JP2003323032A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To electrify a photoreceptor drum at the stable potential by electrification over a long term. <P>SOLUTION: By detecting a current flowing to the electrifying sleeve 2b of a magnetic brush electrifying device 2 by an ammeter 10, controlling the current flowing to the device 2 by a controller 12 based on a detected current value so that its absolute value may be minimum, and setting a voltage value applied to an auxiliary electrifying roller 7 from an auxiliary electrifying bias applying power source 13, the stable potential by electrification is obtained over a long term. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, a printer or a facsimile which forms an image by an electrophotographic method or an electrostatic recording method, and more particularly to an image forming apparatus having a plurality of charging means. Regarding

[0002]

2. Description of the Related Art In an electrophotographic image forming apparatus, a corona charger has heretofore been generally used as a charging means for charging an electrophotographic photosensitive member to a predetermined polarity and potential. In this, a corona charger is placed in non-contact with a photoconductor as a member to be charged, and the photoconductor surface is exposed to the corona emitted from the corona charger to charge the photoconductor surface to a predetermined polarity and potential. Is.

Further, in recent years, since it has advantages such as low ozone and low electric power as compared with the case of using the above-mentioned non-contact type corona charger, a voltage (charging bias) is applied to the photoconductor as the member to be charged. Many contact charging type charging devices that contact a charging member (contact charging member) to charge the surface of the photoreceptor to a predetermined polarity and potential have been proposed and put into practical use.

In such a contact charging type charging device, a roller type (charging roller), a fur brush type, a magnetic brush type, a blade type (charging blade), or the like is used as a charging member to be brought into contact with a photosensitive member as an object to be charged. There are various forms and there are various suggestions for improvement.

Charging mechanism of contact charging (charging mechanism,
In charging principle), there are two types of charging mechanisms, a discharge charging system and a direct injection charging system, which will be described below.

(A) Discharge charging system The discharge charging system is a system in which the surface of the photoconductor is charged by a discharge phenomenon which occurs in a minute gap between the contact charging member and the photoconductor (the body to be charged). Since the discharge charging system has a constant discharge threshold between the contact charging member and the photosensitive member, it is necessary to apply a voltage higher than the charging potential to the contact charging member. Further, compared with a corona charger, the generation amount is remarkably small, but generation of a discharge product is unavoidable in principle, so that a harmful effect due to active ions such as ozone is unavoidable.

(B) Direct Injection Charging System The direct injection charging system is a system in which charges are directly injected from a contact charging member to a photoconductor (charged body) to charge the surface of the photoconductor. It is also called injection charging or charge injection charging. More specifically, the medium-resistance contact charging member comes into contact with the surface of the photoconductor to directly inject the charge into the surface of the photoconductor without a discharge phenomenon, that is, basically without using discharge. Therefore, even if the voltage applied to the contact charging member is equal to or lower than the discharge threshold, the photoconductor can be charged to a potential corresponding to the applied voltage.

In the case of, for example, an organic photoconductor as a charged body that can be directly injected and charged, it is necessary to provide a charge injection layer in which conductive fine particles are dispersed as a charge holding member on the surface of the photosensitive layer. Inorganic photoconductors such as amorphous silicon photoconductors have many trap levels due to crystal defects on the surface without newly providing a charge injection layer,
The injected charges are held in this trap level and injection charging is possible.

Further, since the direct injection charging does not use the discharge phenomenon, the voltage required for charging is only the desired surface potential of the photoconductor, and it is effective in preventing ozone generation and reducing power consumption. Further, the direct injection charging has an advantage that the surface potential of the photoconductor is charged up to the applied voltage in principle and is not easily affected by environmental changes such as humidity.

Further, in the direct injection charging, since the charge is injected only into the region where the contact charging member contacts the surface of the photosensitive member, the contact probability between the contact charging member and the surface of the photosensitive member influences the charging ability. Therefore, when the contact probability is insufficient and there are many uncharged regions, charging ends before the surface potential of the photoconductor reaches the voltage value applied to the contact charging member.

