AddJoining technique uses 3D printing for wood-polymer joining

A research team based out of Graz University of Technology (TU Graz) in Austria has developed two new approaches to joining wood to materials like metals and polymer composites without the use of any adhesives or screws. One of the methods, a technique called AddJoining, leverages additive manufacturing to bond wood materials to polymer composites. The other technique is called Ultrasonic Joining and uses high-frequency vibrations to afix materials to wood.

Both joining methods, which are now patent pending, could be used in industries like aerospace, automotive and furniture to encourage the use of wood in various products and systems. Wood, a renewable natural material that is both strong and lightweight, could therefore become more viable in these industries as a more sustainable alternative to non-renewable materials or materials notoriously challenging to recycle. “Our motivation is clearly environmental protection,” explained Sergio Amancio, the leader of the TU Graz research team.

The AddJoining process consists of 3D printing a polymer composite component directly onto an untreated wood substrate. The joining is the result of the polymer material penetrating into the pores of the wood, which creates a chemical bonding reaction, not unlike glue and wood. According to the researchers, the bonding between the two materials was found to be very strong in mechanical load tests, with researcher Gean Marcatto saying: “After the joint fractured, we were able to find polymer in the wood pores and broken wood fibres in the polymer, which suggests that the fracture occurred in the wood and polymer, but not at the joint.”

The research team believes even more strong and durable joints could be realized by first creating a micro- or nano-structure in the wood using laser texturing or etching. The additional pores could enable more polymer to penetrate the wood surface for enhanced bonding. In the interest of simplicity, however, the researchers were more interested in the bonding that could be achieved using fewer steps. “We can use this technology particularly well with complicated 3D geometries because the components are printed directly onto the surface—in whatever geometry is required,” added Amancio.

The second technique, Ultrasonic Joining, is based on a similar principle to AddJoining, however instead of 3D printing directly onto the wood, the penetration of the polymer or polymer composite is generated by using high-frequency vibrations at a low amplitude. Specifically, a sonotrode is applied to the wood, which is then placed in contact with a polymer or polymer composite material. The friction between the vibrating wood and the synthetic material generates hear and causes the surface of the polymer part to melt, enabling the molten material to seep into the wood’s porous structure. Researcher Awais Awan adds: “This technique is particularly suitable for large components and 2D structures since we achieve a precisely localized spot joint.” If needed, the joints could also be strengthened by texturing the wood using a laser beforehand.

In their work, the researchers tested a number of different materials, including wood varieties like beech and oak, and base materials like carbon-fiber-reinforced polyamide, polyphenylene sulphide, stainless steel 316L and Ti-64 alloys. Currently, the TU Graz team is aiming to work with partners in industries like automotive, aircraft and furniture to further develop the multi-material joining methods.