Additive Manufacturing of Carbon Fiber Reinforced Nylon

All the specimens used within this study were produced on a Markforged Mark Two® equipment with standard parameters from carbon fiber-reinforced tough nylon, commercially known as MarkForged Onyx. The maximum size of the print volume of this printer was 320 × 132 × 154 mm. The Onyx material is tough nylon pre-impregnated with chopped microcarbon fibers in the filament form, combining the toughness of nylon with the thermal properties of carbon [25 (link),26 ]. Various infill strategies can be used with FFF as depicted in Figure 1 [26 ,27 ]. For 100% dense parts, a rectangular infill is generally preferred, and the deposition orientation is varied from layer to layer, whereas for lower densities, honeycomb or triangular infills can be used for weight reduction. This is probably due to the fact that a rectangular pattern allows an infill density of 100% because it does not self-intersect inside the layer [28 ]. The nozzles used in Mark Two® are shown in Figure 2. One of the nozzles is used to print plastic or Onyx fiber whereas the other is used for continuous fiber replacement [29 ]. The fiber nozzle is different from usual filament extrusion heads due to its cutting mechanism for cutting the fiber.
Firstly, the benchmark specimens were built to test the capability of dimensional accuracy and producing proper geometrical features. In order to understand the minimum wall thickness achievable with FFF of Onyx material, as well as other geometrical limitations, a benchmark geometry, which is well known in the AM of metals, was used as shown in Figure 3a [30 ]. There were many features on this benchmark ranging from sharp corners to thin bosses, holes, inclined surfaces, etc. It was manufactured with a 50% triangular infill strategy for maximum dimensional/geometrical accuracy as shown in Figure 3b. On the second benchmark (see Figure 4), walls with thicknesses of 0.3–3.0 mm were produced with a height and width of 12 mm and 100 mm, respectively. Moreover, the stair effect was studied on inclined walls with different inclination angles (5–35 degrees).
Tensile specimens were manufactured from Onyx and tough nylon material in addition to continuous carbon-fiber-reinforced nylon. A total of 30 specimens were produced under six different configurations as shown in Table 1. The first specimens (E_Nylon_R_100_XY) were built as lying specimens from tough nylon only without any reinforcement at 100% density. Moreover, specimens were built in two directions, either lying (XY plane) (A_Onyx_R_100_XY) or standing on their long side (XZ plane) (B_Onyx_R_100_XZ). The other build direction could not be tested due to the maximum build volume of the equipment. The fabricated specimens on the print bed are shown in Figure 5. The use of a brim, the surrounding peripheral deposition around the specimens, was necessary as an anchor of print bed, which is especially critical for parts tending to warp. Brims were used in producing tensile specimens to increase the area of the first layers as a precaution to deformation. In addition to the build direction, the density effect was also tested at 75% (C_Onyx_T_75_XY) and 50% (D_Onyx_T_50_XY) infill density values with lying specimens. Lastly, lying tough nylon specimens (F_Nylon_CF_R_100_XY) were produced by concentric reinforcement of continuous carbon fiber.
During tensile testing, the EN ISO 527-4 standard entitled “Determination of tensile properties of plastics Part 4: Test conditions for isotropic and orthotropic fiber-reinforced plastic composites” was used. The tensile test equipment was a universal Zwick-Roell equipment with a loading capacity of 250 kN. After the tensile testing was complete, the broken specimens were investigated by optical microscopy and scanning electron microscopy. For the hardness testing, a hardness tester from Bareiss Digi Test was used to measure Shore D hardness as per the TS EN ISO 868 standard with a contact pressure force of 5100 g.