Image forming device

1. An image forming device comprising:

a transfer unit transferring a toner image to a sheet;

a resistance measurement member disposed on an upstream side in a sheet carriage direction of the transfer unit and for measuring resistance of the sheet;

a charge elimination unit disposed between the transfer unit and the resistance measurement member in the sheet carriage direction;

a first voltage applying unit applying voltage for resistance measurement to the resistance measurement member; and

a second voltage applying unit applying voltage for charge elimination as a voltage which is reverse bias of the voltage for resistance measurement to the charge elimination unit,

wherein the width of a charge elimination region by the charge elimination member is wider than that of a charged region by the resistance measurement member in a direction perpendicular to the sheet carriage direction, and

the absolute value of the voltage for charge elimination is smaller than that of the voltage for resistance measurement.

2. The image forming device according to claim 1 , wherein the resistance measurement member is made by a pair of resistance measurement rollers which sandwich the sheet,

the charge elimination member is made by a pair of charge elimination rollers which sandwich the sheet, and

a nip width of the sheet by the pair of charge elimination rollers is wider than that of the sheet by the pair of resistance measurement rollers in a direction perpendicular to the sheet carriage direction.

3. The image forming device according to claim 2 , wherein the charge elimination roller has a roller axis part and a roller nip part,

the roller nip part has a first roller part having first electric resistance and a second roller part having second electric resistance which is higher than the first electric resistance, and

the second roller part is disposed on a roller end side more than the first roller part in the center axis direction of the roller axis part.

4. The image forming device according to claim 3 , wherein the resistance measurement roller has a roller axis part and a roller nip part, and

both end positions of the roller nip part of the resistance measurement roller are disposed within a region of the second roller part in the charge elimination roller in a direction perpendicular to the sheet carriage direction.

5. The image forming device according to claim 4 , wherein both end positions of the roller nip part of the resistance measurement roller are disposed in the center position of the second roller part in the charge elimination roller.

6. The image forming device according to claim 1 , wherein the absolute value of the voltage for charge elimination is the half of the absolute value of the voltage for resistance measurement.

7. The image forming device according to claim 1 , wherein a threshold voltage at which poor transfer occurs in the transfer unit is larger than the difference between the absolute value of the voltage for resistance measurement and the voltage for charge elimination.

8. The image forming device according to claim 1 , further comprising a voltage control unit controlling the first and second voltage applying units,

wherein the voltage control unit determines a timing of applying the voltage for charge elimination by the second voltage applying unit on the basis of a timing of applying the voltage for resistance measurement by the first voltage applying unit and carriage speed of the sheet.

9. The image forming device according to claim 1 , wherein a distribution profile of the voltage for charge elimination in a direction perpendicular to the sheet carriage direction has a valley shape.