JP2003307914A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To satisfactorily eject residual toner at the time of transfer from a contact electrifying member. <P>SOLUTION: When an electrifying bias in which an AC voltage overlaps a DC voltage is applied to an electrifying roller 2 from an electrifying bias power source 12, the AC voltage (AC component) of the electrifying bias is applied so that an electric field is intense in the direction of the surface electrification potential of a photosensitive drum 1 and weak in the opposite direction to it. Thus, toner (transfer residual toner) can be satisfactorily ejected from the electrifying roller 2 to the photosensitive drum 1. Additionally, fogging caused by the toner removed from the electrifying roller 2 can also be prevented. <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 performs an image forming apparatus by an electrophotographic method or the like.

[0002]

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

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 which charges the surface of the photoconductor 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.

[0008] This direct injection charging system does not cause the generation of active ions and thus does not cause any adverse effect due to discharge products. However, since it is direct injection charging, the contact property of the contact charging member to the photoconductor is large. It works. Therefore, in order to contact the photoconductor at a higher frequency, the contact charging member has a denser contact point, or the speed difference between the contact charging member and the photoconductor is increased to provide more contact points at the contact point. Will be needed.

In this respect, the roller charging method using a conductive roller (charging roller) as the contact charging member is preferable in terms of charging stability and is widely used. The charging mechanism in conventional roller charging is dominated by the discharge charging system (a).

The charging roller as a conductive roller is made of a conductive or medium-resistance rubber material or foam. Further, there is also one in which these are laminated to obtain desired characteristics.

The charging roller (conductive roller) has elasticity in order to obtain a constant contact state with the photoconductor and has a large friction resistance, so in many cases, it is driven by the photoconductor or has a slight speed difference. Driven by. Then, since there are few contact opportunities, even if it is attempted to directly inject and charge, the absolute charging ability is deteriorated, contact is insufficient, contact unevenness due to the roller shape,
Uneven charging due to the adherents on the photoconductor is unavoidable.

FIG. 10 is a diagram showing an example of the charging characteristics by the contact charging (the relationship between the DC applied voltage and the charging potential of the photoconductor). In FIG. 10, the horizontal axis represents the bias (DC applied voltage) applied to the charging roller (contact charging member), and the vertical axis represents the charging potential of the photoconductor obtained at that time.

In FIG. 10, A is the charging characteristic in the case of charging by discharge. That is, charging starts after the discharge threshold of -500V is exceeded. Therefore, when charging to -500V, a DC voltage of -1000V is applied, or in addition to a charging voltage of -500V DC, an AC voltage of 1200V between peaks is applied so as to always have a potential difference of a discharge threshold value or more. Then, the method of converging the photoconductor potential to the charging potential is general.

More specifically, when the charging roller is brought into pressure contact with the photosensitive member, the surface potential of the photosensitive member begins to rise when a voltage above a certain level is applied, and thereafter. The surface potential of the photoconductor increases linearly with the applied voltage. This threshold voltage is defined as the charging start voltage Vth.

That is, in order to obtain the surface potential Vd of the photosensitive member required for the electrophotographic image forming process, the charging roller needs a direct current voltage of Vd + Vth, which is higher than necessary. In this way, the method of applying only the DC voltage to the charging roller to perform the charging is called "DC charging method".

By the way, in this DC charging system,
Since the resistance value of the charging roller fluctuates due to environmental fluctuations and Vth (charging start voltage) fluctuates when the film thickness changes due to scraping of the surface of the photoconductor, the surface potential of the photoconductor is set to a desired value. It was difficult.

Therefore, in order to make the charging even more uniform, for example, as disclosed in Japanese Patent Laid-Open No. 63-149669, 2 × V is applied to a DC voltage corresponding to a desired Vd.
An “AC charging method” is used in which a voltage obtained by superimposing an AC voltage having a peak-to-peak voltage equal to or higher than th is applied to a charging roller (contact charging member). This is for the purpose of leveling effect of potential by alternating current, and the potential of the photoconductor converges on Vd which is the center of the peak of the alternating voltage, and is not affected by disturbance such as environment.

However, even in such an AC charging type contact charging device, since the essential charging mechanism uses the discharge phenomenon from the charging roller to the photosensitive member, it is applied to the charging roller as described above. Since the voltage is required to have a value higher than the surface potential of the photoconductor, a slight amount of ozone is generated.

When AC charging is performed to make the charging uniform, further ozone is generated, vibration noise (AC charging sound) between the charging roller and the photoconductor due to the electric field of AC voltage, and discharge are generated. Deterioration of the surface of the photoconductor has become remarkable, which has been a new problem.

Further, in order to prevent uneven charging and perform stable and uniform charging, a structure in which powder is applied to the contact surface of the charging roller (contact charging member) with the surface of the photosensitive member is disclosed in Japanese Patent Publication No. 7-9.
It is disclosed in Japanese Patent No. 9442. With such a configuration, the charging roller is driven by the photosensitive member (no speed difference), and the generation of ozone products is significantly less than that of a corona charger such as a scorotron, but the charging principle is as described above. As in the case of roller charging, the discharge charging mechanism is still mainly used. In particular, in order to obtain more stable charging uniformity, a voltage in which an AC voltage is superimposed on a DC voltage is applied, so that ozone products are more likely to be generated by the discharge.

Therefore, when used for a long period of time, adverse effects such as image deletion due to ozone products are likely to occur. Furthermore, when applied to a cleanerless image forming apparatus that does not have a cleaning member that mechanically cleans the transfer residual toner, the applied powder adheres evenly to the charging roller due to the mixing of the transfer residual toner. However, the effect of uniform charging is diminished.

Further, in Japanese Patent Application Laid-Open No. 5-150539, in an image forming apparatus using a contact charging system, toner particles and silica fine particles which cannot be completely cleaned by a cleaning blade during repeated image formation for a long time are charged by a charging roller. In order to prevent charging inhibition due to adhesion and accumulation on the surface of the toner, it is disclosed that the toner contains at least developer particles and conductive particles having an average particle size smaller than the developer particles.

However, the contact charging or the proximity charging used here is based on the above-mentioned discharge charging system and not the direct injection charging system, but has the above-mentioned problems due to discharge charging. Further, when applied to a cleanerless image forming apparatus, a large amount of conductive fine powder and transfer residual toner pass through the charging nip portion (charging area) as compared with a case where a cleaning mechanism is provided, and thus the charging property is improved. No consideration is given to the influence on the development characteristics of the toner due to the influence of the large amount of the conductive fine powder and the transfer residual toner on the developing process, the recovered conductive fine powder and the transfer residual toner.

Further, when the direct injection charging system is applied to the contact charging system, the conductive fine powder is not supplied to the charging roller (contact charging member) in the required amount, and charging failure occurs due to the influence of the transfer residual toner. I will end up.

In the proximity charging, it is difficult to uniformly charge the photoconductor with a large amount of conductive fine powder and transfer residual toner, and the effect of leveling the transfer residual toner pattern cannot be obtained. A pattern ghost is generated to shield the pattern image exposure. Further, when the power is interrupted (OFF) or a jam (paper jam) occurs during image formation, the contamination of the inside of the image forming apparatus by the toner becomes significant.

On the other hand, as disclosed in JP-A-10-307454 and JP-A-10-307457, a speed difference is provided between the charging roller (contact charging member) and the photosensitive member, and the conductive fine powder is used. A configuration has been proposed in which direct injection charging is performed by means of direct injection.

As a specific method for obtaining the speed difference between the charging roller and the photosensitive member, these publications disclose a method of moving the charging roller surface to provide a speed difference between the charging roller and the photosensitive member. It is described that it is preferable that the charging roller is rotationally driven, and further that the rotation direction thereof is rotated in the direction opposite to the moving direction of the surface of the photoconductor.

