1. Introduction
Additive manufacturing (i.e., 3D printing) is constantly evolving due to open source technologies [
1,
2,
3] and the possibility of producing complex geometries with lower costs, faster production times, and less waste [
4,
5,
6] than traditionally manufactured parts [
7]. Moreover, with the increase in available materials, 3D printing has found applications in the aerospace and architectural industries (e.g., creating complex lightweight and structural models) [
8,
9], art fields [
10,
11], and medical fields (e.g., printing tissues and organs) [
12,
13]. However, most plastic 3D printed products are still used primarily as conceptual prototypes rather than functional components because of the poor mechanical and functional properties of the neat polymer used in 3D printing. The development of polymer composites solves these problems [
14]. The incorporation of fibre and nanomaterial reinforcements into polymers allows the manufacture of polymer matrix composites, which are characterized by high mechanical performance and excellent functionality [
14,
15]. One process that has been extensively utilized in the fabrication of polymeric 3D printed parts is fused deposition modelling (FDM), which was developed by Stratasys Inc. In 2009, Stratasys’s FDM printing patent expired, opening up the market for low-cost FDM 3D printers. For non-Stratasys 3D printers, this process is usually referred to as fused filament fabrication (FFF). In FFF, a thin filament wire is extruded through a heated nozzle (
Figure 1). The FFF head moves in the x and y directions, whereas the platform moves in the z direction. The part is manufactured by the sequential build-up of these layered depositions, during which each new layer fuses with material that has already been deposited. The final strength, quality, cost and production time of the parts fabricated by FFF are influenced by some process parameters, such as layer thickness, infill pattern, extruder uniformity and/or build-bed temperature, and by the presence of reinforcing materials (e.g., carbon fibres, glass fibres [
16]).
Some researchers have correlated the build orientation (
Figure 2b) to the mechanical properties of the part [
17,
18,
19,
20,
21,
22]. The variations in quality of the melt between adjacent filaments results in a degradation in mechanical properties (tensile strength, compressive strength, flexural strength, hardness, and elastic modulus), especially for parts tested perpendicular to the direction of layer construction. In addition to the orientation of the part on the building platform, the mechanical properties of FFF parts are also significantly influenced by the process parameters of inherent layering (
Figure 2a,c), such as the layer thickness, raster angle, raster width, infill pattern and air gap [
17,
22,
23,
24,
25]. Ahn et al. [
23] determined that both the air gap and the raster orientation had significant effects on the resulting tensile strength, whereas these factors did not affect the compressive strength. A similar study was carried out by Sood et al. [
17] with varying factors of layer thickness, build orientation, raster angle, raster width, and air gap: the tested factors influenced the mesostructural configuration of the built part and the bonding and distortion within the part. Lužanin et al. [
26] studied the effects of layer thickness, deposition angle and infill percentage on the maximum flexural force in FFF specimens made of polylactic acid (PLA) and concluded that layer thickness has the maximum effect on the flexural strength followed by the interaction between the deposition angle and the infill percentage. Despite its advantages over conventional manufacturing processes, FFF parts often exhibit low mechanical properties. One of the possible methods to improve the strength of FFF parts is adding reinforcing materials, such as carbon fibres, into pure thermoplastic matrix materials to form carbon fibre-reinforced thermoplastic composites [
27,
28,
29]. Several studies have been conducted related to FFF and carbon fibre (CF) composites. A list of various studies of FFF with chopped fibre-reinforced thermoplastics is given in
Table 1.
Love et al. [
30] showed that filaments made from CFs and acrylonitrile butadiene styrene (ABS) polymer significantly increase the strength and stiffness of the final parts: the tensile strength and stiffness of the composite sample were 70.69 MPa and 8.91 GPa, respectively, whereas these values for the pure ABS sample were 29.31 MPa and 2.05 GPa, respectively. They also demonstrated that the addition of CFs decreased the distortion of the printed ABS/CF, which was attributed to the 124% increase in thermal conductivity compared to unfilled ABS. Ning et al. [
31] investigated the material properties of ABS polymer matrices with different CF contents. They concluded that compared with pure ABS specimens, adding CFS into ABS could increase the tensile strength and Young’s modulus but may decrease the toughness, yield strength and ductility. Porosity became the most severe in the specimens with a 10 wt% carbon fibre content. Ivey et al. [
32] analysed specimens produced using a commercial polylactic acid (PLA) filament and a PLA filament reinforced with short-carbon fibres (PLA/CF). The tensile properties of the PLA and PLA/CF filaments showed that the addition of carbon fibres to the PLA filament led to a significant increase in the elastic modulus of the FFF samples. The fracture properties (stress intensity factor and energy release rate) of PLA and its short CF reinforced composites have been studied by Papon and Haque [
33]. Different CF concentrations were printed with two bead lay-up orientations using PLA and CF/PLA composite filaments. The most critical factors for the fracture toughness seem to be the bead layup sequence, fiber pullout, interfacial de-bonding, and void formation. Higher fiber contents did not show improvement in fracture toughness due to higher intra-bead voids, microcracks, and poor interfacial bonding. Yasa [
35] pointed out that the build orientation has a significant influence of carbon-reinforced tough nylon. The impact toughness of specimens built vertically was reduced by 90% in comparison to other directions where the impact was not received in between deposited layers. In 2014, MarkForged© developed the first continuous carbon fibre composite 3D printer. Printed samples generated with MarkForged© printers have been characterized in previous studies [
36,
37,
38,
39]. A summary of some studies focused on the FFF of continuous fibre-reinforced polymers is given in
Table 2. These studies observed some discontinuities in the construction of samples: the carbon fibres were not completely continuous. Experiments showed that discontinuities in the fibres led to premature failure in areas where the fibres were absent, severely reducing the tensile strength of the sample.
This study investigated the performance of parts produced with an FFF 3D printer from chopped carbon fibre-reinforced nylon filaments (nylon-carbon). This filament is composed of nylon 612 with 20% carbon fibres. Nylon 612 (Polyamide 612) gives high resistance to the filament (high impact strength; good resistance to greases, oils, fuels, hydraulic fluids, water, alkalis and saline; good stress cracking resistance; low coefficients of sliding friction; high abrasion resistance; and high tensile and flexural strength [
40]), a relatively low water absorption and a high dimensional stability. The CFs add stability and rigidity, which makes the parts less likely to warp than standard nylon. Due to these characteristics, this filament is ideal for engineering parts, custom end-use production parts, functional prototyping and testing, structural parts, jigs, fixtures, and other tooling. Although the extrusion of preimpregnated fibres does not allow changes in the fibre volume fraction, this approach eliminates the problems associated with poor fibre/matrix interfaces if the impregnation is good and if adequate process parameters are selected. However, the high values of mechanical properties (e.g., tensile modulus of 500–8000 MPa, tensile strength of 54–110 MPa, and hardness of 110 MPa) found in the material datasheets from various manufacturers do not specify the construction conditions, i.e., orientation and filling. This is because some data is obtained by building specimens with other technologies, such as injection molding. In the last year alone, filament manufacturers have been trying to provide mechanical test data by making the specimens using FFF technology. Therefore, the purpose of this study is to analyse the actual mechanical characteristics of a nylon-carbon filament using a low-cost printer.