Photonic technology is the bedrock of additive manufacturing

Additive manufacturing, more commonly known as 3D printing, has transformed all manufacturing sectors from micro-precision medical components to large-scale load-bearing structures. In stark contrast to traditional subtractive machining that relies on removing material from a substrate to produce the final product, additive manufacturing makes 3D physical objects via layer-by-layer deposition.

As a result, additive manufacturing has many advantages over subtractive manufacturing, including reduced waste, elimination of product-specific tooling, and design advantages in terms of increased internal complexity. Technology has even advanced to the point where companies are developing fully 3D-printed engines and rockets. According to their website, Relativity Space (Los Angeles, CA) is now “developing the first autonomous rocket factory and launch services for satellites.”

In addition to the engineering and production advantages of additive manufacturing, it is also integral to the future of sustainable manufacturing. The most apparent way additive manufacturing techniques increase sustainability is through reduced waste compared to subtractive manufacturing, but this is not the only factor. Perhaps even more groundbreaking is the current drive to plant-based photocurable bio-resins (see Fig. 1). The combination of reduced waste, sustainable material production, and increasingly complex production geometries has all but solidified 3D printing as the new standard in advanced manufacturing.

It is impossible to understate the importance that photonics has played in all of this. Not only are lasers, LEDs, and other light sources used in layer deposition for melting and curing, but many advanced additive manufacturing systems also rely on spectral sensors. These sensors can be directly integrated into the printing head to actively monitor the process or be used for spot checks before and after production.

To illustrate how photonics intertwines with modern 3D printing technology, we are taking a 30,000-foot view of various printing techniques in order to give a deeper understanding of how photonics is applied to enable this recent additive manufacturing revolution.

It is impractical to discuss every possible 3D printing variant available on the market. So instead, we will focus our attention on the top two main photonics-based techniques—direct metal deposition and vat photopolymerization. We’ll also briefly look at a relatively new approach, two-photon polymerization, which is capable of producing objects with submicron resolution.

Direct metal deposition

Over the years, we have covered direct metal deposition 3D printing rather extensively, often under the name laser additive manufacturing.^2,3 While the terms direct metal deposition and laser additive manufacturing can be used interchangeably, as more and more laser-based additive manufacturing methods are being developed, the more generic classification has fallen out of favor.

Even the term direct metal deposition itself is a blanket term that can include a wide range of different technologies—the two most popular being laser powder-bed fusion and directed energy deposition. Both rely on high-powered CO₂ or fiber lasers to melt and fuse metal powders layer-by-layer during the print process.

The primary difference between the two approaches is that in directed energy deposition, the laser is focused on a fixed location with powder jets arranged at the appropriate angle to intersect at the focus of the laser (see Fig. 2). The material substrate is placed on an X-Y-Z translation stage and then scanned underneath the laser during fabrication.

FIGURE 2. Picture of a fiber-optic cable and collection optics integrated into a directed energy deposition printing head [4].

By contrast, in laser powder-bed fusion, the whole platform is uniformly covered with a thin layer of powder. Next, the laser is scanned across the bed, using galvo mirrors and a f-theta lens, tracing out each layer of the print fusing the powder as it goes. Finally, the platform is shifted downward, and the process repeats until the construction of the final part is complete.

Directed energy deposition processing is by far the dominant approach for direct metal deposition additive manufacturing because of its high throughput, low waste, and larger build volume. However, it is important to note that laser powder-bed fusion is still used for high-precision applications since the layering height can be better controlled in the powder bed compared to the powder jet.

In addition, the fixed geometry and extreme temperatures generated during manufacturing also make directed energy deposition printing heads more suitable for inline monitoring via optical emission spectroscopy (OSE).

Figure 2 shows an example of a print head that collects emitted light using a fiber-optic cable oriented at a 60° angle relative to the laser’s optical axis. This print head was developed at the University of Brussels (Brussels, Belgium) to monitor the color temperature of the melt pool using an Avaspec-3648-USB2 spectrometer from Avantes (Apeldoorn, Netherlands).⁴

This relatively simple design provides users with a noncontact control system to prevent excessive heat transfer in the substrate, reducing the material stress and decreasing defects. In addition, researchers at Pennsylvania State University (State College, PA) and the University of Michigan (Ann Arbor, MI) developed printing heads with similar optical geometries to monitor atomic emission lines for real-time material characterization and defect detection.^5,6

Vat photopolymerization

Vat photopolymerization relies on photochemical reactions to convert a viscous liquid monomer into a solid polymer. Since this is a photochemical reaction, it requires relatively low optical power, allowing for a diode laser, LED, or other high-brightness UV light sources, making it ideal for benchtop applications. The most important parameter here is the wavelength, which needs to be short enough to provide enough photon energy to initiate the polymerization reaction.

While a bit of an oversimplification, it can be helpful to think of vat photopolymerization as analogous to an inverted laser powder-bed fusion printer. Instead of the part being lowered and filled over with additional powder, in vat photopolymerization, the build platform is pulled out of the resin tank, extruding the part along with it. As such, the z-resolution is almost entirely dependent on the build platform’s stepper motor speed resolution. This parameter is typically adjustable, allowing the user to balance production speed and resolution.

There are two main types of vat photopolymerization 3D printers, commonly referred to as stereolithography (SLA) and digital light processing (DLP) printers. Both approaches utilize the bottom-illuminated resin tanks, but with very different illumination designs. SLA printers use a focused laser and raster scanned across the thin layer of liquid photoresin sandwiched between the build platform and the bottom of the resin tank, again analogous to an inverted powder-bed printer.

