Characteristics, formation mechanisms, and control methods ​of emissions from 3D printing​

Additive manufacturing (AM) has been an active area of research due to its extensive application in many fields such as mechanical engineering, marine engineering, construction, bioengineering, and electronic engineering to manufacture products with complicated geometries and advanced material properties. However, particle emissions from desktop fused deposition modeling (FDM) 3D printers were first reported in 2013 [1]. Since then, most of the studies have focused on the characterizations of the particulate matter (PM) and volatile organic compounds (VOCs) emitted from FDM 3D printers [2], which have adverse health effects on users exposed for prolonged durations. To predict the characteristics of emissions and provide a scientific basis to control emissions, it is necessary to understand the formation of emissions. This work has investigated the characteristics, formation mechanisms, and control methods of hazardous emissions from 3D printers. Since most attention has been focused on desktop FDM printers, particle emissions from other kinds of 3D printing techniques are also of concern to occupational health but have since been less explored. Here, on-site particle concentration levels were examined for polymer filament-based FDM, metal- and polymer-based powder bed fusion (PBF), metal powder-based directed energy deposition (DED), and ink-based material jetting (MJ). Particle concentrations in the operating environments of users were measured using a combination of particle sizers including scanning mobility particle sizer (10-420 nm) and optical particle sizer (0.3-10 µm). The number and mass concentrations of submicron particles emitted from a desktop open-type FDM printer for acrylonitrile-butadiene-styrene (ABS) and polyvinyl alcohol (PVA), approached and significantly exceeded the nanoparticle reference limits (4 × 104 #/cm3), respectively. On the other hand, caution should be taken in the pre- and post-processing of metal and polymer powder. Specifically, one to ten micrometers of particles were observed in the air during the sieving, loading, and cleaning of powder, with transient mass concentrations ranging from 150 to 9000 µg/m3 that significantly exceeded the threshold level (150 µg/m3) suggested for indoor air quality. Automatic systems that enable ‘closed powder cycle’ or ‘powder-free handling’ should be adopted to prevent users from unnecessary particle exposure. While the standard chamber method has been widely adopted to measure particle emissions from an FDM printer, there was obvious inconsistency and uncertainty in terms of particle emission rates (PER, #/min) being measured, owing to different measurement conditions and calculation models used. Here, a dynamic analysis of the size-resolved PER was conducted through a comparative study of the chamber and flow tunnel measurements. Two models to resolve PER from the chamber and a model for flow tunnel measurements were examined. It was found that chamber measurements for different materials underestimated PER by up to an order of magnitude and overestimated particle diameters by up to 2.3 times, while the flow tunnel provided more accurate results. Field measurements of the time-resolved particle size distribution (PSD) in a typical room environment could be predicted well by the flow tunnel, while the chamber measurements could not represent the main PSD characteristics (e.g., particle diameter mode). Secondary aerosols (>30 nm) formed in chambers were not observed in field measurements. Flow tunnel was adopted as a possible alternative for the study of 3D printer emissions to overcome the disadvantages in standard chamber methods and as means to predict exposure levels. For the characterization of emissions from FDM printers, the percentage of PM in total emissions (i.e., the nucleation ratio of evaporated substances) was still unknown. Here, we directly measured particle emission yields from the extrusion process of filaments using an FDM 3D printer in a chamber and at the same time, indirectly measured total evaporated substances yields using a proposed weight-loss method (called TVOCWL). The nucleation ratio of evaporated substances was estimated by comparing the particle and TVOCWL results. It was found that TVOCWL mass yields were 0.03%, 0.21%, and 2.14% for polylactic acid, ABS, and PVA, respectively, at 220℃. Among TVOCWL, particle mass accounted for 1% to 5% of TVOCWL mass depending on the type of filaments used. Important research gaps in the mechanisms leading to the formation of both UFP and VOC from FDM 3D printers remain. Here, we further characterize the formation mechanisms of emissions from polymer filaments commonly used in FDM 3D printing. The temporal relationships between the amount and species of VOCs at different operating thermal conditions were obtained through a combination of evolved gas analysis (EGA) and thermogravimetric analysis (TGA). This is to capture physicochemical reactions, in which the furnace of EGA or TGA closely resembled the heating process of the nozzle in the FDM 3D printer. It was generally observed that emissions initiated at the start of the glass transition process and peaked during liquefaction for filaments. Initial increment in emissions during liquefaction and the relatively constant decomposition of products in the liquid phase were two main VOC formation mechanisms. Also, fumes from an FDM printer were directly captured using a laser imaging method. It was observed that fumes originate from the printer nozzle and newly deposited layers during printing, where control measures should be targeted. Having amassed in-depth knowledge in the formation mechanisms of emissions from FDM printers, control methods were investigated to decrease the amount or change the chemical composition of the emissions. It was found that low heating rates had the potential to restrain the formation of carcinogenic monomer, styrene, from ABS while reusing filaments or pre-conditioning for filaments before use has the potential to decrease the level of emissions. On the other hand, a sucking ring design around a printer nozzle was proposed, that can prevent the diffusion of fumes produced. The removal efficiency of the sucking ring for both particle and VOC emissions was higher than 90%. In summary, this thesis has systematically and comprehensively investigated the characteristics, formation mechanisms, and control methods of particle and VOC emissions from desktop FDM 3D printers. Investigations into on-site particle emissions from a wide spectrum of AM techniques were also conducted, which provided a preliminary understanding of possible particle emissions from a diverse range of 3D printers. For FDM printers, the proposed flow tunnel method provided an alternative to the standard chamber method, to more accurately measure the characteristics of particle emissions. In addition, the proposed TVOCWL measurement method based on the weight-loss analysis provided new insights into the characteristics of the emissions. Separately, investigations in the formation mechanisms contributed to the understanding of the formation of particle and VOC emissions from FDM printers. To minimize the exposure of operators to potentially hazardous emissions, the proposed sucking ring design is a simple and effective method to control emissions from FDM printers. We also recommend the use of closed-type 3D printers with control measures (e.g., use of an enclosure with internal air filtration) and “green materials” with fewer additives. For other industrial-scale 3D printing techniques, additional automatic systems need to be incorporated for pre- and post-processing to achieve ‘powder-free handling’.Doctor of Philosoph