That is, since the charging property of the direct injection charging system depends on the ratio (peripheral speed ratio) of the peripheral speed of the photoconductor to the peripheral speed of the charging roller, it is necessary to obtain the same peripheral speed difference in the opposite direction. This is because the rotation speed of the charging roller in the direction is larger than that in the opposite direction. Therefore, conventionally, a system in which the charging roller is moved in the opposite direction has been used. The above peripheral speed ratio is the peripheral speed ratio (%) = ((charging roller peripheral speed−photoconductor peripheral speed) / photoconductor peripheral speed) × 100 (the charging member peripheral speed is the same as the peripheral speed of the photosensitive member). It is a positive value when the charging roller surface moves in the same direction as the photoconductor surface at the contact portion (charging nip portion)).

On the other hand, the above conductive fine powder is used as charge promoting particles for the purpose of assisting charging. And
Direct injection charging of the photoconductor is performed in a state where the charge promoting particles are present in the contact portion (charging nip portion) between the photoconductor and the charging roller.

Due to the presence of the charge accelerating particles, at the contact portion (charging nip portion) between the photosensitive member and the charging roller, the charging roller can be brought into contact with the photosensitive member at a speed difference with a relatively small frictional resistance. Intimate contact with the photoreceptor via the charge-accelerating particles.

That is, the charge accelerating particles existing in the contact portion between the charging roller and the photosensitive member rub the surface of the photosensitive member with no gap, so that the charge can be directly injected into the photosensitive member. That is,
The direct injection charging is dominant in the charging of the photosensitive member by the charging roller due to the presence of the charging promoting particles.

Therefore, in the case of direct injection charging, FIG.
As in the charging characteristic of B shown in (1), a high charging efficiency which cannot be obtained by the conventional roller charging or the like is obtained, and a potential substantially equal to the voltage applied to the charging roller (contact charging member) is applied to the photoconductor. You can

By the way, in the conventional electrophotographic image forming process, the step of charging the photosensitive member, the step of exposing the photosensitive member, the step of developing the electrostatic latent image, the step of transferring the toner image to the transfer material (paper), and the step after transfer are performed. It is common to repeat a cycle of cleaning the transfer residual toner and eliminating the residual charge on the surface of the photoconductor.

In this process, the transfer residual toner remaining on the surface of the photoconductor after the transfer is removed from the surface of the photoconductor by a cleaner (cleaning device) to become waste toner, and this waste toner also appears from the viewpoint of environmental protection. Not desirable. Therefore, in recent years, a toner recycling process has been adopted in which the cleaner is eliminated, and the transfer residual toner on the photoconductor after transfer is removed from the photoconductor by "development simultaneous cleaning" by the developing device and collected and reused by the developing device. An image forming apparatus has been proposed and put into practical use.

The above-mentioned simultaneous development cleaning means that the toner remaining on the photoconductor after transfer is developed in the subsequent steps,
That is, the photoconductor is continuously charged and exposed to form an electrostatic latent image, and a defoaming bias (a potential difference between the DC voltage applied to the developing device and the surface potential of the photoconductor (hereinafter, According to this method, the transfer residual toner is collected by the developing device and reused after the next step, so that waste toner is eliminated and maintenance is required. In addition, since the cleaner is not used, the space advantage is great, and the image forming apparatus can be significantly downsized.

With reference to FIG. 11, the above-mentioned simultaneous developing cleaning (toner recycling process) will be briefly described. In the image forming apparatus shown in FIG. 11, direct injection charging via conductive fine powder is used between the charging roller 101 and the photoconductor 100 as the charging step. As described above, this charging method is excellent in that it is ozoneless or that high charging efficiency can be obtained. The charging roller (contact charging member) 101 is in contact with the photoconductor 100 (charging nip).
In the above, for the reason described above, the photoconductor 100 is rotated so as to move in the opposite direction.

In the image forming apparatus shown in FIG. 11, the photoreceptor 100 is uniformly charged by applying a voltage by the charging roller (contact charging member) 101. In this case, it becomes negatively charged. Then, the exposure device 102 applies the exposure L of the reversal development method corresponding to the image information to the uniformly negatively charged photoreceptor 100 to form an electrostatic latent image.

Then, the toner t charged with electricity from the developing device 103 is supplied to the surface of the photoconductor 100 in a form corresponding to the electrostatic latent image and visualized as a toner image. In this case, the toner is negatively charged, and a non-contact jumping developing method is used as the developing method. Then, the transfer roller (transfer member) 104 and the transfer material (paper) P conveyed to the transfer nip portion between the photoconductor 100 and the transfer roller 104 to which a transfer bias (a polarity opposite to the toner t) is applied. The image is transferred, fixed by a fixing device (not shown), and discharged to the outside.

At the time of the transfer, a part of the toner remains on the surface of the photosensitive member 100 as a transfer residual toner, and a part of the toner remains in the original polarity by the transfer roller 104 to which a transfer bias having a reverse polarity to the toner t is applied. And reverse toner t1 having the opposite polarity.

The transfer residual toner remaining on the surface of the photosensitive member 100 is re-applied to the same polarity (negative polarity) as the charging polarity including the reversal toner t1 by rubbing between the charging roller 101 and the photosensitive member 100, It is again discharged onto the photoconductor 100. Aligning the reverse toner t1 with the original polarity,
Hereinafter, this is referred to as "toner normalization". The toner t to which the electric charge having the same polarity as the charging polarity is reapplied is developed by the developing device 103 due to the above-described fog removing potential difference Vback of the developing device 103.
It is collected in 3 and reused.

Further, in the image forming apparatus shown in FIG. 11, in order to prevent the charging roller 101 from being contaminated by the toner after being used for a long period of time, an AC component is superposed on the charging roller 101 to transfer residual toner from the charging roller 101. A method of increasing the discharge efficiency is known. Since the injection charging property depends on the contact area of the charging promoting particles with respect to the photoconductor 100, if the ratio of the charging promoting particles on the surface of the charging roller 101 decreases due to the presence of the transfer residual toner, the charging property deteriorates. Good chargeability cannot be obtained.

Further, by superimposing an AC component on the charging roller 101, there is an advantage that a potential leveling effect is obtained and charging uniformity is improved. Examples of commonly used AC voltage waveforms include a rectangular wave (duty 50%), a sine wave, and a triangular wave. In the case of the image forming apparatus shown in FIG. 11, as shown in FIG. 12, a peak-to-peak voltage in a region where discharge is not performed and a duty is 50%: 300 Vpp.
Is used. The frequency was set to 1 KHz. In addition,
In FIG. 12, a is the surface charging potential of the photoconductor 100, and b
Indicates the duty ratio (50%) of the AC voltage (rectangular wave).

Since the voltage rises faster in the rectangular wave than in the sine wave, the triangular wave, etc., the potential difference between the surface potential of the photosensitive member 100 and the potential of the charging roller 101 increases momentarily, and the toner is discharged. Higher efficiency. Further, since the effective current amount also increases, it is also advantageous for minute potential unevenness (spotting unevenness).

The relationship between the peak-to-peak voltage Vpp of the AC voltage of the charging bias and the discharge amount of the transfer residual toner is shown in FIG.
As shown in 3. FIG. 13 shows the increase with respect to the discharge amount when only the DC voltage is applied as the charging bias, and the peak-to-peak voltage of the AC voltage is 100V.
It was confirmed that the discharge amount increased at pp or higher.

