Image forming apparatus and image forming method for determining a transfer voltage value in a transfer section thereof

1. An image forming apparatus comprising:

a transfer section including a transfer roller and a rotatable member facing the transfer roller, the transfer section performing transfer processing in which the transfer section transfers a developer to a recording medium passing between the transfer roller and the rotatable member, the recording medium being either continuous paper or rolled paper;

a voltage generator that applies a voltage to a shaft of the transfer roller;

a current measuring section that measures a current flowing through the shaft of the transfer roller and a shaft of the rotatable member when the voltage is applied to the transfer roller, the current measuring section measuring a first current between the shaft of the transfer roller and the shaft of the rotatable member in a state where the recording medium is absent between the transfer roller and the rotatable member, and a second current between the shaft of the transfer roller and the shaft of the rotatable member in a state where the recording medium is present between the transfer roller and the rotatable member, and a third current between the shaft of the transfer roller and the shaft of the rotatable member in a state where the recording medium is present between the transfer roller and the rotatable member when a predetermined time has passed after measuring the second current; and

a main controller that determines a transfer voltage value for the transfer processing, wherein:

the main controller calculates a first electrical resistance value on a basis of the first current, a second electrical resistance value on a basis of the second current, a medium resistance value by using the first electrical resistance value and the second electrical resistance value, a third electrical resistance value on a basis of the third current, and a fourth electrical resistance value between the shaft of the transfer roller and the shaft of the rotatable member in a state where the recording medium is absent between the transfer roller and the rotatable member when the predetermined time has passed, by using the third electrical resistance value and the medium resistance value; and

the main controller determines the transfer voltage value by using the medium resistance value and the fourth electrical resistance value.

2. The image forming apparatus according to claim 1 , wherein:

the transfer section transfers the developer to the recording medium passing between the transfer roller and the rotatable member; and

the main controller calculates the transfer voltage value each time the transfer section transfers the developer to the recording medium over a predetermined reference length in a conveyance direction of the recording medium.

3. The image forming apparatus according to claim 1 , further comprising an environmental detector that detects at least one of an environmental temperature and an environmental humidity; wherein:

if the main controller determines that the environmental temperature of the transfer section detected by the environmental detector is not lower than a predetermined temperature, the main controller calculates the transfer voltage value.

4. The image forming apparatus according to claim 1 , wherein the main controller calculates the transfer voltage value in a non-transfer period that is a period of time other than a period of time in which the transfer section transfers the developer to the recording medium.

5. The image forming apparatus according to claim 4 , wherein the main controller sets the transfer voltage value as a new voltage value for the transfer processing within the non-transfer period.

6. The image forming apparatus according to claim 1 , further comprising a storage section that stores a target medium current density and a target medium voltage value; wherein:

the target medium current density is a preferable current density of a current that flows through the recording medium;

the target medium voltage value is a preferable electric potential difference between an electric potential of a front surface and an electric potential of a back surface in the recording medium; and

the voltage applied to the transfer roller when the first current is measured is a voltage determined from the target medium current density and the target medium voltage value.

7. The image forming apparatus according to claim 6 , wherein the voltage applied to the transfer roller when the second current is measured is a voltage determined from the target medium current density and the target medium voltage value.

8. The image forming apparatus according to claim 1 , wherein the main controller includes a calculating section that calculates the first electrical resistance value, the second electrical resistance value, the third electrical resistance, the fourth electrical resistance value and the medium resistance value.

9. The image forming apparatus according to claim 1 , further comprising a transfer belt; wherein the transfer section transfers the developer on the transfer belt onto the recording medium.

10. The image forming apparatus according to claim 1 , further comprising a photosensitive body that is capable of carrying an electrostatic latent image on a surface thereof; wherein the transfer section transfers the developer on the photosensitive body onto the recording medium.

11. An image forming method for determining a transfer voltage value in a transfer section, the transfer section including a transfer roller and a rotatable member facing the transfer roller and performing transfer processing, in which the transfer section transfers a developer to a recording medium which is either continuous paper or rolled paper passing between the transfer roller and the rotatable member, the image forming method comprising:

applying a voltage to the transfer roller;

measuring a first current between a shaft of the transfer roller and a shaft of the rotatable member with the voltage applied to the transfer roller in a state where the recording medium is absent between the transfer roller and the rotatable member;

measuring a second current between the shaft of the transfer roller and the shaft of the rotatable member with the voltage applied to the transfer roller in a state where the recording medium is present between the transfer roller and the rotatable member;

measuring a third current between the shaft of the transfer roller and the shaft of the rotatable member with the voltage applied to the transfer roller in a state where the recording medium is present between the transfer roller and the rotatable member when a predetermined time has passed after measuring the second current;

calculating a first electrical resistance value on a basis of the first current;

calculating a second electrical resistance value on a basis of the second current;

calculating a medium resistance value by using the first electrical resistance value and the second electrical resistance value;

calculating a third electrical resistance value on a basis of the third current;

calculating a fourth electrical resistance value between the shaft of the transfer roller and the shaft of the rotatable member in a state where the recording medium is absent between the transfer roller and the rotatable member when the predetermined time has passed, by using the third electrical resistance value and the medium resistance value; and

determining a transfer voltage value for the transfer processing by using the medium resistance value and the fourth electrical resistance value.