Lasers and LEDs Layer on New Capabilities for Additive Manufacturing

HANK HOGAN, CONTRIBUTING EDITOR

Additive manufacturing, aka 3D printing, offers an alternative to conventional manufacturing and assembly methods by building products layer by layer. In addition to enabling heretofore impossible part geometries, the technology also hints at a future in which items could be fabricated anywhere, as needed, from a digital blueprint.

Detail (top) and side view (bottom) of a 3D-printed high-performance shoe midsole produced with an LED projection system in <20 min. Courtesy of LuxCreo.

When applied to polymer materials, the technology is finding application in products ranging from footwear to automobile components. When applied to ceramics, it’s shaping custom dental products. Additive manufacturing is even extending to the fabrication of metal parts designed for medical instruments, cars, and aircraft. Today, most 3D-printing applications target rapid prototyping because the technology can produce sample parts quickly from computer models that can easily be customized and shared. Though less common, the manufacturing technique is increasingly being used to produce finished products, allowing for greater customization, reduced assembly costs, streamlined supply chains, and easier inventory management.

While the underlying bonding mechanism may vary from one platform to another, all additive manufacturing platforms bind raw materials into a finished product by applying some form of energy. Methods such as selective laser sintering (SLS) and digital light processing (DLP), for example, employ photonic energy to shape materials in the x, y, and z axes. Cameras and other photonic sensors also play a role in the inspection and quality control aspects of 3D printing (see sidebar).

Printing metal parts

Given additive manufacturing’s fast-expanding versatility, analysts project strong growth for the technology. According to a November 2020 report from SmarTech Analysis, the market for service bureaus that print metal parts will alone grow from ~$2.2 billion in 2020 to ~$10.2 billion in 2029. Demand from the aerospace industry is a significant driver of this growth.

Denver-based Boom Supersonic, for example, is building Overture, which the company claims will be the world’s fastest passenger aircraft. As befits a speedy and innovative plane, the company is employing advanced technologies to accelerate product development for the XB-1, Boom’s supersonic demonstrator plane that is scheduled to begin flight tests this year.

“Boom has leveraged 3D printing to rapidly manufacture hundreds of printed parts, tools, and prototypes, and saved thousands of hours in work time,” said Mike Jagemann, head of XB-1 production.

The company built 21 flight hardware components using powder bed fusion printers from Velo3D Inc. that incorporate 3D metal printing systems that use two 1-kW lasers. These systems scan the laser beam horizontally over a bed of fine metal powder to melt and fuse the particles into very selective and localized structures. The powder bed is incrementally stepped vertically to fabricate each new layer of the desired part, and fresh metal powder is added as needed.

Challenging demands are made on the lasers used in powder bed fusion 3D printing, according to Roland Spiegelhalder, product manager for additive manufacturing at TRUMPF, which makes its own 3D printers and laser systems.

Fiber lasers emitting around 1020 nm are commonly used for binding metal powders because of the lasers’ beam quality, Spiegelhalder said. They deliver energy through a fiber optic that distributes the beam across a build plate.

Beam qualities such as power level and spot size are interrelated with throughput because of basic physics, he said. “You just need to have a certain volume energy product to melt your material.”

So a beam focused to a 30-µm spot may only need 100 W of power to hit the metal’s melt threshold and ensure consistent density and other physical properties in the final product. A more tightly focused beam further allows 3D printing of finer features. However, a small spot size also requires more passes and a longer processing time to build up the layers of larger parts.

A total of 21 3D-printed titanium components — such as the one shown in the foreground — are on the XB-1, a supersonic demonstrator plane from Boom Supersonic that will undergo flight testing this year. Courtesy of Boom Supersonic.

Moving up in beam size and defocusing the beam may increase spot size to 500 µm, which would permit fewer passes and faster throughput. However, the laser’s power level would need to be increased to kilowatt levels to achieve the necessary melt temperature. Another option is to use multiple beams to speed up the build process, with individual lasers each making a section of the overall component.

Currently, larger parts can take up to five days to print. During that time, the beam spot may traverse tens or hundreds of miles of build track, which makes high reliability of the laser and system critical to success, Spiegelhalder said.

Different material processing needs often require a wide variety of wavelengths, he said. Copper, a key component in the electrical power systems of vehicles, is a highly reflective metal that only absorbs about 3% of incident energy in the infrared. Using a green laser operating at 515 nm results in 50% absorption and makes 3D printing of copper via powder bed fusion much more feasible.

Methods and materials

While the ability to 3D-print metal parts significantly expands the applications for additive manufacturing, most printed parts today use polymers or composites as the medium.

A common method used to fuse polymers together is selective laser sintering, which is a subset of powder bed fusion methods. The printers manufactured by Poland-based Sinterit offer an example of this approach. The company’s systems focus a 5-W, 808-nm diode laser into a 400-µm spot to sinter nylon and other polymer powders together into finished prototypes and products, according Michal Grzymala, Sinterit’s chief technology officer. The diode lasers used are small, stable, and reliable devices — a set of properties that aligns with the advantages offered by SLS technology. “It makes it possible to print objects that are difficult or impossible to be printed in FDM [fused deposition modeling] techniques — the most popular technology in the 3D-printing market,” Grzymala said.