Basically, if the frequency of the AC voltage is the same, the discharge property is improved by increasing the peak-to-peak voltage, but if it is increased too much, discharge occurs or the level of the AC charging sound deteriorates. Not preferable. Further, if the frequency is too low, the surface potential of the photoconductor 100 may have uneven charging depending on the frequency, which may adversely affect the image. On the other hand, if the frequency is set too high, the toner will not be able to respond to the high frequency electric field, and the toner ejection property will deteriorate.

[0046]

By the way, in the conventional image forming apparatus shown in FIG. 11, a solid black toner image is outputted, and after the solid black toner image is outputted, an image having a normal image ratio is outputted. At that time, a fog phenomenon occurred in the white background. This fogging phenomenon is not so-called "development fog on development" but is because the toner (transfer residual toner) discharged from the charging roller (contact charging member) 101 cannot be collected by the developing device 103 by the above-described simultaneous cleaning of development. Alternatively, due to insufficient collection, it may pass through the developing area to reach the transfer nip portion between the photoconductor 100 and the transfer roller 104, and may be transferred to what is originally a white background portion of the transfer material. is there.

At this time, when the charge distribution of the toner discharged from the charging roller 101 was measured, it was found that the distribution was symmetrical with positive and negative signs with a peak at almost zero. That is, since there are many toner particles discharged from the charging roller 101 that are not normalized, they reach the transfer nip portion as they are without being electrostatically collected due to the fog removal potential difference (Vback). It has been found. The toner transferred at this time is not transferred electrostatically but is transferred by the pressing force of the transfer roller 104 and the photoconductor 100.

Originally, the transfer residual toner is the charging roller 101.
Should be rubbed at the contact portion of the photoconductor 100 and normalized by the triboelectric charge. However, when the AC voltage is superimposed on the charging bias, the amount of toner discharged from the charging roller 101 increases, but a large amount of unnormalized toner (positive toner in the image forming apparatus of FIG. 11) is also discharged. It was confirmed that it would end up.

Further, when the route of the toner (transfer residual toner) passing through the charging roller 101 was examined, it was found that it passed as shown in FIG. That is, most of the toner (transfer residual toner) t1 moves while adhering to the surface of the charging roller 101, and
After reaching the downstream side of the contact portion (charging nip portion) with the photoconductor 100 with respect to the rotation direction of 00 (clockwise direction), the toner is discharged again to the photoconductor 100 side.

Further, the charging roller 101 in this case
The relationship between the bias applied to the photosensitive drum 100 and the surface potential of the photoconductor 100 is as shown in FIG. From these facts, the unnormalized toner is ejected by superimposing an AC voltage on the charging application bias, as shown in FIG. 12, because of the relationship between the surface charging potential of the photoconductor after the injection charging and the charging application bias. It is considered that this is because there is a case where there is a potential difference in which the positive toner is also discharged.

However, as described above, by superimposing the AC voltage on the charging application bias, the charging roller (contact charging member) has a long life (prevention of contamination by toner), and it has a great effect on defective charging such as uneven scraping. From that,
AC superposition is considered essential if high durability is required. Further, if the toner is repeatedly rubbed between the charging roller (contact charging member) and the photoconductor until the toner is normalized, the toner deterioration is promoted, and the re-development property is remarkably deteriorated when the transfer residual toner is reused. Will end up. From this point of view, it is necessary to have a structure in which the transfer residual toner from the charging roller is highly discharged.

Further, although the conventional image forming apparatus shown in FIG. 11 is the developing device 103 which uses the non-contact jumping developing method as the developing method, it is coated on the developing sleeve when the contact developing is used. Since the discharged toner is mechanically scraped by the toner, even if the discharged toner has a charge amount of zero to the opposite polarity, the discharged toner is collected by the developing unit. However, unlike the image forming apparatus shown in FIG. When using jumping development, there is only a method of collecting electrostatically. That is, it is necessary to discharge only the normalized toner from the charging roller (contact charging member).

Therefore, in the present invention, in the image forming apparatus using the direct injection charging through the conductive particles and the simultaneous cleaning of development, the contact charging member is prevented from being contaminated by the transfer residual toner and is discharged from the contact charging member for a long time. An object of the present invention is to provide an image forming apparatus capable of preventing the occurrence of fogging due to the toner that is generated.

[0054]

In order to achieve the above-mentioned object, the present invention provides a movable image carrier and a rotatable contact charging member which holds conductive fine powder having conductivity, Means for charging the image bearing member by applying a charging bias in which the contact charging member is brought into contact with the image bearing member in the presence of an electrically conductive fine powder and the AC component is superimposed on the DC component, and the charged image. Exposure means for exposing the carrier to form an electrostatic latent image, developing means for developing the electrostatic latent image with toner to visualize it as a toner image, and the toner image on a transfer material at a transfer portion. Transfer means for transferring, and the developing means causes the contact charging member to electrostatically collect the residual toner remaining on the image carrier after the toner image is transferred onto the transfer material by the transfer means. After being discharged onto the image carrier In a cleanerless image forming apparatus that also serves as a cleaning unit that electrostatically collects toner, the duty ratio of the AC component of the charging bias is on the same polarity side as the charging polarity of the image carrier with respect to the DC component. The peak-to-peak voltage value is larger than the peak-to-peak voltage value on the opposite polarity side.

The AC component of the charging bias has a peak-to-peak voltage value on the same polarity side as the charging polarity of the image carrier with respect to the DC component, and the surface charging potential of the image carrier by the contact charging member. Is set to 100 V or more in absolute value, and the peak-to-peak voltage value on the opposite polarity side is set to 50 V or more in absolute value with respect to the surface charging potential of the image carrier by the contact charging member. It is characterized by doing.

Further, the frequency f (Hz) of the AC component of the charging bias is the moving speed of the image carrier Vd.
(Mm / sec), 1 <(f / Vd) <(50
00 / Vd) is set.

Further, it is characterized in that the contact charging member is rotated so as to move in a direction opposite to the moving direction of the image carrier at the contact portion with the image carrier.

Further, the contact charging member is a roller member having an Asker C hardness of 50 degrees or less.

Further, the volume resistance of the roller member is 10 ³
It is characterized in that it is Ω · cm or more and 10 ⁸ Ω · cm or less.

The volume resistance of the conductive fine powder is 1 ×.
It is characterized in that it is 10 ⁹ Ω · cm or less.

Further, the conductive fine powder is contained as an external additive of the toner, and adheres to the image carrier at the time of development, and at least the contact charging member and the above-mentioned members are moved as the image carrier moves. It is characterized in that it is supplied to the contact portion with the image carrier.

Further, the developing method by the developing means is a non-contact jumping developing method in which the toner is transferred to the electrostatic latent image on the image carrier by the application of a developing bias to develop.

A frictional charging member is brought into contact with the contact charging member.

The triboelectrification member is made of a material which is controlled by triboelectrification so that the toner charged to the opposite polarity to the regular charge polarity has the regular charge polarity.

[0065]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will now be described based on the illustrated embodiments.

<First Embodiment> FIG. 1 is a schematic configuration diagram showing an image forming apparatus according to a first embodiment of the present invention. The image forming apparatus according to the present embodiment is an image forming apparatus such as a copying machine in an electrophotographic image forming process using the simultaneous developing and cleaning system by the direct injection charging via the conductive particles (charge promoting particles) described above.

This image forming apparatus is provided with a drum type electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum) 1 as an image carrier, and the photosensitive drum 1 is a driving device (not shown).
Is driven to rotate in the arrow direction (clockwise direction). Around the photosensitive drum 1, a charging device 20 having a charging roller 2 as a contact charging member in order along the rotation direction thereof,
Exposure device 3, developing device 4, transfer roller 5, separation charger 6
Is provided. Further, a fixing device 7 is disposed on the downstream side of the separation charging device 6 with respect to the transfer direction (arrow direction) of the transfer material (paper) P.

