JP2003280318A - Multicolor image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent the adhesion of toner and the adhesion of carrier in another image non-forming unit in a monochrome mode such as black in an image forming apparatus having a plurality of image forming units (4D or the like). <P>SOLUTION: In the no image forming unit, the DC component or the effective value of developing bias is controlled while performing pre-exposure or exposure by using an image exposure means at least between cleaning and development (which is effective in a machine where a transfer belt and an intermediate transfer belt are not attached/detached in the monochrome mode). <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as an electrophotographic copying machine, a printer or a facsimile having a plurality of image forming units including a plurality of image carriers.

[0002]

2. Description of the Related Art FIG. 1 is a schematic configuration diagram showing an image forming apparatus according to this embodiment (in this embodiment, an inline type full-color image forming apparatus in which four photosensitive drums are arranged in a line).

This image forming apparatus includes an image forming section 30Y for forming a yellow image, an image forming section 30M for forming a magenta image, an image forming section 10C for forming a cyan image, and a black color. The image forming unit 30K includes four image forming units that form an image, and these four image forming units (image forming units) are arranged as an example at regular intervals.

Image forming units 30Y, 30M, 30C, 3
At 0K, the photosensitive drum 1a as an image bearing member,
1b, 1c and 1d are provided. Each photosensitive drum 1
Around 1a, 1b, 1c, 1d, primary chargers 2a, 2
b, 2c, 2d, developing devices 4a, 4b, 4c, 4d, 1
Next transfer rollers 22a, 22b, 22c, 22d, cleaning devices (cleaning blades) 7a, 7b, 7
c, 7d are installed respectively, and the primary charger 2a,
Exposure devices 3a, 3b, 3c, 3d are installed above the developing devices 4a, 4b, 4c, 4d and 2b, 2c, 2d, respectively. Each of the developing devices 4a, 4b, 4c, and 4d contains yellow toner, cyan toner, magenta toner, and black toner, respectively.

Under each photosensitive drum 1a, 1b, 1c, 1d
Endless at each primary transfer part N1, N2, N3, N4
The intermediate transfer belt 23 is in contact. Intermediate transfer bell
Drive 23, tension rollers 25, 2
The drive roller is stretched between the next transfer opposing rollers 26.
It is rotated in the direction of the arrow (counterclockwise) by the drive of 24.
It The resistance of the intermediate transfer belt 23 is low in volume efficiency.
10⁶-10¹²Ω · cm is preferred, urethane
Resin, Fluorine Resin, Nylon Resin, Polyimide Resin
Fatty materials and elastic materials such as silicone rubber and hydrin rubber
Materials and carbon or conductive powder dispersed in them to adjust resistance
And the like can be used. This embodiment
Then, carbon is dispersed in the polyimide resin to reduce the volume effect.
Rate 10 ¹¹Thickness adjusted to Ω · cm and 0.5 mm
The intermediate transfer belt 23 was used.

Primary transfer rollers 22a, 22b, 22c,
22d is composed of a cored bar covered with an elastic member having a medium resistance (actual resistance in the nip formation when 1 KV is applied is 10 ⁶ to 10 ¹⁰ Ω), and each of the primary transfer nip portions N1, N2, N.
The photosensitive drums 1a, 1b, 1c, and 1d contact the photosensitive drums 1a, 1b, 1c, and 1 via the intermediate transfer belt 23 at N3 and N4. The secondary transfer counter roller 26 is in contact with the secondary transfer roller 27 via the intermediate transfer belt 23 to form a secondary transfer portion M. Two
The next transfer roller 27 is installed on the intermediate transfer belt 23 so that it can come into contact with and separate from the intermediate transfer belt 23.

Driving roller 2 outside the intermediate transfer belt 23
A belt cleaning device 28 that removes and collects transfer residual toner remaining on the surface of the intermediate transfer belt 23 is installed in the vicinity of 4. A fixing device 18 having a fixing roller 18a and a pressure roller 18b is installed on the downstream side of the secondary transfer portion M in the conveyance direction of the transfer material P.

Next, the image forming operation of the image forming apparatus will be described.

When the image forming operation start signal is issued, the transfer material (paper) P is sent out one by one and conveyed to the registration roller 17. After that, the image forming units 30Y, 3
The registration roller 17 starts rotating in synchronization with the timing at which 0M, 30C, and 30K start image formation.

