An energy director is widely used in ultrasonic welding to increase the welding speed and quality. In the present work, three different types of energy directors were studied-namely, a triangular, a rectangular, and an innovative semicircular energy director. Experiments were performed using far-field test samples made of amorphous-type (ABS) and semicrystalline-type (PE) thermoplastics. It was found that the weld time is an important parameter of ultrasonic welding for the three types of energy directors studied. Weld pressure has different effects for the types of plastics tested, Increasing the weld pressure will decrease the welding efficiency for ABS. But for PE, increasing the weld pressure to four bars will increase the welding efficiency. The shape of the energy director was found to significantly affect the welding efficiency. In comparison, a semicircular shape was found to yield the highest welding efficiency under the same welding conditions and the triangular shape the lowest. Temperature measurements at the triangular energy director during the welding process indicate that the energy director absorbed 48.5% of the welding energy for ABS and 21.1% for PE. The different energy absorption rates are probably due to the difference in elasticity and viscosity between amorphous (ABS) and semicrystalline (PE) plastics.