JP2003241511A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce phenomena of forming a fogging defective image occurring when a toner of reverse polarity which is remaining developer after transferring is contacted with and transferred to a transfer material depending on the kind of the transfer material in an image forming apparatus which performs cleaning operation simultaneously with development. <P>SOLUTION: The image forming apparatus is provided with: a charging device 2 for performing the primary charging of an image carrier; an electrostatic latent image forming means 3 for forming an electrostatic latent image; a development apparatus 4 which houses the developer 4f including toner and developing the electrostatic latent image; and a transferring apparatus 5 for transferring the developer image to a transfer material P. The remaining developer after transferring which remains on the surface of the image carrier 1 after transferring by the transferring apparatus 5 is collected to the charging member 2A of the charging device 2, the developer collected to the charging member 2A is next discharged onto the image carrier and the developer discharged onto image carrier 1 is collected by the development apparatus 4. A plurality of image forming modes can be switched and toner density in the developer 4f housed in the development apparatus 4 is changed on the basis of the image forming mode. <P>COPYRIGHT: (C)2003,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, a developer image is formed on the image bearing member, the transfer residual developer not transferred by the transfer device and remaining on the surface of the image bearing member is removed, and a cleaning device (cleaner) is attached to the image bearing member. An image of a copying machine, a printer, etc., in which the residual transfer developer is temporarily collected by a charging member that is in contact with the image carrier without being provided, and the collected developer is discharged from the charging member and collected again by the developing device. Forming apparatus

[0003]

2. Description of the Related Art In recent years, electrophotographic and electrostatic recording type image forming apparatuses have been downsized, but the means and devices for image forming processes such as charging, exposure, development, transfer, fixing and cleaning have been developed. There is a limit to the overall size reduction of the image forming apparatus if only each of them is downsized.

Therefore, a blade-shaped or brush-shaped cleaning device (cleaner) facing the image carrier is removed, and the untransferred residual developer on the photoconductor, which is the image carrier, is "cleaned simultaneously with development" by the developing device. An image forming apparatus of a “cleanerless process” has been developed, which has a device configuration in which the photosensitive member is removed from the photoconductor and is collected and reused in a developing device.

However, in the cleanerless process, when the photosensitive member is charged from above the transfer residual developer, the potential of the photosensitive member tends to become non-uniform.

Therefore, first, a charging device for charging an image carrier, particularly a contact charging device, in an image forming apparatus adopting such a cleanerless process will be described.

In an electrophotographic or electrostatic recording type image forming apparatus, a charging device for charging an image carrier such as an electrophotographic photosensitive member or an electrostatic recording dielectric, and other charged members to a predetermined polarity and potential. For this, a corona charging device has been generally used.

Here, the corona charging device is arranged so as to face the photoconductor in a non-contact manner, and the photoconductor surface is exposed to the corona emitted from the corona charging device so that the photoconductor surface has a predetermined polarity.
It is charged to a potential.

However, in recent years, a voltage (charging bias) is applied to the photoconductor as the member to be charged because it has advantages such as low ozone and low power as compared with the above-mentioned non-contact type corona charging device. A contact type charging device for bringing a charging member (contact charging member) into contact and charging the surface of the photoconductor to a predetermined polarity and potential, that is, a contact charging device has been put into practical use.

In particular, a roller charging type roller charging device using a conductive roller (charging roller) as the contact charging member is preferably used from the viewpoint of charging stability.

In addition, contact charging can be performed by using conductive fibers formed in a brush shape (fur brush charging member, charging fur brush), conductive rubber blades made of conductive rubber in a blade shape (charging blade), and the like. It is preferably used as a member.

Further, as the contact charging member, a magnetic brush charging member (charging magnetic brush) having a magnetic brush portion in which magnetic particles are magnetically bound to a carrier is used, and the magnetic brush portion is brought into contact with the photosensitive member. A charging type magnetic brush charging device is also preferably used from the viewpoint of charging stability.

The magnetic brush charging device has magnetic brush portions formed by magnetically binding conductive magnetic particles directly to a magnet or to a magnetic particle carrier such as a sleeve containing the magnet. It is stopped or rotated to bring the magnetic brush portion into contact with the photoconductor, and a voltage is applied to the photoconductor to start charging the photoconductor.

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

The corona charging system is a system in which the surface of the photoconductor is charged by 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 charging bias having a voltage higher than the charging potential to the contact charging member. In addition, the corona charging type of this contact charging device generates a discharge product although the amount of generation is significantly smaller than that of a so-called non-contact type corona charging device.

The charge injection charging system is a system in which the surface of the photoconductor has charge injection charging property, and the surface of the photoconductor is charged by directly injecting charges from the contact charging member to the photoconductor.

That is, the contact charging member of medium resistance is in contact with the surface of the photosensitive member and does not go through the discharge phenomenon, that is, basically no discharge is used, and the charge is directly injected into the surface of the photosensitive member. Therefore, even when the applied voltage of the charging bias of the charging bias 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.

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, in this charge injection charging system, it is necessary to make the contact charging member more dense, have a large speed difference from the photosensitive member, and contact the photosensitive member more frequently.

As a charging device using a contact charging member suitable for such a condition, a magnetic brush charging device, in particular, can perform stable charging.

The charge injection charging mechanism by the magnetic brush charging device can be regarded as equivalent to a series circuit mechanism of a resistor and a capacitor, as described later with reference to FIG. For example, a charge injection layer containing conductive fine particles such as SnO ₂ is provided on the surface of the photoconductor, and these layers function as electrodes of the capacitor. By bringing a conductive contact charging member into contact therewith and applying a charging bias, it is possible to inject charges into the electrodes in the same manner as an ordinary capacitor. In an ideal charging process, it is considered that these capacitors are charged during the time when a certain point on the surface of the photoconductor is in contact with the magnetic brush (charging nip x peripheral speed of the photoconductor), and thus the photoconductor surface potential. Becomes almost the same value as the applied voltage.

In the charge injection charging method (injection charging method) using such a charging mechanism, a charge bias is applied to a conductive contact member to inject charges into the trap level on the surface of the photoconductor. A method of contact charging the photoreceptor,
Further, as the photoconductor, the Sn
If an amorphous silicon photoconductor or the like is used in addition to the one having a surface layer (electron injection layer) in which conductive fine particles such as O ₂ are dispersed, a charge substantially equal to the DC component of the bias applied to the contact charging member is used. It is possible to obtain a potential on the surface of the body to be charged (Japanese Patent Laid-Open No. 6-3921).

The injection charging method is not only environmentally dependent, but does not use discharge, so that the applied voltage to the contact charging member is about the same as the photoconductor potential, and it has the advantage that ozone is not generated. Therefore, it becomes possible to perform ozoneless charging with low power consumption.

Next, the cleaning operation for removing the developer remaining on the image carrier using such a charging device, especially the toner recycling process will be described.

In the cleanerless process, when the photosensitive member is charged from above the transfer residual developer, the potential of the photosensitive member tends to become non-uniform. However, the injection charging type contact charging device described above is used. Then the problem is solved.

By using an injection charging type contact charging device, particularly a magnetic brush charging device, the residual transfer developer is scraped off by the above-mentioned conductive contact charging member and at the same time the photoreceptor is charged during image formation, thereby forming an image. At other times, the transfer residual developer accumulated on the contact charging member can be discharged onto the photoconductor and the toner can be collected again by the developing device.

Therefore, since the photoconductor is not charged from above the transfer residual developer, the potential of the photoconductor can be made uniform, and a so-called ghost image in which the image of the previous circumference appears on the next circumference of the photoconductor is called a ghost. It is possible to realize a cleanerless process that does not occur.

