Respiratory exposure to hazardous chemical substances during additive manufacturing using nylon and alumide
Background: In the additive manufacturing (AM) industry there is very little information available regarding the area emission of and personal exposure to HCSs [particulates, metals and volatile organic compounds (VOCs)] during selective laser sintering (SLS), utilising nylon and alumide. It is possible that HCSs are emitted into the air during the pre-processing, processing and post-processing activities, which could lead to respiratory exposure, followed by the development of adverse health effects. Aims: To determine the physical and chemical composition of both virgin (new) and used nylon and alumide. In addition, the aim was also to determine the area emission of and personal respiratory exposure to HCSs that take place during SLS utilising nylon and alumide powder. Methodology: The physical and chemical composition of virgin and used nylon [nylon-12 (PA2200)] and alumide was determined with particle size distribution (PSD), scanning electron microscopy (SEM) and X-ray fluorescence (XRF) analysis. Area emission of particle size fractions 0.01 μm to > 1μm was determined with a TSI Model 3007 Condensation Particle Counter (CPC) (TSI Inc., Shoreview, Minnesota, USA), while the area emission concentration with a particle size range of 0.3 – 10 μm was determined with a TSI AeroTrakTM Airborne Particle Counter (APC) (TSI Inc., Shoreview, Minnesota, USA). National Institute for Occupational Safety and Health (NIOSH) Methods 0500 and 7300 were used to determine the area emission of and personal exposure to inhalable and respirable sized nylon and alumide dust. A Gillian Gilair Plus sampling pump (Sensidyne, Clearwater, Florida, USA) was calibrated to a flow of 2 L/min and connected to an Institute of Occupational Medicine (IOM) sampler to determine the inhalable and respirable sized nylon and alumide dust. Personal Nanoparticle Respiratory Deposition (NRD) samplers were connected to Gillian Gilair sampling pump calibrated to a flow of 2.5 L/min to determine the personal exposure of AM operators to particles < 300 nm. Traceair VOC badges were used to determine the area emission of and personal exposure to VOCs during all three phases of SLS utilising nylon and alumide. Results: PSD results indicated that all virgin and used nylon and alumide fell into the inhalable particle size range (63.85 - 65.30 μm) and that the measured PSD differed from the particle sizes listed in the Material Data Sheets (56 μm – 60 μm). There were no statistical significant differences (p < 0.05) found between the volume weighted mean particle sizes of virgin (new) and used nylon and alumide powders. The SEM analyses also confirmed that there were no visible differences in the size and shape between virgin and used nylon and alumide. XRF analyses found that virgin powders consisted of 39% of aluminium (Al), while used powders consisted of 51% of Al. An increase in particle number concentration was identified during all three phases of AM, when compared to the corresponding ambient readings. An increase in particle number concentrations were identified for specific activities, such as machine cleaning, powder mixing, machine warm-up, build removal and removal of excess powders from the build. For all APC results, the 0.30 μm particle size fraction indicated the highest concentration compared to 0.5, 1, 3, 5 and 10 μm. During gravimetric area emission sampling of SLS utilising nylon over an entire shift, low concentrations of inhalable and respirable sized dust was detected, while the results for SLS using alumide was below the detection limit. The personal exposure results indicated that the highest concentration of exposure to inhalable sized nylon and alumide dust took place during the post-processing activities (5.52 mg/m3 and 5.32 mg/m3). The personal exposure to respirable sized nylon and alumide dust only took place during post-processing activities (0.18 mg/m3 and 0.59 mg/m3). All personal particulate exposures were below the respective OELs for total inhalable and respirable dust. Small concentrations of particles < 300 nm were also detected during SLS using alumide. Aluminium (Al), iron (Fe), titanium (Ti) and zinc (Zn) were metals found in personal and area samples during SLS with alumide. Acetone, pentane, chloroform, toluene and naphthas were the VOCs detected during area emission and personal exposure during SLS utilising nylon and alumide. All personal exposures to VOCs were well below respective OELs. Conclusions: This study confirmed that HCSs (particulates, metals and VOCs) are emitted into the workplace atmosphere during SLS with nylon and alumide. However, the concentration HCSs emitted were very low according to standards. AM operators experienced personal exposure to these HCSs (particulates, metals, and VOCs) during this process, especially during post-processing. It was recommended that a portable local extraction ventilation (LEV) system should be used, especially for manual handling of powders during pre- and post-processing activities to be able to capture particles before they become airborne. Isolation of machines were also recommended to minimise the concentration of particles emitted. Cleaning protocols and personal hygiene measures are two important administrative control measures which should be revised.
- Health Sciences