JPH11272023A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To restrain the change of density occurring at a part on an image carrier where AC electrification is switched to electrification between colors by setting electrification voltage at the image non-forming time in accordance with the value of bias at the image forming time. SOLUTION: In order to efficiently perform primary transfer, DC bias impressed on a primary transfer roller 12 is gradually increased for every color. Impressing electrification bias on an electrifying roller 5 in an image non- forming area is controlled to perform the same AC electrification as in an image forming area when the value of primary transfer bias at the image pre- forming time is large and to perform the same DC electrification as usual when the value of the primary transfer bias is small. For example, after impressing the primary transfer bias of 150 V for the first color, density irregularity in a switching area does not occur but the density irregularity occurs according to circumstances in the switching area for the second and succeeding colors for which the primary transfer bias of >=550 V is impressed. Therefore, the DC electrification is performed only in the image non-forming area between the first and the second colors and the AC electrification is performed in the image non-forming area other than the above.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus using an electrophotographic technique, such as a printer or a copying machine.

[0002]

2. Description of the Related Art An example of a conventional color image forming apparatus using electrophotography will be described below.

In general, an image carrier for holding toner on an electrostatic latent image is composed of a metal drum or a metal belt having a photosensitive layer composed of a charge generation layer and a charge transport layer. Driven in a fixed direction. Next, a bias (charging bias) is applied to a charging roller, which is a charging unit in contact with the image carrier, to charge up the surface of the image carrier to a certain potential (charging step). In order to charge the image carrier more uniformly, taking an organic photoreceptor (OPC) as an example, the charging bias is usually
A bias in which negative polarity DC (direct current) is superimposed on AC (alternating current) is used. At this time, the peak value of the applied bias and DC
A high AC voltage is applied so that the difference between the components is equal to or higher than the discharge starting voltage. The surface potential of the image carrier thus obtained is referred to as Vd potential.

Further, based on a signal from the controller, the surface of the image bearing member is irradiated while scanning in the longitudinal direction with a laser beam whose ON / OFF is controlled (exposure step). Since the light irradiation position is charged down, an electrostatic latent image is formed on the surface of the image carrier. The surface potential of the exposed portion is represented by Vl
Called potential.

Subsequently, a constant bias (development bias) is applied to a developing device, which is opposite to the image bearing member and is filled with yellow toner, and the toner to which a predetermined charge (negative polarity) is applied is transferred to the image bearing member. The electrostatic latent image is visualized by transferring to the upper electrostatic latent image (developing step). The amount of toner on the image carrier during development is determined by the relationship between the Vl potential and the development bias. The difference between the central value of the developing bias and the VL potential is called a developing contrast. In general, the larger the developing contrast, the larger the amount of the toner to be applied.

Then, a positive bias is applied to an intermediate transfer member, such as a drum or a belt, which is arranged adjacent to the image carrier and moves in the forward direction at substantially the same speed as the image carrier, thereby applying a positive bias. The upper toner is transferred to an intermediate transfer member (primary transfer).

[0007] Residual toner on the image carrier after the primary transfer is removed from the image carrier by an elastic blade arranged in contact with the counter direction with respect to the rotation direction of the image carrier (cleaning step).

The above process is repeated for each of the colors magenta, cyan and black. Thereafter, a positive bias is applied to a secondary transfer device such as a transfer roller or a corona charger, which is disposed opposite the intermediate transfer member, and in that state, the transfer material is interposed between the intermediate transfer member and the secondary transfer device. By passing the paper through, the four color toners carried on the intermediate transfer member are collectively transferred to the transfer material (secondary transfer). At this time, the residual toner on the intermediate transfer member is removed by bringing a cleaner member such as a blade or a fur brush into contact with the intermediate transfer member, similarly to the cleaning of the image carrier.

The transfer material to which the toner has been transferred is conveyed to a fixing device composed of a pair of rollers, where the toner and the transfer material are heated and pressed to permanently fix the toner on the transfer material.

By the way, various members are in contact with the surface of the image carrier, and are always subjected to mechanical force. In particular, an elastic blade or the like for removing transfer residual toner has a large contact pressure against the image carrier. In general, the photosensitive layer on the surface of the image carrier is gradually scraped off by these abutting substances. In particular, when a high-pressure discharge by AC is performed on the image carrier, the surface of the image carrier is damaged, and the amount of scraping tends to increase. If the shaving amount is equal to or more than a predetermined value, sensitivity to exposure is lost, or the surface of the image carrier cannot be uniformly charged to a desired potential, and a clear image cannot be formed. Therefore, in this case, it is necessary to warn of replacement of the image carrier.

Generally, the length of the surface of the intermediate transfer member on which the image can be transferred is longer than the length of the usable transfer material.
At the time of switching of each color, a non-image area exists on the intermediate transfer member and the image bearing member. In order to suppress an increase in the amount of scraping of the image carrier and extend the life of the image carrier, in the non-image area, the above-described AC + DC high-voltage charging bias is not applied, and only the negative DC bias is applied ( Hereinafter, “inter-color charging bias” is performed.
Normally, a value equivalent to VD + | discharge start voltage | is applied as the inter-color charging bias value. It should be noted that the inter-color charging bias is often adopted also when switching the printing paper during continuous printing, and before and after image formation.

