Unfolding device and unfolding method

An unfolding method for unfolding an empty box into a single cross-shaped flat sheet using an unfolding device, the unfolding device including

• a placement frame that provides a placement space for placing the empty box that is open before unfolding,
• a plurality of stress concentrating members configured to unfold a plurality of corners where inner side surfaces of the empty box meet,
• a plurality of movable portions attached to the stress concentrating members, and
• a power transmission portion configured to transmit power for moving a stress concentrating member via a movable portion, the unfolding method comprising:
• placing the empty box by causing the stress concentrating member located in the placement space to contact an inside of the empty box from an open side of the empty box;
• and
• transmitting the power for moving the stress concentrating member, wherein in the transmitting the power for moving the stress concentrating member,
• the power for moving the stress concentrating member is transmitted to the stress concentrating member so as to bring the stress concentrating member into contact with a corner, and the power for moving the stress concentrating member is further transmitted to the stress concentrating member so as to cause the stress concentrating member to unfold the corner by tearing and opening the corner, and
• the stress concentrating member includes a reverse slope blade that protrudes outward as the blade gets closer to a top of the blade.