a method for forming a strong and flexible bond between two plastic fabric layers requires the controlled use of an ultrasonic welding apparatus . control of the time ultrasonic energy is delivered to the two overlapped plastic fabric layers , and of the pressure placed upon those layers during that time , is exercised to ensure that no melting of the yarns from which the two fabric layers are woven occurs . rather , the yarns are softened by the application of ultrasonic energy , and are compressed together between the horn and an anvil to force them to adhere to one another . after the ultrasonic energy is turned off , compression is maintained while the overlapped plastic fabric layers cool and harden . a strong , flexible bond , suitable for operation as a seam for an endless belt , is obtained . because the yarns do not melt during the application of the ultrasonic energy , the bond obtained by the practice of the invention is not brittle , and consequently is not susceptible to the cracking and breakage observed in the bonds produced by ultrasonic welding in the prior art .