1. Field of the Invention
The present invention relates to a developing apparatus wherein a developer is conveyed while being agitated by screw-like agitating members having different numbers of threads, as well as an image formation apparatus using the developing apparatus.
2. Description of the Prior Art
In an image formation apparatus, using a developing apparatus which serves as a developer supply means, a developer is fed to an electrostatic latent image formed on an image bearing member to develop the electrostatic latent image. In a developing apparatus using a two-component developer comprising a carrier and a toner, it is desirable that the toner and the carrier be conveyed in a satisfactorily agitated state. In view of this point, there has been proposed a developing apparatus of a twin screw agitating type wherein the interior of a case is divided by a partition wall into a first space positioned on a toner supply side for the image bearing member and a second space into which is fed a replenishing toner, with screw-like agitating members being disposed in parallel within the spaces.
According to the developing apparatus of a twin screw agitating type, a developer is conveyed while being agitated by a first agitating member disposed in the first space and a second agitating member disposed in the second space and the developer thus conveyed is fed into the spaces from delivery portions formed at end portions of the agitating members, thus being circulated. In such a twin screw agitating type developing apparatus, a screw trace, which is caused by, for example, unevenness in the density of the screw pitch, is apt to occur at the time of development of an image having a high image ratio. Therefore, attempts have been made to agitate and convey a developer by using multiple-thread screws as the first and second agitating members.
With such multiple-thread screws as the first and second agitating members, there occurs a new problem although the occurrence of screw traces can he prevented. For attaining a satisfactory development in the twin screw agitating type developing apparatus, it is important that a sufficient developer be fed to the first space having the first agitating member. However, if the multiple-thread screws are used as agitating members, the area occupied by the screws in each space is large and the amount of developer loaded into the developing apparatus becomes smaller, resulting in decrease of the amount of developer fed to the first space positioned on the toner supply side.
If agitating members are screws having an identical number of threads and an identical pitch, it is theoretically possible to convey an identical amount of developer at an identical speed. However, if the screws have areas too large with respect to the aforementioned spaces, the developer becomes more bulky and the agitating members are buried in the developer. If such a state occurs, there is formed an area on which each of the agitating members cannot exhibit its conveying force. This causes a difference in the developer conveying speed as compared with the case where the agitating members are not buried. As a result, the developer circulating balance is lost and the developer stays in the end portions of the spaces.
Even at the same conveying speed, if agitating members have different numbers of threads, i.e., different numbers of blades, the screw having a larger number of threads conveys less developer. For example, if the second agitating member has a smaller number of threads and the first agitating member has a larger number of threads, the developer will stay in the vicinity of a developer delivery portion for delivery of developer from the second space having the second agitating member to the first space having the first agitating member. If the delivery portion with the developer thus staying therein overlaps an image forming area, a larger amount of developer than necessary is fed into the first space and the developer may leak out of the apparatus from near a portion opposed to the image bearing member.
A certain developing apparatus is provided with a toner concentration detecting means for detecting and outputting a toner concentration in a developer. However, if the way in which the developer stays within a space changes, a greater output difference results, disabling to accurately detect a toner concentration.
It is an object of the present invention to provide a developing apparatus and an image formation apparatus in which the occurrence of screw pitch traces and unsatisfactory development caused by an insufficient amount of developer fed are extremely diminished
It is another object of the present invention to provide a developing apparatus and an image formation apparatus capable of preventing leakage of a developer to the exterior of the apparatus.
It is a further object of the present invention to provide a developing apparatus and an image forming apparatus capable of detecting a toner concentration accurately even if there is a change in the way of staying of a developer.
For achieving the above-mentioned objects, the present invention provides a developing apparatus comprising: a case divided into a first space located at a developer supply side for supplying the developer to an image bearing member and a second space for receiving a replenishing toner; a first and a second agitating member arranged in the first and the second spaces in such a manner that the first and the second agitating members are positioned parallel to each other and are driven to rotate to agitate and convey the developer,
wherein the first agitating member arranged in the first space is a screw member having n threads (nxe2x89xa72), and the second agitating member arranged in the second space is a screw member having (nxe2x88x92x) threads (xxe2x89xa70). Since the second agitating member has a smaller number of threads than the first agitating member, the volume thereof in the second space decreases and the bulk (height) of the developer is prevented from becoming larger.
