Development of Thermal Printer

The working principle of the thermal printer is that the printing head is equipped with a semiconductor heating element. After the printing head heats and contacts the thermal printing paper, the desired pattern can be printed. Its principle is similar to that of the thermal fax machine. The image is generated by heating and chemical reaction in the film. The chemical reaction of the thermal printer is carried out at a certain temperature. High temperature will accelerate this chemical reaction. When the temperature is lower than 60 ℃, it takes a long time, even several years, for the paper to become dark; When the temperature is 200 ℃, this reaction will be completed in a few microseconds.

principle

The principle of the thermal printer is to cover a layer of transparent film on the light color material (usually paper), heat the film for a period of time, and then it becomes dark (generally black, also blue). The image is generated by heating and chemical reaction in the film. This chemical reaction is carried out at a certain temperature. High temperature will accelerate this chemical reaction. When the temperature is lower than 60 ℃, it takes a long time, even several years, for the film to become dark; When the temperature is 200 ℃, this reaction will be completed in a few microseconds. The thermal printer selectively heats the thermal paper at a certain position, thereby generating the corresponding graphics. The heating is provided by a small electronic heater on the print head in contact with the thermosensitive material. The heater is logically controlled by the printer in the form of square dots or strips. When it is driven, a figure corresponding to the heating element is generated on the thermal paper. The same logic circuit that controls the heating element also controls the paper feeding, so that graphics can be printed on the entire label or paper.

The most common thermal printer uses a fixed print head with a heated dot matrix. The print head has 320 square dots, each of which is 0.25mm × 0.25mm。 With this dot matrix, the printer can print at any position on the thermal paper. This technology has been used in paper printers and label printers.

Thermal paper
Thermal paper is a kind of special coated paper, its appearance is similar to ordinary white paper. The surface of thermosensitive paper is smooth, which is made of ordinary paper as the paper base, and is coated with a layer of thermosensitive chromogenic layer, which is coated on one side of the surface of ordinary paper. The chromogenic layer is composed of adhesive, developer, colorless dye (or leuco dye), which is not separated by microcapsules, and the chemical reaction is in a "latent" state. When the thermal paper encounters a hot print head, the color developer and colorless dye at the printing place of the print head react chemically to change color, forming a picture and text.
When the thermal paper is placed in an environment above 70 ℃, the thermal coating starts to change color. The reason for its discoloration should also start from its composition. There are mainly two kinds of heat sensitive components in the thermal paper coating: one is colorless dye or leuco dye; The other is color developing agent. This kind of heat sensitive paper is also known as a two group differentiation type heat sensitive recording paper.
Commonly used as colorless dyes are: crystal violet lactone (CVL), fluorane system, colorless benzoyl methylene blue (BLMB) or spiropyran system of triphenyl alkylphthalide system. Commonly used as color developing agents are p-hydroxybenzoic acid and its esters (PHBB, PHB), salicylic acid, 2,4-dihydroxybenzoic acid or aromatic sulfone.

When the thermal paper is heated, the colorless dye reacts with the developer to produce color, so when the thermal paper is used to receive the signal on the fax machine or directly print with the thermal printer, the image and text will be displayed. As there are many varieties used as colorless dyes, the handwriting colors are different, including blue, magenta, black, etc.

Application
Thermal printing technology was first used on fax machines. Its basic principle is to convert the data received by the printer into a lattice signal to control the heating of the thermal unit, and heat and develop the thermal coating on the thermal paper. Thermal printers have been widely used in POS terminal systems, banking systems, medical instruments and other fields. The thermal printer can only use special thermal paper. The thermal paper is coated with a coating that will change color due to chemical reaction when heated, similar to the photographic film, but this coating will change color when heated. Using this feature of the thermal coating, the thermal printing technology emerged.