In addition to these brush charging devices, those having conductive fibers formed in a brush shape and provided (a fur brush charging member, a charging fur brush, etc. are also preferably used as the contact charging member, As a conductive contact charging member of, the fur brush deteriorates the charging property when the hair brush collapses due to long-term use or long-term storage, whereas the magnetic brush charging member using magnetic particles has such a problem. Since the phenomenon does not occur and the density of the magnetic particles contacting the photosensitive member is higher than that of the fur brush, stable and uniform charging can be performed.

Further, since the magnetic brush has a larger surface area and a larger amount of the transfer residual developer is retained, it is advantageous for the cleanerless process.

However, in the above-mentioned cleanerless process, when the charge polarity of the transfer residual developer is the same as the charge polarity of the photoconductor, that is, when it is the normal polarity (here, the negative polarity), the charge injection system charge is used. It becomes difficult to collect them by the apparatus, and the transfer residual developer tends to slip through the charging apparatus, resulting in ghost.

As a method of preventing this, the polarity of the transfer residual developer is set between the transfer device and the charging device around the photosensitive member, that is, in the portion where the transfer residual developer of the image on the front side is mounted. There is known a method of providing a developer polarity adjusting member (hereinafter referred to as “adjusting member”) for reversing (here, to the positive electrode).

Examples of the adjusting member include a conductive brush and an elastic body, which are brought into contact with the photosensitive member and a voltage having a reverse polarity (positive electrode) to the charging polarity in the charging device is applied to transfer the brush. The residual developer is reversed to the opposite polarity (positive electrode). For example, when a conductive brush (adjusting brush) is used as the adjusting member, a transfer residual developer having a normal polarity (negative electrode) is temporarily taken into the adjusting brush to have a reverse polarity (positive electrode).
Since it is charged on the surface of the photosensitive member and then discharged onto the photosensitive member again, it is possible to efficiently prevent the generation of ghost. The transfer residual developer discharged from the adjusting brush to the photoconductor is then taken into the injection charging device, but the toner (collected toner) in the taken transfer residual developer is again of normal polarity due to frictional charging with the magnetic particles. (Negative electrode) is charged.

The collected toner in the charging device, which is charged to the normal polarity (negative polarity), is discharged to the surface of the photoconductor due to the potential difference between the DC component value of the charging bias and the photoconductor potential.

Here, the potential difference between the charging bias DC component value and the photoconductor potential depends on the AC peak-to-peak voltage superimposed on the charging bias. The larger the peak-to-peak voltage, the convergence at the photoconductor potential. Becomes better and the potential difference becomes smaller.

By utilizing this property, the peak-to-peak voltage is increased during the image formation so as not to affect the image, the toner in the charging device is not discharged, and the non-image formation such as the sheet interval or the post-rotation is performed. It is possible to keep the amount of toner in the charging device low by positively ejecting the toner in the charging device by reducing the peak-to-peak voltage.

[0035]

However, in the above-mentioned cleanerless process, even in the magnetic brush injection charging, which is the most excellent in controlling the amount of charge of the collected toner, the charging polarity of the collected toner in the charging device is normally adjusted by frictional charging. It is difficult to keep the polarity, and there is some recovered toner of opposite polarity.

In particular, when the transfer residual developer after being charged to the opposite polarity by the adjusting member is collected, the opposite polarity toner is likely to exist.

Since the collected toner having the opposite polarity has the opposite polarity to the charged photoconductor potential, it is attached to the surface of the photoconductor by an electric force during image formation in which the charge potential converges. As shown in FIG. 7 which is an explanatory diagram showing the surface potential of the photoconductor and the behavior of the toner, the toner T1 having the opposite polarity attached to the region of the charged potential of the photoconductor is directed toward the photoconductor even when the developing bias is applied in the developing device. Because of the force of, it is not collected and reaches the transfer part as it is.

Since the reverse polarity toner T1 has the same polarity as the transfer bias, it hardly appears on the transfer material and is often collected again by the charging device. However, due to the surface property (smoothness, adhesion) of the transfer material. Causes the reverse polarity toner to be transferred onto the transfer material against the electric force (contact transfer). In this case, a fog defect image in which toner is present on the white background portion occurs.

Examples of such a transfer material having a high surface adhesiveness and easily causing fog include a coated paper having a Bekk smoothness of 100 seconds or more. Further, when using a thick paper having a large heat capacity and a basis weight of 100 to 160 g / m ² , etc., the fixing speed is made slower than usual to secure the fixing property, and the transfer speed is also made slower. Reverse polarity toner becomes easier to contact and transfer.

Since the reverse polarity toner is collected again by the charging device unless it is transferred to the transfer material, it tends to accumulate with the number of times of image formation (the number of prints), and the transfer material which transfers the reverse polarity toner by contact is used. The fog becomes very bad when it is on, and the tendency becomes stronger with the number of image formations.

There is also a method of strengthening the agitation of the magnetic particles and the toner in the charging device in order to eliminate the opposite polarity toner in the charging device. It becomes difficult to realize a certain space saving.

Reverse polarity toner is also accumulated in the developing device. Although they can be triboelectrically charged to a regular polarity in the initial stage when the developer has a high charging performance, their charging performance is deteriorated by long-term use, and the reverse polarity toner further increases in the developing device.

Further, in a device having a plurality of process speeds, the driving of the developing device also changes according to the process speed. Therefore, if the driving of the developing device becomes slower than usual, the stirring ability of the developing device will decrease. Reverse polarity toner is more likely to be accumulated. To avoid this, if the driving of the developing device is made constant without depending on the process speed, the deterioration of the developer becomes severe and the life is shortened.

Therefore, an object of the present invention is to provide a cleanerless image forming apparatus which is not provided with a cleaning device for the image bearing member, and which takes the residual transfer developer into the charging device and discharges it again onto the image bearing member. In an image forming apparatus that collects in a developing device and performs a cleaning simultaneous cleaning operation, depending on the type of transfer material, it may have a polarity opposite to the charging polarity of the image carrier, which is the transfer residual developer accumulated in the charging device and the developing device. An object of the present invention is to provide an image forming apparatus in which a phenomenon in which a fog-defective image is formed by transferring a charged opposite polarity toner to a transfer material by contact transfer is reduced.

[0045]

The above object can be achieved by an image forming apparatus according to the present invention. In summary, the present invention is
A charging device that has an image carrier and a charging member that is in contact with the image carrier, applies a charging bias to the charging member to charge the image carrier, and the image carrier charged by the charging device. An electrostatic latent image forming unit that forms an electrostatic latent image on a body and a developer containing toner are contained, and the electrostatic latent image formed on the image carrier is developed by the developer. In an image forming apparatus including a developing device that forms a developer image and a transfer device that transfers the developer image on the surface of the image carrier to a transfer medium, the image remains on the surface of the image carrier after the transfer by the transfer device. The transfer residual developer is collected by the charging member of the charging device, the transfer residual developer collected by the charging member is discharged onto the image carrier, and then the transfer is discharged onto the image carrier. Image forming apparatus for collecting residual developer by the developing device There is provided a plurality of image forming modes that can be switched, and the toner concentration in the developer contained in the developing device is changed based on the image forming modes. .

According to one embodiment of the present invention, the image forming mode is switched depending on the type of the transfer material, and at that time, the image forming mode is such that the toner density of the developer contained in the developing device is predetermined. When the toner density is used, the first image forming mode in which the transfer residual developer on the image carrier does not adhere and the transfer material on which the residual transfer developer on the image carrier adheres are used. Second
When the image forming mode is switched and the first image forming mode is selected, the toner density in the developer included in the developing device is set to the predetermined toner density, and when the second image forming mode is selected. The toner concentration in the developer of the developing device is lower than the predetermined toner concentration.