Particle emission levels in the user operating environment of powder, ink and filament-based 3D printers

Purpose: This study aims to examine on-site particle concentration levels due to emissions from a wide spectrum of additive manufacturing techniques, including polymer-based material extrusion, metal and polymer-based powder bed fusion, directed energy deposition and ink-based material jetting. Design/methodology/approach: Particle concentrations in the operating environments of users were measured using a combination of particle sizers including the TSI 3910 Nano SMPS (10–420 nm) and the TSI 3330 optical particle sizer (0.3–10 µm). Also, fumes from a MEX printer during printing were directly captured using laser imaging method. Findings: The number and mass concentration of submicron particles emitted from a desktop open-type MEX printer for acrylonitrile-butadiene-styrene and polyvinyl alcohol approached and significantly exceeded the nanoparticle reference limits, respectively. Through laser imaging, fumes were observed to originate from the printer nozzle and from newly deposited layers of the desktop MEX printer. On the other hand, caution should be taken in the pre-processing of metal and polymer powder. Specifically, one to ten micrometers of particles were observed during the sieving, loading and cleaning of powder, with transient mass concentrations ranging between 150 and 9,000 µg/m3 that significantly exceeded the threshold level suggested for indoor air quality. Originality/value: Preliminary investigation into possible exposures to particle emissions from different 3D printing processes was done, which is useful for the sustainable development of the 3D printing industry. In addition, automatic processes that enable “closed powder cycle” or “powder free handling” should be adopted to prevent users from unnecessary particle exposure.Ministry of Education (MOE)Nanyang Technological UniversityAccepted versionThis study was funded by the Singapore Ministry of Education MOE2016- T2-1–063, supported by the National Additive Manufacturing – Innovation Cluster @ NTU (NAMIC@NTU) through Grant No. 2020024 and partially supported by the Start-up Grant by NTU M4082022

Impact response and energy absorption of functionally graded foam under temperature gradient environment

As an effective strategy to enhance the energy absorption and impact resistance capacities of foam materials, density-gradient design has been widely proposed as a viable solution. However, the influence of thermal environment on the impact response and energy absorption of functionally graded foam still remains unclear. In the present paper, an analytical model based on the double shock wave theory is presented to examine the impact performance of density-graded foam rods under temperature gradient environments. A density-graded foam with one end supported and the other end struck by a block of mass is presented to evaluate their impact performances. Both the positive and negative density-gradient models with power-law profiles along the axial direction of the foam rod are considered. The proposed theoretical representation is validated against several previous models, including a finite element (FE) simulation. It is demonstrated that the temperature gradient may lead to transformations in the deformation pattern for the positive and negative density-graded foams. Moreover, the temperature gradient can enlarge the impact resistance property in most cases and can reduce the energy absorption capacity of density-graded foams.Nanyang Technological UniversityAccepted versionThe authors appreciate the support of a start-up grant from Nanyang Technological University

Dynamic analysis of particle emissions from FDM 3D printers through a comparative study of chamber and flow tunnel measurements

