1. Field of the Invention
This invention relates to an image forming apparatus such as a copying machine, a facsimile apparatus or a laser beam printer.
2. Related Background Art
In recent years, needs for coloring have been rising in image forming apparatuses such as copying machines and laser beam printers. As a color image forming process, the electrophotographic process is said to be excellent in that the image forming speed is high.
Color image forming apparatuses of the electrophotographic type include (i) a so-called one-drum type color image forming apparatus provided with a plurality of developing devices corresponding to a plurality of colors around an electrophotographic photosensitive member (hereinafter referred to as the “photosensitive member”) as an image bearing member, and (ii) a so-called tandem type color image forming apparatus provided with developing devices discretely for a plurality of photosensitive members.
The one-drum type has a single photosensitive member, and this leads to the merits that it can be relatively downsized and that cost can be reduced. However, image forming is repeated a plurality of times (usually four times) by the use of the single photosensitive member to thereby form a full-color image and therefore, this type is limited in the heightening of a color image forming speed. In the one-drum type color image forming apparatus, there is known (1) one in which images by toners of plural colors as developers are superposedly formed on the photosensitive member, whereafter these toner images are collectively transferred to a recording material, and (2) one in which toner images of different colors are successively formed on the photosensitive member, and each of them is respectively transferred to a recording material borne on a recording material conveying member and are superposed one upon another, or are respectively transferred to an intermediate transfer member and are superposed one upon another, and thereafter are collectively transferred to a recording material.
On the other hand, the tandem type has the merit that the heightening of the color image forming speed is possible. Recently, a speed as high as that for monochromatic image forming is required for color image forming and thus, attention has been paid to the tandem type. In a color image forming apparatus of the tandem type, there is known one in which toner images formed by toners of different colors on a plurality of photosensitive members are transferred to a recording material borne on a recording material conveying member and are superposed one upon another, or one in which the toner images are successively transferred to an intermediate transfer member and are superposed one upon another, and thereafter are collectively transferred to a recording material.
Among these color image forming apparatuses, the intermediate transfer type using the intermediate transfer member is occupying the mainstream, because this type has the merits that this type makes no choice of recording materials and that it is excellent in color registration (suffers little from color misregister).
Describing an image forming apparatus of this intermediate transfer type as an example, any toners residual on the surfaces of the photosensitive member and the intermediate transfer member (hereinafter referred to as the “untransferred toners”) after the transferring step in the image forming process are generally removed by cleaning means. Heretofore, a cleaning device as the cleaning means is provided with a cleaning member such as a fur brush or a cleaning blade, and the toners collected by the cleaning device are carried by toner carrying means provided with a screw, an auger, a belt or the like as a carrying member, for example, in a pipe-shaped carrying path, and are collected into a toner disposal container as toner disposal means.
Usually, the toner disposal container, when filled with the toners, is disposed of by an operator and is replaced with a new empty toner disposal container.
Now, in recent years, downsizing, a higher function, coloring and a higher speed have been advanced for color image forming apparatuses, while on the other hand, there have been rising requirements for improved reliability, system evolution, maintenance-freedom, a low running cost, effective utilization of resources, consideration to environments, etc. Particularly, there are demands for the consideration to environments, the low running cost, etc.
In the conventional color image forming apparatus, however, the untransferred toners are collected into the toner disposal container and are disposed of and therefore, the effective utilization of resources, the consideration to environments and the low running cost have been tasks.
As regards the untransferred toner, in a single-color (usually monochromatic) image forming apparatus, the recycling thereof has been put into practical use, but in a color image forming apparatus, the recycling of the toners of plural colors mixed together has been difficult because of the problem that the color taste of an image changes.
Japanese Patent Application Laid-open No. H08-63067 proposes, in an image forming apparatus of the one-drum type (particularly a full-color image forming apparatus in which multiple developer images formed on a photosensitive member are collectively transferred to a recording material), to supply toners collected from on the photosensitive member by a cleaning device to a developing device for black. In this prior art, the mixing ratio between the collected toners to be supplied to the developing device for black and the black toner is controlled so that [collected toners/(black toner+collected toners)]≦60%. This prior art, however, does not mention that the supply ratio between the collected toners and the black toner when the collected toners with the toners of plural colors mixed together therein are supplied to the developing device for black is made variable.
Japanese Patent Application Laid-open No. H08-248853 proposes, in an image forming apparatus of the one-drum type (particularly an image forming apparatus in which toner images of two colors formed on a photosensitive member are collectively transferred to a recording material), to dispose two kinds of cleaning devices for the disposal and recycling, respectively, of untransferred toners, and determine into which of the two kinds of cleaning devices the untransferred toners on the photosensitive member are collected, from the color information of written-in data in conformity with the mixing ratio of the color toners. In this prior art, from the pixel data percentage of an original image, only when the black image is 100% (or 98% or more), the cleaning device for recycling is operated and the collected toners are carried to a developing device for black and are recycled. However, when the mixing ratio departs from a desired range, the collected untransferred toners have not been recycled but have been disposed of, and have not been effectively utilized. For example, in a case where this prior art is applied to a four-color full-color image forming apparatus, if the number of full-color images is great, there is the undesirable possibility that the toners disposed of may increase.
