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3-D Printing

3-D printer can build meter-tall objects in just a few hours

Cooling system helps printer to build bigger structures in less time

by Mark Peplow
October 25, 2019

20191025lnp3-harp.jpg
Credit: Northwestern University
The cooling system in the bottom of this tank helps to increase the throughput of the 3-D printer.

A 3-D printer that stands 4 m tall can rapidly print large structures thanks to its innovative cooling system (Science 2019, DOI: 10.1126/science.aax1562).

Three-dimensional printing has long been used for making one-off prototypes, but it’s beginning to move into manufacturing applications. The team behind the new printer, led by Chad A. Mirkin of Northwestern University, thinks the technology could help to accelerate that trend, by producing parts for medical devices, cars, airplanes, and other uses.

The device relies on stereolithography, a form of 3-D printing that uses ultraviolet light to trigger polymerization reactions in a liquid resin to create a solid 3-D structure. In conventional stereolithography printers, UV light shines into the resin tank in specific patterns to build the desired structure layer by layer. Speedier stereolithography systems direct the UV light through a transparent window at the bottom of the tank, and use a moving plate as a platform for the build, so that the object grows continuously on the underside of the plate as it rises out of the reservoir.

Credit: Northwestern University
The HARP 3-D printer can produce a 1.2-meter-tall structure in about 3 hours. This timelapse video was sped up 100 times and edited.

One problem with this kind of continuous printing is that the polymerization reactions produce a lot of heat, which can crack or deform the growing structure, or even risk setting fire to the resin. “There’s a huge heat dissipation problem,” Mirkin says. Since faster processes that print larger structures generate even more heat, “people either go slow and big, or small and fast”, he says.

In contrast, Mirkin’s 3-D printer offers a way to build big structures more quickly. In its reservoir, liquid resin floats on top of a layer of fluorinated oil, which doesn’t mix with the resin. This perfluoro-polyether copolymer flows across the tank and through a heat exchange system, to dissipate heat during the printing process. The oil is also filtered to remove fragments of plastic generated during the process, which can scatter light and decrease the resolution of the printing. As with other continuous printing systems, UV light enters the tank through its transparent base.

The researchers used their device to build a 38 cm x 61 cm x 76 cm lattice structure out of urethane acrylate resin, taking just 105 minutes to complete the build. That is the highest throughput achieved by any stereolithography system, Mirkin says.

The printer could also build structures from a stretchy butadiene rubber, and a silicone polymer that was then pyrolized to create a durable silicon carbide ceramic. Thermal imaging showed that the system kept the temperature of the emerging structures below 120 °C, and unlike rival methods the new system is compatible with oxygen-sensitive reagents.

This versatility is a key advantage of the new system, says Timothy F. Scott at Monash University, who works on 3-D printing technology. “It enables printing for pretty much any photopolymerizable resin chemistry—that’s cool,” he says.

However, heat dissipation is not the only challenge these systems face. For example, Scott suspects that Mirkin’s device might struggle with high-viscosity resins, because fresh resin may not be able to flow into place quickly enough to keep up with the speed of the build. “That’s a genuine challenge,” Scott says, who also notes that the fluorinated coolant is very expensive.

Along with former Northwestern colleagues David A. Walker and James L. Hedrick, Mirkin has founded a company called Azul 3D to commercialize the technology, dubbed HARP for high-area rapid printing. He says the company has already improved the technology beyond the capabilities described in their paper. “They are now through several iterations, refining the technology to make it bigger, faster, and increasing the quality of the parts that are produced,” he says.

Mirkin hopes that HARP could ultimately be used for mass manufacturing components. “When you can go this big and this fast, you can completely change the business proposition of 3-D printing,” he says.

“Even if you don’t necessarily need a large cross-sectional area, it enables you to make a whole load of parts all at once,” Scott says. “From an engineering perspective, there’s enormous value there.”

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