Therefore, in recent years, in order to obtain a high contact probability evenly over the entire charging area, a method of contacting a photosensitive member with a magnetic brush made of magnetically restrained conductive magnetic particles to perform charging, A method has been proposed in which conductive fine particles are attached to an elastic roller made of a conductive sponge, and the particles are charged in the contact area between the surface of the elastic roller and the photoconductor to be charged. There is.

In the former case, a rotatable conductive sleeve containing a multi-pole fixed magnet roller is arranged in the vicinity of a photosensitive member, and magnetic particles are held on the sleeve by the magnetic force of the fixed magnet roller, and a regulating blade is provided. In general, the amount of magnetic particles held is regulated and made uniform by, for example, contacting the photosensitive member, and a charging bias is applied to the sleeve for charging.

On the other hand, in the latter case, a conductive sponge roller having fine pores, to which conductive fine particles are attached, is brought into contact with a photosensitive member, and charging is performed by applying a charging bias to the sponge roller. Is. These fine particles increase the electrical contact area with the photoconductor, and at the same time, have the effect of reducing the frictional force with the photoconductor.
By increasing the speed difference between the sponge roller and the photoconductor, the probability of contact with the photoconductor can be further increased.

The charge injection in the above-mentioned direct injection charging is based on the charging phenomenon of the capacitor which uses the contact area between the conductive substrate of the photosensitive member and the contact charging member as an electrode.

Therefore, in order to obtain a desired potential, a certain amount of charging time is required in principle, but as the process speed increases, the charging time becomes shorter and the desired potential cannot be obtained. May come. Further, due to contamination of the contact charging member or the like, the probability of contact with the photoreceptor may be reduced, and uniform charging may not be performed.

In order to solve these problems, an image forming apparatus using a plurality of charging devices (a plurality of charging members) has been proposed in, for example, Japanese Patent Application Laid-Open No. 8-44153. In this charging method, charging failure due to uneven contact or uneven resistance of the charging member can be suppressed by charging a plurality of times.

Further, in order to obtain a desired potential, the amount of electric charge applied to the photoconductor by each charging device is small, so that the potential fluctuation is reduced even if the resistance of each charging member rises due to contamination or environmental fluctuation. It also has an advantage that the life of the charging device can be easily extended as a result.

[0018]

By the way, even in the image forming apparatus using a plurality of charging devices (charging members) described above, due to contamination of the charging member due to long-term use, resistance increase due to deterioration of energization of the charging member, or the like, The fluctuation of the electric potential occurs though it is small. In particular, when the charging device of the injection charging type and the contact type charging device using the conductive roller of the corona charging type are combined, the resistance rises significantly due to the energization of the conductive roller compared to the injection charging, and the potential fluctuation It will be noticeable.

Therefore, an image forming apparatus has been proposed in which the charging bias applied to each charging device is controlled to perform stable and uniform charging.

However, in this case, in order to control the charging bias applied to each charging device, it is necessary to install a potential sensor for measuring the surface charging potential of the photosensitive member for each charging device. The arrangement of each member (developing device, cleaning device, etc.) around the device is restricted, and it is not preferable in terms of cost.

Therefore, according to the present invention, in an image forming apparatus that charges a photoconductor using a plurality of charging devices, there is no restriction on the arrangement of each member (developing device, cleaning device, etc.) around the photoconductor, and the photoconductor is not restricted. It is an object of the present invention to provide an image forming apparatus capable of charging a sheet at a stable charging potential for a long period of time.

[0022]

[Means for Solving the Problems] To achieve the above object
In the present invention, the movable image carrier and the application of voltage
Main charging means for charging the image carrier, and the main charging means
First voltage applying means for applying a voltage to the image carrier and the image carrier
At least upstream of the main charging means with respect to the moving direction of
One of them is also arranged and has the same polarity as the voltage applied to the main charging means.
Sub-charging means for charging the image carrier by applying the voltage
And a second voltage applying means for applying a voltage to the sub-charging means.
An image forming apparatus including a step, the first voltage
When the voltage is applied from the applying means to the main charging means,
The current detection means for detecting the current
The second voltage application based on the current value detected by the means.
Means to set the voltage application condition from the means to the auxiliary charging means.
It is characterized by.

