A Molding unit for the forming of a container includes: a mold provided with a side wall defining a cavity bearing the impression of a portion of the container; an insert having a frontal surface bearing the impression of a local portion of the container, movable between a retracted position and a deployed position, with this insert being penetrated by ducts; a pierced piston to which the insert is attached and which ensures the mobility of the latter between its retracted position and its deployed position; and at least one pierced screw by which the insert is attached to the piston.