angular range u03b1 to pass through at that moment the shoe 18 leaves its bearing point on the abutment 25 of the arm 16 and with the mold base 3 then being detached kinematically from the half mold 2 the latter completes its closing travel alone with its groove 10 coming to cover the projection 9 on the mold base during the first phase of the movement the return force that the spring 20 exerts on the mold base 3 in order to tend to lift the latter toward its raised position for closing the mold keeps the abutment 25 of the arm 16 in contact with the shoe 18 with the result that the mold base 3 accompanies the movement of the half mold 2 of course numerous variants are conceivable it should be emphasized in particular that the position of the pin 19 defining the axis of rotation of the shoe 18 is immaterial which offers the advantage that the positioning of the arm 16 during the manufacturing operation does not have to be of high precision by way of example fig3 in which the mold 1 is shown in its entirety in perspective at a different angle of view illustrates the coupling 15 mounted at the end of an elongate link 26 itself connected at its other end to the arm 16 attached to the mold base 3 so that this link is able to rotate by means of a pin 27 approximately parallel to the axis 8 of the mold similarly fig4 illustrates another embodiment modeled on that of fig3 and in which the connecting rod 13 constituting the aforesaid connection means is interposed between the link 26 and an arm 28 only partly visible rotatably connected to the lugs.