Image forming apparatus

1 . An image forming apparatus comprising:

an image former that forms a toner image;

an image carrier that carries the toner image formed by the image former;

a transferer that transfers the toner image on a surface of the image carrier to a recording medium that passes through a transfer position of the image carrier by using a transfer electric field that is formed by applying a transfer bias;

a measurer that measures a voltage in the transferer; and

at least one processor configured to:

calculate a voltage division ratio γ=Vp/Vm that is a ratio of a recording-medium divided voltage Vp when the recording medium passes in the transferer to a member divided voltage Vm that is applied to a member before the recording medium passes in the transferer; and

cause the image former to change an amount of toner in a band-shaped toner image to be formed on a region of the image carrier that does not face the recording medium or a supply frequency of the band-shaped toner image in accordance with the voltage division ratio γ or the voltage division ratio γ and a length of the recording medium in a transport direction of the recording medium.

2 . The image forming apparatus according to claim 1 ,

wherein the processor is configured to:

increase the amount of toner in the band-shaped toner image or the supply frequency of the band-shaped toner image when the voltage division ratio γ is greater than or equal to a predetermined threshold, than when the voltage division ratio γ is less than the threshold.

3 . The image forming apparatus according to claim 2 ,

wherein the processor is configured to:

cause, when the voltage division ratio γ is less than the threshold, the image former to form the band-shaped toner image on a region of the image carrier between regions of the image carrier each of which faces the recording medium that is passing; and

cause, when the voltage division ratio γ is greater than or equal to the threshold, the image former to form the band-shaped toner image on a region of the image carrier outside in a width direction of the regions of the image carrier each of which faces the recording medium that is passing, in addition to the band-shaped toner image on the region of the image carrier between the regions of the image carrier each of which faces the recording medium that is passing.

4 . The image forming apparatus according to claim 3 , comprising:

a plurality of the image formers that form different-color toner images to form a full-color image; and

a special-color image former that forms a special-color toner image by using a special-color toner,

wherein, when the image former forms the band-shaped toner image on a region of the image carrier that does not face the recording medium,

the special-color image former does not form a band-shaped special-color toner image on the region of the image carrier that does not face the recording medium.

5 . The image forming apparatus according to claim 2 , comprising:

a plurality of the image formers that form different-color toner images to form a full-color image; and

a special-color image former that forms a special-color toner image by using a special-color toner,

wherein, when the image former forms the band-shaped toner image on a region of the image carrier that does not face the recording medium,

the special-color image former does not form a band-shaped special-color toner image on the region of the image carrier that does not face the recording medium.

6 . The image forming apparatus according to claim 1 ,

wherein the processor is configured to:

increase the amount of toner in the band-shaped toner image or the supply frequency of the band-shaped toner image when the length of the recording medium in the transport direction is greater than or equal to a predetermined length, than when the length of the recording medium is less than the predetermined length.

7 . The image forming apparatus according to claim 6 , comprising:

a plurality of the image formers that form different-color toner images to form a full-color image; and

a special-color image former that forms a special-color toner image by using a special-color toner,

wherein, when the image former forms the band-shaped toner image on a region of the image carrier that does not face the recording medium,

the special-color image former does not form a band-shaped special-color toner image on the region of the image carrier that does not face the recording medium.

8 . The image forming apparatus according to claim 1 , comprising:

a power supply that performs an operation of applying the transfer bias with a constant current,

wherein the processor is configured to:

set the member divided voltage Vm to a voltage measured by the measurer when a preset transfer current value is applied after an image forming operation has been started and before the recording medium passes through the transfer position;

set an output voltage Vout to a voltage measured by the measurer while the recording medium is passing through the transfer position when the transfer current value is applied; and

set the recording-medium divided voltage Vp to Vout−Vm, which is a difference between the output voltage Vout and the member divided voltage Vm.

9 . The image forming apparatus according to claim 8 , comprising:

a plurality of the image formers that form different-color toner images to form a full-color image; and

a special-color image former that forms a special-color toner image by using a special-color toner,

wherein, when the image former forms the band-shaped toner image on a region of the image carrier that does not face the recording medium,

the special-color image former does not form a band-shaped special-color toner image on the region of the image carrier that does not face the recording medium.

