AM ResearchVolumetric 3D Printing

The race towards volumetric 3D printing speeds up

The latest efforts from Harvard, Stanford, Berkeley and LLNL focus on glass and... jelly

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3D Printing Business Media has been following the efforts to bring volumetric 3D printing technology to market for some time now. We appear to be in a very exciting period – seeing many technological advancements within this novel technique. Volumetric Additive Manufacturing (VAM), presented as a “near-instant 3D printing technique”, removes the need for the support structures typically required for creating complex designs with more standard printing methods and could make it easier to print increasingly intricate designs while saving time and material.

Volumetric 3D printing with glass

Among the latest efforts Using a new laser-based VAM approach researchers at Lawrence Livermore National Laboratory and the University of California, Berkeley have demonstrated the ability to 3D print microscopic objects in silica glass, part of an effort to produce delicate, layer-less optics that can be built in seconds or minutes. The results are reported in the latest edition of the journal Science.

Nicknamed ‘The Replicator’, after the fictional device in Star Trek that can instantly fabricate nearly any object, the Computed Axial Lithography (CAL) technology developed by LLNL and UC Berkeley is inspired by computed tomography (CT) imaging methods. CAL works by computing projections from many angles through a digital model of a target object, optimizing these projections computationally, and then delivering them into a rotating volume of photosensitive resin using a digital light projector. Over time, the projected light patterns reconstruct, or build up, a 3D light dose distribution in the material, curing the object at points exceeding a light threshold while the vat of resin spins. In an ideal volumetric 3D printing setup, the fully formed object materializes in mere seconds – far faster than traditional layer-by-layer 3D printing techniques – and then the vat is drained to retrieve the part.

Combining a new microscale VAM technique called micro-CAL, which uses a laser instead of an LED source, with a nano-composite glass resin developed by the German company, Glassomer, and the University of Freiburg, UC Berkeley researchers reported the production of sturdy, complex microstructure glass objects with a surface roughness of just six nanometers with features down to a minimum of 50 microns.

The race towards volumetric 3D printing - laser-based and upconversion. The latest efforts from Harvard, Stanford, Berkeley and LLNL.
Taking inspiration from computed tomography, the Computed Axial Lithography (CAL) 3D printing method uses projected photons to illuminate a syrup-like resin, creating a continuously shifting video of projections as the vial rotates. Like a CT scan done in reverse, the projections combine to form a 3D object that is suspended in the resin. Here, a CAL system projects light into a photosensitive resin to produce a component. Photo by Hossein Heidari/UC Berkeley

UC Berkeley Associate Professor of Mechanical Engineering, Hayden Taylor, the project’s principal investigator, said the micro-CAL process, which produces a higher dose of light and cures 3D objects faster and at higher resolution, combined with the nano-composite resins characterized at LLNL proved a “perfect match for each other,” creating “striking results in the strength of the printed objects.”

“Glass objects tend to break more easily when they contain more flaws or cracks or have a rough surface,” Taylor said. “CAL’s ability to make objects with smoother surfaces than other 3D printing processes is, therefore, a big potential advantage.”

“You can imagine trying to create these small micro-optics and complex microarchitectures using standard fabrication techniques; it’s really not possible,” said LLNL co-author, Caitlyn Krikorian Cook, a group leader and polymer engineer in the Lab’s Materials Engineering Division, “And being able to print it ready-to-use without having to do polishing techniques saves a significant amount of time. If you can eliminate polishing steps after forming the optics – with low roughness – you can print a part ready for use.”

A microscopic object 3D printed in silica glass using VAM

Cook and the UC Berkeley researchers said VAM-printed glass could impact solid-glass devices with microscopic features, produce optical components with more geometric freedom and at higher speeds, and could potentially enable new functions or lower-cost products.

Real-world applications could include micro-optics in high-quality cameras, consumer electronics, biomedical imaging, chemical sensors, virtual reality headsets, advanced microscopes, and microfluidics with challenging 3D geometries such as ‘lab on a chip’ applications, where microscopic channels are needed for medical diagnostics, fundamental scientific studies, nano-material manufacturing, and drug screening. Plus, the benign properties of glass lend themselves well to biomaterials, or in cases of high temperature or chemical resistance, Cook added.

Upconverting volumetric jelly

In other University news, Dan Congreve, an assistant professor of electrical engineering at Stanford and former Rowland Fellow at the Rowland Institute at Harvard University, and his colleagues have developed a way to print 3D objects within a stationary volume of resin. The printed object is fully supported by the thick resin (imagine an action figure floating in the center of a block of Jell-O) so it can be added to from any angle – removing the need for support structures. The report on this type of volumetric 3D printing system has been published in the journal Nature.

