Thermal printing is a technique in which an image is printed on thermal paper by means of heat.

The heat is applied by a controlled printhead. The thermal paper, on the other hand, has a heat-sensitive coating that allows the heat to produce the marking.

There are two types of thermal printing methods: direct thermal printing and thermal transfer printing.

In the case of direct thermal printing, the heat from the printhead is applied directly to the thermal paper. This causes a chemical reaction in the special heat-sensitive layer of the laminate material, turning the paper black. It is worth being noted that modern thermal paper is much more resistant to environmental influences than before so that the colour stays fresh for several years.

Thermal transfer printing, on the other hand, uses colour foil, or thermal transfer foil, to get the print on the paper. The printhead is equipped with hundreds of tiny heating elements that can be computer-control activated. The foil runs between the printhead and paper and is melted by the printhead where the heating elements are activated. The plain surface of the foil achieves a fine and sharp print with a slight shine. The advantage of the thermal-transfer system is the longer shelf life of its prints compared to the direct-thermal prints.

Thermal printers that have to print directly on the product packaging often use inkjet printing systems. The main advantage is that these systems are able to print on various surfaces like paper, cardboard packaging, synthetic materials or metal. Inkjet printing is especially used for pharmaceutical, cosmetic and food packaging, as well as in the postal service.

Thermal printers powered with inkjet printing systems use ink cartridges with lots of tiny chambers that can be heated by an electric impulse. The heat provokes the building of a small vapour bubble which pushes the ink through the nozzle. The tension of the vapour bubble, as well as the surface tension of the ink drop, cause the ink to hurl back within a fraction of a second. Precise and high quality prints of text, barcodes and graphics can therefore be achieved.

Another combination of thermal printing with inkjet printing technology is the so-called piezoelectric inkjet printing process, or piezo inkjet printing. In this case, the walls of the ink chamber are heated by an electrical pulse, so that they extend through the heat. The ink is pressed out of the nozzle and onto the object. By stopping the electric impulse, the walls return to their original position. In the chamber a vacuum is created, pulling the rest of the ink that has not been used for the print back into the cartridge.