Sheet stacking apparatus and image forming apparatus

Abstract

A sheet stacking apparatus has a stack tray which can move so that a sheet stack surface keeps a constant position from a discharge outlet and which is a finisher capable of using the stack tray by switching it. When stacking the maximum number of sheets to be stacked on the stack tray and preventing stacking of sheets on the stack tray up to the maximum number of sheets to be stacked and the number of sheets stacked on the stack tray reaches a predetermined number of sheets to be stacked which does not prevent up to the maximum number of sheets from being stacked on the stack tray, stacking of sheets on the stack tray is temporarily prohibited to stack sheets by switching to the stack tray. Thereby, all sheet stack trays can stack up to the maximum number of sheets to be stacked, decrease of the number of stacked sheets is prevented in the whole sheet stacking apparatus and the number of temporal interruptions of a job is decreased.

1 . A sheet stacking apparatus capable of stacking sheets using a plurality of sheet stack trays by switching with each other, comprising: 
 a plurality of sheet stack trays each of which moves to keep an uppermost surface of sheets stacked on the sheet stack tray in a constant position from a sheet discharge outlet when sheets are discharged on each of said plurality of sheet stack trays,    wherein said plurality of sheet stack trays include:    a first sheet stack tray, when the maximum number of sheets which are allowed to be stacked on said first sheet stack tray have been stacked thereon, which is which prevented from stacking on another sheet stack tray at least until another maximum number of sheets which are allowed to be stacked on another sheet stack tray are stacked; and    a second sheet stack tray, when the maximum number of sheets which are allowed to be stacked on said second sheet stack tray have been stacked thereon, which is not prevented from stacking on another sheet stack tray at least until another maximum number of sheets which are allowed to be stacked on another sheet stack tray are stacked,    wherein in a case where the number of sheets stacked on the first sheet stack tray reaches a predetermined number of sheets to be stacked which does not prevent stacking of sheets onto said another sheet stack trays until the number of sheets stacked on said another sheet stack trays reach the maximum number of sheets stacked, stacking of sheets on the first sheet stack tray is temporarily prohibited to stack sheets by switching the present sheet stack tray to a sheet stack tray other than the first sheet stack tray reaching the predetermined number of sheets.    
   
   
       2 . A sheet stacking apparatus according to  claim 1 , 
 wherein when the switched sheet stack tray is the second sheet stack tray and the number of sheets of the second sheet stack tray reaches the maximum number of sheets to be stacked, stacking of sheets on the switched sheet stack tray is prohibited to stack sheets by switching the present stack tray to another sheet stack tray.    
   
   
       3 . A sheet stacking apparatus according to  claim 1 , wherein when the switched sheet stack tray is the first sheet stack tray and the number of sheets stacked on the switched sheet stack tray reaches the predetermined number of sheets to be stacked, stacking of sheets on the switched sheet stack tray is prohibited to stack sheets by switching the present stack tray to another sheet stack tray.  
   
   
       4 . A sheet stacking apparatus according to  claim 2 , wherein when it is necessary to switch the sheet stack tray while discharging sheets for forming a sheet bundle, the sheet stack tray is switched after stacking of all sheets for respectively forming the sheet bundle is completed.  
   
   
       5 . An image forming apparatus comprising: 
 an image forming part which forms an image on a sheet; and    a sheet stacking apparatus which stacks a sheet on which an image is formed by the sheet forming part, wherein said sheet stacking apparatus capable of using a plurality of sheet stack trays by switching with each other, wherein said sheet stacking apparatus comprises:    a plurality of sheet stack trays each of which moves to keep an uppermost surface of sheets stacked on the sheet stack tray in a constant position from a sheet discharge outlet when sheets are discharged on each of said plurality of sheet stack trays,    wherein said plurality of sheet stack trays includes:    a first sheet stack tray, when the maximum number of sheets which are allowed to be stacked on said first sheet stack tray have been stacked thereon, which is prevented from stacking on another sheet stack tray at least until another maximum number of sheets which are allowed to be stacked on another sheet stack tray are stacked; and    a second sheet stack tray, when the maximum number of sheets which are allowed to be stacked on said second sheet stack tray have been stacked thereon, which is not prevented from stacking on another sheet stack tray at least until another maximum number of sheets which are allowed to be stacked on another sheet stack tray are stacked,    wherein in a case where the number of sheets stacked on the first sheet stack tray reaches a predetermined number of sheets to be stacked which does not prevent stacking of sheets onto said another sheet stack trays until the number of sheets stacked on said another sheet stack trays reach the maximum number of sheets stacked, stacking of sheets on the first sheet stack tray is temporarily prohibited to stack sheets by switching the present sheet stack tray to a sheet stack tray other than the first sheet stack tray reaching the predetermined number of sheets.    
   
   
       6 . An image forming apparatus according to  claim 5 , 
 wherein when the switched sheet stack tray is the second sheet stack tray and the number of sheets stacked on the switched sheet stack tray reaches the maximum number of sheets to be stacked, stacking of sheets on the switched sheet stack tray is prohibited to stack sheets by switching the present stack tray to another sheet stack tray.    
   
   
       7 . An image forming apparatus according to  claim 5 , wherein when the switched sheet stack tray is the first sheet stack tray and the number of sheets to be stacked on the switched sheet stack tray reaches the predetermined number of sheets to be stacked, stacking of sheets on the switched first sheet stack tray is prohibited to stack sheets by switching the present stack tray to another sheet stack tray.  
   
   
       8 . An image forming apparatus according to  claim 6 , wherein when it is necessary to switch the sheet stack tray while discharging sheets for forming a sheet bundle, the sheet stack tray is switched after stacking of all sheets for forming the sheet bundle is completed.