JP2003280335A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent fogging by reversed toner caused in the case of using cardboard or coated paper by making it pass as material to be recorded P in a cleaner-less system image forming apparatus using contact electrification. <P>SOLUTION: The image forming apparatus has an opportunity to apply bias for discharging toner intruding in a contact electrifying member 2A to an image carrier 1 in accordance with an integrated image ratio or the number of passing paper, and is provided with a selection means 101 for selecting the material to be recorded P to be used, and an opportunity or time, or the opportunity and the time to discharge the toner intruding in the member 2A are changed in accordance with the selected material to be recorded P. <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 a transfer type image forming apparatus of a contact charging type and a cleanerless process.

More specifically, an image carrier such as an electrophotographic photoreceptor or an electrostatic recording dielectric, and an image carrier having a charging member in contact with the image carrier are provided by applying a charging bias to the charging member. Contact type charging device (contact charging device,
Direct charging device), an image information writing device that forms an electrostatic latent image on the charged surface of the image carrier, a developing device that visualizes the electrostatic latent image with a developer, and the surface of the image carrier. The transfer device that moves the developer to the recording material, and the developer remaining on the surface of the image carrier without being transferred to the recording material by the transfer device are temporarily collected by the charging member that contacts the image carrier of the charging device. The present invention relates to an image forming apparatus such as a copying machine or a printer in which the collected developer is discharged from a charging member and is collected again by a developing device.

[0003]

2. Description of the Related Art (a) In a contact charging electrophotographic type or electrostatic recording type image forming apparatus,
Conventionally, a corona charger has been generally used as a charging means for charging an image bearing member such as an electrophotographic photosensitive member or an electrostatic recording dielectric member and other members to be charged to a predetermined polarity and potential.

This is because a corona charger is placed opposite to an image bearing member (hereinafter referred to as a photoconductor) in a non-contact manner, and the photoconductor surface is exposed to the corona discharged from the corona charger so that the photoconductor surface has a predetermined polarity. It is charged to a potential.

In recent years, since it has advantages such as low ozone and low power as compared with the case of using the above-mentioned non-contact type corona charger, as described above, a voltage (charging bias) is applied to the photoconductor as the member to be charged. ) Has been put into practical use to bring a charging member (contact charging member) into contact with the surface of the photoconductor to charge the surface of the photosensitive member to a predetermined polarity and potential.
In particular, a roller charging type device using a conductive roller (charging roller) as a charging member is preferably used from the viewpoint of charging stability.

As the contact charging member, a magnetic brush charging member (charging magnetic brush, hereinafter referred to as magnetic brush charger) having a magnetic brush portion in which magnetic particles are magnetically restrained on a carrier is used. A magnetic brush charging device for bringing the magnetic brush portion of the brush charger into contact with the photoconductor is also preferably used from the viewpoint of the stability of the charging device. The magnetic brush charger has magnetic brushes formed by magnetically binding conductive magnetic particles directly to a magnet or on a sleeve containing the magnet to form a magnetic brush portion. The portion is brought into contact with the photoconductor and a voltage is applied to the photoconductor to start charging of the photoconductor.

Also, a contact charging member such as a brush having conductive fibers formed (fur brush charging member, charging fur brush), a conductive rubber blade (charging blade) made of conductive rubber in a blade shape, etc. It is preferably used.

Charging mechanism of contact charging (charging mechanism,
In the charging principle), two types of charging mechanisms, a corona charging system and a charge injection (direct charging) system, coexist, and each characteristic appears depending on which one is dominant.

The corona charging system is a system in which the surface of the photoconductor is charged with a discharge product due to a corona discharge phenomenon generated in a minute gap between the contact charging member and the photoconductor. Since corona charging has a constant discharge threshold between the contact charging member and the photoconductor, it is necessary to apply a voltage higher than the charging potential to the contact charging member. Further, compared with the corona charger, the amount of generation is significantly smaller, but discharge products are generated.

The charge injection charging system is a system in which the surface of the photoconductor is charged by directly injecting the charge from the contact charging member into the photoconductor. 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 applied voltage to the contact charging member is equal to or lower than the discharge threshold value, the photoconductor can be charged to a potential corresponding to the applied voltage. This charge injection charging system does not generate ions. However, since the charge injection charging is used, the contact property of the contact charging member to the photosensitive member greatly affects the charging property. Therefore, it is necessary to make the contact charging member more dense and to have a speed difference with the photoconductor so as to contact the photoconductor more frequently. In this respect, a magnetic brush charger is particularly used as the contact charging member. Can perform stable charging.

The charge injection charging by the magnetic brush charger can be regarded as equivalent to a series circuit of a resistor and a capacitor. In an ideal charging process, the time during which a point on the surface of the photoconductor is in contact with the magnetic brush ( The condenser is charged to (charging nip × peripheral speed of the photoconductor), and the photoconductor surface potential becomes almost the same value as the applied voltage.

There is a method in which a voltage is applied to a conductive contact member to inject a charge into a trap level on the surface of the photosensitive member to contact-charge the photosensitive member. When a photosensitive member having a surface layer (charge injection layer) in which conductive fine particles are dispersed on an ordinary organic photosensitive member or an amorphous silicon photosensitive member is used, the direct current of the bias applied to the contact charging member is reduced. It is possible to obtain a charging potential on the surface of the photoconductor that is substantially the same as that of the components (Japanese Patent Laid-Open No. 6-3921).

The injection charging method is not only environmentally dependent and does not use discharge, so that the voltage applied to the contact charging member is sufficient to be approximately equal to the potential of the photoconductor, and there is an advantage that ozone is not generated. It enables complete ozoneless and low power consumption type charging.

(B) Cleanerless process (toner recycling process) In recent years, the image forming apparatus has been downsized, but each means and device of the image forming process such as charging, exposure, development, transfer, fixing and cleaning. However, there is a limit to the overall size reduction of the image forming apparatus just by making each size smaller. Further, the transfer residual toner (residual developer) on the photoconductor after the transfer is collected by a cleaning means (cleaner) and becomes waste toner, but it is preferable that the waste toner does not appear from the viewpoint of environmental protection.

Therefore, in the "cleanerless process", the cleaner is removed, and the transfer residual toner on the photoconductor is removed from the photoconductor by "development simultaneous cleaning" by the developing means and collected and reused by the developing means. Image forming apparatuses have also appeared.

Simultaneous development cleaning is a fog removal bias (potential difference between the DC voltage applied to the developing means and the surface potential of the photoconductor) for the toner slightly remaining on the photoconductor after the transfer during the subsequent development process. Potential difference Vback)
It is a method of collecting by.

According to this method, since the transfer residual toner is collected by the developing means and used in the subsequent steps, it is possible to eliminate the waste toner and reduce the troublesome maintenance. Further, the cleaner-less structure has a great advantage in terms of space, and the image forming apparatus can be significantly downsized. In the case where the charging device of the photoconductor is of the contact charging type, the transfer residual toner is once collected by the charging member which is in contact with the photoconductor, and is discharged again onto the photoconductor and collected by the developing device.

In a system without a cleaning device for removing the transfer residual toner from the member to be charged, that is, in a so-called cleanerless system, a large amount of the transfer residual toner is mixed or adhered to the contact charging member, so that the image During the formation durability, the resistance value of the contact charging member changes.

