a welding method controls the welding temperature , welding cycle time , and welding pressure for fabrication of composite structural parts . a welding unit welds multiple plies of thermoplastic prepreg into welded multi - ply assemblies by concentrating welds along the thermoplastic prepreg plies at localized and strategic positions . two platens weld a plurality of the multi - ply assemblies into a layup using a plurality of heated pins . the heated pins are hollow to reduce surface heating during welding and are non - uniformly shaped to allow indexing of the layup . after the layup is cut into a plurality of thermoplastic prepreg segments along a layup pattern , the thermoplastic prepreg segments are indexed according to the weld marks impressed by the non - uniformly shaped heated pins . the thermoplastic prepreg segments are formed into kits or wedges by stacking smaller thermoplastic prepreg segments on top of larger thermoplastic prepreg segments .