According to another embodiment of the present invention, the second
When the image forming mode is selected, before the image is formed on the transfer material, the developer latent image is developed by the developing device and transferred to a member other than the transfer material to form the developer image. Formed on the body, the toner in the developing device is consumed, and the toner concentration of the developer in the developing device is lowered.

According to another aspect of the present invention, it is preferable that the charging member has magnetic particles and a magnetic particle carrier, the image carrier has charge injection charging property, and further, the image carrier. It is preferable that the body is an electrophotographic photoreceptor having on its surface a charge injection layer in which conductive fine particles are dispersed in an insulating binder.

[0049]

BEST MODE FOR CARRYING OUT THE INVENTION An image forming apparatus according to the present invention will be described below in more detail with reference to the drawings.

Embodiment 1 First, the overall structure of the image forming apparatus according to the present invention will be described. FIG. 1 is a schematic configuration diagram of this embodiment of an image forming apparatus according to the present invention. The image forming apparatus of this embodiment is a laser beam printer using a transfer type electrophotographic process, a charge injection charging system, and a cleanerless process.

The drum-shaped photosensitive member (photosensitive drum) 1 of this embodiment, which is a rotary drum type electrophotographic photosensitive member as an image bearing member, has a negative charging property and a charge injection charging property, as described later in detail. OPC photoconductor (organic photoconductive photoconductor) of 150 mm / sec. Is driven to rotate at the process speed (peripheral speed).

As a charging device for uniformly charging the surface of the photosensitive drum 1 to a predetermined polarity and potential, a magnetic brush charging device 2 which is a contact charging device is provided in the present embodiment, and the rotating photosensitive drum 1 is provided. Electric charges are directly injected into the surface of No. 2 by the magnetic brush charging device 2 and uniformly charged to about −700 V by the charge injection charging method.

In the present specification, this charging device 2
In order to distinguish the charging of the photosensitive drum 1 before the formation of the electrostatic latent image at the time of image formation from the charging of the residual developer at the time of non-image formation by the other apparatus, it is referred to as “primary charging”.

As the image information exposure means (exposure device) which is the electrostatic latent image forming means, the laser beam scanner 3 is provided in this embodiment. The laser beam scanner 3 has a semiconductor laser, a polygon mirror, an F-θ lens, and the like, and is input 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 beam scanning exposure, an electrostatic latent image corresponding to desired image information is formed on the peripheral surface of the rotary photosensitive drum 1.

In this embodiment, as the developer 4f, a highly releasable spherical toner (toner) prepared by a polymerization method so that the toner remaining on the photosensitive drum 1 as a transfer residual developer is reduced, and a magnetic material. Two-component contact developing type developing device 4 containing a two-component developer in which a carrier is mixed.
Is equipped with. Then, the developing device 4 reversely develops the electrostatic latent image formed on the surface of the rotary photosensitive drum 1 as a developer image (toner image).

A transfer device 5 is disposed below the photosensitive drum 1. In the present embodiment, the transfer device 5 is of a transfer belt type and is a transfer belt (for example, an endless transfer material carrier). Polyimide belt with a film thickness of 75 μm) 5a
However, it is suspended around the drive roller 5b and the driven roller 5c, and is rotated in the forward direction of the rotation direction of the photosensitive drum 1 at substantially the same peripheral speed as the rotation speed of the photosensitive drum 1. A conductive blade 5d, which constitutes a transfer charging device and is arranged inside the transfer belt 5a, 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. Is forming. Conductive blade 5d
Together with the transfer bias applying power source E5 constitute a transfer charging device.

A transfer material P such as paper is stacked and stored in the paper feed cassette 6, and one transfer material P stacked and stored in the paper feed cassette 6 is separately fed by driving the paper feed roller 7. The sheet is fed to the transfer nip portion t between the rotary photosensitive drum 1 and the transfer belt 5a of the transfer device 5 at a predetermined control timing through a sheet path 9 including a conveyance roller 8 and the like.

The transfer material P fed to the transfer nip portion t is
The rotary photosensitive drum 1 and the transfer belt 5a are nipped and conveyed, and a predetermined transfer bias is applied to the conductive blade 5d from the transfer bias applying power source E5 during that time, and the transfer material P is transferred.
From the back surface of the toner is charged with the opposite polarity to the toner in the developer. As a result, the toner image on the surface of the rotating photosensitive drum 1 is sequentially electrostatically transferred to the surface of the transfer material P passing through the transfer nip portion t.

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

Since the printer of this embodiment employs the cleanerless process, the transfer nip portion t
Therefore, a dedicated cleaning device (cleaner) for removing the transfer residual developer remaining on the surface of the rotary photosensitive drum 1 without being transferred to the transfer material P is not arranged.

As will be described later, the transfer residual developer reaches the position of the charging device (magnetic brush charging device) 2 using the magnetic brush other charging system by the subsequent rotation of the photosensitive drum 1, and the photosensitive drum 1 shown in FIG. Is temporarily collected by the magnetic brush portion 2c of the magnetic brush 2A serving as a contact charging member, and the collected toner is again discharged onto the surface of the photosensitive drum 1 and finally collected by the developing device 4 to be exposed to light. The drum 1 is repeatedly used for image formation.

Here, between the transfer device 5 and the magnetic brush charging device 2, it is brought into contact with the photosensitive drum 1 and an AC bias or a DC bias having a polarity opposite to that at the time of primary charging before electrostatic latent image formation, or , AC to DC bias of opposite polarity to primary charging
A conductive brush 12, which is an adjusting member to which a bias is applied, is provided. The conductive brush 12 is
At the same time as smoothing the surface potential of the photosensitive drum 1 immediately before the primary charging by the magnetic brush charging device 2, the transfer residual developer is discharged.
Alternatively, the photosensitive drum 1 is charged with a polarity opposite to that of the primary charging (here, positive polarity) to facilitate collection by the magnetic brush charging device 2A at the magnetic brush portion.

Here, a process of starting up the printer having the above-mentioned configuration, forming an image on a plurality of transfer materials P, and ending the operation will be described here.

The image forming operation of the above printer is as follows.
This is performed both during image formation in which an image is formed on the transfer material P and during non-image formation in which the operation of the image forming apparatus is performed before and after forming an image on the transfer material P, and at the time of image formation and during non-image formation. The cleaning operation of collecting the transfer residual developer of the drum 1 and collecting it in the developing device 4 is shown in FIG. 2 in the operation sequence of (a) to (f).

(A) Pre-multi-rotation step: As a starting operation period (starting operation period, warming period) of the printer, the main power switch is turned on to drive the main motor of the apparatus to rotationally drive the photosensitive drum 1. , Execute a preparatory operation of a predetermined process device.

(B) Pre-rotation step: As a period for executing the pre-printing operation, this pre-rotation step is executed subsequent to the pre-multi-rotation step when a print signal is input during the pre-multi-rotation step.

When the print signal is not input, the drive of the main motor is temporarily stopped after the pre-multi-rotation step is finished, the rotational drive of the photosensitive drum 1 is stopped, and the printer stands by until the print signal is input. Hold) state. When the print signal is input, such a pre-rotation process is executed.

