Feature first published in the January/February 2022 issue of 3D ADEPT Mag. Interviews, roles and examples reflect that period.
Multi-material 3D printing combines materials in a single build. It can bring together different properties or reduce some assembly operations, but its value depends on the application, material compatibility and the cost of the complete process chain. This 2022 feature examines adoption barriers with Aerosint and KU Leuven.
Multi-material describes a manufacturing capability rather than a single process family. How it is achieved depends on the technology and material combination.
In this feature, it means the controlled use of several materials in one build, including, where relevant, a material intended for removal after printing.
Multi-material approaches are studied or offered in material extrusion, including fused filament fabrication (FFF), vat photopolymerisation, material jetting, binder jetting, directed energy deposition (DED) and powder bed fusion, including selective laser sintering (SLS). This list does not mean that every machine in these families has that capability.
Interestingly, the Aerosint team splits the difference and considers MM3DP as both a procedure and a new type of manufacturing process:
“The new multi-material aspect can be seen as adding a degree of freedom (and complexity) to existing techniques. However, being able to use 2 materials in a single bed also offers other, perhaps somewhat unexpected, possibilities.
For instance, using the multi-material deposition to deposit virgin powder alongside recycled powder. This can make it possible to only use the virgin powder to build the parts, fill the rest of the bed, and build support, with recycled used material. Perhaps this angle is more ready for industrial adoption.
Likewise, as in FDM, perhaps a second material can be used for printing just supports.
Finally, our focus is on the material deposition. While it is indeed useful for powder bed-based AM, powdered materials are used in other production techniques as well. I already mentioned traditional powder metallurgy (PM) for one. Technically nothing stops an end-user from trying to control which powder is located where in a die using selective powder deposition to make multi-material parts in this manner. If you will, this falls somewhat between AM and PM.”
In the 2022 context, the contributors identify several barriers to wider adoption. This feature examines:
- The manufacturing process itself
- The materials’ standpoint
- The software perspective.
- The market reality
Several technologies enabling multi-material 3D printing will be cited as examples. Moreover, to address this topic, two organizations joined us: the Aerosint team (Bram Neirinck, Maxime Schoenmakers, Edouard Moens de Hase) that brings the manufacturer’s point of view and the KU Leuven AM team (Prof. Brecht Van Hooreweder, Dr. Louca Goossens & Dr. Jitka Metelkova) that brings the academic and user perspective.
The manufacturing standpoint
A shorter overall lead time is a possible benefit, particularly when assembly operations can be avoided. It does not automatically mean faster or more accurate printing.
“From an end-user perspective, the total lead-time can be further reduced by using multi-material printing. Especially when it comes to assemblies, which typically require joining operations such as welding, bolting, gluing, etc.”, the KU Leuven team comments.
However, the arguments of speed and precision can easily be highlighted in other AM processes. In the case of multi-material 3D printing, comparison is usually done with a single material manufacturing process – which in our opinion, can be a double-edged sword.
Machine manufacturers should pay attention to the way they qualify their multi-material 3D printing processes, especially when targeting industrial prospects, because not only can the latter have different benchmarks in mind, but if these requirements are not met, they could easily disqualify their systems.
The Aerosint team therefore stresses that a multi-material approach is not automatically faster:
“This depends on what shapes and applications you compare, and which manufacturing methods you want to compare.
Much like with single material AM, it is hard to compare with traditionally machined parts and methods. As we have more design freedom / other design rules, often the focus will be more on optimizing the design and functionality of the parts. As a result, parts made for the same application might look completely different.
In our case specifically, our benchmark is the layering of powder with a standard scraper. Currently our recoater is still slower, but we are working hard on getting the deposition speed with our multi-material recoater in the same speed range, and even potentially going beyond this.
As we don’t push a heap of powder forward over the already printed parts and powder bed, we don’t experience the same kind of friction effects resulting in rippling, undulations or defects caused by a coater hitting parts. So there is a potential for going faster if we can optimize the patterning.”
In July 2020, John Whitehead and Hod Lipson at Columbia University presented an inverted laser-sintering prototype. An upward-pointing laser enables successive selection of different powders; the published demonstrators are a feasibility result, not general production qualification.
In conventional SLS, a laser selectively consolidates powder spread in successive layers across a build bed. Using one material throughout the bed makes local placement of different powders more difficult.
Columbia’s prototype uses separate glass plates carrying thin powder layers. The platform is lowered onto powder and a laser passes upwards through the glass to attach selected material. The platform then moves to another plate. The team demonstrated a combination of nylon and TPU using this approach.
Inkbit announced Vista in February 2021, using optical-feedback-controlled material jetting, called Vision-Controlled Jetting. Layer measurements allow subsequent deposition to be adjusted to compensate for geometric deviations. This is process control, not a guarantee that the final part is defect-free.
Intended for production, Inkbit develops a machine that can produce parts that contain, for instance, both a soft and a rigid area, in the same build.
These capabilities make us question the functionalities of these parts. Can we legitimately say a multi-material approach leads to multi-functional 3D printed parts? “Besides for its multi-functionality, multi-material can also be used for single material components with the extra benefit of adding support structures in another material (e.g. water soluble support), or of manufacturing micro channels filled with an easy to remove filler material”, the KU Leuven team draws attention on.
What about the mechanical performances of the part?
Performance depends on each material combination, its interface and manufacturing parameters. Results from a material-jetting process cannot be generalised to powder bed fusion or to every multi-material part.
