Image forming apparatus

摘要

In a constitution provided with no primary-transfer roller, in order to ensure a contact length between each of photosensitive drums and an intermediary transfer belt, the photosensitive drums are disposed with projecting amounts of a plurality of levels with respect to a stretching surface of the intermediary transfer belt, and therefore an apparatus is liable to be increased in a height direction. In order to avoid this, four image bearing members are arranged, along the intermediary transfer belt, with the same projection amount with respect to the stretching surface of the intermediary transfer belt, and a depressing member for depressing a belt surface between two central image bearing members toward an outside is provided.

主权项

1. An image forming apparatus comprising:
a movable endless intermediary transfer belt; a plurality of supporting rollers, including a first supporting roller and a second supporting roller, configured to support said intermediary transfer belt; first to fourth photosensitive drums which are arranged along said intermediary transfer belt between the first supporting roller and the second supporting roller in the order of said first photosensitive drum, said second photosensitive drum, said third photosensitive drum and said fourth photosensitive drum from an upstream side toward a downstream side with respect to a direction in which said intermediary transfer belt moves, and which contact said intermediary transfer belt, wherein toner images transferred from said first, second, third and fourth photosensitive drums onto said intermediary transfer belt at first, second, third and fourth primary-transfer portions, respectively, are transferred from said intermediary transfer belt onto a recording material at a secondary-transfer portion, and wherein the first to fourth primary-transfer portions are disposed in a phantom region demarcated by an assumed position of said intermediary transfer belt stretched by the first supporting roller and the second supporting roller in a hypothetical case wherein said first to fourth photosensitive drums are not provided; a constant voltage element which is electrically connected between said intermediary transfer belt and a ground potential and which maintains a predetermined voltage by passing of a current therethrough; a voltage source configured to apply a voltage to a transfer member for transferring a toner image from said intermediary transfer belt onto the recording material to pass a current through said constant-voltage element thereby to form a secondary-transfer electric field at the secondary-transfer portion and a primary-transfer electric field at the first to fourth primary-transfer portions; and a depressing member, provided in contact with an inner peripheral surface of said intermediary transfer belt between the second primary-transfer portion and the third primary-transfer portion, configured to depress said intermediary transfer belt from the inner peripheral surface toward an outer peripheral surface of said intermediary transfer belt.