On the other hand, the image forming sections 30Y, 30M, 30
In C and 30K, when the image forming operation start signal is issued, the photosensitive drums 1a, 1b, 1c and 1d, which are rotationally driven at a predetermined process speed, are respectively charged by the primary charger 2.
A, 2b, 2c and 2d uniformly charge the negative polarity. The exposure devices 3a, 3b, 3c, and 3d respectively convert the input color-separated image signals into optical signals at a laser output section (not shown), and charge the converted laser signals, which are laser beams. Each photosensitive drum 1a, 1b, 1
Scanning exposure is performed on each of c and 1d to form an electrostatic latent image.

Then, first, the electrostatic latent image formed on the photosensitive drum 1a is applied with a developing bias of the same polarity as the charging polarity (negative polarity) of the photosensitive drum 1a. Toner is attached to make a toner image visible. This yellow toner image is rotated at the primary transfer portion N1 by the primary transfer roller 22a to which a primary transfer bias (polarity (positive polarity) opposite to that of the toner) is applied from a primary transfer bias power source (not shown). The primary transfer is performed on the intermediate transfer belt 23.

The intermediate transfer belt 23 to which the yellow toner image is transferred is rotated toward the image forming section 30M. Then, also in the image forming unit 30M, the magenta toner image formed on the photosensitive drum 1b in the same manner as described above is superimposed on the yellow toner image on the intermediate transfer belt 23, and is transferred by the primary transfer unit N2. To be done.

Thereafter, the cyan and black toner images formed by the photosensitive drums 1c and 1d of the image forming sections 30C and 30K are formed on the yellow and magenta toner images which are transferred on the intermediate transfer belt 23 in the same manner. Primary transfer portion N3,
A full-color toner image is formed on the intermediate transfer belt 23 by sequentially superimposing at N4.

Then, in accordance with the timing when the front end of the full-color toner image on the intermediate transfer belt 23 is moved to the secondary transfer portion M, the transfer material P is transferred to the secondary transfer member P by the registration roller 17.
The full-color toner image is collectively transferred to the transfer material P by the secondary transfer roller 27 to which the secondary transfer bias (reverse polarity to toner (positive polarity)) is applied to the transfer material P.
Next is transcribed.

The transfer material P on which the full-color toner image is formed is conveyed to the fixing device 18, and the transfer material P is heated and pressed at the fixing nip portion between the fixing roller 18a and the pressure roller 18b. After heat fixing on the surface, it is discharged to the outside.

During the above-mentioned primary transfer, the primary transfer residual toner remaining on the photosensitive drums 1a, 1b, 1c and 1d is removed and collected by the cleaning devices 7a, 7b, 7c and 7d. Further, the secondary transfer calculation toner remaining on the intermediate transfer belt 23 after the secondary transfer is removed and collected by the belt cleaning device 28.

A full-color image can be formed by using the above-mentioned image forming apparatus. However, when the image to be output is a black single color, it is not necessary to use the yellow, magenta and cyan image forming units. Therefore, for example, a contact / separation mechanism is provided on the intermediate transfer belt so that only the intermediate belt and the black photosensitive drum 1d are brought into contact with each other in the black monochromatic mode so that they are separated from the photosensitive drums of the other stations and the photosensitive drum is rotated and charged. A configuration in which it is stopped can be considered. However, this configuration has the following drawbacks.

Cost increase It is necessary to provide a contact / separation mechanism on the intermediate transfer belt to switch the position between the full color mode and the single color mode.

As a result, the number of parts increases, the cost increases, and the reliability decreases. Further, when the cleaning member for the belt is provided, it is necessary to install the belt so that the performance can be satisfied regardless of the position of the belt, and the degree of freedom in design is limited.

If the color image and the black monochromatic image are mixed in the document to be output, it is necessary to control the contact / separation of the intermediate transfer belt every time the document is changed. That is, the belt must be separated when the next black image of the color image comes. Therefore, it is necessary to change the position using a motor or the like, and to start the next image formation of a black image after the belt state is stabilized.
Further, when the color image comes next to the black image, it is necessary to return the separated belts to the original state. It takes time for the operation and the stability of the belt. Further, if the contacting / separating operation is performed before the secondary transfer ends, the behavior of the belt becomes unstable, and the image is disturbed during the secondary transfer. Therefore, the contacting / separating operation needs to be performed after the secondary transfer ends. is there. Therefore, when the black image comes after the color image, the belt does not come into contact with or separate from the next image forming operation until the secondary transfer is completed after the formation of the magenta image at the trailing end of the image, and it just waits. .

That is, if neither color images nor black images are mixed, the number of sheets that can output, for example, 30 sheets per minute will be reduced to half.

