Patent Application
20060078357
This application is based on Japanese Patent Application No. 2004-298410, the content of which incorporated herein by reference.
The present invention relates to a developing device to be used for electrophotographic image formation apparatuses such as printers and copiers.
In electrophotographic image formation apparatuses such as printers and copiers, there has conventionally been used a developing device which develops with toner an electrostatic latent image formed on a surface of a photoconductor to visualize the image. This developing device generally has a developing roller or developing sleeve which rotates while holding charged toner on the outer peripheral surface in a thin layer state, so that toner is fed onto the photoconductor from the developing roller or the like.
Meanwhile, as printers have been wide-spreading in offices rapidly in recent years, users have been becoming increasingly more conscious of image quality and cost. One of image noise is ‘fogging’ which occurs due to deposition of the toner at portions other than the latent image on the photoconductor. This occurs more often due to a deterioration of toner charge amount resulting from a deterioration of the toner within the developing device during endurance time. The deterioration of toner charge amount due to endurance causes not only image deteriorations but also increases in toner consumption, which substantially causes larger burdens of cost on users.
For electric charging of the toner in the developing device, as is a general method, a blade-like member is brought into press contact against the developing roller, and the toner held on the developing roller in a thin layer state is frictionally rubbed by the blade-like member, thereby electrically charged. However, a nip width formed between the blade-like member and the developing roller, if small, would make it hard to impart a sufficient charge amount to deteriorated toner. Thus, For obtainment of larger nip widths, there have been proposed methods for bringing a film-like member into press contact with the developing roller in Japanese Patent Laid-Open Publications Nos. S63-155065, H05-11583, H10-31358, H10-133474 and H11-272069.
However, in the case where the nip width is broadened by using a film-like member, there is a tendency that although the toner charge amount gradually increases while passing through within the nip, yet the toner becomes less charged in downstream-side part within the nip with respect to the rotational direction of the developing roller, so that the charge amount reaches the ceiling and a desired charge amount cannot be obtained. This tendency is considerable particularly with deteriorated toner, causing a problem that the fogging involved in endurance cannot be improved.
Accordingly, an object of the present invention is to provide a developing device which is capable of imparting a sufficient charge amount even to deteriorated toner so that occurrence of the fogging can be reduced.
In order to achieve the above object, in a first aspect of the present invention, there is provided a developing device comprising a voltage application member which is in contact via a toner layer with a developing roller that rotates while holding the toner layer on its outer peripheral surface, wherein
to the toner layer, the voltage application member applies a relatively low voltage in an upstream-side contact portion with respect to a rotational direction of the developing roller, and applies a relatively high voltage in a downstream-side contact portion thereof.
With the developing device of this constitution, toner can be charged to an appropriate level by applying such a relatively low voltage that no leakage occurs in the upstream-side contact portion, and a desired toner charge amount can be obtained by applying such a relatively high voltage that the toner does not reach the ceiling but can be sufficiently charged in the downstream-side contact portion. Thus, a desired charge amount can be obtained even with deteriorated toner, and the occurrence of fogging can be improved.
In the developing device of the first aspect of the invention, the voltage application member may comprise one member which makes contact over a specified angle range with respect to the rotational direction of the developing roller, and resistivity of the upstream-side contact portion is larger than resistivity of the downstream-side contact portion.
Also in the developing device of the first aspect of the invention, the voltage application member may be so formed as to be thicker in the upstream-side contact portion and thinner in the downstream-side contact portion. In this case, thickness of the contact portion of the voltage application member may be either varied in a step-by-step manner or continuously varied.
Also in the developing device of the first aspect of the invention, the upstream-side contact portion and the downstream-side contact portion of the voltage application member may be either formed of an identical material or different in material from each other.
Also in the developing device of the first aspect of the invention, it is possible that the voltage application member, by being pressed toward the developing roller by an electrically conductive backup member, is brought into contact with the developing roller over a specified angle range with respect to the rotational direction, and the voltage application member comprises one member to which a voltage is applied via the backup member, and wherein an upstream-side portion of the backup member with respect to the rotational direction of the developing roller is larger in resistivity than a downstream-side portion thereof.
Also in the developing device of the first aspect of the invention, the voltage application member may comprise one member which makes contact with the developing roller over a specified angle range with respect to the rotational direction of the developing roller, and a voltage is applied to the voltage application member from the downstream-side contact portion side.
