Process for manufacturing the seal (1) for packaging by injection, that consists of a frame (8) connecting to the packaging, a cap (9), a hinged connection (10), coupling and inviolability means, with the following phases: closure of the mould (2), forming a moulding space (11) with the cap (9) open and located on the frame (8) in the same vertical plane; injection of the seal (1); pre-expulsion of the seal (1) from the fixed part (2.1); opening of the mould (2); freeing of the cap (9) from the moving part (2.2) and its descent on the frame (8) through the impulsion of compressed air (14); and expulsion of the seal (1) and collection by an expulsion robot that presses the cap (9) on the frame (8), closing the coupling and inviolability means.