A Promising Remedy for a Major Flaw in 3D Printing

Material extrusion (ME) 3D printing is the technique that revolutionized the 3D printing of thermoplastic parts. However, this process is somewhat flawed—the parts printed using plastics, perhaps the most popular 3D printing material, are mechanically weak due to the imperfect bonding between individual printed layers that make up the 3D part. This leads to weak tensile strength in the build direction.

Although many methods have been proposed to address this flaw, many fall short of a solution that is production-ready. In a new study published in the journal Nano Letters on February 27, Texas A&M researchers, in collaboration with scientists from the company Essentium, Inc., report that they have “developed the technology needed” to overcome this flaw.  

 

Inadequate Bonding

In the ME 3D printing technique, known technically as fused-deposition modeling, molten plastic is squeezed out of a nozzle that prints parts layer-by-layer. As the printed layers cool down, they fuse together to create the final 3D part. 

However, previous studies have shown that these layers bond inadequately; the final printed parts when the ME method is used are notably weaker than identical parts made by injection molding where melted plastic assumes the shape of a pre-set mold when it cools down. 

"Finding a way to remedy the inadequate bonding between printed layers has been an ongoing quest in the 3D printing field," said Micah Green, associate professor in the Artie McFerrin Department of Chemical Engineering. "We have now developed a sophisticated technology that can bolster welding between these layers all while printing the 3D part."

To join the various interfaces of a 3D part printed via the ME technique more thoroughly, additional heating is required, but heating printed parts using a furnace-like solution has a major drawback. "If you put something in an oven, it's going to heat everything, so a 3D-printed part can warp and melt, losing its shape," Green said. "What we really needed was some way to heat only the interfaces between printed layers and not the whole part."

 

Texas A&M and Essentium researchers claim to have developed a more effective method of welding adjacent printed layers together, increasing the quality of parts. Image credited to Essentium

 

Using Carbon Nanotubes to Promote Inter-Layer Bonding

The team’s solution? Integrating plasma science and carbon nanotube technology into standard 3D printing to weld adjacent printed layers more effectively, increasing the overall reliability of the final part. 

Since carbon particles heat in response to electrical currents, the researchers coated the surface of each printed layer in them. Similar to how a microwave heats up food, the team found that the carbon nanotube coatings can be heated using electric currents, thereby enabling the printed layers to bond together. 

To apply electricity as the part is being printed, the electrical current must overcome a tiny air gap between the printhead and the 3D part. One option is to use metal electrodes that directly touch the plastic part, however, this has the potential to cause damage to it.

Instead, the team collaborated with Associate Professor David Staack to generate a beam of charged air particles, or plasma, that could carry an electrical charge to the printed part’s surface. This technique enables the currents to pass through the printed part, heating the nanotubes and welding the layers together. 

 

Matching the Strength of Molded Parts

The Texas A&M team and Essentium researchers then added both these components—the plasma technology and the carbon nanotube-coated material—to a conventional 3D printer. When the researchers tested the strength of the 3D printed parts, they found that it was comparable to the strength of injection-molded parts. 

"The holy grail of 3D printing has been to get the strength of the 3D-printed part to match that of a molded part," Green said. "In this study, we have successfully used localized heating to strengthen 3D-printed parts so that their mechanical properties now rival those of molded parts.”