[0012]

However, in practice, the switching from AC charging to inter-color charging is not performed instantaneously, but gradually changes over a certain period of time. Therefore, in this switching process, a desired high pressure has not been obtained.

On the other hand, during the primary transfer, a positive charge flows from the intermediate transfer member to the image carrier. Most of the positive charges that have flowed in recombine with negative charges on the surface of the image carrier and disappear, but a part thereof is trapped by impurities or the like on the surface of the image carrier. Some of the trapped positive charges recombine with the negative charges provided by the charging roller at the next charging. Therefore, when the ratio of the trapped positive charge to the negative charge provided by the charging roller is large, recombination is frequently performed. In such a case, the surface potential decreases (dark decay) while the image carrier moves from the charging position to the developing position. The decrease in the surface potential means that the VL potential decreases.
This leads to an increase in development contrast. That is, an increase in the amount of loaded toner (increase in density) is caused.

The total discharge amount of the image carrier during AC charging and during inter-color charging is set to be substantially the same.
On the other hand, when switching from AC charging to inter-color charging,
For the above reason, the discharge is excessively large or excessively small. It should be noted that whether the discharge is too large or too small depends on
It is determined by the switching timing. This means that the remaining amount of trapped positive charges in the portion of the image carrier where the switching from AC charging to inter-color charging is different from other portions. As a result, at the time of forming the next image, this portion causes a phenomenon that the density becomes lighter or darker than the other portions.

Accordingly, an object of the present invention is to provide a charging means for the image carrier without significantly increasing the amount of shaving of the photosensitive layer on the surface of the image carrier during use and greatly reducing the life of the image carrier. It is an object of the present invention to provide an image forming apparatus capable of suppressing a change in density at a portion where AC charging is switched from AC charging to inter-color charging.

[0016]

The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides:
An image carrier having a photosensitive layer on the surface and rotatably arranged,
A charging unit that is disposed in contact with the image carrier and uniformly charges the surface of the image carrier by applying a predetermined charging voltage; and a charging unit that uniformly charges the image carrier by the charging unit. Exposure means for forming a latent image, a developing device for attaching toner corresponding to the electrostatic latent image formed on the image carrier, and a toner image formed on the image carrier, A transfer unit for transferring to a transfer material or an intermediate transfer body by applying a predetermined transfer voltage, and a bias in which a first DC component is superimposed on an AC component during image formation upon application of a charging voltage by the charging unit; In an image forming apparatus having a unit capable of applying any one of AC + DC, AC only, and DC only during non-image formation, the voltage is applied to the transfer unit during image formation. Depending on the value of the bias, an image forming apparatus characterized by setting the charging voltage to be applied to said charging means that during non-image formation immediately after.

A means for detecting the amount of transfer current flowing from the transfer means to the image carrier is provided, and during image formation, a non-image forming operation immediately after the bias is applied to the transfer means in accordance with the amount of transfer current. In some cases, it is preferable to set a charging voltage to be applied to the charging unit.

According to another aspect of the present invention, a rotatable image carrier having a photosensitive layer on its surface and a predetermined charging voltage applied to the image carrier by applying a predetermined charging voltage to the image carrier are provided. Charging means for uniformly charging the surface of the image carrier,
Exposure means for forming an electrostatic latent image on the image carrier, which is uniformly charged by the charging means, and attaching toner corresponding to the electrostatic latent image formed on the image carrier. A developing device, a transfer unit for transferring a toner image formed on the image carrier to a transfer material or an intermediate transfer member by applying a predetermined transfer voltage, and applying a charging voltage by the charging unit. At the time of image formation, A
An image forming apparatus having a bias in which the first DC component is superimposed on the C component, and a unit capable of applying any one of AC + DC, AC only, and DC only during non-image formation. An image forming apparatus is provided in which the charging voltage applied to the charging unit during non-image formation immediately before is set in accordance with the value of the DC component of the charging voltage applied to the charging unit.

The charging voltage to be applied to the charging means during non-image formation is set by superposing the first DC component on the same AC component as during image formation, or by applying the second DC voltage without applying AC.
It is preferable to switch the application to the application of only the C component.
The setting of the charging voltage to be applied to the charging unit during non-image formation is preferably to change the value of the AC component superimposed on the first DC component.

In the above invention, the charging means is preferably a charging roller.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an image forming apparatus according to the present invention will be described in more detail with reference to the drawings.

Embodiment 1 FIG. 1 shows a first embodiment of an image forming apparatus according to the present invention.
This will be described with reference to FIG.

In this embodiment, the diameter of the image carrier is 40.
mm photosensitive drum 1 is used. As shown in FIG. 2, the photosensitive drum 1 has a charge generation layer 1 on an aluminum cylinder 1a.
and a photosensitive layer 1d including a charge transport layer 1c and the like. The photosensitive layer 1d is usually an insulator, and has a characteristic that it becomes a conductor when irradiated with light of a specific wavelength. This is because the light irradiation generates hole-electron pairs in the charge generation layer 1b, and these serve as carriers of the charge flow. The charge generation layer 1b has a thickness of 0.2
The charge transport layer 1c is composed of a polycarbonate in which a hydrazone compound having a thickness of about 25 μm is dispersed.