(C) Printing process (image forming process, image forming process): When a predetermined pre-rotation process is completed, the charging / exposure / rotation of the photosensitive drum 1 is continued.
An image forming process such as development is executed, the toner image formed on the surface of the rotary photosensitive drum 1 is transferred to the transfer material P, and the toner image is fixed by a fixing device, so that the transfer material P on which the toner image is formed is formed. Is printed out. In the case of the continuous printing (continuous printing) mode, the above printing process is repeatedly executed for a predetermined set number of prints with (d) the paper interval process interposed therebetween.

(D) Paper interval process: In the continuous printing mode, after the rear end of a certain transfer material P passes through the transfer nip portion t, until the leading end of the next transfer material P reaches the transfer nip portion t. Is a non-sheet passing state period of the transfer material P in the transfer nip portion t.

During this period, the area of the rotating photosensitive drum 1 that has passed through the transfer nip t, that is, the area on which the residual transfer developer is mounted is leveled by the conductive brush 12, and the charging device 2 is charged at the charging nip portion n. After the toner is collected, while passing through the charging nip portion n, the smaller the peak-to-peak voltage of the AC component is, the more the transfer residual developer is discharged, so in this embodiment, the application of the AC component of the primary charging bias is stopped. The transfer residual developer temporarily collected by the magnetic brush charging member 2A is discharged to the surface of the rotary photosensitive drum 1.

(E) Post-rotation step: Even after the printing step of the final transfer material P is completed, the main motor is continuously driven for a while, the photosensitive drum 1 is rotationally driven, and a predetermined post-operation is performed. This is the period to be executed.

Also in this period, as in the step (d) of the sheet interval process, by stopping the application of the AC component of the charging bias, the transfer residual developer temporarily collected by the magnetic brush charging member 2A is removed. Exhale on one side.

(F) Standby: When the predetermined post-rotation process is completed, the drive of the main motor is stopped, the rotational drive of the photosensitive drum 1 is stopped, and the printer is kept in the standby state until the next print start signal is input. Be drunk

From the above (a) multi-rotation step,
In the case of printing only one sheet before reaching the (f) standby process, after the printing is completed, the printer goes through the (e) post-rotation process and goes into the (f) standby state. (F) When a print start signal is input in the standby state, the printer shifts to (b) the pre-rotation step.

(C) The printing step is image formation,
(A) Multi-rotation process before, (b) Pre-rotation process, (d) Paper interval process, (e) Post-rotation process is non-image forming (non-image forming)
Is.

As described above, in the image forming apparatus of the present invention, in the cleaning of the photosensitive drum 1 in the cleanerless process in which the cleaner of the photosensitive drum 1 is not provided, the transfer residual developer is charged by the magnetic brush charging device. After being collected by the device 2, it is discharged onto the photosensitive drum 1 again during non-image formation, and according to the rotation of the photosensitive drum 1,
It is sent to the position of the developing device 4 and collected in the developing device 4,
This is a process for cleaning and removing the photosensitive drum 1 by removing and recovering by simultaneous cleaning with development.

Now, the respective structures of the charging device 2 and the developing device 4 for cleaning the photosensitive drum 1 in this printer will be described in detail.

The magnetic brush charging device 2 will be described with reference to FIGS.

FIG. 3 is an enlarged cross-sectional 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) 2A, a charging container (housing) 2B containing the magnetic brush 2A and conductive magnetic particles (charging carrier) 2d, and a magnet. Brush 2
A charging bias application power source E2 for A and the like.

In this embodiment, the magnetic brush 2A is of a sleeve rotation type, and is a roller-shaped magnet (magnet roll) 2a and is fitted onto the magnet roll 2a.
A conductive non-magnetic stainless sleeve (charging sleeve) 2b which is a magnetic particle carrier, and magnetic particles formed and held on the outer peripheral surface of the charging sleeve 2b by magnetically restraining them by the magnetic force of the magnet roll 2a inside the sleeve 2b. 2d magnetic brush part 2c.

The magnet roll 2a is a non-rotating fixing member, and the sleeve 2b is the magnet roll 2a.
Of the outer periphery of the outer peripheral surface of the outer peripheral surface of the outer peripheral surface of the outer peripheral surface of the peripheral portion of the peripheral portion of a predetermined peripheral speed of 225 mm / sec. It is driven to rotate at the peripheral speed of.

Further, the charging sleeve 2b is attached to the photosensitive drum 1 by means of a spacer roller (not shown) or the like,
It is arranged with a gap of about m.

Further, a magnetic brush layer thickness regulating blade (regulating blade) 2e made of non-magnetic stainless steel is attached to the charging container 2B, and a gap with the surface of the charging sleeve 2b is 90.
It is arranged to be 0 μm.

The magnetic particles 2d in the charging container 2B are partly magnetically restrained by the magnetic force of the magnet roll 2a inside the charging sleeve 2b on the outer peripheral surface of the charging sleeve 2b, which is a magnetic particle carrier, to serve as a magnetic brush portion 2c. Retained. The magnetic brush portion 2c rotates as the charging sleeve 2b rotates. At this time, the layer thickness of the magnetic brush portion 2c is
It is regulated to a uniform thickness by the regulation blade 2e.

Then, the regulation layer thickness of the magnetic brush portion 2c (9
00 μm) is larger than the interval (500 μm) between the facing gaps between the charging sleeve 2b and the photosensitive drum 1, the magnetic brush portion 2c has a predetermined distance from the photosensitive drum 1 at the facing portion between the charging sleeve 2b and the photosensitive drum 1. Form and contact a nip of width. This contact nip is the charging nip n
Is.

Therefore, the rotating photosensitive drum 1 has a brush portion 2c composed of magnetic particles 2 which rotate at the charging nip portion n as the charging sleeve 2b of the magnetic brush 2A rotates.
Rubbed by. In this case, in the charging nip portion n, the moving direction A of the photosensitive drum 1 and the moving direction B of the magnetic brush 2c are opposite to each other, and the relative moving speed becomes faster. A predetermined charging bias (primary charging bias) is applied from the power source E2 to the charging sleeve 2b and the regulation blade 2e.

Then, the photosensitive drum 1 is rotationally driven, the charging sleeve 2b of the magnetic brush charging device 2A is rotationally driven, and a predetermined primary charging bias is applied from the power source E2, whereby the circumference of the rotary photosensitive drum 1 is applied. In the case of the present embodiment, the surface is contact-charged uniformly by the injection charging method to a predetermined polarity and potential.

In other words, in the charging nip portion n,
Photosensitive drum 1 by magnetic brush portion 2c of magnetic brush 2A
By rubbing the surface and applying a charging bias to the magnetic brush 2A, electric charges are applied to the photosensitive drum 1 from the charging magnetic particles 2d forming the magnetic brush portion 2c, and the surface of the photosensitive drum 1 has a predetermined polarity. (Here, negative electrode) and the potential are uniformly contact-charged.

The magnet roll 2c fixedly arranged in the sleeve 2b has an angle θ between the horizontal plane and the normal surface of the magnetic brush 2A passing through the closest position c of the sleeve 2b to the photosensitive drum 1, and A upstream side 20 °
It is desirable to be in the range of 10 ° to the downstream side, and more preferably 15 ° to 0 ° on the upstream side.

If it is downstream, a main developing pole (here, N
The magnetic particles 2d are attracted to the (pole) position, and the magnetic particles 2d are apt to accumulate on the downstream side of the charging nip portion n in the rotation direction A of the photosensitive drum 1, and when the magnetic particles 2d pass through the charging nip n too far upstream. The transportability of 2d deteriorates, and retention easily occurs.

