Image forming apparatus, sheet finisher, and method for driving stacking tray

This application is based upon and claims the benefit of priority from U.S. provisional application 61/015,182, filed on Dec. 19, 2007, and U.S. provisional application 61/016,944, filed on Dec. 27, 2007, the entire contents of each of which are incorporated herein by reference.

The present invention relates to an image forming apparatus, a sheet finisher, and a method for driving a stacking tray, and, more particularly to an image forming apparatus, a sheet finisher, and a method for driving a stacking tray for stacking a sheet bundle to be subjected to stapling.

Recently, image forming apparatuses of an electrophotographic system such as a laser printer, a digital copying machine, and a laser facsimile include, for example, a sheet finisher disclosed in JP-2007-76893-A1 that staples a sheet bundle with a staple and sorts the sheet bundle.

As disclosed in, for example, JP-5-305786-A1, if a sheet bundle subjected to stapling is stacked on a stacking tray, in a sheet finisher, there is a technique for changing a lowering amount of the stacking tray more if a sheet bundle is discharged than if unstapled sheets are stacked on the stacking tray. The use of the technique can improve alignability during stacking on the stacking tray.

As disclosed in, for example, JP-11-310362-A1, there is a technique for providing, in a sheet finisher, an upper surface detection sensor for detecting an upper surface of a sheet bundle stacked on a stacking tray and lowering, if an upper surface of the stacking tray or the sheet bundle is detected, the stacking tray by a fixed amount.

However, if a large number of sheet bundles subjected to stapling are stacked on the stacking tray, a portion subjected to the stapling swells compared with a portion not subjected to the stapling. Therefore, an upper surface of the stacked sheet bundles cannot be normally detected. The problem is more conspicuous, for example, if sheet bundles subjected to the stapling in one place at one end of sheets are stacked than if sheet bundles subjected to the stapling in plural positions symmetrically are stacked.

If the sheet bundles subjected to the stapling in one place at one end of sheets are stacked as explained above, a portion subjected to the stapling swells. Therefore, even if an upper surface of the sheet bundles is detected near the center of the sheet bundles, if a large number of sheet bundles are stacked, a top surface of the stacked sheet bundles cannot be accurately detected.

Therefore, since an original upper surface is present in a position higher than a detectable upper surface and a swelling portion closes a discharge port of a processing tray, a jam occurs. If a large number of sheet bundles are stacked, the sheet bundles are unaligned because, for example, the sheet bundles bump against sheet bundles already stacked. The stacked sheet bundles fall from the stacking tray. This problem is more conspicuous if bundles of two sheets are stacked than if, for example, bundles of fifty sheets are stacked.

The present invention has been devised in view of the circumstances and an object of the present invention is to provide an image forming apparatus, a sheet finisher, and a method for driving a stacking tray that can stack a large umber of sheet bundles subjected to stapling on the stacking tray and stack the sheet bundles with high alignability.

In order to attain the object, an image forming apparatus according to an aspect of the present invention includes: a printer configured to print image data on a sheet; a stapler configured to staple a sheet bundle, the sheet bundle including the sheet; a stacking tray configured to receive the sheet bundle at a standby position; a driving mechanism configured to lift and lower the stacking tray; a detecting unit configured to detect an upper surface of the sheet bundle on the stacking tray; a storage unit configured to store number of sheet bundles on the stacking tray; and a controller configured to control the driving mechanism to increase a distance from the detecting unit to the stacking tray at the standby position according to an increase in the number stored in the storage unit.

In order to attain the object, a sheet finisher according to another aspect of the present invention includes: a stapler configured to staple a sheet bundle, the sheet bundle including a sheet; a stacking tray configured to receive the sheet bundle at a standby position; a driving mechanism configured to lift and lower the stacking tray; a detecting unit configured to detect an upper surface of the sheet bundle on the stacking tray; a storage unit configured to store number of sheet bundles on the stacking tray; and a controller configured to control the driving mechanism to increase a distance from the detecting unit to the stacking tray at the standby position according to an increase in the number stored in the storage unit.

A method for driving a stacking tray according to still another aspect of the present invention includes acts of: stapling a sheet bundle, the sheet bundle including a sheet; and controlling driving of a stacking tray to increase a distance from a detecting unit that detects an upper surface of the sheet bundle on the stacking tray to the stacking tray at a standby position according to an increase in number of sheet bundles on the stacking tray.

DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

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