Thermal printing

Thermal printing (or direct thermal printing) is a digital printing process which produces a printed image by passing paper with a thermochromic coating, commonly known as thermal paper, over a print head consisting of tiny electrically heated elements. The coating turns black in the areas where it is heated, producing an image. Most thermal printers are monochrome (black and white) although some two-color designs exist. Thermal transfer printing is a different method, using plain paper with a heat-sensitive ribbon instead of heat-sensitive paper, but using similar print heads. A thermal printer typically contains at least these components: Thermal head: Produces heat to create an image on the paper Platen: A rubber roller which moves the paper Spring: Applies pressure to hold the paper and printhead together Thermal paper is impregnated with a solid-state mixture of a dye and a suitable matrix, for example, a fluoran leuco dye and 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. This process is usually monochrome, but some two-color designs exist, which can print both black and an additional color (often red) by applying heat at two different temperatures. In order to print, the thermal paper is inserted between the thermal head and the platen and pressed against the head. The printer sends an electric current to the heating elements of the thermal head. The heat generated activates the paper's thermochromic layer, causing it to turn a certain color (for example, black). Thermal print heads can have a resolution of up to 1,200 dots per inch (dpi). The heating elements are usually arranged as a line of small closely spaced dots. Early formulations of the thermo-sensitive coating used in thermal paper were sensitive to incidental heat, abrasion, friction (which can cause heat, thus darkening the paper), light (which can fade printed images), and water.

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