On the other hand, when the belts are not brought into contact with or separated from each other, the image forming operation is performed in all the image forming sections regardless of the kind of the image. However, for example, in the case of a black monochromatic image, image exposure of other magenta, cyan, and yellow stations is not performed. Therefore, if the ratio of black monochromatic images is high, ozone is generated from the primary charger and damages the drum and other parts, even though other stations are not forming images, and the life of parts is reduced. Will be shortened. As a countermeasure, there is a method of turning off the primary charging bias in a station that is not forming an image, which is widely used in some full color printers.

Since the photosensitive drum is rotating at this time, the developing bias is also controlled so that the developing nip portion does not develop or fog toner adheres.

[0024]

However, the problem here is that when the primary charging bias is turned off,
For example, the photosensitive drum may be charged due to frictional charging with the cleaning blade, and the contact state is not always uniform due to eccentricity of the photosensitive drum, uneven thickness of the photosensitive layer, uneven abrasion, blade straightness, and the like. Since it is not present, the triboelectric charge potential also becomes uneven. Therefore, even if the developing bias is controlled, the toner may be developed or the carrier may be attached depending on the location on the photosensitive drum. In particular, if the carrier adheres to the photosensitive drum, it reaches the cleaning blade portion, and the blade edge is chipped to cause problems such as poor cleaning and eventually defective images.

The above problems are not limited to the case where the intermediate transfer belt is used, but the case where the transfer belt as the transfer material carrier is used. The same applies when a contact charging device such as a charging roller or a proximity charging device is used as the primary charging device.

Therefore, an object of the present application is to provide a multi-color image forming apparatus having a plurality of image forming units including a plurality of image carriers, with an image carrier of a station which does not require image formation in the monochromatic image forming mode. It is to obtain a stable image for a long period of time by controlling the potential of a certain photosensitive member to prevent toner adhesion and controlling the life of each component to be as long as possible.

[0027]

[Means for Solving the Problems] To achieve the above object,
A first invention according to the present application is a multicolor image forming apparatus having a plurality of image forming units including a plurality of image carriers, in the case of a single color image forming mode in which an image is formed using only one image forming unit. The image carrier of the other image forming unit (non-image forming unit) is exposed by using the pre-exposure means or the image exposure means, and the DC component or the effective value of the developing bias applied to the developing device is set. It is characterized by controlling.

With the above structure, the uniformity of the surface potential in the developing device of the image carrier of the non-image forming unit is improved,
Adhesion of toner and carrier can be prevented. Therefore, when a monochromatic image is formed, a so-called fog phenomenon in which a small amount of toner of another color adheres is suppressed. Further, when the carrier adheres, the cleaning edge is damaged to cause cleaning failure, resulting in image failure. However, it is possible to prevent the carrier adhesion from occurring fundamentally and prevent cleaning failure and image failure.

Further, at this time, if the primary charging bias is completely turned off, ozone generation due to discharge and deterioration of peripheral parts due to ozone exposure can be prevented when the corona charger is used, and discharge products can be prevented. Since the wire is not contaminated due to such reasons, the life of the primary charging device is also improved. Further, it is possible to suppress the abrasion due to the durability of the surface layer of the image carrier, which is remarkable especially in the contact type charging device. Although the detailed mechanism has not been clarified, this phenomenon damages the molecular structure such as breaking the molecular chains on the surface of the image carrier by the discharge energy generated in the minute gap between the contact charging means and the surface of the image carrier. It is considered that the cleaning blade is used to gradually scrape the surface where the intermolecular force is weakened. The reason why the contact type charging device has a relatively large amount of abrasion is that the discharge amount and the ozone generation amount are smaller than those of the corona charging device, but the density is large because it is concentrated locally. Therefore, the amount of abrasion of the image carrier greatly differs depending on whether the primary charging device is turned on or off, and even if the image carrier rotates, the life can be greatly extended by turning off the primary charging device.

A second aspect of the present invention is a multicolor image forming unit having a plurality of image forming units including a plurality of image carriers, and a single color image forming unit using only one image forming unit. In image forming mode, other image forming units (non-image forming units)
The image carrier is once charged by the primary charging device with a primary charging bias different from that during normal image formation, and then exposed by using the image exposing means, and the DC component or effective value of the developing bias applied to the developing device. It is characterized by controlling the value of.

The above-mentioned structure is different from the first invention in that the primary charging device of the non-image forming unit is turned on,
Uniformity of the charging potential is further improved by once charging to a predetermined potential and then exposing to reduce the potential. here,
The primary charging bias applied to the primary charging device is made weaker than that during normal image formation, or the normal AC + DC bias is limited to DC. The harmful effect of can be suppressed.