The charging nip portion n described here indicates the entire region where the magnetic particles of the magnetic brush portion 2c are in contact with the photosensitive drum 1 during charging. Further, when the magnetic pole of the magnet 2c is not located in the charging nip portion n, it is obvious that the magnetic particles 2d are less constrained by the sleeve 2b and the magnetic particles tend to adhere to the photosensitive drum 1. Therefore, in this embodiment, in the magnet roll 2a, a magnetic pole of about 900 G is provided at a position 10 ° downstream of the horizontal plane in the rotating direction B of the sleeve 2b, that is, in the vicinity of the closest contact c which is the upstream side in the rotating direction A of the photosensitive drum 1. The developing magnetic pole (here, N pole) was arranged.

As the primary charging bias applied to the sleeve 2b and the regulating blade 2e by the power source E2, the bias in which the AC component is superimposed on the DC component is used in this embodiment.

Thus, since the photosensitive drum 1 is provided with the charge injection layer 1f (FIG. 9) on the surface thereof, as will be described later in detail, the charging device 2 charges the photosensitive drum 1 by charge injection charging. The primary charging process of No. 1 is performed.

That is, a charge injection layer containing SnO ₂ or the like is provided on the surface of the photoconductor, and these layers function as electrodes of the capacitor. By bringing a conductive contact charging member into contact therewith and applying a voltage thereto, it is possible to inject charges into the electrodes in the same manner as an ordinary capacitor. Therefore,
The surface of the photosensitive drum 1 is charged to a potential corresponding to the DC component of the charging bias DC + AC. The charging speed of the sleeve 2b tends to be better as the rotation speed is higher.

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

[0096]   Vd = V0 (1-exp (T0 / (Cp · r))) ... 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.

In the image forming apparatus of the present embodiment, the peak-to-peak voltage Vpp of the AC component during image formation (image formation) is 100 v or more and 2000.
Among v or less, 300v or more and 1200v or less are especially preferable. When the peak-to-peak voltage Vpp is lower than that, the effect of improving charging uniformity and potential rise is small, and when it is higher than that, retention of the magnetic particles 2d and adhesion to the photosensitive drum 1 are deteriorated.

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. If it is less than that, the effect of worsening the adhesion of the magnetic particles 2d to the photosensitive drum 1 and the effect of improving the charging uniformity and the rising characteristic of the potential becomes small. Become. The AC waveform is preferably a rectangular wave, a triangular wave, a sin wave, or the like. In this embodiment,
The peak-to-peak voltage Vpp was 700 v.

Magnetic particles 2 constituting the magnetic brush portion 2c
d is a sintered ferromagnetic material (ferrite) in this embodiment.
Was subjected to a reduction treatment, but was also kneaded with a resin and a ferromagnetic powder to form a particle, or a mixture of conductive carbon or the like for resistance adjustment,
A surface-treated product can be used as well.

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 1, and the charging current concentrates on defects such as pinholes generated on the photosensitive drum 1. The magnetic brush charging member 2A and the photosensitive drum 1 must also have a role of preventing energization damage due to the above-mentioned destruction.

Therefore, the electric resistance value of the magnetic brush 2A is
It is preferably 1 × 10 ⁴ Ω to 1 × 10 ⁹ Ω, and particularly preferably 1 × 10 ⁴ Ω to 1 × 10 ⁷ Ω.
When the electric resistance value of the magnetic brush 2A is less than 1 × 10 ⁴ Ω, pinhole leakage tends to occur, and 1
If it exceeds × 10 ⁹ Ω, it tends to be difficult to inject favorable charges.

In order to control the resistance value within the above range,
Has a volume resistance value of the magnetic particles 2d of 1 × 10^FourΩcm
1 x 10⁹Ω · cm is desirable, and especially 1
× 10 ^FourΩ · cm ~ 1 x 10⁷More than Ω · cm
preferable.

The electric resistance value of the magnetic brush 2A used in this embodiment is 1 × 10 ⁶ Ω · cm, and the charging bias D is D.
By applying -700v as the C component, the surface potential of the photosensitive drum 1 also became -700v.

The volume resistance value of the magnetic particles 2d was measured by the circuit shown in FIG. That is, the cell 25 is filled with the magnetic particles 2d, and the main electrode 17 is contacted with the filled magnetic particles 2d.
And an upper electrode 18 are arranged, a voltage is applied between the electrodes 17 and 18 from a constant voltage power source 22, and a current flowing at that time is
It was determined by measuring with an ammeter 20.

In the circuit shown in FIG. 5, the magnetic particles 2d in the cell 25 fixed to the guide ring 24 are blocked by the insulator 19, and the voltmeter 21 is wired in parallel.

The measurement condition is that the contact area S of the filled magnetic particles 2d with the cell is S = 2c in an environment of 23 ° C. and 65%.
m ² , thickness d = 1 mm, upper electrode 18 load 10 kg,
The applied voltage is 100V.

The peaks in the measurement of the average particle size and the particle size distribution of the magnetic particles 2d are in the range of 5 to 100 μm to prevent the charge deterioration due to the contamination of the particle surface and to prevent the magnetic particles from adhering to the surface of the photosensitive drum 1. From the viewpoint of. The average particle diameter of the magnetic particles 2d is indicated by the maximum chord length in the horizontal direction, and the measuring method is to randomly select 300 or more magnetic particles by a microscopic method, measure the diameter, and take the arithmetic mean.

The untransferred residual developer on the photosensitive drum 1 has a mixture of normal polarity (negative) and reverse polarity (positive) due to peeling discharge during transfer.

Since the printer of this embodiment is a cleanerless printer, the transfer residual developer remaining on the photosensitive drum 1 after the toner image is transferred onto the transfer material P and having the mixed polarity is a conductive member which is an adjusting member. After being charged to the positive polarity (reverse polarity) by the brush 12 and facilitated to be taken into the charging device 2 which is the magnetic brush contact charging device described above,
It is carried to the charging nip portion n of the photosensitive drum 1, mixed with the brush portion 2c of the magnetic brush 2A of the charging device 2 and temporarily collected.

The transfer residual developer is taken into the magnetic brush portion 2c of the magnetic brush 2A by the transfer residual developer by applying the AC component to the magnetic brush 2A as described above. This can be more effectively performed by the effect of the oscillating electric field between the photosensitive drums 1.

Most of the transfer residual developer taken into the magnetic brush portion 2c is triboelectrically charged to the normal polarity (negative polarity) at the time of primary charging, and the AC component of the charging bias is applied at the time of non-image formation. And is discharged onto the photosensitive drum 1. The transfer residual developer having the same polarity and discharged onto the photosensitive drum 1 reaches the developing portion m shown in FIG.
By the development 4b of the development device 4, the fogging removal electric field at the time of development collects the toner by simultaneous 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 developer simultaneously proceeds with other image forming steps such as charging, exposure, development and transfer. Done in.

As a result, the transfer residual developer is collected in the developing device 4 and used in the subsequent steps, so that the waste toner can be eliminated. Further, there is a great advantage in terms of space, and the image forming apparatus can be greatly downsized.

Here, the toner contained in the transfer residual developer usually has a relatively high electric resistance. The mixing of such toner particles having high electric resistance into the magnetic brush portion 2c of the magnetic brush 2A is a factor that increases the electric resistance of the magnetic brush portion 2c and reduces the charging ability. Therefore, when the amount of mixed toner is relatively large, in order to reduce the amount of toner particles mixed in the magnetic brush portion 2c, the AC bias of the primary charging bias applied from E2 is stopped for a long time, and at the time of non-image formation. Good discharge is maintained by discharging a large amount of toner.

