According to the invention, the device comprises at least one flap spreading head (12a, 12b), the flap spreading head being vertically mobile for insertion thereof into a container (3) to be processed and including first and second flap spreading blades (1211, 1212) mounted on pivots and an actuator (1221, 1222). The actuator is adapted to command rotation of the flap spreading blades that on rotating come into contact with flaps (R1a, R1b) and bring about spreading thereof by folds toward the exterior of the container.