Further, in the developing device of the first aspect of the invention, the voltage application member may be divided into the upstream-side contact portion and the downstream-side contact portion. In this case, it is possible that resistivity of the upstream-side contact portion is larger than resistivity of the downstream-side contact portion, and identical voltages are applied to the upstream-side contact portion and the downstream-side contact portion, respectively, or that the upstream-side contact portion and the downstream-side contact portion are of an identical resistivity, and a voltage applied to the downstream-side contact portion is higher than a voltage applied to the upstream-side contact portion.
In a second embodiment of the present invention, there is provided a developing device comprising:
a developing roller which, while holding a toner layer on an outer peripheral surface thereof, is placed opposite to a photoconductor on a surface of which a latent image is to be formed;
a holding member having an opposing face opposed to the outer peripheral surface of the developing roller;
a power supply for applying a voltage to the holding member; and
a voltage application member which is fixed to the opposing face of the holding member so as to be in surface contact with the outer peripheral surface of the developing roller, and an upstream-side contact portion of the voltage application member with respect to the rotational direction of the developing roller is higher in resistivity than a downstream-side contact portion thereof.
In the developing device of the second aspect of the invention, thickness of the voltage application member may be larger in the upstream-side contact portion than in the downstream-side contact portion. In this case, thickness of the contact portion of the voltage application member with the developing roller may be either varied in a step-by-step manner or continuously varied.
Also in the developing device of the second aspect of the invention, the upstream-side contact portion and the downstream-side contact portion of the voltage application member may be either formed of an identical material or different in material from each other.
In a third aspect of the present invention, there is provided a developing device comprising:
a developing roller which, while holding a toner layer on an outer peripheral surface thereof, is placed opposite to a photoconductor on a surface of which a latent image is to be formed;
a holding member having an opposing face opposed to the outer peripheral surface of the developing roller;
a power supply for applying a voltage to the holding member; and
a voltage application member which has a specified length with respect to a rotational direction of the developing roller and whose downstream-side end portion area is fixed to the opposing face of the holding member so as to be in noncontact with the developing roller and whose upstream-side end portion area is in surface contact with the outer peripheral surface of the developing roller.
In the developing device of the third aspect of the invention, the voltage application member may be formed of a film.
With the use of the developing device of the present invention, a sufficient charge amount can be imparted even to deteriorated toner, and the occurrence of fogging can be improved.
The present invention will be further described with reference to the accompanying drawings wherein like reference numerals refer to like parts in the several views, and wherein:
In the casing 12, a feed roller 16 is disposed in contact with the developing roller 14. As the feed roller 16 is driven into rotation along a direction of arrow B, the toner T is fed to the developing roller 14 so that the a thin toner layer is formed on the outer peripheral surface of the developing roller 14.
on top of the casing 12 is fixed a voltage application member 18 formed of, for example, an electrically semiconductive flexible resin film. The voltage application member 18, which is formed of one member, is pressed against the developing roller 14 by a backup member (holding member) 20 which is attached to an electrode 22 and formed of, for example, an electrically conductive sponge. As a result, the voltage application member 18 is in contact with the developing roller 14 over a specified angle range via the toner layer held on the outer peripheral surface. To the voltage application member 18, a voltage is to be applied via the backup member 20 from the electrode 22 connected to a power supply 21.
It is noted that the voltage application part 18 may be fixed to an opposing face of the backup member 20 confronting the outer peripheral surface of the developing roller by, for example, adhesion or the like, or otherwise may be only held by being pressed by the opposing face without being fixed. The voltage to be applied to the voltage application member 18 is not limited to a DC voltage and may also be a voltage in which an AC voltage is superimposed on a DC voltage.
A contact portion of the voltage application member 18 with the developing roller 14, as shown in
It is noted that although the voltage application member 18 of this embodiment is varied in thickness in three steps, yet the thickness variation at the contact portions with the developing roller 14 may be in two steps or four or more steps.
Also, although the voltage application member 18 is varied in thickness in a step-by step manner by bonding the three films with their forward end positions shifted from one another, yet it is also possible that one film is molded so as to vary in thickness in a step-by-step manner, or that one film is molded so as to be continuously varied in thickness like a film 24 shown in
Further, the voltage application member 18 has its upstream-side contact portion 18a, intermediate contact portion 18c and downstream-side contact portion 18b composed of one identical material. However, it is also possible, like a voltage application member 26 shown in
Furthermore, the voltage application member 18 is varied in resistivity value between the upstream-side contact portion 18a and downstream-side contact portion 18b by its own thickness being varied. However, it is also possible that, as shown in
Next, operations of the developing device 10 having the above constitution are described.
