Lattice structures are those geometric shapes grouped together to create repeatable patterns for engineering purposes. The idea of lattices has been around for quite some time. You see these repeating patterns on buildings and bridges. For design engineers, lattices can deliver specific mechanical properties to an object, often in the form of a stiffness-to-mass ratio.
Lattice structures have not been used as often as they could be because manufacturing methods that could produce them easily did not exist until the development of 3D printing / Additive Manufacturing. Injection molding and machining are often cost-prohibitive when it comes to producing these shapes.
Today, though, 3D printing easily creates lattice structures in helmets, saddles, shoes, nasal swabs, and other designs. For example, lattices are being studied for use in soft robotics and in understanding how pulmonary airways work.
The additive company Carbon recently released a lattice design generator called Design Engine that automates the process of creating conformal, single-zone lattices. Hardik Kabaria, Director of Engineering at Carbon, discusses the importance of lattice structures.
Hardik leads the development of computational design software at Carbon. He graduated from Stanford University with a degree in Mechanical Engineering, with a focus on Mechanical Analysis in Design from a 3D printing perspective. He is considered a leading engineering researcher on Computational Geometry, as it relates to 3D design. He’s an expert in lattice design and has a Ph.D. from Stanford University on his research that marked the first development of 3D universal meshing and opened the door for making 3D printing possible.
Here are key points from the interview.
Using lattices in your design can “basically explode your design freedom significantly,” notes Kabaria.
Lattices can be made from metal or polymer materials. If you use the Carbon Design Engine, you’ll work with stereolithography and polymer materials.
According to Kabaria, the Carbon Digital Light Synthesis system opened the door to more easily work with lattice structures. “The initial application we found and have had success with is in the area where, traditionally, you were using foam. So, if you’re using foam for shoes, or helmets, or saddles, those are the places where you can replace the foam with polymeric lattice parts.
“And the basic advantage there is you can achieve mechanical properties not easily achievable with foams. But we have also found some areas that are significantly different than foam replacements. And the example I would give is COVID-19 nasal swabs.
“I’ve been working on a lattice-design computational-geometry algorithm that is dedicated towards these types of applications for quite some time. I would not have imagined that COVID-19 swabs are something that we would be able to make using lattice design ideas.
“There are more applications out there than what we, who live and breathe lattices, have ever imagined. And that is one of the reasons why we realized that we needed to create a software program that would give access to more mechanical engineers in our ecosystem.”
Doing more with lattices
Lattices are generally complex designs. You can have hundreds of thousands of these geometric structures in a design. There are generally two problems designers face with lattices. Traditional CAD tools do not make it easy to draw hundreds or thousands of these structures in a design. Second, traditional manufacturing methods, such as injection molding or machining are too costly to use to mold or cut that number of geometric shapes.
“So, you have a problem both on the design software side, as well as the manufacturing side,” says Kabaria. “We at least solved the manufacturing side with additive manufacturing and polymer materials. So, you can print and create an end quality part. But we also realized that’s only part of solution. We developed a manufacturing method, but we didn’t give you a way to make amazing parts on that manufacturing method. So, that’s why we started working on the design engine to enable our customers to create really kick-ass products. The most beautiful part of this is that we have enabled customers to put parts in production.”
The software can be used by mechanical and industrial design engineers, as well as the manufacturing technician on the floor that may not have access to the high compute power needed for CAD-produced lattices.
Design for additive manufacturing principles are also needed when working with lattices for a design.
Be sure you can make the structure on the chosen 3D printer or additive system. One question to consider is will you have a good yield on the printer? “Because in the end, some manufacturing projects, almost all of them really, are about unit economics. You can only have so many failed parts, right?”
Be sure the lattice structures are not too small, clustered, or densely packed together. Such designs will inhibit resin flow, which will slow down the printing process. And slow printing affects unit economics.
