An apparatus (10) for welding materials has a flat anvil (12) having a width of at least two inches and a length of at least eighteen inches. A welding apparatus (14) having at least one welding head (22) is mounted on a rail (16) which is parallel to said anvil (12). The apparatus (10) may be mobile. In an illustrated embodiment, welding apparatus (14) is an ultrasonic welding unit, and apparatus (10) is used to weld thermoset materials. As illustrated, the method is used to weld thermoset plastic monofilament fabric (34) to provide a continuous method of treating the monofilament fabric (34).