Elsevier

Additive Manufacturing

Volume 28, August 2019, Pages 814-820
Additive Manufacturing

Full Length Article
Effects of variable gravity conditions on additive manufacture by fused filament fabrication using polylactic acid thermoplastic filament

https://doi.org/10.1016/j.addma.2019.06.018Get rights and content

Abstract

The capability to manufacture items in space is an exploration enabling advancement, and will be crucial for sustainable human exploration as we progress beyond Earth orbit. The extrusion based Fused Filament Fabrication (FFF) method using thermoplastics represents a robust and simple methodology applicable to printing parts for both current and future human spaceflight exploration missions. Understanding the performance and behaviour of the FFF process under varying gravity loads is therefore an important knowledge gap that needs to be addressed in order to fully appreciate the characteristics of space manufactured elements. At present, it is not well understood how gravity can influence the characteristics of such elements fabricated in variable gravity environments. In this study, we detail an experiment conducted on a parabolic flight campaign (PFC) wherein we produced a number of FFF polylactic acid (PLA) polymer test articles and compared them to terrestrially fabricated articles. We report on the methodology and the operational parameters used, as well as presenting an analysis of the samples via optical microscopy and tomography. Compressive, tensile and other technical properties are reported herein. A number of explanations are presented to explain the variance in specimens relative to terrestrial reference samples.

Introduction

Additive Manufacturing (AM) has been acknowledged by multiple space agencies and commercial aerospace organisations as an exploration enabling technology, falling under a wider umbrella of what is colloquially known as in-space manufacturing (ISM). With the potential to manufacture parts or mission elements in space, a fundamental obstacle to sustainable space exploration is overcome, and the currently employed logistics paradigm for human space missions is open to a dramatic re-interpretation. To illustrate, in-situ manufacturing of parts could potentially enhance aspects of crew safety, enable new mission capabilities and address maintainability and replacement challenges. On a larger scale, whole surface habitat elements could be constructed on site using only local surface resources and AM processes [1,2].

A variety of approaches can be envisaged to attain ISM and space based AM capability, and there is much activity in achieving this worthwhile objective. Since 2014, extrusion based Fused Filament Fabrication (FFF), a robust and simple approach to 3D printing, has been demonstrated on the ISS under microgravity conditions [3,4]. This 3D printer payload, known as 3DP (3D Printer) and operated by the company Made in Space (MIS), was delivered to the ISS on Space X-5 and is part of a larger, incremental build-up of technology and experience necessary to enable future ISM missions and capabilities [5,6]. Following on from 3DP, the Italian space agency ASI (POP3D – Portable Onboard Printer 3D) printer has also been flown to the station [7]. MIS have since delivered the Additive Manufacturing Facility (AMF) to the ISS as a continuing payload and a commercial service [8].

Undoubtedly, AM with metals will also be an enabling technology for exploration and a number of payloads and experiments are being developed to demonstrate this potential [9], however the continued use case for thermoplastic FFF will remain strong. Additionally, thermoplastics also have an important, hitherto undemonstrated in-orbit characteristic – the ability to be recycled as feedstock and then printed again. The recycling of thermoplastics could highlight the promise of ISM as an exploration enabling development by demonstrating the fabricate-recycle-fabricate loop [10].

Among the most commonly used thermoplastic polymers, particular focus exists on the materials acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA). Both of these polymer families have extrusion melting temperatures low enough to be safely utilised in a controlled environment such as the ISS, while also maintaining their shape at common usage temperatures (i.e. average cabin temperature). It has been shown that the basic tensile strength and elastic modulus of ABS and PLA printed components to be ˜28.5 MPa for ABS and ˜56.6 MPa for PLA (depending on print parameters, see [11]), showing that terrestrially printed ABS/PLA thermoplastics are mechanically robust. PLA generally displays a lower level of shrinkage in printed elements, as well as a lower melting temperature. This temperature leads to lower residual stresses and the shrinkage advantage leads to reduced layer issues (e.g. delamination) and thus a more productive AM process. Low durability at higher temperatures versus ABS is acceptable when considering PLA in the context of crew operations within a cabin where temperature is a comfortable working range.

Another advantage that PLA displays over ABS are gaseous products and aerosols. Terrestrially, FFF can be hazardous owing to byproducts of the thermoplastic heating process in the absence of precautionary measures. Concentrations of nanoparticles and Volatile Organic Compounds (VOC), with examples being aldehydes (such as formaldehyde, which is a Group 1 carcinogen), acetaldehyde, isovaleraldehyde, toluene and ethylbenzene [12,13], have been reported. FFF with ABS also leads to the by production of styrene nanoparticles, which is a group 2B carcinogen according to International Agency for Research on Cancer (IARC). A convergent observation across the research into this issue reveals that PLA based emissions are generally two orders of magnitude lower (4.89 × 108 ea/min for PLA vs 1.61 × 1010 ea/min for ABS). Pyrolysis on PLA produces fewer emissions than other plastics, making it the most desirable material for health and environmentally conscious 3D printing, and an important consideration for on-orbit printing within an enclosed habitat volume [14].

However, while polymer based FFF is widely used terrestrially, a large number of unknowns remain around the fundamental material characteristics of printed parts on orbit. Even terrestrially, while widespread adoption of AM processes is underway, many aspects relating to batch to batch uniformity and standardisation remain. The aforementioned 3DP, POP3D and AMF have been efforts to better understand the fundamentals and underlying behaviour of 3d printing via FFF in the microgravity environment of the ISS (as well as providing a commercial service for the ISS).

In this study, we produced a number of test article specimens under varying gravity load during a parabolic flight. We report on the manufacture of test articles under a parabolic flight regime using the FFF process, specifically discussing the tensile and compressive properties of produced samples, as well as tomography and optical microscopy. It is our aim that this work may allow for a greater foundational understanding of the FFF process and how it is affected by the varying gravity environment, and potentially form an input into planned future ISM activities using this methodology for fabrication.

Section snippets

Methodology

For this experiment, a commercial-off-the-shelf (COTS) Makerbot Replicator 2 was used – such a system can utilise ABS and PLA feedstock interchangeably. Filament of red-dyed 1.75 mm PLA was used as the feedstock material for all samples. Since thermoplastics are known to experience degradation owing to a hygroscopic nature, we ensured that only a newly opened spool was used as feedstock for the printer. Between the loading of the fresh spool and the PFC, 14 days elapsed. In this time the

Results and discussion

Three experimental runs were conducted corresponding to the three flight opportunities within the campaign. The initial flight proceeded nominally, with only a small irregularity of some buildup of thermoplastic material near the nozzle during the preheat stage. This was quickly cleared away and did not affect the produced sample however. The printing continued through the 0 g and 2 g phases. For the second flight, an attempt was made to limit the printing to only the 1 g and 0 g phases,

Conclusion

We present the results of operating a FFF 3d printing system during the course of a PFC. Our primary research question was to understand the effect of varying the gravity load on the printer during flight to see the effect on specimens fabricated by the printer during the flight. Specimens produced during the flights were then mechanically, optically and structurally analysed and compared to ground reference specimens fabricated from the same setup. It is likely that significant impacts to a

Conflict of interest

None.

Acknowledgments

This work was carried out in the framework of the Spaceship EAC initiative in close collaboration with DLR colleagues. In addition, the authors would like to thank Ana Brandão and Ugo Lafont for their assistance with measurements carried out at ESTEC.

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