Further, it has a control means for controlling the voltage application from the second voltage applying means to the sub-charging means, and the control means has the main charging based on the current value detected by the current detecting means. The absolute value of the current flowing through the means is controlled so as to be the minimum, and the voltage application condition to the sub-charging means is set.

Further, the main charging means interposes conductive fine particles attached to the contact charging member to which a voltage is applied from the first voltage applying means between the main charging means and the image carrier, and injects the charge. It is characterized in that the image carrier is charged.

The conductive fine particles are composed of conductive magnetic particles magnetically bound to the contact charging member,
It is characterized in that the conductive fine particles are brought into contact with the image carrier to charge them by injecting charges.

The sub-charging means brings a conductive roller member into contact with the image carrier and applies a voltage from the second voltage applying means to the roller member to charge the image carrier. It is characterized by that.

Further, a charge removing exposure means for exposing the image carrier to remove residual charges on the surface of the image carrier is provided upstream of the sub-charging means with respect to the moving direction of the image carrier. It has a feature.

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will now be described based on 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 has a magnetic brush charging device as main charging means and a sub charging means. As an auxiliary charging roller.

This image forming apparatus comprises a drum type electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum) 1 as an image bearing member.
And a magnetic brush charging device 2 as a main charging device, a developing device 3 and
Transfer roller 4, charge eliminating light lamp 5, cleaning device 6,
Also, an auxiliary charging roller 7 as a sub charging means is installed. Further, a fixing device 15 is installed on the downstream side of the transfer nip portion N formed between the photosensitive drum 1 and the transfer roller 4 in the transfer material conveying direction.

The photosensitive drum 1 is a negatively charged organic photosensitive drum in the present embodiment, and has a charge generation layer in which a disazo pigment is dispersed in a resin and an hydrazone in a polycarbonate resin, which are dispersed on an aluminum cylinder having a diameter of 30 mm. It has a charge-transporting layer, and further has an organic photosensitive layer consisting of a charge-injecting layer in which ultrafine SnO ₂ is dispersed in a photocurable acrylic resin in the surface layer (outermost layer), and a driving device (not shown). )
Is driven to rotate in the arrow direction (clockwise direction) at a peripheral speed of 100 mm / sec. The resistance of the surface layer of the photosensitive drum 1 is 10 ⁹ to 10 ¹⁴ Ω · m.

The injection charging type magnetic brush charging device 2 is
A charging container 2a, a charging sleeve 2b as a rotatable charging contact member having a diameter of 30 mm with a fixed magnet roller 2c disposed therein, and a charging sleeve 2b which is carried on the charging sleeve 2b and contacts the photosensitive drum 1 to inject charges. Magnetic particles M, a stirring screw 2d, and a regulating blade 2e for coating the surface of the charging sleeve 2b with the magnetic particles M to a uniform thickness.
have.

The charging sleeve 2b includes the photosensitive drums 1 and 50.
They are provided at intervals of 0 μm, and are rotationally driven in the arrow direction (clockwise direction) at a peripheral speed of 150 mm / sec. The charging sleeve 2b has a charging bias applying power source (S
1) From 11, peak-to-peak voltage of 500 Vpp, frequency of 1 kHz
A charging bias in which a DC voltage Vm of −600 V is superimposed on the alternating electric field of z is applied. Fixed magnet roller 2c
Has five magnetic pole peaks in the rotation direction (clockwise direction) of the charging sleeve 2b, and has a repulsive pole structure having adjacent magnetic pole peaks of the same polarity. In the present embodiment, the magnetic flux density of the magnet roller 2c on the charging sleeve 2b is 950 × 10 ⁻⁴ T.

