (188 mm), the straightness of the photosensitive drum is reduced in this case. It can be said that this has had a significant effect on the gap.

Actually, the surface of the photosensitive drum and the surface of the elastic roller portion of the charging roller are swollen in a banana-shape, with the central portion in the longitudinal direction of the surface protruding from both ends. Or the central part in the longitudinal direction is in the shape of a constricted drum compared to both ends, so the combination of the photosensitive drum and the charging roller shows the size of the maximum gap at that time, and that appears. The position is different. As described above, the gap between the surface of the photosensitive drum and the surface of the charging roller is greatly affected by the straightness of each of the photosensitive drum and the charging roller, except for the effect of sudden vibration.

In this charging device, the charging roller 8 shown in FIG.
The average value of the gap at each position in the direction) and the lateral direction (the direction of arrow C) is 10 μm or more, and the variation of the gap is 10 μm or more with respect to the average value. Further, in this charging device, a voltage having an AC component is applied between the charging roller 8 and the photosensitive drum 1. The voltage having the AC component is determined based on experimental results described later. Of the charging start voltage value at the maximum gap (Gmax in FIG. 4) between
The peak-to-peak voltage value (peak-peak value) is twice or more.

Next, a preferred example of a non-contact type (proximity charging type) charging device for forming a minute gap between the charging roller 8 and the photosensitive drum 1 will be described with reference to FIG. FIG. 8 shows charging characteristics showing the relationship between the applied voltage and the charging potential of the photoconductor surface. The charging characteristics show the characteristics when the photosensitive drum is driven to rotate at a linear speed of 230 mm / sec and the charging roller is brought into contact with the surface thereof, and when a minute gap is formed therebetween. The charging roller is when a DC bias (DC constant voltage) is applied.

The experimental results shown below are all performed under the following experimental conditions, except for those specified in each case. Line speed of imaging process: 230 mm / sec Diameter of photosensitive drum (OPC): φ60 Diameter of charging roller: φ16 Roller resistance of charging roller: 1 × 10 ⁵ Ω Charging start voltage (in case of contact): −651 V (gap: 53 μm) ): -745 V (in the case of a gap of 87 μm): -875 V (in the case of a gap of 106 μm): -916 V

As is apparent from the charging characteristics, the photosensitive member is charged at a threshold voltage (-651 V, -7 V).
45V, -875V, -916V) or more,
No charging is performed with an applied voltage having an absolute value smaller than the charging start voltage. The charging potential on the surface of the photoconductor when an applied voltage higher than the charging start voltage is applied has a slope of approximately 1 with respect to the applied voltage regardless of whether the charging roller is in contact with the photoconductor drum or not. It has a linear relationship.

Next, a change in charging characteristics when the charging roller is gradually separated from the photosensitive drum will be described with reference to FIG. At the time of this measurement, a Teflon tube is wound around both ends of the charging roller as described with reference to FIG. 3 to form a minute gap between the charging roller and the photosensitive drum, and the charging roller is inserted through the Teflon tube. The photosensitive drum was pressed against the surface.

That is, the maximum value of the gap between the charging roller and the photosensitive drum was set to correspond to the thickness of the Teflon tube. And in the experiment, 53μm, 87μ
m, 106 μm and three types of Teflon tubes having different thicknesses were prepared, and in each case, the charging characteristics when a DC constant voltage bias was applied to the charging roller were measured.
The measurement result was added to the data (data of gap 0) in the case of contact charging described above with reference to FIG.

According to the experimental results, as the gap is increased, the charging start voltage increases in absolute value with a substantially constant (≒ 1) gradient. Then, a region where the gap is very small (53 μm)
(at around m or less), the change of the charging start voltage with respect to the gap increment is relatively small, but when the gap becomes larger than about 53 μm, the relationship between the gap and the charging start voltage becomes a linear relationship having a certain slope.

This means that when Paschen's discharge law is equal to or greater than 8 μm, it can be approximated to a linear approximation (charging start voltage = 312 + 6.2 × gap). The phenomenon can be inferred from the fact that the phenomenon occurs at a place somewhat away from the photoconductor nip (a place where the gap is 8 μm or more).

The following can be said from the charging characteristics shown in FIG. That is, under a fixed DC voltage condition, the charging potential of the photoconductor depends on the gap between the charging roller and the photoconductor drum. The nature of the charging potential depending on the gap can be understood from Paschen's discharge law.

FIG. 10 shows the simulation results obtained by calculating the relationship between the gap between the charging roller and the photosensitive drum and the charging potential of the photosensitive member, and the results of experiments actually performed.
Shown in FIG. 10 shows a case where the applied DC voltage (DC bias) is fixed at -1600 V.
The simulation results and the experimental results agree very well.