According to the third aspect of the present invention, among the image forming units, the image forming unit that does not form an image in the monochromatic image forming mode is at least a carrier using magnetic particles and a resin powder. Toner 2
It has a two-component developing device using a two-component developer containing one as a developer.

In the above-mentioned constitution, since it is possible to prevent the carrier from adhering to the image carrier, which causes cleaning failure and image failure due to the adherence on the image carrier, a two-component developer using a two-component developer is particularly preferable. It is particularly effective for a developing device.

According to a fourth aspect of the present invention, among the image forming units, an image forming unit that does not form an image in the monochromatic image forming mode is in contact with the image carrier or has a very small gap. It is characterized in that it has a contact charging device or a proximity charging device that charges the surface of the image carrier uniformly in close proximity.

The present invention is applicable to an image forming apparatus using a contact charging device or a proximity charging device, which has a large amount of damage to the surface of the image carrier when the primary charging bias is turned on and has a large scraping amount. It is valid. When the above charging device is used in an image forming unit that does not perform image formation during monochromatic image formation, the primary charging bias is turned off during monochromatic image formation or is switched to a bias that causes less damage to the image carrier than usual. This makes it possible to prevent the life of both the charging device and the image carrier from being shortened. Further, by performing exposure after that, it is possible to reduce unevenness of the surface potential of the image bearing member, suppress toner adhesion and carrier adhesion, and prevent image defects.

In a fifth aspect of the present invention, the bias applied to the contact charging device or the proximity charging device is AC + DC during image formation, only DC when image formation is not performed in the single color mode, or during image formation. It is characterized by superposing weaker AC.

In the case of the image forming apparatus using the above-mentioned charging device, which causes a large damage to the image carrier when the primary charging bias is turned on and a large scraping amount, when the image formation is not performed during the monochromatic image formation. By switching to the primary charging bias that causes less damage to the image carrier, it is possible to prevent the life of both the charging unit and the image carrier from being shortened.

[0038]

BEST MODE FOR CARRYING OUT THE INVENTION (First Embodiment) This embodiment can be embodied by the multicolor image forming apparatus of FIG. 1 described in the section [Prior Art] above. Therefore, the description of the configuration and operation is omitted.

When a black monochromatic image is to be output in the above multicolor image forming apparatus, each of the yellow, magenta and cyan image forming stations is not necessary, but since there is no contact / separation mechanism for the intermediate transfer belt, the photosensitive drum is not provided. Can't stop spinning. If stopped, a part of the photosensitive drum will be concentrated and the intermediate transfer belt will be damaged. Further, since it is not necessary to charge the photosensitive drum, the primary charging bias may be turned off. Therefore, in each of the yellow, magenta, and cyan stations, only the photosensitive drum in each image forming unit is in a so-called idle rotation state. Further, the developing bias power source should be controlled in accordance with the surface potential Vs of the photosensitive drum at that time, or may be completely turned off in some cases to prevent toner / carrier adhesion to the photosensitive drum.

Now, the photosensitive drum potential Vs and the developing property will be described. First, a photosensitive drum is a negatively charged organic photosensitive member, and a so-called digital reversal development type image forming apparatus in which toner is developed on a portion where the potential is lowered by laser exposure. FIG. 2 schematically shows a state in which toner is developed by laser exposure on a photosensitive drum charged to a predetermined potential of −700 V by a primary charger. The potential Vd after primary charging and before laser exposure is −70.
0V, the post-exposure potential Vl becomes -200V as an example,
If the DC component Vdc of the developing bias at this time is -550V, the toner is developed at the potential Vl. A further continuous representation of this is the relationship between the relative drum surface potential (Vdc-Vs) and the density of the image developed with toner (so-called VD curve) shown in FIG. That is,
If Vdc, which is the DC component of the developing bias, is constant, more toner is developed as the amount of laser light increases and the photosensitive drum surface potential decreases. On the contrary, in order to prevent the toner from being applied to the white background portion of the image, that is, to prevent fog, it is necessary to make Vdc smaller than the potential of the white background portion by about 100 to 150V. Regarding the amount of carrier adhesion, as shown in FIG. 4, if the relative drum surface potential is increased in the negative direction, the carrier will adhere in reverse even if the developing bias is only DC. This characteristic differs as shown by A, B, and C depending on the distance between the image carrier and the developing sleeve in the developing device, the respective curvatures, the characteristics of the carrier, and the magnetic pole pattern of the magnet in the developing sleeve. With the characteristic B, it can be seen that neither the toner nor the carrier adheres to the photosensitive drum if the relative drum surface potential is set to about −100 to −150V.

