JP2002357939A - Image forming method and image forming apparatus - Google Patents

Abstract

[PROBLEMS] To provide an image forming method and an image forming apparatus capable of preventing image quality deterioration due to reverse transfer of toner to an image carrier without impairing the high speed of the apparatus. SOLUTION: Each photoconductor 21BK, 21M, 21Y, 2
1C, the surface potential of the photoconductor and the primary transfer electric field are set so that no discharge occurs between the surface of the photoconductor not involved in image formation immediately before the primary transfer and the toner on the intermediate transfer belt 22. To control the potential difference from the surface potential of the intermediate transfer belt 22 due to the above. As a result, the charge amount of the toner on the intermediate transfer belt 22 does not change, and the reverse transfer of the toner to the photoconductor can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, a printer, a facsimile, and the like.
The present invention relates to an image forming apparatus for obtaining a multicolor image by multiplex-transferring toner images formed on a plurality of latent image carriers by an image forming process such as an electrostatic recording method or an electrophotographic method.

[0002]

2. Description of the Related Art In recent years, a large number of color documents have been handled in offices, and it has been desired to provide a higher-speed image forming apparatus such as a full-color printer and a full-color copying machine. 2. Description of the Related Art In recent years, a color laser printer, which has become widespread, generally includes a plurality of developing devices arranged in contact with a single photoconductor, creates a toner image of each color for each rotation of the photoconductor, and prints the toner image on paper or the like. A color toner image is created by transferring the image onto a transfer material such as an OHP sheet.
Drum configuration is the mainstream.

In addition, as a one-drum type image forming apparatus, a toner image of each color on a photoreceptor is primarily transferred onto an intermediate transfer member and superimposed, and then a secondary transfer is collectively performed on a transfer material. There are a transfer method and a direct transfer method in which a toner image of each color on a photoconductor is sequentially transferred to transfer paper held on a transfer drum or the like to create a color toner image.

The direct transfer type image forming apparatus has the advantages that the structure is simple and the cost is low, and that the image forming speed can be increased. However, in this direct transfer type image forming apparatus,
In the process of sequentially transferring the toner images of the respective colors, the resistance and the water content of the transfer paper change and the transfer conditions are different, so that stable transfer is difficult. On the other hand, in the intermediate transfer type image forming apparatus, the toner image can be transferred to the transfer material only once, so that stable transfer can be performed. Further, in an image forming apparatus of a direct transfer system in which a transfer material is wound around a transfer drum, the thickness and type of the transfer material are limited. On the other hand, the intermediate transfer type image forming apparatus has an advantage that the versatility of the transfer material is high without any limitation on the thickness and type of the transfer material.

However, in the above-described one-drum image forming apparatus, in either system, in order to obtain a color image in which four color toner images are superimposed, for example, a photosensitive image is formed for forming each color toner image. Since the body needs to be rotated at least four times, it takes a long time to form a color image, and productivity is not improved.

Therefore, conventionally, in order to cope with high-speed color image formation, a plurality of photoconductors (usually three or four) are juxtaposed along a transfer material conveying path, and each photoconductor is individually provided. After the formed latent images of each color are formed into toner images by a developing device mounted on each photoreceptor, these toner images of each color are successively superimposed and transferred onto a conveyed transfer material to obtain a color image. A so-called tandem type or in-line type image forming apparatus configured as described above has been proposed.
For example, Japanese Patent Application Laid-Open No. Sho 53-74037 (U.S. Pat. No. 4,162,843) discloses that in order to speed up the color image output, a plurality of photoconductors are stacked and the transfer material is belt-shaped. There has been proposed an image forming apparatus for sequentially transferring a toner image in a multiplex manner while being conveyed by a conveying means.

In the tandem type or in-line type image forming apparatus, if the peripheral speed of the photosensitive member is the same, it is possible to form an image at four times or more the speed of the one-drum type image forming apparatus. is there. However, since a normal tandem type or in-line type image forming apparatus is configured to directly transfer a toner image on a photoreceptor onto a transfer material, as described above, transfer of a toner image to transfer paper is performed. There are many problems such as unstable image quality and difficulty in positioning the transfer material when transferring the toner image. In order to solve such a problem, an image forming apparatus of a so-called tandem intermediate transfer system using a tandem system and an intermediate transfer member has been proposed (Japanese Utility Model Application Laid-Open No. 59-59).
192159).

[0008]

However, a tandem method in which a plurality of photoconductors as described above are used to transfer multiple toner images onto a transfer material such as transfer paper or a transfer target such as an intermediate transfer body in a multiplex manner. The following problems may occur in the image forming apparatuses of the image forming method, the in-line method, and the tandem intermediate transfer method. That is, in this type of image forming apparatus, a photoconductor as a latent image carrier, a charging device as charging means for charging the photoconductor, and a latent image forming means for forming an electrostatic latent image on the photoconductor are provided. Latent image writing device,
An image forming unit including a developing device as a developing device for developing the electrostatic latent image and a transfer device as a transferring device for transferring a toner image formed by the development onto a transfer target is required for image formation. A plurality of toners need to be provided according to the number of toner colors. For example, when a full-color image is formed by using four color toners of cyan toner, magenta toner, yellow toner, and black toner, it is necessary to provide four image forming units.

On the other hand, in an image forming apparatus equipped with such four image forming units, an image is formed using one color toner, for example, by selectively using each image forming unit to form an image. It is possible to provide an image forming mode such as a single color mode, a two-color mode for forming an image with two-color toner, a color mode for forming an image with three-color toner, and a full-color mode for forming an image with all toners. . For example, in the two-color mode, a two-color image composed of magenta toner and cyan toner is formed using only the magenta unit for magenta toner and the cyan unit for cyan toner among the image forming units. Is possible.

Here, for example, in the two-color mode using the black toner and the yellow toner, the black unit for the black toner and the yellow unit for the yellow toner among the image forming units are formed according to a predetermined image forming process. If only the unit operates, image formation can be performed even when other magenta toner magenta units and cyan toner cyan units are stopped.

However, this type of image forming apparatus is generally configured such that the object to be transferred sequentially passes through the transfer areas of each of the image forming units facing the respective photosensitive members. For this reason, for example, when all the operations of the image forming processes of the magenta unit and the cyan unit that are not involved in the image formation are stopped during the execution of the two-color mode, the black unit and the yellow unit cause the image to be transferred onto the transfer target. When the already formed black image and yellow image pass through the transfer area of the magenta unit and the cyan unit, they are rubbed by the surface of the photoreceptor, and the image quality of the image already transferred on the transfer target body May be significantly impaired. In addition, when all operations of the image forming process of the image forming unit that are not involved in the image forming are stopped, the image forming unit is configured such that the transfer material passes through the transfer area of the stopped image forming unit. In the apparatus, the transfer material is likely to be jammed in the transfer area.

As a method for solving such a problem,
An image forming unit not involved in image formation may be operated. However, in this method, although the above problem can be avoided, since the image forming unit which does not need to be operated always operates, the life of the parts of the image forming unit is shortened, and the running cost is reduced. It will rise and extra power will be wasted.

Thus, in this type of image forming apparatus,
For example, when the two-color mode using the black toner and the yellow toner is executed, when the transfer material passes through the transfer area of the magenta unit and the cyan unit, the black toner and the yellow toner already transferred on the transfer material are used. Some of the toner may be reversely transferred to the magenta unit and cyan unit photoconductors. In such a case, the image on the transfer material is remarkably deteriorated, which causes a problem such as image unevenness, density reduction, or color balance shift.

In order to solve such a problem of image deterioration due to reverse transfer to a photosensitive member, for example, Japanese Patent Application Laid-Open No.
As described in Japanese Patent No. 0866, urging and release of the transfer belt to the photoreceptor of each image forming unit are performed by
It is conceivable to use a method of performing control so as to be selectively performed according to the image forming mode. But with this method,
It is necessary to provide a pressure release mechanism for selectively bringing the transfer belt into and out of contact with each photoconductor, for each image forming unit. Therefore, when the pressure release mechanism is operated at the time of image formation, there is a possibility that the transferred image on the transfer object may be adversely affected by mechanical vibration generated during the operation of the pressure release mechanism. Further, in order to avoid such an adverse effect due to the vibration of the pressure release mechanism during image formation, the pressure release mechanism is operated during non-image formation, and the image forming operation is performed until the operation of the pressure release mechanism is completed. Cannot be started, and the high-speed performance of the device is impaired.

Even in an image forming apparatus having such a pressure release mechanism, in the case of a full-color mode in which a full-color image is formed by using all image forming units, the toner images of the second and subsequent colors are formed. At the time of transfer, the toner that has already been transferred onto the transfer target body may be reversely transferred to the photoconductors of the image forming units for the second and subsequent colors. When such a reverse transfer of the toner occurs, when the residual toner remaining on the photoconductor is removed and conveyed by the photoconductor cleaner and reused in the developing device, the reverse transfer to the photoconductor is performed. The toner mixes with the original toner in the developing device, causing a problem of toner color mixing. Such color mixture of the toner causes a remarkable reduction in image quality, and thus poses a serious problem when forming a color image.

