3D printing metamaterials
Imagine wanting to construct a personalised bike saddle, but the materials at your disposal are either too stiff for some parts of the buttocks or too soft for others… Creating a product that has different parts, each with unique requirements, often requires the use of different materials. But innovations in 3D printing are changing this by making it possible to create objects with spatial gradations in surface and volumetric properties – in other words, functionally graded materials (FGMs). These engineered metamaterials go beyond the characteristics found in naturally occurring materials. For his PhD, Tim Kuipers explored how toolpath generation and a manufacturing technique called fused filament fabrication (FFF) can be used to create more complex objects with varying material properties.
In 3D printing, the toolpath has a direct influence on the printing time, material cost, and mechanical properties of the printed object. Prior to his PhD, Kuipers spent three years as a software engineer at Ultimaker, a 3D printer manufacturer. There, he worked on algorithms for toolpath generation, also known as slicing.
A critical part of the 3D printing process, this software is responsible for converting a 3D model into specific instructions for the printer. “Normally, software just stays in a computer, but with 3D printing it becomes something real,” says Kuipers. “That’s what really drew me to 3D printing.”
The beauty of FGMs
Manufactured functionally graded materials (FGMs) have a lot of potential. Rather than a homogeneous block of material, FGMs consist of a fine-scale geometry of one or more base materials and can have varying properties such as colour or stiffness. Manufacturing products with FGMs requires fewer components and materials because a single material component can have different properties in different regions. That means you can actually improve a product’s performance by optimising the spatial gradation of material properties throughout the product. And the potential applications for FGMs are broad, ranging from personalised footwear to medical devices to airplane wings.
While each 3D printing method has its own advantages and constraints, fused filament fabrication (FFF) has exploded in popularity in recent decades. This is in part because it is relatively low-tech (easy to use) and less expensive than some other methods. In FFF, thermoplastic material is extruded through a heated nozzle to create objects layer by layer.
As Kuipers explained in his thesis, a 3D model is converted into toolpaths for the 3D printer, which describe the geometry of the extrusion lines the printer’s nozzle should traverse. But there are several manufacturing constraints to take into account in this process. For example, you have to consider the maximum overhang angle to prevent printing in mid-air as well as use chemically compatible materials to prevent a multi-material print job from falling apart during the manufacturing process.
Exploring different perspectives
Kuipers investigated four different perspectives on how toolpaths can be used in FFF to create FGMs that addressed some of the constraints and resulted in innovative contributions. The first cycle of his research focused on properties of the product such as colour (grayscale) and surface roughness. Next, he researched how foams can be printed with flexible material which allows for varying densities and stiffness. Furthermore, he explored printing structures with chemically incompatible materials and proposed a mechanism for interlocking two materials using a repeating lattice structure at the interface between the two bodies.
Finally, he worked on generating toolpaths for variable width geometry and introduced a more reliable method for printing thin geometry with a spatially graded thickness. Such structures can be used to create FGMs with a functionally graded stiffness. “I think what I’m most proud of is that this framework is now being integrated into the commercial software called Cura, which is an open-source slicer for 3D printing that is not only used by Ultimaker but by loads of printers all over the world,” he said. “There are potentially millions of people I’m reaching with this research so that’s really cool.”
The big picture
If FFF technology can be used to manufacture FGMs, Kuipers said that designers and researchers with access to such additive manufacturing systems could design more complex products with improved performance. At the same time, using FFF technology means that FGMs can be produced at a lower cost and with a wider range of materials, unlocking a broader range of applications. In otherwords, the future looks bright for 3D printed metamaterials.
After his PhD, Kuipers plans to take a year-long sabbatical to travel and consider what the next phase of his life will look like.