A joining assembly comprises and a threaded element of plastic material comprising a core and an external threaded portion. A joining assembly (1) comprises a support member (2) of plastic material having a receiving bore defined by a bore wall, and a threaded element (4) of plastic material comprising a core (8) and an external threaded portion (10) which, when the threaded element is threaded into the wall of the receiving bore of the support member forms an internal threaded portion in the receiving bore by a cutting and/or deforming operation. The external threaded portion of the threaded element comprises a thread extending for revolutions and being, in axial cross-section, of a profile having straight flanks that are joined by a rounded crest. The core of the threaded element has, between the revolutions of the thread, a cylindrical external surface, which, in an axial cross-section, is joined to the flanks of the thread by rounded corners. The revolution of the thread defines a helical gap of a volume exceeding that of the thread. Independent claims are also included for: (1) a tool for making a joining assembly, comprising a tool body comprising an axially extending mandrel and an abutment portion, the mandrel comprising an axially extending pin having a short threaded portion adjacent the abutment portion and a cylindrical support portion adjacent the threaded portion; (2) a threaded element for a joining assembly, comprising a screw; and (3) a method for making a joining assembly, comprising threading the threaded element into the receiving bore of the support member at a high speed so that the plastic material of the support member is plastified by friction and the plastified plastic material is displaced from the external thread of the threaded element in a direction opposite to the threading direction to provide an encapsulating enclosure of the external thread when the plastified plastic material has been solidified.