18 Industrial Laser Solutions MAY/JUNE 2015 www.industrial-lasers.com
FIGURE 1. Laser engraving screens, where print color is
squeezed through the mesh.
Laser applications for printing
WHAT DO A BEVERAGE CAN,
A BANKNOTE, AND THE INTERIOR
OF A CAR HAVE IN COMMON?
The industrial market for processing large-scale films has seen dramatic hanges since the 1980s and has almost completely been replaced by lasers and digital processes. A com- monly used technology for engrav- ing screens in the printing industry,
well known since then, is the use of RF-excited CO2 lasers with
a beam power up to about 1k W modulated in accordance to
the pattern to be engraved (FIGURE 1). A mesh is covered with a
thin polymer layer and the modulated laser beam engraves this
layer, where holes in the mesh have to be opened. This is a very
efficient way to produce printing plates and cylinders, especially when it comes to high-volume printing. Almost all printed
textiles, carpets, wallpapers, and some features of banknotes
use this technique.
Direct modulation of CO2 lasers is limited to about 10kHz,
which is mainly due to metastable nitrogen—a major part of
the laser gas mixture. Current printing technologies used in
tube and can printing demand a much higher pulse frequency
of some-hundred kilohertz. The reason for that is, to a lesser
extent, the higher resolution rather than the need for real 3D
structures in the material. Whereas engraving meshes is basically a 2D process, engraving printing plates and rollers of polymer or rubber is a 3D engraving process with complex structures. Each direct engraved structure needs a solid foundation
for stabilization during printing and may have a sophisticated
geometry on top, such as a well-defined plateau and an undercut to compensate for dot gain.
Future needs for high-security printing (banknotes, security
papers, passports, etc.; FIGURE 2) will require at least half a megahertz or even more, and industry now wants photorealistic pictures in packaging design, which requires a similar performance.
Acousto-optic modulators (AOMs) offer the possibility to
control the laser beam in a much faster way than by direct
modulation of the discharge of the laser with the RF sources.
But AOMs have limits due to their absorption in the germanium