By Geoff Shannon, Miyachi Unitek Corp.
Laser marking, a fast and clean marking technology, is rapidly replacing older product marking technology, in particular, inkjet marking systems. A key factor in the widespread adoption of laser marking is the exponential increase in the use of direct marking of part information to aid in tracking and traceability, from medical devices to automotive and aerospace parts. The part information may be in the form of human readable alphanumerics and barcode or Data-MatrixTM codes. Easy and flexible automation, improved environmental profile, and low cost of ownership add to the benefits of the technology. A variety of laser technologies are available, depending upon the application, mark type needed, and material to be marked.
Laser marking basics
With laser marking, focused light from a laser interacts with a material to produce a high quality, permanent mark on an object. Hardware steers the focused beam using two mirrors mounted on high speed precision galvo motors to produce the mark. Each mirror moves along a single axis. These galvos move extremely quickly with very little inertia, and can therefore write marks at high speeds. The beam is focused using an F-Theta lens that produces a focus across the mark plane. The mark "appears" when the material on which it is being made is changed by the interaction with laser light. Other than the mark itself, the surface of the part being marked remains unchanged. FIGURE 1 is a generalized diagram of a typical laser marker.
|FIGURE 1. Typical laser marker.|
A laser marking system is usually made up of a highly flexible and controllable laser source that is coupled via control software and hardware to a very fast and accurate motion system. Frequently, laser marking systems not only mark, but offer machining capabilities, for example, cutting, drilling, polishing, scribing, and skiving.
The laser is equipped with software that enables the marking of text, graphics, logos, barcodes and Data-Matrix codes. Automation features enable part serialization, date coding, variable text inputs, remote programming, input/output control and many other programming features.
Lasers can make very precise, clean marks on both metals and plastics, at high speeds, making them suitable for most manufacturing processes. They can also be used for engraving, a similar process, but one that alters the part’s surface, leaving a mark that can be both seen and felt.
Laser marking is a non-contact process that uses no consumables, making it clean and energy-efficient. It is also an environmentally friendly process that uses no inks and chemicals like those used in other marking options, including inkjet or chemical etching. The process produces a highly flexible mark that can adapt to changing demands. It is also a high speed process, appropriate for a wide range of markable materials, including steels, aluminum, polyolefins, engineering and medical plastics, silicon, and many others.
The laser marking process is very well-suited to the marking of both barcodes and two-dimensional barcodes on many of these materials. The code quality and resolution can be tuned for the space available and the quality of the reader. Specifically, the two dimensional codes offer excellent data density and minimize the space needed for the code.
Selecting the right laser marker for an application
Several laser marking technologies are on the market and each one has specific marking characteristics that align best with particular materials and applications. The major choices are ytterbium:fiber (Yb:fiber); neodymium: vanadate (Nd:YVO4); green (532 nm); ultraviolet (UV); and carbon dioxide (CO2). In addition, neodymium-doped yttrium aluminum garnet (Nd:YAG) is an older technology that is still being used in the high power market, but has largely been superseded by one of the others listed.
Each of these lasers and types of laser materials create light at different wavelengths and with different optical properties, for example, peak power and pulse width. Both the green and UV lasers use the Nd:YVO4 laser engine, but employ additional materials to change the light wavelength from 1064 nm to 532 nm and 355 nm, respectively. Each laser wavelength is suited to different marking requirements based upon material, contrast, size of characters and thermal input to the part. FIGURE 2 provides an overview of laser marking technology by wavelength.
|FIGURE 2. Wavelengths suitable for laser marker technologies.|
Yb:fiber: Cost effective with good mark quality
The fiber laser, the latest laser design, uses a laser that is generated in an ytterbium-doped silica fiber. The pump energy is supplied by single emitter telecom style diodes, which have an extremely long estimated life, up to 100,000 hours. High energy efficiency and low maintenance requirements give the fiber laser the lowest cost of ownership. The Yb:fiber marker is probably the most commonly used laser marking technology on the market. The fiber marker offers high quality, contrasting marks on metals and plastics, engraving of metals, and machining of a wide variety of materials. FIGURE 3 shows a typical example of a fiber laser mark; the example shown is of glass-filled plastic.
|FIGURE 3. Fiber laser mark on plastic.|
Nd:YVO4: Fine laser marking
This laser marker is designed for high resolution fine marking, where very fine detail or small character sizes are needed. With excellent beam quality and a minimum focused spot size of less than 0.001 inches (25 microns), Nd:YVO4markers make fine contrasting marks possible on metals, plastics, and ceramics. The laser architecture enables very high peak powers — greater than 30 kW — and very short pulse widths — less than 20 ns, providing the level of control needed for high resolution marking.
Green laser markers: For certain plastics, silicon and reflective metals
Lasers in the visible green spectrum operate at a 532 nm wavelength, which provides increased contrast on plastics that do not have pigmentation, the ability to soft mark silicon, and high quality marking of such reflective materials as gold and silver.
UV: Extremely high resolution/contrast for plastics and corrosion resistant marking
The UV laser marker can be thought of like a green marker on steroids. Its 355 nm wavelength offers unique marking characteristics, providing excellent contrasting marks on many plastics that other lasers are unable to mark, for example those used for medical tubing, and highly reliable corrosion resistant marking of 17-X stainless steels. Compared to fiber and Nd:YVO4 lasers — and even the green laser — UV offers the best mark quality on almost all materials. The 355 nm wavelength further increases the range of plastics that can be marked with contrast, and also offers tight control of heat input when marking metals.
CO2: Best solution for printed circuit boards, paper and wood
The sealed CO2 laser has a laser medium that is a combination of gases sealed into an aluminum tube under reduced pressure. The laser pump energy is provided by radio frequency (RF) energy. The CO2 laser can mark plastics, however, the marks are engraved, with no surface contrast. The CO2 laser marker, operating at the 10604 nm wavelength, is used extensively for marking such organic material as paper and wood, as well as printed circuit board (PCB) material and glass. It is not suited for marking bare aluminum, copper or brass or producing quality marks on steels. FIGURE 4 shows the technology in use on a PCB.
FIGURE 4. Carbon dioxide laser marker.
Nd:YAG: Older laser engraving technology
Nd:YAG laser markers are predominately used for large area metal marking (surface effect) and deep engraving (depth effect). For these applications, raw power is the requirement. This technology has largely been superseded by other laser marking technologies, but is still widely used in high power (50-100 W) applications. Fiber laser technology is increasing in power, and is beginning to replace the older Nd:YAG in the high power market as the technology becomes obsolete and for the significant reduction in the cost of ownership.
With a variety of choices for use in almost any application that requires direct marking or engraving, laser marking technology offers an updated solution for marking processes. Benefits include superior permanent mark quality; highly flexible marks that can incorporate text, graphics, logos and data codes; and a wide range of markable materials. In addition, laser marking is a high speed process that is easily integrated into the manufacturing line. Safety is enhanced because laser marking is a non-contact process with good standoff distance, and the lack of chemicals makes it environmentally preferable.
Geoff Shannon, PhD, is Miyachi Unitek Corp.’s laser technology manager. He can be reached at ph 626/303-5676.