From this diagram, it can be seen that when the DC constant voltage control is performed, if the gap between the charging roller and the photosensitive drum is 20 μm or more, the relationship between the gap and the charged potential on the photosensitive member surface is about 6 V / It can be seen that it has a change rate of μm. The potential unevenness allowed when a charging device of the proximity charging type in which the charging roller is opposed to the photosensitive drum via a small gap as described above is mounted on an actual machine of an image forming apparatus is ± 0.5% in the case of a monochrome machine. 30 V and ± 10 V in the case of a color machine.

When this is converted into the gap value between the charging roller and the photosensitive drum, the allowable width of the gap is 10 μm in the case of the monochrome machine and 3.times. In the case of the color machine.
3 μm. Thus, in order to dispose the charging roller with a very small gap with respect to the photosensitive drum, the charging roller must be disposed with a very high precision with respect to the gap deviation. In view of the combination of the longitudinal deflections of the body drums and their tolerances such as surface roughness and waviness, it seems difficult in practice.

Then, in the case of such a proximity charging type charging device, the applied voltage is applied to a DC bias,
Consider the case where an AC bias is superimposed. FIG.
FIG. 4 is a diagram (charging characteristic) showing an experimental result when a voltage applied is a DC constant voltage + AC constant voltage superposition in a charging device of a proximity charging method using a minute gap. In this experiment,
-700 V is applied as a DC constant voltage.

According to the results of this experiment, AC constant voltage
When a constant voltage is superimposed, the charged potential on the photoconductor surface is
In each of the gaps between the charging roller and the photosensitive drum, that is, in any of the gaps of 0 μm, 53 μm, 87 μm, and 106 μm, the charging start voltage when a DC constant voltage is applied (see FIG. 8 for the charging start voltage of each gap) )
It can be seen that a charging potential substantially equal to the DC applied voltage (−700 V) can be obtained by applying an AC peak-to-peak voltage that is approximately twice as large to the charging roller.

Next, FIG. 12 shows an experimental result when the AC bias superimposed on the DC constant voltage (DC bias) is controlled at a constant current. According to the experimental results, by controlling the AC bias superimposed on the DC constant voltage with a constant current, regardless of the size of the gap between the charging roller and the photosensitive drum,
It was found that the relationship between the total current and the charged potential on the photoreceptor surface was substantially constant.

Next, a description will be given of the results of an experiment on the output of a halftone image performed to confirm density unevenness due to charging unevenness. The results are shown in Tables 3 to 5. Table 1
FIG. 8 shows image evaluation results when the charging roller and the photosensitive drum are opposed to each other and confirmed in a state where there is no gap deviation between them.

[0049]

[Table 3]

According to the experimental results, when the control is performed by applying only the DC constant voltage, when the gap between the charging roller and the photosensitive drum is 53 μm or more, and when the AC bias is superimposed ( The gap is 106 μm for both AC constant voltage control and AC constant current control.
Under the condition of m or more, white spots occur due to abnormal discharge (indicated by X in Table 3), resulting in an NG image. For this reason, in the proximity charging method, A is applied to the DC constant voltage application.
The effect of superimposing the C bias is shown.

Next, in consideration of the case of actual use, a result of a study on a case where the charging roller has a gap deviation from the photosensitive drum will be described. Table 4 shows image evaluation results when the AC bias was changed under the condition that the gap between the charging roller and the photosensitive drum was varied at each position in the longitudinal direction of the charging roller.

[0052]

[Table 4]

In this experiment, the gap between the elastic roller portion of the charging roller and the photosensitive drum at the right end (indicated by R in Table 4) in the longitudinal direction was set to 0 μm (contact state), and the left end was set. The gap between the photoconductor drums (indicated by L in Table 4) is set to the gap maximum value (53 μm, 87 μm, 1
(3 types of 06 μm are prepared), thereby giving a deviation to the gap.

According to the results of this experiment, a good image was obtained by superimposing a DC bias with an AC peak-to-peak voltage twice or more the charging start voltage value (see FIG. 8) at the maximum gap value. It should be noted that in Table 4, the mark “可能” was evaluated to be usable because the density unevenness was somewhat acceptable, but was within an allowable range. In addition, the mark ○ indicates a good image without any density unevenness.

From these results, the target bias conditions were almost determined. Finally, an experiment for image output was performed for each of the three current control conditions, and the evaluation results are summarized in Table 5. When only the DC bias is applied, as shown in the results of the previous simulation, since the gap of the charging potential is very large, if there is a deviation in the gap, unacceptable image unevenness (indicated by x)
There has occurred.