However, when the photosensitive drum is idled, the photosensitive drum surface potential Vs at the developing sleeve position is, for example, as shown in FIG. 5 due to contact / separation between the intermediate transfer belt and the photosensitive drum, the cleaning blade and the photosensitive drum, friction and the like. To fluctuate. This is because, as described above in [Problems to be Solved by the Invention], the respective contact states are not always uniform. Further, FIG. 5 shows a temporal change in the potential measured at one point in the thrust direction, but in reality, the thrust direction is not uniform and similarly has unevenness. Therefore,
At this time, the amount of toner and carrier adhered to the photosensitive drum is determined depending on which level the development Vdc is adjusted to. If the developing Vdc is set to 0 V, it can be seen from FIG. 3 that the toner hardly adheres. On the other hand, assuming that the carrier has the characteristic B as shown in FIG. 4, the relative drum surface potential (| Vdc | − | Vs ) Is −
Carrier adhesion development occurs in the vicinity of points A and A'in FIG. As a result, cleaning failure and image failure will occur.

Even in a station where no image is formed, the photosensitive drum surface potential Vs becomes small as a whole as shown in FIG. 6 by irradiating the laser exposure used for the image exposing means. Therefore, at this time, by setting the developing bias Vdc to + 100V, the relative drum surface potential (| Vdc |-| Vs |) is in the range of -100 to -150V, and it can be seen from the B curves in FIGS. 3 and 4. In addition, neither toner adhesion nor carrier adhesion occurs. Therefore, it is possible to prevent the fog phenomenon on the image, the cleaning failure, and the image failure.

Although the V-D curve has been described based on the curve (solid line) in FIG. 3 in the above description, it may be controlled to turn off the AC component of the developing bias of the non-image forming station. If possible, the characteristics change as shown by the broken line in the figure, and the amount of toner developed is reduced as a whole.
Further, if the developing bias is turned off for both AC and DC, the amount of fog toner becomes small as indicated by points B → C in the figure. Also, when controlling to apply only DC,
The relative drum surface potential where the toner hardly adheres also changes from point D to point E in the figure. These points and characteristics are
Although it varies depending on the configuration of the developing device used and the gap between the developing device and the photosensitive drum, it has qualitatively the above characteristics. Therefore, by turning off the AC bias and applying only the DC bias, the range of the relative drum surface potential where neither toner nor carrier adheres can be widened.

Even if the relative drum surface potential is such that the toner is slightly developed, if the rotation of the developing sleeve can be controlled so as to stop, only the toner in the nip area facing the photosensitive drum is developed first. However, it is more preferable to do this because the toner will never adhere to it thereafter. However, in this case, the developing bias power supply and the sleeve rotating mechanism of the BK developing device and the other station developing devices need to be separately controlled so that they can be independently controlled.

Further, the toner in the nip portion on the developing sleeve opposed to the photosensitive drum is from the end of the color image formation to the start of the next black image formation (so-called paper interval).
In addition, by skipping over the photosensitive drum, it is possible to prevent the fog phenomenon from occurring in the next image. The sequence of each part of the non-image forming unit at this time is shown in FIG. It shows that color image formation ends at time t ₁ and then black monochromatic image formation starts at time t ₂ . When the color image formation is completed, the primary charging bias, the laser exposure, and the development A in the non-image forming unit at the time of forming the black image of yellow, magenta, cyan, etc.
The C bias and the development sleeve rotation are turned off, and the value of the development DC bias Vdc is switched to a smaller value.
After this, the time t ₃ until the time t ₅ when the next image formation starts is performed.
In ~t _4, by temporarily turning on the development AC bias and laser exposure, it is possible to skip the toner on the developing sleeve opposite to the photosensitive drum to the photosensitive drum. Therefore, it is possible to completely prevent the development of toner fog during the subsequent image formation. It should be noted that, due to the recent increase in the speed of the image forming apparatus, the rotation speed of the photosensitive drum and the transfer material transport speed have been increased, and the distance between transfer materials during continuous image formation has been shortened. It can be difficult to do this between pages. In this case, if the toner in the nip portion is positively attempted to be blown to the photosensitive drum, the toner may be caught in the image portion.
In such a case, it is difficult to adopt this control, but as described above, it is possible to prevent toner adhesion and carrier adhesion only by controlling the laser exposure and the development Vdc. Since the carrier remains on the developing sleeve even after the toner in the nip portion is blown between the sheets to the photosensitive drum, the control of the laser exposure and the developing Vdc is effective at this time as well, and the toner may be blown thereafter. There is no development V
The control value of dc may be set with an emphasis on prevention of carrier adhesion.

