Additive manufacturing (AM), also known as solid freeform fabrication (SFF), binder jetting or 3-D printing, is the direct fabrication of three-dimensional products and components, layer upon layer with the guidance of a digital design.
AM has existed for decades (3-D printing was developed within the Massachusetts Institute of Technology in the late 1980s) (Crawford, 2011) but became something of a buzz word in the late 2000s as technological developments opened up the potential for the technique to transform the manufacturing industry.
It is changing how manufacturing companies design parts, components, products and structures by enabling new levels of control, freedom and flexibility not afforded by traditional processes.
Additive manufacture is an umbrella term that refers to a group of technologies used to build three-dimensional objects. Currently the most common technologies are Fused Deposition Modelling (FDM) and Selective Laser Sintering (SLS).
Fused Deposition Modelling
Although there are professional and industrial FDM printers, most printers of this type are consumer-grade and subsequently the most affordable type of 3D printer. FDM works by laying down layers of material from either plastic or metal filaments. The filament is fed through a heated moving head that melts and extrudes it, layer after layer, into the desired shape. Myriad polymeric materials are available e.g. ABS (acrylonitrile butadiene styrene), PLA (polylactic acid), Polycarbonate (PC), PA (polyamide), PS (polystyrene), with different trade-offs between strength and temperature properties. Filament types will therefore be selected based on the desired end properties of the finished part. During FDM, the hot molten polymer is typically exposed to air. Operating the process within an inert atmosphere can prevent oxidation and significantly increase the inter-layer adhesion and overall mechanical properties.
Selective Laser Sintering
In contrast to the hot extrusion of filaments in FDM, SLS uses a laser to sinter powdered material at points in space defined by the 3D model, binding the material together to form a solid structure. Plastic, metal, ceramic or glass powders can be used to manufacture the desired three-dimensional shape. The physical process can be full melting (SLM, selective laser melting), partial melting or liquid-phase sintering. Depending on the material, up to 100% density can be achieved with material properties comparable to those from conventional manufacturing methods. In many cases large number of parts can be packed within the powder bed, resulting in high productivity and efficiency.
FDM vs SLS
As noted previously FDM is a relatively inexpensive technology allowing affordable access to the personal market. In contrast the use of high power lasers, combined with the complexity of the instruments and associated high costs, SLS is typically only seen in the industrial environment although efforts continue to be made by companies focused on bringing this technology to the individual consumer. A distinct advantage of SLS is that it is fully self-supporting (by the powder), allowing for parts to be built within other parts as well as overhangs i.e. complex geometries that are not possible with FDM. SLS can offer a variety of properties such as strength, durability, chemical resistance and functionality, making it an excellent substitute for plastic parts typically manufactured by injection moulding.
Due to the variety of materials that can be used in SLS (plastics, metals, ceramics or glass), SLS is being increasingly used in industries where small quantities of high value parts are needed e.g. aerospace, automotive.
When adopting any new technology in its relative infancy, it is recommended to take the necessary safety precautions. With any rapidly developing technology, it can be a challenge to identify and implement safety standards that adequately address specific risks. Whilst established industries rely on decades of data to develop and support risk management plans, an industry such as additive manufacture is too early in its development to have substantive health and safety studies to benefit from.
In 2017 the European Agency for Safety and Health at Work (EU-OSHA) published a discussion paper on the processes and materials involved in 3D printing, potential implications of this technology for occupational safety and health and options for controlling potential hazard . Most concerns involve gas and material exposures, in particular nanomaterials, material handling and static electricity.
Due to the closed and controlled nature of modern SLS instruments, and the availability of low cost desk top 3D printers, the majority of investigations have been focused on FDM. Various studies have noted particle emissions from a fused filament can include a large number of ultrafine particles (diameters less than 100 nm) and volatile compounds where concentration peaked a few minutes after printing started and did not return to baseline levels until 100 minutes after printed had finished.
A further study was undertaken which utilised a combination of state-of-the-art instrumentation to map particle concentration and morphology (CPC, SMPS, OPS) and physical sampling for offline analysis (chemical speciation, morphology, elemental composition) using specialised analytical techniques (ATR-FTIR, TEM, EDXS). Results further indicated that 3D printing generates high number concentrations of particles in the ultrafine and nanoscale range. The report suggests that, as an area with limited standard analytical techniques, toxicological implications and regulatory guidance, future studies should target the development of analytical techniques, ventilation recommendations, and establishing suitable printer locations with respect to occupied locations.
Whilst studies are more limited for the SLS process, some consideration can, and should still be given to the following:
Exposure to Metals
The use of powdered metals in SLS continues to grow and it is therefore important to understand the risks of exposure to these materials. In the additive manufacturing industry the average particle size is typically within the 25 – 150 micron range (Berretta et al. 2013), requiring special handling and storage. Exposure to metal powder and metal fume represents a genuine risk, not only through inhalation exposure but also exposure into open cuts and the eyes.
Monitoring Gas Levels
SLS machines run in inert atmospheres where the inert gas displaces oxygen. SLS machines are typically located in small rooms, installing an oxygen sensor is recommended wherever these machines are placed.
Depending on the materials, SLS can expel dangerous gases during operation, requiring ventilation to the outside of the building. Safety training should cover off-gassing and management of volatile organic compounds (VOCs) and inorganic compounds (e.g. metal fume). Companies are recommended to develop an air management plan as part of their risk-reduction strategy.
Handling of Materials
It is recommended that companies develop delivery, handling and storage procedures for SLS powders, particularly metals where powdered metals can be explosive.
SAFENANO is involved in supporting the development, commercialisation and sustainability of advanced materials and technologies in a number of collaborative EU-funded and commercial projects, through our expertise in hazard assessment, exposure assessment and risk management.
For further information on SAFENANO’s Services, click here.
Berretta, S., Ghita, O., Evans, K. E., Anderson, A., Newman, C., 2013, Size, shape and flow of powders for use in Selective Laser Sintering (SLS). University of Exeter, College of Engineering, Mathematics and Physical Sciences, Exeter, United Kingdom and Invibio® Biomaterials Solutions, Thornton-Cleveys, United Kingdom
Parham, A., Zhao, D., Pouzet, C., Crain, N. E., Stephens, B., 2016, Emissions of Ultrafine Particles and Volatile Organic Compounds from Commercially Available Desktop Three-Dimensional Printers with Multiple Filaments. Environ. Sci. Technol., 50(3) pp 1260 – 1268
Stefaniak, A. B., LeBouf R. F., Yi, J., Ham, J., Nurkewicz, T., Schwegler-Berry, D. E., Chen, B. T., Wells, J. R., Duling, M. G., Lawrence, R. B., Martin Jr., S. B., Johnson, A. R., Virji, M. A., 2017, Characterisation of chemical contaminants generated by a desktop fused deposition modeiling 3-dimensional printer. Journal of Occupational and Environmental Hygiene., 14(7) pp 540 – 550
Wojtyła, S., Klama, P., Baran, T., 2017, Is 3D printing safe? Analysis of thermal the thermal treatment of thermoplastics: ABS, PLA, PET and nylon. Journal of Occupational and Environmental Hygiene 14(6) pp D80 – D85