JP2002148900A - Image forming device and process cartridge - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent an image carrier and a contact electrostatic charging member from partially deteriorating. SOLUTION: In equation (1) providing W=(DC-2Y)/x, where DC is the outside diameter of an electrostatic charging roller 2, DD the outside diameter of a photoreceptor drum 1, Z the nip width between the both, Y the intrusion quantity of the electrostatic charging roller 2, and X the absolute value of the peripheral speed ratio of the both, the relation is satisfied that MIN (A, B) is a larger numeral between A and B, and LCM (A, B) is the least common multiple between A and B. Consequently, the frequency at which the same parts of the photoreceptor drum 1 and electrostatic charging roller 2 come into contact with each other is decreased below a certain value and the photoreceptor drum 1 and electrostatic charging roller 2 are prevented from partially deteriorating.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile, and a process cartridge for forming an image by an electrophotographic system or an electrostatic recording system.

[0002]

2. Description of the Related Art In an electrophotographic image forming apparatus,
As a charging means for charging an electrophotographic photosensitive member to a predetermined polarity and potential, a corona charging device has been generally used. In this method, a corona charging device is disposed opposite to an electrophotographic photosensitive member in a non-contact manner, and the surface of the electrophotographic photosensitive member is exposed to a corona emitted from the corona charging device to charge the surface of the electrophotographic photosensitive member to a predetermined polarity and potential. Things. However, a large amount of ozone is generated in the corona charging device, and 10 KV is applied between the corona charging device and the electrophotographic photosensitive member.
It was necessary to apply such a high voltage.

As a charging means for solving these problems,
In recent years, a so-called contact charging device for uniformly charging an electrophotographic photosensitive member by applying a voltage to a charging member that directly contacts the electrophotographic photosensitive member has been proposed and put into practical use. However, this method is the same as the above-described corona charging device in that charging is performed using a discharge phenomenon, and although it is greatly suppressed as compared with the corona charging device, ozone is still generated. This ozone produces nitric oxide (NOx), and when this nitric oxide adheres to the electrophotographic photosensitive member,
Since the resistance is low, it causes image failure due to charging failure.

Therefore, a charging process that does not have the above-described problem of ozone generation and that can further reduce the voltage applied to the charging member has been proposed in Japanese Patent Application Laid-Open No. Hei 6-3921.

The feature of this charging process is that the surface potential of the electrophotographic photosensitive member can be made substantially the same as the voltage applied to the charging device. This is because the electrophotographic photosensitive member that has contacted the charging member without using a discharge phenomenon is used. This is made possible by injecting charges into the electrophotographic photosensitive member by directly transferring charges to and from the body surface.

An example of charging by a charging sponge roller will be described as charging means for realizing the above injection charging process. In this charging method, as shown in FIG. 4, a charging sponge roller (hereinafter, referred to as a charging roller) 2 in contact with a drum-shaped electrophotographic photoreceptor (hereinafter, referred to as a photosensitive drum) 1 serving as an image carrier is provided. A relatively low-resistance conductive fine powder m is adhered to the portion, and a charging bias of -600 V is applied to the charging roller 2 by a charging bias power supply S1.

For this reason, the portion of the photosensitive drum 1 which is in contact with the charging roller 2 tends to have substantially the same potential as the charging bias. At this time, when electric charges are injected into the photosensitive drum 1 from the charging roller 2 over the energy barrier on the surface of the photosensitive drum 1, the photosensitive drum 1 is charged. Also, if the charge cannot exceed the energy barrier on the surface of the photosensitive drum 1 or if the charge moves from the photosensitive drum 1 to the charging roller 2 again when the charging roller 2 separates from the photosensitive drum 1, the charging occurs. Absent.

While this phenomenon is largely due to the energy barrier on the surface of the photosensitive drum 1 and the ability to retain electric charges, the frequency of opportunities for the charging roller 2 to come into contact with the photosensitive drum 1 is important when considered as a competitive reaction. Become. In order to increase the frequency, the conductive fine powder m is adhered to the vacant portion on the surface of the charging roller 2 to increase the adhesion between the charging roller 2 and the photosensitive drum 1, and the rotation direction of the charging roller 2 ( It is effective to increase the relative number of times of contact with the photosensitive drum 1 by, for example, increasing the relative speed by reversing the clockwise direction of the photosensitive drum 1 (clockwise direction).

