At the manufacturing plant, paper is coated with a chemical that darkens when heat is applied to it. In fact, it is possible to make the printed side of heat-transfer paper completely black by placing it in an oven, under a heating lamp, or on the dashboard of a car parked in direct sunlight.
Thermal paper printers and fax machines use "thermal heads" that deliver small amounts of heat to the paper. Those thermal heads have hundreds of tiny elements that heat and cool rapidly and are activated in the correct sequence to create letters, numbers, or images onto the thermal paper fed into it. To ensure that the paper doesn't move, as well as receives the correct amount of heat, a backing roller, or "platen roll," keeps the paper moving and places the paper at an ideal distance from the thermal head
When the printer or fax is activated, the paper feeds into the space between the roller and the thermal head. Precisely-timed electrical impulses are sent by the computer built into the printer or fax machine, causing the thermal head to heat and cool in a certain pattern. That way, a clearly-printed product is produced without the use of ink or mechanical pressure.