In some cases, another material may be dispersed in the charge transport layer 1c. One example of such a material is Teflon (trade name). Teflon is dispersed for the purpose of reducing the frictional force against a cleaning blade described later, and its particle size is about 0.3 μm, and the dispersion amount is preferably 20% or less. If the amount of dispersion is too large, Teflon becomes an obstacle, and laser light described later is scattered. This is because formation of a sharp latent image may be hindered.

In FIG. 1, a charging roller 5 is used as charging means for uniformly charging the surface of the photosensitive drum 1. Briefly, the charging roller 5 is formed by attaching an elastic rubber layer 5 to a metal cored bar 5a.
b. The charging roller 5 applies a spring pressure to the core metal 5a at both ends thereof, brings the elastic rubber layer 5b into contact with the photosensitive drum 1, and rotates the photosensitive drum 1 with the driven rotation.

When a DC bias equal to or greater than a threshold value is applied to the metal core 5a of the charging roller 5, a discharge is generated near the nip between the charging roller 5 and the photosensitive drum 1. As a result, the photosensitive drum 1 is charged to the same polarity as the bias applied to the charging roller 5. As a result, the surface of the photosensitive drum is charged up to -500V. Hereinafter, this potential is referred to as a Vd potential.
Details of the applied bias will be described later.

The surface of the photosensitive drum 1 maintained at the potential Vd by the discharge of the charging roller 5 is subjected to scanning exposure while controlling the light source on and off based on a signal from the controller using the exposure device 7 to form an electrostatic latent image. It is formed. That is, the surface of the photosensitive drum 1 at the light irradiation position on the photosensitive drum 1 becomes a conductor, and the absolute value of the surface potential of the photosensitive drum 1 decreases. In this embodiment, a semiconductor laser is used as the light source, and the potential on the photosensitive drum 1 at the light irradiation position is approximately -1.
The light amount was controlled so as to be 50V. Hereinafter, the surface potential on the photosensitive drum 1 at this time is referred to as V1 potential.

Thereafter, the yellow developing device 8a is arranged at a position facing the photosensitive drum 1. Yellow developing device 8a
Inside, yellow toner is stored. The yellow toner in the yellow developing device 8a is
It is always negatively charged due to the rubbing with the member in a. Then, the negatively charged yellow toner is thinly coated on the sleeve 9 disposed adjacent to the photosensitive drum 1 and rotating in the forward direction with respect to the photosensitive drum 1 rotation direction. Generally, the sleeve 9 is made of a metal roller. At this time, an appropriate bias between Vd and Vl (hereinafter referred to as “developing bias”) is applied to the sleeve 9. Thereby, an electric field is generated between the photosensitive drum 1 and the sleeve 9,
Only the toner on the sleeve 9 corresponding to the Vl portion on the photosensitive drum 1 jumps onto the photosensitive drum 1 and the development is completed.

However, if this method is adopted, excess toner often adheres to the Vd portion on the photosensitive drum 1 as well. Therefore, the AC is simultaneously applied to the developing bias.
When a bias is applied, the toner can be converged while reciprocating between the sleeve 9 and the photosensitive drum 1 many times, and can be developed more clearly than when only the DC component is applied. That is, when the AC component is applied at the same time, extra toner can be suppressed from adhering to the Vd portion on the photosensitive drum 1. Therefore, the normal developing bias uses the AC + DC bias.

An intermediate transfer belt 4 is arranged in contact with the photosensitive drum 1. The toner adhering to the photosensitive drum 1 is applied to a primary transfer roller 12 disposed in contact with the back surface of the intermediate transfer belt 4 at a portion facing the photosensitive drum 1 so that a positive DC bias is applied thereto. The yellow toner on the photosensitive drum 1 is transferred onto the intermediate transfer belt 4 by the electric field generated in the transfer roller 12.

The primary transfer roller 12 includes a metal core 12a.
And a low-resistance (10 ⁵ Ωcm or less) elastic body 1 covering it
2b. As the intermediate transfer belt 4, a rubber material or a resin material is mainly used. A typical example is a base layer made of rubber coated with a medium-resistance surface layer. A metal core may be embedded in the rubber layer to prevent expansion and contraction. The intermediate transfer belt 4
Is supported by three rollers: a driving roller 14, a secondary transfer opposing roller 15 and a tension roller 13. An appropriate tension is maintained, and the driving roller 14 is driven to drive the photosensitive drum 1 in a forward direction. Rotates at approximately the same speed.

Incidentally, in the current technology, the photosensitive drum 1
100% of the toner cannot be transferred to the intermediate transfer belt 4. If the toner remaining on the photosensitive drum 1 is left as it is, it is transferred to the intermediate transfer belt 4 at the next rotation, and the image is disturbed. In order to prevent this, in the present embodiment, the cleaning blade 11 is brought into contact with the counterclockwise direction of the photosensitive drum 1 to mechanically remove the transfer residual toner from the photosensitive drum 1. The removed toner is collected by the waste toner collecting unit 6 by the cleaning blade 11.