Japanese Patent Application Laid-open No. 2000-35703 proposes, in an image forming apparatus of the tandem type (particularly, a full-color image forming apparatus in which toner images are multiplexly transferred from a plurality of photosensitive-members to a recording material borne on a recording material conveying member), to provide a developing device for a recycled developer discrete from a developing device for black used in a color image forming process. Untransferred toners of respective colors are gathered at a location and these toners are utilized as recycled toners in the developing device for the recycled developer. That is, in this method, the collected toners are not returned to the developing device for black, but are recycled as pseudo black. This prior art, however, collects Y, M and C toners in equal amounts and mixes them to thereby make the mixture into a pseudo black toner and therefore, cannot recycle the toners when the consumed amounts of the respective toners are not equal to one another. This prior art gives no consideration to controlling the mixing ratio of the collected toners and a new toner in conformity with the ratio of the toner of each color in the collected toners which becomes important when the collected toners with plural colors mixed together therein are utilized for color image forming.
Japanese Patent Application Laid-open No. 2001-337503 proposes, in a cleanerless image forming apparatus of the tandem type (particularly a full-color image forming apparatus in which toner images multiplexly transferred from a plurality of photosensitive members to an intermediate transfer member are collectively transferred to a recording material and untransferred toners are collected into developing devices for respective colors), a method of controlling color mixing in the developing devices for respective colors so that the untransferred toners may get mixed in the developing device located on the downstream side with respect to the moving direction of the intermediate transfer member, within a predetermined allowable value of color mixing. This prior art, however, recycles the toners collected from respective photosensitive members by cleaning devices provided correspondingly thereto in developing devices for respective colors provided correspondingly to the respective photosensitive members, and does not collectively recycle the collected toners with plural colors mixed together therein, in the developing device for black.
Japanese Patent Application Laid-open No. 2003-15494 proposes, in a color image forming apparatus of the tandem type (particularly a full-color image forming apparatus in which toner images multiplexly transferred from a plurality of photosensitive members to an intermediate transfer member are collectively transferred to a recording member), to return to respective developing devices the toners collected from the respective photosensitive members by cleaning devices provided correspondingly to the respective photosensitive members and recycle these toners. Also, in this publication, it is described that in conformity with the mixing rate of the toners collected in the respective cleaning devices, the supply amounts of these toners and a new toner are made variable. This prior art, however, recycles the toners collected from the respective photosensitive members by the cleaning devices provided correspondingly thereto in the developing devices for respective colors provided correspondingly to the respective photosensitive members, and further relates to the mixing of two colors in which the toner one color upstream of the other, and does not collectively recycle the collected toners with plural colors mixed together therein, in the developing device for black.
To recycle the toner collected by each cleaning device in the developing device for each color, as in Japanese Patent Application Laid-open No. 2001-337503 and Japanese Patent Application Laid-open No. 2003-15494, a complicated construction and control are required to suppress the influence of the color mixing of the toners collected by the respective cleaning devices upon the color taste of an image because of the reverse transfer or the like to the respective photosensitive members. The toners of the other colors (color toners) such as yellow, magenta and cyan than black affect the color taste of the image because a relatively small amount of toner of other color is mixed therewith. Or the techniques described in the aforementioned Japanese Patent Application Laid-open No. 2001-337503 and Japanese Patent Application Laid-open No. 2003-15494 cannot be applied to a color image forming apparatus of the one-drum type. Further, according to this prior art, there is the problem that the degree of freedom of design is restricted by the necessity of disposing on the most upstream side the developing device for yellow which is low in the color mixing rate of a different color toner (the limit of the color mixing rate) with an inherent color toner when a change in the hue of the final image has reached an allowable limit level. Also, when the color mixing rate of the toners collected by the respective cleaning devices is great, there is the undesirable possibility that the consumed (recycled) amount of the toners decreases and the cleaning devices become full of the toners. Also, these prior arts do not mention the recycling of untransferred toners with the toners of four colors mixed together therein on the intermediate transfer member or the recording material conveying member.
Hereinafter, the new toner supplied to the developing device will be referred to as the “fresh toner”, and the toner collected by the cleaning device and returned to the developing device and recycled thereby will be referred to as the “recycled toner”.
As a result of the study assiduously made by the inventor, it has been found that it is very advantageous and very efficient to return the recycle toner with toners of plural colors mixed together collected from the photosensitive member and the image conveying members (such as the intermediate transfer member and the recording material conveying member) or the image conveying member as the black toner to the developing device for black and recycle it.