What is the working principle of thermal printer? There is a so called semiconductor heating element welded onto the print head. This print head heats and contacts thermal papers, then certain images or contents print out. The images and contents are generated by heated and aspired from chemical reaction in the film. This chemical reaction of thermal printing is operated under certain temperatures. High temperature will accelerate the chemical reaction. If temperature is less than 60°C, it takes a long time, even several years, for the paper to become dark. But if temperature is up to 200°C, the printing work is completed in a few microseconds.