Mixing materials is certainly nothing new. Samurai swordsmiths knew that to make the famous katana swords used by its warriors, it needed to combine several steels in various layers to produce the curved blades that are hard, flexible and sharp. Getting the right mix of materials, in the right place, was a skill that was developed through the ages.
It’s a philosophy that is perhaps unknowingly being applied to the modern day. As 3D print manufacturers expand on the multi-material capabilities of printers, many are looking to build up databases of ‘meta-materials’, known recipes that combine various polymers in different ways to produce very honed and specific properties, which are often quite different to the building block base materials used.
Among those carrying out this work is Hod Lipson, director of the Creative Machines Lab at Columbia University in the US. He says: “You can print flexible materials, optically transparent materials, biocompatible materials, hard materials – and as this technology expands, we should expect to see more materials become available by blending different ones together.”
The evolution of 3D printers is fast moving and a decade ago the technology was normally restricted to a single off-white soft plastic. Today, printers such as Stratasys’ J750 allows six materials to be used at once, offering some 360,000 colours. It means that functional parts can be printed, for example scissors that are sharp enough to cut right away, materials that maintain heat can be used for injection moulding and real structural improvements mean parts are no longer flimsy but representative.
We are all intuitive about colours and know that mixing different colours gives you different shades. However, the ability to combine multiple materials, hard and soft, transparent and coloured – layer by layer – to give rise to a huge range of new materials and properties, is not something that is currently thought about.
“We are beginning to explore the ability to generate new materials as we print,” says Lipson. “New mechanical and optical combinations and behaviours that we’ve not been able to access before.
“We’re not just talking about feeding multiple materials in to the same printer and printing a part out of multiple materials, that’s been done. I’m talking about combining very different materials together, to create infinite ‘shades’ of materials, and create a whole range of meta-materials.”
The thinking is that printing using a hard plastic, for example, in a particular 3D arrangement could make a part appear soft. Or, it could be made stiff in one direction, flexible in another. The printer is creating properties by exploiting its freedom to produce complex shapes with complex material distribution.
Like printing grey scale using black ink, where the void and the pattern creates the illusion of grey shades, meta-materials exploit the printer’s ability to control individual layers of material, its orientation, and use of different materials on a single layer as it’s built up. Provided software can do the calculations and analysis and the 3D printers are capable of printing multiple materials in these ways, then the added complexity required doesn’t actually add any extra cost to the production of parts.
“We are now at the point where we can start making these things for ourselves,” Lipson enthuses. “But, if you think additive manufacturing is a revolution now, wait until you can print active materials.”
Passive vs active materials
The possibility of merging materials is something Lipson is pursuing. However, with a background in robotics, he is keen to cross the parapet between printing passive materials to active ones. Active materials include the use of magnets and wires to enable the printing of flexible batteries, actuators, sensors and motors.
While the technology is not there yet to print the likes of a mobile phone, there are some promising early examples. Last year, a working telegraph machine was printed with the wires to the magnet all produced additively. There has also been an example of a loud speaker, again with all the wires and magnets 3D printed. Admittedly, the speaker is essentially rubbish and expensive. However, it does serve as a tantalising preview of what is coming.
CAD tools still lacking for 3D print users
Topological optimisation is fast gaining popularity as a tool to optimise the geometry of materials under particular load conditions. These tools are now becoming available with mainstream CAD packages, gradually, but other tools that engineers are asking for are not yet developed.
“Software companies are behind the curve on this, CAD companies need to do more to keep up,” says Hod Lipson, director of the Creative Machines Lab at Columbia University in the US. “Software is holding up exploring the true potential of 3D printed parts and meta-materials.
“You don’t design this organic geometry by pointing and clicking, you need much more sophisticated generative design tools to enable you to do these things. These tools are not mature or popular. The software needs to catch up.”
Computer led optimisation of both geometry and the lay-up of meta-materials is likely to soon become an activity led by computation rather than a designer, that will more likely specify an outcome, rather than how to do it.
“The use of AI will come in to play, and much like topological optimisation that optimises geometry, similar tools are being developed to simulate the best ways to merge different materials in different combinations,” says Lipson. “We’ve asked software to combine hard and soft materials to create a bending beam that bends in different ways. This is something most engineers can’t do, combine materials to get different bending profiles. AI can do that.”