At the time of image formation, the surface of the photosensitive drum 1 is uniformly charged by the charging roller 2 to have a negative polarity in this embodiment. Then, the scanning exposure L is performed by the laser light emitted from the exposure device 3, the electric charges in the laser light irradiation portion are removed, and the electrostatic latent image according to the image information is formed. The electrostatic latent image on the photosensitive drum 1 is developed by the toner (charged to have a negative polarity) of the developing device 4 in the developing unit, and visualized as a toner image.

The developed toner image on the photosensitive drum 1 is
A transfer roller 5 in which a transfer bias (having a polarity opposite to that of toner) is applied to a transfer material P such as a sheet that is conveyed to a transfer nip portion N between the photosensitive drum 1 and the transfer roller 5 at a predetermined timing.
Transcribed by The transfer material P after the toner image transfer is separated from the surface of the photosensitive drum 1 by the separation charging device 6 and is conveyed to the fixing device 7. Transfer material P conveyed to fixing device 7
Is heated and pressed in the fixing nip portion between the fixing roller 7a and the pressure roller 7b of the fixing device 7, the toner image is fixed on the surface of the transfer material P, and then discharged to the outside.

Further, this image forming apparatus is a simultaneous developing and cleaning system which does not have a dedicated cleaning member for removing the transfer residual toner remaining on the surface of the photosensitive drum 1 after the toner image is transferred onto the transfer material P. is there.

That is, after the toner image is transferred onto the transfer material P, the photosensitive drum 1 reaches the contact portion (charging nip portion) A with the charging roller 2 with the transfer residual toner adhered to the surface thereof, (2) The toner charge whose charge polarity is reversed by frictional charging with the charge promoting particles M carried on the surface is discharged to the photosensitive drum 1 after being aligned to the normal polarity (negative polarity). The discharged toner is
Due to the above-described fog-removal potential difference Vback of the developing device 4, the developing device 4 recovers and reuses.

Hereinafter, the above-mentioned members (photosensitive drum 1, charging device (charging roller 2) 20, exposure device 3, developing device 4,
Details of the transfer roller 5 and the fixing device 7) will be described.

(Photosensitive Drum 1) The photosensitive drum 1 according to the first embodiment of the present invention is charged by the direct injection charging method. As the photosensitive drum 1 of the direct injection charging type, a commonly used organic photoconductor or the like can be used, but desirably, the resistance is 10 ⁹ to 10 ¹⁴ on the organic photoconductor.
By using a material having a surface layer having a material of Ω · cm, an amorphous silicon-based photoconductor, or the like, charge injection charging can be realized, which is effective in preventing ozone generation and reducing power consumption. In addition, it becomes possible to improve the charging property.

In this embodiment, as the photosensitive drum 1,
Using a negative-polarity a-Si photosensitive drum having a diameter of 30 mm, it is rotationally driven in the arrow direction (clockwise direction) at a process speed (peripheral speed) of 210 mm / sec. This photosensitive drum 1 is made of negative-chargeable amorphous silicon (a-S).
It has a photosensitive layer composed of i), and further has an amorphous carbon (a-C) protective film having a higher hardness than a-Si on its outermost surface layer.

The optimum thickness of the aC film can be determined from the relationship between the amount of wear of the surface layer and the life of the image forming apparatus, but is generally 0.01 μm to 10 μm, preferably 0 μm. ．
The range of 1 μm to 1 μm is desirable. The thickness of the surface layer is 0.
If it is less than 01 μm, the mechanical strength may be impaired, and if it is more than 10 μm, the residual potential may increase. In the present embodiment, in consideration of optimum conditions such as translucency, a- of 0.2 μm is used.
The C surface layer was used.

(Exposure Device 3) The exposure device 3 is a laser beam scanner (exposure device) including a laser diode (semiconductor laser), a polygon mirror, etc. in the present embodiment, and a time-series electric digital pixel of target image information. Laser light whose intensity is modulated according to the signal is output, and the uniformly charged surface of the surface of the photosensitive drum 1 is subjected to scanning exposure L with this laser light. By this scanning exposure L, an electrostatic latent image corresponding to target image information is formed on the surface of the photosensitive drum 1.

The light source used in the exposure apparatus 3 may be an LED array other than the above laser diode. In this case, the LE at the position corresponding to the target image information is used.
D is turned on and an electrostatic latent image is formed on the surface of the photosensitive drum 1.
Further, in the image forming apparatus (copying machine) of the present embodiment, the exposure device is only the light source used for forming the electrostatic latent image, and the neutralization light is not provided.

In this embodiment, the surface of the photosensitive drum 1 is uniformly charged to -400V by the charging roller 2. Then, with the laser light of the wavelength of 680 nm emitted from the laser diode (semiconductor laser) of the exposure apparatus 3, 600
Scanning exposure is performed at dpi (dot / inch), the electric charge in the laser light irradiation portion is removed, and an electrostatic latent image is formed.

(Developing Device 4) The developing device 4 includes a developing roller 8 on the surface of which the toner t is applied in a thin layer, and a developing device 4.
It has stirring members 10a and 10b for stirring the toner t therein and moving it toward the developing roller 8, and a regulating blade 9 for regulating the layer thickness of the toner t applied on the surface of the developing roller 8.

The gap between the developing roller 8 and the regulating blade 9 is set to 200 μm. The surface of the developing roller 8 is coated with a phenol resin as a binder and a resin containing carbon and graphite as a pigment, and the composition weight ratio (binder: pigment) is 2.5: 1.0. is there.

In the developing device 4 of this embodiment, the toner t
Is applied to the surface of the developing roller 8 and the toner t whose layer thickness is regulated by the regulating blade 9 to a layer thickness of about 1.0 mg / cm ² while being triboelectrically charged with the coating resin on the surface of the developing roller 8 is The toner is carried on the surface and is conveyed to a developing position (developing section) facing the surface of the photosensitive drum 1 as the arrow rotates (counterclockwise).

The developing device 4 of the present embodiment has a gap (hereinafter referred to as SD gap) between the developing roller 8 and the photosensitive drum 1 at the developing position, and is a non-contact developing type jumping developing system. And This SD gap is formed by a photosensitive drum abutting roller (not shown) rotatably supported by a rotation shaft (not shown) of the developing roller 8.
It is set to 180 to 300 μm.

Further, as the toner t in the present embodiment, a magnetic one-component insulating toner (negative toner) having a volume average particle diameter of 7 μm and externally added with hydrophobized silica and charge promoting particles is used.

At the time of development by the developing device 4 of the present embodiment, the absolute value of the alternating bias voltage of the developing bias applied to the developing roller 8 is 500 V or more, preferably considering the leak to the photosensitive drum 1, 500-1,500
V is good. However, this leak varies depending on the setting of the gap between the developing roller 8 and the photosensitive drum 1.

The frequency of the alternating bias is
It is preferably in the range of 1,000 to 5,000 Hz. That is, when the frequency of the alternating bias is less than 1,000 Hz, the gradation of the obtained image is improved, but it is difficult to eliminate the background fog. In the low frequency region where the number of reciprocating movements of the toner is small, the pressing force of the toner against the photosensitive drum 1 due to the developing bias electric field becomes too strong even in the non-image portion, so the toner peeling force due to the reverse developing side bias electric field. It is considered that the toner adhered to the non-image portion cannot be completely removed even by the above.