After the above steps, the developing rotary 16 rotates, and the magenta developing device 8b, the cyan developing device 8c, and the black developing device 8d are arranged at positions facing the photosensitive drum 1, and magenta toner, cyan toner, and black toner are The same operation is repeated for each of the four colors on the intermediate transfer belt 4.

Thereafter, the secondary transfer roller 10 disposed opposite to the secondary transfer opposing roller 15 with the intermediate transfer belt 4 interposed therebetween is brought into contact with the intermediate transfer belt 4, and a bias having a polarity opposite to that of the toner is applied. In this state, the transfer material is passed between the intermediate transfer belt 4 and the secondary transfer roller 10, so that the four color toners carried on the intermediate transfer belt 4 are transferred to the transfer material at once.

At this time, if the charge amounts of the four color toners are different, the transferability of each color is different. Therefore, in some cases, the toner on the intermediate transfer belt 4 may be recharged by the corona charger 22 or the like before the secondary transfer, so that the charge amounts of the toners of the respective colors may be uniformed. The secondary transfer roller 10 is formed of a metal core and an elastic body covering the metal core.

The remaining toner on the intermediate transfer belt 4 is
The intermediate transfer belt 4 is removed by the cleaning blade 19 which is in contact with the intermediate transfer belt 4 in the counter direction with respect to the moving direction. Further, the surface of the intermediate transfer belt 4 is neutralized by bringing the neutralizing roller 20 into contact with the intermediate transfer belt 4 and applying an AC bias to the neutralizing roller 20.

The transfer material to which the toner has been transferred is
The toner is conveyed to a fixing device 17 composed of a fixing roller 17a and a pressure roller 17b, heated and pressurized together with the toner, and the toner is permanently fixed on the transfer material, and then discharged to a tray 26 via a discharge roller 24. Is done.

Hereinafter, the bias value applied to the charging roller 5 will be described in detail.

When an image is actually formed by one of the four developing devices 8a to 8d, the DC component is added to Vd by 10 V for dark attenuation to -510 V, and the difference between the peak value and Vd is further reduced. Discharge starting voltage (about 550
V) An AC bias (sine wave) that is greater than or equal to V) is applied to charge the surface of the photosensitive drum 1 more uniformly.

The elastic rubber 5 of the charging roller 5 due to environmental fluctuations
Since the resistance b changes, the AC bias is controlled at a constant current in order to always uniformly charge the surface of the photosensitive drum 1.
In this embodiment, the IAC and the frequency are each 1000 μA,
The frequency was set to 1200 Hz. At this time, under a normal temperature and normal humidity environment, Vpp becomes about 1.8 kV. As a result, when the charging roller 5 shown in this embodiment is used, the surface of the photosensitive drum 1 converges to approximately -500V.

The length h of the intermediate transfer belt 4 needs to be equal to or greater than the sum of the maximum usable transfer material length and the length determined by the product of the time required for switching the developing device and the moving speed of the intermediate transfer member. There is. At this time, the length k of the transfer material corresponds to the image forming area on the intermediate transfer belt 4, and the area obtained by subtracting the length k of the transfer material from the length h of the intermediate transfer belt 4 is a non-image area. Become. In this embodiment, the maximum usable transfer material is the longitudinal direction of A4 size paper, and the length h of the intermediate transfer belt 4 is 400 mm. FIG. 3A shows that A4
The relationship between the image area and the non-image area when two sheets of paper are continuously printed is shown.

Further, FIG. 3B shows a DC voltage applied to the primary transfer roller 12 in a region corresponding to FIG.
Bias values are shown. The surface of the intermediate transfer belt 4 is gradually charged up to the negative polarity every one rotation due to the inflow of charges from the photosensitive drum 1. Further, as the primary transfer of the first color and the second color proceeds, it becomes necessary to superimpose the toner on a place where the toner is already on the intermediate transfer belt 4. Therefore, in order to perform the primary transfer efficiently, it is necessary to gradually increase the primary transfer bias for each color. In the present embodiment, the first color 150 V, the second color 5
It was found that transfer was most efficient when the voltage was 50 V, the third color was 650 V, and the fourth color was 750 V. However, since the optimum value fluctuates considerably depending on the temperature and humidity of the atmosphere in the machine, a member for detecting the temperature and humidity may be provided in the machine and variable control may be performed based on the output value.

The charging bias value in the non-image area shown in FIG. 3A will be described below. Basically, in order to reduce the amount of scraping of the charge transport layer 1c on the surface of the photosensitive drum 1, in the non-image area, only a negative DC bias is applied to the charging roller 5 (color Charge bias). As the inter-color charging bias value, V
The dark decay 10V and the discharge starting voltage 550V to 500V of d
Add to -1060V. However, as described in the section “Problems to be Solved by the Invention”, AC
In the process of switching between charging and inter-color charging, there is a region where a desired high-voltage output cannot be obtained.

FIG. 4 shows an example of such a case.
From AC charging to DC charging between colors, (b) From DC charging between colors to A
During the switching of the C charging, the discharge amount from the charging roller 5 to the photosensitive drum 1 is too small. That is, in the switching region, the AC bias V
This indicates that there is a state where the DC bias has not yet sufficiently increased even when the pp becomes small and becomes equal to or lower than the discharge starting voltage.

