Abstract: A plastic straddling or expansion plug having a closed plug neck and a closed plug foot both with a longitudinal aperture into which a screw can cut a thread. The plug includes a shank having a thin plug jacket on which a two-flight helix is integrally formed. In the region between the helices, the plug jacket is provided with longitudinal slits. On the outside of the helix, further ribs are provided which each extend over a 180.degree. arc and are integrally formed with the helix, alternating in orientation by a 90.degree. angle. The plug neck includes flange lugs oriented in the shape of a cross and adapted to be folded over.

Abstract: A rotary welding member made of synthetic resin material comprising a post portion, and a disk-shaped flange portion integrally mounted on one end of the post portion and having a plurality of through holes in the direction of wall thickness thereof for the dissipation of an excessive portion of frictional heat generated during a rotary welding process whereby deleterious or degrading effects upon a base plate, to which the rotary welding member is welded, are effectively prevented.