On the other hand, the frequency of the alternating bias is 5,000.
When the frequency exceeds Hz, the bias electric field on the reverse developing side is applied before the toner sufficiently contacts the photosensitive drum 1, so that the developability is deteriorated. That is, the toner itself cannot respond to the high frequency electric field. According to the study by the present inventor, it has been confirmed that optimum image quality is exhibited in the present embodiment when the frequency of the alternating bias is set to 1,500 to 3,000 Hz.

In the present embodiment, considering the above reason,
2000Hz, peak-to-peak voltage 800Vpp, Duty5
A developing bias in which a DC voltage of −280V is superimposed on an AC voltage of 0% is applied to the developing roller 8. This allows
The electrostatic latent image on the surface of the photosensitive drum 1 is subjected to reversal development, toner is attached and visualized as a toner image. Note that, in order to suppress toner scattering, no AC bias is applied when the photosensitive drum 1 and the developing roller 8 are driven to rotate during non-image formation such as paper interval and pre-rotation.

(Transfer Roller 5) The transfer roller 5 is a roller member having a medium resistance, and forms a transfer nip portion N by being pressed against the photosensitive drum 1. A transfer material P such as a sheet is transferred to the transfer nip portion N from a paper feeding portion (not shown) at a predetermined timing.
By feeding a sheet of paper and applying a predetermined transfer bias voltage to the transfer roller 5, the toner image on the surface of the photosensitive drum 1 is transferred to the surface of the transfer material P by electrostatic force and pressing force.

(Fixing Device 7) The fixing device 7 has a fixing roller 7a containing a heater (not shown) and a pressure roller 7b. The transfer material P conveyed to the fixing device 7 is fixed to the fixing roller 7a. The toner image is fixed on the surface of the transfer material P by being heated and pressed in the fixing nip portion between the pressure roller 7b and the pressure roller 7b.

(Charging Device 20) The charging device 20 is equipped with a charging roller 2 and a charging bias power source 12, and the charging roller 2 has a flexibility arranged in contact with the photosensitive drum 1 with a predetermined pressing force. This is a conductive elastic roller as a contact charging member.

The charging roller 2 has a contact portion (charging nip portion) A.
Is pressed against the photosensitive drum 1 to form a predetermined contact width. The charging roller 2 is preliminarily coated with and carries the charge promoting particles M on its outer peripheral surface. The charge promoting particles M are also present in the contact portion A. Further, the charging roller 2 is configured to rotate in the opposite direction to the photosensitive drum 1 at the contact portion A, and the charging roller 2 contacts the photosensitive drum 1 with a predetermined speed difference.

By applying a predetermined charging bias from the charging bias power source 12 to the charging roller 2, the outer peripheral surface of the photosensitive drum 1 is uniformly contact-charged to a predetermined polarity and potential by the injection charging method. It In the present embodiment, the charging roller 2 has an outer peripheral surface of the photosensitive drum 1 of approximately −40.
A charging bias is applied from the charging bias power source 12 so that the charging process is uniformly performed at 0V.

In the image forming apparatus according to the present embodiment, the transfer residual toner of which the polarity is reversed at the contact portion A between the charging roller 2 and the photosensitive drum 1 has its toner charge regularly aligned by frictional charging, so that the photosensitive drum 1 can be obtained. Exhaled again.

The contact width of the contact portion (charging nip portion) A between the charging roller 2 and the photosensitive drum 1 is determined by the roller hardness and roller diameter of the charging roller 2 and the pressing force applied to the photosensitive drum 1. The larger the contact portion width, the larger the area in contact with the charging promoting particles M, and the better the charging property. Further, the charging uniformity depends on the peripheral speed ratio of the charging roller 2 to the photosensitive drum 1, and the larger the peripheral speed ratio, the higher the charging uniformity.

On the other hand, the abrasion of the surface of the photosensitive drum 1 tends to increase linearly with the contact width of the contact portion (charging nip portion) A between the charging roller 2 and the photosensitive drum 1 and the increase of the peripheral speed ratio. Therefore, it is not preferable to unnecessarily increase each value. In the present embodiment, the charging roller 2 and the photosensitive drum 1 are provided under the condition that good chargeability is obtained in the initial state.
The contact width of the contact portion (charging nip portion) A is set to 1 mm, and the peripheral speed ratio is set to 100% (the charging roller 2 moves in the opposite direction at the contact portion with the photosensitive drum 1). ).

The charging roller 2, the charging promoting particles M, the injection charging, the replenishment of the charging promoting particles M, and the normalization of the transfer residual toner in this embodiment will be described in more detail below.

The charging roller 2 as the contact charging member in this embodiment is formed by forming a medium resistance layer of rubber or foam on a core metal. The medium resistance layer was formulated with a resin (for example, urethane), conductive particles (for example, carbon black), a sulfiding agent, a foaming agent, etc., and was formed in a roller shape on the core metal. Then, the surface was polished as needed.

When the roller resistance of the charging roller 2 of the present embodiment was measured, it was 100 kΩ. The roller resistance is measured by applying 100V between the core metal and the aluminum base in a state where the charging roller 2 is pressure-bonded to an aluminum base having an outer diameter of 30 mm so that a weight of 9.8 N is applied to the core of the charging roller 2. did.

It is important that the charging roller 2 described above functions as an electrode. That is, it is necessary to impart elasticity to obtain a sufficient contact state with the photosensitive drum 1 and at the same time have a resistance sufficiently low to charge the moving photosensitive drum 1. On the other hand, it is necessary to prevent voltage leakage when the photosensitive drum 1 has a low breakdown voltage defective portion such as a pinhole.

Therefore, the roller resistance of the charging roller 2 is
10 ⁴ to 10 ⁷ Ω in order to obtain sufficient chargeability and leak resistance
Is desirable. Further, it is desirable that the surface of the charging roller 2 has micro unevenness so that the charging promoting particles M can be held.

If the hardness of the charging roller 2 is too high, not only the contact portion (charging nip portion) A cannot be secured between the charging roller 2 and the photosensitive drum 1, but also the microscopic contact with the surface of the photosensitive drum 1 is poor. Therefore, the Asker C hardness is preferably 50 degrees or less.

Further, the material of the charging roller 2 is not limited to the elastic foam, and the material of the elastic body is
EPDM, urethane, NBR, silicone rubber, IR
A rubber material in which a conductive material such as carbon black or a metal oxide is dispersed for resistance adjustment, or a material obtained by foaming these materials can be used. It is also possible to adjust the resistance by using an ion conductive material without particularly dispersing the conductive substance.

(Charge promoting particles M) In the present embodiment, as the charge promoting particles M, conductive zinc oxide particles having a specific resistance of 10 ⁷ Ω · cm and an average particle size of 1.5 μm were used. The charge promoting particles M not only exist in the state of primary particles,
There is no problem that the secondary particles exist in an aggregated state. Whatever the aggregated state, the form is not important as long as the function as the charge promoting particles M can be realized as an aggregate.

The particle size of the charge-accelerating particles M is defined as the average particle size of the agglomerates when the particles form agglomerates. To measure the particle size, 100 or more particles were extracted by observation with an optical or electron microscope, the volume particle size distribution was calculated with the maximum chord length in the horizontal direction, and the 50% average particle size was determined.

Further, the resistance value of the charging promoting particles M is 2 × 10.
^{When it was 12} Ω · cm or more, the charging property was impaired. Therefore, the resistance value needs to be 1 × 10 ¹² Ω · cm or less, and more preferably 1 × 10 ⁹ Ω · cm or less. In the present embodiment, the one having 1 × 10 ⁷ Ω · cm is used.

