Air pollutant emissions from multi jet fusion, material-jetting, and digital light synthesis commercial 3D printers in a service bureau

Highlights

• Building air exchange rates and localized exhaust important to emissions in a 3D service bureau.

• Projet 5500X exhibited periodic PM spikes (3.8 ± 0.53 $x$ 10⁵ min^{\protect \relax \special {t4ht=−}1}) most likely from fan activity.

• High airborne concentration of solvents found in Multi Jet Fusion ink.

Abstract

As additive manufacturing becomes more prevalent in industrial manufacturing facilities, so does human interaction with these machines. In this study, we characterized the size-resolved concentrations of particulate matter (PM, from $\approx$ 11 to 20,000 nm in size) and volatile organic compounds (VOC) resulting from a material-jetting (Projet 5500X), multi jet fusion (HP MJF 4200), and photopolymerization (Carbon 3D continuous liquid interface production). During all processes, PM concentrations were below the OSHA limits and ultrafine PM ($P{M}_{0.1}$) concentrations were well below levels commonly observed for fused deposition modeling printers. PM and total volatile organic compound (VOC) concentrations during printing were close to background levels for the HP MJF printer and HP MJF post processing station. However, the mean particle size of the powder used (PA 12) was above the upper detection limit of the instrument (20 $\mu m$). Carbon 3D total VOC levels were highest during part post processing. Cyclic PM emissions from the Projet 5500X printer suggested higher PM emissions during cooling fan activity. VOC analysis revealed high concentrations of HP ink solvents (2-Pyrrolidinone and Triethylene glycol), potential exposure to fluorinated compounds and photoresin VOCs during Carbon 3D post-processing, and exposure to photo-resin compounds during Projet 5500X printing. This study indicates that proper exhaust and facility air exchange can limit concentrations of PM and VOCs and therefore particular attention to the building design and ventilation system should be taken into consideration for additive manufacturing facilities to mitigate potential human exposure and associated health risks.

Introduction

Additive manufacturing (AM) has infiltrated hobbyist and industrial markets at a growing pace. Home 3D printing sale revenues increased 31.5% to $610.5 million in 2017 [1]. The AM market as a whole is expected to reach $11 billion by 2022 growing at a rate of 27% from 2016–2022 [2]. With such sustained growth and increased market share of additive manufacturing, occupational and recreational exposure to printer emissions will grow. Technological centers of excellence housing prototype printers or warehouses of manufacturing printers (service bureaus) will become more common with printing research advancements. Air quality research has focused on airborne emissions of fused-deposition modeling (FDM) with little research on industrial printers such as HP’s Multi Jet fusion [3], ExOne’s powder-binder jetting (PBJ) printers [4], 3D Systems’ Projet equipment [5], or Carbon’s continuous liquid interface [6]. While many companies conduct internal assessments of emissions for safety and PPE recommendations, these assessments are often not made publicly available. Therefore, it is important for air quality researchers to understand how various AM techniques might lead to potential air quality concerns and disseminate these data.

Although there are many studies published to date on PM and VOC emissions from FDM-type 3D printers [7], [8], [9], [10], [11], [12], there has been less emphasis non-FDM 3D printer emissions [13], [14]. These include but are not limited to powder binder jetting printers [15], [16], [17], material jetting [18], and stereolithography printers [19]. Väisänen et al. [13] found high dust mass concentrations (0.03–2.72 mg ${\mathrm{m}}^{-3}$) during post-processing of the HP MJF 4200 machine with a DustTrak DRX particle sizer. As the nominal PA 12 powder size is 58 $\mathrm{\mu }\mathrm{m}$ and detection limit 20 $\mathrm{\mu }\mathrm{m}$, results from Väisänen et al. suggest large dust creation in post processing leads to release of larger particles below the nominal diameter, which account for high mass concentrations.

