Thermal printing

In a thermal printing process, the desired print results " pressure " is achieved by selective heat generation rather than by mechanical stop or pressure.

The technique

For printing, the paper (or other medium) and a thermal print head or a line, or the box consists of a small heating elements moved past each other. This module is called by the manufacturers as a thermal array. Therefore are mainly used in medicine simple direct thermal printers that are integrated into medical measuring devices, also referred to synonymously as a thermal printer. Through sufficiently high heat dissipation a sufficiently small thermal time constant of the heating elements is achieved in order to print on a paper web in a continuous motion with high resolution can. The print density is here both in dots per inch (dpi ) and sometimes listed in Dot Per millimeters ( dpmm ).

Conventional printing densities are 6, 8, 12 or 24 dpmm (corresponding to 150, 200, 300 or 600 dpi ), but there are still finer. The print heads are sensitive to pollution mainly by metal-containing dust, the debris can cause a short circuit between the thermal resistances.

A significant contribution to the invention and exploration of thermal printing made ​​Rolf Gerstenberger with his thesis " Studies on the thermal printing with special consideration of the recording material " (TU Dresden 1974) and his research there in the 1970s.

Many thermal and thermal transfer printers are particularly prepared for printing bar codes. Most models can have its own printer language (such as ZPL Zebra, Easy plug at Avery Dennison, JScript in cab or TPCL Toshiba ) are addressed. The instruction sets contain special commands for printing bar codes. Thank this type thermal printer is also known as "Barcode Printer" familiar.

For thermal printers, there are essentially three printing techniques:

Direct thermal

With a thermal print head or a thermal strip it is printed directly on a thermosensitive special paper, which turns black when heated. The prints are mostly monochrome; Grayscale rasterized usually because they can not adequately grade through temperature control. Rarely dichromatic paper is used with two different colors of different temperature sensitivity.

The first thermal (direct) printers were used for inexpensive fax machines. For today's applications besides include the printing of receipts, invoices, shipping labels, parking tickets, admission, public transport, scientific and medical examination protocols, barcode Awards, price tags (eg self-service fruit and vegetable scales) and many kinds of labels. In the private sector thermal printers only come in inexpensive fax machines used, however, were formerly more common than printers for home computers or calculators.

Classic thermal paper is also light and temperature sensitive. Therefore, tax offices require permanent copies of thermal print purchase documents in different countries. When suitable thermal papers, however, the prints are still legible in light-and heat -proof storage even after many years. Today's paper also shows significantly improved resistance to environmental influences. This is achieved in part by applying a protective layer ( topcoat ), but also through improved formulations.

The direct thermal printing has the advantage that it manages, such as ribbons without consumables. Because the printer has few moving parts, its life is considerably higher than that of, for example, dot matrix printers and inkjet printers. In addition, equipment for paper roll be used, apart from refilling the paper roll, reliably without any maintenance from. This also nearly vandal-resistant and weather-resistant for outdoor use is possible. In addition, so much compact designs are possible.

In thermal copying the template has to be even irradiated only with a strong lamp. The dark colored areas warm up more, and are mapped in direct contact with thermal paper on itself.

Special Direct thermal printers can also be used for thermal transfer printing (see below).

Thermal transfer printing

In thermal transfer printing, a special, coated with temperature-sensitive color film between the paper and a thermal print head is guided through which owns hundreds of computer-controlled heating elements that transfer the printed image. A heating element whose head is driven and heated, the ink layer melts in the foil and is transferred to the paper. The smooth surface ensures a precise color imprint and achieves precise print quality. This kind of pressure is easily recognizable by a higher surface brightness than most other printing processes. During direct thermal printing grayscale in poor quality possible, halftones can transfer printing generally only screened on the paper can be downloaded because the color layer can only dissolve completely from the film.

Since all printouts remain permanently legible on the used thermal transfer film, there is a concrete data security risk for safety-critical applications in banks, industry and government.

The print medium needs to be not necessarily (normal) of paper in this method. It is merely the foil and the temperature of the medium to be printed are matched, then certain plastic surfaces can be printed.

A thermal transfer printer is often used for printing durable labels that are applied, for example on long-life assets for part identification with serial numbers and should be durable for the life of this part.

Dye sublimation

A similar method is the dye sublimation printing. The difference is that the applied to the carrier film dyes are evaporated by application of heat. Here, the dye goes directly from solid to gaseous state ( sublimation). The gaseous dye penetrates into the material to be printed ( paper) or reflected (plastic) on it. In dependence on the pressure point of each amount of energy supplied and the amount of transferred ink ( up to 64 gradations of each color ) is controlled, whereby a high color resolution is achieved and brilliant color result. However, disadvantages are the slow printing speed at high costs, since each printing process, only one color at the same time can be applied. In many cases, the printed image in the last pass with a transparent protective layer must be covered in order to reach the required durability, and abrasion resistance. Since the method is used particularly for printing digital images, each (color) pixel up to four times the same print position has to be precisely controlled, to produce with the conventional four standard printing inks ( cyan, magenta, yellow, and black ), the desired color. ( - Defined as the color shift or range ), the carrier film to the follow - squad transported to the next color Because the four colors in sequence are separated from each other and periodically on the color carrier film in each case exactly equal intervals available, usually after completion of Einfarbausdruck a print line is and the process is repeated with this, to be done every four.

The print results are very similar to those of the highest quality and a color photo. Application areas were in the early days of digital photography, especially photography studios, today they are used because of the excellent print quality in a compact and inexpensive construction still mainly for home use, and soft here increasingly the cheaper inkjet technology. Some devices can be connected ( via Bluetooth or infrared interface ) of digital cameras and used as a mobile printing solution you're.

Is to consider the safety point of that color on the thin carrier film remain color separation negatives of prints. A problem that was already there analog in the films writing bands for ( IBM ) ball typewriters.

Limitations

The heating and cooling of the heating elements is the ( edge of) focus in the printing direction - compared to the transverse direction - significantly reduced. In particular bar codes should therefore always be positioned so that the longitudinal beams are outputted to the printing direction. However, this is again the risk that one or more faulty elements white lines ( defects) occur parallel to the direction of the bar code lines that make the barcode also unusable.

The print resolution can be quite unbalanced in some models. The resolution transverse to the direction of pressure is limited by the density of the heating elements, but in the compression direction, the resolution can be influenced by the pressure and speed during heating, the dynamics of the individual elements. Thus, resolution of 300 × 600 dpi or 200 × 120dpi are not uncommon.

Due to the principle can direct thermal and thermal transfer printers only monochrome printing, as the print head a pressure point either heated or not. With direct thermal printers, this limitation can be circumvented by special paper, in which discolour certain surfaces differently. Thermal transfer printers use different color thermal transfer ribbons and so also print special colors, but are limited to one color per printing. A two-color printing is possible either through synchronized printer or a printer with two printing units.