The resistance was measured by the tablet method and normalized. That is, a powder sample of about 0.5 g was placed in a cylinder having a bottom area of 2.26 cm ² , and 147 N of pressure was applied to the upper and lower electrodes, and at the same time, a voltage of 100 V was applied to measure the resistance value, and then normalized. Then, the specific resistance was calculated.

Further, the charge accelerating particles M are preferably close to white or transparent so that they do not interfere with the exposure of the electrostatic latent image, and are therefore preferably non-magnetic. Further, considering that the charge promoting particles M are partially transferred from the photosensitive drum 1 to the transfer material P, colorless or white particles are desirable in color image formation. Also, if the particle size is not about 1/2 or less of the particle size of the toner (developer), the scanning exposure L of the exposure device 3 may be blocked.

Therefore, it is desirable that the particle size of the charge promoting particles M is smaller than 1/2 of the particle size of the toner. The lower limit value of the particle size of the electrification promoting particles M is considered to be 10 nm as a particle that can be stably obtained as particles. Also,
Although zinc oxide is used as the material of the charge promoting particles M in the present embodiment, the material is not limited to this, and other materials such as titanium oxide and alumina, a mixture of conductive inorganic particles and an organic substance. Alternatively, various conductive particles such as surface-treated particles can be used.

(Injection Charging) As in the present embodiment, the contact portion (charging nip portion) A between the photosensitive drum 1 and the charging roller 2
By interposing the charge accelerating particles M in the
Even if the charging roller 2 has a large frictional resistance and it is difficult to bring the photosensitive roller 1 into contact with the photosensitive drum 1 with a speed difference due to the lubricant effect, it cannot be applied to the surface of the photosensitive drum 1. It becomes possible to easily and effectively bring them into contact with each other with a speed difference, and the charging roller 2 comes into close contact with the surface of the photosensitive drum 1 via the charging accelerating particles M, resulting in a higher frequency. Thus, the surface of the photosensitive drum 1 is contacted.

As described above, the charging roller 2 and the photosensitive drum 1 are
By providing a speed difference between the charging roller 2 and the photosensitive drum 1, the chances of the charging promoting particles M coming into contact with the photosensitive drum 1 at the contact portion (charging nip portion) A between the charging roller 2 and the photosensitive drum 1 are significantly increased, and high contactability Can be obtained. Therefore, the charge accelerating particles M existing in the contact portion A between the charging roller 2 and the photosensitive drum 1 rub the surface of the photosensitive drum 1 without a gap, so that the charge can be directly injected into the photosensitive drum 1, and the charging roller 2 The contact charging of the photosensitive drum 1 by means of the injection charging mechanism is predominant due to the presence of the charging promoting particles M.

Therefore, a high charging efficiency, which has not been obtained by the conventional roller charging or the like, can be obtained, and the charging potential substantially equal to the voltage applied to the charging roller 2 can be applied to the photosensitive drum 1. In this way, the charging roller 2 is used as the contact charging member.
Even when the charging roller is used, the charging application bias necessary for charging the charging roller 2 is sufficient to be a voltage corresponding to the charging potential required for the photosensitive drum 1, and stable and safe contact charging without using a discharge phenomenon is realized. You can

When the image forming apparatus is used for the first time, the charging accelerating particles M are preliminarily carried on the contact portion (charging nip portion) A between the charging roller 2 and the photosensitive drum 1 and the surface of the charging roller 2. Good charging performance can be obtained from the initial stage.

(Replenishment of the contact portion (charging nip portion) A of the charging promoting particles M and the charging roller 2) At the initial stage, a sufficient amount of the charging promoting particles M is applied to the contact portion A of the photosensitive drum 1 and the charging roller 2. Even if the charging-promoting particles M are applied or the charging-promoting particles M are applied to the charging roller 2 in a sufficient amount, the charging-promoting particles M come from the contact portion A and the charging roller 2 in accordance with the image forming operation of the image forming apparatus. The chargeability is lowered due to the decrease or the deterioration of the charge promoting particles M. Therefore, when the charging property is deteriorated, it is necessary to replenish the contact portion A and the charging roller 2 with the charging accelerating particles M.

Therefore, a charging promoting particle supply device (not shown) for supplying the charging promoting particles M to the surface of the charging roller 2
However, in the present embodiment, the charging promoting particles M are supplied into the developing device 4 together with the toner, and the charging promoting particles M are replenished from the developing device 4 through the surface of the photosensitive drum 1. . In the developing device 4, the charge accelerating particles M are an external additive as a charge assisting agent for the toner, and are supplied to the photosensitive drum 1 side by application of a developing bias to the developing roller 8 during development and reach the charging roller 2. Is.

(Normalization of Transfer Residual Toner) The transfer residual toner remaining on the surface of the photosensitive drum 1 after the transfer is rubbed at the contact portion A between the photosensitive drum 1 and the charging roller 2, and the original charging is performed by the frictional charging. It is returned to the polarity (negative polarity) (normalization). In the present embodiment, since the reversal development is performed using the negative toner, the frictional charging characteristics of the toner t and the charging promoting particles M are such that the toner t becomes negative due to friction between them. .

In the image forming apparatus according to this embodiment, the charging roller 2 is provided with the charging bias as described below.
It was applied to.

Charging bias power source 1 in the present embodiment
2, the charging bias applied to the charging roller 2 is a DC voltage to an AC voltage (rectangular wave, duty 50) as shown in FIG.
%), The AC voltage has a large peak voltage value (150 V) in the charging potential direction with respect to the surface charging potential of the photosensitive drum 1 (a in the figure), and the peak voltage value on the opposite side is small. (30V). 2B is the duty of the AC voltage (rectangular wave).
It shows 50%.

Since the process speed of this embodiment is 210 mm / sec, the frequency of this AC voltage needs to be in a range in which frequency unevenness does not occur in the surface charging potential of the photosensitive drum 1. In this embodiment, The frequency of the alternating voltage was 1 KHz.

The frequency unevenness depends on the process speed,
The faster the process speed, the higher the frequency needs to be. Further, when the frequency (f) and the process speed (Vd) were investigated by shaking, it was found that the condition that f / Vd = 1 was the lower limit of the frequency at which the frequency unevenness started to be seen.

If the frequency is set too high, the toner will not be able to follow the high frequency electric field and the dischargeability of the toner will be deteriorated (the toner is pulled back before being firmly attached to the photosensitive drum 1). ). In the experiments conducted by the present inventor, a decrease in toner ejection property began to be observed at f = 5 KHz or higher, so this value was made the upper limit.

FIG. 3 shows the relationship between the process speed and the frequency of the AC voltage in this embodiment.
When the frequency in the shaded area in the figure was used, no decrease in toner ejection property was observed.

Further, in this embodiment, the non-image portion potential (solid white potential) on the surface of the photosensitive drum 1 is -400 V, the developing voltage _VDC is -280 V, and the fog removal potential difference (Vba).
ck) was set to 120V.

When only the DC voltage is applied to the charging roller 2, the transfer residual toner is poorly discharged, and the transfer residual toner remains relatively attached to the charging roller 2 and relatively moves along the charging roller 2. When the AC voltage is superimposed on the DC voltage, an oscillating electric field is applied to the toner, and the amount of toner discharged from the charging roller 2 to the photosensitive drum 1 increases,
The components that rotate around on the charging roller 2 are reduced.

That is, by applying the charging bias in which the AC voltage is superimposed on the DC voltage to the charging roller 2, it is possible to prevent the toner from accumulating on the surface of the charging roller 2 and always keep it in a clean state. Further, by superimposing an AC voltage, a potential leveling effect can be obtained, and charging uniformity can be improved.

