FIELD OF THE INVENTION AND RELATED ART

The present invention relates to an image forming apparatus for forming a toner image on a recording material.

Conventionally, as the image forming apparatus using the electrophotographic type, there is an image forming apparatus using an endless belt as an image bearing member for bearing a toner image. As such a belt, for example, there is an intermediary transfer belt used as a second image bearing member for feeding a sheet-like recording material such as paper from a photosensitive member or the like as a first image bearing member.

In the image forming apparatus using the intermediary transfer belt, a toner image formed on the photosensitive member or the like is primary-transferred onto the intermediary transfer belt at a primary transfer portion. Then, the toner image primary-transferred on the intermediary transfer belt is secondary-transferred onto the recording material at a secondary transfer portion. By an inner member (inner secondary transfer member) provided on an inner peripheral surface side and an outer member (outer secondary transfer member) provided on an outer peripheral surface side, a secondary transfer portion (secondary transfer nip) which is a contact portion between the intermediary transfer belt and the outer member is formed. As the inner member, an inner roller (inner secondary transfer roller) which is one of a plurality of stretching rollers for stretching the intermediary transfer belt is used. As the outer member, an outer roller (outer secondary transfer roller) which is provided in a position opposing the inner roller while nipping the intermediary transfer belt between itself and the inner roller and which is pressed toward the inner roller is used in many instances. Further, a secondary transfer voltage of a polarity opposite to a charge polarity of toner is applied to the outer roller (or a voltage of the same polarity as the charge polarity of the toner is applied to the inner roller), so that the toner image is secondary-transferred from the intermediary transfer belt onto the recording material in the secondary transfer portion. In general, with respect to a feeding direction of the recording material, on a side upstream of the secondary transfer portion, a feeding guide for guiding the recording material to the secondary transfer portion is provided. Incidentally, as regards the recording material, a “leading end” and a “trailing end” refer to those with respect to a recording material feeding direction.

Here, depending on rigidity of the recording material, behavior of the recording material changes in the neighborhoods of the secondary transfer portion on sides upstream and downstream of the secondary transfer portion with respect to the recording material feeding direction, and has an influence on an image which is a product in some instances.

For example, in the case where the recording material is “thin paper” which is an example of the recording material with small rigidity, in the neighborhood of the secondary transfer nip on the side downstream of the secondary transfer portion with respect to the recording material feeding direction, the intermediary transfer belt and the recording material stick to each other, so that a jam (paper jam) occurs in some instances due to improper separation of the recording material from the intermediary transfer belt.

On the other hand, in the case where the recording material is “thick paper” which is an example of the recording material with large rigidity, when a trailing end portion (trailing end or region close to the trailing end) of the recording material passes through the feeding guide, a tailing end portion of the recording material with respect to the recording material feeding direction collides with the intermediary transfer belt in some instances. By this, with respect to the recording material feeding direction, an attitude of the intermediary transfer belt in the neighborhood of the secondary transfer portion on the upstream side is disturbed, so that an image defect (a stripe-shaped image disturbance or the like extending in a direction substantially perpendicular to the recording material feeding direction) occurs at the trailing end portion of the recording material in some instances. In recent years, in a commercial printing market required to meet diversifying recording materials, these problems become apparent in many instances.

Therefore, a constitution in which a shape (position) of the secondary transfer portion is changed depending on a kind of the recording material has been proposed (Japanese Laid-Open Patent Application (JP-A) 2014-134718).

In order to realize improvement in separating property of the recording material from the intermediary transfer belt and suppression of the image defect at the trailing end portion of the recording material, as disclosed in JP-A 2014-134718, it is effective that the shape (position) of the secondary transfer portion is changed depending on the kind of the recording material. This change in shape (position) of the secondary transfer portion can be made by changing a relative position (represented by an “offset amount” described later) between the inner roller and the outer roller with respect to a circumferential direction of the inner roller through movement of the inner roller or the outer roller.

However, when the offset amount is changed depending on a kind of the recording material, at the same time, a position of the stretching roller for the intermediary transfer belt, typically, a position of a tension roller imparting tension to the intermediary transfer belt changes in a region from the primary transfer portion to the secondary transfer portion. When the position of the tension roller changes, a length from the primary transfer portion to the secondary transfer portion with respect to a rotational direction of the intermediary transfer belt also changes. As a result, a phenomenon such that timing when the image on the intermediary transfer belt is fed to the secondary transfer portion deviates and thus a leading end position of the image formed on the recording material deviates from a desired (original) position (herein, this phenomenon is also referred to as “leading end misregistration”) occurs in some instances. When the leading end misregistration occurs, for example, there is a possibility that a leading end or a trailing end of the image protrudes from the recording material and that a printing position is out of an entry frame or overlaps with a frame line in the case where the image is printed in the entry frame pre-printed on a sheet or in the like case.

In the above, conventional problems were described taking, as an example, the secondary transfer portion which is a transfer portion of the toner image from the intermediary transfer belt onto the recording material, but there are similar problems also as to another transfer portion of the toner image from another belt-shaped image bearing member such as a photosensitive belt onto the recording material.

SUMMARY OF THE INVENTION

A principal object of the present invention is to provide an image forming apparatus capable of suppressing an occurrence of leading end misregistration due to a change in offset amount.

The object has been accomplished by the image forming apparatus according to the present invention.

According to an aspect of the present invention, there is provided an image forming apparatus comprising: an image bearing member configured to bear a toner image; an image forming portion configured to form the toner image on the image bearing member; an endless belt onto which the toner image formed on the image bearing member is transferred at a primary transfer portion; a plurality of stretching rollers including an inner roller and configured to stretch the endless belt; an outer roller configured to form a secondary transfer portion, in cooperation with the inner roller, where the toner image is transferred from the endless belt onto a recording material; a position changing mechanism configured to change a position of the secondary transfer portion with respect to a circumferential direction of the inner roller by moving the inner roller, wherein the position changing mechanism is capable of changing a position of the inner roller to a plurality of positions including a first position and a second position positioned downstream of the first position with respect to a rotational direction of the endless belt; a feeding member configured to feed the recording material to the secondary transfer portion; a feeding member driving portion configured to drive the feeding member; and a controller configured to control a feeding start timing of the recording material by the feeding member, wherein the feeding start timing of the recording material by the feeding member depends on the position of the inner roller during transfer of the toner image onto the recording material.

Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