Therefore, there is known a method of delaying the deterioration of the charging member by applying a charging member cleaning bias corresponding to the durable number of sheets and the image ratio, for example, during non-image formation. In other words, in contact charging, there are bias conditions and hardware configuration conditions that make it easy to eject toner that has mixed or adhered to the magnetic brush or roller that is the contact charging member onto the photoconductor, and can extend the life of the contact charging member. It is possible.

Particularly in the case of the magnetic brush injection charging device, the transfer residual toner, which is considered to have toner of both positive and negative polarities, is once collected by the magnetic brush to erase the history of the previous image, and the contact friction between the magnetic particles and the collected toner. Thus, by charging all the toner to the regular polarity, it becomes possible to control the toner to be returned onto the photoconductor again.

Further, since the toner discharged from the magnetic brush portion to the photoconductor is in a very evenly dispersed state and the amount thereof is small, it does not substantially affect the next image exposure process. . Further, there is no occurrence of a ghost image due to the transfer residual toner pattern, and the normal polarity toner can be reliably recovered in the non-image area due to the difference between the developing application bias and the charging potential.

In the case of charging using a conductive roller as a contact charging member, in order to restore the opposite polarity toner adhered to the roller to the normal polarity, a method such as contacting a sheet in consideration of friction series with the roller is available. is there.

[0023]

However, the cleaner-less image forming apparatus using such a contact charging system has the following problems.

That is, as described above, the transfer residual toner is collected in the developing device by aligning it with the regular polarity, that is, the same polarity as the charging polarity in the charging device, but it has little polarity or remains reverse polarity. In some cases, there is a toner in the state (hereinafter, referred to as a reverse toner).
Usually, the amount of the reversal toner is extremely small, and even if the reversal toner is not collected by the development and stumbles on the photoconductor, it is not transferred under the normal transfer conditions, and therefore no particular problem occurs.

However, depending on the thickness and surface property of the recording material such as paper used, fog may occur on a white background portion on the image. Although the reason for this is not clear, it is considered that when the thickness of the recording material is large, the contact pressure of the transfer blade or the transfer roller in the transfer portion is increased, so that the transfer is performed with the pressure although the polarity is opposite.

Further, in a recording material having a high smoothness such as coated paper, since the adhesiveness between the recording material and the photoconductor is enhanced, there is a gap for preventing the toner from being pressed against the recording material at the same time. However, it is considered that the adhesion is increased and the transfer is performed by pressure.

Toners having opposite polarities and toners having almost no polarities may be present in a certain ratio by nature, but especially in a high humidity environment where the toner is unlikely to have a regular charging polarity, or when mixed or attached to the contact charging member. Deteriorated toner increases due to a temporary increase in the amount of toner, which makes it difficult to be contact-charged, or when external additives are embedded while rotating around the photoconductor without being transferred for a long time. It occurs remarkably when doing. The reversal toner accumulates without being discharged from the contact charging member, and when it accumulates in a certain amount, it gradually leaks onto the photoconductor and may be pressure-transferred without being collected in the development.

Further, when a thick paper or coated paper is used as the recording material, in the case of an apparatus having a mode for slowing the process speed in order to maintain the fixing property, the transfer section receives a pressure for a long time. Therefore, the phenomenon becomes more remarkable.

Therefore, an object of the present invention is to provide an image forming apparatus of a cleanerless system using contact electrification, which maintains a high quality image without causing fog due to reversal toner regardless of which recording material is used. A forming device is provided.

[0030]

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

(1) A charging member is brought into contact with the image bearing member and a bias voltage is applied to the charging member to charge the image bearing member, a latent image is formed on the charged surface, and the latent image is formed. After being developed as a toner image by a developing means, the toner image is transferred to a recording material, and then the residual toner particles remaining on the image carrier are once collected by the charging member that contacts the image carrier, and the collected toner particles are collected. In an image forming apparatus that also serves as a cleaning unit that returns the toner particles returned from the charging member to the image carrier by the developing unit, collects the recovered toner particles from the charging member onto the image carrier. It has a developer returning bias different from that at the time of image formation applied to the charging member for returning, and has a selecting means for selecting a recording material to be used, and the developer returning according to the selected recording material. Bias An image forming apparatus comprising altering opportunities or time, or the opportunity and time to pressure.

(2) The image forming apparatus according to (1), wherein the charging member comprises magnetic particles and a magnetic particle carrier.

(3) The image forming apparatus according to (1) or (2), wherein the image carrier is a photoconductor having a charge injection layer on the surface.

(4) The image forming apparatus described in any one of (1) to (3), wherein the charging bias conditions applied to the image forming area and the non-image forming area are different.

(5) The image forming apparatus described in any one of (1) to (4), wherein the charging bias is a DC bias or a bias in which an AC bias is superimposed on the DC bias.

(6) A predetermined threshold value for determining at least one of the opportunity and time for applying the discharge bias to the charging member is determined by the image ratio cumulative calculation value during continuous image formation, which has means for calculating the image ratio. The image forming apparatus as described in any one of (1) to (5) above.

(7) An environment detecting means is provided, and a predetermined threshold value for determining at least one of an opportunity and a time to apply the discharge bias to the charging member is calculated from the environment detecting means. The image forming apparatus according to any one of (1) to (6), which is dependent on output.

(8) A means for counting the total number of image formations is provided, and a predetermined threshold value for determining at least one of the opportunity and time for applying the discharge bias to the charging member with respect to the image ratio cumulative calculation value is set. The image forming apparatus according to any one of (1) to (7), wherein the image forming apparatus depends on the total number of images to be formed.

(9) A predetermined threshold value for determining at least one of the opportunity and time for applying the discharge bias to the charging member with respect to the cumulative image ratio calculation value, or the total toner consumption amount of at least 1 The image forming apparatus according to any one of (1) to (8), which is characterized in that

(10) The amplitude of the AC component of the discharge bias is smaller than the amplitude of the AC component of the charging application bias at the time of image formation (1) to (9)
(11) The image forming apparatus described in any one of (1) to (10), wherein the process speed is changed according to the selected recording material.

(12) In any one of (1) to (11), the means for forming a latent image on the charged surface of the image carrier is an image exposure means for the charged surface of the image carrier. The image forming apparatus described.

<Operation> The opportunity and time for applying the bias for discharging the transfer residual toner mixed in the charging member onto the image bearing member are changed according to the recording material used. That is, there is an opportunity to apply a bias for discharging the mixed toner of the contact charging member onto the image carrier according to the integrated image ratio and the number of passed sheets, and a selection unit for selecting a recording material to be used is provided. Depending on the recording material, by changing the opportunity or time or the opportunity and time to discharge the contact charging member depending on the recording material, for thick paper and coated paper, the reversal toner accumulated on the contact charging member is sufficiently restored to the normal polarity. It is possible to prevent the fog caused by the reversal toner by being charged.

[0043]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS (1) Example of Image Forming Apparatus (FIG. 1) FIG. 1 shows a schematic configuration diagram of the image forming apparatus. The image forming apparatus of this embodiment is a laser beam printer using a transfer type electrophotographic process, a charge injection charging type, and a cleanerless process.

Reference numeral 1 denotes a rotary drum type electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum) as an image bearing member. The photosensitive drum 1 of this embodiment is an OPC having a negative charging property and a charge injection charging property.
It is a photoconductor (organic photoconductive photoconductor), and is 150 mm / sec. Is driven to rotate at the process speed (peripheral speed).

