How lasers for 3D printing differ from cutting lasers
Experts at TRUMPF, Amada, EOS North America, and 3D Systems discuss the differences and similarities of lasers for additive and subtractive manufacturing
Over the past four decades laser cutters have become the must-have machine tool for the overwhelming majority of sheet metal fabricators.
From CO2 lasers to their newer and more capable solid-state cousins, what began as a 1960s-era laboratory experiment is now the preferred method of quickly and accurately cutting virtually any metal less than 1 inch thick.
It wasn’t long after lasers started appearing in fab shops everywhere, however, that another use was found for collimated light, one seemingly destined to play an equally important role in manufacturing.
In 1987, 3D Systems co-founder Chuck Hull sold the first SLA-1, a machine that uses an ultraviolet laser to cure thin layers of photoreactive resin. Additive manufacturing (AM), better known as 3D printing, was born.
Since that time 3D printing has grown from a photopolymer-only prototyping process into a viable method for end-use part production. With a host of available materials, including engineering-grade plastics, superalloys such as titanium and INCONEL® alloy, maraging steels, tool steels, and stainless steels, there’s little that 3D printers can’t produce. And while laser light isn’t the only technology used to cure, sinter, melt, or otherwise join these various materials, it’s definitely the leader of the AM pack.
Viva La Difference?
So how does a laser that can zip through 12-gauge steel at several hundred inches per minute differ from one used for on-demand printing of press brake tooling, assembly fixtures, and robot end effectors?
“For metal-based 3D printing, the industry mainly uses fiber lasers with a wavelength of 1,070 nanometers, which is in the infrared range, whereas a continuous-wave, solid-state fiber or disk laser with a wavelength in the range of 1,030 to 1,080 nm is typically used for cutting,” explained Dave Locke, sales specialist for additive manufacturing at TRUMPF Inc., Farmington, Conn.
Clearly the wavelengths are similar, but that’s not the case with wattages. The lasers installed in TRUMPF’s metal powder-bed-fusion (PBF) printers max out at 500 watts. Those used in the company’s lineup of lasers for cutting, conversely, generate up to 6 kilowatts—12 times the output. If that kind of laser power were placed in a 3D printer, it would burn a hole through the bottom of the machine.
It’s important to recognize, though, that laser power is only one operating parameter among many, regardless of whether metal is being fused together or ripped apart. Besides power, builders also strive to offer versatility. They go to great lengths, for example, to make their products “tunable” (adjustable to accommodate a range of materials), based on the alloy and thickness of the material being cut. TRUMPF’s BrightLine and CoolLine brands are tunable, as is Buena Park, Calif.-based competitor Amada America Inc.’s Ensis technology.
“A bell-shaped Gaussian waveform like that generated by fiber lasers provides a very high spot density and is what gives them the ability to rip through thinner-gauge materials,” explained Dustin Diehl, laser division product manager at Amada America. “This small spot size becomes less effective when you get into materials 1/2 in. and above, however, and it is the reason why CO2 lasers have long offered better speeds and higher edge quality in thicker materials.
“But as the technology has evolved, Amada (and others) has found ways to adjust the laser’s beam diameter and waveform to create a larger spot—one shaped more like a top hat. This, among other reasons, is why Amada has switched entirely to fiber for its standard lineup of laser cutting machines,” said Diehl.
Shining Through Snow
Does this mean that CO2 lasers are headed for the history books? Hardly. “Polymers are largely transparent to a fiber laser’s shorter wavelengths, so using one in a 3D printing application is a little like shining a flashlight through snow,” said Damien Gray, principal laser optical engineer at EOS North America Inc., Pflugerville, Texas. “It goes right through. The light from a CO2 laser, on the other hand, is strongly absorbed by most polymers.”
That’s good news for selective laser sintering (SLS)—metal PBF printing’s plastic alter ego. But there is a caveat: CO2 lasers offer lower print resolution and can’t make features as small as fiber lasers can.
David Cullen, director of applications engineering at 3D Systems Inc., Rock Hill, S.C., explained that the CO2 laser found in his company’s SLS printers has a wavelength of around 10,600 nm, which is 10 times that of a typical fiber laser. And as wavelength increases, so does spot size.
“You’re looking at 475 microns, or about half-a-millimeter print resolution— substantially larger than that found in metal powder bed machines,” he said. “The upside is that SLS print speeds are among the fastest in the industry.”
Two Spots Beat One
Like makers of laser cutting equipment, manufacturers of 3D printers use advanced optics and electronics for on-the-fly modification of laser parameters—at least in some instances.
The SLS machine just discussed has a fixed beam size, but Cullen said that stereolithography (SLA)—the technology on which his company was founded—now offers two spot sizes.
“By tweaking the crystal orientation in the Nd:YAG laser, we can generate a big spot for rapidly scanning large areas on the inside of the part, and then dynamically switch to a small spot for fine features and for tracing the outline,” he said. “The result is better part quality and faster build speeds.”
Ankit Saharan, manager of applications development and research and development at EOS, agreed with the importance of controlling spot size, but added that there’s far more to the additive laser story than the size of the spot and the wavelength or type of laser used to create it.
“A small spot size is better than a larger one, because the smaller the spot, the smaller the melt pool, [meaning] less stress in the workpiece,” said Saharan. “We start at around 45 µm on smaller platforms and go up to 100 µm for larger platforms.”
However, smaller spot sizes also mean lower deposition rates, so a balance must be maintained between process stability and cost. There are numerous other factors at play, too, like layer thickness, powder grain size, powder delivery and application mechanisms, reflectivity of the raw material, and assorted machine parameters.
Saharan said, “It’s a very complex scenario, and a high-quality laser—though important—is only part of what’s needed to create a stable [additive] process.”