The magnetic particles M are the fixed magnet roller 2c.
It is held on the charging sleeve 2b by the magnetic restraining force of and the layer thickness is regulated by the regulating blade 2e. If the peripheral speed of the charging sleeve 2b is too slow, the contact probability between the surface of the photosensitive drum 1 and the magnetic particles M becomes insufficient, which causes image defects such as uneven charging, and if the peripheral speed of the charging sleeve 2b is too high, the magnetic particles It causes the scattering of M.

The peripheral speed of good charging depends on the outer diameter of the charging sleeve 2b and the distance from the photosensitive drum 1, but the peripheral speed of the charging sleeve 2b in the present embodiment is 50 to 250 mm / sec. Is preferred.

A reservoir 2f for the magnetic particles M is formed on the upstream side of the regulating blade 2e in the rotating direction of the charging sleeve, and the stirring screw 2e is provided for the magnetic particles M in the reservoir 2f.
Is stirred along the generatrix direction of the charging sleeve 2b. The stirring screw 2e is configured by attaching elliptical blades (not shown) in alternate directions, and the magnetic particles M in the reservoir 2f are formed.
Can be stirred without biasing.

The magnetic particles M include the following (a):
Those of (b) and (c) are preferably used.

(A) Kneaded resin and magnetic powder such as magnetite into particles, or mixed with conductive carbon for adjusting resistance value, (b) Sintered magnetite, ferrite Or, those whose resistance value is adjusted by reducing or oxidizing them, (c) Coated with a resistance-adjusted coating material of the above magnetic particles (such as phenol resin dispersed with carbon), or a metal such as Ni Those that have been plated to have an appropriate resistance value, etc.

If the resistance value of these magnetic particles M is too high, electric charges cannot be uniformly injected into the photosensitive drum 1, resulting in a fog image due to a minute charging failure. If it is too low, when there are pinholes on the surface of the photosensitive drum 1, current concentrates on the pinholes, the charging voltage drops, and the surface of the photosensitive drum 1 cannot be charged, resulting in charging nip-shaped charging failure.

Therefore, the electric resistance value of the magnetic brush charging device 2 is preferably 1 × 10 ⁴ Ω to 1 × 10 ⁹ Ω,
Particularly, it is preferably 1 × 10 ⁴ Ω to 1 × 10 ⁷ Ω. The electric resistance value of the magnetic brush charging device 2 is 1 × 10 ⁴ Ω
If it is less than 1, pinhole leakage tends to occur, and if it exceeds 1 × 10 ⁹ Ω, it tends to be difficult to inject favorable charges. Further, in order to control the resistance value within the above range, the volume resistance value of the magnetic particles M is 1 × 10 ⁴ Ω · c.
It is preferably m to 1 × 10 ⁹ Ω · cm, and particularly preferably 1 × 10 ⁴ Ω · cm to 1 × 10 ⁷ Ω · cm.

The magnetic particles M used in the present embodiment have a volume average particle diameter of 30 μm, an apparent density of 2.0 g / cm ³ , a resistance value of 1 × 10 ⁶ Ω, and a saturation magnetization of 58 A · m ² / kg. The particle size of the magnetic particles M affects the charging ability and the uniformity of charging. That is, if the particle size is too large, the contact ratio with the photosensitive drum 1 decreases, which causes uneven charging. On the other hand, when the particle size is small, both the charging ability and the uniformity are improved, but the magnetic force acting on one particle is reduced, and the particles are likely to adhere to the photosensitive drum 1.

Therefore, the particle size of the magnetic particles M is 5
Those having a thickness of up to 100 μm are preferably used. Further, the total amount of the magnetic particles M used in the present embodiment is 200 g, and the magnetic particles M as a whole are gently stirred due to the stirring effect of the repulsive poles of the stirring screw 2d and the fixed magnet roller 2c. .

