Process for manufacturing a seal for packaging by injection in a mould, in which the mould consists of a fixed part (2.1) with a first fixed base, a floating plate and a first moulding plate and a moving part (2.2) with a second fixed base, an expulsion assembly and a second moulding plate, the seal consisting of a frame connecting to the packaging, a cap, a hinged connection between both, means of coupling the cap to the frame and means for its inviolability comprising the following phases: closure of the moving part (2.2) and the fixed part (2.1) of the mould, forming between the first and the second moulding plates (5, 7) a moulding space for the seal with the cap in the open position with respect to the frame, where the cap is located over the frame in the same vertical plane; and injection of the material to make the seal into the moulding space; whereas the process further comprises the steps of: pre-expulsion of the seal from the fixed part (2.1) of the mould; the pre-expulsion further involving a first horizontal movement of the floating plate towards the moving part (2.2) of the mould during which the joint movement of this moving part (2.2) of the mould is generated, while the first moulding plate remains fixed, joined to the first fixed base; opening of the mould through the separation of the moving part (2.2) in which the seal is held from the fixed part (2.1); freeing of the cap from the moving part (2.2) of the mould and its descent on the frame through the impulsion of compressed air through a compressed air circuit inside the mould wherein as the cap descends the hinged connection between the cap and the frame is folded while the seal is still fixed to part of the mould and is still hot; and wherein the impulsion of compressed air takes place on the upper level of the cap; final expulsion of the seal from the mould and its collection by an expulsion robot; in which during the expulsion this robot presses the cap on the frame so that it is fixed to it by coupling means and the inviolability means are closed. closure of the moving part (2.2) and the fixed part (2.1) of the mould, forming between the first and the second moulding plates (5, 7) a moulding space for the seal with the cap in the open position with respect to the frame, where the cap is located over the frame in the same vertical plane; and injection of the material to make the seal into the moulding space; whereas the process further comprises the steps of: pre-expulsion of the seal from the fixed part (2.1) of the mould; the pre-expulsion further involving a first horizontal movement of the floating plate towards the moving part (2.2) of the mould during which the joint movement of this moving part (2.2) of the mould is generated, while the first moulding plate remains fixed, joined to the first fixed base; the pre-expulsion further involving a first horizontal movement of the floating plate towards the moving part (2.2) of the mould during which the joint movement of this moving part (2.2) of the mould is generated, while the first moulding plate remains fixed, joined to the first fixed base; opening of the mould through the separation of the moving part (2.2) in which the seal is held from the fixed part (2.1); freeing of the cap from the moving part (2.2) of the mould and its descent on the frame through the impulsion of compressed air through a compressed air circuit inside the mould wherein as the cap descends the hinged connection between the cap and the frame is folded while the seal is still fixed to part of the mould and is still hot; and wherein the impulsion of compressed air takes place on the upper level of the cap; wherein as the cap descends the hinged connection between the cap and the frame is folded while the seal is still fixed to part of the mould and is still hot; and wherein the impulsion of compressed air takes place on the upper level of the cap; final expulsion of the seal from the mould and its collection by an expulsion robot; in which during the expulsion this robot presses the cap on the frame so that it is fixed to it by coupling means and the inviolability means are closed.