An electropyrotechnic initiator includes a pyrotechnic charge (6), the charge (6) containing at least one compound. It also includes a box (1) of plastic material including two sub-assemblies. The first sub-assembly (2) includes a plastic wall (4) integral with a bottom (5) also made of plastic and forming a content. The second plastic sub-assembly (3) having a main axis (10) is traversed by at least two pins (11, 12) along the direction of the axis (10). These pins (11, 12) are connected together by an electric bridge (13) on a face (14) of the sub-assembly (3). This face (14) is hollowed symmetrically over a height H and a width L. The second sub-assembly (3) forms a socket. The first (2) and second (3) sub-assemblies are assembled hermetically by ultrasonic welding.