[0056]

[Table 5]

According to the simulation result, the allowable gap deviation was about 10 μm or less. Therefore, the gap amount was precisely measured in the direction having the gap gradient.
The correspondence between the gap deviation amount and the image unevenness was examined. As shown in Table 6, under the condition that only the DC bias was applied, the allowable limit value of the gap deviation was about 10 μm as predicted from the simulation result. Appeared, resulting in an evaluation result of NG (indicated by x).

[0058]

[Table 6]

On the other hand, under the condition that the AC bias is superimposed on the DC bias, the bias between the AC peaks that is twice or more the charging start voltage value of the gap maximum value is superimposed by the constant voltage control, and the charging start of the gap maximum value is performed. In any case where the constant current control is performed at a current value at which a peak-to-peak voltage value equal to or more than twice the voltage value is obtained, the limit value of the gap deviation is:
Under the condition that there is no gap deviation, it is almost equal to the limit value of the abnormal image gap due to white spots, and under conditions of about 100 μm or less, a good image is always obtained regardless of the magnitude of the gap deviation.

As described above, the charging device 2 shown in FIG.
A voltage having an AC component (a voltage obtained by superimposing AC on a DC constant voltage) is applied between the charging roller 8 and the photosensitive drum 1, and the AC component is applied to the maximum gap between the charging roller 8 and the photosensitive drum 1. By having a peak-to-peak voltage value that is at least twice the charging start voltage value, it is possible to prevent the occurrence of density unevenness caused by uneven charging and obtain a good image.

In this way, it is also possible to solve the following problems which have been problems in the conventional contact charging type charging device. That is, by making the charging roller 8 out of contact with the photosensitive drum 1, it is possible to prevent the photosensitive drum 1 from being contaminated by the charging roller 8. If the charging roller 8 is not in contact, the film of the photosensitive drum 1 is scraped, the leakage margin for the pinhole of the photosensitive drum 1 is further reduced, and the charging roller 8 is brought into contact with the photosensitive drum 1. The resulting banding can also be prevented.

In the charging device according to the present invention, in addition to the case where the entire area of the charging member (charging roller) is in a non-contact state, a part of the charging member is in contact with the photosensitive member and the other part is not. It is needless to say from the results in Table 5 that the present invention can be similarly applied to a case where contact and non-contact coexist as in the case where is in a non-contact state. In the above experiment, in the experiment in which only the DC bias was applied, the DC bias was set to -1300 V, and the developing bias was set to -650 V.

Further, in the experiment of DC constant voltage + AC constant voltage control, the DC bias was set to -600 V, and the AC bias was set to 2 kV (more than twice the charging start voltage at the maximum gap of 106 μm). Furthermore, D
In the experiment of C constant voltage + AC constant current control, the DC bias was set to −600 V, and the AC bias was set to 2 times the charging start voltage.
A current value of 2.5 mA at which a peak-to-peak voltage value that is twice or more can be obtained.
(F = 2 kHz).

[0064]

As described above, according to the present invention, the following effects can be obtained. According to the charging device of the first to tenth aspects and the image forming apparatus of the eleventh aspect, the charging member is brought into non-contact with the member to be charged at least within the charging region by providing a predetermined gap therebetween. Generation can be suppressed, and transfer of dirt from the charging member to the member to be charged can be prevented. Therefore, it is possible to prevent the occurrence of an abnormal image due to the contamination of the member to be charged. Then, a voltage having an AC component is applied between the charging member and the member to be charged, and the AC component has a peak-to-peak voltage value at least twice the charging start voltage value at the maximum gap between the charging member and the member to be charged. , It is possible to obtain a good image in which density unevenness caused by charging unevenness does not occur.

According to the charging device of the eighth aspect, in the case of the charging device provided such that a portion where the charging member contacts the member to be charged in the charging region and a portion which does not contact the charging member are mixed. In general, when the surface resistance of the charging member is low, the gap between the charging member and the member to be charged has a deviation depending on the location, and the surface of the charging member cannot maintain a specified potential. However, a voltage having an AC component is applied between the charging member and the member to be charged, and the AC component has a peak more than twice the charging start voltage value at the maximum gap between the charging member and the member to be charged. Since the intermediate voltage value is used, a good image free from density unevenness caused by charging unevenness can be obtained.

Further, according to the charging device of the fourth and ninth aspects, it is possible to relatively easily manage the gap between the charging member and the member to be charged. According to the charging device of the tenth aspect, since the maximum gap value can be managed by the thickness of the gap management member, the gap management is further facilitated.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram illustrating an image forming unit of an image forming apparatus including a charging device according to the present invention.