He would like to see better beam quality from the lasers used, he said, because this would help improve the manufacturing process and the resulting final product.

LuxCreo Inc. of Belmont, Calif., is another 3D printer manufacturer targeting polymer-based applications, but the company’s systems rely on LEDs instead of lasers, according to Michael Strohecker, the company’s chief revenue officer. LuxCreo achieved this capability by developing its own proprietary photoinitiated polymer that responds to light ranging from 385 to 405 nm, which allows its printers to project 405-nm LED light in a 2D cross section to cure the photopolymer layer by layer.

Unlike raster scanning, projection exposes a large area all at once. “The result is an order of magnitude or more increase in speed,” Strohecker said.

One of LuxCreo’s projection methods bounces the LED light off of a digital light processing micromirror chip to achieve a pattern of 75-µm spots, each with an irradiance of ~12 mW/cm2. In another approach, the LED shines through an LCD panel, resulting in an equivalent spot size but each with about half the intensity. Photopolymer properties, as well as exposure intensity and wavelength, determine throughput.

The company’s technology can print a pair of high-performance shoe midsoles in less than 20 min but cannot achieve the look and feel of traditional textiles, Strohecker said. However, he predicted that a smaller spot size would improve the resolution of printed features and potentially lead to 3D products similar in appearance to fabric.

In addition to speeding the prototyping process for fueling and flight hardware, additive manufacturing allows aerospace manufacturers to fabricate parts for cabin interiors, flight decks, and galley applications. Courtesy of Boom Supersonic.

Another innovation Strohecker hopes to see is an efficient UV source that is able to operate below 405 nm with greater intensity and better resolution than current technology. This would enable LuxCreo’s projection 3D printers to support a broader range of materials.

Vienna-based Lithoz uses a similar DLP-based projection technology for photoinitiated 3D-printing of ceramics, according to Katharina Hofhansl, from the company’s sales and marketing team. Lithoz uses a blue LED-based light engine — with a pixel size as small as 25 µm — in its systems. Applications for this technology include dental crowns and veneers, as well as medical device components, bone replacements, and heat exchangers.

“Our machines are mostly used for mass customization in medical and dental applications, to small series — up to 100,000 parts — production, to proto- typing and research,” Hofhansl said.

High-end projection systems are a must for Lithoz’s 3D-printing technology, she added. Such systems make it possible to achieve consistently high quality over the entire construction area, thereby enabling the manufacture of reproducible precision components.

Shaping the future

Photonic sources are not the only or even the fastest-growing engines that are powering industrial-grade 3D printers. However, photonic-based additive manufacturing methods — such as SLS- and especially DLP-based printers — are reliable, offer good economies of scale, and are comparatively fast.

3D-printed copper parts fabricated using a green laser. Highly reflective metals, such as copper, absorb green light better than infrared, making green lasers the favored source for 3D-printing shiny materials. Courtesy of TRUMPF.

The layers deposited by SLS technology exhibit strong adhesion, making it an appealing option for producing structural or functional parts with complex geometries. And SLS offers the flexibility to print custom parts from metals, ceramics, and composite materials, in addition to polymers. While DLP systems cannot print as many materials, this additive manufacturing approach can also produce strongly adhering layers.

Conversely, SLS and DLP systems can be among the more expensive and complex options for additive manufacturing. This is largely due to the photonic components and the nature of the processes.

But while these drawbacks raise barriers to the 3D printer market for the home and hobbyist, such shortcomings are less relevant to industrial users who have the resources and know-how to benefit from the more functional and complex parts that photonics-based techniques enable.

Additive Manufacturing Needs Vision

As with any manufacturing process, 3D-printing systems must produce repeatable and exacting parts. This often requires quality inspection and process control that, when combined, help ensure the proper linking together of microscopic volumes, along with optimal performance in the final product. Sensors for inspection and control can range from simple photodiodes to sophisticated vision systems, according to Shuchi Khurana, CEO of Addiguru.

A part made by a laser powder bed fusion 3D-printing process. Anomalies (marked by red boxes) are detected using visible imaging and AI algorithms. Courtesy of Addiguru.

The company provides a software platform that enables real-time monitoring of additive manufacturing processes when coupled with 15-plus-MP cameras and optics that enable resolutions of 30 µm per pixel. With this setup, Addiguru’s system spots pore irregularities and other anomalies, either during the development of a production process for a part, or as a post-mortem after a part failure.

While Addiguru’s current solution relies on cameras operating in the visible spectrum, the company is considering how to integrate IR cameras, Khurana said. The temperature profile for 3D-printed parts could help predict the microstructure of metals and other materials, thereby allowing the forecast of parameters such as fatigue life.

“Additive manufacturing processes can be like cars driving at night without headlights,” Khurana said about the importance of sensors in process development and control.

Innovations in photonics-based sensors and sources thus play an essential role in the success of 3D-printing.