There are two methods of thermal overprinting onto labels 

Direct Thermal - direct thermal label stock will change colour (usually black) when heated. A heating element in the shape of letters or images can be used to create an image on the label. Custom labels can be easily be made on location in this way. A disadvantage is durability, because another heat source can ruin or obscure the image, or it may fade completely over time

Thermal Transfer for applications that cannot use Thermal (Thermal Direct) label material because of heat source proximity or short label life, a more widely used material is Thermal Transfer Label printer. This material has the advantage of a much longer readable life and does not fade with time or heat. Most major manufacturers of Thermal Printers can be used for either Thermal Transfer (TT) or Thermal (DT) labels. A thermal transfer ribbon will be required to print the labels. The cost of the ribbons + TT labels is similar to that of the DT labels on their own.

Thermal Transfer Ribbon Types

 Wax is the most popular with some smudge resistance, and is suitable for matte and semi-gloss paper labels.

Wax / Resin is smudge resistant, suitable for semi-gloss paper and some synthetic labels.

Resin is scratch and chemical resistant, suitable for coated synthetic labels.

How does a thermal printer works

A thermal printer (or direct thermal printer) 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 turns black in the areas where it is heated, producing an image. Two-colour direct thermal printers are capable of printing both black and an additional colour (often red), by applying heat at two different temperatures.

Thermal transfer printing is a related method that uses a heat-sensitive ribbon instead of heat-sensitive paper

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 thermo-sensitive paper

 Controller boards — for controlling the mechanism

In order to print, one inserts thermo-sensitive paper between the thermal head and the platen. The printer sends an electrical current to the heating resistor of the thermal head, which in turn generates heat in a prescribed pattern. The heat activates the thermo-sensitive colouring layer of the thermo-sensitive paper, which manifests a pattern of colour change in response. Such a printing mechanism is known as a thermal system or direct system.

The paper is impregnated with a solid-state mixture of a dye and a suitable matrix; a combination of a fluoran leuco dye and an octadecylphosphonic acid is an example. When the matrix is heated above its melting point, the dye reacts with the acid, shifts to its coloured form, and the changed form is then conserved in metastable state when the matrix solidifies back quickly enough. See thermochromism.

Controller boards are embedded with firmware to manage the thermal printer mechanisms. These controller boards’ features are designed to meet the needs in terms of functionalities and specifications.

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 like paper low, paper out, door open, top of form etc., and they are available with the most commonly used interfaces (RS232, Parallel, USB.