There are two types of thermographic printing;
The simplest method uses a substrate which has been coated with a heat sensitive material that changes color exposed to heat. This type of thermography when called direct thermal printing and was originally used in fax machines and shop receipt printers.
The second thermographic printing method (which has more decorative qualities) works by conventionally printing the image and then coating the wet ink with a fine thermographic powder.
The printed and powdered image is exposed to a heat source and the ink and the powder fuse together which allows the powder to flow, forming a coating over the ink which is then cooled, allowing the coating to set and maintain the raised image. (see Figure 9.1)
Thermographic imaging is not recommended for heavy solids or fine text.
Figure 9.1 - Thermography - close up of printed result
The equipment used to produce the thermographic result consists of three units linked with a conveyor
Figure 9.1 - Thermography - close up of printed result system (see Figure 9.2).
Figure 9.2 - Thermographic imaging process
The first section is the printing operation which prints the image requiring thermographic processing. The press is usually a conventional litho, letterpress or screen printing press and the image is printed with slow-drying inks that do not contain drying agents and therefore remain open/wet.
The next stage of the process is the application of the thermosetting powder.
This unit applies the powder to the entire sheet and the powder sticks to the wet ink. The powder is delivered via a hopper which can be adjusted to alter the amount of powder being delivered to the sheet or web.
The amount is governed by the size of the printed image. For a small printed image less powder is required and more powder for a larger printed image. The excess powder is then removed using a low volume vacuuming system to extract all the powder from the non-imaged areas but leave the powder on the imaged areas.
The vacuum strength can be adjusted to ensure that the powder is not removed from the image area whilst the non-image area is powder free.
The thermosetting powder is manufactured from polymer resins. The most common polymers used are polyester and polyurethane. The process of manufacturing the powders involves mixing polymer granules with hardener and pigments. The mixture is heated in an extruder and then rolled flat, cooled and broken into fine chips which are roller milled into a fine powder.
The third part of the process is the application of heat to the sheet/web. The conveyor belt transfers the substrate into the heating unit where it is exposed to high temperatures for a period of approximately 3 seconds. The heat is conducted through the substrate, raising the temperature of the powder and the powder melts and fuses with the ink. It is important that the temperature is correctly set and maintained. The substrate then leaves the heater unit and is cooled, which solidifies the ink/powder creating the raised image.
Image printed (ink remains wet and tacky)
Thermographic powder applied to inked image
Image exposed to heat (ink and powder fuse together)
Image cooled leaving raised image with sharp edges
Some important points for achieving good thermographic results are identified below;
Excessive heat will create irregular edges in the image, bubbling and pin holing
Early melting of the powder during the heating process will cause the liquid powder to run over the edge of the image/type and distort the result
The substrate cannot be exposed to a sudden intense heat as this induces considerable curl. A slower build-up of heat does not create as much substrate curl. A typical example would be to keep all the heater elements at a low temperature, bringing the powder up to the melt temperature much more slowly instead of a rapid high temperature exposure.
ADVANTAGES AND DISADVANTAGES OF THE THERMOGRAPHIC PROCESS
Low cost method of creating a relief image
No dies or tooling required
Exposing substrate to high temperatures