Developing device having a voltage application member for applying varied voltages

1. A developing device comprising:

a voltage application member configured to maintain contact with a toner layer; and

a developing roller configured to concurrently maintain contact with the toner layer and to rotate while holding the toner layer on its outer peripheral surface,

wherein the voltage application member is configured to apply a relatively low voltage to the toner layer in an upstream-side contact portion with respect to a rotational direction of the developing roller,

wherein the voltage application member is configured to apply a relatively high voltage to the toner layer in a downstream-side contact portion thereof, and

wherein the voltage application member comprises 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.

2. The developing device as claimed in claim 1 , wherein the voltage application member is so formed as to be thicker in the upstream-side contact portion and thinner in the downstream-side contact portion.

3. The developing device as claimed in claim 2 , wherein thickness of the contact portion of the voltage application member is varied in a step-by-step manner.

4. The developing device as claimed in claim 2 , wherein thickness of the contact portion of the voltage application member is continuously varied.

5. The developing device as claimed in claim 1 , wherein the upstream-side contact portion and the downstream-side contact portion of the voltage application member are formed of an identical material.

6. The developing device as claimed in claim 1 , wherein the upstream-side contact portion and the downstream-side contact portion of the voltage application member are different in material from each other.

7. A developing device comprising:

a voltage application member configured to maintain contact with a toner layer; and

a developing roller configured to concurrently maintain contact with the toner layer and to rotate while holding the toner layer on its outer peripheral surface,

wherein the voltage application member is configured to apply a relatively low voltage to the toner layer in an upstream-side contact portion with respect to a rotational direction of the developing roller,

wherein the voltage application member is configured to apply a relatively high voltage to the toner layer in a downstream-side contact portion thereof, and

wherein 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.

8. A developing device comprising:

a voltage application member configured to maintain contact with a toner layer; and

a developing roller configured to concurrently maintain contact with the toner layer and to rotate while holding the toner layer on its outer peripheral surface,

wherein the voltage application member is configured to apply a relatively low voltage to the toner layer in an upstream-side contact portion with respect to a rotational direction of the developing roller,

wherein the voltage application member is configured to apply a relatively high voltage to the toner layer in a downstream-side contact portion thereof,

wherein the voltage application member is divided into the upstream-side contact portion and the downstream-side contact portion, and

wherein 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.

9. 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.

10. The developing device as claimed in claim 9 , wherein thickness of the voltage application member is larger in the upstream-side contact portion than in the downstream-side contact portion.

11. The developing device as claimed in claim 10 , wherein thickness of the contact portion of the voltage application member with the developing roller is varied in a step-by-step manner.

12. The developing device as claimed in claim 10 , wherein thickness of the contact portion of the voltage application member with the developing roller is continuously varied.

13. The developing device as claimed in claim 9 , wherein the upstream-side contact portion and the downstream-side contact portion of the voltage application member are formed of an identical material.

14. The developing device as claimed in claim 9 , wherein the upstream-side contact portion and the downstream-side contact portion of the voltage application member are different in material from each other.