There is described a molding unit (1) for producing opening devices (2); the unit (1) is fed with a web (3) having a number of holes; the opening device (2) has a spout (4) closed by a membrane (9), a neck portion (8) projecting from the spout (4), and a pull ring (11) extending from the membrane (9) inside the neck portion (8); the unit (1) has first and second mold means (15, 16) cooperating, at one of the holes, with the web (3) to define a closed cavity (19), and movable so as to be detached from the web (3); the unit (1) also has a first (20) and a second (21) matrix which can be set to a configuration in which they are located inside the cavity (19) to define a first and second volume (28, 29); the matrixes (20, 21) are movable with respect to the web (3) to disengage the opening device (2) from the volumes (28, 29) and permit removal of the opening device by feeding forward the web (3); and the unit (1) also has a single actuator (22), which moves one (16) of the mold means (15, 16) and the matrixes (20, 21) to/from the web (3).