1. Field of the Invention
The present invention relates to image forming apparatuses such as copying machines, facsimile machines and printers which form an electrostatic latent image on an image bearing member by an electrophotographic or electrostatic recording method or otherwise, and turns this electrostatic latent image into a visible image (toner image) with a developer accommodated in a developing device.
2. Related Background Art
In a conventional color image forming apparatus of an electrophotographic type, for example, toner images each of a different color, namely yellow, cyan, magenta and black, are formed on an image bearing member such as a photosensitive member (photosensitive drum), and these toner images of different colors are transferred and superposed on a recording sheet to have a full color image formed on the recording sheet in turn. This is known as a multi-layer transfer process.
Since it is required in this multi-layer transfer process to form toner images of different colors on the photosensitive drum in turn, one at a time of a plurality of developing devices, each containing a toner of one color or another, is selectively set in a developing position opposite the photosensitive drum, and all other developing devices have to be set in waiting positions away from the image bearing member.
For this reason, this conventional color image forming apparatus uses a so-called rotary developing unit, in which the plurality of developing devices are arranged at equal intervals on a movable member. The movable member has a circumferential surface of a cylindrical rotating body and is rotatable(hereinafter referred to as developing rotary). The movable member selectively sets one or another of the four-color developing devices in the developing position. By the movable member, any desired developing device is set in the developing position by the rotation of the developing rotary, while the other developing devices are kept away from positions close to the photosensitive drum.
Incidentally, a two-component developing system which uses a mixture of a non-magnetic toner and a magnetic carrier as the developer is extensively used in image forming apparatuses which form color images in particular among electrophotographic image forming apparatuses. The two-component developing system has advantages in the stability of image quality and the durability of the apparatus among other respects over other developing systems which have been proposed so far.
In the developing using two-component developers, out of the toner and the carrier contained in each developing device, only the toner is consumed by the development of electrostatic latent images, and accordingly the developing device held by the developing rotary has to be replenished with a new supply of toner in turn. Therefore, in order to maintain the developed density of electrostatic latent images at the desired level all the time, the quantity of the toner with which each developing device is replenished with should be kept under strict control.
There is another problem. Every time the developing rotary is turned to change the developing device set in the developing position, a developing device, a toner replenishment container and a toner replenishment passage have to turn around the rotation axis of the developing rotary and, even in a state in which a conveyance screw disposed in the toner replenishment passage is at halt, the toner moves along the spiral blades of the conveyance screw and automatically conveyed within the toner replenishment passage, inviting an excess or shortage of the toner for replenishment.
As a proposed solution to these problems, as shown in FIG. 5 appended to the present application, Japanese Patent Application Laid-Open H10-149012 discloses a rotary developing unit in which a developing rotary 100 is mounted with a developing device 101, and toner conveyance means 104 for conveying the toner from a toner replenishment container 102 to the developing device 101 within a toner replenishment passage 103 is provided. When the toner conveyance means 104 is stopped and the developing rotary 100 is turned, the toner within the toner replenishment passage 103 is conveyed from a developing device discharge outlet 105 to a toner replenishment container receptacle 106, and the toner replenishment quantity is controlled according to the rotational speed of the developing rotary 100.
However, when the developing device is sent in the developing position opposite the photosensitive drum, the vibration at the time of stopping causes the toner remaining in the toner replenishment passage to slip sideways, and the toner gradually moves toward the developing device to be eventually fed to,the developing device.
When this phenomenon occurs, no large quantity of toner moves into the developing device in a single turn of a developing rotary 18 around a rotation axis 18a. However, in an image forming apparatus which has developing devices for yellow, cyan, magenta, black, light magenta and light cyan, for instance, and can operate in either a four-color mode or a six-color mode, the rotation of the developing rotary around its rotation axis accelerates the movement of toners remaining in the toner replenishment passage into the developing devices for light magenta and light cyan, neither used in the four-color mode. Therefore, unintended feeding of toners to the developing devices for light magenta and light cyan is especially accelerated, resulting in an increased density of these toners and a corresponding variation in development performance.