Additive manufacturing (AM), or 3D printing, is an integrated processing technique that unlocks the ability to design and fabricate objects in a parallel operation. This flexibility of material and shape is something that is simply out of reach for traditional manufacturing techniques. An added benefit is the significant reduction in process related waste materials, due to the precise nature of application of material. AM can alleviate environmental concerns in this respect by promoting the reduction of material use. However, AM also has a potential role in the development of a much needed circular economy. The how and why for the known methods of 3D-printing shall be illustrated further.
Vat polymerization is a type of additive manufacturing that does not utilize a flow of material to produce three-dimensional objects. Instead a volume of liquid photopolymer is selectively subjected to ultraviolet (UV) light that results into a cured resin. A platform carrying the object will move along the height of the volume to allow for the addition of another layer of photocured resin. Due to a lack of support from the medium itself, a bracing structure is necessary. Two variants are currently widely applied in rapid prototyping, namely, stereolithography (SLA) and digital light processing (DLP). The difference being the surface area of the UV-light, where SLA uses a point source and DLP applies a large surface area where selectivity is achieved by reflecting the light via a digital mirror that has the ability to vary reflectivity along its surface area. This method is usually applied to rapid prototyping due to its speed of application, high resolution and comparatively higher cost1,2.
The objects produced with vat polymerization are generally disposed of after curing. These thermosetting materials usually being difficult to reuse due to the non-reversible network formation. The unused feedstock is reused in subsequent cycles however, which corresponds to a highly material efficient production process. Oftentimes the technology is applied to prototyping, which means that the volume of throughput is considerably lower compared to other AM techniques, let alone industrial production. Low volume means a bad business case for recyclers and thus it makes recycling of vat polymerized prototyping articles a more difficult proposition. However, local reuse by the protyper itself shows growing potential with developments in photosensitive vitrimers. These vitrimers exhibit chemistry where network formation is mobile or even reversible. Recycling with retention of mechanical properties is a proven reality and full reversibility and reuse in vat polymerization might become actuality with further developments3. Here at Ecoras, we are currently working on improving on this concept in collaboration with NHL Stenden. The goal of this innovation-project is the development of a vitrimer with appropriate properties for defined applications and circular utilization.
Material extrusion, or fused filament fabrication (FFF), is probably the most well-known type of additive manufacturing due to its high user friendliness and easy accessibility. The process is most commonly applied with thermoplastic filaments that are extruded with a heated print nozzle. The cartesian mobility of this small extruder provides the ability for layer after layer application of heated thermoplastic. Bracing is usually added to the structure by the nozzle due to need for structural support of the object. Advantages of the technique are low investment and operational costs and high automation. The resolution of printed objects is usually lower compared to more precise additive manufacturing methods and thus some post-processing, such as sanding, is often required. Binding between the different layers is also mechanically weak, due to the deposition of molten material on top of already cooled substrate. This results in weaker mechanical properties for the object in z-direction (height). Material extrusion is more recently also applied with thermoplastic granulates, which removes the need for the extra processing step of fabricating the filament 4,5.
Material extrusion shows the highest potential for facilitating circular economy of all other additive manufacturing techniques. While other additive manufacturing methods, such as the fusion based methods, loan themselves well for the repair of articles, material extrusion is exceptionally well suited for the use of recycled plastics6,7. In fact, in the fused filament fabrication industry, filaments from recycled plastics are currently already commercially available. In this way material extrusion can provide a decentralized infrastructure that can use recycled plastics at a significant scale for the production of tailored and on demand articles. We at Ecoras are currently developing a large scale granulate printing technique with our partners and developing boundary conditions for the use of recycled plastics.
Sheet lamination is the odd one out out of all the other additive manufacturing techniques, with the need for extensive post-processing. Thin layers of polymer, paper or metal are stacked, supplied by a roller and bonded together. This bond can be provided by a binding adhesive for paper and plastic layers or heat and pressure or ultrasonic vibrations for metal sheets. The piled up sheets are then subjected to precise cutting with CNC machines to produce the three-dimensional object. Sheet lamination is a cost effective technique to quickly produce moulds or protypes without moving parts and allows for the easy addition of reinforcing materials to produce composites. The technique is simple and straightforward, but the post-processing produces a significant amount of waste compared to the other processes4.
Sheet lamination has the lowest potential for improving circularity of supply chains compared to the other additive manufacturing methods. This can be attributed to the large amounts of process waste associated with the extensive post-processing. For non-metal feedstock the process is usually executed without the necessity of heating, which provides an energetically favourable process. However, literature does not provide a route for the use of recycled content.