FIG. 2 is a schematic configuration diagram showing the entire image forming apparatus.

FIG. 3 is a perspective view showing a state in which Teflon tubes are attached to both ends of a charging roller provided in the charging device of FIG.

FIG. 4 is a schematic diagram for explaining a maximum gap Gmax between a surface of a photosensitive drum and an elastic roller portion of a charging roller.

FIG. 5 is a schematic diagram for explaining that a gap Gc at an end of a region on both sides of a discharge region formed between a photosensitive drum and a charging roller is not the maximum gap.

FIG. 6 is a schematic diagram showing a state in which a maximum gap Gmax is formed at a position b at a certain moment due to rotation of a charging roller and variations in straightness.

FIG. 7 is a schematic diagram showing a state where the maximum gap Gmax is formed at a position c.

FIG. 8 is a diagram illustrating charging characteristics showing a relationship between an applied voltage and a charging potential.

FIG. 9 is a diagram showing a change in charging characteristics when the charging roller is gradually separated from the photosensitive drum.

FIG. 10 is a diagram showing both a simulation result and an experimental result obtained by calculating a relationship between a gap between a charging roller and a photoconductor drum and a charging potential on a photoconductor surface.

FIG. 11 is a graph showing charging characteristics when a voltage applied by a charging device of a proximity charging method using a small gap is a DC constant voltage + AC constant voltage superposition.

FIG. 12 is a diagram showing an experimental result when an AC bias superimposed on a DC constant voltage is controlled by a constant current.

[Explanation of symbols]

 1: photosensitive drum (charged object) 2: charging device 8: charging roller (charging member) 12: power supply unit (power supply) 14: Teflon tube (gap management member)

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Megumi Daen 1-3-6 Nakamagome, Ota-ku, Tokyo F-term in Ricoh Co., Ltd. 2H003 AA18 BB11 CC00 DD03 EE19

Claims (11)

[Claims] 1. A charging member provided close to a member to be charged so as to form a predetermined gap at least in a charging region, and an AC voltage is superimposed on a DC constant voltage from a power supply on the charging member. A charging device that charges the member to be charged by applying the applied voltage, wherein the AC component of the voltage applied to the charging member is a peak-to-peak voltage that is at least twice the charging start voltage value at the maximum gap of the predetermined gap. A charging device characterized by having a value. 2. The charging device according to claim 1, wherein a gap between the member to be charged and the charging member is non-uniform and has a deviation depending on a position. 3. The charging device according to claim 1, wherein a gap between the member to be charged and the charging member varies. 4. The charging device according to claim 3, wherein said charging member is a rotating roller. 5. The charging device according to claim 3, wherein the member to be charged is a member that rotates or rotates. 6. The gap according to claim 1, wherein the gap between the charged member and the charging member is a gap having a charging start voltage different from a charging start voltage when the gap is zero. Charging device. 7. A charging member provided in close proximity to a member to be charged so as to form a predetermined gap at least in a charging region, wherein the charging member has a DC voltage controlled by a constant voltage from a power source and a DC voltage. The object to be charged is charged by the application of an AC voltage, and the average value of the gap at each position in the longitudinal direction and the lateral direction of the charging member in the charging area is 10 μm or more, and In a charging device having a variation of 10 μm or more with respect to the average value, the voltage applied to the charging member is such that a voltage having an AC component has a peak-to-peak voltage that is at least twice the charging start voltage value at the maximum gap of the predetermined gap. A charging device having a voltage value. 8. A charging member provided in such a manner that a portion to be charged and a non-contacting portion in the charging area are mixed with the member to be charged, and the charging member has a DC voltage controlled by a constant voltage from a power supply. The charged object is charged by applying a voltage and an AC voltage, and the charged object and the non-contact portion in the charging region at the longitudinal and lateral positions of the charging member in the non-contact portion are charged with the charged object. The average value of the gap with the charging member is 10
μm or more, the variation of the gap is 10 μm or more with respect to the average value, the voltage applied to the charging member, the voltage having an AC component is the charging start voltage value in the maximum gap of the gap A charging device having a peak-to-peak voltage value that is twice or more. 9. The charging device according to claim 7, wherein the charging member is a rotatable elastic roller. 10. The charging device according to claim 7, wherein the gap is formed by interposing a gap management member between the member to be charged and the charging member. The gap is determined by the thickness of the gap management member. 11. An image forming apparatus comprising the charging device according to claim 1. Description: