Try building a 3D immersive stereo experience with all monaural components. The result is likely to be suboptimal.
A similar challenge is facing developers of digitally manufactured products. When it comes to selecting materials to deliver the desired properties and product features in a 3D-printed (aka additively manufactured) product, designers and engineers face a challenge. Nearly all material data has been developed with traditional manufacturing processes such as injection molding in mind.
The solution, says additive manufacturing (AM) expert David Tucker, is something called unit cell design and testing. Tucker, who is co-founder and an instructor with SPE’s Implement AM, believes the future will involve building a detailed database of unit cell testing, to complement the existing material databases based upon the testing of the ubiquitous tensile bars known informally (given their shape) as “dog bones.”
With this data, the designer will be able to select validated design features to use in their product. This process is similar to what is done today, however, the design freedom capabilities are far greater in AM than in injection molding.
DI Labs used an HP MJF 5420 printer to make this lattice ball out of nylon 12, which they dyed red. The cell size is 15 mm with a strut diameter of 2 mm. (Courtesy DI Labs)
What exactly are unit cells?
Purdue University describes unit cells –– the simplest repeating unit in the crystal –– in the following way:
“The structure of solids can be described as if they were three-dimensional analogs of a piece of wallpaper. Wallpaper has a regular repeating design that extends from one edge to the other. Crystals have a similar repeating design, but in this case the design extends in three dimensions from one edge of the solid to the other.
We can unambiguously describe a piece of wallpaper by specifying the size, shape, and contents of the simplest repeating unit in the design. We can describe a three-dimensional crystal by specifying the size, shape, and contents of the simplest repeating unit and the way these repeating units stack to form the crystal.”
DI Labs in Minnesota 3D printed this intricate lattice out of nylon 12 on an HP MultiJet Fusion 5420 machine. (Courtesy DI Labs)
“The simplest repeating unit in a crystal is called a unit cell. Each unit cell is defined in terms of lattice points the points in space about which the particles are free to vibrate in a crystal.”
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Accurate characterization of properties is key
Tucker also is founder and principal of New Wave MFG, a Detroit-based product development firm focused on digital manufacturing technology. “As engineers,” he says, “we need to be able to test the product features that we want to include in our products. We need to have confidence that we can make them and that they will perform as desired.” Doing so involves accurately characterizing the properties of a 3D-printed part.
Posedla in the Czech Republic is 3D printing customized bike saddles using BASF Forward AM‘s Ultrasint TPU Powder Material. (Courtesy of BASF)
He notes that unit cells are advancing the use of AM in industrial markets. “They are doing this by creating new material structures to improve properties such as flexibility or stiffness per unit density. Using AM processes, we can now create energy-absorbing or rebounding structures at lower densities as compared to traditional manufacturing technologies.”
Wilson’s new 3D-printed, airless basketball
Wilson Sporting Goods provided a recent commercial example of this technology when it rolled out its Airless Gen 1 3D-printed basketball. It redesigned this product using an AM process to imitate the performance of basketball. By implementing a characterized unit cell, the Wilson team created a ball that bounces without using air.
Wilson Sporting Goods Co. recently announced the release of its Wilson Airless Gen1 –– a first-of-its-kind, 3D-printed basketball that never needs to be inflated
This is not an easy task. They needed to characterize the unit cell using a computer program that can design the lattice structures, such as Simply Rhino Ltd.’s Grasshopper 3D or Carbon Inc.’s Lattice Design Engine. These are typically surface modeling programs that create a network of floating points. The program then connects these points to form the surface geometry of the part, typically using a B-spline. This approach creates objects that can be designed using mathematical functions that represent the lattice structure.
“Rather than being inflated,” Wilson notes, “the Airless Gen1 relies on a 3D-printed polymer lattice structure to replicate the bounce, flight, and feel of a traditional basketball. The form of the ball features eight panel-like lobes and a familiar seam structure. Hexagonal holes across the surface allow air to pass through freely.” (It touts the development in this brief video.)
Current material selection databases
These are metamaterials, Tucker says, that are not typically seen in current material selection databases. As noted earlier, the material data collected by manufacturers today consists primarily of an industry-standard dog bone, as outlined in ASTM standard D638-14.
The ASTM bars today are based on the “unit cell” representation of the injection molding process, where the test bar is simply a representation of a standard wall thickness in the part. This can be visualized by cutting the dog bone from any manufactured product. The testing and qualification of this method enables the designer to manufacture components with material properties that are known, and with greater confidence that the product is also manufacturable.
The recent characterization of unit cells is enabling 3D printing to be deployed in the product development process, enabling products such as footwear and basketballs to simulate or improve performance, explains Tucker. This allows the creation of unique geometries that are effectively measured material attributes per unit density. This ratio is commonly referred to as specific modulus or stiffness/density.
Developing unit cell design guidelines
A pair of researchers at the University of Maryland –– I’Shea Boyd and Mohammad Fazelpour –– have written a paper, published by the American Society of Mechanical Engineers, called “Design for Additive Manufacturing: Effectiveness of Unit Cell Design Guidelines As Ideation Tools.”
Periodic cellular materials consist of repeatable unit cells. They wrote: “Due to outstanding effective properties of the periodic cellular materials such as high flexibility or high stiffness at low relative density, they have a wide range of applications in lightweight structures, crushing energy absorption, compliant structures, among others.”
Advances in additive manufacturing, continue Boyd and Fazelpour, have led to opportunities for making complex unit cells. “A recent approach introduced four unit cell design guidelines and verified them through numerical simulation and user studies.”
The unit cell design guidelines aim to guide designers to redesign the shape or topology of a unit cell for a desired structural behavior. While the guidelines were identified as ideation tools, they concede that the effectiveness of the guidelines as ideation tools had not a few years ago been fully investigated.
For their research, to evaluate the effectiveness of the guidelines as ideation tools, they considered four objective metrics –– novelty, variety, quality, and quantity. The results of their study revealed that the unit cell design guidelines can indeed be considered ideation tools. The guidelines, they say, are effective in aiding engineers in creating novel unit cells with improved shear flexibility while maintaining the effective shear modulus.
Additional resources of interest:
BASF’s Ultrasim 3D Lattice Design
General Lattice’s Computational Design and Engineering
Carbon Inc.’s Carbon Lattices
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