Thermal printing

Thermal printing is a digital printing process that creates images without using liquid ink. Instead, it relies on a chemical reaction triggered by heat. This method is most commonly known as direct thermal printing. It works by passing paper with a special coating, called thermal paper, over a heated print head.

This technology is vital for many modern tasks because it is fast, quiet, and requires very little power. Because these machines do not need ink cartridges, they are often smaller and lighter than other printers. This makes them perfect for portable devices used in retail or medical settings.

To understand the mechanism, we must look at the chemistry of the paper. Thermal paper is impregnated with a solid-state mixture. This mixture contains a dye, such as a fluoran leuco dye, and a matrix like octadecylphosphonic acid. The process relies on thermochromism, which is a change in color caused by temperature. When the matrix is heated above its melting point, the dye reacts with the acid. This causes the dye to shift into its colored form.

If the matrix solidifies quickly enough, this new color is conserved in a metastable state. Most printers produce monochrome images, but some two-color designs can print red by applying specific temperatures.

The physical hardware of the printer is also essential to the process. A thermal printer typically contains a thermal head, a platen, and a spring. The thermal head consists of tiny, closely spaced heating elements arranged in a line.

These elements can have a resolution of up to 1,200 dots per inch (dpi). The platen is a rubber roller that moves the paper through the machine. A spring applies pressure to hold the paper and the printhead tightly together. When the printer operates, it sends an electric current to the heating elements. This creates the heat necessary to activate the thermochromic layer on the paper.

There are different types of thermal printing methods. Direct thermal printing uses the heat-sensitive paper described above. However, there is also thermal-transfer printing. This method uses plain paper instead of special thermal paper. Instead, it uses a heat-sensitive ribbon made of wax-based materials. The ribbon adheres to the paper during the printing process. Because of this, users must load both labels and a ribbon, which acts as an alternative ink system. While the mechanisms differ, both methods utilize similar thermal print heads to apply heat.

The history of this technology shows how much the process has improved. Thermal printing was first used in the 1930s for electrocardiograph recorders. In 1950, the company 3M used the technology in the Thermofax copy machine.

Early versions were somewhat unreliable. They worked by using heat to destroy a white dye layer to reveal a colored layer underneath. This was later replaced by a more stable process using leucopigments. NCR introduced this improved method for military communications in the 1960s. This advancement made the technology much more dependable for widespread use.

Today, thermal printers are used in many specialized fields. In commercial settings, they are found at filling station pumps, information kiosks, and point-of-sale systems. They are also used to print shipping labels and live rhythm strips on hospital cardiac monitors. In the field of science, they are used for seafloor exploration and engineering geology.

These printers are ideal for creating continuous reels of real-time records from side scan sonar or seismic imagery. Even in the 1990s, fax machines commonly used thermal paper rolls for transmitting documents.

While highly efficient, thermal printing has specific limitations and concerns. The printed text is sensitive to light, water, and friction, which can cause darkening. While modern coatings are more stable, text should remain legible for at least 50 days. Users must also be careful with heat; for example, a laminator will darken the entire page of thermal paper. Additionally, studies in the 2000s found that the chemical bisphenol A (BPA) is often mixed into thermal papers. This has led to pressure from organizations like the Environmental Working Group to remove certain versions from the market.