Last updated: 22.1.2020
Additive manufacturing is set to revolutionise conventional approaches to design, materials, processes and products. The potential for innovation will change and diversify accordingly.
Traditionally, draughtspeople produced technical drawings of structures based on specifications provided by architects and engineers. In the 1980s they began to use computer-aided design (CAD) instead of pens and paper. As CAD systems developed, and computers became more powerful, three-dimensional structures could also be realised in silico (3D-CAD). This led to more innovative AM systems, as computer models could be virtually sliced into layers and each layer manufactured. Such computer models made it possible to customise, redesign and produce any physical part.
Innovative and aesthetic designs can be protected by a variety of intellectual property rights. However, only structures that elicit a technical effect fulfil the requirements of patentability. AM processes create structures that were previously considered impossible or too costly to produce. For example, AM enables the accurate production of medical implants tailored to the anatomy of the individual patient. The design of such structures is the first innovative stage in the AM value chain.
Over the years, AM has been applied to all sorts of materials, starting with polymers, then metals and ceramics, and more recently biomaterials, composites and cements.
Different systems use different materials. Stereolithography uses a photosensitive liquid material that is polymerised by an ultraviolet laser in the same way that dentists harden modern tooth-filling materials. Recent developments allow ceramic or metal particles to be suspended in the liquid.
Powder bed systems can accommodate a wide range of materials, most of which are commercially available. Recent efforts to further develop the method have focused on the flowability of the powder and the density of the bed.
Mixtures of different materials that vary in composition or shape, as well as hybrid materials, can also be used to create an alloy or composite. Combinations of materials can be incorporated into the design of functionally graded products with locally optimised mechanical, chemical and/or physical properties.
Innovation in the area of materials for AM processes will be the key to manufacturing products with properties that match or exceed those of products made using established manufacturing methods.
The selection of polymers for AM purposes is determined mainly by the type of AM technology.
For material extrusion methods, a range of thermoplastic polymers can be used. These are melted before application and harden on cooling. Resins based on PLA, PC, and styrenics are most commonly used in this field.
Acrylates, epoxy resins, and polyurethanes, on the other hand, are the preferred resins for photopolymerisation techniques and binder jetting.
In powder bed fusion technology, polyamide resins are usually applied (e.g. PA6, PA11, PA12) as well as other polymer materials such as PEEK or TPU.
These polymers are usually specially developed for use in AM. They are also the subject of much research, since the end products currently suffer from drawbacks compared to those produced using traditional manufacturing methods - especially when it comes to dimensional stability, mechanical properties, porosity, speed requirements and sufficient resolution.
Different types of metallic powder have been developed for additive manufacturing: steels, particularly stainless steels and tool steels; aluminium alloys for aerospace applications; nickel and cobalt based alloys for turbine parts; titanium alloys for implants; copper alloys for heatsinks and heat exchangers; and noble metal alloys for jewellery.
Material optimisation focuses on two distinct aspects:
- the fine-tuning of alloy composition to improve interaction between the powder alloy and the energy beam
- powder rheology optimisation - in particular powder morphology, particle size, size distribution and flowability - to simplify and speed up the deposition of even powder layers and adjust the density of the final product
Furthermore, additively manufactured parts do not require a final sintering step when produced using powder bed fusion and direct energy deposition, for example. Therefore, they have a different microstructure from parts produced by conventional techniques like casting, forging or injection moulding. This means that specific heat treatments are currently under development in order to tailor the properties of the final AM parts to specific product requirements.
In the field of ceramics, selective laser sintering (SLS) is the most common technique for creating a 3Dform. Organic binders are generally added to the ceramic powder at the start to obtain a stable layer.
In other methods, layers of ceramic powder without a binder are deposited and a binder is then sprayed selectively on each new surface. This process is known as binder jetting; the object holds together where the binder is added and the rest disintegrates. The resulting 3D-object can then be sintered.
The most common ceramic material in additive manufacturing is zirconia (ZrO2), which is used for making teeth, crowns and other tailor-made dental objects. Alternatively, alumina (Al2O3) can be used for these purposes. Ceramic bone-like materials are generally made of phosphate- or silica-based materials. Silicon carbide (SiC) is the most commonly used non-oxide ceramic material in AM. It mainly features in the high-temperature side of turbine components. Here, infiltration techniques are often employed during or after additive manufacturing.
The field of biomaterials centres on the chemical aspects of implants. The materials and products involved must have certain mechanical, degradation or stability properties as well as a desired shape or ability to be processed. They must also interact appropriately with proteins, cells and tissues, and often be conducive to releasing drugs too.
Biocompatible compositions used for the additive manufacture of tissues are called "bio-inks" and comprise materials that mimic natural cellular matrix components. These form three-dimensional porous or hydrogel structures that support or stimulate tissue-regrowth. In hydrogel materials, the cells intended to form the new tissue are also often part of the bio-ink. Additive manufacturing allows these gels to be printed in complex shapes. In the porous materials, the cells are seeded after the structure is formed or the cells grow into the structure after implantation. One of the 3D-printing techniques used here is stereolithography, which uses bio-inks based on known biocompatible polymers that are functionalised to allow photo-crosslinking. These bio-inks can form structures with a high porosity and interconnectivity that are appropriately shaped to fill the tissue defect. It is even possible to provide structures with a printed capillary network to ensure that the cells in such scaffolds have enough nutrients and oxygen to grow into a new tissue.
