Joshua Evans, Applications Engineer and Head of Learning at BOFA International, takes a look at best practices for maintaining a healthy and productive environment in additive manufacturing settings.
Joshua Evans, Applications Engineer and Head of Learning at BOFA International.
This time last year additive manufacturing (AM) entered the consciousness of the wider public at the onset of the pandemic, when advanced manufacturers, along with 3D printing hobbyists, answered the call to create protective equipment for frontline health and care workers.
Of course, we know that AM has long been a cornerstone of advanced manufacturing in sectors such as automotive, defence, aerospace, medical devices and electronics, by virtue of its ability to shorten production cycles, lower tooling costs and reduce waste material.
With AM’s growing adoption across a range of materials and applications, it is worth reminding manufacturers of the need to keep any associated health risks to operatives from airborne contaminants under regular review. This is particularly important, not just for reasons of corporate responsibility, but to meet existing and emerging occupational exposure standards.
Some health, safety and environment legislation and guidance has been re-purposed to accommodate this evolving industry, and there are likely to be further changes emerging over time.
So, ‘watch this space’, as they say, because many studies confirm the presence of fume, gases and particulate in AM processes, highlighting the need for effective practices to capture potentially harmful emissions.
For example, Vat polymerisation and material jetting typically use either UV light or digital light processing to cure photopolymer resin. The resin is photoreactive, which then hardens as it is hit by the light. This process presents several opportunities for gaseous release. Firstly, there is the resin itself, which may contain some compounds which cannot wait to evaporate, even at room temperature. Then, some printers heat the resin which obviously elevates the temperature and provides more energy for other noxious gases to make their escape. And finally, there is the photoreaction itself where even more energy being applied can lead to further unwanted chemical by products (A. B. Stefaniaka, 2019).
Some of these released gases, such as acrylic acid and cumene hydroperoxide can be hazardous to humans. Some gaseous emissions are categorised as a mild irritant and others as fully toxic. These chemicals can have a wide range of effects from headaches to much more serious health conditions.
In another example, powder bed fusion (PBF) and directed energy deposition processes use a high energy source to melt specific areas of material. This can be in the form of a powder bed or solid wire in a variety of different materials from polymers all the way to titanium. When the high energy source hits the material, a very fine emission of particles can result. An example would be the laser from a PBF process hitting a metal powder bed. The ablation mechanism from the laser will instantaneously boil and then condense the metal powder, creating a cloud of ultra-fine particulate.
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To keep these processes controlled, typically the atmosphere inside the printer is sealed and sometimes composed of an inert gas. Or, where this isn’t viable, an inert source is aimed at the target area for deposition.
Due to the enclosed nature of the process, the risk to user health during printing is substantially decreased – however, this can result in particulate residue on equipment, which can compromise quality. Additionally, any build-up of particulate can lead to premature durability concerns as well as a risk to the user after printing when handling any finished component. Similar risks can be present when post processing, for example de-powdering.
In material extrusion, material is forced through a heating nozzle so it becomes pliable and the printer then layers the molten material until the final object is complete. The combined shear force and heating process of polymers breaks down the material but, unfortunately, emits a fume which presents a health risk to operatives. The particle sizes emitted are very small, well below 1 micron (Health and Safety Executive, 2019), and the emission rate of particulate increases with nozzle temperature.
Which brings me to a critical factor that AM users need to take into account - the size of particles emitted from the process. This is key to understanding the potential impact on health, notably how far into the human body any given airborne contaminate can penetrate.
Particles of 30 microns are roughly what you can see with the human eye; at 10 microns particles enter your mouth and nasal cavity; at 5 microns particles enter your respiratory tract; at 2.5 microns they can enter your lungs and particles around 1 micron will reach the extremities of your lungs (Praznikar, 2012).
Nanoparticles are worthy of special mention because, if not captured in an extraction process, they possess the ability to pass through membranes into the human body (Ostiguy C, 2008).
The key take-away here is to specify extraction technology that can demonstrably capture fumes, particulate and nanoparticles associated with the materials being worked, for example by reference to data sheets. Likewise, the volume of airborne particulate being generated must be considered to determine the filtration system best suited to the application.
And remember, from a productivity perspective, if you do not control particulate, this can also negatively impact AM printer efficiency and increase the risk of product contamination, for example through a build-up of sticky plastic droplets on critical components. This can lead to quality and reliability issues, costly unscheduled downtime and, in a worst-case scenario, the need to replace equipment.
So, with these risks in mind, I would urge manufacturers to choose a fume extraction system that incorporates a pre-filter (to remove larger particulate and, therefore, protect the more expensive main filter), a HEPA filter (to remove nanoparticles) and an activated carbon filter (to remove vapours and gases). Smart operating systems can also regulate airflow and monitor filter condition to optimise filter life and ensure timely exchange.
It is also worth considering the high temperatures involved in some AM processes. Here, a sealed filter exchange design can remove the risk of thermal events in pyrophoric material operations. And, under certain circumstances, manufacturers might also consider whether an application would benefit from fire-resistant materials for casings and filters, a spark arrestor and thermal cut-out protection to mitigate the risk of burning particulate entering the extraction system.
References
- A. B. Stefaniaka, L. N. (2019). Particle and vapor emissions from vat polymerization desktopscale 3-dimensional printers.
- Health and Safety Executive. (2019). RR1146 - Measuring and controlling emissions from polymer filament desktop 3D printers. London: Crown.
- Ostiguy C, S. B. (2008). Health Effects of Nanoparticles.
- Praznikar. (2012). The effects of particulate matter air pollution on respiratory health and on the cardiovascular system.
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