Reference numeral 2 is a contact charging device for uniformly charging the surface of the photosensitive drum 1 to a predetermined polarity and potential. In this embodiment, a magnetic brush charging device is used, and the surface of the rotating photosensitive drum 1 is uniformly charged by the magnetic brush charging device 2 to a charge injection charging method of approximately -700V.

Reference numeral 3 denotes an image information exposure means (exposure device), which is a laser beam scanner in this embodiment. The laser beam scanner 3 includes a semiconductor laser, a polygon mirror, an F-θ lens, and the like, and inputs from an unillustrated host device such as a document reading device having a photoelectric conversion element such as a CCD, an electric calculator, a word processor, or the like. The laser light L modulated corresponding to the time-series electric digital image signal of the target image information is emitted, and the uniformly charged surface of the rotary photosensitive drum 1 is subjected to laser light scanning exposure.
By this laser light scanning exposure, an electrostatic latent image corresponding to the target image information is formed on the peripheral surface of the rotary photosensitive drum 1.

Reference numeral 4 is a developing device. In this embodiment, a two-component contact developing type developing device is used which is made by a polymerization method and is composed of a highly releasable spherical toner containing less transfer residual toner and a developer mixed with a magnetic carrier. Then, the electrostatic latent image on the surface of the rotary photosensitive drum 1 is reversely developed as a toner image.

A transfer device 5 is arranged below the photosensitive drum 1, and the transfer device of this embodiment is a transfer belt type. 5a is an endless transfer belt (for example, a film thickness of 75 μ
m polyimide belt), which is stretched around the driving roller 5b and the driven roller 5c, and rotates forward in the rotational direction of the photosensitive drum 1 at substantially the same peripheral speed as the rotational speed of the photosensitive drum 1. To be done. Reference numeral 5d is a conductive blade disposed inside the transfer belt 5a, and presses the ascending-side belt portion of the transfer belt 5a against the lower surface portion of the photosensitive drum 1 to form a transfer nip portion T as a transfer portion. 5e is a cleaner for cleaning the surface of the transfer belt 5a.

Reference numeral 6 denotes a paper feed cassette in which a recording material P such as paper (hereinafter referred to as a transfer material) P is stacked and stored. By driving the sheet feeding roller 7, one sheet of the transfer target material P that is stacked and stored in the sheet feeding cassette 6 is separated and fed, and passes through the sheet path 9 including the conveying roller 8 and the like, and the rotating photosensitive drum 1 at a predetermined control timing. And the transfer belt 5a of the transfer device 5 is fed to the transfer nip portion T.

The transfer material P fed to the transfer nip portion T is nipped and conveyed between the rotary photosensitive drum 1 and the transfer belt 5a, and a predetermined transfer bias is applied from the transfer bias applying power source E5 to the conductive blade 5d during the transfer. When applied, the material having the opposite polarity to the toner is charged from the back surface of the transfer material P. As a result, the toner image on the surface of the rotary photosensitive drum 1 is sequentially electrostatically transferred to the surface side of the transfer material P passing through the transfer nip portion T.

The material P to which the toner image has been transferred through the transfer nip portion T is sequentially separated from the surface of the rotary photosensitive drum 1 and passes through the sheet path 10 to a fixing device (for example, a heat roller fixing device) 11. After being introduced, the toner image is fixed and printed out.

(2) Cleanerless Process The printer of this embodiment is a cleanerless process and is dedicated to removing the toner remaining on the surface of the rotating photosensitive drum 1 without being transferred to the transfer material P at the transfer nip portion T. Although no cleaner is arranged, the residual toner after transfer reaches the position of the magnetic brush charging device 2 by the subsequent rotation of the photosensitive drum 1, and the magnetic brush of the magnetic brush charger 2A as a contact charging member in contact with the photosensitive drum 1. The collected toner is temporarily discharged to the image forming section, and the collected toner is again discharged to the surface of the photosensitive drum 1 and finally collected in the developing device 4, and the photosensitive drum 1 is repeatedly used for image formation.

12 is a transfer device 5 and a magnetic brush charging device 2
Is a conductive brush in which an AC bias, a DC bias having a reverse polarity to the charging, or a DC bias having a reverse polarity to the charging superimposed with the AC bias is applied by a power source E6 between the photosensitive drum 1 and Brush charging device 2
At the same time as smoothing the surface potential of the photosensitive drum immediately before charging by means of, the transfer residual toner is discharged or charged in a polarity opposite to that of the charging of the photosensitive drum to facilitate collection by the magnetic brush portion of the magnetic brush charger 2A.

To further explain, since the printer of this embodiment is a cleanerless process, the toner (transfer residual toner) remaining on the photosensitive drum 1 after the toner image is transferred onto the transfer material P is transferred to the charging nip of the photosensitive drum 1. It is carried to the part N and mixed in the magnetic brush part of the magnetic brush charger 2A of the magnetic brush contact charging device 2 to be temporarily collected.

The transfer residual toner on the photosensitive drum 1 is often mixed with positive and negative polarities due to peeling discharge during transfer. The transfer residual toner having the mixed polarity reaches the charging nip portion N and is mixed into the magnetic brush portion of the magnetic brush charger 2A to be temporarily collected. The transfer residual toner is taken into the magnetic brush portion of the magnetic brush charger 2A more effectively by applying an AC component to the magnetic brush charger 2A by the oscillating electric field effect between the photosensitive drums 1 of the magnetic brush charger 2A. Can be done.

Then, the transfer residual toner taken into the magnetic brush portion is charged to the negative polarity and the photosensitive drum 1
Exhaled on. The transfer residual toner, which has the same polarity and is discharged onto the photosensitive drum 1, reaches the developing portion m and is collected by the developing 4b of the developing device 4 by the fogging electric field at the time of development by the simultaneous development cleaning.

When the image area in the rotational direction is longer than the peripheral length of the photosensitive drum 1, the simultaneous development and recovery of the transfer residual toner proceeds simultaneously with other image forming steps such as charging, exposure, development and transfer. Done. As a result, the transfer residual toner is collected in the developing device 4 and used again, so that the waste toner can be eliminated. In addition, the advantage of space is great, and the image forming apparatus can be significantly downsized.

By using a highly releasable spherical toner prepared by the polymerization method as the toner t of the developer, the amount of transfer residual toner generated can be reduced, and the toner is discharged from the magnetic brush charger 2A. It is possible to improve the collectability of the toner in the developing device 4. The use of the two-component contact developing type developing device 4 also improves the collectability of the toner discharged from the magnetic brush charger 2A to the developing device 4.

Here, since the toner usually has a relatively high electric resistance, the mixing of such toner particles in the magnetic brush portion of the magnetic brush charger 2A raises the electric resistance of the magnetic brush portion and the charging ability. When the amount of mixed toner is relatively large, a large amount of toner is discharged during non-image formation, so that good charging can be maintained.

Here, the toner discharge will be briefly described. When toner is mixed in the magnetic brush part, the electric resistance of the magnetic brush gradually increases, so that sufficient charge transfer does not occur while passing through the charging nip, and the photoconductor surface potential after passing through the charging nip is higher than the applied voltage. It gets smaller. Below, the potential difference between the photoreceptor surface potential and the applied voltage is Δ
V. When the toner taken into the magnetic brush charger is charged with the same polarity as the potential of the photoconductor by contact with the magnetic brush carrier, the mixed toner is transferred from the magnetic brush to the surface of the photoconductor by the electric field generated by the potential difference ΔV. Be exhaled. As disclosed in Japanese Unexamined Patent Publication No. 9-96949 and the like, the amplitude Vpp of the AC component (AC component) of the charging bias is reduced by utilizing this phenomenon during non-image formation (non-image formation). A method is known in which the potential difference ΔV is increased by stopping the application of the component and the toner is positively discharged to suppress the increase in the electric resistance of the magnetic brush.

