Lasers Make Their Mark on and in Glass
Traditional methods of marking glass include stamping or molding the part while it’s in the molten stage, abrasive removal of material and acid etching. Lasers can also be used to mark glass.
Laser marking of glass is a straightforward process if the glass is highly absorptive of the laser beam’s wavelength. The photons are absorbed within a very short distance from the surface. Depending on the laser used, the marks—when viewed with a high-power microscope—will appear to either contain microcracks or be pristine. Microcracks are induced by heat, which means that marking was done with a long-wavelength, long-pulse laser. Pristine marks mean the material was removed with a nonthermal laser, like a short-wavelength, short-pulse model.
Subsurface lasing is increasingly being used for part traceability and anti-counterfeiting purposes. Image courtesy Cerion.
Laser-produced marks, as well as those made by other methods, require no masks. Simple programming is all that’s needed, and setup time is minimal. Alphanumeric characters, bar codes and 2-D dot-matrix codes can be easily made, as can characters sized well below 100µm.
An interesting thing about lasers is that, unlike other marking tools, they can be used to place an indelible, highly visible mark within glass—but not all glass. Subsurface laser marking is only possible if the glass is transparent, or at least partially so. To prevent the laser light from simply passing through the material, the beam must be focused to a tight spot inside the glass.
There are at least two different explanations of how the subsurface-marking mechanism works. One is that applying the right laser and energy density changes the work material’s refractive index, which is the measure of how a light ray changes as it passes through a medium. The result is a visible mark. This technique is used extensively in the laser industry to “write” waveguides onto the optical fibers that serve as oscillators in fiber lasers.
The second explanation is that a laser can deliver so much heat to an immediate and small area that a “micro-explosion” occurs, which affects the immediate structure but not the bulk material. These micro-explosions create voids that are surrounded by denser areas, which combine to change the light-scattering characteristics of the material and result in a mark.
Example of a laser-made surface mark. Letters, digits, logos and machine-readable codes can be placed onto or into glass in just a few seconds, reports laser manufacturer Cerion GmbH (cerion-laser.com). The laser markings cannot be removed or rubbed off and are inconspicuous. Image courtesy Cerion.
Colored markings can be made inside glass by locally changing the oxidation state of metal ions within the glass. (This does not weaken the glass.) Colors such as mauve, yellow, red, brown and gray have been observed. The glass has to be manufactured with these ions in it, of course, and the glass and metal ions must be compatible.
In addition to lasing marks within glass, it’s also possible to make 3-D “marks.” This is accomplished using a galvanometer scanner with Z-axis capability. Three-dimensional “crystal” sculptures can range in size from small to large and incorporate simple or complex features.
A manifestation of this process is In-volume Selective Laser Etching (ISLE), which was discussed in the July/August 2011 “Laser Points” column. A direct-laser-writing technique, it involves applying a focused femtosecond-laser pulse that changes the chemical structure of the workpiece material such that subsequent immersion in a wet etching agent promotes selective etching of the exposed area. The ISLE technique can be used to manufacture parts or create holes and channels inside of a material.
Manufacturing microfluidic devices is of major interest to those who laser-mark glass, and, arguably, it represents the most “glamorous” area of subsurface lasing. More-common applications for subsurface marking are applying marks on parts and devices for traceability or anti-counterfeiting purposes.
Pharmaceutical devices—syringes, ampoules, drug containers, etc.—are marked primarily for traceability. Bottles for holding luxury items like perfumes, cosmetics and high-end wines are marked primarily to discourage counterfeiting.
Surface marks produced with a thermal laser will appear to contain microcracks when viewed with a high-power microscope (left). Nonthermal lasers produce pristine surface marks (right). Images courtesy PhotoMachining.
In an optimal setup, devices and containers are marked at high speeds on automated production lines, with the laser occupying one station. However, whether high-speed automation is possible or not depends on the glass being marked. Even with the “correct” glass and ideal laser setup, this method can prove too slow and the acceptable bandwidth too small for high-volume production runs. It’s difficult to mark more than a few bottles per second using galvo-based scanners.
Though internal laser marking does not stress glass as much as external marking, there is still the possibility of inducing microcracks that can undermine mechanical stability. Also, internal-marking techniques lead to light scattering, which appears to
the observer as white pixels when illuminated.
Despite this and other drawbacks, lasers are excellent tools for marking glass both externally and internally. Cosmetically, a laser can produce a mark that’s superior to any other method in terms of quality and resolution. However, unless a masking procedure is used and the marks can be made in a single pulse on the surface, laser-marking speed, in general, will be insufficient for extremely high-volume production runs. µ
— R. Schaeffer