1. Field of the Invention
The present invention relates to a developing apparatus for use in image formation adopting an electrophotographic process and an image forming apparatus provided with this developing apparatus.
2. Related Background Art
As shown in FIG. 10, as an example of such a developing apparatus, a developing apparatus adopting one-component toner (referred to as toner hereunder) which is a developer has been known and come into practical use.
Such a developing apparatus is provided with a developing sleeve 3 which is a non-magnetic developer bearing member formed of a pipe made of aluminum or stainless steel, and a magnet 4 having a plurality of magnetic poles N and S alternately formed in the peripheral direction thereof is fixedly arranged inside the developing sleeve 3.
The surface of the developing sleeve is processed so as to have an appropriate surface roughness so that desired amount of toner can be born and carried.
An elastic blade 8 formed of, for example, urethane rubber or silicon rubber is fixed as a developer regulating member to a supporting sheet metal (not shown) on the developing sleeve 3 and abuts on the peripheral surface of the developing sleeve 3 by a predetermined pressure.
In such a developing apparatus, toner 7 attracted on the developing sleeve 3 by the magnetic force of the magnet 4 is carried on the developing sleeve 3 to be frictionally charged; an appropriate amount of the toner 7 is slid between the developing sleeve 3 and the elastic blade 8 to be frictionally charged while being regulated by the elastic blade 8; and the toner 7 having an appropriate electric charge given thereto is then carried to a developing area in the vicinity of a portion in which the developing sleeve 3 is opposed to a photosensitive drum 1 as a latent image bearing member so that the toner 7 is subjected to development.
On the other hand, the toner which has not been subjected to development moves to the upper portion of the elastic blade 8 as the developing sleeve 3 rotates and returns again into a toner container 6 which is a developer container so that it is circulated in a direction indicated by arrows in FIG. 10.
An agitating member 10 as agitating means is a rod member having a crank-like shape, and the both ends thereof serve as a center of rotation so that the agitating member is rotated in a direction shown in FIG. 10.
Incidentally, it is common, for example, to reduce the rotational driving force from a driving source for the developing sleeve 3 to an appropriate rotational speed by using a gear train so that the agitating member 10 is driven to be rotated and utilized.
In such a developing apparatus adopting the magnetic one-component toner, it is known that the toner having a relatively smaller particle diameter in the toner contained in the toner container is consumed by priority.
As disclosed in, for example, Japanese patent application laid-open No. 1-52182, there is proposed formation of a partition in the toner container as a countermeasure so that a small chamber on the developing sleeve side and a replenishing chamber for replenishing the toner to the small chamber are formed.
This can cause the toner having a smaller particle diameter to be consumed by priority in the initial stage and increase the average particle diameter of the toner in the small chamber, but the toner in the small chamber is balanced with the toner in the replenishing chamber to achieve stable transition when the particle diameter of the toner in the small chamber reaches a predetermined level, thereby preventing the particle diameter of the toner in the replenishing chamber from being increased after an endurance.
However, when providing a partition in the developing apparatus adopting the magnetic one-component toner, the toner circulation in the vicinity of the developing sleeve 3 is enlarged as shown in FIG. 11, which reduces the advantage of the partition 20.
That is because the large circulation of the toner 7 in the toner container 6 at the rear of the developing sleeve 3 causes the toner flow and the new toner to flow toward the developing sleeve 3 and counterchanging of these types of toner becomes prominent to provoke inflow of the toner from the upper portion of the partition 20.
Therefore, installation of the partition 20 is difficult in such a developing apparatus.
Further, in the above-described developing apparatus, recent reduction in the particle diameter of the toner aiming to increase high-quality image the reproducibility per dot leads to such a tendency as that the image density is lowered. FIG. 12 is a graph showing an initial image density transition caused due to a difference in central particle diameter of the toner in the above-mentioned developing apparatus.
As shown in FIG. 12, although the initial image density tends to be lowered (which will be referred to as the initial density lowering hereinafter) with respect to any particle diameter in such a developing apparatus, this tendency becomes remarkable if the central particle diameter is smaller.
The initial density low level is not very prominent when the central particle diameter is not less than 8 xcexcm, but it can be considered that this level should be improved when the central diameter is less than 8 xcexcm or more specifically not more than 7 xcexcm.
According to examination of the present inventors, as a cause of the initial density lowering, it has been revealed that the toner having a relatively small particle diameter in the toner contained in the developing apparatus tends to be concentrated on the developing sleeve at the stage of bringing the toner into use and the triboelectricity distribution of the toner coated on the developing sleeve becomes thereby broad (a ratio of the toner having the optimum triboelectricity for development is reduced), thus reducing the developing ability. Since existence of the toner having a small particle diameter becomes a problem, this phenomenon remarkably occurs as the central particle diameter of the toner becomes smaller.
Furthermore, is has been also discovered that such a phenomenon becomes prominent as images of patterns with a small consumption of the toner are continuously formed (for example, the solid black becomes further thin immediately after the solid white images are continuously formed).
That is because an amount of the toner fine powder coated on the developing sleeve is increased as the toner consumption is smaller.
Therefore, there may be unstableness such that a line width, the density and others of the printed image may vary depending on patterns of images to be formed in the above-described developing apparatus.
As means for preventing such unstableness, uniformization of the toner particle diameter (cutting the particle diameter on the fine power side during manufacture) can be considered, but this deteriorates a yield ratio during the toner manufacture to increase the cost, which is not a realistic means for preventing unstableness.
It is an object of the present invention to provide a developing apparatus capable of performing stable development by using a developer having a weight average particle diameter is not more than 7 xcexcm and an image forming apparatus having this developing apparatus.
It is another object of the present invention to provide a developing apparatus which can prevent an image density from being lowered and constantly obtain an image density irrespective of a pattern of an image to be formed even though a developer having a weight average particle diameter of not more than 7 xcexcm is adopted or provide an image forming apparatus having such a developing apparatus.
It is still another object of the present invention to provide a developing apparatus comprising: a developer containing portion for containing therein a developer having a weight average particle diameter of not more than 7 xcexcm; a developer bearing member for bearing a developer; and an agitating member for agitating the developer contained in the developer containing portion, wherein the agitating member intermittently moves.
It is yet another object of the present invention to provide an image forming apparatus comprising: an image bearing member for bearing a latent image; and a developing apparatus for developing the latent image, the developing apparatus including: a developer containing portion for containing therein a developer having a weight average particle diameter of not more than 7 xcexcm; a developer bearing member for bearing a developer; and an agitating member for agitating the developer contained in the developer containing portion, wherein the agitating member intermittently moves.
Other objects and advantages of the present invention will become clear by the following detailed description read in conjunction with the accompanying drawings.