Image forming apparatus

1 . An image forming apparatus comprising:

a circulating belt configured to circulate while carrying a toner image on a peripheral surface;

a transfer member configured to nip, at a nip part, a recording medium that is transported and the circulating belt, and configured to transfer the toner image on the circulating belt to the recording medium with a voltage being applied to the transfer member; and

a controller configured to control a transfer scheme in accordance with which a voltage is to be applied to the transfer member,

wherein the controller is configured to switch the transfer scheme between constant-voltage control and constant-current control, and

wherein the controller is configured to cause an image forming section configured to form an image on the circulating belt to start an image forming operation under constant-voltage control, and not constant-current control, selected as the transfer scheme in response to the controller determining that a length ratio, which is defined as a ratio of a length of the recording medium in an axial direction to a length of the nip part in the axial direction, is smaller than a predetermined ratio.

2 . The image forming apparatus according to claim 1 , wherein the controller is configured to, in response to the controller determining that the length ratio is greater than or equal to the predetermined ratio:

cause the image forming section to start an image forming operation under constant-current control as the transfer scheme in response to the controller determining that the recording medium has a basis weight that is smaller than a predetermined basis weight; and

cause the image forming section to start the image forming operation under constant-voltage control as the transfer scheme in response to the controller determining that the recording medium has a basis weight that is greater than or equal to the predetermined basis weight.

3 . The image forming apparatus according to claim 2 , wherein the controller is configured to, in response to the controller determining that the length ratio is greater than or equal to the predetermined ratio and the basis weight is within a predetermined range:

select as the transfer scheme constant-current control in response to the controller determining that an in-apparatus humidity is lower than or equal to a predetermined humidity; and

select as the transfer scheme constant-voltage control in response to the controller determining that the in-apparatus humidity is higher than the predetermined humidity.

4 . The image forming apparatus according to claim 1 , wherein the controller is configured to:

select constant-voltage control in response to the controller determining that a voltage division ratio γ is greater than or equal to a predetermined voltage division ratio; and

select constant-current control in response to the controller determining that the voltage division ratio γ is smaller than the predetermined voltage division ratio, the voltage division ratio γ being calculated by Expression (1):

γ=( Vp−Vm )/ Vm   (1)

where Vm denotes a first transfer voltage generated before an image forming operation is started and when a predetermined amperage of current is supplied to the transfer member with no recording medium being nipped at the nip part, and Vp denotes a second transfer voltage generated in a case where a predetermined environmental change occurs after the image forming operation is started and when the predetermined amperage of current is supplied to the transfer member with a recording medium being nipped at the nip part.

5 . The image forming apparatus according to claim 4 , wherein the predetermined environmental change is a change in an in-apparatus humidity,

wherein the controller is configured to acquire the voltage division ratio γ in response to the controller determining that an amount of change in the in-apparatus humidity reaches a predetermined value, and

wherein the controller is configured to select constant-voltage control or constant-current control in accordance with the acquired voltage division ratio γ.

6 . An image forming apparatus comprising:

a circulating member for circulating while carrying a toner image on a peripheral surface;

transfer means for nipping, at a nip part, a recording medium that is transported and the circulating member, and for transferring the toner image on the circulating member to the recording medium with a voltage being applied to the transfer means; and

a controller means for controlling a transfer scheme in accordance with which the voltage is to be applied to the transfer means,

wherein the controller means is for switching the transfer scheme between constant-voltage control and constant-current control,

wherein the controller means is for causing an image forming section configured to form an image on the circulating member to start an image forming operation under constant-voltage control, and not constant-current control, selected as the transfer scheme in response to the controller means determining that a length ratio, which is defined as a ratio of a length of the recording medium in an axial direction to a length of the nip part in the axial direction, is smaller than a predetermined ratio.

7 . An image forming apparatus comprising:

a circulating belt configured to circulate while carrying a toner image on a peripheral surface;

a transfer member configured to nip, at a nip part, a recording medium that is transported and the circulating belt, and configured to transfer the toner image on the circulating belt to the recording medium with a voltage being applied to the transfer member; and

a controller configured to control a transfer scheme in accordance with which a voltage is to be applied to the transfer member,

wherein the controller is configured to switch the transfer scheme between constant-voltage control and constant-current control,

wherein the controller is configured to cause an image forming section configured to form an image on the circulating belt to start an image forming operation under constant-voltage control selected as the transfer scheme in a case where a length ratio, which is defined as a ratio of a length of the recording medium in an axial direction to a length of the nip part in the axial direction, is smaller than a predetermined ratio,

wherein the controller is configured to:

select constant-voltage control in a case where a voltage division ratio γ is greater than or equal to a predetermined voltage division ratio; and

select constant-current control in a case where the voltage division ratio γ is smaller than the predetermined voltage division ratio, the voltage division ratio γ being calculated by Expression (1):

γ=( Vp−Vm )/ Vm   (1)

where Vm denotes a first transfer voltage generated before an image forming operation is started and when a predetermined amperage of current is supplied to the transfer member with no recording medium being nipped at the nip part, and Vp denotes a second transfer voltage generated in a case where a predetermined environmental change occurs after the image forming operation is started and when the predetermined amperage of current is supplied to the transfer member with a recording medium being nipped at the nip part,

wherein the predetermined environmental change is a change in an in-apparatus humidity,

wherein the controller is configured to acquire the voltage division ratio γ in a case where an amount of change in the in-apparatus humidity reaches a predetermined value, and

wherein the controller is configured to select constant-voltage control or constant-current control in accordance with the acquired voltage division ratio γ.