Researchers from the National Taiwan University of Science and Technology have 3D printed new lattice structures using Fused Deposition Modeling (FDM) that do not require any support structures.
The shell-shaped lattice structures are based on the morphology of a sea urchin, which has a mechanically stable and load-bearing shape. The researchers aimed to emulate these properties in their printed lattice structures in order to eliminate the need for support structures, thereby reducing the amount of material, energy, and time required during the printing process, and subsequent post-processing steps.
The sea urchin-inspired lattices have been designed with specific stiffness and energy absorption properties and could have potential applications within end-use consumer products such as low-cost shoe midsoles and ski boots. The lattices could also be deployed within the automotive sector in the form of tires and automotive crush boxes, or indeed “any other energy-absorbing” structure, according to the researchers.
Removing unwanted supports
Printing lattice structures using the FDM process requires a support structure to ensure each lattice element is printed defect-free and without distortion or sagging. This support structure is then removed during a post-processing step through mechanical or chemical methods.
When printing with elastic materials like TPU, the same material must be used as the support, making the removal of this support from the lattice by chemical or mechanical means near impossible due to the structure’s intricate nature. This extensive post-processing work, coupled with the extra material, energy, and processing time required to print the supports in the first place, prompted the researchers to embark upon designing stable, self-supporting TPU lattice structures that could be produced via FDM.
The researchers also wanted to harness particular mechanical properties such as stiffness and energy absorption to yield decent compression results when loading and unloading the structures, to ensure they would be suitable for footwear and other energy-absorbing applications.
Previous research from China’s Southeast University has embarked upon creating an algorithm for stronger and more efficient objects printed using the FDM process, using a consistent infill pattern similar to that of a lattice. Elsewhere, a team from UC Berkeley has developed a way to incorporate 3D printed polymer octet lattices into concrete structures to act as reinforcers. The lattice structures significantly decreased the overall weight of the structure and also proved to be just as load-bearing as traditional cement.
Printing the lattices with FDM
The researchers decided upon the FDM 3D printing method due to its low operational cost and maintenance. They hope to make the FDM process more efficient through achieving higher print speeds and better build quality by removing the need for support structures when printing lattice structures.
During FDM, supports are used when lattice structures have steep overhangs and parallel ledges. The researchers discovered that in order to print self-supporting lattices, they needed to ensure a maximum overhang angle of 45 degrees, and eliminate the structure’s parallel ledge.
The proposed lattice is made up of an interconnected network of solid struts which form basic building blocks that can then be tessellated in a periodic structure. The structure is based on a cellular design inspired by the dome of sea urchins, which is economical in materials and is able to transfer compressive stress effectively from the dome’s surface to its margin. This makes the structure stronger and more stable, and has previously been used within architectural design.
Several variations of the lattice were printed with TPU filament on a Flashforge Beaver 3 and Ultimaker FDM 3D printer, with unit lattice dimensions of 8x8x8mm. The unit lattices were then tessellated into a cylinder with a diameter of 38mm and a thickness of 16 mm. After the printing of the lattice structures was complete, no post-processing steps were required, and testing of the lattices’ mechanical properties could commence.
Evaluating the lattice structures
The researchers found that their 3D printed support-free lattice structures could improve the overall speed of the FDM 3D printing process for building customized parts, while the lattices themselves offer “great potential” for the fabrication of parts with a wide range of volumes, densities, and sizes.
Visual inspection of the 3D printed lattices found no imperfections or breaks within the structure, proving that support-free lattice structures can be manufactured for end-use products, such as midsoles and tires. The structures also exhibited a near constant energy return under cyclic loading.
According to the researchers, their structure significantly outperformed a benchmark body-centered cubic (BCC) lattice and ethylene vinyl acetate (EVA) foams in terms of energy absorption and stiffness. Additionally, the printing of the support-free lattice took three hours less than those printed with supports and removed around 11 hours of post-processing time. Printing the lattices also costs three times less than comparable structures printed with supports.
Going forwards, the researchers expect their support-free lattice structures to be applied in the development of customized midsoles of sports shoes and ski boots using FDM. They will also investigate the effect of flow rate on the accuracy of printing the lattices using a fixed nozzle diameter.
Further information on the study can be found in the paper titled “Supportless lattice structures for energy absorption fabricated by Fused Deposition Modeling”, published in the 3D Printing and Additive Manufacturing journal. The paper is co-authored by A. Kumar, S. Verma, J. Jeng.
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Featured image shows the researchers based their lattice structures on the design of sea urchin shells. Photo via Halef/Pixabay.