DLP printers, on the other hand, use a divergent light source projected on the bottom of the resin tank with a MEMS mirror array. Projecting a “digital” image of the entire layer to cure the whole layer without needing a scanning laser simultaneously. DLP offers a massive speed advantage over SLA printers, but at the cost of build volume since it is limited to the size of the projected image.

Even though the illumination mechanisms are different—spot size of a focused fiber-coupled 405 nm laser vs. the minimum pixel size of a DLP projector—both technologies offer comparable X-Y resolution on the order of 25 to 100 µm. The primary “print quality” differentiation of these two approaches comes in terms of surface finish. Since DLP projectors produce a 2D pixelized image at the resin, it results in a 3D pixelized print surface, often referred to as a voxel in graphical design, resulting in a blocky or stepped surface finish. It should be noted that this effect is only noticeable on small and/or highly detailed parts.⁷

Joseph Stanzione and his research team at Rowan University (Glassboro, NJ) demonstrated the efficacy of this method to 3D print with bio-resins using the Form 2 SLA 3D printer with a Formlabs (Somerville, MA) fiber-coupled 250 mW 405 nm laser (see Fig. 3). To validate their results, they used near-infrared (near-IR) spectroscopy to monitor the rate-of-cure under different conditions. They quantitated this by rationing the absorbance peak at 6165 cm^-1 (polymerizable methacrylate) to an unreactive control peak at 5900 cm^-1 measured with a Nicolet iS50 FT-IR from Thermo Fisher Scientific.

FIGURE 3. Images of Form 2 SLA 3D printer from Formlabs (a), a 3D-printed owl using a biobased vinyl ester resin (b), and near-IR spectra of the resin under different curing conditions (c) [1].

After comparing the spectra of the pre-cured resin (1P2S MV-GMA Precure) and thermally cured resin (1P2S MV-GMA thermal) with the 3D-native printed part (1P2S MV-GMA AM) and a post-cured 3D printed part (1P2S MV-GMA AM-FC), the team was able to determine that the optimal cure (88% ±1%) occurred after the printed part was post cured for 2 hours at 80°C under 405 nm excitation.

While this work was all done ex vivo using a Fourier transform interferometer, there is no fundamental reason that a reflective-based dispersive near-IR spectrometer could not be integrated into the print head itself, like the OES monitoring system shown in Figure 2.

Two-photon polymerization

One of the newest and most exciting additive manufacturing processes is two-photon polymerization. This process takes advantage of the fact that most photoresins require an excitation source at roughly 400 nm to polymerize. Therefore, a low average-power near-IR ultrafast laser such as a Ti:sapphire or mode-locked fiber laser can be focused through a resin, only curing it at the focus, where the intensity is great enough to second harmonic generation.

While less mature than the previous technologies, it has received a lot of attention in the photonics community after a team at the University of Stuttgart (Stuttgart, Germany) recently took minimization to a whole new level when producing ultra-miniature 3D-printed spectrometers with a less than 100 × 100 × 300 μm³ total footprint.^8,9

In 2019, a team at The Chinese University of Hong Kong (Shatin, Hong Kong) succeeded in combining two-photon polymerization with a MEMS-based digital micromirror device to produce a 3D printer containing multiple diffraction-limited laser foci.¹⁰ Their system was capable of printing objects with ~500nm resolution at an astonishingly high print speed of 22.7 kHz.

According to the authors, this is the highest fabrication speed without negatively impacting resolution to date. The authors explained that their “system presents distinctive advantages in precisely controlling the focus position (~100 nm) and laser dosage (i.e., grayscale control), thereby enabling the design and creation of complex 3D photonic structures, topologically optimized mechanical devices, and many other structures (for example, overhanging structures) that are difficult to fabricate through conventional raster-scanning-based systems, bringing significant impact to the world of nanomanufacturing.”

Figure 4 shows a stunning example of just how powerful two-photon polymerization 3D printing can be, comparing a CAD model of the London Bridge with the scanning electron microscopy (SEM) image of the 3D-printed final part. The total size of the 3D-printed part was only 120 × 14 × 60 µm³.

FIGURE 4. CAD model of the microscale London Bridge (a) and an SEM image of the two-photon polymerization-fabricated London Bridge with a 10 µm scale bar (b) [10].

Looking to the future

When a group of industry experts was asked their views on the future of additive manufacturing Naresh Shanker, CTO at Xerox, summed it up nicely by saying: “Over the next decade, 3D printing will be fully integrated with traditional manufacturing—additive manufacturing will be a mainstream element of most assembly lines. Faster, more robust 3D printing that creates consistent, high-quality output will power large-scale production on par with traditional techniques like casting and injection molding.”¹¹

Between the cost savings, expanded design capability, and sustainability offered by 3D printers, there is little doubt that Shanker’s prediction will come true, just as there is little doubt that photonics will continue to be a key factor in the successful development and refinement of additive manufacturing techniques for many years to come.

ACKNOWLEDGMENT

The author would like to thank Joseph Stanzione, Associate Professor of Chemical Engineering and Director of the Advanced Materials and Manufacturing Institute at Rowan University, for his input on the current state of additive manufacturing technology and several of the images provided for this article.

REFERENCES

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11. See https://bit.ly/LFW-AdditiveRef11.