The main air quality concern during the operation of powder-based systems is associated with respirable dust emissions. Airborne particles ¡10 $\mathrm{\mu }\mathrm{m}$ in diameter can penetrate into the respiratory system increasing the potential for health impacts. Airborne powder ¿30 $\mathrm{\mu }\mathrm{m}$ can deposit in the upper head airways through impaction and interactions with nasal cilia and are removed quickest through mucus movement [20]. Based on rat in vivo studies, large particles in the tracheobronchial region have been shown to clear after approximately half a day, while particles in the alveolar region were shown to clear after about 10 days, and particles from secondary macrophage clearing took about 100–200 days [21]. Morrow et al. [22] proposes that inhalation overload of particles of any size has health implications. Therefore, even powder larger than the respirable limit, broadly accepted as 10 $\mathrm{\mu }\mathrm{m}$, may present potential for health impacts after prolonged exposure. Mohajer et al. [15] found that the continuous operation of a ZCorp powder-binder printer led to high PM mass concentrations, with 344 $\frac{\mathrm{\mu }\mathrm{g}}{{\mathrm{m}}^3}$ measured for $P{M}_{2.5}$ and 474 $\frac{\mathrm{\mu }\mathrm{g}}{{\mathrm{m}}^3}$ for $P{M}_{10}$.

3D System’s Projet MJP 5500X is a material-jetting printer that deposits droplets of a photopolymer from piezoelectric printheads to deposit a layer. The layer is cured with UV lamps and the process repeated to build up a part. By jetting from piezoelectric printheads, a wide variety of feedstock materials are possible including gradient-shore hardness materials and wax. During the jetting process, low vapor pressure solvents in the ink can volatilize out leading to human exposure. 3D Systems SDS for Visijet CE-BK (a black elastomer) identifies monofunctional aliphatic urethane acrylate, isobornyl methacrylate, and phenylbis (2,4,6-trimethyl benzoyl)- phosphine oxide as dangerous components with skin irritant, eye irritant, and single-exposure organ toxicity [23]. Material-jetting was found to emit isobornyl acrylate (1325–2076 $\frac{\mathrm{\mu }\mathrm{g}}{{\mathrm{m}}^3}$), 2-furanpropanoic acid (127–164 $\frac{\mathrm{\mu }\mathrm{g}}{{\mathrm{m}}^3}$), and butylated hydroxytoluene (61–113 $\frac{\mathrm{\mu }\mathrm{g}}{{\mathrm{m}}^3}$) [13].

HP’s Multi Jet Fusion 4200 is a powder-based system with a unique thermoplastic fusing system in which inkjet-printed detailing and fusing agents are deposited under careful temperature control and selectively fused under an IR heat lamp [3]. This differs from traditional powder bed fusion processes using a scanning laser power source. In the HP system, powder is spread by a vane system where fluidized powder is brought up from the trolley and spread by a counter rotating roller over the bed.

Carbon’s M2 printer utilizes a subset of vat polymerization (continuous liquid interface production, CLIP) technology [24]. In this method, photoinitators in a tuned photoresin similar to SLA and DLP methods are used to selectively cure the resin. However, CLIP uses an oxygen-permeable window to create a dead zone due to oxygen-inhibited photopolymerization. This dead zone prevents the need to raise and lower the stage allowing continuous production and isotropic parts. SDS information from photoresin used in Preform printers list Methacrylated oligomers and monomers as type 1 and 2 skin and eye irritants and Diphenyl(2,4,6-trimethylbenzoyl) phosphineoxide as a fertility risk causing atrophy of the testes [25]. Human health concerns are mainly in uncured photoresin, for as soon as it is cured the resin can be made bio-compatible [26]. However, due to proprietary blends and complex chemistry making these photoresins, much is unknown as to their consumer risks. Lago et al. reviewed contamination of foodstuffs by photoinitiator migration even in the cured state concluding more research needs to be done before legislation can determine high risk photoiniators [27].

The objective of this work was to determine size-resolved PM and total VOC emissions from commercial 3D printers in a service bureau. A service bureau is defined as a third-party company that provides 3D printed functional parts for a consumer. Specific objectives included isolating the printers from other emissions in the complex service bureau environment through the use of temporary enclosures. An additional objective was to determine speciated VOC concentrations through offline GC–MS analysis to determine estimate operational exposure to specific VOCs of concern. In this study, we analyzed PM and VOC emissions of three different industrial and advanced prototyping machines: HP’s Multi Jet Fusion 4200 [3], Carbon’s M2 printer [6], and 3D System’s Projet MJP 5500X [5] during their normal operation in a service bureau, while semi-isolated from other operations by an enclosure designed for this work. This work presents size-resolved particle concentrations and emissions calculations for one of the printers, as well as total VOC (TVOC) and VOC composition results along with a comparison with current permissible exposure limits (PEL) and occupational health considerations.