It should be noted that the laser exposure amount of the non-image forming unit in this embodiment does not necessarily require strong light as long as the fluctuation is within the range of the surface potential of the photosensitive drum which can prevent toner adhesion and carrier adhesion. It may be designed according to the life of the laser light emitting element and the width of the surface potential unevenness of the photosensitive drum after exposure.

As described above, by controlling the value of the developing Vdc at the same time when the laser exposure of the non-image forming unit is turned on, the adhesion of toner and carrier is prevented and stable image formation can be performed for a long period of time. Become. Furthermore, development AC bias control, development sleeve rotation control,
A more stable image formation becomes possible by combining the control of skipping the toner on the developing sleeve at the portion facing the photosensitive drum between the sheets.

(Second Embodiment) In the present embodiment, as shown in FIG. 8, the potential (A) of the photosensitive drum surface which has been rubbed with the cleaning blade is once higher than the potential (A) by the primary charging roller. After charging to B),
Similar to the first embodiment, the laser light is irradiated to control so that the potential is finally lowered to (C). When the primary charging roller is further charged in the state where there is uneven charging in the initial period (A), the potential rises as a whole, but the lower potential portion is more easily charged. Becomes slightly smaller from (A) to (B). Next, by irradiating laser light, the potential drops from (B) to (C),
As a result, the width of the final potential unevenness becomes smaller than that of the first embodiment, and it is possible to more reliably prevent toner adhesion and carrier adhesion in the developing section.

Note that FIG. 8 is a sine wave representation of the time change of the surface potential Vs accompanying the rotation of the photosensitive drum at a certain point in the thrust direction. It may be considered that there is a fluctuation of.

(Third Embodiment) In the first and second embodiments, the drum charging potential is lowered between the position of the cleaning blade for cleaning the photosensitive drum and the position of the primary charging means to prevent the memory during the previous image formation. Although there is no pre-exposure means to do so, it is assumed in the present embodiment. A fuse lamp array in which several to ten fuse lamps are arranged in the thrust direction, an LED chip array in which a large number of LED chips are arranged on a substrate, and a light made of resin for LED light are commonly used as pre-exposure means. Any type such as a type that is guided onto the photosensitive drum by a guide can be used.

Description will be made with reference to FIG.

(A): Potential immediately after the cleaning blade: Potential unevenness is large.

(B): After pre-exposure irradiation: A large amount of light is irradiated against normal image exposure to prevent memory, so that the potential is greatly reduced and at the same time, the magnitude of potential unevenness (potential width) is considerably reduced.

(C): After passing the primary charging: The primary charging roller or the like recharges. At this time, AC + DC has better charging uniformity than DC, and charging unevenness is small, but damage to the photosensitive drum is also large, and the surface layer of the drum is scraped and life is shortened. It may be designed with a balance between the size of the unevenness and the relative drum surface potential width that can prevent toner / carrier adhesion. Further, it is not always necessary to charge the battery as high as the potential for performing normal image formation.

(D): After light irradiation by the image exposure means: The potential is lowered by light irradiation using a laser element that performs image exposure during normal image formation, and finally obtained and passes through the development area. When doing so, the potential unevenness can be almost eliminated as in (D). Therefore, it is possible to suppress both toner adhesion and carrier adhesion at the yellow, magenta, and cyan stations in the black monochromatic mode, and it is possible to perform stable image formation without fog toner and for a long period of time.

Although the primary charging roller has been described as an example of the primary charging means, other means such as a corona charger can be used.

(Fourth Embodiment) In the monochromatic image forming mode, the pre-exposure is turned on and the developing bias is controlled in the non-image forming unit. This will be described with reference to FIG.

(A): Potential immediately after the cleaning blade: Potential unevenness is large.

(B): Potential immediately after pre-exposure: Since the memory is prevented by irradiating a stronger light amount than in the normal image exposure, the potential is greatly reduced and at the same time the potential unevenness (potential width). Will be much smaller.

This embodiment does not include the latter half of the control of the third embodiment (primary charging and exposure by laser), but even in this state, the potential unevenness can be considerably reduced, so that it is effective in suppressing toner adhesion and carrier adhesion. Target. Further, when a corona charger is used as the primary charging means, ozone is not generated and the ozone deterioration of peripheral parts can be prevented, and at the same time, the problem of shortening the life of the charger due to discharge does not occur. In addition, it is possible to avoid the problem of shortening the life due to damage to the photosensitive drum and an increase in the amount of scraping, which is conspicuous when a contact type charging means (such as a charging roller) is used. Also, resistance fluctuation due to energization of the charging roller or the like can be avoided in the single color mode. Further, since the laser element used for image exposure is not used as the light irradiation means, there is an advantage that the life of the laser element itself is not shortened.