In order to solve the above-mentioned problem due to the reverse transfer of the toner, Japanese Patent Application Laid-Open No. 9-146334 discloses that the surface of the image carrier has a contact angle of 8 to water.
There has been proposed a method of forming a surface having a temperature of 5 degrees or more. However, this method can prevent the reverse transfer of the toner due to the releasability of the toner on the surface of the photoconductor to some extent, but cannot prevent the reverse transfer of the toner due to the surface potential of the photoconductor. However, it is difficult to sufficiently solve the problem of reverse transfer as described above.

The present invention has been made in view of the above problems, and an object of the present invention is to prevent deterioration of image quality due to reverse transfer of toner to an image carrier without impairing the high-speed operation of the apparatus. It is an object of the present invention to provide an image forming method and an image forming apparatus capable of performing the above.

[0018]

In order to achieve the above object, the present invention provides a latent image carrier for carrying an electrostatic latent image and an electrostatic latent image on the latent image carrier. Latent image forming means for forming, developing means for developing the electrostatic latent image to form a toner image, and transfer means for transferring the toner image on the latent image carrier to a transfer object A plurality of image forming means having a plurality of image forming means, and a transfer medium transferring means for transferring the transfer medium so as to sequentially contact the surface of each latent image carrier of the plurality of image forming means. An image forming method in an image forming apparatus, wherein a potential difference between a surface potential of a latent image carrier that is not involved in image formation among the latent image carriers and a surface potential of the transfer target is determined. Controlling the potential difference so that the toner transferred thereon does not reversely transfer to the latent image carrier. It is.

If the potential difference between the surface potential of the photosensitive member as the latent image carrier and the surface potential of the transfer paper or the intermediate transfer belt as the transfer target is small, the potential difference between the surface potential of the transfer paper and the intermediate transfer belt is reduced. The toner transferred to the photosensitive member is not reversely transferred onto the photosensitive member. Therefore, in this image forming method, the potential difference between the surface potential of the latent image carrier that is not involved in image formation among the latent image carriers and the surface potential of the transfer target is set on the transfer target. The potential difference is controlled so that the transferred toner does not reversely transfer to the latent image carrier. As a result, the reverse transfer of the toner on the transfer medium to the latent image carrier that is not involved in the image formation can be avoided, and the image quality deterioration due to the reverse transfer of the toner to the latent image carrier is prevented. Become like Further, in an image forming apparatus in which the residual toner on the latent image carrier is collected and reused in the developing device, color mixing of the toner in the developing device is prevented.

According to a second aspect of the present invention, there is provided the image forming method according to the first aspect, wherein the latent image carrier among the latent image carriers not involved in image formation is contacted with the transfer target immediately before the contact.
The surface potential of the latent image carrier is controlled, and the potential difference between the surface potential of the latent image carrier and the surface potential of the transfer object is determined by the toner transferred onto the transfer object. This is characterized in that the potential difference is controlled so as not to reversely transfer to the image carrier.

In this image forming method, the surface potential of the latent image carrier that is not involved in image formation among the latent image carriers is controlled, so that the surface potential of the latent image carrier is The potential difference from the surface potential of the transfer object is controlled to a potential difference at which the toner transferred on the transfer object does not reversely transfer to the latent image carrier.

According to a third aspect of the present invention, in the image forming method of the second aspect, a charging step for charging a surface of the latent image carrier not involved in image formation among the latent image carriers is not performed. In this way, the surface potential of the transfer object and the surface potential of the latent image carrier are controlled.

In this image forming method, the surface of the latent image carrier immediately before the latent image carrier which is not involved in the image formation among the respective latent image carriers is brought into contact with the transfer-receiving body is formed. It is no longer charged. Thereby, the potential difference between the surface potential of the latent image carrier and the surface potential of the transfer object is controlled to a potential difference at which the toner transferred on the transfer object does not reversely transfer to the latent image carrier. You.

According to a fourth aspect of the present invention, in the image forming method of the second aspect, a charging potential in a charging step for charging a surface of the latent image carrier which is not involved in image formation among the latent image carriers. To control the surface potential of the transfer object and the surface potential of the latent image carrier.

In this image forming method, the surface of the latent image carrier immediately before the latent image carrier that is not involved in the image formation among the latent image carriers is brought into contact with the object to be transferred is removed. The charging potential in the charging step for charging is controlled. Thereby, the potential difference between the surface potential of the latent image carrier and the surface potential of the transfer object is controlled to a potential difference at which the toner transferred on the transfer object does not reversely transfer to the latent image carrier. You.

According to a fifth aspect of the present invention, in the image forming method of the third or fourth aspect, development by a developing unit not involved in image formation of each of the image forming units is not performed. .

In the case where the surface potential of the latent image carrier is controlled as in the image forming method of the third or fourth aspect, the developer supplied from the developing means may adhere to the surface of the latent image carrier. There is. Such adhesion of the developer to the latent image carrier not involved in image formation not only results in wasteful consumption of the developer but also transfers the developer to the transfer-receiving member and transfers the developer to the transfer-receiving member. Cause contamination. Therefore, in this image forming method, development by a developing unit not involved in image formation is not performed. This prevents the developer from adhering to the latent image carrier that is not involved in image formation.

According to a sixth aspect of the present invention, in the image forming method according to the fifth aspect, the developing means contacts a surface of the latent image carrier with a developer in a development area facing the latent image carrier. The developing device, wherein the developing device is configured to perform the development, and the development unit does not perform the development by cutting off the contact between the surface of the latent image carrier and the developer in the development area. is there.

In this image forming method, the contact between the surface of the latent image carrier and the developer in the developing area is cut off in order to prevent development by a developing means not involved in image formation. This prevents the developer from adhering to the latent image carrier that is not involved in image formation.

According to a seventh aspect of the present invention, in the image forming method of the sixth aspect, the surface of the latent image carrier in the development area is separated from the surface of the latent image carrier by separating the latent image carrier and the developing means. It is characterized in that the contact with the agent is cut off.

In this image forming method, the latent image carrier and the developing means are separated from each other in order to prevent development by a developing means not involved in image formation. As a result, the contact between the surface of the latent image carrier and the developer in the development area is cut off, and the adhesion of the developer to the latent image carrier is prevented.

According to an eighth aspect of the present invention, in the image forming method of the sixth aspect, the developing means has a developer carrying member for supplying the developer to the developing area by carrying the developer and rotating. The supply of the developer on the developer carrier to the developing region is cut off, whereby the contact between the surface of the latent image carrier and the developer in the developing region is cut off. .

In this image forming method, the supply of the developer on the developer carrier to the developing area is cut off in order to prevent the development by the developing means not involved in the image formation. As a result, the contact between the surface of the latent image carrier and the developer in the development area is cut off, and the adhesion of the developer to the latent image carrier is prevented.

According to a ninth aspect of the present invention, in the image forming method of the eighth aspect, the developer carrier is reversed to stop supplying the developer on the developer carrier to the developing area. Is what you do.

In this image forming method, the developer carrier is reversed so as to cut off the supply of the developer on the developer carrier not involved in image formation to the developing area. As a result, the contact between the surface of the latent image carrier and the developer in the development area is cut off, and the adhesion of the developer to the latent image carrier is prevented.

According to a tenth aspect of the present invention, in the image forming method of the eighth aspect, a regulating plate is provided so as to be able to contact and separate from the surface of the developer carrier, and the regulating plate is attached to the surface of the developer carrier. The supply of the developer on the developer carrier to the development area is stopped by contacting the developer.

In this image forming method, the regulating plate is brought into contact with the surface of the developer carrier in order to cut off the supply of the developer on the developer carrier not involved in the image formation to the development area. I do. As a result, the contact between the surface of the latent image carrier and the developer in the development area is cut off, and the adhesion of the developer to the latent image carrier is prevented.

According to an eleventh aspect of the present invention, in the image forming method of the eighth aspect, the developer carrier is stopped, and the supply of the developer on the developer carrier to the developing area is stopped. Is what you do.

In this image forming method, the developer carrier is stopped in order to cut off the supply of the developer on the developer carrier not involved in the image formation to the developing area. As a result, the contact between the surface of the latent image carrier and the developer in the development area is cut off, and the adhesion of the developer to the latent image carrier is prevented.

According to a twelfth aspect of the present invention, in the image forming method of the fifth aspect, the developing means is configured to perform development by applying toner to the surface of the latent image carrier by applying a developing bias, By controlling the developing bias to a value at which the toner does not adhere to the latent image carrier, development by the developing means is not performed.

