Thermographic printing refers to two types of printing, both of which rely on heat to create the letters or images on a sheet of paper.
The simplest type is where the paper has been coated with a material that changes colour on heating. This is called thermal printing and was used in older model fax machines and is used in most shop till receipt printers. This is called direct thermal.
More complex is thermal ink transfer printing that melts print off a ribbon and onto the sheet of paper.
Thermography is also the name of a post print process that is achieved today using traditional printing methods coupled with thermography machines. Thermography machines are constructed with three sections connected by a through conveyor. The first section applies powdered polymer to the entire sheet followed by a gentle vacuuming to remove the excess powder from the non imaged and dry ink areas. The areas selected for raised printing are printed with inks that do not contain dryers or hardeners so that they remain wet during the application of powder. This ink will be dried or hardened later during the heating process. The second section of the process is a vacuum system that removes excess powder from those portions of the paper surface that have not been inked . The sheet is then conveyed through a radiant oven system and exposed to temperatures of 900 to 1300 degrees Fahrenheit. The heating process takes on the order of 2.5 to 3 seconds. During this time, the substrate (usually a paper product) has a peak in IR radiant absorption at the frequency (temperature) used in this process. Through conduction from the contact with the paper, the powder temperature rapidly increases and starts filming at the edges of the selected raised printing areas. When the center of the largest filmed areas reaches a sufficient quality level, the process speed adjustment and heater intensity are such that the product will be exiting the heater. When the polymer filming is at the proper quality and before combustion occurs, the sheet is immediately run into a convection cooling section where the filming action is stopped at that quality level.
This process is sometimes produced using a hand powdering process. The substrate with the wet ink areas selected for the effect are dipped into the powdered polymer. The sheet is tilted back and forth, rolling the powder across the image. The excess powder is removed by raising the substrate to a verical position and lightly tapping the back. The powdered sheet is then fed into a radiant heating system as above at a speed to achieve a quality filming. In the case of craft applications, heating can even be achieved by carefully using a hair dryer.
It is commonly used on letterheads, business cards, greetings cards, gift wrap, packaging and can also be used to print braille text. It is even sometimes used in diploma printing as an attractive alternative to the more expensive engraving option.