The effects of spin-friction pressure and timing on the friction welding between parts of the same materials, PP, HDPE, and nylon-6, are discussed. The heat-affected zone is divided into three regions: the plasticized region (Zpl), the partly plasticized region (Zpd), and the undeformed region (Zud). The tensile fracture morphology has three main sections: the nonplasticized central section (Fud), the plasticized peripheral section (Fpl), and the partly plasticized section between Fud and Fpl (Fpd). Friction pressures (spinning timing) of 1.17 MPa (16 s) in PP-PP welding, 1.77 MPa (8 s) in HDPE-HDPE welding, and 0.98 MPa (8, 10, and 12 s) in nylon-6-nylon-6 welding attains better bonding strength because their components have larger Fpd and Fpl regions, which have no Fud in the central part. (C) 2004 Wiley Periodicals, Inc.