In this image forming method, the developing bias is controlled to a value at which the toner does not adhere to the latent image carrier in order to prevent development by a developing means not involved in image formation. This prevents toner from adhering to the surface of the latent image carrier that is not involved in image formation.

According to a thirteenth aspect of the present invention, in the image forming method according to the second aspect, the surface of the latent image carrier which is not involved in the image formation among the latent image carriers is subjected to exposure and elimination to remove the latent image carrier. The potential difference between the surface potential of the holding body and the surface potential of the transfer receiving body is controlled to a potential difference at which the toner transferred onto the transfer receiving body does not reversely transfer to the latent image carrier. .

In this image forming method, the surface of the latent image carrier immediately before the latent image carrier that is not involved in the image formation among the latent image carriers and the object to be transferred comes into contact with each other. As a result of the exposure and static elimination, the potential difference between the surface potential of the latent image carrier and the surface potential of the transfer object does not cause the toner transferred on the transfer object to reversely transfer to the latent image carrier. Controlled by the potential difference.

According to a fourteenth aspect of the present invention, in the image forming method according to the second aspect, a latent image carrier not involved in image formation among the latent image carriers is provided with a surface potential opposite to the surface potential of the latent image carrier. By applying a polarity charge, the potential difference between the surface potential of the latent image carrier and the surface potential of the transfer receiving body is changed by the toner transferred on the transfer receiving body to the latent image holding body. It is characterized in that the potential difference is controlled so as not to be changed.

In this image forming method, the latent image carrier, which is not involved in the image formation among the latent image carriers, is contacted with the latent image carrier immediately before the transfer object comes into contact with the latent image carrier. By applying a charge having a polarity opposite to that of the surface potential of the carrier, the potential difference between the surface potential of the latent image carrier and the surface potential of the transfer object is changed by the toner transferred onto the transfer object. Is controlled to a potential difference that does not cause reverse transfer to the latent image carrier.

According to a fifteenth aspect of the present invention, in the image forming method according to the first aspect, by controlling the surface potential of the transfer-receiving member, a latent image carrier not involved in image formation among the latent image carriers is controlled. Wherein the potential difference between the surface potential of the body and the surface potential of the transfer receiving body is controlled to a potential difference at which the toner transferred onto the transfer receiving body does not reversely transfer to the latent image carrier. .

In this image forming method, the surface potential of the latent image carrier which is not involved in the image formation among the latent image carriers is controlled by controlling the surface potential of the transfer object. Is controlled to a potential difference at which the toner transferred onto the transfer-receiving member does not reversely transfer to the latent image carrier. As a result, the reverse transfer of the toner on the transfer medium to the latent image carrier that is not involved in the image formation can be avoided, and the image quality deterioration due to the reverse transfer of the toner to the latent image carrier is prevented. Become like Further, in an image forming apparatus that collects the residual toner on the latent image carrier into the developing device, the color mixture of the toner in the developing device is prevented.

According to a sixteenth aspect of the present invention, in the image forming method of the first aspect, at least two of the image forming methods of the second to fifteenth aspects are simultaneously used.

In this image forming method, at least two of the image forming methods of the second to fifteenth aspects are used simultaneously, so that the latent image carriers are not involved in image formation. The potential difference between the surface potential of the latent image carrier and the surface potential of the transfer object is controlled to a potential difference at which the toner transferred on the transfer object does not reversely transfer to the latent image carrier. As a result, the reverse transfer of the toner on the transfer-receiving member to the latent image carrier that is not involved in image formation can be more reliably avoided.

According to a seventeenth aspect of the present invention, in the image forming method of the first aspect, the object to be transferred is an intermediate transfer member.

In this image forming method, reverse transfer of the toner on the intermediate transfer member to the latent image carrier not involved in image formation can be avoided. Further, in the image forming method of the intermediate transfer method using such an intermediate transfer body, the transfer of the toner image to the transfer material only needs to be performed once, so that stable transfer can be performed, and the thickness and the thickness of the transfer material can be improved. The versatility of the transfer material is improved without any limitation on the type and the like.

According to an eighteenth aspect of the present invention, in the image forming method of the first aspect, the object to be transferred is a transfer material.

In this image forming method, the reverse transfer of the toner on the transfer material to the latent image carrier not involved in the image formation can be avoided. Further, in such a direct transfer type image forming method of sequentially transferring the toner images on each latent image carrier onto a transfer material, simplification and cost reduction of the apparatus and speeding up of image formation are to be achieved. Will be able to

According to a nineteenth aspect, in the image forming method according to the first aspect, the average circularity of the toner is 0.94 or more.

In this image forming method, the reverse transfer of the toner on the transfer-receiving member to the latent image carrier which is not involved in the image formation can be avoided, and an image with good image quality can be obtained. Become.

According to a twentieth aspect, in the image forming method according to the nineteenth aspect, the toner is obtained by polymerizing a monomer composition containing at least a polymerizable monomer and a colorant in a dispersion medium. It is characterized by having.

In this image forming method, the reverse transfer of the toner on the transfer-receiving member to the latent image carrier which is not involved in the image formation can be avoided, and an image with good image quality can be obtained. Become.

According to a twenty-first aspect, in the image forming method according to the nineteenth aspect, the toner is obtained by melting, kneading, pulverizing, and classifying a toner raw material containing at least a binder resin and a colorant. Is obtained by performing a spheroidizing process on.

In this image forming method, the reverse transfer of the toner on the transfer-receiving member to the latent image carrier not involved in the image formation can be avoided, and an image of good quality can be obtained. Become.

According to a twenty-second aspect of the present invention, in the image forming method of the first aspect, a lubricant is applied to the surface of the latent image carrier.

In this image forming method, by applying a lubricant to the surface of the latent image carrier, the friction coefficient of the surface of the latent image carrier is reduced. As a result, the reverse transfer of the toner on the transfer-receiving member to the latent image carrier that is not involved in image formation can be more reliably avoided, and an image with good image quality can be obtained.

According to a twenty-third aspect of the present invention, there is provided a latent image carrier for carrying an electrostatic latent image, a latent image forming means for forming an electrostatic latent image on the latent image carrier, and Developing means for developing the latent image to form a toner image, and a plurality of image forming means having transfer means for transferring the toner image on the latent image carrier onto a transfer-receiving member; and 23. The image forming apparatus according to claim 1, further comprising a transfer medium transporting means for transporting the transfer medium so as to sequentially contact the surface of each latent image carrier of the plurality of image forming means. Out of the way,
It is characterized in that at least one image forming method is used.

In this image forming apparatus, there is provided an apparatus capable of obtaining a high-quality image without the toner on the transfer-receiving member being reversely transferred to the latent-image-bearing member not involved in image formation. become able to.

[0064]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A color printer (hereinafter simply referred to as a "printer") as an image forming apparatus according to the present invention will be described below.
An embodiment applied to the present invention will be described. FIG. 1 shows an overall schematic configuration of a copier equipped with a printer 10 according to the present embodiment. The printer 10 includes an image writing unit 12, an image forming unit 13, a paper feeding unit 14, and the like for forming a color image by an electrophotographic method. FIG.
FIG. 1 is a schematic configuration diagram illustrating a configuration of only the image forming unit 13 of the printer 10.

First, the schematic configuration and operation of the printer 10 will be described. 1 and 2, an image writing unit 12 of a printer 10 stores an image signal, which has been subjected to image processing in the image processing unit 1 of the copying machine on the basis of the image signal, in black (BK) for image formation and magenta. (M), yellow (Y), and cyan (C) are transmitted after being converted into respective color signals. The image writing unit 12 includes, for example, a laser light source, a deflector such as a rotating polygon mirror, a scanning imaging optical system, a laser scanning optical system including a mirror group, and a large number of one-dimensional or two-dimensional mirrors. It consists of an LED array with LEDs and an imaging optical system.
The four writing optical paths 12BK, 12Y, 12M, 1 corresponding to the respective color signals are constituted by an LED writing system or the like.
2C. As shown in FIG. 2, the image writing unit 12 includes four photoconductors 21BK, 21M, 21Y, and 21 of four image forming units for each color provided in the image forming unit 13.
21C, each writing optical path 12BK, 12Y, 12M, 12
Through C, writing of an image corresponding to each color signal is performed.

Each of the photoconductors 21BK, 2 for black (BK), magenta (M), yellow (Y), and cyan (C) of each image forming unit provided in the image forming section 13.
As the 1M, 21Y, and 21C, an OPC photoconductor is usually used. Further, each photoconductor 21BK, 21M, 21Y,
As shown in FIGS. 2 and 3, the charging devices 16BK, 16M, 16Y, and 16C, the image writing unit 12
Developing units 20BK, 20M, 20Y, 2 for black, magenta, yellow, and cyan colors
0C, primary transfer bias roller 2 as primary transfer means
3BK, 23M, 23Y, 23C, cleaning device 3
0BK, 30M, 30Y, 30C, and a static eliminator 24BK, 24M, 24Y, 2 for eliminating charges on the surface of the photoconductor.
4C, etc. are provided.