By doing so, the surface potential of the photosensitive drum 1 becomes substantially the same as -600 V applied to the charging roller 2, and uniform charging can be achieved even in a micro part without charging unevenness.

Further, by setting the charging polarity of the conductive fine powder m to be opposite to the polarity of the developer T of the developing device 3, the conductive fine powder m is transferred to the transfer material P such as paper at the transfer nip N by the transfer roller 4. This remains on the photosensitive drum 1 without being recovered, and is collected on the charging roller 2 to always obtain a new injection site. For this reason, the developer T is
Even if it accumulates on the top, if the conductive fine powder m is supplied more than that, the occurrence of charging failure can be suppressed.

In the image forming apparatus shown in FIG. 4 in which charging is performed using the conductive fine powder m, a toner image formed on the photosensitive drum 1 by image exposure L from an exposure device (not shown) is transferred to a transfer bias. Transfer bias from power supply S3 (positive polarity)
After being transferred to the transfer material P at the transfer nip N by the transfer roller 4 to which the transfer roller 4 is applied, the transfer residual developer remaining on the photosensitive drum 1 is removed from the charge roller 2 on which the charging process is performed. During the passage between the conductive fine powder m and the photosensitive drum 1, charge injection is performed in the same manner as in the photosensitive drum 1.

As a result, the transfer residual developer can have an appropriate charge.
When a developing sleeve 3a to which a developing bias is applied passes through a region where an electrostatic latent image formed on the photosensitive drum 1 by the image exposure L is developed (reversed development), the developing device does not pass through. 3 will be collected. Accordingly, since the transfer residual developer can be collected in the developing device 3 by the above-described process, a cleanerless electrophotographic process without a cleaner for removing the transfer residual developer can also be realized.

[0013]

In the above-described conventional image forming apparatus, if the photosensitive drum 1 and the charging roller 2 are in contact with a speed difference in order to increase the charging efficiency, the image forming apparatus is When starting and stopping, a large load is applied to the photosensitive drum 1 and the charging roller 2.

At this time, if foreign matter such as paper powder of the transfer material (paper) P or an external additive of the developer T is attached to the charging nip portion between the photosensitive drum 1 and the charging roller 2 or in the vicinity thereof, Start-up·
Due to the load at the time of stoppage, minute scratches, so-called pinholes, are likely to occur on the surfaces of the photosensitive drum 1 and the charging roller 2. Pinholes cause charging failure due to leaks and the like, and furthermore, if the pinholes of the photosensitive drum 1 and the charging roller 2 come into contact with each other, the deterioration at this contacting portion accelerates. An improvement is desired so that the setting is made so that the same portions of the two do not frequently contact.

Accordingly, the present invention suppresses partial deterioration of the image carrier and the charging member by reducing the frequency of contact between the same portion of the image carrier and the same portion of the charging member to a certain ratio or less, thereby providing a good image for a long period of time. It is an object of the present invention to provide an image forming apparatus and a process cartridge capable of forming.

[0016]

In order to achieve the above object, the present invention is directed to a rotatable image carrier, which is brought into contact with the image carrier with a predetermined nip width and is brought into contact with the image carrier by applying a charging bias. An image forming apparatus comprising: a rotatable contact charging member having flexibility to charge a body, wherein the outer diameter of the contact charging member is DC (mm), and the outer diameter of the image carrier is D.
D (mm), the nip width between the image bearing member and the contact charging member is Z (mm), the penetration amount of the contact charging member into the image bearing member is Y (mm), and the contact charging to the image bearing member is When the absolute value of the peripheral speed ratio of the member is X, the image carrier and the contact charging member are

(Equation 2) (However, MIN (A, B) selects the larger value of A and B, and LCM (A, B) satisfies the relationship of LCM (Least common multiple of A and B).

In addition, a conductive fine powder having conductivity is present in a charging nip where the contact charging member and the image carrier abut, and charge is injected into the image carrier by application of a charging bias. It is characterized by doing.

