1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer or a facsimile apparatus, and more particularly to an image forming apparatus utilizing an electrophotographic process.
2. Description of the Related Art
A schematic constitution of a general image forming apparatus utilizing the electrophotographic process will be described with reference to FIG. 7.
An image forming apparatus 700 is equipped with a sheet cassette 50. Sheets P contained in the sheet cassette 50 are advanced by a pickup roller 8a. On thus advanced sheet P, a desired image is formed in an image forming portion 101. The image forming portion 101 includes detachable process cartridges 7a to 7d. The process cartridges 7a to 7d include photosensitive drums 1a to 1d, charging rollers 2a to 2d for charging the photosensitive drums 1a to 1d, developing devices 4a to 4d for forming toner images on the photosensitive drums 1a to 1d. 
A transfer-conveyor belt 9 conveys the sheet P, advanced by the pickup roller 8a, to the image forming portion 101. Transfer rollers 5a to 5d are disposed at positions opposed to the photosensitive drums 1a to 1d. The transfer rollers 5a to 5d press the sheet P toward the photosensitive drums 1a to 1d across the transfer-conveyor belt 9 and are given a voltage of a polarity opposite to that of the toner image, whereby the toner images are transferred onto the sheet P.
At a downstream side of the image forming portion 101 in the sheet conveying direction, there is provided a fixing unit 40 for fixing the toner images, transferred onto the sheet P, to the sheet P. The sheet P having received the transfer of toner images in the image forming portion 101 is conveyed to the fixing unit 40, in which the toner images are fixed. The sheet P, on which the toner images are fixed in the fixing unit 40, is discharged onto a stacking tray 301, by a discharge roller 302 and a discharge idler 303 opposed to the discharge roller 302.
The discharged sheets P are piled up on the stacking tray 301. Usually the user recovers the discharged sheets P at each discharge or at a suitable timing. However, in the case that the sheets are not recovered, for example because the user forgets to execute the recovery, there has resulted a situation where the sheets P are stacked in excess of the capacity of the stacking tray 301. Thus, such over-stacking of the sheets P has lead to a sticking-out of the sheet P from the stacking tray 301, a dropping of the sheet P from the image forming apparatus 700, or a sheet jamming.
Therefore, in order to avoid an over-stacking of the discharged sheets on the stacking tray, there are proposed a conveying apparatus and an image forming apparatus, equipped with a sheet stack amount detection unit for detecting the stacking of sheets of a prescribed amount (for example cf. Japanese Patent Application Laid-Open No. H09-249334).
However, when the sheet P is let to stand on the stacking tray 301, sheet P may be curled (rounded up) by moisture absorption on the stacking tray 301. Under an environment of a high temperature or a high humidity, the sheet P left on the stacking tray tends to be curled more easily. Also in the case that the distance from the fixing unit 40 to the discharge portion is short, the sheet P is discharged without being cooled sufficiently, so that, particularly in a sheet of a low stiffness such as an OHT (Over Head Transparency) sheet, the sheet P may be curled up on the stacking tray 301. Also in the case that the sheet P is not sufficiently cooled before being discharged, the toner image may be discharged before being completely fixed to the sheet P. In such case, the unfixed toner image on the sheet P is stacked on and adheres to a sheet P already discharged onto the stacking tray 301, whereby the trailing end of the sheet P is not completely discharged from the discharge port but the sheet P remains in the discharge portion.
In case of such abnormal state where the sheet P is curled on the stacking tray 301 or remains in an incomplete discharge state, the detection by the sheet stacking amount detection unit is not executed property, thus leading to a sheet jamming.