Both technologies employ a printing head which, through the use of heat, produces an image on the paper.
The main difference is direct thermal printing is based on a chemical reaction of the components of the paper, which react with the heat of the head and produce an image.
In the case of thermal transfer printing, the ink is in a polyester ribbon that comes into contact with the paper. The ink is wax- or resin-based depending on the required durability of the print. When the head is heated, the ink melts; contact with the cold paper results in the transfer of the image.
Due to the fact that direct thermal printing does not require ink, toner or a transfer ribbon, these printers are more economical and easier to operate than thermal transfer, inkjet, laser or dot-matrix machines.