The auxiliary charging roller 7 is installed on the upstream side with respect to the rotation direction of the photosensitive drum 1 of the magnetic brush charging device 2. The auxiliary charging roller 7 is made by dispersing a carbon black having a thickness of 3 mm on a stainless steel core bar having a diameter of 6 mm, and
A PDM is formed, a coating layer is formed by a dipping method, and the coated layer is heated and dried at 150 ° C. for 30 minutes to form a roller member having an outer diameter of 12 mm, which has an elastic layer and a surface layer as a resistance control body.

The elastic layer of the auxiliary charging roller 7 is not limited to this, but urethane, SBR, EV may be used.
A, SBS, SEBS, SIS, TPO, EPM, NB
There are R, IR, BR, silicone rubber, epichlorohydrin rubber, etc., and depending on the required resistance value, for example, carbon black, carbon fibers, metal oxides, metal powders, solid electrolytes such as hydrogen peroxide, surfactants, etc. The conductivity-providing agent may be added.

The material of the resistance control body is, for example, polyamide, polyurethane, fluorine, polyvinyl alcohol, silicon, NBR, EPDM, CR, IR, B.
There are resins such as R and hydrin rubber, rubbers, etc., and conductive or insulating fillers and additives may be mixed therein. The material as described above is used, and the electric resistance value of the auxiliary charging roller 7 is finally 1 × 10 ³ to 1 ×.
The combination of the above materials is not particularly limited as long as it has a value of 10 ¹⁰ Ω. The electric resistance value of the auxiliary charging roller 7 used in the present embodiment is 1 × 10 ⁸ Ω.

Both ends of the core of the auxiliary charging roller 7 are urged toward the photosensitive drum 1 by urging members (not shown), and are pressed against the surface of the photosensitive drum 1 with a predetermined pressing force. Thus, a strip-shaped charging nip portion is formed between the photosensitive drum 1 and the photosensitive drum 1. Further, the auxiliary charging roller 7 does not have a drive mechanism and is driven to rotate in the arrow direction (counterclockwise direction) as the photosensitive drum 1 rotates.

A direct current voltage is applied to the core of the auxiliary charging roller 7 from the auxiliary charging bias applying power source (S3) 13. The auxiliary charging bias applying power source (S3) 13 can be controlled by the controller (CPU) 12, and the DC voltage Vs applied to the auxiliary charging roller 7 is -600 to -2 kV.
Can be controlled so as to be applied in the range.

The developing device 3 is provided with a rotatable developing sleeve 3b containing a fixed magnet roller 3c in the opening of the developing container 3a, and the developer (toner) T in the developing container 3a is regulated by a regulating blade 3d. Then, a thin layer is coated on the developing sleeve 3b, and the developing sleeve 3b is conveyed to the developing unit facing the photosensitive drum 1. The developing sleeve 3b is rotationally driven in the arrow direction (clockwise direction) at a peripheral speed of 150 mm / sec in this embodiment by the driving of a driving device (not shown).
The developer T in the developing container 3a is conveyed to the developing sleeve 3b side while being uniformly stirred by the rotation of the stirring members 3e and 3f.

In the present embodiment, the developer T in the developing container 3a is a two-component mixture of a toner having a negatively chargeable particle size of 8 μm and a magnetic carrier having a positively chargeable particle size of 50 μm at a weight toner concentration of 5%. It is a developer. The toner concentration is controlled based on detection information by an optical toner concentration sensor (not shown), and toner (not shown) in the toner hopper 3g is supplied to the developing container 3a by driving the supply roller 3h at a proper time. Adjust the concentration constant.

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

At the time of image formation, the photosensitive drum 1 is moved by a driving device (not shown) in the direction of arrow (clockwise) to 100 mm / mm.
It is rotationally driven at a peripheral speed of sec and is uniformly charged to a predetermined negative polarity potential by the injection charging by the magnetic brush charging device 2 described above. Then, laser light (emission wavelength 680n) corresponding to the image signal input from the exposure device (not shown)
The exposure L by m) is applied, and the potential on the photosensitive drum 1 is lowered to form an electrostatic latent image.

