Whenever someone picks up one of the various 3D prints I have on my desk, they usually exclaim over how light it is. While a lot of that has to do with the fact that all of my prints are pretty small, it’s also thanks to the infill pattern and percentage; for instance, my 3D printed raven has a standard honeycomb pattern inside. It can be tough to find the perfect balance between weight and strength when it comes to infill optimization, but earlier this year a group of researchers from the Technical University of Munich, the Technical University of Denmark, and Delft University of Technology developed a new type of infill, inspired by the naturally occurring structure of trabecular bone, that gives an object durable strength without making it too heavy.

L-R: Cross-section of a human femur showing cortical structures on the shell and trabecular structures in the interior; illustration of principle stress directions under major mechanical loads; cross-section of the optimized porous infill in a 3D bone model; the 3D printed bone model

Trabecular bone is less strong and dense than cortical bone, also found in the human body, but it’s flexible and lightweight, made up of tiny lattice structures. The research team’s work built on voxel-wise topology optimization, and they designed a complex mathematical algorithm to generate an infill that resembles bone’s porous structure. Using a boundary shell, the algorithm works out a porous infill for the domain which the shell encloses. As noted, the infill mimics trabecular, or spongy, bone, as it’s seen in nature.

The team determined that its trabecular bone-like structure was promising for 3D printing, especially of large, lightweight parts, and the 3D printed structures held up well under tests for robustness and strength. However, the researchers did note that further work needed to be done in order to further optimize their new bone-like infill, and recently made a big step forward in their work.

Assistant Professor of Advanced Manufacturing at TU Delft Dr. Jun Wu, together with Anders Clausen and Ole Sigmund with TU Denmark, published a research paper on their breakthrough, titled “Minimum Compliance Topology Optimization of Shell-Infill Composites for Additive Manufacturing,” in the mechanics journal Computer Methods in Applied Mechanics and Engineering.

The abstract reads, “Additively manufactured parts are often composed of two sub-structures, a solid shell forming their exterior and a porous infill occupying the interior. To account for this feature this paper presents a novel method for generating simultaneously optimized shell and infill in the context of minimum compliance topology optimization. Our method builds upon two recently developed approaches that extend density-based topology optimization: A coating approach to obtain an optimized shell that is filled uniformly with a prescribed porous base material, and an infill approach which generates optimized, non-uniform infill within a prescribed shell. To evolve the shell and infill concurrently, our formulation assigns two sets of design variables: One set defines the base and the coating, while the other set defines the infill structures. The resulting intermediate density distributions are unified by a material interpolation model into a physical density field, upon which the compliance is minimized. Enhanced by an adapted robust formulation for controlling the minimum length scale of the base, our method generates optimized shell-infill composites suitable for additive manufacturing. We demonstrate the effectiveness of the proposed method on numerical examples, and analyze the influence of different design specifications.”

A shell-infill composite obtained from optimizing a simply supported beam.

This time, the team breaks their original assumption of a fixed boundary shell for their new algorithm. Instead, the shell is optimized at the same time with the optimization of the porous, bone-inspired infill – this kind of structure is known as a shell-infill composite, and could have applications for engineering design, where there are no predefined boundary shells.

There are some unique geometric patterns in the optimized shell-infill: you can see elongated infill in the uni-axially loaded bars, and crossing infill at the joints connecting the bars. Thanks to the flexibility and iterative processes that 3D printing offers, it’s likely that tailor-made structures like these will have many applications in industrial sectors. Discuss in the Infill forum at 3DPB.com.

 

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