a method of manufacturing rectangular frames made of plastic materials , in which at least two four - head welding machines arranged on a turntable are alternatively fitted out with four plastic shaped pieces cut to a predetermined length with corresponding mitres , and clamped together . a frame is welded in a respective welding machine by applying heat and pressure to all mitre surfaces to form welding beads thereat . the welded frame is then transferred to a cleaning machine for treatment of protruding weld bead portions .