Here, since the toner discharged from the magnetic brush portion 2c to the photosensitive drum 1 is in an extremely evenly dispersed state and the amount thereof is small, it adversely affects the next image exposure process. It has no effect. Further, no ghost image is generated due to the transfer residual developer pattern.

The transfer residual developer discharged onto the photosensitive drum 1 by the charging device 2 is collected by the developing device 4 by the simultaneous cleaning of development according to the rotation of the photosensitive drum 1, is agitated in the developing device 4, and is newly agitated. It is recycled together with the development of a large electrostatic latent image.

In the developing device 4, the electrostatic latent image toner developing method used here is generally classified into the following four types (1) to (4).

(1) Non-magnetic toner is coated on a sleeve with a blade or the like, and magnetic toner is coated by a magnetic force to be conveyed and developed in a non-contact state with a photoconductor (one-component non-contact development). .

(2) A method in which the toner coated as described above is developed in contact with the photoconductor (one-component contact development).

(3) A method in which a mixture of toner particles and a magnetic carrier is used as a two-component developer, which is conveyed by magnetic force and is developed in contact with the photoconductor (two-component contact development).

(4) A method of developing the above two-component developer in a non-contact state (two-component non-contact development).

Of the developing methods described in (1) to (4), the two-component contact developing method (3) is often used from the viewpoint of high image quality and high stability.

FIG. 6 is an enlarged cross-sectional view of the developing device 4 of this embodiment using such a two-component contact developing method. The developing device 4 in this embodiment uses a mixture of a highly releasable spherical non-magnetic toner (toner) prepared by a polymerization method and a magnetic carrier (magnetic particles for development, developing carrier) as a developer 4f. The developer 4f is held by the developer carrying member (developing member) 4b as a magnetic brush layer 4f 'by a magnetic force and conveyed to the developing portion m, and is brought into contact with the surface of the photosensitive drum 1 to convert the electrostatic latent image into a toner image. It is a reversal developing device 4 of a two-component magnetic brush contact developing system for developing.

The developing device 4 includes a developing container 4a, a developing sleeve 4b as a developer bearing member, a magnet (magnet roller) 4c as a magnetic field generating means fixedly arranged in the developing sleeve 4b, and a developing sleeve 4b. The developer layer thickness regulating blade 4d for forming a thin layer of the developer, the developer stirring and conveying screw 4e, and the two-component developer 4f housed in the developer container 4a are used. It is a mixture of developing carriers.

The developing sleeve 4b has a closest distance (gap) to the photosensitive drum 1 at least during development.
Is arranged so as to be about 500 μm, and the developing sleeve 4
The developer magnetic brush thin layer 4f ′ carried on the outer surface of b is set so as to contact the surface of the photosensitive drum 1. The contact nip portion m between the developer magnetic brush thin layer 4f ′ and the photosensitive drum 1 is a developing area (developing portion).

The developing sleeve 4b has an internal fixed magnet 4c.
Of the magnetic brush 4f 'of the developer 4f by the magnetic force of the fixed magnet 4c on the outer surface of the sleeve 4b in the developing container 4a.
Is formed. The developer magnetic brush 4f 'is conveyed along with the rotation of the sleeve 4b, and is regulated by the regulation blade 4d to a layer thickness of the developer magnetic brush thin layer 4 having a predetermined layer thickness.
As f ′, it is carried out of the developing container 4a and conveyed to the developing section m, comes into contact with the surface of the photosensitive drum 1, and is subsequently returned to the inside of the developing container 4a and collected by the rotation of the sleeve 4b.

The developer in the developer magnetic brush thin layer 4f 'on the developing sleeve 4b is the toner existing at the facing portion of the exposed portion which is an electrostatic latent image on the photosensitive drum 1 when it comes into contact with the surface of the photosensitive drum 1. T moves to the photosensitive drum 1 to develop the electrostatic latent image.
It is returned to the developing container 4a. At this time, the transfer residual developer discharged from the charging device 2 to the photosensitive drum 1 is also collected in 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 the developing bias applying power source E4. In the developing characteristics in this embodiment, fog occurs when the difference between the charging potential (-700 v) of the photosensitive drum 1 and the DC component value of the developing bias is 200 v or less,
If it is 350 V or more, the development carrier adheres to the photosensitive drum 1, so the DC component of the development bias is -400.
v.

The toner concentration (mixing ratio with the developing carrier) of the developer 4f in the developing container 4a becomes lower because the toner is consumed for developing the electrostatic latent image and is successively consumed.

The toner concentration of the developer 4f in the developing container 4a is detected by a detection means (not shown), and when it falls to a predetermined lower limit concentration, the toner replenishing section 4g removes the toner from the developing container 4a.
The toner T is replenished to the developer 4f therein, and the toner replenishment is controlled so that the toner concentration of the developer 4f in the developing container 4a is always kept within a predetermined allowable range.

By using a highly releasable spherical toner prepared by a polymerization method as the toner T contained in the developer 4f, it is possible to reduce the amount of transfer residual developer generated, and to reduce the magnetic force. The collectability of the toner discharged from the brush charging device 2 to the developing device 4 can be improved.

The use of the two-component contact developing type developing device 4 also improves the collectability of the toner discharged from the magnetic brush charging device 2 to the developing device 4.

Here, most of the reverse polarity (positive electrode) toner temporarily collected by the charging device 2 is charged to the normal polarity (negative electrode) by frictional charging with the magnetic particles 2d, but a part of the reverse polarity (negative electrode) is charged. As it is, the photosensitive drum 1 is driven by the AC voltage of the charging device 2.
Adhere to the surface.

As shown in FIG. 7, in the regular polarity toner on the photosensitive drum 1 charged by the charging device 2, the toner T4 in the exposed portion is the same as the toner T in the developing device 4 in the photosensitive drum. Toner 1 of the toner T of the unexposed portion
5 is collected by the developing device 4. On the other hand, when the reverse polarity toner on the photosensitive drum 1 reaches the developing device 4, the reverse polarity toner T1 adheres to the surface of the photosensitive drum 1 in the area not subjected to image exposure, and the reverse polarity toner in the exposed area. The toner T2 is collected by the developing device 4.

Reverse polarity toner T2 collected in the developing device 4.
Is stirred in the developing device 4 and charged to a regular polarity,
When the chargeability of the developer is lowered due to long-term use, the developer accumulates in the developing device 4 with the opposite polarity.

The opposite polarity toner T3 again adheres to the area of the charging potential of the photosensitive drum 1 and reaches the transfer portion. Since the transfer bias has the same polarity as that of the reverse polarity toner T3, it is usually difficult to transfer it. However, when the paper has a smooth surface such as coated paper, it is transferred to the paper against the electric field of the transfer bias due to the attraction force of the contact. The toner adheres to a place that should be a white background, resulting in a fog image.

For example, in the experiment using the above coated paper, the coated paper is thick paper (209 g / m ² ), and the heat capacity is large and sufficient heat is applied for fixing, so the process speed is 1/2 (150 ÷ 2 = 75 mm / sec.). Under this condition, since the developing driving speed is also halved, the stirring ability of the developer is also reduced, and the reverse polarity toner is often accumulated and fog is likely to occur.

In this state, the fog density was measured when the toner density in the developer 4f was changed. The result is shown in FIG. As the toner concentration in the developer 4f decreases,
The fog reflection density is also decreasing. There are two possible causes for this.

One of the reasons is that the total toner amount is reduced and the absolute amount of the opposite polarity toner is also reduced, so that the fog is reduced.