Here, the developing devices 20BK, 20M,
As 20Y and 20C, a two-component magnetic brush developing type developing device is used. Also, each photoconductor 21BK, 21
M, 21Y, 21C and each primary transfer bias roller 23
An intermediate transfer belt 22 is stretched so as to be interposed between the intermediate transfer belts BK, 23M, 23Y, and 23C. On the intermediate transfer belt 22, toner images of respective colors formed on respective photoconductors are sequentially superimposed. Transcribed together.

On the other hand, after the transfer paper P is fed from the paper feed unit 14 of the printer 10 or the paper feed bank (see FIG. 1) of the copying machine, the registration roller 17 shown in FIG.
Is carried on the transfer / conveying belt 50 via the And
When the intermediate transfer belt 22 and the transfer conveyance belt 50 come into contact with each other, the toner image transferred onto the intermediate transfer belt 22 is transferred to a secondary transfer bias roller 60 as a secondary transfer unit.
Is subjected to secondary transfer (collective transfer). Thereby, a color image is formed on the transfer paper P. The transfer paper P on which the color image is formed in this manner is transported to the fixing device 15 by the transfer transport belt 50. And the transfer paper P is
After the image transferred thereto is fixed by the fixing device 15, the image is discharged outside the printer main body.

The residual toner remaining on the intermediate transfer belt 22 without being transferred onto the transfer paper P at the time of the secondary transfer is removed from the intermediate transfer belt 22 by the belt cleaning device 25. A lubricant application device 26 is provided downstream of the belt cleaning device 25. The lubricant applying device 26 includes a solid lubricant 26a and a conductive brush 26b that rubs the intermediate transfer belt 22 to apply the solid lubricant 26a. The conductive brush 26b
Is in constant contact with the intermediate transfer belt 22 to apply the solid lubricant 26a to the intermediate transfer belt 22. The solid lubricant 26a has a function of improving the cleaning property of the intermediate transfer belt 22, preventing occurrence of filming, and improving durability.

Each of the photoconductors 21BK, 21M, 21Y,
Before the image writing is performed, the respective surfaces of the respective charging devices 16BK, 16M, 1C provided at the upstream portions of the respective writing optical paths 12BK, 12Y, 12M, 12C before the image writing is performed.
It is charged to about -700 V by 6Y and 16C.
The charging device 16BK of the printer 10 according to the present embodiment,
As 16M, 16Y and 16C, conductive rubber rollers are used. The charging devices 16BK, 16M, 16
Each of the rubber rollers as Y and 16C is provided with a corresponding one of the photoconductors 21B.
K, 21M, 21Y, and 21C are installed at a distance of about 50 μm so as to perform non-contact charging. An AC voltage of about 1 kHz and a peak-to-peak voltage of 2 kV is applied to each rubber roller, and the center value thereof is set to be about -800 V. As a result, the surfaces of the photoconductors 21BK, 21M, 21Y, and 21C are uniformly charged to about -700V.

The photoconductors 21BK, 21M, 21
The charging means for charging the surfaces of Y and 21C is not limited to the above-described non-contact charging means. For example, a conductive rubber roller may be used for each of the photoconductors 21BK, 21M and 21C.
Y, 21C is installed in contact with and charged, contact charging, AC + DC charging, DC bias roller charging for charging each photoconductor by applying only a DC bias of about -1400V without applying an AC bias, or conventionally, Charging means such as corona charging or brush charging using a corotron or scorotron can be used.

As described above, each of the photosensitive members 21BK, 21BK
After the surfaces of M, 21Y and 21C are charged, each photoconductor 2
The image writing unit 12 writes an image on the surfaces of 1BK, 21M, 21Y, and 21C. Thereby, on the surface of each photoconductor 21BK, 21M, 21Y, 21C,
An electrostatic latent image corresponding to each color image of black, magenta, yellow, and cyan is formed. These electrostatic latent images are developed by a developing device 20BK for each of black, magenta, yellow, and cyan colors.
Developed by 20M, 20Y, 20C.

Each of the developing devices 20BK, 20M, 20
Y and 20C are, as shown in FIG.
Doctor blade 202, two screws 203, 20
4, a toner density sensor 205, a developing case 206, and the like. Here, the positional relationship between the developing roller 201 and the screws 203 and 204 is such that the screws 203 and 204 are positioned diagonally below the developing roller 201. Also, two screws 203, 2
Numerals 04 are arranged in parallel in the horizontal direction. Further, the developing case 206 has two screws 20.
A partition plate 206a for partitioning the developing case 20 from the developing case 20 is provided.
6 is divided into two rooms. Also, the partition plate 206a
Are cut out so that the developer in each chamber of the developing case 206 is circulated and conveyed by the two screws 203 and 204.

An opening 206b is formed in a portion of the developing case 206 facing the photosensitive member, and a part of the developing roller 201 is exposed from the opening 206b. Further, the developing roller 201 and the screw 2
3, the doctor blade 201 is arranged so that the space above the screw 204 of the developing case 206 is slightly larger, as shown in FIG. Developing case 206 of each developing device 20BK, 20M, 20Y, 20C
Contains developer of each color of black, magenta, yellow, and cyan for developing an electrostatic latent image corresponding to each color image. Here, a two-component developer in which a non-magnetic toner and a magnetic carrier are dispersed and mixed is used as the developer.

Each developing device 20BK, 20M, 20Y, 2
The 0C developer is agitated by two screws 203 and 204 rotating in opposite directions so as to constantly circulate in each chamber of the developing case 206 through the notch on the rear side and the front side of the partition plate 206a. It is conveyed while. Then, the developer is supplied to the developing roller 201 by a screw 205 which stirs and transports the developer while circulating. The developing roller 201 includes a magnet roller 201a as a magnetic field generating means, and a non-magnetic developing sleeve 201b rotatably mounted so as to cover the outer periphery of the magnet roller 201a.

The developer supplied to the developing roller 201 as described above is applied to the developing sleeve 20 by the magnetic force of the magnet roller 201a and the rotation of the developing sleeve 201b.
It is pumped to the surface of 1b and held in the shape of a magnetic brush. The developer held in a magnetic brush shape on the surface of the developing sleeve 201b is conveyed toward the opening 206b of the developing case 206 while rotating along with the rotation of the developing sleeve 201b. The developer is cut by a doctor blade 202 in front of the opening 206b and measured so as to have an appropriate amount, and then the surface of the developing roller 201 exposed from the opening 206b and the photosensitive member It is sent to the development area between the surface.

The developer which has been cut off by the doctor blade 202 and which has been prevented from proceeding to the above-described developing area is moved along the outer periphery of the magnetic brush-like developer held on the surface of the developing sleeve 201b. , Screw 2
05, and returns to the circulating conveyance path of the developing case 206 by its own weight. The developer returned to the circulating conveyance path is again stirred and conveyed by the two screws 203 and 204, and then supplied to the developing roller 201 again by the screw 205.

On the other hand, the developer sent to the developing area
The toner is transferred to the electrostatic latent image formed on the photoreceptor to visualize the electrostatic latent image, thereby forming a toner image on the photoreceptor. That is, a developing bias composed of an AC voltage of 2.25 kHz and a peak-to-peak voltage of about 1 kV is applied to the developing sleeve 201b with a center value of -500V. Due to this developing bias, a potential difference from an exposed area (charge potential of about -150 V) on the photoreceptor causes
The toner in the developer held on the developing sleeve 201b shifts to an electrostatic latent image formed on the photoconductor.

The surplus developer consisting of the toner and the carrier not consumed when the electrostatic latent image is visualized is transferred into the developing case 206 while being held on the developing sleeve 201b. Will be returned. Then, the portion of the surface of the developing sleeve 201b where the magnetic force of the magnet roller 201a is not acting is separated from the developing sleeve 201b,
It falls on the screw 205 by its own weight. As a result, the surplus developer is recovered in the circulating transport path of the developing case 206, is again agitated and transported by the two screws 203 and 204, and is again supplied to the developing roller 201 by the screw 205.

As described above, the developer is agitated and conveyed by the two screws 203 and 204, whereby
While circulating in the developing case 206, the developing sleeve 20
Supply and recovery for 1b are repeated. Here, when the developing process for visualizing the electrostatic latent image on the photoreceptor is repeatedly executed and toner consumption in the developer proceeds,
The toner concentration of the developer contained in the developing case 206 gradually decreases. Therefore, each developing device 2
In 0BK, 20M, 20Y, and 20C, the toner density of the developer contained in the developing case 206 is detected by the toner density sensor 205. Then, based on the detection result of the toner density sensor 205, a new toner supply device (not shown) supplies a new toner to the developing case 206 by a toner replenishing device (not shown) so that the toner density of the developer in the developing case 206 is always constant. The toner is configured to be replenished in a timely manner.

