3D printing, which consists of extruding material and then additive manufacturing, is an up-and-coming technique to fabricate thermoplastic components. However, one the main problems is that successive filament traces are only weakly welded together, something that often leads to delamination and mechanical failure of the finished product.

“3D-printed thermoplastic parts are usually very weak in the vertical direction because each successive layer does not weld to the layer below,” explains team leader Micah Green. “Simple heating cannot solve this problem but by coating the 3D printer filaments with multi-walled carbon nanotubes (MWCNTs) that heat up when irradiated with microwaves, we can locally heat the sample at each weld and create high strength materials.”

LIRF welding

Green and colleagues’ new technique for welding 3D-printed thermoplastic interfaces, dubbed “locally induced radio-frequency (LIRF) welding”, improves the weld fracture strength by as much as 275%.

In their experiments, the researchers coated thermoplastic filaments with a MWCNT-rich polymer film. Once these filaments have been 3D printed into macroscopic structures, the CNTS are localized only at the interfaces between each filament trace. When exposed to microwaves, the CNT-loaded interfaces selectively heat up, causing the polymer to flow and then entangle across the interface. Once it has cooled back to room temperature, the polymer has a stronger structure overall.

Scalable technology

The team, which also includes scientists from Essential Materials and TriFusion LLC in Texas, fabricated its new coaxial filament structure in a bath coating process in which a thin MWCNT “ink” is applied to the exterior of an engineering-grade polylactide (PLA) 3D-printer filament feedstock. A similar type of coated filament structure may be produced by coextruding a CNT melt master batch over a neat polymer core. This means that the technology can be scaled up to produce industrial amounts of the materials. The ink itself is made by sonicating the MWCNT/polymer mixture in chloroform.

The low melt temperature of the PLA allowed the researchers to perform the experiments on a standard open-air desktop 3D printer. Optical microscopy images confirmed that the as-printed structure indeed contained MWCNT-loaded interfaces.

Load-supporting structures with complex shapes now possible

“Being able to print materials with increased weld strength opens up a wide range of new application areas for 3D-printed thermoplastics,” Green tells nanotechweb.org. “The increase in 3D-printed weld strength means that MWCNT coatings on polymer filaments, followed by LIRF heating, can be used to make structures with properties approaching those made in conventional manufacturing processes, something that will allow for load-supporting structures with complex shapes (see image).”

The researchers, reporting their work in Science Advances DOI: 10.1126/sciadv.1700262, say that they are now looking at how they can ingrate the radio-frequency heating hardware into the 3D printer itself. Here is a preview of their FlashFuse technology.

“Essentium Materials and TriFusion LLC is also already commercializing this technology and using the 3D-printed materials in applications such as prosthetic limbs,” reveals Green.