The above-mentioned ejection during non-image formation is performed by the interval between papers or the post-rotation after the image formation operation is completed, so that the amount of the mixed toner in the magnetic brush can be kept below a certain level in the long-term use. Becomes

Further, since the toner discharged from the magnetic brush portion to the photosensitive drum 1 is in a very evenly dispersed state and the amount thereof is small, it does not substantially affect the next image exposure process. Absent. Further, no ghost image is generated due to the transfer residual toner pattern.

(3) Example of printer operation sequence (FIG. 2) Next, a normal copying machine paper, that is, generally called plain paper, has a basis weight of about 100 g / m ² or less, and is particularly surface-coated. An operation sequence when a non-transferred sheet is used as the transfer material P will be described.

A. Pre-multi-rotation step: This is the starting operation period (starting operation period, warming period) of the printer. When the main power switch is turned on, the main motor of the apparatus is driven to rotationally drive the photosensitive drum, and the preparatory operation of a predetermined process device is executed.

B. Pre-rotation step: A period during which the pre-printing operation is executed. The pre-multi-rotation step is executed subsequent to the pre-multi-rotation step when a print signal is input during the pre-multi-rotation step. When there is no print signal input, the main motor drive is temporarily stopped after the previous multi-rotation process is finished, the photosensitive drum rotation drive is stopped, and the printer is kept in a standby state until a print signal is input. . When the print signal is input, the pre-rotation process is executed.

C. Printing process (image forming process, image forming process): When the predetermined pre-rotation process is completed, the image forming process is subsequently performed on the rotating photosensitive drum, and the toner image formed on the surface of the rotating photosensitive drum is transferred to the transfer material. The toner image is fixed by the transfer and fixing means, and the image formed product is printed out. In the case of the continuous printing (continuous printing) mode, the above printing process is repeated for a predetermined set number of prints.

D. Paper interval process: In the continuous printing mode, in the transfer nip portion after the rear end of one transfer material passes through the transfer nip portion and before the front end of the next transfer material reaches the transfer nip portion. This is a non-sheet passing state period of the transferred material. While the area of the rotating photoconductor passing through the transfer nip during this period passes through the charging nip portion before that, the amplitude of the AC component of the charging bias is reduced and the transfer residual toner temporarily collected by the magnetic brush charging member. Is discharged to the surface of the rotating photosensitive drum.

E. Post-rotation step: This is a period in which the main motor continues to be driven for a while to rotate the photosensitive drum even after the last printing step of the material to be transferred is completed, and a predetermined post-operation is executed. Also in this period, the amplitude of the AC component of the charging bias is reduced in the same manner as in the sheet interval process, so that the transfer residual toner temporarily collected by the magnetic brush charging member is discharged to the surface of the rotating photosensitive drum.

F. Standby: When a predetermined post-rotation process is completed, driving of the main motor is stopped, rotation of the photosensitive drum is stopped, and the printer is kept in a standby state until the next print start signal is input.

In the case of printing only one sheet, after the printing is completed, the printer goes through a post-rotation step and goes into a standby state. When the print start signal is input in the standby state, the printer shifts to the pre-rotation step.

The printing step of c is the image formation, and the pre-multi-rotation step of a, the pre-rotation step of b, the sheet interval step of d, and the post-rotation step of e are non-image formation (non-image formation). become.

(4) Photosensitive Drum (FIG. 3) The photosensitive drum 1 of this embodiment is an OPC photosensitive member having a negative charging property and a charge injecting property as described above, and is formed on the drum base 1a made of aluminum having a diameter of 30 mm. ~ The fifth functional layer is provided in order from the bottom.

First layer 1b: an undercoat layer, which is a conductive layer having a thickness of about 20 μm, which is provided to smooth defects such as the aluminum drum substrate 1a and to prevent moire due to reflection of laser exposure. Is.

Second layer 1c: a positive charge injection preventing layer, which plays a role of preventing the positive charges injected from the aluminum drum substrate 1a from canceling out the negative charges charged on the surface of the photosensitive member, and the amylan resin and methoxy. The thickness is adjusted to about 10 ⁶ Ω · cm by methylated nylon, and the thickness is about 1
It is a medium resistance layer of μm.

Third layer 1d: a charge generation layer, which is a layer having a thickness of about 0.3 μm in which a disazo pigment is dispersed in a resin, and positive and negative charge pairs are generated by laser exposure.

Fourth layer 1e: a charge transport layer, which is a polycarbonate resin in which hydrazone is dispersed, and is a P-type semiconductor. Therefore, the negative charges charged on the surface of the photoconductor cannot move in this layer, and only the positive charges generated in the charge generation layer 1d can be transported to the surface of the photoconductor.

Fifth layer 1f: a charge injection layer, which has a particle size of 0.03 μm obtained by doping a photocurable acrylic resin as a binder with antimony as a light-transmissive conductive filler to reduce resistance (conductivity). Is a coating layer of about 3 μm of a material obtained by dispersing 1 g of tin oxide SnO ₂ ultrafine particles of 70% by weight in a resin. The electric resistance value of the charge injection layer 1f needs to be 1 × 10 ¹⁰ to 1 × 10 ¹⁴ Ω · cm, which is a condition that does not cause sufficient chargeability and image deletion. In this embodiment, a photosensitive drum having a surface resistance of 1 × 10 ¹¹ Ω · cm was used.

(5) Developing Device 4 (FIG. 7) As a toner developing method for an electrostatic latent image, the following a to d are generally used.
It is roughly divided into four types.

A. Non-magnetic toner is coated on the sleeve with a blade or the like, magnetic toner is coated by magnetic force and conveyed, and developed in a non-contact state with the photoconductor (one-component non-contact development). B. A method of developing the toner coated as described above in a state of being in contact with the photoreceptor (one-component contact development) c. A method in which a mixture of toner particles and a magnetic carrier is used as a developer, which is conveyed by magnetic force and is developed in a contact state by being exposed to light (two-component contact development). D. Method of developing the above two-component development with the agent in a non-contact state (two-component non-contact development) Among them, the two-component contact development method of c is often used from the viewpoint of high image quality and high stability. Has been.

The developing device 4 in this embodiment uses a mixture of a highly releasable spherical non-magnetic toner prepared by a polymerization method and a magnetic carrier (magnetic particles for development, developing carrier) as a developer. Two-component magnetic brush contact development in which a developer carrying member (developing member, developing device) is held as a magnetic brush layer by magnetic force and is conveyed to a developing section and brought into contact with the surface of a photosensitive drum to develop an electrostatic latent image as a toner image. System reversal developing device.

Reference numeral 4a is a developing container, 4b is a developing sleeve as a developer carrying member, 4c is a magnet (magnet roller) as a magnetic field generating means fixedly arranged in the developing sleeve 4b, and 4d is a developer on the surface of the developing sleeve. A developer layer thickness regulating blade for forming a thin layer of 4; 4e, a developer stirring and conveying screw; 4f: a developer container 4a
The component developer is a mixture of the non-magnetic toner t and the development carrier c as described above.

