A system and method for welding thermoplastic components by positioning and moving a heated plate between the components to melt their respective faying surfaces, and as the plate moves, pressing the components together so that the melted faying surfaces bond together as they cool and re-solidify, thereby creating a composite structure. The plate has a heated portion which is positioned between and heated to melt a portion of the first and second faying surfaces. A manipulator mechanism moves the plate along an interface from between the portion to between a series of subsequent portions of the first and second faying surfaces, thereby welding the thermoplastic components along the entire interface to create the composite structure. The heated portion may contact the faying surfaces and melt them through conduction, or may be suspended between them and melt them through radiation and convection.