As described above, when a multicolor image forming apparatus is used to form a monochromatic image, for example, a black monochromatic image,
In the image forming units of stations other than black, when the unevenness of the photosensitive drum surface potential Vs at the developing nip position is large, the developing bias is controlled, and exposure between cleaning and developing is performed to reduce the unevenness of potential. Since the charging unevenness can be reduced, toner adhesion and carrier adhesion in the non-image forming unit can be prevented, and stable image formation can be performed for a long period without fog.

[0062]

As described above, the first aspect of the present application
In a multicolor image forming apparatus having a plurality of image forming units including a plurality of image carriers, another invention is used in the case of a single color image forming mode in which an image is formed using only one image forming unit. The image carrier of the forming unit (non-image forming unit) is exposed by using the pre-exposure means or the image exposure means, and the DC component or the effective value of the developing bias applied to the developing device is controlled. To do.

With the above structure, the uniformity of the surface potential in the developing device of the image carrier of the non-image forming unit is improved,
Adhesion of toner and carrier can be prevented. Therefore, when a monochromatic image is formed, a so-called fog phenomenon in which a small amount of toner of another color adheres is suppressed. Further, when the carrier adheres, the cleaning edge is damaged to cause cleaning failure, leading to image failure. However, it is possible to prevent the carrier from being fundamentally caused to prevent cleaning failure and image failure.

At this time, if the primary charging bias is completely turned off, when a corona charger is used, it is possible to prevent ozone generation due to discharge and deterioration of peripheral parts due to ozone exposure. Since the wire is not contaminated due to such reasons, the life of the primary charging device is also improved. Further, it is possible to suppress the abrasion due to the durability of the surface layer of the image carrier, which is remarkable especially in the contact type charging device.

That is, the amount of abrasion of the image carrier greatly differs depending on whether the primary charging device is turned on or off. Even if the image carrier rotates, the life can be greatly extended by turning off the primary charging device.

The second aspect of the present invention is a multicolor image forming unit having a plurality of image forming units including a plurality of image carriers, and a single color image forming unit using only one image forming unit. In image forming mode, other image forming units (non-image forming units)
The image carrier is once charged by the primary charging device with a primary charging bias different from that during normal image formation, and then exposed by using the image exposure means, and the DC component of the developing bias applied to the developing device or effective. It is characterized by controlling the value of the value.

The above-mentioned structure is different from the first invention in that the primary charging device of the non-image forming unit is turned on.
Uniformity of the charging potential is further improved by once charging the battery to a predetermined potential and then exposing the film to reduce the potential. Here, the primary charging bias applied to the primary charging device is made weaker than during normal image formation, or normally AC + DC.
By using a method in which only DC is used as the bias, it is possible to suppress the adverse effects such as the shortening of the life due to the abrasion of the image carrier as described above.

In the third invention according to the present application, among the image forming units, the image forming unit that does not form an image in the monochromatic image forming mode is at least a carrier using magnetic particles and a resin powder. Toner 2
It has a two-component developing device using a two-component developer containing one as a developer.

In the above-mentioned constitution, since it is possible to prevent the carrier from adhering to the image carrier, which causes the cleaning failure and the image failure due to the adherence on the image carrier, the two-component developer using the two-component developer is particularly used. It is particularly effective for a developing device.

According to the fourth aspect of the present invention, among the image forming units, the image forming unit that does not form an image in the monochromatic image forming mode is in contact with the image carrier or has a very small gap. It is characterized in that it has a contact charging device or a proximity charging device that charges the surface of the image carrier uniformly in close proximity.

In the above-described structure, the present invention is applied to the case of an image forming apparatus using a contact charging device or a proximity charging device in which the surface of the image carrier is largely damaged when the primary charging bias is on and the amount of abrasion is large. It is valid. When the above charging device is used in an image forming unit that does not perform image formation during monochromatic image formation, the primary charging bias is turned off during monochromatic image formation or is switched to a bias that causes less damage to the image carrier than usual. This makes it possible to prevent the life of both the charging device and the image carrier from being shortened. Further, by performing exposure after that, it is possible to reduce unevenness of the surface potential of the image bearing member, suppress toner adhesion and carrier adhesion, and prevent image defects.

In the fifth invention of the present application, the bias applied to the contact charging device or the proximity charging device is AC + DC during image formation, only DC when image formation is not performed in the single color mode, or during image formation. It is characterized by superposing weaker AC.