As described above, each of the photosensitive members 21Bk, 21B
The toner images of each color formed on Y, 21M and 21C are
The intermediate transfer belt 2 that rotates while being in contact with the surface of each photoconductor by each of the primary transfer bias rollers 23BK, 23M, 23Y, and 23C arranged corresponding to each photoconductor.
2 is superimposed sequentially on the surface of No. 2 and primary-transferred. That is, each primary transfer bias roller 23B provided opposite to each photoconductor so as to sandwich the intermediate transfer belt 22 is provided.
K, 23M, 23Y, and 23C cause the toner image on the photoconductor to be transferred to the intermediate transfer belt 22 by the transfer electric field generated in the primary transfer area between each photoconductor and the intermediate transfer belt 22.
Is transferred electrostatically onto the substrate. In this embodiment, a voltage of about 1.5 kV is applied to a primary transfer bias roller made of conductive foamed EPDM rubber (rubber hardness: JIS-A, 30 degrees, volume resistance: 108 Ωcm) to generate a transfer electric field. I have.

As the intermediate transfer belt 22, various materials can be used.
Polyurethane rubber layer is formed on polyimide belt with excellent durability and high Young's modulus, Pvdf belt with excellent surface smoothness, or polyurethane resin layer,
Further, it is preferable to use a multilayer belt or the like in which a coating layer containing a fluorine component is formed on the polyurethane rubber layer and an elastic layer is provided on the surface. In particular, a multilayer belt having an elastic coat layer provided on a polyurethane rubber layer has an elastic surface, so that it has good adhesion to a photoreceptor surface and a transfer material surface, and has excellent primary transferability and secondary transferability. ing.
Further, as the intermediate transfer belt 22, 10 ¹⁰ to 10
^{A material} having a volume resistance of about ¹² Ωcm and a surface resistance of a portion on which the toner is applied and having a characteristic value of 10 ¹² / □ or more and having excellent transferability is preferable.

As described above, each photoconductor 21Bk, 2B
The toner images of the respective colors formed on 1Y, 21M, and 21C are sequentially superimposed on the surface of the intermediate transfer belt 22 so that 1
By the next transfer, 4
A full-color toner image composed of color toners is formed.
Then, the full-color toner image formed on the intermediate transfer belt 22 is fed by the registration roller 16 and transferred onto the transfer paper P carried on the transfer conveyance belt 50 by the secondary transfer bias roller 60. Next transfer (collective transfer) is performed. The transfer paper P on which the full-color image is secondarily transferred in this manner is conveyed to the fixing device 15 by the transfer conveyance belt 50, and after the secondary transfer image is fixed by the fixing device 15, is discharged outside the printer main body. . The residual toner remaining on the intermediate transfer belt 22 during the secondary transfer is
Intermediate transfer belt 2 by belt cleaning device 25
2 removed. After that, the next image is formed by the image forming units of each color of the image forming unit 13.

Next, a cleaning device 30BK for removing the toner remaining on the photoreceptor after the primary transfer is provided.
30M, 30Y, and 30C will be described. As shown in FIG. 3, the cleaning device of the present embodiment includes a cleaning blade 30 made of an elastic polyurethane rubber.
1 and a fur brush 302 having conductivity are used. An electric field roller 303 made of metal is arranged in contact with the fur brush 302. A scraper 304 is disposed in contact with the electric field roller 303.

In FIG. 3, the toner remaining on the photoconductor is first scraped off from the photoconductor by the fur brush 302 rotating in the direction opposite to the rotation direction of the photoconductor (counter direction). At this time, the fur brush 30
2 adheres to the electric field roller 303 rotating in the counter direction with respect to the fur brush 302 and is removed. Further, the toner attached to the electric field roller 303 is scraped off by the scraper 304 and collected in the cleaning case 305. Here, a cleaning bias is applied to the electric field roller 303, and the residual toner on the photoreceptor moves from the fur brush 302 to the electric field roller 303 by the electrostatic force by the cleaning bias, and is scraped from the electric field roller 303 by the scraper 304. It is scraped off.

Thus, the cleaning case 305
The toner collected therein is sent by a collecting screw 306 to a waste toner bottle (not shown) or a developing device of an image forming unit in which the cleaning device is mounted. In the printer of the present embodiment, the toner collected from the inside of the cleaning case 305 by the collecting screw 306 is returned to the corresponding developing device and reused.

Further, in the cleaning device of each of the image forming units described above, the location of the collection screw 306 is as follows.
The developing case 2 above the screw 203 of the developing device of the image forming unit adjacent to the downstream side of the cleaning device
06 are arranged so as to overlap with each other. Thus, the image forming units can be arranged close to each other, and the size of the printer body can be reduced.

As described above, in a printer equipped with four image forming units, in addition to the full-color mode in which a full-color image is formed using all of the four color toners, for example, yellow and cyan A single color mode in which a single color image is formed using only toner, a green image in which yellow and cyan are combined, a red image in which yellow and magenta are combined, and a two-color toner in which black characters are placed on cyan or yellow. Image formation can be performed in an image formation mode such as a two-color mode for forming a color image.

However, as shown in FIG. 2, in a printer in which the surface of the intermediate transfer belt 22 is always in contact with the surface of each photoconductor, for example,
When an image is formed in the single-color mode or the two-color mode using black or yellow toner, the intermediate transfer belt 22
The black or yellow toner that has already been primary transferred onto
The toner image that is primarily transferred onto the intermediate transfer belt 22 by contacting the surface of the photoreceptor of the image forming unit that forms a magenta or cyan toner image that is not involved in the image formation and reversely transferring onto the photoreceptor Image quality may be degraded. Therefore, in order to improve image quality in this type of printer, reverse transfer of the toner on the intermediate transfer belt 22 to the photoconductor due to contact with the photoconductor of the image forming unit that is not involved in image formation is avoided. It is important to:

The intermediate transfer belt 22 on such a photosensitive member
It is considered that the reverse transfer of the upper toner is caused by a change in the charge amount of the toner on the intermediate transfer belt 22 due to a discharge generated between the surface of the photoconductor and the toner on the intermediate transfer belt 22. . Therefore, in the printer of the present embodiment, the surface potential of the photoconductor and the surface potential of the photoconductor are controlled so that no discharge occurs between the surface of the photoconductor not involved in image formation immediately before the primary transfer and the toner on the intermediate transfer belt 22. And controlling a potential difference from the surface potential of the intermediate transfer belt 22 due to the primary transfer electric field. As a result, the charge amount of the toner on the intermediate transfer belt 22 does not change, and the reverse transfer of the toner to the photoconductor can be prevented.

Here, the case where the present invention is applied to a printer using the intermediate transfer belt 22 has been exemplified.
The present invention may be applied to an intermediate transfer drum type image forming apparatus in consideration of the layout of the apparatus main body, required accuracy, size, and the like. In addition, the present invention may be applied to an image forming apparatus of a direct transfer system in which a transfer sheet is transported by a belt or a drum and is directly transferred from each photoconductor.

The surface potential of the photosensitive member and the intermediate transfer belt 2
There are two methods for controlling the potential difference from the surface potential of No. 2. One method is to control the surface potential of the photoconductor immediately before the photoconductor on which the toner image is not formed contacts the intermediate transfer belt 22. Another method is to control the surface potential of the intermediate transfer belt 22 (details will be described later).

As the method of controlling the surface potential of the photosensitive member, a method of controlling the surface potential before the developing step is performed by the developing device and a method of controlling the primary transfer step after the developing step are performed. Immediately before the surface potential is controlled. For an image forming unit of a non-image forming color which is determined not to form an image based on the image data, the toner is transferred immediately before the primary transfer so that the toner is not reversely transferred to the photosensitive member of the image forming unit. The surface potential of the photoconductor is controlled to a desired potential.

Here, the surface potential of the photoconductor undergoes a so-called dark decay, in which the charging potential decreases with time. Therefore, in consideration of the moving time from the charged portion of the photoconductor to the primary transfer portion, the surface potential of the photoconductor becomes a charged potential that reaches a target surface potential just before the primary transfer due to the dark decay. Thus, the charging device of the corresponding image forming unit is controlled. FIG. 8 shows the relationship between the surface potential of the photoconductor immediately before the primary transfer and the amount of the reverse transfer toner when the charging device of the corresponding image forming unit is controlled as described above. As is apparent from FIG. 8, in this method, the lower the absolute value of the charged potential of the photoconductor surface immediately before the primary transfer, the smaller the amount of toner to be reversely transferred to the photoconductor.

Therefore, for example, when the target charging potential may be in the vicinity of ± 0 V, there is an option that the surface of the photosensitive member of the corresponding image forming unit is not charged. However, depending on the charging device, the frequent application of ON / OFF of the bias may affect the life of the photoconductor and the charging device itself. Accordingly, a method of controlling the charging device of the corresponding image forming unit is more costly, but is more desirable with less fluctuation of the applied bias.

