Elsevier

Additive Manufacturing

Volume 16, August 2017, Pages 146-152
Additive Manufacturing

Full length article
Fabrication of continuous carbon, glass and Kevlar fibre reinforced polymer composites using additive manufacturing

https://doi.org/10.1016/j.addma.2017.06.004Get rights and content

Abstract

This study evaluates the performance of continuous carbon, Kevlar and glass fibre reinforced composites manufactured using the fused deposition modelling (FDM) additive manufacturing technique. The fibre reinforced nylon composites were fabricated using a Markforged Mark One 3D printing system. The mechanical performance of the composites was evaluated both in tension and flexure. The influence of fibre orientation, fibre type and volume fraction on mechanical properties were also investigated. The results were compared with that of both non-reinforced nylon control specimens, and known material property values from literature. It was demonstrated that of the fibres investigated, those fabricated using carbon fibre yielded the largest increase in mechanical strength per fibre volume. Its tensile strength values were up to 6.3 times higher than those obtained with the non-reinforced nylon polymer. As the carbon and glass fibre volume fraction increased so too did the level of air inclusion in the composite matrix, which impacted on mechanical performance. As a result, a maximum efficiency in tensile strength was observed in glass specimen as fibre content approached 22.5%, with higher fibre contents (up to 33%), yielding only minor increases in strength.

Introduction

Additive Manufacturing (AM) is widely used for the fabrication of polymer components ranging from prototypes to ‘final products’ [1]. Various AM techniques for polymer manufacture have been developed, including; Stereolithography (SLA) applied using photopolymer liquids [2], Selective Laser Sintering (SLS) involves the use of polymer powders [3], while Fused Deposition Modelling (FDM) uses polymer filaments [4]. The latter is the most widely utilised system for polymer AM manufacture due to its relative low cost, low material wastage and ease of use [5]. The FDM process most often utilises a continuous polymer filament as a feedstock material. The polymer filament feeds into an extruder and is heated to a quasi-liquid state, enabling it to pass through a heated extruding orifice where it is fused in-place on a print surface. This extruding apparatus is typically mounted onto an X-Y CNC (Computer Numerical Control) gantry, allowing the printing of complex geometric patterns. Once a layer pattern is complete, the print platform drops down, or extruding orifice rises by the layer thickness to deposit a subsequent layer of material. Through the deposition of successive layers, 3D objects are fabricated [6]. At present thermoplastics are the most frequently utilised feedstock materials for FDM due to their low cost, and low melting temperatures [7]. These include Polycarbonate (PC), Polylactic acid (PLA), Acrylonitrile butadiene styrene (ABS) and Polyamide (PA or Nylon). The FDM technique can result in the formation of porous inner structures in the fabricated component which leads to poor mechanical strength, a further issue can be poor surface finish due to the ‘stair stepping’ effect [8], [9], [10]. These limitations have hampered the wider adoption of 3D printed components for use as final products, leaving prototyping as the primary application.

Attempts have been made to overcome the poor mechanical performance of 3D printed parts with the addition of fibre or particle reinforcement. This has been utilised in the polymer industry to enhance structural strengths in traditional composites, forming what are known as fibre reinforced polymers (FRP) [11]. Zhong et al. for example incorporated chopped glass fibres into ABS polymers using an FDM printing process with the aim of increasing tensile strength [12]. This study demonstrated that interlayer bond strength was increased with increased fibre contents, due to ‘bridging’ of fibres across layers. A further study by Ning et al. also investigated ABS composites but this time with chopped carbon fibre materials demonstrating an increase in both tensile strength and stiffness [13]. Samples reinforced with 7.5 wt.% carbon fibre achieved a 27% increase in tensile strength over pure ABS specimen. Interestingly, it was also reported that fibre contents of 10 wt.% or higher resulted in a decrease in tensile strength. This was attributed to a reduction in fibre-matrix contact, as well as an increase in fibre–fibre contact, as the fibre content increased.

