Abstract: For implant molding of toothbrushes pre-cut and pre-rounded fiber of thermoplastic material are used. Loose fibers are presented to an endrounding tool in a tuft or bundle and the free ends of the fibers are exposed to a working surface of the tool. Relative movement between the fiber ends and working surface causes the material of the fibers to be heated by friction of a predetermined intensity. The fibers while they are exposed to the working surface of the tool are permitted to flex laterally and are held in an axial direction of the tuft or bundle with their free ends commonly defining a surface different from a plane which is perpendicular to the axial direction, thereby achieving a substantially consistent flexing resistance throughout the cross-section of the tuft or bundle with respect to lateral deflection upon engagement by the endrounding tool.