Another cause is that since the amount of toner with respect to the developing carrier decreases, the probability that the toner contacts the carrier increases. Therefore, the possibility that the reverse polarity toner is also charged to the regular polarity is increased, the reverse polarity toner is reduced, and the fog is reduced.

From this result, as a characteristic part of the present invention,
In this embodiment, an ordinary image forming mode and an image forming mode when fogging is apt to be selected as the transfer material P are provided. When the image forming mode is selected, the toner concentration in the developer is set. Changed the control value of.

In this embodiment, the user selects a predetermined image forming mode on the basis of information about the type of transfer material from an operation unit such as a host computer or a control panel, and the normal first transfer material is plain paper. In the 1-image forming mode, the toner density in the developing device 4 is a normal predetermined toner density, here 8% wt. In contrast, the toner replenishment is controlled so that the toner concentration is 7% wt. To control.

As a result, the fog reflection density is reduced to about 0.2 when the process speed is lowered, where the reverse polarity toner tends to accumulate.
It could be reduced by 5%.

That is, in this way, the transfer residual developer is once collected by the charging brush 2A of the charging device 2, the transfer residual developer is discharged from the charging brush 2A, and is collected again by the developing device 4. In the image forming apparatus, changing the toner concentration in the developer depending on the type of image forming mode is a feature of the present invention.

In this embodiment, the image forming mode is the type of transfer material. Here, the normal first image forming mode is used in which the transfer material is plain paper, that is, the transfer material in which the transfer residual developer is not transferred even when the toner density is a predetermined normal density, and the transfer residual developer is attached at the predetermined normal density. Fog on the coated paper that has a Beck smoothness of 100 seconds or more, or on thick paper that has a large basis weight that requires a slow fixing speed, that is, a basis weight of 100 to 160 g / m ² The second image forming mode in which the transfer material is easily used is switched.

Further, the toner concentration of the developer 4f in the developing container 4a of the developing device 4 is detected by a detecting means (not shown),
When the toner concentration falls to a predetermined lower limit, toner is replenished from the toner replenishing section 4g to the developer 4f in the developing container 4a.

Therefore, the control of the toner density in the developer is
As described above, in the present exemplary embodiment, when the transfer material that easily causes fogging is selected and the toner density is changed, the control unit (not shown) provides the toner density control value.
By lowering the allowable lower limit concentration of the developer 4f, the toner concentration of the developer in the developing device 4 is lowered as the toner T is consumed.

Further, in the second image forming mode, by lowering the toner concentration, the absolute amount of the opposite polarity toner is reduced, so that the fog is improved, but the driving speed of the development is lowered with the process speed. If the developer agitation ability is lowered, the effect becomes more effective. Therefore, it is also effective in the method of lowering the process speed in the second image forming mode as compared with the process speed in the first image forming mode.

Here, 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 as shown in the partial sectional view showing the layer structure in FIG.
The drum base 1a made of aluminum having an outer diameter of 30 mm is provided with the first to fifth functional layers 1b to 1f in order from the bottom.

First layer: a subbing layer 1b, which is a conductive layer having a thickness of about 20 μm, which is provided for smoothing defects in the aluminum drum substrate and for preventing moire due to reflection of laser exposure. Is.

Second layer: Positive charge injection prevention layer 1c,
The positive charge injected from the aluminum drum base 1a is exposed
Prevents the negative charge on the surface of the drum 1 from being canceled
Plays a role in
Volume resistance 10 by Ron ⁶Resistance adjustment to about Ω · cm
This is a medium resistance layer having a uniform thickness of about 1 μm.

Third layer: The charge generation layer 1d 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 receiving laser exposure.

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

Fifth layer: charge injection layer 1f, which has a grain size of 0.000, which is made by doping a photo-curable acrylic resin as a binder with antimony as a light-transmissive conductive filler to reduce resistance (conductivity). This is a coating layer formed by coating a material in which conductive fine particles, which are ultrafine particles of tin oxide SnO ₂ having a thickness of 03 μm, are dispersed in a resin in an amount of 70% by weight, for about 3 μm. The electric resistance value of the charge injection layer 1f is 1 × 10 ¹⁰ to 1 × 10 ¹⁴ which is a condition that does not cause sufficient chargeability and image deletion.
It must be Ω · cm. In this example, the photosensitive drum 1 having a surface resistance of 1 × 10 ¹¹ Ω · cm was used.

With the above-described structure, even in an image forming apparatus which uses a charging device and a developing device to practice simultaneous cleaning with development without providing a cleaner for space saving, the photoconductor in the transfer residual developer is not affected. On the other hand, it is possible to realize the image forming apparatus in which the toner having the opposite polarity is prevented from being accumulated in the charging device and the developing device and adhering to the transfer material.

Here, in this embodiment, the fog of the image is
Since it occurred in a specific type of transfer material, the image forming mode was set according to the type of transfer material. However, as the transfer material in which fog is particularly likely to occur, a coated paper having a Beck smoothness of 100 seconds or more and a fixing speed are set. An example is thick paper that has a large basis weight that needs to be slowed down, that is, a basis weight of 100 to 160 g / m ² .

Further, even with the same type of transfer material, image fog may occur due to environment such as humidity and temperature. Therefore, the image forming mode may be set according to such environment.

Since image fog often occurs especially when an image is directly transferred from a photoconductor to a transfer material, here, the transfer material, which is a transfer medium, is carried on a transfer belt to directly transfer the image to the transfer material. However, the transfer material carrier may be a drum shape instead of a belt shape, and is widely used in color image forming apparatuses and the like. The present invention can also be applied to an image forming apparatus of an intermediate transfer system in which the image is once transferred to an intermediate transfer body which is a drum-shaped transfer medium and then collectively transferred from the intermediate transfer body to a transfer material.

Second Embodiment In the first embodiment, the toner density is adjusted by lowering the toner allowable lower limit density at which toner replenishment is started. Therefore, the paper which is apt to cause fog is selected and the control toner density is set. If the toner in the developer is not consumed by forming an image on several sheets after lowering the toner density, the actual toner density in the developing device 4 does not decrease and the fog cannot be reduced.

Therefore, in this embodiment, the toner image which is not transferred to the transfer material on the photosensitive drum 1 by using the developer in the developing device 4 is used before the first image formation in which the paper which is apt to cause fog is selected. Was formed, and the toner in the developer was consumed to lower the target toner concentration, and then image formation was performed.

This embodiment is a printer having the same configuration as the printer shown in FIG. 1 in Embodiment 1, and the total amount of the developer is 200 g, and the normal toner density in the first image forming mode using plain paper is 8% wt. Is. In the second image forming mode using the paper that is apt to cause fogging, the toner concentration is set to 7% wt. 200 g × (0.08-
0.07) = 2 g of toner must be developed.

The peripheral speed of the photosensitive drum 1 is 150 mm / se.
c, the developing width is 300 mm, and the weight per unit area of the toner image developed on the photosensitive drum 1 is 0.5 mg / cm ² , so the photosensitive drum 1 is primarily charged before the image formation, and the exposure device 3 sets 9 After solid exposure for 2 seconds, the developing operation was performed by the developing device 4.

The toner developed on the photosensitive drum 1 was transferred onto the transfer belt 5a and was collected by a transfer belt cleaner (not shown). However, if the charging device 2 has a sufficient toner holding capacity, the charging device 2 temporarily removes the toner. After the collection, the toner may be discharged to the photosensitive drum 1 and collected again by the developing device 4.

Therefore, all the images formed by the developer having a high toner density are transferred to the transfer belt 5a until the transfer material which easily causes the fog is selected and the toner density of the developing device 4 is lowered to a predetermined density. It is transferred and never transferred to a transfer material.

