Improvements to the forming of boxes containers

Apparatus for forming containers from plastic blanks, comprising: - a mold having a wall defining a cavity for receiving the blank; - an insert (11) of boxing movably mounted relative to the mold between a retracted position wherein the insert (11) is retracted into the wall, and an extended position wherein the insert (11) protrudes by relative to the wall, inwardly of the cavity; - a plate (15) on which the insert (11) is rigidly fixed; - a support (14) on which the plate (15) is detachably mounted; - a system (23) for rapid fixing of the plate (15) to the support (14).