3D printing of composites and multi-materials

Definition

A composite is a substance comprising two or more bonded materials that, when combined, have properties that pertain to the individual components. If composite materials are additively manufactured, i.e. 3D-printed, the process can be compared to that of an inkjet printer, albeit in three dimensions.

In multi-material printing, several different materials are arranged separately, but in a three-dimensionally defined manner. In this way, composite materials can be produced. Among multi-materials, one variety that has emerged specifically from additive manufacturing consists of so-called ‘functionally graded materials’. These are substances whose properties change continuously along one or more three-dimensional axes, according to the part’s geometry or loads. Such a material gradient can be achieved if the process parameters or material composition are modified point-by-point, line-by-line or layer-by-layer, during additive manufacturing.

So far, the only industrial uses of additive manufacturing with composites or multi-materials have involved polymers, i.e. plastic. 3D printing of composites or multi-materials from ceramics, metal or other materials, such as elastomers, is still at the research stage and nowhere near product-ready. This article deals exclusively with 3D printing of composites and multi-materials based on substances other than polymers.

Various additive procedures are used, such as extrusion and binder jetting. In extrusion, molten filaments (thread-like structures) are pressed through a nozzle and solidified. Here, support structures sometimes have to be printed to secure the resulting product to the substrate or to stabilise overhangs. However, they have to be removed once the product has hardened. In binder jetting, thin layers of powder are applied in a powder bed, then selectively hardened by means of a liquid binder, which is applied by a print head. The resulting workpiece must then be debindered and sintered. Both procedures thus require downstream steps to obtain a stable and visually appealing final product.

Opportunities

Combinations such as ceramics in metal, metal with hard materials like tungsten carbide, or new types of iron and nickel alloys, make materials harder and less susceptible to wear. If such composites are made using conventional procedures like casting, there are shape restrictions. This is where 3D printing brings major advantages, as complex geometries can be realised. Additive manufacturing is particularly promising when it comes to making heavy-duty tools and, in general, components that are subjected to considerable mechanical stress or high temperatures over a long period of time.

Today, the first applications of 3D printing with non-polymer-based multi-materials can be found in the jewellery and dental sectors: Two-colour rings with gradual colour changes, for example, or bridges and crowns with natural colouring can be printed with customised shapes. Initial applications are also emerging in the energy sector, where high-voltage capacitors and photovoltaic cells can already be successfully printed using multi-materials.

Future applications of 3D printing with multi-materials are to be found in many industrial sectors. In medical technology and/or collaborative robotics, the construction of prostheses or gripper arms that combine soft materials like elastomers with pressure-resistant sensors to replicate fingers or other body parts is presenting itself as a possibility. Here, the sensor elements are printed simultaneously, rather than subsequently assembled. These functional structures are thus, so to speak, twins of the real body parts, which include the latter’s main functions, and can be used for training doctors and other healthcare professionals. Applications in the pharmaceutical industry are also conceivable, where 3D printing with multi-materials could be used to produce tablets with multiple active ingredients. These active ingredients are released at different locations in the digestive tract or at different times, depending on the material properties of the substances surrounding them. A similar approach is being pursued in the food industry (see 3D printing of food).

More applications are also to be expected in the construction and electronics sectors. With multi-material printing, it is possible to prefabricate ready-made building elements that already contain pipes and cables and which can be combined very easily on the construction site. While the printing of small-scale power electronic components, such as chips, is not to be expected in the coming years due to the inadequate resolution, larger electronic components could be manufactured using multi-material printing. Preliminary experiments for printing radio-frequency identification (RFID) chips have already been conducted successfully.

The possibilities that the printing of composites and multi-materials brings have not yet been recognised in industry. 3D printing is still a niche, partly because the procedure will not be suitable for mass production in the near future, even though the profitable batch size has increased greatly in recent years. However, the current focus on the production of special geometries and on customisation already offers numerous opportunities in many industrial sectors and in society. In industry, the opportunities are to be found on both the user side and the service-provider side. Processes for manufacturing new or existing products can be optimised (e.g. for lightweight construction) and manufacturing on demand becomes a reality, thus shortening supply chains, and eliminating transport and storage costs. Multiple materials can also be combined within one component in a single step. The technology is also a great opportunity for service providers, especially in the context of large structures.

In society, the increasing demand for personalised products can be satisfied: not only in medicine, but also in decoration and jewellery. In medicine, patients benefit from personalisation and (indirectly) from the new possibilities that the technology offers for the training of healthcare professionals.

Challenges

Printing structures made of more than one substance involves processing different materials simultaneously. This places new requirements on print heads, which can only be met by means of further development. At the same time, there is also a need for research and development on the materials side, so as to optimise printability and understand material properties. In gradients with metallic multi-materials, for example, a new alloy is created in the transition zone, the properties of which are not simply a combination of the feedstock materials’ properties and are barely predictable. This also affects the choice of printing parameters, such as temperature and printing speed, which have to be adapted to the new alloy, so as to avoid cracks and structural weaknesses in the product. Interdisciplinary teams have to rise to these challenges: Materials scientists need to look at the structural and manufacturing issues, and mechanical engineers have to acquire knowledge about materials.

Additively manufactured products based on metal or ceramics have to be compacted after printing: So-called ‘sintering’ transforms a porous component into an almost completely impermeable final product by bonding the layers and eliminating the pores between powder particles. During processing, the components are heated to a temperature below the material’s melting point so as to maintain the shape of the workpiece. This is not an issue for single materials with a defined melting point. For multi-materials, however, sintering is challenging because different materials sinter at different temperatures. One possible solution is selective sintering, where the sintering takes place continuously throughout the process rather than just at the end. However, this requires the processes to be digitalised, so that intervention is possible at any time.

As in all instances of mixed materials, the question of recycling arises: How can the materials used be recycled as homogeneously as possible? It is relatively simple to recycle components made by extruding molten filaments: These can be mechanically shredded and turned into filaments again, from which new components can then be printed. In the case of binder jetting, a more complex approach is needed because the powder mixture that remains in the powder bed, and has to be separated again, is also involved. Without homogeneous recycling, additive manufacturing cannot fulfil the promise of being more resource-efficient and sustainable than conventional procedures.

Building up expertise is important for SMEs

For SMEs, looking into the topic of 3D printing and building up a certain amount of expertise is worthwhile, even though there are very few situations in which purchasing the hardware would pay off. It is important to interact with service providers or universities at an early stage, so as to print prototypes and to test their quality and functionality. 3D printing services based on metals are being offered, but none based on other materials like ceramics. 3D printing of non-polymeric composites and multi-materials is not yet available as an industrial service, but does occur on the basis of cooperation with research institutions.

Specific example applications are described in the articles Sustainability thanks to weight reduction and Foot orthoses from the 3D printer.