The jelly container approach is not entirely new, with MIT and BMW working on an other similar technology for producing urethane parts. The key component in the Harvard researchers’ novel volumetric 3D printing design is turning red light into blue light by adding what’s known as an upconversion process to the resin, the light reactive liquid used in 3D printers that hardens into plastic.

Congreve’s lab specializes in converting one wavelength of light to another using triplet fusion upconversion. With the right molecules in close proximity to each other, the researchers can create a chain of energy transfers that, for example, turn low-energy red photons into high-energy blue ones.

The race towards volumetric 3D printing - laser-based and upconversion. The latest efforts from Harvard, Stanford, Berkeley and LLNL.
Changing the contents of the nanocapsules controls the power of red light required to cure the resin and allows for different types of volumetric printing. Two types of printing shown here: scanning and tracing an image using a high-powered laser (on left) and projecting an image all at once using a low-powered LED (on right). (Image credit: Dan Congreve/Tracy H. Schloemer/Arynn O. Gallegos)

“I got interested in this upconversion technique back in grad school,” Congreve said. “It has all sorts of interesting applications in solar, bio, and now this 3D printing. Our real specialty is in the nanomaterials themselves – engineering them to emit the right wavelength of light, to emit it efficiently, and to be dispersed in resin.”

Through a series of steps (which included sending some of their materials for a spin in a Vitamix blender), Congreve and his colleagues were able to form the necessary upconversion molecules into distinct nanoscale droplets and coat them in a protective silica shell. Then they distributed the resulting nano-capsules, each of which is 1000 times smaller than the width of a human hair, throughout the resin.

“Figuring out how to make the nano-capsules robust was not trivial – a 3D-printing resin is actually pretty harsh,” said Tracy Schloemer, a postdoctoral researcher in Congreve’s lab and one of the lead authors on the paper. “And if those nano-capsules start falling apart, your ability to do upconversion goes away. All your contents spill out and you can’t get those molecular collisions that you need.”

The race towards volumetric 3D printing - laser-based and upconversion. The latest efforts from Harvard, Stanford, Berkeley and LLNL.
a–d, Side and top views of our final benchmark boat (Benchy) print cured using monovoxel excitation printing, sitting on a dime for scale. The scale bar denotes 5 mm. e, f, Top and side views of the Benchy STL file. The side and top views of the final print show the faithful reproduction of the main features. g, h, Top views of the Stanford logo (g) and gear (h) prints cured using large-area, two-dimensional parallel excitation printing, sitting next to a dime for scale. The scale bars denote 5 mm. i, j, Microscope images of lines printed in about 0.1 ml of resin (i) with light projected through the United States Air Force Target Test mask (j). The scale bar denotes 500 μm. Source: Nature Journal

“What we were wondering is, could we actually print entire volumes without needing to do all these complicated steps?” said Congreve. “Our goal was to use simply a laser moving around to truly pattern in three dimensions and not be limited by this sort of layer-by-layer nature of things.”

In 3D printing, resin hardens in a flat and straight line along the path of the light. Here, the researchers use nano-capsules to add chemicals so that it only reacts to a certain kind of light — a blue light at the focal point of the laser that’s created by the upconversion process. This beam is scanned in three dimensions, so it prints that way without needing to be layered onto something. The resulting resin has a greater viscosity than in the traditional method, so it can stand support-free once it’s printed.

“We designed the resin, we designed the system so that the red light does nothing,” Congreve said. “But that little dot of blue light triggers a chemical reaction that makes the resin harden and turn into plastic. Basically, what that means is you have this laser passing all the way through the system and only at that little blue do you get the polymerization, [only there] do you get the printing happening. We just scan that blue dot around in three dimensions and anywhere that blue dot hits it polymerizes and you get your 3D printing.”

The researchers, who included Christopher Stokes from the Rowland Institute, plan to continue developing the system for speed and to refine it to print even finer details. The potential of volumetric 3D printing is seen as a game-changer because it will eliminate the need for complex support structures and dramatically speed up the process when it reaches its full potential.

“We’re really just starting to scratch the surface of what this new technique could do,” Congreve said.

Research 2021
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Edward Wakefield

Edward is a freelance writer and additive manufacturing enthusiast looking to make AM more accessible and understandable.

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