Joining {or sealing} of preformed parts {, e.g. welding of plastics materials}; Apparatus therefor by heating, with or without pressure using heated elements which remain in the joint, e.g. "verlorenes Schweisselement" heated by induction {characterised by the type of elements heated by induction which remain in the joint comprising single particles, e.g. fillers or discontinuous fibre-reinforcements comprising discontinuous fibre-reinforcements}