The developing sleeve 4b has a closest distance (gap) to the photosensitive drum 1 of about 5 at least during development.
The thin layer of the developer magnetic brush carried on the outer surface of the developing sleeve 4b is arranged so that the photosensitive drum 1 has a thickness of 00 μm.
Is set to touch the surface of. The contact nip portion m between the developer magnetic brush thin layer 4 and the photosensitive drum 1 is a developing area (developing portion).

The developing sleeve 4b is driven around the inside of the fixed magnet 4c in the counterclockwise direction of the arrow at a predetermined rotational speed, and the magnetic force of the fixed magnet 4c is applied to the outer surface of the sleeve in the developing container 4a by the magnetic force of the developer 4f (t + c). A brush is formed. The developer magnetic brush is conveyed along with the rotation of the sleeve 4b, is regulated by the blade 4d to a layer thickness, and is taken out of the developing container as a developer magnetic brush thin layer 4 having a predetermined layer thickness and conveyed to the developing section m to be exposed to light. The drum 1 comes into contact with the surface of the drum 1, and the sleeve 4b is subsequently rotated to be conveyed back into the developing container 4a.

A predetermined developing bias in which a DC component and an AC component are superposed is applied to the developing sleeve 4b by a developing bias applying power source E4. Regarding the developing characteristics in the present embodiment, fog occurs when the difference between the charging potential of the photosensitive drum 1 and the DC component value of the developing bias is 200 v or less, and when the difference is 350 v or more, the development carrier c adheres to the photosensitive drum 1. occured.

Regarding the toner concentration of the developer 4f (t + c) in the developing container 4a (mixing ratio with the developing carrier c), the toner is consumed for developing the electrostatic latent image and is successively consumed. When the toner concentration of the developer 4f in the developing container 4a is detected by a detection unit (not shown) and drops to a predetermined lower limit concentration, the toner t is replenished from the toner replenishing section 4g to the developer 4f in the developing container 4a. Developer 4f in developer container 4a
The toner replenishment control is performed so that the toner density is always kept within a predetermined allowable range.

(6) Magnetic Brush Charging Device (FIGS. 1 and 4) FIG. 4 is an enlarged cross-sectional model view of the magnetic brush charging device 2. The magnetic brush charging device 2 of this embodiment is roughly divided into a magnetic brush charging member (magnetic brush charging device) 2A as a contact charging member, a magnetic brush charging device 2A and conductive magnetic particles (charging carrier) 2d. The container (housing) 2B, the charging bias applying power source E4 for the magnetic brush charger 2A, and the like.

The magnetic brush charger 2A of this embodiment is a sleeve rotating type, and is a magnet roll (magnet).
2a, a non-magnetic stainless sleeve (referred to as an electrode sleeve, a conductive sleeve, a charging sleeve, etc.) 2b fitted on the magnet roll, and a magnetic force of the magnet roll 2a inside the sleeve on the outer peripheral surface of the sleeve 2b. It is composed of a magnetic brush portion 2c of magnetic particles 2d which are magnetically constrained and formed and held. The magnet roll 2a is a non-rotating fixed member, and the sleeve 2b is the magnet roll 2a.
a in the direction of arrow b by a drive system (not shown) at a predetermined peripheral speed, 225 mm / sec. It is driven to rotate at the peripheral speed of.

The sleeve 2b is arranged with a gap of about 500 μm with respect to the photosensitive drum 1 by means of a spacer roller or the like. 2e is attached to the container 2B,
This is a magnetic brush layer thickness regulating blade made of non-magnetic stainless steel, and is arranged so that the gap with the surface of the sleeve 2b is 900 μm. A part of the magnetic particles 2d in the container 2B is magnetically bound to the outer peripheral surface of the sleeve 2b by the magnetic force of the magnet roll 2a inside the sleeve and held as a magnetic brush portion 2c.

The magnetic brush portion 2c rotates together with the sleeve 2b in the same direction as the sleeve 2b as the sleeve 2b is driven to rotate. At this time, the layer thickness of the magnetic brush portion 2c is regulated to a uniform thickness by the blade 2c.

Since the thickness of the regulation layer of the magnetic brush portion 2c is larger than the gap between the sleeve 2b and the photosensitive drum 1 facing each other, the magnetic brush portion 2c is located in the photosensitive drum at the facing portion between the sleeve 2b and the photosensitive drum 1. A nip portion having a predetermined width is formed to contact with 1. This contact nip portion is the charging nip portion N.

Therefore, the rotary photosensitive drum 1 is rubbed at the charging nip portion N by the magnetic brush portion 2c which rotates as the sleeve 2b of the magnetic brush charger 2A rotates. In this case, in the charging nip portion N, the moving direction of the photosensitive drum 1 is opposite to the moving direction of the magnetic brush portion 2c, and the relative moving speed is high. A predetermined charging bias is applied from the power source E4 to the sleeve 2b and the magnetic brush layer thickness regulating blade 2e.

Then, the photosensitive drum 1 is rotationally driven, the sleeve 2b of the magnetic brush charger 2A is rotationally driven, and a predetermined charging bias is applied from the power source E4. In the case of the present embodiment, the contact charging process is performed uniformly by the injection charging method to a predetermined electrode and potential.

The magnet roll 2a fixedly arranged in the sleeve 2b has an angle θ between the sleeve 2b and the closest position c of the photosensitive drum 1 at the upstream side 20 in the photosensitive drum rotation direction.
It is hoped that the angle falls within the range of 10 ° from the downstream side. If it is downstream, magnetic particles are attracted to the main pole position, and the magnetic particles are likely to stay on the downstream side of the charging nip portion N in the rotation direction of the photosensitive drum. Transportability deteriorates, and retention easily occurs. Further, when there is no magnetic pole in the charging nip portion N, it is apparent that the magnetic particles are less constrained to the sleeve 2b and the magnetic particles are more likely to adhere to the photosensitive drum 1. The charging nip portion N described here
Indicates an area where the magnetic particles of the magnetic brush portion 2c are in contact with the photosensitive drum 1 during charging. In this embodiment, the upstream side 1
A magnetic pole of about 900 G was arranged at a position of 0 °.

Magnetic particles 2 constituting the magnetic brush portion 2c
d is a sintered ferromagnetic material (ferrite) in this embodiment.
Was used after reduction treatment, but other than that, resin and ferromagnetic powder were kneaded and molded into particles, or this was mixed with conductive carbon or the like to adjust the resistance value, or the surface The processed product can be used in the same manner.

The magnetic particles 2d of the magnetic brush portion 2c play a role of favorably injecting charges into the trap level on the surface of the photosensitive drum, and the charging current is concentrated on defects such as pinholes formed on the photosensitive drum. It must also have a role of preventing the electric current destruction of the charging member and the photoconductor caused by it. Therefore, the electric resistance value of the magnetic brush charger 2A is preferably from ^{1 × 10 4 Ω~1 × 10 9} Ω, and particularly preferably from ^{1 × 10 4 Ω~1 × 10 7} Ω. The electric resistance value of the magnetic brush charger 2A is 1 × 10 ⁴
If it is less than Ω, 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 2d is 1 ×.
10 is preferably a ^{4 Ω · cm~1 × 10 9 Ω} · cm, and more particularly preferably from ^{1 × 10 4 Ω · cm~1 ×} 10 7 Ω · cm.

The electric resistance value of the magnetic brush charger 2A used in this embodiment is 1 × 10 ⁶ Ω · cm, and the surface potential of the photosensitive drum 1 is changed by applying −700 V as the DC component of the charging bias. Also became -700v.

