1. Field of the Invention
This invention relates to an image forming apparatus in which, after forming an unfixed image on a recording material carried by a recording-material carrying member (such as a transfer drum or the like), the image on the recording material, separated from the recording-material carrying member, is fixed.
2. Description of the Related Art
Recently, in accordance with an increasing demand for color image forming apparatuses, a method has been adopted in which, after sequentially transferring multiple images onto a recording material carried by a transfer drum, the images are simultaneously (at one time) fixed.
In an approach proposed in U.S. Pat. No. 4,549,803, the fixing speed is changed in accordance with the detected type of the recording material so that fixability is invariable irrespective of the properties of the recording material.
For example, when forming an image on an OHP (overhead projector) sheet comprising a resin film, the fixing speed is reduced to about half that of ordinary paper so that the fixing time is doubled.
FIG. 8 is a cross-sectional view of a full-color copier which includes a transfer drum, which can change its fixing speed, and which therefore provides the background of the present invention.
A copier 1 includes a photosensitive drum (an image bearing member) 2 which rotates in the direction of arrow R1, and a charging unit 3, a developing device 6 comprising developing units 6M, 6C, 6Y and 6BK for magenta, cyan, yellow and black toners, respectively, and a cleaning unit 7, sequentially disposed in the direction of the rotation of the photosensitive drum 2.
After an exposure unit 5 sequentially forms electrostatic latent images corresponding to respective colors on the photosensitive drum 2 charged by the charging unit 3, the developing unit 6 sequentially forms toner images.
The toner images are sequentially transferred onto a printing medium P wound around a transfer drum (transfer member) 9. The printing medium P is accomodated in a paper-feeding cassette 10 provided at a lower portion of the image forming apparatus, and is supplied to the transfer drum 9 via a paper-feeding roller 11, registration rollers 12 and a pressing roller 13. The leading-end portion of the supplied printing medium P is grasped by a gripper (not shown) of the transfer drum 9, and the entirety of the printing medium P is wound around the surface of the transfer drum 9 by an electrostatic force. The toner images are sequentially transferred onto the printing medium P at a transfer position M1. That is, a single-color toner image is transferred onto the printing medium P at the transfer position M1 at every rotation of the photosensitive drum 2. Four full-color toner images are transferred onto the printing medium P by four rotations of the photosensitive drum 2. The leading-end portion of the printing medium P on which the images have been transferred is released from the gripper, and a separation charger (a separation unit) 15 separates the printing medium P from the photosensitive drum 2 at a separation position M2 facing the separation charger 15. The printing medium P separated from the transfer drum 9 is guided to a fixing unit 17 by a conveying unit 16 including a conveying belt 16a rotatable in the direction of arrow R3.
A pair of fixing rollers 17a rotating in the direction of arrow R5 are provided in the fixing unit 17. The toner images on the printing medium P are mixed and fixed on the printing medium P as a full-color image by a sufficient pressure of the pair of fixing rollers 17a and by heating the medium P. The printing medium P on which the toner images have been fixed is discharged onto a paper-discharge tray 20 by paper-discharging rollers 19.
The above-described fixing operation of the toner images is performed at a fixing speed which differs in accordance with the properties of the printing medium P. For example, if the printing medium P comprises a thick film, such as an OHP sheet, it is necessary to sufficiently press and heat the toner images. For that purpose, the fixing time is elongated by reducing the fixing speed. The fixing speed is reduced by reducing the rotating speed of the fixing rollers 17a.
When the fixing speed is reduced in such an apparatus, in order to prevent the occurrence of deviations between toner images or corrugation in the printing paper P caused by a speed difference between the transfer drum 9 and the pair of fixing rollers 17a, the distance Ls between the separation position M2 (where the printing medium P is separated from the transfer drum 9) and the fixing position (which is at a nip of the pair of fixing rollers 17a) is set to be longer than the maximum length of the used printing medium P.
This hinders a reduction of the size of the copier 1. Furthermore, since a conveying force is not provided to the printing medium P until the printing medium P reaches the fixing unit 17 after leaving the transfer drum 9, it is necessary to provide the conveying unit 16 between the transfer drum 9 and the pair of fixing rollers 17a.