Further, in the charging nip where the image carrier and the contact charging member are in contact with each other, the moving direction of the image carrier and the moving direction of the contact charging member are opposite to each other.

A rotatable image carrier, and a rotatable contact charging member having a predetermined nip width and being in contact with the image carrier and having flexibility to charge the image carrier by applying a charging bias. And at least a process cartridge detachably attached to the image forming apparatus,
The image bearing member and the contact charging member are the image bearing member and the contact charging member according to claim 1, 2 or 3.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the illustrated embodiments.

FIG. 1 is a schematic configuration diagram showing an image forming apparatus according to an embodiment of the present invention. The image forming apparatus according to the present embodiment is a cleaner-less laser beam printer using a contact charging method and simultaneous development and cleaning. The members having the same functions as those of the conventional image forming apparatus shown in FIG.

This image forming apparatus includes a photosensitive drum 1 as an image carrier, and a conductive elastic roller (hereinafter, referred to as a charging roller) 2 as a contact charging means, an exposure device (not shown), and a developing device around the photosensitive drum 1. 3, a transfer roller 4 is provided.
Further, a fixing device 5 is provided on the transfer nip portion N formed between the photosensitive drum 1 and the transfer roller 4 on the downstream side in the transport direction of the transfer material P.

The photosensitive drum has a diameter of 30 in this embodiment.
mm OPC photoreceptor (negative photoreceptor), 50 mm / s in the direction of the arrow (clockwise) by driving means (not shown).
It is rotationally driven at a peripheral speed of ec (process speed).

The charging roller 2 has a medium resistance roller member 2b made of a flexible elastic foam or the like on a cored bar 2a. The roller member 2b is formulated with a resin material (for example, urethane), conductive particles (for example, carbon black), a sulfide agent, a foaming agent, and the like. The charging roller 2 is
In this embodiment, the diameter is 12 mm and the length in the longitudinal direction is 250.
mm and the resistance is 10 ⁵ Ω (total load pressure 9.8 N, applied voltage 100 V). A conductive fine powder m is attached to the surface of the charging roller 2.

The material of the roller member 2b is not limited to the above-mentioned elastic foam, but may be a conductive material such as carbon black or metal oxide for adjusting the resistance to EPDM, urethane, NBR, silicone rubber or IR. Rubber materials in which a conductive substance is dispersed, and foamed materials thereof. Further, it is also possible to adjust the resistance by using an ionic conductive material without dispersing the conductive substance.

It is important that the charging roller 2 functions as an electrode. That is, it is necessary to obtain a sufficient contact state with the photosensitive drum 1, which is a member to be charged, by providing elasticity, and at the same time, to have a sufficiently low resistance to charge the moving (rotating) photosensitive drum 1. On the other hand, when a defective portion such as a pinhole is present on the photosensitive drum 1, it is necessary to prevent voltage leakage, and a resistance of 10 ⁴ to 10 ⁷ Ω is desirable to obtain sufficient chargeability and leakage resistance. .

If the hardness of the charging roller 2 is too low, the shape is not stable, so that the contact with the photosensitive drum 1 is deteriorated. If the hardness is too high, the charging nip M is secured between the charging roller 2 and the photosensitive drum 1. Not only is it impossible, but also the microscopic contact with the surface of the photosensitive drum 1 is deteriorated.
5 to 50 degrees is a preferable range.

The charging roller 2 is rotatably pressed against the photosensitive drum 1 with a predetermined pressing force against the elasticity of the photosensitive drum 1 to form a charging nip portion M having a predetermined width (for example, 3 mm). In the section M, the charging roller 2 is rotationally driven in the direction of the arrow (clockwise) at 75 mm / sec so that the surface of the charging roller 2 and the surface of the photosensitive drum 1 move in opposite directions. That is, the charging roller 2 is rotated in the opposite direction with a speed difference from the photosensitive drum 1.

Further, a DC voltage of -700 V was applied as a charging bias from the charging bias power source S1 to the core metal 2a of the charging roller 2. Thus, in the present embodiment, the surface of the photosensitive drum 1 is uniformly charged by the direct injection charging method to a potential (−680 V) substantially equal to the voltage applied to the charging roller 2.

