Sheet postprocessing apparatus for use with image forming apparatus and folding method

A sheet postprocessing apparatus which executes postprocessing including a punching process, a folding process, and a binding process midway along a sheet convey path for a sheet on which an image is transferred/formed by an image forming apparatus and which is discharged from the image forming apparatus, comprising

• a sheet folding section including
• first and second folding sections which perform folding processes for a sheet discharged from the image forming apparatus,
• a binding section which is disposed on a sheet convey path located downstream of the first folding section and upstream of the second folding section in a sheet convey direction, on which a sheet bundle constituted by a plurality of sheets is stacked, aligned, and subjected to a binding process, and
• control means for controlling driving operation of the first and second folding sections,
• wherein the first folding section includes a pair of folding rollers which rotate in tight contact with each other, a driven roller which is detachably brought into tight contact with one of the pair of folding rollers to be driven/rotated, a driven roller which is brought into tight contact with the other of the pair of folding rollers to be driven/rotated, and a sensor which detects passage of a leading end portion of a sheet guided into the first folding section,
• the second folding section includes a folding plate member which can be moved in a direction perpendicular to a sheet surface, and a pair of folding rollers which are in tight contact with each other, and
• when an internal three-fold process is set for a sheet, the control means performs control to convey the sheet while releasing tight contact between an upstream driven roller which opposes an upstream folding roller of the pair of folding rollers of the first folding section, stop the sheet at a first predetermined position on the basis of a sheet leading end passage detection signal obtained by the sensor, bring the upstream driven roller into tight contact with the upstream folding roller, form a first fold by driving the pair of folding rollers in reverse, and form a second fold by moving the folding plate member of the second folding section on a sheet surface so as to push a second predetermined position of the sheet having the first fold to a nip point of the pair of folding rollers, and driving the pair of rollers in reverse.

A folding method for a Z-fold process method using first and second folding sections of a sheet postprocessing apparatus including a pair of folding rollers which are in tight contact with each other to form a nip point and rotate in predetermined opposite directions, a driven roller which is brought into tight contact with one of the pair of folding rollers to be driven/rotated, and a driven roller which is brought into tight contact with the other of the pair of folding rollers to be driven/rotated, comprising:

• the step of forming a first fold using the first folding section;
• the step of buckling the sheet and making the sheet be caught between the pair of folding rollers at the nip point by rotating the pair of folding rollers in the predetermined directions while the sheet on which the first fold is formed is clamped between the pair of folding rollers and the respective driven rollers and a leading end of a flap portion of the sheet is in contact with a surface of one roller, thereby forming a second fold;
• the step of bringing back the second fold caught at the nip point and the leading end of the flap portion of the sheet from the nip point to a nip point releasing position by reversing the pair of rollers in directions opposite the predetermined directions;
• and
• the step of causing the sheet with the second fold and the leading end of the flap portion of-the sheet being in tight contact with each other to pass through the nip point again by rotating the pair of folding rollers in the predetermined directions again.