At the time of the primary transfer, positive charges flow into the photosensitive drum 1 from the intermediate transfer belt 4 (hereinafter referred to as "primary transfer current"), and a part thereof is trapped by impurities. In particular, in this embodiment, Teflon is intentionally mixed into the outermost charge transport layer 1c of the photosensitive drum 1 in order to reduce the frictional force between the cleaning blade 11 and the photosensitive drum 1. Such a material tends to be a positive charge trapping source, so that the amount of positive charges trapped further increases. Usually, most of these positive charges disappear due to the discharge of negative charges by the charging roller 5.

However, in the switching area shown in FIG. 4, since the amount of discharge from the charging roller 5 to the photosensitive drum 1 is too small, the amount of erasing the trapped positive charges is reduced in other areas (the image forming area and the accurate area). (A non-image forming area where a DC high voltage output is obtained). That is, in the switching area on the photosensitive drum 1, the ratio of the remaining positive charge is larger than that in the other areas, so that the surface of the photosensitive drum 1 caused by recombination with the negative charge given by the charging roller 5 at the time of the next image formation. A decrease in the potential (both Vd and Vl) (so-called dark decay) occurs, and as a result, a horizontal band image in which the density is increased only in that portion is output.

Such a defective image is generated when the primary transfer current is large. If the amount of primary transfer current is suppressed to some extent, even in the switching area, the amount of residual positive charge can be reduced to about the same level as other areas during AC charging at the time of next image formation, so that extreme dark decay does not occur, No horizontal band image is formed.

As a matter of course, the larger the value of the primary transfer bias is, the larger the amount of the primary transfer current is. Therefore, in the present embodiment, as the charging bias application control for the non-image forming area, when the value of the primary transfer bias at the time of forming the previous image is large, the same AC charging as that of the image forming area is performed, and the value of the primary transfer bias is reduced. If smaller, the same D
A method of performing C charging is adopted.

In the present configuration in which the primary transfer bias is applied as described above, after the application of the primary transfer bias of 150 V for the first color, density unevenness does not occur in the switching area. In the switching region for the second and subsequent colors to which the primary transfer bias of 550 V or more is applied, a large amount of primary transfer current flows, and in some cases, density unevenness occurs in the switching region. Therefore, in the present embodiment, only the non-image area portion between the first color and the second color is DC
Charging is performed, and in the other non-image areas, the same AC charging as in the image forming area is performed.

However, the resistance change of the photosensitive layer of the photosensitive drum 1, the intermediate transfer belt 4 and the primary transfer roller 12 due to the temperature and humidity in the apparatus, and the electric capacity change of the photosensitive layer of the photosensitive drum 1 due to the thickness of the photosensitive layer of the photosensitive drum 1. Due to the above factors, the primary transfer current is not constant even if the primary transfer bias is the same. Therefore, in the method of determining the charging method for the non-image area according to the value of the primary transfer bias, the AC charging is performed in spite of the fact that the primary transfer current hardly flows. It is possible. In such a case, the shaving of the photosensitive layer of the photosensitive drum 1 is unnecessarily increased. Also, conversely,
For some reason, DC charging may occur even though the primary transfer current has flowed considerably. In this case, density unevenness at the time of switching will occur.

The following methods may be used as means for improving these adverse effects. As a charging bias application method for the non-image forming area, a means for detecting a primary transfer current value is provided in the apparatus, and the same AC charging as that for the image forming area is performed in the area where a large amount of primary transfer current has flowed during the previous image formation. In the area where the small primary transfer current has flowed,
A method of performing DC charging similar to the conventional method is employed.

Specifically, in the case of the present configuration shown above,
It has been found that DC charging is performed when the primary transfer current is 8 μA or less when the surface potential of the photosensitive drum 1 is Vd, and AC charging is performed when the primary transfer current is higher than 8 μA.

The thickness of the photosensitive layer of the photosensitive drum 1 is 45 μm.
3C and 3D show an example of the primary transfer current amount and the charging bias control at that time.
FIGS. 3E and 3F show the results in a low-temperature and low-humidity (15 ° C., 10%) environment under an environment of 0 ° C. and 80%.

As a specific control method, the power supply
At the time of N and after a fixed time or a fixed number of prints, an image forming operation of a solid white image is performed without passing the transfer material, and a primary transfer current at that time is detected. A method of controlling the charging bias value of the non-image portion based on the output value is sufficient.

As described above, in this embodiment, according to the value of the bias applied to the transfer roller during image formation,
By controlling the charging voltage applied to the charging roller during non-image formation, the amount of shaving of the photosensitive layer on the photosensitive drum surface during use is extremely increased, and the life of the image bearing member is not significantly reduced. The change in density that occurs at the portion where the charging roller switches from AC charging to inter-color charging can be suppressed.

Embodiment 2 FIG. 5 shows a second embodiment of the image forming apparatus according to the present invention.
This will be described with reference to FIG. The image carrier, the charging unit, the exposure device, the developing device, the transfer device, the cleaning device, and the fixing device are all configured the same as in the first embodiment.