The developing device 3 includes a developer (in this embodiment, a magnetic one-component insulating toner (hereinafter, referred to as a toner)) t and a conductive fine powder m (used in the charging roller 2). A mixture of the same conductive fine powder m) is attached to the electrostatic latent image formed on the photosensitive drum 1, and is visualized as a toner image by reversal development.

The non-magnetic developing sleeve 3a rotated at substantially the same speed as the photosensitive drum 1 is arranged so that the closest distance (gap) to the photosensitive drum 1 is about 500 μm, and is fixed in the developing sleeve 3a. Magnet roll 3 included
b, the developer (toner t)
+ Conductive fine powder m) T is magnetically bound. The developer (toner t + conductive fine powder m) T is stored in a developer container 3d. The developer T carried on the surface of the developing sleeve 3a
The layer is regulated by the regulating blade 3c and charged, and is conveyed to the developing section (developing area) S, which is the facing section between the photosensitive drum 1 and the developing sleeve 3a, by the rotation of the developing sleeve 3a.

A developing bias in which a DC component and an AC component are superimposed is applied from a developing bias power source S2 to the developing sleeve 3a. In this embodiment, the DC component of the developing bias is −500 V, the AC component is a peak-to-peak voltage of 1600 V,
A rectangular wave having a frequency of 1800 Hz was used.

The developer T is a mixture of the toner t and conductive fine powder (charge assisting particles) m. The toner t is a polymerizable monomer and a colorant (further as necessary, a polymerization initiator, a crosslinking agent,
After uniformly dissolving or dispersing the charge control agent and other additives into a monomer composition, the monomer composition is placed in a continuous layer containing the dispersion stabilizer using a suitable stirrer. It is prepared by a suspension polymerization method in which a polymerization reaction is carried out at the same time as a dispersion, and a conductive fine powder m and a fluidizing agent are added thereto as an external additive.

The toner t used in the present embodiment has a weight average particle diameter of 7 μm. In the present embodiment, the conductive fine powder m has an average particle diameter of 3 μm and a specific resistance of 10 ⁶ Ω · cm. Zinc particles were used. In the present embodiment, 2 parts by weight of the conductive fine powder m is externally added to 100 parts by weight of the toner t. In addition, as the material of the conductive fine powder m, various conductive particles such as a mixture with conductive inorganic particles such as other metal oxides and organic substances other than the above-described conductive zinc oxide particles can be used.

A transfer roller 4 having a medium resistance as a contact transfer means contacts the surface of the photosensitive drum 1 with a predetermined pressing force to form a transfer nip N, and a transfer bias (toner and toner) applied from a transfer bias power source S3. The toner image formed on the surface of the photosensitive drum 1 at the transfer nip N is transferred to a transfer material P such as paper by the reverse polarity (positive polarity bias).

The fixing device 5 has a fixing roller 5a and a pressure roller 5b.
The toner image transferred onto the surface of the transfer material P such as paper is heat-pressed and heat-fixed while nipping and transporting the transfer material P in the intermediate fixing nip.

Further, in this embodiment, as shown in FIG. 2, the photosensitive drum 1, the charging roller 2 and the developing device 3 are integrally formed into a cartridge to form a process cartridge 6, which is detachably attached to the image forming apparatus. Be attached.

Next, an image forming operation by the image forming apparatus will be described.

When image information is input from a host computer (not shown), the photosensitive drum 1 is driven to rotate at a predetermined peripheral speed in a direction indicated by an arrow (clockwise) by driving a main motor (not shown). Charging bias power supply S at M
The surface is uniformly charged to a predetermined potential (-680 V) by the conductive fine powder m on the surface of the charging roller 2 to which the charging bias (-700 V) is applied from 1. Then, an image exposure L according to image information is given to the charged photosensitive drum 1 from an exposure device (not shown), so that the potential on the photosensitive drum 1 decreases at the image exposed L portion. Then, an electrostatic latent image corresponding to the input image signal is formed.

