Image forming apparatus for enabling easy separation of recording sheets from photosensitive member

• US 6,167,229 A
• Filed: 11/03/1999
• Issued: 12/26/2000
• Est. Priority Date: 07/15/1997
• Status: Expired due to Term

• Current Assignee: Kabushiki Kaisha Toshiba (Toshiba Corporation), Toshiba Corporation
• Original Assignee: Kabushiki Kaisha Toshiba (Toshiba Corporation)
• Inventors: Watanabe, Takeshi, Izumi, Takao
• Primary Examiner(s): Grimley, Arthur T.
• Assistant Examiner(s): NGO, HOANG X
• Application Number:
  US09/432,598
• Time in Patent Office: 419 Days
• Field of Search: 399/299, 399/303, 399/312, 399/313, 399/314, 399/297, 399/302, 430/126
• US Class Current: 399/313
• CPC Class Codes: G03G 15/167 at least one of the recording member or the transfer member being rotatable during the transfer

Abstract

A copying machine is provided with a transfer belt which is in contact with the surface of a photosensitive drum and which is made to run in an endless manner. A transfer roller and an auxiliary roller are arranged inside the region defined by the transfer belt. The transfer roller is in contact with the transfer belt, defining a transfer nip therebetween. The transfer nip is part of a conveyance nip defined between the drum surface and the transfer belt, and is located downstream of the region just under the photosensitive drum. The auxiliary roller is located downstream of the conveyance nip. The contact between the drum surface and the transfer belt defines a posterior-transfer conveyance nip located downstream of the transfer nip with respect to the running direction of the transfer belt, and also defines a prior-transfer conveyance nip located upstream of the transfer nip with respect to the same direction. The width of the posterior-transfer conveyance nip is less than that of the prior-transfer conveyance nip. With this structure, satisfactory transfer characteristics are maintained, and yet the separation characteristics of recording sheets with reference to the drum surface can be improved. Accordingly, images of good quality can be output.

Claims

1. An image forming apparatus comprising:

a cylindrical image bearer rotatable in a predetermined direction;

an image formation unit for forming a visible image on a circumferential surface of the image bearer;

a transfer belt which is stretched between a pair of rollers located at a level lower than that of the image bearer and being away from each other in a horizontal direction and which is arranged in contact with the circumferential surface of the image bearer to define a conveyance nip, said transfer belt being made to run in a direction corresponding to a rotating direction of the image bearer and conveying a transfer medium through the conveyance nip;

a transfer roller arranged in the conveyance nip and kept in contact with a reverse side of the transfer belt to define a transfer nip, said transfer roller serving to transfer the visual image from the image bearer to the transfer medium which is conveyed through the conveyance nip; and

an auxiliary unit for bringing that portion of the transfer belt which has passed through the transfer nip into contact with the circumferential surface of the image bearer, such that a a portion of the transfer belt which contacts the image bearer and has passed through the transfer nip is smaller in length than a portion of the transfer belt which is about to enter the transfer nip, and such that the portion of the transfer belt that contacts the image bearer and has passed through the transfer nip has a length of 1 mm or less.

2. An image forming apparatus according to claim 1, wherein said auxiliary unit guides that portion of the transfer belt which has passed through the transfer nip such that the portion of the transfer belt that contacts the image bearer and has passed through the transfer nip is zero in width.

3. An image forming apparatus according to claim 2, wherein said transfer roller is pressed against the reverse side of the transfer belt such that the transfer nip has a width of 2 mm or less.

4. An image forming apparatus according to claim 1, wherein said transfer roller is pressed against the reverse side of the transfer belt such that the transfer nip has a width of 2 mm or less.

5. An image forming method comprising the steps of:

forming a visible image on a circumferential surface of a cylindrical image bearer when the image bearer is being rotated in a predetermined direction;

driving a transfer belt to run in a direction corresponding to a rotating direction of the image bearer, said transfer belt being arranged in contact with the circumferential surface of the image bearer to define a conveyance nip;

conveying a transfer medium through the conveyance nip defined between the image bearer and the transfer belt; and

transferring the visible image from the circumferential surface to the transfer medium conveyed through the conveyance nip by applying a bias to a transfer roller, said transfer roller being arranged in the conveyance nip and being kept in contact with a reverse side of the transfer belt, with a transfer nip defined,

wherein that portion of the transfer belt which has passed through the transfer nip is brought into contact with the circumferential surface of the image bearer, and the transfer medium is guided in a tangential direction of the image bearer when that transfer medium has passed through the transfer nip, whereby a portion of the transfer belt that contacts the circumferential surface of the image bearer and which has passed through the transfer nip is smaller in length than a portion of the transfer belt that contacts the circumferential surface of the image bearer and that is about to enter the transfer nip.