an apparatus for forming bristle sub - assemblies includes a vertically oriented slot , which is formed by two endless belts . a wheel disposed above the slot pushes a bristle string into the slot . the bristle string has a plurality of bristles connected to a base string in two rows . the bristle rows can be horizontally oriented , so that all of the bristles are co - planar in a horizontal plane , or they can be angularly oriented with the base string at the vertex of the angle . when the bristles are pushed into the slot , they bend to adopt a new , substantially parallel orientation . an ultrasonic heater heats the bent bristles to re - set the thermoplastic material from which the bristles are made so that after cooling , the parallel orientation is maintained .