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 turnsblack in the areas where it is heated, producing an image. Two-colour direct thermal printers can print both black and an additional colour(often red) by applying heat at two different temperatures.
Thermal transfer printing is a very different method that uses a heat-sensitive ribbon instead of heat-sensitive paper, but uses similar thermal print heads.[1]
Thermal printing is notable for being the only[verification needed] form of (non-embossing) printing which involves no ink or toner.
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 theheating 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 anoctadecylphosphonic 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.
Applications[edit]
Thermal printer used in seafloor exploration
Thermal printers print more quietly and usually faster than impact dot matrix printers. They are also smaller, lighter and consume less power, making them ideal for portable and retail applications. Roll-based printers can be rapidly refilled. Commercial applications of thermal printers include filling station pumps, information kiosks, point of sale systems, voucher printers in slot machines, print on demand labels for shipping and products, and for recording live rhythm strips on hospital cardiac monitors.
Many popular microcomputer systems from the late 1970s and early 1980s had first-party and aftermarket thermal printers available for them - such as the Atari 822 printer for the Atari 8-bit systems, the Apple Silentype for the Apple II and the Alphacom 32 for theSinclair ZX Spectrum and ZX81. They often used unusually-sized supplies (10CM wide rolls for the Alphacom 32 for instance) and were often used for making permanent records of information in the computer (graphics, program listings etc.), rather than for correspondence.
Through the 1990s many fax machines used thermal printing technology. Toward the beginning of the 21st century, however, thermal wax transfer, laser, and inkjet printing technology largely supplanted thermal printing technology in fax machines, allowing printing on plain paper.
Thermal printers are still commonly used in seafloor exploration and engineering geology due to their portability, speed, and ability to create continuous reels or sheets. Typically, thermal printers found in offshore applications are used to print realtime records of side scan sonar and sub-seafloor seismic imagery. In data processing, thermal printers are sometimes used to quickly create hard copies of continuous seismic or hydrographic records stored in digital SEG Y or XTF form.
The Game Boy Printer, released in 1998, was a small thermal printer used to print out certain elements from some Game Boy games.
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. Later thermal coating formulations are far more stable; in practice, thermally printed text should remain legible at least 50 days.[citation needed]
In many hospitals in the United Kingdom, many common ultrasound sonogram devices output the results of the scan onto thermal paper. This can cause problems if the parents wish to preserve the image by laminating it, as the heat of most laminators will darken the entire page—this can be tested for beforehand on an unimportant thermal print. An option is to make and laminate a permanent ink duplicate of the image.
Health concerns[edit]
A phenol-free thermal cash receipt
Reports began surfacing of studies in the 2000s finding the oestrogen-related chemical Bisphenol A ("BPA") mixed in with thermal (and some other) papers. While the health concerns are very uncertain, various health and science oriented political pressure organizations such as the Environmental Working Group have pressed for these versions to be pulled from market, but BPA-free and totally phenol-free thermal paper is available.[2][3]
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