Taking example on their technology, Bram Neirinck, Manager Sintered Applications at Aerosint, states:
“I consider multi-metal AM in this aspect to be at the stage where L-PBF was when the first fibre lasers capable of yielding dense parts became available. Though the potential is there to yield parts of which the properties can compare to billet materials welded together, the work developing this process is just getting started. One advantage we have is that L-PBF is already much better understood, therefore, we expect developments to be carried out with an accelerated rate compared to the first metal AM efforts.”
The materials standpoint
The issue is not simply the number of available materials, but compatibility between them and qualification within a shared process.
For the two teams that contribute to this dossier, material compatibility is the major issue for main processes.
Furthermore, to address the material compatibility issue, the entire process chain has to be developed and secured from scratch: “this includes the basics like lasing strategies to post-processing such as heat treatments, and this for every single material combination”, the Aerosint team explains.
Specific considerations should be taken into account when defining the material compatibility with a multi-material 3D printing system. For metals, the KU Leuven Team notes the importance of the following items: “melting points, coefficient of thermal expansion, electrochemical compatibility, solubility and the final application. E.g.: heat cycling of a component. This can then significantly influence the stress level in the component, leading in the extreme case to crack initiation and fatigue failure.”
“Because we are combining materials, it will rely a lot more on fundamental metallurgy. We are directly involved in a project which is just looking on the effect of miscibility and chemical reactivity on the potential to process materials”, the Aerosint team adds, speaking of their technology.
Dissimilar metals in electrical contact and exposed to an electrolyte can undergo galvanic corrosion. The risk depends on the couple and its environment; using several materials alone does not establish it. KU Leuven adds: “There is also a risk of materials mixing, especially for powder bed-based technologies, rendering the unconsolidated leftover powder useless. An approach to solve this is to add an extra filler material and only deposit the material needed for fabrication. The filler material can then be recycled for reuse”, the KU Leuven completes.
As seen across other segments, one of the key solutions to address these materials issues is by fostering collaborations between material producers and machine manufacturers.
The software perspective
STL primarily describes triangulated geometry without standardised material assignment. A multi-material workflow can use separate meshes. OBJ can reference appearance properties through an MTL file; these are not themselves a complete manufacturing-material definition. The 3MF Consortium’s comparison distinguishes rendering data from manufacturing properties.
3MF can carry material and property information, including through its dedicated extension. Its use depends on software and machine support. VRML/X3D can convey appearance information in some workflows, but support must be checked; no format alone guarantees interoperability across the chain.
For PolyJet, regions intended to receive different materials must be identifiable in the preparation workflow. STEP is not universally mandatory: the 2020 Stratasys design guide also describes workflows using separate STL files. The choice depends on software, versions and printer model.
The Aerosint multi-material process on the other hand, requires sliced scanning files as used for normal single material AM. “Only these are now split up in zones for the different materials with different parameters. We are working with the Autodesk NetFabb team on slicing specifically for this, but technically any OEM slices should work if you can load the different sections and slice them individually. The deposition is handled by a separate sliced file indicating the required material zones. But we are using an open file format for this, and even stacked image files could be used”, they outline.
Beyond the file formats, Aerosint deplores the fact that “software tools, both for the design and modelling of parts, are often overlooked heroes of additive manufacturing. Such tools are a tremendous aid in getting the most out of the design freedom offered by AM, while circumventing limitations of different techniques. Adding an extra design parameter, like a second or third material, only fuels the imagination of the people developing these tools. In that light we’ve already printed demonstrators for Gen3D, Hyperganic and Additive Flow, and every one of these approached the potential of multi-metal parts from a different angle, truly demonstrating their creativity.”
The market reality
If you are a regular reader of 3D ADEPT Media, you certainly know we focus on applications. We believe that’s one of the most tangible ways to approach a manufacturing solution. Multi-material 3D printing can be used through various processes, and as such, this approach makes room for a myriad of possibilities.
“To get a grasp of this, you only need to consider the number of applications where people have optimized material properties in parts to increase their properties. It’s like hardening the teeth on steel gears, applying chrome plating to make something rust or scratch resistant, over-moulding TPU rubber onto tool handles to make them more comfortable, … Even in AM Scaffold sections have been used to control the E-modulus locally in medical implants to avoid stress shielding. And that is without changing the material itself. A lot of these can be driven to the next step if the actual material composition can be controlled locally”, Neirinck enthuses.
The KU Leuven team also mentions several examples which involve tools with ductile impact absorbing core and hard & wear resistant outer layer; heat exchangers with conductive but non-structural zones combined with strong and stiff regions for load absorption as well as structural parts with local gradient in stiffness, conductivity, magnetic properties.
In 2022, the approaches described had different maturity levels. Some already served commercial applications; others remained in development or targeted research. FFF and DED did not represent the entire multi-material offering, which also included material-jetting solutions.
R&D investment reflects interest in these possibilities, as Boeing’s Melissa Orme explains in the cited interview. The amount invested does not mechanically determine the user’s purchase price. Economic value should be assessed against total cost and application benefits. Neirinck highlights “high end use cases with often stringent requirements”.
As Neirinck recalls: “We must remember that we are, with additive manufacturing, competing with manufacturing techniques that have been around for at least the better part of a century”. So, there is reason for optimism when we look at how far the AM community has come.
This feature was first published in the January/February 2022 issue of 3D ADEPT Mag.
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