Here, before the development step is performed,
When the above-described method of controlling the surface potential of the photoconductor is employed, it is necessary to prevent unnecessary toner from adhering to the photoconductor of the corresponding image forming unit in the development area. As a method for preventing the unnecessary adhesion of the toner to the photoconductor, there is a method of physically separating the photoconductor from the developing roller 201 of the developing device. As described above, as a method of physically separating the photosensitive member and the developing roller 201, the developing case 206 of the developing device is configured to be able to move toward and away from the photosensitive member so as to be movable.
And a method of removing the developer from the developing roller 201.

In the former method, for example, as shown in FIGS. 4A and 4B, the developing case 206 of the developing device can be freely moved toward and away from the photosensitive member.
Each of the developing devices 20BK, 20M, 20Y, and 20C is rotatably supported about a rotation shaft 207, and the developing case 206 of the developing device is urged to abut on the contact / separation cam 208. As a result, the rotation of the contact / separation cam 208 causes the developing case 206 of the developing device to move as shown in FIG.
The developing roller 201 and the photoconductor are separated from each other by rotating about the rotation shaft 207 from the contact position illustrated in FIG. 4A to the separated position illustrated in FIG.

On the other hand, as described above, the developing roller 201
As a method for removing the developer from above, the developing roller 2
01, the developer held on the developing sleeve 201b is cut off to remove the developer from the developing sleeve 201b, or the doctor blade 202 is attached to the developing sleeve 201b.
b, so that the developer is not supplied to the developing area.

That is, as described above, the magnetic brush-like developer pumped onto the developing sleeve 201b is:
The blade is cut into appropriate amounts by a doctor blade 202 and sent to a developing area facing the photoconductor. Therefore, by changing the gap between the doctor blade 202 and the developing sleeve 201b, it becomes possible not to supply the developer to the developing area. Therefore, for example, as shown in FIG.
2 is arranged so as to be in contact with or separated from the developing sleeve 201b. If necessary, the supply of the developer held on the developing sleeve 201b to the developing area is stopped, and the photosensitive Prevent unnecessary toner from adhering to the body.

The developer that has passed through the developing area is
The portion where the magnetic force of the magnet roller 201a does not work is separated from the developing sleeve 201b and collected by the screw 204. Since there is no developer on the developing sleeve 201b ahead of the portion where the magnetic force of the magnet roller 201a does not work, when the developing sleeve 201b is rotated in reverse, the developer already on the developing sleeve 201b is The developer is pulled back to the pumping portion by the force, and the developer does not exist on the developing sleeve 201b in the developing area facing the photoconductor. Such an operation is called “reversal head cutting”
I'm calling Therefore, as shown in FIG. 5, by performing the “reverse spinning” by rotating the developing sleeve 201 b in the reverse direction, the developer is removed from the developing sleeve 201 b, and the predetermined image forming unit is not required to be attached to the photosensitive member. It is possible to prevent the adhesion of an unusual toner.

As another method for preventing unnecessary toner from adhering to the photosensitive member of a predetermined image forming unit, there is a method of stopping the rotation of the developing sleeve 201b.
As another method, there is a method of controlling a developing bias applied to a developing device to prevent unnecessary toner from adhering to a photoconductor not involved in image formation. For example,
As these developing biases, a developing bias having a DC component of about -500 V and having an AC frequency superimposed thereon is usually applied. The development is performed by the potential difference between the DC component and the exposed portion on the photoconductor, and the background portion on the photoconductor is prevented from being developed by the potential difference of about −700 V in the non-image portion. Therefore, the developing bias is not applied to the surface potential of the photoconductor with respect to the condition for not developing, for example, the surface potential of the photoconductor is-
Unnecessary toner development can be prevented by setting the voltage to about +50 if the voltage is 200 V and to about +250 V if the photoconductor surface potential is 0 V.

As described above, as a method of controlling the surface potential of the photosensitive member, there is a method of controlling the surface potential of the photosensitive member immediately after the development process is performed and immediately before the primary transfer is performed. This method can be roughly divided into two. The first method is, for example, as shown in FIG.
In this method, the surface potential of the photoreceptor is controlled by exposing the surfaces of the photoconductors 21Y, 21M, and 21C to light using an exposing member 27 such as an LED, LD, or xenon lamp (quenching lamp). That is, the relationship between the current or applied voltage flowing through the exposure member 27 composed of an LED, an LD, and the like and the amount of light emission (exposure) is grasped in advance. Thus, by adjusting the amount of light emitted from the exposure member 27 and controlling the amount of charge removed on the surface of the photoconductor,
It is possible to control the surface potential of the photoconductor immediately before the primary transfer to an arbitrary value.

The second method is, for example, as shown in FIG. 7, a discharging member comprising a corona charger such as a scorotron or a corotron, a charging brush, a charging roller or a charging blade on which an AC or DC bias is superimposed. 2
8, photoconductors 21Bk, 21Y, 21M, 21C
This is a method of applying a charge having a polarity opposite to that of the surface potential. In this method, the surface potential of the photoconductor is controlled by applying a charge having a polarity opposite to the surface potential of the photoconductor to the surface of the photoconductor by the charge removing member 28 and controlling the amount of charge on the surface of the photoconductor. It can be controlled to any value.

When the surface potential of the photosensitive member immediately before the primary transfer is controlled in the printer according to the present embodiment by using the above two methods, the amount of toner reversely transferred to the photosensitive member and the photosensitive member immediately before the transfer are controlled. FIG. 8 shows the relationship with the body surface potential. Here, an LED as the exposure member 27 is installed at a position immediately before the primary transfer area of the printer,
This shows the result when the surface potential of the photoconductor is controlled by adjusting the light amount of D. according to this,
When the absolute value of the surface potential of the photoconductor is reduced, reverse transfer of the toner to the photoconductor can be suppressed. In this printer, by controlling the surface potential of the photoconductor immediately before primary transfer to | -200 V | or less, reverse transfer of toner was prevented, and a good image without image disturbance was obtained.

Here, when there is a toner image on the photoconductor, if the surface of the photoconductor is completely discharged, the potential for holding the toner image on the photoconductor disappears, and the toner image is disturbed. That is, so-called toner dust occurs. In this printer, since no toner image is formed on the photoconductor whose surface potential is controlled as described above, there is no need to worry about such a situation. The surface of the photoconductor can be neutralized. However, regardless of the above-described exposure and charge elimination, excessive exposure or charge may damage the photoconductor and significantly shorten its life. Therefore, it is preferable to set the charge in an appropriate range.

On the other hand, by controlling the surface potential of the intermediate transfer belt 22 (transfer paper P in the case of the direct transfer system) as the transfer object, the photosensitive members not involved in image formation among the photosensitive members are controlled. The potential difference between the surface potential of the transfer object and the surface potential of the transfer object can be controlled to a potential difference at which the toner transferred on the transfer object does not reverse-transfer to the photoconductor. Therefore, the present inventor has proposed that each of the photoconductors 21Bk, 21Bk.
Y, 21M and 21C transfer the image to the intermediate transfer belt 22 by 1
The primary transfer electric field intensity applied for the next transfer,
The relationship with the reverse transfer of the toner was measured. FIG. 9 shows the relationship between the primary transfer bias applied to the primary transfer area between the photoconductor and the primary transfer bias roller and the amount of toner reversely transferred onto the photoconductor after the primary transfer. . FIG. 9 shows the measurement of the reverse transfer amount of the yellow toner to the photoconductor of the image forming unit for magenta when the first color is yellow and the second color is magenta. At the same time, the transfer remaining amount of the magenta toner was measured.

According to this, a sufficient transferred image can be obtained,
Although there is no primary transfer bias that does not cause reverse transfer of the toner, in order to prevent the reverse transfer of the toner to the photosensitive member of the image forming unit which is not involved in the image formation, which is the subject of the present invention, the transfer of the toner image is Since it is not necessary to consider the efficiency, it is possible to set the primary transfer bias to a primary transfer bias value that prevents the reverse transfer of the toner. In the printer according to the present embodiment, the reverse transfer of the toner to the photoconductor is prevented by suppressing the primary transfer bias in the primary transfer area of the image forming unit which is not involved in image formation to about +400 V or less. It was found that a good image without image disturbance could be obtained.

As described above, there are various methods for preventing the reverse transfer of the toner to the photoreceptor of the image forming unit which is not involved in the image formation. By using two or more of these methods simultaneously, the reverse rotation of the toner can be achieved. This makes it possible to more reliably eliminate image disturbance due to copying. Further, the above-described various transfer methods are not limited to the intermediate transfer method such as the printer described above, and are sufficiently effective in an image forming apparatus of a direct transfer method. In this case, an appropriate transfer method may be selected according to the characteristics described above.