In a color image forming apparatus, since colors are reproduced by superimposing four color developers, if the image density fluctuates due to various conditions such as a change in the use environment and the number of prints, an original correct color tone cannot be obtained. Would. Further, an excessive increase in image density causes toner scattering due to an excessive amount of toner. Scattering refers to a state in which the action of air pressure, electromagnetic force, or the like, on a toner carrier such as an image carrier, an intermediate transfer body, or a transfer material, exceeds the toner carrying force, causing the toner image to collapse and spread. It is. When scattering occurs, the electrostatic latent image is not reproduced, and an image with a very poor appearance is output. Thus, in the color image forming apparatus, 4
Since the colors are superimposed, the toner is easily scattered. That is, in the color image forming apparatus, unlike the black and white image forming apparatus, a slight change in density is not allowed.

Therefore, in the present embodiment, a toner image (patch) for density detection is formed on the photosensitive drum 1 on a trial basis, and the density of the toner image is detected by an optical density detection sensor 18 arranged adjacent to the photosensitive drum 1. , And image density control for applying feedback to the image forming conditions such as the developing bias for each of the four colors based on the detection results.

Here, the optical density detection sensor 18 is LE
D light emitting element 18a and photodiode light receiving element 1
8b, the infrared light from the light emitting element 18a is radiated to the patch on the photosensitive drum 1 at an angle of 45 ° to the photosensitive drum 1, and the diffused reflected light therefrom is irradiated to the photosensitive drum 1.
The density of the patch is measured by measuring with the light receiving element 18b arranged to be perpendicular to the patch. If the density detection sensor 18 becomes dirty due to toner scattering or the like, the accuracy of density control deteriorates. Therefore, the density detection sensor 18 is preferably disposed at a position where the influence of toner scattering is as small as possible. The actual execution of the density control is preferably performed about every 100 prints when the main body is turned on. Further, in order to prevent the patches from being disturbed, a mechanism for separating the photosensitive drum 1 from the intermediate transfer belt 4 is provided so that the photosensitive drum 1 and the intermediate transfer belt 4 are separated when the density control is executed.

The developing bias consists of AC + DC high voltage, and the central value contributes to the density. When the central value of the developing bias is shifted to the Vd side, the density increases, and when the central value is shifted to the Vl side, the density decreases.

However, if control is performed by changing only the central value of the developing bias, various adverse effects may occur. Examples include the following: In the developing method of this embodiment, the toner is reciprocated between the sleeve 9 and the photosensitive drum 1 by applying an AC bias.
This is to make the adhesion of the toner converge only on the Vl portion.

However, if the central value of the developing bias is shifted too much to the Vd side, the electric field for returning the toner that has flown to the Vd portion on the photosensitive drum 1 to the sleeve 9 becomes small. May not be collected. In addition, some of the toners are positively charged, although a part of them. If the central value of the developing bias is shifted too much to the Vl side, that is, if the difference between the central value of the developing bias and the Vd potential becomes large, these positively charged toners develop on the Vd potential portion on the photosensitive drum 1. . In any of these cases, the toner is applied to the Vd portion of the photosensitive drum 1, and as a result, a defective image in which the base portion of the transfer material is colored is obtained.

In order to suppress these adverse effects, in the present embodiment, Vd is controlled so that the difference from the center value of the developing bias is within a certain range in conjunction with the change of the center value of the developing bias.
Image density control for changing the image forming conditions will be performed. In the configuration of the present embodiment, if the difference between the central value of the developing bias and the Vd potential is within the range of 100 to 200 V,
It has been found that the above problem does not occur.

Since the above-described image density control is performed for each color, the center value of the developing bias and the Vd value naturally differ for each color. The laser light amount of the exposure device 7 is
Since it is always constant, Vl automatically changes with Vd.
Will also fluctuate.

In the present embodiment, as shown in FIG. 6, the center value of the developing bias for each color is made variable in the range of -550 V to -250 V, so that the density is kept constant, and Vd Is variable in the range of -650 V to -450 V. Since the toner material differs considerably for each color, these fluctuation ranges may be provided with optimum ranges respectively.

Hereinafter, the bias value applied to the charging roller 5 will be described in detail.

When an image is actually formed by any of the four developing devices 8a to 8d, the DC component is a value obtained by adding 10 V to the dark attenuation to Vd, and the AC component is the value described in the first embodiment. A bias similar to the contents is applied. The so-called AC charging is performed. As a result, when the charging roller 5 shown in this embodiment is used, the surface of the photosensitive drum 1 converges to almost the potential Vd.

The length h of the intermediate transfer belt 4 is 400 mm as in the first embodiment. FIG. 7A shows the relationship between the image area and the non-image area when two continuous sheets of A4 paper are printed.

The charging bias value in the non-image area shown in FIG. 7A will be described below. Basically, in order to reduce the shaving amount of the charge transport layer 1c on the surface of the photosensitive drum 1, in the non-image area, only the negative DC bias is applied to the charging roller 5 (between colors) as described in the conventional example. Charging bias). The inter-color charging bias value is Vd-560 V obtained by adding 10 V of dark decay and 550 V of the discharge starting voltage to the value of Vd at the time of the next image formation.