Ultrafine particle emissions originating from fused deposition modeling (FDM) three-dimensional (3D) printers have received widespread attention recently. However, the obvious inconsistency and uncertainty in particle emission rates (PERs, #/min) measured by chamber systems still remain, owing to different measurement conditions and calculation models used. Here, a dynamic analysis of the size-resolved PER is conducted through a comparative study of chamber and flow tunnel measurements. Two models to resolve PER from the chamber and a model for flow tunnel measurements were examined. It was found that chamber measurements for different materials underestimated PER by up to an order of magnitude and overestimated particle diameters by up to 2.3 times, while the flow tunnel measurements provided more accurate results. Field measurements of the time-resolved particle size distribution (PSD) in a typical room environment could be predicted well by the flow tunnel measurements, while the chamber measurements could not represent the main PSD characteristics (e.g., particle diameter mode). Secondary aerosols (>30 nm) formed in chambers were not observed in field measurements. Flow tunnel measurements were adopted for the first time as a possible alternative for the study of 3D printer emissions to overcome the disadvantages in chamber methods and as a means to predict exposure levels.Ministry of Education (MOE)National Research Foundation (NRF)Accepted versionThis study was funded by the start-up grant from Nanyang Technological University, Ministry of Education MOE2016- T2-1-063, NAMIC@NTU through Grant No. 2018242, and the National Research Foundation, Prime Minister’s Office, Singapore under its Medium-Sized Centre funding scheme

A CFD-sectional algorithm for population balance equation coupled with multi-dimensional flow dynamics

A novel CFD-sectional algorithm is developed to address the challenges in the existing sectional simulations coupled with multi-dimensional fluid dynamics, including solution of complex sectional coefficients, serious computational burden for lots of coupled partial differential equations, and nonlinear source terms. The sectional coefficients are specified by a numerical quadrature with adaptive integration limits, which proves to be computationally efficient and accurate. The inter-equation coupling is treated by hybrid-segregated procedures and the source term is linearized by the operator splitting method. The CFD-sectional algorithm is validated against a self-preserving solution of particles undergoing Brownian coagulation. The acoustic agglomeration in a standing wave is simulated as a representative case. It has been demonstrated that the predictions regarding the particle size distribution and agglomeration process agree well with the experimental data, which verifies the capability of the developed CFD-sectional algorithm in simulating the spatially inhomogeneous population balance equation coupled with multi-dimensional flows.Ministry of Education (MOE)Accepted versionThis study is supported by the Republic of Singapore's Ministry of Education MOE2016-T2-1-063

Investigation of CFD-PBM simulations based on fixed pivot method : influence of the moment closure

The fixed pivot method can only conserve two moments while other moments suffer from inherent errors caused by internal inconsistency. In this work, we present a comprehensive investigation regarding the influence of moment closure on population balance and hydrodynamics in the coupled CFD-PBM simulations. The CFD-PBM model, which conserves the surface area and volume (second and third moment, i.e. CFD-PBM-SV model), and the number and volume (zeroth and third moment, i.e. CFD-PBM-NV model), has been developed based on a two-fluid model. To assess the accuracy of Sauter Mean Diameter (SMD) with different moment closures, a transient and homogeneous case is first simulated by the single PBM model, which conserves the surface area-volume (i.e. PBM-SV model) and number-volume (i.e. PBM-NV model), respectively. It shows that in comparison with the analytical solution, the SMD predicted by the PBM-SV model shows higher accuracy than the PBM-NV model with identical sectioning resolution, and the PBM-NV model can give satisfactory results only on very fine sectioning grids. A rectangular bubble column is then simulated by the CFD-PBM-SV and CFD-PBM-NV model, respectively. It is found that both models can capture the oscillating bubble plume of the gas-liquid flow inside the column reactor. The flow features predicted by the CFD-PBM-SV model show better agreement with experimental data, in terms of the time-averaged vertical liquid velocity, gas hold-up and plume oscillation period, than the CFD-PBM-NV model. It is speculated that the better performance of the CFD-PBM-SV model is ascribed to more accurate predictions of interfacial forces and momentum transfer between the two phases due to internal consistency of the local SMD compared to the CFD-PBM-NV model.MOE (Min. of Education, S’pore)Accepted versio

