Thermal printing (or direct thermal printing) is a digital printing process which produces a printed image by selectively heating coated thermochromic paper, or thermal paper as it is commonly known, when the paper passes over the thermal print head. The coating turns black in the areas where it is heated, producing an image. Two-color direct thermal printers can print both black and an additional color (often red) by applying heat at two different temperatures.
Thermal transfer printing is a related method that uses a heat-sensitive ribbon instead of heat-sensitive paper.
Thermal printing is notable for being the only form of (non-embossing) printing which involves no ink.
A thermal printer comprises these key components:
- Thermal head: generates heat; prints on paper
- Platen: a rubber roller that feeds paper
- Spring: applies pressure to the thermal head, causing it to contact the thermosensitive paper
- Controller boards: for controlling the mechanism
In order to print, thermo-sensitive paper is inserted between the thermal head and the platen. The printer sends an electrical current to the heating elements of the thermal head, which generate heat. The heat activates the thermo-sensitive coloring layer of the thermosensitive paper, which changes color where heated. Such a printing mechanism is known as a thermal system or direct system. The heating elements are usually arranged as a matrix of small closely spaced dots—thermal printers are actually dot-matrix printers, though they are not so called.
The paper is impregnated with a solid-state mixture of a dye and a suitable matrix; a combination of a fluoran leuco dye and an octadecylphosphonic acid is an example. When the matrix is heated above its melting point, the dye reacts with the acid, shifts to its colored form, and the changed form is then conserved in metastable state when the matrix solidifies back quickly enough (a process known as thermochromism).
Controller boards are embedded with firmware to manage the thermal printer mechanisms. The firmware can manage multiple bar code types, graphics and logos. They enable the user to choose between different resident fonts (also including Asian fonts) and character sizes. Controller boards can drive various sensors such as paper low, paper out, door open and so on, and they are available with a variety of interfaces, such as RS-232, parallel, USB and wireless. For point of sale application some boards can also control the cash drawer.
An image of thermal printer output-