Incorporates means for maintaining a constant tension in the printing ribbon consisting of a loading drum placed after the thermal print head that attracts the ribbon to define a loop, thereby maintaining a constant tension throughout the printing stage to ensure the quality of this operation. The printing method consists of attracting or suctioning the printing ribbon against the loading drum, using an electrostatic system that involves creating an electric field between conductors placed on the loading drum, or a pneumatic system that involves attracting the ribbon by Venturi tubes located inside the loading drum.