10 . The image forming apparatus according to claim 1 , comprising:

a power supply that performs an operation of applying the transfer bias with a constant voltage,

wherein the processor is configured to:

set, based on a preset value of a reference current value that is set in accordance with a movement speed of the image carrier, the member divided voltage Vm to a voltage measured by the measurer when the reference current value is applied after an image forming operation has been started and before the recording medium passes through the transfer position;

calculate, from the member divided voltage Vm, an output voltage Vout in accordance with information about the recording medium and temperature and humidity; and

set the recording-medium divided voltage Vp to Vout−Vm, which is a difference between the output voltage Vout and the member divided voltage Vm.

11 . The image forming apparatus according to claim 10 , comprising:

a plurality of the image formers that form different-color toner images to form a full-color image; and

a special-color image former that forms a special-color toner image by using a special-color toner,

wherein, when the image former forms the band-shaped toner image on a region of the image carrier that does not face the recording medium,

the special-color image former does not form a band-shaped special-color toner image on the region of the image carrier that does not face the recording medium.

12 . The image forming apparatus according to claim 1 , wherein the image former forms the band-shaped toner image on a region of the image carrier that is between regions of the image carrier each of which faces the recording medium and that is outside in a width direction of the regions.

13 . The image forming apparatus according to claim 12 , comprising:

a plurality of the image formers that form different-color toner images to form a full-color image; and

a special-color image former that forms a special-color toner image by using a special-color toner,

wherein, when the image former forms the band-shaped toner image on a region of the image carrier that does not face the recording medium,

the special-color image former does not form a band-shaped special-color toner image on the region of the image carrier that does not face the recording medium.

14 . The image forming apparatus according to claim 1 , wherein the image former forms the band-shaped toner image on a region that is included in a transferable region of the image carrier and that is outside in a width direction of a region of the image carrier that faces the recording medium that is passing.

15 . The image forming apparatus according to claim 14 ,

wherein the processor is configured to:

perform adjustment of the amount of toner in the band-shaped toner image by one or more of toner density, a unit area of the band-shaped toner image, and a number of the band-shaped toner images.

16 . The image forming apparatus according to claim 15 , comprising:

a plurality of the image formers that form different-color toner images to form a full-color image; and

a special-color image former that forms a special-color toner image by using a special-color toner,

wherein, when the image former forms the band-shaped toner image on a region of the image carrier that does not face the recording medium,

the special-color image former does not form a band-shaped special-color toner image on the region of the image carrier that does not face the recording medium.

17 . The image forming apparatus according to claim 14 , comprising:

a plurality of the image formers that form different-color toner images to form a full-color image; and

a special-color image former that forms a special-color toner image by using a special-color toner,

wherein, when the image former forms the band-shaped toner image on a region of the image carrier that does not face the recording medium,

the special-color image former does not form a band-shaped special-color toner image on the region of the image carrier that does not face the recording medium.

18 . The image forming apparatus according to claim 1 , comprising:

a plurality of the image formers that form different-color toner images to form a full-color image; and

a special-color image former that forms a special-color toner image by using a special-color toner,

wherein, when the image former forms the band-shaped toner image on a region of the image carrier that does not face the recording medium,

the special-color image former does not form a band-shaped special-color toner image on the region of the image carrier that does not face the recording medium.

19 . An image forming apparatus comprising:

image forming means for forming a toner image;

image carrying means for carrying the toner image formed by the image forming means;

transferring means for transferring the toner image on a surface of the image carrying means to a recording medium that passes through a transfer position of the image carrying means by using a transfer electric field that is formed by applying a transfer bias;

measuring means for measuring a voltage in the transferring means; and

at least one processing means for:

calculating a voltage division ratio γ=Vp/Vm that is a ratio of a recording-medium divided voltage Vp when the recording medium passes in the transferring means to a member divided voltage Vm that is applied to a member before the recording medium passes in the transferring means; and

causing the image forming means to change an amount of toner in a band-shaped toner image to be formed on a region of the image carrying means that does not face the recording medium or a supply frequency of the band-shaped toner image in accordance with the voltage division ratio γ or the voltage division ratio γ and a length of the recording medium in a transport direction of the recording medium.