The present inventor has found that the adhesive force between the photoconductor and the toner directly affects the amount of the toner to be reversely transferred to the photoconductor. In other words, the degree of aggregation of the toner is high,
When a developer containing toner with a large toner adhesion to the photoconductor is used, the reverse transfer of the toner to the photoconductor often occurs, and the friction coefficient of the surface of the photoconductor causes the toner to transfer to the photoconductor. It has been found that the amount of toner for reverse transfer is different.
As a result, it is clear that, as the toner used in the above-described image forming method, the toner having an average circularity of 0.94 or more and having a nearly spherical shape can prevent the reverse transfer of the toner to the photoconductor. Was.

The circularity of the toner can be determined as follows. That is, the degree of circularity of the toner is determined by observing with a scanning electron microscope or an optical microscope and arbitrarily selecting the shape of a large number of toner particles using a commercially available image analyzer or flow-type particle image analyzer (for example, Sysmex Corporation). FPIA-1000). These devices capture thousands of particles in a liquid and perform image analysis and particle size analysis. The shape of toner particles is given by the following formula. Circularity = Σ [(4 · π · Si) / Li2] / N (Formula 1) In Formula 1, Li is a perimeter of each particle in a projected image,
Si represents the projected area of each particle, and N represents the total number of evaluated particles.
As the degree of circularity approaches 1, the toner approaches a true sphere.

Here, the spherical toner having an average circularity of 0.94 or more has less reverse transfer to the photosensitive member because the adhesion between the toner and the photosensitive member greatly contributes to the reverse transfer. ,
This is because the closer the toner is to a spherical shape, the smaller the van der Waals force with the photoconductor surface. That is, the van der Waals force generally decreases as the contact area with the opposing object (in this case, the photoconductor) decreases.
Therefore, for example, as shown in FIG. 10, the closer the toner is to a spherical shape, the smaller the contact area of the toner itself, and the higher the fluidity of the toner with respect to the photoconductor surface. Also,
Usually, additives such as silica and titanium oxide are mixed into the toner in order to reduce the adhesion of the toner to the photoreceptor surface. In a toner in which such an additive is mixed, as shown in FIG. 11, the probability that the surface portion of the additive adhering to the surface of the toner comes into contact with the photoreceptor surface increases. The particle size of the additive is sufficiently smaller than the particle size of the toner. Therefore, in the case of a toner containing this additive, the apparent van der Waals force between the surface of the photoconductor and the toner is reduced.

In order to investigate the relationship between the circularity of the toner as described above and the rate of reverse transfer of the toner to the photoreceptor, the inventor of the present invention carried out the reversal of the toner with various average circularities on the photoreceptor. An experiment was performed to evaluate the copy ratio. Table 1 shows the results of this experiment.
Shown in

[Table 1]

The above experiment was conducted using a cyan toner of a digital color copier (ImagioColor4000) manufactured by Ricoh. The average circularity of the toner is 0.
919, the average particle size is 6.8 μm. This toner is melted and rounded in a high-temperature airflow, and four types of toners having different average circularities are prototyped by changing the processing temperature and processing time.
The average circularity and reverse transfer rate of the five sample toners shown in Table 1 were measured. From the results of this experiment, as shown in Table 1,
When the toner has an average circularity of 0.94 or more, the reverse transfer to the photoconductor is improved as compared with the toner having an average circularity of 0.94 or less. Accordingly, it can be seen that a desirable toner shape is a toner having an average circularity value of 0.94 or more.

Here, as a method for producing a toner having a nearly spherical shape as described above, a method of polymerizing a monomer composition containing at least a polymerizable monomer and a colorant in a dispersion medium, a method of forming a binder resin and a method of coloring, And a method in which toner particles obtained by melting, kneading, pulverizing, and classifying at least a toner material containing at least an agent are subjected to spheroidizing treatment. In any method, a toner having an effect of preventing reverse transfer to a photoreceptor can be obtained.
The production method of the toner may be selected in consideration of cost and the like.

It is also known that the smaller the coefficient of friction of the photoreceptor surface, the less the reverse transfer of toner to the photoreceptor. FIG. 12 shows the relationship between the coefficient of friction of the photoreceptor surface and the amount of reverse transfer toner to the photoreceptor. FIG.
The graph shown in FIG. 2 is a digital color copier made by Ricoh (I
magioColor4000)
A toner image formed on the intermediate transfer belt 22 as a first color, and then the toner reversely transferred to the photosensitive member when the yellow toner image on the intermediate transfer belt 22 is brought into contact with the photosensitive member three times without image formation It is a comparison of the amounts. In order to suppress the friction coefficient of the photoreceptor with time, the ImageioColor 4000 as the experimental machine has a cleaning unit for the photoreceptor, similar to the lubricant application device 26 shown in FIG.
A lubricant application device for applying zinc stearate to the surface of the photoreceptor is provided.

The coefficient of friction of the surface of the photoreceptor with the lubricant application device attached was 0.25, and the coefficient of friction of the surface of the photoreceptor with the lubricant removed was 0.54. As shown in FIG. 12, it can be seen that the amount of the reverse transfer toner on the photoconductor varies depending on the difference in the friction coefficient of the surface of the photoconductor. Therefore,
By using a lubricant applying means for applying a lubricant such as zinc stearate or PVDF resin powder to the photoconductor as described above, it is possible to reduce the friction coefficient of the photoconductor surface by reversing the toner on the photoconductor. This is effective in preventing copying.

As described above, in the image forming apparatus constructed by using the above-described various image forming methods, the reverse transfer of the toner to the photoreceptor which is not involved in the image formation is reduced. A good image without disturbing the image can be obtained. Further, since the measures for preventing the reverse transfer of the toner to the photoreceptor which is not involved in the image formation do not lower the image forming speed,
Good image formation can be performed without lowering the high-speed productivity of the apparatus. In the present embodiment, the developing device using a two-component developer is shown, but the developing device may use a one-component developer.

[0118]

According to the first, second and fifteenth aspects of the present invention, there is no unnecessary reverse transfer of the toner to the image carrier which is not involved in the image formation, and the image is not disturbed by the reverse transfer toner. There is an excellent effect that formation can be performed.

According to the third and fourth aspects of the present invention, there is an excellent effect that the charging potential can be controlled without adding many new mechanisms.

Further, according to the fifth and sixth aspects of the present invention, there is an excellent effect that image disturbance due to toner adhesion in the developing step of the image carrier for preventing reverse transfer of toner can be prevented.

According to the seventh and tenth aspects of the present invention, an excellent effect that excess toner can be reliably prevented from being developed in the image carrier development step for preventing reverse transfer of toner. There is.

According to the thirteenth and fourteenth aspects,
By controlling the surface potential of the image carrier immediately before the primary transfer,
There is an excellent effect that the surface potential of the image carrier can be controlled without worrying about unnecessary development of toner in the development area.

According to the fifteenth aspect of the present invention, the reverse transfer of the toner to the image bearing member can be prevented at low cost without worrying about the developing and charging steps and without adding a new mechanism. There is an excellent effect that you can.

According to the sixteenth aspect of the present invention,
By using two or more of the above-described methods simultaneously, there is an excellent effect that reverse transfer can be more reliably prevented and image disturbance can be eliminated.

According to the seventeenth aspect of the present invention, since the intermediate transfer system is used, color misregistration is small, the versatility of paper types is excellent, and the toner can be transferred onto the image carrier while maintaining high productivity. There is an excellent effect that reverse transfer can be prevented.

According to the eighteenth aspect of the present invention, since the direct transfer system is used, the number of mechanisms is smaller than that of the intermediate transfer system, and a high-speed and low-cost color image can be transferred to the image carrier. There is an excellent effect that it can be obtained while preventing reverse transfer.

Further, according to the present invention, the occurrence of reverse transfer of toner to the image carrier can be more reliably prevented, and a better image can be obtained. There is an excellent effect.

According to the twenty-third aspect of the present invention, the reverse transfer of the toner to the image carrier not involved in the image formation is small.
There is an excellent effect that it is possible to provide an image forming apparatus capable of forming a good image in which the toner image is not disturbed without reducing the high-speed productivity.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing an overall schematic configuration of a copier equipped with a printer according to an embodiment of the present invention.

FIG. 2 is a schematic configuration diagram illustrating a configuration of only an image forming unit of the printer.

FIG. 3 is a schematic configuration diagram illustrating a configuration of an image forming unit provided in the image forming unit.

FIGS. 4A and 4B are schematic configuration diagrams showing an example of a configuration in which a developing case of a developing device can freely move toward and away from a photoconductor of the printer.

FIG. 5 is a schematic configuration diagram for explaining a method of removing a developer from a developing roller of the developing device.

FIG. 6 is a diagram showing a state in which the surface of the photoreceptor is subjected to exposure and elimination with an exposure member;
FIG. 2 is a schematic configuration diagram for explaining a method of controlling the surface potential of the photoconductor.

