Direct Thermal Printing - What is direct thermal printing?
Definition of DIRECT THERMAL PRINTING:
A method of printing; this process creates an image by selectively heating and cooling a set of dots, which are then dragged over heat sensitive paper that turns dark in the heated areas. The image carrier or imaging device in direct thermal printers is a thermal print head, which contains heating elements (usually pins) arranged in a matrix (meaning that direct thermal printing is a form of dot matrix printing). The pins are heated selectively to form a pattern that represents the image being printed, which means that direct thermal printing is also a form of digital printing; the image or document being printed must be in a bitmap format (the image is represented by a grid of coloured dots, or pixels), which can be recreated by the matrix of pins in the print head.
When the print head passes over heat sensitive paper, the areas that come into contact with the pins change colour, and the image is created on the paper. This process requires a special fine paper known as thermal (or thermochromic) paper; the paper is coated with a mix made from a dye and a developer (one or more organic acids); when the developer is heated above its melting point, the dye reacts with the acid and changes colour before cooling and solidifying. The mix may also include sensitizers (to facilitate the mixing of the dye and the acid, and to optimise the melting temperature of the mixture) and stabilisers (to prevent the dyes from losing their colour in hot or humid conditions).
Direct thermal printing is almost always used to print monochromatic images; the coating will usually turn black, although there are coatings available with dyes that turn red or blue. It is possible to create multi-coloured images; this requires thermal paper with multiple coatings where each coating has a different melting point. The pins are heated to different temperatures, which causes one particular layer to react and creates the required colour in that area.
Direct thermal printing involves printing substrates that are produced on rolls, and common uses include printing receipts and vouchers, shipping labels, rhythm slips on cardiac monitors, sonograms, and all kinds of data strips and images (e.g. sonar images of the seafloor or records of seismic activity).Go Back to Glossary