In the developing device 10, as the developing roller 14 is driven into rotation along the direction of arrow A, the feed roller 16 is rotationally driven along the direction of arrow B. The toner T housed in the casing 12 is fed to the developing roller 14 by the rotating feed roller 16, by which a thin toner layer is formed on the outer peripheral surface of the developing roller 14.
The toner layer on the outer peripheral surface of the developing roller 14 is moved to a contact area with the voltage application member 18 as the developing roller 14 rotates. During the passage through this contact area, the toner is electrically charged to a desired charge amount. The electric charging is carried out as follows.
To the voltage application member 18, a specified voltage (e.g., −300 V) is applied from the electrode 22 via the backup member 20. However, the resistivity value of the contact portion of the voltage application member 18 with the developing roller 14 is larger at the upstream-side contact portion 18a, mid-level at the intermediate contact portion 18c and smaller at the downstream-side contact portion 18b. Therefore, the effective voltage to be applied to the toner layer on the developing roller 14 is relatively lower at the upstream-side contact portion 18a, mid-level at the intermediate contact portion 18c and relatively higher at the downstream-side contact portion 18b.
While the toner on the developing roller 14 passes through in contact with the upstream-side contact portion 18a of the voltage application member 18, the toner is moderately charged with such a relatively low voltage that no leakage occurs between the voltage application member 18 and the developing roller 14. Subsequently, while the toner on the developing roller 14 passes through in contact with the intermediate contact portion 18c of the voltage application member 18, the toner is further charged with application of a voltage higher than at the upstream-side contact portion 18a, so that the charge amount is increased. Then, while the toner on the developing roller 14 passes through in contact with the downstream-side contact portion 18b of the voltage application member 18, the toner is sufficiently charged with application of a voltage even higher than at the intermediate contact portion 18c, so that the toner does not reach the ceiling but comes to have a desired charge amount.
It is noted that although relatively higher voltages are applied at the intermediate contact portion 18c and the downstream-side contact portion 18b than at the upstream-side contact portion 18a, yet there occurs no leakage because the toner charge amount gradually increases along with the passage through the contact area with the voltage application member 18 so that the electric potential heightens.
The toner, which has passed through the contact area with the voltage application member 18 and has thereby been charged up to a desired charge amount, is moved to an opposite area to the photoconductor 15 along with the rotation of the developing roller 14 and provided for development of the latent image on the surface of the photoconductor 15.
As described above, according to the developing device 10 of this embodiment, toner can be sufficiently charged up to a desired charge amount. Therefore, a desired charge amount can be imparted even to toner that has deteriorated due to endurance, so that the occurrence of fogging can be improved.
Next, an experiment which was performed to verify the working effects of the developing device 10 of this embodiment is described.
In a Working Example of the developing device 10, a low-resistivity film having a volume resistivity of 2.5×103 Ω·cm with its thickness continuously varying from 200 μm in the upstream-side contact portion to 50 μm in the downstream-side contact portion as shown in
As shown in the graph of
In addition, it would be conceivable, in the Comparative Example, to select a film having a large resistivity to prevent the leakage at the upstream-side contact portion. However, in the case where a film having a resistivity of, for example, 104 Ω·cm is selected as shown in the graph of
On the other hand, it is also conceivable to elongate the nip width to increase the charge injection time for toner so that the toner charge amount is increased. However, in the Comparative Example, as shown in the graph of
Next, modification examples of the developing device 10 are described with reference to FIGS. 9 to 11.
As shown in
In the developing device of this modification example, the effective voltage for toner charging in an upstream-side contact portion 32a of the voltage application member 32 becomes lower than in a downstream-side contact portion 32b by an extent corresponding to a voltage drop due to the resistivity of the voltage application member 32 in a direction along the rotational direction of the developing roller. As a result, with the voltage application member 32, a relatively low voltage is applied to the toner layer on the developing roller 14 in the upstream-side contact portion 32a while a relatively high voltage is applied to the toner layer on the developing roller 14 in the downstream-side contact portion 32b, so that the same working effects as with the developing device 10 are produced.
In the modification example shown in
Also, in the modification example shown in
Further, in the modification example shown in
In addition, although the above-described modification examples have been described on a case where the voltage application member is divided into two, the voltage application member may also be divided into three or more.
Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-298410 | Oct 2004 | JP | national |
