A thermal bonding assembly for sheets made of polymeric material includes at least one pair of jaws that can be moved mutually closer, at least one of which is provided with a heater at its surface for abutment against a corresponding complementary surface of the other jaw of the pair. The assembly includes a shaft that can rotate by virtue of the action of a respective motor. The rotatable shaft includes at least two eccentric rings which are arranged side by side so that the eccentric portion that protrudes the most with respect to the rotation axis is substantially mutually opposite. Each jaw of the pair includes a respective through hole for the accommodation of the shaft. The shaft is accommodated in the hole of a respective jaw with the interposition of a rotatable disk.