The present invention relates to an image forming apparatus, such as duplicating machines, printers, facsimile apparatuses, and the like, for forming a color image with the use of electro-photography, and more particular, to an electro-photographic apparatus for forming a color image making use of a plurality of color toners.
With an electrophotographic system, light from exposure means is irradiated on an evenly charged photosensitive body to form an electrostatic latent image corresponding to image data, toner is caused to adhere to the electrostatic latent image on the photosensitive body to develop a toner image, and the toner image is transferred to a recording medium to be fixed thereto.
Here, while an explanation will be given to the case where a recording medium is a paper sheet, the recording medium includes all sheet-shaped recording media formed of various materials such as plastics, and the like, as well as paper.
In forming a color image, a plurality of color toners such as yellow Y, magenta M, cyan C, black B, or the like are superposed to form the image.
Color image forming systems include a repetitive developing system of repetitively developing respective color toners on a single photosensitive body to form a color image, and a simultaneous developing system of simultaneously developing respective color toners on a plurality of photosensitive bodies to form a color image.
The repetitive developing system is one, in which a single photosensitive body is used to form a color image, and a four-revolution system is an example thereof.
The four-revolution system is one, in which a plurality of developing means for developing respective different color toners and an intermediate transfer body are arranged around a photosensitive body and color images formed on the photosensitive body are successively transferred color by color to the intermediate transfer body (see, for example, JP-A-8-137179).
Such transfer is repeated every color to superpose a plurality of color toner images on the intermediate transfer body, and then the color toner images on the intermediate transfer body are transferred to a medium to fix a color image thereon.
With the four-revolution system, color toner images of, for example, yellow Y, magenta M, cyan C, and black B are successively formed color by color on a photosensitive body and superposingly transferred to an intermediate transfer body, so that image formation takes time four times that taken in the case where a monochrome image is formed.
The simultaneous developing system is one, in which toner images are substantially simultaneously formed on a plurality of photosensitive bodies corresponding to several colors, and are transferred correspondingly to conveyance of a sheet to form a color image, and which is also called a tandem system (see, for example, JP-A-2001-356548).
With the tandem system, image forming means including a photosensitive body, charging means, exposure means, developing means, and cleaner means is independently provided every color, so that four sets of image forming means must be provided in the case where a color image is formed with the use of color toners of yellow Y, magenta M, cyan C, and black B.
With the tandem system, four sets of independent image forming means are used to form toner images substantially simultaneously, and the toner images are transferred to an intermediate transfer body, or a sheet. With the tandem system, color images are simultaneously superposed on one another, so that a color image can be formed in substantially the same time as that taken in the case where a monochrome image is formed, and so the system is suited for high-speed printing.
In recent years, demands for colored documents have been greatly increased in offices and color printers are being rapidly spread. Also, high-speed printing is wanted, and attention is paid to color printers of the tandem system.
Since color printers of the tandem system comprise four sets of image forming means, however, they are difficult to be miniaturized and large in size as compared with color printers of the repetitive developing system.
An example of conventional color printers of the tandem system is of a construction, in which four sets of independent image forming means making use of LED arrays as exposure means are stacked vertically and arranged, as disclosed in JP-A-2001-356548. In this example, exposure means are fixedly mounted in order to ensure accuracy in exposure, and developing means, that is, process cartridges are susceptible to taking out and putting in.
There has been proposed a construction, in which exposure means are fixedly mounted centrally of a body of an electrophotographic apparatus and light beams from the exposure means are irradiated on photosensitive bodies through spaces formed between charging means, developing means, cleaner means, and so on, which can be mounted to and dismounted from the bodies (see, for example, JP-A-2001-296713).
As to a toner regulatory blade for forming a thin toner layer on a developing roller, there is disclosed a configuration of a toner regulatory blade comprising an elastic blade made of a silicone compound or a fluorine compound and mounted on a tip end of a leaf spring (see, for example, JP-A-5-11584).
There is disclosed a configuration of a toner regulatory blade having a tip end thereof once bent so that a divergence, 30°≦θ≦90°, is formed between the blade tip end, which comes into contact with a developing roller in a direction following rotation of the developing roller, that is, in the same direction as a direction, in which a surface of the developing roller moves, and the developing roller (see, for example, JP-A-11-344858).
There is disclosed a configuration of a toner regulatory blade having a tip end thereof once bent so that a predetermined divergence is formed between the blade tip end, which comes into contact with a developing roller in a direction against rotation of the developing roller, that is, a direction opposed to a direction, in which a surface of the developing roller moves, and the developing roller (see, for example, JP-A-11-167278).
There is disclosed a configuration of a toner regulatory blade, in which a portion having an arcuate-shaped cross section is formed integrally at a tip end of the blade in contact with a developing roller in a direction against rotation of the developing roller (see, for example, JP-A-2-135470).
In the tandem system disclosed in JP-A-2001-356548, a pitch of photosensitive bodies is approximately 2.4 times a diameter of the photo-sensitive bodies. In the case where such pitch of photosensitive bodies is reduced to at most 2 times the photosensitive-body diameter, image forming means arranged vertically interfere with one another, and therefore a limit is placed on miniaturization of an area around the photosensitive bodies.
In the example disclosed in JP-A-2001-296713, exposure means are fixedly mounted centrally of a body of an electrophotographic apparatus and light beams from the exposure means are irradiated on photosensitive bodies through spaces formed between charging means, developing means, cleaner means, and so on, which can be attachably and detachably mounted to the bodies, so that a whole image forming apparatus becomes large in size and a limit is placed on miniaturization of image forming means.
No configuration suited to miniaturization of whole developing means is disclosed in the examples of JP-A-5-11584 and JP-A-11-344858.
In the system disclosed in JP-A-11-167278, in which a tip end of a toner regulatory blade is brought into contact with a developing roller in a direction against rotation of the developing roller, a fixing member for fixation of the tip end of the regulatory blade is in most cases arranged above the developing roller. When the spring is lengthened so as to decrease a spring constant of the regulatory blade to reduce dispersion in pressing forces, a larger space above the developing roller is needed, and so developing means must be thickened, thus making miniaturization of image forming means impossible.
In the system disclosed in JP-A-2-135470, in which a tip end of a toner regulatory blade having an arcuate-shaped cross section is brought into contact with a developing roller in a direction against rotation thereof, such system is of the “against” system and not suited to miniaturization of a developing means. Also, in the case where a toner regulatory blade having an arcuate-shaped cross section at a tip end thereof is diverted to a system involving contact in a direction following rotation of a developing roller and arranged substantially horizontally, there are generated a toner filming phenomenon, in which regulated toner accumulates between the toner regulatory blade and the developing roller to adhere to the regulatory blade or the developing roller, and a phenomenon, in which insufficiency of toner regulating forces causes an increase in a passing toner and insufficiency of electrification of toner.