The developing sleeve of the developing device 3 in which a developing bias having the same polarity as the charging polarity (negative polarity) of the photosensitive drum 1 is applied to the electrostatic latent image formed on the photosensitive drum 1 by the developing unit S By 3a, the toner t charged to the same polarity as the charging polarity (negative polarity) of the photosensitive drum 1 is adhered, and the toner t is subjected to reverse development (visualization) as a toner image. At this time, the developing sleeve 3a supplies -500 V as a DC component and a peak-to-peak voltage 160 as an AC component from the developing bias power source S2.
A developing bias in which a rectangular wave of 0 V and a frequency of 18000 Hz is superimposed is applied.

When the toner image on the photosensitive drum 1 reaches the transfer nip portion N between the photosensitive drum 1 and the transfer roller 4, a transfer material P such as paper in a cassette (not shown) is supplied at this timing. The sheet is conveyed to the transfer nip N by a paper roller (not shown). Then, the transfer material P conveyed to the transfer nip N by the transfer roller 4 to which a predetermined transfer bias of the opposite polarity (positive polarity) to the toner t is applied.
The toner image on the photosensitive drum 1 is transferred by an electrostatic force generated between the photosensitive drum 1 and the transfer roller 4.

Then, the transfer material P on which the toner image has been transferred is conveyed to the fixing device 5, where the toner image is heated and pressed on the transfer material P by a fixing nip between the fixing roller 5a and the pressure roller 5b, and thermally fixed. Then, the sheet is discharged outside, and a series of image forming operations is completed.

The transfer residual toner remaining on the photosensitive drum 1 after the above-mentioned transfer reaches the developing unit S via the charging nip M with the rotation of the photosensitive drum 1, and reaches the developing sleeve 3a of the developing device 3. In the development after the next step, the toner is collected by a fog removing bias (a fog removing potential difference which is a potential difference between a DC voltage applied to the developing sleeve 3a from the developing bias power source S2 and the surface potential of the photosensitive drum 1) (simultaneous development cleaning). The collected transfer residual toner (residual developer) can be used after the next step to eliminate waste toner.

By the way, the conductive fine powder m contained in the developer T used in the developing device 3 is used for developing the electrostatic latent image on the photosensitive drum 1 by the developing sleeve 3 a of the developing device 3. At the same time, an appropriate amount moves to the photosensitive drum 1 side together with the toner t. The toner image on the photosensitive drum 1 is pulled toward the transfer material P by the transfer bias applied from the transfer bias power source S3 to the transfer roller 4 in the transfer nip N, and is positively transferred. Since the conductive fine powder m is conductive, it does not positively transfer to the transfer material P side, and remains substantially adhered and held on the photosensitive drum 1. Further, the effect of improving the transfer efficiency of the toner image from the photosensitive drum 1 side to the transfer material P side can be obtained by the presence of the conductive fine powder m substantially adhered and held on the surface of the photosensitive drum 1.

Since the above-described image forming apparatus for simultaneous cleaning with development does not use a cleaner for removing the transfer residual toner, the transfer residual toner and the conductive fine powder m remaining on the surface of the photosensitive drum 1 after the transfer are removed. The photosensitive drum 1 is carried as it is to the charging nip M between the drum 1 and the charging roller 2 by the rotation of the photosensitive drum 1 and adheres to and mixes with the charging roller 2. Accordingly, contact charging of the photosensitive drum 1 is performed in a state where the conductive fine powder m exists in the charging nip portion M. In the initial stage of image formation, the conductive fine powder m is not supplied to the surface of the charging roller 2 and charging cannot be performed. Therefore, the conductive fine powder m is applied to the surface of the charging roller 2 in advance.

In the present embodiment, since the conductive fine powder m is previously applied to the surface of the charging roller 2,
Even if the toner t adheres to or mixes with the photosensitive drum 1, the contact property of the charging roller 2 to the photosensitive drum 1 and the contact resistance can be maintained, so that the contact charging member is a simple member such as the charging roller 2, and Irrespective of contamination by the transfer residual toner of the roller 2, direct charging of the photosensitive drum 1 by the charging roller 2 can be performed.

