1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a facsimile machine, a printer and a multifunction machine, and, moreover, to a sheet stacking device stacking a sheet (recording medium) formed with an image, and a sheet processing device performing post processing of a sheet.
2. Description of the Related Art
A sheet processing device with the following configuration has been well known as a sheet processing device into which a sheet with an image formed in an image forming apparatus is conveyed. The sheet processing device has a buffer roller through which, when the sheet processing device receives sheets, which have been formed with an image, and have been discharged from an image forming apparatus main body, the received sheets are superimposed for temporary waiting before the sheets are conveyed to a post processing mechanism such as a stapling machine and a saddle stitching machine. That is, while a preceding sheet bundle is processed in a processing tray, first several sheets of the succeeding sheet bundle are on the buffer roller for temporary waiting. When the preceding sheet bundle, which has been processed, is discharged from the processing tray, the succeeding several sheets, which have been delivered from the buffer roller, are conveyed to the processing tray. A brief explanation of a sheet post-processing device with the above-described configuration will be given, referring to FIG. 9A through FIG. 9C.
A plurality of sheets P1, P2, . . . are superimposed one on top of another and wrapped around a buffer roller 5 to form a wrapping path 32. For example, three sheets P1, P2, and P3 are superimposed one on top of another, delivered from the path 32 after temporary waiting, conveyed, and conveyed to a processing tray 101 through a discharge roller 7, bundle discharge rollers 180a, and 180b. When the rear ends of the sheets pass the discharge roller 7, the bundle discharge rollers 180a and 180b rotate in the reverse direction in such a way that the sheet bundle of three sheets P1, P2, and P3 is returned in the direction in which the sheets abut against a rear-end stopper 3 of the processing tray 101. Alignment is performed in such a way that the bundle discharge roller 180b is separated from the bundle discharge roller 180a just before the rear end of the sheet bundle abuts against the rear-end stopper 3 and the sheet bundle abuts against the rear-end stopper 3 by moving inertia. At this time, alignment in a direction perpendicular to the conveyance direction is performed, using aligning plates.
When all the sheets of the first sheet bundle are aligned on the processing tray 101 in such a manner, a swinging guide 150 is lowered and the bundle discharge roller 180b sits atop the sheet bundle to perform stitching processing of the sheet bundle, and the like, using a processing machine such as a stapling machine indicated by a reference number 4 in FIG. 9A.
According to the above-described procedures, a first plurality of sheets of the succeeding second sheet bundle are wrapped around the buffer roller 5 as a temporary accumulating unit for waiting until processing for the first sheet bundle is completed. Thereby, a high-speed image forming apparatus by which sheets are discharged from the main body of an image forming apparatus at a small interval may be realized. Incidentally, the varieties of the quality and the size of sheets have been further increased in recent years. But the sheet processing device shown in FIG. 9A through FIG. 9C may hardly treat sheets, for example, special paper such as coated paper, the surface of which is treated, thick one, and large-sized one.
Even if these kinds of sheets may be surely aligned one by one, it is difficult to align a plurality of the sheets in a state in which the sheets are superimposed. That is, the rear ends of a plurality of the sheets with a special quality, or with a special sheet size is run into the rear-end stopper 3 on the processing tray 1. In this case, there is generated a state in which all the three sheets P1, P2, and P3 with a large coefficient of friction, such as that of coated paper, are not completely returned to the rear end of the stopper 3. Especially, it is serious that the sheet P2 such as the second sheet of the superimposed ones is incompletely or faultily returned, that is, the quality of the post processing is deteriorated, and, consequently, the productivity is reduced. Moreover, when a plurality of sheets such as a thick one, and a large-sized one are superimposed and moved, there is caused larger inertia than the one caused in a case in which one sheet is moved. Accordingly, there is a case in which non-aligning is caused, because the sheets are vigorously run into the rear-end stopper 3 and bound. Moreover, there is a possibility that the end portion of the sheet buckles, and is damaged.