Sheet stacker and image forming apparatus

A sheet stacker, comprising:

• a tray on which a sheet is stacked;
• a transporter that transports the sheet in a sheet-transporting direction towards the tray;
• a guide that retains an edge portion of the sheet transported by the transporter in a standby position, followed by moving in the sheet-transporting direction to guide transporting of the sheet, the edge portion of the sheet retained by the guide being detached from the guide in a detaching position using a difference between a sheet-transporting speed of the transport and a traveling speed of the guide, the detached sheet being allowed to fall and to be stacked on the tray;
• a plurality of blowers that discharge air flows towards the sheet transported to the tray, the air flows being directed towards a top plane of the tray, and a first blower of the plurality of the blowers being disposed at a first position corresponding to the detaching position;
• and
• a controller configured to
• control motion of the plurality of the blowers and air volume from the plurality of the blowers based on length information indicating a length of the transported sheet,
• operate, among the plurality of the blowers, the first blower before or at a time when the edge portion of the sheet is detached from the guide,
• operate, among the plurality of the blowers, one or more blowers disposed upstream of the first blower in the sheet-transporting direction, and
• control the plurality of the blowers to gradually reduce the air volume of the plurality of the blowers from upstream to downstream in the sheet-transporting direction before or at a time when the edge portion of the sheet is detached from the guide.