INCORPORATION BY REFERENCE

This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Application No. 2021-008928 filed on Jan. 22, 2021, the entire contents of which are hereby incorporated by reference.

BACKGROUND

The present disclosure relates to a transfer unit for transferring to a recording medium a toner image formed on an image carrying member such as a photosensitive drum or an intermediate transfer belt. The present disclosure also relates to an image forming apparatus incorporating such a transfer unit.

Conventionally, there is a known intermediate transfer-type image forming apparatus including an endless intermediate transfer belt that rotates in a prescribed direction and a plurality of image forming portions provided along the intermediate transfer belt. In the image forming apparatus, by the image forming portions, toner images of respective colors are primarily transferred to the intermediate transfer belt by being sequentially superimposed on each other, after which the toner images are secondarily transferred by a secondary transfer roller to a recording medium such as paper.

In such intermediate transfer-type image forming apparatuses, adhesion of toner to the surface of the secondary transfer roller accumulates due to durable printing. In particular, to improve the color development and the color reproducibly, it is necessary to execute calibration for correcting the image density and the color displacement with predetermined timing, and the patch image formed on the intermediate transfer belt during execution of calibration is, instead of being transferred to the recording medium, removed by a belt cleaning device. This causes, as the patch image passes through the secondary transfer roller, part of the toner transferred to the intermediate transfer belt to adhere to the secondary transfer roller.

Conventionally, the secondary transfer roller is cleaned by applying a reverse transfer voltage (a voltage with the same polarity as the toner) to the secondary transfer roller during a non-image forming period to move the toner deposited on the secondary transfer roller back to the intermediate transfer belt. However, this method is disadvantageous in that cleaning of the secondary transfer roller takes time, resulting in longer printing wait time.

To cope with that, there have been proposed methods for improving productivity by permitting switching of the secondary transfer roller to the one of the size appropriate to the recording medium, and, for example, there is a known image forming apparatus that includes a plurality of secondary transfer rollers having different lengths in the axial direction, a rotary member having a supporting portion that rotatably supports the plurality of secondary transfer rollers and that is pivotable about an axis parallel to the axial direction, and a control portion that selects one roller out of the plurality of secondary transfer rollers in accordance with the width of the recording medium and rotates the supporting portion to arrange the roller opposite the intermediate transfer belt.

SUMMARY

According to one aspect of the present disclosure, a transfer unit includes a transfer roller having a metal shaft and an elastic layer laid around the circumferential face of the metal shaft to form a transfer nip by keeping the elastic layer in pressed contact with an image carrying member, and transfers a toner image formed on the image carrying member to a recording medium as it passes through the transfer nip. The transfer unit includes, as transfer rollers, a first roller and a second roller, a first bearing member, a second bearing member, a roller holder, a first urging member, a second urging member, a switching cam, a driving mechanism, and a first pre-transfer guide. The second roller has an elastic layer longer in the axial direction than that of the first roller. The first bearing member rotatably supports the first roller. The second bearing member rotatably supports the second roller. The roller holder has a first bearing holding portion and a second bearing holding portion that respectively hold the first and second bearing members slidably in directions toward and away from the image carrying member. The first urging member is arranged between the first bearing holding portion and the first bearing member and urges the first bearing member in the direction toward the image carrying member. The second urging member is arranged between the second bearing holding portion and the second bearing member and urges the second bearing member in the direction toward the image carrying member. The switching cam has a guide hole with which a first engaging portion formed on the first bearing member and a second engaging portion formed on the second bearing member engage. The driving mechanism drives the roller holder and the switching cam to rotate. The first pre-transfer guide is swingably supported on a unit frame in an upstream-side part of the transfer nip in the conveying direction of the recording medium. By rotating the roller holder, one of the first and second rollers is arranged opposite the image carrying member and, by rotating the switching cam to change the positions at which the first and second engaging portions engage with the guide hole respectively, the first or second roller arranged opposite the image carrying member is arranged either at a reference position at which, by being kept in pressed contact with the image carrying member, the first or second roller forms a transfer nip or at a released position at which the first or second roller lies away from the image carrying member. When the first or second roller is arranged at the reference position, as the switching cam rotates, the first pre-transfer guide is arranged at a first guide position where the first pre-transfer guide can guide the recording medium to the transfer nip.