Then, the electrostatic latent image is reverse-developed by the negative-polarity developer (toner) coated on the developing sleeve 3b of the developing device 3 in a thin layer, and visualized as a toner image. At this time, in the developing sleeve 3b, in the present embodiment, the developing bias applying power source (S2) 14 applies an alternating electric field of a peak-to-peak voltage of 2 kVpp and a frequency of 2 kHz to -50.
A developing bias on which a DC voltage Vde of 0 V is superimposed is applied.

When the toner image on the photosensitive drum 1 reaches the transfer nip portion N, the transfer material P such as paper is conveyed to the transfer nip portion N at this timing. Then, by the transfer roller 4 to which a transfer bias having a polarity (positive polarity) opposite to that of the toner is applied, the transfer material P conveyed to the transfer nip portion N is exposed by the electrostatic force generated between the photosensitive drum 1 and the transfer roller 4. The toner image on the drum 1 is transferred.

Then, the transfer material P on which the toner image is transferred is conveyed to the fixing device 15, and the fixing roller 1 of the fixing device 15 is transferred.
At the fixing nip portion between 5a and the pressure roller 15b, the toner image is heated and pressed on the transfer material P to be thermally fixed, and then discharged to the outside.

On the other hand, the surface of the photosensitive drum 1 after the transfer is
After the charge removal light (center wavelength 660 nm) is emitted from the charge removal light lamp 5 to remove the charge, the transfer residual toner remaining on the surface of the photosensitive drum 1 is scraped off by the cleaning blade 6a of the cleaning device 6, and the conveyance screw 6b.
Is rotated to collect in a waste toner container (not shown).

The photosensitive drum 1 from which the transfer residual toner has been removed is previously charged to a predetermined negative potential by the auxiliary charging roller 7 to which a voltage (auxiliary charging bias) is applied from the auxiliary charging bias power source (S3) 13. After that, the next image forming process is performed again, and the magnetic brush charging device 2 charges the layers one by one.

At this time, in the present embodiment, the voltage application conditions for the auxiliary charging roller 7 are set as follows.

That is, in the present embodiment, the ammeter 10 is applied when the voltage (charging bias) is applied from the charging bias applying power source (S1) 11 to the charging sleeve 2b of the magnetic brush charging device 2.
The charging current is detected by, and the voltage application to the charging sleeve 2b is controlled by the controller (CPU) 12 so that the absolute value of the charging current is minimized.

The auxiliary charging roller 7 in this embodiment is a corona charging type of contact charging, and the charging potential linearly increases at the discharge threshold Vth as shown in FIG. In FIG. 2, the horizontal axis represents the voltage applied to the auxiliary charging roller and the vertical axis represents the charging potential.

Therefore, assuming that the charging potential of the auxiliary charging roller 7 is Vd1 and the voltage Vs applied to the auxiliary charging roller 7, the relationship of Vd1 = Vth + Vs is established. Here, two values are set so that Vd1 is Vs where V2 becomes a linear region and V2> V1. Then, when the DC voltage Vm is applied to the charging sleeve 2b of the magnetic brush charging device 2, the flowing charging current Im is proportional to the potential of the photosensitive drum immediately before the charging section.

Therefore, as shown in FIG. 3, the charging current Im
Can be linearly approximated to Vs in the linear region of the charging potential Vd1. In FIG. 3, the horizontal axis represents the voltage applied to the auxiliary charging roller, and the vertical axis represents the charging current flowing in the charging sleeve 2b. Then, by utilizing this characteristic, the voltage applied to the auxiliary charging roller 7 at which the absolute value of the charging current Im is minimized is set.

Specifically, the auxiliary charging bias applying power supply 13 is applied with the DC voltage Vm applied to the charging sleeve 2b of the magnetic brush charging device 2 from the charging bias applying power supply 11.
From the above, voltages V2 and V1 are applied to the auxiliary charging roller 7, and the charging currents I1 and I2 flowing through the charging sleeve 2b at that time are detected by the ammeter 10, respectively.