That is, the charging roller 2 comes into close contact with the photosensitive drum 1 via the conductive fine powder m, and the conductive fine powder m present in the charging nip portion M rubs the surface of the photosensitive drum 1 without gaps. As a result, the charging of the photosensitive drum 1 by the charging roller 2 is controlled by stable and safe direct injection charging without using a discharge phenomenon due to the presence of the conductive fine powder m. Charging efficiency is obtained,
A potential substantially equal to the voltage applied to the charging roller 2 can be given to the photosensitive drum 1.

The transfer residual toner adhering to and mixing into the conductive fine powder m on the charging roller 2 is gradually discharged from the charging roller 2 onto the photosensitive drum 1 so that the developing unit S And the developing device 3 performs cleaning (collection) at the same time as development. As described above, in the simultaneous cleaning with development, the transfer residual toner remaining on the photosensitive drum 1 after the transfer is developed in the next image forming step, that is, the photosensitive drum 1 is charged and exposed to form an electrostatic latent image. At the time of developing the electrostatic latent image, the electrostatic latent image is collected by a fog removing bias of the developing device 3, that is, a fog removing potential difference which is a potential difference between a DC voltage applied from the developing bias power source S2 to the developing sleeve 3a and a surface potential of the photosensitive drum 1. Things. Since the developing device 3 of the present embodiment is a reversal developing, the simultaneous cleaning of the developing is performed by the electric field for collecting the toner from the dark portion potential (unexposed portion) of the photosensitive drum 1 to the developing sleeve 3a and the electric field of the photosensitive drum 1 from the developing sleeve 3a. This is performed by the action of an electric field that causes the toner to adhere to the bright portion potential (exposed portion).

Even if the conductive fine powder m drops from the surface of the charging roller 2, the conductive fine powder m contained in the developer T of the developing device 3 is removed by the image forming operation of the image forming apparatus. In S, the photosensitive drum 1 moves to the surface, and the photosensitive drum 1 rotates, passes through the transfer nip N, and the charging nip M
, And is continuously supplied to the charging roller 2 sequentially, so that good chargeability due to the presence of the conductive fine powder m is stably maintained.

As described above, in the image forming apparatus described above, regardless of the contamination of the charging roller 2 used as the contact charging member by the transfer residual toner, uniform charging by the ozone-less direct injection charging at a low applied voltage is stable for a long time. , And it is possible to prevent troubles due to ozone products, troubles due to poor charging, and the like.

In the image forming apparatus of the present embodiment, since the photosensitive drum 1 and the charging roller 2 are contacted with a speed difference in order to increase the charging efficiency, when the image forming apparatus is started and stopped, A large load is applied to the photosensitive drum 1 and the charging roller 2 at the contact portion (charging nip portion M). At this time, the pinhole causes a charging failure due to leakage or the like, and furthermore, if the pinholes of the photosensitive drum 1 and the charging roller 2 come into contact with each other, the deterioration accelerates. It is important to set so that the same part does not frequently contact.

However, in the present image forming apparatus, in order to increase the charging efficiency, the charging roller 2 is driven so that the charging roller 2 and the photosensitive drum 1 are moved in opposite directions to each other in the charging nip M. Therefore, when starting and stopping the image forming apparatus, a large load is applied to the photosensitive drum 1 and the charging roller 2 at the contact portion (charging nip portion M). At this time, the photosensitive drum 1 and the charging roller 2
If foreign matter such as paper dust of the transfer material (paper) P or an external additive of the developer T is attached to the charging nip portion M or in the vicinity thereof, the photosensitive drum 1 or the charging member is charged by the load at the time of starting and stopping. Minute scratches, so-called pinholes, and the like are likely to occur on the surface of the roller 2.

If a pinhole is formed in the photosensitive drum 1 or the charging roller 2, a leak occurs, which causes charging failure. In addition, when the locations where the pinholes of the photosensitive drum 1 and the charging roller 2 are in contact with each other, a large amount of current flows and the deterioration accelerates, which may shorten the life of the photosensitive drum 1 and the charging roller 2.

For this reason, in the present invention, the photosensitive drum 1 and the charging roller 2 are changed according to the following formula (1) so that the frequency at which the same portion of the rotating photosensitive drum 1 and the same portion of the charging roller 2 come into contact with each other is less than a certain ratio. Set to.

