Mechanical Properties of PLA/TPU Composite Filaments via Novel Core-Shell Fabrication and Re-Extrusion Method in Additive Manufacturing

Additive manufacturing (AM) has revolutionized the manufacturing industry by enabling the creation of complex structures with tailored properties. However, conventional materials used in material extrusion additive manufacturing, such as PLA, often suffer from low mechanical performance, limiting their application in functional and load-bearing components. The development of multi-material printing techniques has aimed to address these limitations by integrating different materials to enhance the mechanical properties and functionality of printed parts. Despite the potential of multi-material printing, existing methods often require specialized equipment and complex processes, which increases costs and limits accessibility. New paper published in Journal of Manufacturing Processes and conducted by Dr. Anni Cao, Di Wan, Chao Gao, and led by Professor Christer Westum Elverum from the Department of Mechanical and Industrial Engineering, Norwegian University of Science and Technology (NTNU) in Norway, developed a novel method that utilizes a commercially available 3D printer to fabricate core-shell composite filaments composed of PLA and TPU.   The first stage involves printing composite filaments with a PLA core and a TPU shell using a standard 3D printer. The team developed an algorithm to accurately slice the composite filament, ensuring precise geometrical design and material distribution. The composite filaments were fabricated with varying volume fractions of the PLA core, allowing for a comprehensive evaluation of the effects of geometrical design on mechanical properties.  In the second stage, the fabricated composite filaments were re-extruded into single-layer composite specimens using the same 3D printer. This re-extrusion process maintained the core-shell configuration of the composite filament, enabling the team to study the local material behavior and mechanical properties of the re-extruded specimens. The study employed various experimental techniques, including microscopy analysis, nanoindentation, and tensile testing, to evaluate the mechanical properties and failure mechanisms of the composite specimens.  Microscopic images revealed that the re-extrusion process effectively eliminated voids within the composite filaments, resulting in a more homogeneous material distribution. The core-shell configuration was maintained throughout the re-extrusion process, with the TPU shell wrapping around the PLA core due to differences in viscosity between the two materials. The reduced modulus of the PLA core and TPU shell was measured across the multi-material interface. The results showed a gradual transition in the reduced modulus between the TPU and PLA, indicating effective bonding at the interface. The zigzag pattern applied to some of the specimens increased the effective contact area between the two materials, improving the mechanical properties of the composite. The stress-strain curves of the re-extruded composite specimens exhibited a pseudo-ductile behavior, characterized by multiple local peak stress points. The toughness of the composite specimens was significantly enhanced compared to neat TPU and PLA, with the highest performing design achieving a 63% increase in toughness over neat TPU and a 27-fold increase over neat PLA. The study also introduced a novel parameter, the average peak strength, to assess the mechanical performance of the composite specimens during the local failure stage.

The authors highlighted the potential of the proposed fabrication method to produce composite materials with superior mechanical properties using a commercially available 3D printer. The re-extrusion process, combined with the core-shell filament design, allows for greater flexibility in material configuration and design, enabling the creation of parts with tailored properties for specific applications.  Moreover, the success of the composite fabrication method hinges on the quality of the multi-material interface. The study demonstrated that the interfacial bonds between the PLA core and TPU shell play a crucial role in determining the mechanical properties of the composite. Strong interfacial bonds lead to improved toughness and resistance to delamination, while weak bonds can result in premature failure. Furthermore, the geometrical design of the composite filaments, particularly the volume fraction of the PLA core and the application of a zigzag pattern at the interface, was shown to significantly influence the mechanical properties of the re-extruded specimens. Higher volume fractions of PLA resulted in increased strength, but also introduced challenges in maintaining interfacial integrity. The zigzag pattern, on the other hand, enhanced the mechanical performance by increasing the effective contact area and reducing shear stress at the interface. Additionally, the team study identified three distinct failure scenarios for the re-extruded composite specimens: local failure of the PLA core without delamination, local failure with dispersed delamination, and catastrophic delamination. The choice of geometrical design and fabrication parameters influenced the failure mode, with stronger interfacial bonds and optimized geometrical designs leading to more favorable failure mechanisms.

In conclusion, Professor Christer Westum Elverum and his team  developed a novel method of fabricating PLA/TPU composite filaments using a standard 3D printer offers a cost-effective and accessible solution for producing high-performance multi-material structures. The ability to tailor the mechanical properties of the composite through careful control of geometrical design and fabrication parameters opens up new possibilities for rapid prototyping and the development of functional parts for a wide range of applications.

Future work in this area could explore the application of this method to other material combinations and further refine the analytical models used to predict failure mechanisms and mechanical properties. The integration of advanced printing toolpaths and more complex material configurations could also enhance the versatility and performance of the fabricated composites, paving the way for more widespread adoption of multi-material 3D printing in industrial applications.