Could additive manufacturing be the future of the plastics industry?

There has been a significant revolution in the world of materials as 3D printing has proved its potential for innovation. Now it must be put into practice. While 3D printers are increasingly present in prototyping workshops, they have been slow to replace the processes traditionally used in the plastics industry. This is because these technologies are too restrictive, in terms of both available materials and performance, which is still low for parts having specific uses.

Article written by Jérémie Soulestin, researcher at IMT Lille Douai.

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[dropcap]P[/dropcap]olymers now represent the vast majority of materials used in 3D printing for all purposes. But the use of additive manufacturing for industrial needs has remained primarily confined to metals. For industry professionals, the use of polymers is limited to prototyping. This reluctance to combine 3D printing and plastic materials on an industrial scale can be explained by the low diversity of polymer materials compatible with this process. The most widely-used technologies, such as stereolithography, rely on thermosetting resins which polymerize during the process. Yet these resins represent only a small share of the polymer materials used by the plastics industry. Fused deposition modeling (FDM) is slightly better-suited to industrial needs. It is based on thermoplastic polymers, which soften when heated during the process, then harden when they return to room temperature. This technical difference makes it possible to make use of materials which are more commonly used in the plastic industry and are therefore better aligned with market demand.

FDM technology was invented in 1989 by Scott Crump, founder of Stratasys, which is now one of the leading manufacturers of 3D printers. It is a very straightforward process. A filament made from thermoplastic polymer is fed into the machine. It is pushed through a heated nozzle to produce a malleable string measuring a few micrometers in diameter. The 3D part is obtained by continuously depositing this string, layer by layer, by moving the nozzle or the printer table, or both in all directions. The simplicity of the FDM process, coupled with the expiry of Stratasys’s patent for the technology, has led to exponential growth in 3D desktop printers. These small-scale machines are mainly intended for the general public and the maker community but have also made their way into design offices and corporate fablabs. Prices range from €220 for 3D printers for beginners who want to discover additive manufacturing to more than €2,000 for 3D desktop printers designed to create prototypes.

FDM technology: turning hope into reality

Although the FDM process is probably the best-suited solution for producing short-run plastic parts, it is not able to miraculously meet the needs of the plastics industry. Even if professional machines that are much more expensive than 3D desktop printers were to be used, there are drawbacks with the thermoplastic polymers available which limit the potential for FDM applications. The philosophy of FDM machine manufacturers – especially manufacturers of professional machines – is still based on the use of proprietary materials. In other words: for a certain brand of printer, only the polymers sold by that brand or its partners are compatible. The process is configured to ensure the quality of parts made using proprietary materials. Users have little control in terms of changing the settings or using other materials.  Even though some brands offer a wide range of materials with different characteristics (hard, soft, translucent, chemically-resistant, biocompatible etc.), they are still limited to a few thermoplastic polymers. The proliferation of suppliers of filaments for 3D printers has not made up for gaps in the catalogue of usable materials. For specific industrial applications – especially high-performance parts like automobile or aircraft components – but even for less technical applications, manufacturers have not yet been won over by this technology. The reason that thousands of different kinds of plastics exist is undoubtedly linked to the fact that each application requires specific properties.

On top of that, there are inherent problems with the FDM process. In general, parts made using FDM are more porous and rougher on the surface than those made with conventional processes like extrusion or injection. This is due to the fact that strings are deposited layer by layer. Because they have a cylindrical shape, a space is created between two strings placed next to one another. As a result, the surface of the part is not smooth and some cavities are located inside. This porosity can be controlled by applying high pressure on the material as the string is being deposited so that the cylinders are compressed and less space is left. But that still may not be enough to meet specifications for high-performance parts. Moreover, it does not solve the problem of surface roughness, since no layer can be compressed on the top layer. The only way to reduce this roughness would be to decrease the diameter of the cylinders and therefore the outlet of the filament nozzle, which would reduce the flow rate of material – and consequently increase production time.

Other promising technologies

These drawbacks of FDM could certainly be overcome by using machines based on polymer pellets instead of filaments. Manufacturers are taking steps in this direction in order to theoretically make it possible to use pellets of any thermoplastic polymer. The Freeformer technology marketed by Arburg, a German injection-moulding specialist, does just that. It is based on two injection units that can melt the polymer pellets, which makes it possible to create parts which require both hard and soft materials or use soluble materials. The molten polymer is deposited in the form of droplets to build the part, rather than in cylindrical threads. In this way, the process is somewhat different from FDM, but it is essentially based on the same principle, and is better-suited to meet the needs of the plastics industry. But research and development work must still be carried out to better understand the possibilities offered by this new process and tap into its full potential.

Future advances in 3D printing technology will depend on the use of additive manufacturing for mass production. For now, this technology is not suited to most applications that require high mechanical performances, but that could change. The aeronautics industry is a good example of the challenges that lie ahead for additive manufacturing. Although aircraft parts are still produced with conventional materials and processes, manufacturers are increasingly using composite materials. These composites are produced with techniques that resemble 3D printing. Combining them with technologies similar to FDM on robotic arms would be one way to greatly improve the performance of parts obtained through additive manufacturing. If such hybrid processes proved to be effective, the performance of 3D printed parts could improve significantly. It would then be possible to use additive manufacturing to produce structural aircraft components, instead of just their prototypes.