Further, in the present embodiment, as shown in FIG. 2, the electric field of the AC voltage (AC component) of the charging bias is made stronger in the surface charging potential direction of the photosensitive drum 1 and weakened in the opposite direction. Therefore, it is possible to effectively eject the normalized toner and hold the non-normalized toner or the charge promoting particles M. That is, in the present embodiment,
The AC voltage (AC component) of the charging bias is the photosensitive drum 1
The peak-to-peak voltage value on the same polarity side as the charging polarity of the photosensitive drum 1 with respect to the surface charging potential was set to be larger than the peak-to-peak voltage value on the opposite polarity side.

The amount of toner discharged from the charging roller 2 and the toner discharged from the charging roller 2 in the image forming apparatus of the present embodiment (Embodiment 1) in which the charging bias set as described above is applied to the charging roller 2. The evaluation experiment of the fog caused by the was carried out. FIG. 9 shows the evaluation result.

The toner discharge amount is evaluated by taping the toner discharged after the solid black toner image is output from the surface of the photosensitive drum 1 and comparing the densities thereof. (If the image ratio is low, the transfer residual toner amount is small. Therefore, it is difficult to compare the amount of discharged toner). In the evaluation of the amount of toner discharged, ⊚ is very good (toner is discharged well), ◯ is good (no problem at a practical level), and Δ is bad.

Further, the evaluation of fog is numerically expressed as fog by measuring the amount of reflected light by a fog reflection densitometer and expressed in%, and the level thereof is ◯ (fog value is less than 1.0%)
Δ (fogging value is 1.0 to 1.5%) and x (fogging value is more than 1.5%) are shown.

As is clear from the evaluation results shown in FIG. 9, in the present embodiment (Embodiment 1), both the discharge toner amount evaluation and the fog evaluation are good, and the average discharge amount of the discharge toner is -6 μC / It was g.

Further, in the evaluation result shown in FIG.
The “conventional example” is a conventional image forming apparatus configured to apply the conventional charging bias shown in FIG. 11, and the average charge amount of the discharged toner was −0.5 μC / g. In this “conventional example”, the evaluation of the amount of discharged toner was good, but the evaluation of fog was poor.

Further, in the evaluation result shown in FIG.
In "Comparative Example 1" and "Comparative Example 2", charging biases with superimposed AC voltages as shown in FIGS. 4 and 5 were used.
In FIGS. 4 and 5, “a” indicates the surface charging potential of the photosensitive drum 1, and “b” indicates the duty 50% of the AC voltage (rectangular wave).

The AC voltage (AC component) of the charging bias of "Comparative Example 1" shown in FIG. 4 has a peak voltage value of 80 V in the charging potential direction with respect to the surface charging potential of the photosensitive drum 1 (a in the figure). And has a peak voltage value of 10 V on the reverse side. Other conditions and configurations are the same as those in the first embodiment. In "Comparative Example 1", the evaluation of the amount of discharged toner was bad, but the evaluation of fog was good. The average charge amount of the discharged toner was −5.5 μC / g.

The AC voltage (AC component) of the charging bias of "Comparative Example 2" shown in FIG. 5 is a peak voltage of 110 V in the charging potential direction with respect to the surface charging potential of the photosensitive drum 1 (a in the figure). And has a peak voltage value of 60 V on the reverse side. Other conditions and configurations are the same as those in the first embodiment. In "Comparative Example 2", the evaluation of the amount of discharged toner was good, but the evaluation of fog was not so good. The average charge amount of the discharged toner was −3 μC / g.

As described above, in the present embodiment, the charging roller 2 is configured to apply the charging bias in which the AC voltage is superimposed on the DC voltage, and the AC voltage (AC component) of the charging bias is applied to the surface of the photosensitive drum 1. By increasing the electric field in the charging potential direction and weakening the opposite direction, the toner (transfer residual toner) can be satisfactorily discharged from the charging roller 2 to the photosensitive drum 1, and due to the toner discharged from the charging roller 2. Since it is possible to prevent the occurrence of fogging, it is possible to perform good image formation for a long period of time.

<Embodiment 2> In the present embodiment, FIG.
As shown in FIG. 5, the AC voltage (rectangular wave, duty 50%) that weights the charging bias applied to the charging roller 2 has a large peak voltage value in the charging potential direction with respect to the surface charging potential of the photosensitive drum 1 (a in the figure). (100V), and the peak voltage value on the reverse direction side is small (10V). Other conditions and configurations are the same as those in the first embodiment. In addition, b of FIG. 6 is the duty of the AC voltage (rectangular wave).
It shows 50%.

The discharge toner amount evaluation and the fog evaluation in the present embodiment (Embodiment 2) are good in both the discharge toner amount evaluation and the fog evaluation as shown in FIG. 9, and the average charge amount of the discharge toner is − It was 6 μC / g.

As described above, also in the present embodiment, the toner (transfer residual toner) can be satisfactorily discharged from the charging roller 2 to the photosensitive drum 1 as in the case of the first embodiment, and discharged from the charging roller 2. Since it is possible to prevent the occurrence of fogging due to the toner that is generated, it is possible to perform good image formation for a long period of time.

<Third Embodiment> In the present embodiment, FIG.
As shown in FIG. 5, the AC voltage (rectangular wave, duty 50%) that weights the charging bias applied to the charging roller 2 has a large peak voltage value in the charging potential direction relative to the surface charging potential of the photosensitive drum 1 (a in the figure). (100 V), and the peak voltage value on the reverse direction side becomes small (50 V). Other conditions and configurations are the same as those in the first embodiment. 7B shows the duty of the AC voltage (rectangular wave).
It shows 50%.

In the present embodiment, the peak voltage value on the reverse direction side is set to 50V, but as shown in FIG. 12, when the peak-to-peak voltage of the AC voltage (AC component) is 100V or more, the discharge effect of the AC voltage is improved. Since it starts to be seen, the effect is obtained when the amplitude on one side is 50 V or more. That is, in the present embodiment, the peak-to-peak voltage in the direction of discharging the unnormalized toner (positive toner) is 50 V, so this toner (positive toner) is not discharged from the charging roller 2 to the photosensitive drum 1, and only the negative toner is discharged. Will be exhaled.

In the discharge toner amount evaluation and the fog evaluation in the present embodiment (Embodiment 3), as shown in FIG. 9, both the discharge toner amount evaluation and the fog evaluation are good, and the average discharge amount of the discharge toner is −. It was 6 μC / g.

As described above, also in the present embodiment, as in the first embodiment, the toner (transfer residual toner) can be satisfactorily discharged from the charging roller 2 to the photosensitive drum 1 and discharged from the charging roller 2. Since it is possible to prevent the occurrence of fogging due to the toner that is generated, it is possible to perform good image formation for a long period of time.

<Embodiment 4> In the case of continuously outputting a large amount of images having a high image ratio such as a solid black toner image,
Even in the image forming apparatus according to the first embodiment (or the second and third embodiments), the discharge capability of the transfer residual toner from the charging roller 2 cannot catch up with the charge roller 2, and the charge capability may decrease.

Therefore, in the present embodiment, as shown in FIG. 8, the frictional charging member 11 is brought into contact with the surface of the charging roller 2. Other conditions and configurations are the same as those in the first embodiment.

Any material can be used as the material of the frictional charging member 11 as long as it has the ability to normalize the reversely charged toner (same polarity as the developing toner). In this embodiment,
As the triboelectrification member 11, a sheet-like urethane was used which was subjected to a coupling treatment and had a property of imparting a negative to the toner.

In the image forming apparatus having the configurations of the first to third embodiments described above, the normalized toner is the charging roller 2
Is effectively discharged to the photosensitive drum 1, but the unnormalized toner is repeatedly rubbed at the contact portion (charging nip portion) A between the charging roller 2 and the photosensitive drum 1 until the toner is normalized.