Porosity (air inclusions) has been cited as a major concern for fibre reinforced AM composites. The maximum achievable strength of these composites is limited by the fibre matrix interaction in porous areas [14], [15], [16]. Authors have addressed this issue through the addition of fillers such as expanding microspheres [15], and/or flake/particle reinforcement in place of chopped fibre reinforcement [16]. These approaches have been successful in reducing porosity to below 10%. Several studies including that of the previously mentioned Tekinalp et al., compared the porosity of FDM printed and compression moulded CF reinforced ABS [7]. Porosity levels of up to 20% were obtained for FDM specimens, compared to a negligible porosity in moulded parts incorporating chopped fibre (0.2–0.4 mm). It was interesting to note that the strength difference between the two types of fibre reinforced samples was minor, with 0–90° fibre alignment in FDM samples, versus the random orientation fibres in moulded samples, compensating for some of the loss of strength from porosity. A recent AM study by Ning et al. investigated the effect of print layer height on void content [17], with the smallest layer height of 0.15 mm resulting in the lowest porosity and the highest tensile strength and modulus. The thinner layers were found to facilitate overlap resulting in the filling of air voids by extra material extrusion. It was also reported that higher printing temperatures also resulted in higher porosity, leading to a decrease in tensile properties.

There have been a number of reports on the fabrication of continuous fibre reinforced composites, one example being Namiki et al. [18], which utilised a unique dual extrusion method. This study, in which a print head was custom built, yielded continuous carbon fibre PLA composites with strengths of up to 190 MPa in tensile, and 133 MPa in flexure. Their values were 435% and 316% higher respectively, compared with the PLA only composites. Despite the enhanced tensile performance, of the continuous fibre AM composites, issues have been highlighted with the reinforcing fibre as a stress concentrators. For example, through the formation of cracks between carbon fibres and a PLA matrix [19].

A study by Van der Klift et al. [20] assessed carbon fibre reinforced specimen produced by the Markforged Mark One, in order to obtain details relating to the mechanical performance of the proprietary fibre filament. Tensile specimens were produced and a 9-fold increase in strength was observed. A factor that may have affected the results in this study however, is that the test specimens were cut post-printing in order to remove ‘discontinuities’ and excess matrix material which may influence the strength results. These authors observed large deviations between individual test results, with standard deviations in tensile strength as high as 22%, obtained from a 10-layer thick specimen containing 6 layers of CF reinforcement.

In this study the performance of the Markforged Mark One system is also evaluated for the fabrication of composites with continuous fibre reinforcement. This study involved the fabrication of nylon composites with carbon, Kevlar® and glass fibres (Sourced from Markforged) and the mechanical performance of all three composite types were compared. To date, Markforged materials have been assessed individually, and under differing testing conditions and standards. This study presents a comprehensive tensile and flexural characterisation of all fibres commercially available for the Markforged system under uniform conditions, facilitating their direct comparison. In addition, the influence of fibre volume fraction (VF), fibre placement and fibre orientation, on the mechanical performance of the composite were evaluated.

Section snippets

Materials

Nylon filament was supplied by Markforged, Cambridge, MA, USA, it is their proprietary blend, and has a diameter of 1.75 mm. Prior to use, this polymer was stored in a moisture-sealed Pelican 1430 modified dry box to prevent deterioration of the filament due to moisture absorption during storage [15]. The reinforcing glass fibre (GF), carbon fibre (CF) and Kevlar® fibre (KF) were also supplied by Markforged. While these are referred to as ‘fibres’, these are composed of fibre bundles infused

Conclusions

Continuous glass, carbon and Kevlar® fibres reinforced nylon composites were fabricated by AM. Up to a 6.3 fold and 5-fold enhancement in the tensile and flexural strengths respectively were observed for each of the printed composites relative to that obtained for the nylon control samples. The FDM composites were found to exhibit tensile strengths that were superior to that of aluminium [21]. Comparing the fibre reinforcing investigated in this study it was found that the nylon composite

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