However, as in the example shown in FIG.
In the process of switching between AC charging and DC charging between colors, there is an area where a desired high-voltage output cannot be obtained. As described in the first embodiment, FIG. 4 shows that the discharge amount from the charging roller 5 to the photosensitive drum 1 during the switching from (a) AC charging to inter-color DC charging and (b) inter-color DC charging to AC charging. Indicates that it is too low. In this state, for the same reason as described in the first embodiment, a horizontal band image in which the density is increased only in the switching area is output. In other words, in the switching area, the amount of disappearing positive charges trapped by the discharge of negative charges from the charging roller 5 is smaller than in the other areas, and the dark decay of the surface potential of the photosensitive drum 1 during the next image formation is increased. , The concentration increases.

However, even in such a switching area, if the amount of discharge of negative charges during AC charging at the time of next image formation is increased, a considerable amount of positive charges trapped during charging can be eliminated. And the amount of positive charges remaining in other regions can be made not so different. in short,
If the absolute value of the DC component of the AC charge at the time of the next image formation is large, the amount of remaining positive charge can be reduced during the AC charge even in the switching area, so that extreme dark decay does not occur, and the horizontal band image is not formed. It is not formed.

Therefore, in this embodiment, as the control of the application of the charging bias to the non-image forming area, when the absolute value of the DC component of the charging bias at the time of forming the next image is small, the same AC charging as in the image forming area is performed (the DC component is The DC bias value at the time of the next image formation is used.) When the absolute value of the DC component of the charging bias at the time of the next image formation is large, the above-described DC charging method is employed.

Specifically, in the case of the present configuration shown above,
The absolute value of the DC component of the charging bias at the time of forming the next image is 61
When the voltage is 0 V or more, DC charging is performed.
It turned out that C charging should be performed.

FIG. 7B shows, as an example, the charging bias during non-image formation when Vd is 450 V, 550 V, 650 V, and 500 V in order from the first color.

As described above, in this embodiment, the charging voltage to be applied to the charging roller at the time of non-image formation immediately before is set according to the value of the DC component of the charging voltage applied to the charging roller at the time of image formation. Thereby, the same effect as in the first embodiment can be obtained.

Embodiment 3 Next, a third embodiment of the present invention will be described with reference to FIGS. The image forming apparatus has the same configuration as that of the second embodiment.

By performing the control described in the first embodiment and the second embodiment, it is possible to suppress the density unevenness occurring in the charging bias switching region. However, it is undeniable that the amount of scraping of the photosensitive drum 1 is increased to some extent. In the present embodiment, the photosensitive drum 1 is controlled while suppressing density unevenness occurring in the charging bias switching area.
The control for further suppressing the increase in the amount of shaving will be described.

In this embodiment, the image density control is executed by the same control as the control described in the second embodiment. After executing the image density control, the photosensitive drum 1 and the intermediate transfer belt 4
, And the primary transfer current detection described in the first embodiment is executed.

The bias applied to the charging roller 5 at the time of image formation is an AC + DC bias as in the second embodiment.
The C component is a value obtained by adding 10 V to the dark attenuation to Vd. The charging bias in the non-image forming area is based on the primary transfer current value at the time of forming the previous image and the absolute value of the DC component of the charging bias at the time of forming the next image. Only decide what to do.

The inter-color DC charging here is the same as that described in the second embodiment, and the bias applied to the charging roller 5 is a value obtained by adding 560 V to Vd at the time of forming the next image. Further, the DC component of the AC charging with the DC superimposed is the same as the DC component applied to the charging roller 5 at the time of forming the next image. Then, a sine wave is used as the AC component, and the difference between the peak value and Vd is the discharge starting voltage (about 550).
V) is controlled so as to slightly exceed V). In this embodiment, the IAC is 150 μA from the time of image formation.
850 μA, the frequency is 1 which is the same as that for image formation.
The frequency was set to 200 Hz. At this time, under a normal temperature and normal humidity environment, Vpp becomes about 1.6 kV.

When the charging roller 5 shown in the present embodiment is used, the surface of the photosensitive drum 1 converges to almost the potential Vd even though the AC + DC bias is applied, though it is not uniform. In the non-image area, it is not necessary to uniformly charge as in the image forming area, and it is only necessary to eliminate the positive charges trapped on the photosensitive drum to a certain extent, so such a bias application is sufficient.

As described above, by making the constant current value of the AC bias when applying an AC + DC bias during non-image formation smaller than the constant current value of the AC bias during image formation, damage to the photosensitive drum 1 due to AC discharge can be prevented. Thus, the shaving of the photosensitive layer of the photosensitive drum 1 can be reduced to a lesser extent than during image formation.

FIG. 8 shows a method of selecting the form of the charging bias in the actual non-image area in the present configuration. The upper left part in the figure is an area where density unevenness does not occur on an image even when switching between AC charging and DC charging is performed.
Apply a bias.

On the other hand, the lower right portion is an area in which density unevenness occurs on an image when switching between AC charging and DC charging is performed. In this area, the above-described AC + DC charging is performed.