AM is becoming a transformative technology for the construction sector, allowing architects and engineers to design more complex structures.
A similar technology to inkjet printing called contour crafting was the first method developed for the additive manufacture of building structures. This method uses larger nozzles and high pressure to extrude concrete paste.
Further processes use binder jetting technology, which is based on the deposition of a layer of reactive material such as Portland cement over a layer of sand.
Portland cement or calcium aluminate cement can be used as the powder bed with an aqueous solution of lithium carbonate as the binder.
Alternatively, 3D printed powder structures in a geopolymer system have been developed, wherein the powder bed consists of ground blast furnace slag, sand and ground anhydrous sodium silicate (an alkali activator).
The 3D printing of wet concrete poses several challenges. These include regulating the pumpability and the properties of the fresh concrete in order to have sufficient workability and open time for extrusion, as well as developing its structural properties and strength in particular. Such properties are of major importance when it comes to the complexity and size of the objects printed.
At least seven AM process categories have been described so far in the ISO/ASTM standard 52900:2015. They are binder jetting, directed energy deposition, material extrusion, material jetting, powder bed fusion, sheet lamination and vat photo polymerisation.
Depending on whether the material is immediately consolidated or not, a distinction can be made between a one- and a two-step process.
In the one-step process, the material is directly consolidated by curing, sintering or melting. Typical examples include directed energy deposition, powder bed fusion and material extrusion, which are also referred to as selective laser melting (SLM), electron beam melting (EBM), fused filament fabrication (FFF), etc.
In the two-step process the material is first bonded temporarily using a binder or "glue". In a separate step, the part is heated to induce its final consolidation. A typical two-step process is 3D printing or binder jetting.
Other distinctions in the process relate to the form in which the (solid) material is introduced and deposited: powder bed, direct deposition of powder or molten droplets, wire extrusion.
Whereas the first proprietary AM machines used either a curable liquid or a sinterable powder bed, current versions allow for more diverse forms of material and supply methods. For example, fused metal deposition (FMD) feeds the material in wire form. It is then melted locally and forms drops of material that solidify on deposition. Material can also be applied locally by using a nozzle through which powder is fed and then directly melted by the energy beam in flight or within the melt pool formed by the beam. In both cases, material is fed with high accuracy to where it is needed in the amounts required.
In the technique of binder jetting or 3D printing, the precise patterning of additive manufacturing is combined with a more classical heating process applied to the final part. As these heating processes have been used for many years, the product properties obtained are more predictable. The product can be built faster, as no heating or melting of the material is required during the process. As both processes (building and heating) take place in different installations, the production output can be organised more flexibly.
Depending on the materials and the application, requirements may arise concerning heating and/or cooling, as well as creating an inert atmosphere or sterile conditions.
AM was once used mainly for rapid prototyping. Now that it is increasingly used to manufacture end-use products, increased speed, quality, productivity, and process control have become more important.
Regarding speed, increasing the number of energy sources and/or material distributors has been an important focus of recent developments. Alternatively, various handling units may be combined to generate a "continuous process" approach.
In addition to productivity, the quality of end products is important, especially in fields such as aerospace and medical technology. By introducing such elements as cameras, sensors, control units and calibration means into the build chamber, quality control can be done in situ and corrective measures taken as required.
Historically, AM or 3D-printing was mainly used for the production of prototypes, particularly in the automotive and aerospace industries. There is now a growing number of private individuals who produce and print their own objects at home. However, AM is also increasingly used in the commercial manufacture of end products.
AM is currently particularly attractive to two main types of manufacturer: those providing customised or individualised products, and those producing objects that would otherwise be technically or financially impossible to make.
- Personalised medicine
Additive manufacturing has already had a positive impact on people's everyday lives. Customisable tools or implants as well as new forms of medication are improving the health of millions. 3D-printing technology for producing prostheses and implants adapted to the anatomy of the individual patient is well established.
Depending on the type of implant, both biodegradable or resorbable materials and permanent materials can be used. In addition, a skeleton of an implant such as a heart valve may be printed and subsequently colonised with cells. There are also promising developments in biotechnology-related applications.
In orthodontics, customised brackets or even archwires, but more often mouthpiece aligners, can be tailored to a patient's needs. Clear aligners became available in the orthodontics industry in the early 2000s. Users benefit from the aligners' discreet appearance and greater flexibility.
- Cardiovascular implants
In the area of cardiovascular implants, 3D-printing technology has become an established means of producing implants such as stents, grafts and heart valves tailored to the anatomy of the individual patient. For this purpose, patients are usually scanned (MRI, CT), the dynamics of the vasculature are calculated and, based on this data, an individual prosthesis or implant is designed and printed.
