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Thermal Printing

Thermal printing (or direct thermal printing) is a digital printing process which produces a printed image by selectively heating coated thermochromic paper, commonly known as thermal paper, 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

Paper is impregnated with a solid-state mixture of a dye and a suitable matrix, for example, combining a fluoran leuco dye with an octadecylphosphonic acid. 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). In order to print, now thermo-sensitive paper is inserted between the thermal head and the platen. The printer sends an electric current to the heating elements of the thermal head. The heat generated then activates the paper's thermochromic layer. Such a printing mechanism is known as a thermal system or direct system. The heating elements are usually arranged as a line of small closely spaced dots.