the joint increased from 30 pounds to 80 pounds when the surface confronted by the energy director was roughened as contrasted with a smooth surface obtained from a standard mold surface fig3 shows another typical joint configuration used for welding a thermoplastic cover to a container again the surface 40 contacted by the energy director 42 is provided with a rough texture fig4 for the sake of completeness depicts a typical apparatus for welding workpieces two workpieces 46 and 48 are in stacked position disposed on a base 50 from which extends a vertical column 52 an electroacoustic converter unit 54 is mounted for reciprocating vertical motion along column 52 the converter unit is coupled to receive electrical high frequency energy from a generator 56 via a cable 58 responsive to the receipt of high frequency electrical energy typically a frequency in the range from 16 khz to 60 khz the converter unit provides mechanical high frequency vibrations to a horn 60 coupled to the converter unit for achieving welding the converter unit is lowered and the horn 60 is brought into forced contact with the workpiece 46 subsequently the generator 56 is energized for a fraction of a second to cause the transfer of ultrasonic energy to the workpieces to effect a flow of thermoplastic material at the joint between the workpieces 46 and 48 then for a brief dwell period the workpieces are retained under a compressive force by the horn while the ultrasonic energy is stopped thereby permitting the softened thermoplastic material to rigidify for forming a weld subsequently the converter unit is raised along the column 52 and the workpieces now welded can be removed from the platform while there has been described and illustrated.