The primary transfer current has an upper limit of 20 μA and a limiter is provided. If the primary transfer current exceeds 20 μA when the primary transfer current is detected, the primary transfer is performed by changing the primary transfer bias to a value such that the primary transfer current is about 20 μA. At the same time, the primary transfer current is 20μ
If it exceeds A, it is considered that there is a defect in the photosensitive drum 1, the intermediate transfer belt 4, the primary transfer roller 12, or other components, so that a warning is issued to the user indicating that maintenance is required.

FIG. 9 shows an example of actual control. In a high-temperature, high-humidity (30 ° C., 80%) environment, when the total thickness of the photosensitive layer of the photosensitive drum 1 is 45 μm, Vd is 450 V, 550 V, 650 V, and 500 V in order from the first color. The primary transfer bias is 15 from the first color in order.
0 V, 550 V, 650 V, and 750 V.

[0087]

As is apparent from the above description, according to the image forming apparatus of the present invention, the current value flowing into the image carrier from the transfer means at the time of image formation or the charging voltage applied to the charging means at the time of the next image formation. By controlling the charging voltage applied to the charging means during non-image formation in accordance with the value of the DC component, the amount of shaving of the photosensitive layer on the surface of the image carrier during use is extremely increased, and the life of the image carrier is reduced. Can be suppressed without significantly reducing the density change occurring at the switching portion from the AC charging to the inter-color charging by the charging means on the image carrier, and a good image can be obtained.

[Brief description of the drawings]

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

FIG. 2 is a configuration diagram illustrating layers of the photosensitive drum according to the first embodiment.

FIG. 3 is an explanatory diagram showing a control mechanism in the first embodiment.

FIG. 4 is a diagram illustrating a high-voltage output value when the charging bias is switched in the first embodiment.

FIG. 5 is a configuration diagram illustrating an image forming apparatus according to a second embodiment of the present invention.

FIG. 6 is an explanatory diagram of image density control according to a second embodiment.

FIG. 7 is an explanatory diagram of a control selection method in the second embodiment.

FIG. 8 is an explanatory diagram of a control selection method in a third embodiment.

FIG. 9 is an explanatory diagram of control according to a third embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Photosensitive drum (image carrier) 4 Intermediate transfer belt (intermediate transfer member) 5 Charging roller (charging means) 7 Exposure device 8a-8d Developing device 10 Secondary transfer roller 12 Primary transfer roller (transfer means)

Claims (6)

[Claims] 1. An image bearing member having a photosensitive layer on its surface and rotatably arranged, and an image bearing member disposed in contact with the image bearing member and applying a predetermined charging voltage to uniformly uniform the surface of the image bearing member. A charging unit for charging, an exposure unit for forming an electrostatic latent image on the image carrier uniformly charged by the charging unit, and a charging unit corresponding to the electrostatic latent image formed on the image carrier. A transfer device for transferring a toner image formed on the image carrier to a transfer material or an intermediate transfer member by applying a predetermined transfer voltage; and a charging device for charging the toner image. When applying voltage,
Means for applying a bias in which the first DC component is superimposed on the AC component at the time of image formation and applying any one of AC + DC, AC only, and DC only at the time of non-image formation. In the image forming apparatus, a charging voltage to be applied to the charging unit at the time of non-image formation immediately thereafter is set according to a value of a bias applied to the transfer unit at the time of image formation. 2. An image forming apparatus according to claim 1, further comprising: a detecting unit for detecting an amount of transfer current flowing from the transfer unit to the image carrier.
2. The image forming apparatus according to claim 1, wherein a charging voltage to be applied to the charging unit at the time of non-image formation immediately thereafter is set according to a transfer current amount when a bias is applied to the transfer unit. 3. An image bearing member having a photosensitive layer on its surface and rotatably disposed, and being disposed in contact with said image bearing member, and applying a predetermined charging voltage to uniformize the surface of said image bearing member. Charging means for charging the image carrier, an exposing means for forming an electrostatic latent image on the image carrier uniformly charged by the charging means, and an electrostatic latent image formed on the image carrier. A developing device for applying the toner in a corresponding manner, a transfer unit for transferring a toner image formed on the image carrier to a transfer material or an intermediate transfer member by applying a predetermined transfer voltage, and the charging unit At the time of application of the charging voltage, the bias is obtained by superimposing the first DC component on the AC component at the time of image formation, and AC + DC, A at the time of non-image formation.
An image forming apparatus having a unit capable of applying any one of a charging voltage of only C and only DC, according to a value of a DC component of a charging voltage applied to the charging unit during image formation, An image forming apparatus for setting a charging voltage to be applied to the charging unit during non-image formation. 4. The setting of a charging voltage to be applied to the charging unit during non-image formation is performed by setting the first AC component to the same AC component as during image formation.
4. The image forming apparatus according to claim 1, wherein switching is performed between application in which the DC component is superimposed and application of only the second DC component without applying AC. 5. The method according to claim 1, wherein the setting of the charging voltage to be applied to the charging unit during non-image formation is to change a value of an AC component superimposed on the first DC component.
2 or 3 image forming apparatuses. 6. An image forming apparatus according to claim 1, wherein said charging means is a charging roller.