As a result of the above, it was possible to reduce the fogging even in the first image formation by selecting the transfer material which is likely to cause the fogging. Therefore, the paper as the transfer material is not wasted, and the cost reduction and the workability increase are achieved.

When the toner density lowered by this operation is returned to the normal mode, it is possible to restore the original toner density by supplying the toner before the first image formation.

Further, although the image density is lowered by lowering the toner density in the developer, the adjustment of the primary charging potential, the developing bias, etc., the rotating speed of the developing sleeve, the rotating speed of the photosensitive member, that is, the process speed. By adjusting, it is possible to form substantially the same image.

Unless otherwise specified, the dimensions, materials, shapes, and relative positions of the components of the image forming apparatus described above are intended to limit the scope of the present invention thereto. Not the one.

[0170]

As described above, the image forming apparatus of the present invention is an image forming apparatus that forms an image on a transfer material with a developer containing toner, and remains on the surface of the image carrier after transfer by the transfer apparatus. The transfer residual developer collected is collected by the charging member of the charging device, the transfer residual developer collected by the charging member is discharged onto the image carrier, and the transfer residual developer discharged onto the image carrier is collected by the developing device. An image forming apparatus that collects and collects images by changing a plurality of image forming modes, and changing the toner concentration in the developer accommodated in the developing device based on the image forming mode. Since it is a device, even when using a transfer material in which the opposite polarity toner is contact-transferred to the image carrier accumulated in the developing device to cause fog, the toner concentration in the developer is reduced and the triboelectric chargeability is improved. The Rukoto, charged with reverse polarity toner to the normal polarity to the image bearing member can be prevented fog could be continued and stable good image formation.

[Brief description of drawings]

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

FIG. 2 is a sequence showing an image forming operation by the image forming apparatus according to the present invention.

FIG. 3 is a cross-sectional view showing a charging device according to the present invention.

FIG. 4 is an explanatory diagram showing a relationship between a charging device and a photoconductor according to the present invention.

FIG. 5 is a circuit diagram showing a circuit configuration for measuring the volume resistance value of magnetic particles according to the present invention.

FIG. 6 is a sectional view of the developing device according to the present invention.

FIG. 7 is an explanatory diagram showing the behavior of the opposite polarity toner at the surface potential of the photoconductor.

FIG. 8 is a graph showing the relationship between the amount of fogging of an image and the toner concentration of the developer in the developing device.

FIG. 9 is a partial cross-sectional view of a photoconductor according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Photosensitive drum (image bearing member) 2 Charging device 2A Magnetic brush charging member (charging member) 2B Housing (charging container) 2a Magnet roll 2b Charging sleeve (magnetic particle carrier) 2c Magnetic brush part 2d Magnetic particles 2e Regulation blade 3 Laser Beam scanner, exposure device (electrostatic latent image forming means) 4 developing device 4f developer 4f 'magnetic brush 4g toner replenishing section 5 transfer device 5a transfer belt (transfer material carrier) 5d conductive blade (transfer charging device) 12 conductive Roller (adjustment member) T Toner T1, T2, T3 Reverse polarity toner

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) G03G 15/16 103 G03G 21/00 21/00 384 384 15/08 507B F term (reference) 2H027 DC02 EA06 ED02 ED30 EE03 FA30 FA35 2H068 AA03 AA05 AA08 AA21 AA33 BB06 BB59 CA37 CA60 FA11 2H077 AA37 AC16 AD06 AD36 DB08 DB25 EA03 2H134 GA01 GB02 HF13 JA11 KG01 GA01 GA45 GA21 GA21 GA16 GA21 MA21 MA16 MA21 MA16 MA16 MA11 MA11 MA11 MA11 MA11 MA11 MA11 MA11 MA11 MA11 MA15 MA11 MA11 MA11 MA11 MA11 MA11 MA11 MA11 MA11 MA11 MA11 MA15 MA11 MA11 MA15 HB12 HB14 HB17 HB22 HB45 HB48 JA02 JB06 JC01 MA01 NA06

Claims (11)

[Claims] 1. A charging device having an image carrier and a charging member in contact with the image carrier, for charging the image carrier by applying a charging bias to the charging member, and charging by the charging device. An electrostatic latent image forming means for forming an electrostatic latent image on the image carrier, and a developer containing a toner, and the electrostatic latent image formed on the image carrier is used as the developer. In an image forming apparatus including a developing device that develops a developer image to form a developer image, and a transfer device that transfers the developer image on the surface of the image carrier to a transfer medium, the image carrier after the transfer by the transfer device. The transfer residual developer remaining on the body surface is collected by the charging member of the charging device, and then the transfer residual developer collected by the charging member is discharged onto the image carrier, and then on the image carrier. Collect the transfer residual developer discharged onto the developing device An image forming apparatus capable of switching a plurality of image forming modes, and changing the toner concentration in the developer accommodated in the developing device based on the image forming mode. Forming equipment. 2. The transfer device includes a transfer material carrier that conveys a transfer material as the transfer medium, and a transfer charging device that charges the transfer material carrier. The image forming apparatus according to claim 1, wherein the developer image is transferred onto a transfer material carried by the transfer material carrier. 3. The transfer device includes an intermediate transfer member as the transfer medium, and a transfer charging device for charging the intermediate transfer member, and transfers the developer image on the surface of the image carrier to the intermediate transfer member. The image forming apparatus according to claim 1, wherein the image is transferred to a body, and the developer image transferred to the intermediate transfer body is transferred to a transfer material. 4. The image forming mode is switched depending on the type of transfer material.
Alternatively, the image forming apparatus of 3. 5. The image forming mode uses a transfer material to which the residual transfer developer on the image carrier does not adhere when the toner density of the developer accommodated in the developing device is a predetermined toner density. When the first image forming mode is selected, the first image forming mode is switched to the second image forming mode in which the transfer material on which the residual transfer developer on the image carrier adheres is used. When the toner concentration in the developer included in the developing device is set to the predetermined toner concentration and the second image forming mode is selected, the toner concentration in the developer included in the developing device is set lower than the predetermined toner concentration. The image forming apparatus according to claim 4, wherein 6. When the second image forming mode is selected, the electrostatic latent image is further developed by the developing device and transferred to a member other than the transfer material before the image is formed on the transfer material. 6. The image forming method according to claim 5, wherein a developer image to be formed is formed on the image carrier to consume the toner in the developing device and to reduce the toner concentration of the developer in the developing device. apparatus. 7. Further, based on the image forming mode,
The process speed is changed, and the process of claim 1 is performed.
The image forming apparatus according to any one of 6 above. 8. The image forming mode uses a transfer material to which the transfer residual developer on the image carrier does not adhere when the toner density of the developer of the developing device is a predetermined toner density. The first image forming mode and the second image forming mode in which the transfer material to which the untransferred residual developer on the image carrier adheres are switched, and when the first image forming mode is selected, the development is performed. When the toner density in the developer included in the apparatus is set to the predetermined toner density and the second image forming mode is selected, the toner density in the developer included in the developing apparatus is set lower than the predetermined toner density, The image forming apparatus according to claim 7, further comprising setting the process speed to be slower than when the first image forming mode is selected. 9. The image forming apparatus according to claim 1, wherein the charging member has magnetic particles and a magnetic particle carrier. 10. The image forming apparatus according to claim 1, wherein the image carrier has a charge injection charging property. 11. The electrophotographic photosensitive member according to claim 1, wherein the image bearing member has a charge injection layer on the surface of which conductive fine particles are dispersed in an insulating binder.
The image forming apparatus according to any one of 0.