Next, as shown in FIG. 3, the charging current I1,
The slope of Im (first-order expression) passing through I2 is calculated, and V0 is calculated from the intersection of Im and charging current = 0. The value of V0 is set as the voltage Vs applied to the auxiliary charging roller 7 from the auxiliary charging bias power source (S3) 13.

Further, the discharge threshold value Vth of the auxiliary charging roller 7 may change depending on the change of the environment.
It is desirable that 1 and V2 are set to sufficiently large values that are not affected by them.

The voltage application control to the auxiliary charging roller 7 may be performed every time an image is formed, or may be performed every certain number of sheets.

In the image forming apparatus of the present embodiment for applying the voltage set as described above to the auxiliary charging roller 7,
An evaluation experiment was conducted on the relationship between the charging potential and the number of durable sheets (the number of output images). 4 shows the relationship between the charging potential and the number of durable sheets (the number of output images) when the control according to the present embodiment is performed, and b in FIG. 4 shows the control according to the present embodiment. 5 shows the relationship between the charging potential and the number of durable sheets (the number of image outputs) in the comparative example in the case of not performing. In the present embodiment shown in a of FIG. 4, the control is performed every 100 sheets. Further, the comparative example shown by b in FIG. 4 is the same as the configuration of the present embodiment except that -1 kV is constantly applied to the auxiliary charging roller.

As is clear from the evaluation results shown in FIG. 4, when the control according to the present embodiment is performed, there is almost no change in the charging potential even when the durable number (image output number) is increased. . On the other hand, in the case of the comparative example in which the control according to the present embodiment is not performed, the charging potential decreases due to the effect of resistance increase due to deterioration of energization of the auxiliary charging roller as the number of durable sheets (image output number) increases.

As described above, in the present embodiment, the ammeter 10 is used.
The controller 12 controls the current value flowing through the charging sleeve 2b of the magnetic brush charging device 2 based on the current value detected by the controller 12 to minimize the absolute value of the current, and sets the voltage application condition to the auxiliary charging roller 7. This makes it possible to obtain a stable charging potential for a long period of time.

Further, in the present embodiment, it is not necessary to install a potential sensor for measuring the surface charging potential of the photosensitive drum 1 for each charging device (magnetic brush charging device 2, auxiliary charging roller 7). There is no restriction on the arrangement of each member (the developing device 3, the cleaning device 6 and the like) around one.

In this embodiment, the magnetic brush charging device 2 is used as the main charging means and the auxiliary charging roller (roller charging member) 7 is used as the sub charging means. However, the structure is not limited to this. In addition to the above, the present invention can be similarly applied to the case where a conductive fur brush, a blade, or a sponge roller is used as the main charging unit and the sub charging unit.

Further, in the present embodiment, the voltage applied to the magnetic brush charging device 2 as the main charging means has a predetermined fixed value, but it can be changed according to the characteristic changes of the photosensitive drum 1 and the developing device 3. It may be allowed to.

In this embodiment, the organic photosensitive drum is used as the photosensitive drum 1. However, the present invention is not limited to this, and other than that, for example, a surface made of amorphous silicon is used. It is also possible to use a photosensitive drum having a layer or a photosensitive drum having a surface layer made of amorphous carbon.

[0073]

As described above, according to the present invention, the voltage applying condition from the second voltage applying means to the sub charging means is set based on the current value flowing through the main charging means detected by the current detecting means. By doing so, it becomes possible to obtain a stable charging potential for a long period of time, and it is possible to obtain a good image.

[Brief description of 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 diagram showing a relationship between a voltage applied to an auxiliary charging roller (voltage applied to the auxiliary charging roller) and a charging potential.

FIG. 3 is a diagram showing a relationship between a voltage applied to the auxiliary charging roller (voltage applied to the auxiliary charging roller) and a charging current flowing through a charging sleeve of the magnetic brush charging device.

FIG. 4 is a diagram showing the relationship between the charging potential and the number of durable sheets (the number of output images) when the control according to the present embodiment is performed.

