Point of sale printers
Introduction
Thermal printing (or direct thermal printing) is a digital printing process that produces a printed image by passing paper with a thermochromic coating, commonly known as thermal paper, over a print head consisting of small electrically heated elements. The coating turns black in areas where it is heated, producing an image. [2].
Most thermal printers are monochrome (black and white), although some multi-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.
Design
A thermal printer typically contains at least these components:.
Thermal paper is impregnated with a solid-state mixture of a dye and a suitable base, for example a fluorane leuco dye and an octadecylphosphonic acid. When the matrix is heated above its melting point, the dye reacts with the acid, changes to its colored form, and the changed form is preserved in a metastable state when the matrix solidifies quickly enough, a process known as thermochromism.
This process is usually monochromatic, but there are some two-color designs, which can print in both black and an additional color (often red) by applying heat at two different temperatures. [3].
To print, thermal paper is inserted between the thermal head and the roller and pressed against the head. The printer sends an electrical current to the heating elements of the thermal head. The heat generated activates the thermochromic layer of the paper, causing it to acquire a certain color (for example, black).
Thermal print heads can have a resolution of up to 1200 dots per inch (dpi). Heating elements are usually arranged as a line of small, closely spaced dots. [4].
Early formulations of the thermosensitive coating used on thermal paper were sensitive to incidental heat, abrasion, friction (which can cause heat and darken the paper), light (which can fade printed images), and water. Subsequent thermal coating formulations are much more stable; In practice, thermally printed text should remain legible for at least 50 days.