1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, printer, or facsimile device, and more particularly to an image forming apparatus or the like which forms images on both sides of a recording medium using a so-called one-pass system.
2. Description of the Background Art
The so-called switch back system and one-pass system are known as conventional systems used to form an image on both sides of a recording medium such as paper. In the switch back system, an image is formed on one side of a sheet of paper by passing the paper through transfer means and fixing means, whereupon the paper is reversed and switched back through the transfer means and fixing means such that an image is formed on the other side. However, the switch back system suffers from a large number of problems that need to be solved, including high costs due to the need to provide complicated mechanisms, the length of time required for image formation by means of switch back, jams caused by switching back paper that has curled due to the heat applied by the fixing means, and so on. Thus a method of solving these various problems has been disclosed in Japanese Unexamined Patent Application Publication H1-209470. In this method, toner images are formed on both sides of a sheet of paper, whereupon fixing is performed only once. More specifically, a first image formed on a photosensitive body serving as an image carrier is transcribed onto a transfer belt by first transfer means, then a second image formed on the photosensitive body is transcribed onto one side of the paper by the first transfer means, and then the first image on the transfer belt is transcribed onto the other side of the paper by second transfer means, thus forming images on both sides of the paper.
In this method proposed in the prior art, however, images are formed on both sides of a sheet of paper using a belt-form intermediate transfer body. This results in an additional transfer operation, and moreover, the thickness of the paper greatly affects the image. Hence, a problem arises in that high image quality cannot be achieved. Another problem lies in that a charger is used as the second transfer means, resulting in the generation of ozone which stains the wires.
In the one-pass method, on the other hand, images are formed on both sides of a sheet of paper without switching the paper back by transcribing images onto both sides of the paper using double-sided transfer means, and then passing the paper through fixing means. Since the problems in the switch back method described above can be solved, this method can be considered superior thereto.
However, a problem in this type of one-pass method lies in that the images can easily be disturbed when the paper is conveyed from the double-sided transfer means to the fixing means after images have been transcribed on both sides. This image disturbance arises in the following manner. That is, when the paper is removed from the double-sided transfer means and passed over to the fixing means, one of the unfixed images on the two sides of the paper may scratch against a guiding member or the like disposed between the double-sided transfer means and fixing means, thus disturbing the image.
Hence, an image forming apparatus has been proposed in Japanese Unexamined Patent Application Publication H10-142869 in which a spur which is capable of being driven to rotate and comprises a plurality of protrusions on its periphery is provided between the double-sided transfer means and fixing means such that a sheet of paper is guided by the spur from the double-sided transfer means to the fixing means. In this image forming apparatus proposed in the prior art, the spur is rotated in accordance with the motion of the paper, with the protrusions impinging on the rear surface of the paper, and thus the paper can be guided from the double-sided transfer means to the fixing means without disturbing the image on the rear surface.
However, even when the protrusions on the spur are sharp, if they pierce the paper, the unfixed image will be considerably disturbed. Moreover, it is extremely difficult to clean this type of spur, having a complex form with a plurality of protrusions, every time a sheet of paper passes through, and hence it is highly likely that toner which has become adhered to the protrusions will be transferred onto a subsequent sheet of paper, thus disturbing the image on that sheet of paper.