Aerosols from speaking can linger in the air for up to nine hours

Airborne transmission of respiratory diseases has been under intense spotlight in the context of coronavirus disease 2019 (COVID-19) where continued resurgence is linked to the relaxation of social interaction measures. To understand the role of speech aerosols in the spread of COVID-19 globally, the lifetime and size distribution of the aerosols are studied through a combination of light scattering observation and aerosol sampling. It was found that aerosols from speaking suspended in stagnant air for up to 9 h with a half-life of 87.2 min. The half-life of the aerosols declined with the increase in air change per hour from 28 to 40 min (1 h-1), 10-14 min (4 h-1), to 4-6 min (9 h-1). The speech aerosols in the size range of about 0.3-2 μm (after dehydration) witnessed the longest lifetime compared to larger aerosols (2-10 μm). These results suggest that speech aerosols have the potential to transmit respiratory viruses across long duration (hours), and long-distance (over social distance) through the airborne route. These findings are important for researchers and engineers to simulate the airborne dispersion of viruses in indoor environments and to design new ventilation systems in the future.Nanyang Technological UniversityAccepted versionThis study was funded by the start-up grant under Nanyang Technological University (04INS000329C160). Shirun Ding would like to acknowledge Nanyang Technological University for funding his Ph.D

Comparative study of combustion and emissions of kerosene (RP-3), kerosene-pentanol blends and diesel in a compression ignition engine

Aviation Piston Engines for small general aviation aircrafts are currently facing a transition from being powered by AVGAS (aviation gasoline) to being powered by heavy fuels (diesel or kerosene). The present study compared the combustion and emission characteristics of diesel, aviation kerosene rocket propellant 3 (RP-3) and RP-3-pentanol blends in a single cylinder compression ignition (CI) engine. Heat release rate, indicated thermal efficiency, ignition delay, combustion duration, and coefficient of variation (COV) of indicated mean effective pressure were experimentally determined to reflect the engine combustion performance. The results demonstrated the feasibility of RP-3 and its mild pentanol blend (20% by volume) in modern CI engines whilst further optimisation of the injection strategy is needed if a higher ratio of pentanol (40% by volume) is used. The discrepancy in terms of combustion and emissions between diesel, RP-3 and its pentanol blends are appreciable, especially for ignition delay, combustion duration and soot emissions. Compared with diesel, RP-3 improved the indicated thermal efficiency by 1.4–12.4%, but pentanol addition decreased that by 1–6.5%. RP-3 and its pentanol blends reduced the soot emissions by nearly an order of magnitude at high engine loads compared with diesel without evident impact on nitrogen oxide (NOx) emissions. Meanwhile, Carbon monoxide (CO) and total hydrocarbon (THC) emissions of RP-3 and its pentanol blends experienced a significant increase at low loads, but CO showed a slight decrease at high loads

The characteristics and formation mechanisms of emissions from thermal decomposition of 3D printer polymer filaments

Ultrafine particles (UFP) and volatile organic compounds (VOC) emitted from fused deposition modelling (FDM) 3D printing have received widespread attention. Here, we characterize the formation mechanisms of emissions from polymer filaments commonly used in FDM 3D printing. The temporal relationship between the amount and species of total VOC (TVOC) at any desired operating thermal condition is obtained through a combination of evolved gas analysis (EGA) and thermogravimetric analysis (TGA) to capture physicochemical reactions, in which the furnace of EGA or TGA closely resembles the heating process of the nozzle in the FDM 3D printer. It is generally observed that emissions initiate at the start of the glass transition process and peak during liquefaction for filaments. Initial increment in emissions during liquefaction and the relatively constant decomposition of products in the liquid phase are two main TVOC formation mechanisms. More importantly, low heating rate has the potential to restrain the formation of carcinogenic monomer, styrene, from ABS. A TVOC measurement method based on weight loss is further proposed and found that TVOC mass yield was 0.03%, 0.21% and 2.14% for PLA, ABS, and PVA, respectively, at 220 °C. Among TVOC, UFP mass accounts for 1% to 5% of TVOC mass depending on the type of filaments used. Also, for the first time, emission of UFP from the nozzle is directly observed through laser imaging.NRF (Natl Research Foundation, S’pore)ASTAR (Agency for Sci., Tech. and Research, S’pore)MOE (Min. of Education, S’pore)Accepted versio

Experimental study of polycyclic aromatic hydrocarbons (PAHs) in n-Heptane laminar diffusion flames from1.0 to 3.0 bar

As PAHs (Polycyclic Aromatic Hydrocarbons) are the main precursor of soot formation during the combustion, the investigation of PAHs formation is essential for the understanding of the soot formation and soot reduction in combustion. In this study, a specially designed burner and the corresponding fueling system was used to stabilize a laminar diffusion flame of n-heptane up to 3.0 bar before it becomes unstable. Using the combination of LII (Laser Induced Incandescence) and LIF (Laser Induced Fluorescence) techniques, the PAHs and soot formation and their distributions in the studied flames were obtained and explained. The results showed that PAHs were almost surrounded by soot and were present in the lower part of the flame. Moreover, the integral soot and PAH intensities exhibited a power law dependence on the pressure, being proportional to pn with n of 1.38 ± 0.32 and 1.49 ± 0.25 respectively under the pressure range of 1.0–3.0 bar