FIG. 7 is a schematic configuration diagram for explaining a method of applying a charge having a polarity opposite to the surface potential of the photoconductor using a charge removing member to control the surface potential of the photoconductor to an arbitrary value.

FIG. 8 is a graph showing the relationship between the amount of toner reversely transferred to a photoconductor and the surface potential of the photoconductor immediately before transfer when the surface potential of the photoconductor is controlled immediately before primary transfer.

FIG. 9 illustrates a relationship between a primary transfer bias applied to a primary transfer area between a photoconductor and a primary transfer bias roller and an amount of toner reversely transferred onto the photoconductor after the primary transfer. Graph.

FIG. 10 is a schematic enlarged view for explaining a contact state of toner on the surface of the photoconductor.

FIG. 11 is a schematic enlarged view for explaining a contact state of a toner mixed with an additive on the surface of the photoconductor.

FIG. 12 is a graph showing the relationship between the coefficient of friction of the photoconductor surface and the amount of reverse-transferred toner on the photoconductor.

[Explanation of symbols]

Reference Signs List 10 printer 12 image writing unit 13 image forming unit 14 paper feeding unit 15 fixing device 16BK, 16M, 16Y, 16C charging device 17 registration roller 20BK, 20M, 20Y, 20C developing device 21BK, 21M, 21Y, 21C photoconductor 22 intermediate Transfer belt 23BK, 23M, 23Y, 23C Primary transfer bias roller 24BK, 24M, 24Y, 24C Static eliminator 25 Belt cleaning device 26 Lubricant coating device 27 Exposure member 28 Static elimination member 30BK, 30M, 30Y, 30C Cleaning device 50 Transfer conveyance Belt 60 Secondary transfer bias roller 201 Developing roller 201a Magnet roller 201b Developing sleeve 202 Doctor blade 206 Developing case

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G03G 9/087 G03G 15/00 303 2H134 15/00 303 15/02 102 2H200 15/02 102 15/06 101 15/06 101 15/08 503A 15/08 503 504Z 504 506A 506 15/16 507 21/00 15/16 15/08 507H 21/00 9/08 381 384 F term (reference) 2H005 AB04 AB06 EA10 2H027 DA02 DE05 DE07 EA01 EA04 EA05 EA08 EA10 EB04 EC07 EC11 EF09 2H030 AA04 AB02 AD01 AD02 AD16 BB33 BB34 BB42 BB44 BB46 BB53 BB54 BB63 2H073 AA03 AA05 BA02 BA13 BA23 BA25 BA43 BA45 CA03 BA03 AD02 BA03 AD03 BA03 AD03 GA01 GA06 GA10 GB02 HB03 HB12 HD19 KD08 KF03 KG04 KG07 KH01 KH15 KJ02 LA01 2H200 FA05 GA12 GA16 GA23 GA33 GA34 GA47 GA59 GA66 GA69 GB02 GB12 GB13 GB26 HA02 HA13 HA29 HB12 HB45 HB48 JA02 JB07 JC04 JC15 JC16 LA17 LA24 MA03 MA04 MA20 MB04 MB05 NA06 NA09 NA10 PA05 PA10

Claims (23)

[Claims] A latent image carrier for carrying an electrostatic latent image;
Latent image forming means for forming an electrostatic latent image on the latent image carrier, developing means for developing the electrostatic latent image into a toner image, and toner image on the latent image carrier A plurality of image forming means having a transfer means for transferring the image on the transfer object, and the transfer means so as to sequentially contact the surface of each latent image carrier of the plurality of image formation means. An image forming method in an image forming apparatus provided with a transfer medium transporting means for transporting a body, wherein the surface potential of the latent image carrier not involved in image formation among the latent image carriers and the transfer medium An image forming method comprising: controlling a potential difference from a surface potential of the toner to a potential difference at which toner transferred onto the transfer-receiving member does not reversely transfer to the latent image carrier. 2. The image forming method according to claim 1, wherein said latent image carrier immediately before said latent image carrier, which is not involved in image formation among said latent image carriers, comes into contact with said transfer object. By controlling the surface potential of the image bearing member, the potential difference between the surface potential of the latent image carrier and the surface potential of the image receiving member is reversed by the toner transferred on the image receiving member to the latent image bearing member. An image forming method characterized by controlling the potential difference so that no image is taken. 3. The image forming method according to claim 2, wherein a charging step for charging a surface of the latent image carrier that is not involved in image formation among the latent image carriers is not performed. An image forming method, comprising: controlling a surface potential of a transfer target body and a surface potential of the latent image carrier. 4. The image forming method according to claim 2, wherein a charging potential in a charging step for charging a surface of the latent image carrier that is not involved in image formation among the latent image carriers is controlled. An image forming method comprising controlling the surface potential of the transfer object and the surface potential of the latent image carrier. 5. The image forming method according to claim 3, wherein the development by a developing unit not involved in the image formation of each of the image forming units is not performed. 6. An image forming method according to claim 5, wherein said developing means performs development by bringing a developer into contact with a surface of said latent image carrier in a development area facing said latent image carrier. An image forming method, wherein the developing means does not perform development by cutting off the contact between the surface of the latent image carrier and the developer in the developing area. 7. The image forming method according to claim 6, wherein the contact between the surface of the latent image carrier and the developer in the development area is performed by separating the latent image carrier from the developing means. An image forming method characterized by cutting off. 8. The image forming method according to claim 6, wherein said developing means has a developer carrying member for carrying the developer and supplying the developer to the developing area by rotating the developer. An image forming method, wherein the supply of the developer on the carrier to the developing area is cut off, whereby the contact between the surface of the latent image carrier and the developer in the developing area is cut off. 9. The image forming method according to claim 8, wherein the developer carrier is reversed to stop supplying the developer on the developer carrier to a developing area. 10. The image forming method according to claim 8, further comprising: providing a regulating plate which can be freely attached to and detached from the surface of the developer carrying member, and bringing the regulating plate into contact with the surface of the developer carrying member. An image forming method, wherein supply of a developer on a developer carrier to a development area is stopped. 11. The image forming method according to claim 8, wherein the developer carrier is stopped to stop supplying the developer on the developer carrier to a developing area. 12. An image forming method according to claim 5, wherein said developing means is configured to apply a developing bias to cause toner to adhere to a surface of said latent image carrier to perform development. An image forming method, wherein development is not performed by the developing unit by controlling the toner so that the toner does not adhere to the latent image carrier. 13. The image forming method according to claim 2, wherein a surface of the latent image carrier that is not involved in image formation among the latent image carriers is subjected to exposure and static elimination, and a surface potential of the latent image carrier is exposed. An image forming method, wherein the potential difference between the toner image and the surface potential of the transfer member is controlled so that the toner transferred on the transfer member does not reversely transfer to the latent image carrier. 14. An image forming method according to claim 2, wherein a charge having a polarity opposite to the surface potential of said latent image carrier is applied to one of said latent image carriers that is not involved in image formation. By doing so, the potential difference between the surface potential of the latent image carrier and the surface potential of the transferred body is controlled to a potential difference at which the toner transferred on the transferred body does not reversely transfer to the latent image carrier. An image forming method comprising: 15. The image forming method according to claim 1, wherein the surface potential of the latent image carrier that is not involved in image formation among the latent image carriers is controlled by controlling the surface potential of the transferred object. An image forming method, wherein a potential difference from a surface potential of the transfer object is controlled to a potential difference at which toner transferred onto the transfer object does not reversely transfer to the latent image carrier. 16. An image forming method according to claim 1, wherein at least two of the image forming methods according to claim 2 to 15 are simultaneously used. 17. The image forming method according to claim 1, wherein the object to be transferred is an intermediate transfer member. 18. The image forming method according to claim 1, wherein the object to be transferred is a transfer material. 19. The image forming method according to claim 1, wherein the average circularity of the toner is 0.94 or more. 20. The image forming method according to claim 19, wherein the toner is obtained by polymerizing a monomer composition containing at least a polymerizable monomer and a colorant in a dispersion medium. Image forming method. 21. The image forming method according to claim 19, wherein the toner is formed into a spherical shape by melting, kneading, pulverizing and classifying a toner raw material containing at least a binder resin and a colorant. An image forming method characterized by being obtained by the above method. 22. The image forming method according to claim 1, wherein a lubricant is applied to the surface of the latent image carrier. 23. A latent image carrier for carrying an electrostatic latent image, a latent image forming means for forming an electrostatic latent image on the latent image carrier, and developing the electrostatic latent image. A plurality of image forming units each having a developing unit for forming a toner image on the latent image carrier and a transfer unit for transferring a toner image on the latent image carrier onto a transfer-receiving body; 23. An image forming apparatus comprising: a transfer object transporting unit configured to transport the transfer target so as to sequentially contact the surface of each latent image carrier of the forming unit. An image forming apparatus using at least one image forming method.