That is, as shown in FIG. 3, the outer diameter of the charging roller 2 is DC (mm), and the outer diameter of the photosensitive drum 1 is DD (m).
m), the nip width between the photosensitive drum 1 and the charging roller 2 is Z
(Mm), the amount of penetration of the charging roller 2 into the photosensitive drum 1 is Y (mm), and the absolute value of the peripheral speed ratio of the charging roller 2 to the photosensitive drum 1 is X,

(Equation 3) (However, MIN (A, B) selects the larger value of A and B, and LCM (A, B) satisfies the relationship of ACM and B.)

(DC-2Y) is a value corresponding to the apparent outer diameter of the charging roller 2 reduced by taking into account the amount of penetration of the charging roller 2 into the photosensitive drum 1. Also, Z
Is the value obtained by dividing it by the absolute value X of the peripheral speed ratio.
Is a value corresponding to the outer diameter of the charging roller 2 when it is assumed that the charging roller 2 is driven relative to the photosensitive drum 1. For example, when the peripheral speed ratio of the charging roller 2 to the photosensitive drum 1 is larger than 1, the charging roller 2 rotates quickly, and the interval until the same portion on the surface of the charging roller 2 returns to the same position becomes short. That is, when viewed from the photosensitive drum 1 side, the same effect as when the apparent outer diameter of the charging roller 2 is reduced is obtained.

Next, the above equation (1) will be described in more detail from the left side. The two numbers in parentheses of the MIN symbol (A, B) are obtained by multiplying W and DD by 10 each to obtain a significant figure to one decimal place, and dividing their least common multiple by W and DD, respectively. Yes, the resulting numbers are relatively prime natural numbers.

For example, if these numbers are 5 and 3, the same portions of the charging roller 2 and the photosensitive drum 1 are repeatedly contacted every 5 rotations of the charging roller 2 and every 3 rotations of the photosensitive drum 1. Become. Accordingly, the larger the number is, the longer the time interval until the same portion of the charging roller 2 and the photosensitive drum 1 come into contact with each other, and the probability that the damaged portions of the charging roller 2 and the photosensitive drum 1 come into contact again. Is smaller.

On the other hand, the right side of the equation (1) is obtained by dividing the smaller peripheral length of the photosensitive drum 1 and the charging roller 2 by the nip width Z between the photosensitive drum 1 and the charging roller 2, and directly Indicates how many nip widths Z can be taken on the circumference. The larger the number is, the larger the charging roller 2 and the photosensitive drum 1 are.
Means that the probability of contact between the damaged parts again decreases.

That is, after the photosensitive drum 1 and the charging roller 2 once come into contact with each other, the number of rotations from when the photosensitive drum 1 and the charging roller 2 come into contact again at the same position can be taken on the circumference of the smaller diameter of the photosensitive drum 1 and the charging roller 2. If the number is larger than the number of the widths, the probability that the damaged portions come into contact with each other can be minimized.

Specifically, in the present embodiment, DC = 12
mm, DD = 30 mm, Z = 3 mm, Y = 0.2 mm,
Since X = 1.5 and W = 7.73 mm from these values, the left side of Expression (1) is 77 and the right side is 12.6.
Satisfying the expression (1).

Then, a durability test of outputting 12,000 continuous images was performed using the image forming apparatus of the present embodiment that satisfies the above expression (1), and the pinholes of the charging roller 2 and the photosensitive drum 1 were compared. There was no occurrence of charging failure due to contact with the toner.

Next, the effect of the image forming apparatus of the present embodiment satisfying the above-mentioned expression (1) was evaluated by the following image forming apparatus for a comparative example.

(Comparative Example 1) In this comparative example, the charging roller 2
Was changed to an outer diameter of 12.4 mm. Other configurations and numerical values are the same as those of the first embodiment. In this comparative example,
DC = 12.4 mm, DD = 30 mm, Z = 3 mm, Y
= 0.2 mm and X = 1.5. From these values, W =
Since it is 8.0 mm, the left side of Expression (1) is 4, and the right side is 13.0, which does not satisfy Expression (1).

When a durability test of continuous image output was performed using the image forming apparatus of this comparative example, charging failure occurred due to contact between the charging roller 2 and the pinholes of the photosensitive drum 1 on about 4,500 sheets. did.

