A modular system for providing additional injection units, to be used as a complementary multi-use tool for multi injection molding systems with molds for an injection machine, the modular system comprising: a metal structure; a first clamping plate, the structure clamping plate, adapted to be coupled to an injection machine; a second clamping plate, the front mold clamping plate, adapted to be coupled to a mold; an injection nozzle extension having a first end and a second end; wherein: the first end, the nozzle extension inlet, is disposed on the first clamping plate and is adapted to be coupled to the injection machine and to receive a first material from the injection machine; and the second end is disposed on the second clamping plate and is adapted to be coupled to an inlet of the mold and to inject the first material into the mold; an additional injection assembly (6, 7) disposed between the structure clamping plate and the front mold clamping plate, the additional assembly being coupled to the first clamping plate, the additional injection assembly comprising at least one additional nozzle (9, 10) adapted to inject at least one additional material into the mold; the structure clamping plate and the front mold clamping plate comprise a structure spacer block assembly interposed, which is positioned and fastened to the structure clamping plate or is integral therewith; and wherein the structure clamping plate is disposed in one of the following arrangements: clamped to a stationary plate of the main injection machine, clamped to a movable plate of the main injection machine, or interposed between the mold bodies. a metal structure; a first clamping plate, the structure clamping plate, adapted to be coupled to an injection machine; a second clamping plate, the front mold clamping plate, adapted to be coupled to a mold; an injection nozzle extension having a first end and a second end; wherein: the first end, the nozzle extension inlet, is disposed on the first clamping plate and is adapted to be coupled to the injection machine and to receive a first material from the injection machine; and the second end is disposed on the second clamping plate and is adapted to be coupled to an inlet of the mold and to inject the first material into the mold; the first end, the nozzle extension inlet, is disposed on the first clamping plate and is adapted to be coupled to the injection machine and to receive a first material from the injection machine; and the second end is disposed on the second clamping plate and is adapted to be coupled to an inlet of the mold and to inject the first material into the mold; an additional injection assembly (6, 7) disposed between the structure clamping plate and the front mold clamping plate, the additional assembly being coupled to the first clamping plate, the additional injection assembly comprising at least one additional nozzle (9, 10) adapted to inject at least one additional material into the mold; the structure clamping plate and the front mold clamping plate comprise a structure spacer block assembly interposed, which is positioned and fastened to the structure clamping plate or is integral therewith; and wherein the structure clamping plate is disposed in one of the following arrangements: clamped to a stationary plate of the main injection machine, clamped to a movable plate of the main injection machine, or interposed between the mold bodies.