Also called intaglio printing, the lines to be printed are cut into a metal plate by means of a cutting tool called a burin, held in the hand – in which case the process is called engraving; or through the corrosive action of acid – in which case the process is known as etching. In etching, for example, the plate is covered in a resin ground or an acid-resistant wax material. The plate is then dipped into acid. The acid bites into the surface of the plate where it was exposed.
To print with an engraving plate, ink is applied to the surface of the plate by the press and pushed into the recessed lines, or grooves. The plate is then wiped clean on the surface with a paper material so that ink only resides in the etched impression of the plate, the intended image. A piece of paper is fed into the press and is placed on top of the plate and is squeezed using immense pressure into the plate’s ink-filled grooves. The ink is transferred from the etched area of the plate to the paper and the result is a printed sheet of paper with a raised ink impression.
Thermography is also the name of a post print process that is achieved today using traditional printing methods coupled with thermography machines. Thermography machines consist of three sections with a through conveyor.
The first section applies thermographic/embossing powder, made from plastic resins, to the substrate (normally paper). The areas selected for raised printing are printed with slow-drying inks that do not contain dryers or hardeners so that they remain wet during the application of powder. This ink is dried and hardened later during the heating process.
The second section of the process is a vacuum system that removes excess powder from uninked areas of the substrate.
The third section of the process conveys the product through a radiant oven where it is exposed to temperatures of 900 to 1300 degrees Fahrenheit. The heating process takes on the order of 2.5 to 3 seconds. The substrate (usually paper) has a peak in IR absorption at the wavelength used. Through conduction from the paper, the powder temperature rapidly increases and starts melting. When the process is correctly adjusted, the center of the largest filmed areas reach sufficient quality level as the product exits the heater. The melted ink then solidifies as the product cools.