a method and apparatus for forming a honeycomb structure in which a plurality of thermoplastic layers are fused together at selected locations . the thermoplastic layers at each of the selected locations are melted together to form a welded portion which includes first and second exterior surfaces . the welding of the thermoplastic layers is controlled so that no more than one of the exterior surfaces is melted . this partial melting of one layer prevents undesirable welding to adjacent layers . an apparatus for carrying out the method for welding thermoplastic honeycomb structures is also disclosed .