Lattices can require a lot of memory to render, represent, and transfer to the printer, …. “there are challenges in every one of those stages. Like, our design software works on AWS. We can use the cloud infrastructure and high-performance computing to generate the designs, but that’s sort of the first step. Then, we have to successfully transfer the design to the printer, make sure we don’t run into any problems during that process, ensure it’s sliceable so, … we slice the part and convert it into like images or voxels, and that process has to be foolproof because initially, you had kind of a simple parts, but now you have a very complex geometry.”
In terms of computational geometry algorithms, there are challenges on the design front to make sure that the part is not just a nicely rendered geometry, but that it is something you can print and have it in your hand.
Keep in mind that every printer has a resolution, so don’t design a lattice structure smaller than the resolution, or the pixel size, of the printer.
For some 3D printing systems, materials can be a limitation. Each material has a viscosity. If the viscosity is high and you have little space between the lattice struts, the resin will not flow through. You will only print a few lattice struts, and after that, there’ll be nothing there, or it will be a full blob of resin.
Then, there are a few process limitations to consider. For example, how fast will the printer move? Will heat be an issue?
“Generally, we worry about three things, hardware limitation, material-specific limitation, and the limitation of the process. And we try to account for that in the design.”
There are thousands of useful geometric shapes suitable for lattices. One of the challenges is determining which structure is right for the project. “We wanted to make that part easy. You are a mechanical engineer, the most important knowledge that you have is figuring out if a mechanical response you’re looking for, let’s say for a saddle, has the right stress distribution, so you don’t have a peak pressure at your sit bones. So, that’s the most important part that you as a mechanical engineer for saddle industry know. The customer cares about the mechanical response, our software will help you find the lattice structures.
“While we know there are hundreds of thousands of different lattice structures, we initially launched with just five different structures. If you’re looking for a polymeric foam-like response, which is what we call non-linear response, we’ll give you a lattice structure.
“If you’re looking for a memory foam-like response, it’s a different lattice structure…. Often, you’re looking for something very linear, which is actually not achievable with foam easily, but those are the kinds of things you really get in shoes, we have different lattice structures.
“Our goal was to enable custom manufacturing. As in the example of helmets, each head shape is different from person to person.
“You have a part that you want to populate with lattice structures that you decided on based on the mechanical response you wanted. We want that process to be flawless, ….so users do not have to worry about broken structures that could lead to part failure. We don’t want users to deploy mechanical engineers to even inspect that there are no broken structures or open structures, it’s just not going to work. It’s just not going to scale.”
Making it easier to include lattice structures in a design opens the door for new applications. If customers know that a design choice achieves a non-linear foam-like response, but they would like a very plateau-like stress versus strain response, which is almost like a flat curve, “we have a mechanical response wave structure for them. We call it the tetrahedral unit cell. They can choose that, and they can start populating it with the geometry and start testing it. The engineering team of one of our customers, CCM, has been using this tool and has been able to find a new application on its own.
Customers can work with Carbon engineers to design parts. However, the Design Engine enables customers to design without any aid.
“This is possible because of two things. We don’t have to teach everybody which structures are the best, which model to use it with. And the second is, you have a part, you choose the structure, we’ll be able to populate it so that they can quickly throw it on a printer, print it, iterate it, and the cycle continues.
Currently, the Design Engine program is not directly connected to CAD programs. “It’s not as smooth of a link as we want in the future. Today, you use your SolidWorks, or CATIA, or whichever is your CAD design tool that is your favorite, and you can export it from there, the part you want a lattice in a triangle mesh.
Then you bring it to our tool, which is browser-based, so you can access it through your browser on your computer. You upload it. All the real computation happens on our AWS cloud infrastructure, so you are not really utilizing the computing power you have and more importantly, we are not limited by it. So if we want to do some heavy computation, we can run it on the cloud, and then send it back to your browser for you.
“So, our idea has been that anything that you want to do with the lattice design, we’ll allow you to do it in our tool base, in our software.”
Examples include creating different zones, creating a “skin” on a surface, creating a texture that is more comforting. Anything that is related to the 3D printing specific design, we’ll build it in our tool base, but you can still export it back into the SolidWorks for revision or for version control purposes.