The machine (1) comprises: - one supporting base (2); - at least one lateral welding device (3) associated with the supporting base (2) and provided with one lateral heating plate (13) which is adapted to heat one main profiled element (5) and a respective lateral profiled element (6); and - at least one intermediate welding device (4) associated with the supporting base (2) and provided with one intermediate heating plate (18) adapted to heat the main profiled element (5) and a respective transverse profiled element (7), wherein said main profiled element (5) comprises at least one pair of intermediate areas to be welded (19) and the transverse profiled element (7) comprises one pair of transverse areas to be welded (21) wherein the intermediate welding device (4) comprises intermediate removal means (42) to make one groove (43) on one peripheral edge of the intermediate areas to be welded (19) and/or of the transverse areas to be welded (21).