In the case of heart valves, for example, a skeleton of an implant may be printed and subsequently colonised with cells. However, 3D-printing technology is not yet widely used for the serial production of standard-sized implants as traditional technologies are still more cost-effective and/or deliver an end product that offers better mechanical properties.
Additive manufactured vascular stents on the other hand can be customised with regard to shape, choice of material, wall thickness and radial strength. Depending on the type of implant, biodegradable or -resorbable materials (polymers and metals, e.g. magnesium) as well as permanent materials are used.
The role of stents is limited to the moment of intervention and shortly thereafter, i.e., until healing and re-endothelialisation are achieved. However, conventional 3D-printed stents are usually made of non-resorbable metals (stainless steel, titanium, nitinol). They therefore remain in situ even after vascular repair and can damage the vessels. To overcome this, bioresorbable stents (BRSs) were introduced.
The latest stents exhibit some significant advantages over the conventional metal stents, such as complete bioresorption, mechanical flexibility or the production of no imaging artefacts in non-invasive imaging modalities. But conventional ways of manufacturing BRSs, such as weaving or laser cutting, have proven problematic. AM is therefore being explored as a means of producing this new type of stent.
The technology is also used for patient-specific operation planning and for training surgeons, as well as for the further development of standardised implants. Here, the vasculature of a patient is printed using materials with different properties to simulate e.g. calcification.
- Prosthetic bone implants
Additive manufacturing is well established as a means of providing prostheses adapted to the needs of the individual patient. Since the implant is individually shaped, much more of the host bone can remain in place as it does not need to accommodate a standard, off-the-shelf implant. Moreover, the proportions and surfaces of joint prostheses can be tailored to an individual knee, hip or shoulder.
In traumatology, where larger areas of lost bone must be replaced, implants can be customised such that an individual patient's physical appearance can be reconstructed.
The stiffness of the implants can also be customised. An open-porous structure can allow for bone ingrowth too, a particular advantage of AM. The technology is currently being applied to cranial, jaw or facial implants and bone fixation plates, screws or nails.
Recently, 3D printing has been used directly in the human body. For example, cartilage prostheses can be manufactured inside the body. Examples of this technology are disclosed in patent applications such as WO2017205663, WO2014110590 and WO2017080646. Under the EPC, however, surgical and therapeutic methods practised on the human or animal body are excluded from patentability.
The use of additive manufacturing to print organs is in research and development. The next step would be to use living material to replicate and replace organs or blood vessels. For example, skin or cartilage could be printed and then grafted on to a living organism.
Additive manufacturing enables the production of objects with a complex shape in a single piece body structure. Bicycle frames represent a great opportunity for the application of metal additive manufacturing, and selective laser melting in particular. Carbon fibre can also be used in the form of a towpreg deposited by a robotic arm. Another advantage of AM is that design improvements can be implemented right up to production. Further, topology optimisation deposits the material only where it is needed, thus allowing the frame to be customised to the body shape of the cyclist and incorporate other unique features too.
Extensive use of CAD algorithms ensures that the frame is as efficient and light weight as possible whilst maintaining the required structural strength. Recently, the companies Carbon and Specialized collaborated to produce a lightweight, breathable 3D-printed bicycle saddle with a hollow lattice structure (made from EPU 41) that enables the product to rebound quickly. Not only did AM technology enable the production of a design that would otherwise have been impossible to realise. The saddle was also designed and developed in half the time, a process that still involved testing over 70 versions.
The prospect of producing lightweight parts without compromising safety is of the utmost importance in the aviation industry. Further development of additive manufacturing technologies will enable more and more aeroplane parts to meet these needs. Jet engines are already being built using additive-manufactured parts, of which there are numerous further examples: borescope bosses, door latch components, fuel nozzles, turbine blades, double-walled pipes, single-piece leading edges with integral duct coupling and many others.
Turbine blades represent a very promising field of application for AM due to their complex geometry. Those made of nickel superalloy powder offer a recent case in point. Additive-manufactured blades with an improved internal cooling system constitute another success story. AM also has the potential to reduce the lead time for such products to a few months.
Additive manufacturing can deliver complex forms that were previously scarcely available in the construction sector. Furthermore, whenever a hurricane or an earthquake destroys infrastructure, leaving thousands of people homeless, 3D printers can be used to quickly rebuild bridges, highways and residences.
Thanks to their low cost and the speed with which they can be constructed, 3D-printed houses can present a practical option when it comes to social housing. A single-storey 60 m2 house can be built in 12 to 24 hours and at a lower cost than traditional technologies offer. Given that the technique can be automated, the construction of buildings in hazardous environments (or even on the surface of other planets) can be undertaken without exposing builders to danger.
The 3D printing of buildings represents a large-scale AM process that can take place either in situ or in a factory. For this purpose, new large scale 3D printing systems are being developed, such as 3D printing heads and 3D printing units, whether stationary or mobile. Ultra-high performance concretes are another focus area of recent innovation. Such ventures demand the interdisciplinary cooperation of professionals from the fields of civil, architectural, chemical, mechanical, electrical and computer engineering.