FAQ# 1076: What is the difference between direct thermal and thermal transfer?
What is the difference between direct thermal and thermal transfer?
Unlike impact printers, which use physical force to press ink onto paper, thermal printers, both direct and transfer, work by applying electrical current to a heating element, which either reacts with chemical compounds in the paper or melts a wax or resin compound onto the paper.
The former of these two methods is direct thermal. A direct thermal printer requires thermal paper, which is impregnated with a special sort of dye which, prior to printing, remains colorless. However, when heat is applied to this dye, it changes color; black is the most common color, but there are other colors available, such as red. (In most cases, non-black dyes will function as dual-color thermal paper: A light application of heat may turn the dye red, whereas a stronger application of heat may turn the dye a dark enough shade of red so as to appear black.) You may have been told that thermal printers are unsuitable for certain applications, such as in a restaurant kitchen, where the temperatures the paper will be subjected to are quite high. As the entire paper is impregnated with dye, the high heat of certain environments will, in fact, change the color of the entire paper, thus obscuring the text on the page.
Conversely, thermal transfer functions more similarly to a traditional impact printer: Heat is applied to a special wax or resin ribbon, and this material is thus bonded to the paper. This sort of printing technology is most common in label printers: Direct thermal, while cheaper, is subject to heat distortion, and, even in the absence of heat, may yellow or fade over time. Direct thermal is also unsuitable for use in sunny conditions, where surfaces may be subject to significantly higher heat than the ambient temperature. In situations where heat or longevity are real concerns, thermal transfer is ideal. Because thermal transfer printing applies wax or resin to paper, longevity is significantly higher than that of direct thermal printing. And, while documents printed with impact printers have many of these same benefits -- such as heat immunity and some resistance to fading -- only thermal transfer is capable of doing these things while being moisture-resistant and printing with great speed. Where conditions are unsuitable to either direct thermal or impact printing, thermal transfer will nearly always meet your needs.