Inkjet printing is an additive manufacturing technique that is inspired by the basic printing of paper-documents and uses an inkjet nozzle. Small amounts of material can be precisely applied to a substrate in a highly reproducible and non-contact manner, such that the technique is ideal for applying graphical patterns to objects and build circuit boards for electronic devices89. It is also an upcoming technology in the medical industry, such as for stem cell printing, due to the prevention of cross-contamination as a result of the non-contact property10. While the high resolution and reproducibility are major benefits, the technique suffers from a low printing speed and high cost 8.
The niche application range of injet printing in the electronic and medical industries make it a less suitable candidate for closing the loop. Literature does not report on its role within the circular economy. Low volumes and low tolerances for exceeding boundary conditions shape this result.
Binder jetting is a composite printing technology that employs two phases to achieve structural integrity. First a thin layer of powder is deposited and compacted over a movable build platform and then a nozzle selectively applies a liquid binder that essentially glues the powder together. The movable platform subsequently moves downwards, which allows for the addition of another layer of powder. The powder that occupies the ‘un-glued’ volume provides bracing for the printed objects during processing and can be easily removed and re-utilized during later processes. This does restrict the addition of pores or other problematic hollow structures, due to the difficulty of removing residual powder11,12.
Binder jetting, as with most additive manufacturing techniques, benefits from a high material efficiency. Up to 96% of not consumed powder, can be reused through several cycli with an insignificant loss of properties for the printed object, with the best results achieved with depowdering with soft-brushes instead of air13. The composite nature of articles produced with binder jetting makes recycling of printed volumes difficult. However, literature reports on the possibility of separating thermormoplastic binders from the fillers with aqueous washing (depending on the filler) or selective dissolution. Such routes are not yet possible with cured binders, due to the irreversibility of formed networks. Solutions for this route can be found in chemistries with reversible or dynamic covalent networks (vitrimers)14. Literature does not report on the use of recycled thermoplastics used as binders, as of yet.
Powder bed fusion shows significant similarities when compared to binder jetting, however, the method of powder consolidation differs. Instead of using a liquid binder, the powder is fused together with the application of heat. This heat is usually applied by the use of a laser or a thermal pinhead. Powder bed fusion processes using a laser are usually called selective laser melting (SLM), selective laser sintering (SLS) or Direct metal laser sintering (DMLS), between which the differences are minute. Thermal pinhead processes usually come in the form of selective heat sintering (SHS). This process is variable in the use of printing mediums by allowing for the fabrication of both metal and thermoplastic objects out of their respective powders15,4.
Directed energy deposition is an additive manufacturing process that is applied uniquely to metals only. In contrast to metal powder bed fusion, a powder or a wire of metal is deposited with a movable nozzle, while simultaneously applying energy to fuse the material on top of a fixed object. A variety of energy sources are known to be applied, but most commonly utilized are laser and electron beams or plasma arcs. Drawbacks are the necessity for the use of inert gas to prevent oxidation and post-processing with CNC machining to improve resolution16,17.
Powder bed fusion and directed energy deposition, like all other AM techniques, benefit from a high material efficiency by producing less process-waste. An effect that can be further compounded with unused powder recycling18 for directed energy deposition. A thing that is already commonplace for powder bed fusion.
Its low volumetric throughput and precise application of feedstock make it an excellent option for the repair, refurbishment and remanufacturing of metal articles used in f.e. the automotive or turbine industry. This application is enhanced by the associated lower distortion and warpage and the excellent bonding between layers when compared to traditional welding techniques19. A similar thing can be observed with powder bed fusion, where the speed of manufacturing is a benefit by offering on demand spare parts and reducing down-time significantly. An effect that is magnified by the ability to repair said spare parts when defects occur20. The key for these processes is the increase in lifetime of metal articles. Powder bed can extend even further with ability to deposit wear or corrosion resistant coatings21.
To recap; most techniques provide and excellent opportunity to increase material efficiency and high variability within manufacturing. Especially powerful within the framework of circular economy are the vat polymerization and material extrusion techniques. Though, material extrusion is most suitable for large scale industry with its capacity for a high throughput of material. This analysis shows the potential for circular economy, but neglects the impact on a macro-economic level. To take a deep dive into this crucial aspect of circular economy, a follow-up piece will be posted soon.
Article by Wybren Kalsbeek
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