In the case of an image forming apparatus using the above-mentioned charging device, which causes a large damage to the image carrier when the primary charging bias is turned on and a large amount of scraping occurs, when an image is not formed during monochromatic image formation. By switching to the primary charging bias that causes less damage to the image carrier, it is possible to prevent the life of both the charging unit and the image carrier from being shortened.

Although the image forming apparatus using the intermediate transfer member has been described above as an example, the present invention can be applied to an image forming apparatus using a transfer belt carrying and conveying a transfer material.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a multicolor image forming apparatus that can embody each embodiment of the present invention.

FIG. 2 is an explanatory diagram of a photosensitive drum surface potential, a realization Vdc, and a developing property in a reversal developing system.

FIG. 3 Relative drum surface potential (| Vdc | − | Vs |)
And FIG. 5 is a view showing a relationship between image density and image density (VD curve).

FIG. 4 Relative drum surface potential (| Vdc | − | Vs |)
FIG. 3 is a diagram showing a relationship between a carrier adhesion amount on a photosensitive drum and a photosensitive drum.

FIG. 5 is a surface potential of the photosensitive drum before control in the first embodiment.

FIG. 6 is a photosensitive drum surface potential after control in the first embodiment.

FIG. 7 is a sequence diagram of each unit according to the first embodiment.

FIG. 8 is a control example of the photosensitive drum surface potential in the second embodiment.

FIG. 9 is a control example of the photosensitive drum surface potential in the third embodiment.

FIG. 10 shows an example of control of the photosensitive drum surface potential in the fourth embodiment.

[Explanation of symbols]

1: 1a to 1d Photosensitive drum (image carrier) 2: 2a to 2d Primary charger (charging means) 3: 3a-3d Laser element (exposure device) 4: 4a-4d Developing device 7: 7a-7d Cleaning blade 9: 9a to 9d Development sleeve (developer carrier) 22: 22a to 22d Primary transfer roller 23 Intermediate transfer belt 27 Secondary transfer roller

─────────────────────────────────────────────────── ─── Continued Front Page (51) Int.Cl. ⁷ Identification Code FI Theme Coat (Reference) G03G 15/02 102 G03G 15/02 102 15/04 111 15/04 111 15/06 101 15/06 101 21 / 00 384 21/00 384 F-term (reference) 2H027 EA01 EA05 EB04 EC20 ED03 ED06 ED09 EE07 EF09 FA28 2H073 AA03 BA04 BA22 CA03 CA22 2H076 AB02 DA11 EA01 2H200 FA08 FA02 GA12 GA23 GA34 GA45 GA02 HA12 GB02 GA29 GA02 HA12 GB02 PA26 PB01 2H300 EA10 EB04 EB07 EB12 EC02 EC05 EG02 EG03 EG05 EG13 EH16 EJ01 EJ09 EJ33 EJ35 EJ39 EJ46 EJ47 EJ51 FF02 FF05 FF08 GG01 GG02 GG03 GG11 GG29 GG31 GG37 TT06QQ32 TT37

Claims (5)

[Claims] 1. In a multicolor image forming apparatus having a plurality of image forming units including a plurality of image carriers, in the case of a single color image forming mode in which an image is formed using only one image forming unit, other The image carrier of the image forming unit (non-image forming unit) is exposed by using the pre-exposure means or the image exposure means, and the DC component or effective value of the developing bias applied to the developing device is controlled. And a multicolor image forming apparatus. 2. A multicolor image forming unit having a plurality of image forming units including a plurality of image carriers, and in the case of a monochromatic image forming mode in which an image is formed using only one image forming unit, other The image carrier of the image forming unit (non-image forming unit) is once charged by the primary charging device with a primary charging bias different from that during normal image formation, and then exposed by the image exposing means and applied to the developing device. DC of developing bias
A multicolor image forming apparatus characterized by controlling the value of a component or an effective value. 3. An image forming unit that does not form an image in the single color image forming mode among the image forming units contains at least two carriers, that is, a carrier using magnetic particles and a toner which is a resin powder. The multicolor image forming apparatus according to claim 1, further comprising a two-component developing device that uses a two-component developer. 4. An image forming unit that does not form an image in the single-color image forming mode among the image forming units is in contact with the image carrier or is close to the image carrier with a very small gap to make the surface of the image carrier uniform. The multicolor image forming apparatus according to any one of claims 1 to 3, further comprising a contact charging device or a proximity charging device that is charged to the surface. 5. The bias applied to the contact charging device or the proximity charging device is AC + DC during image formation, DC only when image formation is not performed in the single color mode, or AC weaker than that during image formation is superimposed. The multicolor image forming apparatus according to claim 1.