The developing apparatus may further comprise: a first delivery portion for feeding the developer from the first space into the second space; and a second delivery portion for feeding the developer from the second space into the first space, which secondary delivery portion is positioned outside an image formation area of the image bearing member. According to this construction, the position where the developer is delivered from the second space to the first space lies exteriorly of the image forming area of the image bearing member, with no delivery of the developer within the image forming area.
For diminishing the developer conveying capacity of the second agitating member, a non-screw portion may be formed at a portion of the second agitating member located at a predetermined interval in the convey direction of the developer conveyed by the second agitating member, thereby suppressing the convey force.
Likewise, for diminishing the developer conveying capacity of the second agitating member, rotation speed V1 of the first agitating member is set greater than rotation speed V2 of the second agitating member. This relation can be established by rotating the first and second agitating members each individually with use of different drive sources for rotation or, in case of using a single drive source for rotation, with use of gears having a different number of teeth or pulleys different in speed change ratio in an integrally rotatable manner with the first and second agitating members. As a result, the amount of developer conveyed per unit time by the second agitating member is kept smaller than that by the first agitating member.
Further, for diminishing the developer conveying capacity of the second agitating member, it is preferable to narrow the screw pitch of the second agitating member. This is also effective because the amount of developer conveyed per unit time decreases.
In the case where the developing apparatus includes toner concentration detecting means to detect and output the concentration of toner contained in the developer, the toner concentration detecting means having a detection surface which faces the interior of the second space, it is preferable that the toner concentration detecting means be disposed closer to the first delivery portion than to the second delivery portion with respect to the center of the image forming area. According to this construction, even if the way of staying of the developer changes on the second delivery portion side, this can be made less influential on the toner concentration detecting means.
According to another aspect of the present invention, there is provided an image forming apparatus having an image bearing member and developer supply means for supplying a developer to an electrostatic latent image formed on the image bearing member to develop the electrostatic latent image, wherein a developing apparatus is used as the developer supply means, the developing apparatus having a screw member of n threads (nxe2x89xa72) as a first agitating member disposed within a first space and a screw member of (nxe2x88x92x) threads (xxe2x89xa70) as a second agitating member disposed within a second space to receive the supply of a replenishing toner. According to this construction, since the number of threads of the second agitating member is smaller than that of the first agitating member, the volume of the second agitating member in the second space decreases.
In case of using as the developer supply means a developing apparatus having a first delivery portion for feeding the developer from the first space to the second space and a second delivery portion for feeding the developer from the second space to the first space, the second delivery portion being positioned on an outer side with respect to an image forming area of the image bearing member, the position where the developer is delivered from the second space to the first space lies exteriorly of the image forming area of the image bearing member, with no delivery of the developer within the image forming area.
In case of using a developing apparatus as the developing supply means, the developing apparatus having non-screw portion at a portion of the second agitating member which portion is positioned in a predetermined section in the direction of conveyance of the developer conveyed by the second agitating member, the developer conveying capacity of the second agitating member is kept low by the non-screw portion.
In case of using as the developer supply means a developing apparatus wherein if the rotational speed of the first agitating member is V1 and that of the second agitating member is V2, there is established a relation of V1 greater than V2, the developer conveying capacity of the second agitating member decreases.
In case of using as the developer supply means a developing apparatus wherein the screw pitch of the second agitating member is set narrow, the amount of the developer fed per unit time decreases.
Further, as the developer supply means there may be used a developing apparatus including a toner concentration detecting means for detecting and outputting the concentration of toner contained in the developer, the toner concentration detecting means having a detection surface which faces the interior of the second space and being disposed closer to the first delivery portion rather than the second delivery portion with respect to the center of the image forming area. In this case, even if the way of staying of the developer changes on the second delivery portion side, this can be made less influential on the toner concentration detecting means.