The volume resistance value of the magnetic particles 2d was measured by an apparatus whose outline is shown in FIG. That is, the cell A is filled with the magnetic particles 2d, the main electrode 17 and the upper electrode 18 are arranged in contact with the filled magnetic particles 14, and a voltage is applied between the electrodes 17 and 18 from the constant voltage power source 22. The current flowing at that time was measured by an ammeter 20. 19 is an insulator, 21 is a voltmeter, and 24 is a guide ring. The measurement conditions are 23 ° C., 65% environment, and filled magnetic particles 2 d
The contact area S with the cell of S = 2 cm ² , the thickness d = 1 mm, the load of the upper electrode 18 is 10 kg, the average particle size of the magnetic particles 14 with an applied voltage of 100 V and the peak in the particle size distribution measurement are in the range of 5 to 100 μm. Is preferable from the viewpoint of preventing charge deterioration due to contamination of the particle surface, and preventing magnetic particles from adhering to the surface of the photosensitive drum 1. The average particle size of the magnetic particles 14 is indicated by the maximum chord length in the horizontal direction. The measuring method is to randomly select 300 or more magnetic particles by a microscopic method, measure the diameters, and take the arithmetic mean.

The charging bias is applied to the sleeve 13 and the regulating blade 15 by the power source E2. In this embodiment, D
A bias in which the AC component is superimposed on the C component is used.

In the charging nip portion N, the magnetic brush portion 2c is constituted by rubbing the surface of the photosensitive drum 1 by the magnetic brush portion 2c of the magnetic brush charger 2A and applying a charging bias to the magnetic brush charger 2A. Charging magnetic particles 2
An electric charge is applied to the photosensitive drum 1 from d and the photosensitive drum 1
The surface is uniformly contact-charged to a predetermined polarity and potential.

In the case of this example, as described above, since the photosensitive drum 1 is provided with the charge injection layer 1f (FIG. 3) on its surface, the charging process of the photosensitive drum 1 is performed by the charge injection charging. . That is, the surface of the photosensitive drum 1 is charged to a potential corresponding to the DC component of the charging bias DC + AC. The sleeve 13 tends to have better charge uniformity as the rotation speed is higher.

Photosensitive drum 1 by magnetic brush charger 2A
The charge injection charging can be regarded as a series circuit of a resistor R and a capacitor C as shown in the equivalent circuit of FIG.
In the case of such a circuit, if the resistance value is r, the electrostatic capacity of the photoconductor is Cp, the applied voltage is V0, and the charging time (the time at which a point on the surface of the photosensitive drum passes through the charging nip portion N) is T0, The surface potential Vd of the photosensitive drum is expressed by equation (1).

[0102]

[Formula 1]

In the charging bias DC + AC, the DC component has the same value as the required surface potential of the photosensitive drum 1, that is, -700 v in this embodiment.

When the peak-to-peak voltage of the AC component at the time of image formation (at the time of image formation) is small, the effect of improving the charging uniformity and the rise of the potential is small, and when it is too large,
The retention of magnetic particles and the level of adhesion to the photosensitive drum deteriorate. Also, the upper and lower limits of Vpp are
It changes depending on the mixing amount of the transfer residual toner, the deterioration state of the magnetic particles, or the external environment. That is, when the amount of mixed toner is large, when the toner or external additive adheres to magnetic particles to deteriorate due to long-term paper feeding durability, or when the external environment is low humidity, the resistance value of the magnetic particles increases. Therefore, in order to perform proper charging, it is necessary to increase Vpp and increase the amount of current. However, the adhesion level of the magnetic particles to the photoconductor, which is considered as an adverse effect when Vpp is increased, tends to decrease as the resistance value increases. This is because the larger the amount of charges injected into the magnetic particles, the more easily they are affected by the potential difference between the applied bias and the photoconductor, and the more easily they are attached. That is, depending on the amount of current, the appropriate AC bias amplitude shifts in the same direction in both upper and lower limits. Therefore, conversely, when the resistance value is low, the adhesion of magnetic particles tends to increase, but a sufficient amount of current can be obtained for charging even if Vpp is set to a small value.

The frequency is preferably 100 Hz or more and 5000 Hz or less, and particularly preferably 500 Hz or more and 2000 Hz or less.
Below that, the adhesion of the magnetic particles to the photosensitive drum becomes worse,
The effect of improving the charging uniformity and the rising property of the potential becomes weak, and even if it is more than that, the effect of improving the charging uniformity and the rising property of the potential becomes difficult to be obtained. The AC waveform is preferably a rectangular wave, a triangular wave, a sin wave, or the like.

(7) Charging Bias Control Here, the characteristics of the bias applied to the magnetic brush charger 2A which is the contact charging member of the charging device 2 at the time of image formation and non-image formation will be described. Magnetic brush charger 2A for DC electric field only
When applied to the magnetic brush charger 2A, the discharge property of the toner mixed in the magnetic brush charger 2A to the photosensitive drum 1 is improved and the carrier is maintained for a long time without being deteriorated. However, the carrier is slightly deteriorated as compared with the case where the AC electric field is applied. Also, the charging property is reduced. For example, when the charging potential target is 700 V, the initial value is 700
Even if V is applied, the potential on the photosensitive drum is about 690V. As the durability of the magnetic brush 2c deteriorates, the potential further decreases, and the difference from the case where an AC electric field is applied gradually widens.

Therefore, in the non-image area, the developing bias D
The necessary reverse potential with respect to the C value is not maintained, resulting in fogging. Further, when the potential drop exceeds a certain value with respect to the initial set potential, the density of the output image exceeds the allowable level due to the fluctuation of the exposed portion potential. Therefore, it is desirable to continuously apply the AC bias with an appropriate amplitude to the paper passing portion through which the paper passes, and it is necessary to discharge the paper between the paper and during the forward and backward rotation.

However, when images having a relatively high image ratio (ratio of the print area in the image) are continuously formed, the transfer residue mixed per image is larger than the amount of toner discharged onto the surface of the photoconductor between the sheets. The amount of toner becomes larger, and the amount of toner in the magnetic brush may exceed a certain value in the short term. Or, if images with a partially high image ratio are continuously imaged, the amount of mixed toner in the magnetic brush in the region with a high image ratio increases and the partial toner amount in the magnetic brush exceeds a certain value. Occurs.

Further, in a high humidity environment in which the magnetic particles and the electric resistance of the photosensitive drum become low, the above-mentioned ΔV for discharging the toner becomes small, so that the toner discharged from the magnetic brush even if the above-mentioned toner discharging operation is performed. In some cases, the amount of toner decreases and the amount of toner in the magnetic brush exceeds a certain value. On the contrary, in a low humidity environment, since the magnetic particles and the electric resistance of the photosensitive drum are slightly increased, the above ΔV is increased and the amount of toner in the magnetic brush is decreased, but the magnetic brush for performing good charging in this environment. The toner allowance in is also reduced. Therefore, it is necessary to keep the amount of toner in the magnetic brush below the allowable amount in any environment.

In order to solve these problems, Japanese Patent Laid-Open No.
As disclosed in Japanese Patent Application Laid-Open No. 0-314993, Japanese Patent Application Laid-Open No. 2000-75602, and the like, stable images are obtained by performing control in which the image ratio and environment are predicted.

