An injection mold of a plastic preform comprising a first part and a second part, fixed to each other, and a third part, wherein the third part is provided with a molding cavity defining an axis (Y') and is adapted to be integrally fixed to a first surface of a supporting element of the mold, wherein the second part is arranged between the third part and the first part and is apt to slide along said axis (Y') in order to close or open the molding cavity, wherein the first part comprises a longitudinal rod apt to slide along said axis (Y') through a second surface of the supporting element, opposite to and spaced apart from the first surface, wherein the second part is provided with - a sliding guiding cage for guiding the sliding comprising a first plate, provided with a through hole through which the longitudinal rod can slide and configured for abutting on said second surface of the supporting element; a base structure (18"); parallel guiding rods (16', 16", 16"', 16 iv) fixed at a first end thereof to said first plate and fixed at a second end thereof to said base structure (18"), - an assembly capable of sliding inside said sliding guiding cage, fixed at a first end thereof to said longitudinal rod, and comprising at a second end thereof a punch capable of sliding inside the base structure (18") and defining a first complementary component of the molding cavity delimiting the inner shape of the preform, wherein the base structure (18") is provided with two half-collars (66', 66") and is configured to define, together with the punch, a cam system for opening or closing the two half-collars (66', 66") which define, when they are closed, a second complementary component of the molding cavity defining the outer shape of a neck zone of the preform, characterised in that the third part is adapted to be integrally fixed to said first surface by means of a first bayonet coupling provided on a supporting frame and cooperating with a clutch arranged at the base of the molding cavity, in that the assembly is fixed to the longitudinal rod by means of a second bayonet coupling, and in that there is provided at least one extension which can be either added or removed for a change of format for the preforms to be molded having different length or format, said at least one extension being provided between the second bayonet coupling and the assembly and/or is provided on the longitudinal rod. wherein the third part is provided with a molding cavity defining an axis (Y') and is adapted to be integrally fixed to a first surface of a supporting element of the mold, wherein the second part is arranged between the third part and the first part and is apt to slide along said axis (Y') in order to close or open the molding cavity, wherein the first part comprises a longitudinal rod apt to slide along said axis (Y') through a second surface of the supporting element, opposite to and spaced apart from the first surface, wherein the second part is provided with - a sliding guiding cage for guiding the sliding comprising a first plate, provided with a through hole through which the longitudinal rod can slide and configured for abutting on said second surface of the supporting element; a base structure (18"); parallel guiding rods (16', 16", 16"', 16 iv) fixed at a first end thereof to said first plate and fixed at a second end thereof to said base structure (18"), - an assembly capable of sliding inside said sliding guiding cage, fixed at a first end thereof to said longitudinal rod, and comprising at a second end thereof a punch capable of sliding inside the base structure (18") and defining a first complementary component of the molding cavity delimiting the inner shape of the preform, - a sliding guiding cage for guiding the sliding comprising a first plate, provided with a through hole through which the longitudinal rod can slide and configured for abutting on said second surface of the supporting element; a base structure (18"); parallel guiding rods (16', 16", 16"', 16 iv) fixed at a first end thereof to said first plate and fixed at a second end thereof to said base structure (18"), - an assembly capable of sliding inside said sliding guiding cage, fixed at a first end thereof to said longitudinal rod, and comprising at a second end thereof a punch capable of sliding inside the base structure (18") and defining a first complementary component of the molding cavity delimiting the inner shape of the preform, wherein the base structure (18") is provided with two half-collars (66', 66") and is configured to define, together with the punch, a cam system for opening or closing the two half-collars (66', 66") which define, when they are closed, a second complementary component of the molding cavity defining the outer shape of a neck zone of the preform, characterised in that the third part is adapted to be integrally fixed to said first surface by means of a first bayonet coupling provided on a supporting frame and cooperating with a clutch arranged at the base of the molding cavity, in that the assembly is fixed to the longitudinal rod by means of a second bayonet coupling, and in that there is provided at least one extension which can be either added or removed for a change of format for the preforms to be molded having different length or format, said at least one extension being provided between the second bayonet coupling and the assembly and/or is provided on the longitudinal rod.