When an image with a high image ratio such as a solid black toner image is continuously output, the transfer residual toner amount also increases, so that the unnormalized toner accumulates on the surface of the charging roller 2 and thus the surface of the charging roller 2 Charge promoting particles M
Occupies less area, and sufficient injection charging performance cannot be obtained. Further, if the toner occupying area on the surface of the charging roller 2 increases, the probability that the transfer residual toner is rubbed with the charging roller 2 decreases, so that normalization becomes difficult and the transfer residual toner becomes difficult to be discharged from the charging roller 2. There is a possibility that it will fall into a vicious circle.

Therefore, by bringing the frictional charging member 11 into contact with the surface of the charging roller 2 as in the present embodiment, the toner on the surface of the charging roller 2 can be positively normalized, so that the embodiment is realized. By applying the charging bias in No. 1, it is possible to obtain excellent toner ejection property.

In the discharge toner amount evaluation and the fog evaluation in the present embodiment (Embodiment 4), as shown in FIG. 9, the discharge toner amount evaluation was very good, and the fog evaluation was also good. The average charge amount of the discharged toner was −7 μC / g.

As described above, in the present embodiment, the toner (transfer residual toner) can be satisfactorily discharged from the charging roller 2 to the photosensitive drum 1 even when images having a high image ratio such as a solid black toner image are continuously output. In addition, since it is possible to prevent the occurrence of fog due to the toner discharged from the charging roller 2, it is possible to perform good image formation for a long period of time.

[0150]

As described above, according to the present invention, the peak-to-peak voltage value on the same polarity side as the charging polarity of the image carrier is opposite to the DC component of the duty ratio of the AC component of the charging bias. By setting the voltage higher than the peak-to-peak voltage value on the side, the residual toner can be satisfactorily discharged from the contact charging member to the image carrier, and the fog caused by the toner discharged from the contact charging member is generated. Since this can be prevented, good image formation can be performed for a long period of time.

[Brief description of drawings]

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

FIG. 2 is a diagram showing a waveform of an AC voltage of a charging bias according to the first embodiment of the present invention.

FIG. 3 is a diagram showing an appropriate range of an AC component frequency of a charging bias with respect to a process speed in the first embodiment of the present invention.

FIG. 4 is a diagram showing a waveform of an AC voltage of a charging bias in Comparative Example 1 of the present invention.

FIG. 5 is a diagram showing a waveform of an AC voltage of a charging bias in Comparative Example 2 of the present invention.

FIG. 6 is a diagram showing a waveform of an AC voltage of a charging bias according to the second embodiment of the present invention.

FIG. 7 is a diagram showing a waveform of an AC voltage of a charging bias according to the third embodiment of the present invention.

FIG. 8 is a schematic configuration diagram showing a main part of an image forming apparatus according to a fourth embodiment of the present invention.

FIG. 9 is a diagram showing evaluation results in the first to fourth embodiments of the present invention.

FIG. 10 is a diagram showing a relationship between a DC voltage applied to a charging roller (charging bias) and a charging potential of a photoconductor (charging surface potential of a photosensitive drum).

FIG. 11 is a schematic configuration diagram showing an image forming apparatus in a conventional example.

FIG. 12 is a diagram showing a waveform of an AC voltage of a charging bias in a conventional example.

FIG. 13 is a diagram showing an increase amount of the discharged toner amount with respect to an AC applied voltage to the charging roller (voltage between charging bias AC component peaks).

FIG. 14 is a diagram for explaining a path when the transfer residual toner passes through the charging roller.

[Explanation of symbols]

1 Photosensitive drum (image carrier) 2 Charging roller (contact charging member) 3 exposure equipment 4 Developing device 5 Transfer roller 6 Separation charger 7 fixing device 8 developing roller 9 regulation blade 11 Friction charging member 12 Charging bias power supply 20 Charging device t toner M Charge accelerating particles (conductive fine powder)

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 2H073 AA03 BA04 BA13 BA43 CA02                       CA14                 2H077 AA37 AB03 AB14 AB18 AC16                       AD06 AD13 AD31 AD36 DB08                       DB12 EA11 EA16 GA04                 2H200 FA08 FA14 FA18 GA16 GA18                       GA23 GA30 GA33 GA46 GA49                       GB37 HA03 HA21 HA29 HB17                       HB45 HB46 HB47 HB48 LB03                       LB15 MA03 MA08 MA14 MB04                       MC02 NA06 PA05 PA08 PA09                       PB14

Claims (11)

[Claims] 1. A movable image carrier and a rotatable contact charging member which holds conductive fine powder having conductivity, and the conductive fine powder is allowed to exist so that the contact charging member is supported by the image carrier. A charging unit that contacts the body and charges the image carrier by applying a charging bias in which an AC component is superimposed on a DC component, and an exposing unit that exposes the charged image carrier to form an electrostatic latent image. A developing unit that develops the electrostatic latent image with toner to visualize it as a toner image; and a transfer unit that transfers the toner image onto a transfer material at a transfer site. After the toner image is transferred onto the transfer material by the transfer means, the residual toner remaining on the image carrier is once electrostatically collected by the contact charging member, and then the residual toner discharged onto the image carrier is removed. Also serves as a cleaning means for electrically collecting In a cleanerless image forming apparatus, the duty ratio of the AC component of the charging bias is the same as the charging component of the image carrier relative to the DC component. An image forming apparatus characterized by being larger than a voltage value. 2. The AC component of the charging bias has a peak-to-peak voltage value on the same polarity side as the charging polarity of the image carrier with respect to the DC component, and the surface charging potential of the image carrier by the contact charging member. Is set to 100 V or more in absolute value, and the peak-to-peak voltage value on the opposite polarity side is set to 50 V or more in absolute value with respect to the surface charging potential of the image carrier by the contact charging member. The image forming apparatus according to claim 1, wherein: 3. The frequency f (Hz) of the AC component of the charging bias is the moving speed of the image carrier Vd (mm
/ Sec), it is set so as to satisfy 1 <(f / Vd) <(5000 / Vd). 3. The image forming apparatus according to claim 1, wherein 4. The contact charging member is rotated so as to move in a direction opposite to the moving direction of the image carrier at the contact portion with the image carrier. The image forming apparatus according to any one of 1. 5. The image forming apparatus according to claim 1, wherein the contact charging member is a roller member having an Asker C hardness of 50 degrees or less. 6. The volume resistivity of the roller member 10 ³ Omega ·
The image forming apparatus according to claim 5, wherein the image forming apparatus has a size of not less than 10 cm and not more than 10 ⁸ Ω · cm. 7. The volume resistance of the conductive fine powder is 1 × 10.
The image forming apparatus according to claim 1, wherein the image forming apparatus has a resistance of ⁹ Ω · cm or less. 8. The conductive fine powder is contained as an external additive for a toner, adheres to the image carrier during development, and moves at least with the contact charging member and the toner as the image carrier moves. The image forming apparatus according to any one of claims 1 to 7, wherein the image forming apparatus is supplied to a contact portion with an image carrier. 9. The non-contact jumping developing method, wherein the developing method by the developing means transfers the toner to an electrostatic latent image on the image bearing member by applying a developing bias to develop the electrostatic latent image. Item 9. The image forming apparatus according to any one of items 1 to 8. 10. The image forming apparatus according to claim 1, wherein a frictional charging member is brought into contact with the contact charging member. 11. The frictional charging member is made of a material that is controlled by triboelectrification so that the toner charged to a polarity opposite to the regular charging polarity has a regular charging polarity. The image forming apparatus according to item 1.