However, as described above, when there is toner having almost no polarity or reversal toner on the photosensitive drum, in the magnetic brush 2c of the magnetic brush body 2A or in the developing device, and when it reaches the transfer portion, There is a problem that pressure is transferred to thick paper or coated paper. Since the non-polarized toner and the reversal toner existing on the photosensitive drum are rarely removed by the action of the transfer bias or the developing bias, they are often accumulated inside the magnetic brush 2c of the magnetic brush body 2A. Magnetic brush body electricity 2A
The reverse toner and the normal polarity toner both exist as toner discharged from the photosensitive drum 1 onto the photosensitive drum 1. However, if the reverse toner is discharged during image formation, fogging occurs on thick paper or coated paper. It has been found that this occurs even in the presence of a very small amount of reverse toner.

Therefore, when using thick paper or coated paper, it is necessary to restore even a slight amount of the reversal toner to the normal polarity, which extends the rotation time of the magnetic brush to promote contact triboelectric charging between the reversal toner and the magnetic carrier. It is realized by doing.

Therefore, in this embodiment, the charging AC bias at the time of image formation is set to 700 V, and the set value is 400 at the time of non-image formation,
In a normal mode (plain paper mode) using a so-called plain paper having a basis weight of about 100 g / m ² or less,
Every time the count value of the sheet passing cumulative number counting means reaches 500 sheets in A4 size conversion, the mixed toner discharge charging operation of AC400V is performed for 60 seconds continuously after the post-rotation operation, while the thick paper or coated paper mode ( Special paper mode)
When is selected, the discharging operation is performed for 60 seconds for every 100 sheets.

That is, the selection switching between the plain paper mode and the special paper mode is performed by the operator operating the mode selection switching key 101 (FIG. 1) provided on the operation unit of the printer. The mode selection switching key 101 is a selection means for selecting a recording material to be used.

According to the mode selected by the mode selection switching key 100, that is, when the selected recording material is plain paper, the printer control unit 100 has a paper passing total provided in the printer control unit 100. Every time the count value of the number counting unit (count function unit) 102 reaches 500 sheets in A4 size conversion, the mixed toner discharge charging operation of AC400V is controlled so as to be continuously performed for 60 seconds after the post-rotation operation. When the paper is a special paper such as paper, AC400V is supplied every time the count value of the cumulative sheet passing number counting means 102 reaches 100 sheets in A4 size conversion.
The charging operation for discharging the mixed toner is controlled so that the post-rotation operation is continuously performed for 60 seconds.

Selection means 101 for selecting the recording material
Can be an automatic paper type detection device provided in the recording material conveyance path upstream of the transfer nip portion T in the recording material conveyance direction. In this case, the recording material used by the printer is usually used. Whether it is paper or special paper such as thick paper or coated paper is automatically detected by the above-described paper type automatic detection device, and the printer control unit 100 responds to the magnetic brush charger 2A as described above according to the detection information. The mixed toner discharge charging operation is executed by changing the opportunity and time for applying the developer discharge bias.

The mixed toner discharge charging operation described above is not limited to the setting for continuously performing the post-rotation operation in the present embodiment, and may be performed during the pre-multi-rotation operation, the pre-rotation operation, the paper interval, the standby time, or the like. You can also set it to run. Further, it is needless to say that the execution time of the charging operation for discharging the mixed toner can be arbitrarily set.

FIG. 8 shows a case where a continuous paper feed is performed using a document having an image ratio of 10%, and a thick paper having a basis weight of 207.5 g is fed in such a setting that the mixed toner discharging operation is performed once for every 500 sheets. Change in fogging reflectance due to reversal toner (A in the figure) when fusing, and fogging due to reversal toner when a thick paper having a basis weight of 207.5 g is set to be ejected once per 100 sheets Transition of reflectance (B in the figure)
Is shown.

Here, it is judged whether or not the fog on the paper is the toner of the normal polarity, but it is assumed that the reverse toner is hardly transferred on the plain paper, and the fog on the plain paper is the fog of the normal polarity. The fogging on the thick paper when no fogging occurs is regarded as the fogging by the reverse toner, and if the fogging occurs on the plain paper, the fogging amount of the regular polarity toner is subtracted and converted.

From this result, the magnetic brush charger 2A, which is a contact charging member, is increased by increasing the number of discharge operations.
It is possible to increase the probability that the reversal toner and the uncharged toner mixed in the magnetic brush portion 2c are reversed to the normal polarity, and it is possible to reduce the reversal toner fog on the thick paper.

(Second Embodiment) In this embodiment, the timing of the discharging operation is determined according to the calculated value of the integrated image ratio. Specifically, the printer control unit 100
Is an integrated image ratio calculation unit (integrated image ratio calculation function unit) 10 for calculating an integrated image ratio, that is, an image ratio × the number of sheets.
In the case of a plain paper having a value of 2, when the value exceeds 2000%, the mixed toner discharge charging operation is performed for 60 seconds to obtain an image without fog. On the other hand, in the case of thick paper, the charging operation for discharging the mixed toner is performed for 60 seconds when it exceeds 500%. The integrated image ratio calculation means 102 is reset at 2000% and restarts the calculation.

FIG. 9 shows a case in which a continuous toner feed is performed using a document having an image ratio of 10%, a mixed toner discharge operation is performed once each time the integrated image ratio exceeds 2000%, and the basis weight is 105 g. The transition of the fog reflectance due to the reversal toner when the coated paper is passed (A in the figure), and the mixed toner discharge operation is performed once every time the integrated image ratio exceeds 500%, and the basis weight of 207.5 g is set. 7 shows the transition of the fog reflectance (B in the figure) due to the reversal toner when a thick paper is passed.

From this result, it is possible to increase the probability that the reversed toner or uncharged toner mixed in the magnetic particles is reversed to the normal polarity by increasing the number of ejection operations, and it is possible to reduce the reversed toner fog on the thick paper. .

An AC bias may be used during image formation, and a DC bias may be used during non-image formation. Although the ejection property at the time of non-image formation is improved, regular fog is apt to occur at the time of non-image formation in the latter half of the durability, and the fog toner that has passed through the transfer portion causes stains on the charger. Therefore, a slight AC
It is better to apply a bias.

An environment detecting means is provided, and a predetermined threshold value for determining at least one of an opportunity and a time for applying the discharge bias to the charging member with respect to the image ratio cumulative calculation value depends on an output from the environment detecting means. You can also let it. More specifically, the absolute water content converted from the temperature and humidity sensor output values is divided into the following 7 areas, and for each environment,
One discharge opportunity is provided for the number shown on the right.

Environment 1 0 to 2 (less than) (g / kg dry air) 1000 sheets Environment 2 2 to 6 (less than) (g / kg dry air) 800 sheets Environment 3 6 to 9 (less than) (g / kg dry) air) 650 sheets Environment 4 9 ~ 13 (less) (g / kg dry air) 500 sheets Environment 5 13 ~ 16 (less) (g / kg dry air) 300 sheets Environment 6 16 ~ 20 (less) (g / kg Dry air) 200V Environment 7 20 or more (g / kg dry air) 100V Also, it has a means to count the total number of image formations, and the opportunity and time to apply the discharge bias to the charging member for the cumulative image ratio calculation value. It is also possible to make the predetermined threshold value for determining at least one of the above-mentioned values dependent on the total number of image formations. More specifically, in the initial stage, the discharge extension mode is set to once every 500 sheets, whereas when the number of commuted sheets exceeds 30,000, it is set to 1 in 300 sheets.
The mode of times, etc.

