Contents lists available at ScienceDirect Additive Manufacturing journal homepage: www.elsevier.com/locate/addma Full Length Article Two coatings that enhance mechanical properties of fused filament- fabricated carbon-fiber reinforced composites S. Barone, P. Neri, S. Orsi, A. Paoli, A.V. Razionale, F. Tamburrino* University of Pisa, Department of Civil and Industrial Engineering, Largo Lucio Lazzarino, 1, 56126 Pisa, Italy ARTICLE INFO Keywords: Material extrusion Fused filament fabrication Short fiber-reinforced polymers Moisture absorption Mechanical properties enhancement ABSTRACT Moisture absorption degrades the mechanical properties of polymeric parts that are 3D-printed by fused filament fabrication (FFF). This limitation is particularly significant for short fiber-reinforced polymers because the mechanical enhancement obtained by the fiber reinforcement can be compromised by the plasticizing effect introduced by water absorption. Therefore, the present work investigates the effects of two different coatings, a UV cured acrylate resin and an acrylic varnish, on the moisture absorption of FFF 3D-printed samples consisting of polyamide reinforced by short carbon fibers. Water content (CI) and open porosity (OP) were estimated through water absorption tests in distilled water for 2, 24, and 168 h, and after reconditioning. The coating effects were evaluated by conducting tensile tests to compare the Young’s modulus, yield stress, and ultimate stress of the coated and uncoated specimens. The results demonstrated a significant reduction of CI and OP with both the acrylic and UV resin coatings, as well as considerable enhancements of these samples’ mechanical properties. Stress-strain curves evidenced a strain reduction after water immersion, which can be ascribed to a greater stability against different moisture conditions. These findings indicate the significant potential of the proposed coating processes to extend the use of FFF 3D-printed composite materials to a broader range of applications. 1. Introduction Fused filament fabrication (FFF) is one of the most popular additive manufacturing techniques because of its potential for fabricating com- plex shapes, at low cost and in a wide range of materials [1]. FFF is based on the extrusion of a thermoplastic polymer by a temperature- controlled head with a nozzle. The extrusion follows a raster pattern and the process is repeated layer-by-layer to create complex shapes, in a process that significantly improves design flexibility with respect to traditional manufacturing technologies (e.g., casting and machining) and with minor material waste. When a new layer is extruded onto the previous one, the material is in a semi-molten state and its surface re- melts the previous layer, creating a polymer bond. The most common materials used in this type of process are amorphous or semi-crystalline thermoplastic filaments, including acrylonitrile butadiene styrene, polycarbonate, polylactide, polyamide, or blends of different thermo- plastic materials [2,3]. The manufactured parts exhibit poor mechan- ical properties, which significantly limits their use in industrial appli- cations [4]. Fiber-based reinforcement has been a focus of research aimed at enhancing the mechanical properties of 3D printed parts with polymeric matrices [5]. Although continuous fiber composites may offer better mechanical performance [6,7], their processing is based on complex and non-robust procedures [2,8]. Short fiber-reinforced poly- mers are the most straightforward approach to fabricating low-cost composite parts with improved mechanical properties [9], and pre- blended materials obtained by adding discontinuous fibers to the polymeric matrix have also been intensely investigated as a suitable alternative to multi-head printers with complex and costly designs. Carbon, glass, kevlar, and natural fibers (e.g., wood, jute) are the most commonly used fibers for increasing component strength [4,10–16]. However, compared to pure polymer 3D printed samples, short fiber- reinforced polymers contain a higher percentage of unintentional voids, which can be attributed to the presence of fibers in the filament [14]. Process parameters, such as layer thickness, nozzle temperature, raster angle, and infill speed, strongly affect the mechanical properties of these materials [17]. Another important parameter is the hygro- scopicity of the polymeric matrix and the interface between the fibers and matrix (i.e., moisture sensitivity), which is a crucial consideration for predicting and optimizing mechanical performance during the de- sign process. Incompatibility issues can cause delamination at the fiber- matrix interface, and moisture absorption is thus a variable that https://doi.org/10.1016/j.addma.2020.101105 Received 2 September 2019; Received in revised form 20 December 2019; Accepted 30 January 2020 ⁎ Corresponding author. E-mail address: francesco.tamburrino@ing.unipi.it (F. Tamburrino). Additive Manufacturing 32 (2020) 101105 Available online 31 January 2020 2214-8604/ © 2020 Elsevier B.V. All rights reserved. T