[Explanation of symbols]

1 Photosensitive Drum (Image Carrier) 2 Magnetic Brush Charging Device (Main Charging Means) 2b Charging Sleeve (Contact Charging Member) 3 Developing Device 3b Developing Sleeve 4 Transfer Roller 5 Electrification Light Lamp (Electrification Exposure Means) 6 Cleaning Device 6a Cleaning Blade 7 Auxiliary Charging Roller (Sub-Charging Means) 10 Ammeter (Current Detecting Means) 11 Charging Bias Applying Power Supply (First Voltage Applying Means) 12 Control Device (Control Means) 13 Auxiliary Charging Bias Applying Power Supply (Second Voltage Applying Means) ) 14 developing bias applying power supply 15 fixing device 15a fixing roller 15b pressure roller

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) G03G 5/147 503 G03G 5/147 503 21/08 21/00 342 (72) Inventor Ryo Nakamura Ota, Tokyo 3-30-2 Maruko Shimoshita Canon Inc. F term (reference) 2H035 AA09 AA10 AB02 2H068 AA04 AA05 AA08 CA37 DA03 DA12 DA17 2H200 FA02 GA23 GA34 GA45 GA49 GB02 HA03 HA21 HA28 HA29 HA30 HB12 HB17 HB22 H45 HB43 HB41 HB43 HB41 HB43 HB48 LA12 LA14 LA23 LA40 LC09 MA03 MA14 MB01 MB06 MC11 MC13 MC15 NA06 NA10 PA03 PA22 PB05

Claims (11)

[Claims] 1. A movable image carrier, a main charging unit that charges the image carrier by applying a voltage, a first voltage applying unit that applies a voltage to the main charging unit, and the image carrier. At least one upstream of the main charging means with respect to the moving direction of the main charging means, and a sub-charging means for charging the image carrier by applying a voltage having the same polarity as the voltage applied to the main charging means, An image forming apparatus comprising: a second voltage applying unit that applies a voltage to the sub-charging unit; and a current detection that detects a current flowing when a voltage is applied from the first voltage applying unit to the main charging unit. An image forming apparatus comprising: means for setting a voltage application condition from the second voltage applying means to the sub-charging means based on a current value detected by the current detecting means. 2. A control means for controlling the voltage application from the second voltage application means to the sub-charging means, wherein the control means is based on the current value detected by the current detection means. The image forming apparatus according to claim 1, wherein the voltage application condition to the sub-charging unit is set by controlling the absolute value of the current flowing through the unit to be minimum. 3. The main charging means interposes conductive fine particles attached to a contact charging member, to which a voltage is applied from the first voltage applying means, between the main charging means and the image carrier, and injects the charge. The image forming apparatus according to claim 1 or 2, wherein the image carrier is charged. 4. The conductive fine particles are made of conductive magnetic particles magnetically restrained by the contact charging member, and the conductive fine particles are brought into contact with the image carrier to perform charge injection. The image forming apparatus according to claim 3, wherein: 5. The sub-charging means brings a conductive roller member into contact with the image carrier and applies a voltage from the second voltage applying means to the roller member to charge the image carrier. The image forming apparatus according to claim 1 or 2, wherein: 6. A static elimination exposure unit is provided upstream of the sub-charging unit with respect to the moving direction of the image carrier to expose the image carrier to remove residual charges on the surface of the image carrier. The image forming apparatus according to any one of claims 1 to 5, wherein: 7. The image forming apparatus according to claim 1, wherein the image carrier has a photosensitive layer and a surface layer, and the surface layer has a resin and conductive fine particles. 8. The image forming apparatus according to claim 7, wherein the conductive fine particles are SnO ₂ . 9. The surface layer has a resistance of 10 ⁹ to 10 ¹⁴ Ω.
The image forming apparatus according to claim 7, wherein the image forming apparatus is m. 10. The image forming apparatus according to claim 1, wherein the image carrier has a surface layer made of amorphous silicon. 11. The image forming apparatus according to claim 1, wherein the image carrier has a surface layer made of amorphous carbon.