Further, a predetermined threshold value for determining at least one of the opportunity and time for applying the discharge bias to the charging member with respect to the cumulative image ratio calculation value, at least one of the total number of image formations or the total toner consumption amount. Can be made dependent on. More specifically, it is usually once every 500 sheets, but once every 100 sheets, the cumulative image ratio of the past 100 sheets is read, and if it is 2000% or more, the post-rotation is extended at that time. is there.

Further, the amplitude of the AC component of the discharge bias is set to be smaller than the amplitude of the AC component of the charging application bias during image formation. More specifically, the amplitude is 600 V when forming an image and 400 V when ejecting the image.

(Others) 1) Although the magnetic brush injection charging device has been described as an example of the contact charging device in the above embodiment, various other contact charging devices can be used, and the discharging operation condition is that the charging high voltage is A.
Obviously, all alternatives are possible for the C component, frequency, waveform, timing, etc.

2) The toner developing system and means for the electrostatic latent image are arbitrary. The reversal development method or the regular development method may be used.

3) The transfer means is not limited to the belt transfer of the embodiment, but may be roller transfer, blade transfer, or other contact transfer charging system, or a non-contact transfer charging system using a corona charger.

4) The present invention can be applied not only to monochromatic image formation but also to an image forming apparatus for forming multicolor or full color images by multiple transfer or the like.

5) It is preferable that the photoconductor as an image bearing member has a low resistance layer having a surface resistance of 10 ⁹ to 10 ¹⁴ Ω · cm from the viewpoint of realizing charge injection charging and preventing ozone generation. Organic photoreceptors other than the above may be used. That is, the contact charging may be either a charge injection charging method or a contact charging system in which the discharge phenomenon is dominant.

6) The image forming process of the image forming apparatus is not limited to that of the embodiment and is arbitrary. Also, other auxiliary process equipment may be added if necessary.

7) The image exposure means for forming an electrostatic latent image is not limited to the laser scanning exposure means for forming a digital latent image as in the embodiment, but a normal analog image. Other light-emitting elements such as exposure and LEDs may be used, or a combination of a light-emitting element such as a fluorescent lamp and a liquid crystal shutter may be used as long as it can form an electrostatic latent image corresponding to image information.

The image carrier may be an electrostatic recording dielectric or the like. In this case, after the primary surface of the dielectric surface is uniformly charged to a predetermined polarity and potential, the target electrostatic latent image is written and formed by selectively neutralizing with a neutralizing means such as a neutralizing needle head or an electron gun.

8) An optional process cartridge such as an image carrier, a charging device, a developing device and the like can be collectively attached to and detached from the main body of the image forming apparatus so that the process cartridge can be attached and detached.

[0138]

According to the present invention, in a cleanerless, contact charging type image forming apparatus, when thick paper or coated paper is used, by increasing the discharge operation time or timing, the reverse toner or polarity specific to the thick paper or coated paper can be obtained. It is possible to prevent fogging due to toner having no toner.

[Brief description of drawings]

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

FIG. 2 is a process explanatory diagram illustrating sequence control in the embodiment.

FIG. 3 is a schematic diagram of the configuration of the photoconductor used in the examples.

FIG. 4 is a schematic configuration diagram of a charging device used in an example.

FIG. 5 is an equivalent circuit diagram of the charging circuit.

FIG. 6 is an explanatory diagram of a procedure for measuring the electric resistance value (volume resistance value) of magnetic particles (charged carriers).

FIG. 7 is a schematic configuration diagram of a developing device used in the examples.

FIG. 8 is an explanatory diagram showing a transition of the fogging reflectance on paper during paper running durability in an example.

FIG. 9 is an explanatory diagram showing changes in charging potential of magnetic brush injection charging.

[Explanation of symbols]

1 .. Photosensitive drum (image carrier) 2..Charging device (magnetic brush injection charging device) 3 ... Image exposure device 4 ... Developer 5 ... Transfer device 6 ... Paper cassette P ... Recording material 11 ... Fusing device 12 ... Conductive brush

Continued front page    F-term (reference) 2H077 AA37 AB02 AB14 AB15 AC02                       AC16 AD06 AD13 AD18 AD31                       AD36 BA07 DA10 DB03 DB12                       DB14 EA01 FA19 GA01 GA17                 2H134 GA01 GB02 HF13 KH01 KH11                       KH17 MA02 MA11                 2H200 FA03 FA14 FA17 FA18 GA23                       GA34 GA45 GA53 GA59 GB22                       GB37 HA03 HA28 HA29 HA30                       HB07 HB12 HB14 HB17 HB22                       HB48 JA02 JB06 NA02 NA06                       PA03 PA19 PA20 PB17 PB27                       PB28 PB33 PB34

Claims (12)

[Claims] 1. A charging member is brought into contact with the image carrier, and a bias voltage is applied to the charging member to charge the image carrier, a latent image is formed on the charged surface, and the latent image is developed. The toner image is developed by a means, and after the toner image is transferred to the recording material, the residual toner particles remaining on the image carrier are once collected by the charging member that contacts the image carrier, and the collected toner particles are charged. In the image forming apparatus, which returns the toner particles from the charging member to the image carrier, the developing unit also serves as a cleaning unit that collects the toner particles returned from the charging member to the image carrier, and returns the collected toner particles from the charging member to the image carrier. Has a developer return bias different from that at the time of image formation applied to the charging member, and has a selection unit for selecting a recording material to be used, and the developer return bias according to the selected recording material. Apply That opportunity or time, or the image forming apparatus characterized by changing the opportunity and time. 2. The image forming apparatus according to claim 1, wherein the charging member comprises magnetic particles and a magnetic particle carrier. 3. The image forming apparatus according to claim 1, wherein the image carrier is a photoconductor having a charge injection layer on the surface thereof. 4. The image forming apparatus according to claim 1, wherein the charging bias conditions applied to the image forming area and the non-image forming area are different. 5. The image forming apparatus according to claim 1, wherein the charging bias is a DC bias or a bias in which an AC bias is superimposed on a DC bias. 6. A predetermined threshold value for determining at least one of the opportunity and time for applying the discharge bias to the charging member is provided by means of calculating an image ratio, and is determined by an image ratio cumulative calculation value during continuous image formation. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus. 7. An environment detecting means is provided, and a predetermined threshold value for determining at least one of an opportunity and a time to apply the discharge bias to the charging member is output from the environment detecting means with respect to the image ratio cumulative calculated value. The image forming apparatus according to any one of claims 1 to 6, wherein the image forming apparatus is dependent on the following. 8. A means for counting the total number of image formations, wherein a predetermined threshold value for determining at least one of the opportunity and the time to apply the discharge bias to the charging member is added to the cumulative image ratio calculation value. The image forming apparatus according to claim 1, wherein the image forming apparatus depends on the number of images to be formed. 9. A predetermined threshold value for determining at least one of an opportunity and a time to apply a discharge bias to a charging member with respect to an image ratio cumulative calculation value, at least one of a total number of image formations or a total toner consumption amount. 9. The image forming apparatus according to claim 1, wherein the image forming apparatus is dependent on 10. The image forming apparatus according to claim 1, wherein the amplitude of the AC component of the discharge bias is smaller than the amplitude of the AC component of the charging application bias during image formation. 11. The image forming apparatus according to claim 1, wherein the process speed is changed according to the selected recording material. 12. The image according to claim 1, wherein the means for forming a latent image on the charged surface of the image carrier is image exposure means for the charged surface of the image carrier. Forming equipment.