Comparative Example 2 In this comparative example, the outer diameter of the charging roller 2 was changed to 13.0 mm. Other configurations and numerical values are the same as those of the first embodiment. In this comparative example, D
C = 13.0 mm, DD = 30 mm, Z = 3 mm, Y =
0.2 mm, X = 1.5, and W =
Since it is 8.4 mm, the left side of Expression (1) is 7, and the right side is 13.6, which does not satisfy Expression (1).

When a durability test of continuous image output was performed using the image forming apparatus of this comparative example, charging failure occurred due to contact between the charging roller 2 and the pinholes of the photosensitive drum 1 on about 8,500 sheets. did.

As described above, the diameter of the photosensitive drum 1, the diameter of the charging roller 2, the nip width between the photosensitive drum 1 and the charging roller 2, and the width of the charging roller 2 with respect to the photosensitive drum 1 satisfy the expression (1). By setting the amount of intrusion and the peripheral speed difference between the photosensitive drum 1 and the charging roller 2, it is possible to minimize the contact between the damaged portions of the charging roller 2 and the photosensitive drum 1 again. Thus, it is possible to prevent the photosensitive drum 1 and the charging roller 2 from deteriorating at an accelerated rate, and it is possible to form a good image over a long period.

[0069]

As described above, according to the present invention, the frequency of contact between the same portion of the image carrier and the contact charging member can be reduced to a certain ratio or less. Thus, it is possible to minimize the probability that the damaged portions come into contact with each other again, prevent the image carrier and the contact charging member from being acceleratedly degraded, and perform good image formation over a long period of time.

[Brief description of the drawings]

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

FIG. 2 is a schematic sectional view showing a process cartridge of the image forming apparatus according to the embodiment of the present invention.

FIG. 3 is a diagram showing a contact state between a photosensitive drum and a charging roller of the image forming apparatus according to the embodiment of the present invention.

FIG. 4 is a schematic configuration diagram showing an image forming apparatus in a conventional example.

[Explanation of symbols]

 Reference Signs List 1 photosensitive drum (image carrier) 2 charging roller (contact charging member) 2a cored bar 2b roller member 3 developing device 3a developing sleeve 4 transfer roller 5 fixing device 5a fixing roller 5b pressure roller 6 process cartridge

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hiroshi Sato 3-30-2 Shimomaruko, Ota-ku, Tokyo Inside Canon Inc. (72) Inventor Masahiro Yoshida 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Incorporated (72) Inventor Yasushi Shimizu 3-30-2 Shimomaruko, Ota-ku, Tokyo F-term (reference) 2H003 AA18 BB11 CC05 EE16 EE19

Claims (4)

[Claims] A rotatable image carrier, and a flexible rotatable contact charging member that contacts the image carrier with a predetermined nip width and charges the image carrier by applying a charging bias. Wherein the outer diameter of the contact charging member is DC (mm), the outer diameter of the image carrier is DD (mm), and the nip width between the image carrier and the contact charging member is Z. (Mm), when the amount of penetration of the contact charging member into the image carrier is Y (mm) and the absolute value of the peripheral speed ratio of the contact charging member to the image carrier is X, the image carrier and the The contact charging member is expressed as follows: (However, MIN (A, B) selects the larger one of A and B, and LCM (A, B) satisfies the relationship of LCM (Least common multiple of A and B). apparatus. 2. A charging nip where the contact charging member and the image carrier are in contact with each other has conductive fine powder present therein, and charges are injected into the image carrier by applying a charging bias to the image carrier. The image forming apparatus according to claim 1, wherein: 3. A charging nip where the image carrier and the contact charging member are in contact with each other, wherein a moving direction of the image carrier and a moving direction of the contact charging member are opposite to each other. The image forming apparatus according to claim 1. 4. An image carrier, and a flexible rotatable contact charging member which contacts the image carrier with a predetermined nip width and charges the image carrier by applying a charging bias. 4. A process cartridge provided at least and detachably mounted on an image forming apparatus, wherein the image carrier and the contact charging member are the image carrier and the contact charging member according to claim 1, 2, or 3. And process cartridge.