1. Field of the Invention
The present invention relates to a sheet inverting unit that inverses a sheet for image forming on a reverse surface and to a double-side image forming apparatus, which is equipped with a single-side image forming device, that employs the sheet inverting unit.
2. Description of the Related Art
Currently available, to satisfy a demand for plain paper image recording, are image forming apparatuses, such as copying machines, printers, and facsimiles, that employ a latent image forming recording apparatus similar to an electrophotographic apparatus. According to this image forming principle, a photosensitive drum, which is used as an image carrier, is charged, and the photosensitive drum is then exposed to a light image to form an electrostatic latent image on it. When the electrostatic latent image on the photosensitive drum is developed by a developing unit, a toner image is formed on the photosensitive drum. Then, the toner image is transferred from the photosensitive drum to a sheet of paper.
Such an image forming apparatus, however, normally forms an image on only one surface of a sheet, but there has recently arisen a demand for image forming apparatuses that can perform double-side image forming. If, however, paired image forming mechanisms were provided to print both sides of a sheet, the size of an image forming apparatus would increase and its per unit manufacturing cost would rise. A sheet inverting unit is therefore incorporated in an image forming apparatus to enable double-side image forming. After one surface of a sheet has been printed, this sheet inverting unit inverts the sheet, returns it to an image forming mechanism to print an image on the reverse of the sheet, and discharges it.
The above described image forming apparatus should be made compact. Particularly desirable is a small image forming apparatus that can perform double-side image forming while keeping its size.
FIGS. 1A and 1B are explanatory diagrams for conventional techniques. Of the conventional double-side image forming apparatuses, one type, described in, for example, U.S. Pat. No. 4,814,743, incorporates a sheet inverting unit, and another, described in, for example Japanese Unexamined Patent Publication No. 138262/1986 and No. 42774/1986, has an attached a sheet inverting unit.
A general arrangement for a double-side image forming apparatus that incorporates a sheet inverting unit will now be described while referring to FIG. 1A. In single-side printing, an image forming apparatus 1 feeds a sheet from a hopper 153, which protrudes from the image forming apparatus 1, along a feeding path to an image forming process mechanism 151, and prints the obverse side of the sheet. The sheet is then discharged to a tray 152.
In double-side printing, however, the image forming apparatus 1 employs a switching lever LV to guide a single side printed sheet to a feedback path FP where the sheet is inverted. The inverted sheet is fed to and switched by an externally provided inverting guide 154, and is refed to the image forming process mechanism 151. After the reverse of the sheet has been printed, the sheet is discharged to the tray 152.
A general arrangement for an image forming apparatus that has an externally attached sheet inverting unit will now be explained while referring to FIG. 1B. In single-side printing, a sheet inverting unit 155 is provided beneath an image forming apparatus 150. A sheet from the hopper 153, which protrudes from the image forming apparatus 150, is guided along a feeding path to the image forming process mechanism 151 and the obverse of the sheet is printed. The printed sheet is then discharged to the tray 152.
In double-side printing, however, the single-side printed sheet is guided by a switching lever LV from the discharge port of the image forming apparatus 150 to a sheet inverting unit 155. After the sheet is switched back by a sheet inverting mechanism 156 of the sheet inverting unit 155, it is guided to a feedback path FP and inverted. The inverted sheet is carried to the insertion port of the image forming apparatus 150, and from there is refed to the image forming process mechanism 151. Printing is performed on the reverse of the sheet and the double-side printed sheet is discharged to the tray 152.
According to the well known techniques, as the sheet feedback path extends beyond either side of the image forming apparatus 150, the size of a double-side image forming apparatus is increased. Also, since mechanisms for double-side image forming, such as a switching lever, must be installed within the image forming apparatus 150, the cost of the image forming apparatus 150 will rise and its size will increase. Further, because of the long feeding path provided for the image forming apparatus shown in FIG. 1A, more time is required for double-side image forming. And as the image forming apparatus shown in FIG. 1B cannot be opened to provide access to the feedback path FP in the sheet inverting unit 155, it is difficult to remove a jammed sheet from the sheet inverting unit 155.